Preparation And Electrochemical Performance Of Micro/NanoHierarchical Tungsten,Molybdenum Oxides

In recent years,micro/nano hierarchical structure materials have aroused wide attention due to the advantages of size effect of nano-structure and structural stability of micro-structure.In this paper,the h-WO3·0.28H2O micro-disks assembled by nanorods were successfully prepared via a hydrothermal method.Similarly,the MoO3 micro-flowers assembled by nano-sheets were also prepared via a solvothermal method.The structure and electrochemical performance of the samples were deeply discussed in this paper.Besides,we also investigated the formation process of h-WO3·0.28H2O micro-disks and h-MoO3-x micro-flowers.The main research contents are as follows:1.The h-WO3·0.28H2O micro-disks were prepared by a hydrothermal method.Using sodium tungstate as the tungsten source and NaCl as the directing agent,a hexagonal phase h-WO3 with micro/nano hierarchical structure was prepared by a simple one-step hydrothermal method.The structure water in h-WO3 was characterized by Raman spectrum,scanning and X-ray photo electron spectroscopy(XPS).According to the TG test results,the product was characterized to be h-WO3·0.28H2O.Due to the presence of hexagonal tunnels as well as the presence of structural water in the crystal structure and the unique hierarchical architecture,the hydrothermally synthesized h-WO3·0.28H2O exhibits excellent electrochemical performance with a high specific capacitance of about 390.2 F g-1 at a current density of 0.5 A g-1 and 297.8 F g-1 at 10 A g"1 and a good cycle stability.The capacity remains basically unchanged after even 2000 cycles at 10 A g-1.2.The h-MoO3-x micro-flowers were prepared by a solvothermal method.Using ammonium heptamolybdate as the molybdenum source,the a-MoO3 nanobelt was prepared by a hydrothermal method.The h-MoO3-x micro-flowers with hierarchical structure were prepared by adding ethanol to the solvent.The product was analyzed and a small amount of hexavalent molybdenum was found in the h-MoO3-X micro-flowers synthesized in a water/alcohol solvent.A small amount of oxygen vacancies was found in the product due to being reduced to pentavalent under the solvothermal condition.Electrochemical tests of the products with two morphologies revealed that the h-MoO3-x micro-flowers with a small amount of oxygen vacancies exhibit fairly good electrochemical performance.At a current density of 0.5 A g-1,the specific capacity of 80.9 F g-1,higher than that of a-MoO3 nanobelts(47.6 F g-1).To further optimize the electrochemical performance,the molybdenum trioxide samples with two different morphologies were annealed under an Ar/H2 atmosphere.The results showed that the sample was introduced a large amount of oxygen vacancies after calcination and its specific capacity was greatly improved.The specific capacity of R-MoO3-x micro-flowers is increased from 80.9 F g-1 to 495.4 F g-1 at a current density of 0.5 Ag-1.Also,benefiting from the micro/nano structure,the materials exhibited good cycle performance with a capacity retention of 87.3%after 2000 cycles at a current density of 10 A g-1.Besides,the specific capacity of R-MoO3-X nanobelts is also increased from 47.6 F g-1 to 440.0 F g-1 at a current density of 0.5 A g-1 and 78.6%capacitance retention after 2000 cycles at 10 Ag-1.