Carbon-based H2-production photocatalytic materials

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 27 (2016) 72–99

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews journal homepage: www.elsevier.com/locate/jphotochemrev

Invited review

Carbon-based H2 -production photocatalytic materials Shaowen Cao a , Jiaguo Yu a,b,∗ a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China b Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 5 February 2016 Accepted 15 April 2016 Available online 20 April 2016 Keywords: Carbon materials Semiconductor photocatalysis Photocatalytic hydrogen production

a b s t r a c t Photocatalytic hydrogen production from water splitting is of promising potential to resolve the energy shortage and environmental concerns. During the past decade, carbon materials have shown great ability to enhance the photocatalytic hydrogen-production performance of semiconductor photocatalysts. This review provides a comprehensive overview of carbon materials such as CNTs, graphene, C60 , carbon quantum dots, carbon fibers, activated carbon, carbon black, etc. in enhancing the performance of semiconductor photocatalysts for H2 production from photocatalytic water splitting. The roles of carbon materials including supporting material, increasing adsorption and active sites, electron acceptor and transport channel, cocatalyst, photosensitization, photocatalyst, band gap narrowing effect are explicated in detail. Also, strategies for improving the photocatalytic hydrogen-production efficiency of carbonbased photocatalytic materials are discussed in terms of surface chemical functionalization of the carbon materials, doping effect of the carbon materials and interface engineering between semiconductors and carbon materials. Finally, the concluding remarks and the current challenges are highlighted with some perspectives for the future development of carbon-based photocatalytic materials. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Roles of carbon materials in photocatalytic H2 production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 2.1. Supporting material for enhanced structure stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 2.2. Increasing adsorption and active sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 2.3. Electron acceptor and transport channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.4. Cocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.5. Photosensitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 2.6. Photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.7. Band gap narrowing effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Strategies for tailoring the properties of carbon materials for photocatalytic H2 production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 3.1. Surface chemical functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 3.2. Doping effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.3. Interface engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Concluding remarks and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

∗ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China. E-mail address: [email protected] (J. Yu). http://dx.doi.org/10.1016/j.jphotochemrev.2016.04.002 1389-5567/© 2016 Elsevier B.V. All rights reserved.

S. Cao, J. Yu / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 27 (2016) 72–99

Shaowen Cao received his BS in Geochemistry in 2005 from the University of Science and Technology of China, and his PhD in Materials Chemistry & Physics in 2010 from the Shanghai Institute of Ceramics, Chinese Academy of Sciences. Then he worked as a Research Fellow at the School of Materials Science and Engineering, Nanyang Technological University until Feb 2014. He is now a Professor at Wuhan University of Technology. His current research interests include the design and fabrication of photocatalytic materials for energy and environmental applications. See more details on: http://www. researcherid.com/rid/I-8050-2013. Jiaguo Yu received his BS and MS in chemistry from Central China Normal University and Xi’an Jiaotong University, respectively, and his PhD in Materials Science in 2000 from Wuhan University of Technology. In 2000, he became a Professor at Wuhan University of Technology. He was a postdoctoral fellow at the Chinese University of Hong Kong from 2001 to 2004, a visiting scientist from 2005 to 2006 at University of Bristol, a visiting scholar from 2007 to 2008 at University of Texas at Austin. His current research interests include semiconductor photocatalysis, photocatalytic hydrogen production, CO2 reduction to hydrocarbon fuels, and so on. See more details on: http://www.researcherid.com/rid/G4317-2010.

1. Introduction The development of modern industrial civilization heavily relies on the fast consumption of fossil energy. However, in the past decades, the governments, scientists and the public have observantly realized the depletion of fossil resources with accompanying environmental issues. Nowadays, more and more attention has been paid with specific priority on the exploration of renewable energy resources. Solar energy is such a renewable and the most abundant source of clean energy. The environmental friendly, economic and efficient solar-to-fuel conversion has been considered as one of the most promising techniques to resolve the afore-mentioned energy shortage and environmental issues [1–3]. Particularly, photocatalytic hydrogen production from water splitting provides a potentially sustainable strategy to fulfill the future energy demand without environmental disruption, since hydrogen is a clean energy carrier, while water and solar energy is inexhaustible [4–7].

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Photocatalysis simply starts from the photo-absorption of semiconductor photocatalysts, to excite a number of electrons to the conduction band (CB) and leave behind the equal number of holes in the valence band (VB). These charge carriers then undergo a separation and migration process, and finally reach the surface of the photocatalysts to react with targeting reagents [8,9]. Therefore, to achieve an ideal photocatalytic performance, a semiconductor photocatalyst should have a suitable band gap to harvest sufficient solar energy. Thereafter, an efficient separation and prompt migration of the photo-excited electrons and holes is required. While on the photocatalyst surface, abundant adsorption sites and active centers must be assured. Specific to photocatalytic hydrogen production from water splitting, its thermodynamic requirement shows that a more negative CB minimum than the reduction potential of water is essential for hydrogen evolution, and a more positive VB maximum than the oxidation potential of water is necessary for oxygen evolution [10,11]. In this regard, semiconductors such as Fe2 O3 [12], WO3 [13], BiVO4 [14] etc. are usually excluded in the particle photocatalytic systems for hydrogen production, due to their insufficient potential of CB. In the past 40 years, TiO2 [15,16], ZnO [17,18], Cu2 O [19,20], SrTiO3 [21,22], CdS [23,24], graphitic carbon nitride (g-C3 N4 ) [25,26] etc. have shown exciting prospect for hydrogen production from photocatalytic water splitting. However, the relevant achievements are still far behind the requirement for practical applications. Even for a short-term goal, i.e., a visible-light-driven photocatalytic system with high efficiency and stability, these semiconductors are struggling against their own challenge. For instance, TiO2 , ZnO and SrTiO3 are only active in the UV region, which occupies only ∼4% in the solar spectrum, while ZnO, Cu2 O and CdS suffer from photo-corrosion. Moreover, most of the photoexcited electron-hole pairs tend to recombine in the semiconductor photocatalysts, thus leading to a reduction of the photocatalytic performance. To date, many approaches have been applied to overcome the above-mentioned shortcomings. Particularly, elemental doping has been used to extend the light absorption range and adjust the redox potential of a semiconductor [27]; dye sensitization has also been utilized to extend the light absorption range [28]; facet engineering has been employed to provide highly active surface [29]; creation of mesoporous structure has been adopted to enlarge the surface area for more adsorption sites and active centers [30]; construction of semiconductor heterostructures [31] and

Fig. 1. Illustration of structures for typical nanocarbon materials.

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Fig. 2. (a) Roles of carbon materials and (b) strategies for tailoring the properties of carbon materials for H2 production from photocatalytic water splitting.

semiconductor–metal hybrids [32] has been applied to suppress the recombination of photo-excited electron-hole pairs. The discovery of carbon nanotubes (CNTs, Fig. 1) in 1991 [33] and graphene (Fig. 1) in 2004 [34] as the shining stars in the research of chemistry and material science has offered new opportunities in designing efficient photocatalytic materials with high stability [35–38]. These sp2 bonded carbon materials possess large surface area, excellent conductivity, and high chemical and thermal stability. The incorporation of CNTs and graphene with semiconductor photocatalysts can not only increase the adsorption sites and active centers towards the reagents, but also serve as an electron acceptor or transport channel to suppress the recombination of photo-excited electron-hole pairs. Moreover, these dimensionally confined 1D and 2D carbon materials can act as a host matrix to various semiconductor photocatalysts, which consequently reduce the particle aggregation and improve the dispersion. In addition, the light absorption would also be extended, resulting in a photo-sensitization and/or photo-thermal effect. All these advantages could enhance the photocatalytic performance of semiconductor photocatalysts through the whole process

of a photocatalytic reaction. Generally, graphene exhibits some superiority in comparison with CNTs for the application of photocatalytic H2 production in several aspects. First, the theoretical specific surface area of graphene (∼2600 m2 g−1 ) is much higher than that of CNTs (∼1300 m2 g−1 ), which essentially means more adsorption sites and active centers on the surface of graphene. Second, the 2D structure feature of graphene can assure better dispersion of semiconductor nanostructures on its surface and larger contact interface between semiconductors and graphene, as compared to the 1D featured CNTs. Third, due to the flatter extended ␲-aromatic structure of graphene, the electrical conductivity and electron mobility of graphene are much higher than that of CNTs, which is beneficial for the electron capture and migration during photocatalytic H2 -evolution process. Actually, another two nanocarbon materials (Fig. 1), C60 [39,40] and carbon quantum dots [41], have also been widely investigated and compared with CNTs and graphene in photocatalytic applications. These 0D carbon materials are more capable of being a modifier on the surface of semiconductor photocatalysts rather than a supporting substrate for semiconductor nanoparticles. Of par-


