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Tungsten

, Volume 1, Issue 1, pp 19–45 | Cite as

Recent progress of tungsten- and molybdenum-based semiconductor materials for solar-hydrogen production

  • Songcan Wang
  • Lianzhou WangEmail author
Review Paper

Abstract

Semiconductor-based solar water splitting is regarded as one of the most promising technologies for clean hydrogen production. The rational design of semiconductor materials is critically important to achieve high solar-to-hydrogen (STH) conversion efficiencies towards practical applications. A rich family of tungsten- and molybdenum-based materials have been developed as both photocatalysts and cocatalysts for solar-hydrogen production in the past years, providing more opportunities to achieve high solar-to-hydrogen (STH) efficiencies. In this review article, we comprehensively review the recent progress of tungsten- and molybdenum-based materials for solar-hydrogen production. In particular, the strengths and drawbacks of each material system are critically discussed, followed by an overview of the emerging strategies to improve their performances. Finally, the key challenges and possible research directions of tungsten- and molybdenum-based materials are presented, which would provide useful information for the design of efficient semiconductor materials for solar-hydrogen production.

Keywords

Semiconductor Tungsten Molybdenum Photocatalyst Solar energy conversion Water splitting Hydrogen production 

1 Introduction

Currently, the global energy consumption is dominated by fossil fuels such as petroleum, natural gas, and coal. The consumption of non-renewable fossil fuels is generally accompanied by the significant emissions of CO2, which would inevitably cause an energy crisis and environmental issues. Therefore, the exploration of alternative clean and renewable energy sources is urgently needed. The photocatalytic/photoelectrochemical (PEC) process of the semiconductor that converts earth-abundant solar energy into fuels such as hydrogen by water splitting is regarded as a promising technology to address the energy and environmental issues in the future sustainable society [1, 2, 3, 4]. Since the first demonstration of PEC water splitting in 1972 [5], intensive efforts have been devoted to enhancing the STH conversion efficiency in a cost-effective manner, and a number of semiconductor photocatalysts such as TiO2 [6, 7, 8, 9], WO3 [10, 11, 12, 13], Fe2O3 [14, 15, 16, 17], Cu2O [18, 19, 20], BiVO4 [21, 22, 23, 24], Ta3N5 [25, 26, 27, 28, 29], and g-C3N4 [30, 31, 32, 33, 34, 35] have been developed. However, the solar fuel production technology is still far from maturity, mainly due to the lack of proper materials.

Tungsten and molybdenum generally form d0 metal cations (W6+, Mo6+) in compounds that comprise a large family of the earth-abundant photocatalyst and cocatalyst materials for solar fuel production, and these compounds have attracted considerable attention in recent years. For example, binary oxides such as WO3 and MoO3 are photocatalytic materials for the oxygen evolution reaction (OER), while their chalcogenides such as WS2 and MoS2 are excellent cocatalysts for the hydrogen evolution reaction (HER) [36, 37, 38, 39]. By incorporating another metal cation into the binary oxides, ternary tungsten- and molybdenum-based materials such as CuWO4 [40], Bi2WO6 [41], and Bi2MoO6 [42], with various optoelectronic properties can be achieved. Moreover, other more complicated quaternary semiconductors such as RbNbWO6, RbTaWO6, CsNbWO6, and CsTaWO6 can also be generated [43], providing more opportunities for the rational design of materials for efficient solar fuel production.

Although a number of excellent review articles focusing on solar fuel production are published in recent years, only a few of them partially mentioned some types of tungsten- and molybdenum-based materials [44, 45, 46, 47, 48, 49]. A comprehensive review fully covering this material family still lacks. In this review article, we aim to critically discuss the emerging tungsten- and molybdenum-based photocatalysts and cocatalyst materials for solar-hydrogen production including photocatalytic and PEC water splitting. Other photocatalytic applications such as CO2 reduction, environmental remediation, and value-added chemical production are beyond this scope and will not be discussed. Some recent review articles for more information are provided in Refs. [50, 51, 52, 53]. The emerging tungsten- and molybdenum-based oxides and chalcogenides are discussed with critical analyses on the strengths and drawbacks of each material. In addition, the strategies to enhance the solar-hydrogen production performances of the tungsten- and molybdenum-based semiconductors are reviewed. Finally, the main challenges and possible research directions for the tungsten- and molybdenum-based semiconductors are presented. We hope to provide useful information to accelerate the research progress of the efficient tungsten- and molybdenum-based materials for solar fuel production.

2 Principle requirements of materials for solar-hydrogen production

Since light absorption is the first step for solar fuel production, the semiconductor photocatalyst must possess a small bandgap to absorb a wide spectrum range of sunlight to achieve a high theoretical STH efficiency. It's aimed at driving the spontaneous water splitting reactions in a single semiconductor. However, the band-gap minimum is confined by both thermodynamic and kinetic limitations [54]. The Gibbs free energy required for water splitting is 237.13 kJ mol−1, corresponding to a minimum potential of 1.23 V at 298 K [3]. Taking the overpotential and energy loss into account, the optimal bandgap of a semiconductor should be ~ 2.0 eV, which can absorb the sunlight spectrum up to ~ 620 nm [54]. To achieve the efficient charge separation and transfer, the semiconductor should have a high charge mobility and a long charge carrier diffusion length. In addition, the crystallinity, morphology, and doping concentration are important factors in affecting the charge separation and transfer processes. To drive the final step of the surface redox reactions, the conduction band (CB) and valence band (VB) of a semiconductor should straddle the water splitting redox reaction potentials, which is thermodynamically required (Fig. 1).
Fig. 1

Schematic of the thermodynamic requirement of a photocatalyst for water splitting

It should be emphasized that the reaction kinetics are also critically important for the final step. For example, the water oxidation is a four-electron process and generally requires a high overpotential, which is the bottleneck for water splitting [55]. Therefore, decorating the semiconductor surfaces with cocatalysts to accelerate the reaction kinetics is indispensable. Since cocatalysts not only function as the active sites for the targeted redox reactions but also form heterojunctions with the photocatalyst to extract the photogenerated charge carriers from the photocatalyst, the band alignment between the semiconductor photocatalyst and the cocatalyst is important for the charge transfer and separation. In addition, cocatalysts can also suppress the photocorrosion and increase the stability of the semiconductor photocatalysts [56].

Therefore, the rational design of photocatalyst and cocatalyst materials to achieve the efficient light harvesting, charge separation and transfer, and surface reactions is critically important to obtain high STH efficiencies for solar water splitting.

3 Emerging tungsten- and molybdenum-based semiconductors

The band structures of the emerging tungsten- and molybdenum-based semiconductors for solar-hydrogen production are shown in Fig. 2. According to the anions in the compounds, tungsten- and molybdenum-based materials can be divided into two main categories: oxides and chalcogenides. In this section, we will review the recent progress of tungsten- and molybdenum-based oxides and chalcogenides as photocatalysts and cocatalysts for solar water splitting. In particular, the challenges of each material for solar water splitting and the emerging strategies to enhance the performances will be critically discussed.
Fig. 2

Band structures of the emerging tungsten- and molybdenum-based semiconductors for solar-hydrogen production

3.1 Oxides

Oxides are a category of important semiconductor materials for solar water splitting, due to their low cost and high stability in the reaction conditions. In this subsection, we will focus on the main emerging tungsten- and molybdenum-based oxides for solar-hydrogen production.

3.1.1 Binary oxides

WO3 WO3 is composed of perovskite units with a bandgap of 2.5–2.8 eV, which can absorb the solar spectrum range up to ~ 500 nm [36]. The hole diffusion length of WO3 (~ 150 nm) is two orders of magnitude longer than that of α-Fe2O3 (2–4 nm), while the electron mobility of WO3 (~ 12 cm2 V−1 s−1) is 40 times higher than that of TiO2 (0.3 cm2 V−1 s −1) [57, 58], indicating that the charge separation and transfer properties in WO3 are good. In addition, WO3 is stable in aqueous solutions with a pH < 4 [59]. However, the CB of WO3 is ~ 0.4 eV vs. the reversible hydrogen electrode (RHE), which is too positive for HER. Interestingly, single-crystal WO3 nanosheets with a thickness of 4–5 nm exhibited a negative shift of the CB due to the size-quantization effects, which can even photocatalytically reduce CO2 to CH4 in the presence of water [60]. Likewise, the crystal facet engineering of WO3 crystals with the predominant {002} facet can also shift the CB, enabling the photocatalytic reduction of CO2 [61]. Thus, the proper CB position of WO3 may be achievable by carefully tuning the nanocrystal size and exposed facets.

