MoS2 as a co-catalyst for photocatalytic hydrogen production from water

Abstract

Solar-to-hydrogen conversion based on photocatalytic water splitting is a promising pathway for sustainable hydrogen production. The photocatalytic process requires highly active, inexpensive, and earth-abundant materials as photocatalysts. As a presentative layer-structured transition metal dichalcogenides, molybdenum disulfide (MoS₂) is attracting intensive attention due to its unique electro and photo properties. In this article, we comprehensively review the recent research efforts of exploring MoS₂ as a co-catalyst for photocatalytic hydrogen production from water, with emphasis on its combination with CdS, CdSe, graphene, carbon nitride, TiO₂, and others. It is shown that MoS₂–semiconductor composites are promising photocatalysts for hydrogen evolution from water under visible light irradiation.

Introduction

Global demand for energy is predicted to be more than double by 2050 [1]. If only fossil fuels are exploited to meet this requirement, the content of carbon dioxide in the atmosphere will be more than double, thereby enhancing global warming. Therefore, it is a great challenge to meet the growing energy requirement without contribution to environmental issues. The sun is the only source that is capable of providing enough energy [2]. Sunlight energy, however, is intermittent and requires an efficient storage [1, 3-5]. Sunlight-driven water splitting to produce hydrogen is widely considered as one of the most attractive methods for solar energy storage [6].

The overall water splitting reaction can be expressed as follows:

$H_{2}O\rightarrow H_{2}+{\frac {1}{2}}O_{2}\quad \Delta G_{298K}^{0}=$$237.1\quad {\mbox{kJ}}\quad {\mbox{mol}}^{-1}$

(1)

The input energy of this reaction corresponds to a thermodynamic voltage requirement of 1.23 V [7, 8]. Without bias, the conduction band (CB) minimum of semiconductor photocatalyst should be more negative than the H⁺/H₂ redox potential and the valence band (VB) maximum should be more positive than the O₂/H₂O redox potential. When a photon with energy of hv matches or exceeds the band gap energy (E_g) of the semiconductor, an electron in the VB can be excited into the CB, leaving a positive hole in the VB. With the generated electrons, H₂ evolution reaction takes place at the cathode, while the generated holes participate in the oxygen evolution reaction at the anode, which are shown below as they occur in acidic electrolyte:

• Hydrogen evolution reaction (HER): 2H⁺ + 2e⁻ → H₂
• Oxygen evolution reaction (OER): ${\textstyle H_{2}O\rightarrow {\frac {1}{2}}O_{2}+2H^{+}+2e^{-}}$

Fujishima and Honda reported the first photoelectrochemical (PEC) cell for water splitting, in which a rutile TiO₂ was used as a photoelectrode and Pt as a counter electrode [9]. Under UV light irradiation on TiO₂, O₂ was formed at TiO₂ surface, while water was reduced to produce H₂ at the Pt counter electrode. When heterojunctions of expensive group III–V materials (such as a GaInP₂ and n-p-GaAs junctions in series) were employed as a photoelectrode with a Pt counter electrode, a high energy conversion efficiency of 12.4% was achieved [10]. In this tandem configuration, each semiconductor only needs to supply part of the photovoltage required to electrolyze water and some with smaller band gaps can be used to absorb the visible and near-infrared lights of solar. A low-cost Z-scheme tandem device, which is based on two photosystems connected in series, was also developed [11, 12]. A thin film of nanocrystalline WO₃ [12] or Fe₂O₃ [13] serves as the short-wavelength-irradiation absorber where water was oxidized into O₂, while dye-sensitized nanocrystalline TiO₂ cells absorb long-wavelength-irradiation to generate H₂. Recently, it was demonstrated that the activity of silicon photocathodes could be enhanced by depositing hydrogen evolution catalysts on the protected Si surface [13].

Compared to the PEC configuration, a photocatalytic process generally exhibits a low efficiency for overall water splitting due to the back reaction. Pt/TiO₂ is one of well-investigated catalysts. However, it works only under UV irradiation and its efficiency for overall water splitting is extremely low. In the 1980s, CdS [14] and WO₃ [15] were among few visible light photocatalysts for H₂ and O₂ evolution. With continuous and intensive efforts, many photocatalyst systems have been developed. For example, an efficient water splitting was achieved using a powdered photocatalyst of NiO/NaTaO₃:La under UV irradiation [16]. Furthermore, powdered photocatalyst systems, such as Cr_xRh_2-xO₃/GaN:ZnO [17] and Z-scheme systems, such as Ru/SrTiO₃:Rh-BiVO₄ [18], were developed for visible light water splitting. Besides metal oxide and metal nitride, sulfide solid solution photocatalysts (AgInS₂–CuInS₂–ZnS) were also demonstrated to be active for H₂ evolution under visible light irradiation [19]. Very recently, we reported a highly efficient temperature-induced visible light photocatalytic hydrogen production from water, which exhibited a high photohydrogen yield with a large apparent quantum efficiency (QE) for the entire visible light range at 280°C [20, 21].

Platinum is mostly used with semiconductors as photocatalysts for HER from water due to their negligible overpotential and excellent kinetics. However, because the widespread use of platinum would be limited by its scarcity and high cost [22], it is necessary to develop nonnoble metal alternatives. For example, nickel-based catalysts showed high activity but instability in acidic conditions [23]. Molybdenum is a transition metal that is relatively abundant and therefore much more economical than platinum. Furthermore, MoS₂-based materials offer significant advantages for photohydrogen production over noble metal catalysts. Several excellent review articles have been published for the structures, properties, synthesis, and catalytic performance of MoS₂ [24-33]. In this article, we attempt to provide a comprehensive review on MoS₂ as a co-catalyst for photocatalytic hydrogen production from water.

Structures and Preparation of MoS₂

Crystalline MoS₂ occurs in nature as mineral molybdenite. In 1923, Dickson and Pauling took a Laue photograph of mineral molybdenite with the incident beam normal to the basal plane and found it possessed a hexagonal axis and six symmetry planes [34]. In the 1960s, Frindt et al. [35] showed that the micromechanical peeling technique can be employed to obtain thin sheets of MoS₂ with thicknesses of 1.2–1.5 nm, corresponding to two molecular layers. In 1980s, efforts were made to obtain single layers of MoS₂ using lithium intercalation followed by exfoliation in water [36]. The absence of the (002), (103), and (105) peaks in the X-ray diffraction pattern is characteristic of one-molecule-thick sheets. Furthermore, Koma and Yoshimura [37] obtained a single-layer MoS₂ and MoSe₂ by van der Waals epitaxy. Frindt et al. carried out a detailed structural investigation of single-layer MoS₂ prepared by exfoliation of lithium-intercalated MoS₂ powder [38]. So far, three crystal structures were revealed for MoS₂. The unique layered structure endows MoS₂ with many promising properties, such as anisotropy, chemical stability, and antiphotocorrosion.

Crystal structures of MoS₂

As shown in Figure 1, the single layer of MoS₂ possesses the Lamellar S–Mo–S structure with a thickness of ~0.7 nm [39]. Each 2D crystal layer of MoS₂ consists of a hexagonal plane of Mo atoms and two S atoms of hexagonal planes modulated by covalent interactions in the form of a trigonal prismatic form. Adjacent atomic sandwich units are joint by weak van der Waals forces. The Mo-S length, the crystal lattice constant, and the distance (between the upper and lower sulfur atoms) are 2.4, 3.2, and 3.1 Å, respectively [40, 41]. MoS₂ has two main types of phases: 2H and 3R phases. The former contains two layers per unit cell stack in the hexagonal symmetry with trigonal prismatic coordination, and the latter can be described by three layers per unit cell stack in the rhombohedral symmetry with trigonal prismatic coordination [42]. The 2H phase is dominant in nature and more stable than the 3R phase. Furthermore, Frindt and co-workers exploited X-ray diffraction for the detailed structural evaluation of single-layer MoS₂ prepared by exfoliation of lithium-intercalated MoS₂ powder and discovered the 1T metallic phase, in which Mo atoms are coordinated in an octahedral manner (Fig. 1) [38]. The metallic phase possesses unique electrical properties [43-46]. As a metastable structure, the 1T phase can transform into the 2H phase by heating or aging.

Figure 1. Crystal structure of MoS2: Octahedral (1T), Trigonal prismatic (2H) and Trigonal prismatic (3R) unit cell structures. Reproduced with permission [47].

Electrical structures of MoS₂

Photocatalysts must possess high stability and suitable energy band structures. MoS₂ has an excellent stability against the photocorrosion in solution due to its antibonding state (formed from an interaction between molybdenum ${\textstyle d_{z}^{2}}$, and sulfur p_z orbital at the top of the valence band) [48]. The conduction band position of bulk MoS₂ is slightly more positive than that for the HER, indicating that bulk MoS₂ cannot evolve hydrogen under light illumination without a negative bias or quantum confinement. However, the band gap of MoS₂ varies with the number of its layers [49]. As the thickness of MoS₂ decreased to the size of a monolayer, a transformation from the indirect band gap (1.3 eV) to the direct band gap (1.9 eV) was observed [50-52]. Such an interesting transformation was explained as follows: The VB top edge and the CB bottom edge of MoS₂ are located at the Γ point almost halfway along the Γ-Κ direction, respectively, which constitutes the indirect band gap transition (Fig. 2) [53, 54]. With decreasing layer number, the bottom edge of the CB moves upward, increasing the overall band gap. Since the CB states at the Κ point are mainly associated with the Mo d-orbitals and relatively unaffected by interlayer interactions, the direct band gap at the Κ point only increases about 0.05–0.1 eV [55]. The states near the Γ point on the CB that are due to the hybridization between S p_z-orbitals and Mo d-orbitals, are strongly dependent on the interlayer interaction. Thus, the bands at Γ are remarkably affected by decreasing layer number [49, 55]. As a result, for the monolayer, the indirect transition gap is larger than the direct transition gap, and the direct band gap (about 1.9 eV) at Κ point becomes the smallest gap.

Figure 2. Band structure of MoS2 (A) showing the direct and indirect band gap, as well as the A and B excitons. (B) Transition of the band structure of MoS2 from indirect to direct band gap (a → d). Reproduced with permission [51, 56].

Quantum confinement, which usually affects the band gap in semiconductors was also employed to tune the band gap of MoS₂ and thus its photocatalytic activity. For example, in the visible light-driven photo-oxidation of the organic pollutant molecules phenol and pentachlorobenzene, MoS₂ with size of 8–10 nm exhibited negligible activity, whereas MoS₂ with size of 4.5 nm had reasonable activity [57]. The increased activity was attributed to the quantum confinement giving a lager oxidation potential as a result of the valence band lying at a more positive value.

