Study On Gas Sensing Properties And Sensing Mechanism Of Silicon Based SnO₂/ZnO Materials

Synthesis of novel nanomaterials with heterogeneous structure and developing a simple and inexpensive process which is capable of mass production and compatible with IC technology to fabricate silicon based micro sensors are the focus of the study on gas sensors.Due to the lack of general gas sensing mechanism model as theoretical guidance,the selection of gas sensing materials currently needs to carry out a large number of experiments.Therefore,it is an important subject to study how to prepare multi level heterogeneous nanomaterials,to develop the fabrication technology of silicon based micro gas sensors and to establish the gas sensing mechanism model.The research purposes of this dissertation are mainly divided into three aspects.Firstly,basing on the principles of structure control and component regulation,hydrothermal synthesis and electrospinning are used to explore new heterogeneous materials with high performance via the methods of doping and composition.The second aspect is about the study of growing hierarchical composite nanomaterials on silicon substrates selectively.Lastly it is about establishing a complete gas absorption-desorption mechanism model to provide theoretical guidance for further experiments of exploring new materials and to verify the experimental phenomenon of gas sensing tests.In this work,SnO₂/ZnO system was selected as gas sensing material.It can be treated as typical n-type or n-n type semiconductor oxide in the study of gas sensing mechanism.Electrospinning was used to prepare SnO₂ nanofibers loaded with different concentrations of noble metal Pd.The gas sensing tests showed that when the Pd loading concentration was 1.5 mol%,the response of the gas sensors to 100 ppm acetone was as high as 98.8 and the working temperature was as low as 275?.If the load concentration of Pd was too low or too high,the gas sensing sensitivity was reduced.Flow-like hierarchical ZnO gas sensors was fabricated by hydrothermal synthesis.At the working temperature of 275?,the flow-like ZnO sensors showed good selectivity to toluene.SnO₂/ZnO composite nanofibers were synthesized by electrospinning method.The effect of different Zn content on the morphology,structure and gas sensing properties of the composites were also studied.It showed that when the molar ratio of Sn to Zn is 1:1,the SnO₂/ZnO composite nano fibers had highest response to methanol.With Zn content increasing,the response of S1Z2 and S0Z1 sensors decreased.Silicon based Zn₂SnO₄/SnO₂ micro gas sensors were prepared by spin coating method.The sensors had good selectivity to formaldehyde at 200? and the detection concentration of formaldehyde was as low as 100 ppb.The response and recovery time were 66 s and 27 s,respectively.SnO₂/ZnO composite nanofibers were directly synthesized on silicon substrates by stepwise-heating electrospinning to fabricate silicon based SnO₂/ZnO micro gas sensors.This method solved the problem of poor contact and bad mechanical stability between nanofibers and silicon substrates.The adsorption mechanisms of methanol on SnO₂?110?surface,reduced SnO₂?110?surface and Zn doped reduced SnO₂?110?surface were investigated by the density functional theory.The doping of Zn promoted the formation of bridge oxygen vacancy O₂c on SnO₂?110?surface and enhanced its ability of adsorbing oxygen.When exposed to methanol,the methanol would react with bridge oxygen O₂c,planar O3c and pre adsorbed oxygen vacancy on the lattice surface.The-CH₃ and-OH of methanol molecule would both lose one H atom.The lost H atoms bonded with oxygen at the adsorption sites.The final products were HCHO and H₂O.Electrons were transferred from methanol to the lattice surface to reduce the resistance of semiconductor gas sensitive materials.