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The surfaces of carbon nanotubes (CNTs) were covered with thin silica layers through the application of 3-aminopropyltriethoxysilane (APTES) in order to enhance the deposition of precious metal and transition metal oxide nanoparticles. Pt metal particles smaller than 2 nm in diameter could be deposited, using a conventional impregnation method, on CNTs coated with silica layers, whereas Pt particles supported on bare CNT surfaces were approximately 3 nm in diameter. Thus, coating CNTs with thin silica layers enhanced the catalytic activity of the Pt catalysts. In addition, CNTs with thin silica layers could be uniformly covered with transition metal oxide layers (ZrO 2 , Nb 2 O 5 and Ta 2 O 5 ) by hydrolysis of the corresponding metal alkoxides. In contrast, metal oxides were very difficult to deposit on bare CNTs. The thin silica layers produced on the CNTs via the use of APTES evidently function as adsorption sites for precursor metals and metal oxides, as well as nucleation sites for metals and metal oxides, allowing the formation of precious metal-CNT and transition metal oxide-CNT composites.

The catalytic performance of Pt metal at the tips of carbon nanotubes (CNT@Pt), which was prepared by ethylene decomposition over Pt/MgO, was compared with that of Pt metal on the outer surface of CNT (Pt/CNT). CNT@Pt showed excellent catalytic performance toward the hydrogenation of α, β-unsaturated aldehydes to unsaturated alcohols, which was similar to that of Pt/CNT and resulted from a strong interaction between Pt metal and the CNT. Pt metal particles at the tips of CNTs showed excellent activity for the selective hydrogenation of α, β-unsaturated aldehydes to unsaturated alcohols similarly to Pt metal particles supported on the outer surface of CNTs.

Pd metal supported on carbon nanotubes (Pd/CNT) for use as a cathode catalyst in polymer electrolyte fuel cells was covered with silica layers a few nanometers thick to improve the durability of the catalyst under severe cathode conditions. The silica-coated Pd/CNT cathode catalyst showed similar activity for the oxygen reduction to Pd/CNT despite coverage of the Pd metal with a silica insulator. In addition, the silica-coated Pd/CNT was highly durable during potential cycling between 0.05 and 1.20 V (vs. reversible hydrogen electrode) in aqueous HClO4 while Pd/CNT suffered serious deactivation under the same conditions. The silica layers in the silica-coated Pd/CNT prevent the diffusion of dissolved Pd species from the Pd metal out of the layers. Therefore, silica-coated Pd/CNT was highly durable under the cathode conditions. Furthermore, coverage of Pd/CNT with silica layers inhibited the two-electron reduction of oxygen to H2O2.

Ethylene decomposition was performed over supported Pt catalysts to fabricate composites of Pt metal nanoparticles and carbon nanotubes (CNTs). All supported Pt catalysts (Pt/carbon black, Pt/CNT, Pt/MgO, Pt/Al2O3 and Pt/SiO2) showed catalytic activity for ethylene decomposition at 973 K to form CNTs. Pt metal particles were found at tips of CNTs. These results indicate that Pt metal particles have catalytic activity for growth of CNTs through hydrocarbon decomposition. A broad range (5–50 nm) of CNT diameters were formed from the use of supported Pt metal catalysts although Pt metal particles in the catalysts before ethylene decomposition were relatively uniform in size (2–5 nm). These results imply that Pt metal particles in the catalysts aggregated during ethylene decomposition at 973 K. Aggregation of Pt metal particles in catalysts during ethylene decomposition could be suppressed by covering catalysts with silica layers that were a few nanometers thick. Silica-coated Pt catalysts showed high activity for ethylene decomposition to form CNTs with uniform diameters (8–10 nm) despite the uniform coverage of Pt metal particles with silica layers.

