Synthesis and oxygen reduction activity of shape-controlled Pt(3)Ni nanopolyhedra

Highly open rhombic dodecahedral PtCu nanoframes

An efficient approach that enables the direct creation of highly open PtCu nanoframes was developed for the first time.

Facile synthesis of oleylamine-capped silver nanowires and their application in transparent conductive electrodes

Silver nanowires capped with oleylamine molecules have been prepared in high purity for the first time.

Element-specific anisotropic growth of shaped platinum alloy nanocrystals

Morphological shape in chemistry and biology owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-corrected scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chemical degradation pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.

Multimetallic core/interlayer/shell nanostructures as advanced electrocatalysts

The fine balance between activity and durability is crucial for the development of high performance electrocatalysts. The importance of atomic structure and compositional gradients is a guiding principle in exploiting the knowledge from well-defined materials in the design of novel class of core-shell electrocatalysts comprising Ni core, Au interlayer, and PtNi shell (Ni@Au@PtNi). This multimetallic system is found to have the optimal balance of activity and durability due to the synergy between the stabilizing effect of subsurface Au and modified electronic structure of surface Pt through interaction with subsurface Ni atoms. The electrocatalysts with Ni@Au@PtNi core-interlayer-shell structure exhibit high intrinsic and mass activities as well as superior durability for the oxygen reduction reaction with less than 10% activity loss after 10,000 potential cycles between 0.6 and 1.1 V vs the reversible hydrogen electrode.

Gold-promoted structurally ordered intermetallic palladium cobalt nanoparticles for the oxygen reduction reaction

Considerable efforts to make palladium and palladium alloys active catalysts and a possible replacement for platinum have had a marginal success. Here we report on a structurally ordered Au10Pd₄₀Co₅₀ catalyst that exhibits comparable activity to conventional platinum catalysts in both acid and alkaline media. Electron microscopic techniques demonstrate that, at elevated temperatures, palladium cobalt nanoparticles undergo an atomic structural transition from core-shell to a rare intermetallic ordered structure with twin boundaries forming stable {111}, {110} and {100} facets via addition of gold atoms. The superior stability of this catalyst compared with platinum after 10,000 potential cycles in alkaline media is attributed to the atomic structural order of PdCo nanoparticles along with protective effect of clusters of gold atoms on the surface. This strategy of making ordered palladium intermetallic alloy nanoparticles can be used in diverse heterogeneous catalysis where particle size and structural stability matter.

Synthesis and characterization of Pd@Pt-Ni core-shell octahedra with high activity toward oxygen reduction

The oxygen reduction reaction (ORR) on the cathode of a polymer electrolyte fuel cell requires the use of a catalyst based on Pt, one of the most expensive metals on the earth. A number of strategies, including optimization of shape or facet, formation of alloys with other metals, and incorporation of a different metal into the core, have been investigated to enhance the activity of a Pt-based catalyst and thus reduce the loading of Pt. This article reports the synthesis and characterization of Pd@Pt-Ni core-shell octahedra with high activity toward ORR. The octahedra with an edge length of 8 nm were obtained by directly depositing thin, conformal shells of a Pt-Ni alloy on Pd octahedra of 6 nm in edge length. The key to the success of this synthesis is the use of an amphiphilic solvent to ensure good compatibility between the solvents typically used for the syntheses of Pd and Pt-Ni nanocrystals. The core-shell structure was confirmed by a number of techniques, including scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy mapping, in situ X-ray diffraction under H2 and He, and electrochemical measurements. Relative to the state-of-the-art Pt/C catalyst, the Pd@Pt-Ni/C catalyst showed mass and specific ORR activities enhanced by 12.5- and 14-fold, respectively. The formation of a core-shell structure helped increase the electroactive surface area in terms of Pt and thus the mass activity. During an accelerated durability test, the mass activity of the Pd@Pt-Ni/C catalyst only dropped by 1.7% after 10,000 cycles.

