# Green Method of Doping Photochromic TiO2
## 1 Introduction  
Titanium dioxide is widely known as a photocatalyst, although TiO2 has a wide band gap (3.0-3.2 eV) [1]. The doping method is used to increase the photocatalytic properties under visible light irradiation. Both metals and nonmetals are used as doping agents [2]. In addition to its photocatalytic properties, titanium dioxide has photochromic properties [3]. The main problem with using TiO2 as a photochromic material lies in the low rate of coloring-bleaching processes under light irradiation. The structure of the resulting titanium dioxide may play a key role in this process. For example, the best photochromic characteristics appear in the amorphous structures of tungsten oxide [4]. Moreover, in experiments with TiO2, the amorphous structure has a negligibly low photochromic response [5]. Since TiO2 has a rather low photoresponse, its photochromic properties need to be improved by creating binary structures, doping, and making composites based on polymers and oxide structures. Currently, Ag, Cu, Mo, Nb, V, and W, as ions and oxides, are being intensively studied as doping agents that increase the photochromic characteristics of titanium dioxide. Silver nanoparticles are chosen as dopants because of their pronounced localized surface plasmon resonance in the visible region of the spectrum [6, 7]. Copper ions are used because color changes are easily controlled by the reduction process (Cu2+\(\rightarrow\)Cu+) [8]. The choice of transition metal ions (Mo, Nb, V, W) is determined by the fact that the oxide structures of these materials themselves have photochromic properties [9, 10, 11]. In a series of works by [PERSON] et al., the influence of transition metal ions (W\({}^{6+}\), Mo\({}^{6+}\), and V\({}^{5+}\)) on the photochromic properties of TiO2 (anatase) was studied [12, 13, 14]. The bleaching time decreased in the V \(>\) Mo \(>\) W series. The authors explained this effect in terms of the charge/radius of the doped ion, charge-separation efficiency, and surface acidity. The process of discoloration (oxidation) is explained by the slow release of trapped electrons. [PERSON] et al. explained the slow release of the electron by the fact that the electrons are delocalized and stabilized by the extra positive charge of Nb\({}^{5+}\)[15].  
The photochromic properties of titanium dioxide in the anatase phase or anatase/rutile mixture, which are the most stable and easily synthesized phases, have been studied. For a long time, TiO2 brookite was believed to be an unstable phase. Owing to the unique crystal lattice structure of brookite and its electronic properties, it is an ideal material for photonics,as well as for electroph photochemical applications [16; 17]. Recent studies have shown that brookite can be obtained via hydrothermal methods [18].  
Among the physical and chemical methods for producing TiO\({}_{2}\), one can distinguish the sol-gel method, which allows one to obtain an oxide of a given structure (anatase, rutile, brookite, or mixed phase), particle size, and textural properties [19]. As a doping method, the use of low-temperature plasma is the most promising [20; 21]. In this case, the electrode material acts as a source of ions or oxide structures.  
This paper presents the results of the application of low-temperature alternating-current plasma in contact with liquid as a method for doping photochromic titanium dioxide. The electric discharge excited by an alternating current source allows for the avoidance of additional designs of the rectification circuit, which simplifies its industrial applications. In this report, the doping process was carried out via underwater AC diaphragm discharge plasma. The novelty of this work lies in the plasma treatment of photochromic sol TiO\({}_{2}\) (pure brookite). In addition, a comparison of the effects of the doping element on the photochromic characteristics of the sols was carried out, and possible reasons for the improved photochromism were proposed.