Graphene-Based Photocatalysts for CO2 Reduction to Solar Fuel

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Perspective 2

Graphene-Based Photocatalyst for CO Reduction to Solar Fuel Jingxiang Low, Jiaguo Yu, and WingKei Ho J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01610 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 8, 2015

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The Journal of Physical Chemistry Letters

Submit it to JPC Letter as a perspective paper

Graphene-based Photocatalyst for CO2 Reduction to Solar Fuel Jingxiang Low,5 Jiaguo Yu,*,a,c and Wingkei Ho*,b a

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, P. R. China. b

Department of Science and Environmental Studies, Centre for Education in

Environmental Sustainability, The Hong Kong Institute of Education, Tai Po, Hong Kong, China. c

Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia

1

ACS Paragon Plus Environment


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ABSTRACT Recently, photocatalytic CO2 reduction for solar fuel production has attracted much attention because of its potential for simultaneously solving energy and global warming problems. Many studies have been conducted to prepare novel and efficient photocatalysts for CO2 reduction. Graphene, a two-dimensional material, has been increasingly used in photocatalytic CO2 reduction. In theory, graphene shows several remarkable properties, including excellent electronic conductivity, good optical transmittance, large specific surface area, and superior chemical stability. Attributing to these advantages, fabrication of graphene-based materials has been known as one of the most feasible strategy to improve the CO2 reduction performance of photocatalyst. This perspective mainly focuses on the recent important advances in the fabrication and application of graphene-based photocatalysts for CO2 reduction to solar fuels. The existing challenges and difficulties of graphene-based photocatalyst are also discussed for future application.

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Table of Contents

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Given the increasing challenges in energy demands and environmental pollutions caused by fossil fuel consumption, developing renewable and green technologies for energy production, such as photovoltaic and photocatalytic H2 production, has caused widespread concern in the past several decades. Among various proposed technologies, photocatalytic CO2 reduction has been known as one of the most important strategies for solving both global energy and environmental problems due to its low cost, clean and environmental friendliness.1,2 Specifically, photocatalytic CO2 reduction can transform harmful greenhouse gases (such as CO2) into valuable solar fuels, such as CH4 and CH3OH, by using solar energy.3,4 After years of research and development, many semiconductors have been used as photocatalysts for CO2 reduction including TiO2, CdS, g-C3N4, ZnO, and Bi2WO6.5–9 However, the practical applications of these photocatalyst for CO2 reduction are still limited by the low CO2 conversion efficiency due to fast charge carrier recombination and low light utilization. Diverse strategies such as band gap tuning, morphology controlling and metal loading have been explored for enhancing the photocatalytic CO2 reduction performance. Graphene, a single-layer carbon 2D nanosheet with hexagonal packed lattice structure material, has received great deal of attention in scientific and engineering fields since its breakthrough discovery by Geim and Novoselov in 2004.10-14 This wonder material of the century has exhibited many extraordinary properties, such as large surface area, good conductivity, and high flexibility.15,16 Therefore, the coupling of graphene with the photocatalyst has offered great opportunities for improving photocatalytic CO2 reduction efficiency to meet the practical requirements. Since the first report by Liang et al., coupling of graphene with semiconductor has been known as one of the most feasible ways to improve the photocatalytic CO2 reduction