Synthesis of MoS₂

The synthesis and exfoliation of MoS₂ materials were reviewed elsewhere [58]. Two synthesis strategies were employed: (1) the top-down approach, such as mechanical exfoliation [59, 60], and chemical exfoliation [61-63]; and (2) the bottom-up approach, including chemical vapor deposition on substrates [63, 64] and chemical synthesis [65-67]. MoS₂ has been prepared as a co-catalyst with various methods:

• Impregnation method is widely used for the dispersion of nanoparticulate MoS₂ (2H) on photosensitizer (e.g., CdS, TiO₂) for both powdered photocatalysis and PEC processes. For example, MoS₂ on CdS can be prepared by impregnating CdS with an aqueous solution of (NH₄)₂MoS₄, followed by treatment in H₂S flow at high temperature [68].
• Hydrothermal method could use low toxic thiourea as a sulfur source to react with molybdenum salt (such as Na₂MoO₄), together with semiconductor powders [69].
• Ball-milling method was employed to mix (NH₄)₂MoS₄ and semiconductor powder in the presence of ethanol, followed by high temperature calcination in inert atmosphere [70].
• Photodeposition approach could be used to decorate MoS₂ nanocrystals on TiO₂ under UV irradiation with (NH₄)₂MoS₄ as precursor in ethanol/water solution [71].
• Anion exchange reaction was applied to synthesize MoS₂ nanotube photoelectrode from Mo₃O₁₀(C₂H₁₀N₂) nanowires and l-cysteine by heating at 200°C for 14 h [72].
• Chemical exfoliation process was usually exploited to prepare 1T metallic MoS₂ catalysts by ion intercalation [73].

More detail for the synthesis of MoS₂ co-catalysts will be discussed in the following sections.

MoS₂-based Composites as Photocatalysts for the Splitting of Water to H₂

A standard photocatalyst for hydrogen evolution consists of platinum and a semiconductor, but platinum is very expensive with a limited source [22]. Therefore, the exploration of nonnoble metal alternatives for platinum is an important topic. Nanostructured nickel phosphide [74], cobalt phosphide [75], and molybdenum sulfides [76] are among the best reported systems with demonstrated high activity for HER, and some of them showed excellent stability under controlled conditions. Furthermore, Mo-based complexes offer significant advantages over noble metal catalysts as co-photocatalysts for H₂ production (Table 1).

Table 1. Summary of photocatalytic activity of MoS₂-based materials

Catalyst	Light source	Sacrificial reagent	H2 yield (mmol g−1 h−1)	Reaction time	Reference
MoS2/CdS
0.1 g MoS2/CdS	300 W Xe lamp λ > 420 nm	10% lactic acid in water	5.30		[68]
0.1 g MoS2/CdS	300 W Xe lamp λ > 420 nm	10% lactic acid in water	13.15	5 h	[70]
0.05 g MoS2/CdS	300 W Xe lamp λ > 400 nm	Na2S/Na2SO3 aqueous solution	3.84	5 h	[69]
0.02 g CdS/graphene/MoS2	UV lamp 280–320 nm, 500 W	10% lactic acid in water	6.86		[77]
0.2 g MoS2–CdS/γ-TaON hollow composites	300 W Xe lamp λ > 420 nm	0.35 mol L−1 Na2S/0.25 mol L−1 Na2SO3 aqueous solution	3.14	5 h	[78]
0.02 g 2D MoS2/CdS p-n nanohybrids	300 W Xe lamp λ > 420 nm	10% lactic acid in water	6.85	4 h	[79]
0.05 g rGO/CdS/MoS2	350 W Xe lamp λ > 420 nm (34 mW cm−1)	10% lactic acid in water	1.98	5 h	[80]
0.1 g MoS2/CdS	300 W Xe lamp λ > 400 nm	10% lactic acid in water	4.06	5 h	[81]
MoS2/Graphene
0.02 g Eosin dye Y sensitized MoS2/RGO	300 W Xe lamp λ > 420 nm	15 vol. % TEOA aqueous solution	4.19	6 h	[82]
0.08 g TiO2 NPs/MoS2/graphene	300 W Xe lamp	25 vol. % ethanol in water	2.07	3 h	[48]
0.01 g p-MoS2/n-N-doped RGO	AM 1.5G solar simulator (40−50 mW cm−2)	50 vol. % ethanol aqueous solution	0.025	20 h	[83]
0.1 g MoS2/graphene/CdS nanorods	300 W Xe lamp λ > 400 nm	20 vol. % lactic acid aqueous solution	2.32		[84]
0.2 g MoS2/Graphene/CdS	300 W Xe lamp λ > 420 nm	20 vol. % lactic acid aqueous solution	9	5 h	[85]
0.2 g MoS2/graphene	Xe lamp (λ > 390 nm) (1.3 W m−2)	15% TEOA aqueous solution with Eosin Y dye	1.21		[86]
0.01 g MoSxCy (C3)	300 W Xe lamp λ > 420 nm	Triethylamine, water, [Ru(bpy)3](PF6)2 in acetonitrile	12	16 h	[87]
0.01 g MoSxCy (C3)	300 W Xe lamp λ > 420 nm	Triethylamine, water, [Ir(pyb)2(bpy)] (PF6) in acetonitrile	19	16 h	[87]
0.01 g CdSe/MoS2	300 W Xe lamp λ > 400 nm (450 mW cm−2)	0.1 mol L−1 Na2S/0.1 mol L−1 Na2SO3 aqueous solution	0.9	300 min	[88]
MoS2/TiO2
1 g MoS2/SiO2/TiO2	Medium pressure Hg lamp	KOH, water methanol	0.86		[89]
0.03 g MoS2/TiO2	300 W Xe lamp	5% formic acid aqueous solution	0.03	5 h	[71]
0.0016 g MoS2/TiO2	300 W Xe lamp (600 mW cm−2)	0.35 mol L−1 Na2S/0.25 mol L−1 Na2SO3 aqueous solution	1.6		[90]
0.05 g MoS2/TiO2	300 W Xe lamp AM 1.5G filter	20 vol. % methanol aqueous solution	0.12	4 h	[91]
0.2 g MoS2/TiO2	300 W Xe lamp (250–380 nm)	15% methanol aqueous solution	0.75	6 h	[92]
MoS2/TiO2	300 W Xe lamp λ > 420 nm	0.35 mol L−1 Na2S/0.25 mol L−1 Na2SO3 aqueous solution	0.49	5 h	[93]
MoS2/Carbon nitride
0.1 g MoS2/g-C3N4	300 W Xe lamp λ > 400 nm	25% methanol aqueous solution	0.231	6 h	[94]
0.02 g MoS2/mesoporous g-C3N4	300 W Xe lamp λ > 420 nm	10 vol. % lactic acid aqueous solution	1.4	5 h	[95]
MoS2/Zn-based material
0.05 g MoS2/ZnIn2S4	300 W Xe lamp λ > 420 nm	0.5 mol L−1 Na2SO3/0.43 mol L−1 Na2S aqueous solution	3.06	5 h	[96]
0.1 g MoS2/ZnIn2S4	300 W Xe lamp λ > 420 nm	10 vol. % lactic acid aqueous solution	8.047	5 h	[97]
MoS2/Graphene/ZnS	300 W Xe lamp (125 mW cm−2)	0.005 mol L−1 Na2S/0.005 mol L−1 Na2SO3 aqueous solution	2.26		[98]
MoS2/transition metals or other materials
Amorphous MoS3 on a CdSe seeded CdS nanorod	450 nm single wavelength (40 mW cm−2)	TEOA aqueous solution	100	50 min	[99]
0.02 g nioblate/graphene/MoS2	500 W mercury vapor lamp	10% methanol aqueous solution	2.12	2 h	[100]
0.01 g Cr/MoS2/CdS	300 W Xe lamp λ > 420 nm	0.25 mol L−1 Na2S/0.35 mol L−1 Na2SO3 aqueous solution	38	4 h	[101]
0.01 g Ag/MoS2/CdS	300 W Xe lamp λ > 420 nm	0.25 mol L−1 Na2S/0.35 mol L−1 Na2SO3 aqueous solution	107	4 h	[101]
0.2 g MoS2/SrZrO3	100 W mercury lamp	0.35 mol L−1 Na2S/0.25 mol L−1 Na2SO3 aqueous solution	26.55		[102]
MoS2/Sensitizer
2 g Colloidal MoS2	300 W Xe lamp λ > 420 nm	Ascorbic acid, [Ru(bpy)3](PF6)2 in acetonitrile/methanol	0.21	6 h	[103]
0.001 g MoS2 nanosheet/TiO2 nanowire with Eosin Y dye	300 W Xe lamp λ > 420 nm	5 vol. % TEOA aqueous solution	16.7	12 h	[104]

CdS/MoS₂

Cadmium sulfide (CdS), which is an n-type semiconductor with a band gap of 2.4 eV, was reported as a visible light active photocatalyst for H₂ production 30 years ago [14]. However, CdS suffers photocorrosion during the photoreaction where CdS can be oxidized by photo-generated holes. To stabilize the CdS nanoparticles, sacrificial electron donors have been employed. Since CdS alone showed very poor catalytic activities for HER, co-catalysts (such as noble metals) are necessary. However, noble metals are expensive with limited source. The earth abundant p-type MoS₂ is promising as a co-catalyst because of its suitable band structure, high thermal stability, and electrostatic integrity [39, 105]. Furthermore, MoS₂ and CdS share the same hexagonal crystalline structure, ensuring that an intimate heterojunction can be formed. Li et al. reported a number of MoS₂/CdS systems, in which MoS₂ nanoparticles were deposited onto CdS as both a colloidal system and a photocathode [68, 103, 106]. In the colloidal system, electron donors (such as lactic acid, glycerol, ethanol, and methanol) were introduced to inhibit the photocorrosion of CdS. As shown in Figure 3, one can see a higher rate of hydrogen evolution on MoS₂/CdS than on metal/CdS, indicating a favorable interaction between MoS₂ and CdS [106].

Figure 3. The rate of H2 evolution on CdS loaded with 0.2 wt. % of different co-catalysts. (Xe lamp, 0.1 g catalyst, 10 vol % lactic solution). Reproduced with permission [68].

