Functionalized-Graphene Composites - American Chemical Society

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Functionalized-Graphene Composites: Fabrication and Applications in Sustainable Energy and Environment Xuezhong Gong,† Guozhen Liu,‡,§ Yingshun Li,∥ Denis Yau Wai Yu,∥ and Wey Yang Teoh*,† †

Clean Energy and Nanotechnology (CLEAN) Laboratory, Joint Laboratory for Energy and Environmental Catalysis, School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong, S.A.R. ‡ Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, Hubei, P. R. China § ARC Centre of Excellence in Nanoscale Biophotonics (CNBP), Department of Physics and Astronomy, Macquarie University, North Ryde, New South Wales, Australia ∥ Battery and Energy Storage Technologies Laboratory, School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong, S.A.R. ABSTRACT: Graphene, including pristine graphene and its analogues of graphene oxide and reduced graphene oxide, is revolutionizing the way we design high performance devices, particularly in the areas of sustainable energy and environmental technologies. From environmental remediation and sensing to energy conversions and storage, there are many successful cases of graphene applications. Instead of being a standalone working material, graphene is almost always coupled with another active material as a composite. With its high surfaceto-bulk ratio, efficient heat transfer, and electron conduction, the interfacing with graphene not only helps to overcome such limitations in the bare working material but actually accentuates them. To achieve this, the strategy of surface functionalization of graphene, with either soft matters (e.g., organics, molecular linkers, proteins) or solid inorganic matters (e.g., metal nanoparticles, oxide semiconductors), holds the key to enabling the fabrication of high performance composites. The resultant architectures, in which the graphene is applied to, yield the highest achievable properties and should be unique to the specific applications. This Review provides a bottom-up account encompassing the functionalization of graphene to the design of graphene-based composites and also their selected applications in high performance systems relevant to energy and the environment. such as the highest known electron conductivity (200 000 cm2 V−1s−1) arising from the delocalized network of π orbitals,7 excellent thermal conductivity (∼5000 W m−1 K−1),8 and high Young’s modulus (0.1 TPa, fracture strength of 130 GPa) owing to the inherent strong σ bonds between two equivalent sublattices of carbon atoms.9 With a physical thickness of ∼0.35 nm,10,11 the theoretical specific surface area of a graphene monolayer is 2630 m2 g−1 or 6 times larger than that of carbon nanotubes (CNTs).12 As represented among others under the category of “General Material Science” in Figure 1, much effort has been devoted toward understanding the intriguing properties of graphene and its synthesis. These studies laid the foundations for the applications of graphene and its analogues, mostly notably in photovoltaics, electronics, catalysis, environmental remediation, fuel cells, sensors, and batteries (Figure 1). In terms of fabrication, studies are focused on the synthesis of large area defects-free graphene sheets and their scalable productions. Among the mainstream syntheses

1. GRAPHENE, GRAPHENE OXIDE, AND REDUCED GRAPHENE OXIDE The discoveries of fullerenes (Nobel Prize, 1996)1 and carbon nanotubes2 are the landmark achievements in Nanoscience, representing the zero- and one-dimensional carbon nanomaterials, respectively, while the stability of graphene, the twodimensional analogue, has long been disputed prior to its discovery by Geim, Novoselov, and co-workers (Nobel Prize, 2010).3 This is despite the fact that research on graphite derivatives such as graphite oxide and intercalated graphite has been developed as early as the 20th century, and over the following decades, methods for preparing graphitic oxide have been improved steadily.4 In 1962, ultrathin graphite flake was obtained for the first time by chemically reducing graphitic oxide,5 and it took another two decades in 1986 for the term “graphene” to be designated as a single layer of graphite sheet.6 However, it was not until the breakthrough by Geim, Novoselov, and co-workers that stable monolayers of graphene were successfully isolated for the first time using the now classic tape stripping method.3 Arguably the most intensively researched material since its discovery, pristine graphene possesses extraordinary properties © 2016 American Chemical Society

Received: April 11, 2016 Revised: September 29, 2016 Published: October 7, 2016 8082

DOI: 10.1021/acs.chemmater.6b01447 Chem. Mater. 2016, 28, 8082−8118

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Chemistry of Materials

Figure 1. Number of papers published annually in topics related to graphene and the analogues, as well as the landmark breakthroughs in the field. Literature search was carried out with ISI Web of Science database. Accessed March 2016.

reported to date include chemical vapor deposition (CVD),13,14 mechanical exfoliation,15 and the surface epitaxial growth on SiC.16,17 However, none of these techniques are yet capable of fast and large-scale production of high quality graphene to meet the industrial demand. While pristine monolayer graphene with almost no impurities and defects are ideal, if not necessary, for the applications in high precision microelectronics,18−20 other applications are either less sensitive to the defects carried by imperfect graphene or have at least adapted to these imperfections by taking advantage of the often less meticulous fabrication. As a matter of fact, free-standing graphene sheets are almost always accompanied by “intrinsic” ripples and vacancies that alter its optical and electronic properties.21,22 Moreover, the presence of basal oxygen groups on the carbon network inevitably disrupts the C−C sp2 network and, hence, its electron conductivity.23,24 Where the graphene is extensively oxidized, it is more appropriately referred to as graphene oxide (GO). Synthetically, single sheets of GO can be derived directly from graphite powders using the now popular Hummers method that involves first the chemical oxidation by concentrated H2SO4/ HNO3, KMnO4, and H2O2 and is followed by the process of sonication−exfoliation.4 According to the most popular model on the structure of GO proposed by Lerf and Klinowisk and coworkers,25,26 hydroxyl oxide and epoxy groups populate the basal graphene plane, while carboxylic groups are located at the edges. Because of the rich oxygenated moieties, GO exhibits strong hydrophilicity that allows it to be easily dispersed in polar solvents and functionalized using conventional aqueous chemistry routes. For the same reason, GO sheets are versatile platforms for anchoring a secondary nanomaterial (e.g., metal clusters, semiconductor nanoparticles)27,28 or dispersing in functional medium (e.g., polymers)29 to obtain graphene-based hybrids. The ease in synthesizing and processing GO has attracted the possibilities of converting it to graphene by restoring the sp2 structure and its physicochemical and electronic properties.

Chemical reductions, such as those using NaBH4,30,31 N2H4,32 and thermal treatments,33,34 can eliminate a significant fraction of the oxygenated moieties but never quite completely. As such, the term reduced graphene oxide (rGO) is given to distinguish it from pristine graphene. Typically, the extent of reduction of oxygen content in rGO is approximately 70%,32 resulting in an electron conductivity of ∼80 S m−1.31 Further removal of the residual hydroxyl and carboxylic groups can improve the conductivity, but this requires extreme treatments such as those with concentrated sulfuric acid and high temperature annealing (≥1000 °C) under H2 gas.31 Depending on the applications, it is not always necessary to remove these residual oxygenated groups, as they can be useful as adsorption sites35 or functionalization and anchoring points for the fabrication of composite materials (Sections 2 and 3).36,37 In fact, the facile and robust processing of rGO has attracted many efforts in utilizing this material for wide ranging applications, from transparent conductive layer,38,39 chemical and biosensing,40−43 cancer therapy,44,45 photovoltaic solar cells,46,47 and supercapacitors and batteries48,49 to composite photocatalysts.27,50 Often enough, the applications of graphene and its analogues (i.e., GO and rGO) require some form of chemical functionalization such that their functional properties can be integrated in the target products. In this Review, the protocols will be categorized as “soft functionalization” when using soft matters such as organic and linker molecules (Section 2) and “hard functionalization” when using solid inorganic materials (Section 3). More specifically, the functionalization by soft matters proceed through covalent (e.g., acylation, isocyanate esterification, diazotization) or noncovalent interaction (e.g., π−π stacking, hydrophilic and hydrophobic interactions). Hard functionalization includes the interfacing of graphene and the analogues with noble metal (e.g., Au, Pt, Rh) or semiconductor particles (e.g., TiO2, CdS, ZnO, BiVO4). A brief account on the functionalization routes as well as the integration of graphenebased composites in energy- and environmental-related applications will be presented (Section 4). For convenience, 8083


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Figure 2. Schematic illustration of covalent functionalization routes targeting the different sites of graphene: acylation reaction (Route I), nucleophilic ring-opening reaction (Route II), isocyanate/esterification reaction (Route III), diazotization (Route IV), and cycloaddition reaction (Route V).

the term “graphene” will be used in the most general sense to incorporate pristine graphene, GO, and rGO, unless otherwise stated. Instead of presenting an exhaustive list of applications, here, we aim to provide some key energy and environmental application of graphene composites to exemplify the current research trend and, more importantly, the fundamental principles in which they are used. On the basis of these principles, it is possible to extend them to various other applications not readily covered in this Review.

