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.