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.

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.

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.

Effect of the Air/Water Interfacial Properties of Amine Derivatives on the in Situ Fabrication of Microsized Gold Sheets.

Development of new methods for producing large-area nanocrystals with specific shapes is crucial for advancements in various fields. In this study, submillimeter-sized sheet-structured gold crystals with nanoscale thicknesses were fabricated by chemical reduction of HAuCl4 in the presence of long-chain amidoamine-derived surfactants (C nAOH; n = 12, 14, 16, or 18) in aqueous solutions. Using the C18AOH system at 30 °C, large-area sheet-structured crystals with widths of ∼100 µm and thicknesses of 30 nm were effectively obtained at the air/water interface. The crystal size depended on the temperature and the alkyl-chain length of the surfactant. An investigation of the relationship between the crystal growth and the interfacial properties of C nAOH revealed that large-area crystals were obtained when densely packed molecular layers of long-chain C nAOH were formed at the air/water interface. The interfacial molecular layer of C18AOH showed most effective soft-templating effect and contributed in promoting the growth of sheet-structured gold crystals.

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.

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.