DOI: 10.1002/cssc.201501203

Reviews

Noble-Metal-Free Molybdenum Disulfide Cocatalyst for Photocatalytic Hydrogen Production Yong-Jun Yuan,*[a] Hong-Wei Lu,*[a] Zhen-Tao Yu,*[b] and Zhi-Gang Zou*[b]

ChemSusChem 2015, 8, 4113 – 4127

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Reviews Photocatalytic water splitting using powered semiconductors as photocatalysts represents a promising strategy for clean, low-cost, and environmentally friendly production of H2 utilizing solar energy. The loading of noble-metal cocatalysts on semiconductors can significantly enhance the solar-to-H2 conversion efficiency. However, the high cost and scarcity of noble metals counter their extensive utilization. Therefore, the use of alternative cocatalysts based on non-precious metal materials is pursued. Nanosized MoS2 cocatalysts have attracted considerable attention in the last decade as a viable alternative to im-

1. Introduction Over-use of fossil fuels resulted in human societies being close to energy and environmental crises. The largest technological and scientific challenge of this century is to find ways to replace the supplies of fossil fuels with renewable energy resources.[1] H2, with an energy density of 140 MJ kg¢1, can be used as a clean and sustainable alternative to fossil fuels to cheaply and efficiently power vehicles equipped with fuel cells without producing any pollutants.[2] The development of energy-efficient, cost-effective, and clean processes for H2 production is, however, a major challenge to successfully shift from a fossilfuel-based economy to a fuel economy based on H2. Water is the most promising source for the production of H2 as it is carbon-free, widely available at almost no cost. In addition, solar energy as one of the renewable energies is clean and could be used in most regions on the surface of the earth, which is expected to play a key role in the development of a sustainable economy. Therefore, a highly desirable strategy is to convert the energy provided by the sun to hydrogen fuel to build a clean and sustainable energy cycle.[3] 2 H 2 O ! 2 H2 þ O 2

DE ‘ ¼ 1:23 V

ð1Þ

2 H 2 O ! 4 Hþ þ O 2

E ‘ ¼ þ0:82 V, pH 7

ð2Þ

4 H þ ! 2 H2

E ‘ ¼ ¢0:41 V, pH 7

ð3Þ

Since the pioneering work by Fujishima and Honda in 1972 who used a TiO2 electrode for photoelectrochemical water [a] Dr. Y.-J. Yuan, Dr. H.-W. Lu College of Materials and Environmental Engineering Hangzhou Dianzi University Hangzhou, Zhejiang 310018 (P. R. China) E-mail: [email protected] [email protected] [b] Dr. Z.-T. Yu, Prof. Dr. Z.-G. Zou Ecomaterials and Renewable Energy Research Center Jiangsu Key Laboratory for Nano Technology College of Engineering and Applied Science Nanjing University Nanjing, Jiangsu 210093 (P. R. China) E-mail: [email protected] [email protected] ORCID(s) from the author(s) for this article is/are available on the WWW under http://dx.doi.org/10.1002/cssc.201501203.

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prove solar-to-H2 conversion efficiency because of its superb catalytic activity, excellent stability, low cost, availability, environmental friendliness, and chemical inertness. In this perspective, the design, structures, synthesis, and application of MoS2based composite photocatalysts for solar H2 generation are summarized, compared, and discussed. Finally, this Review concludes with a summary and remarks on some challenges and opportunities for the future development of MoS2-based photocatalysts.

splitting,[4] solar H2 generation via photoelectrochemical/photocatalytic water splitting has attracted considerable attention because it proposes a cost-effective means to convert incident solar light to hydrogen fuel.[5] The overall water-splitting reaction is a thermodynamically uphill reaction with a positive free energy change of DG = + 237.12 kJ mol¢1, corresponding to a minimum energy of 1.23 eV per electron [Eq. (1)], which involves multiple electron-transfer processes.[6] The photocatalytic path to water splitting involves two half reactions: the oxidation of water to evolve O2 and the reduction of water to produce H2 ; the two half reactions with their corresponding standard reduction potential E8 at pH 7 with respect to the standard hydrogen electrode (SHE) are shown in Equations (2) and (3), respectively.[7] Designing a complete water splitting system is a daunting task, only a few photocatalysts reported in literature successfully decomposed water into H2 and O2 in a photocatalytic system.[8] To screen highly efficient photocatalyst for the assembly of overall photocatalytic water splitting systems, it is often necessary to investigate H2 evolution and O2 evolution half reactions by replacing the other half reaction with an appropriate sacrificial reagent (SR) as an electron donor or electron acceptor, respectively.[9] Solar H2 generation from water has been achieved using different approaches, including semiconductor-based devices and transition-metal complexes in homogeneous or heterogeneous systems.[10–12] The research area dealing with this area is expanding very rapidly and attracting scientists from different disciplines: 1) Chemists that design and synthesize suitable molecular light-harvesting photosensitizers and water-reduction catalysts and study the structure–property relationships of homogeneous photocatalytic H2-evolution systems; 2) physicists that build novel semiconductor devices and understand the fundamental photophysical processes; and 3) materials scientists that develop new photocatalytic materials with novel structures and that characterize and optimize the performances. The synergy between all disciplines will play a major role for future advancements in this area. It has been known that the performance of homogeneous photocatalytic H2-evolution systems, typically consisting of transition-metal complexes as light-harvesting photosensitizers and molecular H2- evolution catalysts, is limited by the poor stability of the molecular materials used.[13] However, semiconductor-based heterogeneous photocatalysts are relatively stable during the photocatalytic H2-evolution reaction. Thus,

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Reviews the development of efficient heterogeneous solar H2-evolution systems is of considerable importance to advance the science of artificial photosynthesis. During the past 40 years, numerous semiconductor photocatalysts were developed for solar H2 generation from water, such as metal oxides (TiO2,[14]ZnO,[15] Cu2O,[16] Ga2O3,[17] SrTiO3[18]), sulfides (CdS,[19] ZnIn2S4,[20] CuInS2,[21] Cu2ZnSnS4[22]), selenides (CdSe[23]), oxynitrides (TaON,[24] BaTaO2N[25]), inorganic solid solutions (ZnO:GaN[26]), and organic polymers (g-C3N4).[27] In general, the overall photocatalytic H2-evolution reaction involves three major processes: 1) absorption of light by a light-harvesting semiconductor to generate electron–hole pairs, 2) charge separation and migration to the surface of the semiconductor, and 3) water reduction reaction on the surface of semiconductor to evolve H2. The overall efficiency of photocatalytic H2 evolution is determined by all of the above three processes.[28] For most semiconductor photocatalysts, the photogenerated electron–hole pairs can quickly recombine in bulk or on surface of the semiconductor to release energy in the form of unproductive heat or photons, resulting in a low charge-separation efficiency. One of the efficient methods to

improve the charge-separation efficiency is loading cocatalyst on semiconductor photocatalysts (Figure 1), which not only acts as an electron sink to suppress the recombination of photogenerated electron–hole pairs, but also provides a large amount of active sites for the proton-reduction reaction.[29]

Yong-Jun Yuan obtained his BSc degree in Chemistry from Nanchang Hangkong University, PR China, in 2009 and received his Ph.D. in Materials Science in 2014 from Nanjing University under the supervision of Professors Zhi-Gang Zou and Zhen-Tao Yu. Currently, he is a lecturer at Hangzhou Dianzi University at the School of Materials and Environmental Engineering. His research focuses on the design and development of nanosized composite photocatalysts for photocatalytic H2 production.

