Core-shell Octahedra Research Articles

Strain-Induced Corrosion Kinetics at Nanoscale Are Revealed in Liquid: Enabling Control of Corrosion Dynamics of Electrocatalysis

Summary Corrosion of nanoparticles during electrocatalysis, such as oxygen reduction reaction (ORR), occurs at the nanoscale and is vital for catalyst stability. Here, using liquid cell (LC) transmission electron microscopy (TEM), we study the corrosion process of palladium@platinum (Pd@Pt) core-shell octahedra in real time and reveal that both the static local strain and the evolving curvature synergistically control the nanoscale corrosion kinetics. Specifically, in locations with tensile strain and high local curvature, the etching process is much faster. Density functional theory (DFT) calculation suggests that the dissolution potential of the Pd nanocrystal decreases as the strain increases; meanwhile, Pd atoms tend to be corroded more easily on a surface under tensile strain than on one under compressive strain. With these insights on the corrosion mechanism at the nanoscale, we subsequently designed and synthesized nanoparticles with smaller strain, and these showed higher durability in both an in situ LC study and an ex situ ORR stability test.

Strain-Induced Corrosion Kinetics at Nanoscale Revealed in Liquid: Enabling Control of Corrosion Dynamics of Electrocatalysis

SummaryCorrosion at the nanoscale is vital for the stability of nanoparticles. Understanding the corrosion mechanism can offer insights on how to design more stable nanoparticles during electrocatalysis, such as oxygen reduction reaction (ORR). Here, using a liquid cell (LC) transmission electron microscopy (TEM) technique, we study the corrosion process of palladium@platinum (Pd@Pt) core-shell octahedra at real time. The results revealed that the nanoscale corrosion kinetics was determined synergistically by both the quasi-static factor, local strain, and dynamic factor, local curvature. Specifically, in locations with tensile strain and high local curvature, the etching process is much faster than other places. Density functional theory (DFT) calculation suggested that the dissolution potential of the nanoscale Pd nanocrystal is decreased by the increasement of the strain and meanwhile the calculation indicated that the nanoscale surface Pd atoms trended to be corroded more easily under tensile strain than that under compressive strain. Through the investigation of the nanoscale corrosion mechanisms above, we subsequently designed and synthesized a nanoparticle with smaller strain, which showed higher durability both in in-situ liquid cell and ex-situ ORR stability test.

Theoretical and Experimental Understanding of Hydrogen Evolution Reaction Kinetics in Alkaline Electrolytes with Pt-Based Core-Shell Nanocrystals.

The free energy of H adsorption (ΔGH) on a metallic catalyst has been taken as a descriptor to predict the hydrogen evolution reaction (HER) kinetics but has not been well applied in alkaline media. To assess this, we prepare Pd@Pt and PdH@Pt core-shell octahedra enclosed by Pt(111) facets as model catalysts for controlling the ΔGH affected by the ligand, the strain, and their ensemble effects. The Pt shell thickness is adjusted from 1 to 5 atomic layers by varying the amount of Pt precursor added during synthesis. In an alkaline electrolyte, the HER activity of core-shell models is improved either by the construction of core-shell structures or by the increased number of Pt shells. These experimental results are in good agreement with the ΔGH values calculated by the first-principles density functional theory with a complex surface strained core-shell slab model. However, enhanced HER activities of Pd@Pt and PdH@Pt core-shell nanocrystals over the Pt catalyst are inconsistent with the thermodynamic ΔGH scaling relationship only but can be explained by the work function and apparent ΔGH models that predict the interfacial electric field for the HER.

Synthesis and Catalytic Activities of Metal Shells (Monolayer, Bilayer, and Alloy Layer)-Coated Gold Octahedra toward Catalytic Reduction of Nitroaromatics

Different types of metal shells such as monolayer, bilayer, and alloy layer of gold (Au) and/or silver (Ag) on gold octahedra (AuOh) nanoparticles were synthesized using N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TPDT) without employing any external reducing agents. TPDT, an alternative to sodium borohydride, was used to prepare 15 different core–shell metal nanostructures. The direct colloidal synthesis and stabilization of silver octahedra nanoparticles is still a challenging task. The proposed methodology is an alternative synthetic route for Ag or Au nanoshell coated onto the AuOh core. UV–vis absorption spectroscopy, high-resolution electron transmission microscopy analyses with selected area electron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, and X-ray diffraction techniques were used to characterize the prepared core–shell octahedra nanostructures. High-angle annular dark field scanning transmission electron microscopy–energy-dispersive X-...

