The Preparation Of One-dimensional Hollow SnO₂ And ZnO Nanomaterials And Gas Sensing Properties Of Acetone And Ethanol

With the development of economy and society, the natural resource consumption is increasing and environmental pollution is becoming increasingly serious. The leakage of poisonous and harmful gases, the greenhouse effect, the explosion of the combustible gas, food security and other issues are threatening people’s life and health. People can avoid from the gas leakage and contamination accident by monitoring the harmful gases in the surrounding environment. Hence, researching and developing the gas sensor is one of the important topics of current research. Both SnO₂ and ZnO are the important gas sensing materials. Due to the advantage of thermostability, anticorrosion and low cost, these materials have been widely studied. However, a lot of problems still exist. Low response, high operating temperature, long response/recovery time, poor selectivity restrain the applications of gas sensors. Gas sensing reaction is one kind of gas-solid interface reactions, which occurs between gas molecules and the surface of gas sensing materials. The composition, microstructure, surface defect, surface modification of gas sensing materials play important roles in improving the sensing performance. Therefore, the study of factors which are affecting sensing properties has guiding significance in the practice. The detailed information of the dissertation is listed as follows.1. The preparation of hollow SnO₂ nanobelts and their application in acetone sensor. In this paper, hollow SnO₂ nanobelts were prepared by electrospinning method and then followed by calcination. The formation of hollow nanobelts is attributed to solvent evaporation rate, phase separation and collapse. In the process of electrospinning, the ethanol forms concentration gradient along the radius of the fiber. Therefore, the Sn ions and PVP molecules move to the surface and then occupy the outside surface. It makes the molecules all concentrate on the surface and residual ethanol concentrate in the center. After calcination at 550 °C, ethanol and PVP decompose, and the hollow SnO₂ nanobelts are finally achieved by the collapse of the nanotubes. The structure of hollow SnO₂ nanobelts is characterized by XRD and Raman analysis, which confirm the sample with rutile phase of SnO₂. Hollow structure and mesoporous structure of samples are investigated by SEM and TEM. At the operating temperature of 260 °C, the response of hollow SnO₂ nanobelts is up to 52 when the sensor is exposed to 100 ppm acetone, which is almost 5 times larger than the bulk materials of SnO₂. In addition, the sensor performs good saturation concentration, sensitivity and response/recovery time.2. The preparation of Pr-doped SnO₂ hollow nanofibers and their ethanol gas sensing properties. 0 wt%, 0.3 wt%, 0.6 wt%, and 1.0 wt% Pr-doped SnO₂ hollow nanofibers are prepared by electrospinning method using stannous chloride, praseodymium（III） nitrate and PVP as raw materials, ethanol and DMF as stabilizers. The properties of as-synthesized nanofibers are characterized by XRD, XPS, EDS, SEM, TEM and BET analysis. With the increase of doping concentration, the diameter of SnO₂ nanotubes is increased firstly and then decreased. The Pr ions（Pr3+ and Pr4+） are successfully doped into the lattice of SnO₂, which also produce a large amount of oxygen vacancies. Then Pr-doped SnO₂ hollow nanofibers are coated on a ceramic tube to form gas sensors. The results indicated that Pr doping brings the good response of SnO₂ sensor to ethanol. In addition, the ethanol sensing mechanism which is based on operating temperature is also discussed.3. The preparation of Ce-doped SnO₂ nanotubes and their ethanol gas sensing properties. Pure and Ce doped SnO₂ hollow nanofibers were prepared by electrospinning method using stannous chloride, Cerium nitrate and PVP as raw materials, ethanol and DMF as stabilizers. The microstructure of as-synthesized nanotubes are characterized by XRD, XPS and SEM analysis. The SEM images show the hollow structure of SnO₂ nanotubes. The XPS analysis indicates Ce3+/Ce4+ ions are successfully doped into the lattice of ZnO, and produce a large number of free electrons. The sensor based on Ce doped SnO₂ nanotubes are followed by test. Compared with pure sample, Ce doped ZnO nanotubes exhibit improved sensing performance to acetone at 260 °C, which is attributed to the change of the electronic and structural properties by Ce doping.4. The preparation of ZnO/SnO₂ core-shell nanotubes and their ethanol gas sensing properties. The hollow SnO₂ nanotubes are first synthesized by using the electrospinning method, and then the ZnO shell is subsequently grown on the fibers via the hydrothermal method. The ZnO-SnO₂ core-shell structure is confirmed by XRD, EDS, SEM, TEM, XPS and elemental mapping analysis. The results indicate that the ZnO shell uniform covered the SnO₂ core and the thickness of shell is about 12 nm. The gas sensing behaviors of the fabricated sensors are systematically investigated. Under optimum operating temperature（200 °C） at 100 ppm ethanol, the response of ZnO-SnO₂ sensor is 392.29. The improvement in gas sensing performance is attributed to unique hollow structure, oxygen vacancies and n-n heterojunction.