Faculty Profiles - Yamakata Akira

Electronically cross-linked PbS quantum dot networks using polymers as surface ligands Reviewed

Wei Lü, Yue Yang, Akira Yamakata, Itaru Kamiya

Nano 8 ( 2 ) 1350013.1-1350013.7 2013.4

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

Bilayer structures of colloidal PbS quantum dots (QDs) with different sizes are fabricated by two methods: chemical bond anchoring and electrostatic adsorption. Unlike the sample prepared by chemical bond anchoring, we find that the electronic connection between different layers is suppressed in the samples prepared by electrostatic adsorption with charged polyelectrolytes. To clarify the electronic coupling between the QDs and the polyelectrolytes, QDs directly stabilized by charged polyelectrolytes were synthesized. Transmission electron microscopy investigation shows that cross-linked QD networks are formed. Time-resolved photoluminescence measurements indicate that the carrier lifetimes of QDs in networks were greatly reduced. The photoluminescence peak energy increases with a third root of the excitation power. All these observations can be consistently explained considering spatially separated photoexcited carriers in a type-II band alignment. The abnormal behaviors of electrostatic adsorption sample are discussed based on these results. © 2013 World Scientific Publishing Company.

DOI： 10.1142/S1793292013500136

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Cobalt-Modified Porous Single-Crystalline LaTiO2N for Highly Efficient Water Oxidation under Visible Light Reviewed

Fuxiang Zhang, Akira Yamakata, Kazuhiko Maeda, Yosuke Moriya, Tsuyoshi Takata, Jun Kubota, Katsuya Teshima, Shuji Oishi, Kazunari Domen

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 134 ( 20 ) 8348 - 8351 2012.5

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

Highly efficient water oxidation utilizing visible photons of up to 600 nm is a crucial step in artifical photosynthesis. Here we present a highly active photocatalyst for visible-light-driven water oxidation, consisting of single-crystalline meso- and macroporous LaTiO2N (LTON) with a band gap of 2.1 eV, and earth-abundasnt cobalt oxide (CoOx) as a cocatalyst. The optimized CoOx/LTON had a high quantum efficiency of 27.1 +/- 2.6% at 440 nm, which substantially exceeds the values reported for previous particulate photocatalysts with a 600-nm absorption edge.

DOI： 10.1021/ja301726c

Infrared Spectroscopic Study of the Potential Change at Cocatalyst Particles on Oxynitride Photocatalysts for Water Splitting by Visible Light Irradiation Reviewed

Xuwang Lu, Athula Bandara, Masao Katayama, Akira Yamakata, Jun Kubota, Kazunari Domen

JOURNAL OF PHYSICAL CHEMISTRY C 115 ( 48 ) 23902 - 23907 2011.12

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

Visible light-induced changes in Pt/(Ga1-xZnx)(Ni1-xOx), Pt/TaON, and Pt/LaTiO2N powder photocatalysts were investigated using infrared spectroscopy with adsorbed CO probe molecules on Pt cocatalysts. Among these photocatalyst materials, only the (Ga1-xZnx)(N1-xOx) photocatalyst has achieved overall water splitting with visible-light irradiation, although all of these materials have suitable band gaps for the reaction. For Pt/(Ga1-xZnx)(N1-xOx) and Pt/TaON, the C-O stretching vibrations of adsorbed CO on Pt shifted to a lower frequency under visible light irradiation. However, no frequency shift was observed for Pt/LaTiO2N. This lower-frequency shift indicates a negative electronic potential shift of the Pt cocatalysts, which results in hydrogen evolution. Differences in the light intensity dependence of the potential shifts of these photocatalysts were observed, and the involvement of a different recombination mechanism of photoexcited carriers in these photocatalysts will be discussed. These results clarify some of the reasons for the activities of various photocatalytic materials for overall water splitting.

