# Green Method of Doping Photochromic TiO2
## 3 Results
### Sol Characteristics  
Information on the phase composition of TiO\({}_{2}\) was obtained via XRD and Raman spectroscopy (Figure 3). Figure 3a shows the diffraction patterns of the samples before and after plasma treatment. All the peaks belong to the brookite phase (JCPDS 29-1360). There are no peaks related to other phases or impurities. A change in the ratio of the main brookite peaks intensity is noted at 12.4\({}^{\circ}\) and 24.1\({}^{\circ}\). This means the formation of lattice structure defects due to the incorporation of metal ions (doping). Therefore, plasma treatment leads to the doping of titanium dioxide.  
The Raman bands in the region of 100-850 cm\({}^{-1}\) (172, 259, 326, 396, 578, and 850 cm\({}^{-1}\)) belong to the titanium dioxide phase brookite (Figure 3b) [24]. The bands at 899 and 935 cm\({}^{-1}\) are attributed to the PVP [25]. Plasma treatment does not result in the appearance of new bands but does cause a Raman shift in the main bands. This indicates the incorporation of metal ions into the TiO\({}_{2}\) structure and the absence of oxide phases on its surface [14].  
The chemical composition and valance states of the elements present in the undoped and doped TiO\({}_{2}\) samples were analyzed via XPS. Figure 4a,b show the high-resolution XPS spectra of O 1s for undoped and Mo-doped TiO\({}_{2}\) (as an example) samples. The high-resolution spectrum of O 1s for undoped TiO\({}_{2}\) (Figure 4a) shows two components: the major peak at a binding energy 531.4 eV, which is assigned to surface hydroxyl group (O-H). The peak at  
Figure 3: XRD patterns (**a**) and Raman spectra (**b**) of undoped and doped TiO\({}_{2}\) sols.  
530.2 eV is attributed to the O\({}^{2-}\) 1s lattice ion. In the doped TiO\({}_{2}\) (Mo-doped TiO\({}_{2}\) as example), these peaks are at 530.8 eV and 529.6 eV (Figure 4b). This indicates the formation of a new charge transfer bridge during the doping process (Me-O-Ti). Figure 4c shows the XPS spectra of Ti 2p for the doped samples. The peak positions for Ti 2p\({}_{1/2}\) and Ti 2p\({}_{3/2}\) are 464.51 and 458.58, respectively, which correspond to Ti\({}^{4+}\) (Ti(OOH)) [26; 27]. In addition, there are peaks related to Ti\({}^{3+}\) (most likely in the form of Ti\({}_{2}\)O\({}_{3}\)) with binding energies of 463.21 and 457.28 eV. The XPS spectra for Nb 3d for the Nb/TiO\({}_{2}\) sample are shown in Figure 4d. Since the concentration of Nb is low, the peaks are unresolved. The major peak position is 207.31 eV, and the minor peak position is 210.2 eV. This peak is assigned to Nb 3d\({}_{5/2}\) and 3d\({}_{3/2}\) of Nb\({}^{5+}\). Figure 4e presents the XPS spectra of Mo for the Mo-doped TiO\({}_{2}\) sample. The spectra display Mo\({}^{6+}\) 3d\({}_{5/2}\) and 3d\({}_{3/2}\), Mo\({}^{5+}\) 3d\({}_{5/2}\) and 3d\({}_{3/2}\), and Mo\({}^{4+}\) 3d\({}_{5/2}\) and 3d\({}_{3/2}\) peaks. This confirms the formation of nonstoichiometric molybdenum oxide (Mo\({}_{4}\)O\({}_{11}\)) during the plasma-doping process [28]. The W 4f XPS spectra of the W/TiO\({}_{2}\) sample are shown in Figure 4f. The spectrum shows a resolved doublet owing to the W 4f\({}_{7/2}\) and 4f\({}_{5/2}\) components. Deconvolution of the peaks revealed that, in addition to the W\({}^{6+}\) state, tungsten is also present as W\({}^{5+}\) and W\({}^{4+}\). This also confirms the formation of nonstoichiometric tungsten oxide (W\({}_{18}\)O\({}_{49}\)) during plasma treatment [29].  
Figure 4: High-resolution XPS spectra of O 1s (for undoped TiO\({}_{2}\) (**a**) and Mo-doped TiO\({}_{2}\) as an example (**b**)), Ti 2p (**c**), Nb 3d (**d**), Mo 3d (**e**), and W 4f (**f**).