Zhen-Tao Yu received his BSc and PhD degrees in Chemistry from Jilin University, PR China, in 1999 and 2004, respectively. After that he worked as a postdoctoral researcher at the Humboldt Universit•t Berlin in 2004–2005, at the Technische Universit•t CaroloWilhelmina Braunschweig in 2005– 2006, and at the Institute of Physical and Chemical Research (RIKEN, Japan) in 2007–2009. He joined the Ecomaterials and Renewable Energy Research Centre of Nanjing University in 2009 as an Associate Professor. His research interests are the development of the design, synthesis, and characterization of metal complexes for solar H2 production.

Hong-Wei Lu received his BSs and PhD degrees in Chemistry from Sun Yat-Sen University, PR China, in 2003 and 2008, respectively. After working as a postdoctoral researcher at Zhejiang University, he joined the faculty of the School of Materials and Environmental Engineering in Hangzhou Dianzi University as an Associate Professor. His current research is focused on photocatalytic water splitting.

Zhi-Gang Zou is the director of Ecomaterials and Renewable Energy Research Center, Nanjing University. He received his BSc and MSc degrees from Tianjing University, PR China, in 1982 and 1986, respectively. He worked as a lecturer in Tianjing University from 1986 to 1991. Then, he joined the Research Laboratory of Engineering Materials at the Tokyo Institute of Technology as a visiting Researcher. After he received his PhD degree from the University of Tokyo (Japan) in 1996, he was a researcher at the Photoreaction Control Research Center of National Institute of Advanced Industrial Science and Technology (AIST), Japan. Since 2003 he is a full professor at Nanjing University, PR China. His current research focuses on photocatalytic and photoelectrochemical water splitting, solar cells, and fuel cells.

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Figure 1. Photocatalytic H2 production from water over cocatalyst-loaded semiconductor photocatalyst.

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Reviews Good cocatalysts for photocatalytic H2 evolution should work with a high catalytic activity and a low overpotential as well as be cheap and robust. Currently, most of the heterogeneous photocatalytic H2evolution systems developed so far utilize noblemetal-based cocatalysts to achieve high photocatalytic activities. For example, a series of noble metals, such as Pt,[30]Ru,[31] Rh,[32] Pd,[33] Au,[34] and Ag[35] have been extensively investigated as efficient cocatalysts for photocatalytic H2 evolution. Among them, Pt metal with the largest work function is considered to be the best candidate cocatalyst for the photocatalytic H2-evolution half reaction. High quantum efficiencies above 50 % have been achieved when Pt was loaded as cocatalyst on a CdS semiconductor in the presence of Na2S and Na2SO3 as sacrificial reagents.[36] Unfortunately, the above noble-metal-based cocatalysts are too scarce and expensive to be used for Figure 2. (a) Three-dimensional representation of the structure of a MoS2 monolayer. Relarge-scale energy production. Therefore, the devel- printed with permission from Ref. [53]. Copyright 2011, Nature Publishing Group. (b) Schematics of structural polytypes: including 2H, 3R, and 1T-MoS2 polytypes and opment of noble-metal-free cocatalysts with high ef- (c) band structures calculated from first-principles density functional theory for bulk and ficiencies and low cost is highly desirable. monolayer MoS2. Reprinted with permission from Ref. [57]. Copyright 2012, Nature PubIn recent years, many types of novel cocatalysts lishing Group. (d) Energy diagrams of the conduction band (CB) and valence band (VB) constructed from inexpensive and earth-abundant el- edge potentials in different sized MoS2. ements such as Cu(OH)2,[37] Ni(OH)2,[38] Co(OH)2,[39] FeS,[40] CoS,[41] NiS,[42] NiS2,[43] CuS,[44] MoS2,[45] MoS3,[46] and of 0.65 nm, a single layer of Mo atoms is sandwiched between WS2[47] have been developed for photocatalytic H2 evolution. two layers of sulfur atoms; this structure can be prepared Among them, nanosized MoS2 is thought to be the most reusing Li-based intercalation compounds[54] or scotch tape.[55] nowned H2-evolution catalyst because of its low cost, excellent Large-area MoS2 nanosheets can also be produced using the chemical stability, nontoxicity, and high reactivity. It has been liquid exfoliation method.[56] MoS2 has three main structural known that bulk MoS2 is an excellent catalyst for hydrodesulfupolytypes: 2H (two layers per repeat unit, hexagonal symmerization.[48] However, bulk MoS2 was not considered to be an eftry), 3R (three layers per repeat unit, rhombohedral symmetry), ficient H2-evolution catalyst for a long time.[49] Since the group and 1T (one layer per repeat unit, tetragonal symmetry).[57] As of Nørskov reported that nanosized MoS2 was an active elecshown in Figure 2 b, the most common two-dimensional form trocatalyst for H2 production from water in 2005,[50] a significant of MoS2 has the 2H structure, in which the two sulfur lattice amount of research on MoS2-catalyzed H2 production from planes are staggered with regard to the Mo site with a trigonal water in both electrochemical and photochemical systems has prismatic coordination. For the rhombohedral 3R-MoS2, there been performed in the past decade. Recently, some reviews are three layers per repeat unit with a trigonal prismatic coorsummarized and discussed the development of MoS2 catalysts dination. However, the tetragonal structure of 1T-MoS2 is differfor electrochemical H2 evolution.[51, 52] However, no prior work ent from both 2H- and 3R-MoS2, in which the tetragonal strucregarding an in-depth discussion of the role of nanosized MoS2 ture has Mo atoms at every alternate site and a pair of S atoms for the enhanced photocatalytic H2-evolution activity has been centered at each of the other sites in octahedral coordination. reported to date. In this report, we systematically review the The electronic structures of bulk and single-layer MoS2 calculatrecent development of MoS2-based photocatalysts for solar H2 ed from first principles calculations are illustrated in Figure 2 c. generation. This Review focuses on the structure, photophysiAt the G-point, the bandgap transition is direct for the monocal and electrochemical properties, and photocatalytic H2-evolayer MoS2, but gradually shifts to be indirect for the bulk malution activities of MoS2 cocatalysts. In the last section, we will terial.[58, 59] The change in the band structure of MoS2 with the introduce a strategy to design potential MoS2-based photocalayer numbers can be attributed to quantum confinement and talysts for highly efficient solar H2 generation applications. the resulting change in hybridization between Mo 4dz2 orbitals and S 3pz orbitals. For MoS2, the theoretical calculations show the conduction-band states at the K-point are mainly due to the in-plane orbitals (i.e., Mo 4dxy and Mo 4dx2 ¢y2 ), which are rel2. Basic Properties of MoS2 atively unaffected by interlayer coupling. However, the states 2.1. Structure and electronic properties of MoS2 near the G-point are assigned to the combinations of out-ofMoS2 is a typical layered transition-metal dichalcogenide. As plane orbitals (i.e., Mo 4dz2 and S 3pz) and have a strong intershown in Figure 2 a, bulk MoS2 is composed of vertically layer coupling effect.[57, 60–62] Therefore, as the layer numbers stacked, weakly interacting layers held together by van der decrease, the direct excitonic states near the K-point are relaWaals interactions.[53] For single-layered MoS2 with a thickness tively unchanged, but the transition of states at the G-point ChemSusChem 2015, 8, 4113 – 4127

