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Oxygen Vacancy in TiO2: Production Methods and Properties

Written By

Javid Khan and Lei Han

Submitted: 06 January 2023 Reviewed: 06 April 2023 Published: 02 May 2023

DOI: 10.5772/intechopen.111545

Updates on Titanium Dioxide IntechOpen
Updates on Titanium Dioxide Edited by Bochra Bejaoui

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Updates on Titanium Dioxide

Edited by Bochra Bejaoui

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Abstract

Titanium dioxide (TiO2) is a versatile material used in a variety of applications, including photocatalysis, photovoltaics, sensing, and environmental remediation. The properties of TiO2 are influenced by its defect disorder, with oxygen vacancy (V0) being a prominent defect that has been widely studied. Defective TiO2 materials, particularly those containing V0 defects, are of interest for the development of next-generation semiconducting nanomaterials. Several methods, including high-temperature calcination, ion implantation, and chemical doping, are used to produce defective TiO2 with varying degrees of V0 defects. The properties of defective TiO2, including optical, electronic, and structural characteristics, are essential for determining the material’s suitability for various applications. Modification of the defect structure of TiO2 through doping with impurities can enhance the photocatalytic activity of the material. Researchers continue to investigate the impact of factors such as crystal structure and the presence of other defects on the properties of TiO2-based materials, further enhancing their potential for various applications. Overall, a deeper understanding of defect disorder and the development of production methods for defective TiO2 will play a crucial role in the design and production of next-generation semiconducting nanomaterials.

Keywords

  • TiO2
  • defects
  • oxygen vacancies
  • structural
  • electronic
  • optical properties

1. Introduction

As a wide bandgap semiconductor material with industrially significant applications, titanium dioxide (TiO2) is commonly used in catalysts [1, 2, 3], ointments [4, 5, 6], paints [7, 8, 9], sunscreens [10, 11, 12], and toothpaste [13, 14, 15]. Intense study has been put into TiO2 materials since Honda and Fujishima [16] discovered the phenomenon that TiO2 can be used for photocatalytic water splitting. This has allowed TiO2 to be used in photoelectrochemical cells [17, 18, 19], photovoltaics [20, 21, 22], and photocatalysis [23, 24, 25]. TiO2 offers several benefits over other semiconductor materials, including its low toxicity, resistance to photocorrosion, abundance on Earth, and chemical and thermal stability [26]. However, due to its significant recombination rate and broad band gap (3.2 eV), poor quantum efficiency as well as inadequate exploitation of visible light during photocatalytic reaction TiO2’s applications is severely limited [27]. Therefore, a variety of methods have been used to alter the TiO2 in an effort to narrow the band gap and lengthen the lifetime of photogenerated charge carriers [28]. These methods include co-doping with metal ion/nonmetal ions, coupling TiO2 with a semiconductor with a small band gap, encasing noble metal cores in a TiO2 shell to create metal core@TiO2 shell composite photocatalysts, noble metal deposition, and surface sensitization by organic dyes [29, 30, 31, 32]. Recent research has shown that the defect disorder of TiO2 may influence several of its physical and chemical characteristics, including selectivity, photocatalytic reactivity, and light absorption, among others [33, 34, 35].

One of the most prominent defects observed in TiO2 is oxygen vacancies (V0), which are also considered to be common defects in metal oxides and have been studied extensively by using both experimental and theoretical characterizations [36, 37, 38]. The V0 has the potential to function as active sites and adsorption points during heterogeneous catalysis [39, 40, 41]. The electrical structure, charge transport, surface properties, and other photocatalytic characteristics of metal oxides based on TiO2 have also been demonstrated to be intimately connected to V0 [42, 43, 44]. It is theoretically possible that Ti3+ centers or unpaired electrons (e−), which could lead to the creation of donor levels in TiO2’s electronic structure, are produced as a result of the production of V0 on TiO2 [45, 46, 47]. Additionally, it is thought that V0 alters the recombination rate of electron-hole pairs during photocatalysis, which alters the chemical processes that rely on charge transfer from either hole (h+) or e− [48, 49, 50]. According to theoretical and experimental findings, the excess e− on V0 states impacts the reactivity and surface adsorption of important adsorbates like H2O or O2 on TiO2. In order to take use of their special features for photocatalytic applications, controlled synthesis of V0 incorporating TiO2 is of utmost importance [51, 52, 53].

