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Journal Articles

Effect of water for the oxygen adsorption on surface of PtCo catalysts

Cui, Y.-T.*; Harada, Yoshihisa*; Niwa, Hideharu*; Oshima, Masaharu*; Hatanaka, Tatsuya*; Nakamura, Naoki*; Ando, Masaki*; Yoshida, Toshihiko*; Ishii, Kenji*; Matsumura, Daiju

NanotechJapan Bulletin (Internet), 11(4), 6 Pages, 2018/08

no abstracts in English

Journal Articles

Wetting induced oxidation of Pt-based nano catalysts revealed by ${{it in situ}}$ high energy resolution X-ray absorption spectroscopy

Cui, Y.-T.*; Harada, Yoshihisa*; Niwa, Hideharu*; Hatanaka, Tatsuya*; Nakamura, Naoki*; Ando, Masaki*; Yoshida, Toshihiko*; Ishii, Kenji*; Matsumura, Daiju; Oji, Hiroshi*; et al.

Scientific Reports (Internet), 7(1), p.1482_1 - 1482_8, 2017/05

 Times Cited Count:15 Percentile:48.4(Multidisciplinary Sciences)

Journal Articles

Electronic structure of Pt and Pt-Co nanoparticles with O$$_{2}$$ and O$$_{2}$$/H$$_{2}$$O adsorption revealed by in situ XAFS and hard X-ray photoelectron spectroscopy

Cui, Y.*; Harada, Yoshihisa*; Hatanaka, Tatsuya*; Nakamura, Naoki*; Ando, Masaki*; Yoshida, Toshihiko*; Ikenaga, Eiji*; Ishii, Kenji*; Matsumura, Daiju; Li, R.*; et al.

ECS Transactions, 72(8), p.131 - 136, 2016/10

 Times Cited Count:1 Percentile:51.89

Journal Articles

Thickness-dependent magnetic properties and strain-induced orbital magnetic moment in SrRuO$$_{3}$$ thin films

Ishigami, Keisuke*; Yoshimatsu, Kohei*; Toyota, Daisuke*; Takizawa, Masaru*; Yoshida, Teppei*; Shibata, Goro*; Harano, Takayuki*; Takahashi, Yukio*; Kadono, Toshiharu*; Verma, V. K.*; et al.

Physical Review B, 92(6), p.064402_1 - 064402_5, 2015/08

 Times Cited Count:36 Percentile:82.83(Materials Science, Multidisciplinary)

Journal Articles

Spin and orbital magnetic moments of Fe in the $$n$$-type ferromagnetic semiconductor (In,Fe)As

Kobayashi, Masaki*; Anh, L. D.*; Hai, P. N.*; Takeda, Yukiharu; Sakamoto, Shoya*; Kadono, Toshiharu*; Okane, Tetsuo; Saito, Yuji; Yamagami, Hiroshi; Harada, Yoshihisa*; et al.

Applied Physics Letters, 105(3), p.032403_1 - 032403_4, 2014/07


 Times Cited Count:3 Percentile:14.76(Physics, Applied)

Journal Articles

Thickness-dependent ferromagnetic metal to paramagnetic insulator transition in La$$_{0.6}$$Sr$$_{0.4}$$MnO$$_3$$ thin films studied by X-ray magnetic circular dichroism

Shibata, Goro*; Yoshimatsu, Kohei*; Sakai, Enju*; Singh, V. R.*; Verma, V.*; Ishigami, Keisuke*; Harano, Takayuki*; Kadono, Toshiharu*; Takeda, Yukiharu; Okane, Tetsuo; et al.

Physical Review B, 89(23), p.235123_1 - 235123_5, 2014/06


 Times Cited Count:15 Percentile:57.95(Materials Science, Multidisciplinary)

Journal Articles

Unveiling the impurity band induced ferromagnetism in the magnetic semiconductor (Ga,Mn)As

Kobayashi, Masaki*; Muneta, Iriya*; Takeda, Yukiharu; Harada, Yoshihisa*; Fujimori, Atsushi*; Krempask$'y$, J.*; Schmitt, T.*; Oya, Shinobu*; Tanaka, Masaaki*; Oshima, Masaharu*; et al.

