## 2 Materials and Methods  
TiHO samples were deposited using a custom modified [PERSON] physical vapor deposition system equipped with cryo pump and one unbalanced Torrus 3 magnetron. The magnetron had a titanium target (76 mm diameter, 6 mm thickness, 99.99% purity) and was powered by a 300 W DC power source. Ar, O\({}_{2}\) and H\({}_{2}\) gases (all 99.999% purity) were supplied by three mass flow controllers that were operated by a Nova Fabrica FloTron X multi-channel process control system. Three different Ar-O\({}_{2}\)-H\({}_{2}\) gas mixtures were used to deposit titanium oxyhydride films with varying hydrogen concentrations. The hydrogen concentration was increased, going from sample B1 to sample B3 (more details are provided in Table 1). In addition, an Ar-O\({}_{2}\) gas mixture was used to deposit conventional titanium dioxide films (samples A). For the deposition of later samples, the same experimental set-up was used. The only modifications to the process were the annulment of hydrogen gas supply and introduction of a pulsed direct-current (p-DC) power source. In the Ar-O\({}_{2}\) gas mixture, the p-DC power source allowed us to prevent titanium target charging/poisoning and improved reactive magnetron sputtering process stability (in the Ar-O\({}_{2}\)-H\({}_{2}\) gas mixture, the Ti target was continuously reduced by hydrogen; therefore, for TiHO deposition, the p-DC power source was not needed). Deposition time for all group B samples was the same--1 h, but their deposition rate varied significantly depending on the working gas mixture composition. Accordingly, the ultimate thickness of group B samples reached approximately 700 nm for B1, 900 nm for B2 and 1500 nm for B3. Group A samples were deposited for 2 h, and their thickness was approximately 1500 nm.  
Crystal structures of the deposited films were determined by X-ray diffraction (XRD, Bruker D8). Elemental composition and chemical states of the TiHO films were analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5000 Versaprobe). To minimize the extent of additional sample surface oxidation and contamination by adventitious hydrocarbons, XPS analysis was carried out straight after the samples were transferred from the deposition chamber to the XPS instrument. Optical transmission and reflectance of the films were acquired by UV-VIS spectrophotometer. To estimate band gap energies of the samples, corresponding Tauc plots were drawn. Since it was not easy to split the Kubelka-Munk spectrum into spectra of individual components (in this case, films and their substrates), a simplified band gap estimation method described in [16] was applied. By this method, at first, the traditional linear part of the plot was extrapolated to the \(x\)-axis, and an additional linear fit was applied for the slope below the fundamental absorption. Then, a band gap value was determined at the intersection of the corresponding two linear fittings. Photocatalytic activity of TiO\({}_{2}\) and TiHO samples was estimated using methylene blue (MB) bleaching test (adopted version of ISO 10678:2010 standard [17]). For these tests, the MB solution volume was 15 mL, and initial concentration 15 mg/L. Photocatalysis was initiated with collimated 365 nm LED light source (Thorlabs M365 LP1-C1).