Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Katsuro Hayashi Last modified date:2021.06.11

Professor / Functional Materials Chemistry (Applied Fine Chemistry) / Department of Applied Chemistry / Faculty of Engineering


Papers
1. George Hasegawa, Takaya Yano, Hirofumi Akamatsu, Katsuro Hayashi, Kazuki Nakanishi, Variation of meso- and macroporous morphologies in resorcinol-formaldehyde (RF) gels tailored via a sol?gel process combined with soft-templating and phase separation, Journal of Sol-Gel Science and Technology, 10.1007/s10971-020-05236-9, 2020.02.
2. Wang, He; Hasegawa, George; Akiyama, Yuto; Yamamoto, Takahisa; Inoishi, Atsushi; Akamatsu, Hirofumi; Inada, Miki; Ishihara, Tatsumi; Hayashi, Katsuro, A highly conductive Na3V2(PO4)3 ceramic sheet prepared by tape-casting method, Electrochimica Acta, 10.1016/j.electacta.2019.03.057, 305, 10, 197-203, 2019.03.
3. He Wang, Keisuke Okubo, Miki Inada, George Hasegawa, Naoya Enomoto, Katsuro Hayashi, Low temperature-densified NASICON-based ceramics promoted by Na2O-Nb2O5-P2O5 glass additive and spark plasma sintering, Solid State Ionics, 10.1016/j.nanoen.2018.04.074, 322, 54-60, 2018.09.
4. George Hasegawa, Shizuka Moriya, Miki Inada, Masaaki Kitano, Masaaki Okunaka, Takahisa Yamamoto, Yuko Matsukawa, Kazuma Nishimi, Kazunari Shima, Naoya Enomoto, Satoru Matsuishi, Hideo Hosono, Katsuro Hayashi, Topotactic Synthesis of Mesoporous 12CaO·7Al2O3 (C12A7) Mesocrystalline Microcubes, Chemistry of Materials, 2018.07.
5. Keisuke Okubo, He Wang, Katsuro Hayashi, Miki Inada,Naoya Enomoto, George Hasegawa, Takashi Osawa, Hitoshi Takamura, A Dense NASICON Sheet Prepared by Tape-Casting and Low Temperature Sintering, Electrochimica Acta, 10.1016/j.electacta.2018.05.020, 278, 176-181, 2018.07.
6. Feng Liang, Xuechao Qiu, Qingkai Zhang, Yao Kan, Alicia Koo, Katsuro Hayashi, Kunfeng Chen, Dongfeng Xue, Kwun-Nam Hui, Hossein Yadegari, Xueliang Sun, A liquid anode for rechargeable sodium-air batteries with low voltage gap and high safety, Nano Energy, 10.1016/j.nanoen.2018.04.074, 574-579, 2018.05.
7. Kenta Hongo, Shinji Kurata, Apichai Jomphoak, Miki Inada, Katsuro Hayashi, Ryo Maezono, Stabilization mechanism of tetragonal structure in hydrothermal synthesized BaTiO3 nanocrystal, Inorganic Chemistry, 10.1021/acs.inorgchem.8b00381, 2018.04.
8. Hiroshi Kageyama, Katsuro Hayashi, Kazuhiko Maeda, J. Paul Attfield, Zenji Hiroi, James M. Rondinelli, Kenneth R. Poeppelmeier, Expanding frontiers in materials chemistry and physics with multiple anions, Nature Communications, 9, Article No. 772, 2018.02.
9. Yao Kang, Da Zou, Jianyong Zhang, Feng Liang, Katsuro Hayashi, He Wang, Dongfeng Xue, Kunfeng Chen, Keegan R.Adair, Xueliang Sun, Dual–phase Spinel MnCo2O4 Nanocrystals with Nitrogen-doped Reduced Graphene Oxide as Potential Catalyst for Hybrid Na–Air Batteries, Electrochimica Acta, DOI: 10.1016/j.electacta.2017.05.100, 244, 222-229, 2017.08.
10. Zhu Yang, George Hasegawa, Kazuyoshi Kanamori, Tsutomu Kiyomura, Hiroki Kurata, Katsuro Hayashi, Kazuki Nakanishi, Nanostructured Titanium Phosphates Prepared via Hydrothermal Reaction and Their Electrochemical Li- and Na-ion Intercalation Properties, CrystEngComm, DOI: 10.1039/C7CE01123G, 19, 4551-4560, 2017.08.
11. Naoya Enomoto, Shingo Hirata, Miki Inada, Katsuro Hayashi, Crystallization Behavior of Iron-based Amorphous Nanoparticles Prepared Sonochemically, Ultrasonics Sonochemistry, doi.org/10.1016/j.ultsonch.2016.04.033, 35B, 4, 563-568, 2017.03.
12. Miki Inada, Naoya Enomoto, Jyunichi Hojyo, Katsuro Hayashi, Structural analysis and capacitive properties of carbon spheres prepared by hydrothermal carbonization, Advanced Powder Technology, DOI: 10.1016/j.apt.2016.12.014, 53, 12, 804-809, 2017.03.
13. Feng Liang, Takayuki Watanabe, Katsuro Hayashi, Yaochun Yao, Wenhui Ma, Bin Yang, Yongnian Dai, Liquid exfoliation graphene sheets as catalysts for hybrid sodium-air cells, MATERIALS LETTERS, 10.1016/j.matlet.2016.10.054, 187, 32-35, 2017.01.
14. Yao Kang, Feng Liang, Katsuro Hayashi, Hybrid Sodium-Air Cell with Na[FSA-C(2)C(1)im][FSA] Ionic Liquid Electrolyte, ELECTROCHIMICA ACTA, 10.1016/j.electacta.2016.09.113, 218, 119-124, 2016.11.
15. Katsuro Hayashi, Hideo Hosono, Green apatites: hydride ions, electrons and their interconversion in the crystallographic channel
, Physical Chemistry Chemical Physics , DOI: 10.1039/C6CP00515B, 18, 8186-8919, 2016.02.
16. Taijyu Hashimoto, Katsuro Hayashi, Aqueous and Nonaqueous Sodium-Air Cells with Nanoporous Gold Cathode, Electrocimica Acta, doi:10.1016/j.electacta.2015.09.153, 182, 10, 809-814, 2015.10.
17. M. Inada, N. Enomoto, K. Hayashi, J. Hojo, S. Komarneni, Facile synthesis of nanorods of tetragonal barium titanate using ethylene glycol, Ceramics International, Volume 41, Issue 4, May 2015, Pages 5581–5587, doi:10.1016/j.ceramint.2014.12.137, 2015.05.
18. Feng Liang, Katsuro Hayashi, Mixed-Aprotic-Aqueous Sodium-Air Cell with a NASICON Ceramic Separator and a Porous Carbon Electrode, The Journal of Electrochemical Society, 162, 7, A1215-A1219, 2015.04.
19. K. Koganei, T. Oyama, M. Inada, N. Enomoto, K. Hayashi, C-axis oriented β″-alumina ceramics with anisotropic ionic conductivity prepared by spark plasma sintering, Solid State Ionics, Volume 267, 1 December 2014, Pages 22-26, DOI: 10.1016/j.ssi.2014.09.003, 2014.12.
20. Katsuro Hayashi, Peter V. Sushko, Yasuhiro Hashimoto, Alexander L. Shluger, Hideo Hosono, Hydride Ions in Oxide Hosts Hidden by Hydroxide Ions, Nature Communications, 6, Article No. 4515, 2014.03, The true oxidation state of formally 'H(-)' ions incorporated in an oxide host is frequently discussed in connection with chemical shifts of (1)H nuclear magnetic resonance spectroscopy, as they can exhibit values typically attributed to H(+). Here we systematically investigate the link between geometrical structure and chemical shift of H(-) ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches, in an attempt to resolve this issue. We demonstrate that the electron density near the hydrogen nucleus in an OH(-) ion (formally H(+) state) exceeds that in an H(-) ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H(-) and the distance from the H(-) ion to the coordinating electropositive cation. This relationship is pivotal for resolving H(-) species that are masked by various states of H(+) ions.
21. Yingchao Dong, Hideo Hosono, Katsuro Hayashi, Formation and quantification of peroxide anions in nanocages of 12CaO·7Al2O3, RSC Advances, 10.1039/c3ra42521e, 3, 40, 18311-18316, 2013.10, The concentration of peroxide (O22-) ions in the cage structures of the mayenite (12CaO·7Al2O3 or C 12A7) crystal lattice has been determined quantitatively by Raman spectroscopy combined with volumetry and electron paramagnetic spin resonance. The formation process of the O22- ion was also investigated using in situ Raman spectroscopy under controlled atmospheres. Although room temperature Raman spectroscopy showed that the equilibrium concentration of O22- ions in the cage structures increased with increased annealing temperatures (especially below 800 °C), no O22- signal was observed at very high temperatures (1350 °C) during the in situ measurements. We explain this apparent inconsistency by hypothesizing that transient atomic oxygen, possibly originating from the thermal dissociation of atmospheric O2, is formed at interstitial or cage sites within C12A7 at high temperatures and subsequently combines with O2- ions in the cages to form O22- during the cooling process..
22. Katsuro Hayashi, Kazunari Shima, Fumiaki Sugiyama, A mixed aqueous/aprotic sodium/air cell using a NASICON ceramic separator, Journal of the Electrochemical Society, 10.1149/2.067309jes, 160, 9, 2013.07, An aqueous (mixed aqueous/aprotic) Na-air cell is fabricated and its discharge properties are examined experimentally. This design is attractive because Na is not only more abundant and cheaper than Li, but also potentially advantageous for enhancing cell output. The theoretical energy density of the aqueous Na-air cell exceeds that of a Li one under conditions where precipitation of hydroxides in the aqueous electrolyte is not allowed. Na 3Zr2Si2PO12 (NASICON) ceramics, which has a Na+ ion conductivity of 2 × 10-3 S · cm-1 at 50°C, is used as a solid electrolyte separator in the Na-air cell. Involvement of O2 gas in the cell reaction is verified by changing cell voltage in the presence or absence of O2 flow through the aqueous electrolyte. The Na-air cell exhibits an electrochemical discharge of ∼600 mAh· g-1 and energy density of ∼1500 Wh· kg-1 based on the weights of reacted Na and H2O. Its maximum areal output power is 11 mW· cm -2 and depends on the thickness of the NASICON ceramic separator, the concentration of NaOH in the aqueous electrolyte and the type of cathode, suggesting that cell performance can be improved by optimization of these components..
23. Toshihiro Yoshizumi, Katsuro Hayashi, Thermionic electron emission from a mayenite electride-metallic titanium composite cathode, Applied Physics Express, 10.7567/APEX.6.015802, 6, 1, 2013.01, A composite thermionic cathode consisting of 12CaO·7Al 2O3 (C12A7) electride and metallic Ti (70 : 30 vol %) was fabricated as an electron emitter, and the thermionic electron emission properties were evaluated. A high emission current density of ∼1:4mAcm -2 was achieved at 700 °C with an electric field of 4:0 × 104 Vcm-1. The work function evaluated from the Richardson-Dushman equation was 2:1 ± 0:3 eV, which coincides with the value for pure C12A7 electride and is lower than that for LaB6. Unlike the pure material, the composite has ohmic contact with metallic materials, and can be heated directly by electrical current..
