Stannous Tungstate Semiconductor for Photocatalytic Degradation and Photoelectrochemical Water Splitting: A Review

Abstract

The use of photocatalysis and photoelectrocatalysis is expected to achieve the efficient utilization of solar energy to alleviate and even solve the problems of energy depletion and environmental pfollution. At present, stannous tungstate materials have attracted extensive attention in the fields of photocatalysis and photoelectrocatalysis as favorable candidates for such utilization because of their narrow band gap energy (which is ~1.7 eV for the α phase and ~2.7 eV for the β phase, respectively) and unique band structure (which covers the oxidation and reduction potential of water). However, their practical application is still limited by excessive electron–hole recombination and poor stability. In this review, basic information (crystal and electronic structures) related to photocatalysis and photoelectrocatalysis is presented. Additionally, various strategies to enhance the photocatalytic and photoelectrochemical properties of stannous tungstate materials, such as morphological modification, crystal facet engineering, doping modification, and multicomponent compositing, are summarized. Furthermore, the achievements and difficulties of the relevant studies are discussed. The information presented in this review can provide a reference for subsequent research on the photocatalytic and photoelectrochemical performance of tungstate-based materials.

1. Introduction

With the rapid development of technology, energy has become a necessity for social operation, and the human consumption of non-renewable traditional energy (from sources such as oil and coal) is decreasing. The energy crisis has become an urgent problem that needs to be solved. Therefore, researchers are enthusiastically looking for alternative renewable clean energy sources. Among the many potential energy sources, solar energy has attracted significant attention due to its large reserves and reproducibility. Although solar energy exhibits a variety of excellent properties, its dispersion, instability and intermittency have led to its low utilization. Therefore, exploring efficient ways to better use solar energy is necessary for the generation of alternative power. Photocatalytic and photoelectrochemical methods show promise for degrading pollutants and generating new kinds of energy (such as hydrogen and chemical) via the use of solar energy. All the relevant reactions are completed by the photo-generated holes and electrons; thus, they follow a similar principle. Since the first report of TiO₂ in 1972 [1], different kinds of materials with a smaller band gap than that of TiO₂ (3.0–3.2 eV) have been explored [2].

One of these materials is stannous tungstate (SnWO₄), which is a potential catalyst due to its narrow band gap, suitable band position, and nontoxicity [3]. However, due to its low charge separation efficiency and poor stability [4], its photocatalytic and photoelectrochemical performance has not met theoretical expectations. To solve this problem, many researchers are committed to obtaining a better catalytic performance and higher solar energy efficiency by means of morphology control, doping, and multi-component compositing. In this paper, we describe the synthesis method, physical characteristics, and activities regarding the photocatalysis and photoelectrocatalysis of stannous tungstate. In addition, the challenges and future outlook regarding the use of stannous tungstate in photoelectrochemical water splitting are summarized.

2. Structure, Properties, and Synthesis of SnWO₄

In the 1970s, Jeitschko and Sleight reported the crystalline structure and morphology of the low-temperature (α) and high-temperature (β) phases of stannous tungstate materials [5,6]. Orthorhombic α-SnWO₄(Figure 1a) exhibits a stable structure below 670 °C, and it tends to transform into cubic β-SnWO₄ (Figure 1b) above 670 °C [7]. A reverse transition from the β-phase to the α-phase may occur under negative pressure, and the W is replaced by Mo [8]. In the crystal of α-SnWO₄, W and O form WO₆ octahedral structures that are interconnected in four corners, while O binds to Sn in a regular octahedral form [5]. For β-SnWO₄, the W atoms of β- form WO₄²⁻ tetrahedrons with O and disperse within the crystal, whereas near the Sn atoms, O is bound to it as a twisted octahedron [6].