In the presence of redox couples (e.g., IO3/I, I3/I, and Fe3+/Fe2+) or solid-state electron mediators (e.g., Au and Ag), WO3 is generally used as an oxygen evolution photocatalyst (OEP) to couple with another hydrogen evolution photocatalyst (HEP) such as SrTiO3, TaON, and CdS to form Z-scheme systems for overall water splitting [44, 62]. For example, using IO 3 /I redox couples as an electron mediator, a Z-scheme system composed of Pt/ZrO2/TaON as the HEP and Pt/WO3 as the OEP was constructed, which could achieve overall water splitting under visible light illumination [63]. In addition, a direct Z-scheme system for overall water splitting can be formed using WO3 as the OEP and Ru/SrTiO3:Rh as the HEP without an electron mediator [64]. Interestingly, overall water splitting can also be achieved using WO3-based photocatalysts without any HEPs. For example, in the presence of the Fe3+/Fe2+ redox couples, H2 and O2 gas evolution in a stoichiometric ratio of 2:1 was achieved using the RuO2-WO3 photocatalysts under the illumination of UV light and visible light (λ < 460 nm) [65]. A two-step photoexcitation reaction mechanism similar to the Z-scheme reaction was proposed. In the first step, holes generated in the RuO2-WO3 photocatalysts oxidized water for O2 evolution, while the photogenerated electrons reduced Fe3+ ions to Fe2+ ions. In the second step, the Fe2+ ions were oxidized back to Fe3+ ions by UV light, which was accompanied by the H2 evolution. The recent progress of Z-scheme systems using WO3 as the OEP is summarized in Table 1.
Table 1

Recent progress of WO3 based Z-scheme for overall water splitting

OEP

HEP

Mediators

Efficiency or activity

References

PtOx/WO3

Pt/NiS/La5Ti2AgS5O7

I/I3

AQY = 0.12% (420 nm)

[66]

WO3·H2O

g-C3N4

None

AQY = 6.2% (420 nm)

[67]

PtOx/WO3

Ru/SrTiO3:Rh

[SiW11MoVIO40]4−/[SiW11MoVO40]5−

H2: 4.4 µmol h−1; O2: 2.3 µmol h−1; 300 W Xe lamp (400 < λ < 800 nm)

[68]

PtOx/WO3

Pt/SrTiO3:(La,Cr)

I/IO3

H2: 10.4 µmol h−1; O2: 4.9 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[69]

RuO2·H2O/H2WO4

Pt/SrTiO3:Rh

I/IO3

H2: 2.2 µmol h−1; O2: 0.9 µmol h−1; 300 W Xe lamp (λ > 400 nm)

[70]

Fe-H-Cs-WO3

Ru/SrTiO3:Rh

VO2+/VO2+

STH = 0.06%

[71]

PtOx/WO3

Pt/Ta3N5/BaTaO2N

I/IO3

H2: 2.7 µmol h−1; O2: 1.3 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[72]

PtOx/H-Cs-WO3

Pt/Sm2Ti2S2O5

I/I3

H2: 4.1 µmol h−1; O2: 1.6 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[73]

Pt/WO3

Pt/SrTiO3:(Cr,Ta)

I/IO3

AQY = 1.52% (420 nm)

[74]

PtOx/WO3

Ru/SrTiO3:Rh

[SiW11O39MnII(H2O)]6−/[SiW11O39MnIII(H2O)]5−

H2: 3.3 µmol h−1; O2: 1.0 µmol h−1; 300 W Xe lamp (400 < λ < 800 nm)

[75]

PtOx/Ni(OH)2/WO3

Pt/TiO2/C3N4

I/IO3

AQY = 4.01% (405 nm)

[76]

H2WO4

Ru/SrTiO3:Rh

Fe3+/Fe2+

H2: 4.5 µmol h−1; O2: 2.1 µmol h−1; 300 W Xe lamp (λ > 400 nm)

[77]

WO3

Pt/SiC

I/IO3

AQY = 0.021% (420 nm)

[78]

WO3

Pt/C3N4

rGO

AQY = 0.9% (420 nm)

[79]

Pt-IrO2/WO3

Dye-adsorbed Pt/H4Nb6O17

I/I3

H2: 2.2 µmol h−1; O2: 0.9 µmol h−1; 300 W Xe lamp (410 < λ < 800 nm)

[80]

PtOx/WO3

Pt/BaZrO3-BaTaO2

I/IO3

H2: 22.5 µmol h−1; O2: 9.3 µmol h−1; 300 W Xe lamp (350 < λ < 800 nm)

[81]

WO3

Ru/SrTiO3:Rh

[Co(bpy)3]3+/2+/[Co(phen)3]3+/2+

H2: 14/15 µmol h−1; O2: 0.5/0.4 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[82]

PtOx/H-Cs-WO3

Pt/SrTiO3:(Cr,Ta)

I/IO3

H2: 32 µmol h−1; O2: 16 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[83]

Pt/WO3

Pt/ZrO2/TaON

I/IO3

AQY = 6.3% (420.5 nm)

[63]

WO3

Pt/SrTiO3:Rh

Fe3+/Fe2+

H2: 2.2 µmol h−1; O2: 0.8 µmol h−1; 500 W halogen lamp (400 < λ < 1100 nm)

[84]

WO3

Ru/SrTiO3:Rh

None

H2: 5.7 µmol h−1; O2: 2.4 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[64]

Pt/WO3

Pt/ZrO2/TaON

I/IO3

H2: 4.3 µmol h−1; O2: 1.5 µmol h−1; 300 W Xe lamp (λ > 420 nm)

[85]

Pt/WO3

Pt/SrTiO3:(Cr,Ta)

I/IO3

AQY = 0.1% (420.7 nm)

[86]

AQY apparent quantum yield

In addition to the Z-scheme systems, WO3 was intensively investigated as the photoanodes for PEC water splitting. WO3 was first reported as a visible light responsive photoanode for PEC water splitting in 1976 [87]. The WO3 photoanodes were synthesized by either thermally treating the tungsten metal to form a WO3 layer, or by spraying ammonium tungstate on a glass substrate coated with a gold layer and heating at 500 °C. It was found that WO3 photoanodes converted from tungsten metal could generate higher photocurrents, possibly due to the better interfacial contact for the charge transfer. However, the long-term stability of WO3 photoanodes was not investigated in this work. In fact, WO3 photoanodes suffer from a gradual loss of activity during long-term light illumination, which was attributed to the formation and accumulation of the peroxo species at the WO3 surfaces [88]. By coupling a WO3 photoanode with a thick Co-Pi oxygen evolution cocatalyst (OEC) layer (Fig. 3a, b), the photocurrent to the O2 conversion efficiency was increased from ~ 61% to ~ 100%, leading to almost no photocurrent decay at 0.8 V vs. Ag/AgCl under consecutive air mass (AM) 1.5 global (G) illumination for 12 h (Fig. 3c) [59]. Nevertheless, the photocurrent density of the Co-Pi/WO3 photoanode is lower than that of its bare WO3 counterpart, indicating that there is room to further modify the OEC layer. By carefully tuning the photo-assisted electrodeposition time to deposit a FeOOH OEC layer on the surface of a WO3 photoanode, both the enhanced activity and stability can be achieved (Fig. 3d) [89]. In addition, the stoichiometric H2 and O2 evolution was observed with a Faraday efficiency of ~ 95% (Fig. 3e). Further analysis revealed that the p-type FeOOH not only acted as an OEC to accelerate the surface kinetics but also formed a p-n junction with the n-type WO3, leading to the increased charge separation and transfer. Similarly, other OECs such as CoOx [90], NiFe layer double hydroxide (LDH) [91], and Mn-based catalysts [92] can also enhance the PEC performances of the WO3 photoanodes. Interestingly, HfO2 coated on the WO3 surfaces as a passivation layer can also reduce the recombination of the photogenerated electron-hole pairs, resulting in better PEC performances [93, 94].
Fig. 3

SEM images of a cross-sectional view and b top view of a WO3/Co-Pi photoanode; cI-t curves of I: a bare WO3 photoanode and II: a WO3/Co-Pi photoanode. ac Reproduced with permission from Ref. [59] Copyright 2011 American Chemical Society (ACS); dI-t curves of bare WO3 and WO3/FeOOH photoanodes at 1.23 V vs. RHE; e the corresponding H2 and O2 evolution. The y axes of Fig. 3c, d refer to the photocurrent density. Reproduced with permission from Ref. [89] Copyright 2016 Elsevier

