Optimization Methods of Tungsten Oxide-Based Nanostructures as Electrocatalysts for Water Splitting

Electrocatalytic water splitting, as a sustainable, pollution-free and convenient method of hydrogen production, has attracted the attention of researchers. However, due to the high reaction barrier and slow four-electron transfer process, it is necessary to develop and design efficient electrocatalysts to promote electron transfer and improve reaction kinetics. Tungsten oxide-based nanomaterials have received extensive attention due to their great potential in energy-related and environmental catalysis. To maximize the catalytic efficiency of catalysts in practical applications, it is essential to further understand the structure–property relationship of tungsten oxide-based nanomaterials by controlling the surface/interface structure. In this review, recent methods to enhance the catalytic activities of tungsten oxide-based nanomaterials are reviewed, which are classified into four strategies: morphology regulation, phase control, defect engineering, and heterostructure construction. The structure–property relationship of tungsten oxide-based nanomaterials affected by various strategies is discussed with examples. Finally, the development prospects and challenges in tungsten oxide-based nanomaterials are discussed in the conclusion. We believe that this review provides guidance for researchers to develop more promising electrocatalysts for water splitting.


Introduction
With the rapid development of global modernization, the excessive consumption of non-renewable energy sources, such as oil and coal, has resulted in the crisis of greenhouse effect, energy shortage, and severe environmental pollution [1][2][3][4][5]. It is urgent to develop clean and sustainable energy to alleviate energy pressure and ameliorate environmental problems. Therefore, sustainable clean energy resources, such as wind, solar, tidal, and hydropower, have been extensively studied [6][7][8][9][10][11]. However, these energy sources have the disadvantages of uneven geographical distribution and intermittency, which seriously restrict their popularization and application [12]. Hydrogen fuel is expected to play a significant role in developing sustainable clean energy due to its high energy density, high energy yield (122 kJ/g), and environmentally friendly characteristics [13][14][15]. However, it is estimated that nearly 96% of worldwide hydrogen comes from the conversion of fossil fuels, where the pollution byproducts cause environmental problems, such as climate warming [14,[16][17][18][19]. Electrochemical water splitting, as a sustainable method of producing hydrogen with simple operation, mild reaction conditions, environmental protection, and low cost, has attracted much attention from researchers [13,[20][21][22]. The water-splitting process involves two half-reactions, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, due to the slow-kinetic four-electron transfer process with a high reaction barrier, the potential required for water splitting is always higher than the theoretical decomposition potential of water (1.23 V), resulting in additional electrical power consumption [23,24]. Effective electrocatalysts are required to reduce semiconductor−WOx, WOx−C, and metal-WOx heterostructures, have been develo [54][55][56][57]. These fantastic efforts substantially promoted the development of tungsten ide−based electrocatalysts. Tungsten oxides are widely utilized in energy storage [58][59][60][61][62][63], sensors [64][65][66][67][68], c ysis [69][70][71][72][73][74], and other fields because of their adjustable valence states (W 4+ , W 5+ , and and band gaps [31,75,76], various morphologies from zero to three dimensions [44 and different crystal phases. Here, the research progress of tungsten oxide−based ele catalysts for water splitting over the last few years is reviewed. Emphatically discu are the impacts of crystal phase, morphology, defect engineering, and heterojunctio fects on the electronic structure and catalytic activity of tungsten oxide−based nanoma als. We also present a brief summary and outlook of the tungsten oxide catalyst resea with the intention that this review may provide some insights into the constructio high−efficiency oxide catalysts.

Phase Control
Controlling the crystal phase of tungsten oxide and optimizing its physical chemical properties have been proven effective in improving catalytic perform [41,42]. The WOx is not simply composed of W 6+ and O 2− ions, which mainly consi hybrid conduction and valence band states of W 5d and O 2p [32]. The electronic struc of WOx in different crystal phases, such as monoclinic, orthorhombic, and hexag phases, is affected by the W-O bond length [32,78,79]. Consequently, it is possible to timize their catalytic performances by carefully adjusting the crystal phases [30,32,80] relatively stable monoclinic and metastable hexagonal phases have attracted extensiv search for electrocatalytic performances due to their tunnel structure and rich inter tion [81]. The synthesis methods and the properties (electrolyte, over−potentials at 10 cm −2 , and Tafel slopes) of tungsten oxide−based electrodes with different phases are s marized in Table 1. Tungsten oxides are widely utilized in energy storage [58][59][60][61][62][63], sensors [64][65][66][67][68], catalysis [69][70][71][72][73][74], and other fields because of their adjustable valence states (W 4+ , W 5+ , and W 6+ ) and band gaps [31,75,76], various morphologies from zero to three dimensions [44,77], and different crystal phases. Here, the research progress of tungsten oxide-based electrocatalysts for water splitting over the last few years is reviewed. Emphatically discussed are the impacts of crystal phase, morphology, defect engineering, and heterojunction effects on the electronic structure and catalytic activity of tungsten oxide-based nanomaterials. We also present a brief summary and outlook of the tungsten oxide catalyst research, with the intention that this review may provide some insights into the construction of high-efficiency oxide catalysts.

