Hui Zhang

Shape-Controlled Synthesis of Pd Nanocrystals and Their Catalytic Applications

Palladium is a marvelous catalyst for a rich variety of reactions in industrial processes and commercial devices. Most Pd-catalyzed reactions exhibit structure sensitivity, meaning that the activity or selectivity depends on the arrangement of atoms on the surface. Previously, such reactions could only be studied in ultrahigh vacuum using Pd single crystals cut with a specific crystallographic plane. However, these model catalysts are far different from real catalytic systems owing to the absence of atoms at corners and edges and the extremely small specific surface areas for the model systems. Indeed, enhancing the performance of a Pd-based catalyst, in part to reduce the amount needed of this precious and rare metal for a given reaction, requires the use of Pd with the highest possible specific surface area. Recent advances in nanocrystal synthesis are offering a great opportunity to investigate and quantify the structural sensitivity of catalysts based on Pd and other metals. For a structure-sensitive reaction, the catalytic properties of Pd nanocrystals are strongly dependent on both the size and shape. The shape plays a more significant role in controlling activity and selectivity, because the shape controls not only the facets but also the proportions of surface atoms at corners, edges, and planes, which affect the outcomes of possible reactions. We expect catalysts based on Pd nanocrystals with optimized shapes to meet the increasing demands of industrial applications at reduced loadings and costs. In this Account, we discuss recent advances in the synthesis of Pd nanocrystals with controlled shapes and their resulting performance as catalysts for a large number of reactions. First, we review various synthetic strategies based on oxidative etching, surface capping, and kinetic control that have been used to direct the shapes of nanocrystals. When crystal growth is under thermodynamic control, the capping agent plays a pivotal role in determining the shape of a product by altering the order of surface energies for different facets through selective adsorption; the resulting product has the lowest possible total surface energy. In contrast, the product of a kinetically controlled synthesis often deviates from the thermodynamically favored structure, with notable examples including nanocrystals enclosed by high-index facets or concave surfaces. We then discuss the key parameters that control the nucleation and growth of Pd nanocrystals to decipher potential growth mechanisms and build a connection between the experimental conditions and the pathways to different shapes. Finally, we present a number of examples to highlight the use of these Pd nanocrystals as catalysts or electrocatalysts for various applications with structure-sensitive properties. We believe that a deep understanding of the shape-dependent catalytic properties, together with an ability to experimentally maneuver the shape of metal nanocrystals, will eventually lead to rational design of advanced catalysts with substantially enhanced performance.

Platinum Concave Nanocubes with High-Index Facets and Their Enhanced Activity for Oxygen Reduction Reaction†

