Intermetallic Nanocrystals: Syntheses and Catalytic Applications

Intermetallic Nanoparticles: Synthetic Control and Their Enhanced Electrocatalysis

Intermetallic nanoparticles (NPs) described in this Account are a class of metallic alloy NPs within which metal atoms are bonded via strong d-orbital interaction and ordered anisotropically in a specific crystallographic direction. Compared to the common metallic alloy NPs with solid solution structure, intermetallic NPs are generally more stable against chemical oxidation and etching. The strict stoichiometry requirement, well-defined atom binding environment and layered atomic arrangement also make intermetallic NPs an ideal model for understanding their physical and catalytic properties. This account summarizes the synthetic principles and strategies developed to obtain monodisperse intermetallic NPs, especially tetragonal L1₀-NPs. The thermodynamics and kinetics involved in the conversion between disordered and ordered structures are briefly discussed. The synthetic methods are grouped into two slightly different categories: solution-phase synthesis followed by solid state annealing and direct solution-phase synthesis. In the former method, high-surface-area supports are often needed to disperse NPs and to prevent them from aggregation, while in the latter method such supports are not required since the structure conversion temperature is lowered to a level that the conversion can proceed in the solution reaction condition. In any of these two synthetic approaches, various factors influencing intermetallic structure formation should be carefully controlled to ensure more complete structural transition within NPs. Using representative synthetic examples, we highlight the strategies explored to facilitate the formation of intermetallic structure, including the introduction of vacancies/defects within NP structures and the control of atom addition rate/seed-mediated diffusion to lower the energy barrier. These strategies illustrate how the concept of thermodynamics and kinetics can be used to design the synthesis of intermetallic NPs. Additionally, to correlate NP structure and catalysis, we introduce briefly the d-band theory to explain how the electronic, strain and ensemble effects can be used to tune NP catalysis. We focus specifically on Pt-, Pd-, and Au-based L1₀-NPs and demonstrate how these L1₀-NPs could be prepared to show much enhanced catalysis for electrochemical reactions, including oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), formic acid oxidation reaction (FAOR), and thermo-oxidation reaction of CO. Due to the enhanced metal atom stability in the "sandwich"-type structure, the roles of the first-row transition metal atoms in catalysis are better understood to achieve catalysis optimization. This concept can be extended to other alloy NPs, demonstrating great potentials in using intermetallic structures to control NP reduction and oxidation catalysis for important chemical and energy applications.

Tweaking the Interplay among Galvanic Exchange, Oxidative Etching, and Seed-Mediated Deposition toward Architectural Control of Multimetallic Nanoelectrocatalysts

Nanoscale galvanic exchange confined by metallic nanoparticles is an intriguing structure-remodeling process that transforms geometrically simple solid nanoparticles into multimetallic hollow nanoparticles with increased structural complexity and compositional diversity. Using liquid polyols with intrinsic reducing capabilities as the reaction medium for nanoparticle-templated galvanic exchange represents an interesting paradigm shift, allowing us to interface galvanic exchange with oxidative etching and seed-mediated deposition without introducing any additional oxidizing or reducing agents. By kinetically maneuvering the interplay among galvanic Cu-Pt exchange, oxidative Cu etching, and seed-mediated Pt deposition, we have been able to selectively transform AuCu₃ alloy nanoparticles into two architecturally distinct multimetallic heteronanostructures, namely, Au-Pt alloy skin-covered spongy nanoparticles and Pt nanodendrite-covered hollow nanoparticles, both of which exhibit unique structural features highly desirable for high-performance electrocatalysis. Using the formic acid oxidation and hydrogen evolution reactions in acidic electrolytes as model electrocatalytic reactions, we show that the multimetallic nanoparticles derived from AuCu₃ alloy nanoparticles through polyol-mediated galvanic exchange reactions markedly outperform the commercial Pt/C benchmark catalysts in terms of both activity and durability. This work not only provides important mechanistic insights on how galvanic exchange dynamically interplays with other redox processes to rigorously dictate the versatile structural transformations of multimetallic nanoparticles but also sheds light on the detailed structure-property relationships underpinning the intriguing electrocatalytic behaviors of architecturally complex multimetallic heteronanostructures.

