Thermodynamic And Structural Properties Of Bimetal Nano-Alloys

Bimetallic nanoclusters have received extensive attention due to their excellent stability,selectivity,and magnetic and catalytic properties,which are tunable by changing the nanoparticle morphology,surface atom distribution,and particle size.However,the thermodynamic properties and structural characteristics of nanoparticles need to be further investigated.Therefore,to explore the structural characteristics,structural stability,and thermodynamic properties of nanoclusters,taking several metal nanoclusters as the research object,the structural stability,growth law,structural phase transition,surface segregation,and melting characteristics of nanoclusters are systematically studied from the atomic scale by molecular dynamics(MD)simulation with the embedded atom(EAM)potential.Firstly,some basic concepts and general properties of nanoclusters are briefly introduced.The melting behavior of Fe and Al nanoparticles,in which the icosahedral(ICO),decahedral(DEC),cuboctahedral(CUB)structure and the number of the atom are“magic number”(N=147,309 and 561),was studied by the MD method with EAM potential.Some technology analysis method,such as potential energy and common neighbor analysis is used to study the nanoparticles The simulation results show that Fe and Al nanoparticles of the ICO structure with the atomic number of 147,309,and 561,retain their original structure before melting,while the Al nanoparticles of DEC and CUB structure with the atomic number of 147,transform to the ICO structure before melting.Therefore,for Al nanoparticles with a small size(N?147),the ICO structure has higher thermal stability than the CUB and DEC structures.Secondly,the sintering reaction between metal nanoparticles determines the resultant structure of the active sites where reactions(e.g.catalysis)actually take place,i.e.facets,edges,vertices,or protrusions.The sintering–alloying processes of Ni,Fe,and Mg with Al nanoparticles were studied by MD simulation with the EAM potential.Potential energy,mean heterogeneous coordination number NA~B,and surface atomic number Nsurf-A were used to monitor the sintering-reaction processes.The effects of surface segregation,the heat of formation,and melting point on the sintering–alloying processes were discussed.Results revealed that sintering proceeded in two stages.First,atoms with low surface energy diffused onto the surface of atoms with high surface energy;second,metal atoms diffused with one another with increased system temperature to a threshold value.Under the same initial conditions,the sintering reaction rate of the three systems increased in the order Mg Al<Fe Al<NiAl.Depending on the initial reaction temperature,the final core-shell(Fe Al and Mg Al)and alloyed(NiAl and Fe Al)nano-configurations can be observed.Thirdly,NiAl nanoalloys possess high-energy density and excellent high-temperature mechanical properties and are considered an important material.However,the differences in the diffusion behavior of Al adsorbed atoms on different Ni substrate surfaces and the effects of different diffusion mechanisms on the deposition growth of Al atoms on the Ni substrate surface are highly desired to be clarified.Therefore,in the present work,the diffusion behavior of single Al adsorbed atoms and nanoparticle cluster growth on the Ni substrate surface of DEC,CUB,and ICO structures are systematically studied by MD and the nudged elastic band(NEB)method.The diffusions of Al adsorption atoms on the surfaces of three Ni substrates are realized by two mechanisms,namely exchanging or hoping,and the lowest Ehrlich-Schwoebel(ES)barrier is 0.38 e V for exchange CUB(111)?(100),0.52 e V for exchange DEC(111)?(100),and 0.52 e V for hoping ICO(111)?(111).The exchanging mechanism supports Al adatoms diffusing from(111)to(100)facet on the three Ni substrates,while the diffusion between two adjacent(111)facets is mainly driven by the hoping mechanism.The deposited Al atoms first tend to diffuse near the edges of the steps and the vertices.The deposited Al atoms begin to aggregate into islands with the increase of their number.For Al atoms on the Ni cluster,a good Ni-core/Al-shell structure can be obtained by depositing Al atoms on the surface of Ni substrate at lower temperatures.For the ICO substrate,the corresponding defect number of core-shell clusters is smaller than for the CUB and the DEC substrate.The surface of Ni-Al bimetal is gradually alloyed with the increase of growth temperature.Fourth,bimetallic NiCu nanoparticles are used as catalysts in critical chemical reactions such as methane decomposition,ethanol steam reforming,methanol oxidation,and water-gas shift reaction.The catalytic performance of NiCu bimetallic nanoalloys is strongly dependent on the concentration,distribution,and structure of the metal atoms.In this article,the surface segregation and structural features of NiCu bimetallic nanoparticles and the deposition growth and surface diffusion of Cu adsorbed atoms on the Ni surface substrates were studied using MD and MC method combined with EAM potential.The results show that the Cu atom has a strong tendency for surface segregation.With the increase of concentration of Cu atoms,Cu atoms preferred to occupy the vertex,edge,(100),and(111)facet of nanoparticles,and finally formed perfect Ni-core/Cu-shell nanoparticles.When T=400 K,the Ni-core/Cu-shell structure formed is the best.The diffusion energy barrier of Cu adsorbed atoms on the Ni substrate surface was calculated by using the nudged elastic band method.The results show that Cu adsorbed atoms need to overcome a large ES barrier for both exchange and diffusion,making it difficult to diffuse between the surfaces of Ni substrates in the temperature range T=200?800 K.In contrast to Ni substrates,Ni atoms deposited on Cu substrates can easily migrate from the(111)plane to the(100)plane,and at the current simulated temperature,Ni adsorbed atoms are unable to migrate on the(100)surface,resulting in a growth configuration toward an octahedral shape with its eight apex angles almost occupied by Ni atoms.