Photoluminescence imaging of defects in TiO2: The influence of grain boundaries and doping on charge carrier dynamics

Abstract Understanding the mechanisms of charge generation and their recombination in rutile TiO2 is of key importance in the design of optoelectronic and photocatalytic devices. In this study, we investigate the impact of both extrinsic and intrinsic defects on photoluminescence (PL) decay dynamics. The exploitation of two-photon fluorescence lifetime imaging microscopy (FLIM) enabled differentiation to be made between photoluminescence originating from dislocations and bulk in Nb-doped rutile TiO2. It was found that dislocations pinned at grain boundaries feature lower photoluminescence intensity and faster decay times (by 100 ps) than those in the bulk. This can be reversed upon reduction, whereby trap states are preferentially formed near to dislocation sites. We also evaluated the dependence of the extrinsic doping level on the charge carrier dynamics of rutile, and show that PL lifetimes are governed by predominant Auger processes that are insensitive to reduction, unlike dislocations. We believe that the oxygen deficiency suppression of charge carrier recombination at grain boundaries is the key factor in improving the photocatalytic activity of real TiO2-based materials.