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

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 [Mo₇O₂₄]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 [Mo₇O₂₄]⁶⁻ may lead to the in situ generation of few analogous [Mo_xO_y]ⁿ⁻ clusters, proved by Raman study, which can also participate in stabilizing the γ-FeO(OH) nanocore, Mo_xO_y@FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mo_xO_y]ⁿ⁻, they are covalently linked via Mo^VI-μ₂O-Fe^III bridging to γ-FeO(OH) core in Mo_xO_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_xO_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_xO_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).