Impact Statement

Developing highly efficient photocatalysts through defect engineering is of great significance for solar energy conversion, carbon neutrality, and environmental sustainability. Although extensive studies have explored defect-mediated photocatalysis, a unified understanding of the atomic-scale mechanisms governing photocatalytic CO₂ reduction and related reactions remains elusive due to the complexity of charge transfer and surface reaction pathways. In this mini-review, we systematically summarize recent advances in atomic defect engineering of WO₃-based photocatalysts, with particular emphasis on the atomic-scale origins of defect-induced catalytic activity. We discuss how various defect configurations, including oxygen vacancies, tungsten vacancies, disordered lattices, and ordered line defects, modulate the local electronic structures, surface coordination environments, and interfacial charge-transfer dynamics of WO₃ photocatalysts. Special attention is devoted to correlating defect-induced electronic reconstruction with molecular adsorption/activation behaviors and reaction selectivity in photocatalytic CO₂ reduction, water oxidation, H₂O₂ production, and selective organic transformations. Furthermore, advanced atomic-resolution characterization techniques and theoretical simulations are used to reveal the dynamic evolution of defect structures under realistic reaction conditions. This review also addresses key challenges toward practical implementation, including controllable defect fabrication, long-term structural stability, and operando identification of active sites. By bridging atomic-scale defect chemistry with photocatalytic functionality, this work provides mechanistic insights and rational design principles for next-generation WO₃ photocatalysts and other defect-engineered semiconductor systems for sustainable solar-to-chemical energy conversion.

2. The Phase Structures of Defective Tungsten Trioxides

The crystal structures of WO₃ are composed of the corner-sharing [WO₆] octahedra [Reference Al Mohammad and Gillet51], as shown in Figure 1a, where the centered W atom is bonded with six adjacent O atoms [Reference Ramana, Utsunomiya, Ewing, Julien and Becker52]. Given that the corner-sharing [WO₆] octahedra exhibit various tilting and rotation angles under different environments, the phase structures of WO₃ can be divided into tetragonal α-WO₃, orthorhombic β-WO₃, monoclinic γ-WO₃, triclinic δ-WO₃, and monoclinic ε-WO₃ [Reference Zheng, Ou and Strano44,Reference Di Valentin and Pacchioni53]. As shown in the phase diagram of the WO₃ sample as a function of temperature in Figure 1b, increasing the temperature from 0 °C under an oxygen-rich environment induces a phase transition from triclinic δ-WO₃ to tetragonal α-WO₃, showing temperature-dependent evolution of the crystal structures [Reference Salje, Rehmann and Pobell54,Reference Vogt, Woodward and Hunter55]. With the increased O-W-O bond angles along the [100], [010], and [001] directions, the crystal symmetry of WO₃ improves until it approaches an “ideal” cubic structure [Reference Granqvist56]. Among these structures, the monoclinic γ-WO₃ is the common and stable phase that exists at room temperature [Reference Zheng, Ou and Strano44,Reference Di Valentin and Pacchioni53]. As depicted in Figure 1c, the yellow WO₃ consists of corner-sharing distorted [WO₆] octahedra with a band gap of 2.8 eV, whose band structure positions meet the requirements of overpotentials for some photocatalytic redox reactions [Reference Li, Da and Zhang57,Reference Mi, Zhanaidarova, Brunschwig, Gray and Lewis58]. However, the apparent quantum yield of the pristine γ-WO₃ photocatalysts is poor compared to the natural photosynthesis reaction efficiency without any cocatalysts and modifications [Reference Li, Chen and Wang59,Reference Wang, Xue and Huang60]. To expand the light absorption in visible light and construct more active sites at the surface, various chemical reduction methods introduced diverse defects within the γ-WO₃ catalysts, including hydrogenation [Reference Yella, Tahir and Meuer61,Reference Wang, Ling and Wang62], liquid-phase reduction [Reference Santato, Odziemkowski, Ulmann and Augustynski63,Reference Polleux, Pinna, Antonietti and Niederberger64], etc [Reference Chandra, Saito, Yui and Yagi65].

Figure 1.

(a) The structural units of [WO₆] octahedra in tungsten oxides. Reproduced with permission of Ref. [Reference Ramana, Utsunomiya, Ewing, Julien and Becker52]. Copyright 2006, American Chemical Society. (b) The phase diagram of tungsten trioxides as a function of temperature, with an emphasis on 21 wt% oxygen content (redrawn from Ref. [Reference Vogt, Woodward and Hunter55]). (c) The atomic structures of the yellow tungsten oxide, which corresponds to the γ-WO_3, consisting of corner-sharing [WO₆] octahedra. (d) The phase structure of (upper panel) the blue tungsten oxide and its atomic configuration (lower panel) along the [010] zone axis [Reference Li, Liu and Pan74]. (e) The crystal structures of violet WO_2.72 (upper panel) and its structures from the top view (lower panel) [Reference Sundberg, Zakharov and Zibrov75]. The blue and red spheres refer to the W and O atoms, respectively.

Figure 1. Long description

Panel a shows a single WO₆ octahedron with a central light blue sphere representing W and six red spheres at the vertices representing O.

Panel b is a phase diagram with a vertical temperature axis in degrees Celsius. From bottom to top, the phases are delta-W O₃ starting at 17, gamma-WO ₃ above 17, beta-WO₃ above 330, and alpha-WO₃ above 740.

Panel c displays the yellow tungsten oxide gamma-WO₃ structure, consisting of a three-dimensional network of corner-sharing octahedra.

Panel d shows the blue tungsten oxide WO_2.9. The upper sub-panel is a perspective view of the lattice. The lower sub-panel shows the atomic configuration along the 0 1 0 zone axis with a brown dashed line indicating a shear plane across the octahedra.

Panel e shows the violet tungsten oxide WO_2.72. The upper sub-panel is a perspective view. The lower sub-panel is a top view along the c and a axes, highlighting pentagonal rings formed by the octahedra with brown dashed outlines.