In this instructional chapter, we list the methods for producing TiO2 with V0, go over their characteristics, and touch on some of the uses for photocatalysis. The preparation technique is the main division used to classify the syntheses of TiO2 nanomaterials with V0. The readers may consult the relevant literature for comprehensive directions for each synthesis. In Section 3, the reductive, adsorption, optical, and structural characteristics of the TiO2 nanomaterials containing V0 are discussed. To create highly effective photocatalysts and increase the functional applications of photocatalysis, it is intended that this chapter would be a beneficial resource for engineers who want to create defective semiconductors.

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2. Synthetic methods

2.1 Treatment under hydrogen

A common technique to alter the photoelectrochemical and surface characteristics of TiO2 is the hydrogen treatment [54, 55, 56]. When TiO2 is heated, the oxygen (O) atoms in the lattice are interacted with by hydrogen (H) atoms to generate V0 and alter the material’s surface characteristics [57]. Three stages may be distinguished between the interaction among H and TiO2 throughout this process: The elimination of the adsorbed oxygen ESR signals in Figure 1 is evidence that hydrogen physically interacts with the gas at temperatures below 300°C. Additionally, at temperatures above 300°C, O atoms in the TiO2 lattice get e− that were previously held by H atoms. Then, the surface of TiO2 has its lattice O extracted, causing the O atom to separate from the H atom and create H2O. As a result, TiO2’s surface develops V0, as seen in Figure 1. Thirdly, as the temperature reaches 450°C, the contact between the two substances happens more significantly. In order to produce Ti3+ defects, the e− in the H atoms are transported to the Ti4+ ions of the TiO2. The V0 states’ e− are forced away and moved to Ti4+ as the temperature rises to 560°C, where they remain until 600°C. As a consequence, the V0 states’ ESR signal strength decreases and that of Ti3+ rises.

Figure 1.

ESR spectra of Ti3+ and V0 in TiO2 after treatment with H2. Reproduced with permission from ref. [57] Copyright Elsevier.

Notably, the hydrogen treatment used to reduce TiO2 results in the formation of Ti interstitials as well as V0 in the matrix of TiO2 [58]. The optical band gap of TiO2 reduces when the amount of Ti exceeds that of O. According to Morgan and Watson [59], V0 creation is to some extent more favorable in rutile compared to anatase, whereas Ti interstitials formation occurs more in rutile. However, V0 is the preferred defect type in oxygen-rich environments. Still, the formation energies of both defect types are high. However, both defect types are stabilized in O-poor environments. Additionally, it is proposed that vacuum annealing and high-temperature type harsh conditions are needed for Ti ions interstitials formation than V0 [60]. Moreover, V0 are common defects in many oxides and not just significant defects in TiO2, which has a significant impact on the physicochemical properties of such oxides. As a result, V0 has received a lot of attention and may be more interesting than Ti interstitials.

2.2 Bombardment with high-energy particles

Numerous studies have demonstrated that oxygen ions and neutral atoms can be selectively desorbed from TiO2 surfaces, leading to the creation of vacancies [61, 62, 63]. Knotek and Feibelman [64] discovered that an interatomic Auger recombination practice enables e− having energies of more than 34 eV to knockout surface oxygen. In their research, a potential mechanism for the formation of V0 when irradiated with e− was also put forth. They suggested that the formation of V0 is caused by the removal of O+ from the surface of TiO2 due to e− induced desorption. The benefit of using this technique for defect production is that e− with moderate energies are bombarded to cause slight surface destruction and utterly create V0. Even exposing the electron-irradiated surfaces at low temperatures to molecular oxygen can result in the creation of V0 [65].

