Molecular excited states at conductive and semiconductive interfaces were found to transfer an electron to the oxide (injection) or accept an electron from the oxide (hole transfer). The direction of this electron transfer was determined by the metal oxide electronic structure and the electronic coupling. Potentiostatically controlled mesoporous thin films based on a nanocrystalline conductive metal oxide (tin-doped indium oxide (ITO)) and semiconducting metal oxides (TiO2 and SnO2) were utilized with the sensitizers (S) [Ru(bpy)2(P)]Br2, RuP, and [Ru(bpz)2(P)]Br2, Ru(bpz), where bpy is 2,2’-bipyridine, bpz is 2,2’-bipyrazine, and P is 2,2′-bipyridyl-4,4′-diphosphonic acid. For dye-sensitized TiO2, excited state injection (TiO2|S* → TiO2(e–)|S+) was exclusively observed, and the injection yield decreased at negative applied potentials. In contrast, evidence for both injection (ITO|S* → ITO(e–)|S+) and hole transfer (ITO|S* → ITO(h+)|S–) are reported for ITO and SnO2. Hole transfer became more efficient with negative applied potentials. The direction of electron flow between metal oxide electrodes and excited state sensitizers was correlated with the oxide electronic structure, specifically the density of low energy redox active states, and the electronic coupling between the sensitizer and the oxide surface. The data reveal that control of the Fermi level enables conductive oxides to function as photocathodes or as a photoanodes for solar energy conversion applications.
Bangle, R. E.; Meyer, G. J. Factors that Control the Direction of Excited State Electron Transfer at Dye-Sensitized Oxide Interfaces. J. Phys. Chem. C 2019, 123 (42), 25967 – 25976. http://dx.doi.org/10.1021/acs.jpcc.9b06755