Thermally induced alloying processes in a bimetallic system at the nanoscale: AgAu sub-5 nm core-shell particles studied at atomic resolution

Abstract

Alloying processes in nanometre-sized Ag@Au and Au@Ag core@shell particles with average radii of 2 nm are studied via high resolution Transmission Electron Microscopy (TEM) imaging on in situ heatable carbon substrates. The bimetallic clusters are synthesized in small droplets of superfluid helium under fully inert conditions. After deposition, they are monitored during a heating cycle to 600 K and subsequent cooling. The core-shell structure, a sharply defined feature of the TEM High-Angle Annular Dark-Field images taken at room temperature, begins to blur with increasing temperature and transforms into a fully mixed alloy around 573 K. This transition is studied at atomic resolution, giving insights into the alloying process with unprecedented precision. A new image-processing method is presented, which allows a measurement of the temperature-dependent diffusion constant at the nanoscale. The first quantification of this property for a bimetallic structure <5 nm sheds light on the thermodynamics of finite systems and provides new input for current theoretical models derived from bulk data.

Fig. 1. Transmission electron microscopy HAADF scans of a single Ag@Au core@shell cluster as a function of temperature (upper images) and a Au@Ag core@shell cluster scanned at the same temperatures (lower images). With increasing temperature, a softening of the contrast borders between Ag and Au is detected.

Fig. 2. Linear HAADF intensity profiles of a Ag@Au (upper row) and Au@Ag (lower row) cluster as a function of the observation temperature. For each temperature a fit of the calculated intensity profile and the measurements is performed. Each measured, temperature-dependent intensity profile is plotted and compared to the calculated fit obtained from eqn (4).

Fig. 3. Diffusion constant as a function of temperature. Black squares are derived from HAADF measurements, the red dashed line is a fit based on eqn (5), the blue line is based on a revised model from ref. 49 derived from bulk values with a mean radius of the cluster of r = 1.95 nm and atomic binding length of h = 0.2889 nm.

Fig. 4. Phase diagram for the AgAu system as a function of the particle radius as suggested in ref. 32, compared to selected measurements of crucial temperatures in pure and mixed-metallic systems, see text for details.