Metal oxides can be roughly divided into transition metal oxides and main group metal oxides according to their components. Based on different experimental methods, oxygen vacancy defects can be formed on both types of metal oxides. In metal oxides, if the oxygen atom leaves the equilibrium position in the perfect metal oxide crystal and enters the interstitial site, forming a pair of holes and interstitials, the OV formed in this way is called Frankel type (F-OV).If the oxygen atom leaves the original equilibrium position and escapes to the outside of the system, a hole is formed in the original position, and the OV formed in this way is called Schottky type (S-OV)^[22]. F-OV and S-OV can be transformed into each other under suitable conditions. For example, when Lin et al studied the combustion of carbonaceous particulate matter on a MnO_x-CeO₂, they found that there was a transformation process between F-OV and S-OV, which had a strong promoting effect on the catalytic activity of carbonaceous particulate matter oxidation^[39].

According to the position of OV, it can be divided into surface type and bulk type. Most of the surface-type OV originates from the conversion of metal cations from high valence to low valence, for example, Jiang et al. Reported Co-doped CeO₂ materials and found that Co doping enhanced charge transfer, resulting in the appearance of Ce³⁺ as well as a large amount of surface OV, and Zhu et al. Studied the conversion of Co₃O₄ to CoO, in which part of Co³⁺ was reduced to Co²⁺, resulting in the generation of surface OV^[40][41][42]. Compared with the surface-type OV, most bulk-type OV is derived from the migration of lattice oxygen. For example, in 2014, Behm et al. Confirmed that the proper introduction of Au NPs in the process of CO oxidation on TiO₂ can lead to the migration of lattice oxygen to produce bulk-type OV^[43]. During the synthesis of CeO₂-ZrO₂ nanotubes by Liu et al., it was found that the addition of Zr⁴⁺ caused the shift of lattice oxygen in CeO₂, resulting in bulk OV in CeO₂^[44].

According to the size classification of OV, it can be divided into single OV, OV pair and OV cluster. Taking OV of different size types in CeO₂ as an example, the conversion from Ce⁴⁺ to Ce³⁺ produces single OV, as shown in Figure 1A^[45]. The migration of lattice oxygen from the normal position into the gap of other ions produces a gap-type single OV, as shown in fig. 1b; When dopants are introduced, a change in the valence state of the element also occurs to produce a single OV, as shown in Figure 1C. When two single OVs are aggregated, an OV pair is generated; When multiple OVs aggregate, OV clusters can be formed, such as triangular defect clusters and linear defect clusters, as shown in Figure 1 d, e.

According to the number of localized charges, OV can be divided into the type of localized double charge (F center, effective charge number is 0), the type of localized single charge (F⁺ center, effective charge number is + 1) and the type of nonlocal charge (effective charges number is + 2)^[46,47]. When Cronemeyer studied the semiconductor properties of reduced rutile TiO₂, he obtained the OV of different kinds of localized electrons by treating them in hydrogen reduction atmosphere for different time. It was found that the number of localized charges of OV was closely related to the ionization energy and conductivity of the material^[48].

The type of OV is often closely related to its formation process, and different formation methods will cause different types or mixed types of OV. Common formation methods and the elements involved are shown in Figure 2. The formation of oxygen vacancy defects mainly occurs in two stages: the treatment during the material synthesis and the surface treatment after the material synthesis.

Thermal treatment, Reduction processing (chemical agents such as H₂, NH₃, and NaBH₄ can be used to form OV at low temperature), and Cation/anion doping are the methods for forming OV through treatment during material synthesis^{[49,50][51][52][53][54⇓~56]}. Both heat and reduction treatments reduce the transition metal cation (Mⁿ⁺) to a lower valence state (M^(n-1)+) to obtain a suitable defect concentration, and the substitution of high-valence ions in metal oxides by low-valence ions or the use of dopants (which can reduce the formation energy of defects) can also introduce OV into metal oxides^[15,16].

Plasma treatment, Flame treatment, Laser irradiation, Exfoliation, and Adopting templates are the methods of forming OV by surface treatment after material synthesis. The above post-treatment methods all produce OV by promoting the escape of O atoms in metal oxide crystals^[16].

With the deepening of research, the regulation of OV has become an effective strategy to optimize the performance of catalysts. The existing OV control strategies are based on the formation of OV, including temperature control, atmosphere control, atom, ligand or other defect doping control, reaction solution concentration control and pressure control.

Temperature can directly affect the formation energy of OV to achieve regulation. In general, the higher the heat treatment temperature, the lower the OV formation energy and the higher the OV concentration. Tan et al. Found that the OV concentration of the SrTiO₃ annealed at 375 ° C was higher than that of the sample annealed at 300 ° C, and the extension of heating time further promoted the formation of OV^[57]. However, too high temperature will cause the crystal structure distortion, which is not conducive to the reaction. For example, Han et al. Found that the TiO₂ annealed at 800 ℃ contains more OV than the samples annealed at 700 ℃ and 900 ℃, showing the best NRR catalytic activity^[58].

Reducing atmosphere, which is usually required for high temperature calcination and annealing, is also one of the prerequisites for the formation of OV, among which low oxygen atmosphere (such as vacuum, He, Ar, N₂) is more conducive to the formation of OV, so atmosphere control is widely used in experiments. In 2016, Xu et al. Prepared VO₂ nanosheets on Si substrate by DC magnetron sputtering at room temperature and vacuum annealing, and increasing Ar pressure during annealing produced more OV^[50]. In 2019, Zhang et al. Prepared a series of one-dimensional InO_x nanobelts (without reducing atmosphere, under air reducing atmosphere and under H₂ reducing atmosphere) for the catalytic process of CO₂ electrochemical reduction (CO₂electroreduction,CO₂ER).It was found that the InO_x nanobelts under the reducing atmosphere of H₂ contained the most OV, and the Faraday efficiency (FE) of HCOOH was as high as 90.2%, showing excellent catalytic activity^[59].

Doping of atoms, ligands, or other defects can affect the internal structure of the crystal and thus the OV formation process. In 2020, Zhao et al. Used different contents of Ni doping to generate different concentrations of OV in AgFe_1-xNi_xO₂ to catalyze PMS degradation^[60]. Wu et al. Found that the A-site cation (A-site atom in ABO₃ formula compound) defect on LaFeO₃ increases the valence state of Fe atom (B-site atom), which is beneficial to the generation of OV^[61].

