Faculty Profiles - Yamakata Akira

Papers - Yamakata Akira

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Effects of accumulated electrons on the decay kinetics of photogenerated electrons in Pt/TiO2 photocatalyst studied by time-resolved infrared absorption spectroscopy Reviewed

A Yamakata, TA Ishibashi, H Onishi

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 160 ( 1-2 ) 33 - 36 2003.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

The effects of accumulated electrons on the decay kinetics of photogenerated electrons in Pt/TiO2 photocatalyst were examined using time-resolved infrared (TR-IR) absorption spectroscopy. When the catalyst was irradiated by a UV pulse in the presence of methanol gas, the lifetime of the electrons was drastically elongated through the effective hole-consuming reaction by the adsorbed methoxy groups. When the catalyst was further irradiated by a second UV pulse in the presence of these long-lived electrons, the number of electrons generated by the second pulse was smaller than that generated by the first pulse though these pulse energies were the same. By increasing the interval between these two pulses, the number of electrons generated by the second pulse was gradually restored to equal that generated by the first pulse. These changes were ascribed to the enhancement of the rate of electron-hole recombination due to the long-lived electrons. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S1010-6030(03)00217-X

Microsecond kinetics of photocatalytic oxidation on Pt/TiO2 traced by vibrational spectroscopy Reviewed

A Yamakata, T Ishibashi, H Onishi

CHEMICAL PHYSICS LETTERS 376 ( 5-6 ) 576 - 580 2003.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

2-Propanol oxidation to acetone was examined by time-resolved infrared spectroscopy on a Pt/TiO2 photocatalyst in an aqueous solution. Holes generated by the band-gap excitation were found to attach to the adsorbed reactant within the first 0.5 mus. Subsequent rearrangement of atoms in the hole-attached reactant was observed on a series of time-resolved vibrational spectra. The C=O stretching band of a reaction intermediate to be converted to acetone appeared at time delays of 0-20 mus. The observed wavenumber of the intermediate, 1640 cm(-1), was compared with theoretically predicted C=O stretching frequencies of possible species. An acceptable agreement was obtained with the anion radical of acetone adsorbed on the catalyst. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0009-2614(03)01034-0

Photophysics and electron dynamics in dye-sensitized semiconductor film studied by time-resolved mid-IR spectroscopy Reviewed

Kan Takeshita, Yutaka Sasaki, Masahiro Kobashi, Yuki Tanaka, Shuichi Maeda, Akira Yamakata, Taka-Aki Ishibashi, Hiroshi Onishi

Journal of Physical Chemistry B 107 ( 17 ) 4156 - 4161 2003.5

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

Photophysics and electron dynamics in dye-sensitized semiconductor film were studied by transient mid-IR spectroscopy, in particular on the nano- to millisecond time scale. As sensitizers, a Ru complex and 9-phynol xanthane derivatives were used. We simultaneously observed electrons injected into the conduction band of a semiconductor and the change of vibration bands due to formation of a cation of a dye molecule. The transient absorption components derived from these two species decayed differently. From this observation, we found that injected electrons decay through two paths
one was back electron transfer, and the other was decay to deep trap sites that cannot be detected by mid-IR light. We could also estimate the amount of electron injection for different dye-sensitized TiO2. Our transient mid-IR spectroscopy in long time scale introduces a semi-new approach to the slow dynamics in dye-sensitized semiconductors including back electron transfer.

DOI： 10.1021/jp0275645

Kinetics of the photocatalytic water-splitting reaction on TiO2 and Pt/TiO2 studied by time-resolved infrared absorption spectroscopy Reviewed

A Yamakata, T Ishibashi, H Onishi

JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL 199 ( 1-2 ) 85 - 94 2003.5

