Graphitic Carbon Nitride - ACS Publications - American Chemical

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Graphitic Carbon Nitride (g‑C3N4)‑Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Wee-Jun Ong,*,† Lling-Lling Tan,† Yun Hau Ng,§ Siek-Ting Yong,† and Siang-Piao Chai*,† †

Multidisciplinary Platform of Advanced Engineering, Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 47500 Selangor, Malaysia § Particles and Catalysis Research Group (PARTCAT), School of Chemical Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia ABSTRACT: As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The construction and characteristics of each classification of the heterojunction system will be critically reviewed, namely metal-g-C3N4, semiconductorg-C3N4, isotype g-C3N4/g-C3N4, graphitic carbon-g-C3N4, conducting polymer-g-C3N4, sensitizer-g-C3N4, and multicomponent heterojunctions. The band structures, electronic properties, optical absorption, and interfacial charge transfer of g-C3N4-based heterostructured nanohybrids will also be theoretically discussed based on the first-principles density functional theory (DFT) calculations to provide insightful outlooks on the charge carrier dynamics. Apart from that, the advancement of the versatile photoredox applications toward artificial photosynthesis (water splitting and photofixation of CO2), environmental decontamination, and bacteria disinfection will be presented in detail. Last but not least, this comprehensive review will conclude with a summary and some invigorating perspectives on the challenges and future directions at the forefront of this research platform. It is anticipated that this review can stimulate a new research doorway to facilitate the next generation of gC3N4-based photocatalysts with ameliorated performances by harnessing the outstanding structural, electronic, and optical properties for the development of a sustainable future without environmental detriment.

CONTENTS 1.0. Introduction 2.0. Pristine g-C3N4 and Its Functionalization 2.1. Discovery and Development History of gC3N4 2.2. Synthesis and Properties of g-C3N4 2.2.1. Influence of Nitrogen-Rich Precursors and Reaction Parameters 2.2.2. Nanostructure Design of g-C3N4 2.2.3. Exfoliation of Bulk g-C3N4 2.3. Functionalization of g-C3N4 2.3.1. Elemental Doping of g-C3N4 2.3.2. Copolymerization of g-C3N4 3.0. Basic Principles of Photocatalytic Enhancement Using g-C3N4-Based Heterojunction Photocatalysts

4.0. State-of-the-Art Accomplishments of Engineering g-C3N4-Based Heterojunction Photocatalysts 4.1. Design and Construction of Metal/g-C3N4 (M-CN) Heterojunctions 4.1.1. Metal/g-C3N4 Hybrid Heterostructures 4.1.2. Noble Metal/g-C3N4 Hybrid Heterostructures 4.1.3. Bimetallic/g-C3N4 Hybrid Heterostructures 4.2. Design and Construction of Semiconductorg-C3N4 (S-CN) Heterojunctions 4.2.1. Earth-Abundant Metal Oxide and Metal Hydroxides/g-C3N4 Hybrid Heterostructures

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DOI: 10.1021/acs.chemrev.6b00075 Chem. Rev. 2016, 116, 7159−7329

Chemical Reviews 4.2.2. Metal Sulfide/g-C3N4 Hybrid Heterostructures 4.2.3. Metal Complex/g-C3N4 Hybrid Heterostructures 4.2.4. Complex Compound/g-C3 N 4 Hybrid Heterostructures 4.2.5. Metal Organic Framework (MOF)/g-C3N4 Hybrid Heterostructures 4.2.6. Other Systems 4.3. Design and Construction of Isotype g-C3N4/ g-C3N4 Heterojunctions 4.4. Design and Construction of Carbon-g-C3N4 (C-CN) Heterojunctions 4.4.1. C60/g-C3N4 Hybrid Heterostructures 4.4.2. CNT/g-C3N4 Hybrid Heterostructures 4.4.3. Graphene/g-C3N4 Hybrid Heterostructures 4.4.4. Other Carbonaceous-Modified g-C3N4 Hybrid Heterostructures 4.5. Design and Construction of Conducting Polymer-g-C3N4 (P-CN) Heterojunctions 4.5.1. PANI/g-C3N4 Hybrid Heterostructures 4.5.2. P3HT/g-C3N4 Hybrid Heterostructures 4.5.3. Other Derivatives of Conducting Polymer/g-C3N4 Hybrid Heterostructures 4.6. Design and Construction of Sensitizer-gC3N4 Heterojunctions 4.7. Design and Construction of Multicomponent Heterojunctions 5.0. Photocatalytic Applications of g-C3N4-Based Nanocomposites for Artificial Photosynthesis and Environmental Remediation 5.1. Photocatalytic Water Splitting for H2 and O2 Generation 5.2. Photocatalytic Reduction of CO2 to Renewable Hydrocarbon Fuels 5.3. Photocatalytic Degradation of Pollutants and Bacteria Disinfection 5.3.1. Gas Phase Degradation of Pollutants 5.3.2. Liquid Phase Degradation of Pollutants 5.3.3. Bacterial Disinfection 6.0. Conclusions, Perspective, and Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments Abbreviations and Acronyms References

Review

engineering have been pursued to overcome the obstacle for effective energy conversion and environmental protection.3 Among various renewable energy projects, semiconductorbased photocatalysis, in which the inexhaustible and clean solar energy can be harvested as a feasible technology,4−18 has gained considerable interdisciplinary attention for its diverse potential in energy and environmental applications. Until now, the direct conversion of solar energy to energy fuels and chemical energy has been regarded as one of the green sustainable avenues to address the energy and environmental crisis in the future.19−31 In addition to the solar light as a driving force, photocatalysis requires an appropriate semiconductor to carry out numerous catalytic reactions such as water splitting to produce H2 and O2,32−47 reduction of CO2 to hydrocarbon fuels,48−62 degradation of organic pollutants,63−68 bacteria disinfection,69−72 and selective synthesis of organic compounds.73−75 The landmark event of photocatalytic water splitting using TiO2 electrodes under ultraviolet (UV) light was ignited by the pioneering study coauthored by Fujishima and Honda in 1972.76 In 1976, the photocatalytic degradation of organic pollutants was reported by Carey et al. in the presence of TiO2 in aqueous suspensions.77 Three years later, Inoue and coworkers investigated the photocatalytic reduction of CO2 to various organic compounds using semiconductor powders such as TiO2, ZnO, SiC, GaP, and CdS in aqueous solution.78 Since then, there has been substantial development in the fabrication of highly efficient semiconductor-based photocatalysts, as reported in numerous reviews.79−109 From the literature survey, the potential UV-active and visible-light-active photocatalysts include TiO2,110−121 ZnO,122,123 Fe2O3,124,125 CdS,126−129 Bi 2 WO 6 , 130−132 BiVO 4 , 133−135 Ta 2 O 5 , 136,137 Ta 3 N 5 , 138 TaON,139,140 and many more. To date, developing a high quality semiconductor photocatalyst for surmounting the remediation of energy shortages and environmental threats has become a hot research area. Very recently, the design of visible-light-responsive photocatalysts is vastly pursued by researchers for effective utilization of the solar spectrum, which comprises a large fraction of visible light (ca. 43%).141−144 The large band gap energy of semiconductor photocatalysts, such as the traditional TiO2, remains the bottleneck to satisfy the requirements of visiblelight applications, due to the low utilization of solar energy.145−153 Therefore, in the search for robust and visiblelight-active semiconductor photocatalysts, a polymeric semiconductor, namely graphitic carbon nitride (g-C3N4), has elicited ripples of excitement in the research communities as the next generation photocatalyst, due to its facile synthesis, appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature.154−158 Importantly, gC3N4 is easily fabricated by thermal polymerization of abundant nitrogen-rich precursors such as melamine,159−167 dicyandiamide,168−176 cyanamide,177−181 urea,182−185 thiourea,186−189 and ammonium thiocyanate.190 Carbon nitride (C3N4) is not new at all, and it is considered one of the oldest reported artificial polymers in the scientific literature. The history of C3N4 polymers and their precursors could be traced back to 1834, where the embryonic form of melon, which is a linear polymer of interconnected tri-s-triazine through secondary nitrogen, was first developed by Berzelius and named by Liebig.191,192 The utilization of g-C3N4 in the heterogeneous catalysis arena started around 10 years ago in 2006.193 The discovery of g-C3N4 as a metal-free conjugated semiconductor photocatalyst for H2 evolution was first reported by Wang et al.

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1.0. INTRODUCTION The increasing challenges in energy demands and environmental concerns due to the consumption of fossil fuels have invigorated growing awareness in the past few decades. With industrialization and rapid growth of the population, it is projected that the globe will require two times its current energy supply by 2050.1,2 At present, the world’s energy demand is largely dependent on fossil fuels, such as petroleum, coal, and natural gas, which are rapidly being depleted. The consumption of fossil fuels will inevitably lead to harmful emissions that are detrimental to the environment. As a result, novel discoveries and frontlines in materials science and 7160


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Figure 1. (a) Triazine and (b) tri-s-triazine (heptazine) structures of g-C3N4.

in 2009.194 This potentially shifted the research exploration from inorganic to polymeric conjugated semiconductor photocatalysts.195 There are generally seven phases of C3N4, which are α-C3N4, β-C3N4, cubic C3N4, pseudocubic C3N4, g-h-triazine, g-otriazine, and g-h-heptazine with band gaps of around 5.49, 4.85, 4.30, 4.13, 2.97, 0.93, and 2.88 eV, respectively.196 It was found that the basic tectonic units to establish allotropes of g-C3N4 are triazine (C3N3) and tri-s-triazine/heptazine (C6N7) rings (Figure 1).197,198 In this case, the size of the nitride pores and the different electronic environments of the N atom contribute to various energetic stabilities. Among all phases, tri-s-triazinebased g-C3N4 was energetically favored and was the most stable phase of C3N4 at ambient conditions.199 This was in line with the first-principles density functional theory (DFT) calculations performed by Kroke et al.200 Most works indicated that the polycondensation of melamine, cyanamide, dicyandiamide, or urea formed a melon polymer from the melem units,201−205 manifesting that the tecton was the most stable pattern. Thus, the tri-s-triazine is generally recognized as the building block for the typical formation of g-C3N4. Since polymeric g-C3N4 consists of earth-abundant carbon and nitrogen elements, it is versatile for providing reactions to alter its surface activity without manifestly changing the theoretical structure and composition. Due to the polymeric feature of g-C3N4, the surface chemistry can be easily modulated by means of surface engineering at the molecular level. Furthermore, it has the lowest band gap among the seven phases of C3N4, owing to the presence of sp2-hybridized carbon and nitrogen, establishing the π-conjugated electronic structures.177 In comparison with TiO2, g-C3N4 has a moderate band gap of 2.7−2.8 eV, resulting in an onset visible light absorption of around 450−460 nm.206 As the most stable allotrope in all C3N4 structures, g-C3N4 is thermally stable up to ca. 600 °C in air, as evidenced by thermogravimetric analysis (TGA), which can be ascribed to the aromatic C−N heterocycles.155 In addition to that, g-C3N4 is also chemically stable and not dissolved in acid, alkali, or organic solvents, rendering it a robust material under ambient conditions.207 Thus, with all the interesting properties possessed by the g-C3N4, it is regarded as a promising metal-free semiconductor photocatalyst, which could provide vast changes in the photocatalytic applications. However, practical applications are still hindered by several obstacles and shortcomings of bare g-C3N4. Owing to the high recombination rate of charge carriers, low electrical con-

ductivity, and the lack of absorption above 460 nm in pristine gC3N4,208,209 several modifications of bare g-C3N4, including synthesis techniques, electronic structure modulation, and nanostructure design have been systematically conducted to optimize the photoactivity.172,210−218 As a matter of fact, copolymerization and doping are effective approaches to introduce impurities into the g-C3N4 matrix to modify the electronic structure and energy band configuration.219−225 In terms of nanostructure design, controllable morphologies of gC3N4 with different geometrical shapes have been engineered by multifarious synthetic routes such as soft templating, hard templating, supramolecular preorganization, and exfoliation strategies.226−232 The development of modified g-C3N4-based heterojunction photocatalysts with improved physicochemical properties for remarkable photocatalytic activity is an increasing requirement to advance g-C3N4 for target-specific applications. To date, researchers have attempted to enhance the photocatalytic performance of g-C3N4 via noble metal deposition,233−237 metal doping,238−244 the incorporation with carbonaceous nanomaterials,245−254 the coupling with other semiconductors,255−263 and many others. Due to its appealing features as described above, many inorganic and organic compounds and even metal nanoparticles could easily anchor on the g-C3N4 surface, forming hybrid nanocomposites, thus further affecting the activity of g-C3N4.259,264−270 Among various modification routes, the formation of an appropriate band structure at the heterojunction interface is the most important prerequisite condition to enhance charge separation efficiency for increased photocatalytic performance.269,271−273 This is advantageous in the formation of coupling hybridization, resulting in a spatially efficient separation of charge carriers on different sides of the heterojunction.274,275 Many important and interesting findings on the g-C3N4-based heterojunction photocatalysts have been widely reported, especially during the past three years. The past few years have witnessed a surge of interest in the development of a myriad of heterojunction photocatalysts by taking advantage of the versatile platform of g-C3N4. The gC3N4-driven “gold rush” has been significantly flourishing in photocatalysis, considering the exceptional electronic, structural, and optical features of g-C3N4. The explosive interests in the arena of g-C3N4-based photocatalysts can be undoubtedly evidenced by a quick search using the terms “graphitic carbon nitride”, “g-C3N4”, and “photocatal*” as the topic keywords in the ISI Web of Science database. From Figure 2, the number of 7161


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fact that the progress on this hotspot research topic is encountering a challenging bottleneck at the moment. At present, there are some inspiring reviews on g-C3N4 describing synthesis techniques, characteristic features, promising applications, and so forth.199,276−282 However, functionalization of gC3N4, such as doping, copolymerization, and supramolecular preorganization for redox reactions, is not thoroughly discussed thus far. To the best of our knowledge, there is no review article hitherto which critically presents the rational design and construction of g-C3N4-based hybrid heterojunction photocatalysts from the perspective of engineering strategies. Till now, there are no review articles that have covered all the reported types of functionalization of g-C3N4, heterojunction photocatalysts, photoredox applications for solar energy conversion, and environmental remediation, as well as the key roles of g-C3N4 and its heterostructured nanocomposites in increasing the photocatalytic activity. This leads to our belief in the need for another focused review. Furthermore, previous reviews have frequently neglected and paid less emphasis to the emerging strategies for the photocatalytic enhancement of CO2 reduction, which is of huge impact for diversifying the application scope of g-C3N4based materials in solar energy conversion.154,155,283−288 Therefore, we are confident that this comprehensive review on the rapid-progressing pace of the research subject is timely not only to highlight the advancements in this booming research field, but also to inspire new concepts and present a good reference to engineer g-C3N4-based photocatalysts for energy and environmental applications. Countless research is available daily, and it is forecasted that the quantity of scientific literature on the g-C3N4-based hybrid photocatalysts will grow promptly over the next five years or even beyond. A tremendous amount of undiscovered prospects are still present on the subject of applications of the g-C3N4-based nanohybrids; thus, no review can be far-reaching at this moment. In this regard, this has spurred our enormous interest in and urgent need for further underlining another important section of gC3N4-based heterojunction systems for the requirements of energy technologies in order to understand the core principles of photocatalytic enhancement. On account of the huge accomplishments in this field of gC3N4 by numerous research groups since 2009, we herein present a comprehensive and updated review on this emerging hot research topic. Considering the objective of delivering a panorama of the latest progress of g-C3N4-based photocatalysts, all the aspects of g-C3N4-based nanomaterials will be critically reviewed. This includes the synthesis pathways and functionalization of g-C3N4, the classification and engineering of gC3N4-based heterojunction photocatalysts, the multifaceted roles g-C3N4 and its heterostructured nanoarchitectures played in photocatalysis, the charge carrier dynamics, the theoretical simulations of band structures, and also the potential photocatalytic applications toward artificial photosynthesis and environmental decontamination. Apart from photocatalysis, photoelectrochemical devices using g-C3N4-based nanomaterials are also one of the crucial parts of artificial photosynthesis for the production of solar fuels via water splitting and CO2 reduction with the interaction of both electrochemical systems and light.289−295 The significant advances in these photochemistry fields have attracted remarkable attention for the energy conversion and storage processes to aim for real life applications in industry.296,297 Specifically, the review begins with the discovery history,

Figure 2. (a) Number of annual publications using “photocatal*” as a topic keyword since 1990. The annual collections of (b) journal publications and (inset of b) citations concerning the combined “graphitic carbon nitride”, “g-C3N4”, and “photocatal*” subjects since 2009. Adapted from ISI Web of Science, dated 20th April 2016.

publications on photocatalysis has increased dramatically over the past 25 years, and more than 1000 papers have been published since 2003. With respect to both g-C3N4 and photocatalysis topics, there is a significant growth in the number of publications since 2009, right after the first seminal report on g-C3N4 photocatalysts in the H2 evolution was published. This signifies the importance and attraction of this hot research field at the moment. Notably, g-C3N4 serves as a new research dimension for the construction of a novel photocatalytic system, enabling it to become a new family of next generation light-driven composite nanomaterials for applications in solar energy conversion. However, it should be noted that the exponential increase in the number of papers on g-C3N4-based photocatalysts has not been well-matched by the knowledge that we gained on how to fully exploit the extraordinary characteristics and properties of g-C3N4 in developing highly efficient g-C3N4-based heterojunction photocatalysts. As a result, it is of particular importance to accentuate the means of unleashing the intriguing properties of g-C3N4, especially the visible-lightresponsiveness, in the g-C 3 N 4-based nanomaterials, to effectively increase the photocatalytic activities. The fascinating properties of g-C3N4 are still available for us to fully explore, and that requires more time and strength from the scientific communities to get an overview picture of the recent development and future direction of this emerging field of research. With such rapidly accelerating number of literature reports in g-C3N4-based photocatalysts, it is the ripe and right period to provide a comprehensive and state-of-the-art review on g-C3N4based nanomaterials from a broad perspective in view of the 7162


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Figure 3. Carbon- and nitrogen-containing materials: (a) melamine, (b) melam, (c) melem, and (d) melon obtained from the thermolysis of mercury(II) thiocyanate as presented by Liebig.

(K3C6N7NCN) by heating potassium ferricyanide and sulfur in a crucible.298 The structures of C3N4 and K3C6N7NCN were unclear because of the lack of advanced analytical tools during that time. The structure of the compounds was then closely restudied and reinvestigated by Franklin as early as 1922.299 He discovered that the carbonic nitride (C3N4)x could be the final polymerization product of heating melon; albeit, the real evidence was still ambiguous. Franklin also suggested the possible structures, but the proposals were merely estimations in the absence of X-ray crystallography studies. In addition, he revealed that the empirical composition of melon derivatives using mercuric thiocyanate as the precursor altered with the synthesis methods and the content of hydrogen ranged from 1.1 to 2.0 wt %. In 1937, a coplanar tri-s-triazine unit as the elementary structural motif of various polymeric derivatives was first suggested by Pauling and Sturdivant, as evidenced by an Xray crystallography study.300 Three years after that, Redemann and Lucas discovered that a formal resemblance existed between graphite and melon and that the molecules were markedly large.301 They inferred that 2,5,8-triamino-tris-striazine (C126H21N175), as an oligomeric condensation product, best described Franklin’s C3N 4. Following from these discoveries, they deduced that a single crystal structure should not be assigned to melon, since it comprised a combination of various architectures and sizes. In 1982, the first crystal structure of a cyameluric derivative was reported by Leonard and co-workers via the synthesis of the cyameluric nucleus (C6N7H3) by a bottom-up assembly using 2,4-diamino-1,3,5triazine and methyl N-cyanomethanimidate as the starting materials. Interestingly, a coplanar arrangement was observed for the crystal structure, confirming Pauling’s structure which was proposed 45 years ago.302 The melon-based C3N4 had been forgotten for a long period of time as an unconfirmed species due to its chemical inertness and insolubility in most solvents.197 Surprisingly, 150 years

preparation methods, and functionalization of g-C3N4 (Section 2.0). This is followed by a consideration of the fundamental processes for photocatalytic enhancement by employing gC3N4-based heterojunction photocatalysts (Section 3.0). Next, the design and construction of each type of unique heterojunction system will be discussed in-depth, such as metal-g-C3N4, semiconductor-g-C3N4, isotype g-C3N4/g-C3N4, carbon-g-C3N4, conducting polymer-g-C3N4, sensitizer-g-C3N4, and multicomponent heterojunctions (Section 4.0). The emerging roles of g-C3N4 and its heterostructured nanocomposites will also be included. Furthermore, the recent developments in the photocatalytic applications of the g-C3N4based heterojunction for artificial photosynthesis and environmental applications will be systematically discussed (Section 5.0). Last but not least, this review will be concluded with a summary of and an outlook on the major challenges, opportunities, and some invigorating perspectives for future research in this emerging frontier based on the pioneering studies in this direction (Section 6.0).

2.0. PRISTINE G-C3N4 AND ITS FUNCTIONALIZATION 2.1. Discovery and Development History of g-C3N4

As briefly discussed in Section 1.0, the history of C3N4 and its corresponding precursors originated in very early days in 1834, which was reported by Berzelius and Liebig (Figure 3).191 Liebig discovered melamine, melam, melem, and melon, which were recognized as heptazine- and triazine-based molecular compounds. Based on his work, a yellow, amorphous, and insoluble product named as melon was yielded by the pyrolysis of ammonium chloride with potassium thiocyanate. Due to the high purity of the product, the first characterization of the C3N4 compounds was conducted by elemental analysis. The problem was that not all of these compounds were well developed and characterized. Furthermore, he could not establish a reproducible chemical formula even with the aid of elemental analysis. In 1835, Gmelin found potassium hydromelonate 7163


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Figure 4. Schematic illustration of the synthesis process of g-C3N4 by thermal polymerization of different precursors such as melamine,317 cyanamide,318 dicyanamide,319 urea,311,320 and thiourea.188 The black, blue, white, red, and yellow balls denote C, N, H, O, and S atoms, respectively.

Figure 5. Reaction pathway for the development of g-C3N4 using cyanamide as the precursor.

mention that, in this present context, we do not emphasize a perfect crystalline g-C3N4 material, since the defect-containing polymeric g-C3N4 is by far more reactive, as proven from the experiments. In most of the heterogeneous catalysis, surface defects and terminations are the active sites for various applications, while crystalline perfection with ideal condensation of g-C3N4 “only” leads to the properties of bulk materials, namely graphitic structure and high chemical and thermal stability.278,308 More importantly, understanding polymeric gC3N4 allows various synthesis techniques to be applied, such as supramolecular and copolymerization with identical tectons and nanoarchitecture design, or a template-mediated approach to yield large porosity and specific surface area. Based on our opinion and the pioneering studies, these techniques have indeed served as a new paradigm for advanced photocatalysis than the search for the single perfect crystal. The detailed summary of various types of synthesis techniques will be further elaborated in the latter sections.

later, in the 1990s, the research interest in C3N4 was remotivated and reinspired by the theoretical expectation in which the dense phase of sp3-hybridized C3N4 (denoted as βC3N4) exhibited high bulk hardness and modulus values, which were comparable with or greater than that of diamond.196,303 The drawback was that it was extremely challenging to synthesize single-phase sp3-bonded C3N4 (β-C3N4), owing to the low thermodynamic stability.304 Although g-C3N4 is the most stable allotrope among all types of C3N4, the research on its synthesis and characterization is still underway to date, and until now, a huge number of experimental procedures have been performed.305,306 The detailed description of the structural part of C3N4 has been reported by Kroke307 and Antonietti.278 Thus far, defective and polymeric components were yielded by the typical condensation routes of cyanamide to dicyandiamide, followed by melamine and other related C/N materials. However, it is difficult to develop a perfect and crystalline g-C3N4 from the viewpoint of the materials structures. In spite of many attempts, the synthesis protocols using the C/N/H-containing species have yet to produce a desired, ideally condensed, crystalline g-C3N4, which has been broadly recognized as a kinetic problem.285 Because of the relatively low crystallinity and a high disorder degree of the fabricated g-C3N4, the acquisition of correct numbers beyond stoichiometry and composition remains indefinable. Based on the perspective of applied photocatalysis, we can firmly

2.2. Synthesis and Properties of g-C3N4

2.2.1. Influence of Nitrogen-Rich Precursors and Reaction Parameters. 2.2.1.1. Types of g-C3N4 Precursors, Reaction Temperature, and Reaction Duration. Since Wang and co-workers first demonstrated g-C3N4 as a promising visible-light photocatalyst for H2 evolution in 2009,194 strenuous efforts have been devoted to synthesizing g-C3N4 by thermal treatment of nitrogen-rich precursors such as urea, thiourea, melamine, cyanamide, dicyandiamide, and so forth 7164


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Figure 6. (a) XRD patterns of C3N4 developed at different temperatures. Reprinted with permission from ref 194. Copyright 2009 Nature Publishing Group. (b) Calculated energy diagram for the development of C3N4 using cyanamide as the precursor. Cyanamide was condensed to melamine. Further condensation proceeded by a triazine route (dash-dot line) or a tri-s-triazine path (dashed line). Reprinted with permission from ref 278. Copyright 2008 Royal Society of Chemistry.

Figure 7. (a) XRD patterns of g-C3N4 synthesized at various temperatures. Transmission electron microscopy (TEM) images of g-C3N4 prepared at (b) 550 °C, (c) 600 °C, and (d) 650 °C. (e) Ultraviolet−visible (UV−vis) diffuse reflectance spectra and (f) transient photocurrent responses of gC3N4 developed at different calcination temperatures. Inset of (e) shows the color of the g-C3N4 photocatalysts from light yellow to dark orange synthesized at different temperatures. Reprinted with permission from ref 327. Copyright 2015 Royal Society of Chemistry.

(Figure 4).309−316 Based on the pioneering work by Wang et al.,194 cyanamide was employed as the precursor of g-C3N4. The combination of both TGA and X-ray diffraction (XRD) techniques was employed to characterize the reaction intermediate compounds. The synthesis was a combination of polyaddition and polycondensation, in which the cyanamide molecules were condensed to dicyandiamide and melamine at temperatures of ca. 203 and 234 °C. This was followed by a condensation phase where ammonia was removed. Essentially, all the melamine-based products were found when the temperature was around 335 °C. Further heating to ca. 390 °C resulted in the formation of tri-s-triazine units via rearrangements of melamine. Lastly, the polymeric g-C3N4 occurred at ca. 520 °C through the further condensation of the unit, and it became unstable at above 600 °C. Beyond 700 °C, this resulted in the disappearance of g-C3N4 to “residuefree” through production of nitrogen and cyano fragments. The molecular arrangement is depicted schematically in Figure 5.

The structural phase change from cyanamide to g-C3N4 during the transition of the polycondensation step is supported by the XRD patterns of C3N4 obtained at various heating temperatures (Figure 6a). In addition to the experimental support for the mechanisms, the reaction steps of the combined polyaddition and polycondensation processes can also be further evidenced and proven by the ab initio calculations. The first-principles DFT calculations were performed using a plane wave basis set with a 550 eV energy cutoff.278 Based on the calculations, the cohesive energy of the molecules increased following the polyaddition pathway (Figure 6b), confirming that melamine was produced upon heating the cyanamide. The final process of the polymerization was the C3N4 sheets formation by fusing the melon units. The successful preparation of g-C3N4 photocatalysts upon thermal polymerization of nitrogen-rich precursors can be generally confirmed by several analytical measurements, namely X-ray photoelectron spectroscopy (XPS), XRD, and Fourier transform infrared (FTIR) 7165


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Figure 8. (A) Reaction pathway for the self-polymerization of thiourea into g-C3N4 at high temperature in air. (B) UV−vis spectra of the thioureaderived g-C3N4 (CN-Tx, where x refers to the synthesis temperatures of 450, 500, 550, 600, 650 °C). Reprinted with permission from ref 188. Copyright 2012 Royal Society of Chemistry.

spectroscopy.188,194,311,318,320−324 The effects of precursors and the synthesis parameters will be compared and discussed to advance g-C3N4 for various properties. Due to the high condensation degree and the presence of the heptazine ring structure of g-C3N4, g-C3N4 exhibits several advantages, such as a high physicochemical stability and unique electronic band structure. Layered g-C3N4 exhibits a C/N ratio near 0.75, which can be developed by suitable selection of nitrogen-rich precursors and condensation approaches. It is widely reported that precursor types and reaction conditions are crucial factors, which influence the physicochemical properties of g-C3N4, such as C/N ratio, specific surface area, porosity, absorption edge, and nanostructures.325 Yan et al. prepared g-C3N4 with various proportions of C/N by calcining melamine at different heating temperatures in a semiclosed system.317 The authors reported that when the reaction temperatures increased from 500 to 580 °C, the ratio of C/N was found to increase from 0.721 to 0.742 with decreasing band gaps from 2.8 to 2.75 eV. This inferred that amine functional groups existed with about 2% of hydrogen content when the molar ratio of C/N was smaller than that of the ideal g-C3N4 (0.75), as a result of structural defects and incomplete condensation.311 It is challenging to fabricate an ideal, fully condensed g-C3N4 with a stoichiometric C/N ratio of 0.75 as further lowering the hydrogen content by a facile condensation approach is difficult to achieve. As such, the presence of a trace amount of amine groups was actually found to be advantageous to increase the g-C3N4 surface activeness in order to exhibit better interactions with reactant molecules.190,326 However, extreme defects owing to the incomplete condensation evidenced by a low C/N stoichiometric ratio could undesirably hinder the charge transportation and separation, resulting in a low photocatalytic activity. In another similar work, Mo et al. studied the influence of calcination temperature on the crystal structure, morphology evolution, and band gap engineering of melamine-derived gC3N4.327 It was demonstrated that g-C3N4 could only be fully formed when the calcination temperature was greater than 500 °C, as evidenced from the two major XRD diffraction peaks (Figure 7a). Surprisingly, the characteristic peaks of the sample prepared at 450 °C were different compared to the other samples, which accounted for the melem derivatives. With increasing temperatures, the structure of g-C3N4 became more slack, thinner, and fluffier with the appearance of abundant pores on the g-C3N4 surface at 600 and 650 °C (Figure 7b−d). Meanwhile, an obvious red-shift in the absorption band edge

ranging from 470 to 570 nm was shown in Figure 7e along with the change of color from light yellow to dark orange (inset of Figure 7e), implying enhanced visible light absorption for samples with increasing temperatures.327 The red-shift was as a result of the degree of π-plane conjugation and the improvement of the degree of polymerization. Based on the transient photocurrent responses (Figure 7f), g-C3N4 prepared at 650 °C exhibited the highest current density with a good reproducibility and stability, which was favorable to photoactivity. In the same year, Gu et al. modified the melamine-derived g-C3N4 photocatalysts with tailored structures by varying the calcination temperature through air-assisted thermal polymerization and etching processes.328 The structures of the samples synthesized at different temperatures underwent a series of evolution from bulk g-C3N4 to nanosheets, rolled nanosheets, tailored nanotubes, nanoflakelets, and even nanoparticles. Similar studies have also been reported by Hollmann et al.329 to explore the effect of the structural features of sol−gel derived C3N4 (SG-CN) materials pyrolyzed at different temperatures (350−600 °C). In situ electron paramagnetic resonance (EPR) was utilized for the first time to visualize the charge separation, electron transfer, and trapping in SG-CN samples. Upon irradiation, a substantial rise of the EPR signal owing to the surface trapped conduction band (CB) electrons was noted for samples pyrolyzed between 450 and 600 °C, which further clarified the point of increasing polymerization degree and hence an improved charge separation in the SG-CN samples. This study is anticipated to pave a new research direction for the use of advanced in situ EPR spectroscopy to analyze the charge separation processes and detect paramagnetic species, such as radicals and CB electrons during the light irradiation to further comprehend the structure−activity relationships in any semiconductor photocatalysts. In addition to cyanamide, dicyandiamide, and melamine as the precursors for the g-C3N4 synthesis, thiourea, which is a sulfur-containing compound, has also been thermally transformed into g-C3N4.186,187,323,330 Previous works by Ang and Chan demonstrated that in the presence of TiO2 or SiO2 as inorganic substrates, a melon structure was formed at a low temperature of 400 °C using thiourea as the precursor.331 Nevertheless, the low extent of polymerization and the incomplete formation of an electronic band structure afford moderate photocatalytic performance of g-C3N4. Following the problem statement, the fabrication of well-condensed g-C3N4 from thiourea without the aid of any substrates is of huge interest in the research community. Zhang et al.188 successfully 7166


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Figure 9. (A) Reaction step for the self-polymerization of urea into g-C3N4 at high temperatures in air. Reprinted with permission from ref 188. Copyright 2012 Royal Society of Chemistry. TEM images of (B) urea-derived g-C3N4, (C) thiourea-derived g-C3N4, and (D) dicyandiamide-derived g-C3N4. Reprinted with permission from ref 311. Copyright 2012 Royal Society of Chemistry. (E) UV−vis spectra of g-C3N4 prepared from different types of precursors. Reprinted with permission from ref 343. Copyright 2014 John Wiley & Sons, Inc. (F) PL spectra for g-C3N4 synthesized using different precursors. Reprinted with permission from ref 320. Copyright 2015 Royal Society of Chemistry.

form when urea, which is an oxygen-containing compound, is utilized as the precursor? To answer the above question, urea has been recently found to be an excellent precursor for synthesizing sheet-like g-C3N4 with high specific surface area and high porosity.336−342 For example, Zhang et al. reported a direct strategy of obtaining urea-derived g-C3N4 with selfsupported gas in the absence of additive assistance at 550 °C for 3 h.311 During the thermal treatment process, the generated gases such as NH3 at a low temperature (1.3 cm2 V−1 S1−), the photoexcited electrons from g-C3N4 are promptly migrated and accumulated on C60 nanoparticles to enhance the charge separation rate for effective redox reactions. 7239


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Figure 110. (a) Synthesis pathway of the graphene/g-C3N4 hybrid nanocomposites via a one-pot impregnation−thermal reduction approach. (b) TEM image and (c) EELS of the graphene/g-C3N4 sample. Reprinted with permission from ref 320. Copyright 2015 Royal Society of Chemistry.

