Faculty Profiles - TORIMOTO Tsukasa

Papers - TORIMOTO Tsukasa

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Controlling Surface Reactions of CdS Nanocrystals: Photoluminescence Activation, Photoetching and Photostability under Light Irradiation Reviewed

Keiichi Sato, Satoshi Kojima, Shinya Hattori, Taeko Chiba, Keiko Ueda-Sarson, Tsukasa Torimoto, Yasuhiro Tachibana, and Susumu Kuwabata

Nanotechnology Vol. 18 page： 465702 2007

Language：English Publishing type：Research paper (scientific journal)

Photoluminescence enhancement, photoetching and photostability of CdS nanocrystals were investigated under light irradiation. Strongly photoluminescent nanocrystals were obtained when the nanocrystal was weakly photoexcited in an aqueous solution at pH = 11 in the presence of oxygen. With the support of XPS measurements, the following photoactivation mechanism is proposed: Cd2+ ions are released from the CdS surface owing to slow photocorrosion in the presence of oxygen, and Cd-OH bond formation occurs on the CdS surface under the alkaline conditions, removing the surface trap states. The wavelength of the irradiating light and the pH of the solution were determined as key parameters for nanocrystal surface modification. For the stability measurements the nanocrystals were extracted with an ammonium salt in a non-polar solvent. The photoluminescence quantum yield for the nanocrystals in the non-polar phase reached approximately 30%. The extracted nanocrystals were remarkably stable even under UV light irradiation, and the photoluminescence intensity was maintained for several months.

CdS Quantum Dots Sensitized TiO2 Sandwich Type Photoelectrochemical Solar Cells

Y. Tachibana, H. Y. Akiyama, Y. Ohtsuka, T. Torimoto, and S. Kuwabata

Chem. Lett. Vol. 36 page： 88-89 2007

Effect of Ionic Surfactants on the Iridescent Color in Lamellar Liquid Crystalline Phase of a Nonionic Surfactant

X. Chen, H. Mayama, G. Matsuo, T. Torimoto, B. Ohtani, and K. Tsujii

J. Colloid Interface Sci. Vol. 305 page： 308-314 2007

Language：English Publishing type：Research paper (scientific journal)

A nonionic surfactant, n-dodecyl glyceryl itaconate (DGI), self-assembles into bilayer membranes in water having a spacing distance of submicrometer in the presence of small amounts of ionic surfactants, and shows beautiful iridescent color. Ionic surfactants have strong effects on this iridescent system. We have interestingly found that the iridescent color changes with time after mixing DGI and ionic surfactants and the color in equilibrium state changes greatly with concentration of the ionic surfactants. The time-dependent color change results from the transformation of DGI aggregate structure after being mixed with ionic surfactant. It is first found that the iridescent color of this nonionic system can be changed
from red to deep blue by altering the concentration of ionic surfactants added even though the total concentration of surfactant is almost constant. Such large blue shift of the iridescent color in equilibrium state cannot be fully explained by the ordinary undulation theory applied so far for this phenomenon. The flat lamellar sheets tend to curve by increasing the concentration of ionic surfactants to form separated onion-like and/or myelin-like structures. These separated structures of lamellar system result in the decrease of spacing distance between bilayer membranes because
some vacant spaces necessarily appear among these structures.

Double-Beam Photoacoustic Spectroscopic Studies on Transient Absorption of Titanium(IV)Oxide Photocatalyst Powders Reviewed

Naoya Murakami, Orlando Omar Prieto Mahaney, Ryu Abe, Tsukasa Torimoto, and Bunsho Ohtani

J. Phys. Chem. C Vol. 111 page： 11927-11935 2007

Language：English Publishing type：Research paper (scientific journal)