Moving Frontiers in Transition Metal Catalysis: Synthesis, Characterization and Modeling

Nanosized transition metal particles are important materials in catalysis with a key role not only in academic research but also in many processes with industrial and societal relevance. Although small improvements in catalytic properties can lead to significant economic and environmental impacts, it is only now that knowledge-based design of such materials is emerging, partly because the understanding of catalytic mechanisms on nanoparticle surfaces is increasingly improving. A knowledge-based design requires bottom-up synthesis of well-defined model catalysts, an understanding of the catalytic nanomaterials "at work" (operando), and both a detailed understanding and a prediction by theoretical methods. This article reports on progress in colloidal synthesis of transition metal nanoparticles for preparation of model catalysts to close the materials gap between the discoveries of fundamental surface science and industrial application. The transition metal particles, however, often undergo extensive transformations when applied to the catalytic process and much progress has recently been achieved operando characterization techniques under relevant reaction conditions. They allow better understanding of size/structure-activity correlations in these systems. Moreover, the growth of computing power and the improvement of theoretical methods uncover mechanisms on nanoparticles and have recently predicted highly active particles for CO/CO₂ hydrogenation or direct H₂ O₂ synthesis.

Thermally induced transformations of Au@Cu2O core–shell nanoparticles into Au–Cu nanoparticles from temperature-programmed in situ powder X-ray diffraction

Temperature-programmed in situ X-ray diffraction with whole-powder-pattern modeling is used to investigate the reaction of Au@Cu2O core–shell nanoparticles to form nanocrystalline bimetallic Cu x Au1−x alloys (x = 0, 0.25, 0.5, 0.75, 1.0) in a reducing atmosphere. The mechanisms of the reactions are key to informed design of tailored non-equilibrium nanostructures for catalytic and plasmonic materials. The Au@Cu2O reaction is initiated by reduction of the Cu2O cuprite shell to form nanocrystalline metallic Cu at about 413 K. Alloying begins immediately upon formation of metallic Cu at 413 K, with the nucleation of an Au-rich alloy phase which reaches the nominal Cu content of the overall system stoichiometry by 493 K. All bimetallic alloys form a transient ordered Cu3Au intermetallic compound at intermediate temperatures, with the onset of ordering and subsequent disordering varying by composition. No evidence for an ordered Au3Cu intermetallic is found for any composition. Significant crystal growth in the bimetallic phase is apparent at higher temperatures, with the onset temperature increasing with Cu concentration and initial Cu-shell thickness. The reduction of the cuprite phase is slowed by the presence of the core–shell interface, and crystal growth in the Cu shell is completely suppressed within the alloy systems.

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.

Magnetically Hollow Pt Nanocages with Ultrathin Walls as a Highly Integrated Nanoreactor for Catalytic Transfer Hydrogenation Reaction

Fabricating efficient and stable nanocatalysts for chemoselective hydrogenation of nitroaromatics is highly desirable because the amines hold tremendous promise for the synthesis of nitrogen containing chemicals. Here, a highly reactive and stable porous carbon nitride encapsulated magnetically hollow platinum nanocage is developed with subnanometer thick walls (Fe3O4@snPt@PCN) for this transformation. This well-controlled nanoreactor is prepared via the following procedures: the preparation of core template, the deposition of platinum nanocage with subnanometer thick walls, oxidative etching, and calcination. This highly integrated catalyst demonstrates excellent performance for the catalytic transfer hydrogenation of various nitroaromatics and the reaction can reach >99% conversion and >99% selectivity. With the ultrathin wall structure, the atom utilization of platinum atoms is highly efficient. The X-ray photoelectron spectroscopy results indicate that partial electrons transfer from the iron oxides to Pt nanowalls, and this increases the electron density of snPt nanoparticles, thus promoting the catalytic activity for the transfer hydrogenation of nitroaromatics. For the reduction of 4-nitrophenol, the reaction rate constant K app is 0.23 min-1 and the turnover frequency (TOF) is up to 3062 h-1. Additional reaction results illustrate that this magnetic nanoreactor can be reused more than eight times and it is a promising catalytic nanoplatform in heterogeneous catalysis.

Heterobimetallic Single-Source Precursors: A Springboard to the Synthesis of Binary Intermetallics

Intermetallics are atomically ordered crystalline compounds containing two or more main group and transition metals. In addition to their rich crystal chemistry, intermetallics display unique properties of interest for a variety of applications, including superconductivity, hydrogen storage, and catalysis. Because of the presence of metals with a wide range of reduction potentials, the controlled synthesis of intermetallics can be difficult. Recently, soft chemical syntheses such as the modified polyol and ship-in-a-bottle methods have helped advance the preparation of these materials. However, phase-segregated products and complex multistep syntheses remain common. Here, we demonstrate the use of heterobimetallic single-source precursors for the synthesis of 10-15 and 11-15 binary intermetallics. The coordination environment of the precursor, as well as the exact temperature used play a critical role in determining the crystalline intermetallic phase that is produced, highlighting the potential versatility of this approach in the synthesis of a variety of compounds. Furthermore, we show that a recently developed novel plasma-processing technique is successful in removing the surface graphitic carbon observed in some of the prepared compounds. This new single-source precursor approach is a powerful addition to the synthesis of atomically ordered intermetallic compounds and will help facilitate their further study and development for future applications.

