Blinking photoluminescence properties of single TiO2 nanodiscs: interfacial electron transfer dynamics.

Blinking photoluminescence was observed in single TiO2 nanodiscs (NDs) by using a laser scanning confocal microscope (LSCM)-coupled steady-state and ps-time-resolved photoluminescence (PL) spectroscopic system, while it was not significantly observed for TiO2 quantum dots (QDs). Analysis of the PL blinking time trajectories revealed single-exponential kinetics with the average lifetimes of on-state (approximately 286 ms) and off-state (approximately 58 ms), implying the existence of inherent surface-trap sites which can be filled by photogenerated electron or hole. The PL spectra of single TiO2 NDs exhibited broad surface emissions with four decay times, which may be due to diffusion of the energies of electron or hole trap states related to surface structural changes by modification of TiO2 QDs. These results and the surface structural analysis (IR and XPS) suggests a simple model for the PL blinking of single TiO2 NDs that is based on repetitive interfacial electron transfer to the inherent surface trap sites (4Ti4+-OH) with Auger-assisted hole trapping in the multiple surface states as modified by the diffusive coordinate model and the surface-trap-filling model. Based on this blinking mechanism and kinetics, the rates of the interfacial electron transfer and the back electron transfer in TiO2 NDs were determined to be 18 ns and 58 ms, respectively, which are slow enough to keep the polarization of e-h pairs at the surface for efficient photocatalysis and photovoltaic activities. The present methodology and results may be applicable to obtain surface exciton dynamics of various photoelectronic semiconductor nanostructures.