ticular interest, C60 and carbon quantum dots are both excellent electron acceptors and electron donors, which enrich the functionality of carbon materials in photocatalytic applications. Other carbon materials, including carbon fibers, activated carbon, and carbon black, are also important members of the nanocarbon family, and have played significant roles in the design and preparation of high-performance photocatalytic materials. Obviously, the enhancement of the photocatalytic performance of semiconductorcarbon hybrid photocatalysts depends on many factors such as the size of the carbon materials, the number of walls/layers, the type and density of defects, the surface chemical functional groups of the carbon materials, and the interfacial contact area between semiconductor and carbon material. Hence in this review, we provide a comprehensive overview of carbon-based photocatalytic materials for H2 production from photocatalytic water splitting. As there is vast number of literatures focusing on carbon-containing materials including carbon nitride and boron carbon nitride [42–57], which have been intensively investigated as new class of semiconductor photocatalysts, we could not cover all the literatures though they are very important carbon-containing materials. Indeed, the carbon materials discussed here are not the generalized carbon-containing materials, but limited to CNTs, graphene, C60 , carbon quantum dots, carbon fibers, activated carbon, carbon black, etc. We first systematically discuss the roles of carbon materials in enhancing the performance of semiconductor photocatalysts for H2 production from photocatalytic water splitting (Fig. 2a), in order of supporting material, increasing adsorption and active sites, electron acceptor and transport channel, cocatalyst, photosensitization, photocatalyst, and band gap narrowing effect. Another section of this review focuses on the strategies for tailoring the properties of carbon materials for H2 production from photocatalytic water splitting (Fig. 2b), by surface chemical functionalization of the carbon materials, doping effect of the carbon materials, and interface engineering between semiconductors and carbon materials. Finally, the concluding remarks, the current challenges and perspectives are discussed.

2. Roles of carbon materials in photocatalytic H2 production 2.1. Supporting material for enhanced structure stability Carbon materials, especially CNTs [58–60], carbon fibers [61,62], graphene [63,64] and activated carbon [65] are frequently applied as supporting materials to anchor small nanoparticles of semiconductor photocatalysts. This is attributed to several advantages of these carbon materials (Fig. 3). First, they provide a structure with a large surface area over which nanoparticles can be distributed and immobilized. Second, these carbon materials are chemically inert and thermally stable, thus can preserve unchanged structure and properties during the process to couple with nanoparticles. Third, defect sites and oxygen-containing groups on the surface of carbon materials are capable of supplying abundant nucleation sites for the growth and anchor of uniform nanoparticles. Fourth, the carbon materials possess a light specific weight, which is important for a supporting material. Last, carbon is abundant on the earth; nowadays, activated carbon and carbon fibers, and even CNTs and graphene can be easily prepared in a large scale in an economic way. The presence of these carbon materials can retard the aggregation and improve the structure stability of semiconductor nanoparticles through affecting the nucleation and growth processes [66–71]. In this section, we aim to emphasize the effect of carbon materials on stabilizing the supported nanostructures. Thus the enhanced structure stability is referring to the geometric structures and mor-

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Fig. 3. Advantages of carbon materials for serving as a supporting material of semiconductor photocatalysts.

phologies of the supported units. In other words, such enhanced structure stability is directly related to the good dispersion and stabilization of the ultra-small nanoparticles, clusters or other nanostructures. Such benefit subsequently leads to an enhanced surface area with increased surface active sites on semiconductor nanoparticles, and an enhanced contact interface between semiconductors and carbon materials for improved charge transfer. As a result, more efficient photocatalytic H2 production can be achieved on the semiconductor-carbon hybrid photocatalysts, as compared to that on pristine semiconductor photocatalysts. Note that the interaction between semiconductor nanoparticles and carbon materials is extremely influenced by the surface chemistry of carbon materials. Wang et al. [60] first treated the multiwalled CNTs with mixed solution of HCl and HNO3 to improve the hydrophilic property of CNT surface, through the introduction of oxygen-containing groups such as hydroxyl, carboxyl, carbonyl and epoxy groups (Fig. 4a). The functionalized CNTs were then used to support the Znx Cd1−x S nanoparticles through a solvothermal process using Zn(AC)2 ·2H2 O, CdCl2 ·2/5H2 O, and thiourea as the raw materials of Znx Cd1−x S. As a typical example, the Zn0.83 Cd0.17 S nanoparticles with a diameter of ∼100 nm were well assembled on the CNT surface (Fig. 4b). Otherwise, these nanoparticles would aggregate together in the absence of CNTs (Fig. 4c). The good dispersion endows the Zn0.83 Cd0.17 S/CNTs nanocomposite with an improved interfacial area. Since the CB potential of Zn0.83 Cd0.17 S is more negative than the Fermi level (or greater work function) of CNTs, the photoexcited electrons will transfer from the CB of Zn0.83 Cd0.17 S to the surface of CNTs. Therefore, the above-mentioned benefit can induce an efficient separation of photoinduced charge carriers on the interface between Zn0.83 Cd0.17 S and CNTs. In addition, a band gap tuning to smaller value also occurred for Zn0.83 Cd0.17 S/CNTs nanocomposite as compared to pure Zn0.83 Cd0.17 S, which is indicated by the UV–vis absorption spectra (Fig. 4d). Therefore, a photocatalytic H2 -production rate of 6.03 mmol h−1 g−1 under the wavelength illumination from 300 to 800 nm was achieved for Zn0.83 Cd0.17 S/CNTs nanocomposite, 1.5 times that of pure Zn0.83 Cd0.17 S.

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Fig. 4. (a) Illustration of surface functionalization of CNTs by acid treatment; SEM images of Zn0.83 Cd0.17 S/CNTs (b) and pure Zn0.83 Cd0.17 S (c); (d) UV–vis absorption spectra of Zn0.83 Cd0.17 S/CNTs and pure Zn0.83 Cd0.17 S. Reprinted with permission from Ref. [60]. Copyright 2013 Royal Society of Chemistry.

CdS-cluster-decorated graphene nanosheets were prepared by Li et al. [64] via a solvothermal process in the presence of graphene oxide (GO) as the support, Cd(Ac)2 as the source of Cd2+ , and DMSO as the source of S2− and solvent. Fig. 5a and b clearly show that well-dispersed CdS clusters formed on the graphene surface. The reduction of GO to graphene and the formation of CdS clusters on the graphene surface occurred simultaneously during the sovothermal process. Contrarily, in the absence of graphene, CdS aggregated particles of ∼100 nm were obtained (Fig. 5c). Normally, factors such as physisorption, electrostatic binding and charge transfer interaction could strengthen the interaction between nanoparticles and graphene, and thus retard the aggregation of nanoparticles. As a result of the uniform distribution of CdS clusters on the graphene surface, a larger specific surface area was obtained for CdS/graphene photocatalyst. Moreover, the uniform distribution of CdS clusters also led to a more efficient transfer of photoinduced electrons from CdS to graphene. Obviously, a high H2 -production rate of 1.12 mmol h−1 was obtained at an optimal graphene content of 1.0 wt%, which was 4.87 times higher than that of pure CdS particles (Fig. 5d). The corresponding apparent quantum efficiency (QE) at the wavelength of 420 nm was 22.5% for this optimal CdS/graphene photocatalyst. In another work, Shen et al. [71] also reported the utilization of reduced graphene oxide (RGO) to stabilize the ultrathin nanorods of Zn0.5 Cd0.5 S to enhance the photocatalytic H2 evolution.

2.2. Increasing adsorption and active sites The above section indicates that carbon materials can improve the dispersion and increase the surface areas of supported semiconductor nanoparticles for more adsorption and active sites on the nanoparticle surface and the semiconductor-carbon interface. On the other hand, carbon materials themselves such as CNTs and graphene are also able to increase the adsorption and active sites of a photocatalytic system for H2 production due to their large specific surface area [72–84]. This is because photocatalytic reactions occur not only on the interface between semiconductors and carbon materials but also on the surface of carbon materials around the interface. Theoretical studies have indicated that carbon materials can promote the adsorption and dissociation of water molecules, which is beneficial for hydrogen production [85–90]. Especially for those carbon materials with modified surface, the substantial surface functional groups such as COOH and OH can provide additional reaction centers to enhance the photocatalytic H2 -production activity [91–95], because these functional groups can act as anchoring sites towards the reagents. Zhang et al. [77] prepared graphene supported TiO2 photocatalyst from the precursor of tetrabutyl titanate and graphite oxide, through a sol–gel method with a subsequent calcination process at 450 ◦ C. XRD results showed that the average crystal size of TiO2 (anatase) was ∼11 nm for all samples with various graphene content. However, their specific surface areas


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Fig. 5. (a) TEM and (b) HRTEM images of sample GC1.0, inset of (b) showing the SAED pattern of CdS clusters decorated on graphene sheet; (c) SEM image of sample GC0; (d) visible-light (␭ ≥ 420 nm) photocatalytic H2 -production activity of various samples in 10 vol% lactic acid aqueous solution with 0.5 wt% Pt as the cocatalyst. GCx represents the graphene-CdS sample with x wt% graphene content. Reprinted with permission from Ref. [64]. Copyright 2011 American Chemical Society.

