Fig. 2: Correlating optical, hard and soft X-ray spectroscopies to identify Ir and O oxidation processes.
a, Difference ultraviolet–visible absorbance spectra of IrOx during a linear sweep scan from 0.66 VRHE to 1.48 VRHE in 0.1-M HClO4 at a scan rate of 1 mV s−1 (iR corrected). The absorption changes are calculated with respect to the absorption at 0.66 VRHE. Absorption changes were recorded after five cyclic voltammetry cycles, at every 1 mV and shown at every 5 mV. Arrows indicate the dominant wavelengths at which the absorbance increases with potential, representing three different redox transitions. b, Ir L3-edge XANES region of electrodeposited IrOx on FTO substrate measured at different potentials in 0.1-M HClO4. Data adapted from our previous work21. c, Insets: steady-state operando NEXAFS O K-edge spectra of amorphous IrOx deposited on gold-coated SiNx substrates measured at various applied potentials in 0.1-M HClO4 electrolyte. Potential values are given versus a reversible hydrogen electrode (RHE). The data are normalized to the intensity at around 540 eV and calibrated using the pre edge of water at 535 eV (refs. 37,38). The main figure shows a zoomed-in view with energy ranging from 526 to 531 eV. Dashed lines show two peak positions at an increase of around 528.9 eV and 529.3 eV with the applied potentials. d, Comparison between the iridium oxidation state determined from the white-line position in b (left y axis), the sum of deconvoluted densities of redox transitions 1, 2 and 3 from optical spectroscopy (black, right y axis) and the fluorescence intensity at 529 eV in the O K edge over two cyclic voltammetry cycles. The y axis scales are adjusted for visual clarity.
