| Due to the rapid development of the chemical industry and economy,a large number of toxic and harmful gases were increasingly emitted,which to a large extent affects air quality,public safety,life and health of living organism.NO2 is a toxic gas that is mainly released from fossil fuels,engine work,and coal combustion.It will cause great harm to human health and the environment,such as human respiratory diseases and heart disease.At the same time,NO2 can also pollute our environment by causing acid rain and photochemical smog.Therefore,the detection of low-concentration NO2 gas(ppb level)in the environment is very necessary.Recently,based on metal oxide semiconductor(SMO)gas sensors have attracted great attention in detecting a large number of toxic gases.However,this type of sensor operates at high temperature,the power consumption of the sensor will increase and there is a risk of explosion,which limits the practical application of the gas sensor.Recently,graphite-like nitrogen carbide(g-C3N4)as a two dimensional material has been widely used as a gas sensing material operating at low temperatures or even at room temperature(RT,25 ?).This outstanding gas sensing performance might be related to its best catalytic performance and excellent 2D material properties.However,the low specific surface area,small conductivity,and poor dispersion of pure g-C3N4 largely limit its practical application as a gas sensing material.Thus,in order to overcome these limitations and improve the performance of high-efficiency gas sensors,it is necessary to modify pure g-C3N4.Therefore,in the present research works,we have been adopted various strategies to overcome these drawbacks,which will not only enhance the surface area but will also enhanced electron diffusion across the heterostructure(improved gas sensing ability at RT).The different methods used were controllable synthesis of three-dimensional g-C3N4 and formation of different heterostructure nanomaterials by the combination with semiconductor oxide such as NiO,ZIF-67 and ZIF-67 derived Co3O4 etc to design g-C3N4-based gas sensing materials.The detail formation procedure was given as follows:(1).In this work,we have synthesized the three-dimensional g-C3N4/NiO(Ni CN)nanocomposite materials by simple hydrothermal method,which were used as a sensitive gas sensing material for the effective detection of NO2 gas at room temperature(RT).Among them,the designed and synthesized NiCN-2 nanocomposite sensor shows a large response value of 25.4 to 50 ppm NO2 gas under RT,fast response and recovery time(0.53 and 25.06 s)performance).In addition,the NiCN-2 gas sensor has undergone continuous gas-sensitive response detection for up to 12 weeks,showing good long-term stability,a low detection limit of 10 ppb and high selectivity.These excellent gas-sensing properties of the NiCN-2 gas sensor can be attributed to the strong synergistic effect of nanocomposite material with a unique three-dimensional hierarchical structure,large specific surface area,large porosity,p-n heterostructure and defect structure,which is beneficial for the adsorption of large number of gas molecules and electron transport across the nanocomposite structure.(2).The Co3O4 derived from ZIF-67 was combined with thin g-C3N4 nanosheets by hydrothermal method and calcination technology to prepare Co3O4/g-C3N4 heterostructure nanocomposite materials for the detection of NO2 gas at RT.The nanostructure composed of Co3O4 nanoparticles has the same polyhedral morphology and large porosity as ZIF-67,and is combined with 2D g-C3N4 nanosheets to form a considerable interface and p-n heterojunction.The obtained Co3O4/g-C3N4 heterostructure(Co CN-1 nanocomposite)has large specific surface area,large porosity and fast electron transport,respectively.When the Co CN-1 nanocomposite gas sensor is exposed to 60 ppm NO2 gas,its response value is 17.83 with short response time of 1.06 s,and fast recovery time of 26.6 s at room temperature.Further,it also shows a high level of NO2 gas selectivity and long-term stability.(3).In this work,Pd-doped ZIF-67 and g-C3N4 nanosheet composite materials were synthesized by a simple hydrothermal method,and a gas sensor was assembled which is used for the detection of NO2 gas at RT.Pd-ZIF-67/g-C3N4(Pd-ZC)nanocomposite material(especially Pd-ZC-2)gas sensor shows excellent gas sensor performance: the response to 100 ppm NO2 at RT is 23.8.The response time and recovery time are only 0.53 and 21.7 s.The gas sensor also showed a low detection limit of 10 ppb and excellent stability(10 weeks).These excellent gas sensing properties of the Pd-ZC-2 sensor can be attributed to porous hierarchical structure and large specific surface area,which can provide more accessible active sites.The interaction of Pd nanoparticles,ZIF-67 and g-C3N4 nanosheets form p-n heterogeneity,which can help in the rapid transmission of electrons across the heterostructure.The synergy of many factors makes the Pd-ZC-2 gas sensor show excellent gas sensing performance at room temperature. |
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