| Semiconductor gas sensors have serious application in gas detection due to their low cost, simple detection method and easy to carry. SnO2 and In2O3, as surface-control type material, draw lots of attentions because of their high and stable sensitivity. In this thesis, SnO2 and In2O3 nanofibers are synthesized by electrospinning. Metal and metallic oxides are used to decorate on SnO2 and In2O3 nanofibers. The gas sensing properties of pure and decorated materials were compared and the gas sensing mechanism was analyzed.Use immersion method to prepare different concentration of PdO-decorated SnO2. Before and after PdO decorated SnO2 material were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS). Fabricate indirectly heated gas sensors and test their gas sensing properties in a static gas sensing test system. PdO-decorated SnO2 having Pd/Sn atomic ratio 3 at% exhibited the highest response to toluene. Spill-over effect and catalytic effect are responsible for improvement of PdO-decorated SnO2 gas sensitivity. PdO-decorated SnO2 sensor had higher toluene response than PdO-doped SnO2 sensor.Ag decorated SnO2 was obtained by irradiated AgN03 aqueous solution with UV lamp. The Ag content decorated on SnO2 was controlled by changing the time of irradiation. XRD of Ag decorated SnO2 showed diffraction peaks coinciding with Ag. Ag decorated SnO2 revealed higher response to formaldehyde compared with pure SnO2 and Ag decorated SnO2 with Ag/Sn atmoic ratio 3 at% showed the highest response. Moreover, after decorated Ag, the optimum operating temperature of the material droped from 300 ℃ to 250 ℃. In air, partial Ag was oxidized to Ag2O forming Schottky contact with SnO2, which resulted in wider depletion layer and caused improvement of gas sensing properties.Sputtering was used to prepare Au decorated SnO2. Then anneal Au decorated SnO2 material with different temperature. When the anneal temperature was 300℃, Au decorated SnO2 showed the highest response to formaldehyde. Au can be observed directly on the surface of SnO2 nanofibers whose size was around 7 nm. In addition, the optimum operating temperature of SnO2 droped from 300℃ to 200 ℃ after decorated with Au. The shift of the binding energy of Sn 3d should be mainly ascribed to the interaction between Au and SnO2. When Au and SnO2 attach together, the electrons transfer from Au to Sn, which broaden the depletion layer. The improvement of gas sensing was also related to the increasement of absorbed oxygen and catalysis of nano-Au.CdO was decorated on the SnO2 and InO3 nanofibers with the same concentration. Their responses to formaldehyde were increased by more than 4 times. But CdO decoration had minimal effect on ethanol and acetone sensing and leaded better selectivity. The influence of CdO is more significant on SnO2 than on In2O3. Probably because When CdO decorated on In2O3, CdIn2O4 generated.Both theories and experiments suggest that decoration is one of the efficient way to improve the gas sensing properties of semiconducting material. |
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