Influencing factors on electrochromic hysteresis performance of ruthenium purple produced by a WO3/Tris(2,2′-bipyridine)ruthenium(II)/polymer hybrid film

A [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine)/WO3 hybrid (denoted as Ru-WO3) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy)3]2+ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, FeIII4[RuII(CN)6]3, denoted as FeIII-RuII) layer was electrodeposited on a neat WO3 film or a Ru-WO3 film from an aqueous RP colloid solution to yield a WO3/RP bilayer film or a Ru-WO3/RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that FeII-RuII is oxidized to FeIII-RuII by a geared reaction of [Ru(bpy)3]2+/3+ and FeIII-RuII is reduced by a geared reaction of HxWO3/WO3 in the Ru-WO3/RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO3/Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO3/RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO3 and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO3 and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO3 and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy)3]2+ and terminal [Ru(CN)6]4− moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy)3]2+ in the Ru-WO3 layer from 1.03 to 0.61 V by formation of the RP layer.