# Green Method of Doping Photochromic TiO2
## 3 Results
### Photochromic Properties  
Figure 5 shows the absorption spectra of the photocolored titanium oxide sols in the visible and near-infrared regions. In the visible region of the spectrum, the presence of a maximum of 530-540 nm and an unresolved band (highlighted fragment) in the region of 640-740 nm is registered. All these absorption bands are attributed to their own F color centers. This is inherent in an external defect arising from oxygen vacancies that capture two electrons [30; 31]. Notably, the doping process shifts the absorption maximum to longer wavelengths. This is typical for structures of doped titanium dioxide [32]. Two absorption bands in the near-IR region belong to the reduced state (Ti\({}^{3+}\)). A comparison of the photocoloring intensities revealed that the influence of the dopant material on the coloring intensity can be traced. The highest coloring efficiency is recorded in soils of titanium oxide doped with niobium, despite the lower content of doped material compared with that of Mo/TiO\({}_{2}\). As for the explanation of the increase in photochromism upon doping, we can assume the following. The key role in the photochromic process is played by localized defects, dislocations, and impurities. An important factor is the intervalence electron transfer between titanium ions and doping elements with different oxidation states (Ti\({}^{4+}\)-Mo\({}^{4+}\), Ti\({}^{4+}\)-Nb\({}^{5+}\), Ti\({}^{4+}\)-W\({}^{5+}\), Ti\({}^{4+}\)-Mo\({}^{5+}\), Ti\({}^{4+}\)-Nb\({}^{6+}\), Ti\({}^{4+}\)-W\({}^{5+}\), etc.) The energy level of Me\({}^{n+}\)/Me\({}^{(n-1)+}\) lies below the conduction band edge. Doping with Mo, Nb, or W ions creates energy levels below the conduction band edge that act as photoelectron traps. In addition, it is necessary to take into account that in the case of incorporation of Mo, Nb, or W ions into the TiO\({}_{2}\) structure, longer Me-O bonds are formed compared to Ti-O [33]. This also affects the intensity of photocoloration. In the case of Nb-doped TiO\({}_{2}\), the more intense coloration can be explained as follows. According to [34], the formation of Nb\({}^{4+}\) is accompanied by the formation of additional oxygen vacancies near Nb\({}^{4+}\), which act as photoelectron traps. Figure 6 presents the kinetics of the photocoloring and bleaching of the TiO\({}_{2}\) sols without and after plasma treatment. The results show that exposing the sol to a plasma accelerates the photocoloring process (Figure 6a). The process of decolorization of sols in the presence of atmospheric oxygen occurs more slowly (Figure 6b). Compared with those of sols of tungsten and molybdenum oxides, the dynamics of the discoloration of TiO\({}_{2}\) sols are slower [35]. Our results revealed that approximately half of the staining in sols after exposure to plasma disappears within the first 3 h. This is faster than the results obtained previously for W\({}^{6+}\)/TiO\({}_{2}\) (50% discoloration for 4 h) [14]. Complete discoloration was  
Figure 5: Visible and NIR absorption spectra of the irradiated sol before and after plasma treatment. recorded after 20 h. A previous work reported that titanium dioxide doped with tungsten bleaches in 24-26 h [14] and that Mo\({}^{6+}\)/TiO\({}_{2}\) bleaches for 3 days [13]. Our data contradict previously published results [13; 14; 15]. The reason for the discrepancy in our results may be a combination of several factors: (1) the phase composition of TiO\({}_{2}\) itself, (2) the degree of oxidation of the doping agent, and (3) the modification of the sol during plasma treatment. Early studies investigated the effect of doping on the photochromic characteristics of titanium dioxide with an anatase structure or mixed-phase titanium dioxide (anatase, rutile, or brookite) [4; 6; 7; 12; 13; 14]. This work demonstrates for the first time the photochromic effect of pure brookite. On the other hand, the oxidation state of the doping agent can also affect the photochromic properties.