The reaction solution (such as ethanol solution, ethylene glycol and liquid alloy) usually has strong reducibility, which significantly affects the production of OV, so the change of the concentration of the reaction solution will also directly affect the content of OV. In 2019, Yu et al. Used ethanol as a reducing agent to obtain a Sr₂Bi₂Nb₂TiO₁₂ containing OV, and found that when the ethanol content was 0.5 mL, the OV content in the sample was the highest, and the efficiency of catalytic reduction of CO₂ to produce CO was the highest^[8]. In 2021, Wang et al. Obtained BiOF (101) surface containing a large amount of OV by adjusting the concentration of ethylene glycol (EG) solution for catalytic removal of perfluorooctanoic acid pollutants. When the concentration of EG was 50%, the sample had the highest OV content and the strongest catalytic activity^[62].

Strain control is mostly used in some perovskite material systems. The different lattice constants between the substrate and the epitaxial film in perovskite materials will lead to the lattice mismatch of the epitaxial film, resulting in epitaxial strain. Strain can reduce the formation energy and migration energy of OV, and effectively control the concentration of OV on the crystal surface. Usually, the greater the external pressure, the greater the strain, the easier the formation of OV, but too much pressure will cause lattice distortion of the material, which is not conducive to the stability of the catalytic system. As early as 2013, Yang et al studied the effect of strain on the formation and migration of OV in BaTiO₃, and found that compressive strain and tensile strain can increase the formation energy of OV, but compressive strain can increase the migration barrier of OV, thus slowing down the degradation of ferroelectric properties caused by OV migration, while tensile strain has no such effect^[63]. In 2021, Rawat et al. Reviewed the latest progress in the field of interface at the intersection of interface strain and OV in complex oxides, and clearly proposed that the interface strain would reduce the formation energy of OV, so the formation of OV could be regulated by the interface strain generated by pressure regulation^[64].

In order to better evaluate the relationship between defects and catalytic effect, determine the quantitative relationship between material structure and properties, and clarify the definition and energy calculation method of OV, it is necessary to analyze the thermodynamic and kinetic characteristics of OV formation from a theoretical point of view. Since the formation of OV involves the destruction of MO bonds in metal oxides, and the enthalpy of formation is an indicator of the average strength of MO bonds, the formation energy of E_vac(OV) has a close relationship with the enthalpy of formation of metal oxides. In addition, the formation of OV is also largely dependent on other structural factors of metal oxides, such as crystal phase, crystal plane, crystallinity and strain^[65,66]. There are two existing formulas for calculating E_vac:

In formula (1), E_b and E_r are the bond energy and relaxation energy of the original oxygen atom at the oxygen vacancy binding to the system, respectively. The E_b depends on the strength of the MO bond, including ionic electrostatic interaction and covalent bonding interaction. The ionic electrostatic interaction of the MO bond is related to the intrinsic quantum initial orbital effect of the metal in the metal oxide, where CeO > ThO > HfO > ZrO > TiO; the covalent bonding interaction of the MO bond depends on the overlap of the metal orbital and the oxygen orbital^[67]. E_r can be divided into electronic ( {E_{\mathrm{r}}}^{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}}) and geometric ( {E_{\mathrm{r}}}^{\mathrm{g}\mathrm{e}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{y}}) relaxation energies, which correspond to the transfer of the remaining electron when the defect is formed and the geometric relaxation when there is no defect, respectively. In equation (2), E_total, E_perfect, and E_{O_2} are the energies of the OV-containing surface after structure optimization, the perfect surface after structure optimization, and oxygen, respectively. Although the expressions of the two formulas are different, because E_b=E_v+ \frac{1}{2} E_{O_2} -E_perfect,E_r=E_total-E_v in (1), where E_v is the energy of the unoptimized OV-containing surface, formulas (1) and (2) can be transformed into each other and are essentially the same.

In the field of thermal catalysis, OV can not only be used as the main Reaction site to adjust the adsorption free energy of catalytic intermediates, stabilize intermediates, reduce the activation energy of Reaction determining step (RDS), but also weaken competitive reactions and promote catalytic activity. There are many examples of using OV for thermal catalytic oxidation reactions, such as NO_x (nitrogen oxides) oxidation, CO oxidation, formaldehyde oxidation, benzene combustion, HMF oxidation, ozonolysis, CO₂ thermal catalytic conversion, syngas conversion, etc.

As one of the industrial waste gases, NO_x can stimulate the lungs and cause respiratory diseases, and is also an important cause of photochemical smog and acid rain. At present, NH₃ is often used as a reducing agent to selectively reduce (Selective catalytic reduction,SCR)NO_x to achieve denitrification. At present, the V₂O₅ in the V₂O₅-WO₃(MoO₃)/TiO₂ of commercial denitration catalyst has carcinogenic risk, so it is necessary to develop other kinds of catalysts. In 2016, Liu et al. Studied the reaction mechanism of selective catalytic reduction of NO by NH₃ over W-doped CeO₂ catalyst, and summarized the reaction into 4 steps, which are Lewis acid site reaction, BrØnsted acid site reaction, oxygen vacancy defect reaction, and catalyst regeneration reaction. The microscopic action mechanism is shown in Fig. 3^[35]. Theoretical calculations show that W doping reduces the formation energy of OV (from 2.39 eV to 1.62 eV). OV plays a key catalytic role in the NH₃-SCR reaction: on the one hand, surface OV facilitates the introduction of two Ce³⁺ ions, and on the other hand, OV promotes the adsorption of NH₃ to generate N₂ {{\mathrm{O}}_2}^{2-} species, which becomes the precursor of SCR reaction and opens up a unique reaction path. Electron transfer from Ce³⁺ to N₂ {{\mathrm{O}}_2}^{2-} enhances the interaction between nitrogen atoms and greatly promotes the formation of N₂. In 2018, Ma et al. Doped Rh into the OV of CoO (011) surface to form a Rh₁Co₃ cluster, and studied the feasibility and microscopic mechanism of ammonia synthesis on the catalyst by DFT calculation. It was found that H₂ and N₂ could be adsorbed on the bimetallic sites of Rh₁Co₃/CoO(011) at the same time, and the adsorbed hydrogen could be activated on the two metal sites to produce active hydrogen, promote the alternative hydrogenation of N₂, and directly convert it into NNH intermediate^[14]. The cluster in the cluster-type catalyst can be used as a charge buffer to greatly reduce the energy barrier of N ≡ N bond cleavage, which provides a new reaction mechanism for ammonia synthesis. In 2021, Zhang et al. Reviewed the application of rare earth elements in denitration catalysts and their action mechanism, and summarized that OV is beneficial to the storage and release of oxygen species, the promotion of reactant adsorption, the acceleration of electron transfer, and the improvement of the redox capacity of the catalyst, so it plays a great role in the SCR reaction^[68]. In 2022, Ma et al. Used DFT method to construct a series of metal doping on CoO (011) surface for dinitrogen hydrogenation reaction, and found that the late transition metal (M = Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) doping is easier to form M₁Co_n/CoO_xSCCs, in which the Pd₁Co₄/CoO_xSCC formed by Pd doping into OV is the SCCs with the highest activity and the lowest energy barrier in the first step of hydrogenation of N₂^[69]. When foreign metal M is doped into the OV of CoO, the bimetallic cluster of M₁Co_n can be used as a charge buffer, and there is a lot of electron transfer between the d orbital of the metal and the π^* orbital of the N₂.It favors the co-adsorption of H₂ and N₂ and the hydrogenation of N₂ to produce the important NNH * intermediate.

