Building Durable Multimetallic Electrocatalysts from Intermetallic Seeds.

ConspectusWhen combined with earth-abundant metals, Pt-based alloy nanoparticles (NPs) can be cost-effective electrocatalysts. However, these NPs can experience leaching of non-noble-metal components under harsh electrocatalytic conditions. The Skrabalak group has demonstrated a novel NP construct in which Pt-based random alloy surfaces are stabilized against non-noble-metal leaching by their deposition onto intermetallic seeds. These core@shell NPs are highly durable electrocatalysts, with the ability to tune catalytic performance by the core@shell architecture, surface alloy composition, and NP shape. This versatility was demonstrated in a model system in which random alloy (ra-) PtM surfaces were deposited onto ordered intermetallic (i-) PdCu seeds using seed-mediated co-reduction (SMCR). In the initial demonstration, ra-PtCu shells were deposited on i-PdCu seeds, with these core@shell NPs exhibiting higher specific and mass activities for the oxygen reduction reaction (ORR) when compared to similarly sized ra-PtCu NPs. These NPs also showed outstanding durability, maintaining ∼85% in specific activity after 5000 cycles. Characterization of the NPs after use revealed minimal loss of Cu. The activity enhancement was attributed to the strained surface that arises from the lattice mismatch between the intermetallic core and random alloy surface. The outstanding durability was attributed to the ordered structure of the intermetallic core.The origin of this durability enhancement was investigated by classical molecular dynamics simulations, where Pt atoms were found to have a lower potential energy when deposited on an intermetallic core than when deposited on a random alloy core. Also, ordering of Cu atoms at the core@shell interface appears to enhance the overall binding between the core and the shell materials. Inspired by this initial demonstration, SMCR has been used to achieve shells of different random alloy compositions, PtM (M = Ni, Co, Cu, or Fe). This advance is significant because ligand effects vary as a function of PtM identity and Pt/M ratio. These features also influence the degree of surface strain imparted from the lattice mismatch between the core and shell materials. Like the initial demonstration, standout features of these core@shell NPs were high durability and resistance to non-noble metal leaching.Moving forward, efforts have been directed toward integrating shape-control to this core@shell NP construct. This integration is motivated by the shape-dependent catalytic performance of NPs derived from the selective expression of specific facets. Considering the initial i-PdCu@ra-PtCu system, NPs with a cubic shape have been achieved by judicious selection of capping ligands during SMCR. Evaluation of these NPs as catalysts for the electrooxidation of formic acid found that the nanocubic shape enhances catalytic performance compared to similar core@shell NPs with a spherical morphology. We envision that SMCR can be applied to other NP systems to achieve highly durable catalysts as the syntheses of monodisperse and shape-controlled intermetallic seeds are advanced. This Account highlights the role of intermetallic cores in providing more durable electrocatalysts. More broadly, the versatility of SMCR is highlighted as a route to integrate architecture, alloy surfaces, and shape within one NP system, and how this achievement is inspiring new high-performance and robust catalysts is discussed.

Mechanochemical Syntheses of Ln(hfac)3(H2O)x (Ln = La-Sm, Tb): Isolation of 10-, 9-, and 8-Coordinate Ln(hfac)n Complexes.

Volatile lanthanide coordination complexes are critical to the generation of new optical and magnetic materials. One of the most common precursors for preparing volatile lanthanide complexes is the hydrate with the general formula Ln(hfac)3(H2O)x (x = 3 for La-Nd, x = 2 for Sm) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato). We have investigated the synthesis of Ln(hfac)3(H2O)x using more environmentally sustainable mechanochemical approaches. Characterization of the products using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, elemental analysis, and powder X-ray diffraction shows substantial differences in product distribution between methods. The mechanochemical synthesis of the hydrate complexes leads to a variety of coordination compounds including the expected hydrate product, the known retro-Claisen impurity, and hydrated protonated Hhfac ligand depending on the technique employed. Surprisingly, 10-coordinate complexes of the form Na2Ln(hfac)5·3H2O for Ln = La-Nd were also isolated from reactions using a mortar and pestle. The electrostatic bonding of lanthanide coordination complexes is a challenge for obtaining reproducible reactions and clean products. The reproducibility issues are most acute for the large, early lanthanides whereas for the mid to late lanthanides, reproducibility in terms of product distribution and yield is less of an issue because of their smaller size and greater charge to radius ratio. Ball milling increases reproducibility in terms of generating the desired Ln(hfac)3(H2O)x along with hydrated Hhfac (tetraol) and free Hhfac products. The results illustrate the dynamic behavior of lanthanide complexes in solution and the solid state as well as the structural diversity available to the early lanthanides.