distribution and well-connected porous network of CNT grafted with g-C3N4 nanosheets were further confirmed by the TEM image (Figure 109c). 4.4.3. Graphene/g-C3N4 Hybrid Heterostructures. Waltzing with graphene in the development of graphenebased nanostructures has emerged as one of the most interesting hot research topics in multifarious fields of photoredox catalysis.871−877 The 2D graphene has received tremendous attention at present, owing to its extraordinary electrical, mechanical, and thermal properties, such as excellent mobility of charge carriers, large surface area, high thermal conductivity, optical transparency, and good chemical stability.878−882 By considering the exceptional properties of graphene and g-C3N4 nanosheets as well as the geometric advantages of 2D/2D face-to-face interaction, there are increasing numbers of research reports on the construction of graphene/g-C3N4 hybrid nanoarchitectures for the last two years in the platform of energy conversion and environmental decontamination.248,883−889 One of the facile approaches in developing graphene/g-C3N4 nanohybrids is through a onestep thermal annealing of a mixture of GO and nitrogen-rich precursors.245,890−892 In one of the pioneer studies, metal-free graphene/g-C3N4 nanocomposites were synthesized by a combined impregnation−chemical reduction approach followed by thermal calcination.883 In this work, hydrazine hydrate was used as a reducing agent to reduce GO to graphene. With the hybridization of g-C3N4 with graphene, a layered composite was formed by immobilizing the g-C3N4 on the graphene surface. With increasing graphene contents, the composites exhibited higher surface areas and splendid broad background absorption in the visible-light region. The incorporation of graphene sheets with g-C3N4 behaves as electron-conducting channels to separate the photogenerated charge carriers to hinder the recombination process for the enhancement of H2 production. In another study, Li and coworkers fabricated cross-linked reduced graphene oxide (rGO)/g-C3N4 nanocomposites with tunable band structures by calcining a mixture of cyanamide and GO under an Ar environment.884 It was found that the band gap of the rGO/gC3N4 was reduced with increasing graphene loadings, thus modifying the band structures (VB and CB) of the nanohybrids. Interestingly, a positively shifted VB potential was

protonation treatment of g-C3N4, the white g-C3N4 possessed positive polarity (+3.3 mV), which would be favorable to couple with the negatively charged CNT. Evidently, the CNT/ white g-C3N4 sample showed a reduction in the zeta potential (+1.6 mV) compared to the white g-C3N4 (+3.3 mV), signifying that the white g-C3N4 surface was successfully loaded with CNT. The synthesis process of unmodified CNT/g-C3N4 and CNT/white g-C3N4 is pictorially depicted in Figure 108a− b. Interestingly, the color of CNT/white g-C3N4 suspension was uniform, but for the unmodified CNT/g-C3N4 sample, the black CNT appeared on the upper part and the yellow powder corresponding to the pristine g-C3N4 covered at the bottom. This highlighted that both CNT and white g-C3N4 integrated to form a hybrid composite (inset of Figure 108c), whereas CNT and unmodified g-C3N4 were isolated from each other without forming a strong bonding. This strongly proves that the electrostatically driven self-assembly between CNT and white g-C3N4 plays a decisive role for the efficient separation of photogenerated charge carriers due to the well-contacted interfaces in the CNT/white g-C3N4 sample by transporting photoexcited electrons to the highly conductive CNT. What is of interest is that a significant decrease of the N content was observed during the high temperature annealing of the nitrogen-rich precursors in the presence of CNT to form gC3N4 in the carbon matrix.249 Thus, engineering well-interacted CNT and g-C3N4 at relatively moderate temperatures during the synthesis is of paramount significance. Employing a similar concept, Ma and co-workers developed a strongly coupled gC3N4 nanosheets−CNT composite through a facile lowtemperature self-assembly technique driven by electrostatic attractions and π−π stacking interactions (Figure 109a).249 Briefly, the HCl-protonated g-C3N4 not only exfoliated the gC3N4 nanosheets by disrupting the interplanar cohesion forces of bulk g-C3N4 (e.g., hydrogen bonding and van der Waals forces), but also changed the surface zeta potential from a negative to positive polarity, for ease of coupling between two oppositely charged surfaces. Therefore, the spontaneous selfassembly between the positively charged g-C3N4 and the oxygen-containing groups of CNTs (e.g., COO−) is mainly afforded by electrostatic attraction and π−π stacking. From the SEM image, a 3D porous interconnected network was formed, consisting of thin g-C3N4 nanosheets and embedded CNTs within the interlayer region (Figure 109b). The homogeneous 7240


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observed with an appropriate rGO loading in the hybrid sample, inferring an improved degree of oxidation. Our research group employed a facile one-pot impregnation−thermal reduction process under atmospheric pressure to engineer sandwich-like graphene/g-C3N4 hybrid nanostructures with GO and urea as the starting materials (Figure 110a).320 The graphene/g-C3N4 sample demonstrated a more compact sheet-on-sheet morphology (Figure 110b) through the thermal polycondensation of urea molecules pregrafted on the GO sheets. Notably, the CO bonds activated the amino groups in urea, which eased the adsorption of urea molecules onto the GO sheets. Importantly, the C and N atoms were sp2hybridized within a graphitic network indicated by the electronic transitions of 1s → π* in both C and K edges, as shown in the electron energy loss spectrum (EELS) of the graphene/g-C3N4 sample (Figure 110c). With the incorporation of graphene, there was a slight red-shift in the absorption band edge, which was accredited to the presence of a covalent cross-linker, with C−O−C bonds formed between graphene and g-C3N4 during the thermal annealing process, as confirmed by the XPS analysis. In addition to the above one-pot synthesis, another typical method of preparing graphene/g-C3N4 is by immobilizing the as-prepared g-C3N4 into the GO suspension.893 Dai et al. reported the delamination of bulk g-C3N4 in water under sonication by utilizing GO as a substrate.248 Upon sonication of g-C 3 N 4 in the presence of GO, the exfoliated g-C 3 N 4 nanosheets were anchored on the GO to form the 2D/2D layered hybrid structures. Similarly, the resulting GO/g-C3N4 composites exemplified extended background absorption into the visible-light range, a lower PL emission intensity and enhanced reduction power of the electrons due to the incorporation of GO compared to the pristine g-C3N4. A similar synthesis and morphological structure were also achieved by Liao et al.885 and Li et al.894 Through the innovative nanostructure design of a 3D porous framework, Tong and co-workers have just reported a 3D porous g-C3N4/GO aerogel through a hydrothermal treatment by employing GO and g-C3N4 nanosheets as building blocks followed by freeze-drying (Figure 111a).895 In this case, g-C3N4 served as a photocatalyst, while GO promoted the 3D network as well as accelerated the electron conductivity. It could be seen that a highly interconnected structure was formed in the gC3N4/GO aerogel with abundant macropores and also a robust sheet-on-sheet structure constructed the pore wall of the sample (Figure 111b−c). Concretely, the 3D architectures facilitated the reactant adsorption and multireflection of incident light to promote more charge carriers. Additionally, a coherent heterointerface between g-C3N4 and GO was clearly indicated (Figure 111d), confirming the formation of welldeveloped heterostructures, which is beneficial for improved electron transfer from g-C3N4 to GO, thereby ameliorating the photoredox catalysis. Similar to what have been observed for unmodified g-C3N4 and CNT that possess negative polarity, the negatively charged g-C3N4 and GO will not be strongly coupled due to the mutual electrostatic repulsion in the solution. Furthermore, reduction of GO to rGO is essential to restore the electro-conductive network system of graphene since GO is known to be an insulator with poor electrical conductivity because of the presence of oxygen functional groups.878,896 To this end, our research group reported a 2D/2D hybrid nanostructure by coupling the negatively charged rGO and positively charged

Figure 111. (a) Synthesis procedure of a 3D porous g-C3N4/GO aerogel. (b−c) SEM and (d) TEM images of a g-C3N4/GO aerogel. The inset of (d) shows the enlarged image of the white dotted box from the panel (d). Reprinted with permission from ref 895. Copyright 2015 American Chemical Society.

protonated g-C3N4 (pCN) by means of a sonication-assisted and surface charge modification route followed by a NaBH4reduction process (Figure 112a).777 The well-contacted 2D/2D heterojunction interface of metal-free rGO/pCN composites was achieved by the electrostatic and π−π stacking interactions (Figure 112b−d). The presence of rGO in the hybrid could be manifested by the SAED pattern, which comprised a 6-fold symmetry diffraction pattern, corresponding to the (1100) crystalline plane of rGO. It could be further viewed that few layers of exfoliated pCN (ca. 7−15 layers) were dispersed on the rGO sheets, confirming excellent interfacial contact (Figure 112c). However, for the rGO/g-C3N4 sample using the unmodified g-C3N4 as a comparison, aggregation of g-C3N4 without anchoring on the rGO sheets was observed (Figure 112e). Additionally, the PL emission intensity of rGO/pCN was found to be lower than that of rGO/g-C3N4, resulting in prolonged lifetime of charge carriers with reduced recombination rate of electron−hole pairs. Thus, this elucidates the significant role of the surface charge modification between rGO and g-C3N4 layered nanomaterials for distinguished heterointerfaces with efficient charge separation, instead of integrating the rGO “gold rush”. Recently, graphene derivatives such as chemically doped graphene have been explored to hybridize with g-C3N4 to form a heterojunction photocatalyst. Among various types of dopants, doping of nitrogen elements in the graphene matrix is one of the appealing methods to modulate the optical, electronic and chemical reactivity of 2D graphene by taking the advantage of the lone pair of electrons on nitrogen and the πconjugated electrons on graphene.897 Besides exhibiting improved electronic conductivity associated with the nitrogen doping, the N-doped graphene also inherits the unique characteristics of graphene (e.g., large surface area). As a result, it is anticipated that incorporating N-doped graphene with gC3N4 will certainly improve the photoactivity to a certain extent. For example, N-doped graphene/g-C3N4 hybrid nanostructures were designed by Wang et al.898 and Duan et al.899 using urea and NH3 as the nitrogen doping agent for enhanced photodegradation and H2 evolution, respectively. 7241


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Figure 112. (a) Synthesis process of rGO/pCN hybrid nanocomposites driven by surface charge modification. (b) TEM, (c) HRTEM, and (d) SEM images of rGO/pCN sample. The insets of (b) and (d) show the SAED pattern and enlarged SEM view corresponding to the green rectangle in (d) of rGO/pCN nanocomposites, respectively. (e) TEM image of the rGO/CN sample. Blue and red dotted lines denote the boundaries of g-C3N4 and rGO, respectively. Reprinted with permission from ref 777. Copyright 2015 Elsevier.

113e−g). Besides having 2D/2D layered nanojunctions, the resulting composites showed a significant red-shift at the absorption edge of ca. 534 nm with improved background absorption of visible light, greatly harvesting the larger span of solar spectrum. This work offers new idea of introducing codoped graphene as an excellent electron reservoir coupled with g-C3N4 to develop highly efficient metal-free photocatalysts. In short, the above-mentioned literature provides new inroads into the development of metal-free graphene/g-C3N4 composite materials with a cornucopia of synthesis strategies for improved charge transfer and separation in the photocatalytic applications. This could certainly advance the rational fabrication of smart g-C3N4-based nanoarchitectures in the near future. It is expected that the innovative fabrication processes described herein can be further extended to other novel layered nanostructures as well as multicomponent heterojunction photocatalysts. 4.4.4. Other Carbonaceous-Modified g-C3N4 Hybrid Heterostructures. Apart from C60, CNTs and graphene for the modification of g-C3N4-based heterojunction photocatalysts, other carbonaceous materials such as carbon nanodots, graphene quantum dots, carbon fiber, ordered mesoporous carbon and so forth have been incorporated into the gC3N4 framework at present for improved physicochemical properties.397,869,901−905 Carbon nanodots, which are composed of sp2-bonded graphitic carbon with a diameter of less than 10 nm, have gained huge attention due to the excellent optical and electronic features, i.e. quantum effect with broad optical absorption, light absorbers and electron reservoirs.906 Several interesting findings on the metal-free carbon nanodots/g-C3N4 using different synthesis methods have been extensively explored by many researchers.906−910 For example, Xia et al. employed a one-pot hydrothermal treatment process to synthesize NIR light-driven metal-free photocatalysts by incorporating g-C3N4 nanosheets with carbon quantum dots (Figure 114a).911 The carbon quantum dots, which were

Other than that, the incorporation of metal-free sulfur-doped g-C3N4 (CNS) with sulfur/selenium codoped graphene (S-SeGr) to form the CNS:S-Se-Gr nanohybrids was reported by Shinde et al. in 2016 (Figure 113a).900 GO was thermally

Figure 113. (a) Typical synthesis route of CNS:S-Se-Gr hybrid nanocomposites. DCDA and BDS denote dicyandiamide and benzyl disulfide, respectively. SEM images of (b) g-C3N4 and (c) sulfur-doped g-C3N4 (CNS). (d) TEM image of CNS. (e) SEM and (f−g) HRTEM images of CNS:S-Se-Gr nanohybrids. Reprinted with permission from ref 900. Copyright 2016 Elsevier.

reduced to graphene. Subsequently, the S and Se elements were doped into the graphene matrix followed by thermal polymerization of dicyandiamide and benzyl disulfide for constructing CNS:S-Se-Gr photocatalysts. Both g-C3N4 and CNS possessed 2D slate-like structures (Figure 113b−d). With the incorporation of S-Se-Gr, the CNS nanosheets were found to grow on the large S-Se-Gr sheets to develop 2D multilayered nanostructures with intimate interfacial interactions (Figure 7242


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Figure 114. (a) Synthesis of the carbon quantum dots/g-C3N4 hybrid photocatalysts. (b) TEM and HRTEM images of (b, c) carbon quantum dots and (d, e) carbon quantum dots/g-C3N4. (f) Upconversion PL spectra of carbon quantum dots. (g) Transient photocurrent density of g-C3N4 nanosheets (black curve) and 10 wt% carbon quantum dots/g-C3N4 (red curve) under the illumination of a 808 nm laser. Reprinted with permission from ref 911. Copyright 2015 Royal Society of Chemistry.

fabricated by pyrolysis of EDTA-2Na•2H2O at 350 °C under N2 flow, possessed a diameter of 5−10 nm with an interplanar spacing of 0.283 nm (Figure 114b−c). However, as a result of the undistinguishable contrast of carbon quantum dots and gC3N4, the deposition of carbon quantum dots on the g-C3N4 nanosheets was not vividly observed (Figure 114d−e). Due to the unique nature of carbon quantum dots as spectral converters, the infrared light was upconverted to visible light by the carbon quantum dots (as evidenced by PL spectra in Figure 114f) and then the emitted light was utilized by the gC3N4 for various photoredox reactions. This could also be evidenced by the transient photocurrent change of carbon quantum dots/g-C3N4 under the irradiation of a 808 nm laser (Figure 114g). In a similar work by Guo et al., they employed D-glucose as the source of carbon quantum dots for coupling with g-C3N4 for efficient degradation of methyl orange under infrared light (λ > 800 nm).906 Therefore, the present works pave a new doorway for synthesizing NIR-responsive photocatalysts for energy conversion requirements. Thus far, the fundamental mechanism on the interfacial electron coupling and charge transfer in the graphene quantum dots/g-C3N4 hybrid composites is still lacking, unclear and unexplored. Ma and co-workers theoretically examined the interfacial electronic structure in the graphene quantum dots/gC3N4 nanohybrids.912 Five types of graphene dots with dissimilar thicknesses and sizes were used as models to interact with the buckled g-C3N4 monolayer (Figure 115a−e). Surprisingly, the band gap of the hybrid composites was modified by engineering the lateral size of graphene quantum dots (Figure 115f−k). Specifically, an ideal band gap of 1.92 eV was found in the C24H12/g-C3N4 sample, which was favorable to harness a huge portion of solar light in the visible region. From the 3D charge density plots (Figure 115l−p), substantial charge redistribution occurred at the heterojunction interface of graphene quantum dots/g-C3N4 composites due to intimate contact. It could be deduced that charge redistribution resulted

in electron accumulation (yellow region) below the g-C3N4 and electron depletion (cyan region) above the graphene quantum dots, creating the build-in electric field to give rise to the separation of electron−hole pairs. Meanwhile, increasing sizes of the graphene quantum dots markedly enhanced the hybridization between g-C3N4 and graphene quantum dots, improving the charge transfer across the interface (Figure 115q). As such, a Type II band alignment was formed at the heterojunction interface of the graphene quantum dots/g-C3N4 composites, which is a promising property to spatially separate the charges to retard the recombination of charge carriers. Ordered mesoporous carbon is another type of carbon material, consisting of a tubular structure with a large surface area, a huge pore size, and a monodispersed mesopore space, which can act as an excellent electron transporter due to the unique properties of carbon. Thus, the integration of ordered mesoporous carbon with g-C3N4 will be promising for photocatalysis. In view of that, ordered mesoporous carbon/ g-C3N4 hybrid nanocomposites were developed by Shi et al.913 The ordered mesoporous carbon was deposited on the g-C3N4 surface, which formed clear heterointerfaces, facilitating the ease of electron transfer from g-C3N4 to the ordered mesoporous carbon. It should be noted that the ordered mesoporous carbon was not incorporated into the g-C3N4 lattice, instead it was immobilized on the surface. Not only that, the introduction of ordered mesoporous carbon promoted visible-light absorption, which is analogous to the typical graphene-based hybrid nanostructures.777,914 As we can see till now, the modification of g-C3N4 with carbon materials has been widely employed in the field of photocatalysis owing to their high affinity with g-C3N4 as well as remarkable charge carrier mobility. With the advances toward carbon materials, it is documented that self-polymerization of a biomolecule (e.g., dopamine) under a designated tris-buffer condition (pH 8.5) can form a continuous polydopamine (PDA), which can practically deposit on any 7243


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melamine surface at room temperature and a subsequent thermal carbonization and condensation process (Figure 116a).917 Briefly, dopamine was added into the tris-buffer solution containing melamine with pH 8.5, followed by the formation of PDA coating on the melamine surface. Since PDA has catechol-derivative functional groups, which can anchor strongly and irreversibly to the −NH2 groups of melamine, this renders the production of a continuous polymer layer through the covalent interactions (Figure 116b). Upon thermal treatment under Ar environment, polycondensation of melamine and the carbonization of PDA yielded the resulting carbonized PDA/g-C3N4 hybrid composites (Figure 116c). Interestingly, the carbonized PDA layer was observed with a layer thickness of ca. 0.342 nm, corresponding to the typical distance of graphite layers (Figure 116d). Taking the advantages of PDA coating, which can promote heat and gas diffusion barriers, this leads to a more condensed g-C3N4 due to better polymerization of melamine. In essence to the typical characteristics of carbon nanomaterials, the hybrid nanocomposite established a higher surface area, increased visiblelight absorption and a slight red-shift compared to the pure gC3N4. Not only that, due to the high electrical conductivity of carbonized PDA, the charge carrier transfer and separation were distinctly promoted via the coherent heterojunction interface. Indeed, this work provides a reference pathway for engineering novel carbon-modified g-C3N4 heterojunction nanostructures for versatile photocatalytic applications in the arena of science and technology. In another study of carbon composites, carbon black/g-C3N4 intercalation compound hybrid nanomaterials were synthesized by a one-step molten-salt method utilizing melamine and carbon black as the starting materials in the presence of NaCl, KCl and LiCl as the solvent for intercalation.918 As expected, the polycrystalline carbon black with a high surface area to volume ratio functioned as an electron-conductive bridge to store the photoexcited electrons from g-C3N4 to retard the charge recombination. Moreover, the 1D carbon fiber and CNT exhibit comparable electrochemical properties. The former, which acts as another viable electron-transporting channel, can be fabricated more facilely using an electrospinning method in the presence of inexpensive polymers, thus rendering the process more economical and practical.919 Zhang and Huang reported a two-step synthesis encompassing electrospinning and a thermal calcination process to develop carbon fiber/g-C3N4 hybrid photocatalysts with a splendid

Figure 115. Structure models for graphene quantum dots: (a) C6H6, (b) C16H10, (c) C24H12, (d) 2C24H12, and (e) 3C24H12. Different layer levels are represented by different colors. Band structures employing the HSE03 functional for (f) pure g-C3N4, (g) C6H6/g-C3N4, (h) C16H10/g-C3N4, (i) C24H12/g-C3N4, (j) 2C24H12/g-C3N4, and (k) 3C24H12/g-C3N4. The Fermi level is shown by a horizontal dashed line. Side view of 3D charge density difference plots for the graphene quantum dots/g-C3N4 nanohybrids: (l) C6H6/g-C3N4, (m) C16H10/gC3N4, (n) C24H12/g-C3N4, (o) 2C24H12/g-C3N4, and (p) 3C24H12/gC3N4. The isosurface value is 0.00067 e Å−3. Cyan and yellow regions denote electron depletion and accumulation, respectively. The crosssection of the supercell is indicated in the gray section. (q) Planaraveraged charge density difference Δρ(z) for the graphene quantum dots/g-C3N4. Positive and negative values denote electron accumulation and depletion, respectively. The central positions of g-C3N4 and graphene quantum dots are shown by the vertical dash lines. Reprinted with permission from ref 912. Copyright 2016 Royal Society of Chemistry.

surfaces including g-C3N4.915,916 Thus, the coating of PDA thickness can be regulated by controlling the concentration of dopamine and the duration of polymerization. By considering the merits of the ease of preparation and flexible adhesion of conductive carbon materials (e.g., PDA), He et al. employed the coating of a multilayer graphene-like morphology of PDA on the g-C3N4 surface by polymerization of dopamine on the

Figure 116. (a) Synthesis route of the carbonized PDA-coated g-C3N4 hybrid heterostructures. TEM images of (b) PDA/melamine nanocomposites before thermal treatment, showing a continuous PDA coating layer on the melamine surface and (c) carbonized PDA/g-C3N4 hybrid photocatalysts. (d) HRTEM image of carbonized PDA/g-C3N4. C-PDA denotes carbonized PDA. Reprinted with permission from ref 917. Copyright 2015 Royal Society of Chemistry. 7244


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visible-light absorption and enhanced H 2 evolution.250 However, in most situations for carbonaceous materials which include C60, CNT and graphene, excessive amounts of the carbon hybridization will be detrimental to the photoactivity albeit enhanced visible-light response.320,777,865,883,920,921 This is because of the negative shielding effect induced by the excessive carbon contents, which compete with the light harvesting gC3N4 photocatalysts. The shielding effect in influencing the photoactivity will be elucidated in detail with specific cases of the photoredox applications in Section 5.0. To cut a long story short, interface engineering is of paramount significance to optimize the charge migration and separation across the heterojunction formed between the carbon materials and g-C3N4 to inhibit the electron−hole recombination. Nevertheless, there is always an optimum loading of carbon to be introduced in the g-C3N4-based hybrid system in order to avoid the shielding effect caused by the excessive carbon. Therefore, optimizing the g-C3N4-based hybrid nanostructures with the modification of appropriate carbon contents is undeniably important based on the idea of a system-level material engineering. In other words, the research should begin from the design of individual material components to the heterointerface construction and optimization process for advanced functional applications (Figure 117).

poly(3-hexylthiophene) (P3HT),925 and poly(3,4-ethylenedioxythiophene) (PEDOT).926 4.5.1. PANI/g-C3N4 Hybrid Heterostructures. Among all the conducting polymers, PANI is one of the most extensively reported conducting polymers, which possesses nontoxicity, high chemical stability, exceptional electron and hole conducting properties, corrosion resistant and cost-effective synthesis.927 Importantly, PANI acts as both an electron donor and a remarkable hole acceptor upon the light illumination. Not only that, a high absorption coefficient of PANI in the visiblelight region (ca. 5 × 104) accompanied by superb charge carrier mobility are the two most promising features to harness more photons and to transport electrons and holes for various photocatalytic reactions.657 Inspired by the formation of all polymeric nanocomposites as well as the aforementioned unique properties of PANI, Ge et al. synthesized the PANI/g-C3N4 composite photocatalysts by an in situ deposition oxidative polymerization of an aniline monomer together with g-C3N4 in an ice bath.923 The small PANI particles were deposited on the g-C3N4 surface (Figure 118a), forming the composite polymeric structures with a stronger visible-light absorption. Similar to the metal oxide semiconductor photocatalysts, the PANI absorbed photons to create π−π* transition, migrating the excited electron to the π*-orbital. On the basis of the energy levels of g-C3N4 (VB and CB) and PANI (π-orbital and π*-orbital), the edge potentials of π-orbital and π*-orbital were more negative than VB and CB edge potentials of g-C3N4 (Figure 118b).928 Thus, the excited electrons from PANI could be easily injected into the CB of gC3N4, while the holes on the VB of g-C3N4 transferred to the πorbital of PANI, leading to effective charge separation. As a result, the synergistic effect between PANI and g-C3N4 formed a staggered gap of Type II alignment. In another work by Zhang et al., hierarchical nanocomposites of PANI nanorods arrays were deposited on the g-C3N4 nanosheets synthesized by a dilute polymerization under −20 °C.657 As a result of the nucleation process of PANI on the g-C3N4, its surface was rougher depicted by the SEM and TEM images (Figure 118c− d). It is noted that more effective utilization of visible light can be achieved from this hybrid hierarchical nanostructure due to the multiple light scattering and reflections in the interior void. 4.5.2. P3HT/g-C3N4 Hybrid Heterostructures. P3HT is regarded as a p-type polymer semiconductor that exhibits a band gap of 1.9−2.1 eV and several unique properties such as high stability, good dissolubility and excellent hole carrier mobility (10−4−10−3 cm2 V−1 s−1).925 In one of the early studies, Yan and Huang reported a polymer nanocomposite of g-C3N4 and P3HT by impregnating g-C3N4 with a chloroform solution of P3HT overnight and a subsequent evaporation process using a water bath.925 In another study with a different synthesis protocol, Bai et al. employed a ball milling technique to assemble n-type g-C3N4 nanoplates with p-type P3HT particles to design a p-n P3HT/g-C3N4 heterojunction nanostructure.929 The resulting composite showed different morphologies, such as lamellar structures, to be more disordered and bundled shapes through stacking forces by the strong adsorption of P3HT on the g-C3N4 surface, thus developing an intact interface between g-C3N4 and P3HT. Additionally, Zhang and co-workers fabricated a polymer/ polymer surface heterojunction by a rotary evaporation process comprising P3HT chloroform solution and g-C3N4 particles.656 All the above three studies elucidate a Type II heterojunction alignment, favoring effective charge transfer. Moreover, the

Figure 117. State-of-the-art system materials engineering triangle. Reprinted with permission from ref 922. Copyright 2012 Nature Publishing Group.

This can certainly benefit all the ongoing research in the development of g-C3N4-based heterojunction photocatalysts, e.g. the rational design, construction, and optimization of the individual and the composite interface nanomaterials. 4.5. Design and Construction of Conducting Polymer-g-C3N4 (P-CN) Heterojunctions

Conducting polymer is regarded as one of the main organic semiconductors with visible-light absorption and π-conjugated electronic systems. By taking the advantage of the aromaticbased conjugated system and π-electronic structures of the gC3N4 polymer, it facilitates the hybridization of g-C3N4 with conducting polymers to form organic−organic or polymer− polymer surface heterojunctions. So far, the widely studied conducting polymers include polyaniline (PANI),923 polypyrrole (PPy),658,924 graphitized polyacrylonitrile (g-PAN),647 7245


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Figure 118. (a) TEM image of PANI/g-C3N4. (b) Schematic of the charge transfer and separation in the visible-light-driven PANI/g-C3N4 system. Reprinted with permission from ref 923. Copyright 2012 Royal Society of Chemistry. (c) SEM and (d) TEM images of PANI nanorods/g-C3N4. Reprinted with permission from ref 657. Copyright 2014 Elsevier.

extended π-conjugation promotes the migration of charge carriers at the heterointerface. As a result, the redistribution of electrons and holes markedly minimized the energy-wasteful electron and holes recombination for enhanced photoactivity. 4.5.3. Other Derivatives of Conducting Polymer/gC3N4 Hybrid Heterostructures. PPy is another common conducting polymer, which possesses high conductivity and a band gap of 2.2−2.4 eV. Sui et al. loaded PPy nanoparticles on the g-C3N4 nanosheets.924 The surface heterojunction constructed by the g-C3N4 and the hole-oxidized p-doped PPy enabled rapid separation of electron−hole pairs, leading to an increase in the number of photoinduced electrons on g-C3N4. As such, the mobility of charge carriers and surface electronic conductivity of the composite were markedly improved due to the deposition of PPy, which facilitated the transfer of electrons from PPy to g-C3N4 to reduce H2O to H2. In addition, g-PAN, having a well-matched π-conjugated structure to the g-C3N4, has also been employed for the development of g-PAN/g-C3N4 hybrid photocatalysts.647 The g-PAN/g-C3N4 photocatalyst was fabricated by means of a onepot thermal condensation of polyacrylonitrile (PAN) and melamine. Upon heat treatment above 600 °C, PAN resulted in graphitization, displaying a graphite-like network structure consisting of several layer sheets. After the modification with gPAN, the surface area of the nanohybrids was increased with the presence of porous structures. As a proof-of-concept, the incorporation of g-PAN improves the charge transport ability for enhanced activity. In this scenario, the role of g-PAN is to serve as an electron transfer channel from the g-C3N4 as H+ reduction sites for H2 generation, effectively prolonging the lifetime of charge carriers. This research provides a new doorway into the design of composite structures modified with polymer components to form polymer/polymer heterojunction interfaces for solar energy conversion applications.

4.6. Design and Construction of Sensitizer-g-C3N4 Heterojunctions

Hitherto, several strategies including elemental doping, copolymerization, supramolecular preorganization and incorporation of semiconductors have been employed to extend the optical absorption to cover more visible-light flux as thoroughly described in the above sections. Despite the above endeavors, absorption of g-C3N4-based nanohybrids with a longer wavelength irradiation remains a challenge. At present, the photoabsorption in the lower energy region of solar light is still limited for the modified g-C3N4 samples even though with a red-shift of the absorption edge. To further expand the absorption of visible-light response toward a higher wavelength range, sensitization is another typical appealing approach to efficiently harvest solar energy.930 A case in point, dye organic molecules such as Eosin Y,931,932 Rose Bengal,760 Erythrosin B,933,934 fluorescein,935 dibromofluorescein,936 indole-based Dπ-A937 and so forth have been widely used as a sensitizer by absorbing light with wavelengths longer than 600 nm for enhanced photocatalytic activity, which are generally incorporated during the photocatalytic system for the H2 evolution and photodegradation process. This will be further discussed in a specific case in Section 5.0. The utilization of such dyes for the sensitization of g-C3N4 is essential for improving the electron injection from the LUMO of dye organic molecules to the CB of g-C3N4 to suppress the recombination of charge carriers for various photocatalytic functions. Besides that, zinc phthalocyanine and its derivatives demonstrate a large response toward the visible-light region (600−800 nm), which can be integrated with other compounds through coordination or loading.938 Thus, combining g-C3N4 with zinc phthalocyanine as a sensitizer to form a hybrid composite is expected to extend the entire visible-light range.939 Very recently, Lu et al. employed this excellent idea by immobilizing zinc tetracarboxyphthalocyanine (ZnTcPc) onto 7246


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Figure 119. (a) Synthesis procedure of g-C3N4/ZnTcPc. ZnTcPc, PyBOP, and DIEA denoting zinc tetracarboxyphthalocyanine, hexafluorophosphate, and N,N-diisopropylethylamine, respectively. (b) UV−vis spectra of ZnTcPc, g-C3N4/ZnTcPc, and physically mixed gC3N4 + ZnTcPc. (b) UV−vis absorption spectra of different samples with increasing amounts of ZnTcPc. Reprinted with permission from ref 940. Copyright 2016 Elsevier.

Figure 120. (a) Schematic illustration of synthesis preparation of CuTCPP/g-C3N4 hybrid heterostructures. (b) UV−vis spectra of g-C3N4, CuTCPP, and CuTCPP/g-C3N4 samples with increasing amounts of CuTCPP. (c) Charge transfer and separation at the heterojunction interface of CuTCPP/g-C3N4 nanohybrids under visible-light irradiation (λ > 420 nm). Reprinted with permission from ref 645. Copyright 2015 Elsevier.

which increasing the amount of ZnTcPc in the composite led to the enhancement of spectral absorption at ca. 700 nm. This infers that a synergistic interaction between ZnTcPc and gC3N4 is induced as a result of the strong conjugated interaction between both components, thus increasing the electron transfer from the LUMO of ZnTcPc to the CB of g-C3N4 to decrease the recombination probability. A similar phenomenon has also been observed by Zhang and co-workers using a highly asymmetric zinc phthalocyanine derivative with a strong absorption in visible/NIR (650−800 nm) as a promising sensitizer.938

the g-C3N4 via a covalent bond to form a g-C3N4/ZnTcPc hybrid photocatalyst (Figure 119a).940 The formation of an amide bond between ZnTcPc and g-C3N4 could be confirmed by the red-shift of the absorption of the g-C3N4/ZnTcPc hybrid by 10 nm in comparison to the pure ZnTcPc and the physically mixed g-C3N4 and ZnTcPc (Figure 119b). Importantly, the gC3N4/ZnTcPc exhibited a wide absorption band covering the whole visible-light region by extending from 450 nm (for pure g-C3N4) to more than 800 nm with the maximum absorption peak centered at ca. 700 nm (Figure 119c). All the g-C3N4/ ZnTcPc samples had marvelous visible-light absorption, in 7247


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Porphyrins are another excellent photosensitizer for photocatalysts due to their exceptional chromophore activities over a large span of solar photon flux and a superb electron donor as a result of a large system of p-electrons. Principally, metalloporphyrin modified photocatalysts are formed by covalent bonding between the functional groups in the porphyrin moiety (e.g., −OH and −COOH) and the photocatalyst. It is known that the transfer of electrons can be greatly improved between photocatalysts and porphyrins through the channels developed by the covalent bonds, resulting in the delocalization of p* orbitals of porphyrins.941 Nevertheless, the covalent interaction between the photocatalysts and porphyrins is challenging, causing limited absorption of porphyrins on the photocatalyst surface. Considering the advantage of g-C3N4 having layer structures with π-conjugated electrons, it is anticipated to couple g-C3N4 with porphyrins through hydrogen bonding, π−π and electrostatic interactions. Chen et al. hybridized gC3N4 with Cu (II) meso-tetra (4-carboxyphenyl) porphyrin (CuTCPP) molecules as sensitizers to develop CuTCPP/gC3N4 heterostructures for phenol degradation (Figure 120a).645 Due to the structure-matching of CuTCPP molecules with gC3N4, the intermolecular interactions (e.g., π−π and electrostatic interactions) were formed after coating the CuTCPP on the g-C3N4 nanosheets. Apparently, the successful formation of π−π stacking between both components was ascribed to the 2D 18-π-electron aromatic conjugation of porphyrin molecules with the compatible π-electronic structure of g-C3N4. Besides the π-stacking and electrostatic interactions, the resulting CuTCPP/g-C3N4 nanohybrids were also formed by an amide bond via covalent bonding through the interaction of −NH2 groups on the edge of g-C3N4 with the −COOH groups on the periphery of CuTCPP molecules. Thus, all these interactions significantly promoted the extension of visible light to 450−600 nm with the highest absorption peak at ca. 538 nm, corresponding to the Q-band of porphyrin, rendering more effective visible-light utilization efficiency to generate more electrons and holes (Figure 120b). Additionally, through the well-contacted heterointerfaces and well-matched overlapping band potentials of g-C3N4 and CuTCPP, this leads to efficient charge separation, i.e. electron injection from CuTCPP to gC3N4 and hole transfer from g-C3N4 to CuTCPP (Figure 120c). In a work by Wang et al., they employed another type of porphyrin, μ-oxo dimeric iron (III) porphyrin (FeTPP)2O to react with g-C3N4 to design (FeTPP)2O/g-C3N4 hybrid heterostructures by means of π−π stacking and Fe-amine interactions.185 With the introduction of (FeTPP)2O, numerous nanoparticles were loaded on the g-C3N4 surface while still maintaining the 2D layer-by-layer morphology (Figure 121a− d). The hybridization of (FeTPP)2O and g-C3N4 was validated by the EDX analysis to confirm the distribution of Fe, C, N, and O elements over the hybrid heterostructures (Figure 121e). Similar to the CuTCPP porphyrin molecules as reported above,645 (FeTPP)2O plays dual roles: (1) as a photosensitizer to absorb light in the range of 400−700 nm, and (2) as a charge promotor by migrating photoexcited electrons to g-C3N4 to hinder the charge recombination process. In addition, a visible/NIR-triggered g-C3N4-based heterojunction photocatalyst was designed by integrating upconversion nanoparticles (NaYF4:Yb) into g-C3N4 nanosheets by an electrostatic attraction (Figure 122a).942 Since NaYF4:Yb exhibits a hydrophobic property due to the presence of surface adsorbed oleic acids, it is impossible to integrate NaYF4:Yb with

Figure 121. (a) SEM, (b) TEM, and (c) HRTEM images (inset shows the SAED pattern), (d) TEM image and the corresponding EDX elemental analysis (right), and (e) TEM image and the corresponding EDX mapping images of (FeTPP)2O/g-C3N4 hybrid photocatalysts. Reprinted with permission from ref 185. Copyright 2016 Royal Society of Chemistry.

g-C3N4 to construct a well-contacted heterojunction. To surmount the shortcoming, an ionic CTAB was employed to modify the surface of NaYF4:Yb from hydrophobic to hydrophilic with positive charges. Consequently, negatively charged exfoliated g-C3N4 nanosheets and NaYF4:Yb were combined electrostatically to form a NaYF4:Yb/g-C3N4 hybrid nanocomposite. Due to the benefit of upconversion capability of NaYF4:Yb, it functions as a light converter to provide additional light by absorbing NIR light, followed by upconverting the NIR to UV and visible light for the photoexcitation in g-C3N4 to create more electron−hole pairs (Figure 122b). As such, this broadens the whole solar spectrum for the effective utilization of sunlight for photocatalysis. A similar work on the use of upconversion nanocrystals for modifying g-C3N4 to realize the full use of visible/NIR light via energy gap engineering in the hybrid nanocomposites was also reported by Li and co-workers in 2016 (Figure 122c).225 The gC3N4 was first formed by a copolymerization technique using dicyandiamide and an organic monomer ABN as the starting materials. As discussed earlier in the copolymerization section, the molecularly grafted g-C3N4 endowed several intriguing properties such as increased surface area, extended πconjugated electronic structures, enhanced light utilization with a remarkable red-shift, decreased band gap and ameliorated charge separation. Such appealing features of the improved g-C3N4 nanosheets were then assembled on the upconversion phosphors microrods (NaYF4:Yb, Tm) (Figure 122d−g), which can convert NIR to visible light to fully utilize the NIR in photocatalysis. Therefore, with the ingenious energy gap match of g-C3N4 and upconversion phosphors, this allows the energy transfer from the NIR-excited upconversion phosphors to g-C3N4. 7248


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Figure 122. (a) Synthesis pathway of NaYF4:Yb, Tm/g-C3N4 hybrid nanocomposites by electrostatic attraction. (b) Working mechanism of NaYF4:Yb, Tm/g-C3N4 hybrid photocatalysts with the upconversion property exhibited by NaYF4:Yb, Tm. Reprinted with permission from ref 942. Copyright 2015 Elsevier. (c) Formation process of upconversion phosphors (NaYF4:Yb, Tm)/g-C3N4 hybrid nanostructures. (d and e) SEM images of (d) NaYF4:Yb, Tm and (e) (NaYF4:Yb, Tm)/g-C3N4 samples. (f) TEM and (g) HRTEM images of (NaYF4:Yb, Tm)/g-C3N4 sample. The inset of (g) shows the enlarged HRTEM image from panel (g). CNX0.1 denotes the g-C3N4 copolymerized by an organic monomer, 2-aminobenzonitrile (ABN). Reprinted with permission from ref 225. Copyright 2016 Royal Society of Chemistry.

