| Summary: | 博士 === 國立東華大學 === 物理學系 === 106 === Transition metal oxides are well known for their large variety of physical and chemical properties, such as electronic, thermoelectric, magnetic, optical, and electrochemical properties. Among the transition metal oxides, molybdenum oxide and vanadium oxide are two of the interesting materials due to their stable and metastable oxidation states with varying valences. The morphology of molybdenum oxide and vanadium oxide can be satisfied into a variety of forms, including zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) nanostructures. Among all nanostructured materials, 1D and 2D nanostructures of molybdenum oxide and vanadium oxide possess versatile and excellent electrical, optical, and electrochemical properties. Due to their versatile optical, electronic, and electrochemical properties, 1D and 2D nanostructures of molybdenum and vanadium oxide have the potential for use in practical devices such as electrochromic devices, field emission devices, photocatalysts, photodetectors, gas sensors, batteries, and supercapacitors.
One-dimensional nanorods of MoO2, MoO3, Magnéli-phase Mo4O11, and two-dimensional nanosheets of VO2 on conducting indium-tin-oxide (ITO) thin films coated on glass substrates were effortlessly prepared using the hot-filament metal oxide vapor deposition technique. Thermal reduction and oxidation were then used to process 1D Magnéli-phase Mo4O11 nanorods and 2D V2O5 nanosheets into 1D MoO2 nanorods and 2D VO2 nanosheets, while 1D MoO3 nanorods were prepared by using the thermal oxidation process of 1D Magnéli-phase Mo4O11 nanorods. The nanostructures prepared at higher synthesis temperatures were thinner and longer. The 1D Magnéli-phase Mo4O11 nanorods consisted of various combinations of two orthorhombic (α) and monoclinic (η) crystals and varying mixtures of Mo4+, Mo5+, and Mo6+ (3d5/2 and 3d3/2) cations. The 1D MoO2 nanorods were comprised of only monoclinic crystals and various complex mixtures of Mo4+, Mo5+, and Mo6+ (3d5/2 and 3d3/2) cations. The 1D MoO3 nanorods contained only orthorhombic crystals and varying mixtures of Mo5+ and Mo6+ (3d5/2 and 3d3/2) cations. The 2D VO2 nanosheets possessed a monoclinic structure and only V4+ (2p3/1 and 2p1/2) cations.
The optical properties of 1D Magnéli-phase Mo4O11 nanorods synthesized at varying synthesis temperatures were studied through absorbance and transmittance measurements. The bandgap of 1D Magnéli-phase Mo4O11 nanorods can be acquired by using the absorbance spectra. The bandgap decreases linearly with the elevation of temperature, meaning that the bandgaps of the 1D Magnéli-phase Mo4O11 nanorods can be tuned or tailored without doping with other materials. Obviously, the bandgap tuning occurs due to varying combinations of two orthorhombic (α) and monoclinic (η) phases and various mixtures of the Mo4+, Mo5+, and Mo6+cations.
The electrochemical characterization, charge storing properties, and capacitive performance of the 1D MoO2, MoO3, Magnéli-phase Mo4O11 nanorods, and 2D VO2 nanosheets was examined through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurement and electrochemical impedance (EI) spectroscopy. The synthesis of 1D MoO2, MoO3, Magnéli-phase Mo4O11 nanorods, and 2D VO2 nanosheets at higher temperatures improves their capacitive behavior. The electrochemical results verify that the capacitive performance of the 1D MoO2 nanorods and 2D VO2 nanosheets are superior to that of the 1D MoO3 or Magnéli-phase Mo4O11 nanorods, making them suitable for the electrode materials in supercapacitors.
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