| Some vanadium oxides such as vanadium dioxide (VO2) and vanadium pentoxide (V2O5) were chosen as research objects. In the dissertation, we focused on the preparation and morphologies control of VO2and V2O5nanomaterials via hydrothermal and CVD methods, their growth mechanisms were analyzed and their applications in humidity sensing, field emission and Li-ion battery were also emphatically investigated. The main results obtained in this dissertation are as follows:1. Synthesis of VO2nanostructures via hydrothermal and CVD methodsVO2(B) nanostructures were synthesized via a facile hydrothermal process using V2O5or NH4VO3as source material and oxalic acid (H2C2O4) as reductant, the influence of the concentration of the reductant were discussed, and the morphology control method of VO2(B) nanostructures were investigated. It was demonstrated that the morphologies of as-fabricated VO2(B) nanostructures were closely related to the concentration of oxalic acid. When NH4VO3are used as source materials, different VO2(B) nanostructures such as nanorods, nanoflakes, nano-spiky-balls and hollow nanospheres are synthesized by adjusting the concentration of oxalic acid. When V2O5are used as source materials, three nanostructures of nanorods, nanocarambolas and nanobundles were found existing in the products, and a continuous changing of morphology was found in the synthesis process, during which the proportion of these three types of nanostructures can be adjusted by altering the concentration of oxalic acid. Moreover, VO2(M1) nanowires and VO2/ZnO core-shell nanostructures were also successfully synthesized via a CVD route, and four growth stages such as melting, reduction, nucleation and growth of VO2(M1) nanowires, and the corresponding growth mechanism are also discussed.2. Humidity sensing properties of VO2nanostructuresFlower-like VO2(B) nanostructures having large surface area were synthesized via hydrothermal method, and VO2(M1) nanostructures were obtained by heating VO2(B) nanostructures in an inert atmosphere. The subsequent SEM research indicated that the morphologies were not destroyed by the heat-treatment, which was very similar before and after the heat-treatment. Two sensors based on VO2(B) and nanoflowers were constructed and their humidity properties were studies. It was found that the resistances of these two sensors changed linearly with the relative humidity, but showing a reverse variation trend each other. The resistance of the VO2(B) sensor increased linearly with relative humidity, but the resistance of the V02(M) sensor decreases linearly with relative humidity. Static and dynamic measurements indicated that the two sensors exhibited fast response and recovery, perfect reproducibility and good stability. The VO2(B) type sensor had a higher sensitivity at low RH, while the VO2(M1) type sensor is more sensitive at high RH, indicating that VO2(B) and VO2(M1) type sensor could be used for low and high humidity detection, respectively.3. Field emission properties of VO2nanostructuresVO2(B) nanomaterials were synthesized via a hydrothermal route, VO2(M1/R) nanobundles were then obtained through a heat-transition, and field emission properties of the two types of VO2nanostructures were investigated for the first time. Field emission properties of different VO2nanostructures which have different morphologies were studied. The influence of the morphologies on field emission properties was also analyzed. Moreover, the temperature-dependent field emission of VO2(M1) nanostructures were emphatically studied. It is demonstrated that field emission properties were significantly improved by increasing the ambient temperature, with the turn-on field decreasing from4.65V/μm (298K, M1phase) to2.1V/μm (383K, R phase). About a three-orders-of-magnitude increasing of the emission current density has been observed at a fixed field of6V/μm. The temperature dependent work function of these VO2(M1/R) nanostructures measured by UPS analysis, showed that the work function decreases when the temperature increases, and there was a decrease of～0.4eV (from333to343K) near the phase transition temperature. Therefore, the electron may be emitted more easily from R phase nanostructures. Density-functional theory calculations indicated that the decrease of work function with temperature is the main cause of the improvement of FE properties. In the insulating and metallic phases, the decrease of work function was mainly caused by the gap narrowing and the upward shift of Fermi level respectively. These characteristics make VO2(M1/R) a candidate material for new type of temperature-controlled field emitters.4. Synthesis of V2O5nanostructures and their application in humidity sensing and Li-ion batteriesV2O5nanostructures having large surface area were firstly mass-synthesized via a modified chemical vapor deposition process. The structural characterization indicated these nanostructures were aggregated with large quantities of V2O5particles with size of30-120nm, forming1D hierarchical V2O5nanostructures or V2O5nanotubes with porous wall. A humidity sensor based on a single porous V2O5micro/nano-tube was constructed, and humidity sensing measurements indicated fast response/recovery. Due to the mixed conductive mechanism of V2O5, the sensitivity exhibited nonlinear variation with the relative humidity. Because of the large surface area, the porous V2O5micro/nano-tubes were used as a cathode material for lithium-ion batteries that exhibited great improvement of electrochemical performance such as high lithium-storage capacity of～290mA h g"1, excellent cycling stability and reversibility. Further cyclic voltammetry measurements under different scanning rates confirmed that the intercalation/de-intercalation is of high efficiency and the kinetic performance of the battery is significiantly improved. The porous structure and large surface area of these porous V2O5nanotubes are favorable in reducing the diffusion distance of the solid-state lithium ion. Thus, the intercalation and extraction processes are of much higher efficiency, and the high rate capability of the lithium-ion battery can be achieved.
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