Observation of an optical phonon band in situ in TiO2 electrochemistry: a possible indicator of strongly trapped intermediates in the O2 evolution reaction

This Letter reports in situ Fourier transform infrared (FTIR) spectroscopic data on thermal TiO2 films fabricated by heating titanium plates in air at 475, 700 and 800 °C. The films were studied in the dark and under UV-irradiation in aqueous 0.1 M NaClO4 in the presence and absence of 0.1 M Na2(OOC)2 and at 10, 25 and 50 °C. The film fabricated at 800 °C showed a broad feature near 1580 cm−1 under UV-irradiation that was not observed in the dark, whilst the films fabricated at lower temperatures, 475 and 700 °C, showed no such feature. This feature appears to be associated with the accumulation of surface-mobile holes at the complex, porous film–electrolyte interface and the capacity of such holes to enhance the absorption cross-section of optical phonons characteristic of the rutile crystal form at and near the surface of the TiO2/electrolyte interface.