## 3 Results
Notwithstanding the initial assumptions and significantly different appearance of TiHO samples, all photoelectron spectra of titanium, oxygen and valence-band electrons were nearly identical (Figure 2). More specifically, Ti 2p photoelectron spectra for all TiHO samples had two almost fully symmetrical peaks at approximately 458.7 eV and 464.4 eV. These peaks fit well with the standard Ti 2p\({}_{3/2}\) and Ti 2p\({}_{1/2}\) peak positions of Ti(IV) oxide [28]. Similarly, all acquired O 1s spectra had a strong peak at 530.0 eV with a small shoulder at 531.0-531.5 eV. The main O 1s peak corresponded well with the oxygen from Ti(IV) oxide [28], whereas the higher energy shoulder was attributed to C-O [30] and O-H groups [31] presumably coming from the adventitious surface contamination and moisture. Based solely on the XRD and XPS data, it might be assumed that the only form of titanium in TiHO samples is Ti\({}^{4+}\) and it makes up only Ti(IV) oxide. But the evident t  
Figure 2: XPS spectra of the as-deposited TiHO films: B1—red, B2—blue, B3—green. B2 and B3 samples suggest that this should not be the case. It is likely that other forms of titanium also exist, but they were not detected due to the limitations of the XRD and XPS methods. As mentioned above, the blue tint in Ti-O system traditionally is associated with the presence of Ti\({}^{3+}\)[18,19]. Considering the limits of the XRD and XPS methods, it can be concluded that other forms of titanium (presumably Ti\({}^{3+}\) next to the oxygen vacancies, or amorphous Ti\({}_{2}\)O\({}_{3}\) regions at grain boundaries) should be present in the bulk of the film. In defected TiO\({}_{2}\) and mixed TiO\({}_{2}\)-Ti\({}_{2}\)O\({}_{3}\) materials, Ti\({}^{3+}\) cations introduce new electron levels just bellow the conduction band of TiO\({}_{2}\) and result in the strong absorbance (mostly witnessed as a fall in light transmittance) above 500-600 nm [19,21,32]. At the same time, these materials demonstrate an increase in reflectance starting from 400-600 nm (the position of the lower end of reflectance increase depends on the oxygen content [21]). In order to test if as-deposited TiHO films have these optical characteristics of Ti\({}^{3+}\) oxides, optical transmittance and reflectance measurements were performed (Figure 3). Typically for standard TiO\({}_{2}\) films, above 400 nm, sample A had a stable transmittance level of nearly 80% and reflectance of approximately 20%. Films deposited with low and medium hydrogen concentrations (samples B1 and B2) had slightly lower optical transmittance, but it remained relatively stable over the whole tested range. Sample B3, on the other hand, had a considerably lower transmittance, which peaked out at 500 nm and reached just over 30%. Such transmittance spectra are typical for Ti(III) oxide [22], but there was no rise in reflectance spectra, and this feature distinguished the B3 samples from other reported blue tinted defected TiO\({}_{2}\) and/or Ti\({}_{2}\)O\({}_{3}\) films [4,21]. Another interesting characteristic of the current TiHO films is that higher hydrogen content resulted in slightly higher optical band gap values (samples B1 and B3 had band gap values of 3.25 and 3.26 eV, respectively, Figure 4) than that of standard TiO\({}_{2}\) (sample A had a band gap of 3.22 eV). Meanwhile, the reported band gap values of reduced or defected TiO\({}_{2}\) materials had a tendency to be lower [4]. Additionally, as a common trend, reduced or defected TiO\({}_{2}\) oxides have higher photocatalytic activity [27]. However, in the current study, photocatalytic MB bleaching efficiency using tinted TiHO films (samples B2  
Figure 3: Optical transmittance (**red**) and reflection (**blue**) of TiHO films. and B3) was weaker in comparison to that with samples A and B1, which were deposited using lower hydrogen fractions in the gas mixture (Figure 5).