Article pubs.acs.org/JPCC
Facet Selectivity of Ligands on Silver Nanoplates: Molecular Mechanics Study Zhiye Tang, Qiao Zhang,† Yadong Yin, and Chia-en A. Chang* Department of Chemistry, University of California, Riverside, California 92521, United States S Supporting Information *
ABSTRACT: Colloidal nanomaterials with well-defined shapes have wide applications in many fields. However, the exact role of capping ligands, which often dictates the shape of products in colloidal syntheses, is often unclear. Here we use a classical molecular-mechanics force-field method, mining minima (M2), to compute the binding free energy of the ligands such as citrate, monocarboxylates, dicarboxylates, and tricarboxylates to both (111) and (100) facets of silver and to investigate the mechanisms of the anisotropic growth of silver nanoplates. The distribution of partial charges on a ligand, the geometry complementation in the complex, and the entropic penalty on binding played crucial roles in discriminating the two facets and determining a good or poor ligand. Our finding allows rational design of capping ligands that may perform as well as citrate in promoting the anisotropic growth of nanoplates; however, designing a compound that outperforms citrate is found to be challenging.
1. INTRODUCTION
Previous studies modeled interactions between ligand molecules and metal nanoplates with ab initio calculations or classical molecular mechanics (MM) methods. Density functional theory (DFT) was used to study interactions between ligands and Ag nanoplates. For example, the DFT electronic structure calculations were applied to study the binding of citric acid on Ag nanoplates.30 The calculation showed preferential binding of citric acid to the (111) rather than (100) facet, with stronger binding to the (111) facet promoting crystal growth along the (100) facet. The work also provided a structure of citric acid on the Ag nanoplate surface. However, the DFT calculations need to assume that citric acid is in its neutral form, which differs from actual synthesis conditions where the ligand molecules are in the deprotonated form.29 In addition, the DFT calculation focused on only one conformation of citric acid, not considering possible contributions from other molecular conformations. The DFT method was also used to study the role of polyvinylpyrrolidone (PVP) in the shape-selective synthesis of Ag nanoplates.31 In contrast to citrate, PVP binds more strongly to the (100) than (111) facet.32 The DFT calculations suggested that electrostatic or van der Waals (vdW) forces govern the ligand−Ag nanoplate binding. Although experimentally, the synthesis of Ag nanoplates was carried out in solvent, DFT studies treated the ligands and silver plate systems in vacuum and neglected the influence of solvents. DFT has also been used to explain the surfaceenhanced Raman scattering (SERS) results of adenine33,34 and 5-amino tetrazole35 absorptions on a Ag or gold (Au) surface. In these works, the interaction between adenine and the noble metal surface was treated as the interaction between adenine
Inorganic nanostructures often display shape-dependent physiochemical properties,1 which promise them for many elegant and practical applications in fundamental sciences and industry.2 Silver nanoplates, a classic example of shapecontrolled nanostructures, show tunable surface plasmon resonance2,3 and can find uses in biomedicine,4 catalysis,5 microelectronics and data storage,6 single-molecule labelingbased biological assays,7,8 LED materials,9 and lasers.7 Because of the numerous applications, substantial efforts have been spent on synthesis of silver nanoplates with well-controlled dimensions. Reported methods include ligand-assisted chemical reductions,10−12 electrochemical synthesis,13,14 photoinduced method,3,15−20 sonochemical routes,21 solvothermal method,22,23 and templating method.24−26 The ligand-assisted chemical reduction method has been the most popular one because of its high yield and relatively simple setup. Citrate, often considered as a “magic” reagent in the solution phase synthesis of silver nanoplates, has been studied for years, but its role in directing plate growth remains unclear. Selective surface protection is one of the hypotheses for explaining the mechanism.27 In this theorem, a core with both (111) and (100) facets is used, and citrate preferentially protects the (111) facet in a nonbonded manner; thus, Ag atoms deposit on other facets such as (100) and (110) selectively.27,28 During the process, (111) facet actually grows thicker to some extent, so the interaction between citrate and the Ag surface is noncovalent rather than covalent or ionic.28 Several other dicarboxylates or tricarboxylates are eligible ligands and presumably play the same roles as citrate in protecting a facet selectively.29 However, one needs to show that good ligands can tightly bind to the Ag nanoplate noncovalently and discriminate one facet for anisotropic plate growth. © 2014 American Chemical Society
Received: April 3, 2014 Revised: August 19, 2014 Published: August 25, 2014 21589
dx.doi.org/10.1021/jp503319s | J. Phys. Chem. C 2014, 118, 21589−21598
The Journal of Physical Chemistry C
Article
molecules have a basic form (COO−). In normal experimental conditions yielding Ag nanoplates, the pH of the solution was typically 9.0 initially and increased to ∼9.7 near the end of the experiments. No Ag nanoplate was synthesized if the pH of the solution was