Platinum nanoparticles are widely used as the primary catalysts in a myriad of industrial processes such as CO/NOx oxidation in catalytic converters, nitric acid production, petroleum cracking, as well as hydrogen (or alcohol) oxidation and oxygen reduction reactions in fuel-cell technology. For most catalytic reactions, it has been shown that high-index planes, which are associated with large numbers of atomic steps, edges, and kinks hold the key to the enhancement of catalytic performance in terms of activity and/or selectivity. A number of protocols have been demonstrated for generating Pt nanoparticles enclosed by high-index facets, including those based on electrochemical reduction and heat treatment. For example, Sun and co-workers have reported the synthesis of tetrahexahedral (THH) Pt nanocrystals with high-index facets, such as {730}, {210}, and {520}, by applying a square-wave potential to polycrystalline Pt microspheres supported on a glassy carbon electrode. Although these Pt nanocrystals have been shown to have high catalytic activity, their sizes are still relatively too large and the method of preparation is rather limited in terms of production volume. It still remains a challenge to produce Pt nanocrystals with high-index facets by using a simple, scalable route based on wet chemical reduction. Over the past several years, kinetic control has been demonstrated as a simple and versatile approach to the shapecontrolled synthesis of noble-metal nanocrystals in the solution phase. In general, kinetic control can be achieved by: 1) substantially slowing down the formation rate of atoms, 2) using a weak reducing agent, 3) introducing an oxidation process, and 4) taking advantage of Ostwald ripening. When the concentration of metal atoms in the solution is low, the atoms tend to add to the edges and corners of a seed rather than the entire surface, thus leading to the formation of nanocrystals with thermodynamically unfavorable morphologies, including rods, plates, multipods, and dendritic structures. In recent years, nanocrystals with concave rather than flat faces have attracted attention because of their high-index facets. To this end, Zheng and co-workers have demonstrated the synthesis of concave Pd polyhedral nanocrystals with high electrocatalytic activity for formic acid oxidation. Mirkin and co-workers have also reported the synthesis of concave cubic Au nanocrystals, and demonstrated higher chemical activity compared to octahedra enclosed by lowindex {111} facets. Herein we report the first synthesis of Pt concave nanocubes enclosed by high-index facets including {510}, {720}, and {830} by slowly adding an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 into deionized water by using two syringe pumps. In this synthesis, the formation of a Pt pyrophosphato complex (that is formed by mixing K2PtCl4 and Na2H2P2O7) and the slow addition of this precursor by a syringe pump are believed to play a key role in the formation of Pt concave nanocubes. In this case, the seeds selectively overgrow from corners and edges, and the Br ion serves as a capping agent to block the growth of the h100i axis. The Pt concave nanocubes exhibited substantially enhanced specific activity (per unit surface area) relative to those of Pt nanocubes, cuboctahedra, and commercial Pt/C catalysts that are bounded by low-index facets such as {100} and {111} toward the oxygen reduction reaction (ORR), which is the ratedetermining step in a proton-exchangemembrane (PCM) fuel cell. In a typical synthesis, an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 were prepared separately and then injected simultaneously at an injection rate of 67 mLmin 1 by using two syringe pumps into deionized water maintained at 95 8C. The color of the solution immediately turned from light pink to black upon the addition of the reactant solutions, thus indicating rapid reduction of PtCl4 2 into elemental Pt by NaBH4. Figure 1a shows a typical transmission electron microscopy (TEM) image of the product that contains Pt nanocubes with a concave structure. [*] Dr. T. Yu, D. Y. Kim, Prof. H. Zhang, Prof. Y. Xia Department of Biomedical Engineering Washington University Saint Louis, MO 63130 (USA) E-mail: xia@biomed.wustl.edu D. Y. Kim Department of Chemical and Biomolecular Engineering (BK21 graduate program) Korea Advanced Institute of Science and Technology (KAIST) 335 Gwahangro, Yuseong-gu, Daejeon 305-701 (Korea)

Synthesis of Pd-Pt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction

This article describes a systematic study of the galvanic replacement reaction between PtCl(6)(2-) ions and Pd nanocrystals with different shapes, including cubes, cuboctahedrons, and octahedrons. It was found that Br(-) ions played an important role in initiating, facilitating, and directing the replacement reaction. The presence of Br(-) ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br(-) ions on this crystallographic plane. The site-selective galvanic replacement resulted in the formation of Pd-Pt bimetallic nanocrystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The Pd-Pt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the largest specific electrochemical surface area and was found to be four times as active as the commercial Pt/C catalyst for the ORR in terms of equivalent Pt mass.

Noble-metal nanocrystals with concave surfaces: synthesis and applications.

Metal nanocrystals with concave surfaces are interesting for a wide variety of applications that are related to catalysis, plasmonics, and surface-enhanced spectroscopy. This interest arises from their high-index facets, surface cavities, and sharp corners/edges. Two major challenges are associated with this novel class of nanocrystals: 1)u2005how to generate a concave surface with negative curvature, which is not favored by thermodynamics owing to its higher energy than the convex counterpart; and 2)u2005how to stabilize the morphology of a nanocrystal with concave structures on the surface. Recently, a number of different procedures have been developed for the synthesis of noble-metal nanocrystals with concave surfaces. This Review provides a brief account of these developments, with the aim of offering new insights into the growth mechanisms. We focus on methods based on two general strategies: 1)u2005site-specific dissolution through etching and galvanic replacement; and 2)u2005directionally controlled overgrowth by facet-selective capping, kinetic control, and template-directed epitaxy. Their enhanced catalytic and electrocatalytic properties are also described.