In situ photoabsorption properties of titanium(IV) oxide (TiO2) powders under continuous ultraviolet irradiation
were investigated by double-beam photoacoustic (PA) spectroscopy. This PA measurement enabled observation
of two kinds of ultraviolet-light-induced intermediate species appearing on various kinds of TiO2 powder samples. Most of the samples (type 1) exhibited photoabsorption due to the production of trivalent titanium (Ti3+) species, while transient absorption assigned to trapped holes or surface peroxy species was also observed for anatase samples with a relatively large specific surface area (type 2). Time-resolved measurements and analyses of the kinetics of photoinduced Ti3+ species suggest that electrons accumulated in type-2 samples have high reactivity toward molecular oxygen compared to type-1 samples. Saturation limits of intensity of the PA signal attributed to Ti3+ species under deaerated conditions in the presence of surface-adsorbed methanol were estimated for both types of samples, and their linear relation with density of Ti3+ species estimated by a conventional photochemical technique was observed. This suggests that the present double-beam PA technique is an alternative feasible method for estimation of density of Ti3+ species, which is a potential measure of density of crystalline defects.

Pyrene-to-porphyrin excited singlet energy transfer in LBLdeposited LDH nanosheets Reviewed

Vol. 11 page： 428 2007

Microscopic Structure of Separately Accommodated Phorphyrins and Viologens in Mesoporous Silica and Titania Nanosheet Hybrid Films Reviewed

Tatsuo Yui, Tsutomu Kajino, Takako Tsuchino, Ken-ichi Okazaki, Tsukasa Torimoto, Yoshiaki Fukushima, and Katsuhiko Takagi

Trans. Mater. Res. Soc. Jap. Vol. 32 page： 449-452 2007

Language：English Publishing type：Research paper (scientific journal)

The microscopic structures of two nano-structured inorganic films, the one based on titania nanosheets
(TNS) and the other on mesoporous silica (MPS), were investigated. Clear stacking structure of the TNS and MPS films was observed for the XRD, SEM, and TEM analyses. Both porphyrin derivatives and viologens were separately and selectively adsorbed into the MPS nano-cavities and the TNS nano-layers, respectively by means of their absorption and energy-dispersive spectroscopies (EDS).

Photo-Switching Behavior of CdS Nanoparticles Doped in a Polymer Film Reviewed

Y. Ohara, T. Nakabayashi, K. Iwasaki, T. Torimoto, B. Ohtani, and N. Ohta

Compt. Rend. Chim. Vol. 9 page： 742-749 2006

Language：English Publishing type：Research paper (scientific journal)

Light-emission intensity of CdS nanoparticles doped in a PMMAfilm is enhanced by a UV irradiation in air, but the emission intensity is strongly de-enhanced at a low pressure of 10−5 torr. Atmospheric gas as well as UV irradiation is essential for the enhancement, and these enhancement and de-enhancement processes are almost completely reversible. The emission of CdS nanoparticles doped in a PMMA film is also quenched by applying an external electric field, suggesting that the non-radiative
decay rate from the emissive exciton state is accelerated by an external electric field.

Photochemical Fine-Tuning of Luminescent Color of Cadmium Selenide Nanoparticles: Fabricating a Single-Source Multi-Color Luminophore

T. Torimoto, S.-y. Murakami, M. Sakuraoka, K. Iwasaki, K.-i. Okazaki, T. Shibayama, and B. Ohtani

J. Phys. Chem. B Vol. 110 page： 13314-13318 2006

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Size-selective photoetching was applied to silica-coated cadmium selenide (SiO2/CdSe) nanoparticles to precisely control their photoluminescence properties. The absorption spectra of CdSe was blue-shifted by irradiation of monochromatic light, and finally, the absorption onset agreed with the wavelength of irradiation light, indicating that CdSe particles were photoetched to smaller ones until the irradiated photons were not absorbed by the photoetched particles and that the SiO2 shell layer surrounding the CdSe core prevented coalescence between the photoetched particles. Although as-prepared SiO2/CdSe did not exhibit photoluminescence, the application of size-selective photoetching to SiO2/CdSe resulted in the development of the band gap emission, with the degree being enhanced with progress of the photoetching. The peak wavelength of photoluminescence decreased with a decrease in the wavelength used for the photoetching, so that the
luminescence color could be tuned between red and blue. Partial photoetching of SiO2/CdSe nanoparticle
films produced intense band gap emission of CdSe at the photoetched area, while the remainder of the SiO2/
CdSe films did not exhibit detectable photoluminescence, resulting in the formation of a clear photoluminescence
image under UV irradiation. This technique makes it possible to produce a multicolored photoluminescence image by irradiation with monochromatic lights having various wavelengths using a single source material.

