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.
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| http://dx.doi.org/10.1038/s41467-018-06141-0 | DOI Listing |
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