This difference in structure leads to different properties for the α and β phases. The W-O bond in the WO₄²⁻ tetrahedron (~1.75 Å) [5] is shorter than that in the WO₆ octahedrons (~1.80–2.15 Å) [6]. It leads to a larger crystal field-splitting energy and a higher orbital energy of W in β-SnWO₄. The conduction band edge of the α phase is mainly composed of a W 5d orbital, while the β phase contains an Sn 5p orbital, in addition to the W 5d orbital (Figure 2). The valence bands of both SnWO₄ are formed by hybridizing the O 2p and Sn 5s orbitals, and the Sn-O bonds are almost identical in length (~2.20–2.80 Å) [5,6]. Thus, the positions of the valence bands are similar. These characteristics result in a narrower band gap for α-SnWO₄ (~1.7 eV) than for β-SnWO₄ (~2.7 eV) [3], and α-SnWO₄ has a wider range of light absorption and a higher theoretical photocurrent density (~17 mA cm⁻² [9]) than β-SnWO₄. In addition, the β phase is a direct band gap and the α phase is an indirect band gap [3], which leads to a higher carrier separation efficiency for the α phase than the β phase [10]. Due to the wider band gap, the photo-generated electrons (holes) of β-SnWO₄ show a more significant reduction (oxidation) activity. Thus, β-SnWO₄ was used in photocatalysis, and only α-SnWO₄ has been used in photoelectrochemical water splitting and sulfite oxidation. However, in addition to the excellent optical properties mentioned above, the use of stannous tungstate exhibits problems that urgently need to be solved. Since Sn is divalent and oxidizable in SnWO₄, it is susceptible to photocorrosion, which is attributed to stannous, and the following reaction occurs: SnWO₄ + H₂O + 2h⁺ → SnO₂ + WO₃ + 2H⁺. Therefore, SnWO₄ has poor stability, which limits its performance in practical applications [4]. At the same time, a recombination of carriers occurs in the diffusion process of SnWO₄. Its diffusion length is short, which limits the number of photo-generated carriers that can arrive at the surface of the crystal, leading to unsatisfactory photocatalytic and photoelectrochemical performance.

Various methods have been developed to synthesize SnWO₄ materials, and their differences in thermodynamic stability lead to changes in the synthesis of α-SnWO₄ and β-SnWO₄. Because the α phase is a kind of crystalline phase at low temperatures, its synthesis conditions are milder. Many methods can be used to synthesize the α phase, e.g., hydrothermal methods, solid–solid reaction methods [3,11,12,13], and magnetron sputtering methods [10,14,15]. Each synthesis method has its own advantages. The hydrothermal method can be used to obtain crystals with a complete crystal phase, small particle size, and uniform distribution. A magnetron sputtering method can obtain dense and uniform high-quality thin films. The solid–solid reaction method possesses a simple process and a large output. However, the synthesis of the high-temperature crystalline β-phase is relatively harsh because it requires higher temperature conditions (which should be obtained by rapid cooling at more than 940 K). It has been reported that most β-SnWO₄ powders are synthesized with a high-temperature solid phase method [11,12,16,17]. β-SnWO₄ can also be obtained by using NaWO₄ as a raw material through kinetic control under more mild conditions [18]. Jan Ungelenk and Claus Feldmann synthesized β-SnWO₄ using a microemulsion method at low temperatures [19]. When W and O combine to form a discrete WO₄²⁻ tetrahedron in NaWO₄, the structure is similar to that of β-SnWO₄, so Na₂WO₄ is likely to be directly converted to β-SnWO₄. Additionally, assisted by hydrothermal [20,21] and microwave-assisted hydrothermal methods [22], α-SnWO₄ can also be synthesized using NaWO₄ and SnCl₂ as the precursors.

To synthesize the film for photoelectrochemical water spliting, an indirect or direct coating method can be used. In indirect coating, as-prepared SnWO₄ powders are drop-cast onto conductive substrates. This is an easy-to-perform method, but the performance of the photoanode is weak due to the poor charge transfer caused by bad connections between the particles. For direct coating, Moritz Kölbach et al. employed a pulsed laser-deposited method that used an α-SnWO₄ target formed by annealing WO₃ (99.99%) and SnO (99.99%) powders [23]. Bozheyev et al. reported on the use of a magnetron co-sputtering method in which Sn (99.99%) and W (99.95%) were the targets, and the deposition was performed with the existence of O₂ [24]. Recently, Gottesman used a new method—rapid thermal processing—to synthesize an α-SnWO₄ film. The technique could be used to treat a sample at higher temperatures, without destroying the glass-based F:SnO₂ (FTO), and a desired crystallinity with few defects was obtained [25]. Liu et al. used a chemical vapor deposition to convert WO₃ into α-SnWO₄, and the reaction is shown in Equation (1) [26]. An in situ hydrothermal conversion method was also reported, in which WO₃ was used as a precursor to react with Sn²⁺ in the solution [27]. The possible reactions occurring in this method are illustrated as Equations (2)–(3) [28]. As a result, the α-SnWO₄ film exhibits similar nanostructured array morphology to that of WO₃ films due to the inherited behavior (Figure 3) [28]. The anion in hydrothermal conversion solutions may also affect the morphology of α-SnWO₄ films [29].