24. Toshihiro Yoshizumi, Yoji Kobayashi, Hiroshi Kageyama, Katsuro Hayashi, Simultaneous quantification of hydride ions and electrons incorporated in 12CaO·7Al 2O 3 cages by deuterium-labeled volumetric analysis, Journal of Physical Chemistry C, 10.1021/jp2106742, 116, 15, 8747-8752, 2012.04, An extended volumetric method, combined with quadrupole mass spectroscopy (QMS), is proposed. This method enables us to distinguish and simultaneously quantify hydride (H -) ions and electrons (e -) incorporated in cages of 12CaO·7Al 2O 3 (C12A7), which is accomplished upon annealing with CaH 2. When a sample is dissolved in a deuterium chloride solution, most of the H - ions and electrons released from cages react to form HD and D 2 molecules, respectively. These isotope-labeled molecules are then detected by QMS. We have used this method to follow the concentrations of H - ions and electrons in C12A7-treated CaH 2 over the thermal treatment time. We found that during the initial seven days of treatment, both concentrations increased. Thereafter, the electron concentration begins to decrease, while the H - ion concentration continues to increase toward the theoretical maximum. This diverging behavior is due to differences in their diffusion ratios and thermodynamic stabilities..
25. Yoji Kobayashi, Olivier J. Hernandez, Tatsunori Sakaguchi, Takeshi Yajima, Thierry Roisnel, Yoshihiro Tsujimoto, Masaki Morita, Yasuto Noda, Yuuki Mogami, Atsushi Kitada, Masatoshi Ohkura, Saburo Hosokawa, Zhaofei Li, Katsuro Hayashi, Yoshihiro Kusano, Jung Eun Kim, Naruki Tsuji, Akihiko Fujiwara, Yoshitaka Matsushita, Kazuyoshi Yoshimura, Kiyonori Takegoshi, Masashi Inoue, Mikio Takano, Hiroshi Kageyama, An oxyhydride of BaTiO3 exhibiting hydride exchange and electronic conductivity, Nature Materials, 10.1038/nmat3302, 11, 6, 507-511, 2012.01, In oxides, the substitution of non-oxide anions (F -S 2-N 3- and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H-) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO3, as an O 2-/H-solid solution with hydride concentrations up to 20% of the anion sites. BaTiO3-x Hx is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400°C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state..
26. Daiki Ojima, Tetsuya Chiba, Kazunari Shima, Hidenori Hiramatsu, Hideo Hosono, Katsuro Hayashi, Solid-state source of atomic oxygen for low-temperature oxidation processes
Application to pulsed laser deposition of TiO2:N films, Review of Scientific Instruments, 10.1063/1.3683571, 83, 2, 2012.01, An atomic oxygen (AO) source has been redesigned to coordinate with a pulsed laser deposition system and used to grow nitrogen-doped TiO2 films by deposition of TiN and simultaneous irradiation of the substrate with AO. The AO source uses an incandescently heated thin tube of zirconia as an oxygen permeation media to generate pure AO of low kinetic energy. The emission flux is calibrated using a silver-coated quartz crystal microbalance. The thin shape of the probe and transverse emission geometry of this emission device allow the emission area to be positioned close to the substrate surface, enhancing the irradiation flux at the substrate. AO irradiation is crucial for formation of TiO2 phases via oxidation of the deposited TiN laser plume, and is effective for decrease of the substrate temperature for crystallization of anatase phase to as low as around 200 C..
27. Katsuro Hayashi, Heavy doping of H- ion in 12CaO·7Al2O 3, Journal of Solid State Chemistry, 10.1016/j.jssc.2011.04.008, 184, 6, 1428-1432, 2011.06, 12CaO·7Al2O3 (C12A7, mayenite), which has a nanoscale porous structure that can accommodate extraframework species such as hydride (H-), oxide (O2-), hydroxide (OH-) ions, and electrons, has been doped with H- ions to investigate its effects as dominant extraframework species. Chemical doping with CaH2 enables the concentration of H- ions to reach almost the theoretical maximum. The concentration of H- ions is characterized by optical absorption intensity ascribed to photoionization of H- ions, and 1H magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Persistent electron generation, which is accompanied by the formation of an F absorption band and electrical conductivity, by irradiation with ultraviolet light at room temperature increases as the H- ion doping increases until it reaches half the theoretical maximum and then decreases as the H- ion concentration increases further. This dependence indicates that both H- and O2- ions are necessary for the generation of persistent electrons..
28. Katsuro Hayashi, Kinetics of electron decay in hydride ion-doped mayenite, Journal of Physical Chemistry C, 10.1021/jp202428s, 115, 22, 11003-11009, 2011.06, Persistent electrons are generated in hydride (H-) ion-doped 12CaO·7Al2O3 (C12A7) by UV irradiation at room temperature. The kinetics of thermal electron decay is examined using experimental data collected at 200-295 °C. The observed reaction order together with activation energy shifts approximately from one-half to first depending on the electron concentration and temperature. The data is explained by a two-electron transfer reaction (H- + O2- ↔ OH- + 2e-) with H0 and H+ as reaction intermediates and diffusion of these hydrogen species. The observed one-half-order kinetics originates from a zero-order reaction influenced by inhomogeneity of the electron concentration. The electron decay rate is principally determined by the supply of H+ ions liberated from OH- ions. The large activation energy (1.9 ± 0.3 eV) of this process is responsible for the persistence of electrons at room temperature. As the residual electron concentration decreases, the contribution of electron trapping by the intermediate hydrogen species becomes dominant for the rate-determination, shifting the reaction order and activation energy..
29. Hideo Hosono, Katsuro Hayashi, Toshio Kamiya, Toshiyuki Atou, Tomofumi Susaki, New functionalities in abundant element oxides
Ubiquitous element strategy, Science and Technology of Advanced Materials, 10.1088/1468-6996/12/3/034303, 12, 3, 2011.06, While most ceramics are composed of ubiquitous elements (the ten most abundant elements within the Earth's crust), many advanced materials are based on rare elements. A 'rare-element crisis' is approaching owing to the imbalance between the limited supply of rare elements and the increasing demand. Therefore, we propose a 'ubiquitous element strategy' for materials research, which aims to apply abundant elements in a variety of innovative applications. Creation of innovative oxide materials and devices based on conventional ceramics is one specific challenge. This review describes the concept of ubiquitous element strategy and gives some highlights of our recent research on the synthesis of electronic, thermionic and structural materials using ubiquitous elements..
30. Toshihiro Yoshizumi, Satoru Matsuishi, Sung Wng Kim, Hideo Hosono, Katsuro Hayashi, Iodometric determination of electrons incorporated into cages in 12CaO·7Al2O3 Crystals, Journal of Physical Chemistry C, 10.1021/jp1054364, 114, 36, 15354-15357, 2010.09, The accurate determination of the concentration of electrons incorporated into cages in 12CaO·7Al2O3 (C12A7) has been examined using iodometry. Preliminary experiments confirmed that iodine is reduced in preference to protons by electron-enriched C12A7, even in aqueous solution. The electron concentrations in several C12A7 samples are measured using a procedure where the iodine molecules in an I2-HCl solution are partially reduced by electrons released from the samples, followed by titration of the residual iodine molecules. The accuracy of this procedure is confirmed by the excellent agreement of the results with those from thermogravimetric analysis. The reevaluated maximum electron mobility is nearly twice as high as that reported previously, and the critical electron concentration for the metal-insulator transition is revised slightly because of the accuracy of the iodometry results..
31. Jiang Li, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Field-assisted sustainable O- ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability, Solid State Ionics, 10.1016/j.ssi.2009.04.018, 180, 17-19, 1113-1117, 2009.07, We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O-) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al2O3 (C12A7) with a composition of (12 - x)CaO·7Al2O3·xCaF2 (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO{bullet operator}3Al2O3 (C5A3) and 3CaO{bullet operator}Al2O3 (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F- ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O- ions (∼ 92%) together with a small amount of O2- ions (∼ 4%) and F- ions (∼ 4%). The absence of an O2 gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O- ions and electrons with a ratio of < 1/1. The O2 gas supply markedly enhanced the O- ion emission, and suppressed the electron emission. A sustainable and high-purity O- ion emission with a current density of 11 nA cm- 2 was achieved at 830 °C with the supply of 40 Pa O2 gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F- ion-substituted C12A7 is a promising material for higher intensity O- ion emission at higher temperatures..
32. Katsuro Hayashi, Tetsuya Chiba, Jiang Li, Masahiro Hirano, Hideo Hosono, Intense atomic oxygen emission from incandescent zirconia, Journal of Physical Chemistry C, 10.1021/jp902159p, 113, 22, 9436-9439, 2009.06, Particle emissions from an oxide-ion conducting solid electrolyte (tetragonal zirconia polycrystal, 3% Y 2O 3-doped ZrO 2) have been examined using a high vacuum evaluation system. Oxygen gas with a pressure range from 10 to 3 × 10 3 Pa was fed into a 2 mm outer diameter zirconia tube, whose central region of 10 mm length was heated to 1400-1800 °C by direct Joule heating. The emission species were identified by appearance potential spectroscopy using a quadrupole mass spectrometer. Atomic oxygen (AO) is the dominant species that is radiantly emitted from the high-temperature region. Temperature dependence of the AO emission intensity exhibits far less activation energy (̃0.3-1.0 and 0.7 eV, on average) than electric field-assisted thermal O - ion (̃2 eV) and electron (̃6 eV) emission from zirconia reported before. High AO emission flux (in the order of 10 17 atoms.cm -2.s -1) is promising for use as practical AO sources..
33. Hideo Hosono, Katsuro Hayashi, Koichi Kajihara, Peter V. Sushko, Alexander L. Shluger, Oxygen ion conduction in 12CaO·7Al2O3
O2- conduction mechanism and possibility of O- fast conduction, Solid State Ionics, 10.1016/j.ssi.2008.10.015, 180, 6-8, 550-555, 2009.05, 12CaO·7Al2O3 (C12A7) with a unique nano-porous structure and free O2- ions entrapped in sub-nanometer-sized cages is a fast oxygen-ion-conducting material. These free O2- may be replaced by various oxygen-related species, OH-, O2- and O-, by tuning the atmosphere during the heat treatment. We examined the conduction mechanism for stoichiometric C12A7 (C12A7:O2-), in which O2- ions exist as counter anions in sub-nanometer-sized cages, by Raman measurement of C12A7:O2- annealed in a dry 18O2 atmosphere. It was revealed that the primary ion conducting species is an O2- ion which diffuses via exchange with O2- in the cage wall. An experimental result on the sample containing O- ions implied that O- is more mobile than O2- in C12A7. Ab initio calculations on the diffusion paths of O2- and O- ions in C12A7 supported the above experimental results..
34. Katsuro Hayashi, Hiroki Muramatsu, Satoru Matsuishi, Toshio Kamiya, Hideo Hosono, Humidity-sensitive electrical conductivity in Ca12 Al14-x Six O32 Cl2+x (0≤x≤3.4) ceramics, Electrochemical and Solid-State Letters, 10.1149/1.3032908, 12, 2, 2009.01, The effect of humidity on the electrical conductivity in porous and dense ceramics of Ca12 Al14-x Six O32 Cl2+x (x=0, 1.7, 3.4) with a mayenite structure was examined at room temperature. Surface ionic conductivity was enhanced by increasing the humidity. The sample with x=3.4, which had a porosity of 30%, exhibited the largest conductivity change of about 4 orders of magnitude over the relative humidity range of 10-90% at 25°C. The presence of Cl- ions on the surface most likely enhances the formation of hydronium ions which are responsible for proton transfer in physisorbed water layers..
