Takao A. Yamamoto

Osaka University, Suika, Ōsaka, Japan

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Publications (67)107.22 Total impact

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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.
    Journal of Synchrotron Radiation 09/2014; · 2.19 Impact Factor
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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.
    Journal of Nanoparticle Research 01/2014; 16(3). · 2.18 Impact Factor
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    Journal of Materials Science 07/2013; 48(14):5047-5054. · 2.31 Impact Factor
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    ABSTRACT: In order to investigate the effect of transition metal addition to platinum with different support materials on preferential CO oxidation, structure and chemical properties of supported bimetallic catalysts prepared by electron beam irradiation method were correlated to the catalytic performance. On Al2O3, decoration of Pt by small amount of Co (Co/Pt ∼ 0.03) drastically increased CO and O2 conversions while addition of equimolar Cu to Pt increased them only above 100 °C, where the rate-controlling factor was suggested to change from oxygen transport to CO activation. On CeO2, either addition of Co or Cu to Pt had minor or negative effect on high O2 conversion inherent to high oxygen transport at Pt–CeO2 interface. On Pt–Cu/CeO2, however, metal-CuOx interface dominates the reaction characteristics to give improved selectivity, which is suitable for deep CO removal in excess O2/CO condition. The order of selectivity above 100 °C, Pt–CoOx > Pt(alloy)–CuOx > Pt–CeO2 interfaces, was derived from structural analysis and catalytic tests.
    International Journal of Hydrogen Energy 04/2013; 38(11):4456–4465. · 3.55 Impact Factor
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    ABSTRACT: This paper describes a novel synthesis method of stabilizer-free Pt/TiO2 composite nanoparticles using electron beam irradiation. The chemical compositions were analyzed by inductively coupled plasma-atomic emission spectroscopy. The microstructures of the samples were observed by using transmission electron microscope. Pt nanoparticles with the sizes of 2–4 nm were deposited on TiO2 without any use of stabilizers. The concentrations of Pt ions and 2-propanol notably affected the size and shape of Pt nanoparticles. Their reactions of preferential CO oxidation were measured in temperature region from 60 to 140 °C. The Pt/TiO2 catalyst with spherical Pt nanoparticles exhibited a 67% of CO conversion rate and 100% of selectivity at a low temperature of 60 °C.
    Materials Research Bulletin. 04/2013; 48(4):1347–1351.
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    ABSTRACT: The structure of bimetallic nanoparticles has a great impact on catalytic performance. By radiation-induced reduction of metal ions in the aqueous phase, supported bimetallic Pt–Cu catalysts were synthesized with various Cu loadings, support materials, and copper sources. These parameters had a great impact on the structural properties of Pt–Cu nanoparticles. On carbon black, Pt–Cu alloy nanoparticles were readily formed with little oxide phase. On CeO2 support, Pt–Cu was highly oxidized for the low-Cu region (Cu/Pt < 1), and Pt–Cu alloy nanoparticles were formed together with CuOx for the high-Cu region (Cu/Pt > 1). The structure of CeO2-supported catalysts obtained from two different copper sources (CuSO4 and CuCl2) suggested that such a drastic change in oxidation state is the result of two competing effects, CeO2 to oxygenate metals and the sulfate ion to stabilize them in the metallic state. The reaction characteristics in preferential CO oxidation reflected the Pt–Cu structure (oxidation state) determined by those parameters in the synthesis stage. The low-Cu oxidic samples showed excellent light-off performance, which was attributed to high oxygen transport from CeO2 to metals through the metal–CeO2 interface. The high-Cu samples comprising a Pt–Cu alloy and CuOx showed excellent selectivity, which was attributed to the metal–CuOx interface predominating over metal–CeO2. The latter exhibited 100% CO conversion in a wide temperature range in excess O2 conditions.
    The Journal of Physical Chemistry C. 03/2013; 117(11):5742–5751.
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    Journal of Materials Science 03/2013; 48(5):2142-2150. · 2.31 Impact Factor
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    ABSTRACT: La0.666Sr0.373Mn0.943Cu0.018O3 spheres (average diameter: 2.4 mm) were used as magnetic hyperthermia mediators. They were prepared by the polymerized complex method and alginate gelation. A starch paste containing nine such spheres (0.1034 g) was maintained at a constant temperature when it was exposed to an alternating current magnetic field (frequency: 1 MHz; magnetic field strength: 20–55 Oe-rms). After exposure to a magnetic field with a field strength of 55 Oe-rms for 60 min, a temperature of T60=73.5 °C was obtained. T60 for field strengths of 40 and 55 Oe-rms differed by only 2.6 °C. La0.666Sr0.373Mn0.943Cu0.018O3 spheres satisfy the following important requirements for heating mediators: their amount can be controlled in tumors, their temperature can be controlled without monitoring, and their heating ability is independent of the field strength and direction. Thus, La0.666Sr0.373Mn0.943Cu0.018O3 spheres are promising heating mediators for inducing magnetic hyperthermia.
