Shape Transformation and Surface Melting of Cubic and Tetrahedral Platinum Nanocrystals

We report transmission electron microscopic studies of in-situ temperature-induced shape transformation and melting behavior of polymer-capped cubic and tetrahedral nanocrystals. Our results indicate that the surface-capping polymer is removed by annealing the specimen at temperatures between 180 and 250 °C. The particle shapes show no change up to ∼350 °C. In the temperature range between 350 and 450 °C, a small truncation occurs in the particle shapes but no major shape transformation is observed. The particle shapes experience a dramatic transformation into spherical-like shapes when the temperature is raised above ∼500 °C, where surface diffusion or surface premelting (softening) takes place. Above 600 °C, surface melting becomes obvious leading to coalescence of the surfaces of neighboring nanocrystals and a decrease in the volume occupied by the assembled nanocrystals. The surface melting forms a liquid layer a few atomic layers deep around the still solid core of the nanocrystal. This temperature is much lower than the melting point of bulk metallic platinum (1769 °C). The reduction in the melting temperature is discussed in terms of the surface tension of the solid−liquid interface (γSL). For an 8 nm diameter Pt nanocrystal, γSL is calculated to be 2.0 N m-1 at 650 °C, which is smaller than that of the bulk solid−vapor metal surface tension (γSV). This reduction is proposed to be due to the compensation of the increase in γSV of the nanocrystal by the wetting effect at the solid−liquid interface.