Tungsten oxide-based visible light-driven photocatalysts: crystal and electronic structures and strategies for photocatalytic efficiency enhancement

Abstract

Photocatalysis (PC) technology has received global attention due to its high potential of addressing both environmental and energy issues using only solar light as energy input. However, large-scale commercialization of PC technology is still far from expectation, which is primarily limited by low efficiency. The development of PC technology depends crucially on photocatalyst materials. In the past half century, TiO₂ has been mostly investigated and developed as a benchmark photocatalyst. However, TiO₂ responds intrinsically only to UV light, which has limited its efficient utilization of solar energy and restrained its applications to where UV light is not available, e.g., indoor air purification. The development of novel intrinsically visible light-driven photocatalysts has been a new trend of PC technology. Amongst the various visible-light responsive candidates, tungsten oxides (WO_X, X ≤ 3) have attracted much attention due to their diversely tunable stoichiometries and structures, suitable band gaps, chemical stability and Earth-abundance. However, bare WO_X exhibits comparatively low efficiency because of the fast recombination rate of photogenerated electrons and holes. Various strategies have been developed to enhance the photocatalytic efficiency of WO_X, including the controls in the crystal phase, stoichiometry/oxygen-vacancy, active facet and morphology, elemental doping, loading of noble metal nanoparticles, hybridization with carbon materials and coupling with other semiconductors to construct heterojunctions. This review summarizes firstly the fundamentals of WO_X (i.e., crystal and electronic structures and optical properties) and then highlights the strategies for the enhancement of the photocatalytic efficiency of WO_X-based photocatalysts. The related synthesis methods are also briefly discussed. It is anticipated that this paper could offer a comprehensive understanding of WO_X-based photocatalysts and serve as a guideline for future designs of highly active visible light-driven WO_X-based photocatalysts.

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Introduction

In the past half century, photocatalysis (PC) technology has attracted global attention due to its high potential of addressing both environmental and energy issues using only solar light as energy input. To date, PC technology has been widely applied to various fields (see Fig. 1), such as water splitting for H₂ and O₂ generation,^1–6 CO₂ reduction for fuels and to alleviate the global warming crisis,^7,8 N₂ fixation for ammonia,⁹ wastewater treatment,^10,11 air purification,^12,13 soil remediation,¹⁴ self-cleaning surfaces,¹⁵ anti-bacteria/virus,^16,17etc. The merits of PC technology are fundamentally due to the strong redox ability of photocatalysts induced by proper illumination. However, large-scale commercialization of PC technology is still far from expectation, which is primarily limited by low efficiency.

Fig. 1 Applications of photocatalysis.

The development of PC technology depends crucially on photocatalyst materials, which are generally semiconductors composed of a valence band (VB) filled with electrons and a conduction band (CB) empty of electrons. When irradiated by photons with energy higher than or equal to the band gap (i.e., hv ≥ E_g), a photocatalyst could be excited with part of electrons jumping from the VB to the CB, leaving excited electrons and holes in the CB and VB, respectively (see Fig. 2(i)).^18,19 The photogenerated electrons and holes, also called charge carriers, could transfer to the surface of the photocatalyst and then react with the adsorbed electron-acceptors (A) and electron-donors (D), respectively, initiating both photocatalytic reduction and oxidation reactions (Fig. 2(ii) and (iii)).^18,19 However, before participating in photocatalytic redox reactions, the photogenerated electron–hole pairs might have recombined quickly in bulk or on the surface of the photocatalyst (Fig. 2(iv) and (v)), as they are subjected to a strong Coulomb force.²⁰ This is one main reason for the low efficiency of photocatalysis.

Fig. 2 Schematic photoexcitation in a solid photocatalyst followed by deexcitation (A: electron-acceptor; D: electron-donor).

In the past several decades, TiO₂ has been mostly investigated and developed as a benchmark photocatalyst.^21,22 However, as a wide band gap semiconductor (3.0–3.2 eV), TiO₂ responds intrinsically only to UV light that occupies merely 3–5% of the solar spectrum.²³ This has limited its efficient utilization of solar energy and restrained its applications to where UV light is not available, e.g., indoor air purification. Although many efforts have been made to extend light absorption of TiO₂,^24–28 the efficiency of TiO₂-based photocatalysts under visible light irradiation is still unsatisfactory. In recent years, the development of novel intrinsically visible light-driven photocatalysts has been a new trend of PC technology. To date, various narrow bandgap (<3.0 eV) semiconductors have been developed as visible-light sensitive photocatalysts, such as WO₃,²⁹ α-Fe₂O₃,³⁰ CdS,³¹ BiVO₄,³² Bi₂WO₆,^33,34 Ag₃PO₄,³⁵ g-C₃N₄,³⁶etc. Amongst them, tungsten oxides (WO_X, X ≤ 3) have received increasing interest (a publication survey is shown in Fig. 3) due to their easy preparation, diversely tuneable stoichiometries and structures, suitable band gaps (2.4–2.8 eV, responsive to ∼12% of the solar spectrum), strong photocatalytic oxidizing ability, high chemical stability, nontoxicity, and Earth-abundance.³⁷ Furthermore, WO_X possesses a much higher carrier mobility (10–12 cm² V⁻¹ s⁻¹) than TiO₂ (0.3 cm² V⁻¹ s⁻¹) and a comparatively longer hole diffusion length (150–500 nm) when compared with α-Fe₂O₃ (2–4 nm), both of which are essential for the transfer and separation of photogenerated charge carriers.^38,39 However, bare WO_X exhibits a relatively low efficiency because of the fast recombination rate of photogenerated charge carriers. This may have a close relationship with the low CB level (0.3–0.7 V vs. NHE) of WO_X that is not negative enough for the single-electron reduction of oxygen (e.g., O₂ + e⁻ = O₂˙⁻ (aq), −0.284 V; O₂ + H⁺ + e⁻ = HO₂˙ (aq), −0.046 V vs. NHE), which in turn, however, is important for the scavenging of photogenerated electrons.⁴⁰ As the photogenerated electrons could not be consumed efficiently, they accumulate and lead to the increased recombination rate of charge carriers.

Fig. 3 Publication survey in Web of Science using “tungsten oxide” and “photocatalytic” as keywords (since 2000).

In the literature, many strategies have been developed to improve the photocatalytic behaviour of WO_X, such as the controls in the crystal phase,^41–43 stoichiometry/oxygen-vacancy,^38,44,45 active facet^46–48 and morphology,^49–51 elemental doping,^52–54 loading of noble metal nanoparticles (NPs),^55–57 hybridization with carbon materials^58–60 and coupling with other semiconductors to construct heterojunctions.^61–63 According to the basic PC processes (Fig. 2), any improvement in the following aspects can lead to an enhancement of the photocatalytic efficiency: (i) extend light absorption, (ii) facilitate charge transfer, (iii) inhibit the recombination of charge carriers, (iv) shorten the diffusion length for charge carriers, (v) increase the reactive sites on the surface of the photocatalyst, and (vi) increase the overall reaction surface area. The relationships between the various strategies and their possible resulted enhancements are summarized in Fig. 4.

