Shape-Controlled Synthesis of Pt3Co Nanocrystals with High Electrocatalytic Activity toward Oxygen Reduction

Singular behaviour of atomic ordering in Pt-Co nanocubes starting from core-shell configurations

The ordered structure of platinum-cobalt (Pt-Co) alloy nanoparticles has been studied actively because the structure influences their magnetic and catalytic properties. On the Pt-Co alloy's surface, Pt atoms preferentially segregate during annealing to reduce the surface energy. Such surface segregation has been shown to promote the formation of an ordered structure near the surface of Pt-Co thin films. Although this phenomenon seems also useful to control the nanoparticle structure, this has not been observed. Here, we have studied the ordered structure in annealed Pt@Co core-shell nanoparticles using a scanning transmission electron microscope. The nanoparticles were chemically synthesized, and their structural changes after annealing at 600 °C, 700 °C, and 800 °C for 3 h were observed. After being annealed at 600 °C and 800 °C, the particles contained the L1₂-Pt₃Co ordered structure. The structure seems reasonable considering an initial Pt : Co ratio of ∼4 : 1. However, we found that the L1₀-PtCo structure was formed near the nanoparticle surface after annealing at 700 °C. The L1₀-PtCo structure was thought to be formed from the surface segregation of Pt atoms and insufficient diffusion of Pt and Co atoms to mix them in the particle overall.

In Situ Growth of Pt-Co Nanocrystals on Different Types of Carbon Supports and Their Electrochemical Performance toward Oxygen Reduction

Carbon-supported Pt-M (M = Co, Ni, and Fe) alloy nanocrystals are widely used as catalysts toward oxygen reduction, a reaction key to the operation of proton-exchange membrane fuel cells. Here we report a colloidal method for the in situ growth of Pt-Co nanocrystals on various commercial carbon supports. The use of different carbon supports resulted in not only variations in size and composition for the nanocrystals but also their catalytic activity and durability toward oxygen reduction in acidic media. Among the nanocrystals, those grown on Vulcan XC72 and Ketjenblack EC300J showed the highest specific and mass activities in the 0.1 M HClO4 and 0.05 M H2SO4 electrolytes, respectively. Additionally, the catalysts also showed different durability depending on the strength of the interaction between the nanocrystals and the carbon support. Our analysis demonstrated that the difference in catalytic performance could be ascribed to the distinct effects of carbon support on both the synthetic and catalytic processes.

Enhancement of Activity and Development of Low Pt Content Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O₂-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H₂O₂-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO₂, WO₃, Ta₂O₅, Nb₂O₅, and ZrO₂), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt-Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal-support interactions and interfacial structural changes affecting adsorption and activation of O₂-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional.

An Overview on Pt3 X Electrocatalysts for Oxygen Reduction Reaction

The activity of Pt towards oxygen reduction reaction (ORR) can be enhanced by alloying it with secondary metals. They can be grouped into three different classes: alloys, bimetallics and intermetallics. Although alloys and bimetallics exhibit enhanced performance, often they are limited by metal dissolution and resulted in poor durability. This invokes the need on the development of ordered intermetallics. In this minireview we comprehensively present the recent progress and developments of Pt₃ X alloys and intermetallics towards ORR. Additionally, major technical challenges and possible future research directions to overcome these challenges are discussed to facilitate further research in this area.

PtCo-excavated rhombic dodecahedral nanocrystals for efficient electrocatalysis

Platinum (Pt)-based catalysts have shown excellent catalytic performance in many fields, particularly for the oxygen reduction reaction (ORR) and direct oxidation of small fuel molecules. Further development of Pt-based electrocatalysts relies on the morphology design of Pt-based alloy nanocrystals (NCs) with highly accessible and active surface sites to optimize Pt atomic utilization. In this work, we reported PtCo-excavated rhombic dodecahedral (ERD) NCs consisting of the self-assembly of 24 ultrathin nanosheets synthesized by a simple wet chemical method. The morphology can be regulated from convex to excavated polyhedra by controlling the amount of formaldehyde and the molar ratio of the Co/Pt precursor. The as-prepared PtCo ERD NCs/C catalyst exhibits excellent ORR performance, which has about 12 times higher specific activity and 6 times higher mass activity than the commercial Pt/C catalyst. It also displays good electrocatalytic ability towards methanol oxidation, in which the specific activity and mass activity are about 6 times higher and 2 times higher than the commercial Pt/C, respectively. Their enhanced activity is attributed to the excavated structure and alloy feature.