35. Jiang Li, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Sustainable thermionic O- emission from stoichiometric 12CaO7 Al2O3 with nanoporous crystal structure, Journal of the Electrochemical Society, 10.1149/1.2999055, 156, 1, 2009.01, We have examined electric field-assisted thermionic emission of oxygen radical anion (O-) and electron from a stoichiometric 12CaO7 Al 2O3 (C12A7). Lattice framework of C12A7 is composed of subnanometer-sized cages that can accommodate extra- framework species with negative charge, such as O-, O2- ions, and electrons. Absence of oxygen gas supply leads to a steady coemission of O- ion and electron with a ratio of ∼3:7. However, the oxygen gas supply markedly enhances the O- ion emission and suppresses the electron emission, realizing a sustainable and high-purity O- ion emission with a current density up to 11 nA cm-2 at 825°C. The O- ion emission accompanies a weak O2- ion emission, which is smaller by ∼130. TheO2 pressure at the rear side controls the concentrations of O- ion and electron on the emission surface as well as their concentrations in the bulk, and in turn, determines their emission intensities..
36. Katsuro Hayashi, Naoto Ueda, Satoru Matsuishi, Masahiro Hirano, Toshio Kamiya, Hideo Hosono, Solid state syntheses of 12SrO·7Al2O3 and formation of high density oxygen radical anions, O- and O 2 -, Chemistry of Materials, 10.1021/cm800666p, 20, 19, 5987-5996, 2008.10, A new synthetic route via a solid-state reaction, the crystal structure of 12SrO·Al2O3 (S12A7) incorporated with OH- ions, and the formation of oxygen radical anions in S12A7 have been examined. Investigations using X-ray powder diffraction and thermogravimetric and evolved gas combined analyses demonstrate that homogenization promoted by molten Sr(OH)2 hydrates and the presence of Sr-hydrogarnet precursors play crucial roles in the formation of S12A7. Rietveld analyses confirm the lattice framework of S12A7 has a structure identical to that of the nanoporous crystal 12CaO·7Al2O3 (C 12A7, a mayenite structure) and demonstrate that two Sr2+ ions (pole Sr2+ ions) on the S4 axis of a cage displace toward the cage center owing to the accommodation of an OH - ion. An electron paramagnetic resonance study demonstrates that oxygen radical anions, O2 - and O-, are generated in the cages with concentrations on the order of 1020 cm-3 each by oxygen annealing. From the g-values observed at 77 K, the oxygen radical anions are interpreted as being sandwiched between the two pole Sr2+ ions rather than adsorbed on one side..
37. Y. Nishio, K. Nomura, M. Miyakawa, Katsuro Hayashi, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Fabrication and transport properties of 12CaO · 7Al 2O 3 (C12A7) electride nanowire, Physica Status Solidi (A) Applications and Materials Science, 10.1002/pssa.200778901, 205, 8, 2047-2051, 2008.08, Electron-doped 12CaO · 7Al 2O 3 (C12A7) nanowires (NWs) were fabricated by two approaches. First, spatially selective electron doping of C12A7 films was examined by using a chemical method specific to C12A7. However, the conductive NW was not formed due to diffusion of oxygen in the C12A7 films to the conductive area. Secondly, a combination of dry etching and electron doping was examined. NWs with conductivities ∼6 S/cm at 300 K were successfully fabricated by this method. The temperature dependence of the conductivity in the NWs exhibited almost degenerated conduction down to 5 K..
38. Sung Wng Kim, Satoru Matsuishi, Masashi Miyakawa, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Fabrication of room temperature-stable 12CaO · 7Al 2 O 3 electride
A review, Journal of Materials Science: Materials in Electronics, 10.1007/s10854-007-9183-y, 18, SUPPL. 1, 5-14, 2007.10, 12CaO · 7Al 2O 3 (C12A7) electride, which is synthesized by replacing free oxygen ions in cages with electron anions, has distinct advantages over electrides reported so far in respect of thermal and chemical stability and flexible preparation of various sample forms including single crystal, thin film, polycrystalline bulk and powder. These advantages, together with the fact that the concentration of the electron anions is controlled in a wide range according to a synthetic process, make the C12A7 electride attract growing attentions from both scientific and practical points of views. This paper reviews several chemical and physical synthetic processes of the C12A7 electride including thermal treatment of C12A7 under metal vapor and reducing gas atmospheres, hot Ar + ion implantation, solidification of the strongly reduced C12A7 melt, and crystallization of the reduced glass in vacuum. Each process, having its own suitability for a specific form of the electride, has unique advantages such as a completeness of anion replacement, mass production capability and controllability of the electron-doped area. Electronic and optical properties of the resulting electrides prepared by the different process are briefly discussed in terms of the feature of the processes..
39. Koichi Kajihara, Satoru Matsuishi, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Vibrational dynamics and oxygen diffusion in a nanoporous oxide ion conductor 12CaO·7Al2O3 studied by18O labeling and micro-Raman spectroscopy, Journal of Physical Chemistry C, 10.1021/jp074248n, 111, 40, 14855-14861, 2007.10, Nanoporous oxide 12CaO·7Al2O3 (C12A7) is a fast oxide-ion conductor that may encage various types of anionic oxygen species such as O2-, O2 -, O2 2-, and O- in its crystallographic cages. The C12A7 single crystals thermally annealed in 18O2 were examined by micro-Raman spectroscopy. The symmetric properties and the origin of Raman bands were analyzed by the isotope shift and the depolarization on the bases of the crystal symmetry of C12A7, and the Raman band due to the stretching mode of extraframework O2 2- was identified. Spatially resolved measurements of the isotope shift of the Raman bands in a partially 18O-substituted sample indicated that the local fraction of 18O is equal between the framework and extraframework sites, demonstrating that the substitution of O2- ions in the C12A7 framework is as fast as the formation of extraframework oxygen species by thermal oxygen loading. It strongly suggests that the extraframework oxygen species diffuse by replacing oxygens in the framework sites, rather than simply permeating through intercage openings..
40. Takatoshi Nomura, Katsuro Hayashi, Yoshiki Kubota, Toshio Kamiya, Masahiro Hirano, Masaki Takata, Hideo Hosono, Anion incorporation-induced cage deformation in 12CaO·7Al 2O 3 crystal, Chemistry Letters, 10.1246/cl.2007.902, 36, 7, 902-903, 2007.07, Effects of O 2- or OH - ion incorporation on the cage geometry in 12CaO·7Al 2O 3 (C12A7) crystals are examined by the maximum entropy method (MEM)/Rietveld analyses of synchrotron powder X-ray diffraction data. Presence of an extraframework anion in a cage causes inward displacements of two Ca 2+ ions axially coordinating to the anion. The OHion locates at the cage center and induces smaller symmetric displacements of the two Ca 2+ ions, while the O 2- ion likely locates on off-axis positions and induces larger asymmetric Ca 2+ ion displacements..
41. Sung Wng Kim, Satoru Matsuishi, Takatoshi Nomura, Yoshiki Kubota, Masaki Takata, Katsuro Hayashi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Metallic state in a lime-alumina compound with nanoporous structure, Nano Letters, 10.1021/nl062717b, 7, 5, 1138-1143, 2007.05, We report a metallic state in a nanostructured porous crystal 12CaO-7Al2O3 by incorporating electrons in the inherent subnanometer-sized cages, in which a three-dimensionally closely packed cage structure acts as an electronic conduction path. High-density electron doping (∼2 x 1021 cm-3), which was achieved by a thermal treatment in Ti metal vapor at ∼1100 °C, induces homogenization of the cage geometry to a symmetric state, resulting in an insulator-metal transition with a sharp enhancement of the electron drift mobility from ∼0.1 to 4 cm2 V-1 s-1. The results provide an approach for the realization of electroactive functions in materials composed only of environmentally benign elements by utilizing the appropriate nanostructures..
42. Hideo Hosono, Katsuro Hayashi, Masahiro Hirano, Active anion manipulation for emergence of active functions in the nanoporous crystal 12CaO·7Al2O3
A case study of abundant element strategy, Journal of Materials Science, 10.1007/s10853-006-1316-9, 42, 6, 1872-1883, 2007.03, This article reviews our approach to render 12CaO·7Al 2O3 (C12A7) electronically active using a new concept of 'active anion manipulation', where nanostructures embedded within the C12A7 crystal lattice are intentionally utilized to generate chemically unstable ('water-free active') anions. Anionic active oxygen radicals, O- and O 2 - , are formed efficiently in C12A7 cages under high oxygen activity conditions. The configuration and dynamics of O 2 - in cages are revealed by a combination of continuous-wave and pulsed electron paramagnetic resonance (EPR). It is demonstrated that metal-loaded C12A7 is a promising oxidation catalyst for syngas (CO + H 2) formation from methane. Furthermore, the O- ion, the strongest oxidant among active oxygen species, can be extracted from the cage into an external vacuum by applying an electric field with thermal assistance, generating a high-density O- beam in the order of μA cm -2. In contrast, heat treatment of C12A7 in a hydrogen atmosphere forms H- ions in the cages. The resultant C12A7:H- exhibits a persistent insulator-conductor conversion upon ultraviolet-light or electron-beam irradiation. The irradiation-induced conversion mechanism is examined by first-principle theoretical calculations. Furthermore, the presence of a severely reducing environment causes the complete substitution of electrons for anions in the cages. The resulting C12A7:e-, which exhibits excellent stability and an electrical conductivity greater than 100 S cm -1, is regarded as an 'electride', an ionic compound in which electrons serve as anions. The C12A7 electride exhibits a high potential for applications involving cold cathode and thermal field electron emissions due to its small work function. Electride fabrication methods suitable for large-scale production via melt processing are described. It is also demonstrated that proton or inert gas ion implantations into C12A7 thin films at elevated temperatures are effective for both H- and electron doping..
43. Katsuro Hayashi, Peter V. Sushko, David Muñoz Ramo, Alexander L. Shluger, Satoshi Watauchi, Isao Tanaka, Satoru Matsuishi, Masahiro Hirano, Hideo Hosono, Nanoporous crystal 12CaO·Al2O3
A playground for studies of ultraviolet optical absorption of negative ions, Journal of Physical Chemistry B Materials, 10.1021/jp065793b, 111, 8, 1946-1956, 2007.03, A novel nanoporous material 12CaO·Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>10 21 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2 2-, and O2 2-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of ∼4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments..
44. Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Functionalities of a nanoporous crystal 12CaO·7Al2O 3 originating from the incorporation of active anions, Bulletin of the Chemical Society of Japan, 10.1246/bcsj.80.872, 80, 5, 872-884, 2007.01, A cage structure formed in a positively charged lattice framework of 12CaO-7Al2O3 (C12A7) is utilized to incorporate and release chemically unstable anions, imparting chemical and electronic activities to this material. High concentrations of oxygen radical anions, i.e. O - and O2
-, are formed efficiently in these cages under a high oxygen activity condition. The O-ion, which is the strongest oxidant among all the reactive oxygen species, can be extracted from the cages into an external vacuum by applying an electric field with thermal assistance. In contrast, hydrogen-reduction processes allow the formation of hydride (H-) ions in the cage. H- ion-incorporated C12A7 exhibits a persistent insulator-conductor conversion upon irradiation with ultraviolet-light or electron-beam. The irradiation-induced conversion mechanism and electronic transport characteristics are discussed on the basis of experimental evidence and first-principle theoretical calculations. Electrons released from the H- ions are trapped by the cages and subsequent inter-cage hopping of the electrons is responsible for the electrical conduction. Furthermore, a severe reducing environment enables us to substitute electrons almost completely for anions in the cages, forming a stable inorganic electride. Finally, the formation of these active anions in this material is examined in terms of electronic structures, thermodynamics, and structural features of the cage..