Fig. 4 Relationship between the strategies and their main possible resulted enhancements.

Although there have been some review papers concerning WO_X-based photocatalysts, they paid attention to only limited aspects, e.g., nanostructured WO₃ thin films for photoelectrocatalytic (PEC) water oxidation⁶⁴ and nanostructured-based WO₃ for wastewater treatment.⁶⁵ A comprehensive review regarding WO_X-based photocatalysts, especially involving the advancements in the past few years, is still in need. This review summarizes firstly the fundamentals of WO_X (i.e., crystal and electronic structures and optical properties) and then highlights the strategies for the improvement of the photocatalytic efficiency of WO_X-based photocatalysts. It is anticipated that this paper could serve as a guideline for future designs of highly active visible light-driven WO_X-based photocatalysts.

Fundamentals

Tungsten oxide (WO_X) is a big family that is composed of stoichiometric WO₃ and various sub-stoichiometric tungsten oxides (WO_3−x, 0 < x ≤ 1). As tungsten oxide hydrates (WO_X·nH₂O) have a close relationship with their dehydrated counterparts and exhibit photocatalytic activity, they are considered herein as part of WO_X-based photocatalysts. In this section, the crystal and electronic structures and optical properties of WO_X and their hydrates are discussed.

Crystal structures

The ideal crystal structure of WO₃ is identical to the cubic ReO₃ type or ABO₃ perovskite structure in the absence of an A cation, i.e., a three-dimensional network formed by corner-sharing WO₆ octahedra.⁶⁶ As illustrated in Fig. 5(a), the ideal cubic WO₃ has W atoms at the corners of a cube, each surrounded by six octahedrally coordinated oxygens; four oxygens lie in a plane containing the W atoms and there is one oxygen above and one below this plane for each W atom; each oxygen is common to two octahedra, giving the formula of WO₃. Cubic cavities with constant edges (about 2.7 Å, the length of O–O bonds) form the interstices of the network of WO₆ octahedra (regular four-membered rings can be seen in the [100], [010] and [001] directions, respectively, as shown in Fig. 5(c)). The ideal cubic WO₃ has never been observed experimentally.⁶⁷ Bulk WO₃ undergoes at least five reversible phase transitions upon heating or cooling (Fig. 6).^68–71 This involves the tetragonal (t- or α-WO₃),^70,72 orthorhombic (o- or β-WO₃),⁷³ monoclinic I (m- or γ-WO₃),⁷¹ triclinic (tr- or δ-WO₃),⁶⁸ and monoclinic II (ε-WO₃) phases.⁶⁶ All these phases of WO₃ have a similar crystal structure to the ideal cubic WO₃, however, with a lowered symmetry owing to three possible types of distortions: displacement of the W atom from its octahedron and distortion and tilting of WO₆ octahedra.⁶⁶ According to Corà et al., the lowering of the symmetry, especially by the off-central displacement of W atoms, results in an increase in the covalence between tungsten and its nearest oxygen, which thus leads to a more stable structure.⁶⁷ At room temperature, m-WO₃ has been reported as the most stable phase, with tr-WO₃ also being observed.⁷⁴ A hexagonal phase of WO₃ is also frequently reported,^75–77 which was firstly obtained by the dehydration of WO₃·0.33H₂O in 1979.⁷⁸ The hexagonal h-WO₃ is also build up from corner-sharing WO₆ octahedra but their arrangement results in three- and six-membered rings in the ab-planes and leads to the formation of large hexagonal tunnels (∼5.6 Å) in the c-axis (see Fig. 5(c)). In the ac- and bc-planes, four-membered rings formed by WO₆ octahedra, as in other phases, are also the case in h-WO₃. According to Gerand et al., the unit cell of h-WO₃ contains six molecules and shows lattice constants of a = 7.298 Å and c = 7.798 Å (Fig. 5(b)).⁷⁸ More information about the crystal structures and lattice parameters of polymorphic WO₃ can be found in Fig. 5(b) and (c).

Fig. 5 Crystal structures of polymorphic WO₃: cubic WO₃ and its WO₆ octahedron (a), structural parameters (b), and stick–ball and polyhedral representations (c).

Fig. 6 Classical phase transitions of bulk WO₃ upon heating or cooling.^68–71

The lattice of WO₃ can sustain considerable amounts of oxygen vacancies, however, this is accompanied by structural changes according to the degree of reduction. In a sub-stoichiometric WO_3−x with a low degree of reduction (x < 0.01), single oxygen vacancies can be dispersed in the lattice in low concentration. However, with increasing x the lattice tends to eliminate single oxygen vacancies by a crystal shear (CS) mechanism, resulting in groups of edge-sharing WO₆ octahedra arranged along some crystallographic planes (shear planes, SPs). For moderate x, these SPs are isolated and can be regarded as extended defects.⁷⁹ With further increase in x, the SPs begin to interact and tend to align in parallel. If the SPs are all in parallel and equidistant, a crystal phase with a defined structure arises, which can be recognized as the Magnéli W_nO_3n−2 series.⁸⁰ One such typical example is W₂₀O₅₈ (WO_2.9), the structure of which is demonstrated in Fig. 7(a). When x ≥ 0.13, structures with pentagonal columns (PCs) become dominant. These structures contain two kinds of coordination polyhedra, viz., WO₆ octahedra and WO₇ pentagonal bipyramids. The WO₇ bipyramids share their equatorial edges with five WO₆ octahedra to form groups of PCs (generally parallel to the monoclinic b-axis). The variations of such structures depend on the modes of the linking of the PCs.⁸¹ One such typical example is W₁₈O₄₉ (WO_2.72), as shown in Fig. 7(b). Other frequently investigated WO_3−x with defined structures, also known as the Magnéli phases, include W₃₂O₈₄ (WO_2.625), W₃O₈ (WO_2.67), W₁₇O₄₇ (WO_2.76), W₅O₁₄ (WO_2.8), W₂₅O₇₃ (WO_2.92), etc.⁸²

Fig. 7 Idealized structures of W₂₀O₅₈ (a) and W₁₈O₄₉ (b) looking down from the [010] direction.

