Study Of The Interaction Potential Between Metal Atoms And Its Application In Nanoparticles

Metal nanoparticles are widely used in chemical industry,energy,environmental protection,medicine and other fields.The structural characteristics of nanoparticles,such as shape,size,thermal vacancy and element segregation,have important influences on their properties.Because it is difficult to obtain detailed information about the structure of nanoparticles from experiments,it is also a feasible method to study the microstructure and properties of nanoparticles through computational simulation.It mainly includes:first principles,molecular dynamics and Monte Carlo.The results of first principles are more accurate,but it consumes too much computing resources,and is usually used to deal with systems with less than 100 atoms.For nanoparticles with more than 100 atoms,molecular dynamics and Monte Carlo simulation are commonly used.The validity of molecular dynamics and Monte Carlo simulation results depends on the accuracy of the interatomic interaction potential,but there is still a lack of high-precision interatomic interaction potential(especially alloy potential).In this paper,high precision interatomic interaction potentials of metals and their alloys were constructed by fitting the experimental data or first principles calculation values of material properties.For metals with positive Cauchy discrepancy,the potential function is in the form of EAM potential which is widely used.For metals with negative Cauchy discrepancy,the potential function is in the form of ADP potential(an improved model of EAM potential)because the EAM potential can not describe their elastic constants correctly.Then,the obtained potentials were used to study the properties of metal nanoparticles,including the effects of size and shape on the stability,the distribution of vacancy concentration and the segregation of elements.Firstly,a new EAM potential was obtained by fitting the experimental data of bulk Pt,and the effectiveness of this potential in Pt bulk and Pt nanoparticles was verified.Then,the effects of size and shape on the structural stability and thermal stability of Pt nanoparticles were systematically studied by using the EAM potential.The cohesive energy,surface energy,structure evolution during the heating process and melting point of Pt nanoparticles with different shapes and sizes were calculated.The results show that the cohesive energy and melting point of Pt nanoparticle can be expressed as a function of the particle size and average coordination number.It should be noted that the shape of Pt nanoparticles changes during the melting process,resulting in the change of average coordination number.To describe the effect of the shape change on the melting point,a deformation factor was introduced into the melting point function.Secondly,the fitting method of EAM potential mentioned above was improved,and the parameters to be fitted are reduced from 23 to 13.A new EAM potential was obtained by fitting the experimental data of bulk Pt,and the effectiveness of this potential was verified.Then,the distribution and size effect of vacancy concentration(VC)of films and nanoparticles were studied by this potential.It was found that the VC in the subsurface of the film is smaller than that of the bulk,and the VC in the surface and the body and the average VC in the entire film are larger than the VC in the bulk.The VC in each part of the film increases with the decrease of thickness except for the surface.The VCs in the body and the subsurface of the nanoparticle are lower than that of the bulk,and the VC in the surface and the average VC in the whole nanoparticle are higher than that of the bulk.The VCs in the body and the subsurface of the nanoparticle decrease as the particle size decreases.The average VC in the whole nanoparticle increases with the decrease of particle size.Finally,the pure element potential parameters of Au and Rh were obtained by fitting the experimental data of bulk Au and bulk Rh,and then the cross potential parameters were obtained by fitting the first principles data of formation energy of Au-Rh compounds.Then,the ADP potential for the Au-Rh system was obtained and the validity of this potential was tested.Then,the element segregation of Au-Rh nanoparticles at 300 K was studied by Monte Carlo simulation using this potential.The results show that the preferential segregation behavior of atoms in Au-Rh nanoparticles is not affected by the particle size.On the surface,due to the large surface energy difference between Au and Rh,the element segregation is mainly induced by the surface energy,and Au atoms preferentially occupy the lower coordination sites.In the body,Au atoms tend to occupy the sites with small local pressure to release strain energy,while the whole system tends to reduce the interface area to decrease the interface energy.The element segregation is primarily induced by the strain energy,and the interface energy also participates in the competition.The final structure is the result of the competition between strain energy and interface energy.