RhOx cocatalyst for efficient water oxidation over TaON photoanodes in wide pH range under visible-light irradiation

Abstract

Because the oxidation of water is both the key and a common process in artificial photosynthesis systems, coupled with the reduction of water or carbon dioxide, the development of photocatalysts or photoelectrodes that can efficiently oxidize water to oxygen molecules (O2) under a wide range of conditions (e.g., pH) is crucial. Although the loading of CoOx cocatalysts on oxynitride (e.g., TaON) semiconductor photoanodes has been proven to boost O2 evolution significantly under basic conditions (pH > 8), their performance has been found to decrease significantly with decreasing pH, mainly owing to the dissolution of CoOx; thus, their use in CO2 reduction systems that are generally carried out under acidic solutions has been limited. Here, we demonstrate that nanoparticulate RhOx species can function as an effective cocatalyst to boost O2 evolution on TaON photoanodes under visible-light irradiation, affording not only superior activity to previous CoOx systems but also versatile performance even under acidic conditions. Although the photocurrent on the RhOx/TaON photoanode under acidic conditions (1.5 < pH < 5) was reduced to some extent (ca. 70% of that under pH 8) compared to those under basic conditions, the RhOx/TaON photoanode generated a photocurrent stably accompanied by almost stoichiometric O2 evolution (i.e., ca. 100% of Faradaic efficiency) even under the conditions where CoOx/TaON cannot (pH 1.5 and 3). In addition, the loading of the RhOx cocatalyst shifted the onset potential of the photocurrent to a more negative potential (ca. + 0.1 V vs. RHE) compared to CoOx/TaON (ca. + 0.3 V vs. RHE), consequently enabling much more efficient photoelectrochemical water splitting even at a relatively low applied bias (0.6 V vs. Pt) in the two-electrode system.