Shape-Selective Syntheses of Gold and Copper Nanostructures: Insights from Density-Functional Theory and Molecular Dynamics

Open Access
Author:
Liu, Shih Hsien
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 06, 2016
Committee Members:
  • Kristen Ann Fichthorn, Dissertation Advisor
  • Kristen Ann Fichthorn, Committee Chair
  • Michael John Janik, Committee Member
  • Robert Martin Rioux Jr., Committee Member
  • Raymond Edward Schaak, Outside Member
Keywords:
  • hexadecylamine
  • copper
  • nanostructure
  • force field
  • molecular dynamics
  • density-functional theory
  • polyvinylpyrrolidone
  • gold
  • reconstruction
Abstract:
Density-functional theory (DFT) and molecular dynamics (MD) were used to resolve the origins of shape-selective syntheses of {111}-faceted Au nanostructures mediated by polyvinylpyrrolidone (PVP) as well as {100}-faceted Cu nanostructures mediated by hexadecylamine(HDA) seen in experiment. For the work in PVP on Au surfaces, the hexagonal reconstruction of Au(100) was considered. DFT results indicate that the Au(111) surface covered by the PVP segment, 2-pyrrolidone (2P), has a lower surface energy than the 2P-covered (5 × 1) Au(100)-hex surface, and that PVP may exhibit a binding affinity for Au(111) comparable to or greater than (5 × 1) Au(100)-hex. With MD, it is shown that the PVP-covered Au(111) surface has a lower surface energy than the PVP-covered (5 × 1) Au(100)-hex surface, and that the atactic PVP isosamer chains have a binding affinity for Au(111) comparable to (5 × 1) Au(100)-hex. Also, the (5 × 1) Au(100)-hex surface may have a higher flux of Au atoms than the Au(111) surface. Therefore, the Au(111) surface would be thermodynamically and kinetically favored in PVP-mediated syntheses, leading to {111}-faceted Au nanostructures. For the work in HDA on Cu surfaces, DFT results show that the HDA-covered Cu(100) surface has a slightly higher surface energy than the HDA-covered Cu(111) surface. However, HDA has a significant binding preference on Cu(100) over Cu(111). Therefore, the Cu(100) surface would be kinetically favored in HDA-mediated syn- theses, leading to {100}-faceted Cu nanostructures. Further, a metal-organic many-body (MOMB) force field for HDA-Cu interactions was developed based on the DFT work, and the force field was used to resolve the HDA binding patterns on Cu(100) at molecular level. With MD, it is found that decylamine (DA) may be used as an effective capping agent in the synthesis of {100}-faceted Cu nanostructures since DA as well as HDA are organized on Cu surfaces and have the same binding preference on Cu(100) over Cu(111). It is also found that the HDA structures on Cu surfaces remain intact in aqueous solution due to hydrophobicity of alkyl tails and long alkyl chains in the HDA molecules, which could prevent Cu oxidation during the synthesis.