Gold and rhodium are two types of *** and they have many applications in jewelry, electronics, and catalysis. However, they are also immiscible, meaning they don't mix well together. In fact, if you try to fuse them together, they will separate into two different phases, just like oil and water. This limits the possibility of creating new materials with novel properties from these two elements.
However, scientists have recently discovered a way to overcome this challenge and allow gold and rhodium to be completely mixed at the nanoscale. In an article published in the journal Nature Nanotechnology, a team of researchers from the University of California, Berkeley, and Lawrence Berkeley National Laboratory reported that they were able to prepare nanoparticles of different sizes and compositions from gold and rhodium, and observed that when the size of the nanoparticles decreased to less than 2 nanometers (nm), the transition from phase separation to alloy occurred.
Details: Nanoparticles are tiny particles that are only a few nanometers, or billionths of a meter, in size. At this scale, the properties of the material can vary dramatically depending on the size, shape, and composition of the nanoparticles. For example, gold nanoparticles can appear red, blue, or purple, depending on their size, and can also exhibit different catalytic, optical, and electronic behaviors.
The researchers used a special technique, called polyol synthesis, to prepare gold and rhodium nanoparticles of different sizes and compositions. They used a polymer called polyvinylpyrrolidone (PVP) to control the growth and shape of the nanoparticles, and a solvent called ethylene glycol to reduce the metal precursors. By changing the ratio, reaction time, and temperature of gold and rhodium precursors, they can adjust the size and composition of nanoparticles.
To characterize the structure and composition of nanoparticles, the researchers used a powerful technique called high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM). This technique allows them to image individual atoms in nanoparticles and measure their intensity, which is proportional to the atomic number of the element. By comparing the strength of the atoms in the nanoparticles with the strength of the atoms of pure gold and pure rhodium, they were able to determine the distribution of the two elements in the nanoparticles.
The researchers found that nanoparticles exhibit different mixing behaviors depending on their size and composition. For nanoparticles larger than 2 nm, they observed phase separation, meaning that the nanoparticles consist of two distinct regions: one rich in gold and one rich in rhodium. The shape and location of these regions depend on the composition of the nanoparticles. For example, for nanoparticles containing 50% gold and 50% rhodium, they observed a core-shell structure in which the nucleus part is rich in gold and the shell part is rich in rhodium. For nanoparticles containing 15% gold and 85% rhodium, they observed a crown-jewel structure in which the core part is rich in rhodium and the shell part is decorated with gold atoms.
However, for nanoparticles smaller than 2 nm, they observed a different phenomenon: completely miscible, meaning that the nanoparticles were a homogeneous mixture of gold and rhodium with no phase separation. This is true for all compositions, from 15% gold to 85% gold. The researchers explained that this immiscible-to-miscible transition is driven by the size of the particles, their composition, and the possible presence of surface adsorbents under synthetic conditions. They used density functional theory (DFT) calculations, a computational method for studying the electronic structure and energy of materials, to support their experimental observations and reveal underlying mechanisms.
The researchers also studied the catalytic properties of nanoparticles and found that they exhibited different activity and selectivity for the electrochemical reduction of carbon dioxide (CO2), a reaction that can convert CO2 into useful chemicals and fuels. They found that phase-separated nanoparticles had higher activity and selectivity for carbon monoxide (CO) production, while alloyed nanoparticles had higher activity and selectivity for formic acid (HCOOH) production. They attribute these differences to the electronic and geometric effects of nanoparticles, which affect the binding and activation of CO2 and its intermediates.
The researchers concluded that their research demonstrates the possibility of creating new materials with tunable structures and properties from immiscible elements at the nanoscale. They also suggest that their approach could be extended to other immiscible systems, such as platinum and gold, or copper and silver, to explore the rich phase diagrams and functions of multi-element nanoparticles. They hope that their work will inspire more research and applications of nanoscience and nanotechnology.
chen, pc., gao, m., mccandler, c.a. et al. complete miscibility of immiscible elements at the nanometre scale. nat. nanotechnol. (2024).