Preparation Of Tungsten Oxide Supported Gold And In Situ Electron Microscopy Study Of Its Gas Sensing Mechanism

Detection and control of inflammable,explosible and pollution gases are the key parts in fuel gas and a clean energy production.High sensitivity with fast echo detector is crucial tool to detect harmful and dangerous gases in air.The core component of a gas sensor is its materials,and its properties decides its applications.A facile preparation method to obtain high sensitive materials is still a technical problems need to be overcame.Nobel metal supported oxide exhibits better sensitive performance than its base oxide.The improved gas-sensitive mechanism of metal supported oxide is complicated and debatable due to the absence of experimental data.Understanding the sensoring mechanism is key step for developing high performance materials.Lots of calculations predicts the microstructure variation in metal/oxide interface,but there is still absent of experimental data especially in situ observable scene during reaction.Recently developed environmental transmission electron microscopy realized the atomic scale observation on variation of nanomaterials in gaseous environment and disclosed some debatable point on gas-solid interface.Enhanced sensoring mechanism to gas was estimated to revealed from atomic scale with the aid of first principle calculations.Research of this thesis focused on preparation,characterization of an Au/WO_2.7 and Nitrogen doped graphene(NP)Au/NG/WO_2.7compounds in order to accelerate its application and disclose its physical mechanism.Preparation,performance test and atomic scale in situ experiments on mechanisms were characterized and works and conclusions mainly contain following parts:(1)Nano scale Au were prepared using Na BH₄ and relationships between dose and Au particle size were studied and optimized for our work.Particle size of Au is best about 4-8nm with moderate dispersion degree.Au/WO_2.7 materials were prepared using sol-gel method.Morphology and structure of Au/WO_2.7 compounds were analyzed by x-ray diffraction,transmission electron microscope.Factors of dose,temperature,reaction time which affect Au loading rate were discussed,and their connection type between Au and WO_2.7 was revealed.(2)Gas sensitivity of pure WO_2.7and Au/WO_2.7compound were characterized to H₂,ethanol and acetone,formaldehyde,CO,ammonia,Au/WO_2.7compound exhibits better sensitivity than pure oxide with enhanced sensitivity,decreased recovery time.Loading rate of Au affect the gas sensitivity and the loading rate of 1%of Au is best and it behaves selective response to H₂,ethanol and acetone.The best working temperature decreased from 280?to 240?after Au loading.(3)Structure of Au/WO_2.7 interface was revealed by high resolution transmission electron microscopy.The Au nano particles exhibit polygon shapes with exposed to the air.The binding between Au nanoparticles and base is rigid with embedded mode into the base and usually located on the steps of nano sheets.In situ environmental transmission electron microscopy was used to study its thermal and gas thermal behavior on Au/WO_2.7component in H₂atmosphere in a TEM chamber.Thermal effect only makes the Au nanoparticle changes from polyhedral to flat ball-like structure at500 ^oC.The Au particles inserted into the WO_2.7 base with elevated temperature from350 ^oC with increase contacting interfaces.Atoms in Au particles surface transferred to the Au/WO_2.7 interface and stirred and inserted into the WO_2.7base with flatten of Au particles.Atomic models had been established according to the in situ environmental TEM data.Connection energy was calculated based on first principle density of states(DOS),and it indicates that hydrogen atoms has preference to adsorb to Au(110)plane.First principle calculation on banding indicates that narrow of the band and elevated Fermi level are the main reason attribute to enhanced sensitivity.(4)Nitrogen doped graphene NG/WO_2.7 and NG/Au/WO_2.7 components were further prepared and gas-sensitive tests were made.Their responses to H₂,ethanol,acetone,methanol,formaldehyde,CO,ammonia and other gases exhibit enhanced sensitivity.The enhance sensitivity can be attribute to the creation of new bond of N-C which accelerate the combination and transfer between electron and holes along N-C-O-W bond.Increased electron transfer improved the conductivity and then enhanced the sensitivity.In total,Au/WO_2.7is a kind of high sensitivity,fast echo,facile preparation and stability compound and it has good prospective applications in gas sensors.Atomic scale in situ experiments provide a direct evidence for reveal enhancement mechanism,and provide reference for further response improvement.