研究者詳細 - 徳永　智春

論文 - 徳永　智春

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Size-Tunable Alumina-Encapsulated Sn-Based Phase Change Materials for Thermal Energy Storage 査読有り

Zhu, SL; Nguyen, MT; Tokunaga, T; Yonezawa, T

ACS APPLIED NANO MATERIALS 2 巻 ( 6 ) 頁： 3752 - 3760 2019年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Applied Nano Materials

The most commonly used phase change materials (PCMs), like organic compounds and inorganic salts, were limited in application by their low thermal conductivity. Herein, for the first time, alumina-encapsulated metallic Sn-based PCMs, named Sn@Al2O3, were successfully fabricated with tunable size (60-2000 nm) by a facile process from low-cost chemicals. The robust fabrication process consists of a surfactant-free solvothermal synthesis of SnO2 spheres, boehmite treatment on SnO2 spheres, calcination in the air, and the final hydrogen reduction to transform SnO2 to metallic Sn, endowing the PCMs with high potential for mass production. The as-obtained Sn@Al2O3 showed a core-shell structure, in which a main metallic Sn core located in the center covered with an Al2O3 shell with small Sn nanoparticles distributed inside. The boehmite treatment, in which the penetration of aluminum species into SnO2 spheres played an important role, was found to be responsible for the unique structure formation of final Sn@Al2O3. The understanding of structure formation mechanism gives the possibilities of a new facile way for the synthesis of metal nanoparticles and particle-distributed nanostructures. The obtained Sn@Al2O3 particles not only have high PCM content (92.37 wt %) but also show a stable thermal behavior and morphology during 100 melt-freeze cycles in the air atmosphere. Furthermore, the low melting temperature of the PCM core, combined with high thermal conductivity of both core material (Sn, 66.8 W m-1 K-1) and shell material (Al2O3, 35 W m-1 K-1), makes Sn@Al2O3 potentially suitable for rapid thermal energy storage in the range 100-300 °C.

DOI： 10.1021/acsanm.9b00649

Topologically immobilized catalysis centre for long-term stable carbon dioxide reforming of methane 査読有り国際誌 Open Access

Shoji, S; Peng, XB; Imai, T; Kumar, PSM; Higuchi, K; Yamamoto, Y; Tokunaga, T; Arai, S; Ueda, S; Hashimoto, A; Tsubaki, N; Miyauchi, M; Fujita, T; Abe, H

CHEMICAL SCIENCE 10 巻 ( 13 ) 頁： 3701 - 3705 2019年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Chemical Science

Methane reforming at low temperatures is of growing importance to mitigate the environmental impact of the production of synthesis gas, but it suffers from short catalyst lifetimes due to the severe deposition of carbon byproducts. Herein, we introduce a new class of topology-tailored catalyst in which tens-of-nanometer-thick fibrous networks of Ni metal and oxygen-deficient Y2O3 are entangled with each other to form a rooted structure, i.e., Ni#Y2O3. We demonstrate that the rooted Ni#Y2O3 catalyst stably promotes the carbon-dioxide reforming of methane at 723 K for over 1000 h, where the performance of traditional supported catalysts such as Ni/Y2O3 diminishes within 100 h due to the precluded mass transport by accumulated carbon byproducts. In situ TEM demonstrates that the supported Ni nanoparticles are readily detached from the support surface in the reaction atmosphere, and migrate around to result in widespread accumulation of the carbon byproducts. The long-term stable methane reforming over the rooted catalyst is ultimately attributed to the topologically immobilized Ni catalysis centre and the synergistic function of the oxygen-deficient Y2O3 matrix, which successfully inhibits the accumulation of byproducts.

DOI： 10.1039/c8sc04965c

Nanoporous Nickel Composite Catalyst for the Dry Reforming of Methane 査読有り国際誌 Open Access

Fujita, T; Peng, XB; Yamaguchi, A; Cho, Y; Zhang, YZ; Higuchi, K; Yamamoto, Y; Tokunaga, T; Arai, S; Miyauchi, M; Abe, H

ACS OMEGA 3 巻 ( 12 ) 頁： 16651 - 16657 2018年12月

記述言語：英語出版者・発行元：ACS Omega

The development of efficient catalysts with high activities and durabilities for use in the dry reforming of methane (DRM) is desirable but challenging. We report the development of a nanoporous nickel composite (nanoporous Ni/Y2O3) via a facile one-step dealloying technique, for use in the DRM. Focusing on the low-temperature DRM, our composite possessed remarkable activity and durability against coking compared with conventional particle-based Ni catalysts. This was attributed to the aluminum oxides present on the Ni surface, which suppress pore coarsening. In addition, the inert bundled Y2O3 nanowires are suitable for use as substrates for nanoporous Ni.

