Explicit Detection of the Mechanism of Platinum Nanoparticle Shape Control by Polyvinylpyrrolidone

The irreversible adsorption of polyvinylpyrrolidone
(PVP) on a
series of well-defined platinum single crystal electrode surfaces
has been investigated using voltammetry, ex situ XPS and DFT calculations.
It is found that the adsorption of PVP is highly structure sensitive
with strong adsorption exhibited by step and {100} terrace sites with
only weak interactions observed between PVP and Pt{111} terraces,
at least at low PVP surface concentrations. Subsequent investigations
using CO electrooxidation confirmed that blocking of platinum surface
sites by PVP toward CO chemisorption was marked for Pt{100} terraces
but hardly occurred at all at Pt{111} terraces. Density Functional
Theory calculations also confirmed that the monomer of PVP interacts
more strongly with Pt{100} compared to Pt{111} sites (−142
and −125 kJ mol–1 respectively). Ex situ
XPS studies suggested that the main PVP–Pt interaction is associated
with charge transfer from the carbonyl substituent of PVP toward the
metal surface in accordance with earlier studies of PVP adsorbed on
polycrystalline platinum surfaces. Irreversible adsorption of Pt adatoms
onto Pt­{hkl} surfaces with and without PVP–surface
modification demonstrated a marked preference for {100} facet formation
on Pt{100} surfaces but no corresponding preferential {111} facet
growth on Pt{111} when PVP was present. Hence, the shape control exhibited
by PVP in expediting the formation of cubic Pt nanoparticles is explicitly
confirmed as arising from relatively weak PVP chemisorption on Pt{111}
facets at low PVP surface loading.