Nanoarchitectonics and Catalytic Performance of Au-Pd Nanoflowers Supported on Fe2O3.

Supported anisotropic bimetallic nanocrystals are attractive owing to their potential for novel catalytic applications. Au-Pd nanocrystals are expected to have higher catalytic activity for alcohol oxidation than Au nanocrystals. However, only a few studies have reported the application of anisotropic Au-Pd nanocrystals as alcohol-oxidation nanocatalysts. Support materials such as Al2O3 and Fe2O3 influence the catalytic activity of spherical Au nanoparticles. Thus, optimization of the support is expected to improve the catalytic activity of anisotropic Au-Pd nanocrystals. Herein, we report the synthesis and catalytic performances of Al2O3- and Fe2O3-supported Au and Au-Pd nanoflowers. Au99-Pd1 NFs supported on Fe2O3 exhibited the highest catalytic activity for 1-phenylethyl alcohol oxidation.

Water-phase synthesis of Au and Au-Ag nanowires and their SERS activity.

Metal nanowires (NWs) with a diameter of a few nanometers have attracted considerable attention as a promising one-dimensional nanomaterial due to their inherent flexibility and conductive properties and their weak plasmon absorption in the visible region. In a previous paper, we reported the synthesis of ultrathin 1.8 nm-diameter Au NWs using toluene-solubilized aqueous solutions of a long-chain amidoamine derivative (C18AA). This study investigates the effect of different organic solvents solubilized in C18AA aqueous solutions on the morphology of the Au products and demonstrates that solubilizing methylcyclohexane yields thick 2.7 nm-diameter Au NWs and 3.3 nm-diameter Au-Ag alloy NWs. Further, the surface-enhanced Raman scattering sensitivity of ultrathin Au NWs, thick Au NWs, and thick Au-Ag alloy NWs were assessed using 4-mercaptopyridine and found that their enhancement factors are 104-105 and the order is Au-Ag NWs > thick Au NWs > ultrathin Au NWs.

Hole, Convex, and Silver Nanoparticle Patterning on Polystyrene Nanosheets by Colloidal Photolithography at Air-Water Interfaces.

Colloidal photolithography is a versatile advanced technique for fabricating periodic nanopatterned arrays, with patterns carved exclusively on photoresist films deposited on solid substrates in a typical photolithographic process. In this study, we apply colloidal photolithography to polystyrene (PS) films half-covered with poly(methyl methacrylate) (PMMA) colloids at the air-water interface and demonstrate that periodic hole structures can be carved in PS films by two processes: photodecomposing PS films with ultraviolet (UV) light and removing PMMA colloids with a fluorinated solvent. Nonspherical holes, such as C-shaped and chiral comma-shaped holes, are also fabricated by regulating the UV illumination conditions. Furthermore, in addition to holes, convex patterns on PS films are realized by combining weak UV illumination with solvent treatment. We also demonstrate that actively using the water surface as the UV illumination field enables periodic silver nanoparticle spots to be deposited on PS films simply by dissolving silver ions in the water phase.

Thermally Tunable Structural Coloration of Water/Surfactant/Oil Emulsions.

Stimuli-responsive structural color in nature has fascinated scientists, directing them to develop artificial coloration materials that adjust colors in response to external stimuli. Many stimuli-responsive structural color materials have been realized. However, only a few have reported on all-liquid-type materials, which have a particularly desirable feature because they impart their function to the device of any shape. We have previously reported the development of a consistent structural color within a narrow temperature range for all-liquid-type emulsions comprising a long-chain amidoamine derivative (C18AA) and tetraoctylammonium bromide (TOAB). In the present study, we demonstrate that introducing NaCl as an electrolyte affords a highly thermo-sensitive color-changing ability to the emulsions. The structural color of the emulsions can be controlled from red to blue by tuning the temperature. Furthermore, the C18AA and TOAB concentrations can independently regulate the color and coloring-temperature, respectively, realizing that the desired color can develop at a given temperature.

Preparing Alumina-Supported Gold Nanowires for Alcohol Oxidation.

The development of shape-controlled noble metal nanocrystals such as nanowires (NWs) is progressing steadily owing to their potentially novel catalytic properties and the ease with which they can be prepared by reducing the metal ions in a particular solution as capping agents. Recently, many reports have been presented on the preparation of shape-controlled Au nanocrystals, such as nanostars and nanoflowers, by a one-pot method using 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) as capping and reducing agents. The catalytic activity is depressed due to the adsorption of the capping agent onto a Au surface. Since HEPES has low binding affinities on the Au surface, shape-controlled nanocrystals obtained using HEPES are effective for application as nanocatalysts because HEPES was easily removed from the Au surface. In this study, we report the preparation of AuNWs, with an average diameter of 7.7 nm and lengths of a few hundred nanometers, in an aqueous solution containing HEPES and sodium borohydride. A γ-Al2O3-supported AuNW (AuNW/γ-Al2O3) catalyst was obtained using catalytic supporters and a water extraction method that removed HEPES from the Au surface without morphological changes. AuNW/γ-Al2O3 was then utilized to catalyze the oxidation of 1-phenylethyl alcohol to acetophenone. The formation rate of acetophenone over AuNW/γ-Al2O3 was 3.2 times that over γ-Al2O3-supported spherical Au nanoparticles (AuNP/γ-Al2O3) with almost the same diameter.