DOI： 10.1021/jp207484q

Effect of Photoexcited Electron Dynamics on Photocatalytic Efficiency of Bismuth Tungstate Reviewed

Fumiaki Amano, Akira Yamakata, Kohei Nogami, Masatoshi Osawa, Bunsho Ohtani

JOURNAL OF PHYSICAL CHEMISTRY C 115 ( 33 ) 16598 - 16605 2011.8

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

Photoexcited carrier dynamics of bismuth tungstate (Bi(2)WO(6)) photocatalysts was investigated by time-resolved infrared (IR) absorption spectroscopy. Monotonic absorption at the mid-IR region, which is attributable to absorption by photoexcited electrons, was monitored as a function of time delay from the microsecond to millisecond range after photoexcitation. Bi(2)WO(6) particles with different crystalline content were prepared by hydrothermal reaction at several temperatures and used to elucidate the relation between density of photoexcited carriers and steady-state photocatalytic efficiency. Photocatalytic efficiency was tested using two reactions: oxidative decomposition of acetic acid in an aqueous solution (reaction 1) and oxidative decomposition of acetaldehyde in air (reaction 2). Crystallization of Bi(2)WO(6) particles suppressed the fast recombination of photoexcited electrons and holes within 1 its. In the case of crystallized particles, the density of the photoexcited electron increased with an increase in the crystalline content, and the photocatalytic efficiency for reaction 1 strongly depended on the crystalline content, indicating that photoexcited electrons remaining in the submillisecond time range significantly affect the reaction rate. On the other hand, photocatalytic efficiency for reaction 2 showed a proportional relation with specific surface area rather than crystalline content. The difference in a decisive factor depending on reaction condition is considered to be the slower rate of reaction of photoexcited electrons with molecular oxygen, which might occur within a time range between 200 mu s and 3 ms over Bi(2)WO(6).

DOI： 10.1021/jp2051257

Potential-Dependent Recombination Kinetics of Photogenerated Electrons in n- and p-Type GaN Photoelectrodes Studied by Time-Resolved IR Absorption Spectroscopy Reviewed

Akira Yamakata, Masaaki Yoshida, Jun Kubota, Masatoshi Osawa, Kazunari Domen

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 133 ( 29 ) 11351 - 11357 2011.7

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

Recombination kinetics of photogenerated electrons in n-type and p-type GaN photoelectrodes active for H-2 and O-2 evolution, respectively, from water was examined by time-resolved IR absorption (TR-IR) spectroscopy. Illumination of a GaN film with UV pulse (355 nm and 6 ns in duration) gives transient interference spectra in both transmittance and reflection modes. Simulation shows that the interference spectra are caused by photogenerated electrons. We observed that recombination in the microsecond region is greatly affected by the applied potentials, the lifetime becoming longer at negative and positive potentials for n- and p-type GaN electrodes, respectively. There is a good correlation between potential dependence of the steady-state reaction efficiency and that of the number of surviving electrons in the millisecond region. We also performed potential jump measurement to examine the shift in Fermi level by photogenerated charge carriers. In the case of n-type GaN, the electrode potential jumps to the negative side by accumulation of electrons in the bulk. However, in the case of p-type GaN, the electrode potential first jumps to the negative side within 20,us and gradually shifts to the positive side in a few milliseconds, while the number of charge carriers is constant at >0.2 ms. This two-step process is ascribed to electron transport from the bulk to the surface of GaN, because the electrode potential is sensitive to the number of electrons in the bulk. The results confirm that TR-IR combined with potential jump measurement provides useful information for understanding the behavior of charge carriers in photo electrochemical systems.

