Preparation, Characterization And The Physical Properties Of The A, B And M Phases Of VO₂

Vanadium dioxide(VO2) undcrgos a reversible metal-insulator phase transition at 341K, from a insulator phase under low temperature to a metallic phase duner high temperature. Accompanied with the first-order phase transition, its electrical and magnetic properties exhibit a sudden invertible transformation and the optical transmittance of VO2 also changes a lot. The macrophysical properties change suddenly with the alter of the temperature makes VO2 nano material a great candidate in the field of smart materials.In this work, we take VO2 as the research target. Applying the hydrothermal approach, VO2(B) and VO2(A) were synthesized in a V2O5-H2C2O4-H2O system by controlling the reaction condition. VO2(M) was prepared by annealing in the later stage. The products were characterized through various testing and analytical methods. The phase transition, optical,electrical, magnetic and electrochemical properties were studied systematacially.The compositon of the sample and oxidation state of the vanadium was confirmed by X-ray photoelectron spectroscopy(XPS). The phase structure was examined using X-ray diffraction(XRD). The surface morphology, growth orientation and degree of crystallinity were detected employing scanning electron microscopy(SEM) and transmission electron microscope(TEM). The optical properties of VO2(A) were researched using fourier transform infrared spectroscopy(FT-IR), ultraviolet and visible spectrum (UV), photoluminescence spectroscopy (PL) and raman spectrum. The magnetic susceptibility of the VO2(A) sample were measured by a Physical Property Measurement System(PPMS) from Quantum Design Company. We test the electrical resistivity of VO2(A) using a four probe method. At last, the electrochemical property of the VO2(B), VO2(A) and VO2(M) were tested applying an electrochemical workstation. The major work and results are listed in detail as following:1.Taking V2O5 as the resource, under a condition of relatively lower temperature and filling ratio,well crystallized VO2(B) nanobelts and V02(A) nanorods were synthesized through controlling the molar ratio between V2O5 and oxalic acid. A mechanism of transformation, self-assembling,attaching and rccrystallization was proposed to explain the growth process of VO2(A) nanorods.2.The temperature dependence of the phase transition temperature, electrical resistivity and magnetic conductivity were studied during the phase transition process.The optical switching property was found by infrared spectra at various temperatures.Using ultraviolet absorption spectrum and photoluminescence spectrum, the bandgap and emission gap information were inspected.3.The electrochemistry property of VO2(B) and V02(A) nano materials were measured and analysised.From the cyclic voltammogram, it was found that electrodes made by V02(B) and VO2(A) have good electrochemical properties and reversibility.4.The transition temperature of V02(M) was controlled by doping with boron and tungsten respectively that widen its range of application as smart materials.