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:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): H_2O\rightarrow H_2+\frac{1}{2}O_2\quad \Delta G_{298K}^0=

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(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 (Eg) 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): Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): {\textstyle H_2O\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 CrxRh2-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.


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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://test.scipedia.com:8081/localhost/v1/":): {\textstyle d_z^2} , and sulfur pz 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 pz-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.


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.


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.


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.


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.


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.


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.


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.


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.


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 (Eg = 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.


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.


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.


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.


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.

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