研究者詳細 - 徳永　智春

論文 - 徳永　智春

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Excess oxygen-vacancy formed by FAST regime of direct-current electric field during flash sintering for 3 mol%-10 mol% Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> 査読有り Open Access

Morisaki, N; Tokunaga, T; Kobayashi, K; Kodaira, A; Yamamoto, T

CERAMICS INTERNATIONAL 48 巻 ( 9 ) 頁： 12091 - 12097 2022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：Ceramics International

The amount of excess oxygen vacancies that are generated during FAST regime of direct-current flash sintering was estimated semi-quantitatively for 3 mol%–10 mol% Y2O3-doped ZrO2 (3–10YSZ) using the temperature dependence of the electrical conductivity. For the estimation, the electrical conductivity in the temperature range, where shrinkages are below a few percent, was used to avoid the effect of large microstructural changes on the electric conductivity. For all Y2O3 contents except 3YSZ, the amount of excess oxygen vacancies increased above a compact temperature of approximately 840 °C, which depended on Y2O3 content, and was highest in 8YSZ. Approximately 0.13% excess oxygen vacancy was generated by direct-current flashing in 8YSZ in the present electric field condition. In contrast, the increase in the excess oxygen vacancies is negligible in 3YSZ. The dependence of the amount of excess oxygen vacancies on the Y2O3 content was related possibly to the dependence of the electrode overvoltage on the Y2O3 content.

DOI： 10.1016/j.ceramint.2022.01.069

Anelasticityinduced by AC flash processing of cubic zirconia 査読有り Open Access

Masuda, H; Morita, K; Tokunaga, T; Yamamoto, T; Yoshida, H

ACTA MATERIALIA 227 巻頁： 117704 - 117704 2022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：Acta Materialia

The defect physics is important for understanding the athermal effect of electric fields and/or currents on the enhanced ionic diffusion during flash sintering of ceramics. In this study, conventionally sintered 8 mol% yttria-stabilized zirconia with a fluorite cubic structure was processed by AC flash and subsequently quenched to partially freeze the flash-induced defect structures. The dynamic and quasi-static mechanical properties of flashed and non-flashed samples were evaluated by a sound velocity measurement and a nanoindentation technique to characterize the mechanical responses attributed to the flash-induced defects. A negligible change was confirmed in the dynamic elastic constants even after flash, while the quasi-static elastic properties of the flashed sample exhibited a considerable rate dependence (i.e., the slower the loading rate, the lower the measured contact modulus). The contact modulus of the flashed sample tested at the slowest loading rate in this study was reduced to approximately 72% of the non-flashed one, where the contact modulus was rate-independent. The time-dependent mechanical behavior was also confirmed during nanoindentation creep. Such rate- and time-dependent mechanical properties are characterized as anelasticity (i.e., viscoelasticity or pseudoelasticity in other terms), which can be attributed to stress-induced and thermally activated recoverable motions of point defects. The flash process of cubic zirconia possibly induced point defects associated with short-range structural change, which could be the origin of anelasticity.

DOI： 10.1016/j.actamat.2022.117704

Anisotropic Growth of Copper Nanorods Mediated by Cl<SUP>-</SUP> Ions 査読有り国際共著 Open Access

Saw, MJ; Nguyen, MT; Kunisada, Y; Tokunaga, T; Yonezawa, T

ACS OMEGA 7 巻 ( 8 ) 頁： 7414 - 7420 2022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：ACS Omega

Anisotropic growth to form Cu particles of rod and wire shapes has been obtained typically in a complex system that involves both organic capping agents and Cl- ions. However, the sole effect of Cl- ions on the formation of Cu wires has yet to be fully understood, especially in an organic system. This present work determines the effect of Cl- ions on the morphologies of Cu particles in an organic phase without any capping agents. The results revealed that anisotropic Cu rods could be grown with the sole presence of Cl- ions. The rods have the (011) facets as the long axis, the (111) facets as the tip, and the (100) facets as the side surface. By increasing the Cl- ion concentration, more Cu atoms contributed to the formation of Cu rods and the kinetic growth of the length and the diameter of the rods varied. This suggests that Cl- ions have preferential adsorption on the (100) Cu surfaces to promote the anisotropic growth of Cu. Meanwhile, the adsorption of Cl- to the (111) and (100) surfaces at high Cl- concentrations regulates the relative growth of the particle length and diameter.