Facile One-Pot Synthesis of Pd@Pt1L Octahedra with Enhanced Activity and Durability toward Oxygen Reduction

A successful strategy for reducing the content of Pt without compromising the activity of a Pt-based catalyst is to deposit Pt as an ultrathin overlayer on the surface of another metal. Here, we report a facile one-pot synthesis of Pd@Pt1L (1L: one atomic layer) core–shell octahedra using a solution-phase method. The success of this method relies on the use of metal precursors with markedly different reduction kinetics. In a typical synthesis, the ratio between the initial reduction rates of the Pd(II) and Pt(II) precursors differed by almost 100 times, favoring the formation of Pd–Pt bimetallic octahedra with a core–shell structure. The reduction of the Pt(II) precursor at a very slow rate and the use of a high temperature allowed the deposited Pt atoms to spread and cover the entire surface of Pd octahedral seeds formed in the initial stage. More importantly, we were able to scale up this synthesis using continuous-flow reactors without compromising product quality. Compared to a commercial Pt/C catalyst, the Pd@Pt1L core–shell octahedra showed major augmentation in terms of catalytic activity and durability for the oxygen reduction reaction (ORR). After 10000 cycles of accelerated durability test, the core–shell octahedra still exhibited a mass activity of 0.45 A mg–1 Pt. We rationalized the experimental results using DFT calculations, including the mechanism of synthesis, ORR activities, and possible Pd–Pt atom swapping to enrich the outermost layer with Pd. Specifically, the as-synthesized Pd@Pt1L octahedra tended to take a slightly mixed surface composition because the deposited Pt atoms were able to substitute into Pd upon deposition on the edges; ORR energetics were more favorable on pure Pt shells as compared to significantly mixed Pd–Pt shells, and the activation energy barriers calculated for the Pd–Pt atom swapping were too prohibitive to significantly alter the surface composition of the as-synthesized Pd@Pt1L octahedra, helping sustain their activity for prolonged operation.

Transformation of carbon-supported Pt-Ni octahedral electrocatalysts into cubes: toward stable electrocatalysis.

Octahedral Pt-Ni catalyst nanoparticles (NPs) are predicted to exhibit high activity for the oxygen reduction reaction. However, until now this class of catalysts has been limited by its long-term performance, as a result of compositional and morphological instabilities of the NPs. In situ transmission electron microscopy (TEM) is a powerful technique for understanding morphological and compositional evolution under controlled conditions. It is of great importance to study the evolution of the morphology and elemental distribution in bimetallic NPs and their interaction with the support in reducing and oxidizing treatments at the atomic scale for the rational design of catalysts. Here, we use in situ TEM to follow dynamic changes in the NP morphology, faceting and elemental segregation under working conditions in previously unreported Pt-Ni core-shell octahedral structures. We follow changes in the Pt-Ni catalyst from a segregated structure to an alloyed shell configuration and then a more spherical structure as a function of temperature under reducing conditions. Exposure to an oxidizing environment then leads to oxidation of the C support, while the spherical NPs undergo a cycle of transformations into cubic NPs followed by the reaction to spherical NPs. The formation of the cubic NPs results from CO formation during C oxidation, before it is finally oxidized to CO2. Our observations may pave the way towards the design of optimized structure-stability electrocatalysts and highlight the importance of TEM visualization of degradation and transformation pathways in bimetallic Pt-Ni NPs under reducing and oxidizing conditions.

Facile Synthesis of Ru-Based Octahedral Nanocages with Ultrathin Walls in a Face-Centered Cubic Structure

Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hexagonal close-packed (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temperature were all instrumental to the formation of Pd@Ru core–shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five atomic layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent dens...

Structural Evolution of Sub-10 nm Octahedral Platinum-Nickel Bimetallic Nanocrystals.

Octahedral Pt alloy nanocrystals (NCs) have shown excellent activities as electrocatalysts toward oxygen reduction reaction (ORR). As the activity and stability of NCs are highly dependent on their structure and the elemental distribution, it is of great importance to understand the formation mechanism of octahedral NCs and to rationally synthesize shape-controlled alloy catalysts with optimized ORR activity and stability. However, the factors controlling the structural and compositional evolution during the synthesis have not been well understood yet. Here, we systematically investigated the structure and composition evolution pathways of Pt-Ni octahedra synthesized with the assistance of W(CO)6 and revealed a unique core-shell structure consisting of a Pt core and a Pt-Ni alloy shell. Below 140 °C, sphere-like pure Pt NCs with the diameter of 3-4 nm first nucleated, followed by the isotropic growth of Pt-Ni alloy on the seeds at temperatures between 170 and 230 °C forming Pt@Pt-Ni core-shell octahedra with {111} facets. Owing to its unique structure, the Pt@Pt-Ni octahedra show an unparalleled stability during potential cycling, that is, no activity drop after 10 000 cycles between 0.6 and 1.0 V. This work proposes the Pt@Pt-Ni octahedra as a high profile electrocatalyst for ORR and reveals the structural and composition evolution pathways of Pt-based bimetallic NCs.

