Faculty Profiles - Yamakata Akira

Papers - Yamakata Akira

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Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction Reviewed

Ryo Kuriki, Chandana Sampath, Kumara Ranasinghe, Yasuomi Yamazaki, Akira Yamakata, Osamu Ishitani, Kazuhiko Maeda

J. Phys. Chem. C. 122 ( 29 ) 16795 - 16802 2018.6

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

We have previously developed photocatalytic CO, reduction systems using graphitic carbon nitride (g-C3N4) and a Ru(II) mononuclear complex (e.g., trans(Cl)-[Ru-II{4,4'-(H2PO3)(2) bpy}(2)(CO)(2)Cl-2] bpy = 2,2'-bipyridine, abbreviated as RuP) hybrids and demonstrated its high activities under visible light (lambda > 400 nm). To understand the excited-state dynamics of C3N4 and electron-transfer process to RuP, here we examined the photophysical properties of g-C3N4 as well as mesoporous g-C3N4 (mpg-C3N4) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C3N4 (as well as mpg-C3N4) has at least three emissive excited states with different lifetimes (g-C3N4; 1.3 +/- 0.4, 3.9 +/- 0.9, and 15 +/- 4 ns at 269 nm photoexcitation) in aqueous suspension. These excited states were not quenched upon addition of a hole scavenger (e.g., disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (RuP), even though photochemical electron-transfer processes from/to g-C3N4 has been experimentally confirmed by photocatalytic reactions. On the other hand, TR-IR spectroscopy clearly indicated that mobile electrons photogenerated in mpg-C3N4, which are shallowly trapped and/or free electron in the conduction band, are able to move into RuP with a timescale of a few picoseconds. These results suggest that main emission centers and reaction sites (including charge-transfer interfaces) are separately located in the C3N4 materials, and that electron transfer from C3N4 to RuP progresses through less- or non-luminescent sites, in which mobile electrons exist with a certain lifetime.

DOI： 10.1021/acs.jpcc.8b03996

Homogeneous Electron Doping into Non-Stoichiometric Strontium Titanate Improves Its Photocatalytic Activity for Hydrogen and Oxygen Evolution Reviewed

Shunta Nishioka, Junji Hyodo, Junie Jhon, M. Vequizo, Shunsuke Yamashita, Hiromu Kumagai, Koji Kimoto, Akira Yamakata, Yoshihiro Yamazaki, Kazuhiko Maeda

ACS Catal. 8 ( 8 ) 7190 - 7200 2018.6

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

Water splitting using a semiconductor photocatalyst has been extensively studied as a means of solar-to-hydrogen energy conversion. Powder-based semiconductor photocatalysts, in particular, have tremendous potential in cost mitigation due to system simplicity and scalability. The control and implementation of powder-based photocatalysts are, in reality, quite complex. The identification of the semiconductorphotocatalytic activity relationship and its limiting factor has not been fully solved in any powder-based semiconductor photocatalyst. In this work, we present systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution using a model strontium titanate powder/aqueous solution interface in a half reaction. The electron density was controlled from 10(16) to 1(02)0 cm(3) throughout the strontium titanate powder by charge compensation with oxygen nonstoichiometry (the amount of oxygen vacancy) while maintaining its crystallinity, chemical composition, powder morphology, and the crystal and electronic structure of the surface. The photocatalytic activity of hydrogen evolution from aqueous methanol solution was stable and enhanced by 40-fold by the electron doping. The enhancement was correlated well with increased Delta absorbance, an indication of prolonged lifetime of photoexcited electrons, observed by transient absorption spectroscopy. Photocatalytic activity of oxygen evolution from aqueous silver nitrate solution was also enhanced by 3-fold by the electron doping. Linear correlation was found between the photocatalytic activity and the degree of surface band bending, Delta Phi, above 1.38 V. The band bending, potential downhill for electronic holes, enlarges the total flux of photoexcited holes toward the surface, which drives the oxygen evolution reaction.

