Evolution of surface properties of titanium oxide thin films

Abstract

Titanium oxide thin films were deposited by rf magnetron sputtering of Ti metallic target in flow-controlled O2 + Ar atmosphere. Comprehensive investigations of crystallographic structure, phase composition, morphology and optical properties were performed by means of X-ray diffraction, X-ray reflectivity (XRR), scanning electron microscopy, optical spectrophotometry, and X-ray photoemission spectroscopy. It was found that as the O2/(O2 + Ar) flow ratio increased, the microstructure of thin films changed from TiO, Ti2O3, to rutile TiO2, rutile-anatase mixture and, finally, to pure anatase. Special emphasis was given to soft X-ray absorption spectroscopy in surface-sensitive total electron yield mode, which provided simultaneous information on electronic occupancy and local atomic structure. Studies of titanium L23 edge revealed systematic changes in crystal field strength, increasing upon rutile formation and decreasing when anatase became the dominant phase in sputtered films. Diffuse reflectance and XRR were employed to investigate the light scattering effects related to the surface roughness. The highest value (5.3 nm) of XRR roughness was observed for pure rutile sample. Performance of films as photoanodes in photoelectrochemical cells was assessed by photocurrent vs potential and time measurements. Results revealed that crystallized, stoichiometric films containing a mixture of anatase and rutile were promising photoanode materials for green hydrogen generation.