KAIST, Ewha researchers find 3D atomic mapping of ligands on palladium nanoparticles

Researchers at Korea Advanced Institute of Science and Technology (KAIST) and Ewha Womans University have revealed the three-dimensional distribution of ligand molecules on the surface of nanoparticles using atom probe tomography, the KAIST said Monday.

A capping ligand is an organic molecule that occurs when metal nanoparticles (NPs) synthesize. It prevents NP agglomeration and controls the size, shape, and functionality of NP. The importance of which is growing in synthesizing and designing NPs.

The study was published in the July 14 online edition of the journal Nature Communications titled “Three-dimensional atomic mapping of ligands on palladium nanoparticles by atom probe tomography.”

From left, Professor Choi Pyuck-pa of KAIST, Professor Lee Sang-heon of Ewha Womans University, Jang Kyu-seon, a Ph.D. candidate at KAIST, and Doctor Kim Se-ho of Max-Planck-Institut in Germany. The joint research team has revealed the 3D distribution of ligand atoms on the surface of nanoparticles using atom probe tomography for the first time.

Professor Choi Pyuck-pa of the Department of Materials Science and Engineering at KAIST and Professor Lee Sang-heon of the Department of Chemical Engineering and Materials Science at Ewha Womans University led the joint research. Chang Kyu-seon, a Ph. D. candidate at KAIST, and Doctor. Kim Se-ho at Max-Planck-Institut in Germany co-authored the paper.

To analyze the spatial distribution of capping ligand composed of organic molecules with complex structure, one should have the technology that can make the spatial resolution of atomic units, have high detection sensitivity to lightweight elements, and make a three-dimensional analysis. Unfortunately, due to the absence of such technology, there have been no cases of the three-dimensional observation of the distribution of capping ligand on the surfaces of nanoparticles. Therefore, the behaviors of ligands in synthesizing process of particles have remained a mystery to a considerable extent.

For instance, bromide ion, a halide ligand, has been known to facilitate the formation of cube-shaped metal NPs, but many studies are reporting different results these days.

Professor Choi’s team succeeded in observing the three-dimensional distribution atom map of Cetrimonium chloride ligand on the surface of two types of palladium synthesized through different halide ligands NPs using atom probe tomography (ATP.

APT reconstructs the 3D atomic distribution map of a specimen. The atoms on the surface of the needle-shaped specimen evaporate with ultrahigh vacuum and intense electric field. These evaporated atoms collide into the detector one after another. Using the collided point and order of atoms recorded in the detector and comparing the mass of collided atoms and electricity, the 3D atom map is attained.

Recently, APT that can make 3D atomic analysis, quantitative chemical analysis, has a spatial resolution of angstrom unit, and ppm-level detection sensitivity for all atoms is gaining a lot of attention in material analysis.

The research team calculated the density of Cetrimonium ligand present on the surface of each NPs from monolayer filmed data of the 3D distribution of ligand. Through this study, the team had found the existence of interaction between Cetrimonium ligand and halide ligand. This interaction between different ligands determines the final shape of NPs and the characteristics of oxidation resistance. It also explained why existing study results were not in accord with one another.

“The research is meaningful by providing experimental and theoretical results that can embrace the existing conflicting research results,” Professor Choi said. “We expect the research result to help the fundamental understanding synthesis of NPs and be applied to mapping NPs with excellent characteristics.”