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Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts Nan Zhang,†,‡ Min-Quan Yang,†,‡ Siqi Liu,†,‡ Yugang Sun,*,§ and Yi-Jun Xu*,†,‡ †

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, P.R. China ‡ College of Chemistry, New Campus, Fuzhou University, Fuzhou 350108, P.R. China § Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States

Downloaded by FLORIDA ATLANTIC UNIV on September 3, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.chemrev.5b00267

S Supporting Information *

7.6. System-Level Optimization 8. Charge Carrier Dynamics in the Graphene-Based Composite Photocatalysts 9. Photostability of Graphene in the GrapheneBased Composite Photocatalysts 10. Comparison between Graphene-Based and Other Carbon Allotropes-Based Composite Photocatalysts 11. Potential Application of Graphene-Based Composite Photocatalysts 11.1. Photocatalytic “Nonselective” Processes 11.1.1. Elimination of Pollutants 11.1.2. Disinfection 11.1.3. Water Splitting 11.2. Photocatalytic “Selective” Transformations 11.2.1. CO2 Reduction 11.2.2. Nitroaromatics Reduction 11.2.3. Alcohols Oxidation 11.2.4. Other Reactions 12. Strategies for Utilizing Graphene-Based Composite Photocatalysts toward Practical Application 12.1. Graphene-Based Composite Film Materials 12.2. Other Possible Development Strategies 12.2.1. Graphene-Based Composite Ceramic Materials 12.2.2. Graphene-Based Composite Concrete Materials 12.2.3. Graphene-Based Composite Plastic Materials 12.2.4. Photocatalytic Reactor Design 13. Concluding Remarks and Future Outlook Associated Content Supporting Information Author Information Corresponding Authors Notes Biographies Acknowledgments Abbreviations and Acronyms References

CONTENTS 1. 2. 3. 4.

Introduction Category of Different Types of Graphene Synthesis of Graphene Remark on the Classification of Graphene-Based Composite Photocatalysts 5. Synthesis and Category of Graphene-Based Composite Photocatalysts 5.1. Basic Preparation Methods 5.1.1. In Situ Synthesis Procedure 5.1.2. Ex Situ Synthesis Procedure 5.2. Types of Graphene-Based Composite Photocatalysts 5.2.1. Graphene-Organics Photocatalysts 5.2.2. Graphene-Semiconductor Photocatalysts 5.2.3. Graphene-Metal Photocatalysts 5.2.4. Graphene Oxide or Graphene Photocatalysts 6. Fundamental Roles of Graphene in the Graphene-Based Composite Photocatalysts 6.1. Photoelectron Mediator and Acceptor 6.2. Enhancing Adsorption Capacity 6.3. Tuning Light Absorption Range and Intensity 6.4. Photothermal Effect 6.5. Macromolecular Photosensitizer 7. Key Factors on Enhancing the Photoactivity of Graphene-Based Composites 7.1. Defect Density of Graphene 7.2. Chemical Modification of Graphene 7.3. Dimensionality Effect 7.4. Interfacial Contact Effect 7.5. Interfacial Junction Effect © XXXX American Chemical Society

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Received: May 4, 2015

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DOI: 10.1021/acs.chemrev.5b00267 Chem. Rev. XXXX, XXX, XXX−XXX


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1. INTRODUCTION The efficient and cost-effective direct conversion of solar energy to chemical energy and solar fuels has been considered as one of the most perspective strategies to solve the environment and energy problems in the future. Photocatalysis, in which the inexhaustibly abundant and clean solar energy can be harnessed as viable technologies, offers us a promising avenue with the tenet of sustainable chemistry toward solar energy conversion. The massive research interest was ignited by the seminal report on photoelectrochemical water splitting to produce H2 over TiO2 electrode in 1972.1 Since then, increasing research attention has been paid to the development of novel efficient photocatalysts and the exploration of different approaches to improve the performance of semiconductor-based artificial photocatalytic redox processes. The successful isolation of a single layer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb crystal structure, graphene, in 20042 triggered ripples of excitement in the scientific and technological communities. Suddenly then, the graphene-driven “gold rush” has been prospering in various areas due to the unique and outstanding structural, optical, and electronic properties of graphene.3−15 Considering the integrative unique properties of graphene, including the superior electrical conductivity and mobility, theoretically high specific surface area, excellent optical transmittance, and high chemical stability, it is not surprising at all to see the entry of graphene into the field of photocatalysis.16−21 The past several years have witnessed an explosive interest in constructing a myriad of composites based on the versatile platform of graphene. The introduction of graphene into the matrix of traditional semiconductors mainly aims to improve the photocatalytic performance of semiconductors via harnessing the cocatalyst role of graphene to boost the separation and transfer of charge carriers photogenerated from semiconductors, which is the critical step for photocatalytic reactions.22−33 The high importance and tremendous interests in the fields of photocatalysis and graphene can be clearly exemplified by a simple search of the ISI Web of Knowledge database using the terms “photocatal*” and “graphene” as the topic keywords, respectively. It can be seen from Figure 1 that, over the past two decades, the number of publications on photocatalysis has exhibited a significant growth rate, and more than 1000 publications on photocatalysis per year can be found from the

year 2003. Since 2005, the year after the successful isolation of graphene,2 the increase in papers on graphene-related research has maintained a marvelous rate. With regard to the combined topic of these two areas, an exponential growth in the number of publications is observed from 2009 when graphene was applied to the photocatalytic applications16 (see the inset of Figure 1), suggesting the extreme significance and attraction of this subject. Notably, since 2010, shortly after the first report on graphene-based photocatalysts, the increased rate of publications on photocatalysis is accelerated more significantly. Obviously, graphene provides a new growth point for the development of new photocatalytic materials, rendering the promising scope of becoming a new family of next-generation photocatalytic composite materials for harvesting solar energy. On the other hand, we should learn more about the development status of the “graphene&photocatalysis” research field behind these statistics. That is, since 2009, when graphene was first used as the cocatalyst to improve the photoactivity of semiconductor TiO2,16 the easily obtainable “low-hanging fruits” on the “graphene-photocatalysis tree” have been picked. Thus far, the number of the papers on the “graphene&photocatalysis” topic has been over 1000. However, the exponential increase in publications on graphene-based photocatalysts has not been well matched by the increase in our knowledge regarding how to make better use of the remarkable properties of graphene in designing more efficient graphene-based composite photocatalysts and thus advancing the sufficient realization of graphene’s potential in constructing highperformance graphene-based photocatalysts for practical applications in solar energy conversion.22 After passing the zenith of praise or hype on graphene, it should be increasingly rational to be conscious of the high importance of focusing on how to sufficiently unleash the fascinating properties of graphene, particularly the electrical conductivity, in graphenebased composites, aiming to improve their photocatalytic performance more effectively.22 The “high-hanging fruits” are still waiting for us to reach, which not only needs more effort and time, but also obliges the communities to get a whole picture of the current status as well as the future development direction of this research field. As such, we think it is the ripe and right time to present a comprehensive, systematic, and up-to-date review on graphenebased photocatalysis from a whole scenario of viewpoint, considering the fast growth of literature reports in this field and that the development of this field is likely confronted with a bottleneck period. Although there have been some reviews and book chapters on this topic,22−37 they only focus on summarizing the preparation methods and/or some selected photocatalytic reactions of graphene-based composites. None of the previous reviews and book chapters22−37 have covered all the reported photoredox types over the graphene-based composite photocatalysts, the elaborate understanding of the fundamental roles of graphene in promoting the photocatalytic performance and, particularly, charge carrier dynamics in the graphene-based composites, the comparison between graphenebased and other carbon allotropes-based composite photocatalysts, the existing problems and challenges in this field, and how and what we should rationally consider for applying the graphene-based composite photocatalysts to practical applications in solar energy conversion. Thus far, graphene-based composite photocatalysts have been utilized for various redox reactions, including photocatalytic degradation of pollutants and bacteria, water splitting,

Figure 1. Number of yearly publications with topic keywords of “photocatal*” and “graphene” since 1990; the inset shows the number of papers published per year on the combined “photocatal*&graphene” subject since 2009. B



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very popular and familiar to the scientific and technological communities. However, it has been used loosely in the literature.22−38 The oversimplified and vague naming of “graphene” with regard to graphene-based materials under the umbrella of “graphene” inevitably results in the misunderstanding of differences in physicochemical properties for different kinds of graphene among researchers, particularly who are not intimately involved in graphene-related research. The issue becomes more prominent in the community of graphene-based photocatalysis, for which most of the graphene is derived from the reduction of graphene oxide (GO) that is a widely used precursor of graphene in the synthesis of graphenebased composite photocatalysts.22−37 Notably, the abundance of oxygenated functional groups endows GO with the flexible, easily accessible wet-chemistry processability.39 However, the oxygenated moieties inevitably cause considerable disruption of the electronic structure of graphene via breaking the 2D πconjugation of the original graphene sheet. Although most of the oxygenated functional groups can be removed by reduction, large defect populations still remain,22−24,26−31,33,40 which significantly change the electronic (e.g., electrical conductivity and energy gap) and optical (e.g., transmittance and photoluminescence) properties of GO-derived graphene.41−46 The so-called unique, excellent electronic, optical, and physicochemical properties of graphene are intrinsically associated with the single-layer and defect-free 2D graphene sheet, instead of the GO-derived graphene.23 Furthermore, during the wet-chemistry synthesis of graphene-based composites, GO-derived graphene often suffers from irreversible aggregation, which further results in a striking change in its properties, including the decrease of electrical conductivity, surface area, and optical transparency of graphene.14,47,48 Thus, there is a distinct gap of significant differences between the physicochemical properties of the pristine graphene at an ideal state and the graphene derived from different precursors; for example, reduced graphene oxide (RGO) is often called “graphene” because of the lack of unified classification in the graphene-related photocatalysis. However, in the literature, researchers are inclined to simply attribute the activity enhancement of graphene-based composite photocatalysts to “the unique properties of graphene, particularly its outstanding electrical conductivity”.22−37 Clearly, this vague but “panacea” expression does not benefit the readership (i) with an objective and rational viewpoint on the role of graphene in promoting the activity of graphene-based composite photocatalysts, and (ii) with focusing on how to better transform the power of different kinds of graphene into the graphene-based composite photocatalysts. It is worth noting that the properties of graphene in the composites can change dramatically when it interacts with the surrounding environment or its structure and morphology are changed.23,49,50 Nowadays, the various available chemical and physical techniques have made “graphene” beyond the pristine graphene as defined originally. The real world of graphene materials is not the description of just simple and well-defined molecules, but materials with complex and variable structures that accordingly bestow diverse physicochemical properties upon graphene materials.5 Given such a situation, in this graphene-based photocatalysis review, we want to emphasize to the readers that the use of the term “graphene” is intended to be a generalized concept rather than the rigorously defined pristine graphene. We should not

reduction of CO2, and selective organic transformations. It can be seen from Figure S1 (Supporting Information) that, thus far, the photocatalytic degradation of pollutants and bacteria and photocatalytic water splitting have received the majority of the research interest in graphene-based composite photocatalysts (accounting for 78.6% and 15.5% of all the photocatalytic applications of graphene-based composites, respectively), whereas the photocatalytic reduction of CO2 and photocatalytic selective organic transformations have been paid less attention, whose proportions are 4.2% and 1.7%, respectively. The previous reviews and book chapters often neglect or give little attention to the latter two types of important photocatalytic applications,26−37 which are of great significance for widening the application scope of graphene-based composites in solar energy conversion.22−25 In addition to the above situations, other fundamental and critical issues, including the normative classification of graphene materials used in the photocatalysts, the systematic summary of the key factors on enhancing the photoactivity of graphenebased composites, the photostability of graphene during the photocatalytic processes, and the strategies for utilizing graphene-based composite photocatalysts for practical application, have not been discussed comprehensively in an individual review. On the basis of the progress achieved by various research groups in this field, since the year of 2009, we herein present such an up-to-date review on this hot-spot research topic from a whole scenario viewpoint. With the purpose of presenting a panorama of the current status of graphene-based photocatalysis, all the aspects of graphene-based photocatalysis will be attempted to be elaborated systematically, including the category and synthesis of different types of graphene, the classification of graphene-based composite photocatalysts, their preparation methods, the multifaced roles of graphene played in the photocatalysis, the key factors on enhancing the photoactivity of composites, the charge carrier dynamics in the graphene-based composite photocatalysts, the photostability of graphene in the composite photocatalysts, the comparison between graphene-based and other carbon allotropes-based composite photocatalysts, and the potential photocatalytic applications of graphene-based composites. Furthermore, the possible future development trend of graphene-based photocatalysts for practical applications including the proposed possible strategies will also be summarized based on the pioneering works in this direction. Finally, this review will be concluded with highlighting the important opportunities and challenges in this booming research area. It is kindly anticipated that this review would be able to act both as an objective, scientific interest introduction for newcomers in this field and as a reference for experienced researchers at the forefront of this research area. In addition, we do hope this review could make its contribution to the design and construction of the next-generation artificial photosynthesis systems with high performance based on graphene-based composites, and to stimulating further developments and sparkling ideas in related research areas, particularly the fabrication and applications of functional materials related to graphene.

2. CATEGORY OF DIFFERENT TYPES OF GRAPHENE In view of the fact that the graphene-driven “gold rush” has been flourishing in various areas since the isolation of a single-layer pristine graphene sheet in 2004,2 the term “graphene” seems C


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Figure 2. Schematic illustration of the fabrication of GO-derived graphene (often called RGO) through oxidation−exfoliation−reduction of graphite.

give excessive praise to “graphene” because it is just a promising material, not a miracle one.22,49 The delivery of the promising “potential” of graphene into practical applications of photocatalysis is a long-term story.22,49,51 In order to help the readers to discriminate different types of graphene in graphene-based composite photocatalysts, we roughly name each specific graphene with a corresponding acronym depending on which type of graphene precursor is used to fabricate the graphenebased composite photocatalysts. Before moving to this categorization of different kinds of graphene for the graphene-based composite photocatalysts (section 4), we give a concise summary of the different approaches to graphene synthesis in the following section. The characteristics, including the advantages and disadvantages of each type of graphene obtained from different synthetic approaches, are briefly recapped, from which it is apparent that, in order to better review the state of graphene-based composite photocatalysts, a rough but unified criterion for separating different types of graphene needs to be clarified.

composite photocatalysts is nearly always prepared in the liquid phase, therefore, this review mainly focuses on the description of the solution-based methods for synthesis of graphene (see the Supporting Information for the details of the typical nonsolution-based methods for graphene synthesis). Thus far, oxidation−exfoliation−reduction of graphite has been the most promising solution-based method for bulk production of graphene in various fields (e.g., widely used in the synthesis of graphene-based composite photocatalysts), which can be dated back to the 19th century.94,95 The typical procedure is depicted in Figure 2. Graphite is first oxidized to a water dispersible intermediary, i.e., graphite oxide, in the presence of strong oxidants and intercalating compounds (e.g., H2SO4, HNO3, KMnO4, KClO3, and NaClO2).7,82−84,94−97 Graphite oxide can be exfoliated by sonication, thermal treatment, microwave, or arc discharge to give single- or fewlayer hydrophilic graphene oxide (GO) with abundant oxygencontaining functional groups (i.e., hydroxyl and epoxy groups on the basal plane and carboxyl, carbonyl, phenol, lactone, and quinone at the sheet edges).81,98 Subsequently, GO can be reduced by chemical, thermal, electrochemical, sonochemical, microwave, or photocatalytic methods.99−106 The graphene materials through this procedure are often referred to as reduced graphene oxide (RGO). Notably, the strong oxidation process unavoidably produces structural defects in the graphene materials and the reduction of GO is generally not complete, which is concomitant with a remarkable change of the electronic, optical, mechanical, and electrochemical properties of RGO as compared to the pristine graphene.41−45 In addition, the RGO obtained by reduction of GO prepared according to the Staudenmaier method may contain a certain amount of metallic impurities (e.g., Fe, Cu, Ni, and Cu) which are derived from the precursor graphite.107,108 Although such a situation possibly limits the direct application of RGO in electrically active materials and devices, this approach for graphene production is quite promising and advantageous, considering its low cost, facile preparation, massive scalability, and flexible solution processability of GO. Therefore, it is amenably accessible to produce RGO in bulk with wide utilization in many variable architecture forms, catering to the need for potential applications.4,109 Liquid exfoliation of graphite is an alternative solution-based technique used for graphene synthesis (Figure 3).85−87 Being different from the oxidation−exfoliation−reduction approach, the liquid exfoliation method includes neither the strong oxidation of graphite nor the subsequent reduction process. It often involves sonication of graphite in the presence of a specific organic solvent which has a surface energy close to that of graphite, such as N-methyl-pyrrolidone (NMP), N,Ndimethylformamide (DMF), γ-butyrolactone (GBL), and 1,3dimethyl-2-imidazolidinone (DMEU).86,87,110 The interfacial energy between the solvent and graphene as well as the van der

3. SYNTHESIS OF GRAPHENE The past few years have witnessed a plethora of fabrication routes for graphene4,52,53 after the pioneering “Scotch Tape” method.2 Aiming to prepare pristine graphene with high quality, numerous non-solution production methods have been developed, such as epitaxial growth on SiC,3,54−62 chemical vapor deposition (CVD),3,55,63−72 arc discharge,73−76 segregation growth,77 unzipping carbon nanotubes by plasma etching,78 etc.79−81 The graphene materials prepared via these non-solution-based approaches often feature excellent electronic and optical properties and, thus, are able to serve as desirable candidates for research on studying the electrical, mechanical, and thermal conductivity of graphene. Whereas, for the cases in which graphene is utilized as the additional constituent (e.g., cocatalyst) to enhance the performance of the active component for specific applications, the processability of graphene and its compatibility with other ingredients must be taken into account, which would determine the microscopic properties of the resulting composites and thus the functional performance. Considering that most of the fabrication procedures for graphene-based composites are conducted in the liquid phase, the solution-based techniques for the synthesis of graphene with flexible solution processability have been developed, mainly including reduction of graphene oxide (GO),7,22−24,82−84 liquid exfoliation of graphite,85−87 and synthesis from organic precursors.88−93 In terms of the phase in which the fabrication procedure for graphene proceeds, the commonly used approaches for synthesis of graphene can be classified into two categories, i.e., non-solution-based and solution-based methods. In view of the fact that the graphene used to construct graphene-based D


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ularly the synthesis of graphene-based composite photocatalysts.3,114 Obviously, there have been diverse approaches for the synthesis of graphene, and each method has innate advantages and disadvantages in terms of the cost, resultant quality, quantity, and applicability of the as-obtained graphene materials.3−5,13,22,72 No single methodology is suitable for all potential applications of graphene, and the choice of method depends heavily on the intended use. In view of the utilization of graphene in constructing graphene-based composite photocatalysts, the graphene obtained by solution-based techniques is more beneficial, especially that derived from reduction of GO due to the low cost, facile preparation, massive scalability, and flexible solution processability of GO. Briefly, to achieve efficient graphene-based composites independent of which type of graphene is adopted, we should best bring the advantageous property side of graphene materials into full play and simultaneously alleviate the intrinsic disadvantages, thereby maximizing the net efficiency of graphene for improving the functional performance of graphene-based composites toward specific applications.22


Figure 3. Schematic illustration of liquid exfoliation of graphite in a specific organic solvent or in water−surfactant solution.

Waals forces between graphene sheets would be minimized, and ultrasonic agitation supplies the energy to cleave the graphene precursor.86 Besides, the exfoliation of graphite to produce graphene can also be realized by ultrasonication in water using surfactants to stabilize the exfoliated graphene sheets, such as sodium dodecylbenzenesulfonate and sodium cholate.111,112 In the presence of surfactants, stable encapsulation layers can be formed on each side of the graphene sheets, and graphite flakes are dispersed in the aqueous surfactant solution and transformed into monolayer or multilayer graphene by the application of ultrasound.111,112 The success of ultrasonic cleavage of graphite depends on the appropriate choice of solvents, surfactants, and relevant ultrasound parameters, such as the sonication frequency, amplitude, and time.110,113 The resulting graphene would be a mixture of single and multilayered defect-free or defect-few graphene sheets. This approach is easy to realize and gives a higher quality of graphene materials than those derived from GO. However, this type of graphene cannot be made in large quantities and it possesses low solution processability, which accounts for its relatively limited use in the wet chemistry synthesis of graphene-based composite photocatalysts as compared to GO. Besides the above solution-phase-based methods for graphene synthesis using graphite as the starting material, the chemical synthesis of graphene-like polycyclic aromatic hydrocarbons (PAHs) directly from the organic precursors has also been attracting great research attention, which provides a possible alternative route to graphene synthesis.88−93 Stepwise organic chemistry strategies enable graphene-based materials to be produced in a massively parallel fashion with very high structural precision. Thus far, various PAHs have been synthesized, including dendronized polyfluorenes, hexa-perihexabenzocoronene, superfluorene, graphite ribbons, welldefined ribbon-like graphitic structures, “propeller’’ molecules, and graphite discs.88−93 These graphene-like PAHs occupy an interesting place between “molecular” and “macromolecular” structures and can be substituted with a range of aliphatic chains to modify the solubility.3,93 However, the major drawback of PAHs lies in their limited size range. This is due to the fact that increasing molecular weight generally decreases the solubility of PAHs and increases the occurrence of side reactions.3,93 Under such a situation, the preservation of dispersibility and a planar morphology for large PAHs has been very challenging. If the size range of PAHs can be further extended, it could provide a clean synthetic route with wide scope of applications of the graphene-type materials, partic-

4. REMARK ON THE CLASSIFICATION OF GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS Currently, there are no standard criteria to categorize the graphene-based composite photocatalysts reported in the literature, especially different kinds of graphene materials. The term “graphene” and acronym “GR” have been used widely and loosely for various graphene materials.22−33 In fact, in addition to “GR”, other acronyms have also been used to name the graphene materials. This is conceptually undesirable, given that (i), strictly speaking, the majority of the graphene materials reported so far cannot be indiscriminately defined as pristine graphene; (ii) the precursors of graphene used in the literature are dissimilar, even significantly different; and (iii) thus, the graphene materials prepared by diverse methods have vastly different chemical structures and physicochemical properties. Such a situation is unfavorable for the comparison and discussion among different graphene-related photocatalysts for the benefit of the readership. Therefore, before summarizing the progress of this hot-spot research field, we would like to remark on the classification of graphene-based composite photocatalysts distinctly in this section to circumvent the possible misunderstanding and maintain uniformity and simplicity, hence making this review easily understandable and readable. Thus far, there are mainly three types of graphene materials applied to construct graphene-based photocatalysts, i.e., the graphene produced by the reduction of graphene oxide (GO), the graphene synthesized by liquid exfoliation of graphite in suitable solvents, and the graphene obtained through organic synthesis. Herein, we name the different graphene materials according to their preparation methods to distinguish them. For the first type of graphene, we name it as reduced graphene oxide, abbreviated as RGO, despite the variation of the specific oxidation and/or reduction strategies. The second one is referred to as solvent exfoliated graphene, acronymized as SEG; the last type of graphene materials is termed as organic synthesis graphene, denoted as OSG. Besides, the acronym GO is referred to as graphene oxide. In addition, to avoid confusion on the above categorization, namely RGO, SEG, or OSG, we will not use the acronym “GR” for graphene that only is a E



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Figure 4. Schematic illustrations of the fabrication of graphene-based composite photocatalysts: (A) in situ synthesis procedure and (B) ex situ synthesis procedure.

precursors of photoactive species can be transformed into specific nanostructures anchored on the resulting graphene surface, thereby forming the graphene-based composite photocatalysts (Figure 4A). When GO is chosen as the precursor of graphene, the rich assortment of oxygenated functional groups, including carboxylic, hydroxyl, and epoxide on the surface of GO nanosheets, can serve as nucleation sites to tune the size, shape, morphology, and crystallinity of the in situ derived photoactive nanostructured components. In addition, during such in situ synthesis processes, GO can be simultaneously reduced to graphene (RGO), thus forming the RGO-based composite photocatalysts. In a similar way, when small organic molecules are used as the precursor of graphene, they are also capable of mixing with the precursor of photoactive components, forming a homogeneous dispersion. Upon chemical or thermal treatment, the graphitization of organic molecules into graphene is accompanied by the formation of a photoactive component. This often ensures sufficient and intimate interfacial contact between the different components in the composites, which is typically afforded by such in situ synthesis of graphene-based composite photocatalysts. The above two synthesis processes generally avoid the use of extralinker molecules or protecting surfactants, thereby implying an easily accessible experimental procedure and clean interface connection between graphene and the photoactive component. On the contrary, when SEG obtained from exfoliation of graphite in a suitable solvent is employed to construct graphene-based composites, the surface modification of SEG

generalized nomenclature of graphene materials regardless of how it is prepared.

5. SYNTHESIS AND CATEGORY OF GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS 5.1. Basic Preparation Methods

The flexibly tunable surface properties of graphene via the chemical modification process offer plentiful opportunities to synthesize graphene-based composite photocatalysts. A variety of synthesis strategies, including solution mixing, hydrothermal/solvothermal synthesis, sol−gel method, combustion synthesis, sonication-assisted deposition, microwave-assisted preparation, electrochemical deposition, photoassisted reduction, etc.,22−24,26,31,33,115−119 have been exploited to couple graphene as a desirable two-dimensional (2D) supporting substrate with diverse photoactive materials to form graphenebased composite photocatalysts. Although these methods have been put forward differently from each other, they can conceptually be divided into two general categories, i.e., in situ and ex situ synthesis procedures. 5.1.1. In Situ Synthesis Procedure. The in situ synthesis procedure is a commonly used method for preparing graphenebased composite photocatalysts. Generally, the precursors of graphene, e.g., GO, organic molecules, or functionalized SEG, and the soluble precursor of photoactive materials (e.g., metal salts) are mixed together in an appropriate solvent. Followed by the chemical, thermal, optical, or ultrasonic treatment, the F


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Figure 5. Schematic illustrations of the charge carrier transfer for (A) graphene-organics and (B) graphene-MOFs photocatalysts.

can be carried out in a controlled framework during which the differences in the size, shape, and morphology between blank photoactive ingredient and composite are excluded.121−128 On the contrary, for the case of the in situ synthesis procedure, such differences often cannot be strictly ruled out due to the direct nucleation and growth of photoactive components on the 2D platform of graphene, which usually changes the morphology and structure of the photoactive component in the composite in comparison to blank photoactive materials.

with functional molecules is often necessary because of the hydrophobic nature of the carbon basal plane of SEG. The modification process can not only improve the processability of pristine SEG in solvents, but also uniformly functionalize the SEG nanosheets with specific groups,24,120 which thus endows SEG with suitable functionality to finely tune the growth of photoactive species on the 2D graphene nanosheets and enhances the interactions between graphene sheets and photoactive components. Therefore, the in situ synthesis procedure involves the formation of composite ingredients in the presence of graphene precursors (GO, organic molecules, or modified SEG). Depending on the specific synthesis conditions, the photoactive ingredients in situ formed on the graphene surface can be of different structure and morphology, such as nanoparticles, nanowires, nanorods, nanoplates, or nanospheres.116,119 The photoactive components (e.g., semiconductor) comprised in graphene-based composites are often intimately integrated with the graphene nanosheets during this in situ synthesis process, which thereby results in good interfacial contact. 5.1.2. Ex Situ Synthesis Procedure. In the ex situ synthesis (a.k.a., hard integration) procedure, the presynthesized or commercially available photoactive components, usually semiconductors, instead of their soluble precursors are mixed with the precursors of graphene (GO, organic molecules, or functionalized SEG) to synthesize graphene-based composite photocatalysts, as illustrated in Figure 4B. During the synthesis process, in order to reduce the graphene precursors and/or strengthen the interfacial interaction between the photoactive material and the as-obtained graphene that both constitute graphene-based composites, the pretreatment of the components (e.g., surface charge modification) and/or the post-treatment of the graphene precursor-photoactive material (e.g., chemical, optical, or thermal treatment) are often necessary, which thus yields various graphene-based composite photocatalysts. From the viewpoint of synthesizing composites with sufficient interfacial contact between graphene and photoactive components, the ex situ synthesis method is often less efficient than the in situ integration of graphene with a soluble precursor of photoactive components. However, it is worth mentioning that the specific advantage of this method is the precise control of the microscopic structure of graphene-based composite photocatalysts in a uniform manner by preselection of desirable photoactive components before assembling with the graphene sheets to form graphene-based composites.121−128 Besides, during the ex situ integration process, the size, shape, and morphology of the photoactive components in the composites can be maintained almost the same as those of the original forms. This enables that the photoactivity comparison between blank photoactive materials and graphene-based composites

5.2. Types of Graphene-Based Composite Photocatalysts

To date, a large number of graphene-based photoactive composites have been synthesized and applied in different artificial photocatalytic redox processes.22−24,26,31,33,115−119,129 In general, these graphene-based composite photocatalysts are typically constructed by using three kinds of photoactive component materials (i.e., organics, semiconductors, and metals) as the light harvesters and graphene as the cocatalyst. Beyond that, some reports have proven that graphene oxide (GO) and graphene can also exhibit photoactivity in some given reaction systems under light irradiation by photoexcitation or promoting the electrons relay from the substrate (e.g., dye), respectively. Therefore, in terms of the different photoactive components, the graphene-based photoactive composite materials reported in the literature can be classified into four main different groups: graphene-organics photocatalysts, graphene-semiconductor photocatalysts, graphenemetal photocatalysts, and graphene oxide or graphene photocatalysts, which will be discussed separately in the following parts. 5.2.1. Graphene-Organics Photocatalysts. Organic compounds and metal organic complexes with suitable absorption bands in the ultraviolet−visible (UV−vis) range are an important class of promising materials that are capable of participating in various photochemical charge transfer and energy transfer processes. Some organics, such as porphyrins, polythiophene, and phthalocyanines, as well as dye molecules,130−133 have been employed to integrate with graphene as composite photocatalysts to harvest solar energy. The photocatalytic processes over these graphene-organics photocatalysts can generally take place via the following two mechanistic pathways. In the first type of pathway, as illustrated in Figure 5A, under light irradiation, the organics photocatalyst (represented by O) absorbs light with the appropriate wavelength to reach the excited state (represented by O*). The absorption of light is mainly determined by the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the organics photocatalyst.134 Then, the photoexcited organics (O*) can inject electrons to graphene, which would react with an electron acceptor or specific reactant and, thus, trigger the G



