Structural and architectural evaluation of bimetallic nanoparticles: a case study of Pt-Ru core-shell and alloy nanoparticles

3D Atomic Arrangement at Functional Interfaces Inside Nanoparticles by Resonant High-Energy X-ray Diffraction

With current science and technology moving rapidly into smaller scales, nanometer-sized materials, often referred to as NPs, are produced in increasing numbers and explored for numerous useful applications. Evidence is mounting, however, that useful properties of NPs can be improved further and even new NP functionality achieved by not only controlling the NP size and shape but also interfacing chemically or structurally distinct entities into single, so-called "composite" NPs. A typical example is core-shell NPs wherein the synergy of distinct atoms at the core\shell interface endows the NPs with otherwise unachievable functionality. However, though advantageous, the concept of functional interfaces inside NPs is still pursued largely by trial-and-error. That is because it is difficut to assess the interfaces precisely at the atomic level using traditional experimental techniques and, hence, difficult to take control of. Using the core\shell interface in less than 10 nm in size Ru core-Pt shells NPs as an example, we demonstrate that precise knowledge of the 3D atomic arrangement at functional interfaces inside NPs can be obtained by resonant high-energy X-ray diffraction (XRD) coupled to element-specific atomic pair distribution function (PDF) analysis. On the basis of the unique structure knowledge obtained, we scrutinize the still-debatable influence of core\shell interface on the catalytic functionality of Ru core-Pt shell NPs, thus evidencing the usefulness of this nontraditional technique for practical applications.

The formation mechanism of bimetallic PtRu alloy nanoparticles in solvothermal synthesis

An understanding of the nucleation and growth mechanism of bimetallic nanoparticles in solvothermal synthesis is important for further development of nanoparticles with tailored nanostructures and properties. Here the formation of PtRu alloy nanoparticles in a solvothermal synthesis using metal acetylacetonate salts as precursors and ethanol as both the solvent and reducing agent has been studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXRD). Unlike the classical mechanism for the synthesis of monodisperse sols, the nucleation and growth processes of bimetallic PtRu nanoparticles occur simultaneously under solvothermal conditions. In the literature co-reduction of Pt and Ru is often assumed to be required to form PtRu bimetallic nanocrystals, but it is shown that monometallic Pt nanocrystals nucleate first and rapidly grow to an average size of 5 nm. Subsequently, the PtRu bimetallic alloy is formed in the second nucleation stage through a surface nucleation mechanism related to the reduction of Ru. The calculated average crystallite size of the resulting PtRu nanocrystals is smaller than that of the primary Pt nanocrystals due to the large disorder in the PtRu alloyed structure.

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.

Synthesis and X-ray Characterization of Cobalt Phosphide (Co2P) Nanorods for the Oxygen Reduction Reaction

Low temperature fuel cells are clean, effective alternative fuel conversion technology. Oxygen reduction reaction (ORR) at the fuel cell cathode has required Pt as the electrocatalyst for high activity and selectivity of the four-electron reaction pathway. Targeting a less expensive, earth abundant alternative, we have developed the synthesis of cobalt phosphide (Co2P) nanorods for ORR. Characterization techniques that include total X-ray scattering and extended X-ray absorption fine structure revealed a deviation of the nanorods from bulk crystal structure with a contraction along the b orthorhombic lattice parameter. The carbon supported nanorods have comparable activity but are remarkably more stable than conventional Pt catalysts for the oxygen reduction reaction in alkaline environments.

Surface Segregation in Bimetallic Nanoparticles: A Critical Issue in Electrocatalyst Engineering

Bimetallic nanoparticles are a class of important electrocatalyst. They exhibit a synergistic effect that critically depends on the surface composition, which determines the surface properties and the adsorption/desorption behavior of the reactants and intermediates during catalysis. The surface composition can be varied, as nanoparticles are exposed to certain environments through surface segregation. Thermodynamically, this is caused by a difference in surface energy between the two metals. It may lead to the enrichment of one metal on the surface and the other in the core. The external conditions that influence the surface energy may lead to the variation of the thermodynamic steady state of the particle surface and, thus, offer a chance to vary the surface composition. In this review, the most recent and important progress in surface segregation of bimetallic nanoparticles and its impact in electrocatalysis are introduced. Typical segregation inducements and surface characterization techniques are discussed in detail. It is concluded that surface segregation is a critical issue when designing bimetallic catalysts. It is necessary to explore methods to control it and utilize it as a way towards producing robust, bimetallic electrocatalysts.