Yellow WO₃ is a typical substoichiometric compound at room temperature, having a certain concentration of V_O in bulk [Reference Polaczek, Pekala and Obuszko66,Reference Makarov and Trontelj67]. As the concentration of V_O rose in bulk, the substoichiometric ratio between O and W atoms would be lower than 3, forming oxygen-deficient tungsten oxide materials [Reference Bandi and Srivastav68]. These defects would produce a defect level in the forbidden band below the conduction band, where the position is dependent on the defect concentrations [Reference Migas, Shaposhnikov, Borisenko and oxides69]. Meanwhile, the existence of the V_O in bulk WO₃ produces plentiful dangling bonds to increase the concentrations of photoexcited electrons, resulting in the Burstein–Moss shift [Reference Niklasson, Berggren and Larsson70]. The narrowed band gap and up-shifted Fermi level of the γ-WO_3-x crystals contribute to broadening their absorption ranges in visible and infrared light due to the additional defect levels and local surface plasmon resonance (LSPR) [Reference Manthiram and Alivisatos71,Reference Lu, Jiang, Gao, Wang and Chen72]. When V_O continually rises, it would be gathered and reconstructed to produce well-known Magnéli phases [Reference Ekanayake, Mai, Chen and Caruso73], including WO_2.92, WO_2.9 (blue tungsten oxide), WO_2.72 (violet tungsten oxide), WO_2.67, etc [Reference Yan, Wang and Wu41,Reference Al Mohammad and Gillet51]. As seen in the upper panel of Figure 1d, the crystal structure of the blue tungsten oxide was composed of corner-sharing and edge-sharing [WO₆] octahedra, which is different from WO_2.92 structures with V_O [Reference Polaczek, Pekala and Obuszko66]. From the atomic configuration of the WO_2.9 phase along [010] direction (the lower panel of Figure 1d), half of [WO₆] octahedra to the right of the dotted brown line have shifted 1/2 distances of O-W-O bond length along the (200) and (002) planes as marked by brown arrows, making corner-sharing connected [WO₆] octahedra changed to edge-sharing connected [WO₆] octahedra, thereby inducing the formation of stacking faults [Reference Ye, Wei and Xue24,Reference Li, Liu and Pan74]. This structure reconstruction is accompanied by the formation of many three-coordinated O atoms to reduce the stoichiometric ratio of tungsten oxides, producing WO_2.9 Magnéli phases [Reference Ye, Wei and Xue24].

For WO_2.72, its phase structures show different polyhedron connection patterns as compared to the blue tungsten oxide in the upper panel of Figure 1e [Reference Sundberg, Zakharov and Zibrov75]. In the atomic model of the WO_2.72 phase from the [010] direction, we found that the [WO₆] octahedral structural units show varying degrees of distortion in the bond length and bond angles in the a, b, and c directions that are connected by corner-sharing at the structures [Reference Frey, Rothschild and Sloan76]. Meanwhile, surrounded [WO₆] octahedra have been tilted and rotated to form edge-sharing connections, producing two [WO₇] decahedra as boxed by brown dashed lines [Reference Migas, Shaposhnikov, Borisenko and oxides69]. Due to the distorted structures of the monoclinic WO_2.72 phase, the WO_2.72 phase shows metal-like band structures [Reference Migas, Shaposhnikov, Borisenko and oxides69]. The edge-sharing connections over [WO₆] octahedra result in delocalized electronic structures, increasing carrier concentration, and thereby improving the electrical conductivity of WO₃ [Reference Gerosa, Di Valentin, Onida, Bottani and Pacchioni77,Reference Gillet, Lemire, Gillet and Aguir78].

2.1. The Evolution Mechanism of Various Defect Types

Defects originating from the crystal structures that break the periodicity of atom stacking in materials were classified into three types according to their dimensions: point, line, and planar defects [Reference Böer and Pohl79]. These defects can be generally constructed by employing various reduction methods with specific reaction environments to form Magnéli phases of WO₃ materials, including gas phase reduction [Reference Zhang, Gong and Liu80,Reference Li, Bando and Golberg81], liquid phase reduction [Reference Böttger-Hiller, Lungwitz and Seifert82], and solid phase reduction [Reference Liu, Wang and Lin83]. The basic principle of constructing defects is applying the reducibility of reaction agents to seize the lattice oxygen, forming different defect types in various Magnéli phases.

Yu et al. have investigated the dynamic evolution mechanism of defect types with increased temperature under a 5 wt% H₂/Ar atmosphere by employing in situ transmission electron microscopy (TEM) [Reference Ye, Wei and Xue24]. As shown in Figure 2a, hydrogen reduction generates a certain concentration of V_O in WO₃ nanosheets, and their concentration increases positively with the heat treatment temperature. When the temperature reached 350 °C, the V_O would gather to trigger the rotation of [WO₆] octahedra, resulting in the appearance of the {120} stacking faults in a unique zigzag form (Figure 2b). With the temperature increased to 450 °C, we found that the [WO₆] octahedra have been tilted by 45° to form {200}-1X (Figure 2c) and {200}-2X (Figure 2d) stacking faults along [100] directions. These results indicate that γ-WO₃ materials show temperature-dependent defect types.

Figure 2.

(a–d) The microstructure of the γ-WO₃ crystal at the increased temperature from 200 °C to 500 °C under the 5% H₂/Ar atmosphere. Reproduced with permission of Ref. [Reference Ye, Wei and Xue24]. Copyright 2021, American Chemical Society.

Figure 2. Long description

The diagram consists of four main sections labeled a through d.

Panel a shows the initial state at 200 to 350 degrees Celsius. It features a high-resolution transmission electron microscopy micrograph of a regular grid of atoms and a corresponding ball-and-stick model of corner-sharing octahedra. Red dashed circles indicate point defects scattered throughout the lattice.

Panel b illustrates the transition at 350 to 450 degrees Celsius. White arrows and green curved arrows indicate the migration and rotation of octahedra. A dashed black line outlines a shifting boundary. To the right, the migration results in a line or planar defect, shown in a micrograph where a red arrow points to a vertical disruption in the atomic grid.

Panel c shows a transition at 450 to 500 degrees Celsius. Multiple white arrows point diagonally downward toward a central horizontal layer highlighted in yellow. The resulting structure on the right shows a planar defect where octahedra are more densely packed along a horizontal axis, confirmed by a micrograph showing a distinct horizontal line of contrast.

Panel d also shows a transition at 450 to 500 degrees Celsius but with a different migration pattern. White arrows point both up and down toward a central yellow-highlighted zone. The final state on the right shows a planar defect characterized by larger open channels between octahedral layers, visible as dark voids in the corresponding micrograph.

Except for the approaches mentioned above, the photochemical reduction method is an effective strategy to obtain abundant V_O defects inside the WO₃ particles. For instance, Wang et al. used photo-excited electrons to reduce the bulk of WO₃ materials for constructing V_O under a solution with electron donors [Reference Zhou, Wang and Lin84]. Meanwhile, multiple-step reduction methods have been employed to optimize the surface atomic structures for changing the selectivity of products [Reference Shen, Ye and Wang85]. It is worth noting that the influence of defect types on photocatalytic properties is complex, since their terminations on the surface can modulate surface atomic structures and concurrently affect the transport behavior of photogenerated carriers [Reference Li, Zhao, Luo, Du and Zeng86].

The crystal surface, defined as the boundary interface terminating periodic atomic arrangements, exhibits various structural imperfections when defects from the bulk material terminate at this surface [Reference Bollmann87]. These surface defects dependent on their dimensions, can also be categorized into three primary classifications [Reference Böer and Pohl79]: (1) surface point defects, manifested as adatoms (surface-adsorbed atoms) or vacancies (missing lattice atoms) [Reference Ida, Kim, Ertekin, Takenaka and Ishihara88]; (2) surface linear defects, including step edges and kink sites [Reference Ren, Song and Lee89]; and (3) surface planar defects [Reference Liu, Jing, Zhou and Guo90], typically characterized by atomically rough surfaces with disrupted periodicity.