45. Sung Wng Kim, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Isao Tanaka, Electron carrier generation in a refractory oxide 12CaO·7Al 2O3 by heating in reducing atmosphere
Conversion from an insulator to a persistent conductor, Journal of the American Ceramic Society, 10.1111/j.1551-2916.2006.01213.x, 89, 10, 3294-3298, 2006.10, Electron carriers were generated in refractory oxide 12CaO·7Al 2O3 (C12A7) through a thermal treatment in a reducing CO/CO2 atmosphere, and the reduction process was thermodynamically analyzed. Electrical-conductive C12A7, which had an electron concentration of ∼8 × 1019 cm-3 and an electrical conductivity of ∼4 S/cm at 300 K, was obtained by extracting free O2- ions in sub-nanometer-sized cages in the C12A7 lattice and leaving electrons behind. The enthalpy for this substitutive reaction is +425 kJ/mol, suggesting that stoichiometric C12A7 is more stable in air than the C12A7 electride..
46. Kazuhisa Kurashige, Yoshitake Toda, Satoru Matstuishi, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Czochralski growth of 12CaO·7Al2O3 crystals, Crystal Growth and Design, 10.1021/cg0600290, 6, 7, 1602-1605, 2006.07, Single crystals of nanoporous 12CaO·7Al2O3 (C12A7) as large as 27 mm in diameter, 120 mm in length, and 207 g in weight have been successfully grown by the Czochralski method. We found that a 2.0% oxygen-containing nitrogen atmosphere and an iridium crucible are the critical factors for success. The crystal ingot exhibited an orange color resulting from the incorporation of Ir4+ ions of ca. 5 × 1017 cm-3, which presumably occupy Ca2+ sites. X-ray diffraction analysis indicated that the ingot obtatined was a single crystal with few domains..
47. Sung Wng Kim, Yoshitake Toda, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Synthesis of a room temperature stable 12CaO·7Al2O 3 electride from the melt and its application as an electron field emitter, Chemistry of Materials, 10.1021/cm052367e, 18, 7, 1938-1944, 2006.04, A room temperature (RT) stable electride was realized by thermally annealing an insulating 12CaO·TAl2O3 (C12A7) single crystal in a calcium metal vapor. Here we report a simple and direct method for synthesizing polycrystalline C12A7 electride (C12A7:e-); the solidification of a "melt" in a reducing atmosphere and the crystallization of a "glass" with an oxygen-deficient composition in a vacuum. The carbon-related anion (C2
2-) presumably serves as the template for the formation of the C12A7 phase in the solidification process and may be spontaneously released from the lattice during the cooling process, leaving mobile electrons in the lattice. Also the C2 2 ions accommodated in the glass may play a significant role in the formation of C12A7:e- during the crystallization. The polycrystalline C12A7:e- exhibits an electrical conductivity up to 5 S·cm-1 at 300 K, which corresponds to an electron concentration of ∼3 × 1019 cm-3 and a nearly equal mobility of ∼0.1 cm2·W-1·s -1 to that of the single crystalline C12A7:e-. A large current density of ∼12 μA·cm-2 is obtained in the electron field emission from a flat surface of the polycrystalline C12A7:e - with a work function of ∼0.6 eV for an applied electric field of 2.4 × 105 V·cm-1. These simple synthetic methods make it possible to produce efficiently the electride in large volume, which will facilitate the C12A7:e- research, paving a way for various applications including cold electron emission..
48. Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Mechanisms of oxygen ion diffusion in a nanoporous complex oxide 12CaO7 Al2 O3, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.73.014101, 73, 1, 2006.02, We performed a theoretical analysis of O2- diffusion mechanisms in a nanoporous complex oxide 12CaO7 Al2 O3 (C12A7). This material can be viewed as a positively charged framework, arranged in subnanometer sized cages, hosting extra-framework O2- ions occupying one in six cages. Using both classical molecular-dynamics simulations and ab initio calculations we demonstrate that the diffusion of O2- species is dominated by the exchange of framework and extra-framework O2- ions rather than by an interstitial diffusion mechanism. The results allow us to rationalize the origins of the experimentally observed high oxide ion conductivity of C12A7 and the stability of its lattice under positive ion-beam irradiation..
49. Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Role of hydrogen atoms in the photoinduced formation of stable electron centers in H-doped 12CaO·7 Al2 O3, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.73.045120, 73, 4, 2006.02, In this work we investigate a variety of chemical and photoinduced processes in which different hydrogenous species including H2 molecules, H- ions, and H0 atoms interact with the bulk of a complex nanoporous oxide 12CaO·7 Al2 O3. Our results provide a detailed and consistent explanation of the recently observed phenomenon of photoinduced conversion of the insulating H-doped 12CaO·7 Al2 O3 to a conductor [K. Hayashi, Nature (London) 419, 462 (2002)]. The formation of a large and thermally stable concentration of electron centers in this process is facilitated by a large concentration (up to 1020 cm-3) of extraframework O2- naturally present in this material and homogeneously distributed in its bulk. We show that these species are able to split H2 molecules into pairs of H+ and H- ions and convert H0 atoms into H+ and e- promoting the photoinduced conversion process. The similarity of the mechanisms described in this work to those known for low-coordinated sites at MgO surfaces indicates that the formation of electronic centers in oxides interacting with hydrogenous species could be a generic feature..
50. Toshio Kamiya, Shouzou Aiba, Masashi Miyakawa, Kenji Nomura, Satoru Matsuishi, Katsuro Hayashi, Kazushige Ueda, Masahiro Hirano, Hideo Hosono, Field-induced current modulation in nanoporous semiconductor, electron-doped 12CaO·7Al2O3, Chemistry of Materials, 10.1021/cm051904s, 17, 25, 6311-6316, 2005.12, 12CaO·Al2O3 (C12A7) has a unique crystal structure composed of positively charged cages ∼0.4 nm in inner diameter and a free oxygen ion (O2-) clathrated in one-sixth of the cages. C12A7 can be converted to inorganic electride by replacing the clathrated oxygen ions with electrons, and the electride exhibits degenerate-type conduction with room-temperature conductivities >100 S cm-1. In intermediate states, semiconductive C12A7 can be obtained by controlling electron density. In this study, we examined effects of electric field on carrier transport properties of the semiconductive C12A7 using a field-effect transistor (FET) structure targeted for future mesoscopic devices and electrochemical devices that will utilize the quantum-dot-like cage structures and chemically active clathrated anions in C12A7. FETs were fabricated using two types of samples, (i) single-crystalline bulk and (ii) polycrystalline thin films, for channels. First, conditions to form good contacts for source and drain electrodes were examined because the semiconductive C12A7 has a small work function, and it was difficult to form good electrical contacts with metals. It was found that Pt was the best metal with the lowest contact resistance to C12A7, and thermal annealing at ≥300 °C improved its non-ohmic characteristics. Electrical conductivity was modulated by 1-2 orders of magnitude by applying gate voltage. Apparent field-effect mobilities were 0.02-0.08 cm2 (V s) -1, which were comparable with the drift mobilities of the semiconductive C12A7. This is the first demonstration of a semiconductor device using an electride..
51. Katsuro Hayashi, Peter V. Sushko, Alexander L. Shluger, Masahiro Hirano, Hideo Hosono, Hydride ion as a two-electron donor in a nanoporous crystalline semiconductor 12CaO·7Al2O3, Journal of Physical Chemistry B Materials, 10.1021/jp053990p, 109, 50, 23836-23842, 2005.12, The 12CaO·7Al203 (C12A7) crystal with a nanoporous lattice framework exhibits high electrical conductivity with an activation energy of ∼ 1.5 eV when equilibrated in a hydrogen atmosphere above ∼800°C. The high conductivity is preserved in a quenched state below ∼600°C with a reduced activation energy of ∼0.8 eV, Such complex behavior in electrical conductivity is associated with incorporation of hydride ions (H-) in cages of the lattice framework. Electromotive force measurements reveal that the major carrier for the conductivity is electron with a small contribution by proton (H+), ruling out the possibility of direct intercage migration of the H- ion. A combination of these observations with the ab initio calculations leads to the conclusion that the electrons are thermally generated from the H- ion by the dissociation into two electrons and an proton, which is further converted to an OH- ion via reaction with an extraframework oxide ion (02-). The energy difference between the initial (H- + O2-) and the final (2e- + OH-) states as evaluated by the theoretical calculation is as small as ∼1 eV, which agrees well with an experimentally obtained enthalpy change, ∼1.4 eV. Thus, internal equilibration between the extraframework hydrogen and the oxygen species is responsible for the thermal generation of the carrier electron. It is also suggested that the same conductive (2e- + OH-) state is reached by the photoirradiation of H--containing C12A7. In this case the photoionization of H- forms an electron and an H o atom, which then forms an OH- ion and another electron with thermal assistance. The persistence of photoinduced conductivity is explained by the slow kinetics of the reverse process at room temperature..
52. Yoshitake Toda, Sung Wng Kim, Katsuro Hayashi, Masahiro Hirano, Toshio Kamiya, Hideo Hosono, Takeshi Haraguchi, Hiroshi Yasuda, Intense thermal field electron emission from room-temperature stable electride, Applied Physics Letters, 10.1063/1.2149989, 87, 25, 1-3, 2005.12, Thermal field emission (TFE) from a flat surface of 12CaO·7 Al2 O3 (C12A7) electride was examined at temperatures up to 900 °C and applied external voltages of 0-6 kV in a 10-5 Pa vacuum. TFE started to occur at ∼650 °C and steeply increased at ∼900 °C to reach ∼80 μA (∼1.5 A cm-2) when an extraction field of 105 V cm-1 (an extraction voltage of 6 kV) was applied. The work function estimated from the Richardson-Dushman equation was ∼2.1 eV, which is rather smaller than that of La B6 (∼2.7 eV). It is experimentally confirmed that the emission with a current of ∼50 μA (from a 80 μm diameter area) was stably sustained for more than 90 h. The efficient and stable emission at relatively low temperature in a moderate vacuum atmosphere strongly suggests that the C12A7 electride has high potential for TFE applications..
53. Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Localisation assisted by the lattice relaxation and the optical absorption of extra-framework electrons in 12CaO•Al 2O 3, Materials Science and Engineering C, 10.1016/j.msec.2005.06.047, 25, 5-8, 722-726, 2005.12, The crystalline framework of a complex oxide 12CaO•7Al 2O 3 (C12A7) is built from positively charged sub-nanometer cages able to trap electrons. An embedded cluster approach is used to study the electronic structure and optical properties of electron-containing C12A7 for different concentration of extra-framework electrons. Our results suggest that the conduction band of the C12A7 consists of a relatively narrow band of empty cage states (cage conduction band) with one-electron energies between 5.5 and 6.1 eV above the top of the valence band (VB), and a framework conduction band at about 6.7 - 7.0 eV above the to of the VB. The gap between the cage conduction band and the framework conduction band is estimated to be about 1 - 1.5 eV. The energies of the lowest optical transitions from the top of the valence band to the cage conduction band are predicted to be at 5.2 - 5.5 eV. The extra-electrons occupy cage states, which split from the cage conduction band to about 4 eV above the top of the VB. The extra-framework electrons introduce two absorption bands with the experimental peaks at 0.4 eV and 2.8 eV. These bands are assigned to the inter-cage charge transfer and the intra-cage s-p transitions respectively. The changes in the optical absorption spectra as a function of the extra-electrons concentration are also discussed..
