Research Highlight: Mirkin
Shape regulation of high-index facet nanoparticles by dealloying
Nanoparticle shape control is extremely important in catalysis as catalytic activity and selectivity are highly dependent on the arrangement of surface atoms. High-index facet nanoparticles are especially interesting due to the high-density of low-coordination surface steps and edges, which are catalytically favorable sites. However, challenges in their synthesis greatly hinder their commercial use. Conventional electrochemical methods are low-throughput, and solution-phase approaches, while higher yielding, result in ligand-passivated surfaces that negatively affect activity. Thus, a straightforward, ligand-free, and generalizable method for synthesizing high-index facet nanoparticles at scale has been needed.
Inspired by the observation that the underpotential deposition of trace amounts of shape-regulating metal elements can be used to synthesize high-index facet nanoparticles during solution phase synthesis, the Mirkin group developed a bulk-scale and ligand-free method for synthesizing Pt, Pd, Rh, Ni, Co, and bimetallic high-index facet tetrahexahedral (THH) nanoparticles using solid state foreign metal (Sb, Bi, Pb or Te) modification (Fig.1). To understand how and why these structures form, they studied this process via electron microscopy, X-ray spectroscopy, and modeled using density functional theory calculations that revealed that the trace foreign metal stabilized the {210} high-index facets of the resulting THH particles. A study of the PtSb system showed that the THH shape resulted from the evaporative removal of Sb from the initial Sb-rich PtSb alloy, a shape-regulating process that is fundamentally different from solution-phase, ligand-dependent processes. Remarkably, THH particles form regardless of the initial shape of the particles, making this method attractive for recycling waste catalysts that are not in industrially useful forms. Finally, Pt-M (M = Sb, Bi, Pb, Te) and Pd-Bi THH nanoparticles synthesized on carbon black powder were evaluated as catalysts for the electrooxidation of formic acid, a promising alternative fuel for fuel cells. Both high-index facet and foreign metal modification were determined to be favorable for enhancing the catalyst performance, and the synthesized THH catalysts were determined to be superior to state-of-the-art commercial Pt/C and Pd/C catalysts. Importantly, the current density of the THH Pt particles made from a commercial Pt/C catalyst is 20 times higher than that for the as-purchased catalyst at a fixed overpotential of 0.5 V. This work was recently reported in Science (2019, 365, 1159-1163).
Fig.1. (A) Scheme showing a library of monometallic and bimetallic alloy tetrahexahedra nanoparticles. (B) A false color scanning electron microscopy image of tetrahexahedral nanoparticles.