Carbon nanotube (CNT)-supported Pt metal (Pt/CNT) was covered with silica layers using hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and/or tetraethoxysilane (TEOS). The hydrolysis of APTES resulted in a uniform coverage of Pt/CNT with silica layers, but the silica layers were very thin. Pt/CNT could be coated with silica layers of a few nanometers thickness by hydrolysis of TEOS, but the thickness was not uniform. In contrast, the successive hydrolysis of APTES and TEOS resulted in a uniform coverage of Pt/CNT with silica layers of a few nanometers thickness. Pt metal particles in silica-coated Pt/CNT adsorbed CO chemically despite uniform coverage of Pt metal particles with silica layers. Therefore, silica-coated Pt/CNT demonstrated high catalytic activity in methane combustion.

Carbon nanotube (CNT)-supported Pt metal nanoparticles were covered with silica layers by utilizing the successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt hydroxide. The CNT-supported Pt metal particles covered with silica layers (denoted as SiO2/Pt/CNT) were used as the electrocatalysts. SiO2/Pt/CNT electrocatalysts showed a high stability for the repeated potential cycling experiment, whereas Pt/CNT electrocatalysts were deactivated seriously for the experiment because of the growth of Pt metal particles in size. The silica layers in SiO2/Pt/CNT prevent the dissolution of Pt metal particles as well as the migration and agglomeration of Pt metal particles on the supports, which results in the improvement of the stability of Pt/CNT electrocatalysts.

Silica-supported Co catalysts were prepared by using microemulsion. The structure and state of the Co species in the catalysts were characterized by TEM, XRD, Co K-edge XANES/EXAFS, and diffuse reflectance UV−vis spectroscopy. Co species in the catalysts prepared using microemulsion were atomically dispersed as Co2+ with tetrahedral symmetry (CoO4), even after reduction of the catalysts with hydrogen at 773 K, while Co metal particles were supported on the silica surface for the catalysts prepared by a conventional impregnation method. The Co catalysts prepared using microemulsion selectively formed multi-walled carbon nanotubes with a uniform diameter by the decomposition of ethylene at 973 K, whereas silica-supported Co catalysts prepared by the impregnation method formed carbon nanofibers with various diameters. In addition, the Co catalysts prepared using microemulsion formed bundles of single- or double-walled carbon nanotubes by methane decomposition at 1073 K. Contact of the Co catalysts with hydro...

Amorphous Cr-silicide films are prepared by electron-beam-deposition on a Cu-substrate at room temperature. The microstructures of as-grown films are investigated using transmission and analytical electron microscopy. The morphology of the amorphous films is dendritic. In addition, one-dimensional concentration modulations are observed. The modulated structure is revealed not only by TEM imaging but also by a series of satellite spots at the centre of the diffraction pattern. This concentration modulation is confirmed by analytical electron microscopy.

SiO2 nanotubes could be synthesized by hydrolysis of tetraethyl orthosilicate (TEOS) on the surface of carbon nanofibers (CNF), followed by removal of CNF. In the synthesis of SiO2 nanotubes, CNF acted as templates. The wall thickness and the internal diameter of SiO2 nanotubes were changed by hydrolysis time of TEOS and the diameter of CNF templates, respectively. The catalysts for the hydrolysis of TEOS were functional groups on the surface of CNF. The methane decomposition was carried out over SiO2 nanotubes containing Ni metal particles, so that carbon nanotubes with uniform diameter (ca. 8 nm) were formed at the inside of SiO2 nanotubes. This result strongly indicated that SiO2 nanotubes worked as nanoscale reactors.

Catalytic performance of Co catalysts supported on different supports (MgO, Al2O3, SiO2, and TiO2) for the formation of carbon nanofibers through methane decomposition was investigated. Catalytic activity and the life of supported Co catalysts for methane decomposition at 773 K strongly depended on the type of catalytic supports; i.e., the catalytic performances of Co/Al2O3 and Co/MgO were superior to those of Co/TiO2 and Co/SiO2. All the Co catalysts produced carbon nanofibers with relatively uniform diameters (10−30 nm) through the reaction at 773 K irrespective of the kind of catalytic supports, although an average crystallite size of Co metal was different for these catalysts. These results implied that Co metal particles with diameters from 10 to 30 nm grew carbon nanofibers from methane preferentially, while ones larger than 30 nm were inactive for the reaction. The structures of carbons formed by methane decomposition over Co/Al2O3 depended on the reaction temperatures. Multiwalled carbon nanotubes...

