Research On Photoexcited Gas Sensor Based On Zinc Oxide Nanostructures

In recent years,gas sensors have shown good application prospects in air quality monitoring,food safety management,home intelligent control,medical and health diagnosis and other aspects.The research and development of new gas sensors are becoming more and more urgent,and have received extensive attention.The semiconductor oxide gas sensor is the frontier direction and research hotspot in the field of gas sensor because of its advantages such as all solid state,simple structure,low cost,small size and adjustable performance.At present,semiconductor oxide gas sensors are still mainly thermal excitation,with excellent gas sensitive performance,but also cause insufficient stability and some safety risks,and is not conducive to the application of gas sensors in wearable devices.ZnO nanomaterials,with excellent physical,chemical and optical properties,have become an important sensitive material.In this academic dissertation,ZnO is used as the matrix material and excited by light instead of thermal.The performance of gas sensors is improved by means of nanostructure regulation and heterostructure modification/recombination.The details are as follows:Since precious metal loading is a common method to improve the gas sensitivity of semiconductor oxides and reduce the optimal operating temperature,in this academic dissertation,Au nanoparticles loaded ZnO nanorods were prepared by solvothermal method.Gas sensitive test results show that the 2 mol%Au modification increased significantly the response of ZnO nanorods.The response to 4 ppm n-pentanol is 71.8at 260?,and the response time is 1 s.In addition,it also has excellent stability,selectivity and wet resistance.Although the gas-sensitive response of ZnO nanorods can be effectively improved by Au loading,the optimal operating temperature of the sensor cannot be reduced.Therefore,subsequent studies are devoted to the development of room temperature gas sensors using photoexcitation methods.Light excitation can activate the surface of sensitive materials,increase carrier concentration and reduce their initial resistance.But the gas sensors excited by light still has problems such as low sensitivity and slow response and recovery speed.Based on this,the relationship between different low-dimensional nanostructures,such as ZnO nanorods,nanospheres and nanoflowers,and the gas-sensing properties were compared,and the sensing mechanism was analyzed,which provided the experimental basis and data support for further improving the properties of the photoexcited room temperature sensor.In this academic dissertation,we prepared porous ZnO nanospheres doped with Co by water bath method,which has a large specific surface area.Meanwhile,it was found that Co doping promoted the adsorption of oxygen on the ZnO surface,and the surface band bending caused by oxygen adsorption further improved the separation efficiency of photogenerated charge.The gas sensitive test results show that the gas-sensitive response of 1.5 mol%Co doped ZnO to 50 ppm HCHO excited by ultraviolet light(6 m W/cm²)can reach 111.6.The detection limit is as low as 100 ppb,and the response time and recovery time are 32 s and 44 s,respectively.In order to compare the gas-sensitive properties of different ZnO nanostructures,ZnO nanorods and ZnO nanoflowers were synthesized by hydrothermal method,and porous ZnO nanospheres were synthesized by water bath method.XRD,SEM,BET,XPS,UV-Vis and impedance spectroscopy were used to characterize the surface and electronic properties of three kinds of ZnO nanostructures.ZnO nanospheres exhibit the highest gas-sensitive response to NO₂ gas because of their larger specific surface area and more surface oxygen adsorption.ZnO nanorods exhibit the fastest response and recovery speed due to their high crystallinity,low defect density and unique unidirectional electron transport characteristics.The gas-sensitive response,response and recovery rate of ZnO nanoflowers are between ZnO nanorods and ZnO nanospheres.Therefore,the combination of one-dimensional nanostructure and zero-dimensional nanometer particles can improve the sensitivity of the sensor and obtain a fast response and recovery speed.In₂O₃ is a kind of oxide with good selectivity to NO₂ and is matched with the band structure of ZnO.We have modified In₂O₃ nanoparticles on the surface of one-dimensional ZnO nanorods by in-situ growth method,and obtained a room temperature NO₂ gas sensor with both high sensitivity and fast response and recovery rate.The heterojunction formed between In₂O₃ and ZnO not only introduces an extra depletion layer,but also promotes the separation efficiency of photogenerated carriers under the action of interfacial electric field,thus improving the gas-sensitive performance.In₂O₃-ZnO composites can effectively improve the sensitivity and recovery speed of the sensor to NO₂ under the excitation of ultraviolet light,and the response time and recovery time are 100 s and 31 s,respectively.The response to 700 ppb NO₂is up to117.0,and the detection limit is 50 ppb.Finally,we realize the room temperature detection under visible light excitation.UV-LED is expensive and harmful to human body.Therefore,visible light excitation gas sensor has a better application prospect.2D/2D ZnO/g-C₃N₄ heterocomposites were synthesized by ultrasonic mixing and subsequent calcination.The gas-sensitive properties of ZnO/g-C₃N₄ heterocomposites to NO₂ were investigated under UV and visible light excitation.When the ZnO/g-C₃N₄ composite is irradiated by a blue light source at 460 nm wavelength,its gas-sensitive response to NO₂ reaches the highest,and the response time to NO₂ at 7 ppm reaches 44.8.The response time is 142 s,the recovery time is 190 s,and the detection limit is 38 ppb.Moreover,it has good selectivity and repeatability.The excellent gas-sensitive properties of ZnO/g-C₃N₄ composites under visible light are mainly attributed to the excellent absorbance of g-C₃N₄ in the visible light region and the effective photogenerated charge separation between ZnO and g-C₃N₄ heterointerfaces.