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Reviews shifts significantly from being indirect to a larger, direct one. As a result, MoS2 is expected to undergo an indirect-to-direct bandgap transformation with decreasing layer numbers, and the bandgap of MoS2 is increased from 1.2 eV (bulk MoS2) to 1.9 eV (monolayer MoS2). Figure 2 d summarizes the calculated conduction band (CB) and valance band (VB) edge potentials for MoS2 with different layers. In the bulk MoS2, the CB edge potential of bulk MoS2 is ¢0.16 V, which is higher than that of the H + /H2 couple redox (¢0.41 V vs. SHE, pH 7). Therefore, the bulk MoS2 is an inactive catalyst to reduce protons to produce H2. Because of the quantum size effect, a MoS2 monolayer has a lower CB edge potential (¢0.53 V) than the H + /H2 redox couple, indicating that the MoS2 monolayer can reduce protons to produce H2. On the other hand, as a H2-evolution catalyst, the CB edge potential should be more positive than that of photocatalyst and more negative than that of the H + /H2 redox couple. For example, the CB edge potential of monolayer MoS2 is more negative than that of TiO2 (¢0.50 V vs. SHE, pH 7), making the electron transfer from the TiO2 CB to the MoS2 CB thermodynamically infeasible. Contrarily, few-layered MoS2 nanosheets have a suitable CB edge potential more positive than that of TiO2 and more negative than that of the H + /H2 redox couple; they have been used as a cocatalyst to enhance the photocatalytic H2-evolution activity of TiO2. To reveal the microstructure of MoS2 at the atomic level, MoS2 clusters were characterized by scanning tunneling microscopy (STM) capable of resolving the individual atoms of closepacked surfaces and clusters on a routine basis.[62, 63] Figure 3 a

Figure 4. Ball model of a bulk-truncated hexagonal MoS2 nanocluster exposing the low-index (1010) Mo and (1010) S edges (S: yellow; Mo: blue) and also the most stable edge structures. gMo and gS are the edge free energies of (1010) Mo edge and (1010) S edge, respectively. If gS < 2 Õ gMo, the result is a triangle terminated exclusively by the S edge, or vice versa for the Mo edgeReprinted with permission from Ref. [64]. Copyright 2007, Elsevier.

2.2. Identification active site of MoS2 for H2 evolution To identify the active edge sites of MoS2 for the H2-evolution reaction, Jaramillo et al. systematically prepared MoS2 nanoparticles with different sizes.[68] The variation in surface-site distribution on MoS2 nanoparticles was quantified by means of STM. The electrocatalytic H2-evolution measurements showed that the catalytic activity correlated linearly with the number of edge sites on the MoS2 catalyst. Therefore, the edge sites of MoS2 were identified as active sites for the H2-evolution reaction. To gain further insight into the catalytic nature of the MoS2 edge, the Gibbs free energy of atomic H bonding to the catalyst was calculated.[69] As shown in Figure 5, the DFT-calcu-

Figure 3. (a) STM and (b) atomically resolved STM image of MoS2 nanoclusters synthesized on Au(111). Adapted with permission from Ref. [63]. Copyright 2007, Nature Publishing Group.

shows a typical STM image of MoS2 clusters on a single-crystalline Au(111) substrate, on which MoS2 nanoclusters consisting of a single S–Mo–S trilayer oriented with the (0001) basal plane were dispersed uniformly.[63] With respect to the morphology of the MoS2 nanoclusters, the atomically resolved STM image shown in Figure 3 b reveals that the clusters are of triangular shape under sulfiding conditions. As shown in Figure 4, there are two types of low-index edge terminations of a MoS2 hexagon for the bulk MoS2 structure: the (1010) Mo and the (1010) S edges.[64] With respect to the MoS2 nanoclusters with a triangular shape, only one edge type is energetically favored.[65, 66] Both computational and an atom-resolved STM image confirmed that the most stable edge type is the (1010) Mo edge covered with 50 or 100 % S atoms.[64, 67] ChemSusChem 2015, 8, 4113 – 4127

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Figure 5. Volcano plot of the exchange current as a function of the DFT-calculated Gibbs free energy of adsorbed atomic hydrogen for pure metals and MoS2 nanoparticles. Reprinted with permission from Ref. [68]. Copyright 2007, American Association for the Advancement of Science.

lated Gibbs free energy of absorption of H atoms was determined to be + 0.08 V for the MoS2 edge. Metals with strong bonds to atomic hydrogen (such as Nb, W, Re, Ni, Mo, and Co) are not good catalysts because the hydrogen release step will be slow. On the other hand, metals (Au, Ag, and Cu) with a relatively high Gibbs free energy, which do not bind to atomic hydrogen, are also excluded because the H2-evolution reaction step will be thermodynamically uphill and therefore slow. As a result, the criterion for a material to be a good catalyst is that the Gibbs free energy of absorption of H atoms is close to

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Reviews that of the product (that is, H2 and DG = 0). This principle can explain available experimental observations regarding noble metals (such as Pt, Pd, Rh, and Ir) and MoS2 as highly efficient catalysts for the H2-evolution reaction. Theoretical calculations and high-resolution STM studies regarding MoS2 indicated that the catalytic activity of MoS2 is localized on few surface sites. However, bulk MoS2 is relatively inert, the precise molecular structures and modes of action of these sites remain elusive. Karunadasa et al. reported the synthesis of a side-on bound MoIV-disulfide complex [(PY5Me2)MoS2]2 + [PY5Me2 = 2,6-bis(1,1-bis[2-pyridyl]ethyl)pyridine] (Figure 6), which is a well-defined molecular analogue of

ly inert, the reaction pathways also remained inconclusive. In the past decade, most studies with the aim to improve the catalytic activity of the MoS2 catalyst focused on the preparation of two-dimensional layered MoS2 with high exposure of edges.[72–75] In addition, the combination of MoS2 and electronically dissimilar materials, such as graphene,[76–78] and CoSe2,[79] to achieve contact on the nanoscale is another efficient method to develop highly efficient MoS2-based water-reduction catalysts.

3. MoS2-based materials for solar H2 evolution 3.1. MoS2/sulfide photocatalysts

Figure 6. Synthesis of [(PY5Me2)MoS2]2 + and a model of the layered structure of MoS2 highlighting the proposed surface reconstruction to yield disulfideterminated edge sites. Reprinted with permission from Ref. [70]. Copyright 2012, American Association for the Advancement of Science.

the proposed MoS2 edge structure.[70] Electrochemical H2 generation from acetic acid acetonitrile solution as well as from aqueous acetic acid solution was observed when using the [(PY5Me2)MoS2]2 + complex as catalyst, lending support to the proposed active-site morphology in the solid MoS2 catalyst. Controlling the surface structure of MoS2 on the atomic scale is paramount to develop highly efficient MoS2 catalysts. Kibsgaard et al. engineered the surface structure of MoS2 to preferentially expose edge sites to effect improved catalysis by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS2 bicontinuous network with nanoscale pores.[71] The high surface curvature of this catalyst mesostructure exposes a large fraction of edge sites, which, along with its high surface area, leads to excellent activity for electrocatalytic H2 evolution. The work elucidates how morphological control of MoS2 on the nanoscale can significantly impact the surface structure on the atomic scale, enabling new opportunities for enhancing surface properties for catalysis and other important technological applications. Although both computational and experimental results confirmed that the catalytic sites of the S–Mo–S layers are associated with the S edges while their basal planes were catalyticalChemSusChem 2015, 8, 4113 – 4127

