Nanostructured Ternary Metal Tungstate-Based Photocatalysts for Environmental Purification and Solar Water Splitting: A Review
KeywordsTernary metal tungstates Micro- and nanostructures Photocatalysis Environmental purification Water splitting
A series of ternary tungstate-based photocatalysts and their applications in solar energy conversion and environmental purification are systematically introduced.
The relationship between intrinsic structures and unique properties of ternary tungstate-based photocatalysts is discussed and summarized in detail.
Various new concepts and innovative strategies are employed to enhance the photocatalytic performance of ternary tungstate-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 . For example, in photocatalytic water splitting, when the VB edge position of the semiconductor photocatalyst is more positive than the potential of H2O/O2 (1.23 V vs. NHE) and the CB position is more negative than the potential of H2/H2O (0 V vs. NHE), the water splitting reaction can occur [44, 45, 46, 47].
Band gaps, crystal sizes, and effective ionic radii of different MWO4 materials
Ionic radius of cation M (Å)
Band gap Eg (eV)
Crystalline sizes (nm) via Scherrer equation
Herein, we provide a comprehensive review of the evolution and current state of the development and application of ternary MWO4-based photocatalysts in environmental purification and solar water splitting. First, we discuss the fundamentals of ternary MWO4 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 MWO4 are systematically summarized. Finally, challenges and future developments of ternary MWO4-based photocatalysts are discussed. We believe that this review provides information on recent progress in ternary MWO4-based photocatalysts for environmental and energy applications and insight into future perspectives, which will aid the design of highly efficient semiconductor-based photocatalysts.
3 Ternary MWO4 Photocatalysts (M = bivalent metal cations)
The photocatalytic activity of semiconductor photocatalysts is known to be closely related to their crystal and electronic structures . In this section, an overview of the crystal and electronic structures of ternary MWO4 is presented and the factors influencing their photocatalytic performance is explored.
3.1 Crystal Structure
3.2 Electronic Structure
Generally, tungstates are considered derivatives of H2WO4 and WO3 because of their similar elemental constitutions and crystal structures [89, 90, 91]. The DFT  and ab initio  calculations indicate that the CB of MWO4 consists of W 5d orbitals in tungstates, as in WO3, 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 .
Apart from CuWO4, the electronic structures of other MWO4 materials have also been previously studied. For example, Rajagopal and co-workers [103, 104] used X-ray emission spectroscopy and DFT computation to study the electronic structures and related properties of FeWO4 and CoWO4 photocatalysts. The theoretical calculation results demonstrated that O 2p orbitals mainly contributed to the VB of tungstates, while the unoccupied Fe 3d orbitals and Co 3d orbitals were dominantly dedicated to the CB of FeWO4 and CoWO4, respectively. In addition, the density of states showed that Co 3d and W 5d orbitals also contributed to the VB regions of CoWO4, similar to that of FeWO4. Hence, the VBs of FeWO4 and CoWO4 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 FeWO4 and CoWO4. In addition, the electronic structure of NiWO4 was obtained. For NiWO4, 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 . It was found that the VB composition of NiWO4 is different from those of FeWO4 and CoWO4, 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 MWO4 systems mainly depend on the position of the M2+ cation in the periodic table, which affects the outer electronic arrangements and hybridized electrons of the atomic orbital to the M2+ cation.
4 Strategies for Enhanced Photocatalytic Activity
As described in Sect. 1, ternary MWO4 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 MWO4 photocatalysts, many research groups have endeavored to develop various techniques to enhance the separation and transfer efficiency of photoexcited charge carriers inside MWO4 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 MWO4-based photocatalysts.
4.1 Morphological Control
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 CdWO4 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  prepared a series of FeWO4 with different morphologies, including nanoparticles, NRs, and nanosheets, by varying the Fe precursors. The nanosheet-like FeWO4 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 FeWO4 nanosheets showed higher photocatalytic activity than other control samples for the degradation of organic dyes under visible-light irradiation.
4.2 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.  investigated the effects of intentional electron-beam irradiation on the crystal phase, size, and surface properties of CaWO4 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 CaWO4 photocatalyst was irradiated by an electron beam, the surface atomic layers of CaWO4 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 band-gap energy were observed in the irradiated CaWO4 sample. In addition, Lin et al.  prepared a visible-light-driven Ag2WO4 photocatalyst through a laser irradiation route in liquid using commercial Ag2WO4 as a starting material for organics degradation and H2 evolution. Because of the laser irradiation, the crystal structure was recrystallized and a lattice defect was introduced in Ag2WO4, leading to the formation of intermediate energy levels with a decrease of 0.44 eV in the band gap. The synthesized Ag2WO4 exhibited a photocatalytic H2 evolution rate as high as 13.73 μmol (hg)−1 under visible-light illumination, while no H2 evolution was observed in the unirradiated commercial Ag2WO4 sample, which was ascribed to a large band gap of 3.22 eV for bulk Ag2WO4.
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  reported the synthesis of NiWO4 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 NiWO4 nanoparticles, which allowed the NiWO4 nanoparticles to separate well from each other. In comparison to the bare NiWO4 nanoparticles, the A. indica-coated NiWO4 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 NiWO4 presented higher antimicrobial activity as compared with pure NiWO4 nanoparticles.
4.3 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 . 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 MWO4 materials are reviewed and their roles discussed in detail.