Figure 123. (a) TEM, (b) HRTEM, and (c−d) SEM images, and EDX elemental mapping images of TiO2-In2O3@g-C3N4 ternary nanocomposites. (e) Charge transfer process of TiO2-In2O3@g-C3N4 ternary hybrids under visible-light irradiation. Reprinted with permission from ref 948. Copyright 2015 Elsevier.

The above discussion exemplifies the possibilities of utilizing a large span of solar spectrum by virtue of a feasible sensitization strategy for the g-C3N4-based heterojunction photocatalysts. In addition to the role of a sensitizer for the enhancement of solar light absorption, the well-matched band structures between g-C3N4 and a sensitizer is expected to facilitate charge separation at the closely contacted interfaces. All in all, this will provide an excellent reference and cornerstone to the materials community in designing energy

matched candidates for more effective utilization of visible and NIR light as well as reduced electron−hole recombination for advanced photoredox functions. 4.7. Design and Construction of Multicomponent Heterojunctions

Up to now, there are plenty of research studies focusing on the g-C3N4-based binary nanocomposites for improved visible-light photocatalysis as depicted in Sections 4.1 to 4.6. To further 7249


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Figure 124. (a) Synthesis process of g-C3N4/nitrogen-doped graphene/MoS2 ternary nanocomposites. (b) TEM and (c) HRTEM images (the inset shows the SAED pattern) of g-C3N4/nitrogen-doped graphene/MoS2. (d) PL and (e) transient photocurrent response of various photocatalysts. (f) Schematic showing band potentials and charge transfer of MoS2/g-C3N4 binary nanocomposite (left) and g-C3N4/nitrogen-doped graphene/MoS2 ternary hybrids (right). Reprinted with permission from ref 434. Copyright 2013 John Wiley & Sons, Inc.

junction was successfully fabricated by Hou et al. via a combined pyrolysis and hydrothermal approach (Figure 124a− c).434 Different layers of MoS2 indicated by parallel-aligned dark lines were observed on the g-C3N4/N-doped rGO (Figure 124b). A distinguished contact interface among N-doped rGO, g-C3N4, and MoS2 was formed, leading to effective electronic transfer within the ternary nanostructures (Figure 124c). This could be further evidenced by the lowest PL quenching effects and the highest photocurrent density for the g-C3N4/nitrogendoped graphene/MoS2 photocatalysts compared to the other samples (Figure 124d−e). In this hybrid nanoarchitecture, the large surface area of g-C3N4 absorbed visible light, while the layered MoS2 improved light harvesting by producing more photoexcited electrons. This was accompanied by the charge transfer and separation at the heterointerfaces of MoS2/g-C3N4. Meanwhile, the nitrogen-doped graphene behaved as an electron mediator for shuttling electron−hole pairs between MoS2 and g-C3N4 sheets (Figure 124f). The effective short diffusion distance of the charge carriers across the 2D ternary nanojunction promoted the overall photoactivity. Similar observations for the formation of p-n heterojunctions using the g-C3N4/rGO/MoS2 ternary nanohybrids have also been reported by Hu et al.951 Employing the same concept of the electron mediator with the aid of graphene, Liu and co-workers integrated graphene into the Fe(III)/g-C3N4 binary nanocomposite.952 In the presence of graphene, well-distributed and a relatively small size of Fe (III) clusters were found in the ternary structure. This was ascribed to the strong electron affinity force and synergistic interaction between graphene and Fe (III) species. The resulting composite showed improved visible-light absorption and decreased recombination rate of charge carriers taking advantage of the graphene for rapid electron transfer. In another study, Zhang et al. fabricated 1D Ag@AgVO3 nanowires/2D rGO/2D protonated g-C3N4 (Ag@AgVO3/ rGO/pCN) hybrid nanocomposites driven by electrostatic attraction and a subsequent photochemical reduction process (Figure 125a).209 An interconnected porous 3D hybrid morphology was observed for the Ag@AgVO3/rGO/pCN

advance the physicochemical properties of g-C3N4, a multicomponent complex heterojunction g-C3N4-based system consisting of two or more components has been designed.943−946 It is projected that a ternary composed heterojunction g-C3N4 system can prolong the lifetime of charge carriers. For example, Chai et al. designed a three component system of g-C3N4-Pt-TiO2 by a facile chemical adsorption-calcination process for dramatically enhanced visible-light H2 production.947 The deposition of Pt cocatalysts as well as the synergistic effect and intimate interfacial connections between g-C3N4 and Pt-TiO2 readily separated the photogenerated charge carriers in space to lengthen the lifetime of electron−hole pairs. Besides that, Jiang et al. employed a facile solvothermal route to develop a novel TiO2-In2O3@g-C3N4 multifunctional ternary system.948 The successful development of the ternary hybrid nanostructures was demonstrated by TEM, HRTEM, SEM, and EDX mapping (Figure 123a−d). It was found that the TiO2 and In2O3 nanoparticles with single-crystalline nature were densely decorated on the g-C3N4 nanosheets (Figure 123a−b). Additionally, well-defined with a sharp contrast was obtained for the elemental mapping images of C, N, Ti and In (Figure 123c−d). Similar to most of the previous reports,949,950 the ternary nanohybrids led to the rapid migration of photogenerated charge carriers to suppress the recombination rate (Figure 123e). Upon the visible-light irradiation, both g-C3N4 and In2O3 were excited from the VB to the CB of g-C3N4 and In2O3, leaving holes at VB. Due to the well-matched electronic potentials of g-C3N4, In2O3, and TiO2, the photoinduced electrons on the CB of g-C3N4 were quickly injected to that of In2O3, followed by further migration into the CB of TiO2 to increase the charge carriers lifetime. Meanwhile, the holes on the VB of In2O3 were transported to that of g-C3N4. Evidently, the TiO2-In2O3@g-C3N4 ternary sample possessed the slowest decay kinetics with an average lifetime of 18 ns compared to those of g-C3N4 single component (7 ns) and TiO2@g-C3N4 binary composite (11 ns). Taking advantage of 2D graphene as a catalyst support, a novel g-C3N4/nitrogen-doped graphene/MoS2 ternary nano7250


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co-workers,954,955 it has drawn tremendous attention by worldwide scientists to design g-C3N4-based heterojunction nanocomposites with effective reduction and oxidation reaction pathways. It is documented that PEDOT, as a superb conducting polymer, is regarded as a hole conductor owing to its excellent hole mobility in dye-sensitized solar cells.956,957 With this mind, Xing and co-workers constructed a unique type of g-C3N4-based photocatalyst by coloading with dual cocatalysts, i.e. PEDOT as a hole transport channel and Pt as an electron reservoir on the g-C3N4 (Figure 126).926 Briefly, PEDOT-poly(styrenesulfonate) (PEDOT-PSS) is utilized, which accounts for two roles: (1) as the counterion of the positively charged PEDOT and (2) as the dispersing agent in the solution. Since PEDOT-PSS is highly soluble in water, the impregnated g-C3N4-PEDOT hybrid is further reacted with ethylene glycol to hinder the detachment of PEDOT and to enhance the conductivity and stability of PEDOT. Subsequently, an in situ photoreduction technique in the presence of triethanolamine (TEOA) yields the deposition of Pt nanoparticles on the g-C3N4-PEDOT to finally develop g-C3N4PEDOT-Pt ternary nanoarchitectures. Upon light irradiation, the photoexcited electrons are migrated to Pt, whereas the photogenerated holes are transferred to PEDOT. As a whole, this work accentuates the necessary functions of the holeconducting polymer and electron-conducting Pt for spatially separating the oxidative and reductive sites toward enhanced photocatalysis. Apart from the aforementioned ternary nanocomposites, a series of attempts have been dedicated to exploit other candidates by hybridizing with g-C3N4 to form multicomponent heterojunction systems. These include graphene/ g-C3N4/P3HT,958 Ag/AgBr/g-C3N4,959 Ag/AgCl/g-C3N4,849 AgBr/g-C 3 N 4 /nitrogen-doped graphene, 960 Ag/Ag 2 O/gC3N4,961 Ag/AgVO3/g-C3N4,962 Au/g-C3N4/Fe2O3,963 PtTiO2/g-C3N4-MnOx,964 ZnO/In2O3/g-C3N4,965 g-C3N4/Ag/ TiO2,966 g-C3N4/CNT/NiS,967 NiS/CdS/g-C3N4,968 g-C3N4/ carbon black/NiS,969 g-C3N4/CdS/rGO,970 g-C3N4/TiO2/ MoS2,971 Ag/g-C3N4/rGO,972 Ag/MoS2/g-C3N4,973 Au/gC3N4/rGO,974 Sb2S3/g-C3N4/g-C3N4 quantum dots,975 AuPtO/g-C3N4,976 Ni/NiO/g-C3N4,266 Ni/CdS/g-C3N4,977 Ni(OH)2/CdS/g-C3N4,978 g-C3N4/rGO/S8,397 BiPO4/TiO2/gC3N4,271 and many more. They have been categorized as a new class of effective visible-light photocatalysts with improved charge transfer and separation for environmental decontamination and solar energy conversion. A few selected photoredox applications using the multicomponent heterojunction systems will be further detailed in Section 5.0. With the significant advancements and achievements on the g-C3N4-based binary

Figure 125. (a) Synthesis process of Ag@AgVO3/rGO/pCN ternary nanocomposites through electrostatic self-assembly followed by a photochemical reduction approach. (b) SEM (inset shows magnified SEM image) and (c) TEM images of the Ag@AgVO3/rGO/pCN sample. Reprinted with permission from ref 209. Copyright 2015 Royal Society of Chemistry.

ternary photocatalysts, comprising thin layers of rGO and pCN with 2D structures and 1D Ag@AgVO3 nanowires decorated with Ag of diameter in the range of 2−5 nm (Figure 125b−c). The enlarged SEM image elucidated the well-dispersion of Ag@AgVO3 between rGO and pCN, confirming that the nanowires and nanosheets were successfully assembled by the electrostatic self-assembly approach (inset of Figure 125b). As a result of strong interaction in the ternary architecture, the Ag@ AgVO3 nanowires served as photosensitizers to absorb more solar photon flux and ameliorated the migration and separation of electron−hole pairs at the interface of Ag@AgVO3/pCN. Similar as above, rGO plays a crucial role as an electron mediator to enhance the charge separation between pCN and Ag@AgVO3 to significantly prohibit the electron−hole recombination for improved photoredox efficiency. This work opens a new vista in materials research to discover promising prospects for constructing more 3D hybrid heteroconjugates with fascinating properties for photochemistry applications. Engineering a photocatalyst with spatially separated oxidative and reductive sites is significantly vital and gaining more interest.953 Motivated by the excellent piece of work on the roles of {010} and {110} facets of BiVO4 nanomaterials as reductive and oxidative sites, respectively, as reported by Li and

Figure 126. Schematic of the g-C3N4-PEDOT-Pt ternary nanocomposites and the charge separation with spatially separating oxidative and reductive sites. Reprinted with permission from ref 926. Copyright 2014 Royal Society of Chemistry. 7251


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and multicomponent heterojunction nanocomposites, this will lead to unprecedented breakthroughs in materials science, interface engineering, and photocatalytic applications. The present work can pave the new doorway to construction of other novel g-C3N4-based photocatalytic systems with appropriate band energy levels for optimizing the visible-light absorption, enhancing charge transfer and separation, and increasing numbers of active sites for various redox reactions for progress in sustainable energy and environmental arenas. We would like to highlight that the current state-of-the-art advancement of nanoscale-architecturing of g-C3N4-based heterojunction photocatalysts lies in stimulating our rational thinking to fully utilize the structural, optical, and electronic features of the g-C3N4 in order to devise and engineer utterly smart light-harvesting g-C3N4-based hybrid nanostructures. In regard to this, it requires great effort from all parties, including academia and industry for commercializing g-C3N4-based nanomaterials in the future. Therefore, we should be hopeful, looking at the bright side from now on, putting more efforts and being more rational in the journey of delivering the prospects of g-C3N4 for practical benefits in the field of photocatalysis.

Figure 127. Schematic of photocatalytic water splitting for H2 and O2 evolution under light irradiation using pristine g-C3N4 as a reference photocatalyst.

Oxidation:

5.0. PHOTOCATALYTIC APPLICATIONS OF G-C3N4-BASED NANOCOMPOSITES FOR ARTIFICIAL PHOTOSYNTHESIS AND ENVIRONMENTAL REMEDIATION In this section, we will present the advancement of photocatalytic redox processes such as water splitting for H2 and O2 evolution (Section 5.1), reduction of CO2 into hydrocarbon fuels (Section 5.2), and degradation of pollutants and bacterial disinfection (Section 5.3). In view of the fact that these potential redox applications are elucidated independently in different review articles thus far,154,199,261,287 in this present review, we majorly accentuate describing a concise and recapitulative summary of the photocatalytic applications using g-C3N4-based hybrid nanocomposites to give a rapid understanding of these photochemistry applications for the benefits of all readers. A number of selected examples based on the most recent findings without prejudice on research studies from a specific research group have been described.

H 2O + 2h+ → 2H+ + 1 2 O2

(1)

Reduction:

2H+ + 2e− → H 2

(2)

Overall reaction: H 2O → H 2 + 1 2 O2

(3)

Table 3 summarizes the recent advances in the photocatalytic water splitting for H2 and O2 generation using g-C3N4-based nanocomposites. In the pioneering work of Wang and co-workers, the authors prepared g-C3N4 by heating cyanamide to temperatures between 673 and 873 K for 4 h and applied it in photocatalytic H2 and O2 evolution from water splitting under visible-light irradiation.194 The production of H2 over bare g-C3N4 was observed to fluctuate and demonstrated discrepancy from batch to batch (0.1−4 μmol h−1). By modifying the sample with 3 wt % Pt, the amount of H2 evolved reached 770 μmol after 72 h, which far exceeded the amount of C6N8 (tri-s-triazine) units or the deposited Pt nanoparticles. The authors also explored the evolution of O2 from the surface of g-C3N4 by introducing RuO2, which is a typical oxidation catalyst for O2 generation, using AgNO3 as a sacrificial electron acceptor. Upon ultraviolet illumination (λ > 300 nm), the total O2 evolution after 8 h of photoactivity was 53 μmol.194 Following this work, Yang et al. demonstrated the synthesis of free-standing g-C3N4 nanosheets by liquid phase exfoliation.493 The sample modified with 3 wt% Pt showed an average rate of H2 evolution of 93 μmol h−1 and a quantum yield of 3.75% at 420 nm.493 The photoactivity of g-C3N4 toward photocatalytic H2 evolution can be improved through several means. First, the solution pH is an important factor for the activity of g-C3N4. Wu et al. demonstrated that alkalization of solution to a pH of 13.3 resulted in a photocatalytic H2 evolution rate of 2.23 mmol h−1 g−1 (Figure 128A).322 The AQY achieved in this work was determined to be 6.67% (Figure 128B). Figure 128C shows the photocurrent scans on g-C3N4 films immersed in neutral or basic electrolyte solution, with or without added methanol.

5.1. Photocatalytic Water Splitting for H2 and O2 Generation

The generation of H2 from water by employing a photocatalyst and solar energy is a desirable future energy source, without relying on fossil reserves.979,980 H2 makes a promising case to serve as an alternative fuel, as it represents a chemical fuel with the highest energy density (140 MJ kg−1) on a gravimetric basis.981 This value is significantly higher as compared to that for most hydrocarbon fuels, such as petrol and diesel, which range from 40−50 MJ/kg.982 Over the past few years, a large amount of g-C3N4-based nanomaterials with enhanced activity for the photocatalytic splitting of water have been developed.264,337,341,565,675,684,747 Figure 127 shows a simplified schematic of the photocatalytic water splitting for H2 and O2 evolution under light irradiation over pristine g-C3N4. Upon light illumination, photogenerated electron−hole pairs will migrate to the bulk of the g-C3N4 photocatalyst toward the reaction sites on the surface. The charge carriers, which flow to the surface of g-C3N4 without recombination, can reduce and oxidize, respectively, adsorbed water molecules to produce gaseous H2 and O2 by reactions 1−3. 7252


7253

PEDOT (2 wt%)

Pt (1 wt%)

Ppy (1.5 wt%) Pt (3 wt%) g-PAN (5 wt%) Pt (1.5 wt%) P3HT (3 wt%) Pt (1 wt%)

PEDOT/g-C3N4/Pt

PMDA/g-C3N4/Pt

Ppy/g-C3N4

P3HT (3 wt%)

UiO-66 (50 wt%) Pt (0.5 wt%)

P3HT/g-C3N4

UiO-66/g-C3N4/Pt

P3HT/g-C3N4/Pt

g-PAN/g-C3N4/Pt

Pt (3 wt%)

Pt (3 wt%) BA (5 wt%), Pt (3 wt%)

ATCN modified g-C3N4

BA-modified g-C3N4

20 mL AA solution (0.1 M)

10 mL of water containing AA

600 mL of aqueous Na2S (0.25 M) and Na2SO3 (0.25 M)

300 mL of TEOA (10 vol%)

100 mL of DI water

400 mL of methanol (10 vol%)






ABN (5 wt%) Pt (3 wt%) ATCN (5 wt%)

ATCN modified hollow gC3N4 nanosphere


Phenylurea (70 mg) Pt (3 wt%)

Covalent C3N4 by copolymerization of urea and phenylurea ABN-functionalized gC3N4

120 mL of solution containing 25% methanol by volume

N/A

Dopant/cocatalysts

CoPi/g-C3N4

Composite type

Reactant solution and sacrificial agent

P3HT/g-C3N4 H2: 6.2 μmol h−1 g-C3N4 H2: 0.80 × 10−6 M h−1

H2: 3045 μmol h−1 H2: 14.11 × 10 −6 M h −1

H2: ∼550.0 μmol h−1

300 W high pressure Hg lamp with a cutoff filter (λ = 420 ± 10 nm)

300 W Xe lamp with a 420 nm cutoff filter


H2: 37.0 μmol h−1

Pure g-C3N4 H2: 7.8 μmol Pure g-C3N4 H2: 0.7 μmol h−1 Pt/g-C3N4 H2: 1.8 μmol h−1

H2: 7.0 μmol h−1 O2: 0.8 μmol h−1

Pure g-C3N4 H2: 85 μmol h−1 Pure g-C3N4 H2: 8.18 μmol h−1 Pure g-C3N4

H2: 6.5 μmol h−1 (λ > 420 nm)

Pure g-C3N4 H2: 18 μmol h−1 Pure hollow gC3N4 nanosphere H2: 100 μmol h−1 Pure g-C3N4 H2: 148.2 μmol h−1 (λ > 300 nm)

H2: 1.46 μmol g−1 h−1 O2: 4.27 μmol g−1 h−1 Pure g-C3N4 H2: 60.2 μmol h−1

Pure g-C3N4

H2: 385.15 μmol

H2: 20.6 μmol h−1 O2: 7.7 μmol h−1

H2: 32.7 μmol h−1

H2: 85 μmol h−1

H2: 253.1 μmol h−1 (λ > 300 nm) H2: 29.4 μmol h−1 (λ > 420 nm)

H2: 278 μmol h−1

H2: 147 μmol h−1

H2: 535 μmol h−1

H2: 194.8 μmol g−1 h−1 O2: 29.04 μmol g−1 h−1

Activity (unit)




300 W Xe lamp with a cutoff filter (λ > 400 nm)


λ > 420 nm

500 W Xe lamp with a cutoff filter: λ > 300 nm


300 W Xe lamp with a working current of 15 A with a cutoff filter (λ > 420 nm) 300 W Xe lamp with a cutoff filter (λ > 420 nm)

300 W Xe arc lamp with a UV cutoff filter (λ > 400 nm)

Light source

Reference photocatalyst and its activity (unit)

-

-

-

-

-

-

-

-

H2: 1.71

-

-

-

-

UV

H2: 17.64

H2: 491.13

H2: 305.56

H2: 52.86

H2: 49.38

O2: 9.63

H2: 3.00

H2: 4.00

H2: 8.60

H2: 4.52

H2: 2.78

H2: 8.17

H2: 9.00

O2: 6.80

H2: 112.8

Vis

Enhancement factor over ref photocatalyst

Table 3. Representative summary of the photocatalytic activity enhancement of g-C3N4-based photocatalysts toward water splittinga

50% activity lost after 15 h 70% activity after 23 days N/A

N/A

>50 h

>30 h

N/A

N/A

>25 h

>16 h

>20 h

>16 h

>48 h

Stability

N/A

77.4% (420 nm)

2.9% (420 nm)

N/A

O2: 0.2% (420 nm) N/A

H2: 0.3%

N/A

8.8% (420 nm)

N/A

N/A

N/A

N/A

N/A

AQY (%)

846 (2015)

656 (2015)

925 (2011)

647 (2014)

924 (2013)

219 (2013)

926 (2014)

576 (2014)

573 (2010)

212 (2015)

358 (2012)

223 (2013)

685 (2013)

ref (Year)

Chemical Reviews Review


ATCN (0.01g) DCDA (3g) Pt (3 wt%) Hydrogenase (50 pmol)

DCDA/ATCN/Pt

7254

20 mL phosphate buffer solution (0.1 M, pH 7) containing 10% (v/v) methanol

Loading of Co0 nanoparticles by weight (10%)

CoS content (1.0 at %)

Co (1 wt%)

NiS2 (2 wt%)

NiS (1 wt%)

CoPi/mpg-CNx

CoS/mpg-CN

Co/g-C3N4

NiS2/g-C3N4

NiS/g-C3N4


100 mL of solution containing AgNO3 (0.01 M) and La2O3 (0.2 g) 10 mL of TEOA (15 vol%)



N/A


Solution of EDTA (0.1 M, 3 mL)


5 vol% TEOA acetonitrile aqueous (9/1, v/v)

70 mL water and 10 mL TEOA


g-C3N4/KCl

[Ni(TEOH)2]Cl2/g-C3N4

NiCl2 (1 wt%)

N/A

C2/g-C3N4

g-C3N4/ hydrogenase/NiP

N/A

Dopant/cocatalysts

C3N4/Ni-Tu-TETN

Composite type

Table 3. continued





H2: 626.4 μmol g−1 h−1

Visible light (100 mW/cm2, λ > 400 nm)

H2: 4.20 μmol h−1

H2: 4.06 μmol h−1

O2: 10.5 μmol h−1

O2: 1012 μmol g−1 h−1 H2: 36.5 μmol h−1

H2 (HER): 0.332 mmol g−1 h−1

H2 (TOF): 12.4 mol h−1 (mol Ni)−1

H2 (TON 4 h): 9135

H2: 131 μmol h−1

H2 (TON): 281

H2: 51 μmol h−1

Activity (unit)

300 W Xe lamp with a water cooling filter (λ > 420 nm)


1000 W halogen lamp with a AM 1.5G filter (100 mW/cm2, λ > 300 nm)



300 W Xe lamp with a solar simulator filter

Light source

1 wt% Pt/g-C3N4 H2: 1.35 μmol h−1 NiS/bulk C3N4 H2: 1.60 μmol h−1

mpg-CN H2: 3 μmol g−1 h−1 g-C3N4 O2: 4.5 μmol h−1

H2: Trace O2: Trace

[Fe(TEOH)2]Cl2/ g-C3N4 H2 (TOF): 0.46 mol h−1 (mol Ni)−1 [Co(TEOH)2]Cl2/ g-C3N4 H2 (TOF): 2.6 mol h−1 (mol Ni)−1 g-C3N4 H2 (HER): 0.072 mmol g−1 h−1 C3N4/NaCl H2 (HER): 0.229 mmol g−1 h−1 mpg-CNx

g-C3N4 H2: Trace

g-C3N4 H2: Trace C1/g-C3N4 H2 (TON): 234 C3/g-C3N4 H2 (TON): 195 Pt/g-C3N4 H2: 13.4 μmol h−1


-

-

-

-

-

-

-

-

-

-

-

UV

H2: 2.63

H2: 3.00

O2: 2.33

H2: 12.17

O2: -

H2: -

H2: 1.45

H2: 4.61

H2: 4.77

H2: 26.96

H2: -

H2: 9.78

H2: 1.20 H2: 1.44

H2: -

Vis


>30 h

>4 h

N/A

>20 h

>24 h

N/A

N/A

N/A

N/A

N/A

N/A

Stability

1.4% (420 nm)

N/A

N/A

N/A

N/A

7.20% (420 nm)

0.07% (365 nm) 0.005% (465 nm) 1.51% (400 nm)

N/A

0.2% (420 nm) N/A

AQY (%)

684 (2015)

785 (2014)

240 (2015)

786 (2014)

983 (2013)

553 (2014)

792 (2012)

797 (2014)

575 (2014)

794 (2014)

796 (2014)

ref (Year)



7255

Zn (10 wt%) Pt (0.5 wt%) NiS (1.5 mol%)

Co(OH)2 (3 wt%)

N/A

Ni(OH)2 (0.5 mol %)

NiS (1.1 wt%)

Ni(dmgH)2 (3.5 wt %)

WS2 (0.3 at%)

Ni (0.1 wt%)

Ni/NiO (2 wt%)

KCl/ g-C3N4 weight ratio (10:1) Pt (0.5 wt%) Ph4BNa (5 mg) Pt (3 wt%) C/N (0.766)

Pt (1.5 wt%) Fe (0.5%)

S/C (0.012) N/C (1.239)

Zn/g-C3N4

NiS/g-C3N4

Co(OH)2/g-C3N4

Au/g-C3N4

Ni(OH)2/g-C3N4

NiS/g-C3N4

Ni(dmgH)2/g-C3N4

WS2/g-C3N4

Ni/g-C3N4

Ni/NiO/g-C3N4

K/g-C3N4

C-doped g-C3N4

Fe/P/g-C3N4

S-doped and N-deficient g-C3N4

B-doped g-C3N4

Cu(OH)2 (0.34 mol %)

Dopant/cocatalysts

Cu(OH)2/g-C3N4

Composite type

Table 3. continued

TEOA

100 mL of aqueous methanol (10 vol%)

80 mL of aqueous methanol (25 vol %)




50 mL solution containing TEOA (10 vol%)

10 mL of lactic acid (10 vol%)





100 mL of solution containing AgNO3 (0.01 M) and La2O3 (0.2 g)


50 mL of methanol and 220 mL of H2O

80 mL mixed solution of methanol and water (volume ratio 1:3)


Visible light (λ > 420 nm)

250 W high pressure Na lamp (λ = 400−800 nm)

300 W Xe lamp with a water filter (λ > 400 nm)

300 W Xe lamp with a water filter (λ > 420 nm)


H2: 121 μmol h−1

H2: 150.6 μmol h−1

H2: 25.3 μmol h−1

H2: 278 μmol h−1

H2: 102.8 μmol h−1

H2: 10 μmol h−1

H2: 5.5 μmol h−1

Xe lamp (125 mW/cm2) with a 420 nm cutoff filter 300 W Xe lamp with a 420 nm cutoff filter

H2: ∼12 μmol h−1

H2: 1.18 μmol h−1

H2: 48.2 μmol h−1

H2: 7.60 μmol h−1

O2: 7.10 μmol h−1 (Vis) H2: 10.70 μmol h−1

O2: 27.4 μmol h−1 (UV−vis)





500 W HBO lamp with a water filter (λ > 420 nm)

300 W Xe lamp with a working current of 15 A

H2: 44.77 μmol h−1

H2: 59.5 μmol h−1

200 W Xe lamp with a 420 nm cutoff filter 300 W Xe lamp with a 420 nm cutoff filter

H2: 48.7 μmol g−1 h−1

Activity (unit)


Light source

−1

H2: 17.8 μmol h Pure g-C3N4 H2: 30.2 μmol h−1 Pure g-C3N4 H2: 10.08 μmol h−1

Pt/g-C3N4

H2: 7.3 μmol h−1 Pt/g-C3N4 H2: 111 μmol h−1

O2: 5.00 μmol h−1 (UV−vis) O2: 1.01 μmol h−1 (Vis) 1.0 wt% Pt/g-C3N4 H2: 2.30 μmol h−1 Pure g-C3N4 H2: Trace Pure g-C3N4 H2: 0.2 μmol h−1 Pure g-C3N4 H2: Trace mpg-CN H2: Trace Pure g-C3N4 H2: Trace Pure g-C3N4 H2: 30 h

N/A

>250 min

>24 h

>16 h

>16 h

>24 h

>4 h

>18 h

>24 h

>12 h

>15 h

N/A

>15 h

>48 h

>28 h

Stability

N/A

8.5% (400 nm)

N/A

N/A

N/A

N/A

2.6% (420 nm)

N/A

N/A

1.9% (440 nm)

1.1% (420 nm)

N/A

N/A

N/A

3.2% (420 nm)

N/A

AQY (%)

985 (2015)

562 (2014)

522 (2012)

222 (2013)

560 (2014)

266 (2015)

984 (2015)

784 (2014)

795 (2014)

339 (2013)

336 (2013)

706 (2010)

337 (2015)

310 (2014)

550 (2011)

759 (2014)

ref (Year)



NaOH conc (0.001 M) Pt (3 wt%) Ammonium iodine conc (1.0 g) Pt (3 wt%) I (0.34 at%)

Surface H-bonding network/g-C3N4

7256

S-doped g-C3N4

S-doped g-C3N4 microrods

S-doped mpg-CN

S-doped g-C3N4

P-doped g-C3N4

P-doped g-C3N4

Pt (3 wt%)

S (0.5 wt%)

O (7.98 at%) Pt (1.2 wt%) O (1.5 at%) Pt (3 wt%) HCCP, GndCl (10 wt%) Pt (3 wt%) Melamine: HEDP mass ratio (12:1) Pt (3 wt%) Thiourea (10 wt%) Pt (1 wt%) S (0.8 wt%) Pt (3 wt%) Pt (1 wt%)

O-doped g-C3N4

O-doped g-C3N4

Pt (3 wt%)

N self-doped g-C3N4

N-vacant/g-C3N4

N-deficient g-C3N4 (gC3N4−x)

I-doped g-C3N4 nanosheets

Pt (3 wt%) N/C atomic ratio (1.319) Pt (1 wt%) Pt (3 wt%)

F (0.5 at%)

F-doped g-C3N4

I-doped g-C3N4

NH4F (2.0 g) Pt (3 wt%)

Dopant/cocatalysts

Fluorinated polymeric C3N4

Composite type

Table 3. continued




120 mL solution containing methanol (25 vol%)











200 mL of aqueous methanol (20 vol%)

















300 W Xe lamp (UV−vis)


Light source

-

H2: 14 μmol h−1 I-free g-C3N4 nanosheets H2: 19.5 μmol h−1 Pure g-C3N4

H2: 140.5 μmol h−1 O2: 20.1 μmol h−1

H2: 5000 μmol g−1 h−1

H2: 136.0 μmol h−1

H2: 12.16 μmol h−1

H2: 104.1 μmol h−1

H2: 50.6 μmol h−1

H2: 60.2 μmol h−1

H2: 37.5 μmol h−1

H2: 44.28 μmol h−1

H2: 123 μmol g−1 h−1

H2: 31.6 μmol h−1

H2: 44.5 μmol h−1

H2: 11.2 μmol h

−1

H2: 11.2 μmol h−1 Pt/g-C3N4 H2: 2.03 μmol h−1 Pt/g-C3N4 H2: 4.50 μmol h−1 Pt/g-C3N4 H2: 538 μmol g−1 h−1 Pt/g-C3N4

-

H2: 17.45 μmol h−1 Pt/g-C3N4

-

-

-

-

-

-

-

-

-

H2: 10.5 μmol h Pt/g-C3N4 H2: 69 μmol g−1 h−1 Pt/g-C3N4 H2: 7.86 μmol h−1 Pt/g-C3N4 H2: 15.2 μmol h−1 Pt/g-C3N4 H2: 9.87 μmol h−1 Pt/g-C3N4

