Photoelectrochemistry and interfacial energetics of titanium dioxide photoelectrodes in fluoride-containing solutions

In a preliminary report, the authors described the photoelectrochemical production of fluorine from anhydrous hydrogen fluoride solutions using single-crystal TiO₂ electrodes. It was found that fluorine doping substantially enhanced the stability of n-TiO₂ photoelectrodes in HF/NaF. They now report a fuller characterization of these effects and a study of the energetics of the semiconductor/fluoride-containing solution interface by ac impedance techniques. This study reveals an extremely low density of midgap surface states, which allows the electrode to develop a large open circuit photovoltage and to evolve fluorine efficiently, and without substantial photoanodic corrosion, from anhydrous hydrogen fluoride solutions. This same low density of surface states, and a shift of the valence band-edge to more negative potentials, gives rise to a complete lack of photoactivity in fluoride-containing acetonitrile solutions.