Takao A. Yamamoto

Osaka University, Suika, Ōsaka, Japan

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Publications (103)183.7 Total impact

  • Yuji Ohkubo · Satoshi Seino · Takashi Nakagawa · Koji Ueno · Takao A. Yamamoto
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    ABSTRACT: To clarify the influence of pH on performance of sodium phosphinate (NaPH2O2) for decreasing particle size of Pt nanoparticles, carbon-supported Pt nanoparticles that contained phosphorus (P–Pt/C) were synthesised by an electron-beam irradiation reduction method (EBIRM) under four different pH conditions (pH = 3, 6, 9, and 12) and under five different NaPH2O2 concentrations (0.0, 0.5, 1.0, 3.0, and 5.0 mM). The relationship among pH, NaPH2O2 concentration, average particle size of Pt nanoparticle, and Pt loading weight was investigated in this study. The average particle size of Pt nanoparticles was in the range of 0.8–3.4 nm and lower in the order; pH12 > pH3 > pH6 > pH9; for example, under the same NaPH2O2 concentrations of 3.0 mM, the particle size of P–Pt/C prepared at pH = 12 and pH = 9 was 1.5 and 0.8 nm, respectively. In addition, Pt loading weight was also lower in the order; pH12 > pH3 > pH6 > pH9. In summary, these results indicated that the pH in the precursor solution affected the performance of NaPH2O2. These findings would be useful for controlling the particle size of monometallic Pt and Pt-based bimetallic nanoparticles supported on carbon particles for fuel cell applications.
    No preview · Article · Feb 2016 · Journal of Experimental Nanoscience
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    ABSTRACT: Antiviral activity of metallic Ag nanoparticles immobilized on textile fabrics were investigated. The Ag nanoparticles synthesized by radiochemical process are firmly immobilized on the surface of support textile fabrics of cotton. Small Ag particles of about 2–4 nm were observed together with relatively large particles of more than 10 nm. The Ag nanoparticles showed antiviral activity against Influenza A and Feline Calicivirus. The antiviral activity significantly depended on the concentration of the Eagle’s minimal essential medium. It was implied that the surface passivation by inhibitory agent lead to the deactivation of metallic Ag nanoparticles.
    No preview · Article · Jan 2016
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    ABSTRACT: PtCu nanoparticles were synthesized with different pH and support conditions using radiochemical process. The nanoparticle structures were characterized by transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, X-ray absorption spectroscopy, and X-ray diffraction techniques. The nanoparticle structure was relevant to the pH of the precursor solutions. The lattice parameter of PtCu alloy increased in high pH samples, which indicates the critical effect of metal ion adsorption in precursor solution on nanoparticle structure.
    No preview · Article · Jan 2016
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    ABSTRACT: Bimetallic nanoparticle catalysts of PtRh supported on carbon were synthesized using an electron beam irradiation reduction method. The PtRh nanoparticle catalysts were composed of particles 2–3 nm in size, which were well dispersed on the surface of the carbon support nanoparticles. Analyses of X-ray diffraction and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy revealed that the PtRh nanoparticles have a randomly alloyed structure. The lattice constant of the PtRh nanoparticles showed good correlation with Vegard’s law. These results are explained by the radiochemical formation process of the PtRh nanoparticles. Catalytic activities of PtRh/C nanoparticles for ethanol oxidation reaction were found to be higher than those obtained with Pt/C.
    No preview · Article · Jan 2016
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    ABSTRACT: This paper presents a new technique for synthesizing silver nanoparticles immobilized on textile fabrics using a radiochemical process. In this process, the irradiation of a high-energy electron beam on an aqueous solution containing silver ions induces a reducing reaction that forms metallic silver nanoparticles. Small Ag particles of about 2–4 nm were observed together with relatively large particles of more than 10 nm. These nanoparticles are firmly immobilized on the surface of a support textile fabric without the need for any binder or surfactant. The amount of silver nanoparticles immobilized was found to depend on the water content of the support textile fabric, suggesting that the silver ions are reduced not only by radiochemical species generated by the radiolysis of water, but also by radiochemical species generated in the irradiated support fabric itself. The silver nanoparticles that were immobilized on the support textile fabric exhibited an excellent antibacterial activity across a wide antibacterial spectrum, even after a durability test involving washing the fabric 100 times.