Cover Picture: Facile Synthesis of Pd@Pt3–4L Core–Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction (ChemCatChem 3/2017)

Cover Picture: Facile Synthesis of Pd@Pt_{3<b>–</b>4L} Core–Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction (ChemCatChem 3/2017)

Facile Synthesis of Pd@Pt3–4L Core–Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction

AbstractThe front cover artwork for Issue 3/2017 is provided by the Xia group at the Georgia Institute of Technology. The artwork shows a strategy for the synthesis of Pd@Pt3‐4L core–shell octahedra with a clean surface and their transmission electron microscopy images. See the Full Paper itself at https://doi.org/10.1002/cctc.201601438.

Facile Synthesis of Pd@Pt3–4L Core–Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction

AbstractThe presence of a capping agent or stabilizer in the synthesis of colloidal metal nanocrystals will compromise their performance if employed as electrocatalysts. Herein we demonstrate the synthesis of Pd@Pt3–4L core–shell octahedral nanocrystals with greatly enhanced activity toward the oxygen reduction reaction by eliminating the use of any capping agent or stabilizer. This was achieved by employing Pd octahedral seeds with well‐defined {1 1 1} facets and by dispersing them on a carbon black support prior to Pt deposition. Upon optimization of the reaction conditions, Pt ultrathin shells could be conformally deposited on the Pd octahedral seeds in a layer‐by‐layer fashion without involving self‐nucleation or island growth for the Pt atoms. The as‐obtained octahedral Pd@Pt3–4L/C catalyst exhibited a specific activity 50 % greater than that of a reference sample prepared in the presence of a polymer stabilizer such as poly(vinyl pyrrolidone). The polymer‐free catalyst also showed 5‐fold enhancement in specific activity if benchmarked against a commercial Pt/C catalyst.

Molybdenum-Doped PdPt@Pt Core-Shell Octahedra Supported by Ionic Block Copolymer-Functionalized Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst.

Development of highly active and durable electrocatalysts that can effectively electrocatalyze oxygen reduction reactions (ORR) still remains one important challenge for high-performance electrochemical conversion and storage applications such as fuel cells and metal-air batteries. Herein, we propose the combination of molybdenum-doped PdPt@Pt core-shell octahedra and the pyrene-functionalized poly(dimethylaminoethyl methacrylate)-b-poly[(ethylene glycol) methyl ether methacrylate] ionic block copolymer-functionalized reduced graphene oxide (Mo-PdPt@Pt/IG) to effectively augment the interfacial cohesion of both components using a tunable ex situ mixing strategy. The rationally designed Mo-PdPt@Pt core-shell octahedra have unique compositional benefits, including segregation of Mo atoms on the vertexes and edges of the octahedron and 2-3 shell layers of Pt atoms on a PdPt alloy core, which can provide highly active sites to the catalyst for ORR along with enhanced electrochemical stability. In addition, the ionic block copolymer functionalized graphene can facilitate intermolecular charge transfer and good stability of metal NPs, which arises from the ionic block copolymer interfacial layer. When the beneficial features of the Mo-PdPt@Pt and IG are combined, the Mo-PdPt@Pt/IG exhibits substantially enhanced activity and durability for ORR relative to those of commercial Pt/C. Notably, the Mo-PdPt@Pt/IG shows mass activity 31-fold higher than that of Pt/C and substantially maintains high activities after 10 000 cycles of intensive durability testing. The current study highlights the crucial strategies in designing the highly active and durable Pt-based octahedra and effective combination with functional graphene supports toward the synergetic effects on ORR.

Synthesis of compositionally tunable, hollow mixed metal sulphide CoxNiySz octahedral nanocages and their composition-dependent electrocatalytic activities for oxygen evolution reaction.