54. Satoru Matsuishi, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Hydride ion as photoelectron donor in microporous crystal, Journal of the American Chemical Society, 10.1021/ja053568m, 127, 36, 12454-12455, 2005.09, Atomic hydrogen (H0) and trapped electrons generated by UV illumination (λ 330 nm) at 4 K were observed using electron paramagnetic resonance (EPR) in a 12CaO·7Al2O3 (C12A7) crystal heated in a hydrogen atmosphere. The concentration ratio of generated H0 to the electrons encaged in the subnanometer-sized cages of C12A7 (F+ centers) is almost 1:1, providing direct evidence that a hydride ion, H-, accommodated in the cage by the heat treatment was dissociated to a pair of an H0 and an electron by a UV photon: H- → H0 + e- (F+). After annealing at 300 K, H0 was completely annihilated, while 60% of the trapped electrons survived. The remaining electrons can hop between neighboring cages and give electrical conductivity to C12A7. The hyperfine splitting of the EPR spectrum of H0 in C12A7 (48.6 mT) is 4% smaller than that of the neutral hydrogen atom (50.6 mT), implying that H0 is trapped at the interstitial sites among the cages..
55. Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Thermodynamics and kinetics of hydroxide ion formation in 12CaO·7Al 4O 3, Journal of Physical Chemistry B Materials, 10.1021/jp050807j, 109, 24, 11900-11906, 2005.06, We have examined the thermodynamics and kinetics of hydroxide (OH) ions that formed in cages of 12CaO·7Al 2O 3 (C12A7) with nanoporous structures. It is confirmed using thermogravimetric-evolved gas analyses (TG-EGA) that hydration in C12A7 is mediated by a reaction between an oxide (O 2-) ion in the cage and an H 2O molecule in the atmosphere to form two OH - ions in the cages. To simply and exactly quantify the OH - content from infrared absorption measurements of OH-stretching band, we propose a method combined with a thermodynamic analysis, allowing the simultaneous determination of the molar extinction coefficient of the OH-band, enthalpy, and entropy for the hydration. Hydration enthalpy in C12A7 is extremely high compared with other oxides and was enhanced by the marked instability of O 2- ion in the cage. Consequently, high solubility of OH - ion is retained up to unusually high temperatures. Furthermore, we determined diffusion coefficients of species relevant to the hydration process and demonstrated that inward diffusion of OH - ions is the rate-determining process..
56. Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Excess oxygen in 12CaO·7Al2O3 studied by thermogravimetric analysis, Chemistry Letters, 10.1246/cl.2005.586, 34, 4, 586-587, 2005.04, Oxygen radial (O2-, O-)-accommodated 12CaO·7Al2O3 (CI2A7) releases oxygen molecules at about 800 C when heated in helium. The corresponding weight loss nearly accords with that calculated from an amount of the accommodated radicals, providing solid evidence that they are formed by an oxygen gas uptake from the atmosphere. Small difference between the observed and calculated losses suggests the presence of peroxide ion, O2
2-, which is confirmed by the enhancement of 770 cm-1 band in Raman spectra of the radical-accommodated samples..
57. Takahisa Yamamoto, Yukio Sato, Tomohito Tanaka, Katsuro Hayashi, Yuichi Ikuhara, Taketo Sakuma, Electron transport behaviors across single grain boundaries in n-type BaTiO3, SrTiO3 and ZnO, Journal of Materials Science, 10.1007/s10853-005-6505-4, 40, 4, 881-887, 2005.02, In some electroceramic materials, their unique electrical properties are due to potential barriers, i.e., double Schottky barriers (DSBs), formed at grain boundaries. So far, some researchers have revealed that the electrical properties of DSB are closely related to grain boundary characters, especially grain boundary coherency. For example, highly coherent boundary does not give PTCR or varistic property, while random types exhibit clear resistivity jump or abrupt current increment. Therefore, a concept of grain boundary design will be required for future device manufacturing, even in bulk materials. But it has not been clarified yet why the electron transport behaviors depend on them. In order to address this question, it is necessary to carry out a systematic experiment focusing on single grain boundaries using well-defined bicrystals. In the present study, we have summarized our studies with a special interest in electron transport behavior across single grain boundaries for n-type BaTiO 3, SrTiO3 and ZnO..
58. Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Photoinduced generation of electron anions in H-doped nanoporous oxide 12CaO·7 Al2 O3
Toward an optically controlled formation of electrides, Applied Physics Letters, 10.1063/1.1871359, 86, 9, 1-3, 2005.02, We suggest, on the basis of detailed ab initio calculations of both ground and photoexcited states of H-doped 12CaO·7 Al2 O3 (C12A7:H), that stable high-conductivity regions can be optically generated in this insulating system. Each H- ion in C12A7:H can, under photoirradiation at 3.8-4.5 eV, produce up to two electrons and a proton that binds to the lattice network and forms OH-. Mobile electrons play a role of anions in this system. They move over subnanosized cages of the C12A7 lattice via polaron hopping. The insulating state is reversibly restored upon heating to 300 °C..
59. SungWng Kim, Masashi Miyakawa, Katsuro Hayashi, Takashi Sakai, Masahiro Hirano, Hideo Hosono, Simple and efficient fabrication of room temperature stable electride
Melt-solidification and glass ceramics, Journal of the American Chemical Society, 10.1021/ja043990n, 127, 5, 1370-1371, 2005.02, Electrides are ionic compounds in which electrons act as anions. These compounds are expected to have interesting properties arising from their exotic structure. The fatal drawbacks of the thermal and chemical instability of organic electrides were resolved by the synthesis of a room temperature (RT) stable electride using single crystalline 12CaO·7Al2O3 (C12A7) with a nanoporous structure and the chemical treatments for a long duration. However, an innovative fabrication method is obviously required for practical applications such as cold electron-emitter and thermionic devices. Herein we report a simple synthesis for polycrystalline C12A7 electrides with a moderate electronic conductivity via a strongly reducing C12A7 "melt", i.e., direct solidification of the melt or crystallization of the transparent glass. Generation of carrier electrons and precipitation of the C12A7 phase from the strongly reducing melt and glass are likely associated with the incorporation of carbon-related anions for stabilizing the C12A7 phase and keeping the mobile electrons in C12A7. These findings will be broadly utilized for applications by mass production in a desired shape and dimension, facilitating the research of electrides..
60. Katsuro Hayashi, Yoshitake Toda, Toshio Kamiya, Masahiro Hirano, Minako Yamanaka, Isao Tanaka, Takahisa Yamamoto, Hideo Hosono, Electronic insulator-conductor conversion in hydride ion-doped 12CaO·7Al2O3 by electron-beam irradiation, Applied Physics Letters, 10.1063/1.1852723, 86, 2, 2005.01, We report formation of persistent carrier electrons in hydride ion (H-) -incorporated 12CaO·7Al2O3 (C12A7) by electron-beam irradiation. The electrical conductivity of H- -doped C12A7 single crystals increases with the electron-beam irradiation dose, accompanied with a green coloration attributable to a carrier electron formation. A 25 keV electron beam with a dose of ~500 μC cm-2 fully converts the conductivity in surface layers to the depth of ~4 μm. Carrier electron formation is most likely due to electron-hole pairs generated in the electron excitation volume and subsequent energy transfer to the H- ions. The estimated carrier formation yield per an incident electron is ~30. These findings may enable a fine patterning of the conductive area without photomasks and photoresists..
61. Masashi Miyakawa, Yoshitake Toda, Katsuro Hayashi, Masahiro Hirano, Toshio Kamiya, Noriaki Matsunami, Hideo Hosono, Formation of inorganic electride thin films via site-selective extrusion by energetic inert gas ions, Journal of Applied Physics, 10.1063/1.1829151, 97, 2, 2005.01, Inert gas ion implantation (acceleration voltage 300 kV) into polycrystalline 12CaO·7 Al2 O3 (C12A7) films was investigated with fluences from 1× 1016 to 1× 1017 cm-2 at elevated temperatures. Upon hot implantation at 600 °C with fluences greater than 1× 1017 cm-2, the obtained films were colored and exhibited high electrical conductivity in the as-implanted state. The extrusion of O2- ions encaged in the crystallographic cages of C12A7 crystal, which leaves electrons in the cages at concentrations up to ~1.4× 1021 cm-3, may cause the high electrical conductivity. On the other hand, when the fluence is less than 1× 1017 cm-2, the as-implanted films are optically transparent and electrically insulating. The conductivity is enhanced and the films become colored by irradiating with ultraviolet light due to the formation of F+ -like centers. The electrons forming the F+ -like centers are photo released from the encaged H- ions, which are presumably derived from the preexisting OH- groups. The induced electron concentration is proportional to the calculated displacements per atom, which suggests that nuclear collision effects of the implanted ions play a dominant role in forming the electron and H- ion in the films. The hot ion implantation technique provides a nonchemical process for preparing electronic conductive C12A7 films..
62. Shuwu Yang, Junko N. Kondo, Katsuro Hayashi, Masahiro Hirano, Kazunari Domen, Hideo Hosono, Partial oxidation of methane to syngas over promoted C12A7, Applied Catalysis A: General, 10.1016/j.apcata.2004.09.030, 277, 1-2, 239-246, 2004.12, Partial oxidation of methane into syngas (CO and H2) was investigated over 12CaO·7Al2O3 (Cl2A7) promoted by metals such as Ni, Co, Pt, Pd and Ru. The effect of space velocity and metal loading was studied, and the catalytic behavior of Ni/Cl2A7 was compared with nickel catalysts supported on CaO, α-Al2O3 and other calcium aluminates with high CaO/Al2O3 ratio. On Ni, Pt and Pd/Cl2A7, this reaction readily happened at temperatures as low as 500 °C and attained thermodynamic equilibrium. Five samples were investigated at 800 °C and at a space velocity of 240,000 ml g-1 h-1. Their activity values decreased in the following order: 1% Pt/Cl2A7 > 5% Co/C12A7 > 5% Ni/C12A7 > 1% Ru/C12A7 > 1% Pd/Cl2A7. The activity and selectivity of Ni/Cl2A7 and Pt/Cl2A7 increased with metal loading, and the activity of 10% Ni/C12A7 has been comparable to that of 1% Pt/C12A7. Ni/C12A7 exhibitedlow coke formation rate and the material was more active than nickel supported on CaO, α-Al2O3, 3CaO·Al2O3 (C3A) or CaO·Al2O3 (CA), owing to the good dispersion of NiO on C12A7 and the existence of active oxygen ions incorporated in its nanocages..
63. Satoru Matsuishi, Katsuro Hayashi, Masahiro Hirano, Isao Tanaka, Hideo Hosono, Superoxide ion encaged in nanoporous crystal 12CaO-7Al2O 3y continuous wave and pulsed electron paramagnetic resonance, Journal of Physical Chemistry B Materials, 10.1021/jp046963s, 108, 48, 18557-18568, 2004.12, Continuous wave and pulsed electron paramagnetic resonance studies were performed on the Superoxide ion, O2-, encaged in the nanoporous crystal 12CaO'7Al2O3 (C12A7). Hyperfine splitting due to 17O in O2- was observed in C12A7 heated in 40% 17O-enriched O2 gas at 550 °C, providing solid evidence that encaged O2derives from gaseous O2 in the atmosphere via reactions with free oxygen ions in the cage. Angular variations of g values, 17O hyperfine splitting in the single crystal at 20 K, and 27Al electron spin-echo envelope modulation pattern for the powder at 4 K clarified that an O2- is encaged in a subnanometer-sized cage of C12A7 and adsorbed on a Ca2- ion forming a part of the cage wall. Further, two oxygen atoms in the O2ion are equivalent in the cage, indicating that the O2- ion takes a "side-on" configuration. Then, the O-O bond is perpendicular to the 2-fold rotation axis (C2 \\ (100)) across the center of the cage and directed to two oxygen ions forming the cage wall. Variation in the g values with temperature indicates that the O2- may be regarded as solidlike below ∼20 K and above this temperature the rocking motion of the O-O bond about the C2 axis is activated with an increase in temperature, but the anisotropy in the motion remains even at 400 K. This result may be explained by a view that Coulombic interaction of the Ca2- with the fully occupied π orbital in the O2 - is stronger than that with the semioccupied π orbital, controlling the dynamics..
