Ionic γ‐FeO(OH) Nanocrystal Stabilized by Small Isopolymolybdate Clusters as Reactive Core for Water Oxidation

Abstract

Abstract At near neutral to basic pH, hydrolysis‐induced aggregation to insoluble bulk iron‐oxide is often regarded as the pitfalls of molecular iron clusters. Iron‐oxide nanocrystals are encouragingly active over the molecular clusters and/or bulk oxides albeit, stabilizing such nanostructures in aqueous pH and under turnover condition remain a perdurable challenge. Herein, an Anderson‐type [Mo 7 O 24 ] 6− isopolyanion, a small (dimension ca. 0.85 nm) isolable polyoxometalate (POM) possessing only {31} atoms, has been introduced for the first time as a covalent linker to stabilize an infinitely stable and aqueous‐soluble γ‐FeO(OH) nanocore. During the hydrothermal isolation of the material, a partial dissociation of the parent [Mo 7 O 24 ] 6− may lead to the in situ generation of few analogous [Mo x O y ] n− clusters, proved by Raman study, which can also participate in stabilizing the γ‐FeO(OH) nanocore, Mo x O y @FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mo x O y ] n− , they are covalently linked via Mo VI ‐μ 2 O‐Fe III bridging to γ‐FeO(OH) core in Mo x O y @FeO(OH), established by numerous spectroscopic and microscopic evidence. Such bonding mode is more likely as precedent from the coordination motif documented in the transition metal clusters stabilized by this POM. The γ‐FeO(OH) nanocore of Mo x O y @FeO(OH) behaves as potent active center for electrochemical water oxidation with a overpotential, 263 mV @ 10 mA cm −2 , lower than that observed for bare γ‐FeO(OH). Despite of some molybdenum dissolution from the POM ligands to the electrolyte, residual anionic POM fragments covalently bound to the OER active γ‐FeO(OH) core of the Mo x O y @FeO(OH) makes the surface predominantly ionic that results in an ordered electrical double layer to promote a better charge transport across the electrode‐electrolyte junction, less likely in bulk γ‐FeO(OH).

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