Thin Colour TiOx Films as Photoanodes in PEC Cells for Green Hydrogen Production

This study focusses on the surface and bulk properties of Ti–O thin film photoanodes for water splitting to generate green hydrogen. Here, TiOx thin films were deposited by reactive RF magnetron sputtering of Ti in an Ar + O2 atmosphere. The oxygen flow rate ηO2, was varied to grow a sequence of TiO, Ti2O3 and TiO2 layers, as determined by X-ray diffraction. The spectral dependence of the optical absorption coefficient reveals a significant colour evolution, which is due to the interference of light, as well as black appearance, resulting from strong absorption within the visible range. Electrical resistivity from impedance spectroscopy increased from 5.2 × 10−2 for black TiO (ηO2 = 5%) to 9 × 104 ohm cm for transparent anatase TiO2 (ηO2 = 30%). X-ray photoelectron spectra were collected at different photon energies, 200 and 1200 eV above the O 1s and Ti 2p core levels, probing the surface and subsurface states, respectively. The depth distribution of the OH–Ti3+ defects indicated their increased surface/ subsurface concentration at higher ηO2. X-ray absorption spectroscopy (XAS) showed that the crystal field splitting increased from 1.7–2.1 eV to 2.2–2.3 eV as the amount of Ti3+ states decreased from 20% to 10%. Surface photovoltage (SPV) and the photoelectrochemical performance were correlated. The anatase/rutile mixture or pure anatase TiO2 photoanodes with the highest SPV values of about 270 mV demonstrated the best combination of high negative flat band potential (−650 mV), photocurrent density (350 mA cm−2 at 0 V vs. Ag/AgCl) and a reasonable shape factor (0.75). These findings highlight the critical role of surface-sensitive characterization in optimizing TiOx photoanodes for efficient solardriven hydrogen development.