Niobium-doped (001)SrTiO3 (Nb:STO) single crystal, and epitaxial anatase (001)TiO2 and (001)SrO films, which were subsequently deposited on the Nb:STO by pulsed laser deposition, were electrochemically investigated with ionic liquid (IL) in a vacuum. After being prepared by cleaning and/or depositing the films, the electrode surfaces were characterized by observing the reflection of high-energy electron diffraction, prior to the electrochemical measurements. Electrochemical impedance spectroscopy as well as cyclic voltammetry were then performed, and the electrical double-layer (EDL) capacitance at their interfaces with the IL were successfully evaluated. The EDL capacitance of IL was several μF cm−2 for Nb:STO and gradually increased to over 10 μF cm−2 with an increase of the thickness of the deposited TiO2 films. In contrast, as SrO was being deposited, the EDL capacitance gradually decreased, approaching a constant value of about 1.4 μF cm−2, which was significantly smaller than those values achieved for TiO2 films and single-crystal Nb:STO. For different oxide/IL interfaces, a possible correlation between their measured EDL capacitance and the ionicity of the oxides is discussed.
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