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Nanosized MoS2 water-reduction catalysts have been widely studied for electrochemical H2 evolution from water. Since 2008 Zong et al. synthesized a composite material consisting of MoS2 nanosheets grown on the surface of CdS as a highperformance noble-metal-free photocatalyst for visible-lightdriven H2 evolution (Figure 7 a).[80, 81] The nanosized MoS2 was loaded on the CdS surface by impregnating CdS with an aqueous solution of (NH4)2MoS4 followed by a treatment in a H2S flow at high temperatures. Through optimization of the loading amount of the MoS2 cocatalyst, the 0.2 wt % MoS2/CdS hybrid composite showed the highest photocatalytic H2 production rate of 5.4 mmol h¢1 g¢1 in the presence of lactic acid as the sacrificial reagent, which is about 36 times that of bare CdS. More importantly, the MoS2/CdS composite exhibited a considerably higher activity compared to noble metals such as Pt, Pd, Rh, Ru, and Au loaded on CdS photocatalysts under the same reaction conditions (Figure 7 b). The excellent H2 activation behavior of MoS2 and the intimate junction between MoS2 and CdS were supposed to be responsible for the superior photocatalytic performance of MoS2/CdS. These results not only suggested that MoS2 could act as a substitute for noble metals in solar-to-H2 conversion, but also provided an important strategy to obtain a more efficient interelectron transfer through an intimate junction between the cocatalyst and semiconductor. Despite nanosized MoS2 being an excellent cocatalyst for photocatalytic H2 production, it is a fact that its cocatalytic activity is restricted by the poor electrical conductivity of MoS2. Recently, some studies showed that the electrical conductivity and activity of MoS2 for solar H2 generation can be improved by adding other conductive materials. Chang and co-workers investigated the photocatalytic H2 generation ability of MoS2/RGO–CdS (RGO: reduced graphene oxide) composites prepared through a solution–chemistry method using CdCl2, Na2MoO4, and graphene oxide (GO) as precursors (Figure 7 c).[82] By optimizing the content of each component, the highest H2-evolution rate (9.0 mmol h¢1 g¢1) was achieved for the MoS2/RGO–CdS composites when the content of the MoS2/ RGO cocatalyst was 2.0 wt % and the molar ratio of MoS2 to graphene was 1:2 under visible-light irradiation in the presence of Na2S and Na2SO3 as sacrificial reagents, which corresponds to an apparent quantum efficiency (AQE) of 28.1 % at 420 nm. As illustrated in Figure 7 d, layered graphene acts as an electron-conducting electron transport “highway’’, which can ac-

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Figure 7. (a) High resolution transmission electron microscopy (HRTEM) image of 1 wt % MoS2/CdS and (b) the rate of H2 evolution on CdS loaded with 0.2 wt % of different cocatalysts. Reprinted with permission from Ref. [80]. Copyright 2008, American Chemical Society. (c) HRTEM image of the MoS2/RGO-CdS composite. (d) Schematic illustration of the charge transfer in the MoS2/RGO-CdS composite under visible light irradiation. Graphene provides a template for photogenerated electron transfer. Reprinted with permission from Ref. [82]. Copyright 2014, American Chemical Society. (e) Schematic illustration of photogenerated carrier transfer in the hydrogen evolution reaction utilizing Cr-MoS2 hybrid nanosheets as the cocatalyst. (f) The H2 production as a function of irradiation time utilizing MoS2/CdS, CrMoS2/CdS and Ag-MoS2/CdS as photocatalysts. Reprinted with permission from Ref. [84]. Copyright 2014, American Chemical Society.

celerate the transfer of photogenerated electrons from the CB of CdS to MoS2 nanosheets, and then the electrons on the MoS2 surface react with the adsorbed H + ions to produce H2. The structure of the MoS2/graphene cocatalyst created a notable synergetic effect, which suppresses charge recombination, improves interfacial charge transfer, and increases the number of active sites for photocatalytic H2 evolution. Apart from RGO, the metal–organic framework UiO-66 (consisting of zirconium oxoclusters and terephthalates) was also used as an ideal support for the well-dispersed growth of CdS and MoS2. Shen et al. prepared a highly efficient MoS2/UiO-66/ CdS photocatalyst through the dual modification of CdS with metal–organic framework UiO-66 and MoS2.[83] It is believed that the positive synergetic effect between MoS2 and UiO-66 can efficiently improve interfacial charge transfer, suppress charge recombination, and provide a large number of active sites and H2 evolution reaction centers. Even without a noblemetal cocatalyst, the obtained MoS2/UiO-66/CdS composite photocatalyst reached a high H2-evolution rate of 32 500 mmol h¢1 g¢1 under visible-light irradiation using lactic acid as a sacrificial reagent when the content of UiO-66 is 50 wt % and MoS2 was 1.5 wt %, which was nearly 60 times ChemSusChem 2015, 8, 4113 – 4127

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higher than the H2-evolution rate of pure CdS and also exceeded that of Pt/UiO-66/CdS under the same reaction conditions. This study clearly demonstrates that the metal–organic framework material can act as an ideal support to enhance the photocatalytic activity and stability of the semiconductor. Although the MoS2 cocatalyst can effectively enhance the photocatalytic H2-evolution activity of CdS, the energy conversion efficiencies are limited by charge recombination and still low for practical applications; a further improvement in stability and activity of MoS2/CdS photocatalyst is still needed. However, the development of highly efficient MoS2/CdS photocatalysts for photocatalytic H2 production is still largely hindered by the ultrafast charge recombination occurring in the material. To improve the charge carrier separation efficiency of MoS2/ CdS photocatalysts for solar H2 generation, Yang and co-workers introduced metal nanoparticles (Cr, Ag) on the surface of MoS2 nanosheets, which acted as trapping sites for photogenerated electrons (Figure 7 e).[84] The carrier dynamics and photoluminescence studies showed that the Cr and Ag nanoparticles can reduce the electron–hole recombination probability in MoS2/CdS. As cocatalysts, both Cr–MoS2 and Ag–MoS2 composites exhibited much higher catalytic activities for photocatalytic H2 production than pure MoS2 nanosheets (Figure 7 f), and the average rate of H2 evolution increased from 18 000 (MoS2/ CdS) to 38 000 and 107 000 mmol h¢1 g¢1 for the Cr–MoS2/CdS and Ag–MoS2/CdS photocatalysts, respectively. For most MoS2-based photocatalysts, nanosized MoS2 was usually prepared through high-temperature calcination under protective gas or using solvothermal or photodeposition techniques using (NH4)2MoS4 or Na2MoO4 as a precursor.[80, 85, 86] Andrew et al. showed for the first time that MoS2 nanosheets obtained from bulk MoS2 by chemical exfoliation is an effective cocatalyst for photocatalytic H2 evolution from water under visible light when coupled with CdSe as the light-harvesting material. Electrochemical measurements revealed that MoS2 activated the MoS2/CdSe photocatalyst by lowering the electrochemical proton reduction overpotential.[87] The introduction of only a small amount of MoS2 (0.50 wt %) as cocatalyst on the CdSe surface could significantly enhance the photocatalytic H2 production rate (890 mmol h¢1 g¢1), which is almost 3.7 times that of pure CdSe. An optimum MoS2 loading was 0.5 wt %, and a further increase in the loading amount of MoS2 resulted in decreased photocatalytic H2 evolution. The decreased H2 production activities of those samples with a high MoS2 loading (> 0.5 % mass) could be attributed to the shading effect of MoS2, that is, MoS2 blocking light absorption by CdSe. As a consequence, an appropriate amount of MoS2 is important for optimizing the photocatalytic H2-evolution activities of MoS2loaded semiconductor photocatalysts. For MoS2 crystals, both DFT and experimental studies showed that the exposed edge sites are active for the H2 evolution reaction because the adsorbed S atoms at the edge are unsaturated.[50, 68] As a result, protons can be adsorbed on these unsaturated S atoms and then the protons are reduced by electrons from the light-harvesting semiconductor to produce H2. However, the amorphous MoS2 is lacking such welldefined edge sites and the active sites for H2-evolution reac-