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photocatalytic reactions. Simultaneously, radical cation O•+ is formed, which returns back to the ground state by accepting electrons from electron donors (e.g., reductants or reactants) in the reaction system, thereby completing the overall cycle of the photocatalytic process. In another type of pathway (Figure 5B), the organics photocatalysts behave as semiconductors and can produce charge carriers (electrons and holes) under light irradiation with a specific wavelength. Taking the metal−organic frameworks (MOFs) as an example, MOFs are a class of crystalline hybrid inorganic−organic materials with structures which are composed of clusters of a few metallic atoms held in a threedimensional (3D) structure connected by organic linkers.135−139 Upon light irradiation, electrons and holes are generated in the MOFs photocatalysts with reductive and oxidative power, respectively,137,140 which can separate and migrate to graphene and/or substrate reactants, resulting in light-driven redox processes. The organic photocatalysts possess a wide structural and species diversity, which provides versatile possibilities to construct appropriate grapheneorganics composite photocatalysts for different applications. But it is notable that in spite of the advances in grapheneorganics photocatalysis, the extent of relevant work undertaken in this regard is significantly much less than that of conventional graphene-semiconductor composites, which are the most frequently studied type of graphene-based composite photocatalysts. 5.2.2. Graphene-Semiconductor Photocatalysts. Semiconductors are well-known photocatalysts with an appropriate band gap, which thus far have been most widely investigated to couple with graphene to form graphene-semiconductor composite photocatalysts. The basic action mechanism of semiconductor photocatalysts is based on their unique electronic structure, consisting of a filled valence band (VB) and an empty conduction band (CB), which plays a key role in the photocatalytic processes. When a semiconductor photocatalyst is irradiated with light illumination, the semiconductor can absorb photons with energy matching or exceeding its band gap energy Eg (i.e., the energy difference between the CB and VB).141,142 Then the electrons in the VB are photoexcited to the CB, leaving positive holes in the VB. The photogenerated electron−hole pairs can (i) separate from each other and migrate to catalytically active sites at the semiconductor surface where they reduce the electron acceptors or oxidize the electron donor species, thereby driving the photocatalytic redox reactions; and (ii) recombine at the surface or in the bulk of the semiconductor and release the energy in the form of heat or photon.141,142 The recombination of electron−hole pairs is a deactivation process and would lower the efficiency of the photocatalytic reaction over semiconductor. Therefore, accelerating the separation and migration of the photogenerated electron−hole pairs to avoid their recombination is fundamentally important for the photocatalytic processes. Graphene, as a unique 2D electron conductive platform with lower Fermi level, is able to perform as a cocatalyst to accept and shuttle electrons generated from the band gap-photoexcitation of semiconductors, thus boosting the separation and transfer of charge carriers to participate in the photoredox processes (Figure 6). So far, there have been more than 100 different kinds of graphene-semiconductor composite photocatalysts, as summarized in Table S1 (Supporting Information), including (i) graphene-single semiconductor composites and (ii) the combination of graphene-single semiconductor

Figure 6. Schematic illustration of the charge carrier transfer for graphene−semiconductor photocatalysts.

composites with additional functional components (e.g., metals, second semiconductor, organics, and so on) which leads to multinary composites with multilevel/route electron transfer. For these composite photocatalysts, semiconductor components often act as the light harvesters while the role of graphene is cocatalyst. The fabrication and application of graphenesemiconductor composite photocatalysts are the most active academic research focus in the area of graphene-based photocatalysis. 5.2.3. Graphene-Metal Photocatalysts. In heterogeneous photocatalysis, the traditional function of metals, especially noble metals, is primarily to play as cocatalyst to (i) enhance the separation efficiency of electron−hole pairs and extend the lifetime of the energetic charge carriers generated from excitation of photoactive materials (e.g., semiconductor or organics photocatalyst);143−146 and (ii) facilitate the catalytic process by providing chemically active sites where relevant chemical transformations can take place with lower activation barriers than that on the photoactive materials,147−149 which thereby improves the activity of the composite photocatalysts. Notably, in recent years, the nanosized metals (e.g., Au, Ag, Cu, and Al) have been widely demonstrated to display an extraordinary surface plasmon resonance (SPR) effect under light irradiation,150−161 which is often absent from the photoabsorption spectrum of their bulk materials, enabling them to perform as promising light-responding photocatalysts, i.e., the SPR-induced photocatalysis.151,162−165 The SPR effect in metal is a physical behavior that describes the collective oscillation of conduction electrons.155,166,167 The metals, such as Au, Ag, Cu, and Al, can be treated as freeelectron systems whose electronic and optical properties are determined by the conduction electrons alone.166 Such a metal is denoted as plasma in the Drude−Lorentz model, which contains equal numbers of positive ions (fixed in position) and conduction electrons (free and highly mobile).166 When the metals are illuminated by light with the frequency of photons matching the natural frequency of surface electrons oscillating against the restoring force of positive nuclei, a Coulombic restoring force is then established and the charge density coherently oscillates like a harmonic oscillator in phase with the incident light. The excited plasmonic metal nanostructures can transfer the energetic electrons formed in the process of the SPR excitation to the nearby substrates (e.g., semiconductors and carbon materials) and thus drive the photocatalytic reaction.162 In graphene-metal plasmonic composite photocatalysts, the charge carriers (e.g., hot electrons) are photogenerated from the SPR excitation of plasmonic metal under light irradiation, and then injected into the graphene substrate,165,168−171 H


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thereby driving appropriate chemical reactions,147 as portrayed in Figure 7. In this photocatalyst system, the plasmonic metal

semiconductor for photocatalytic applications, which has been proven by some preliminary observations.176−181 In the GO nanosheets, the CB minimum is formed by the antibonding π* orbital, while the VB maximum is composed of the π orbital179 or the O 2p orbital176,178 that varies with the oxidation degree. Under light irradiation, an electron in the VB of GO is excited into the π* orbital of the CB and a hole is created in the VB. Then, the photogenerated electron and hole contribute to reduction and oxidation reactions, respectively (Figure 8A). Obviously, under light photoexcitation, the GO nanosheets can act as a semiconductor and exhibit photoresponse or photoreactivity when the energy of irradiated light exceeds their band gap. Notably, as for GO photocatalysts, there are two issues to be paid attention. One is that, during the photocatalytic process, GO itself can be reduced, which inevitably alters the band gap of the photocatalyst by tuning the proportion of the sp2 and sp3 fractions, thus causing the photostability concern of GO photocatalysts. The other is that the photocatalytic reaction mechanism at the atomic scale for GO photocatalysts has been still unclear due to the heterogeneous electronic structure and complex configurations of GO.40,182,183 These will be discussed in detail with the specific cases in section 11. In addition, in some given photocatalytic reaction systems where the reaction substrate can be photoexcited to generate electrons, graphene by itself can also be directly used as special photocatalyst to promote the electrons relay, thereby retarding the recombination of charge carriers and improving the photocatalytic performance.184−187 For example, if the reaction substrate is the specific organic dye featuring a photosensitization effect, the light irradiation with suitable wavelength will enable the dye (e.g., rhodamine B) to be photoexcited from the ground state (dye) to the excited state (dye*). Subsequently, dye* injects an electron into graphene, with dye* being converted to the radical cation dye•+, which can react with the reactive oxygen species (ROSs) and/or molecular oxygen to yield intermediates, or to be mineralized into CO2 and H2O, as illustrated in Figure 8B.184−187 The presence of electron conductive graphene is able to boost the electron relay, thereby promoting the separation of charge carriers and the photocatalytic degradation of dyes. The results in this context demonstrate the potential of directly developing GO or graphene with variable electrical conductivity as a new type of metal-free photocatalysts for harvesting solar energy.


Figure 7. Schematic illustration of the charge carrier transfer for graphene−metal photocatalysts.

serves as photoactive component, and graphene acts as cocatalyst to facilitate the transfer of charge carriers. The research on SPR-induced photocatalysis has attracted everincreasing attention and potentially opened a promising avenue for the construction of a new class of photocatalysts with tailored light absorption for various reactions, but without the limitations on band gap-photoexcitation of traditional semiconductor materials.162 5.2.4. Graphene Oxide or Graphene Photocatalysts. As discussed above, the oxidized graphene, i.e., graphene oxide (GO), is an important precursor of graphene for the synthesis of graphene-based composite photocatalysts.22−33 It is covalently decorated with oxygen-containing functional groups (epoxy and hydroxyl groups on the basal plane and carboxyl groups at the edges of graphene sheets). The abundant oxygenated functional groups allow GO to be well dispersed in solution, showing versatile multifaced roles in a solution phase,99,172 for example, acting as highly anisotropic colloid or amphiphile and providing anchoring points for introducing selective functionality. Besides, the carbon−oxygen covalent bonds in GO break the extended sp2 conjugated network and confine the π-electrons within the isolated sp2 domains, thus resulting in the change of the electronic states of graphene from a zero-gap semiconductor to a semiconductor with a finite band gap, as reflected by the theoretical and experimental observations.43,44,46,99,173,174 The oxidation process also provides a wealth of opportunities for further tailoring the electronic and optical properties by the manipulation of the size, shape, and relative fraction of the sp2-hybridized domains of GO.40,172,175 In view of these statements, it is reasonable to infer that GO with appropriate oxidation degree can be utilized as a

6. FUNDAMENTAL ROLES OF GRAPHENE IN THE GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS Regarding the reported graphene-based composite photocatalysts in the literature, the most-widely recognized role of

Figure 8. Schematic illustrations of the charge carrier transfer for (A) GO and (B) graphene photocatalysts. I


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Figure 9. (A) Scanning transmission electron microscopy (STEM) image showing Ag and TiO2 nanoparticles at distinct sites on a RGO sheet (the inset is the energy dispersive X-ray spectra recorded at different spots); (B) photographs showing the color changes observed during stepwise transfer of electrons. Reprinted with permission from ref 191. Copyright 2010 American Chemical Society.191

colored TiO2 suspension until no blue color remains, during which GO is reduced to RGO and stores the excess electrons. The gray-colored solution is due to the formation of RGO. In the third step, following the addition of deaerated AgNO3 solution into the above suspension, the stored electrons in RGO reduce Ag+ to Ag nanoparticles (red color) at different sites on the RGO surface. Xu and co-workers have investigated the effects of the introduction of RGO on the photoactivity and physicochemical properties of RGO-CdS composites.192 The transient photocurrent responses of blank CdS and the optimal 5%RGO-CdS under intermittent visible light illumination (Figure 10A) show that the addition of RGO is able to enhance the photocurrent of the sample, indicating a more efficient separation of the photoexcited electron−hole pairs for 5%RGO-CdS than blank CdS. This result can also be confirmed by the photoluminescence (PL) spectra, which are often employed to

graphene is regarded as an electron reservoir to accept, transport, and shuttle electrons photogenerated from the excitation of photoactive components in the composites through its electronically conductive 2D structure.22−33 However, considering the fact that, in general, the real state of graphene in the graphene-based composite photocatalysts is not the well-defined and ideal 2D single atomic carbon sheet, especially for the GO-derived graphene (RGO), the surface, electronic, and optical properties of graphene in the composites are complex and variable. For example, the amount of oxygenated functional groups on the graphene surface and the chemical modification of graphene with different organics both have significant influence on the electronic and optical properties of graphene and, thus, the roles of graphene played in the graphene-based composite photocatalysts,6,188−190 which have been found to be diverse and worthy of continuous research. Besides performing as a conductive media for accepting and transporting photogenerated electrons, graphene has some other roles, such as enhancing the adsorption capacity, and tuning the light absorption range and intensity of the graphene-based composite photocatalysts. Additionally, the novel photothermal effect and the macromolecular photosensitizer role of graphene have also been reported recently. In the following section, we would summarize these fundamental roles of graphene in graphene-based composite photocatalysts and elaborate them separately with selected typical examples. 6.1. Photoelectron Mediator and Acceptor

The role of graphene as a photoelectron mediator and acceptor in graphene-based composite photocatalysts has been widely confirmed by the elucidation of charge transfer between the photoactive component materials (e.g., semiconductor) and graphene by different research groups.22−33 For example, using reduced graphene oxide (RGO) as a 2D support, Lightcap et al. have found that the electrons photogenerated from UVirradiated TiO2 can be transported across RGO to reduce silver ions (Ag+) into Ag nanoparticles at a location distinct from the TiO2 anchored site, thus selectively anchoring semiconductor TiO2 and Ag nanoparticles at separate sites of RGO support, as displayed in Figure 9A.191 The ability of RGO to store and shuttle electrons has also been visualized via a stepwise electron transfer process (Figure 9B). The first step involves the excitation and storing of electrons in TiO2 nanoparticles by irradiating the deaerated ethanol suspension of TiO2 with UV light (λ > 300 nm) for 30 min. In the second step, a deaerated ethanol suspension of GO is added incrementally into the blue-

Figure 10. (A) Transient photocurrent response of blank CdS and the optimal 5%RGO-CdS composite in 0.2 M Na2SO4 aqueous solution without bias versus Ag/AgCl under the irradiation of visible light (λ > 420 nm); (B) photoluminescence (PL) spectra of blank CdS and the optimal 5%RGO-CdS composite (λexc. = 358 nm). Reprinted with permission from ref 192. Copyright 2011 American Chemical Society.192 (C) Fluorescence emission spectra (λexc. = 418 nm) and (D) fluorescence decay profiles (λexc. = 408 nm) of GO-H2P (black) and free H2P (red) in DMF. Reprinted with permission from ref 194. Copyright 2011 Royal Society of Chemistry.194 J



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structures, thus contributing to improving the adsorption of reaction substrates. The increased adsorption capacity is favorable for the enrichment of target reactants from the bulk solution onto the surface of photocatalyst. As a result, the adsorbed reaction substrates can effectively react with the photogenerated active species (e.g., electrons, holes, hydroxyl radicals, and superoxide radicals) on the surface of composites, thus contributing to the improved photoredox activity. For example, the adsorption capacity of RGO-P25 with different weight content of RGO has been investigated.17 The adsorption equilibrium in the dark shows that the introduction of RGO into the matrix of TiO2 significantly improves the adsorption capacity of RGO-P25 toward substrates, i.e., benzene and methylene blue (MB), as shown in Figure 12. The increased adsorption of benzene and MB on the surface of the RGO-P25 composites has proven to contribute to improving the photocatalytic activity toward gas-phase degradation of benzene and liquid-phase degradation of MB. In addition, Ng et al. have synthesized the RGO-TiO2 composites via a solution-based method and investigated the photocatalytic activity of blank TiO2 and RGO-TiO2 for degradation of 2,4-dichlorophenoxyacetic acid (2,4-D).196 The RGO-TiO2 composite displays higher photodegradation rate than blank TiO2 under UV light irradiation. The enhanced photocatalytic activity is attributed to the addition of RGO, which not only improves the charge separation efficiency but also enhances the adsorption and the accumulated concentration of 2,4-D near the surface of RGO-TiO2.196 Similarly, the role of graphene in improving the adsorption capacity has also been observed for RGO-Au composites toward rhodamine B (RhB), which results from the strong π−π interaction between RGO and dye chromophores of RhB.186

study surface processes involving the electron−hole fate of semiconductors.193 As displayed in Figure 10B, the PL intensity obtained over 5%RGO-CdS is weaker than that of blank CdS, suggesting the more efficient inhibition of the recombination of electron−hole pairs photogenerated in the 5%RGO-CdS composite. In addition, it has been demonstrated that GO, a widely used precursor of graphene, can store and transfer electrons.194 The fluorescence spectra of GO and GO-H2P (5-(4-aminophenyl)10,15,20-triphenyl-21,23H-porphyrin) upon excitation wavelength at 418 nm (Figure 10C) show the effective emission quenching of porphyrin covalently linked with GO, which can be attributed to the interaction of the singlet excited state of the porphyrin (1H2P*) with GO.194 This electronic communication between the two species has been further confirmed by the fluorescence decay profiles (Figure 10D). The fluorescence lifetime of the photoexcited porphyrin in GO-H2P is evaluated to be significantly shorter than that of the intact porphyrin, demonstrating the charge separation and transfer scenario from 1 H2P* to GO for GO-H2P composites under light irradiation. 6.2. Enhancing Adsorption Capacity

Apart from performing as a photoelectron media, enhancing the adsorption capacity toward reaction substrates is another important and fundamental role of graphene, as observed in graphene-based composite photocatalysts.17,186,195,196 Owing to its unique 2D structure, versatile surface properties, and high theoretical surface area, graphene and its derivatives are ideal supports to increase the specific surface area and enhance the adsorption capacity of composites. Depending on the specific reaction system, the adsorption interaction types of target reactants (e.g., heavy metals, dyes, and aromatic compounds) on graphene-based composites are different, which can be attributed to physical adsorption, electrostatic attraction, or chemical interaction.195 As shown in Figure 11, in addition to physical adsorption of target reactants on the surface of graphene-based composites, the presence of functional groups (e.g., carbonyl, epoxy, hydroxyl, and amino groups) on the surface of graphene enables the composite to interact with a wide variety of molecules and metal ions. Moreover, the aromatic regions of graphene can form a π−π stacking interaction with organic pollutants containing aromatic

6.3. Tuning Light Absorption Range and Intensity

To fabricate efficient composite photocatalysts for solar energy conversion, the photocatalysts should be sensitive to visible light, which accounts for about 43% of the total solar energy.197,198 However, up to now, an important proportion of photocatalysts under investigation are wide-band gap semiconductors (e.g., TiO2 and ZnO) that are only photoactive under UV light. This greatly limits their wide practical applications because the UV light only constitutes ca. 4% in the whole solar spectrum.199 To extend the light absorption range from the UV to visible light region, the combination of wide-band gap semiconductor photocatalysts with graphene has proven to be a promising approach.18,209−212 By choosing appropriate synthesis methods, sufficient interfacial contact is able to be realized for graphene-semiconductor composites, and in some cases, the chemical bonding between semiconductors and graphene can be formed.18,220−223 These consequently enable the extension of the light absorption edge and the improvement of the light absorption intensity for the graphenebased composite photocatalysts. Zhang et al. have extended the light response range of semiconductor TiO2 (P25) into the visible light region by integrating P25 nanoparticles with RGO through a one-step hydrothermal method.18 During the hydrothermal reduction process, graphene oxide (GO) can firmly interact with the surface hydroxyl groups of P25 nanoparticles, thus forming the chemically bonded RGO-P25 composites. It can be seen from Figure 13A that the low frequency band around 690 cm−1 in the Fourier transform infrared (FTIR) spectrum of P25 sample corresponds to the vibration of Ti−O−Ti bonds whereas, for

Figure 11. Schematic illustration of different types of interactions involved in the adsorption of reactants on graphene-based composite photocatalysts. Reprinted with permission from ref 195. Copyright 2014 Royal Society of Chemistry.195 K


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Figure 12. Bar plots showing the remaining reactants after reaching the adsorption−desorption equilibrium in the dark over the RGO-TiO2 composites for (A) the gas-phase degradation of benzene and (B) liquid-phase degradation of methylene blue (MB), with the corresponding pictures of the MB solution (the bottom panel of B). Reprinted with permission from ref 17. Copyright 2010 American Chemical Society.17

Figure 13. (A) Fourier transform infrared (FTIR) spectra of (1) P25, (2) RGO obtained by hydrothermal reduction of GO, and (3) RGO-P25 in the range of 4000−450 cm−1; (B) diffuse reflectance absorption spectra of (1) P25 and (2) RGO-P25. Reprinted with permission from ref 18. Copyright 2010 American Chemical Society.18

the FTIR spectrum of RGO-P25, it displays a broad and shifted absorption peak below 1000 cm−1 in comparison with that of blank P25, which can be ascribed to the combination of the Ti−O−Ti vibration and the Ti−O−C vibration (798 cm−1). The formation of Ti−O−C bonds between P25 and RGO results in the band gap narrowing of P25 and extends the light response range of P25 into the visible light region, as reflected by the diffuse reflectance absorption spectra in Figure 13B. This is similar to the case of other carbon-doped TiO2 composites. Furthermore, the addition of RGO also improves the absorption intensity of P25-RGO in the visible light region. The tuning of the light absorption range and the intensity of semiconductors induced by the introduction of graphene has also been observed in the composites of wide-band-gap ZnO and RGO.200,201 For instance, Tien et al. have fabricated RGOZnO sphere composites by a simple microwave-assisted solvothermal process.200 They found that the absorption edge of the ZnO sphere in RGO-ZnO can be extended to the visible light region, which might be ascribed to the formation of Zn− O−C chemical bonds in the RGO-ZnO composites; however, the authors did not give direct experimental evidence to support the speculation on the formation of Zn−O−C bonds.

proposed that the strong photothermal effect (PTE) of RGO plays an important role in improving the photoactivity of RGOTiO2 (P25) composites toward degradation of methylene blue (MB).202 Based on the results of photocatalytic degradation experiments under different reaction conditions (Figure 14A), they found that (i) no obvious MB degradation by RGO-P25 has been observed under irradiation of near-infrared (NIR) light (curves 1), indicating that no chemically reactive charge carriers are generated by the NIR light irradiation. (ii) Under

6.4. Photothermal Effect

Figure 14. (A) Photocatalytic degradation of MB on P25 and RGOP25; (B) HRTEM image of a RGO sheet (the RGO sheet contains graphene islands (black circles) separated by disordered regions). Reprinted with permission from ref 202. Copyright 2014 American Chemical Society.202

In addition to the widely recognized three common functions of graphene in the graphene-based composite photocatalysts as discussed above, recently, Gan et al. have for the first time L



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Figure 15. (A) UV−vis diffuse reflectance spectroscopy (DRS) results of RGO-ZnS composites and blank ZnS; (B) their photoactivity for selective oxidation of benzyl alcohol under visible light irradiation (λ > 420 nm) for 4 h. Reprinted with permission from ref 204. Copyright 2012 American Chemical Society.204 (C) DRS spectra of blank ZnO, RGO-ZnO with different addition ratios of RGO, and blank ZnO; (D) photocatalytic performance of bare RGO, blank ZnO, RGO-ZnO composites, and 10% RGO-blank-ZnO toward the reduction of Cr(VI) aqueous solution under visible light irradiation (λ > 400 nm). Reprinted with permission from ref 205. Copyright 2013 American Chemical Society.205

catalytic activity could be assisted by rationally harnessing the photothermal effect associated with graphene.

the illumination of UV−vis light (curves 2 and 3) or the entire wavelength range (curves 4 and 5), the MB degradation rates over RGO-P25 are both higher than blank P25. Of particular note is that the gap between curves 4 and 5 is much larger than that between curves 2 and 3, which indicates that the NIR irradiation plays an auxiliary but important role in enhancing the photoactivity of RGO-P25 composites. Through a series of controlled experiments and calculations, the contribution of the PTE of RGO is found to be as high as ∼38% for the photodegradation of MB by RGO-P25 composite. This is mainly because the chemically derived RGO sheet is constituted by a structurally and electronically inhomogeneous system. As revealed by the high-resolution TEM (HRTEM) analysis in Figure 14B, a single RGO sheet contains both graphene islands and disordered regions. The charge carriers in the graphene islands act like delocalized state carriers, but for these disordered regions with localized states, charge conduction is mainly realized by hopping. The conductance of the RGO sheet would be influenced by the temperature; the faster charge carrier mobility is expected at higher temperature. Therefore, under solar light illumination, the NIR heats up the RGO sheets because of PTE and thus increases the local temperature around the RGO-P25 composite. Simultaneously, both P25 and MB are photoexcited to generate charge carriers, and the separation of electrons and holes can be favored due to the shuttling of RGO sheets. The PTE of RGO causes the electrons to obtain more energy and move faster on the hot RGO sheets. As a result, PTE promotes the degradation of MB over RGO-P25. This work reveals that the photothermal effect could be a vital factor contributing to the activity of graphenebased composite photocatalysts.202 The design and construction of graphene-based composites with improved photo-

6.5. Macromolecular Photosensitizer

In 2011, Du et al. employed large-scale density functional calculations to study the interfacial interaction of the graphenerutile TiO2 (110) model.203 Along with the experimental proof from the wavelength-dependent photocurrent study on a graphene-TiO2 photoanode, they proposed that graphene can act as a photosensitizer for TiO2. Namely, under visible light irradiation, the valence electrons may be expected to become directly photoexcited from graphene to the conduction band (CB) of TiO2. However, the photocurrent measurement alone cannot be sufficient enough to prove whether or not graphene acts as a photosensitizer for a semiconductor during an actual photocatalytic process. The direct and robust experimental evidence on disclosing the photosensitizer role of graphene for semiconductors toward photocatalytic applications has not been available until the experimental work regarding RGO-ZnS composite photocatalysts for selective oxidation of alcohols and alkenes under visible light irradiation, as reported by Xu’s group.204 By a two-step wet chemistry process, the RGO-ZnS composites featuring an intimate interfacial contact between ZnS nanoparticles and the RGO sheet have been prepared.204 Figure 15A shows that the addition of RGO cannot sufficiently narrow the band gap of ZnS to the visible light region, suggesting that the ZnS in RGO-ZnS is not able to be band gap-photoexcited under visible light irradiation. Moreover, the bare RGO sheets also do not exhibit visible light photoactivity. However, the RGO-ZnS composites do exhibit visible light photoactivity toward selective oxidation of alcohols and alkenes (as exemplified in Figure 15B). The structure−photoactivity M



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correlation and reaction mechanism study using different radical scavengers reveal that the role of graphene in RGOZnS is to act as a macromolecular organic dye-like photosensitizer, with which the photogenerated electrons from graphene upon visible light irradiation can transfer to the CB of ZnS. However, such a photosensitizer role of graphene cannot be observed for the RGO-ZnS composites that are prepared by mechanical mixing of ZnS nanoparticles and the RGO sheets, indicating the crucial effect of intimate interfacial interaction on observing the photosensitizer role of graphene in the specific graphene-semiconductor composite photocatalysts. This case is similar to the well-known photosensitization process of semiconductors by selective matched adsorption of organic dyes. An analogous photosensitizer role of graphene has also been observed in a subsequent work on RGO-ZnO composites toward liquid-phase reduction of heavy metal ions under visible light irradiation, as shown in Figure 15C and D.205 In particular, three basic principles for experimentally investigating the photosensitizer role of graphene in graphene-semiconductor composite photocatalysts under visible light have been proposed. (i) The possibility of band gap-photoexcitation of the semiconductor in the graphene-semiconductor composites should be excluded under visible light irradiation. For example, Manga and co-workers206 have once proposed that the electron transfer mechanism in RGO-TiO2 film composites is driven by the absorption of visible light (400 nm) by RGO, followed by the injection of photoexcited electrons into the CB of TiO2. However, this statement is not convincing because even the slight band gap narrowing of TiO2, which results from its hybridization with RGO, can make TiO2 in the RGO-TiO2 composites respond to visible light (400 nm), thereby inducing band gap-photoexcited charge carriers. In such a case, it is difficult to determine the exact transfer direction of charge carriers across the interface between RGO and TiO2. (ii) The proper interfacial interaction manner between graphene and the semiconductor should be achieved because it will affect the charge carrier lifetime and transfer efficiency across the interface between graphene and the semiconductor.205 (iii) The suitable probe reactions should be chosen, which needs to avoid the self-induced photosensitization effect (e.g., organic dye photosensitization for semiconductor),207,208 and can effectively capture the photogenerated electrons from graphene photoexcitation in graphene-semiconductor composites under visible light irradiation. On the other hand, recently, it has been shown that graphene quantum dots (GQDs), as an important graphene derivative, are able to be photoexcited under visible light and act as a photosensitizer for semiconductors.209−211 Engineering the size and shape of a nanosized graphene system makes it possible to tune the band gaps of GQDs. When the GQDs are integrated with the large band gap semiconductor such as TiO2, they are able to absorb the visible light and be excited to generate charge carriers (electron−hole pairs) while the semiconductor TiO2 is unable to be photoexcited, as discussed by Feng et al.210 and Qu et al.211 The photogenerated electrons in GQDs can transfer to the semiconductor, leaving holes on the surface of GQDs. Then, the separated electrons and holes react with the electron acceptor and donor, respectively, resulting in the visible light driven activity of GQDs-TiO2 composites, as illustrated in Figure 16. Notably, bare GQDs do not display photocatalytic activity under visible light irradiation. Thus, the role of GQDs with broad visible light absorption actually acts as

Figure 16. Schematic illustration of the photocatalytic mechanism of GQDs-TiO2 composites under visible light irradiation. Reprinted with permission from ref 210. Copyright 2014 Royal Society of Chemistry.210

a photosensitizer for the wide-band gap TiO2, which transforms semiconductor TiO2 to exhibit the visible light activity.

