Regulating Surface Faceting as a Kinetic Switch for Core-Shell Nanoparticle Crystallization Pathways

Nanoparticle morphology is a critical determinant of physical and chemical properties, yet the fundamental mechanisms governing how specific shapes emerge during crystallization remain elusive. In this work, we reveal that the final morphology of binary core-shell nanoparticles is governed by a kinetic bifurcation in nucleation modes: surface-nucleation mode and inner-nucleation mode. Using binary Pt@Au core-shell nanoparticles as a representative model system, we identify a "kinetic switch" regulated by the shell-to-core atomic ratio. At low shell concentrations, the surface faceting effect of the core remains, acting as a template for surface nucleation that yields 5-fold twinned structures. As the shell concentration increases, the gold atoms progressively disrupt the surface faceting of the platinum core, shifting the nucleation site to the core interior and resulting in single-crystalline morphologies. We demonstrate that this transition is driven by shell-induced disruption of surface faceting rather than thermodynamic stability alone, and we establish a mechanistic link between nucleation position and final morphology. By elucidation of these two intrinsic crystallization pathways and the origin of their bifurcation, this work provides a predictive framework for the rational design and kinetic control of binary nanomaterials.