Study of the potassium ion insertion of the electrodeposited electrochromic tungsten trioxide thin films

Various types of dynamic adjustable optical shutters have been proposed; they are called as "smart windows". The electrochromic smart window utilizes the phenomenon of electrochromism. Electrochromism is the property of a material such that its color is changed by an electrochemical redox reaction. Numerous inorganic and organic electrochromic materials have been examined. During the color-bleach process, the redox reaction of the host material causes injection or ejection of both cation (or anion) and an electron (or hole). So it behaves as a mixed ion conductor and hence it has recently attracted interest in the field of solid state ionics. At present, several prototypes of electrochromic smart windows have been proposed and some of them are commercially available. The cathodic electrochromic oxides consist of n-type semiconductors such as TiO2 V2O5, WO3 and MoO3. Among them, WO3 has been intensively examined and used for most electrochromic devices. These materials are cathodically colored in blue. The electrochromic reaction is expressed by xA(+) + MOy (transparent) + x e(-) double left right arrow A(x)MO(y) (blue) (A = H, Li, Na, K...., MOy = metal oxide) where the cation A(+) and electron e- are co-injected into the host oxide MOy which results in the formation of the nonstiochiometric compound A(x)MO(y). Here the electrodeposition method is used to deposite WO3 films under by galvanostatic conditions. The parameters like deposition time, deposition temperature, electrolyte concentration, pH and bath temperature are optimized. The XRD results show the triclinic structure for the as-deposited film. The film shows the same structure after intercalation but the peak intensity is different. The tungsten trioxide thin films show the colour change during the intercalation and retrace the original colour. The diffusion coefficient for K+ ion is calculated by using the Randles-Servick equation. In this paper the electrochromic properties, the optical properties of the as deposited, ion intercalated and deintercalated WO3 films and the diffusion coefficient values are presented.