First-Principles Calculations of the Role of PVP In the Controlled Synthesis of Colloidal Ag Nanostructures

We employ first-principles, density-functional theory (DFT) calculations to resolve the role of polyvinylpyrrolidone (PVP) in the controlled synthesis of colloidal Ag nanostructures. This polymer plays a central but poorly understood role in the selective formation of a large variety of metal nanostructures.  One hypothesis is that PVP is an effective structure-directing agent because of its surface-sensitive binding.  Examining the case of Ag, it has been proposed that PVP binds more strongly to Ag(100) than to Ag(111).  To study this binding, we divide the PVP repeat unit into two logical submolecules:  2-pyrrolidone and ethane.  While the interaction of ethane with Ag surfaces is almost purely van der Waals (vdW) in nature, the interaction of 2-pyrrolidone with Ag surfaces involves both direct bonding and vdW interactions. We compare two methods for including vdW interactions in DFT:  Grimme's method and the Tkatchenko-Scheffler method and find the latter method to be the most suitable.  We find that the binding of 2-pyrrolidone is stronger on Ag(100) than on Ag(111), consistent with experimental expectations.  Our studies indicate that a rich interplay between direct bonding, vdW attraction, and Pauli repulsion is responsible for this surface sensitivity.