In Situ Infrared Analysis for the Process of Water Photo-oxidation on Monoclinic Bismuth Vanadate

The process of the water photo-oxidation on monoclinic bismuth vanadate (m-BiVO4) was investigated in detail by measuring in situ infrared spectra. To measure the spectra, a setup of internal reflectance was employed with a photo-electrochemical cell. A porous m-BiVO4 electrode using a transparent oxide conductive substrate was contacted with an attenuated total reflectance crystal. Electrolytes penetrated at the interface between the electrode and the crystal, due to the porous morphology of the electrode. A potential was applied under irradiation of an incident light from the back side of the electrode during the measurement of the spectra. Absorption peaks were observed in the spectra for the electrolyte of water (H216O) with generating the photo-current by the oxidation of H216O. By considering the positions of the peaks in the spectra for the electrolytes of heavy oxygen water (H218O) and deuterium oxide (D216O), it was considered that the peaks were generated by production of hydroperoxo structures on the surface of m-BiVO4 (BiOOH and/or VOOH). The structures can be related to the generation of oxygen (O2), hydrogen peroxide (H2O2), and a hydroxyl (OH) radical in the water oxidation. Finally, a plausible process was proposed for the water oxidation on m-BiVO4.