DOI： 10.1021/ja203639q

Quantum Dots and Wells for Novel Light-Energy Conversions

YAMAKATA Akira

触媒 53 ( 3 ) 201 - 201 2011.4

Structure and Behavior of Water at the Electrochemical Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy Reviewed

Masatshi Osawa, Akira Yamakata

BUNSEKI KAGAKU 60 ( 1 ) 1 - 9 2011.1

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC ANALYTICAL CHEMISTRY

How do water molecules behave at the electrochemical interface (the charged electrode-electrolyte interface)? This is one of the most fundamental and important questions in electrochemistry, and has been discussed extensively since the first proposal of the electric double layer model by Helmholtz in 1853. In the 1960s, a general consensus was reached that water molecules orient along the electric field at the interface and change the orientation depending on the applied potential (i.e., surface charge). However, details had long been unclear. Recent developments of computational theoretical chemistry and in situ surface analytical techniques have enabled a further detailed study of the interface at a molecular level and renewed our understanding. Recent progresses has been reviewed in the manuscript by focusing on experimental data provided by surface-enhanced infrared absorption spectroscopy (SEIRAS).

DOI： 10.2116/bunsekikagaku.60.1

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Destruction of the Water Layer on a Hydrophobic Surface Induced by the Forced Approach of Hydrophilic and Hydrophobic Cations Reviewed

Akira Yamakata, Masatoshi Osawa

JOURNAL OF PHYSICAL CHEMISTRY LETTERS 1 ( 9 ) 1487 - 1491 2010.5

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

Structural change of the water layers is the primary step for the direct interactions among the hydrated substances in aqueous solutions. We have studied how the water layers on a solid surface is destroyed by the approach of several hydrophilic and hydrophobic cations, which are used as model substances. The combined use of electrochemical system and IR spectroscopy enables the study of the destruction process of water layers during the close contact among them. We have observed for the first time that hydrophilic cations cause greater destruction of the interfacial water structure than hydrophobic cations. This result is ascribed to the different orientations of water molecules around the cations; H-bonds around hydrophilic cations expand toward the outer hydration shells, while they are closed inside the primary shell in the case of hydrophobic cations. These different orientations of water molecules cause different effects on the interfacial water structures.

DOI： 10.1021/jz100424e

Correlation between Photoexcited Electron Lifetimes and Photocatalytic Activities of Bismuth Tungstate Particles Reviewed

AMANO Fumiaki, YAMAKATA Akira, NOGAMI Kohei, OSAWA Masatoshi, OHTANI Bunsho

触媒 52 ( 2 ) 128 - 130 2010.3

Dynamics of photogenerated charge carriers on photocatalysts for the water splitting reaction : relation between the carrier dynamics and the steady state reactions Reviewed

YAMAKATA Akira

分光研究 = Journal of the spectroscopical research of Japan 59 ( 1 ) 12 - 19 2010.2

ATR-SEIRAS Investigation of the Fermi Level of Pt Cocatalyst on a GaN Photocatalyst for Hydrogen Evolution under Irradiation Reviewed

Masaaki Yoshida, Akira Yamakata, Kazuhiro Takanabe, Jun Kubota, Masatoshi Osawa, Kazunari Domen

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 131 ( 37 ) 13218 - 13219 2009.9

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

The interaction of photogenerated carries in GaN photocatalyst with Pt cocatalyst for hydrogen evolution under irradiation was investigated from in situ ATR-SEIRAS measurement by following the CO vibrational frequency. After irradiation, the CO frequency shifted higher, indicating that the Fermi Level of Pt particles was positively shifted by the photogenerated holes. Thereafter, a Lower frequency peak appeared, indicating that the Fermi level of some Pt particles was negatively shifted to the hydrogen evolution potential by photogenerated electrons, which is the essential function of cocatalysis for hydrogen evolution.

