Vacancy-driven twinning in metal nanoparticles: from bulk morphology transformation to optical and electrochemical effects.

Stacking defects in noble metal nanoparticles influence their optical, catalytic, and electrical properties, but the mechanisms behind these changes are in many cases not well understood. While some mechanisms behind the formation of stacking defects have been studied, the possibility to deliberately manipulate the nanoparticle bulk morphology remains largely unexplored. Here, we introduce a vacancy-driven twinning mechanism that transforms face-centered cubic (fcc) gold nanoparticles into locally hexagonal close-packed (hcp) structures upon reaching a critical vacancy concentration. Atomistic simulations reveal that this transformation leads to multiply twinned morphologies, including cross-twinning patterns and stable hcp domains spanning 2-6 atomic layers. Experimentally, we validate this mechanism through thermal treatment of Au-Pt alloy nanoparticles, with multidomain X-ray diffraction (MDXRD) confirming controllable ordering and disordering transitions in the bulk structure. Furthermore, we investigate how the nanoparticles' bulk morphology influences their optical properties. Using time-dependent density-functional theory with Hubbard U corrections (TDDFT+U), we analyze the plasmonic response of models with specific defect motifs demonstrating that structural defects influence the absorption profile. Finally, we discuss how these morphology-induced changes in the electronic structure and plasmonic behavior could potentially modulate electrochemical activity in catalytic systems.