Structure and activity of photochemically deposited “CoPi” oxygen evolving catalyst on titania

Abstract

The cobalt phosphate “CoPi” oxygen evolving catalyst (OEC) was photochemically grown on the surface of TiO2 photoanodes short-circuited to a Pt wire under bandgap illumination in the presence of Co(NO3)2 and sodium phosphate (NaPi) buffer. Extended photodeposition (15 h) using a hand-held UV lamp readily permitted quantitative structural and electrochemical characterization of the photochemically deposited CoPi OEC on titania. The formed catalytic material was characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy experiments, illustrating the production of easily visualized micrometer scale clusters throughout the titania surface containing both cobalt and phosphate. X-ray absorption fine structure (XAFS) and X-ray absorption near edge structure (XANES) studies indicated that the newly formed material was structurally consistent with the production of molecular cobaltate clusters composed of a cobalt oxide core that is most likely terminated by phosphate ions. The oxidation state, structure, and the oxygen evolution activity of this CoPi catalyst photochemically grown on titania were quantitatively similar to the analogous electrodeposited materials on titania as well as those produced on other electroactive substrates. From pH-dependent electrochemical measurements, proton-coupled electron transfer was shown to be an important step in the oxygen evolution mechanism from the photodeposited OEC clusters on TiO2 in agreement with previous reports on other materials. Similarly, the utilization of NaClO4 as electrolyte during the controlled potential electrolysis experiments failed to maintain an appreciable current density, indicating that the catalyst was rendered inactive with respect to the one immersed in NaPi. The requirement of having phosphate present for long-term catalytic activity implied that the same “repair” mechanism might be invoked for the hybrid materials investigated here. The OEC catalyst operated at Faradaic efficiencies close to 100% in controlled potential electrolysis experiments, indicating that the holes relayed to the photodeposited CoPi are indeed selective for promoting water oxidation on titania.