Controlling Nanocrystals

Nanocrystals come in a wide variety of shapes that can affect applications. In some cases, the distribution of products generated by catalytic nanoparticles depends on the particle shape. And the effectiveness of nanoparticle-based chemical sensors can also depend on crystal shape. But controlling these shapes as the tiny particles grow during synthesis remains challenging.

Researchers may soon be able to exert tighter control over the final shapes of metal and other inorganic nanocrystals thanks to a study that examined the dependence of crystal shape on fundamental crystal growth processes. The investigation revealed that for seed crystals of noble metals growing in solution, the crystal’s shape depends on the relative rates of two easily controlled parameters: the rate at which metal atoms adsorb on certain crystal faces and how quickly the atoms diffuse to other crystal locations (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1222109110).

The investigation results were presented by Younan Xia of Georgia Institute of Technology during a symposium sponsored by the Division of Colloid & Surface Chemistry at the American Chemical Society national meeting in New Orleans.

Xia’s research team developed a model based on surface energetics and diffusion kinetics to predict the final shapes of particles grown from cube-shaped palladium seed crystals with truncated corners and edges. He explained that those crystals can be prepared uniformly and treated with bromide ions to control Pd deposition and diffusion and thereby test the model.

The model predicts that if the rate of Pd adsorption on the corners is much faster than the rate at which the atoms diffuse away from those sites and settle on the main crystal faces and along the edges, the crystals will form octapods. If diffusion occurs much faster than deposition, the model predicts cubeoctahedrons will form. And for cases in between those two extremes, the model predicts the crystals will form concave nanocubes and nanocubes with slight truncation at the corners.

To test the model, the team injected an aqueous solution of Na₂PdCl₄ (a Pd precursor) into a suspension containing the seed crystals, a fast-acting reductant (ascorbic acid) to liberate the Pd atoms, and a polymer stabilizer. The group controlled the deposition rate by adjusting the injection rate and controlled the diffusion rate by tuning the reaction temperature.

Xia reported that microscopy analysis shows that the crystals indeed assume the shapes predicted by the model. He also noted that the group applied the same techniques to control the shapes of Pd–Pt crystals.

“Understanding and controlling seed-mediated crystal growth mechanisms is important for future catalyst design, development, and testing,” said Shouheng Sun, a nanocrystal specialist at Brown University. He added that the crystals described in this study could serve as valuable model systems for further research.

Pennsylvania State University’s Raymond E. Schaak remarked that many complex and competing factors influence the formation of metal nanocrystals. “This work offers important insights into the role that surface diffusion plays in crystal growth, he added.”