(286h) The Influence of Sequenced Peptoids in Controlling Five-Fold Twinned Au Nanostar Formation

Five-fold twinned metal nanocrystals have attracted much attention due to their unique plasmonic properties, and a collection of five-fold twinned nanostructures have been synthesized in solution for Au, Ag and Cu, with the aid of chemical additives. It has been widely demonstrated that these chemical additives are critical in achieving such shape-controlled syntheses, and their roles may vary significantly with materials, exact structures, and solid-liquid interfacial properties. Peptoids, or N-substituted glycines, with highly tunable sequence, and consequently functionality, contributes to a rising group of promising structure-directing chemical additives. We find that an amphiphilic peptoid Nce3Ncp6, composed of three carboxyl side chains and six chlorobenzyl side chains, can effectively assist the formation of Au nanostars in water. These nanostars adopt an unconventional five-fold twinned structure, comparing to a classical decahedron, as the twin planes extend to the grooves of the star, instead of the vertices on a decahedron. Since these nanostars are majorly {111}-faceted, it is hypothesized that these shapes are formed as a result of a preferred binding to Au(111) facets from peptoid Nce3Ncp6.

We use all-atom molecular dynamics (MD) simulations, together with enhanced sampling methods, such as Parallel Bias metadynamics (PBMetaD) and umbrella sampling, to study the adsorption of peptoid Nce3Ncp6 at the Au-water interface and unravel how surface adsorption can affect shape revolution. Using PBMetaD, we find that Nce3Ncp6 strongly prefers Au(111) over Au(100), and the origin of this binding preference can be dissected into two aspects. First, while water forms layers near both surfaces, the packing of the first-layer water molecules is much more ordered on Au(100) than Au(111), which protects the Au(100) facet from molecular adsorptions. Second, aromatic rings significantly favor Au(111) surface due to surface electron density distribution, and their energetic preference is the major contribution to the energetic selectivity of Nce3Ncp6. The carboxyl side chains on the peptoid, however, bind weakly and non-selectively to either facet. As a result, the adsorption of Nce3Ncp6 can largely passivate the Au(111) facet thermodynamically, counterbalance the energy associated with the twin and stabilize the five-fold twinned structure. Due to the amphiphilic and facet-selectivity nature of the peptoid, its packing on surfaces can influence the deposition of solution-phase atoms towards different facets. Using umbrella sampling, we investigated the steric effect from the peptoids adsorbed on Au(100) and Au(111). Weak adsorption results in loosely packed peptoids on Au(100), allowing a higher deposition flux towards Au(100), and thus grows the vertices on the nanostars. In sum, the formation of five-fold Au nanostar is regulated both thermodynamically and kinetically by the strongly selective binding of peptoid Nce3Ncp6 on Au surfaces.