Electrochromic Hysteresis Performance of a Prussian Blue Film Arising from Electron-Transfer Control by a Tris(2,2′-bipyridine)ruthenium(II)-Doped WO3 Film as Studied by a Spectrocyclic Voltammetry Technique

A [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine)‐doped WO3 film was prepared as a base layer on a substrate by cathodic electrodeposition from a colloidal triad solution containing peroxotungstic acid (PTA), [Ru(bpy)3]2+, and poly(sodium 4‐styrenesulfonate) (PSS). A Prussian blue (PB; FeII‐FeIII) film was cathodically electrodeposited on the [Ru(bpy)3]2+‐doped WO3 film or neat WO3 film from an aqueous Berlin brown (BB; FeIII‐FeIII) colloid solution to yield a [Ru(bpy)3]2+‐doped WO3/PB bilayer film or WO3/PB bilayer film. For the spectrocyclic voltammogram (SCV) of the WO3/PB film, a redox response of Prussian white (PW; FeII‐FeII)/PB was observed at 0.11 V, however, further oxidation of PB to BB was not allowed by the interfacial n‐type Schottky barrier between the WO3 and PB layers. For the [Ru(bpy)3]2+‐doped WO3/PB film, any electrochemical response assigned to the redox of PB was not observed in the cyclic voltammogram, however, the in situ absorption spectral change recorded simultaneously showed the significant redox reactions based on PB. The SCV revealed that PW on the [Ru(bpy)3]2+‐doped WO3 film is completely oxidized to PB by a geared reaction of RuII/RuIII at 1.05 V, and that 32 % of PB formed is further oxidized to BB by the same geared reaction in the potential scan to 1.5 V. PB was completely re‐reduced to PW by a geared reaction of HxWO3/WO3 at −0.5 V in the reductive potential scan. These geared electrochemical reactions produced an electrochromic hysteresis performance of the PB film layered on the [Ru(bpy)3]2+‐doped WO3 film. Film doping: A new electrochromic [Ru(bpy)3]2+‐doped WO3/Prussian blue (PB) bilayer film was developed by a two‐step electrodeposition. The redox of the layered PB film was controlled by [Ru(bpy)3]2+‐doped WO3/PB. Prussian white (PW) was oxidized to PB by a geared reaction of RuII/RuIII and PB was re‐reduced to PW by a geared reaction of HxWO3/WO3. A novel electrochromic hysteresis based on PW/PB was produced by the geared cycle redox reactions of the [Ru(bpy)3]2+‐doped WO3/PB film (as shown in the scheme).