Synthesis And Performance Application Research Of New Type Bismuth Tungstate Composite Material

With the rapid development of industry and the significant progress of science and technology,people's material life has been greatly enriched and satisfied,the demand for a beautiful living environment has also become stronger.The emission of industrial exhaust gas and automobile exhaust not only pollutes the natural and living environment,but also endangers human health.Therefore,it is of great significance to develop detection technologies that can effectively and real-time detect harmful gases.Nowadays,a variety of NO₂ detection methods have been developed.Among them,semiconductor gas sensors have gradually become the most widely researched and applied gas sensors due to their advantages of real-time,high sensitivity,rapid response,and good portability.However,the reported high-performance semiconductor gas sensors still have some urgent challenges to be solved,such as achieving room temperature operation and improving the selectivity of target gas.In response to the above problems,this thesis builds a Bi₂WO₆/reduced graphene oxide(rGO)composite material nitrogen dioxide(NO₂)gas sensor to achieve high selectivity and high response detection of NO₂ gas at room temperature.In addition,the photochromic phenomenon of Bi₂WO₆ material was observed in the development stage of the gas sensor,and then the mechanism of bismuth tungstate photochromic was deeply studied.The main research content of this thesis includes.1.Studies on the Preparation and Gas-sensing Properties of Bi₂WO₆/rGO Composite Material NO₂ Gas Sensor:Bi₂WO₆/rGO composite material was prepared by one-step hydrothermal method,and the successful combination of rGO and Bi₂WO₆was proved by three characterization methods:raman spectroscopy,scanning electron microscopy and transmission electron microscopy.The gas sensor was fabricated using Bi₂WO₆/rGO composite material and interdigital electrode design,the gas-sensing properties was systematically tested using a home-made gas sensor test device,which mainly included the following parts:response,response-recovery curve,selectivity,effect of ultraviolet light and temperature on properties,repeatability,long-term stability,detection limit,etc.The results showed that the Bi₂WO₆/rGO composite material gas sensor had a response of 28.35%to 47.03 mg/m³ NO₂ at room temperature,the response time was 55 s,the recovery time was 549 s;in the study of selectivity,the response of the Bi₂WO₆/rGO composite material gas sensor to low concentrations of NO₂ was much higher than that of ammonia,methanol,ethanol,isopropanol and acetone with concentration of several tens of times NO₂,indicating that the gas sensor had good selectivity;after 4 repeatable cycles,the gas response of the sensor had not decayed;after 4 weeks of long-term stability testing,the properties of the gas sensor still had no major fluctuations,RSD is 2.7%,in the calculation of detection limit,detection limit of the Bi₂WO₆/rGO composite material gas sensor was 0.062 mg/m³.By combining Bi₂WO₆ and rGO,the detection of NO₂ by Bi₂WO₆ composite material at room temperature was realized.2.Studies on the Photochromic Properties of Bi₂WO₆ Composite Material:Bi₂WO₆with flower-like superstructure was synthesized by hydrothermal method and baking method.Bi₂WO₆ was dispersed in ethylene glycol to prepare the Bi₂WO₆ ethylene glycol suspension,and Bi₂WO₆ and nano titanium dioxide were compounded on the surface of glass beads by coating method to prepare Bi₂WO₆ composite material.The pure Bi₂WO₆ powder turned from light yellow to light blue after ultraviolet light irradiation,Bi₂WO₆ powder in the ethylene glycol suspension turned light yellow to black after ultraviolet light irradiation,and Bi₂WO₆ composite material turned light yellow to black after ultraviolet light irradiation.The light absorption performance of Bi₂WO₆ composite material in the visible light region gradually increased as the color deepened,and the band gap width also decreased.The X-ray photoelectron spectroscopy of Bi₂WO₆ composite material proved that about 27.2%of W elements were reduced from+6 valence to+5 valence after photochromism.At the same time,the X-ray absorption fine structure(XAFS)spectroscopy method in the Shanghai Synchrotron Radiation Source(BL14W line station)was used to study the photochromic mechanism of Bi₂WO₆ material.The preliminary results showed that the W-O bond length increased from 1.41(?)to 1.43(?)after the photochromism.Based on the above experiments:in the black Bi₂WO₆ material after the photochromism,the light-excited electrons were located on the W atom,which changed the valence state of W and W-O bond length,resulting in the lattice distortion of the WO₆ octahedron,this process provided a possible explanation for the inorganic photochromic mechanism,and the photochromic process increased the energy level of W⁵⁺as an impurity and improved the materials utilization of visible light.