Nanostructured Ternary Metal Tungstate-Based Photocatalysts for Environmental Purification and Solar Water Splitting: A Review

Visible-light-responsive ternary metal tungstate (MWO4) photocatalysts are being increasingly investigated for energy conversion and environmental purification applications owing to their striking features, including low cost, eco-friendliness, and high stability under acidic and oxidative conditions. However, rapid recombination of photoinduced electron–hole pairs and a narrow light response range to the solar spectrum lead to low photocatalytic activity of MWO4-based materials, thus significantly hampering their wide usage in practice. To enable their widespread practical usage, significant efforts have been devoted, by developing new concepts and innovative strategies. In this review, we aim to provide an integrated overview of the fundamentals and recent progress of MWO4-based photocatalysts. Furthermore, different strategies, including morphological control, surface modification, heteroatom doping, and heterojunction fabrication, which are employed to promote the photocatalytic activities of MWO4-based materials, are systematically summarized and discussed. Finally, existing challenges and a future perspective are also provided to shed light on the development of highly efficient MWO4-based photocatalysts.

Desired photocatalysts should have a suitable band gap to achieve a high harvesting efficiency of sunlight, sufficient quantum yield, and an appropriate position of the band edge to trigger redox reactions [40][41][42]. For an ideal photocatalyst, the conduction band (CB) edge should be sufficiently negative to drive the photo-reduction reaction. In contrast, the valence band (VB) edge should be sufficiently positive to trigger the photo-oxidation reaction [43]. For example, in photocatalytic water splitting, when the VB edge position of the semiconductor photocatalyst is more positive than the potential of H 2 O/O 2 (1.23 V vs. NHE) and the CB position is more negative than the potential of H 2 /H 2 O (0 V vs. NHE), the water splitting reaction can occur [44][45][46][47].
Recently, ternary tungstate-based complex oxides have been developed as potential candidates for efficient photocatalytic applications [48][49][50]. Tungstates are described by the general formula MWO 4 (M denotes a bivalent cation) [51] and are widely used in the luminescence, microwave ceramics, and catalytic fields, given their selfactivating fluorescence effect, microwave, and optical properties [52][53][54][55]. Owing to the variety of bivalent cations, the crystal structure of MWO 4 is dependent on the size of cationic radii. According to the literature [56], MWO 4 typically has a monoclinic wolframite structure for small M 2? cations (M = Fe, Co, Sn, and Ni) and a tetragonal scheelite structure for large M 2? cations (M = Ca, Ba, Pb, and Sr), as shown in Fig. 2 [57]. During the past few decades, MWO 4 with large radii cations, including CaWO 4 [58,59], BaWO 4 [60,61], PbWO 4 [62], and SrWO 4 [63], has been prepared using different synthesis approaches. However, the band gaps of these MWO 4 photocatalysts are much larger than that of TiO 2 and are not suitable for practical photocatalytic applications. In contrast, the band gaps of MWO 4 with small-radii cations are considerably smaller than that of TiO 2 and could be a promising choice for the efficient utilization of solar energy [64][65][66][67][68][69][70][71][72]. The band-gap energies of representative MWO 4 are summarized in Fig. 3 and Table 1. In addition, the band-gap energies of various MWO 4 and the radii of the metal cations are plotted in Fig. 4. It can be seen that a flock of MWO 4 is in the yellow area, in which each MWO 4 has a smaller metal cation radius (\ 0.73 Å ) and narrower band-gap energy (\ 3.2 eV) than those of the others. In contrast, MWO 4 with a larger cation radius have larger band-gap energy. Notably, for specific MWO 4 , the band gaps presented in Table 1 are not the only ones, because the absorption light range of the semiconductors can be affected and controlled by various factors, including morphology, size, doping, and defects, thus resulting in one semiconductor material possessing several band-gap energies. By comparing the requirements of novel photocatalysts, MWO 4 with small-radius cations is more advantageous and can be further developed as highly efficient photocatalysts. In addition, it is clearly observed that ternary MWO 4 systems with a narrow band gap have transition metals as the cation component, which is earthabundant, cost effective, and low-toxicity, benefiting wide usage in the future. However, for these visible-light-responsive pristine MWO 4 , photocatalytic activities remain inadequate for practical applications because of the rapid recombination of photogenerated holes and electrons.
Herein, we provide a comprehensive review of the evolution and current state of the development and application of ternary MWO 4 -based photocatalysts in environmental purification and solar water splitting. First, we discuss the fundamentals of ternary MWO 4 systems, including the crystal composition, electronic structure, and relationship between the intrinsic structures and properties. Subsequently, versatile reported strategies to improve the photocatalytic activities of pristine MWO 4 are systematically summarized. Finally, challenges and future developments of ternary MWO 4 -based photocatalysts are discussed. We believe that this review provides information on recent progress in ternary MWO 4 -based photocatalysts for environmental and energy applications and insight into future perspectives, which will aid the design of highly efficient semiconductor-based photocatalysts.

Ternary MWO 4 Photocatalysts (M = bivalent metal cations)
The photocatalytic activity of semiconductor photocatalysts is known to be closely related to their crystal and electronic structures [82]. In this section, an overview of the crystal and electronic structures of ternary MWO 4 is presented and the factors influencing their photocatalytic performance is explored.