Proper nanostructures can shorten the migration distance of the photogenerated electron-hole pairs, and provide more active sites for the surface reactions, which are effective to reduce the charge recombination. For example, the WO3 nanoflakes grown on fluorine-doped tin-oxide (FTO) substrates showed enhanced photocurrent densities than that of the WO3 nanowire arrays (Fig. 4a–c) [95]. Etching the WO3 nanostructures can increase the surface reaction sites, leading to enhanced PEC performances [58, 96]. In particular, a dual etching and reducing process could increase the surface roughness and create oxygen vacancies, resulting in an enhanced photocurrent density of ~ 1.10 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination (Fig. 4d–f). Similarly, other WO3 nanostructures such as porous nanorod arrays [97] and hexagonal nanoflowers [98] also exhibited an enhanced PEC performance. It should be mentioned that the growth of WO3 nanostructures on conductive substrates generally requires the deposition of a WO3 nanoparticle layer serving as a seed layer. Nevertheless, the grain boundaries in the seed layer generally cause charge recombination [99]. To avoid energy loss in the seed layer, a seed-free process was developed to directly growing WO3 nanostructures such as nanoplate arrays [100], nanorod arrays [101], and nanoneedles [102] on the conductive substrates.
Fig. 4

a SEM images of WO3 nanowires, inset: cross-sectional view; b SEM images of WO3 nanoflakes, inset: cross-sectional view (upper right corner) and the thickness of a nanosheet (lower left corner); cI-V curves for annealed WO3 nanowires, and two flake samples (NF1 and NF2). ac Reproduced with permission from Ref. [95] Copyright 2011 ACS; SEM images of d WO3 nanoflakes and e dual-etched/reduced WO3 nanoflakes; fI-V curves for pristine, etched, reduced and dual-etched/reduced WO3 nanoflake photoanodes. df Reproduced with permission from Ref. [58] Copyright 2014 ACS; g SEM images of WO3 with the modified (002) facet, inset: cross-sectional view; hI-t curves of WO3 photoanodes enriched with (200), (002), and modified (002) facets at 1.23 V vs. RHE under AM 1.5 G illumination; i DFT calculations of free energy change during water oxidation on (200) and (002) facets of WO3. The y axes of Fig. 4c, f, h refer to the photocurrent density. gi Reproduced with permission from Ref. [11] Copyright 2016 Elsevier

Interestingly, the exposure of different crystal facets in the nanostructure also plays a pivotal role in affecting the PEC performance of the WO3 photoanodes. For example, a WO3 photoanode with highly exposed reactive facets of {002} exhibited enhanced photocurrent densities [103, 104, 105, 106, 107, 108]. In particular, the WO3 nanoplate arrays with dominant {002} facets could achieve a high photocurrent density close to its theoretical value under AM 1.5 G illumination [11]. By tuning the hydrothermal conditions, the WO3 nanoplate arrays with the same film thickness but different enriched facets of {002} and {200} were prepared, respectively. It revealed that the photocurrent density of a WO3 photoanode with enriched {002} facet is around 1.5 times that of its counterpart with enriched {200} facets. In addition, a WO3 photoanode with enriched {002} facets prepared by an optimized hydrothermal process exhibited an excellent photocurrent density of 3.7 mA cm−2 at 1.23 V vs. RHE without any OECs or sacrificial agent (Fig. 4g, h). The enhanced photocurrent densities and stability achieved in the WO3 film with enriched {002} facets are attributed to the exposure of highly reactive {002} facets and the suppressed formation of the hydroxyl groups on the {002} facets, as evidenced by the density functional theory (DFT) calculations (Fig. 4i).

Doping WO3 with other anions or cations can change the optoelectronic properties [109]. For example, incorporating N2 into the WO3 matrix causes a significant reduction of the bandgap from 2.6 to 1.9 eV (Fig. 5a), which is attributed to the deformation of the WO3 host lattice and the weak electronic interactions between the trapped N2 and the WO3 matrix [110]. On the other hand, Ti-doped WO3 photoanodes showed a better charge separation efficiency and a positive shift of the flat-band potential [111]. Likewise, the PEC performance of the WO3 photoanodes could also be improved by Gd or Sm doping [112, 113]. Interestingly, hydrogen-treated WO3 photoanodes at elevated temperatures could induce the formation of oxygen vacancies as the self-dopants, resulting in an order of magnitude of enhanced photocurrents compared to their pristine counterparts (Fig. 5b) [114]. Impressively, a hydrogen-treated WO3 photoanode without any OCEs also exhibited an excellent stability for PEC water splitting without obvious decay of the photocurrent for 7 h (Fig. 5c). In addition, the oxygen vacancies generated by electrochemical reduction could also enhance the PEC performance of the WO3 photoanodes [115]. By introducing dual oxygen and tungsten vacancies on the surfaces of the WO3 photoanodes, the charge transfer efficiency as well as the charge carrier density was improved, leading to the significant enhanced photocurrent densities and a remarkable cathodic shift of the onset potential (Fig. 5d–f) [116]. The creation of oxygen vacancies becomes an important strategy to improve the PEC performance of photoanodes, which has attracted increasing attention in recent years [16, 21, 22, 23, 117, 118].
Fig. 5

a Diffuse reflectance spectra of i: pure WO3; ii: 0.034N2‧WO3 annealed at 750 °C; iii: 0.039N2‧WO3 annealed at 550 °C and iv: 0.039N2‧WO3 annealed at 420 °C. Reproduced with permission from Ref. [110] Copyright 2012 ACS; bI-V curves of pristine WO3 and hydrogen-treated WO3 at different temperatures; cI-t curves of pristine WO3 and hydrogen-treated WO3 at 350 °C. b, c Reproduced with permission from Ref. [114] Copyright 2012 The Royal Society of Chemistry(RSC); HRTEM images of d pristine WO3 and e dual vacancy WO3; fI-V curves of pristine (WO3 0 s), dual vacancy (WO3 20 s), and reannealed dual vacancy (WO3 20 s reannealed) WO3 photoanodes. The y axes of Fig. 5b, c, f refer to the photocurrent density. df Reproduced with permission from Ref. [116] Wiley-VCH

Owing to the relatively low CB position and the excellent electron mobility, WO3 could be coupled with many other materials to form type-II or p-n heterojunctions with enhanced light absorption and charge separation efficiencies [119]. A typical example is the WO3/BiVO4 heterojunction, which was intensively studied for both PEC water splitting and CO2 reduction [120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131]. The film thickness and nanostructures of the WO3 are critically important to achieve a PEC performance. For example, a WO3/BiVO4/CoPi core-shell nanostructured photoanode achieved a world record photocurrent density of 6.72 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination (Fig. 6a, b) [132]. The delicate design on the WO3/ITO interface and the WO3 nanorod array structure, and the photoanode surface modification with Co-Pi OECs lead to the excellent PEC performance. In addition to the nanostructure, the exposure of crystal facets of WO3 also affected the PEC performance of the WO3/BiVO4 heterojunction photoanodes, as evidenced by a recent report that a {002}-facet enriched WO3/BiVO4 film exhibited a higher hole injection efficiency than its counterparts with other exposed facets [133].
Fig. 6

a SEM image of a WO3-NRs/BiVO4 core-shell photoanode; bI-V curves of a WO3/BiVO4/CoPi core-shell photoanode under the illumination of different light intensities. a, b Reproduced with permission from Ref. [132] Copyright 2015 Nature Publishing Group. Distributed under a Creative Commons Attribution (CC-BY) 4.0 license; Schematics of the charge transfer and separation in a WO3/BiVO4 heterojunction with c only BiVO4 photoexcited and d both WO3 and BiVO4 photoexcited. The y axis of Fig. 6b refers to the photocurrent density. c, d Reproduced with permission from Ref. [135] Copyright 2018 ACS

It is generally accepted that the photogenerated electrons in the CB of BiVO4 are transferred to the CB of WO3, leaving the photogenerated holes in the VB of BiVO4. As a result, the photogenerated electron-hole pairs in the WO3/BiVO4 heterojunction can be effectively separated, leading to the enhanced PEC performance. However, the detailed charge transfer mechanisms between WO3 and BiVO4 under light illumination were not well understood for years. Until recently, using the ultrafast transient absorption spectroscopy and transient absorption (TA) mid-infrared (mid-IR) spectroscopy [134, 135], it was found that if only the BiVO4 is photoexcited, electrons can be transferred to the WO3 and the charge recombination is eliminated (Fig. 6c). However, if both WO3 and BiVO4 are photoexcited, a recombination channel between the electrons in the CB of WO3 and the holes in the VB of BiVO4 occurs (Fig. 6d). Thus, to achieve efficient charge separation in the WO3/BiVO4 heterojunction, it is necessary to avoid the photoexcitation of WO3.