Phase Control
Controlling the crystal phase of tungsten oxide and optimizing its physical and chemical properties have been proven effective in improving catalytic performance [41,42]. The WO x is not simply composed of W 6+ and O 2− ions, which mainly consist of hybrid conduction and valence band states of W 5d and O 2p [32]. The electronic structure of WO x in different crystal phases, such as monoclinic, orthorhombic, and hexagonal phases, is affected by the W-O bond length [32,78,79]. Consequently, it is possible to optimize their catalytic performances by carefully adjusting the crystal phases [30,32,80]. The relatively stable monoclinic and metastable hexagonal phases have attracted extensive research for electrocatalytic performances due to their tunnel structure and rich intercalation [81]. The synthesis methods and the properties (electrolyte, over-potentials at 10 mA cm −2 , and Tafel slopes) of tungsten oxide-based electrodes with different phases are summarized in Table 1. Heat treatment is an effective way to control the crystal phase of WO x -based nanomaterials, in which the temperature is a vital factor. For instance, Guninel et al. annealed orthotropic WO 3 ·H 2 O in air, and they found that the crystal structure transformed to monoclinic WO 3 with the disappearance of water molecules in the structure. The detailed dehydration process is shown in Figure 2a [83]. Pradhan et al. also have prepared the monoclinic WO 3 by annealing orthotropic tungsten oxide hydrate at 400 • C in the air (Figure 2b) [82]. It shows that the double-layer capacitance (C dl ) of monocline WO 3 is 2.83 times that of the original WO 3 ·H 2 O, providing more active surfaces during the catalytic reaction. As a result, the monoclinic WO 3 exhibits an over-potential of 73 mV at 10 mA cm −2 in 0.5 M H 2 SO 4 , which is much lower than that of orthorhombic tungsten oxide hydrate (147 mV). The density functional theory (DFT) results ( Figure 2c) proved that the hydrogen proton adsorption energy on P2 1/n monocline WO 3 (200) is more suitable than that of Pt (111). Halder's group investigated the effect of heat treatment temperatures on the phase transition. With the increase in calcination temperature, the crystal phase of WO 3 changed from hexagonal to monoclinic and then to cubic phase [84]. The phase transformation process from hexagonal to monoclinic at 550 • C was further observed by in situ transmission electron microscopy (TEM). The monoclinic WO 3−x obtained at this temperature exhibits the best HER activity because of the highest oxygen vacancy concentration.
Certain additives also have an impact on the crystal phase of tungsten oxide during the preparation process. Song's team precisely prepared the orthorhombic WO 3 ·0.33H 2 O and monoclinic WO 3 ·2H 2 O by utilizing ethylene diamine tetra acetic acid and DL-malic acid at room temperature, respectively (Figure 2d) [40]. It demonstrated that a lower over-potential (117 mV) and Tafel slope (66.5 mV dec −1 ) of monoclinic WO 3 ·2H 2 O were required to reach a current density of 10 mA cm −2 in 0.5 M H 2 SO 4 ( Figure 2e).
Hajiahmadi et al. explored the reaction mechanism and adsorption model of hex-WO 3 (001) in acid oxygen evolution reaction (OER) (Figure 3) [85]. There are six adsorption models involved: (1)  Certain additives also have an impact on the crystal phase of tungsten oxide during the preparation process. Song's team precisely prepared the orthorhombic WO3·0.33H2O and monoclinic WO3·2H2O by utilizing ethylene diamine tetra acetic acid and DL-malic acid at room temperature, respectively (Figure 2d) [40]. It demonstrated that a lower over−potential (117 mV) and Tafel slope (66.5 mV dec −1 ) of monoclinic WO3·2H2O were required to reach a current density of 10 mA cm     orthorhombic unit cell of tungstite, which converts to the monoclinic  tungsten oxide unit cell by dehydration at elevated temperatures (≥300 °C). Reprinted with permission from [83], with permission from Springer, 2014. (b) XRD patterns of orthorhombic WO3·H2O and monoclinic WO3. (c) Calculated energy landscapes of the HER on WO3 (200) and Pt (111). Reprinted with permission from [82], with permission from American Chemical Society, 2017. (d) Synthesis process and (e) linear sweep voltammograms at 2500 of hydrated tungsten oxide (WO3·nH2O, n values 0.33, 1.00, or 2.00) at room temperature. Reprinted with permission from [40], with permission from American Chemical Society, 2020.
Certain additives also have an impact on the crystal phase of tungsten oxide during the preparation process. Song's team precisely prepared the orthorhombic WO3·0.33H2O and monoclinic WO3·2H2O by utilizing ethylene diamine tetra acetic acid and DL-malic acid at room temperature, respectively (Figure 2d) [40]. It demonstrated that a lower over−potential (117 mV) and Tafel slope (66.5 mV dec −1 ) of monoclinic WO3·2H2O were required to reach a current density of 10 mA cm