Crystal Structure
As ABO 4 -type compounds, ternary MWO 4 complex materials possess a typical wolframite-type monoclinic crystal structure and scheelite-type tetragonal structure. In the scheelite crystal structure, one W atom coordinates with four O atoms to form the WO 4 tetrahedral unit. In contrast, in the wolframite crystal structure, one W atom is encircled by six oxygen atoms to form the WO 6 octahedral unit. For example, Yan et al. [83] reported a tetragonal structure in CdWO 4 material, in which the W atom is situated in the center of the tetrahedra, forming four W-O bonds of the same bond length of 1.758 Å , with the coordination number of the Cd atom being eight. However, the monoclinic structure of CdWO 4 is similar to that of previously reported MnWO 4 [84]. Both W(VI) and Cd(II) have octahedral O coordination, in which each octahedron shares two corners with its neighbors. However, the configuration of the WO 6 octahedron leads to severe distortion in which two W-O bonds are much shorter than the other four W-O bonds. The two crystal phases of CdWO 4 are shown in Fig. 5. An investigation of methyl orange (MO) degradation showed that the photocatalytic performance of monoclinic CdWO 4 was much higher than that of tetragonal CdWO 4 and commercial TiO 2 under UV light irradiation, which can be ascribed to the lower lattice symmetry of the monoclinic CdWO 4 . Furthermore, the electronic structures of the wolframite-and scheelite-type CdWO 4 were investigated by theoretical computations and simulations based on density functional theory (DFT). As shown in Fig. 5b-d, both monoclinic and tetragonal CdWO 4 are indirect-type semiconductors because the calculated bottom of the CB is not situated in the same line as the top of the VB. The calculated band gap of monoclinic CdWO 4 was larger than that of tetragonal CdWO 4 , which was not consistent with the photocatalytic results. Actually, the monoclinic CdWO 4 consisted of distorted WO 6 octahedra, leading to the generation of dipole moments in the WO 6 octahedral units, while the tetragonal CdWO 4 comprised normal WO 4 tetrahedra, forming a highly symmetric lattice in the absence of dipole moments. Based on its crystal and geometric structures, the monoclinic structure of CdWO 4 is considered a highly efficient photocatalyst, and its catalytic performance has been extensively explored [74,85,86].
In addition, it has also been reported that SnWO 4 has two crystal structures: a-SnWO 4 and b-SnWO 4 [87]. The orthorhombic a-SnWO 4 is comparatively stable in the structure below 670°C, whereas the cubic b-SnWO 4 is a steady structure above 670°C. In the crystal unit of  orthorhombic a-SnWO 4 , a single W atom is bonded with six O atoms to constitute typical corner-shared WO 6 octahedra, while the unshared WO 4 tetrahedra are composed of a crystal unit of b-SnWO 4 . Owing to the lone-pair effects on the Sn(II) ion, distorted SnO 6 octahedra are formed in both polymorphs. Cho et al. [88] prepared the aforementioned SnWO 4 materials with different types of crystal structures and investigated the relationship of the crystal structure with the optical and catalytic properties of SnWO 4 (Fig. 6). It was demonstrated that a difference in atom arrangement could result in an apparent difference in electronic distribution. The VB of SnWO 4 is constituted through high hybridization between the Sn 5 s orbital and O 2p orbital, resulting from the strong interaction between the atomic orbitals with closer energy. Meanwhile, the Sn 5 s orbitals contribute to the lower and upper energy levels of the VB, while the O 2p orbitals are dedicated to the middle energy levels of the VB. Therefore, the VB and CB electronic structures of SnWO 4 are totally different from those of the pristine WO 3 , in which the VB and CB comprise filled O 2p orbitals and empty W 5d orbitals, respectively. The band gap of a-SnWO 4 was calculated to be 1.65 eV, which was lower than that of WO 3 (1.77 eV), thereby accounting for the broadening effect of the VB, which can be attributed to the contribution of the Sn 5 s orbitals. In contrast, the calculated band gap of 3.45 eV for b-SnWO 4 was apparently much larger than that of a-SnWO 4 and WO 3 . Although the Sn 5 s orbitals also contribute to the VB of b-SnWO 4 (Fig. 6b), the increase in the band gap stems from the decreased length of the W-O bonds and enhanced crystal field splits, thus resulting in the upshifting of the CB position. Furthermore, the experimental results showed that both a-SnWO 4 and b-SnWO 4 exhibited higher photocatalytic performance for the degradation of Rhodamine B (RhB), as compared to other visible-light-response photocatalysts, such as bulk-or nanoWO 3 and nanoTiON x (Fig. 6d). Moreover, the b-SnWO 4 showed a higher photocatalytic activity for H 2 evolution in the presence of methanol, accompanying Pt as a co-catalyst under visible-light irradiation (Fig. 6e), which can be mainly ascribed to the higher CB edge of b-SnWO 4 compared to that of a-SnWO 4 . Based on the aforementioned analysis, one can conclude that the MWO 4 photocatalysts usually possess more than two types of crystal structure, thus leading to a difference in geometries and local lattice distortions. The non-bonded electrons in the metal ion should be considered an important factor in analyzing the crystal field, particularly in an MWO 4 -based asymmetric coordination environment. The distortion of the local crystal structure influences the electronic structure and band distribution, thus affecting the catalytic performances of MWO 4 -based photocatalysts.

Electronic Structure
Generally, tungstates are considered derivatives of H 2 WO 4 and WO 3 because of their similar elemental constitutions and crystal structures [89][90][91]. The DFT [92] and ab initio [93] calculations indicate that the CB of MWO 4 consists of W 5d orbitals in tungstates, as in WO 3 , while the O 2p orbitals only comprise the upper part of the VB because the bivalent metal cation in tungstates contributes differently to the VB and CB positions given different outer electronic arrangements [55]. For instance, visible-light-driven CuWO 4 is employed for photoelectrochemical (PEC) water splitting because of its suitable band gap of 2.3 eV [94][95][96]. For most binary metal oxides, the VB is mainly constituted of O 2p orbitals; thus, the VB energy is usually in the range of 2.5-3.0 eV, indicating that the top of the VB is not significantly shifted in the metal oxides. For ternary metal oxides, however, the mixing atomic orbitals between the O 2p orbitals and metal orbitals could result in an apparent shift in the top position of the VB [97]. Thus, for CuWO 4 , the VB shifts upward, in comparison to that of WO 3 , accounting for the hybridization between the O 2p orbitals and Cu 3d orbitals (Fig. 7a). The upward movement in the VB position indicates a decrease in the band gap, which results in an increased visible-light absorption range [98]. However, the composition of the CB is still a topic of hot debate. DFT  calculations show that the VB of CuWO 4 is composed of O 2p orbitals with a small portion of Cu 3d, whereas the CB of CuWO 4 is composed of W 5d orbitals [99,100]. Moreover, the Cu 3d orbitals may contribute to the CB of CuWO 4 except for the top of the VB [101]. Experimental results demonstrate that the CB shift of CuWO 4 relative to that of WO 3 can be ascribed to the incorporation of Cu 3d orbitals into the energy level of the CB [102]. Yet, strong evidence of an accurate contribution proportion of Cu 3d orbitals to the CB of CuWO 4 has not been obtained. Although the CB composition of CuWO 4 is unclear, the CuWO 4 photoanode presented a high photocatalytic activity with a photocurrent density of up to 0.07 mA cm -2 at 1.23 V and a high stability under AM 1.5G illumination (Fig. 7b, c), indicating that the CuWO 4 photoanode has a thermodynamic potential for water oxidation. In this system, a physical model of the photogenerated charge carrier pathways in CuWO 4 is proposed as shown in Fig. 7d. It shows that when the photogenerated electrons are transferred from a solution medium to a mid-gap state, the thermodynamic potential of the mid-gap state can be utilized to determine which elementary reaction is favored to occur and which is the rate-limiting reaction.
