Heat Treatment Of VO₂ Gel Film In A Closed Environment And Synthesis And Properties Of N-doped VO₂ Thin Films

M1 phase VO₂ thin film with metal-insulator-transition?MIT?property has attracted extensive research around the world,due to its transition temperature?TC?is nearest to the room temperature among the inorganic phase transition material,which can be used in many field,such as thermochromic smart window for energy saving.However,the controllable preparation of VO₂?M1?is very difficult on account of the multivalence of V element.Hence,we need to employ facile and low-cost method to fabricate pure VO₂?M1?thin film,for the sake of its potential large-scale application on architectures and automobiles.During the heat treatment process of VO₂?M1?thin film,the annealing atmosphere is a crucial parameter to form stoichiometry VO₂.To date,the most used inert atmospheres to synthesize VO₂ thin films are flowing N₂ and Ar.However,the flowing atmosphere concumes a lot of gas during the whole annealing process.Herein,a facile method based on sol-gel process with good repeatability using closed NH3 atmosphere annealing is employed to synthesize VO2?M1?thin films.Interestingly,it is found that N is doped into VO₂ thin film through this method.This paper characterized the microstructure of VO₂ thin films using grazing incident X-ray diffraction?GIXRD?,Raman scattering spectra,X-ray photoelectron spectra?XPS?,field emission scanning electron microscopy?FESEM?and Fourier transform infrared spectroscopy?FTIR?,and the optical properties were tested by UV-Vis-NIR spectrometer.The effects of N-doping on structures and properties of VO₂?M1?thin films were also studied,and the band structures of N-doped VO₂ with different modes were calculated by the first-principle theory.Moreover,Soda-lime glass and borosilicate glass were also employed as substrate to fabricate VO₂ thin films in consideration of the high cost of quartz glass substrate.The main results of this dissertation are outlined as follows.1.The thermochromic M1-phase VO₂ thin films have been successfully synthesized by sol-gel process using a facile annealing method in a closed vacuum furnace containing a small amount of NH₃.As the concentration of VO²⁺and PVP in precursor increases,the thickness of VO₂ thin film is increased,facilitating the growth of VO2 grain,while the luminous transmittance?Tlum?is gradually reduced.2.VO₂ thin films with high purity and good repeatability can be prepared bycontrolling the base pressure,temperature and duration of heat treatment.Thermochromic M1-phase VO₂ thin films can be fabricated under the base pressure of 600-2000 Pa with the annealing temperature at 470-590°C for more than 0.5 h.Moreover,increasing the concentration of NH₃ during annealing process is helpful to improve the purity of VO₂?M1?thin film.By synthetically considering their luminous transmittance(T_lum),solar modulation efficiency(?T_sol),and modulation efficiency at2000 nm(?T_2000nm),the most appropriate heat treatment condition is annealing under1000²000 Pa at 500⁵30°C for 0.5¹ h,and the optimal optical property exhibited in this paper is:T_lum?39⁴1%,?T_sol?7.7%,?T_{2000 nm}?50%.3.The N-doped VO₂?M1?thin films have been successfully synthesized through annealing in a closed NH₃ atmosphere.The N-doping in VO₂?M1?thin film contains substitutional doping mode and interstitial doping mode,and the doping level varies by NH3 concentration during the annealing process.With the increase of NH3concentration,the T_C of VO₂ thin film decreases from 57.8 to 48.7°C,and the width of hysteresis loop is narrowed by 17°C.The experiment and theory calculation results consistently suggest that both substitutional and interstitial N-doping can decrease the T_C due to the reduction of Eg₁.The N introduced by PVP in precursor is mainly absorbed on the surface or in the pores of VO₂ thin film.4.We can hardly synthesize VO₂ thin film but vanadium bronze on soda-lime glass,due to the diffusion of Na⁺ions from soda-lime glass substrate.Using borosilicate glass substrate may produce VO₂ thin film with well thermochromic property,but it is also affected by the formation of vanadium bronze companyed with VO₂?M1?phase.The best optical property of VO₂ thin film fabricated on borosilicate glass substrate is:T_lum?46.8%,?T_sol?5.5%,?T_{2000 nm}?33.5%.