Photochemistry on the TiO2 Surface-Watching Electrons and Holes at Work

John T. Yates, Jr.
Department of Chemistry 
University of Virginia
Charlottesville, VA 22904 USA

     Titanium dioxide is widely used to harvest solar energy for both photovoltaic cells and in environmental remediation where solar-driven oxidation chemistry of organic molecules is employed. The photochemistry induced on TiO₂ surfaces requires the understanding of the activation of electron-hole pairs by a photon and the destruction of these charge carriers by recombination processes at defect sites in the semiconductor.

     We have employed a simple photochemical process, the hole-mediated desorption of molecular O₂ from TiO₂(110) surfaces, as a means of understanding the essential photo-production and destruction of electrons and holes in the crystal. By these studies, we have been able to separate recombination processes in the bulk from surface recombination processes.

     The transfer of electrons and holes from the TiO₂ bulk to an adsorbed molecule can be profoundly modified by controlled band bending and examples of such modification by adsorbates such as Au atoms will be presented. In addition, by manipulating band bending upwards or downwards, we have been able to show that the oxidation chemistry of adsorbed CO on TiO₂(110) is mediated by electrons, probably through the production of a surface peroxide species.

    The talk will end with a discussion of the photoluminescence (PL) of powdered TiO₂. Photo-luminescence occurs when photoexcited electrons and holes recombine. Here the photoluminescence signal is strongly affected by band bending effects which modify the depletion layer thickness. We have combined PL spectroscopy with IR spectroscopy of adsorbed molecules to understand the details of band bending effects caused by well controlled adsorption as well as by surface charging.