Apart from CuWO 4 , the electronic structures of other MWO 4 materials have also been previously studied. For example, Rajagopal and co-workers [103,104]    VB of tungstates, while the unoccupied Fe 3d orbitals and Co 3d orbitals were dominantly dedicated to the CB of FeWO 4 and CoWO 4 , respectively. In addition, the density of states showed that Co 3d and W 5d orbitals also contributed to the VB regions of CoWO 4 , similar to that of FeWO 4 . Hence, the VBs of FeWO 4 and CoWO 4 are composed of O 2p, W 5d, and Fe/Co 3d orbitals. However, X-ray emission spectroscopy results revealed that the W 5d orbitals and O 2p orbitals are dedicated to the entire VB of the tungstates, in which the O 2p orbitals are dedicated to the upper region of the VB and W 5d orbitals are dedicated to the lower region of the VB. The theoretical results agreed well with the experimental results for FeWO 4 and CoWO 4 . In addition, the electronic structure of NiWO 4 was obtained. For NiWO 4 , the CB predominantly consists of W 5d orbitals and Ni 3d orbitals, while the VB primarily consists of Ni 3d orbitals and O 2p orbitals [105]. It was found that the VB composition of NiWO 4 is different from those of FeWO 4 and CoWO 4 , which is related to the energy level distribution of the orbital electrons around the metal ions. Therefore, based on the aforementioned results, one can conclude that the electronic structures of ternary MWO 4 systems mainly depend on the position of the M 2? cation in the periodic table, which affects the outer electronic arrangements and hybridized electrons of the atomic orbital to the M 2? cation.

Strategies for Enhanced Photocatalytic Activity
As described in Sect. 1, ternary MWO 4 systems can act as highly promising photocatalysts for environmental purification and solar water splitting. However, among the major limiting factors in enhancing their photocatalytic activities is the rapid recombination of photogenerated electron and hole pairs. To overcome this problem and improve the overall catalytic performance of MWO 4 photocatalysts, many research groups have endeavored to develop various techniques to enhance the separation and transfer efficiency of photoexcited charge carriers inside MWO 4 or at the interface between different components. In this section, an overview of the developed strategies is provided to offer insights on their effects for the separation efficiency of photogenerated charge carriers and the corresponding catalytic performance of MWO 4 -based photocatalysts.

Morphological Control
As is known, the morphology, facet exposure, and dimensions of photocatalysts have a significant influence on the photocatalytic performance. For example, nanostructured photocatalysts can exhibit excellent photocatalytic activities because the morphologies and particle sizes of the photocatalysts have a significant influence on the optical and electronic properties, which in turn affect the photocatalytic activities. For example, Yu et al. [106] prepared FeWO 4 samples with different morphologies, including nanoparticles, flakes, nanorods (NRs), and a mixture of NRs with flakes and tiny granules, by varying pH values during the hydrothermal process, and systematically investigated their optical properties. The results indicated that the band-gap values of FeWO 4 are correlated with specific morphologies. Hosseinpour-Mashkani and coworkers [75] synthesized ZnWO 4 nanoparticles with different sizes through a precipitation route using different polymeric surfactants. The photocatalytic degradation experiments of MO demonstrated that the ZnWO 4 with the smallest size of 27 nm exhibited the highest photoactivity, compared with other ZnWO 4 samples under UV light illumination. It has been demonstrated that particle size can affect the band gap of semiconductors because of quantum size effects. The exposed surface area of nanoparticles increases with a decrease in nanoparticle size, which can provide more active sites for a surface catalytic reaction. In addition to morphologies and particle sizes, size-related crystallinity of MWO 4 has an important influence on the photophysical and photocatalytic properties. For example, Tong et al. [74] reported nanostructured CdWO 4 with controllable particle sizes via a hydrothermal process and studied the effects of the particle sizes on the lattice symmetry and crystallinity. It was found that the decreased size of the CdWO 4 nanoparticles resulted in an expanded lattice, lowered crystallinity, and broadened band gap. Meanwhile, the decrease in particle sizes caused an apparent decrease in the photocatalytic activity of CdWO 4 . These results illustrate that size-related properties are closely correlated with the photocatalytic activity of MWO 4 . Apart from the effects of the aforementioned factors on photoactivity, the crystal phase of MWO 4 is also regulated by various synthetic routes to obtain the desired physiochemical and optical properties, because different crystal phases can lead to distinct differences in the exposure of crystal facets and reactivities. For instant, different crystal phases of CdWO 4 nanocrystals were prepared by adjusting the used solvents for photocatalytic H 2 evolution (Fig. 8) [86]. The solvent significantly affected the chelation and growth of the CdWO 4 material, thus leading to apparent differences in the crystal phase. m-CdWO 4 nanocrystals with particle sizes ranging from 4.4 to 31 nm can be prepared using short-chain solvents, while t-CdWO 4 nanocrystals can be easily prepared utilizing long-chain solvents. The obtained m-CdWO 4 nanocrystals exhibited much higher H 2 production than the t-CdWO 4 nanocrystals.