These types of defects at the surface fundamentally change the local atomic coordination and the surface energy landscape, significantly impacting the material’s chemical reactivity and physical properties [Reference Rousseau, Glezakou and Selloni91]. Understanding how surface defects drive the adsorption and polarization of molecules is essential. Furthermore, the spatial distribution of these defects is crucial in determining their roles. Bulk defects typically serve as recombination centers for photogenerated carriers, whereas surface defects, rich in dangling bonds, act as active sites that facilitate surface reaction dynamics. The specific roles of these surface defects in photocatalytic processes will be discussed in detail below.

2.2. The Role of the Surface Point Defect

Surface point defects contain surface adatoms [Reference Yu, Yu and Wang92,Reference Zhang, Wang and Zuo93,Reference Zhang, Jalil and Wu94], W interstitials [Reference Zhu, Vasilopoulou and Davazoglou23], W vacancies, and O vacancies [Reference Meng, Lin and Chen95,Reference Yang, Wang and Wang96,Reference Liu, Sun and Lin97]. Among them, surface V_O is the most frequently reported in the literature, since it is easily generated during experiments. The construction of surface V_O causes the exposure of W atoms, acting as a surface-active site for adsorbing the reactants and polarizing the electronic structures of molecules in the WO₃ photocatalyst [Reference Shi, Wang, Song, Yu and Wu98]. Zhou et al. [Reference Zhou, Wang and Lin84] reported that the WO₃-hν-1.0 h sample generated abundant V_O on the {002} surface of photocatalysts under irradiation, which effectively enhanced the adsorption of oxygen and the formation of superoxide radicals (Figure 3a–e). What’s more, the production of V_O facilitates the spatial separation of photoexcited charge carriers, establishing dual activation pathways to activate benzene under visible light and improve the phenol yield, as shown in Figure 3f,g. However, excessive surface V_O concentration could impair the photocatalytic performance, because the high density of V_O serves as a recombination center for photocarriers. The influence of oxygen vacancies on photocatalytic performance is highly dependent on their concentration [Reference Corby, Francàs, Kafizas and Durrant21]. At low to moderate concentrations, oxygen vacancies introduce defect states within the band gap, broaden visible-light absorption, increase carrier density, and facilitate charge transport [Reference Sachs, Park and Pastor99,Reference Li, Tang, Zhang and Zhang100]. Simultaneously, surface oxygen vacancies expose coordinatively unsaturated W sites and alter the local electronic structure, thereby enhancing the adsorption and activation of reactant molecules [Reference Kato, Uemura, Asakura and Yamakata42]. The accompanying local surface reconstruction can further create favorable adsorption configurations that stabilize reaction intermediates and reduce surface reaction barriers [Reference Xue, Wei and Shen19,Reference Meng, Lan, Castelli and Zheng101]. In contrast, excessive oxygen-vacancy concentrations may introduce deep traps or recombination centers, intensify carrier scattering, and trigger pronounced lattice distortion or structural reconstruction, ultimately suppressing charge separation and deteriorating structural stability [Reference Kato, Uemura, Asakura and Yamakata42]. Therefore, optimizing the concentration of oxygen vacancies, rather than simply maximizing their density, is critical for achieving a balanced enhancement of light harvesting, charge transport, surface reactivity, suppression of carrier recombination, and long-term structural stability [Reference Corby, Francàs, Kafizas and Durrant21].

Figure 3.

The atomic structure of (a) WO₃-hν-1.0 h samples, showing that the oxygen vacancies were produced on the (002) surface under illumination. (b) Integrated differential phase contrast (iDPC) image of the WO₃-hν-1.0 h sample along the [001] zone axis. (c) The enlarged iDPC image of the WO₃-hν-1.0 h sample at the surface area and (d) its corresponding intensity-line profiles. (e) DFT calculation of O₂ adsorbed on WO₃-hν-1.0 h sample. (f) The photocatalytic properties of the WO₃ and WO₃-hν-1.0 h samples over the benzene-to-phenol process under visible light. (g) Schematic illustration of O₂ and H₂O activation over WO₃-hv-1.0 h [Reference Zhou, Wang and Lin84]. Reproduced with permission of Ref. [Reference Zhou, Wang and Lin84] Copyright 2024, Wiley-VCH.

Figure 3. Long description

A multi-panel scientific display.

Panel a: A 3 D ball-and-stick model of a WO ₃ lattice under a sun icon. White dashed circles on the (002) surface indicate photo-induced oxygen vacancies, labeled V _O.

Panel b: An i D P C microscopy image showing a crystalline lattice with two regions outlined in yellow dashed lines labeled Disorder. A scale bar indicates 2 nanometers. Lattice planes (020) and (200) are marked.

Panel c: A magnified view of the lattice from panel b. Two dark spots are circled and labeled O V for oxygen vacancies.

Panel d: An intensity-line profile graph corresponding to panel c. The y-axis shows intensity peaks. Red arrows point to lower intensity troughs labeled OV, while a black arrow points to a higher peak labeled O.

Panel e: A D F T calculation model showing an O₂ molecule adsorbed on the surface with a bond length of 1.23 Angstroms.

Panel f: A line graph comparing Phenol yield percentage on the y-axis against time in hours on the x-axis. WO₃-hv-1.0 h (red circles) shows a linear increase reaching 20 percent, while pristine WO₃ (black squares) reaches 8 percent. A pink arrow indicates a 2-fold increase.

Panel g: A schematic illustration of the photocatalytic process. Under sunlight, electrons and holes facilitate the conversion of H₂O and O₂ into hydroxyl radicals, superoxide radicals, and hydrogen peroxide on the WO₃ surface.

V_O can also be engineered under high-pressure environments to regulate the distribution of V_O [Reference Wang, Xue and Huang60]. Since the γ-WO₃ sample was terminated with oxygen atoms, a great lattice compression was introduced on the sample under a pressure of 1.2 gigapascals (GPa) to facilitate the migration of surface oxygen atoms, constructing the abundant surface V_O on WO_3-x photocatalysts (Figure 4a–e) [Reference Wang, Xue and Huang60]. These surface-defined V_O on the WO₃ sample showed improved photocatalytic O₂ activation, further promoting the production of H₂O₂ (Figure 4f). It is concluded that precisely constructing surface V_O is a significant means to fortify the spatial charge separation and serves as an active site for the photocatalytic activation of O₂ under visible light. Recent studies further demonstrate a shift from the nonspecific generation of defects toward the spatial and chemical regulation of defect environments in WO₃-based photocatalysts. Pressure- and photo-mediated strategies have been employed to regulate the distribution of surface oxygen vacancies, while metal-vacancy engineering has been shown to modify the electronic structure and carrier transport behavior [Reference Granqvist56,Reference Li, Bando and Golberg81,Reference Ren, Song and Lee89,Reference Liu, Sun and Lin97]. These advances highlight the importance of controlling not only defect concentration, but also defect location, local coordination environment, and electronic structure.

