Study On The Preparation And Performance Of Bi₂WO₆-Based Photochromic Materials

Currently,commercial automotive photochromic smart window is made of naphthalopyran,which has fast coloring time（<30 s）,high short bleaching time（a few minutes）and low transmittance after coloration（40%）.The transmissibility（30%）of transition metal oxides such as tungsten oxide（WO₃）and molybdenum oxide（Mo O₃）after coloration have reached the standard of photochromic smart window,however,the bleaching efficiency（>10 h）and coloration efficiency（>1 min）can not meet the practical requirements.Inorganic photochromic materials with fast response,highly reversible and significant color changing effect are the inevitable choice for practical applications.In this paper,we chose bismuth tungstate（Bi₂WO₆）as the photochromic material due to its narrow band gap and visible or solar light responsiveness.Based on three photochromic processes,the effects of defects and morphologies,heterojunctions and conducting polymers on the separation,migration and lifetime of photogenerated carriers in light absorption were studied to design and synthesize inorganic photochromic materials with rapid and reversible visible or solar light response.The main research contents are as follows:（1）Bi₂WO₆ ultrathin nanosheets with abundant“Bi-O”vacancy pairs were prepared via hydrothermal method using cetyltrimethylammonium bromide（CTAB）.The results show that the horizontal size and thickness of the Bi₂WO₆ ultrathin nanosheet is about30～80 nm and 1.29 nm（close to 0.8 nm for monolayered layer）,which can not only maximally increase the irradiation area and exposed active site,but also greatly shorten the migration distance of photogenerated electrons from inside to surface.Moreover,the“Bi-O”vacancy pairs on the surface can capture photogenerated electrons and improve the separation efficiency of photogenerated carriers,which is beneficial to the surface photochemical reduction reaction(W⁶⁺to W⁵⁺).Compared with the pristine Bi₂WO₆（2.86 e V;17.97 m²/g）,the optical band gap（2.73 e V）and the specific surface area（22.74 m²/g）of Bi₂WO₆ ultrathin nanosheets are decreased and increased,respectively,which could improve the light absorption.As a result,Bi₂WO₆ultrathin nanosheets exhibit excellent photochromic behavior upon solar light irradiation（5 s）and under dark atmospheric treatment（35 min）compared with that of WO₃photochromic materials.However,the low reflectance after coloration（67.7%）indicates that the color change is not significant.（2）In order to solve the problem of lighter coloration for Bi₂WO₆ ultrathin nanosheets upon solar light irradiation.,WO₃ quantum dots（WO₃QDs）and Bi₂WO₆ultrathin nanosheet heterojunction material was prepared,which could increase the active sites on the nanosheet surface,realize the effective separation of Bi₂WO₆photogenerated carriers through the interface contact between the two,and prolong the carrier life.The results show that WO₃QDs and Bi₂WO₆ ultrathin nanosheets have matching conduction and valence band energy levels.The formation of Z-scheme heterojunction in the Bi₂WO₆-WO₃QDs heterojunction with 2D-0D structure can effectively inhibit the recombination of photogenerated electron-hole pairs of Bi₂WO₆ultrathin nanosheets.The specific surface area of Bi₂WO₆-WO₃QDs heterojunction is significantly higher than that of the Bi₂WO₆,which endures higher light absorption and more surface active sites for photochromic behavior.Based on the above factors,under solar light irradiation for 5 min,the reflectance at 500 nm and b^*color parameter of Bi₂WO₆-WO₃QDs heterojunction materials is decreased to 42.8%and-9.04,respectively,compared with that of Bi₂WO₆（65.9%;-5.07）,which has basically met the demand of smart windows.Due to the enhanced color depth after irradiation,compared with that of Bi₂WO₆ ultrathin nanosheets（5 s;35 min）,the solar light responsiveness（10 s）and bleaching time（60 min）with the dark air treatment of the composite coating are slightly decreased,but still superior to the WO₃.（3）Considering the direct correlation between the separation and migration of photocarriers and Bi₂WO₆ photochromic performance,in this study,Bi₂WO₆-PEDOT-Bi OBr ternary composite photochromic material with 2D-2D heterojunction were obtained via one-step hydrothermal method.On the one hand,PEDOT has strong electron-donating property and high conductivity,which provides more electrons and accelerates the migration of photogenerated carriers.On the other hand,the necessary chemical environment is provided for the formation of Bi OBr compounds on the surface of Bi₂WO₆ nanosheets.The results show that compared with that of the Bi₂WO₆（thickness is 1.29 nm）,the nanocomposite still maintains the morphology of the ultrathin nanosheet（thickness is 1.26 nm）,but its horizontal size（20～40 nm）was decreased.In addition,the Bi OBr compounds grow on the surface of Bi₂WO₆nanosheets,forming a close contact interface between them,namely a 2D-2D nano-heterojunction,which can increase the contact surface area and accelerate the transfer of photogenerated carriers in the 2D-2D interface regions.Furthermore,due to the synergistic enhancement of PEDOT and 2D-2D heterojunction,the photogenerated carrier separation and migration efficiency of the nanocomposite material become higher.Thus,upon solar light irradiation for 5 min,compared with that of Bi₂WO₆ultrathin nanosheets（65.8%;-2.78）,the reflectance at 500 nm and the b^*color parameter of Bi₂WO₆-PEDOT-Bi OBr ternary composite are reduced to 42.6%and-6.81,respectively,indicating the enhanced photochromic performance.Furthermore,the coloration and bleaching time of the nanocomposite coating are 15 s（upon solar light irradiation）and 80 min（with dark air treatment）.The Bi₂WO₆ material showed high responsiveness（15 s）and color depth（42.6%）,reaching the standard of organic smart windows.Although the reversibility of photochromism（80 min）was not as good as that of naphthalopyran（several minutes）,it was much better than that of WO₃,Mo O₃（>10 h）.This study provides the possibility to develop inorganic photochromic smart windows.