3D Atomic Structure of Pt Nanocrystals Related to Their Catalytic Activity and Surface Ligand Adsorption

The surface structure of nanocrystals determines catalytic activities and ligand adsorption behavior. However, it is challenging to precisely investigate the physicochemical behavior because the surface structure of synthesized nanocrystals consists of intricate and low-symmetry surface structures. Here, we revealed 3D atomic structure of sub 3-nm Pt nanocrystals and analyzed surface-related characteristics of the synthesized nanocrystals. First, we constructed 3D maps of the coordination number (CN) and generalized CN (GCN) for individual surface atoms to directly correlate types of constituent surface structures and catalytic activities, respectively. The results reveal that the synthesized Pt nanocrystals are enclosed by islands with nonuniform shapes that lead to complex surface structures, including a high ratio of low-coordinated surface atoms, reduced domain size of low-index facets, and various types of high-index facets. In addition, these intricate surface structures give rise to a wide range of GCN meaning various active sites, which explains the origin of high catalytic performance of small Pt nanocrystals in important reactions such as oxygen reduction reaction. Second, we applied a machine-learning-accelerated ab-initio calculation into the 3D atomic structures of Pt nanoparticles to analyze the complex adsorption behavior of polyvinylpyrrolidone (PVP) ligands on synthesized nanoparticles. Different angular configurations of the large-sized ligands on low-symmetry surfaces are thoroughly investigated. It is revealed that two components of binding affinity, short-range direct bonding and long-range van der Waals interaction, of PVP on synthesized Pt nanoparticles are differently affected by the coordination number of the binding atomic site and surface-exposed local geometry surrounding it. In addition, the adsorption directions of the large-sized ligand have a tendency depending on the types of adsorption centers, despite the low-symmetry surfaces. The results highlight the important contribution of vdW interactions to adsorption behavior of large-sized ligands on nanoparticle surfaces.