Ion sputtering, specifically argon ion (Ar+) sputtering, produces V0 on the surface of TiO2, much like electron bombardment does [66]. When exposed to oxygen at low temperatures, the defects at the surface due to Ar+ sputtering do not go away. This shows surface bridging V0 on Ar+ sputtered surfaces along with other subsurface defects are suggested to be more highly reduced surface species. However, only by treating under oxygen at low temperatures, these kinds of defects cannot be repaired.

Additionally, reducing gas atmosphere plasma treatment at low temperatures is frequently used to produce V0 on the surface of metal oxides [67, 68]. Species with high energies like radicles, atoms, and e− are used under low-temperature plasma. Due to moderate reaction conditions, the outer layer of metal oxides is changed, while the bulk materials are unaffected.

2.3 Doping

In the lattice of TiO2, V0 often occur when they are doped with a nonmetal or metal ions. For instance, Krol and Wu have shown that the production of V0 in the TiO2 lattice may occur when Fe3+ ions are substituted for Ti4+ ions in the lattice [69]. Additionally, Domen’s group [70] revealed that aliovalent cations can be used to successfully dope and improve the defects of the photocatalyst. They proved that the extrinsic V0 are introduced with the help of a cation that has a lower valence as compared to the parent cation (Ti), preventing the creation of Ti3+, as seen in Figure 2a. As a lower valence cation, trivalent cation (M3+) doping occupies the Ti4+ sites. As seen in the equation of Figure 2a, this causes the production of V0 to be aided without producing Ti3+ species. In contrast, as seen in Figure 2b, the Ti3+ is stabilized by the higher valence cations without developing V0. Similarly, the equation in Figure 2b shows that Ti4+ sites have been occupied by the pentavalent cation (M5+) so Ti3+ sites would be created and the development of V0 would be prevented when a higher valence cation is doped.

Figure 2.

Schematic diagram of doping of (a) trivalent cations and (b) pentavalent cations in SrTiO3. Replicated with consent from Ref. [70] Copyright American Chemical Society.

Similar to how doping with metal ions may produce V0 in the TiO2 lattice, doping with nonmetal ions like fluorine or nitrogen can also do so [51, 71, 72, 73, 74]. According to calculations using density functional theory (DFT), adding N to bulk TiO2 results in a significant decrease in the energy required to generate V0. This shows that N doping makes V0 more likely to occur. Additionally, N doping is often performed in a decreasing environment. TiO2 may be partially reduced by this reducing environment, which will lead to the development of V0.

2.4 Through different reaction conditions

Another possible effect of the lattice oxygen participation in the thermally driven catalytic reaction of organic molecules is oxygen removal from the surface of TiO2 [75]. A surface vacancy is created as a consequence of this process, which involves oxidizing organic materials on the surface of oxides while losing oxygen atoms from the surface lattice. For example, as shown in Figure 3, Morris and Panayotov [76] showed that methoxyl groups could be burnt thermally by activated lattice oxygen, leading to shallow donor states (Ti3+ and V0) below the conduction band of TiO2.

Figure 3.

Schematic illustration of (A) thermally activated oxygen leaving Ti3+-V0-Ti3+ donors in the bridge lattice. (B) Methoxyl groups attached to CUS Ti4+ Lewis acid sites burn on the particle surface where the oxygen atoms diffuse to. Reprinted with permission from ref. [76]. Copyright American Chemical Society.

On the surface of several semiconductors, photochemically induced oxidation reactions, the mechanism of reaction-driven V0 production is also at work [77, 78, 79, 80, 81]. For instance, under the circumstances of a photocatalytic process, Xu et al. [82] have only recently discovered that V0 are photoinduced formed on TiO2. According to the detailed production method, when UV light is applied, molecular oxygen absorbs the photo-generated e−, while the h+ diffuses to the surface of the TiO2 and are trapped by the lattice oxygen. Consequently, the lattice oxygen and Ti atom’s binding link become weaker due to the trapped h+, and this bond is broken by the adsorbed molecule benzyl alcohol. The oxygen from the lattice is subsequently removed from the surface of TiO2, creating V0 defects on the catalyst’s surface.