Figure 4.

(a) Formation of V_O in WO₃ at ambient pressure (upper panel) and under 1.2 GPa (lower panel). (b) V_O formation energy of WO₃ as a function of oxygen layers. (c) Relative energy profiles of diffusion of the O from the bulk to the surface of WO₃. (d) The observed atomic structures of pressure-induced surface oxygen vacancies, and (e) their corresponding intensity-line profiles. (f) Photocatalytic H₂O₂ production rates of the WO₃ and V_O-rich WO₃ samples under visible irradiation [Reference Wang, Xue and Huang60]. Reproduced with permission of Ref. [Reference Wang, Xue and Huang60]. Copyright 2025, American Chemical Society.

Figure 4. Long description

Panel a shows two parallel rows of atomic models. The top row shows WO₃ at ambient pressure transitioning through heating to a random distribution of oxygen vacancies, labeled V_O. The bottom row shows compressed WO₃ at 1.2 G P a transitioning through heating to a surface distribution of vacancies. Grey spheres represent W, red represent lattice O, and cyan represent O migration.

Panel b is a line graph of O V formation energy in e V versus oxygen layers 1 through 5. Compressed WO ₃ (blue circles) shows a higher energy requirement, increasing sharply at layer 4, compared to standard WO₃ (pink squares).

Panel c is a line graph of relative energy in e V from bulk to surface. Both curves peak in the middle, but compressed WO ₃ maintains higher relative energy throughout the diffusion path.

Panel d is a high-resolution transmission electron microscopy image with a 2 nm scale bar, showing atomic lattice fringes. Two dashed boxes highlight regions L 1 and L 10.

Panel e shows the intensity line profile corresponding to panel d. Ten peaks are labeled L 1 through L 10. Red arrows point to troughs at L 1 and L 3, identifying them as V_O sites.

Panel f is a bar chart showing photocatalytic H ₂ O₂ production. Black bars represent selectivity (Sel. / %) which remains near 100 percent for all samples. Red bars represent conversion (Con. / %). The WO ₃-1.2 G P a sample, highlighted with a green box, shows the highest conversion rate, significantly outperforming the other four samples.

V_O can also play a vital role in tailoring the band structures of photocatalysts. Recently, Xie et al. reported that ultrathin cubic-WO₃ layers with rich oxygen structures (as boxed by a red dashed frame in Figure 5a,b) exhibited an intermediate band (IB) in the band gap, thereby lowering the band gap of the substrates and absorbing infrared light, which enables the infrared light-driven photocatalytic performances for CO₂ reduction reaction (Figure 5c,d) [Reference Liang, Li and Sun43]. In addition to the anion vacancies, cation vacancies were generated through thermal evaporation and other methods in a reducing atmosphere. These defects play a crucial role in tuning the band structures by creating new electronic states near the valence band of the N-type semiconductor. This enhancement significantly improves the conductivity and bolsters carrier mobility within the photocatalyst. Liu et al. prepared WO₃ with W vacancies (V_W) through a chemical bath method, which can delocalize the valence band structures to reduce the effective mass of photogenerated holes for accelerating charge transport, thereby improving the photoelectrochemical water splitting performance (Figure 5e–h) [Reference Wang, Liu and Liao102]. In conclusion, constructing surface point defects with appropriate types, concentrations, and distributions on γ-WO₃ not only introduces defect levels in band structures to expand the absorption range of photocatalysts in visible light and enhance the dynamic transport of the photogenerated carrier, but also serves as a reaction site to capture photogenerated electrons for facilitating the activation of the molecules.

Figure 5.

(a) Atomic model, (b) High-resolution TEM (HRTEM) image, (c) band structure illustration, and (d) photocatalytic properties of the ultra-thin WO₃ nanosheets with rich oxygen vacancies [Reference Liang, Li and Sun43]. Reproduced with permission of Ref. [Reference Liang, Li and Sun43]. Copyright 2018, Elsevier. (e) The atomic configuration, (f) Aberration-corrected high-angle annular dark field (AC-HAADF) image, and (g) band structure of the WO₃ sample with W vacancies. (h) The PEC water-splitting properties of the pristine WO₃ and V_W-WO₃ samples [Reference Wang, Liu and Liao102]. Reproduced with permission of Ref. [Reference Wang, Liu and Liao102]. Copyright 2025, Nature Publishing Group.

Figure 5. Long description

The figure consists of eight panels arranged in two rows.

Top row:

Panel a: Atomic ball-and-stick model of V_O-rich WO₃. Blue spheres represent W and red spheres represent O. Dashed circles indicate missing oxygen atoms (vacancies).

Panel b: HRTEM micrograph showing a lattice with 0.371 nanometer spacing in the (100) and (010) planes. A red dashed box highlights a region of O vacancies.

Panel c: Band structure diagram on a vertical axis of Potential versus N H E (V) at p H equals 0. It shows the C B and V B of W O₃ shifting upward in the V_O-rich sample, with an arrow pointing to the CO ₂ / C O redox potential.

Panel d: Bar chart of evolution rates in micromoles per gram per hour under 800 to 1700 nanometer irradiation. Red bars (V_O-rich) show significantly higher C O and O₂ production compared to blue (V _O-poor) and black (pristine WO₃).

Bottom row:

Panel e: Atomic model of V_W-WO ₃. Yellow dashed circles indicate missing tungsten atoms.

Panel f: AC-HA A D F image showing a grid of bright spots. A pink rectangle and yellow dashed circles highlight the W vacancies.

Panel g: Band structure comparison on an E versus R H E axis. WO₃ has a 2.72 V gap, while V sub W-WO₃ shows an upward shift to a 2.49 V gap with pink-shaded band-tail states. Effective mass m_h super star values are 2.69 m₀ and 0.95 m₀ respectively.

Panel h: P E C water-splitting graph showing current density J in milliamperes per square centimeter versus Potential in V versus R H E. The pink stepped curve for V_W-WO₃ reaches approximately 4 units, while the blue curve for pristine WO₃ stays below 1 unit.

2.3. The Role of the Surface Line Defect

Surface line defects on photocatalysts are mainly derived from the surface steps and the surface terminations of planar defects (e. g., stacking faults) that existed in bulk [Reference Wang, Wang and Arandiyan103]. They show lower-coordinated surface atoms to polarize the electronic structures of molecules as compared to the surface points [Reference Mascaró, Koper and Rost104,Reference Liu, Shih and Deng105]. These higher-concentration defects may produce continuous defect energy levels to broaden the light absorption range of the materials to the visible or even infrared region [Reference Liang, Li and Sun43,Reference Mahmood, Lin, Xie and Wang106]. The surface line defects with one-dimensional geometry ^[Reference Arandiyan, Mofarah and Sorrell107 ^] can provide a continuous electron transport path to promote the directional migration of photogenerated carriers and restrict electron–hole recombination. Furthermore, the continuous structure of the surface line defect can disperse the reaction intermediates [Reference Yi, Choi and Jang108], reducing the risk of the active site being covered or deactivated on the isolated active site of the surface point defect [Reference Páez, Pereira, Rodrigues and Peres109,Reference Schweizer, Dolle and Spiecker110].

