Factors that Control the Direction of Excited-State Electron Transfer at Dye-Sensitized Oxide Interfaces

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 energetic overlap of the metal oxide and sensitizer redox-active states and their 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 and [Ru(bpz)2(P)]Br2, 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-] is reported for ITO and SnO2. Hole transfer became more efficient with negative applied potentials. The direction of electron flow between the metal oxide and excited state sensitizer was correlated with the energetic overlap and the electronic coupling as predicted by Marcus-Gerischer theory. The data reveal that control of the Fermi level enables conductive oxides to function as a photocathode or as a photoanode for solar energy conversion applications.