To improve photocatalytic activity, various preparation methods were exploited to engineer nanostructures of MoS₂–CdS composites. Chen et al. [70] developed a simple ball-milling method with calcination to prepare MoS₂/CdS photocatalysts for visible light-driven H₂ evolution. A suitable extent of ball-milling provides a uniform dispersion of MoS₂ (prepared from (NH₄)₂MoS₄) on CdS nanoparticles. This intimate contact of heterojunction can facilitate the electron transfer between MoS₂ and CdS, resulting in the remarkable increase in H₂ evolution rate (Fig. 4A). However, further increasing ball-milling time could cause a gradual decrease in the H₂ evolution rate due to the creation of too many defects, which act as electron-hole recombination centers. The calcination treatment is also important, because it can complete the decomposition of (NH₄)₂MoS₄ to form an effective MoS₂ co-catalyst for CdS. Furthermore, increase in calcination temperature can eliminate the defects to improve the crystallinity of photocatalysts, leading to higher photocatalytic activity. However, the calcination at too high temperatures could cause a decrease in surface area and a change in morphology, leading to a remarkable decrease in photocatalytic activity (Fig. 4B). Recently, a hot-injection method was developed to prepare MS₂/CdS (M=W or Mo) nanohybrids, in which single-layer MS₂ nanosheets with lateral size of 4–10 nm grew on the Cd-rich (0001) surface of wurtzite CdS nanocrystals. A large number of edge sites in the MS₂/CdS nanohybrids are active sites for HER. The photocatalytic performances over MoS₂/CdS nanohybrids reached 1472 μmol h⁻¹ g⁻¹ hydrogen yield for HER under visible light irradiation (>420 nm), which is about 12 times that of pure CdS. Furthermore, the MS₂/CdS nanohybrids showed an enhanced stability, namely, after a reaction of 16 h, 70% of catalytic activity still remained.

Figure 4. (A) The H2 evolution rate of 0.9 mol% MoS2/CdS (pretreated) photocatalysts (prepared with different ball-milling time followed by calcining at 673 K for 2 h) under visible light (λ > 420 nm) with lactic acid as electron donor. (B) The H2 evolution rate based on 0.9 mol% MoSx/CdS and 0.9 mol% MoSx/CdS (pretreated) photocatalysts (prepared by calcining the (NH4)2MoS4/CdS and (NH4)2MoS4/CdS (pretreated) precursors at different temperatures for 2 h) under visible light (λ > 420 nm) with lactic acid as a sacrificial reagent. Reproduced with permission [70].

A green hydrothermal method was explored for the synthesis of MoS₂/CdS photocatalysts for H₂ production under visible light irradiation [69]. This approach, which uses low toxic thiourea as a sulfur source to react with Na₂MoO₄ at 200°C, is better than traditional methods, which are based on an annealing process at relatively high temperature (above 400°C) with toxic H₂S as a reducing agent. The H₂ evolution rate of the MoS₂/CdS is 17 times larger than that of CdS alone. The morphologies of obtained MoS₂/CdS samples were feather shaped (Fig. 5A). Furthermore, transmission electron microscopy (TEM) images showed the layer of flocculent material coated on the surface of CdS catalyst (Fig. 5B), indicating that MoS₂ was uniformly dispersed on CdS [81].

Figure 5. (A) SEM image of feather-shaped MoS2/CdS samples. Reproduced with permission [65]. (B) TEM images of MoS2/CdS catalyst synthesized by loading 2.0 wt. % MoS2 on CdS. Reproduced with permission [81].

The coupling of p-type MoS₂ and n-type CdS to form p-n junctions at heterostructure interfaces was found helpful for enhancing photo-generated electron-hole separation. Mott–Schottky (M-S) plot of CdS film showed a positive slope in the linear region, indicating an n-type characteristic. In contrast, MoS₂ film exhibited p-type behavior, reflected by a negative slope in its M-S plot [107]. The p-n junctions of MoS₂/CdS could accelerate the effective separation of photo-generated carriers by the internal electrostatic field at the junction interface. The excited electrons on the conduction band of the MoS₂ transfer to that of CdS and further to the counter electrode to produce hydrogen (Fig. 6). Simultaneously holes are transferred to VB of MoS₂ and accumulate there, which inhibit photocorrosion of CdS and thus enhance its photostability. Furthermore, the effect of MoS₂/CdS p-n heterojunctions on PEC was evaluated. MoS₂/CdS p-n heterojunction films, which were prepared by electrodeposition followed by chemical bath deposition, showed much higher visible light PEC activity and higher stability for water splitting than pure CdS film [107]. The highest photocurrent of the MoS₂/CdS film was seven times higher than that of the pure CdS film. The improved PEC performance of the MoS₂/CdS heterojunction film was attributed to the enhanced visible light absorption by MoS₂ and the formation of a p-n junction between CdS and MoS₂. The IPCE measurement revealed that MoS₂/CdS heterojunction not only contributed to the improvement of the electron-injection efficiency in the CdS absorption region (λ < 510 nm), but also broadened the utilization range of the solar spectrum to red light region (λ > 510 nm). In contrast, MoS₂ or CdS alone exhibited a very low photocatalytic activity. A comparison of PL spectra between CdS and MoS₂/CdS indicates effective charge transfer between n-type CdS and p-type MoS₂ under illumination, which results from the lower recombination probability of photo-generated electrons and holes. Furthermore, the holes transferred from CdS to MoS₂ prevented the photocorrosion of CdS, leading to better stability. 2D MoS₂ nanosheets were also utilized to fabricate 2D MoS₂/CdS with a sandwich-like p-n heterojunction through a one-pot solvothermal process [79]. Transient photocurrent tests revealed that the photocurrent is enhanced ~4.5-fold compared with that of pure CdS nanoparticles.

Figure 6. Formation of a p-n junction when MoS2 contacts CdS. Reproduced with permission [107].

Recently, other semiconductor materials were explored to combine with MoS₂–CdS composites for the improvement of photocatalytic activity [78]. Three-dimensional MoS₂–CdS–γ-TaON hollow composites were successfully synthesized by anchoring MoS₂/CdS nanocrystals on the surfaces of γ-TaON hollow spheres with a hydrothermal approach [78]. The photo-excited electrons in the CB of CdS can migrate and be injected into TaON, followed by transfer to the MoS₂ nanosheets to generate H₂, which inhibits the charge carrier recombination. Even without a noble-metal co-catalyst, the hollow-structured MoS₂-CdS-γ-TaON with 1 wt. % MoS₂/CdS co-catalyst (0.2 wt. % MoS₂) decorated on its surface produced a high photocatalytic H₂ production rate of 628.5 μmol h⁻¹, which is about 70 times higher than that of Pt on pristine TaON. The PEC performance was examined for the system with TiO₂/CdS/ZnS as a photoelectrode and TiO₂/CdS/MoS₂ heterointerface as a counter electrode (Fig. 7) [108]. This tandem electrode configuration showed higher hydrogen fuel generation of 1.47 mL h⁻¹ cm⁻² at 1 V versus RHE applied potential under visible light, which is 1.5 times higher than that using Pt as a counter electrode. Furthermore, 1T MoS₂ sheet passivation at the TiO₂/CdS interface could reduce charge recombination with the electrolyte through uncovered CdS sites on TiO₂.

Figure 7. Schematic illustration of the double-sided photoelectrode with TiO2/CdS/ZnS (anode) and TiO2/CdS/MoS2 electrodes (cathode). Reproduced with permission [108].

In summary, a promising efficiency of photocatalytic H₂ production from water splitting under visible light irradiation has been achieved over MoS₂–CdS composites. This can be attributed to the matched energy band alignment, p-n heterojunction, and uniform dispersion of co-catalyst, which promotes the charge transfer and suppresses the photoelectron/hole recombination.

CdSe/MoS₂

Bulk CdSe semiconductor does not have photocatalytic activity for HER, even with sacrificial electron donors, because its conduction band is below the water reduction potential. However, reducing thickness of CdSe nanomaterial can cause strong quantum confinement, leading to a larger band gap and higher conduction band potential. Nano-CdSe possesses a raised CB flat band potential (from −0.10 V to −0.55 V vs. NHE) that results from the increased band gap (from 1.7 to 2.7 eV) [88]. Under visible light irradiation, this quantum confined CdSe-nanoribbons exhibited an attractive H₂ production rate from aqueous sodium sulfite/sulfide solution with a quantum efficiency of 9.2% at 440 nm, whereas bulk CdSe is not active for the reaction. Furthermore, the chemical linking of the CdSe nanoribbons to MoS₂ nanoplates increased the activity by almost four times. Cyclic voltammetry reveals that such an enhancement by the MoS₂ nanoplates was due to a decrease in the H₂ evolution overpotential.

Microwave-deposited MoS₃ on CdSe-seeded CdS nanorode was investigated as light harvester (Fig. 8) [99]. The CdSe/CdS nanorod heterostructure could control the pathways of charge migration where photo-generated holes are spatially confined to CdSe core and electrons can delocalize over the entire structure. This heterogeneous spatial distribution, which is the result of the staggered band alignments from CdSe and CdS (that are controlled by seed and rod diameters), decreased electron-hole overlap and increased the exciton lifetime with increasing the rod length or decreasing the seed diameter. Deposition of MoS₃ thin film on CdSe/CdS nanorods was demonstrated as a photocatalytic active system for H₂ generation, namely, yield of 100 mmol h⁻¹ g⁻¹ of H₂ was obtained over the catalyst of MoS₃-coated 60 nm CdS rods containing 2.8 nm CdSe seeds, leading to an apparent quantum efficiency of 10% at 450 nm light. In contrast, irradiation of a solution of MoS₃ or as-synthesized CdSe/CdS nanorods alone in water showed a negligible rate for H₂ production. Therefore, one can concluded that intimate contact between the MoS₃ layers and the CdSe/CdS nanorods is vital for efficient charge transfer and thus beneficial for efficient photocatalytic H₂ production. However, the stability test showed a decrease in H₂ production over time, which was attributed to the gradual dissolution of MoS₃ from the surface of the rods.

Figure 8. (A) MoS3 deposition on a CdSe-seeded CdS nanorod, with photocatalytic H2 production in the visible light range using triethanolamine (TEOA) as a sacrificial reagent. (B) Bright-field TEM image of CdSe-seeded CdS nanorods (length 60 nm). Reproduced with permission [99].