Language：English Publisher：ELSEVIER SCIENCE BV

The present account describes how the water-splitting reaction on TiO2 (P-25) and Pt/TiO2 catalysts is traced by time-resolved infrared (IR) absorption spectroscopy. The ac-coupled amplification of the IR signal allows detection of transient absorbance-change as small as 10(-6) with a time-resolution of 50 ns. The TiO2 or Pt/TiO2 catalyst irradiated by a 355 run UV-pulse at time = 0 presents a transient IR absorption which monotonically increases in intensity with decreasing wavenumber from 3000 to 1000 cm(-1). Photogenerated electrons trapped in shallow mid-gap states are proposed to cause the absorption. The decay kinetics of the electrons traced by monitoring the IR absorbance at 2000 cm(-1) is sensitive to vapor atmospheres. The electrons recombine with the complementary holes along a multi-exponential rate law in the catalysts placed in a vacuum. Dioxygen arriving from the gas-phase captures the electrons at delay time of 10-100 mus following the UV irradiation. In the presence of water vapor, holes in the TiO2 catalyst are captured by an adsorbed reactant (probably hydroxyl species) within 2 mus and the recombination is obstructed thereafter. The excess electrons cannot transfer to another adsorbate to be reduced (probably proton) and hence reduce the catalyst itself. On the Pt/TiO2 catalyst exposed to water, the electron transfer takes place at 10 mus or later, following the hole transfer completed within 2 mus. The hole transfer on Pt/TiO2 is insensitive to the pressure of water vapor whereas the rate of the electron transfer is enhanced with the increasing pressure at 1-10Torr. Transient response of an adsorbate vibration is also observed. The O-H stretching band of an adsorbed hydroxyl species at 3677 cm-1 thermally shifted to the low-wavenumber side, when the TiO2 catalyst is irradiated by the UV-pulse. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S1381-1169(03)00021-9

Photophysics and electron dynamics in dye-sensitized semiconductor film studied by time-resolved mid-IR spectroscopy Reviewed

K Takeshita, Y Sasaki, M Kobashi, Y Tanaka, S Maeda, A Yamakata, T Ishibashi, H Onishi

JOURNAL OF PHYSICAL CHEMISTRY B 107 ( 17 ) 4156 - 4161 2003.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

Photophysics and electron dynamics in dye-sensitized semiconductor film were studied by transient mid-IR spectroscopy, in particular on the nano- to millisecond time scale. As sensitizers, a Ru complex and 9-phynol xanthane derivatives were used. We simultaneously observed electrons injected into the conduction band of a semiconductor and the change of vibration bands due to formation of a cation of a dye molecule. The transient absorption components derived from these two species decayed differently. From this observation, we found that injected electrons decay through two paths; one was back electron transfer, and the other was decay to deep trap sites that cannot be detected by mid-IR light. We could also estimate the amount of electron injection for different dye-sensitized TiO(2). Our transient mid-IR spectroscopy in long time scale introduces a semi-new approach to the slow dynamics in dye-sensitized semiconductors including back electron transfer.

DOI： 10.1021/jp0275645

Studies on the electron transfer reaction between TiO_2 and adsorbed dyes by time-resolved infrared spectroscopy

TAKESHITA Kan, SASAKI Yutaka, KOBASHI Masahiro, TANAKA Yuki, MAEDA Shuichi, YAMAKATA Akira, ISHIBASHI Taka-aki, ONISHI Hiroshi

触媒 45 ( 2 ) 181 - 183 2003.3

Electron-and Hole-transfer from TiO_2 Particles to Adsorbates Studied by Time-Resolved Infrared Absorption Spectroscopy Reviewed

YAMAKATA Akira, ISHIBASHI Takaki, ONISHI Hiroshi

J. Surf. Sci. Soc. Jpn. 24 ( 1 ) 46 - 52 2003.1

Language：Japanese Publisher：The Surface Science Society of Japan

Behavior of the electrons and holes photogenerated in TiO<Sub>2</Sub> particles was observed by time-resolved infrared absorption spectroscopy in the presence of oxygen, water, and methanol vapor. The electrons photogenerated by a band-gap excitation displayed a structureless, broad absorption of IR light from 3000 to 900 cm<Sup>−1</Sup>, which was assigned to the intra-conduction-band transition and/or excitation from mid-gap traps to the conduction band. This electron-induced absorption was probed as a function of time delay after the photoexcitation. Electron decay caused by the recombination with holes and by the charge-transfer reactions with adsorbates were kinetically analyzed. The electron decay was accelerated in the presence of oxygen gas due to an electron-capture reaction at the interface, whereas was decelerated in methanol vapor due to an effective hole-capture by methoxy groups. On platinized TiO<Sub>2</Sub> particles exposed to water vapor, a hole-capture reaction completed within 2 μs after band-gap excitation, whereas the electron-capture reaction occurred in 10 μs or later. These results demonstrate the effectiveness of this method to identify individual steps of photo-induced reactions at interfaces.

