Thermodynamic study on the melting of nanometer-sized gold particles on graphite substrate

Surface tension plays an important role in lowering the melting temperature of nanometer-sized particles, but whether the surface tension determined in macro scale is valid for the nanometer-sized particles is unclear. Moreover, the melting of the nanometer-sized particles formed on solid substrates can be affected by interfacial tension, but no research has been reported on the effect of substrates on the melting temperature. Therefore, in order to predict the melting temperature of nanometer-sized metallic particles on solid substrates, thermodynamic parameters such as surface tension and interfacial tension should be properly estimated. In the present work, thermodynamic assesment is given on the melting temperature of gold particles in nanometer-size placed on a graphite substrate. Surface tension of liquid gold and the contact angle between liquid gold and the graphite substrate are measured by the constrained drop method and the sessile drop method in macro scale, respectively. Then, the effect of the graphite substrate on the melting temperature of nanometer-sized gold particles are examined by thermodynamic calculations minimizing the total Gibbs free energy, the sum of bulk, surface and interface energies. It is found that the graphite substrate has negligible effect on the melting temperature of nanometer-sized gold particles. Thermodynamic assessments provide that the surface tension of solid gold is 1.339 N/m at 1373 K and that the decrease in the surface tension of liquid gold with size will be considerable for the particles smaller than ~5 nm.