Conductive nanosheets produced by UV irradiation of a Ag nanoparticle monolayer at the air-water interface.

In a previous study, we demonstrated that conductive Au nanosheets can be prepared by UV irradiation of an Au nanoparticle monolayer spreading on water. In this study, we applied this UV irradiation technique to inexpensive Ag nanoparticles (NPs) to expand their versatility. UV irradiation of Ag NPs on water resulted in the formation of large Ag NPs and was ineffective for preparing conductive Ag films. The solubilization of additives in the water phase, however, resulted in the conversion of the large Ag NPs into a nanosheet, and the solubilization method was highly effective for preparing transparent conductive Ag films with an optical transmittance of above 70%.

Magnetic Fe3O4-Supported Gold Nanoflowers with Lattice-Selected Surfaces: Preparation and Catalytic Performance.

Nanoflowers (NFs)-shape-controlled noble metal nanocrystals-have garnered significant attention because of their novel catalytic properties and applicability. In this paper, we report the preparation and catalytic performance of a magnetic Fe3O4-supported AuNF catalyst with a clean surface. The magnetically supported AuNFs were obtained by using magnetic Fe3O4 as the support. However, when nonmagnetic γ-Al2O3 was utilized as the support, the AuNFs did not exhibit a magnetic response. These supported AuNFs were utilized to catalyze the oxidation of 1-phenylethyl alcohol to acetophenone using air (1 atm) as the oxidant. The rate of formation of acetophenone using supported AuNFs was 8-fold higher than that of acetophenone using supported spherical Au nanoparticles of comparable size. In addition, the Fe3O4-supported AuNFs exhibited a higher rate of formation of acetophenone than the Al2O3-supported AuNFs. The Fe3O4-supported AuNFs were recovered using a magnet, and the recovered catalyst was reused under identical catalytic reaction conditions. The rate of formation of acetophenone using recovered Fe3O4-supported AuNFs remained unchanged, demonstrating no loss of catalytic activity.

Synthesis of water-dispersible, plate-like perovskites and their core-shell nanocrystals.

Shape-controlled halide perovskite nanocrystals are attractive as an emerging functional material; however, these nanocrystals are prepared using organic solvents containing alkylamines and there are few reports on the synthesis of water-dispersible halide perovskite nanocrystals. We report a simple method to prepare water-dispersible, plate-like perovskite nanocrystals by mixing a long-chain amidoamine derivative (C18AA) and potassium tetrachloropalladate (K2PdCl4) in water. The obtained nanocrystals have a 2D layered perovskite structure represented by the chemical formula (C18AAH2)PdCl4. Furthermore, because seed-mediated growth is useful for preparing shape-controlled nanocrystals, such as rods, plates, wires and cubes, we used the water-dispersible (C18AAH2)PdCl4 nanocrystals as seeds to grow (C18AAH2)PdCl4@Pt core-shell nanocrystals. The core-shell nanocrystals have rough surfaces due to the deposition of Pt on the (C18AAH2)PdCl4 seeds. In addition, plate-like (C18AAH2)PdCl4@Au core-shell nanocrystals were easily obtained using this seed-mediated growth method.

Fabrication of Flexible and Transparent Conductive Nanosheets by the UV-Irradiation of Gold Nanoparticle Monolayers.

Conductive films that are highly transparent and flexible are extremely attractive for emerging optoelectronic applications. Currently, indium-doped tin oxide films are the most widely used transparent conductive films and much research effort is devoted to developing alternative transparent conductive materials to overcome their drawbacks. In this work, a novel and facile approach for fabricating transparent conductive Au nanosheets from Au nanoparticles (AuNPs) is proposed. Irradiating an AuNP monolayer at the air-water interface with UV light results in a nanosheet with ≈3.5 nm thickness and ≈80% transparency in the UV-visible region. Further, the so-fabricated nanosheets are highly flexible and can maintain their electrical conductivity even when they are bent to a radius of curvature of 0.6 mm. Fourier-transform infrared and X-ray photoelectron spectroscopy characterizations reveal that the transformation of the monolayer of AuNPs into the nanosheet is induced by the photodecomposition and/or photodetachment of the dodecanethiol ligands capping the AuNPs. Further, the UV-irradiation of a hybrid monolayer consisting of AuNPs and silica particles affords the patterning of Au nanosheets with periodic hole arrays.

