Yoshiro Imura

Preparation and Catalytic Activity of Pd and Bimetallic Pd–Ni Nanowires

This article describes the preparation and catalytic property of Pd and Pd-Ni nanowires with network structure. A soft template with network structure formed by long-chain amidoamine derivative (C18AA) was essential to preparing Pd and Pd-Ni nanowires because of the preparation of only spherical nanoparticles using octadecylamine, which does not form a network structure as a soft template, instead of C18AA. Furthermore, this soft-template method demands a slow reduction rate for the metal ion, the same as the general preparation method for novel metal nanowires. The distinguishing features of the present method is that the nanowires are a few nanometers in diameter and there are no byproducts such as nanoparticles. In addition, the bimetallic Pd-Ni nanowires show very high catalytic activity for the hydrogenation of p-nitrophenol as compared to Pd nanowires, Pd nanoparticles, and Pd-Ni nanoparticles.

Thermal-sensitive viscosity transition of elongated micelles induced by breaking intermolecular hydrogen bonding of amide groups.

A heat-induced viscosity transition of novel worm-like micelles of a long alkyl-chain amidoamine derivative (C18AA) bearing intermolecular hydrogen-bonding group was investigated by cryo-TEM, FT-IR, and rheological measurements. At lower temperature, C18AA forms straight elongated micelles with a length on the order of micrometers due to strong intermolecular hydrogen-bonded packing of the amide groups, although the micelles rarely entangle and have low value of zero-shear viscosity. The straight elongated micelles likely became flexible and underwent a morphological transition from straight structure to worm-like structure at a certain temperature, which caused a drastic increase in viscosity due to entanglement of the micelles. This morphological transition was caused by a defect of intermolecular hydrogen bonding between the amide groups on heating. Furthermore, addition of LiCl, which acts as hydrogen-bond breaker, also promoted the viscosity transition, leading to a lowering of the transition temperature.

Room-temperature synthesis of two-dimensional ultrathin gold nanowire parallel array with tunable spacing.

A series of long-chain amidoamine derivatives with different alkyl chain lengths (CnAA where n is 12, 14, 16, or 18) were synthesized and studied with regard to their ability to form organogels and to act as soft templates for the production of Au nanomaterials. These compounds were found to self-assemble into lamellar structures and exhibited gelation ability in some apolar solvents. The gelation concentration, gel-sol phase transition temperature, and lattice spacing of the lamellar structures in organic solvent all varied on the basis of the alkyl chain length of the particular CnAA compound employed. The potential for these molecules to function as templates was evaluated through the synthesis of Au nanowires (NWs) in their organogels. Ultrathin Au NWs were obtained from all CnAA/toluene gel systems, each within an optimal temperature range. Interestingly, in the case of C12AA and C14AA, it was possible to fabricate ultrathin Au NWs at room temperature. In addition, two-dimensional parallel arrays of ultrathin Au NWs were self-assembled onto TEM copper grids as a result of the drying of dispersion solutions of these NWs. The use of CnAA compounds with differing alkyl chain lengths enabled precise tuning of the distance between the Au NWs in these arrays.

Preparation of Silica-Coated Ultrathin Gold Nanowires with High Morphological Stability

We demonstrated a preparation method of silica-coated straight ultrathin Au nanowires (NWs). Water-dispersive ultrathin Au NWs capped with a long-chain amidoamine derivative (C18AA) were used for silica coating. The Au NWs were partially covered with 3-mercaptopropanoic acid by the ligand exchange method, and silica coating of the Au NWs was carried out by the hydrolysis of tetraethoxysilane (TEOS) at pH > 6.7 because the shape of the Au NWs was changed under acidic conditions. The thickness of the silica layer depended on the concentration of TEOS, and the layer was able to decrease to 6-10 nm thick. We also demonstrated that the silica-coated Au NWs had high morphological stabilities against external stimuli such as a TEM electron beam, heat, and pH compared with the bare Au NWs.

Neuron-shaped gold nanocrystals and two-dimensional dendritic gold nanowires fabricated by use of a long-chain amidoamine derivative.

We report the synthesis of two-dimensional (2D) dendritic Au nanowires (DNWs) with diameters of 100-200 nm in an aqueous solution of long-chain amidoamine derivative (C18AA), which acted as both capping and reducing agent, and the preparation of large 2D DNWs with diameters of 400-700 nm by seeded growth of the original DNWs. The seeded growth method in the presence of C18AA enables the fabrication of novel neuron-shaped Au nanostructures consisting of two DNWs dangling from both ends of an ultrathin Au nanowire.