Kinetics, energetics, and size dependence of the transformation from Pt to ordered PtSn intermetallic nanoparticles

The outstanding catalytic activity and chemical selectivity of intermetallic compounds make them excellent candidates for heterogeneous catalysis. However, the kinetics of their formation at the nanoscale is poorly understood or characterized, and precise control of their size, shape and composition during synthesis remains challenging. Here, using well-defined Pt nanoparticles (5 nm and 14 nm) encapsulated in mesoporous silica, we study the transformation kinetics from monometallic Pt to intermetallic PtSn at different temperatures by a series of time-evolution X-ray diffraction studies. Observations indicate an initial transformation stage mediated by Pt surface-controlled intermixing kinetics, followed by a second stage with distinct transformation kinetics corresponding to a Ginstling-Brounstein (G-B) type bulk diffusion mode. Moreover, the activation barrier for both surface intermixing and diffusion stages is obtained through the development of appropriate kinetic models for the analysis of experimental data. Our density-functional-theory (DFT) calculations provide further insights into the atomistic-level processes and associated energetics underlying surface-controlled intermixing.

Engineering Surface Structure of Pt Nanoshells on Pd Nanocubes to Preferentially Expose Active Surfaces for ORR by Manipulating the Growth Kinetics

Synthesis of Pt nanoshells on substrates can increase the utilization efficiency of Pt atoms and reduce the amount of Pt used in the applications. However, it is still an enormous challenge in tailoring the required crystal facets of Pt nanoshells on a given substrate. In this work, we demonstrate a facile and convenient approach capable for generating Pt octahedral islands with tunable sizes and densities on Pd nanocubes by manipulating the deposition rate. The key to this synthesis is the fine control over the deposition rate of Pt on Pd seeds. Because of the different reactivities at the surface sites, the deposition of Pt can be controlled at a certain site by carefully tuning the deposition rate. With a low concentration of reductant (8.33 mg/mL of glucose), surface diffusion dominates the process, and thus the Pt cubic shells form on Pd cubic seeds. In contrast, when a higher amount of the reductant (16.67 mg/mL of glucose) is added, the deposition starts to dominate the growth of Pt shells. In this case, the deposition would be controlled at the corners, forming eight large Pt octahedra on a cubic Pd seed. Further increasing the deposition rate can induce much higher deposition rates, in which case, the deposition of Pt would likely take place not only at the corners, but also the edge and surface sites of the seeds. Not surprisingly, this growth habit can result in the formation of high-density octahedral islands on Pd cubic seeds. With the same amount of precursor supply, the higher the densities of Pt islands, the smaller the size of the octahedral islands on Pd nanocubes. Unlike other synthetic methods, the size of the octahedral islands on Pd seeds can be even controlled to be smaller than 3 nm by controlling the amount of the Pt precursor. Considering the excellent performance of {111} facets of Pt catalysts toward ORR, the Pt nanocages with small octahedral islands on the surfaces can exhibit a high activity with a mass activity 0.68 A/mg, as high as 5.2 times of that of commercial Pt/C.

Monodisperse ordered indium-palladium nanoparticles: synthesis and role of indium for boosting superior electrocatalytic activity for ethanol oxidation reaction

The slow kinetics of ethanol oxidation reaction (EOR) has limited its widespread use for fuel cells. Bimetallic catalysts with optimized surface compositions can considerably govern rate-determining steps through selectivity for CH3COOH formation or by facilitating the adsorption of OHadsvia the bifunctional effect of an alloy to increase the EOR's kinetic rates. Here, we reported monodisperse ordered In-Pd nanoparticles as new bimetallic high-performance catalysts for EOR. In-Pd nanoparticles, i.e., In3Pd2 and In3Pd5 were prepared using arrested precipitation in solution, and their composition, structures, phase and crystallinity were confirmed using a variety of analyses including TEM, XPS, EDS and XRD. In-Pd nanoparticles were loaded on carbon black (Vulcan XC-72) as electrocatalysts for EOR in alkaline media. In3Pd2 and In3Pd5 nanoparticles exhibited 5.8 times and 4.0 times higher mass activities than commercial Pd/C, which showed that the presence of indium greatly boosts electrocatalytic reactivity for EOR of Pd catalysts. This performance is the best among those of bimetallic nanoparticles reported to date. Such high performance of In-Pd nanoparticles may be attributed to the following two reasons. First, In-Pd nanoparticles exhibited excellent CO anti-poison ability, as confirmed by CO striping experiments. Second, as revealed by DFT calculations of metals with OHads adsorption, In atoms on In3Pd2 surface exhibited the lowest energy (-1.659 eV) for OHads adsorption as compared to other common oxophilic metals including Sn, SnPt, Ag, Ge, Co, Pb, and Cu. We propose that the presence of indium sites promoted efficient free OH radical adsorption on indium sites and resulted in a faster reaction rate of acetate formation from acetaldehyde (the rate determining step for EOR on Pd sites). Finally, a single direct ethanol fuel cell (DEFC) with Pd/C anode was prepared. Compared to the results for a commercial Pd/C anode, the open circuit voltage (OCV) of In3Pd2/C improved by 0.25 V (from 0.64 to 0.89 V) and the power density improved by ∼80% (from 3.7 to 6.7 mW cm-2), demonstrating its practical uses as Pt or Pd catalyst alternatives for DEFC.

