Details of a Researcher - Einaga, Yasuaki

Papers - Einaga, Yasuaki

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Homoepitaxial single-crystal boron-doped diamond electrodes for electroanalysis

Kondo Takeshi, Einaga Yasuaki, Sarada Bulusu V., Rao Tata N., Tryk Donald A., Fujishima Akira

Journal of the Electrochemical Society 149 ( 6 ) 2002.06

ISSN 0013-4651

In order to study the properties of single-crystal boron-doped diamond electrodes for electroanalysis, (100) and (111) boron-doped homoepitaxial single-crystal and polycrystalline diamond thin films were deposited by means of microwave plasma-assisted chemical vapor deposition. Features in the cyclic voltammograms (CVs) for aqueous H₂SO₄ supporting electrolyte and Fe(CN)₆^3-/4- in aqueous Na₂SO₄ for polycrystalline electrodes were dominated by behaviour typical of (111) single-crystal facets, rather than (100) facets. Apparent heterogeneous electron-transfer rate constants (k⁰) for various redox systems were estimated with CV simulation at polycrystalline, (111) and (100, off-axis, 4°). Based on comparison with the Marcus model, the behavior of the k⁰ values, except for that of Fe(CN)₆^3-/4-, and indicates apparent outer-sphere electron-transfer, but with a lower density of states compared to glassy carbon or typical metal electrodes. The (111) homoepitaxial film performed better than a polycrystalline film in the cyclic voltammetric detection of serotonin, with signal-to-background ratios of 5 and 2, respectively. Single-crystal diamond may thus be an even more optimal electrode material for electroanalysis than polycrystalline diamond, which has been shown to have superior characteristics as an electrode material.

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Fabrication and electrochemical characterization of boron-doped diamond microdisc array electrodes

Tsunozaki Kentaro, Einaga Yasuaki, Rao Tata N., Fujishima Akira

Chemistry Letters ( 5 ) 502 - 503 2002.05

ISSN 0366-7022

Electrochemical glucose detection using nickel-implanted boron-doped diamond electrodes

Ohnishi Keisuke, Einaga Yasuaki, Notsu Hideo, Terashima Chiaki, Rao Tata Narasinga, Park Soo Gil, Fujishima Akira

Electrochemical and Solid-State Letters 5 ( 3 ) 2002.03

ISSN 1099-0062

Nickel-implanted boron-doped diamond electrodes (Ni-DIA) were fabricated in view of their application for carbohydrate detection. This electrode produced well-defined and reproducible voltammograms for 1 mM glucose in alkaline media. The electrode exhibited excellent electrochemical stability with low background current even after ultrasonic treatment, indicating the strong bonding of nickel with carbon. These results indicate the promising use of Ni-DIA for the detection of carbohydrates and amino acids, and thus, an application of ion implantation as the surface modification method is effective for controlling the electrochemical properties of polycrystalline diamond thin films.

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Factors controlling the electrochemical potential window for diamond electrodes in non-aqueous electrolytes

Yoshimura M., Honda K., Kondo T., Uchikado R., Einaga Y., Rao Tata N., Tryk D. A., Fujishima A.

Diamond and Related Materials 11 ( 1 ) 67 - 74 2002.01

ISSN 0925-9635

The electrochemical behavior of as-deposited diamond was examined in various non-aqueous solvents and supporting electrolytes. The potential for the onset of anodic current was found to be dependent on the nature of the organic solvent, whereas that for the onset of cathodic current was dependent on the type of supporting salt cation. These results indicate that outer-sphere decomposition reactions of the organic solvents are the most important in determining the oxidation potential limit, and those involving cations are the most important in determining the reduction potential limit. These oxidations and reductions were correlated to the HOMO and LUMO energies of the solvents and the supporting electrolyte cations, respectively. The correlations were found to be linear. Electrical double-layer capacitance values were also found to be dependent on the type of electrolyte, and correlations were observed between the capacitance values and the molecular volumes of the solvent as well as between the capacitance and the volumes of the supporting electrolyte cations. The experimental results clearly indicate that the electrical double-layer capacitance values decrease, whereas the thickness of the Helmholz layer increases with increasing volume of the solvated cations. © 2002 Elsevier Science B.V. All rights reserved.

