Theoretical study of the thermal behavior of free and alumina-supported Fe-C nanoparticles

Abstract

The thermal behavior of free and alumina-supported iron-carbon nanoparticles is investigated via molecular-dynamics simulations, in which the effect of the substrate is treated with a simple Morse potential fitted to ab initio data. We observe that the presence of the substrate raises the melting temperature of medium and large ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ nanoparticles ($x=0--0.16$, $N=80--1000$, nonmagic numbers) by $40--60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; it also plays an important role in defining the ground state of smaller Fe nanoparticles $(N=50--80)$. The main focus of our study is the investigation of Fe-C phase diagrams as a function of the nanoparticle size. We find that as the cluster size decreases in the $1.1--1.6\text{\ensuremath{-}}\mathrm{nm}$-diameter range, the eutectic point shifts significantly not only toward lower temperatures, as expected from the Gibbs-Thomson law, but also toward lower concentrations of C. The strong dependence of the maximum C solubility on the Fe-C cluster size may have important implications for the catalytic growth of carbon nanotubes by chemical-vapor deposition.