(−R) on graphene oxide sheets through amide bonding (Figure 2, Route I). The amine-terminated organic molecules used to functionalize rGO can alter the hydrophobic−hydrophilic characteristic of the graphene for dispersion in different solvents, a critical step that potentially leads to low-cost solution processing of graphene. For example, the amidation of GO using octadecylamine (ODA, C18−NH2) allows the dispersion in tetrahydrofuran (THF), carbon tetrachloride, and 1,2-dichloroethane.51 Other amine-terminated molecules (e.g., polyethylene glycol (PEG),55 DNA41) have also been successfully grafted onto GO through the amidation reaction. The resulted hybrids exhibit outstanding stability and biocompatibility in biological matrix that can be potentially used for drug delivery and DNA detection.58 The conjugation of graphene with optoelectronically active materials is promising in the field of optoelectronics and photovoltaics. As such, GO and rGO sheets have been successfully decorated with fullerene (C60),59,60 porphyrins,52,53,61−63 phthalocyanines, and polythiophenes through the amidation reaction. Prior to the amidation process, amine groups are to be introduced to the pyrrolidine ring through chemical modification.59,61 Amidation reaction proceeds between the amine-modified C60 and SOCl2-activated GO with the formation of amide bond between pyrrolidine rings of C60 and the carboxylic groups on GO.60 The strong amide linkage is favorable for fast electron transfer from graphene to the C60 acceptor, thus affording better electronic and optical properties in the C60/rGO hybrids. In the amine-terminated porphyrin-functionalized graphene hybrid,52 the fluorescence of photoexcited triphenyl porphyrin was effectively quenched by the electron-transfer process, confirming that such a covalent bond is beneficial for efficient charge transfer. 2.1.2. Epoxy Ring-Opening Reaction by Nucleophilic Attack. Epoxy groups located on the graphene basal are susceptible to ring-opening reaction upon nucleophilic attack at the α-carbon by amine.64−69 The formed linkage can be amide or ester bond, depending on the nucleophilic reagent (R−NH2

2. “SOFT” FUNCTIONALIZATION WITH ORGANIC MOLECULES 2.1. Covalent Functionalization. There are two types of target sites on graphene and its analogues that are suitable for covalent functionalization. The first is the oxygenated groups (−OH and −COC on the basal plane and −COOH at the edge) where attachment of new moieties can be carried out by condensation, esterification, or reactions targeting the lone pair electrons of the oxygen atom. The second is the −CC sites on the basal plane of the graphene requiring the attacks by free radicals or dienophiles species. The following subsections provide an overview of five of the more commonly employed covalent functionalization routes targeting the different sites of graphene: (1) acylation reaction; (2) nucleophilic ring-opening reaction; (3) isocyanate/esterification reaction; (4) diazotization; (5) cycloaddition reaction. These reaction routes and some typical organic reagents are illustrated in Figure 2. 2.1.1. Acylation Reaction. The carboxylic groups at the edges of rGO and GO can be modified by amine groups via amidation upon nucleophilic acyl substitution, resulting in the formation of amide bond. Prior to that, however, the carboxylic groups need to be “activated” by active reagents such as thionyl chloride (SOCl2),51−54 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC)/N-hydroxysulfosuccinimide (NHS),55 and N,N-dicyclohexylcarbodiimide (DCC).56,57 Subsequent reaction with amine-terminated organic molecules (NH2−R) eventually results in the covalently attached functional moieties 8084


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with electron-rich substrates such as that by electrophilic aromatic substitution. Upon heating of diazonium salt, highly reactive radicals are produced to attack the sp2-hybridized carbon atoms to form covalent bonds. This reaction has been widely applied to functionalize sp2-hybridized carbon materials, such as glassy carbon,77 highly ordered pyrolytic graphite (HOPG),78 and carbon nanotubes79,80 as well as graphene.81,82 More recently, exfoliated graphene on silica wafer was modified by 4-nitrophenyl through the diazotization reaction.81 Because the conjugated carbon atoms were covalently bonded with the aryl groups (Figure 2, Route IV), the overall conductivity of the functionalized graphene inevitably decreased since the sp2hybridized carbon network was disrupted. Nevertheless, the azo-coupled graphene has potential application as a tunable semiconductor material since bandgap is introduced within the sp2/sp3 hybrid network.82 Since rGO layers are prone to aggregation through π−π stacking during the reduction of GO by NaBH430,58 or N2H4,32 and are thus less dispersible in water, functionalization with organic linkers using the diazotization reaction on GO can effectively prevent the restacking of the rGO sheets during the reduction process.78,83,84 For example, the introduction of pphenyl-SO3H on GO (prepared via diazotization) gave a better dispersibility of the resultant rGO suspension in polar aprotic solvents compared to that without functionalization.30 The well-dispersed rGO is an important feature in facilitating the blending of graphene and polymers. As an example, the grafting of radical initiator (NH2−C6H4−OH) on rGO surface by diazotization (resulting in hydrophilic rGO due to the hydroxyl group) and followed by in situ polymerization yielded the polystyrene/rGO composites.85 The general methodology can be applied to the fabrication of reinforced polymer/graphene composites with high tensile strength. 2.1.5. Cycloaddition Reaction. The cycloaddition reaction on CC induces the transformation of sp2-C to sp3-C and has been extensively used for the chemical modification of carbon nanotubes (CNTs)86,87 and fullerenes.88,89 This process is energetically favored at the tips and defects of CNTs due to the pronounced curvature. Since the exfoliated graphene does not have the same extent of surface curvatures, the defects on graphene can instead function as the reactive sites.36 The only setback is the disruption of the conjugated network of graphene upon cycloaddition reaction, rendering a lower electron conductivity.22 Regarding nitrene cycloaddition on the graphene surface, it is claimed that the cycloaddition reaction only uses a small fraction of sp2 bonds,90,91 and thus, the conjugation network has less impact on the electrical properties. In principle, a diversity of available reagents of various functional moieties (e.g., hydroxyl, carboxyl, amino, bromide, and long alkyl chain) and polymers (e.g., PEG, polystryene) can be employed for the cycloaddition reaction on graphene (Figure 2, Route V).92,93 Among its many potential applications, such functionalized graphene can be used as substrates for depositing inorganic nanoparticles (e.g., magnetic nanoparticles for separation and magnetic resonance imaging)94,95 and as dispersible conductive nanofillers in polymer matrix to fabricate graphene−polymer composites.93 2.2. Noncovalent Functionalization. The noncovalent functionalization of graphene can take place via π−π stacking, hydrophobic interaction, van der Waals forces of attraction, and electrostatic attraction, among others. Such approach is attractive because it does not disrupt the conjugated sp2 hybridized carbon network of graphene, hence retaining its

or R−OH, respectively). For example, poly(allylamine) with a large number of reactive amine groups can react with epoxy groups of the graphene oxide sheets through ring-opening reaction, forming the amide bonds;65 while poly(vinyl alcohol) can be grafted onto GO via the ester bond.66 However, the amidation process cannot be excluded due to the presence of carboxylic groups and may take place simultaneously with the epoxy ring-opening reaction if the conditions are appropriate. This is a positive note for the fabrication of more stable graphene-based hybrids by utilizing more reactive sites, i.e., both epoxy and carboxylic groups. It has been long suspected that the amidation of edge −COOH groups may not sufficiently furnish all the side chains to make the large sheets dispersible.64 Therefore, the parallel amidation and epoxy ringopening reactions may provide more linkage points, among which can be beneficial for the mechanical reinforcement of graphene oxide when cross-linked.67,68 Silane moieties can be grafted onto GO via the ring-opening reaction between epoxy groups of GO and the amine groups of 3-aminopropyltriethoxysilane (APTES) (Figure 2, Route II).68 Although the side amidation was not mentioned in the work,68 one can expect that additional silane moieties would be bonded to the carboxylic groups if the amidation is adopted, allowing a higher reinforcement effect. To produce polydisperse and stable graphene sheets in different solvents, ionic liquid (IL) such as the 1-(3-aminopropyl)-3-methylimidazolium bromide terminated with an amine group has been proposed for the functionalization of the epoxy ring-opening reaction (Figure 2).67 The resulting composites are well dispersed in water, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), as assisted by the wide solubility of the ILs. The functionalization of GO via the treatment with sodium azide gives rise to a graphene-like material with amino moieties (GO-NH2) that can be easily modified by 1-octadecyne for dispersion in organic solvents.69 2.1.3. Isocyanate/Esterification. Organic compounds that contain an isocyanate group (−NCO) are reactive toward a variety of nucleophiles (R−OH) to form the urethane (ROC(O)N(H)R′) linkage.70 The isocyanate esterification is an efficient method to remove hydroxyl groups on GO and, in the process, alters the hydrophilicity of GO. Isocyanate compounds (R−NCO)71 have been demonstrated to functionalize GO via amides72 or carbamate esters73 bond. The formation of carbamate esters removes the surface hydroxyls and edge carboxyl groups that engage in hydrogen bonding, thus making GO sheets less hydrophilic and more compatible with polar aprotic solvents.74 The ester bond formation also occurs from the chemical reaction between −OH and −COOH groups, which is similar to the amidation process.75 For example, the −COOH on GO activated by SOCl2 was able to bind with CH2−OH terminated poly(3-hexylthiophene) (P3HT) via ester bond (Figure 2, Route III), resulting in the highly dispersible P3HT/GO hybrids in common organic solvents such as DMF and DMSO.76 A stepforth work was presented by combining the P3HT/GO product with C60 to fabricate an organic photovoltaic device with 200% increase in the power conversion efficiency compared to pure P3HT/C60 due to the extended electron delocalization that occurred after the covalent attachment of P3HT with GO.76 2.1.4. Diazotization. The use of diazonium salts (R−N2+ 2X−) is by far the most widely practiced technique in azo coupling, where the diazonium compound is reacted or coupled 8085