Microbial synthesis of Pd-Pt alloy nanoparticles using Shewanella oneidensis MR-1 with enhanced catalytic activity for nitrophenol and azo dyes reduction

Bimetallic nanoparticles (NPs) often exhibit improved catalytic performance due to the electronic and spatial structure changes. Herein, a novel green biosynthesis method for Pd-Pt alloy NPs using Shewanella oneidensis MR-1 was proposed. The morphology, size and crystal structure of Pd-Pt alloy NPs were studied by a suite of characterization techniques. Results showed Pd-Pt alloy NPs were successfully synthesized inside and outside the cell. The biosynthesized Pd-Pt alloy NPs were polycrystalline and face-centered-cubic structure with the particle size ranged from 3-40 nm. Furthermore, the catalytic experiment demonstrated that the Pd-Pt alloy NPs exhibited the highest performance for the catalytic reduction of nitrophenol and azo dyes compared with the as-synthesized Pd and Pt monometallic NPs. This enlarged catalytic activity resulted from the synergistic effect of Pd and Pt element. Thereby, this paper provided a simple biosynthesis method for producing bimetallic alloy nanocatalyst with superior activity for contaminant degradation.

Revealing the atomic ordering of binary intermetallics using in situ heating techniques at multilength scales

Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). The ordered phase is generally identified based on the existence of superlattice ordering peaks in powder X-ray diffraction (PXRD). However, after employing a widely used postsynthesis annealing treatment, we have found that claims of "ordered" catalysts were possibly/likely mixed phases of ordered intermetallics and disordered solid solutions. Here, we employed in situ heating, synchrotron-based, X-ray diffraction to quantitatively investigate the impact of a variety of annealing conditions on the degree of ordering of large ensembles of Pt3Co nanoparticles. Monte Carlo simulations suggest that Pt3Co nanoparticles have a lower order-disorder phase transition (ODPT) temperature relative to the bulk counterpart. Furthermore, we employed microscopic-level in situ heating electron microscopy to directly visualize the morphological changes and the formation of both fully and partially ordered nanoparticles at the atomic scale. In general, a higher degree of ordering leads to more active and durable electrocatalysts. The annealed Pt3Co/C with an optimal degree of ordering exhibited significantly enhanced durability, relative to the disordered counterpart, in practical membrane electrode assembly (MEA) measurements. The results highlight the importance of understanding the annealing process to maximize the degree of ordering in intermetallics to optimize electrocatalytic activity.

An efficient thin-walled Pd/polypyrrole hybrid nanotube biocatalyst for sensitive detection of ascorbic acid

Controllable fabrication of novel and uniform noble metal nanoparticles on a specific support with a superior catalytic or electrocatalytic performance is of significantly importance for practical applications. In this report, we demonstrated an effective way to fabricate uniform thin-walled Pd/polypyrrole (PPy) hollow nanotubes. The prepared Pd/PPy hybrid nanotubes exhibited an excellent peroxidase-like activity to oxidize a typical peroxidase substrate such as 3,3',5,5'-tetramethylbenzidine in comparison with traditional Pd/C and Pd black catalysts. The outstanding catalytic activity of the Pd/PPy hybrid nanotubes for peroxidase mimicking could be resulting from their unique hollow characteristic and an interfacial effect between PPy and Pd components. Based on the favorable catalytic property of the Pd/PPy hybrid nanotubes, a convenient and rapid colorimetric way to sensitively determine ascorbic acid has been presented. The detection limit was around 0.062 μM and an excellent selectivity was also achieved. The developed detection system in this study could be extended to the fields of bioscience and biotechnology with promising prospects.

