Transient Kinetics of O₂ Evolution in Photocatalytic Water-Splitting Reaction

Abstract

Water splitting to produce H 2 and O 2 is a fundamental reaction for artificial photosynthesis on semiconductor photocatalysts. The mechanism of the multistepped reaction, especially four-electron oxidation to O 2 , has not yet been understood. Although some intermediate states have been detected in transient spectroscopy, O 2 evolution kinetics remain unknown at the end of consecutive reaction steps. We apply operando O 2 detection with a microelectrode to determine the absolute evolution rate on a highly efficient SrTiO 3 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 SrTiO 3 surface and then the latter photons drove steady O 2 evolution. When excitation light stopped, the O 2 evolution rate exponentially decayed in seconds. The microelectrode method demonstrated herein will be useful for understanding many other reaction kinetics at liquid−solid interfaces.

Microelectrode-based transient amperometry of O2adsorption and desorption on a SrTiO3photocatalyst excited under water

“…1(A) increased and then decreased in a similar manner to what was reported in ref. 27 Hence, the response in Fig. 1(A) was ascribed to photocatalytic O 2 evolution on the irradiated film.…”

“…We had observed gradual current increases and decreases in response to UV light irradiation on the same photocatalyst, 27 where the whole setup was purged with N 2 to remove background O 2 in the solution. The current changes observed in the N 2 atmosphere were quantitatively interpreted with photocatalytic O 2 evolution on the irradiated film followed by O 2 Fig.…”

“…28 Details of the experimental setup are described in ref. 27 and its ESI. A millimeter-sized photocatalyst-coated glass plate was placed in a KCl aqueous solution (0.1 mol l À1 , pH = 7) that was exposed to air in this study.…”

“…The proportional relation of O 2 concentration and electrode current was checked and confirmed. 27 The Ag wire served as a quasi Ag/AgCl reference electrode in the KCl solution. On the other hand, the oxidative reaction on the wire, Ag + h + -Ag + , as the counter electrode caused silver contamination in the solution to a limited extent.…”

“…Our simple idea is to shorten the physical distance from the place of reaction to the electrode for detection, as was conducted in scanning electrochemical microscopy (SECM) suitable for O 2 concentration mapping. [22][23][24][25][26] In our latest study, 27 we used diffusion simulations and electrochemical detection on a microelectrode to determine the absolute O 2 evolution rate with a time resolution of 0.1 s on a SrTiO 3 photocatalyst film immersed in a nitrogenpurged, aqueous KCl solution.…”