Palladium Concave Nanocubes with High-Index Facets and Their Enhanced Catalytic Properties†

NSF[DMR-0804088, ECS-0335765]; Washington University in St. Louis; China Scholarship Council (CSC)

Synthesis of flower-like ZnO nanostructures by an organic-free hydrothermal process

Flower-like ZnO nanostructures, which consisted of sword-like ZnO nanorods, have been prepared by an organic-free hydrothermal process. The XRD pattern indicated that the flower-like ZnO nanostructures were hexagonal. The SAED and HRTEM experiments implied that the sword-like ZnO nanorods were single crystal in nature and preferentially grew up along the [001] direction. The effects of temperature, pH value and mineralizer on the morphology have been also investigated. It is considered that pH value is the main factor to influence the morphology because of its effect on the initial nuclei and growth environment of ZnO. Finally, the mechanism for organic-free hydrothermal synthesis of the flower-like ZnO nanostructure is discussed.

Shape‐Controlled Synthesis of Copper Nanocrystals in an Aqueous Solution with Glucose as a Reducing Agent and Hexadecylamine as a Capping Agent

NSF [DMR 0804088, 1104614, ECS-0335765]; Washington University in St. Louis; China Scholarship Council (CSC)

Palladium nanocrystals enclosed by {100} and {111} facets in controlled proportions and their catalytic activities for formic acid oxidation

This article reports a seed-mediated approach to polyhedral nanocrystals of Pd with controlled sizes, shapes, and different proportions of {100} to {111} facets on the surface. The success of this synthesis relies on the use of Pd nanocubes with different sizes as the seeds and the use of formaldehyde as a relatively mild reducing agent. By controlling the ratio of Pd precursor to the seed, we obtained uniform polyhedrons such as truncated cubes, cuboctahedrons, truncated octahedrons, and octahedrons in a purity approaching 100%. The sizes of these polyhedrons were determined by the edge length of the cubic seeds. Since these Pd polyhedrons were characterized by different proportions of {111} to {100} facets, they could serve as model catalysts to uncover the correlation between the surface structure and the catalytic performance for formic acid oxidation. Our measurements indicate that Pd nanocubes exhibited the highest maximum current density in the forward anodic scan, but the peak position was also located at a potential higher than those of the other polyhedrons. When both the current density and the operation potential are taken into consideration, Pd nanocubes with slight truncation at the corners become the best choice of catalyst for formic acid oxidation. Our study also revealed that the size of Pd polyhedrons had essentially no effect on the activity for formic acid oxidation.

Facile Synthesis of Pd–Pt Alloy Nanocages and Their Enhanced Performance for Preferential Oxidation of CO in Excess Hydrogen

This article describes a new method for the facile synthesis of Pd-Pt alloy nanocages with hollow interiors and porous walls by using Pd nanocubes as sacrificial templates. Differing from our previous work (Zhang, H.; Jin, M. S.; Wang, J. G.; Li, W. Y.; Camargo, P. H. C.; Kim, M. J.; Yang, D. R.; Xie, Z. X.; Xia, Y. Synthesis of Pd-Pt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction. J. Am. Chem. Soc.2011, 133, 6078-6079), we complemented the galvanic replacement (between Pd nanocubes and PtCl(4)(2-)) with a coreduction process (for PdCl(4)(2-) from the galvanic reaction and PtCl(4)(2-) from the feeding) to generate Pd-Pt alloy nanocages in one step. We found that the rate of galvanic replacement (as determined by the concentrations of Br(-) and PtCl(4)(2-) and temperature) and the rates of coreduction (as determined by the type of reductant and temperature) played important roles in controlling the morphology of resultant Pd-Pt alloy nanocages. The Pd-Pt nanocages exhibited both enhanced activity and selectivity for the preferential oxidation (PROX) of CO in excess hydrogen than those of Pd nanocubes and the commercial Pt/C thanks to the alloy composition and hollow structure. In addition, as the sizes of the Pd-Pt nanocages decreased, they exhibited higher CO conversion rates and lower maximum conversion temperatures due to the increase in specific surface area.

Controlling the Nucleation and Growth of Silver on Palladium Nanocubes by Manipulating the Reaction Kinetics

NSF (DMR) [0804088, 1104616]; Washington University in St. Louis; World Class University (WCU) through the National Research Foundation of Korea; Ministry of Education, Science and Technology [R32-20031]; NSF [ECS-0335765]; U.S. Department of Energy, Basic Energy Sciences, by the Materials Sciences and Engineering Division [DE-AC02-98CH10886]