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Radio frequency GDOES depth profiling analysis of a B-doped diamond film deposited onto Si by microwave plasma CVD

Shimizu K., Einaga Y., Ohnishi K., Fujishima A., Habazaki H., Skeldon P., Thompson G. E.

Surface and Interface Analysis 33 ( 1 ) 35 - 40 2002.01

ISSN 0142-2421

Analysis of a boron-doped diamond film, ∼13 μm thick, deposited onto a mirror-finished n-Si(100) substrate using microwave plasma chemical vapour deposition (CVD), demonstrates the suitability of radiofrequency glow discharge optical emission spectroscopy (rf-GDOES) for rapid depth profiling analysis. The distributions of boron and hydrogen are revealed clearly. Intrestingly, the sputtering of the diamond film is accompanied by smoothing of the initial rough, faceted surface, implying that the ridges sputter more rapidly than the valleys during analysis.

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Assembly of highly ordered three-dimensional porous structure with nanocrystalline TiO₂ semiconductors

Meng Q. B., Fu C. H., Einaga Y., Gu Z. Z., Fujishima A., Sato O.

Chemistry of Materials 14 ( 1 ) 83 - 88 2002

ISSN 0897-4756

In this paper, we report on the assembly of a highly ordered three-dimensional porous structure with nanosized crystalline TiO₂ particles by a cooperative assembly method in which the fabrication of the template and the infiltration of the voids of the template are carried out at the same time and the related experimental parameters for the assembly, including temperature, humidity, and concentrations and concentration ratio of the colloid mixture. SEM (scanning electron microscope) images and transmission spectra of these samples demonstrate that these films have a highly ordered three-dimensional structure. The Bragg law was used to calculate the diameter of the spheres of air in the porous TiO₂ structure. A good agreement between the calculated results for the diameter of the spheres of air and those measured by SEM further confirms the high quality of the films fabricated using this simple method. Additionally, based on these experimental results, a detailed mechanism of the simple method is also discussed.

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The electrooxidation of sulfur-containing compounds at boron-doped diamond electrode

Chailapakul O., Aksharanandana P., Frelink T., Einaga Y., Fujishima A.

Sensors and Actuators, B: Chemical 80 ( 3 ) 193 - 201 2001.12

ISSN 0925-4005

The electrooxidation of homocysteine, glutathione (GSH), 2-mercapto ethanesulfonic acid and cephalexin at boron-doped diamond (BDD) electrodes and glassy carbon (GC) electrodes was investigated by cyclic voltammetry. The oxidation of these sulfur-containing compounds exhibited well-defined irreversible responses. This preliminary study has shown that BDD has better sensitivity than GC. Concentration dependence has been studied and indicated the promise of using BDD electrodes for quantitative determination. All of the compounds displayed recognizable oxidation peaks at BDD electrodes at millimolar concentration levels. Scan rate dependence of GSH has been examined. It was shown that there was no adsorption on the surface of the BDD electrode for the low concentration. © 2001 Elsevier Science B.V. All rights reserved.

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Pressure-stabilized low-spin state for binuclear iron(III) spin-crossover compounds

Hayami Shinya, Hosokoshi Yuko, Inoue Katsuya, Einaga Yasuaki, Sato Osamu, Maeda Yonezo

