Preparation of electrochemically active hydrate and hydroxide thin films by sputtering

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Hydrated Ta_2O_5 is a proton-conducting solid-electrolyte and Ni oxyhydroxide (NiOOH) is an electrochromic (EC) material. Thin films of hydrated Ta_2O_5 and NiOOH were prepared by reactive sputtering, which is one of the most widely used dry processing methods.1) A Ta metal target was reactively sputtered using H_2O as a reactive gas. Substrate temperature was cooled to -20℃ during sputtering to decrease the desorption of H_2O molecules from the films surface. The amount of hydrogen atoms incorporated into the Ta_2O_5 films was estimated by Ratherford Back Scattering (RBS) and Hydrogen Forward Scattering (HFS). It is found that hydrogen content in the Ta_2O_5 films increased with decreasing substrate temperature and a maximum H/Ta atomic ratio of 2.2 was obtained at a substrate temperature of -20℃. The film density was found to decrease with decreasing substrate temperature by X-ray Reflectivity (XRR) measurement. Corresponding to the increase of the hydrogen content and the decrease of film density, the ion-conductivity of the Ta_2O_5 films increased and a maximum ion-conductivity of 4x10^<-8> S/cm was obtained at a substrate temperature of -20℃.2) A Ni metal target was reactively sputtered using O_2+H_2O mixed gas, and the formation of NiOOH thin films was confirmed at H_2O flow ratios above 5% by Fourier Transform Infrared Spectroscopy (FTIR). EC properties of the NiOOH thin films were studied in a KOH aqueous electrolyte and a coloration efficiency of approximately 30 cm^2/C was obtained.3) All-solid-state reflective-type EC devices were fabricated using the hydrated Ta_2O_5 solid-electrolyte and the NiOOH EC layers, and reflectance change of 5:1 was obtained.From these results, it is clarified that hydrate and hydroxide thin films can be prepared by the sputtering method. In the future, we will search for new solid electrolyte materials with high-ion-conductivity and develop high-performance thin-film-electrochemical-devices.