Hydrated tungsten oxides WO_X·nH₂O have been frequently obtained as intermediate products during the synthesis of WO_X using a wet chemical route before annealing. The most widely investigated WO_X·nH₂O photocatalysts include monoclinic WO₃·2H₂O (dihydrate),⁸³ orthorhombic WO₃·H₂O (monohydrate),^84,85 cubic pyrochlore-type WO₃·0.5H₂O (hemihydrate),⁸⁶ and orthorhombic WO₃·0.33H₂O.⁸⁷ The crystal structures of WO₃·nH₂O are highly dependent on their water content. WO₃·2H₂O is built up from layers of corner-sharing WO₅(OH₂) octahedra and interlayer water molecules (Fig. 8(a)).⁸⁸ Each WO₅(OH₂) octahedron consists of one W atom at the centre, one terminal oxygen, one coordinated water and four bridging oxygens, with which WO₅(OH₂) octahedra are connected to each other in the ac-planes forming neutral WO₃·H₂O layers. The interlayer water molecules connect with WO₅(OH₂) octahedra through hydrogen bonds.⁸⁸ WO₃·H₂O can be obtained by removing the interlayer water from WO₃·2H₂O (Fig. 8(b)).^89,90 WO₃·0.5H₂O is less documented and believed to have a structure of cubic pyrochlore-type, where the water molecules are presented in tunnels along the [110] direction, constructed by six-membered rings of corner-sharing WO₆ octahedra (Fig. 8(c)).⁹¹ WO₃·0.33H₂O has been reported to consist of both WO₆ and WO₅(OH₂) octahedra.⁹² A typical representation of orthorhombic WO₃·0.33H₂O is demonstrated in Fig. 8(d), where three- and six-membered rings are formed by corner-sharing WO₆ and WO₅(OH₂) octahedra in the ab-planes.⁹³

Fig. 8 Crystal structures of tungsten oxide hydrates: monoclinic WO₃·2H₂O (a),⁸⁸ orthorhombic WO₃·H₂O (green balls represent H atoms) (b),⁹⁰ projection of cubic pyrochlore-type WO₃·0.5H₂O in the [110] direction (small circles represent water molecules) (c),⁹¹ and orthorhombic WO₃·0.33H₂O (d).⁹³ Reprinted with permission from ref. 88, 90, 91 and 93. Copyright 2004 Elsevier, 2012 the Royal Society of Chemistry, 1989 Elsevier, and 2008 American Chemical Society, respectively.

Electronic band structures and optical properties

WO₃ is an n-type semiconductor with an indirect bandgap E_g characterizing the energy difference between the VB (E_VB) and the CB (E_CV), as shown in Fig. 9(a). The VB of WO₃ is formed by filled O 2p-orbitals while the CB is composed mainly of empty W 5d-orbitals.⁹⁴ The relationships between E_VB, E_CB and E_g follow the equations: (1) E_CB = χ − E^e − 0.5E_g and (2) E_g = E_VB − E_CB, where χ is Mulliken's electronegativity of the material (6.59 eV for WO₃) and E^e is the energy of a free electron on the hydrogen scale (4.5 eV).⁸⁵ This indicates that the positions of the VB and the CB for a specific material are influenced directly by the bandgap E_g. Bulk WO₃ has a typical E_g of 2.6 eV at room temperature, corresponding to a light absorption threshold at 477 nm determined by λ = 1240/E_g and E_VB and E_CB at +3.39 eV and +0.79 eV, respectively.⁹⁵ This implies that the photogenerated holes in the VB of WO₃ is highly oxidizing, which is strong enough to decompose water (E(O₂/H₂O) = +1.23 V vs. NHE) and almost all organic compounds, and/or react with water and surface hydroxyl (OH⁻) to produce ˙OH (E(˙OH/OH⁻) = +1.99 V, E(˙OH/H₂O) = +2.72 V vs. NHE).⁹⁶ However, the photogenerated electrons in the CB of WO₃ are comparatively weak, which are not negative enough to photo-reduce H⁺ for H₂ (E(H⁺/H₂) = 0 V vs. NHE) and oxygen to O₂˙⁻ (E(O₂˙⁻/O₂) = −0.284 V vs. NHE).⁹⁷ The E_g of WO₃ is obviously affected by the phase transitions, which in turn is a function of temperature. In general, E_g decreases and becomes increasingly diffuse as the temperature increases, indicating a redshift of the light absorption edge.⁶⁶ At the nanoscale, especially when the particle size is close to or smaller than the exciton Bohr radius of the material (∼3 nm for WO₃),⁹⁸E_g might increase significantly with decreasing particle size owing to the quantum confinement (QC) effect.⁹⁹ The particle size effect on E_g can be estimated by Brus’ equation,¹⁰⁰ as shown in Fig. 9(b).¹⁰¹ It indicates that the E_g of WO₃ would be increased up to ∼3.0 eV when the particle size is reduced to ∼3 nm, implying that WO₃ quantum dots (QDs, sizes smaller than the exciton Bohr radius) need UV light for excitation. The E_CB of WO₃ QDs would be lifted upwards due to the expansion of the bandgap, resulting in enhanced photo-reducing ability. An experimental measurement of bandgap expansion and obviously uplifted E_CB of WO₃ QDs has been reported by Watanabe et al., where the WO₃ QDs with sizes at the sub-nano scale show E_g values up to 3.7 eV and achieve single-electron reduction of molecular oxygen.¹⁰²

Fig. 9 Electronic band structure of stoichiometric bulk WO₃ (a), effect of the particle size on E_g for WO₃ predicted by Brus’ equation with and ( and are the effective mass of an electron and a hole, respectively; m₀ is the electron rest mass) (b),¹⁰¹ electronic band structure and electron excitation modes in sub-stoichiometric WO_3−x (c), typical light absorptions of WO_X with different extents of oxygen vacancies (d).³⁸ Reprinted with permission from ref. 38. Copyright 2019 Elsevier.

Partial loss of oxygen from WO₃ has similar consequences to the insertion of donors.¹⁰³ Both experimental and simulation studies have revealed that the density of free charge carriers (N_D) in WO_3−x increases with the number of oxygen vacancies. As reported by Migas et al., N_D increases from 2.90 × 10²¹ to 1.62 × 10²² cm⁻³ when the sub-stoichiometry is varied from WO_2.92 to WO_2.625.⁸² The introduction of oxygen vacancies would lead to partial reduction of WO₃ (W⁶⁺ → W^5+/4+) in order to match the charge balance. The presence of W^5+/4+ creates new states closely below the CB of WO₃ (W⁶⁺), as shown in Fig. 9(c). The injected electrons by oxygen vacancies would be firstly trapped in the W 5d-orbitals in the W^5+/4+ sites, and then polarize the surrounding lattice to generate polarons.¹⁰⁴ WO_3−x has been reported to possibly absorb light ranging from the UV to near-infrared (NIR) regions due to three distinct modes of electron excitation: (i) VB-to-CB transition, (ii) VB-to-W^5+/4+ state transition, and (iii) polaron-induced localized surface plasmon resonance (LSPR) (Fig. 9(c)).³⁸ A typical light absorption of WO_3−x was demonstrated by Kalanur et al., as shown in Fig. 9(d).³⁸ The strong absorption in the visible and NIR regions (above 500 nm) has been ascribed to the third mode of electron excitation, i.e., the polaron-induced LSPR. The intensity of LSPR absorption has been proved to be correlated with N_D, where a large N_D generally leads to a strong LSPR absorption. Some recent reports also suggest that the oxygen deficiency results in the formation of oxygen-vacancy (V_O) states above and partly overlap with the VB.^105,106 This leads to an extension of the VB and narrows the band gap, which then expands the photo-response of WO_3−x toward the longer wavelength range.