WO3/C3N4 is another efficient heterojunction photoanode for PEC water splitting. For example, loading C3N4 nanosheets on the branched WO3 nanosheet arrays to form a heterojunction photoanode exhibited the enhanced PEC performance [136]. Moreover, modifying the surfaces with a CoOx OEC can further enhance the surface kinetics, leading to an excellent photocurrent density of 5.76 mA cm−2 at 2.1 V vs. RHE under AM 1.5 G illumination. In addition, other WO3-based heterojunction photoanodes such as WO3/Sb2S3 [137], WO3/Bi2S3 [138], WO3/α-Fe2O3 [139], WO3/Cu2O [140], WO3/ZnxBi2S3+x [141], WO3/carbon quantum dots (CQDs) [142], and WO3/Ag nanoparticle (AgNP) [143] also show enhanced PEC performances.

MoO3 MoO3 is an n-type semiconductor with a bandgap of 3.1–3.4 eV [37, 144, 145]. Owing to the wide bandgap, the investigation of MoO3 for solar-hydrogen production is much less compared to that of WO3. MoO3 is generally coupled with other narrow band-gap semiconductor materials to form heterojunctions, promoting the charge separation. For example, MoO3-CdS core-shell nanospheres exhibited a significant enhanced photocatalytic activity for hydrogen production under visible light illumination (λ > 400 nm) [146]. In particular, the optimized MoO3-CdS core-shell nanospheres exhibited a hydrogen production rate of 5.25 mmol h−1 g−1 and an AQY of 28.86% at 420 nm without any cocatalysts. A WO3/MoO3 heterojunction photoelectrode exhibited a significant improvement of PEC performances than those of pure WO3 and MoO3 photoelectrodes for water splitting and CO2 reduction [147]. Similar to WO3, MoO3 can also be coupled with BiVO4 to form heterojunction photoelectrodes for PEC water splitting. By depositing MoO3 nanoparticles on the BiVO4 nanoflake arrays, the obtained MoO3/BiVO4 heterojunction photoanode exhibited a photocurrent density sixfold than that of the bare BiVO4 photoanode [148]. Owing to the difference of band-edge positions and conductivity between MoO3 and BiVO4, the photogenerated electrons and holes can be physically separated and transferred through the MoO3/BiVO4 heterojunction, leading to significantly enhanced photocurrent densities. Post-treating the MoO3/BiVO4 heterojunction photoanode in an Ar atmosphere could generate oxygen vacancies, resulting in a higher photocurrent density of 4.1 mA cm−2 at 1.1 V vs. saturated calomel electrode (SCE) under AM 1.5 G illumination [149]. The presence of oxygen vacancies can accelerate the interfacial charge transfer and separation, leading to a high activity as well as an excellent stability for PEC water splitting. The recent progress of MoO3-based photocatalysts for solar-hydrogen production is summarized in Table 2.
Table 2

Recent progress of MoO3 based photocatalysts for solar-hydrogen production

MoO3-based photocatalysts

Electrolyte

Light source

Efficiency or activity

References

MoO3@Ni2P

Eosin (20 mg) + 30 mL of triethanolamine aqueous solution (15%, pH = 10)

5 W LED white light

H2: 37.7 µmol h−1 (10 mg of photocatalyst)

[150]

MoO3/BiVO4 photoanode

0.1 mol L−1 Na2SO4

AM 1.5 G

4.1 mA cm−2 (bias: 1.1 V vs. SCE)

[149]

1T’-MoS2/MoO3

Deionized water (90 mL) + triethanolamine (10 mL) + eosin (100 mg)

300 W Xe lamp (λ > 420 nm)

H2: 88.4 µmol h−1 (4 mg of photocatalyst)

[151]

MoS2-MoO3-x/AgIn5S8

10 mL of aqueous solution containing ascorbic acid

300 W Xe lamp (λ > 420 nm)

H2: 117 µmol h−1 (10 mg of photocatalyst)

[152]

MoOx/Pt/SrTiO3

100 mL of Milli-Q water

300 W Xe lamp (300 < λ < 800 nm)

H2: 17.1 µmol h−1; O2: 8.8 µmol h−1 (50 mg of photocatalyst)

[153]

MoO3/BiVO4 photoanode

0.1 mol L−1 Na2SO4

AM 1.5 G

0.54 mA cm−2 (bias: 0.8 V vs. SCE)

[148]

MoS2-MoO3/CdS photoanode

Na2SO3 (0.01 mol L−1) + Na2S (0.1 mol L−1)

AM 1.5 G

1.6 mA cm−2 (bias: 0.2 V vs. SCE)

[154]

TiO2 nanotube @ MoO3/Au photoanode

1 mol L−1 NaOH

532 nm laser

2 µA cm−2 (bias: 0.8 V)

[155]

Bi2MoO6/MoO3 photoanode

0.1 mol L−1 Na2SO4

300 W Xe lamp (UV)

2.75 mA cm−2 (bias: 0.4 V vs. Ag/AgCl)

[145]

MoO3/conjugated polymer

Water (360 mL) + methanol (40 mL)

300 W Xe lamp (λ > 420 nm)

H2: 4.1 µmol h−1 (20 mg of photocatalyst)

[156]

MoO3-x nanoparticles

H2O (5 mL) + NH3BH3 (20 µmol)

Visible light irradiation (λ > 420 nm)

H2: 68 µmol h−1 (20 mg of photocatalyst)

[157]

Pt/MoO3/TiO2

300 mL of methanol solution (20 vol. %)

75 W Xe lamp (UV)

H2: 16.9 µmol h−1 (100 mg of photocatalyst)

[158]

MoO3-CdS core-shell structure

Aqueous solution (400 mL) + Na2SO3 (1.5 mol L−1) + Na2S (0.2 mol L−1)

300 W Xe lamp (λ > 400 nm)

H2: 1050 µmol h−1 (200 mg of photocatalyst)

[146]

Although MoO3 doesn’t attract too much attention for solar water splitting, the very high carrier mobility (1100 cm2 V −1 s −1) of the two-dimensional (2D) MoO3 with oxygen vacancies indicates excellent charge separation properties in the bulk MoO3 [159]. The main limitation of MoO3 for solar water splitting is the relatively large bandgap that can only absorb UV light. Future research on MoO3 should be focused on reducing the bandgap while keeping the excellent carrier mobility.

3.1.2 Tungsten-based ternary oxides

CuWO4 CuWO4 has a narrow bandgap of 2.3 eV, corresponding to a light absorption edge of ~ 550 nm [40]. The VB of CuWO4 is composed of Cu 3d and O 2p orbitals that cause an upward shift, whereas the CB is similar to that of WO3 [160]. Compared to WO3 that is only stable in acidic solutions, CuWO4 shows an excellent stability in aqueous solutions with a pH range of 7–9 [161, 162, 163]. However, the obtained PEC performances of the CuWO4 photoanodes are still far from their theoretical values considering the bandgap of 2.3 eV, and the major limitation is attributed to the low charge carrier mobility (~ 0.006 cm2 V−1 s−1) and short charge diffusion length (~ 30 nm) that cause poor charge separation in the bulk CuWO4 [164, 165]. In addition, the surface states also affect the PEC water oxidation performance of the CuWO4 photoanodes [166, 167].

