Structure Regulation And Performance Optimization Of Reduced Graphene Oxide Based Gas Sensors

With low power consumption and high accuracy,chemresistive gas sensors play an increasingly important role in different gas sensing applications,including environmental monitoring,industrial and agricultural production,pharmaceuticals,military and public safety.In recent years,a great deal of progress has been made in the research of resistive gas sensors for various low-dimensional nanomaterials.Among these materials,graphene is one of the most studied materials.Graphene,as a typical two-dimensional material,has the advantages of single atomic layer,two-dimensional conjugation structure,superconductivity and high specific surface area.It has been widely used in various frontier research and also caused a great deal of attention in the field of gas sensing.On the one hand,the room temperature conductivity allows graphene to work at room temperature.On the other hand,its high surface area can provides a considerable number of active sites for gas sensing.However,graphene also encounter some problems in the field of gas sensing.Firstly,graphene can adsorb many kinds of gases at the same time,so it will not show a good selectivity for different kinds of gases.Secondly,graphene is also vulnerable.Once damaged,devices based on graphene are difficult to be repaired.It will greatly reduce the overall stability of the device and limit the practical applications of graphene based gas sensors.Finally,it is not easy to obtain sensing devices uniformly coated by few layers of graphene.The unavoidable agglomeration of nanomaterials occurs during evaporating process.The well-known coffee ring effect also makes an uneven distribution of nanomaterials on the surface of electrodes,which will do a serious impact on the performance of sensing devices.In order to solve the above problems,this dissertation presents a three-dimensional core-shell conductive network based on reduced graphene oxide for gas sensing.The structure and composition of the conductive network are adjusted and optimized for different gases.The research results in detail are as follows:1.The three-dimensional core-shell structure of SiO₂@graphene oxide?SiO₂@GO?was designed for the first time,and reduced by simple thermal reduction for gas sensing.Compared with the gas sensing performance of thermally reduced graphene?TRGO?,the response value of SiO₂@TRGO device towards 50 ppm NH₃ in 850 s is as high as12%,about 8 times higher than that of bare TRGO.For the NO₂ case,the response value of SiO₂@TRGO reaches 172.5%towards 50 ppm NO₂ in 250 s,which is times higher than pure TRGO under the same conditions.Due to its three-dimensional structure,the thickness of the gas-sensitive film is less affected.Even if the sample concentration is increased by 8 times,the response decrease rate is less than 30%.And much less than that of the pure TRGO?90%?.In addition,SiO₂@TRGO composite material has good stability.The sensing performance of SiO₂@TRGO shows no significant attenuation after 4 cycles.The impact of humidity to the conductivity of SiO₂@TRGO devices less than 4%.As a result,the SiO₂@TRGO has very high practical value.2.The graphene shell of the nanosphere graphene core-shell structure is optimized,and the 2-aminothiophenol is used to reduce SiO₂@GO at room temperature.The obtained SiO₂@CRGO has a response value of 31.5%towards 50 ppm of NH₃ in 850s,which is higher than pure TRGO,pure CRGO,and SiO₂@TRGO.In addition to the enhancements brought by structural optimization,chemical reduction forms a layer of modifying oligomers on the surface of SiO₂@CRGO that can be doped/de-doped according to the adsorption/desorption of NH3,affecting the electron transfer between the oligomer and graphene,thus affecting the gas sensing performance.3.Tin dioxide?SnO₂?nanospheres were used to construct the three-dimensional core-shell structure of SnO₂@TRGO,which greatly improved the sensing performance of graphene based gas sensor towards NO₂.The gas sensitivities of SnO₂@TRGO towards 10 ppm of NO₂ in 250 s is as high as 172.5%,which in significantly enhanced compared with that of pure TRGO and SiO₂@TRGO.This response value is 16 times that of pure TRGO.This gas-sensing performance improvement can be attributed to two reasons.Firstly,with the structural optimization,the specific surface area of SnO₂@TRGO sensing material has been improved.Secondly,the heterojunctions formed between SnO₂ nanospheres and graphene plays a very important role in regulating the TRGO conductive channel when NO₂ is adsorbed.The theoretical simulation results show that NO₂ has lower adsorption energy on the surface of SnO₂/GR composite than on pure graphene.From this result,the good recoverability of SnO₂@TRGO is well explained.In summary,the proposed three-dimensional core-shell network based on reduced graphene oxide in this paper can not only be easily modified for the graphene shell for enhanced selectivity,but also can improve the overall performance of the sensing device by tuning the core to form heterojunction.This three-dimensional core-shell network can serve as a cost-effective,stable and flexible universal template to enhance the performance of a gas sensor in practical application.