An Experimental and Theoretical Investigation of the Inversion of Pd@Pt Core@Shell Dendrimer-Encapsulated Nanoparticles

Bimetallic PdPt dendrimer-encapsulated nanoparticles (DENs) having sizes of about 2 nm were synthesized by a homogeneous route that involved (1) formation of a Pd core, (2) deposition of a Cu shell onto the Pd core in the presence of H₂ gas, and (3) galvanic exchange of Pt for the Cu shell. Under these conditions, a Pd@Pt core@shell DEN is anticipated, but detailed characterization by <i>in</i>-<i>situ</i> extended X-ray absorption fine structure (EXAFS) spectroscopy and other analytical methods indicate that the metals invert to yield a Pt-rich core with primarily Pd in the shell. The experimental findings correlate well with density functional theoretical (DFT) calculations. Theory suggests that the increased disorder associated with <∼2 nm diameter nanoparticles, along with the relatively large number of edge and corner sites, drives the structural rearrangement. This type of rearrangement is not observed on larger nanoparticles or in bulk metals

Electrochemical Activity of Dendrimer-Stabilized Tin Nanoparticles for Lithium Alloying Reactions

The synthesis and characterization of Sn nanoparticles in organic solvents using sixth-generation dendrimers modified on their periphery with hydrophobic groups as stabilizers are reported. Sn²⁺:dendrimer ratios of 147 and 225 were employed for the synthesis, corresponding to formation of Sn₁₄₇ and Sn₂₂₅ dendrimer-stabilized nanoparticles (DSNs). Transmission electron microscopy analysis indicated the presence of ultrasmall Sn nanoparticles having an average size of 3.0–5.0 nm. X-ray absorption spectroscopy suggested the presence of Sn nanoparticles with only partially oxidized surfaces. Cyclic voltammetry studies of the Sn DSNs for Li alloying/dealloying reactions demonstrated good reversibility. Control experiments carried out in the absence of DSNs clearly indicated that these ultrasmall Sn DSNs react directly with Li to form SnLi alloys

Doped Amorphous Ti Oxides To Deoptimize Oxygen Reduction Reaction Catalysis

The oxygen reduction reaction (ORR) is a major factor that drives galvanic corrosion. To better understand how to tune materials to better inhibit catalytic ORR, we have identified an <i>in silico</i> procedure for predicting elemental dopants that would cause common, natively formed titanium oxides to better suppress this reaction. In this work, we created an amorphous TiO₂ surface model that is in good agreement with experimental radial distribution function data and contains reaction sites capable of replicating experimental ORR overpotentials. Dopant performance trends predicted with our quantum chemistry model mirrored experimental results, and our top three predicted dopants (Mn, Al, and V, each present at doping concentrations of 1%) were experimentally verified to lower ORR currents under alkaline conditions by up to 77% vs the undoped material. These results show the robustness of calculated thermodynamic descriptors for identifying poor, TiO₂-based ORR catalysts. This also opens the possibility of using quantum chemistry to guide the design of coating materials that would better resist the ORR and presumably galvanic corrosion

In Situ Probing of the Active Site Geometry of Ultrathin Nanowires for the Oxygen Reduction Reaction

To create truly effective electrocatalysts for the cathodic reaction governing proton exchange membrane fuel cells (PEMFC), namely the oxygen reduction reaction (ORR), necessitates an accurate and detailed structural understanding of these electrocatalysts, especially at the nanoscale, and to precisely correlate that structure with demonstrable performance enhancement. To address this key issue, we have combined and interwoven theoretical calculations with experimental, spectroscopic observations in order to acquire useful structural insights into the active site geometry with implications for designing optimized nanoscale electrocatalysts with rationally predicted properties. Specifically, we have probed ultrathin (∼2 nm) core–shell Pt∼Pd₉Au nanowires, which have been previously shown to be excellent candidates for ORR in terms of both activity and long-term stability, from the complementary perspectives of both DFT calculations and X-ray absorption spectroscopy (XAS). The combination and correlation of data from both experimental and theoretical studies has revealed for the first time that the catalytically active structure of our ternary nanowires can actually be ascribed to a PtAu∼Pd configuration, comprising a PtAu binary shell and a pure inner Pd core. Moreover, we have plausibly attributed the resulting structure to a specific synthesis step, namely the Cu underpotential deposition (UPD) followed by galvanic replacement with Pt. Hence, the fundamental insights gained into the performance of our ultrathin nanowires from our demonstrated approach will likely guide future directed efforts aimed at broadly improving upon the durability and stability of nanoscale electrocatalysts in general