Charge Recombination Dynamics in Sensitized SnO2/TiO2 Core/Shell Photoanodes

Studies have been conducted to examine the mechanisms of charge recombination in dye-sensitized SnO2/TiO2 core/shell films. Nanostructured SnO2/TiO2 core/shell films varying in TiO2 shell thicknesses were prepared via atomic layer deposition and sensitized with a phosphonate-derivatized ruthenium chromophore [Ru(bpy)(2)(4,4'-(PO3H2)(2)bpy)](2+). Transient absorption spectroscopy was used to study the interfacial charge recombination dynamics for these core/shell materials. Charge recombination for sensitized, as-deposited SnO2/TiO2 core/shell systems is dominated by a tunneling mechanism for shell thicknesses between 0 and 3.2 nm, with beta = 0.25 angstrom(-1). For shell thicknesses greater than 3.2 nm, recombination primarily proceeds directly via electrons localized in the relatively thick TiO2 shell. Annealing the SnO2/TiO2 core/shell structure at 450 degrees C affects the recombination dynamics substantially; charge recombination dynamics for the annealed films do not show a dependence on shell thickness and are comparable to ZrO2/TiO2 control samples, suggesting the annealing process perturbs the core/shell interface. This analysis of charge recombination dynamics indicates that there is an optimum shell thickness to maximize charge separation lifetimes in dye-sensitized core/shell photoanodes and that the nature of the core/shell interface influences the efficacy of these materials.