MODULATION SPECTROSCOPY OF SUBBANDGAP LIGHT IN REDUCED TIO2 ELECTRODES

The charge transfer dynamics across a TiO2 aqueous electrolyte interfa ce was studied by modulation spectroscopy of subbandgap light. A chopp ed light from a He-Ne laser was illuminated, and the resultant signals were detected by a phase-sensitive detector (a lock-in amplifier). Al though light energy(similar to 1.97 eV) is too small to generate elect ron-hole pairs by a band-to-band transition of TiO2 with a bandgap ene rgy of 3.05 eV, photocurrent could be detected by the lock-in amplifie r. The real and imaginary parts of the photocurrent could be well char acterized by an are plot similar to the Cole-Cole plot, from the analy sis of which it is concluded that the electron transfer takes place fr om 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.