Pt-Co truncated octahedral nanocrystals: a class of highly active and durable catalysts toward oxygen reduction

We report a facile and scalable synthesis of Pt-Co truncated octahedral nanocrystals (TONs) by employing Pt(acac)2 and Co(acac)2 as precursors, together with CO molecules and Mn atoms derived from the decomposition of Mn2(CO)10 as a reductant and a {111} facet-directing agent, respectively. Both the composition and yield of the Pt-Co TONs could be varied through the introduction of CHCl3. When tested at 80 °C using membrane electrode assembly (MEA), the 4 nm Pt2.6Co TONs gave a mass activity of 294 A gPt-1 at beginning-of-life (BOL) and it increased to 384 A gPt-1 during recovery cycles. The mass activity at BOL only dropped by 24% after 30 000 voltage cycles at end-of-life (EOL) in a metal dissolution accelerated stress test. The Pt2.6Co/C catalyst outperformed the commercial TKK Pt3Co/C (230 A gPt-1 at BOL and 40% loss after 30 000 cycles at EOL) in terms of both activity and durability. Our systematic analysis suggested that the enhancement in activity can be attributed to the combination of small, uniform size and well-defined {111} facets. This new class of catalysts holds promise for applications in proton-exchange membrane fuel cells.

Preserving the Exposed Facets of Pt3Sn Intermetallic Nanocubes During an Order to Disorder Transition Allows the Elucidation of the Effect of the Degree of Alloy Ordering on Electrocatalysis

Controlling which facets are exposed in nanocrystals is crucial to understanding different activity between ordered and disordered alloy electrocatalysts. We modify the degree of ordering of Pt₃Sn nanocubes, while maintaining the shape and size, to enable a direct evaluation of the effect of the order on ORR catalytic activity. We demonstrate a 2.3-fold enhancement in specific activity by 60- and 30%-ordered Pt₃Sn nanocubes compared to 95%-ordered. This was shown to be likely due to surface vacancies in the less-ordered particles. The greater order, however, results in higher stability of the electrocatalyst, with the more disordered nanoparticles showing the dissolution of tin and platinum species during electrocatalysis.

One-Pot Synthesis of Concave Platinum-Cobalt Nanocrystals and Their Superior Catalytic Performances for Methanol Electrochemical Oxidation and Oxygen Electrochemical Reduction

Exploring highly efficient electro-catalysts is of significant urgency for the widespread uptake of the direct methanol fuel cells (DMFCs). Pt-Co nanocrystals have attracted considerable attentions because of their superior catalytic performance toward both methanol oxidation and oxygen reduction in the preliminary assessments. This Research Article presents a Pt-Co bimetal catalyst that is synthesized through a facile coreduction strategy. The Pt-Co nanocrystals have concave cubic shape with a high uniform size of 7-9 nm and Pt-rich surfaces. The catalysis of the concave cubic Pt-Co nanoparticles toward both methanol electrochemical oxidation reaction (MOR) and oxygen electrochemical reduction reaction (ORR) is evaluated. In comparison with the commercial Pt/C catalyst (Johnson Matthey), the present concave cubic Pt-Co catalyst displays superior performances in not only catalytic activity but also durability. The concave Pt-Co catalyst also shows higher activities than spherical and cubic Pt-Co nanoparticles. The dramatic enhancement is mainly attributed to its alloyed composition, Pt-rich surface and the concave nanostructure. The results of our research indicate that the concave Pt-Co nanocrystal could be a promising catalyst for both MOR and ORR. The present work might also raise more concerns on exploiting morphology and composition of nanocrystal catalysts, which are expected to provide high catalytic performance in electrochemical reactions.