Doping is a commonly used strategy to change the optoelectronic properties of semiconductors to improve the charge separation efficiency. A successful example is the doping of Fe into CuWO4 photoanodes, which can increase the photocurrent density by 1.5-fold and the charge separation efficiency by 50% at 1.23 V vs. RHE under AM 1.5 G illumination (Fig. 7a) [164]. In addition, the incorporation of Mo atoms into CuWO4 to form the CuW1−xMoxO4 solid solution photoanodes can enhance the carrier concentration as well as the photon utilization, resulting in over 6.5-fold increase of the photocurrent density at 1.23 V vs. RHE under AM 1.5 G illumination (Fig. 7b, c) [168]. Interestingly, a solid solution of CuW0.35Mo0.65O4 exhibited a significantly reduced bandgap of 2.0 eV (Fig. 7d, e), which is promising for PEC water splitting [169]. Alternatively, the generation of oxygen vacancies can also improve the PEC performance of the CuWO4 photoanodes. For example, thermal treating the CuWO4 photoanodes in a nitrogen atmosphere increased the concentration of oxygen vacancies, leading to the enhanced charge carrier separation [170]. Similarly, oxygen vacancies could also be generated by annealing the CuWO4 photoanodes in a hydrogen atmosphere [171]. In particular, the optimized H-treated CuWO4 photoanode exhibited a photocurrent density of 0.75 mA cm−2 at 1.0 V vs. Ag/AgCl, which was threefold higher than that of its untreated counterpart.
Fig. 7

aI-V curves of pristine and Fe-doped CuWO4 photoanodes. Reproduced with permission from Ref. [164] Copyright 2015 the Physical Chemistry Chemical Physics (PCCP) Owner Societies; b SEM image of CuW0.5Mo0.5O4; cI-V curves of pristine and Mo-doped CuWO4 photoanodes. b, c Reproduced with permission from Ref. [168] Copyright 2018 Springer Nature; d SEM image of CuW0.35Mo0.65O4; e diffuse reflectance spectra of i: pure CuWO4; ii: CuW0.55Mo0.45O4; iii: CuW0.35Mo0.65O4; iv: a mixture of CuW1−xMoxO4, CuMoO4, and Cu3Mo2O9; and v: a mixture of CuMoO4 and Cu3Mo2O9. The y axes of Fig. 7a, c refer to the photocurrent density. de Reproduced with permission from Ref. [169] Copyright 2013 RSC

On the other hand, proper nanostructures can reduce the migration distance of the photogenerated charge carriers and thus enhance the charge separation efficiency. For example, the CuWO4 porous films were shown to be promising photoanodes for PEC water splitting [172]. Using WO3 nanoflake arrays as the templates, the CuWO4 nanoflake arrays were prepared, which exhibited a photocurrent density of ~ 0.4 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination [173]. It was believed that the advantageous microstructures promote the charge separation, resulting in increased incident-photon-to-current-efficiencies (IPCEs) and photocurrent densities which are twice higher than that of the conventional CuWO4 polycrystalline films. In addition, the hydrogen treatment on the CuWO4 nanoflake array films generated oxygen vacancies to increase the electron density, leading to both the high photocurrent density and stability in a weak alkaline borate buffer solution (pH = 9) [174].

Constructing heterojunctions is another way to improve the PEC performance. For example, CuWO4 can be deposited on the surfaces of WO3 to form type-II heterojunctions [175, 176, 177]. In particular, in situ formation of CuWO4/WO3 heterojunction films (Fig. 8a) was achieved by dipping WO3 films in an ethanol solution containing 50 mmol L−1 of Cu(CH3COO)2, followed by a thermal treatment at 500 °C [178]. As shown in Fig. 8b, the CuWO4/WO3 heterojunction photoanode exhibits a photocurrent density of 1.21 mA cm−2 at 1.5 V vs. Ag/AgCl, which is much higher than that of most reports using CuWO4 as a single light absorber [160, 161, 174]. It should be mentioned that the PEC performances in CuWO4-based heterojunctions may be mainly contributed from the other coupled semiconductors rather than CuWO4 itself. Likewise, other CuWO4-based heterojunction photoanodes such as CuWO4/Ag2NCN [179], CuWO4/BiVO4 [180], CuWO4/CuO [181], and CuWO4/CdS [182] also show enhanced PEC performances. In addition to semiconductors, coupling CuWO4 with plasmonic metal nanoparticles such as Au and Ag can also improve the PEC performance due to the surface catalytic effect and plasmonic effect [183, 184]. So far, the best photocurrent density of 1.5 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination for CuWO4 photoanodes was achieved by embedding Ag nanowires in the CuWO4 matrix (Fig. 8c, d) [184]. The dual functions of Ag to increase the carrier mobility of the CuWO4 photoanode and act as a cocatalyst are attributed to the significantly enhanced PEC performance.
Fig. 8

a SEM image of CuWO4/WO3 heterojunctions; bI-V curves of WO3 and CuWO4/WO3 heterojunction photoanodes. a, b Reproduced with permission from Ref. [178] Copyright 2015 Elsevier; c SEM image of Ag nanowires embedded CuWO4; dI-V curves of CuWO4 photoanodes embedded with different percentages of Ag nanowires. The y axes of Fig. 8b, d refer to the photocurrent density. c, d Reproduced with permission from Ref. [184] Copyright 2015 RSC

It is clear that even though the bandgap of CuWO4 is smaller than that of WO3, the best photocurrent density achieved for the CuWO4 photoanodes is still much lower than that of WO3. Thus, a more fundamental understanding in the PEC behaviour of CuWO4 is still required and other more efficient strategies to improve its performance should be developed.

Bi2WO6 Bi2WO6 is a layer structured material, which consists of perovskite-like [WO4]2− layers sandwiched between the [Bi2O2]2+ layers. Such structure can generate internal electric fields between the slabs, promoting the separation of photogenerated charge carriers. The VB of Bi2WO6 is composed of O 2p and Bi 6p hybrid orbitals, whereas the CB of Bi2WO6 is formed by the W 5d orbital, leading to a narrow bandgap of 2.8 eV [185]. In addition, the CB position of Bi2WO6 is close to the hydrogen reduction potential [161], which means that the photogenerated electrons in Bi2WO6 can be used for hydrogen production with little or even no external bias. For example, the photocatalytic hydrogen production of Bi2WO6 loaded with 1 wt.% of Pt was observed in an aqueous methanol solution under the illumination of a 450-W high-pressure mercury lamp [186]. So far, the Bi2WO6 photocatalyst is mainly used for organic pollutant degradation and the application for the solar-hydrogen production is still rare [41].

The performance of Bi2WO6 is still limited by its severe charge recombination [187]. Nanostructures play a pivotal role in affecting the charge transfer and separation properties of Bi2WO6. For example, porous Bi2WO6 photoanodes were beneficial for the light absorption, charge transfer, and electrolyte infiltration, exhibiting much higher photocurrent densities than that of their nonporous counterparts [188, 189]. In particular, the photocurrent density of a porous Bi2WO6 photoanode is approximately fivefold higher than that of its nonporous counterpart under visible light illumination (λ > 420 nm), as shown in Fig. 9a, b. In addition, Bi2WO6 photoanodes with an inverse-opal structure showed an almost threefold increase of the STH efficiency for PEC water splitting under visible light illumination [190]. It is believed that the continuous porous structure of the inverse-opal structure can improve the light harvesting as well as the charge separation efficiency, resulting in an enhanced PEC performance. Similarly, nanovoid Bi2WO6 2D ordered arrays also exhibited enhanced photocurrent densities and STH efficiencies [191]. The enhanced PEC performance is attributed to the excellent light scattering properties and favourable hole diffusion within the 2D array structure. Remarkably, Bi2WO6 three-dimensional (3D) hierarchical architectures composed of single-unit-cell (SUC) layers exhibited a superior photocatalytic H2 production rate that is 14-fold higher than that of the bulk Bi2WO6 under visible light illumination (λ > 420 nm, Fig. 9c) [192]. The enhanced photocatalytic activity is attributable to the substantially enhanced carrier density and the charge separation efficiency. In addition, Bi2WO6 square nanoplates with well-defined {001} facets exhibited a significantly enhanced photocatalytic activity under visible light illumination [193], indicating that the exposed facets also affected the photocatalytic performances.
Fig. 9

a SEM image of porous a CuWO4 photoanode; bI-V curves of nonporous and porous CuWO4 photoanodes. a, b Reproduced with permission from Ref. [188] Copyright 2009 Wiley-VCH; c photocatalytic H2 evolution curves of Bi2WO6 composed of SUC layers (denoted as SUC-Bi2WO6) and bulk Bi2WO6 (denoted as Bi2WO6). Reproduced with permission from Ref. [192] Copyright 2017 Elsevier; dI-V curves of pristine Bi2WO6 and 12 at.% Zn doped Bi2WO6. The y axes of Fig. 9b, d refer to the photocurrent density. Reproduced with permission from Ref. [194] Copyright 2013 ACS

Doping 12 at.% of Zn into the Bi2WO6 photoanode showed 80% enhancement of the photocurrent densities compared to its pure counterpart at low external potentials (Fig. 9d) [194]. However, such a high level of doping may generate impurities such as Bi2O3 and ZnO in the Bi2WO6 photoanode. By investigating the electronic structures of the N- and Mo-monodoped and N/Mo-codoped Bi2WO6 based on DFT calculations, it was found that N/Mo-codoped Bi2WO6 could reduce the bandgap by 0.19 eV without changing the CB position, which might be a better candidate for photocatalytic water splitting [195]. Since the band-edge positions of a semiconductor obtained from DFT calculations may be different compared to the experimental values, experimental works should be conducted to further confirm the feasibility of the N/Mo-codoped Bi2WO6 for water splitting.

