Toward Efficient Electrochromic NiO x Films: A Study of Microstructure, Morphology, and Stoichiometry of Radio Frequency Sputtered Films

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Abstract
In this work, films were deposited by reactive radio frequency sputtering, varying either the oxygen flow or the power during deposition. The morphology was studied by atomic force microscopy. The Ni/O and Ni/H ratios were determined by Rutherford backscattering spectroscopy and forward recoil spectroscopy. The microstructure of pristine and colored or bleached films were investigated by means of X‐ray diffraction. The electrochromic and mechanical behavior in aqueous alkaline electrolyte were investigated in situ. Samples deposited at low oxygen flow (or high power) are transparent, and have the highest Ni/O ratio and the lowest Ni/H ratio. The effective area is also greater for these samples. Samples deposited at high oxygen flow (or low power) are dark brown and present the lowest Ni/O ratio and the highest Ni/H ratio. For all samples, the Ni/O ratio is lower than 1. All films were cubic NiO, with preferred orientation in the (111) direction. Lattice parameters and crystalline grain size increase with the increase of oxygen flow (or decrease of power) during deposition. The highest optical contrast and lowest stress changes upon intercalation were obtained for samples deposited at low oxygen flow. Nonintentional, low‐size dopants, like H, favor the mechanical stability of the films.

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