Synthesis And Gas Sensing Properties Of Hollow Nanomaterials（ZnSnO₃,SnO₂）

Hollow nanostructures with high surface, penetrable shell, and superior mass transfer properties have attracted tremendous attention owing to a growing range of surface applications in catalysis, gas sensors, etc. However, the synthesis and formation mechanism of non-spherical hollow nanostructures are still restricted by limited templates and preparation methods.This dissertation is to study the synthesis, growth mechanism, and gas-sensing properties of hollow nanomaterials（ZnSn O₃, Sn O₂）, and the main research results are as follows.1. ZnSn O₃ hollow microspheres with a hierarchical structure composed of small nanorods as building blocks are synthesized by using a facile one-pot hydrothermal method. The time-dependent hollowing evolution of ZnSn O₃ microspheres reveals that reaction time is a key facor to the formation of hollow structure. Moreover, ZnSn O₃ spheres with hollow interiors and permeable surfaces are exploited as gas sensors, exhibiting improved sensing performances to ethanol. The response value of the sensor based on these ZnSn O₃ hollow microspheres to 50 ppm ethanol is about 27.8, and the response and recovery time of sensor is 0.9 and 2.2 s, respectively, when the sensor is exposed in ethanol at the optimal operating temperature of 270 oC. Compared with other reported pure ZnSn O₃-based nanostructures, the hierarchical ZnSn O₃ hollow spheres in this work exhibit a faster response and recovery time to ethanol. The significant increase in response is attributed to the hierarchically hollow structure and the significant decrease in response and recovery time is attributed to these rodlike building blocks with ultra-thin diameters.2. ZnSn（OH）₆ polyhedral（cube, truncated cube, cuboctahedron, octahedron） templates are synthesized by using a fcalie co-precipitation method at room temperature without employing any capping agents or surfactants, transforming into monocrystalline ZnSn（OH）₆ polyhedrons with hollow ineriors via a Na OH etching at room temperature. These polyhedrons will be used as sacrificial templates in our following study.3. Cubic（cuboctahedron, octahedron） Sn O₂ hollow cages with double-shelled have been successfully prepared by using a simple “anneal-etching” method, in which ZnSn（OH）₆ hollow polyhedrals are used as sacrificial templates. These Sn O₂ hollow particles composed of small nanoparticles as building blocks, have the same morphology with ZnSn（OH）₆ templates. Annealing treatment is a key factor to the formation mechanism of these novel double-shelled nanostructures. Meanwhile, cubic Sn O₂ hollow cages with double-shelled exhibits a high response to toluene, and the response is 33.4 to 20 ppm toluene at 250 oC. This good sensitivity could be ascribed to the multilevel and highly mesoporous shells, and face-to-face contact between adjacent particles of our Sn O₂ hollow cages.4. A new type of nonspherical yolk-shell structures for Sn O₂ has been successfully prepared by using a simple “anneal-etching” method, in which ZnSn（OH）₆ polyhedrals are used as sacrificial templates. The Sn O₂ yolk-shell particle has a hierarchical architecture with penetrable multi-walled surfaces, in which the thickness of outer shell is about 45-60 nm, and the inner core is about 200 nm in radius. The annealing and the etching process are crucial to the formation of the final yolk-shell hollow structure. The sensor based on Sn O₂ products has been fabricated and exhibit excellent sensing performances for toluene. It is found that the response time and recovery time of our Sn O₂ yolk-shell particles to 20 ppm toluene at the optimal working temperature of 250 oC, is within 1.8 and 4.1 s, respectively. Compared with normal hollow structures for Sn O₂, the yolk-shell configuration has a good structural robustness to maintain multi-walled and penetrable surfaces.5. The Au-loaded Sn O₂ hollow multilayered nanosheets have been prepared via a simple “anneal-etching” method. Flowerlike Sn O₂ nanosheets are assembled from hexagonal mesoporous andmultilayered walls, which can further be subdivided into Sn O₂ nanoparticles as structural subunits. Furthermore, The Au-loaded Sn O₂ hollow nanosheets show excellent gas-sensing properties for CO, The response is 36.5 to 50 ppm CO at 250 oC, which is five times as high as other Sn O₂-based sensors. Besides structural merits including hollow and easily penetrated multilayered walls, the significant improvement of the sensing properties is also attributed to the catalytic effect of Au nanoparticles.