In 2018, Liu et al. Found that the catalytic activity of Au/TB-ZnO containing OV for CO oxidation was 153 times higher than that of Au/NR-ZnO, and the surface OV promoted the formation of peroxide or superoxide, and accelerated the dissociation of oxygen molecules by weakening the O — O bond, thus accelerating the CO oxidation reaction^[70]. In 2019, Yin et al. Developed a novel OV-rich nitrogen-doped Co₃O₄ catalyst using the urea-assisted method^[71]. Nitrogen doping enhances the consumption rate of hydroxyl groups on the surface of Co₃O₄, increases the number of OV, and enhances the activity of lattice oxygen. Compared with pure Co₃O₄ and less N-doped Co₃O₄, the catalytic activity and hydrophobicity of the OV-rich N-doped Co₃O₄ system for CO oxidation are significantly enhanced. Yang et al. Successfully loaded the ordered macroporous OV-rich perovskite La_0.8Sr_0.2CoO₃(Vo-OM LSCO) catalyst onto Commercial cordierite for catalytic CO oxidation reaction by in situ solution assembly and NaBH₄ reduction method^[72]. As shown in Fig. 4, the oxidation reaction of CO on the pristine La_0.8Sr_0.2CoO₃ surface follows the Eley-Rideal (E-R) mechanism, while the oxidation reaction on the OV-rich La_0.8Sr_0.2CoO₃ surface follows the Langmuir-Hishelwood (L-H) mechanism. The molecular O₂ favors the adsorption and activation on the surface OV to form oxygen ions (O^-) through a single electron transfer process; At the same time, OV also enriches the O^-, promotes the adsorption of CO, and reduces the activation energy of the rate-determining step on the perovskite surface. Since CO does not react with surface {{\mathrm{O}}_2}^{-} (and has a tendency to move away from {{\mathrm{O}}_2}^{-}) but reacts with O^-, OV also promotes the interaction of reactants (CO * and O^-), thus significantly enhancing the low-temperature oxidation activity of CO.

Formaldehyde is the main indoor pollutant, which has high toxicity and carcinogenicity. Thermal catalytic formaldehyde is not easy to cause secondary pollution, so its research is increasingly widespread. In 2020, Zha et al. Grew OV-rich Co₃O₄ nanowires in situ on Ni foam (r-Co₃O₄NW@Ni foam) and used them to catalyze formaldehyde oxidation^[73]. The presence of OV on the r-Co₃O₄NW@Ni foam catalyst decreased the adsorption energy of O₂ and induced more active oxygen adsorption and storage, thus promoting the formation of formaldehyde reaction intermediate. In 2021, Su et al. Reviewed the research progress of formaldehyde oxidation catalysts, and proposed that the OV on the surface of the catalyst could effectively adsorb formaldehyde or O₂, promote the migration of active oxygen species and then react with formaldehyde, so the preparation of OV-containing catalysts played an important role in formaldehyde removal reaction^[74]. In 2021, Ma et al. Reported that OV-mediated Ag/CeO₂-Co₃O₄ catalyst promoted benzene combustion, as shown in Fig. 5, Ag doping improved interfacial electron transfer, induced exposed Co³⁺ sites and structural distortion on the surface of Co₃O₄,Abundant OV is produced, which contributes to the rapid generation of active oxygen, improves the low-temperature reducibility and mobility of oxygen species, promotes the adsorption of benzene and the dissociation of oxygen, and then promotes the process of oxidation reaction^[75]. In the same year, Liu et al. Proposed a simple green vitamin C assisted solid-state grinding method to synthesize mesoporous manganese-cobalt spinel oxides for the catalytic oxidation of 5-hydroxymethyl-furfural (HMF) to 2,5-furoic acid (FDCA)^[76]. The strength of Mo — O bond is the weakest in the manganese-cobalt spinel oxides with high concentration of OV. OV reduces the adsorption energy of O₂, promotes the adsorption and activation of O₂, increases the electron density of lattice oxygen, reduces the binding energy of lattice oxygen, and enhances the activity of lattice oxide, which is beneficial to the catalytic oxidation reaction. Li et al. Focused manganese oxides, summarized the role of OV in the catalytic decomposition of ozone at room temperature in the review.Involved in the reaction (where V_o represents a localized two negatively charged oxygen vacancy defect) :(1)O₃+V_o→O^2-+O₂;(2)O₃+O^2-→ {{\mathrm{O}}_2}^{2-} +O₂;(3) {{\mathrm{O}}_2}^{2-} →V_o+O₂^[77]。 OV can be used as adsorption sites, reaction active sites, and can be recycled and regenerated in the reaction, so manganese oxides with OV usually have excellent ozone removal activity.

For the thermal catalytic conversion of CO₂, in 2017, Gao et al. Developed a bifunctional catalyst composed of In₂O₃ and zeolite, which has a high selectivity (78.6%) for gasoline hydrocarbons (C₅₊ hydrocarbons) and a very low selectivity (1%) for methane^[1]. As shown in Fig. 6a, the formation mechanism of CH₃OH at OV on the surface of In₂O₃ catalyst mainly includes four steps: (1) CO₂ adsorption at OV; (2) Adsorbed CO₂ is successively hydrogenated to produce CH₃OH;(3)CH₃OH desorption, resulting in no OV on the surface; (4) OV is regenerated by surface hydrogenation. The OV on the surface of form zeolite socony mobil activates CO₂ and H₂O to form CH₃OH, and then the CH₃OH is transferred to HZSM-5 (H-form zeolite socony mobil-5) zeolite, and C — C coupling occurs in the zeolite pores (as shown in Fig.Hydrocarbon pool mechanism means that methanol first forms large molecular weight hydrocarbons on the surface of the catalyst, and then the large molecular weight hydrocarbons continuously react with methanol to introduce methyl groups.On the one hand, the dealkylation reaction is continuously carried out to produce olefins such as ethylene and propylene, and the number of catalytic centers remains stable after the reaction reaches a certain level, resulting in gasoline hydrocarbons with high octane number. In 2018, Liu et al. Doped Zr atoms into the lattice of CeO₂ to obtain fluorite-like solid solution, which promoted the formation of OV^[78]. The results showed that Zr_0.1Ce nanorods had the highest OV content and the highest activity for the synthesis of dimethyl carbonate. CO₂ can adsorb on OV to form bidentate carbonate and participate in the reaction as an intermediate. The surface OV can act as a Lewis acid site to interact with the O atom of CO₂. Methanol adsorption near OV can generate an intermediate, which adsorbs CO₂ to form dimethyl carbonate and regenerates OV during further reaction. In the same year, Gu et al. Designed partially reduced copper oxide nanodendrites (CuO_x-OV) for the reduction of CO₂ to produce C₂H₄, and the study proved the importance of OV for the generation of C₂H₄: on the one hand, OV provided beneficial Lewis acid sites and donated electrons to form CO₂·^- intermediates, and on the other hand, OV could also be recycled and regenerated during the reaction^[2]. The CuO_x-OV system can affinity the * CO and * COH intermediates and repel the *CH₂ intermediate, resulting in a highly efficient C₂H₄ yield. In 2019, Zhang et al. Used DFT calculations to explore the different faces of CeO₂ nanoparticles, namely (100), (110) and (111), for CO₂ hydrogenation to methanol.It was found that the oxidation of H₂ produced H₂O and a large number of coordinately unsaturated oxygen atoms, and the adjacent Ce⁴⁺ was converted to Ce³⁺ to produce OVs^[79]. The oxidation activity of H₂ on CeO₂(100) surface is the highest, and CO₂ can be adsorbed linearly on the surface OV (OV can increase the surface basicity and enhance the interaction between CO₂ and surface).After hydrogenation, bi-H₂COO^* is produced, followed by the filling of OV to produce bi-H₂CO* (an important intermediate for methanol), and then hydrogenation continues to produce H₂COH* to regenerate H₃COH*.