Acid-tolerant intermetallic cobalt–nickel silicides as noble metal-like catalysts for selective hydrogenation of phthalic anhydride to phthalide

Intermetallic Co–Ni silicide catalyst embedded in a carbon matrix with a unique synergistic effect exhibits excellent activity, selectivity, and acid corrosion resistance in hydrogenation of phthalic anhydride to phthalide, which matches noble metal catalysts.

Surface and interface design for heterogeneous catalysis

Recent progresses in catalytic nanocrystals with uniform and well-defined structures, in situ characterization techniques, and theoretical calculations are facilitating the innovation of efficient catalysts via surface and interface designs, including crystal phase design, morphology/facet design, and size design, followed by controlled synthesis.

Intermetallic Pd3Pb square nanoplates as highly efficient electrocatalysts for oxygen reduction reaction

Intermetallic Pd₃Pb square nanoplates were synthesized and exhibited excellent performance for the oxygen reduction reaction in alkaline solution.

The importance of direct reduction in the synthesis of highly active Pt–Sn/SBA-15 for n-butane dehydrogenation

Supported Pt–Sn bimetallic catalysts directly reduced by H₂ are highly active for the dehydrogenation of n-butane, while the catalysts calcined in air, followed by H₂ reduction are totally inactive.

High temperature self-assembly one-step synthesis of a structurally ordered PtFe catalyst for the oxygen reduction reaction

Herein, we present a high-temperature self-assembly strategy that directly allows the transformation of adsorbed Pt(NH₃)₄²⁺ and Fe³⁺ sources into structurally ordered face-centered tetragonal (fct)-PtFe alloy NPs (2.6 ± 0.2 nm).

Control of crystallographic phases and surface characterization of intermetallic platinum tin nanoparticles

Colloidal synthesis and characterization of intermetallic tin-rich platinum–tin nanoparticles with detailed surface characterization.

Detailed Characterization of the Surface and Growth Mechanism of Monodisperse Ni3Sn4 Nanoparticles

Synthesis of most tin-based bimetallic nanoparticles is a challenging task because of the differences in the redox potential and the melting point between both components. This article presents a co-reduction synthesis of monoclinic Ni₃Sn₄ nanoparticles. Varying time and temperature gives the possibility to control the size of the nanoparticles in the range of 4-12 nm. The products were characterized by X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy measurements. Although the synthesis was conducted entirely oxygen free, the postsynthetic treatment undertaken under air leads to the formation of an amorphous oxide shell. The oxide shell consists of an outer tin-rich region and a nickel-rich region at the interface to the metallic Ni₃Sn₄ core. On the basis of the investigation of the particles at different stages of the synthesis, we propose a growth mechanism for the Ni₃Sn₄ nanocrystals. These results can be a guidepost for the synthesis of other tin-based bimetallic nanoparticles.

L12 Atomic Ordered Substrate Enhanced Pt-Skin Cu3Pt Catalyst for Efficient Oxygen Reduction Reaction

Constructing Pt skin on intermetallics has been confirmed as an efficient strategy to boost oxygen reduction reaction (ORR) kinetics. However, there still lacks a systematic study on revealing the influence of low-Pt-content intermetallic substrates (L1₂-PtM₃). In this paper, Pt skin-encapsulated low-Pt-mole-fraction L1₂ Cu₃Pt has been constructed (denoted as Pt-o-Cu₃Pt/C) and compared with its disordered analogue (denoted as Pt-d-Cu₃Pt/C). The L1₂ substrate shows a contracted lattice structure and provides Pt-o-Cu₃Pt/C with an excellent specific activity of 1.73 mA cm^-2, which is 1.4- and 8.4-fold higher than that of Pt-d-Cu₃Pt/C and commercial Pt/C, respectively. Density functional theory calculations reveal that this superior performance is attributed to the more favorable oxygen adsorption energy of surface Pt atoms. Furthermore, the lower formation energy of L1₂ Cu₃Pt combined with the enhanced antioxygenation of Pt provide Pt-o-Cu₃Pt/C with a superior durability, showing only a 12.5% loss in mass activity after 5000 potential cycles. Therefore, it is suggested that L1₂ atomic ordered structure with a low Pt fraction is a promising substrate for building high-performance Pt-skin catalysts for ORR.

