Inflating hollow nanocrystals through a repeated Kirkendall cavitation process.

The Kirkendall effect has been recently used to produce hollow nanostructures by taking advantage of the different diffusion rates of species involved in the chemical transformations of nanoscale objects. Here we demonstrate a nanoscale Kirkendall cavitation process that can transform solid palladium nanocrystals into hollow palladium nanocrystals through insertion and extraction of phosphorus. The key to success in producing monometallic hollow nanocrystals is the effective extraction of phosphorus through an oxidation reaction, which promotes the outward diffusion of phosphorus from the compound nanocrystals of palladium phosphide and consequently the inward diffusion of vacancies and their coalescence into larger voids. We further demonstrate that this Kirkendall cavitation process can be repeated a number of times to gradually inflate the hollow metal nanocrystals, producing nanoshells of increased diameters and decreased thicknesses. The resulting thin palladium nanoshells exhibit enhanced catalytic activity and high durability toward formic acid oxidation

Facile syntheses and electrocatalytic properties of porous Pd and its alloy nanospheres

Pd-based catalysts are of great interest both for fundamental research and applications. It still remains a challenge to develop reliable and versatile approaches to prepare Pd-based nanostructures with higher performance and better stability. In this article, a facile solution route has been developed to prepare Pd nanospheres, as well as Pd-Pt, Pd-Ag and Pd-Pt-Ag alloy nanospheres. SEM and TEM investigations revealed that the as-prepared nanospheres were three-dimensionally interconnected porous networks with primary nanoparticles as building blocks. The chemical composition of these nanospheres can be easily adjusted by controlling the molar ratio of precursors. Electrochemical measurements indicated that the electrocatalytic activity of these nanospheres towards formic acid oxidation depended on the composition of the nanospheres. By judiciously adjusting the composition of the Pd based alloy nanospheres, the performance of the Pd based catalysts, i.e., the onset potential of the formic acid oxidation, the corresponding peak current density and the ability to tolerate CO, can be optimized.National Natural Science Foundation of China[20725310, 21021061, 21073145]; Key Scientific Project of Fujian Province of China[2009HZ0002-1]; National Basic Research Program of China[2007CB815303, 2009CB939804]; NCET in Fujian Province Universit

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

Redox reaction induced Ostwald ripening for size- and shape-focusing of palladium nanocrystals

We report here that size- and shape-focusing can be achieved through the well-known Ostwald ripening process to produce high-quality metal nanocrystals (NCs). Using Pd as an example, we show that the addition of small NCs of appropriate sizes could help in modulating the growth of larger NCs and enable excellent control over both the size and shape uniformity of the products. A detailed mechanistic study showed that the self-focusing of Pd NCs relied on a dissolution and regrowth process induced by redox reaction of HCHO. With the assistance of HCHO, injection of small sacrificial nanocrystals (SNCs), with sizes below a critical value, into larger seeds results in the dissolution of the SNCs and subsequent deposition onto the larger ones, thus allowing the formation of monodisperse Pd NCs. We have identified the critical radius of the SNCs to be ∼5.7 nm for Pd, and verified that SNCs with sizes larger than that could not effectively support the growth of larger seeds. More interestingly, since Ostwald ripening involves matter relocation, this synthetic approach could even break the self-termination growth habits of metal NCs and produce nanocrystals with sizes that are not conveniently accessible by direct growth

Structure Sensitivity of Alkynol Hydrogenation on Shape- and Size-Controlled Palladium Nanocrystals: Which Sites Are Most Active and Selective?

The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational catalyst design, especially if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a solution-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The observed activity and selectivity suggested that two types of active sites were involved in the catalysis those on the planes and at edges which differ in their coordination numbers. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallographic orientation, Pd-(111) and/or Pd-(100), whereas overhydrogenation occurred mainly at the edge sites. The experimental data can be fit with a kinetic modeling based on a two-site Langmuir Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3-5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design

Size and Shape-controlled Pd Nanocrystals on ZnO and SiO2: When the Nature of the Support Determines the Active Phase

The structure sensitivity of acetylene hydrogenation was studied over size-controlled cubic and octahedral Pd nanocrystals (NCs) supported on ZnO and SiO2. Acetylene hydrogenation was found to be structure sensitive [with Pd(111) in octahedra being more active than Pd(100) in cubes]. However, the response was found to vary depending on the redox properties of the support. The catalytically active and selective palladium carbide (PdCx) phase was readily formed on the SiO2-supported Pd NCs (much faster on cubes than on octahedra), whereas a markedly less active PdZn phase was preferentially formed on the ZnO-supported Pd nanocrystals. Here we show that the structure sensitivity of a chemical reaction is a complex phenomenon that originates from the reconstruction of the active phase in response to the reactive environment, which is ultimately determined by the support used

Selectivity on Etching: Creation of High-Energy Facets on Copper Nanocrystals for CO₂ Electrochemical Reduction

Creating high-energy facets on the surface of catalyst nanocrystals represents a promising method for enhancing their catalytic activity. Herein we show that crystal etching as the reverse process of crystal growth can directly endow nanocrystal surfaces with high-energy facets. The key is to avoid significant modification of the surface energies of the nanocrystal facets by capping effects from solvents, ions, and ligands. Using Cu nanocubes as the starting material, we have successfully demonstrated the creation of high-energy facets in metal nanocrystals by controlled chemical etching. The etched Cu nanocrystals with enriched high-energy {110} facets showed significantly enhanced activity toward CO₂ reduction. We believe the etching-based strategy could be extended to the synthesis of nanocrystals of many other catalysts with more active high-energy facets

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

Direct synthesis of silver/polymer/carbon nanocables via a simple hydrothermal route

National Natural Science Foundation of China [20725310, 20721001, 20673085, 20671078]; National Basic Research Program of China [2007CB815303]; Key Scientific Project of Fujian Province of China [2005HZ01-3]High-yield silver/polymer/carbon nanocables were synthesized via a one-step simple hydrothermal route by using silver chloride and glucose as precursors. High-resolution TEM and element mapping proved that as-prepared nanocables consist of a silver nanowire core, a polymer inner shell, and a graphitic carbon outer shell. A three-step growth mechanism was proposed to explain the growth of Such three-layer nanocables, i.e. the formation of silver nanowires, the glycosidation of glucose molecules on silver nanowire surface and the carbonization of the outmost glycosidation layer. We believe that reaction temperature plays the key role in the polymerization of glucose and sequent surface-carbonization. (C) 2008 Elsevier Inc. All rights reserved

Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties

Anatase TiO2 nanosheets with highly reactive (001) facets exposed have been successfully synthesized by a facile hydrothermal route, taking advantage of a specific stabilization effect of fluorine ion on (001) facets. The percentage of highly reactive (001) facets in such TiO2 nanosheets is very high (up to 89%). In addition, the as-prepared TiO2 nanosheets exhibit excellent activity in the photocatalytic degradation of organic contaminants.National Natural Science Foundation of China [20725310, 20721001, 20673085, 20801045]; National Basic Research Program of China [2007CB815303, 2009CB939804