Electron Transfer Dynamics at Dye-Sensitized SnO2/TiO2 Core-Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures

The dynamics and efficiency of photoinduced electron transfer were measured at dye-sensitized photoanodes in aqueous (acetate buffer), nonaqueous (acetonitrile), and mixed solvent electrolytes by nanosecond transient spectroscopy (TAS) and ultrafast optical-pump terahertz-probe spectroscopy (OPTP). Higher injection efficiencies were found in mixed solvent electrolytes for dye-sensitized SnO2/TiO2 core/shell electrodes, whereas the injection efficiency of dye-sensitized TiO2 electrodes decreased with the increasing acetonitrile concentration. The trend in injection efficiency for the TiO2 electrodes was consistent with the solvent-dependent trend in the semiconductor flat-band potential. Photoinduced electron injection in core-shell electrodes has been understood as a two-step process involving ultrafast electron trapping in the TiO2 shell followed by slower electron transfer to the SnO2 core. The driving force for shell-to-core electron transfer increases as the flat band potential of TiO2 shifts negatively with increasing concentration of acetonitrile. In acetonitrile-rich electrolytes, despite the larger driving force, electron injection is suppressed. Interestingly, a net negative photoconductivity in the SnO2 core is observed in mixed solvent electrolytes by OPTP. We hypothesize that an electric field is formed across the TiO2 shell from the oxidized dye molecules after injection. The intrinsic conduction band electrons in SnO2 are trapped at the core-shell interface by the electric field, resulting in a negative photoconductivity transient. The overall electron injection efficiency of the dye- sensitized SnO2/TiO2 core/shell photoanodes, measured on longer timescales, is optimized in mixed solvents. The ultrafast transient conductivity data illustrate the crucial role of the electrolyte in regulating the driving forces for electron injection and charge separation at dye-sensitized semiconductor interfaces.