As shown in Figure 6a–c, Xue et al. constructed the surface-ordered line defects identified as the terminations of the {013} stacking faults on the {002} surface of the WO_3-x photocatalysts (blue tungsten oxide) [Reference Xue, Wei and Shen19]. The integrated differential phase contrast (iDPC) image showed that the surface-ordered line defects expose plentiful quadri-coordinated W atoms, serving as the active sites for the adsorption and activation of CO₂ molecules, promoting the methanation desorption (Figure 6d). When the concentration of the surface-ordered line defect was increased by optimizing the heat treatment temperature, the photocatalytic properties of the monoclinic WO_2.9 sample were further improved.

Figure 6.

(a) AC-HAADF image, (b) enlarged iDPC image, (c) surface atomic structure of WO-550 sample at the termination of {002} surfaces, and (d) the reaction pathway of photoreduction of CO₂ molecules on the WO-550 sample with surface-ordered line defect for activating CO₂ molecules [Reference Xue, Wei and Shen19]. With permission of Ref. [Reference Xue, Wei and Shen19]. Copyright 2024, National Academy of Sciences. (e) AC-HAADF image, (f) enlarged iDPC image, and (g) atomic configuration of the WO_3-x nanosheets with hexagonal-line defects. (h) The reaction pathway of photoreduction of CO₂ molecules on the WO_3-x nanosheets with hexagonal-line defects [Reference Shen, Ye and Wang85]. With permission of Ref. [Reference Shen, Ye and Wang85] Copyright 2024, American Chemical Society.

Figure 6. Long description

A multi-panel display of atomic-resolution microscopy and chemical reaction pathways.

Panel a is an AC-HAADF micrograph of WO-550 showing a blue and yellow atomic lattice with white lines tracing stepped surface defects under a (00 2) plane label.

Panel b is an enlarged iDPC micrograph showing a specific surface vacancy site with blue and red atomic overlays.

Panel c is a ball-and-stick model of the surface structure with W super 4 plus sites and dashed red circles indicating vacancies.

Panel d shows a horizontal reaction pathway for CO₂ reduction on WO-550. It starts with a W super 4 plus surface, adds CO₂, then proceeds through multiple H plus and e minus steps, indicated by gray arrows, to eventually release CH₄. A legend identifies H as gray, C as black, W as light blue, O of WO-550 as red, and O of CO₂ as dark blue.

Panel e is an A C H A A D F micrograph of WO₃ minus x nanosheets with a square grid lattice and labels for (200) and (020) planes.

Panel f is an enlarged iDPC micrograph showing hexagonal-line defects highlighted with red outlines and white curved arrows.

Panel g is an atomic configuration model showing W⁵⁺ sites within a hexagonal defect arrangement.

Panel h shows the reaction pathway for CO₂ reduction on the hexagonal-line defect nanosheets. The sequence follows CO₂ adsorption and H plus and e minus addition, leading to the release of C O. The legend identifies W as red, O as gray, H as pink, and C as black.

To regulate the selectivity of CO products, Shen et al. reconstructed the surface-ordered line defect of the WO_2.9 sample to hexagonal-line defects (Figure 6e–g) [Reference Shen, Ye and Wang85]. The terminations of hexagonal-line defects on the {002} surfaces of the WO_3-x nanosheets varied the redox pathways of the *COOH intermediates, of which photogenerated carriers broke the C-O bonds, resulting in the main CH₄ product being changed to CO gas (Figure 6h). These results indicate that photocatalytic reaction pathways are strongly dependent on the atomic configurations and termination structures of surface defects. Recent studies published in 2024 further reveal that the local coordination environment generated by surface defect terminations is a key factor governing CO₂ photoreduction selectivity. Surface-ordered line defects terminating on the {002} facets of WO_3-x expose tetra-coordinated W sites, which promote cooperative CO₂ adsorption and favor deep hydrogenation toward CH₄ [Reference Xue, Wei and Shen19]. In contrast, reconstruction into hexagonal line defects produces distinct penta-coordinated W environments and modifies the conversion pathway of *COOH intermediates, thereby shifting the major product from CH₄ to CO [Reference Shen, Ye and Wang85]. These findings demonstrate that product selectivity is determined not simply by defect concentration, but also by the atomic configuration, termination structure, and local coordination environment of the defects.

2.4. The Role of Other Surface Defects

Generally, surface planar defects refer to the rough surface of photocatalysts, including nanoscale protrusions [Reference Sun, Liu and Gao111], nanopores [Reference Li, Jin and Xing112], or nanopits [Reference Sun, Liu and Gao111]. In addition to greatly increasing the specific surface area to offer more reactive sites for reactant molecule activations or heteroatom loading, these microstructures also improve light energy utilization by extending the light path through multiple scattering effects [Reference Li, Salvador and Rohrer113]. Meanwhile, the surface planar defects can separate the redox reaction sites on the exposed facet to hinder the occurrence of back reactions [Reference Liu, Liang and Xiao114] and decrease the diffusion length of the photogenerated carrier [Reference Zhao, Pan and Li115] for achieving spatial separation.

A representative example of the direct catalytic role of surface pits was reported by Liu et al., who constructed pothole-rich ultrathin WO₃ nanosheets for photocatalytic nitrate synthesis from N₂ [Reference Liu, Cheng and He116]. The potholes disrupted the local surface periodicity and generated abundant dangling bonds and electronically active adsorption environments. These sites enhanced N₂ adsorption and polarization, thereby facilitating N ≡ N bond activation and subsequent nitrate formation. Under ambient conditions, the pothole-rich WO₃ nanosheets achieved an average nitrate-production rate of 1.92 mg g⁻¹ h⁻¹ without sacrificial agents or noble-metal cocatalysts (Figure 7a–d). This work demonstrates that surface potholes are not merely morphology-related features, but can generate distinct local coordination and electronic environments that directly participate in the activation of chemically inert molecules.

Figure 7.