DOI： 10.1021/acsomega.8b02023

Intergranular amorphous films formed by DC electric field in pure zirconia 査読有り

Morisaki, N; Yoshida, H; Kobayashi, T; Tokunaga, T; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 101 巻 ( 8 ) 頁： 3282 - 3287 2018年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

It is difficult to obtain pure ZrO2 sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO2 can be consolidated without shattering using flash-sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High-resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash-sintering at 1350°C for 5 minutes. Monoclinic ZrO2 flash-sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash-sintering.

DOI： 10.1111/jace.15485

Ligand free green plasma-in-liquid synthesis of Au/Ag alloy nanoparticles 査読有り

Cempel, D; Nguyen, MT; Ishida, Y; Tokunaga, T; Yonezawa, T

NEW JOURNAL OF CHEMISTRY 42 巻 ( 8 ) 頁： 5681 - U9 2018年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：New Journal of Chemistry

Au/Ag alloy nanoparticles (NPs) were successfully prepared by a one step synthesis using HAuCl4 and [Ag(NH3)2]+ as the corresponding precursors in a microwave-induced plasma in liquid process (MWPLP) without the addition of any organic protecting or reducing agents. The time dependent reduction and particle formation during the 140 min plasma discharge were studied with TEM, UV-vis extinction spectra, and ICP-OES. After 140 min, the HAADF-STEM and EDX results indicated that the obtained Au/Ag NPs were alloy NPs. Varying the input molar ratios of the metal precursors allowed for control of the composition of the Au/Ag alloy NPs.

DOI： 10.1039/c7nj05154a

Sputter Deposition toward Short Cationic Thiolated Fluorescent Gold Nanoclusters: Investigation of Their Unique Structural and Photophysical Characteristics Using High-Performance Liquid Chromatography 査読有り国際誌

Ishida, Y; Morita, A; Tokunaga, T; Yonezawa, T

LANGMUIR 34 巻 ( 13 ) 頁： 4024 - 4030 2018年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Langmuir

We herein present the preparation of short, bulky cationic thiolate (thiocholine)-protected fluorescent Au nanoclusters via sputter deposition over a liquid polymer matrix. The obtained Au nanoclusters showed near-infrared fluorescence and had an average core diameter of 1.7 ± 0.6 nm, which is too large compared to that of the reported fluorescent Au nanoclusters prepared via chemical means. We revealed the mechanism of formation of this unique material using single-particle electron microscopy, optical measurements, X-ray photoelectron spectroscopy (XPS), and high-performance liquid chromatography fractionations. The noncrystallized image was observed via single-particle high-angle annular dark-field scanning transmission electron microscopy observations and compared with chemically synthesized crystalline Au nanoparticle with the same diameter, which demonstrated the unique structural characteristic speculated via XPS. The size fractionation and size-dependent fluorescence measurement, together with other observations, indicated that the nanoclusters most probably contained a mixture of very small fluorescent species in their aggregated form and were derived from the sputtering process itself and not from the interaction between thiol ligands.

DOI： 10.1021/acs.langmuir.8b00067

Au/Cu Bimetallic Nanoparticles via Double-Target Sputtering onto a Liquid Polymer 査読有り国際誌

Nguyen, MT; Zhang, H; Deng, LL; Tokunaga, T; Yonezawa, T

LANGMUIR 33 巻 ( 43 ) 頁： 12389 - 12397 2017年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Langmuir

Alloy nanoparticles (NPs) of a bimetal system, Au/Cu, that form intermetallic compounds in a bulk state have been successfully produced using a double-target sputtering technique onto a low-cost and biocompatible liquid polymer (polyethylene glycol, PEG). The formation of an Au/Cu solid solution alloy in individual NPs was revealed by scanning transmission electron microscopy-energy-dispersive X-ray elemental mapping analysis. Altering the sputter currents for Au and Cu targets resulted in a tailored NP composition, but the particle sizes did not significantly vary. We found similar structures, sizes, and optical properties of Au/Cu NPs obtained by double-head sputtering on carbon-coated transmission electron microscopy grids or PEG and by Au/Cu alloy target sputtering. Random alloy formation occurred in matrix sputtering using double-target heads. This method is advantageous for manipulating the alloy composition through highly independent control of sputter parameters for each metal target.