7. KEY FACTORS ON ENHANCING THE PHOTOACTIVITY OF GRAPHENE-BASED COMPOSITES In the preparation of graphene-based composite photocatalysts, it has been well recognized that the random integration of graphene with the photoactive component is hard to sufficiently harness the unique properties of graphene for enhancing the photoactivity of the resultant composites. The defect density and chemical modification of graphene, the dimensionality of graphene and the photoactive component, the interfacial contact, and the system-level engineering of the whole composite system all have been demonstrated to be crucial factors that significantly influence the utilization of the structure and electrical conductivity of graphene, which determines the charge carrier transfer pathway and efficiency and thus the net efficacy of graphene for improving the photoactivity of the graphene-based composites. Therefore, in order to design and fabricate high-performance graphene-based photoactive composite materials, these factors should be rationally considered and optimized. 7.1. Defect Density of Graphene

An overview of the literature on graphene-based composite photocatalysts reveals that GO, synthesized from the Hummers’ method-based strong oxidation process,96 is the most commonly used precursor of graphene.22−33 However, the GO-derived graphene (RGO) inevitably contains residual few amounts of oxygenated functional groups and a particularly large population of defects, which results in considerable disruption of the 2D π-conjugation of the electronic structure of graphene. The charge carrier mobility and electrical conductivity of RGO are lower than those of an ideal pristine graphene sheet.41,42 Therefore, from the viewpoint of more efficient utilization of the electrical conductivity of graphene to separate and transfer photogenerated charge carriers, the employment of graphene with less defects and thus higher electrical conductivity would be a straightforward strategy for improving the activity of graphene-based composite photocatalysts. In this regard, the synthesis of defect-few or defect-free graphene with high quality through dispersion and exfoliation of graphite in organic solvents with appropriate surface energy matching that of graphene has been developed.86,87,111 This type of solvent exfoliated graphene (SEG) with higher electrical conductivity than RGO has shown promise for enhancing the N



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Figure 17. (A) Intensity-normalized Raman spectra of SEG and RGO films annealed at 400 °C for 30 min in air; (B) sheet resistances of SEG and RGO thin films formed via vacuum filtration as a function of mass density. Reprinted with permission from ref 21. Copyright 2011 American Chemical Society.21 (C) Transient photocurrent response and (D) electrochemical impedance spectroscopy (EIS) Nyquist plots of 5%SEG-TiO2, 5%RGO-TiO2, 1%SEG-P25, and 1%RGO-P25 under visible light irradiation (λ > 400 nm). Reproduced from ref 212 with permission from the PCCP Owner Societies.212

demonstrate that the as-synthesized SEG-TiO2 and SEG-P25 both display higher visible light photoactivities as compared to their counterparts of RGO-TiO2 and RGO-P25 toward aerobic oxidation of alcohols in a liquid phase. This activity enhancement indicates that SEG, with lower defect density than RGO, can make more efficient use of the electrical conductivity of graphene for both of the cases of in situ and ex situ synthesis processes, as supported by the transient photocurrent and Nyquist impedance analysis in Figure 17C and D. These studies demonstrate that the defect has a significant influence on the electrical conductivity of graphene, which affects the efficiency of graphene for separating and transporting the charge carriers in the composites and thus determines the photoactivity enhancement of graphene-based composite photocatalysts.

photoactivity of graphene-semiconductor composite photocatalysts.21,212 Hersam et al. have carried out a comparative study between SEG-P25 and RGO-P25 composite photocatalysts using SEG and GO as the precursor of graphene, respectively, in an effort to elucidate the role of graphene defect in affecting the activity of composites.21 The Raman spectra of SEG obtained via ultrasonic treatment of natural graphite in N,N-dimethylforamide (DMF) and the solvent-reduced graphene oxide (RGO) in Figure 17A show that the intensity ratio of the D and G bands (ID/IG) for SEG (0.17) is much lower than that for RGO (ID/IG = 0.82). The ID/IG value is a measure of the relative concentration of sp3 hybridized defects compared to the sp2 hybridized graphene domains; the lower ID/IG of SEG indicates that it has a much lower defect density than RGO. As a result, the sheet resistance of SEG films is lower than that of RGO films at the same area mass density (Figure 17B). The SEG sheet with lower defect density and sheet resistance displays higher electronic coupling with TiO2 than RGO. In addition, the enhanced electrical mobility of SEG compared to RGO implies a longer electronic mean free path, which enables the photoexcited energetic electrons to diffuse farther from the SEG-TiO2 interface, thereby decreasing the recombination of electron−hole pairs and increasing their likelihood of interaction with adsorbed reactants. Consequently, the SEGTiO2 composites demonstrate higher photoactivities than the RGO-TiO2 counterparts toward photocatalytic oxidation of CH3CHO and reduction of CO2 under both UV and visible light irradiation. The advantage of SEG over RGO toward improving the charge carriers separation has been further evidenced in the graphene (SEG and RGO)-TiO2 composites prepared by an in situ synthesis procedure using SEG/GO as the precursor of graphene and TiF4 as the precursor of TiO2.212 Moreover, the composites of SEG-P25 and RGO-P25 by a “hard” ex situ integration of solid P25 nanoparticles with SEG and GO, respectively, have also been prepared. The photoactivity tests

7.2. Chemical Modification of Graphene

Other than decreasing the defect density of graphene, chemical doping with foreign atoms is another important approach to tailor the physicochemical properties of graphene, which has opened the possibilities of tuning the activity of graphene-based composite photocatalysts. Generally, the chemical doping of graphene can be divided into the following two categories: surface transfer doping and substitutional doping.213,214 Surface transfer doping, also called adsorbate-induced doping, can be achieved by electron exchange between graphene and dopants that are adsorbed on the graphene surface. This type of doping does not disrupt the structure of graphene and is reversible in most cases.215 Substitutional doping would introduce heteroatoms with different number of valence electrons, such as nitrogen atoms (N) and boron atoms (B), into the carbon lattice of graphene.215 The doping of the carbon network with heteroatoms of N and B can minimize the change of the conjugation length in the graphitic basal plane.215 Moreover, depending on the doping site, type, and amount that vary along with the doping methods, the introduction of heteroatoms into graphene is able to enhance the electrical/thermal conductivities, facilitate the charge carrier transfer property of graphene, O


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Figure 18. (A) High-resolution N 1s XPS spectra of N/RGO-ZnSe and the mixed product of [ZnSe](DETA)0.5 nanobelts and GO. Reprinted with permission from ref 217. Copyright 2012 American Chemical Society.217 (B) N 1s XPS spectrum of N/RGO. Reprinted with permission from ref 218. Copyright 2011 American Chemical Society.218

Figure 18A and B, which may explain the distinctly different roles of N/RGO in the corresponding composite photocatalysts. Besides N element, boron (B) is another important type of doping element used to modulate the electronic properties of graphene. As an electron-deficient dopant, the doping of boron can increase the electrical conductivity and work function of graphene due to the increased density of states (DOS) value near the Fermi level,185,219 which is beneficial for the photogenerated charge carrier transfer in graphene-based photocatalysts. Recently, Tang and co-workers185 have synthesized B-doped reduced graphene oxide (B-RGO) via a facile one-step reflux route,220 and found that the B-RGO displays higher activity than the undoped RGO toward degradation of rhodamine B (RhB) under visible light irradiation. The enhanced photoactivity of B-RGO is because the B doping decreases the defect density and increases the electrical conductivity and photogenerated electron transfer efficiency of B-RGO, as demonstrated by the Raman spectra and electrochemical impedance spectroscopy (EIS) plots of BRGO and RGO.

and even open the band gap and convert graphene into a semiconductor.216 Therefore, in graphene-based composite photocatalysts, the chemical modification of graphene is mainly focused on the substitutional doping. Chen et al. have synthesized N-doped graphene/ZnSe (N/ RGO-ZnSe) composites through a one-pot hydrothermal process using GO nanosheets as the precursor of graphene and ZnSe-(diethylenetriamine)0.5 ([ZnSe](DETA)0.5) nanobelts as the source of ZnSe nanoparticles and N element.217 Because the N element has comparable atomic size and contains five valence electrons available to form strong valence bonds with carbon atoms, the N doping of graphene can be easily obtained. It can be seen from the N 1s X-ray photoelectron spectroscopy (XPS) results in Figure 18A that the main peak at 399.5 eV is observed for the mixed product of [ZnSe](DETA)0.5 and GO. Regarding N/RGO-ZnSe, the main N 1s XPS peaks are located at 399.9 and 400.2 eV, which can be attributed to the CN and CN (sp3 hybridization) bonding configurations, respectively. The resultant N/RGOZnSe displays significantly enhanced electrochemical performance and photocatalytic activities in comparison with blank ZnSe.217 The improved photoactivities of N/RGO-ZnSe are attributed to the fact that the N doping can open the band gap of graphene and convert graphene into an n-type semiconductor. Therefore, the N/RGO-ZnSe composite is a heterosystem with two semiconductors (N/RGO and ZnSe) forming p−n junctions, which promotes charge collection and separation at the interfaces and thus enhances the photoactivity of the composite.217 In another work, N-doped graphene (N/RGO) has been fabricated by the calcination of GO in NH3 atmosphere, which is then integrated with CdS via an ex situ solution synthesis process followed by calcination in N2 atmosphere.218 As shown in Figure 18B, the N 1s XPS peaks of N/RGO located at ca. 398.6 and 401.5 eV correspond to pyridinic N (sp2 hybridization) and quaternary N (also called “graphitic N”), respectively. The N/RGO-CdS exhibits higher photocatalytic activity toward H2 evolution than the RGO-CdS counterpart without N doping prepared by the same solution synthesis method as that for N/RGO-CdS. This is ascribed to the fact that the N doping enhances the electrical conductivity of RGO, which facilitates the photoinduced electron transport, thereby preventing the recombination of electron−hole pairs and thus enhancing the photoactivity. Notably, in this report,218 the N atoms doped into RGO have different bonding configurations from the previous one,217 as revealed by the XPS results in

7.3. Dimensionality Effect

With regard to fabrication of graphene-based composite photocatalysts with improved activity, manipulation of the dimensionalities of individual graphene and photoactive components has been demonstrated to be a significant factor that influences the photocatalytic performance of the graphenebased composite photocatalysts. Recent works have proposed that when 2D graphene is used as catalyst support, the dimensionality of the photoactive component would influence the efficiency of photogenerated electron transfer across the interface between the components and, thus, the activity of the composite photocatalyst.221−223 For example, Sun et al. have constructed a TiO2 nanosheetRGO 2D-2D composite system with a well-defined heterojunction interface by a solvothermal method.222 The comparison with TiO2 nanoparticle-RGO zero-dimensional (0D)-2D and TiO2 nanotube-RGO 1D-2D composites reveals that the TiO2 nanosheet-RGO 2D-2D hybrid exhibits the highest photoactivity toward the degradation of RhB and 2,4dichlorophenol under UV light irradiation. By analyzing the interfacial electron transfer kinetics, it has been disclosed that the TiO2 nanosheet-RGO 2D-2D system with stacked layer structure possesses a strong electronic and physical coupling effect, which is believed to increase the contact area between the TiO2 component and RGO sheets and shorten the diffusion P


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Figure 19. Schematic illustration of 2D-2D layered composites in comparison with other kinds of composites with different dimensionality (0D-2D and 1D-2D). Reprinted with permission from ref 224. Copyright 2014 Royal Society of Chemistry.224

The construction of 3D graphene-based scaffold provides a feasible and potential strategy to optimize the structural and electronic properties of graphene-based composite photocatalysts by providing multidimensional electron transport pathways and large accessible surface area. Furthermore, the 3D architectures impart other favorable properties, such as aggregation resistance, interconnected conductive frameworks, fast mass kinetics, and a special microenvironment.22 These merits would have a synergistic influence on the optimization of the photoactivity of graphene-based composites for target reactions.

distance of photogenerated charge carriers, thus facilitating fast electron transfer across the heterojunction interface and resulting in the enhancement of photoactivity. Therefore, coupling 2D layered photoactive materials with 2D graphene into a hybrid system is able to enhance the activity of the graphene-based composite photocatalysts due to the increased contact surface and charge transfer rate, especially compared with other photoactive material-graphene composites with different dimensionality (e.g., 0D-2D and 1D-2D),222,224 as illustrated in Figure 19. In addition to the dimensionality of the photoactive component in the composites, the dimensionality of graphene is also a key factor that affects the photocatalytic performance of graphene-based composites.22,225,226 Besides the typical 2D graphene nanosheets, other different dimensionalities of graphene have also been developed, including 0D graphene quantum dots, 1D graphene nanoribbons, and 3D graphene architectures. Some of them have been used to integrate with photoactive materials to form composite photocatalysts. For instance, ultradispersed mesoporous TiO2 nanocrystals with (001) facets have grown on 3D-structured RGO aerogel through a simple in situ hydrothermal method using glucose as the linker and face-controlling agent (Figure 20A).226 The

7.4. Interfacial Contact Effect

For the graphene-based composite photocatalysts, since the photogenerated charge carrier transfer predominantly occurs across the interfacial domain between graphene and photoactive components in the composite system, it is intuitively understandable that increasing the interfacial contact between them would be a viable approach to promote the charge carrier transfer efficiency and achieve a more efficient graphene-based composite photocatalyst. In this respect, using GO as the precursor of graphene has specific advantages to achieve good interfacial contact. The large amount of oxygenated functional groups endows GO with the superior and easily accessible “structure-directing” role in a solution phase, which leads to the even nucleation and in situ growth of photoactive components on the graphene surface. As a result, intimate interfacial contact between graphene and the photoactive component can be achieved. For instance, by taking TiO2 semiconductor as an example, the prominent advantage of the “structure-directing” role of GO on strengthening the interfacial contact and thus enhancing the photocatalytic activity of the semiconductor has been conceptually demonstrated.20 With TiF4 as a soluble precursor of TiO2 in an aqueous phase and GO as the precursor of graphene, the semiconductor TiO2 can be in situ grown on the GO sheet surface. Followed by a hydrothermal reduction of GO to RGO, the RGO-TiO2 composites with an intimate interfacial contact can be obtained, as shown in Figure 21A. However, for the case of replacing GO with carbon nanotube (CNT), such intimate interfacial contact cannot be achieved in the counterpart of CNT-TiO2 prepared via the same approach as that of RGO-TiO2, as displayed in Figure 21B. As a result, RGO-TiO2 exhibits much higher activity than CNT-TiO2 without sufficient interfacial contact. This study clearly signifies that to improve the interfacial contact between the photoactive component and the graphene sheet is a feasible strategy for the more effective utilization of the electrical conductivity of graphene, which in turn inhibits the recombination of photogenerated charge carriers and improves the photoactivity of graphene-based composites more effectively. Besides the direct utilization of GO in a solution phase to guide the in situ growth of photoactive components on the

Figure 20. (A) Appearance illustration of 3D TiO2-RGO composites involving glucose; (B) SEM images of RGO-TiO2 (67 wt %) synthesized in the presence of glucose. Reprinted with permission from ref 226. Copyright 2014 American Chemical Society.226

introduction of conductive 3D RGO into the matrix of TiO2 efficiently facilitates the photogenerated electron transfer, thus remarkably boosting the photocatalytic performance. Additionally, the macroporous 3D structure (Figure 20B) of RGO-TiO2 composites has a high surface area of 204 m2 g−1. The hierarchical channels and high specific surface area are able to improve the adsorption of reactants, which also contributes to improving the activity of the resulting 3D RGO-TiO 2 composite photocatalysts. Furthermore, the hydrophobic property and massive appearance of the 3D RGO-TiO2 aerogel contribute to its excellent recyclable activity that is advantageous for photocatalytic applications. Q


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the reduction of GO to RGO and the change of the property of the graphene sheets from negatively charged to positively charged surface are accomplished simultaneously. Driven by the strong electrostatic attraction, the negatively charged semiconductor (e.g., SnNb2O6 nanosheets125 and WO3 nanorods124) can rapidly adhere to the surface of the positively charged RGO in aqueous solution, forming the corresponding RGO-semiconductor composites with intimate interfacial contact, as exemplified in Figure 22B. The as-prepared RGOsemiconductor (SnNb2O6 or WO3) composites are able to display higher visible light photoactivity than blank semiconductor toward dye degradation under visible light irradiation. In addition, constructing graphene-based composite photocatalysts with specific structure also provides an alternative way for increasing the interfacial contact between graphene and the photoactive components.227−236 For instance, Choi’s group has prepared size-controlled nano-graphene oxide (nano-GO; 510 nm), which are the p-type and n-type photoresponses, respectively. When the irradiation wavelength extended to the UV light region, photocurrent conversion from cathodic to anodic can be observed for GOT-A, and no photocurrent conversion is found for GOT-E. Such wavelength-dependent photocurrent conversion of GOT-A suggests the formation of a p−n junction, which contributes to the enhanced visible light photoactivity of GOT-A for degradation of methyl orange. However, in this work, there are two issues unaddressed: (i) the XPS results indicate that GO has been reduced remarkably during the preparation procedure. Therefore, it should be regarded as RGO instead of GO. (ii) Considering that GO often features ptype semiconductor characteristics due to the large electronegativity of oxygen atoms as compared to carbon atoms,238,241 the explanation for how the concentration of GO in the starting solution affects the semiconductor type of GO has been still unavailable, which needs to be further studied. Substitutional nitrogen doping has proven to be an efficient approach to convert graphene into an n-type semiconductor due to its similar atomic size and larger valence electron number as compared to those of carbon atoms.237,245 Therefore, N-doped graphene has been integrated with ptype semiconductor to construct graphene-based composite photocatalysts with a p−n junction.217,243,244 For instance, Ndoped RGO has combined with p-type ZnSe through a one-pot hydrothermal process using GO nanosheets as the precursor of graphene and ZnSe-(diethylenetriamine)0.5 ([ZnSe](DETA)0.5) nanobelts as the source of ZnSe nanoparticles and N element.217 It has been found that the as-prepared N/RGOZnSe composites exhibit remarkably enhanced photocatalytic activities for the bleaching of methyl orange under visible light irradiation, which can be partially ascribed to the p−n junction formed in the N/RGO-ZnSe heterosystem.217 In addition, ptype MoS2 nanoplatelets have been deposited on the n-type Ndoped RGO nanosheets to form nanoscale p−n junction featured composites (Figure 25A).243 The photoelectrochemical measurement shows that the heterostructure in MoS2/NRGO greatly enhances the charge generation and suppresses the charge recombination, which is responsible for enhancement of solar hydrogen generation, as illustrated in Figure 25B. In addition to p−n junction, Schottky junction has also been claimed to form between graphene and the semiconductor. Liu

of nano-RGOT exhibits higher H2 production rate and photocurrent than large-RGOT under UV light irradiation, which is ascribed to the fact that the presence of the nano-GO shell on the surface of TiO2 facilitates the interfacial electron transfer due to the maximized direct interfacial contact between nano-RGO and TiO2 in such a core−shell structure. The above selected examples highlight the significant influence of interfacial contact on the photoactivity of the graphene-based composite photocatalysts. To construct efficient graphene-based composite photocatalysts with high performance, strengthening the interfacial interaction between graphene and the photoactive component (e.g., semiconductor) is an important strategy for utilizing the electrical conductivity of graphene and boosting the charge carrier separation across the interfacial domain.


7.5. Interfacial Junction Effect

We can learn from the above discussion in section 5.2.4 that GO/RGO with appropriate oxidation degree and chemical modified graphene are able to act as finite-gap semiconductors.43,44,46,174 The type of conductivity (i.e., n- or ptype) for the graphene-related materials is determined by the electronegativity of the atom bonded to carbon atoms in the graphene skeleton or the valence electron number of the doped heteroatoms relative to that of the carbon atom.237,238 It has been recognized that the junction formed at the interface between semiconductor and another component (e.g., metal and secondary semiconductor to result in Schottky and p−n junction, respectively) in the semiconductor-based composite photocatalysts benefits more efficient separation and transfer of photogenerated charge carriers, thus enhancing the photocatalytic performance of the composites.239 Inspired by such advances achieved in the field of semiconductor-based composite photocatalysts, the graphene-related materials featuring semiconductor characteristics have been integrated with other components to construct a specific junction to improve the photoactivity of the resultant composites.217,218,240−244 For example, a series of GO-TiO2 composites (GOT) with p−n junction have been prepared by using GO and TiCl3 as the reactants through a wet-chemistry approach followed by calcination treatment (as exemplified by GOT-A in Figure 24A).240 The GO in the composites acts as a sensitizer for TiO2 even under the visible light irritation with wavelengths longer

Figure 24. (A) TEM image of GOT-A (the inset shows the corresponding selected area electron diffraction pattern); (B) transient photocurrents of GOT-A and GOT-E under different kinds of irradiation with an electrode potential of 0 V vs Ag/AgCl. Reprinted with permission from ref 240. Copyright 2010 American Chemical Society.240

Figure 25. (A) HRTEM image of the MoS2/N-RGO composite; (B) schematic illustration of the charge separation and transfer in the MoS2/N-RGO composite for photocatalytic H2 generation. Reprinted with permission from ref 243. Copyright 2013 American Chemical Society.243 S



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Figure 26. (A) Schematic illustration for the fabrication of (RGO-M)-CdS (M = Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) composites in which metal ions are introduced into the interfacial layer matrix between RGO and semiconductor CdS; (B) photoluminescence (PL) spectra and (C) electrochemical impedance spectroscopy (EIS) Nyquist plots of blank CdS, RGO-CdS, and (RGO-M)-CdS composites with different weight addition ratios of RGO. Reprinted with permission from ref 252. Copyright 2014 American Chemical Society.252 (D) Schematic diagram of the charge carrier transfer in the ternary (RGO-Pd)-CdS composites under visible light irradiation. Reprinted with permission from ref 253. Copyright 2014 Royal Society of Chemistry.253

performance for a specific graphene-based composite system should be at the system level. This concept highlights the importance of a system-level planning effort beginning from individual components, followed by interface optimization, which are rationally integrated in the entire composite system.22,251 In other words, the investigation of isolated components within a device or catalyst system is not sufficient to solve the technological challenges involved in the development of an environmentally benign energy infrastructure. The integration of individual components into a complete and functioning system while simultaneously optimizing the resulting interfaces is inevitably needed. Recently, a simple and general approach to improve the photoactivity of RGO-semiconductor CdS composites by the addition of a tiny amount of metal ions as the generic interfacial mediator has been reported, which is able to optimize the interfacial atomic charge carrier transfer pathway and efficiency resulting from the rational synergy interaction between respective individual components integrated in the composites.252 Figure 26A displays the typical synthesis procedure of (RGO-M)-CdS (M = Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) composites. A tiny amount of metal ions is introduced into the interfacial layer matrix between RGO and CdS, which is achieved by the premodification of the GO sheet through the electrostatic attractive interaction between positively charged metal ions and negatively charged oxygenated functional groups on the GO surface in water. The premodification process does not have an influence on the subsequent formation of CdS on the RGO sheet, and the intimate interfacial contact between CdS and RGO can still be retained the same as that for RGO-CdS without the addition of metal ions. The presence of metal ions in the interlayer matrix between CdS and RGO is able to optimize the charge carrier transfer pathway from CdS to the electron conductive RGO sheets, by which the separation and transfer of charge carriers are improved (Figure 26B and C). This in turn leads to the remarkably higher activity of (RGO-M)-CdS composites than

et al. used the Schottky junction model to explain the charge transfer process in RGO-TiO2 composites.246 In the proposed model, the Fermi level of TiO2 is regarded to move toward the vacuum level after the UV light irradiation; the semiconductorto-graphene barrier decreases and electrons in the conduction band of TiO2 transfer to RGO. However, it should be noted that although pristine graphene is a semimetal with a zero band gap which makes the formation of a metal−semiconductor like junction between graphene and semiconductors possible,11,247 the contact between semiconductors and RGO is not the case due to the significantly different properties of RGO as compared to the pristine graphene as discussed above. Therefore, it is misleading to use Schottky junction to describe the interfacial contact between RGO and semiconductors. In contrast, the high-quality graphene obtained by non-solutionbased methods (e.g., CVD and epitaxial growth on SiC) can be used to construct Schottky junction with semiconductor materials.248,249 7.6. System-Level Optimization

All of the aforementioned strategies often focus on only considering the isolated constituents (either graphene or photoactive components such as semiconductor), instead of optimizing the entire system of graphene-based composites. However, this is not the full story of fabricating graphene-based composites with high activity for potential practical applications. As is well-known, photocatalysis is an artificial technique that tries to mimic the natural photosynthesis observed in green plants and a few other micro-organisms, thus achieving the direct conversion of solar energy into chemical energy. The biological systems in the natural world enjoy the highest degree of complexity with exquisite functions.250 To fully understand the biological systems, the system level consideration of the structure and dynamics of cellular and organismal function is of crucial importance.250 Analogously, in order to design efficient artificial photosynthetic materials systems to meet practical applications, the optimization of the overall photocatalytic T



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scopic dynamic process of the generation, recombination, separation, and transfer of charge carriers in any photocatalyst lies in the center of affecting the photocatalytic performance.254,255 In this section, the characterization techniques, including femtosecond transient absorption spectroscopy, X-ray absorption spectroscopy, time-resolved fluorescence spectroscopy, open-circuit potential decay, transient photocurrent decay, Nyquist impedance plots, the transient photovoltage (TPV) technique, etc., that can be used to study the charge carrier dynamics in the graphene-based composites will be discussed briefly with selected examples and with prospected for further extension. The electron transfer process, including the electron transfer pathway and the kinetics, is closely related to the photocatalytic performance of the graphene-based composites. Some pioneering works have been reported to investigate the microscopic electron transfer process in the graphene-based composite photocatalysts by using different characterization techniques.222,256−262 For example, Wojcik et al. have used femtosecond transient absorption measurements to monitor the interaction and electron transfer between the 5,10,15,20tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) molecule and RGO.256 As shown in Figure 28A, the spectra recorded immediately after the 387 nm laser pulse excitation exhibit the absorption bleaching below 480 nm, which results from ground-state depletion as the TMPyP molecules are excited with the laser pulse. The absorption with a maximum around 515 nm corresponds to the formation of an excited singlet state 1(TMPyP)*. The transient absorbance behaviors in time at 450 and 515 nm (Figure 28B) reveal that the initial decay of the 1(TMPyP)* at 515 nm corresponds well with the ground-state bleach recovery at 450 nm, and the residual absorbance appears in the traces recorded at 515 and 450 nm. The presence of a long-lived absorption is indicative of the fact that the interaction of 1(TMPyP)* with RGO leads to the formation of a long-lived transient. The appearance of a transient at long delay time is attributed to the porphyrin radical cation (TMPyP)•+ formed during the electron transfer from 1(TMPyP)* to RGO. X-ray spectroscopy, particularly soft X-ray absorption spectroscopy (XAS) for 3d transition metals, is able to provide sensitive measurements of the metal spin state and the electronic structure at an atomic resolution,263,264 which thus can be used to investigate the electron transfer of materials.265 The electron transfer properties of anatase TiO2/RGO composites have been studied by soft XAS measurement.257 As displayed in Figure 28C, the Ti L edge XAS spectra of highly reactive TiO2 (HR-TiO2) with exposed facets and TiO2/RGO consist of two sets of peaks (L3 and L2) in the energy range of 454−470 eV due to spin−orbit coupling splitting of the initial 2p states into 2p3/2 and 2p1/2. Both of the L3 and L2 features further split into t2g (formed by dxy, dxz, dyz orbitals) and eg (formed by dx2−y2 and dz2 orbitals) features because of the low symmetry of the Oh ligand field compared to the spherical field.266 The XAS spectra of the samples indicate that the introduction of RGO leads to a shift of the first two peaks (L3 t2g and L3 eg) to high energy, which is often ascribed to the change of the metal’s chemical state or oxygen vacancies.267 There is no corresponding change in the O K edge for TiO2/ RGO in the XAS spectra. Therefore, these results indicate that the electron transfers from Ti 3d orbitals in the conduction band of TiO2 to the C 2s orbitals of graphene in the TiO2/ RGO composite.257

the optimal 5%RGO-CdS toward visible-light-driven photocatalytic selective redox processes. Furthermore, for the optimal (RGO-M)-CdS photocatalysts, the weight addition ratio of RGO is significantly increased to 10 or 30%, which is higher than the usually optimal weight addition ratio of graphene (≤5%) in the graphene-semiconductor composite photocatalysts.22−33 Such an “interfacial-mediator” strategy has also been adopted to construct (RGO-Pd)-CdS composites, which shows remarkably enhanced visible light photoactivity as compared to the optimal binary RGO-CdS.253 The photocatalytic performance enhancement of (RGO-Pd)-CdS is ascribed to the optimized spatial charge carrier transfer across the interface resulting from the introduction of Pd nanoparticles as mediator into the interfacial layer between RGO and CdS, as pictorially shown in Figure 26D. Besides, the negative light “shielding effect” of graphene can be partially counterbalanced in the (RGO-Pd)-CdS composites. Therefore, the results demonstrate that the interfacial composition engineering not only optimizes the photogenerated charge carrier transfer pathway across the interface between graphene and semiconductor but also partially offsets the “shielding effect” induced by a high addition of graphene in the graphene-based photoactive composite systems.252,253 This strategy manifests the importance and encouraging promise of optimizing the graphene-based composite photocatalysts from the angle of a system-level materials engineering concept; that is, the research effort should start from individual components followed by interface design and optimization, as illustrated in Figure 27. Ongoing efforts in fabricating graphene-based

Figure 27. Systems materials engineering triangle. Reprinted with permission from ref 251. Copyright 2012 Nature Publishing Group.251

composite photocatalysts can benefit from this system-level concept, i.e., precisely and rationally focusing on the design and optimization of individual components and the interface.

8. CHARGE CARRIER DYNAMICS IN THE GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS The introduction of graphene to the photocatalytic systems has often focused on utilizing the tunable structural and electronic properties of graphene to improve the activity of graphenebased composite photocatalysts by improving the probability of charge carriers to participate in the photoredox reactions.22−33 Therefore, the in-depth study on the underlying charge carrier dynamics in the graphene-based composites is fundamentally very crucial for guiding us to design and synthesize the more efficient composite photocatalysts. This is because the microU


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Figure 28. (A) Time-resolved transient absorption spectra and (B) normalized absorption time profiles taken at 450 and 515 nm recorded for TMPyP adsorbed on RGO films upon a 387 nm laser pulse excitation. Mean laser power during all the experiments: 41 mW/cm2. Reprinted with permission from ref 256 Copyright 2010 American Chemical Society.256 (C) Ti L edge XAS spectra of TiO2 and TiO2/RGO composite. Reprinted with permission from ref 257. Copyright 2011 American Chemical Society.257

where RhB → x indicates the emission quenching of RhB to x. The ket values corresponding to C-TiO2, C-TiO2/μRGO, and nRGO@C-TiO2 are calculated to be 8.04 × 106 s−1, 1.50 × 107 s−1, and 1.94 × 107 s−1, respectively. These results indicate that the incorporation of nRGO into C-TiO2 can facilitate the interfacial electron transfer process more efficiently as compared to μRGO, which could contribute to the enhanced photoactivity of nRGO@C-TiO2. As a competitive process of electron transfer, the recombination kinetics of electrons with photogenerated holes has also been investigated by different decay spectra, including the open-circuit potential decay and transient photocurrent decay curves.268−270 Choi and co-workers have measured the open-circuit potential (OCP) decay kinetics to evaluate the recombination properties of the graphene oxide sheets-incorporated TiO2 nanofibers (referred as GO-TiO2 NF).269 Figure 30 shows the OCP decay profiles in TiO2 NF

Additionally, the time-resolved spectroscopy technique (e.g., time-resolved fluorescence spectroscopy and transient absorption spectroscopy) affords an effective approach to study the electron transfer kinetics in the graphene-based composite photocatalysts.222,258−262 For instance, the interfacial charge transfer rates of C-doped TiO2 nanoparticles (C-TiO2), microsized RGO supported C-TiO2 (C-TiO2/μRGO), and nanosized RGO (nRGO) wrapped C-TiO2 (nRGO@C-TiO2) have been measured through time-resolved fluorescence experiments using rhodamine B (RhB) as an indicator dye.260

Figure 29. Time-resolved fluorescence spectra of 1 μM rhodamine B (RhB) in the presence of bare C-TiO2, 0.1%nRGO@C-TiO2, and CTiO2/0.1%μRGO. Reprinted with permission from ref 260. Copyright 2014 Elsevier.260

As shown in Figure 29, the time-resolved fluorescence spectra are fitted with a biexponential decay function eq 1. I(t ) = A1e−t / τ1 + A 2 e−t / τ2

Figure 30. Time profiles of the open-circuit voltage decay (after UV light irradiation was turned off), for TiO2 NF and GO-TiO2 NF electrodes. Reprinted with permission from ref 269. Copyright 2014 Elsevier.269

(1)

which contains two lifetimes (τ1 and τ2), and the corresponding amplitudes (A1 and A2). The lifetime provides information on the quenching behavior in the testing system. When electron transfer from RhB to adjacent material (x) is the dominant process, the electron-transfer rate (ket) can be estimated from the emission lifetime using eq 2. 1 1 − ket(RhB → x) = τRhB → x τRhB (2)

and GO-TiO2 NF after light was turned off. Obviously, GOTiO2 NF exhibits a much slower OCP decay profile than TiO2 NF. The OCP signals are normalized, and the average recombination rate constant (kr) can be estimated by fitting the decay profile to a first-order kinetic model eq 3.271,272 E − Eph E0 − Eph V

= 1 − exp( −krt )

(3) DOI: 10.1021/acs.chemrev.5b00267 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 31. (A) Nyquist impedance plots of TiO2 nanorods-RGO and TiO2 nanorods (the inset is the equivalent circuit). Reprinted with permission from ref 273. Copyright 2013 Royal Society of Chemistry.273 (B) TPV spectra of RGO, anatase TiO2, and RGO-anatase TiO2. Reprinted with permission from ref 274. Copyright 2011 Royal Society of Chemistry.274

Figure 32. (A) Open-circuit voltage decay curve and (B) corresponding electron lifetime in ZnO hollow spheres and 3.56%RGO-ZnO. Reprinted with permission from ref 277. Copyright 2012 American Chemical Society.277

The TiO2 nanorods-RGO shows a larger τn of 2.06 s as compared to the TiO2 nanorods (0.11 s) which implies a longer electron lifetime in TiO2 nanorods-RGO.273 TPV technique provides a useful approach to study the charge carrier transfer properties of semiconductor materials.279 A photovoltage (PV) arises when light induced excess charge carriers are separated in space.279 This technique has been used to characterize the transfer of photogenerated charge carriers as well as the electron lifetime in the RGO-TiO2 composites.256,273,274 It can be seen from Figure 31B that the TPV spectrum of blank TiO2 displays a low photovoltaic response of ca. 0.03 mV, while no obvious photovoltaic response has been observed over pure RGO.274 After the integration of anatase TiO2 with RGO, the photovoltaic response is significantly increased about 5-fold to reach 0.16 mV. The positive signal and improved photovoltaic response of RGO-anatase TiO2 imply that once the anatase TiO2 is photoexcited, the generated electrons will transfer from anatase TiO2 to RGO, leaving holes in anatase TiO2, thus resulting in the efficient separation of electron−hole pairs. The comparison of the photovoltaic response between RGO-TiO2 and anatase TiO2 also shows that the mean lifetime of electron−hole pairs is prolonged from ∼10−7 s to ∼10−5 s. As a commonly used quantitative method to determine the electron lifetime, open-circuit voltage (VOC) decay measurement has been utilized to investigate the RGO-hierarchical ZnO hollow sphere composites.277 Figure 32A compares the VOC decay curves of 3.56%RGO-ZnO and ZnO hollow spheres, showing that the VOC decay rate of 3.56%RGO-ZnO is lower than that of ZnO hollow spheres. The electron lifetime (τ) can be calculated according to eq 5.

where E represents the OCP at any time, E0 the stationary OCP value in the dark, and Eph the photostationary OCP value.271,272 The fitted kr for GO-TiO2 NF equals 1.88 × 10−3 s−1, which is much smaller than that for TiO2 NF (9.84 × 10−3 s−1). This result indicates that the charge recombination is remarkably retarded by the incorporation of GO into TiO2 NF. The results of competition between electron transfer and charge carrier recombination can be directly reflected by the electron lifetime, which is one of the most common parameters to evaluate the efficacy of graphene to accept and shuttle the photoexcited electrons in the graphene-based composite photocatalysts. Thus far, there have been some qualitative and quantitative measurements to assess the electron lifetime in the graphene-based composite photocatalysts, including the Nyquist impedance plots,268,273 the transient photovoltage (TPV) technique,257,274,275 and the decay of the open-circuit voltage upon termination of irradiation.276,277 For instance, Gao et al. have evaluated the effect of RGO hydrogel on the electron lifetime of TiO2 nanorods-RGO hydrogel composites based on the Nyquist plots, as show in Figure 31A.273 The charge transfer resistance between the electrodes and the electrolyte interface (Rct) of TiO2 nanorodsRGO is ∼3.51 kΩ, which is smaller than that for the TiO2 nanorods (∼11.82 kΩ), indicating a better electrical conductivity between the TiO2 nanorods-RGO electrode and electrolyte. Furthermore, the time constant (τn) related to the electron lifetime of the sample is determined by Rct and chemical capacitance (Cμ) using eq 4.278 τn = Rct × Cμ

(4) W



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Figure 33. (A) Peak deconvolution of the C 1s XPS core level of TiO2-GO/RGO thin films (a) as-deposited, (b) annealed at 400 °C in air, reduced by the solar light-assisted photocatalytic reduction for (c) 0.5, (d) 1, (e) 2, and (f) 4 h in ethanol, and (g) after photocatalytic degradation of the bacteria in aqueous phase under solar light irradiation for 80 min. Reprinted with permission from ref 16. Copyright 2009 American Chemical Society.16 (B) Peak deconvolution of the C 1s XPS core level of TiO2-GO/RGO sheets after (a) 0, (b) 1, (c) 2, (d) 4, (e) 10, and (f) 24 h UV light irradiation time. Reprinted with permission from ref 288. Copyright 2010 American Chemical Society.288

τ=−

−1 kBT ⎛ dVoc ⎞ ⎜ ⎟ e ⎝ dt ⎠

dynamics of exciton or electronic excited states in other systems can also be adopted. The progress achieved in the field of X-ray spectroscopy, ultrafast spectroscopy, and their combination in recent years offers more possibilities for elucidating the charge carrier dynamics in the graphene-based composites. In particular, the extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and resonant inelastic X-ray scattering (RIXS), which describe the atomic and electronic structure and the changes induced by the optical excitation of the valence orbitals,265,284,285 can be used to investigate the electronic excited-state properties, the electron transfer pathway, and the trapping sites for photoelectrons in the graphene-based composite photocatalysts. On the other hand, the X-ray free electron lasers (XFEL) with the capability to produce high intensity X-rays on the pico- and femtosecond time scales make single shot experiments feasible,263,286 which may provide more specific information on the time-resolved charge carrier and structural dynamics in the graphene-based composite photocatalysts. Time-resolved multidimensional coincidence imaging, especially time-resolved photoelectron angular distributions (TRPADs), offers a sensitive probe of the electronic evolution of the excited state,287 which would also be expected to reveal the subtle details of the nonadiabatic electronic dynamics in the graphenebased composite photocatalysts. Besides, the combination of the theoretical investigations, such as molecular dynamics (MD) simulations and time-dependent density functional theory (TDDFT) calculations,263 with the sophisticated experimental techniques is expected to deepen our fundamental knowledge on the charge carrier dynamics in the graphenebased composite photocatalysts, which we believe would further facilitate the elegant design of graphene-based composites with enhanced photoactivity for target redox processes.