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activity.17 It was found that graphene nanosheet can significantly enhance the electron–hole separation rate and specific surface area of the photocatalyst. Moreover, the coupling of graphene and semiconductor suggests the possibility of fabricating new multicomponent composite materials that exhibit synergistic physicochemical properties. Generally, the advantages of graphene-based photocatalysts for CO2 reduction can be categorized into 6 aspects (see Fig. 1): (1) Suppressing photogenerated carrier recombination: Graphene has a single-atomthick nanosheet of sp2 hybridized planar structure arranged in a honeycomb lattice with excellent conductivity, making it a good electron acceptor during photocatalytic reaction.18 In addition, given the Fermi level of graphene (0 V vs NHE) is lower than the conduction band of most photocatalysts, a rapid electron transfer from photocatalyst to graphene can be achieved on graphene-based photocatalysts because of the band alignment. Therefore, under light irradiation, the photoinduced electrons on photocatalyst can be transferred to the graphene immediately for reduction process; meanwhile, photoinduced holes can be remained on photocatalyst for oxidation process, thereby resulting in a high spatial separation of the photoinduced electron–hole pairs. Furthermore, the electron density on the graphene nanosheet can be also enhanced, which favors multi-electron reactions for photocatalytic CO2 reduction. (2) Increasing specific surface areas: Other than its electronic properties, graphene nanosheets are known for its ultra-large theoretical specific surface area.19 The specific surface area of graphene is widely accepted as the highest among all the explored materials because of its one atom-thick structure. Given this remarkable specific surface area, the coupling of graphene with photocatalyst can significantly increase the specific surface area of the photocatalyst. The enhanced specific surface

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area can increase the surface active site on sample which is beneficial for enhancing photocatalytic CO2 reduction activity. (3) Increasing CO2 adsorption and activation: Graphene exhibits a large 2D πconjugated structure. Meanwhile, CO2 molecules also contain delocalized πconjugated binding π43 . Thus, π–π conjugation interaction can be established between graphene and CO2. This unique π–π conjugation interaction can significantly enhance the adsorption of CO2 molecules on the graphene-based photocatalysts, thereby improving the photocatalytic CO2 reduction activity. Notably, this strong π–π conjugation interaction between graphene and CO2 can also cause destabilization and activation of CO2 molecules, thereby leading to an easier reduction of CO2 during the photocatalytic CO2 reduction reaction.20 (4) Enhancing photostability: Given its extraordinary mechanical and chemical stability, graphene nanosheets have been shown as effective supporting materials in photocatalysis for enhancing the photostability of the photocatalyst.20,21 Particularly, graphene can wrap on the target photocatalysts with low photostability, such as Cu2O and CdS to avoid the attack of active species especially •OH radicals on the photocatalyst.20 Therefore, the graphene-based photocatalysts always have good stability for long-term photocatalytic CO2 reduction application. (5) Improving nanoparticles (NPs) dispersion and reducing NPs size: Graphene prepared by chemical method consists of large volume of surface functional groups, which can function as the anchoring site and allow the photocatalyst to grow on its surface.22 Therefore, photocatalyst can be uniformly deposited on the surface of graphene nanosheet, thereby inhibiting the aggregation and growth of photocatalysts. Furthermore, the graphene nanosheet can also function as a capping agent for NPs, thereby limiting the growth of the NPs. Thus, the photocatalyst can be restrained into

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smaller sizes to obtain a higher surface area, which is beneficial for the photocatalytic reaction. (6) Enhancing light absorption: Graphene can absorb almost whole spectrum of solar light because of its black color and zero band gap. Although such good light absorption ability does not produce active electrons or holes for the photocatalytic reaction, it can increase the temperature around the photocatalyst to create a local photothermal effect.21,23 It was proved that this local photothermal effect will enhance movement of reaction and product molecules, thereby enhancing photocatalytic CO2 reduction effciency.23 Moreover, the photothermal effect can also improve the charge carrier separation by creating high energy electrons on the semiconductor. Under the light irradiation, these high energy electrons can be easily transferred to the graphene for photocatalytic reaction instead of recombining with the hole. However, when the loading amount of graphene on the photocatalyst is too high, graphene can cause a negative effect on the photocatalytic reaction. The overloading of graphene will shield the photocatalyst and prohibit the photocatalyst to absorb incident light. As a result, the generation rate of photoinduced electron–hole pairs will be reduced. Therefore, finding the optimal loading amount of graphene on the photocatalyst is imperative.

