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

Nickel diacetate tetrahydrate, [Ni(acetate) 2·4H 2O] and nickel diacetate dimethylaminoethanol, [Ni(acetate) 2(dmaeH) 2] were successfully used to deposit NiO x 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 2O] in dmaeH at 1000 rpm and by dip coating methods. The NiO x 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 NiO x 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 NiO x 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.