Investigation of the Chemical Ordering and Structural Properties of the Trimetallic (PtNi)@Ag Nanoalloys

Abstract

The effect of systematic Ni atom addition on the chemical ordering properties of the nanoalloys has been studied for PtNi@Ag core–shell systems in which Ag atoms full occupy the shell of the icosahedral structure. We concentrate only on the chemical ordering optimization within a given geometric structure and we have core–shell system compositions (Pt13-nNinAg42 and Pt55-nNinAg92 for PtNi@Ag) with size of 55 and 147. Local relaxations were performed by using Monte Carlo Basin-Hopping algorithm within Gupta potential. Results show that it is energetically favourable to substitute Ni atoms for Pt13-nNinAg42 and Pt55-nNinAg92 systems. Excess energy variations were calculated to compare the relative stability. The atomic mixing degrees of Pt, Ni and Ag atoms were discussed by using order parameter (R) indicator. It is observed that the Ni atoms prefer to locate with closer to Pt atoms than Ag atoms due to tendency of occupying the core regions of the icosahedral structures. The local pressures of the atoms were also examined to clarify the composition effect on atomic stress and pressure. It is also observed that in trimetallic PtNi@Ag core–shell nanoalloys, the strain on the core atoms can be released by substituting Pt atoms with Ni atoms.