DOI： 10.1021/acs.langmuir.7b03194

In-Situ TEM Study of a Nanoporous Ni-Co Catalyst Used for the Dry Reforming of Methane 査読有り Open Access

Fujita, T; Higuchi, K; Yamamoto, Y; Tokunaga, T; Arai, S; Abe, H

METALS 7 巻 ( 10 ) 頁： 406 2017年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Metals

We performed in-situ transmission electron microscopy (TEM) on a dealloyed nanoporous NiCo catalyst used for the dry reforming of methane (DRM) to investigate the origin of the catalytic activity and structural durability. The in-situ observations and local chemical analysis indicated that the DRM induced chemical demixing of Ni and Co accompanied by grain refinement, implying possible “synergic effects” in a general bimetallic NiCo catalyst when used for the DRM.

DOI： 10.3390/met7100406

Electric current-controlled synthesis of BaTiO<sub>3</sub> 査読有り

Nakagawa, Y; Yoshida, H; Uehashi, A; Tokunaga, T; Sasaki, K; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 100 巻 ( 9 ) 頁： 3843 - 3850 2017年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

In BaTiO3, flash-sintering associated with a surge of the specimen electric current sometimes results in an inhomogeneous microstructure including Ti-excess secondary phases because of discharging. We applied field-assisted sintering technique (FAST) under precisely controlled specimen current that was set just below the threshold value for the occurrence of flash event for BaTiO3, to avoid the occurrence of the discharging. As a result, uniform and fine-grained compacts were obtained without any secondary phases. A relative density of approximately 92% was achieved under FAST condition of 100 V/cm with a limiting current of 72 mA and soaking time of 3 hours at 1070°C. The voltages during sintering under a constant current of 72 mA were found to decrease during the soaking process. Electron energy loss spectroscopy revealed the generation of excess oxygen vacancies at/near grain boundaries. The excess oxygen vacancies induced by application of DC electric fields were confirmed to reduce the voltages and to retard the shrinkage rate in a final sintering stage.

DOI： 10.1111/jace.14938

Consolidation of undoped, monoclinic zirconia polycrystals by flash sintering 査読有り

Morisaki, N; Yoshida, H; Tokunaga, T; Sasaki, K; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 100 巻 ( 9 ) 頁： 3851 - 3857 2017年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

Consolidated, monoclinic ZrO2 polycrystal was produced from undoped ZrO2 powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO2 was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal-to-monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO2 body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO2 without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain-boundary second phase in the flash-sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash-sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that <001> directions of the monoclinic ZrO2 grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash-sintered specimen for 5 minutes. The alignment of ZrO2 grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.

DOI： 10.1111/jace.14954

Structural Control Parameters for Formation of Single-Crystalline ss-Sn Nanorods in Organic Phase 査読有り

Lyn Marie Z. De Juan, Mai Thanh Nguyen, Tetsu Yonezawa, Tomoharu Tokunaga, Hiroki Tsukamoto, Yohei Ishida

CRYSTAL GROWTH & DESIGN 17 巻 ( 9 ) 頁： 4554 - 4562 2017年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER CHEMICAL SOC

For the first time, investigation on the effect of lowering the synthesis temperature below room temperature (10 to -10 degrees C) and increasing the viscosity of the reaction solution at certain temperatures has been used to trigger the formation of single-crystalline ss-Sn nanorods (NRs) in a chemical synthesis using an organic solvent. These two parameters govern the energy of the particles in the solution, thereby mediating the selective attachment to the nuclei and the preferential growth of Sn along the c-axis. The length and aspect ratio of the NRs were inversely proportional to the synthesis temperature and directly proportional to the viscosity of the reaction solution. In addition, we found that poly(vinylpyrrolidone) with a high molecular weight aided the formation of longer and higher aspect ratio NRs via the bridge flocculation effect.

