Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO₂ Photoanode for Water Oxidation in Acid

We constructed a photoanode comprising the homogeneous water oxidation catalyst (WOC) Na₈K₈[Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃] (CoPOM) and nanoporous n-type TiO₂ photoelectrodes (henceforth "TiO₂-CoPOM") by first anchoring the cationic 3-aminopropyltrimethoxysilane (APS) ligand on a metal oxide light absorber, followed by treatment of the metal oxide-APS with a solution of the polyoxometalate WOC. The resulting TiO₂-CoPOM photoelectrode exhibits a 3-fold oxygen evolution photocurrent enhancement compared to bare TiO₂ in aqueous acidic conditions. Three-element (Co 2p, W 4f, and O 1s) X-ray photoelectron spectroscopy and Raman spectroscopy studies before and after use indicate that surface-bound CoPOM retains its structural integrity throughout all photoelectrochemical water oxidation studies reported here. Extensive charge-transfer mechanistic studies by photoelectrochemical techniques and transient absorption spectroscopy elucidate that CoPOM serves as an efficient WOC, extracting photogenerated holes from TiO₂ on the picosecond time scale. This is the first comprehensive mechanistic investigation elucidating the roles of polyoxometalates in POM-photoelectrode hybrid oxygen evolution reaction systems.