Yoshio Kobayashi

Direct coating of gold nanoparticles with silica by a seeded polymerization technique.

Gold nanoparticles prepared through a conventional citrate-reduction method were directly coated with silica by means of a seeded polymerization technique based on the Stöber method. The method required no surface modification. The addition of tetraethylorthosilicate and water prior to ammonia was found to be critical to obtain a proper coating. The silica shell thickness was varied from 30 to 90 nm for TEOS concentrations of 0.0005-0.02 M at 10.9 M of water and 0.4 M of ammonia. The optical spectra of the core-shell gold-silica composite particles agreed with predictions of Mie theory.

A transparent alumina film doped with laser dye and its emission properties

Abstract Laser dye (Rhodamine 6G, Rhodamine B or Oxazine 4) was doped in a matrix of alumina gel glass by the sol-gel technique using aqueous sol solution. The undesirable dye aggregation usually observed in aqueous solution is largely reduced in the film, even at a concentration as high as 10 −2 M. When this film was irradiated with a N 2 laser, the film gave laser emission with a beam divergence of about 0.1 rad. The width of the dye laser emission is 10 nm which is typical of a dye laser. The calculated conversion efficiency is 2.1%. A decrease of power is linearly proportional to the shot number, which indicates a one-photon process.

The high dispersion of organic dye into a transparent alumina film and its applications to photochemical and non-photochemical hole burnings☆

Abstract The dyes active in PHB (photochemical hole burning) and NPHB (non-PHB) were embedded in matrices of transparent alumina film by the sol-gel process using hydrous alumina sol derived from aluminum chloride. The dyes doped were sodium resorufin, 5,8-dihydroxy-1,4-naphthoquinone (DNQ) and α,β,γ,δ-tetrakis(4-sulphophenyl) porphine (TPPS). The undesirable dye aggregation usually observed in aqueous solution is largely reduced in the film even at a concentration as high as 10 −2 M. Spectral shifts were observed in the absorption spectra of the films similarly to those in polar matrices. The spectrum of TPPS doped into the film corresponds to that in aqueous solution at pH 2.5. It seems thus that the protonation occurs for resorufin and TPPS in the films. Reflecting this fact, the spectrum of the DNQ film contains the band active in PHB while the spectra of the resorufin and TPPS films do not. The latter dyes reveal active bands on exposure to NH 3 gas because of deprotonation. The thermal durability of the hole profile was investigated by the temperature cycling method. Some discussion is given on the thermal durability in the alumina film compared to those in polystyrene (PS) and poly(methyl methacrylate) (PMMA).

Template‐Synthesized Nanotubes for Chemical Separations and Analysis

We have developed a new class of synthetic membranes that consist of a porous polymeric support that contains an ensemble of gold nanotubes that span the thickness of the support membrane. The support is a commercially-available microporous polycarbonate filter with cylindrical nanoscopic pores. The gold nanotubes are prepared by electroless deposition of Au onto the pore walls, that is, the pores acts as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These nanotube membranes can be used to cleanly separate small molecules on the basis of molecular size. Furthermore, use of these membranes as a novel electrochemical sensor is also discussed. This new sensing scheme involves applying a constant potential across the Au nanotube membrane and measuring the drop in the transmembrane current upon the addition of the analyte. This paper reviews our recent progress on size-based based transport selectivity and sensor applications in this new class of membranes.

Size Effect on Crystal Structures of Barium Titanate Nanoparticles Prepared by a Sol-Gel Method

Barium titanate nanoparticles were synthesized by the hydrolysis of complex alkoxide precursor that was prepared in a reflux of metallic barium and tetraethylorthotitanate in solvent. The hydrolysis was performed by the addition of water/ethanol solution to the precursor solution. As reflux time increased, particle sizes, which were measured with transmittance electromicroscopy, became smaller followed by sharpening of size distribution. As water concentration and benzene content in the hydrolysis increased, the particle size increased with crystallite size that was determined with X-ray diffractometry. No significant difference was observed between the particle and crystallite sizes up to 30 nm. Over the size of 30 nm, the particle size was larger than the crystallite size because of generation of polycrystallites. Annealing treatments in air at 400 and 1000°C also increased both the particle and crystallite sizes. The crystallite sizes estimated from the (111) peaks were smaller than those from (110) peaks in a range of sizes larger than 40 nm, which indicated that the critical crystallite size of transformation from cubic to tetragonal structures was approximately 40 nm.

Preparation of tin oxide monolith by the sol-gel method from inorganic salt

Tin oxide monolith was prepared by a sol-gel process using inorganic salt as a starting material. The tin oxide prepared had no aggregations visible to the naked eye and was very transparent in the visible wavelength even after heating in air at 600 °C. It was confirmed from X-ray diffraction results that the crystal structure of the tin oxide was of the rutile type. The crystal size changed from 1.4 nm to 23.5 nm as the heating temperature increased from room temperature to 700 °C, indicating a possibility to control the crystal size.

Direct Drawing of Ag Microwiring by Laser-Induced Pyrolysis of Film Prepared from Liquid-Dispersed Metal Nanoparticles

Ag microwiring with submicron resolution was fabricated by the laser-induced pyrolysis of a film prepared from liquid-dispersed Ag nanoparticles. The linewidth of the wiring fabricated by this laser direct-write maskless technique can be controlled flexibly by the laser beam focusing conditions. The resistivity of the Ag wiring fabricated by this technique (1.9×10-6 Ω cm) is comparable with that of bulk Ag (1.6×10-6 Ω cm).

Optical nonlinearities of Al2O3 glasses with CdS microcrystallines

Fabrication of Al2O3 glasses with CdS microcrystallites by means of the sol-gel method, as well as their linear and nonlinear optical characteristics, is reported for the first time. The shift in the wavelength of the absorption edge through a quantum size effect was clearly observed for the CdS-doped Al2O3 prepared by changing the annealing temperature. A large nonlinear absorption and refractive index were obtained because a high CdS microcrystallite concentration was achieved through the sol-gel method.

Preparation of tubular alumina membrane with uniform straight channels by anodic oxidation process

We prepared alumina tubes by an anodic oxidation process. The tube obtained was mechanically tough against a pressure of about 1 MPa from the outside of the tube. The tube was characterized by scanning electron microscopy (SEM) and a gas permeability measurement. The SEM observations revealed that this has uniform straight channels penetrating through the tube wall. The channels are vertical to the tube surface and their diameter is about 150 nm. The permeability of gas was measured with H2, He, N2, O2 and CO2 in the range of 0–100 kPa. The gas flow was mainly governed by Knudsen flow. This indicates the absence of big cracks in the tube.

Control of Shell Thickness in Silica-Coating of AgI Nanoparticles

Silica-coating of AgI nanoparticles with a Stöber method was carried out to find out reaction conditions for control of the shell thickness. The AgI nanoparticles were prepared from AgClO4 and KI with the use of 3-mercaptopropyltrimethoxysilane (MPS) as a silane coupling agent and dimethylamine (DMA) catalyst for alkoxide hydrolysis. The silica-coating was performed at 4.5×10-6-4.5×10-5 M MPS, 11-20 M water, 0.002-0.1 M DMA and 0.005-0.04 M tetraethylorthosilicate at AgI concentrations of 0.1-1 mM. Consequently, AgI-silica core-shell particles could be prepared with the use of 4.5×10-5 M MPS, 20 M water, 0.01 M DMA and 1 mM AgI. Silica shell thickness could be varied from 15 to 28 nm with an increase in the TEOS concentration from 0.005 to 0.04 M.