Graphene/MoS₂

Graphene, a single layer of sp²-bonded carbon atoms tightly packed into a two-dimensional honeycomb structure, has attracted a lot of attention since its discovery in 2004 [109, 110]. Especially, graphene possesses an excellent mobility of charge carriers at room temperature (2 × 10⁵ cm² V⁻¹ sec⁻¹) and a high theoretical surface area (~2600 m² g⁻¹) [111]. Those unique properties make graphene an efficient electron acceptor to enhance the photo-induced charge transfer and to inhibit the backward reaction by separating the evolution sites of hydrogen and oxygen. MoS₂ itself shows very low HER due to its low surface area and insufficient charge separation. However, the combination of MoS₂ with graphene as catalysts is very promising for solar hydrogen generation, because graphene can provide a channel for charge transport and promote the growth of highly dispersed MoS₂ nanoparticles on it. The most widely used procedure for the preparation of MoS₂–graphene composites is the mixing of presynthesized graphene oxide (GO) with molybdate salts, which are hydrotreated in autoclave in the presence of a sulfur containing compound, typically thiourea [73, 82]. It was reported that, in the presence of graphene during the solvothermal process, MoS₂ quantum dots (QD) instead of layered MoS₂ nanosheets were formed [112]. The formation of MoS₂ QDs was attributed to the interactions between functional groups on GO sheets and Mo precursors in a suitable solvent environment. No MoS₂ QDs were obtained in the process without GO.

The combination of MoS₂ and reduced GO (RGO) sheets was reported in application of water electrolysis where a low over potential of 0.1 V and a small Tafel slope ~41 mV per decade were showed [113]. In contrast, free MoS₂ particles or RGO alone exhibited little HER activity. Very recently, MoS₂–graphene composites were explored for photocatalytic and PEC H₂ evolution. In most cases, the MoS₂/RGO composite was coupled with light absorber such as semiconductors or molecular dyes for HER. For co-deposition of Eosin Y dye and the MoS₂ nanoparticles on the RGO nanosheets as a photocatalyst, the RGO sheets not only provide a confined substrate for the growth of limited-layer MoS₂ (a co-catalyst with a large number of exposed catalytic sites), but also form the interconnected conductive networks for efficiently transferring photo-generated electrons from excited dye to the catalytic active sites of MoS₂ [82]. This can suppress the electron-hole recombination and enhance the photocatalytic efficiency of HER. As a result, a greatly improved HER activity compared to that of the pristine MoS₂ was achieved, namely, a high apparent quantum efficiency of 24.0% at 460 nm was obtained over an Eosin Y-sensitized MoS₂/RGO photocatalyst. However, the MoS₂/RGO system showed no photocatalytic activity in the absence of a photosensitizer (Eosin Y dye), indicating that MoS₂ provided reaction sites for HER, but did not harvest solar energy.

The photocatalytic activity for H₂ evolution was obtained using MoS₂/graphene hybrid as a co-catalyst on TiO₂ semiconductor nanocrystals [48]. The photo-generated CB electrons of TiO₂ can easily migrate into the graphene sheets because the graphene/graphene⁻ redox potential is slightly lower than the CB of TiO₂. Such charge migration from TiO₂ to the MoS₂–graphene composite could efficiently inhibit charge recombination, improve interfacial charge transfer, and increase active adsorption sites and photocatalytic reaction centers. A high H₂ production rate of 165.3 μmol h⁻¹ was achieved when the content of MoS₂/graphene co-catalyst is 0.5 wt. % and the content of graphene in this co-catalyst is 5 wt. %. The quantum efficiency reached 9.7% at 365 nm in water/ethanol mixture solution, which is 39 and four times larger than those on TiO₂ alone and TiO₂ with MoS₂ co-catalyst, respectively. This impressive HER reaction was attributed to the high mobility of electrons on the graphene sheets and the highly active nanoscale MoS₂ as a result of the quantum-confinement effect.

Without additional light harvester, the combination between nanoplatelet p-MoS₂ and nitrogen-doped n-RGO can create significant photocatalytic activity for HER in the wavelength range from ultraviolet light through the near-infrared light [83]. MoS₂ acts not only as a catalytic site but also as a photocenter for absorbing solar light to generate charge carriers. The quantum confinement of the nano-sized MoS₂ enlarges its band gap and thus makes it thermodynamically possible for water reduction. The thin nanoplatelet geometry of MoS₂ can increase the edge sites and the contact area with the RGO sheets. Furthermore, nitrogen-doped n-type RGO nanosheets also act as a support for the growth of p-type MoS₂ nanoplatelets to form many nanoscale p-n junctions on each RGO nanosheet. 24.8 μmol g⁻¹ h⁻¹ was achieved by p-MoS₂/n-RGO junction under simulated solar light irradiation in the water/ethanol mixture solution, whereas the MoS₂–RGO composite without nitrogen-doping has only 7.4 μmol g⁻¹ h⁻¹ (Fig. 9). Furthermore, MoS₂ alone has negligible photocatalytic activity due to its insufficient charge separation. In the MoS₂–RGO composite, the RGO increases the energy conversion efficiency as a passive charge extraction layer. When a nanoscale p-n junction is formed in the MoS₂–n-RGO composite, the space charge layer creates a built-in electric field and separates the electrons and holes upon light illumination.

Figure 9. (A) HRTEM image of the p-MoS2/n-rGO. (B) Hydrogen generated by pure MoS2, the MoS2/RGO, and the p-MoS2/n-RGO photocatalysts. Reproduced with permission [83].

Single-layer 1T-MoS₂ and few-layer 2H-MoS₂ with heavily nitrogenated RGO (N content ca. 15%) showed excellent performance in the production of H₂ under visible light illumination [73]. Nitrogen incorporation in graphene could improve the catalytic activity of the composite with 2H-MoS₂ layers since it enhances the electron donating ability of the graphene. Furthermore, for photocatalytic process with water (40 mL) and triethanolamine (15% v/v; 8 mL) as a sacrificial, the composite materials were sensitized by Eosin Y as catalysts. The yield of H₂ for nitrogen-doped RGO–2H-MoS₂ composite (10.8 mmol g⁻¹ h⁻¹) is nearly 3.5 times higher than that found with unmodified RGO/2H-MoS₂ (Fig. 10A) and about 200 times higher than 2H MoS₂ alone. In contrast, 1T phase of MoS₂ showed highly efficient for photocatalytic HER even without graphene due to its metallic conducting property. The H₂ yielding of 1T-MoS₂ prepared by Li-intercalation of bulk MoS₂ followed by exfoliation in water is as high as 30 mmol g⁻¹ h⁻¹ (Fig. 10B), which is 600 times higher than that of the few layer 2H-MoS₂.

Figure 10. (A) Time course of H2 evolved by (1) Nitrogen-doped RGO-MoS2 and (2) RGO-MoS2 per gram of catalyst. After 10 h of H2 evolution, the vessel was purged. The rate of H2 evolution remains constant after purging. (B) Time course of H2 evolved by freshly prepared 1T-MoS2. Reproduced with permission [73].

From the above discussion, one can conclude that graphene is an active component in photohydrogen production system, which can improve all three vital steps in photocatalysis: charge separation, migration, and recombination. The MoS₂/Graphene heterostructure is a promising candidate for the development of efficient visible light photocatalysts for H₂ evolution, which contains only earth-abundant, nontoxic, and inexpensive materials.

MoS₂/Carbon nitride

Graphitic carbon nitride (g-CN), a polymeric melon semiconductor with a layered structure analogous to graphite, meets the essential requirements as a sustainable solar energy transducer for water redox catalysis. These requirements include being abundant, highly-stable, and responsive to visible light [114]. g-CN has a suitable electronic structure (E_g = 2.7 eV, CB at −0.8 V and VB at 1.9 V vs. RHE) covering the water-splitting potentials [115]. However, g-CN, which alone showed very poor photocatalytic activities, relies on co-catalysts to provide active sites for HER. MoS₂ and g-CN have similar layered structures, which would minimize the lattice mismatch and facilitate the planar growth of MoS₂ slabs over the g-CN surface [116, 117]. A thin layer can reduce the light blocking effect of the co-catalyst to improve the light absorption of g-CN. Importantly, the more negative position of CB in g-CN than in MoS₂ provides the possibility of the directional migration of photo-generated electrons from g-CN to MoS₂, while keeping sufficient chemical potential in the electrons for water reduction at active sites of MoS₂ [118]. Therefore, the combination of MoS₂ and g-CN can facilitate the charge separation, increase the lifetime of the photo-generated electron/hole pair, and lower activation barriers for H₂ or O₂ evolution. Namely, MoS₂ is a promising co-catalyst for the g-CN photocatalyst.

An organic–inorganic-layered heterojunctions for photocatalytic hydrogen production were obtained by gas-controlled growth of thin-layered MoS₂ on a mesoporous g-CN (mpg-CN) surface [95]. As the content of mpg-CN increased for the MoS/mpg-CN catalyst, the rate of photo hydrogen evolution increased to reach a maximum at about 0.2 wt. % MoS₂ content and then decreased. The activity decrease at large MoS₂ contents is due to the serious shielding effect. Furthermore, thin-layered MoS₂ can reduce the poor charge transport from layer to layer and shorten the electron transport time and distance, leading to the efficient utilization of photo-generated electrons for hydrogen production. The best performance of 0.2 wt. % MoS₂/mpg-CN showed an apparent quantum yield of 2.1% measured at 420 nm with lactic acid as an electron donor. The hydrogen evolution rate over 0.5 wt. % MoS₂/mpg-CN reached 1.03 mmol h⁻¹ g⁻¹ which is higher than that of 0.5 wt. % Pt/mpg-CN (0.24 mmol h⁻¹ g⁻¹) under visible light. In addition, PEC activity of MoS₂–g-CN composite was also tested, revealing that 0.5 wt. % MoS₂/g-C₃N₄ sample exhibited the highest catalytic activity with a H₂ evolution rate of 0.23 mmol h⁻¹ g⁻¹, which is 11.3 times higher than that of g-C₃N₄ without MoS₂ [94].

Ternary g-CN/GO/MoS₂ hybrid photocatalyst were synthesized by noncovalent doping of graphite-like carbon nitride (g-CN) with ultrathin GO and MoS₂ nanosheets using a facile sonochemical method [119]. In the hybrid, layered MoS₂ and GO nanosheets with a large surface area enhance light absorption to generate more photoelectrons [120]. Furthermore, it possesses binary p-n heterojunctions at the g-CN/MoS₂ and g-CN/GO interfaces, which can promote charge separation and transfer of electron-hole pairs. As a result, the ternary hybrid photocatalyst exhibits improved PEC and photocatalytic activities under visible light irradiation compared to other reference materials.

In summary, the g-C₃N₄–MoS₂ composite photocatalysts can be synthesized with a facile impregnation method. The favorable electron transfer from g-C₃N₄ to well dispersed MoS₂ layers can inhibit charge recombination and enhance the H₂ evolution activity under visible light irradiation.