(5)

where kB is the Boltzmann constant, T is temperature, and e is the elementary charge.276 The calculated τ is plotted in Figure 32B. It is obvious that τ of the 3.56%RGO-ZnO is longer than that of ZnO hollow spheres, which quantitatively indicates that the introduction of graphene can slow the charge carrier recombination rate in the RGO-ZnO composites. Besides the experimental techniques as discussed above, some theoretical simulations have also been conducted to evaluate the charge carrier dynamics in the graphene-based composite photocatalysts.280−283 For instance, Long et al. have investigated the electronic structure and mechanisms of the photoinduced interfacial charge transfer, energy relaxation, and energy transfer in a hybrid graphene-TiO2 system in real time and at the atomistic level by a time-domain ab initio analysis.280 They found that the electron can be injected from graphene into TiO2 on an ultrafast time scale due to strong donor− acceptor coupling and that the photoinduced electron transfer occurs several times faster than the electron−phonon energy relaxation. This result suggests that graphene-TiO2 interfaces can form the basis for photovoltaic and photocatalytic devices using visible light.280 In addition, finite element analysis has been performed for RGO-BiVO4 composite to study the effects of RGO on the photoelectrochemical performance of BiVO4.281 The digital simulation parameters, including the hole and electron transfer rate constants, the surface recombination rate constants, the electron/hole mobility, and the electron/hole recombination lifetimes of BiVO4, indicate that the addition of RGO into BiVO4 can improve the charge carrier diffusion length and mobility, and reduce the recombination rate for the RGO-BiVO4 composites. In the future extension of further understanding of microscopic charge carrier dynamics in the graphene-based composite photocatalysts, the methodologies used to study the X


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Figure 34. C 1s XPS spectra and its peak deconvolution of (A) fresh TiO2-5% RGO and (B, C) TiO2-5% RGO after visible irradiation for 4 and 10 h in the solvent of benzotrifluoride, respectively. Reprinted with permission from ref 20. Copyright 2011 American Chemical Society.20

9. PHOTOSTABILITY OF GRAPHENE IN THE GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS In addition to understanding and taking advantage of the favorable physicochemical properties of graphene in enhancing the photocatalytic performance of graphene-based composites, the photostability of graphene in the composites should also be taken into account when constructing efficient and stable graphene-based composite photocatalysts. Thus far, there have been some studies involving the photostability of graphene during the photocatalytic process, which are always focused on the investigation related to RGO, the most common form of graphene in the graphene-based composite photocatalysts.16,20,288−294 Nevertheless, this significant issue has not been exclusively discussed in previous review articles.22−33 In this section, we will briefly sum up the research works concerning the photostability of graphene upon light irradiation, and discuss the possible influential factors. Accordingly, the efficient strategies to prevent graphene in the graphene-based composite photocatalysts from photodegradation will be also proposed. In 2009, Akhavan et al. reported the use of RGO for enhancing the photocatalytic performance of semiconductor TiO2 toward degradation of Escherichia coli (E. coli) bacteria in aqueous solution under solar light irradiation.16 It can be seen from Figure 33A(a, b) that the Ti−C bonds are formed between the RGO platelets and their beneath TiO2 thin film in the TiO2-RGO films after the postannealing of TiO2-GO films at 400 °C in air. The carbon skeleton of GO sheets has proven to be chemically stable not only during the photocatalytic reduction of GO in ethanol upon solar light irradiation, but also after solar-light-driven photocatalytic degradation of the bacteria in aqueous phase, as revealed by the X-ray photoelectron spectroscopy (XPS) results in Figure 33A(b−g).16 However, in a subsequent work, they observed the photodegradation of GO in the TiO2-GO composite under UV light irradiation.288 As shown in Figure 33B, after the effective photocatalytic reduction of GO sheets by TiO2 nanoparticles in ethanol, the carbon content of RGO gradually decreases along with increasing the UV light irradiation time.288 It is worth noting that, in this TiO2-GO composite, GO is physically attached to the TiO2 and there are no Ti−C bonds formed, which should be the key reason explaining the different photostability of graphene in these two composite-based photocatalytic systems. 16,288 Obviously, with regard to graphene-based composite photocatalysts, the strong interfacial

interaction between graphene and the other component (e.g., semiconductors) plays a crucial role in stabilizing graphene under light-irradiated photocatalytic reaction conditions, which would provide a feasible approach for preventing graphene in the composites from photodegradation upon light irradiation. Afterward, the RGO nanomeshes have been synthesized through the reduction and local degradation of the GO sheets by using the vertically aligned ZnO nanorods arrays under the irradiation of a UV-LED source.289 It has been proposed that the local degradation of RGO sheets is induced by the highly reactive hydroxyl radicals (•OH), which are produced by the reaction of photoexcited holes and water/hydroxyl groups on the surface of the semiconductor ZnO nanorods. The role of • OH radicals played in photodegrading graphene has also been confirmed by the research works from Liu’s group291 and Kamat’s group.290,292 However, in some other graphene-based composite photocatalytic systems where •OH radicals are present, no obvious degradation of graphene occurs during the photocatalytic process, as reflected by the high stability of the c o m p o s i t e s i n t h e s u c c e s s i v e r e c y c l i n g a c t iv it y tests.17,122,124,295−301 For example, Ghosh et al. have reported that the reused CdSe-RGO/TiO2 composite does not show apparent loss of photoactivity toward degradation of organic dyes (methyl orange and rhodamine B) in an aqueous phase with visible light irradiation for 12 h in the presence of •OH radicals, indicating the good photostability of the composite photocatalyst.298 Through comparing these works,17,122,124,289−292,295−301 it can be found that the constituents contained in the compositebased photocatalytic systems should have a great effect on the photostability of graphene. Regarding the systems where the photodegradation of graphene is observed,289−292 there are no additional compositions existing to react with •OH radicals except graphene, which leads •OH radicals to exclusively oxidize and degrade graphene. In contrast, the presence of substrates (e.g., dyes and alcohols) that are able to react with • OH radicals can effectively prevent graphene in the composites from photodegradation.17,122,124,295−301 Therefore, the addition of other species in the specific photocatalytic systems to readily and timely consume •OH radicals is another viable approach to enhance the photostability of graphene in the graphene-based composite photocatalysts. As for the various graphene-semiconductor composites applied for photocatalytic selective redox processes, no obvious degradation of graphene has been observed.20,123,192,204,212,252,253,293,302−309 For example, the photoY


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Figure 35. (A) Recycling photocatalytic selective reduction of 4-nitroaniline over the optimal 5%RGO-ZnO nanorods-CdS composite under visible light irradiation (λ > 420 nm) with the addition of ammonium formate as quencher for photogenerated holes and N2 purge at room temperature in an aqueous phase; SEM images of (B) fresh and (C) used 5%RGO-ZnO NRs-CdS. Reprinted with permission from ref 293. Copyright 2015 John Wiley & Sons, Inc.293

Figure 36. (A) Photographs of RGO-TiO2 aqueous suspension after various UV light irradiation times (the concentrations of RGO and TiO2 in the suspension are 0.5 mg/mL and 1 mM, respectively); (B) schematic depiction of the oxidative fragmentation and mineralization of RGO during UV light irradiation of RGO-TiO2 aqueous suspensions; (C) comparison of absorbance changes at 600 nm following UV-irradiated (a) TiO2 and (b) RGO-TiO2 suspensions containing naphthol blue black (NBB) dye (the concentrations of dye, RGO, and TiO2 in the suspension are 45 μM, 0.05 mg/mL, and 0.1 mM, respectively); the inset of C is the visual depiction of color changes for the suspensions. Reprinted with permission from ref 290. Copyright 2014 American Chemical Society.290

stability of RGO in the TiO2-RGO composites prepared by an in situ synthesis procedure during the photocatalytic selective oxidation of alcohols in the solvent of benzotrifluoride (BTF) has been investigated.20 It is easy to see from Figure 34 that the normalized intensity of the main peak of C 1s XPS spectra for the fresh TiO2-5% RGO and TiO2-5% RGO with visible light irradiation for 4 or 10 h shows almost no change, which demonstrates that the RGO sheets in the TiO2-5% RGO composite are chemically stable in the photocatalytic reaction system. Notably, being different from the case in water, there are no • OH radicals generated in the solvent of BTF.20,192,204,212,252,253,302−305,310,311 Therefore, it is understandable that, under such photocatalytic reaction condition, the occurrence of photodegradation of RGO caused by •OH radicals attack can be ruled out, which thus accounts for the excellent photostability of RGO observed in the reaction system.20,192,204,212,252,253,302−305

Hitherto, it has been well established that the photodegradation of graphene is induced by the direct oxidation of graphene by •OH radicals, which are generally formed by the reaction of the photoexcited holes and the surrounding water/ hydroxyl or photolysis of H2O2, rather than other oxidative species (e.g., holes and H2O2).289−292 Therefore, the use of organic solvent can exclude the presence of water/hydroxyl, by which •OH radicals are not formed in the photocatalytic system. This effectively prevents the photodegradation of graphene for graphene-based composite photocatalyst.20,192,204,212,252,253,302−305 In addition, in order to prohibit the formation of •OH radicals, it can be considered to add hole scavengers (e.g., ammonium formate) to the photocatalytic system while the photogenerated electrons timely participate in the reduction of reactant substrates in the photocatalytic system.123,253,293,306−309,312 In this way, the inhibition of graphene photodegradation can also be well achieved, as evidenced by Z



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addition ratio of graphene in most of the graphene-based composite photocatalysts (i.e., often less than 5 wt %).22−33 Under such conditions, the exposure area of RGO should be quite large. Considering that the uncovered graphene sheets may be more susceptible to the •OH radicals attack, the larger exposure area of RGO further increases the vulnerability of RGO sheets to the oxidation by the attack of •OH radicals. Furthermore, the very poor interfacial interaction resulting from the physical mixture of RGO and TiO2 also increases the susceptibility of RGO toward •OH radicals promoted by UVirradiated TiO2 aqueous suspensions. On the other hand, the phenomenon observed in the RGO-TiO2 suspensions containing dye under UV light irradiation conveys useful information to us that the presence of certain concentrations of reactant substrate (i.e., dye) in the photocatalytic reaction system is able to compete with RGO to consume •OH radicals. This may provide a viable approach for preventing graphene from photodegradation during the photocatalytic redox process, which is consistent with the discussions above. The influence of the exposure area of graphene on its photostability has also been evidenced by the results from Min’s group.294 In their study, RGO-TiO2 films have been fabricated by UV-assisted incorporation of TiO2 nanoparticles (NPs) onto RGO films. For comparison, the control RGOTiO2 films have also been prepared through the same strategy except the exclusion of UV light irradiation. They found that UV light irradiation can facilitate the incorporation of TiO2 NPs onto RGO films by enhancing the interaction between TiO2 and RGO. Therefore, the RGO-TiO2 films possess higher coverage of TiO2 NPs on the RGO films than control RGOTiO2 films, as revealed by the XPS results in Figure 37A. The

the excellent recycling activity test on visible-light-driven selective photoreduction of nitroaromatics over graphenesemiconductor composites with the presence of hole scavengers in water.123,253,293,306−309,312 For instance, RGO-ZnO nanorods (NRs)-CdS ternary hierarchical nanostructures have been prepared via a simple, low-temperature synthesis approach and utilized for visible-light-driven photocatalytic selective reduction of nitroaromatics with the addition of ammonium formate as hole scavenger and N2 purge in an aqueous phase.293 As shown in Figure 35A, no significant loss of photoactivity is observed for the optimal 5%RGO-ZnO NRs-CdS during four successive recycling activity tests for reduction of 4-nitroaniline under controlled experimental conditions. Besides, the comparison between the morphology of the fresh and used 5%RGO-ZnO NRs-CdS (Figure 35B and C, respectively) reveals that no obvious morphological change occurs in the composite photocatalyst nanostructures after the photocatalytic reaction. Considering that, in the RGO-ZnO NRs-CdS composite, CdS sensitized ZnO NRs arrays are formed on the RGO substrate, the stability in its morphology indicates no obvious degradation of the RGO substrate during the photocatalytic process, as also reflected by the excellent repeated photoactivity of 5%RGO-ZnO NRs-CdS. In such controlled reaction conditions, the efficient consumption of photogenerated holes and electrons by hole scavenger (i.e., ammonium formate) and the adsorbed nitroaromatics, respectively, prevents the RGO sheets from photodegradation under visible light irradiation in the aqueous phase. More recently, Kamat’s group has demonstrated the mineralization of RGO induced by the attack of •OH radicals generated at the TiO2 surface in an aqueous phase under UV light irradiation.290 It can be seen from Figure 36A that, with the illumination of UV light for 30 min, the initial dark brown TiO2-RGO aqueous suspension becomes a light brown one, indicating the oxidative transformation of RGO to GO in the suspension. The prolonged irradiation time induces further increase in light transmittance of the suspension, and finally, after 75 min of UV light irradiation, the solution is colorless, suggesting the mineralization of RGO. The possible mechanism for the mineralization of RGO in RGO-TiO2 suspensions has been proposed according to the changes of absorbance and total organic carbon (TOC) content of the suspensions upon the UV light irradiation. As illustrated in Figure 36B, the initial • OH attack leads to scission of the RGO sheets into polycyclic aromatic hydrocarbons (PAHs)-like compounds and the continued UV light irradiation promotes the mineralization of these fragments to CO2 and H2O. Additionally, they have also compared the photodegradation rate of dye naphthol blue black (NBB) in TiO2 and RGO-TiO2 suspensions under UV light irradiation. It is easy to see from Figure 36C that the presence of RGO in the suspension (curve b in Figure 36C) can change the degradation rate of NBB by competing with the dye for reacting with •OH radicals. The initial rapid change in absorbance (zone I) is due to the breakdown of conjugation in dye, and the slower change in absorbance in zone II is the degradation of residual dye and RGO. Continuous irradiation can induce the degradation of both the dye and RGO, as shown in the inset of Figure 36C. It should be noted that, in this photocatalytic testing system,290 the weight ratio of RGO to TiO2 in the aqueous suspension is calculated to be ca. 6.2:1; that is, the content of RGO with regard to the RGO-TiO2 is as high as ca. 86 wt %, which is much higher than the commonly optimum weight

Figure 37. (A) Surface elemental composition of C, O, and Ti from the RGO-TiO2 and control RGO-TiO2 films determined from the XPS spectra; (B) changes of the average sheet resistance of RGO, RGOTiO2, and control RGO-TiO2 films as a function of UV exposure time. Reprinted with permission from ref 294. Copyright 2013 Royal Society of Chemistry.294

changes of average sheet resistance of these films have been investigated under different UV exposure times. As shown in Figure 37B, after 9 h of UV exposure, the significant increase and slight increase in the average sheet resistance are observed for the RGO films and the control RGO-TiO2 films, respectively. These results indicate that the RGO in the control AA



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RGO-TiO2 films is partially protected from suffering UV light induced photodegradation in comparison with the bare RGO films. In contrast, the average sheet resistance of the RGO-TiO2 films decreases from 1.0 × 106 to 6.1 × 105 Ω per sq after 1 h of UV exposure, and this value is maintained with continuous UV exposure. The slight decrease in average sheet resistance of the RGO-TiO2 films is attributed to the further reduction of RGO films by photoexcited electrons trapped on the surface of TiO2 NPs under UV light irradiation.294 The results demonstrate that the more TiO2 NPs incorporated onto the RGO films result in the better protection of the underneath RGO films from photodegradation, partially by preventing the direct UV exposure of RGO.294 Therefore, decreasing the exposure area of isolated graphene sheets in the graphene-based composite photocatalysts is beneficial for increasing the photostability of graphene. As such, we can see that there are several factors that are able to affect the photostability of graphene in the graphene-based composite photocatalysts during the given photocatalytic processes, which include the interfacial interaction between graphene and other components in the composites, the specific reaction conditions of photocatalytic testing systems, and the exposure area of isolated graphene. Although the above discussion is based on the observations for RGO-based photocatalytic systems, it would be applicable to the cases of SEG and OSG to some extent. Accordingly, the effective strategies to inhibit the photodegradation of graphene in graphene-based composites, which is mainly induced by the oxidation of •OH radicals, can be proposed as the following. (i) The achievement of appropriate interfacial interaction between graphene and other components in graphenebased composite photocatalysts can be used as an effective approach to enhance the photostability of graphene under light irradiation. (ii) The photostability of graphene in graphene-based composite photocatalysts can be significantly improved when the strongly oxidative •OH radicals are consumed effectively or absent, which is able to be achieved by the fine control of the reaction parameters, i.e., by introducing other species to compete with graphene for reacting with •OH radicals, or by excluding the existence of water/hydroxyl or by adding hole scavengers, respectively. (iii) Decreasing the exposure area of isolated graphene in graphene-based composite photocatalysts by reasonably controlling the weight addition ratios of graphene and/or improving the preparation methods can diminish the direct contact of graphene with •OH radicals, which thus reduces the possibility of photodegradation of graphene. The above discussions manifest that the experimental observation for photodegradation of graphene is conditional. We can finely control the specific reaction conditions to prevent the photodegradation of graphene in the graphenebased composites during photocatalytic redox processes. The reasonable regulation of the whole graphene-based composite photocatalytic system, including the composition and microscopic structure (especially the interfacial interaction manner of the respective components) of graphene-based composites, the probe reaction, and the specific experimental parameters (e.g., the solvent and scavengers), would aid in guaranteeing the photostability of graphene in the graphene-based composite photocatalysts under light irradiation.

10. COMPARISON BETWEEN GRAPHENE-BASED AND OTHER CARBON ALLOTROPES-BASED COMPOSITE PHOTOCATALYSTS The emergence of graphene has triggered a huge research upsurge in scientific and technological communities,22−37 which spontaneously recalls the familiar situations to us when fullerene (C60) and carbon nanotube (CNT) first appeared.51,313−316 Looking back at the development history of the carbon family, we can find that the utilization of graphene in various areas seems to have a quite similar start to its carbon forebears (e.g., C60 and CNT), especially in the field of photocatalysis.317−320 Since graphene emerges as a new allotrope of carbon, the question naturally arises: what advantages does graphene have over its carbon forebears in improving the performance of the photoactive components? This is a good question but simultaneously rather difficult to answer, although several comparison studies between graphenebased composite photocatalysts and its carbon forebears (e.g., C60 and CNT)-based counterparts have been attempted since 2010. In 2010, Li et al. synthesized a chemically bonded TiO2 (P25)-RGO composite photocatalyst using a one-step hydrothermal treatment of solid P25 nanoparticles with GO.18 They found that P25-1%RGO shows moderately higher photoactivity than P25-1%CNT composite with the same carbon content toward the photodegradation of methylene blue (MB) under both UV and visible light irradiation, which is largely ascribed to the giant 2D planar structure of graphene favorable for dye adsorption and charge carrier transportation. However, it should be noted that some fundamental issues remain unaddressed in this work. First, the effect of different addition ratios of carbon materials on the photoactivity of the P25carbon (RGO or CNT) composite has not been taken into account. Second, the comprehensive comparison (e.g., adsorptivity, light absorption, and charge carrier separation and transfer) between P25-RGO and P25-CNT is absent. Thus, it remains unclear whether the as-claimed unique attributes, including the increased adsorptivity of pollutants, extended the light absorption range, and boosted charge transportation and separation as found for P25-RGO composite, can be similarly observed for P25-CNT composite. In order to address the above issue and timely obviate the exaggeration on the role of graphene in promoting the photocatalysis of graphene-semiconductor composite, Xu’s group has subsequently attempted to perform a relatively systematic comparison between graphene-semiconductor and CNT-semiconductor composite photocatalysts.17 Two series of P25-carbon composites, i.e., P25-RGO and P25-CNT, with different weight addition ratios of RGO/CNT have been prepared via a “hard” integration of solid P25 nanoparticles and GO/CNT that is similar to the previous work.17,18 Taking gasphase degradation of benzene and liquid-phase degradation of dyes as model reactions, it has been found that the key features for P25-RGO composite photocatalysts, including the increased adsorptivity of pollutants, enhanced light absorption range and intensity, promoted charge separation and transportation, and enhanced photoactivity with low content of RGO, can be analogously observed in the P25-CNT counterparts. In particular, the photoactivity between P25-RGO and P25CNT does not make a significant difference. This manifests that, at least for carbon-semiconductor composites prepared by this “hard” integration method, the RGO sheet is in essence AB



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Notably, in the above two works,20,321 the RGO-semiconductor composites have more sufficient interfacial contact than the CNT-semiconductor counterparts. Therefore, more efficient separation and transfer of charge carriers across the interface domain are expected, which makes it easily understandable to observe the higher photoactivity of the RGOsemiconductor composite than the CNT-semiconductor counterpart. However, one may naturally ask if the carbonsemiconductor composite photocatalysts feature intimate interfacial contact in a similar way, what will be the difference of their photoactivity? As a continued effort to answer the above question, Xu and co-workers have synthesized a series of CdS-carbon (C60, CNT, and graphene) composites, all with analogous and intimate interfacial contact via a facile solvothermal approach.302 A comparison among these CdS-carbon composites demonstrates that the optimal CdS-5%RGO does not exhibit superior photoactivity as compared to the optimal CdS-5%CNT and CdS-10%C60 toward selective oxidation of alcohols. Such a similar phenomenon has also been observed for the TiO2carbon composite photocatalysts.304 However, when SEG with lower defect density and higher electrical conductivity than RGO is used as the precursor of graphene, the photoactivity of optimal CdS-5%SEG can be further enhanced to be moderately higher than CdS-5%CNT and CdS-10%C60, but still not exhibit the as-expected prominent advantage over its analogues.302 Thus, this comparison (i) verifies the importance of intimate interfacial contact on influencing the photoactivity of carbonsemiconductor composites; (ii) verifies that the interfacial contact is not the only factor determining the overall photoactivity of graphene-semiconductor composites; (iii) and importantly suggests that there is a wide scope to improve the activity of graphene-based composite photocatalysts by rationally optimizing the precursor of graphene, e.g., improving the electrical conductivity of graphene by virtue of adopting different precursors of graphene, structure/morphology architecturing of graphene, interface engineering, and the systemlevel optimization of graphene-based composites as mentioned in section 7. In fact, the complexity for drawing a definite conclusion for the question “what advantages does graphene have over its carbon allotropes in boosting the photocatalysis?” begins from the moment when we put forward this seemingly simple but actually difficult question at the very start of this section. Such difficulty is not only reflected by the aforementioned pioneering comparison studies in this regard, but also directly mirrored by the versatile, variable physicochemical properties of different carbon materials by themselves. As stated in sections 2−4, the graphene material used for constructing graphene-based composite photocatalyst is far beyond the pristine graphene. There is a distinct gap of significant difference between the physicochemical properties of pristine graphene at an ideal state and graphene derived from different precursors of graphene. Even for the most widely used RGO for fabricating graphenebased composite photocatalysts, its electronic and optical properties depend on many factors, including the number of layers, lateral size, edge structure, defect density, and reduction degree of GO. During the hybridization process of assembling RGO with photoactive components to obtain RGO-based composites, the physicochemical properties of graphene in the composites can further change dramatically when it interacts with the surrounding environment or its structure and morphology are changed.

quite similar to CNT in enhancing the photocatalysis of semiconductor P25 nanoparticles toward degradation of pollutants. The significance of this work is 2-fold. One is to avert the misleading message in the previous report18 to the readership, i.e., the exaggerated unique, advantageous role of graphene over its carbon allotrope (CNT) in boosting the photocatalysis. The other aim is the wish to timely remind the research communities to rationally evaluate the role of graphene in the graphene-based composite photocatalysts and, more importantly, focus on how to make better use of the unique structural and electronic properties of graphene in designing more efficient graphene-based composite photocatalysts, rather than simply “hard” integration of graphene with a photoactive component (e.g., semiconductor) and subjectively imposing hype on the excellent electrical conductivity of graphene. Therefore, one of the key issues on fabricating efficient graphene-based composite photocatalysts is how to utilize the electrical conductivity of graphene effectively. Since the photogenerated charge carrier transfer predominantly occurs across the interfacial domain between graphene and photoactive components in the composites,22−37 enhancing the interfacial contact would be a viable approach to promote the charge carrier transfer efficiency and achieve more efficient graphene-based composite photocatalysts. However, the sufficient interfacial contact cannot be readily achieved by the above ex situ “hard” integration approach.17,18,24 In view of this, in the further work,20 the in situ “soft” integration synthesis approach has been adopted using soluble TiF4 as the precursor of TiO2 instead of solid P25 nanoparticles. This synthesis approach can take advantage of the easily accessible “structuredirecting” role of GO in the liquid phase and thus lead to an intimate interfacial contact between TiO2 and RGO. In sharp contrast, the TiO2−CNT counterparts prepared through the same “soft” integration method feature relatively poor interfacial contact. The optimal TiO2-RGO exhibits a much more active visible light activity than TiO2−CNT and P25RGO toward selective oxidation of alcohols, which is ascribed to the more sufficient interfacial contact of TiO2-RGO than TiO2−CNT and P25-RGO, and thus the more effective separation of photogenerated electron−hole pairs in TiO2RGO.20 These results indicate that (i) the intimate interfacial contact between graphene and semiconductor plays a key role in improving the photoactivity of the composites; (ii) more efficient graphene-based composite photocatalysts can be developed through rational design and engineering of the interface between graphene and the photoactive ingredients. The importance of interfacial contact/interaction between the carbon materials and semiconductors for affecting the photoactivity of carbon-based composites has been further confirmed by the report from Wang’s group.321 They have fabricated CdS-carbon (RGO and CNT) composites by an in situ hydrothermal method using Cd(Ac)2, Na2S, and GO/CNT as the precursors of semiconductor and carbon. The incorporation of RGO or CNT into CdS is able to enhance the photoactivity for both the evolution of H2 and the degradation of methyl orange (MO) under the irradiation of visible light. Under the optimized mass ratios, the CdS-RGO composite exhibits higher photoactivity than the CdS-CNT counterpart, which is attributed to the stronger interaction or larger interfacial contact between CdS and RGO and thus the more efficient transfer of photogenerated electrons from CdS to the RGO sheet than that in CdS-CNT.321 AC



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A similar situation does also exist for CNT. The term “CNT” covers a plethora of different materials.322 Besides the obvious differences in number of walls, i.e., single-walled CNT (SWCNT) and multiwalled CNT (MWCNT), additional dissimilarities in the properties of CNT can be induced by other parameters, including the length, diameter, structure, chirality, electronic type, purity, modification, defect density, and level of agglomeration. 323−326 However, the fine homogeneity control of these factors for CNT remains out of reach. Moreover, the commonly used CNT for fabricating CNT-based composite photocatalysts is often synthesized by chemical vapor deposition (CVD) which generally involves the use of metal nanoparticles (e.g., Fe, Co, Ni, Cu, Au, and Pt) as catalyst. 327,328 Such CVD-grown CNT always contains impurities, e.g., metallic compounds or nanoparticles derived from the catalysts used in nanotube growth, which can be still retained in the matrix of CNT even after extensive acid washing.329,330 These metallic impurities are likely to affect the pathway and efficiency of photogenerated charge carrier transfer and thus the photocatalytic performance of the composites via acting as the mediator.252,253 Therefore, the heterogeneity of carbon materials resulting from their preparation processes makes the strict, absolute comparison between graphene- and CNT-based composite photocatalysts almost impossible to be conducted in a reasonable framework. In addition to the diverse forms of graphene and CNT materials, the possible changes of their properties during the synthesis process for the composites also need to be considered. For example, the stacking or aggregation, cleaving into smaller sheets and edges change of graphene could happen in the process of synthesizing graphene-based composites. A similar situation, e.g., the agglomeration and the variations in the length of CNT, also occurs during the fabrication of CNTbased composites. These changes will inevitably alter the optical and electronic properties of the carbon materials, which further affect the performance of the resultant carbon-based composite photocatalysts. All of the above factors add the difficulty and uncertainty of the comparison investigation between graphene-based and other carbon allotropes-based composite photocatalysts. Furthermore, as discussed in section 7, the overall performance of any composite photocatalysts is determined not just by an individual component but also by the synergistic results of each component and its interfacial composition and interaction manner.22,251 From such a viewpoint of system-level consideration, when comparing different carbon-based composite photocatalysts, besides the physicochemical properties of carbon materials, the properties (e.g., crystal phase, crystallinity, size, shape, and morphology) of other components and the interfacial composition and interaction, which would affect the overall photocatalytic performance of carbon-based composites in a collective way, should also be taken into account together. Thus far, it has still remained rather difficult to draw a conclusion on the genuine advantage of graphene over its carbon allotropes in promoting photocatalysis by performing a head-to-head comparison between graphene-based composite photocatalysts and their counterparts of carbon allotropes. However, after a critical examination of the progress in graphene-based composite photocatalysis in the past several years and now, we at least can draw one distinct advantage when using GO as the precursor of graphene to construct graphene-based composite photocatalysts. Namely, the versatile wet chemistry processability of GO enables the wide flexibility

of tuning the structure, morphology, dimensionality, and nanoscale-architecturing of graphene-based composites, thereby providing the easily tunable accessibility of optimizing the photocatalytic performance of graphene-based composites. In this sense, although the answer to the proposed question is still open, we want to emphasize that the key far-reaching point of comparison between graphene-based and other carbon allotropes-based composite photocatalysts lies in promoting our rational thinking and making better use of the structural and electronic properties of different graphene materials to design truly smart graphene-based composite photocatalysts. In view of the fact that it typically takes 20 years or more for any new material to emerge from the lab and be commercialized and the development path of graphene’s forebears,22,49,51 we should be optimistic but more rational about the long story of the delivery of graphene’s potential to practical applications in the area of graphene-based composite photocatalysis.

