Modulation spectroscopy of subbandgap light in reduced TiO2 electrodes

The charge transfer dynamics across a TiO2 aqueous electrolyte interface was studied by modulation spectroscopy of subbandgap light. A chopped light from a He-Ne laser was illuminated, and the resultant signals were detected by a phase-sensitive detector (a lock-in amplifier). Although light energy(similar to 1.97 eV) is too small to generate electron-hole pairs by a band-to-band transition of TiO2 with a bandgap energy of 3.05 eV, photocurrent could be detected by the lock-in amplifier. The real and imaginary parts of the photocurrent could be well characterized by an are plot similar to the Cole-Cole plot, from the analysis of which it is concluded that the electron transfer takes place from the valence band via ch midgap states to the semiconductor bulk. It is also concluded that the defect doping under a hydrogen atmosphere introduces deep levels, penetrating deep into the TiO2 bulk.