Shape-controlled Metal Nanocrystals Research Articles

Facet-dependent catalytic activity of shape-controllable palladium nanocrystals with clean surface in acetophenone selective hydrogenation

Shape-controlled metal nanocrystals have wide applications in catalysis. However, the capping agents playing significant roles in the shape-controlled synthesis often remain on the nanocrystal surfaces and are difficult to be removed completely, which not only causes the decrease in activity but also hinders the establishement of the correlation between various facets and intrinsic activity. In our present work, surface-clean Pd nanocrystals enclosed by different facets without capping agents were successfully synthesized by simply tuning the reduction kinetics in ascorbic acid mediated reduction of H2PdCl4 in water system. It was found that the turnover frequency (TOF) in acetophenone selective hydrogenation decreased with the decrease of the percentage of Pd{100} facet exposed in the Pd nanocrystals. The TOF value on Pd{100} facet was found 10 times higher than that on Pd{111} facet, while Pd{110} facet was found not favorable for acetophenone hydrogenation. Density functional theory (DFT) calculations reveal the more negative adsorption energy of acetophenone, the relatively shorter OPd distance between carbonyl group and the Pd surface, along with the much lower energy barrier in the acetophenone hydrogenation reaction, which leads to the Pd{100} surface exhibiting much higher TOF value than the other two surfaces. As for the Pd{110} surface, the long OPd distance, together with the large energy barrier makes it not favorable for the acetophenone hydrogenation.

Anisotropic growth of gold nanocrystals induced by concerted adsorption using a photochemical approach

Photochemical reduction of noble metal ions in the presence of stabilizers that selectively adsorb on different crystal planes is a fascinating method for synthesizing shape-controlled metal nanocrystals (NCs). In this study, we demonstrate the photoreduction of Au3+ ions in the presence of polyvinylpyrrolidone (PVP) and Br− ions, that are classified into different categories of protective agents. Au nanoplates with atomically flat surfaces are obtained by the photoirradiation of an aqueous solution containing Au3+, PVP and Br−. Notably, PVP preferentially adsorbs along the perimeter, while Br− ions tend to adsorb on the flat top and bottom (111) faces. The selective adsorption of these structurally distinct stabilizers promotes two-dimensional growth of the Au nanoplates. PVP serves not only as a protective agent but also as a reduction agent in the photoreduction process. At high concentrations of Br− ions, the adsorption of the Br− ions also occurs at the perimeter, which kinetically traps the side growth of the nanoplates and leads to the formation of polygonal Au nanoplates. In other words, competitive adsorption of PVP and Br− at the facets and their density kinetically affect the shape of the formed Au NCs. This concept was successfully applied to control the photoreduction of Au3+ ions into icosahedral Au nanoparticles.

Insights into Reaction Intermediates to Predict Synthetic Pathways for Shape-Controlled Metal Nanocrystals.

Understanding nucleation phenomena is crucial across all branches of physical and natural sciences. Colloidal nanocrystals are among the most versatile and tunable synthetic nanomaterials. While huge steps have been made in their synthetic development, synthesis by design is still impeded by the lack of knowledge of reaction mechanisms. Here, we report on the investigation of the reaction intermediates in high temperature syntheses of copper nanocrystals by a variety of techniques, including X-ray absorption at a synchrotron source using a customized in situ cell. We reveal unique insights into the chemical nature of the reaction intermediates and into their role in determining the final shape of the metal nanocrystals. Overall, this study highlights the importance of understanding the chemistry behind nucleation as a key parameter to predict synthetic pathways for shape-controlled nanocrystals.

Direct Synthesis of in-Situ Chirally Modified Palladium Nanocrystals without Capping Agents and Their Application in Heterogeneous Enantioselective Hydrogenations

Shape-controlled metal nanocrystals possess the advantages of specific atomic arrangement on the surface and uniform size, which can act as ideal model catalysts for heterogeneous enantioselective catalytic studies. Nevertheless, capping agents are commonly retained on the surface of the as-synthesized metal nanocrystals, which may hinder the necessary interaction between the substrate and the chiral modifiers coadsorbed on the metal surface for chiral recognition. In this paper, we directly synthesized dendritic or cubic palladium nanocrystals in-situ modified by chiral modifiers such as cinchonidine (CD) or S-proline through a one-pot strategy by replacing the conventional capping agents with the chiral modifiers. The as-prepared CD-modified Pd nanodendrite catalyst already exhibits enantioselectivity in the asymmetric hydrogenation of (E)-α-phenylcinnamic acid without subsequent chiral modification processes. The in-situ S-proline-modified Pd nanocube catalyst shows higher enantioselectivity than nanoc...