DOI： 10.1021/acs.cgd.7b00227

Small Nanosized Oxygen-Deficient Tungsten Oxide Particles: Mechanistic Investigation with Controlled Plasma Generation in Water for Their Preparation 査読有り国際誌 Open Access

Ishida, Y; Motono, S; Doshin, W; Tokunaga, T; Tsukamoto, H; Yonezawa, T

ACS OMEGA 2 巻 ( 8 ) 頁： 5104 - 5110 2017年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ACS Omega

Production of oxygen-deficient tungsten oxide nanoparticles with a diameter of around 10 nm have been successfully developed using a microwave-induced plasma in liquid technique. The prepared blue-green nanoparticles exhibit strong absorption in the visible region; thus, these could be efficient visible-light photocatalysts. The high-angle annular dark-field images revealed the dislocation of tungsten, which causes oxygen deficiencies.

DOI： 10.1021/acsomega.7b00986

Water transport phenomena through membranes consisting of vertically-aligned double-walled carbon nanotube array

Matsumoto, H; Tsuruoka, S; Hayashi, Y; Abe, K; Hata, K; Zhang, SL; Saito, Y; Aiba, M; Tokunaga, T; Iijima, T; Hayashi, T; Inoue, H; Amaratunga, GAJ

CARBON 120 巻頁： 358 - 365 2017年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Carbon

Nanofluidics in CNTs is argumentative though it is theoretically calculated by various reports. It is because only a few of experimental reports are available, and the measured permeability is not so large as that suggested from the theoretical calculations. Also, water motion suppression in the confined space has not been exhibited by flux measurement. The present work explores these yet-unsolved discrepancies using the measurable size membrane of vertically aligned double-walled carbon nanotube array, which is borne with durability and flexibility, and a conventional measurement method is applied to the membranes. Water motion suppression occurs in the CNT confined space significantly, depending on temperature. Additionally, it is confirmed that the obtained permeability correlates to the reported experimental results with regard to the relationship between CNT length and permeability, and the correlation does not agree with permeability calculated from the Hagen-Poiseuille law. These results pose an insight into the inherent water transport characteristics in the CNT confined space.

DOI： 10.1016/j.carbon.2017.05.034

Nanophase-separated Ni<sub>3</sub>Nb as an automobile exhaust catalyst 国際誌 Open Access

Tanabe, T; Imai, T; Tokunaga, T; Arai, S; Yamamoto, Y; Ueda, S; Ramesh, GV; Nagao, S; Hirata, H; Matsumoto, S; Fujita, T; Abe, H

CHEMICAL SCIENCE 8 巻 ( 5 ) 頁： 3374 - 3378 2017年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Chemical Science

Catalytic remediation of automobile exhaust has relied on precious metals (PMs) including platinum (Pt). Herein, we report that an intermetallic phase of Ni and niobium (Nb) (i.e., Ni3Nb) exhibits a significantly higher activity than that of Pt for the remediation of the most toxic gas in exhaust (i.e., nitrogen monoxide (NO)) in the presence of carbon monoxide (CO). When subjected to the exhaust-remediation atmosphere, Ni3Nb spontaneously evolves into a catalytically active nanophase-separated structure consisting of filamentous Ni networks (thickness < 10 nm) that are incorporated in a niobium oxide matrix (i.e., NbOx (x < 5/2)). The exposure of the filamentous Ni promotes NO dissociation, CO oxidation and N2 generation, and the NbOx matrix absorbs excessive nitrogen adatoms to retain the active Ni sites at the metal/oxide interface. Furthermore, the NbOx matrix immobilizes the filamentous Ni at elevated temperatures to produce long-term and stable catalytic performance over hundreds of hours.

DOI： 10.1039/c6sc05473k

Methane decomposition for hydrogen production by catalytic activity of carbon black under low flow rate conditions Open Access

Tokunaga, T; Kishi, N; Yamakawa, K; Sasaki, K; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 125 巻 ( 4 ) 頁： 185 - 189 2017年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Ceramic Society of Japan