    Journal of Magnetism and Magnetic Materials 03/2013; 329:49–52. · 2.00 Impact Factor
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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
    Catalysis Letters 01/2013; · 2.24 Impact Factor
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    ABSTRACT: A nanoparticle catalyst of PtRuAu/C was synthesized by including an Au precursor in the radiolytic process for preparing a PtRu/C catalyst. Their methanol oxidation activity and electrochemical durability were measured by linear sweep voltammetry before and after potential cycling treatment. PtRuAu/C had a significantly higher durability than PtRu/C while maintaining a comparable high activity. The morphology and substructure of the nanoparticles were investigated by energy-dispersive x-ray spectroscopy, x-ray diffraction, and x-ray absorption fine structure spectroscopy. Metallic nanoparticles with diameters of about 2 nm were obtained; they probably had Pt-core/PtRu-shell structures. Transmission electron microscopy observations after potential cycling revealed that 2-nm-diameter nanoparticles containing Au did not coarsen, whereas nanoparticles without Au coarsened significantly to 3.7 nm. Some crystal defaults were observed in the coarsened particles, implying that the coarsening was caused by Ostwald ripening. The Au addition to catalyst particles consisting of PtRu inhibits coarsening and consequently improves the electrochemical durability.
    Journal of Materials Research. 04/2012; 27(07).
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    ABSTRACT: Gold magnetic particles (GMP) are magnetic iron oxide particles modified with gold nanoparticles. The gold particles of GMP specifically bind to cysteine and methionine through Au-S binding. The aim of the present study was to establish a quick and easy protein purification system using novel peptide tags and GMP. Here, we created a variety of peptide tags containing methionine and cysteine and analyzed their affinity to GMP. Binding assays using enhanced green fluorescent protein (EGFP) as a model protein indicated that the tandem methionine tags comprising methionine residues had higher affinity to the GMP than tags comprising both methionine and cysteine residues. Tags comprising both methionine and glycine residues showed slightly higher affinity to GMP and higher elution efficiency than the all-methionine tags. A protein purification assay using phosphorylcholine-treated GMP demonstrated that both a tandem methionine-tagged EGFP and a methionine and glycine-tagged EGFP were specifically purified from a protein mixture with very high efficiency. The efficiency was comparable to that of a histidine-tagged protein purification system. Together, these novel peptide tags, "methionine tags", specifically bind to GMP and can be used for a highly efficient protein purification system.
    Bioconjugate Chemistry 05/2011; 22(5):887-93. · 4.58 Impact Factor
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    ABSTRACT: Au–Pd bimetallic nanoparticles supported on carbon particles were synthesized by reduction of precursor ions in an aqueous solution irradiated with a high energy electron beam. The composition of the samples was analyzed by the inductively coupled plasma atomic emission spectrometry (ICP-AES), and the morphology of the samples was observed by the transmission electron microscope (TEM). TEM micrographs indicated that Au–Pd particles of ca. 5-nm were well dispersed on the surface of carbon particles of ca. 30-nm without any serious agglomeration. Addition of citric acid to the initial solution and high pH were found to be effective for formation of random alloy structure in the resultant bimetallic nanoparticles. The change in the bimetallic structure from core–shell to random alloy was identified by techniques of the X-ray diffraction (XRD) and the extended X-ray absorption fine structure (EXAFS).
    Materials Letters - MATER LETT. 01/2011; 65(14):2165-2167.
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    ABSTRACT: PtRu nanoparticle catalysts with 2-nm size supported on carbon were synthesized by a radiolytic process, assisted with the addition of dl-tartaric acid. Gradual alloying with the addition was confirmed by structural analyses with techniques of the X-ray diffraction and the X-ray absorption fine structure. Their methanol oxidation activities evaluated by the linear sweep voltammetry were higher than that of the commercial one, and found to be enhanced in accordance with the gradual change in the bimetallic structure from the Pt-core/Ru-shell to the random alloy. A good correlation was found between the catalytic activity and the indicators, which stands for the occurring frequency of the Pt–Ru bonds, calculated from the coordination numbers determined by the extended X-ray absorption fine structure analysis. It was confirmed that the addition of dl-tartaric acid in the radiolytic process certainly promoted the random alloy formation of PtRu bimetallic nanoparticles and enhanced their methanol oxidation activity. KeywordsMethanol oxidation–PtRu catalyst–Radiolytic process– dl-tartaric acid–Fuel cells
