Gas Sensing Properties Of Electrospun Ni-doped SnO₂ Nanofiber

Metal oxide semiconductor（MOS）is widely studied and used in the field of gas sensing nowdays.Tin oxide（SnO₂）is one of the most widely used MOS for gas sensing due to its low cost,excellent stability and high sensitivity.However,one major flaw of SnO₂ is that it can interact with a variety of gases at the same time,leading to“low selectivity”.To address the problem,the project tried chemical modification of SnO₂ with Ni dopant to to realize selective response to different air pollutants（i.e.NO₂,H₂S）.In view of the unique characteristics and great prospects of one-dimensional nanomaterials,the work chose Ni-doped SnO₂ nanofiber as the object of study.In addition,facile process of electrospinning is employed to synthesis the samples for gas sensing test.The first chapter introduces the research background of this work:the harmfulness of different air pollutants.Then,the gas sensing mechnism of MOS sensors was introduced,followed with the application of SnO₂ materials for gas sensing.Next,the drawback of SnO₂ as gas sensing materials was discussed,followed with approaches for improvement.Then,emphasis was placed on the origin,principle and applications of the electrospinning technology.The goal and contents of this research were summarized in the end.The second chapter used the electrospinning method to synthesize Ni-doped SnO₂nanofibers with different doping concentrations.XRD,SEM,TEM and XPS were carried out to analyze the effect of Ni doping on the crystal structure,morphology and near-surface composition of the nanofiber samples.The influence of Ni concentration on the gas sensing properties of samples was explored according to the gas sensing tests,and the optimum Ni concentration point was determined to be 8 mol.%.Test results from NO₂and CO sensing were compared,showing that the Ni doping can improve the sensing selectivity for NO₂.In the third chapter,on the basis of the optimum doping concentration of Ni,the Ni-doped SnO₂ nanofiber array was prepared by a modified electrospinning process.SEM,TEM,XRD and XPS were used to characterize the nanofiber array.At 250 ^oC,the sensor to 20 ppm NO₂ was 90.3,with response and recovery times of 40 s and 18 s.The NO₂ sensing properties of ordered and disordered nanofibers were compared to verify that the orientational arrangement of one-dimensional nanostructure benefits to gas adsorption and carrier transmission,resulting in improved gas sensing performance.The response mechanism was investigated by FT-IR and XPS,and it was found that the large change of sensor resistance was due to direct capture of the electrons by NO₂ from the conduction band of SnO₂.In the fourth chapter,on account of using n-p type metal oxide composites for the detection of reducing gases,SnO₂-NiO nanofibers were prepared by electrospinning.Pure SnO₂ and NiO nanofibers were prepared under similar conditions.Their differences in the crystal structure and morphology were compared.Preliminary results from gas sensing tests showed that introduction of n-p junction improves the H₂S sensitivity of SnO₂nanofibers.The fifth chapter is the summary and outlook of the project.