G. Nogami et al., MODULATION SPECTROSCOPY OF SUBBANDGAP LIGHT IN REDUCED TIO2 ELECTRODES, Journal of the Electrochemical Society, 143(11), 1996, pp. 3600-3606
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.