SURFACE CHELATION OF SEMICONDUCTORS AND INTERFACIAL ELECTRON-TRANSFER

The effect of surface chelation of oxide semiconductors on interfacial electron transfer was investigated. The studies employed phenylfluorone (2,6,7-trihydroxy-9-phenylisoxanthen-3-one), PF, as a chelating agent and titanium dioxide as a semiconductor. Pt forms a chelate with surface titanium ions which was characterized by UV-vis and IR spectroscopy. With respect to the free ligand, it exhibits a red-shifted and strongly enhanced absorption in the visible. Marked changes occur also in the IR spectrum of the phenylfluorone upon surface chelation, the vibrational modes characteristic for carbonyl, hydroxyl, and carbon-carbon double bonds being most strongly affected. Electronic excitation in the visible absorption band of the chelate results in extremely rapid and efficient electron injection in the conduction band of the semiconductor. A lower limit for the rate constant of interfacial electron transfer was determined as 108 s-1. The recapture of the injected conduction band electron by PF+ was found to occur with a specific rate of 2.8 X 105 s-1. Highly efficient charge injection from the electronically excited chelate into the semiconductor was confirmed by photoelectrochemical experiments. Laser photolysis and cyclic voltametric investigations indicate that surface chelation by PF drastically accelerates the electron transfer from the conduction band of TiOz to solution acceptors such as methylviologen. © 1990, American Chemical Society. All rights reserved.