Preparation Of P-type Metal Oxide Semiconductor Nanostructures For Gas Sensing Application

As an extension of the olfactory function,gas sensors are able to identify different gases and convert the related information into readable signals,which has been widely used in the fields of environmental monitoring,public safety,disease detection and the military defense.With the rapid development of semiconductor fabrication technology,the gas sensors are facing the great challenge of miniaturiziton,portability and multifunction.As for various gas sensors,sensitive materials as basic block buildings directly affect the intrinsic properties and final performance.In recent years,metal oxide semiconductor has attracted numerous attention owning to its unique advantages of low-cost,easy preparation and integration.However,gas sensors based on metal oxide semiconductors possess higher operating temperature and poor selectivity,which limits their large-scale practical application.In order to overcome the above disadvantages,the p-type oxide semiconductor with higher reactivity and relatively low working temperature is taken as the research object.The gas sensing performance of p-type oxide semiconductor is improved through controllable synthesis of novel nanostructures,construction of heterojunction and compositional modulation.At the same time,the relationship between the nanostructure of sensitive materials and gas sensing enhancement mechanism are also explored.The experiment results in this dissertation provide new ways for building high-performance oxide semiconductor gas sensors and the main research contents are shown as follows:1.A novel nanostructure of crescent-shaped porous NiO nanoplates,has been synthesized via hydrothermal approach combining with calcination treatment.The volume ratio of ethanol to distilled water and the adding amount of polyvinylpyrrolidone（PVP）have been investigated in detail for precisely controlling the morphology of NiO nanostructure.Subsequently,the gas sensing performance further indicates that the crescent-shaped NiO nanoplates based gas sensors demonstrate excellent ethanol gas sensing performance at 130°C.The enhanced ethanol sensing performance is mainly attributed to its stable porous structure with fine grain size（approximate 10.3 nm）.2.With the assistance of glucose,hierarchical flower-like Co₃O₄nanostructures with diameter ranging from 1.5?2μm were synthesized through hydrothermal method with subsequent calcination treatment.The adding amount of glucose and hydrothermal reaction time have been investigated in detail for explaining the role of glucose in the hydrothermal process and the feasible formation mechanism of flower-like Co₃O₄nanostructures.The fabricated hierarchical flower-like Co₃O₄ sensor exhibites the highest response of 48.1 toward 100 ppm acetone at 130°C.The enhanced acetone sensing performance is mainly attributed to its stable porous flower-like structure and large specific surface area of 80.8 m² g^?1.3.With the assistance of glucose,a flower-like hierarchical NiO-Zn O（G-NiO-Zn O）heterostructure has been synthesized through a one-step hydrothermal method with subsequent calcination treatment.That is to say,the p?n heterojunction is used to improve the gas-sensitive properties of composite materials.Furthermore,G-NiO-Zn O based gas sensors exhibit a response of 142.0 toward 100 ppm glycol at low operating temperature about 140°C,which is 3.2 time and 8.6 time of pure NiO and Zn O,respectively.Besides,its response to 100 ppm glycol is much higher than other common volatile organic compounds（VOCs）gases.The enhanced sensing performance of G-NiO-Zn O to glycol is mainly attributed to its large specific surface area of 183.7 m² g^?1,the synergistic effects of p?n heterojunction,and the massive oxygen vacancies produced during the synthesis process with assistant of glucose,which can promote the sensing performance.4.Novel p–p heterojunction Cu O-NiO nanotubes have been controllably synthesized for high performance glycol gas sensors through one-pot synthesis approach combined with certain calcination treatment,which means improving the gas-sensitive properties of composite materials by establishing the p?p heterojunction.In order to optimize element ratio of Cu to Ni for high performance gas sensors,the feeding ratio of Cu/Ni in the synthesis process has been systematically studied.The gas sensing test indicates that the gas sensor based on optimal hybrid nanotubes with Cu/Ni molar ratio of 13:7（Cu O-NiO（13:7））shows the highest sensitivity toward100 ppm glycol at 110°C.The response is 10.35 and the response/recovery time is 15 and 45 s,respectively.The enhanced sensing properties to glycol are mainly attributed to the synergistic effects of p–p heterojunction and massively produced oxygen vacancies under the optimized element ratio of Cu/Ni that can peovide more reactive sites.In summary,the study on the controllable synthesis of novel nanostructures,construction of heterojunction and compositional modulation of p-type oxide semiconductors can provide a value reference for the design of high-performance gas sensors.