Physical Review B, 89(20), p.205204_1 - 205204_8, 2014/05

 Times Cited Count:70 Percentile:92.82(Materials Science, Multidisciplinary)

Journal Articles

Electronic structures of N-doped graphene with native point defects

Hou, Z.*; Wang, X.*; Ikeda, Takashi; Terakura, Kiyoyuki*; Oshima, Masaharu*; Kakimoto, Masaaki*

Physical Review B, 87(16), p.165401_1 - 165401_16, 2013/04

 Times Cited Count:97 Percentile:95.46(Materials Science, Multidisciplinary)

We have performed the DFT calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. Compared with the case of an isolated N dopant in perfect graphene, the electrons donated by substitutional N dopants would be localized significantly when a N-N pair is formed. The N-N interaction and the interference between native point defect and N dopant strongly modify the role of N doping regarding the free carrier production in the bulk $$pi$$ bands. Our results are qualitatively consistent with the experimental observation that the concentration of free electrons introduced by N dopants would be lower than that of doped N.

Journal Articles

Probing carbon edge exposure of iron phthalocyanine-based oxygen reduction catalysts by soft X-ray absorption spectroscopy

Niwa, Hideharu*; Saito, Makoto*; Kobayashi, Masaki*; Harada, Yoshihisa*; Oshima, Masaharu*; Moriya, Shogo*; Matsubayashi, Katsuyuki*; Nabae, Yuta*; Kuroki, Shigeki*; Ikeda, Takashi; et al.

Journal of Power Sources, 223, p.30 - 35, 2013/02

 Times Cited Count:18 Percentile:54.18(Chemistry, Physical)

To design non-platinum, inexpensive, but high performance carbon-based cathode catalysts for polymer electrolyte fuel cells, it is important to elucidate the active site for oxygen reduction reaction (ORR). However, it is difficult to directly identify the active site by applying conventional structural or electronic probes to such complex systems. Here, we used C 1${it s}$ X-ray absorption spectroscopy (XAS) to observe electronic structure of carbon in iron phthalocyanine-based catalysts, and found a signature of edge exposure below the $$pi^{ast}$$ edge, whose intensity is well correlated with the ORR activity. These results demonstrate that C 1${it s}$ XAS can be used to characterize the ORR activity of carbon-based cathode catalysts in terms of the edge exposure.

Journal Articles

Theoretical characterization of X-ray absorption, emission, and photoelectron spectra of nitrogen doped along graphene edges

Wang, X.*; Hou, Z.*; Ikeda, Takashi; Oshima, Masaharu*; Kakimoto, Masaaki*; Terakura, Kiyoyuki*

Journal of Physical Chemistry A, 117(3), p.579 - 589, 2013/01

 Times Cited Count:33 Percentile:80.08(Chemistry, Physical)

${it K}$-edge X-ray absorption (XAS), emission (XES), and photoelectron (XPS) spectra of nitrogen doped along graphene edges are systematically investigated by using first-principles methods. In this study we considered pyridinium-like, pyridine-like, cyanide-like, and aminelike nitrogens at armchair and zigzag edges and pyrrole-like nitrogen at armchair edge as well as graphite-like nitrogen at graphene interior site. Our results indicate that nitrogen configuration and its location (armchair or zigzag edge) in nitrogen-doped graphene can be identified via the spectral analysis. Furthermore, some controversial spectral features observed in experiment for N-doped graphene-like materials are unambiguously assigned. The present analysis gives an explanation to the reason why the peak assignment is usually made differently between XPS and XAS.

Journal Articles

Study on the oxygen adsorption property of nitrogen-containing metal-free carbon-based cathode catalysts for oxygen reduction reaction

Kiuchi, Hisao*; Niwa, Hideharu*; Kobayashi, Masaki*; Harada, Yoshihisa*; Oshima, Masaharu*; Chokai, Masayuki*; Nabae, Yuta*; Kuroki, Shigeki*; Kakimoto, Masaaki*; Ikeda, Takashi; et al.