DOI： 10.1021/acsomega.2c00359

Direct determination of a topmost atomic layer of (100) surfaces on (La<sub>0.3</sub>Sr<sub>0.7</sub>)(Al<sub>0.65</sub>Ta<sub>0.35</sub>)O<sub>3</sub> single crystals annealed in air by high-resolution scanning transmission electron microscopy 査読有り

Tokuda, Y; Irimoto, T; Nishikawa, N; Katsuyama, Y; Kobayashi, S; Tokunaga, T; Yamamoto, T

SURFACE AND INTERFACE ANALYSIS 54 巻 ( 3 ) 頁： 212 - 217 2022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：Surface and Interface Analysis

Herein, we investigated the surface of an (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) single crystal via scanning transmission electron microscopy (STEM) after annealing in air. We found amorphous mounds and atomically flat surfaces formed on the surface of the (100) LSAT single crystal subsequent to heat treatment. These surface mounds mainly constituted SrO, including ~50 at.% Al. The atomically flat surface showed a B-site termination determined by the direct surface observation at the atomic scale through HAADF-STEM. The atomic arrangements on the surface were accommodated by surface-mound formation and La evaporation. Consequently, the ordered structure disappeared because of the increase in the Ta content at the terminated B-site layer.

DOI： 10.1002/sia.7043

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/sia.7043

Alginate-Stabilized Gold Nanoparticles Prepared Using the Microwave-Induced Plasma-in-Liquid Process with Long-Term Storage Stability for Potential Biomedical Applications 査読有り Open Access

Liu, HR; Ikeda, K; Nguyen, MT; Sato, S; Matsuda, N; Tsukamoto, H; Tokunaga, T; Yonezawa, T

ACS OMEGA 7 巻 ( 7 ) 頁： 6238 - 6247 2022年2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

A one-step preparation of alginate-stabilized gold nanoparticles (Au NPs) using the microwave-induced plasma-in-liquid process (MWPLP) was reported. Effects of alginate with various concentrations on the preparation and properties of the synthesized Au NPs, including reaction rate, morphology, size, and optical absorption property, were studied. The introduction of alginate (1) accelerated the reaction rate, (2) prevented aggregation and precipitation due to long time discharge in MWPLP, and (3) provided long-term colloidal stability. An abnormal size change (from large to small) of Au NPs during particle growth, which was opposite to the typical change in bottom-up chemical reduction, was observed and a possible mechanism was proposed based on the dynamical and thermodynamical instability of particles during growth. The strategy of drying and redispersion of Au NPs in alginate solution was also studied. The drying and redispersion process had an imperceptible effect on the Au NPs. As a consequence, this strategy might be an effective technique for the long-term storage of Au NPs and other metal NPs. The alginate-stabilized Au NPs without the addition of toxic reducing or stabilizing agents can be appropriate to biomedical applications.

DOI： 10.1021/acsomega.1c06769

Nanoporous ultra-high-entropy alloys containing fourteen elements for water splitting electrocatalysis Open Access

Cai, ZX; Goou, H; Ito, Y; Tokunaga, T; Miyauchi, M; Abe, H; Fujita, T

CHEMICAL SCIENCE 12 巻 ( 34 ) 頁： 11306 - 11315 2021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：Chemical Science

High-entropy alloys (HEAs) are near-equimolar alloys comprising five or more elements. In recent years, catalysis using HEAs has attracted considerable attention across various fields. Herein, we demonstrate the facile synthesis of nanoporous ultra-high-entropy alloys (np-UHEAs) with hierarchical porosityviadealloying. These np-UHEAs contain up to 14 elements, namely, Al, Ag, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Rh, Ru, and Ti. Furthermore, they exhibit high catalytic activities and electrochemical stabilities in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic media, superior to that of commercial Pt/graphene and IrO2catalysts. Our results offer valuable insights for the selection of elements as catalysts for various applications.