In the hydrogenation reaction, in 2017, Werner et al. Pointed out that the presence of OV in CeO₂ is the key to the hydrogenation reaction^[36]. DFT calculations show that OV promotes the adsorption of H on the surface and the penetration of H into the bulk, which is beneficial to the stabilization of hydrogen species and the promotion of the reaction. In the WGS reaction, in 2018, Xu et al. Prepared Ni nanoparticles supported on reducible TiO_2-x (denoted as Ni@TiO_2-x) by structural topological transformation of NiTi-layered double hydroxides (NiTi-LDHs) precursor, which showed good catalytic performance for the WGS reaction^[9]. The theoretical study shows that the interfacial sites of Ni@TiO_2-x catalyst are the best sites for water dissociation, and the OV on them is beneficial to the adsorption and dissociation of water, which is the main active site.

In the thermal conversion of syngas, Huang et al. Proved through theoretical research that OV can play a key role in the activation of Frustrated Lewis Pairs and CO through the formation of Frustrated Lewis Pairs (FLPs)^[80]. As shown in the right of Figure 7, after removing two O atoms on CeO₂(110), two Ce cations next to OV can act as Lewis acid, and one adjacent O anion next to OV can act as Lewis base, which together construct the FLP site. Activation of the H₂ on FLPs proceeds through a heterolytic dissociation mechanism, forming hydrides or hydrogen protons. CO is activated by binding to the basic site (O atom) on the FLP site to form C {{\mathrm{O}}_2}^{2-}, and the stable hydride on the FLP also contributes to CO hydrogenation. There are four syngas conversion pathways on FLPs, which are as follows :(1)CO*+2H*+CO(g)+H₂(g)→CH₂CO(g)+H₂O(g);(2)CO*+CO*+2H₂→CH₂CO(g)+H₂O(g);(3)CO(g)+2H*+H₂(g)→CH₃OH(g);(4)CO*+H₂(g)→CH₃OH(g), where (1) and (2) produce enones, and (3) and (4) produce methanol.

In the field of electrocatalysis, OV changes the local coordination environment of electrocatalysts by affecting the band structure of materials, thus affecting their electrochemical properties (such as energy density, conductivity, etc.). Specifically, the effects of OV can be listed as follows: (1) In semiconductor materials, OV can reduce the hybridization level and thus reduce the band gap; (2) OV can introduce a built-in electric field in the lattice, and then promote ion migration through Coulomb force, thereby increasing the conductivity of the system; (3) OV can be used as a charge carrier to promote charge transfer; (4) OV can provide more additional active sites for the electrode reaction^[81,82]. Because of its special electronic properties, OV has been widely used in OER, HER, ORR, NRR, The preferential CO oxidation, The preferential CO oxidation (CO-PROX), and lithium storage reactions, all of which have good catalytic activity^{[16][42][79][10,11][83][84][85]}.

Oxygen evolution reaction (OER) plays an important role in electrochemical water splitting, but it is limited by low Oxygen evolution reaction yield, so it is necessary to find ways to introduce reinforcing catalysts into the reaction, to which Oxygen vacancy defects can make a great contribution. In 2017, using NaBH₄ as a reducing agent, Zhuang et al. Prepared OV-rich iron-cobalt oxide nanosheets Fe_xCo_y-ONS(x/y representing the molar ratio of Fe/Co) for catalytic OER^[86]. When the overpotential is 350 mV, the mass activity of Fe₁Co₁-ONS reaches 54.9 A·g^-1 and the Tafel slope is 36.8 mV·g^-1, both of which are better than those of industrial RuO₂, crystalline Fe₁Co₁-ONP and most reported OER catalysts. The thin layer composed of atoms in this material can promote electron transfer, and the surface OV can improve the conductivity and promote the adsorption of H₂O onto the nearby Co³⁺ sites.

In 2019, Ge et al. Synthesized a Carbon cloth-supported ultrafine defective RuO₂ electrocatalyst (UfD-RuO₂/CC) using a two-step method^[87]. With a low overpotential (179 mV) and high stability at 10 mA·cm^-2, UfD-RuO₂/CC outperforms most of the currently reported OER catalysts in acidic media. Zn doping in RuO₂ leads to the substitution of Zn²⁺ for Ru⁴⁺, which produces OV, and then the Zn species are removed by acid treatment, finally obtaining defective RuO₂ (one vacancy contains one Ru-O₂ unit). As shown in Fig. 8, the active site on the original RuO₂(110) plane is Ru0, while new active sites Ru1 and Ru2 are added on the defect RuO₂(110)) plane. DFT calculation shows that the rate-determining step energy barriers of OER on Ru0, Ru1 and Ru2 sites are 2.08, 1.83 and 1.83 eV, respectively. The hydrogen bond between the H in the * OOH on Ru1 and the nearby doubly coordinated O atom is stronger, and the Ru1 center has more positive charges than the Ru0 center (the more positive charges the Ru center has, the stronger the oxidation ability is, and the better the OER performance is), so the reaction barrier is reduced; The hydrogen bond between the H in * OOH on Ru2 and the nearby three-coordinate O atom is relatively weak, but the Ru2 center has more positive charges, so the reaction barrier does not change much. It can be seen that the presence of OV not only helps to produce more active sites, but also changes the electronic structure of Ru atom (from 6-coordinated Ru to 5-coordinated Ru2, the number of positive charges on Ru center changes), thus improving the intrinsic catalytic activity.

