SHORT COMMUNICATION Ce´sar O. Avellaneda Æ Luis O.S. Bulho˜es Electrochromic properties of WO 3 and WO 3 :P thin films Received: 10 June 2002 / Accepted: 11 June 2002 / Published online: 7 August 2002 Ó Springer-Verlag 2002 Abstract WO 3 and WO 3 :P (5 mol% H 3 PO 4 ) thin films were prepared using the sol-gel route and the electro- chromic properties of the films were investigated using in situ spectroelectrochemical methods. The measurements were performed in propylene carbonate solution with 0.1 M LiClO 4 as electrolyte. During the cathodic po- larization at –0.8 V a blue coloration is observed with a reversible variation between 14% and 84% of the transmittance at k=633 nm. The kinetics for the bleaching process is faster for the WO 3 :P film than for the undoped WO 3 film. Keywords Electrochromism Æ Tungsten oxide Æ Sol-gel Introduction Certain materials, referred to as electrochromic materi- als, are known to change their optical properties in re- sponse to the application of an electric current or an electric potential. This property has been used to pro- duce electrochromic devices which can be controlled to transmit optical energy selectively. Such electrochromic devices typically have a structure consisting of sequential layers including an electrically conducting material, an active electrode formed by an electrochromic material, an ionic conductive layer, a counter electrode film and another electrically conductive layer. Electrochromism in transition metal oxides and electrochromic devices (ECD) have been extensively in- vestigated since their discovery by Deb [1] in 1973. Tungsten trioxide (WO 3 ) film is the most investigated electrochromic material and it is considered a viable material for use in electrochromic devices, such as smart windows, sunroofs, rear and side view mirrors [2, 3, 4]. Several papers have described the fabrication of tung- sten oxide thin films for electrochromic applications. These films are typically made by evaporation [5], sputtering [6], anodization [7] and the sol-gel method [8, 9, 10, 11, 12, 13, 14, 15, 16], the latter methodology being very attractive owing to its low cost and to the formation of homogenous films. Several sol-gel routes have been developed for the fabrication of WO 3 thin films, and the earliest alterna- tive to produce sol-gel WO 3 films was the acidification of sodium tungstate [8, 9]. The major advantage of this method is the formation of WO 3 at room temperature; however, the stability of the solution, which is an im- portant parameter for industrial production, was not found to be adequate. The second method is by means of peroxopolytungstic acid [10, 11], in which tungsten and tungsten carbide powders are dissolved in an aqueous solution of hydrogen peroxide. Another route uses tungsten alkoxide [12], the classical sol-gel route, but this method is expensive and consequently is not useful for industrial applications. The reaction of tungsten oxychloride (WOClO 4 ) with isopropanol, proposed by Livage [13], produces a sol that is stable for several months and the process is inexpensive. Finally, two in- teresting results about the improvement of WO 3 film production have been published. Cronin et al. [14, 15] patented the preparation of a WO 3 electrochromic coating solution by reacting metallic tungsten with hy- drogen peroxide and acetic acid. The resulting product, a tungsten peroxy acid, was then esterified by reacting it with alcohol to produce a tungsten peroxy ester deriv- ative (PTE). Denesuk et al. [16] proposed a novel methodology for the synthesis of WO 3 thin films. The precursor sol was prepared using the route proposed by Cronin et al. [14] and to the final solution a 32 mol% solution of dehydrated oxalic acid was added. These films displayed a homogeneous amorphous/crystalline hybrid structure. J Solid State Electrochem (2003) 7: 183–186 DOI 10.1007/s10008-002-0302-7 C.O. Avellaneda Æ L.O.S. Bulho˜es (&) LIEC – Laborato´rio Interdisciplinar de Eletroquı´mica e Ceraˆmica, Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, C.P. 676, CEP 13565-905, Sa˜o Carlos-SP, Brazil E-mail: dlob@power.ufscar.br Tel.: (55) (16) 2608214 Fax: (55) (16) 2615215