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Figure 3. Schematic illustration of noncovalent functionalization of graphene including π−π stacking, surfactant, and ionic functionalization.

functionalize rGO. This approach can also be extended to obtain stable dispersion of rGO in organic solvents. Ionic functionalization of graphene can be divided into two types, namely, (1) the addition of charge to the graphene surface such as the introduction of oxygenated groups by oxidation (GO can be regarded as one of the typical examples) and (2) the adsorption of a new functional group of opposite charges through electrostatic attraction. Potassium intercalated graphite can be readily exfoliated to a few-layer graphene sheet with a negatively charged surface in an aqueous system.108 When the positively charged surfactants (e.g., quaternary ammonium salt) adsorb electrostatically on the negatively charged graphene, the graphene will transform from hydrophilic to hydrophobic and can be readily dispersed in organic solvents, such as chloroform (Figure 3).109

superior intrinsic electron and thermal conductivity. This can be achieved while at the same time improving the dispersibility of the functionalized graphene in targeted solvents for potential utilizations in electronic and chemical/biological systems. 2.2.1. π−π Stacking. Molecules and polymers containing aromatic structures can be attached onto graphene through the strong π−π interaction. A typical example is the functionalization of graphene or rGO with 7,7,8,8-tetracyanoquinodimethane (TCNQ), where the formed TCNQ−graphene hybrids maintained excellent conductivity without disruption to the sp2-C network of graphene (Figure 3).96 At the same time, the adsorbed TCNQ anion on graphene prevents the inter- and intra-π−π stacking of graphene, rendering it highly dispersible in water, DMF, and DMSO. By capitalizing on the nondisrupted sp2 structure and hence preserving its intrinsic electron conductivity, graphene modified using the π−π stacking methodology is suitable for the fabrication of transparent conductive thin films with potential photonic applications,97 including polymer solar cells.98 The chemical modification of rGO sheets by pyrene and its derivatives96−101 remarkably improves their dispersibility in organic solvents and polymers and can be readily fabricated as thin films. With minimal parasitic absorption in the visible region, the pyrene butanoic acid succidymidyl ester (PBASE)functionalized rGO improved the power conversion efficiency of the polymer solar cell it is applied on compared to that of pristine graphene.102 When the rGO film laminated on PDMS is functionalized with 1-pyrenecarboxylic acid (PCA), the composite exhibits unique characteristics of ultraviolet blocking and visible light transmittance that are otherwise absent in pristine graphene film.103 2.2.2. Surfactant/Ions-Assisted Hydrophobic/philic Interaction. As an amphiphilic molecule, the hydrophobic part of the surfactant can be conjugated onto the planar graphene via hydrophobic interactions, while the hydrophilic part is capable of improving its dispersibility in polar solvents. Ruoff and coworkers104 first reported the homogeneous surfactant-assisted rGO suspension by using polystyrene alkyl sulfonate as the surfactant (Figure 3). Following this, other surfactants such as sodium dodecylbenzenesulfonate83,105 and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer,106 as well as surfactants containing phospholipid107 were also successfully applied in a similar manner to

3. “HARD” FUNCTIONALIZATION WITH INORGANIC MATERIALS The overarching motivation of functionalizing graphene with foreign inorganic materials, e.g., metallic nanoparticles and oxide semiconductor, is to enhance the properties of the inorganic material or to combine the properties of the two entities. More specifically, the benefits of graphene can be summarized in the following aspects: (1) Graphene possesses ultrathin 2D structure that provides an ideal platform for anchoring inorganic nanomaterials of various sizes and shape and (2) the graphene support augments the interfacial heat and/or electron transfer liberated by the anchoring inorganic materials. To date, a large number of strategies have been developed to interface inorganic nanomaterials with graphene. In general, inorganic material can be either nucleated and grown in situ on graphene or grafted (if using prefabricated inorganic materials) by physical or covalent attachment on the graphene sheet. 3.1. Interfacing with Noble Metal Nanoparticles. Graphene is an ideal platform for anchoring metal nanoparticles given their high surface to volume ratio. Among the many different types of metals, the deposition of Au,110,111 Ag,112 Pt,113 Pd,114 and Cu,115 as well as their alloys on graphene has been reported and used in various applications including surface enhanced Raman scattering (SERS), catalysis, and electrochemical sensors.116−119 With their large specific surface area populated by oxygenated groups, the GO and rGO provide abundant nucleation sites for metal nanoparticles growth. The 8086


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Figure 4. TEM images of rGO/semiconductor composites: (a) rGO/ZnO (Reprinted with permission from ref 142. Copyright 2011 Elsevier); (b) rGO/SnO2 (Reprinted with permission from ref 145. Copyright 2011 Wiley-VCH); (c) rGO/Fe2O3 (Reprinted with permission from ref 144. Copyright 2011 Royal Society of Chemistry); (d) rGO/ZnS (Reprinted with permission from ref 150. Copyright 2010 Springer); (e) rGO/Co3O4 (Reprinted from ref 147. Copyright 2011 American Chemical Society); (f) rGO/CdS (Reprinted from ref 149. Copyright 2013 American Chemical Society); (g) rGO/TiO2 (Reprinted with permission from ref 141. Copyright 2011 Royal Society of Chemistry); (h) rGO/Bi2WO6 (Reprinted with permission from ref 148. Copyright 2011 Royal Society of Chemistry).

first report of Au/rGO nanocomposites was based on the simultaneous chemical reduction of HAuCl4 and GO by NaBH4 in THF medium.120 Mild reducing agents such as ethylene glycol and H2 are often required to prepare rGO-supported Au,121 Pt,122,123 and Pd114 nanocomposites. Surfactants such as hexadecyl-trimethylammonium bromide and polyvinylpyrrolidone can be introduced as additives to control the morphology of the supported nanoparticles as demonstrated for Au nanorod,124,125 cubic Au nanosheet,111 and dendritic Pd/Pt.113 In the electrochemical deposition approach, graphene sheet is coated onto electrode and coreduced in an electrolyte containing the desired metal precursor. The deposition of Au126−129 and Pt130,131 are the most commonly prepared using this method because of their facile reduction and resistance to chemical oxidation. Galvanic reduction−deposition of metal cations by rGO is also possible for metals with reduction potentials more positive than that of the GO/rGO potential (+0.38 V vs SHE) as has been demonstrated for the deposition ° +/Ag0 = +0.73 V vs SHE),110 Au (EAuCl ° 4−/Au0 = +1.002 of Ag (EAg

V vs SHE),111 and Pd (E°PdCl42−/Pd0 = +0.83 V vs SHE),132 but it is not possible for Cu (ECu2+/Cu0 = +0.38 V vs SHE).133 Microwave-assisted coreduction of GO and metal ions is another popular technique to synthesize the metal/rGO composites,115 where the deposition of a series of metals (Au, Ag, Cu, Pd) and metal alloys (CuPd, NiCo) can be carried out in the presence of reductants or in reducing solvents such as oleylamine and ethylene glycol.134,135 In some cases, the reduction of Au salts can be readily induced by microwave synthesis without the presence of reducing agents since the electromagnetic radiation significantly reduces the barrier of metal ion reduction and nucleation, while the oxy-functional groups on GO stabilizes the Au nuclei for further deposits growth.136 In general, the method of attaching prefabricated metallic particles on graphene is preferable if precise size, morphology, and/or distribution control cannot be easily achieved by the nucleation−deposition method as described above. In most 8087


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Figure 5. (a) Schematic illustration of spatial deposition of Pt nanoparticles on rGO/TiO2 through Pt(OH)x precipitation and the UV-assisted photocatalytic reduction process and TEM images of the different photocatalysts, namely, (b) rGO/TiO2, (c) Pt−TiO2, (d) Pt−rGO/TiO2, (e) Pt− TiO2/rGO, and (f) Pt−rGO/Pt−TiO2. The difference in (d)−(f) lies in their spatial decoration of Pt, where Pt is exclusively deposited on rGO in (d) Pt−rGO/TiO2 or on TiO2 in (e) Pt−TiO2/rGO, while Pt is randomly deposited on both TiO2 and rGO in (f) Pt−rGO/Pt−TiO2 Adapted with permission from ref 222. Copyright 2015 Elsevier.

