Yutaka Ohya

Synthesis of transparent aqueous sols of colloidal layered niobate nanocrystals at room temperature.

Transparent aqueous sols of colloidal tetramethylammonium niobate nanocrystals were synthesized by mixing tetramethylammonium hydroxide (TMAOH), niobium ethoxide, and water at TMAOH/Nb≥0.7 at room temperature. The X-ray diffraction patterns of the thin films prepared by evaporating the colloidal solutions on a glass substrate indicated that the colloidal niobate had a layered crystalline structure. Two types of layered structures are known as a layered niobate, i.e. M(4)Nb(6)O(17)·nH(2)O and MNb(3)O(8) (M=H, H(3)O, or alkaline metal). Raman spectra and electron diffraction suggested that the niobate nanocrystals were similar in crystal structure to M(4)Nb(6)O(17)·nH(2)O compounds. Moreover, when niobium oxide thin films were fabricated from the niobate colloidal solutions by the sol-gel method, oriented T-Nb(2)O(5) thin films, whose c-axis was parallel to the substrate surface, were obtained. The orientation of the thin films was probably attributed to the layered structure of the colloidal niobate nanocrystals.

Complex changes in the framework of endohedrally Na-doped type II Si clathrates with respect to Na content

Crystal structures of endohedrally Na-doped type II silicon clathrates with variable Na content were refined by Rietveld analysis. Type II Si clathrates have two types of cages: small and large. The large cages were preferentially occupied by Na atoms. Upon occupation by Na in the large cages, the lattice constant of the clathrates decreased slightly. The attractive interaction of the Na atoms in the large cages with a framework that caused displacement of the Na atoms from the center might influence lattice shrinkage. Moreover, in the region where almost large cages contained Na atoms and the small cages were partially occupied by Na, the lattice constant increased with the Na content; however, the relationship between these features was complex. The lattice constant demonstrated a linear relationship with the size of the small cages; however, the enlargement of the small cages was not linear with respect to its Na occupancy, resulting in complex changes in the lattice constant with respect to the Na content. It was inferred that enlargement of the small cages by Na insertion may be dependent on the Na occupancy of neighboring small cages.

Phase Transition between Layered Tungstates and Polyoxotungstates in Aqueous Solutions

Aqueous sols of colloidal layered tetramethylammonium (TMA) tungstate nanocrystals were obtained by diluting an aqueous suspension of TMA polyoxotungstate precipitates. The slow evaporation of the colloidal-layered tungstate sols led to the deposition of TMA polyoxotungstate and significantly decreased the amount of layered tungstate nanocrystals. Moreover, the increase in the amount of TMA(+) in the sols also facilitated precipitation of the polyoxotungstates because of a common ion effect. Thus, the dissolution and formation of polyoxotungstate were reversible phenomena. The reaction between the dissolved species, which were formed by the dissolution of TMA polyoxotungstate, likely provided the highly water-dispersible layered tungstate nanocrystals. Furthermore, transmission electron microscopy observation suggested that the layered tungstate nanocrystals had a layer structure similar to those of H2WO4 and H2WO4·H2O, which is a layered tungstic acid.

Synthesis of layered tantalate nanocrystals by aqueous process at room temperature

Colloidal suspensions of highly water-dispersible tantalate were prepared by the acid–base reaction of tetramethylammonium hydroxide (TMAOH) and tantalic acid, which was formed via the hydrolysis of tantalum ethoxide, in water at a molar ratio of TMAOH/Ta ≥ 0.7. Despite the preparation at room temperature, the obtained TMA tantalate was a crystalline compound with a layered hexatantalate structure, TMA4Ta6O17·nH2O. Many of the tantalate nanocrystals were about 50 nm in lateral size and consisted of 6 to 12 tantalate layers. The tantalate structure was independent of the TMAOH/Ta ratio although other layered tantalates, such as MTaO3 (M+ = alkaline metal ion or H+), are known. Moreover, at TMAOH/Ta ≤ 0.6, the prepared sols were turbid and the precipitates were amorphous. Thus, the TMAOH/Ta ratio required for the formation of the highly water-dispersible tantalate nanocrystals likely has a relation to the chemical composition, TMA4Ta6O17·nH2O (TMA/Ta = 0.67).

