Higher Surface Energy of Free Nanoparticles

We present an accurate online method for the study of size-dependent evaporation of free nanoparticles allowing us to detect a size change of 0.1 nm. This method is applied to Ag nanoparticles. The linear relation between the onset temperature of evaporation and the inverse of the particle size verifies the Kelvin effect and predicts a surface energy of $7.2\mathrm{J}/{\mathrm{m}}^2$ for free Ag nanoparticles. The surface energy of nanoparticles is significantly higher as compared to that of the bulk and is essential for processes such as melting, coalescence, evaporation, growth, etc., of nanoparticles.

Figure 1

(a) Size distributions of Ag nanoparticles as measured by a DMA at the outlet of the sintering furnace. The initial mobility-equivalent diameter $d_{M0}$ as selected by a DMA is 24.0 nm. (b) $d_M$ as a function of the sintering temperature for different values of $d_{M0}$.

Figure 2

TEM micrographs of Ag agglomerates/nanoparticles sintered at different temperatures ($d_{M0}=16.0\mathrm{n}\mathrm{m}$). The micrographs (a)–(d) correspond to sintering temperatures of 913, 933, 1023, and 1043 K.

Figure 3

(a) The reduced mobility-equivalent diameter as a function of the sintering temperature. The particle size and the onset temperature of evaporation are obtained by an interpolation technique shown in the figure. (b) The onset temperature of evaporation versus the inverse of particle size.