cases, unless electrostatic attraction is favorable, molecular linkers are required to attach the nanoparticles to the graphene sheet, for example, using the biocompatible bovine serum albumin (BSA).137 On the other hand, pyridine-functionalized Au nanoparticles with their aromatic moieties allow for noncovalent π−π stacking on the basal plane of rGO sheet,138 while DNAs with both purine and pyrimidine bases have been used to stabilize Au nanoparticles on the rGO sheet.139,140 3.2. Interfacing Semiconductor Oxide with Graphene. The interfacing of graphene with semiconductor oxide has wide reaching implications to the applications in heterogeneous

catalysis and energy storage, where intimate contact between graphene and the semiconductor is highly desirable to facilitate interfacial charge transfer. For this purpose, a range of synthetic techniques for fabricating graphene-based composite has been developed including hydrothermal/solvothermal, coprecipitation, photoassisted reduction, and sol−gel processing. The hydrothermal and solvothermal syntheses involve treating the semiconductor nanoparticles (or their precursors) and GO in aqueous and organic solvent-based medium, respectively, and under moderately elevated temperature and vapor pressure. The formed nanoparticles are often wellcrystallized and do not require postannealing, which is well 8088


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Figure 6. (a−e) Absorption of dodecane by spongy graphene (SG) at the intervals of 20 s, where dodecane (stained with Sudan red 5B) floating on artificial seawater was completely absorbed within 80 s. (f) Comparison of the mass of SG required for the complete absorption of dodecane in artificial seawater. (g) Contact angle of SG surface (upper panel) and the absorption of dodecane (lower panel). SEM images of the (h) microporous (fusiform) structure of SG and (i) the graphene skeleton, and (j) TEM image of the graphene skeleton. Adapted with permission from ref 35. Copyright 2012 Wiley VCH.

manipulation of the photocharge transport and hence the overall quantum efficiencies (Figure 5; see more in Section 4.3).222

suited to GO and rGO-containing composites since the graphene can be easily removed at high temperature annealing in the slightest oxygen pressure. As shown in Figure 4, among the various semiconductor oxides/graphene composites that have been synthesized using the hydrothermal/solvothermal method include those of TiO2,141 ZnO,142,143 Fe2O3,144 SnO2,145,146 Co3O4,147 and Bi2WO6.148 Chalcogenides such as CdS,149,150 CdTe,151 and ZnS150,152 on rGO have also been demonstrated (Figure 4f), and for these, S or Te precursors often function as the reducing agents during the hydrothermal/ solvothermal synthesis.149,152 For example, in the synthesis of rGO−CdS composite, the DMSO serves not only as a solvent but also as a source of sulfur and reductant (upon decomposition).149 The abundant oxygenated groups on GO are the sites to anchor chemically coordinated metal ions for further precipitation by raising the pH or temperature of the suspensions to form supported metal hydroxide/oxide. Precipitation of Fe3+/Fe2+ ions and GO in alkaline medium produces GO−Fe3O4 nanocomposites, which can be further reduced to rGO/Fe3O4 nanocomposites by hydrazine.153 A one-pot method was reported for the synthesis of rGO/TiO2 composite using TiCl3 as both the precursor of TiO2 and the reducing agent of GO.154 A similar procedure can be seen in the preparation of SnO2/rGO composites.141,155 In the case of graphene-chalcogenide nanocomposites, the in situ production of H2S from S precursors acts as both the sulfur source and reducing agent, as demonstrated for the fabrication of rGO/ (ZnS, CdS) composites.156 The direct photocatalytic reduction of GO by semiconductor photocatalyst is another common methodology for producing semiconductor/rGO composites. As first demonstrated by Kamat and co-workers, the photoassisted preparation of rGO/ TiO2 composite makes use of the photogenerated electrons (from ultraviolet-irradiated TiO2) as the reduction source, while ethanol was introduced as the hole scavenger.157 The facile technique paves the way for the fabrication of other graphene-based semiconductor composites, which besides TiO2,141 includes ZnO158,159 as well as visible light-driven WO3 and BiVO4.160 By firstly precipitating Pt(OH)x on either TiO2 or GO and followed by photocatalytic reduction, we recently demonstrated the spatial deposition of Pt exclusively on TiO2 or rGO of the rGO/TiO2 composite that strongly affects the resultant photocatalytic efficiencies due to the direct

4. APPLICATIONS OF FUNCTIONALIZED GRAPHENE-BASED COMPOSITES 4.1. Decontamination of Water through Physical/ Chemical Adsorption. Like activated carbon and other carbon allotropes, graphene is an excellent adsorbent of aqueous pollutants by virtue of its large specific surface area available for pollutants sequestration.161 Particularly, GO obtained from the oxidation and exfoliation of graphite contains functional groups, such as −COOH, −CO and −OH, which can be utilized as the anchoring sites to bind metal cations by both electrostatic and/or coordinated interactions.162−164 Furthermore, the oxygenated groups enhance the GO hydrophilicity, allowing it to be applied readily in aquatic and biological environments.165 A large number of studies have been carried out on graphene and its composites for the adsorption of hazardous pollutants, ranging from small molecules,166 heavy metal ions,167−169 dyes,170,171 pesticides,172,173 and aromatic pollutants174 to antibiotics175 in an aqueous environment. The adsorption capacity of graphene depends on the nature of the adsorption sites (e.g., presence of defects and surface oxygenated groups) and pore structure of the graphene aggregates, as well as the pH of the aquatic environments. The different modes of pollutants adsorption on graphene include (1) electrostatic attraction and coordination with the lone pair electrons on the oxygenated groups especially that of the carboxylic groups;176−178,180,183 (2) binding of heavy metal ions on the π electrons on the graphene basal plane (Lewis base) to form the electron donor−acceptor complexes;179,181 (3) adsorption on externally introduced functional moieties or decorated nanoparticles on graphene sheets, for example, the adsorption on MnO2-decorated graphene.182 The introduction of microporosity in graphene-based materials is an effective method to increase the adsorption capacity such as that demonstrated for porous chitosan− gelatin/GO composite that shows high affinity toward Cu2+ and Pb2+.182 Amino-rich chitosan has a significant advantage in adsorbing heavy metal ions due to the chelation ability of the amines. In fact, the amine group also serves as anchoring points for the covalent attachment to GO. The ordered architecture of 8089


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Figure 7. (a) Structure of EDTA-functionalized GO by silanization and (b) its interactions with heavy mention cations on the carboxylate moieties. Adapted from ref 188. Copyright 2012 American Chemical Society.

Figure 8. (a) Schematic diagram on the fluorescence sensing mechanism for the detection of Pb2+ ions. Au nanoparticles react with thiosulfate and formed Au(S2O3)23− complexes, which could be rapidly be dissolved to form Au(2-ME)2− complexes after adding Pb2+ and 2-ME. The fluorescence of the rGO−Au increased owing to the gradual dissolution of Au nanoparticle. Adapted from ref 201. Copyright 2012 American Chemical Society. (b) Schematic illustration of the DNAzyme-GO-based fluorescence sensor for Pb2+ detection. Upon the addition of Pb2+, the DNAzyme was activated and cleaved the substrate strand, releasing a short FAM-linked oligonuleotide fragment and hybridizing with another substrate strand, inducing the second cycle of cleavage by binding Pb2+ and providing an amplified detection signal for Pb2+. Adapted from ref 203. Copyright 2011 American Chemical Society.

desorption by HCl can reach >90% within 1 h at below pH 2, as the protons disrupt and displace the coordination spheres of chelated metal ions on the GO surface.190 For practical applications, the toxicity of the graphene sorbents should be taken into consideration. If uncontained, these ultrafine graphene sheets may find their way into the open environment and potentially result in (yet-to-be-assessed) environmental damage since the sharp edges of GO and rGO are known to be damaging to the cell membranes of microbes.192,193 Thus, besides meeting the requirements for high stability and reusability, it is necessary to contain the release of graphene-based sorbents especially during prolonged usages. 4.2. Graphene-Based Sensors for the Detection of Aqueous Heavy Metal Ions. The most common types of graphene-based sensors that have been developed for aqueous contaminants sensing are those based on the electrochemical, fluorescence, and field-effect transistor (FET) sensing. In particular to this subsection, we will focus mainly on heavy metal ions. For other biological and chemical graphene-based sensors, readers are referred to the excellent review by Liu et al.194 Although the direct adsorption of heavy metal ions on the oxygenated groups of GO and rGO can take place readily, the soft functionalization with DNAzyme, polymers, acids, amines, and peptides can be exploited as smart sensing platforms as will be exemplified below. Fluorescence sensors rely on the LUMO−HOMO transition of GO as given by the coexistence of the sp2/sp3 carbon domains.195 The rGO, on the other hand, with a relatively high charge mobility and large detection area as well as tunable ambipolar field-effect characteristics, is the best candidate for

