Surfactant-free synthesis of mixed-phase (cubic + hexagonal) gold and gold-graphene core-shell nanowires

In recent years, hexagonal close-packed (HCP) gold nanostructures have attracted considerable interest due to their unconventional crystalline phase. The HCP phase facilitates the controllable preparation of large-sized gold crystals, which was challenging for conventional synthesis methods. We report a surfactant-free wet-chemical approach for the synthesis of hybrid-phase Au nanowires in a graphene shell, with diameters exceeding 20 nm and reaching up to 70 nm. Electron microscopy and X-ray diffraction analyses confirm the stable coexistence of both hcp and face-centered cubic (fcc) phases in these large-diameter nanowires. Based on time-dependent experiments, a growth mechanism is proposed involving the confined reduction of the chloroauric acid in curled graphene layers followed by the oriented attachment of gold crystallites. The work reveals the critical role of the graphene sheath. It not only promotes hcp phase formation by reducing the high interfacial energy between gold crystallites and the graphene sheath during the initial growth stages but also provides essential mechanical stabilization for the metastable hcp phase through spatial confinement, enabling it to exist in large-diameter nanowires. This study elucidates a novel growth mechanism for core–shell nanowires, while offering new perspectives on the potential for manipulating the properties of metastable materials by carbon nanoconfinement.