Hollow nanostructures such as nanocages and nanoframes can serve as advanced catalysts with their enlarged active surface areas, and hence they have been of widespread interest. Despite the recent progress in the synthesis of this class of nanomaterials, hollow nanostructures with tunable compositions and controlled morphologies have rarely been reported. Here, we report a facile synthetic route to a series of compositionally tunable, hollow mixed metal sulphide (CoxNiySz) octahedral nanocages. The sulfidation of CoO octahedral nanoparticles generates CoO@CoxSy core-shell octahedra, and the in situ etching of the CoO core and annealing yield Co9S8 (pentlandite) octahedral nanocages (ONC). The addition of a Ni precursor during the etching/annealing process of CoO@CoxSy core-shell octahedra progressively yields hollow ONC structures of Co9-xNixS8, Ni9S8, Ni9S8/β-NiS, and Ni3S2/β-NiS via cation exchange reactions. Mixed cobalt/nickel sulphide, Co9-xNixS8 ONC, shows superior oxygen evolution reaction activity to monometallic sulphide ONC structures, demonstrating the synergy between different metal species.

Seed‐Mediated Growth of Colloidal Metal Nanocrystals: Scaling up the Production through Geometric and Stoichiometric Analyses

AbstractDespite the remarkable progress in shape‐controlled synthesis of noble‐metal nanocrystals, there still exists a major gap between academic studies and industrial applications. This gap can be largely attributed to the inability to produce nanocrystals in large quantities while retaining uniformity and good control over both size and shape. Here we provide a new perspective on increasing the volume of production during a synthesis of colloidal nanocrystals via seed‐mediated growth. We demonstrate that the yield of Pd octahedra and Pd@Pt3−4L (L: atomic layers) core–shell octahedra with different sizes can be increased from ca. 2 mg to over 500 mg without compromising the product quality. Specifically, we apply geometric and stoichiometric analyses to calculate the minimum amounts of reagents needed for the complete evolution of Pd cubic seeds into Pd octahedra, and subsequently for the formation of Pd@Pt3−4L core–shell octahedra through conformal Pt coating. The simple analyses allow us to obtain uniform products by avoiding homogeneous nucleation while minimizing the waste of reagents.

Quantitative Analysis of the Reduction Kinetics Responsible for the One-Pot Synthesis of Pd-Pt Bimetallic Nanocrystals with Different Structures.

We report a quantitative understanding of the reduction kinetics responsible for the formation of Pd-Pt bimetallic nanocrystals with two distinctive structures. The syntheses involve the use of KBr to manipulate the reaction kinetics by influencing the redox potentials of metal precursor ions via ligand exchange. In the absence of KBr, the ratio between the initial reduction rates of PdCl4(2-) and PtCl4(2-) was about 10.0, leading to the formation of Pd@Pt octahedra with a core-shell structure. In the presence of 63 mM KBr, the products became Pd-Pt alloy nanocrystals. In this case, the ratio between the initial reduction rates of the two precursors dropped to 2.4 because of ligand exchange and, thus, the formation of PdBr4(2-) and PtBr4(2-). The alloy nanocrystals took a cubic shape owing to the selective capping effect of Br(-) ions toward the {100} facets. Relative to the alloy nanocubes, the Pd@Pt core-shell octahedra showed substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). Specifically, the specific (1.51 mA cm(-2)) and mass (1.05 A mg(-1) Pt) activities of the core-shell octahedra were enhanced by about four- and three-fold relative to the alloy nanocubes (0.39 mA cm(-2) and 0.34 A mg(-1) Pt, respectively). Even after 20000 cycles of accelerated durability test, the core-shell octahedra still exhibited a mass activity of 0.68 A mg(-1) Pt, twice that of a pristine commercial Pt/C catalyst.

Atomic layer-by-layer deposition of platinum on palladium octahedra for enhanced catalysts toward the oxygen reduction reaction.

We systematically evaluated two different approaches to the syntheses of Pd@PtnL (n = 2-5) core-shell octahedra. We initially prepared the core-shell octahedra using a polyol-based route by titrating a Pt(IV) precursor into the growth solution containing Pd octahedral seeds at 200 °C through the use of a syringe pump. The number of Pt atomic layers could be precisely controlled from two to five by increasing the volume of the precursor solution while fixing the amount of seeds. We then demonstrated the synthesis of Pd@PtnL octahedra using a water-based route at 95 °C through the one-shot injection of a Pt(II) precursor. Due to the large difference in reaction temperature, the Pd@PtnL octahedra obtained via the water-based route showed sharper corners than their counterparts obtained through the polyol-based route. When compared to a commercial Pt/C catalyst based upon 3.2 nm Pt particles, the Pd@PtnL octahedra prepared using both methods showed similar remarkable enhancement in terms of activity (both specific and mass) and durability toward the oxygen reduction reaction. Calculations based upon periodic, self-consistent density functional theory suggested that the enhancement in specific activity for the Pd@PtnL octahedra could be attributed to the destabilization of OH on their PtnL*/Pd(111) surface relative to the {111} and {100} facets exposed on the surface of Pt/C. The destabilization of OH facilitates its hydrogenation, which was found to be the rate-limiting step of the oxygen reduction reaction on all these surfaces.