During recent years, the construction of hierarchical structures to tune the morphologies of MWO 4 photocatalysts has attracted more attention, because hierarchical structures can offer more exposed surfaces and/or active sites. For example, highly efficient hydrophobic CdWO 4 microspheres were synthesized by Hou et al. [107] through a microwave-assisted interfacial hydrothermal strategy. The CdWO 4 microspheres showed enhanced photocatalytic activity for the degradation of MO under mercury lamp irradiation (Fig. 9). The advantages of a hierarchical structure for the enhanced photocatalytic activity of MWO 4 are supported by several research groups [108][109][110][111]. Chen et al. [112] prepared FeWO 4 microspheres by using an ethylene glycol-assisted solvothermal approach, in which ionic1-octyl-3-methylimidazolium tetrachloroferrate was used as one of the starting materials and played an important role as both a reactant and template. The microsphere-like FeWO 4 material consisted of numerous nanosheets and exhibited a much better photo-Fenton activity in water purification (Fig. 10) because of the generation of hydroxyl radicals, which are produced from the chemical reaction between Fe 2? on the surface of FeWO 4 and H 2 O 2 . The formed Fe 3? was further reduced by the photoinduced electrons to generate Fe 2? , which is a virtuous cycle, to maintain high photocatalytic performance. Recently, Zhou et al. [113] synthesized an urchinlike MnWO 4 hierarchical structure through a facile hydrothermal process with the assistance of cetyltrimethylammonium bromide (CTAB) as surfactant. The introduction of CTAB as the surfactant had significant effects on the morphology and magnetic properties of the MnWO 4 nanocrystals. Subsequently, Xing et al. [114] provided a new route for the synthesis of a complex threedimensional (3D) MnWO 4 nanostructure. In this case, a 3D flower-like MnWO 4 nanocomposite was synthesized using a microemulsion-based solvothermal approach. Moreover, Chen et al. [115] discussed the effect of the morphology of b-SnWO 4 on its photocatalytic performance, in which the b-SnWO 4 with a multi-armed architecture and hexahedral symmetry displayed much higher photocatalytic activities Hence, more surface reaction sites induced by the hierarchically multi-armed architecture and the band structure reframing caused by incorporation of the Sn atom into WO 3 contributed to the excellent photocatalytic activity. Previously, crystal facet exposure was viewed as an effective strategy to regulate the surface physicochemical and photophysical properties, thus optimizing the reactivity and selectivity of photocatalysts. In general, a crystal facet with a high percentage of unbonded atoms has superior reactivity in comparison to that with a low ratio of unpaired atoms. In addition, crystal facets possessing high surface energy are usually unstable during preparation. It is desirable to synthesize photocatalysts with a high exposure of high-energy crystal facets to enhance the photocatalytic reactivity and selectivity. Using DFT calculations, Qiu and co-workers [116] reported the atom distributions and electronic properties of MnWO 4 and FeWO 4 with specific facets. The calculated results showed that the {010} and {100} facets have the lowest surface energies in wolframite-type FeWO 4 and MnWO 4 , respectively. Meanwhile, it was observed that the Fe and Mn atoms on the {010} and {001} planes as absorption sites can be used to absorb anions. These results indicated that the exposed {100} facet in MnWO 4 and FeWO 4 can provide a path for improving their photoactivities, while the other exposed facets can offer a certain selectivity to a specific reaction, such as a dichlorination reaction. Recently, Ungelenk et al. [117] synthesized phase-pure b-SnWO 4 with truncated rhombic dodecahedrons using a microemulsion technique with CTAB as a co-surfactant with n-hexanol (Fig. 11). Benefitting from the rapid Na 2 WO 4 -induced nucleation and slow crystal growth controlled by the CTAB-mediated microemulsion reaction, the as-prepared b-SnWO 4 with truncated rhombic dodecahedrons was encircled with highly exposed {100} and {110} facets. e Photocatalytic mechanism for CdWO 4 microspheres. Reproduced with permission from Ref. [107]. Copyright 2014 Royal Society of Chemistry daylight irradiation, which was far better than that of other photocatalysts, including bulk non-faceted b-SnWO 4 and spherical-like b-SnWO 4 nanoparticles. In addition, given the slight difference in specific surface area between the faceted b-SnWO 4 and non-faceted b-SnWO 4 , it was concluded that optimized facet exposure was the predominant reason for the distinct photocatalytic activity. In addition, Tian et al. [118] reported a facile solvothermal method for the synthesis of hierarchical FeWO 4 nanosheets with an exposed {100} facet, which exhibited excellent peroxidaselike catalytic activity for oxidizing the peroxidase substrate of 3,3 0 ,5,5 0 -tetramethylbenzidine (TMB) because of the formation of hydroxyl radicals in the presence of H 2 O 2 . The surface Fe 2? in FeWO 4 can activate the H 2 O 2 molecule to produce active hydroxyl radicals for the oxidation of TMB. The results clearly indicated that the {100} facet of FeWO 4 had a much higher ratio of Fe atoms than that of the {001} and {010} facets, which explained the enhanced catalytic activity of FeWO 4 with the exposed {100} facet.
