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Gold–silica (Au–SiO2) nanohybrids are of great technological importance, and it is crucial to develop facile synthetic protocols to prepare Au–SiO2 nanohybrids with novel structures. Here we report the bioinspired synthesis of pomegranate-like SiO2@Au nanoparticles (P-SiO2@Au NPs) via one-step aqueous synthesis from chloroauric acid and tetraethyl orthosilicate mediated by a basic amino acid, arginine. Effects of chloroauric acid, tetraethyl orthosilicate, and arginine on the morphology and optical property of the products are investigated in detail. The P-SiO2@Au NPs achieve tunable plasmon resonance depending on the amount of chloroauric acid, which affects the size and shape of the P-SiO2@Au NPs. Finite-difference time-domain simulations are performed, revealing that the plasmon peak red-shifts with increasing particle size. Arginine serves as the reducing and capping agents for Au as well as the catalyst for SiO2 formation and also promotes the combination of Au and SiO2. Formation process of the P-SiO2@Au NPs is clarified through time-course analysis. The P-SiO2@Au NPs show good sensitivity for both colloidal and paper-based surface-enhanced Raman scattering measurements. They achieve enhancement factors of 4.3 × 107–8.5 × 107 and a mass detection limit of ca. 1 ng using thiophenol as the model analyte., The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.0c00334.

Hexagonal cesium tungsten bronze (Cs0.32WO3) nanoparticles are well known as near-infrared (NIR) shielding materials. However, one of the critical issues for industrial applications involves a photochromic instability, i.e., a blue color change of this material under ultraviolet (UV) irradiation. This color change is attributed to the formation of HxWO3 phase through doping of H+ ions into Cs-deficient sites present at the surface of Cs0.32WO3 nanoparticles. Therefore, a new approach was adopted to prevent the color change by synthesizing a Cs0.32WO3 nanoparticle with less Cs-deficient sites using spray pyrolysis followed by heat treatment and short-time milling. The Cs0.32WO3 nanoparticles of spray pyrolysis route exhibited an improved photochromic stability under UV irradiation compared to those synthesized through a solid-state-reaction route. Detailed analysis using atomic resolution transmission electron microscopy revealed that the Cs0.32WO3 nanoparticles of spray pyrolysis route retain Cs ions up to a depth of less than 1 nm from the surface. This work demonstrates that the less-Cs-deficient Cs0.32WO3 nanoparticles are beneficial to improve the photochromic stability.

Requirements for nanoparticle processing based on energy and cost-effective technologies have increased in recent years. Flame synthesis is widely used on an industrial scale and is superior to gas...

The metastable state hexagonal-tungsten oxide (h-WO3) has been attracting attention over the past decade because of its high reactivity that arises from the hexagonal channels in its crystal structure. Simplification of the process used to synthesize h-WO3 is an important step to facilitate the industrial applications of this material. In this study, we addressed this challenge by developing a spray pyrolysis process to synthesize highly crystalline h-WO3. The ratio of the monoclinic to the hexagonal phase was controlled by adjusting the segregation time. Single-phase h-WO3 nanorods were synthesized using a carrier gas flow rate of 1 L/min, which was equivalent to a segregation time of 18.4 s. The ability of the h-WO3 nanorods to adsorb nitrogen and carbon dioxide was evaluated to confirm the presence of hexagonal channels in the crystal structure.

Metal oxide supports have attracted much attention for improving the performance and durability of polymer electrolyte fuel cells. In this study, niobium-doped tin oxide (NTO) nanoparticles with a network structure were synthesized and Pt catalysts deposited on them using a flame aerosol technology. Because the flame method involves a variety of particle formation processes, which depend on the type of raw material and combustion method used, the effect of these processes on the particle structure of the flame-made Pt/NTO was evaluated. When Pt/NTO nanoparticles were prepared by flame-assisted spray pyrolysis (FASP) using an inorganic raw material (metal chloride) with lower combustion enthalpy, the precursor liquid burned incompletely and Pt aggregation was observed. However, when flame spray pyrolysis (FSP) using a combustible precursor solution was used, the precursor rapidly evaporated in spray flames, and NTO with uniformly dispersed Pt was synthesized through gas-to-particle conversion. The electrochemical surface area and mass activity values of 5.44 wt% Pt-loaded NTO particles prepared by FSP were 48.2 m2 g−1 Pt and 338 mA mg−1 Pt, respectively. Flame aerosol synthesis of supported metal catalysts can be widely used as a dry one-step process that does not involve any complicated post-treatment. The two types of noble metal loading processes revealed in this study provide important insights for determining particle design guidelines for in-situ production of supported metal catalysts in flames.