Bulletin of the Chemical Society of Japan 74 ( 12 ) 2361 - 2368 2001.12

ISSN 0009-2673

Binuclear iron(III) spin-crossover complexes with salten ligand [Fe ₂(salten) ₂(L)](BPh ₄) ₂ were synthesized and characterized by single-crystal X-ray diffraction, M6ssbauer spectra, magnetic susceptibilities and electronic spectra, where H ₂salten is a pentadentate ligand derived from salicylaldehyde and 3,3′-diaminodipropylamine and L is a didentate axial ligand (az = azobis(4-pyridine) and cc = 4,4′-vinylenebis(pyridine)). The structures of [Fe ₂(salten) ₂(az)](BPh ₄) ₂ (1) and [Fe ₂(salten) ₂(cc)](BPh ₄) ₂ (2) were determined at both 100 K and 298 K. Crystal data for 1 at 100 K: C ₉₈H ₉₄ O ₄N ₁₀B ₂Fe ₂, monoclinic, space group P2 ₁/c, Z = 2, a = 15.986(2) Å, b = 15.825(2) Å, c = 16.441(2) Å, β = 107.902(2)°, V = 3957.8(8) Å ³. Crystal data for 2 at 100 K: C ₁₀₀H ₉₆ O ₄N ₈B ₂Fe ₂, monoclinic, space group P2 ₁/a, Z = 2, a = 16.4296(8) Å, b = 15.747(1) Å, c = 16.637(1) Å, β = 109.2375(9)° and Z = 2. The complexes 1 and 2 exhibited the spin-crossover behavior depending on temperature within the temperature range of 200 to 350 K, and rapid spin equilibrium behavior was observed by means of M6ssbauer spectroscopy at 293 K. The complexes 1 and 2 also exhibited a pressure-induced spin transition between the rapid spin equilibrium state and low-spin state at 300 K. Such a pressure-induced spin transition has not been reported earlier.

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Varying the optical stop band of a three-dimensional photonic crystal by refractive index control

Gu Zhong Ze, Kubo Shoichi, Qian Weiping, Einaga Yasuaki, Tryk Donald A., Fujishima Akira, Sato Osamu

Langmuir 17 ( 22 ) 6751 - 6753 2001.10

ISSN 0743-7463

A method was developed to continuously adjust the position of the stop band of inverse opal by controlling the refractive index. The inverse opal films were fabricated using a coassembly of monodispersed polystyrene spheres and nanoparticles of silica and titania from aqueous suspensions followed by a calcination treatment. Varying the ratio of silica and titania in the suspensions gave the ability to finely adjust the refractive indices, which as a result changed the stop band.

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Crystal structure and photo-reaction of a new coordination polymer

Gu Zhong Ze, Sato Osamu, Einaga Yasuaki, Kai Masahiro, Iyoda Tomokazu, Fujishima Akira, Hashimoto Kazuhito

Bulletin of the Chemical Society of Japan 74 ( 9 ) 1617 - 1622 2001.09

ISSN 0009-2673

A new photo-responsive coordination polymer of [Ni(en) ₂] ₄[Fe(CN) ₅NO] ₂ [Fe(CN) ₆]·6H ₂O synthesized. In this compound, Ni-CN-Fe forms one-dimensional standing-wave-like chains, onto which photo-functional groups of pentacyanonitrosylferrate(II) are appended. Photo-irradiation with blue light at less than 200 K transformed the pentacyanonitrosylferrate(II) from the ground state to a metastable state, which could be reverted to the original state by irradiation with red light.

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Preparation of graded-morphology diamond thin films

Einaga Yasuaki, Kim Gyu Sik, Ohnishi Keisuke, Park Soo Gil, Fujishima Akira

Materials Science and Engineering B: Solid-State Materials for Advanced Technology 83 ( 1-3 ) 19 - 23 2001.06

ISSN 0921-5107

Graded-morphology diamond thin films were prepared in a single experimental run using a simple technique during the chemical vapor deposition (CVD) process: one end of the substrate was elevated. We showed that the substrate temperature had a profound effect on the morphology and grain size of the as-deposited lightly boron-doped diamond films. We have discussed the morphological dependence of electrochemical properties as one example of the excellent function of the graded film. © 2001 Elsevier Science B.V.