DOI： 10.1039/d1sc01981c

Flash sintering for BaTiO<sub>3</sub> with square alternating current field including zero-field duration 招待有り査読有り Open Access

Umemura, R; Tokunaga, T; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 128 巻 ( 12 ) 頁： 1018 - 1023 2020年12月

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

The shrinkage behavior of barium titanate (BaTiO3) green compacts during flash sintering with a square alternating current (AC) field including a zero-field duration (AC-pulse wave) was investigated. It was found that an AC-pulse wave is effective for the densification of BaTiO3 green compacts without reduction and discharge, which is confirmed by transmission electron microscopy and electron energy loss spectroscopy. BaTiO3 sintered polycrystal was obtained at a furnace temperature of 1000 °C by the optimal AC-pulse conditions of 285 V/cm at 250 Hz and a duty ratio of 30 %. A waveform with alternating polarity and the inclusion of a zero-field duration is shown to be possibly useful for flash sintering of oxide materials including altervalent cations.

DOI： 10.2109/jcersj2.20181

H<sub>2</sub> production from methane decomposition by fullerene at low temperature 査読有り

Tokunaga, T; Kuno, K; Kawakami, T; Yamamoto, T; Yoshigoe, A

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 45 巻 ( 28 ) 頁： 14347 - 14353 2020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：International Journal of Hydrogen Energy

Carbon materials have previously been reported to work as catalysts for hydrogen (H2) production from hydrocarbons. Mechanisms of the catalytic behavior of graphite and carbon black (CB) have often been discussed in literature. Graphite and CB is constructed from mainly 6-membered rings with sp2 bonds. To understand the catalytic behavior of carbon materials for H2 production by methane (CH4) decomposition, the catalytic behavior of fullerenes with 6-membered rings and also those comprising 5- and 7-membered rings with sp2 bonds and their associated mechanisms should be investigated. In this study, the fullerene catalyst activity has been investigated using gas chromatography and the electronic states and nanoscale structures have been analyzed. H2 production started at 400 °C and the H2 production rate gradually increased with time, and the activation energy of the fullerene for H2 production by CH4 decomposition was found to be 166 kJ/mol. Moreover, in situ heating X-ray photon spectroscopy (XPS) measurements showed that the π-π∗ transition signal becomes stronger with increasing temperature above the threshold of 300 °C. The transition of the π electrons to π∗ orbitals upon heating is expected to decompose CH4 absorbed on fullerene. Moreover, transmission electron microscopy (TEM) analysis revealed that the generated carbon atoms from the CH4 decomposition were deposited onto the surfaces of the fullerenes, forming amorphous and layered concentric sphere carbon. Amorphous carbon is reported to not work as a catalyst for CH4 decomposition at around 400 °C. From XPS analysis and TEM observations of these two structures, it is anticipated that the ring structures without 6-membered rings in carbon materials with sp2 bonding contribute to this catalytic behavior for CH4 decomposition at a low temperature of 400 °C.

DOI： 10.1016/j.ijhydene.2020.03.178

Blue photo luminescence from 3 mol%Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> polycrystals sintered by flash sintering under an alternating current electric field 査読有り Open Access

Yamashita, Y; Kurachi, T; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 40 巻 ( 5 ) 頁： 2072 - 2076 2020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：Journal of the European Ceramic Society

We investigated the photoluminescence (PL) properties of 3 mol%Y2O3-ZrO2 (3YSZ) polycrystals. The 3YSZ polycrystals were prepared by an isothermal flash sintering technique in which an alternating electric field was applied at a predetermined furnace temperature. After flash sintering, clear, visibly blue PL at 2.82 eV (440 nm) was confirmed under an ultraviolet light (3.4 eV (365 nm)); however, no visible PL was observed from 3YSZ polycrystals conventionally sintered using the same raw powders. Thus, the emission of blue PL was induced by the electric field applied during sintering. The obtained PL band was related to excitation at 3.87 eV (320 nm), which is possibly assigned to the singly ionized associated oxygen vacancy defects (AOD + centers). Furthermore, the blue emission was stable after high temperature annealing at 1400 °C for 1 h in air. We possibly attribute the emission of this PL to the stabilization of AOD + centers by the electric field loading during flash sintering.

DOI： 10.1016/j.jeurceramsoc.2019.12.060

Suppression of nitridation of yttria-doped zirconia during flash sintering 査読有り国際共著

Kurachi, T; Yamashita, Y; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 103 巻 ( 5 ) 頁： 3002 - 3007 2020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：Journal of the American Ceramic Society

The effect of direct current (DC) and alternating current (AC) on nitridation of 3 mol% Y2O3-doped ZrO2 (3YSZ) after keeping in a flash state for 1 hour was investigated. The inside of the DC-flashed compact was confirmed to exhibit blacking. Scanning transmission electron microscopy, electron energy loss spectroscopy, and X-ray diffraction analysis revealed that zirconium nitrides formed in the blackened area. In contrast, a uniformly densified compact without blackening was obtained by AC fields. No zirconium nitrides formed in the compacts exposed to AC fields even when the flash state was maintained for 1 hour. Therefore, AC fields are effective to suppress nitridation of 3YSZ during flash sintering.