Owing to the high surface area and large absorption cross section it provides, a low-dimensional nanostructure can be constructed to manipulate and regulate optical, electrical, and magnetic properties [119,120]. Low-dimensional nanostructures including those that are one-dimensional (1D) or two-dimensional (2D) cause the growth of a crystal along one or two directions, which can contribute to more exposure of specific surface facets. For example, 1D CdWO 4 NRs were prepared using microwave or hydrothermal approaches and exhibited excellent photocatalytic activity for environmental treatments, as compared to nanoparticles [121][122][123]. Kovacs and co-workers [111] prepared a series of FeWO 4 with different morphologies, including nanoparticles, NRs, and nanosheets, by varying the Fe precursors. The nanosheet-like FeWO 4 with band-gap energy of * 2.2 eV exhibited excellent absorption ability throughout the UV and visible-light range, which was attributed to the formation of a cavity assembled by nanosheets that resulted in enhanced photon harvest. Therefore, the FeWO 4 nanosheets showed higher

Surface Modification
Considering that the photocatalytic reaction proceeds on the surface of semiconductors, the surface physiochemical properties of semiconductor-based photocatalysts are important for improvement of photocatalytic activity. Several strategies, including chemical etching, surface coverage, and co-catalyst attachment, have been developed to tune the surface properties of semiconductor-based photocatalysts.
The purpose of etching techniques, such as chemical etching and laser or electron-beam irradiation, is to form non-stoichiometric or metal/oxygen vacancies on the surface of an inorganic semiconductor. The formation of metal or oxygen vacancies has proven to have an apparent influence on the electronic distribution because of the introduction of a new defect energy level, thus affecting the light absorption range and photocatalytic activity. For example, Aloysius-Sabu et al. [124] investigated the effects of intentional electron-beam irradiation on the crystal phase, size, and surface properties of CaWO 4 that was prepared through chemical precipitation and heat treatment. The experimental results showed that the high-energy electron beam significantly affected the crystal size and surface structure, but not the crystal phase. When the CaWO 4 photocatalyst was irradiated by an electron beam, the surface atomic layers of CaWO 4 underwent stretching and compressive strain, which resulted in the formation of surface defects and a new energy level in the band gap. Therefore, an apparent absorption tail and narrowed bandgap energy were observed in the irradiated CaWO 4 sample. In addition, Lin et al. [125] prepared a visible-light-driven Ag 2 WO 4 photocatalyst through a laser irradiation route in liquid using commercial Ag 2 WO 4 as a starting material for organics degradation and H 2 evolution. Because of the laser irradiation, the crystal structure was recrystallized and a lattice defect was introduced in Ref. [117]. Copyright 2012 Royal Society of Chemistry commercial Ag 2 WO 4 sample, which was ascribed to a large band gap of 3.22 eV for bulk Ag 2 WO 4 . However, to enhance the solar conversion efficiency of tungstates, surface coverage has been adopted to increase the charge transfer efficiency by means of passivating the surface via deposition of a protective layer. For example, Karthiga and co-workers [71] reported the synthesis of NiWO 4 nanoparticles modified by a plant extract, A. indica, as a capping agent through a precipitation route for enhanced photocatalytic activity. The introduction of A. indica, which possesses rich water-soluble heterocyclic groups, led to the formation of a passivation layer on the surface of the NiWO 4 nanoparticles, which allowed the NiWO 4 nanoparticles to separate well from each other. In comparison to the bare NiWO 4 nanoparticles, the A. indica-coated NiWO 4 exhibited a much higher photocatalytic activity for the degradation of organic contaminants under visible-light irradiation because of the formation of the passivation layer of the plant extract, which significantly suppressed the recombination of photoinduced electrons and holes. Meanwhile, modified NiWO 4 presented higher antimicrobial activity as compared with pure NiWO 4 nanoparticles.
Apart from chemical etching and surface coverage strategies, attachment of noble metal co-catalysts (such as, Pt, Au, and Ag) is another effective means to tune the surface photophysical properties of photocatalysts because of the surface plasmon resonance (SPR) effect [126,127]. The SPR effect not only significantly enhances visible-light absorption, but also produces a localized electromagnetic field, thus improving the separation efficiency of the photogenerated charge carriers at the interfaces between the metal and semiconductor [128,129]. Furthermore, the metal-semiconductor heterostructure could efficiently suppress the recombination of photogenerated electrons and holes because of the formation of Schottky barriers at the contacted interface, thus enhancing photocatalytic performance [126]. Based on the aforementioned features, the introduction of a noble metal into MWO 4 is a feasible approach to realize enhancement of its photocatalytic performance. Recently, Liu et al. [130] prepared Ag nanoparticles (NPs)/a-SnWO 4 nanosheets through microwave-assisted anchoring of Ag NPs on SnWO 4 nanosheets. The loading amount of the deposited Ag NPs was welltuned by adjusting the initial concentration of the Ag ? precursor and CTAB surfactant (Fig. 12a). The obtained Ag NPs/a-SnWO 4 hybrid showed enhanced light absorption ability and photocatalytic activity for the degradation of MO, compared to pure a-SnWO 4 , under visible-light irradiation. Moreover, the hybridized Ag NPs/a-SnWO 4 system exhibited improved transient photocurrent density in comparison to that of pristine a-SnWO 4 (Fig. 12c), indicating that more photoinduced charge carriers could be produced and further participated in the redox reaction. In addition, Yan et al. [131] synthesized Ag-loaded CdWO 4 NRs using a photo-assisted co-precipitation method with the addition of the PEG-100 surfactant, which exhibited a higher photocatalytic activity than that of pure CdWO 4 NRs because of the SPR effect. In another study, Au NPs were utilized to construct Schottky barriers in an MWO 4based system. For instance, Au NRs/MnWO 4 , with a diameter of 20-40 nm, was reported by Chakraborty et al. [132] to enhance the photocatalytic decomposition of 2-propanol and phenol. The Au NPs as co-catalysts in the Au/MnWO 4 hybrid were beneficial to multi-electron O 2 reduction and hole oxidation.

