Enhancing the photoelectrochemical properties of titanium dioxide by thermal treatment in oxygen deficient environment

Highlights: • Oxygen vacancies in TiO₂ thin films were introduced by the thermal treatment in oxygen deficient environment. • Oxygen vacancies are effective to shift the optical absorption in visible region. • The up-shift of the Fermi level and more negative flat band potential were obtained in thermal treated TiO₂ films. • Thermal treated TiO₂ thin films exhibits fast charge separation at the semiconductor/electrolyte interface. • Enhanced photoelectrochemical performance of TiO₂ thin films in the presence of oxygen vacancies. - Abstract: The effect of thermal treatment on TiO₂ thin films under oxygen deficient environment (5% H₂ in Ar) at partial pressure of 2 × 10⁻² Torr have been studied for photoelectrochemical (PEC) water splitting application. Thermal treatment in anatase TiO₂ thin films exhibits a shift in optical absorption from UV to visible region and activates TiO₂ for water splitting application under visible light. X-ray photoelectron spectroscopy results showed that the thermal treated thin films contain oxygen vacancies, which suggests improved charge transport. Optical absorption, X-ray spectroscopy (XPS) and Kelvin probe force microscope (KPFM) studies show reduction in band gap by 0.36 eV, shift in valence band maximum by 0.49 eV towards the Fermi level and work function values by 0.3 eV towards the vacuum level. The pristine TiO₂ thin films exhibit very less photoactivity in terms of photocurrent density, whereas thermally treated thin films displayed a markedly enhanced photocurrent density of ∼2.41 mA/cm² at 0.23 V vs. Ag/AgCl. Higher values of photocurrent density in thermal treated TiO₂ films have been explained in terms of change in the optical and electrical properties along with energy band diagram.