Morphology Control
Due to their flexible crystal structures, WO x nanomaterials with rich morphologies exhibit different physical and chemical properties. Reasonable design of catalyst morphology may increase the contact area between catalyst and electrolyte, thus improving the electrocatalytic performance. The impact of morphology on the catalytic performance of WO x has been studied recently, and some advancements have been made [42,86,87].
One-dimensional nanostructures, such as nanorods [38] and nanowires [88][89][90], have been extensively studied in tungsten oxide-based nanomaterials. For example, Liang's group prepared WO 3 nanowires with rich oxygen vacancy (WO 3 -V O NWs) by hydrothermal method combined with plasma sputtering [89]. The WO 3 -V O NWs grew along the [001] direction (c axis) and exhibited stable electrocatalytic oxygen evolution activity under acidic conditions. Two-dimensional nanostructures have also attracted extensive attention due to their increased surface area, abundant active sites, and appropriate adsorption of intermediates in recent years. Pradhan et al. prepared tungsten oxide hydrate nanoplates with apparent gaps between the stacked nanoplates using the hydrothermal method [82]. Guo's group prepared hierarchical WO 3 nanowire arrays on nanosheet arrays (WO 3 NWA-NSAs) by the hydrothermal method for alkaline OER [29]. The WO 3 NWA-NSAs electrocatalyst only requires an over-potential of 230 mV to reach the current density of 10 mA cm −2 because of its unique hierarchical structure. By changing the composition of surfactants and synthesis parameters of hydrothermal processes, Rajalakshmi et al. prepared various WO 3 nanomaterials, including one-dimensional nanowires and nanorods, two-dimensional nanoflakes and nanobelts, and three-dimensional nanoparticles, which are star-like and globule-like structures (Figure 4a-g). The influence of morphology on the band gap width and hydrogen evolution performance was also investigated [44]. It was found that the band gaps of tungsten oxide with different morphologies are inconsistent. Among them, WO 3 nanorods have a higher aspect ratio and better bandgap and adsorption energy in conjunction with the precise cutting of crystal facets along the (001) direction. As a result, the HER performance of one-dimensional WO 3 nanorods exceeds other morphologies in an acidic electrolyte. For the three-dimensional nanostructure, our group investigated the effect of the etching agent concentration on the morphologies and the alkaline OER performances of Ni-WO 3 nanostructures [91]. The Ni-WO 3 octahedral structure (Figure 4h) was in situ etched with (NH 4 ) 2 SO 4 , and the serrated Ni-WO 3 (Figure 4i) was obtained. It was found that the crystal phase of Ni-WO 3 was unaffected by the concentration of the etching agent, but the serrated Ni-WO 3 dramatically improves OER performance (an over-potential of only 265 mV at 10 mA cm −2 ) compared with octahedral Ni-WO 3 (365 mV).

Defect Engineering
Defect engineering, including oxygen vacancy construction and hetero atom doping, reduces the atomic coordination numbers in the material and, thus, regulates the electronic structure [89,92]. The band gap and the adsorption−free energy of the catalyst could be optimized by regulating the electronic structure of the catalyst, thus reducing the catalytic reaction barrier and improving the reaction kinetics. These doping and defect sites can also potentially increase the number of active sites and the concentration of free carriers, which are essential for reducing the reaction barrier and increasing the electron transfer efficiency [93]. The synthesis methods and the properties (electrolyte, over−potentials

Defect Engineering
Defect engineering, including oxygen vacancy construction and hetero atom doping, reduces the atomic coordination numbers in the material and, thus, regulates the electronic structure [89,92]. The band gap and the adsorption-free energy of the catalyst could be optimized by regulating the electronic structure of the catalyst, thus reducing the catalytic Nanomaterials 2023, 13, 1727 7 of 26 reaction barrier and improving the reaction kinetics. These doping and defect sites can also potentially increase the number of active sites and the concentration of free carriers, which are essential for reducing the reaction barrier and increasing the electron transfer efficiency [93]. The synthesis methods and the properties (electrolyte, over-potentials at 10 mA cm −2 , and Tafel slopes) of tungsten oxide-based electrodes modified by defect engineering are summarized in Table 2.

