Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition

Fast and earth-abundant-element polyoxometalates (POMs) have been heavily studied recently as water oxidation catalysts (WOCs) in homogeneous solution. However, POM WOCs can be quite unstable when supported on electrode or photoelectrode surfaces under applied potential. This article reports for the first time that a nanoscale oxide coating (Al₂O₃) applied by the atomic layer deposition (ALD) aids immobilization and greatly stabilizes this now large family of molecular WOCs when on electrode surfaces. In this study, [{Ru^IV₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₄)₂]^10- (Ru₄Si₂) is supported on hematite photoelectrodes and then protected by ALD Al₂O₃; this ternary system was characterized before and after photoelectrocatalytic water oxidation by Fourier transform infrared, X-ray photoelectron spectroscopy, energy-dispersive X-ray, and voltammetry. All these studies indicate that Ru₄Si₂ remains intact with Al₂O₃ ALD protection, but not without. The thickness of the Al₂O₃ layer significantly affects the catalytic performance of the system: a 4 nm thick Al₂O₃ layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. Al₂O₃ layers thicker than 6.5 nm appear to completely bury the Ru₄Si₂ catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.