Evidence for an Electronic State at the Interface between the SnO2 Core and the TiO2 Shell in Mesoporous SnO2/TiO2 Thin Films

A comparative spectroelectrochemical study of micron thick mesoporous thin films composed of TiO2 and SnO2 nanocrystallites was undertaken to identify the location and identity of electronic state(s) in core/shell SnO2/TiO2 films that have emerging applications in energy science. The mesoporous TiO2 and SnO2 films were synthesized by sol–gel techniques, and the core/shell material was created by atomic layer deposition of a TiO2 shell on a SnO2 core. Electrochemical reduction of these materials in aqueous electrolytes resulted in a blue shift of the fundamental VB → CB absorbance as well as a broad absorbance across the visible region. For SnO2, three unique spectra were required to model the potential dependent data while only one unique spectrum was needed to model the corresponding data for SnO2/TiO2 and TiO2. Significantly, standard addition of the spectra measured for SnO2 and TiO2 did not model that for SnO2/TiO2. In other words, upon reduction of SnO2/TiO2 there was no spectroscopic evidence for electrons residing in the SnO2 core or in the TiO2 shell. Instead, the appearance of a single potential-independent absorption spectrum was observed and attributed to electrons present within an interfacial region between the core and the shell. The charge extraction technique was used to determine that the density of SnO2/TiO2 states was more similar to that of TiO2 than SnO2. The potential onset followed the trend SnO2/TiO2 > SnO2 > TiO2, with TiO2 closest to the vacuum level and SnO2/TiO2 the furthest away. The implications of these results for solar energy conversion are discussed.