Synthesis Of Vanadium Dioxides Micro-nanostructures And Their Smart Properties

Vanadium oxide nanomaterials have exhibited promising applications in sensors, optics, magnetism and energy storage and energy saving. In this dissertation, preparation of nanomaterials with well-defined size, morphology, dimensionality and diversity has been developed through novel synthesis approaches and stratages. The valuable explorations of their novel structures and properties have been carried out based on the structural analysis and theoretical guidance of vanadium oxides; meanwhile, the general synthetic routes have been developed to synthesize vanadium oxides nanostructures, especially for those two dimension structure of ultrathin nanosheets, along with their formation mechanisms, emphasizing on the structural-related properties. The main parts of the results are summarized briefly as follows:1. New-phased VO2micro/nanostructures built by nanoflakes have been first synthesized by a hydrothermal method with NH4VO3as precursor in the presence of poly (vinyl pyrrolidone)(PVP). The combined structural analysis of X-ray powder diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy determined the crystal structure as a new-phased vanadium dioxide, which is the isostructure of monoclinic NiWO4and designated as VO2(D). In particular, electron spin resonance (ESR) measurement provides the direct evidence of vanadium ion at the four oxidation state. The formation energy of VO2(D) was estimated and showed a very close value to rutile-type V02(R), which guided the preparation of V02(R/M) by making use of the structural transformation from VO2(D) to VO2(R) at320℃, which was a comparatively lower temperature from other vanadium oxide, such as VO2(B). The obtained VO2(R) shows the reversible metal-to-insulator transition (MIT) near critical temperature (Tc) which is associated with clear changes in differential scanning calorimetry (DSC) curves. In addition, the temperature-dependent DC electrical conductivity of the new-phased VO2(D) exhibits Arrhenius-type behaviour, which reveals its semiconducting character with a band gap of0.33eV. ESR and temperature-dependent magnetic susceptibility measurements were employed to obtain information for the magnetic properties of VO2(D), which correspond to one-dimentional Heisenberg system.2. A convenient room-temperature intercalation-deintercalation strategy was proposed for the first time to synthesize layered compounds with strong interlaminar covalent bonds. As an example, VO2(B) single layers with atomic thickness of～0.69nm were first synthesized and time-dependent experiments suggested the involved intercalation-hydration-exfoliation process. This VO2(B)single layer possesses a more symmetric atomic structure with slight lattice expansion, as revealed by synchrotron radiation X-ray absorption fine structure (XAFS) explained the unique electronic structure and excellent structural stability. First-principle calculation indicated that the resulted VO2(B)single layers showed a band gap of⊿Eg=0.19eV higher than that of bulk counterpart, which is in good agreement with experimental UV-vis spectroscopy. This work opens the door for extending the two-dimensional nanomaterials with atomic-thickness aside from graphene, providing more possibilities for the energy-level engineering in photo voltaics and catalyst and may spark new discoveries in condensed matter physics and nanoelectronics.3. M phased VO2ultrathin nanosheets with thickness of only-3nm have been successfully synthesized by a developed lithiation-exfoliation method, which was called M phase lithiation-R phase exfoliation method. This method provided a novel way to elaborate VO2(M) nanostructures with highly crystalline under soft condition. The variable temperature FT-IR spectra of the VO2(M/R) ultrathin two-dimensional structure revealed their excellent infrared through/blocking performance in the infrared range during the phase transition. Additionally, the UV-visible-near infrared diffuse reflectance spectroscopy was first proposed to identify the changes of the energy band gap of the VO2(M) samples and revealed the features of its enhanced metallization. In particular, the local structures of VO2(M) ultrathin nanosheets were studied through synchrotron radiation X-ray absorption fine structure (XAFS), combined with first-principle calculation took use of structure differences revealed by XAFS, which explained the excellent phase transition performance in the infrared range and its enhanced metallization owing to its ultra-thin two-dimensional structure. VO2(M) ultrathin nanosheets may also be an attractive candidate for the use of smart window coatings because of their excellent optical transparency and thermochromic property due to the ultrathin2D nanostructures.