(393al) First-Principles Calculations of the Role of PVP and PEO in the Controlled Synthesis of Colloidal Ag Nanostructures

Inorganic colloidal nanomaterials exhibit size- and shape-dependent
properties with enormous potential in a variety of applications
including sensing, catalysis, and electronics.  Solution-phase polyol
synthesis has become a popular method for preparing noble-metal
nanostructures with well-defined morphologies.  The key step in the
polyol process is in the presence of a polymeric capping agent that
determines the relative growth rates of the different crystallographic
facets of the nanoparticles. This capping mechanism depends on the
nature of the structure-directing agent and its weight percentage with
respect to the polyol.  We use density-functional theory to elucidate
the roles of polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO)
polymers as structure-directing agents in the shape-selective
synthesis of Ag nanostructures.  At the polymer segment level, we
identify several different binding conformers on Ag(100) and Ag(111),
and show they both have energetic preference for the {100} facet. This
preference results from a surface-sensitive balance between direct
binding and van der Waals attraction.  At the polymer chain level,
correlated segment binding leads to a strong preference for PVP bind
to Ag(100) but not for PEO.  Our study underscores differences between
small-molecule and polymeric structure-directing agents, and explains
why PVP is a better structure directing agent than PEO.