Synthesis And Electrochromic Properties Of Nano Cesium Tungsten Bronze Cs_xWO₃

The smart window can adjust the indoor brightness and temperature by responding to the changes of external light,electricity,heat and other conditions which fulfils energy conservation and emission reduction.Nano cesium tungsten bronze Cs_xWO₃has found its extensive applications in the field of smart windows due to its excellent near-infrared shielding performance.Although its application prospects are very bright,the chromatic instability of nano-coating Cs_xWO₃ under atmosphere is still a barrier for the further application.Electrochromic technology is one of the possible ways to solve this problem.Therefore,it is great significant to study the electrochromic properties of Cs_xWO₃ for its wider application.Adjustments of the crystal structure and microscopic morphology are effective methods to improve electrochromic properties of Cs_xWO₃.Therefore,it is of practical significance to regulate these two parameters through experimental methods and investigate the effects of them on the electrochromic properties of Cs_xWO₃.The main results of this paper are listed as follows:（1）Using tungsten chloride,cesium hydroxide monohydrate,ethanol absolute and pure acetic acid as raw materials,the hexagonal nano cesium tungsten bronze was prepared by solvothermal method and the fluidity of the reaction solution was changed by adding magnetic stirring during the synthesis process.The effects of the fluidity of the reaction solution on the crystal structure and microscopic morphology of the samples have been investigated.With increasing the speed of magnetic stirring bar（v_r）from 500 rpm to 800 rpm,the cell parameters a,c and V of the samples firstly increase and then decrease.When v_r is 700 rpm,they reach the maximum value,and the sizes of hexagonal and trigonal channels in WO6 framework are also the largest.With the increase of v_r,the grain shapes gradually change from the mixture of rod-like and granular ones to rod-like or granular ones,and the average grain sizes increase first and then decrease.As v_r is 700 rpm,clusters（～100 nm）composed of rod-like grains are evenly distributed in the sample.These results show that the crystal structure and microscopic morphology of the samples can be regulated by changing the fluidity of the reaction solution.（2）The electrochromic properties of samples were characterized by optical fiber spectrometer and electrochemical workstation.The results show that the electrochromic performance of sample with v_r=700 rpm is more excellent than those of other samples.The transmittance modulation range and coloration efficiency of this sample is up to 42.7%and 20.2 cm² C^-1,and about 3～6 and 2 times than those of other samples,respectively.By analyzing the transmittance curves of samples changes little during 150 electrochromic cycles including 1500 s continuous excitation of positive and negative voltage,which indicates their stability of electrochromic properties.（3）In order to further optimize the synthesis process of nano Cs_xWO₃ and obtain samples with different Cs content,the molar ratio of cesium hydroxide monohydrate to tungsten chloride（Cs/W）was changed as 0.4,0.5,0.6 and 0.7 in raw materials and the fluidity of the reaction solution was invariant for all samples during their synthesis processes.The effects of Cs/W on the electrochromic properties of samples have been investigated.The results of X-ray diffraction show that the samples with pure hexagonal phase are obtained.The effects of Cs/W in raw materials on the Cs content of samples are nonlinear according to the results of X-ray photoelectron spectroscopy.When Cs/W is 0.5,the Cs content x（=0.29）of the sample is the highest.The transmittance modulation range and coloration efficiency of this sample are also the largest among all samples.