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A novel LIESST iron(II) complex exhibiting a high relaxation temperature

Hayami S., Gu Z. Z., Einaga Y., Kobayasi Y., Ishikawa Y., Yamada Y., Fujishima A., Sato O.

Inorganic Chemistry 40 ( 13 ) 3240 - 3242 2001.06

ISSN 0020-1669

The spin-crossover iron(II) complex [Fe(L)(CN)₂·H₂O (1) has been investigated. Complex 1 has a pentagonal bipyramidal structure which is seven-coordinated, containing an N₃O₂ macrocycle and two CN^- ligands. Water molecules bind the complex by means of hydrogen bonding. It was found that the complex 1 exhibited a light-induced excited spin state trapping (LIESST) effect. The relaxation temperature is above 130 K, which is the highest temperature for LIESST compounds reported so far.

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A study of the crystalline growth of highly boron-doped CVD diamond

Einaga Yasuaki, Kim Gyu Sik, Park Soo Gil, Fujishima Akira

Diamond and Related Materials 10 ( 3-7 ) 306 - 311 2001.03

ISSN 0925-9635

An investigation was made of graded-morphology diamond thin films deposited on Si substrates by use of the microwave plasma chemical vapor deposition (CVD) technique. The preparation of graded diamond thin films not only offers new perspectives into functional materials, but it also gives clues to the growth mechanism of CVD diamond. Although it is clear that the substrate temperature and the deposition time affect the grain size in particular, the difference depending on the boron concentration in the growing process was studied in the present work. As a result, in highly boron-doped (10⁴ ppm) diamond films, the Raman peak which was ascribed to the boron-doping was not observed at the very initial growth stage. However, the crystal growth process was almost irrelevant for the boron-doping quantity. Furthermore, we showed the morphology dependence of electrochemical properties as one example of the excellent functions of the graded-morphology highly boron-doped diamond film. © 2001 Elsevier Science B.V. All rights reserved.

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Fabrication of structured porous film by electrophoresis [2]

Gu Z. Z., Hayami S., Kubo S., Meng Q. B., Einaga Y., Tryk D. A., Fujishima A., Sato O.

Journal of the American Chemical Society 123 ( 1 ) 175 - 176 2001.01

ISSN 0002-7863

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Photo- and dehydration-induced charge transfer processes accompanied with spin transition on CoFe(CN)₅NH₃·6H₂O

Gu Z. Z., Einaga Y., Sato O., Fujishima A., Hashimoto K.

Journal of Solid State Chemistry 159 ( 2 ) 336 - 342 2001

ISSN 0022-4596

The coexistence of different valences of iron and cobalt in cobalt pentacyanoferrate, CoFe(CN)₅NH₃·6H₂O, was confirmed by the infrared spectrum, in which two distinct peaks for the C-N stretch at 2162 cm^-1 and 2117 cm^-1 were assigned to CN in Fe^III-CN-Co^II and Fe^II-CN-Co^III respectively, and by the Mössbauer spectrum, in which Fe^III and Fe^II exhibited doublets with different isomer shifts and different quadrupole splitting. Photo-irradiation of this compound with red light changed the population of these two electronic states due to the internal electron transfer from Fe^II to Co^III. As a result, the ferrimagnetic properties of this pentacyanometalate at low temperature can be changed by photo-irradiation similar to the case of the hexacyanometalate, K_0.2Co_1.4Fe(CN)₆·6.9H₂O reported previously (O. Sato et al., Science 272, 704 (1996)). Moreover, the fraction of these two electronic states in this pentacyanoferrate is sensitive to the number of water molecules, which induces a charge transfer from Co^II and Fe^III. As a result, dehydration increases the fraction of Fe^II-CN-Co^III and causes the compound to be paramagnetic even at low temperature. © 2001 Academic Press.

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Reversible photoinduced switching of magnetic properties at room temperature of iron oxide particles in self-assembled films containing azobenzene

Einaga Y., Gu Z. Z., Hayami S., Fujishima A., Sato O.

