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.

本研究采用伏安法、离线X射线光电子能谱（ex situ XPS）与密度泛函理论（DFT）计算，探究了聚乙烯吡咯烷酮（polyvinylpyrrolidone, PVP）在一系列结构明确的铂单晶电极表面的不可逆吸附行为。研究发现，PVP的吸附具有显著的结构敏感性：台阶位点与{100}晶面平台位点吸附作用极强，而在低PVP表面浓度条件下，PVP与Pt{111}晶面平台仅存在微弱相互作用。后续通过CO电氧化实验进一步验证，PVP对铂表面位点的阻塞效应在Pt{100}晶面平台上极为显著，但在Pt{111}晶面平台上几乎未发生。密度泛函理论计算同样证实，PVP单体与Pt{100}位点的相互作用强度高于Pt{111}位点（结合能分别为−142与−125 kJ·mol⁻¹）。离线X射线光电子能谱分析表明，PVP与铂的主要相互作用机制为PVP的羰基取代基向金属表面发生电荷转移，这与此前关于PVP在多晶铂表面吸附的研究结果一致。通过探究在有无PVP表面修饰的Pt{hkl}表面上铂吸附原子的不可逆沉积行为，发现PVP修饰的Pt{100}表面会显著偏好{100}晶面的形成，但当存在PVP时，Pt{111}表面并未出现对应的{111}晶面优先生长现象。综上，PVP在促进铂纳米立方体形成过程中所展现的形貌调控效应，可明确归因于低PVP表面负载量下，PVP在Pt{111}晶面上的化学吸附作用相对较弱。