Oxygen Vacancy
Tungsten oxide-based nanomaterials with abundant oxygen defects show great potential for water splitting. Abundant oxygen vacancies can improve the conductivity and promote the adsorption of OH − , thus effectively increasing the oxygen evolution activity [89]. For example, Guo's research group prepared WO 3 with rich oxygen vacancies by hydrothermal method, and they explored its oxygen evolution properties in the alkaline electrolyte [29]. The abundant oxygen vacancy not only improves the conductivity of WO 3 , but also modifies its electronic structure, so that WO 3 only needs 230 mV overpotential to reach the current density of 10 mA cm −2 .
Effect of oxygen vacancy on morphology and properties of WO x . Two extra electrons are produced when an O atom is removed from the WO 3 structure. One or two electrons can be transferred to a neighboring W atom to form the W 5+ or W 5+ -W 5+ centers [32]. Additionally, introducing the O-vacancy modifies the splitting of the W-O bond, increases the W-W distance at the O-vacancy position, and narrows the WO 3 band gap accordingly (Figure 5a-d) [79]. The electrode's conductivity is improved as a result of abundant oxygen vacancies, which can turn an n-type WO x semiconductor into a degenerate semiconductor with metallic characteristics (Figure 5e) [35,89]. Besides, a high oxygen vacancy concentration will increase the material's surface roughness and the area in contact with the electrolyte. For example, the surface of tungsten oxide with a smooth hexahedral structure (Figure 5f) turns rough after being annealed in a H 2 atmosphere, forming a porous structure (Figure 5g) [121]. As shown in Figure 5h,i, the porous WO 2 HN/NF has a BET surface area, pore size, and specific volume of 22.8 m 2 g −1 , 10-100 nm, and 0.138 cm 3 g −1 , respectively. Owing to the highly concentrated oxygen vacancies that provide more active sites and narrower band gaps, the porous WO 2 HN/NF electrode showed lower potential and excellent catalytic stability in HER, OER, and overall water splitting. Oxygen vacancy recognition. First, the O vacancy can be directly observed by atomic high−resolution TEM (HRTEM), where the variation in atomic column intensity indicates the variation in oxygen atomic occupation. As shown in Figure 6a, the tiny pits shown by the arrows indicate the presence of oxygen vacancies [89]. The variations in intensity and contrast were further highlighted in the colored image and the line profile ( Figure 6b). Second, UV−vis diffuse reflectance spectroscopy and UV−vis absorption spectroscopy are also used to identify oxygen vacancy defects. As shown in Figure 6c,d, a stronger photo−response in the visible to infrared region indicates a higher O vacancy concentration in the material [35,40,122]. Besides, electron paramagnetic resonance (EPR), a technique for detecting the chemical environment of unpaired electrons in atoms or molecules, can be used to confirm the existence of oxygen vacancies (Figure 6e). When oxygen vacancies capture electrons, symmetrical EPR signals appear at the position g ≈ 2.002. The stronger the intensity of the EPR signal, the higher concentration of oxygen vacancies present in the material [35,89]. Notice that the oxygen vacancy also changes the metal valence in the metal oxide. Therefore, in addition to the direct characterization methods of oxygen vacancies, alternative techniques, such as X−ray photoemission spectroscopy (XPS) and synchrotron X−ray absorption fine structure (XAFS), can also be employed to infer the existence of oxygen vacancies. As shown in Figure 6f, after the oxygen vacancy was introduced, the XPS peaks of W 4f were moved to the lower binding energy region [35,38,122]. For O1s XPS, the peak at ~531.3 eV corresponds to the oxygen vacancy [35,[123][124][125]. Oxygen vacancy recognition. First, the O vacancy can be directly observed by atomic high-resolution TEM (HRTEM), where the variation in atomic column intensity indicates the variation in oxygen atomic occupation. As shown in Figure 6a, the tiny pits shown by the arrows indicate the presence of oxygen vacancies [89]. The variations in intensity and contrast were further highlighted in the colored image and the line profile ( Figure 6b). Second, UV-vis diffuse reflectance spectroscopy and UV-vis absorption spectroscopy are also used to identify oxygen vacancy defects. As shown in Figure 6c,d, a stronger photoresponse in the visible to infrared region indicates a higher O vacancy concentration in the material [35,40,122]. Besides, electron paramagnetic resonance (EPR), a technique for detecting the chemical environment of unpaired electrons in atoms or molecules, can be used to confirm the existence of oxygen vacancies (Figure 6e). When oxygen vacancies capture electrons, symmetrical EPR signals appear at the position g ≈ 2.002. The stronger the intensity of the EPR signal, the higher concentration of oxygen vacancies present in the material [35,89]. Notice that the oxygen vacancy also changes the metal valence in the metal oxide. Therefore, in addition to the direct characterization methods of oxygen vacancies, alternative techniques, such as X-ray photoemission spectroscopy (XPS) and synchrotron X-ray absorption fine structure (XAFS), can also be employed to infer the existence of oxygen vacancies. As shown in Figure 6f, after the oxygen vacancy was introduced, the XPS peaks of W 4f were moved to the lower binding energy region [35,38,122]. For O1s XPS, the peak at~531.3 eV corresponds to the oxygen vacancy [35,[123][124][125]. Effect of oxygen vacancy concentration on performance. To further investigate th impact of oxygen vacancy concentration on catalytic performance, Thomas et al. explore the relationship between the electrocatalytic performance of Meso−WO2.83 and surface ox idation degree brought on by exposure to air (Figure 6g,h) [35]. With the increase in expo sure time, the plasmon resonance of Meso−WO2.83 was weakened, accompanied by a re shift, and its electrocatalytic activity gradually decreased, as well. This indicates tha abundant oxygen vacancies are advantageous for enhancing catalytic activity. Zeng et a demonstrated that abundant oxygen vacancies could optimize the hydrogen adsorptio Gibbs free energy (ΔGH*) using the DFT (Figure 6i) [94].
Method of producing oxygen vacancy. Oxygen vacancies are widespread in trans tion metal oxides because of their low formation energy [35,39]. Representative method of producing oxygen vacancies in metal oxides include heat treatment, reductive trea ments, and other methods [38,126]. The first method is heat treatment, which involve removing a small amount of lattice oxygen from metal oxides in low−oxygen condition at high temperatures without causing bulk phase transition. The oxygen vacancy concen tration can be adjusted by controlling the heat treatment temperature or the inert gas flow rate. For instance, Halder's group thermally treated WO3 in a vacuum environment t produce WO3−x with rich oxygen vacancies [84]. The second method of producing oxyge vacancies in tungsten oxide is reductive treatments, which create oxygen vacancies wit the aid of reducing agents, such as H2 [123], NaBH4 [127,128], and sodium dodecyl sulfat [29]). Taking the reduction in hydrogen as an example, with the extension of reductio  (Figure 6g,h) [35]. With the increase in exposure time, the plasmon resonance of Meso-WO 2.83 was weakened, accompanied by a red shift, and its electrocatalytic activity gradually decreased, as well. This indicates that abundant oxygen vacancies are advantageous for enhancing catalytic activity. Zeng et al. demonstrated that abundant oxygen vacancies could optimize the hydrogen adsorption Gibbs free energy (∆G H* ) using the DFT (Figure 6i) [94].
Method of producing oxygen vacancy. Oxygen vacancies are widespread in transition metal oxides because of their low formation energy [35,39]. Representative methods of producing oxygen vacancies in metal oxides include heat treatment, reductive treatments, and other methods [38,126]. The first method is heat treatment, which involves removing a small amount of lattice oxygen from metal oxides in low-oxygen conditions at high temperatures without causing bulk phase transition. The oxygen vacancy concentration can be adjusted by controlling the heat treatment temperature or the inert gas flow rate. For instance, Halder's group thermally treated WO 3 in a vacuum environment to produce WO 3−x with rich oxygen vacancies [84]. The second method of producing oxygen vacancies in tungsten oxide is reductive treatments, which create oxygen vacancies with the aid of reducing agents, such as H 2 [123], NaBH 4 [127,128], and sodium dodecyl sulfate [29]). Taking the reduction in hydrogen as an example, with the extension of reduction time, WO 3 gradually evolved into WO 3−x , WO 2 , and finally, metallic W 0 [129]. Thomas et al. prepared Meso-WO 2.83 with oxygen-rich vacancies by replacing bulk materials with mesoporous materials to increase the reduction rate in the hydrogen atmosphere [35]. Due to the slow diffusion of H 2 molecules in bulk materials, the use of mesoporous WO 3 with a higher surface area and thin nanoscale pore wall allows H 2 molecules to diffuse and migrate more easily on its surface and inside. The results show that the mesoporous structure not only dramatically reduces the H 2 reduction temperature, but also selectively generates WO 2.83 . Other methods of producing oxygen vacancies include plasma treatment, flame, mechanical exfoliation, and hydrothermal methods [29,89,130,131]. For example, as a surface treatment technology, plasma has a certain reduction ability, and the oxygen vacancy generated by it only exists on the surface of the sample [131].

