Connecting the Twisting of Ultrathin <i>FCC</i> Au Nanowires to the Formation of 4H Phase

The formation mechanism of the 4H Au phase remains elusive, despite its great importance and intensive studies. This work reports an unexpected connection between the formation of the 4H phase from fcc Au nanowires and their twisting behavior to give double helices. With Ag deposition taken as a model, slow reduction by H₂O₂ leads to double helices, whereas rapid Ag deposition with stronger reductants (hydrazine or hydroquinone) results in heterophase nanowires with 4H segments. For the latter, slower reduction by using lower concentrations again gives double helices, confirming the critical role of deposition kinetics. Careful characterization confirms the 4H phase upon deposition of either Au, Ag, or Pd, and detailed control experiments show that the choice of metal shell, ligand and solvent environments, pH, and auxiliary reagents, are not the critical factor. Hence, it really matters if the nanowires have sufficient time to twist or not. It is proposed that the 4H phase may result from failed twisting transformation, which is driven by the metal deposition and subsequent lattice reorganization for stabler surface atoms. This unprecedent example of fcc-to-4H phase transformation offers a rare perspective on the underlying mechanisms, and more importantly, new avenues for phase engineering of nanomaterials.