Research on Visible Light Activated Oxygen Vacant ZnO1x Based Room-temperature NO2 Gas Sensors
NO2 is one of the important pollutants affecting air quality.It is highly desired to develop NO2 gas sensors with high sensitivity,low cost and easy maintenance.Among these semiconductors,ZnO is the most promising NO2 gas sensing material.At present,one important research direction is to reduce the operating temperature,that is,to develop room-temperature semiconductor gas sensors.It has been shown that UV light illumination can effectively reduce the working temperature of ZnO gas sensors to room temperature.However,UV light requires a customed light source,and may cause aging of polymer components of sensor device,and even cause decomposition or chemical reaction of the target gases.Therefore,it is necessary to develop visible light activated ZnO gas sensors working at room temperature.The thesis is aimed at developing high-performance visible light activated ZnO room temperature NO2 gas sensors.Based on the three core problems of ZnO,including narrow visible light absorption range,low electron concentration,and poor room-temperature NO2 adsorption activity,the thesis adopts oxygen vacancy self-doping to solve these problems,and thus improving the room temperature NO2 sensing response.The visible light absorption ability,electron concentration,and room temperature NO2 adsorption activity are characterized by UV-Vis absorption spectrum,base resistance measurement,and gas sensing test.The enhanced gas sensing mechanism is analyzed by DFT simulation in terms of adsorption energy and electron transfer.Apart from oxygen vacancy self-doping,the thesis also adopts four strategies,like synthesizing special nanostructures,constructing homogeneous heterojunctions,introducing narrow bandgap semiconductors,and rGO to further improve the NO2 sensing performance.The main research contents are as follow:(1)The NO2 sensing response of highly porous nanostructured ZnO1-x was studied.The highly porous structure was favorable for increasing the specific surface area and porosity,which increased the number of adsorption sites and adsorption activity.The NO2 sensing performance was improved by combining oxygen vacancy self-doping and highly porous nanostructure.The highly porous nanostructured ZnO1-x sensor was prepared by solution precursor plasma spray(SPPS)technique.Three characteristics of SPPS including ultra-high temperature,extremely fast cooling rate and reducing atmosphere,generated rich oxygen vacancies.The presence of oxygen vacancy increased the visible light absorption range,electron concentration,and NO2 adsorption activity of ZnO.The SPPS ZnO1-x sensor exhibited obvious sensing response to ppm level NO2 under visible light irradiation at room temperature.However,the sensor response was a little bit low,besides,the response and recovery times were relatively long.Other methods should be adopted to further improve the sensing performance.The light wavelength and intensity played important roles in the sensing performance.When wavelength reduced or intensity increased,the sensing performance improved.10 mW/cm2 blue light was the optimul excitation source.(2)Effect of oxygen vacancy concentration on NO2 sensing response of ZnO1-x with DFT simulation was studied.In order to study the evolution of NO2 sensing performance with the oxygen vacancy concentration,ZnO1-x sensors with different oxygen vacancy concentration were prepared by thermal decomposition of ZnO2.The oxygen vacancy concentration was adjusted by changing the ZnO2 concentration.When oxygen vacancy concentration increased,the bandgap reduced,the visible light absorption range extended,the electron concentration increased,and the NO2 sensing performance enhanced.The sensor response was higher,additionally,the response time and recovery time were shorter.The effects of oxygen vacancy concentration on the bandgap,adsorption energy and electron transfer were studied by DFT simulation.The DFT calculations showed that bandgap was reduced due to the valance band maximum moved up and widened.Besides,the adsorption energy reduced and number of electrons transferred increased,which facilitated the adsorption reaction and improved the sensing performance.(3)The NO2 sensing response of hierarchical ZnO1-x nanosheets was studied.The NO2 sensing performance was improved by combining oxygen vacancy self-doping and hierarchical nanostructure.First hierarchical ZnO 2D nanosheet powder was synthesized by direct precipitation method.Afterwards,hierarchical 2D ZnO1-x nanosheets were deposited on flexible polypropylene substrates by suspension flame spray(SFS)technique.Similar to SPPS,SFS technique also had three characteristics,including ultra-high temperature,extremely fast cooling rate and reducing atmosphere.Therefore,SFS ZnO1-x coatings also had high oxygen vacancy concentration.Compared with highly porous ZnO1-x,the sensing performance of hierarchical ZnO1-x sensor was much better.The enhanced sensing mechanism was due to the fact that hierarchical structure can overcome the agglomeration of 0D nanoparticles,which greatly increased the specific surface area,the number of adsorption sites,and surface reactivity.In addition,most of techniques preparing flexible gas sensors were medium and low-temperature methods.Herein,flexible ZnO gas sensors were prepared by a high-temperature deposition method.(4)The NO2 sensing response of highly porous nanostructured ZnO1-x heterojunction composites were studied.The NO2 sensing performance was improved by combining oxygen vacancy self-doping and constructing heterojunctions.The experiment was based on two strategies.The first one was introducing SnO1-α and SnO2-γ to construct homogeneous heterojunction.Due to the energy level matching,composting ZnO1-β with SnO1-α and SnO2-γ could effectively separate the electron-hole pairs,which greatly increased the electron concentration.The increased electron concentration facilitated the electron transfer between sensitive material and NO2,which improved the sensor response.The second one was to introduce narrow bandgap semiconductors of Cu2O1-α and CuO1-γ to enlarge the visible light absorption range of ZnO,thereby accelerating the adsorption and desorption rate of NO2 gas molecules.The SnO1-α@ZnO1-βp@SnO2-γ and Cu2O1-a@ZnO1-β@CuO1-γ sensors were successfully prepared by solution precursor plasma spray(SPPS)technique.Compared with ZnO1-x,the sensing performance of SnO1-α@ZnO-β@SnO2-γ and Cu2O1-α@ZnO1-β@CuO1-γ sensor were significantly enhanced.(5)The NO2 sensing response of rGO@ZnO1-x nanosheets with MD simulation was studied.Based on rich oxygen vacancies,the NO2 sensing response of ZnO1-x sensor was further enhanced by introducing rGO to take advantage of its high specific surface area,good conductivity and fast electron transfer.The GO suspension was synthesized by Hummer method,and then the rGO@ZnO1-x composites were synthesized by hydrothermal method using zinc nitrate and GO suspension as raw materials.rGO-wrapped ZnO nanosheets were found in the morphological studies.Large numbers of ZnO nanosheets were uniformly loaded on the rGO sheets,forming many nanoscale heterojunctions.The rGO@ZnO1-x composites had strong absorption in the whole visible light region.Compared with ZnOi-x,the rGO@ZnO1-x composite sensor exhibited better sensing performance.Even to ppb-level NO2,the rGO@ZnO1-x sensor also showed good response at room temperature with visible light illumination.The detection limit was low,and the response and recovery process were fast.MD simulation indicated that the introduction of graphene reduced the adsorption energy of NO2 gas by two-thirds,which made the adsorption reaction happen easier.