Efficient NiSe-Ni3Se2/Graphene Electrocatalyst in Dye-Sensitized Solar Cells: The Role of Hollow Hybrid Nanostructure

Hollow and hybrid nanomaterials are excellent electrocatalysts on account of their novel electrocatalytic properties compared with homogeneous solid nanostructures. In this report, NiSe-Ni3Se2 hybrid nanostructure with morphology of hollow hexagonal nanodisk was synthesized in situ on graphene. A series of NiSe-Ni3Se2/RGO with different phase constitutions and nanostructures were obtained by controlling the durations of solvothermal treatment. Because of their unique hollow and hybrid structure, NiSe-Ni3Se2/RGO hollow nanodisks exhibited higher electrocatalytic performance than NiSe/RGO and solid NiSe-Ni3Se2/RGO nanostructure for reducing I3(-) as counter cell (CE) of dye-sensitized solar cells (DSSCs). Additionally, NiSe-Ni3Se2/RGO hollow nanodisks achieved much lower charge transfer resistance (Rct = 0.68 Ω) and higher power conversion efficiency (PCE) (7.87%) than those of Pt (Rct = 1.41 Ω, PCE = 7.28%).

Composition-tunable synthesis of Pt–Cu octahedral alloy nanocrystals from PtCu to PtCu3via underpotential-deposition-like process and their electro-catalytic properties

Octahedral Pt–Cu alloy nanocrystals with tunable composition from PtCu to PtCu₃ was successfully synthesized via UPD-like process.

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.

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.

One-pot synthesis of trimetallic Au@PdPt core-shell nanoparticles with high catalytic performance

The development of an efficient synthesis method to produce multimetallic nanoparticles (NPs) with a desirable structure is strongly required to clarify the structure-composition-property relationship of NPs and to investigate their possible applications. However, the controlled synthesis of NPs consisting of multiple (n ≥ 3) noble metal components has been relatively unexplored in comparison to bimetallic NPs. In the present work, we have demonstrated a facile one-pot aqueous approach for the controlled synthesis of trimetallic Au@PdPt core-shell NPs with a well-defined octahedral Au core and a highly crystalline dendritic Pd-Pt alloy shell (Auoct@PdPt NPs). The simultaneous reduction of multiple metal precursors with dual reducing agents, namely, ascorbic acid and hydrazine, gave a fine control over the nucleation and growth kinetics of NPs, resulting in the formation of novel Auoct@PdPt NPs. The prepared NPs showed excellent catalytic performance for methanol electrooxidation, which can be attributed to their optimized binding strength toward adsorbate molecules due to the improved charge transfer between core and shell of the NPs. The present strategy can offer a convenient and valuable way to fabricate multicomponent nanostructures with desired structures and functions.

Shape-control and electrocatalytic activity-enhancement of Pt-based bimetallic nanocrystals

