Nickel oxide sol-gel films from nickel diacetate for electrochromic applications

Nickel diacetate tetrahydrate, [Ni(acetate)(2).4H(2)O] and nickel diacetate dimethylaminoethanol, [Ni(acetate)(2)(dmaeH)(2)] were successfully used to deposit NiOx thin films on conductive glass substrates by sol-gel techniques for large area electrochromic applications. Homogeneous one layer films 100 nm thick were deposited by spin coating 0.5 M [Ni(acetate)(2).4H(2)O] in dmaeH at 1000 rpm and by dip coating methods. The NiOx films were characterised by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The thin film electrochromic performances were characterised by means of optical (transmittance) and electrochemical (cyclic voltammetry) methods. On early cycling NiOx thin films present an activation period, related to an increase in capacity. The electro-optical data show an increase in the electrochromic response (i.e. an increase in contrast and colouration efficiency) upon cycling. Following this initial activation period a steady state is reached in which the thin films reversibly switch from transparent to brown. The anodically coloured NiOx thin films are therefore suitable for use in a complete electrochromic cell with tungsten oxide as the cathodic colouring layer. However, the films are not fully stable with long cycling. (C) 2002 Elsevier Science B.V. All rights reserved.