Highly crystalline and hexagonal single-phase cesium tungsten bronze (Cs0.32WO3) nanoparticles were successfully synthesized by a flame-assisted spray pyrolysis followed by annealing under a reducing gas atmosphere. The resulting Cs0.32WO3 nanoparticles featured a pure hexagonal Cs0.32WO3 phase with a high crystallinity and homogeneous chemical composition. Unlike conventional methods, the proposed process in this paper has several advantages, including a short reaction time and the ability to yield products with high purity and good energy efficiency. Furthermore, the Cs0.32WO3 nanoparticles produced in this research showed a remarkable near-infrared shielding ability with a 97.7% cut-off at 1500 nm.

In this work, Pt nanoparticles were deposited onto the surface of Magneli phase titanium suboxide (TiOx) nanoparticles using a microwave-assisted deposition method. The effect of different concentrations of Pt nanoparticles was investigated to evaluate the strong metal-support interactions (SMSIs) between Pt and TiOx based on their performance for the degradation of organic pollutant molecules. The adsorption and catalytic performance of the as-synthesized Pt/TiOx nanoparticles were evaluated with respect to the degradation of rhodamine B (RhB) molecules without any external energy source. The Pt/TiOx nanoparticles with Pt loading at 10 wt% (10%Pt/TiOx) exhibited a remarkable performance. The XPS, CV, and FTIR analyses confirmed the presence of RhB degradation reactions under dark condition. This remarkable performance of the Pt/TiOx nanoparticles was attributed to the SMSIs between Pt and Ti3+ atoms, which improves their performance compared with Pt/TiO2 nanoparticles, and high density of active sites due to their nanometer size, which results in better performance compared with that of Pt/TiOx submicron particles.

Rare-earth-doped yttrium aluminum garnet (YAG:RE) phosphors have good photoluminescence (PL) properties and are widely used in light-emitting diodes. However, the RE elements used in these phosphors are expensive and in short supply. It is therefore important to develop phosphors that contain smaller amounts of RE materials. One strategy is to produce nanocomposite phosphors in which a cheaper and more readily available material is used as a matrix for an RE oxide. In this study, we produced a YAG:Ce/SiO2 nanocomposite using a sol–gel method; poly(ethylene glycol) and urea were added to promote micelle formation and agglomeration, respectively. The nanocomposites were characterized using X-ray diffraction, scanning and transmission electron microscopies (TEM), and energy-dispersive X-ray spectroscopy. We determined the concentration of SiO2 that provided maximum PL enhancement, and used geometrical models as well as the characterization results to propose an explanation for this enhancement. Our results showed that an SiO2 concentration of 10 vol% provided a PL intensity 120% that of pure YAG:Ce. TEM analysis showed that SiO2 nanoparticles covered the voids between the single grain boundaries of the YAG:Ce crystals, thereby inhibiting light scattering, resulting in enhanced PL. This method will be useful for large-scale synthesis of low-RE, high-PL phosphors.

The effects of deformation temperature on texture and thermoelectric properties of p-type Bi0.5Sb1.5Te3 sintered materials were investigated. The sintered materials were prepared by mechanical alloying and hot-press sintering. The hot-press deformation was performed at 723 K and 823 K by applying mechanical pressure in a graphite die. Then, the materials were extruded in the direction opposite to the direction of applied pressure. X-ray diffraction and electron backscattered diffraction patterns showed that the hexagonal c-plane tended to align along the extruded direction when the samples were deformed at high temperatures. The thermoelectric power factor was increased by high-temperature hot-press deformation because of the low electrical resistivity that originated from the c-plane orientation.