11. POTENTIAL APPLICATION OF GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS As one of the “advanced oxidation processes” (AOPs),331 heterogeneous photocatalysis has often been considered to be a highly unselective process because of the generation of very reactive species such as hydroxyl radicals (•OH) with strong oxidation capability, which generally degrade organic and inorganic reactants to water, carbon dioxide, small molecules, and salts.332 However, recent studies conducted on the application of photocatalysis for organic synthesis by selective redox reactions have proven that the photocatalytic selective processes can be achieved through the selection of appropriate photocatalytic material and control of the reaction conditions (e.g., choosing appropriate reaction solvent and adding radical scavenger/sacrificial agent).24 In the following section, we will demonstrate the typical progress of photocatalytic redox applications of graphene-based composites, including the “nonselective processes” (e.g., elimination of pollutants, disinfection, and water splitting) and “selective” synthesis reactions (such as CO2 reduction, nitroaromatics reduction, alcohols oxidation, etc.). Considering that there have been some reviews and book chapters to separately elaborate these reactions,22−37 in this review, we mainly focus on presenting a recapitulative summary of the reported application types of graphene-based composites to facilitate a quick understanding of the versatile potential photocatalytic applications of graphene-based composites for the benefit of the readership. A collection of some typical examples without bias on research works from specific research groups has been discussed as follows. 11.1. Photocatalytic “Nonselective” Processes

11.1.1. Elimination of Pollutants. With a rapid population growth and heavy industrialization, a wide range of toxic and hazardous pollutants are continually released into the surrounding environment, which intensively threatens the health and life of humans.22 In order to protect the living environment and realize the sustainable development of human society, the degradation of pollutants and remediation of a contaminated environment represent a significant and wideimpact concern of the scientific community. In this respect, graphene-based composite photocatalysts, as a new type of promising photocatalysts with high electron mobility, high adsorption capacity, and large specific surface area, have been extensively studied for degradation of pollutants. The available AD



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Figure 38. Time-online data for gas phase photocatalytic degradation of benzene over P25 and RGO-P25 composites under UV light irradiation. Reprinted with permission from ref 17. Copyright 2010 American Chemical Society.17

Figure 39. (A) Comparison of the photocatalytic activity of RGO-TiO2 composites and the P25 samples for the photocatalytic degradation of acetone in gas phase under UV light irradiation. Reprinted with permission from ref 333. Copyright 2012 Elsevier.333 (B) Plots of the decrease in NO concentration vs irradiation time in the presence of BiOBr, RGO-BiOBr composites, and a mixture of BiOBr and RGO (molar ratio 20:1) under visible light irradiation. Reprinted with permission from ref 335. Copyright 2011 American Chemical Society.335

and stable photocatalytic performance of RGO-P25 for the degradation of benzene under UV light irradiation is primarily attributed to its increased adsorptivity of pollutant, enhanced light absorption intensity, and promoted charge carrier separation efficiency, which facilitate the formation of the larger amount of radical species with strong oxidation capability (e.g., hydroxyl radical and superoxide radical species), and thus promote the degradation of benzene. Hierarchical macro-/mesoporpous RGO-TiO2 composites prepared by an in situ one-pot hydrothermal method have been utilized for UV-light-driven photocatalytic degradation of acetone in gas phase.333 The 0.05%RGO-TiO2 composite presents the highest reaction rate, which is 1.7- and 1.6-fold of blank TiO2 and commercial Degussa P25, respectively, as displayed in Figure 39A. The enhanced photoactivity of RGOTiO2 has been ascribed to the addition of RGO as an electron acceptor and transporter to inhibit the recombination of photogenerated electron−hole pairs instead of the adsorption capacity. However, the effect of the introduction of RGO into TiO2 on the optical properties and thus the photocatalytic performance of the samples has not been taken into account. Besides volatile organic compounds, photocatalytic removal of gaseous nitrogen monoxide (NO), which contributes to the

photocatalytic degradation reactions over graphene-based composites in the literature generally contain two types: gas phase degradation of pollutants (Table S2 in the Supporting Information) and liquid phase elimination of organic pollutants and toxic ions (Table S3 and Table S4 in the Supporting Information, respectively). 11.1.1.1. Gas Phase Degradation of Pollutants. Xu and coworkers have for the first time reported the application of RGO-P25 composite photocatalysts for the degradation of monocyclic aromatic benzene in the gas phase under UV light irradiation.17 The results show that the RGO-P25 composites can degrade the toxic benzene into harmless CO2 and display higher photocatalytic performance than bare P25. As shown in Figure 38, during the photoactivity test process, the conversion of benzene over the optimal 0.5%RGO-P25 photocatalyst is maintained at 6.4% and the average amount of produced CO2 is 67 ppm, implying that the average mineralization ratio of the converted benzene is 76.2%. However, over the bare P25, the conversion of benzene significantly decreases from the initial 5.8% to 1.2% and the produced amount of CO2 decreases to only 12 ppm. Obviously, the integration of RGO with P25 leads to an evident increased photoactivity and photostability of RGO-P25 composites as compared with bare P25. The high AE


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Figure 40. (A) Photocatalytic activity of RGO-TiO2 samples with different mass ratios of RGO to tetrabutyl titanate (the precursor of TiO2) at 0.04%, 0.2%, and 1% toward degradation of methylene blue (MB) under UV light irradiation for 80 min; (B) the proposed UV-light-driven photocatalytic reaction mechanism of RGO-TiO2 composites for degradation of MB dye. Reprinted with permission from ref 336. Copyright 2013 American Chemical Society.336

Figure 41. (A) Photocatalytic degradation for RhB under different experimental conditions with catalysts of RGO-CNT and P25; (B) energy diagram showing the proposed mechanism of photosensitized degradation of RhB over RGO−CNT under visible light irradiation. Reprinted with permission from ref 184. Copyright 2010 American Chemical Society.184

organic pollutants by these ROSs.185 For instance, Gu et al. have fabricated RGO-TiO2 composites with exposed TiO2 (001) facets via a facile hydrothermal route for photocatalytic degradation of methylene blue (MB) under UV light irradiation.336 As shown in Figure 40A, the photocatalytic degradation percentage of MB over the optimal RGO-TiO2 is 96% under UV light irradiation for 80 min, which is higher than that of blank TiO2 (73%). The photocatalytic reaction mechanism is as discussed above and illustrated in Figure 40B. The presence of conductive RGO in RGO-TiO 2 effectively promotes the separation and transfer of charge carriers photogenerated from TiO 2 and enhances the adsorption of MB molecules, thus improving the photocatalytic activity of the composites. The second mechanism for photocatalytic degradation of organic pollutants in a liquid phase is that the organic pollutants (e.g., organic dye) are photoexcited by themselves under light irradiation, followed by electron transfer from the dye* (excited state of the dye) to the graphene-based composites with energy level matching to the dye*.184,185 Then, the electron is trapped by surface adsorbed O2 to generate various ROSs, thus promoting the self-degradation of the dye or the degradation by ROSs. For example, Zhang and co-workers have prepared RGO-CNT composites with RGO platelets pillared by single-walled CNT using a chemical vapor deposition (CVD) method with acetonitrile as the carbon source and Ni nanoparticles as the catalyst.184 The resulting 3D RGO-CNT displays higher visible light photocatalytic performance than P25 toward the degradation of RhB dye in water (Figure 41A). The degradation of RhB dye over RGO-CNT is caused by the dye photosensitization process. In detail, under

formation of acid rain formation, has also attracted increasing attention.334 For example, Ai and co-workers have fabricated RGO-BiOBr composites by a facile solvothermal route and applied them to photocatalytic removal of NO.335 It has been found that the introduction of RGO efficiently enhances the photocatalytic performance of BiOBr (Figure 39B). The NO removal rate over the optimal RGO-BiOBr sample is 2-fold of blank BiOBr under visible light irradiation. The characterization results demonstrate that the enhanced photoactivity of the RGO-BiOBr composites is attributed to the more effective charge separation and transportation aroused from the strong chemical bonding between BiOBr and RGO, not to the light absorption extension in the visible region and higher surface area of the composites.335 11.1.1.2. Liquid Phase Elimination of Organic Pollutants and Toxic Ions. In addition to the photodegradation of pollutants in gas phase, the studies on the photocatalytic elimination of organic pollutants and toxic ions in liquid phase are another major research focus in graphene-based photocatalysis. Table S3 (Supporting Information) has summarized various graphene-based composite photocatalysts for degradation of different organic pollutants in liquid phase. Although the photoactive components and the target pollutants in these reports are diverse, these photocatalytic degradation reactions generally follow two typical mechanisms. The first one is that, upon light irradiation, the photoactive components (e.g., semiconductor) in graphene-based composites are excited to generate charge carriers (i.e., electron−hole pairs), which react with the electron acceptor (e.g., O2) and electron donor (e.g., H2O) to form various reactive oxidative species (ROSs, e.g., O2•− and •OH) and then oxidize the AF


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Figure 42. (A) Photocatalytic degradation of Cr(VI) by RGO-(α-FeOOH) composites with different weight contents of RGO under visible light irradiation; (B) effect of pH on the photoactivity of degradation of Cr(VI) over the optimal 3%RGO-(α-FeOOH) photocatalyst with visible light irradiation for 180 min. Reprinted with permission from ref 337. Copyright 2014 Royal Society of Chemistry.337

Figure 43. (A) Number of bacteria cultured from the viable E. coli on the surface of the RGO-TiO2 thin films reduced by a solar light-assisted photocatalytic process for (a) 0, (b) 0.5, (c) 1, (d) 2, and (e) 4 h irradiation time, as compared to (f) number of bacteria on blank TiO2 thin film and (g) on GO-glass film, under solar light irradiation; (h) number of bacteria on the GO applied in part g but in the dark, as a control sample. Reprinted with permission from ref 16. Copyright 2009 American Chemical Society.16 (B) Viability of E. coli treated on different samples after 12 h of ambient visible light illumination; the insets are the corresponding photographs of bactericidal effects. Reprinted with permission from ref 338. Copyright 2013 Elsevier.338

visible light irradiation, the RhB dye is photoexcited to RhB*. The electrons transfer from excited RhB* to RGO and then to CNT and Ni substrate due to their matched energy level, as schematically illustrated in Figure 41B. Such multilevel electron transfer can spatially separate the RhB•+ radical and the electron, thus effectively decreasing the recombination of charge carriers. The electrons moved to the Ni can be trapped by O2 to produce various ROSs, and the RhB•+ would be degraded by itself and/or by the highly oxidative ROSs. Besides organic pollutants, heavy metal ions such as bivalent cadmium, bivalent lead, and hexavalent chromium, symbolized as Cd(II), Pb(II), and Cr(VI), respectively, are a typical type of inorganic pollutants existing in a liquid phase. They are widely distributed in the environment from wastewater disposal of some industries, including electroplating activities, paint, electric equipment, and nylon and plastic factories, and they have shown seriously harmful effects on human physiology and other biological systems when exceeding the tolerance level. Therefore, continuous efforts have been devoted to removing these metal ions from water using photocatalysis, including that driven by graphene-based composite photocatalysts. In this regard, the research attention has been mainly focused on

elimination of Cr(VI), as summarized in Table S4 (Supporting Information). RGO-(α-FeOOH) nanorod composites prepared by a onestep hydrothermal route have been utilized for photocatalytic reduction of Cr(VI) under visible light irradiation.337 As shown in Figure 42A, the integration of different weight contents of RGO with α-FeOOH nanorod can enhance the photoactivity of α-FeOOH. The sample of 3%RGO-(α-FeOOH) shows the highest reduction efficiency (94%) of Cr(VI), whereas that over blank α-FeOOH nanorod shows only 26%. The roles of RGO in enhancing the photoactivity of α-FeOOH nanorod can be ascribed to three aspects: (1) decreasing the size of α-FeOOH nanorod, which may favor the transfer of electrons from bulk to its surface; (2) increasing the light harvesting capacity of αFeOOH nanorod; and (3) promoting efficient transfer and separation of photogenerated electron−hole pairs. In addition, the effect of pH on the reduction of Cr(VI) has also been studied in the range of pH 2−10. It is observed that the photocatalytic activity of the composite decreases with increasing the pH of the solution (Figure 42B). This is because, at low pH, Cr species exist as HCrO4− and the surface of the photocatalyst becomes highly protonated and more AG



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Figure 44. (A) Hydrogen evolution rates of Sr2Ta2O7−xNx and RGO-Sr2Ta2O7−xNx using Pt as the cocatalyst under simulated solar light irradiation and (B) schematic diagram for the corresponding photocatalytic mechanism over RGO-Sr2Ta2O7−xNx-Pt. Reprinted with permission from ref 342. Copyright 2011 American Chemical Society.342 (C) Photocatalytic H2 production activity over RGO-Zn0.8Cd0.2S (abbreviated as GS), Pt-Zn0.8Cd0.2S (1 wt % of Pt), and RGO under simulated solar irradiation. Reprinted with permission from ref 343. Copyright 2012 American Chemical Society.343 (D) The time courses of hydrogen evolution over EY-Pt, RGO-EY, and RGO-EY-Pt photocatalysts under visible light irradiation; the inset is the proposed photocatalytic mechanism for H2 evolution over RGO-EY-Pt. Reprinted with permission from ref 344. Copyright 2011 American Chemical Society.344

positive, which is beneficial for the accumulation of HCrO4− ions and their reaction with photogenerated electrons. As the pH increases, Cr species change to Cr2O72− and the surface of the photocatalyst becomes negative, which tends to repel the Cr2O72− ions and hence decreases the photocatalytic activity.337 Besides, RGO sheets have been incorporated with commercial ZnO particles by a solvothermal reaction to form RGO-ZnO composites for photocatalytic degradation of Cr(VI) in water under UV light irradiation.128 The RGOZnO composite exhibits enhanced photoactivity as compared to blank ZnO sample. Moreover, the recycling activity test shows that the photocorrosion of ZnO is efficiently inhibited by the hybridization with RGO. The improved photocatalytic performance of RGO-ZnO composite is ascribed to the strong and efficient hybridization interaction between the ZnO and RGO with a π-conjugative 2D system, which efficiently separates the photogenerated charge carriers and reduces the activation of surface oxygen atoms in ZnO, thus enhancing the photoactivity and photostability of RGO-ZnO composites. 11.1.2. Disinfection. Graphene-based composite photocatalysts have also been utilized in the photoinactivation of bacteria, as listed in Table S5 (Supporting Information). It has been demonstrated that the graphene sheets play a key role in promoting the photocatalytic disinfection efficacy of graphenebased composites. For example, Akhavan et al. have prepared RGO-TiO2 thin films via deposition of GO sheets on TiO2 thin films followed by annealing and a UV−vis light-assisted reduction process, and utilized them for photoinactivation of E. coli in an aqueous solution under solar light irradiation.16 With the appropriate addition of RGO, the RGO-TiO2 composite displays significantly enhanced bactericidal activity in comparison with blank TiO2, as shown in Figure 43A. The RGO-TiO2 sample obtained via photoreduction for 4 h exhibits the highest antibacterial activity, and its activity is improved by a factor of about 6 and 7.5 as compared to the activities of GO-

TiO2 and blank TiO2 thin films, respectively. This is attributed to the electron acceptor role of RGO to inhibit the recombination of electron−hole pairs photogenerated from semiconductor TiO2. Besides, the Ti−C bonds formed between TiO2 and RGO after the annealing treatment are able to stabilize RGO during the photocatalytic antibacterial tests, although it is claimed that such chemical bonds do not induce significant change of the optical band gap energy of TiO2. Cao and co-workers have reported the preparation of RGOTiO2 composites through the redox reaction between TiCl3 and GO for photocatalytic inactivation of E. coli.338 The photoinactivation activities of the RGO-TiO2 composites are much higher than that of blank TiO2 nanoparticles (Figure 43B), which is ascribed to formation of a Ti−C bond between TiO2 and RGO that extends the light absorption range and boosts the separation of photogenerated electron−hole pairs from TiO2 upon visible light irradiation. The difference in the role of Ti−C bonds in these two cases may result from their dissimilar synthesis methods. Besides the photodegradation of the typical bacteria of E. coli over graphene-based TiO2 composites, some other composites (e.g., RGO-WO3,339 GO-CdS,340 and RGO-ZnFe2O4-polyaniline341) have also been reported for degradation of different bacteria and viruses (e.g., bacteriophage MS2 viruses and Bacillus subtilis). The integration of RGO/GO with the photoactive materials can lead to the improved photoinactivation efficiency, which is often assigned to the enhanced separation and transfer of the charge carriers in the composites and the interaction between RGO/GO and the other components. 11.1.3. Water Splitting. 11.1.3.1. Hydrogen (H2) Evolution. Besides the environmental contamination, the energy crisis depending on fossil fuels is another serious issue that significantly threatens the sustainable development of human society. To seek renewable and clean energy, photocatalytic AH


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Figure 45. (A) Time course of photocatalytic H2 evolution from an aqueous solution of 20% methanol (v/v for methanol/water) or pure water with suspended photocatalysts (GO or GO-Pt) under mercury lamp irradiation. Reprinted with permission from ref 176. Copyright 2010 John Wiley & Sons, Inc.176 (B) Comparison of photocatalytic H2 evolution rates from an aqueous solution of 20% methanol over GO, GO-NiO, and GO-Ni. Reprinted with permission from ref 345. Copyright 2012 Royal Society of Chemistry.345

23.4% at 420 nm, which is enhanced by a factor of 4.5 as compared with that of blank Zn0.8Cd0.2S. The photoactivity of 0.25%RGO-Zn0.8Cd0.2S is even higher than that of the optimal Zn0.8Cd0.2S-Pt under the same reaction conditions, demonstrating that RGO can act as a promising substitute for noble metals in photocatalytic H2 production. It is claimed that RGO as an effective cocatalyst can not only promote the separation and transfer of charge carriers photogenerated from Zn0.8Cd0.2S, but also reduce H+ to H2 molecule on its surface due to the appropriate potential of graphene/graphene•− (−0.08 V vs standard hydrogen electrode (SHE), pH = 0). However, it should be noted that, as stated in section 2, the GO-derived graphene (RGO) always contains surface defects and residual few amounts of oxygenated functional groups, which inevitably induces considerable disruption of the electronic structure of graphene via breaking the 2D πconjugation of the original graphene sheet. Consequently, there should be a distinct difference of Fermi level for RGO and the pristine graphene. It has been reported that GO/RGO with appropriate oxidation degree may feature p- or n-type semiconductor characteristics due to the Fermi level detuning away from the Dirac point.46 In this regard, the position of the Fermi level for various graphene materials should be seriously considered, which is a general issue for the reported literature on graphene-based composite photocatalysts. Min et al. have designed the RGO-Eosin Y-Pt (RGO-EY-Pt) composite system for photocatalytic H2 evolution from water with the addition of triethanolamine (TEOA) as sacrificial donor.344 Upon the illumination of visible light, the RGO-EYPt exhibits much higher activity than the EY-Pt system. The introduction of conductive RGO greatly facilitates the electron transfer from the excited dye of EY* to Pt cocatalyst, which promotes the photocatalytic reduction of water and improves the H2 evolution activity. As shown in Figure 44D, the rate of photocatalytic H2 evolution over RGO-EY-Pt is about 10.17 μmol h−1, which exceeds the rate of the EY-Pt system (ca. 1.37 μmol h−1) by a factor of 7.4. Moreover, the apparent quantum yield of 9.3% for photocatalytic H2 evolution has been achieved at 520 nm over this RGO-EY-Pt system. However, under the optimal conditions, the rate of H2 evolution over RGO-EY-Pt declines in the consecutive runs of stability testing, which is probably caused by the consumption of electron donor and the partial degradation of dye. The instability of dye molecules in the photocatalytic reaction environment limits the application

water splitting to generate hydrogen (H2), which possesses the highest energy density (120 MJ/kg) known for any fuel and no carbon footprint, is regarded as an ultimately promising solution for meeting future fuel needs. Over the past few years, a large number of graphene-based composites with improved activity for H2 generation have been developed, as summarized in Table S6 (Supporting Information), which represents another research emphasis in graphene-based photocatalysis. The advantages of graphene for improving the efficiency of graphene-based composites for photocatalytic H2 generation from water splitting are generally ascribed to two aspects.30 First, graphene with low Fermi level and unique electrical conductivity can serve as an efficient electron relay mediator, which promotes the charge carrier separation and facilitates the electron transfer from excited photoactive material to the cocatalyst, thus enhancing the activity for photocatalytic H2 evolution. Second, graphene by itself can serve as an alternative cocatalyst to the commonly used noble metal cocatalyst (e.g., Pt) because of the more negative reduction potential of graphene/graphene•− than that of H+/H2.30 For example, Mukherji and co-workers have reported the synthesis of RGO-Sr2Ta2O7−xNx-Pt composites via a combined calcination-photoreduction method and applied them for photocatalytic H2 evolution from water under simulated solar light irradiation.342 The H2 evolution rate reaches 293 μmol h−1 over the optimal 5%RGO-Sr2Ta2O7‑xNx-Pt sample with methanol as a sacrificial agent in water (Figure 44A), and the quantum efficiency is 6.45% in the wavelength region of 280 to 550 nm, which is improved by ∼51% as compared with that of Sr2Ta2O7−xNx-Pt (4.26%). The improved photoactivity of RGO-Sr2Ta2O7−xNx-Pt is ascribed to the fact that RGO is able to capture and transfer the photoelectrons generated from the band gap excitation of Sr2Ta2O7−xNx to the Pt cocatalyst with abundant active sites (Figure 44B). This significantly inhibits the recombination of charge carriers and facilities the reduction of proton to generate H2, thus enhancing the photocatalytic performance. Additionally, RGO-ZnxCd1−xS composites have been applied to photocatalytic H2 evolution under simulated solar light irradiation without the noble metal cocatalyst.343 As shown in Figure 44C, 0.25%RGO-Zn0.8Cd0.2S photocatalyst displays the highest H2 evolution rate of 1824 μmol h−1 g−1 in the Na2S/ Na2SO3 solution and the apparent quantum efficiency reaches AI


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Figure 46. (A) Normalized O2 evolution plots of the α-Fe2O3 and RGO-(α-Fe2O3) under simulated solar light irradiation; (B) transient absorption data of α-Fe2O3, RGO-(α-Fe2O3), and RGO samples measured at an excitation pulse of 400 nm and probed at a 700 nm wavelength; (C) schematic illustration of charge transfer in the RGO-(α-Fe2O3) composite. Reprinted with permission from ref 270. Copyright 2013 American Chemical Society.270

photocatalytic activity of GO-Ni than that of GO-NiO is due to the fact that the trapping of electrons generated from the photoactive GO over Ni cocatalyst is more efficient than that over NiO. It has been found that there is no H2 evolution over GO in the aqueous methanol solution under visible light irradiation, which is ascribed to the fact that the GO specimens do not effectively absorb visible light in a manner appropriate for H2 evolution. However, this statement is inconsistent with the proposed band structure of GO-Ni.345 From the above discussion, we can find that, despite the observations of photocatalytic H2 production over GO under specific light irradiation, the underlying photocatalytic mechanism for GO is still exclusive and needs further and extensive studies. The investigation on this issue is complicated by the intrinsic varied surface properties of GO. This can be reflected by the reported photoluminescence (PL) features of GO43,44,346 because the PL originates from the recombination of photogenerated charge carriers that are the active species in the photocatalytic reactions.347 Thus far, the origins of PL from GO have been found to be diverse, which may be ascribed to the quasi-molecular fluorophores (i.e., sp2 fragments consisting of small numbers of aromatic rings with oxygen functional groups), bond distortions, disorder-induced states, localized sp2 clusters, or electronic band structure of GO.43−45,346−349 These species are likely to contribute to the observed photoactivity of GO. Obviously, to identify the specific photoactive ingredients in GO still requires significant attention of the research communities. 11.1.3.2. Oxygen (O2) Evolution. Besides boosting the H2 evolution, photocatalytic oxygen (O2) evolution from water over graphene-based composite photocatalysts has also been reported, as summarized in Table S7 (Supporting Information). Meng et al. have synthesized the RGO-(α-Fe2O3) composites via a thermal hydrolysis-calcination approach.270 Under simulated solar light irradiation and with the addition of AgNO3 as the sacrificial electron scavenger, the O2 generation rate is measured to be 387 μmol g−1 h−1 and 752 μmol g−1 h−1 for α-Fe2O3 and RGO-(α-Fe2O3) composite, respectively (Figure 46A). It has been revealed that, with the introduction of RGO into α-Fe2O3, the photoelectrons generated from the band gap-excitation of α-Fe2O3 can quickly transfer to the RGO sheet and diffuse into trap states in the RGO, which thus suppresses the recombination of electron−hole pairs. As shown in Figure 46B, the photoexcited charge carriers in α-Fe2O3 decay within ∼1 ns, whereas the decay time of charge carriers in RGO-(α-Fe2O3) composite is greater than 6 ns. The photogenerated electrons trapped by RGO as long-lived carriers can react with the Ag+, while the accumulated holes in the

of dye-sensitized photocatalysts, which may be alleviated by some approaches, such as strengthening the covalent interaction between dye molecules and other components, adding appropriate sacrificial agent, or encapsulating the dye molecules in some porous materials.239 On the other hand, some preliminary reports have demonstrated that graphene oxide (GO), the widely used precursor of graphene, with appropriate oxidation degree is able to directly perform as effective photoactive material for H2 generation.176,178,238,345 For example, Yeh and co-workers have synthesized the GO photocatalyst with an apparent band gap of 2.4−4.3 eV by the modified Hummers’ procedure and utilized it for photocatalytic H2 evolution.176 As shown in Figure 45A, under the mercury light irradiation for 6 h, the total evolution amount of H2 over the GO photocatalyst without noble metal as cocatalyst from pure water and aqueous methanol solution is ca. 280 μmol and 17 000 μmol, respectively. This has been ascribed to the fact that GO sheets are molecule-like and highly dispersed in water and the carbon atoms on the sheets are therefore accessible to protons that can readily transform to H2 by accepting electrons generated from the photoexcitation of GO. It is claimed that, for a reaction period of 50 h, the GO photocatalyst exhibits no noticeable decrease of activity for H2 evolution from the aqueous solution of methanol under the mercury lamp irradiation, which is explained by the fact that the oxygen functional groups responsible for the photocatalytic reaction are stable during the photocatalytic reaction.176 However, the Fourier-transform infrared (FTIR) spectrum of GO after the photocatalytic process shows reduced absorption intensities for all the oxygen functional groups on GO as a result of irradiation-induced reduction, which is contradictory with the explanation for the stable photoactivity of GO. Such a situation leads us to wonder what the real photoactive species for the H2 production are, which needs further and more detailed studies. In addition, Agegnehu et al. have deposited Ni and NiO cocatalysts on the GO sheets prepared via the modified Hummers’ method, which significantly enhances the photocatalytic H2 evolution activity of GO from an aqueous solution of methanol under UV−vis light illumination.345 As displayed in Figure 45B, the total H2 evolution amounts of GO-NiO and GO-Ni photocatalysts under the light irradiation of 8 h reach about 320 μmol and 650 μmol, respectively, which are both much higher than that of blank GO (ca. 100 μmol). The improved photoactivities of GO-Ni and GO-NiO are attributed to the inhibited recombination of electron−hole pairs resulting from the efficient transfer of photoelectrons generated from the photoactive GO to the Ni and NiO cocatalysts. The higher AJ



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Figure 47. (A) Photocatalytic O2 evolution under visible light irradiation from an AgNO3 aqueous solution over different catalysts: (from bottom to top) bare Ag/AgBr, Ag3PO4, RGO-Ag3PO4, Ag3PO4−Ag/AgBr, and RGO-Ag3PO4−Ag/AgBr; rates are based on a linear fit of the performance in the first 4 h; (B) model of the synergistic increase of the photocatalytic activity of Ag3PO4 upon functionalization with Ag/AgBr and RGO. Reprinted with permission from ref 350. Copyright 2012 American Chemical Society.350 (C) Visible light induced production of O2 over (a) pristine Zn−CrLDH, (b) GO, (c) RGO, the self-assembled composites of (d) 0.05%GO-(Zn−Cr-LDH-GO), (e) 0.1%GO-(Zn−Cr-LDH-GO), (f) 0.05%RGO(Zn−Cr-LDH-GO), and (g) 0.1%RGO-(Zn-Cr-LDH-GO), (h) the physical mixture of Zn-Cr-LDH and GO; and (i) the physical mixture of Zn-CrLDH and RGO. Reprinted with permission from ref 351. Copyright 2013 Royal Society of Chemistry.351

Table 1. Overall Water Splitting under Visible Light Irradiation by the (Ru/SrTiO3:Rh)-(BiVO4) System in the Presence and Absence of Photoreduced RGOa Entry

H2 photocat.

1 2 3 4 5 6 7 8 9 10

Ru/SrTiO3:Rh Ru/SrTiO3:Rh

reduced GOb

pH

H2 (μmol)

O2 (μmol)

BiVO4 BiVO4 BiVO4 BiVO4 BiVO4 BiVO4 BiVO4 BiVO4

3.5 3.5 3.5 3.5 3.5 7.0 3.5 3.5 7.0 3.5

0.9 0.9 0 0 3.7 0.8 1.4 11 1.1 4.8

0 0 0 0 1.9 0.5 0.6 5.5 0.6 2.3

RGO(Ru/SrTiO3:Rh) RGO(BiVO4)

Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh

O2 photocat.