Hetero Atom Doping
Hetero atom doping is also an effective strategy to prepare tungsten oxide-based nanomaterials with abundant defects, which achieve a significant leap in catalytic performance by regulating the electronic coordination environment, the number of active sites, and the adsorption strength of intermediates [48,113]. In practical applications, atom-doped materials usually contain both hetero atoms and oxygen vacancies, which can cooperatively promote the catalytic activity [93,118,132]. The effect of doping on catalytic performance can be adjusted by changing the type [48,105] and concentration [115,118,133] of hetero atoms. Atom doping could be divided into noble metal atomic doping and non-noble metal atomic doping. For the former, the noble metal atoms typically exist as single atoms or atomic clusters to reduce the costs and improve the utilization rate of noble metal atoms [112,113,[134][135][136]. For example, Hou et al. reported Pt sing-atoms supported on monolayer WO 3 (Pt-SA/ML-WO 3 ) for HER [104]. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Figure 7a,b) and ICP result showed that 0.20 wt% Pt atoms were immobilized on ML-WO 3 . HRTEM images demonstrate the existence of defects, including lattice distortion, as well as W and O vacancies (Figure 7c,d). The HER performance of Pt-SA/ML-WO 3 (over-potential of 22 mV at 10 mA cm −2 , Tafel slope of 27 mV dec −1 ) was even better than that of 20% Pt/C (over-potential of 34 mV at 10 mA cm −2 , Tafel slope of 28 mV dec −1 ). The improved performance is mainly attributed to the strong interaction between Pt single atoms and ML-WO 3 , which drastically tunes the electronic structure of the catalyst, endowing Pt-SA/ML-WO 3 with a strong conductivity and an adequate ∆G H* (Figure 7e). Sun et al. investigated the overall water-splitting performance of the iridium-doped tungsten trioxide array (Ir-doped WO 3 ) in acidic conditions [100]. Ir can preempt some of the electrons in the WO 3 matrix to optimize its electronic structure because Ir 4+ has a smaller radius and a higher atomic number than W 6+ . The Ir-doped WO 3 exhibited low cell voltages of 1.56 and 1.68 V to drive the current densities of 10 and 100 mA cm −2 , respectively. Ma's group analyzed the effects of oxygen vacancy and Ru doping on electronic states and d-band center of WO 3 by density functional theory (DFT) calculations [136]. As shown in Figure 7f, when only oxygen vacancies exist, the electrons produced by oxygen vacancies are transferred to neighboring W atoms. After doping Ru atom, electrons are mainly transferred from Vo site to adjacent Ru site, and a few electrons are transferred to an adjacent W site. Therefore, Ru sites with sufficient electrons in Vo-WO 3 /Ru SAs may be active centers for adsorption intermediates in acidic media. In alkaline and neutral solutions, the oxygen atoms in water molecules are more easily captured by the electron-deficient W site, and the generated H* migrates to the nearby Ru site to form H 2 . In addition, the d-band center of Vo-WO 3 /Ru SAs (−5.180 eV) is much lower than that of WO 3 (−3.252 eV) and Vo-WO 3 (−4.133 eV), indicating that the strength of H* bond is weakened (Figure 7g), which is conducive to the desorption of H* from the surface and the promotion of HER. In addition to doping the noble metal atoms, doping of non−noble metal at Fe, Ni, F, and Mo) could also regulate the physicochemical properties of the cata effectively improve the catalytic performance of tungsten oxide−based nanom [48,107,111,115,116,133,137,138].  (Figure 8b). Therefore, Ni−WO2 only needs 83 mV the current density of 10 mA cm −2 in an alkaline environment. To investigate the Ni atom doping on the localized structure of tungsten oxide, the author compar L3−side X−ray absorption near side structures (XANES) and further proved that tion of Ni atoms can effectively reduce the d−band occupation state on W atoms. ing the whole contour plots of wavelet transform (WT) (Figure 8c,d) and the cha sity difference slices (Figure 8e,f) of WO2 and Ni−WO2, it was found that the R− about k = 12.2 Å −1 in the WT spectrum of Ni−WO2 is increased, and the charge between Ni-W is significantly decreased. All these results indicate that Ni dopin fectively regulate the coordination strength between metal atoms and reduce th to capture electrons of W atoms, which is closely related to the absorption/desor havior on the catalyst surface. In addition to doping the noble metal atoms, doping of non-noble metal atoms (Co, Fe, Ni, F, and Mo) could also regulate the physicochemical properties of the catalyst and effectively improve the catalytic performance of tungsten oxide-based nanomaterials [48,107,111,115,116,133,137,138]. For example, Xiao et al. simulated the d-orbital hybridization of WO 2 by a series of transition metal heteroatoms using first principles and explored HER properties of M-WO 2 (M = Fe, Co, Ni, and Cu atoms) [105]. The Fe, Co, Ni, or Cu heteroatoms replace one of the W atoms (W-M bond) to form higher d-band packed atomic coordination, resulting in increased filling of W-M bond antibonding orbitals and weakening of bond strength (Figure 8a). The DFT result showed that the Ni-W site has modest hydrogen adsorption (∆G H* = −0.43 eV) due to the dynamic transfer of some bonded electrons from the W-W/Ni bonds to the Ni-O bonds after the substitution of W atoms by Ni, which reduces the free energy, weakens the metal-metal bond, and increases the bond length of W-W/Ni (Figure 8b). Therefore, Ni-WO 2 only needs 83 mV to reach the current density of 10 mA cm −2 in an alkaline environment. To investigate the effect of Ni atom doping on the localized structure of tungsten oxide, the author compared the W L 3 -side X-ray absorption near side structures (XANES) and further proved that the addition of Ni atoms can effectively reduce the d-band occupation state on W atoms. Comparing the whole contour plots of wavelet transform (WT) (Figure 8c,d) and the charge density difference slices (Figure 8e,f) of WO 2 and Ni-WO 2 , it was found that the R-value at about k = 12.2 Å −1 in the WT spectrum of Ni-WO 2 is increased, and the charge density between Ni-W is significantly decreased. All these results indicate that Ni doping can effectively regulate the coordination strength between metal atoms and reduce the ability to capture electrons of W atoms, which is closely related to the absorption/desorption behavior on the catalyst surface.