Alloy Cu₃Pt nanoframes through the structure evolution in Cu-Pt nanoparticles with a core-shell construction

Noble metal nanoparticles with hollow interiors and customizable shell compositions have immense potential for catalysis. Herein, we present an unique structure transformation phenomenon for the fabrication of alloy Cu₃Pt nanoframes with polyhedral morphology. This strategy starts with the preparation of polyhedral Cu-Pt nanoparticles with a core-shell construction upon the anisotropic growth of Pt on multiply twinned Cu seed particles, which are subsequently transformed into alloy Cu₃Pt nanoframes due to the Kirkendall effect between the Cu core and Pt shell. The as-prepared alloy Cu₃Pt nanoframes possess the rhombic dodecahedral morphology of their core-shell parents after the structural evolution. In particular, the resulting alloy Cu₃Pt nanoframes are more effective for oxygen reduction reaction but ineffective for methanol oxidation reaction in comparison with their original Cu-Pt core-shell precursors.

Morphological control and evolution of octahedral and truncated trisoctahedral Pt-Au alloy nanocrystals under microwave irradiation

Uniform and well-defined truncated trisoctahedral and octahedral Pt-Au alloy nanocrystals were fabricated by co-reducing H(2)PtC(l6)-HAuC(l4) with tetraethylene glycol (TEG) under microwave irradiation for only 140 s. Iodide ions were critical to the morphological control and evolution of Pt-Au alloy nanostructures. The as-prepared Pt-Au alloy nanocrystals exhibited efficient electrocatalytic activities.

A comparison of atomistic and continuum approaches to the study of bonding dynamics in electrocatalysis: microcantilever stress and in situ EXAFS observations of platinum bond expansion due to oxygen adsorption during the oxygen reduction reaction

Microcantilever stress measurements are examined to contrast and compare their attributes with those from in situ X-ray absorption spectroscopy to elucidate bonding dynamics during the oxygen reduction reaction (ORR) on a Pt catalyst. The present work explores multiple atomistic catalyst properties that notably include features of the Pt-Pt bonding and changes in bond strains that occur upon exposure to O2 in the electrochemical environment. The alteration of the Pt electronic and physical structures due to O2 exposure occurs over a wide potential range (1.2 to 0.4 V vs normal hydrogen electrode), a range spanning potentials where Pt catalyzes the ORR to those where Pt-oxide forms and all ORR activity ceases. We show that Pt-Pt surface bond strains due to oxygen interactions with Pt-Pt bonds are discernible at macroscopic scales in cantilever-based bending measurements of Pt thin films under O2 and Ar. Complementary extended X-ray absorption fine structure (EXAFS) measurements of nanoscale Pt clusters supported on carbon provide an estimate of the magnitude and direction of the in-operando bond strains. The data show that under O2 the M-M bonds elongate as compared to an N2 atmosphere across a broad range of potentials and ORR rates, an interfacial bond expansion that falls within a range of 0.23 (±0.15)% to 0.40 (±0.20)%. The EXAFS-measured Pt-Pt bond strains correspond to a stress thickness and magnitude that is well matched to the predictions of a mechanics mode applied to experimentally determined data obtained via the cantilever bending method. The data provide new quantitative understandings of bonding dynamics that will need to be considered in theoretical treatments of ORR catalysis and substantiate the subpicometer resolution of electrochemically mediated bond strains detected on the macroscale.

On the dual roles of ligands in the synthesis of colloidal metal nanostructures

Eloquent routes to colloidal metal nanostructures have emerged in recent years, and a central component to any successful nanosynthesis is the initial selection of metal complexes with an appropriate ligand environment. This local ligand environment may be predetermined by the coordination complex selected as the metal precursor; however, recent studies reveal that the ligand environment of coordination complexes can be modified through exchange with other components for the synthesis that include solvent molecules, capping agents, anions, and even reducing agents. Importantly, ligands can often play multiple roles in a synthesis and direct the outcome by manipulating the rates of precursor reduction and particle coalescence, providing colloidal and facet stabilization and even serving as reducing agents themselves. This Feature Article highlights examples in which the ligand environments of metal precursors and nanoparticles contribute to product formation in multiple ways. Acknowledgment of the dual roles of ligands in nanomaterial synthesis will enable new strategies for nanostructures by decoupling the often contradictory roles of ligands.