4WO₃ + 3SnCl₂ ⇌ 3SnWO₄ + WCl₆

(1)

$${\mathrm{WO}}_3 + {\mathrm{H}}_2\mathrm{O}\rightleftharpoons 2{\mathrm{H}}^{+} + {{\mathrm{WO}}_4}^{2-}$$

(2)

$$\mathrm{W}{{\mathrm{O}}_4}^{2-} + {\mathrm{Sn}}^{2+}\rightleftharpoons {\mathrm{SnWO}}_4$$

(3)

Generally speaking, SnWO₄ has good optical properties, such as a narrow band gap and a special band structure, which results in excellent application prospects for the fields of photocatalysis and photoelectrocatalysis (especially in the field of water splitting). However, it is necessary to pay attention to the instability and short carrier diffusion length of the material itself. Furthermore, because of the differences in crystal structure and band structure between the two crystal phases (α phase and β phase), their properties and synthesis methods are different. For instance, the photo-generated electrons (holes) of the β phase have more significant reduction (oxidation) activity than that of the α phase, so the β phase is suitable for photocatalytic degradation. The α phase has a wider light absorption range and higher carrier separation efficiency, so it has potential for use in photoelectrocatalysis.

3. Research on Modification of SnWO₄

Since Cho et al. first reported its optical properties in 2009 [3], SnWO₄ has been widely used in photocatalytic degradation [30] and photoelectrochemical water splitting [31]. Researchers have been working to modify SnWO₄ via morphological modification, facet regulations, doping, and the construction of multicomponent composites in order to obtain better photocatalytic degradation rates of organic pollutants (

Table 1. Recently reported SnWO₄-based materials for photocatalysis.