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Reviews tion are different from those of crystalline MoS2. Wei et al. reported an enhancement of the photocatalytic H2-evolution activity of hexagonal ZnIn2S4 using amorphous MoS2 as a cocatalyst; the amorphous MoS2 was shown to exhibit an excellent promoting effect for the solar-to-H2 conversion for the first time.[88] The promoting effect played by amorphous MoS2 can be ascribed to the existence of many defect sites in amorphous MoS2 that can act as adsorption sites for H atoms and eventually lead to H2 evolution. The defect sites in amorphous MoS2 could be assigned to structurally unsaturated S atoms, which adsorb protons to evolve H2.[51] The optimum MoS2 loading amount was found to be 0.6 wt %, giving rise to a high H2evolution rate of 3060 mmol h¢1 g¢1 in the presence of Na2S and Na2SO3 as the sacrificial reagents under visible light irradiation. Moreover, the photocatalytic results showed that the MoS2/ZnIn2S4 nanocomposites exhibited considerably higher activity compared to Pt/ZnIn2S4 under similar reaction conditions. Chen et al. prepared a MoS2/ZnIn2S4 photocatalyst for visible-light-driven H2 production, which was obtained using hydrothermal method to afford fluorated ZnIn2S4 microspheres, and the MoS2 nanosheets were loaded on ZnIn2S4 surface using a facile in situ photodeposition method.[89] The photocatalytic results showed that the deposition method of the MoS2 cocatalyst dramatically influences the photocatalytic activities of MoS2/ZnIn2S4. The highest H2-evolution rate was 8047 mmol h¢1 g¢1 under visible light irradiation (l > 420 nm) using a 0.375 % MoS2 loading, which is 28 times higher than that of untreated ZnIn2S4. These studies demonstrate a high potential for the development of environmental friendly and noble-metal-free cocatalyst using MoS2 for solar H2 generation. 3.2. MoS2/oxide photocatalysts Although the photocatalytic H2 production activity of CdS can be significantly enhanced by using MoS2 as cocatalyst, Cd is a widespread environmental pollutant that is toxic and harmful to human beings. Kanda et al. reported the fabrication of MoS2 nanocrystals on TiO2 through in situ photodeposition using (NH4)2MoS4 as a precursor.[90] Unfortunately, the MoS2/TiO2 photocatalyst exhibited a relative low photocatalytic activity with a H2-evolution rate of approximately 73 mmol h¢1 g¢1 under ultraviolet light irradiation (l > 300 nm) in the presence of HCOOH as sacrificial agent. For MoS2-based cocatalysts, the MoS2 nanosheets would be more active than MoS2 nanoparticles because of the large amount of exposed edge sites. Recently, few-layer MoS2-nanosheet-coated TiO2 nanobelt with 3 D hierarchical structures were reported as an active photocatalyst for solar H2 generation.[91] The scanning electron microscope (SEM) and transmission electron microscopy (TEM) characterization demonstrated that only low-density MoS2 nanosheets were uniformly coated on the surface of TiO2 nanobelts (Figure 8 a and b), and the lattice fringes of MoS2 and TiO2 can be seen clearly in the HRTEM image (Figure 8 c). The matched energy band of the TiO2/MoS2 heterostructure favors the charge transfer and suppresses the photogenerated electron– hole recombination between MoS2 and TiO2, leading to the enhanced photocatalytic H2 production activity; the highest H2 ChemSusChem 2015, 8, 4113 – 4127

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Figure 8. (a) SEM and (b, c) TEM images of few-layer MoS2 nanosheet-coated TiO2 nanobelt heterostructures. Reprinted with permission from Ref. [91]. Copyright 2013, Wiley-VCH.

production rate (1.6 mmol h¢1 g¢1) was obtained when using the TiO2/MoS2 heterostructure with 50 wt % of MoS2 as the photocatalyst. MoS2-loaded semiconductor composite photocatalysts are usually obtained through high-temperature calcination in protective gas or solvothermal methods using (NH4)2MoS4 or Na2MoO4 as a precursor. However, the abovementioned methods have some weaknesses such as the complexity of the reaction process, high cost, considerable secondary pollution, and incomplete reduction. Accordingly, it is of great importance to develop a facile, environmentally friendly, and cost-effective method for the preparation of MoS2/TiO2 photocatalysts. Liu et al. introduced an effective method for the facile one-step synthesis of highly crystalline MoS2/TiO2 nanocomposite photocatalysts—the solvothermal treatment of (NH4)2MoS4 and tetrabutyl titanate in diethylene glycol.[92] Although the efficient interfacial electron transfer from TiO2 to MoS2 and the decrease of H2 reduction overpotential were observed for MoS2/TiO2 nanocomposite, the MoS2/TiO2 composite photocatalysts show poor photocatalytic activities with a maximum H2-evolution rate of 119 mmol h¢1 g¢1. In 2014, Zhu et al. proposed a simple method to synthesize MoS2/TiO2 composite photocatalysts through mechanochemistry by using MoS2 as a direct precursor. Photoelectrochemical measurements confirmed that the photogenerated electrons on the CB of TiO2 could be easily transferred to the MoS2 cocatalyst, which promoted efficient charge separation and improved the photocatalytic performance.[93] The 4.0 % MoS2/TiO2 composite photocatalyst had the highest photocatalytic activity with a H2-evolution rate of 753 mmol h¢1 g¢1, which was 48.6 times higher than that of pure TiO2. Owing to the high specific surface area and superior electron mobility of two-dimensional (2 D) graphene, which can also act as a support for the growth and coupling of MoS2, producing a highly-efficient 2 D graphene-MoS2 composite cocatalyst for the water reduction reaction. In 2012, Xiang et al. synthesized a ternary composite material consisting of TiO2 nanoparticles grown on the surface of layered MoS2/RGO hybrid as a highly efficient and noble-metal-free photocatalyst for solar H2 generation.[94] The TiO2/RGO/MoS2 composite photocatalyst was prepared through a two-step simple hydrothermal process using tetrabutyl titanate as the TiO2 precursor and Na2MoO4, (NH2)2CS, and graphene oxide as precursors for MoS2/RGO. Through optimization of content of each component, a high H2-evolution rate (2066 mmol h¢1 g¢1) could be achieved with 0.5 wt % MoS2/graphene as cocatalyst and a molar ratio of MoS2 to graphene of 95:5, with the apparent

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Reviews a promising substitute for noble metals in solar H2 generation system. Furthermore, the MoS2–ZnO heterostructure photocatalyst exhibited an excellent stability during the photocatalytic reaction. The intimate contact can efficiently promote charge transfer between ZnO and MoS2 in the heterostructure, which was considered to give rise to the high performance and stability of the MoS2–ZnO heterostructure photocatalyst. Similar results were observed in a MoS2/Cu2O-catalyzed H2-evolution system, in which MoS2 as cocatalyst loaded on the surface of Cu2O can greatly improve the photostability of Cu2O by forming a nanojunction.[96] 3.3. MoS2/organics photocatalysts Recently, the polymeric graphitic carbon nitride (g-C3N4) with a bandgap of 2.7 eV has emerged as a new type of visible-light photocatalyst showing catalytic activity for solar H2 generation.[27, 97, 98] More importantly, g-C3N4 has a layered structure analogous to MoS2. Coupling different 2D-layered materials such as g-C3N4 and MoS2 can greatly enhance the photocatalytic performance of 2D composite materials because of an increased contact surface and charge transfer rate. Hou et al. synthesized MoS2/g-C3N4 composite photocatalyst by impregnating g-C3N4 with an aqueous solution of (NH4)2MoS4, and subsequent sulfidation with H2S gas at 350 8C (Figure 10 a).[99]