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Figure 1. Superior advantages of the graphene-based photocatalyst for CO2 reduction. Given its remarkable properties, graphene provides a wide range of opportunities to prepare diverse forms of composite materials with extraordinary properties for photocatalytic CO2 reduction. Particularly, preparing fine-tuned and robust graphenebased photocatalytic material is necessary to meet the practical requirement of CO2 reduction. In this perspective, we aim to prepare a short overview of the recent progress of preparing graphene-based photocatalyst for CO2 reduction and provide insights about its unique properties.

Preparation of high-performance graphene-based photocatalyst for CO2 reduction Several works have been carried out to highlight the high-performance graphene-

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based photocatalyst with excellent CO2 reduction activity.21-26 Coupling of the graphene with photocatalyst can provide the photocatalyst with several advantages, including large surface area, abundant surface active sites, and high electron–hole separation rate. For example, our group reported that reduced graphene oxide (RGO)– CdS (RGO-CdS) nanorod composites for photocatalytic CO2 reduction.21 It was shown that the prepared RGO-CdS composite photocatalyst exhibited good electronhole separation rate due to the high electron conductivity and proper band structure of RGO. Moreover, the addition of RGO nanosheets can also enhance adsorption of CO2 molecules because of the π–π conjugation interaction between RGO and CO2 molecules. This π–π conjugation interaction can significantly enhance the adsorption and activation of CO2 molecules, thereby accelerating the photocatalytic reduction of CO2. Moreover, given its local photothermal effect, RGO can speed up the chemical reaction. Therefore, the electron–hole separation and photocatalytic CO2 reduction rate can be significantly enhanced. Finally, the prepared RGO-CdS composite exhibited an enhanced photocatalytic CO2 reduction rate 10 times higher than that of pure CdS nanorods in producing CH4 due to the presence of graphene (see Fig. 2).

Figure 2. Schematic illustration for the photocatalytic CO2 reduction mechanism of CdS nanorod/graphene composites. 9



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Furthermore, Wang et al. reported the fabrication of graphene–WO3 (G-WO3) nanobelt composites via a facile in situ hydrothermal method.24 It was found that the conduction band minimum value of WO3 can be elevated for photocatalytic CO2 reduction by graphene loading. During the photocatalytic CO2 reduction test, WO3 did not exhibit any photocatalytic CO2 reduction activity because of its low conduction band minimum value. However, G-WO3 nanobelt showed photocatalytic CO2 reduction activity by yielding 0.89 µmol of CH4 after 8 h of continuous visible light irradiation. According to the UV–vis diffuse reflection and X-ray photoelectron spectroscopy (XPS) valence band characterization, this enhanced photocatalytic activity of G-WO3 was attributed to the rise in the conduction band position of WO3 by graphene loading. The CB position of WO3 rose to −0.24 V, resulting in the production of CH4 on the graphene–WO3 composite under light irradiation. The rise of conduction band level of WO3 is due to the electron transfer from graphene to WO3 because of the prepared graphene oxide with higher Fermi level than WO3. Of course, the detailed electron transfer mechanism needs to be further investigated. More recently, An et al. reported RGO supported Cu2O for photocatalytic CO2 reduction.25 A novel one-step microwave-assisted chemical method was used for the preparation of Cu2O/RGO hybrids. Given the good electron conductivity and large specific surface area of the RGO, Cu2O/RGO exhibited a low electron–hole recombination rate and large amount of surface active sites. As a result, the prepared Cu2O/RGO composite showed a photocatalytic activity six times higher than that of the optimized Cu2O for CO2 reduction. More interestingly, the photocorrosion problem of Cu2O could be also effectively prohibited by RGO loading. In details, Cu2O has a high leaching Cu ion concentration of 2670 ppm after 3 h of light irradiation, as determined via inductively coupled plasma optical emission 10


