Preparation And Electrochromic Properties Of TiO₂/WO₃ Composite Film

Tungsten trioxide?WO₃?,as a very important inorganic electrochromic material,was extensively researched.WO₃ thin films are widely used in fields of display,variable reflectivity mirror,color-changing smart window and variable emissive surface.The preparation methods of WO₃ thin film are divided into chemical method and physical method.Magnetron sputtering as one of the physical methods has a lot of advantages.Thin films deposited by magnetron sputtering show various structures and stable composition,as well as strong adhesion.The electrochromic mechanism of WO₃ thin films is explained by ion/electron double injection,which alters film's color from transparent to dark blue.However,the colorization efficiency and cyclic stability of WO₃ thin films could not satisfy the requirement of practical application,thus in this paper TiO₂/WO₃ composite thin films were fabricated in order to improve the electrochromic properties.Firstly,WO₃ thin film and TiO₂/WO₃ composite thin film were prepared by conventional magnetron sputtering technique.The surface of the composite film was smooth and dense.No obvious delamination was observed from the cross section images.The electrochemical voltammetry results show the driving potentials of WO₃ film and TiO2/WO3 composite film were all above-0.8 V.The color of the films turned to dark blue with the increase of potential.The ions diffusion rates(D_in and D_de)of the two films were at the same order of magnitude.But the value of the composite film was slightly lower.The optical modulation amplitude and optical density of the composite film were also relatively low.However,cyclic stability of the composite film was improved because of the surface protection of TiO₂.Secondly,nano-column structure WO₃ films were prepared by glancing angle magnetron sputtering.The films deposited at?=80^o possess nano-column structure.The surface was loose and porous which facilitates the injection and extraction of the ions.The values of Din and Dde of nano-column structure WO3 films are close to each other,which indicate the processes are highly reversible.The D_dee of nano-column structure WO₃ films is one order magnitude faster than that of the dense film.Furthermore,the nano-structured WO₃ film exhibits higher cycle reversibility,light modulation amplitude and optical density.Finally,TiO₂ was deposited on nano-structured WO₃ film by glancing angle magnetron sputtering to obtain TiO2/WO3 composite film.The nano-structured TiO2/WO3 composite film maintained a loose and porous structure.The driving potential was as low as-0.4 V.The presence of TiO₂ on the nanostructure WO₃ has little effect on the ion diffusion rates.The nano-structured TiO₂/WO₃ composite film shows good cyclic reversibility and stability,optical modulation amplitude,optical density and colorization efficiency.