DOI： 10.1111/jace.16990

<i>In situ</i> TEM observation of liquid-state Sn nanoparticles vanishing in a SiO<sub>2</sub> structure: a potential synthetic tool for controllable morphology evolution from core-shell to yolk-shell and hollow structures 査読有り国際共著 Open Access

Zhu, S; Nguyen, MT; Tokunaga, T; Wen, CY; Yonezawa, T

NANOSCALE ADVANCES 2 巻 ( 4 ) 頁： 1456 - 1464 2020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：Nanoscale Advances

Precise design of hollow nanostructures can be realized via various approaches developed in the last two decades, endowing nanomaterials with unique structures and outstanding performances, showing their usefulness in a broad range of fields. Herein, we demonstrate the formation of SnO2@SiO2 hollow nanostructures, for the first time, by interaction between liquid state Sn cores and SiO2 shell structures inside Sn@SiO2 core-shell nanoparticles with real-time observation via in situ transmission electron microscopy (TEM). Based on the in situ results, designed transformation of the nanoparticle structure from core-shell Sn@SiO2 to yolk-shell Sn@SiO2 and hollow SnO2@SiO2 is demonstrated, showing the controllable structure of core-shell Sn@SiO2 nanoparticles via fixing liquid-state Sn inside a SiO2 shell which has a certain Sn containing capacity. The present approach expands the toolbox for the design and preparation of yolk-shell and hollow nanostructures, thus providing us with a new strategy for fabrication of more complicated nanostructures.

DOI： 10.1039/c9na00782b

Highly Correlated Size and Composition of Pt/Au Alloy Nanoparticles via Magnetron Sputtering onto Liquid 査読有り国際誌

Deng, LL; Nguyen, MT; Shi, JM; Chau, YTR; Tokunaga, T; Kudo, M; Matsumura, S; Hashimoto, N; Yonezawa, T

LANGMUIR 36 巻 ( 12 ) 頁： 3004 - 3015 2020年3月

記述言語：英語出版者・発行元：Langmuir

Pt/Au alloy nanoparticles (NPs) in a wide composition range have been synthesized by room-temperature simultaneous sputter deposition from two independent magnetron sources onto liquid PEG (MW = 600). The prepared NPs were alloyed with the face-centered cubic (fcc) structure. In addition, the particle sizes, composition, and shape are strongly correlated but can be tailored by an appropriate variation of the sputtering parameters. No individual particle but large agglomerates with partial alloy structure formed at Pt content of less than 16 atom %. Highly dispersed NPs with no agglomeration were observed in PEG when the quantity of Pt is more than 26 atom %. On the other hand, a small amount of Pt could terminate the agglomeration of Au when sputtering on the grids for transmission electron microscope observation. Our experiment and computer simulation carried out by two different methods indicate that the composition-dependent particle size of Pt/Au can be explained by the atomic concentration, formation energy of the cluster, and interaction between different metal atoms and the PEG molecule.

DOI： 10.1021/acs.langmuir.0c00152

Synthesis of composition-tunable Pd-Cu alloy nanoparticles by double target sputtering

Chau, YTR; Nguyen, MT; Zhu, MB; Romier, A; Tokunaga, T; Yonezawa, T

NEW JOURNAL OF CHEMISTRY 44 巻 ( 12 ) 頁： 4704 - 4712 2020年3月

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

In this work, we introduce a green synthesis technique, double-target sputtering into a liquid polymer-polyethylene glycol (PEG, M. W. = 600), to synthesize palladium-copper (Pd-Cu) alloy nanoparticles (NPs) dispersed in PEG. The composition of Pd and Cu in the alloy NPs can be controlled and varied by applying different sputtering currents for Pd and Cu targets. The particle sizes of the Pd-Cu NPs are maintained in the range of 2.4-2.5 nm for various Pd-Cu compositions. Oxidation issues and crystallinity enhancement are discussed.