Disorder-to-Order Transition Mediated by Size Refocusing: A Route toward Monodisperse Intermetallic Nanoparticles.

Intermetallic nanoparticles are remarkable due to their often enhanced catalytic, magnetic, and optical properties, which arise from their ordered crystal structures and high structural stability. Typical syntheses of intermetallic nanoparticles include thermal annealing of the disordered counterpart in atmosphere (or vacuum) or colloidal syntheses, where the phase transformation is achieved in solution. Although both methods can produce intermetallic nanoparticles, there is difficulty in achieving monodisperse nanoparticles, which is critical to exploiting their properties for various applications. Here, we show that overgrowth on random alloy AuCu nanoparticles mediated by size refocusing yields monodisperse intermetallic AuCu nanoparticles. Size refocusing has been used in syntheses of semiconductor and upconverting nanocrystals to achieve monodisperse samples, but now we demonstrate size refocusing as a mechanism to achieve the disorder-to-order phase transformation in multimetallic nanoparticles. The phase transformation was monitored by time evolution experiments, where analysis of reaction aliquots with transmission electron microscopy and powder X-ray diffraction revealed the generation and dissolution of small nanoparticles coupled with an increase in the average size of the nanoparticles and conversion to the ordered phase. This demonstration advances the understanding of intermetallic nanoparticle formation in colloidal syntheses, which can expedite the development of electrocatalysts and magnetic storage materials.

Defect-Directed Growth of Symmetrically Branched Metal Nanocrystals.

Branched plasmonic nanocrystals (NCs) have attracted much attention due to electric field enhancements at their tips. Seeded growth provides routes to NCs with defined branching patterns and, in turn, near-field distributions with defined symmetries. Here, a systematic analysis was undertaken in which seeds containing different distributions of planar defects were used to grow branched NCs in order to understand how their distributions direct the branching. Characterization of the products by multimode electron tomography and analysis of the NC morphologies at different overgrowth stages indicate that the branching patterns are directed by the seed defects, with the emergence of branches from the seed faces consistent with minimizing volumetric strain energy at the expense of surface energy. These results contrast with growth of branched NCs from single-crystalline seeds and provide a new platform for the synthesis of symmetrically branched plasmonic NCs.

Identification of Nanoscale Processes Associated with the Disorder-to-Order Transformation of Carbon-Supported Alloy Nanoparticles.

Due to their ordered crystal structures and high structural stabilities, intermetallic nanoparticles often display enhanced catalytic, magnetic, and optical properties compared to their random alloy counterparts. Intermetallic nanoparticles can be achieved by thermal annealing of their disordered (random alloy) counterparts. However, high temperatures and long annealing times needed to achieve the disorder-to-order transition often lead to a loss of sample monodispersity and an increase in the average size of nanoparticles. Here, we performed ex situ powder X-ray diffraction (XRD) and in situ annealing transmission electron microscopy (TEM) experiments to elucidate nanoscale processes that contribute to the ordering of carbon-supported PdCu nanoparticles as a model system. Random alloy PdCu nanoparticles supported on carbon were thermally annealed for various lengths of time at the disorder-to-order phase transition temperature, where changes in nanoparticle size and the crystal phase were monitored. The nanoparticles were only completely transformed to the intermetallic phase by undertaking measures to deliberately increase their size by increasing the number of nanoparticles on the carbon support. In situ annealing TEM experiments reveal nanoscale processes that account for the disorder-to-order phase transformation. Five different processes were observed at 400 °C. Isolated nanoparticles remained in the random alloy phase or underwent a phase transformation to the intermetallic phase. Nanoparticles fused with neighboring nanoparticles resulting in no change in phase or conversion to the intermetallic phase. Evidence of vapor transport was also observed, as some isolated nanoparticles were found to diminish in size upon heating. These variable processes account for the heterogeneity often observed for intermetallic nanoparticle samples achieved through annealing and motivate the development of synthetic routes that suppress particle-particle coalescence, as well as investigating metal-support interactions to facilitate the disorder-to-order phase transformation under mild conditions. Overall, this work furthers our knowledge of the formation of intermetallic nanoparticles by thermal annealing approaches, which could accelerate the development of electrocatalysts and the application of intermetallic nanoparticles in magnetic storage devices.