Figure 9. (a) Photocatalytic H2 evolution for TiO2/RGO/MoS2 composites with different MoS2 and RGO contents in the RGO/MoS2 hybrid used as cocatalyst under ultraviolet irradiation (denoted as T/95M5.0G, which contains 99.5% TiO2 and 0.5% cocatalyst consisting of MoS2 (95%) and graphene (5.0%). (b) Schematic illustration of the charge transfer in TiO2/RGO/MoS2 composites. Reprinted with permission from Ref. [94]. Copyright 2012, American Chemical Society.

quantum efficiency reaching 9.7 % at 365 nm (Figure 9 a). The high H2 production rate was mainly attributed to the incorporation of graphene as an electron acceptor and transporter to separate photogenerated electron–hole pairs in the light-harvesting semiconductor (Figure 9 b). It is well known that some oxides such as ZnO and Cu2O usually suffer from serious photocorrosion during the photocatalytic reaction. Interestingly, the utilization of MoS2 as a cocatalyst led to remarkable improvement in stability of these oxide photocatalysts. Yuan and co-workers synthesized a MoS2-nanosheet-coated ZnO heterostructure photocatalyst through a simple hydrothermal method for splitting of water into H2 in the presence of Na2S and Na2SO3 as sacrificial reagents.[95] The 1.00 wt % MoS2/ZnO heterostructure photocatalyst had the highest H2 production rate (768 mmol h¢1 g¢1), which is 14.8 times higher than that of pure ZnO. The study found that the MoS2/ZnO with intimate contact between the two components exhibited considerably higher photocatalytic H2 production activities than noble metal (Pt, Rh, Ru, and Au)-loaded ZnO photocatalysts, suggesting that the 2D MoS2 could act as ChemSusChem 2015, 8, 4113 – 4127

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Figure 10. The procedure for gas-phase sulfidation and the idealized structural model of the resultant MoS2/g-C3N4 layered junction photocatalyst for solar H2 generation. Reprinted with permission from Ref. [99]. Copyright 2013, Wiley-VCH.

The 2D–2D MoS2/g-C3N4 hybrid structure with graphene-like thin layered heterojunctions minimizes the lattice mismatch and facilitates planar growth of MoS2 slabs over the g-C3N4 surface owing to their analogous layered structures. The distinct nanoscale structure of MoS2/g-C3N4 has some advantages, including an increase in accessible area around the planar interface of the MoS2 and g-C3N4 layers, reduction in the barriers of electron transport through the cocatalyst, and facile interfacial charge transfer across the interface through electron tunneling. The photocatalytic results showed that the H2 production activity of g-C3N4 under visible light was significantly improved by growing MoS2 thin layers: 0.5 wt % MoS2/g-C3N4 performed better than 0.5 wt % Pt/g-C3N4 under identical reaction conditions (Figure 11 a). Unfortunately, the activity test revealed that the MoS2/g-C3N4 composite photocatalyst became deactivated during prolonged operations (Figure 11 b), which was attributed to the oxidation or photocorrosion of MoS2 by photogenerated holes. Ge et al. also prepared MoS2 and g-C3N4 composite

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Reviews talysts for the solar H2 generation in a heterogeneous system has rarely been considered. Yuan et al. developed a simple molecular organic material [6,13-pentacenequinone (PQ)] for the water-reduction reaction driven by visible light for the first time in the presence of MoS2 as a cocatalyst and triethanolamine (TEOA) as an electron-donating sacrificial reagent.[103] The MoS2 loading amount significantly affected the photocatalytic behavior of the MoS2/PQ composite photocatalysts. The PQ sample with a loading of 0.5 wt % MoS2 afforded the best catalytic performance and resulted in a H2 generation rate of 79.5 mmol h¢1 g¢1 at a pH value of 7.0 with good stability. These results represent a potential and prospective application of MoS2-modified molecular organic materials for solar H2 generation, which will open new pathways for simulating the natural photosynthesis to convert sunlight into H2 energy. 3.4. MoS2/RGO photocatalysts

Figure 11. (a) Rate of H2 production over g-C3N4 loaded with different amounts of MoS2 or Pt, and (b) cycle runs for the photocatalytic H2 production over 0.2 wt % MoS2/g-C3N4. Reprinted with permission from Ref. [99]. Copyright 2013, Wiley-VCH.

photocatalysts using a facile impregnation method.[100, 101] Unfortunately, the 0.5 wt % MoS2/g-C3N4 sample showed low catalytic activity with a maximum H2-evolution rate of 231 mmol h¢1 g¢1. It has been known that graphene can be used as an electron mediator to accelerate the electron transfer from light-harvesting semiconductor to cocatalyst, resulting in an improved photocatalytic activities.[82, 94] Hou synthesized a 2D porous g-C3N4 nanosheet/nitrogen-doped graphene/layered MoS2 ternary nanojunction by using a simple pyrolysis process followed by a hydrothermal treatment.[102] The 2D ternary nanojunction exhibited significantly enhanced photoelectrochemical and photocatalytic activity. As discussed above, MoS2 has been incorporated with different semiconductors such as CdS, CdSe, ZnIn2S4, TiO2, ZnO, Cu2O, and g-C3N4 to form heterostructure that improve the photocatalytic activity for H2 evolution (see Table 1). However, the limitations of these photocatalysts are low photocatalytic activity, poor chemical durability, or high toxicity. Therefore, it is important to develop new light-harvesting materials to obtain visible-light-responsive, non-toxic, stable, and highly reactive MoS2-based photocatalysts. It is well known that the band structure of organic solids can be precisely tailored by simply varying the chemical structure. However, the practical application of simple organic molecular materials as photocaChemSusChem 2015, 8, 4113 – 4127

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MoS2 is a nontoxic, inexpensive, earth-abundant material, which has been widely used as an electrocatalyst for H2 production from water. Although MoS2 strongly absorbs in both ultraviolet and visible light, on its own it is inactive for the photocatalytic H2 evolution and requires assistance from lightharvesting semiconductors. It remains unclear why MoS2 is an efficient electrocatalyst but inactive photocatalyst for water reduction; the possible reasons could be possibly assigned to insufficient charge separation in MoS2 caused by the defects present in MoS2 and the poor electrical conductivity of MoS2.[82] Promoting the charge transfer in MoS2 was proposed to be an efficient strategy to improve its photocatalytic H2-evolution activity. To separate the excited electron of MoS2 for the H2-evolution reaction, Meng et al. synthesized 5–20 nm-sized p-type MoS2 nanoplatelets, which were deposited on n-type Ndoped reduced graphene oxide (n-NRGO) nanosheets to form multiple nanoscale p–n junctions (Figure 12 a and b).[104] Photoelectrochemical measurements showed that the p-MoS2/nNRGO heterostructure can greatly suppress charge recombination and enhance charge generation. As shown in Figure 12 c, the photocatalytic results revealed that the p-MoS2/n-NRGO junction was an efficient photocatalyst for photocatalytic H2 production in the wavelength range from ultraviolet to nearinfrared, giving a H2-evolution rate of 24.8 mmol h¢1 g¢1 in the presence of ethanol as an sacrificial reagent. Notably, the pMoS2/n-NRGO composite photocatalyst exhibited considerably higher activities than both MoS2 and MoS2/RGO composite. In the p-MoS2/n-NRGO, NRGO was transformed from a passive support to an active component of the heterostructure through doping, which greatly enhanced charge generation and suppressed charge recombination (Figure 13). 3.5. Dye-sensitized MoS2-based photocatalysts Use of semiconductors as the light-harvesting materials imposes a limitation on the bandgap of the semiconductors, especially for semiconductors that have a relatively large bandgap. For example, MoS2/TiO2 composite is a promising photocatalyst owing to its low cost, high stability, high efficiency, and

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Reviews Table 1. MoS2-based composite photo-cocatalysts for photocatalytic H2 production.