MoS₂/TiO₂

As one of the most investigated functional material in semiconductor photocatalysis, titanium dioxide (TiO₂) has been widely used for photocatalysis due to its excellent photocatalytic properties of nontoxicity, effectiveness, low cost, and chemical stability [121]. Nevertheless, TiO₂ has a band gap larger than 3.0 eV and thus cannot absorb visible light. The employment of earth abundant MoS₂ as co-catalyst and photosensitizer has become an attractive strategy to increase solar energy harvesting and photocatalytic activity for TiO₂. Cyclic voltammograms (CV) measurements revealed that TiO₂ modified with 1.0 wt. % MoS₂ can reduce hydrogen reduction overpotential, thereby enhancing photocatalytic activity (Fig. 11) [91]. Furthermore, smaller Nyquist radii of the MoS₂–TiO₂ composite than that of TiO₂ under UV light irradiation indicate that the doping of MoS₂ into TiO₂ led to an efficient separation of the photo-generated electrons and holes [92].

Figure 11. Cyclic voltammograms of bare glassy carbon, pure TiO2, and 1.0 wt. % MoS2/TiO2 in 0.5 mol L−1 H2SO4 versus Ag/AgCl at a scan speed of 20 mV sec−1. Reproduced with permission [91].

Various methods were employed to load MoS₂ on TiO₂, including hydrothermal reaction [90, 91, 93], photodeposition [71, 122], and mechanochemistry [92]. Furthermore, different shapes of TiO₂ were employed as templates for the synthesis, such as nanobelt [90], nanowire [104], and nanofibers [93]. It was found that TiO₂ nanobelts used as a synthetic template could inhibit the growth of MoS₂ crystals along the c-axis, resulting in a few-layer MoS₂ nanosheet coating on the TiO₂ nanobelts [90]. Small TiO₂ nanoparticles possess a short diffusion distance of electron-hole pair, leading to efficient interfacial electron transfer from TiO₂ nanoparticles to MoS₂ surface [91]. Porous TiO₂ nanowire hybrid nanostructures can increase surface area and thus provide more chances to expose the edges of the MoS₂ nanosheets to the reactants [104].

The MoS₂ content in the MoS₂–TiO₂ composite could affect its photocatalytic performance. Zhou et al. [90] demonstrated that the optimal MoS₂ loading is 50 wt. %, leading to a hydrogen production rate of 1.6 mmol h⁻¹ g⁻¹. This happened because too low loading of MoS₂ suffers an insufficient visible light absorption, whereas the excessive loading of MoS₂ can block the photoelectron transfer between the core part of the TiO₂ nanobelt and the shell part of MoS₂ nanosheets. It was suggested that the contact between the light absorber and MoS₂ is crucial for the electron transfer between two components [68]. Vertically standing single or few-layer MoS₂ nanosheets on porous TiO₂ nanofibers (TiO₂@MoS₂) were successfully prepared via a simple hydrothermal reaction [93]. Due to plenty of pores in the TiO₂ nanofibers, the MoS₂ nanosheets vertically grew from the inside to the outside, and the growth mode of the MoS₂ nanosheets rooting into the TiO₂ nanofibers endowed not only intimate contact between the TiO₂ and MoS₂ for fast electrons transfer, but also high structural stability of TiO₂@MoS₂. The vertical orientation of MoS₂ nanosheets enables the active edge sites of MoS₂ to be maximally exposed. Without using an expensive Pt co-catalyst, the TiO₂@MoS₂ heterostructure achieved high photocatalytic H₂ production rates of 1.68 and 0.49 mmol h⁻¹ g⁻¹ under UV-vis and visible light illumination, respectively. In addition, the TiO₂@MoS₂ catalyst exhibited a high durability as evidenced by the invariable H₂ production rate after continuous illumination over 30 h.

In conclusion, tuning TiO₂ nanostructure can improve photocatalytic H₂ production rate of MoS₂–TiO₂ composite by increasing surface area and deceasing electron-hole pair diffusion distance. The TiO₂–MoS₂ hybrid nanostructure of the composite possesses the intimate interaction between MoS₂ and TiO₂ and the superior structure stability of the MoS₂–TiO₂ composite, leading to enhanced photocatalytic activity.

MoS₂ with Zn-based materials

Zn-related sulfides have shown remarkable versatility of fundamental properties and important applications. Bulk ZnS has a large band gap of ~3.72 eV for cubic zinc blende and ~3.77 eV for hexagonal wurtzite ZnS. It is well known that ZnS is an attractive photocatalyst due to its rapid photogeneration of electron-hole pairs; its highly negative potentials of CB (about −0.91 vs. SHE) results in superior HER activity [123, 124], even without any assistance of noble metal co-catalysts [125]. However, ZnS is unstable in aqueous solutions where it suffers anodic photocorrosion with the formation of sulfur and/or sulfate ions. Furthermore, the rapid electron-hole recombination also limits its practical application. To solve those issues, graphene and MoS₂ nanosheets were exploited to modify ZnS nanoparticles (Fig. 12) [98]. The ZnS–graphene–MoS₂ nanocomposites exhibited significantly enhanced photocatalytic activity for H₂ evolution from water splitting. When the ZnS–graphene–MoS₂ nanocomposite contained 0.25 wt. % graphene and 2 atom% MoS₂, it exhibited a high H₂ evolution rate of 2258 μmol h⁻¹ g⁻¹, which is about two times that of ZnS alone under a 300 W Xe lamp in Na₂S/Na₂SO₃ aqueous solution. This can be attributed to the synergistic effect of co-catalysts, namely, graphene serves as an excellent electron acceptor and transporter, and MoS₂ nanosheets provide a source of active sites.

Figure 12. Typical TEM (A) and (B) and HRTEM images (C) of ZnS/Graphene/MoS2, where (B) is the detail of the square frame in (A) and (C) is that in (B). Reproduced with permission [98].

ZnIn₂S₄ is a ternary chalcogenide with unique properties, such as a suitable band gap (2.34–2.48 eV) corresponding to the visible light absorption, low toxicity, considerable chemical stability, and facile preparation process [126]. ZnIn₂S₄ exhibits two distinct polymorphs based on cubic and hexagonal lattices. Both MoS₂ and hexagonal ZnIn₂S₄ possess a similar layered structure, which might allow MoS₂ to easily grow on the ZnIn₂S₄ surface. The conduction band position of hexagonal ZnIn₂S₄ (−1.1 eV vs. NHE) is more negative than that of MoS₂ (−0.5 and −0.9 vs. NHE [127]), providing a possibility for the transfer of photo-generated electrons from ZnIn₂S₄ to MoS₂. The electrons transferred to CB of quantum confined MoS₂ still maintain enough chemical potential for HER reaction. Furthermore, the MoS₂/ZnIn₂S₄ nanocomposite can be synthesized by impregnating ZnIn₂S₄ with (NH₄)₂MoS₄ aqueous solution, followed by high-temperature treatment in H₂S flow [96]. It can also be prepared via an in situ photo-assisted deposition process [97]. The photocatalytic activity of the MoS₂–ZnIn₂S₄ nanocomposite is comparable to that of Pt/ZnIn₂S₄ and 10 times higher than that of pure ZnIn₂S₄ [96, 97]. This can be explained by the intimate contact between ZnIn₂S₄ and MoS₂, which promotes the formation of junctions between the two components to improve the charge separation and prolong the mean lifetime of the electron-hole pairs. In addition, the MoS₂ in the MoS₂–ZnIn₂S₄ nanocomposite possesses an amorphous state, in which there are many defect sites that can act as adsorption sites for hydrogen atoms [96].

In summary, the combination of Zn-based semiconductors (such as ZnS and ZnIn₂S₄) with MoS₂ can create promising photocatalysts for H₂ evolution under visible light irradiation, which is due to their similar layered structures, favorable electron transfer at the heterojunction interface, and the defect sites over MoS₂ which act as adsorption site for hydrogen atoms.

MoS₂ with transition metals or other metals

Nakagawa et al. [100] prepared single molecular sheets of niobate (K₄Nb₆O₁₇) by a hydrothermal approach using niobium ethoxide with the aid of triethanolamine (TEOA) as a structural modifier (Fig. 13). The highly stable molecular entities against self-assembly allows them to mix well with other colloids and facilitates their extensive electronic interactions. The 2D niobate molecular sheets can absorb light and generate excited electrons and holes under UV irradiation. MoS₂ has a lower conduction band position than those of niobate, but its edge sites can offer lower activation energy for H₂O reduction while conjugated graphene acts as an electron transfer mediator. A rapid electron transfer from niobate nanosheets to MoS₂ through the 2D graphene nanosheets is expected to reduce the electron-hole recombination in niobate molecular sheet, leading to enhanced photocatalytic activity. Indeed, though MoS₂ exhibited negligible photocatalytic activity, the optimized H₂ evolution rates of MoS₂/niobate/TEOA/Graphene reached 2.12 mmol h⁻¹ with methanol as a sacrificial reagent, which is much larger than that of niobate/TEOA alone.

Figure 13. (A) TEM image of niobte/TEOA and (B) HRTEM image of discrete niobate/TEOA single molecular sheets. Reproduced with permission [100].

It was demonstrated that the introduction of metal nanoparticles (Cr, Ag) to the surface of MoS₂ nanosheets could increase the photo-generated carrier separation and suppress the electron-hole recombination for HER applications [101]. The work function difference can drive electron transfer from Cr to MoS₂ to build an electric field at the thin metal/semiconductor interfacial barrier layer (Fig. 14A), leading to an efficient separation of photo-generated carriers. This was supported by PL measurement, in which with MoS₂ as an electron sink, the intensity of the photoluminescence of Cr-MoS₂ hybrid nanosheets was extensively quenched. With Cr-MoS₂ as co-catalyst for CdS photocatalyst, the average rate of hydrogen evolution is 38,000 μmol g⁻¹ h⁻¹ under visible light with a sacrificial reagent, which is higher than that of MoS₂ as a single co-catalyst without Cr. Furthermore, significant enhancement is achieved for photocatalytic HER in the case of Ag-MoS₂ as co-catalyst, leading to an average rate of 107,000 μmol g⁻¹ h⁻¹, which is six times higher than that of MoS₂ co-catalyst (Fig. 14B). Furthermore, the composite showed an excellent stability of 12 h against photocorrosion.

Figure 14. (A) Energy band-edge alignment of Cr-MoS2 hybrid structure. EF,Cr and EF,MoS2 are the Fermi Levels of Cr and MoS2, respectively. ΦB is the electron barrier potential in the hybrid Cr–MoS2 system. ECB and EVB are the conduction and valence band-edges of MoS2, respectively. A thin interfacial layer is inserted between Cr and MoS2 to indicate a nonepitaxial contact. (B) The H2 production as a function of irradiation time. As co-catalysts combined with photocatalyst of CdS, the plots represent 1.0 wt. % Cr–MoS2 (in red), 1.0 wt. % Ag–MoS2 (in blue), and 1.0 wt. % MoS2 (in black). Reproduced with permission [101].