the GO was maintained by the cross-linking chitosan that prevented the aggregation of GO when complexed with metal ions. For the same reasons, poly(amidoamine)-modified GO exhibits superior performance in adsorbing heavy metal ions, including Cu 2+, Zn 2+, Fe 3+ , Pb 2+ , and Cr 3+ .183 When constructed as a 3D spongy graphene (SG), it can be a versatile and recyclable sorbent material with efficient adsorption of petroleum products, fats, and toxic solvents as shown in Figure 6.35 The addition of magnetic functionality to graphene sorbent is advantageous for the ease in recovery and reuse. For example, by attaching GO or rGO onto magnetic Fe3O4 nanoparticles, the composite can be easily recovered using a permanent magnet after adsorbing toxic trivalent and pentavalent arsenic, hence circumventing the needs for energy-intensive separation such as centrifugation or membrane filtration.183−186 The introduction of chitosan-functionalized GO attached to magnetic particles can enhance adsorption of methyl blue184 and Cu2+185 by virtue of the rich amine moieties of the chitosan, while retaining the aqueous dispersibility of the GO composite. In a less straightforward adsorption, magnetically recoverable Co@graphene core@shell nanocomposite efficiently recovers aqueous AuCl4− through the galvanic reduction by leached Co2+ to form metallic gold deposits on the graphene shell.186 Furthermore, it is possible to functionalize the graphene with chelates, e.g., EDTA, to adsorb a wider variety of heavy metal ions (Figure 7).187,190 Once recovered, the saturated graphene sorbent can be regenerated by treating with typical desorbing agents, such as HCl189,190 and thiourea−HCl solution.191 In the example case of Pb2+ on EDTA-functionalized GO (by silanization reaction, Figure 2, Route II), 8090


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Figure 9. (a) Stepwise modification of gold electrode with L-cysteine functionalization followed by the EDC/NHS-mediated coupling of activated GO. Au surface was modified by self-assembled L-cysteine and further coupled with a carboxylic group of graphene oxide in the presence of EDC/ NHS through an amide bond. Adapted with permission from ref 211. Copyright 2014 Royal Society of Chemistry. (b) Schematic of the preparation of GO modified sensing interfaces, where Au electrodes were first modified with 4-aminophenyl and then converted either to the Au−Ph−GO interface through aryl diazonium salt chemistry or to Au−Ph−NH−CO−GO through the coupling reaction, for detection of heavy metals. Adapted with permission from ref 212. Copyright 2015 Wiley VCH.

the applications as FET196 and electrochemical sensors.197 The ability to modulate the fluorescence of GO sheet by surface functionalization and size manipulation resulted in its numerous applications as fluorescence sensors.198 An effective platform for the selective sensing of Pb2+ was demonstrated by using rGO−Au nanoparticle (Figure 8a), where the presence of Pb2+ accelerated the leaching rate of Au (by galvanic oxidation) in the presence of thiolsulfate and alkanethiol, resulting in “turn-on” fluorescence due to the suppressed fluorescence resonance energy transfer (FRET) between rGO and Au nanoparticle.199,200 The rGO−Au sensor is capable of ultralow detection limit of 10 nM with excellent selectivity and repeatability.201 More recently, Mohan and Neeroli Kizhakayil202 took advantage of the duality of graphene to bind Rhodamine by π−π stacking and dispersive interactions to fabricate a FRET-based sensor for the selective detection of Hg2+ as low as 380 ppt. The Rhodamine is an example molecule that exhibits altered fluorescence properties upon specific binding with Hg2+. For more general heavy metal ions sensing, a fluorophore carboxyfluorescein can be attached to the end of a sacrificial substrate strand that is in turn bound to the DNAzyme−GO hybrid sensor.203 The presence of Pb2+ activates the DNAzyme upon the addition of Pb2+ and cleaves the substrate strand into two parts, thereby releasing the short fluorophore-linked oligonuleotide fragment (Figure 8b). This allows turn-on fluorescence, even with a mere 300 pM Pb2+ that was otherwise quenched when the fluorophore was attached onto the GO. By capitalizing on the specificity of the DNA

sequence, a similar DNAzyme−rGO sensor, formed by the electrostatic interaction between the amino-functionalized rGO and the DNA sequences, can be fabricated for the simultaneous detection of Pb2+ (limit 7.8 pM) and Hg2+ (limit 5.4 pM).204 The sensor is highly stable and promising for the application in environmental monitoring. In terms of electrochemical sensors, graphene-based composite electrodes are well-known to exhibit better performance than the conventional graphite sensors due to the larger specific surface area of the former.205 As part of the electrode fabrication, the graphene-based composites can be fixed onto the electrode substrate by Nafion adhesive. The resultant electrochemical sensor shows high sensitivity toward Pb2+, Cd2+, Zn2+, and Cu2+ (at concentrations as low as 350 nm, Hg arc lamp

activity (percentage of degradation or rate constant)

ref.

85% in 55 min; 65% in 65 min 0.211 min−1 98.8% in 80 min 82% in 180 min

229 230 231

0.0285 min−1 0.067 min−1 35% in 180 min

225 232 233

0.0102 min−1 90% in 50 min 65% in 90 min 0.0341 min−1

224 226

235

≥400 nm, cutoff filter, Xe arc lamp 500 W Hg arc lamp 500 W Xe arc lamp 90% in 10 min 0.07 min−1

245

0.6

self-assembly

methylene blue

365 nm, 24 W UV lamp

0.05 mg/cm2

photoreduction and dropcasting

≥320 nm, cutoff filter, 450 W Xe arc 0.008 min−1 lamp

246

G-TiO2/ MCM-41 GO/TiO2 rGO/ZnO rGO/CoFe2O4

0.15

223

0.855 min−1 0.098 min−1 0.01483 min−1

228 247 248

N-rGO/ZnSe

N/A

solvothermal

methylene blue

90% in 4.5 min

249

GO/Ag/AgBr GO/Ag/AgCl rGO/SnO2

N/A

water/oil emulsion

methyl orange

360 nm, water filter, 100 W Hg arc lamp 254 nm, 11 W UV lamp 254 nm, 8 W UV lamp ≥420 nm, cutoff filter, 500 W Xe arc lamp ≥400 nm, cutoff filter, 500 W Xe arc lamp ≥400 nm, cutoff filter, 500 W Xe arc lamp

45% conversion in 24 h

N/A 2.0 40

hydrothermal and impregnation self-assembling hydrazine reduction hydrothermal

2,4dichlorophenolacetatic acid 2-propanol

50%

hydrothermal

rhodamine B

rGO/Bi2WO6

N/A

hydrothermal

rhodamine B

rGO/InNbO4 rGO/ZnFe2O4

N/A 20%

hydrothermal hydrothermal

methylene blue methylene blue

a

methylene blue methylene blue methylene blue

85% in 40 min 71% in 40 min ≥420 nm, cutoff filter, 300 W Xe arc 0.091 min−1 lamp ≥420 nm, cutoff filter, 500 W Xe arc 90% in 4 min lamp 99% in 90 min lamp

250 251 148 252 253

Only the optimum loadings of graphene that give the highest photocatalytic efficiencies are shown.

acetic acid.222 More importantly, we demonstrated the decoupling of the Schottky barrier induced-charge separation (requiring direct Pt−TiO2 contact) and the cocatalytic effects (Pt deposited on rGO), showing unambiguously the superior effect of the former.222 On the basis of this, a new configuration of spatially deposited Pt and rGO on TiO2 was achieved that exhibited more than 3-fold higher activity than the conventional Pt−TiO 2 benchmark photocatalyst (Figure 11). While

graphene-based photocatalysts have been shown to be favorable in the decomposition of gas phase organic compounds such as propanol,223 acetone,224 and formaldehyde,225 mechanistically, it has been difficult to probe the generation of active radical species especially the superoxide radicals formation on the attached graphene (where surface electron transfer via the graphene sheet to adsorbed O2 is expected). In principle, one can expect to obtain indirect information on the effects of the 8093


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Figure 11. (a) Photocatalytic activity of Pt−TiO2 (black square) as a function of Pt loading and that deposited exclusively on the rGO of the optimal 2% Pt−TiO2/rGO (red circle). (b) Schematic illustration showing the Schottky barrier effect (Pt on TiO2) combined with electron extraction by rGO and catalytic electron transfer (Pt on rGO) for oxygen reduction, enhancing the overall photocatalytic activity. Adapted and modified with permission from ref 222. Copyright 2015 Elsevier.