(a) Atomic-resolution HAADF-STEM image of a pothole-rich WO₃ nanosheet (inset is corresponding low-magnification HAADF image). (b) The charge difference density of the pothole-rich WO₃ nanosheet around the pothole region. (c) Schematic illustration of photoconversion of N₂ into nitrate at ambient conditions. (d) Nitrate formation rate calculated by the nitrate concentration profiles [Reference Liu, Cheng and He116]. Reproduced with permission of Ref. [Reference Liu, Cheng and He116]. Copyright 2019, Wiley-VCH. (e) HRTEM image of the Mo-doped W₁₈O₄₉ sample demonstrating rod-like morphology. (f) The adsorption configurations and energies of the surface-active sites of the W₁₈O₄₉ and Mo-doped W₁₈O₄₉ samples for capturing N₂ molecules. (g) Its reaction mechanism. (h) Photocatalytic ammonia production rates. Light irradiation conditions: full-spectrum or visible-NIR light (λ > 400 nm), 200 mW cm⁻², Na₂SO₃ as a sacrificial agent [Reference Zhang, Jalil and Wu94]. Reproduced with permission of Ref. [Reference Zhang, Jalil and Wu94]. Copyright 2018, American Chemical Society. (i) AC-HAADF image of Co-decorated W₁₈O₄₉ ultrathin nanowires. Calculated PDOS of (g) the pristine W₁₈O₄₉ and (k) Co-decorated W₁₈O₄₉ samples. (l) The photocatalytic CO₂ reduction performances of the Co-decorated W₁₈O₄₉ sample under various experimental conditions [Reference Zhang, Wang and Zuo93]. Reproduced with permission of Ref. [Reference Zhang, Wang and Zuo93]. Copyright 2021, American Chemical Society.

Figure 7. Long description

Panel a is a high-resolution HAADF-STEM photo showing a dark pothole defect in a WO ₃ lattice with a 1 nanometer scale bar. Panel b is a charge difference density map showing electron transfer between W 0, W 1, and W 2 atoms. Panel c is a schematic of a blue nanosheet surface converting N₂ and H₂O into NO ₃ minus under light. Panel d is a bar chart showing NO₃ minus yield is highest for pothole-rich nanosheets at nearly 2.0 milligrams per gram per hour. Panel e is an H R T E M photo of a Mo-doped W₁₈O ₄₉ nanorod with a 2 nanometer scale bar. Panel f shows adsorption energy bar graphs where Mo-doping increases energy from minus 1.65 to minus 2.48 electron volts. Panel g is a molecular mechanism diagram showing N₂ capturing at a Mo site. Panel h is a bar chart of NH₃ production rates, peaking for M W O minus 1 under full spectrum light. Panel i is an A C H A A D F photo of Co-decorated W₁₈O₄₉ nanowires. Panels j and k are density of states graphs showing a d-band center upshift from minus 2.475 to minus 1.169 electron volts upon Co-decoration. Panel l is a bar chart showing gas generation rates for H₂ and CO under seven different reaction conditions, with the highest C O production occurring for W ₁₈ O ₄₉ at Co.

Zhang et al. reported Mo-doped W₁₈O₄₉ ultrathin nanowires with high-density V_O to promote the reduction of nitrogen under light irradiation (Figure 7e) [Reference Zhang, Jalil and Wu94]. These surface Mo adatoms integrated with adjacent W atoms exhibit asymmetric electronic structures, showing a large adsorption energy of nitrogen and converting to ammonia (Figure 7f–h). Zhang et al. have designed the Co-decorated W₁₈O₄₉ ultrathin nanowires to modify the energy configurations of substrates, making a significant upshift of the d-band center to produce more antibonding states above the Fermi level, thereby effectively adsorbing CO₂ molecules (Figure 7i–l) [Reference Zhang, Wang and Zuo93]. In fact, few investigations have exploited the roles of surface planar defects (rough surface) on photocatalytic reactions due to the limited strategies for precisely regulating the types and distribution of surface planar defects on photocatalysts. We believe that the development of chemical strategies to tailor the surface planar defect configuration has great applications in improving photocatalytic conversion efficiency.

Compared with point and line defects, the photocatalytic roles of extended surface irregularities remain less systematically understood because their atomic configurations and spatial distributions are difficult to regulate and characterize precisely. The pothole-rich WO₃ example demonstrates that local surface pits can generate chemically active coordination environments for molecular adsorption and bond activation. In parallel, heteroatom incorporation provides a complementary strategy for modifying the local electronic structure and adsorption energetics of oxygen-deficient tungsten oxide surfaces. However, changes in neighboring W coordination environments depend on the dopant identity, defect configuration, and surface structure and should not be generalized as a universal effect. Future studies should distinguish the intrinsic contributions of surface roughness and potholes from those of isolated heteroatom sites using atomic-resolution and operando characterization.

The product selectivity of CO₂ photoreduction over defect-engineered WO₃-based photocatalysts is determined by the combined effects of defect configuration, surface termination, local W coordination, and intermediate adsorption geometry, rather than by defect dimensionality or concentration alone. Surface oxygen vacancies on selected WO₃ facets commonly expose penta-coordinated W sites, which facilitate *COOH formation and favor CO evolution. Ordered line defects derived from stacking-fault terminations expose tetra-coordinated W sites and enable cooperative W–O–C–W–O interactions, thereby enhancing CO₂ polarization and promoting deeper hydrogenation toward CH₄. Reconstruction into hexagonal line defects generates a distinct penta-coordinated W environment and modifies the conversion pathway of *COOH intermediates, redirecting product selectivity toward CO. In contrast, excessive structural reduction may induce reconstruction into stable, catalytically less active polyhedral configurations. These observations indicate that the atomic configuration and termination structure of surface defects are more reliable descriptors of product selectivity than defect dimensionality or concentration alone.

Defect engineering in WO₃ nanomaterials is crucial for enhancing photocatalytic performance by precisely tailoring their electronic structures, which in turn govern light absorption [Reference Yan, Wang and Wu41,Reference Chen, Gao, Yasumori, Kuroda and Sugahara117], charge separation [Reference Liu, Liang and Xiao114,Reference Liu, Han and Zhou118], and reaction kinetics [Reference Liang, Li and Sun43]. Unlike bulk defects, whose coordination environments are determined by the crystal lattice, bulk defects terminating at the surface generate unique coordination structures governed by the surface termination. These structure-specific environments dictate molecular adsorption and reactivity, making the termination of bulk defects critical for understanding photocatalytic reaction pathways in WO₃. It is therefore essential to clarify the role of unique surface geometric configurations—particularly those involving coordinated W atoms originating from low-dimensional defects—in governing reaction pathways, as summarized in Table 1 [Reference Zafar, Yi and Li9,Reference Xue, Wei and Shen19]. Typically, surface V_O on {002} facets exposes penta-coordinated W atoms, which activate molecular oxygen to form superoxide radicals [Reference Zhang, Li and Ye119]. In CO₂ photoreduction, such vacancies on flat surfaces promote CO production via the formation of single W–O bonds [Reference Liang, Li and Sun43], while on high-index planes containing under-coordinated atoms, they induce CO₂ polarization through dual tilted W–O interactions. Line defects arising from stacking fault terminations on the {002} surface of WO_2.9 create tetra-coordinated W atoms at step edges, leading to the formation of W–O–C–W–O intermediates that bend the CO₂ configuration and favor CH₄ generation [Reference Xue, Wei and Shen19]. A shift to penta-coordinated W atoms within such defects redirects product selectivity toward CO [Reference Shen, Ye and Wang85]. Conversely, excessive reduction of the W coordination number (N ≤ 4) at stacking fault terminations on WO_2.72 surfaces induces structural reconstruction into stable [WO₇] decahedra, thereby deactivating the photocatalyst [Reference Xue, Wei and Shen19]. Hence, loading heteroatoms onto WO_2.72 surfaces represents an effective strategy for tuning the local coordination geometry of surface W atoms, enabling efficient photocatalytic conversion of target molecules [Reference Zhang, Wang and Zuo93,Reference Zhang, Jalil and Wu94].