On the other hand, coupling Bi2WO6 with other materials to form heterojunctions is another approach to improve the performances. For example, depositing Co3O4 on the Bi2WO6 nanoplate arrays can synergistically increase the surface OER kinetics and charge transport properties of the bulk, leading to over twofold enhancement of the photocurrent density at 1.23 V vs. RHE, compared to that of the bare Bi2WO6 nanoplate array photoanode [196]. Other Bi2WO6-based heterojunctions such as In2O3/Bi2WO6 [197], Bi2WO6/reduced graphene oxide (RGO) [198], Bi2WO6/Co(OH)x [199], Bi2WO6/g-C3N4 [200], and Bi2WO6/Cu3P [201] are also reported for solar-hydrogen production.

α-SnWO4 The bandgap of α-SnWO4 is ~ 1.64–2.1 eV, corresponding to a theoretical maximum photocurrent density of 24.4 mA cm−2 under AM 1.5 G illumination [202]. The VB of α-SnWO4 is formed by the Sn 5s and O 2p hybrid orbitals, whereas the CB is mainly contributed by W 5d. It is reported that the flat-band potential of α-SnWO4 is − 0.14 to 0.05 V vs. RHE [203, 204], indicating that the CB of α-SnWO4 is located at a more negative potential than that of 0 V vs. RHE (Fig. 10a). Thus, the band-gap and band-edge positions of α-SnWO4 are ideal for solar water splitting. However, the relatively low absorption coefficients in the visible light region (> 450 nm), low charge carrier mobility (~ 0.017 cm2 V−1 s−1), and short charge diffusion length (~ 30 nm) cause severe contradiction between the light absorption efficiency and the charge separation efficiency, resulting in the low PEC performance of the α-SnWO4 photoanodes [202]. As shown in Fig. 10b, a film thickness of ~ 10 µm is proposed to be necessary for α-SnWO4 to achieve a theoretical photocurrent density of 15 mA cm−2 [202], while the thickness of ~ 10 µm is two orders of magnitude higher than its charge diffusion length, which would cause severe charge recombination in the bulk α-SnWO4. Another issue for α-SnWO4 is that Sn2+ tends to be oxidized to Sn4+ during the water oxidation process or even the material synthesis process, which would generate defect states at the interface [202, 203]. In addition, the oxidation of Sn2+ may form a SnO2 layer on the surfaces of the α-SnWO4 that blocks the transfer to photogenerated holes to the electrolyte [202]. By depositing a NiOx protection layer, the α-SnWO4/NiOx photoanode exhibited a record photocurrent density of ~ 0.75 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination in the presence of Na2SO3 [202]. To further improve the performance of α-SnWO4, efforts should be focused on improving the charge separation efficiency in the bulk α-SnWO4.
Fig. 10

a Schematic of the band-edge positions of α-SnWO4. The positions of the valence band, Fermi level EF and the conduction band with respect to the vacuum level and the NHE potential are depicted; b light absorption ability of α-SnWO4 films with various thicknesses calculated based on the absorption coefficients. Inset: maximum achievable photocurrent density vs. film thickness of α-SnWO4 films. a, b Reproduced with permission from Ref. [202] Copyright 2018 ACS

Fe2WO6 Fe2WO6 has a bandgap of 1.5–1.7 eV [205, 206], which is suitable for solar water splitting in terms of the light absorption. However, the VB position of Fe2WO6 is ~ 1.7 eV below the Fermi level and the flat-band potential is 0.6–0.65 vs. RHE [205], suggesting that the CB position is located at ~ 0.6–0.65 V vs. RHE. Such a positive CB theoretically requires a large external bias for PEC water splitting using Fe2WO6 as the photoanode. In addition, the very short carrier diffusion length (< 10 nm) is mismatched with the light penetration depth of ~ 300 nm [205], which would cause severe charge recombination in the material. Therefore, Fe2WO6 is not a promising material for water splitting. Since the photogenerated holes in the VB of Fe2WO6 have relatively strong oxidation power, Fe2WO6 may be suitable for other photocatalytic applications such as organic pollutant degradation once the bulk charge recombination issue can be effectively addressed.

Ag2WO4 Ag2WO4 has a bandgap of ~ 3.1 eV with a CB and VB located at − 0.42 and 2.68 eV vs. normal hydrogen electrode (NHE) at pH = 7, respectively [207], which can be applied for the photocatalytic organic pollutant degradation and hydrogen production under UV light illumination. Interestingly, the bandgap of Ag2WO4 can be significantly reduced to 2.78 eV upon laser irradiation in liquid, enabling Ag2WO4 to be activated by visible light for hydrogen production [208]. It was believed that laser irradiation could generate abundant [WO6] cluster distortions in the crystal lattice and significantly increase the defect density, forming uneven intermediate energy levels between the CB and VB. Thus, Ag2WO4 showed a reduced bandgap and could be directly activated by visible light. Owing to the very large bandgap, Ag2WO4 is unlikely to be a promising photocatalyst for solar-hydrogen production. However, the negative CB position of Ag2WO4 is beneficial for achieving solar water splitting. Future research on this material should be focused on effectively reducing the bandgap, enabling Ag2WO4 to absorb a broad range of visible light.

3.1.3 Molybdenum-based ternary oxides

Unlike tungsten-based ternary oxides, the reported molybdenum-based ternary oxides for solar-hydrogen production are much few. A typical example is Bi2MoO6, which exhibits a typical Aurivillius structure consisting of the perovskite-like [MoO4]2− layers sandwiched between the [Bi2O2]2+ layers. Interestingly, the VB of the low-temperature Bi2MoO6 is mainly contributed by O 2p orbitals, while the CB is composed of Mo 4d and Bi 6p hybrid orbitals, forming a bandgap of 2.7 eV [209]. Similar to Bi2WO6, the main application of Bi2MoO6 is also for the photocatalytic organic degradation [42]. The main drawbacks of Bi2MoO6 are the severe bulk recombination and low charge separation efficiencies [210].

The heterojunction is the main strategy to improve the photocatalytic performance of Bi2MoO6. For example, a photoanode composed of Bi2MoO6@Bi2Mo2O9 exhibited a photocurrent density that is 3.6- and 8-fold higher than those of the pristine Bi2MoO6 and Bi2Mo2O9 photoanodes, respectively (Fig. 11a) [211]. The band alignment between the Bi2MoO6 and Bi2Mo2O9 improves the charge separation and transfer properties (Fig. 11b, c), leading to an enhanced PEC performance. Impressively, a Bi2MoO6/WO3 heterojunction photoanode exhibited a remarkable photocurrent density of 2.2 mA cm−2 at 0.8 V vs. SCE under simulated visible light illumination, which is double than that of the WO3 photoanode (Fig. 11d) [212]. The enhanced PEC performance is attributed to the fast charge separation in the interfaces of the Bi2MoO6/WO3 heterojunction. Similarly, other heterojunctions such as Bi2MoO6/g-C3N4 [213, 214], Bi2MoO6/CdS [215], Bi2MoO6/RGO [216], Bi2MoO6/MoO3 [145], Bi2MoO6/ZnIn2S4 [217], and Bi2MoO6/Si [218] are also reported.
Fig. 11

aI-V curves, b charge separation efficiencies, and c surface oxidation efficiencies of Bi2MoO6, Bi2Mo2O9, and Bi2MoO6@Bi2Mo2O9 photoanodes. Reproduced with permission from Ref. [211] Copyright 2018 Elsevier; dI-V curves of WO3 and WO3/Bi2MoO6 photoanodes. The y axes of Fig. 11a, d refer to the photocurrent density. Reproduced with permission from Ref. [212] Copyright 2016 Elsevier

Compared to WO3, the development of tungsten- and molybdenum-based ternary oxides for solar-hydrogen production is only in the very early stage. Although the bandgaps of most tungsten- and molybdenum-based ternary oxides discussed above are very close to or even lower than that of WO3, their PEC performances are much lower than that of the state-of-the-art WO3 photoanodes [11, 13, 219]. A fundamental understanding of the photocatalytic behaviours of the ternary oxides is still needed. Since nanostructures, heterojunctions, and doping are not effective to significantly enhance the PEC performance of the tungsten- and molybdenum-based ternary oxides, other novel strategies to overcome the severe bulk charge recombination issue should be developed.