Photoacoustic Spectroscopic Analysis of Photoinduced Change in Absorption of Titanium(IV) Oxide Photocatalyst Powders: A Novel Feasible Technique for Measurement of Defect Density

N. Murakami, O. O. P. Mahaney, T. Torimoto, and B. Ohtani

Chem Phys. Lett. Vol. 426 page： 204-208 2006

Language：English Publishing type：Research paper (scientific journal)

Photoabsorption properties of titanium(IV) oxide powders were investigated by photoacoustic (PA) spectroscopy. The photoacoustic spectra obtained under steady-state ultraviolet irradiation showed appearance and growth of two kinds of PA signal bands, which were attributable to trivalent titanium species (Ti3+) and trapped holes or surface peroxide species. The saturation limit of the PA signal was estimated by analyzing the time course of Ti3+ signals and compared to the density of Ti3+ determined by a photochemical method. The results suggest that the photoacoustic spectroscopic technique is a feasible alternative method for estimation of Ti3+ as an empirical measure of defect density.

Fabrication of Jingle-Bell-Shaped Core–Shell Nanoparticulate Films and Molecular-Size-Responsive Photoluminescence Quenching of Cadmium Sulfide Cores Reviewed

K. Iwasaki, T. Torimoto, T. Shibayama, T. Nishikawa, and B. Ohtani

Small Vol. 2 page： 854-858 2006

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

A monoparticulate layer assembly of silica (SiO2)-coated cadmium sulfide (CdS) nanoparticles (SiO2/CdS) was prepared at an air-water interface and accumulated as thin films on solid substrates by the Langmuir-Blodgett technique. Size-selective photoetching of the CdS core by irradiation of monochromatic light induced a blue shift of diffuse reflectance spectra of the nanoparticulate films, indicating the formation of jingle-bell-shaped CdS core-SiO2 shell nanoparticles in the film. Photoluminescence of thus-photoetched SiO2/CdS nanoparticulate films was quenched by the addition of trialkylamines, the degree being dependent on their molecular size, while little change in the emission intensity was observed in the original films, regardless of the size of trialkylamines added. These facts were attributable to the molecular-size-selective penetration of quencher molecules from the bulk solution to the void space through the pores of the SiO2 shell of the jingle-bell-shaped nanoparticles.

Observation of Ionic Liquid by Scanning Electron Microscope Reviewed

S. Kuwabata, A. Kongkanand, D. Oyamatsu, and T. Torimoto

Chem. Lett. Vol. 35 page： 600-601 2006

Analysis of Quantum Dot Fluorescence Coupled with a Microsphere Resonator

H. Takashima, Hideki Fujiwara, J.-i. Hotta, S. Takeuchi, K. Sasaki, S.-y. Murakami, T. Torimoto, B. Ohtani

Jap. J. Appl. Phys. Vol. 45 page： 6917-6921 2006

Language：English Publishing type：Research paper (scientific journal)

The emission spectrum of CdSe/ZnS quantum dots adhering to the surface of a microsphere resonator is measured and
discussed. Only certain peaks are observed, and the order numbers of the modes (l = 5 to 7) of the observed peaks are
different from those previously reported (l = 1 and 2). [Nano Lett. 1 (2001) 399]. A detailed analysis is presented using the cavity mode efficiency , which is a function of surface roughness, the absorption coefficient of the quantum dots, and mode parameters. The difference between the observed order number modes is explained to be due to the ratio of the rate of absorption by the quantum dots to that of the leakage rates of the modes. The surface roughness of a quantum-dot-coated microsphere and the absorption coefficient of the quantum dots on the sphere are estimated in the analysis.