Au-Ag Nanoflower Catalysts with Clean Surfaces for Alcohol Oxidation.

Shape-controlled metal nanocrystals, such as nanowires and nanoflowers, are attractive owing to their potentially novel catalytic properties and bimetallic nanocrystals composed of two distinct metals are expected to act as highly active catalysts. However, their catalytic activities are limited because of the capping agents adsorbed on the metal surfaces, which are necessary for the preparation and dispersion of these nanocrystals in solvents. Therefore, the preparation of bimetallic shape-controlled noble metal nanocrystals with clean surfaces, devoid of almost all capping agents, are expected to have high catalytic activity. Herein, we report the preparation of bimetallic Au-Ag nanoflowers using melamine as the capping agent. The bimetallic Au-Ag nanoflowers with a clean surface were subsequently obtained by a support and extraction method. The bimetallic nanoflowers with a clean surface were then used for the aerobic oxidation of 1-phenylethyl alcohol and they exhibited high rates for the formation of acetophenone compared to Au nanoflowers and spherical nanoparticles with almost the same size and Au/Ag ratio. We also show that Au-Ag nanoflowers containing only 1 % Ag (Au99 -Ag1 NFs) exhibit the highest rate of acetophenone formation among Au-Ag nanoflowers with different Au/Ag ratios owing to an increase in the electron density of the Au atoms that act as active sites for the oxidation of 1-phenylethyl alcohol.

Citrate-Coated, Size-Tunable Octahedral Platinum Nanocrystals: A Novel Route for Advanced Electrocatalysts.

The development of green and scalable syntheses for the preparation of size- and shape-controlled metal nanocrystals is of high interest in many areas, including catalysis, electrocatalysis, nanomedicine, and electronics. In this work, a new synthetic approach based on the synergistic action of physical parameters and reagents produces size-tunable octahedral Pt nanocrystals, without the use of catalyst-poisoning reagents and/or difficult-to-remove coatings. The synthesis requires sodium citrate, ascorbic acid, and fine control of the reduction rate in aqueous environment. Pt octahedral nanocrystals with particle size as low as 7 nm and highly developed {111} facets have been achieved, as demonstrated by transmission electron microscopy, X-ray diffraction, and electrochemical methods. The absence of sticky molecules together with the high quality of the surface makes these nanocrystals ideal candidates in electrocatalysis. Notably, 7 nm bismuth-decorated octahedral nanocrystals exhibit superior performance for the electrooxidation of formic acid in terms of both specific and mass activities.

Exploring the Multifunctionality of a Novel Long-Chain Amidoamine Amphiphile.

In this review, we demonstrate that the practical applications of a novel long-chain amidoamine derivative with a simple molecular structure (C18AA) are equivalent to or higher in number than those of supramolecules with complex molecular structures. Molecular assemblies of C18AA exhibit distinctive thermal responsiveness; namely, C18AA can form a gel with apolar organic solvents, while O/W emulsions of C18AA act as a heat-induced gelator that undergoes a phase transition from solution to gel upon heating. We also show that C18AA emulsions containing a quaternary ammonium salt develop an iridescent color over a specific temperature range. Further, molecular assemblies of C18AA serve as high-performance soft templates for the preparation of shape-controlled metal nanocrystals such as ultrathin Au nanowires and Pd nanowires.

Shape-Controlled Metal Nanocrystals for Heterogeneous Catalysis.

The ability to control the shape of metal nanocrystals allows us to not only maneuver their physicochemical properties but also optimize their activity in a variety of applications. Heterogeneous catalysis, in particular, would benefit tremendously from the availability of metal nanocrystals with controlled shapes and well-defined facets or surface structures. The immediate benefits may include significant enhancements in catalytic activity and/or selectivity along with reductions in the materials cost. We provide a brief account of recent progress in the development of metal nanocrystals with controlled shapes and thereby enhanced catalytic performance for several reactions, including formic acid oxidation, oxygen reduction, and hydrogenation. In addition to monometallic nanocrystals, we also cover a bimetallic system, in which the two metals are formulated as alloyed, core-shell, or core-frame structures. We hope this article will provide further impetus for the development of next-generation heterogeneous catalysts essential to a broad range of applications.

