Oriented attachment mechanism of triangular Ag nanoplates: a molecular dynamics study.

We use molecular-dynamics simulations to probe the experimentally observed aggregation of PVP-covered triangular Ag nanoplates to form 2D sheets in solution. We find lateral plate attachment is the most favorable aggregation pathway - consistent with experiment. The mechanism is general and suggests new processing strategies for creating 2D architectures in solution-phase syntheses.

Revisiting the Polyol Synthesis of Silver Nanostructures: Role of Chloride in Nanocube Formation.

Chloride (Cl-) is often used together with polyvinylpyrrolidone (PVP) in the polyol synthesis of Ag nanocubes. In the literature, shape control is attributed predominantly to the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets, whereas the role of Cl- has not been well studied. Several hypotheses have been proposed regarding the role of Cl-; however, there is still no consensus regarding the exact influence of Cl- in the shape-controlled synthesis of Ag nanocubes. To examine the influence of Cl-, we undertook a joint theoretical-experimental study. Experimentally, we examined the influence of Cl- concentration on the shape of Ag nanoparticles (NPs) at constant H+ concentration. In the presence of H+, in situ formed HNO3 etches the initially formed Ag seeds and slows down the overall reduction of Ag+, which promotes the formation of monodisperse Ag NPs. Ex situ experiments probed the evolution of Cl- during the growth of Ag nanocubes, which involves the initial formation of AgCl nanocubes, and their subsequent dissolution to release Cl-, which adsorbs onto the surfaces of single crystal seeds to impact shape evolution through apparent thermodynamic control. The formation of cubes is independent of the source of AgCl, indicating temporal control of the Cl- chemical potential in solution leads to high-yield synthesis of Ag nanocubes. Increasing the concentration of Cl- alone leads to a progression in shape from truncated octahedra, to cuboctahedra, truncated cubes, and ultimately cubes, directly demonstrating the importance of Cl- in Ag NP shape control. We used ab initio thermodynamics calculations based on density functional theory to probe the role of Cl- in directing shape control. With increasing Cl chemical potential (surface coverage), calculated surface energies γ of Ag facets transition from γ111 < γ100 to γ100 < γ111 and predict Wulff shapes terminated with an increasing (100) contribution, consistent with experimental observations. The combination of theory and experiment is beneficial for advancing the understanding of nanocrystal formation.

Self-assembled monolayer structures of hexadecylamine on Cu surfaces: density-functional theory.

We used dispersion-corrected density-functional theory to probe possible structures for adsorbed layers of hexadecylamine (HDA) on Cu(100) and Cu(111). HDA forms self-assembled layers on these surfaces, analogous to alkanethiols on various metal surfaces, and it binds by donating electrons in the amine group to the Cu surface atoms, consistent with experiment. van der Waals interactions between the alkyl tails of HDA molecules are stronger than the interaction between the amine group and the Cu surfaces. Strong HDA-tail interactions lead to coverage-dependent tilting of the HDA layers, such that the tilt angle is larger for lower coverages. At full monolayer coverage, the energetically preferred binding configuration for HDA on Cu(100) is a (5 × 3) pattern - although we cannot rule out incommensurate structures - while the pattern is preferred on Cu(111). A major motivation for this study is to understand the experimentally observed capability of HDA as a capping agent for producing {100}-faceted Cu nanocrystals. Consistent with experiment, we find that HDA binds more strongly to Cu(100) than to Cu(111). This strong binding stems from the capability of HDA to form more densely packed layers on Cu(100), which leads to stronger HDA-tail interactions, as well as the stronger binding of the amine group to Cu(100). We estimate the surface energies of HDA-covered Cu(100) and Cu(111) surfaces and find that these surfaces are nearly isoenergetic. By drawing analogies to previous theoretical work, it seems likely that HDA-covered Cu nanocrystals could have kinetic shapes that primarily express {100} facets, as is seen experimentally.

Predicting kinetic nanocrystal shapes through multi-scale theory and simulation: Polyvinylpyrrolidone-mediated growth of Ag nanocrystals.

In the shape-controlled synthesis of colloidal Ag nanocrystals, structure-directing agents, particularly polyvinylpyrrolidone (PVP), are known to be a key additive in making nanostructures with well-defined shapes. Although many Ag nanocrystals have been successfully synthesized using PVP, the mechanism by which PVP actuates shape control remains elusive. Here, we present a multi-scale theoretical framework for kinetic Wulff shape predictions that accounts for the chemical environment, which we used to probe the kinetic influence of the adsorbed PVP film. Within this framework, we use umbrella-sampling molecular dynamics simulations to calculate the potential of mean force and diffusion coefficient profiles of Ag atom deposition onto Ag(100) and Ag(111) in ethylene glycol solution with surface-adsorbed PVP. We use these profiles to calculate the mean-first passage times and implement extensive Brownian dynamics simulations, which allows the kinetic effects to be quantitatively evaluated. Our results show that PVP films can regulate the flux of Ag atoms to be greater towards Ag(111) than Ag(100). PVP's preferential binding towards Ag(100) over Ag(111) gives PVP its flux-regulating capabilities through the lower free-energy barrier of Ag atoms to cross the lower-density PVP film on Ag(111) and enhanced Ag trapping by the extended PVP film on Ag(111). Under kinetic control, {100}-faceted nanocrystals will be formed when the Ag flux is greater towards Ag(111). The predicted kinetic Wulff shapes are in agreement with the analogous experimental system.

How Structure-Directing Agents Control Nanocrystal Shape: Polyvinylpyrrolidone-Mediated Growth of Ag Nanocubes.

The importance of structure-directing agents (SDAs) in the shape-selective synthesis of colloidal nanostructures has been well documented. However, the mechanisms by which SDAs actuate shape control are poorly understood. In the polyvinylpyrrolidone (PVP)-mediated growth of {100}-faceted Ag nanocrystals, this capability has been attributed to preferential binding of PVP to Ag(100). We use molecular dynamics simulations to probe the mechanisms by which Ag atoms add to Ag(100) and Ag(111) in ethylene glycol solution with PVP. We find that PVP induces kinetic Ag nanocrystal shapes by regulating the relative Ag fluxes to these facets. Stronger PVP binding to Ag(100) leads to a larger Ag flux to Ag(111) and cubic nanostructures through two mechanisms: enhanced Ag trapping by more extended PVP films on Ag(111) and a reduced free-energy barrier for Ag to cross lower-density films on Ag(111). These flux-regulating capabilities depend on PVP concentration and chain length, consistent with experiment.