Fig. 6. (a) SEM and (b) TEM images of 1.0 wt%-RGO/ZnIn2 S4 nanocomposite; (c) comparison of photocatalytic H2 -production activity over 1.0 wt%-RGO/ZnIn2 S4 nanocomposite and pure ZnIn2 S4 . Reprinted with permission from Ref. [84]. Copyright 2014 American Chemical Society.

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increased with increasing the graphene content, which was 15.9 and 24.1 m2 g−1 for pure TiO2 and 5 wt% graphene modified TiO2 , respectively. As a result, the 5 wt% graphene modified TiO2 exhibited a hydrogen evolution rate of 8.6 ␮mol h−1 , 1.9 times higher than that of P25 (4.5 ␮mol h−1 ). Of course, the larger surface area was not the only factor that enhanced the photocatalytic activity of TiO2 /graphene. Another reason was the excellent electronic conductivity of graphene that suppressed the recombination of photoinduced electrons and holes, which will be discussed in the following sections. A sheet-on-sheet structure composed by RGO and ZnIn2 S4 nanosheets was prepared by Ye et al. [84] via a one-pot solvothermal route, in a mixed solvent of N,N-dimethylformamide and ethylene glycol (Fig. 6a and b). It was found that even a small amount of 1.0 wt% RGO could lead to a significant increase of specific surface area. The 1.0 wt%-RGO/ZnIn2 S4 nanocomposite possessed a specific area of 150 m2 g−1 , much higher than that of pure ZnIn2 S4 (99.8 m2 g−1 ). In addition, the highly reductive RGO also acted as an efficient electron acceptor and mediator. Along with the co-catalytic activity of RGO, the 1.0 wt%-RGO/ZnIn2 S4 nanocomposite showed a hydrogen evolution rate of 40.9 ␮mol h−1 , while the rate of pure ZnIn2 S4 was only 9.5 ␮mol h−1 , under visible-light irradiation, using lactic acid as the sacrificial agent (Fig. 6c). 2.3. Electron acceptor and transport channel Single-component photocatalysts are usually hard to gain a high efficiency of photocatalytic H2 production. One of the key issues is the rapid recombination of photoinduced electrons and holes, without migrating to the catalyst surface and/or further to reactive sites. The high conductivity of carbon materials enables them to capture photoinduced electrons from the CB of a variety of semiconductors [96–131] or the lowest unoccupied molecular orbitals (LUMO) of dyes [132–150]. This is because the carbon materials such as CNTs and graphene have a high work function, while the CB of the coupled semiconductors or LUMO of the coupled dyes is more negative as compared to the work function of CNTs (∼−0.30 eV vs. NHE) [100] and graphene (∼−0.08 eV vs. NHE) [151], as illustrated in Fig. 7a. These captured electrons are then accumulated on the surface of carbon materials, or rapidly transport across the conductive surface of carbon materials to reactive sites. For instance, Dong et al. [131] performed a theoretical study on the visiblelight induced charge transfer pathway of graphene-supported CdS quantum dots based on first-principle calculations. Their results indicate that photo-excited electrons of CdS quantum dots tend to inject into the LUMO of graphene and further transport across

Fig. 7. (a) Comparison of work function of CNTs and graphene with the CB of several typical semiconductor photocatalysts. (b) Schematic illustration of the role of graphene and carbon nanotube as electron acceptor and transport channel in photocatalytic H2 production.

the surface of graphene through ␲* orbitals, assuring an efficient interfacial charge separation. By this way, the carbon materials serve as excellent electron acceptors and transport channels to suppress the recombination of photoinduced electrons and holes, and thus enhancing the efficiency of photocatalytic H2 production [151–158], as shown in Fig. 7b. Acid-treated multiwalled carbon nanotubes were coupled with Cd0.1 Zn0.9 S solid solution through a hydrothermal method at the pH value of 8, reaction temperature of 160 ◦ C, and reaction time of 8 h, using Zn(Ac)2 ·2H2 O, Cd(Ac)2 ·2H2 O and thioacetamide as the source materials of Cd0.1 Zn0.9 S [159]. Since there were abundant oxygen-containing groups such as hydroxyl groups (–OH), carboxyl groups (–COOH) and carbonyl groups ( C O) on the surface of acidtreated CNTs, Cd0.1 Zn0.9 S nanoparticles were easily anchored on the surface of CNTs (Fig. 8a). It was observed that at an optimal amount of ca. 0.25 wt% CNTs, the CNT/Cd0.1 Zn0.9 S exhibited a highest visible-light (␭ ≥ 420 nm) photocatalytic H2 -production rate of 78.2 ␮mol h−1 . The corresponding apparent quantum efficiency was 7.9% at 420 nm. Note that such excellent activity was achieved in the absence of any noble metal cocatalyst. In comparison, the rate

Fig. 8. (a) TEM image of 0.25 wt% CNT-Cd0.1 Zn0.9 S; (b) photocatalytic mechanism for CNT-Cd0.1 Zn0.9 S composite under visible-light irradiation using Na2 S and Na2 SO3 as sacrificial reagents. Reprinted with permission from Ref. [159]. Copyright 2012 Royal Society of Chemistry.


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Fig. 9. (a) TEM and (b) HRTEM image of RGO/NiS/Zn0.5 Cd0.5 S composite; (c) photocatalytic mechanism for RGO/NiS/Zn0.5 Cd0.5 S composite; (d) comparison of photocatalytic H2 -production activity under simulated solar irradiation over various photocatalysts. Reprinted with permission from Ref. [160]. Copyright 2014 John Wiley and Sons.

of pure Cd0.1 Zn0.9 S was only 24.1 ␮mol h−1 . As shown in Fig. 8b, when the chemically bonded Cd0.1 Zn0.9 S nanoparticles on the CNT surface was photoexcited, electrons on the CB of Cd0.1 Zn0.9 S could quickly transfer to and accumulate on the CNT surface due to the high electrical conductivity and long-range electronic conjugation of CNTs. Thus the recombination of photoinduced charge carriers

was effectively suppressed, and the photocatalytic H2 -evolution activity was successfully enhanced. RGO/NiS/Zn0.5 Cd0.5 S composite photocatalyst was prepared by a co-precipitation method with subsequent hydrothermal treatment [160]. Cd(Ac)2 ·2H2 O, Zn(Ac)2 ·2H2 O, Ni(Ac)2 ·2H2 O and thioacetamide were used as the source materials of Zn0.5 Cd0.5 S

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Fig. 10. Comparison of photocatalytic H2 -production activity over CuO/CF/TiO2 composite, TiO2 and CuO/TiO2 photocatalyst [104].

and NiS, while RGO was obtained from GO during the hydrothermal treatment at 180 ◦ C for 12 h. In this ternary composite, the three components were well connected with each other, as shown in Fig. 9a and b. Such connection enables RGO to be an effective electron acceptor and transporter to capture photoinduced electrons from the CB of Zn0.5 Cd0.5 S and simultaneously to be reduction active centers for H2 evolution. Meanwhile, NiS served as oxidation active centers with accumulated holes from the VB of Zn0.5 Cd0.5 S due to a p-n junction effect between NiS and Zn0.5 Cd0.5 S (Fig. 9c). This efficient separation of photoinduced electron-hole pairs led to excellent photocatalytic H2 -production performance of the RGO/NiS/Zn0.5 Cd0.5 S composite photocatalyst under simulated solar irradiation, without any noble metal cocatalyst. At the optimal amount of 0.25 wt% RGO and 3 mol% NiS, the ternary composite photocatalyst exhibited a H2 -production rate of 375.7 ␮mol h−1 , with a high apparent QE of 31.1% at 420 nm. The activity is much higher than the single- or two-component photocatalysts composed by RGO, NiS, and Zn0.5 Cd0.5 S (Fig. 9d). Carbon fibers (CF) were coupled with TiO2 (P25) and CuO via a wet impregnation and calcination route, to construct a CuO/CF/TiO2 composite photocatalyst [104]. The small amount of CuO acted as a cocatalyst for H2 evolution, while the carbon fibers played the role of electron transport channel to reduce the recombination of electron-hole pairs. As a result, the CuO/CF/TiO2 composite photocatalyst with 1 wt% CF showed much higher photocatalytic H2 -evolution rate (2000 ␮mol h−1 g−1 ) in 10 vol% ethanol solution under the irradiation of a 300 W Xenon lamp, 45 and 2 times that of TiO2 and CuO/TiO2 , respectively (Fig. 10). C60 clusters were embedded into CdS quantum dot-decorated TiO2 mesoporous architectures through an evaporation-induced self-assembly combined with an ion-exchanged route [97]. Under visible-light irradiation, the 0.50 wt% C60 -CdS/TiO2 photocatalyst exhibited a H2 -production rate of 6.03 ␮mol h−1 , while the value of CdS/TiO2 was only 0.71 ␮mol h−1 . Moreover, the C60 -CdS/TiO2 photocatalyst showed a good photostability after cycling photocatalytic tests. This is mainly due to the fact that the C60 clusters could effectively capture and accumulate photoinduced electrons from the CdS/TiO2 semiconductor composite and subsequently promote the charge separation process (Fig. 11). An additional factor is that the C60 clusters could enhance the light absorption for photocatalytic reactions. Carbon dots were prepared from natural vegetables such as guava, red pepper, peas and spinach by Wang et al. [101]. These vegetables were hydrothermally treated at 180 ◦ C for 4 h. The obtained carbon dots (CDs) were further loaded onto TiO2 nanoparticles (NPs) and nanotubes (NTs) by another hydrothermal process. Obviously, these carbon dot-modified TiO2 nanostructures exhibited superior photocatalytic H2 -production activity as compared to the unmodified counterparts. Particularly, under the irradia-