A hot-press deformation study of Bi 0.5 Sb 1.5 Te 3 was undertaken to obtain polycrystals with a preferred textural orientation for use in thermoelectric applications. Source materials were prepared by mechanical alloying (MA) as well as hot pressing (HP) and they were deformed at 673-823 K by applying mechanical pressure in a graphite die. The materials were extruded in a direction opposite to the direction of the applied pressure. X-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analyses indicated that the hexagonal c-planes of the samples that were deformed at high temperatures preferably aligned along the direction of the extrusion. The Hall mobility increased as the deformation temperature increased and it showed a good correlation to the textural orientation.

The preparation of the highly refractive transparent TiO 2 -dispersed microspheres with uniform size was studied. These nanocomposite microspheres were prepared via TiO 2 dispersion in an acrylic monomer and the emulsification by the Shirasu porous glass (SPG) membrane technique. Two kinds of the dispersion methods (bead mill dispersion with fine beads and ultrasonic dispersion) were carried out. The suspension prepared by bead mill included well-dispersed TiO 2 nanoparticles and led to the uniformly sized TiO 2 -dispersed microspheres. It was indicated that the dispersibility of TiO 2 in a monomer was the important factor in this process.

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.

Gallium nitride (GaN) nanoparticles were successfully produced from nano-sized gallium oxide (Ga 2 O 3 ) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared by salt-assisted spray pyrolysis (SASP). Highly crystalline Ga 2 O 3 nanoparticles with an average diameter of approximately 10 nm were obtained at various temperatures when a flux salt (LiCl, 5 mol/l) was added to the precursor solution. The effects of the crystallinity of the Ga 2 O 3 particles and nitridation time on transformation to GaN were characterized using X-ray diffraction and scanning/transmission electron microscopy. Highly crystalline GaN nanoparticles with a mean size of 23.4 nm and a geometric standard deviation of 1.68 nm were obtained when Ga 2 O 3 nanoparticles with relatively low crystallinity were used as the starting material. The resulting GaN nanoparticles showed a photoluminescence peak at 364 nm under UV excitation at 254 nm.

High luminance Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) nanoparticles were prepared from urea-added nitrate aqueous precursor by flame-assisted spray pyrolysis (FASP). The addition of urea into nitrate precursor plays an important role in YAG:Ce nanoparticle formation and in improving its optical performance. The decomposition and combustion of urea in the flame zone provides additional heat to the particles, which coupled with the evolution of large volumes of gasses, contributes to nanoparticle formation. The as-prepared nanoparticles are hexagonal YAlO 3 , that are nearly spherical, rough on the surface and dense—and they can be converted to YAG:Ce after being annealed at 1200 °C for 4 h. The heat-treated particles are single crystalline, smooth in surface and dense with an average size around 50 nm. The optimum cerium-doping concentration of YAG:Ce nanoparticles is 4.0 at.%, which exhibits quantum efficiency of 45.0%. This quantum efficiency is comparable with that of YAG:Ce nanoparticles produced from other processes. The efficient emission of YAG:Ce nanoparticles also originates from a relatively good distribution of Ce ions incorporated into the host material of YAG as evidenced from the elemental mapping analysis.

We developed TiO2-dispersed nanocomposite polymer films, which have a high concentration of 65 wt. % TiO2, high refractive index of 1.85, and high transmittance of 81.5% (thickness: 5 μm). Finely dispersed TiO2 nanoslurry was obtained by using a bead mill filled with fine beads, where two monomers (either neopentyl glycol dimethacrylate or divinylbenzene) and a single organic solvent (butylacetate) were used as a disperse medium. Up to 20 wt. % TiO2 could be added into a polymer film by the direct dispersion of TiO2 in the monomer. Furthermore, it was possible to incorporate high-concentration TiO2 (above 60 wt. %) in the polymer film by replacing the disperse medium with the monomer after the dispersion of TiO2 in the organic solvent. Visually, the surface of these films was smoother than that of other films prepared by direct dispersion. Further, these films were transparent and had a high refractive index.

