Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO2 Films: Site Specific Kinetics Revealed by Single-Particle Spectroscopy

Abstract

The presenting surface of TiO2 is one of the key factors that influence the photoinduced charge injection process from covalently bound chromophores. However, the dependence of electron transfer (ET) on TiO2 surface properties (structure, defects, and facets) remains poorly understood due to the difficulties of deconvoluting the signal from a multitude of surface binding sites in highly heterogeneous ET systems. In an effort to correlate TiO2 surface features with ET, we compare the photoinduced ET dynamics from single quantum dots (QDs) to polycrystalline TiO2 thin films (pc-TiO2) grown by atomic layer deposition (ALD) with that of porous TiO2 nanoparticle films (np-TiO2) by utilizing single-particle fluorescence spectroscopy. Unlike the broad distribution of ET rates (deduced from fluorescence lifetimes) on np-TiO2, QDs on pc-TiO2 exhibit two narrowly distributed ET rates that we attribute to reduced site heterogeneity. Variable temperature pc-TiO2 annealing studies suggest that the double-peaked distri...