Faculty Profiles - Yamakata Akira

KCl flux-induced growth of isometric crystals of cadmium-containing early transition-metal (Ti4+, Nb5+, and Ta5+) oxides and nitridability to form their (oxy)nitride derivatives under an NH3 atmosphere for water splitting application Reviewed

Mirabbos Hojamberdiev, Hajime Wagata, Kunio Yubuta, Kenta Kawashima, Junie Jhon M. Vequizo, Akira Yamakata, Shuji Oishi, Kazunari Domen, Katsuya Teshima

APPLIED CATALYSIS B-ENVIRONMENTAL 182 626 - 635 2016.3

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

In this study, we have attempted to experimentally validate the results of previous theoretical calculations predicting the possible formation of the CdTiO3-xNy, CdNbO2N, and CdTaO2N phases by applying conventional one- and two-step fabrication methods under an NH3 flow. For the two-step method, CdTiO3, Cd2Nb2O2, and Cd2Ta2O7 crystals were first grown by a KCl flux method, and the effects of solute concentration and cooling rate on the crystal growth were studied. The formability of their (oxy)nitride derivatives was investigated by changing the nitridation temperature (750-950 degrees C) and time (1-10 h) of oxide precursors. It was found that the CdTiO3-xNy, CdNbO2N, and CdTaO2N phases cannot be formed by the applied conventional methods due to the low volatilization temperature of cadmium and the susceptibility of titanium and niobium to reduction under an NH3 atmosphere. Under high-temperature NH3 atmosphere, only Cd2Ta2O2 was fully converted to single-phase Ta3N5. The results from the photocatalytic O-2 evolution test of bare and CoOx-loaded Ta3N5 crystalline structures, converted from Cd2Ta2O2 (Cd-Ta3N5) and Na2CO3-treated Ta2O3 (Na-Ta3N5) and Cd2Ta2O2 (Na-Cd-Ta3N5) crystals by nitridation at 850 degrees C for 20h under an NH3 flow, revealed that the CoOx-loaded Ta3N5 showed more than two times higher O-2 evolution rate (655 mu mol), whereas the CoOx-loaded Cd-Ta3N5 and Na-Cd-Ta3N5 exhibited nearly four (501 mu mol) and three (422 mu mol) times higher O-2 evolution rates at 5 h compared with their bare counterparts. An improved photocatalytic activity for O-2 evolution is related to the higher density of nucleation centers of CoOx nanoparticles in the form of dangling bonds in porous Ta3N5 structures and long-lived photogenerated holes, as attested by time-resolved absorption spectroscopy. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/bj.apcatb.2015.10.002

Transient Absorption and FT-IR Analysis of the Carrier Dynamics of TiO2 Photocatalysts

Vequizo Junie Jhon, Matsunaga Hironori, Okamura Shou, Ohno Teruhisa, Yamakata Akira

表面科学学術講演会要旨集 36 ( 0 ) 217 - 217 2016.1

Language：English Publisher：The Surface Science Society of Japan

The behavior of photocarriers in anatase, rutile, and brookite TiO<sub>2</sub> was studied by using transient absorption and FT-IR spectroscopy. In anatase, free electrons survive > 1 ms. In contrast, most of the electrons in rutile are deeply trapped at the defects. In brookite, some of the electrons are deeply trapped, but some also survive in the conduction band. FT-IR results revealed that the rate of charge accumulation is faster in anatase but slower in rutile and brookite.

DOI： 10.14886/sssj2008.36.0_217

CiNii Article

Flux-mediated doping of SrTiO3 photocatalysts for efficient overall water splitting Reviewed

Yeilin Ham, Takashi Hisatomi, Yosuke Goto, Yosuke Moriya, Yoshihisa Sakata, Akira Yamakata, Jun Kubota, Kazunari Domen

