ABSTRACT
Metal nanocrystals (NCs) produced by colloidal synthesis have a variety of structural features, such as different planes, edges, and defects. Even from the best colloidal syntheses, these characteristics are distributed differently in each NC. This inherent heterogeneity can play a significant role in the properties displayed by NCs. This manuscript reports the use of electrochemistry to synthesize Au NCs in a system evaluated to track individual NC growth trajectories as a first step toward rapid identification of NCs with different structural features. Au nanocubes were prepared colloidally and deposited onto a glassy carbon electrode using either electrospray or an airbrush, resulting in well-spaced Au nanocubes. The Au nanocubes then served as seeds as gold salt was reduced to deposit metal at constant potential. Deposition at constant potential facilitates overgrowth on the Au nanocubes to achieve new NC shapes. The effects of applied potential, deposition time, precursor concentration, and capping agents on NC shape evolution were studied. The outcomes are correlated to results from traditional colloidal syntheses, providing a bridge between the two synthetic strategies. Moreover, scanning electron microscopy was used to image the same NCs before and after deposition, linking individual seed features to differences in deposition. This ability is anticipated to enable tracking of individual growth trajectories of NCs to elucidate sources of heterogeneity in NC syntheses.
ABSTRACT
Galvanic replacement (GR) of monometallic nanoparticles (NPs) provides a versatile route to interesting bimetallic nanostructures, with examples such as nanoboxes, nanocages, nanoshells, nanorings, and heterodimers reported. The replacement of bimetallic templates by a more noble metal can generate trimetallic nanostructures with different architectures, where the specific structure has been shown to depend on the relative reduction potentials of the participating metals and lattice mismatch between the depositing and template metal phases. Now, the role of reaction stoichiometry is shown to direct the overall architecture of multimetallic nanostructures produced by GR with bimetallic templates. Specifically, the number of initial metal islands deposited on a NP template depends on the reaction stoichiometry. This outcome was established by studying the GR process between intermetallic PdCu (i-PdCu) NPs and either AuCl2- (Au1+) or AuCl4- (Au3+), producing i-PdCu-Au heterostructures. Significantly, multiple Au domains form in the case of GR with AuCl2- while only single Au domains form in the case of AuCl4-. These different NP architectures and their connection to reaction stoichiometry are consistent with Stranski-Krastanov (SK) growth, providing general guidelines on how the conditions of GR processes can be used to achieve multimetallic nanostructures with different defined architectures.
ABSTRACT
Colloidally prepared core@shell nanoparticles (NPs) were converted to monodisperse high entropy alloy (HEA) NPs by annealing, including quinary, senary, and septenary phases comprised of PdCuPtNi with Co, Ir, Rh, Fe, and/or Ru. Intraparticle heterogeneity, i.e., subdomains within individual NPs with different metal distributions, was observed for NPs containing Ir and Ru, with the phase stabilities of the HEAs studied by atomistic simulations. The quinary HEA NPs were found to be durable catalysts for the oxygen reduction reaction, with all but the PdCuPtNiIr NPs presenting better activities than commercial Pt. Density functional theory (DFT) calculations for PdCuPtNiCo and PdCuPtNiIr surfaces (the two extremes in performance) found agreement with experiment by weighting the adsorption energy contributions by the probabilities of each active site based on their DFT energies. This finding highlights how intraparticle heterogeneity, which we show is likely overlooked in many systems due to analytical limitations, can be leveraged toward efficient catalysis.