Apart from the introduction of non-metal elements, transition metal elements can also be a potential dopant. For example, Su et al.  prepared Zn2+-doped SnWO4 nanocrystals, and reported that the morphological alteration of SnWO4 nanocrystals from nanosheets to nanowires can be controlled by Zn2+ doping. Consequently, the Zn2+-doped SnWO4 exhibited a greater Brunauer–Emmett–Teller surface area (54 and 100 m2 g−1 for pure SnWO4 and Zn2+-doped SnWO4, respectively), narrowed band gap (2.68 and 2.64 eV for pure SnWO4 and Zn2+-doped SnWO4, respectively), and highly enhanced photocatalytic performance for the degradation of MO (95% and 30% for pure SnWO4 and Zn2+-doped SnWO4, respectively) compared to that of pure SnWO4. In addition, Song et al.  reported the synthesis of Zn-doped CdWO4 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 CdWO4 NRs was also systematically investigated. Compared to the undoped sample, the Zn-doped CdWO4 NRs exhibited much higher photocatalytic activity, which was assigned to the narrowed band gap due to Zn-doping. Heteroatom doping could be an effective means to tune the distribution of the energy level and further enhance the photocatalytic performance of MWO4 without consuming excess foreign substances.
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 MWO4 . Phuruangrat et al.  synthesized Ce-doped ZnWO4 using a hydrothermal process and investigated the influence of Ce doping on the crystal phase, morphology, electronic structure, and photocatalytic activity of ZnWO4. After the introduction of Ce atoms, the photocatalytic activity of ZnWO4 improved with the increase in Ce content, owing to the following two reasons. First, the introduction of the Ce3+ dopant led to the generation of defects on the surface of Ce-doped ZnWO4. Second, the Ce4+ ions on the surface of ZnWO4 could efficiently trap electrons at the CB by the reduction of Ce4+ into Ce3+ ions, thus efficiently suppressing the recombination of photoinduced electrons and holes in the Ce-doped ZnWO4. 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.
4.4 Heterojunction Fabrication
4.4.1 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.
Furthermore, a MOx/MWO4 hybrid as a smart-built heterojunction was fabricated using a facile one-pot synthetic strategy to enhance the interaction between MOx and MWO4, in which M is generally reported to be Fe, Ni, Co, or Cu [165, 166, 167, 168]. Cao et al.  fabricated a novel p-n heterojunction consisting of Fe3O4 NPs and FeWO4 nanowires. The calculated band gap of the FeWO4/Fe3O4 heterojunction was 2.50 eV, lower than that of pristine FeWO4 nanowires. The FeWO4/Fe3O4 heterojunction exhibited enhanced photo-Fenton activity compared to that of the bare FeWO4 nanowires under UV–visible-light irradiation with the addition of H2O2. In addition, a α-SnWO4/SnO2 heterostructure was synthesized with CTAB as the surfactant  and displayed enhanced photocatalytic performance compared to that of pure α-SnWO4. Considering that WO3 can be obtained by dehydration from tungstate acid, WO3 is considered to be simultaneously produced during the synthesis of MWO4 and is likely to form a MWO4/WO3 heterojunction, such as CoWO4/WO3 [171, 172], NiWO4/WO3 [173, 174], or CuWO4/WO3 [175, 176]. Aslam et al.  prepared a CdWO4/WO3 heterojunction using a hydrothermal and chemisorption method, and reported enhanced photocatalytic activities toward the degradation of organic pollutants, compared with pure CdWO4 and WO3. This section clearly demonstrates in detail that the construction of MWO4-based heterojunction systems is an effective and controllable method for enhancing the photocatalytic activities of MWO4-based semiconductors.
4.4.2 Hybridization with Carbon-Rich Materials
Carbon-rich materials, including carbon nanotubes (CNTs), graphene, and graphitic carbon nitride (g-C3N4), 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 π structure and unique sp2/sp3 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.  synthesized a novel photoelectrode consisting of CuWO4 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 CuWO4 photoelectrode, the resistance and photocurrent density of the CuWO4/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 CuWO4 layer. Compared to CNTs, graphene nanosheets produced via the chemical oxidation treatment of graphite have more sp3 hybridized edge structure because of the destroyed perfect sp2 structure. It is well-known that a perfect sp2 carbon structure (CNT) is beneficial for rapid charge mobility, and that an sp3-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.  designed a ZnWO4/graphene hybrid, demonstrating that graphene could act as a photo-sensitizer in the hybrid and improve the production of ·OH and ·O2− radicals. The excited photogenerated electrons at the CB of ZnWO4 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 ZnWO4/graphene hybrid, in which more holes and electrons can participate in the production of active species, in comparison to bare ZnWO4. Xu et al.  reported a CdWO4/graphene hybrid using a hydrothermal process, in which the formed heterojunction showed significantly enhanced photocatalytic activity compared to that of the bare ZnWO4. However, it was found that excessive graphene could have a negative effect on the photocatalytic performance because of the reduced light absorption efficiency of CdWO4 with the addition of the superfluous graphene.
5 Summary and Outlook
This review summarized the development of novel strategies to enhance the photocatalytic performance of MWO4-based 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 MWO4-based oxides, challenges remain that need to be addressed. First, the recombination rate of photogenerated charge carriers for MWO4-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 MWO4-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 MWO4-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 MWO4 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 MWO4 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 MWO4-based photocatalysts for applications in environmental purification and solar water splitting will be made in future studies.
Y. Hou thanks the support of NSFC 51702284, Fundamental Research Funds for the Central Universities (112109*172210171) and the Startup Foundation for Hundred-Talent Program of Zhejiang University (112100-193820101/001/022). J. Ke thanks the support of the NSFC 21501138, the Science Research Foundation of Wuhan Institute of Technology (K201513).
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