DOI： 10.1039/d0nj00288g

Blue photoluminescence at room temperature from Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> polycrystals sintered by flash sintering

Yamashita, Y; Itoh, A; Tokunaga, T; Yoshida, H; Yamamoto, T

APPLIED PHYSICS EXPRESS 13 巻 ( 3 ) 2020年3月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Applied Physics Express

Photoluminescence band variation of Y2O3-doped ZrO2 (YSZ) polycrystals was investigated for flash-sintered YSZ polycrystals. Nearly full density could be obtained by flash sintering under an alternating current field of 100 V cm-1, 100 Hz, limiting current of 1000 mA and at 1200 °C for 5 min. The flash-sintered YSZ polycrystals emitted blue photoluminescence even at room temperature under excitation of a handheld ultraviolet light, which could be observed with the naked eye. By contrast, conventionally sintered YSZ polycrystals emitted no photoluminescence despite using the same raw powders. In addition, the photoluminescence bands were tunable by varying the Y2O3 content.

DOI： 10.35848/1882-0786/ab7710

Green Synthesis of Size-Tunable Iron Oxides and Iron Nanoparticles in a Salt Matrix 査読有り

Nguyen, MT; Yu, K; Tokunaga, T; Boonyaperm, K; Kheawhom, S; Arita, M; Yonezawa, T

ACS SUSTAINABLE CHEMISTRY & ENGINEERING 7 巻 ( 21 ) 頁： 17697 - 17705 2019年11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 Sustainable Chemistry and Engineering

This research is devoted for the synthesis of Fe oxide and Fe nanoparticles (NPs) via, respectively, thermal decomposition of iron(III) oleate and H2 reduction of either iron(III) oleate or iron oxide NPs in a NaCl matrix. Lowering the weight ratios of metal precursor to NaCl and synthesis temperature results in smaller particle sizes. A higher uniformity of Fe NPs can be achieved with a two-step synthesis, that is, thermal decomposition of iron oleate followed by H2 reduction of iron oxides, with the milling of iron oxides and NaCl prior to the reduction step. Fe NPs obtained in this study are stable in the air for long-time storage. It is found that thin oxide and C layers on the particle surface are responsible for their stability.

DOI： 10.1021/acssuschemeng.9b03950

Technique to control specimen electric current during a flash state with alternating current electric fields 査読有り Open Access

Yamashita, Y; Kurachi, T; Tokunaga, T; Yoshida, H; Yamamoto, T

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 127 巻 ( 11 ) 頁： 849 - 851 2019年11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

A simple technique to control the specimen electric current during flash sintering was developed using AC electric fields in 3 mol% Y2O3-doped ZrO2. Flash sintering is characterized by the occurrence of a flash event, i.e., a spike in the specimen electric current, which makes it difficult to control the specimen electric current at/ after the flash event for AC electric fields. However, AC electric fields are very attractive from the viewpoint of a suppression of the severe reduction arising from the unidirectional ionic flow during the flash event, which is a fatal phenomenon with DC electric fields. In this study, we develop a simple technique to control the AC electric fields at/after the flash event by limiting the AC electric fields under certain values. As a result, the flash state after the flash event is successfully maintained. Further, it is found that 3 mol% Y2O3-doped ZrO2 compacts prepared by DC- and AC-flash sintering emit blue fluorescence (FL) under 254 nm ultraviolet light. The blue FL is not obtained in the compacts prepared by conventionally-sintering. The application of electric fields is confirmed to result in the blue florescent properties of 3 mol% Y2O3-doped ZrO2 compacts.

DOI： 10.2109/jcersj2.19144

One-Minute Joule Annealing Enhances the Thermoelectric Properties of Carbon Nanotube Yarns via the Formation of Graphene at the Interface 査読有り

Hada, M; Hasegawa, T; Inoue, H; Takagi, M; Omoto, K; Chujo, D; Iemoto, S; Kuroda, T; Morimoto, T; Hayashi, T; Iijima, T; Tokunaga, T; Ikeda, N; Fujimori, K; Itoh, C; Nishikawa, T; Yamashita, Y; Kiwa, T; Koshihara, S; Maeda, S; Hayashi, Y