Structural investigation of Ru/Pt nanocomposite films prepared by plasma-enhanced atomic layer depositions

We have fabricated Ru and Pt nanocomposite films using plasma-enhanced atomic layer deposition (PE-ALD), and characterized their structure by means of analytical electron microscopy. Pt and Ru were deposited in Ar/O(2) plasma using trimethyl(methylcyclopentadienyl) platinum(IV) and bis(cyclopentadienyl)Ru(II) or bis(ethylcyclopentadienyl)Ru(II) as precursors, respectively. The resistivity of a Pt film deposited on a Si substrate at 300°C was 16.2μΩcm, and that of a Ru film was as low as 11μΩcm, showing the film to be metallic and not oxidized. It was revealed that the film prepared by successive PE-ALDs of Pt and Ru on a thin amorphous carbon substrate for electron microscopy analysis is a nanocomposite of Ru ribbons and PtRu (7:3) alloy ribbons with 2-3 nm in width. The Ru ribbon comprised small particles with poor crystallinity of the hcp A3 structure and the PtRu ribbon comprised crystallites with good crystallinity of the fcc Al structure. The atomic layer deposition would be one of potential techniques to produce Ru/Pt nanocomposites for fuel cell catalysts.

Sub-nanometer dimensions control of core/shell nanoparticles prepared by atomic layer deposition

Bimetallic core/shell nanoparticles (NPs) are the subject of intense research due to their unique electronic, optical and catalytic properties. Accurate and independent control over the dimensions of both core and shell would allow for unprecedented catalytic performance. Here, we demonstrate that both core and shell dimensions of Pd/Pt core/shell nanoparticles (NPs) supported on Al2O3 substrates can be controlled at the sub-nanometer level by using a novel strategy based on atomic layer deposition (ALD). From the results it is derived that the main conditions for accurate dimension control of these core/shell NPs are: (i) a difference in surface energy between the deposited core metal and the substrate to obtain island growth; (ii) a process yielding linear growth of the NP cores with ALD cycles to obtain monodispersed NPs with a narrow size distribution; (iii) a selective ALD process for the shell metal yielding a linearly increasing thickness to obtain controllable shell growth exclusively on the cores. For Pd/Pt core/shell NPs it is found that a minimum core diameter of 1 nm exists above which the NP cores are able to catalytically dissociate the precursor molecules for shell growth. In addition, initial studies on the stability of these core/shell NPs have been carried out, and it has been demonstrated that core/shell NPs can be deposited by ALD on high aspect ratio substrates such as nanowire arrays. These achievements show therefore that ALD has significant potential for the preparation of tuneable heterogeneous catalyst systems.

Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles: in situ observation of the linear composition-strain-activity relationship

Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt-Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques. The PtxCo/C NPs with high Co content possess a Co concentration gradient such that Co is concentrated in the core and gradually depletes in the near-surface region, whereas the PtxCo/C NPs with low Co content possess a relatively uniform distribution of Co with low Co population in the near-surface region. Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt). The RPt-Pt is further shown to contract linearly with the increase in Co/Pt composition. These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

Electrospun polyacrylonitrile nanofibers supported alloyed Pd–Pt nanoparticles as recyclable catalysts for hydrogen generation from the hydrolysis of ammonia borane

Electrospun polyacrylonitrile nanofibers supported alloyed Pd–Pt nanoparticles were synthesized as recyclable catalysts for hydrogen generation from the hydrolysis of ammonia borane.

Ag–Pd and CuO–Pd nanoparticles in a hydroxyl-group functionalized ionic liquid: synthesis, characterization and catalytic performance

Heteronuclear Ag–Pd and CuO–Pd nanoparticles with a controllable Ag : Pd or Cu : Pd ratio were easily synthesized through thermal decomposition of their acetate salts in a functionalized ionic liquid, [C₂OHmim][NTf₂].

Influence of Sn content on the hydrogenation of crotonaldehyde catalysed by colloidally prepared PtSn nanoparticles

For increasing tin concentrations PtSn nanoparticles of similar size show a monotonically increasing selectivity towards crotylalcohol and a volcano like activity.

Clarifying the role of Ru in methanol oxidation at Rucore@Ptshellnanoparticles

Ru_core@Pt_shellNPs with a controlled Pt_shellcoverage and two different Ru_coresizes (2 and 3 nm) were synthesized to determine the precise role of Ru in the MOR activity at Ru_core@Pt_shellNPs.

