**2. Synthetic methods**

### **2.1 Treatment under hydrogen**

A common technique to alter the photoelectrochemical and surface characteristics of TiO2 is the hydrogen treatment [54–56]. 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 *Oxygen Vacancy in TiO2: Production Methods and Properties DOI: http://dx.doi.org/10.5772/intechopen.111545*

### **Figure 1.**

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

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.

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 [61–63]. 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<sup>+</sup> ) 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<sup>+</sup> sputtering do not go away. This shows surface bridging V0 on Ar<sup>+</sup> 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+

### **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.*

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.

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, 71–74]. 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.

On the surface of several semiconductors, photochemically induced oxidation reactions, the mechanism of reaction-driven V0 production is also at work [77–81]. 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

### **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.*

detailed production method, when UV light is applied, molecular oxygen absorbs the photo-generated e�, while the h<sup>+</sup> 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<sup>+</sup> , 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:

$$\mathbf{O}\_0 \overset{\text{TiO}\_2}{\leftrightarrow} \mathbf{V}\_0 + \frac{1}{2}\mathbf{O}\_2(\mathbf{g}) + 2\mathbf{e}^- \tag{1}$$

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

$$K = [\mathbf{V}\_0] \mathbf{n}^2 p(\mathbf{O}\_2)^\ddagger \tag{2}$$

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

$$\mathbf{[V\_0]} = K \mathbf{n}^{-2} \mathbf{p(O\_2)}^{\frac{1}{2}} \tag{3}$$

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.

$$\text{Fe}\_2\text{O}\_3 \stackrel{\text{TiO}\_2}{\rightarrow} 2\text{Fe}\_{\text{Ti}} + \text{3O}\_0 + \text{V}\_0 \tag{4}$$

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.
