THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF CHEMICAL ENGINEERING MOLECULAR DYNAMICS SIMULATION OF THE ORIENTED ATTACHMENT OF SILVER NANOPLATES IN PRESCENCE OF POLYVINYLPYRROLIDONE

In the experimental synthesis of silver nanoplates, the solution consists of small triangular Ag plate, Polyvinylpyrrolidone (PVP) and Polyethylene glycol (PEG) as solvent. The scope of this paper only focus on the interactions between Ag plate and PVP, where we investigated the Ag plate and PVP system in three different set ups of MD simulations. First is the MD simulations of single-plate, aiming to look at PVP chain length effects as well as Ag plate configuration effects. Second is the unbiased MD simulations of two-plates, which is to investigate how two Ag plates will interact with each other in the presence of PVP. Third is the biased MD simulations that pull two plates together to form attachment, then Umbrella Sampling was used to calculate the Potential Mean Force(PMF) of plate attachment with the presence of PVP. Each set up of the simulations gave different collective insights to aid the knowledge about the Oriented Attachment mechanism. From single plate equilibration simulations, the system with the least binding preference was the short PVP 5-mer chains on type A plate with slanted side. System that displayed highest binding preferences towards the plate flat faces over the plate sides was the system with long polymer chains (PVP 100-mer) on type B plate with HCP sides. The PVP chains preferred to bind on the flat surface because there is a larger number of possible conformations and the chains have larger degree of freedom or entropy. This way, the plate sides are less crowded and can be approached by another plate and undergo Oriented Attachment. From unbiased two-plates MD simulations, there were no successful attachments in any of the 8 systems run with PVP 100-mer even at ramped up temperature from T = 900 K to T = 1200 K. Insights from pull simulations and umbrella sampling showed that Major-Minor side attachment systems (similar to Oriented attachment) required smaller force constant of 1400 eV/Å. While the plate face-face attachment used larger force constant of 1500 eV/Å. The PMF calculated the energy barrier to be 57.0 ± 1.8 kcal/mol for Major-Minor side attachment system and 70.2 ± 3.0 kcal/mol for parallel face-face attachment. Confirming that Major-Minor side attachment or Oriented Attachment has lower energy barrier and is the more likely mechanism. The PMF for both systems showed energy local minima before the energy barriers. This could explain why the two-plates system unbiased runs were less successful because the systems were stuck at the local energy minimum. Possible future work include doing umbrella sampling and analysis for systems at T = 1200 K. To further study PVP binding behavior onto surface and examine how a single PVP 100-mer chain lay itself onto the plate can help predict the binding preferences. Also, PEG can be added to investigate solvent effects. Plate-PVP-PEG system is more similar to experimental system and therefore will describe the system more accurately. In a far future where the system is more well understood, it is possible to switch the force field to reactive force fields, which are more representative of experimental systems.