掺钨VO_(2)纳米粉双相界面自组装成膜及微结构的光学性质

Optical Properties of Tungsten-doped VO_(2) Films and Microstructure Formation by Nano-powders Using Double-phase-interface Self-assembled Method

VO_(2)的相变温度68℃限制了其在室温环境中的应用和安全性,采用W^(6+)离子掺杂的方法可有效降低VO_(2)相变温度。利用涂有真空硅脂的聚苯乙烯模具和添加表面活性剂的W_(x)V_(1-x)O_(2)纳米溶液,在玻璃基底上通过双相界面自组装方法将水热法合成的W_(x)V_(1-x)O_(2)纳米粉末制备成薄膜,在此基础上加装直径1μm的聚乙烯线制备W_(x)V_(1-x)O_(2)/glass微结构。直接观察到液/固双相界面上毛细管流的运动过程,以及微结构两侧对称出现半月形的现象。液-固-气系统复杂的动态变化的过程,遵循流体动力学和热力学的规律。X射线衍射表征结果表明,自组装W_(x)V_(1-x)O_(2)薄膜为多晶结构,主要成分为M相VO_(2)和少量钒的其他价态氧化物及少量钨钒的氧化物(WVO_4);近红外透过率测试显示W_(x)V_(1-x)O_(2)/glass薄膜随温度变化规律与理论模拟计算的趋势吻合;其温度-透过率曲线“突变”特性优于旋涂法制备的薄膜;可见光的微结构衍射图样类似光栅衍射图样,表明此种薄膜和微结构制备方法可行。研究结果可应用于防护涂层制备和微结构光学调控领域。

Vanadium dioxide(VO_(2))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 VO_(2) is limited by the transition temperature at 68℃.Ion doping is an effective method to effectively reduce the VO_(2) phase transition temperature,and W6+doping can reduce the VO_(2) 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,W_(x)V_(1-x)O_(2)/glass films were prepared by bi-phase interfacial self-assembly method.Under hydrothermal conditions,pure W-doped VO_(2)(B)nano-powder was synthesized by hydrothermal crystallization using oxalic acid,tungstic acid and VO_(2) as raw materials,and then the crystalline transformation from VO_(2)(B)to VO_(2)(M)powder was realized by annealing process.The W_(x)V_(1-x)O_(2) nano-solution was synthesized by ultrasonic mixing of VO_(2) 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.W_(x)V_(1-x)O_(2)/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 W_(x)V_(1-x)O_(2) film at the bi-phase interface,Polyethylene(PE)wire with diameter of 1μm was added into the mold,and the microstructure of W_(x)V_(1-x)O_(2) film was fabricated by a simple method.The optical transmittance of W_(x)V_(1-x)O_(2)/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 VO_(2)(M).Although the polycrystalline state,VO_(2)(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%W_(x)V_(1-x)O_(2)/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 VO_(2).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.