The effect of annealing temperature on the electrochromic properties of nanostructured NiO films

The nickel oxide (NiO) films have been prepared by sol–gel dip coating route. They were treated at different temperatures ranging from 250 to 450 °C at an optimum number of layers (8 layers) on a conducting substrate (FTO) and glass plate. The X-ray diffraction (XRD) spectrum reveals that the crystallanity increases along the planes (1 1 1), (2 0 0) and (2 2 0) and the crystallite size increases from 7 to 26 nm as the temperature increased from 250 to 450 °C. The FTIR spectrum confirms the formation of Ni–O bond. The SEM images indicate the formation of nanorods in the temperature range 350–450 °C. The electrochromic properties have been studied using cyclic voltammetric (CV) and chronoamperometric (CA) techniques. The Ni(II)/Ni(III) transformation is the possible cause for the reversible colour change from transparent to dark brown. The film prepared at 300 °C with a thickness of 306 nm exhibits maximum anodic/cathodic diffusion coefficient of 11×10−12 cm2/s/6.44×10−12 cm2/s and the same film exhibits the maximum colour change of 68% with a photopic contrast ratio of 5.17. The chronoamperogram reveals that the colouration/decolouration response time decreases with the increase in preparation temperature. The films treated at 300 °C exhibit the optimum electrochromic behaviour.