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.
Highlights
Core@shell nanoparticles represent a class of materials with unique physical properties and various fine-tuning possibilities with respect to size, morphology and the variety of composition
When comparing images taken at 448 K to those taken at 573 K, an intact core can still be seen at the former temperature but disappears completely at the latter
The nature of the alloying process was studied via atomic resolution Transmission Electron Microscopy (TEM) imaging
Summary
Core@shell nanoparticles represent a class of materials with unique physical properties and various fine-tuning possibilities with respect to size, morphology and the variety of composition. The combination of gold and silver has highly desirable functionality with possible applications in catalysis and medicine.[9,10] both elements are highly resistant to oxidation under ambient conditions as well as during TEM measurements, and their phase diagram does not show a miscibility gap. Their inertness completely removes any oxidation or contamination issues during the synthesis of the core@shell structure. This set of properties makes the AgAu system ideal for a detailed study of alloying at the nanoscale
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