Stimulation of electro-oxidation catalysis by bulk-structural transformation in intermetallic ZrPt₃ nanoparticles

Although compositional tuning of metal nanoparticles (NPs) has been extensively investigated, possible control of the catalytic performance through bulk-structure tuning is surprisingly overlooked. Here we report that the bulk structure of intermetallic ZrPt3 NPs can be engineered by controlled annealing and their catalytic performance is significantly enhanced as the result of bulk-structural transformation. Chemical reduction of organometallic precursors yielded the desired ZrPt3 NPs with a cubic FCC-type structure (c-ZrPt3 NPs). The c-ZrPt3 NPs were then transformed to a different phase of ZrPt3 with a hexagonal structure (h-ZrPt3 NPs) by annealing at temperatures between 900 and 1000 °C. The h-ZrPt3 NPs exhibited higher catalytic activity and long-term stability than either the c-ZrPt3 NPs or commercial Pt/C NPs toward the electro-oxidation of ethanol. Theoretical calculations have elucidated that the enhanced activity of the h-ZrPt3 NPs is attributed to the increased surface energy, whereas the stability of the catalyst is retained by the lowered bulk-free-energy.

Current advances in precious metal core-shell catalyst design

Precious metal nanoparticles are commonly used as the main active components of various catalysts. Given their high cost, limited quantity, and easy loss of catalytic activity under severe conditions, precious metals should be used in catalysts at low volumes and be protected from damaging environments. Accordingly, reducing the amount of precious metals without compromising their catalytic performance is difficult, particularly under challenging conditions. As multifunctional materials, core-shell nanoparticles are highly important owing to their wide range of applications in chemistry, physics, biology, and environmental areas. Compared with their single-component counterparts and other composites, core-shell nanoparticles offer a new active interface and a potential synergistic effect between the core and shell, making these materials highly attractive in catalytic application. On one hand, when a precious metal is used as the shell material, the catalytic activity can be greatly improved because of the increased surface area and the closed interfacial interaction between the core and the shell. On the other hand, when a precious metal is applied as the core material, the catalytic stability can be remarkably improved because of the protection conferred by the shell material. Therefore, a reasonable design of the core-shell catalyst for target applications must be developed. We summarize the latest advances in the fabrications, properties, and applications of core-shell nanoparticles in this paper. The current research trends of these core-shell catalysts are also highlighted.

Seed-mediated synthesis of bimetallic ruthenium–platinum nanoparticles efficient in cinnamaldehyde selective hydrogenation

The two-step synthesis of small ruthenium–platinum nanoparticles leads to the formation of a core–shell structure. The catalytic results provide supplementary evidence of the core–shell structure.

Neuron-like polyelectrolyte–carbon nanotube composites for ultra-high loading of metal nanoparticles

We report a simple protocol for the fabrication of multiwalled carbon nanotubes (MWCNTs) with a neuron-like structure for loading ultra-high densities of metal nanoparticles (NPs).

Surface platinum-rich CuPt bimetallic nanoparticles supported by partially unzipped vapor grown carbon fibers and their electrocatalytic activities

Here we prepare the CuPt/PUVGCF catalyst made of surface platinum-rich CuPt nanoparticles anchored on partially unzipped vapor grown carbon fibers (PUVGCFs) as an anode catalyst for fuel cells.

Novel electrochemical fingerprinting methods for the precise determination of Ptshell coverage on Rucore nanoparticles

The electrochemical signatures of Ru oxide formation and CO stripping were used, for the first time, to determine the Pt shell coverage (from 0–2 monolayers) on Ru core nanoparticles, also revealing the chemical and electronic interactions that exist between these two phases.

Nucleation and aggregative growth process of platinum nanoparticles studied by in situ quick XAFS spectroscopy

The early stage in the nucleation and subsequent aggregative particle growth of the colloidal platinum (Pt) dispersions produced by photoreduction in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) was quantitatively investigated by means of in situ quick XAFS (QXAFS) measurements. The stages of the reduction-nucleation and the association process (aggregative particle growth and Ostwald ripening) of Pt atoms to produce Pt nanoparticles was successfully discriminated in course of the photoreduction time. The present QXAFS analysis indicated that Pt nuclei (i.e., (Pt(0))(m) nucleates approximately m = 4) were continuously produced in the reduction-nucleation process at the early time, followed by the aggregative particle growth with the autocatalytic reduction of Pt ionic species on the surface of Pt nuclei to produce Pt nanoparticles. Subsequently the particle growth proceeded via Ostwald ripening, resulting in the production of larger Pt nanoparticles at a later time. It was also found that the aggregative particle growth follows a sigmoidal profile well described either by the solid-state kinetic model or by the chemical-mechanism-based kinetic model, specifically the Avrami-Erofe'ev or Finke-Watzky models. The difference in terms of the formation mechanism was observed between the reduction of Pt(IV)Cl(6)(2-) and Pt(II)Cl(4)(2-) as a source material. Also presented is that the addition of the photoactivator such as benzoin, benzophenone, and acetophenone in the system is very effective to enhance the rate for the formation of Pt nanoparticles.