Preparation,Structure And Properties Investigation Of Electrochromic Nanocrystal Composited Tungsten Oxide Films

Most of the building energy consumption is lost through the glass doors and windows.As the static energy-saving glasses used now cannot be timely regulated according to changes of temperature,light or human comfort conditions,they do not show energy saving of cooling or heating energy consumption to the best advantage.The optical properties of Electrochromic glazing(darken/lighten)can reversibly change by applying small electric voltage,which are recognized as an effective energy saving technology because electrochromic glazing can dynamically control the in or out radiation heat transfer,even better than low-emissivity glazing.EC glass is currently the most promising energy-saving smart glass for large-scale commercial application.At present,the research and application of electrochromic technology in foreign countries is in a leading position,while electrochromic technology of our country falls behind.There is still a lack of deep understanding of the coloration mechanism of electrochromic materials and the change of optical constants in the coloring process.The shortcomings of electrochromic materials such as low light optical modulation and slow response time also limit the development of electrochromic technologies.In this dissertation,cathode electrochromic material based on WO3 was studied.The coloration mechanism of amorphous and crystalline WO3 films was investigated by spectroscopic ellipsometry.In order to overcome the shortcomings of conventional WO3 electrochromic materials such as low light optical modulation,slow response time and other issues,nanocrystallization and nano-compositing were studied to improve the perfomance of the WO3 films.The effect of material structure on electrochromic performance was discussed in depth.Ammonium metatungstate,as a non-expensive industrial raw material,was taken as precursor to prepare electrochromic W03 films by a simple method.The WO3 thin films exhibit clear electrochromic performance.The coloration process of amorphous and crystalline WO3 films under different voltages was investigated by spectroscopic ellipsometry.The refractive indices of both amorphous and crystalline WO3 films decrease while extinction coefficients increase during the coloration process.But the optical parameters of crystalline WO3 are much more sensitive to the H+ ions injected compared to the amorphous WO3 during the coloration process.The absorption coefficient of amorphous W03 increases almost linearly throughout the visible light band,while the absorption of crystalline WO3 is more pronounced over the long wavelength band.This shows that the coloration mechanism of amorphous WO3 is small polaron absorption,while that of crystalline W03 is Drude free electron absorption similar to heavily doped semiconductor.The electrochromic mechanism of amorphous and crystalline WO3 is very different.Monodispersed tungsten oxide nanorods were synthesized by solvothermal method.By controlling the concentration of oleylamine in the precursor,the average length and diameter of the monodispersed tungsten oxide nanorods increase from 38×3.2 nm to 89×4.5 nm.Electrochromic WO3 films with the size controlled nanorods were successfully fabricated and the size effect of the nanorods on electrochromic performance was studied.The WO3 film consisting of smaller size nanorods has a faster electrochromic response.By simply changing the size of the nanorods,the response time of the nanorod films can be regulated nearly by about five times.The fastest coloring and bleaching time is 1.9 s and 1.0 s.The WO3 film composed of larger nanorods has higher optical modulation.With the film thickness of 130 nm,the optimal optical modulation of the W03 nanorod film at 633 nm is 49%.All the samples showed good cycling stability and reversibility,and the difference in nanorod size had little effect on the cycle stability.ITO@WO3 and TiO2@WO3 nanocrystals embedded composite films were successfully prepared respectively.The WO3 in the composite film was amorphous.Nanocrystals embedded structure lead to significant change in the microscopic morphology of the film.The nano crystals are embedded in the film to form a composite interfacial area,providing more paths for ion diffusion and active sites reaction position for the electrochromic reaction.Compared with the pure W03 film,the electrochromic optical modulation of the nanocrystal composite films is significantly enhanced and the electrochromic response is faster.Reversibility and coloring efficiency are also improved.Through the study of TiO2@WO3 composite films,a model was established to distinguish the faradaic charge and the capacitive charge in the electrochemical reaction,demonstrating that the capacitive charge does not participate in the electrochromic effect and that the faradaic charge is closely related to the electrochromic behavior.The research work is conducive to a better understanding of the coloring mechanism of WO3,and also provides a new strategy for improving the performance of electrochromic materials,which is helpful for realizing large-area and low-cost preparation of WO3 electrochromic films.