In-situ TEM observation of the seed-free growth of five-fold twinned Au decahedra

Five-fold twinned decahedral nanoparticles exhibit exceptional properties due to their unique symmetry, yet seed-free formation mechanisms remain elusive despite extensive research relying on pre-existing twinned seeds or tetrahedral building blocks. Here, we report the seed-free evolution of polycrystalline Au nanoparticles into five-fold twinned decahedra using in-situ high-resolution transmission electron microscopy, revealing a new multi-stage growth mechanism involving reversible phase transitions. The pathway proceeds through: curvature-driven atomic diffusion initiating particle necking; subsequent localized atomic rearrangements yield coherent twin boundaries; and reversible fcc–2H-polytype hcp–fcc phase transitions within twin domains enable lattice reconstruction through coordinated atomic sliding. Transient stacking faults act as pivotal structural mediators that facilitate atomic migration, significantly reducing energy barriers and stabilizing the emergent twinned lattice. These findings advance the mechanistic understanding of seed-free formation of five-fold twinned Au nanostructures and facilitate the rational design of multiply nanoparticles.