Effect of voltage treatment applied on photoacoustic and photoelectrochemical current spectra in final preparation processes of porous TiO2 electrodes

We report the effect of different voltages in a concentrated KCl electrolyte applied to highly porous, polycrystalline TiO2 films during their final preparation processes on photoacoustic (PA) and photoelectrochemical (PEC) current spectra. Below the band-gap energy region, the PA signal intensities of the TiO2 films with different voltage treatment applied are higher than those without treatment and they increase with an increase of applied voltage. These experimental results suggest an innerband transition from an increase in carrier concentration due to the formation of donor levels by partially reduced Ti ions (Ti4+-->Ti3+) by the voltage treatment. The PEC current spectra for applied voltage treatment over -1.5 V show two peaks above the band-gap energy region whose intensities increase rapidly above applied voltage of -1.5 V (eight times larger for voltage of -3.5 V than those without treatment), which is similar to that of the PA intensity at a photon energy of 2.0 eV. The modulation frequency dependence of the PEC current intensity at the peak position shows different characteristics depending on the voltage treatment applied, indicating the difference in PEC current response due to the formation of donor levels by the treatment. (C) 2003 American Institute of Physics.