Melting Behavior Of Mixed Metal Clusters And Their Catalytic Mechanisms For Carb On Materials

With the rapid development of computer science and technology,the software and hardware of computer are constantly updated.It makes the computational simulation become more and more important in the fields of physics,chemistry,biology and so on.Due to high accuracy and moderate computational time,the first principle method based on density functional theory and molecular dynamics method have become one of the most commonly used research methods in current computational physics,materials science and other fields.In recent years,due to the excellent properties of carbon nanotubes?CNT?,especially in the field of microelectronics,the potential application value is difficult to measure.The experimental investigations have revealed that some alloy?multicomponent?transition metal particles?i.e.,Co-Mo,Fe-Mo and Fe-Co?often lead to improved CNT growth as compared to the pure transition metal catalysts.For example,as comparing with the pure Co or Mo catalyst,the mixed Co-Mo nanoparticles show significant advancement towards the controlled production of single-walled cabon nanotubes?SWCNTs?,and the capability of the Co-Mo catalysts depends on the Co-Mo component ratio.Therefore,it is a very interesting thing to study the relationship between the size,composition and catalytic properties of the mixed clusters.This paper mainly consists of two parts: Firstly,the molecular dynamics method was used to study the Ag-Ni bimetallic clusters with different sizes?55,147,and 309?and components;Secondly,based on the VASP software package and the first principle calculation,with using the plane wave pseudopotential basis sets,the geometries and electronic structures of Co-Mo clusters are systematically investigated by the GGA-PW91 exchange-correlation functional.The main contents and conclusions of this paper are as follows:?1?The annealing processes of the Ag-Ni bimetallic clusters with different sizes and compositions are systematically investigated by using the constant temperature molecular dynamics simulation method.The segregation behaviours of the annealing clusters and their correlation with the different sizes and different components of the annealing clusters were discussed.Our results show that for the annealing structures,when the number of Ag atoms is less than that of Ni atoms,all Ag atoms distribute on the surface of the clusters;With the increase of the number of Ag atoms,and thenumber of atoms of Ag and Ni are close to each other,Ag atoms are still distributed on the surface of mixed clusters;When the number of Ag atoms is more than that of Ni atoms,the surfaces of clusters are almost entirely occupied by Ag atoms.?2?The geometries,energetics and electronic structures of Co₁3,Mo₁3,Co₁2 Mo and Mo₁2 Co clusters are systematically investigated by using the first principles method combined with the genetic algorithm.A new candidate is found for the ground-state geometry of Mo₁3.Compared to the ground-state geometry of Co₁3 possessing high stability,there are many isomers energetically closer to the groundstate of Mo₁3.The relatively high stability of the pure Co₁3 can be reduced by doping,but the isomerization near the ground-state of the pure Mo₁3 can be suppressed by doping.With respect to that of the central doping Mo atom in the high symmetric close-packed geometry,there are significantly more d electrons near the Fermi energy of the Mo atom in the ground-state Co₁2 Mo clusters and as a result the d-d hybridization between the doped atom and the matrix is significantly enhanced.On the contrary,compared to the excited structure with a relatively higher energy,the d-d hybridization near the Fermi energy between the doped atom and the matrix of the ground-state Mo₁2 Co clusters is significantly decreased,which implies that the d-d hybridization near the Fermi energy between the doped atom and the matrix is strongly influenced by the component ratio and geometry of the clusters,and the competition between them can have a crucial impact on the catalytic property of the mixed clusters.An explanation is proposed to the excellent catalytic abilities of the Co-Mo alloy nanoclusters with approaching component ratios in the catalytic growth of carbon nanotubes.