Clara Morita

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.

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.

Characterization of colloidal crystal film of polystyrene particles at the air-suspension interface

An aqueous suspension of polystyrene (PS) particles gave rise to a distinct color when a water-soluble volatile solvent, such as methanol or acetone, was added and left to evaporate. The color was caused by the formation of a colloidal crystal film of PS particles at the air-suspension interface. The colloidal crystal formation was possibly induced by the shear flow of particles and condensation of the suspension due to evaporation of the solvent. The colloidal crystal was estimated to consist of about 120 layers of PS particles. PS particles in the colloidal crystals formed a periodic array among the dispersion media, and the interparticle distance of the particles depended on the conductivity of the suspension. Further, the color of the colloidal crystal film as observed by the naked eye was complementary to the direct reflection peak.