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 degrees 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 (111), (200) and (220) and the crystallite size increases from 7 to 26 nm as the temperature increased from 250 to 450 degrees 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 degrees 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 degrees C with a thickness of 306 nm exhibits maximum anodic/cathodic diffusion coefficient of 11 x 10(-12) cm(2)/s/6.44 x 10(-12) cm(2)/sand 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 degrees C exhibit the optimum electrochromic behaviour. (C) 2008 Elsevier B.V. All rights reserved.