Type of Catalyst	Organic Pollutant	Light	Degradation Efficiency	Ref.
Nest-like α-SnWO4	methyl orange (MO)	300 W Xe arc lamp > 420 nm	96.1% (60 min)	[32]
Flower-like α-SnWO4	MO	300 W tungsten-halogen lamp > 420 nm	95% (80 min)	[33]
α-SnWO4 nanostrips + small nanosheets	MO	500 W Xe lamp	90.4% (30 min)	[34]
α-SnWO4 synthesized with different solvents	MO	300 W Xe lamp > 420 nm	98% (80 min)	[35]
leaf-like β-SnWO4	MO	250 W tungsten-halogen lamp	60% (2 h)	[36]
Bi3+-doped α-SnWO4	MO	500 W Xe lamp > 420 nm	95% (2 h)	[37]
α-SnWO4/Ag-NPs	MO	500 W Xe lamp > 420 nm	97% (70 min)	[38]
α-SnWO4/ZnFe-LDH	MO	500 W Xe lamp	95.1% (40 min)	[39]
SnS/α-SnWO4	MO	500 W Xe lamp	95.6% (90 min)	[40]
CTAB-α-SnWO4	MO	500 W Xe lamp	58.7% (30 min)	[20]
h-BN/α-SnWO4	MO	300 W Xe lamp > 420 nm	94.7% (90 min)	[41]
α-SnWO4/SnO2	MO	500 W Xe lamp > 420 nm	97% (40 min)	[42]
β-SnWO4-GO	MO	tungsten-halogen lamp > 420 nm	90% (25 min)	[12]
α-SnWO4-GO	MO	300 W tungsten-halogen lamp > 420 nm	41.2% (6 h)	[43]
Zn2+ doped α-SnWO4 nanowires	MO	300 W Hg lamp > 420 nm	~100% (90 min)	[44]
β-SnWO4 with different morphology	rhodamine B (RhB)	AM 1.5 G solar light (100 mW cm−2)	~22% (2 h)	[45]
Cube β-SnWO4	RhB	8 W UV-lamp with a monowavelength of 366 nm	TOF:1.14	[46]
Spike-cube β-SnWO4	RhB	8 W UV-lamp with a monowavelength of 366 nm	TOF:2.77	[46]
SnS/α-SnWO4	RhB	500 W Xe lamp	97.62% (2 h)	[40]
α-SnWO4/g-C3N4	RhB	500 W Xe arc lamp > 420 nm	91% (80 min)	[47]
1D ZnWO4@β-SnWO4	RhB	300 W Xe lamp > 420 nm	~100% (2 h)	[48]
β-SnWO4-GO	RhB	under visible light	91% (25 min)	[12]
α-SnWO4	RhB	100 W tungsten-halogen lamp > 420 nm	~82% (4 h)	[3]
β-SnWO4	RhB	100 W tungsten-halogen lamp > 420 nm	~97% (4 h)	[3]
BiOBr/α-SnWO4	RhB	sunlight	97.9% (1 h)	[49]
spike-cube β-SnWO4	methylene blue (MB)	8 W UV-lamp with a monowavelength of 366 nm	TOF:2.65	[43]
β-SnWO4-rGO nanocomposite	MB	tungsten-halogen lamp > 420 nm	94% (2 h)	[50]
β-SnWO4/ZnO	MB	70 W sodium lamps	82.9% (2 h)	[51]
β-SnWO4 truncated rhombic dodecahedrons	MB	150 W halogen lamp	100% (20 min)	[19]
β-Sn1−nWO4·nα-Sn	MB	halogen bulb (3300 K)	~90% (1 h)	[52]
h-BN/α-SnWO4	tetracycline (TC)	300 W Xe lamp > 420 nm	82.2% (4 h)	[41]
SnS/α-SnWO4	TC	500 W Xe lamp	57.0% (3 h)	[40]
β-SnWO4-rGO nanocomposite	rose bengal (RB)	tungsten-halogen lamp > 420 nm	98% (2 h)	[50]
β-SnWO4 NPs	RB	300 W tungsten bulb	94.6% (2.5 h)	[53]
α-SnWO4/UiO-66(NH2)/g-C3N4	ibuprofen (IPF)	high pressure Xe lamp source simulated sunlight	95.5% (2 h)	[54]
β-Sn1−nWO4·nα-Sn	basic green 4 (BG)	halogen bulb (3300 K)	~63% (1 h)	[52]
BiOBr/α-SnWO4	BG	sunlight	95.5% (45 min)	[49]
β-Sn1−nWO4·nα-Sn	methyl red (MR)	halogen bulb (3300 K)	~71% (1 h)	[52]

), as well as improved photoelectrochemical water splitting and sulfite oxidation performance (

Table 2. Recent report on the performance of α-SnWO₄ based photoanodes in PEC water splitting.

Photoanode	Photocurrent Density (at 1.23 V vs. RHE)	Electrolyte	Ref.
α-SnWO4 nanowires	0.032 mA cm−2	0.1 M KPi buffer (pH ≈ 5)	[27]
α-SnWO4 porous nanostructure	0.080 mA cm−2	0.5 M KPi buffer (pH ≈ 7)	[31]
α-SnWO4 nanocrystalline particles	~0.750 mA cm−2	0.5 M KPi buffer and 0.5 M Na2SO3 (pH ≈ 7)	[4]
α-SnWO4 microcrystalline particles	0.375 mA cm−2	0.5 M Na2SO4 (pH ≈ 7)	[24]
α-SnWO4 coral-like morphology	0.420 mA cm−2	KOH/H3BO3 buffer and 0.2 M Na2SO3 (pH ≈ 9)	[55]
2D nanosheets α-SnWO4	0.411 mA cm−2	0.2 M KPi buffer (pH ≈ 7)	[56]
Nail-like α-SnWO4	0.300 mA cm−2	0.2 M KPi buffer (pH ≈ 7)	[29]
2D long-plate α-SnWO4	0.790 mA cm−2	0.2 M KPi buffer (pH ≈ 7)	[28]
NiOx coated RTP-α-SnWO4	0.950 mA cm−2	0.5 M KPi buffer with 0.5 M Na2SO3 (pH ≈ 7)	[25]

).