As for WO_X·nH₂O photocatalysts, they generally show smaller E_g than their dehydrated counterparts due to the weaker binding energy, thus exhibiting larger light absorption ranges.^41,85 For instance, Ke et al. have synthesized WO₃·H₂O via a hydrothermal process and then obtained WO₃ by calcining the as-synthesized sample at 500 °C.⁸⁵ Their WO₃·H₂O and WO₃ samples exhibit light absorption edges at 530 nm (E_g = 2.44 eV) and 472 nm (E_g = 2.64 eV), respectively.

Enhancement strategies

In order to improve the photocatalytic behaviours of WO_X, many enhancement strategies have been reported. In this section, various enhancement strategies will be summarized from the aspects of the controls in the crystal phase, stoichiometry/oxygen-vacancy, active facet and morphology, elemental doping, loading of noble metal NPs, hybridization with carbon materials and coupling with other semiconductors to construct heterojunctions.

Phase control

Apart from m-WO₃, nanostructured WO₃ in hexagonal,^42,48 orthorhombic^107,108 and triclinic phases^109,110 have also been reported to retain phase stability and exhibit photocatalytic activity at room temperature. Although some studies have reported that m-WO₃ exhibits better photocatalytic behaviours than other phases of WO₃,^42,107 it is still difficult to make a conclusive comparison, for the photocatalytic efficiency of WO_X is influenced simultaneously by many factors.

Recently, a phase junction photocatalyst constructed by different phases of the same semiconductor has attracted much attention due to its simplicity, controllability and great photocatalytic activity. WO_X is a polymorphic semiconductor that consists of many crystal phases and abundant hydrates, which has offered great possibility to construct phase junctions between different phases. Some WO_X-based phase junction photocatalysts have been reported, such as h-WO₃·0.33H₂O/c-WO₃·0.5H₂O,⁴³ h-WO₃/m-WO₃ ¹¹¹ and o-WO₃·0.33H₂O/h-WO₃.⁴¹ The enhancement mechanism depending on the phase junction is mainly due to the improved electron–hole separation between the different phases which show unequal band structures. A typical phase junction photocatalyst with the corresponding charge transfer mechanism is demonstrated in Fig. 10.

Fig. 10 Schematic illustration of charge transfer and separation in a typical phase junction constructed by o-WO₃·0.33H₂O and h-WO₃.

In general, a phase junction could be obtained by applying the intermediate synthesis conditions located between those for the synthesis of single-phase tungsten oxides. The mass ratio between the combined phases can be tuned by shifting the conditions toward the synthesis of the phase that is expected to increase. For instance, Li et al. have prepared a phase junction photocatalyst of o-WO₃·0.33H₂O/h-WO₃via a hydrothermal method by adjusting the amount of NaCl (a capping agent).⁴¹ They obtained pure o-WO₃·0.33H₂O in the absence of NaCl and pure h-WO₃ with the addition of 0.4 g NaCl. Their phase junction was obtained when NaCl was applied between 0.1 and 0.2 g and the mass ratio of o-WO₃·0.33H₂O to h-WO₃ decreases when the amount of NaCl increases.

Stoichiometry/oxygen-vacancy control

In recent years, many efforts have been made to improve the photocatalytic efficiency of WO_X by tuning its oxygen vacancies (e.g., number and distribution). Many studies have confirmed that the photocatalytic efficiency of WO_3−x increases as the number of oxygen vacancies increases, due to enhanced optical absorption and reduced recombination rate of charge carriers. However, an over-abundance of oxygen vacancies (i.e., more than the optimal level) can act as recombination or trap centres for the photogenerated electrons and holes, thus lowering the photocatalytic activity.^38,45 Some studies suggest that the distribution of oxygen vacancies (i.e., in bulk or on the surface) also matters to the photocatalytic activity, however, in different ways.^{44,45,112,113} Wang et al. have reported that bulk oxygen vacancies mainly promote visible light harvesting and slightly restrain the recombination of electrons and holes by narrowing the band gap, while the surface oxygen vacancies significantly increase the charge separation efficiency by lowering the VB edge.⁴⁵

Oxygen vacancies in WO₃ can be introduced by several means, including annealing under oxygen-deficient atmospheres (e.g., hydrogen⁴⁵ and vacuum¹¹⁴), hydrogen peroxide treatment,¹¹² etching and by specific routes.^39,98,115 By annealing, which is the most common way, the extent and distribution of oxygen vacancies can be tuned by varying the temperature, duration and atmosphere. In general, the extent of oxygen vacancies increases as the reducibility of the atmosphere and the thermal treatment time increase. A moderate reductive atmosphere (e.g., 20% H₂ in N₂) is generally beneficial for generating surface oxygen vacancies, while thermal treatment under a highly reductive atmosphere (e.g., 100% H₂) favours the generation of bulk oxygen vacancies. Oxygen vacancies generally propagate from the surface into the bulk as the thermal treatment proceeds. The oxygen vacancies can also be introduced by annealing in air, because, depending on the crystal structure and annealing temperature, the critical phase transition and nanoscale inhomogeneous deformation (during annealing) in the WO₃ lattice can also create oxygen vacancies.¹⁰⁵

Amongst the various oxygen-deficient tungsten oxides, W₁₈O₄₉ has attracted much attention due to its stable defect structure, strong LSPR absorption and good photocatalytic performance.^116–118 W₁₈O₄₉ nanowires preferentially growing along the [010] direction have been frequently reported and applied to various photocatalytic reactions (e.g., CO₂ reduction to CH₄,¹¹⁹ degradation of organic dyes¹²⁰ and H₂ generation¹¹⁴). They have been synthesized by various methods, such as a solvothermal reaction followed by vacuum drying¹¹⁹ and solution combustion synthesis.¹²⁰