DOI： 10.1021/ja904991p

Photoexcited processes on metal surfaces : from gas/solid toward liquid/solid interfaces Reviewed

YAMAKATA Akira, OSAWA Masatoshi

Photochemistry 40 ( 2 ) 78 - 84 2009.8

Destruction of the hydration shell around tetraaolkylammonium ions at the electrochemical interface Reviewed

Akira Yamakata, Masatoshi Osawa

Journal of the American Chemical Society 131 ( 20 ) 6892 - 6893 2009.5

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

The structure and behavior of the hydration shells around tetraethyl-, -propyl-, and -butylammonium ions (Et4N+, Pr4N+, and Bu4N+, respectively) at a CO-covered Pt electrode was studied by surface-enhanced IR absorption spectroscopy. Selective concentration of cations at the electrochemical interface by controlling the applied potential facilitates the observation of the hydration shells without interference from hydrated anions. We report for the first time that the hydration shells around Pr4N+ and Bu4N+ are decomposed at sufficiently negative potentials because of the strong electrostatic attraction with the surface. On the other hand, the hydration shell around Et4N+, the smallest ion, is more stable and does not decompose in the potential range examined. The difference in the stability can be ascribed to the difference in the size of the central cation. On the other hand, the hydration shells are easily deconstructed on a bare Pt surface because of the stronger interaction with the surface. Copyright © 2009 American Chemical Society.

DOI： 10.1021/ja8100374

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Acceleration of electrolysis of water by the adsorption of 4, 4'-bipyridine on Ag electrode : creation of reaction center by organic molecules Reviewed

UCHIDA Taro, YAMAKATA Akira, SASAKI Youichi, OSAWA Masatoshi

觸媒 = Catalyst 51 ( 2 ) 129 - 131 2009.3

Electrocatalytic Reactions Studied by Time-resolved Surface-enhanced Infrared Spectroscopy Reviewed

SAMJESKE Gabor, YAMAKATA Akira, UCHIDA Taro, OSAWA Masatoshi

Journal of the Surface Science Society of Japan 30 ( 2 ) 68 - 74 2009.2

Visible Light Responsive Pristine Metal Oxide Photocatalyst: Enhancement of Activity by Crystallization under Hydrothermal Treatment Reviewed

Fumiaki Amano, Akira Yamakata, Kohei Nogami, Masatoshi Osawa, Bunsho Ohtani

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 130 ( 52 ) 17650 - + 2008.12

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

Photocatalytic activities of amorphous and crystal bismuth tungstate (Bi(2)WO(6))were investigated using oxidative decomposition of gaseous acetaldehyde under visible light irradiation (>400 nm). Here, for the first time, negligible photocatalytic activity of amorphous Bi(2)WO(6) owing to the fast recombination of electron-hole pairs and the high quantum efficiency of Bi(2)WO(6) crystallites under visible light were demonstrated by action spectrum analysis and time-resolved infrared absorption measurements. Crystallization of the amorphous phase provided a red shift of the photoabsorption edge and marked increase in the lifetime of photoexcited electrons, resulting in an increase of photocatalytic activity.

DOI： 10.1021/ja807438z

Dynamics of the Electric Double Layer of CO-covered Pt Electrode Reviewed

YAMAKATA Akira, OSAWA Masatoshi

J. Surf. Sci. Soc. Jpn. 29 ( 9 ) 526 - 531 2008.9

Language：Japanese Publisher：The Surface Science Society of Japan

The dynamics of the water molecules on a CO-covered Pt electrode has been studied by laser-induced potential transient measurements. This is a model system to elucidate the behavior of water molecules in the electric double layer during electrochemical reactions. At the potentials where CO is stably adsorbed, the Pt surface is negatively charged and water molecules are oriented pointing the hydrogen atoms toward the surface. The laser irradiation heats the interface and disturbs the orientation of water, which induces a potential jump toward the negative direction. We found that the amplitude of the potential jump on the CO-covered surface is much larger than on the CO-free surface. The finding suggests that the water molecules are more flexible on the CO-covered surface due to the weaker water-CO interaction than water-Pt interaction. Stronger laser pulse irradiation additionally induces the desorption of CO, which result in the adsorption of water with the hydrogen atom pointing toward the surface and induces a positive potential shift. The potential shift takes ∼70 microseconds to reach the maximum, which becomes slower as the electric field becomes intense. The intense field and hydrogen-bonded cations affect water structure in a long range and the collective motion of the water layers.

DOI： 10.1380/jsssj.29.526

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