Solution-State Catalysis of Visible Light-Driven Water Oxidation by Macroanion-Like Inorganic Complexes of γ-FeOOH Nanocrystals
Yan Duan, Biswarup Chakraborty, Chandan Kumar Tiwari, Mark Baranov, Tal Tubul, Nitai Leffler, Alevtina Neyman, Ira A. Weinstock
Abstract
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 CoOxHy and NiOxHy electrocatalysts. 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-PW11O39FeIII]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 {H3IVIIO6}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 {H3IVIIO6}2–, indicative of a chain mechanism. A solvent kinetic isotope effect (KIE), kH/kD, of 1.32 was assigned to the rate-limiting trapping of photoexcited electrons by {H3IVIIO6}2–, which initiates a radical-chain process inhibited by added iodate [IVO3]−. 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.