Galvanic replacement of intermetallic nanocrystals as a route toward complex heterostructures.

Galvanic replacement reactions are a reliable method for transforming monometallic nanotemplates into bimetallic products with complex nanoscale architectures. When replacing bimetallic nanotemplates, even more complex multimetallic products can be made, with final nanocrystal shapes and architectures depending on multiple processes, including Ostwald ripening and the Kirkendall effect. Galvanic replacement, therefore, is a promising tool in increasing the architectural complexity of multimetallic templates, especially if we can identify and control the relevant processes in a given system and apply them more broadly. Here, we study the transformation of intermetallic PdCu nanoparticles in the presence of HAuCl4 and H2PtCl6, both of which are capable of oxidizing both Pd and Cu. Replacement products consistently lost Cu more quickly than Pd, preserved the crystal structure of the original intermetallic template, and grew a new phase on the sacrificial template. In this way, atomic and nanometer-scale architectures are integrated within individual nanocrystals. Product morphologies included faceting of the original spherical particles as well as formation of core@shell and Janus-style particles. These variations are rationalized in terms of differing diffusion behaviors. Overall, galvanic replacement of multimetallic templates is shown to be a route toward increasingly exotic particle architectures with control exerted on both Angstrom and nanometer-scale features, while inviting further consideration of template and oxidant choices.

Synthesis of monodisperse high entropy alloy nanocatalysts from core@shell nanoparticles.

High-entropy alloy (HEA) nanoparticles (NPs) hold great promise in electrocatalysis because of their nearly unlimited compositions, tailorable active sites, and high durability. However, the synthesis of these compositionally complex structures as monodisperse NPs remains a challenge by colloidal routes because the different rates of metal precursor reduction lead to phase separation. Here, we report the conversion of core@shell NPs into HEA NPs through annealing, with conservation of sample monodispersity. This potentially general route for high-quality HEA NPs was demonstrated by preparing PdCu@PtNiCo NPs via seed-mediated co-reduction, wherein Pt, Ni, and Co were co-deposited on PdCu seeds in solution. These multimetallic NPs were then converted to single-crystalline and single-phase PdCuPtNiCo NPs through annealing. On account of their small particle size, highly dispersed Pt/Pd content, and low elemental diffusivity, these HEA NPs were found to be a highly efficient and durable catalyst for the oxygen reduction reaction. They were also highly selective for the four-electron transfer pathway. We expect that this new synthetic strategy will facilitate the synthesis of new HEA NPs for catalysis and other applications.

Kinetically Controlled Sequential Seeded Growth: A General Route to Crystals with Different Hierarchies.

The organization of natural materials into hierarchical structures accounts for the amazing properties of many biological systems; however, translating the structural motifs present in such natural materials to synthetic systems remains difficult. Inspired by how nature creates materials, this work demonstrates that kinetically controlled sequential seeded growth is a general bottom-up strategy to prepare hierarchical inorganic crystals with distinct compositions and nanostructured forms. Specifically, 85 distinct hierarchical crystals with different shape-controlled features, compositions, and overall symmetries were readily achieved by altering the kinetics of metal deposition in sequential rounds of seeded growth. These modifications in the deposition kinetics were achieved through simple changes to the reaction conditions (e.g., pH or halide concentration) and dictate whether concave or convex features are produced at specific seed locations, much in the manner that the changing atmospheric conditions account for the hierarchical and symmetrical structures of snow crystals. As such, this work provides a general paradigm for the bottom-up synthesis of hierarchical crystals regardless of inorganic material class.