RGO(Ru/SrTiO3:Rh) RGO(BiVO4) RGO(BiVO4) N2H4-RGO

a Conditions: photocatalysts (0.03 g each) in water or H2SO4 (aq) (120 mL); light source, 300 W Xe lamp (λ > 420 nm); top-irradiation cell with a Pyrex window. bRGO(Ru-SrTiO3:Rh) and RGO(BiVO4) denote graphene oxide (GO) photoreduced by Ru-SrTiO3:Rh and BiVO4, respectively; N2H4-RGO denotes GO reduced by hydrazine. Reprinted with permission from ref 353. Copyright 2011 American Chemical Society.353

semiconductor α-Fe2O3 are able to oxidize water to generate O2 (Figure 46C). The efficient transfer and trap of electrons by RGO have resulted in the enhanced photocatalytic O2 evolution activity of the RGO-(α-Fe2O3) composite. Hou and co-workers have investigated the visible light photocatalytic O2 evolution activity of RGO-Ag3PO4-Ag/AgBr composite prepared by a photoassisted deposition-precipitation method, followed by a hydrothermal treatment.350 The result shows that the photocatalytic O2 evolution rates over blank Ag3PO4, Ag3PO4-Ag/AgBr, RGO-Ag3PO4, and RGO-Ag3PO4Ag/AgBr from an aqueous solution containing AgNO3 as the electron scavenger are 38, 48, 43, and 76 μmol h−1, respectively, while the blank Ag/AgBr shows no O2 evolution activity under identical reaction conditions (Figure 47A). The increase in photocatalytic activity caused by addition of Ag/AgBr and RGO to Ag3PO4 has been ascribed to the following two key factors: (1) the depletion of the conduction band of the as-synthesized Ag3PO4 by contact with the Ag nanoparticles leads to a longer lifetime of photogenerated holes; (2) a downward shift of the Ag3PO4 valence band due to charge transfer from Ag3PO4 to Ag and subsequently to RGO results in a higher water oxidation power, as schematically illustrated in Figure 47B. Besides, the characterization results demonstrate that the RGO-Ag3PO4-Ag/

AgBr composite is stable during the photocatalytic O2 evolution process. In addition, Gunjakar et al. have reported the fabrication of mesoporous 2D-2D RGO-layered double hydroxide (RGO(Zn−Cr-LDH)) composites by an electrostatically derived selfassembly of Zn−Cr-LDH nanoplates with 2D RGO sheets.351 The RGO-(Zn−Cr-LDH) samples display strong visible light harvesting ability, effective inhibition of electron−hole pairs recombination, and highly porous structure with an expanded surface area. These integrative features endow the optimal 0.05%RGO-(Zn−Cr-LDH) composite with higher activity (∼1.20 mmol h−1 g−1) than blank Zn-Cr-LDH (∼0.67 mmol h−1 g−1) for the visible light induced photocatalytic O2 generation from aqueous solution with AgNO3 as electron scavenger (Figure 47C). In addition, the physical mixed samples of Zn-Cr-LDH with GO and RGO are much less active for the visible light driven O2 evolution. The result implies that the efficient interfacial contact between the photoactive component and graphene is essential for effectively utilizing the electrical conductivity of graphene to facilitate the separation and transfer of photogenerated charge carriers, thus enhancing the photoactivity of graphene-based composite photocatalysts. AK



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11.1.3.3. Overall Water Splitting. According to the above discussion, it can be seen that most of the studies over graphene-based composite photocatalysts for water splitting often focus on half reaction of photocatalytic H2 or O2 evolution by using sacrificial reagents as electron donors or acceptors, respectively.101,176,270,342−345,351,352 This greatly hinders the solar energy conversion efficiency due to the inadequate utilization of photogenerated charge carriers. In addition, some sacrificial agents are expensive and require a persistent supply during the photocatalytic reactions, which also increases the cost of the photocatalytic water splitting process. Therefore, some efforts have been devoted to constructing graphene-based composite photocatalysts to realize the overall water splitting to generate H2 and O2 simultaneously. Iwase and co-workers have synthesized a series of Z-scheme photocatalysts using BiVO4 as O2-evolving photocatalyst, Ru/ SrTiO3:Rh as H2-evolving photocatalyst, and RGO as an electron mediator.353 Table 1 shows the visible light activity of the constructed Z-scheme systems using various combinations of Ru/SrTiO3:Rh, BiVO4, and RGO. The results demonstrate that only when the Ru/SrTiO3:Rh and BiVO4 are integrated, both H2 and O2 can evolve. Additionally, when RGO obtained from photoreduction of GO by BiVO4 is used to combine BiVO4 with Ru/SrTiO3:Rh (Entry 8), the (Ru/SrTiO3:Rh)RGO-BiVO4 displays 3-fold enhancement in the gas evolution as compared to the Ru/SrTiO3:Rh-BiVO4 composite system (Entry 5). This is mainly because, under visible light illumination, the photoelectrons generated from the excitation of BiVO4 can be injected from the conduction band to the RGO sheet and then transferred to Ru/SrTiO3:Rh and combined with the photogenerated holes. The left electrons on Ru/SrTiO3:Rh and the holes on BiVO4 subsequently react with water, forming H2 and O2, respectively, thus accomplishing a complete water splitting cycle, as illustrated in Figure 48.

RGO (Ru/SrTiO3:Rh)-BiVO4 systems (Entry 5). This is because the photoreduction of GO by Ru/SrTiO3:Rh (10% O-bound carbon) is more efficient than that by BiVO4 (28% Obound carbon). The efficient reduction of GO is desirable in most cases, but, in the present reaction system, it strongly decreases the hydrophilicity of the RGO-Ru/SrTiO3:Rh particles and makes them immiscible in water. During the reaction process, it has been observed that the RGO-Ru/ SrTiO3:Rh component floats on the water surface, while BiVO4 particles are suspended in the water, which significantly decreases the interfacial contact between the H2 and O2 photocatalysts, thus resulting in the poor photocatalytic activity. The hydrophilicity effect of RGO in this Z-scheme photocatalytic system has been further evidenced by employing the N2H4 reduced hydrophobic RGO as the mediator (Entry 10). The photoactivity is similar to that of the non-RGO system, and the poor dispersity of the N2H4 reduced RGO in water has also been observed. The results disclose the importance of RGO miscibility in water in addition to good electron transporter. Furthermore, the pH of the reaction solution also has an important influence on the photoactivities of the resultant composite photocatalysts (Entries 5−6 and Entries 8−9). This is mainly due to the fact that the pH is able to change the surface charges of Ru/SrTiO3:Rh and RGO/BiVO4, which thus determines the electrostatic interaction between the H2 and O2 photocatalysts. More specifically, at low pH of 3.5, the surface charge between Ru/SrTiO3:Rh and RGO/BiVO4 is opposite and they would attract with each other to form an intimate interfacial contact. However, at high pH of 7, the Ru/ SrTiO3:Rh and RGO/BiVO4 are repulsive due to their same surface charge, resulting in the poor interfacial contact between them. Consequently, the photoelectron transfer from BiVO4 to RGO and then to Ru/SrTiO3:Rh is not efficient, which leads to the lower photoactivity. The role of RGO as solid-state electron mediator has also been demonstrated in other Z-scheme photocatalytic systems in which various metal sulfides are chosen as H2-evolving photocatalyst and TiO2 is adopted as O2-evolving photocatalyst.354 It has been proven that when the n- or i-type semiconductors (e.g., ZnS, AgGaS2, AgInS2, Ag2ZnGeS2, and Ag2ZnSnS2) are employed as the H2-evolving photocatalyst (Entries 1−5, Table 2), only H2 evolves. In contrast, when ptype semiconductors (e.g., CuGaS2, CuInS2, Cu2ZnGeS2, and Cu2ZnSnS2) are used (Entries 6−11, Table 2), H2 and O2 evolve nearly in stoichiometric amounts. The results indicate that, through RGO as solid state electron mediator, photogenerated electrons in TiO2 with n-type semiconductor characteristics are able to transfer to metal sulfides with ptype semiconductor character, whereas this electron transfer process proceeds with difficulty between the n-type TiO2 and n- or i-type semiconductor. Furthermore, taking the optimal Pt/CuGaS2-RGO-TiO2 composite system as an example, the controlled experiments have shown that, without RGO, the photocatalytic activity is low, and H2 and O2 do not evolve in a stoichiometric amount, which demonstrates that RGO is necessary for the smooth electron transfer from O2- to H2evolving photocatalysts. However, due to the hydrolysis and photocorrosion of CuGaS2, the rates of gases evolution over the composite become slower with the irradiation time.

Figure 48. Schematic illustration of water splitting in a Z-scheme photocatalysis system consisting of Ru/SrTiO3:Rh and RGO-BiVO4 under visible light irradiation. Reprinted with permission from ref 353. Copyright 2011 American Chemical Society.353

The minimum turnover number (TON), calculated as the number of moles of reactive electrons per mole of RGO (assumed to contain pristine graphitic carbon), is ca. 3.2, which is comparable with the value of the system using Fe3+/Fe2+ as the shuttle mediator in the same reaction period, suggesting that RGO is suitable to serve as effective alternative solid redox mediator. Besides, in comparison with the traditional Z-scheme photocatalyst system of employing ionic Fe3+/Fe2+ and IO3−/I− redox couples as electron mediators for shuttling electrons from the O2- to the H2-evolving photocatalyst, the RGO solid electron mediator is more favorable in terms of recovery of the photocatalyst and reclamation of clean water. It is notable that when the RGO obtained from photoreduction of GO by Ru/SrTiO3:Rh is used to combine BiVO4 with Ru/SrTiO3:Rh (Entry 7), the photoactivity is much lower than that of (Ru/SrTiO3:Rh)-RGO-BiVO4 (Entry 8) and non-

11.2. Photocatalytic “Selective” Transformations

11.2.1. CO2 Reduction. CO2, a major greenhouse gas that comes from the anthropogenic sources such as the combustion AL


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Table 2. Overall Water Splitting Using Z-Scheme Photocatalyst Systems Consisting of Various Metal Sulfide Photocatalysts as a H2-Evolving Photocatalyst and RGOTiO2 (Rutile) Composite of an O2-Evolving Photocatalysta


Initial activity (μmol h−1)

Entry

H2-photocatalyst (Metal sulfide photocatalyst)

Amount of Pt loaded (wt %)

1 2 3 4 5 6 7

ZnS AgGaS2 AgInS2 Ag2ZnGeS4 Ag2ZnSnS4 CuGaS2 CuGaS2

0.3 0.3 0.3 0.3 0.3 0.3b 0.3b

8 9 10 11

CuGaS2 CuInS2 Cu2ZnGeS4 Cu2ZnSnS4

0.1b 0.3 0.3 0.3

Loading method

H2

O2

Photodeposition Photodeposition Photodeposition Photodeposition Photodeposition Photodeposition Impregnation + H2 red.c Adsorption Photodeposition Photodeposition Photodeposition

0.8 12.5 0.3 6.1 0.4 19.8 7.7

0 0 0 0 0 10.3 3.3

56.2 9.9 17.4 6.3

25.0 4.5 7.8 2.9

Figure 49. (A) Photocatalytic CH4 (dots) and CO (squares) evolution amounts for (a) (RGO-Ti0.91O2)5 hollow spheres, (b) (Ti0.91O2)5 hollow spheres, and (c) P25 under the irradiation of a xenon arc lamp; (B) average formation rates of the products. Reprinted with permission from ref 355. Copyright 2012 John Wiley & Sons, Inc.355

spheres and P25, respectively (Figure 49B). Obviously, the integration of RGO with Ti0.91O2 not only improves the photocatalytic performance, but also influences the products distribution for photoreduction of CO2. The improved activity of RGO-Ti0.91O2 composite can be attributed to its hollow structure, the ultrathin nature of Ti0.91O2 nanosheets, and their sufficiently compact stacking with the RGO sheets. These lead to the more efficient, permeable absorption and scattering of light, and allow more efficient spatial separation and transfer of the charge carriers onto the photocatalyst surface to participate in the photoreduction of CO2. With regard to the generation of different products, the formation of CO and CH4 follows twoelectron and eight-electron transfer processes, respectively. The generation of CH4 is a major product over titania catalyst through the reaction route: CO2 → CO → C· → CH2 → CH4. In the RGO-Ti0.91O2 composite system, CO is dominantly produced. The possible reason may be explained as the following: the transferred electrons to RGO can diffuse quickly on a large area of RGO, benefiting from the enhanced electron mobility of RGO. This restrains the accumulation of the electrons and decreases local electron density, which is favorable for two-electron reaction to form CO. In addition, Hersam’s group has synthesized a series of RGOP25 and SEG-P25 photocatalysts by a vacuum filtration method for reduction of CO2 to CH4.21 The SEG-P25 composites exhibit higher activity than P25 and RGO-TiO2 counterparts under both UV and visible light illumination. As shown in Figure 50A, under UV light irradiation, the optimal 0.27%SEGTiO2 composite produces 4.5 times more CH4 than P25, whereas no improvement in the photoactivity is observed for RGO-TiO2 as compared to P25. Under visible light irradiation, the photocatalytic activity of the optimum 0.55%SEG-P25 is 7.2 times higher than that of P25, while a maximum enhancement of 2.3-fold is achieved for the 0.41%RGO-P25 composite as compared to P25. The obvious differences in photoactivity between SEG-P25 and RGO-P25 composites are due to the SEG sheets with lower defect density and sheet resistance having better electronic coupling with TiO2 than the RGO sheets. This enables the photoexcited energetic electrons to diffuse farther on the SEG sheet and increases their likelihood of interaction with the adsorbed CO2 (Figure 50B), thus resulting in the higher photoreduction activity of the SEG-P25 composites than their RGO-TiO2 counterparts. This work demonstrates the correlation between the electron mobility of the graphene component and the photoactivity of the resulting graphene-based composite photocatalysts. The use of defect-

a

Conditions: 0.05 g each; water without pH adjustment (120 mL); 300 W Xe lamp full-arc; top-irradiation cell with a Pyrex window. b Determined by ICP. cHeat-treated at 473 K for 1 h in H2. Reprinted with permission from ref 354. Copyright 2015 American Chemical Society.354

of fossil fuels, has been accumulated rapidly in the atmosphere accompanied by development of industry and society. The high atmospheric levels of CO2 have caused great damage to the global climate. Therefore, mitigating the effect of increasing CO2 emissions through its sequestration has been studied intensively. In this sense, photocatalytic conversion of CO2 into fuels and chemicals is considered to be a fascinating solution, which is able to mitigate the problem of global warming and achieve solar energy conversion simultaneously. Thus far, some graphene-based composites have been successfully utilized for photocatalytic reduction of CO2 into renewable fuels, as summarized in Table S8 (Supporting Information). The possible reactions that could occur during the photocatalytic process for reduction of CO2 through protonassisted multiple-electron transfer are diverse. The gas products may include CO, CH4, and C2H6, and the liquid hydrocarbons may contain HCOOH, HCHO, and CH3OH. The product selectivity is strongly influenced by the photoactive materials in the graphene-based composite photocatalysts. Moreover, the presence of graphene also plays an important role in determining the product distribution of photocatalytic reduction of CO2. Zou et al. have reported the layer-by-layer assembly of hollow RGO-Ti0.91O2 spheres consisting of alternating Ti0.91O2 nanosheets and RGO nanosheets for photocatalytic reduction of CO2.355 As shown in Figure 49A, in the presence of H2O, the formation average rates of CO and CH4 over the RGO-Ti0.91O2 composite are 8.91 and 1.14 μmol g−1 h−1, respectively, whereas for blank Ti0.91O2, CH4 is observed as an exclusive product with a rate of 1.41 μmol g−1 h−1, and no CO is detected. For comparison, the photocatalytic reduction of CO2 over commercial P25 shows that CH4 and CO are obtained with the rates of 0.69 μmol g−1 h−1 and 0.16 μmol g−1 h−1, respectively. The total conversion of CO2 over RGO-Ti0.91O2 is about 5- and 9-times higher than that of blank Ti0.91O2 hollow AM



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reduction of CO2 to CH4, the photostability of the composites has not been evaluated. Since the photocatalytic reduction of CO2 over RGO-CdS composites is performed without sacrificial reagent, the well-known photocorrosion of semiconductor CdS in the composites should occur during the photocatalytic process. Apart from the photocatalytic conversion of CO2 into gaseous CO and CH4, the reduction of CO2 into CH3OH and HCOOH has also been reported over graphene-based composite photocatalysts. RGO-NiOx-Ta2O5 composites have been utilized for selectively photocatalytic reduction of CO2 or CO2/NaHCO3 in the presence of H2O to CH3OH and H2.357 Under UV−vis light irradiation, the sample of 1%RGO-NiOxTa2O5 displays the highest photoactivity for reduction of CO2 to CH3OH, producing 3.4 times more CH3OH and 2.3 times more H 2 than blank NiO x -Ta 2 O 5 photocatalyst. The introduction of RGO as an electron collector and transporter efficiently facilitates the photoreduction of CO2 to yield CH3OH. Additionally, GO has proven to be a promising photocatalyst to catalyze reduction of CO2 into CH3OH under simulated solar light irradiation.181 Through adopting different oxidation conditions, various GO samples have been synthesized and exhibit tunable photoactivity for CO2 conversion to CH3OH, as displayed in Figure 51A. The photocatalytic CH3OH formation rate over GO-3 obtained with appropriate amounts of the graphite powder and oxidants181 can be achieved up to 0.172 μmol g cat−1 h−1, which is 6-fold higher than the pure TiO2 P25. To confirm the methanol formation through CO2 reduction, isotope tracer analyses of GO with 13CO2 have been performed. The mass spectroscopy (MS) results (Figure 51B) show that when replacing 12CO2 by 13CO2 for photocatalytic reduction with GO-3 in a closed system under irradiation for 2 h, the distinct peak associated with 13CH3OH (m/z 33) is observed instead of 12CH3OH (m/z 32). This indicates that methanol is produced directly from the photocatalytic reduction of CO2 instead of any photodissociation of the carbon containing catalyst. Besides, the photocatalytic reaction mechanism has been described by the band structure of GO. However, due to the photoinduced reduction of GO during the photocatalytic process, the detailed photocatalytic mechanism over GO still needs further studies, as stated in the cases of photocatalytic H2 evolution over GO above.176,345 A photocatalyst-enzyme coupled system consisting of RGO coupled multianthraquinone substituted porphyrin (RGO-

Figure 50. (A) Photocatalytic activity of the SEG-P25 and RGO-P25 composites for CO2 photoreduction under UV and visible light illumination; (B) the proposed photocatalytic mechanism for graphene (SEG, RGO)-TiO2 composites; the color scheme is carbon (gray), hydrogen (white), oxygen (red), and titanium (blue), hole (green). Reprinted with permission from ref 21. Copyright 2011 American Chemical Society.21

free graphene would be a straightforward strategy to exploit the electron conductive properties of graphene, which is able to further improve the activity of graphene-based composite photocatalysts, as discussed in section 6. Yu et al. have reported the visible-light-driven photocatalytic reduction of CO 2 to CH 4 over RGO-CdS nanorod composites,356 which are prepared via a one-step microwavehydrothermal method in ethanolamine-water solution. The optimal RGO-CdS exhibits the CH4 production rate of 2.51 μmol h−1 g−1, which exceeds the rate over blank CdS nanorods by more than 10 times (0.21 μmol h−1 g−1) and is also higher than that of the optimum Pt-CdS nanorod composite photocatalyst (1.52 μmol h−1 g−1) under identical reaction conditions. The introduced RGO not only acts as an electron acceptor and transporter to efficiently separate the charge carriers generated from the photoexcitation of CdS nanorod, but also enhances the adsorption and activation of CO2 molecules over the RGO-CdS composite, thus speeding up the photocatalytic reduction of CO2 to CH4. However, although RGO-CdS has proven to be active for photocatalytic

Figure 51. (A) Photocatalytic methanol formation (RMeOH) on different GO samples and TiO2 using a simulated solar light source; (B) mass spectroscopy (MS) results of methanol produced by photocatalytic reduction of 13CO2 over GO-3. Reprinted with permission from ref 181. Copyright 2013 Royal Society of Chemistry.181 AN



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MAQSP), nicotinamide adenine dinucleotide (NADH), and organometallic rhodium complex has been constructed for photocatalytic reduction of CO2 to formic acid (HCOOH).358 In this artificial photosynthesis system, the RGO-MAQSP is used as a photocatalyst. Under visible light irradiation, the absorption of photon occurs as a transition between localized orbitals around chromophore (MAQSP), which generates electrons and transfer to the rhodium complex via RGO. The reduced organometallic rhodium complex can abstract protons from aqueous solution and transfer electrons and hydrides to NAD+, which gets converted to NADH, thus forming the photocatalysis cycle (Figure 52A). In this way, the rhodium

have proven that this process can be achieved over graphenebased composite photocatalysts. Zou et al. have fabricated RGO-TiO2 composite photocatalysts in a binary ethylenediamine (En)/H2O solvent via the in situ simultaneous reduction-hydrolysis technique,359 during which the reduction of GO to RGO by En and the formation of TiO2 nanoparticles loaded onto RGO through chemical bonds (Ti−O−C bond) can be achieved simultaneously. Owing to the reducing role of En, abundant Ti3+ is formed on the surface of TiO2 in the RGO-TiO2 composites. As shown in Figure 53A,

Figure 52. (A) Schematic illustration of graphene-based photocatalyst catalyzed artificial photosynthesis of formic acid from CO2 under visible light illumination; (B) photocatalytic activities of RGOMAQSP, MAQSP, and W2Fe4Ta2O17 in visible light driven artificial photosynthesis of formic acid from CO2. Reprinted with permission from ref 358. Copyright 2012 American Chemical Society.358

Figure 53. (A) Comparison of photocatalytic activity of RGO-TiO2 with different weight contents of RGO. The molar ratio of C2H6 to CH4 increases from 0.71 (for blank TiO2), 2.09 (1%RGO-TiO2), 2.10 (2%RGO-TiO2), to 3.04 (5%RGO-TiO2); photocatalytic (B) CH4 and (C) C2H6 evolution amounts for RGO-TiO2. Reprinted with permission from ref 359. Copyright 2013 John Wiley & Sons, Inc.359

complex shuttles as an electron mediator between RGOMAQSP photocatalyst and NAD+, which facilitates the regeneration of the NADH. Finally, NADH is consumed by the CO2 substrate for its enzymatic (formate dehydrogenase) conversion to HCOOH. The NAD+ released from the enzyme can undergo the photocatalysis cycle in the same way, leading to the photoregeneration of NADH. These two catalysis cycles thus couple integrally to work together, ultimately yielding HCOOH from CO2. As shown in Figure 52B, the HCOOH yield increases linearly with the reaction time when RGOMAQSP is used as the photocatalyst. The yield for the production of HCOOH over RGO-MAQSP within 2 h is 110.55 μmol, while those of the reference samples W2Fe4Ta2O17 and blank MAQSP are only 14.25 and 46.53 μmol, respectively. This work demonstrates the possibility of using graphene-based photoactive materials to construct a photocatalyst-biocatalyst coupled system for photocatalytic reduction of CO2 to value-added chemicals. In the above discussion, the products of CO, CH4, CH3OH, and HCOOH are all single-carbon molecules. The reports on the direct conversion of CO2 to multi-carbon compounds by photocatalysis are relatively scanty. Notably, some recent works

2%RGO-TiO2 composite displays enhanced photocatalytic performance (8 μmol g−1 h−1 CH4 and 16.8 μmol g−1 h−1 C2H6) as compared with blank TiO2 (10.1 μmol g−1 h−1 CH4 and 7.2 μmol g−1 h−1 C2H6) and commercial P25 (0.69 μmol g−1 h−1 CH4, and minor CO 0.16 μmol g−1 h−1, C2H6 is absent). Moreover, it is notable that, with the increase of weight ratio of RGO in the RGO-TiO2 composites, the production rate of CH4 slowly decreases but the production rate of C2H6 increases to some degree (Figure 53B and C). The preferential generation of C2H6 over the RGO-TiO2 composites can be ascribed to the presence of surface-Ti3+ and RGO, which are able to couple with the •CH3 and stabilize the •CH3 species via π-conjugation between the unpaired electrons of the radical and aromatic regions of the RGO, respectively. The increasing accumulation of •CH3 over RGO-TiO2 significantly raises the opportunity of formation of C2H6 by the coupling of •CH3 and restrains the combination of •CH3 with H+ and e− into CH4. In addition, SEG-porphyrin composite films have been utilized for photocatalytic reduction of CO2 to CH4 and acetylene (C2H2) under visible light irradiation.360 In this system, cobalt tetrahydroxyphenyl porphyrin (CoTHPP) and meso-tetrahydroxyphenyl porphyrin (THPP) serve as the light AO


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Figure 54. (A) Plots of hydrocarbon generation vs time for the SEG-porphyrin catalysts obtained using the gas−solid phase reduction method; (B) plots of hydrocarbon generation vs time for THPP and CoTHPP, and control experiments of the SEG-porphyrin catalysts in O2 or in H2O; (C) the charge distribution and distance between the positive charges and molecules: (top) THPP+ with CO2, (down) CoTHPP+ with CO2. Reprinted with permission from ref 360. Copyright 2014 Royal Society of Chemistry.360

11.2.2. Nitroaromatics Reduction. Catalytic reduction of nitroaromatics to corresponding amines is an important chemical transformation in synthetic organic chemistry.361−364 The nitroaromatics are often used in the production of pesticides, herbicides, insecticides, and synthetic dyes and have been found to be refractory pollutants for water.361,362,365−367 However, their derivates of aromatic amines, obtained from catalytic reduction reaction, are potent intermediates in the industrial synthesis of biologically active compounds, pharmaceuticals, rubber chemicals, and photographic and agricultural chemicals.365,368,369 Therefore, selective reduction of nitroaromatics to corresponding amines represents a promising approach to restore the environment and realize the sustainable organic synthesis simultaneously. Photocatalytic selective reduction of nitroaromatics to amines with high selectivity has been studied over graphenebased composites with the control of reaction conditions, as summarized in Table S9 (Supporting Information). For instance, RGO-TiO2 composites have been applied to photocatalytic selective reduction of 4-nitrophenol and 4-nitroaniline to their corresponding aromatic amines.370 Upon UV light irradiation under N2 atmosphere with the addition of hole scavenger, the addition of RGO has significantly enhanced the photoactivity of TiO2, and 1%RGO-TiO2 displays the optimal performance. This has been ascribed to the fact that RGO can effectively minimize the recombination of photogenerated charge carriers derived from the irradiated TiO2 and facilitate the separated electrons to participate in the reactions. The effect of hole scavengers on the photocatalytic reduction activity of RGO-TiO2 composites has also been studied in this work. The results show that (i) without the use of hole scavengers (Entry 1, Table 3), no effective photocatalytic reduction reaction occurs; (ii) the photoactivity of 1%RGOTiO2 improves gradually with increasing dosage of oxalic acid (Entries 2−5, Table 3); (iii) in the absence of suitable hole scavenger, the nitroaromatics are unable to be selectively converted to their amines (Entries 6−8, Table 3). This directly demonstrates the importance of controlling reaction conditions for promoting the photocatalytic selective reduction of nitroaromatics. Chang and co-workers have fabricated RGO-ZnO nanospheres-Au nanorods (RGO-ZnO-Au) composites via a simple one-step hydrothermal method for photocatalytic reduction of nitrobenzene (NB) to aniline under N2 atmosphere in the

harvester due to their suitable light absorption property and high photostability. It can be seen from Figure 54A and B that the photoactivities of SEG-THPP and SEG-CoTHPP are both higher than THPP and CoTHPP in terms of the yield of hydrocarbon generation. This is mainly because the SEG sheet with high electrical conductivity can greatly increase the lifetime of photogenerated charge carriers in the composite photocatalysts, and improve the adsorption of CO2 molecules, thereby enhancing the photoactivity of the SEG-THPP and SEG-CoTHPP composites. In addition, the production rates of C2H2 and CH4 on the SEG-THPP are about 113 μmol m−2 h−1 and 57 μmol m−2 h−1, respectively, which are higher than those over SEG-CoTHPP (ca. 42 μmol m−2 h−1 and 38 μmol m−2 h−1). The possible reason for such an observation has been studied by theoretical simulations, as shown in Figure 54C. It is proposed that the central ring of the porphyrin molecule is the active center for electron transfer in the photocatalytic process. For the CoTHPP, its central ring is not a planar structure. In such a situation, the large π structure will be broken, which is adverse for the transfer of electrons to graphene for the reduction of CO2. Moreover, the control experiment on THPP without SEG shows that only very small amounts of CH4 and trace amounts of H2 can be detected, while C2H2 is not detected, demonstrating that the addition of SEG plays the key effect on the generation of C2H2 in this composite system. The above works have proven the great promise of converting CO2 into more valuable high-grade multi-carbon compounds (such as C2H2, C2H6) over graphene-based composite photocatalysts. It provides a new doorway for the potential application of graphene in promoting the activity toward photocatalytic C−C coupling reactions. It is worth noting that, as for the studies on utilizing graphene-based composite photocatalysts for CO2 reduction, the confirmation of the carbon source for the hydrocarbon compounds formed in the reaction system is indispensable, considering that, besides CO2, graphene in the composites or carbon-containing solvent could also serve as carbon source. In this regard, the isotopic labeling analysis with 13CO2 provides an efficient approach to identify the hydrocarbon derived through CO2 reduction instead of photo-dissociation of the carbon-containing catalyst or solvent.181 However, this key issue has often been unaddressed in the related reports,21,355,356,358−360 and it should be paid more attention in the future studies. AP


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can be obtained in the absence of photocatalyst or in the presence of RGO has not been available. In addition to the reported UV light responsive graphenebased composites, a series of visible-light-driven graphenebased photocatalysts (e.g., RGO-CdS,192,309,312,372,373 RGOIn2S 3,123 RGO-ZnIn 2S 4,306 RGO-Bi 2 WO 6,374 RGO-CdSTiO2,308 RGO-CdS-ZnO,293 RGO-Pd-CdS,253 and RGO-AgCdS375) have also been fabricated for selective reduction of nitroaromatics to their corresponding amines. Xu and co-workers have utilized the RGO-CdS nanowire (NW) composites with 1D-2D structure192 and RGO-CdS nanosphere (NSP) composites with quasi core−shell structure309 for photocatalytic selective reduction of nitroaromatics with different substituent groups to their corresponding amines under visible light irradiation, as illustrated in Scheme 1. The

Table 3. Effects of Hole Scavengers on Photocatalytic Activities for Reduction of 4-Nitrophenol to 4-Nitroaniline over 1%RGO-TiO2 Samples upon UV Light Illumination for 1 ha Entry

Scavenger

Dosage (mmol)

Conversion (%)

Yield (%)

1 2 3 4 5 6 7 8

none H2C2O4 H2C2O4 H2C2O4 H2C2O4 Methanol Na2C2O4 (NH4)2C2O4

0 0.08 0.4 0.8 1.6 0.8 0.8 0.8

18.5 75.0 82.2 100 100 18.8 16.3 16.9

0 53.6 59.5 94.5 93.9 0 0 0


a Reprinted with permission from ref 370. Copyright 2013 Royal Society of Chemistry.370

Scheme 1. Photocatalytic Selective Reduction of Nitroaromatics to Corresponding Amines in Water with the Addition of Ammonium Formate (HCOONH4) as Hole Scavenger under N2 Atmosphere

presence of methanol as hole scavenger under UV light irradiation.371 As shown in Figure 55, the RGO-ZnO-Au

photoactivity tests have demonstrated that the RGO-CdS NW and RGO-CdS NSP composites exhibit higher visible light photoactivity than their counterparts of bare CdS NW and CdS NSP, respectively. This is mainly because the addition of RGO sheets significantly enhances the adsorptivity toward substrates and promotes the charge carrier separation of the resulting RGO-CdS NW and RGO-CdS NSP composites, which thus increases the probability of photogenerated electrons participating in the photocatalytic reduction of nitroaromatics. In addition, the photocorrosion of CdS is efficiently inhibited by the proper control of reaction conditions, i.e., the use of ammonium formate (HCOONH4) as hole scavenger and N2 atmosphere. Recently, Xiao et al. have constructed well-defined RGO nanosheets-CdS quantum dots (RGO-CdS QDs) multilayered films312 via a layer-by-layer self-assembly methods for visible light photocatalytic selective reduction of nitroaromatics to corresponding amines. The architecture and photoelectrochemical and photocatalytic properties of the composite films can be easily tuned by the simple control of deposition cycles. With the addition of hole scavenger (HCOONH 4 ) in N 2 atmosphere, the integration of CdS QDs with RGO can improve the photoactivity of CdS QDs, as reflected by two representative examples in Figure 56. The photoactivity enhancement is attributed to the judicious integration of CdS QDs with RGO in an alternative stacking manner, which well utilizes the 2D nanoarchitecture of graphene and boosts the charge separation and transport in the RGO-CdS QDs composite films. Moreover, it is notable that, after annealing treatment of RGO-CdS QDs composite films in an inert environment, the photocatalytic performance of the composite is further improved. This can be ascribed to better intimate interfacial contact between RGO and CdS QDs by attenuating the steric effect caused by organic molecules grafted on the verge of the RGO framework. Besides, the RGO-CdS QDs composite film exhibits good photostability during the three successive recycling tests period, which confirms that the photocorrosion of CdS and the possible photodegradation of