64. Katsuro Hayashi, Naoto Ueda, Masahiro Hirano, Hideo Hosono, Effect of stability and diffusivity of extra-framework oxygen species on the formation of oxygen radicals in 12CaO-7Al2O3, Solid State Ionics, 10.1016/j.ssi.2004.07.057, 173, 1-4 SPEC. ISS., 89-94, 2004.09, The thermodynamics and kinetics of oxygen radical formation process in a nanoporous crystal, 12CaO-7Al2O3, were examined by isothermal annealing experiments in dry atmospheres. It is clarified that the formation of oxygen radicals, O-2 and O-, in the crystal occurs in the following three steps; outward diffusion of extra-framework oxide ions, its oxidation by absorbed oxygen molecules to form O- and O-2 at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extra-framework oxide ions is essentially the driving force for the process. Further, the analyses allow to predict the total oxygen radical content for a given annealing condition..
65. Hayato Kamioka, Hidenori Hiramatsu, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Photonic materials utilizing naturally occurring nanostructures, Journal of Photochemistry and Photobiology A: Chemistry, 10.1016/j.jphotochem.2004.04.033, 166, 1-3, 141-147, 2004.08, We report distinct optical/electrical properties inherent to build-in nanostructures in transparent oxide based materials, oxychalcogenides LaCuOS and LaCuOSe having naturally formed multi-quantum well structures and 12CaO·7Al2 O3 (C12A7) with a unique nanoporous structure. LaCuOSe doped with Mg ions exhibit degenerative p-type conduction presumable due to "modulation doping mechanism" with a high mobility of 4.0 cm2 V-1 s-1 even at high hole concentration (∼2×1020 cm-3). The room temperature exciton, having a large binding energy (∼50 meV) due to the confinement effect of carriers in the CuS or CuSe layer, induces a large optical nonlinearity at room temperature (RT) (χ(3) = 2-4×10-9 esu) near the absorption band. The H- ion entrapped in nanosize cages in C12A7 lattice causes an persistent conversion at RT from an electric insulator (<10-10 S cm-1) to a semiconductor (0.3-100 S cm-1) by capturing electrons photo-ionized from H- ions in the cages. This is a first electronic conductor based on main group light metal oxides..
66. Katsuro Hayashi, Satoru Matsuishi, Masahiro Hirano, Hideo Hosono, Formation of oxygen radicals in 12CaO·7Al2O3
Instability of extraframework oxide ions and uptake of oxygen gas, Journal of Physical Chemistry B Materials, 10.1021/jp037916n, 108, 26, 8920-8925, 2004.07, The thermodynamics and kinetics of oxygen radical formation in cages of a nanoporous crystal, 12CaO·7Al2O3, were examined during isothermal annealing in a dry oxygen atmosphere. The formation of oxygen radicals, O2
- and O-, involves the outward diffusion of extraframework oxide ions, their oxidation by absorbed oxygen molecules to form O- and O2
- at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extraframework oxide ions provides the driving force for this process. Evaluating the radical formation enthalpy and entropy enabled us to determine the temperature and oxygen partial pressure dependences of equilibrium oxygen radical concentration. The rate-limiting process for the radical formation is not the surface reaction, but the total ionic diffusion process, in which the smaller diffusivity of O2
- likely dominates the process..
67. Toshio Kamiya, Hiromichi Ohta, Hidenori Hiramatsu, Katsuro Hayashi, Kenji Nomura, Satoru Matsuishi, Kazushige Ueda, Masahiro Hirano, Hideo Hosono, Natural nanostructures in ionic semiconductors, Microelectronic Engineering, 10.1016/j.mee.2004.03.003, 73-74, 620-626, 2004.06, Nanostructures naturally formed in oxide semiconductor crystals are discussed in relation to their electronic structure and properties. Three different crystals are studied: (i) layered oxychalcogenides LnCuOCh (Ln = lanthanide, Ch = chalcogen), (ii) homologous series layered oxides RMO 3(ZnO)m (R = In, Lu etc., M = In, Ga etc., m = integer), and (iii) nanoporous crystal 12CaO7 · Al2O3 (C12A7). Coexistence of moderately large mobility, intense photoluminescence arising from room-temperature exciton and high-density hole is explained by a natural modulation doping structure in LnCuOCh. C12A7 exhibits different properties by replacing free O2- ions anions clathrated in its subnanometer-sized cages with O-, H-, and electron. The electronic structures of the clathrated anions were investigated by ab initio calculations and discussed in terms of possible quantum structures and applications..
68. Yoshitake Toda, Satoru Matsuishi, Katsuro Hayashi, Kazushige Ueda, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Field emission of electron anions clathrated in subnanometer-sized cages in [Ca24Al28O64]4+(4e-), Advanced Materials, 16, 8, 685-689+665, 2004.04, [Ca24Al28O64]4+(4e-) is a new inorganic electride, which is air-stable even at ∼ 300°C. Its electron field-emission properties originate from electron anions encaged in subnanometer-sized cages embedded in its crystal structure. und the emission current is controlled by Fowler-Nordheim field emission at high electric fields (see Figure). Field-emission display devices demonstrate bright light emission clearly visible under ambient light (inset)..
69. Katsuro Hayashi, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Direct characterization of grain-boundary electrical activity in doped (Ba0.6Sr0.4)TiO3 by combined imaging of electron-beam-induced current and electron-backscattered diffraction, Journal of the American Ceramic Society, 10.1111/j.1551-2916.2004.01153.x, 87, 6, 1153-1156, 2004.01, Simultaneous measurements of remote electron beam induced current (REBIC) and orientation imaging microscopy (OIM) in a scanning electron microscope (SEM) have been applied to a polycrystalline (Ba0.6Sr0.4)TiO3 with a positive temperature coefficient of resistivity (PTCR) to elucidate a grain-boundary character dependence of the potential barrier formation. The absence of electrical activity in a coherent Σ3 twin boundary is clearly imaged. The resistivity of individual grain boundaries estimated from a resistive contrast image is interpreted in terms of geometrical coherency, which is defined by the degree of coincidence in the reciprocal lattice points..
70. Shuwu Yang, Junko N. Kondo, Katsuro Hayashi, Masahiro Hirano, Kazunari Domen, Hideo Hosono, Formation and Desorption of Oxygen Species in Nanoporous Crystal 12CaO·7Al2O3, Chemistry of Materials, 10.1021/cm034755r, 16, 1, 104-110, 2004.01, The formation and desorption mechanisms of oxygen species in nanoporous crystal 12CaO· 7Al2O3 (C12A 7) are studied by temperature-programmed desorption (TPD), electron-spin resonance (ESR), and Raman spectroscopy. Various oxygen ion species, including O-, O2-, and O 22-, are observed to exist in C12A7 at high concentrations (∼1020 cm-3). The desorption of these oxygen species in C12A7 is proposed to proceed in three steps: (1) diffusion of oxygen species from bulk to the surface (rate-limiting step), (2) decomposition of oxygen species into oxygen molecules, and (3) release of molecular oxygen into the gas phase. Oxygen species in C12A7 can be regenerated by heating in an O2 atmosphere at higher than 400 °C, and the concentrations of O- and O2- increase with temperature. More O2 - is formed than O- at low activation temperatures, whereas nearly equal amounts of O- and O2- are formed at high temperatures around 1350 °C. At 400-700 °C, a considerable amount of molecular oxygen, O2, is suggested to be present together with other oxygen species. The formation of oxygen ion species occurs in a manner reverse to the desorption processes, with the first step involving diffusion of oxygen from the atmosphere into nanocages of C 12A7..
71. Masashi Miyakawa, Katsuro Hayashi, Yoshitake Toda, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Persistent electronic conduction in 12CaO·7Al2O 3 thin films produced by Ar ion implantation
Selective kick-out effect leads to electride thin films, Quaternary International, 811, 385-389, 2004.01, A new method to convert 12CaO·7Al2O3 (C12A7) thin films to electronic conductor by hot Ar+ ion implantation has been developed and its mechanism is discussed. It was found that hot Ar + ion implantation extruded free O2- ions in C12A7 films by kick-out effects at fluences higher than 1×1017 cm -2, which left electrons in the cages embedded in C12A7 crystal and produced high concentration F+-like centers (∼1.4×10 21 cm-3). The resulting films show coloration and persistent electronic conduction with conductivities up to ∼1 Scm -1. On the other hand, fluences less than 1×1017 cm-2 kept the films transparent and insulating..
72. Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Hopping and optical absorption of electrons in nano-porous crystal 12CaO·7Al2O3, Thin Solid Films, 10.1016/S0040-6090(03)01156-8, 445, 2, 161-167, 2003.12, Recently it has been discovered that a nano-porous main group oxide 12CaO·7Al2O3 (C12A7) can be converted from a wide-gap insulator to a good transparent conductor. Using ab initio modelling we explain good conductivity of this material by very small barriers for hopping of localised electrons between neighbouring positive cages. We show that optical absorption of C12A7 in infrared region and at energies higher than 2.7 eV is due to inter-cage and intra-cage electron transitions, respectively. The proposed mechanisms can be useful in further search for conducting transparent media..
73. Yoshitake Toda, Masashi Miyakawa, Katsuro Hayashi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Thin film fabrication of nano-porous 12CaO·7Al2O 3 crystal and its conversion into transparent conductive films by light illumination, Thin Solid Films, 10.1016/S0040-6090(03)01170-2, 445, 2, 309-312, 2003.12, Thin film fabrication of crystalline 12CaO·7Al2O 3 (C12A7) with zeolitic structure was examined, and their electrical and optical properties were measured. Polycrystalline thin films were prepared by post-annealing of amorphous films in oxygen atmosphere at temperatures above 800 °C. Choice of substrates was crucial for obtaining single-phase thin films. Although various oxide substrates (single crystals of Al 2O3, Y-stabilized ZrO2, MgO and silica glass) were examined, single-phase films were obtained only for MgO substrates and the other substrates reacted with the CaO component in the films during post-annealing. The optical band gap of C12A7 was evaluated to be 5.9 eV. Hydride ions were incorporated into the film by a thermal treatment in a hydrogen atmosphere at 1200 °C. The resulting transparent thin films were converted into transparent persistent electronic conductors exhibiting an electrical conductivity 6.2×10-1 Scm-1 at 300 K by ultraviolet light illumination. This is the first example of transparent conductive thin film in which conductive areas can be patterned directly by light..
74. Satoru Matsuishi, Yoshitake Toda, Masashi Miyakawa, Katsuro Hayashi, Toshio Kamiya, Masahiro Hirano, Isao Tanaka, Hideo Hosono, High-density electron anions in a nanoporous single crystal
[Ca24Al28O64]4+ (4e-), Science, 10.1126/science.1083842, 301, 5633, 626-629, 2003.08, We removed ∼100% of clathrated oxygen ions from the crystallographic cages in a single crystal of 12CaO·7Al2O3, leading to the formation of high-density (∼2 x 1021 cm-3) electrons highly localized in the cages. The resulting electron forms a structure that we interpret as an F+ center and migrates throughout the crystal by hopping to a neighboring cage with conductivity ∼100 siemens per centimeter, demonstrating that the encaged electron behaves as an anion. The electron anions couple antiferromagnetically with each other, forming a diamagnetic pair or singlet bipolaron. The resulting [Ca24Al28O64]4+(4e-) may be regarded as a thermally and chemically stable single crystalline "electride"..
