Tetsuya Kodaira

Defects in Anatase TiO2 Single Crystal Controlled by Heat Treatments

Optical absorption and electron paramagnetic resonance (EPR) spectra were measured for synthetic anatase TiO 2 single crystals annealed under oxygen or hydrogen atmosphere at the desired temperatures. The as-grown crystal changed its color from pale blue through yellow to colorless by annealing under oxygen pressure. On the other hand, the contact of the colorless crystal with hydrogen gas resulted in the change of its color to pale blue or dark blue depending on annealing temperature. On repetitive heat-treatment under oxygen atmosphere, the dark blue crystal became dark green, yellow and colorless. Since the change of the crystal colors reflects difference in the defect states, repetitive control of the defect states was established in this work. The EPR spectra revealed absence of paramagnetic species in colorless crystal and presence of the conduction electrons in pale blue, dark blue and dark green crystals. The EPR spectra of pale blue crystal were asymmetric because of the existence of conduction e...

Synthesis of AlPO4-5 powder by microwave heating : Influence of starting gel pH and reaction time

Abstract Extremely pure AlPO4-5 powder was synthesized using a microwave heating method. The yield as well as the size distribution of the AlPO4-5 crystals depended on the pH value of the starting gel. On reducing the pH value from 7.0 to 4.0, the average size of the product crystal was found to increase, the size distribution became wider, and the yield increased. The pH value of the supernatant liquid after the synthesis was always ca. 7.5, independent of the pH values of the starting gels. During the first step of crystallization, disk-like crystals are formed, and they grow in the c-axis direction resulting in the formation of a hexagonal rod-like shape. The channels of the crystals are completely open, and, therefore, the crystals obtained are suitable for use as a host to stabilize guest materials.

Influences of aliphatic alcohols on crystallization of large mordenite crystals and their sorption properties

The influences of aliphatic alcohols (ROH) on the synthesis of mordenite (MOR) type zeolite were investigated using a starting synthesis mixture with the chemical compositions (0–3 ROH∶0.035 Na2O∶0.005–0.01 Al2O3∶SiO2∶40 H2O). Highly crystalline single crystals of MOR were successfully synthesized using ethanol, 1-propanol and 1-butanol. Under well-optimized conditions, pure uniform and large single crystals up to ca. 50 μm in length were obtained. From sorption experiments of nitrogen, water, benzene and Se, it was found that the single large MOR obtained is not the large-port type MOR and that the effective sorption capacity is markedly influenced by the carbon chain length of aliphatic alcohol used in the crystallization.

Possibility of quantum effect in micropore filling of Ne on AlPO4-5

Abstract Adsorption isotherms of neon at 27, 30, and 33 K on aluminophosphate AlPO 4 -5 were measured. The experimental adsorption isotherms and isosteric heat of adsorption for neon on AlPO 4 -5 were compared with those calculated from the density functional theory (DFT) treating the classical Lennard-Jones systems. The classical DFT results exhibited the first layer filling on the pore wall and the second filling in the center of the pore. However the experimental isotherms showed only one layer transition, suggesting that quantum contributions cannot be neglected for neon adsorption on the micropores of AlPO 4 -5 at low temperatures.

Neutron powder diffraction study of potassium clusters in zeolite K-LTA

Abstract The crystal structures of K-doped K-LTA below and above the Curie temperature of 8 K were analyzed by neutron powder diffraction on the basis of space group F23. We found alternate ordering of Si and Al in a distorted framework and different K clusters in two kinds of α-cages. Additional K + ions are distributed mainly in four- and eight-membered rings of the framework and partially occupy the center of an α-cage. K + ions near the center of a six-membered ring form two kinds of tetrahedral assemblies occupied statistically. Our structural data suggest that the K clusters in the adjacent α-cages have different magnetic moments. This is consistent with a spontaneous magnetic moment, which is observable even if the K clusters interact antiferromagnetically.

K+ ion distribution in zeolite ZK-4’s with various Si/Al ratios and the contribution of K+ ions to K cluster formation

Abstract The detailed distribution of K + ions on dehydrated K-type zeolite ZK-4’s having various Si-to-Al atomic (Si/Al) ratios was determined using a high-resolution X-ray powder diffraction technique. The space group changes from Fm 3 c ( a ∼24.6 A) to Pm 3 m ( a ∼12.1 A) at an Si/Al ratio of between 1.1 and 1.5 as the increases. The site occupancy of the K + ion at the six-membered ring (6-MR) decreases as the Si/Al ratio increases, whereas that at the eight-membered ring (8-MR) is close to unity, independent of the Si/Al ratio. The K + ion site at the 6-MR was split into two independent positions, which were displaced in α- and β-cage sides. The K clusters can be stabilized in the α-cages of ZK-4 having a high Si/Al ratio by K atom adsorption onto the ZK-4 powder. The mechanism of stabilization of the K clusters in the α-cage is strongly correlated with the K + ion distribution, i.e., the high occupancy of the K + ions at the 8-MR site.

