Effect of voltage treatment on modulation frequency dependence of the photoacoustic and photoelectrochemical current spectra of highly porous, polycrystalline TiO2 electrodes

We report the effect of voltage on a concentrated KCl electrolyte applied to a highly porous TiO2 electrode during its final preparation process on the photoelectrochemical current (PEC) spectra. Optical absorption measurements were monitored by a photoacoustic (PA) method. The PA spectrum of the TiO2 electrode with the voltage treatment is similar to that without the treatment above the band-gap region. Below the band-gap region, the intensities of the PA spectrum of the TiO2 electrode with the voltage treatment are higher than those without the treatment, suggesting an increase of carrier concentration by the voltage treatment. The PEC spectra without the voltage treatment show a monotonic increase with the photon energy above the band-gap region. The PEC intensity with the voltage treatment increases rapidly immediately above the band-gap region and shows saturated values. The PEC intensity of the TiO2 electrode with the voltage treatment is approximately twice that without the treatment above the band-gap region. The increase of the PEC intensity implies an increase of carrier concentration due to partially reduced Ti ions. The modulation frequency dependence of the PEC intensity of TiO2 electrodes with and without the voltage treatment shows two types of exponential decay. The decay rate in the TiO2 electrode with the voltage treatment is slower than that without the treatment below the modulation frequency of 100 Hz. The decay rate with the voltage treatment agrees with that without the treatment above the modulation frequency of 100 Hz.