| The key factor of gas sensor is sensing material,the contact and reaction between the gas-sensing material and the analyte gas causes the change in the physical orchemical properties of the gas-sensing material,and thus provide feedbackinformation of anayte gas.Among different kinds of gas sensors,gas sensor based onmetal semiconductor oxide are widely used due to its simple preparation method,reliable repeatability,low cost and excellent stability.However,the gas sensor based on metal semiconductor oxide usually requires high working temperature to achieve the best gas-sensing performance,extended operation at elevated temperature can have serious consequence for the repeatability and stability of gas-sensing material and increase the cost of testing.Therefore,researchers have paid more attention to improve the gas-sensing performance while lowering the operation temperature.The improvement mainly includes the material composition,surface morphologic structure and surface modification.In addition,in the traditional gas sensor prototypes preparation process,the powders are mixed with adhesive and printed on prefabricated electrodes substrates.But this process is complex and extraordinary random,and easy to cause uneven distribution and agglomeration,leading to reduced gas-sensing performance.Therefore,the gas sensor based on nanoarray materials that directly growing on the substrates are of particle interest,because it not only avoids the traditional coating process,but also provides open space allowing for easy diffusion and contact of the gas molecular into sensing materials..In this thesis,the metal semiconductor oxides with special morphology and structure were synthesized by hydrothermal method and template method.By comparing and characterizing the precursors and gas-sensing materials,we analyze its growth process and mechanism and construct gas sensor prototype for gas-sensing test.In view of the requirement of gas sensors used to non-invasive health testing,the main target gases are set as the volatile organic gases related to human metabolism,and achieve low-level gas detection at low temperature.The main contents in this thesis include:1.The gas sensor based on ultrathin porous Co3O4 nanosheets has been successfully synthesized through a hydrothermal technique and a subsequent controlled annealing route,showing excellent performances for acetone detection at a much lower operating temperature.The response to 100 ppm acetone reached 11.4 at 150℃.What’s more,the detection limit of Co3O4 nano sheet-based sensors is lower than the 1.8 ppm that is exhaled from diabetes patients,and exhibited good reproducibility and stability.Its high-performance is due to the ultrathin structure,meso-porosity,and large specific surface area.These results indicate that the gas sensor based on ultrathin porous Co3O4 nanosheets is very promising for making a daily life initial judgment on diabetes in human beings.However,it must be mentioned that there are lots of problems which should be overcome before putting the sensor into practical use.2.Speical Co3O4 crossed nanosheet arrays were synthesized via a facile fluorine-assisted hydrothermal method.The growth mechanism has been comprehended by studying the morphology of precursor with the increment of the reaction time.The Co3O4 crossed nanosheet arrays are interdigitated and perpendicular to the substrate.Due to the good Ohmic contact with the electrodes,and intensive contact,it could directly serve as gas sensor prototypes without the conventional film formation process.This highly open and porous structure not only prevents agglomeration but also leads to more active sites to absorb O2,and the open space among the neighboring nanosheets allows fast absorption and desorption reactions.The gas sensors based Co3O4 crossed nanosheet arrays show great performance for detecting acetone.What’s more,sensors can detect acetone in a wide range of concentrations starting with a low detection limit and exhibit good reproducibility and stability.The high-performance results from the unique structure of crossed nanosheet arrays with a variety of excellent features,mesoporous structure and large specific surface.The fabrication of the high-performance gas sensors possesses the capability of constructing complex architectures with the Co3O4 crossed nanosheet arrays as the building block.3.The porous Co3O4/Ta2O5 heterostructure hollow nanospheres prepred with the assistance of carbon colloidal nanospheres and following annealing.The Ta2O5 nanostructures with ultrathin hollow spherical shell acted as the supporting model,hindering the aggregation of Co3O4 nanoparticles and possessing more specific surface area that supplies gas molecule adsorption with more active sites.Numerous Co3O4 nanoparticles attached on the Ta2O5,forming heterojunction in the contact surface.Owning to the pyrolysis of precursor and release of CO2 and H2O in the time of thermal conversion,numerous pores formed on the surface of the hollow nanospheres.In gas sensing application,Co3O4/Ta2O5 hollow nanospheres exhibited excellent gas sensing performance to ethanol at room temperature.The enhanced gas sensing properties at room temperature is attributed to the formation of heterojunction between Co3O4 and Ta2O5 and porous hollow nanospheres structure.We can confirm that the Co3O4/Ta2O5 hollow nanospheres heterostructure is promising candidates for high performance room temperature gas sensor.4.Fe-doped WO3 mesoporous hollow nanospheres with different concentrations were synthesized by a facile method,which have small grain size,high crystalline quality,and ultrahigh surface area.Fe doping leading to the smaller cell parameters as compared to pure WO3 and the slight distortion in the crystal lattice produces a number of defects,making it a better candidate for gas sensing,in addition,there are more oxygen vacancies than pure WO3 hollow nanospheres,which is beneficial to the adsorption of oxygen and NO2 and the surface reaction.The NO2 sensing properties of as-prepared samples in the concentration range 10-1000 ppb have been investigated.Comparing with pure WO3 hollow nanospheres,Fe-doped WO3 shows better NO2 sensing performance,especially the 5.2%Fe-WO3.The reason for that is attributed to the more oxygen vacancies and surface chemical state changed by the doping of Fe,resulting in more gas adsorption,diffusion,and reaction and attaining the enhanced gas-sensing performance. |
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