In 2020, Ma et al. Obtained OV-rich CoFe₂O₄/ graphene (r-CFO/rGO) composite by citric acid-assisted sol-gel method, heat treatment process, and sodium borohydride (NaBH₄) reduction^[88]. The r-CFO/rGO has a high specific surface area (108 m²·g^-1), low crystallinity and abundant OVs, and its electrocatalytic activity is superior to that of the commercial RuO₂ catalyst, which is characterized by a small Tafel slope (68 mV·dec^-1), low overpotential (300 mV), high current density (10 mA·cm^-2) and high durability. The OV-rich makes the delocalized electrons easily excited to the conduction band, improving the conductivity of r-CFO/rGO. Li et al. Constructed a novel CoP/CeO₂ heterostructure modified by CeO₂ nanoparticles by selective phosphating of Co(OH)₃/CeO₂ precursor, and the test proved that the CoP/CeO₂ catalyst had stable performance and was superior to the existing RuO₂ catalyst, showing good OER electrocatalytic activity^[37]. In addition, the CoP/CeO₂+Pt/C based battery has long-life cycling stability, which is far superior to the RuO₂+Pt/C mixture. During the experiment, the presence of OV was confirmed by ESR and XPS, which was attributed to the in-situ incorporation of O atoms into the crystalline phase of CoP due to the phosphating of Co(OH)₃ to CoP, and the addition of interface-modified CeO₂ particles resulted in the conversion of Ce⁴⁺ to Ce³⁺, and the amount of OV was further increased. On the one hand, OV is beneficial to the adsorption and desorption of oxygen species; On the other hand, the charge redistribution of CoP caused by the interface of CoP/CeO₂ reduces the charge transfer resistance between CoP and CeO₂. On the other hand, the porous CoP nanosheets can provide more active sites, as shown in Fig. 9. The above structural features significantly enhance the OER process. Subsequently, Li et al. Uniformly constructed two-dimensional (2D) nanoscale RuO₂ heterostructures with defects on graphene (2D-RuO₂/G), which showed good stability and excellent OER performance in both acidic and alkaline solutions^[89]. The theoretical study shows that the bridge site OV on the surface of RuO₂(110) can significantly enhance the oxygen adsorption and hydrogen dissociation on the top Ru atom, and reduce the energy barrier of the rate-determining step. In 2021, Wang et al. Prepared manganese- and nitrogen-doped cactus-like porous cobalt oxide nanostructures (N-Mn-Co₃O₄) on nickel foam as an electrocatalyst for OER by hydrothermal method and N₂ plasma treatment^[90]. N-Mn-Co₃O₄ exhibited low overpotentials of 302 and 320 mV at current densities of 50 and 100 mA·cm^-2, respectively, and stability in alkaline environment for more than 40 H. Heteroatom doping introduces OV, which increases the number of exposed active sites, enhances the conductivity of the intermediate, and optimizes the adsorption free energy of the intermediate, thereby improving the electrocatalytic performance of N-Mn-Co₃O₄.

For the Hydrogen evolution reaction (HER), in 2020 Jiang et al. Reported a thermal decomposition exfoliation method to synthesize 2D CeO₂ nanosheets from the laminated structure of Hydrogen evolution reaction^[42]. Cobalt salt additives promote the formation of two-dimensional structures, introducing beneficial cobalt doping. In alkaline HER, 2D Co-doped CeO₂ nanosheets showed excellent catalytic performance, in which Co doping produced a large number of OVs by reducing the OV formation energy. The high defect concentration optimizes the Hydrogen bonding energy (HBE) and increases the number of active sites, while the Co-modification of Co and OV can reduce the Hydrogen bonding energy, thus optimizing the hydrogen adsorption and hydrolysis.

For Oxygen reduction reaction (ORR), in 2020, Sun et al. Introduced a large amount of OV into Oxygen reduction reaction {{\mathrm{O}}_{5+}}_{\delta } (NBCO) by partially replacing cobalt to promote the electrochemical activity of ORR^[81]. At 700 ° C, the cathodic polarization impedance of NdBaCo_1.8Sc_0.2 {{\mathrm{O}}_{5+}}_{\delta } (NBCSc-2) with higher OV concentration decreased to 0.035Ω·cm² at 700 ° C, which was about 35% of that of NBCO. Compared with the impedance decay rate of NBCO cathode (3.21×10^-4Ω·cm²·h^-1), the impedance decay rate of NBCSc-2 cathode (1.18×10^-4Ω·cm²·h^-1) is lower, which shows higher stability. Sc substitution causes a decrease in the average valence of Co in the perovskite, resulting in a shift of the chemical equilibrium, which ultimately leads to an increase in OV concentration. The high concentration of OV plays a vital role in promoting the kinetic process of surface oxygen exchange and bulk oxygen transport.

For Nitrogen reduction reaction (NRR), in 2020, Lv et al. Simultaneously introduced OV and hydroxyl (OH) on the surface of Nitrogen reduction reaction to generate visible diffuse reflectance spectra, which was used as an electrocatalyst for dinitrogen reduction, and confirmed the existence of OV by EPR and Ultraviolet-visible diffuse reflectance spectra (UV-vis DRS)^[91]. The performance of OV-Bi₄O₅I₂-OH is superior, with an average yield of NH₃ up to 20.44μg·h^-1·mg^-1cat in 0.1 M Na₂SO₄ and FE up to 32.4% under ambient conditions. As shown in fig. 10a, OV and OH are simultaneously introduced into the Bi₄O₅I₂, and OV induces the generation of paired unsaturated vicinal Bi atoms (except for Bi³⁺, which adds Bi^3+δ and Bi^3-δ); As shown in Fig. 10B, compared with the case of only OV, the simultaneous presence of OV and OH enhances the charge transfer from the coordinatively unsaturated Bi atom to OH, so that the Bi atom has both empty and occupied orbitals, and there is a strong interaction between OV and OH; As shown in Figure 10C, OV with OH induces Bi2 to produce an empty orbital, which can imitate the "πback-donation" to accommodate the lone pair electrons of N₂ molecules, thus reducing the protonation energy barrier of N₂ (from 1.96 eV to 1.28 eV) and generating * NNH, thus promoting the whole dynamic process of NRR. In 2022, Wen et al. Used the protonated brucite precursor (Methyltri phenylphosphonium bromide), Methyltri phenylphosphonium bromide (MPB), and a modified liquid exfoliation method to prepare mesoporous MnO₂ nanosheets containing a large number of Mn³⁺-Mn³⁺ pairs for catalytic NRR. The average yield of NH₃ of the material was as high as 10.5% FE at 147.2μg·h^-1·mg^-1cat,-0.75 V (compared with the standard hydrogen electrode), and the material still had high stability after six cycle tests^[92]. The theoretical calculation shows that the introduction of OV leads to the generation of Mn³⁺-Mn³⁺ pairs, and there is a strong electron exchange between Mn and N₂ in Mn³⁺-Mn³⁺ pairs.A large number of electrons are injected into the π^* orbital of N from the d orbital of the Mn³⁺, which makes the N₂ active and weakens the triple bond of nitrogen and nitrogen, thus making the reaction of nitrogen hydrogenation easier and greatly improving the activity of NRR.

