Preparation And Optimization Of Layered Double Hydroxide-based Gas Sensor

At present,more and more people begin to pay attention to environmental protection and health problems,the demand for gas-sensitive sensors is gradually increasing.NO₂ and methanol are two toxic gases that seriously endanger human health,so that the development of high-performance gas sensor to achieve rapid and micro detection is of great significance.Layered double hydroxides（LDHs）is a new type of two-dimensional semiconductor material that has received extensive attention from researchers in the field of gas sensing owing to its unique layered structure,high porosity,and large specific surface area.However,the poor conductivity and serious stacking of LDH layers have limited its application in rarefied gas detection.In view of the structural modification mode of LDHs,this paper designs and constructs layered double hydroxides/reduced graphene oxide（LDHs/r GO）based hybrid and silver nanoparticles/single layered double hydroxide（Ag/LDH）composite structure gas sensors respectively from the direction of double and single LDHs modification.The purpose of different modification methods is to explore the improvement effect of NO₂and methanol gas sensitive reaction.First,we design and construct a hybrid of Zn Ti-LDHs/r GO with ZIF-8 oriented grown on GO as precursor and self-sacrificial templates.In the hybrid system,the derived Zn Ti-LDHs well inherits the cage-shaped geometry of the ZIF-8 templates,which significantly improved the aggregation phenomenon of LDHs and increased its specific area and porosity.At the same time,the conductivity and heterojunction modulation introduced by r GO promotes the sensing responses and dynamic characteristic of LDHs-based sensor.The results point out that Zn Ti-LDHs/r GO hybrid gas sensor can detect a trace amount of NO₂ at 50 ppb at room temperature,with a 9-fold enhanced response compared to pure Zn Ti-LDHs.Moreover,the hybrid sensor shows instantaneous response characteristic,stability,and selectivity.Next,the adsorption characteristics of methanol molecules based on silver modified monolayer ZnAl-LDH gas sensor are studied.Based on first-principles calculation,a gas adsorption model of methanol-Ag/ZnAl-LDH was established.Then the adsorption structure is analyzed in depth,including the calculation of the corresponding adsorption energy and bond length.It is found that the modification of Ag atom resulted in more charge transfer,which significantly improves the adsorption performance of pure monolayer ZnAl-LDH for methanol gas.Based on the conclusion of theoretical calculation,monolayer ZnAl-LDH nanosheets are prepared by inverse microemulsion method.Then silver nanoparticles are introduced onto LDH surface by glucose reduction method.The results show that the response to 25 ppm methanol is 21.9%,indicating that the monolayer structure of LDH is favorable for gas adsorption.The Ag/ZnAl-LDH composite structure gas sensor can not only reduce the lower limit of methanol detection,which is reflected in the response value of 3.35%for 500 ppb methanol,but also can greatly improve the response value,which is proved in the response value of 64.4%in 10 ppm methanol environment.In addition,the Ag modification also achieves fast response-recovery characteristics with response time less than 10 s and recovery time less than 20 s,as well as good stability and selectivity.Finally,the internal mechanism of Ag modification to improve LDH gas sensitivity is studied based on microstructure and first-principles calculation.