# Green Method of Doping Photochromic TiO2
## 3 Results
### Photochromic Properties
In general, photochromic can be described as follows:  
\[TiO_{2}+h\upsilon\ \to\ TiO_{2}\left(e_{CB}^{-}+h_{VB}^{+}\right)\]  
\[Me_{surf}^{n+}+e_{CB}^{-}\ \to\ Mc_{surf}^{(n-1)+}\ \ \text{or}\ Mc_{surf}^{n+}+h_{VB}^{+}\ \to\ Mc_{surf}^{(n+1)+}\]  
\[Tt^{4+}+e_{CB}^{-}\ \to\ Ti^{3+}\]  
When irradiated with light, an electron and a hole are formed, which accumulate in the conduction band or participate in redox processes. If the TiO\({}_{2}\) matrix contains doping agents with maximum degrees of oxidation (for example, W\({}^{6+}\) or Mo\({}^{6+}\)), only photogenerated electrons participate in the redox process. The electrons are delocalized near the excess positive charges of dopant ions and have less affinity to participate in oxidation reactions (bleaching process) [15]. In the case of the presence of structures in which Mo\({}^{n+}\)-Me\({}^{(n-1)+}\) states exist, both a photoelectron and a photohole participate simultaneously in the oxidation-reduction process. This is confirmed by the presence of two absorption maxima in the visible region. The absorption in the region of 650 nm is responsible for trapped electrons, and the presence of an absorption maximum in the region of 430-530 nm corresponds to trapped holes [36]. Figure 6: Kinetics of the photocoloration (**a**) and dynamics of bleaching (**b**) TiO\({}_{2}\) sols before and after plasma treatment. It was previously established that underwater diaphragm discharge leads to the formation of structures of nonstoichiometric oxides of tungsten and molybdenum (W\({}_{18}\)O\({}_{49}\), Mo\({}_{4}\)O\({}_{11}\)) [35]. The presence of Mo\({}^{4+}\) in our samples was confirmed by the XPS data. This may be the reason for the increased color and accelerated discoloration (oxidation). Plasma modification can be reflected in changes in the physicochemical properties of sols. Notably, the action of the plasma led to a decrease in the viscosity of the solutions. For PVP, which forms the basis of the sol, the molecular weight is related to the viscosity via the following relationship [37]:  
\[[\eta]=4.1\cdot 10^{-5}\cdot M^{0.85} \tag{1}\]  
Measurements of the viscosity of solutions after plasma treatment made it possible to estimate changes in the molecular weights of PVP (Table 1). The action of the plasma with Mo electrodes leads to a greater effect on the Mw. A decrease in the molecular weight of PVP indicates that a depolymerization process occurred, which is induced by the action of the plasma. According to a previous work [38], PVP is effectively depolymerized in the presence of H\({}_{2}\)O\({}_{2}\). Numerous studies of the kinetics of hydrogen peroxide accumulation in plasma systems in contact with liquids have shown that the electrode material can be both a catalyst and an inhibitor of the decomposition reaction of hydrogen peroxide [39; 40; 41; 42]. The results of measuring the concentration of hydrogen peroxide under conditions of a diaphragm discharge action in distilled water with Mo, Nb, and W electrodes are also presented in Table 1. According to the obtained results, the highest concentration of hydrogen peroxide is recorded in experiments after plasma exposure with molybdenum electrodes. The lowest concentration value accumulated when using niobium electrodes. This correlates with the molecular weight values of PVP after plasma treatment with different electrodes. Based on this, it can be concluded that the depolymerization process during plasma treatment occurs with the participation of H\({}_{2}\)O\({}_{2}\). Reducing the molecular weight of PVP leads to an increase in the permeability of the polymer to oxygen [43], which accelerates the bleaching process.