*Sputter Deposition onto Ionic Liquids: Simple and Clean Synthesis of Highly Dispersed Ultrafine Metal Nanoparticles

T. Torimoto, K.-i. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, and S. Kuwabata

App. Phys. Lett. Vol. 89 page： 243117 2006

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Sputter deposition of gold Au onto ionic liquids ILs resulted in the formation of highly dispersed Au nanoparticles without additional chemical species, such as reducing and/or stabilizing agents. The Au nanoparticles in 1-ethyl-3-methylimidazolium tetrafluoroborate had an average diameter (dav) of 5.5 nm with a standard deviation (s) of 0.86 nm, while sputter deposition onto N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide resulted in the formation of much smaller Au nanoparticles with dav of 1.9 nm and s of 0.46 nm. Prolongation of sputtering time results in a higher concentration of Au nanoparticles in ILs, but did not cause a remarkable change in their size.

Origin of Chiral Discrimination by a Two-Dimensionally Dhiral Self-Assembled Monolayer: A Quantum Chemical Study

T. Nishikawa, T. Torimoto, T. Nakanishi, T. Osaka, B. Ohtani

Chem. Phys. Lett. Vol. 432 page： 502-507 2006

Language：English Publishing type：Research paper (scientific journal)

Structural and energetic investigations based on semiempirical PM5 and ab initio HF and MP2 calculations suggested that the selfassembled monolayer of an atropisomeric compound, (R)- or (S)-1,10-binaphthalene-2,20-dithiol (BNSH), on a gold (111) surface selectively
adsorbs one enantiomer of phenylalanine (Phe), resulting in chiral discrimination, through hydrophobic, cation(NHþ3
Þ–p and NH(Phe)–p (BNSH) interactions in a nanometer-sized screw-hole-like pocket composed of three BNSH molecules in the monolayer.

Electric-Field-Induced Changes in Absorption and Emission Spectra of CdS Nanoparticles Doped in a Polymer Film

Y. Ohara, T. Nakabayashi, K. Iwasaki, T. Torimoto, B. Ohtani, T. Hiratani, K. Konishi, N. Ohta

J. Phys. Chem. B Vol. 110 page： 20927-20936 2006

Language：English Publishing type：Research paper (scientific journal)

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the fieldinduced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.

Ligand-Free Platinum Nanoparticle Encapsulated in a Hollow Porous Carbon Shell as a Highly Active Heterogeneous Catalyst in Hydrogenation Reaction

S. Ikeda, S. Ishino, T. Harada, N. Okamoto, T. Sakata, H. Mori, S. Kuwabata, T. Torimoto, M. Matsumura

Angew. Chem. Int. Ed. Vol. 45 page： 7063-7066 2006

Photochemical Electron Transfer through the Interface of Hybrid Films of Titania Nano-Sheets and Mono-Dispersed Spherical Mesoporous Silica Particles

T. Yui, Y. Kobayashi, Y. Yamada, T. Tsuchino, K. Yano, T. Kajino, Y. Fukushima, T. Torimoto, H. Inoue, and K. Takagi

Phys. Chem. Chem. Phys. Vol. 8 page： 4585-4590 2006

Language：English Publishing type：Research paper (scientific journal)

Photochemical Electron Transfer (ET) between an organic dye, the porphyrin derivative TMPyP, and an electron acceptor, methyl viologen MV2+, have been investigated at the interface of two different inorganic films, i.e., layered titania nano-sheets (TNS) and a monolayer film of spherical and mono-dispersed mesoporous silica (sMPS) particles (ca. 0.5 mm). TMPyP ions were intercalated within the sMPS nano-cavities to form (TMPyP–sMPS) while MV2+ ions were intercalated into the TNS interlayers to form (MV2+–TNS). The (TMPyP–sMPS) and (MV2+–TNS) films were then stacked on a silica substrate in this order to form a (MV2+–TNS)/(TMPyP–sMPS) film and, upon UV light irradiation, ET could be induced. However, when this
film was stacked inversely, i.e., for the (TMPyP–sMPS)/(MV2+–TNS) films on a silica substrate, no photoinduced ET were observed. Interestingly, however, even for this photo-inactive inversely stacked film, ET could be generated by inserting a gold vapor-deposited layer between the
(MV2+–TNS) and (TMPyP–sMPS) films. The conjugation conditions at the interface of the inversely stacked (TMPyP–sMPS)/(MV2+–TNS) hybrid film were, thus, confirmed to strongly affect the photoinduced electron transfers and their efficiencies.

