Study On Preparations And Properties Of Electrochromic Thin Films Based On Titanium Via Sol-gel Routes

Electrochromic (EC) materials are one kind of functional materials, which have very good application future. In recent years, there has been a vast amount of research and development conducted at both university and company level into electrochromism, due to the variety of potential applications, which exist for electrochromic devices, i.e., antiglare rearview mirrors, information displays and smart windows. The ability to achieve variable thermal emittance can also be utilized for temperature control of satellites and in military camouflage technology.There are several promising electrochromic materials, but among them, WO₃ due to its high coloration efficiency, quick response time and long life, is of great interests. Although molybdenum trioxide has comparatively lower coloration efficiency for a given inserted electrical charge, the closer position of its optical absorption peak to the human eye sensitivity peak makes this material very attractive for many applications. Mixed molybdenum and tungsten oxides are of great interests since they are expected to exhibit a broadened optical absorption band to offer an additional possibility to tailor the optical absorption spectra giving a more neutral color. This effect may originate from the larger amount of defects in the two-component material. This can be achieved by changing the fraction of the two components. Higher coloration efficiency is expected, due to increased electron transitions between two kinds of metal sites with different valences (W⁶⁺, W⁵⁺, W⁴⁺, Mo⁶⁺, Mo⁵⁺, Mo⁴⁺).The commercial viability of electrochromic devices (ECD) depend signifycantly on its cost effectiveness. The materials used and the processes adopted play a crucial role. Since the discovery of the electrochromic (EC) phenomenon in WO₃ films in 1969 by Deb., different techniques have been used to deposit electrochromic films e.g. sputtering, vapor deposition, electrodeposition and sol-gel. In recent years, however, there has been an orientation to use chemical rather than physical techniques which involve high capital investments. The sol-gel synthesis methods offer many advantages for the preparation of high value added materials in addition to being economic. Therefore, in this work a modified sol-gel route was developed under consideration of criteria such as sol stability,film structure and the potential for scaling up, which are relevant for the applications of large area. The simplicity of this route, the use of relatively inexpensive materials and equipments and the room temperature preparation attracted many researchers to follow this technique.In present study, the hydrolysis of Ti(OC₄H₉)₄ in the alcohol was controlled by hydrogen peroxide and oxalic acid dehydrate. The sol produced by this way is very stable and can form high quality TiO₂ counter electrode films. From the experiment, it is found that the optimum sol is very stable which can be stored more than one year at room temperature. Adding H₂O₂ and H₂C₂O₄ can not only improve the stabilities of the TiO₂ sol, but also can control the grain size and micro structure of the TiO₂ film. The TiO₂ thin films which were vacuum-annealed at 180°C for 2h on ITO/glass showed excellent photoelectric performances.Working as a promising electrochromic material, WO₃ is usually doped with one or two efficient additives to improve its properties. We choose the commercially available products: sodium tungstate dihydrate, sodium molybdate dihydrate, and cation exchange resin as reagents to produce doped WO₃ sols and films. MoO₃ doped WO₃ sols were formed upon acidification of sodium tungstate aqueous solution with sodium molybdate through a proton exchange resin column, adding the H₂O₂ and CH₃COOH to form acetylated peroxo tungstic acid and molybdic acid, and then dissolved in ethanol with oxalic acid. For preparation of TiO₂, MoO₃ doped WO₃ sols and films, TiO₂ sols were directly dissolved in the foregoing MoO₃ doped WO₃ sols. Adding H₂O₂ and H₂C₂O₄ can not only improve the stabilities of the doped WO₃ sol, but also improve the quality of the doped WO₃ film. H₂C₂O₄ can control the micro structure of the doped WO₃ electrochromic film, especially. The doped WO₃ thin films which were vacuum-annealed at 180°C for 2h on ITO/glass showed excellent photoelectric performances.We have demonstrated that films derived from the sols with optimum mol ratios of component possess excellent electrochemical and physical characteristics quite suitable for application in electrochromic devices.In this paper, we report our results on the components influence of the sols stabilities and films quality. Also, the electrical, optical and electrochromic properties of films were studied. Detailed infrared and visible spectrophotometric, XRD and electrochromic characterization data are presented.