−1

-

H2: 33 μmol h−1 Pt/g-C3N4

H2: 38 μmol h−1

UV H2: 2.70

Vis

O2: 4.28

H2: 12.54

H2: 9.29

H2: 30.2

H2: 6.00

H2: 9.29

H2: 2.90

H2: 6.10

H2: 2.47

H2: 5.63

H2: 1.78

H2: 3

H2: 2.28

H2: 2.71

H2: 2.21

H2: 8.61 (UV−vis)

-

H2: 0.327 mmol h−1 H2: 73 μmol h−1

-

Pt/g-C3N4 H2: ∼4.74 μmol h−1 g-C3N4 H2: 0.038 mmol h−1 Pt/g-C3N4

H2: ∼12.8 μmol h−1

Activity (unit)



N/A

>60 h

>72 h

>8 h

>16 h

>15 h

N/A

>24 h

>16 h

N/A

>30 h

>12 h

>20 h

>28 h

>32 h

N/A

Stability

N/A

N/A

5.8% (440 nm)

2.6% (420 nm)

N/A

N/A

7.8% (420 nm)

N/A

N/A

N/A

N/A

3.0% (420 nm)

2.4% (420 nm)

N/A

4.1% (420 nm)

N/A

AQY (%)

357 (2011)

544 (2014)

359 (2012)

545 (2013)

541 (2015)

540 (2015)

537 (2015)

536 (2012)

160 (2015)

378 (2012)

372 (2013)

533 (2015)

169 (2014)

987 (2013)

986 (2015)

201 (2010)

ref (Year)



MWCNT (0.5 wt%)

CQD (1.0 wt%)

MWCNT (0.2 wt%) Pt (1.2 wt%) Carbon black (0.5 wt %) Pt (3.0 wt%) C-PDA (1.5 wt%) Pt (1.5 wt%) Pt (1.0 wt%)

MWCNT/g-C3N4

CQD/g-C3N4 nanosheets

MWCNT/g-C3N4

7257

Ag2O (0.83 wt%)

Pt (1.0 wt%) MgFe2O4 (150 mg) g-C3N4 (2 wt%)

Ag2O/g-C3N4

MgFe2O4/g-C3N4

Nano-InVO4 (20 wt %)

Pt (1.0 wt%)

Pt (1.0 wt%) ZnFe2O4

Nano-InVO4/g-C3N4

N-CeOx/g-C3N4

ZnFe2O4/g-C3N4

CdS/g-C3N4

C, N-TiO2 (3.0 wt %)

C, N-TiO2/g-C3N4

g-C3N4 quantum dots

C-PDA/g-C3N4

Carbon black/g-C3N4

Pt (6 wt%)

Dopant/cocatalysts

S-doped g-C3N4 (C3N4−xSx)

Composite type

Table 3. continued



200 mL of methanol (20%)

80 mL of 0.35 M Na2S and 0.25 M Na2SO3






100 mL of solution containing methanol

10 mL of TEOA

10 mL of solution containing methanol (20 vol%)

10 mL of water and 3 mL of methanol












Visible light (λ > 420 nm)




300 W Xe lamp

Light source

H2: ∼200 μmol g−1 h−1

H2: 292.5 μmol g−1 h−1

H2: 212 μmol g−1 h−1

H2: 4152 μmol g−1 h−1

H2: 30.09 μmol h−1

H2: 32.88 μmol g−1 h−1

H2: 39.18 μmol g−1 h−1

H2: 137.84 μmol h−1

H2: 81.1 μmol h−1

H2: 68.9 μmol h−1

H2: 39.4 μmol h−1

H2: 50.5 μmol g−1 h−1

H2: 75.0 μmol h−1 (λ > 420 nm) H2: 42 μmol g−1

H2: 160 μmol h−1 (λ > 300 nm)

Activity (unit)

H2: 21.5 μmol h Pt/g-C3N4 H2: 9.0 μmol h−1 Pt/g-C3N4 H2: 48.05 μmol h−1 C, N-TiO2 H2: 3.59 μmol g−1 h−1 g-C3N4 H2: 1.84 μmol g−1 h−1 g-C3N4 H2: 0.12 μmol h−1 Pt/g-C3N4 H2: 10.03 μmol h−1 CdS H2: 2001 μmol g−1 h−1 g-C3N4 H2: ∼120 μmol h−1 Pt/g-C3N4 H2: 134.5 μmol g−1 h−1 Pt/g-C3N4 H2: ∼10 μmol g−1 h−1

−1

Pure g-C3N4 H2: ∼22 μmol g−1 g-C3N4 nanosheets H2: 9.71 μmol g−1 h−1 Pt/g-C3N4 H2: 16.4 μmol h−1 Pt/g-C3N4

-

-

-

-

-

-

-

-

-

-

-

-

-

H2: 7.2

O2: 4.7 μmol h−1 Pt/g-C3N4 H2: 22.22 μmol h−1 (λ > 300 nm) H2: 9.38 μmol h−1 (λ > 420 nm)

UV


H2: 20

H2: 1.2

H2: ∼1.8

H2: 2.07

H2: ∼3

H2: 274

H2: 21.3

H2: 10.9

H2: 2.87

H2: 9.01

H2: 3.20

H2: 2.40

H2: 5.20

H2: 1.91

H2: 8.0

Vis


>40 h

>40 h

>20 h

>12 h

N/A

>8 h

>32 h

N/A

>10 h

N/A

>40 h

N/A

N/A

N/A

Stability

N/A

N/A

4.9% (420 nm)

4.3% (420 nm)

1.8% (420 nm)

N/A

N/A

N/A

2.3% (420 nm)

N/A

N/A

EQE: 1.4% (405 nm)

N/A

N/A

AQY (%)

674 (2014)

642 (2015)

650 (2015)

639 (2013)

651 (2015)

272 (2015)

611 (2015)

485 (2014)

917 (2015)

918 (2014)

865 (2014)

911 (2015)

988 (2012)

381 (2010)

ref (Year)



Pt (1.0 wt%)

Zn-tri-PcNc/g-C3N4

7258

Pt (1.0 wt%) Au/g-C3N4 (0.5 g) S8 (2.0 mmol) Cd(ClO4)2· xH2O (4.0 mmol) ZnCl2 (0.136 g) InCl3·4H2O (0.586 g) g-C3N4 (0.047 g) Pt (1.0 wt%) g-C3N4-rGO (1000 mg) Na2WO4·2H2O (260 mg)

CdS/Au/g-C3N4

100 mL of AgNO3 aqueous solution

100 mL of water and 10 mL of TEOA


10 mL methanol and 50 mL water

300 mL isopropanol/H2O (1:11 v/ v)



10 mL of TEOA

80 mL solution containing 25 vol% methanol

10 mL of water, and 88 mg of AA (50 mM)

10 mL of TEOA


LED white light source

250 W iron doped metal halide UV−vis lamp with a 420 nm cutoff filter

Four low power UV-LEDs (3W, 420 nm)

300 W UV−vis lamp with a 420 nm cutoff filter

300 W UV−vis lamp with a 420 nm cutoff filter




Four low power UV-LEDs (3W, 420 nm)


300 W Xe lamp (Solar light)

Light source

O2: 20 μmol L−1

O2: 0.73 μmol h−1 -

WO3/g-C3N4 H2: 1.50 μmol h−1

H2: 2.84 μmol h−1 O2: 1.46 μmol h−1

Bare ZnIn2S4 H2: 14.71 μmol h−1

H2: 4.49 μmol mg−1 h−1 Pt/TiO2 H2: 5.5 mmol g−1 h−1 Pt/g-C3N4 H2: 0.151 μmol h−1

mpg-C3N4 H2: 3.52 μmol h−1 1.5 wt% NiS/gC3N4 H2: 395 μmol g−1 h−1 CdS/g-C3N4

H2: ∼23 μmol h−1 g-C3N4 H2: 0.18 μmol h−1

Commercial (FeTPP)2O H2: ∼0.1 μmol h−1 Pt/g-C3N4

H2: 29.97 μmol h−1

H2: 19.02 μmol g−1 h−1

H2: 7.6 mmol g−1 h−1

H2: 115.18 μmol mg−1 h−1

H2: 992 μmol g−1 h−1

H2: 521 μmol g−1 h−1

H2: 6.2 μmol h−1

H2: ∼68 μmol h−1

H2: ∼14.5 μmol h−1

Activity (unit)


-

-

-

-

-

-

-

-

-

-

-

UV

-

O2: 2.01

H2: 1.89

H2: 2.04

H2: 125.8

H2: 1.38

H2: 25.7

H2: 2.51

H2: 148

H2: 34.2

H2: 2.96

H2: 140

Vis


N/A

N/A

N/A

N/A

N/A

>40 h

>15 h

>15 h

N/A

>30 h

N/A

Stability

N/A

0.9% (420 nm)

N/A

N/A

N/A

16.7% (450 nm)

N/A

N/A

N/A

1.85% (700 nm)

0.0415% (420 nm)

AQY (%)

618 (2015)

669 (2015)

676 (2015)

989 (2015)

964 (2014)

978 (2016)

969 (2015)

967 (2015)

948 (2015)

938 (2014)

185 (2016)

ref (Year)

AQY: apparent quantum yield; CoPi: cobalt-oxide-phosphate; ABN: 2-aminobenzonitrile; ATCN: 2-aminothiophene-3-carbonitrile; BA: barbituric acid; PEDOT: poly(3,4-ethylenedioxythiophene); PMDA: pyromellitic dianhydride; Ppy: polypyrrole; DI: denionized; g-PAN: graphitized polyacrylonitrile; TEOA: triethanolamine; P3HT: poly-3-hexylthiophene; AA: ascorbic acid; Ni-Tu-TETN: nickelthiourea-N(CH2CH3)3; C1: carboxy-functionalized cobaloxime; C2: pyrene-functionalized cobaloxime; C3: nonfunctionalized cabaloxime; DCDA: dicyandiamide; EDTA: ethylenediaminetetraacetic acid; HER: hydrogen evolution rate; mpg-CN: mesoporous graphitic carbon nitride; e-C3N4: exfoliated graphitic carbon nitride; HCCP: hexachlorotriphosphazene; GndCl: guanidiniumhydrochloride; HEDP:

a

Ag3PO4/g-C3N4

WO3/g-C3N4/rGO

g-C3N4/ nanocarbon/ ZnIn2S4

g-C3N4 (200 mg)

Ni(OH)2 (4.76 wt %) g-C3N4/ CdS ratio (4%) Pt (1.0 wt%) MnOx (1.0 wt%)

Ni(OH)2/ CdS/g-C3N4

Pt-TiO2/g-C3N4-MnOx

Carbon black (0.5 wt %) NiS (1.5 wt%)

g-C3N4/NiS/ carbon black

NiS/mpg-C3N4/CNT

Zn-tri-PcNc (3 mL) g-C3N4 (0.5 g) TBOT (0.2 mL) In(NO3)3 (0.5 mmol) NiS (1 wt%)

(50 wt%) (FeTPP)2O (5 wt%)

(FeTPP)2O/g-C3N4

TiO2/In2O3/g-C3N4

Dopant/cocatalysts

Composite type

Table 3. continued



Review

Upon adding methanol, the photocurrent increased from 0.6 to 1.2 μA cm−2. A further increase to 4.2 μA cm−2 was found after the addition of base to bring the pH to 12.8. This observation showed that hole transfer into the solution was enhanced by the addition of methanol, particularly at high pH. The flatband and band-edge of g-C3N4 with respect to the methanol (and proton) redox potential are depicted in Figure 128D. Overall, SPV spectra and electrochemical measurements revealed increased driving force for photochemical methanol oxidation at high pH, which stemmed from the low acidity of the amine terminated g-C3N4 surface.322 In addition to solution pH, the literature has inferred that significantly higher activity of g-C3N4 can be achieved through the introduction of disorder in the g-C3N4 structure. Such disorder can be induced from stacking defects, grain boundaries, surface termination sites, as well as heteroatom substitution.986 Lau and co-workers demonstrated the control of the polymerization process for melon synthesis to improve the intrinsic photocatalytic activities.373 By decreasing the synthesis temperature, the length of the melon polymer chain could be reduced, which led to a H2 evolution activity three times higher than that of the polymer under AM 1.5 conditions. The photocatalytic performance was observed to increase with shorter oligomer length to a certain extent, consistent with the periphery of the heptazine moiety being involved in the reaction.373 In a paper by Martin and co-workers, the authors examined the effects of protonation and polymerization of pristine gC3N4.343 The as-developed g-C3N4 photocatalyst, prepared from abundant and inexpensive urea, displayed a H2 evolution rate of 20000 μmol h−1 g−1 and 3300 μmol h−1 g−1 under full arc and visible-light irradiation, respectively (Figure 129a−b). Moreover, the sample retained a stability of more than 30 h and an internal quantum yield of 26.5% under visible light. To elucidate the effects of polymerization and protonation on the H2 evolution rates, protonations by DFT simulations were carried out using periodic supercells (Figure 129c−d). The DOS, depicted in Figure 129e, evidently shows the downshifting of the CB edge (CBE) of the protonated system by 0.34 eV. The downshift signified the modification of electrochemical properties, wherein a lower overpotential was provided for reduction reactions. The distribution of the lowest-energy exciton for both C3N4 models is shown in Figure 129f−g. In the deprotonated model, the exciton is distributed relatively homogeneously over the cluster, with transitions from occupied N pz orbitals to empty C pz* orbitals. In contrast, a more heterogeneous pattern is observed for protonated gC3N4; the photohole on the protonated heptazine ring and the photoelectron are evenly distributed on the other two heptazine rings. Despite the better spatial separation between the two, both charge carriers are more localized around the central N3 site, and are therefore not as available to take part in the photochemical reactions. Furthermore, this localization will serve to increase the exciton-recombination rate, thereby hindering the efficient utilization of charge carriers.343 Inspired by the shape-directed functionality in nanoscale, incessant research interest has been directed toward the synthesis of photocatalysts with specific morphology and microstructure, which can enhance sunlight harvesting and charge carrier separation.12,990 Hierarchical nanoporous microspheres of g-C3N4 were successfully synthesized by Gu et al. via a template-free solvothermal approach with postheating treatment.476 In comparison to bulk g-C3N4, the as-prepared

(hydroxyethylidene)diphosphonic acid; MWCNT: multiwalled carbon nanotube; CQD: carbon quantum dot; EQE: external quantum efficiency; C-PDA: carbonized polydopamine; (FeTPP)2O: μ-oxo dimeric iron(III) porphyrin; Zn-tri-PcNc: zinc phthalocyanine.

Table 3. continued

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Figure 128. (A) Amount of H2 production using 30 mg of D52 sample in methanol aqueous solution (20 vol%) at different pH values under visiblelight (λ > 400 nm) illumination. (B) H2 evolution over D52 or UT52 (30 mg) in basic (pH = 13.3) methanol aqueous solution (20 vol%) under 400 nm LED light irradiation for AQY calculation. (C) Transient photocurrent response for a g-C3N4 film in an aqueous NaOH solution (pH = 5.6, 5.7 and 12.8 for (a) D52, (b) D52-MeOH, and (c) D52-MeOH-NaOH, respectively). D52 and UT52 denote g-C3N4 synthesized using dicyandiamide and stoichiometric mixtures of urea and thiourea, respectively. (D) Energy diagrams of g-C3N4 at pH 5.7 and 12.8 solution. Reprinted with permission from ref 322. Copyright 2014 Royal Society of Chemistry.

Wang and co-workers prepared g-C3N4 quantum dots directly from bulk g-C3N4 by a thermal-chemical etching process.485 The as-developed g-C3N 4 quantum dots demonstrated excitation wavelength-dependent PL spectra. As shown in Figure 130a, when the excitation wavelength was varied from 340 to 420 nm, the PL peak shifted to longer wavelengths. Interestingly, the g-C3N4 quantum dots were observed to display obvious upconversion properties when excited by long wavelength light. As depicted in Figure 130b, illuminating gC3N4 quantum dots with 705 to 862 nm light would produce emissions in the range of 350 to 600 nm, which covered a wide spectrum of the visible-light range. Therefore, these results indicate that g-C3N4 quantum dots have the ability to convert NIR light to visible light, which renders the material useful as a universal energy-transfer component in a photocatalytic system. The g-C3N4 quantum dot (CNQD)−g-C3N4 suspension was employed for the generation of H2 from water splitting in the presence of 1 wt% Pt as a cocatalyst. As shown in Figure 130c, CNQD−g-C3N4 with 10 mL g-C3N4 quantum dots addition (CN-10) showed a H2 generation efficiency of 137.84 μmol h−1, which was 2.9 times higher than that of bulk g-C3N4. Surprisingly, it is noted that no H2 production was observed when using pure g-C3N4 quantum dots as photocatalysts. This implies that g-C3N4 quantum dots do not exhibit intrinsic activity, but instead play a crucial role in accelerating photocatalytic reactions. The proposed mechanism, as depicted in Figure 130d, suggested the utilization of both short and long wavelengths in the light spectrum. Upon irradiation, visible

porous microspheres exhibited narrowed bandgap and lower resistance, which allowed for better harvesting of visible light and higher efficiency of charge carrier transport and separation. The sample displayed a H2 generation rate of 1.80 μmol h−1 (an estimated AQY of 1.62% at 420 nm), which was 2.3 folds higher as compared to bulk g-C3N4. Both the hierarchical porous structure and high-degree condensation play significant roles in the improvement of photoactivity. In the proposed mechanism, the sesame-like structure of g-C3N4 particles created homojunctions, which led to the directional transfer of photoinduced charge carriers from core to surfaces, thereby suppressing their recombination.476 Other unique architectures reported in literature include that of a seaweed-like g-C3N4, which was developed by thermal heating of the freeze-drying assembled, hydrothermally treated dicyandiamide fiber structure.210 The seaweed structure of highly porous g-C3N4 was indeed promosing for efficient light harvesting, charge separation and utilization active sites. As compared to its gC3N4 bulk counterpart, the g-C3N4 seaweed demonstrated a superior H2 generation rate of 9900 μmol h−1 g−1 accompanied with an AQY of 7.8% at 420 nm and an excellent turnover number of 385 after 6 h of reactions.210 In most of the papers published to date, the g-C3N4-based materials are generally submicrometer-sized nanospheres, nanorods and nanoplates. Quantum dots-based photocatalysts have garnered tremendous research interests owing to its excellent optical properties, which result from strong quantum confinement and edge effects when sizes are down to 10 nm.991−993 In this context, 7260


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Figure 129. H2 production with a 300 W Xe lamp and 3 wt% Pt under (a) full arc and (b) λ ≥ 395 nm. Optimized geometric structures of (c) gC3N4 sheet and (d) protonated g-C3N4. (e) TDOS for a g-C3N4 sheet (black line) and protonated g-C3N4 (red dashed line). Energy is given with respect to the zero of the simulation for the g-C3N4 sheet. The DOS of the protonated g-C3N4 has been shifted so that the corresponding zero points align. (f) Lowest energy exciton of the C18N28H12 cluster. (g) Lowest energy exciton of the C18N28H13 cluster. Orange isosurface indicates distribution of photohole upon photoexcitation; green isosurface indicates distribution of photoelectron upon photoexcitation. Blue, gray, and white spheres denote hydrogen, carbon, and hydrogen atoms, respectively. Reprinted with permission from ref 343. Copyright 2014 John Wiley & Sons, Inc.

light with wavelength λ < 600 nm directly excited g-C3N4, while longer wavelength (λ > 600 nm) was upconverted by g-C3N4 quantum dots for the subsequent excitation of g-C3N4 to generate electron−hole pairs.485 As discussed earlier, introducing structural distortion to gC3N4 photocatalysts can dramatically modify their electronic structures, leading to efficient separation of charge carriers and attaining high photocatalytic performance. Wang et al. theoretically examined the orbitals of F-doped g-C3N4, along with pristine g-C3N4, to elucidate the reason behind the higher photoactivity of the former.986 As depicted in Figure 131A(a− b), the CB and VB of pristine g-C3N4 are both derived from the hybridization of p orbitals between carbon and nitrogen. In contrast, the VB of the F-modified g-C3N4 sample is derived from fluorine and nearby carbon and nitrogen, while the CB is contributed by the hybrid of other carbon and nitrogen atoms (Figure 131A(d−e)). Evidently, the CB and VB of F-doped gC3N4 are separated from one another, which would ultimately reduce the recombination of photoinduced electron−hole pairs and favor the photoactivity of F-doped g-C3N4. By employing XPS valence spectra (Figure 131B) and UV−vis spectra, it was deduced that both F-doped g-C3N4 and pristine g-C3N4 exhibited nearly identical VB with the edge of the maximum energy at ca. 1.56 eV. Meanwhile, the CB minimum of F-doped g-C3N4 upshifted about 0.10 eV relative to pristine g-C3N4, as schematically illustrated in Figure 131C. Such difference in band structure could enhance the reduction ability of photoinduced electrons over F-doped g-C3N4. The modified sample displayed the highest photoactivity toward H 2 generation (Figure 131D), achieving an enhancement factor

of 8.6 over pristine g-C3N4. Furthermore, additional structural distortion of g-C3N4 via heating was also observed to enhance the photocatalytic activity as depicted in Figure 131E. These findings highlight the importance of structural distortion of gC3N4, which can be achieved through two strategies, i.e. elemental doping as well as an external heat treatment.986 An important strategy to improve the performance of g-C3N4 is to engineer its electronic structure by introducing dopants and hybridization with nonmetals such as B, S, O, P, and I, as detailed in Section 2.3.1.169,451,539 This strategy plays an essential role in tuning the band gap structure, extending the light absorption range, enhancing charge transfer mobility as well as generating more active sites.537,541,542 Recently, Ran et al. fabricated novel porous P-doped g-C3N4 by combining Pdoping and thermal exfoliation.543 The as-synthesized samples exhibited high visible-light photocatalytic H2-generation activity of 1596 μmol h−1 g−1 and an AQY of 3.56% at 420 nm (Figure 132a).543 The empty midgap states, created by P doping, significantly extended the light-responsive range up to 557 nm, as demonstrated from DFT and experimental studies (Figure 59e−g), while the macroporous structure promoted the masstransfer process and improved light harvesting.543 Figure 59f−g clearly shows that, after P-doping, the intrinsic band gap of gC3N4 reduced from 2.98 to 2.66 eV based on the earlier discussion. As delineated in Figure 132b, the exact position of midgap states for P-doped g-C3N4 was determined to be at −0.16 eV vs SHE, which is higher than that for H2 evolution (2H+ + 2e = H2, E° = 0.0 V vs SHE at pH = 0). The results suggested that photoinduced electrons excited from VB to the midgap states were thermodynamically capable of reducing 7261


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Figure 130. (a) PL spectra of g-C3N4 quantum dots at various excitation wavelengths and (b) the upconversion PL spectra of the g-C3N4 quantum dots. (c) Photocatalytic H2 evolution over g-C3N4, g-C3N4 quantum dot (CNQD)−g-C3N4, and pure g-C3N4 quantum dots with 1 wt% Pt under visible-light irradiation (λ > 420 nm). CN-5 and CN-10 denote g-C3N4 with the addition of 5 and 10 mL of g-C3N4 quantum dot solution, respectively. (d) Photocatalytic mechanism for CNQD−g-C3N4 under visible light. Reprinted with permission from ref 485. Copyright 2014 Royal Society of Chemistry.

protons to produce H2.543 In another paper, Guo and coworkers synthesized P-doped hexagonal tubular g-C3N4 with layered stacking structures, which achieved a H2 evolution rate of 67 μmol h−1 under visible light and an AQY of 5.68% at 420 nm.542 The tubular structure favored the enhancement of light scattering and provided additional active sites, while the doping of P resulted in reduced band gap energy and increased electric conductivity.542 Other than P-doping, Lin et al. demonstrated that an ionic liquid, [Bmim][BF4] could function as a multifunctional modifier for the self-polymerization of urea to prepare B, F-containing g-C3N4 porous nanosheets.451 The asobtained sample achieved a 3.9 times higher photocatalytic H2 production rate than that of pristine g-C3N4. Zhang et al. developed I-doped g-C3N4 solids, which displayed a H2 evolution rate of ca. 38 μmol h−1, which was 2 times greater than that of pure g-C3N4.169 Fascinatingly, the I-doped g-C3N4 samples showed photocatalytic activity despite increasing the wavelength to 600 nm, while pristine g-C3N4 was inactive at 500 nm.169 This observation clearly indicates the advantage of nonmetal doping to optimize the band structure and texture of the polymeric photocatalyst for the solar energy production. To make g-C3N4 an economically feasible photocatalyst for water splitting, considerable efforts have been made to render the photocatalyst more efficient. However, in most of the strategies reported to date, noble metal Pt is essentially the cocatalyst required, which greatly limits the application of gC3N4 in water splitting. In the past few years, the Pt-free approach has been successfully demonstrated for H2 evolution by combining g-C3N4 with NiS,310 Ni(OH)2,336 ZnO,490 MoS2783 and so forth. Chen and co-workers conducted surface functionalization to introduce a single Ni active site onto the surface of g-C3N4.984 The resulting catalyst family (with less

than 0.1 wt% of Ni) was shown to generate H2 with a rate close to that attained with a 3 wt% Pt photodeposited g-C3N4. In another paper by Bi et al., the authors synthesized a series of Ni-doped g-C3N4 by a solvent thermal method using melamine and acetylacetone nickel as precursors.242 Work function measurements (Figure 133a) and Mott−Schottky plots (Figure 133b) confirmed the surface band bending change of g-C3N4 when contacted with Ni. The work functions of g-C3N4 and Ni were calculated to be 4.34 and 4.86 eV, respectively (Figure 133c). When g-C3N4 was brought in contact with Ni, the free electrons could be transferred from g-C3N4 to Ni until the Fermi levels of two semiconductors aligned, resulting in an interface electric field orienting from g-C3N4 to Ni (Figure 133d). The results obtained indicated that Ni-coating deepened the surface band bending of g-C3N4, thereby resulting in higher separation efficiency of photoinduced electron−hole pairs and improved H2 generation activity.242 In a most recent paper by Indra et al., the nature of active species in Ni-containing sol−gel prepared mesoporous g-C3N4 (sg-CN) for the photocatalytic H2 evolution in the presence of sacrificial agent (e.g., TEOA) was extensively investigated.241 The studies conducted clearly proved that Ni2+ had undergone reduction to Ni0 nanoparticles during the photocatalytic reaction and served as a cocatalyst to reduce protons to H2. The generation of H2 over the Nimodified g-C3N4 is summarized in eqs 4−7.

7262

sg‐CN + hv → esg‐CN‐CB− + h sg‐CN‐VB+

(4)

TEOA + h sg‐CN‐VB+ → TEOA+

(5)

2H+ + 2esg‐CN‐CB− → H 2

(6) DOI: 10.1021/acs.chemrev.6b00075 Chem. Rev. 2016, 116, 7159−7329

Chemical Reviews

Review

Figure 131. (A) Kohn−Sham orbitals for the VB and the corresponding CB of (a−b) pristine g-C3N4, (d−e) F-doped g-C3N4 and (g−h) structural distorted g-C3N4. (c, f, and i) Atomic structural model of pristine g-C3N4, F-doped g-C3N4, and structurally distorted g-C3N4 projected along a random direction, respectively. (B) VB XPS spectra of pristine g-C3N4 and F-modified g-C3N4. (C) Schematic illustration of the band structure of pristine g-C3N4 and F-modified g-C3N4. (D) Photocatalytic activities of as-developed samples toward H2 generation and (E) H2 evolution rate for gC3N4 and F-modified g-C3N4 at different times of heat treatment. Reprinted with permission from ref 986. Copyright 2015 Royal Society of Chemistry.

Figure 132. (a) Quantum efficiency (QE) of P-doped g-C3N4 nanosheets (PCN-S) at 420 nm, 460 nm, 500 nm, and 540 nm. (b) Electronic band structures of bulk g-C3N4 (CN-B), g-C3N4 nanosheets (CN-S), bulk P-doped g-C3N4 (PCN-B), and PCN-S. Reprinted with permission from ref 543. Copyright 2015 Royal Society of Chemistry.

Ni 2 + + 2esg‐CN‐CB− → Ni0

Na2SO3, and ethylenediamine tetraacetic acid (EDTA) as a sacrificial reagent resulted in a H2 production activity of 812, 104, 57, and 44 μmol h−1 under visible-light irradiation, respectively. Based on Figure 134b, it is evident that each sacrificial reagent used possessed sufficient thermodynamic electron donating ability to facilitate the regeneration of excited P3HT after electron injection to g-C3N4. However, the authors noted that the photocatalytic performance had no distinct relation to the redox potentials of the sacrificial reagents, and that their oxidation processes displayed higher impacts on the photoactivity.656 In addition, a comparison of photoactivity of g-C3N4 and 3 wt% P3HT/g-C3N4 in ascorbic acid solution was carried out under different visible-light irradiation (Figure

(7)

Other transition metal, metal oxides, or hydroxides, including Ni(OH)2,336 Ni@NiO,266 TiO2,608 Cu(OH)2,759 Cu2O,644 etc., have also been reported to be economical and effective cocatalysts for H2 production from water splitting. In a paper by Zhang et al., a robust polymer/polymer surface heterojunction catalyst for wide visible-light-driven production of H2 was successfully prepared by rotary evaporation of P3HT solution containing g-C3N4.656 The photoactivity of the asproduced P3HT/g-C3N4 catalyst could be enhanced to certain degrees by employing different sacrificial reagents. From Figure 134a, it can be observed that ascorbic acid, TEOA, Na2S + 7263


Chemical Reviews

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oxidation reaction of ascorbic acid could support the production of H2 to the highest extent. Figure 134f shows the photoactivity and AQY values for H2 production over the catalyst under different monochromatic lights. Results indicated that the photoactivity should be dominated by the light absorption of P3HT, generating charge carriers that could further migrate to the P3HT/ g-C3N4 interfaces. Moreover, the catalyst showed activity and AQY exceeding 10% in the whole visible-light region (400−660 nm), with a good red/NIR lightresponsive ability with an AQY of 3.2% and 0.68% under 700 and 800 nm light irradiation, respectively (Figure 134e). A plausible reaction mechanism was proposed as shown in Figure 134f. Overall, P3HT plays dual roles of sensitization and heterojunction formation: (1) extending the light absorption to visible/NIR regions of g-C3N4, and (2) forming a heterojunction with g-C3N4, which facilitates the charge transfer process at the interface junction. Owing to its high activity, earth-abundant composition, inexpensiveness, and robustness, MoS2 has emerged as a promising cocatalyst for g-C 3 N 4 in photocatalytic H 2 generation.434 In this connection, the effective combination of g-C3N4 and MoS2 is essential for the rapid migration of electrons, which is among the key factors for photocatalytic efficiency.330 In 2013, Hou et al. grew thin-layered MoS2 onto mesoporous g-C3N4 by a thermal deposition process, and high photoactivity for H2 evolution was obtained.652 The asdeveloped photocatalyst showed an AQY of 2.1% measured at 420 nm. In a separate paper, the enhanced photocatalytic mechanism for the hybrid MoS2/g-C3N4 nanocomposites were investigated by extensive DFT calculations.783 The top and side views of MoS2/g-C3N4 are depicted in Figure 135a−b. Due to the presence of MoS2, g-C3N4 displayed clear geometric distortion. The calculated band alignment between the g-C3N4

Figure 133. (a) Schematic showing the working functions of g-C3N4 and Ni. (b) Mott−Schottky plots of g-C3N4 (Ni0) and Ni-doped gC3N4 (Ni10) in the dark. (c) Energy level diagram of g-C3N4 and Ni. (d) Interfacial electron transfer between Ni-doped g-C3N4 and Ni under light irradiation. Reprinted with permission from ref 242. Copyright 2015 Royal Society of Chemistry.

134c). The results indicated that g-C3N4, as an electron acceptor, was necessary in the surface heterojunction catalyst, and the photoactivity under λ ≥ 500 nm light was primarily due to the light absorption of P3HT and its electron injection toward λ ≥ 500 nm g-C3N4. Furthermore, the concentration of ascorbic acid was vital in the photocatalytic activity and stability of P3HT/g-C3N4 (Figure 134d−e). At an optimum level, the

Figure 134. (a) Effects of different sacrificial reagents on the photoactivity of 3 wt% P3HT/g-C3N4 under light irradiation (λ ≥ 420 nm). (b) Energy levels of P3HT and g-C3N4 and redox potentials of the sacrificial reagents employed. Conditions: 10 mg of photocatalyst with Pt loadings (1 wt%), 10 mL of sacrificial reagent solution (0.25 M Na2S + 0.35 M Na2SO3; 50 mM ascorbic acid (AA); 10 mM EDTA or 10 vol% TEOA) without pH adjustment. (c) Photocatalytic performance of pure g-C3N4 and 3 wt% P3HT/g-C3N4 in 50 mM ascorbic acid under different wavelengths of light irradiation. (d) Influence of ascorbic acid concentration on the H2 production over 3 wt% P3HT/g-C3N4 under light irradiation (λ ≥ 500 nm). (e) Wavelength-dependent activity and AQY plot of 3 wt% P3HT/g-C3N4 in 250 mM ascorbic acid solution. (f) Charge transfer of surface heterojunction (SHJ) catalyst for H2 production. Reprinted with permission from ref 656. Copyright 2015 American Chemical Society. 7264


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Review

evolution reaction activity was obtained by spatially separating the charge carriers on each side of photocatalyst. Despite the promising results obtained from g-C3N4-based photocatalytic systems consisting of sulfides (e.g., CdS), it has been well established that sulfides suffer from self-oxidation and poor photocorrosion,338,639,773 which will dramatically reduce their stability. To overcome these drawbacks, Hu and coworkers prepared nano-InVO4/g-C3N4 heterojunction-type photocatalysts via a hydrothermal process.650 The as-obtained nanocomposite with a mass ratio of 80:20 resulted in a maximum H2 evolution rate of 212 μmol h−1 under visible-light irradiation (Figure 136a). In addition, the photoactivity achieved was also observed to be higher compared to InVO4 microspheres/g-C3N4. It was suggested that the reducing power of nano-InVO4/g-C3N4 composites was much stronger than that of InVO4 microspheres/g-C3N4 due to the different energy bands of InVO4 nanoparticles and InVO4 microspheres, which was demonstrated by EPR data (Figure 136b). The nanocomposites could not only enhance the charge separation efficiency, but also achieve a strong driving force for H2 production compared to bare g-C3N4 and micro-InVO4/gC3N4 composites, respectively (Figure 136c). In the past three years, carbonaceous nanomaterials with πconjugative structures have been employed to fabricate hybrids with g-C3N4 for improved photocatalytic H2 production.900,911 The π-conjugated networks have the capability of accepting, transporting, and storing photogenerated electrons and hence suppressing the recombination of charge carriers on gC3N4.248,320,895 In a paper by Chen et al., g-C3N4 hybridized with a small amount of MWCNTs were synthesized using cyanamide as precursor.865 The hybrid with ca. 0.2 wt% of

Figure 135. (a) Top and side views of the MoS2/g-C3N4 nanohybrids. (c) The calculated band structure and (d) the calculated absorption coefficients of the g-C 3 N 4 monolayer and the MoS 2 /g-C 3 N 4 photocatalysts are labeled with the black and red lines, respectively. Reprinted with permission from ref 783. Copyright 2014 Royal Society of Chemistry.

monolayer and MoS2 sheets revealed that the CB minimum and VB maximum of the g-C3N4 monolayer were higher by about 0.83 and 0.15 eV, respectively than those of the MoS2 sheet (Figure 135c, see also Figure 96a). In addition, the calculated optical absorption curve also confirmed that the layered nanocomposite exhibited good light-harvesting properties (Figure 135d). Owing to these unique features, high H2

Figure 136. (a) Photocatalytic H2 evolution for g-C3N4 and different InVO4/g-C3N4 samples under visible-light irradiation. (b) EPR signals of the DMPO−•O2− with irradiation for 20 s in CH3OH solution. (c) Charge transfer of InVO4/g-C3N4 nanohybrids for H2 evolution. Reprinted with permission from ref 650. Copyright 2015 American Chemical Society. 7265


Chemical Reviews

Review

Figure 137. (A) HOMO and LUMO of carbon nanodots/g-C3N4 from both sides and top views. Color code: carbon (gray), nitrogen (blue), hydrogen (white). “Carbon nanodots” is omitted from the labels. (B) The calculated VB maximum (HOMO) and CB minimum (LUMO) potential vs NHE of bare g-C3N4 and carbon nanodots by the HSE06 approach. Color code: carbon (gray), nitrogen (blue), hydrogen (white). Reprinted with permission from ref 909. Copyright 2015 Royal Society of Chemistry.

