Surface and bulk properties of black, blue and transparent TiOx thin-film photoanodes for green hydrogen generation by water splitting

Abstract

This study focusses on the surface and bulk properties of Ti–O thin film photoanodes for water splitting to generate green hydrogen. Here, TiO_x thin films were deposited by reactive RF magnetron sputtering of Ti in an Ar + O₂ atmosphere. The oxygen flow rate η_O2, 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 (η_O2 = 5%) to 9 × 10⁴ ohm cm for transparent anatase TiO₂ (η_O2 = 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 η_O2. 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 μA 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 TiO_x photoanodes for efficient solar-driven hydrogen development.