JOURNAL OF MATERIALS CHEMISTRY A 4 ( 8 ) 3027 - 3033 2016

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

SrTiO3 is a photocatalyst that is well known for its activity for the overall water splitting reaction under UV light irradiation. In this study, the effects of SrCl2 flux treatments and Al doping on the photocatalytic properties of SrTiO3 were investigated. The SrTiO3, which showed an apparent quantum efficiency of 30% at 360 nm in the overall water splitting reaction, the highest value reported so far, was prepared by SrCl2 flux treatments in alumina crucibles. Scanning electron microscopy and X-ray diffractometry revealed that the flux-treated SrTiO3 consisted of well-crystalline particles with a cubic shape reflecting the perovskite-type structure. Inductively coupled plasma optical emission spectroscopy revealed that Al ions from the alumina crucibles were incorporated into the SrTiO3 samples. The SrTiO3 that was treated with SrCl2 flux in Al-free conditions showed a marginal improvement in photocatalytic activity despite the high crystallinity and the clear crystal habit. Doping SrTiO3 with Al improved the photocatalytic activity even without SrCl2 treatment. These results suggested that Al doping was a principal factor in the dramatic improvement in the water splitting activity of the flux-treated SrTiO3. The effects of flux treatments and Al doping on the morphology and water splitting activity of SrTiO3 were discussed separately.

DOI： 10.1039/c5ta04843e

Femto-second time-resolved absorption study on photocatalysts

Matsunaga Hironori, Vequizo Junie Jhon, Yamakata Akira

Abstract of annual meeting of the Surface Science of Japan 36 218 - 218 2016

Behavior of photogenerated charge carriers on metal-particle loaded photocatalysts studied by time-resolved absorption spectroscopy

Yamakata Akira

Abstract of annual meeting of the Surface Science of Japan 36 216 - 216 2016

Morphology-sensitive trapping states of photogenerated charge carriers on SrTiO3 particles studied by time-resolved visible to Mid-IR absorption spectroscopy: The effects of molten salt flux treatments Reviewed

Akira Yamakata, Ham Yeilin, Masayuki Kawaguchi, Takashi Hisatomi, Jun Kubota, Yoshihisa Sakata, Kazunari Domen

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 313 168 - 175 2015.12

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

The effects of the morphology-change of SrTiO3 particles on the behavior of photogenerated charge carriers are studied by time-resolved absorption (TA) spectroscopy from the visible to mid-IR region. In the case of as-purchased defect-rich commercial SrTiO3 particles, most of the charge carriers are deeply trapped, showing a transient absorption peak at 11,000 cm(-1). Scanning electron microscopy reveals that the irregular-shaped primary particles are heavily aggregated and that the photocatalytic activity for the water splitting reaction is negligibly small. However, when this powder is flux-treated by SrCl2, fine cubic SrTiO3 crystals exposing well-defined surfaces are formed and the photocatalytic activity is greatly improved. TA measurements show that the spectral shape is changed dramatically: the number of deeply trapped electrons is reduced, and that of shallowly trapped electrons producing the absorption band at 2500 cm(-1) is increased. The change in electron trap depth, observed upon flux treatment, is due to the decrease in the number of defects. We also found that further flux treatment in an Al2O3 crucible (i) induces Al doping into SrTiO3, (ii) enhances the photocatalytic activity, (iii) changes the spectral shape, and (iv) prolongs the lifetime of shallowly trapped electrons. The increase in photocatalytic activity is presumably due to the change in lifetime. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jphotochem.2015.05.016

Distinctive Behavior of Photogenerated Electrons and Holes in Anatase and Rutile TiO2 Powders Reviewed

Akira Yamakata, Junie Jhon M. Vequizo, Hironori Matsunaga

JOURNAL OF PHYSICAL CHEMISTRY C 119 ( 43 ) 24538 - 24545 2015.10

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

TiO2 powders have been widely used for photocatalysts; however, why anatase shows higher activity than ruffle has been a long-standing question. Here, we have elucidated the difference in the behavior of photogenerated electrons and holes by time-resolved visible to mid-IR absorption spectroscopy. In anatase TiO2, a considerable number of free electrons survive longer than 1 ms, but they are deeply trapped within a few picoseconds in the case of ruffle TiO2. The longer lifetime of free electrons is responsible for the higher activity for reduction processes on anatase TiO2. However, deep electron trapping in ruffle TiO2 elongates lifetime of holes and promotes multihole processes such as water oxidation. However, the low reactivity of deeply trapped electrons fails to increase the overall activity. These peculiar behaviors of electrons and holes are induced by defects on the powder particles and less sensitive to the physical properties such as particle size and specific surface area.

