Observation and Analysis of Surface States on TiO₂ Electrodes in Aqueous Electrolytes

When a electrode in an aqueous electrolyte is biased enough positive of its flatband potential and then photoexcited with light of wavelength shorter than 400 nm, photocurrent is observed. If the electrode is subsequently swept to more negative voltages in the dark a negative current is observed which is in excess of the normal dark current observed on a subsequent sweep with no photoexcitation between sweeps. The excess current is observed to have a maximum at a voltage positive of the flatband potential. In this paper, the growth and decay of this reduction peak are investigated experimentally. The peak is analyzed as a reduction by conduction band electrons of surface states that were oxidized by valence band holes during the photoexcitation. The observations favor the interpretation that these states are intermediates of the reaction leading to evolution. This interpretation, however, is not unequivocally established. It is clear that the conduction‐band electron reduction of a state that was previously oxidized by a valence‐band hole is in effect an electron‐hole recombination. This recombination controls the onset of photocurrent with voltage. There are 10¹³–10¹⁴ of these states per cm² and the cross section for electron interaction with this surface state is estimated to be 10⁻¹⁶–10⁻¹⁷ cm² based on the analysis used to describe the peak. The usefulness of this analysis in investigating these states and surface states due to surface coatings is discussed.