High thermal stability of core–shell structures dominated by negative interface energy
Nanoscale core/shell structures are of interest in catalysis due to their superior catalytic properties. Here we investigated the thermal stability of the coherent core–shell structures in a thermodynamic way by considering the impact from the core with the bulk melting point Tm(∞) lower or higher than the shell. When a low-Tm(∞) core is adopted, core–shell melting induced by the melting depression of the core does not occur upon heating because of the superheating, although the melting depression of the core can be triggered ultimately by the preferential melting of the high-Tm(∞) shell for small cores. The superheating of the core is contributed by the negative solid–solid interface energy, while the depression is originated from the positive solid–liquid interface energy. Owing to the presence of the negative interface energy, moreover, the low-Tm(∞)-core structure possesses a low difference in thermal expansion between the core and the shell, high activation energy of outward atomic diffusion from the core to shell, and low heat capacity. This result is beneficial for the core–shell structure design for its application in catalysis.