Electron Traps and the Stark Effect on Hydroxylated Titania Photocatalysts

Abstract

Electric fields generated by photoexcited charge carriers in TiO_2 (anatase) produce Stark effect intensity and wavelength shifts for surface TiO−H stretching vibrations. Deep electron-trapping states affect a single type of TiO−H stretch (Ti(IV)O−H at 3647 cm^(-1) ⇒ Ti(III)O−H at 3716 cm^(-1)). Shallow electron-trapping states, observed as broad absorption bands above 3000 cm^(-1), produce an apparently homogeneous electric field. Intensity changes and corresponding wavelength shifts for ν(TiO−H) are proportional to the magnitude and polarity of the electric field. O_2 is shown to reversibly abstract electrons from shallow trapping states. The results suggest that shallow electron traps are not associated with localized structures, but rather are delocalized across the TiO_2 surface. This behavior may explain the photon-dependent wettability of TiO_2 surfaces.

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