75. Masashi Miyakawa, Katsuro Hayashi, Masahiro Hirano, Yoshitake Toda, Toshio Kamiya, Hideo Hosono, Fabrication of highly conductive 12CaO·7Al2O3 thin films encaging hydride ions by proton implantation, Advanced Materials, 10.1002/adma.200304925, 15, 13, 1100-1103, 2003.07, A method to obtain a polycrystalline thin film, where an amorphous C12A7 film was first deposited was developed. The effects of hydrogenation using thermal treatment in H2 atmosphere or proton implantation were investigated. The results indicated that the H-loaded C12A7 films provide a unique capability to realize semi-transparent electric circuits directly patterned by UV-light irradiation..
76. Katsuro Hayashi, Satoru Matsuishi, Naoto Ueda, Masahiro Hirano, Hideo Hosono, Maximum incorporation of oxygen radicals, O- and O2
-, into 12CaO·7Al2O3 with a nanoporous structure, Chemistry of Materials, 10.1021/cm020959g, 15, 9, 1851-1854, 2003.05, The effects of oxygen partial pressure (pO2) up to 400 atm on the generation of active oxygen radicals in 12CaO·7Al2O3 (C12A7) was examined using a hot isostatic pressing technique. Concentrations of superoxide (O2
-) and oxygen anion radicals (O-), which were analyzed from a combination of electron paramagnetic spin resonance (EPR) and Raman spectroscopy, increased with pO2. The total concentration of the radicals reached 1.7 × 1021 cm3, which is comparable to the theoretical maximum, confirmed that dominant radical incorporation process is O2- (cage) + O2 (atmosphere) → O- (cage) + O2
- (cage). The radical incorporated C12A7 has a large magnetic susceptibility that obeys Curie's law down to 2.5 K without magnetic ordering, although a weak exchange interaction was suggested by the line narrowing of the EPR signal..
77. Quanxin Li, Hideo Hosono, Masahiro Hirano, Katsuro Hayashi, Masateru Nishioka, Hideo Kashiwagi, Yoshifumi Torimoto, Masayoshi Sadakata, High-intensity atomic oxygen radical anion emission mechanism from 12CaO · 7 Al2O3 crystal surface, Surface Science, 10.1016/S0039-6028(02)02575-X, 527, 1-3, 100-112, 2003.03, 12CaO·7Al2O3 (C12A7) crystal is an interesting material where a large amount of O-(c) and O2-(c) can be formed and trapped in the cages when it is sintered in dry oxygen. The emissive features of this material including the emission species, temperature dependence, field dependence, emission branch ratio, emission stability, the effects of water on the emission intensity, have been investigated. The temperature and oxygen pressure dependences of the electrical conductivity have also been measured. The mechanisms of the O- emission have been discussed based on the investigations of the emission and electrical conductivity features..
78. Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Electron localization and a confined electron gas in nanoporous inorganic electrides, Physical Review Letters, 10.1103/PhysRevLett.91.126401, 91, 12, 2003.01, The nanoporous main group oxide [Formula presented] (C12A7) can be transformed from a wide-gap insulator to an electride where electrons substitute anions in cages constituting a positive frame. Our ab initio calculations of the electronic structure of this novel material give a consistent explanation of its high conductivity and optical properties. They show that the electrons confined in the inert positive frame are localized in cages and undergo hopping between neighboring cages. The results are useful for the understanding of behavior of confined electron gas of different topology and electron-phonon coupling, and for designing new transparent conductors, electron emitters, and electrides..
79. Katsuro Hayashi, Satoru Matsuishi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Photo-induced insulator-semiconductor transition in 12CaO·7Al 2O3 (C12A7), Materials Research Society Symposium - Proceedings, 747, 247-255, 2003.01, We report the first electronic conduction in main group metal oxides (MGOs) or light-metal oxides, which are represented by alkaline-earth oxides, alumina, and silica. They are believed to be never converted to an electronic conductor. One of the MGOs, 12CaO·7Al2O3 (C12A7), has optical transparency, but electrical insulation limits intrinsic form. It is characterized by sub-nanometer sized cages in the lattice framework. Hydride ion, H- was incorporated into the cage by a thermal treatment of C12A7 single crystals or ceramics in hydrogen atmosphere. The product, C12A7:H, was colorless, transparent and a good insulator having electronic conductivity less than 10-10 S·cm-1. We found that the C12A7:H exhibits a coloration of yellowish green corresponding to optical absorptions at 2.8 and 0.4 eV with simultaneous conversion into an electronic conductor with 0.3 S·cm-1 at 300 K upon a irradiation of ultraviolet light, The conductive state continued even after the irradiation was stopped. Inversion to the insulator occurred when heated more than ∼320°C accompanying with rapid decay of the optical absorptions. When temperature rose above 550°C, H2 gas was released from the sample and the photosensitivity was lost. We consider that high concentration of F +-like centers are created by photo-released electrons from the H- anions being captured by empty cages. Further, a migration of the electrons at the F+-like centers may be responsible for the conduction. The visible light absorption loss is estimated to be only 1% for a 200 nm thick conductive C12A7:H films. The present properties provides novel applications such as direct optical writing of conducting wires on insulating transparent media..
80. Katsuro Hayashi, Satoru Matsuishi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, Light-induced conversion of an insulating refractory oxide into a persistent electronic conductor, Nature, 10.1038/nature01053, 419, 6906, 462-465, 2002.10, Materials that are good electrical conductors are not in general optically transparent, yet a combination of high conductivity and transparency is desirable for many emerging opto-electronic applications. To this end, various transparent oxides composed of transition or post-transition metals (such as indium tin oxide) are rendered electrically conducting by ion doping. But such an approach does not work for the abundant transparent oxides of the main-group metals. Here we demonstrate a process by which the transparent insulating oxide 12CaO·7Al2O3 (refs 7-13) can be converted into an electrical conductor. H- ions are incorporated into the subnanometre-sized cages of the oxide by a thermal treatment in a hydrogen atmosphere; subsequent irradiation of the material with ultraviolet light results in a conductive state that persists after irradiation ceases. The photo-activated material exhibits moderate electrical conductivity (∼0.3 S cm-1) at room temperature, with visible light absorption losses of only one per cent for 200-nm-thick films. We suggest that this concept can be applied to other main-group metal oxides, for the direct optical writing of conducting wires in insulating transparent media and the formation of a high-density optical memory..
81. Katsuro Hayashi, M. Hirano, Q. X. Li, M. Nishioka, M. Sadakata, Y. Torimoto, S. Matsuishi, H. Hosonoa, Electric field emission of high density O- ions from 12CaO·7Al2O3 engineered to incorporate oxygen radicals, Electrochemical and Solid-State Letters, 10.1149/1.1498970, 5, 9, 2002.09, We succeeded in extracting intense O- ion beam by applying dc voltage to a microporous crystal 12CaO·7Al2O3 (C12A7) engineered to incorporate extremely large amounts of O- (1.3 × 1020 cm-3) and O2- (2.7 × 1020 cm-3) in crystallographic cages. The O- ion beam is monochromatic and the current density (∼2 μA/cm2) is higher by three orders of magnitude than the maximum value recorded for yttrium-stabilized ZrO2 (YSZ). A surface metal electrode, essential for the YSZ to dissociate a drifted O2- in the bulk into an O- and an electron on the surface, is not needed for the C12A7. The presence of extremely large concentrations of O- and O2- ions and fast oxygen-ion-conductive nature of C12A7 allow extraction of O- ions directly from the surface..
82. Q. X. Li, Katsuro Hayashi, M. Nishioka, H. Kashiwagi, M. Hirano, Y. Torimoto, H. Hosono, M. Sadakata, Absolute emission current density of O- from 12CaO7Al 2O3 crystal, Applied Physics Letters, 10.1063/1.1476958, 80, 22, 4259-4261, 2002.06, Strong and high purity O- negative ion emission has been observed from a synthesized crystal 12CaO7Al2O3 surface. A μA/cm2-level O- emission from this material has been achieved at the surface temperature of 800°C and the extraction field over 1000 V/cm, which is about three orders of magnitude higher than the current density emitted from the Y2O3-stabilized ZrO2 electrolyte surface. The strong emissivity of this material, as well as easy and economical fabrication, may provide a useful tool to generate the O- negative ion, which is expected to be one of the most important radicals for chemical syntheses and material modifications..
83. Quanxin Li, Katsuro Hayashi, Masateru Nishioka, Hideo Kashiwagi, Masahiro Hirano, Yoshifumi Torimoto, Hideo Hosono, Masayoshi Sadakata, Reproducibility of O- negative ion emission from C12A7 crystal surface, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 41, 5, 2002.05, Reproducibility, stability and oxygen-pressure dependence of O- negative ion emission from a synthesized crystal 12CaO · 7Al2O3 (C12A7) surface have been investigated. We found the continuous O- implantation into the C12A7 crystal can be simply realized by supplying electron and oxygen on the Au-deposited C12A7 surface via electrocatalytic reactions. The O- emission from the consumed C12A7 surface was recovered after the implantation operation (O2:0.13 Pa; 770°C) for 30 minutes. In contrast, the implantation influence on electron emission is negligible. The O- emission intensity is nearly constant in the O2 pressure lower than 1 × 10-3 Pa. The O- implantation mechanism will be addressed..
84. Katsuro Hayashi, Masahiro Hirano, Satoru Matsuishi, Hideo Hosono, Microporous crystal 12CaO·7Al2O3 encaging abundant O- radicals, Journal of the American Chemical Society, 10.1021/ja016112n, 124, 5, 738-739, 2002.02, Extremely high concentrations (>1020 cm-3) of active oxygenic radicals, O- and O2
-, have been created in the zeolitic crystal, 12CaO·7Al2O3 (C12A7), which can accommodate anions in its cavities. An increase in oxygen pressure and a decrease in water vapor pressure at high temperature enhance the formation of the radicals. The oxidation of Pt is observed on the surface of the material as a result of reaction with the active oxygens..
85. Satoshi Watauchi, Isao Tanaka, Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Crystal growth of Ca12Al14O33 by the floating zone method, Journal of Crystal Growth, 10.1016/S0022-0248(01)02038-3, 237-239, 1 4 I, 801-805, 2002.01, We have grown single crystals of Ca12Al14O33 (C12A7), a congruently melting compound, by the floating zone method. With the conventional method, the shape of the solid-liquid interface was concave, and many bubbles and cracks remained in the grown crystal. To reduce them, the crystal growth at a lower growth rate and the control of the shape of the solid-liquid interface were examined. For the growth rate <0.3 mm/h, the amount of bubbles was reduced considerably. However, in such a condition, a significantly large bubble was often formed in the molten zone, which disturbed the stability of the molten zone. Introducing an alumina tube as a heat reservoir at a suitable position in the heated area, the shape of the solid-liquid interface could be changed from concave to a convex one. This may suggest that the effect of the concave interface could be eliminated and crystals with high quality may be grown at a higher growth rate..
86. H. Hosono, H. Ohta, Katsuro Hayashi, M. Orita, M. Hirano, Near-UV emitting diodes based on a transparent p-n junction composed of heteroepitaxially grown p-SrCu2O2 and n-Zno, Journal of Crystal Growth, 10.1016/S0022-0248(01)01951-0, 237-239, 1 4 I, 496-502, 2002.01, A near-UV emitting diode was fabricated by successive heteroepitaxial growth of In2O3:Sn(ITO), n-electrode, n-ZnO, and p-SrCu2O2 (SCO) on an extremely flat surface of yttria-stabilized ZrO2 single crystalline substrate using a pulsed-laser deposition technique. Upon current injection at room temperature, the resulting diode emitted near-UV light with a peak at 382 nm. The threshold voltage for emission was ∼ 3 V, which is in agreement with the energy difference between the ZnO donor level and the SCO acceptor level. The epitaxial relationship between SCO and ZnO depends on the deposition temperature of SCO on ZnO. The inhomogeneity of the SCO crystal was improved by raising the deposition temperature from 350°C to 600°C. The threshold current for near-UV emission from the diode fabricated by deposition of the SCO film at 600°C was reduced to ∼ 1/5 of that at 350°C. A reason why this heteroepitaxial growth occurs for oxides with large lattice misfit is explained in terms of the high ionicity of chemical bonds in oxides..
