Structural Regulation Of Mesoporous VO₂ Nanocrystals And Investigation Of Their Thermochromic Properties

The application of smart windows is a promising way to solve the problem of energy conservation for buildings.And the monoclinic phase（M phase）VO₂ is an ideal material for smart windows due to its phase transition and a dramatic change in the optical properties near room temperature.However,pure VO₂ has following major problems:The low luminous transmittance(T_lum),undesirable solar modulation efficiency and the phase transition temperature（Tc）is much higher than room temperature,which greatly limits its application on smart windows.In order to solve this problems,the mesopores is introduced into VO₂ crystal in this work,and the air-filled mesopores which are considered to be a secondary component in film can improve the luminous transmittance of VO₂ film,simultaneously W doping is employed to reduce the phase transition temperature,revealing the inner link between the structure and properties.The main results are as follows:1.The mesoporous VO₂ nanoparticles are successfully prepared by the method with cotton as template.The pure VO₂（M）crystals are completely obtained after vacuum annealing at 450℃.Pure mesoporous VO₂（M）crystals can be synthesized at hydrothermal temperatures of 120℃,140℃,160℃,180℃,but the size of VO₂（M）crystals and mesopores are different.As the hydrothermal temperature increases,the better crystallinity of VO₂（M）crystals obtained,and the crystal size increase continuously,while the mesopore size in the crystal increase first and then decrease.2.The optical properties is closely related to the structure of mesoporous VO₂nanoparticles including the pore size and grain size,and high luminous transmittance is in corresponding with the large pore size,Simultaneously the large grain size can promote the improvement of solar modulation efficiency.Therefore,as the hydrothermal reaction temperature increases,the luminous transmittance of the VO₂film increases first and then decreases,and the solar modulation efficiency increases continuously.The VO₂ film obtained under the hydrothermal temperature of 180℃ has the best optical properties due to its large particle size and pore size.which solar modulation efficiency(ΔT_sol)is up to 12.9%and luminous transmittance(T_lum)is56.0%.3.The W-doped mesoporous VO₂ particles are successfully synthesized using hydrothermal method with ammonium tungstate as W dopant.The phase transition temperature of VO₂ decreases from 65.3℃ to 39.5℃ with the W content（0.2 at%,0.4at%,0.6 at%）increasing,and luminous transmittance of VO₂ films increases first and then decreases,and solar modulation efficiency continuously decreases,the best performance is obtained at the doping level of 0.4 at%.Moreover,W doping efficiency（the ratio of W in obtained samples to that in the initial solution）is very promising,and phase transition temperature is greatly reduced.The 0.4 at%W-doped mesoporous VO₂film show excellent overall thermochromic performance with promising solar modulation efficiency(ΔT_sol=11.4%),suitable luminous transmittance(T_lum=61.6%),as well as a low phase transition temperature（Tc=43℃）.