3.1.4 Quaternary oxides

In addition to binary and ternary oxides, tungsten and molybdenum can form quaternary oxides by coupling with other two cations. A typical example is CsTaWO6, which shows a defect-pyrochlore structure with two different cations (Ta5+, W6+) randomly distributing on the B sites, forming a formula of AB2X6 [220]. The structural disorder in the defect-pyrochlore structure is beneficial for the fast ion conduction, which promotes the charge separation and transfer on the surfaces of the photocatalyst [221]. The CB of CsTaWO6 is composed of the W 5d and Ta 5d hybrid orbitals, while the VB is mainly contributed by O 2p orbitals [43]. Attractively, the CB of CsTaWO6 is located at approximately − 0.37 V vs. NHE [222], indicating that the photogenerated electrons in CsTaWO6 have sufficient reductive potential for the hydrogen production. However, the bandgap of CsTaWO6 is as large as 3.8 eV [43], which limits the light absorption in a very narrow UV range.

Encouragingly, nitrogen-doped CsTaWO6 can significantly reduce the bandgap from 3.8 to 2.3 eV due to the hybridization of the N 2p and O 2p orbitals [223]. In particular, the 0.3 wt.% N-doped CsTaWO6 nanoparticles with sizes of ~ 100 nm exhibited a nearly 100% increase of the photocatalytic hydrogen production rate compared to their pure counterparts under the similar reaction conditions (Fig. 12a, b). Moreover, sulphur and nitrogen codoped CsTaWO6 can further reduce the bandgap to 2.06 eV, which is very close to the ideal bandgap of 2.0 eV for solar water splitting [224]. As shown in Fig. 12c, the light absorption edge can be greatly expanded to ~ 600 nm. In addition, the photocatalytic activity of CsTaWO6 can be improved by delicately tuning the nanostructure [225, 226, 227]. Therefore, the proper doping and nanostructure engineering may synergistically boost the photocatalytic activity of CsTaWO6.
Fig. 12

a SEM image of CsTaWO6−xNx; b hydrogen evolution rate of CsTaWO6-xNx, CsTaWO6, and P25. a, b Reproduced with permission from Ref. [223] Copyright 2011 Wiley-VCH; c UV-Vis spectra of CsTaWO6, CsTaWO6-xSx, and CsTaWO6-x-ySxNy. Insets: digital images of the corresponding samples. Reproduced with permission from Ref. [224] Copyright 2011 RSC; d band-gap evolution of CsTaMoxW1−xO6 with Mo substitution. Reproduced with permission from Ref. [228] Copyright 2014 Wiley-VCH

Interestingly, the substitution of Ta by Nb can hardly change the bandgap, while the partial substitution of W by Mo to form CsTaMoxW1−xO6 can effectively reduce the bandgap (Fig. 12d) [228]. In particular, a new quaternary oxide of CsTaMoO6 can be formed by totally replacing W with Mo, which exhibits a much small bandgap of 2.9 eV. The reduced bandgap is mainly caused by the downward shift of the CB, as confirmed by both the DFT calculations and the Mott-Schottky plots [228]. According to the Mott-Schottky plot, the flat-band potential of CsTaMoO6 is ~ 0.1 V vs. RHE [228], indicating that the CB of CsTaMoO6 is very close to the H+/H2 reduction potential. Consequently, CsTaMoO6 is not able to produce hydrogen.

Thus, ideal strategies to reduce the bandgap of CsTaWO6 should not affect the CB position so as to keep the sufficient power of the photogenerated electrons to reduce protons. In this regard, non-metal doping may be a promising strategy to further modify CsTaWO6 for efficient solar-hydrogen production [229].

3.2 Chalcogenides

Transition metal dichalcogenides (TMDCs) such as WS2, WSe2, MoS2, and MoSe2 are layered materials with strong in-plane bonding and weak out-of-plane interactions. Therefore, these TMDCs can be exfoliated into 2D single-layer nanosheets with versatile applications [230]. In addition, tungsten- and molybdenum-based ternary chalcogenides can also be formed by incorporating another proper cation [231]. A typical example is Cu2MX4 (M = W or Mo; X = S, Se or S/Se), which can be synthesized by a solvothermal process [232]. DFT calculations predict that the bandgaps of Cu2MX4 (M = W or Mo; X = S, Se or S/Se) are 2.03–2.48 eV [233], indicating that the ternary chalcogenide family is theoretically suitable for solar-hydrogen production. In this section, we will briefly introduce the application of tungsten- and molybdenum-based chalcogenides as the light absorbers for solar energy conversion, and the cocatalysts to accelerate surface reactions.

3.2.1 Light absorber

The PEC performances of n- and p-type WSe2 single electrodes in aqueous solutions containing various redox couples were systematically investigated, demonstrating that layer-type semiconductors could be used for solar-hydrogen production [234]. Impressively, a crystalline p-type WSe2 cathode coated with Pt/Ru mixed cocatalysts exhibited a high photocurrent density of ~ 15 mA cm−2 under AM 1.5 G illumination for 2 h in an acid solution (pH = 2), as shown in Fig. 13a, b [235]. The energy conversion efficiency for hydrogen production is as high as 7%. Similarly, single-crystal p-type WS2 photocathodes deposited with Pd or Pt as the cocatalysts showed an excellent performance for PEC hydrogen production, with a solar conversion efficiency of ~ 7% [236]. In addition, unassisted HI splitting into H2 and HI3 was achieved using a crystalline n-type WSe2 film as the photoelectrode, delivering a high photocurrent density of ~ 12 mA cm−2 [237]. Interestingly, the crystalline WSe2 monolayers with a direct bandgap of ~ 1.65 eV were deposited on the large-area amorphous SiOx substrates by a molecular beam epitaxial growth method (Fig. 13c), which achieved a photon conversion efficiency of over 12% for overall water splitting without any additives (Fig. 13d) [238].
Fig. 13

aI-V curves of p-WSe2/Pt/Ru photoelectrodes measured in different electrolytes; bI-t curves of p-WSe2/Pt/Ru photoelectrodes measured in acidic and basic electrolytes. a, b Reproduced with permission from Ref. [235] Copyright 2012 ACS; c schematic of overall water splitting on a WSe2 single layer. Inset: band-edge positions of a WSe2 single layer; d overall water splitting performances of the WSe2 monolayer in pure water. The y axes of Fig. 13a, b refer to the photocurrent density. c, d Reproduced with permission from Ref. [238] Copyright 2018 Elsevier

Although the single-crystal WSe2 and WS2 materials show excellent solar-hydrogen production properties, the fabrication cost is extremely high, which is not suitable for the large-scale applications. Encouragingly, the solution-processed WSe2 thin films prepared by a space-confined self-assembled (SCSA) thin-film deposition method using the solvent-exfoliated few-layer WSe2 flakes as the precursor, exhibited a p-type photocurrent density of 1.0 mA cm−2 at 0 V vs. RHE under AM 1.5 G illumination [239]. Using Pt-Cu as the cocatalyst, a solution-processed 2D WSe2 nanoflake photocathode exhibited a new benchmark photocurrent density of 4.0 mA cm−2 for solar-hydrogen production with an internal quantum efficiency of over 60% at the wavelength of 740 nm [240]. MoS2/WS2 heterojunction photoanodes prepared by restacking the WS2 and MoS2 monolayers on FTO substrates exhibited a photocurrent density of 0.45 mA cm−2 and the oxygen evolution under simulated solar irradiation [241]. Owing to the band alignment formed at the interfaces that promotes the charge separation, the MoS2/WS2 heterojunction photoanode exhibits a tenfold increase in the IPCE compared to that of its MoS2 counterpart. In addition to photoelectrodes, the liquid phase exfoliated WS2 monolayers can be directly used for solar-hydrogen production [242]. Using ascorbic acid as the hole scavenger, the carrier lifetime was increased by threefold, leading to a 14-fold increase in hydrogen production. Likewise, solution-processed nanoscale p-n junctions composed of 5–20 nm sized p-type MoS2 nanoplatelets and n-type nitrogen-doped RGO exhibited a significantly high photocatalytic activity for hydrogen production in a wide range of wavelengths up to the near-infrared light [243].