Fig. 11. Schematic illustration of the charge transfer of C60 -CdS/TiO2 composite photocatalyst under visible-light irradiation. Reprinted with permission from Ref. [97]. Copyright 2015 American Chemical Society.

Fig. 12. Comparison of photocatalytic H2 -production activity over carbon dotmodified TiO2 nanostructures [101].

tion of a 300W Xe lamp, the optimal carbon dot-modified TiO2 nanoparticles and nanotubes possessed H2 -production rates of 75.5 and 246.1 ␮mol h−1 g−1 , respectively, 21.6 and 3.3 times that of pure TiO2 nanoparticles and nanotubes (Fig. 12). The crucial factor for such performance enhancement of carbon dot-modified TiO2 nanostructures was the outstanding electron transfer property of carbon dots. Lu’s group has studied the role of carbon materials as the electron mediator in dye-sensitized photocatalytic systems for H2 production [133,161,162]. For example, they decorated NiSx catalyst on graphene by an in situ chemical deposition method and used Eosin Y (EY) as a photosensitizer [161]. For EY-NiSx photocatalytic system, the evolved H2 within 5.5 h was 293.5 ␮mol under visiblelight (␭ ≥ 420 nm) irradiation. For EY-NiSx -graphene photocatalytic system in which the mass ratio of NiSx to graphene was 46.7%, the evolved H2 within 5.5 h was 599.1 ␮mol, with an apparent QE of 32.5% at 430 nm. It was found that graphene greatly improved the electron transfer from excited EY dye to the NiSx cocatalyst in terms of electron-mediator effect. The improved charge transfer process was evidenced by time-resolved transient photoluminescence decay curves. Zhu et al. [135] grew Ni3 S2 nanosheets on CNT backbone and used Erythrosin Yellowish (ErY) as a photosensitizer for photocatalytic H2 production under visible light (␭ ≥ 420 nm). First, CNT@SiO2 was prepared through a simple coating process. This CNT@SiO2 composite then served as a substrate for growing nickel silicate (NiSilicate) nanosheets. The obtained CNT@SiO2 @NiSilicate hybrid structures were finally transformed to CNT@Ni3 S2 nanostructures by hydrothermal treatment in the presence of Na2 S, with the conversion of NiSilicate nanosheets to Ni3 S2 nanosheets and


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Fig. 13. (a) Schematic illustration of the preparation of CNT@Ni3 S2 nanocomposite; (b) FESEM and (c) TEM images of CNT@Ni3 S2 nanocomposite; (d) schematic illustration of photocatalytic H2 production by dye-sensitized CNT@Ni3 S2 photocatalyst. Reprinted with permission from Ref. [135]. Copyright 2012 John Wiley and Sons.

higher than that of ErY-NiS system (2.54 mmol h−1 g−1 ). The corresponding apparent QE was 11.1% at 420 nm. Such superiority of ErY-CNT@Ni3 S2 for photocatalytic H2 production is because the CNTs could quickly transport the photoinduced electrons from ErY to Ni3 S2 nanosheets, as shown in Fig. 13d. Another fascinating function of carbon materials is that graphene can be used as an electron mediator to transfer electrons from O2 -evolution photocatalyst to the H2 -evolution photocatalyst via a Z-scheme mechanism, giving rising to abundant photoinduced electrons for the reduction of H+ [163–165]. For example, Iwase et al. [163] found that photoreduced graphene oxide could mediate photoinduced electrons from BiVO4 (O2 -evolution photocatalyst) to Ru/SrTiO3 :Rh (H2 -evolution photocatalyst) under visible-light irradiation, greatly improving the electron-hole separation in the water splitting reaction. The photocatalytic system was thus a Zscheme system, leaving holes in the VB of BiVO4 and accumulating electrons on the surface or Ru/SrTiO3 :Rh (Fig. 14). As a result, the visible-light photocatalytic activity was enhanced to a 3-fold level as compared to that without RGO. Fig. 14. Z-scheme photocatalytic mechanism over BiVO4 -RGO-Ru/SrTiO3 :Rh system for water splitting.

2.4. Cocatalyst simultaneous removal of intermediate silica layer (Fig. 13a–c). It was observed that the average photocatalytic H2 -production rate of ErY-CNT@Ni3 S2 within 12 h was 5.32 mmol h−1 g−1 , much

Introduction of cocatalysts into a photocatalytic system is usually a rational strategy to enhance the H2 -production efficiency. Normally, noble metals such as Pt, Au or Pd have shown excellent

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Fig. 15. Advantages of carbon materials as cocatalysts for photocatalytic H2 production.

performance when used as cocatalysts, due to their higher work function in comparison with that of semiconductors [166]. The function of a cocatalyst in photocatalytic H2 -production includes improving the charge separation, providing catalytic sites, reducing the overpotential and lowering the activation energy for H2 evolution [166,167]. However, it is commonly realized that noble metals are rare and expensive. Thus alternative cocatalysts with low cost are highly desirable for the development of economic photocatalytic system. Carbon materials are such candidates and have been widely investigated as cocatalysts for photocatalytic H2 production [168–186], because the Fermi level of these carbon materials are lower than the CB of coupled semiconductors but higher than the reduction potential of H+ /H2 . As shown in Fig. 15, besides the advantages similar with that of noble metal cocatalysts, the carbon materials can provide some additional merits such as more adsorption sites due to the large surface area, and a localized photothermal effect due to the extended light absorption [187–191]. Khan et al. [168] loaded chemically oxidized (acid-treated) multiwalled CNTs onto the CdS/TiO2 /Pt composite photocatalyst, in which Pt and CdS were grown on TiO2 by photodeposition and hydrolysis methods, respectively. The resultant CdS/TiO2 /Pt/CNTs composite photocatalyst was investigated for photocatalytic H2 production under visible-light (␭ ≥ 420 nm) irradiation, using Na2 S and Na2 SO3 as sacrificial agents. An electron-hole separation firstly occurred on the heterojunction of CdS and TiO2 . The photoinduced electrons on the CB of TiO2 further transferred to CNTs and Pt catalysts. Since the Fermi level of CNTs is lower than the CB of TiO2 , it can act as a cocatalyst together with Pt nanoparticles. The photocatalytic results showed that either Pt or CNTs exhibited efficient catalytic activity to catalyze the photocatalytic H2 production with CdS/TiO2 photocatalysts. Importantly, the co-loading of Pt (0.4 wt%) and CNTs (4 wt%) could enhance the performance by more than 50%, which means that around 80–90 wt% of Pt could be replaced by CNTs without losing the high catalytic performance. Graphene/MoS2 /TiO2 composite photocatalyst was prepared and used for high-performance photocatalytic H2 production without the addition of noble metal cocatalysts [185]. Graphene/MoS2 layered heterostructure was first prepared by hydrothermal treatment of sodium molybdate, thiourea, and graphene oxide in aqueous solution at 210 ◦ C for 24 h. After that, further hydrothermal treatment of the obtained graphene/MoS2 hybrid with tetrabutyl titanate in ethanol/water solvent resulted in the formation of graphene/MoS2 /TiO2 composite photocatalyst (Fig. 16a and b). It was found that both graphene and MoS2 served as highly active cocatalysts for photocatalytic H2 production under xenon arc lamp