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spectrometry. Surprisingly, the leaching of Cu ions was significantly suppressed to a concentration of 96 ppm by the loading of graphene nanosheets. This is because the presence of RGO can avoid the direct contact of Cu2O with the water, which can prevent the attack of active species on Cu2O. This work indicated that the loading of graphene not only improved the photocatalytic activity of the photocatalyst but also enhanced the photostability of the photocatalyst. Moreover, it was found that graphene has a great effect on tuning the size and morphology of the photocatalyst. Li and co-workers reported that the RGO had an important function in reducing the NPs size and prohibiting the self-aggregation of ZnO.26 ZnO-RGO was prepared via a simple hydrothermal reaction. In comparison with the size of spherical ZnO NPs, the size of the obtained ZnO NPs on RGO nanosheet was significantly reduced. This is because the surface functional groups on the RGO act as the anchoring sites for the ZnO NPs and thus reduce the ZnO NPs self-aggregation. Since ZnO NPs were uniformly distributed on the graphene, the prepared ZnO-RGO sample has a large specific surface area, and thus providing more reaction sites for photocatalytic reactions. Given the superior properties of the graphene and improved crystallinity of ZnO, the photocatalytic CO2 reduction activity of the ZnO-RGO was enhanced by 75% in comparison with that of the pure ZnO to produce CH3OH. Furthermore, Tu and co-workers fabricated sandwich-like graphene–TiO2 (G/TiO2) hybrid via a simultaneous reduction-hydrolysis (SRH) technique.27 Particularly, the prepared graphene oxide was simultaneously reduced into graphene by ethylenediamine (En) during the formation of TiO2 NPs through the hydrolysis of titanium (IV) (ammonium lactato) dihydroxide. TiO2 NPs can be in situ loaded onto the graphene nanosheet through the chemical bonds via the SRH technique to form a

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sandwich-like structure (see Fig. 3). Moreover, G/TiO2 hybrid has a slit-like pore structure. This unique slit-like pore structure of G/TiO2 can efficiently prevent the restacking of graphene nanosheets and the aggregation of TiO2 NPs. Therefore, the specific surface area of the composite can be optimized. Consequently, the specific surface area of G/TiO2 is significantly higher than that of commercial P25; thus, more adsorption and reaction sites are generated on the photocatalyst surface. In addition, more Ti3+ could be found on the surface of the G/TiO2 hybrid. These Ti3+ could function as photoinduced electron trapping sites and improve the electron–hole separation rate on the photocatalyst. As a result of these advantages, G/TiO2 exhibited a higher photocatalytic CO2 reduction activity (16.8 µmolg−1h−1 C2H6) than pure TiO2 (7.2 µmolg−1h−1 C2H6) to produce C2H6.

Figure 3. SEM image (a,b) and schematic illustration (c) of sandwich-like G-TiO2. Reprinted with permission from ref. 27. Copyright 2013, John Wiley & Sons, Inc. 12


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Graphene exhibited unique properties when it was hybridized with other materials to function as co-catalyst on the photocatalyst. Lately, Baeissa reported the graphene and tourmaline co-loaded titania (GT/T) nanocomposites.28 GT/T exhibited significantly improved photocatalytic CO2 reduction activity than the grapheneloaded titania, tourmaline-loaded titania, and pure titania. This enhancement is attributed to the synergic effect of graphene and tourmaline. Specifically, both graphene and tourmaline can improve electron–hole separation; meanwhile, graphene can reduce the bandgap of TiO2. As a result, GT/T exhibited the enhanced methanol production rate via photocatalytic CO2 reduction, which was 21 times higher than that of pure TiO2. In comparison with the simple coupling of graphene with semiconductor, the rational design of the graphene-based photocatalyst was shown to be more efficient for enhancing CO2 reduction activity. Graphene is a 2D nanosheet with a layered structure. When graphene was coupled with NPs, only a small part of the NPs will be connected to the graphene. Consequently, only a small contact area can be created between this 0D–2D composite, thereby limiting the electron transfer from the NPs to the graphene.29 To fully utilize the 2D properties of graphene, a graphene-based 2D– 2D composite photocatalyst can be fabricated to enhance CO2 reduction efficiency (see Fig. 4). Given the large contacting area of 2D–2D composite, the interaction between graphene and photocatalyst is expected to be improved, thereby enhancing the photocatalytic CO2 reduction activity.