3.1. Morphological Modification

By regulating the synthesis technique, which is a universal method for enhancing the catalytic performance of a material, a target material with a specific morphology can be obtained. The morphology of a material is usually designed for the following purposes [57]: (1) to increase the specific surface area and reduce the nanometer size, as a high specific surface area can provide more active sites, and a small nanometer size can provide a shorter charge transfer path that improves carrier separation efficiency and enhances light absorption efficiency; (2) to improve the crystallinity of materials, as a high crystallinity can reduce the recombination center and improve carrier separation efficiency, but it is difficult to simultaneously improve crystallinity and specific surface area; (3) to obtain special structural shapes (such as 0D, 1D, 2D, and 3D materials), as different dimensions have diverse characteristics. Thus, directional design can improve the performance of the materials.

In the study of SnWO₄ for photocatalysis, there have been many attempts to obtain improved properties by adjusting the morphologies of the substrate materials. Zhu et al. obtained flower-like α-SnWO₄ powders under neutral conditions with a DTAB-assisted hydrothermal method, degrading 95% of methyl orange (MO) within 80 min [33]. Want et al. also discussed the solvent-dependent morphology of α-SnWO₄ powders [35], and Liu et al. synthesized nanostrips and nanosheets by changing the water/ethylene glycol ratio through a solvothermal method [34]. Meanwhile, the layered multi-hollow spherical α-SnWO₄ powders [37] prepared by Zhu et al. and the nested nanostructure α-SnWO₄ [32] prepared by Zhang et al. showed an improved catalytic performance after controlling their morphologies. The increased specific surface area enabled by morphology controlling exposes the active sites and shortens the length needed for photo-generated carriers to arrive at a surface. For a β-SnWO₄ photocatalyst, Chen et al. prepared cubes and spike-cube shaped particles, with the latter showing strong photocatalytic activity [46]. Warmuth et al. formed tetrahedral β-SnWO₄, which demonstrated better photodegradation activity than a β-SnWO₄ material, with truncated octahedra and a spike-cube morphology (Figure 4a) [45]. Ungelenk et al. synthesized β-SnWO₄ powders with a microemulsion method. The powders had a rhombohedral dodecahedron shape, and they could complete the degradation of methylene blue (MB) in 20 min [19]. Raj et al. also noted the better MO degradation performance of leaf-like β-SnWO₄ compared with that of sphere-like and irregularly structured β-SnWO₄ [36]. Alharthi et al. synthesized honeycomb-like β-SnWO₄ with a hydrothermal method and calcined it at 700 °C, which enabled the photodegradation of rose bengal in 150 min [53]. For photoelectrochemical water splitting, it is difficult to control the morphology of the powder present on an electrode. Accordingly, Zhu et al. synthesized a porous α-SnWO₄ film through the hydrothermal conversion of WO₃ films [31], and they obtained a photocurrent density of 0.08 mA cm⁻² at a potential of 1.23 V_RHE. In contrast, topography optimized nails (Figure 4b) [27,29], nanosheets (Figure 4c) [56], and long-plates [28] have presented better PEC activity than α-SnWO₄, with a powder stacked morphology because the array structures provide pathways for charge transfer, with less recombination.

3.2. Crystal Facet Engineering

During morphology regulation, the ratio of exposure facets may change as well [58]. It is noted that reaction activities vary for different facets [59,60]. Thus, charge transfer ability can be modified by controlling the anisotropic growth of crystals in a certain direction. The β-SnWO₄ enclosed by (100) and (110) facets obtained the strong photocatalytic degradation of methylene blue [19]. Harb et al. found that the (001) facet of α-SnWO₄ and the (100) facet of β-SnWO₄ showed strong charge separation and transport capabilities via calculations with the HSE06 code [10]. They also discussed the (121), (210), (111), (200), and (040) exposed facets of α-SnWO₄ using DFT, and the (210) and (121) facets were shown to perform oxygen evolution reaction due to the positive position of their valence band (Figure 5a) [61]. They also noted that the (001) facet had a high surface energy, but it may be thermodynamically less stable than the other five facets. Wang et al. discussed the oxygen evolution performance of α-SnWO₄ (010) facets with different kinds of termination, and O-Sn termination showed better OER activity than did the others [62]. These DFT researchers have demonstrated that crystal facet engineering is another viable method for obtaining a good SnWO₄ substrate material. Inspired by these projects, our group synthesized an α-SnWO₄ array and added F ions to its precursor solution to increase the ratio of the (001) facet (Figure 5b). The facet-controlled film had a higher photocurrent than the uncontrolled film due to the better OER performance (Figure 5c) [29], and a ~1.9 times greater photocurrent was obtained in an unbiased PEC cell.