Figure 55. Photocatalytic reduction of NB (5 mM) (a) in the absence of any photocatalyst and (b) in the presence of RGO (0.5 mg mL−1), (c) Au nanorods (0.5 mM), (d) ZnO nanospheres (18 mM), (e) P25 (18 mM), (f) RGO-Au nanorods (4 mg mL−1), (g) RGO-ZnO nanospheres (4 mg mL−1), and (h) RGO-ZnO-Au composite (4 mg mL−1). Reprinted with permission from ref 371. Copyright 2013 American Chemical Society.371

composite exhibits the highest photocatalytic activity toward reduction of NB among the testing samples, including RGO, Au nanorods, ZnO nanospheres, commercial P25, RGO-Au, and RGO-ZnO. The improved photoactivity of RGO-ZnO-Au can be ascribed to the rapid and efficient photoelectron transfer from ZnO to RGO and then to Au due to their matchable energy level, which decreases the recombination of electron− hole pairs and facilities the reduction of NB. The surface assisted laser desorption/ionization mass spectrometry has confirmed that aniline is the major product after the photocatalytic reaction. The controlled experiments of using methanol, ethanol, and isopropyl alcohol as different hole scavengers reveal that the primary alcohol is the most efficient scavenger for capturing the photogenerated holes. Moreover, it has also been found that the maintenance of N2 atmosphere (without O2) is favorable for the photocatalytic reduction of NB to aniline. This is because the presence of O2 would compete with NB for reacting with the photogenerated electrons, which is undesirable for the photocatalytic reduction process. However, the reason why 30−40% conversion of NB AQ


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Figure 56. Photocatalytic reduction of substituted nitroaromatics over CdS QDs film (15 cycles), as-assembled RGO-CdS QDs composite film (15 cycles), and calcined RGO-CdS QDs composite films (15 cycles) under visible light irradiation, with the addition of HCOONH4 as quencher for photogenerated holes and N2 purge under ambient conditions: (A) 4-nitroaniline, (B) 4-nitrophenol. Reprinted with permission from ref 312. Copyright 2014 American Chemical Society.312

Figure 57. Photocatalytic performance of CdS NW, RGO-CdS NW, and RGO-CdS NW-TiO2 for reduction of (A) 4-nitrotoluene and (B) 1-bromo4-nitrobenzene under visible light irradiation with the addition of ammonium formate as hole scavenger in N2 atmosphere. Reprinted with permission from ref 308. Copyright 2014 Royal Society of Chemistry.308

and derivatives, such as aldehydes and ketones obtained from the oxidation processes, are widely utilized in the fragrance, confectionary, and pharmaceutical industries.376−378 Traditionally, the classical industrial oxidation processes involve environmentally and corporally harmful or corrosive strong oxidants (e.g., chromate and permanganate) and need harsh reaction conditions (e.g., high temperature and high pressure), which produces quantities of hazardous wastes. Therefore, developing alternative catalytic processes for promoting selective oxidation of alcohols under milder reaction conditions is necessary for safe and clean organic synthesis. In this context, photocatalysis has aroused great interest in recent years and has proven to be effective for selective oxidation of alcohols to target products (e.g., aldehydes and acids). In comparison with the traditional method, the photocatalysis technique is directly driven by clean solar energy and performed under room temperature and ambient atmosphere with benign oxidants, such as O2. Consequently, the process is more mild and sustainable. The reports on the utilization of graphene-based composite photocatalysts for selective oxidation of alcohols are summarized in Table S10 (Supporting Information). Xu’s group has made some progress in this respect.20,192,204,212,302,304,305,379−381 For instance, RGO-TiO2 composites with intimate interfacial contact have been prepared via a facile “soft” wet-chemistry approach, during which the structure-directing role of GO in solution is employed to induce the in situ growth and nucleation of TiO2 nanoparticles on the surface of graphene.20 The RGO-TiO2 composites have been used for the selective oxidation of various benzylic alcohols and allylic alcohols to corresponding aldehydes in benzotrifluoride (BTF) solvent under visible light irradiation.

graphene can be efficiently inhibited by the proper control of reaction conditions. Apart from the binary graphene-based composites, ternary graphene-based composites have also been fabricated for photocatalytic selective reduction of nitroaromatics. For example, the flat structured RGO-CdS nanowire-TiO2 (RGOCdS NW-TiO2) composites have been utilized for visible light photocatalytic selective reduction of nitroaromatics to amines.308 As exemplified in Figure 57, the ternary RGO-CdS NW-TiO2 composites display higher visible light activity toward the probing reactions than CdS NW and the binary RGO-CdS NW. This can be ascribed to the specific nanostructure and composition of RGO-CdS NW-TiO2 composites. In detail, the introduction of negatively charged TiO2 nanoparticles on both the surfaces of CdS NW and RGO sheets efficiently prevents the RGO sheets from being curly or aggregated through an electrostatic repulsion. This large 2D flat structure leads to an increased optical absorption of visible light and enhanced electrical conductivity of the RGO-CdS NW-TiO2 composites as compared to the curly RGO-CdS NW composites synthesized without the introduction of TiO2. Moreover, the TiO2 nanoparticles on CdS NW are also able to further boost the separation and transfer of charge carriers in the ternary RGO-CdS NW-TiO2 composites due to the suitable energy level match between TiO2 and CdS, which improves the lifetime of electron−hole pairs photogenerated from excitation of CdS NW upon visible light irradiation. 11.2.3. Alcohols Oxidation. Selective oxidation of alcohols is another type of reaction widely studied in both industrial and laboratory syntheses, which play an important role in the production of a wide range of chemicals.146,331 The products AR


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site exhibits enhanced photoactivity as compared to blank CdS because the hybridization of RGO with CdS is able to tune the morphology of CdS, enhance the light absorption intensity, and promote the separation and transfer of photoinduced electron− hole pairs of the RGO-CdS composite. In addition, the effect of reaction solvents on the conversion and selectivity has been investigated. As shown in Figure 58 it is found that when the

The choice of solvent BTF is because of its inertness to oxidation and high solubility for molecular O2.20,204,252,302,303,310,311 In addition, the photocatalytic reactions performed in the inert BTF are able to prevent the generation of strong and nonselective hydroxyl radical (•OH) species, which would improve the selectivity of photocatalytic oxidation reactions, as listed in Table 4. Moreover, in


Table 4. Selective Oxidation of a Range of Alcohols over the 5%RGO-TiO2 Photocatalyst under the Visible Light Irradiation (λ > 400 nm) for 20 ha

Figure 58. Photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over the 5%RGO-CdS composite under the irradiation of visible light for 4 h in different solvents. Reprinted with permission from ref 192. Copyright 2011 American Chemical Society.192

a

Reprinted with permission from ref 20. Copyright 2011 American Chemical Society.20

nonfluorinated solvents, such as toluene, acetonitrile, and water, are used with other conditions unchanged, the conversion of benzyl alcohol obtained over 5%RGO-CdS composite is lower than that in BTF. This is ascribed to the higher solubility for molecular oxygen of benzotrifluoride. Meanwhile, the selectivity for benzaldehyde decreases with the increase in the polarity of solvents, which may be due to the solvolysis of O−O bands in polar media.192,382 On the basis of the above work, the ternary RGO-CdS-TiO2 composites have been further constructed using an in situ growth strategy, for which TiO2 nanoparticles are uniformly covered on the surface of the binary RGO-CdS.305 The introduction of TiO2 into the RGO-CdS substrate displays no obvious influence on the morphology and porosity properties of the samples. The results on photocatalytic oxidation of alcohols show that the ternary RGO-CdS-TiO2 composites display higher activity than the binary RGO-CdS counterparts under identical reaction conditions, as reflected in Figure 59. The improved photocatalytic performance of RGO-CdS-TiO2 can be ascribed to the synergistic effects of enhanced separation and transfer of photogenerated electron−hole pairs, and larger surface area resulting from the introduction of TiO2. Additionally, the layered RGO-C3N3S3 polymer composites have also been used as visible light photocatalysts for selective aerobic oxidation of benzylic alcohols to corresponding aldehydes in BTF solvent.383 The layered structure of the RGO-C3N3S3 polymer is able to prevent RGO sheets from restacking and boost the separation and transfer of photogenerated charge carriers from C3N3S3 during the organic transformation. This leads to optimal 0.3%RGO-C3N3S3 polymer displays having much higher photoactivity than blank C3N3S3 photocatalyst. Besides studying photocatalytic selective oxidation reactions in the organic BTF solvent, driven by the typical tenet of green chemistry, the application of graphene-based composite photocatalysts, including graphene (RGO, SEG)-CdS and graphene (RGO, SEG)-TiO2 composites, for aerobic oxidation

comparison with the CNT-TiO2 composites prepared by the similar procedure, the RGO-TiO2 composites exhibit higher photoactivity. This is mainly due to the fact that the “structuredirecting” role of GO leads to a more intimate interfacial contact between RGO and TiO2 than that between CNT and TiO2, which determines that the separation and transfer of the photogenerated charge carriers in RGO-TiO2 are more efficient than that in CNT-TiO2. As a result, the RGO-TiO2 composite exhibits higher photoactivity than CNT-TiO2. When using SEG with better electrical conductivity than RGO as the precursor of graphene to fabricate the SEG-TiO2 composites, the photoactivity for selective oxidation of different benzylic alcohols and allylic alcohols to aldehydes under visible light irradiation in BTF solvent can be further enhanced.212 The photoactivities of SEG-TiO2 are much higher than the RGOTiO2 prepared via the similar “soft” wet-chemistry approach and SEG-P25 and RGO-P25 prepared by the “hard” integration method. The improved photoactivity of SEG-TiO2 is attributed to the fact that, as compared to the composites of RGO-TiO2, SEG-P25, and RGO-P25, the SEG-TiO2 composites are able to make more sufficient use of the electrical conductivity of graphene due to the decreased defects of SEG and the improved interfacial contact between SEG and TiO2 nanoparticles. As a result, the transfer and lifetime of photoexcited charge carriers can be improved more efficiently, which in turn leads to the enhanced photoactivity of SEG-TiO2. This work reveals that decreasing the defect density of graphene along with strengthening interfacial contact is a feasible approach to improve the photocatalytic performance of graphene-based composite photocatalysts. In addition to graphene-TiO2, the RGO-CdS composites with different weight addition ratios of RGO have also been synthesized for photocatalytic selective oxidation of various benzylic and allylic alcohols to corresponding aldehydes under visible light irradiation.192 The optimal 5%RGO-CdS compoAS


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Figure 59. Time-online photocatalytic selective oxidation of alcohols to aldehydes over the optimal 5%RGO-CdS-10%TiO2 and 5%RGO-CdS photocatalysts under visible light irradiation: (A) benzyl alcohol and (B) 2-buten-1-ol. Reprinted with permission from ref 305. Copyright 2012 American Chemical Society.305


Table 5. Photocatalytic Selective Oxidation of Various Benzylic Alcohols to Aldehydes and Acids over Blank CdS, 5%RGO-CdS, and 5%SEG-CdS Composites in Water under Visible Light Irradiation for 4 h

blank CdS

5%RGO-CdS

Yield (%)

a

5%SEG-CdS

Yield (%)

Yield (%)

Entry

Ra

Conv. (%)

aldehyde

acid

Conv. (%)

aldehyde

acid

Conv. (%)

aldehyde

acid

1 2 3 4 5 6

p-H p-methyl p-methoxyl p-nitro p-fluoro p-chloro

30 24 36 29 30 23

13 17 17 6 6 8

17 7 19 23 24 15

35 28 50 37 41 32

25 21 32 8 10 12

10 7 18 29 31 20

67 55 68 54 50 72

47 40 45 12 15 19

20 15 23 42 35 53

R represents the different substituent groups. Reprinted with permission from ref 390. Copyright 2013 Elsevier.390

of alcohols in water has been investigated recently.299,379 It has been found that the graphene-CdS and graphene-TiO2 composites are active for selective oxidation of benzylic alcohols to aldehydes and acids in the green solvent of water under visible light irradiation, as exemplified in Table 5. The introduction of an appropriate amount of graphene can effectively enhance the photoactivity of the graphene-CdS and graphene-TiO2 composites. In addition, the selectivity distribution of the products is changed with the variation of graphene content in the composites. The high weight addition of graphene in graphene-CdS and graphene-TiO2 composites leads to a decrease of selectivity for benzaldehyde, with the main byproduct of benzoic acid. This is mainly because with the high content of graphene, the target product of aldehyde may not be easily desorbed from the surface of the photocatalysts, which would lead to a deep oxidation of aldehyde and result in the high yield of acid. Moreover, it has been found that the SEG-CdS and SEG-TiO2 composites both exhibit higher photoactivity than the RGO-CdS and RGO-TiO2 counterparts, respectively. This is in accordance with the result of the previous work that the SEG, with lower defect density, has higher electrical conductivity than RGO, which can promote more efficient separation of photogenerated charge carriers and enhance the photoactivity more effectively. 11.2.4. Other Reactions. 11.2.4.1. Phenol Oxidation. Das and co-workers have reported the application of RGO-Ag3VO4 photocatalysts prepared via a one-pot in situ photochemical synthesis route for photocatalytic selective hydroxylation of phenol.384 Without using any hydroxylating agents, the RGO-

Ag3VO4 photocatalysts have effectively converted phenol into catechol (CAT) and hydroquinone (HQ) under visible light illumination. As shown in Table 6, after visible light irradiation of 2 h, the introduction of RGO into the matrix of Ag3VO4 gives rise to a prominent photoactivity enhancement. This is mainly due to the fact that during the photocatalytic hydroxylation process over the RGO-Ag3VO4 composites in aqueous medium, the RGO sheets are able to accept electrons Table 6. Catalytic Activity and Product Selectivity over Various RGO-Ag3VO4 Photocatalystsa Selectivity (%) Entry

Catalyst

Conversion (%)

CAT

HQ

1 2 3 4 5 6 7 8 9 10

No catalyst Graphite GO RGO Ag3VO4 1%RGO-Ag3VO4 2%RGO-Ag3VO4 4%RGO-Ag3VO4 8%RGO-Ag3VO4 4% RGO (HT)b-Ag3VO4

0 0 0 0 18.82 36.47 66 100 100 70.24

32.9 40.25 80.23 89.38 50.49

90 67.1 59.75 19.77 10.62 49.51

a Reaction conditions: [catalyst] = 2 g L−1, [phenol] = 20 mg L−1, visible light irradiation time = 2 h. bRGO (HT) refers to RGO obtained from the hydrothermal reduction process. Reprinted with permission from ref 384. Copyright 2012 Royal Society of Chemistry.384

AT



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easily, forming the superoxide radicals (O2•−) that might be subsequently transformed to hydroxyl radicals (•OH). With an increase in the weight ratio of RGO to Ag3VO4, the generation of •OH can be boosted on the surface of the catalysts, which thus facilitates the hydroxylation reaction (Entries 6−9, Table 6). However, the reason for why the increase of RGO content improves the selectivity for CAT has not been clear. The 4% RGO (HT)-Ag3VO4 obtained by assembling of Ag3VO4 on the surface of hydrothermal reduced GO (RGO) exhibits lower photoactivity as compared to the 4%RGO-Ag3VO4 composite prepared by the in situ reduction dispersed GO photoirradiation process (Entries 8 and 10, Table 6). This might be due to the insufficient interfacial contact in the 4%RGO (HT)-Ag3VO4. This work provides a green and effective process for the synthesis of catechol from hydroxylation of phenol in water under visible light irradiation. 11.2.4.2. Primary C−H Bonds Oxidation. Selective oxidation of saturated sp3 C−H bonds for fine chemicals synthesis is of crucial importance for the sustainable exploitation of available feedstocks. In a recent study, a highly selective ternary RGO-CdS-TiO2 composite photocatalyst has been reported for visible light driven oxidation of saturated primary C−H bonds using benign O2 as oxidant in BTF solvent under ambient conditions.303 The ternary RGO-CdS-TiO2 is fabricated by assembling RGO and TiO2 as two cocatalysts into the semiconductor CdS matrix to form intimate spatial integration and sheet-like structure. The visible light harvester in this ternary composite is a highly active cubic phase, sheet structured CdS semiconductor.385 Table 7 shows the results of selective oxidation of toluene and other substituted toluenes to corresponding aldehydes over blank CdS, the optimum binary 5%RGO-CdS and ternary 5%RGO-CdS-10%TiO2 composites under visible light irradiation. It can be seen that the 5%RGOCdS composite displays higher activity than blank CdS

photocatalyst, while the introduction of the second cocatalyst TiO2 further improves the activity of CdS under identical reaction conditions. This is because, in the ternary RGO-CdSTiO2 composite, the introduction of RGO and TiO2 cocatalysts into the CdS matrix efficiently optimizes and improves the separation and transfer of charge carriers over the light harvester CdS under visible light irradiation, by which the photoactivity of CdS for aerobic selective C−H activation is enhanced. This work further enriches the promising application scope of the rationally designed graphene-based composites in photocatalytic selective oxidation of saturated C−H bonds for fine chemicals synthesis. 11.2.4.3. Tertiary Amines Oxidation. Recently, the incorporation of visible light active organocatalyst with graphene oxide (GO) has been demonstrated to improve the photoactivity toward organic synthesis reactions. Pan and co-workers have reported the visible light induced oxidative C−H functionalization of tertiary amines catalyzed by the GO-Rose Bengal (RB) photocatalyst, in which GO is proposed to behave as a cocatalyst.386 Table 8 displays the photoactivities for αTable 8. α-Cyanation of Tertiary Amine in the Presence of RB and GOa

Table 7. Photocatalytic Selective Oxidation of Toluene and Other Substituted Toluenes to Corresponding Aldehydes over Blank CdS, 5%RGO-CdS, and 5%RGO-CdS-10%TiO2 under Visible Light Irradiation of 10 ha

Entry

Carbon catalyst (wt %)

Yield (%)b

1 2 3 4 5 6d 7e 8f 9g

none 50% GOc 50% graphite 50% activated carbon 200% GO 50% GO 50% GO 50% GO 50% GO

60 97 65 56 96 94 (63) 10 0 94

a

Reaction was performed using 0.05 mmol of 1a and 0.125 mmol of TMSCN in 0.5 mL of CH3CN. bIsolated yield. cGO was prepared according to the reported method. dRu(bpy)3Cl2 was used instead of RB. The number in parentheses is the yield without GO. eReaction was performed without RB. fReaction was performed in dark. g Recycled GO was used. Reprinted with permission from ref 386. Copyright 2011 Royal Society of Chemistry.386

cyanation of tertiary amine over different photocatalysts under green light irradiation. The result shows that blank RB is able to catalyze the reaction and the combination of GO with RB further enhances the activity, whereas the hydrophobic graphite and activated carbon have no significant effects on the performance of RB. In addition, the controlled experiments show that no desired product or trace amount of product can be obtained when the reaction is conducted in the dark or without RB, demonstrating that the light and photocatalyst are both essential for this reaction. The recycled experiment shows that the GO-RB composite has no obvious deactivation during the photocatalytic process, implying that it can perform as a stable photocatalyst for α-cyanation of tertiary amine. Moreover, this photoactivity enhancement has also been observed for photocatalytic α-cyanation of other cyclic tertiary amines and trialkyl amines over GO-RB, which may be ascribed to the

a

Reprinted with permission from ref 298. Copyright 2013 Nature Publishing Group.303 AU


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Table 9. Oxidative Mannich Reaction between N-Aryl-tetrahedroisoquinoline and Acetonea

Entry

Catalyst

Amount (wt %)

Yield (%)b

1 2 3 4 5 6

no catalyst GO r-GO-P3HT GO-P3HT P3HT P3HT

2.5 2.5 2.5 2.5 0.5

47 44 64 93 74 65


a The reaction was performed using 0.1 mmol of 1 in the mixed solvent of 0.5 mL of acetone and 0.5 mL of CHCl3. bIsolated yield. Reprinted with permission from ref 387. Copyright 2012 American Chemical Society.387

Figure 60. (A, C) Schematic illustrations of drop-casting and dip-coating processes, respectively. (B) SEM image of RGO-TiO2 composite (the inset is the photograph of the film obtained by drop-casting). Reprinted with permission from ref 196. Copyright 2010 American Chemical Society.196 (D) SEM image of the RGO-TiO2 layer formed by the dip-coating method. Reprinted with permission from ref 392. Copyright 2011 Elsevier.392

intermediate O2•− is stabilized by the aromatic scaffold in RGO (GO) and the oxygenated groups in the RGO (GO) surface may undergo electrostatic interaction with the amine radical cation, thus facilitating the reaction with O2•− to form iminium. As compared to GO-P3HT, the smaller population of oxygenated species on the RGO-P3HT scaffold is suggested to be the possible reason for its lower photoactivity.

stabilization of iminium intermediate by slightly acidic GO and the intrinsic high surface area of GO. 11.2.4.4. Mannich Reaction. Wang et al. have constructed the GO-poly(3-hexylthio-phene-2,5-diyl) (P3HT) and RGOP3HT composites via π−π interaction between the nonpolar regions of GO/RGO and the molecular layers of P3HT for the photocatalytic oxidative Mannich reaction under mild conditions.387 In these composite systems, the semiconducting conjugated polymer of P3HT can be photoexcited to generate charge pairs and then inject into GO/RGO across the interface of the RGO-P3HT (GO-P3HT), thus facilitating the charge carrier separation and improving the photoactivity. As shown in Table 9, under solar light irradiation, the photoactivity results show that the RGO-P3HT improves the reaction yield for the target product from 40 to 60% as compared to blank P3HT, while the GO-P3HT shows the highest yield of 93%. The reason for different photocatalytic performance between GOP3HT and RGO-P3HT has been proposed as follows. During the photocatalytic process, the tertiary amine is oxidized by the positive hole on the HOMO of P3HT via single electron transfer to form the radical cation. At the same time, the excited electron is injected from the LUMO of P3HT into RGO (GO), which is then used to activate molecular O2 to form O2•−. The

12. STRATEGIES FOR UTILIZING GRAPHENE-BASED COMPOSITE PHOTOCATALYSTS TOWARD PRACTICAL APPLICATION As summarized above, so far, large quantities of various graphene-based composite photocatalysts (ca. 200 different types) have been reported for extensive potential photocatalytic redox processes with enhanced performance.22−33 Meanwhile, it should be rationally realized that there is still a long way to go to implement these graphene-based composite photocatalysts into practical applications.22,49,51 To this end, one of the pivotal issues is to immobilize the graphene-based composite photocatalysts on appropriate supports/substrates to circumvent the time-consuming and costly separation and recovery processes (e.g., centrifugal sedimentation and filtration). However, the research in this respect has received relatively less attention, AV


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Figure 61. (A) Schematic illustration of the spin-coating process. (B) TEM image of RGO-La1−xSrxMnO3 composite film photocatalyst obtained by the spin-coating method. Reprinted with permission from ref 395. Copyright 2014 Elsevier.395

60B). In addition, the dip-coating method has been adopted to fabricate the RGO/Fe3+−TiO2 composite film on glass391 and RGO-TiO2 composite films on indium tin oxide (ITO)392 or FTO393 glass toward photocatalytic degradation processes, as exemplified in Figure 60D. The advantages of drop-casting and dip-coating methods lie in their easy operation procedures, including the simple available apparatus, mild operating conditions, and low demand on the surface properties and shape of the substrates. However, the relatively nonuniform coverage of the resultant films due to the agglomeration of the composite during the solvent evaporation process is the main drawback associated with these two methods.406 To alleviate the nonuniformity issue of the composite films prepared by drop-casting and dip-coating methods, some other strategies have been developed and utilized for the fabrication of graphene-based composite film photocatalysts, such as spincoating394,395 and electrostatic spray deposition (ESD).396 Besides, in some cases, strategies such as the introduction of some additive agents (e.g., binder and plasticizer) in the coating material suspension and a postannealing process can be applied to improve the quality of film materials by strengthening the adhesion between graphene-based composite films and substrates, and increasing the crystallinity of the semiconductor and/or the reduction degree of RGO.388,391,392,394−396 The typical spin-coating procedure is illustrated in Figure 61A. A small amount of coating material is applied on the center of the substrate, and then the substrate is rotated at high speed in order to spread the coating material by centrifugal force. After drying, the film material is formed. For instance, RGO-La1−xSrxMnO3 film photocatalysts on glass substrate have been synthesized through a spin-coating method along with a sol−gel process.395 The coating materials consist of La(NO3)3, Mn(NO3)2, Sr(NO3)2, and RGO aqueous solution. Besides, some additives, including citric acid, alkylphenol polyoxyethylene, poly(vinyl alcohol), and polyethylene glycol are added to the solution as chelating agent, dispersing agent, agglomerant, and plasticizer, respectively. The above mixture is adjusted to pH = 9 using aqueous ammonia and then used for spin-coating. After aging for 36 h by the spin-coating method, the solution is coated on the pretreated glass, and dried to form the gel film. With the heat treatment process, the RGO-La1−xSrxMnO3 film photocatalysts are obtained (Figure 61B). The advantage of spin-coating over drop-casting and dipcoating is its capability to give a relatively uniform film. However, only flat substrates can be used for the spin-coating method, which limits its wide application for the preparation of graphene-based composite film materials. Graphene-ZnO composite thin films on ITO glass have been produced by the ESD technique for photocatalytic degradation

which has not been addressed specifically in previous review articles.22−33 In this section, we will review the progress on the reported graphene-based composite film photocatalysts with discussion on the pros and cons of the synthetic routes based on selected typical examples, and then some possible development strategies for the preparation of fixed graphenebased composite materials combined with the design of practical photocatalytic reactors are proposed. These aspects are expected to bring closer the widespread practical implementation of the graphene-based composites in artificial photocatalysis. 12.1. Graphene-Based Composite Film Materials

The preparation of graphene-based composite film materials on proper substrates is a favorable approach to fix the photocatalysts because of the large surface area of film materials and the 2D structural advantage of graphene nanosheets. Some graphene-based composite film materials with specific photocatalytic applications have been fabricated by diverse methods, as summarized in Table 11 (Supporting Information).16,196,235,262,312,339,388−405 The synthetic strategies for these film photocatalysts mainly include drop-casting,196,339,388−390 dip-coating,391−393 spin-coating,235,394,395 electrostatic spray deposition (ESD),396 vacuum filtration,262,397 layer-by-layer (LbL) self-assembly,312,390,398 confinement self-assembly,399 radio frequency (RF) magnetron sputtering,400 electron-beam evaporation,401 electrodeposition,402,403 electrophoretic deposition (EPD),404 chemical vapor deposition (CVD) 405 and their combined processes.16,235,400−405 Among various methods for the synthesis of graphene-based composite film photocatalysts, drop-casting and dip-coating are two of the most convenient and commonly used approaches. In their typical procedures, the coating material suspension is deposited on the substrate surface by a dropping or dip-lift method, and then dried until all solvents evaporate, leaving the composite films formed on the substrate, as illustrated in Figure 60A and C, respectively. Thus far, through the drop-casting approach, RGO-TiO2,196,388 RGO-BiVO4,389 RGO-WO3339 and RGO-Ti0.87O2390 composite film photocatalysts have been fabricated on different substrates, such as optically transparent electrode (OTE), fluorine-doped tin oxide (FTO) glass, quartz glass, and Si wafer. For example, Kamat’s group has prepared RGO-TiO2 (P25) film composites on OTE by the drop-casting method and utilized them for photocatalytic decomposition of 2,4-dichlorophenoxyacetic acid under UV light irradiation.196 In a typical preparation process, the methanol suspension of RGO-TiO2 (P25) composite is deposited onto OTE with dropwise addition under flowing air followed by a calcination process at 400 °C for 1 h, which leads to the formation of RGO-TiO2 composite films (Figure AW


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Figure 62. (A) Experimental schematic used for electrostatic spray deposition of graphene-ZnO thin films; (B) SEM surface view of 0.1 wt % graphene−ZnO thin film. Reprinted with permission from ref 396. Copyright 2014 Elsevier.396

Figure 63. (A) Schematic illustration for preparing the RGO-TiO2 NTs hybrid film; SEM images of RGO-TiO2 NTs hybrid films with different electrodeposition cycles: (B) 5 cycles and (C) 26 cycles. Reprinted with permission from ref 402. Copyright 2011 Elsevier.402

of dye.396 As shown in Figure 62A, the precursor coating solution containing zinc acetate dihydrate and graphene (commercial) is administered from a syringe pump to yield a stable Taylor cone, which produces small droplets under the given operating conditions. An appropriate voltage (12−14 kV) is supplied between the needle (anode) and the grounded substrate (cathode). The colloidal particles can migrate under the influence of an electric field (electrophoresis) and are deposited onto the stationary ITO substrate whose temperature is maintained at 90 °C to ensure pyrolysis and subsequent formation of a solid thin film (Figure 62B). In this case, the propylene glycol (PG) as plasticizer and a two-step postannealing have been adopted to improve the adhesion of graphene-ZnO composite films to ITO and increase the crystallinity of ZnO, respectively.396 As compared to other techniques for layer deposition, such as radio frequency (RF) magnetron sputtering,400 and electronbeam evaporation,401 the ESD technique has the specific advantage that it does not require powerful magnets or an electron beam to bombard the target materials under high vacuum conditions to form the film, which thus reduces the manufacturing costs. In contrast to spin-coating and electrophoretic deposition (EPD)404 methods, ESD has less demand on the flatness and conductivity of the substrates. Consequently, it is expected to find a wide scope of applications for

coating graphene-based composite photocatalysts on diverse substrates. On the other hand, it deserves more efforts to moderate the susceptibility of the ESD process to the operation temperature and surrounding humidity, which have great effects on the quality of the as-obtained film materials. Besides adopting the single approach for the synthesis of graphene-based composite film photocatalysts, strategies combining two synthetic methods have also been developed to fabricate graphene-based composite films, such as dipcoating and drop-casting,16 RF magnetron sputtering and spincoating,400 electron-beam evaporation and spin-coating,401 anodization and electrodeposition,402,403 electrophoretic deposition and anodization,404 plasma enhanced chemical vapor deposition (PECVD) and RF magnetron sputtering,405 and hydrothermal reaction and spin-coating.235 In the typical process of these joint strategies, one component (semiconductor or graphene) is first deposited on the substrate via a specific synthetic method, and then the second component is coated on the preformed film surface through a complementary approach, thus giving rise to the composite films.16,400−405 For instance, Liu et al. have reported the synthesis of RGOTiO2 nanotube arrays (NTs) hybrid films by a two-step strategy.402 First, the TiO2 NTs are grown on a Ti foil through anodization. In the second step, the RGO films are formed on the surface of TiO2 NTs through electrodeposition and AX


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Figure 64. (A) Low magnification TEM image of the PECVD-grown graphene (the inset shows the SAED pattern taken from the region marked by the red circle); (B) AFM micrographs of (PECVD−grown graphene−TiO2)3 films; (C) the magnified view of the region indicated by the blue box in (B); (D) the corresponding particle size distribution histograms. Reproduced from ref 405 with permission from the Centre National de la Recherche Scientifique (CNRS) and Royal Society of Chemistry.405

Figure 65. (A) Schematic illustration for LbL self-assembly of RGO-CdS QDs composite film; (B) TEM and (C) AFM images of RGO-CdS QDs composite film with five deposition cycles [denoted as (RGO-CdS QDs)5] peeled off from FTO substrate; (D) 3D AFM image of (RGO-CdS QDs)5 composite films assembled on silicon substrate. Reprinted with permission from ref 312. Copyright 2014 American Chemical Society.312