Figure 138. Electron transfer mechanisms of the (a) ZnIn2S4/g-C3N4 (Type II) and (b) g-C3N4/nanocarbon/ZnIn2S4 (Z-scheme). Reprinted with permission from ref 676. Copyright 2015 Royal Society of Chemistry. (c) Double-charge transfer and (d) Z-scheme mechanisms of Ag3PO4/g-C3N4 photocatalysts. Reprinted with permission from ref 618. Copyright 2015 John Wiley & Sons, Inc.

recombination of electron−hole pairs. More recently, Gao’s group carried out extensive DFT calculations to examine the interactions between g-C3N4 and trigonal/hexagonal shaped carbon nanodots.909 Figure 137A shows the HOMO and LUMO of carbon nanodots/g-C3N4. The primary difference between trigonal carbon nanodots/g-C3N4 and that of hexagonal form was the charge distribution on the HOMO. The HOMO of trigonal carbon nanodots/g-C3N4 was more delocalized, owing to the net spin on the trigonal carbon nanodots, i.e. the unpaired electrons. Although the electrons in

MWCNT demonstrated a 2.4-fold enhancement in photocatalytic water splitting over pure g-C3N4. Xiang et al. interfaced graphene with g-C3N4 to produce a metal-free graphene/gC3N4 system via a combined impregnation−chemical reduction strategy.883 The graphene/g-C3N4 composite with 1.0 wt% of graphene resulted in a H2 evolution rate of 451 μmol h−1 g−1, which was more than 3.07 times higher than that of pure gC3N4. Both PL and photocurrent response characterizations verified the role of graphene as a good acceptor of photogenerated electrons, which effectively suppressed the 7266


Chemical Reviews

Review

Figure 139. (a) Rate of H2 evolution of Ag nanoparticles, 3 wt% Ag/g-C3N4 (3SCN) by photodeposition, 3SCN by chemical reduction, and 3SCN by chemical reduction with fluorescein (0.005 g) in a 70 mL aqueous solution with and without 10 mL of sacrificial agent (3 h of reaction). (b) Stability test of the 3SCN by chemical reduction with fluorescein (0.005 g) and 10 vol% TEOA for photocatalytic H2 generation (20 h of reaction). (c) Schematic illustrating of the charge migration and separation over Ag/g-C3N4 with fluorescein. Reprinted with permission from ref 935. Copyright 2016 Royal Society of Chemistry.

catalytic H2 production due to its excellent charge separation and transfer efficiency. Figure 138a−b depicts the electronic transfer mechanisms of binary ZnIn2S4/g-C3N4 and ternary gC3N4/nanocarbon/ZnIn2S4 nanocomposites. For the case of ZnIn2S4/g-C3N4, a slight enhancement was observed due to the close VB and CB position values for g-C3N4 (ca. 1.57 and −1.12 eV) and ZnIn2S4 (ca. 1.63 and −0.80 eV), which led to an unfavorable driving force for charge transfer between the semiconductors (Figure 138a). However, for g-C3N4/nanocarbon/ZnIn2S4, when nanocarbon was coated onto ZnIn2S4, due to high visible-light-activity and conductivity of graphitic phase nanocarbon, the nanocarbon could not only improve the light absorption performance of the photocatalyst, but also serve as a conducting medium to improve the transfer rates of both electrons and holes in g-C3N4/nanocarbon/ZnIn2S4. Under visible-light irradiation, photogenerated electrons migrate from the CB of ZnIn2S4 to the nanocarbon and then g-C3N4, before combining with the holes stemming from the VB of g-C3N4 (Figure 138b). As a consequence, a larger amount of photoinduced electrons at g-C3N4 are available to take part in the reduction of protons. Thus, this resulted in the enhancement of the activity of the Z-scheme photocatalytic system toward H2 evolution by ca. 3.4 and 3.2 times compared to those of pure ZnIn2S4 and ZnIn2S4/g-C3N4, respectively.676 In another paper by Yang and co-workers,618 the authors demonstrated the formation of an in situ Z-scheme process by the evolution of small Ag nanoparticles in the heterointerface of Ag3PO4/g-C3N4 photocatalysts under the light irradiation for O2 evolution. In contrast to the conventional double-charge transfer mechanism (Figure 138c), the Ag nanoparticles

hexagonal carbon nanodots/g-C3N4 were all paired, the HOMO and LUMO in both systems were segregated, which was suggested to favor the separation of photoinduced electron−hole pairs spatially. Using the hybrid functional technique, the authors calculated the band alignment of isolated carbon nanodots and g-C3N4 as shown in Figure 137B. Interestingly, the Type II band alignment of carbon nanodots and g-C3N4 indicated that carbon nanodots acted as spectral sensitizers in the hybrid composite for water splitting. Additionally, the electrons from the HOMO of carbon nanodots could be directly photoexcited into the CB of the g-C3N4 by visible light, which separated the charge carriers into each of the different materials. It was concurred that these extraordinary properties were responsible for the enhancement of oxidation and reduction of water into O2 and H2.909 Artificial Z-scheme photocatalytic systems, which mimic the natural photosynthesis process, provide a great potential to realize superior photocatalytic performance in H2 evolution by water splitting under visible light.588,589,601,994,995 This is due to the remarkable redox power found in the Z-scheme photocatalytic system, which can be attained by integrating two semiconductors with a narrow-bandgap to suppress the electron−hole recombination.598,682 Hence, the ability to design a Z-scheme photocatalytic system with excellent photoactivity is highly desirable. Recently, Shi et al. employed nanocarbon as a solid-state electron mediator to construct the Z-scheme mechanism, whereby g-C3N4 and ZnIn2S4 were chosen as the two photocatalysts.676 In comparison to pure gC3N4, ZnIn2S4, ZnIn2S4/g-C3N4 and nanocarbon/ZnIn2S4, the as-synthesized Z-scheme system displayed enhanced photo7267


Chemical Reviews

Review

Figure 140. Time dependent evolution of H2 and O2 via direct water splitting over the g-C3N4 modified with 3 wt% Pt, PtOx, and 1 wt% CoOx cocatalysts under the illumination of (a) UV−vis (λ > 300 nm) and (b) visible light (λ > 420 nm). Reprinted with permission from ref 237. Copyright 2016 Royal Society of Chemistry.

react with H+ for easier production of H2. At the same time, the photogenerated holes of fluorescein and g-C3N4 could be packed by electrons from TEOA for reuse.935 Up to now, the evolution of H2 and O2 by means of overall water splitting in the absence of sacrificial agents remains a key science challenge. The reason behind it is due to the fact that the direct water splitting is not only dependent on the textural features of photocatalysts, but also on the rational development of the hybrid composites to accelerate the reaction kinetics of the process.997,998 As a matter of fact, the lack of surface redox catalytic sites on the pure g-C3N4 significantly limits in catalyzing the overall water splitting. Thus, water half-splitting reactions for the H2 and O2 generation can be ultimately achieved and markedly improved by engineering the structural morphology of g-C3N4 and utilizing appropriate redox cocatalysts.215,337,999 In light of this, most recently, Wang’s group successfully developed g-C3N4 decorated with H2 and O2 redox cocatalysts (Pt, PtOx, and CoOx) for the direct water splitting through a four-electron pathway without sacrificial agents.237 The simultaneous production of H2 and O2 was obtained in a stoichiometric ratio of 2:1. Indeed, the Pt served as an excellent H2 evolution cocatalyst, whereas PtOx were active for enhancing the water oxidation rate. In addition, the presence of CoOx species also selectively promoted the water oxidation reaction. Amazingly, the Pt-Co/g-C3N4 sample exhibited a robust stability for 510 h of reactions with no noteworthy deactivation (Figure 140), inferring the high resistance of the light and water corrosion at the heterojunction interface of the composites. Based on the direct water splitting, the AQY was estimated to be ca. 0.3% at 405 nm. Although the value was relatively smaller than that of the inorganic photocatalysts (Ga1−xZnx)(N1−xOx) (AQY = 2.5% at 420− 440 nm),33 this work was considered to be pioneer in exploring the photocatalytic overall water splitting on polymeric semiconductors. Moreover, considerably greater AQY and solar-tohydrogen energy conversion efficiency were reported on single/ binary nanomaterials, namely CoO and carbon nanodots/gC3N4 for photocatalytic water splitting,1000,1001 albeit extensive studies are urgently indispensable to ensure reproducibility results.32 Indeed, further optimization of the direct splitting of water using organic photocatalysts is ongoing to expedite the progress of next generation solar-fuel nanocomposites. Due to increasing technological attention for realizing “green” energy storage and conversion, the water splitting process is a good starting point for the investigation of a theoretical simulation on g-C3N4 photocatalysts.1002 It is vital to elucidate the mechanism behind the interaction between water

facilitate the separation of charge carriers within the composite material by serving as a storage and recombination center for electrons and holes from Ag3PO4 and g-C3N4, respectively (Figure 138d). In the presence of AgNO3 as a sacrificial agent, the optimal Ag3PO4/g-C3N4 exhibited higher O2 evolution (25 μmol L−1) than the pristine g-C3N4 (negligible O2 evolution) and Ag3PO4 (8 μmol L−1). Similar findings have also been reported by the same research group in another report of Ag3PO4/g-C3N4, whereby they examined the variation of gC3N4 materials (e.g., silica templating and supramolecular preorganization) for the improvement of H2O oxidation by a Zscheme mechanism.619 It was found that the incorporation of Ag3PO4 with supramolecular-derived g-C3N4 demonstrated the highest O2 evolution by ca. 2- and 1.7-fold increments as compared to the bulk g-C3N4 and silica templating-derived gC3N4, respectively. As a matter of fact, organic dyes have been amply used in dye-sensitized solar cells for exploiting the efficient utilization of solar energy. Over the past few years, there has been gradual use of dyes toward the photocatalytic H2 evolution process as a visible-light-harvesting molecule to extend the possibility of the reaction.996 In 2016, Qin and co-workers reported the improvement in photocatalytic H2 generation of Ag/g-C3N4 nanocomposites through dye-sensitization under visible-light illumination.935 Fluorescein was introduced as the photosensitizer and the H2 production reached 2014.2 μmol h−1 g−1, which was approximately 4.8 times higher than that of 3 wt% Ag/g-C3N4 in the absence of dye molecules (Figure 139a). Furthermore, as can be seen from the recycling test of Ag/gC3N4 obtained by chemical reduction with fluorescein, the catalytic activity remained fairly consistent in four consecutive cycles, which indicated high photocatalytic stability (Figure 139b). Figure 139c manifests the electron transfer mechanism of Ag/g-C3N4 heterostructures. It is noted that the LUMO of fluorescein is more negative than the CB of g-C3N4, while its HOMO is more positive than the redox potential of the sacrificial reagent, TEOA.935 Upon the irradiation of visible light, both fluorescein and g-C3N4 were excited, and the electrons from fluorescein migrated to the CB of g-C3N4 easily. Subsequently, the electrons rapidly migrated to the Ag nanoparticles with the Fermi level at + 0.4 eV vs NHE, which matched well with the energy level of g-C3N4. The electrons then accumulated onto Ag nanoparticles to create a Schottky barrier, which could hinder the recombination of electron−hole pairs. The strong local electromagnetic field induced by the SPR effect of Ag could also improve the captured electron energy and transfer rate, allowing them to 7268


Chemical Reviews

Review

Figure 141. (A) Optimized adsorption geometries of intermediates during the H2O oxidation using the PBE functional. Bond lengths/atom distances: (a) r (N−O) = 1.28 Å; (b) r (N−O) = 2.28 Å, r (O−H) = 0.99 Å; (c) r (O−C) = 2.89 Å, r (O−O) = 1.34 Å. (B) Optimized adsorption geometries of intermediates during the H+ reduction using the PBE functional. Bond lengths/atom distances: (a) r (N−H) = 1.05 Å; (b) r (N−H) = 1.04 Å, r (O−C) = 2.81 Å, hydrogen bonds (dotted red lines) = 2.85 Å. Reprinted with permission from ref 1002. Copyright 2014 Royal Society of Chemistry.

E′: *H 2O + H+ + e− → *H3O

molecules and g-C3N4 during the photocatalytic water-splitting process.1003 Wirth and co-workers systematically examined the process by means of DFT and density functional based tightbinding.1002 Figure 141A shows the optimized adsorption geometries (PBE approach) of intermediates during the half reaction of water oxidation (eqs 8−12, where * denotes bare surface). 2H 2O → O2 + 4H+ + 4e−

(8)

A: * + H 2O → *OH + H+ + e−

(9)

B: *OH → *O + H+ + e−

(10)

C: *O + H 2O → *OOH + H+ + e−

(11)

D: *OOH → * + O2 + H+ + e−

(12)

Similarly, the optimization of adsorption geometries of intermediate species (H atom, H3O radical) was performed on the hollow site (Figure 141B (a)), which yielded large adsorption energies of several eV. This was attributed to the strong covalent bonding, which was created between adsorbate H and substrate N atom, even subtracting one H atom in the case of H3O radical. As a result, the remaining water molecule was present in a hydrogen bonded arrangement (Figure 141B(b)).1002 In another paper by Srinivasu and Ghosh, DFT was also carried out to elucidate the reaction mechanism of the overall photocatalytic water splitting reaction on s-triazine based gC3N4.1004 The calculated overpotential for the evolution of O2 was determined to be ca. 0.93 V, and the holes in the VB were at a potential of 2.64 eV with respect to the NHE. These results indicated that g-C3N4 exhibited the potential to facilitate the reaction without the aid of any cocatalyst, which is highly desirable for designing an active photocatalyst. On the other hand, studies on H2 evolution revealed that the process had an overpotential of approximately 1.0 eV. As the photoinduced electrons in the CB were located just 0.26 eV above the H2 reduction level, this indicated that the reduction reaction would only be possible in the presence of a cocatalyst.1004 Gao and coworkers reported that the binding free energy of H atom (ΔG°H*) on g-C3N4 was very sensitive to mechanical strain, which in turn led to substantial tuning of the H2 evolution rate performance of g-C3N4 at different coverages.1005 It was observed that ΔG°H* of g-C3N4 decreased with strain, which could be realized by substituting the C atom in g-C3N4 with isoelectronic Si atom, upon which ΔGH* ° = 0. The authors proposed that the dynamic strain promoted the binding energy

The geometries all came with high adsorption energies, inhibiting potential diffusion processes and therefore indicating stable configurations on the time scale of transfer of electrons. The intermediates of OH and OOH radicals were stabilized primarily through the interaction of electrostatic and hydrogen bonding (Eads = 0.61 and 0.66 eV, respectively). On the other hand, the single oxygen intermediate interacted with a substrate nitrogen through covalent bonding, forming an N-oxide group, which was evidenced by its high adsorption energy of −2.73 eV. A similar approach was also used to study the second halfreaction, i.e. H+ reduction (eqs 13−15) on the same substrate. 4H+ + 4e− → 2H 2

(13)

E: * + H+ + e− → *H

(14)

(15)

7269


7270

Mass ratio of urea and Ti (OH)4 (70:30)

Mass ratio of BmimBF4 to 300 W Xe lamp with a 420 nm cutoff filter DCDA: 6 (8 h reaction) Molar ratio of TiO2 to Bdoped g-C3N4: 6

ZnO (6 wt%)

N-TiO2/g-C3N4

TiO2/B-doped g-C3N4

ZnO/g-C3N4

500 W Xe lamp with a 420 nm cutoff filter (4 h reaction)

300 W Xe lamp (UV−vis) (12 h reaction)


300 W simulated solar Xe arc lamp (4 h reaction)

Pd (11.6 wt%)

15 W energy-saving daylight bulb (10 h reaction)

300 W Hg lamp (UV light) (8 h reaction)

Pd nanotetrahedrons/gC3N4

Molar ratio of Mo to melamine: 4%

Mo-doped g-C3N4


Pt (0.75 wt%)

-

g-C3N4 nanosheets (melamine precursor)


Pt/g-C3N4

-

g-C3N4 nanosheets (DCDA precursor)


Pt (2 wt%)

-

g-C3N4 (urea precursor)


Light source (Reaction duration)

Pt/g-C3N4

-

Dopant/cocatalysts

g-C3N4 (melamine precursor)

Composite type g-C3N4 (melamine hydrochloride precursor) CO: 0.41 mM h−1 g-C3N4 (melamine precursor) CH3OH: Trace


CO2 reduction rate: 45.6 μmol g−1 h−1

CH4: 106 μmol

CO: 14.73 μmol

CH4: 0.18 μmol g−1 h−1 H2: 6.50 μmol g−1 h−1 CO: 14.90 μmol g−1 h−1 C2H5OH: 1.72 μmol g−1 h−1

CH4: ∼0.10 μmol CH3OH: ∼0.12 μmol HCHO: ∼0.05 μmol

CH4: 13.02 μmol g−1

CH4: 123 μmol g−1

CH4: 6.7 μmol B-doped g-C3N4 CH4: 68 μmol g-C3N4 CO2 reduction rate: 9.4 μmol g−1 h−1

CO: ∼400 μmol g−1 CH4: ∼40 μmol g−1 Pure g-C3N4 CH4: 2.55 μmol g−1 Pure g-C3N4 CH4: ∼0.028 μmol CH3OH: ∼0.042 μmol HCHO: ∼0.020 μmol g-C3N4 nanosheets CH4: Trace H2: 2.30 μmol g−1 h−1 CO: 0.70 μmol g−1 h−1 C2H5OH: 0.40 μmol g−1 h−1 P25 CO: 3.68 μmol g-C3N4

CH3OH: 6.28 μmol g−1 h−1 C2H5OH: 4.51 μmol g−1 h−1 O2: 21.33 μmol g−1 h−1 C2H5OH: 3.64 μmol g−1 h−1 O2: 10.29 μmol g−1 h−1 CH4: ∼0.12 μmol Bulk g-C3N4 (DCDA precursor) CH4: ∼0.02 μmol CH4: 7.47 μmol g−1 Bulk g-C3N4 (melamine precursor) CH4: 2.42 μmol g−1 CO: 887 μmol g−1 g-C3N4

CO: 1.9 mM h−1

Activity (unit)

CH4: H2: 2.83 CO: 21.29 C2H5OH: 4.30

CH4: 3.57 CH3OH: 2.86 HCHO: 2.50

CH4: 5.10

-

CH4: 3.09

CH4: 6

O2: 2.07

C2H5OH: 1.24

CH3OH: -

CO: 4.63

Vis

-

-

CO2 reduction rate: 4.9

CH4: 15.82 (g-C3N4) CH4: 1.56 (Bdoped g-C3N4)

CO: 4.00 (UV−vis)

-

-

-

CO: 2.22 CH4: 3.08

-

-

-

-

UV

Enhancement factor over reference photocatalyst

Table 4. Representative Summary of the Photocatalytic Activity Enhancement of g-C3N4-Based Photocatalysts toward CO2 Reductiona

>24 h

>8 h

>36 h

N/A

N/A

>30 h

N/A

>26 h

N/A

>12 h

>11 h

Stability

N/A

1.68% (420 nm)

N/A

N/A

N/A

N/A

N/A

N/A

N/A

0.18% (420 nm)

N/A

AQY

1015 (2015)

530 (2016)

609 (2014)

703 (2014)

186 (2014)

701 (2015)

1017 (2016)

1020 (2015)

1019 (2014)

350 (2013)

353 (2012)

ref (Year)



7271

Ru complex (25 mL) g-C3N4 (50 mg)

CoCl2 (1 μmol) bpy (15 mg)

CoOx/g-C3N4 with COCl2 and bpy

Ru (3.9 μmol g−1 added)

RuP/g-C3N4

Ru complex-modified mesoporous g-C3N4

Ru (39.2 μmol g−1 added)

Co-ZIF-9 (1 mg)

Co-ZIF-9/g-C3N4

Ru complex-modified mesoporous g-C3N4

WO3 (∼65 wt%)

WO3/g-C3N4

CeO2 (3 wt%)

CeO2/g-C3N4

Molar ratio of Ag3PO4 to g-C3N4: 30%

In2O3 (10 wt%)

In2O3/g-C3N4

Ag3PO4/g-C3N4

g-C3N4 (10 wt%)

UiO-66/g-C3N4 nanosheets

SnO2‑x (42.2 wt%)

BiOI (7.4 wt%)

BiOI/g-C3N4

SnO2‑x/g-C3N4

g-C3N4 (0.1 g)

Dopant/cocatalysts

Bi2WO6/g-C3N4

Composite type

Table 4. continued


400 W Hg lamp with a NaNO2 solution filter (λ > 400 nm) (5 h reaction) 400 W high pressure Hg lamp with a 400 nm cutoff filter (1 h reaction)

450 W Xe lamp with a NaNO2 solution filter (5 h reaction)

CO: 10.2 μmol H2: 2.8 μmol

HCOOH: 8.8 μmol CO: 0.7 μmol H2: 0.1 μmol

HCOOH: 1854 nmol

H2: 1482 nmol CO: 2499 nmol HCOOH: 17248 nmol

CO: 20.8 μmol H2: 3.3 μmol

CH3OH: ∼1400 nmol

LED lamp (λ ∼435 nm) (24 h reaction) Xe lamp with a 420 nm cutoff filter (2 h reaction)



CH4: 1.1 μmol

CO: 1.35 μmol

CH4: 76.7 ppm

CO: 59.4 μmol g−1

CO: 17.23 μmol g−1 CH4: 0.82 μmol g−1 H2: 1.85 μmol g−1 O2: 9.45 μmol g−1

CO: 5.19 μmol g−1 h−1

Activity (unit)









HCOOH: Trace RuH/g-C3N4 HCOOH: 0.8 μmol CO: Trace H2: 0.1 μmol g-C3N4 CO: Trace

CH4: ∼0.025 μmol g-C3N4 CO2 reduction rate: 5.32 μmol g−1 h−1 g-C3N4 CO2 reduction rate: 9.4 μmol g−1 h−1 g-C3N4 CH3OH: ∼600 nmol g-C3N4 CO: N/A H2: N/A Mesoporous g-C3N4 H2: 354 nmol CO: Trace HCOOH: Trace Bulk g-C3N4

g-C3N4 CO: 0.24 μmol g−1 h−1 Bi2WO6 CO: 0.81 μmol g−1 h−1 g-C3N4 CO: 1 μmol g−1 CH4: Trace H2: 4.31 μmol g−1 O2: 2.70 μmol g−1 g-C3N4 nanosheets CO: 17.1 μmol g−1 g-C3N4 CH4: 22.9 ppm g-C3N4 CO: 0.98 μmol


3.47

CO: 17.23 CH4: H2: 0.43 O2: 3.5

CO: 6.4 (Bi2WO6)

CO: 22 (g-C3N4)

Vis

-

-

-

-

-

-

-

CO: H2: 28

HCOOH: 11.0 CO: H2: 1

HCOOH: -

H2: 4.19 CO: HCOOH: -

CO: H2: -

CH3OH: 2.33

CO2 reduction rate: 6.12

CO2 reduction rate: 4.27 (UV−vis)

CO: 1.38 (UV−vis) CH4: 44 (UV−vis)

CH4: 3.35 (UV−vis)

-

-

-

UV


>8 h

>20 h

N/A

N/A

>42 h

N/A

>24 h

N/A

>24 h

>16 h

>18 h

>15 h

>36 h

Stability

0.25% (420 nm)

5.7% (400 nm)

N/A

1.5% (400 nm)

0.9% (420 nm)

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

AQY

1024 (2014)

1023 (2015)

1022 (2014)

1021 (2013)

841 (2014)

670 (2014)

617 (2015)

662 (2015)

258 (2016)

649 (2014)

161 (2015)

598 (2016)

628 (2015)

ref (Year)



7272

Graphene (15 wt%)

Graphene (15 wt%)

Graphene/g-C3N4

rGO/pCN





CH4: 13.93 μmol g−1

CH4: 5.87 μmol g−1

CH4: 10.92 μmol g−1

CH4: 3.7 μmol CO: 0.1 μmol C2H6: 0.06 μmol H2: 9.6 μmol

CH4: 0.25 μmol h−1

CH4: 6.40 μmol g−1 h−1 CH4: 0.8 μmol g−1 h−1 Pt/g-C3N4 CH4: 0.06 μmol h−1 Pd/g-C3N4 CH4: 1.4 μmol CO: Trace C2H6: Trace H2: Trace Pure g-C3N4 CH4: 2.55 μmol g−1 Pure g-C3N4 CH4: 2.55 μmol g−1 Pure g-C3N4 CH4: 2.55 μmol g−1

r-P CH4: 145 μmol g−1 h−1 Pt/g-C3N4

CH4: 107 μmol g−1 h−1

H2: 30 h

>10 h

>40 h

>72 h

>24 h

N/A

N/A

Stability

0.560%

N/A

N/A

0.093% (420 nm)

N/A

N/A

N/A

AQY

777 (2015)

320 (2015)

850 (2016)

1016 (2014)

1014 (2015)

1025 (2014)

659 (2013)

ref (Year)

AQY: apparent quantum yield; N/A: not available; BA: barbituric acid; Ru: ruthenium complex; cis,trans-[Ru(4,4′-(CH2PO3H2)2-2,2′-bipyridine)(CO)2Cl2]; r-P: red phosphor; Co-ZIF-9: cobaltcontaining zeolitic imidazolate framework; LDH: layered double hydroxide; pCN: protonated g-C3N4; rGO: reduced graphene oxide; bpy: bipyridine; DCDA: dicyandiamide.

a

Mass ratio of AgBr to pCN: 30 wt%

300 W Xe lamp with a 420 nm cutoff filter (8 h reaction) 500 W Hg (Xe) lamp (24 h reaction)


(No mention of reaction time)

Pt (0.5 wt%)

Mass ratio of NaNbO3 and melamine (1:4) Pt (0.5 wt%) KNbO3 (25 wt%) Pt (0.5 wt%) Mg-Al-LDH (10 wt%) Pd (0.5 wt%)



g-C3N4 (30 wt%)

Dopant/cocatalysts

Ag/AgBr/pCN

Pd/Mg-Al-LDH/g-C3N4

Pt-g-C3N4/KNbO3

Pt/g-C3N4/NaNbO3

Pt/r-P/g-C3N4

Composite type

Table 4. continued



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Figure 142. Schematic illustration of photocatalytic reduction of CO2 with H2O to various solar fuels under light irradiation using pristine g-C3N4 as a reference photocatalyst.

Figure 143. (a) Schematic illustration of the production of CH3CHO and CH4 on bulk g-C3N4 and g-C3N4 nanosheets for the CO2 photoreduction. (b) Band structures of g-C3N4 nanosheets (left panel) and bulk g-C3N4 (right panel) with respect to the redox potentials of the reactions. Reprinted with permission from ref 1019. Copyright 2014 Royal Society of Chemistry.

half-reaction of CO2 fixation and the oxidative half-reaction of H2O to accomplish a carbon neutral cycle.1010−1013 In the past few years, g-C3N4 has been successfully employed as an effective photocatalyst to facilitate the photocatalytic CO2 reduction (as summarized in Table 4) because its high CB minimum favors the reduction half-reaction (Figure 142).187,213,1014−1018 It is shown that CO2 photoreduction process is not solely a single-step reaction. It involves a protonassisted multielectron process to yield diverse products. From the viewpoint of thermodynamic, CO2 is reduced progressively by obtaining multiple (two, four, six, or eight) electrons and hydrogen radicals to produce gaseous and liquid hydrocarbons in the order from HCOOH (liquid), CO (gas), HCHO (liquid), CH3OH (liquid), to CH4 (gas), as shown in eqs 16−20. The selectivity of the desired product is significantly affected by the nanostructure design and the choice of the photoactive components in the g-C3N4-based nanomaterials.

of hydrogen to the C3N4, where part of the mechanical energy was converted into chemical reaction energy.1005 5.2. Photocatalytic Reduction of CO2 to Renewable Hydrocarbon Fuels

The continuously increasing concentration of carbon dioxide (CO2), which is a major greenhouse gas in the atmosphere, is one of the most serious problems, contributing to global warming and climate change. Thus far, the anthropogenic emission of CO2 mostly comes from the combustion of fossil fuels in energy use as a result of the development of industry and society. Since the global energy economy will continue to rise steadily due to the growing demand on fossil resources for the next few decades, it is undoubted that mitigation of increasing CO2 emissions is pivotal to reduce it on a large scale.1006 Till now, many great strides are made to reduce the regional and global CO2 emissions. Motivated by the natural photosynthesis in green plants, photocatalytic conversion of CO2 into energy-bearing products has been regarded as an alternative option to address the diminishing of fossil resources and mitigate the greenhouse gas effect to attain solar energy conversion.1007,1008 This acts as killing two birds with one stone in view of protecting the environment and simultaneously supplying energy.1009 Naturally, the photosynthesis of plants converts CO2 with H2O to carbohydrates and O2 at room temperature under the sunlight. For the photocatalytic reduction of CO2 with H2O, it combines both the reductive

CO2 + 2H+ + 2e− → HCOOH (Eoredox = −0.61V vs NHE at pH 7)

(16)

CO2 + 2H+ + 2e− → CO + H 2O (Eoredox = −0.53 V vs NHE at pH 7)

(17)

CO2 + 4H+ + 4e− → HCHO + H 2O (Eoredox = −0.48 V vs NHE at pH 7) 7273

(18)


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Figure 144. (a) Yields of CO and H2 from the photocatalytic reduction of CO2 using CNU-BAx with different amounts of barbituric acid integrated with g-C3N4. (b) Wavelength-dependent generation of CO and H2 from the reaction system. (c) Stability test of the production of CO and H2 over the CNU-BA0.03 sample. (d) Effects of different comonomers-modified g-C3N4 for the formation of CO and H2 under visible light. (e−f) Mass spectroscopy results of the generated CO from the photoreduction of 13CO2 over the CNU-BA0.03 sample. Reprinted with permission from ref 221. Copyright 2015 Elsevier.

the design of highly efficient photocatalysts with a high selectivity for the reaction. Besides that, Wang et al. fabricated sulfur-doped g-C3N4 photocatalysts by employing thiourea as the precursor of sulfur for the reduction of CO2 to CH3OH.187 As confirmed by the DFT studies, the electrons can easily be excited from the VB to the impurity state or from the impurity state to the CB of sulfur-doped g-C3N4 owing to the incorporation of sulfur doping, which induced additional electrons, resulting in the spin polarization. As the band gap was narrowed from 2.7 to 2.63 eV, the light absorption was broadened in the sulfur-doped g-C3N4, generating more electrons and holes under the light irradiation. Thus, a higher CH3OH yield (1.12 μmol g−1) was obtained in the sulfur-doped g-C3N4 relative to the unmodified g-C3N4 (0.81 μmol g−1). Qin and co-workers applied barbituric acid-modified g-C3N4 (CNU-BAx, x is weight-in amount of barbituric acid) for the conversion of CO2 to CO.221 Besides ameliorating the physicochemical properties of g-C3N4 with enhanced visiblelight absorption, the copolymerization of urea and barbituric acid significantly facilitated the charge migration and separation via the construction of surface molecular heterojunctions. As depicted in Figure 144a, the optimal CNU-BA0.03 exhibited a drastically increased photocatalytic CO evolution, exceeding 15 times (31.1 μmol h−1) over bare g-C3N4 (2.1 μmol h−1). However, excessive doping of barbituric acid led to the decrease in the CO evolution rate, which was attributed to the destruction of g-C3N4 polymeric framework by the barbituric acid. This clearly emphasizes the importance of appropriately controlling the degree of polymerization in increasing the photoredox efficiency. Meanwhile, the trend of the products (CO and H2) corresponded well with the optical absorption of CNU-BA0.03 based on the action spectrum (Figure 144b) and also the sample possessed a high stability without an evident reduction in the product yield (Figure 144c). To further reveal the role of copolymerization, other comonomers such as ABN, ATCN and DAMN were also employed to conjugate with urea

CO2 + 6H+ + 6e− → CH3OH + H 2O (Eoredox = −0.38 V vs NHE at pH 7)

(19)

CO2 + 8H+ + 8e− → CH4 + 2H 2O (Eoredox = −0.24 V vs NHE at pH 7)

(20)

In one of the early reports, the photoreduction of CO2 to CO in the presence of H2O vapor was demonstrated by Dong and Zhang over high porosity of g-C3N4 nanomaterials synthesized from melamine hydrochloride.353 Furthermore, Niu et al. reported that the nature of the major CO2 reduction product in the presence of water vapor was modulated by the band structure of bulk g-C3N4 and g-C3N4 nanosheets.1019 The g-C3N4 nanosheets were obtained by the top down synthesis route as described in Section 2.2.3, namely thermal delamination of bulk g-C3N4 in air. It was shown that g-C3N4 nanosheets with a band gap of 2.97 eV yielded the major product of CH4, whereas bulk g-C3N4 with a smaller band gap of 2.77 eV formed the main product of CH3CHO (Figure 143a). This elucidated that the nanosheets had a larger band gap by 0.2 eV, leading to a lower VB edge by 80 meV and a higher CB edge by 120 meV. Therefore, the nanosheets provided a larger thermodynamic diving force for the hole and electron transfer by means of a greater difference in energy level between redox potentials of the reactants and band edges (Figure 143b). This indirectly led to a larger proportion of long-lived charge carriers for the nanosheets in contrast to the bulk, which was consistent with their previous findings.226 As a consequence, the formation of CH4 was more favorable due to rapid transfer of photoexcited electrons in the nanosheets to the intermediate species. This phenomenon is in good agreement with another recent investigation by Huang et al., in which the hydroxyl-rich g-C3N4 nanosheets prepared via liquid exfoliation of bulk g-C3N4 in water formed CH4 from the CO2 reduction.1020 These results provide a stepping stone into 7274


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Figure 145. Photocatalytic reduction of CO2 to (a) CO and (b) CH4 over the photocatalysts under UV−vis irradiation. CT-x denotes N-TiO2/gC3N4 with different mass ratios of urea and Ti(OH)4. (c) Charge transfer mechanism for the photoreduction of CO2 with H2O over N-TiO2/g-C3N4 sample. Reprinted with permission from ref 609. Copyright 2014 Elsevier.