DOI： 10.1021/acs.jpcc.5b09236

NH3-Assisted Flux Growth of Cube-like BaTaO2N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity Reviewed

Mirabbos Hojamberdiev, Kunio Yubuta, Junie Jhon M. Vequizo, Akira Yamakata, Shuji Oishi, Kazunari Domen, Katsuya Teshima

CRYSTAL GROWTH & DESIGN 15 ( 9 ) 4663 - 4671 2015.9

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

As the 600 nm-class photocatalyst, BaTaO2N is one of the promising candidates of the perovskite-type oxynitride family for photocatalytic water splitting under visible light. The oxynitrides are routinely synthesized by nitriding corresponding oxide precursors under a high-temperature NH3 atmosphere, causing an increase in the defect density and a decrease in photocatalytic activity. To improve the photo catalytic activity by reducing the defect density and improving the crystallinity, we here demonstrate an NH3-assisted KCl flux growth approach for the direct synthesis of BaTaO2N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO2N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO2N crystals could only be grown with the solute concentrations of >= 10 mol % using the KCl flux, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO2N crystals with an average size of about 125 nm and exposed {100} and {110} faces at 950 degrees C for 10 h. The time- and temperature-dependent experiments were also performed to postulate the direct growth mechanisms of cube-like BaTaO2N submicron crystals. The BaTaO2N crystals modified with Pt and CoOx nanopartides showed a reasonable H-2 and O-2 evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH3-assisted KCl flux method.

DOI： 10.1021/acs.cgd.5b00927

NH3-Assisted Flux-Mediated Direct Growth of LaTiO2N Crystallites for Visible-Light-Induced Water Splitting Reviewed

Kenta Kawashirna, Mirabbos Hojamberdiev, Hajirne Wagata, Kunio Yubuta, Junie Jhon M. Vequizo, Akira Yamakata, Shuji Oishi, Kazunari Domen, Katsuya Teshima

JOURNAL OF PHYSICAL CHEMISTRY C 119 ( 28 ) 15896 - 15904 2015.7

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

Photocatalytic overall water splitting on (oxy)nitrides under visible light is one of the interesting approaches to fulfill the growing demand for clean and renewable energy. The improvement of the fabrication method is however important for reducing the defect density of (oxy)nitride crystals. The present study aims to investigate the direct growth of the LaTiO2N (LTON) crystallites with less defect density by an NH3-assisted flux method and to demonstrate the visible-light-induced photocatalytic water oxidation activity in relation to their crystallite morphology. Single-phase LaTiO2N crystallites (average size of 120 +/- 39 nm) in round shape with smooth surface and high crystallinity were grown by an NH3-assisted flux method using the KCl flux with the solute concentration of 5 mol % at 950 degrees C for 10 h. The photocatalytic water oxidation activity of bare and CoOx-loaded LaTiO2N crystallites grown directly by an NH3-assisted flux method (1-step-LTON) was evaluated under visible light by comparing with the LaTiO2N crystallites fabricated by a two-step method (2-step-LTON), converting La2Ti2O7 to LaTiO2N by high-temperature nitridation. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O-2 evolution rates of bare and CoOx-loaded 1-step-LTON crystallites were 82 mu mol.h-(1) and 204 mu mol.h(-1), respectively, which are much higher than that of bare and CoOx-loaded 2-step-LTON crystallites (37 mu mol.h(-1) and 177 mu mol.h(-1)) due to less defect density of the LaTiO2N crystallites achieved by a direct fabrication route using KCl flux. An NH3-assisted flux growth is a promising route for the direct fabrication of the LaTiO2N crystallites with less defect density that is beneficial for the enhancement of photocatalytic water oxidation half-reaction.

DOI： 10.1021/acs.jpcc.5b03718

時間分解可視中赤外分光法を用いた光励起キャリアーの反応活性評価

山方啓, 久保田純, 堂免一成

触媒 115 52 - 54 2015.3

Behavior and Energy State of Photogenerated Charge Carriers in Single-Crystalline and Polycrystalline Powder SrTiO3 Studied by Time-Resolved Absorption Spectroscopy in the Visible to Mid-Infrared Region Reviewed

Akira Yamakata, Junie Jhon M. Vequizo, Masayuki Kawaguchi

JOURNAL OF PHYSICAL CHEMISTRY C 119 ( 4 ) 1880 - 1885 2015.1

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

The effects of defects on the behavior of photogenerated charge carriers in SrTiO3 (STO) are studied by time-resolved absorption spectroscopy from the visible to mid-IR region. In the case of defect-free single-crystalline STO, free and shallowly trapped electrons are dominant, but they recombine within 50 ns. By contrast, in the case of defect-rich powder STO, the electron lifetime is much longer than 1 ms. The transient absorption spectra show that most of the charge carriers in powder STO are trapped in the defects, which elongates their lifetime. We found that these trapped carriers are nevertheless reactive toward O-2 or CH3OH that depends on the trap depth. The steady-state photocatalytic activity is strongly correlated with the lifetime and the reactivity of the trapped charge carriers: the energy state of electrons can be deduced from the spectral shape, especially in the mid-IR region.

