Thermal behaviour of Pd clusters inside carbon nanotubes: insights into the cluster-size, tube-size and metal–tube interaction effects

Abstract

Molecular dynamics simulations were used to investigate the cluster-size, tube-size and metal–tube interaction effects on the melting of Pd clusters encapsulated inside carbon nanotubes (CNTs). The second moment approximation to the tight-binding potential was used to model Pd–Pd metal–metal interaction and the Tersoff potential was used for C–C interactions. Pd–C interaction was modelled by the typical weak van der Waals Lennard-Jones (VDW-LJ) potential to understand the cluster-size and tube-size effects on the thermal behaviour of supported Pd clusters. Linear decrease in cluster melting point with the inverse in cluster diameter is predicted for the CNT containing Pd clusters, well known as Pawlow's law. It is also found that the melting temperature of the supported Pd cluster is much lower than that of free one, and the rearrangement and transformation of the cluster at higher temperatures before melting are responsible for this lowering. In this case, the downward shift is independent of the CNT diameter for the same Pd cluster. In addition, the Pd–C interaction was redefined to assess the metal–tube interaction effect on the thermal evolution of the CNT-containing Pd clusters by fitting to first-principle calculations. Using the fitted strong density functional theory-Morse Pd–C potential, deformation for the CNT and structural transformation from the icosahedral to the stacked for the Pd cluster inside the CNT are found, which is not shown by using the VDW-LJ potential.

Keywords:

• molecular dynamics
• melting
• metal cluster
• carbon nanotubes

Acknowledgements

We would like to thank Marc Hou for helpful comments and suggestions. This work was supported by the Foundation of Excellent Doctoral Dissertation of Beijing City (No. 200918).