Heterostructure Construction
Constructing heterostructured nanomaterials composed of tungsten oxide and another material is an effective strategy to improve the catalytic performance of tungsten oxide [139][140][141]. The heterostructured nanomaterials reaches the catalytic performance of 1 + 1 > 2 by exposing more active sites and promoting interfacial electron transfer, which is called the "spillover" mechanism of the system [52,141]. The synthesis methods and the catalytic properties of tungsten oxide−based heterostructure electrodes with different morphologies (such as nanowires, nanoflakes, nanospheres, urchin−like, and so on) are summarized in Table 3. Here, tungsten−oxide heterostructures are divided into three types according to the different components: (1) semiconductor-WOx; (2) WOx-C; and (3) metal-WOx.

Heterostructure Construction
Constructing heterostructured nanomaterials composed of tungsten oxide and another material is an effective strategy to improve the catalytic performance of tungsten oxide [139][140][141]. The heterostructured nanomaterials reaches the catalytic performance of 1 + 1 > 2 by exposing more active sites and promoting interfacial electron transfer, which is called the "spillover" mechanism of the system [52,141]. The synthesis methods and the catalytic properties of tungsten oxide-based heterostructure electrodes with different morphologies (such as nanowires, nanoflakes, nanospheres, urchin-like, and so on) are summarized in Table 3. Here, tungsten-oxide heterostructures are divided into three types according to the different components: (1) semiconductor-WO x ; (2) WO x -C; and (3) metal-WO x .

Semiconductor-WO x
The interaction between tungsten oxide and another semiconductor is conducive to adjusting its electronic structure. When tungsten oxide comes into contact with a semiconductor with different Fermi levels, the electrons will spontaneously diffuse from the semiconductor with a high Fermi level to another component until the chemical potential of the two parts is equal, thus forming a semiconductor-WO x heterojunction structure [172]. Consequently, net charges accumulate at the contacting interface, which lowers the initially higher Fermi level and raises the initial lower Fermi level. Meanwhile, the electronic band of the contacting semiconductor bends over subject to the movement of Fermi levels, generating different types of band alignments. Due to the synergistic effect and electronic effect between the components, the catalytic performance of the composites is improved. Wang and his colleagues prepared the Ni 2 P-WO 3 nanoneedle structure on carbon cloth using a combination of in situ electrodeposition and phosphating treatment methods [151]. The XPS results demonstrate the electrons transfer from Ni to P in Ni 2 P-WO 3 . Benefiting from the heterojunction structure, Ni 2 P-WO 3 exhibits excellent HER catalytic performance in both acidic (over-potential of 107 mV at a current density of 10 mA cm −2 ) and alkaline (over-potential of 105 mV at a current density of 10 mA cm −2 ) environments. Peng et al. designed a Fe 2 P-WO 2.92 heterostructure on nickel foam by a facile consecutive three-step synthesis method [72]. The oxygen vacancies and the synergistic effect between WO 2.92 and Fe 2 P facilitated a drastic reduction in over-potential for the catalytic OER performance of Fe 2 P-WO 2.92 /NF (over-potential of 215 mV at 10 mA cm −2 in 1.0 M KOH solution).
Moreover, the interfacial richness of the two phases in the semiconductor-WO x heterostructure directly affects the number of active sites. Yang's group prepared Ni 17 W 3 /WO 2 heterojunction on nickel foam (WO 2 /Ni 17 W 3 /NF) by the hydrothermal and annealing method (Figure 9a) [150]. WO 2 /Ni 17 W 3 heterojunctions increase the exposure of active edge sites and facilitate the water dissociation and H intermediates association during HER kinetics. Therefore, WO 2 /Ni 17 W 3 /NF demonstrated high catalytic efficiency for HER with a low over-potential of 35 mV at 10 mA cm −2 . Following this work, Liu's group prepared R-Ni 17 W 3 /WO 2 catalysts on Ni foam and explored the effect of Ni 17 W 3 particle size decorated on the NiWO 4 /WO 2 substrate for hydrogen evolution reaction (Figure 9b) [152]. The R-Ni 17 W 3 /WO 2 with larger Ni 17 W 3 particles exhibited superior HER catalytic activity (over-potential of 48 mV at 10 mA cm −2 ), resulting from more interfaces and more active sites (Figure 9c) (Figure 9d). Our group also prepared a TA-Fe@Ni-WO x hierarchical structure by the interfacial coordination assembly process. After the introduction of the TA-Fe layer, the electrons transfer from W and Ni to TA-Fe, and as a result, the TA-Fe@Ni-WO x has an upward-moving Fermi energy, a smaller ionization potential, and a more electron-rich environment, which is more conducive to OER [167].