The optimum ratio of graphene in composite photocatalysts is typically found to be 5 wt %,245,256−259 beyond which parasitic light absorption becomes significant.228 Hydrothermally prepared composite has been shown to be one of the most active photocatalysts toward H2 evolution (compared to rGO/TiO2 prepared by photoreduction260 and chemical reduction by N2H4)261 due to the intimate interfacial ohmic contact to facilitate electrons transfer.262 To further maximize the amount of interfacial contact, TiO2@rGO core@shell structure was shown to exhibit significantly higher H2 evolution rate compared to the typical TiO2-loaded rGO sheets (Figure 12).260 The rGO-supported TaON@CdS core@shell photocatalyst exhibits a remarkable enhancement of 141 times in photocatalytic H2 evolution compared to the pristine TaON.263 While the CdS−TaON heterojunction enhances the photocharge separation, the rGO serves as an electron acceptor that further prolonged the photoelectrons lifetime for H 2 evolution.263 The effect can also be evidenced through the heterojunction array of α-Fe2O3@BiV1−xMoxO4 core@shell nanorods, where rGO in the interlayer facilitated fast charge separation for photoelectrochemical water oxidation on BiV1−xMoxO4 and electron transport to the back of the electrode through the α-Fe2O3 cores (Figure 13a).264 In fact, depending on the quality of the contact between rGO connector and the photocatalysts, it is possible to achieve Zscheme photoexcitation as shown between CdS and TiO2 (i.e., net photoelectron on CdS and photoholes on TiO2) instead of interfacial charge trapping or the more commonly observed sensitization effects (net photoelectrons on TiO2).147 This requires the rate of interfacial recombination of photoelectron (from TiO2) and photohole (from CdS) to be faster than the rate of sensitization (Figure 13b). Geometrically, the sheet-on-sheet sandwich-type of BiVO4graphene heterostructure gives maximum charge transfer interface, rendering high photocatalytic efficiency in O2 evolution.265 In a similar construct, ZnIn2S4 nanosheet grown in situ on graphene enhances the H2 generation and stability since the efficient electron extraction by graphene suppresses the photoreduction of In3+ and Zn2+.266 Kamat and co-workers112 reported rGO as a common support for both TiO2 photocatalyst and Ag cocatalysts, demonstrating the efficient photoelectron transfer across the rGO sheet and paving the way for the development of multifunctional graphene-based nanocomposites. For example,

degradation mechanism by following the dynamics of the state change of graphene during the photocatalytic process, such as the C/O ratio, size, defects, and morphology. Clarification on the mechanisms of graphene-based photocatalysts shall allow for the rational design of more efficient architecture to target different pollutants, both in air and water. Although the two-dimensional structure of graphene has been claimed to be beneficial for charge transfer and pollutants adsorption,220 comparison of rGO/TiO2 and CNT/TiO2 in the photocatalytic degradation of aqueous methylene blue and gaseous benzene found that both composite photocatalysts are essentially the same when the adsorption effect is excluded.226 The appropriate target substrate is important to correctly reflect the difference between the two carbon allotropes in promoting photocatalytic activity.227 In a relatively remote case when carbon is doped into the TiO2 lattice (forming a Ti−O−C bond), the photoresponse of the photocatalyst can be extended up to 440 nm.220 The effect can be traced to the localized occupied states in the TiO2 bandgap as a result of substitutional C doping in the lattice O sites and/or by interstitial doping.228,229 4.4. Photocatalytic H2 Evolution and Water Splitting. The photocatalytic generation of H2 is one of the pillar technologies in achieving a sustainable hydrogen economy in the foreseeable future.254 To achieve water splitting in the absence of potential bias, the conduction band (CB) edge potential of the bare photocatalyst needs to be more negative than that required for the reduction of protons (E°H+/H2 = 0 V vs RHE, pH 0), while and valence band (VB) edge potential needs to be more positive than that for the oxidation of water (EH2O/O2° = +1.23 V vs RHE, pH 0). In the hydrogen evolution reaction, sacrificial electron donors (e.g, biomass and wastewater) can be added to enhance electron−hole separation to overcome the sluggish 4-electron transfer water oxidation step or if the oxidation potential is less positive than +1.23 V vs RHE. Interfacing rGO with irradiated photocatalyst allows efficient extraction of photoelectrons given the high work function of graphene (Φ = 4.42 eV),38 comparable to Ag (Φ = 4.26 eV) and Cu (Φ = 4.7 eV),255 that resulted in enhanced interfacial charge separation. Besides, the planar structure of graphene serves as reactive sites for H2 evolution. A number of synthetic approaches have been developed in recent years to yield efficient graphene/photocatalyst composites, both for H2 evolution and water splitting (Table 2). 8094


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Chemistry of Materials Table 2. Graphene-Based Semiconductor Photocatalyst for H2 Evolutiona photocatalyst

graphene loading (wt %)

preparation method

rGO/TiO2

5.0

sol−gel

rGO/TiO2

0.7

rGO/TiO2 rGO/TiO2

0.2 2.0

photocatalytic reduction hydrothermal hydrothermal

rGO/TiO2

1

rGO/TiO2

N/A

rGO/C3N4

N/A

rGO/CdS

2.0

impregnation and reduction sol−gel

rGO/CdS

1.0

solvothermal

rGO/NiO N-GO/QDs

97 N/A

wet chemical NH3-treatment

GO/SiC

1

solution mixing

rGO/AgBr rGO/Cu2O

N/A 3.8

solution mixing in situ growth

rGO/CuInZnS

2.0

solvothermal

N-rGO/CdS

2

solution mixing

rGO/ZnIn2S4

5

solvothermal

rGO/ZnCdS

0.25

rGO/ZnxCd1‑xS

0.5

coprecipitation and hydrothermal hydrothermal

rGO/ Sr2Ta2O7−xNx rGO/TiO2/MoS2 MoS2-rGO/ZnS

5 5.0 0.25

GO/CdS/Pt

N/A

rGO/Cu/TiO2 rGO/RuO2− TiSi2 rGO/NiTi-LDH

2.0 1

solution mixing and reduction hydrothermal precipitation

2

coprecipitation

rGO/ CdS@TaON rGO/CdS/Al2O3 rGO/CdS/ZnO BiVO4-rGO-Ru/ SrTiO3:Rh

1

hydrothermal

1

solid state milling

5.0

photocatalytic reduction

rGO/EY-MoS2

N/A

hydrothermal

CTAB/TPPH/ rGO Ru(dcbpy)3− rGO/Pt [Ru(bpy)3]2+/ rGO EY−rGO/Pt

2/3

RB-rGO/Pt EY−rGO

N/A 50

hole scavengers

light source

Solid Photocatalyst−Graphene Composites 500 W Xe arc lamp 0.1 M Na2S/0.4 M Na2SO3 10 vol % methanol ≥320 nm, cutoff filter, 300 W Xe arc lamp 25 vol % methanol 500 W Xe arc lamp 0.1 M Na2S, 0.04 M 500 W Xe arc lamp Na2SO3 25 vol % methanol 350 W Xe arc lamp

Cocatalyst

H2 evolution rate (μmol/h)

ref.

none

8.6

256

Pt

50

260

Pt none

43.8 5.4

282 267

none

736

283

none

20

284

Pt

451

285

none

∼5

286

Pt

1120

268

Ni none

∼70 0.6

287 288

none

4.2

289

none Pt

0.44 265

290 291

none

3800

278

none

210

292

none

82

266

none

1824

293

none

0.053

294

Pt

293

295

MoS2 rGO, MoS2 Pt

165 226

259 277

275

257

300 W Hg arc lamp ≥420 nm, cutoff filter, 150 W Xe arc lamp >420 nm, cutoff filter, 300 W Xe arc 0.01 M AgNO3 lamp 0.1 M Na2S, 0.04 M ≥420 nm, cutoff filter, 300 W Xe arc Na2SO3 lamp 0.35 M Na2S, 0.25 M 500 W tungsten halogen lamp Na2SO3

Cu RuO2

1275 9.8

296 297

none

98

298

Pt

633

263

none

299

Ru

none

84

300

solution mixing

≥420 nm, cutoff filter, 300 W Xe arc lamp Dye-Sensitized Graphene Composites 15 vol % TEOA ≥420 nm, cutoff filter, 300 W Xe arc lamp 10 vol % TEA 150 W Xe arc lamp

350 751 11

Pt

2240

281

4/5

solution mixing

5 vol % TEOA

Pt

2533, 118

276

N/A

solution mixing

20 vol % methanol

none

3290

280

18

in situ photoreduction solution mixing solution mixing

10 vol % TEOA

10

272

14 14

274 273

microwave-assisted hydrothermal hydrothermal

photocatalytic reduction hydrothermal hydrothermal

0.1 M Na2S, 0.04 M Na2SO3 25 vol % methanol

UV−vis, 150 mW cm−2

≥400 nm, cutoff filter, 350 W Xe arc lamp 0.05 M Na2S, 0.07 M 450 W Hg arc lamp Na2SO3 10% lactic acid ≥420 nm, cutoff filter, 350 W Xe arc lamp 20 vol % methanol ≥250 nm, 400 W Hg arc lamp pure waterb 420 nm < λ < 800 nm, cutoff filter, 300 W Xe arc lamp 0.3 M KI ≥420 nm, cutoff filter, 300 W Xe arc lamp 33 vol % methanol 300 W Xe arc lamp 20 vol % methanol ≥400 nm, cutoff filter, 1500 W Xe arc lamp 0.1 M Na2S, 1.2 M ≥420 nm, cutoff filter, 800 W Xe−Hg arc lamp Na2SO3, 0.1 M Na2S, 0.1 M ≥420 nm, cutoff filter, 300 W Xe arc Na2SO3 lamp 0.35 M Na2S, 0.25 M ≥420 nm, cutoff filter, 300 W Xe arc Na2SO3 lamp 0.35 M Na2S, 0.25 M AM 1.5, 100 mW cm−2 Na2SO3 0.35 M Na2S, 0.25 M ≥400 nm, cutoff filter, 150 W Xe arc Na2SO3 lamp 20 vol % methanol 300 W Xe arc lamp 25 vol % ethanol 0.005 M Na2S, 0.005 M Na2SO3 20 vol % methanol

