Application Of Two-Dimensional Organic Frameworks In Chemical Gas Sensing

Chemical gas sensor,as a device that converts chemical information into digital information,is closely related to our daily life and personal health.Among them,chemiresistive gas sensor has the advantages of low power consumption,low cost,and easy integration in standard electronic equipment,compared with the currently widely used electrochemical sensors and optical chemical sensors.Commonly used chemiresistive sensing materials include conductive polymers,ceramics,metal oxide semiconductors,carbon-based materials,etc.These materials have their own characteristics,but they rarely meet the requirements of low power consumption,low cost,high selectivity,and high stability at the same time.For example,ceramics and metal oxides often need to be tested based on surface chemical reactions under high temperature conditions,conductive polymer materials have poor long-term stability,and new types of nanomaterials such as carbon nanotubes generally have poor selectivity.It is still a great technological challenge to design new high-performance sensing materials suitable for practical and commercial applications.To this end,the two-dimensional metal/covalent organic frameworks(2D MOF/COF)are new types of porous material with diversified and adjustable characteristics in terms of chemical structure,physical properties and material functions.They also have large specific surface area,adjustable diameter,and diverse structures.These advantages have been widely used in various electrical fields as soon as they appeared,and they also have shown great potential in chemical sensing.However,at present,the 2D MOF/COF used in chemiresistive sensors are limited by conductivity,at the same time,2D MOF/COF-related devices with better conductivity have problems such as high cost and relatively slow response,which are not conducive to promotion in actual application scenarios.Breaking through the conductivity limitation of frame materials while optimizing the molecular recognition process is an urgent problem that needs to be solved to expand the application of 2D MOF/COF in chemiresisive gas sensing and promote the development of related prototype devices.In this work,we used a simple ligand oxidation method to introduce defects into the 2D conductive MOF material(a M-HAB,M = Ni,Co,Cu,Fe),and prepared a "missing ligand" 2D conductive MOF material.Due to the defects in ligand,a Ni-HAB has many hydroxyl defect sites on the basis of maintaining the original framework structure and conductivity.The introduction of multiple hydroxyl defect sites damages the crystallinity of a Ni-HAB,but compared to its c Ni-HAB,a Ni-HAB shows a faster adsorption and desorption speed for water molecules,so it shows good sensitivity,selectivity,corresponding linear range in humidity sensing,excellent stability,and unique high response/recovery rate.What’s more,in view of the adsorption characteristics of a Ni-HAB for water molecules,we have achieved further improvement and regulation of sensor performance through Nafion functionalization.Transient current measurement and density functional theory(DFT)calculations as well as the comprehensive performance of isomorphic 2D a M-HAB(M = Cu,Fe,Co)MOF show that the activation energy of the hydrogen bond generated by the hydroxyl group introduced at the defect site is important for enhancing humidity response.At the same time,it also shows that defect engineering produced by defective ligand is a universal and effective method to adjust the sensing performance of conductive MOF.This method also reduces the synthesis cost of MOF materials and provides new ideas for commercial MOF-based gas sensing.In addition,we also choose 2D COF material with the same porous,high specific surface area and higher stability as the sensing layer,and use the excellent conductivity of SWCNT to prepare SWCNT@COF-1heterojunctions to achieve selective response to specific gas molecules.This work lays a foundation for further expanding the application of weak-conductive 2D MOF/COF in chemiresistive gas sensing.The specific content of this study is as follows:(1)Through the preliminary exploration of a series of 2D MOF in chemiresistive gas sensing,2D MOF Ni-HAB with suitable conductivity and morphology was selected.Through the ligand pretreatment,a simple hydrothermal method is used to synthesize the defective "missing ligand" 2D conductive MOF material a Ni-HAB,and use its hydroxyl defects to quickly adsorb and desorb the water molecules to design the humidity sensor.Sensing performance shows that the 2D MOF a Ni-HAB with defects has an excellent linear response to water molecules in the range of 20%-90% relative humidity,and the response time is 5.43±1.28 s,while the corresponding recovery time is 1.31±0.22 s at 80% relative humidity.This performance is significantly faster than c Ni-HAB(response time: 36.77 s,recovery time 2.26 s).This fast response/recovery time is unprecedented in organic-based sensing materials.In addition,we can also improve the humidity performance of MOF materials through Nafion functionality,and verify the universality of defect engineering through the synthesis of a series of MOF materials with similar structures.(2)Through the transient current measurement and density functional theory calculation of the corresponding crystalline materials and defective MOF materials,we can further understand the mechanism for rapid response of defective materials to humidity.Transient current measurement shows that the conduction current in a Ni-HAB includes ion conduction(due to the exponential decay of the electrode interface charge)and electronic conduction(steady-state current),and humidity sensing may be mainly based on the proton transport of the material.The humidity sensing signal in the crystalline Ni-HAB may come from electron transport(water adsorption and electronic interaction).Secondly,through density functional theory calculations,we found that the introduction of multiple hydroxyl groups significantly increases the number of water adsorption sites and the binding energy of the corresponding sites to adsorb water in a Ni-HAB,which makes the sensor work in a low relative humidity with higher relative response.More importantly,the hydrogen bond introduced by the hydroxyl group at the defect site will increase the transition path during surface adsorption,reduce the activation energy,and make it easier for adsorption to reach equilibrium to show a faster response speed.(3)A heterogeneous structure is constructed through the π-π interaction between2 D COF material and SWCNT.The unique redox effect of COF-1,lewis acid-base effect of COF-5 and excellent conductivity of SWCNT are used to constract heterojunctions.The enhancing selectivity of specific molecules by heterojunctions lays the foundation for further research on the application of poor conductive 2D MOF/COF materials in chemiresisive gas sensing.