irradiation, using TiO2 as the photocatalyst and ethanol as the sacrificial agent. At an optimal amount of 5.0 wt% graphene and 0.5 wt% MoS2 , the graphene/MoS2 /TiO2 composite photocatalyst exhibited a high H2 -production rate of 165.3 ␮mol h−1 (Fig. 16c), with an apparent QE of 9.7% at 365 nm. This is reasonable because graphene possesses a high work function (∼−0.08 eV vs. NHE) to capture photoinduced electrons from the CB of TiO2 [151,167] (Fig. 16d). In another work, only RGO was coupled with Znx Cd1−x S photocatalysts by a coprecipitation-hydrothermal reduction technique (Fig. 17a), and also showed an excellent cocatalytic performance for photocatalytic H2 production (Fig. 17b) [186]. Under simulated solar irradiation and with Na2 S and Na2 SO3 as the sacrificial agents, the photocatalytic H2 -production rate of the optimized RGO-Zn0.8 Cd0.2 S photocatalyst was as high as 1824 ␮mol h−1 g−1 when the RGO content was 0.25 wt%, with an apparent QE of 23.4% at 420 nm. The performance was even better than that of optimized Pt-Zn0.8 Cd0.2 S under the same reaction conditions (Fig. 17c). These works indicate the promising potential of graphene to replace noble metals as a cocatalyst in specific photocatalytic systems for H2 production. Liu et al. [173] incorporated carbon dots into g-C3 N4 through heat treatment of ammonia-treated carbon dots and urea at 550 ◦ C for 3 h (Fig. 18a–d), in which the carbon dots was prepared firstly by an electrochemical method with subsequent hydrothermal treatment with ammonia. The carbon dot-C3 N4 hybrid photocatalyst was found to be effective in overall water splitting for H2 and O2 evolution under visible-light irradiation. At an optimal content ratio of carbon dots to C3 N4 , i.e. 4.8 × 10−3 gCDots /gcatalyst , the highest QE of 16% was achieved at a 420 ± 20 nm (Fig. 18e). An overall solar energy conversion efficiency of 2.0% was obtained by using an AM 1.5 G solar simulator as the light source for 6 h irradiation, in the presence of carbon dot-C3 N4 hybrid photocatalyst (4.8 × 10−3 gCDots /gcatalyst , 80 mg catalyst) in 150 mL water. It was proposed the photocatalytic overall water splitting in this study followed a stepwise two-electron/two-electron process. In particular, at the first step, water was split into H2 O2 and H2 by the photocataltyic behavior of g-C3 N4 . Then at the second step, H2 O2 was decomposed into H2 O and O2 , catalyzed by the carbon dots. 2.5. Photosensitization In some cases, carbon materials exhibited semiconductor-like or dye-like properties, and thus could act a photosensitizer to extend the photo-response of a wide bandgap semiconductor photocatalyst with additional photoinduced electrons [192–201], when the LUMO of nanocarbon is more negative than the CB of the coupled semiconductor so that the photoinduced electrons in nanocarbon can be injected into the CB of the coupled semiconductor; or when the emission of the nanocarbon overlaps the absorption of the coupled semiconductor so that the fluorescence resonance energy transfer from nanocarbon to the coupled semiconductor can occur. Wang et al. [194] investigated the visible-light (␭ > 420 nm) photocatalytic H2 production over graphene supported ZnS nanoparticles, which were prepared through a two-step hydrothermal method, with zinc chloride, sodium sulfide, and graphite oxide as the source materials. The optimal composite photocatalyst with 0.1 wt% graphene showed a H2 -production rate of 7.42 ␮mol h−1 g−1 , which is 8 times more than that of pure ZnS photocatalyst. Since ZnS could not be excited by visible light, it is expected that photoinduced electrons for water reduction came from graphene, which was first excited from the highest occupied molecular orbital (HOMO) to the LUMO and subsequently transferred into the CB of ZnS (Fig. 19). In another work, RGO was used as an additional photosensitizer together with a natural light-capturing membrane complex (bacteriorhodopsin) to harvest visible light for Pt/TiO2 photocatalyst [192]. The presence of RGO


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Fig. 16. (a) TEM and (b) HRTEM images of graphene/MoS2 /TiO2 composite, with the interlayer spacings of 0.62 and 0.35 nm belonging to MoS2 and graphene, respectively; (c) photocatalytic H2 production of TiO2 /MG (M = MoS2 , G = Graphene) composites, where T/(xM)(yG) represents graphene/MoS2 /TiO2 composite with x% MoS2 and y% graphene; (d) schematic illustration of photocatalytic H2 production over graphene/MoS2 /TiO2 composite photocatalyst, showing graphene and MoS2 as cocatalysts. Reprinted with permission from Ref. [185]. Copyright 2012 American Chemical Society.

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Fig. 17. (a) TEM image of RGO-Zn0.8 Cd0.2 S photocatalyst; (b) schematic illustration of photocatalytic H2 production over RGO-Zn0.8 Cd0.2 S photocatalyst, showing graphene as the cocatalyst; (c) photocatalytic H2 -production activity over various photocatalysts, where GSx represents RGO-Zn0.8 Cd0.2 S photocatalyst with x wt% of RGO content. Reprinted with permission from Ref. [186]. Copyright 2012 American Chemical Society.

led to a photocatalytic H2 -production rate of 11.24 mmol H2 (␮mol protein)−1 h−1 . The interfacial charge transfer from photoexcited RGO to Pt/TiO2 photocatalyst was evidenced by electron paramagnetic resonance and transient absorption spectroscopy. Carbon dots, typically sized in the range of 2–10 nm, usually exhibit strong photoluminescence and good photo-absorption. Such optical property enables carbon dots to be promising photosensitizers for photocatalytic reactions. For instance, Martindale et al. [196] constructed a quasi-molecular photocatalytic system for H2 production under full solar spectrum irradiation, using carbon dots as photosensitizers combined with a Ni-bis(diphosphine) catalyst (NiP) (Fig. 20a). The sodium carboxylate capped carbon dots of 6.8 ± 2.3 nm were prepared by thermolysis of citric acid under air at 180 ◦ C for 40 h and subsequent neutralization with aqueous NaOH (Fig. 20b and c). Due to the ␲–␲* (C C) and n−␲* (C O) transitions, these carbon dots showed a broad UV absorption with a tail in the near-visible region. Drifting photoluminescence emission peak was also found from 464 to 532 nm with shifting the excitation wavelength from 360 to 460 nm. A photocatalytic H2 -production rate of 398 ␮mol H2 (g CQD)−1 h−1 was reached in the presence of 0.5 mg (2.2 nmol) of carbon dots and 30 nmol of NiP, in 3 mL of 0.1 M EDTA solution at pH 6. The corresponding internal QE was ∼1.4% at 360 ± 10 nm. Another unique property of carbon dots is the photoluminescence upconversion property, which endows the carbon dots with the ability to harvest near-infrared photons and convert them for semiconductor photocatalysis [197–201]. Tian et al. [197] constructed a UV–visible–NIR active photocatalyst, carbon quantum dots/hydrogenated TiO2 nanobelt heterostructures (CQDs/H-TiO2 ) by hydrogenation of the as-synthesized TiO2 nanobelts with subsequent bath reflux loading of carbon quantum dots (Fig. 21a–c). This composite photocatalyst exhibited excellent photocatalytic H2 -production performance in a broad spectrum range from UV to NIR under the irradiation of a 300 W Xe arc lamp, in the presence

of methanol as the sacrificial agent and platinum as the cocatalyst. The H2 -production rate was 7.42 mmol h−1 g−1 , higher than that of hydrogenated TiO2 nanobelts (6.01 mmol h−1 g−1 ). Here the NIR photocatalytic activity is attributed to the photoluminescence upconversion property of carbon quantum dots which could convert the NIR light into visible light and transfer to hydrogenated TiO2 nanobelts (Fig. 21d). 2.6. Photocatalyst Theoretical and experimental work has also demonstrated that carbon materials can be employed as photocatalysts for H2 production [202–213], because of the semiconducting nature of CNTs, GO, RGO, C60 and carbon dots. And the semiconducting nanocarbons can possess more negative LUMO positions than the reduction potential of H+ /H2 . A typical candidate is RGO [214–217], whose CB minimum is mainly composed by anti-bonding ␲* orbital, corresponding to a potential of −0.52 eV vs. NHE, pH = 0 [151]. Note that the O 2p orbital contributes mainly to VB maximum of RGO. As such, the band gap of RGO could be tuned by varying the degree of oxidation (Fig. 22). In this regard, Jiang et al. [211] applied density functional theory calculations to investigate the electronic structures of GO by varying the coverage and relative ratio of the surface epoxy and hydroxyl groups. It was found that at a coverage of 40–50% and OH:O ratio of 2:1, or a coverage of 33–67% and OH:O ratio of 1:1, the electronic structures can assure both the photocatalytic hydrogen evolution and oxygen evolution. Teng’s group has revealed that GO prepared with a modified Hummers’ procedure exhibited apparent direct band gap of 3.3–4.3 eV and indirect band gap of 2.4–3.0 eV (Fig. 23a–c) [213]. This is because the GO is composed by graphene molecules with various oxidation degrees. It was confirmed by chemical composition analysis that the carbon content was 49%. Continuous H2 production was achieved on the GO under either UV or visible-


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Fig. 18. (a) TEM of carbon dots modified g-C3 N4; (b) magnified TEM image the marked area in (a) (c) HRTEM image of a single carbon dot in the composite (d) FFT pattern of (c) with hexagonal crystalline structure of the carbon dots (e) QE for carbon dots (CDots) in different concentrations. Reprinted with permission from Ref. [173]. Copyright 2015 The American Association for the Advancement of Science.

light irradiation. Particularly, around 17000 ␮mol H2 was obtained after 6 h irradiation of a mercury lamp in a 20 vol% methanol aqueous solution, in the presence of 0.5 g bare GO without any cocatalyst (Fig. 23d). Meng et al. [215] deposited p-type MoS2 nanoplatelets of 5–20 nm onto the surface of n-type nitrogendoped RGO. As expected, the p-MoS2 /n-RGO hybrid photocatalyst exhibited much higher photocatalytic hydrogen-evolution activity than that of individual MoS2 and common MoS2 /RGO composite, in ethanol aqueous solution under simulated solar light irradiation. This is due to the efficient charge separation process of the Fig. 19. Schematic illustration of photocatalytic H2 production over ZnS-graphene composite. Reprinted with permission from Ref. [194]. Copyright 2015 IOP Publishing Ltd.

