(363b) Chloride Directs the Growth of Cu Nanowires

Fine control of the shape and size of colloidal metal nanocrystals is of great interest in various fields ranging from catalysts to various plasmonic and electronic devices, since the properties of the nanocrystals are exquisitely sensitive to their shapes. While most solution-phase syntheses require halide ions to direct well-defined metal nanostructures, the role played by halides in growth is poorly understood. Our group recently found that solution-phase Cl^- can induce the anisotropic growth of five-fold twinned Cu nanowires, with aspect ratios around 100, by selectively displacing hexadecylamine (HDA) monolayers and leading to a 14.7 times faster reduction rate of Cu ions on the end {111} facets of the nanowires. However, the study does not explain how such high aspect ratios could be achieved, since around 100 of high aspect ratios needs approximately 100 times faster reduction (growth) rate of the end {111} facet than the side {100} facet based on our group’s theoretical calculation, and it is unclear why the growth of the nanowires was still observed at the high concentration of Cl^- where the facet-selective passivation effect of HDA does not exist. This indicates that there should be another role of Cl^- that contributed to the high aspect ratio growth. In this study, using quantum density-functional theory (DFT), we focused on the binding strength and surface diffusion of Cu adatoms on Cu surfaces in the presence of Cl at 1/2 monolayer(ML) coverage corresponding to the high Cl^- concentration in experiment and compared them with those on bare Cu surfaces. Upon the adsorption of Cl, we observed a higher surface energy of Cl-Cu(111) than Cl-Cu(100), and thus a stronger binding of Cu adatoms on Cl-Cu(111). We also discovered that absorbed Cl significantly decreases the surface diffusion-energy barrier of Cu adatoms from 0.55 eV – on bare Cu(100) to 0.15 eV – on Cl-Cu(100), but increases the barrier from 0.04 eV – on bare Cu(111) to 0.23 eV – on Cl-Cu(111). Most importantly, a model based on the theory of absorbing Markov chains indicates that absorbed Cl allows a much lower diffusion barrier (0.22 eV) of Cu adatoms from {100} to {111} than that (0.69 eV) for {111} to {100}. Thus, we find a much faster inter-facet diffusion rate from {100} to {111} than the opposite direction at the temperature ranges for most experimental Cu nanowire syntheses. Indeed, our predicted nanowire aspect ratios can be over 1000, depending on the synthesis conditions, and are comparable to those observed in experiments. This can well explain why experiments including Cl^- could obtain Cu nanowires with high aspect ratios. It is also worthy to note that all of these results in the presence of Cl are totally opposite to those for bare Cu surfaces, in which the growth of isotropic Cu nanoparticles is observed experimentally. Therefore, this study demonstrates that Cl^- significantly influences thermodynamics and kinetics on Cu surfaces and can direct the growth of Cu nanowires.