(136f) Molecular Dynamics Simulation of Melting and Structural Evolution of Pd-Cu, Pd-Pt and Pd-Rh Bimetallic Nanowires

Bimetallic nanoclusters and nanowires find applications in catalysis and sensing. In sensing applications, the increased surface area and surface segregation of these nanoclusters/nanowires results in superior selectivity, sensitivity and speed of response. Pd and Pd-alloy nanowires find use in hydrogen sensing. Understanding the thermal characteristics of these bimetallics is critical to the design and synthesis of these nanowires. Complex phenomena such as surface segregation and micromixing make them difficult for theoretical studies, and have resulted in much less attention being paid to bimetallic nanosystems.

The thermal characteristics of different bimetallic transition metal nanowires (Pd-Cu, Pd-Pt and Pd-Rh) with diameters ranging from 2.3 nm to 6 nm have been studied using molecular dynamics utilizing the quantum Sutton-Chen potential function. Monte-Carlo simulations employing bond order simulation model were used to generate the initial configurations. These initial configurations consisted of surface segregated structures. The atoms with low surface energy segregated to the lower coordination sites, resulting in contrasting segregation profiles of Pd for the three nanoclusters. The melting temperatures were found by studying the variations in thermodynamic properties such as potential energy and heat capacity and structural properties such as bond orientational order parameter and Wigner values. Structural transformations associated with melting phenomenon can also be identified using bond orientational order parameter values. Literature (Wang et al., 2002) suggests onset of melting in nanowires differs from that in nanoclusters. Shell-based diffusion coefficients were used to gain insights into this aspect. Components of velocity autocorrelation function and deformation parameters were used to characterize the movement of metal atoms and the associated shape changes. The effect of composition of the bimetallic on the thermal characteristics of nanowires was also investigated. Comparisons with nanocluster melting were made to identify the underlying differences in the melting behavior. Different starting configurations (helical multiwalled, hcp) were employed to identify precursors to nanowire (hcp in case of nanoclusters (Sankaranarayanan et al., 2005)) melting. Repeated heating-cooling cycles with a glassy structure as starting configuration for subsequent runs can give insights into the hysteresis effect as well as the stability of the resulting nanowires. Finally, a comparison to available melting theories will be carried to predict the size and composition dependence on nanowire melting.

References:

Sankaranarayanan, S. K.R.S., Bhethanabotla. V. R., Joseph, B. Molecular dynamics simulation study of the melting of Pd-Pt nanoclusters. Physical Review B: Condensed Matter and Materials Physics (2005), 71 (19).

Wang, J.; Chen, X.; Wang, G.; Wang, B.; Lu, W.; Zhao, J. Melting behavior in ultrathin metallic nanowires. Physical Review B: Condensed Matter and Materials Physics (2002), 66(8).