Aerosol-assisted chemical vapor deposition
(AACVD) was used for the first time in the preparation of thin-film
electrochromic nickel(II) oxide (NiO). The as-deposited films were
cubic NiO, with an octahedral-like grain structure, and an optical
band gap that decreased from 3.61 to 3.48 eV on increase in film thickness
(in the range 500–1000 nm). On oxidative voltammetric cycling
in aqueous KOH (0.1 mol dm–3) electrolyte, the morphology
gradually changed to an open porous NiO structure. The electrochromic
properties of the films were investigated as a function of film thickness,
following 50, 100, and 500 conditioning oxidative voltammetric cycles
in aqueous KOH (0.1 mol dm–3). Light modulation
of the films increased with the number of conditioning cycles. The
maximum coloration efficiency (CE) for the NiO (transmissive light
green, the “bleached” state) to NiOOH (deep brown, the
colored state) electrochromic process was found to be 56.3 cm2 C–1 (at 450 nm) for films prepared by AACVD
for 15 min followed by 100 “bleached”-to-colored conditioning
oxidative voltammetric cycles. Electrochromic response times were
<10 s and generally longer for the coloration than the bleaching
process. The films showed good stability when tested for up to 10 000
color/bleach cycles. Using the CIE (Commission Internationale de l’Eclairage)
system of colorimetry the color stimuli of the electrochromic NiO
films and the changes that take place on reversibly oxidatively switching
to the NiOOH form were calculated from in situ visible spectra recorded
under electrochemical control. Reversible changes in the hue and saturation
occur on oxidation of the NiO (transmissive light green) form to the
NiOOH (deep brown) form, as shown by the track of the CIE 1931 xy chromaticity coordinates. As the NiO film is oxidized,
a sharp decrease in luminance was observed. CIELAB L*a*b* coordinates were also used to quantify the electrochromic color
states. A combination of a low L* and positive a* and b* values quantified the perceived
deep brown colored state.
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