Biotemplate Syntheses Of Tubular ZnO Hierarchical Structures And Their Sensing Performance Towards Nitrogen Oxides

Nitrogen oxides(NOx)are forerunner for the formation of air pollution,such as acid rain,ozone hole and PM2.5,which greatly damage the ecosystems on the earth.Meanwhile,prolonged exposure to low concentration of NOx environment can cause mucosal and respiratory damage in humans,and even life-threatening.Thus,real-time and accurate monitoring of NOx can be of great practical significance for environmental protection and human health.As a wide band-gap semiconductor sensing material,ZnO has been extensively investigated in recent years for detecting toxic NO2.However,these materials still have defects of high energy consumption,low sensitivity and large detection limit.In contrast,there are fewer reports on ZnO-based NO gas sensor.Especially,dual-sensing detection of the above two harmful gases has not been achieved so far.Meanwhile,the traditional chemical methods for synthesizing tubular sensing materials(hydrothermal,electrostatic spinning,etc.)suffer from complicated operation,high cost,low yield and poor reproducibility,and the utilization of multiple chemical reagents also generates environmental pollution.Accordingly,in this thesis,green and renewable natural biomasses were chosen as templates to controllably synthesize biomorphic ZnO materials with different tubular structures by simple zinc salt immersion and air calcination method.These ZnO sensing materials separately achieve high selectivity to NO and NO2 gases and their dual-sensing detection at low temperature.The main research contents and results include the following parts:(1)Discarded chestnut mesocarp(CM)was employed as biotemplate and immersed in zinc salt solution.Then,one-dimensional(1D)ZnOCM-6 tubes assembled from uniform nanoparticles were simply and controllably prepared by calcining the immersed precursors at 600℃.The fabricated sensor achieves a response value of 365 towards 10 ppm NO2 at 92℃,which is 30.4 times higher than that(S=12)of template-free sample.Meanwhile,it is also the highest response value among reported 1D ZnO-based sensors.Furthermore,this sensor has a short recovery time(22 s).(2)Hemp(HP)fiber was utilized as biotemplate to simply synthesize ZnO hierarchical tubes by calcining zinc salt solution immersed precursors at different temperatures.Amongst,ZnOHP-6 tubes obtained by calcination at 600℃ were cross-linked by-50 nm nanoparticles.The ZnOHP-6 sensor shows a response value of 78.54 towards 10 ppm NO at 92℃ with response/recovery times being 56 s and 92 s.In particular,its detection limit of 5 ppb is the lowest among NO sensors reported so far.(3)Biomorphic ZnOcB-6 hierarchical tubes constructed from quasi-spherical nanoparticles were obtained by calcining zinc salt solution-immersed cajuput bark(CB)precursors at 600℃.Due to the existence of rich oxygen vacancy(Vo)and interstitial zinc defects,the fabricated sensor exhibits an enhanced response value of 118 towards 10 ppm NO at 92 ℃,which is 5.4 times higher than that of template-free ZnO nanoparticles.Meanwhile,its recovery time shortens to 52 s,being superior to currently reported NO sensors.(4)Two ZnO hierarchical tubes were prepared by simple and controllable zinc salt immersion and air calcination method using loofah sponge(LS)as biotemplate.Amongst,biomorphic ZnOLs-6 structure obtained by calcination at 600℃ is cross-linked by nanorods and quasi-spherical nanoparticles,which possesses relatively narrow mesopore distribution,large specific surface area and abundant Vo defects.The ZnOLs-6 sensor has response values of 100 and 342 towards 10 ppm NO and NO2 at 92℃.The response/recovery times are separately 46 s/12 s(NO)and 61 s/15 s(NO2).This is the first time for conductometric sensor to achieve low-temperature and dual-sensing detection to the above NOx gases.Moreover,this sensor shows satisfactory detection results in detecting real samples from self-service gas stations.(5)Compositions and microstructural features of above tubular ZnO sensing material were carried out in detail.Meanwhile,x-ray photoelectron spectroscopy and mass spectrometry were employed to analyze the change in valence,content of surface adsorbed oxygen species before and after contacting with NOx,as well as the composition of gaseous products after redox reactions.Combined with the changes in sensor resistance and response in air and high-purity N2,the sensing mechanism of ZnO material to NOx is investigated in depth.