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Figure 4. Advantages of 2D–2D graphene-based composites in comparison with other kinds of graphene-based composites. Tu et al. explored the potential of graphene–titania-based 2D-2D composite photocatalyst by preparing a unique hollow structure of alternating titania nanosheet and graphene nanosheet (G-Ti0.91O2) via a layer-by-layer assembly technique.30 Since both titania nanosheet and graphene nanosheets are 2D structure, G and Ti0.91O2 has a close and large contact surface area. This close and large contact interface allowed the photoinduced electron to migrate fast from the titania nanosheet to the graphene surface, resulting in the spatial separation of photoinduced electron–hole pairs. In addition, the hollow structure of the composite also has an important function in further improving the photocatalytic activity of the composite. Given the unique light scattering ability of the hollow structure, the incident light can be trapped within the photocatalyst structure, thereby leading to enhanced light utilization. The prepared samples exhibited a high CO formation rate via photocatalytic CO2 reduction, which is nine times higher than that of the commercial P25 TiO2 due to its fast electron–hole separation and good light utilization (see Fig. 5).

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Figure 5. Proposed mechanism for photocatalytic CO2 reduction activity enhancement in the titania/graphene hollow sphere composite system under light irradiation. Liang et al. further compared the effect of single-wall carbon nanotube (SWCNT)/TiO2 nanosheet (1D–2D structure) and graphene/TiO2 nanosheet (2D–2D structure) on the photocatalytic CO2 reduction activity.31 They found that the loading of SWCNT or graphene on the TiO2 nanosheet will significantly enhance the CO2 reduction activity of the photocatalysts because the loading of the carbon-based SWCNT or graphene on the titania nanosheet can improve photoinduced electron– hole separation, increase specific surface area, and enhance number of surface active sites. However, graphene-loaded titania nanosheet exhibited a higher photocatalytic CO2 reduction rate than the SWCNT-loaded titania nanosheet for CH4 production because the 2D–2D structure of the graphene-loaded titania nanosheet has larger contact interface than the 1D–2D structure of the SWCNT-loaded titania nanosheet, thereby resulting in a faster electron migration rate. Recently, 2D layered materials such as g-C3N4 have attracted wide attention because of their large surface area, good conductivity, and tunable bandgap structure.

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Much effort has been carried out to build a 2D–2D layered composite photocatalyst by using 2D layered materials and graphene. Lately, Ong and coworkers reported a sandwich-like

graphene-g-C3N4 (GCN)

composite

prepared

via

a

one-pot

impregnation–thermal reduction approach with enhanced visible light photocatalytic CO2 reduction activity.32 They found that the GCN (2.7 eV) composite exhibited a red shift of absorption band edge in comparison with the pure g-C3N4 (2.82 eV). According to the XPS and Fourier transform infrared spectroscopy (FTIR) characterization results, this band edge shift was attributed to the formation of the C– O–C bond as a covalent crosslinker between graphene and g-C3N4. The modification of the electronic structure of g-C3N4 by graphene is beneficial in improving its visible light absorption ability. Moreover, as shown in the TEM images (see Fig. 6), the combination of 2D graphene and 2D g-C3N4 resulted in a large contact interface, which is good for electron migration. As a result, the GCN composite showed a 2.3fold increase in CH4 production activity in comparison with that of pure g-C3N4 in photocatalytic CO2 reduction. Overall, a well-defined nanocomposite interface, such as a 2D–2D composite, is important in preparing highly efficient graphene-based photocatalysts.

