Nickel oxide is a promising electrochromic (EC) material, and it is a material that changes color upon undergoing electrochemical oxidation and reduction. In this study, we prepared nickel oxide thin films by sputtering a nickel metal target using water vapor (H2O) as a reactive gas, and investigated the effects of substrate cooling and water vapor flow ratio in the atmosphere of argon (Ar) gas, RH2O = H2O/(Ar + H2O). High and constant target voltage and plasma emission peaks of Ni atoms were obtained up to RH2O = 75% at ‒80 °C, indicating that the Ni target maintained a metallic state. The deposition rate increased with increasing RH2O when the substrates were cooled, and a maximum deposition rate of 34.6 nm min−1 was obtained at RH2O = 50%. The nickel oxide thin films that were formed in the metallic target mode showed high transmittance in the as-deposited state and good EC properties.
Nickel hydroxide [Ni(OH)2] is an electrochemically-active material used for rechargeable batteries, electrochemical capacitors, and electrochromic devices. Although there have been some studies on nickel hydroxide thin films deposited by sputtering, the Ni(OH)2 formation has not been fully confirmed. In this study, a Ni metal target was reactively sputtered in atmospheres of O2 and Ar + H2O at substrate temperatures of room temperature (RT, around 20 °C), −80 °C, and −170 °C, and the aging treatment effects in the air at RT were studied. From optical, X-ray diffraction, and infrared absorption measurements, β-Ni(OH)2 thin films were found to be formed after aging the films deposited at −80 °C in Ar + H2O, however, NiO thin films were formed at RT. These results corresponded well with a thermodynamic consideration of Ni(OH)2. At −170 °C, mixed metal and oxide films were formed, presumably because of insufficient Ni oxidation.
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