Surface modification of photoluminescent CdS nanocrystals inducing spontaneous phase transfer reaction Reviewed

S. Kuwabata, K. Sato, S. Hattori, T. Chiba, K. Ueda-Sarson, Y. Tachibana, and T. Torimoto

Chem. Lett. Vol. 34 ( 9 ) page： 1300-1301 2005.9

Language：English Publishing type：Research paper (scientific journal)

A facile one-step method to modify the surface of luminescent US nanocrystals by alkylamines, without their luminescence intensity loss, was established. The interfacial modification inducing spontaneous transfer of the US from an aqueous to a non-polar organic phase was found to be controlled simply by choosing the alkyl group of the amine. Analysis of the absorption and emission spectra revealed the phase transfer yield of ca. 100%.

Synthesis of metal-cadmium sulfide nanocomposites using jingle-bell-shaped core-shell photocatalyst particles Reviewed

B. Pal, T. Torimoto, K. Iwasaki, T. Shibayama, H. Takahashi, and B. Ohtani

J. Appl. Electrochem. Vol. 35 ( 7-8 ) page： 751-756 2005.7

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Photocatalytic deposition of gold (Au) and silver (Ag) nanoparticles was investigated using jingle-bell-shaped silica (SiO2)-coated cadmium sulfide (CdS) nanoparticles (SiO2/CdS), which each had a void space between the CdS core and SiO2 shell, as a photocatalyst. A size-selective photoetching technique was used to prepare the jingle bell nanostructure of SiO2/CdS. Nanoparticles of Au and Ag were deposited by irradiation of the photoetched SiO2/CdS in the presence of the corresponding metal complexes under deaerated conditions. Chemical etching of Au-deposited particles enabled the selective removal of CdS without any influence on the surface-plasmon absorption of Au. TEM analyses of the resulting particles suggested that some particles were encapsulated in hollow SiO2 particles, while other Au particles were deposited on the outer surface of the SiO2 shell. Emission spectra of the photoetched SiO2/CdS showed that the metal deposition developed a broad emission with a peak around 650 nm originating from surface defect sites, the degree being dependent on the kind of metal nanoparticles and their amount of deposition. This fact can be explained by the formation of metal-CdS binary nanoparticles having defect sites at the interface between metal and CdS.

Photocatalytic preparation of encapsulated gold nanoparticles by jingle-bell-shaped cadmium sulfide-silica nanoparticles Reviewed

B. Pal, T. Torimoto, S. Ikeda, T. Shibayama, K. Sugawara, H. Takahashi, and B. Ohtani,

Top. Cat. Vol. 35 ( 3-4 ) page： 321-325 2005

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Gold (Au) nanoparticles were deposited inside silica: (SiO2) shells containing cadmium sulfide (CdS) nanoparticles through photocatalytic reduction of potassium dicyanogold(I) by CdS. Photocatalytic Au deposition occurred only when core-shell nanoparticles having a void space between the core and shell, i.e., a jingle-bell-shaped structure, were used. These core-shell nanoparticles were prepared by size-selective photoetching of SiO2 -covered CdS nanoparticles. The size of Au nanoparticles could be controlled by adjustment of the void space in SiO2-covered CdS. Dissolution of CdS by acid treatment from the Au-deposited jingle-bell nanoparticles did not have any effect on the surface-plasmon absorption by Au. These facts indicate that Au nanoparticles of adjustable size can be prepared in an SiO2 shell that prevents mutual coalescence of Au nanoparticles but allows permeation of molecules and ions.

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Papers - TORIMOTO Tsukasa