    No preview · Article · Oct 2015 · Journal of Nuclear Science and Technology
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    ABSTRACT: In order to clarify the effect of reduction enhancer on the nanoparticle formation process and their structural and catalytic properties, carbon-supported Pt–Cu nanoparticles were synthesized by electron beam irradiation on an aqueous precursor solution in the presence/absence of reduction enhancer. In the absence of reduction enhancer, tetravalent platinum oxide particles of approximately 1 nm in diameter were formed on carbon support with copper barely precipitated, while in the presence of 2-propanol or ethylene glycol or glucose both platinum and copper precipitated as few-nanometer-sized alloy particles together with copper oxides. It was suggested that the metal nuclei produced upon electron beam irradiation do not have enough lifetime without reduction enhancer due to fast oxidation of the nuclei by oxidizing radicals, while the reduction enhancer scavenges these oxidizing radicals preventing oxidation of metallic clusters and prolonging their lifetime. Ethylene glycol gave smaller and better alloyed particles with less copper oxides compared to 2-propanol since the carbonyl compounds derived from oxidation of ethylene glycol protect metallic clusters from oxidation further prolonging their lifetime. In the electrochemical measurements, the methanol oxidation activities of Pt–Cu/C catalysts were well explained by their structural characteristics.
    No preview · Article · Jun 2015 · Journal of Nanoparticle Research
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    ABSTRACT: An electron-beam irradiation reduction method (EBIRM) is a technique to reduce metal ions in an aqueous solution via irradiation with a high-energy electron beam. In this study, an EBIRM is improved to develop a technique for the mass production of highly loaded and highly dispersed PtRu/C catalysts for use as direct methanol fuel cell anodes. An increase in the Pt and Ru input concentrations increased the loading weight from 9 to 37 wt%; however, the dispersibility of the PtRu nanoparticles on the carbon particles decreased. To improve the low dispersibility, sodium phosphinate was added to the precursor solution and the input amount of carbon particles was decreased. These changes resulted in not only highly loaded but also highly dispersed PtRu/C catalysts. The catalytic activity of the highly loaded and highly dispersed PtRu/C catalysts for methanol oxidation was at least 1.6 times higher than that of the lowly loaded and lowly dispersed PtRu/C catalysts in all voltage range. More than 6000 mg of highly loaded and highly dispersed PtRu/C catalysts were relatively easily obtained, and the average particle size of the PtRu nanoparticles was 1.8 nm. These results demonstrated that the improved EBIRM is effective for the mass production of carbon-supported, highly loaded, and highly dispersed metal nanoparticles.
    Full-text · Article · Apr 2015 · Journal of Experimental Nanoscience
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    ABSTRACT: Catalytic activities of sonochemically prepared Au-core/Pd-shell-structured bimetallic nanoparticles (NPs) immobilised on TiO2 were evaluated. Comparing with the mixture of monometallic Au and Pd NPs on TiO2, core/shell-immobilised catalysts exhibited higher activities for the partial reduction of nitrobenzene (NB) to aniline (AN), suggesting that the synergistic effect originating from the core/shell structure enhanced the catalytic activities. In the case of high Au/Pd ratios, where the Pd-shell thickness was calculated to be 0.5 nm or lower, infrared spectroscopic measurements of adsorbed CO showed that the Au cores were successfully covered with Pd shells. It was found that a thin Pd shell of one layer or two layers of Pd atoms effectively catalysed the reduction of NB under ambient temperature, whereas the formation of AN was not confirmed on monometallic Au NP-immobilised catalysts.