Perovskite-type SrZrO₃ with a band gap of 5.6 eV has been considered as a promising photocatalysts due to its high stability, more photocatalytic active sites and nontoxicity. Recently, Tian et al. [102] synthesized a novel heterojunction of a MoS₂/SrZrO₃ photocatalyst (via a simple hydrothermal process) and applied it for photocatalytic H₂ evolution under UV light irradiation. The rate of H₂ evolution for pure SrZrO₃ photocatalyst (with Na₂S/Na₂SO₃ as a sacrificial agent) was 9.75 mmol h⁻¹ g⁻¹ whereas the rate was increased 2.7 times to reach 26.55 mmol h⁻¹ g⁻¹ by introducing 0.05 wt. % MoS₂ into SrZrO₃. The enhancement was attributed to the junction between SrZrO₃ and MoS₂, which can suppress the recombination of the photo-generated electron-hole.

In summary, the incorporation of metal nanoparticles (Cr, Ag) into MoS₂ can lead to the improvement in photocatalytic H₂ production, which is attributed to the reduction in photo-generated electron/hole recombination in MoS₂ nanosheets by Cr or Ag nanoparticles. Single molecular niobate sheets can facilitate the assembly with graphene, forming a promising MoS₂/niobate/TEOA/Graphene photocatalyst, in which electrons can rapidly transfer from niobate nanosheets to MoS₂ through the 2D graphene nanosheets to reduce electron/hole recombination in niobate molecular sheets.

MoS₂/dye sensitizer

A molecular photon absorber, ruthenium trisbipyridine (Ru(bpy)₃), was added to colloid amorphous MoS₂ nanoparticles with diameter <10 nm [103]. With ascorbic acid as an electron donor, the HER activity of the colloidal system was evaluated under λ > 420 nm visible light, revealing the optimized H₂ yield of about 500 μmol H₂ with 12.5 μmol MoS₂ in 6 h. Furthermore, colloidal MoS₂ nanoparticles (NPs) were also combined with a series of cyclometalated Ir (III) sensitizers for multicomponent photocatalytic water reduction systems [77]. The existence of metal-carbon sigma bonds in cyclometalated Ir (III) emissive species improves their photostability compared to Ru(II) diimines under light illumination. The introduction of the carboxylate anchoring groups in the iridium complexes can allow the species to be chemically adsorbed onto MoS₂ NPs for the increase in electron transfer, resulting in enhancement of H₂ evolution. The highest apparent quantum yield was as high as 12.4% for H₂ evolution at λ = 400 nm with triethanolamine (TEOA) as an electron-donating agent, indicating that the combination between MoS₂ and dyes provides a promising approach to improve the photocatalytic activity of MoS₂.

Outlook

As a repressive layered transition metal dichalcogenides (TMDs), MoS₂ has been widely investigated as a co-catalyst for the photocatalytic hydrogen from water. Its combination with various other materials, such as CdS, graphene, and TiO₂, exhibited excellent photocatalytic performance. It is still an important topic to find new MoS₂-based composite materials. For example, it was theoretically predicted that MoS₂ and AlN(GaN) can form an van der Waals (vdW) heterostructured composite as an efficient photocatalyst for water splitting under visible light irradiation [128]. The MoS₂/AlN(GaN) vdW catalyst can be expected to separately produce hydrogen and oxygen at the opposite surfaces, where the photo-excited electrons transfer from AlN(GaN) to MoS₂ during the photocatalysis process.

Although bulk MoS₂ cannot evolve hydrogen under light illumination due to slightly more positive CB than that for the HER, its CB varies with the number of its layers. Therefore, it needs intensive research to explore a single layer or few-layer MoS₂ samples as a single catalyst with both functions of absorbing light and catalyzing HER. Furthermore, the band structure of MoS₂ may also be tuned by creating hybrid-layered structures with other TMDs, such as WS₂, MoSe₂, and WSe₂. The hybrid TMD materials would be promising photocatalysts.

Conflict of Interest

None declared.

