Study On Microstructure Regulation And Structure-Activity Relationship Of WO₃ Electrochromic Materials

The optical properties of electrochromic materials or devices can be changed reversibly and continuously under a certain applied voltage,which gives them wide application prospects in smart windows,low-energy displays,anti-glare mirrors,smart phones,and other fields.Tungsten trioxide（WO₃）is one of the earliest materials discovered,studied and used in electrochromic devices.It has many advantages,such as high color contrast,stable physical chemistry properties and low cost of raw materials,which attract the attention of many researchers.At present,WO₃-based electrochromic devices are still difficult having large optical modulation,short response time and long cycle stability,which restrict their large-scale application in the electrochromic field.To understand deeply the discoloration principle of tungsten oxide and the process of electron conduction and ion diffusion in the electrochromic process,and to understand the structure-activity relationship between the morphology,electronic structure and crystal structure of tungsten oxide and its electrochromic properties,it is the key point and breakthrough point to design and fabricate WO₃ electrochromic materials with large optical modulation,short response time and long cycle stability.However,the structure-activity relationship of WO₃-based electrochromic materials has not been thoroughly elucidated,and most of the modification research is still based on a lot of experiments.Therefore,the relationship between WO₃ electrochromic materials with different morphologies,crystal（electronic）structures and their electrochromic properties have been analyzed in detail in this paper,it provides a theoretical basis for improving the properties of electrochromic materials.The specific research contents are as follows:（1）Structure-activity relationship between morphology and electrochromism of WO₃.WO₃ powders with different morphologies were prepared by a simple sol-gel method,and the electrochromic films were prepared by simply spray-coating on FTO（fluorine-doped Sn O₂ conductive glass）substrate.The formation mechanism of different WO₃ morphologies was analyzed,and the effects of two morphologies on the electrochromic properties were studied.The results indicate that dispersed nanosheets with higher specific surface area exhibit a larger optical modulation range（78%at 633 nm）,fast switching speed（coloring time is 14 s and bleaching time is 5.4 s at 633 nm）,higher coloration efficiency,and demonstrated strong broad spectral modulation ability compared to nanosheet stacking structures.（2）Structure-activity relationship between crystal structure and electrochromism of WO₃.The structure-activity relationship between the typical WO₃ crystal structure and the electrochromic properties was studied systematically,three representative crystal structures（monoclinic phase,orthorhombic phase with structural water and hexagonal phase）were selected for the experimental and theoretical analysis of phase reconstruction,it was found that the subnanoscale tunnels or cavities constructed by WO₆ octahedral molecular unit in hexagonal phase tungsten trioxide（h-WO₃）and its special electronic structure provided effective ion and electronic transport for electrochemical and electrochromic reactions,it has a super-large cavity（18.2%）and narrow band gap（1.29 e V）.Based on this,a novel modification strategy was developed to obtain h-WO₃ under mild experimental conditions and a large-area uniform electrochromic film was prepared.Experiments and Density functional theory（DFT）analysis showed that h-WO₃ films exhibited excellent electrochromic properties thanks to the structure-activity relationship,86%at 633 nm（90%at1000 nm）of the optical modulation range,8.7 s of coloring time,1.9 s of bleaching time and 86.4%of the initial optical modulation remain after 3000 cycles.In addition,the unique insights between H⁺adsorption/desorption and the response of electrochromic broad spectrum at different bands are provided.The thermodynamic analysis provides theoretical support for the excellent memory performance of the material in the near-infrared region,and the charge storage mechanism of capacitance behavior confirms the unique response of the material in the near-infrared region at high potential.