Studies On Preparation, Structure And Performances Of Electrochromic WO₃ Films

Tungsten trioxide film is a kind of functional material with wide application challenges. This kind of film can be used as the electrochromic display, electrochromic smart-window and rear-view mirror. There are different rules for the film in different application fields. Although the research is very widely for the WO3 film, there are many problems in the practical application. In this dissertation, we have prepared two kinds of WO3 films by the Sol-Gel technology based on the works of other researchers. The microstructure, light property, electrochemical property and electrochromic property of the films were characterized with many technologies and we have obtained some useful results. This dissertation mainly consists of the following several parts:1. The Tungsten trioxide films with different crystal phases were prepared by the Dip-Coating-Pyrolysis technology. The precursor composed of ammonium metatungstate solution and Pluronic P123 as the template. The annealing temperature(300℃, 400℃and 500℃)was determined according to the TG-DSC and FT-IR results of the precursor. The Nitrogen adsorption-desorption test suggest that the films possess ruleless pore structure and small specific surface. The X-ray diffraction data indicate the films heated in different temperatures have different kinds of crystal phases. From the AFM and FSEM test results, we can know the surface of the films is rough but the grains have uniformly distribution. The crystal grains' size and crystal lattice stripe are shown in their TEM spectrums.2. The electrochromic property and light property of the films prepared with Dip-Coating-Pyrolysis technology were investigated deeply. The electrochemical cyclic voltammetry results point out that all the films embody some electrochromic property and the film annealed at 400^C has the best electrochromic reversibility and stability. The coloring responds time for the films heated at 400℃and 500℃isabout 23 s and the bleaching responds time is about 7～9 s. The UV-visible spectroscopy test results show that the film prepared at 300^C has hardly optical modulation ability but the films annealed at 400℃and 500℃attain better modulation ability and their electrochromic coloring efficiency reach to 19.86 cm2/C and 40.98 cm²/C, respectively. The optical band gap Eg for the films is approximately 3.2 eV. The results of Mott-Schottky can prove the films prepared by this method belong to n-type semiconductor and their flat-band and donor density is calculated.3. The Tungsten trioxide films with different mesoporous structure were prepared by the Evaporation-Induced Self-Assembly process. In this method, the precursor composed of the ethanol solution of WCl₆ and the Pluronic P₁₂₃ as the structure direct template. The annealing temperature (300℃and 400℃) was determined following the TG-DSC and FT-IR results of the precursor. The Nitrogen adsorption-desorption results suggest that the films annealed at 300℃possess mesoporous structure, but the films prepared at 400℃, have a ruinate mesoporous structure. The X-ray diffraction data reveal that the films annealed at 300℃are mainly amorphous and the films prepared at 400℃create triclinic WO₃. The AFM and FSEM test results show the surface of the films is not smooth and but the distribution of the particles is uniformly. The films become more and more compactly with the temperature rising. The TEM results indicate the annealing temperature and proportion of Pluronic P₁₂₃ have notable effect on the crystal structure, the morphology and the mesoporous structure.4. The electrochromic property and light property of the films prepared with by the Evaporation-Induced Self-Assembly process were investigated deeply. The electrochemical cyclic voltammetry results suggest that all the films embody good electrochromic property and the film annealed at 300℃have better electrochromic reversibility and stability than those of the films prepared at 400℃。The film prepared from sample C at 300℃has the best electrochromic reversibility. The results of the UV-visible spectroscopy test demonstrate the coloring efficiency for the film annealed at 300℃is higher than that of the film prepared at 400℃。The optical band gap of the film in the three states(virgin state, colored state and bleached state) has the same value, but the band gap of the film prepared at 400℃is lower than that of the film annealed at 300℃。The results of Mott-Schottky prove the films prepared by this method belong to n-type semiconductor and their flat-band and donor density is calculated.5. Four kinds of classical theoretical models about the electrochromism of the tungsten trioxide film were introduced, including the "color center" model, double injection/extraction model, small polaron and energy band model. The valence state change of W and O in the WO₃ film was studied at the virgin state and colored state through the XPS technology. We try to explain the electrochromism of the WO₃ films prepared in this dissertation and verify the double injection/extraction model.Through the above works, we investigated the preparing method for tungsten trioxide films and enriched the details preparing technology. Two kinds of WO₃ films have been prepared. The research on the mesoporous tungsten trioxide film has enhanced the order of the mesoporous structure and improved the electrochromic property of the film.