Effect of O_2 Flow Concentration during Reactive Sputtering of Ni Oxide Thin Films on Their Electrochemical and Electrochromic Properties in Aqueous Acidic and Basic Electrolyte Solutions

摘要

Thin films of Ni oxide were deposited by reactive sputtering in argon/oxygen gas mixtures using O_2 flow concentrations ranging from 6 to 100% and their electrochemical and electrochromic properties were examined using dilute acidic (1M KCl + 0.5 mM H_2SO_4) and basic (1 M KOH) aqueous electrolyte solutions. An electrochromic coloration efficiency of 32 ± 5 cm~2/C was obtained for all the Ni oxide films regardless of O_2 concentration in both KCl + H_2SO_4 and KOH. The charge capacity and resultant change in the optical density of the Ni oxide films increased with O_2 concentration owing to a decrease in crystal grain size and the resultant increase in the active surface area of the NiO crystal grains. Although the interfacial capacitances of the Ni oxide films in KCl + H_2SO_4 are 2-3 times less than those in KOH, a maximum change in optical density of 0.57 was obtained in KCl + H_2SO_4 for a fine-grained Ni oxide film with a thickness of 400 nm sputtered in 100% O_2.