Study On Synthesis,Thermal Expansion And Optical Properties Of Nano Hydrogen Tungsten Bronze H_XWO₃

Energy and environmental are always the focus of many researches.The investigation of new functional materials is one of the effective ways to solve such problems.Nano hydrogen tungsten bronzes H_xWO₃?0<x<1?are a kind of materials with excellent optical and electrical properties,and have been widely used in many fields,such as photoelectrocatalyst,gas sensors,supercapacitors,electrochromic devices,etc.However,the current preparation methods of H_xWO₃ have the disadvantages of high energy consumption and complex preparation process,which restricts the further application of H_xWO₃.Therefore,it is important to develop a simple and convenient synthesis method for the research and application of nano hydrogen tungsten bronze.In this thesis,the cubic nano hydrogen tungsten bronzes have been synthesized by microwave method together with subsequent gas reduction.The effects of hydrogen content x on the crystal structure,near-infrared light absorption and thermal expansion properties of the samples have been studied.The main results are listed as follows:?1?Cubic nano hydrogen tungsten bronze has been synthesized by microwave method together with subsequent gas reduction.The sample was characterized by X-ray diffractometer,X-ray photoelectron spectrometer,scanning electron microscope and Raman spectrometer.Its molecular formula can be written as H_0.25WO₃.The space group and unit cell parameters are Im-3 and a=7.642?3??,respectively.The micro-morphology of sample shows the uniform disk-shaped particles with about 50 nm thickness and 100?200 nm length.Comparing with the methods reported in references,this microwave-assisted synthesis method has the advantages of time-and energy-saving and simplicity.?2?By changing the time during gas reduction process,samples with differenthydrogen contents have been obtained.Their molecular formulas are H_0.2WO₃,H_0.25WO₃,H_0.29WO₃,and H_0.34WO₃,respectively.According to the results of scanning electron micrograph and Scherrer formula calculation,all samples have irregular disk-shaped micro-particles with 100?200 nm length and grains with size in the range of13?16 nm.This indicates that the micro-morphology and grain size of samples change little with the increase of hydrogen content.The results of Rietveld structure refinement at room temperature show that as changing x from 0.20 to 0.34,the cell parameter a increases from 7.636?1??to 7.654?2??.The W-O bond length of the WO₆ octahedron in the lattice also increases with hydrogen content.?3?The near infrared?NIR?absorption spectra of samples with different hydrogen content have been measured using an ultraviolet-visible-near infrared spectro-photometer.The results show that as increasing the hydrogen content x,the NIR absorption peaks gradually increase.When changing x from 0.20 to 0.34,the intensity of the absorption peaks increases by nearly five times.The rise of the absorption peaks is due to the increase of W⁵⁺ions in the sample,which enhances the NIR absorption including small polarons and local surface plasmon resonance absorption.?4?The crystal structure of samples H_xWO₃?x=0.20,0.25,0.29,0.34?at different temperatures has been characterized by variable temperature X-ray diffraction.The cell parameters at different temperatures are obtained by Rietveld structure refinement.The results show that the cell volume increases as x=0.20,and decreases as x=0.25,0.29,0.34 with raising temperature,indicating that with the increase of hydrogen content,the hydrogen tungsten bronzes undergo the transition from positive thermal expansion to negative thermal expansion.The calculation results of the expansion coefficient?show that as increasing x from 0.20 to 0.34,?gradually decreases from 2.28×10^-6K^-1 to-12.23×10^-6K^-1.The further experimental and theoretical researches are needed to clarify the negative thermal expansion mechanism of hydrogen tungsten bronze and the dependence of the expansion coefficient on x.