Effects Of Doping And Pore-making On The Gas Sensing Properties Of The Usual Low Dimensional Nanomaterials

With the improvement of peoples’ life and the enhancement of environmentalprotection awareness, the fast and intelligent detection of indoor Vocs （such as xyleneand formaldehyde）；combustible and explosive gases （such as ethanol, hydrogen andmethane） and poisonous gases （like nitrogen oxides） make the development ofhigh-performance gas sensors an necessity. The low-dimensional metal oxidesemiconductor nanomaterials were extensively investigated due to theiranti-aggregation, high surface-to-volume ratios and oriented electron conduction. Thispaper firstly discussed the gas sensing mechanism, and then introduced the method ofgas sensing-enhancement: increasing the surface-to-volume ratios, doping andpore-making to the matrix materials. Decreasing the particle size and developing theone-dimensional （1D） nanostructure can increase the surface-to-volume area;making-pore to the matrix material can not only increase the surface-to-volume ratios,but also is beneficial for gases fast adsorption, diffusion, penetration, leave-away andfinally improved the response and recovery rate. Doping to the matrix material canincrease the active surface areas. So we improve the gas sensing performance throughpreparing the1D nanomaterials, doping and pore-making to the matrix materials. Inthis paper, the usual low-dimensional （zero and one-dimension） nanomaterials ZnO,In₂O₃, LaCoxFe_1-xO₃and Co₃O₄were prepared by sol-gel and electrospinning method.Their sensing enhancements were also investigated through one-dimensioality, dopingand pore-making to the matrix materials. The results are shown as follow:In chapter2, the ZnO nanofibers were firstly prepared by an electrospinningmethod. The effects of Ni doping on the TMA sensing performance were alsoinvestigated. The results showed that NiO as a second phase formed on the surface ofthe ZnO nanofibers, Ni-doping greatly improved the TMA sensing performance ofZnO nanofibers by forming p-n junction together with the enhanced interactionbetween NiO and TMA gases. The NiO-ZnO nanofibers exhibited higher sensitivity,wider linearity and better selectivity: the sensitivity to100ppm TMA at280oC increased from103（ZnO） to492（NiO-ZnO）, the linearity range increased from1-200ppm to1-500ppm，moreover, the response times were also decreased from9s to5s.In chapter3, the cubic In₂O₃nanoparticles were firstly prepared by a sol-gelmethod and their ethanol sensing properties were also investigated. The resultsshowed that the In₂O₃nanopaticles with small particle size of11nm aggregated,which greatly affected their ethanol sensing properties. Then the In₂O₃nanofiberswere prepared by an electrospinning method. In order to improve the ethanol sensingproperties, P-type NiO were introduced to N-type In₂O₃nanofibers and formed Stablesolid solution In₂xNixO₃nanofibers, which resulted in the formation of p-n junctionand increasing of oxygen vacancies. At the same time, the average diametersdecreased from95nm （In₂O₃） to60nm （In₂xNixO₃）, the particle size decreased from14.02nm to11.28nm, which indicated the nanofibers after doping, owns higheraspect ratio and large surface to volume ratios. All of which greatly enhanced theethanol sensing performance: the sensitivity of In₂O₃nanofibers to100ppm ethanol at180°C were35, after Ni-doping, the sensitivity increased to80. Moreover, goodselectivity, fast response and recovery rate （<3s and <2s）, and excellent linearity in arelatively wide range of1–500ppm were also observed. In the last part of chapter3,the In₂O₃nanotubes were prepared by an electrospinning method together with anoriented-contraction calcination scheme. The ethanol sensing results showed that theIn₂O₃nanotubes exhibited much poor performance, which on one hand, may becausethat the rapid heating rate has a positive influence on crystallinity, on the other hand,the tube morphology were destroyed in the sensor fabrication process.In chapter4, LaCoxFe₁xO₃nanoparticles （x=0,0.1,0.2, and0.3） are preparedby a sol–gel method, and the effects of Co-doping, calcination temperature, andcarbon nanotube （CNT）-treatment on their ethanol sensing properties are investigated.The results showed that stable LaCoxFe₁xO₃solid state solution were formed after Codoping, their ethanol sensing performance decreased with the doping rate, the optimalratio is0.1, the average particle size increased with the calcination temperature, thehighest response is found based on the LaCo0.1Fe0.9O₃nanoparticles calcined at600°C, and the sensing properties of this sample can be further improved by adding CNT in the precursor. The responses of un-treated and CNT-treated LaCo0.1Fe0.9O₃nanoparticles are about120.1and137.3–500ppm ethanol at140°C, respectively.Simultaneously, by CNT-treatment, the response time is decreased from56s to10s,and the recovery time is decreased from95to35s.In chapter5, Co₃O₄nanofibers were prepared by an electrospinning method. Theeffects of pore-making by calcination on the Xylene sensing properties of theas-prepared nanofibers were also investigated. The results showed that the averageparticle size increased with the calcination temperature （400°C，500°C，600°C，700°C）. The pore size increased with calcination temperature. The Co₃O₄nanofiberscalcined at500°C exhibits the highest response to xylene in a wide concentrationrange. At255°C, the response of Co₃O₄nanofibers calcined at500°C to100ppmxylene were10.14, moreover, these fibers also exhibited good selectivity, fastresponse （15s） and recovery （22s） rate. These properties make the fabricatednanofibers good candidates for xylene detection.