For CO₂ activation, Varandili et al. Demonstrated that the OV site on the Cu/CeO_2-x cooperates with Cu.The CO₂ intermediate can be adsorbed on the surface of the Cu/CeO_2-x as a double ligand, thereby promoting the activity of catalyzing the CO₂ and improving the selectivity of generating the CH₄^[4]. Zhang et al. Designed OV-rich InO_x for electrocatalytic reduction of CO₂ to formic acid (HCOOH), which has high HCOOH selectivity (91.7%) and high HCOOH local current density in a wide potential range, and still has some stability after 20 H of operation^[59]. Geng et al. Introduced OV into ZnO nanosheets to promote the electroreduction of CO₂, and proved that OV increased the yield of CO by increasing the binding strength of CO₂ and reducing the activation barrier of CO₂, while OV was also an electron donor and improved the conductivity of the catalyst^[3].

For lithium storage, in 2017, Li et al. reviewed the mechanism of ion doping, surface coating and surface oxygen vacancy modification in lithium-rich materials for lithium-ion batteries and their combined mechanism, and systematically explained that OV has the following functions in lithium-rich materials: (1) inhibiting the removal of lattice oxygen and improving the deterioration of crystal structure during charge and discharge^[93]; (2) It can not only reduce the interfacial impedance between the electrode and electrolyte, but also act as a Li⁺ ion migration channel, thus significantly increasing the rate of Li⁺ ion migration. In 2021, Liu et Al. Used a hydrothermal method to prepare an Al-doped 2D Li₄Ti₅O₁₂(LTO) nanosheet with excellent lifetime and charge capacity^[85]. As shown in Fig. 11, selective doping of Al³⁺ into the LTO lattice (Al³⁺ doping into octahedral Li⁺/Ti⁴⁺16d sites, i.e., octahedral interstitial sites, charge redistribution occurs resulting in the conversion of Ti⁴⁺ to Ti³⁺. Li exists in the Li⁺/Ti⁴⁺8a site, namely the tetrahedral interstitial site), a large number of OVs are introduced in the LTO bulk phase, the Ti — O bond length and the distorted Li polyhedron are increased by OV, a more open and shorter Li⁺ ion diffusion channel is provided, the energy barrier of Li⁺ ion diffusion is significantly reduced, and a large number of reaction sites are provided by OV^[94]; The 2D structure shortens the ion diffusion channel, so the OV and 2D structures jointly promote the efficient diffusion of Li⁺ ions and ensure the ultrafast lithium storage.

In the field of photocatalysis, OV is ubiquitous on the surface of nano-transition metal oxides, which is by far the most widely studied type of photocatalyst defect. It plays an important role in improving the adsorption of semiconductor photocatalysts in the process of light harvesting, charge separation and reduction^[95]. Both theoretical calculations and experimental work show that the OV in the photocatalyst can localize a large number of charges, which can act as photoexcited electron capture sites and promote the separation of electrons and holes. OV can form an intermediate band between the valence band and the conduction band, effectively reduce the band gap, promote surface charge transfer, and promote the adsorption and activation of target molecules, thereby further enhancing the photocatalytic properties in the visible light field, and has wide application in the processes of photocatalytic nitrogen fixation, oxygen evolution, and CO₂ activation conversion^[96].

In the field of nitrogen fixation, in 2015, Li et al. Introduced OV on the { 001 } surface of BiOBr nanosheets, which can localize a large number of electrons and has the following advantages: (1) promoting the adsorption and activation of N₂ molecules^[12]; (2) trapping electrons or holes and inhibiting carrier recombination; (3) to reduce the energy barrier for interfacial charge transfer. In 2018, Xue et al. Added a certain amount of polyvinylpyrrolidone (PVP) surfactant in the synthesis process, which significantly improved the photocatalytic nitrogen fixation performance of OV on ultrathin BiOBr nanosheets^[13]. The negatively charged — C O bonds of surfactant PVP molecules tend to combine with the unsaturated positively charged Bi atoms on the surface of BiOBr, reducing the surface energy, thus forming a large number of OVs. OV plays the following roles in this system: (1) effectively narrowing the band gap, enhancing the optical absorption, and changing the position of the conduction band and valence band; (2) capturing photoelectrons in the BiOBr nanosheet to participate in nitrogen reduction reaction, promoting charge separation, delocalizing electrons, enhancing charge transfer, reducing interface resistance, and promoting the separation of photoelectrons and holes, thereby increasing the reaction rate; (3) favor that adsorption and activation of N₂. In 2020, Ren et al. Designed an ultrathin W₁₈O₄₉ nanowire with distorted surface structure, containing abundant surface OVs, for photocatalytic nitrogen fixation to generate NH⁺₄ and NO^-₃^[97]. As a catalytic site, surface OV is very important for the efficient immobilization of N₂, which can not only promote the absorption of light from the visible region to the near-infrared region, but also improve the separation ability of photoexcited electrons and holes.At the same time, it can also be used as an active site for chemical adsorption of N₂, and can also be used as a bridge connection site between photogenerated carriers and N₂ molecules, so that the disproportionation reaction of N₂ molecules is more energetically favorable.

In the field of OER, similar to electrocatalysis, OV under photocatalysis can promote the generation, adsorption and activation of oxygen species, which is also an important reason for the increase of absorbance in the visible region. In 2017, Cui et al. Synthesized BiOCl nanosheets containing a large number of OVs by a simple solvothermal method for visible-light-driven photocatalytic oxygen evolution reaction^[98]. The results show that a new electron donating level appears in the band gap of BiOCl after the introduction of OV, which extends the optical absorption from the ultraviolet region to the visible region. Meanwhile, the presence of OV realizes a high-intensity visible photocurrent, which promotes the photoinduced charge separation and charge transport, so the OV-rich BiOCl nanosheets have high photooxidation activity. In 2019, Bhatt et Al. Doped Al (3,5,7 wt%) into the ZnO lattice to prepare ZnO photoconductors with high transparency, high performance and low cost^[99]. Al doping increases the stacking defects on the surface of ZnO, increases the bending of the semiconductor band, produces a large number of OV, increases the surface roughness, and enhances the photoresponse, thus improving the photocatalytic activity. In 2020, Mao et al. Demonstrated that double defects (oxygen vacancy defects and interstitial oxygen defects) can synergistically promote the chemisorption of surface O₂, the separation and transfer of bulk charge, thus achieving efficient photocatalytic O₂ activation^[100]. Experiments and theoretical calculations show that the surface OV can lower the conduction band and act as an active site to capture electrons from bulk oxygen and chemisorbed O₂ molecules to participate in the reaction. In 2021, Li et al. Designed an OV-rich AgI/CeO₂ heterojunction system, which can activate O₂ into reactive oxygen species and use them for photocatalytic disinfection and tetracycline degradation^[101]. A large number of OVs can not only act as oxygen adsorption sites and accelerate the chemical adsorption of O₂, but also promote charge separation and transfer and accelerate the formation of active oxygen species (such as H₂O₂, · {{\mathrm{O}}_2}^{-} and · OH).

