Optical Properties of Tungsten-doped VO2 Films and Microstructure Formation by Nano-powders Using Double-phase-interface Self-assembled Method
Vanadium dioxide(VO2)has a phase transition between the high-temperature metal state and the low-temperature semiconductor state,accompanied by significant optical property changes.However,the applicability of pure VO2 is limited by the transition temperature at 68℃.Ion doping is an effective method to effectively reduce the VO2 phase transition temperature,and W6+ doping can reduce the VO2 phase transition temperature to 20～30 K/wt%.The dual-interface self-assembly method based on nanomaterials is environmentally friendly and simple to operate,and has the potential of large-scale production.In this study,WxV1-xO2/glass films were prepared by bi-phase interfacial self-assembly method.Under hydrothermal conditions,pure W-doped VO2(B)nano-powder was synthesized by hydrothermal crystallization using oxalic acid,tungstic acid and VO2 as raw materials,and then the crystalline transformation from VO2(B)to VO2(M)powder was realized by annealing process.The WxV1-xO2 nano-solution was synthesized by ultrasonic mixing of VO2 nano-powder with tungsten(W)atom ratio of 0%,1%,2%and 3%,high purity anhydrous ethanol and Polyethylpyrrolidone(PVP,concentration of 98%)at 1∶1∶28 weight ratio.WxV1-xO2/glass film samples with tungsten concentration of 0%,1%,2%and 3%were prepared by vacuum silicone(Polydimethylsiloxane,PDMS)coated with self-made Polystyrene(PS)mold and nano-sized tungsten-doped vanadium dioxide solution added with surfactant on glass substrate,respectively.At the same time,the capillary flow movement and film formation process at the liquid/solid dual phase interface,as well as the double"coffee ring"pattern were directly observed.The complex dynamic change process of liquid-solid-gas system follows the laws of fluid dynamics and thermodynamics.On the basis of preparing WxV1-xO2 film at the bi-phase interface,Polyethylene(PE)wire with diameter of 1 μm was added into the mold,and the microstructure of WxV1-xO2 film was fabricated by a simple method.The optical transmittance of WxV1-xO2/glass thin film was measured by using a laser with wavelength of 980 nm as the light source.Ultima IV equipment of Rigaku in Japan was used to test the crystal structure of the film by X-ray Diffraction(XRD)and analyze the crystallization of the sample.The surface morphology of the thin film samples was measured by the German ZEISS GeminiSEM 300 Scanning Electron Microscope(SEM).By comparing the XRD pattern with the standard card,the main crystal structure of the sample is VO2(M).Although the polycrystalline state,VO2(M)has the largest(011)crystal plane diffraction peak intensity,which is the component that has the greatest influence on the results of subsequent property tests.With the SEM photos,it can be observed that most of the surface of the films are cuboid grains with some small grains attached.It can be seen that the film is a preferentially oriented polycrystalline film,which is mutually confirmed by XRD results.Compared with the near-infrared optical transmittance of 2%WxV1-xO2/glass prepared by the spin-coating method,the optical transmittance of the filmprepared by the bi-phase interface self-assembly method is better than that of the previous methods in decreasing trend with increasing temperature.The microstructure diffraction pattern was observed by He-Ne laser.The visible light microstructure diffraction pattern is similar to the grating diffraction pattern,which indicates that the preparation method of the film and microstructure is feasible.The electromagnetic wave frequency domain module in COMSOL software is used to simulate the theoretical transmittance of VO2.This module uses finite element method to solve Maxwell equations.In the near infrared light band,the transmittance remains relatively high at low temperature,but drops sharply at high temperature,which is the same as the transmittance curve of vanadium dioxide described in the literature.By multiplying the experimental data of transmittance of thin film sample with W doping concentration of 0%by 10 times and comparing with the simulation data,it can be seen that the simulation results are consistent with the experimental ones.It is proved that a low cost and easy to implement dual-interface self-assembly method can be used to prepare microstructures for regulating optical properties,which provides a new solution for the self-assembly preparation of nano-powder films.The results can be applied in the fields of protective coating preparation and microstructure optical control.
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