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.

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.

Poly(styrene-co-acrylonitrile) Particles Prepared by Phase Inversion of W/O Emulsions.

In this paper, we describe a particle preparation method that combines phase inversion of water in oil (W/O) emulsions with solidification of polymers dissolved in the emulsion droplets, induced by introducing a poor solvent, in order to prepare poly(styrene-co-acrylonitrile) (SAN) particles with a relatively narrow size distribution. W/O emulsions containing SAN are prepared by adding an aqueous solution of polyvinylalcohol (PVA) as a protective colloid into a methylethylketone (MEK) solution of SAN. Sufficient addition of the aqueous solution causes the precipitation of SAN particles from MEK followed by phase inversion from W/O emulsions to oil in water (O/W) emulsions. We also demonstrate that the use of sorbitan fatty acid esters as dispersion agents is effective for controlling size and polydispersity of SAN particles. They depend strongly on the concentrations and kinds of sorbitan fatty acid esters; a low concentration of sorbitan monolaurate resulted in an average particle size of 1.2 µm with a narrow size distribution.

Ion-specific Effect on Oil-in-water Emulsion Gels Containing a Stimuli-responsive Fibrous Assembly of Amidoamine-derivative Hydrogelator.

Amidoamine derivative C18AA forms a highly viscous oil-in-water (O/W) emulsion, called an "emulsion gel." Previously, it was showed that the viscosity sensitively changes with the C18AA concentration, temperature, pH changes, and salt addition. In this work, C18AA concentration in the continuous aqueous phase ([C18AA]w) was investigated at different compositions, and it was found that a threshold concentration value of 16.5 wt% was required to obtain a highly viscous emulsion gel below 45°C. C18AA formed micellar networks in the aqueous phase, and stimuli or additives increase the value of [C18AA]w, so that the C18AA micellar network could grow in the continuous aqueous phase.

Stimuli-Responsive Extraction and Ambidextrous Redispersion of Zwitterionic Amphiphile-Capped Silver Nanoparticles.

Citrate-stabilized silver nanoparticles (AgNPs) were functionalized with a pH-responsive amphiphile, 3-[(2-carboxy-ethyl)-hexadecyl-amino]-propionic acid (C16CA). At pH ∼ 4, the zwitterionic C16CA assembled into lamellar structures due to the protonation of the amine groups of the amphiphile that neutralized the anionic charge of the carboxylate groups. The lamellar supramolecules incorporated the AgNPs into their 3D network and extracted them from water. C16CA supramolecules dissolved into water (at pH > 6) and organic solvents; consequently, the recovered C16CA-AgNPs were redispersed not only to water but also to chloroform and tetrahydrofuran without any additional functionalization. C16CA acted as a pH-responsive stabilizer of AgNPs and formed a solvent-switchable molecular layer such as a bilayered structure in water and densely packed monolayer in chloroform and tetrahydrofuran. Redispersion of the AgNPs was achieved in different solvents by changing the solvent affinity of the adsorbed C16CA molecular layer based on the protonation of the amine groups of the pH-responsive amphiphile. The morphology of redispersed AgNPs did not change during the recovery and redispersion procedure, due to the high steric effect of the network structure of C16CA supramolecules. These observations can lead to a novel solvent-exchange method for nanocrystals without aggregation and loss of nanocrystals, and they enable effective preparations of stimuli-responsive plasmonic nanomaterials.

Recovery and redispersion of gold nanoparticles using the self-assembly of a pH sensitive zwitterionic amphiphile.

The pH-responsive self-assembly of zwitterionic amphiphile C16CA was expanded to the recovery of gold (Au) nanoparticles for environmentally friendly chemistry applications. Multilayered lamellae at pH ∼ 4 were successfully incorporated into nanoparticles by dispersion. Redispersion of nanoparticles was achieved under basic conditions by the transition of self-assembly.

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.

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.

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.

High organogelation ability and soft-templating for ultrathin Au nanowires of long-chain amidoamine derivatives.

Two kinds of long-chain amidoamine derivatives (C13A2AA and C17A2AA) bearing three amide moieties and terminal amine moieties were synthesized and their gelation abilities in apolar solvents were compared with the previously reported amidoamine gelator containing two amide moieties (C18AA). The derivatives acted as organogelators in the same organic solvents. XRD and FT-IR measurements revealed that C13A2AA and C17A2AA formed lamellar-like aggregates in the organogels, in which amide moieties were in the strong intermolecular hydrogen bonding state. In addition, C17A2AA was found to have an outstanding capacity as a soft template for Au nanowires, allowing the successful preparation of Au NWs at a low (0.3 wt%) concentration of [C17A2AA].

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.