Analysis of the Crystallization Process of Pt Single Atoms on Graphene

Crystallization is a widely observed phenomenon in daily life, within biological systems, and in industrial processes, but its detailed mechanism at the atomic level remains a topic of ongoing discussion. We have achieved single Pt atoms adsorbed on graphene by plasma sputtering. Graphene films were synthesized by a CVD method on a copper substrate (Alfa Aesar, 99.8%, thickness: 25 μm). After the etching of Cu substrate by 100 mM ammonium peroxodisulfate solution, the specimen was rinsed thoroughly with distilled water. Next, it was transferred on a carbon-supported Cu TEM grid. We deposited dispersed Pt single atom on graphene by magnetron spattering. We use a pure Pt thin film (t = 0.1mm) for the spatter. The chamber was evacuated to 4.5 Pa in air. The metal Pt atoms were spattered by strong magnetic and electric fields between a Pt target and the TEM grid. We deposited single-atom Pt dispersion by short-pulsed spattering. In order to observe the translational motion of Pt single atoms on graphene, this study conducted continuous imaging using aberration-corrected STEM. We have shown that Pt atoms are dominantly adsorbed near the step edges of nanoscale graphene stacked on monolayer pristine graphene, which is called nanographene, and hardly adsorbed on the terraces. In addition, we found that the Pt 5dxy-orbital forms a bond to the step edge. However, Pt atoms may cause aggregation from single atoms into 2D clusters or 3D nanoparticles at high concentrations. Single Pt atoms on graphene moved across the surface under electron beam irradiation and also gradually aggregated. We found that graphene supporting Pt single atoms becomes damaged and its structure changes under electron beam irradiation, leading to increased mobility of the Pt single atoms. On the other hand, when the structure of the graphene remains stable, the movement distance of many Pt single atoms between images is smaller than the nearest-neighbor distance of carbon atoms in graphene (1.4 Å). Furthermore, the motion of Pt single atoms was suppressed when nitrogen was doped in graphene. By measuring the number of Pt single atoms before aggregation, we were able to analyze how many atoms are required to form Pt nanoparticles with a three-dimensional structure. Nanoparticles with a three-dimensional structure incorporated additional Pt single atoms and crystallized on graphene. We were able to directly observe the entire process from assembly of single atoms to crystallization. On the day of the presentation, we also plan to report on the analysis results regarding nucleation and the critical nucleus.