Preparation,Gas Sensing Properties And Theory Study Of Hierarchical Nanostructure SnO₂

The surface configuration, adsorption energy, density of states and population analysis of the undoped SnO₂ （110） surface and rare earth La doped SnO₂ （110） surface models adsorbed by different gases were calculated based on the primary principle of density functional theory to study the gas adsorption characteristics of the SnO₂ （110） surface and explore the related gas sensing mechanism. The population analysis results showed that La doping Snsc site caused O3c site electronic defects, and La doping Sn6c site caused O₂c site electronic defects. Both of them significantly improved the activity of SnO₂ （110） surface, which can promoted the surface gas-sensing properties to the gases. The calculation results of the La doped SnO₂（110） adsorbed by oxygen, carbon monoxide, hydrogen, ethanol, isopropanol and acetone demonstrated that the La doping can effectively reduce the band gap and decrease the gas adsorption energy. It promotes the gas absorption, which increases the electron transfer between the gases and the surface, thus contribute to enhance the SnO₂ gas-sensing properties.In this thesis, a simple hydrothermal method was explored to synthesize hollow square hierarchical SnO₂ nanostructure which was made of nanorods arrays. This method was also efficient, green environmental protection, without substrates or the surfactants. Rare earth La doped SnO₂ hierarchical nanostructures were successfully prepared by adding rare earth salt La chlorate into hydrothermal precursor. Then, the gas sensing mechanism and properties of rare earth La doped hierarchical nanostructures was explored.X-ray powder diffraction （XRD） spectrum, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, high-resolution transmission electron microscope （HRTEM） and raman spectrum were employed to characterize the morphology and structure of undoped and La doped SnO₂ products, respectively. The influence of the various parameters, such as tin source concentration, hydrothermal temperature, hydrothermal time and alcohol volume on structure and morphology of hollow square SnO₂ were researched. The optimum synthesis parameters of preparation hollow square SnO₂ hierarchical structure were obtained. At the same time, the possible growth mechanism of hollow square SnO₂ products was explored. The XRD characterization results of the rare earth La doped SnO₂ nanostructures products showed that the main components of the products were still rutile phase, while the 3% and 5% molar ratio of La doped products gradually appeared La（OH）3 phase. TEM characterization results showed that the 5% molar ratio of La doped SnO₂ nanostructure appeared spherical morphology, which were made of nanorods with about 10 nm diameter and 50 nm length.The undoped and La doped SnO₂ products were made into gas sensors. Then, the gas sensitive properties of 12 kinds gases, including alcohol, acetone, isopropyl alcohol, methanol, formaldehyde, ethylene glycol, aldehyde, ether,93# gasoline, benzene, ammonia and carbon monoxide were tested. The test results showed that the sensitivity of undoped SnO₂ gas sensors to 500 ppm ethanol was more than 90. The respond time and recovery time of gas sensor to 200 ppm methanol were 5 s and 2 s, respectively, which exhibited perfect resoponse and recovery properties. The La doped hollow hierarchical SnO₂ nanostructure gas sensor showed good selectivity to isopropyl alcohol, ethanol and acetone. Compared with 4.5 V for undoped gas sensor, the optimum working voltage of La doped gas sensor for ethanol and acetone dropped by 0.5 V. When the ethanol concentration reached 1000 ppm, the sensitivity of the 5% molar ratio of La doped sensor was more than 450, which is far higher than undoped one. The response time of the 1% molar ratio of La doped sensor to 200 ppm actone was 7.5 s and the recovery time was 12 s, which was significantly reduced compared with 70 s recovery time of undoped SnO₂.