Size dependence of nanoparticle dissolution in a matrix: Gold in bismuth

We discuss the dissolution of Au nanoparticles in a Bi matrix. The particles were produced by vapor deposition at room temperature, and they burrowed to minimize their free energy. Using in situ transmission electron microscopy, we measured the rate of dissolution as a function of particle size and deduced the activation energy for this process. We also followed the structural changes in the system by high-resolution electron microscopy and nanobeam diffraction. This study develops an important understanding of the dependence of size on the stability and reactivity of nanoparticles.

Figure 1

TEM images of Au particles embedded in Bi at $160°\text{C}$ showing that they dissolved and ultimately disappeared. The disappearance time was roughly proportional to the particle size. Changes to the Bi film were negligible.

Figure 2

HRTEM images of a Au particle at $130°\text{C}$ collected over a 25 min period. The particle orientation, as measured by the spacing in the moiré pattern in the first two images, and the in-plane spacing do not change.

Figure 3

(Color online) NBED data during a constant rate heating experiment. The data are plotted as normalized intensity vs distance, vertically displaced for clarity. The Au particles were formed by depositing 1 nm of the material on 20 nm Bi. The annealing rate was $3°\text{C}/\text{min}$. Initially, at room temperature, peaks corresponding to Bi and Au are visible. With an increase in temperature Au peaks go down in intensity and ultimately disappear, while new peaks corresponding to ${\text{Au}}_2\text{Bi}$ are formed. No visible changes are observed in the Bi peaks.

Figure 4

(Color online) Plot of the disappearance time as a function of particle radius and temperature. The disappearance time defined as the first TEM image in which the particle is no longer visible is obtained from TEM images such as those of Fig. 1. The time increases nonlinearly with particle size but decreases with temperature for particles of a given size. The lines through the data points represent fits obtained using Eq. (4).

Figure 5

The slope of $\text{ln}\left[D{C}_I\left(\infty \right)\right]$ vs ${\left(k_{B}T\right)}^{-1}$ gives the activation energy of dissolution, where $D{C}_I\left(\infty \right)$ was obtained from the fits of the dissolution time to the particle radius at different temperatures, using Eq. (4). The energy of 0.69 eV represents the total energy for an Au atom to detach from the particle and diffuse into the matrix.