Electrochimica Acta, 82(1), p.291 - 295, 2012/10

 Times Cited Count:12 Percentile:32.36(Electrochemistry)

We study the characteristics of oxygen adsorption on metal-free carbon-based cathode catalysts derived from nitrogen-containing polyamide (PA) and nitrogen-free phenolic resin (PhRs). Electrochemical analysis and Raman spectroscopy showed higher 2-electron oxygen reduction reaction (ORR) activity and more defect sites in PA than PhRs. The increase in the amount of adsorbed oxygen in PA was also identified by oxygen adsorption isotherms. ${it In-situ}$ X-ray photoelectron spectroscopy reveals that graphite-like nitrogen contributes to oxygen adsorption and C=O components are dominant in PA. These experimental results indicate that the adsorbed C=O components near the graphite-like nitrogen can be assigned as active sites for 2-electron ORR.

Journal Articles

Indirect contribution of transition metal towards oxygen reduction reaction activity in iron phthalocyanine-based carbon catalysts for polymer electrolyte fuel cells

Kobayashi, Masaki*; Niwa, Hideharu*; Saito, Makoto*; Harada, Yoshihisa*; Oshima, Masaharu*; Ofuchi, Hironori*; Terakura, Kiyoyuki*; Ikeda, Takashi; Koshigoe, Yuka*; Ozaki, Junichi*; et al.

Electrochimica Acta, 74, p.254 - 259, 2012/07

 Times Cited Count:52 Percentile:83.6(Electrochemistry)

The electronic structure of the residual metal atoms in FePc-based carbon catalysts, prepared by pyrolyzing a mixture of FePc and phenolic resin polymer at 800$$^{circ}$$C, before and after acid washing have been investigated using XAFS spectroscopy to clarify the role of Fe in the ORR activity. The decomposition analyses for the XAFS spectra reveal that the composition ratio of each Fe component is unaltered by the acid washing, indicating that the residual Fe components were removed by the acid washing irrespective of their chemical states. Because the oxygen reduction potential was approximately unchanged by the acid washing, the residual Fe itself does not seem to contribute directly to the ORR activity. The residual Fe can act as a catalyst to accelerate the growth of the sp$$^{2}$$ carbon network during pyrolysis. The results imply that light elements are components of the ORR active sites in the FePc-based carbon catalysts.

Journal Articles

Quantitative measurement of hard X-ray spectra for high intensity laser produced plasma

Zhang, Z.*; Nishimura, Hiroaki*; Namimoto, Takura*; Fujioka, Shinsuke*; Arikawa, Yasunobu*; Nishikino, Masaharu; Kawachi, Tetsuya; Sagisaka, Akito; Hosoda, Hirokazu*; Orimo, Satoshi; et al.

Review of Scientific Instruments, 83(5), p.053502_1 - 053502_5, 2012/05

 Times Cited Count:18 Percentile:65.08(Instruments & Instrumentation)

X-ray line spectra ranging from 17 to 77 keV were quantitatively measured with a Laue spectrometer, composed of a cylindrically curved crystal and a detector. Either a visible CCD detector coupled with a CsI phosphor screen or an imaging plate can be chosen, depending on the signal intensities and exposure times. The absolute sensitivity of the spectrometer system was calibrated using pre-characterized laser-produced X-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an open code for X-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency for specific X-ray line emissions, is derived as a consequence of this work.

Journal Articles

Interplay between nitrogen dopants and native point defects in graphene

Hou, Z.*; Wang, X.*; Ikeda, Takashi; Terakura, Kiyoyuki*; Oshima, Masaharu*; Kakimoto, Masaaki*; Miyata, Seizo*

Physical Review B, 85(16), p.165439_1 - 165439_9, 2012/04

 Times Cited Count:118 Percentile:96.44(Materials Science, Multidisciplinary)

To understand the interaction between nitrogen dopants and point defects in graphene, we have studied energetic stability of N-doped graphene with vacancies and Stone-Wales defect by performing the density functional theory calculations. Our results show that N substitution energetically prefers to occur at the carbon atoms near the defects, especially for those sites with larger bond shortening, indicating that the defect-induced strain plays an important role in the stability of N dopants. In the presence of mono-vacancy, the most stable position for N is the pyridine-like configuration while for other point defects studied N prefers a site in the pentagonal ring. While the N doping is endothermic in defect-free graphene, it becomes exothermic for defective one. Our results imply that the point defect and N dopant attract each other, which means that substitutional N dopants would increase the probability of point defect generation and vice versa.

