Electron Localization and Transport in SnO₂/TiO₂ Mesoporous Thin Films: Evidence for a SnO₂/SnTi_1-O₂/TiO₂ Structure

A study of SnO₂/TiO₂ core/shell films was undertaken to investigate the influences of shell thickness and post deposition sintering on electron localization and transport properties. Electrochemical reduction of the materials resulted in the appearance of a broad visible-near IR absorbance that provided insights into the electronic state(s) within the core/shell structures. As the shell thickness was increased from 0.5 to 5 nm, evidence for the presence of a SnTi_1-O₂ interfacial state emerged that was physically located between the core and the shell. The lifetime of photoinjected electrons increased with the shell thickness. Electron transport occurred through the SnO₂ core; however, when materials with shell thicknesses ≥2 nm were annealed at 450 °C, a new electron transport pathway through the shell was evident. The data indicate that these materials are best described as SnO₂/SnTi_1-O₂/TiO₂ where electrons preferentially localize in a SnTi_1-O₂ interfacial state and transport through SnO₂ and annealed TiO₂ (if present). The implications of these results for applications in solar energy conversion are discussed.