A one-step green route to synthesize copper nanocrystals and their applications in catalysis and surface enhanced Raman scattering

A nontoxic, simple, inexpensive, and reproducible strategy, which meets the standard of green chemistry, is introduced for the synthesis of copper nanocrystals (Cu NCs) with olive oil as both reducing agent and capping agent. By changing the reaction parameters, the shape, size and surface structure of the Cu NCs can be well controlled. The obtained Cu nanocubes show excellent catalytic properties for the catalytic reduction of dyes and CO oxidation. Moreover, the prepared Cu nanocubes as substrates exhibit surface enhanced Raman scattering (SERS) activity for 4-mercaptopyridine (4-Mpy). Therefore, this facile route provides a useful platform for the fabrication of Cu NCs which have the potential to replace noble metals for certain applications.

Phase-segregated Pt-Ni chain-like nanohybrids with high electrocatalytic activity towards methanol oxidation reaction

The phase-segregated Pt-Ni chain-like nanostructures, composed of monometallic counterparts attached to each other, were synthesized via a modified polyol process with the assistance of a small amount of PVP. The molar ratio between Pt and Ni was tuned by simply adjusting the feed ratio of the precursors. High-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS) results reveal that atomic diffusion occurred at the interface of the granular subunits. The negative shift of the Pt4f7/2 peak in the XPS spectra indicates the electron transfer from Ni to Pt atoms, while the strong peaks at around 855.7 eV suggest the surface oxidation of the Ni nanoparticles, which was further confirmed by the cyclic voltammetry (CV) measurement. The electrocatalytic activities of the methanol oxidation reaction (MOR) were found to be higher for the phase-segregated structures relative to those for pure Pt nanoparticles, and the activities followed the sequences of Pt1Ni1 > Pt3Ni1 ∼ Pt2Ni3 > pure Pt. We believe that the modified electronic structures and the existence of nickel hydroxide both contributed to the improved catalytic activities.

Controlling the size and composition of nanosized Pt-Ni octahedra to optimize their catalytic activities toward the oxygen reduction reaction

Electrocatalysts based on Pt-Ni alloys have received considerable interest in recent years owing to their remarkable activities toward the oxygen reduction reaction (ORR). Here, we report the synthesis of nanosized Pt-Ni octahedra with a range of controlled sizes and compositions in an effort to optimize their ORR activities. If we employed benzyl ether as a solvent for the synthesis, we could readily control the edge lengths of the Pt-Ni octahedra in the range of 6-12 nm and keep the Pt/Ni atomic ratio at around 2.4 by varying the amount of oleylamine added into the reaction system. If we adjusted the amount of Ni precursor, the atomic ratio of Pt to Ni in the Pt-Ni octahedra could be controlled in the range of 1.4-3.7 and their edge lengths were kept at 9 nm. For the catalysts with a Pt/Ni atomic ratio around 2.4, their specific ORR activities (per unit surface area) increased monotonically as the edge length increased from 6 to 12 nm. However, the mass activities (per unit mass of Pt) of these Pt-Ni octahedra showed a maximum value at an edge length of 9 nm. The specific and mass activities for the Pt-Ni octahedra with an edge length of 9 nm but different compositions both showed peak values at a Pt/Ni atomic ratio of 2.4.

Boron nitride nanosheet on gold as an electrocatalyst for oxygen reduction reaction: theoretical suggestion and experimental proof

Boron nitride (BN), which is an insulator with a wide band gap, supported on Au is theoretically suggested and experimentally proved to act as an electrocatalyst for oxygen reduction reaction (ORR). Density-functional theory calculations show that the band gap of a free h-BN monolayer is 4.6 eV but a slight protrusion of the unoccupied BN states toward the Fermi level is observed if BN is supported on Au(111) due to the BN-Au interaction. A theoretically predicted metastable configuration of O2 on h-BN/Au(111), which can serve as precursors for ORR, and free energy diagrams for ORR on h-BN/Au(111) via two- and four-electron pathways show that ORR to H2O2 is possible at this electrode. It is experimentally proved that overpotential for ORR at the gold electrode is significantly reduced by depositing BN nanosheets. No such effect is observed at the glassy carbon electrode, demonstrating the importance of BN-substrate interaction for h-BN to act as the ORR electrocatalyst. A possible role of the edge of the BN islands for ORR is also discussed.

