Solution-State Catalysis of Visible Light-Driven Water Oxidation by Macroanion-Like Inorganic Complexes of γ-FeOOH Nanocrystals

Recent investigations reveal that by providing active sites for O-O bond formation, Fe(III) oxyhydroxides (FeOOH) dramatically enhance the oxygen evolution activities of iron-containing abundant-earth CoOxHyand NiOxHyelectrocatalysts. In contrast to α-Fe₂O₃(hematite), however, little detailed information is available concerning fundamental reactivities of the Fe(III) oxyhydroxides themselves. We here report a macroanion-like polyoxometalate cluster-anion complex of 2.6 nm γ-FeOOH nanocrystals, 1 , that not only catalyzes visible light-driven water oxidation with no need for added photosensitizers but also whose unique stability and solubility facilitate investigation of oxygen evolution using the toolbox of solution-state methods typically reserved for molecular catalysis. The γ-FeOOH active centers of 1 are comprised of ca. 250 Fe atoms and coordinated by an average of six oxo-donor ligands, [α-PW₁₁O₃₉Fe^III]^4--μ-O^-, each with a formal charge of 5-, giving freely diffusing macroanion-like hexacoordinate complexes readily observed in their native, vitreous water solution state by cryogenic TEM. With a bandgap energy of 2.3 eV and valence- and conduction-band (VB and CB) energies of 2.34 and 0.04 V vs NHE, 1 catalyzes visible light-driven water oxidation by orthoperiodate {H₃I^VIIO₆}^2-at pH 8, at a rate similar to that documented for hematite nanocrystals. Kinetic data show the reaction to be one-half order in concentrations of both 1 and {H₃I^VIIO₆}^2-, indicative of a chain mechanism. A solvent kinetic isotope effect (KIE),k_H/k_D, of 1.32 was assigned to the rate-limiting trapping of photoexcited electrons by {H₃I^VIIO₆}^2-, which initiates a radical-chain process inhibited by added iodate [I^VO₃]⁻. In contrast to the rate-determining O-O bond formation typical of metal-oxide electrocatalysts and of many molecular catalysts, chain mechanisms initiated by the rate-limiting trapping of excited-state electrons may prove a general feature of water oxidation by freely diffusing photoactive nanocrystals.