The Mechanism Research And Doping Modification Of Noble Metal-ZnO Core-shell Nanoparticles In Gas Sensing

With the increasing demand for health and safty,gas sensitive sensors gradually play an increasingly important role in the detection of environmental air,industrial exhaust gas monitoring,flammable and toxic gas detection,exhalation gas detection and other areas.At present,the gas sensor has some problems,such as unsatisfactory sensitivity and poor selectivity.Therefore,it is particularly important to develop highly sensitive and selective gas sensor.This research is aiming on designing and synthesizing highly sensitive and selective gas sensor materials.Through a series of characterization and performance test,we found out the role of metal-ZnO hetero-interfaces in enhancing sensing performance.Based on the finding,we synthesized a series of highly sensitive and selective Metal@ZnO core-shell nanoparticles.The detail content and results were listed below.(1)The adjustment of Au-ZnO heterogenous interface and its role in gas sensing.Au@ZnO nanoparticles were firstly synthesized through a facile hydrothermal reaction and subsequently sintering treatment.During the hydrothermal reaction,CTAB was added to the mix solution,in order to control the growth rate of ZnO.The gas sensing performances of the as-synthesized core-shell nanoparticles were evaluated comparatively in detecting low-concentration benzene(less than 5 ppm).Due to the unique structure,the effects of Au-ZnO hetero-interfaces were analyzed and discussed.The work function differences between Au and ZnO which adjusted the electronic state of the ZnO shell is the main reason of enhanced sensing performance.After doping with Pd and Pt,the Au-Pd@ZnO and Au-Pt@ZnO nanoparticles showed further improvement of sensing performances.(2)The role of metal-ZnO heterogenous interface in gas sensing.The core-shell Metal@ZnO nanoparticles(Metal=Au,Pd,Pt)were synthesized via hydrothermal reaction and thermal decomposing.The formation,morphological,and compositional properties of the typical core@shell structure were confirmed by using various techniques.The obtained core-shell Metal@ZnO nanoparticles(M=Au,Pd,Pt)were applied to sensor devices and their gas sensing properties towards ppb-ppm level benzene in comparison to the pure singular ZnO nanoparticles were examined.The core-shell Metal@ZnO based sensor performed better sensing performance than pure ZnO based sensor towards low concentration benzene.In addition,Pt@ZnO based sensors displayed significantly high sensitivity,ultralow detection limit(10 ppb),high selectivity and long-term stability.The sensitivity sequence of various core-shell nanoparticles and pure ZnO towards benzene is Pt@ZnO>>Pd@ZnO>Au@ZnO>ZnO,and is well consistent with the work function differences(Pt@ZnO>Pd@ZnO?Au@ZnO>ZnO).It is implied that the bigger difference of work function between metal and ZnO exists,the higher sensitive performance performs.(3)Synergistic effect between doping and metal-ZnO heterogenous interface.Doping seems to be an efficient way to further improve the sensing performance of Metal@ZnO core-shell nanoparticles.Appropriate element selection and doping amount can keep original core-shell structure unchanged and change the morphology and performance of ZnO shell.Core-shell 3 mol%Al doped Pt@ZnO,4 mol%Al doped Pt@ZnO and 5 mol%Al doped Pt@ZnO nanoparticles were synthesized via hydrothermal reaction and thermal decomposing.Various techniques were conducted to characterize the compositional properties and analyze the sensing properties of all nanoparticles.Among those nanoparticles,4 mol%A1 doped Pt@ZnO based sensor presents excellent acetone sensitive.The response of the sensor to 1 and 100 ppm acetone is as high as 9.26 and 128,and the detect limit towards acetone is as low as 20 ppb.Moreover,the 4%-Al-Pt@ZnO based sensor presents excellent selectivity and stability toward acetone at low and high concentration range.A1 doping induces structure adjustment of ZnO shells and ZnO shells could provide more active sites for gas molecules.The synergistic effect between doping and metal-ZnO heterogenous interface enhanced sensing property of metal@ZnO core-shell nanoparticles.The doping of A1 was proved as a feasible method for fabricating acetone sensor,with high sensitivity and selectivity.(4)1 mol%Ga doped Pt@ZnO based sensor presents excellent acetone sensitive and interesting reversible switching from P-to N-type NO2 sensing.The response of the 1 mol%Ga doped Pt@ZnO based sensor to 1 ppm and 50 ppm acetone is as high as 13.6 and 296,and the detecting concentration limit to acetone and NO2 is as low as 10 ppb and 20 ppb.The analogical binary p-n transition phase diagram was established to understand the NO2-sensing reversible switching as a function of operating temperature(T)and NO2 gas concentration(C).The role of Ga doping in enhancing the sensing property and the mechanism of NO2 p-n sensing transition is elaborated.With the help of classical Lennard-Jones(LJ)potential function,there is a possible T-C transition mechanism which was established to understand the NO2-sensing reversible switching as a function of operating temperature(T)and NO2 gas concentration(C).