Light-harvesting material

Cocatalyst

CdS CdS CdS CdS

MoS2 MoS2/RGO MoS2/UiO-66 Cr-MoS2 Ag-MoS2 CdSe MoS2 ZnIn2S4 MoS2 ZnIn2S4 MoS2 MoS2 TiO2 TiO2 MoS2 TiO2 MoS2 TiO2 MoS2 TiO2 MoS2/RGO ZnO MoS2 Cu2O MoS2 C3N4 MoS2 C3N4 MoS2 PQ MoS2 MoS2/NRGO (photocatalyst) Eosin Y MoS2/RGO Eosin Y MoS2/NRGO

Loading method

Light source

Sacrificial reagent

H2 evolution activity quantum [mmol h¢1 g¢1] efficiency [%]

Ref.

impregnation–sulfidation hydrothermal in situ photodeposition hydrothermal

300 W Xe 300 W Xe 300 W Xe 300 W Xe

lamp, l > 420 nm lamp, l > 420 nm lamp, l > 420 nm lamp, l > 420 nm

lactic acid Na2S + Na2SO3 lactic acid Na2S + Na2SO3

[80] [82] [83] [84]

chemical exfoliation impregnation-sulfidation in situ photodeposition in situ photodeposition hydrothermal solvothermal mechanochemistry hydrothermal solvothermal solvothermal impregnation-sulfidation hydrothermal hydrothermal solvothermal hydrothermal solvothermal

300 W Xe lamp, l > 400 nm 300 W Xe lamp, l > 420 nm 300 W Xe lamp, l > 420 nm 300 W Xe lamp, l > 300 nm 300 W Xe lamp 300 W Xe lamp 300 W Xe lamp 350 W Xe lamp 300 W Xe lamp 300 W Xe lamp 300 W Xe lamp, l > 420 nm 300 W Xe lamp, l > 420 nm 300 W Xe lamp, l > 420 nm 300 W Xe lamp 300 W Xe lamp, l > 420 nm 400 W halogen lamp

Na2S + Na2SO3 Na2S + Na2SO3 lactic acid HCOOH Na2S + Na2SO3 CH3OH CH3OH CH3CH2OH Na2S + Na2SO3 CH3OH lactic acid CH3OH TEOA CH3CH2OH TEOA TEOA

5400 9000 32 500 38 000 107 000 890 3060 8047 73 1600 119 753 2066 748 560 1030 231 79 25 4910 42 000

non-toxicity. However, the practical application of MoS2/TiO2 for solar H2 generation is still limited by the wide bandgap of TiO2 (3.2 eV), which results in MoS2/TiO2 responding only to ultraviolet light (about 4 % the solar spectrum). The key to obtaining highly efficient MoS2/TiO2-based photocatalytic systems lies in the design and synthesis of photocatalysts capable of utilizing visible light, which accounts for about 43 % of the solar spectrum. It has been known that photosensitization of TiO2 by dyes is a promising strategy to shift its optical response from the ultraviolet to the visible light region. As shown in Figure 14 a, Yuan et al. developed a visible-light-responsive solar H2-generation system using ZnII-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (ZnTCPP)-sensitized MoS2/TiO2 (ZnTCPP–MoS2/TiO2) composites as photocatalyst.[105] The ZnTCPP–MoS2/TiO2 composite has an apparent adsorption in the visible light region (Figure 14 b), making them active for visible-light-driven H2 evolution from water. On illumination with visible light, the electrons in the highest occupied molecular orbital (HOMO) of the ZnTCPP dye are first excited to the lowest unoccupied molecular orbital (LUMO) to form an excited dye. The excited dye has a lower oxidation potential than the CB of TiO2, providing ample driving force for efficient electron transfer from the excited ZnTCPP molecules to the CB of TiO2 through an oxidative quenching pathway for ZnTCPP*. The electrons in the CB of TiO2 were transferred to the MoS2 cocatalyst, and then the electron-rich MoS2 catalyzed the reduction of water to H2. On the other hand, the ZnTCPP dye has an oxidation potential that is higher than that of TEOA, ensuring that the regeneration of ZnTCPP dye by TEOA is energetically permitted. Even without any noble metal, the ZnTCPP–MoS2/TiO2 photocatalyst loaded with 1.00 wt % MoS2 exhibited the highest H2-evolution rate (10.2 mmol h¢1), and the turnover number with respect to the ZnTCPP dye reached 261 ChemSusChem 2015, 8, 4113 – 4127

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28.1 (420 nm)

9.2 (440 nm)

9.7 (365 nm)

2.1 (420 nm)

24 (460 nm)

[87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [99] [100, 101] [103] [104] [107] [108]

after irradiation with visible light for 12 h (Figure 14 c). Unfortunately, the dye-sensitized photocatalyst exhibited poor photostability during the photocatalytic H2 production reaction. The photocatalytic activity significantly decreased with increasing irradiation time, which can be assigned to the fact that the ZnTCPP dye desorbed from the surface of TiO2. Furthermore, the ZnTCPP–MoS2/TiO2 composite showed considerably higher H2-evolution activity than ZnTCPP–Pt/TiO2 under the same conditions. Similar results were observed in a heterogeneous photocatalytic H2 evolution system using a ZnTCPP-sensitized MoS2/ZnO (ZnTCPP–MoS2/ZnO) composite as the photocatalyst.[106] Min and Lu reported a highly active cocatalyst with limited-layered MoS2 grown on RGO sheets for H2 evolution in dye-sensitized photocatalytic systems using Eosin Y as a dye.[107] The Eosin Y–MoS2/RGO photocatalyst exhibited a considerably higher activity than Eosin Y–MoS2 under the same reaction conditions. The highest H2-evolution rate (4190 mmol h¢1 g¢1) was achieved in the Eosin Y-sensitized MoS2/RGO system under visible light irradiation (l Š 420 nm), and an apparent quantum efficiency of 24 % was obtained at 460 nm. It has been known that the poor electrical conductivity of MoS2 could be a key factor in determining its low activity for photocatalytic for H2 production.[82] It would be more rewarding if the MoS2 layer itself could be made more conducting. With this in mind, Matitra et al. prepared MoS2 by Li intercalation followed by exfoliation, which yielded single layers of the metallic 1T-MoS2.[108] The as-prepared 1T-MoS2 showed extraordinary results in photocatalytic H2 evolution with a high H2-evolution rate of 30 mmol h¢1 g¢1 and a high turnover frequency (TOF) of 6.25 h¢1 in the presence of Eosin Y as a photosensitizer and TEOA as an electron donor, which is 600 times higher than that of few-layer 2H-MoS2. Furthermore, they also found that Eosin Y sensitized few-layer 2H-MoS2 with heavily

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Figure 12. SEM (a) and TEM (b) images obtained from p-MoS2/n-NRGO heterostructure. (c) Photocatalytic H2 production using MoS2, MoS2/n-NRGO, and p-MoS2/n-NRGO photocatalysts. Reprinted with permission form Ref. [104]. Copyright 2013, American Chemical Society.