Journal Articles

Selective nitrogen doping in graphene; Enhanced catalytic activity for the oxygen reduction reaction

Wang, X.*; Hou, Z.*; Ikeda, Takashi; Huang, S.-F.*; Terakura, Kiyoyuki*; Boero, M.*; Oshima, Masaharu*; Kakimoto, Masaaki*; Miyata, Seizo*

Physical Review B, 84(24), p.245434_1 - 245434_7, 2011/12

 Times Cited Count:31 Percentile:77.27(Materials Science, Multidisciplinary)

The structural and electronic properties of N-doped zigzag graphene ribbons with various ratios of di- to monohydrogenated edge carbons are investigated within the density functional theory framework. We find that the stability of graphitic N next to the edge, which is claimed to play important roles in the catalytic activity in our previous work, will be enhanced with increasing the concentration of di-hydrogenated carbons. Furthermore, the di-hydrogenated edge carbons turn out to be easily converted into mono-hydrogenated ones in the presence of oxygen molecules at room temperature. Based on our results, we propose a possible way to enhance the oxygen reduction catalytic activity of N-doped graphene by controlling the degrees of hydrogenation of edge carbons. The characteristic features in the X-ray absorption and emission spectra for each specific N site considered here will also be given.

Journal Articles

Role of residual transition-metal atoms in oxygen reduction reaction in cobalt phthalocyanine-based carbon cathode catalysts for polymer electrolyte fuel cell

Kobayashi, Masaki*; Niwa, Hideharu*; Harada, Yoshihisa*; Horiba, Koji*; Oshima, Masaharu*; Ofuchi, Hironori*; Terakura, Kiyoyuki*; Ikeda, Takashi; Koshigoe, Yuka*; Ozaki, Junichi*; et al.

Journal of Power Sources, 196(20), p.8346 - 8351, 2011/10

 Times Cited Count:32 Percentile:69.85(Chemistry, Physical)

The electronic structure of Co atoms in CoPc-based carbon catalysts, which were prepared by pyrolyzing a mixture of CoPc and phenol resin polymer up to 1000$$^circ$$C, has been investigated using XAFS analysis and HXPES. The Co K XAFS spectra show that most of the Co atoms are in the metallic state and small quantities of oxidized Co components are present in the samples even after acid washing to remove Co atoms. Based on the difference in probing depth between XAFS and HXPES, it was found that after acid washing, the surface region with the aggregated Co clusters is primarily composed of metallic Co. Since the electrochemical properties remain almost unchanged even after the acid washing process, the residual metallic and oxidized Co atoms themselves will hardly contribute to the ORR activity of the CoPc-based carbon cathode catalysts, implying that the active sites of the CoPc-based catalysts primarily consist of light elements such as C and N.

Journal Articles

Effect of hydrogen termination on carbon $$K$$-edge X-ray absorption spectra of nanographene

Hou, Z.*; Wang, X.*; Ikeda, Takashi; Huang, S.-F.*; Terakura, Kiyoyuki*; Boero, M.*; Oshima, Masaharu*; Kakimoto, Masaaki*; Miyata, Seizo*

Journal of Physical Chemistry C, 115(13), p.5392 - 5403, 2011/03

 Times Cited Count:36 Percentile:73.17(Chemistry, Physical)

Carbon ${it K}$-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations in order to obtain the information on the edge termination by hydrogen. Our results show that different edge terminations significantly affect the binding energy of 1s core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the $$pi^{ast}$$ peak at relatively different positions with respect to the $$pi^{ast}$$ peak position in the theoretical spectra of zigzag graphene nano-ribbons, depending on the ratio of mono-hydrogen- to di-hydrogen-terminations. We also point out that the two additional features observed between the $$pi^{ast}$$ and $$sigma^{ast}$$ peaks of an ideal graphene originate from the $$sigma^{ast}$$ states of C-H bonding and C-H$$_{2}$$ bonding at the edges.