Due to the increasing worldwide energy demand and environ-mental concerns, the need for alternative energy sources is growing stronger, and platinum catalysts in fuel cells may help make the technologies a reality. However, the pursuit of highly active Pt-based electrocatalysts continues to be a challenge. Scientists developing electrocatalysts continue to focus on characterizing and directing the construction of nanocrystals and advancing their electrochemical applications. Although chemists have worked on Pt-based bimetallic (Pt-M) preparations in the past, more recent research shows that both shape-controlled Pt-M nanocrystals and the assembly of these nanocrystals into supercrystals are promising new directions. A solution-based synthesis approach is an effective technique for preparing crystallographic facet-directed nanocatalysts. This is aided by careful selection of the metal precursor, capping ligand, reducing agent, and solvent. Incorporating a secondary metal M into the Pt lattice and manipulating the crystal facets on the surface cooperatively alter the electrocatalytic behavior of these Pt-M bimetallic nanocrystals. Specifically, chemists have extensively studied the {111}- and {100}-terminated crystal facets because they show unique atomic arrangement on surfaces, exhibit different catalytic performance, and possess specific resistance to toxic adsorbed carbon monoxide (COads). For catalysts to have maximum efficiency, they need to have resistance to COads and other poisonous carbon-containing intermediates when the catalysts operate under harsh conditions. A necessary design to any synthesis is to clearly understand and utilize the role of each component in order to successfully induce shape-controlled growth. Since chemists began to understand Pt nanocrystal shape-dependent electrocatalytic activity, the main obstacles blocking proton exchange membrane fuel cells are anode poisoning, sluggish kinetics at the cathode, and low activity. In this Account, we discuss the basic concepts in preparation of Pt-M bimetallic nanocrystals, focusing on several immaculate examples of manipulation at the nanoscale. We briefly introduce the prospects for applying Pt-M nanocrystals as electrocatalysts based on the electronic and geometric standpoints. In addition, we discuss several key parameters in the solution-based synthesis approach commonly used to facilitate Pt-M nanocrystals, such as reaction temperature and time, the combination of organic amines and acids, gaseous adsorbates, anionic species, and solvent. Each example features various nanoscale morphologies, such as spheres, cubes, octahedrons, and tetrahedrons. Additionally, we outline and review the superior electrocatalytic performances of the recently developed high-index Pt-M nanostructures. Next, we give examples of the electrocatalytic capabilities from these shape-defined Pt-M architectures by highlighting significant accomplishments in specific systems. Then, using several typical cases, we summarize electrochemical evaluations on the Pt-based shape-/composition-dependent nanocatalysts toward reactions on both the anode and the cathode. Lastly, we provide an outlook of current challenges and promising directions for shape-controlled Pt-M bimetallic electrocatalysts.

Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high activity of 3.3 A/mg(Pt) for the oxygen reduction reaction

Nanoscale Pt-Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt-Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt(-1) at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt(-1)). Our analysis suggests that this great enhancement of ORR activity could be attributed to the presence of a clean, well-preserved (111) surface for the Pt-Ni octahedra.

77Se solid-state NMR of As2Se3, As4Se4 and As4Se3 crystals: a combined experimental and computational study

(77)Se NMR parameters for three prototypical crystalline compounds (As2Se3, As4Se4 and As4Se3) have been determined from solid-state NMR spectra in the framework of an investigation concerning AsxSe(1-x) glass structure understanding. Density functional NMR calculations using the gauge including projector augmented wave methodology have been performed on X-ray and optimized crystal structures for a set of selenium-based crystals. These theoretical results have been combined with the experimental data in order to achieve a precise assignment of the spectral lines. This work and the high sensitivity of solid-state NMR to local order show that the structure of As4Se3 should be reinvestigated using state-of-the-art diffraction techniques. Calculations performed on several molecules derived from the crystal structures have demonstrated the limited effect of interlayer or intermolecular interactions on the isotropic chemical shifts. These interactions are therefore not responsible for the unexpected large chemical shift difference observed between these three systems that could mostly be attributed to the presence of short rings.

Octahedral PtNi nanoparticle catalysts: exceptional oxygen reduction activity by tuning the alloy particle surface composition

We demonstrate how shape selectivity and optimized surface composition result in exceptional oxygen reduction activity of octahedral PtNi nanoparticles (NPs). The alloy octahedra were obtained by utilizing a facile, completely surfactant-free solvothermal synthesis. We show that the choice of precursor ligands controls the shape, while the reaction time tunes the surface Pt:Ni composition. The 9.5 nm sized PtNi octahedra reached a 10-fold surface area-specific (~3.14 mA/cm(Pt)(2)) as well as an unprecedented 10-fold Pt mass based (~1.45 A/mg(Pt)) activity gain over the state-of-art Pt electrocatalyst, approaching the theoretically predicted limits.

