Electrochromic response, structure optimization and ion transfer behavior in viologen adsorbed titanium oxide films

Abstract

Nanostructured TiO₂ films have been fabricated using a simple wet chemistry route and the post-deposition annealing temperature has been optimized on the basis of crystal structure, particle and pore size, reflectance (diffuse) and band gap. The film annealed at 700 °C with a rutile structure, equipped with mesopores of 15 to 30 nm dimension, high diffuse reflectance (R_max = 91%) and an indirect band gap of 2.5 eV was found to be most suitable for the adsorption of bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride (viologen). The electroactive behavior of the viologen adsorbed TiO₂ film as a function of reduction level and in oxidized and neutral states has been followed by cyclic voltammetry and electrochemical impedance spectroscopy through equivalent circuits and compared with the response of blank TiO₂. It has been shown that interfacial charge transfer resistance governs ion intercalation during coloration with little contribution from the charge transfer and double layer capacitive components. The remarkable coloration efficiency (203 cm² C^{− 1}, λ = 720 nm) and good reversibility of the color-bleach process for viologen adsorbed TiO₂ films have been found to be closely linked to the microstructure of the oxide layer.