Figure 6. TEM images of (a) pure g-C3N4 (white dotted circles indicate pores) and (b) GCN. Reprinted with permission from ref. 32. Copyright 2015, The Royal Society of Chemistry. To further explore the potential of graphene, several methods were proposed to 16


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modify the graphene nanosheet. Such methods include minimizing the defects on graphene surface and doping the graphene. Fine-tuned graphene nanosheets are beneficial in optimizing the electron conductivity of graphene, thereby enhancing photocatalytic CO2 reduction activity. For example, Liang et al. recently reported that the synthesis of graphene-based nanocomposites by using less defective solventexfoliated graphene (SEG) showed significantly improved photocatalytic CO2 reduction activity than RGO.17 Particularly, two synthesis routes with different exfoliation

methods

were

applied

for

the

preparation

of

graphene–TiO2

nanocomposites, which are reduced graphene oxide–TiO2 (RGO-TiO2) and solventexfoliated graphene–TiO2 (SEG-TiO2). The I(D)/I(G) values of the Raman spectra of RGO-TiO2 and SEG-TiO2 indicated that RGO (I(D)/I(G)=0.82) exhibited a higher defect density than SEG (I(D)/I(G)=0.17). Moreover, it was found that SEG-TiO2 nanoplatelets showed relatively less basal plane defects than RGO-TiO2. The electrical conductivity and mobility of SEG-TiO2 were higher than those of RGOTiO2 because the former has fewer defects. Thus, electrons on the photocatalyst could be effectively diffused away from the graphene–TiO2 interface, thereby reducing the probability of their recombination with holes on TiO2 and thus promoting the migration of electrons to the reactive sites. As a result, a more superior photocatalytic CO2 reduction activity can be observed on SEG-TiO2 in comparison with RGO-TiO2 and pure TiO2 for the production of CH4. Doping is another efficient way to tune the physical, optical, and physicochemical properties of graphene. Xing et al. reported on boron-doped graphene (BG) nanosheets loaded on TiO2 NPs to improve the photocatalytic CO2 reduction activity.33 Highly dispersed TiO2/BG composite were successfully prepared via a vacuum activation and ultraphonic method. The doping of boron on the graphene

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nanosheet can effectively reduce the size of the graphene nanosheet by cutting graphene nanosheets into nanoribbons due to the macro-residual stress induced by the boron substitutional doping. It is interesting to note that the cutting of graphene nanosheet by boron doping introduces ZZ-edges and AC-edges on the B-GR. Therefore, B-GR exhibited semimetallic properties with high conductivity. Furthermore, by coupling B-GR with the P25 NPs, the chemical bonds (Ti-O-C) between P25 and B-GR can also introduce macro-residual stress on the surface of BGR and further cut B-GR nanoribbons into smaller nanosheets. The smaller size of BGR nanosheets indicates the increasing of exposed edges of B-GR, which can provide more reactive sites for the photocatalytic reaction. Furthermore, in comparison with other boron-doped graphene reported previously, the TiO2/BG samples exhibited low defect and excellent conductivity because the nanosized structure of TiO2/BG can greatly reduce the local density of defects, which were generated by boron substitutional doping. Thus, the superior conductivity of graphene can be kept in TiO2/BG. As a result, TiO2/BG showed a higher CH4 production rate via photocatalytic CO2 reduction than pure TiO2 and TiO2/graphene composite for CH4 production. This work suggested that the photocatalytic CO2 reduction activity and properties of the graphene-based photocatalyst can be improved by the doping of graphene with other elements.