2.5 Thermal treatment under oxygen deficient conditions

Another method for producing V0 is to anneal TiO2 in the pure form above 400°C under Ar, N2, or He gas atmosphere, or in a vacuum [83]. The following equilibrium may be used to explain how V0 arises at high temperatures, using the common Kroger-Vink notation:

O0↔TiO2V0+12O2g+2e–E1

Following is an expression for the equilibrium constant of this reaction:

K=V0n2pO212E2

As a function of p(O2), Eq. (2) may be transformed to indicate the V0 concentration:

V0=Kn–2pO2–12E3

Where O0 signifies the lattice oxygen; [V0] signifies the concentration of V0, V0 the number of oxygen vacancies, and p(O2) the oxygen pressure. From Eq. (3), we may infer that the concentration of V0 rises as O2 pressure falls, i.e., the oxygen deprived state during thermal annealing would promote the production of V0.

Reaction (1) is reversible even at room temperature, and thus, the V0 as they have generated will gradually vanish when the TiO2 is exposed to air [69]. The TiO2 nanoparticles may be doped with foreign ions working as accepter-like Fe dopants, to stabilize the V0.

Fe2O3→TiO22FeTi+3O0+V0E4

There is no way to undo this breakdown process. Positively charged V0 in the TiO2 lattice would be made up for by the inclusion of Fe3+ [52]. The amount of free e− in TiO2 is subsequently reduced as a consequence. Since Fe3+ ions are doped into TiO2, V0 are so stabilized.

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3. Properties

3.1 Structural properties

3.1.1 Electronic structure

One or two e− localize in a V0 state when an oxygen atom from the bulk or surface of TiO2 is absent. The highly ionic crystal’s Madelung potential is the main factor behind the e-'s localization in the V0 state [60]. One or two “free” e− in the flawed crystal take the O2 anion’s position from the normal lattice in this fashion, reducing the energy cost of vacancy creation. As illustrated in Figure 4, these e− on the V0 states directly affect the electrical structure of TiO2 by producing a donor level below the conduction band. Localized donor states generated from V0 have an energy level between 0.75 and 1.18 eV below TiO2’s conduction band [84]. Additionally, the elimination of oxygen atoms to create V0 may also result in the redistribution of extra e− to the Ti atoms in the neighborhood of the V0 site, leading to the formation of shallow donor states below the conduction band derived from Ti 3d orbits [85]. These donor states are shown to rise with increasing V0 in both rutile and anatase TiO2. For anatase TiO2 with very low oxygen content, they may even overlap the conduction band [86]. These results indicate that the formation of V0 leads to a large shift of the Fermi level of TiO2 toward higher energies.

Figure 4.

Projected band structure model for V0 in anatase TiO2. Copied with approval from Ref. [84] Copyright Elsevier.

3.1.2 Geometric structure

In addition to altering the electrical structure of TiO2, V0 also changes the material’s geometric structure [87, 88, 89]. The formation of V0 changes the surface structure of TiO2, as shown by Park et al. [90] who also found that the generation of V0 resulted in an upshift of the Eg mode of the Ti-O bond in the Raman spectrum. Due to the existence of V0, this may be ascribed to atomic rearrangement. In order to strengthen their connection with the remaining lattice, the three Ti atoms closest to an ejected O atom have a tendency to move away from the vacancy [91]. Similarly, Watson et al. reported that the bond length of the Ti-O bond is reduced as a result of outward relaxation, which also reduces the overlap between the three Ti dangling bonds. According to Dal Santo et al. [58] experimental electron diffraction data support this conclusion. They have seen a TiO2 lattice shrinkage brought on by V0.

