| As the air pollution becomes more and more serious and the need for h?unan safety,it is becoming more and more important to produce and manufacture the real-time gas monitoring sensor.Nano-sized metal oxide semiconductor is a very promising functional materials,as a core sensitive material in metal oxide semiconductor gas sensor shows the satisfactory performance.However,the sensitivity,selectivity and responding/recovering time of such materials can not meet the need of real-time monitoring of volatile organic gases.Therefore,it is urgent to improve the comprehensive performance of metal oxide semiconductor gas sensors.From the perspective of nanomaterials,the integrated performance gas sensors not only rely on the development of advanced electronic circuit system,"how to design and synthesize the metal oxide nanostructure with superior gas sensing performance based on the theoretical calculation" is also indispensable.The nanoscale porestructure not only increases the specific surface area of the sensitive materials to provide more active sites for chemical reactions but also provide the channel for the transfer and diffusion of gas molecules,gas molecules can reach the active site of the sensitive materials quickly with the nano pores.But in the actual application of gas sensors,the higher heating temperature will cause the intergrowth polymerization between the nanoparticles.The irregularity or closure of the pores will let the gas molecules through haphazard or hinder its transmission directly.Secondly,in the actual application of gas sensor,the higher heating test temperature will lead to the mutual agglomeration and growth polymerization of the oxide semiconductor nanoparticles.What is more,based on the theoretical calculation,the size of nanoparticles will fundamentally affect the sensitivity of n-type metal oxide semiconductor gas sensors,but the grain sizes and contact configuration affect the propertis of p-type metal oxide semiconductor gas sensors deeply.Thus,this thesis design the preparation of porous nanomaterials which can effectively promote the transport of gas molecules within the nanomaterials,eventually shorten the responding and recovering time of gas sensor.We design ultrathin nanosheets assembled hollowed-out hierarchical nanostructure based on the research of nanomaterials with porous structure,further improved the responding/recovering speed and sensitivity of gas sensor.At the same time,we design ultrathin nanosheets assembled hollowed-out hierarchical nanostructure and nanowires assembled hollowed-out nanostructure,benefitied the sensitivity and responding/recovering speed of p-type gas sensor from the aspect of nanostucture.This thesis mainly oriented to high sensitivity,rapid responding and recovering gas sensor,based on the theoretical calculation,systematically carried out the preparetion of metal oxide semiconductor materials with various special micro-nano structures,and study the relationship between gas sensing properties and nanostructure.In the field of material preparation,the paper used oxalate and metal alcohol salts as precursors in the preparation of metal oxide nanomaterials with special micro-nano structures,and he nanoparticles size of p type metal oxide in this research is far less than Debye length when the previous theoretical calculation about metal oxide semiconductor gas sensor started from the premise that nanoparticles size is much less than Deby length.The main contents in this thesis are included as follows:1.Synthesis of porous a-Fe2O3 microrods for gas sensorsFor n type semiconductor material,in order to keep the small size of nanoparticles and excellent porestructure at the same time,surrounding the sensor's high sensitivity,rapid responding and recovering speed,we prepared porous ?-Fe2O3 microrods by a nonsurfactant assisted hydrothermal method and calcined decomposition conversion,analyzed the sensing properties of porous ?-Fe2O3 microrods and close packing ?-Fe2O3 nanoparticles gas sensors.The results showed that the sensitivity of the porous ?-Fe2O3 microrods gas sensor to 100 ppm ethanol was 8.0,and the responding/recovering time was 7/7 s.Compared with the close packing ?-Fe2O3 nanoparticles,we have found that the well dispersed pores and the interconnection of smaller nanoparticles can provide not only through gas transmission channel,but also greater specific surface area and the corresponding active site,it ensure the responding/recovering speed and high sensitivity of gas sensors.2.Synthesis of ultrathin nanosheets assembled hollowed-out hierarchical ?-Fe2O3 nanorods for gas sensorsBased on the research of porous ?-Fe2O3 microrods gas sensor,in order to further improve the sensitivity of ?-Fe2O3 gas sensor,shorten the responding and recovering time.We synthesized ultrathin nanosheets assembled hollowed-out hierarchical ?