Electron Transfer Dynamics at Dye-Sensitized SnO 2 /TiO 2 Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures.

The dynamics of photoinduced electron transfer were measured at dye-sensitized photoanodes in aqueous (acetate buffer), nonaqueous (acetonitrile), and mixed solvent electrolytes by nanosecond transient absorption spectroscopy (TAS) and ultrafast optical-pump terahertz-probe spectroscopy (OPTP). Higher injection efficiencies were found in mixed solvent electrolytes for dye-sensitized SnO 2 /TiO 2 core/shell electrodes, whereas the injection efficiency of dye-sensitized TiO 2 electrodes decreased with the increasing acetonitrile concentration. The trend in injection efficiency for the TiO 2 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 TiO 2 shell followed by slower electron transfer to the SnO 2 core. The driving force for shell-to-core electron transfer increases as the flat band potential of TiO 2 shifts negatively with increasing concentrations of acetonitrile. In acetonitrile-rich electrolytes, electron injection is suppressed due to the very negative flat band potential of the TiO 2 shell. Interestingly, a net negative photoconductivity in the SnO 2 core is observed in mixed solvent electrolytes by OPTP. We hypothesize that an electric field is formed across the TiO 2 shell from the oxidized dye molecules after injection. Conduction band electrons in SnO 2 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 SnO 2 /TiO 2 core/shell photoanodes 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.