Figure 146. Schematic illustration of the proposed charge transfer mechanism in the CeO2/g-C3N4 heterojunction photocatalysts upon light irradiation. Reprinted with permission from ref 258. Copyright 2016 Elsevier.

optimal In2O3/g-C3N4 hybrid nanostructure with 10 wt% of In2O3 possessed a considerable increase in the CH4 evolution, which was more than 4 times greater than that of pure In2O3 and more than 3 times higher than that of pure g-C3N4. This was accredited to the rapid interfacial charge transfer and separation of photoinduced charge carriers between In2O3 and g-C3N4, as evidenced by the transient PL spectroscopy. Since the CB of In2O3 (−0.60 eV vs NHE) was less negative than that of g-C3N4 (−0.60 eV vs NHE), the electrons from the CB of g-C3N4 could be rapidly migrated to that of In2O3. On the other hand, the VB of g-C3N4 (+1.6 eV vs NHE) was less positive than that of In2O3 (+2.2 eV vs NHE). This led to the transfer of holes from In2O3 to g-C3N4, thus oxidizing water to protons and O2. With the presence of protons, the accumulated electrons from the CB of In2O3 could effectively reduce CO2 to CH4. Benefitting from the well-contacted interfaces and wellmatched band structures of In2O3/g-C3N4, this prolonged the charge carrier lifetime for enhanced photocatalytic activity. In another study, the synthesis of nitrogen-doped TiO2 (NTiO2) with g-C3N4 was reported by Zhou et al.609 The optimal N-TiO2/g-C3N4 (denoted as CT-50) displayed higher CO and CH4 production yields of 5.71 and 3.94 μmol (over 0.1 g sample) and a high stability at room temperature compared with the pure g-C3N4 under the irradiation of UV−vis light (Figure 145a−b). Similarly, under light irradiation, the transfer of electrons and holes occurred at the heterointerface of NTiO2 and g-C3N4, spatially separating the photogenerated electrons and holes from both components, owing to the

precursor. Interestingly, a remarkably high photocatalytic performance in all comonomer-modified g-C3N4 samples was noted compared with the pure g-C3N4 (Figure 144d). This signifies that ample choices of organic comonomers can be integrated with g-C3N4 through the universal copolymerization method to improve the photoreduction of CO2 process. To verify the production of CO from the CO2 reduction, a 13Clabeled isotope tracer analysis using 13CO2 was conducted (Figure 144e−f). Based on the mass spectroscopy results, an individual peak referring to the 13CO (m/z value of 29) was obtained instead of 12CO (m/z value of 28). This concluded that the formation of CO directly stemmed from the photocatalytic reduction of CO2 rather than the dissociation of carbon impurities from the catalyst surface. Apart from the nanostructure engineering of g-C3N4, such as structure modifications, copolymerization, and elemental doping, the photocatalytic activity of CO2 reduction using gC3N4-based hybrid nanocomposites can be further enhanced via the means of constructing heterojunctions between g-C3N4 and other components to develop a distinguished and coherent interface as detailed in Section 4.0. The development of semiconductor heterojunctions by hybridizing two different semiconductors with suitable band edge potentials is considered as one of the effective avenues for accelerating the CO2 photoreduction process. For example, Cao and co-workers presented a prototype 0D-2D heterojunction system by the in situ growth of In2O3 crystals on the g-C3N4 through a solvothermal approach.649 Under visible-light irradiation, the 7275


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Figure 147. (a) Time dependent CO production over the photocatalysts. (b) CO2 adsorption isotherms of the studied samples. (c) Electron transfer process across the heterojunction interface in the UiO-66/g-C3N4 nanosheets under visible-light irradiation. (d) Time-resolved PL decay profiles of g-C3N4 nanosheets and UiO-66/g-C3N4 nanosheets. Bulk CN and CNNS denote bulk g-C3N4 and g-C3N4 nanosheets, respectively. Reprinted with permission from ref 161. Copyright 2015 John Wiley & Sons, Inc.

Figure 148. (a) Schematic illustration of the charge transfer process and reaction mechanism for CO2 photoreduction in TiO2/B-doped g-C3N4. (b) SS-SPS responses in N2 and (c) TS-SPS responses under a 322 nm laser excitation of pristine g-C3N4 (CN), B-doped g-C3N4 (6B-CN), and TiO2/ B-doped g-C3N4 (6T/6B-CN) samples. Reprinted with permission from ref 530. Copyright 2016 American Chemical Society.

CeO2, the inner electric field drove the transfer of photoinduced electrons from g-C3N4 to CeO2 as a result of energy band bending in the space charge region. On the contrary, the holes were migrated from CeO2 to the g-C3N4. Upon electron transfer to CeO2, Ce4+ trapped the photogenerated electrons to form Ce3+. As a consequence, the Ce3+ interacted with CO2 to produce CO2− and, subsequently, with increasing numbers of electrons, accompanied with the assistance of H+, CO and CH4 were generated. Therefore, the recombination of charge carriers was markedly prevented and, more importantly, the asdeveloped Ce3+ from Ce4+ reduction facilitated the formation of products from CO2−, thus speeding up the photoreduction process. As described earlier, UiO-66, which is a zirconium-based MOF with a high CO2 uptake as well as the presence of active sites for the CO2 reduction,1026 has been incorporated with gC3N4 through an electrostatic self-assembly approach.161 The resulting UiO-66/g-C3N4 nanosheets participated in photocatalyzing CO2 conversion with more than 3-fold enhancement of CO evolution (59.4 μmol g−1) in comparison with the g-

presence of a staggered gap Type II offset (Figure 145c). In the same year, Shi et al. successfully fabricated the NaNbO3/gC3N4 nanowire photocatalysts, which demonstrated a remarkably enhanced photoactivity for the CH4 production in comparison to the single-phase g-C3N4 and NaNbO3.1025 As such, the photocatalytic activity of NaNbO3/g-C3N4 hybrid heterojunctions for the CH4 evolution (6.4 μmol h−1 g−1) was almost 8 times higher than that of the individual g-C3N4 (0.8 μmol h−1 g−1). Such an enhancement of photoactivity was originated from the well-aligned overlapping band structures of g-C3N4 and NaNbO3, forming an intimate heterojunction that exhibited a Type II band alignment. In 2016, Li and co-workers reported mesostructured CeO2/ g-C3N4 (CeO2/g-C3N4) using SiO2 nanoparticles as a hard template, and Ce(NO3)3·6H2O and urea as the precursors of CeO2 and g-C3N4 by a hard-template approach.258 The enhanced photocatalytic reduction of CO2 with H2O vapor to CO and CH4 was accredited to the synergistic influence of mutual activations of g-C3N4 and CeO2 (Figure 146). Due to the construction of a heterojunction between g-C3N4 and 7276


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C3N4 nanosheets (17.1 μmol g−1) after 6 h of reactions under visible-light irradiation (Figure 147a). The superior photoactivity was due to a few concomitant factors such as a higher ability for CO2 adsorption in UiO-66/g-C3N4 nanosheets than g-C3N4 nanosheets (Figure 147b) and also a favorable electron separation from g-C3N4 to UiO-66 through an intimate interface (Figure 147c). As a result, the average lifetime of UiO-66/g-C3N4 nanosheets was strikingly longer (846.3 ns) than that of g-C3N4 nanosheets (481.4 ns) by performing the fitting to the decay curves with a triexponential function (Figure 147d). The isotope experiment using 13CO2 was carried out, and the results indicated that a m/z value of 29 associated with 13CO was observed to confirm the production of CO from CO2 to rule out any possibilities of contamination. By combining the positive effects of boron doping in the gC3N4 framework as well as the hybridization of B-doped gC3N4 with TiO2 semiconductors, Raziq and co-workers successfully fabricated TiO2/B-doped g-C3N4 hybrid photocatalysts through a copolymerization method followed by thermal calcination.530 As a result of boron doping in the gC3N4 by substituting the C atoms in the heptazine unit (refer to Section 2.3.1), the optical absorption was progressively extended from 450 nm (for g-C3N4) to ca. 500 nm (for TiO2/B-doped g-C3N4). More importantly, the increased charge separation efficiency is resulted from the trapping of positively charged holes by the boron surface states near the VB maximum and also the transfer of electrons from boron-doped g-C3N4 to TiO2 (Figure 148a). This could be further manifested by both steady-state surface photovoltage spectroscopy (SS-SPS) and transient-state surface photovoltage spectroscopy (TS-SPS) (Figure 148b−c). As illustrated in Figure 148b, the highest SS-SPS response was observed in the TiO2/ B-doped g-C3N4 sample compared with the pure g-C3N4 and Bdoped g-C3N4, establishing the most enhanced charge separation. In contrast with pure g-C3N4, B-doped g-C3N4 showed a higher response, confirming the role of B-doping to trap the photogenerated holes. Similarly, in Figure 148c, following the same trend as the SS-SPS, the TiO2/B-doped gC3N4 sample exhibits the most negative with respect to the other two samples, reflecting the longest lifetime of photogenerated charge carriers to participate in the redox reactions. As a result, the CH4 evolution from the CO2 reduction using the TiO2/B-doped g-C3N4 photocatalyst was found to be conspicuously greater, which was about 16 times more than the pristine g-C3N4, with a high AQY of 1.68% at 420 nm. In contrast with the common type of heterojunctions, which will minimize the redox power of the photoinduced holes and electrons, a solid-state Z-scheme heterojunction has been constructed to overcome the shortfalls for photoreduction of CO2.587,677 For example, Ohno et al. reported the visible-light photoconversion of CO2 to CH3OH over WO3/g-C3N4 nanocomposites, which were developed by a planetary mill.670 They concluded that the significant improvement in the photoactivity was due to the efficient charge transfer and separation between two types of photocatalysts (WO3 and gC3N4) via a solid-state Z-scheme mechanism, mimicking the artificial photosynthesis. The electron behavior between these two photocatalysts was elucidated and confirmed by a DB-PA spectroscopy. Based on this work, the high reduction ability of g-C3N4 and the high oxidation ability of WO3 were maintained, leading to an enhanced CO2 reduction activity. In another related study, Li et al. obtained Bi2WO6/g-C3N4 composites through an in situ hydrothermal process.628 The

optimal hybrid composites showed a significant visible-light absorption and an efficient charge separation across the heterointerface between g-C3N4 and Bi2WO6, following a Zscheme mechanism (Figure 149a). The overwhelmingly

Figure 149. (a) Schematic of the charge transfer mechanism in Bi2WO6/g-C3N4 by a solid-state Z-scheme heterojunction system. (b) Photocatalytic reduction of CO2 over Bi2WO6/g-C3N4-x with various amounts of g-C3N4. (c) Stability test of photocatalytic CO production over the Bi2WO6/g-C3N4-0.1 sample (mass: 0.1 g catalyst) under visible-light irradiation (λ > 420 nm). Reprinted with permission from ref 628. Copyright 2015 Royal Society of Chemistry.

enhanced activity of the binary nanocomposites for the photoreduction of CO2 to CO was 22 and 6.4 times greater than those of pure g-C3N4 and Bi2WO6, respectively (Figure 149b). However, the photocatalytic stability was not satisfactory evidenced by the reduction in the CO evolution after three cycles (Figure 149c), which was ascribed to the saturation of the adsorption sites on the surface of catalysts with products and intermediate products. In addition, He et al. calcined g-C3N4 and Sn6O4(OH)4 at 400 °C in N2 to obtain a solid-state Z-scheme SnO2−x/g-C3N4 catalyst.662 Without employing any electron mediators, the optimal SnO2−x/g-C3N4 sample with 42.2 wt% of SnO2−x loading revealed the highest photoactivity of the CO2 reduction rate, reaching a total of 22.7 μmol h−1 g−1, which was a 4.3- and 5-fold enhancement compared to those of g-C3N4 and P25, respectively (Figure 150a). The high photoactivity for the CO2 reduction originated from the good separation of charge carriers by a direct Z-scheme mechanism (Figure 150b). Upon irradiation of visible light, both SnO2−x and g-C3N4 would be excited. Due to the Z-scheme mechanism, the electrons from the CB of SnO2−x interacted with the photogenerated holes from the VB of g-C3N4, resulting in a strong reducing power of the excited electrons in g-C3N4 to reduce CO2 to energy fuels such as CO, CH4, and CH3OH. Apart from that, the same group of authors synthesized another efficient Z-scheme hybrid of Ag3PO4/g-C3N4 by an in situ deposition technique for the reduction of CO2 into various energy fuels, such as CH4, CO, CH3OH, and C2H5OH.617 The optimal Ag3PO4/g-C3N4 nanohybrid presented a total CO2 conversion rate of 57.5 μmol h−1 g−1, which exceeded the rate over pristine g-C3N4 by ca. 6.1 times (9.4 μmol h−1 g−1) under the irradiation of simulated sunlight (Figure 150c). On the 7277


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Figure 150. (a) Rate of formation of different products from the CO2 photoreduction over SnO2−x/g-C3N4 hybrid nanocomposites under the irradiation of simulated sunlight. m wt% SC denotes SnO2−x/g-C3N4 with different loadings of SnO2−x. (b) Reaction and charge separation mechanism of the visible-light-driven SnO2−x/g-C3N4 sample. Reprinted with permission from ref 662. Copyright 2015 Elsevier. (c) Formation rate of various products from the photoconversion of CO2 using Ag3PO4/g-C3N4 hybrid photocatalysts. mSC denotes Ag3PO4/g-C3N4 with different molar ratios of Ag3PO4 to g-C3N4. (d) Charge transfer mechanism of Ag3PO4/g-C3N4 nanohybrids through a Z-scheme process. Reprinted with permission from ref 617. Copyright 2015 American Chemical Society.

Figure 151. (A) Time course of the photocatalytic reduction of CO2 to CH4 and (B) total CH4 yield over the g-C3N4 and Pt-loaded g-C3N4 photocatalysts for 10 h of reaction. (C) Charge transfer mechanism for the reduction of CO2 with H2O to CH4 using Pt/g-C3N4 hybrid samples. xPt/CN denotes Pt-loaded g-C3N4 with various loadings of Pt. Reprinted with permission from ref 701. Copyright 2015 Royal Society of Chemistry. (D) Production yield of hydrocarbon with respect to different loadings of Pt (0−2.0 wt%) decorated on the g-C3N4 nanosheets under the irradiation of simulated solar light for 4 h of reaction. Reprinted with permission from ref 186. Copyright 2014 Royal Society of Chemistry.

basis of this finding, a charge transfer mechanism was proposed. There are some contradicting opinions on the charge transfer process of Ag3PO4/g-C3N4 by means of the traditional Type II heterojunction (also known as a double-charge transfer mechanism), as reported earlier by Kumar et al.832 and Jiang

et al.615 However, the present situation of CO2 photoreduction could not be explained if the concept of double-charge transfer mechanism was employed since the CB potential of Ag3PO4 was situated at + 0.45 eV, which was impossible to reduce CO2 to any chemical fuels. Moreover, it is noteworthy that upon 7278


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Figure 152. Adsorption configurations for CO2 on the (a) Pd {100} and (b) Pd {111} facets accompanied with the adsorption energy, Ea, according to the geometry optimization simulations. (c−d) Optimized configurations for the adsorption of CO2 on the Pd {100} and Pd {111} facets. Potential energy along the bond length O−C of the CO2 molecule adsorbed on the Pd {100} and Pd {111} facets under neutral conditions or charged with two electrons, from which the activation energy EB of CO2 is subtracted. (e) Model systems and (f) calculated potentials with work functions for C3N4, Pd {100}, and Pd {111}. (g) Energy level diagrams for C3N4 and Pd before and after forming the interfaces for electron transfer based on first-principles calculations. Ec, Ev, and Ef denote the CB, VB, and Fermi levels. Reprinted with permission from ref 703. Copyright 2014 Royal Society of Chemistry.

by a chemical reduction approach in the ethylene glycol solution.701 In contrast to the frequently employed high-power Xenon arc lamps from the literature, our work rendered the photocatalytic process more economically feasible and practically viable by utilizing a low-power 15 W energy-saving daylight bulk as the light source. As demonstrated in Figure 151A− B, the photocatalytic CH4 production over the 2Pt/g-C3N4 with an optimal 2 wt% content of Pt could be achieved up to 13.02 μmol g−1 after 10 h of light irradiation under ambient conditions, which was 5.1-fold greater than the pure g-C3N4 (2.55 μmol g−1). Hence, the noticeably improved photoactivity of Pt/g-C3N4 was assigned to the increased light absorption and effective interfacial electron transfer from g-C3N4 to Pt due to its lower Fermi level to prohibit the recombination of electron− hole pairs (Figure 151C). Similarly to another previous report by Yu and co-workers, they employed Pt/g-C3N4 hybrid photocatalysts for the reduction of CO2 under the irradiation of 300 W simulated solar light.186 With the optimal content of Pt, CO2 was reduced to single-carbon compounds, i.e. CH4, CH3OH and HCHO in the liquid phase reaction system (Figure 151D). With the introduction of Pt, the overpotential was dramatically reduced, thus facilitating the electron transfer for the photofixation of CO2. In addition to Pt cocatalysts, Bai and co-workers deposited Pd cocatalysts with various facets, namely {100} and {111}, on the 2D g-C3N4 nanosheets to examine the photoreduction of CO2.703 The synthesis procedure has been described earlier in Section 4.1.2. The facet-dependent CO2 reduction process could be theoretically analyzed by the first-principles calculations. For any chemical reactions, the first predominant

light irradiation, Ag was formed due to the photoreduction of Ag+ from Ag3PO4. In view of that, Ag could serve as an electron mediator and a charge transmission bridge to develop a Ag3PO4/g-C3N4 Z-scheme system by interacting the electrons from the CB of Ag3PO4 and the holes from the VB of g-C3N4 (Figure 150d).617 Another interesting argument was that since the Fermi level of metallic Ag was less negative than the CB potential of Ag3PO4, the transfer of electrons from Ag3PO4 to metallic Ag was feasible instead of exhibiting the SPR effect by the Ag metals. In this regard, the high efficiency of Z-scheme process in the Ag3PO4/g-C3N4 hybrid photocatalysts promoted the overall CO2 reduction toward a sustainable solar fuel production. In the above discussion for most of the studies, the reaction products from the CO2 photoreduction are mainly single-carbon molecules, namely CH4, CO and CH3OH. To date, there is still a paucity of reports on the formation of multicarbon compounds from the photoconversion of CO2. Herein, this study successfully achieved the generation of C2H5OH (a higher carbon chain of hydrocarbon) as liquid fuels over the Z-scheme g-C3N4-based system albeit the formation rate is still relatively low compared to CO and CH3OH. Thus, intensive studies in this area to enhance the rate of production of multicarbon molecules from the conversion of CO2 are desirable and underway. The incorporation of noble metals with g-C3N4 is also a fascinating strategy for the development of noble metal/g-C3N4 heterojunction photocatalysts for effective CO2 reduction to value-added chemicals. Recently, our research group has demonstrated an enhanced reduction of CO2 to CH4 using Pt-loaded g-C3N4 hybrid photocatalysts, which were prepared 7279


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step involves an adsorption process between a reactant molecule and the catalyst surface. It was found that the adsorption energy of CO2 on the Pd {100} (Ea = 0.064 eV) was lower than that for Pd {111} (Ea = 0.230 eV), inferring the remarkably improved adsorption of CO2 on Pd {111} facets (Figure 152a−b). Moreover, the activation barrier EB was reduced from 7.15 to 3.98 eV and from 6.79 to 4.15 eV for Pd {111} and Pd {100} facets, respectively, when two electrons were brought to the surfaces to initiate the activation of CO2 (Figure 152c−d). This corroborated that Pd {111} facets were more reactive for the reduction of CO2. Therefore, the CO2 reduction showed a better photoactivity on the Pd {111} facets than Pd {100} facets. In essence similar to the Pt cocatalysts, upon light irradiation, the photoexcitation of electrons took place from the VB to the CB in g-C3N4. Due to larger work functions of Pd {100} (5.05 eV) and Pd {111} (5.23 eV) compared with the g-C3N4 (4.31 eV) (Figure 152e−g), the photoexcited electrons were rapidly shuttled to the Pd cocatalysts to retard the recombination of charge carriers, resulting in increased CO2 reduction to CH4, CO and C2H5OH. Apart from that, Maeda and co-workers hybridized g-C3N4 with a low quantity of a molecular ruthenium complex, cis,trans[Ru{4,4′-(CH2PO3H2)2-2,2′-bipyridine}(CO)2Cl2] (denoted as Ru) as a catalytic center for the photofixation of CO2 under visible light.1021 The photocatalyst system recorded a high production yield of formic acid (HCOOH) with a selectivity of more than 80% and an excellent turnover number of exceeding 200 for 20 h of reactions. As a continuation from the previous work,1021 the same authors further examined the photoactivity of CO2 reduction by investigating the structural properties of g-C3N4 with different pore structures followed by modifying the g-C3N4 with Ru catalyst.1022 It was found that introducing porosity to the g-C3N4 markedly reduced the distance of charge transportation to the catalytic center and also enhanced the density of adsorption sites on the Ru catalyst, thus promoting the CO2 reduction to HCOOH. Similar researchers improved the existing hybrid photocatalytic system by engineering a Ru catalyst with remarkable electron transfer efficiency from g-C3N4 for the generation of HCOOH from the CO2 conversion.1023,1027,1028 As divulged in Figure 153, the Ru complex has dual vital roles: (1) to receive photogenerated electrons from the g-C3N4, and (2) to act as an active catalytic site for the photofixation of CO2. As a result, the structure of the Ru metal complex was critical in affecting the photoactivity. They employed four different types of Ru-based complexes, trans(Cl)-[Ru(bpyX2) (CO)2Cl2] (bpyX2 = 2,2′bipyridine with substituents X in the 4-positions, X = H, CH3, PO3H2 or CH2PO3H2), which were utilized as kinetic promoters for the CO2 reduction.1023 Notably, the adsorption of RuCP and RuP occurred on the g-C3N4 surface ascribed to the acid−base interactions from both components. Nevertheless, RuH and RuMe could not adsorb on the g-C3N4 surface owing to the absence of an anchoring group. Therefore, a strong electrostatic interaction can be formed between amino groups of g-C3N4 and the Ru-based catalyst that contains acid functional groups. This resulted in the fast injection of electrons from g-C3N4 to the Ru complexes, facilitating CO2 conversion reactions with an AQY of 5.7% at 400 nm and a superior turnover number of more than 1000. Similar phenomena have also been reported by Ye et al. for the rapid transfer of photoexcited electrons from g-C3N4 to a Fe complex upon the light irradiation when the g-C3 N 4 was modified with

Figure 153. Photocatalytic reduction of CO2 to HCOOH over a Ru complex coupled with g-C3N4 photocatalyst along with the electron transfer from CB of g-C3N4 to the Ru catalyst. Different types of Rubased complexes are shown. Reprinted with permission from ref 1023. Copyright 2015 John Wiley & Sons, Inc.

ferrocenecarboxaldehyde.566 As such, the photocatalytic performance was distinctly maximized by strengthening the electronic interactions between g-C3N4 and Ru catalyst, which led to high efficiency of electron migration to the active sites of Ru catalyst. This research work paves a new doorway in merging the organic polymeric g-C3N4 photocatalysts with surface organometallic chemistry to modulate the electronic property for artificial photosynthesis. Moreover, it is well-known that separation of charge carriers and reaction kinetics promoted by a cooperative binary catalyst system has been enthusiastically explored. It is the long-term interest by most researchers to pursue a noble metal-free system to replace with a transition metal-based cocatalyst accompanied with the g-C3N4 semiconductor for the photoactivation of CO2. To date, cobalt ions linked with appropriate organic ligands were utilized as effective electron transport channels for enhancing the reduction of CO2 and H+ ions,1029 while other cobalt components such as cobalt-oxide-phosphate (CoPi) and CoOx were functioned as oxidative cofactors for water splitting.685,983,1030,1031 It is aimed to incorporate the redox functions of cobalt species into an individual photocatalytic system comprising both oxidation and reduction reactions to effectively photoconvert CO2 into useful chemicals. In this regard, Lin et al. designed a photochemical system consisting of g-C3N4 as a semiconductor photocatalyst, Co2+ ion coordinated with a bipyridine ligand (Co(bpy)32+) as an electron mediator for supporting the reduction reactions and CoOx as an oxidative cocatalyst for the photochemical conversion of CO2 to CO with an excellent stability (Figure 154a−b).1024 Furthermore, the action spectrum for the formation of CO coincided well with the UV−vis absorption spectrum of g-C3N4 (Figure 154b), inferring that the photoreduction of CO2 was strongly influenced by the 7280


Chemical Reviews

Review

Figure 154. (a) Synergistic interaction of g-C3N4 (g-CN) with Co−bipyridine complex and CoOx for the photochemical reduction of CO2 to CO under visible-light irradiation. (b) Wavelength-dependent CO and H2 production and the UV−vis absorption spectrum of g-C3N4 (dashed line). The inset of (b) shows the time dependence of the rate of formation of CO and H2. Reprinted with permission from ref 1024. Copyright 2014 American Chemical Society. (c) Cooperative effect of g-C3N4 and Co-ZIF-9 for the photocatalytic CO generation from CO2 conversion under visible-light illumination. Reprinted with permission from ref 841. Copyright 2014 Royal Society of Chemistry.

Figure 155. (a−b) Total CH4 evolution from the CO2 reduction over the Ag/AgX/pCN photocatalysts. (c) Stability test for the photoreduction of CO2 over the 30Ag/AgBr/pCN sample. Schematic of the band structures of (d) Ag/AgCl/pCN and (e) Ag/AgBr/pCN with respect to the conversion of CO2 to CH4 under the light irradiation. Reprinted with permission from ref 850. Copyright 2016 Elsevier.

followed by integrating it with g-C3N4, which acts as a sensitizer to reduce CO2. Thus, g-C3N4 serves as a light-absorbing semiconductor photocatalyst, while Co-ZIF-9 behaves as a microporous crystalline cocatalyst to facilitate the capture and conversion of CO2 as well as accelerate the light-triggered charge separation. On the other hand, bipyridine and TEOA function as an electron mediator and an electron donor, respectively. Even in the absence of noble metals, the hybrid system still attained a high stability and an AQY of 0.9% at 420 nm. It is envisioned that engineering a g-C3N4-based ternary nanocomposite will be a potential candidate for the CO2 photoreduction. Presently, potassium niobate (KNbO3) with a ABO3 perovskite assembly and a semiconductor behavior is drawing much research fascination in the areas of solar cells and materials engineering owing to its amazing features including piezoelectric and ionic conductive. In regard of this, Pt/ KNbO3/g-C3N4 ternary heterostructures were constructed by

generation and separation of charge carriers. The hybrid photosystem showed an AQY of 0.25% at 420 nm. This finding puts a crucial step forward for artificial CO2 conversion using metal-free g-C3N4 as a light harvester and CO2 activator, and cobalt species as redox promoters. It is expected that the quantum yield efficiency for this redox catalysis system can be further increased by modifying the pristine g-C3N4 with elemental doping, copolymerization and supramolecular preorganization approach. Hence, exploring novel functionalities of the g-C3N4 nanostructures coupled with redox catalysis is essential in the materials chemistry and science to promote remarkably enhanced photoactivity. In another related work by employing transition metal ions with multiple redox states and organic ligands, Wang et al. integrated g-C3N4 with a cobalt-containing zeolitic imidazolate framework (Co-ZIF-9) for the photochemical reduction of CO2 to CO (Figure 154c).841 It is noted that CO2 can be captured and concentrated by Co-ZIF-9 in the microporous network 7281


Chemical Reviews

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Figure 156. (a−b) Total production of CH4 over the rGO/pCN hybrid photocatalysts with different rGO contents and the 15rGO/CN sample for comparison. (c) Time course of the rate of production of CH4 and also the control experiments. (d) Schematic illustration of charge transfer for the CO2 reduction to CH4 in the rGO/pCN sample under visible-light irradiation. Reprinted with permission from ref 777. Copyright 2015 Elsevier.

AgCl (Figure 155d−e). The considerably increased photoactivity of Ag/AgBr/pCN was due to the metallic Ag possessing the SPR effect and the presence of heterojunction structure between AgBr and pCN to effectively separate the photogenerated charge carriers upon light irradiation. Fascinatingly, the Ag/AgBr/pCN exhibited a staggered gap Type II band alignment in comparison to the Ag/AgCl/pCN with a straddling gap Type I offset. On account of well-aligned electronic band potentials of AgBr and pCN (Type II), the resulting Ag/AgBr/pCN spatially separated the photogenerated electrons and holes apart from each semiconductor to suppress the recombination process. In contrary, for Type I (Ag/AgCl/ pCN), the shortfall would be the accumulation of all charge carriers in a single pCN component, leading to a higher recombination rate without attaining an overall enhancement of the charge separation efficiency. Thus, this lowered the photocatalytic CH4 generation. This study has demonstrated the rational importance of engineering effective g-C3N4-based heterojunction photocatalysts with different charge transfer pathways in view of the band structures and redox potentials of the desired products formed. Up to now, immersed research interest has been devoted to the development of cost-effective g-C3N4-based photocatalytic systems for the conversion of CO2. Considering the advantages of the composite heterojunction, Yuan et al. developed red phosphor (r-P) coupled with g-C3N4 (referred as r-P/g-C3N4) to catalyze reduction of CO2 to CH4 under a 500 W xenon arc lamp.659 They reported a nearly 3-fold improvement in the CH4 production yield for the optimal r-P/g-C3N4 with 30% content of g-C3N4 (295 μmol h−1 g−1) as compared to pure gC3N4 (107 μmol h−1 g−1). The enhancement was ascribed to the effective separation of photoinduced electrons and holes across the r-P/g-C3N4 heterojunction. Their work has provided new inroads on the development of metal-free heterostructured photocatalysts composed by inexpensive, nontoxicity and earthabundant elements for artificial photosynthesis.

Shi et al. and the nanohybrids exhibited superior visible-light photocatalytic performance toward CO2 reduction to CH4 by a factor of 4 when compared with Pt/g-C3N4.1014 This was contributed by the improved electron transfer from g-C3N4 to KNbO 3 and finally to Pt cocatalysts via the formed heterojunction interface in a ternary hybrid. Since silver halide AgX (X = Cl and Br) is a light-sensitive compound, photoreduction of Ag+ to metallic Ag upon the light irradiation can easily happen in the AgX materials. As a consequence, a Schottky barrier is formed at the Ag/AgX interface due to the dissimilar work functions of Ag and AgX. Seeing that AgCl and AgBr possess different band structures and work functions, the electronic charge transfer properties will be certainly different. It is of utmost interesting to investigate the emerging roles of halide ions on the photoreduction of CO2 accompanied with the metallic Ag and gC3N4 in a ternary nanocomposite system. Moreover, less attention is put on comprehending the effect of different categories of AgX hybridized with g-C3N4 with respect to the energy band structures and photoactivity of CO2 reduction. To this end, our research group has designed heterostructured Ag/ AgX/pCN (X = Cl and Br) for the photoconversion of CO2 to CH4 in the presence of H2O vapor under a typical daylight bulb.850 As delineated in Figure 155a, the photocatalytic activity of the optimal 30Ag/AgBr/pCN with a content of 30 wt% AgBr (10.92 μmol g−1) was 4.2 and 34.1 times higher than those of single-phase pCN (2.58 μmol g−1) and AgBr (0.32 μmol g−1), respectively. On the other hand, a maximum improvement of 1.3-fold was achieved for the 30Ag/AgBr/pCN (10.92 μmol g−1) in relative to the optimal 30Ag/AgCl/pCN with a 30 wt% AgCl (8.51 μmol g−1) by comparing different AgX (Figure 155b). The 30Ag/AgBr/pCN sample manifested a high catalytic stability even after four runs of reaction (Figure 155c). Owing to the higher work functions of AgBr (5.3 eV) and AgCl (4.8 eV) than that of Ag (4.25 eV), the electrons were migrated from the plasmonic Ag to the CB of AgBr or 7282


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the improvement of each g-C3N4-based photocatalytic system. However, it is worth mentioning that the current apparent quantum efficiency is still low for the commercial applications. Furthermore, the exact reaction mechanism for the CO2 photoreduction still remains to be unraveled and it is not clear up to now. Less emphasis is placed on understanding the photocatalytic reaction steps. Thus, it is of utmost significance to examine the reaction pathways to fully elucidate the fundamental enhancements toward the CO2 reduction process. Moreover, it should be highlighted that the production of carbon-containing compounds from the CO2 conversion requires confirmation. For that reason, the isotopic labeling analysis using 13CO2 as the reactant is indispensable to prove and verify the obtained hydrocarbons stemmed from the photofixation of CO2 in order to exclude the possibility of photodissociation of the organic impurities or even carboncontaining catalysts.