Utilization of oxidized graphene as photocatalyst for CO2 reduction Moreover, oxidized graphene (i.e., graphene oxide) with an appropriately oxidized level, can function as an efficient photocatalyst. Several research groups have performed the photocatalytic CO2 reduction test to investigate the photocatalytic CO2 reduction ability of oxidized graphene.34,35 Hsu and co-workers reported that graphene oxide (GO) can be used as an efficient photocatalyst for CO2 reduction.32 In

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particular, the GO were prepared via a modified Hummer’s method. Three samples were prepared and named as GO-1 (prepared via conventional Hummer’s method), GO-2 (prepared via modified Hummer’s method with the presence of excess KMnO4 and H3PO4), and GO-3 (prepared via modified Hummer’s method with the presence of excess H3PO4). The prepared GO samples had non-uniform oxidation level; thus, GO-1, GO-2, and GO-3 did not show sharp absorption edges and had bandgap values of approximately 2.9–3.7, 3.1–3.9, and 3.2–4.4 eV, respectively. Gas phase photocatalytic CO2 reduction test was performed to compare the photocatalytic activities of the prepared samples with the commercial TiO2, P25. Surprisingly, all three prepared GO samples exhibited higher photocatalytic activity than P25 under visible light irradiation because the prepared GO samples had proper bandgap energy, which was more suitable for photocatalytic CO2 reduction. Particularly, sample GO-3 showed the highest photocatalytic activity among the three prepared GO samples because GO-3 had more surplus oxygenated components on its basal plane, which was beneficial in increasing the bandgap energy. The results indicated that the oxidized graphene, GO, can be modified and tuned to be an effective solar energy harvesting material. Overall, given its remarkable properties, including high conductivity, large surface area, good flexibility and stability, and single-layer structure, graphene has been known as a promising material for photocatalytic CO2 reduction. Graphenebased photocatalyst can not only improve the photocatalytic CO2 reduction activity but also endow the photocatalyst with several new and unique synergistic properties. However, several theoretical and fundamental issues must be resolved and paid more attention to realize the practical use of graphene-based photocatalyst for CO2 reduction to solar fuels.

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First, the migration path of the photogenerated charge carriers between the graphene and the semiconductor should be carefully investigated. Graphene can effectively reduce the electron–hole recombination rate in the semiconductor by functioning as an electron acceptor, thereby improving the photocatalytic CO2 reduction activity. However, this inference and conclusion were obtained by the result of the photocatalytic reaction and indirect characterization, such as the photocurrent response test. Such superior electron conductivity of graphene should be further investigated via more powerful characterization tools, such as UV photoelectron and time-resolved transient absorption spectroscopy, to clearly investigate the migration path of the photogenerated charge carriers on the photocatalyst/graphene interface and the changes in the number of the photogenerated charge carriers during the photocatalytic reaction. Second, the effect of graphene on the formation of product during the photocatalytic CO2 reduction remains unclear and must be systematically studied. As mentioned above, several kinds of products, such as CH4, HCO2H, CH2O, and CH3OH can be detected after photocatalytic CO2 reduction using graphene-based photocatalyst. It was found that the presence of graphene can influence the formation of the different products during the photocatalytic CO2 reduction reaction. Therefore, the formation mechanism of these products must be further investigated. Moreover, the real-time monitoring of the CO2 concentration during the photocatalytic CO2 reduction or isotope tracer experiments with

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CO2 should be also performed to

confirm that the carbon source in produced solar fuels originated from the CO2 instead of graphene. Third, modification of graphene, such as doping and defect control, can cause significant effects on the photocatalytic CO2 reduction activity. Particularly, the size,

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morphology, and electronic and optical properties of the graphene-based photocatalyst can be tuned to meet the requirement of the photocatalytic CO2 reduction reaction by modification of graphene. Therefore, the modification and optimization of graphene should be always taken into account to prepare high-quality graphene nanosheet for photocatalytic CO2 applications. Fourth, the potential of the graphene-based photocatalyst can be further explored by preparing multicomponent graphene-based composite. Graphene was normally coupled with only one semiconductor to enhance the photocatalytic CO2 reduction activity. Most of the reported graphene-based photocatalysts are mainly focused on binary component composites. Thus, the enhancement of the photocatalytic CO2 reduction activity is limited. The preparation of ternary or multi-component graphenebased composite photocatalysts can further explore the potential of graphene by conferring several extraordinary properties to the photocatalyst. Fifth, the stability of graphene-based photocatalysts against photo-corrosion during photocatalytic reaction should be paid more attention. Recently, Kamat et. al. reported that graphene suffer attack of the OH• radical during the photocatalytic reaction and as a result it can be mineralized into the H2O and CO2.36 Thus, the graphene-based photocatalyst do not have good stability. Moreover, the intermediate product (viz. polyaromatic hydrocarbons-like compounds) of the graphene mineralization can cause the environmental problem. In order to achieve the longterm and environmental friendliness application of the graphene-based photocatalyst, the stability of the graphene-based photocatalyst must also be enhanced. In short, photocatalytic CO2 reduction to produce solar fuels by using solar energy has been proven to be a potential strategy for reducing the CO2 concentration in the environment and producing a sustainable energy source. The activity of the

