Immobilization of a [CoᴵᴵᴵCoᴵᴵ(H₂O)W₁₁O₃₉]⁷⁻ Polyoxoanion for the Photocatalytic Oxygen Evolution Reaction

Abstract

The ongoing transition to renewable energy sources and the implementation of artificial photosynthetic setups call for an efficient and stable water oxidation catalyst (WOC). Here, we heterogenize a molecular all-inorganic [CoᴵᴵᴵCoᴵᴵ(H2O)W₁₁O39]⁷⁻ ({CoᴵᴵᴵCoᴵᴵW₁₁}) Keggin-type polyoxometalate (POM) onto a model TiO₂ surface, employing a 3-aminopropyltriethoxysilane (APTES) linker to form a novel heterogeneous photosystem for light-driven water oxidation. The {CoᴵᴵᴵCoᴵᴵW₁₁}-APTES-TiO₂ hybrid is characterized using a set of spectroscopic and microscopic techniques to reveal the POM integrity and dispersion to elucidate the POM/APTES and APTES/TiO2 binding modes as well as to visualize the attachment of individual clusters. We conduct photocatalytic studies under heterogeneous and homogeneous conditions and show that {CoᴵᴵᴵCoᴵᴵW₁₁}-APTES-TiO₂ performs as an active light-driven WOC, wherein {CoᴵᴵᴵCoᴵᴵW₁₁} acts as a stable co-catalyst for water oxidation. In contrast to the homogeneous WOC performance of this POM, the heterogenized photosystem yields a constant WOC rate for at least 10 h without any apparent deactivation, demonstrating that TiO₂ not only stabilizes the POM but also acts as a photosensitizer. Complementary studies using photoluminescence (PL) emission spectroscopy elucidate the charge transfer mechanism and enhanced WOC activity. The {CoᴵᴵᴵCoᴵᴵW₁₁}-APTES-TiO₂ photocatalyst serves as a prime example of a hybrid homogeneous-heterogeneous photosystem that combines the advantages of solid-state absorbers and well-defined molecular co-catalysts, which will be of interest to both scientific communities and applications in photoelectrocatalysis and CO₂ reduction.