In addition to the binary chalcogenides, the photocatalytic performances of ternary chalcogenides are also explored. For example, Cu2WS4 (bandgap: 2.1 eV) synthesized by a hydrothermal process demonstrated a high activity for hydrogen production under visible light illumination [244]. By loading 1.5 wt.% of Ru as the cocatalyst, Cu2WS4/Ru exhibits a hydrogen evolution rate of ~ 135 µmol h−1 under visible light illumination and an AQY of 11% at the wavelength of 425 nm. Interestingly, Cu2WS4 decahedral photocatalysts showed a high photocatalytic activity with a larger ratio of {001}/{101} facets [245], which is attributed to the surface heterojunction formed between the {001} and {101} facets, leading to the accommodation of photogenerated electrons on the {001} facets for hydrogen production. Similarly, Cu2MoS4 with a small bandgap of 1.61–1.76 eV was also reported solar-hydrogen visible light illumination [246, 247]. The facet-dependent photocatalytic activity of Cu2MoS4 was also confirmed by solar-hydrogen properties of the Cu2MoS4 nanotubes with exposed {010} facets and the 2D Cu2MoS4 nanosheets with exposed {001} facets [248]. In addition, the PEC performance of a Cu2MoS4 photocathode was also observed [249]. The hybridization of CQDs into the Cu2MoS4 photocathode can further improve the PEC performance, as evidenced by the low onset potential and enhanced photocurrent densities.

It is clear that the solar-hydrogen production performance of the solution-processed chalcogenides is still much lower than their single crystalline counterparts. However, the fabrication cost can be significantly reduced. More efforts should be devoted in exploring new solution processes to prepare efficient chalcogenides for practical solar-hydrogen production.

3.2.2 Cocatalysts

Typically, MoS2 and WS2 can be used as the efficient HER cocatalysts for solar-hydrogen production. As shown in Fig. 14a, the hydrogen production activity of CdS was increased by 36-fold after loading 0.2 wt.% of MoS2 on CdS using a lactic acid solution as the sacrificial agent under visible light illumination (λ > 420 nm) [250]. Impressively, the 0.2 wt.% MoS2-loaded CdS showed a better hydrogen production performance than that of the 0.2 wt.% Pt loaded CdS under the same reaction conditions (Fig. 14b), indicating that MoS2 can be potentially used to replace the expensive Pt as an efficient HER cocatalyst for solar-hydrogen production. The heterojunction form between CdS and MoS2 and the excellent HER cocatalyst performance of MoS2 are attributed to the significantly enhanced hydrogen production activities [251]. A two-step hydrothermal process was developed to prepare TiO2-based photocatalysts with MoS2/graphene as the cocatalyst, exhibiting a high photocatalytic hydrogen production rate of 165.3 µmol h−1 and an AQE of 9.7% at the wavelength of 365 nm [252]. The positive synergetic effect between the MoS2 and the graphene sheets is believed to efficiently improve the charge separation efficiency and provide more active sites for solar-hydrogen production. In addition, an amorphous MoS2 layer was deposited on the Cu2O cathodes as a HER cocatalyst for PEC water splitting (Fig. 14c), which generates a photocurrent density of − 5.7 mA cm−2 at 0 V vs. RHE under AM 1.5 G illumination [253]. Moreover, the amorphous MoS2 modified Cu2O photocathode showed a much more stable PEC performance than that of the Cu2O photocathode loaded with Pt in an acidic electrolyte (pH = 1.0, Fig. 14d). Similarly, MoSe2 [254, 255, 256], WS2 [257, 258, 259], and WSe2 [260] can also be used as HER cocatalysts for solar-hydrogen production.
Fig. 14

a Hydrogen evolution performances of CdS/MoS2 with different amounts of MoS2; b hydrogen evolution performances of CdS loaded with different cocatalysts. a, b Reproduced with permission from Ref. [250] Copyright 2008 ACS; c SEM image of a Cu2O/AZO/TiO2/MoS2+x photocathode; dI-t curves of Cu2O photocathodes loaded with Pt and MoS2+x cocatalysts in electrolytes with various pH values. The y axis of Fig. 14d refers to the photocurrent density. c, d Reproduced with permission from Ref. [253] Copyright 2014 Nature Publishing Group

Likewise, tungsten- and molybdenum-based ternary chalcogenides such as Cu2WS4 [261, 262, 263], Cu2MoS4 [264, 265, 266], and Ag2WS4 [267] can also be used as the HER cocatalysts to promote hydrogen production. For example, using Cu2WS4 as a noble metal-free cocatalyst for organic dyes, sensitized TiO2 could double the solar-hydrogen production rate [268]. Impressively, Cu2MoS4/g-C3N4 nanosheets exhibited an excellent hydrogen production rate of 2170.5 μmol h−1 g−1 under visible light illumination, which is 677- and 34-fold higher than that of the bulk g-C3N4 and g-C3N4 nanosheets, respectively [247]. In addition, Cu2MoS4 can also be used as an HER cocatalyst to enhance the solar-hydrogen production performances of the CdS catalysts [269] and Cu2O photocathodes [270].

4 Conclusion and outlook

Semiconductor-based solar-hydrogen production is one of the most promising technologies to preserve the global energy security. However, the practical application is still limited by the low STH efficiency, which is highly dependent on the development of the robust and efficient semiconductor materials. In the past decades, a large family of tungsten- and molybdenum-based materials including oxides and chalcogenides has been developed as the photocatalysts and cocatalysts for solar-hydrogen production, which is critically reviewed in this article. Although single crystalline WSe2 and WS2 show excellent solar water splitting performances, the extremely high fabrication cost hinders their scale-up applications. To reduce the fabrication cost, the solution-processed WSe2 and WS2 are also developed. However, their solar-hydrogen production performances are much lower compared to their single crystalline counterparts. Based on their bandgaps, WSe2 (1.2 eV) and WS2 (1.51 eV) can theoretically generate a photocurrent density of 40 mA cm−2 and 29 mA cm−2 under AM 1.5 G illumination, respectively [271]. Therefore, WSe2 and WS2 would be the promising materials for solar fuel production once the fabrication cost can be significantly reduced without sacrificing their performances. In addition, chalcogenides also show excellent performances as the HER cocatalysts, which can potentially replace the expensive Pt used in solar-hydrogen production.

Compared to chalcogenides, the tungsten- and molybdenum-based oxides generally have larger bandgaps, which may not achieve high STH efficiencies. However, oxides are generally stable in aqueous solutions under light illumination and can be achievable via solution processes, making them as attractive materials for cost-effective solar-hydrogen production. Even though ternary semiconductors such as CuWO4, α-SnWO4, and Fe2WO6 have the small bandgaps of ~ 2 eV, they suffer from very poor charge separation efficiencies in the bulk. To further improve the solar-hydrogen production properties of these materials, the key idea is to improve both the majority and minority carriers to improve their bulk charge separation. In general, the atomic doping and defect engineering (e.g., oxygen vacancies) are two efficient strategies to tune the optoelectronic properties of the semiconductors to improve their charge transport properties, which should be considered to overcome the severe charge recombination issues. It should be emphasized that doping or defect engineering has multiple effects on the solar-hydrogen production properties of the semiconductors. It is critically important to control the doping or defect levels in the semiconductors to achieve the optimized performances.

Among all the available tungsten- and molybdenum-based oxides, WO3 still shows the best PEC performance hitherto and the photocurrent density reaches ~ 3.7 mA cm−2 [11], which is very close to its theoretical value under AM 1.5 G illumination. Compared to other multinary tungsten- and molybdenum-based oxides with smaller bandgaps, the bulk charge separation of WO3 is much better. However, the instability in solutions with a pH > 4 and the relatively large bandgap (~ 2.8 eV) still hinder the application of WO3 as an efficient photoanode for solar-hydrogen production [59]. Proper strategies to improve the stability in a wider pH range and reduce the bandgap to absorb a wide range of the solar spectrum are the key research targets for WO3. Although heavily doped with nitrogen can significantly reduce the bandgap of WO3, the PEC performance is still not promising [110]. Thus, effects on other properties such as the charge separation and surface reactions should also be taken into account when trying to reduce the bandgap of WO3.

Overall, a significant progress is achieved for the development of tungsten- and molybdenum-based semiconductor materials for solar-hydrogen production, even though there are still many challenges. The accumulation of knowledge and deeper fundamental understanding of these materials during the solar water splitting reaction processes are critically important to explore efficient strategies to overcome their limitations. We hope to promote the further development of related fields for efficient solar fuel production.

Notes

Acknowledgements

The work is financially supported from the Australian Research Council through its DP programs and the Queensland node of the Australian National Fabrication Facility (ANFF).

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Copyright information

© The Nonferrous Metals Society of China 2019

Authors and Affiliations

  1. 1.Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and NanotechnologyThe University of QueenslandBrisbaneAustralia

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