Ligand-induced structural evolution of Pt55 nanoparticles: amine versus thiol.

We report the geometric and electronic effects of amine (with one lone pair electron) and thiol (with two lone pair electrons) ligands on the structural transformation of Pt(55) nanoparticles (NPs) by first-principles calculation. Although a cuboctahedral (COh) structure is less stable than an icosahedral (Ih) structure by 1.36 eV for a bare Pt(55) NP, the activation barrier from the COh to the Ih structure is very high, by 1.97 eV, indicating that it would be difficult to observe the structural evolution of a COh structure to an Ih structure for a bare Pt(55) NP at ambient temperature. However, with the help of the adsorption of methylamine, the structural evolution from a COh structure to an Ih structure is accomplished by the Mackay transformation. This transformation is driven by a combination of both the external forces resulting from the adsorption of the ligand, which pull out the Pt atoms on the face sites of NPs in a radial direction, and the contraction forces in a tangential direction. As more methylamine is added, the Ih structure is observed to return to the original COh structure owing to the directional orbital hybridization that occurs between the Pt NPs and the methylamine. In contrast, such structural evolutions are not observed in the case of methylthiol because the sulfur (S) in the ligand has two lone pair electrons, leading to two Pt-S bonds. As a result, the radial-directed external force that the NPs experience because of the adsorption of methylthiols is much lower than that found in methylamine-ligated NPs. Furthermore, the adsorption of methylthiol leads to an expansion (not contraction) in the tangential direction, which does not qualify as a Mackay transformation. Thus, the Pt NPs ligated with methylthiol do not have a driving force strong enough to cause structural change. The methylthiol-stabilized Pt NPs retain their initial COh structure despite an abundance of ligand adsorption. From these results, we suggest that the NP structure can be controlled by varying the amount and species of ligands.