March 28, 2024 – At the bulk scale, gold and rhodium separate like oil and water, but at the nanoscale, they can mix completely. The reason for the miscibility is not what it seems.
Phase diagrams change at the nanoscale. When particles are shrunk down to small-enough sizes, the fraction of atoms on the surface increases, and the free-energy balance of atomic interactions changes. Although that has been common knowledge in the nanoscience community for decades, the details have remained fuzzy. Now Peidong Yang of the University of California, Berkeley, and colleagues have filled in some of those gaps by documenting the nanoscale phase transition of gold–rhodium mixtures down to the 1–5 nm range. In particles as small as 3.8 nm, the metals remain mostly separate. But, as shown in the figure, below 2 nm the thermodynamic balance shifts, and an alloy forms.
The reason the community had been missing that experimental data, explains Yang, is that it’s difficult to systematically produce nanoparticles of 1–5 nm. And once they’ve been made, analyzing their mixing state is also a challenge. Such small particles are easily knocked out of place by the electron beams used to image them. To deal with that challenge, the researchers used a combination of scanning transmission electron microscopy (STEM) at low energies and a random-walk algorithm that measures the domain size and distribution of the gold in the samples.
Gold and rhodium have several qualities that made them an appealing pair to investigate. Nanoparticles of those elements are already used for electrocatalysis reactions, such as the separation of water to make hydrogen fuel. Both elements have cubic lattice structures, so the analysis wouldn’t have to account for structural differences. And gold’s relatively high atomic number, 79, makes it stand out from rhodium, with an atomic number of 45, in the STEM imaging used.
In addition to looking at the effects of particle size, the researchers investigated how different mixing ratios of the two metals affected the results. They found that a 50-50 composition of the elements was the most resistant to mixing. After building a detailed experiment-based phase diagram for the metals’ miscibility, the researchers turned to thermodynamic models to confirm their findings. But they saw a problem: Model after model came back in disagreement with what they had observed. Instead of the evenly mixed alloys they measured, thermodynamic simulations suggested that small nanoparticles of the metals should separate into a core of rhodium surrounded by a shell of gold.
The apparent conflict between theory and observation revealed a piece of the picture the researchers had missed. Polymers used in the nanoparticle synthesis process left behind remnants, such as carbon atoms, that had attached to the surface of the metals. Those remnants, undetectable in the STEM images, changed the nanoparticle’s surface energy. Once the effects of surface passivation were accounted for, thermodynamic models finally agreed with the experimental phase diagram. Only nanoparticles synthesized in UHV would form the core-shell structure that the models predicted. (P.-C. Chen et al., Nat. Nanotechnol. 2024, doi:10.1038/s41565-024-01626-0.)
Original title: When unmixable metals mix
Author: Laura Fattaruso
Link: Physics Today