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photocatalytic CO2 reduction is dependent on five aspects, namely, light absorption ability, photogenerated charge carrier separation efficiency, CO2 absorption capability, CO2 activation capability, and surface reactant kinetics. This perspective reveals that graphene-based photocatalyst endows numerous opportunities for the photocatalytic CO2 reduction field by improving these five aspects. Thus, the potential of graphenebased composite in the photocatalytic CO2 reduction field must be further explored. Finally, the advancements in graphene-based photocatalysts will not only improve the potential of photocatalytic CO2 reduction but also provide new insights into other solar conversion applications.

AUTHOR INFORMATION Corresponding Author *Tel: 0086-27-87871029, Fax: 0086-27-87879468, E-mail: [email protected]. [email protected] Notes The authors declare no competing financial interest. Biographies Jingxiang Low obtained his B.Eng (Hons) from Multimedia University, Malaysia at 2011. He is a Master’s degree candidate under the supervision of Prof. Jiaguo Yu at Wuhan University of Technology. His current research interests include the design, synthesis, and fabrication of photocatalytic materials for energy and environmental applications. Jiaguo Yu received his BS and MS in Chemistry from Huazhong Normal University and Xi’an Jiaotong University, respectively; his PhD in Materials Science in 2000 was from Wuhan University of Technology. In 2000, he became a Professor at Wuhan 22


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University of Technology. His current research interests include semiconductor photocatalysis, photocatalytic hydrogen production, CO2 reduction to hydrocarbon fuels, dye-sensitized solar cells, and so on. See more details on: http://www.researcherid.com/rid/G-4317-2010. Wingkei Ho received his BSc and Ph.D degrees in chemistry from The Chinese University of Hong Kong in 2001 and 2005. In 2015, Dr. Ho became an Associate Professor in the Department of Science and Environmental Studies at The Hong Kong Institute of Education. His current research interests include air pollution control, indoor air quality (IAQ) monitoring and management, heterogeneous catalysis and photocatalysis, nano-technology for environmental applications, science and environmental education. See more details on: http://www.scopus.com/authid/detail.url?authorId=56516040400

ACKNOWLEDGEMENTS This work was partially supported by the 973 program (2013CB632402), NSFC (51320105001, 51372190, 21177100, 51272199 and 21433007), Deanship of Scientific Research (DSR) of King Abdulaziz University (90-130-35-HiCi), Fundamental Research Funds for the Central Universities (WUT: 2014-VII-010), and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1). This research is financially supported by the research grant of Early Career Scheme (ECS 809813) from the Research Grant Council, Hong Kong SAR Government, Internal Research Grant (R3588) and (R3633) from The Hong Kong Institute of Education.

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The fabrication of graphene-based photocatalysts has been known as one of the most feasible strategy to improve the CO2 reduction performance of photocatalyst. Coupling of the graphene with photocatalyst can provide the photocatalyst with several advantages, including large surface area, abundant surface active sites, and high electron–hole separation rate. The existing challenges and difficulties of graphene-based photocatalyst are also discussed.

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Graphene-Based Photocatalysts for CO2 Reduction to Solar Fuel

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