References

1. Lewis, N. S., and D. G. Nocera. 2006. Powering the planet: chemical challenges in solar energy utilization. Proc. Natl Acad. Sci. USA103:15729–15735.
2. PE United Nations Department of Economic and Social Affairs, Population Division. July 2015. Available at: https://esa.un.org/unpd/wpp/Publications/Files/Key_Findings_WPP_2015.pdf
3. Wang, H., and Y. H. Hu. 2012. Graphene as a counter electrode material for dye-sensitized solar cells. Energy Environ. Sci.5:8182–8188.
4. Wei, W., H. Wang, and Y. H. Hu. 2013. Unusual particle-size-induced promoter-to-poison transition of ZrN in counter electrodes for dye-sensitized solar cells. J. Mater. Chem. A1:14350–14357.
5. Wang, H., W. Wei, and Y. H. Hu. 2013. Efficient ZnO-based counter electrodes for dye-sensitized solar cells. J. Mater. Chem. A1:607–611.
6. Crabtree, G., and N. Lewis. 2007. Solar energy conversion. Phys. Today60:37–42.
7. Kudo, A., and Y. Miseki. 2009. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev.38:253–278.
8. Walter, M. G., E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, et al. 2010. Solar water splitting cells. Chem. Rev.110:6446–6473.
9. Fujishima, A., and K. Honda. 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature238:37–38.
10. Khaselev, O., and J. A. Turner. 1998. A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting. Science280:425–427.
11. Khan, S. U. M., and J. Akikusa. 1999. Photoelectrochemical splitting of water at nanocrystalline N-Fe₂O₃ thin-film electrodes. J. Phys. Chem. B103:7184–7189.
12. Santato, C., M. Ulmann, and J. Augustynski. 2001. Photoelectrochemical properties of nanostructured tungsten trioxide films. J. Phys. Chem. B105:936–940.
13. Hou, Y., Abrams, B., Vesborg, P., Björketun, M., Herbst, K., Bech, L. 2011. Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nat. Mater.10:434–438.
14. Matsumura, M., Y. Saho, and H. Tsubomura. 1983. Photocatalytic hydrogen production from solutions of sulfite using platinized cadmium sulfide powder. J. Phys. Chem.87:3807–3808.
15. Maruthamuthu, P., K. Gurunathan, E. Subramanian, and M. Ashokkumar. 1991. Photocatalytic activities of Bi₂O₃, WO₃, and Fe₂O₃: an assessment through decomposition of peroxomonosulfate in visible radiation. Bull. Chem. Soc. Jpn.64:1933–1937.
16. Kato, H., K. Asakura, and A. Kudo. 2003. Highly efficient water splitting into H₂ and O₂ over lanthanum-doped NaTaO₃ photocatalysts with high crystallinity and surface nanostructure. J. Am. Chem. Soc.125:3082–3089.
17. Ohno, T., L. Bai, T. Hisatomi, K. Maeda, and K. Domen. 2012. Photocatalytic water splitting using modified Gan: Zno solid solution under visible light: long-time operation and regeneration of activity. J. Am. Chem. Soc.134:8254–8259.
18. Jia, Q., A. Iwase, and A. Kudo. 2014. BiVO4-Ru/SrTiO3: Rh composite Z-Scheme photocatalyst for solar water splitting. Chem. Sci.5:1513–1519.
19. Tsuji, I., H. Kato, and A. Kudo. 2006. Photocatalytic hydrogen evolution on ZnS−CuInS₂−AgInS₂ solid solution photocatalysts with wide visible light absorption bands. Chem. Mater.18:1969–1975.
20. Han, B., and Y. H. Hu. 2015. Highly efficient temperature-induced visible light photocatalytic hydrogen production from water. J. Phys. Chem. C119:18927–18934.
21. Han, B., W. Wei, L. Chang, P. Cheng, and Y. H. Hu. 2016. Efficient visible light photocatalytic CO₂ reforming of CH₄. ACS Catal.6:494–497.
22. U.S. Geological Survey. 2011. Mineral commodity summaries. U.S. Geological Survey, Reston, VA.
23. McKone, J. R., B. F. Sadtler, C. A. Werlang, N. S. Lewis, and H. B. Gray. 2012. Ni–Mo nanopowders for efficient electrochemical hydrogen evolution. ACS Catal.3:166–169.
24. Merki, D., and X. Hu. 2011. Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts. Energy Environ. Sci.4:3878–3888.
25. Laursen, A. B., S. Kegnaes, S. Dahl, and I. Chorkendorff. 2012. Molybdenum sulfides-efficient and viable materials for electro – and photoelectrocatalytic hydrogen evolution. Energy Environ. Sci.5:5577–5591.
26. Chhowalla, M., H. S. Shin, G. Eda, L.-J. Li, K. P. Loh, and H. Zhang. 2013. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem.5:263–275.
27. Benck, J. D., T. R. Hellstern, J. Kibsgaard, P. Chakthranont, and T. F. Jaramillo. 2014. Catalyzing the hydrogen evolution reaction (Her) with molybdenum sulfide nanomaterials. ACS Catal.4:3957–3971.
28. Morales-Guio, C. G., and X. Hu. 2014. Amorphous molybdenum sulfides as hydrogen evolution catalysts. Acc. Chem. Res.47:2671–2681.
29. Morales-Guio, C. G., L.-A. Stern, and X. Hu. 2014. Nanostructured hydrotreating catalysts for electrochemical hydrogen evolution. Chem. Soc. Rev.43:6555–6569.
30. Yan, Y., B. Xia, Z. Xu, and X. Wang. 2014. Recent development of molybdenum sulfides as advanced electrocatalysts for hydrogen evolution reaction. ACS Catal.4:1693–1705.
31. Zou, X., and Y. Zhang. 2015. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev.44:5148–5180.
32. Li, S.-L., K. Tsukagoshi, E. Orgiu, and P. Samori. 2016. Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors. Chem. Soc. Rev.45:118–151.
33. Mak, K. F., and J. Shan. 2016. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nat. Photonics10:216–226.
34. Dickinson, R. G., and L. Pauling. 1923. The crystal structure of molybdenite. J. Am. Chem. Soc.45:1466–1471.
35. Frindt, R. F.1966. Single crystals of MoS₂ several molecular layers thick. J. Appl. Phys.37:1928–1929.
36. Joensen, P., R. F. Frindt, and S. R. Morrison. 1986. Single-layer MoS₂. Mater. Res. Bull.21:457–461.
37. Koma, A., and K. Yoshimura. 1986. Ultrasharp interfaces grown with van der Waals epitaxy. Surf. Sci.174:556–560.
38. Yang, D., S. J. Sandoval, W. M. R. Divigalpitiya, J. C. Irwin, and R. F. Frindt. 1991. Structure of single-molecular-layer MoS₂. Phys. Rev. B43:12053–12056.
39. Late, D. J., B. Liu, H. S. S. R. Matte, V. P. Dravid, and C. N. R. Rao. 2012. Hysteresis in single-layer MoS₂ field effect transistors. ACS Nano6:5635–5641.
40. Ataca, C., M. Topsakal, E. Aktürk, and S. Ciraci. 2011. A comparative study of lattice dynamics of three- and two-dimensional MoS₂. J. Phys. Chem. C115:16354–16361.
41. Ataca, C., H. S¸ahin, and S. Ciraci. 2012. Stable, single-layer MX₂ transition-metal oxides and dichalcogenides in a honeycomb-like structure. J. Phys. Chem. C116:8983–8999.
42. Beal, A. R., J. C. Knights, and W. Y. Liang. 1972. Transmission spectra of some transition metal dichalcogenides. II. Group via: trigonal prismatic coordination. J. Phys. C: Solid State Phys.5:3540.
43. Shirodkar, S. N., and U. V. Waghmare. 2014. Emergence of ferroelectricity at a metal-semiconductor transition in a 1T monolayer of MoS₂. Phys. Rev. Lett.112:157601.
44. Acerce, M., D. Voiry, and M. Chhowalla. 2015. Metallic 1t phase MoS₂ nanosheets as supercapacitor electrode materials. Nat. Nanotechnol.10:313–318.
45. Cheng, P., K. Sun, and Y. H. Hu. 2016. Memristive behavior and ideal memristor of 1T phase MoS₂ nanosheets. Nano Lett.16:572–576.
46. Cheng, P., K. Sun, and Y. H. Hu. 2016. Mechanically-induced reverse phase transformation of MoS₂ from stable 2H to metastable 1T and its memristive behavior. RSC Adv.6:65691–65697.
47. Kuc, A.2015. Low-dimensional transition-metal dichalcogenides. Pp. 1–29inM. Springborg, and J. Joswig, ed. Chemical modelling: volume 11. The Royal Society of Chemistry, London, UK.
48. Xiang, Q., J. Yu, and M. Jaroniec. 2012. Synergetic effect of MoS₂ and graphene as cocatalysts for enhanced photocatalytic H₂ production activity of TiO₂ nanoparticles. J. Am. Chem. Soc.134:6575–6578.
49. Kadantsev, E. S., and P. Hawrylak. 2012. Electronic structure of a single MoS₂ monolayer. Solid State Commun.152:909–913.
50. Lee, H. S., S.-W. Min, Y.-G. Chang, M. K. Park, T. Nam, H. Kim, et al. 2012. MoS₂ nanosheet phototransistors with thickness-modulated optical energy gap. Nano Lett.12:3695–3700.
51. Mak, K. F., C. Lee, J. Hone, J. Shan, and T. F. Heinz. 2010. Atomically thin MoS₂: a new direct-gap semiconductor. Phys. Rev. Lett.105:136805.
52. Lee, C., H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu. 2010. Anomalous lattice vibrations of single- and few-layer MoS₂. ACS Nano4:2695–2700.
53. Ellis, J. K., M. J. Lucero, and G. E. Scuseria. 2011. The indirect to direct band gap transition in multilayered MoS₂ as predicted by screened hybrid density functional theory. Appl. Phys. Lett.99:261908.
54. Kuc, A., N. Zibouche, and T. Heine. 2011. Influence of quantum confinement on the electronic structure of the transition metal sulfide TS₂. Phys. Rev. B83:245213.
55. Eda, G., H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, and M. Chhowalla. 2011. Photoluminescence from chemically exfoliated MoS₂. Nano Lett.11:5111–5116.
56. Splendiani, A., L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, et al. 2010. Emerging photoluminescence in monolayer MoS₂. Nano Lett.10:1271–1275.
57. Wilcoxon, J. P.2000. Catalytic photooxidation of pentachlorophenol using semiconductor nanoclusters. J. Phys. Chem. B104:7334–7343.
58. Rao, C. N. R., H. S. S. Ramakrishna Matte, and U. Maitra. 2013. Graphene analogues of inorganic layered materials. Angew. Chem. Int. Ed.52:13162–13185.
59. Novoselov, K. S., D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, et al. 2005. Two-dimensional atomic crystals. Proc. Natl Acad. Sci. USA102:10451–10453.
60. Li, H., Z. Yin, Q. He, H. Li, X. Huang, G. Lu, et al. 2012. Fabrication of single- and multilayer MoS₂ film-based field-effect transistors for sensing no at room temperature. Small8:63–67.
61. Coleman, J. N., et al. 2011. Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science331:568–571.
62. Zhou, K.-G., N.-N. Mao, H.-X. Wang, Y. Peng, and H.-L. Zhang. 2011. A mixed-solvent strategy for efficient exfoliation of inorganic graphene analogues. Angew. Chem. Int. Ed.50:10839–10842.
63. Wang, X., H. Feng, Y. Wu, and L. Jiao. 2013. Controlled synthesis of highly crystalline MoS₂ flakes by chemical vapor deposition. J. Am. Chem. Soc.135:5304–5307.
64. Huang, X., Z. Zeng, and H. Zhang. 2013. Metal dichalcogenide nanosheets: preparation, properties and applications. Chem. Soc. Rev.42:1934–1946.
65. Ramakrishna Matte, H. S. S., A. Gomathi, A. K. Manna, D. J. Late, R. Datta, S. K. Pati, et al. 2010. MoS₂ and WS₂ analogues of graphene. Angew. Chem. Int. Ed.49:4059–4062.
66. Altavilla, C., M. Sarno, and P. Ciambelli. 2011. A novel wet chemistry approach for the synthesis of hybrid 2D free-floating single or multilayer nanosheets of MS₂@Oleylamine (M=Mo, W). Chem. Mater.23:3879–3885.
67. Peng, Y., Z. Meng, C. Zhong, J. Lu, W. Yu, Y. Jia, et al. 2001. Hydrothermal synthesis and characterization of single-molecular-layer MoS₂ and MoSe₂. Chem. Lett.30:772–773.
68. Zong, X., H. Yan, G. Wu, G. Ma, F. Wen, L. Wang, et al. 2008. Enhancement of photocatalytic H₂ evolution on CdS by loading MoS₂ as cocatalyst under visible light irradiation. J. Am. Chem. Soc.130:7176–7177.
69. Liu, Y., H. Yu, X. Quan, and S. Chen. 2013. Green synthesis of feather-shaped MoS₂/CdS photocatalyst for effective hydrogen production. Int. J. Photoenergy 2013, 247516–247521.
70. Chen, G., D. Li, F. Li, Y. Fan, H. Zhao, Y. Luo, et al. 2012. Ball-milling combined calcination synthesis of MoS₂/CdS photocatalysts for high photocatalytic H₂ evolution activity under visible light irradiation. Appl. Catal. A443–444:138–144.