Active-facet control

As photocatalytic reactions occur on the surface of the photocatalyst, the surface features (e.g., energy, atomic coordination and electronic structure) influence directly the overall photocatalytic reactivity. It has been widely accepted that high surface energy results in high photocatalytic reactivity due to the more active sites and stronger adsorption ability. From this point of view, {002} is the most active facet for both m- and o-WO₃, for their surface energies follow the order: {002} (1.56 J m⁻²) > {020} (1.54 J m⁻²) > {200} (1.43 J m⁻²)¹²¹ and {002} (1.74 J m⁻²) > {020} = {200} (1.69 J m⁻²),¹²² respectively. This has been confirmed by several experimental measurements where both m- and o-WO₃ with the preferentially exposed {002} facets exhibit better photocatalytic performance than their counterparts without the preferred orientation of the crystal facets.^{46,50,123,124} The properties of exposed facets have a close relationship with the crystal phase which they belong to. For h-WO₃, both {002} and {200} have been reported to be active facets, the photocatalytic enhancement of which was ascribed to the increased charge separation efficiency.^48,125 For o-WO₃·0.33H₂O, however, {020} was reported to be the most active facet, which has been said to be correlated with the unique W=O and O–H groups.⁴⁷

Some studies suggest that the exposed facets have influences on the electronic structure, thus affecting the reduction and oxidation abilities of the photocatalyst. Xie et al. have prepared a quasi-cubic-like WO₃ crystal with a nearly equal percentage of the {002}, {020} and {200} facets and a rectangular sheet-like WO₃ crystal with the predominant {002} facet. Their study demonstrates that the former exhibits a deeper VB maximum, thus showing a much higher O₂ evolution rate in photocatalytic water oxidation, while the latter exhibits an elevated CB minimum, and thus is able to photo-reduce CO₂ to CH₄.¹²¹

Hydrothermal/solvothermal synthesis has been commonly used to control the exposed facets of WO_X. It is well established that solvents, impurities and additives in solution can substantially influence the ultimate shape of the crystals by controlling their growth rate in specific directions. For instance, Liang et al. have synthesized 2D ultra-thin m-WO₃ nanosheets with more than 90% of the exposed {002} crystal facets via using a surfactant (Pluronic P123) as a capping agent.⁵⁰ Since the surface energy of the {002} facet is much higher than that of the {020} and {200} facets, the polar groups of P123 preferentially adsorb onto the {002} facets, thus inhibiting their growth and finally promoting their exposure. Some inorganic salt anions, such as NO₃⁻,¹²¹ BF₄⁻,¹²² Cl⁻ (ref. 46) and SO₄²⁻,⁴⁸ have also been reported as effective capping agents for the preferential exposure of the {002} facets for m- and o-WO₃ due to their preferential adsorption onto the {002} facets.

Morphology control

Morphology, mainly characterizing the features of shape and size, is one of the most important factors influencing the performance of photocatalysts. In general, a morphology that could offer a large specific surface area, large number of active sites, suitable pore features and short diffusion length for charge carriers is desirable.

Various unique morphologies of WO_X photocatalysts have been reported in the literature; a brief summary can be found in Fig. 11. These morphologies can be classified into zero-dimensional (0D, e.g., spherical and pseudo-spherical NPs^{102,126–128}), one-dimensional (1D, e.g., nanorods,¹²⁰ nanowires,¹¹⁴ nanobelts,¹²⁹ nanofibers^130,131 and nanotubes²³), two-dimensional (2D, e.g., nanoplatelets,¹³² nanoplates⁴⁸ and nanosheets¹³³) and three-dimensional (3D, e.g., porous interconnected structures,^134,135 core–shell structures^136–139 and hierarchical structures assembled by low-dimensional building blocks^{46,51,140–144}) according to the dimensionality. A schematic illustration of simplified structures in different dimensionalities is demonstrated in Fig. 12(a). With a specific volume, particles in different shapes show different specific surface areas. A comparison of the specific surface area (S) between nanospheres, nanorods and nanosheets with different aspect ratios is demonstrated in Fig. 12(b). It indicates that, for both 1D nanorods and 2D nanosheets, the specific surface area increases as the aspect ratio increases. The 0D nanosphere shows the smallest specific surface area. It can be concluded that, if only considering the specific surface area, the preferability of morphology follows the order: 2D (high aspect ratio) > 1D (high aspect ratio) > 0D. However, it should always be kept in mind that a large specific surface area tends to result in severe agglomeration due to high specific surface energy, which in turn is not beneficial for the photocatalytic performance. Compromised strategies might be considered when developing efficient photocatalysts for practical applications. Apart from the shape, the size matters as well. In general, a smaller size results in a larger specific surface area and a shorter diffusion length for charge transfer, both of which are desirable for the enhancement of photocatalytic efficiency. When the particle size (at least in one dimension) is reduced to be close to or smaller than the exciton Bohr radius (∼3 nm for WO₃), the band gap of the material would be increased significantly with the CB edge uplifted due to the strong QC effect.

Fig. 11 Typical morphologies of WO_X photocatalysts from the literature: QDs (a),¹²⁶ monodisperse nanoparticles (b),¹²⁸ aggregated nanoparticles (c),⁴² nanorods (d),⁷⁷ nanowires (e),⁹⁸ nanofibers (f),¹³¹ nanosheets (g),¹³³ nanosheets (h),⁵⁰ nanoplates (i),⁴⁸ hollow microspheres (j),¹³⁷ multiple-shell hollow spheres (k),¹³⁹ sphere-in-shell microstructures (l),¹³⁸ hierarchical structures (m),¹⁴¹ flower-like microstructures (n),¹⁴² flower-like microstructures (o),¹⁴³ cylindrical-stack microstructures (p),¹⁴³ hierarchical structures (q),¹⁴⁴ and 3D ordered macroporous structures (r).¹³⁵ Reproduced with permission from ref. 42, 48, 50, 77, 98, 126, 128, 131, 133, 135, 137–139 and 141–144. Copyright 2010 the Royal Society of Chemistry, 2016, 2014 and 2017 Elsevier, 2018 American Chemical Society, 2015 Springer Nature, 2012 American Chemical Society, 2019, 2017 and 2008 Elsevier, 2012 and 2008 John Wiley and Sons, 2013, 2009, 2014, 2018 and 2013 Elsevier, respectively.

Fig. 12 Schematic illustration of simplified morphologies in 0D, 1D, 2D and 3D (a), and the effect of the particle shape and size on the specific surface area for WO₃ (b).

