Controllable Preparationand Property Investigation Of W-doped VO₂ Smart Window Film

As one of the most important functional materials,vanadium dioxide?VO₂?owns a reversible metal-insulator transition?MIT?driven by the temperature of the surroundings.At the critical temperature(T_mit)of 68°C,accompanying the structural conversion from a monoclinic P21/c low temperature phase to a tetragonal P42/mnm high temperature phase,an abrupt change occurs in the electrical and optical properties,whichpromotesmanypotentialapplicationsincluding temperature-sensing elements,laser protection,optical-switching devices,light modulators,optical memory,etc.As the most potential material for“smart winsow”,VO₂ films possess excellent temperature-dependent effect of solar radiation?near infrared?based on changes in ambient temperature.However,the lower visible transmittance and weaker thermochromic properties caused by elemental doping and other methods to reduce the phase transition temperature,the problems severely limit its application on the smart windows.Thus,under the premise of enhance the visible transmittance and reduce the T_mit,how to prepared excellent thermochromic properties has become a hot issue for researchers.In this paper,we choose low cost V₂O₅ and H₂O₂ as raw mraterials to prepare the precursor solution using the sol-gel method.Precursor films were prepared on quartz glass rby a spin coating method.After that,a thermally reduction procedure under the NH₃ atmosphere will be conducted to obtain the porous thermochromic VO₂?M?films.Based on the above,we firstly used high-energy ball milling technique to mill V₂O₅/Wpowder,and prepared the tungsten-doped VO₂ film as the previous method.The optical properties andthermochromic properties were characterized by temperature-dependent UV-Vis-NIR spectra;the crystal structure and surface morphology was detect by XRD and FESEM,we explore the relationship between properties and structures of W-doped VO₂ films.Finally,in order to explore the role of parameters on the properties and structure of VO₂ film coming from NH₃atmosphere,we prepared VO₂ films by increasing the concentration of NH₃ and prolonging the annealing time.The results were shown as follows:?1?The obtained VO₂?M?films prepared by V₂O₅-H₂O₂ sol were porous films with excellent thermochromic properties.The optimal properties are as follows:the visible transmittance T_vis-max reached 70.3%,the integral visible tranmittance T_lum=53.3%,and the solar energy modulating efficiency?T_sol=8.3%.?2?The high-energy ball milling technique firstly used in this experiment can be effectively achieved W atom take sites of the V sublattice as a substitutional dopant,resulting in decreasing T_mit.When the W-doping content is 2 at.%,T_mit reduced to28°C.The doped W should play a role of lowering the thermal energy barrier for the nucleation and growth of nano-particles,leading to the evolution of surface morphology.Due to the effect of porous structure of the W-doped VO₂ films,the T_lumum remained at more than 50%(1 at.%,T_lum=50.8%;2 at.%,T_lum=50.6%).However,the substitutional doping,leading to the destruction of the VO₂ crystal,weakened the thermochromic properties of the film.Compared to the pure VO₂ film,the 1 at.%W doped film exhibits a weakened solar energy modulating efficiency(?T_sol=5.6%).Increasing the amount of W doping,the?T_solis further decreased(?T_sol=3.5%).?3?We prepared VO₂ films by prolonging annealing time to 24 hours in an atmosphere of excess NH₃.The obtained VO₂ film exhibited a better transmittance in visible light,T_lum=66.5%.What's more,the phase transition temperature reduced to43°C.Analyzing based on our characterization methods and conclusions of other researchers,we infered that the N atoms lacate at the substituent position of lattice O,widinging the band gap between the V_3d and O_2p orbitals and thus reducing the intrisinc absorption of VO₂ at wavelength below 500 nm;weakening the interaction with V-V dimmers and thus narrowing the energy gap,thereby reducing the phase transition temperature.