    Journal of Nanoparticle Research 01/2011; 13(10):5275-5287. · 2.18 Impact Factor
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    ABSTRACT: Pt-Cu bimetallic nanoparticles immobilized on γ-Fe2O3 support were synthesized by a unique radiolytic process and their physical properties and catalytic activity for CO preferential oxidation were investigated for various Pt/Cu atomic ratios. The chemical composition of the catalysts measured by inductively coupled plasma revealed most of the platinum and copper aqueous precursors were deposited on to the γ-Fe2O3 support after irradiation with electron beam. Crystallographic analysis of the catalysts by X-ray diffraction showed Pt-Cu alloy is formed for wide range of Pt/Cu ratio. From the lattice parameter of this Pt-Cu alloy, it was found that Cu can be incorporated into Pt lattice up to 30at% with the present irradiation condition. The rest of Cu for high-Cu (low-Pt) samples was identified as divalent oxide with poor crystallinity by XANES spectrum. Detailed STEM-EDX analyses further confirmed that Pt-Cu alloy exists as nanoparticles with a few nanometers in diameter and CuO with low crystallinity distributes on entire surface of the γ-Fe2O3 support. The PROX activity showed different trends between high-Pt and high-Cu samples. The CO conversion decreased as the Pt loading was decreased to 50at%, and it monotonically decreased with increasing temperature. However, as the Pt loading was further decreased, the activity increased with temperature by contraries, and reached the maximum conversion at 100°C. Regardless of the low Pt loading, the sample with 10 at% Pt and 90at% Cu exhibited the highest activity at 100°C, which is preferable for low temperature fuel cell applications. This enhanced activity was attributed to oxygen supply via the copper of low crystallinity from the O2-poor atmosphere to Pt-Cu alloy particles that chemisorbs CO molecules.
    Applied Catalysis A-general - APPL CATAL A-GEN. 01/2011; 406(1):43-50.
  • Yusuke Hirayama, Takashi Nakagawa, Takao A. Yamamoto
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    ABSTRACT: The Curie temperatures TC of the nitrides of the rare earths (Gd, Tb, Dy, Ho, and Er), including binary systems, were investigated. TC was found to be approximately proportional to the de Gennes factor, ξ=(g−1)2J(J+1), where g is the Landé g-factor and J is the total angular momentum quantum number of a trivalent rare earth (RE). This proportionality was significantly improved by introducing a modified de Gennes factor, ξbi. The conventional de Gennes factor ξ indicates the exchange interaction given by the inner product of the effective spin components of ions of the same kind, whereas our modified de Gennes factor ξbi also considers interactions between different kinds of ions and statistical factors calculated on the basis of the binomial distribution. The good proportionality obtained between TC and ξbi indicates that the spin components of RE ions interact with each other. This interaction is considered to be responsible for the ferromagnetism of these nitrides (including binary systems). These considerations were supported by the synthesis of and magnetization measurements on GdxEr1−xN (x=0, 0.25, 0.5, 0.75, 1) samples.
    Solid State Communications 01/2011; 151(21):1602-1604. · 1.53 Impact Factor
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    ABSTRACT: Nanoparticle catalysts of PtRu/C for the direct methanol fuel cell anode were synthesized by a radiolytic process. Bimetallic substructures were controlled by varying irradiation dose rate and by addition of NH4OH or NaH2PO2. Material characterization was performed with the transmission electron microscopy, the X-ray diffraction, the X-ray fluorescence spectroscopy and the X-ray absorption fine structure techniques. Methanol oxidation activity was evaluated by the linear sweep voltammetry. We concluded that the structure of the radiolytically synthesized catalysts has a Pt-rich core/Ru-rich shell structure or incomplete alloy structure. A correlation between the substructures and catalytic activities was found by using a pairing factor defined from coordination numbers determined by the extend X-ray absorption fine structure analysis, which indicates the validity of the bifunctional mechanism in the PtRu nanoparticle system. This radiolytic process is promising for synthesizing advanced PtRu/C catalysts with well-mixed bimetallic substructures enhancing methanol oxidation.
    Applied Catalysis A-general - APPL CATAL A-GEN. 01/2011; 396(1):68-75.