Enhanced oxygen reduction activity of Pt shells on PdCu truncated octahedra with different compositions

Pd@Pt core-shell nanocrystals with ultrathin Pt layers have received great attention as active and low Pt loading catalysts for oxygen reduction reaction (ORR). However, the reduction of Pd loading without compromising the catalytic performance is also highly desired since Pd is an expensive and scarce noble-metal. Here we report the epitaxial growth of ultrathin Pt shells on Pd x Cu truncated octahedra by a seed-mediated approach. The Pd/Cu atomic ratio (x) of the truncated octahedral seeds was tuned from 2, 1 to 0.5 by varying the feeding molar ratio of Pd to Cu precursors. When used as catalysts for ORR, these three Pd x Cu@Pt core-shell truncated octahedra exhibited substantially enhanced catalytic activities compared to commercial Pt/C. Specifically, Pd2Cu@Pt catalysts achieved the highest area-specific activity (0.46 mA cm-2) and mass activity (0.59 mA μgPt -1) at 0.9 V, which were 2.7 and 4.5 times higher than those of the commercial Pt/C. In addition, these Pd x Cu@Pt core-shell catalysts showed a similar durability with the commercial Pt/C after 10 000 cycles due to the dissolution of active Cu and Pd in the cores.

The Sub-Nanometer Scale as a New Focus in Nanoscience

Size is one of the central issues in nanoscience. The practical meaning of the term "sub-nanometric material (SNM)" requires two aspects: (1) its size should be at the atomic level; (2) it shows unique (size-related) properties compared to its nano-counterparts with larger sizes. Here, SNMs in the form of wires (SNWs) and the unique properties arising from their special size are reviewed. First, their polymer-like behavior, including rheological behavior and self-assembly, is dicussed. Their origins may stem from the special size and the ligands around the wire. Even a slight increase in diameter would risk the polymer-like behavior. Meanwhile, the ligands on SNWs are proportional to the inorganic entity at this scale. Consequently, surface ligands should have a profound impact on the properties, like catalysis, self-assembly, optics, etc. To reveal more potential applications, their applications in energy conversion are comprehensively reviewed. To some extent, characterization can greatly influence the way things are observed. Thus, some appropriate characterization techniques are briefly introduced. Finally, another emerging part of SNWs (atomic chain material) is briefly introduced. It is hoped that this review can provide new insights to this special scale.

Highly Durable and Active Pt-Based Nanoscale Design for Fuel-Cell Oxygen-Reduction Electrocatalysts

Fuel cells are one of the promising energy-conversion devices due to their high efficiency and zero emission. Although recent advances in electrocatalysts have been achieved using various material designs such as alloys, core@shell structures, and shape control, many issues still remain to be resolved. Especially, material design issues for high durability and high activity are recently accentuated owing to severe instability of nanoparticles under fuel-cell operating conditions. To address these issues, fundamental understanding of functional links between activity and durability is timely urgent. Here, the activity and durability of nanoscale materials are summarized, focusing on the nanoparticle size effect. In addition to phenomenological observation, two major degradation origins, including atomic dissolution and particle size increase, are discussed related to the activity decrease. Based on the fundamental understanding of nanoparticle degradation, recent promising strategies for durable Pt-based nanoscale electrocatalysts are introduced and the role of each design for durability enhancement is discussed. Finally, short comments related to the future direction of nanoparticle issues are provided in terms of nanoparticle synthesis and analysis.

Size, shape, and compositional effects on the order-disorder phase transitions in Au-Cu and Pt-M (M = Fe, Co, and Ni) nanocluster alloys

Au-Cu and Pt-M (M = Fe, Co, and Ni) nanocluster alloys are currently being investigated world-wide by many researchers for their interesting catalytic and nanophase properties. The low temperature behavior of the phase diagrams is not well understood for alloys with nanometer sizes and shapes. We consider two models for low temperature ordering in the phase diagrams of Au-Cu and Pt-M nanocluster alloys. These models are valid for sizes ∼5 nm and approach bulk values for sizes ∼20 nm. We study the phase transitions in nanoclusters with cubic, octahedral, and cuboctahedral shapes, covering the compositions of interest. These models are based on studying the melting temperatures in nanoclusters using the regular solution, mixing model for alloys. From our data, experiments on nanocubes about 5 nm in size, of stoichiometric AuCu and PtM composition, could help differentiate between the models. Dispersion data shows that for the three shapes considered, octahedra have the highest percentage of surface atoms for the same relative diameter. We summarize the effects of structural ordering on the catalytic activity and suggest a method to avoid sintering during annealing of Pt-M alloys.

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.