    No preview · Article · Feb 2015 · Journal of Experimental Nanoscience
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    ABSTRACT: Carbon-supported Pt–SnO2 electrocatalysts with various Sn/Pt molar ratios were prepared by an electron beam irradiation method. These catalysts were composed of metallic Pt particles approximately 5 nm in diameter together with low crystalline SnO2. The contact between the Pt and SnO2 in these materials varied with the amount of dissolved oxygen in the precursor solutions and it was determined that intimate contact between the Pt and SnO2 significantly enhanced the catalytic activity of these materials during the ethanol oxidation reaction. The mechanism by which the contact varies is discussed based on the radiochemical reduction process.
    Full-text · Article · Nov 2014 · Radiation Physics and Chemistry
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    ABSTRACT: We developed a novel technique for immobilizing enzymes on magnetic nanoparticle surfaces by using a methionine-tag system. Au/Fe-oxide composite nanoparticles synthesized through a radiochemical process were used as magnetic nanocarriers. The C-terminus of Tk-subtilisin, a model enzyme, was modified with a methionine tag and mixed with Au/Fe-oxide composite nanoparticles for immobilization via Au-S bonding. Methionine-tagged immobilized enzymes showed 98% residual specific activity, while the untagged enzymes showed 78%. The methionine-tagged immobilized enzymes retained their activities in a wide temperature range of 30–70 °C. Thus, the methionine-tag system provided orientational immobilization via the formation of Au-S bonds, which resulted in structural stability of the immobilized enzymes.
    No preview · Article · Oct 2014 · Colloids and Surfaces A Physicochemical and Engineering Aspects
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    ABSTRACT: Synchrotron X-ray-induced reduction of Au ions in an aqueous solution with or without support materials is reported. To clarify the process of radiation-induced reduction of metal ions in aqueous solutions in the presence of carbon particles as support materials, in situ time-resolved XANES measurements of Au ions were performed under synchrotron X-ray irradiation. XANES spectra were obtained only when hydrophobic carbon particles were added to the precursor solution containing Au ions. Changes in the shape of the XANES spectra indicated a rapid reduction from ionic to metallic Au in the precursor solution owing to synchrotron X-ray irradiation. In addition, the effects of the wettability of the carbon particles on the deposited Au metallic spots were examined. The deposited Au metallic spots were different depending on the relationship of surface charges between metal precursors and support materials. Moreover, a Au film was obtained as a by-product only when hydrophilic carbon particles were added to the precursor solution containing the Au ions.
    Full-text · Article · Sep 2014 · Journal of Synchrotron Radiation
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    ABSTRACT: The effects of sintering condition to magnetic properties of the SrMg2W-type ferrite (SrMg2Fe16O27) prepared by the Spark Plasma Sintering (SPS) method were investigated. SrMg2W-type ferrite powders with average particle sizes of 0.43 μm were prepared by iron ball-milling for 10 hours. Some kinds of SrMg2W-type ferrite samples were obtained by sintering SrMg2W-type powders by the SPS method at 1073-1173 K for 0-20 min as holding time. Compared to the SrMg2W-type ferrite prepared at 1523 K by the conventional solid reaction method, all the spark-plasma-sintered samples had larger coercivity. Mass magnetization of the SPS samples were almost the same (51.9 ∼ 54.3 Am2/kg @ 1 T) regardless of the sintering condition. In the case of SPS without holding time, the coercivity of the SPS samples increased with increasing the sintering temperature, and reached 191 kA/m at 1173 K. When the samples sintered at 1073 K, the coecivity became the maximum (190 kA/m) at 10 min. From the results of X-ray diffraction, it was found that the coercivity of the samples enhanced as lattice strain of them was small. The influence of the lattice strain on the coercivity was discussed using the Kronmuller equation.