Heteroatom Doping
It has been demonstrated that the introduction of impurities via doping of heteroatoms into a semiconductor can influence the photocatalytic performance [133,134]. However, heteroatom doping might either have positive or negative impacts for the photocatalytic performance of semiconductors, because there are two different doping levels-the shallow level near the surface and deep level inside the body [135]. Deep-level doping can usually provide a recombination center to intensify the meaningless dissipation of absorbed photons, thus undermining the photocatalytic activity. Therefore, appropriate heteroatom doping is vital to increasing the photocatalytic performance of heteroatom-doped photocatalysts. To overcome shortcomings, such as a narrow wavelength range and rapid recombination of photogenerated electron-hole pairs, a few recent reported types of heteroatom doping to enhance the photocatalytic performances of ternary MWO 4 materials are reviewed and their roles discussed in detail.
In heteroatom doping, the dopants are mainly non-metal elements such as B [136], Cl [137], and various transition metals (Zn, Ni, Fe, Co, etc.) [138,139], which exhibit enhanced photocatalytic activity for mineralizing organic pollutants. For instance, Chen et al. [140] synthesized F-doped ZnWO 4 nanocrystals (F i -ZnWO 4 nanocrystals) using a two-step hydrothermal process and investigated its chemical bonds via band structure calculations (Fig. 13). In comparison to undoped ZnWO 4 , the F i -ZnWO 4 nanocrystals exhibited significant red shifts and improved light absorption in the range of UV-visible light, which resulted in enhancement of photocatalytic activity for the degradation of RhB under mercury lamp irradiation. Additionally, the experimental results showed that the crystallinity and morphology of the prepared F i -ZnWO 4 was strongly related to the photocatalytic activity. F i -ZnWO 4 exhibited higher photocatalytic performance for the degradation of organic dyes than F i -ZnWO 4 NPs. Based on the theoretical calculation results, the enhancement of the photocatalytic activity of F i -ZnWO 4 might be ascribed to the following reasons. First, F-doping increased the absorptivity of F i -ZnWO 4 , such that it could absorb more reactants to enhance the photocatalytic activity. Second, a new halffilled state was introduced into the original band gap of ZnWO 4 accompanying the F-doping, which could provide more holes to enhance the photocurrent density of F i -ZnWO 4 . Thus, heteroatom doping could efficiently improve the photocatalytic activities of MWO 4 by introducing a new energy level to regulate the original electronic structure.
Apart from the introduction of non-metal elements, transition metal elements can also be a potential dopant. For example, Su et al. [141] prepared Zn 2? -doped SnWO 4 nanocrystals, and reported that the morphological alteration of SnWO 4 nanocrystals from nanosheets to nanowires can be controlled by Zn 2? doping. Consequently, the Zn 2? -doped SnWO 4 exhibited a greater Brunauer-Emmett-Teller surface area (54 and 100 m 2 g -1 for pure SnWO 4 and Zn 2? -doped SnWO 4 , respectively), narrowed band gap (2.68 and 2.64 eV for pure SnWO 4 and Zn 2? -doped SnWO 4 , respectively), and highly enhanced photocatalytic performance for the degradation of MO (95% and 30% for pure SnWO 4 and Zn 2? -doped SnWO 4 , respectively) compared to that of pure SnWO 4 . In addition, Song et al. [142] reported the synthesis of Zn-doped CdWO 4 NRs using a hydrothermal process to enhance the photocatalytic conversion efficiency of organic pollutants into low-toxicity small molecules under simulated sunlight irradiation. The influence of the Zn-doping amounts on the crystal phase, morphology, and optoelectronic properties of CdWO 4 NRs was also systematically investigated. Compared to the undoped sample, the Zn-doped CdWO 4 NRs exhibited much higher photocatalytic activity, which was assigned to Recently, rare earth element-doped photocatalysts have attracted more attention because of their special 4f electron configuration, which could be beneficial for introducing a suitable energy level into the original band gap of MWO 4 [143]. Phuruangrat et al. [144] synthesized Ce-doped ZnWO 4 using a hydrothermal process and investigated the influence of Ce doping on the crystal phase, morphology, electronic structure, and photocatalytic activity of ZnWO 4 . After the introduction of Ce atoms, the photocatalytic activity of ZnWO 4 improved with the increase in Ce content, owing to the following two reasons. First, the introduction of the Ce 3? dopant led to the generation of defects on the surface of Ce-doped ZnWO 4 . Second, the Ce 4? ions on the surface of ZnWO 4 could efficiently trap electrons at the CB by the reduction of Ce 4? into Ce 3? ions, thus efficiently suppressing the recombination of photoinduced electrons and holes in the Ce-doped ZnWO 4 . Therefore, rare earth elements with variable valence, such as Ce, La, and Eu, are promising dopants to improve the photocatalytic activity by trapping photogenerated electrons, consequently limiting the recombination of photogenerated charge carriers.

Hybridization with Semiconductors
Among the aforementioned approaches, constructing a semiconductor heterostructure is an effective means to obtain highly efficient photocatalysts [145][146][147][148]. When heterojunctions are composed of different semiconductors that have matching band potentials to form type-I or type-II heterojunctions through realignment of the energy level, the excited photogenerated holes and electrons can be moved from one semiconductor to another in opposite directions driven by the formed built-in electric fields [149,150], thus strengthening the separation efficiency of the photoinduced electrons and holes and further enhancing photocatalytic performance.
