Electrochemical Screening of Tungsten Trioxide–Nickel Oxide Thin Film Combinatorial Library at Low Nickel Concentrations

The electrochemical behavior of a tungsten trioxide–nickel oxide (WO₃–NiO) thin film library was investigated using scanning droplet cell microscopy (SDCM) in 0.1 mol dm^–3 sodium perchlorate (NaClO₄) solution. The WO₃–Ni film library was deposited by thermal coevaporation on an indium tin oxide (ITO)-coated glass substrate in an atomic Ni concentration range from 2.8 to 15.6 at. %. After an oxidation/crystallization heat treatment, the Ni was oxidized and the crystal structure of WO₃–NiO was transformed from monoclinic WO₃ (3.5 at. % Ni) to cubic WO₃ (up to 7.1 at. % Ni) and again to monoclinic WO₃ when the Ni amount increased (>11.8 at. %). Proton (H⁺) intercalation (cathodic reaction) and deintercalation (anodic reaction) into the WO₃–NiO mixed phases was induced. Electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S) analysis revealed that the WO₃–NiO film has n-type bilayer capacitive property, with the outer capacitive layer having a higher defect density than the inner capacitive layer. With a Ni concentration of 7.1 at. %, the WO₃–NiO film was the most defective in the library. Introduction of the Ni cation into the WO₃ network was associated with changes of the semiconducting properties of the film.