Preparation,Structure Manipulation Of Graphene Nanomesh And Its Gas-Sensing Properties For NO₂ At Room Temperature

At present,more and more attentions have been paied to air-quality problem,due to the increasingly serious indoor and outdoor air pollution,consequently driving the fast-developments of consumer gas sensor market,such as "smart city","smart home",smart phone and wearable devices,etc.Room temperature?RT?resistance-type semiconductor gas sensor has attracted extensive attentions as a promising commercial sensor,due to the merits of simple fabrication,easy of process integration,low-cost,low-dosage,wide range of application,energy efficiency and safety.Graphene-based materials are a important branch of RT gas-sensing materials,because of their large specific surface area,highly exposed surface atom ratio,and high room-temperature conductivity.It has been proven that pure graphene has amazing detection ability for single NO₂ molecule in the high degree of vacuum,however,there are many common problems of graphene-based gas-sensing materials for RT gas sensor,such as low responsivity,long response-recovery time,poor stability,and so on.Therefore,in this paper,graphene nanomesh?GNM?with abundant surface active sites were prepared,and a series of methods were adopted to optimize the gas-sensing behaviors of GNM in NO₂ detection by changing its surface structure,morphological structure,and decoration with noble metal.Ultimately,we obtained an outstanding three-dimensional?3D?GNM-based RT gas-sensing material,which had excellent comprehensive performance of high structure stability,high responsivity,sensitivity and selectivity,as well as rapid response-recovery rate.Substantial work has been devoted to understanding the inherent relationship between gas-sensing properties and the types of surface active sites,morphological structure,decoration with Pt quantum dots.And the gas-sensing mechanism of Pt QDs-3D HrGNM was elaborated specifically.We do hope this work can provide a reference for the subsequent researches in the field of RT gas-sensing materials,especially for carbonaceous RT gas-sensing materials.Firstly,GNMs were prepared through an efficient modified photo-Fenton method.The nanopore size and surface structure can be regulated conveniently through controlling reaction time.The as-prepared GNMs were rich in edge defects and oxygen functional groups,all of which were effective adsorption and reactive sites for gas molecules.The dual properties of electrophilicity and strong oxidability of hydroxyl radical?HO·?,which was the main product of Fenton reaction,was responsible for chemical etching of graphene oxide?GO?.There were there reactions in the reactive process,that is,the electrophilic addition of HO· on the sp2 domain of GO,the oxidizing reaction of HO· on the sp3 domain of GO,and the decarboxylic reaction of carboxyl.They proceeded simultaneously,competed with each other,all contributing to controllable etching of GO and surface structure modulation of GNM,and consequently modulation of band gap and work function of GNM.Subsequently,the mild and controllable hydrothermal reduction was applied to GNM,using pure water as reductant,to realize the dual regulation of surface structure and RT conductivity of reduced GNM?rGNM?.According to binding energy and charge transfer direction,the active sites of rGNM could be divided into three categories:favourable factor?C-H?sp3 C-C and C-O?,neutral factor?sp2 C=C?and negative factor(C=O,O=C-O.Along with gradual restoration of sp2 domains of rGNM,the RT conductivity increased first slowly and then rapidly.And the optimum responsivity of rGNM could be acquired only when surface structure and RT conductivity reached the best balance.The edge defects of rGNM has distinguished contribution to gas-sensing performance.Both responsivity and sensitvity of rGNM were higher than that of rGO,which had a similar oxygen content with rGNM.When the NO₂ concentration low to 0.5 ppm,rGO had no any responsivity,while the response value of rGNM still reached to 28%.Then,micro-sized 3D hollow rGNM?3D HrGNM?was prepared via a templated method.The three-dimensional transformation of 2D nansheets,not only hindered the recombination of nanosheets in the reduction procss,but also reduced the loss of specific surface area.In addition,benefit from porous 3D morphological structure,ample channels were constructed in the material layer of sensor device,benefiting for the diffusion of gas molecules to the internal surface of material,thus greatly improving the mass transfer rate and availability of material surface.Compared to HrGNM,the response values of 3D HrGNM were increased significantly,especially for higher NO₂ concentration.There was a linear relationship between response values of 3D HrGNM and NO₂ concentration.The response-recovery rates of 3D HrGNM were also improved at certain degree.Finally,by virtue of decoration of Pt quantum dots,the response value of material was significantly increased once again.For 10 ppm NO₂,the response value of 0.2%Pt QDs-3D HrGNM reached to 8.3,about 3.5 times that of 3D HrGNM.Through electrical performance test,it has been proven that the Pt quantum dots had n-type doping effect on 3D HrGNM,and improved responsivity through three aspects:increasing specific surface area and active sites;lowering the intial conductivity?G0?of material system;served as electron warehouses to raise carrier transfer number and the corressponding conductivity?Gg?,through providing large amounts of electrons to NO₂ molecules for chemical adsorption.And the recovery performance of Pt QDs-3D HrGNM was also enhanced,due to the smaller binding energy between NO₂ and new active sites.Ultraviolet?UV?light was loaded in the recovery process to further enhance recovery performance,and 0.2%Pt QDs-3D HrGNM could recover more than 90%in about 30 min.Through photoelectrical performance test in specific atomsphere,it has been known that there were two photon excitation process in the material system,that is,photo-induced electron hole pair,which leading to conductivity increasing,and photo-induced gas molecule desorption,which leading to conductivity decreasing.With the increasing of Pt content,the proportion of photo-induced gas molecule desorption process increased gradually.As a result,the recovery performance of Pt QDs-3D HrGNM was improved gradually.