-Fe2O3 nanorods by the kirkendall reaction of FeC2O4 precursors with the NaOH as medi?m.The characterizition results show that the thickness of the single ultrathin nanosheet is about 3 nm,which is much less than the Debye length of ?-Fe2O3 at the operating temperature.The surface area is 131.5 m2·g-1,and has an abundant pore structure center on 10-40 nm.Gas sensing performance test showed that the sensitivity to the excellent levels of iron oxide gas sensor,especially the responding and recover time to reach second level(the responding/recovering time is 0.4/2.4 and 0.4/2.4 s to 100 ppm ethanol,respectively).We did not introduce other surfactants and template reagents throughout the preparation process.Compared with the traditional surfactant assisted synthesis method,our method has the characteristics of universal,high efficiency and low cost.3.Synthesis of nanosheets assembled hollowed-out hierarchical Co3O4 microrods for gas sensorsWe synthesized homogeneous CoC2O4 microrods precursor by hydrothermalmethod,translate the CoC2O4 microrods precursors into nanosheets assembled hollowed-out hierarchical Co3O4 microrods based on a kirkendal effect.In addition,the transformation method perfectly preserves the geometric shape of the precursors of CoC2O4 microrods.In the process of transformation,CoC2O4 microrods precursors translated to hollowed-out hierarchical Co(OH)2 microrods,which produced the pore structure not only formed the porous structure of hollowed-out structure,but also makes the conversion process of inner cobalt oxalate precursor more thoroughly.The gas sensor performance test results show that the hollowed-out hierarchical Co3O4 microrods gas sensor has high sensitivity and fast responding and recovering speed especially for methanol and ethanol(the responding/recovering time of hollowed-out hierarchical Co3O4 microrods gas sensor to 100 ppm methanol and ethanol are 0.8/7.2 and 0.8/10.8 s,respectively).4.Porous ultrathin nanosheets assembled hollowed-out and hollow hierarchical Co3O4 microspheres for gas sensorsA simple hydrothermal method was adopted to realize the growth of cobalt precursor in aqueous solution.Using the reaction with NaOH solution,the porous ultrathin nanosheets assembled hollowed-out and hollow hierarchical Co3O4 microspheres were prepared rapidly in 10 minutes at room temperature.The average thickness of porous ultrathin nanosheets is about 3 nm,which is much less than the Debye length of Co3O4 at the operating temperature.This nanostructure match the need of high performance p-type metal oxide semiconductor gas sensor at the same time:the reactivity of sensing materials to oxygen,low grain sizes and contact configuration.Gas sensing measurement results show that the hollowed-out and hollow hierarchical Co3O4 microspheres gas sensor show superior gas sensing characteristics:high sensitivity(to 100 ppm ethanol is 38.2)and super fast responding/recovering features(the responding and recovering time to 100 ppm ethanol were 0.1 s and 0.7 s).To the best of our knowledge,this is the first time to use porous ultrathin nanosheets assembled hollowed-out and hollow hierarchical Co3O4 microspheres as the sensitive material of thin film gas sensors,shorten the responding time and recovering time less than one second.This fast-transfomation strategy provides a new solution to the challenge of high sensitivity,ultra-fast responding/recovering and real-time monitoring gas sensors.5.Synthesis of CuO microspheres assembled by parallel nanowires for gas sensorsThrough a simple and convenient solvent reaction and kirkendall reaction,we synthesized CuO microspheres assembled with parallel nanowires,and used it as a kind of high efficient,sensitive active materials of gas sensors.In the process of kirkendall reaction,copper oxalate microspheres not only provided the source of Cu ions for the CuO microspheres assembled with parallel nanowires,but also provided a template for the formation of the hierarchical microspheres.During the evaluation tests of gas sensing materials properties,CuO microspheres assembled with parallel nanowires gas sensors showed a relatively high sensitivity(6.2)and rapid responding/recovering speed(responding/recovering time were 1.2/6.6 s)to 100 ppm n-propanol steam at the operating temperature of 190 ?.This superior gas sensing performance may be attributed mainly to the chemical composition and unique structure of the ordered CuO microspheres assembled with parallel nanowires,as well as their robust overall framework structure.It's worth noting that the average diameter of CuO nanowires is about 15 nm,which is much less than the Debye length of CuO at the operating temperature.The simple strategies in this section can be used for the preparation of many other hierarchical nanomaterials with self-template,and opened up a new way for exploring the sensing materials for real-time gas sensor. |
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