Fabrication of carbon nanotube assemblies on Ni–Mo substrates mimics law of natural forest growth

We fabricated cone-shaped carbon nanotube assemblies, which align in a densely packed geometry like a cedar tree forest, on unpatterned nanocomposite substrate of Ni and Mo by the DC plasma assisted hot filament chemical vapor deposition. Carbon nanotubes initially grew on the nickel domains created during deposition, forming cone-shaped assemblies. These assemblies grew keeping their shape with decreasing their number density by merging together. The relationship between the mean volume and the number density of conical-shaped tree-like assemblies was similar to one of natural plant forest.

Surface treatment- and calcination temperature-dependent adsorption of methyl orange molecules in wastewater on self-standing alumina nanofiber films

A novel form of alumina, free-standing and transparent alumina nanofiber thin films has been synthesized and utilized as highly efficient adsorbents for organic dye removal and decoloration, and their adsorption capacities are sensitively dependent on surface treatment and calcination temperature. Compared with commercial alumina powders, the films exhibited totally different adsorption behaviors and enhanced adsorption capacities, which may originate from their different pore sizes in the two forms. The as-prepared dye-adsorbed alumina nanofiber films may open up new possibilities for some potential applications, e.g., pH indicator paper for pH testing derived from the dye-adsorbed alumina films was enabled based on different dye-desorption characteristics in different solutions.

Layered Hybrid Perovskites with Micropores Created by Alkylammonium Functional Silsesquioxane Interlayers

Layered organic-inorganic hybrid perovskites that consist of metal halides and organic interlayers are a class of low-dimensional materials. Here, we report the fabrication of layered hybrid perovskites using metal halides and silsesquioxane with a cage-like structure. We used a silsesquioxane as an interlayer to produce a rigid structure and improve the functionality of perovskite layers. Propylammonium-functionalized silsesquioxane and metal halide salts (CuCl2, PdCl2, PbCl2, and MnCl2) were self-assembled to form rigid layered perovskite structures with high crystallinity. The rigid silsesquioxane structure produces micropores between the perovskite layers that can potentially be filled with different molecules to tune the dielectric constants of the interlayers. The obtained silsesquioxane-metal halide hybrid perovskites exhibit some characteristic properties of layered perovskites including magnetic ordering (CuCl4(2-) and MnCl4(2-)) and excitonic absorption/emission (PbCl4(2-)). Our results indicate that inserting silsesquioxane interlayers into hybrid perovskites retains and enhances the low-dimensional properties of the materials.

Structure of intrazeolite molybdenum oxide clusters and their catalysis of the oxidation of ethyl alcohol

Molybdenum oxide clusters confined in zeolite supercages were synthesized using a precursor Mo(CO)6 adsorbed in the pores of zeolite and characterized by XAFS, HREM, LRS and XPS. The structure of the intrazeolite Mo oxide clusters and their catalysis of the oxidation of ethyl alcohol to acetaldehyde were investigated. It was found by Mo K-edge XAFS that intrazeolite Mo(VI) oxide dimer species with an octahedral symmetry of Mo are formed by thermal oxidation of Mo(CO)6 entrapped in an NaY zeolite irrespective of the Mo loading up to two Mo atoms per supercage. HREM and XRD detected no degradation of the crystallinity of the host zeolite. The dimer catalyst showed a specific activity for the oxidation of ethyl alcohol 10 times higher than an impregnation catalyst, in which isolated tetrahedral Mo oxide species are formed. The Mo oxide dimer species were transformed to monomeric species by a heat treatment at 673 K. This led to a considerable decrease in catalytic activity. The Mo oxide clusters prepared by thermal oxidation of Mo(CO)6 encaged in the cavity of a high silica FAU zeolite (Si/Al = 630) showed a significantly higher specific activity than the Mo oxide clusters in NaY. From the Mo K-edge EXAFS analysis, it was found that Mo oxide clusters containing several Mo atoms are constructed in the pores of the high silica zeolite. It is concluded that the structure and size of Mo oxide clusters encaged in zeolite are controlled by the zeolite composition and heat treatment and that larger Mo oxide clusters exhibit a higher specific activity for the partial oxidation of ethyl alcohol.