As illustrated in Figure 5, Cheng et al. [89] recently revealed that the prominent surface reorganization is caused by V0 in TiO2 sheets with surface-terminated fluorine. This is supported by two new Raman modes at 155 and 171 cm−1 and the weaker B1g mode at 397 cm−1. By simple calcination of the sample in air, the two modes at 155 and 171 cm−1 entirely vanished when the surface fluorine and V0 were removed from the TiO2 sheets. Additionally, only “fluorine-terminated anatase TiO2 sheets” are capable of producing a novel active mode. This finding suggests that the two new Raman modes are separate from the single-surface fluorine. Additionally, it was shown that the lone V0 only produces a few additional weak modes above 300 cm−1. The surface fluorine and V0 in oxygen-deficient anatase TiO2 have synergistic effects that likely change the bonding length of the Ti▬O▬Ti network and the atomic coordination numbers. It is suggested that the rebuilt surface is made up of Ti atoms having smaller coordination numbers such as Ti with four coordination numbers, which may provide reactants in catalytic processes with more advantageous places to bind to. This is significant because, in contrast to TiO2 sheets without V0, a regenerated surface may boost the contact between the TiO2 matrix and the loaded Pt through a unique e− transfer mechanism induced by V0 that increases the photocatalytic hydrogen generation rate.

Figure 5.

Raman spectra of (a) anatase sheets with V0, (b) anatase sheets without V0 (a), and (c) reference anatase TiO2 in the ranges of (A) 110–700 cm−1 and (B) 300–750 cm−1, respectively. Partial Raman spectra of curves b and c between 100 and 300 cm−1 are shown in the left inset of panel A, while the fitted Eg mode of curve b at 100–200 cm−1 is shown in the right inset of panel A. Copied with permission from Ref. [89] Copyright American Chemical Society.

3.2 Optical properties

To generate e−-h+ pairs for surface reactions, heterogeneous photocatalysis depends on the ability of photocatalysts to harvest light energy. However, TiO2 can only absorb UV light because of its broad band gap. Fortunately, defect engineering allows for the manipulation of TiO2’s optical properties. As local states are created by V0 below the conduction band edge, the light harvesting ability of TiO2 increases from UV to the visible light range. The V0 states that have been developed may participate in a fresh photoexcitation process. In other words, visible light’s energy is used to excite the electron from the valence band to the V0 states, resulting in the usual excitations seen in the visible spectrum. Because of this, V0 are referred to as F centers, which comes from the German word for color, Farbe. Additionally, by interacting with nearby Ti4+, the e− remaining in the V0 may create the Ti3+ species [92]. Just below the conduction band, the Ti3+ defects may generate a shallow donor level that might also affect the sensitivity to visible light.

3.3 Dissociative adsorption properties

Understanding the active locations on TiO2 has been aided by research into defects using adsorbing probe molecules. On TiO2 single-crystal surfaces, small molecules including HCOOH, O2, H2O, N2O, H2, and CO have been employed to study the performance of such defective sites [93, 94, 95]. Some of these molecules’ adsorption properties are discovered to change as a result of defects linked to V0 [96]. During the photocatalytic activity, h+ and e− produced in the crystals of TiO2 under UV light may transfer to the surface and then be transported to the adsorbed species, where they take part in the redox reaction [97]. Even though the microscopic specifics of the mechanism of these e− transfer are still not fully known, it is anticipated that the transfer will be more effective if the surface and adsorbate are closely connected, as well as when the adsorbed materials are detached.

3.3.1 Oxygen adsorption

In areas like gas sensing and heterogeneous catalysis, the interaction of oxygen with TiO2 is essential. TiO2 catalyzes a number of photooxidation processes, where molecular oxygen acts as the oxidizing agent [98]. One crucial stage of the photocatalytic reaction in these systems is the adsorption of molecular oxygen on the surface of TiO2 [99]. O2 does not, however, adsorb on a perfectly neutral TiO2 surface [100]. Only when there is sufficient negative charge available to form O▬Ti bonds can O2 adsorb onto the TiO2 surface; this charge may come from subsurface V0 or photogenerated e−, with adsorption energies of 2.52 and 0.94 eV, respectively, as shown in Figure 6. Superoxide radical groups may be created simultaneously by the O2 adsorbed on the surface of TiO2 and the free e− present on V0 states. Both the charge separation process and the oxidation of organic materials are actively promoted by the production of these radical groups [101].