WO x -C
Creating a heterostructure with another conductive material is a typical way to increase the electrocatalyst's overall electronic conductivity. Carbon materials, including graphene oxide (GO), carbon nanotube (CNT), carbon paper, and carbon cloth, are often used due to their superior electronic conductivity, high specific surface area, and excellent chemical durability [56,[173][174][175][176]. WO x /carbon composites have been widely used in electrocatalytic water splitting [71,142,143,145,146,177]. In this case, carbon-encapsulated WO x with rich oxygen vacancies was synthesized by pyrolyzing carbon/tungsten mixture (Figure 10a,b) [95,144,178,179], which has a favorable impact on enhancing charge transfer and compensating for the weak hydrogen adsorption of the tungsten oxide. The effects of W content [95], annealing time [179], and annealing temperature [178] on HER performance of WO x /C have been deeply explored. For example, Yin et al. studied the effects of different annealing times and temperatures on hydrogen evolution properties of WO x /C in 0.5 M H 2 SO 4 (Figure 10c) [179]. Pan's group introduced 15 nm thick carbon-based shells on the surface of tungsten oxide nanospheres (CTO) and investigated its catalytic performance in alkaline OER [145]. It was found that the overpotential of nanoparticles at 50 mA cm −2 decreased from 360 to 317 mV after the introduction of a carbon-based shell (Figure 10d).
The improved catalytic performance is attributed to the carbon-based shell that speeds up the electron transfer between the catalyst and the reactant, provides the catalytic active site, and promotes the adsorption of the catalyst to the reactant and the dissociation of the O-H bond.

WOx−C
Creating a heterostructure with another conductive material is a typical way to increase the electrocatalyst's overall electronic conductivity. Carbon materials, including graphene oxide (GO), carbon nanotube (CNT), carbon paper, and carbon cloth, are often used due to their superior electronic conductivity, high specific surface area, and excellent chemical durability [56,[173][174][175][176]. WOx/carbon composites have been widely used in electrocatalytic water splitting [71,142,143,145,146,177]. In this case, carbon−encapsulated WOx with rich oxygen vacancies was synthesized by pyrolyzing carbon/tungsten mixture (Figure 10a,b) [95,144,178,179], which has a favorable impact on enhancing charge transfer and compensating for the weak hydrogen adsorption of the tungsten oxide. The effects of W content [95], annealing time [179], and annealing temperature [178] on HER performance of WOx/C have been deeply explored. For example, Yin et al. studied the effects of different annealing times and temperatures on hydrogen evolution properties of WOx/C in 0.5 M H2SO4 (Figure 10c) [179]. Pan's group introduced 15 nm thick carbon−based shells on the surface of tungsten oxide nanospheres (CTO) and investigated its catalytic performance in alkaline OER [145]. It was found that the overpotential of nanoparticles at 50 mA cm −2 decreased from 360 to 317 mV after the introduction of a carbon−based shell ( Figure  10d). The improved catalytic performance is attributed to the carbon−based shell that speeds up the electron transfer between the catalyst and the reactant, provides the catalytic active site, and promotes the adsorption of the catalyst to the reactant and the dissociation of the O-H bond. Reprinted with permission from [179], with permission from Elsevier, 2021. (d) LSV polarization curves measured at a scan rate of 5 mV s −1 in 1 M NaOH. Reprinted with permission from [145], with permission from the American Chemical Society, 2020.

Metal−WOx
Conductive metal is also widely used to increase the electrocatalyst's overall electronic conductivity and adjust the electronic structure of WOx. In order to reduce the cost of the catalyst, the metal part in metal-WOx heterostructure often exists in the form of single atoms, clusters, or nanoparticles [71,120,156]. Li et al. prepared Pt@WO3/C with three−dimensional nano architectures for HER via the water-oil two−phase microemulsion method and the annealing treatment [156]. Pt nanoparticles with a diameter of about 4 nm were monodispersed on the surface of WO3/C structures (Figure 11a,b). The over−potential at 10 mA cm −2 of Pt@WO3/C as HER electrocatalyst was 149 mV (Figure 11c), which was smaller than that of WO3/C (244 mV). The mechanism of HER consists of three main steps, including H2O adsorption, H2O dissociation, and H * desorption (Figure 11d). The Reprinted with permission from [179], with permission from Elsevier, 2021. (d) LSV polarization curves measured at a scan rate of 5 mV s −1 in 1 M NaOH. Reprinted with permission from [145], with permission from the American Chemical Society, 2020.

