| With the rapid development of environmental safety,medical care and science/technology,people pay more and more attention to the research of high-performance intelligent gas sensors.The chemiresistive gas sensors based on metal oxide semiconductors are widely used because of its simpleness,low cost and easy control.However,its practical application is limited by higher working temperatures.In order to improve the performance of this kind of gas sensor,this dissertation puts forward a unique strategy,using low-dimensional quantum dots materials combined with hollow and hierarchical nanostructures,aiming at the ultra-fast response/recovery of room-temperature gas sensors to monitor the flammable,explosive,and toxic gases(e.g.,hydrogen,hydrogen sulfide).The synthesis of sensing materials,enhanced performance and sensing mechanisms are systematically studied and discussed.A large number of theoretical calculations reveal that the size of n-type semiconductor materials can directly affect the response of gas sensors based on them.While the morphology and particle contact configuration of p-type metal oxide semiconductor sensing materials have a determining impact on the performance of the sensors.At first,based on the previous work,the traditional p-type metal oxide semiconductor nanomaterials are designed into a hollow and hierachical structure with open gas molecule diffusion channels,ensuring that the oxygen and target gas reaches the active site and participates in redox reactions quickly,significantly shortening the response/recovery times of the gas sensor.Based on this research,n-type quantum dots are selected and uniformly and continuously loaded on these unique nanostructures.Quantum dots materials are low-dimensional semiconductor materials,whose dimensions are no more than twice the exciton Bohr radius of the corresponding semiconductor materials.Thus,ammounts of the atoms are exposed on the surface of the material.Both specific surface area and the number of surface-residing atoms increase with decreasing particle size so that the coordination of surface atoms was insufficient and the number of unsaturated and dangling bonds increased.Therefore,quantum dots usually possess numerous active sites and high activity at low temperatures.These characteristics are conducive to a large-scale adsorption of the gases at room temperature,which can hugely enhance the response of the gas sensor and reduce the operating temperature.The integration of quantum dots and hollow hierarchical structured metal oxide ensures a larger specific surface area and an effective electron transport channel,and the gas can quickly reach the active sites on the surface of the quantum dots at a lower temperature,therefore,the comprehensive performance of the gas sensors at room temperature can be effectively improved,that is essential for the development of high-performance gas sensors with high response and ultra-fast response/recovery at room temperature.To this end,the main contents in this thesis are included as follows:1.CdS quantum dots supported by hollowed-out Co3O4 microspheres for enhancing roomtemperature H2S sensing performance.In this work,we propose an efficient way to fabricate a high response ultrafast response/recovery rate room-temperature gas sensor by integrating quantum dots and hollowed-out nanostructures.CdS quantum dots supported by hollowed-out Co3O4 microspheres(CdS QD/Co3O4 HMSs)were successfully synthesized via a simple energy-efficient room-temperature in situ growth method and then made into sensing films for a gas sensor device.The CdS QD/Co3O4 HMSs gas sensor has a high response(12.7)and ultra-fast response/recovery rate(0.6/1.0 s)to 100 ppm H2S at 25℃.Additionally,the sensor had good selectivity,a low detection limit(1-5 ppm),excellent reversibility and good longterm stability.The superior gas sensing properties were ascribed not only to the large specific surface area and high activity of interconnected CdS quantum dots at room temperature,but also to the favorable effect of hollow and hollowed-out structures assembled by ultra-thin and porous Co3O4 nanosheets on efficient gas adsorption-desorption and diffusion.2.Synergistic effect of PdO quantum dots and parallel nanowires assembled CuO microspheres enables high performance room-temperature H2S sensor.In this work,we first applied the CuO hierarchical microspheres assembled by parallel nanowires(CuO NWMs)as H2S sensors,enriching the diversity of CuO gas sensor family.Furthermore,to build an ultra-fast response/recovery room-temperature H2S sensor,we proposed a simple impregnation method that utilizing high catalytic activity PdO quantum dots to functionalize CuO NWMs.Specifically,the response based on pristine CuO NWMs sensor was 4.9 to 50 ppm H2S at 150℃,while for PdO decorated samples,1 and 2 wt.%PdO-decorated CuO NWMs based sensors can sense H2S at room temperature.Among them,the 2 wt.%PdO-CuO NWMs sensors demonstrate the best sensing performance,with the response of 6.8 and ultra-short response/recovery times of 1.8/4.1 s to 50 ppm H2S at 30℃.Additionally,the 2 wt.%PdO-CuO NWMs sensor exhibited outstanding anti-humidity ability,H2S selectivity,and long-term stability.The excellent sensing performance is due to the synergistic effect of the high catalytic activity of PdO nanoparticles and the hierarchical nanostructures.3.ZnS quantum dots loaded on MOF-derived Co3O4 nanoboxes boosted ultrasensitive and ultrafast gas sensors.Hollow and hollowed-out Co3O4 nanoboxes assembled by porous ultrathin nanosheets(Co3O4HHNBs)were successfully synthesized through morphology-conserved transformations by MOF precursors,and applied to ultrasensitive toluene and ethanol gas sensors.The Co3O4-HHNBs sensors exhibited high response of 56.6 to 100 ppm toluene at 200℃ and 37.2 to 100 ppm ethanol at 220℃.The response/recovery time is 10/9 s and 0.4/0.5 s to 50 ppm toluene and ethanol,respectively.Based on the unique non-spherical morphology of the Co3O4-HHNBs sensor significantly improve the response/recovery rates of the sensor,high-activity ZnS quantum dots was loaded on the Co3O4-HHNBs through the in-situ growth method,The ZnS QD/Co3O4-HHNBs sensor exhubited a high response of 9.3 to 100 ppm H2 at room temperature of 25℃,while maintaining an ultra-fast response/recovery time(3.8/3.2s).The integration of quantum dots and MOF derived p-type metal oxide semiconductors with hollow hierarchical nanostructures promote the realization of the ultra-fast response/recovery roomtemperature gas sensors. |
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