Preparation, Characterization And Properties Of Thermochromic Vanadium Oxides And Their Composites

Vanadium oxides show excellent electrical, optical, magnetic, and catalytic properties, which make them have lots of potential applications in many fields. One of the most important applications is as "smart window coatings", and these vanadium oxides possess thermochromic properties. The low-cost, facile and large-scale synthesis of thermochromic vanadium oxides are a great challenge for materials scientists. In this thesis, thermochromic vanadium oxides and their composites are designedly synthesized by the hydrothermal method. The aim is to develop the novel method to the synthesis of thermochromic vanadium oxides by looking for the optimal synthesis conditions.VO2(A) micro-nanobelts are successfully prepared by a one-step hydrothermal route. The influence of different synthetic conditions on the synthesis of V02(A) is investigated, and both the reaction mechanism and formation mechanism of the synthesis of VO2(A) is studied. It is found that the phase transition temperature (Tc) of the as-obtained VO2(A) is about160?. It is firstly found that the optical properties of VO2(A) change with its phase transition, and this transition has a good reversibility. These properties reveal VO2(A) can be used as the optical switching materials. The oxidation resistance property of VO2(A) indicates that it has good thermal stability and oxidation resistance below400?in air. V02(M) can be prepared by the irreversible transformation of VO2(A) at700?for2h under the inert atmosphere.In H2O2-EtOH-H2O system, VO2(B) and doped VO2(B) are first synthesized by the hydrothermal route, then VO2(M) and doped VO2(M) are obtained by subsequent calcination under the inert atmosphere. Doped V02(M) is directly synthesized using H2O2-EtOH-H2O and H2C2O4-H2O systems by controlling the synthetic conditions via a one-step hydrothermal route. The influence of different additives on the formation of VO2polymorphs by a facile hydrothermal route was studied, and it is found that F, Ti, Cr, Fe, Mo, Sn, Sb and W atoms can promote the formation of VO2(M), while Mg, Al, Co and Ni atoms are favorable for the synthesis of VO2(B), whereas, Na, Ca, Mn and Zn atoms are favorable for the formation of VO2(A). W, Mo and F atoms can reduce the Tc of VO2(M), whereas Ti, Cr, Fe, Sn and Sb atoms have little effect on the Tc. The characterizations of the as-obtained VO2(M) indicate that the dopant atoms are successfully doped into V02(M) crystal lattice, and the morphology of the samples is micro/nanobelts. The influence of the concentration of W doping on the Tc is studied in detail, and it is found that the Tc of W-doped VO2(M) can be simply tuned by changing the doping concentration of W atom, and even to the designed Tc. The as-obtained V02(M) and doped VO2(M) exhibit good thermochromic properties, which suggests they can be used as smart window materials. In the case of one-step hydrothermal synthesis of doped VO2(M), the additives play the crucial role in the preparing VO2(M):Only VO2(A) is obtained without additives, whereas VO2(M) can be directly synthesized with adding appropriate dopants. All the results suggest this method is low-cost, environmentally friendly and large-scale. The enlarged-scale experiments (20times) for the preparation of doped VO2(M) were carried out, and the similar results are obtained.Highly uniform V2O3microspheres with rough surface are directly synthesized by a facile hydrothermal decomposition of VOC2O4solution, and these microspheres are made up of small particles by self-assembly. The formation mechanism of V2O3microspheres can be explained as that the autogenic CO and CO2gas from the decomposition of VOC2O4can be served as the soft templates. Based on the synthesis of V2O3microspheres, it is found that different metal oxides microspheres can be synthesized by the hydrothermal treatment of the precursor which consist of metal oxides and oxalic acid, such as AIO(OII), Fe3O4,?-Fe2O3, Co3O4, Cr2O3, MoO2, WO3, V-Cr-O, V-Fe-O, and so on. These microspheres have a uniform particle size, rough surface, large specific surface area (80?110m2·g1). This process can be explored as a methodology to synthesize different metal oxides microspheres.Carbon coated on thermochromic vanadium oxides (VOx@C) core-shell structured materials are designed and synthesized:VO2(M)@C and V2O3@C composites are successfully synthesized by the thermal treatment with V3O7·H2O@C composite under the inert atmosphere; and V2O3@C composite is also successfully synthesized by the thermal treatment with VO2(B)@C composite under the inert atmosphere. VO2(M)@C and V2O3@C composites are explored as the cathode materials to apply in lithum-ion batteries, indicating that they exhibit better electrochemical properties than that of pure VO2(M) and V2O3.