Specific Heat, Melting, Crystallization, and Oxidation of Zinc Nanoparticles and Their Transmission Electron Microscopy Studies

The specific heat, C-p, of zinc nanoparticles (size distribution 30-180 nm and peak at 30 nm) was measured, and their melting behavior was investigated as the ZnO shell grew around the metal particles and thickened. Both structural and chemical analyses were performed by using Transmission Electron Microscopy and techniques of energy filtering and energy dispersive X-ray analyses. The C-p of Zn nanoparticles is slightly higher than that of bulk metal. The melting point of Zn nanocrystals confined to the ZnO shell is only 1-2 K less than that of bulk Zn, much less than that expected from the Gibbs-Thomson equation. This is attributed to the increase in pressure on the zinc core because (i) zinc expands more on heating and on melting than ZnO, (ii) the ZnO shell thickens at the expense of the zinc core, and (iii) there is an epitaxial interaction between Zn and the ZnO shell. The enthalpy of melting decreases on thermal cycling. Nanodroplets of Zn supercooled by a few degrees before crystallizing in two steps. The high temperature step is attributed to heterogeneous nucleation at the core-shell interface, and the low temperature step to homogeneous nucleation in the particle core. The amount crystallized on homogeneous nucleation decreased on thermal cycling as the ZnO shell thickened when oxygen diffused through this layer. The enthalpy of crystallization also decreased on thermal cycling.