87. Katsuro Hayashi, Masahiro Hirano, Hideo Hosono, Translucent ceramics of 12CaO · 7Al2O3 with microporous structure, Journal of Materials Research, 10.1557/JMR.2002.0185, 17, 6, 1244-1247, 2002.01, We examined the densification of 12CaO · 7Al2O3 (C12A7), which has a microporous lattice framework with a cavity of approximately 0.4 nm. Fully densified translucent ceramics are obtained when hydrated C12A7 powders prepared as precursors are sintered in a dry oxygen atmosphere at 1300°C. The average transmittance between 400-800 nm for 1-mm-thick samples is improved up to approximately 70% with a 48-h increase in the sintering time. Dissociation of water molecules at the grain-pore interface into grains which entraps hydroxide ions in the crystallographic cage, and the release of the hydroxide ions into the atmosphere during the sintering procedure are considered to play crucial roles in the pore-annihilation processes..
88. Katsuro Hayashi, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Formation of potential barrier related to grain-boundary character in semiconducting barium titanate, Journal of the American Ceramic Society, 83, 11, 2684-2688, 2000.11, Resistance-temperature (R-T) characteristics were measured directly at single-grain boundaries in 0.1-mol%-niobium-doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain-boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low-Σ boundaries exhibit a similar grain-boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R-T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain-boundary character is likely to affect the diffusivity of O2- ions and, hence, is crucial to the formation of the potential barrier..
89. Takahisa Yamamoto, Katsuro Hayashi, Yuichi Ikuhara, Taketo Sakuma, Current-voltage characteristics of Σ1 boundaries with and without cobalt ions in niobium-doped SrTiO3 bicrystals, Journal of the American Ceramic Society, 10.1111/j.1151-2916.2000.tb01423.x, 83, 6, 1527-1529, 2000.01, Current-voltage (I-V) characteristics across Σ1 boundaries of Nb-doped SrTiO3 bicrystals were investigated with a special interest in the effect of an impurity ion, cobalt. The Σ1 boundary with cobalt ions was fabricated by hot-joining technique after evaporating metallic cobalt onto the (001) plane used for a contact plane. High-resolution transmission electron microscopy (HREM) study associated with X-ray energy dispersive spectroscopy (EDS) revealed that cobalt solved and distributed around the Σ1 boundary which was perfectly joined without any second films. It was found that the Σ1 boundary with cobalt ions exhibited a nonlinear I-V relationship while the Σ1 boundary without cobalt ions showed a linear I-V relationship. This result indicates that the presence of cobalt itself at the boundary can form a potential barrier irrespective of grain boundary coherency..
90. Takahisa Yamamoto, Katsuro Hayashi, Yuichi Ikuhara, Taketo Sakuma, Grain boundary structure and electrical properties in Nb-doped SrTiO3 bicrystals, Key Engineering Materials, 181-182, 225-228, 2000.01, Grain boundary structure and electrical properties were examined for Nb-doped SrTiO3 bicrystals. Bicrystals with Σ1, cobalt-segregated Σ1, and small angle boundaries were prepared by hot joining technique. High-resolution transmission electron microscopy (HRTEM) study revealed that two single crystals were perfectly joined without any secondary phases in all kinds of bicrystals. Σ1 boundary exhibited linear I-V characteristic while non-linear I-V behavior appeared in cobalt-segregated Σ1 boundary in spite of the similar coherent boundary. On the other hand, it was found that non-linearity became remarkable by reduction of cooling-rate after joining even in a small angle boundary, whose dislocation structure is stable against annealing treatment. This result suggests that negative-charged point defects must play an important role on the formation of potential barriers..
91. Katsuro Hayashi, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Misorientation dependence of grain boundary resistivity in Nb-doped barium titanate, Key Engineering Materials, 181-182, 51-54, 2000.01, The influence of orientation of grain boundary plane as well as misorientation of grain boundary on positive temperature coefficient of resistivity (PTCR) was examined in 0.1mol%Nb-doped barium titanate. The magnitude of the PTCR effect mainly depends on tilt component of misorientation as compared to twist one. This dependence is considered to be related to the fact that grain boundary with low-index plane has low grain boundary energy, and hence hinders the formation of potential barrier..
92. T. Yamamoto, Katsuro Hayashi, Y. Ikuhara, T. Sakuma, Chemical bonding state at grain boundaries in BaTiO3doped with a small amount of cation, Philosophical Magazine Letters, 10.1080/095008399177165, 79, 6, 327-331, 1999.01, The chemical bonding state at grain boundaries was examined with electron- energy-loss spectroscopy in BaTiO3doped with a small amount of cations at a level of 0.001. In contrast with A-site excess compounds, the oxygen K-edge electron energy-loss near-edge structure spectra were different for grain interiors and for grain boundaries in B-site excess compounds. The difference is explained in terms of the linkage of TiO6octahedra at grain boundaries in the B-site excess BaTiO3..
93. Katsuro Hayashi, Takahisa Yamamoto, Yuichi Ikuhara, Taketo Sakuma, Grain boundary electrical barriers in positive temperature coefficient thermistors, Journal of Applied Physics, 10.1063/1.371140, 86, 5, 2909-2913, 1999.01, Both current-voltage (I-V) and capacitance-voltage (C-V) characteristics were studied in Nb-doped BaTiO3 bicrystals to verify the potential barrier model. The potential barrier of n-i-n structure can reasonably describe the observed data in comparison with that of double Schottky barrier (DSB) model. The difference in the I-V characteristics between the two models is characterized by the appearance of the sub-ohmic behavior with α<1. In the DSB model, the barrier height is kept almost the same because the acceptor states are strongly pinned by the Fermi level. When the acceptor states are completely occupied, the potential barrier will collapse and then the current will increase steeply..
94. Katsuro Hayashi, T. Yamamoto, T. Sakuma, Grain Boundary Potential Barrier in Barium Titanate PTC Ceramics, Key Engineering Materials, 157-158, 199-206, 1999.01, A numerical calculation based on a drift-diffusion model was made to explain the experimental current-voltage (J-V) characteristics at a single grain boundary in Nb-doped BaTiO3. The temperature dependence of breakdown voltage in J-V characteristics can be reasonably explained by taking into account the change in occupation of interface states. The nonlinearity coefficients in J-V characteristics are interpreted in terms of energetic distribution of interface states and spatial distribution of the states near the grain boundary..
95. T. Yamamoto, Y. Ikuhara, Katsuro Hayashi, T. Sakuma, Grain boundaries in BaTiO3 with a small cation nonstoichiometry, Key Engineering Materials, 169, 113-116, 1999.01, Grain boundary structure in BaTiO3 doped with a small amount of dopants was examined. It was found that grain growth behavior was very sensitive to the substituting sites of dopants, i.e., A- or B-site in a perovskite structure ABO3. Abnormal grain growth takes place in B-site type dopant excess BaTiO3 while normal grain growth occurs in A-site type dopant excess BaTiO3. High-resolution electron microscopy (HRTEM) study associated with electron energy-loss spectroscopy (EELS) revealed that the grain boundaries were facetted with {210} type habits and extra Ti-O2 bonds were formed along the facetted grain boundaries in the compounds doped with B-site type dopants. Controlling the doping ratio of A- and B-site type dopants severely, coarse-grained sinters whose grain size was about 1 mm could be obtained..
96. Katsuro Hayashi, T. Yamamoto, Y. Ikuhara, T. Sakuma, Grain boundary character dependence of potential barrier in barium titanate, Materials Science Forum, 294-296, 711-714, 1999.01, Positive temperature coefficient of resistivity (PTCR) effect is directly measured in bicrystal specimens of Nb-doped BaTiO3 to examine its grain boundary character dependence and to clarify the nature of the potential barrier formed at grain boundaries. The coherency of grain boundaries is evaluated from a geometric method based on the coincidence of reciprocal lattice points (CRLP). Larger PTCR effects were obtained for random grain boundaries rather than coherent boundaries. The formation of the potential barrier is discussed from an analysis of the resistance-temperature characteristics..
97. T. Yamamoto, Y. Ikuhara, Katsuro Hayashi, T. Sakuma, Grain boundary structure in BaTiO3 with a small excess of Ti-site dopant, Materials Science Forum, 294-296, 247-250, 1999.01, Grain boundary structure was examined with HRTEM and EELS for BaTiO3 doped with a small amount of cations with a special interest in chemical bonding state. It was found that the grain boundary structure was sensitive to a small excess of Ti-site dopant. The grain boundaries arc facetted with {210} habits and an ordered structure is formed. HRETM and EELS examination combined with molecular orbital calculation clearly revealed that TiO6 octahedral linkage changes from corner-sharing in the grain interior to edge-sharing at the grain boundary..
98. Noriyasu Kataoka, Katsuro Hayashi, Takahisa Yamamoto, Yoshihiro Sugawara, Yuichi Ikuhara, Taketo Sakuma, Direct observation of the double Schottky barrier in niobium-doped barium titanate by the charge-collection current method, Journal of the American Ceramic Society, 81, 7, 1961-1963, 1998.07, Charge-collection (CC) current was measured at a single grain boundary, which exhibited positive temperature coefficient of resistivity (PTCR) effects, in 0.1-mol%-Nb-doped BaTiO 3. The CC current systematically reversed across the grain boundary above the Curie point, which indicated the presence of a double Schottky barrier (DSB) at the grain boundary. In contrast, the CC current was constant across the grain boundary below the Curie temperature. The result obtained from the CC current measurement agrees with the classical DSB model in donordoped BaTiO 3, indicating the PTCR effects..
99. Takahisa Yamamoto, Yuichi Ikuhara, Katsuro Hayashi, Taketo Sakuma, Grain boundary structure in TiO2-excess barium titanate, Journal of Materials Research, 10.1557/JMR.1998.0469, 13, 12, 3449-3452, 1998.01, Grain boundary structure was examined in 0.1 mol% TiO2-excess BaTiO3 by high-resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). Their grain boundaries were mostly faceted with {210} type habit. The faceted boundaries were characterized to be associated with an extra Ti-O2 bond with the rutile-like structure. The grain growth behavior in a small TiO2-excess BaTiO3 is discussed from the viewpoint of grain boundary structure..
100. Katsuro Hayashi, Takahisa Yamamoto, Taketo Sakuma, Grain orientation dependence of the PTCR effect in niobium-doped barium titanate, Journal of the American Ceramic Society, 10.1111/j.1151-2916.1996.tb08780.x, 79, 6, 1669-1672, 1996.04, The positive temperature coefficient of resistivity (PTCR) effect is directly measured in single grain boundaries in 0.1-mol%-Nb-doped BaTiO3 with 1 mm coarse grains. The PTCR effect largely depends on grain boundary structure. Random grain boundaries exhibit the PTCR effect as in polycrystalline samples, but the PTCR effect does not appear in highly coherent boundaries such as small-angle boundaries, twin boundaries, and coincidence site lattice (CSL) boundaries with low ∑ values. For ∑= 3 boundaries, the resistance increase above the Curie temperature is a function of deviation angle. A small PTCR effect is observed in ∑ = 3 boundaries with a deviation angle of about 9° in contrast with ideal ∑ = 3 boundaries and boundaries with a deviation of about 4°..