3.3. Doping Modification

Doping can change the position of an energy band, reduce the width of a band gap, and improve the efficiency of carrier separation and transfer, thus improving the catalytic performance of the materials. For example, Zn²⁺ doping was introduced into α-SnWO₄ by Su et al. [44]. Because Zn and Sn have similar electronegativity values and suitable sizes, Zn²⁺ is used to replace Sn²⁺ in SnWO₄ to form Sn_1−xZn_xWO₄. This ensures that the Zn 4s and Zn 4p orbitals participate in the hybridization of the valence band and the conduction band, respectively, thus changing the position of the energy band and reducing the band gap. The authors also found that Zn²⁺ doping changed the morphology of the material, and the resulting Sn_0.955Zn_0.045WO₄ could degrade nearly 90% of MO in 20 min. Zhu et al. introduced defects into α-SnWO₄ through Bi³⁺ doping [37]. Unlike Zn²⁺ doping, Bi does not participate in the formation of energy band hybridization, but cation vacancies are formed due to changes in the charges. These defects form effective energy levels in the band gap which enhance light absorption, inhibit electron-hole pairing, and improve the photocatalytic properties of the material. Although the mechanisms of Zn² and Bi³ doping are not exactly the same, both of them can improve the performance of SnWO₄ by changing its morphology and band structure. At the same time, because β-SnWO₄ has a wider band and a lower carrier separation efficiency than α-SnWO₄, the doping of the two ions may lead to better performance improvements for β-SnWO₄. In addition to these two kinds of hetero ions, there may be more suitable ions for SnWO₄ modification (such as Mo and Co) that need to be further studied. In theory, Azofra et al. replaced Sn with Ge and substituted W with Mo in DFT calculations, and they obtained the enhanced orbital hybridization of the VBM/CBM electronic states, which amplified the amount of generated holes/electrons on top of the (110)/(100) facets [63]. However, these result have not been experimentally confirmed.

3.4. Multicomponent Composite

The multicomponent composite strategy includes coating a sedimentary protection layer and building heterojunction. Constructed multicomponent catalysts generally have the following advantages over single catalysts: (1) they expand the light absorption spectra, (2) they promote carrier separation, (3) they inhibit carrier recombination, and (4) they prevent photocorrosion.

In photocatalysis, the compositing of reduced graphene oxide (RGO) and β-SnWO₄ can be achieved via microwave heating [12] and hydrothermal methods [50]. In such a composite, RGO, as a supporting material, provides a larger surface area and more active sites than β-SnWO₄, thereby improving the specific surface area and catalytic activity of the material. At the same time, the high conductivity of RGO makes the electrons generated by β-SnWO₄ move towards the RGO, while the holes remain inside β-SnWO₄, which improves the carrier separation efficiency of the materials. The compositing of RGO and α-SnWO₄ was reported by Huang et al. [43], and Wang et al. [41] combined hexagonal boron nitride (2D materials analogous to graphite) with α-SnWO₄. Both groups improved the photocatalytic activity of α-SnWO₄ for degrading methyl orange (MO) and tetracycline. In a composite of Ag-NPs and α-SnWO₄ [38], the photo-absorption efficiency can be increased through the surface plasmon resonance (SPR) of Ag-NPs. Because Ag-NPs have lower Fermi levels than the bottom of the conduction band of α-SnWO₄, the photoelectrons can migrate to Ag, increasing the efficiency of carrier separation. Moreover, Ag can more easily transfer electrons to a solution when it is in direct contact with the solution, which accelerates the transfer of interfacial carriers. The construction of heterojunctions is widely studied in SnWO₄ material research. The formation of heterojunctions is mainly based on the combination of different energy band positions of different materials. Accordingly, photogenerated electrons are easier to transfer to materials with lower conduction bands, while photogenerated holes tend to transfer to materials with higher valence bands. It is possible to enhance the carrier separation efficiency of materials by selecting suitable materials for compositing. For example, the heterojunctions of α-SnWO₄ and g-C₃N₄ were shown to effectively inhibit carrier recombination, thereby increasing the catalytic activity of the material [47]. SnWO₄/ZnO [51], SnWO₄/BiOBr [49], SnWO₄/UiO-66/g-C₃N₄ ternary heterojunction [54], one dimensional ZnWO₄@SnWO₄ core-shell heterojunction [48], and SnS/α-SnWO₄ [40] have also been reported. In addition, α-SnWO₄ and SnO₂ [42], along with β-SnWO₄ [52], can form heterojunctions to improve the efficiency of carrier separation.