Thin Solid Films 374 ( 1 ) 109 - 113 2000.10

ISSN 0040-6090

The use of organized organic assemblies to direct the formation of mesoscopic inorganic structures presents many possibilities in the development of new functional materials. We have designed nano-films containing iron oxide particles, which exhibit magnetic properties at room temperature, and azobenzene by use of an alternate adsorption technique. Photoisomerization of the film was accompanied by a geometrically confined structural change, as reflected by changes in the electrostatic field. As a result, we were able to control the magnetic properties of this film by photoillumination at room temperature.

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First observation of light-induced excited spin state trapping for an iron(III) complex [7]

Hayami Shinya, Gu Zhong Ze, Shiro Motoo, Einaga Yasuaki, Fujishima Akira, Sato Osamu

Journal of the American Chemical Society 122 ( 29 ) 7126 - 7127 2000.07

ISSN 0002-7863

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Hetero metal spin-crossover complex with LIESST iron(II) building block

Hayami Shinya, Gu Zhonge, Einaga Yasuaki, Fujishima Akira, Sato Osamu

Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals 343 2000

ISSN 1058-725X

LIESST iron(II) spin-crossover mononuclear complex [Fe(L)(CN)₂]·H₂O (1) (L is Schiff-base macrocylic ligand derived from the condensation of 2,6-diacetylpyridine with 3,6-dioxaoctane-1,8- diamine) with macro ring and two CN^- bridging ligands in complex 1 can coordinate to other metal compounds, it is possible to build one-dimensional molecular-based photo-magnets. We have synthesized [Fe(L)(CN)₂][Mn(hfac)₂] (2) and investigated it by using magnetic susceptibility, Mossbauer spectra and IR spectra measurements. The complex 2 exhibits spin-crossover behavior (T_1/2=110 K) on the moieties of the iron sites.

Magnetic properties and optical control of electrochemically prepared iron-chromium polycyanides

Ohkoshi Shin Ichi, Einaga Yasuaki, Fujishima Akira, Hashimoto Kazuhito

Journal of Electroanalytical Chemistry 473 ( 1 ) 245 - 249 1999.09

ISSN 0022-0728

The magnetic properties and photo-magnetic effects of the electrochemically prepared metal polycyanide film, Fe_1.5^II[Cr^III(CN)₆]·7.5H₂O, were investigated. Magnetization measurements showed that it was a ferromagnet (parallel spin ordering) having a T_c = 21 K. The spontaneous magnetization decreased optically by the photoexcitation of the intervalence transfer (IT) band between Fe^II and Cr^III. This optically reduced magnetization persisted for a period of several days at 5 K after turning off the light. ⁵⁷Fe Moessbauer experiments showed that the spin state of iron did not change but the strength of a ferromagnetic interaction between Fe^II and Cr^III decreased after the irradiation, forming a paramagnetic metastable state. This metastable state returned to the original state above 40 K, showing that the magnetization of this compound can be controlled reversibly by the combination of optical and thermal methods.

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Photofunctional vesicles containing prussian blue and azobenzene

Einaga Y., Sato O., Iyoda T., Fujishima A., Hashimoto K.

Journal of the American Chemical Society 121 ( 15 ) 3745 - 3750 1999.04

ISSN 0002-7863

The intercalation of inorganic materials into organized organic assemblies presents many opportunities for the development of new functional materials with superior physicochemical properties. We have designed a composite material comprising Prussian blue intercalated into photoresponsive organic molecules (azobenzene-containing multibilayer vesicles). Photoisomerization in the solid system was achieved by diluting the azobenzene-containing bilayer membrane in a poly(vinyl alcohol) matrix. The photoisomerization of the film was accompanied by a geometrically confined structural change within the vesicles, as reflected by changes in the dipole moment and electrostatic field. As a result, we were able to control the magnetic properties of this material by photoillumination.

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