(PECVD) followed by the radio frequency (RF) magnetron sputtering technique.405 The high-quality graphene films (as shown in Figure 64A) are first synthesized by the PECVD technique on Cu foils in a mixture of CH4/Ar/H2 gases at the pressure of 1000 Pa, and then transferred to a quartz substrate. Subsequently, the TiO2 films are deposited on the surface of PECVD-grown graphene through RF magnetron sputtering at room temperature in an Ar atmosphere at 0.5 Pa with 130 W RF power. The commercially available, hot-pressed, and sintered ceramic TiO2 is used as the sputtering target. The obtained graphene−TiO2 films are calcined in flowing N2 at 350 °C for 3 h to improve the crystallinity of TiO2 (Figure 64B−D). In this case, the graphene films perform as the growth substrates for TiO2, which have great effects on the crystalline structure, morphology, specific surface area, and thus photocatalytic activity of the resultant graphene-TiO2 composite films. Consequently, it is reasonable to expect that the morphology of the films deposited on graphene might be

simultaneously electrochemical reduction of GO dispersion by cyclic voltammetry on the TiO2 NTs/Ti electrode. Followed by further photoassisted reduction of the RGO film, the RGOTiO2 NTs hybrid film photocatalysts are obtained (Figure 63A). The surface coverage of RGO can be extended by increasing the cycle numbers, as exemplified in Figure 63B and C, which makes the photoactivity of the RGO-TiO2 NTs hybrid films tunable. Moreover, in this case, the morphology of TiO2 NTs is maintained in the resultant film photocatalysts to make use of the advantages of the 1D TiO2 structure. However, it is worth noting that the contact area between RGO and the TiO2 NTs is limited, and in such an architecture, the inner surface of the TiO2 NTs cannot be sufficiently utilized. If these issues are optimized, further enhancement of the photocatalytic performance of graphene-based composite films would be expected. Recently, graphene−TiO2 multilayer films have been prepared using plasma enhanced chemical vapor deposition AY



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Figure 66. (A) Schematic illustration for the preparation of a macro-mesoporous RGO-TiO2 composite film; (B) SEM and (C, D) TEM micrographs of a macro-mesoporous RGO-TiO2 composite film; the black arrow in (B) indicates the interconnected channels between macropores in the films. Reprinted with permission from ref 399. Copyright 2011 American Chemical Society.399

densely and homogeneously (Figure 65B), and results in an interconnected structure between CdS QDs and RGO in the RGO-CdS QDs composite film (Figure 65C and D). Furthermore, the architecture and photoelectrochemical and photocatalytic performance of RGO-CdS QDs composite films can be appropriately tuned by simple control of the deposition cycles, which takes a significant step forward to meet the application requirements of graphene-semiconductor composite photocatalysts. In addition to the compact graphene-based composite film materials, hierarchically ordered macro-mesoporous RGO-TiO2 composite film photocatalysts have been produced on glass substrate through a confinement self-assembly method within the regular voids of a 3D periodic colloidal crystal, using Pluronic P123 and polystyrene spheres as a mesostructured template and a macrostructure scaffold, respectively, as shown in Figure 66.399 Macro-mesoporous TiO2 films possess a wellordered 2D hexagonal mesostructure and well-interconnected periodic macropores as well as high specific surface areas and large pore volumes. Such an architecture can improve the mass transport through the film, reduce the length of the mesopore channel, and increase the accessible surface area of the thin film. Furthermore, RGO, as an electron-acceptor and electrontransport material, is able to effectively suppress the recombination of charge carriers photogenerated from TiO2 in the films. Therefore, the hierarchically ordered macromesoporous RGO-TiO2 composite films show the enhanced

controlled by changing the type of graphene substrates, which is of significance to achieve controllable film growth.405 Despite the respective advantages of the above synthetic methods, it is still challenging to obtain uniform graphenebased composite films with molecular-level controllable thickness and architecture, which are of paramount importance to meet the photocatalytic application requirements.312 In this respect, a layer-by-layer (LbL) self-assembly approach provides a favorable alternative, which has been applied to prepare RGOCdS,312 RGO-Ti0.87O2,390 and RGO-TiO2398 composite film photocatalysts. In addition, the LbL self-assembly method is often operated with simple procedures under mild conditions (i.e., room temperature and ambient pressure). As a typical example, Liu’s group has fabricated RGO-CdS quantum dots (QDs) composite film photocatalysts using the LbL self-assembly approach with simple benchmark operation.312 As illustrated in Figure 65A, FTO substrate with negatively charged surface after “piranha” solution treatment is first modified by poly(diallyldimethylammonium chloride) (PDDA), which acts as the precursor film. Subsequently, different layers of negatively charged CdS QDs and positively charged RGO-poly(allylamine hydrochloride) (PAH) are deposited onto the precursor film to construct the (RGOCdS QDs)n multilayered film, where n is the number of deposition cycles. Notably, the judicious integration of CdS QDs with RGO nanosheets in an alternating stacking manner allows the monodispersed CdS QDs to cover the RGO sheets AZ


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Figure 67. (A) Digital photo and (B) SEM image of GOT/P25 (the usage of GO sheets in the sample is about 0.2 mg). Reprinted with permission from ref 397. Copyright 2014 American Chemical Society.397

adsorption capacity and photocatalytic activity toward degradation of organic dyes under UV light irradiation, which are potentially suitable for wastewater treatment and air purification applications. It is easy to learn from the above discussion that the graphene-based composite films are often deposited on the rigid substrates (e.g., glass and Ti foil), which exhibit poor processability. Thus far, there have been some reports on the preparation of flexible graphene-based composite film materials through the facile vacuum filtration strategy.262,397 For instance, Xu et al. have prepared GO-based filtration membranes, GO− TiO2−P25 (denoted as GOT/P25) with photocatalytic antifouling function.397 In their study, GO-TiO2 (GOT) particles composite sheets are first prepared by decorating GO sheets with an appropriate amount of TiO2 nanoparticles, which can be assembled into filtration membranes with suitable permeation and retention rates. Then, an additional TiO2 particle layer (P25) is coated on these films by vacuum filtration, forming the hierarchical structure membranes, as shown in Figure 67A and B. Besides, free-standing RGO-porphyrin (meso-tetra(phydroxyphenyl)porphyrin, p-THPP) nanohybrid films have been synthesized via a facile vacuum filtration followed by a gaseous reduction. Typically, the p-THPP nanoparticles (NPs) are integrated into macroscopic RGO films by filtration of the co-colloids of GO and p-THPP NPs through a polyvinylidene fluoride (PVDF) film filter, followed by air drying and peeling from the filter. Then, the freestanding GO-based films are placed in an autoclave with the addition of hydrazine monohydrate (98%) and the autoclave is heated at 90 °C for 6 h to obtain RGO-(p-THPP) films (Figure 68A and B).262 The film thickness can be controlled by adjusting the volume of the colloidal suspension. The RGO-(p-THPP) film photocatalyst can be easily separated and recovered because of its free-standing nature, which might promote its practical utility to eliminate the organic pollutants from wastewater. However, the resultant films are relatively fragile and have to be used with specific protection, as displayed in Figure 68C and D. Therefore, with the purpose of utilizing such flexible graphene-based composite film photocatalysts for practical applications, the strategy to enhance their mechanical strength and durability should be developed in the future study. As such, we can see that some positive attempts have been made to prepare the fixed graphene-based composite film photocatalysts via diverse approaches,16,196,235,262,312,339,388−405 and this is an active topic of research on graphene-based composite photocatalysts, especially for the interest in practical

Figure 68. (A) Digital photo and (B) TEM image of free-standing RGO-(p-THPP) films (the inset of A is the co-colloids of GO and pTHPP NPs); (C) side view and (D) top view of the photocatalysis setup for degrading RhB aqueous solution. Reprinted with permission from ref 262. Copyright 2014 Royal Society of Chemistry.262

applications. Noteworthily, like a coin having two sides, each synthetic strategy has its inherent merits and drawbacks in terms of the cost, availability of the apparatus, scale-up possibility, quality, quantity, processability, and applicability of the resultant graphene-based composite films. The choice of the method should be determined according to the properties of selected substrate materials and the intended specific utilization. With regard to the further development of graphene-based composite photocatalysts in practical applications with satisfying solar-to-energy conversion efficiency, there are two important methodologies. One is the improvement and optimization of the existing synthetic methods by tuning the specific experimental parameters (e.g., the precursors, solvents, additives, surface properties of the substrates, pretreatment and post-treatment processes, etc.). The other is to explore novel fixed graphene-based composites combined with the design of photocatalytic reactors, which is expected to enrich the family of functional graphene-based composite photocatalysts in the field of solar energy conversion, as will be discussed in the following part. BA


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12.2. Other Possible Development Strategies

induced superhydrophilicity (Figure 69B) of the formed TiO2 films to realize the self-cleaning and clean-at-ease effects. More specifically, the adsorbed organic pollutants/bacteria on the surface of the Hydrotect products can be decomposed by the TiO2 coatings under UV light irradiation through photocatalytic redox processes.422 Because the binding energy between the Ti atom and the lattice oxygen atom can be weakened by the hole generated after UV light irradiation, the adsorbed water molecules can break a Ti−O−Ti bond to form two new Ti−OH bonds, resulting in superhydrophilicity, and water tends to spread out the TiO2 films with a very small contact angle instead of beading up.423 TiO2 film is not only hydrophilic but also amphiphilic after UV light irradiation, and thus, the surface may adsorb both polar and nonpolar liquids. When water is rinsed over the surface, contaminations like oil can be washed away.424 The above nutrients obtained from previous precedents enlighten us with the potential to fabricate graphene-based composite ceramic materials through the techniques used to synthesize semiconductor-coated ceramic materials, for which the available strategies include dip-coating, drop-coating, sol− gel process, screen-printing, and electrostatic spray deposition. In particular, as compared to the functions of semiconductorbased ceramic materials, graphene-based composite counterparts exhibit additional advantages because the addition of graphene with an appropriate content can (i) increase the adsorption capacity of composites toward target reactants (e.g., organic pollutants and bacteria), (ii) tune the light absorption range and capture capability, (iii) inhibit the recombination of electron−hole pairs, and thus (iv) enhance the activity of graphene-based composite ceramic photocatalytic materials. Therefore, graphene-based composite ceramic materials could find their wide application scope and benefit practical photocatalytic applications in environmental remediation, indoor furnishing, and building materials. However, it should be noted that, unlike semiconductors with good thermal stability, graphene will undergo decomposition at high temperature in air. Therefore, the process parameters (e.g., temperature and atmosphere) of the postannealing treatment to improve the adhesion of graphene-based composites to the ceramic materials surface must be controlled with rational attention. 12.2.2. Graphene-Based Composite Concrete Materials. Concrete is one kind of the most important and widely used building and pavement materials.425 Considering the quite large exposure areas of the buildings and roads to sunlight, their surfaces are the ideal media for applying photocatalytic materials, which offer an approach to mitigate the time- and cost-consuming cleaning maintenance and may also be a good supplement to the conventional technologies to reduce gaseous exhaust emission (e.g., catalytic converters fitted on the vehicles).419 Under the irradiation of solar light, the adsorbed pollutants can be degraded on the photocatalyst-coated surface of buildings or roads, while the residues and dust can be washed away by rainwater, during which the whole removal process of pollutants is driven by the natural renewable energy alone.419 Driven by the great attraction of the practical applications of photocatalytic concrete materials, the studies in this field have received intense attention from both fundamental and industrial communities,425−433 and several pilot projects have been conducted to verify the effectiveness of the photocatalytic concrete materials in ambient environment.334,434,435 For example, Boonen and his co-workers have carried out a series

The reported literature on the graphene-based composite film photocatalysts provides us with useful and positive information that the graphene-based composites are able to serve as a type of promising photocatalysts with enhanced performance for practical applications. It can be seen from Table S11 (Supporting Information) that, so far, a variety of synthetic methods have been utilized for fabricating graphene-based composite film photocatalysts and that the involved substrates to fix the composites are mainly focused on glass materials.16,196,235,262,312,339,388−405 This has laid the foundation for the production of fixed graphene-based composite photocatalysts on glass supports/substrates and self-cleaning glass products, which is of great significance from the standpoint of practical applications. To expand the practical applications’ scope of graphenebased composite photocatalysts, other possible development strategies should be paid attention and further extended in future studies. The availability of different methods to fix the powder samples,407,408 and the abundant knowledge and experiences on utilizing the most-studied semiconductor photocatalysts (e.g., the well-known TiO2) for practical applications409−418 can facilitate the design and construction of functional graphene-based composite photocatalysts for practical implementation. In this section, some possible development strategies for practical applications of graphenebased composite photocatalysts along with the consideration of photocatalytic reactors design are proposed. 12.2.1. Graphene-Based Composite Ceramic Materials. Ceramic materials, as an important type of inorganic, nonmetallic materials, have been used for many different applications in domestic, industrial, and building products and in water processing, due to their diverse advantages, including high chemical and physical stability, bacteria resistance, high availability, and good durability.411,419 Therefore, the fabrication of graphene-based composite ceramic materials should be one promising development strategy for their practical applications. Thus far, the semiconductor ceramic materials have been prepared through various methods, such as dip-coating,414 sol− gel process,415,420 and screen-printing.413 TiO2-based tiles have even been widely commercialized and applied. As a typical representative, TOTO Ltd. has developed the Hydrotect technique to manufacture the self-cleaning ceramic products for exterior walls and home environments.421 The essence of the Hydrotect technique is to spray a TiO2 suspension or gel on the surface of ceramic tiles with the purpose of making use of the photocatalytic redox reaction induced by the photogenerated electron−hole pairs (Figure 69A) and the photo-

Figure 69. Mechanism illustrations for Hydrotect products (TOTO Ltd.) with the functions of (A) self-cleaning by photocatalytic redox reaction and (B) antifouling by photoinduced superhydrophilicity. BB


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Figure 70. Schematic illustration of photocatalytic air purification pavement. Reprinted with permission from ref 334. Copyright 2014 MDPI AG (Basel, Switzerland).334

Figure 71. Scheme of the photoactivation of the gradient coating system. Reprinted with permission from ref 407. Copyright 2006 Elsevier.407

at the surface are active.334 In contrast, dispersing graphenebased composites at the surface of concrete will provide a more direct action with lower initial cost. However, in this case, the longevity of the photocatalytic action is questioned due to the loss of adhesion to the surface in time.334 Therefore, the optimization of the adhesion of graphene-based composite coatings to the concrete surface is a remarkable topic for bringing closer the practical applications of graphene-based composite concrete materials. 12.2.3. Graphene-Based Composite Plastic Materials. Plastic consists of a wide range of synthetic or semisynthetic organic solids, typically organic polymers of high molecular mass. Because of their relatively low cost, ease of manufacture, versatility, and imperviousness to water, plastic materials have been used in various products, including paper clips, packages, piping, vinyl siding, furniture, and the components in automobiles and even spaceships. Notably, in addition to the better processability and lower cost of plastic materials as compared to glass, ceramic, and concrete materials, the surface properties of plastics are beneficial for the formation of photocatalytic film materials.411 The functional groups (e.g., carboxyl) present on the polymeric chains can form chemical bonds with those (e.g., hydroxyl) on the surface of photocatalysts, thus increasing the adhesion strength between the photocatalytic coatings and the plastic substrates. In view of the wide applications and advantages of plastic materials, coating the photocatalytically active films on their surface has thus drawn great research attention.407−409,417,436 Hitherto, the plastic materials applied for photocatalytic applications (e.g., degradation of the pollutants in gas and

of research works on air purification using the photocatalytic concrete.432,433 Special attention is given to NO and NO2 in the air, because they are for over 50% caused by the exhaust of traffic and are at the base of smog, secondary ozone, and acid rain formation.334 The process of air purification over such photocatalytic concrete is illustrated in Figure 70.334 The photocatalytic material, semiconductor TiO2, is introduced on the top layer of the concrete. Based on their laboratory results, a good efficiency and durability toward NO and NO 2 abatement can be achieved. In Bergamo (Italy), a street in the city center has been repaved by the photocatalytic concrete paving blocks (total area of about 12,000 m2). The results of successive air monitoring for NOx concentration indicate that, during 2 weeks, ca. 45% of NOx can be abated over the photocatalytic concrete roads in day time (from 9 am to 5 pm).434 Therefore, based on the advances in using photocatalytic concrete materials to reduce the pollution level in laboratory research and field work, the development of graphene-based composite concrete materials would be a feasible choice for practical applications of graphene-based composites with enhanced photocatalytic performance. In this regard, two approaches can be considered for introducing graphene-based composites in the concrete. One is to add the composites in the cement, an ingredient of concrete to allow them to mix in the mass of the concrete. The other is to disperse the composites suspension on the surface of concrete by the spraying technique. The former has the advantage of a more durable action, since the graphene-based composite will continuously be present, even after wearing off the top layer. For this approach, the initial cost will be higher and only the composites BC



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Figure 72. Schematic diagrams of (A) fixed bed photocatalytic reactor, (B) annular reactor with an inside lamp, (C) optical fiber photocatalytic reactor, and (D) monolith reactor with light illuminating optical fiber.

graphene-based composites, the design of photocatalytic reactors is also one of the most important issues from the consideration of both further improving the photocatalytic efficacy and industrial feasibility.437 The progress achieved in the aspect of immobilizing the graphene-based composites on a stationary phase (i.e., supports or substrates) can facilitate the integration of photocatalytic reactor design with the fixed graphene-based composite photocatalysts, which is expected to be a combined strategy for promoting the practical applications of graphene-based composite photocatalysts. Thus far, the photocatalytic reactors, which can be considered to be used in specific practical applications for graphene-based composite photocatalysts, mainly include the fixed bed, annular, optical fiber, and honeycomb monolith reactors (Figure 72). When using fixed bed reactors (Figure 72A), the graphenebased composite photocatalysts can be coated on the inner walls of the reactor or a support matrix around the light source. In this case, the photocatalysts recycling process is easily processed. However, the total illuminated area is largely limited by the reactor geometry and the spatial distance between the photocatalyst and light source, because the light irradiation is quickly attenuated by the absorption and scattering by the reaction medium.438 Consequently, the efficiency of photon conversion is relatively low for fixed bed reactors. To improve the photonic efficiency, one direct strategy is making better use of the light irradiation. In this respect, annular, optical fiber, and honeycomb monolith reactors can be considered. The annular reactors usually consist of two concentric cylinders that form an annular space with a certain distance, and the light source is located in the center (Figure 72B).437 The graphene-based composite photocatalyst layer can be coated on the inner wall of the cylinder, and the width of the catalyst layer on the surface of the reactor should be low enough to allow all of the catalysts to be exposed to the light irradiation, which is beneficial for increasing the photonic efficiency. Furthermore, the transmission and uniform distribution of light energy are of vital importance for designing an effective photoreactor. As for the optical fiber reactors, the use of optical fiber can facilitate the high transmission and uniform light distribution inside the reactor.439−442 The graphene-based

liquid phase, and disinfection) are mainly focused on TiO2deposited polymeric films, including TiO2-polythene,436 TiO2polypropylene,436 TiO2-polycarbonate (PC),409,417 TiO2-polyethylene terephthalate (PET),408 and TiO2-polyvinyl chloride (PVC).407 The synthetic methods involve dip-coating, spincoating, impregnation, magnetron sputtering, and plasma spraying.407−409,417,436 However, because plastic materials are malleable and cannot tolerate the high sintering temperature to anchor the photocatalyst layer, the heat treatment process for the production of photocatalytic plastic materials must be controlled with attention. Another problem on photocatalytic plastic materials is that the organic substrates themselves tend to be decomposed by photocatalytic reactions, which not only reduces photocatalytic activity but also induces the structure and strength destructions (i.e., the delamination of the coatings).407 To inhibit the possible degradation of the substrate/support, a barrier/ intermediate layer can be introduced between organic substrate and photocatalyst. For example, Schmidt et al. have presented the transparent coating systems applicable on plastic surfaces by a spray technique.407 The surfaces of TiO2 powders are modified with silanes, which allows for the introduction of the particles in organic/inorganic hybrid NANOMER coating systems. With the evaporation of the solvents in the wet film, a self-organizing gradient layer is formed with an upconcentration of the active particles at the interface layer between coating and air due to the decompatibilization of the coated particles to the matrix. After activation by light irradiation, highly active transparent photocatalytic coatings on the plastics surfaces are obtained (Figure 71). Obviously, the significant progress on TiO2-coating plastic materials has paved the pioneering way for fabricating graphene-based composite plastic materials. On the basis of the reported techniques, it is reasonable to anticipate graphenebased composite plastic materials to be another promising development strategy for the practical applications of graphenebased composite photocatalysts, which deserves to be extended in future study. 12.2.4. Photocatalytic Reactor Design. To advance the practical applications of graphene-based composite photocatalysts, besides the construction of high-performance fixed BD


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Figure 73. Schematic illustration of the continuous mode integrated photocatalytic reactor.

composite photocatalysts can be coated on the casings of the optical fibers (Figure 72C), which enables the efficient light distribution over the surface of photocatalysts. Nevertheless, the configuration of this reactor does not effectively utilize the entire reactor volume. The optical fibers usually take up 20− 30% of the reactor volume and provide relatively low surface area of the coating support, since the optical fiber is usually thin.443 The development of honeycomb monolith reactor provides a promising alternative for efficient photocatalytic reactors. As illustrated in Figure 72D, the honeycomb monolith reactor consists of a definite number of channels, offering a high surface-to-volume ratio and low pressure drop.444,445 The graphene-based composite photocatalysts are layered onto the walls of channels in a very thin wash coat. The coated optical fibers can be employed in the channels of the monolith substrate to develop the reactor configuration.446 Therefore, in this case, the sufficient loading of photocatalysts with enough surface area exposed, efficient light distribution over the surface of photocatalysts, and facility to visible light or direct solar irradiation can be achieved. In addition to the above photocatalytic reactors, the continuous mode integrated photocatalytic reactor, as displayed in Figure 73, can also be considered for specific practical applications of graphene-based composites. The main parts of the designed reactor constitute several consecutive but segmented tubular units. In the center of each tubular unit, a light source with graphene-based composite coated casings is equipped. A circulation pump enables the continuous feeding of the reactant in the whole system. Such configuration can not only allow the uniform light distribution and intensity over the photocatalysts surface without interference/attenuation induced by penetration through the solvent, but also provide sufficient contact area between graphene-based composite photocatalysts and the photons as well as that between the photocatalysts and reactants. These conditions are beneficial for improving the photonic efficiency and the performance of photocatalysts. Finally, we would like to sum up some fundamental considerations when designing an effective photocatalytic reactor, which indeed is not only related to target practical applications of graphene-based composite photocatalysts, but also to any emerging photocatalytic materials as reported in the literature.

(ii) The coverage region (including the area and position) of the photocatalysts should be sufficient to effectively harvest the maximum illumination and intensity. (iii) Efficient contact between the photocatalysts and reactants is able to facilitate the photocatalytic redox processes and thus improve the overall photocatalytic efficiency. (iv) High ratio of the illuminated photocatalyst area to the volume of the reactor is desirable from the economical point of view. (v) The whole setup should be easily scaled up from the consideration of industrial feasibility.

13. CONCLUDING REMARKS AND FUTURE OUTLOOK As such, we have attempted to cast a panoramic review of graphene-based photocatalysis, which has covered a systematic understanding of the multifarious roles of graphene in boosting the photocatalytic performance, the key factors affecting the activity of graphene-based composites, some pioneering efforts on investigating the charge carrier dynamics in the graphenebased composites, the photostability of graphene during the photocatalytic processes, comparison of graphene-based and other carbon allotropes-based composite photocatalysts, all the types of reported photoredox processes over graphene-based composite photocatalysts, the current status and opportunities in this active field, and a perspective on how graphene-based composites can be applied to practical applications in solar energy conversion. To fabricate smart graphene-based composite photocatalysts, it is necessary and still challenging to rationally design graphene-based composites from a system-level engineering consideration, which in analogy to biological systems in nature requires a collective integration of the individual components, interface composition, and fine control of material structure and morphology at the nanoscale. In particular, interface engineering by the combination of tailored individual components that afford special interfacial interaction (e.g., band alignment) such as p−n junctions, heterojunctions, and Zscheme systems would be an effective strategy for improving the photoactivity of graphene-based composites because it predominantly determines the microscopic transfer pathway of charge carriers, the efficiency of the separation and transfer of charge carriers, and thus the photoactivity. In this regard, the joint effort between experiment and theory on the fundamental in-depth understanding of the charge carrier dynamics over the graphene-based composite photocatalysts would be instructive for designing spatially multicomponent composite architectures, thereby enabling more sophisticated and efficient

(i) High transmission and equal distribution of light irradiation in the photocatalytic reactor are advantageous to lower the power consumption. BE



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AUTHOR INFORMATION

photocatalytic systems. Regarding the overall photocatalytic performance of graphene-based composites, it is not the issue of only the electrical conductivity of graphene, but the harmonious combination of the structure and morphology of each component and the connection or interaction manner between them in the whole composite. Further to the above design concept, with regard to the implementation of “potential” of graphene-based composite photocatalysts into “practical” applications, both the construction of sunlight highly reactive fixed graphene-based composite photocatalysts and the design of solar light operating reactors must be taken into account. All of these fundamental and technological issues should essentially be integrated from a system-level consideration, which together determine the overall practical performance of graphene-based photocatalytic systems. In fact, this whole scenario cogitation is of vital importance for any photocatalyst materials to develop their widespread practical applications in artificial photocatalysis. In this sense, the sufficient delivery of potential of graphene-based composite photocatalysts for practical applications requires the joint efforts of both chemists and industrial engineers, instead of subjectively imposing hype on the miracle of graphene. However, considering the trajectory of any material research, it typically takes 20 years or more to emerge from a fundamental understanding to practical applications. Therefore, to be optimistic but rational, the construction of graphene-based composites as the next-generation of photocatalytic systems would take a much longer time than people think. Looking to the future, there is a wide scope of opportunity and challenge simultaneously existing for this hot research topic. We wish that this review would make the following overarching directions for the next decade of research on graphene-based photocatalysis: (1) the field needs to timely communicate the status, opportunities, and challenges at all levels of research and industry communities, particularly for the newcomers and those who are not intimately involved in graphene-related research, which avoids the exaggerated hype on graphene-promoted photocatalysis; and (2) while it is well recognized that graphene-based composites are feasible for boosting photocatalysis, more attention should be paid to the design of graphene-based composites by a system-level method and, particularly, to an in-depth fundamental understanding of the pathway and dynamics of charge carrier transfer associated with such composites by the joint cooperation between experiment and theory. It is hoped that this review would contribute to advancing the achievement of these goals and shaping the development road of graphene-based composites photocatalysis in the posthype era.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Nan Zhang is now pursing her Ph.D. degree with the supervision of Prof. Yi-Jun Xu at State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, P. R. China. Her main research interests include the fabrication of core− shell and carbon-based nanocomposites for potential target applications in heterogeneous photocatalysis.

ASSOCIATED CONTENT S Supporting Information *

Min-Quan Yang is now pursing his Ph.D. degree with the supervision

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrev.5b00267. Additional information on the proportion distribution of different photocatalytic applications of graphene-based composites, non-solution-based methods for graphene synthesis, and summary of the typical examples of graphene-semiconductor composite photocatalysts and their various photocatalytic applications. (PDF)

of Prof. Yi-Jun Xu at State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, P. R. China. His main research interests focus on the synthesis of carbonbased materials, especially graphene-based composite nanomaterials for applications in heterogeneous photocatalysis. BF


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dimension-based, and core−shell structure-based composite photocatalysts, in the field of heterogeneous photocatalysis.

ACKNOWLEDGMENTS The support from the Key Project of National Natural Science Foundation of China (U1463204), the National Natural Science Foundation of China (20903023 and 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation of Fujian Province for Distinguished Young Investigator Grant (2012J06003), the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (No. 2014A05), the first Program of Fujian Province for Top Creative Young Talents, and the Program for Returned High-Level Overseas Chinese Scholars of Fujian province is kindly acknowledged. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC0206CH11357.

Siqi Liu is now pursing his Ph.D. degree with the supervision of Prof. Yi-Jun Xu at State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, P. R. China. His main research interests include the fabrication of one-dimensionbased nanostructures and graphene-based composites and their applications in heterogeneous photocatalysis.

ABBREVIATIONS AND ACRONYMS 0D zero-dimensional 1D one-dimensional 2D two-dimensional 3D three-dimensional 2,4-D 2,4-dichlorophenoxyacetic acid 4-ABT 4-aminobenzenethio 4-NBT 4-nitrobenzenethiol AFM atomic force microscope AOPs advanced oxidation processes APTES 3-amino-propyltriethoxysilane BPEI branched polyethylenimine BTF benzotrifluoride C60 fullerene CAT catechol CB conduction band CMBE carbon molecular beam epitaxy CNT carbon nanotube CoTHPP cobalt tetrahydroxyphenyl porphyrin CVD chemical vapor deposition DETA diethylenetriamine DMEU 1,3-dimethyl-2-imidazolidinone DMF N,N-dimethylforamide DRS diffuse reflectance spectroscopy EIS electrochemical impedance spectroscopy En ethylenediamine EPD electrophoretic deposition ESD electrostatic spray deposition FTIR Fourier transform infrared FTO fluorine-doped tin oxide GBL γ-butyrolactone GO graphene oxide GQDs graphene quantum dots HOMO highest occupied molecular orbital HOPG highly oriented pyrolytic graphite HQ hydroquinone ITO indium tin oxide LbL layer-by-layer LUMO lowest unoccupied molecular orbital MAQSP multianthraquinone substituted porphyrin MB methylene blue MO methyl orange

Yugang Sun is a principal staff scientist at the Center for Nanoscale Materials, Argonne National Laboratory, USA. His current research interests focus on the synthesis of a wide range of nanostructures, including metal nanoparticles with tailored properties, the development of in situ synchrotron X-ray techniques for real-time probing of nanoparticle growth, and the application of these nanomaterials in energy storage, photocatalysis, and sensing.

Yi-Jun Xu is a full professor now working at State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, P. R. China. His current research interests primarily focus on the assembly and applications of semiconductorbased nanostructured materials, such as graphene-based, one BG



Chemical Reviews MOFs MWCNT NADH NB NBB NF NIR NMP NPs NRs NSP NWs OCP OSG OTE P3HT PAHs PAH PC PDDA PECVD PET PFU PG PL PTE p-THPP PVC PVDF RB RF RGO RhB ROSs SEG SERS SHE SPR STEM SWCNT THPP TMPyP TOC TPV UHV VB XAS XPS

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metal−organic frameworks multiwalled carbon nanotube nicotinamide adenine dinucleotide nitrobenzene naphthol blue black nanofiber near-infrared N-methyl-pyrrolidone nanoparticles nanorods nanosphere nanowires open-circuit potential organic synthesis graphene optically transparent electrode poly(3-hexylthio-phene-2,5-diyl) polycyclic aromatic hydrocarbons poly(allylamine hydrochloride) polycarbonate poly(diallyldimethylammonium chloride) plasma enhanced chemical vapor deposition polyethylene terephthalate plaque forming units propylene glycol photoluminescence photothermal effect meso-tetra(p-hydroxyphenyl)porphyrin polyvinyl chloride polyvinylidene fluoride rose bengal radio frequency reduced graphene oxide rhodamine B reactive oxygen species solvent exfoliated graphene surface-enhanced Raman spectroscopy standard hydrogen electrode surface plasmon resonance scanning transmission electron microscopy single-walled carbon nanotube meso-tetrahydroxyphenyl porphyrin 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) total organic carbon transient photovoltage technique ultrahigh vacuum valence band X-ray absorption spectroscopy X-ray photoelectron spectroscopy

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