Strings of Metal Half-Shells Fabricated Using Colloidal Particle Monolayer as a Template

A two-dimensional (2-D) polystyrene (PS) particle monolayer is demonstrated as a template for the fabrication of one-dimensional (1-D) strings of metal shells. Metal is deposited by vacuum evaporation onto the PS particle monolayer, which is tilted at an angle (theta) between the normal of the monolayer plane and the direction of metal deposition. The effects of the metal deposition thickness, the tilt angle (theta) of the particle monolayer, and the type of metal on the formation of 1-D strings of metal shells are investigated. Strings of fan-shaped Au shells are obtained at a given tilt angle and deposition thickness. On the other hand, 1-D strings of Cr and Ag cannot be obtained, and they tend to form 2-D films and half-shell dots of the metal, respectively.

pH-induced recovery and redispersion of shape-controlled gold nanorods for nanocatalysis

The amphiphilic, pH-responsive amine derivative, 3-[(2-carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA) was used for the functionalization of gold nanorods (Au NRs) prepared with cetyltrimethylammonium bromide (CTAB). The Au NRs could also be stabilized with C16CA owing to the selective adsorption of the amino moiety, and the Au NRs were well dispersed following one-step ligand exchange using C16CA. Based on a change in the nature of C16CA, self-assemblies of spherical micelles (pH > 5) or lamellar precipitates (pH 2–5) are formed in dispersion. Au NRs were incorporated into the precipitates at pH 2–5, but could be redispersed by redissolution of C16CA at pH > 5. The pH-induced recovery–redispersion of Au NRs was successfully accomplished without affecting the morphology of the Au NRs, the amount of Au in the dispersion, or the catalytic activity of the Au NRs for the reduction of p-nitrophenol.

Reversible dispersion–precipitation of single-walled carbon nanotubes by pH change and addition of organic components

Single-walled carbon nanotubes (SWCNTs) were dispersed in an aqueous solution of a long-chain amidoamine surfactant (C18AA) at a high concentration (>0.5 wt%). The SWCNT dispersion was very stable in the pH range from 7.0 to 11.5. Addition of chloroform into the aqueous dispersion at pH 11.5 brought about the precipitation of SWCNTs in water, although there was no effect on the dispersibility of SWCNTs at pH 7. We demonstrated that reversible dispersion–precipitation regulation of SWCNTs in water can be achieved by changing pH and adding the C18AA surfactant in a biphasic system of C18AA–water–chloroform.

Highly Stable Silica-Coated Gold Nanoflowers Supported on Alumina

Shape-controlled nanocrystals, such as nanowires and nanoflowers, are attractive because of their potential novel optical and catalytic properties. However, the dispersion and morphological stabilities of shape-controlled nanocrystals are easily destroyed by changing the dispersion solvent and temperature. Methods of support and the silica coating are known to improve the dispersion and morphological stabilities of metal nanocrystals. The silica-coating method often causes morphological changes in shape-controlled nanocrystals because the silica coating is formed in mixed solutions of water and organic solvents such as ethanol, and this results in aggregation due to changes in the dispersion solvent. Furthermore, ligand exchange, designed to improve the dispersion stability in the solvent, often causes morphological changes. This article introduces a method for the preparation of highly stable silica-coated Au nanoflowers (AuNFs) supported on Al2O3. The method of support prevents the aggregation and precipitation of AuNFs when the solvent is changed from water to water/ethanol. Through stability improvement, silica coating of AuNFs/Al2O3 was conducted in water/ethanol without ligand exchange that causes morphological changes. Furthermore, silica-coated AuNFs/Al2O3 exhibit high morphological stability under high-temperature conditions compared to uncoated AuNFs/Al2O3. These results are very useful when preparing highly morphologically stable, silica-coated, shape-controlled nanocrystals without ligand exchange.

Dendritic gold nanowires supported on SiO2 nanoparticles fabricated by a seed growth method

Seed growth methods are useful for the preparation of various shape-controlled nanocrystals, such as rods, cubes, and plates. The seed growth method generally employs seed nanocrystals dispersed in solutions and capping agents with selective adsorption properties. This article describes a novel seed growth method using Au nanoparticles supported on SiO2 nanoparticles with a diameter of ∼160 nm (SiO2@Au NPs) and a long-chain amidoamine derivative (C18AA) with selective adsorption properties on gold. The selective growth of dendritic Au nanowires (Au DNWs) on the SiO2@Au NPs took place, but growth of Au nanocrystals was not observed using bare SiO2 NPs. The disadvantages of this method were a long time-consuming preparation and a lower yield of SiO2@Au DNWs; for this reason, a co-reducing agent was used to increase the formation rate of Au DNWs. We found that the use of ascorbic acid, which has weak reducing powers, brought a considerable improvement to the preparation time and yield of SiO2@Au DNWs. Furthermore, these SiO2@Au DNWs showed high morphological and dispersion stabilities compared with unsupported Au DNWs, and this method is expected to be the preparation method of supported shape-controlled nanocrystals with high morphological and dispersion stabilities.