Table 1.

Representative correlations among surface-defect configuration, local W coordination, CO₂ activation mode, key intermediates, and product selectivity in WO₃-based photocatalysts.

Table 1. Long description

The table consists of 6 columns and 8 rows.

Column headers from left to right:

1. Proposed reaction pathways.

2. C-O bond cleavage and C O desorption.

3. C O₂ polarization and deep hydrogenation.

4. Intermediate conversion suppresses methanation.

5. [WO₇] formation deactivates sites.

6. Local electronic regulation of adsorption energetics.

Row-by-row data breakdown:

- Major product: CO; C H₄; C O; Low activity; C O / H₂.

- Key intermediate: *C O O H; Bent *C O₂ / *C H O; *C O O H; (empty); Depends on dopant.

- CO₂ activation mode: W-O; W-O-C-W-O; W⁵⁺- O; Structural reconstruction; Enhanced chemisorption.

- W coordination: Penta-coordinated W; Tetra-coordinated W; Penta-coordinated W; Highly under-coordinated W; Tunable W coordination.

- Active site: W super 5 plus adjacent to V_O; W⁴⁺ at the termination of ordered line defect; W⁵⁺ located at the hexagonal-tungsten line defect termination; Surface under-coordinated W; under-coordinated W neighboring heteroatom.

- Surfaces geometry: 1 D; 2 D; 1 D; 2 D; 2 D.

- Surface defect type: Oxygen vacancies; Ordered line defect; Hexagonal line defect; Excessively reduced surface; Heteroatom-decorated rough surface.

Identifying the structures and active mechanisms of the surface defects at high spatial and temporal resolution can bridge the gap between atomic structures and catalytic properties. Over recent decades, numerous advances in situ spectroscopy techniques have been developed to probe the vibrational modes of chemical bonds between photocatalysts and adsorbates, facilitating the elucidation of photocatalytic CO₂ reduction mechanisms across diverse surface configurations at a macroscopic level.

Among these, in situ Fourier-transform infrared (FTIR) spectroscopy has been widely employed to identify key intermediates and infer reaction pathways. During the photoreduction of CO₂ coupled with water oxidation, the process typically involves carboxylation to form key *COOH intermediates, which subsequently interact with protons and photoexcited carriers to yield products such as CO, CH₄, and HCOOH. However, in situ FTIR spectroscopy offers limited accuracy in identifying the surface chemical states of photocatalysts. To address this, quasi-in situ or in situ X-ray photoelectron spectroscopy (XPS) has been utilized to monitor molecular adsorption behavior on photocatalyst surfaces under vacuum or controlled environments. These studies reveal that surface metal atoms exhibit high activity in capturing target molecules, and multiple surface atoms with dangling bonds can simultaneously interact with adsorbates via chemisorption on specific surface structures. Moreover, changes in the valence state of surface atoms have been detected when lattice atoms participate in photo-redox reactions. Because high-concentration defects can exert opposite effects on carrier dynamics depending on their type, concentration, and distribution, a combination of complementary characterization techniques is generally required to obtain a comprehensive understanding of charge-carrier behavior. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) are commonly employed to evaluate radiative recombination and carrier lifetimes. Transient photocurrent response and electrochemical impedance spectroscopy (EIS) provide insights into charge separation efficiency and interfacial charge-transfer kinetics (Table 2). Surface photovoltage (SPV) spectroscopy enables direct visualization of photoinduced charge separation and migration near semiconductor surfaces, whereas femtosecond and nanosecond transient absorption (TA) spectroscopy captures ultrafast carrier trapping, transfer, and recombination processes over broad temporal scales (Table 2). The combined application of these techniques allows the intrinsic relationship between defect structures and carrier dynamics to be established more reliably than any individual characterization method alone. Given that the migration of photoexcited carriers occurs on picosecond to nanosecond timescales, ultrafast transient absorption spectroscopy has been employed to trace the transport of carriers from the bulk to the surface. To further probe proton-coupled electron transfer during reactions, in situ Raman spectroscopy has proven effective in identifying the chemical states of electrons and lattice phonons. Preliminary experimental results indicate that protons are injected into the gaps of corner-sharing [WO₆] octahedra when photoexcited electrons migrate to the surface—challenging the conventional view that protons react solely with target molecules on the surface.

Table 2.

Complementary characterization techniques for resolving defect structures, surface chemical states, and charge-carrier dynamics in defect-engineered WO₃ photocatalysts.

Table 2. Long description

The table consists of six columns: Characterization technique, Primary information, Quantitative capability, Spatial level, Temporal scale, and Main limitation.

* iDPC-STEM: Atomic arrangement and local coordination; Mostly qualitative; Single particle/atomic level; Qualitative; Limited to single particle.

* XPS: Chemical state (W⁵⁺ / W⁶⁺, O vacancy); Semi-quantitative; Surface averaged; Semi-quantitative; Surface averaged.

* EPR: Unpaired electrons/oxygen vacancies; Semi-quantitative; Bulk averaged; Semi-quantitative; Bulk averaged.

* Raman / PL spectroscopy: Lattice distortion and radiative recombination; Qualitative to semi-quantitative; Bulk/Local; Qualitative; Bulk/Local.

* CL/ KPFM mapping: Local electronic activity and potential variation; Indirect; Nanoscale mapping; Indirect; Nanoscale mapping.

* TRPL: Carrier lifetime and recombination kinetics; Model-dependent; Bulk averaged; p s - n s; Lifetime does not uniquely identify transport pathway.

* Transient photocurrent: Photoinduced charge extraction response; Comparative; Device/electrode averaged; ms - s; Strongly affected by electrode architecture.

* EIS: Interfacial charge-transfer resistance; Equivalent-circuit dependent; Device/electrode averaged; Frequency domain; Model ambiguity.

* SPV: Surface charge separation and migration; Comparative; Surface averaged/mapped; mus - s; Interpretation depends on surface states.

* Transient absorption: Carrier trapping and transfer; Kinetic modeling; Ensemble averaged; fs - ms; Spectral assignment can be complex.