Intermetallic Pd3Pb nanocubes with high selectivity for the 4-electron oxygen reduction reaction pathway.

Pd-Based nanoparticles are excellent alternatives to the typically used Pt-based materials that catalyze fuel cell reactions. Specifically, Pd-based intermetallic nanomaterials have shown great promise as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media; however, their synthesis remains a challenge and shape-controlled nanoparticles are limited. Here, a low-temperature approach to intermetallic Pd3Pb nanocubes is demonstrated and their electrocatalytic properties evaluated for the ORR. The intermetallic Pd3Pb nanocubes outperformed all reference catalysts, with a mass activity of 154 mA mgPd-1 which is a 130% increase in activity compared to the commercial Pd/C reference and a 230% increase compared to Pd nanocubes. Tafel analysis reveals that the Pd3Pb nanocubes are highly selective for the 4-electron reduction pathway, with minimal HO2- formation. Density functional theory (DFT) calculations show that the increased activity for the intermetallic nanocubes compared to Pd is likely due to the weakening of OH* adsorption, decreasing the required overpotential. These results show that intermetallic Pd3Pb nanocubes are highly efficient for the 4-electron pathway of the ORR and could inspire the study of other shape-controlled intermetallics as catalysts for fuel cell applications.

Achieving Highly Durable Random Alloy Nanocatalysts through Intermetallic Cores.

Pt catalysts are widely studied for the oxygen reduction reaction, but their cost and susceptibility to poisoning limit their use. A strategy to address both problems is to incorporate a second transition metal to form a bimetallic alloy; however, the durability of such catalysts can be hampered by leaching of non-noble metal components. Here, we show that random alloyed surfaces can be stabilized to achieve high durability by depositing the alloyed phase on top of intermetallic seeds using a model system with PdCu cores and PtCu shells. Specifically, random alloyed PtCu shells were deposited on PdCu seeds that were either the atomically random face-centered cubic phase (FCC A1, Fm3m) or the atomically ordered CsCl-like phase (B2, Pm3m). Precise control over crystallite size, particle shape, and composition allowed for comparison of these two core@shell PdCu@PtCu catalysts and the effects of the core phase on electrocatalytic durability. Indeed, the nanocatalyst with the intermetallic core saw only an 18% decrease in activity after stability testing (and minimal Cu leaching), whereas the nanocatalyst with the random alloy core saw a 58% decrease (and greater Cu leaching). The origin of this enhanced durability was probed by classical molecular dynamics simulations of model catalysts, with good agreement between model and experiment. Although many random alloy and intermetallic nanocatalysts have been evaluated, this study directly compares random alloy and intermetallic cores for electrocatalysis with the enhanced durability achieved with the intermetallic cores likely general to other core@shell nanocatalysts.

Random Alloyed versus Intermetallic Nanoparticles: A Comparison of Electrocatalytic Performance.

As synthetic methods advance for metal nanoparticles, more rigorous studies of structure-function relationships can be made. Many electrocatalytic processes depend on the size, shape, and composition of the nanocatalysts. Here, the properties and electrocatalytic behavior of random alloyed and intermetallic nanoparticles are compared. Beginning with an introduction of metallic nanoparticles for catalysis and the unique features of bimetallic compositions, the discussion transitions to case studies of nanoscale electrocatalysts where direct comparisons of alloy and intermetallic compositions are undertaken for methanol electrooxidation, formic acid electrooxidation, the oxygen reduction reaction, and the electroreduction of carbon dioxide (CO2 ). Design and synthesis strategies for random alloyed and intermetallic nanoparticles are discussed, with an emphasis on Pt-M and Cu-M compositions as model systems. The differences in catalytic performance between alloys and intermetallic nanoparticles are highlighted in order to provide an outlook for future electrocatalyst design.