Hiroshi Hirashima

Crystallization and reduction of sol-gel-derived zinc oxide films by irradiation with ultraviolet lamp

Structural changes in sol-gel films with photo-irradiation were investigated using zinc oxide (ZnO) derived from zinc acetate. The exposure of the films to an ultraviolet lamp induced hexagonal ZnO crystals in a relatively dense amorphous structure. On the other hand, the formation of zinc metal was found in a porous gel film. The photo-induced crystallization and reduction are ascribed to the electronic excitation in the metastable non-crystalline states.

Direct preparation of anatase TiO2 nanotubes in porous alumina membranes

Monodisperse hollow nanocylinders consisting of crystalline titania particles have been directly prepared in a porous alumina membrane by a deposition technique using an aqueous solution system of titanium tetrafluoride.

Low-temperature synthesis of anatase thin films on glass and organic substrates by direct deposition from aqueous solutions

Abstract Crystalline titania thin films were obtained on glass and various kinds of organic substrates at 40–70°C by deposition from aqueous solutions of titanium tetrafluoride. Transparent films consisting of small anatase particles (∼20 nm) exhibited excellent adhesion to relatively hydrophilic surfaces. Uniform coatings were successfully prepared on substrates with complex shapes such as cotton and felt fiber. Growth rate and particle size were controlled by both the deposition conditions and the addition of an organic surfactant. Organic dyes were incorporated into the anatase films using organic-dye dissolving solutions and a surfactant. The photo-catalytic property of the titania films deposited in the precursor solutions at relatively low temperatures was identified as that of thermally crystallized anatase.

Preparation of MTMS based transparent superhydrophobic silica films by sol-gel method.

Superhydrophobic surfaces with water contact angle higher than 150 degrees generated a lot of interest both in academia and in industry because of the self-cleaning properties. Optically transparent superhydrophobic silica films were synthesized at room temperature (27 degrees C) using sol-gel process by a simple dip coating technique. The molar ratio of MTMS:MeOH:H(2)O (5 M NH(4)OH) was kept constant at 1:10.56:4.16, respectively. Emphasis is given to the effect of the surface modifying agents on the hydrophobic behavior of the films. Methyl groups were introduced in the silica film by post-synthesis grafting from two solutions using trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) silylating agents in hexane solvent, individually. The percentage of silylating agents and silylation period was varied from 2.5 to 7.5% and 1 to 3 h, respectively. The TMCS modified films exhibited a very high water contact angle (166+/-2 degrees) in comparison to the HMDZ (138+/-2 degrees) modified films, indicating the water repellent behavior of the surface. When the TMCS and HMDZ modified films were heated at temperatures higher than 350 degrees C and 335 degrees C, respectively, the films became superhydrophilic; the contact angle for water on the films was smaller than 5 degrees. Further, the humidity study was carried out at a relative humidity of 85% at 30 degrees C temperature over 30 days. The films have been characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), % optical transmission, humidity tests and contact angle (CA) measurements.

Structural Changes in Sol-Gel Derived SiO2 and TiO2 Films by Exposure to Water Vapor

Structural changes in sol-gel derived thin films by exposure to water vapor were investigated using Fourier transform infrared absorption spectroscopy, ellipsometry and x-ray diffraction. We found that SiO2 gel films were densified with a decrease in OH groups by the exposure at 60°–180°C. The shrinkage and the peak frequency of ω4 (TO) for the exposed films were comparable to those heated in a dry atmosphere at temperatures above 500°C. However, OH groups in the films were not completely removed by the water exposure. A subsequent annealing above 300°C changed the structure of the water-exposed SiO2 films with condensation of the remaining OH groups. Although the exposed SiO2 gel films were amorphous, TiO2 gel films were transformed to anatase with a decrease in OH groups by the treatments at 80°–180°C.

