| Zinc oxide, as an important n-type semiconductor with wide forbiddenband, is widely used in semiconductor devices because of the advantages, suchas good stability, non-toxicity, cheapness, etc. Recently, zinc oxidenanomaterials with various morphologies have been synthesized by differentmethods, and it is found that the performance of these materials is related withtheir compositions and structures. Therefore, preparation of nanomaterialswith special structures and establishment of the relationship between thestructure and property are especially significant to the development offunctional devices. The research of gas sensors is relatively late and the gassensing mechanism is not fully understood. Currently, there is no a definiteand unified theory that can be used to guide the preparation of sensitivematerials. Based on the facts above, the synthesis and gas-sensing propertiesof zinc oxide nanomaterials were chosen to be investigeted. Zinc oxidenanomaterials with different morphologies were synthesized firstly, and thenthe NO2sensing mechanism was studied thoroughly. The relationship betweenthe structure and gas-sensing property of the material was established through the investigation of microstructures of the materials. The analytical methodswere also used to clarify the relationship among chemical compositions,structures and gas-sensing properties of Pt doped zinc oxide nanomaterials.The works of this thesis were as follows:1. A series of zinc oxide materials with different morphologies, such ashollow microspheres, single-hole microspheres, tubes, rods, flakes,arborizations and flowers, were prepared by the hot emulsion, microwavesolvent and microwave hydrothermal methods, and the different morphologiesof the products resulted from the special airtight reaction system and themodification effect of surfactants. The influence of experimental parameterson the morphology of the product was investigated, including types anddosage of surfactant, quantity of the reaction liquid, reactant concentrations,type of the precipitation agents and reaction time. And then controllablesynthesis of zinc oxide nanomaterials with different structures were realizedthrough adjusting the experimental parameters, which was the foundation forthe follow-up performance studies of the materials.2. In-situ diffuse infrared spectrum (DRIFTS) was used to study thereaction kinetic process of nitrogen oxides on the zinc oxide surface in orderto explain the gas-sensing mechanism. It was found that free nitrate, nitrateand nitrite ions were the main species adsorbed on the material surface, and asmall amounts of NO and N2O gases may be generated from thermaldecomposition of the NO-3, NO-and N-2O2species. According to the changes of adsorbed species upon the time and temperature, a more comprehensivegas-sensing mechanism was proposed in the basis of the defect structures ofzinc oxide nanomaterials. Reactions during the response process mainlyincluded: nitrogen dioxide captured an electron from the surface donor of zincoxide and reacted with the adsorbed oxygen to form the nitrate ions, and thatwas the essential reason for the increase of the resistance of zinc oxidesemiconductor nanomaterial. Therefore, the gas sensitivity of nanomaterialsmight increase with the adsorbance of intermediate.3. Tube-like and flower-like zinc oxide nanomaterials were prepared by thehot emulsion method and their NO2sensing properties were investigatedcomprehensively. It was found that the tube-like zinc oxide sensor showedhigher sensitivity to nitrogen dioxide and the sensitivity was about five timeshigher than that of the flower-like zinc oxide. And the detection limit of thereactions was up to50ppb. Response and recovery time of the two kinds ofzinc oxide nanomaterials to nitrogen dioxide were compared and it wasconcluded that the tube-like zinc oxide was in favor of electronic transmission,which led to a shorter response time. While the flower-like zinc oxidenanomaterial had a unique hierarchical structure leading to a shorter recoverytime. Porous zinc oxide nanoflakes, formed by cross linking nanoparticles,with the thickness of about40to80nm and the aperture about30to60nm,were synthesized by the microwave solvent method and their gas-sensingproperties were compared with that of purchased nanoparticles. It was found that gas sensitivities of the porous zinc oxide nanoflakes was about7timeshigher than that of the purchased nanoparticles and to10ppm NO2thesensitivity was up to about1100. The influences of the post-treatmentconditions (such as calcination temperature) on the gas-sensing performanceof zinc oxide nanomaterials were further investigated, and it was found thatthe gas sensitivities of the nanomaterials had no linear relation with theirparticle sizes, but there was an optimal value. The composition, structure andelectro-optical properties of zinc oxide nanomaterials were analyzed by usingthe characterization techniques such as XRD, TEM, PL and XPS, and thereason for the different gas-sensing performance was revealed. In addition, thesemiquantitative relationship between the gas sensitivity and defect structureof the nanomaterial was established according to the results of thefluorescence and X-ray photoelectron spectroscopy.4. Pt-doped zinc oxide nanomaterials were prepared by the impregnationmethod. According to the characteristics of electron transfer in semiconductor,the relationship model of methane sensitivity and catalytic reaction rate onzinc oxide was established through theoretical calculation, and it explained thereasons for why the oxide semiconductor sensor had a best workingtemperature. Products of catalytic conversion of methane were tested by theonline TPD-MS method, and it was concluded that the process of methanegaining or losing electron to form intermediates (CO) may play a key role indetermining its sensitivity. The comparism of the gas-sensing performance between ZnO-Pt and ZnO nanomaterials to CO, C2H5OH, NO2and CH4werecarried out. The electro-optical properties and microstructures of the zincoxide nanomaterials were analyzed by using a series of characterisationtechniques and the different gas-sensing performances were explained. ZnO-Ptsensors, which can selectively response to the four gases, were fabricated bychanging the working temperature. |
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