350 W Xe arc lamp 300 W Xe arc lamp 400 W Hg arc lamp

33 vol % methanol pure water

H2SO4 (pH 3.5)b

150 W Xe arc lamp; ≥400 nm, cutoff filter, 150 W Xe arc lamp 532 nm, laser

≥420 nm, cutoff filter, 150 W Xe arc Pt lamp ≥420 nm, 300 W tungsten halogen lamp Pt ≥420 nm, 300 W tungsten halogen lamp none

15 vol % TEOA 15 vol % TEOA

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Chemistry of Materials Table 2. continued photocatalyst TPA-rGO/Pt a

graphene loading (wt %) N/A

preparation method solution mixing

hole scavengers

light source

Dye-Sensitized Graphene Composites 1.0 M KI 150 W Xe arc lamp

Cocatalyst Pt

H2 evolution rate (μmol/h) 2.3

ref. 279

EY: Eosin Y; RB: Rose Bengal; TPA: triphenylamine; TEOA: triethanolamine. bWater splitting without hole scavengers.

Figure 12. Fabrication of (a) reduced nanosized GO-coated TiO2 nanoparticles (r-NGOT) and (b) r-GO/TiO2 with deposited TiO2 on a larger rGO sheet (r-LGOT) and (c) their rates of photocatalytic H2 evolution compared to bare TiO2. Light source, ≥ 320 nm cutoff filter, 300 W Xe arc lamp. Adapted from ref 260. Copyright 2012 American Chemical Society.

Figure 13. (a) Schematic band structure of α-Fe2O3@ BiV1−xMoxO4 heterojunction and the photocharge separation mechanism with and without rGO interlayer. Adapted from ref 264. Copyright 2012 American Chemical Society. (b) Z-scheme system consisting of a Pt-loaded metal sulfide ptype photocatalyst and rGO/TiO2 composite photocatalyst for water splitting. Adapted from ref 258. Copyright 2011 American Chemical Society.

tions of Rh/CuGa0.8Zn0.4S2 and BiVO4,269 Ru/SrTiO3:Rh and IrOx/SrTiO3:Rh,270 and Ir/CoOx/Ta3N5 and CoOx/Ta3N5.271 As an extension of our earlier discussion on dye sensitization (Section 4.3), graphene on its own (i.e., without particulate photocatalysts) can serve as a platform for the immobilization of dye molecules for the water reduction reaction (Figure 14). To date, the range of dye-sensitized rGO systems that has been investigated for H2 evolution includes immobilized polyar-

decoration of Pt cocatalyst on rGO/CdS improved the H2 evolution activity by 2.5 times relative to that without Pt.268 By capitalizing on the efficient electron transport, rGO can be used as an electron mediator to construct Z-scheme photocatalyst systems such as that between Ru/SrTiO3:Rh H2 evolution photocatalyst and BiVO4 O2 evolution photocatalyst.258 The concept has been extended to the construction of various Zscheme water splitting photocatalysts including the combina8096


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Figure 14. (a) Schematic illustration of the mechanism and (b) the energy level diagram for photocatalytic H2 evolution over Eosin Y sensitized graphene/Pt photocatalyst under visible light irradiation and in the presence of triethanolamine (TEOA) sacrificial donor. (c) The time courses of hydrogen evolution over EY−Pt, EY−rGO, and EY−rGO/Pt photocatalysts. Adapted and modified from ref 272. Copyright 2011 American Chemical Society. (d) Photocatalytic activities of H2 evolution over different dyes sensitized graphene/Pt photocatalysts. Adapted and modified with permission from ref 274. Copyright 2013 Elsevier.

omatic xanthenes dyes such as Eosin Y (EY),272,273 Rose Bengal (RB),274,275 and triphenylamine (TPA)257 on Pt cocatalysts/ rGO.272,274 In some cases, sacrificial donors (TEOA,276,274 S2−/ SO32−,277,278 KI,279 and methanol280) were added to enhance the rate of H2 evolution and to prolong the durability of the dyes. These dye molecules can be grafted onto graphene either covalently279 or by physical interaction.276,270 As an example of the former, TPA molecules were covalently bonded onto rGO by the linkage of the aziridine ring through the 1,3-dipolar cycloaddition,279 which as discussed in Section 2, can potentially disrupt the electron conductivity of the graphene. Alternatively, these polyaromatic dye molecules can be readily attached onto the graphene planar surface via π−π stacking. Photosensitization of the EY-attached rGO was reported to produce 2-fold higher H2 evolution rates compared to that of bare rGO but requiring a sacrificial donor such as that of TEOA (Figure 14c).272 Cosensitization by using the mixture of EY and RB (EY/RB) leads to an enhanced H2 evolution activity compared to the single dye-sensitized catalysts, owing to the enhanced light absorption (Figure 13d).274 In general, the injection of electron from sacrificial donor helps to prolong the stability of the dye against irreversible oxidation/decomposition (Figure 14b).281 4.5. Liquid-Junction and Organohalide Perovskite Solar Cells. A basic liquid junction or photoelectrochemical solar cell comprises a pair of anode and cathode, with either one or both acting as photoelectrode(s) and separated by redox mediator-containing liquid electrolyte. The photoelectrode consists of solar absorbers, most efficiently as organic dyes301 and quantum dots,302 attached on the surface of the mesoporous wide bandgap acceptor. In general, the application of graphene in liquid junction solar cells takes many forms and functions, ranging from the fabrication of conductive substrate, and efficient charge extraction on the photoelectrode layer, to the catalytic surface for the reduction of redox mediator. Just as any graphene-based photonic devices, the enhancement in the charge transport and collection efficiencies arising from the addition of graphene needs to overcome the negating effect of

the broad parasitic wide absorbance inherent to the C−C sp2 network. Here, the purpose of this subsection is to highlight some key directions in the applications of functionalized graphene in liquid-junction solar cells. For more comprehensive reviews on the general applications of graphene, including nonfunctionalized graphene, in photovoltaics, readers may refer to the works of Roy-Mayhew and Aksay303 and Yin et al.304 The motivation of designing graphene-based electrodes stems from the vision to potentially replace the mechanically brittle indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) layers, which are the standards used in the transparent conductive oxide substrates in solar cells, with economical and physically flexible alternatives. Wang et al. first demonstrated the deposition on graphene on quartz by repeated dip coating in GO dispersion, followed by thermal annealing in flowing Ar and/or H2 to produce a thin graphene layer (∼10 nm).38 Although the sheet resistance (Rs ∼ 1 kΩ sq−1) and transmittance (T = 71%) were inferior to the commercial ITO (Rs = 5 Ω sq−1, T = 90%) and FTO (Rs = 15 Ω sq−1, T = 85%),38 the work inspired further efforts to improve the quality and reduce the thickness of the graphene layer. At this point in time, high temperature annealing (≥1000 °C) under vacuum, inert (e.g., Ar) or reducing gas (e.g., H2), appears to be the only option to minimize the oxygen and defect contents, as well as to increase the domain size of the graphene.20 These physicochemical features of graphene are essential to obtain high conductivity of the substrate. Because every graphene monolayer contributes to 2.3% loss in transmittance,305 it is further necessary to optimize its thickness since this directly affects the sheet resistance of the substrate, Rs = (σN)−1, where σ is the conductivity of each graphene layer and N is the number of monolayers. Zheng et al. demonstrated the deposition of ultralarge GO sheets on silicon, glass, quartz, and mica wafers using the Langmuir−Blodgett method, where depending on the applied surface pressure and pulling speed as well as the number of repeated coatings, it was possible to control the thickness and extent of surface coverage as well as the density of wrinkles formed.306 By doing so, the Rs after 8097