Methane, n-hexane, benzene, and cyclopentadiene were decomposed at a relatively mild temperature (773 K) over a Ni catalyst supported on either vapor grown carbon fibers (VGCF) or graphitized carbon fibers (GCF). Transmission electron microscopy showed that the morphology of the fibers changed according to hydrocarbon and particle size. Decomposition of methane and n-hexane produced fishbone-type fibers. The fibers from n-hexane sometimes showed intermittent hollow structures but the diameters of the fibers were widely distributed. Decomposition of benzene and cyclopentadiene mainly produced winding type carbon nanotubes of relatively uniform diameters (10–20 nm). Bidirectional fishbone-type fibers (fibers growing outward from a central catalyst particle) were also observed as a by-product. Small Ni particles (10–20 nm) with stretched tails were present on the tips of tubular fibers, some of which frequently changed growth direction. The varying tubular morphologies can be ascribed to liquid-like Ni particles resulting from the freezing point depression due to a fast dissolution of carbons during decomposition of benzene or cyclopentadiene. The formation of bidirectional fibers was also observed in the decomposition of n-hexane. Relatively large well-faceted Ni particles (diameter 50–110 nm) grew bidirectional fibers.

We examined hydrogen absorption/desorption behaviors in graphite mechanically milled under hydrogen pressures ranging from 0.3 to 6.0 MPa. Experimental results indicated that the chemisorbed hydrogen concentration during 80 h milling decreased from 6.1 to 4.1 wt.% with increasing milling hydrogen pressure, while the physisorption-like hydrogen concentration increased with increasing milling hydrogen pressure and reached more than 0.5 wt.% at 6 MPa. Moreover, higher pressure hydrogen suppressed the fracture rate of the milled graphite more effectively, suggesting that the hydrogen atoms trapped at the edges of the graphene sheets and between the graphene layers near the surface are responsible for preserving the lamellar nanocrystalline structure, which may be the host for physisorption-like hydrogen absorption.

The ternary alloy, MgYNi 4 , was synthesized by mechanical alloying (MA). Its microscopic structures and hydriding properties were studied. The annealing of the alloy at 773 K was also studied. The alloy consists of nanocrystals of less than 10 nm in diameter. The capacity of the hydrogen atoms in the alloy increases with the increasing size of the nanocrystals. The crystal structure was found to be the C15 type Laves phase structure for the MA-treated alloy, while crystal structure of the as annealed alloy was the ordered C15 type structure which is conventionally called C15b type and is formally called AuBe 5 type structure. The hydrogen capacity in the alloy might have a strong relation to the atomic arrangement of this alloy.

Microstructural and microanalytical investigations were carried out for traditional Japanese clay ceramics called Sekishu-kawara. In this study, optical microscopy as well as scanning- and transmission-electron microscopy were employed. Specimens under investigation are (A) the original wet materials dried in air for several days, (B) the materials dried at 773 K, and (C) the same materials fired and sintered at 1493 K in a tunnel kiln. Not only some clear SEM images but also the TEM images, which were taken for the first time by us, for the clay ceramics are presented. TEM mi-crographs of specimen C show that mullite crystals, grown in the firing and sintering process, are in the shape of strip, 5 nm×50 nm×500 nm in largest, and are dispersed, in the amorphous glass. SEM micrographs of specimen C, on the other hand, show that the clay ceramics are porous and contain considerable amount of tunnel-shaped open spaces called continuous pores. The continuous pores are constructed by a complicated, three dimensional shape of amorphous glass, in which the mullite strips exist.