Among various reduction cocatalysts, carbon-based nanostructured material such as graphene has received merit attention due to its high electron mobility. Based on the unique properties of 2D graphene and g-C3N4, the incorporation of g-C3N4 with graphene has spurred tremendous interest for a cornucopia of artificial photosynthesis. Following that, our research group utilized urea and GO as the precursors to synthesize graphene/g-C3N4 nanocomposites via a one-pot impregnation-thermal reduction strategy.320 This was the first report on the visible-light photocatalytic activity of graphene/gC3N4 heterostructures toward CH4 formation from the CO2 reduction. The resulting graphene/g-C3N4 sample showed a 2.3-fold improvement (5.87 μmol g−1 CH4) over pure g-C3N4 (2.55 μmol g−1 CH4). As a whole, this pioneer study has demonstrated a proof-of-concept for the use of graphenemodified g-C3N4 as a metal-free photocatalyst for solar-toenergy applications, in which graphene can be utilized as a reduction cocatalyst to increase the photocatalytic reduction of CO2 to CH4. Based on our former work, the sublimation amount of urea was difficult to be controlled during the thermal annealing process and the main challenge still revolved in the development of an intimate interfacial contact between graphene and gC3N4. Additionally, it was targeted to improve the interfacial coupling between graphene and g-C3N4 to form synergistic 2D/2D heterointerfaces. Therefore, to overcome this bottleneck, further improvements of our former synthesis were subsequently conducted through altering the surface charge of g-C3N4. The g-C3N4 was acid-treated with HCl to form protonated g-C3N4 (pCN) to ease the coupling with rGO to develop rGO/pCN samples driven by electrostatic attraction. For comparison, the rGO/pure g-C3N4 (rGO/CN) constructed without the surface charge modification of g-C3N4 was also prepared. It is interesting to note that the optimal 15% rGO/pCN (15rGO/pCN) photocatalyst exemplified the enhanced photocatalytic reduction of CO2 for CH4 generation (13.93 μmol gcatalyst−1) with a photochemical quantum yield of 0.560%, which was 5.4 and 1.7 times higher than those of pCN (2.58 μmol gcatalyst−1) and 15rGO/CN (8.29 μmol gcatalyst−1) samples, respectively (Figure 156a−b). However, further increasing the rGO content to 20 wt% (20rGO/pCN) resulted in a decrease in the total CH4 evolution. This was ascribed to the excessive rGO content covering the active sites of pCN, shielding them from absorbing visible light to generate more charge carriers. Hence, there always exists an optimum loading of rGO for achieving the maximum photoactivity, which was in principle identical to any dopants such as metal and metal oxide. Meanwhile, control experiments confirmed that no substantial generation of CH4 was evolved for all cases as indicated in Figure 156c, clarifying visible light, photocatalyst and reactant feeds (CO2 and H2O) were indispensable for the photoredox process. The conspicuously improved visible-light photoactivity of rGO/pCN nanoarchitectures over pCN and rGO/CN was because of the controlled ratio of rGO with pCN forming the nanohybrids as well as a well-contacted 2D/2D interfacial region between rGO and pCN at the heterojunction. As such, this led to outstanding charge separation to retard the electron−hole recombination (Figure 156d). In short, the above studies have established unbounded prospects of converting CO2 into energy-bearing solar fuels for the sustainable energy conversion over various types of g-C3N4based hybrid nanostructures. The present research accentuates the scientific aspects of photocatalytic CO2 reduction as well as

5.3. Photocatalytic Degradation of Pollutants and Bacteria Disinfection

With significant industrialization and rapid population growth, a wide range of hazardous and toxic pollutants are endlessly being emitted into the environment, which not only raises environmental concerns, but poses a threat to human health and life.1032 With the aim to realize the sustainable growth of human society and to preserve the environment, the photocatalytic degradation of pollutants and environmental remediation constitute an important and wide-impact concern of the scientific community. In this respect, polymeric g-C3N4, as a metal-free and sustainable visible-light photocatalyst, has garnered incessant research interest due to its promising application in the degradation of pollutants. In general, the photocatalytic degradation reactions over g-C3N4 reported in literature could be categorized into two types: gas phase degradation of pollutants and liquid phase removal of organic pollutants and toxic ions. Table 5 summarizes the recent advances on the photocatalytic pollutant degradation using gC3N4-based nanocomposites. Figure 157 depicts the schematic of photocatalytic degradation of pollutants under light irradiation over pristine g-C3N4. Upon light illumination, highly reactive electrons and holes initiate oxidation and reduction reactions on the surface of g-C3N4. The photoinduced electrons reacts with adsorbed molecular O2 to produce •O2− superoxide anion radical, which could contribute to the production of •OH radicals and H+.921 It is believed that the reaction of •OH radicals with organic pollutants will eventually result in the mineralization of these compounds.145 At the same time, in aqueous systems, the photogenerated holes react with surface OH− groups to give •OH radicals. Detailed mechanisms underlying the photocatalytic degradation of pollutants over gC3N4-based will be described in this section. In addition, the use of g-C3N4 in bactericidal and antifungal applications will also be reviewed. 5.3.1. Gas Phase Degradation of Pollutants. Nitrogen dioxide and nitric oxide, equally denoted as NOx, are categorized as air pollutants with growing environmental concern because they are responsible for atmospheric pollutions such as urban smog and acid rain.1049 Sano and co-workers have for the first time reported the application of gC3N4 photocatalysts for the oxidation of NO or acetaldehyde.506 In their preliminary studies, the authors observed low photocatalytic rates over g-C3N4, which was attributed to its small specific surface area of 7.7 m2 g−1. By subjecting g-C3N4 7283


P3HT (Mass ratio 0.7 wt%)

N/A

Weight ratio of PANI: g-C3N4 (0.5:10) Weight percentage of Ptp to g-C3N4 (1.5%)

P3HT (20% conc) Graphene (30 mg) MIL-125(Ti) (93 wt%)

C/N = 0.72 H content = 1.93 wt% N/A

Molar ratio of C, M, and Mp (1:1:1) Na conc (0.05 M)

K conc (0.05 M)

K mass fraction (0.22)

Fe mole percentage (2 mol%)

80 mL solution containing organic substrates and H2O2 (30%)

Weight ratio of eutectic salts to melamine: 30

PMDA/g-C3N4

P3HT/g-C3N4

PANI nanorod arrays on g-C3N4

PANI/g-C3N4

Graphene/g-C3N4/P3HT (G-g-C3N4P3HT)

7284

MCA/g-C3N4

C-M-Mp/g-C3N4

K/g-C3N4

K/g-C3N4

Fe/g-C3N4

Fe/g-C3N4

K-Na/g-C3N4

Na/g-C3N4

CM/g-C3N4

MIL-125(Ti)/g-C3N4

Ptp/g-C3N4

Mass ratio of PVC and gC3N4 (1:300) N/A

CPVC/g-C3N4

Degradation of RhB (k = 0.017 min−1)

Degradation of RhB by H2O2 (0.05 M) activated by Fe/g-C3N4 (∼95% degradation after 9 h)

Degradation of RhB (∼99% degradation after 30 min)

Degradation of phenol (k = ∼0.04)



250 W high pressure Na lamp (λ: 400−800 nm)

500 W Halogen lamp with a 420 nm cutoff filter

Sunlight irradiation

250 W high pressure Na lamp (λ: 400−800 nm) 250 W high pressure Na lamp (λ: 400−800 nm) 500 W Xe lamp with a 420 nm cutoff filter

LED white light

12 W LED module emitting at 420 nm

Degradation of RhB (Efficiency = ∼90%) Degradation of RhB

1 kW Hg lamp with a cutoff filter (λ > 420 nm)


Philips lamp (40 W/230 V) (full emission)

Degradation of RhB (Efficiency = ∼90%) Degradation of RhB (k = 0.0624 min−1, Efficiency = ∼95.2%) Degradation of RhB (k = 0.062)

250 W high pressure Na lamp (λ: 400−800 nm)

H2O2 Fe/g-C3N4 without H2O2 g-C3N4

Bulk g-C3N4 Pure g-C3N4 nanosheets

g-C3N4

g-C3N4

g-C3N4

Bulk g-C3N4

Pure g-C3N4

DCDA/g-C3N4

Pure g-C3N4

P3HT/g-C3N4 PANI/g-C3N4 P3HT/g-C3N4

Pure g-C3N4


Degradation of RhB (Rate = ∼90%)

Pure g-C3N4

Pure g-C3N4

Pure g-C3N4

Pure g-C3N4

Bulk g-C3N4 synthesized from DCDA (DCDACN) Pure g-C3N4

Unmodified g-C3N4

Reference photocatalyst

500 W Xe lamp with a super cold filter (λ: 400−700 nm)

Decomposition of MB (k = 0.18134 h−1) Degradation of MB (Efficiency = 78.6%) Degradation of MO (Efficiency = 99.8%) Degradation of MB (Rate = 92.8%)


Degradation of RhB (k = 0.0277 mg L−1 min−1) Degradation of MO (k = 3.34 h−1)

1 kW Hg lamp with a cutoff filter (λ > 420 nm)

30 W visible LED

Light source

300 W Xe lamp (output light intensity is 1 sun, AM1.5, 100 mW cm−2) with a 420 nm cutoff filter 300W iodine tungsten lamp with a 400 nm optical filter 300 W Xe lamp with a 420 nm cutoff filter

Degradation of MO (k = 0.1879 h−1)

Degradation of RhB: 60% in 10 min. (k = 0.062)

Hydrogen (1.93 wt%)

Mesoporous hollow spheres comprised tris-triazine based g-C3N4 nanosheets (MCA-CN) DPY-doped g-C3N4 (DPY-CN)

DPY (Mass fraction 7 wt%)

Removal of NO: 59.4%

TAP (1 wt%)

Dopant/cocatalysts

g-C3N4 copolymerized with TAP

Composite type

Evaluation of photocatalytic activity (rate constant, k)

5.24 4.00 2.25

2.00

9.54

2.70

2.16

10

1.49

Vis

-

-

-

-

-

-

-

-

-

-

10.00

19.00 9.50

7.00 4.50

3.30

6.47

3.56

16.00

4.50

5.64

2.09

∼1.13 ∼1.29 3.00 (UV− vis) -

-

-

-

-

-

-

-

-

UV


Table 5. Representative Summary of the Photocatalytic Activity Enhancement of g-C3N4-Based Photocatalysts toward Degradation of Pollutantsa

•OH, •O2−

•OH, •O2−, h+ N/A

•OH, •O2− •OH, •O2− N/A

N/A

•OH, •O2− •OH, •O2− •OH

N/A

N/A

•OH

•OH, •O2−

O2•‑, OH• O2•‑

•O2−

O2•‑, •OOH

•OH, •O2−

N/A

Main active species

556 (2015)

195 (2009)

844 (2015) 466 (2013) 454 (2013) 467 (2014) 557 (2014) 558 (2015) 561 (2015) 551 (2014)

958 (2012)

923 (2012) 658 (2015)

268 (2014) 219 (2013) 929 (2015) 657 (2014)

577 (2015)

568 (2015) 466 (2013)

ref (Year)



Weight content of Na2WO4 to dicyandiamide (1.0 wt%) Europium content (0.38 wt%)

[WO4]2−/g-C3N4

7285

Ag (4 wt%)

Ag (8 wt%)

N/A

Ag (1.0 wt%)

Ag (0.5 wt%)

Pt (2.0 wt%)

Ag (10 wt%)

Pd (1.5 wt%)

Ag (5 wt%)

Ag (3 wt%)

O (7.98 at%)

(NH4)2HPO4 (6 wt%)

HCCP, GndCl (10 wt%)

Ag/g-C3N4

Ag/g-C3N4

Au/g-C3N4

Ag/g-C3N4

Core−shell Ag/g-C3N4

Pt/g-C3N4

Ag/g-C3N4

Pd/mpg-C3N4

Ag/g-C3N4

Ag/mpg-C3N4

O-doped g-C3N4

P-doped g-C3N4

P-doped g-C3N4

Er3+/g-C3N4

Degradation of RhB (∼100% degradation after 10 min)

Degradation of MO (92.6% degradation after 2.5h) Degradation of MO (100% degradation after 3h) Degradation of MO (k = ∼0.85 h−1) Degradation of MO (k = 0.01 h−1) Degradation of PCP (∼100% degradation after 7 h) Removal of NOx in air (Removal ratio = 54.3%) Degradation of bisphenol A (k = ∼0.01 min−1) Degradation of MO (92% degradation after 1 h) Degradation of RhB (100% degradation after 25 min) Degradation of MB (81% degradation after 3 h) Degradation of RhB (k = 0.0466 min−1)

Degradation of RhB (∼100% degradation after 90 min) Degradation of RhB (∼100% degradation after 40 min) Degradation of MB (k = 0.00605 min−1) Degradation of MO (k = 0.01425 min−1) Degradation of NDY-GL (k = 0.01305 min−1) Degradation of RhB (k = ∼0.08 min−1)

5 g of Ni(NO3)2·6H2O, 12 g urea Er content (2 wt%)

NiO/g-C3N4

Fe3+/g-C3N4

g-C3N4/NaYF4: Yb, Tm

Degradation of RhB (∼100% degradation after 50 min) Degradation of RhB (∼80% degradation after 105 min) Degradation of RhB (k = 0.0236 min−1)

Degradation of MB (k = 0.0121 min−1)




Amount of TBOT (0.75 mmol) Weight ratio of g-C3N4 to NaYF4: Yb, Tm (2:1 w/w) Fe3+ content (0.5%)

Ti/g-C3N4

Europium/g-C3N4

Molar content of ZnCl2 (0.580)

Dopant/cocatalysts

Microporous NaCl/ZnCl2-derived C3N4

Composite type

Table 5. continued

300 W tungsten halogen lamp with a 420 cutoff filter 350 W Xe arc lamp with simulated sunlight irradiation 300 W tungsten halogen lamp with a 420 cutoff filter 300 W tungsten halogen lamp with a 420 cutoff filter 500 W halogen tungsten lamp with a 420 cutoff filter 250 W high pressure Na lamp (λ: 400−800 nm) 300 W Xe lamp with a 420 nm cutoff filter


500 W Xe lamp with 420 nm cutoff filter





White LED (λ > 420 nm)

100 W Halogen lamp with UV-stop feature

•OH

∼1.14

g-C3N4

g-C3N4

g-C3N4

mpg-C3N4

g-C3N4

-

-

-

-

-

•O2− N/A

∼3.00

•OH, •O2− •OH, •O2− •OH, •O2− •OH, •O2− •OH, •O2− N/A

•OH, •O2− •OH, •O2−

•O2

−

N/A

4.05

3.38

∼1.43

1.31

5 (UV−vis)

mpg-C3N4

1.37

1.80 2.00 ∼2.00

3.46

3.23

7.00

•O2−

N/A

∼1.11 3.78 7.09 4.98

N/A

1.78 2.48 ∼1.53

•OH, •O2−

N/A

•OH, •O2− N/A

N/A

Main active species

1.25

2.09

1.75 5.45 4.40

Vis

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

UV

g-C3N4

g-C3N4 nanotubes

g-C3N4

g-C3N4

Pure g-C3N4 nanosheets

Pure g-C3N4 nanosheets

g-C3N4

g-C3N4

Fe2O3/g-C3N4 g-C3N4 g-C3N4

g-C3N4

50 W Xe lamp (λ: 400−2500 nm) 250 W high pressure Na lamp (λ: 400−800 nm)

g-C3N4

g-C3N4

ZnO/g-C3N4 g-C3N4 g-C3N4



300 W Halogen lamp with a 420 nm cutoff filter 300 W Xe lamp with a 420 nm cutoff filter

LED module emitting at 420 nm

Light source


700 (2015) 1039 (2015) 702 (2013) 715 (2013) 710 (2014) 536 (2012) 539 (2014) 540 (2015)

1038 (2015) 704 (2013) 714 (2011) 235 (2014)

762 (2014) 465 (2014) 1037 (2015)

1034 (2015) 1035 (2013) 1036 (2015) 942 (2015) 554 (2014)

1033 (2014)

ref (Year)



N deficiency (1.9 at%)

N-deficient g-C3N4

7286

Au (100 vol%) CNT/g-C3N4 mass ratio (60:40) C/N atomic ratio (0.74)

Au/CNT/g-C3N4

Bentonite (5.4 wt%)

MWCNT (5 mg)

Co3O4 (1.5 wt%)

CeO2 (5.0 wt%)

Bentonite/g-C3N4

MWCNT/white C3N4

Co3O4/mpg-C3N4

CeO2/g-C3N4

OMC/g-C3N4

C60/g-C3N4

Mass ratio of C60 to DCDA (0.03) C60 (1 wt%)

C60/g-C3N4

Carbon dots/g-C3N4

B-doped g-C3N4

CQD (5 mL) Melamine (1 g) Carbon dots (0.5 wt%)

Molar ratio C/N (0.8)

C-doped g-C3N4

CQD/C3N4

BmimPF6 (0.1 g)

P-doped g-C3N4

Potassium fluoride (0.2 g)

CNB (25 wt%)

g-C3N4/CNB

F-doped g-C3N4

N/A

O-doped g-C3N4

Molar ratio of phosphorus atom to nitrogen atom: 2 Boron oxide powder (0.1 g)

O (3 wt%)

O-doped g-C3N4@TiO2

Phosphate-modified g-C3N4

Conc of C (8%)

Dopant/cocatalysts

C-doped g-C3N4

Composite type

Table 5. continued

300 W Xe lamp with a 400 nm cutoff filter 300 W Xe lamp with a 400 nm cutoff filter 500 W Xe lamp (Visible light) 300 W Xe lamp with a 400 nm cutoff filter

Degradation of MB (k = 1.64 h−1) Degradation of BPA (k = 0.014 min−1) Degradation of MB (k = 0.1621 min−1)


Degradation of RhB (k = 0.0534 min−1) Degradation of MB (k = 0.74 h−1)

g-C3N4

visible-light source (50 mW/cm2)

g-C3N4

g-C3N4

white C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4 500 W Xe lamp with a 420 nm cutoff filter


Degradation of MB (k = 1.04 h−1)

g-C3N4

g-C3N4 (no activity)

g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4@TiO2

g-C3N4


Degradation of RhB (97% degradation after 60 min) Degradation of RhB (k = 0.0411 min−1)


Infrared lamp

300 W Xe lamp; simulated sunlight



300 W Xe lamp







Light source

Degradation of phenol (k = 0.55 h−1)

Degradation of RhB (100% degradation after 5 min) Degradation of MO (90% degradation after 4 h)

Degradation of RhB (∼100% degradation after 20 min) Degradation of RhB (k = 0.0475 min−1) Degradation of MO (k = 0.1607 h−1) Degradation of PCP-Na (k = 0.0613 h−1) Degradation of MO (93% degradation after 2 h) Degradation of RhB (∼100% degradation after 3 h) Degradation of 4-NP (∼100% degradation after 3 h) Degradation of RhB UV−vis: 100% degradation after 30 min Visible: 100% degradation after 50 min Degradation of liquid phase phenol (Rate = ∼78%) Degradation of RhB (k = 0.199 min−1)


Evaluation of photocatalytic activity (rate constant, k) 4.47

Vis

2.00

1.43

1.33

-

-

-

-

-

-

-

-

-

7.8

4.78

8.10

2.47

10.17

89.35

1.80

3.19

3.67

10 (UV− vis) -

∼2.60 (UV− vis) 3.62

-

-

-

∼1.25 (UV− vis) 17.59 15.30 5.47 2.07

-

UV


•OH, •O2− •OH, •O2− -

•OH, •O2− N/A

•OH, •O2− •OH, •O2− •OH, •O2− •OH, •O2−

N/A

•OH, •O2− N/A

N/A

•OH

N/A

•OH, •O2− N/A

•OH, •O2−

N/A

•O2−

Main active species

913 (2014) 1041 (2014) 870 (2013) 643 (2014) 641 (2015)

907 (2016) 246 (2014) 247 (2014) 867 (2015)

1040 (2014) 546 (2010) 986 (2015) 906 (2015)

220 (2015) 524 (2013) 523 (2015) 378 (2012)

522 (2012) 519 (2014) 520 (2014)

ref (Year)



7287

BaTiO3 (12.0 wt%)

SnS2 (30 wt%)

MoS2 (0.05 wt%)

NiO (6.3 wt%)

YVO4 (25.8 wt%)

Al2O3 (50 wt%)

ZnTcPc (0.64 wt%)

g-C3N4 (0.5 g) TBOT (0.2 mL) In(NO3)3 (0.5 mmol) Au (1 wt%) Graphene (5 wt%) N-rGO (0.25 wt%) CeO2 (2 wt%) Ag/Fe2O3 (5 wt%)

BaTiO3/g-C3N4

SnS2/g-C3N4

MoS2/g-C3N4

NiO/g-C3N4

YVO4/g-C3N4

Al2O3/g-C3N4

ZnTcPc/g-C3N4

TiO2/In2O3/g-C3N4

g-C3N4 (10 wt%)

1D Ag@AgVO3 nanowire/graphene/ protonated g-C3N4

Ag3PO4(111)/g-C3N4

g-C3N4 (5 wt%)

Bi2O3 (1.0 wt%)

Bi(NO3)3·5H2O (0.12 g)

MoO3 (1.5 wt%)

N-doped ZnO/g-C3N4 core−shell nanoplates

Bi2O3/g-C3N4

BiOCl/g-C3N4

MoO3/g-C3N4

CeO2/TiO2/g-C3N4

BiOI (20 wt%) g-C3N4/Fe3O4 (2:1) GO (30 mg) Ag@AgVO3 (80 mg) Porous g-C3N4 (180 mg) g-C3N4 (1 wt%)

g-C3N4/Fe3O4/ BiOI

g-C3N4/Ag/Fe2O3

CeO2/g-C3N4/N-rGO

Au/graphene/ porous g-C3N4

Cu2‑xSe (60.0 wt%)

Dopant/cocatalysts

Cu2‑xSe/g-C3N4

Composite type

Table 5. continued

300 W Xe lamp with a 420 nm cutoff filter 500 W Xe lamp with a 420 nm cutoff filter

Degradation of RhB (k = 0.025 min−1) Degradation of RhB (k = 0.0895 min−1)


300 W Xe lamp


Degradation of RhB (k = 0.0253 min−1) Degradation of RhB (Removal rate = 81.1% after 1 h) Degradation of MO (k = 0.0177 min−1)


g-C3N4

Four fluorescent daylight lamps (6 W) with a radiation spectrum simulating sunlight 300 W Xe lamp with a 400 nm cutoff filter

g-C3N4

g-C3N4

N-doped ZnO g-C3N4 g-C3N4

g-C3N4

g-C3N4

g-C3N4

Porous g-C3N4

0.25% N-rGO/g-C3N4

g-C3N4

TiO2 g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4

g-C3N4



Gas phase photooxidation of toluene (Rate = 3.52 × 10−10 mol s−1 m−1) Degradation of MB (k = 0.6330 min−1)

Degradation of MB (98.9% degradation after 20 min)

50 W LED source (Visible light)



Degradation of RhB (Completely degraded after 180 min)



Degradation of MO (k = 0.4550 h−1)



Degradation of RhB (k = 2.34 h−1)




Degradation of RhB (Removal rate = 95%)


SnS2


200W Xe lamp (Simulated sunlight)

g-C3N4 Cu2‑xSe g-C3N4



Light source

Degradation of MO (76% degradation after 6 h) Photoreduction of Cr(VI) (k = 0.4582 mol−1 dm3 min−1) Degradation of RhB (k = 0.301 min−1)



10.44 4.85 5

2.91

2.27

5.17

3.13

2.74

2.10

7.42

11.5 6.6

1.60

7.30

2.75

2.30

3.60

9.51

2.8 6.1 7.8

Vis

4.06 (UV− vis) 10.4

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

UV


•OH, •O2−

•OH, •O2− + h , •O2−

•OH, •O2−

h+, •O2−

1048 (2014) 633 (2014) 595 (2014)

1047 (2015) 616 (2015) 679 (2014)

209 (2015)

•OH, •O2− N/A

1045 (2015) 1046 (2016)

898 (2015) •OH, •O2− •OH, •O2−

•O2−

•OH, •O2−

•OH, •O2− •OH, •O2− •OH, •O2− •OH, •O2−

974 (2015)

1043 (2015) 1044 (2014) 780 (2014) 655 (2014) 821 (2013) 765 (2015) 940 (2016) 948 (2015)

•OH, •O2− •OH, •O2− •OH

1042 (2015)

ref (Year) •OH, •O2−

Main active species



g-C3N4 (3 wt%)

g-C3N4 (400 mg)

TiO2 nanotube/g-C3N4

Ag3PO4/g-C3N4

a TAP: 2,4,6-triaminopyrimidine; DPY: 2,6-diaminopyridine; CPVC: partially conjugated polyvinyl chloride; PMDA: pyromellitic dianhydride; P3HT: poly-3-hexylthiophene; PANI: polyaniline; Ptp: polythiophene; MIL-125(Ti): Ti-benzenedicarboxylate; MCA: melamine cyanuric acid complex; DCDA: dicyandiamide; CM: cyanuric acid-melamine; C-M-Mp: cyanuric acid-melamine-2,4-diamino-6phenyl-1,3,5-triazine; PTI: poly(triazine imides) intercalated with Li+; RhB: rhodamine B; MB: methylene blue; TBOT: tetrabutyl titanate; MO: methyl orange; NDY-GL: neuatral dark yellow GL; PCP: pentachlorophenol; mpg: mesoporous graphitic; PNP: p-nitrophenol; MR: methyl red; CNQD: carbon nitride quantum dots; HCCP: hexachlorotriphosphazene; GndCl: guanidiniumhydrochloride; CNB: B-modified graphitic carbon nitride; CDQ: carbon quantum dot; CNT: carbon nanotube; OMC: ordered mesoporous carbon; C-PDA: carbonized polydopamine; MWCNT: multiwalled carbon nanotube; BPA: bisphenol A; ZnTcPc: zinc phthalocyanine; LED: light emitting diode.

∼5 LED white-light module

1.24 Visible-light source

1 wt% TiO2 nanotube/gC3N4 g-C3N4

594 (2014) 606 (2015) 618 (2015) •OH, •O2− •OH, •O2− •OH, •O2− 12.05 g-C3N4 (25 wt%) Ag3PO4/g-C3N4

Degradation of MO (Complete decolorization after 15 min) Degradation of aqueous isoniazid (90.8% degradation after 4 h) Degradation of RhB (Complete degradation after 10 min)


g-C3N4

ref (Year) Vis UV Light source Composite type

Dopant/cocatalysts


Table 5. continued

Review

Main active species Reference photocatalyst


Chemical Reviews

Figure 157. Schematic of photocatalytic degradation of pollutants under light irradiation using pristine g-C 3N4 as a reference photocatalyst.

to an alkaline hydrothermal treatment at 90−130 °C, the surface area was markedly increased up to 65 m2 g−1 and the oxidation rate of NO under visible light was enhanced by 8.6 times. It was inferred that the unstable domains of g-C3N4 were eliminated by hydrolysis to produce the mesoporous structure with a higher specific surface area.506 Recently, Wang et al. prepared g-C3N4 with honeycomb structures through thermal polymerization of urea in the presence of water.364 By lengthening the condensation period, the highly porous honeycomb-like g-C3N4 was transformed into a velvet-like morphology with increased surface area. In contrast to the observation by Sano et al., the photocatalytic activity of g-C3N4 toward NO removal was found to decrease with increasing surface area.506 It was reported that large surface areas could induce the formation of more surface defects, which in turn captured charge carriers and hindered their involvement in the photocatalytic reaction.364 The immobilization of a photocatalyst on a support has been reported to be essential for practical environmental applications. In a paper by Dong and co-workers, g-C3N4 photocatalysts immobilized on structured Al2O3 ceramic foam were applied in the photocatalytic removal of NO in air.332 The immobilized g-C3N4 showed a high NO removal ratio of 77.1% and a superb catalytic stability even after six cycles (Figure 158a) under real indoor irradiation of an energy-saving lamp. The formation of reactive radicals for the removal of NO was verified using DMPO spin-trapping EPR measurements in methanol dispersion for DMPO−•O2− (Figure 158b) and aqueous dispersion for DMPO−•OH (Figure 158c). It was deduced that •O2− radicals and photoinduced holes were the main reactive species responsible for the photocatalytic reactions, whereas •OH radicals played a minor role in the photo-oxidation of NO. Figure 158d shows the schematic illustration of Al2O3 displaying L-acid properties, where Al atoms could act as electron acceptors. Indeed, bare Al atoms accepted the lone pairs of nonbonding electrons from the N atoms in g-C3N4. Consequently, a chemical bonding between Al2O3 foam and g-C3N4 was created, which ultimately led to an extremely firm immobilization of g-C3N4 on Al2O3.332 Over the past few years, g-C3N4-based photocatalysts have been extensively explored in photocatalysis-related applications. This is primarily due to the unique 2D structure, environmentally benign characteristics, chemical stability, and tunable electronic properties.347 A composite comprised of g-C3N4 and metal oxide can improve the surface area of the photocatalyst and promote the separation of charge carriers, so as to enhance 7288


Chemical Reviews

Review

Figure 158. (a) Stability test for the photodegradation of NO over the g-C3N4 immobilized on the Al2O3 ceramic foam. (b and c) DMPO spintrapping EPR spectra of immobilized g-C3N4 in (b) methanol dispersion for DMPO−•O2− and (c) aqueous dispersion for DMPO−•OH. (d) Illustration of an electron pair from g-C3N4 accepted by bare Al atoms and exhibiting L-acid properties. Reprinted with permission from ref 332. Copyright 2014 American Chemical Society.

Figure 159. Schematic showing the transfer of charge carriers, generation of radicals, and oxidation of NO on TiO2/g-C3N4 under (a) visible and (b) UV light illumination. Reprinted with permission from ref 270. Copyright 2016 Elsevier.

shows the schematic illustration of the charge transfer, radical generation and NO oxidation on TiO2/g-C3N4 under both light conditions. In another paper, Dong et al. constructed an advanced semimetal−organic Bi spheres/g-C3N4 nanohybrid.689 The nanohybrid demonstrated highly enhanced visible-light photoactivity and stability for NO purification compared to that of pristine g-C3N4. The strengthened photocatalytic performance was ascribed to the cocontributions of (1) improved light harvesting owing to the SPR effects of Bi nanospheres and (2) increased separation efficiency of electron−hole pairs due to the electron trapping effect of Bi in the hybrid system. The local electromagnetic field arising from the SPR effects of Bi was simulated with a rigorous Maxwell’s solver based on the finite integration techniques (Figure 160a), where a significant enhancement of electro-

the photocatalytic performance. In a most recent work published in 2016, Ma et al. carried out the photocatalytic oxidation of NO over TiO2/g-C3N4 composites, which were prepared by a one-step calcination method using commercial P25 and melamine as precursors.270 The hybrid composite with 15 wt% g-C3N4 displayed a NOx conversion of 27%, which was higher than that of bare P25 (17%) and g-C3N4 (7%). It was also found that the synthesis procedure of the composite was important in determining its photocatalytic activity. A mechanically mixed g-C3N4 and TiO2 sample with similar content did not remarkably improve the conversion of NOx, thus confirming that the interaction between g-C3N4 and P25 is vital for the activity. EPR measurements once again indicated that •O2− was the main active species involved in the oxidation of NO under both visible and UV light irradiation. Figure 159 7289


Chemical Reviews

Review

Figure 160. (a) SPR-mediated local electromagnetic field in Bi nanospheres. The Bi particles are irradiated by plane waves with a wavelength of 420 nm that emerges from the z-direction. A 3D view and 2D cross-sections perpendicular to the x, y, and z axes are illustrated. The scale bar indicates the relative increase in field enhancement. (b) Charge separation and proposed photocatalytic mechanism of Bi spheres/g-C3N4 for NOx purification. Reprinted with permission from ref 689. Copyright 2015 American Chemical Society.

Figure 161. (a) Effect of different scavengers on the photocatalytic activity over 5.0 wt% WO3/g-C3N4 (Dosage of scavengers = 0.1 mmol L−1; irradiation duration = 60 min). MB and BF denote methylene blue and fuchsin, respectively. (b−c) Schematic showing photogenerated electron− hole separation processes. Reprinted with permission from ref 667. Copyright 2014 Elsevier.

magnetic field under visible light could be observed. Figure 160b depicts the proposed photocatalysis mechanism over Bi spheres/g-C3N4 for the treatment of NO under visible-light illumination. Apart from NOx, g-C3N4-based photocatalysts have also been used in the removal of other air pollutants as such formaldehyde, acetaldehyde and so forth. Katsumata et al. synthesized WO3/g-C3N4 composite photocatalysts, which showed a marked enhancement in photoactivity toward acetaldehyde gas degradation by 1.4 times compared to pristine g-C3N4.665 In another paper by Yu et al., a direct TiO2/g-C3N4 Z-scheme photocatalyst without an electron mediator exhibited a high photocatalytic performance in the oxidation decomposition of formaldehyde in air.597 In both of these studies, the enhancement in photoactivity was primarily accredited to the improved transfer and separation of photogenerated charge carriers.597,665 5.3.2. Liquid Phase Degradation of Pollutants. In addition to the photodegradation of pollutants in the gas phase, studies on the photocatalytic removal of organic pollutants in the liquid phase are another major research focus in g-C3N4based photocatalysis.260,952 Among the most widely employed pollutants in the evaluation of photoactivity are methyl orange,

methylene blue and Rhodamine B. In the context of water purification, H2O2 is one of the most widely used green oxidants, which typically requires a catalyst for activation into more reactive oxidizing intermediates.1050 In place of the conventional photo-Fenton agents (e.g., Fe2+ and Fe3+) which are limited to low pH ( •OH > h+. In relation to the band gap of WO3 and g-C3N4, the separation processes of photoinduced electrons and holes were described in Figure 161b−c. It was deduced that if the charge carriers of the composite transferred in agreement to the conventional double-charge transfer scheme as shown in Figure 161b, the formation of active species would not be favorable and would result in poorer photocatalytic activities. In contrast, if the charger carriers transferred according to Figure 161c, which is a direct Z-scheme system, a rapid combination was attained between the photogenerated holes in the VB of g-C3N4 and the photoinduced electrons in the CB of WO3. The electrons with a more negative potential in the CB of g-C3N4 reduced the molecular O2 to yield •O2−, while the holes with a more positive potential in the VB of WO3 generated active •OH species. The interfacial charge transfer (IFCT) between the continuous energy levels of solids and the discrete ones of molecular species has been confirmed as an appealing technique in improving the visible-light responsiveness of photocatalysts.1051 In 2016, Liu et al. reported a ternary Fe(III)/graphene/g-C3N4 composite, which displayed high performance in the visible-light degradation of methyl orange.952 As shown in Figure 162a, the kinetic constant of

Generally, in pure g-C3N4-based visible-light systems, the two main reactive species that involve in the degradation of pollutants are •O2− and h+ species. Interestingly, it was found that for Ag/g-C3N4 systems, •OH radicals also took part in the photocatalytic reactions in addition to the aforementioned two species, thus giving the direct evidence to contribute to its increased catalytic performance.715 The same group of researchers later reported the fabrication of Z-scheme plasmonic photocatalyst Ag@AgBr/g-C3N4 to further enhance the photocatalytic activity.682 The high photoactivity achieved in the degradation of methyl orange was attributed to the Zscheme system, which retained the photoinduced electrons with strong reduction power in the CB of g-C3N4 and h+ with strong oxidation power in the VB of AgBr. The subsequent generation of Br0 atoms and superoxide radicals with high oxidizing capabilities further led to the enhancement in the degradation reaction.682 Xu et al. reported the use of upconversion agents to alter g-C3N4 by thermal polymerization of a combination of ErCl3•6H2O and the supramolecular cyanuric acid-melamine precursors.465 The doping of g-C3N4 with Er3+ resulted in high photodegradation rates of Rhodamine B, where the contribution of the upconversion agent was demonstrated by measurements using only a red laser. The improvement in photoactivity was accredited to the energy transfer from the upconversion agent to g-C3N4, as well as additional charge transfer paths besides radiative recombination.465 In another study, Chen and co-workers studied the separation mechanisms of photoinduced electrons and holes for composite photocatalysts WO3/g-C3N4.667 In the process, several scavengers were utilized to explore the reactive species in the photocatalytic oxidation reactions. Ammonium oxalate (AO) was used to remove holes (h+), IPA was employed to impair hydroxyl radical (•OH) and benzoquinone (BQ) was 7291


Chemical Reviews

Review

Figure 163. (a) Steady-state PL measurements of 15 wt% g-C3N4 (CN)/BTO nanocomposites and BTO nanosheets. (b−c) Time-resolved transient PL spectra monitored at 435 nm under the excitation of 340 nm for BTO and 15 wt% CN/BTO samples. (d) EPR signals of the DMPO-•O2− in methanol dispersion upon light irradiation for 20 s. Reprinted with permission from ref 601. Copyright 2015 Royal Society of Chemistry.

