Structural And Reaction Properties Of Cu_nNi_55-n（N=0～55） Nanoparticles

This article mainly studies the structural,magnetic and electronic properties of bimetallic Cu_nNi_55-n（n=0-55）nanoparticles（NPs）,which combine artificial bee algorithm with DFT approaches.Moreover,we also have searched the structures of different ratios and different sizes of CuNi nanoparticles by artificial bee algorithm.We analyses the structure changes of CuNi nanoparticles with the ratio and size changing for CuNi nanoparticles.The conclusions are as follow:1.We discover that all the Cu_nNi_55n（n=0-55）nanoparticles structures which get from ABCluster are all regular icosahedron。It means that the icosahedron Cu_nNi_55-n（n=0-55）structure will be more stable than any other Cu_nNi_55-n（n=0-55）structures.And we can see that the Eexc wil decrease at first,and then increase with the number of Cu atom increasing.Beside,we find that the Eexc of all the Cu_nNi_55-n nanoparticles is low to zero,which means that the pure Cu55 nanoparticle or Ni55 nanoparticle will be more stable if it is doped with Ni atom or Cu atom.2.With analyzing the magnetic property and density of state,we find that when the number of Cu atom increases,the average magnetic moment of Cu atom will increase firstly,and then decrease when the number of Cu atom increases continually.And when the number of Cu atom is equal to 30,the average magnetic moment of Cu atom is the larger than any Other Cu_nNi_55-n structures.Moreover,the overlap of Cu₃d orbital and Ni₃d orbital decide the density of state;we can see the density of state for the whole Cu_nNi_55-n nanoparticles has the same changing trend with the average magnetic moment of Cu atom.When the number of Cu atom is lower than 30,the overlap of Cu₃d orbital and Ni₃d orbital increase with the increase of Cu atom.And the overlap of Cu₃d orbital and Ni₃d orbital will decrease when the number of Cu atoms increase,while the number of Cu atom is larger than 30.Moreover,the total magnetic moment of pure Cu55 is zero;because the density of state for Cu55 is symmetric,pervious study found that while the density of state is symmetric,the metallic atoms would have no magnetism,so the Cu atom is nonmagnetic atom.Compared with the Cu atoms,the density of state for Ni55 nanoparticle is asymmetric,that is the reason why Ni atom is magnetic atom.We also find that the total magnetic moments of Cu_nNi_55-n nanoparticles and the average Ni atom will always decrease when the number of Cu atoms increases.The reason is that the increase of nonmagnetic Cu atoms has a negative influence on the total magnetic moments for Cu_nNi_55-n nanoparticles and average magnetic moments for Ni atoms.3.When the number of Cu atoms increases,we can see that more Bader charge will transfer form Ni atoms to Cu atoms.But as it is known to us,the average Bader charge is influenced by the losing charge and the number of atoms who loses charge.So we find that the average Bader charge of Ni atom will decrease at first and the increases while the number of Cu atom increases.While the number of Cu atom is equal to 42,the average Bader charge of Ni atoms is the lowest of all Cu_nNi_55-n structures.Because the Ni atom is the atom that loses charge,so when the average Bader charge of Ni atom decrease,that means the Ni atom loses more charge.It is noting that when the number of Cu atom is equal to 42,the Cu₄₂Ni₁₃ nanoparticle will form a perfect core-shell structure,and the core-shell structure has the lowest Eexc of all the Cu_nNi_55-n nanoparticles structures and the lowest average Bader charge of Ni atom in all Cu_nNi_55-n nanoparticles structures.We can see a great relationship between the Eexc and the average Bader charge of Ni atom.4.We also can see that there are two stable absorption sites on the Cu₄₂Ni₁₃ nanoparticle for H₂O molecule,and the two absorption sites are located on the top site of Cu₄₂Ni₁₃ nanoparticle.And as we know that the Cu₄₂Ni₁₃ nanoparticle structure is a symmetric icosahedron structure,so there are six atoms on the surface of every face for Cu₄₂Ni₁₃ nanoparticle,and the every face made up by six Cu atoms has six different six sites,two different top site,two different bridge sites,two different hollow sites.By calculating the absorption energy of H₂O,OH and H,we find the stable absorption sites on the Cu₄₂Ni₁₃ nanoparticle for H₂O are the top 1 site and top 2 site,the stable absorption sites on the Cu₄₂Ni₁₃ nanoparticle for OH are the bridge 1 site and top 1 site,the most stable absorption site the Cu₄₂Ni₁₃ nanoparticle for H is the top 2 site,the absorption energy is-3.28 eV,the second stable absorption site is the bridge 1 site,the absorption energy is-3.23 eV.5.Comparing the energy barrier and density of state when H₂O molecule absorbs and reacts on the top 1 site of Cu₄₂Ni₁₃ nanoparticle with the energy barrier and density of state when H₂O molecule absorbs and reacts on the top 2 site of Cu₄₂Ni₁₃ nanoparticle,we find that the H₂O can only absorb on the top 1 site and top 2 site,so we only consider the two sites on which H₂O molecule absorbs and reacts.We find there are two reaction steps for H₂O molecule decomposing on the Cu_nNi_55-n nanoparticle.The first reaction step is that the H₂O molecule decomposes into H and OH,the second step reaction step is that the OH decomposes into H and O.We find that wherever the H₂O molecule reacts on the surface site of Cu2Ni13 nanoparticle,the energy barrier of second reaction step is higher than the energy barrier of first reaction step.Moreover,we find the energy barrier and enthalpy are different when the H₂O molecule reacts on different site of Cu₄₂Ni₁₃ nanoparticle.This means when the H₂O molecule absorbs on the different site of Cu₄₂Ni₁₃ nanoparticle,it will form isomer.And our calculation result indicates that the energy barrier and enthalpy will be lower when the H₂O molecule absorb and decomposes on the top 2 site than which on the top 1 site,which means the structure that H₂O absorbs and reacts on the top 2 site of Cu₄₂Ni₁₃ is the more stable structure.