Functional nanoparticle-dispersed composite polymer materials were prepared. Metal oxide nanoparticles such as BaTiO3 (primary diameter: 20nm) and three sizes of TiO2 (primary diameter: 7, 15, 20nm) were dispersed into acrylic monomer by means of beads mill filled with fine beads (50μm in diameter), and the nanoparticle-dispersed monomers were polymerized to obtain nanocomposite polymers by the suspension polymerization. The morphology, the nanoparticle dispersibility and the compressive strength of prepared polymer were evaluated. As a result, the TiO2-dispersed acrylic polymer spheres were fairly uniform in size with the diameter of several micrometers, and had relatively high compressive strength of 257MPa. It was also confirmed from the transmission electron microscope that the nanoparticles were well-dispersed in the polymer matrix.

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...

As negative electrode materials with high capacity and long cycle lives for advanced Li-ion batteries, Si-based alloy films have been prepared on Cu-foil substrate by use of electron beam deposition method or magnetron-sputtering method at room temperature. In this study, nano-structures of the Si-based alloy films have been investigated by conventional and analytical transmission electron microscopy. Experimental results are summarized as follows: (1) Si-based alloy films consist of a number of nano-fibers of 5 nm in diameter. Electron diffraction patterns from the films show halo. Crystallites are seldom found in high resolution images. (2) One-dimensional modulations of the constituents are formed in the range of 3.6 to 11 nm in the films. These nano-structures of the Si-based alloy films could relax the volume expansion of the Si during Li-insertion, causing the longer cycle lives than that of the Si film as the negative electrodes.

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.

A plastic deformation process and ratio study of p-type Bi 0.5 Sb 1.5 Te 3 was performed. The ingots were grown by the Bridgman method. Disks were cut from the ingots and deformed by either cold-pressing or by hot-pressing under pulse current heating. The plastic deformation ratio was controlled from 55 to 90%. The crystal structures of the deformed samples were identified by X-ray diffraction and pole figure analyses. The diffraction patterns indicate that the surfaces and bottoms of the samples were highly oriented in the hexagonal (00-l) plane. Thermoelectric properties change depending on not the plastic deformation ratio but the kind of plastic deformation process. The power factor for hot-press deformed samples exceeded those for the original ingots and cold-press deformed samples. The results suggest that the process of hot-press deformation enhances the thermoelectric properties of p-type Bi 0.5 Sb 1.5 Te 3 .

Microstructures of graphite mechanically milled under a hydrogen gas or an argon gas atmosphere using zirconia balls as well as chromium steel balls were investigated. Using transmission electron microscopes, high-resolution transmission electron microscopy (HREM), selected area electron diffraction (SAD), electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis (EDS) were carried out to compare the microstructures. Hydrogen concentration reaches up to 7.4 mass% after 80 h milling under a hydrogen gas atmosphere using chromium steel balls and homogeneous distribution of iron nano crystals was observed. No significant difference was observed in microstructures of 80 h milled graphite under the different gas atmosphere using the balls of different materials. This suggests that the significant absorption of hydrogen into the mechanically milled graphite strongly relates to the iron nano crystals included there.

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 pyrolysis of CH 4 to H 2 and carbon fibers over MCM-41 supported Ni catalyst has been investigated at different temperatures. The initial conversion of CH 4 at 873 K is higher than it is at 773 K, but the deactivation at 873 K occurs rapidly. The investigations with SEM and TEM techniques indicate that the pyrolysis of CH 4 generates mainly solid carbon fibers at 773 K but mostly hollow ones at 873 K on Ni/MCM-41. Ni metal particles have hardly been found on the tip of or inside the carbon fibers. Raman spectroscopic studies suggest that the carbon fibers formed at 873 K are more graphitic than those obtained at 773 K. The temperature-programmed reactions with O 2 (TPO) and H 2 (TPH) reveal that the solid carbon fibers formed at 773 K show higher reactivity towards O 2 and H 2 than the hollow ones formed at 873 K.

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.

Ni-supported catalysts on perovskite-type oxides, i.e. , Ni 0.2 /ATiO 3 (A: Ca, Sr or Ba) have been prepared by solid phase crystallization (spc) method and tested for partial oxidation of CH 4 into synthesis gas. Ni 0.2 /SrTiO 3 catalyst showed the highest activity as well as the highest sustainability among the catalysts tested, followed by Ni 0.2 /BaTiO 3 and then Ni 0.2 /CaTiO 3 . The high and stable activity may be due to highly dispersed and stable Ni metal particles (diameter

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.