From single crystal surfaces to single atoms: investigating active sites in electrocatalysis

Electrocatalytic processes will undoubtedly be at the heart of energising future transportation and technology with the added importance of being able to create the necessary fuels required to do so in an environmentally friendly and cost effective manner. For this to be successful two almost mutually exclusive surface properties need to be reconciled, namely producing highly active/reactive surface sites that exhibit long term stability. This article reviews the various approaches which have been undertaken to study the elusive nature of these active sites on metal surfaces which are considered as adatoms or clusters of adatoms with low coordination number. This includes the pioneering studies at extended well defined stepped single crystal surfaces using cyclic voltammetry up to the highly sophisticated in situ electrochemical imaging techniques used to study chemically synthesised nanomaterials. By combining the information attained from single crystal surfaces, individual nanoparticles of defined size and shape, density functional theory calculations and new concepts such as mesoporous multimetallic thin films and single atom electrocatalysts new insights into the design and fabrication of materials with highly active but stable active sites can be achieved. The area of electrocatalysis is therefore not only a fascinating and exciting field in terms of realistic technological and economical benefits but also from the fundamental understanding that can be acquired by studying such an array of interesting materials.

Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles

Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations.

Platinum nanoparticles modified with alkylamine derivatives as an active and stable catalyst for oxygen reduction reaction

Platinum nanoparticles (NPs) protected with octylamine (OA) as well as partially replaced by controlled ratios of alkylamine with pyrene group (PA) were successfully synthesized by a two-phase liquid reduction method of Pt(IV). The NPs without any pretreatment to remove the introduced organic-protected agents have been well-characterized and applied as supported catalysts on carbon black for oxygen reduction reaction (ORR). The modification of the Pt NP surface with OA and PA significantly improved the electrocatalytic activity such as area specific and mass specific activities, whose values increased by an increase in PA ratios. The potential cycle test demonstrates that the PA modification of Pt NPs enhances the stability of the catalyst and sustains high area and mass specific activities of ORR.

Mechanisms for enhanced performance of platinum-based electrocatalysts in proton exchange membrane fuel cells

As a new generation of power sources, fuel cells have shown great promise for application in transportation. However, the expensive catalyst materials, especially the cathode catalysts for oxygen reduction reaction (ORR), severely limit the widespread commercialization of fuel cells. Therefore, this review article focuses on platinum (Pt)-based electrocatalysts for ORR with better catalytic performance and lower cost. Major breakthroughs in the improvement of activity and durability of electrocatalysts are discussed. Specifically, on one hand, the enhanced activity of Pt has been achieved through crystallographic control, ligand effect, or geometric effect; on the other hand, improved durability of Pt-based cathode catalysts has been realized by means of the incorporation of another noble metal or the morphological control of nanostructures. Furthermore, based on these improvement mechanisms, rationally designed Pt-based nanoparticles are summarized in terms of different synthetic strategies such as wet-chemical synthesis, Pt-skin catalysts, electrochemically dealloyed nanomaterials, and Pt-monolayer deposition. These nanoparticulate electrocatalysts show greatly enhanced catalytic performance towards ORR, aiming not only to outperform the commercial Pt/C, but also to exceed the US Department of Energy 2015 technical target ($30/kW and 5000 h).

One-step sonochemical syntheses of Ni@Pt core-shell nanoparticles with controlled shape and shell thickness for fuel cell electrocatalyst

We demonstrate a facile one-step method to synthesize Ni@Pt core-shell nanoparticles (NPs) with a control over the shape and the Pt-shell thickness of the NPs. By adjusting the relative reactivity of the Pt and Ni reagents in ultrasound-assisted polyol reactions, two Ni@Pt NP samples of the same composition (Ni/Pt=1) and size (3-4 nm) but with different particle shape (octahedral vs. truncated octahedral) and different Pt-shell thicknesses (1-2 vs. 2-3 monolayer) are obtained. The control is achieved by using different Ni reagents, Ni(acac)2 (acac=acetylacetonate) and Ni(hfac)2 (hfac=hexafluoroacetylacetonate). A reaction mechanism that can explain all of the observations is proposed. The Ni@Pt NPs show up to threefold higher mass activity than pure Pt NPs in oxygen reduction reaction. Between the two Ni@Pt NP samples, the one composed of octahedral NPs with the thicker Pt-shell has higher activity than the other.