Shape-controlled metal nanocrystals for catalytic applications

Abstract

Pt/Ru/C nanocomposites for methanol electrooxidation: how Ru nanocrystals’ surface structure affects catalytic performance of deposited Pt particles

The surface structure of supporting materials has great effect on the catalytic performance of supported catalysts. In this work, three kinds of Ru nanocrystals with different morphologies and surface structures, namely triangular plates (TPs), capped columns (CCs) and nanospheres (NSs), were used as substrates for the deposition of Pt particles through an aqueous adsorption–reduction approach. By testing the catalytic activity and stability of these composite catalysts towards methanol electrooxidation, the relationship between the surface structure of Ru nanocrystals and the catalytic performances of Pt particles was built up. Pt/Ru TPs/C exhibited higher catalytic activity and stability than Pt/Ru CCs/C, indicating that the closest packed facet of Ru serves as better substrate for the deposition of Pt particles than the high index facets of Ru for methanol electrooxidation. Pt particles and Ru NSs formed a self-supported network structure in Pt/Ru NSs/C, which increased the level of dispersion of Pt particles and guaranteed higher catalytic activity and stability compared with Pt/Ru CCs/C. This work demonstrated how to use shape controlled metal nanocrystals to study the surface structure effect of supports in catalytic reactions.

Single-Molecule Catalysis Mapping Quantifies Site-Specific Activity and Uncovers Radial Activity Gradient on Single 2D Nanocrystals

Shape-controlled metal nanocrystals are a new generation of nanoscale catalysts. Depending on their shapes, these nanocrystals exhibit various surface facets, and the assignments of their surface facets have routinely been used to rationalize or predict their catalytic activity in a variety of chemical transformations. Recently we discovered that for 1-dimensional (1D) nanocrystals (Au nanorods), the catalytic activity is not constant along the same side facets of single nanorods but rather differs significantly and further shows a gradient along its length, which we attributed to an underlying gradient of surface defect density resulting from their linear decay in growth rate during synthesis (Nat. Nanotechnol.2012, 7, 237-241). Here we report that this behavior also extends to 2D nanocrystals, even for a different catalytic reaction. By using super-resolution fluorescence microscopy to map out the locations of catalytic events within individual triangular and hexagonal Au nanoplates in correlation with scanning electron microscopy, we find that the catalytic activity within the flat {111} surface facet of a Au nanoplate exhibits a 2D radial gradient from the center toward the edges. We propose that this activity gradient results from a growth-dependent surface defect distribution. We also quantify the site-specific activity at different regions within a nanoplate: The corner regions have the highest activity, followed by the edge regions and then the flat surface facets. These discoveries highlight the spatial complexity of catalytic activity at the nanoscale as well as the interplay amid nanocrystal growth, morphology, and surface defects in determining nanocatalyst properties.

Controlled growth of uniform noble metal nanocrystals: Aqueous-based synthesis and some applications in biomedicine

Aqueous-dispersed single and binary noble metal nanocrystals have attracted much attention as key materials in many fields, especially in biomedicine, catalysis, etc. Controlled growth of the metal nuclei allow for the manipulation of uniform morphology of final products. This behavior would tailor their unique physiochemical and electronic properties and follows by their practical applications. This review presents an overall picture of kinetic formation of a particle and then summarizes an overview of recent progress in many research groups concerning aqueous- and/or polyol-based syntheses of many types of aqueous-dispersed single metallic and bimetallic nanocrystals with controlled shape. The main advantages in these synthetic approaches for the shape-controlled metal nanocrystals are simple, versatile, environmentally friendly, low cost, pure and single-crystalline products, and high yield. The formed products can be easily dispersed in water medium and compatible for biotechnological field. Particularly the biomolecule (antibody including protein and/or DNA)-conjugated gold nanocrystals have been utilized as an active agent for a broad range of biomedical applications. We expect that this review will have a high potential towards novel materials fabrication and nanotechnological fields.