well-defined Keggin structure.1052,1053 In a paper by Li et al., H3PW12O40-modified g-C3N4 nanotubes exhibited considerably high visible-light photoactivity and stability with respect to methyl orange degradation.648 This high photocatalytic performance was attributed to their unique tubular nanostructure and high light-utilization efficiency, while the catalytic stability was due to the strong chemical bonding between the gC3N4 nanotubes and the Keggin unit.648 Cheng et al. designed 2D/2D heterojunctions between g-C3N4 nanosheets and oxygen vacancies confined in bismuth titanate (BTO) mesoporous nanosheets.601 At an optimal ratio of 15 wt% gC3N4, the 2D/2D heterojunctions served as an efficient visiblelight-responsive photocatalyst for degradation of Rhodamine B and 4-chlorophenol. The superior photoactivity was first ascribed to the synergistic effect between the 2D structure and the oxygen vacancies confined in the BTO nanosheets. To elaborate the photogenerated interfacial charge transfer mechanism, steady-state and transient PL spectra of the composite were recorded (Figure 163a−c). Fascinatingly, the 15 wt% g-C3N4/BTO nanosheet heterojunctions (τ1 = 5.65 ns, A1 = 65.72%; τ1 = 35.19 ns, A2 = 34.62%) yielded the longest decay time in comparison to the pristine BTO nanosheets (τ1 = 5.11 ns, A1 = 64.94%; τ1 = 35.17 ns, A2 = 31.29%), which implied an ameliorated charge transfer mechanism mediated by the modification of g-C3N4.601 In view of the charge transfer of the g-C3N4/BTO, the stored electrons in the CB of BTO were unable to reduce O2 to •O2−, whereas the holes in the VB of gC3N4 did not possess sufficient power to oxidize OH to •OH, as evidenced from the EPR results (Figure 163d). Therefore, based on this charge radical analysis, the charge transfer of gC3N4/BTO is more likely to take place through a Z-scheme pathway rather than the traditional Type II heterojunction. This is in essence similar to the above-discussed WO3/g-C3N4 nanohybrids.667

25%-Fe/graphene/g-C3N4 (Fe/GE/CN) was calculated to be 0.02765 min−1, which was 1.97, 4.82 and 11.28 folds higher than that of 25%-Fe/g-C3N4, 1%-GE/CN and g-C3N4, respectively. In addition to methyl orange, the photoactivity of the samples was also evaluated toward the degradation of phenol (Figure 162b). Similarly, the highest photocatalytic performance was observed in the ternary composite of 25%-Fe/ GE/CN. Figure 162c−d illustrates a plausible photocatalytic process of the Fe/GE/CN ternary composite, along with that of a Fe/CN binary system for comparison purposes. As can be seen from Figure 162c, in the presence of graphene as a conductor, the electrons were rapidly transferred to the surface to form reactive species for the reaction processes. Furthermore, the incorporation of graphene allowed better distributed, smaller-sized and higher content of the Fe(III) species within the ternary nanohybrid than those in the binary system (Figure 162d). As a result, improved optical absorption and suppressed electron−hole recombination were attained by the ternary system, resulting in higher catalytic efficiency.952 In another work on the metal-free isotype heterojunctions, gC3N4/B-modified g-C3N4 hybrid composites were synthesized by employing low-cost and abundant Ph4BNa and urea as starting materials.220 The g-C3N4/B-modified g-C3N4 sample exhibited the highest photoactivity, where the degradation activity for phenol and methyl orange was at least two times greater than that of single-component g-C3N4 and B-modified g-C3N4. Importantly, no appreciable change in performance was detected after four consecutive cycles, inferring high catalytic stability for practical benefits.220 Despite the great advancements made to date, the exploration of hybrid photocatalyst is mainly limited by cost and environmental friendliness. 12-Tungstophosphoric acid (H3PW12O40) is a widely used polyoxometalate in homogeneous photocatalytic reactions due to its low toxicity, inexpensiveness, efficient electron trapping characteristics and 7292


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composites demonstrated noticeably enhanced photoactivity for methylene blue degradation than pure g-C3N4 and ZnWO4 under visible-light illumination. To reveal the charge transfer complex formed at the interface between g-C3N4 and ZnWO4, the imaginary parts of the dielectric function for the ZnWO4 (010) surface, ZnWO4 (011) surface, g-C3N4 (001)/ZnWO4 (010) and g-C3N4 (001)/ZnWO4 (011) interface have been calculated (Figure 165A). In comparison to pure ZnWO4 (010) and ZnWO4 (011) surfaces, the absorption edges for both gC3N4 (001)/ZnWO4 (010) and g-C3N4 (001)/ZnWO4 (011) interfaces displayed a red-shift of approximately 0.8 eV to the visible-light region. The highest occupied surface crystal orbital (HOSCO), the next highest occupied surface crystal orbitals (HOSCO-1 and HOSCO-2), the lowest unoccupied surface crystal orbital (LUSCO) and the next lowest unoccupied surface crystal orbitals (LUSCO+1 and LUSCO+2) of the gC3N4 (001)/ZnWO4 (010) (Figure 165B(a)) and g-C3N4 (001)/ZnWO4 (011) interfaces (Figure 165B(b)) were also calculated at the Γ-point to understand the origin of visiblelight absorption and the mechanism of interface charge transfer. Upon visible-light irradiation, the valence electrons were expected to be directly excited from g-C3N4 into the W 5d orbitals of ZnWO4 in the CB, thereby generating well-separated electron−hole pairs.681 In addition to the aforementioned samples, many other g-C3N4-based photocatalyst systems have been successfully developed for the degradation of dye molecules, which include CaIn2S4/g-C3N4 heterojunction nanocomposites,260 SnNb2O6/g-C3N4,184 BiVO4/g-C3N4,636 polyvinyl chloride (PVC)/g-C 3 N 4 , 268 V 2 O 5 /g-C 3 N 4 , 589 Bi5O7I/g-C3N4,631 CeO2/g-C3N4641 and many more. Liquid phase photocatalytic degradation has also been applied for other pollutants such as bisphenol A, which is used extensively in the manufacturing of consumer goods and products. Owing to its wide application, a large amount of bisphenol A has been released into the environment, leading to ubiquitous presence of bisphenol A in natural water.1054 Therefore, extensive research effort has been devoted to the remediation of this problem in recent years. In a paper by Chang et al., Pd-modified mesoporous g-C3N4 (Pd/mpg-C3N4) was fabricated and applied in the degradation of bisphenol A in water.702 The high photoactivity achieved was attributed to the enhanced light absorbance in the range of UV−vis region, along with the ability of embedded Pd to serve as electron traps and facilitate the separation of charge carriers.702 Most recently, Zhang and co-workers prepared an agar-C3N4 hybrid hydrogel photocatalysts with 3D network structure, which showed a phenol removal rate of approximately 1.3 times higher than that of pure g-C3N4.174 Carbon dots decorated g-C3N4 photocatalysts have also been reported to be efficient in the degradation of phenol under visible-light irradiation.907 In addition to effective separation of charges, the upconverted PL character of carbon dots also extended the visible-light absorption range of the composite, thus leading to a 3.7-fold enhancement in the degradation rate over pristine g-C3N4 under the same light conditions.907 g-C3N4-based materials have also been applied in the photoreduction of Cr(VI).1055 Dong and Zhang reported on the preparation of formate anion comprising g-C3N4 and its drastically improved performance and stability on Cr(VI).1056 Through the inhibition of surface dioxygen adsorption, the incorporation of formate anions promoted the trapping of photoinduced holes to generate free electrons, and at the same time manipulated the two-step superoxide-ions-mediated indirect reduction to a one-step

In another paper, Zhang et al. prepared 2D/2D heterojunction photocatalysts by hybridizing hexagonal SnS2 nanosheets with g-C3N4 for remarkably improved visible-light degradation of organic dyes and phenol.663 To investigate the interfacial charge transfer, PL spectroscopy of the SnS2/g-C3N4 heterojunction nanocomposites was examined and compared to those of pure g-C3N4 nanosheets. From Figure 164A, bare g-

Figure 164. (A) Steady-state PL measurements of (a) g-C3N4 nanosheets and (b) 5 wt% SnS2/g-C3N4. (B and C) Time-resolved transient PL spectra of (B) g-C3N4 nanosheets and (C) 5 wt% SnS2/gC3N4. (D) Charge transfer mechanism of SnS2/g-C3N4 heterojunction nanocomposites. Reprinted with permission from ref 663. Copyright 2015 Elsevier.

C3N4 nanosheets displayed a strong emission peak at ca. 450 nm. When 5 wt% SnS2 nanosheets were added onto the g-C3N4 surface, a significant PL quenching was observed, which inferred that the electron−hole recombination was markedly hindered by the interfacial charge transfer process. This phenomenon could be further elucidated by the time-resolved transient PL spectroscopy (Figure 164B−C). The emission decay results were fitted by triexponential kinetics, leading to the derivation of three components. For 5 wt% SnS2/g-C3N4, the emission lifetimes of all components were found to be smaller relative to those of g-C3N4 nanosheets. In this case, the observed smaller emission lifetime implied the existence of a nonradioactive route from the electronic coupling between gC3N4 and SnS2 nanosheets. The synergistic interaction of the SnS2/g-C3N4 nanohybrids was concluded to have enhanced photoinduced interfacial charge transfer and separation during the reactions (Figure 164D). By employing a combination of experimental and theoretical techniques, Sun and co-workers reported an extensive study on the microscopic mechanism of interface interaction, charge transfer and separation as well as their effect on the photoactivity of g-C3N4/ZnWO4 heterostructured nanostructures.681 HRTEM results and DFT calculations mutually verified the reasonable existence of g-C3N4 (001)/ZnWO4 (010) and g-C3N4 (001)/ZnWO4 (011) interfaces. The 7293


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Figure 165. (A) Calculated imaginary part of the dielectric function for the ZnWO4 (010) surface (black solid line) and g-C3N4 (001)/ZnWO4 (010) interface (red dot line) in part a and the ZnWO4 (011) surface (black solid line) and g-C3N4 (001)/ZnWO4 (011) interface (red dot line) in part b. (B) Γ-point orbital-isoamplitude surface of the HOSCO, HOSCO-1, HOSCO-2, LUSCO, LUSCO+1, and LUSCO+2 for (a) the g-C3N4 (001)/ZnWO4 (010) interface and (b) g-C3N4 (001)/ZnWO4 (011) interface. Gray, azure, red, dark gray, and blue denote Zn, W, O, C, and N, respectively. Reprinted with permission from ref 681. Copyright 2012 Royal Society of Chemistry.

As discussed in previous sections, photogenerated electron− hole pairs can either recombine or migrate to the photocatalyst surface to react with adsorbed molecular oxygen and hydroxyls to generate reactive oxygen species for the mineralization of contaminants. In particular, molecular oxygen reduction by photoinduced electrons on the CB of photocatalyst, which generally proceeds via one or two-electron transfer, plays an indispensable role in the overall reaction process.1057 Zhang et al. demonstrated that the oxygen reduction pathway could be switched by exfoliating g-C3N4 for enhanced photocatalytic phenol degradation.504 On the basis of experimental results, the authors proposed a mechanism of selective O2 activation on both bulk g-C3N4 as well as exfoliated g-C3N4 nanosheets (Figure 167). On bulk g-C3N4, as reported by Shiraishi et al., in the absence of any additional modifiers, an electron generated from the photoexcitation of g-C3N4 would reduce O2 and produce surface-associated superoxo species, which then became protonated and dissociated from the surface to convert to HO2− radicals.179,570 On the other hand, on g-C3N4 nanosheets, some superoxo species would be rapidly reduced by another electron to generate a surface-stabilized intermediate, subsequently converting into H2O2. The quick formation of 1,4-endoperoxide removed two electrons simultaneously from the g-C3N4 nanosheets, which in turn stabilized more holes for the oxidation of phenol.504 The authors also attributed the high photoactivity of g-C3N4 nanosheets to its small thickness of < 2 nm, which provided a much shorter

direct photoinduced electron reduction of Cr(VI) over g-C3N4. Figure 166 illustrates the Cr(VI) photoreduction pathways over both g-C3N4 and formate anion-modified g-C3N4.

Figure 166. Plausible mechanism of the Cr(VI) photoreduction pathways over g-C3N4 and formate anion-modified g-C3N4. Reprinted with permission from ref 1056. Copyright 2013 American Chemical Society. 7294


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Figure 167. Reaction pathways of oxygen reduction process on the surfaces of photoexcited bulk g-C3N4 and g-C3N4 nanosheets. Reprinted with permission from ref 504. Copyright 2015 American Chemical Society.

Figure 168. Photocatalytic disinfection efficiencies over (a) different g-C3N4 photocatalysts and (b) single-layer g-C3N4 with different scavengers. SL g-C3N4 and g-C3N4 NS denote single-layer g-C3N4 and g-C3N4 nanosheets, respectively. TEM images of E. coli before and after photocatalytic disinfection: (c) Before reaction, and after irradiation for (d) 4 h, (e) 8 h, and (f) 12 h. Reprinted with permission from ref 494. Copyright 2014 Elsevier.

efficiency.408 In another paper by Zhao et al., single-layer gC3N4 was applied in the photocatalytic disinfection by inactivation of E. coli under visible-light irradiation.494 Figure 168a shows the disinfection efficiency of E. coli using different photocatalysts, where single-layer g-C3N4 exhibited the highest bactericidal performance. From Figure 168b, it can be seen that the addition of isopropanol or Cr(VI) resulted in ca. 5 log10 of E. coli cell being destructed in 4 h. However, only ca. 2 log10 of E. coli cell were destroyed with the addition of KI. These results suggested that photocatalytic disinfection over g-C3N4 follows a hole oxidation-dominated process under visible-light irradiation. The destruction of the E. coli cell was further verified

distance for the migration of electrons to the catalyst surface for the reduction of superoxo, before being released from the surface.504 5.3.3. Bacterial Disinfection. In 1985, Matsunaga and coworkers first demonstrated photocatalytic sterilization of microbial cells in water.1058 Huang et al. were the first to reveal that g-C3N4 exhibited bactericidal effects on Escherichia coli (E. coli) K-12 in water under visible-light illumination.408 The study of the mechanism revealed that the performance of bacterial inactivation stemmed from the light-induced holes on the surface of g-C3N4. This was confirmed by radical scavenger of •OH, which virtually had no effect on the inactivation 7295


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Figure 169. (A) Schematic illustration of the photocatalytic bacterial inactivation mechanisms of (a) CNRGOS8 and (b) RGOCNS8 under aerobic conditions, and (c) CNRGOS8 and (d) RGOCNS8 under anaerobic conditions. Photocatalytic inactivation efficiency against E. coli K-12 in the presence of the as-prepared samples under (B) aerobic and (C) anaerobic conditions under visible-light irradiation. Reprinted with permission from ref 397. Copyright 2013 American Chemical Society.

effect of Ag plays an essential role in the biocidal action. In this regard, a complex phenomenon involving adsorption, radical attack as well as deactivation process of the interaction between microorganism and photocatalyst took place simultaneously.712 It is noteworthy that the bactericidal action was greatly influenced by both electron and hole-related charge species, which could be confirmed by the EPR measurements of the charge radicals produced from the photoactivity. Metal-free light-driven nanomaterials are of particular importance in photocatalytic inactivation of bacterial since the highly efficient metal-based photocatalytic systems mostly contain Ag0 or Ag+ species, which is unfavorable for “green” water disinfection. In a paper by Wang et al., a novel metal-free heterojunction photocatalyst was developed by cowrapping αsulfur (α-S8) with polymeric g-C3N4 and rGO sheets for the photocatalytic disinfection of bacterial cells under visible-light illumination.397 The authors examined the antibacterial activity toward inactivation of E. coli K-12 cells using two different structures of ternary hybrids (Figure 169A). For the first structure (denoted as CNRGOS8), the rGO sheets were sandwiched between α-S8 and g-C3N4 sheets (Figure 169A(a and c)). On the other hand, for the second structure (denoted as RGOCNS8), the g-C3N4 sheets were situated between rGO and α-S8. Interestingly, the samples synthesized through different wrapping sequences (CNRGOS8 and RGOCNS8) displayed dissimilar photocatalytic inactivation activity toward E. coli K-12 cells under different aerobic and anaerobic environments (Figure 169B−C). As can be seen from Figure 169B, much higher photocatalytic bacterial inactivation efficiency was attained on CNRGOS8 than RGOCNS8 in the aerobic atmosphere. In contrary, RGOCNS8 demonstrated enhanced bactericidal performance under anaerobic conditions compared to CNRGOS8 (Figure 169C). In the absence of O2 (anaerobic environment), the photocatalytic inactivation was dominated by a photocatalytic reductive route rather than an oxidation pathway. This research provides new research

through examining its morphology and microstructure before and after the photocatalytic disinfection. Figure 168c shows the TEM image of E. coli before the photocatalytic reaction, where a well-preserved cell wall could be seen. By the end of 12 h irradiation time, only a small portion of the cell debris could be found, which confirmed a complete destruction of the bacterial cell (Figure 168d−f).494 Overall, it is deduced that the enhancement of photocatalytic performance is due to the low charge transfer resistance and efficient charge separation processes, which has been confirmed in this study by EIS and photocurrent measurements. As shown in Section 4.1.2, noble metal nanoparticles have the ability of increasing the photoenergy conversion efficiency of semiconductors by facilitating the creation of electron−hole pairs and extending the spectral absorption.715 Bing and coworkers successfully established an effective platform based on g-C3N4 nanosheets with embedded Ag nanoparticles (Ag/gC3N4) for improved visible-light-driven antibacterial activity and dispersal of biofilms.713 The reactive oxygen species (ROS) generation of Ag/g-C3N4 nanohybrids was found to be much more effective as compared to bare g-C3N4 nanosheets under visible light. Moreover, the Ag/g-C3N4 hybrid had the ability to degrade proteins, exopolysaccharides, and nucleic acids, which constitute the primary components of biofilms. All in all, the composite was employed as an efficient bactericidal agent for both Gram-negative (G−) E. coli and Gram-positive (G+) Staphylococcus aureus (S. aureus) under visible-light illumination.713 Muñoz-Batista et al. investigated the biocidal capability of Ag/g-C3N4 against E. coli upon UV and visible-light excitation.712 Regardless of the wavelength used to irradiate the samples, the composite system displayed high antibacterial capabilities compared to the single components of Ag and gC3N4. Upon UV light illumination, the Ag/g-C3N4 system was demonstrated to handle charge carriers more efficiently by an effective charge separation process, which left hole-related radicals at the g-C3N4 component and electrons on the Ag nanoparticles. Meanwhile, under visible excitation, the SPR 7296


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tions, conducting polymer-g-C3N4 heterojunctions and gC3N4-based multicomponent nanocomposites. Among all these types, most of the g-C3N4-based nanomaterials possess a Type II heterojunction or a Z-scheme heterojunction system, which have been proven to be outstanding for a cornucopia of potential applications such as water splitting to H2 and O2, conversion of CO2 to energy-bearing fuels, pollutant decontamination and bacteria disinfection. Hence, engineering gC3N4-based heterostructured composites at different nanoscales will undeniably enrich the family of visible-light-driven photocatalysts in a more rational manner. In spite of some promising results reported thus far, the studies in this field are still in preliminary stages and further developments are prominently required. Notably, the works still suffer from the relatively low efficiency and low stability of the hybrid nanocomposites, which are far from the requirements of industrial needs. To date, there are a number of issues, which need to be resolved for promising solar-to-fuel conversion before practical applications at commercial scale are possible in the future. For example, knowledge about effective cocatalysts for H2O splitting and CO2 reduction is still scarce and more work is demanded to increase the quantum efficiency of the reaction. Until now, the design of g-C3N4based nanoarchitectures is of extreme significance for superior photoredox efficiency. These include: (1) to exploit new strategies to increase the light harvesting ability of g-C3N4 to utilize higher wavelengths of light (λ > 500 nm) to imitate natural photosynthesis in plants, (2) to increase the stability of the hybrid photocatalysts, and (3) to develop novel and scalable exfoliation approaches for obtaining uniform single-layered or few-layered g-C3N4 nanosheets for efficient solar-to-chemical applications. It is worth mentioning that even though there have been a number of studies on the successful formation of a monolayer g-C3N4 nanosheet, the obtained surface area is still significantly lower than the theoretical value. Thus, this creates great promise into uncovering new functionalities of the monolayer g-C3N4 for achieving more active sites. It is interesting to document that poly(triazine imide), g-C4N3 and C3N3S3, which are visible-light-responsive for various redox applications, feature analogous polymeric structures to gC3N4,166,176,1059−1062 which can give rise to unexpected and captivating results by modifying the existing molecular configuration of g-C3N4. Apart from that, g-C3N4 nanosheets act as anchoring sites for the decoration of nanoparticles to form a hybrid system. However, it is challenging to control the particle size on the gC3N4 substrate at the molecular level, resulting in severe aggregation. Consequently, the well-contacted heterojunction interface between g-C3N4 and the nanoparticles is difficult to accomplish. This markedly affects the intimate interfacial interaction between g-C3N4 and the other composite for effective charge transport and separation. To address this shortfall, surface functionalization of g-C3N4 to tune the surface charge with specific surface groups will be a positive point to strengthen the anchoring ability of g-C3N4. As such, the design of visible-light-active g-C3N4-based semiconductor photocatalysts with rational design of nanostructures should be given more emphasis to attain a high quantum efficiency and superior turnover number. Not only that, the development of an effective hybrid photocatalyst system without employing noble metal cocatalysts is necessary to achieve a viable and costeffective photocatalyst for practical benefits. Therefore, more works that explore an efficient non-noble-metal cocatalyst have

horizons toward the use of metal-free catalysts for remarkably increased efficiency in the bacteria photoinactivation.

6.0. CONCLUSIONS, PERSPECTIVE, AND OUTLOOK Over the past five years, the effort dedicated to the g-C3N4based hybrid nanocomposites has led to a rich knowledge and database for their smart engineering, characterizations, and versatile energy and environmental-related applications. The boundless breakthroughs in the arena of g-C3N4-based nanomaterials have undoubtedly witnessed novel appealing properties with remarkably ameliorated photocatalytic activity. To this end, the enhancement of the photocatalytic performance of g-C3N4-based nanostructures was by and large recognized by three main far-reaching criteria, including (1) extending the absorption toward the visible-light region (visible-light harvesting), (2) hindering the recombination rate of photoinduced charge carriers (charge migration and separation), and (3) increasing the amount of adsorbed reactant species on the photocatalyst surface (surface adsorption and reaction). Since the pioneering work of Wang et al. in 2009,194 ample studies have put emphasis on the construction of g-C3N4 with a large surface area, a high chemical tunability, prolonged visiblelight absorption, reduced band gap, extended π-conjugated structure, highly porous architecture, and enhanced charge transfer and separation. Seeing that there is an abundance of literature related to g-C3N4-based photoactive materials, this critical review covers a large timeline of scientific literature, mainly for the past four years, as this is essential to elucidate a whole framework and agenda of the progress in this emerging field. Typically, the article presents the state-of-the-art advancements in the development of diverse synthesis strategies to render g-C3N4 with improved crystal structure, increased optical absorption, excellent structural design, superior electronic properties, and optimized band arrangements to promote the photocatalytic applications. Although g-C3N4 can be facilely synthesized by thermal polymerization of nitrogenrich precursors, the bulk counterparts are disadvantageous to the photochemical efficiency, as a result of low surface area, limited surface reactive sites, and inadequate utilization of a wide spectrum of solar light. The aforementioned shortcomings can be alleviated by several means. To name a few, this includes selection of appropriate g-C3N4 precursors, optimized reaction temperatures and durations for the condensation process, an exfoliation-assisted strategy, hard and soft templating approaches, a supramolecular preorganization route, and template-free methods to account for highly porous g-C3N4 nanostructures with controllable morphologies. Equally important, g-C3N4 can also be modulated at an atomic level via elemental non-metal and metal doping and also at a molecular level through copolymerization to modify the band alignments, broaden the visible-light response, improve redox ability, increase electron−hole mobility, as well as create surface dyadic heterostructures at the interfaces. Following that, the development of heterojunction nanocomposites by hybridizing g-C3N4 with another well-matched energy levels of semiconductors or metals as cocatalysts to form different types of heterojunctions is regarded as a new paradigm toward targeting pronounced enhancement of artificial photoredox applications. In this review, we have divided the heterojunction nanohybrids into several major classes, namely metal-g-C3N4 heterojunctions, inorganic semiconductor-gC3N4 heterojunctions, graphitic carbon-g-C3N4 heterojunc7297


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Figure 170. Interrelationship of predominant factors in nanoarchitecture design, functionalization, and heterostructuring of g-C3N4-based hybrid nanocomposites for advanced solar energy conversion and environmental remediation toward achieving sustainability.

star for the next decade of research in the field of g-C3N4-based nanomaterials. We are confident that expanding the knowledge on fundamental aspects via more physical chemistry research accompanied with the experimental results by the joint collaboration between experiment and theory will positively bring new findings in materials science and technology. Without doubt, this will further advance the development road of light-harvesting g-C3N4-based nanohybrids in the posthype era. Therefore, with more advances in materials science and engineering, the bottleneck of energy- and environmental-related subjects can substantially be addressed. Following that, solar energy conversion applications will be enriched by more practical means. As mentioned earlier, joint efforts from both parties, including chemists (academia) and industrial engineers, are mandatory to deliver the prospects of g-C3N4-based nanostructures for practical use to open a revolution of renewable energy. Last but not least, with the synergy from all disciplines of researchers worldwide, including materials scientists, physicists, and chemists, it is highly anticipated that the objectives of accomplishing a clean environment and overcoming the crisis of fossil fuel depletion through the production of chemical fuels from green photocatalysis will no longer be a dream and, more importantly, will turn dreams into reality for heading toward a sustainable future. With that successful accomplishment in years to come, it will unquestionably be very far beyond what has been comprehensively described in this review article.

become an urgent necessity in order to match well or even outperform the widely used noble metals-modified g-C3N4. There have been plenty of recent findings devoted to applications in H2 evolution and degradation of pollutants using g-C3N4-based photocatalysts. Work on the platforms of O2 evolution from the other half-reaction of H2O splitting and CO2 photoconversion has conceivably gained less attention than is warranted. Additionally, the exact reaction mechanism, particularly the CO2 reduction using g-C3N4-based photocatalysts, still remains doubtful and unresolved to date. The investigation of reaction pathways is vital to elucidate the fundamental enhancements and further optimization of the photoactivity in the future. Furthermore, some key issues that account for the high photocatalytic activity, i.e. optical absorption, electronic band structure, and interfacial charge transfer across the g-C3N4-based heterojunction nanocomposites, should be exhaustively investigated to gain theoretical insights by means of first-principles DFT calculations. In terms of experimental work, reactant adsorption sites, charge transfer dynamics, and molecular orbitals should also be further researched. Therefore, the synergy cooperation of both experimental and computational simulations is indispensable to be analyzed together to provide a logical framework to further advance the current state of knowledge on the photocatalysis.1063 As a whole, the relationship of the key factors in nanoarchitecture design, functionalization, and heterostructuring design can be pictorially depicted in Figure 170 as a mind map to all the readers for the state-of-the-art research in g-C3N4-based heterostructure composites. Looking at the future, there is a limitless scope of opportunities and challenges present for this booming research hotspot. We hope that this review article will be a good guiding 7298


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AUTHOR INFORMATION

membrane separation. She was a visiting scholar at NUS from November to December 2012.

Corresponding Authors

Siang-Piao Chai received his Ph.D. degree in Chemical Engineering from the School of Chemical Engineering, Universiti Sains Malaysia, in 2008. He is currently the Deputy Head (Research) for the School of Engineering at Monash University Malaysia. He is an active researcher in the areas of catalysis, reaction engineering, membrane technology, carbon dioxide adsorption and utilization, natural gas processing technology, surface engineering, and nanotechnology. His current research interests are centered on the development of carbon nanomaterials (carbon nanotubes, graphene, and nanoporous carbon) and advanced hybrid nanocomposites for environmental remediation and renewable fuels in photocatalysis. To date, he has published over 100 peer-reviewed scientific papers.

*E-mail address: [email protected] (W.-J. Ong). Research homepage: https://sites.google.com/site/wjongresearch/. *Tel.: +603-551 46234; Fax: +603-551 46207; E-mail address: [email protected] (S.-P. Chai). Notes

The authors declare no competing financial interest. Biographies Wee-Jun Ong received his BEng and Ph.D. degrees in Chemical Engineering from Monash University in 2012 and 2016, respectively. He was the recipient of Best Graduate Award of Bachelor of Engineering in 2012, Excellence Scholarship Award in 2014, and Endeavour Research Fellowship, awarded by the Australian Government Department of Education, in 2015. In 2015, he was a Visiting Research Fellow at the University of New South Wales (with Rose Amal and Yun Hau Ng) and at Monash University Clayton Campus, Australia (with Chenghua Sun). In 2016, he joined the Institute of Materials Research and Engineering (IMRE) under Agency for Science, Technology and Research (A*Star) as a research scientist. He is an early career researcher who is active in the areas of advanced materials, catalysis, and nanotechnology. He received the David Trimm Catalysis Award in 2015 in recognition of his work in the photocatalytic field. His research interests include the development of carbonaceous and two-dimensional-based hybrid nanocomposites (e.g., graphene- and graphitic carbon nitride-based materials) for artificial photosynthesis, environmental remediation, and photoelectrochemical and electrochemical applications. For more details, see: https://sites.google.com/site/wjongresearch/.

ACKNOWLEDGMENTS This work was funded by the Ministry of Higher Education (MOHE) Malaysia and Universiti Sains Malaysia under NanoMITe grant scheme (Acc. no.: 203/PJKIMIA/6720009), and the Ministry of Science, Technology and Innovation (MOSTI) Malaysia under e-Science Fund (ref. no.: 03-02-10SF0244). ABBREVIATIONS AND ACRONYMS 0D Zero-dimensional 1D One-dimensional 2D Two-dimensional 3D Three-dimensional AA Ascorbic acid AAO Aluminum oxide ABN 2-aminobenzonitrile ACN Amorphous carbon nitride AEP 2-aminoethylphosphonic acid AFM Atomic force microscopy AgX Silver halides AO Ammonium oxalate AQY Apparent quantum yield ATCN 2-Aminothiophene-3-carbonitrile BET Brunauer−Emmett−Teller BmimBF4 1-Butyl-3-methylimidazolium tetrafluoroborate BmimCl 1-Butyl-3-methylimidazolium chloride BmimDCN 1-Butyl-3-methylimidazolium dicyanamide BmimPF6 1-Butyl-3-methylimidazolium hexafluorophosphate BQ Benzoquinone BTO Bismuth titanate (C16mim)Br 1-Hexadecyl-3-methylimidazolium bromide C60 Fullerene CB Conduction band CB Conduction band CBE Conduction band edge CMS Chiral mesoporous silica CN-D Dicyandiamide-derived g-C3N4 CNQD g-C3N4 quantum dot CNS Sulfur-doped g-C3N4 CNT Carbon nanotube CN-T Thiourea-derived g-C3N4 CN-U Urea-derived g-C3N4 CNU-BA Barbituric acid-modified g-C3N4 CoPi Cobalt-oxide-phosphate CTAB Cetrimonium bromide CuTCPP Cu(II) meso-tetra(4-carboxyphenyl)porphyrin

Lling-Lling Tan received her BEng degree in Chemical Engineering from Monash University in 2011. She obtained her Ph.D. degree in Chemical Engineering from the Department of Chemical Engineering, School of Engineering, Monash University, in 2015. Currently, she is a Lecturer in Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University. She is an early career researcher who is active in the fields of functional materials, nanotechnology, catalysis, and reaction engineering. Her current research interests primarily focus on the design and synthesis of highly efficient graphene-based semiconductor photocatalysts for energy and environmental applications under visible-light irradiation. Yun Hau Ng received his Ph.D. from Osaka University in 2009. After a brief research visit to Radiation Laboratory at University of Notre Dame (Prashant Kamat’s group), he joined the ARC Centre of Excellence for Functional Nanomaterials led by Rose Amal at UNSW with the Australian Postdoctoral Fellowship (APD) in 2011. He is currently a lecturer in the School of Chemical Engineering at UNSW. His research is focused on the development of novel photoactive semiconductors (particles and thin films) for sunlight energy conversion. He received the Honda−Fujishima Prize in 2013 in recognition of his work in the area of photodriven water splitting. He has published over 60 peer-reviewed research articles and is currently serving as Editorial Board Member for Scientific Reports. Siek-Ting Yong received her Bachelor of Engineering degree in Chemical Engineering from University of Sheffield in 2001. She obtained her Ph.D. degree in Chemical Engineering from National University of Singapore (NUS) in 2008. Currently, she is a Senior Lecturer at Monash University Malaysia. Her current research interests include synthesis of nanostructured materials, fuel processing for fuel cell applications, direct carbon fuel cells, catalytic reactions, and 7299


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SG-CN SHE SHJ SNSs SPR SPV S-Se-Gr SS-SPS STEM TAP TDOS TEM TEOA TGA Ti-BALDH

DAMN DB-PA DFT Dmb DMF DMPO DMSO DOS E. coli EDTA EDTA EDX EELS EIS EPR FDTD (FeTPP)2O FFT FRET FTIR G− G+ GO g-PAN HAADF HCNS HEDP HGCN HOMO HOSCO HRTEM

Diaminomaleonitrile Double-beam photoacoustic Density functional theory Desulfomicrobium baculatum Dimethylformamide 5,5-Dimethyl-1-pyrroline N-oxide Dimethyl sulfoxide Density of states Escherichia coli Ethylenediamine tetraacetic acid Ethylenediaminetetraacetic acid Energy dispersive X-ray Electron energy loss spectrum Electrochemical impedance spectroscopy Electron paramagnetic resonance Finite difference time domain μ-Oxo dimeric iron (III) porphyrin Fast Fourier transform Förster resonance energy transfer Fourier transform infrared Gram-negative Gram-positive Graphene oxide Graphitized polyacrylonitrile High angle annular dark field Hollow g-C3N4 nanospheres (Hydroxyethylidene)diphosphonic acid Holey g-C3N4 Highest occupied molecular orbital Highest occupied surface crystal orbital High resolution transmission electron microscopy IFCT Interfacial charge transfer IPA Isopropanol LED Light emitting diode LUMO Lowest unoccupied molecular orbital LUSCO Lowest unoccupied surface crystal orbital MCA-DMSO Melamine-cyanuric acid complex in DMSO MIL-125(Ti) Ti-benzenedicarboxylate MOFs Metal organic frameworks MSR Mesostructured silica nanorod MWCNTs Multiwalled carbon nanotubes NHE Normal hydrogen electrode NIR Near infrared NMP N-Methyl-pyrrolidone NMR Nuclear magnetic resonance ompg-CN Ordered mesoporous g-C3N4 P3HT Poly(3-hexylthiophene) PAN Polyacrylonitrile PANI Polyaniline pCN Protonated g-C3N4 PDA Polydopamine PDOS Partial density of states PEDOT Poly(3,4-ethylenedioxythiophene) PL Photoluminescence PPy Polypyrrole PRET Plasmon resonance energy transfer PSS Poly(styrenesulfonate) rGO Reduced graphene oxide ROS Reactive oxygen species S. aureus Staphylococcus aureus SAED Selected area electron diffraction SEM Scanning electron microscopy

TS-SPS tz UV UV−vis VB XPS XRD ZIF-8 ZnTcPc

Sol−gel derived C3N4 Standard hydrogen electrode Surface heterojunction Silica nanospheres Surface plasmon resonance Surface photovoltage Sulfur/selenium codoped graphene Steady-state surface photovoltage spectroscopy Scanning transmission electron microscope 2,4,6-Triaminopyrimidine Total density of states Transmission electron microscopy Triethanolamine Thermogravimetric analysis Titanium(IV) bis(ammonium lactato) dihydroxide Transient-state surface photovoltage spectroscopy Tetragonal zircon Ultraviolet Ultraviolet−visible Valence band X-ray photoelectron spectroscopy X-ray diffraction Zeolitic imidazolate framework-8 Zinc tetracarboxyphthalocyanine

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