    No preview · Article · Apr 2014 · Journal of the Japan Society of Powder and Powder Metallurgy
  • Junichiro Kugai · Satoshi Seino · Takashi Nakagawa · Takao A. Yamamoto
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    ABSTRACT: The effects of phosphorus (PH2O2−) and copper (Cu2+) additions to the aqueous precursor solution on the structure of Pt–Cu nanoparticles were investigated for a radiation-induced reduction method. Addition of PH2O2− in the precursor solution reduced the diffraction intensity of Pt or Pt–Cu crystallites due to smaller size and/or lower crystallinity. Both the diffraction intensity and the particle size (measured by an electron microscope) were minimized when Cu/Pt ratio was 0.05–0.25, which was attributed to the effects of copper and phosphorus to stabilize crystallites and particles through the negative heat of mixing. The concomitant increase in phosphorus content suggested that PH2O2− is partly reduced and taken into the Pt lattice. Further increase of copper content caused larger particles and decrease in phosphorus content. These trends were also consistent with electrochemical surface area and oxidation/reduction behavior of Pt surface. The radiation-induced reduction method is suited to produce small Pt–Cu particles uniformly distributed on carbon support, which are potentially served for heat treatment for improved oxygen reduction performance.
    No preview · Article · Mar 2014 · Journal of Nanoparticle Research
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    ABSTRACT: Carbon-supported Pt nanoparticles that contained phosphorus (P–Pt/C) were synthesized by the reduction of Pt ions in an aqueous solution via irradiation with a high-energy electron beam; this synthesis technique is referred to as the electron beam irradiation reduction method (EBIRM). To decrease the size of the Pt nanoparticles supported on the carbon particles, sodium phosphinate (NaPH2O2) was added as a phosphorus precursor to the precursor solution that contained Pt ions. The P–Pt/C samples were observed using a transmission electron microscope. The average particle size of the Pt nanoparticles decreased as the NaPH2O2 concentration in the precursor solution increased. The average particle size was in the range 0.9–3.4 nm. The electrochemically active surface areas (ECSAs) of Pt were estimated using cyclic voltammetry. In contrast to the average particle size, the ECSAs increased as the NaPH2O2 concentration was increased. The decrease in the size of the Pt nanoparticles using NaPH2O2 effectively increased the ECSA. This result indicated that the EBIRM combined with the use of NaPH2O2 to decrease the particle size is a useful and simple tool for the preparation of Pt nanoparticle catalysts with a high specific activity.
    Full-text · Article · Feb 2014 · Journal of Nanoparticle Research

  • No preview · Article · Jan 2014
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    ABSTRACT: In order to understand the formation process of metal–oxide composite in an electron beam irradiation method in aqueous phase, the structure and composition of obtained solid were correlated to the synthesis parameters. Transition metal did not precipitate alone by the electron beam irradiation, but they did in the presence of platinum or support. Due to the relatively high reduction potential, copper underwent reduction to metallic state and readily precipitated by forming Pt–Cu alloy and/or copper oxide on solid surface. In the Pt–Cu/CeO2 system, the structure of Pt–Cu was ruled by two competing factors, growth of alloy nanoparticles promoted by sulfate ion and deposition of metal (alloy) on CeO2 support with their concomitant partial oxidation. CeO2 was suggested to immobilize the metals oxidatively before they coalesce. Iron barely formed alloy with Pt, but it directly precipitated on support as oxide without being reduced to metal due to its oxophilicity. Oxide was formed either via reduction to metallic state (for Pt and Cu) or through direct oxygenation or hydroxylation on solid (for Fe). Under the restriction of reduction potential, the size and composition of alloy nanoparticles and the content of oxide phase were drastically modified by support surface property and anion species in the solution.