Windowless Observation of Evaporation-Induced Coarsening of Au-Pt Nanoparticles in Polymer Nanoreactors

The interactions between nanoparticles and solvents play a critical role in the formation of complex, metastable nanostructures. However, direct observation of such interactions with high spatial and temporal resolution is challenging with conventional liquid-cell transmission electron microscopy (TEM) experiments. Here, a windowless system consisting of polymer nanoreactors deposited via scanning probe block copolymer lithography (SPBCL) on an amorphous carbon film is used to investigate the coarsening of ultrafine (1-3 nm) Au-Pt bimetallic nanoparticles as a function of solvent evaporation. In such reactors, homogeneous Au-Pt nanoparticles are synthesized from metal-ion precursors in situ under electron irradiation. The nonuniform evaporation of the thin polymer film not only concentrates the nanoparticles but also accelerates the coalescence kinetics at the receding polymer edges. Qualitative analysis of the particle forces influencing coalescence suggests that capillary dragging by the polymer edges plays a significant role in accelerating this process. Taken together, this work (1) provides fundamental insight into the role of solvents in the chemistry and coarsening behavior of nanoparticles during the synthesis of polyelemental nanostructures, (2) provides insight into how particles form via the SPBCL process, and (3) shows how SPBCL-generated domes, instead of liquid cells, can be used to study nanoparticle formation. More generally, it shows why conventional models of particle coarsening, which do not take into account solvent evaporation, cannot be used to describe what is occurring in thin film, liquid-based syntheses of nanostructures.

In Situ X-ray Scattering Guides the Synthesis of Uniform PtSn Nanocrystals

Compared to monometallic nanocrystals (NCs), bimetallic ones often exhibit superior properties due to their wide tunability in structure and composition. A detailed understanding of their synthesis at the atomic scale provides crucial knowledge for their rational design. Here, exploring the Pt-Sn bimetallic system as an example, we study in detail the synthesis of PtSn NCs using in situ synchrotron X-ray scattering. We show that when Pt(II) and Sn(IV) precursors are used, in contrast to a typical simultaneous reduction mechanism, the PtSn NCs are formed through an initial reduction of Pt(II) to form Pt NCs, followed by the chemical transformation from Pt to PtSn. The kinetics derived from the in situ measurements shows fast diffusion of Sn into the Pt lattice accompanied by reordering of these atoms into intermetallic PtSn structure within 300 s at the reaction temperature (∼280 °C). This crucial mechanistic understanding enables the synthesis of well-defined PtSn NCs with controlled structure and composition via a seed-mediated approach. This type of in situ characterization can be extended to other multicomponent nanostructures to advance their rational synthesis for practical applications.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals by Replicating the Surface Atomic Structure on the Seed

Controlling the surface structure of metal nanocrystals while maximizing the utilization efficiency of the atoms is a subject of great importance. An emerging strategy that has captured the attention of many research groups involves the conformal deposition of one metal as an ultrathin shell (typically 1-6 atomic layers) onto the surface of a seed made of another metal and covered by a set of well-defined facets. This approach forces the deposited metal to faithfully replicate the surface atomic structure of the seed while at the same time serving to minimize the usage of the deposited metal. Here, the recent progress in this area is discussed and analyzed by focusing on the synthetic and mechanistic requisites necessary for achieving surface atomic replication of precious metals. Other related methods are discussed, including the one-pot synthesis, electrochemical deposition, and skin-layer formation through thermal annealing. To close, some of the synergies that arise when the thickness of the deposited shell is decreased controllably down to a few atomic layers are highlighted, along with how the control of thickness can be used to uncover the optimal physicochemical properties necessary for boosting the performance toward a range of catalytic reactions.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal-support interaction, and metal-reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Galvanic Replacement-Driven Transformations of Atomically Intermixed Bimetallic Colloidal Nanocrystals: Effects of Compositional Stoichiometry and Structural Ordering

Galvanic replacement reactions dictated by deliberately designed nanoparticulate templates have emerged as a robust and versatile approach that controllably transforms solid monometallic nanocrystals into a diverse set of architecturally more sophisticated multimetallic hollow nanostructures. The galvanic atomic exchange at the nanoparticle/liquid interfaces induces a series of intriguing structure-transforming processes that interplay over multiple time and length scales. Using colloidal Au-Cu alloy and intermetallic nanoparticles as structurally and compositionally fine-tunable bimetallic sacrificial templates, we show that atomically intermixed bimetallic nanocrystals undergo galvanic replacement-driven structural transformations remarkably more complicated than those of their monometallic counterparts. We interpret the versatile structure-transforming behaviors of the bimetallic nanocrystals in the context of a unified mechanistic picture that rigorously interprets the interplay of three key structure-evolutionary pathways: dealloying, Kirkendall diffusion, and Ostwald ripening. By deliberately tuning the compositional stoichiometry and atomic-level structural ordering of the Au-Cu bimetallic nanocrystals, we have been able to fine-maneuver the relative rates of dealloying and Kirkendall diffusion with respect to that of Ostwald ripening through which an entire family of architecturally distinct complex nanostructures are created in a selective and controllable manner upon galvanic replacement reactions. The insights gained from our systematic comparative studies form a central knowledge framework that allows us to fully understand how multiple classic effects and processes interplay within the confinement by a colloidal nanocrystal to synergistically guide the structural transformations of complex nanostructures at both the atomic and nanoparticulate levels.

