Mixed phosphotungstic acid/titanium oxide gels and thin solid xerogel films with electrochromic-ionic conductive properties

Abstract

Electrochromic-ion conductive gels (σ = 1.7 × 10−4 S/cm) and thin solid films composed of phosphotungstic acid (PWA) incorporated in titanium oxide xerogel (PWA:Ti = 0.07) were made via the sol-gel route by the dip-coating technique. The electrochromism of the as-deposited films was studied with the help of cyclic voltammetric (CV) measurements in HClO4 electrolyte, in situ ultraviolet-visible spectroelectrochemical measurements and ex situ near-grazing incidence angle (NGIA) reflection spectroscopy. It was demonstrated that the colour change from transparent to blue is about 40% efficient and is accompanied by effective inserted charges up to 35 mC/cm2, giving rise to HxPW12y+O403− species with x = 4.7 and y = 5.6. Ex situ NGIA Fourier transform infrared reflection measurements revealed that proton injection decreases the intensity of the longitudinal optical (LO) modes corresponding to the intra νW-Oc-W and inter νW-Ob-W vibrations of the Keggin's ions, while the νW-Od modes remain unaffected. An intensity decrease of the LO modes of TiO vibrations was also detected in the ex situ NGIA reflection spectra of cathodically coloured pure Ti xerogel films. It was found that the proton insertion process is accompanied by the hydroxylation of titanate ions, while no such effect was noted for PWA/Ti and Ti xerogel films aged in water and the corresponding electrolytes without the applied potential. The electrochromic-ionic conductive properties of mixed PWA/Ti gels were demonstrated in a semi-liquid electrochromic cell.