Study on the Preparation and Characteristic of Light-Activated Gas Sensors at Room Temperature

Author:Zhang Qiu Ping

Supervisor:xie guang zhong xu ming


Degree Year:2019





Environmental pollution,especially air pollution caused by industrial emissions,automobile exhaust emissions and improper waste disposal,as well as the volatilization and leakage of indoor toxic and harmful gases,continuing to endanger human life.In addition,the development of the miniaturization,integration,and flexibility has placed higher demands on the electronic devices operated at room temperature.Therefore,the development of portable gas detection technology at room temperature is imperative.Among them,light-activated gas sensors show great application potentials in gas detection because of their excellent performances at room temperature,low ambient humidity dependence,feasibility in flexible integration and low energy consumption.However,such gas sensing technology started late,and there are still many scientific problems behind the technology.The sensor(especially the room temperature visible light-activated gas sensor)performances need to be further improved for the practical applications.In this regard,based on ZnO nanostructures,this paper reveals in detail the scientific problems related to the light-activated gas sensing technology from the perspective of both experimental observations and theoretical calculations,and develops high performance room temperature light-activated resistance gas sensors according to the proposed theory.The main research contents of the thesis are summarized as follows:1.A UV-activated room temperature chemiresistive gas sensor based on ZnO nanocrystals film was fabricated.The influences of sensing film thickness,and the gas sensing properties of sensor under various conditions were also investigated in detail.Based on the research results,the adsorption behaviors of gas molecules on the surface of ZnO nanocrystals under UV irradiation was uncovered.The chemisorbed oxygen species(O-2(ads)(hv))induced by UV light govern the adsorption and desorption ways of other gas molecules on the surface of ZnO nanocrystal,which is depended on the electron affinity of gas molecules.Gas molecules with higher electron affinity than oxygen molecules can be adsorbed on the surface by the competitive adsorption way,extracting electrons from the surface.Gas molecules with lower electron affinity than oxygen molecules are attracted by the adsorbedO-2(ads)(hv)layer,releasing electrons to the surface.These processes can influence the gas sensing properties of the sensor.2.Based on a novel research route and elaborate experiments,we reveal the surface structure-property relationships of the facet effect,and,in conjunction with first-principle density functional theory(DFT)calculations,identify the defect effects of the dominant exposed(100)surfaces of the ZnO nanorods synthesized by a mild hydrothermal process.The smaller rod diameter and higher density array make the ZnO(100)surfaces more exposure to the surrounding,and surface oxygen vacancy more,increasing O2 and H2O adsorption(the latter preferentially occurs on the ZnO(001)surfaces).The visible light absorption strengthens with raising the valence band maximum position caused by the enhanced surface oxygen vacancies.A fundamental understanding of surface adsorption-reaction-desorption mechanism well explains the essence of light-induced NO2 gas detection application.3.A fast and simple method using co-grinding with Fe powder followed by heat treatment(easy to industrialize)is developed to regulate surface oxygen vacancy content of ZnO nanoparticles.The increased surface oxygen vacancy is conducive to shortening the adsorption and desorption times of NO2 molecules,arising from the enhanced O2 adsorption.Therefore,the response and recovery speeds of ZnO nanoparticle-based sensor under UV irradiation improve.4.Noble metal-metal oxide nanohybrids play an ever-increasing role in photocatalytic applications.Here,a ZnO-Ag nanoparticle prepared by a modified polymer-network gel method was utilized for room temperature light-activated NO2 gas detection.Since a heterojunction forms between the two materials and surface oxygen vacancies increase,the sensitivities of the sensors to NO2 gas(0.5-5 ppm)under various light(λ=365-520 nm)illumination conditions are enhanced in comparison with those of pure ZnO sensor.Localized surface plasmon resonance(LSPR)was found to result in the excellent visible-light performance of this ZnO-Ag nanostructure.More importantly,by changing the Ag loading,and tuning the working wavelength using different LED light sources,we can obtain an optimized sensitivity.When blue-green LED(470 nm,75 mW/cm2)is used,the 3 mol%Ag-loaded ZnO sensor shows the highest sensitivity as well as superior stability and selectivity.In addition,the as-prepared sensor shows a positive influence of humidity on the response to NO2,which can be attributed to the photocatalytic reactions related to water molecule.