Photo-electrochemical Effect in the Amorphous Cobalt Oxide Water Oxidation Catalyst Cobalt–Phosphate (CoPi)

The cobalt-phosphate amorphous oxyhydroxide water-splitting catalyst, CoPi, is widely investigated because of its reactivity and self-repair analogous to the oxygen-evolving catalyst in Photosystem II. CoPi films show optical absorption features analogous to those seen more generally in transition metal oxides and oxygen-evolving catalysts. Possible photocatalytic properties of CoPi have, up until now, not been considered. Herein, we report on the finding of wavelength-dependent photo-electrochemical responses in CoPi. Red light excitation (623 nm) into a manifold of charge transfer and d-d electronic transitions was seen to produce photoanodic current responses that enhance OER, while blue light excitation (415 nm) within the ligand-to-metal charge-transfer transition was found to produce photocathodic responses that inhibit OER and oxidative repair. These results demonstrate intrinsic wave-length-dependent photo-electrochemistry in CoPi and opportunities to use light-excited states in CoPi, and potentially other transition metal oxides, to enhance OER and to track reaction mechanisms using light-triggered, pump-probe techniques.