Preparation And Properties Of Electrochromic Materials Based On Tungsten Oxide

Electrochromic materials are materials that can change its color reversible due to the redox reaction caused by the injection/extraction of ions and charges under the application of an external voltage.Electrochromic materials integrated into the device can achieve reflectance,transmittance,and absorption adjustment.Thus the change of optical performance has a very broad application prospect in the fields of smart windows,displays,automobile rearview mirrors and so on.Tungsten trioxide is the most widely used inorganic electrochromic material.Nano materials have a larger specific surface area and shorter ion transmission path distance.The response speed of the materials can be improved by constructing suitable pore structures and fast ion transmission channels.The sol-gel method can be used to control the morphology and improve the performance of nanomaterials.Among them,nanowire materials have the characteristics of self-winding,and it is easy to form a"bird nest-like"pore structure during the preparation of thin films.The contact area of the liquid promotes ion transport.Moreover,doping is also a way to widely improve performance.Doping heteroatoms can not only improve the morphology of the material,but also can increase the lattice spacing,promote ion transmission,and shorten the response time when incorporated into the crystal structure.Therefore,we first prepared nanorods and nanowires with nano-tungsten oxide materials by hydrothermal method,prepared them into thin films by spraying method,assembled them into devices,and performed material characterization and performance research.The effect of morphology on electrochromic properties was studied,and the results showed that nanowire structured materials have better electrochromic properties than nanorod structured materials,with a larger light modulation rate of 44.2%@630 nm and a coloring response time of 10.6 s,Bleaching response time is 1.6 s,no attenuation within 100 cycles,and has good electrochromic performance.The Nb element is a commonly used electrochromic material,the radius of Nb⁵⁺and W⁶⁺are similar,and Nb is a common doping element.In order to make further study about the effect of Nb doping on the structure and properties of tungsten oxide,Nb-doped WO_x thin films were prepared by solvothermal and spraying techniques.As the Nb doping content increased,the lattice distance of WO_x crystalline was expanded and the ion diffusion rate was increased.So the response time is gradually shortening.The results show that WO_x with 10%Nb doping content has the best electrical properties.It has a large light modulation rate of 66.1%in the wavelength range of 300-1100 nm,a coloring response time of 19.7 s,a bleaching response time of 1.4 s,and the number of cycles exceeds1000 cycles.Magnetron sputtering is an important way to prepare amorphous electrochromic films,and it is also very important to improve the cycle stability of the films by using appropriate materials for compounding.In addition,the thin films prepared by magnetron sputtering are very dense.The use of templates or etching to construct appropriate microporous structures,and faster ion transmission can improve the response speed of the material.Due to niobium oxide is also an excellent cathode electrochromic material;Nb₂O₅-WO₃composite thin film was sputtered on stainless steel substrate by using dual-target magnetron sputtering technology to improve the film performance,especially the cycle stability of the film.Owing to the composite of Nb₂O₅ and WO₃ materials,on the one hand,the light modulation rate of the thin film has improved,and on the other hand,Nb₂O₅ mitigates the attenuation during the thin film cycling and improves the cycle stability;the best fim with best light modulation rate in the range of 400-1600 nm reaches 83.4%,which has very good application potential in spacecraft thermal control.In this study,we prepared Al₂O₃-WO₃ hybrid film by dual-target sputtering,and then etched by hydrochloric acid(HCl)to remove the Al₂O₃to form pores.And the relation between HCl etching time and electrochromic properties has been investigated here.The optimal etching time by 1M HCl solution with 10 min can have the best performance,with a color response time of 6.8 s,a fade response time of 0.8 s,and a light modulation rate of 77.1%.For crystalline films,the etched films show better electrochromic properties.The coloring response time is 10.8 s,the bleaching response time is 4.1 s,and the average light modulation rate at 300-800 nm is 50.7%,and there is almost no attenuation during 1000 cycles.Finally,the assembled electrochromic device was used as a smart window to simulate a building model,and the optical and thermal control capabilities were evaluated.It proved that the electrochromic smart window model can be used as a smart building window to achieve solar thermal management and regulation of radiation and reduce energy consumption.Moreover,the Al₂O₃-WO₃ hybrid film can reduce the use of the precious metal W while keeping good electrochromic performance,saving resources and reducing costs.