Effect of O₂ flow concentration during reactive sputtering of Ni oxide thin films on their electrochemical and electrochromic properties in aqueous acidic and basic electrolyte solutions

Thin films of Ni oxide were deposited by reactive sputtering in argon/oxygen gas mixtures using O₂ flow concentrations ranging from 6 to 100% and their electrochemical and electrochromic properties were examined using dilute acidic (1 M KCl + 0.5 mM H₂SO₄) and basic (1 M KOH) aqueous electrolyte solutions. An electrochromic coloration efficiency of 32 ± 5 cm²/C was obtained for all the Ni oxide films regardless of O₂ concentration in both KCl + H₂SO ₄ and KOH. The charge capacity and resultant change in the optical density of the Ni oxide films increased with O₂ 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 + H₂SO₄ are 2-3 times less than those in KOH, a maximum change in optical density of 0.57 was obtained in KCl + H₂SO₄ for a fine-grained Ni oxide film with a thickness of 400 nm sputtered in 100% O₂.