Journal Articles

X-ray photoemission spectroscopy analysis of N-containing carbon-based cathode catalysts for polymer electrolyte fuel cells

Niwa, Hideharu*; Kobayashi, Masaki*; Horiba, Koji*; Harada, Yoshihisa*; Oshima, Masaharu*; Terakura, Kiyoyuki*; Ikeda, Takashi; Koshigoe, Yuka*; Ozaki, Junichi*; Miyata, Seizo*; et al.

Journal of Power Sources, 196(3), p.1006 - 1011, 2011/02

 Times Cited Count:85 Percentile:91.91(Chemistry, Physical)

We report on the electronic structure of three different types of N-containing carbon-based cathode catalysts for polymer electrolyte fuel cells observed by hard X-ray photoemission spectroscopy. C 1s spectra show the importance of $$sp^{2}$$ carbon network formation for the oxygen reduction reaction (ORR) activity. Samples having high oxygen reduction reaction activity in terms of oxygen reduction potential contain high concentration of graphite-like nitrogen. Based on a quantitative analysis of our results, the oxygen reduction reaction activity of the carbon-based cathode catalysts will be improved by increasing concentration of graphite-like nitrogen in a developed $$sp^{2}$$ carbon network.

Journal Articles

Efficient multi-keV X-ray generation from a high-Z target irradiated with a clean ultra-short laser pulse

Zhang, Z.*; Nishikino, Masaharu; Nishimura, Hiroaki*; Kawachi, Tetsuya; Pirozhkov, A. S.; Sagisaka, Akito; Orimo, Satoshi; Ogura, Koichi; Yogo, Akifumi; Okano, Yasuaki*; et al.

Optics Express (Internet), 19(5), p.4560 - 4565, 2011/02

 Times Cited Count:17 Percentile:65.28(Optics)

$$Kalpha$$ line emission from Mo and Ag plate were experimentally studied by using ultra-high intensity, clean femtosecond laser pulses. Absolutely yield of $$Kalpha$$ X-rays at 17 keV from Mo and 22 keV from Ag were measured as a function of the laser pulse contrast ratio and irradiation intensity. Significant enhancement of $$Kalpha$$ yields were obtained for both Mo and Ag with higher contrast ratios and high irradiance. The conversion efficiencies of 4.28 $$times$$ 10$$^{-5}$$/sr for Mo and 4.84 $$times$$ 10$$^{-5}$$/sr for Ag, the highest values ever obtained, have been demonstarted with the contrast ratio of 10$$^{-10}$$ to 10$$^{-11}$$.

Journal Articles

$${gamma}$$-H2AX and phosphorylated ATM focus formation in cancer cells after laser plasma X irradiation

Sato, Katsutoshi; Nishikino, Masaharu; Okano, Yasuaki*; Oshima, Shinsuke*; Hasegawa, Noboru; Ishino, Masahiko; Kawachi, Tetsuya; Numasaki, Hodaka*; Teshima, Teruki*; Nishimura, Hiroaki*

Radiation Research, 174(4), p.436 - 445, 2010/10

 Times Cited Count:16 Percentile:51.65(Biology)

Usefulness of laser-plasma X-ray pulse for medical and radiation biological studies was investigated and compared with that of conventional sources such as those obtained from a linear accelerator. A cell irradiation system was developed which uses Copper-K$${alpha}$$ (8 keV) lines from an ultra-short high intensity laser to produce plasma. The absorbed dose of the 8 keV laser-plasma X-ray pulse was accurately estimated with Gafchromic EBT film. When the cells were irradiated with approximately 2 Gy of laser plasma X-ray, the circular regions on $${gamma}$$-H2AX positive cells could be clearly identified. Moreover, the number of $${gamma}$$-H2AX and phosphorylated ataxia telangiectasia mutated (ATM) foci induced by 8 keV laser plasma X-ray were comparable to those induced by 4MV X-ray. These results indicate that the laser plasma X-ray source can be useful for radiation biological studies.

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