Abstract
Current-potential and ac impedance measurements were used to characterize the semiconductor/solution interface for TiO{sub 2-x} and TiO{sub 2-x}F{sub x} single-crystal electrodes in anhydrous HF/NaF and acetonitrile/tetraethylammonium fluoride (TEAF) solutions. In HF/NaF, photoelectrochemical evolution of elemental fluorine occurs at potentials positive of the flatband potential, E{sub fb}, which is +0.6 V vs the H{sup +}/H{sub 2} formal potential. Photocorrosion occurs readily in this medium with TiO{sub 2-x} but not with TiO{sub 2-x}F{sub x}. No photocurrent is observed even at very positive potentials in acetonitrile/TEAF, and Mott-Schottky plots show that E{sub fb} is {minus}2.10 V vs NHE in this medium and that the valence band-edge potential is therefore negative of the F{sup {minus}}/F{sub 2} formal potential. The ideal photoelectrochemical behavior and low photocorrosion rate observed with TiO{sub 2-x}F{sub x} are attributed to a low density of interband surface states ((3-7) {times} 10{sup 11} cm{sup {minus}2}), tentatively assigned to Ti{sup 3+} deep levels. No oxygen vacancy states are detected at the TiO{sub 2-x}F{sub x} surface, in either HG/NaF or acetonitrile/TEAF, by the ac impedance technique.
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