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.

真空蒸着法を用いるイオン液体中でのナノ粒子合成 Invited

岡崎健一，桑畑　進，鳥本　司

触媒 Vol. 49 page： 630-635 2007

コア・シェル型粒子の光化学的ナノ構造制御と機能材料への応用 Invited Reviewed

鳥本　司, 大谷文章

Vol. 38 page： 103-110 2007

イオン液体の電顕微鏡観察 Invited

桑畑進, 鳥本司

表面科学 Vol. 28 page： 322-326 2007

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.

*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.

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.

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

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.

Size and structure-dependent photocatalytic activity of jingle-bell-shaped silica-coated cadmium sulfide nanoparticles for methanol dehydrogenation Reviewed

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

J. Phys. Chem. B Vol. 108 ( 48 ) page： 18670-18674 2004.12

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

Silica-coated cadmium sulfide nanoparticles (SiO2/CdS) having a jingle-bell structure were prepared via size-selective photoetching and were used as photocatalysts for dehydrogenation of methanol. Irradiation Of SiO2/ CdS suspended in an aqueous solution containing methanol induced the liberation of hydrogen (1-12), the amount of which increased linearly with increase in the time of irradiation. The observed stable photocatalytic activity was attributed to the prevention of coalescence between CdS core particles by the surrounding SiO2 shells during the photocatalytic reaction. The rate of H, liberation increased with decrease in the wavelength of irradiation light for the size-selective photoetching, that is, the smaller the size of the CdS core, the higher the rate of H-2 liberation, probably because of increased reduction and oxidation abilities of CdS as a result of decrease in their particle size, that is, size quantization effect. Rhodium photodeposited on SiO2/CdS worked as a cocatalyst for the enhancement of dehydrogenation. The photocatalytic activity was reduced by increase in shell thickness because of a decrease in the rate of penetration of chemical species or the transfer of electrons and holes through the SiO2 layer. Also, the close contact between the core and shell retarded the photocatalytic reaction, indicating that the surface of the CdS core for methanol dehydrogenation was covered with a SiO2 shell layer. The results indicate that the jingle-bell-shaped SiO2/CdS nanoparticles can be an efficient and stable photocatalyst with a flexibly tunable structure, in contrast to s urface- modified CdS particles prepared by a conventional technique.

Preparation and characterization of water-soluble jingle-bell-shaped silica-coated cadmium sulfide nanoparticles Reviewed

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

J. Phys. Chem. B Vol. 108 ( 32 ) page： 11946-11952 2004.8

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

Treatment of surface silanol groups with hydrophilic compounds enabled silica-coated cadmium sulfide (SiO2/CdS) nanoparticles of core-shell morphologies to be dissolved in a water/methanol mixture. Size-selective photoetching of the particles was performed by irradiation with monochromatic light at 458 nm, resulting in a blue shift of its absorption onset because of the decreasing size of the CdS. TEM analyses revealed that the SiO2 shell structure was not shrunken by photoetching and that a void space (ca. 2.3 nm) was formed between the US core and the SiO2 shell to give a jingle-bell structure. The emission spectra of photoetched particles showed the development of band-gap emission when cadmium ion (Cd2+) was added and the pH was adjusted to 10, whereas the same treatment of original particles gave no peaks assigned to band-gap emission, indicating that the SiO2 shell was sufficiently porous for small ionic species such as Cd2+ to penetrate through the shell and that the photoetched US core incorporated in the SiO2 shell had a bare surface to be covered with a cadmium hydroxide layer. With increasing shell thickness, the rate of CdS photoetching was reduced as a result of decrease in the rate of scavenging of the photogenerated electrons in US by electron acceptors such as O-2 and methyl viologen (MV2+) in solution. The structure of the SiO2 layer was varied by surface modification of the US with both 3-mercaptopropionic acid (MPA) and 3-mercaptopropyltrimethoxysilane (MPTS) followed by hydrolysis of its trimethoxysilyl group. FT-IR spectra showed that MPA molecules, but not MPTS molecules, attached to the US core surface were eliminated by photoetching of the particles, resulting in the formation of windows in the SiO2 shell. The behavior of US emission quenching suggested that MV2+ is transported through the windows to the void space inside the SiO2 shell to reach the US core.

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Papers - TORIMOTO Tsukasa