DOI： 10.1380/jsssj.24.46

Pressure dependence of electron- and hole-consuming reactions in photocatalytic water splitting on Pt/TiO2 studied by time-resolved IR absorption spectroscopy Reviewed

A Yamakata, T Ishibashi, H Onishi

INTERNATIONAL JOURNAL OF PHOTOENERGY 5 ( 1 ) 7 - 9 2003

Language：English Publishing type：Research paper (scientific journal) Publisher：PHOTOENERGY CENTER AIN SHAMS UNIV,

The decay kinetics of photogenerated electrons in the water splitting reaction on a Pt/TiO2 photocatalyst was studied by time-resolved IR absorption spectroscopy. The decay of the photogenerated electrons within 2 mus was decelerated when the catalyst was exposed to water vapor. The holes were consumed by the reaction with water instead of by the recombination with the electrons. On the other hand, the decay at 10-900 mus was accelerated by the exposure. The electrons were consumed by the reaction with water. The rate of the hole-consuming reaction was independent of the pressure of water vapor, whereas that of the electron-consuming reaction increased with the pressure from 1 to 10 Torr. The different pressure dependences indicate different reactants involved in the oxidative and reductive reactants.

DOI： 10.1155/s1110662x03000047

Time-resolved infrared absorption study of electron- and hole-capture reactions on photoexcited Pt/TiO2 in the presence of methanol-water vapor mixture Reviewed

A Yamakata, TA Ishibashi, H Onishi

SCIENCE AND TECHNOLOGY IN CATALYSIS 2002 145 157 - 160 2003

Language：English Publishing type：Research paper (scientific journal) Publisher：KODANSHA LTD

The decay kinetics of electrons and holes photogenerated in Pt/TiO2 was studied by time-resolved infrared absorption spectroscopy in methanol vapor, water vapor, and a mixture of the two. The holes were exhausted within 50 ns in the presence of methanol and it was concluded that they attached directly to methanol-derived adsorbates without the aid of water-derived hydroxyl radicals.

DOI： 10.1016/s0167-2991(03)80184-2

Microscopic identification of a bimolecular reaction intermediate Reviewed

Hiroshi Uetsuka, Akira Sasahara, Akira Yamakata, Hiroshi Onishi

Journal of Physical Chemistry B 106 ( 44 ) 11549 - 11552 2002.11

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

How a molecule impinging from an ambient phase attaches to and reacts with a preadsorbed reactant was studied by scanning tunneling microscopy (STM). A carboxylate (RCOO-) chemisorbed on a TiO2(110) surface was exchanged by another carboxylate (R′COO-) via an SN2-like mechanism when the RCOO--covered TiO2 surface was exposed to R′COOH vapor at room temperature or below. An R′COOH impinging from the gas phase was temporarily bound to a preadsorbed RCOO- yielding a bimolecular intermediate. The trapped R′COOH was inserted into the monolayer and dissociated to produce an adsorbed R′COO-. One of the carboxylates (RCOO- or R′COO-) was then pushed out of the monolayer to restore the intermediate and released to the vapor phase. The individual reaction steps were resolved using time-lapse imaging.