Photocatalytic CO₂ reduction is also an important field, and its products include CH₄, CO, syngas and C₂ fuels, which have received more and more attention in recent years.

For the generation of CH₄, in 2020, Jia et al. Prepared Co/Al₂O₃ catalyst by MOF template method for CO₂ activation to produce CH₄.The production of OV was induced by light irradiation, which enhanced the adsorption of H₂, C, O₂ and CO species, and inhibited the recombination of photogenerated electrons and holes, thus improving the reactivity^[5]. The results indicated high CH₄ yield (up to 6036μmol·g^-1·h^-1), good selectivity (97.7%), and catalytic durability of the system. In 2021, Jiang et al. Prepared an OV-rich layered structure bismuth oxysulfide (Bi₂O₂S) by heat treatment under N₂ atmosphere for efficient photocatalytic reduction of CO₂ to CH₄ under visible light^[6]. The structures of BiOS and OV-BiOS are shown in Fig. 12. Due to the decrease of the bond energy of the Bi — O bond after heat treatment, the bond length is elongated, resulting in the removal of the O atom connecting the Bi atom and the generation of OV. Both experimental and calculated results show that OV introduces defect levels into BiOS, which are helpful to capture photoexcited electrons and transfer them to CB, and then the electrons are transferred from CB to the surface. OV shortens the band gap (compared to pristine BiOS without OV) and also contributes to the rapid transfer of electrons from the valence band to the conduction band; OV inhibits the recombination of photogenerated electrons and holes and accelerates the high-speed separation of photogenerated carriers; OV can make OV-BiOS have more negative charges (compared with the original BiOS without OV), which is beneficial to the adsorption of CO₂ to promote the subsequent activation of CO₂.It is not conducive to the adsorption of H₂O to weaken the formation of H₂, but also provides more reaction sites for the activation of CO₂, thus jointly promoting the reduction of CO₂.

For the generation of CO, as early as 2018, Chen et al. Used the heat treatment method to dope Cu into OV-rich TiO₂ for CO₂ reduction, and the structure is shown in Fig. 13, and it was found that the CO₂ adsorption energy of this material was − 0.85 and the barrier of -1.30 eV,CO₂ dissociation reaction (CO₂*→CO*+O*) was very low (about 0.10 and 0.19 eV, respectively) when Cu atoms filled OV and Cu atoms were near OV^[7]. Both theoretical calculations and experiments show that the co-existence of Cu-doping and OV on the one hand enhances the charge transfer (from reduced Ti atoms to CO₂), which is beneficial to promote the adsorption of CO₂ (CO₂ can be linear or curved adsorption), and the addition of Cu increases the selectivity of adsorption sites. When Cu fills in or is close to OV, Cu atoms can strongly intercept CO₂ molecules and further enhance the adsorption of CO₂); On the other hand, it is helpful to reduce the energy barrier of CO₂ dissociation (a large number of photoexcited electrons near OV and Cu atoms further activate CO₂stably), promote the spontaneous decomposition of CO₂ (CO₂ dissociation can remove OV and form Cu-CO complex), and accelerate the formation of CO. In 2019, Yu et al. Prepared OV-rich Sr₂Bi₂Nb₂TiO₁₂ nanosheets (SBNT-HR) for photocatalytic preparation of CO, and found that OVs could extend the light absorption region of the material from the ultraviolet region to the visible region, while increasing its CO₂ photoreduction activity^[8]. Liu et al. Used an OV-rich ultrathin TiO₂ modified by highly dispersed Pt nanoparticles to realize the conversion of CO₂ into CH₄ and CO, and the material had excellent photocatalytic activity, in which OV could accelerate the electron transfer from the OV-rich ultrathin TiO₂ to Pt, accelerate the separation of photogenerated electron-hole pairs, and also strongly adsorb CO₂, thus strongly improving the catalytic activity of the system^[102]. In 2020, Liu et al. Introduced OV-forming BTO-UV materials (V refers to vacancy defects) on two-dimensional Bi₄Ti₃O₁₂ ultrathin nanosheets (BTO-U) by a combined hydrothermal and post-reduction process^[83]. As surface active sites, OV can optimize the surface state and electronic structure of Bi₄Ti₃O₁₂ nanosheets, form defect levels in the BTO band structure, and narrow the band gap, thereby expanding the optical absorption range and enhancing the visible light absorption. OV can also capture electrons, improve the separation efficiency of photogenerated electron-hole pairs, increase the number of active sites, and enhance the adsorption of CO₂.

For the generation of syngas (H₂ and CO), in 2020, Pan et al. Prepared a Zn-doped Pt-supported CeO₂ catalyst and coated it with MgO coating for the catalytic conversion of CO₂ to syngas^[103]. Proper Zn doping and photo-irradiation together introduce OV. On the one hand, OV can enhance the light absorption, on the other hand, it can also promote the activation of CO₂ (adsorb C — O species, produce CO and O species to participate in the subsequent reaction), and can also be recycled to enhance the stability of the system.

For the generation of C₂ fuel, in 2021, Sun et al. Designed a nanoribbon material (Zn₂GeO₄) containing asymmetric M₁-O-M₂ triatomic sites in phenolite to accelerate the reduction of CO₂ to C₂ fuel.As shown in Fig. 14, the illumination caused the electron hole separation and initiated the reaction, which converted the CO₂ adsorbed on the metal Zn and Ge into COOH * and CO * intermediates to participate in the subsequent reaction^[104]. The asymmetry of the electron distribution on the triatomic site is enhanced by OV, which leads to a more obvious difference in the charge distribution of the two CO * intermediates adsorbed on Zn and Ge, thus further reducing the carbon-carbon coupling energy barrier. OV also stabilizes the OC-COH * intermediate, lowering the energy barrier of the rate-determining step (RDS, i.e., OC-CO*+e^-+H⁺→OC-COH*).