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.

Facet-dependent optical properties of polyhedral Au–Cu2O core–shell nanocrystals

We fabricated Au-Cu₂O core-shell octahedra, cuboctahedra, and nanocubes having sizes of 90-220 nm using 50 nm octahedral cores. The smaller particle sizes minimize the strong light scattering features from the Cu₂O shells and enable the surface plasmon resonance (SPR) absorption band of the gold cores to be clearly identified. Beyond a lower shell thickness limit, the SPR band positions of the gold cores are independent of the shell thickness, but are strongly dependent on the exposed particle surfaces. The plasmonic band red-shifts from Au-Cu₂O octahedra to cuboctahedra and nanocubes, and differs by as much as 26 nm between the octahedra and the nanocubes. The same facet-dependent optical effects were observed using larger octahedral gold cores and cubic gold cores. In contrast, simulation spectra show progressively red-shifted SPR band positions with increasing shell thickness. The Cu₂O shells are also found to exhibit facet-dependent optical behavior. These nanocrystals can respond to changes in the solvent environment such as solvents with different refractive indices, indicating that the plasmonic field of the gold cores can extend beyond the particle surfaces despite the presence of thick shells. Plane-selective spectral responses to low concentrations of surfactants were also recorded.

The Effect of Lattice Strain on the Catalytic Properties of Pd Nanocrystals

The effect of lattice strain on the catalytic properties of Pd nanoparticles is systematically studied. Synthetic strategies for the preparation of a series of shape-controlled Pd nanocrystals with lattice strain generated from different sources has been developed. All of these nanocrystals were created with the same capping agent under similar reaction conditions. First, a series of Pd nanoparticles was synthesized that were enclosed in {111} surfaces: Single-crystalline Pd octahedra, single-crystalline AuPd core-shell octahedra, and twinned Pd icosahedra. Next, various {100}-terminated particles were synthesized: Single-crystalline Pd cubes and single-crystalline AuPd core-shell cubes. Different extents of lattice strain were evident by comparing the X-ray diffraction patterns of these particles. During electrocatalysis, decreased potentials for CO stripping and increased current densities for formic-acid oxidation were observed for the strained nanoparticles. In the gas-phase hydrogenation of ethylene, the activities of the strained nanoparticles were lower than those of the single-crystalline Pd nanoparticles, perhaps owing to a larger amount of cetyl trimethylammonium bromide on the surface.

Fabrication of Au–Pd Core–Shell Heterostructures with Systematic Shape Evolution Using Octahedral Nanocrystal Cores and Their Catalytic Activity

By using octahedral gold nanocrystals with sizes of approximately 50 nm as the structure-directing cores for the overgrowth of Pd shells, Au-Pd core-shell heterostructures with systematic shape evolution can be directly synthesized. Core-shell octahedra, truncated octahedra, cuboctahedra, truncated cubes, and concave cubes were produced by progressively decreasing the amount of the gold nanocrystal solution introduced into the reaction mixture containing cetyltrimethylammonium bromide (CTAB), H(2)PdCl(4), and ascorbic acid. The core-shell structure and composition of these nanocrystals has been confirmed. Only the concave cubes are bounded by a variety of high-index facets. This may be a manifestation of the release of lattice strain with their thick shells at the corners. Formation of the [CTA](2)[PdBr(4)] complex species has been identified spectroscopically. Time-dependent UV-vis absorption spectra showed faster Pd source consumption rates in the growth of truncated cubes and concave cubes, while a much slower reduction rate was observed in the generation of octahedra. The concave cubes and octahedra were used as catalysts for a Suzuki coupling reaction. They can all serve as effective and recyclable catalysts, but the concave cubes gave higher product yields with a shorter reaction time attributed to their high-index surface facets. The concave cubes can also catalyze a wide range of Suzuki coupling reactions using aryl iodides and arylboronic acids with electron-donating and -withdrawing substituents.