For constructing MWO 4 -based heterojunctions, semiconductors with a narrow band gap were chosen as a counterpart component to form the heterojunction system, which exhibited remarkable advantages for enhancing the photocatalytic performance. Among most semiconductors with a narrow band gap, cadmium sulfide (CdS) has attracted increasing attention for heterojunction fabrication because of its narrow band gap and high CB position, which could be beneficial for photocatalytic H 2 production [151,152]. Therefore, various CdS/MWO 4 heterojunction photocatalysts have been developed, which exhibit enhanced photocatalytic activities in water purification and energy conversion [153,154]. For instance, Yan et al. [155] prepared a CdS/MnWO 4 heterojunction using a facile hydrothermal method to mineralize MB and methyl violet (MV) under visible-light irradiation. Owing to the overlapping of energy bands and tightly contacted interfaces between CdS and MnWO 4 , the holes at the VB of MnWO 4 could transfer to the VB of CdS, and the excited electrons at the CB of CdS could in turn move into the CB of MnWO 4 . This model could efficiently limit the recombination rate of the photogenerated electrons and holes, thus intensifying the separation of the photogenerated charge carriers in the hybrid system (Fig. 14). Nevertheless, it is apparent that the introduced CdS amount was not synchronous with the photocatalytic activities of the heterojunction, while there was an optimal amount of CdS in the hybrid. Excessive CdS amounts caused severe agglomeration of MnWO 4 , damaging the heterojunction and worsening the separation efficiency of the photogenerated electrons and holes. This phenomenon was nearly discovered in the heterojunctions by combining two or more components into one system. In addition, Xu et al. [156] prepared a CdS/ZnWO 4 heterojunction consisting of ZnWO 4 NRs and CdS NPs using a hydrothermal method for the photodegradation of ciprofloxacin antibiotics. Compared to the ZnWO 4 NRs and CdS NPs, the CdS/ ZnWO 4 hybrids showed higher photocatalytic activity than bare ZnWO 4 NRs and CdS NPs, which was ascribed to the highly efficient separation of the photogenerated electrons and holes in the hybrid structure.
The CdS/CdWO 4 heterojunction also exhibited enhanced efficiency in the photocatalytic H 2 production. For instance, Jia [157] and Wang et al. [158] deposited CdS on the surface of CdWO 4 via an ion-exchange and in situ growth route, and the fabricated Z-scheme CdS/CdWO 4 hybrid exhibited significantly enhanced photocatalytic performance for H 2 evolution compared to that of the pure CdWO 4 and CdS. As discusses in the previous section, the MWO 4 with a small M 2? cation has a narrow band-gap energy, which can be considered an efficient solar energy harvester to connect with wide band-gap semiconductors for constructing MWO 4 -based heterojunctions [159][160][161]. Thus, Jiang et al. [162] fabricated an MnWO 4 /TiO 2 heterojunction with excellent mechanical adhesion by the in situ growth of MnWO 4 on a porous TiO 2 film; it presented extremely high photocatalytic performance for degrading MB because of its high crystallinity, large surface area, and strong mechanical properties (Fig. 15). Similarly, Buvaneswari et al. [163] prepared an FeWO 4 / ZnO heterojunction via a simple co-precipitation route; the band gap of the prepared FeWO 4 /ZnO heterojunction was estimated to be 2.12 eV, which was apparently smaller than that of ZnO (3.01 eV). In comparison to pristine ZnO, the photocatalytic activity of the FeWO 4 /ZnO hybrid was substantially enhanced because of the formation of a FeWO 4 /ZnO heterojunction structure. Additionally, the CuWO 4 /ZnO hybrid consisting of ZnO NRs and CuWO 4 NPs fabricated by Mavric et al. [164] showed enhanced photocatalytic activity compared to that of pure CuWO 4 NPs and ZnO NRs.
Furthermore, a MO x /MWO 4 hybrid as a smart-built heterojunction was fabricated using a facile one-pot synthetic strategy to enhance the interaction between MO x and MWO 4 , in which M is generally reported to be Fe, Ni, Co, or Cu [165][166][167][168]. Cao [171,172], NiWO 4 /WO 3 [173,174], or CuWO 4 /WO 3 [175,176]. Aslam et al. [177] prepared a CdWO 4 /WO 3 heterojunction using a hydrothermal and chemisorption method, and reported enhanced photocatalytic activities toward the degradation of organic pollutants, compared with pure CdWO 4 and WO 3 . This section clearly demonstrates in detail that the construction of MWO 4 -based heterojunction systems is an effective and controllable method for enhancing the photocatalytic activities of MWO 4 -based semiconductors.

Hybridization with Carbon-Rich Materials
Carbon-rich materials, including carbon nanotubes (CNTs), graphene, and graphitic carbon nitride (g-C 3 N 4 ), possess unique physical and chemical properties such as a large surface area, high absorption co-efficiency, and chemical stability, ensuring excellent and long-lasting applications in the fields of photochemical and PEC water treatment, photovoltaic devices, and water splitting [178][179][180][181][182][183]. Carbon-rich materials have a conjugative p structure and unique sp 2 /sp 3 hybrid carbon network, which are suitable substrates for constructing hybrid photocatalysts to intensify the separation and transportation of photoinduced charge carriers inside carbon-rich networks, thus improving the photocatalytic performance [184][185][186][187]. Based on this strategy, Gaillard et al. [188] synthesized a novel photoelectrode consisting of CuWO 4 and a multi-wall CNT (MWCNT) to tune the photogenerated charge transfer in the nanocomposite film for enhancing the performance of solar-driven PEC water splitting. Compared to the bare CuWO 4 photoelectrode, the resistance and photocurrent density of the CuWO 4 /MWCNT composite photoelectrode decreased and increased by 30% and 26%, respectively.  This was mainly attributed to the complete dispersion of the MWCNT as an electron collector in the entire CuWO 4 layer. Compared to CNTs, graphene nanosheets produced via the chemical oxidation treatment of graphite have more sp 3 hybridized edge structure because of the destroyed perfect sp 2 structure. It is well-known that a perfect sp 2 carbon structure (CNT) is beneficial for rapid charge mobility, and that an sp 3 -hybridized carbon structure (graphene) can lead to a small band gap in the semiconductor [189,190]. Meanwhile, layer-structure carbon materials have a richer porosity substructure assembly from graphene stacking and surface defects, which could provide more reactive sites, in comparison to tube-like carbon materials. Recently, Bai et al. [191] designed a ZnWO 4 /graphene hybrid, demonstrating that graphene could act as a photo-sensitizer in the hybrid and improve the production of Á OH and Á O 2 radicals. The excited photogenerated electrons at the CB of ZnWO 4 could be easily injected into the lowest unoccupied molecular orbital (LUMO) of graphene, resulting in a beneficial spatial separation between the holes and electrons inside the ZnWO 4 /graphene hybrid, in which more holes and electrons can participate in the production of active species, in comparison to bare ZnWO 4 . Xu et al. [192] reported a CdWO 4 /graphene hybrid using a hydrothermal process, in which the formed heterojunction showed significantly enhanced photocatalytic activity compared to that of the bare ZnWO 4 . However, it was found that excessive graphene could have a negative effect on the photocatalytic performance because of the reduced light absorption efficiency of CdWO 4 with the addition of the superfluous graphene.