In photoelectrocatalysis, Kölbach and Schnell et al. deposited NiO_x on α-SnWO₄ to protect the substrate from photocorrosion [4], which improved material stability and maintained a stable current for sulfite oxidation at 1.23 V vs. RHE in 30 min. Furthermore, α-SnWO₄ is susceptible to photo-corrosion and oxidation during photoelectrochemical water splitting, which results in a lower photocurrent value (<10 μA cm⁻²). However, the photocurrent of the material is greatly increased after protection by NiO_x (~0.75 mA cm⁻²). Another study found that a thin oxide layer (SnO₂) might form at the interface of α-SnWO₄ and NiO_x, which explains the reason for the decreasing photovoltage after the loading of NiO_x [9]. Thus, the exploration of alternative techniques for loading protection layers, such as vacuum evaporation and atomic layer deposition, is recommended.

3.5. Other Techniques

In addition to the aforementioned approaches, photocatalysis and photoelectrocatalysis performance may be influenced by the external environment [64], as has been studied in recent years. Liu et al. discussed the PEC performance of SnWO₄/Sn electrodes at different operating temperatures, and they found that higher photocurrents could be obtained at 70 °C (Figure 6a) [55]. Schnell studied the pH-dependent stability of α-SnWO₄, and pH = 7 (Figure 6b) was suggested to be used for testing due to the formation of the surface oxide layer as a protection layer [65]. Besides, α-SnWO₄, with higher crystallinity, was formed by Cottesman, presenting ~0.95 mA/cm² after loading with NiO_x for sulfite oxidation.

4. Conclusions and Outlook

In general, SnWO₄ is considered a type of potential catalytic material for photocatalysis and photoelectrocatalysis, especially for water splitting, because of its optical properties and special band structure (which covers the oxidation and reduction potential of water). β-SnWO₄ is always used in photocatalytic degradation due to the high reduction (oxidation) activity of its photo-generated electrons (holes), while α-SnWO₄ is employed in both photocatalysis and photoelectrocatalysis. In recent years, many researchers have studied the application of SnWO₄ in photocatalysis and photoelectrocatalysis, and they have improved the properties of materials by means of morphology control, doping, and multicomponent combinations. Compared with photocatalysis, photoelectrocatalysis has the advantage of discrete oxidation and reduction processes. This makes it easier to separate gaseous products, study reaction mechanisms and kinetics, and adjust reaction selectivity. Therefore, the application of α-SnWO₄ in photoelectrocatalysis needs to be further explored. However, it is worth noting that the catalytic performance of α-SnWO₄ and its applications in solar energy have not reached their expected values. For example, the highest reported experimental current for PEC water oxidation is only ~0.79 mA cm⁻² at 1.23 V_RHE, which is far from the theoretical value. Other problems include low efficiency and poor stability. To solve these problems, obtaining a high-performing substrate through morphology control, crystal surface control, and doping may prove to be an effective research route. Then, the PEC performance of α-SnWO₄ can be further upgraded by constructing heterojunctions, depositing protective layers, and depositing cocatalysts. Of these methods, loading cocatalysts is the most promising because the catalysts can also act as protection layers. However, a suitable loading method to avoid oxidizing α-SnWO₄ is required, and pinholes should be excluded to prevent α-SnWO₄ from attaching to the electrolytes.