Figure 14. (a) Visible-light-induced H2 production over ZnTCPP-MoS2/TiO2. (b) UV-vis diffuse reflectance spectra of pure TiO2, MoS2/TiO2 and ZnTCPPMoS2/TiO2 composites. (c) Photocatalytic H2 evolution over ZnTCPP-MoS2/ TiO2 photocatalysts loaded with different amounts of MoS2 under visible light irradiation (l > 420 nm). Reprinted with permission from Ref. [105]. Copyright 2015, Elsevier.

nitrogenated RGO (Eosin Y/2H-MoS2-NRGO) showed a considerably higher H2-evolution rate (42 mmol h¢1 g¢1) under 400 W halogen irradiation in the presence of TEOA as an electron donor, which can be assigned to the strong electronic coupling between NRGO and MoS2. 3.6. Colloidal MoS2 catalyst for solar H2 generation

Figure 13. Schematic illustration of solar H2 generation using a nanoscale p– n junction of p-MoS2/n-NRGO as photocatalyst. Reprinted with permission form Ref. [104]. Copyright 2013, American Chemical Society.

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MoS2 nanosheets are active as cocatalyst for solar H2 generation in heterogeneous photocatalytic systems when combined with an inorganic light-harvesting semiconductor (such as CdSe, CdS or TiO2), and the performance of MoS2 exceeded some commonly used cocatalysts such as Ru, Pt, Pd, or Au.[71, 87, 91] However, utilization of MoS2 as a H2 evolution reaction catalyst for molecular photocatalytic H2 production sys-

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Reviews tems was rarely reported. In 2009, Zong et al. prepared “watersoluble” colloidal MoS2 nanoparticles using a simple solvothermal method for photocatalytic H2 evolution in a three-component molecular system with [Ru(bpy)3]2 + (bpy = 2,2’-dipyridine) as light-harvesting photosensitizer and ascorbic acid (H2A) as sacrificial reagent for the first time.[109] This work demonstrated the possibility of using colloidal MoS2 as a H2-evolution catalyst in a molecular system. As shown in Figure 15 a, the MoS2 col-

which are superior photosensitizers for photocatalytic water reduction.[110] Moreover, the existence of metal¢C bonds in IrIII complexes improved their photostability during the H2 evolution.[111] Many studies showed that the derivatives of [Ir(ppy)2(bpy)] + (ppy = 2-phenylpyridine) were the most successful photosensitizers, which has been most extensively studied in homogeneous photocatalytic H2 production systems.[112, 113] Yuan et al. reported the utilization of water-soluble MoS2 catalysts in molecular photocatalytic water reduction systems in the presence of a cyclometalated IrIII complex as a photosensitizer and H2A or TEOA as a donor in methanol/water solution (Figure 16).[114] The MoS2 catalyst has a considerably higher cat-

Figure 16. Schematic representation of IrIII–MoS2–H2A system. Adapted with permission from Ref. [114]. Copyright 2014, Nature Publishing Group.

Figure 15. (a) TEM image of colloidal MoS2 prepared using a simple solvothermal method from a (NH4)2MoS4 methanol solution at 100 8C. (b) Visiblelight-driven H2 evolution from acetonitrile/methanol solutions (2:1) containing colloidal MoS2 (12.5 mmol) and a) 20, b) 15, c) 10, and d) 5 mmol of [Ru(bpy)3]2 + and H2 evolution from a solution containing 10 mmol [Ru(bpy)3]2 + and MoS2/Al2O3 prepared from e) (NH4)2MoS2/Al2O3 and f) MoO3/Al2O3 precursors. Reprinted with permission from Ref. [109]. Copyright 2009, Royal Society of Chemistry.

loids prepared using (NH4)2MoS4 as precursor consisted of nanoparticles with diameters of less than 10 nm. Under visible light irradiation (300 W Xe lamp, l > 420 nm), the photocatalytic results showed that the colloidal MoS2 nanoparticles can act as a more efficient H2-evolution catalyst than the traditional MoS2/Al2O3 catalyst (Figure 15 b). Unfortunately, these systems display a photocatalytic lifetime of less than 6 h and a low efficiency with a turnover number of 75 with respect to MoS2, which should be ascribed to the photodecomposition of the [Ru(bpy)3]2 + . In contrast to [Ru(bpy)3]2 + , IrIII complexes with a greater field strength resulting from cyclometalated ligands, ChemSusChem 2015, 8, 4113 – 4127

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alytic activity than commonly used water reduction catalysts such as [Rh(dt-bpy)3](PF6)3 (dt-bpy = 4,4’-di-tert-butyl-2,2’-dipyridine), K2PtCl4, [Co(bpy)3]Cl2 and [Co(dmgH)2](H2O)2 (dmgH2 = dimethylglyoxime) under the same test conditions. Under optimal conditions, the total turnover number for H2 evolution was up to 3142 based on MoS2 after 12 h of visible light irradiation in the IrIII–MoS2–H2A system. The highest apparent quantum yield (up to 12.4 %) for H2 evolution was obtained for the IrIII– MoS2–TEOA system using a monochromatized light of l = 400 nm. More importantly, the introduction of carboxylate anchoring groups in the IrIII complexes allows the photosensitizers to be adsorbed onto the MoS2 nanoparticles surface, increasing the electron transfer rate, which results in enhanced H2 evolution. This study advocates further exploration of stable catalyst through the introduction of water-soluble inorganic compounds in molecular photocatalytic H2-evolution systems.

4. Summary and Outlook In this Review, some recent significant advances on the design and efficient utilization of MoS2-based photocatalysts for photocatalytic H2 production have been presented. So far, the most promising MoS2-based composite photocatalyst reached the highest apparent quantum efficiency of 28.1 % at 420 nm when it was coupled with CdS as the light-harvesting semiconductor and RGO as the as the electron-transfer medium, representing one of the most highly active MoS2-based photocatalysts in the absence of noble-metal cocatalysts. Although encouraging progress has been achieved during the past years

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Reviews toward MoS2-based composite photocatalysts for solar-to-H2 conversion, there are still many problems in those systems. One of the crucial problems is that MoS2 alone has negligible photocatalytic activity for H2 evolution despite the fact that nanosized MoS2 has a suitable redox potential for water splitting, and a better understanding on the underlying reasons for the poor photocatalytic activity of MoS2 alone is challenging but crucial for designing novel and highly reactive MoS2 photocatalysts. The other problem is that most semiconductor photocatalysts catalyze H2 production only in aqueous solutions in the presence of additional sacrificial reagents such as methanol, the Na2S–Na2S2O3 pair, triethanolamine, lactic acid, or ethanol as electron donor. The use of sacrificial reagents for the photocatalytic reduction of water to H2 will impede these H2-evolution systems for future practical applications on a large scale. It is highly required to develop overall watersplitting systems that use MoS2-modified semiconductors as photocatalysts. In the future, with the smart design and synthesis of more earth-abundant MoS2-based photocatalysts for photocatalytic water splitting without the use of additional sacrificial reagents can be expected. For example, the construction of a suitable Z-Scheme photocatalytic water-splitting system in which MoS2 is used as the cocatalyst for H2-evolution reaction could be a feasible approach.

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