(201c) Halides Induce Anisotropic Growth of Cu Nanocrystals

Properties of metal nanocrystals are generally controlled by their morphologies. Though Ag nanocrystals are used in commercial applications, the properties of Cu are comparable so that this earth-abundant material has drawn increasing attention. There have been many accounts of experimental, solution-phase syntheses of various Cu nanocrystals, including nanowires and nanoplates – however, the mechanisms of such anisotropic growth are still not fully understood.

It is commonly believed that capping agents, such as hexadecylamine (HDA) can induce anisotropic growth by selective binding to certain crystal facets. However, the emergence of round Cu nanoparticles synthesized in the presence of HDA does not seem to support this. Only when halide is present as an additive during nanocrystal synthesis are distinctive shapes observed. When Cl^- is the additive, nanowires form, but I^- leads to the formation of nanoplates. To elucidate the roles of halides in anisotropic Cu nanocrystal growth, we use density functional theory (DFT) to study the co-adsorption of halides and HDA on Cu surfaces with an analysis based on ad initio thermodynamics.

Both Cl^- and I^- are found to displace a chemisorbed HDA self-assembled monolayer (SAM) on Cu(100) and Cu(111). In the case of Cl^-, HDA SAMs are disrupted at lower Cl coverages on Cu(111) than on Cu(100). This can result in full protection for Cu(100) from Cu ion addition and a lack of protection for Cu(111) and is consistent with the experimentally observed formation of nanowires with long {100} side facets. Ab initio thermodynamics calculations show that, under the conditions for which nanowires are predicted and observed, introducing a trace amount of I^- can displace both Cl^- and HDA on both Cu surfaces, which creates a strong thermodynamic driving force for nanoplates with large {111} facets. The predicted evolution of Cu nanocrystal shape under different synthetic conditions agrees well with experimental results.