    Full-text · Article · Nov 2013 · Journal of Alloys and Compounds
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    ABSTRACT: Pt–Cu supported on CeO2 and mechanically mixed with CeO2 were synthesized using an electron beam irradiation method to probe the active metal–oxide interfaces for catalytic CO preferential oxidation. The lack of activity for the mechanical mixture of Pt with CeO2 showed the metal–CeO2 interface is critical for monometallic Pt. The comparable activity for the CeO2-supported Pt–Cu and mechanical mixture of Pt–Cu with CeO2 suggested platinum–copper contact as a new active site for bimetallic Pt–Cu. A non-linear increase of activity along the Cu content in catalyst and the Cu–O bonds detected in XANES spectra in the reaction condition at 100 °C suggested the presence of CuOx on the Pt–Cu alloy surface as strong chemisorption sites for oxygen. Graphical Abstract
    Full-text · Article · Nov 2013 · Catalysis Letters
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    ABSTRACT: We prepared carbon-supported PtCo bimetallic nanoparticles (PtCo/C) as electrode catalysts for the oxygen reduction reaction (ORR) at the cathodes in polymer electrolyte membrane fuel cells (PEFCs) by an electron-beam irradiation reduction method (EBIRM). An EBIRM allows nanoparticles to be easily prepared by the reduction of precursor ions in an aqueous solution irradiated with a high-energy electron beam. The structures of PtCo/C were characterized by transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, and the techniques of X-ray diffraction and X-ray absorption near edge structure. It found for the first time that both PtCo alloy and Co oxide were prepared simultaneously on the carbon support by an EBIRM. The catalytic activity and durability of PtCo/C were evaluated by linear-sweep voltammetry and cyclic voltammetry, respectively. The addition of Co to Pt/C not only enhanced the catalytic activity for the ORR but also improved the catalytic durability. As the Co concentration increased, both behaviors became pronounced. These improvements are explained by the effects of both PtCo alloy and Co oxide. We demonstrated that an EBIRM can not only synthesize the alloy and oxide simultaneously on the carbon support but also mass-produce the electrode catalysts for PEFC cathodes.
    Full-text · Article · Jul 2013 · Journal of Materials Science
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    ABSTRACT: The influence of surface properties of CeO2 support on the structure of Pt–Cu bimetallic system in an aqueous-phase synthesis process using electron beam irradiation and the resulting impact on PROX performance were investigated. The exposed hydroxyls on CeO2 surface with low hydrogen-bonding water led to cluster-like Pt–Cu phase with low crystallinity and high oxidation state. It was attributed to a strong interaction of CeO2 and Pt–Cu phase. In CO atmosphere, such oxidic or fine Pt–Cu phase was reduced to platinum and partially reduced CuOx, and CeO2 was concomitantly reduced to retain carbonate species on its surface. This in turn affected catalytic performance, i.e., a significant increase in O2 conversion was observed in an oxygen-rich PROX condition compared to a stoichiometric condition while selectivity was retained in higher level than monometallic Pt. The higher oxidation state of Pt in the excess O2 was suggested to be responsible for the drastic change in activity. The surface chemical property of CeO2 was shown to affect PROX activity through the structure and oxidation state of Pt–Cu species which originates from metal–ceria interaction in the synthesis stage.
    Full-text · Article · May 2013 · The Chemical Engineering Journal
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    ABSTRACT: Electrode catalysts composed of carbon-supported PtRu nanoparticles (PtRu/C) for use as a direct methanol fuel cell anode were synthesized by the reduction of precursor ions in an aqueous solution via irradiation with a high-energy electron beam. The effect of pH control in the precursor solution on the PtRu mixing state and the methanol oxidation activity was studied in order to enhance the catalytic activity for methanol oxidation. The PtRu/C structures were characterized by transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, X-ray fluorescence spectrometry, and X-ray diffraction and X-ray absorption fine structure techniques. The methanol oxidation activity was evaluated by linear sweep voltammetry. The initial pH of the precursor solution has little influence on the average grain size for the metal particles (approximately 3.5 nm) on the carbon particle supports, but the dispersibility of the metal particles, PtRu mixing state, and methanol oxidation activity differed. The maintenance of a low pH in the precursor solution gave the best dispersibility of the PtRu nanoparticles supported on the surface of the carbon particles, whereas, a high pH gave the best PtRu mixing state and the highest oxidation current although a low dispersibility of the PtRu nanoparticles supported on the surface of the carbon particles was obtained. The PtRu mixing state strongly correlated with the methanol oxidation current. In addition, a high pH was more effective for PtRu mixing when using an electron beam irradiation reduction method, because the complexation reaction of the chelating agents was improved, which resulted in an enhancement of the catalytic activity for methanol oxidation.
    Full-text · Article · May 2013 · Journal of Nanoparticle Research