Recently, carbon materials were reported to work as catalysts for the production of H3 from hydrocarbons. However, the hydrogen production rate is extremely low. To broaden the scope of carbon materials as catalysts for the decomposition of hydrocarbons, a higher H3 production rate is required. Thus, the hydrogen production rate under conditions employing a low flow rate of hydrocarbon per unit area and a large quantity of carbon materials as the catalyst should be evaluated, and the influence of these conditions on the rate of H3 production by carbon materials should be discussed. Herein, the H3 production rate was measured under conditions employing a low flow rate of CH4 and a large quantity of carbon black (CB) as a catalyst. H3 production started at 600°C and the production rate was less than 0.1%. The transmission electron microscope image of CB heated in CH4 atmosphere demonstrated that a low flow rate of CH4 does not affect the reaction site (i.e., graphite edges) of CB. However, the H3 production rate was extremely low compared to that achieved with a high flow rate. It is proposed that there is an optimal flow rate for maintaining the catalytic activity at the graphite edge of CB for decomposition of CH4.

DOI： 10.2109/jcersj2.16246

Preparation of Au/Pd Bimetallic Nanoparticles by a Microwave-Induced Plasma in Liquid Process

Shirai, H; Nguyen, MT; Cempel, D; Tsukamoto, H; Tokunaga, T; Liao, YC; Yonezawa, T

BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 90 巻 ( 3 ) 頁： 279 - 285 2017年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Bulletin of the Chemical Society of Japan

Au/Pd alloy nanoparticles (NPs) were successfully synthesized using HAuCl4, H2PdCl4, and α-thioglycerol as precursors and a stabilizer, respectively, by microwave-induced plasma in liquid process (MWPLP). After only several minutes of microwave irradiation, metallic NPs were obtained in aqueous environment without any reducing agent. The reduction mechanism of metal ions was confirmed by UV-vis spectral changes. The results from HAADF-STEM and EDS indicated that synthesized NPs were bimetallic Au/Pd. Moreover, XRD patterns of synthesized NPs implied that the composition of bimetallic NPs could be controlled by changing the initial ratio of metal precursors.

DOI： 10.1246/bcsj.20160333

Directly determining the polarity of WC grains in WC-12 wt%Co alloy by ABF-STEM 査読有り

Hayashi, Y; Tokunaga, T; Sasaki, K; Yamamoto, T

INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS 62 巻頁： 9 - 13 2017年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Refractory Metals and Hard Materials

WC grains in WC–Co alloys are shaped as truncated triangular prisms, each with a basal plane of a (0001) truncated triangle and two {11̅00} prismatic planes, one narrower and one wider. This anisotropic shape is related to the polarity of the WC lattice, which comes from the difference in the spacing of the W and C layers in the (0001) projection. We directly determined the relationship between the two {11̅00} prismatic planes and the polarity of the WC lattice by using high-resolution scanning transmission electron microscopy (STEM). High-angle annular dark-field STEM and annular bright-field STEM revealed that the wider prismatic plane had W and C atomic layers with a shorter spacing, while the narrower plane had a longer spacing. The WC/Co interface on the wider prismatic side was atomically flat, while many steps formed on the surface of the narrower prismatic side. This difference in step formation at these interfaces could have come from the difference in how their W[sbnd]C bonds formed.

DOI： 10.1016/j.ijrmhm.2016.10.013

Water transport phenomena through membranes consisting of vertically-aligned double-walled carbon nanotube array

Hidetoshi Matsumoto, Shuji Tsuruoka, Yasuhiko Hayashi, Koji Abe, Kenjiro Hata, Shaoling Zhang, Yoshitaka Saito, Motohiro Aiba, Tomoharu Tokunaga, Toru Iijima, Takuma Hayashi

Carbon 120 巻頁： 358–365 2017年

Nanophase-separated Ni3Nb as Automobile Exhaust Catalysts 査読有り

Toyokazu Tanabe, Tsubasa Imai, Tomoharu Tokunaga, Shigeo Arai, Yuta Yamamoto, Shigenori Ueda, Gubbala V. Ramesh, Satoshi Nagao, Hirohito Hirata, Shin-ichi, Matsumoto, Takeshi Fujita, Hideki Abe

Chemical Science 8 巻頁： 3374-3378 2017年

Preparation of Au/Pd bimetallic nanoparticles by a microwave-induced plasma in liquid process 査読有り

Hiroaki Shirai, Mai Thanh Nguyen, David Čempel, Hiroki Tsukamoto, Tomoharu Tokunaga, Ying-Chih Liao, and Tetsu Yonezawa

Bulletin of the Chemical Society of Japan 90 巻 ( 3 ) 頁： 279-285 2017年

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