Engineering nanointerfaces for nanocatalysis

We focus on recent advances in the delicate design of well-defined nanointerfaces to promote nanocatalysis towards renewable energy.

Twinning boundary-elongated hierarchical Pt dendrites with an axially twinned nanorod core for excellent catalytic activity

Introduction of twinning boundary elongation and lattice mismatch to the hierarchical and dendritic Pt₃Ni@Pt nanostructures by heteroepitaxial twinning transfer from five-fold twinned Pt₃Ni nanorods leads to great enhancement of the electrocatalytic performance in MOR and ORR.

Electrocatalytic activity of various types of h-BN for the oxygen reduction reaction

Enhancement of oxygen reduction reaction activity of a Au electrode by modification with various types of BN nanostructure.

Facile synthesis of 3D Pd–P nanoparticle networks with enhanced electrocatalytic performance towards formic acid electrooxidation

3D Pd–P nanoparticle networks (NNs) have been successfully synthesized using a facile one-step soft-template-assisted method. The as-prepared Pd–P NNs exhibit markedly improved activity and stability towards formic acid electrooxidation over Pd NNs, commercial Pd/C and Pd–P nanoparticle aggregates (NAs).

From Pt-rich dendrites to Ni-rich cuboctahedrons: structural evolution and electrocatalytic property studies

The composition and morphology evolution for Pt_xNi_1−x (0 < x < 1) nanoalloys was achieved by adjusting the growth kinetics, which were found different under different temperatures. The structure-dependent electrocatalytic performance was evaluated with MOR as the model reaction.

Magnetic PEGylated Pt3Co nanoparticles as a novel MR contrast agent: in vivo MR imaging and long-term toxicity study

Magnetic resonance (MR) imaging using magnetic nanoparticles as the contrast agent has been extensively explored in biomedical imaging and disease diagnosis. Herein, we develop biocompatible polymer coated ultra-small Pt3Co magnetic nanoparticles as a new T2-weighted MR imaging contrast agent. A unique class of alloy Pt3Co nanoparticles is synthesized through a thermal decomposition method. After being modified with polyethylene glycol (PEG), the obtained Pt3Co-PEG nanoparticles exhibit an extremely high T2-weighted relaxivity rate (r2) up to 451.2 mM s(-1), which is much higher than that of Resovist®, a commercial T2-MR contrast agent used in the clinic. In vitro experiments indicate no obvious cytotoxicity of Pt3Co-PEG nanoparticles to various cell lines. After intravenous injection of Pt3Co-PEG nanoparticles, in vivo T2-weighted MR imaging of tumor-bearing mice reveals strong tumor contrast, which is much higher than that offered by injecting Resovist®. We further study the long-term biodistribution and toxicology of this new type of MR contrast nanoparticles after intravenous injection into healthy mice. Despite the significant retention of Pt3Co-PEG nanoparticles in the mouse liver and spleen, no appreciable toxicity of these nanoparticles to the treated animals has been noted in our detailed histological and hematological analysis over a course of 60 days. Our work demonstrates that functionalized Pt3Co nanoparticles may be a promising new type of T2-weighted MR contrast agent potentially useful in biomedical imaging and diagnosis.

25th anniversary article: galvanic replacement: a simple and versatile route to hollow nanostructures with tunable and well-controlled properties

This article provides a progress report on the use of galvanic replacement for generating complex hollow nanostructures with tunable and well-controlled properties. We begin with a brief account of the mechanistic understanding of galvanic replacement, specifically focused on its ability to engineer the properties of metal nanostructures in terms of size, composition, structure, shape, and morphology. We then discuss a number of important concepts involved in galvanic replacement, including the facet selectivity involved in the dissolution and deposition of metals, the impacts of alloying and dealloying on the structure and morphology of the final products, and methods for promoting or preventing a galvanic replacement reaction. We also illustrate how the capability of galvanic replacement can be enhanced to fabricate nanomaterials with complex structures and/or compositions by coupling with other processes such as co-reduction and the Kirkendall effect. Finally, we highlight the use of such novel metal nanostructures fabricated via galvanic replacement for applications ranging from catalysis to plasmonics and biomedical research, and conclude with remarks on prospective future directions.