The crystal polymorphism of calcium carbonate is determined by the matrix structure in quail eggs

Two calcified structures, the eggshell and sperm-associated body (SB), are present in the eggs of the Japanese quail, Coturnix japonica. X-ray diffractometry showed that calcium carbonates take the form of calcite in the eggshell and aragonite in the SB. The aim of the present study was to identify the factors that determine the morphology of calcium carbonate crystals. The matrix of EDTA-treated eggshell was a meshwork of vesicles, 200 to 500 nm in diameter, connected by fine fibers or fibrous sheets. The matrix of SB cortex was a radiation of rod-shaped projections approximately 130 nm in width. In vitro crystal formation was achieved by adding dissociated matrix substances to test solutions. When eggshell matrix material was added, formation of calcite crystals, which had many vesicular holes on their surface, was observed. When SB matrix material dissociated by sonication was added, rhombohedral calcite crystals formed at protein concentrations of 100 microg/mL or lower, and elongated and bundled crystals formed at concentrations of 150 microg/mL or higher. When SB matrix material dissociated by pipetting was added, aragonite crystals formed. These observations indicate that the matrix structure is the principal factor in determining the crystal polymorphism of calcium carbonate.

Influence of Si species on intergrowth and anisotropic crystal growth of silicalite-1

Silicalite-1 crystals were hydrothermally synthesized from silica gels prepared under different conditions. The influence of the state of Si species in the silica gels and the concentrations of silicate ions and a template agent in the reaction sols on crystal growth of silicalite-1 was examined. The use of silica gels with a high degree of condensation of Si species resulted in the intergrowth of silicalite-1 crystals, whereas a low degree of condensation of Si species led to coffin-shaped crystals. Thus, the degree of condensation of Si species had significant influence on the intergrowth of silicalite-1 crystals. Moreover, at a low silicate concentration, silicalite-1 crystals elongated along the c-axis were obtained. With increasing silicate concentration in the reaction sols, the aspect ratio of silicalite-1 decreased. Furthermore, with decreasing the amount of N(C3H7)4+ used as a template agent, the silicalite-1 crystals became larger isotropically. Thus, the growth direction of silicalite-1 crystals was dependent on silicate ion concentration in the reaction sols, but not on N(C3H7)4+ concentration.

Orientation of tungsten trioxide thin films fabricated by sol–gel method using aqueous sols of colloidal layered tungstates

Oriented monoclinic WO3 thin films were fabricated by sol–gel method using aqueous sols of colloidal layered tungstates. The colloidal tungstate sols were prepared by reacting different alkylamines with layered tungstic acid H2WO4 in water. With decreasing the alkyl chain length of the alkylamines, the colloidal layered tungstate became smaller. Alkylamines with a short alkyl chain provided transparent aqueous sols. Furthermore, the WO3 thin films fabricated from the obtained aqueous sols had a high (100)-orientation. However, upon annealing H2WO4 crystals applied on a glass substrate with the tungstate layers parallel to the substrate, highly (001)-oriented WO3 layers were obtained. Since both of the A- and C-planes of WO3 have a similar structure to the layers of H2WO4, the orientation of the WO3 thin films and layers probably resulted from the topotactic structural conversion of the tungstates. Interestingly, the preferential orientation of the thin films was dependent on the presence or absence of interlayer alkylamines in the tungstates.

Synthesis and Magnetic Behavior of Nickel Zinc Ferrite Nanoparticles Coated Onto Carbon Microcoils

Magnetic nanoparticles consisted of nickel zinc ferrite (NZF) were chemically synthesized by co-precipitation and simultaneously coated onto carbon microcoils (CMC), and their structure and magnetic properties were investigated. The samples were prepared at various mass ratios of CMC : NZF ranging from 1 : 5 to 5 : 1. According to X-ray diffraction (XRD) measurements, all the samples synthesized in this study exhibit peaks associated with the spinel ferrite structure of NZF nanoparticles along with a fairly broad peak due to the amorphous structure of CMC. The sample synthesized at CMC: NZF = 1 : 1 gives the highest crystallinity and the largest magnetization at 300 K for NZF according to XRD and magnetization measurement using a superconducting quantum interference device magnetometer. Microstructural analysis of the CMC-NZF assemblies using scanning electron microscopy shows that the use of oleic acid and oleylamine improves the coverage of NZF on CMC and that the assembly made at CMC: NZF = 2 : 1 yields the highest coverage of NZF nanoparticles onto the CMC surface.