Surface Interrogation Scanning Electrochemical Microscopy (SI-SECM) of Photoelectrochemistry at a W/Mo-BiVO₄ Semiconductor Electrode: Quantification of Hydroxyl Radicals during Water Oxidation

Reaction kinetics and surface coverage of water oxidation intermediates at a W/Mo-BiVO4 photoanode were studied using surface interrogation scanning electrochemical microscopy (SI-SECM). Adsorbed hydroxyl radicals (OH•) were produced during water oxidation at the semiconductor surface under UV–visible irradiation and were subsequently electrochemically titrated by tip-generated reductant without irradiation. The IrCl62–/3– redox couple was used to determine the surface concentration of OH• in acidic solution. On W/Mo-BiVO4, ∼6% of the absorbed photons generate surface OH• with a coverage of 5.8 mC cm–2. Less than 1% of the irradiated photons were eventually used for water oxidation under high intensity irradiation (∼1 W cm–2) at the photoanode. Assuming that the primary decay mechanism of the adsorbed OH• on W/Mo-BiVO4 is dimerization to produce hydrogen peroxide (H2O2), the rate constant was determined to be 4 × 103 mol–1 m2 s–1. A faster decay rate of OH• was observed in the presence of excess methanol (a radical scavenger) in aqueous solution. In addition, quantitative analysis of the water oxidation processes at W/Mo-BiVO4 along with the quantum efficiency for the oxygen evolution reaction was determined using SECM.