ACS APPLIED ENERGY MATERIALS 2 巻 ( 10 ) 頁： 7700 - 7708 2019年10月

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

Interfaces in nanocarbon materials are highly important, as they determine the properties of carbon-based devices. In terms of carrier and thermal transport properties, the interfacial features are often more important than the intrinsic characteristics. Herein, we describe how 1 min Joule annealing of carbon nanotube (CNT) yarns can convert the interfacial amorphous carbon into graphene fragments. After 1 min Joule annealing, we have obtained multiwalled CNT yarns with extremely high Seebeck coefficients (±100 μV/K) and high thermoelectric power factor (400 and 1000 μW/mK2) at room temperature, both with or without polyethylenimine doping. Theoretical simulations and experimental measurements helped to determine the optimal annealing conditions in terms of a rapid transformation of the interfacial amorphous carbon between the bundled CNTs in the yarn into graphene fragments at â¼2000 K. The present approach represents significant progress in energy materials science, as it provides a guiding principle for the design of interfaces in nanocarbon materials with potential applications in energy-harvesting systems.

DOI： 10.1021/acsaem.9b01736

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode <i>versus</i> the Thermal Mode 査読有り国際誌

Hada, M; Miyata, K; Ohmura, S; Arashida, Y; Ichiyanagi, K; Katayama, I; Suzuki, T; Chen, W; Mizote, S; Sawa, T; Yokoya, T; Seki, T; Matsuo, J; Tokunaga, T; Itoh, C; Tsuruta, K; Fukaya, R; Nozawa, S; Adachi, S; Takeda, J; Onda, K; Koshihara, S; Hayashi, Y; Nishina, Y

ACS NANO 13 巻 ( 9 ) 頁： 10103 - 10112 2019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：ACS Nano

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

DOI： 10.1021/acsnano.9b03060

Porous ZnV<sub>2</sub>O<sub>4</sub> Nanowire for Stable and High-Rate Lithium-Ion Battery Anodes 査読有り

De Juan-Corpuz, LMZ; Nguyen, MT; Corpuz, RD; Yonezawa, T; Rosero-Navarro, NC; Tadanaga, K; Tokunaga, T; Kheawhom, S

ACS APPLIED NANO MATERIALS 2 巻 ( 7 ) 頁： 4247 - 4256 2019年7月

記述言語：日本語出版者・発行元：American Chemical Society ({ACS})

Porous ZnV2O4 nanowires (NWs) were successfully prepared by hydrothermal reaction followed by calcination. Despite the porous structure, these porous ZnV2O4 NWs are single crystal with {220} facets and a wire direction along the c-axis. On the basis of an electrochemical test, these porous ZnV2O4 NWs have better cycling stability and higher specific capacity (i.e., 460 mA h g-1 after 100 cycles and 149 mA h g-1 after 1000 cycles using 1 and 5 A g-1 current densities, respectively) compared to other morphologies (i.e., spherical and coral-like morphologies). As a ternary transition metal oxide, the produced porous ZnV2O4 NWs undergo phase transformation without compromising the resulting capacity. On the other hand, the CV curves at different scan rates indicate a pseudocapacitive electrochemical behavior of the porous ZnV2O4.

DOI： 10.1021/acsanm.9b00703

Preparation and Growth Mechanism of Pt/Cu Alloy Nanoparticles by Sputter Deposition onto a Liquid Polymer 査読有り国際誌

Deng, LL; Nguyen, MT; Mei, S; Tokunaga, T; Kudo, M; Matsumura, S; Yonezawa, T

LANGMUIR 35 巻 ( 25 ) 頁： 8418 - 8427 2019年6月

記述言語：英語出版者・発行元：Langmuir

We use a green sputtering technique to deposit a Pt/Cu alloy target on liquid polyethylene glycol (PEG) to obtain well-dispersed and stable Pt29Cu71 alloy nanoparticles (NPs). The effects of sputtering current, rotation speed of the stirrer, sputtering time, sputtering period, and temperature of PEG on the particle size are studied systematically. Our key results demonstrate that the aggregation and growth of Pt/Cu alloy NPs occurred at the surface as well as inside the liquid polymer after the particles landed on the liquid surface. According to particle size analysis, a low sputtering current, high rotation speed for the stirrer, short sputtering period, and short sputtering time are found to be favorable for producing small-sized single crystalline alloy NPs. On the other hand, varying the temperature of the liquid PEG does not have any significant impact on the particle size. Thus, our findings shed light on controlling NP growth using the newly developed green sputtering deposition technique.

DOI： 10.1021/acs.langmuir.9b01112

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