Transient Kinetics of O2Evolution in Photocatalytic Water-Splitting Reaction

Abstract

Water splitting to produce H2 and O2 is a fundamental reaction for artificial photosynthesis on semiconductor photocatalysts. The mechanism of the multistepped reaction, especially four-electron oxidation to O2, has not yet been understood. Although some intermediate states have been detected in transient spectroscopy, O2 evolution kinetics remain unknown at the end of consecutive reaction steps. We apply operando O2 detection with a microelectrode to determine the absolute evolution rate on a highly efficient SrTiO3 photocatalyst film casted on a glass plate. The evolution rate was determined with a time resolution of 0.1 s, which was improved by 1000 times compared with that in widely used gas-chromatographic detection. The observed rate did not respond instantaneously to excitation light irradiation. When light was turned on, the photocatalyst film was inactive for evolution and light-activated in seconds. It was proposed that the first absorbed photons were consumed to fill trap states on SrTiO3 surface and then the latter photons drove steady O2 evolution. When excitation light stopped, the O2 evolution rate exponentially decayed in seconds. The microelectrode method demonstrated herein will be useful for understanding many other reaction kinetics at liquid-solid interfaces.