Design And Synthesis Of Tungsten-molybdenum-vanadium-manganese-based Oxide Micro-nano Materials And Their Applications In The Energy Field

Energy has become a global social issue. The solar energy is an abundant and clean alternative energy. Photocatalysis is a technology utilizing the solar radiation to decompose the contaminants in water/air, and to convert the solar energy into electricity and chemical energy. Thus, photocatalysis plays an essential role for clean energy production and environmental remediation, where the preparation of high efficient photocatalysts is the key in this area. Because the solar energy is intermittent, energy storage devices such as batteries are required to provide back-up for intermittent solar energy. Lithium ion batteries (LIBs) are developing very fast and have dominated the markets of portable electronics in recent years. The key technology for LIBs is the fabrication of high performance electrode materials.This dissertation aims at the designed synthesis of novel materials with energy applications. A series of nano-and micro-structured W, Mo, V, and Mn based materials have been synthesized, including binary tungsten trioxide (WO3) and molybdenum trioxide (MoO3), ternary molybdenum-tungsten trioxide hydrate (MoxW1-xO3·0.33H2O) and copper vanadium oxide (Cu4V2.15O9.38), quaternary bismuth molybdenum tungsten oxide (Bi2MoxW1-xO6), and lithium manganese oxide spinel (LiMn2O4). The morphology and structure of these materials have been characterized in detail, their applications in LIBs and photocatalysis have also been studied.In Chapter 3, a facile route for the preparation of hexagonal shaped WO3·0.33H2O nanodiscs via hydrothermal treatment of peroxo-polytungstic acid solution has been developed. The orthorhombic WO3·0.33H2O is a metastable phase, calcination at 350 and 550℃leads to the formation of hexagonal h-WO3 nanodiscs and submicrometer sized monoclinic m-WO3 nanoparticles, respectively. The complete absence of protecting agents (e.g. surfactants) on the surface may make the products promising in photocatalysis.In Chapter 4, single-crystallineα-MoO3 nanobelts have been synthesized through a facile hydrothermal treatment method using peroxo-molybdic acid as the precursor. As a cathode material for LIBs, theα-MoO3 nanobelts exhibit high discharge capacity (264 mAh/g at 30 mA/g), excellent rate capability (176 mAh/g at 5000 mA/g), and good cycling performance (a capacity of 114 mAh/g was retained after 50 charge-discharge cycles at 5000 mAh/g). The multi-electron reduction process of Mo6+provides this material with high capacity, while the nanobelts morphology of the products ensures its rate capability and cycling performance.In Chapter 5, a series of MoxW1-xO3·/0.33H2O (x= 0,0.25,0.50,0.75) solid solutions have been synthesized through a facile hydrothermal treatment method, starting from a mixture of aqueous peroxo-polytungstic acid and peroxo-molybdic acid. With the increase of Mo content x, the band gap of MOxW1-xO3·0.33H2O narrowed from 3.25 to 2.77 eV. The increase M5+(M= Mo, W) fraction, which acts as the "color center" in the materials, is responsible for the narrowing of the band gap.In chapter 6, a series of homogeneous Bi2MoxW1-xO6 (x= 0,0.25,0.50,0.75,1.00) solid solutions have been synthesized by a hydrothermal crystallization method. When compared to the most studied Bi2WO6 photocatalyst, the valence band (VB) maximum of Bi2MoxW1-xO6 (x= 0.25,0.50,0.75) is elevated significantly, leading to a narrowed band gap and enhanced visible light harvesting ability. The photocatalytic activities of the Bi2MoxW1-xO6 solid solutions have been evaluated by utilizing the photodecomposition of methylene blue (MB) under visible light (λ> 400 nm) irradiation as a model reaction. Bi2Mo0.25W0.75O6 shows the highest photocatalytic activity for MB photodecomposition among the Bi2MoxW1-xO6 solid solutions. Our study sheds light on designing highly efficient visible-light-driven photocatalysts with controlled band gap and morphology.In Chapter 7, hierarchical Cu4V2.15O9.38 micro-/nanostructures composed of intergrown nanosheets and nanoplates have been synthesized via a facile hydrothermal treatment method. As a cathode material for primary LIBs, it delivers a high discharge capacity of 471 mAh/g above 1.5 V, which is much higher than the theoretical capacity of the commercial Ag2V4O11 cathode (315 mAh/g). The high discharge capacity makes the Cu4V2.15O9.38 a promising cathode material for implantable cardioverter defibrillators (ICDs).In Chapter 8, LiMn2O4 hollow spheres have been prepared using a simple solid state reaction between porous MnO2 microspheres and LiOH·H2O. The fusion of the initial mesopores and the "Kirkendall effect" during the lithiation process are responsible for the formation of the hollow interiors. Both the wall thickness and void size can be tuned by employing a "controlled decomposition" and "selective etching" process. The LiMn2O4-A hollow spheres with a thick wall and small void show much better electrochemical performances than that of LiMn2O4-B with a thin wall and large void. LiMn2O4-A hollow spheres deliver a discharge capacity of～120 mAh/g at 0.1 C,106.7 mAh/g at 5C, and retains 96.6% of the capacity after cycling at 1 C for 105 charge-discharge cycles. When compared to the traditional grounding method, the impregnation method we employed allows for a homogeneous contact of the reagents at the nanoscale, thus lowers the lithiation temperature, and leads to products with higher purity and better electrochemical performances.