**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

### *Oxygen Vacancy in TiO2: Production Methods and Properties DOI: http://dx.doi.org/10.5772/intechopen.111545*

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.

### *3.1.2 Geometric structure*

In addition to altering the electrical structure of TiO2, V0 also changes the material's geometric structure [87–89]. 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

### **Figure 4.**

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

### **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<sup>1</sup> and (B) 300–750 cm<sup>1</sup> , respectively. Partial Raman spectra of curves b and c between 100 and 300 cm<sup>1</sup> are shown in the left inset of panel A, while the fitted Eg mode of curve b at 100– 200 cm<sup>1</sup> is shown in the right inset of panel A. Copied with permission from Ref. [89] Copyright American Chemical Society.*

fluorine. This is supported by two new Raman modes at 155 and 171 cm<sup>1</sup> and the weaker B1g mode at 397 cm<sup>1</sup> . By simple calcination of the sample in air, the two modes at 155 and 171 cm<sup>1</sup> 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<sup>1</sup> . The surface fluorine and V0 in oxygen-deficient anatase TiO2 have synergistic effects that likely change the bonding length of the TidOdTi 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.
