Spectroelectrochemistry in the investigation of sol–gel electrochromic V₂O₅ films

Spectroelectrochemistry can offer important insights into the performance of materials that change properties during electrochemical treatment. The benefits of spectroelectrochemical measurements are shown through the example of an electrochromic sol–gel vanadium(V) oxide (V₂O₅) film. In situ UV-visible absorbance spectroelectrochemistry is indispensable to reveal the optical modulation, which reached about 28% during the first coloration in the unsafe potential range, but then diminished to around 12% during the following cycles. Ex situ IR and Raman spectroscopy were used to probe the structural modifications that occur during the intercalation of small ions into the orthorhombic structure of the crystalline V₂O₅ film. In particular, infrared reflection–absorption (IR RA) and IR absorption spectroelectrochemistry confirmed the red shift of the vanadyl band and the disappearance of the bridging oxygen band due to the puckering of layers of VO₅ square pyramids following intercalation. In addition, Raman spectra reveal the broad and low-intensity bands of the V₂O₅ film after it has been coloured in the unsafe potential range. Electron energy-loss spectroscopy (EELS) revealed the presence of vanadium in oxidation states V⁴⁺, V⁴⁺/V⁵⁺, and V⁵⁺ on different positions on the bleached film.