Photoacoustic and Photoelectrochemical Current Spectra of Highly Porous, Polycrystalline TiO₂ Films Fabricated with Different Applied Voltage Treatments

We report the effect of voltage 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. The PA signal intensities of the TiO₂ films with different applied voltage treatments are higher than those without the treatments below the band-gap region, suggesting an inner-band transition due to an increase in carrier concentration by the voltage treatments. The PA intensity below the band-gap region increases with the increase of the applied voltage, indicating the increase in the carrier concentration due to the formation of donor levels by partially reduced Ti ions (Ti⁴⁺→Ti³⁺). The PEC spectra for the applied voltage treatments with -1.5 V and without the treatments show broad bands at approximately 3.31 eV. The PEC spectra for the applied voltage treatments over -1.5 V show two peaks at approximately 3.31 eV and 3.88 eV. The PEC intensities at each peak position increase rapidly above the applied voltage of -1.5 V (eight times larger for the voltage of -3.5 V than those without the treatments), which is similar to that of the PA intensity at a photon energy of 2.0 eV. The increase of the PEC intensity with different applied voltage treatments implies an increase in carrier concentration due to the formation of donor levels by the treatments. These results suggest that TiO₂ electrodes suitable for the sensitization by dyes or quantum-sized semiconductors can be obtained by applying the voltage treatments.