DOI： 10.1021/jp026253r

Microscopic identification of a bimolecular reaction intermediate Reviewed

H Uetsuka, A Sasahara, A Yamakata, H Onishi

JOURNAL OF PHYSICAL CHEMISTRY B 106 ( 44 ) 11549 - 11552 2002.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

How a molecule impinging from an ambient phase attaches to and reacts with a preadsorbed reactant was studied by scanning tunneling microscopy (STM). A carboxylate (RCOO-) chemisorbed on a TiO2(110) surface was exchanged by another carboxylate (R'COO-) via an S(N)2-like mechanism when the RCOO--covered TiO2 surface was exposed to R'COOH vapor at room temperature or below. An R'COOH impinging from the gas phase was temporarily bound to a preadsorbed RCOO- yielding a bimolecular intermediate. The trapped R'COOH was inserted into the monolayer and dissociated to produce an adsorbed R'COO-. One of the carboxylates (RCOO- or R'COO-) was then pushed out of the monolayer to restore the intermediate and released to the vapor phase. The individual reaction steps were resolved using time-lapse imaging.

DOI： 10.1021/jp026253r

Electron- and hole-capture reactions on Pt/TiO2 photocatalyst exposed to methanol vapor studied with time-resolved infrared absorption spectroscopy Reviewed

Akira Yamakata, Taka-Aki Ishibashi, Hiroshi Onishi

Journal of Physical Chemistry B 106 ( 35 ) 9122 - 9125 2002.9

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

The decay kinetics of photogenerated electrons in a Pt/TiO2 catalyst has been studied by time-resolved infrared absorption (TR-IR) spectroscopy. The electron decay was drastically affected by exposure to methanol vapor, and the electron-hole pair recombination was prevented. A certain fraction of the electrons remained beyond 1 s and slowly decayed from ∼1-9 s. The prevented recombination was interpreted with the effective capture of the holes. The extent of the hole capture was insensitive to the pressure of methanol vapor. An irreversibly adsorbed methoxy groups was proposed to capture the holes on the basis of the steady-state FT-IR spectra of the catalyst exposed to methanol vapor of different pressures. However, the slow decay of the electrons exhibited a positive-order response to the methanol pressure. Undissociated methanol physisorbed and equilibrated with the gas phase, which was identified in the steady-state spectra, should play a key role in the electron capture process. Possible mechanisms of the process are discussed.

DOI： 10.1021/jp025993x

Electron- and hole-capture reactions on Pt/TiO2 photocatalyst exposed to methanol vapor studied with time-resolved infrared absorption spectroscopy Reviewed

A Yamakata, T Ishibashi, H Onishi

JOURNAL OF PHYSICAL CHEMISTRY B 106 ( 35 ) 9122 - 9125 2002.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

The decay kinetics of photogenerated electrons in a Pt/TiO2 catalyst has been studied by time-resolved infrared absorption TR-IR) spectroscopy. The electron decay was drastically affected by exposure to methanol vapor, and the electron-hole pair recombination was prevented. A certain fraction of the electrons remained beyond 1 s and slowly decayed from similar to 1-9 s. The prevented recombination was interpreted with the effective capture of the holes. The extent of the hole capture was insensitive to the pressure of methanol vapor. An irreversibly adsorbed methoxy groups was proposed to capture the holes on the basis of the steady-state FT-IR spectra of the catalyst exposed to methanol vapor of different pressures. However, the slow decay of the electrons exhibited a positive-order response to the methanol pressure. Undissociated methanol physisorbed and equilibrated with the gas phase, which was identified in the steady-state spectra, should play a key role in the electron capture process. Possible mechanisms of the process are discussed.

DOI： 10.1021/jp025993x

Time-resolved infrared absorption studies of surface OH groups on TiO2 particles irradiated by UV pulses Reviewed

A Yamakata, T Ishibashi, H Onishi

BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 75 ( 5 ) 1019 - 1022 2002.5

Language：English Publishing type：Research paper (scientific journal) Publisher：CHEMICAL SOC JAPAN

The shift of vibrational frequency of surface OH groups on TiO2 particles induced by UV pulse irradiation has been studied by time-resolved IR absorption spectroscopy. By the UV irradiation of the TiO2 particles, the absorption intensity at 3696 cm(-1) was decreased and that at 3654 cm(-1) was increased. These changes were gradually recovered on a millisecond time scale. The transient changes were interpreted by a small (<0.1 cm(-1)) shift of a v(O-H) band at 3677 cm(-1). The temperature jump triggered by the UV pulse caused the shift.