Nanotube-shaped PtFe intermetallics: controlled synthesis, crystal structure, and improved electrocatalytic activities

Nanotube-shaped PtFe intermetallics synthesized over charged β-FeOOH by self-assembly and careful heat treatment exhibit higher Pt activities toward methanol electro-oxidation.

Generalized nano-thermodynamic model for capturing size-dependent surface segregation in multi-metal alloy nanoparticles

Multi-metal alloy nanoparticles (NPs) offer new avenues for exploration and design of nanoscale-properties, e.g., catalytic, electronic and optical properties, by virtue of their tunable composition. A method that can aid such exploration by accurately predicting the size-, shape- and composition-dependent elemental distribution associated with nanomaterials is crucially missing. A nano-thermodynamic model based on distribution coefficients Δ is introduced to fill this gap. Δ is employed to predict surface segregation in NPs as a function of the NP size and composition. Interestingly, we find Δ to be independent of size for NPs beyond 2 nm. This key finding motivates the construction of thermodynamic tables for distribution coefficients using segregation observed with one or more NP sizes. The tables can enable accurate prediction of phase diagrams for nanomaterials across a wide-range of sizes. Key concepts of this new theory are demonstrated with Au–Pt–Pd, Ag–Au–Pd and Ni–Pt–Pd, which are found to exhibit complex size-dependent segregation behavior for 2–6 nm NPs and relatively weaker size-dependence beyond 6 nm. Numerically well-converged values of Δ are calculated for small NPs using Monte Carlo simulations in the canonical ensemble. Simulations are based on an embedded atom method (EAM) potential for metal alloys.

Nano-thermodynamic model captures thermodynamic preference of metal species for different regions of a nanoparticle while accounting for size effects.

DFT Study on Intermetallic Pd-Cu Alloy with Cover Layer Pd as Efficient Catalyst for Oxygen Reduction Reaction

Detailed density functional theory (DFT) calculations of the adsorption energies (E_ad) for oxygen on monolayer Pd on top of the Pd–Cu face-centered cubic (FCC) alloy and intermetallic B2 structure revealed a linear correspondence between the adsorption energies and the d-band center position. The calculated barrier (E_barrier) for oxygen dissociation depends linearly on the reaction energy difference (ΔE). The O₂ has a stronger adsorption strength and smaller barrier on the intermetallic Pd–Cu surface than on its FCC alloy surface. The room-temperature free energy (ΔG) analysis suggests the oxygen reduction reaction (ORR) pathways proceed by a direct dissociation mechanism instead of hydrogenation into OOH. These results might be of use in designing intermetallic Pd–Cu as ORR electrocatalysts.

Ligand-Assisted, One-Pot Synthesis of Rh-on-Cu Nanoscale Sea Urchins with High-Density Interfaces for Boosting CO Oxidation

Predictable synthesis of bimetallic nanocrystals with spatially controlled metal distributions offers a versatile route to the development of highly efficient nanocatalysts. Here we report a one-pot synthesis of super branched Rh-on-Cu nanoscale sea urchins (Rh-Cu NSUrs) with a high density of Cu-Rh interfaces by manipulating the ligand coordination chemistry. Structural analysis and UV-vis spectra reveal that ascorbic acid can serve as a Rh-selective coordination ligand in the nonaqueous synthesis to reverse the reduction potentials of Rh³⁺ and Cu²⁺ cations. The sequential reduction of Cu²⁺ and then Rh³⁺ cations, as well as the island epitaxial growth of Rh atoms on Cu cores, leads to the formation of Rh-on-Cu nanostructures mimicking sea urchin. The size of the Cu cores and the density of Rh branches can both be facilely regulated by tuning the mole ratio of Cu to Rh. The Cu-Rh NSUrs show enhanced activity and stability in catalyzing CO oxidation, as the intrinsic Cu-Rh interfaces can act as catalytic hot spots through a bifunctional mechanism. The Cu-Rh two-component system can separate the adsorption and activation of CO and O₂ on the Rh and Cu surfaces, respectively, accelerating the generation of CO₂ at the interfaces.