Electrochromism of phosphotungstic acid incorporated in titanium alkoxide xerogel films

摘要

Electrochromic-ion conductive gels ($sigma $EQ 1.7 $MUL 10$+$MIN@4$/ S/cm) and thin solid films composed of phosphotungstic acid (PWA) incorporated in titanium oxide xerogel (PWA/Ti $EQ 0.07) were made via the sol-gel route by the dip-coating technique. The electrochromism of the as - deposited film was studied with the help of cyclic voltammetric (CV) measurements in HClO$-4$/ electrolyte, in-situ UV-VIS spectroelectrochemical measurements and ex-situ Near-Grazing Incidence Angle (NGIA) reflection spectroscopy. It was demonstrated that the color change from transparent to blue in the electrolytic cell is about 40% and is accompanied with effective inserted charges up to 35 mC/cm$+2$/, giving rise to H$-x$/PW$-12$/$+y$PLU$/O$-40$/$+3$MIN$/ species with x $EQ 4.7 and y $EQ 5.6. Ex-situ NGIA FT-IR reflection measurements revealed that proton injection decreases the intensity of the longitudinal optical (LO) modes corresponding to the intra vW-O$-c$/-W and inter vW-O$-b$/-W vibrations of the Keggin's ions while the vW-O$-d$/ modes remain unaffected. An intensity decrease of the LO modes of Ti-O vibrations was also detected in the ex-situ NGIA reflection spectra of cathodically colored pure Ti-xerogel films. It was found that the proton insertion process is accompanied by the hydroxylation of titanate ions, while no such an effect was noted for PWA/Ti and Ti xerogel films aged in water and the corresponding electrolytes without the applied potential. Electrochromic-ionic conductive properties of mixed PSA/Ti gels were demonstrated in a semi-liquid electrochromic cell (EC) with reflectance modulation and electrochromism of PWA/Ti solid films was shown by the in- situ UV-VIS transmittance measurements of an all-solid state device with PWA/Ti / H$- 3$/PO$-4$/ doped polyvinyl alcohol (PVA) / Sb:SnO$-2$/ configuration.