Efficient Charge Transport through a Metal Oxide Semiconductor in the Nanocomposite Film with Tris(2,2‘-bipyridine)ruthenium(II)

A nanocomposite film of tungsten trioxide (WO₃) and [Ru(bpy)₃]²⁺ (bpy = 2,2‘-bipyridine) (denoted as Ru−WO₃) was prepared from an aqueous colloidal triad solution containing peroxotungstic acid, [Ru(bpy)₃]²⁺, and poly(sodium 4-styrenesulfonate) by an electrodeposition technique. The electrochemical features of the Ru−WO₃ film were investigated using cyclic voltammetry (CV) and potential-step chronoamperospectrometry (PSCAS) techniques, compared with those in a [Ru(bpy)₃]²⁺/Nafion (Ru−Nf) film. PSCAS data spectrophotometrically showed that Ru^II is completely oxidized for 0.3 s in the Ru−WO₃ film, in contrast to the corresponding reaction completed for 30 s in the Ru−Nf film. The apparent diffusion coefficient (D_app = (0.1−1.1) × 10^-7 cm² s^-1) for charge transport (CT) by a Ru^II/Ru^III redox in the Ru−WO₃ film was higher than that (2.4 × 10^-10 cm² s^-1) in the Ru−Nf film by 2 or 3 orders of magnitude. Activation energy (E_a) for CT (14.1 kJ mol^-1) for the Ru−WO₃ film was 3.5 times lower than that (E_a = 49.8 kJ mol^-1) for the Ru−Nf film. The lower E_a could be responsible for the faster CT in the Ru−WO₃ film than the Ru−Nf film. D_app in the Ru−WO₃ film increased linearly with an increase of the final applied potential (E_f) for PSCAS from 1.2 to 1.5 V vs SCE and saturated above E_f = 1.5 V. The mechanism for CT in the Ru−WO₃ film is proposed, in which electrons are injected from [Ru(bpy)₃]²⁺ into the conduction band (CB) of WO₃ and go through there to a collector electrode.