Pd cathode catalysts for polymer electrolyte fuel cells have been covered with silica layers a few nanometres thick. The silica-coated Pd catalysts showed high activity and excellent durability for the oxygen reduction under the severe cathode conditions of PEFCs, while Pd catalysts without silica-coating were seriously deactivated under the same conditions. The coverage of Pd metal with silica prevents the diffusion of Pd species out of the silica layers.

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg6Al2(OH)16CO 2 2− ]·4H2O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd2+ and various trivalent metal ions which can replace each site of Mg2+ and Al3+ in the hydrotalcite. Rh3+ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd2+ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd2+ cannot well replace the Mg2+ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst.

Pt metal particles supported on carriers such as MgO and SiO2 showed high activity for the formation of carbon nanotubes (CNTs) through ethylene decomposition at 973 K. However, these CNTs were not...

Silica-coated Pt-based alloys (Pt–Co and Pt–Pd alloys) were prepared using a water-in-oil-type microemulsion. The silica-coated Pt-based alloys decomposed ethylene to form composites composed of th...

Single-crystalline MoO3 nanobelts were formed by calcination of physical mixtures of carbon nanofibers and H3(PMo12O40) under air at 873 K. In contrast, single-crystalline MoO3 with long wire struc...

Multiwalled carbon nanotube (CNT)-supported Pt nanoparticles (Pt/CNT) were covered with silica layers by successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt metal precursors, followed by reduction with hydrogen. The Pt/CNT covered with silica layers (SiO 2 /Pt/CNT) was used as a cathode catalyst for a proton exchange membrane fuel cell (PEMFC). The activity of SiO 2 /Pt/CNT catalyst for the oxygen reduction reaction in a single-cell PEMFC was similar to that of Pt/CNT, in spite of the uniform coverage of Pt with silica layers, indicating that the coverage of Pt/CNT with silica layers did not appreciably decrease the catalytic activity. In addition, SiO 2 /Pt/CNT electrocatalyst showed high stability during potential cycling from 0.05 to 1.20 V vs reversible hydrogen electrode in an aqueous H 2 SO 4 electrolyte, whereas Pt/CNT significantly deactivated during the experiment. The structural change of Pt species in these electrocatalysts during potential cycling was investigated by transmission electron microscopy images and Pt L III -edge X-ray absorption fine structure. The crystallite size of Pt metal in SiO 2 /Pt/CNT did not change appreciably during the potential cycling, while Pt metal crystallites in Pt/CNT seriously aggregated. Silica layers enveloping Pt metal particles in SiO 2 /Pt/CNT prevent the dissolution and redeposition of Pt metal particles as well as the agglomeration of Pt metal particles on the supports.

A facile, economic and environmentally friendly one-step approach for the preparation of highly luminescent graphene quantum dots (GQDs) was developed using a hydrothermal reaction between citric acid and urea. Unlike previous reports, we focused on the effect of the transient nature of GQD formation on the photoluminescence (PL) properties and molecular structure changes of the products. We found that the GQDs have an optimum reaction time and require an effective precursor to achieve excellent luminescent properties. The PL, ultraviolet-visible (UV-vis) absorption, zeta potential, and nuclear magnetic resonance (NMR) analyses of the GQDs prepared at various reaction times revealed that the molecular structures responsible for the luminescence of the GQDs are aggregates or condensation products of citric acid amides. We found that urea addition to the precursor drastically enhances the PL intensity of the GQDs, and it is 40 times higher than those prepared using the pure citric acid precursor. Additionally, a GQDs–polyvinyl alcohol composite achieved an excellent quantum yield (QY) of 43.6%., This work was supported by JSPS KAKENHI Grant numbers 26709061 and 24656413. We thank Dr Eishi Tanabe from the Hiroshima Prefectural Institute of Industrial Science and Technology for helping with TEM analyses, and Drs Naoya Tochio and Junichi Kakimura from Hiroshima University for NMR analyses.