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Fig. 20. (a) Schematic illustration of photocatalytic H2 production over carbon dot-NiP system; (b) TEM and (c) HRTEM of carbon dots. Reprinted with permission from Ref. [196]. Copyright 2015 American Chemical Society.

Fig. 21. (a) Schematic illustration of the synthetic route of CQDs/H-TiO2 composite; (b) TEM and (c) high-magnification TEM images of CQDs/H-TiO2 composite; (d) schematic illustration of photosensitized behavior over CQDs/H-TiO2 composite. Reprinted with permission from Ref. [197]. Copyright 2015 Elsevier.


Fig. 22. Band structure of RGO with various reduction degrees. Reprinted with permission from Ref. [151]. Copyright 2013 John Wiley and Sons.

p-MoS2 /n-RGO junctions, as demonstrated by the photoelectrochemical measurement. Zhu’s group has investigated both pure carbon dots [207,208] and carbon dot-based composite photocatalysts [206] for H2 production. It was observed that pure carbon dots prepared hydrothermally from multiwalled carbon nanotubes oxide could photocatalyze the hydrogen evolution even in pure water without any cocatalyst, with a H2 -production rate of 423.7 ␮mol g−1 h−1 [208]. In another work, a Z-scheme carbon nanodots/WO3 hybird photocatalyst showed a H2 -production rate of 1330 ␮mol g−1 h−1 under xenon lamp irradiation, in the presence of methanol as the sacrificial agent and Pt as the co-catalyst [206]. 2.7. Band gap narrowing effect It was demonstrated by both theoretical [218,219] and experimental studies [220–226] that specific chemical bonding (e.g. metal O C bonds) could form between semiconductor photocatalysts and carbon materials, once strong interactions were established between each other. Such chemical bonding would lead

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to a band gap narrowing effect on the semiconductor photocatalysts and consequently enhance the photocataltyic H2 -production performance. Ye et al. [223] prepared graphene or CNT-supported CdS nanoparticles (∼35 nm) through a hydrothermal method for photocatalytic H2 production. At an optimal mass ratio of 0.01:1 for graphene:CdS, and 0.05:1 for CNT:CdS, the photocatalytic H2 production rates were 70 ␮mol h−1 and 52 ␮mol h−1 , respectively. The experiments were performed under the irradiation of a 200 W Xe lamp (␭ ≥ 420 nm), using 0.1 g catalyst in 100 mL aqueous solution of 0.1 M Na2 S and 0.05 M Na2 SO3 . Pure CdS only showed a rate of 14.5 ␮mol h−1 . Significant band-gap narrowing was observed for these graphene or CNT modified CdS photocatalysts due to the strong interactions between CdS and graphene or CNT. Combined with the advantage of more efficient charge separation, the graphene or CNT modified CdS photocatalysts thus possessed much better photocatalytic H2 -production performance than pure CdS. Normally, Bi2 WO6 is not suitable for the photocatalytic H2 evolution reaction from water splitting due to its less negative CB, as compared to the reduction potential of water. However, Sun et al. [225,226] have indicated that by incorporation of graphene with Bi2 WO6 nanostructures, the CB of Bi2 WO6 could be raised to a more negative level for feasible ability to photocatalyze H2 evolution reaction. In a typical work [225], Bi2 WO6 nanoparticles sized in 30–40 nm were in situ grown on the surface of graphene via a sonication process in the presence of GO, Bi(NO3 )3 ·5H2 O, (NH4 )10 W12 O41 and HNO3 , with a subsequent calcination at 450 ◦ C for 3 h in nitrogen atomosphere. Both Raman and XPS analysis confirmed the chemical bonding between Bi2 WO6 and graphene. Moreover, as evidenced by Mott–Schottky measurements, the CB of Bi2 WO6 was raised from +0.09 V to −0.30 V vs. NHE after the introduction of graphene (Fig. 24). Therefore, a photocatalytic H2 -production rate of 159.2 ␮mol h−1 was achieved in methanol aqueous solution under visible-light (␭ ≥ 420 nm) irradiation, by using 0.03 g Bi2 WO6 -graphene photocatalyst. In summary, carbon materials are capable of improving the photocatalytic H2 production over semiconductor photocatalysts from

Fig. 23. (a) Optical absorption of GO and irr-GO; (b) plots of (␣E)2 versus the photon energy (E) for GO and irr-GO; (c) plots of (␣E)1/2 versus the photon energy (E) for GO and irr-GO; (d) photocatalytic H2 production over GO. irr-GO represents GO irradiated in methanol solution after 6 h. Reprinted with permission from Ref. [213]. Copyright 2010 John Wiley and Sons.

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Table 1 Possible roles of various carbon materials in this Review. Nanocarbon/Roles

Carbon nanotubes

Graphene

C60

Carbon quantum dots

Carbon fibers

Activated carbon

Carbon black

Supporting material Increasing adsorption and active sites Electron acceptor and transport channel Cocatalyst Photosensitization Photocatalyst Band gap narrowing effect

Yes Yes Yes Yes \ Yes Yes

Yes Yes Yes Yes Yes Yes Yes

\ \ Yes Yes Yes Yes \

\ \ Yes Yes Yes Yes \

Yes Yes Yes \ \ \ \

Yes Yes Yes \ \ \ Yes

\ \ Yes Yes \ \ \

3. Strategies for tailoring the properties of carbon materials for photocatalytic H2 production The improvement of photocatalytic H2 -production performance of carbon material-based photocatalysts is highly related to the structure effects and property effects of carbon materials. These effects include the size of the carbon materials, the number of walls/layers, the type and density of defects, the surface chemical functional groups of the carbon materials, and the interfacial contact area between semiconductor and carbon materials. Therefore, to sufficiently harness the advantages of carbon materials for enhanced photocatalytic H2 production, rational strategies are applied to control the structure effects and property effects of carbon materials (Fig. 2b). 3.1. Surface chemical functionalization

Fig. 24. Mott–Schottky plots of (a) BWO-T and (b) Gr–BWO-T versus Ag/AgCl electrodes, where BWO-T and Gr–BWO-T represent Bi2 WO6 and graphene-Bi2 WO6 , respectively. Reprinted with permission from Ref. [225]. Copyright 2014 Royal Society of Chemistry.

a variety of aspects. The incorporation of different carbon materials with semiconductor photocatalysts can lead to different promoting effect through one or more of the following roles: supporting material for enhanced structure stability, increasing adsorption and active sites towards the reagents, electron acceptor and transport channel to suppress the recombination of photo-excited electronhole pairs, cocatalyst, photosensitization, photocatalyst and band gap narrowing effect. Note that specific carbon material may have multi-functional roles during the whole photocatalytic reaction. On the other hand, specific carbon material may not have the function of all roles due to the texture and property limitation. As such, the possible roles of different carbon materials are listed in Table 1 as a quick reference.