Memory switching of V2O5TeO2 glasses

A dc electric field up to about 106 V/cm was applied to V2O5TeO2 glass film of 2 to 20 μm in thickness, and memory switching was observed. The conductivity increased two orders of magnitude or more. Crystallization of these glasses by heat treatment also caused an increase in conductivity of similar magnitude. The threshold field of switching depended on the temperature but not on the film thickness, L, for L ≲ 5 μm. The dependences of the delay time on the applied field and film thickness were deviated from those predicted from electrothermal switching theory for L ≲ 5 μm or applied fields ≳105 V/cm. It was discussed that in these regions the electronic switching mechanism could be applied.

Preparation of PZT powders from metal alkoxides

Abstract Fine powders of PZT, [Zr]/[Ti] = 53/47 and 55/45 in mol ratio, were prepared from Pb(isoC3H7O)2, Zr(nC4H9O)4 and Ti(isoC3H7O)4 through simultaneous hydrolysis in alcohol solutions. Spherical and X-ray-amorphous powders were obtained by hydrolysis in ethanol solution at temperatures below 76°C. After calcination at 400°C for 72 h, diffraction peaks of PZT were observed. During the calcination, an intermediate phase, cubic Pb2Ti2O6, was found. When the hydrolysis was carried out in n-propanol at 95°C, irregular shaped particles were obtained. After drying at 90°C, diffraction peaks of PZT and of the intermediate phase were observed.

Intrinsic- and extrinsic-defect formation in silica glasses by radiation

Abstract The dose dependence of the defect concentration produced by radiation was studied for many types of silica glass in order to discuss the contribution of ‘extrinsic’ and ‘intrinsic’ processes to the paramagnetic defect formation. A linearly increasing concentration of paramagnetic defects with dose accompanied by a saturating tendency is observed for the ‘extrinsic’ defect formation due to transformation of pre-existing precursors. The concentration of E′ centers substantially equals that of non-bridging oxygen hole centers, and both are approximately proportional to the square-root of the accumulated dose for the ‘intrinsic’ defect formation involving cleavage of the SiO network. Since the dose dependence of the defects is independent of the incident photon energy, electron-hole pairs having the band gap energy of silica are implied to have an essential role for either ‘extrinsic’ or ‘intrinsic’ defect formation.

Preparation of mesoporous TiO2 thin films by surfactant templating

Abstract Porous TiO2 thin films have potential applications in optical-, electronic- and mechanical-components, catalysts, photoelectrodes, etc. A novel method to control pore sizes of sol–gel thin films by surfactant templating was investigated. The coating sol was obtained by hydrolysis of Ti(C4H9O)4 in ethanol solution. The films were prepared by spin-coating on glass substrates and drying at 90°C (xerogel film) or drying after immersion in a cetyltrimethylammoniumchloride (CTAC) or benzyltrimethylammoniumchloride solution for 1–24 h (CTAC- or BTAC-modified film), followed by calcination at 500°C in air. The calcined films were transparent and 30–120 nm in thickness. The refractive indices of the CTAC- and BTAC-modified films were about 1.7 and 1.9, respectively. These were 10–20% lower than those of the xerogel films. A columnar structure was observed by SEM and AFM. The columns were about 30 nm in diameter. The spacing between the columns was about 10 nm, corresponding to the size of CTAC micelles. These results show that the microstructure and pore size of the sol–gel films can be controlled by the surfactant-templating.

Ultraviolet-reduced reduction and crystallization of indium oxide films

Structural changes stimulated by ultraviolet (UV) irradiations of sol–gel-derived indium oxide thin films were investigated. Illumination of incoherent UV photons (4.9 eV) from a low-pressure mercury lamp resulted in formation of crystalline indium metal. Irradiation of coherent UV beams from an ArF excimer laser (6.4 eV) and from the fourth harmonics of a Nd:YAG laser (4.7 eV) was found to be effective in the crystallization of indium oxide, accompanied by a decrease in the sheet resistance. The lowest resistance without a reduction of transmission in the visible region was achieved with a 6.4 eV laser beam at a fluence over 10–20 mJ cm−2 shot−1. The results of x-ray photoelectron spectroscopy revealed that charge transfer from O2− to In3+ was induced by the incoherent and the coherent UV photons. The partial reduction with the incoherent illumination and the crystallization with the laser irradiation are tentatively assumed to be due to electronic excitations in the amorphous network.