In the field of photocatalytic HER, in 2020, Xiao et al. Designed a new Molten salt method (MSM) to prepare Ni atom co-catalyst on Molten salt method nanoparticles for hydrogen evolution reaction^[105]. The results show that Ni atom reduces the formation energy of OV and promotes the formation of OV on the adjacent surface of TiO₂ in the MSM process, which is beneficial to charge transfer and HER. Hydrogen species tend to interact with Ni atoms and neighboring OVs, so OV-containing Ni-a/TiO₂ has a more favorable adsorption free energy. However, when the concentration of OV is too high, it will cause charge recombination, which is not conducive to the occurrence of HER.

In the field of photocatalytic organic oxidation, in 2018, Mao et al. Prepared BiOCl nanosheets with different concentrations of OV by a new method of H₂O₂ treatment coupled with infrared irradiation.The selective oxidation of secondary amine-tert-butylbenzylamine (BA) to n-tert-butylbenzylamine (BI) in acetonitrile under visible light was used as the target reaction, and it was found that the OV concentration on BiOCl (001) significantly affected the atomic and electronic structure of the BiOCl surface, thereby modulating the photocatalytic pathway^[106]. As shown in fig. 15, OV exists on the surface of BiOCl, and the Bi^(3-x)+ species on the surface of BiOCl (001) increase with the increase of OV concentration; The bond length of BI-Bi and Bi-O is shortened (due to the structural distortion caused by OV), and the local electrons are reduced, which is more conducive to the adsorption of O₂, the activation of molecular oxygen, and the formation of {{\mathrm{O}}_2}^{2-} through the two-electron transfer path (compared with the formation of {{\mathrm{O}}_2}^{-} through the one-electron path), while the {{\mathrm{O}}_2}^{2-} is conducive to preventing overoxidation and improving the selectivity of Bi formation. In addition, the increase of OV shortens the band gap and increases the visible light absorption range. Wang et al. Synthesized WO₃ nanorods with uniform surface OV modification by a simple hydrothermal synthesis method for selective oxidation of alcohols to ketones in aqueous medium^[107]. The appropriate amount of surface OV has high specific surface area and small grain size, which can capture photoelectrons, thus improving the separation efficiency of photogenerated electron-hole pairs and ensuring the superior catalytic performance of the system.

In recent years, the removal of pollutants has been an urgent problem to be solved. OV-containing catalysts have been developed in the field of pollutant removal because of their strong catalytic activity and low cost. In 2021, Shang et al. Used DFT theoretical calculations to study the SO₂ oxidation reaction catalyzed by defective TiO₂ mineral particles at the molecular scale^[108]. They found that the surface OVs of the loading system (TiO₂-OV) contributed to the adsorption of SO₂ in the dark and the accumulation of sulfate under sunlight. As shown in Fig. 16, O₂ tends to adsorb on OVs, and one O-filled OV,SO₂ tends to adsorb on three kinds of sites near OV: site1, site2, and site3. In the dark environment, OVs will change the electronic environment of O_2c and Ti_5c with insufficient surface coordination, so that site1 and site3 sites accumulate more charges to adsorb SO₂; Under illumination, OVs can induce the activation of molecular oxygen to produce · {{\mathrm{O}}_2}^{-}, which oxidizes the adsorbed SO₂ into sulfate ( {{\mathrm{SO}}_4}^{2-} species). Aiming at the removal of organic pollutants, in 2019, Qiu et al. Migrated a large number of OVs to the surface of Pt/TiO₂ by vacuum annealing, effectively increasing the number of surface OVs^[109]. The OV-rich enriched the active Pt species (Pt⁰ and Pt-OH) on the surface of Pt/TiO₂, which greatly enhanced the photocatalytic activity of the catalyst under visible light irradiation. The introduction of surface OV effectively optimizes the chemical state of Pt, thereby improving the separation of photogenerated carriers. In 2020, Ni et al. Designed an OV-regulated sandwich-type TiO_2-x/ ultrathin g-C₃N₄/TiO_2-xZ-type heterojunction structure visible-light-driven photocatalyst (S-TUCNov) for effective removal of tetracycline hydrochloride (TCH) in water^[110]. The results show that S-TUCNov has a high TCH removal rate (87.7% in 90 min) by virtue of its special three-dimensional structure and active substances such as · {{\mathrm{O}}_2}^{-}, H⁺ and · OH. In this process, the NaBH₄ reactant first induces the generation of OV, and the surface OV enhances the separation of photoinduced carriers, shortens the band gap of TiO₂, and forms a heterojunction of TiO_2-x and two-dimensional ultrathin g-C₃N₄(UCN); Secondly, the Z-type heterojunction accelerates the charge transfer, promotes the formation of active species, and effectively changes the band gap position, which improves the catalytic performance together with OV. In 2021, also for the TCH degradation reaction, Li et al. Synthesized an OV-rich pencil-like ZnO nanorod by a simple hydrothermal method, and further improved its photocatalytic performance by co-modification of Carbon nanodots (C Dots) and Ag^[111]. C Dots/Ag/ZnO with the highest content of OV had the strongest TCH removal rate (up to 94.95%) and mineralization performance under UV-vis light. The co-existence of the pencil-like morphology and OV enhances the carrier concentration of ZnO and promotes the separation of charge carriers. Subsequently, Yang et al. Used a template-free hydrothermal method to fabricate a bayberry-like Sm-doped CeO₂ material (ASC) immobilized with Ag quantum dots (QDs)^[112]. The activity of this ASC to remove CH₃CHO and produce H₂ was 7.08 and 6.83 times higher than that of pure CeO₂, respectively. The OV introduced by doping can trap photoexcited electrons and construct a doping-dependent transition state between the conduction band (CB) and the valence band (VB), effectively limiting the recombination of photoexcited electrons and holes. In addition, the above trapped electrons can be rapidly transferred to the co-catalytic site anchored with Ag QDs, enhancing the synchronous absorption and utilization of visible light. Li et al. Successfully introduced OV into the sample by designing a reducing atmosphere, and synthesized Bi₅O₇I samples with adjustable ratios of { 314 } and { 008 } faces using an ionic liquid spontaneous combustion method for pollutant degradation and CO₂ reduction^[113]. The results of Photoelectrochemical test show that the synergistic effect of appropriate face ratio and OV can improve the carrier and band structure, thereby reducing the band gap and enhancing the photocatalytic activity. Badreldin et al. Synthesized a novel black V₂O₅ material for photodegradation of cationic methylene blue (MB), neutral quinoline yellow (QY) and anionic methyl orange (MO) using a controllable and environmentally friendly physicochemical reduction method^[114]. The results show that a large number of OVs on the V₂O₅(001) plane can promote electron-hole separation, promote the formation of hydroxyl (· OH) and superoxide radicals (· {{\mathrm{O}}_2}^{-}), and narrow the band gap, improve the visible light activity of pristine V₂O₅, and further enhance its photocatalytic performance in the visible light region.