Stability, Oxidation, and Shape Evolution of PVP-Capped Pd Nanocrystals

A critical aspect in the practical applications and catalytic performance of shape-controlled nanocrystals is their stability and morphology retention under ambient conditions. Changes to the morphology of shape-controlled Pd nanocrystals capped by PVP are assessed by TEM, and surface oxidation was evaluated by X-ray photoelectron spectroscopy (XPS) over 12 months. Surface oxidation of PVP-capped Pd nanocrystals resulted in the loss of edge and corner sites and a transition to spherical morphologies. The shape stability of the nanocrystals was found to follow the trend cubic < cuboctahedra < octahedral concave cubes. For low index planes, {111} surfaces were more resistant to oxidation compared to {100} facets, correlating with the surface free energy of the nanocrystals. Cubic and cuboctahedral nanocrystals transitioned to spherical particles while octahedral nanocrystals retained their morphology. The presence of high-energy {110} facets was observed in the cubic nanocrystals which undergo surface reconstruction. The presence of surface defects may also influence the rate of the structural changes. Concave cubic nanocrystals, which possess high index facets and surface energies, were consistently found to display excellent morphology retention. The concave cubic nanocrystals displayed superior shape stability and reduced oxidation compared to cubic and cuboctahedral nanocrystals. XPS analysis further determined that PVP capping ligands on different Pd surface facets strongly influenced the morphological consistency. The stability of the concave cubes can be attributed to the stronger chemisorption of PVP capping ligands to the high index planes, making them less susceptible to oxidation.