Composition-controlled PtCo alloy nanocubes with tuned electrocatalytic activity for oxygen reduction

Modification of the electronic structure and lattice contraction of Pt alloy nanocatalysts through control over their morphology and composition has been a crucial issue for improving their electrocatalytic oxygen reduction reaction (ORR) activity. In the present work, we synthesized PtCo alloy nanocubes with controlled compositions (Pt(x)Co NCs, x = 2, 3, 5, 7, and 9) by regulating the ratio of surfactants and the amount of Co precursor to elucidate the effect of the composition of nanocatalysts on their ORR activity. Pt(x)Co NCs had a Pt-skin structure after electrochemical treatment. The electrocatalysis experiments revealed a strong correlation between ORR activity and Co composition. Pt₃Co NCs exhibited the best ORR performance among the various Pt(x)Co NCs. From density functional theory calculations, a typical volcano-type relationship was established between ORR activity and oxygen binding energy (E(OB)) on NC surfaces, which showed that Pt₃Co NCs had the optimal E(OB) to achieve the maximum ORR activity. X-ray photoelectron spectroscopy and X-ray diffraction measurements demonstrated that the electronic structure and lattice contraction of the Pt(x)Co NCs could be tuned by controlling the composition of NCs, which are highly correlated with the trends of E(OB) change.

One-pot synthesis of monodisperse 5 nm Pd-Ni nanoalloys for electrocatalytic ethanol oxidation

Highly monodisperse 5 nm Pd-Ni alloy nanoparticles (NPs) were prepared by the reduction of Pd(acac)(2)/Ni(acac)(2) mixtures with tert-butylamine-borane complex (TBAB) in the presence of oleic acid (OA) and oleylamine (OAm). Employing TBAB as an effective reductant and OA/OAm combination as an effective stabilizing agent is crucial to the formation of monodisperse Pd-Ni NPs. Experimental results collectively verify that the Pd-Ni alloy NPs form through the sequential nucleation-interdiffusion process and the simultaneous reduction of both metal precursors by the one-pot protocol is the key to the formation of homogeneous NPs. The Pd-Ni NPs were well-dispersed on carbon supports and chemically dealloyed after acetic acid washing through the selective dissolution of the less noble Ni component. The Pd-Ni NP catalysts exhibited much higher electrocatalytic activity and stability for ethanol oxidation than those of a commercial Pd/C catalyst.

All-metal layer-by-layer films: bimetallic alternate layers with accessible mesopores for enhanced electrocatalysis

We have prepared multilayer mesoporous bimetallic (Pt/Pd) alternating films by layer-by-layer (LbL) electrochemical deposition. Because of the high surface area and heterometallic interfacial atomic contacts, enhanced electrocatalytic activity for methanol oxidation reaction is realized. This novel LbL approach allows optimization of the electrocatalytic performance through precise tuning of the thickness of each layer.

Controlled synthesis of Pd-Pt alloy hollow nanostructures with enhanced catalytic activities for oxygen reduction

Pd-Pt alloy nanocrystals (NCs) with hollow structures such as nanocages with porous walls and dendritic hollow structures and Pd@Pt core-shell dendritic NCs could be selectively synthesized by a galvanic replacement method with uniform Pd octahedral and cubic NCs as sacrificial templates. Fine control over the degree of galvanic replacement of Pd with Pt allowed the production of Pd-Pt NCs with distinctly different morphologies. The synthesized hollow NCs exhibited considerably enhanced oxygen reduction activities compared to those of Pd@Pt core-shell NCs and a commercial Pt/C catalyst, and their electrocatalytic activities were highly dependent on their morphologies. The Pd-Pt nanocages prepared from octahedral Pd NC templates exhibited the largest improvement in catalytic performance. We expect that the present work will provide a promising strategy for the development of efficient oxygen reduction electrocatalysts and can also be extended to the preparation of other hybrid or hetero-nanostructures with desirable morphologies and functions.