Figure 6.

Schematic illustration of photoexcited charge carriers in (a) TiO2 and (b) Pt/TiO2. Reprinted with approval from Ref. [101] Copyright American Chemical Society.

3.3.2 H2O adsorption

As an example of a straightforward surface chemical reaction with considerable applications, water dissociation on TiO2 is of basic importance. It has been thoroughly investigated how H2O and TiO2 interact with surfaces, significantly influencing the photocatalytic processes [102]. It is particularly well known that on the imperfect TiO2 surface, H2O molecules that have been chemically dissociated are energetically preferred, whereas H2O molecules are only physically adsorbed on the ideal TiO2 surface [103, 104, 105]. H2O dissociation only occurs on defect sites linked to V0 at low coverage, according to research by Besenbacher et al. [106] that combines experimental and theoretical methods. They used scanning tunneling microscopy (STM) to show a direct correlation between V0 before water exposure and surface hydroxyl groups after exposure, and they used DFT to show that only the defect sites are energetically capable of supporting water dissociation. It is shown that V0 in the surface layer dissolves H2O by transferring one proton to an oxygen atom nearby, resulting in the formation of two OH groups for every vacancy.

3.3.3 Adsorption of alcohol

The reactive sites on metal oxides, both in powder form and in single crystal, have been intensively probed using alcohols. Both experimental characterizations and theoretical calculations have been used to extensively study the adsorption of alcohols on TiO2 [107]. Using theoretical calculations, Oviedo et al. [108] have shown that methanol dissociation is thermodynamically advantageous on the V0 states. According to Farfan-Arribas and Madix, temperature programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS) are used to investigate the function of V0 in the adsorption of aliphatic alcohols on TiO2. They discovered that the existence of V0 on the surface leads to greater alcohol adsorption on the surface. At room temperature, the adsorbed aliphatic alcohols spontaneously dissolve on the TiO2-(110) surfaces containing V0, generating hydroxide and alkoxide groups [109]. Particularly, it is discovered that the alkoxide species is more photocatalytically reactive than the physisorbed species. Additionally, chemically dissociated alcohols may swiftly scavenge the photogenerated h+, substantially extending the lifespan of photogenerated e− [110].

3.3.4 CO2 adsorption

One of the potential options for lowering CO2 emissions and utilizing CO2 as a building block to produce valuable goods is the photochemical conversion of CO2 into solar fuels by photocatalysts like TiO2. The first stage in CO2’s photo-reduction is its adsorption on TiO2 [39, 111]. According to theoretical research, the physisorption and the most stable chemisorption of CO2 on the neutral charge of perfect anatase TiO2 (001) have adsorption energies of 9.03 and 24.66 kcal mol1, respectively, on the spin-unpolarized TiO2 with V0. This suggests that CO2 is tightly bound by V0 on a TiO2 surface that is deficient. Additionally, it is shown that the CO2 activation barrier on TiO2 with V0 is lower than it is on TiO2 with flawless anatase (001) [112]. Furthermore, it is discovered that the energetically favored conversion of CO2 to CO occurs on the surface of flawed TiO2, with V0 acting as the photocatalyst. Surprisingly, Li et al. [83] have shown that CO2 spontaneously dissociates into CO on a Cu(I)/TiO2x surface created by thermal annealing under an inert atmosphere even when it is dark. According to Figure 7, the surface V0 that provides the electrical charge as well as the locations for the adsorption of oxygen atoms from CO2 is mostly responsible for the spontaneous dissociation of CO2 in the dark. In addition, compared to those in the dark, CO2 activation and dissociation may be markedly enhanced by photoirradiation.

Figure 7.

Proposed mechanism for CO2 dissociation on the surface of Cu/TiO2 under ambient temperature in dark. Copied with permission from Ref. [83] Copyright American Chemical Society.