Metal-WO x
Conductive metal is also widely used to increase the electrocatalyst's overall electronic conductivity and adjust the electronic structure of WO x . In order to reduce the cost of the catalyst, the metal part in metal-WO x heterostructure often exists in the form of single atoms, clusters, or nanoparticles [71,120,156]. Li et al. prepared Pt@WO 3 /C with three-dimensional nano architectures for HER via the water-oil two-phase microemulsion method and the annealing treatment [156]. Pt nanoparticles with a diameter of about 4 nm were monodispersed on the surface of WO 3 /C structures (Figure 11a,b). The overpotential at 10 mA cm −2 of Pt@WO 3 /C as HER electrocatalyst was 149 mV (Figure 11c), which was smaller than that of WO 3 /C (244 mV). The mechanism of HER consists of three main steps, including H 2 O adsorption, H 2 O dissociation, and H * desorption (Figure 11d). The corresponding free energy calculation results show that the introduction of Pt has no obvious effect on water absorption, but it can promote the water dissociation and H * desorption of WO 3 , thus accelerating the HER process (Figure 11e). The XPS results (Figure 11f) also confirm that the peaks of W 4f were positively shifted after the introduction of Pt due to the difference in electronegativity between W and Pt atoms, resulting in the transfer of electrons from W to Pt. In another work, Pt-WO 3−x nanodots were anchored on rGO for water splitting [37]. The optimized composite Pt-WO 3−x @rGO exhibited the highest HER, OER, and overall water-splitting activities in alkaline environments (overpotentials of about 34 mV, 174 mV, and 1.55 V at 10 mA cm −2 , respectively). At the same time, its overall water-splitting performance showed excellent durability at 1.55 V, where 93.3% initial potential could be maintained after 14 h of cycling.

Summary and Outlook
In this review, we focus on the recent research progress of tungsten oxide−based nanomaterials for water splitting, especially revealing the mechanism of structural design and component regulation to improve the electrocatalytic performances, which is expected to guide the preparation of high−performance tungsten oxide−based electrocatalysts. The practical strategies for improving the performance of tungsten−oxide catalysts are discussed in this review. The band gap and contact area of a tungsten oxide−based electrode can be adjusted by controlling the morphology and crystal phase, thus improving its catalytic performance. The electronic structure of WOx can also be optimized through defect engineering to provide more active sites and an excellent electronic environment for catalytic reactions. Additionally, the synergistic effect can boost the electron transfer of tungsten oxide and provide more active boundaries, which is another commonly used effective strategy for enhancing catalytic performance. Due to the adjustment of structure and composition, noble metal single atom/tungsten oxide/carbon, the best tungsten−based catalyst at present, greatly reduces the cost on the premise of excellent catalytic performance. Despite the advancement of these methods, the practical application of tungsten−based materials in water splitting is still in the early stage, and there are still many problems and challenges.
First, defect engineering has been widely used in regulating the electronic structure of tungsten oxide and promoting its catalytic performance. However, the precise control of the defect location and concentration is difficult, and the characterization technology of defect types and concentration is insufficient. Therefore, the improvement of catalytic performance by defect engineering lacks an intrinsic understanding. The development and Reprinted with permission from [156], with permission from Elsevier, 2022.

Summary and Outlook
In this review, we focus on the recent research progress of tungsten oxide-based nanomaterials for water splitting, especially revealing the mechanism of structural design and component regulation to improve the electrocatalytic performances, which is expected to guide the preparation of high-performance tungsten oxide-based electrocatalysts. The practical strategies for improving the performance of tungsten-oxide catalysts are discussed in this review. The band gap and contact area of a tungsten oxide-based electrode can be adjusted by controlling the morphology and crystal phase, thus improving its catalytic performance. The electronic structure of WO x can also be optimized through defect engineering to provide more active sites and an excellent electronic environment for catalytic reactions. Additionally, the synergistic effect can boost the electron transfer of tungsten oxide and provide more active boundaries, which is another commonly used effective strategy for enhancing catalytic performance. Due to the adjustment of structure and composition, noble metal single atom/tungsten oxide/carbon, the best tungsten-based catalyst at present, greatly reduces the cost on the premise of excellent catalytic performance. Despite the advancement of these methods, the practical application of tungsten-based materials in water splitting is still in the early stage, and there are still many problems and challenges.
First, defect engineering has been widely used in regulating the electronic structure of tungsten oxide and promoting its catalytic performance. However, the precise control of the defect location and concentration is difficult, and the characterization technology of defect types and concentration is insufficient. Therefore, the improvement of catalytic performance by defect engineering lacks an intrinsic understanding. The development and utilization of more advanced in situ preparation and characterization technologies may somewhat solve this problem. For example, in situ atmospheric spherical correction transmission electron microscopy can be used to directly observe the defect formation process in nanomaterials and the resulting crystal phase changes.
Besides, the current research on the structure-property relationship mainly adopts the post-analysis method, which compares the electrocatalyst's structure before and after the catalytic reaction and infers the structure evolution during the reaction based on the static characterization results. However, the structure of tungsten oxide dynamically evolves in the catalytic reaction process. The morphology, chemical composition, and electronic structure of tungsten oxide are constantly changing and recycling. As a result, the structure differences before and after long-lasting catalytic reactions may not reflect the real active sites. To further understand the structure-property mechanism of tungsten oxide-based electrocatalysts, it is necessary to develop and utilize in situ characterization methods, such as in situ electron microscope and the in situ XRD technique, to record the dynamic structural changes of the catalysts in real-time during the reaction.
We believe that the growth mechanism of material preparation and structure-property relationship can be better understood by in situ observation of the structure and defect location and concentration, as well as in situ characterization of catalytic processes. Combined with advanced theoretical simulation techniques, it is possible to obtain tungsten oxidebased electrocatalysts with high intrinsic activity. All these developments will significantly contribute to the rapid development of renewable energy storage and conversion.