71. Kanda, S., T. Akita, M. Fujishima, and H. Tada. 2011. Facile synthesis and catalytic activity of MoS₂/TiO₂ by a photodeposition-based technique and its oxidized derivative MoO₃/TiO₂ with a unique photochromism. J. Colloid Interface Sci.354:607–610.
72. Zhuo, S., Y. Xu, W. Zhao, J. Zhang, and B. Zhang. 2013. Hierarchical nanosheet-based MoS₂ nanotubes fabricated by an anion-exchange reaction of MoO₃–amine hybrid nanowires. Angew. Chem. Int. Ed.52:8602–8606.
73. Maitra, U., U. Gupta, M. De, R. Datta, A. Govindaraj, and C. N. R. Rao. 2013. Highly effective visible-light-induced H₂ generation by single-layer 1t-MoS₂ and a nanocomposite of few-layer 2H-MoS₂ with heavily nitrogenated graphene. Angew. Chem. Int. Ed.52:13057–13061.
74. Popczun, E. J., J. R. McKone, C. G. Read, A. J. Biacchi, A. M. Wiltrout, N. S. Lewis, et al. 2013. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J. Am. Chem. Soc.135:9267–9270.
75. Popczun, E. J., C. G. Read, C. W. Roske, N. S. Lewis, and R. E. Schaak. 2014. Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide nanoparticles. Angew. Chem. Int. Ed.53:5427–5430.
76. Jaramillo, T. F., K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, and I. Chorkendorff. 2007. Identification of active edge sites for electrochemical H₂ evolution from MoS₂ nanocatalysts. Science317:100–102.
77. Yuan, Y.-J., Z.-T. Yu, X.-J. Liu, J.-G. Cai, Z.-J. Guan, and Z.-G. Zou. 2014. Hydrogen photogeneration promoted by efficient electron transfer from iridium sensitizers to colloidal MoS₂ catalysts. Sci. Rep.4:4045–4054.
78. Wang, Z., J. Hou, C. Yang, S. Jiao, and H. Zhu. 2014. Three-dimensional MoS₂-Cds-[Gamma]-Taon hollow composites for enhanced visible-light-driven hydrogen evolution. Chem. Commun.50:1731–1734.
79. Zhang, J., Z. Zhu, and X. Feng. 2014. Construction of two-dimensional MoS2/Cds P-N nanohybrids for highly efficient photocatalytic hydrogen evolution. Chemistry20:10632–10635.
80. Li, Y., H. Wang, and S. Peng. 2014. Tunable photodeposition of MoS₂ onto a composite of reduced graphene oxide and Cds for synergic photocatalytic hydrogen generation. J. Phys. Chem. C118:19842–19848.
81. Xu, J., and X. Cao. 2015. Characterization and mechanism of MoS₂/Cds composite photocatalyst used for hydrogen production from water splitting under visible light. Chem. Eng. J.260:642–648.
82. Min, S., and G. Lu. 2012. Sites for high efficient photocatalytic hydrogen evolution on a limited-layered MoS₂ cocatalyst confined on graphene sheets-the role of graphene. J. Phys. Chem. C116:25415–25424.
83. Meng, F., J. Li, S. K. Cushing, M. Zhi, and N. Wu. 2013. Solar hydrogen generation by nanoscale p–n junction of P-type molybdenum disulfide/N-type nitrogen-doped reduced graphene oxide. J. Am. Chem. Soc.135:10286–10289.
84. Liu, M., F. Li, Z. Sun, L. Ma, L. Xu, and Y. Wang. 2014. Noble-metal-free photocatalysts MoS₂-graphene/Cds mixed nanoparticles/nanorods morphology with high visible light efficiency for H₂ evolution. Chem. Commun.50:11004–11007.
85. Chang, K., Z. Mei, T. Wang, Q. Kang, S. Ouyang, and J. Ye. 2014. MoS₂/graphene cocatalyst for efficient photocatalytic H₂ evolution under visible light irradiation. ACS Nano8:7078–7087.
86. Latorre-Sánchez, M., I. Esteve-Adell, A. Primo, and H. García. 2015. Innovative preparation of MoS₂–graphene heterostructures based on alginate containing (NH₄)₂MoS₄ and their photocatalytic activity for H₂ generation. Carbon81:587–596.
87. Djamil, J., S. A. Segler, A. Dabrowski, W. Bensch, A. Lotnyk, U. Schurmann, et al. 2013. The influence of carbon content on the structure and properties of mosxcy photocatalysts for light-driven hydrogen generation. Dalton Trans.42:1287–1292.
88. Frame, F. A., and F. E. Osterloh. 2010. Cdse-MoS₂: a quantum size-confined photocatalyst for hydrogen evolution from water under visible light. J. Phys. Chem. C114:10628–10633.
89. Sobczynski, A.1991. Molybdenum disulfide as a hydrogen evolution catalyst for water photodecomposition on semiconductors. J. Catal.131:156–166.
90. Zhou, W., Z. Yin, Y. Du, X. Huang, Z. Zeng, Z. Fan, et al. 2013. Synthesis of few-layer MoS₂ nanosheet-coated TiO₂ nanobelt heterostructures for enhanced photocatalytic activities. Small9:140–147.
91. Liu, Q., Z. Pu, A. Asiri, A. Qusti, A. Al-Youbi, and X. Sun. 2013. One-step solvothermal synthesis of MoS₂/TiO₂ nanocomposites with enhanced photocatalytic H₂ production. J. Nanopart. Res.15:1–7.
92. Zhu, Y., Q. Ling, Y. Liu, H. Wang, and Y. Zhu. 2015. Photocatalytic H₂ evolution on MoS₂-TiO₂ catalysts synthesized via mechanochemistry. Phys. Chem. Chem. Phys.17:933–940.
93. Liu, C., L. Wang, Y. Tang, S. Luo, Y. Liu, S. Zhang, et al. 2015. Vertical single or few-layer MoS₂ nanosheets rooting into TiO₂ nanofibers for highly efficient photocatalytic hydrogen evolution. Appl. Catal. B164:1–9.
94. Ge, L., C. Han, X. Xiao, and L. Guo. 2013. Synthesis and characterization of composite visible light active photocatalysts MoS₂–g-C₃N₄ with enhanced hydrogen evolution activity. Int. J. Hydrogen Energy38:6960–6969.
95. Hou, Y., A. B. Laursen, J. Zhang, G. Zhang, Y. Zhu, X. Wang, et al. 2013. Layered nanojunctions for hydrogen-evolution catalysis. Angew. Chem. Int. Ed.52:3621–3625.
96. Wei, L., Y. Chen, Y. Lin, H. Wu, R. Yuan, and Z. Li. 2014. MoS₂ as non-noble-metal co-catalyst for photocatalytic hydrogen evolution over hexagonal Znin2s4 under visible light irradiations. Appl. Catal. B144:521–527.
97. Chen, G., N. Ding, F. Li, Y. Fan, Y. Luo, D. Li, et al. 2014. Enhancement of photocatalytic H₂ evolution on ZnIn₂S₄ loaded with in-situ photo-deposited MoS₂ under visible light irradiation. Appl. Catal. B160–161:614–620.
98. Zhu, B., B. Lin, Y. Zhou, P. Sun, Q. Yao, Y. Chen, et al. 2014. Enhanced photocatalytic H₂ evolution on ZnS loaded with graphene and MoS₂ nanosheets as cocatalysts. J. Mater. Chem. A2:3819–3827.
99. Tang, M. L., D. C. Grauer, B. Lassalle-Kaiser, V. K. Yachandra, L. Amirav, J. R. Long, et al. 2011. Structural and electronic study of an amorphous MoS₃ hydrogen-generation catalyst on a quantum-controlled photosensitizer. Angew. Chem. Int. Ed.50:10203–10207.
100. Nakagawa, K., T. Jia, W. Zheng, S. M. Fairclough, M. Katoh, S. Sugiyama, et al. 2014. Enhanced photocatalytic hydrogen evolution from water by niobate single molecular sheets and ensembles. Chem. Commun.50:13702–13705.
101. Yang, L., et al. 2014. Optical properties of metal–molybdenum disulfide hybrid nanosheets and their application for enhanced photocatalytic hydrogen evolution. ACS Nano8:6979–6985.
102. Tian, Q., L. Zhang, J. Liu, N. Li, Q. Ma, J. Zhou, et al. 2015. Synthesis of MoS₂/SrZrO₃ heterostructures and their photocatalytic H2 evolution under UV irradiation. RSC Adv.5:734–739.
103. Zong, X., Y. Na, F. Wen, G. Ma, J. Yang, D. Wang, et al. 2009. Visible light driven H₂ production in molecular systems employing colloidal MoS₂ nanoparticles as catalyst. Chem. Commun.30:4536–4538.
104. Shen, M., Z. Yan, L. Yang, P. Du, J. Zhang, and B. Xiang. 2014. MoS₂ nanosheet/TiO₂ nanowire hybrid nanostructures for enhanced visible-light photocatalytic activities. Chem. Commun.50:15447–15449.
105. Yoon, Y., K. Ganapathi, and S. Salahuddin. 2011. How good can monolayer MoS₂ transistors be?Nano Lett.11:3768–3773.
106. Zong, X., G. Wu, H. Yan, G. Ma, J. Shi, F. Wen, et al. 2010. Photocatalytic H₂ evolution on MoS₂/Cds catalysts under visible light irradiation. J. Phys. Chem. C114:1963–1968.
107. Liu, Y., Y.-X. Yu, and W.-D. Zhang. 2013. MoS₂/Cds heterojunction with high photoelectrochemical activity for H₂ evolution under visible light: the role of MoS₂. J. Phys. Chem. C117:12949–12957.
108. Raja, R., P. Sudhagar, A. Devadoss, C. Terashima, L. K. Shrestha, K. Nakata, et al. 2015. Pt-free solar driven photoelectrochemical hydrogen fuel generation using 1t MoS₂ co-catalyst assembled Cds Qds/TiO₂ photoelectrode. Chem. Commun.51:522–525.
109. Novoselov, K. S., A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, et al. 2004. Electric field effect in atomically thin carbon films. Science306:666–669.
110. Hu, Y. H., H. Wang, and B. Hu. 2010. Thinnest two-dimensional nanomaterial—graphene for solar energy. ChemSusChem3:782.
111. Xiang, Q., J. Yu, and M. Jaroniec. 2012. Graphene-based semiconductor photocatalysts. Chem. Soc. Rev.41:782–796.
112. Gao, W., M. Wang, C. Ran, and L. Li. 2015. Facile one-pot synthesis of MoS₂ quantum dots-graphene-TiO₂ composites for highly enhanced photocatalytic properties. Chem. Commun.51:1709–1712.
113. Li, Y., H. Wang, L. Xie, Y. Liang, G. Hong, and H. Dai. 2011. MoS₂ Nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc.133:7296–7299.
114. Wang, X., K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, et al. 2009. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater.8:76–80.
115. Zhang, J., X. Chen, K. Takanabe, K. Maeda, K. Domen, J. D. Epping, et al. 2010. Synthesis of a carbon nitride structure for visible-light catalysis by copolymerization. Angew. Chem. Int. Ed.49:441–444.
116. Lunt, R. R., K. Sun, M. Kröger, J. B. Benziger, and S. R. Forrest. 2011. Ordered organic-organic multilayer growth. Phys. Rev. B83:064114.
117. Hong, Y. J., and T. Fukui. 2011. Controlled Van Der Waals heteroepitaxy of Inas nanowires on carbon honeycomb lattices. ACS Nano5:7576–7584.
118. Kibsgaard, J., Z. Chen, B. N. Reinecke, and T. F. Jaramillo. 2012. Engineering the surface structure of MoS₂ to preferentially expose active edge sites for electrocatalysis. Nat. Mater.11:963–969.
119. Hu, S. W., L. W. Yang, Y. Tian, X. L. Wei, J. W. Ding, J. X. Zhong, et al. 2014. Non-covalent doping of graphitic carbon nitride with ultrathin graphene oxide and molybdenum disulfide nanosheets: an effective binary heterojunction photocatalyst under visible light irradiation. J. Colloid Interface Sci.431:42–49.
120. Wang, J., Z. Guan, J. Huang, Q. Li, and J. Yang. 2014. Enhanced photocatalytic mechanism for the hybrid g-C₃N₄/MoS₂ nanocomposite. J. Mater. Chem. A2:7960–7966.
121. Gratzel, M.2001. Photoelectrochemical cells. Nature414:338–344.
122. Meng, C., Z. Liu, T. Zhang, and J. Zhai. 2015. Layered MoS₂ nanoparticles on TiO₂ nanotubes by a photocatalytic strategy for use as high-performance electrocatalysts in hydrogen evolution reactions. Green Chem.17:2764–2768.
123. Zhang, J., J. Yu, Y. Zhang, Q. Li, and J. R. Gong. 2011. Visible light photocatalytic H₂-production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer. Nano Lett.11:4774–4779.
124. Zhang, Y., N. Zhang, Z.-R. Tang, and Y.-J. Xu. 2012. Graphene transforms wide band Gap ZnS to a visible light photocatalyst. The new role of graphene as a macromolecular photosensitizer. ACS Nano6:9777–9789.
125. Reber, J. F., and K. Meier. 1984. Photochemical production of hydrogen with zinc sulfide suspensions. J. Phys. Chem.88:5903–5913.
126. Zhang, K., and L. Guo. 2013. Metal sulphide semiconductors for photocatalytic hydrogen production. Catal. Sci. Technol.3:1672–1690.
127. Thurston, T. R., and J. P. Wilcoxon. 1998. Photooxidation of organic chemicals catalyzed by nanoscale MoS₂. J. Phys. Chem. B103:11–17.
128. Liao, J., B. Sa, J. Zhou, R. Ahuja, and Z. Sun. 2014. Design of high-efficiency visible-light photocatalysts for water splitting: MoS₂/Aln(Gan) heterostructures. J. Phys. Chem. C118:17594–17599.
129. Jia, T., A. Kolpin, C. Ma, R. C.-T. Chan, W.-M. Kwok, and S. C. E. Tsang. 2014. A graphene dispersed Cds-MoS₂ nanocrystal ensemble for cooperative photocatalytic hydrogen production from water. Chem. Commun.50:1185–1188.