DOI： 10.1021/acscatal.8b01379

Fabrication of robust TiO2 thin films by atomized spray pyrolysis deposition for photoelectrochemical water oxidation Reviewed

Chandana Sampath Kumara Ranasinghe, Junie Jhon M. Vequizo, Akira Yamakata

Journal of Photochemistry and Photobiology A: Chemistry 358 320 - 326 2018.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

Photoelectrodes are highly essential for the photoelectrochemical water splitting process and development of novel fabrication techniques is vital for further enhancement of activity. In this study, we successfully fabricated highly active TiO2 thin films by using novel atomized spray pyrolysis deposition (ASPD) technique. The ASPD technique utilizes a unique atomization process to produce highly fine aerosols which resulted in a highly crystalline TiO2 nanostructure. The deposition process was optimized by controlling deposition temperatures and precursor amounts. XRD and SEM studies confirmed the formation of anatase TiO2 phase and a highly interconnected nano-flakes on FTO substrate at 550 °C. The photoelectrochemical activity of the optimized thin films showed a photocurrent density of ∼5 mA cm−2 at 1.0 V (vs. Ag/AgCl) in 0.1 M Na2SO4 (aq) under 375 nm (150 mW cm−2) illumination. This photocurrent was much higher than the two other anatases TiO2 thin films fabricated by conventional spray pyrolysis deposition (SPD) using the same precursor and anatase TiO2 powder (particle size ∼21 nm). Transient IR absorption study revealed that the SPD powder based thin films have deeply trapped electrons, whereas ASPD thin films consisted with only free and/or shallowly trapped electrons. Higher crystallinity and enhanced electron conductivity of the TiO2 thin films fabricated by ASPD are responsible for this stable and high photoelectrochemical activity.

DOI： 10.1016/j.jphotochem.2017.09.006

Role of CoOx cocatalyst on Ta3N5 photocatalysts studied by transient visible to mid-infrared absorption spectroscopy Reviewed

Junie Jhon M. Vequizo, Mirabbos Hojamberdiev, Katsuya Teshima, Akira Yamakata

Journal of Photochemistry and Photobiology A: Chemistry 358 315 - 319 2018.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

Tantalum nitride (Ta3N5) photocatalysts attract considerable attention owing to their visible light absorption below 600 nm and suitable band structures for reduction and oxidation of water to generate hydrogen and oxygen gas. Herein, we investigated the effects of CoOx on the dynamics of photogenerated charge carriers in Ta3N5 powder photocatalysts by using microsecond to millisecond transient visible to mid-IR absorption spectroscopy. Transient absorption measurements revealed that most of the electrons in Ta3N5 are deeply trapped at the defects, most likely originating from oxygen vacancies and reduced Ta species. Two important roles of CoOx were elucidated: effective capture of holes to prolong electron lifetime and reduction of defects as recombination centers, which further elongates the lifetime of both electrons and holes.

DOI： 10.1016/j.jphotochem.2017.09.005

Structural changes of water molecules during photoelectrochemical water oxidation on TiO2 thin film electrodes Reviewed

Chandana Sampath Kumara Ranasinghe, Akira Yamakata

Physical Chemistry Chemical Physics 20 ( 5 ) 3388 - 3394 2018.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Royal Society of Chemistry

Behaviors of photogenerated charge carriers and structural changes of water molecules on TiO2 photoelectrodes were investigated by using time-resolved visible to mid-IR absorption spectroscopy. From the spectra measured in the visible to NIR region, it was shown that the lifetime of trapped electrons and holes becomes longer upon applying more positive potentials. This result was reasonably explained by the enhancement of the upward band bending at the water/TiO2 interface. On the other hand, from the spectra measured in the mid-IR region, structural changes of the water molecules were observed. When a TiO2 electrode was photoexcited at the potential where the water oxidation starts, a new absorption peak appeared at 3620 cm-1 with a slight decrease in the intensity of hydrogen-bonded water. This new peak was assigned to the isolated O-H band of water molecules. Usually, TiO2 surfaces exhibit super-hydrophilic properties with strong hydrogen-bonding
however, the obtained result was opposite. Therefore, the appearance of this isolated O-H band was ascribed to the cleavage of the hydrogen-bonding networks resulting from the production of reaction intermediates such as OH radicals or H2O2. The intensity of the isolated O-H decreases when applying more positive potentials, where the O2 evolution proceeds more efficiently. This could be ascribed to the rapid consumption of the reaction intermediates. At these potentials, the intensity of hydrogen-bonded water was also further decreased.