Surface chemical functionalization, such as acid oxidation, and chemical grafting of organic ligands does not destroy the internal structure of carbon materials. However, it could induce a number of surface defects and appropriate amount of oxygenated functional groups, which were beneficial for the enhancement of photocatalytic H2 -production performance [227–239]. In general, such functionalization can generate abundant nucleation sites for the growth and anchor of uniform nanoparticles, thus help to improve the dispersion and qualification of coupled semiconductor nanostructures. Moreover, these surface defects and functional groups can act as anchoring sites towards the reagents during photocatalytic reactions. Li et al. [227] prepared ZnIn2 S4 /CdIn2 S4 3D hierarchical films that were composed by porous nanosheets on carbon cloth (CC) through a simple hydrothermal method. It was found that the addition of l-cysteine was critical for the formation of the hierarchical structures during the hydrothermal process. On one hand, l-cysteine served as a coordinating agent with the inorganic metal ions, since l-cysteine possesses the functional groups of NH2 , COOH, and SH. On the other hand, l-cysteine was used as a sulfur source for the formation of metal sulfides. Such dual effects of l-cysteine enabled the slow release of S2− ions and metal ions, providing the opportunity to form porous nanosheets on the surface of carbon cloth (Fig. 25). As a typical example, the optimized ZnIn2 S4 /CdIn2 S4 (20%)-CC exhibited an excellent photocatalytic H2 -production activity under visible-light (␭ ≥ 420 nm) irradiation in Na2 S/Na2 SO3 aqueous solution. The corresponding rate was 210 ␮mol h−1 . Moreover, such 3D hierarchical films on carbon cloth were easily recycled for repeated tests. This work indicated the introduction of organic ligands was beneficial for the formation of specific structures of semiconductor photocatalysts on the surface of carbon materials, to further improve the photocatalytic H2 -production performance. Sordello et al. [232] found that COOH and NH2 functionalized graphene could act as a shape controller for TiO2 during the hydrothermal synthesis of graphene-TiO2 composite photocatalysts. Particularly, COOH functionalized graphene preferred to bond with the {101} facets for the growth of anatase truncated


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Fig. 25. (a) Formation process of CC-supported ZnIn2 S4 /CdIn2 S4 (20%) nanosheet layers; (b) SEM and (c) high-magnification SEM of CC-supported ZnIn2 S4 /CdIn2 S4 (20%) nanosheet layers. Reprinted with permission from Ref. [227]. Copyright 2015 John Wiley and Sons.

bipyramids, while NH2 functionalized graphene chose to bond with the {100} facets for the growth of belted truncated bipyramids. Fang et al. [230] prepared thiolated graphene nanosheets through the modification of GO with l-cysteine and a subsequent reduction process by hydrazine solution. The thiolated graphene nanosheets showed excellent ability to immobilize CdS quantum dots and Pt nanocrystals with good dispersion (Fig. 26). As a result, a high H2 -evolution rate of 2.15 mmol h−1 was achieved in lactic acid aqueous solution under visible-light (␭ ≥ 420 nm) irradiation, with a QE of 50.7% at 420 nm. Other surface chemical functionalization of graphene (oxide) such as carboxylic acid functionalization with chloroacetic acid [239], and triphenylamine functionalization with 4-(diphenylamino)benzaldehyde [233] was also applied to improve the dispersion of photocatalysts or cocatalysts for enhanced photocatalytic H2 production. 3.2. Doping effect Heteroatom doping is an effective strategy to tailor the electrical conductivity and electronic structure of carbon materials, consequently tuning the electron mobility, charge transfer ability, and metal-like/semiconductor-like property of carbon materials. For example, Kwon et al. [240] investigated the role of metal cations including Na+ , K+ , Mg2+ , and Ca2+ in alkali metal chloride doped graphene. It was found that these metal cations with low work functions could lead to n-type doping by forming interfacial dipole complexes with the oxidized functional groups on graphene, which consequently decrease the conductivity and work function of graphene. However, current studies of heteroatom doping of carbon materials to modify the photocatalytic H2 -production activity almost focus on non-metal doping [241–253]. Teng’s group [242] has prepared nitrogen-doped graphene oxide quantum dots by treating GO in NH3 at 500 ◦ C, with subsequent oxidation via a modified Hummers’ method. Interestingly,

the electrochemical Mott−Schottky analysis indicated that the nitrogen-doped graphene oxide quantum dots showed both pand n-type conductivities, which resulted in an internal Z-scheme charge transfer pathway (Fig. 27). Photocatalytic evaluation in pure water under visible-light (420 nm < ␭ < 800 nm) irradiation suggested that overall water splitting with a H2 :O2 ratio of around 2:1 was achieved over these nitrogen-doped graphene oxide quantum dots. Normally, the p-type conductivity of oxygen functional groupcontaining GO could promote the hydrogen evolution, while the n-type conductivity of nitrogen-doped GO could benefit the oxygen evolution. Although the activity was not as high as that of the Rh2-y Cry O3 /GaN:ZnO composite photocatalyst, the nitrogen-doped graphene oxide quantum dots were totally metal-free candidates for photocatalytic H2 production. Further, the same group also prepared surface intact nitrogen-doped graphene oxide quantum dots by ultrasonic exfoliation of NH3 -treated GO sheets for photocatalytic H2 production in triethanolamine aqueous solution, still in terms of a Z-scheme charge transfer mechanism [241]. Latorre-Sánchez et al. [247] reported the preparation of P-doped graphene for photocatalytic H2 production. Typically, the P-doped graphene was synthesized by thermal decomposition of H2 PO4 − modified alginate at 900 ◦ C under Argon atmosphere. The P doping led to a conversion from zero-bandgap graphene to semiconducting graphene (up to 2.85 eV), which exhibited both UV and visible activity towards photocatalytic H2 production. A H2 -production rate of 282 ␮mol h−1 g−1 could be achieved under UV–vis irradiation in using triethanolamine as the sacrificial agent and Pt as the co-catalyst. 3.3. Interface engineering For carbon material-based semiconductor photocatalysts, there are generally two types of interface effects. In particular, Schottky junction would form between metallic carbon materials and

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Fig. 26. (a) Formation process of thiolated graphene (Gcys ) nanosheets with immobilized CdS quantum dots and Pt nanocrystals; (b) HRTEM image of thiolated graphene nanosheets with immobilized CdS quantum dots and Pt nanocrystals. Reprinted with permission from Ref. [230]. Copyright 2013 Royal Society of Chemistry.

semiconductor photocatalysts, while semiconductor heterojunction such as p-n junction and Z-scheme heterojunction could form between semiconducting carbon materials and semiconductor photocatalysts [35,38]. Since the photoinduced charge transfer mainly occurs across the contact interface between carbon materials and the coupled semiconductors, it is expectable that both the above-mentioned interface effects significantly influence the charge transfer and separation of the carbon material-based semiconductor photocatalysts. Therefore, rationally designing an intimate and large contact interface between the carbon materials and semiconductor photocatalysts would be a critical approach to promote the charge separation efficiency and obtain more efficient

carbon material-based semiconductor photocatalysts. To date, a variety of studies have been focused on such interface engineering of carbon material-based semiconductor photocatalysts [254–288]. In a theoretical study based on density functional theory calculations [267], it was revealed that graphene acted as a sensitizer for SrTiO3 (100) with a termination layer of TiO, while it served as an electron shuttle for SrTiO3 (100) with a termination layer of SrO. While when RGO was coupled, a type II heterojunction could form at the interface, with negatively charged O atoms in the RGO as the active sites. As a result, an enhanced photocatalytic performance could be achieved by tuning the interface for efficient charge separation and optimal active sites.


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Fig. 27. Illustration of electronic structure of nitrogen-doped graphene oxide quantum dots with a Z-scheme charge transfer pathway. Reprinted with permission from Ref. [242]. Copyright 2014 John Wiley and Sons.

Cherevan et al. [261] prepared CNT–Ta2 O5 hybrid photocatalysts through in situ growth of Ta2 O5 onto the surface of multiwalled CNTs by a hydrothermal approach using Ta(OEt)5 as the precursor. Two kinds of CNT–Ta2 O5 hybrids with different CNT to Ta2 O5 weight ratios were prepared, which were 1:4 and 1:2 for thick (sample H1) and thin Ta2 O5 coating (sample H2), respectively (Fig. 28). Such variation consequently led to different interface effects between CNTs and Ta2 O5 . In particular, H2 hybrid was more single-crystalline than H1 hybrid, and thus possessed fewer grain boundaries for improved interface charge transfer. Moreover, the thin growth of Ta2 O5 on CNTs resulted in the formation of a tight Schottky-junction in the H2 hybrid, which promoted the interface charge separation. In contrast, the charge transfer in H1 hybrid with

thick Ta2 O5 layer needed to go through a long-distance tunneling effect, which was less effective than that of H2. In addition, the tight and thin growth of Ta2 O5 in H2 hybrid caused a larger band gap with an increased reduction potential of CB. As a result of the above-mentioned reasons, the H2 hybrid exhibited a much higher photocataltyic H2 -production rate (1600 ␮mol h−1 ) than H1 hybrid (520 ␮mol h−1 ) under UV light irradiation, in the presence of Pt as the cocatalyst and methanol as the sacrificial agent. An effective strategy to obtain intimate and large contact interface is to construct 2D-2D layered junctions to provide abundant surface active sites and achieve efficient interfacial charge transfer [289]. For instance, TiO2 nanosheets with exposed {001} facets were hybridized with graphene, which showed excellent photocat-

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Fig. 28. (a) TEM image of H1 annealed at 500 ◦ C; (b) TEM image of H2 annealed at 500 ◦ C; HRTEM images of H2 annealed at 500 ◦ C showing the Ta2 O5 layer (c) and the tight interface between CNTs and Ta2 O5 (d). Reprinted with permission from Ref. [261]. Copyright 2014 Royal Society of Chemistry.

