Potential‐Modulated Reflectance Study of the Oxidation State of Iridium in Anodic Iridium Oxide Films

Abstract

In an attempt to identify the changes of oxidation state involved in the electrochromic effect of anodic iridium oxide films (AIROF's) we have obtained potential‐modulated reflectance (PMR) spectra of AIROF's at three pH values: 0.9, 6.7, and 13.0, in the wavelength range 200–650 nm and at an angle of incidence of 45°. Depending on pH, the voltammograms show up to four peaks (I–IV) in the anodic scan, the coloration‐bleaching process corresponding to the main voltammetric peak II . The PMR spectra measured at a given voltammetric feature were nearly independent of pH, which shows that (i) the chemical composition of the AIROF is largely independent of the pH of the solution in which it was grown; (ii) the reflectance signal is originated by changes of the oxidation state of iridium in the AIROF. The PMR spectra were independent of the thickness of the AIROF, and therefore were not a trivial interference effect. The PMR maximum at 580–610 nm at voltammetric peak II has been tentatively assigned to an Ir(IV) oxide or oxyhydroxide, as have the PMR maximum at 330–360 nm at the minimum between voltammetric peaks II and III and the PMR maximum at 220–260 nm at potentials at or above voltammetric peak III . Apparently an Ir(IV) oxide or oxyhydroxide predominates in the potential range between voltammetric peaks II and IV , and different transitions become possible in this oxide at increasingly positive potentials.