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Sythesis and Gas Sensing Property of Zinc Oxide Hollow Sphere and Nanrods

Author: MengFan
Tutor: BieLiJian
School: Tianjin University of Technology
Course: Materials Science
Keywords: ZnO porous spheres co-precipitation Gas sensing La-doped PL
CLC: TB383.1
Type: Master's thesis
Year: 2011
Downloads: 102
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Abstract


As an n-type metal oxide semiconductor sensing material, zinc oxide has been under extensive research due to its high chemical stability, low cost and good flexibility in fabrication. Nanofilms, nanowires, nanobelts, nanorods, nanotubes and hollow spheres gas sensors based on ZnO nanostructures were reported in detecting gases, but their sensitivities are not high enough for detecting very low concentration of reductant/oxidant gases. Because zinc oxide semiconductor gas sensor is mainly the surface resistance control type, the sensitivity depends greatly on the surface microstructure, so high surface area and permeable shell structure with a less agglomeration is thought to be good for effective gas diffusion, therefore, sensing materials with either hollow-sphere structure or hierarchically porous structure will enhance the sensor properties. The use of surfactants may affect the growth activity of different crystal faces, selective adsorption of a specific crystal face may change the growth rate of various crystal faces, thus affecting the final morphology of crystals.In this dissertation, ZnO hierarchically porous hollow spheres were prepared by co-precipitation method using chemicals （Zn（NO3）2·6H2O\（CH2）6N4） with sodium citrate surfactant. The morphology of the spheres can be modified by controlling reaction time and the concentration of sodium citrate. The surface area was measured to be 41.84 m2/g. It is found that the sensors show satisfactory sensitivity to both low and high concentrations ethanol, the response reached 9.6 to 10 ppm, 37 to 500 ppm, with response and recovery time were 9s and 11s to 10 ppm, 10s and 19s to 500 ppm, respectively.Extensive studies also have been done in improving the performances by adding catalysts to the ZnO nanospheres. The gas sensing results show that among all doped samples studied, the 3mol% La-doped ZnO spheres sensing unit reaches the sensitivity of 51.3 and 49.4 to 500 ppm ethanol and hydrogen, respectively. Dy-doped ZnO spheres reaches the sensitivity of 48.2 and 39.4 to 500 ppm ethanol and hydrogen, respectively.ZnO:La3+ nanorods and ZnO-Fe2O3 composites show good performance to gases. The gas sensing results show that the ZnO:La3+ nanorods reach sensitivity of 3.6 and 17.2 to 10 ppm and 500 ppm ethanol, respectively. ZnO:Fe2O3 composites reach the sensitivity of 4.3 and 22.5 to 10 ppm and 500 ppm ethanol, respectively.The Photoluminescence （PL） spectrum of ZnO nanospheres mainly consisted of two emission bands: the UV near-band-edge emission （NBE） at about 385 nm, and the visible emission at 470 nm.

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