Collectively, findings from electron microscopy at the microscopic level and in situ spectroscopy at the macroscopic scale establish that the spatial freedom of surface W atoms, governed by coordination number and surface atomic geometry, serves as a key regulator for overcoming rate-determining steps in photocatalytic reactions. This mechanistic insight underscores the need for a comprehensive database correlating defect geometries with reaction pathways, which will facilitate the rational design of defect-engineered photocatalysts and reduce reliance on empirical optimization. To provide a more systematic comparison, representative defect-engineered WO₃-based photocatalysts are summarized in Table 3, including defect type, preparation method, characterization technique, primary active site, electronic/carrier modulation, target reaction, major product, performance enhancement, and photocatalytic stability.

Table 3.

Summary of representative defect-engineered WO₃-based photocatalysts, including defect construction, structural characteristics, electronic regulation, photocatalytic performance, and stability.

Table 3. Long description

The table is divided into two sections summarizing ten different photocatalytic systems. The rows categorize each material by Stability issue, Performance enhancement, Major product, Target reaction, Band-structure or carrier modulation, Primary active site, Main characterization methods, Preparation or defect-construction method, Defect type and location, and Representative material.

Section 1 (Materials 1 to 6):

* Material 1: Stable for 10 cycles; 2-fold enhancement in benzene oxidation to phenol; uses surface V sub O created by photo-reduction.

* Material 2: Stable for 12 cycles; 2.3-fold enhancement in O₂ reduction to H₂O₂; uses surface V_O from high-pressure treatment.

* Material 3: Stable for 9 cycles; enables I R light-driven C O₂ reduction to C O; uses V_O-related states in ultrathin cubic WO₃.

* Material 4: Stable over 120 h; higher photocurrent for water oxidation to O₂; uses W vacancies from chemical-bath synthesis.

* Material 5: Stable for 6 cycles; produces 8.2 micromol CH₄ from CO₂ reduction; uses line defects on {002} planes.

* Material 6: Stable for 9 cycles; switches selectivity to C O in CO₂ reduction; uses hexagonal line defects.

Section 2 (Materials 7 to 10):

* Material 7: Stable over 30 h; 9.8-fold higher nitrate photosynthesis; uses surface potholes in ultrathin W O₃.

* Material 8: Stable for 5 cycles; 3-fold higher NH₃ yield in N₂ fixation; uses Mo-modified defects in W₁₈ O₄₉ nanowires.

* Material 9: Stable for 4 cycles; 10-fold higher CO ₂ reduction to C O and H₂; uses isolated Co sites on W₁₈O₄₉ nanowires.

* Material 10: Stable over 10 h; 3-fold higher phenol yield in benzene oxidation; uses Mo⁵⁺ surface V _O created by thermal treatment and irradiation.

Note: The reported performance values were obtained under different reaction conditions and should not be directly compared across different photocatalytic systems. “Not systematically evaluated” indicates that long-term defect stability or operando structural evolution was not explicitly examined in the corresponding study. Reference numbers should be updated after the final reference list is reordered.

2.5. Summary and Outlook

In this mini-review, we have explored the structures and evolutionary mechanisms of surface defects in WO₃ photocatalysts. The impact of these defects on the photocatalytic reaction process was also investigated, including light-absorption properties, dynamic charge-transport behavior, surface adsorption, and reaction processes. Furthermore, we summarize various types of dimensional surface defects, providing insights into their atomic-level mechanisms in photocatalytic environments. This mini-review aims to offer a comprehensive outlook on defect engineering, addressing four key aspects.

1. 1. Atomic-level regulation of defect structures. To date, the fabrication of surface defects primarily relies on methods such as high-temperature treatment, liquid-phase reduction, ion exchange, plasma etching, and photochemical processes. However, achieving precise control over the type, concentration, and distribution of surface defects remains a significant challenge. Future research may leverage machine learning and high-throughput computational techniques, coupled with advanced atomic-resolution characterization methods (e.g., aberration-corrected transmission electron microscopy and X-ray absorption fine structure spectroscopy), to optimize defect engineering strategies and enable the controlled design of defects in photocatalysts. A major challenge is that the formation of a target defect is often accompanied by defect migration, aggregation, lattice relaxation, surface reconstruction, or the generation of coupled defects. Therefore, independently controlling defect type, concentration, spatial location, and termination structure remains difficult. Future synthesis strategies should move beyond empirical reduction treatments toward quantitatively controlled and site-selective defect engineering.

2. 2. The interactive influence of defect types, concentrations, and distributions. Surface defects disrupt the periodic atomic structures of photocatalysts, leading to the formation of defect energy levels that can facilitate the dynamic behavior of photogenerated charge carriers. However, defects at high concentrations, or bulk defects, may act as recombination centers, thereby hindering charge separation. Thus, it is essential to investigate the relationship between the type, concentration, and distribution of surface defects and the dynamic behavior of photogenerated carriers under operational conditions. Quantitative identification of these defect parameters also remains challenging because most characterization techniques provide either localized atomic-scale information or ensemble-averaged signals. It is therefore difficult to simultaneously determine defect concentration, spatial distribution, local coordination environment, and dynamic evolution under reaction conditions. Combining atomic-resolution imaging with quantitative spectroscopy and operando measurements will be essential for establishing reliable correlations between defect structures and charge-carrier behavior. Such studies are crucial for guiding the rational design of defective photocatalysts.

3. 3. The stability of the surface defects. While defects can enhance photocatalytic activity, certain defects―such as high-concentration oxygen vacancies―may undergo reconstruction or disappear over extended operational periods, leading to a decrease in catalytic activity. Future research should prioritize the investigation of defect stability under realistic operating conditions, employing advanced in situ characterization techniques and theoretical modeling. This approach will provide insights into self-repair mechanisms and dynamic regulation strategies for defects, enabling precise control over defect evolution during photocatalytic cycles. Such advancements will improve the operational longevity and reliability of photocatalysts, thus facilitating their use in sustainable energy and environmental applications. The design of the photocatalyst in the future should go beyond isolated investigations of individual defect types to fully elucidate the synergistic interactions among multiscale defects (e.g., cooperative effects between surface point vacancies and atomic-level step edges). Moreover, integrating these insights with complementary modification strategies, such as heterojunction formation, interfacial engineering, and surface functionalization, will be crucial for advancing photocatalyst performance.

4. 4. Data-driven and operando understanding of dynamic defects. Future research should establish dynamic defect databases that correlate defect configurations, spatial distributions, evolution pathways, and catalytic performance under operating conditions. Integrating these databases with artificial intelligence and high-throughput calculations may enable the prediction, screening, and controllable synthesis of target defect structures. Meanwhile, advanced operando characterization techniques are required to directly monitor the formation, reconstruction, migration, and deactivation of active defects under realistic reaction conditions. Such efforts will facilitate the establishment of quantitative defect–structure–activity relationships and accelerate the rational design of defect-engineered WO₃-based photocatalysts.