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Chemistry of Materials thermal annealing was reduced to 459 Ω sq−1 at 90% transparency. Interestingly, given the right catalytic surface (e.g., Ni), it is possible to form a high quality graphene layer of up to 2 nm in thickness merely from the unintentional residue carbon upon annealing at 800−1000 °C under vacuum. This 2−3 monolayer thick graphene layer can be transferable to a quartz surface, giving Rs of 0.9 ± 0.4 kΩ sq−1 at 94 ± 4% transparency.307 Superior industrial-scale manufacturing of 30 in. electronic-grade flexible graphene/polyethylene terephthalate (PET) was demonstrated using the state-of-the-art roll-toroll transfer of graphene film that was pregrown by the chemical vapor deposition on copper foil. Although tedious, the roll-to-roll technique allows high temperature annealing or synthesis of high quality graphene prior to the transfer to low melting point substrates, i.e., PET. The resultant pristine monolayer graphene/PET substrate has a sheet resistance of 125 Ω sq−1 at 97.4% transmission, while the four-layer HNO3doped graphene/PET further decreased the sheer resistance to 30 Ω sq−1 at 90% transmission.13 It should be mentioned that the examples above involve nonfunctionalized and high temperature-synthesized graphene where the attachment onto the substrate is purely by van der Waals forces. The formulation of stable graphene dispersion in solvent medium relies on the chemical functionalization (see Section 2) that is essential to the low-temperature and solution processing. Zhu and He demonstrated the electrostatic layerby-layer (LbL) deposition of the negatively charged rGO and the positively charged rGO−PDDA + (PDDA = poly(diallyldimethylammonium chloride)) on a polyethylene terephthalate (PET) substrate.308 The latter was obtained via the reduction of GO with hydrazine in the PDDA/aqueous solvent. The LbL technique gives a wide control over the achievable graphene layer thickness where a thick 30 bilayer of rGO−PDDA+/rGO electrode gave low transmittance T = 44% and sheet resistance Rs = 7 kΩ sq−1 after thermal annealing at 550 °C (the temperature limit of PET). The flexible electrode maintains good adhesion and conductivity even after 30 bending cycles, whereas the brittle ITO/PET loses conductivity with each bending cycle due to cracks. Despite its economical and scalable manufacturing, an inherent disadvantage of the low-temperature and solution-processed rGO is its low quality (i.e., high oxygen and defects content, inhomogeneous thickness, and domain size) that in turn limits its conductivity. Incorporating silver nanowires (AgNWs) on graphene can drastically reduce the sheet resistance of such LbL-assembled layer (Rs = 10 Ω sq−1, T = 91%).309 Since the conductivity originates predominantly from the AgNW network, the hermetic graphene layer serves as the protection for AgNWs against a corrosive environment while at the same time also improves the adhesion of the dopamine-functionalized graphene with the base substrate.309,310 Incorporating graphene in photoelectrodes or more specifically blending with the acceptor semiconductor particles can improve the overall charge collection efficiencies. Like those discussed in Sections 4.3 and 4.4, the high work function and conductivity of graphene facilitate the interfacial electron extraction from the acceptor upon photoelectron injection from the adsorbed sensitizers.158 For this purpose, the rGO is more commonly employed to avoid the harsh thermal and chemical conditions required for the conversion to pristine graphene that would otherwise compromise the high surface area of the acceptor. A high extent of interfacial contact with the acceptor, as provided by the 2D rGO as compared to the

1D carbon nanotube, is necessary to collect a large fraction of the short-lived photoelectrons before they recombine at the defect sites or grain boundaries of the acceptor particles.311 When applied in the ruthenium dye-sensitized solar cell, the rGO-blended TiO2 electrode resulted in photoconversion efficiency (PCE) of 6.97%, which is 39% higher than the bare TiO2 acceptor.311 There is a general trend of PCE enhancements for dye-sensitized cells consisting of acceptor−rGO composite photoelectrodes (e.g., TiO2/rGO,312 p-NiO/ rGO,313 ZnO/rGO)314 prepared by various synthesis techniques (e.g., hydrothermal, solvothermal, coagulation).312−315 As in all cases, the rGO surface is capable of additional dye loading on the photoelectrode and this effect should not be discounted when considering the photoconversion enhancement.316 Similar strategies have also been demonstrated for chalcogenide quantum dots-sensitized solar cells through graphene-supported CdS,47 CdSe,317 and PbS318 photoelectrodes. The replacement of Pt counter electrode (CE) with graphene as well as its composites is a recent direction of intense interest. In one of the earliest works that demonstrates the ability of graphene cathode (PCE = 4.99%) to almost match that of the conventional Pt (PCE = 5.48%), RoyMayhew et al. identified the importance of the oxygenated moieties on graphene (optimized by controlled temperature annealing under Ar) in electrocatalysing the reduction of triiodide in the I−/I3− redox mediator.319 This means the highly conductive pristine graphene substrates with minimum oxygen content discussed at the beginning of this subsection may not be suitable as high efficiency CEs despite their low sheet resistance.320 The exact identity of the active oxygenated moieties and their specific mechanisms are still elusive at this moment but can be expected as originating from the localized electron sites where surface charge transfer to triiodide may take place. At the same time, the edge defects have also been identified as the catalytic sites.321 For this reason, small graphene flakes (e.g., commercial graphene nanoplatelets, 5 nm thickness, and 1 V vs Li/Li+), large polarization, and large potential range (from 0 to 3 V vs Li/Li+). In that sense, the overall energy density needs to be considered when coupling these materials with a cathode. 8105


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(9) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (10) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563−568. (11) Stolyarova, E.; Rim, K. T.; Ryu, S. M.; Maultzsch, J.; Kim, P.; Brus, L. E.; Heinz, T. F.; Hybertsen, M. S.; Flynn, G. W. HighResolution Scanning Tunneling Microscopy Imaging of Mesoscopic Graphene Sheets on an Insulating Surface. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 9209−9212. (12) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (13) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ö zyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-in. Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (14) Lee, J. H.; Lee, E. K.; Joo, W.-J.; Jang, Y.; Kim, B.-S.; Lim, J. Y.; Choi, S.-H.; Ahn, S. J.; Ahn, J. R.; Park, M.-H.; Yang, C.-W.; Choi, B. L.; Hwang, S.-W.; Whang, D. Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium. Science 2014, 344, 286−289. (15) Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.; Khan, U.; O’Neill, A.; Boland, C.; Lotya, M.; Istrate, O. M.; King, P.; Higgins, T.; Barwich, S.; May, P.; Puczkarski, P.; Ahmed, I.; Moebius, M.; Pettersson, H.; Long, E.; Coelho, J.; O’Brien, S. E.; McGuire, E. K.; Sanchez, B. M.; Duesberg, G. S.; McEvoy, N.; Pennycook, T. J.; Downing, C.; Crossley, A.; Nicolosi, V.; Coleman, J. N. Scalable Production of Large Quantities of Defect-Free Few-Layer Graphene by Shear Exfoliation in Liquids. Nat. Mater. 2014, 13, 624−630. (16) Berger, C.; Song, Z. M.; Li, T. B.; Li, X. B.; Ogbazghi, A. Y.; Feng, R.; Dai, Z. T.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-Based Nanoelectronics. J. Phys. Chem. B 2004, 108, 19912−19916. (17) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Electronic Confinement and Coherence in Patterned Epitaxial Graphene. Science 2006, 312, 1191−1196. (18) Wang, X.; Zhi, L. J.; Tsao, N.; Tomovic, Z.; Li, J. L.; Mullen, K. Transparent Carbon Films as Electrodes in Organic Solar Cells. Angew. Chem., Int. Ed. 2008, 47, 2990−2992. (19) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Room-Temperature Quantum Hall Effect in Graphene. Science 2007, 315, 1379. (20) Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270−274. (21) Chen, J. H.; Jang, C.; Xiao, S. D.; Ishigami, M.; Fuhrer, M. S. Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2. Nat. Nanotechnol. 2008, 3, 206−209. (22) Fasolino, A.; Los, J. H.; Katsnelson, M. I. Intrinsic Ripples in Graphene. Nat. Mater. 2007, 6, 858−861. (23) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. TwoDimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (24) Yoo, E.; Kim, J.; Hosono, E.; Zhou, H. S.; Kudo, T.; Honma, I. Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries. Nano Lett. 2008, 8, 2277− 2282. (25) He, H. Y.; Klinowski, J.; Forster, M.; Lerf, A. A New Structural Model for Graphite Oxide. Chem. Phys. Lett. 1998, 287, 53−56.

graphene sheet. Without such information, not only is it impossible to clearly pinpoint the source of different effects arising from the variation rGO samples, for example, in the electrocatalytic reduction of redox mediators, but also it prevents meaningful ab initio calculations from being made. Unless this is solved, any hypotheses put forward to explain the experimental observations of rGO using the energy level of pristine graphene or that interpolated from GO remain speculative. Lastly, while uncertainties continue to surround the toxicology of graphene, as with other nanomaterials, the progress in nanotoxicology will at least educate us on ways to limit the free exposure of graphene during the manufacturing as well as in the design of the finished products. Indeed, this is one of the final technological hurdles that ought to be overcome before the general consumers can enjoy the benefits of this advanced material. Having learned from the past lesson of overhyping the research in genetic engineering that led to various media controversies in the late 1990s, the scientific community is fully conscious of the risk in repeating the same mistake. At the same time, it is also better aware of the responsibilities and ethics clearance it requires before introducing any high performance graphene-based devices to the consumer market in large scale.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.L. is thankful of the financial support from the National Natural Science Foundation of China (Grant 21575045), the self-determined research funds of CCNU (CCNU15A02015), and the ARC Centre of Excellence for Nanoscale Biophotonics CE140100003. The work is financially supported by the Research Grant Council of Hong Kong through the Early Career Scheme (Project 104812) and General Research Fund (Project 11302714).

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