The Science
Researchers have long known that the shape of nanoparticles depends on the choice of solvent and temperature during their growth. However, the tiny seed particles that form first and that guide the formation of final nanoparticle shapes are too small to measure accurately. With the help of a supercomputer, researchers have developed a new approach to successfully model seed particles with 100 to 200 atoms. They found that the shapes of the tiny particles depend on the solvent composition and temperature in unexpected ways. Surprisingly, in some cases the shape of the seed particle changes dramatically when only a single atom is added or removed.
The Impact
Metal nanoparticles can be used in catalysis, solar cells, transparent conducting films, electromagnetic shielding, wearable electronics, and more. In these technologies, the nanoparticle shape must be tuned for the best performance. It is a major challenge for scientists to grow metal nanoparticles with controlled shape and size. The ability shown in this research to model seed particle shapes is a key breakthrough. The researchers have shown how both temperature and solvents control nanoparticle shape. These models can suggest promising routes for growing nanoparticles with the desired sizes and shapes.
Summary
A significant challenge in materials synthesis is growing metal nanocrystals with controlled shapes and sizes. Metal nanocrystals are often synthesized in the solution phase, where a metal salt is reduced by solvent or additives. Metal atoms and/or ions then aggregate to form nuclei, which grow to become seeds in the single-nanometer size range. The seeds grow further to form the final nanocrystal shapes. Since the shapes of the seeds determine the final nanocrystal shapes, control of seed-crystal shapes is an important goal.
Researchers used two computational approaches, parallel-tempering molecular dynamics (MD) and partial replica exchange MD, to estimate the most probable shapes of silver nanocrystals in vacuum, in ethylene glycol (EG) solvent, and in EG solvent with a growth-directing chemical (polyvinylpyrrolidone). These studies reveal that nanocrystal shapes can change dramatically with the addition or removal of a single atom at certain critical sizes. In efforts to devise processing routes to achieve a particular nanocrystal shape, it is important for scientists to identify these critical nanocrystal sizes, as these sizes could be shape turning points along the nanocrystal growth trajectory. An additional defining point along the nanocrystal growth trajectory is temperature. In some cases, there are critical sizes at which one shape is the most probable at low temperatures and another shape is the most probable at high temperatures.
Funding
This work was funded by the Department of Energy Office of Science, Office of Basic Energy Sciences, Materials Science Division. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation.
