Published papers

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.

Photochromic magnetic vesicles have been designed by intercalation of Prussian blue into a lipid bilayer membrane which consists of azobenzene. Reversible photoisomerization of the solid system realized reversible photocontrol of the magnetic properties. Here, we have synthesized an amphiphile azo moiety containing a long alkyl chain, and we have prepared photochromic magnetic vesicles consisting of only this azo moiety. As a result, the magnetic property change brought about by visible light illumination was estimated to be ca. 13%, which is much larger than the figure achieved for the similar system that we reported previously (∼1%), which contained the double-chain ammonium amphiphile (J. Am. Chem. Soc. 1999, 121, 3745.).

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)63-/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)63-/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.

Microdisc array (MDA) electrodes of boron-doped diamond (BDD) were fabricated on structured silicon substrates. The BDD-MDA electrodes exhibited sigmoidal voltammetric curves, which show that they function as assemblies of single microelectrodes. The microelectrode behavior was also confirmed with biologically important species such as ascorbic acid and 3,4-dihydroxyphenylacetic acid, indicating its applicability in electroanalysis.

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.

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.

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.

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 FeIII-CN-CoII and FeII-CN-CoIII respectively, and by the Mössbauer spectrum, in which FeIII and FeII 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 FeII to CoIII. 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, K0.2Co1.4Fe(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 CoII and FeIII. As a result, dehydration increases the fraction of FeII-CN-CoIII and causes the compound to be paramagnetic even at low temperature. © 2001 Academic Press.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The photoswitchable magnetic thin films have been designed by alternate depositions of bipolar amphiphilic azobenzene and Fe3O4 nanoparticles by using the layer-by-layer adsorption technique. In these films, reversible photoisomerizations of the azobenzene chromophore affected the electrostatic interaction between the azobenzene chromophore and the surface of Fe3O4 nanoparticles. As a result, we have succeeded in controlling the magnetization of these films by photoillumination at room temperature. Copyright © 2005 The Chemical Society of Japan.