Nanoparticle-host interactions in natural systems

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Nanoparticles in isolation from a host phase

In order to constrain the effects of nanoparticle-host interactions on the stability of natural nanoparticles, we must first study the behaviour of such nanoparticles in isolation from their host phases. It is well known that the physical properties of materials change significantly when the dimensions are reduced to the nanoscale (Halperin, 1986). One particular phenomenon of interest to mineralogy and geochemistry is the dependence of the melting point on particle size, i.e., nanoscale particles have lower melting points than their bulk material.

The phenomenon of melting point depression in nanoscale metal particles has been investigated in detail since the 1960’s, when it was found that small evaporated particles of tin have a lower melting point than bulk tin (Wronski, 1967). The strong dependence of the melting point on particle size was confirmed by Buffat and Borel (1976) by investigating the thermal stability of metallic gold nanoparticles. In this seminal paper, X-ray diffraction techniques were used to monitor the solid-to-liquid phase transition of Au particles of variable size (2-25 nm) during heating in a furnace, thus obtaining the melting point of Au particles as a function of particle size. As seen in Fig. , the decrease in particle size has a dramatic impact on the melting temperature (Tm) when compared to bulk Au (=1064 °C). Thus, an “isolated” Au particle ~4 nm in diameter (i.e. in experimental vacuum, 210-6 Torr) can melt at temperatures as low as 430 °C. In present days, the size-dependent melting point depression of nanomaterials is determined using nanocalorimetric measurements, a technique that allows a precise measurement of the melting temperature and latent heat of fusion (Zhang et al., 2000).

The thermal stability of nanoparticles as “isolated” entities can be explained using a classic thermodynamic approach, in terms of surface/volume ratio, because a decrease in particle size can promote phase instability due to the increase in surface energy (Navrotsky, 2001). Several phenomenological models have been proposed to account for this fact, and almost all of them consider the gold nanoparticle as composed of “bulk” and “surface” atoms. The decrease of the Tm is due to the fact that at the bulk melting temperature (), the surface free energy of the liquid is lower than that of the crystal (Pawlow, 1909). Within this classical approach, the melting-temperature depression T as a function of particle size r can be written as (Zhang et al., 2000, and references therein):

where,, and are the bulk latent heat of fusion, the solid phase density, and a parameter related to the interfacial tension between the solid phase and its environment, respectively (Zhang et al., 2000).

The previous approach is not accurate for explaining melting behaviour of particles smaller than 2-4 nm, or clusters with less than 1000 atoms. In these very small particles, surfaces are far from being ideal owing to the limited size of crystalline faces and to the large number of atoms on edges and vertices. Consequently, surface reconstruction and relaxation effects question the assumption of a size-independent surface energy for explaining particle melting, and a more sophisticated atomistic simulation approach is necessary. During the last decade, several authors have studied the melting of nanoparticles and clusters (<1000 atoms) in detail from a computational perspective (Ercolessi et al., 1991; Lewis et al., 1997; Arcidiacono et al., 2004; Koga et al., 2004; Sun et al., 2004; Wang et al., 2004). Molecular dynamics simulations of melting of small gold particles by Ercolessi et al. (1991) reveal the formation of a liquid skin precursor for clusters bigger than 350 atoms. Their results show that in the size range of 100 to 1000 atoms, simple models for metal cluster melting, constructed in the macroscopic limit, are not suitable since surface effects play a crucial role. In particular, the melting mechanism of small gold particles reveals precursor effects such as an increased diffusivity of atoms located at edges between crystal facets and on facets.
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