Fig. 29. (a) Schematic illustration of photocatalytic H2 production over g-C3 N4 -graphene composite; (b) comparison of the photocatalytic H2 production over various photocatalysts, where GCx represents graphene/g-C3 N4 composite with x wt% graphene. Reprinted with permission from Ref. [281].Copyright 2011 American Chemical Society.

alytic H2 -evolution activity even without Pt cocatalyst [282]. In a more typical work, Xiang et al. [281] hybridized the two star materials, g-C3 N4 and graphene through an impregnation-chemical reduction route, with subsequent thermal treatment at 550 ◦ C in nitrogen atmosphere. Melamine and graphene oxide were used as the precursors of g-C3 N4 and graphene, respectively. And hydrazine hydrate was used as the reducing agent to convert graphene oxide to graphene. The successful formation of 2D–2D layered junctions between g-C3 N4 and graphene led to a very efficient interfacial charge separation, which enabled the spatial accumulation of photoinduced electrons and holes on the sides of graphene and g-C3 N4 , respectively (Fig. 29a). Consequently, with optimizing the interface

effect at the amount of 1.0 wt% graphene, the photocatalytic H2 production rate (451 ␮mol h−1 g−1 ) was much higher than that of pure g-C3 N4 and g-C3 N4 modified with other amount of graphene (Fig. 29b). In summary, the properties of carbon materials for H2 production from photocatalytic water splitting can be optimized by surface chemical functionalization to provide nucleation sites for the growth and dispersion of coupled semiconductor units, as well as anchoring sites for the photocatalytic reagents. The performance can also be tuned by heteroatom doping of carbon materials to modulate the electrical conductivity and electronic structure of carbon materials. Moreover, rational interface engineering between


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Table 2 Representative summary of the photocatalytic H2 -production activity of carbon-based photocatalysts. Photocatalysts

Other cocatalysts

Sacrificial agents

Light source (wavelength/nm)

Activity (mmol h−1 g−1 )a

Quantum efficiency

Refs.

Zn0.83 Cd0.17 S/CNTs CdS/graphene TiO2 /graphene ZnIn2 S4 /RGO CdS/TiO2 /C60 TiO2 /carbon dots CuO/CF/TiO2 ErY+ Ni3 S2 /CNT Cd0.1 Zn0.9 S/CNTs Zn0.5 Cd0.5 S/NiS/RGO EY+ NiSx /graphene TiO2 /MoS2 /Graphene Zn0.8 Cd0.2 S/RGO carbon dots + Ni-bis-(diphosphine) H-TiO2 /CQDs carbon nanodots GO CdS/graphene CdS/CNT Bi2 WO6 /graphene CdS/thiolated graphene Ta2 O5 /CNT g-C3 N4 + graphene

none Pt none none none none none none none none none none none none Pt none none none none none Pt Pt Pt

Na2 S + Na2 SO3 lactic acid Na2 S + Na2 SO3 lactic acid Na2 S + Na2 SO3 methanol ethanol triethanolamine Na2 S + Na2 SO3 Na2 S + Na2 SO3 triethanolamine ethanol Na2 S + Na2 SO3 EDTA methanol none methanol Na2 S + Na2 SO3 Na2 S + Na2 SO3 methanol lactic acid methanol methanol

500 W Xe lamp (300–800 nm) 350 W Xe lamp (≥ 420 nm) 500 W Xe lamp 300 W Xe lamp (≥ 420 nm) Four 3 W LEDs (420 nm) 300 W Xe lamp 300 W Xe lamp 300 W Xe lamp (> 420 nm) 300 W Xe lamp (≥ 420 nm) solar simulator 300 W Xe lamp (≥ 420 nm) 350 W Xe lamp solar simulator solar simulator 350 W Xe lamp 300 W mercury lamp 400 W mercury lamp 200 W Xe lamp (≥ 420 nm) 200 W Xe lamp (≥ 420 nm) 300 W Xe lamp (> 420 nm) 300 W Xe lamp (≥ 420 nm) 450 W mercury lamp 350 W Xe lamp (> 400 nm)

6.03 56 0.086 0.818 0.121 0.246 2 5.32b 1.564 7.514 12.378c 2.066 1.824 0.398d 7.42 0.424 5.667 0.7 0.52 5.307 29.655 32 0.451

\ 22.5% (420 nm) \ \ 2.0% (420 nm) \ \ 11.1% (420 nm) 7.9% (420 nm) 31.1% (420 nm) 32.5% (430 nm) 9.7% (365 nm) 23.4% (420 nm) 1.4% (360 nm) \ \ 2.7% (full spectrum) \ \ \ 50.7% (420 nm) \ 2.6% (420 nm)

[60] [64] [77] [84] [97] [101] [104] [135] [159] [160] [161] [185] [186] [196] [197] [208] [213] [223] [223] [225] [230] [261] [281]

a b c d

All data are normalized based on the mass of photocatalyst. Calculated based on the mass of Ni3 S2 /CNT. Calculated by normalizing the mass of NiSx /graphene. Calculated by normalizing the mass of carbon dots.

the carbon materials and coupled semiconductor units can greatly enhance the photocatalytic H2 production mainly through improving the interfacial charge separation. For reference, the representative summary of the photocatalytic hydrogen evolution activity of some typical carbon material-based photocatalysts discussed in Section 2 and 3 are listed in Table 2.

4. Concluding remarks and perspectives Carbon-based H2 -production photocatalytic materials are promising functional materials for the potential solution to the increasing energy demand. In this review, carbon materials such as CNTs, graphene, C60 , carbon quantum dots, carbon fibers, activated carbon, carbon black, etc. for enhanced H2 production over semiconductor photocatalysts were comprehensively overviewed. A systematical understanding of the roles of carbon materials in enhancing the performance of semiconductor photocatalysts for H2 production from photocatalytic water splitting was summarized in terms of supporting material for enhanced structure stability, increasing adsorption and active sites, electron acceptor and transport channel, cocatalyst, photosensitization, photocatalyst and band gap narrowing effect. The strategies for tailoring the properties of carbon materials for H2 production from photocatalytic water splitting were also discussed in several aspects, including surface chemical functionalization of the carbon materials, doping effect of the carbon materials, and interface engineering between semiconductors and carbon materials. Despite great accomplishment demonstrating the extraordinary potential of carbon-based photocatalytic materials in the field of photocatalytic H2 production, significant challenges still remain in the current stage. First, the performance of the hybrid photocatalytic materials is highly related to the properties of carbon materials, which are greatly affected by the synthetic technique, functionalization and defects. Moreover, affirmative interface effects rely on the good dispersion of semiconductor photocatalysts, high-quality decoration of carbon materials, intimate and large contact interface between semiconductor photocatalysts and carbon materials. In this regard, it is necessary to

develop advanced preparation methods for both pristine carbon materials and carbon-semiconductor hybrids, for tuning the abovementioned factors. Second, the interfacial charge transfer dynamics are not clear enough till now. For example, whether the carbon materials act as electron donors or acceptors, whether the interface junction is traditional heterojunction or Z-scheme junction, are difficult to distinguish. Therefore, accurate trace of charge carriers using high-resolution transient spectroscopy should be intensively applied in the study of carbon-based photocatalytic materials on photocatalytic H2 production. On the other hand, deep insights based on considerable theoretical and mechanistic studies are beneficial for the understanding of the electronic interactions and charge transfer pathways between semiconductor photocatalysts and carbon materials. Third, current studies using carbon-based photocatalytic materials mostly focus on the photocatalytic H2 production in the presence of sacrificial agents. However, for the real application objective, the photocatalytic H2 production based on overall splitting is in fact much closer to the concept of “clean and renewable energy”. Hence it is highly encouraged to popularize the investigation of carbon-based photocatalytic materials on photocatalytic overall water splitting. In addition, relevant exploration should also be expanded to photocatalytic CO2 reduction into hydrocarbon fuels, which is considered as another promising way to gain “clean and renewable energy”.

Acknowledgements This work was supported by the 973 program (2013CB632402), and NSFC (51472191, 21407115, 51272199, 51320105001 and 21433007). Also, this work was financially supported by the Natural Science Foundation of Hubei Province of China (2014CFB164, 2015CFA001), the Special Financial Grant from the China Postdoctoral Science Foundation (2015T80843), Deanship of Scientific Research (DSR) of King Abdulaziz University (90-130-35-HiCi), the Fundamental Research Funds for the Central Universities (WUT: 2015-III-034) and Innovative Research Funds of SKLWUT (2015ZD-1) and a WUT Start-Up Grant.

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