Abstract:

Materials such as metals, semiconductors, and oxides are attractive at nanometer scales due to the physical and chemical property differences with their bulk counterparts as induced by the quantum confinement effect and large surface-to-volume ratios. In particular, heterogeneous nanostructures consisting of semiconductors and noble metals are extremely important because of the synergistic effects occurring at the interfaces between their noble metal and semiconductor domains; these often equip the heterogeneous nanostructures with improved properties compared to those of isolated individual components. Thus far, heterogeneous nanostructures have garnered a considerable research interest, and tremendous development in achieving high degree control over these nanostructures with respect to their domain size, morphology, and composition has been realized. Their immense application potential in optics, catalysis, imaging, and biomedicine render them a field full of original innovation possibilities. Herein, we demonstrate a phenomenon observed in core-shell nanostructures composed of noble metals and silver sulfide (Ag₂S): the inside-out migration of noble metals in Ag₂S nanoparticles. We prepare core-shell nanostructures with noble metals and Ag₂S residing at the core and shell regions, respectively, through various synthetic strategies including seed-mediated growth and galvanic replacement reactions followed by sulfidation. We then characterize the core-shell nanostructures before and after aging them in toluene at room temperature (e.g. 25 ℃) for a period of time up to 72 h. In contrast to the reported diffusion of Au from the outside to the inside of InAs or PbTe nanoparticles, which results in an Au core encapsulated by an amorphous InAs or PbTe shell, the noble metals (Au, Ag, Pd, or Pt) in core-shell nanostructures with noble metals and Ag₂S residing at the core and shell regions, respectively, are found to diffuse from the inside to the outside through the Ag₂S shell. Thus, heterogeneous nanodimers consisting of the corresponding noble metal and Ag₂S are formed. Observations using an electron transmission microscope confirm that the inside-out migration of noble metals in Ag₂S is carried out in a holistic manner. Due to the apparent interface mismatch between face-centered cubic noble metals and monoclinic Ag₂S crystal phases, defects such as vacancies must exist at these interfaces. This makes the migration of noble metals in Ag₂S possible by either a vacancy/substitutional mechanism or by the self-purification mechanism that occurs intrinsically in nanoscale semiconductors. As the migration rate of noble metals in Ag₂S increases with the decrease in the size of the noble metal core and the radius of noble metal atoms, the inside-out migration rates of Ag, Pd, and Pt in Ag₂S are found to be much higher than that of Au because of their smaller particle sizes or atom radii. This scientific phenomenon can be effective in the development of synthetic routes for heterogeneous nanostructures that might not be obtained by conventional methods.

Key words: Nanoparticle, Noble metal, Silver sulfide, Core-shell structure, Migration