Photoacoustic and photoelectrochemical current response of nanostructured TiO2 electrodes

We report the effect of voltage in a concentrated KCl electrolyte applied to nanostructured TiO2 electrodes during their final preparation processes on photoacoustic (PA) and photoelectrochemical (PEC) current spectra and their modulation frequency dependence to clarify their response characteristics. The PA signal intensities of the TiO2 electrodes with different applied voltage treatments are higher than that without the treatments below the fundamental absorption edge. These results suggest an inner-band transition due to an increase in carrier concentration owing to donor levels formed by partially reduced Ti ions (Ti4+ --> Ti3+) following the applied voltage treatments. The PEC spectra for the applied voltage treatments above -1.5 V show broad bands at approximately 3.3 and 3.8 eV. The intensities of all peaks increase rapidly at an applied voltage above -1.5V, which is similar to that of the PA intensity at a photon energy of 2.0eV. The increase in the PEC intensity with different applied voltage treatments implies an increase in carrier concentration due to donor level formation by the treatments. The modulation frequency dependence of the PA signal intensity is related to the applied voltage treatments below a frequency of 60 Hz. This indicates the increase in the interfacial thermal resistance to prevent heat diffusion at the interface between the nanostructured TiO2 film and the Ti substrate with the increase in applied voltage. The modulation frequency dependence of PEC current also depends on the applied voltage treatments, indicating the enhancement of the photoexcited electron diffusion response with the increase in applied voltage.