DOI： 10.1246/bcsj.75.1019

時間分解赤外分光法を用いた二酸化チタン上の光励起電子と吸着分子への電荷移動反応の観察 Reviewed

山方啓, 石橋孝章, 大西洋

触媒 43 ( 6 ) 396 - 398 2001.9

Water- and oxygen-induced decay kinetics of photogenerated electrons in TiO2 and Pt/TiO2: A time-resolved infrared absorption study Reviewed

A Yamakata, T Ishibashi, H Onishi

JOURNAL OF PHYSICAL CHEMISTRY B 105 ( 30 ) 7258 - 7262 2001.8

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

Electrons photogenerated in TiO2 and Pt/TiO2 catalysts were observed by time-resolved IR absorption spectroscopy. Transient IR absorption of an identical spectrum appeared on TiO2 and Pt/TiO2 irradiated by a 355 nm pump pulse. The absorption was attributed to optical transition of photogenerated electrons trapped in shallow midgap states. Electron- and hole-consuming reactions of adsorbates controlled the decay kinetics of the electrons competing with the electron-hole recombination. On TiO2, O-2 from the gas-phase captured the electrons and accelerated the decay rate at a delay time of 10-100 mus. In water vapor, holes reacted with surface hydroxyls within 2 mus, and the recombination decay of the electrons was obstructed. When Pt/TiO2 was exposed to water, the oxidation and reduction steps of the water splitting reaction affected the decay kinetics in different time domains. Photogenerated holes oxidized water within 2 mus, whereas electrons reduced water at 10-900 mus.

DOI： 10.1021/jp010802w

Time-resolved infrared absorption spectroscopy of photogenerated electrons in platinized TiO2 particles Reviewed

A Yamakata, T Ishibashi, H Onishi

CHEMICAL PHYSICS LETTERS 333 ( 3-4 ) 271 - 277 2001.1

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Kinetics of photogenerated electrons in platinized TiO2 (P-25) particles were observed by time-resolved infrared (IR) absorption spectroscopy with AC-coupled detection of cw probe light. The sample irradiated by UV pulse (355 nm and 10 ns) gave transient absorption which monotonously increased in intensity with decreasing wavenumber from 3000 to 900 cm(-1). Photogenerated electrons trapped in shallow mid-gap states were proposed to originate the transient absorption. The decay kinetics of the trapped electrons were fitted to a multi-exponential formula with six lifetimes ranging from 1 x 10(-7) to 3 x 10(-1) s. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(00)01374-9

Transport channels of X-ray beamlines at SPring-8

Goto, S, Ohashi, H, Takeshita, K, Yabashi, M, Yamakata, A, Asano, Y, Ishikawa, T

Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment 467 2001

Transport channels of X-ray beamlines at SPring-8

Akira Yamakata

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 2001

Exchange reaction of formate with gas-phase acetic acid on Ni(110) Reviewed

A Yamakata, J Kubota, JN Kondo, C Hirose, K Domen, F Wakabayashi, K Tamaru

SURFACE SCIENCE 433 210 - 214 1999.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The exchange reaction of formate with gas-phase acetic acid on Ni(110) was studied by time resolved infrared reflection absorption spectroscopy. It was found that the preadsorbed for mate was replaced ill the acetate by the introduction of acetic acid on formate-full covered Ni(110), originated from the post-exposed acetic acid. Acetate was also replaced in formate by the introduction of formic acid on acetate-full covered Ni(110). These exchange reactions took place from 240 to 300 K, where decomposition of neither formate nor acetate was observed. Thus, it was confirmed that the preadsorbed formate or acetate was desorbed as formic or acetic acid accompanied by the transfer of hydrogen atoms from post-introduced acetic or formic acid. The activation energy of the exchange reaction of formate with post-introduced acetic acid was estimated to be 17 +/- 2 kJ mol(-1). The small activation energy of the exchange reaction is discussed on the basis of adsorption assisted desorption. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0039-6028(99)00468-9

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