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    ABSTRACT: Gold/iron oxide magnetic nanoparticles are hybrid nanoparticles containing a core of magnetic iron oxide and surface colloidal gold, which allows for various biomaterials to be immobilized on the surface of the iron oxide nanoparticles via colloidal gold. Here, we developed a novel magnetic resonance (MR) imaging agent to broaden the MR tumor-imaging spectrum of superparamagnetic iron oxide nanoparticles (SPIO), e.g., Feridex(), a clinical MR imaging agent for diagnosing liver cancer. Au/Feridex was synthesized by electron beam irradiation, and thiol-modified poly(ethylene glycol) (PEG-SH) was easily conjugated to its surface via an Au-S bond without the need for any chemical reactions. PEG conjugation of Au/Feridex enhanced its accumulation in Meth-A tumor tissue and decreased its accumulation in normal liver tissue. In addition, MRI using PEG-Au/Feridex, in contrast to MRI using unmodified Au/Feridex and Feridex, detected B16BL6 and Meth-A tumor tissues in vivo. This finding indicates that PEG-Au/Feridex is useful for diagnosing various types of tumors. In addition, because the synthesis of PEG-Au/Feridex is simple and high yields are easily produced, PEG-modified SPIO for tumor diagnosis can be prepared on an industrial scale with low cost.
    Bioconjugate Chemistry 06/2010; 21(6):1026-31. · 4.58 Impact Factor
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    ABSTRACT: Bimetallic composite nanoparticles of PtM (M=Au, Cu or Ni) supported on γ-Fe2O3 were synthesized by a radiolytic method employing a 4.8-MeV electron beam to reduce aqueous ions. These composite particles were obtained at room temperature just by irradiating glass vials containing source materials in an aqueous system without using any surfactant, organic solvent and stabilizer. The irradiation, taking several seconds, formed metallic nanograins of 3nm in diameter stabilized on the support of γ-Fe2O3 particle of 13nm in diameter. XRD patterns of PtAu and PtCu samples showed clear peak shifts indicating occurrence of random alloy phase. X-ray absorption fine structure analysis for the PtCu and PtNi samples indicated significant Pt–M coordination. Catalytic activity of CO oxidation of the samples was evaluated by measuring residual CO contents in air in contact with the sample material with a gas-chromatograph. All the PtM systems exhibited activities higher than those of monolithic Pt on γ-Fe2O3. The correlation between the atomic structures and the catalytic activities indicated that the random alloy structure enhances the activity.
    Applied Catalysis A-general - APPL CATAL A-GEN. 01/2010; 387(1):195-202.
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    ABSTRACT: Gold/iron-oxide MAgnetic Nanoparticles (GoldMAN) imparts useful magnetic properties to various biomolecules. Gold nanoparticles immobilized on the surface of magnetic nanoparticles allow for the conjugation of biomolecules via an Au-S bond. Here, we present a practical application by utilizing GoldMAN and a magnetic field to induce intracellular transduction. This method has great potential for application of the adenovirus gene delivery vector (Ad), widely used for in vitro/in vivo gene transfer, to Ad-resistant cells. We demonstrated that Ad was easily immobilized on GoldMAN and the Ad/GoldMAN complex was introduced into the cell by the magnetic field, which increased gene expression over 1000 times that of Ad alone. The GoldMAN penetrated the plasma membrane directly, independent of the cell-surface virus receptors and endocytosis pathway. This mechanism will contribute to improve the gene expression efficiency of Ad. This technology is a useful tool for extending Ad tropism and enhancing transduction efficiency. GoldMAN also makes possible the effective use of various biomolecules within the cell because of its interesting cell-entry mechanism.
    Biomaterials 02/2009; 30(9):1809-14. · 8.31 Impact Factor
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    ABSTRACT: Composite gold/iron-oxide nanoparticles were synthesized by a sonochemical process and their surface then modified with polyethylenimine (PEI). The dispersibility of the composite nanoparticles in acidic solution was improved by the PEI modification. The composite nanoparticles specifically adsorbed sulfur-containing amino acids and could be picked up by an external magnetic field. Thus, composite nanoparticles modified with PEI carry both the ability to bond Au–S and to disperse well in buffer solutions.
    Journal of Magnetism and Magnetic Materials 01/2009; 321(10):1404-1407. · 2.00 Impact Factor

Publication Stats

270 Citations
107.22 Total Impact Points

Institutions

  • 1998–2014
    • Osaka University
      • Graduate School of Engineering
      Suika, Ōsaka, Japan
  • 2011
    • Doshisha University
      Kioto, Kyōto, Japan
  • 2008
    • Tohoku University
      • Institute for Materials Research
      Sendai-shi, Miyagi-ken, Japan
  • 2005
    • Osaka Prefectural College Of Technology
      Ōsaka, Ōsaka, Japan
  • 2003
    • Mitsubishi Materials Corporation
      Edo, Tōkyō, Japan