Two-site HO photo-oxidation on haematite photoanodes.

HO is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of HO photo-oxidation on haematite (α-FeO) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the HO concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley-Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by HO deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir-Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and HO concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.