Synthesis and Characterization of Cu and N Codoped RF-Sputtered TiO2 Films: Photoluminescence Dynamics of Charge Carriers Relevant for Water Splitting

Cu-N codoped TiO2 anatase thin films with a nanocolumnar architecture have been synthesized by RF-magnetron sputtering and characterized by Raman, scanning electron spectroscopy, and X-photoelectron spectroscopy. Absorption, photoluminescence, and photoluminescence lifetimes of the prepared samples have been investigated to understand the dynamics of the photogenerated carriers in connection to both introduced defects and the modified TiO2 band structure. At low concentrations Cu is mainly present as Cu+, while at higher concentrations the Cu2+ oxidation state prevails. Nitrogen, at low concentration and without the presence of copper dopant, substitutionally replaces oxygen to form a O-Ti-N linkage. With increasing concentration, interstitial nitrogen and Ti-O-N and Ti-O-N-O linkages are observed. In all codoped samples nitrogen is present as both interstitial and substitutional dopant. From photoluminescence spectra it is observed that nitrogen, in cooperation with Cu, more heavily affects the oxide structure, through Ti-N linkages, in such a way to quench the TiO2 exciton luminescence through charge trapping or energy transfer mechanisms. Time-resolved PL analysis evidenced that Cu-N codoping hinders the exciton radiative recombination in the anatase network, giving rise to increase of both the mean lifetime and trapping rate on defects at the nanocolumn surface.