Revisiting the Facet Control in the Growth of Au Nanobipyramids

Abnormalities of Au nanobipyramids (NBPs), such as rough surfaces, variable tip angles, corrugated edges, and curved tips, cannot be explained by traditional facet control. The underlying mechanism and significance of these abnormalities have not been fully recognized. This study revisits the growth process, focusing on the transition from normal to abnormal structures. We propose that both cetyltrimethylammonium bromide (CTAB) and Ag⁺ ions passivate the Au surface. When the passivation exceeds a limit, a nonequilibrium growth regime ensues (active surface growth, ASG), where the dynamic interplay between growth and passivation causes some sites to grow faster and thus become less passivated, leading to focused Au growth at these sites. This positive feedback leads to inequivalent growth of the initially equivalent surfaces, causing abnormalities in the Au NBPs. We believe these insights resolve the long-standing puzzle of why "ligand-specific facet control" does not always lead to flat, well-defined facets.