DOI： 10.1021/jp510647b

Surface-enhanced IR absorption spectroscopy of the KcsA potassium channel upon application of an electric field Reviewed

Akira Yamakata, Hirofumi Shimizu, Shigetoshi Oiki

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 17 ( 33 ) 21104 - 21111 2015

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

Surface-enhanced IR absorption spectroscopy (SEIRAS) is a powerful tool for studying the structure of molecules adsorbed on an electrode surface (ATR-SEIRA). Coupled with an electrochemical system, structural changes induced by changes in the electric field can be detected. All the membrane proteins are subjected to the effect of membrane electric field, but conformational changes at different membrane potentials and their functional relevance have not been studied extensively except for channel proteins. In this contribution, background information of potential-dependent functional and structural changes of a prototypical channel, the KcsA channel, is summarized, and SEIRAS applied to the KcsA channel under the application of the potential is shown. The potassium channels allow K+ to permeate selectively through the structural part called the selectivity filter, in which dehydrated K+ ions interact with backbone carbonyls. In the absence of K+, the selectivity filter undergoes conformational changes to the non-conductive collapsed conformation. To apply the electric field, the KcsA channels were fixed on the gold surface in either upside or reverse orientation. The SEIRA spectrum in K+ or Na+ solution revealed both backbone structural changes and local changes in the OCO-carboxylate groups. Upon application of the negative electric field, the spectrum of OCO was enhanced only in the K+ solution. These results indicate that the negative electric field accumulates local K+ concentration, which turned the collapsed filter to the conductive conformation. ATR-SEIRA serves as an unprecedented experimental system for examining membrane proteins under an electric field.

DOI： 10.1039/c5cp02681d

Behavior and Energy States of Photogenerated Charge Carriers on Pt- or CoOx-Loaded LaTiO2N Photocatalysts: Time-Resolved Visible to Mid-Infrared Absorption Study Reviewed

Akira Yamakata, Masayuki Kawaguchi, Naoyuki Nishimura, Tsutomu Minegishi, Jun Kubota, Kazunari Domen

JOURNAL OF PHYSICAL CHEMISTRY C 118 ( 41 ) 23897 - 23906 2014.10

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

Femtosecond to second time-resolved visible to mid-infrared absorption spectroscopy was applied to investigate the behavior of photogenerated electrons and holes on a Pt- or CoOx-loaded LaTiO2N photocatalyst. CoOx-loaded catalyst exhibits the highest activity for water oxidation under visible light (<600 nm) excitation, and the quantum efficiency reaches up to 30%. Transient absorption spectra suggest that most of the photoexcited electrons in LaTiO2N lose activity by deep trapping in the mid-gap states created at 0.74 eV (6000 cm1) below the conduction band. In this case, Pt loading was not so effective for H-2 evolution because the loaded Pt could not effectively capture the trapped electrons from LaTiO2N. The electron transfer was slow, proceeding in 0100 mu s, and was thus ineffective. However, in the case of CoOx loading, we have clearly observed, for the first time, that the holes are captured rapidly by CoOx in a few picoseconds, and the lifetimes of electrons are dramatically prolonged to the second region. This implies that the photogenerated holes and electrons are separated effectively in CoOx and LaTiO2N, respectively. Furthermore, the electron trap becomes shallower, its depth decreasing from 0.74 eV (6000 cm1) to 0.49 eV (4000 cm1) upon CoOx loading, suggesting that the reactivity of the trapped electrons increases. These perturbations of electrons and holes are what cause the dramatic increase in photocatalytic activity. We expected that coloading of Pt and CoOx would further increase the activity, but it was significantly reduced. It was demonstrated that the undesirable process of recombination is accelerated under high loading and coloading.

