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 (Al 2 O 3 ) 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 IV 4 (OH) 2 (H 2 O) 4 }(γ-SiW 10 O 34 ) 2 ] 10– ( Ru 4 Si 2 ) is supported on hematite photoelectrodes and then protected by ALD Al 2 O 3; 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 Ru 4 Si 2 remains intact with Al 2 O 3 ALD protection, but not without. The thickness of the Al 2 O 3 layer significantly affects the catalytic performance of the system: a 4 nm thick Al 2 O 3 layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. Al 2 O 3 layers thicker than 6.5 nm appear to completely bury the Ru 4 Si 2 catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.