In conclusion, the reactant molecule’s dissociative adsorption would be facilitated by V0 on the TiO2 surface. In the photocatalytic processes, it seems that the dissociative adsorption of the reactant molecule on the TiO2 surface lowers its activation energy and influences the reaction mechanism at the molecular level. Additionally, compared to physisorbed species, chemically separated compounds have higher photocatalytic reactivity. It should be emphasized, nevertheless, that the surface of TiO2 goes through re-oxidation often in conjunction with the dissociated adsorption of the adsorbates. Therefore, before investigating its dissociation adsorption capabilities for photocatalytic application, we first need to stabilize the V0 in reduced TiO2. This may be accomplished by doping the Fe into the TiO2 nanoparticles, a procedure we covered in Section 2.5.

3.4 Reductive properties

In addition to changing the properties of adsorbates, V0 on catalyst surfaces also plays a role in the reduction of a number of these adsorbates. As demonstrated by Lu et al., one can observe the reactivity of thermally created V0 sites for the reduction of NO, CH2O, or D2O by adsorbing a test molecule on the defective surface as well as a fully stoichiometric surface and comparing the results of temperature programming desorption (TPD) [113]. By measuring the TPD, the reductive products (N2O, C2H4, and D2) are identified after adsorbing these adsorbates on the flawed surface. The oxidation of surface defect sites occurs concurrently with the deoxygenation of adsorbates. Therefore, the coverage of surface V0 is inversely correlated with the yield of reduction products. On the surface with no defects, there are no deoxygenation processes seen. It has also been shown that surface V0 sites are active in the reduction of metal ions. Our most recent study has shown that V0 plays a crucial role in the charge transfer from the damaged surface to gold ions. As a result, a very quick, direct development of metallic gold nanoparticles was accomplished on the surface of the semiconductor TiO2 containing V0. Ye and colleagues also saw the metal ions spontaneously reducing on damaged surfaces. They have shown that on the flawed surface of WO2.72, the ions of noble metals are engaged in redox processes where the metal ions partly oxidize the reduced V0 states [114]. As a result, the metal ions are quickly reduced and nucleated on their surface, where they develop into clusters and then nanoparticles. The controlled synthesis of metal/semiconductor hybrid nanomaterials may be accomplished using this approach in a single step without the need for external reducing agents, stabilizing molecules, or pretreatment of the precursors. It is interesting to note that Li et al. [115] found that the sub-stoichiometric WO3-x, which is produced by utilizing hydrogen treatment to create V0 in WO3, is stable thermodynamically at room temperature and has a strong resistance to re-oxidation. So, in the absence of catalysts for oxygen evolution reaction, hydrogen-treated WO3 is stabilized to be used for water oxidation in a neutral media. These ground-breaking findings imply that by carefully selecting the preparation techniques, the property of V0 states may be precisely regulated.

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4. Conclusion

This chapter discusses various methods that can be employed to produce defective TiO2 containing V0. These methods include thermal processing in an oxygen-depleted atmosphere, doping with non-metal or metal ions, bombardment with high-energy particles, and even in situ catalytic processes. These processes remove lattice oxygen from the surface or bulk of TiO2, resulting in a vacancy state. The presence of V0 defects provides defective TiO2 with unique chemical and physical characteristics, including enhanced reductive and dissociative adsorption properties and visible light absorption capabilities. Although the function of V0 in photocatalytic processes is still not completely understood, defective TiO2 has been shown to have benefits for a wide range of applications, such as selective charge separation and visible light response for photocatalytic activities. However, there are also conflicting claims about the role of V0 in the photocatalytic performance of semiconductors. Overall, the intentional introduction of V0 defects into TiO2 has great potential for improving the material’s properties and enhancing its performance in various applications. Further research is necessary to fully understand the impact of V0 defects on TiO2 and to explore the potential of defective TiO2 materials in emerging fields such as sensing, photoelectrochemical water splitting, and photocatalytic air purification.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Javid Khan and Lei Han

Submitted: 06 January 2023 Reviewed: 06 April 2023 Published: 02 May 2023

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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