Study On Hydrogen Sensors Of SnO₂Nanofibers And Thermoelectric Materials

Environmental problems and energy crisis are the two most severe problems incurrent society. Environmental problems, especially air quality problems, haveinfluenced people’s daily life seriously. Due to the poor performance, the traditionalgas sensors cannot satisfy our requirements for higher gas detection level. Manyresearchers devoted themselves to develop new methods to improve the sensingmaterial properties, such as trace metal oxide doping, constructing P-N heterojunction,loading noble metals and so on. Although gas sensing properties have been improvedgreatly, the structure-activity relationship and the enhancement mechanism are stillunclear, which is worth to further research. Energy crisis has also evolved to anotherimportant problem related to our life closely. Besides developing new energy sources,great efforts have been focus on improving the utilization ratio of energy by newtechnology, especially strengthening the use of low grade thermal energy, includinggeothermal, solar, ocean temperature difference, industrial and agricultural wasterheat, etc. Recently, many theoretical prediction and research results have confirmedthe huge potential use of one dimensional nanostructured materials in gas sensing andthermoelectric conversion. Based on the widely application of electrospun nanofibers,plenty of novel ideas have been created to solve the two problems. In this thesis, electrospinning technical has been used to prepare a series ofinorganic nanofibers and inorganic/organic composite nanofibers. Gas sensing andthermoelectric conversion properties have been studied systematically. Besides obtainnovel materials with improved performance, we also engaged in the composition ofnanofibers, structure design and modulation to establish structure-activity relationship,which will extend the application of nanofibers in energy and environment area. Thedetails are presented as follows:1. The enhancements mechanism of trace metal oxide doping was investigatedfirstly. In this chapter, from the perspective of ionic radius (Sn⁴⁺with an ionic radiusof69pm in SnO₂), La₂O₃(La³⁺with an ionic radius of103.2pm) with larger ionicradius and CoO (Co²⁺with an ionic radius65pm) with smaller ionic radius wereselected as dopants. Research the hydrogen sensing performance of SnO₂nanofiberswith different doping levels and analysis the structure changes which are important togas sensors. According to our discussion, with the increase of La₂O₃doping level,their XRD peaks exhibit a blue-shift and then move back in trend. The calculatednanoparticle sizes of these nanofibers also reduced at first and then increased.Nanofibers with maximum XRD peaks displacement and smallest nanoparticles arecorresponding to the2at%La₂O₃doped SnO₂nanofibers, which exhibited best H₂sensing properties. There is a critical doping concentration. Metal ions areincorporated into the SnO₂lattice and occupying the tetragonal Sn⁴⁺cation sites atlower doping concentration. Once it exceeds the critical doping concentration, somemetal ions can not be contained within the SnO₂crystal and expelled from the crystalduring the calcination. The nanocrystal sizes change respectively and influence thegas sensing properties greatly. The ion radius of Co²⁺is smaller and the dopant resultsin a red-shift of the XRD peaks. A similar phenomenon was observed with theincrease of doping levels. The smallest nanocrystal size and best sensing performancewere obtained by the doping level near the critical concentration. All these dataconfirm that trace metal oxide dopant induced the changes in structure andnanocrystal size, which are issue to gas sensing properties of body materials. 2. Research on gas sensing enhancement mechanism of P-N heterojunction. Inthis chapter, we chose P-type NiO as dopant in N-type SnO₂nanofibers to considertheir hydrogen sensing performance, which constructed countless nanoscale P-Njunction. The device based on3at%NiO doped SnO₂nanofibers exhibited a responseof13.5to100ppm H₂and its response and recovery time are about3s.The selectivityand stability are also outstanding. In order to research the synergy effect of P-Njunction more intuitive, especially the relationship between space charge region andgas sensing properties, we successfully synthesized PPy/SnO₂composite nanofibersvia in-situ gas phase polymerization method. Different polymerization time made PPylayer thickness different, which realized the modulation for space charge region.When the polymerization time was1h, the device exhibited largest response toammonia with a detection limit of20ppb. According to our discussion, if thethickness of sensitive layer matched with space charge region of P-N junction, thebest sensing properties can be obtained. If the thickness of sensitive layer is thicker orthinner than space charge region, the sensitivity will reduce respectively. So the spacecharge region of P-N heterojunction is the main factor to influence gas sensingperformance of our materials.3. Loading noble metals is another research hot spot to improve gas sensingperformance. In this chapter we took palladium as the loading noble metal, which hasan effective catalytic property to H₂, and fabricated SnO₂nanofibers loaded with PdOand Pd separately. We also carried out their hydrogen sensing performance. As fornanofibers doped with PdO, due to the ionic radius of Pd²⁺（86pm） is larger than thatof Sn⁴⁺, similar results were obtained. With the increase of doping levels, the responseto hydrogen increased and then decreased. While the optimized operating temperaturewas observed decreased, meaning that the catalytic role played by PdO. After in-situreduction, Pd²⁺was reduced to Pd⁰. When the loading levels of Pd⁰exceeded10at%,materials exhibit a good response to hydrogen at room temperature and its detectionlimit is20ppb. What’s more the selectivity was also improved. On the one hand, Pd⁰loaded in the nanofibers can form schottky barrier with SnO₂, which plays a rectifier effect. On the other hand, the catalytic property to decompose gas molecular of Pd ismuch higher than that of SnO₂, resulting in a low operating temperature for hydrogendetection.4. Flexible thermoelectric materials with high performance is the hot spot anddifficulty during the research in developing low grade energy in recent years. In thischapter, we successfully fabricated Ag₂S/Ag/PAN composite nanofibers viaelectrospinning technology combining with chemical synthesis process and then studyits thermoelectric properties. It was tested at temperatures in the range from305to340K. The seebeck coefficient reached more than10³and the highest ZT value wasreached0.9or so. It’s worth noting that the composite nanofibers retained the flexibleproperty of polymer nanofibers. This method based on one dimension nanofibers canreduced thermal conductivity caused by quantum limit field effect and interfacescattering. What’s more the synergy effect between metal and semiconductor and theflexible property of polymer nanofibers were in favour for improving the integratedthermoelectric performance. It was also helpful to expend avenues to the developmentand application of high performance thermoelectric materials.