The morphology of WO_X can be controlled by using a template-based method or a template-free method. With a template method, the shape and size of WO_X are determined primarily by the structure of the template. Two such typical examples are WO₃ QDs in macro/mesoporous silica^102,126 and 3D ordered macroporous WO₃.^134,135 In a typical synthesis process, a precursor solution is simply introduced into template pores followed by particulation or chemical reactions for conversion to WO_X. The template can be removed by using specific routes depending on the template material (e.g., calcination for removing the polymer template¹⁴⁵) or just kept as a support for the WO_X catalyst.¹⁰² With a template-free method, the shape and size of WO_X can be tuned by varying the synthesis conditions (e.g., type and concentration of the capping agent, pH, and reaction temperature and time), which are influencing factors for the nucleation and growth rate of crystals in specific directions. For instance, Shukla et al. have synthesized monodisperse spherical WO₃ NPs using cationic surfactants (i.e., cetylpyridinium chloride (CPyC), cetylpyridinium bromide (CPyB), hexadecyltrimethyl ammonium bromide (HTAC) and tetradecyltrimethyl ammonium bromide (TTAB)) as a capping agent.¹²⁸ As these cationic surfactants adsorb non-selectively onto the surface of the WO₃ nuclei, the final product of WO₃ is in a spherical shape. When agents that can selectively adsorb onto specific faces of WO₃ are applied, 1D and/or 2D structures might be obtained.

Elemental doping

Elemental doping is an effective way to tune the properties of photocatalysts. The incorporation of foreign ions into the lattice of WO_X may result in changes in the crystal structure, morphology, electronic structure and optical properties depending on the nature and concentration of the dopant and the doping routes. The incorporated ions have two possible positions in the lattice of WO_X, i.e., the W or O sites (substitution) and the interstice between WO₆ octahedra (intercalation). The feasibility and extent of doping depend crucially on the differences in the radius and valence state between the dopant and host atoms. In general, a dopant with a similar radius to that of the host atom is easier for achieving doping and has a higher solubility in the host lattice, and simultaneously, resulting in lighter distortions of the host lattice and smaller changes in the morphology. Compared to anion doping (e.g., I⁻ doping⁵²), cation doping with low valence metal ions is much more frequently reported, such as Mo⁵⁺,^146,147 Ta⁵⁺,¹⁴⁸ Nb⁵⁺,¹⁴⁹ Ti⁴⁺,¹⁵⁰ Sn⁴⁺,¹⁵¹ Bi³⁺,¹⁵² Fe³⁺,^153,154 Yb³⁺,¹⁵⁵ Ce³⁺,¹⁵⁶ La³⁺,¹⁵⁶ Y³⁺,¹⁵⁶ Co²⁺,¹⁵⁷ Cu²⁺,¹⁵⁸ Zn²⁺,¹⁵⁹ Ni²⁺,¹⁶⁰etc. In order to maintain the charge balance, oxygen vacancies are generally created when low valence metal ions are doped, which could result in extra benefit for the improvement of the photocatalytic performance. The doped metal ions on the surface of WO_X could trap and localize electrons around them and enhance the photo-induced electron density on the active sites, so as to improve the electron-giving ability for photocatalytic reactions, e.g., CO₂ reduction¹⁶¹ and N₂ fixation.⁵⁴

Elemental doping is commonly achieved by adding the starting material of the dopant (e.g. related ions or salts) into the precursor that is used for the synthesis of WO_X. The extent of doping could be easily tuned by varying the addition amount of the dopant source. For instance, Wang et al. have performed Mo-doping into WO₃·0.33H₂O by adding Na₂MoO₄·2H₂O into the Na₂WO₄·2H₂O based precursor that is used for the hydrothermal synthesis of WO₃·0.33H₂O and modified the extent of doping by adjusting the stoichiometric ratio of Mo : W from 1% to 5%.¹⁶¹

Noble metal loading

In the past two decades, loading of noble metal NPs (e.g., Pt,¹⁶² Au¹⁶³ and Ag¹⁶⁴) has received increasing interest for the enhancement of the photocatalytic efficiency of WO_X. WO₃ has been thought to be unsuitable for the efficient oxidative decomposition of organic compounds in air or be limited to the reactions with strong electron acceptors, since its CB is not negative enough for the single-electron reduction of oxygen.⁴⁰ In 2008, Abe et al. loaded Pt nanoparticles onto the surface of WO₃ and found that the photogenerated electrons in Pt/WO₃ could reduce O₂ through multi-electron reduction ways (e.g., O₂ + 2H⁺ + 2e⁻ = H₂O₂ (aq), +0.682 V vs. NHE).⁴⁰ In these processes, Pt works as an electron pool to accept the photogenerated electrons from WO₃ and as a cocatalyst to facilitate the multi-electron reduction of O₂ to produce H₂O₂. The study by Kim et al. has revealed that the reductive decomposition of H₂O₂ produced in situ from the reduction of O₂ on the Pt/WO₃ surface is another important path for the generation of ˙OH radicals, which is an important active species for the degradation of organic compounds.¹⁶² This enhancement mechanism has also been accepted for Au/WO_X and Ag/WO_X composites,^56,165 the typical electron–hole transfer and separation process of which is demonstrated in Fig. 13(a).

Fig. 13 Schematic illustration of electron–hole transfer and separation in a noble metal loaded WO₃ photocatalyst: the noble metal works as an electron pool (a) and the LSPR effect dominates (b).

The strong LSPR effect induced by noble metal NPs is another important factor contributing to the enhanced photocatalytic efficiency. Surface plasmon resonance (SPR) is a coherent oscillation of the surface conduction electrons excited by an electromagnetic radiation.¹⁶⁶ For the case of LSPR, light interacts with particles much smaller than the incident wavelength.¹⁶⁶ The plasmon frequency of a noble metal NP is correlated with its shape, size and proximity to other nanoparticles. Generally, decreasing the particle size can lead to a reduction in the plasmon frequency, i.e. resulting in a redshift of the plasmon resonance absorption.¹⁶⁷ The noble metal NPs loaded on the WO_X surface are generally smaller than 10 nm, corresponding to plasmon resonance absorptions in the visible and NIR regions. When the size of noble metal NPs is reduced to around 2 nm or less, the LSPR would disappear as the band structure becomes discontinuous and breaks down into discrete energy levels.¹⁶⁷ Regarding the plasmonic enhancement in photocatalysis, two mechanisms have been frequently discussed: charge transfer and local electric field enhancement. The mechanism of charge transfer was firstly proposed by Tatsuma's group in 2004 for the study of Au- or Ag-loaded TiO₂ systems.¹⁶⁸ In this mechanism, the plasmon resonance excites electrons in noble metal NPs, which are then transferred to the CB of their adjacent semiconductors, namely the noble metal NPs act as electron-donors (see Fig. 13(b)). This charge transfer mechanism has also been accepted by some authors to explain the enhancement behaviour of noble metal/WO_X photocatalysts.¹⁶⁹ As for the mechanism of local electric field enhancement, studies have revealed that intense local electric fields near the surface of noble metal NPs could be generated by irradiating the NPs near their plasmon resonance frequency. Studies of electromagnetic simulations using the finite-difference time-domain (FDTD) method have shown that the electric field intensity of local plasmonic “hot spots” can reach as much as 1000 times that of the incident electric field.¹⁷⁰ In these “hot spots”, the electron–hole pair generation rate is 1000 times that of the incident electromagnetic field. Thus, an increased amount of photoinduced charges is generated locally in the photocatalyst due to the local field enhancement of the plasmonic NPs. This local electric field mechanism has also been adopted by some authors to explain their developed efficient noble metal loaded WO_X photocatalysts.

