SYNTHESIS AND PHYSICAL PROPERTIES OF MICRO-NANOSTRUCTURED V2O5: STRUCTURE, OPTICAL CHARACTERIZATION, AND APPLICATION

Abstract

Bulk V2O5 is a diamagnetic semiconductor with a band gap (Eg) of about 2.30 eV, which is based on the ionic configuration with filled O2p and unoccupied V3d orbitals. However, the special electronic structure of V2O5 forms three bands, including V3d states, V3d split-off states, and mid-gap states, which lead to interesting optical properties of V2O5 micro-nanostructures. They have four main transitions, including the transition between VB and V3d states of CB, the transition between VB and V3d split-off of CB, the transition between VB and the mid-gap defect state, and the transition between mid-gap defect states and CB. Therefore, the band edge absorption and photoluminescence (PL) peak positions of low-dimensional V2O5 materials do not coincide and are distributed over wide ranges of 0.75 to 3.49 eV and 0.73 to 3.30 eV, respectively. The wide ranges of band edge absorption and broad PL of V2O5 micro-nanostructures are clarified in terms of factors such as the morphology, synthesis method, growth conditions, crystal size, micro-nano size, phase transition, and measurement conditions. These factors not only affect the broad emission but also the PL intensity. Fundamental understanding of the optical characteristics plays a key role in V2O5 micro-nano device applications. This thesis summarizes and analyzes the fabrication processes, structure, optical characterization, and photocatalytic activity of V2O5 micro-nanostructures, including thin films (TFs), nanoparticles (NPs), micro-nanorods (NRs), micro-nanowires (NWs), nanospheres (NSs), nanohollows (NHs), and V2O5/RGO nanocomposites. The relations among the separation, diffusion, recombination, and degradation of the electron-hole pairs in V2O5 micro-nanostructures are also discussed. The thesis also demonstrates the role of V2O5 micro-nanostructures and other materials (OMs) in V2O5/OMs heterostructures for slowing down recombination, prolonging lifetime, improving electron-hole separation, and increasing photocurrent to enhance the photocatalytic activity. V2O5 TFs with micro-nanostructure were fabricated by an electrodeposition method using an aqueous solution of NH4VO3. The annealing temperature strongly affects the surface morphology, crystal structure, and photoluminescence (PL) properties. The formation of α-V2O5 structure occurred when the sample was annealed at temperatures below 500 ℃. As the annealing temperature increases, some of the α-V2O5 structures were distorted and restructured to form a high-quality mix of α-β phase V2O5. This leads to wide absorption and enhancement of the visible-light due to the presence of numerous defects on the surface of the V2O5 films. V2O5 nanostructures of NPs, NRs, NWs, NSs, and NHs were prepared by hydrothermal and chemical reaction methods. The morphology measurements showed well-shape nanostructures, the X-ray diffraction (XRD) and Raman results revealed that all nanostructures had an α-V2O5 phase with an orthorhombic structure. A large increase of the V4+ oxidation state in NSs compared to the other nanostructures was observed by X-ray photoelectron spectroscopy (XPS) measurements and confirmed by Raman spectroscopy. The results show that a larger number of V4+ oxidation states of V2O5 NSs strongly enhanced PL intensity compared with other structures that showed weak PL. In particular, V2O5 NSs showed intense ultraviolet (UV) PL near 395 nm (~3.14 eV) due to strong excitation by UV light, while this PL peak was not observed from other nanostructures. The bottom level of the CB of V2O5 has to be less negative than the redox potential of H+/H2 (eV vs. NHE), but electrons in the CB can react in the oxidation reaction with dye solution due to the special electron structure of V2O5. A large amount of charge separation in V2O5 NSs and the large surface contact area in V2O5 NHs and NPs result in more efficient photocatalytic activity than from V2O5 NRs and NWs. Reduced graphene oxide (RGO) was mixed with pure V2O5 nanostructures to form V2O5/RGO nanocomposites. The peak photoluminescence (PL) intensity is around 670 nm in the V2O5/RGO nanocomposites, which is much lower than that of pure V2O5. This seems to be evidence of facile electron transfer from V2O5 to RGO due to the strong adhesion of RGO with pores on the V2O5 surface. This leads to the enhancement of the sunlight photocatalytic activity of the V2O5/RGO nanocomposites. The relation between the separation, diffusion, recombination, and degradation of the electron-hole pairs in the V2O5 nanostructures and V2O5/RGO nanocomposite is discussed.