Apart from graphene, other carbon-rich materials, such as g-C 3 N 4 and mesoporous carbon materials, have been considered as promising candidates to build MWO 4 -based heterojunctions. g-C 3 N 4 , which is regarded as an allotrope of C 3 N 4 , possesses excellent chemical stability, a suitable band gap of 2.7 eV, and a high specific surface area, which are beneficial for anchoring various other semiconductor materials [193][194][195]. For instance, Sun et al. [196] synthesized g-C 3 N 4 /ZnWO 4 NRs via a thermal treatment route, and investigated the microcosmic mechanisms of the  [162]. Copyright 2016 Royal Society of Chemistry enhanced photocatalytic activity of C 3 N 4 /ZnWO 4 in comparison to pristine ZnWO 4 NRs and C 3 N 4 . Together with theoretical calculation results, it was found that the wellmatched energy level alignment of the C 3 N 4 and ZnWO 4 NRs was the primary reason for the enhancement of the photocatalytic performance, in which the electrons at the VB top edge of the C 3 N 4 , when excited by the incident light, quickly jumped to the CB bottom edge of C 3 N 4 and then transferred to the CB of ZnWO 4 because of the staged alignment, thus enhancing the charge transfer and separation efficiency. In addition, Tian et al. [197] synthesized a CdWO 4 /C 3 N 4 hybrid using a hydrothermal process, followed by a mixed-calcination treatment. Compared to pure CdWO 4 , the CdWO 4 /C 3 N 4 heterojunction exhibited higher photocatalytic activity (Fig. 16c), the proposed mechanism for which is shown in Fig. 16d. The matched band structures between the two components contributed to the separation and transfer process, in which the excited electrons at the CB of C 3 N 4 moved to the CB of CdWO 4 following a downward staged band, such that the photoinduced electrons and holes were efficiently separated, thus enhancing the overall photocatalytic activity. Considering its appropriate band energy level, g-C 3 N 4 is usually used to construct a Z-scheme hybrid system to enhance photocatalytic activities. Instead of a type-II heterojunction, the direct Z-scheme configuration not only forms a built-in electronic field to promote the separation and transfer efficiency of photogenerated electrons and holes, but it also maintains the reductive and oxidative ability of electrons and holes [198,199]. Recently, Zhu and co-workers [200] reported a direct Z-scheme heterojunction by combining g-C 3 N 4 with Ag 2 WO 4 through a facile precipitation route. Because of the use of g-C 3 N 4 as a support in the precursor solution, Ag 2 WO 4 was able to nucleate and grow on the surface of the layered C 3 N 4 , thus resulting in the Ag 2 WO 4 evenly distributing on the surface of the layered g-C 3 N 4 -nanosheets. In comparison to the bare Ag 2 WO 4 and g-C 3 N 4 , the Z-scheme of the g-C 3 N 4 /Ag 2 WO 4 hybrid system exhibited  [197]. Copyright 2015 Elsevier a much higher photoactivity for the degradation of MO, owing to the efficient separation between the photoinduced electrons and holes in the direct Z-scheme configuration. Given the aforementioned examples and explanations, it is obvious that carbon-rich materials possess a high charge carrier mobility and large specific surface area, which could efficiently promote the separation efficiency of photoexcited electrons and holes in the MWO 4 , ultimately leading to improvement of the photocatalytic activity.

Summary and Outlook
This review summarized the development of novel strategies to enhance the photocatalytic performance of MWO 4based materials with a special emphasis on their applications in environmental purification and solar water splitting. Although significant improvements have been achieved in the construction of highly efficient ternary MWO 4 -based oxides, challenges remain that need to be addressed. First, the recombination rate of photogenerated charge carriers for MWO 4 -based photocatalysts is still considerably high, accounting for poor reduction ability in the photoexcited electrons at a low potential of the CB edge, which are easily quenched by defects and holes. Second, although morphological engineering could improve the photocatalytic activity of MWO 4 -based systems by regulating the crystal structure, particle size, and surface area, the current synthetic methods are inadequate for large-scale preparation, particularly for nanosized materials, which would significantly improve the separation efficiency of the photogenerated charge carriers. Third, the surface effect, particularly the crystal surface effect on the photocatalytic performance of MWO 4 -based systems, has not been synergistically and comprehensively investigated. It is thought that the atom configurations and surface defects should be paid more attention, to provide important information for designing highly efficient photocatalysts. Fourth, based on this review, it is clear that the MWO 4 materials consisting of a single valence metal ion, such as Cd, Zn, or Sn, have been well-developed in the past, while those composed of a versatile valence metal, for instance, Co, Fe, or Ni, have been insufficiently explored in surface engineering and theoretical computation.
To overcome these challenges, future research needs to focus on the exploration of novel photocatalytic materials. Although the sunlight-harvesting ability and separation efficiency of photogenerated charge carriers could be strengthened by heteroatom doping or heterojunction fabrication as reported by previous literature, traditional material screening, high-throughput screening, and computational materials design can guide the construction of photocatalysts with a proper band edge position and suitable band gap, thereby shortening the experimental period, reducing the workload, and saving experimental cost. In other fundamental studies, combining experiment and theory would enable understanding the photocatalytic principles and screen alternative high-performance photocatalysts. Future work also needs to focus on facile synthetic approaches for constructing stable MWO 4 materials with high active crystal surface and/or quantum size, and developing advanced techniques for large-scale production. It is expected that further progress in ternary MWO 4 -based photocatalysts for applications in environmental purification and solar water splitting will be made in future studies.