A uniform distribution of noble metal NPs on the surface of WO_X is always desirable. An excess loading (i.e., more than the optimal level) may lead to agglomeration of the noble metal NPs, thus deteriorating the photocatalytic performance.¹⁶⁹ The optimal loading depends on various factors. Even for the same noble metal, the optimal loading may vary significantly with the morphology of the as-prepared WO_X.^23,57 Nevertheless, the optimal loading of noble metal NPs on the WO_X surface has always be reported to be less than 5 wt% in the literature.

A popular method for the deposition of noble metal NPs onto the WO_X surface is photo-deposition. In a typical process, commercial or as-prepared WO_X particles are added firstly into the aqueous solution of noble metal ions (e.g., AgNO₃,¹⁷¹ H₂PtCl₆ ⁵⁷ and HAuCl₄ ⁵⁶), which is then subjected to light irradiation for a certain period of time in the presence of an electron donor (generally methanol). The content and size of the loaded noble metal NPs could be tuned by varying the concentration of the noble metal ion in the solution and/or the intensity and time of the light irradiation. Apart from the in situ photo-deposition process, some authors prepared the noble metal colloidal solution firstly, and then immersed the WO_X particles into the as-prepared noble metal colloidal solution. The final noble metal/WO_X composite could be obtained by a post-heat treatment process.²³

Hybridization with carbon materials

In the past decade, the coupling of WO_X with carbon materials to form highly efficient composite photocatalysts has received increasing interest. Various carbon materials with unique structures have been adopted, such as graphene or reduced graphene oxide (RGO),^58,172 carbon nanotube (CNT) or multi-walled carbon nanotube (MWCNT),^59,173 carbon fiber¹⁷² and carbon nanodot.⁶⁰ The carbon material is characterized by excellent electron mobility exceeding ∼15 000 m² V⁻¹ s⁻¹, outstanding chemical and thermal stability and strong mechanical strength, which makes it a superior supporting matrix for photocatalysts. In a WO_X/carbon hybrid under illumination, the photogenerated electrons produced in WO_X could be transferred quickly to the carbon material through the interface, leaving photogenerated holes in WO_X. The photogenerated electrons could then react with adsorbed electron-acceptors on the surface of the carbon material. In this way, efficient charge separation is achieved (see Fig. 14).

Fig. 14 Schematic illustration of the charge transfer and separation in a WO_X/carbon photocatalyst.

Amongst the various carbon materials, graphene and RGO (a single layer or multilayer of sp² bonded carbon atoms with a honeycomb lattice structure) have attracted much attention due to their ultralight-weight and flexible feature and ultra-large specific surface area (∼2600 m² g⁻¹). RGO is usually obtained by the reduction of graphene oxide (GO), which is commonly prepared by the modified Hummers’ method, wherein graphite is used as a starting material and strongly oxidized during a grinding process.^174,175 The oxidation introduces many oxygen-containing functional groups, such as epoxy, hydroxyl, carboxyl and carbonyl groups, on the carbon basal plane, making the obtained GO hydrophilic and easy to disperse stably in water. The oxygen-containing groups on the surface of GO are usually active sites for the growth or deposition of the WO_X catalyst. According to the loading mechanism of WO_X onto the surface of GO, the preparation of WO_X/RGO composites can be categorized into two routes. One is that GO is added into the precursor that is used for the synthesis of WO_X. With this route, WO_X nucleates and grows on the surface of GO during the synthesis process (e.g., a hydrothermal treatment process).¹⁷⁶ Another route is that WO_X is firstly synthesized and then mixed with GO in solution followed by a specific treatment that allows the deposition of WO_X onto the surface of GO. The GO in the as-prepared WO_X/GO composite could then be reduced to RGO by a thermal decomposition or by specific reduction processes (e.g., a chemical reduction using hydrazine vapor at 90 °C for 24 h).¹⁷⁷

Coupling with other semiconductors

Coupling WO_X with other semiconductors having unequal band structures is an effective way to facilitate charge transfer and separation and to improve photo-induced redox ability. In the literature, various semiconductors (e.g., chalcogenides, halogenides, salts and carbon nitrides) have been reported to construct heterojunctions with WO_X to form efficient photocatalysts, such as WO_X/TiO₂,^178–180 WO₃/Fe₂O₃,^181,182 WO₃/Cu₂O,^61,183 WO₃/ZnO,^96,184 WO₃/CdS,^185,186 WO₃/Bi₂S₃,^187,188 WO₃/ZnIn₂S₂,¹⁸⁹ WO₃/AgI,¹⁹⁰ WO₃/BiOCl_0.25Br_0.75,¹⁹¹ WO₃/BiOI,⁹⁵ WO₃/Ag₃PO₄,^62,192 WO₃/BiVO₄,^193,194 WO₃/NiWO₄,¹⁹⁵ WO₃/Bi₂WO₆,^196,197 WO_X/g-C₃N₄,^198–200etc. For the ease of comparison, the electronic band structures of WO₃ and its typical coupled semiconductors are summarized in Fig. 15. According to the mechanism of charge transfer, the WO_X/semiconductor photocatalysts can be categorized into two groups: conventional type-II and Z-scheme (see Fig. 16). In a conventional type-II heterojunction, the photogenerated electrons transfer from the CB of the coupled semiconductor to that of WO_X with the photogenerated holes migrating from the VB of WO_X to that of the coupled semiconductor. Therefore, the photoreduction occurs on the surface of WO_X while the photooxidation takes place on the surface of the coupled semiconductor. With this configuration, efficient spatial separation of electron–hole pairs could be obtained, however, the photo-oxidizing ability of the composite is decreased to some extent when compared to that of bare WO_X. In a Z-scheme heterojunction, the photogenerated electrons from WO_X recombine with the holes in the coupled semiconductor, while the holes in WO_X and the electrons in the coupled semiconductor remain separated and reactive. In this case, the heterojunction retains the strong photo-oxidizing ability of WO_X and possesses a higher photo-reducing ability imparted by the coupled semiconductor. As both types of the heterojunction have a staggered band structure, the charge transfer mechanism for a specific composite needs to be confirmed by experiment, e.g., detection of active species during a photocatalytic reaction. For some WO_X/semiconductor systems (e.g., WO₃/Cu₂O, WO₃/TiO₂ and WO₃/Fe₂O₃), the mechanism of charge transfer may alter depending on the synthesis routes and the applied photocatalytic reactions. For instance, Zhang et al. have prepared a WO₃/Cu₂O photoanode for PEC water splitting via a hydrothermal method followed by electrodeposition.¹⁸³ The charge transfer in this heterojunction has been reported to follow the conventional type-II mode. However, in the study of Shi et al.⁶¹ where a WO₃/Cu₂O composite was synthesized using similar procedures but applied for CO₂ reduction, the charge transfer was confirmed to follow the Z-scheme mechanism.

