Synthesis, Properties And Devices Study Of One-dimensional Nanostructured Molybdenum And Vanadium Oxide

Because of their rich physical and chemical properties, transition metal oxide materials have always been a hot research field. Among these kind of materials, due to the various valences and special features in crystal and electronic structure, molybdenum oxide and vanadium oxide have attracted much interest for important applications such as energy storage, functional devices, etc. In this Ph. D. dissertation, we focuses on the study of sample preparation with the growth mechanism, properties, modification and functional device fabrication of one-dimensional molybdenum and vanadium oxide nanomaterials, to explore the relationship between structure and properties. The main findings and conclusions as follows:1. A sol hydrothermal method has been developed to synthesize orthorhombic MoO3 nanobelts. The effect of synthesis conditions on the morphology has been investigated to explain the condensation-nucleation-aging process for the growth mechanism. Based on these results, face-down floating hydrothermal technology has been developed to prepare MoO3 nanoneedle array on different surfaces, and the array epitaxially grown on the buffer layer which can enhance the bonding strength between the substrate and MoO3 structures.2. MoO2 nanorods have been synthesized using MoO3 nanobelts as self-sacrificing template in reduced atmosphere. The MoO3 nanobelts split along the axial direction to form MoO2 nanorods and the anisotropic bonding strength in accordance with the crystal structure of MoO3 nanobelt responsible for this special formation mechanism. MoO2 nanorods were also obtained by one-pot hydrothermal process using ethanol as a weak reducing agent, and the phase evolution of MoO3→MogO23→MoO2 is demonstrated by theoretical and experimental analysis.3. Combining the hydrothermal and microemulsion technology, a complex of molybdenum oxide nanorods were synthesized and the sustained release growth model was first proposed. Moreover, this process can be utilized as a universal method for preparation of other natrochalcite-type molybdenum oxide nanorods, and the effect of synthesis conditions on the morphology and formation mechanism of nanorods is deeply investigated. We found that the pH value is critical for precursor grain size control in the initial reaction, which provided the primitive for nanorods formation.4. Vertical-standing single-crystal VO2 nanowires were synthesized (aspect ratio> 100) by optimized PVD process, and we found the velocity of carrier gas and tilting angle of substrate are important roles for the formation of free nanowires, and obtained the optimum synthesis conditions. On this basis, ultralong V2O5 nanobelts can be synthesized by mixing a small amount of oxygen in the carrier gas. Both vanadium oxides are very flexible, therefore have the potential application in novel electronic devices, such as strain sensors and fiber waveguides.5. The electrochemical properties of MoO3 nanobelts is studied and the first discharge capacity was up to 300 mAh/g which takes advantage of special layered structure, and we built a theoretical model to explain the capacity decay with cycling. We used a novel secondary hydrothermal technology for lithiation of MoO3 nanobelts, and found the electrochemical cycling stability is significantly improved and found the nanobelt has transformed from semiconducting to semi-metallic with the conductivity increased by 2 orders of magnitude. In addition, similar results of other semiconductor nanomaterials also proved that the lithiation can be a universal way to optimize the electrochemical properties of electrode materials, which provides an effective way of thinking to enhanced the materials performance in the lithium-ion battery.6. AFM was utilized to study the mechanical properties of single MoO3 nanobelts. Compared with the bulk MoO3 (540 GPa), we found one order of magnitude decrease of Young's module (31 GPa) for MoO3 nanobelt, which means the nanobelts is much easier to be bent in the elastic region. In addition, utilizing the largest compressibility along b-axis direction of MoO3 nanobelt due to the presence of van der Waals forces, a sensitive miniature piezoresistive device is fabricated, and a small volume deformation of nanobelt results in a large change in conductivity, which can be used in microelectronic devices to monitor the environmental pressure.7. Electrical transport properties of single MoO2 nanorod were studied under different bias voltages to explore the stability of the device. We found the conductivity of single MoO2 nanorod can reach～190 S/cm at low voltage, and this semi-metallic feature comes from the special electronic structure in MoO2. High electric field (>2500 V/cm) can switch the current transport mechanism into Schottky emission, while even higher 5 V bias would cause the surface-oxidation of rough MoO2 nanorod, thus the resistivity increased. In additional, the electrochemical properties of MoO2 nanorods was studied, which exhibited the reversible and stable phase transition between monoclinic and orthorhombic during the lithium ions insertion and extraction, and the capacity can be up to～300 mAh/g. The initial capacity is inversely proportional to the current density, however the capacity will gradually increase under high discharge current density with the cycle, and the mechanism of this phenomenon is discussed.8. The ratio control of insulating phase between M1 and M2 was achieved by loading the external stress in VO2 nanowire and confocal Raman microscopy was utilized to in-situ observe the relationship between phase-domain and stress, and reveals the competition mechanism between the insulating phase. Based on this principle, first high-sensitivity VO2 nanowire strain sensor was assembled, and the gauge factor can be as higher as 347, much better than traditional metal film and silicon nanowire based strain sensor, and with the characteristics of fast and reversible response. Therefore, it is expected to apply this kind sensor in the field where need to monitor tiny stress such as bridge engineering and optical fiber manufacturing.9. Combining self-heating and external stress these two environmental stimuli, a high-performance electro-mechanical switch was fabricated based on MIT in VO2 nanowire, and the MIT can be controlled through the coupling the bias voltage and applied stress. More importantly, we achieve the phase transition as a single-domain, thus allowing the device with four orders of magnitude change in resistance. This switch is very stable and fast and can be applied in the robot sense, logic gates and other electro-mechanical devices.10. Aligned nanowire film of Mo-doped VO2 has been prepared by simple melting-quenching followed by heat treatment in a vacuum. We found that the amount of doping can be well controlled and the introduction of Mo element changes the growth dynamics and thermodynamics for VO2 formation, and results in the relaxation of V-V Peierls pair, thus reducing the insulating phase stability and decreasing the transition temperature from 64℃down to 41℃. At the same time, as the electron density increased in the nanowires, resulting in the shift of Fermi level to the conduct band in VO2, so that the activation energy reduced. These results provide an important experimental and theoretical basis for the design of room-temperature phase transition devices.