DOI： 10.1021/jp508233z

電極界面におけるイオンの水和殻崩壊過程の時間分解赤外分光観察 Reviewed

山方啓, 大澤雅俊, 森田明弘

Electrochemistry 電気化学および工業物理化学 82 ( 9 ) 771 - 776 2014.9

Behavior of Photogenerated Charge Carriers on Several Photocatalysts Studied by Time-resolved IR Absorption Spectroscopy. Reviewed

天野史章, 山方啓

触媒 56 ( 2 ) 88 - 94 2014.4

Properties of N-H Local Vibration Modes in GaAsN Grown by Chemical Beam Epitaxy

Kazuma Ikeda, Akira Yamakata, Koushiro Demizu, Nobuaki Kojima, Oshio Ohshita, Masafumi Yamaguchi

2014 IEEE 40TH PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC) 1821 - 1824 2014

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IEEE

The structural and electrical properties of N-H complexes in GaAsN grown by chemical beam epitaxy are summarized. Using local vibration mode spectroscopy, some of the vibration frequencies are shown to be the same as those in the material grown by liquid encapsulated Czochralski and metal organic chemical vapor deposition, while no vibration modes are common to those in the material grown by molecular beam epitaxy. The dependencies of the vibration frequencies on measurement temperatures and annealing temperatures, and polarization of the vibration directions are studied to understand the structural properties of N-H complexes. The As source flow rate affects ratios of the defect densities, which indicate that those defects have different formation processes. The charge excitations by irradiation light indicate that the charge relaxation processes caused by those defects are in the orders of micro seconds.

DOI： 10.1109/PVSC.2014.6925276

Effect of Particle Size on the Photocatalytic Activity of WO3 Particles for Water Oxidation Reviewed

Fumiaki Amano, Eri Ishinaga, Akira Yamakata

JOURNAL OF PHYSICAL CHEMISTRY C 117 ( 44 ) 22584 - 22590 2013.11

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

The particle size of a photocatalyst is one of the factors that determine the photon utilization efficiency. Here, we report the effects of the particle size on the photocatalytic activity of tungsten trioxide (WO3) for water oxidation to evolve molecular oxygen in the presence of sacrificial silver nitrate. For the water oxidation, the photocatalytic activity of large aggregates consisting of particles with a small specific surface area (3 m(2) g(-1)) was greater than that of fine particles with a larger specific surface area (11 m(2) g(-1)). Transient infrared absorption spectra revealed that the recombination of photoexcited electrons in the large particles was slower than that in the fine particles. It was found that the fast recombination in the fine particles mostly occurred on the surface. The photocatalytic activity for water oxidation increased with increasing particle size for spherical diameters smaller than 200 nm. These results showed that large particles with a low surface area-to-volume ratio were suitable for photocatalytic oxygen evolution, which requires long-lived holes, because the surface recombination of photogenerated electrons and holes occurred less frequently.

DOI： 10.1021/jp408446u

Real-Time Observation of the Destruction of Hydration Shells Reviewed

Akira Yamakata, Eiji Soeta, Tatsuya Ishiyama, Masatoshi Osawa, Akihiro Morita

J. Am. Chem. Soc. 135 ( 40 ) 977 - 988 2013.10

Real-Time Observation of the Destruction of Hydration Shells under Electrochemical Force Reviewed

Akira Yamakata, Eiji Soeta, Tatsuya Ishiyama, Masatoshi Osawa, Akihiro Morita

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 135 ( 40 ) 15033 - 15039 2013.10

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

Major alteration or even destruction of the hydration shell around interacting molecules and ions in solution is an important process that determines how hydrated substances interact. Therefore, the direct observation of structural changes in hydration shells around solutes in close contact with other solutes or surfaces is important for understanding chemical processes that take place in solution. In the work described in this paper, time-resolved IR absorption measurements were performed to study the interaction of hydrated Na+ or tetrapropylammonium cation (Pr4N+) with a hydrophobic CO-covered Pt surface; the adsorption force between cations and the surface was controlled by using an electrochemical system. We found that the hydrophobic hydration shell of Pr4N+ is initially stabilized on the hydrophobic surface, but application of a strong force to the cation approaching CO destroys the water layers between them. This process is rather slow, taking a few hundred milliseconds. Hydrophilic Na+ behaves quite differently from Pr4N+ due to the different structure of its hydration shell. These experimental results are supported by molecular dynamics simulations.

DOI： 10.1021/ja408326d