Fig. 15 CB and VB energy levels of WO₃ and a number of semiconductors.

Fig. 16 Schematic illustration of electron–hole transfer and separation in conventional type-II (a) and Z-scheme (b) WO_X/semiconductor heterojunction photocatalysts.

Apart from the necessity of unequal band structures, intimate contact is another basic requirement for the efficient separation of charge carriers. In addition, the optimization of the contact surface area between WO_X and the coupled semiconductor is also important to intensify the overall photocatalytic efficiency. This has been achieved by various unique morphology designs, such as the 0D/1D (e.g. Bi₂WO₆ NPs decorated on WO₃ nanorods),¹⁹⁷ 0D/2D (e.g. BiVO₄ NPs anchored on WO₃ nanoplates),¹⁹⁴ 0D/3D (e.g. Ag₃PO₄ NPs dispersed in 3D ordered microporous WO₃),²⁰¹ 1D/2D (e.g. W₁₈O₄₉ nanowires dispersed on g-C₃N₄ nanosheets)¹⁹⁸ and 2D/2D (e.g. WO₃ nanoplates on g-C₃N₄ nanosheets)⁶³ structures, etc.

The construction of WO_X/semiconductor heterojunctions could be achieved by a one-step or two-step preparation method. In a one-step method, the starting materials for both WO_X and the coupled semiconductor are generally mixed in a precursor solution and then used for a synthesis process. For instance, Zhou et al. have prepared a WO₃/BiWO₆ composite via a one-step hydrothermal method using a precursor containing both Bi(NO₃)₃·5H₂O and H₂WO₄.²⁰² Compared to the one-step method, the two-step methods are more commonly adopted. In a two-step method, one semiconductor (i.e. WO_X or the coupled semiconductor) is generally synthesized firstly and then mixed with the precursor solution that is used for the synthesis of the second semiconductor. The deposition and growth of the second semiconductor onto the surface of the first semiconductor could be achieved by using various synthesis methods. For instance, Ye and Wen have prepared a WO₃/ZnIn₂S₄ composite by synthesizing firstly the WO₃ nanorods via a hydrothermal process and then mixed the as-synthesized WO₃ nanorods with the precursor solution containing In(NO₃)₃·4.5H₂O, Zn(AC)₂·6H₂O and C₃H₇NO₂S·HCl·H₂O followed by another hydrothermal treatment, which allows the deposition and growth of ZnIn₂S₄ on the surface of WO₃.¹⁸⁹ The WO_X and the coupled semiconductor could also be synthesized firstly and separately and then mixed in a solution followed by a specific treatment to allow intimate contact. For instance, Lara et al. have performed the coupling of WO₃ and TiO₂ by mixing the as-synthesized WO₃ and TiO₂ in a solution followed by a second-step hydrothermal treatment.¹⁷⁹

Conclusions and perspectives

This review summarizes firstly the fundamentals of WO_X (i.e. crystal and electronic structures and optical properties) and then highlights the strategies for the enhancement of the photocatalytic efficiency of WO_X-based photocatalysts. These include the controls in the crystal phase, stoichiometry/oxygen-vacancy, active facet and morphology, elemental doping, loading of noble metal NPs, hybridization with carbon materials and coupling with other semiconductors to construct heterojunctions.

For nanostructured WO_X, not only the monoclinic I phase but also the hexagonal, orthorhombic and triclinic phases could retain phase stability and exhibit photocatalytic activity at room temperature. Taking advantage of the polymorphic property of WO_X, a phase junction could be formed to facilitate the transfer and separation of photogenerated charge carriers. Compared to stoichiometric WO₃, oxygen-deficient WO_3−x exhibits extended light absorption in the visible and NIR regions and possesses a higher density of free charge carriers, both of which are beneficial for improving the photocatalytic performance. {002} is the active facet of m- and o-WO_X due to their high surface energy. A morphology with a high percentage of exposed active facets and large specific surface area is always desirable, which could be best achieved by a 2D structure with a high aspect ratio. The WO_X with 3D hierarchical structures assembled by 1D and/or 2D building blocks and 3D ordered macroporous structures are also applausive for practical applications due to their high structural stability and less agglomeration. Doping WO_X with low valence metal ions could extend light absorption, promote photo-induced electron–hole separation and improve photocatalytic redox ability due to the increased oxygen vacancies and the introduced defect band. By loading noble metal NPs onto the surface of WO_X, it is possible to extend light absorption into the visible and NIR regions and provide large numbers of “hot electrons” to participate in photocatalytic reactions, which is due to the strong LSPR effect. Graphene is an ideal support for the WO_X photocatalyst, for it provides an ultra-large specific surface area and serves as an excellent conductor for photogenerated electrons migrating from WO_X. Coupling WO_X with other semiconductors having unequal band structures could achieve efficient spatial charge separation via facilitating the charge transfer through the interfaces. By the construction of a Z-scheme heterojunction, the strong photo-oxidation ability of WO_X can be retained while the photo-reduction ability could be enhanced because of the higher CB level of the coupled semiconductor.

However, the photocatalytic efficiency of WO_X-based photocatalysts is still far from expectation, further improvement of which might rely on controls in synergistic effects. Combining different strategies (e.g. controls in the crystal phase, stoichiometry, active facet and morphology and coupling with other materials) in unique ways might lead to incredible results that could not be obtained with single strategies. To achieve the effective control of synergistic effects, the mechanisms of single strategies should be clear, some of which, however, are still vague. For instance, the charge transfer mechanism in the noble metal loaded system is still inconsistent. Furthermore, theoretical guidelines for the construction of Z-scheme heterojunctions are still lacking. The final aim of the development of efficient photocatalysts is to serve human beings by solving practical problems. However, the research of WO_X-based photocatalysts is still limited to laboratories. The efforts contributing to exploit appropriate ways for practical applications of WO_X-based photocatalysts, e.g., design of photo-reactors and integration with other technologies to extend applications, should be highlighted in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by the Ministry of Science and Technology of China (2016YFB0303901-05). Y. Gao acknowledges the funding from the Changjiang Scholars Programs (T2015136). W. Wang acknowledges the National Natural Science Foundation of China (51772312, 21671197).

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