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

摘要

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 electrochemi-cally titrated by tip-generated reductant without irradiation. The IrCl6~(2-/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 × 10~3 mol~(-1) m~2 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.