Visible light sensitized photocurrent generation from electrostatically assembled thin films of [Ru(bpy)3]2+ and the polyoxometalate γ*-[W18O54(SO4)2]4−: Optimizing performance in a low electrolyte medium

Abstract

Under appropriate conditions, photocatalytic oxidation of benzyl alcohol by Dawson polyoxometalates, POM, can be sensitized by ruthenium(II) polypyridyl complexes under visible irradiation of electrostatic adducts formed between these materials. To realize the potential of such sensitization in a photoelectrochemical cell, thin film assembly of at an electrode interface of such materials is required. However, because they are electrostatically associated, the structural integrity of the Ru-POM complex can be compromised by the contacting electrolyte. Herein, thin layers of an electrostatic adduct formed between the polyoxotungstate γ*-[W18O54(SO4)2]4− and the ruthenium polypyridyl complex [Ru(bpy)3]2+ have been self-assembled on platinum electrodes for the first time, using alternate immersion layer-by-layer assembly. SEM and AFM imaging reveal that the adduct films exhibit a highly nanoporous structure with an average pore diameter of approximately 200nm; a film morphology that is clearly distinctive from films comprised of either parent ion; [(Hex)4N]4γ*-[W18O54(SO4)2] or [Ru(bpy)3](PF6)2 alone, and indicate that ion-pairing is what drives film porosity. Cyclic voltammetry of the adduct ([Ru–POM]) films was performed on Pt microelectrodes in the absence of, or presence of low levels of added supporting electrolyte to minimise any electrostatic disruption of the ionic film. Low concentrations of supporting electrolyte (1mM (Bu)4NBF4) decreased the resistance across the cell by an order of magnitude, compared with no electrolyte whilst maintaining the film integrity. Photoelectrochemical studies of [Ru–POM] films using visible light (λ>400nm) and benzyl alcohol as the sacrificial electron donor were performed in the absence of added electrolyte. Resonance Raman spectroscopy, along with the extinction of photocurrent on oxidation of the ruthenium center indicate that the benzyl alcohol oxidation is sensitized in the visible spectral region by the ruthenium center. Increasing the surface coverage by adding more layers resulted in a net decrease in photocurrent density due to ion-transport limitations. However, rates of electrochemical re-oxidation of the photochemically reduced films ([Ru–POM]red) increased exponentially with increasing applied overpotential up to the oxidation potential of the ruthenium center. The overall photocurrent was optimized at 59μAcm−2 in (1mM (Bu)4NBF4), which is almost an order of magnitude higher than currents generated under the same conditions (at 6.8μAcm−2) but in the absence of added electrolyte.