  Capping ligands are crucial to synthesize colloidal nanoparticles with novel
functional properties. However, the interaction between capping ligands and
their interaction with the crystallographic surfaces of nanoparticles during
colloidal synthesis remains a great mystery, due to the lack of direct imaging
techniques. In this study, atom probe tomography was adopted to investigate the
three-dimensional atomic-scale distribution of two of the most common types of
these ligands, cetrimonium and halide (Br and Cl) ions, on Pd nanoparticles.
The results, validated using the density functional theory, demonstrated that
the Br anions adsorbed on the nanoparticle surfaces promote the adsorption of
the cetrimonium cations through electrostatic interactions, stabilizing the Pd
{111} facets.
