IR reflection and raman spectroscopies of the anodic oxide films on tungsten

Abstract

Abstract In order to understand the mechanism of the electrochromic reaction of hydrated WO 3 , the oxide film formed on tungsten in sulphuric and perchloric acid solutions has been studied by in‐situ optical techniques. Ellipsometric measurements reveal that the optical property of the film changes from a transparent substance to a coloured substance at a potential of E = 0.50 V (vs. RHE). The IR reflection absorption Spectroscopy (IRRAS) shows that the anodic oxide film is strongly hydrated forming two types of OH bonds, one corresponding to the incorporated H 2 O and the other to the OH bonded with tungsten ion. The Raman Spectroscopy (RS) shows that the anodic oxide film is an amorphous form of hydrated tungsten oxide including a terminal W(VI) = O bond. The mechanism of electrochromic reaction is discussed on the basis of IR and Raman spectra which change as the potential shifts from the transparent state to the colouring state. The electrochromic process may start with a reaction between the terminal W = O and the incorporated H 2 O: W(VI) = O + H 2 O (film) + e → W(V) — OH + OH − (film) The OH − (film) formed reacts in the film with H + injected from the acid aqueous solution.