DOI： 10.1039/c7cp06646e

Oxygen induced enhancement of NIR emission in brookite TiO2 powders: Comparison with rutile and anatase TiO2 powders Reviewed

Junie Jhon M. Vequizo, Sunao Kamimura, Teruhisa Ohno, Akira Yamakata

Physical Chemistry Chemical Physics 20 ( 5 ) 3241 - 3248 2018.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Royal Society of Chemistry

Brookite TiO2 attracts considerable attention in photocatalysis owing to its superior performance in several photocatalytic reactions. In this work, we investigated the behavior of charge carriers in brookite, rutile, and anatase TiO2 by using photoluminescence (PL) and transient absorption (TA) spectroscopies. PL measurements revealed that brookite TiO2 exhibits a visible and a NIR emission at ∼520 nm and ∼860 nm, respectively. Addition of methanol vapor quenched both the visible and NIR emissions by the hole-consuming reaction of methanol. However, exposure to O2 shows curious behaviors: the visible emission was quenched but the NIR emission was enhanced. These results can be accounted for by the enhancement of upward band bending resulting in the effective separation of electrons and holes into the bulk and the surface, respectively. Furthermore, the shallowly trapped electrons, which are responsible for visible PL, are consumed by O2
hence, the visible emission is quenched. However, in the case of NIR emission, the deeply trapped electrons are responsible and they are mainly located at the surface defects. The O2 adsorption promotes the hole accumulation at the surface and then assists the recombination of these deeply trapped electrons, resulting in the enhancement of the NIR emission. We also found that the lifetime of NIR emission (τ1 = 43 ± 0 ns and τ2 = 589 ± 1 ns) was much longer than that of visible emission (τ1 = 15 ± 0 ns and τ2 = 23 ± 0 ns), since the mobility of these deeply trapped electrons to encounter with holes is lower than that of the shallowly trapped electrons. However, even for this slow NIR emission, the actual lifetime of the deeply trapped electrons estimated by TA (1.5 ± 0.0 μs and 17 ± 0 μs) was one or two orders of magnitude longer, confirming that non-radiative recombination is dominant and it is much slower than radiative recombination: TAS and PL provide detailed information on the radiative and non-radiative recombination processes. The PL of anatase and rutile TiO2 powders was also measured and the difference from brookite TiO2 was discussed.

DOI： 10.1039/c7cp06975h

Visible-light CO2 reduction over a ruthenium(ii)-complex/C3N4 hybrid photocatalyst: The promotional effect of silver species Reviewed

Kazuhiko Maeda, Daehyeon An, Chandana Sampath Kumara Ranasinghe, Tomoki Uchiyama, Ryo Kuriki, Tomoki Kanazawa, Daling Lu, Shunsuke Nozawa, Akira Yamakata, Yoshiharu Uchimoto, Osamu Ishitani

Journal of Materials Chemistry A 6 ( 20 ) 9708 - 9715 2018

Language：English Publishing type：Research paper (scientific journal) Publisher：Royal Society of Chemistry

Hybrid photocatalysts constructed with a mononuclear Ru(ii)-complex (RuP), silver nanoparticles, and carbon nitride nanosheets (NS-C3N4) photocatalyze CO2 reduction to selectively form formate under visible light. The structure of the nanoparticulate silver species, which worked as promoters for the reaction, was characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, high-resolution transmission microscopy, and X-ray absorption fine-structure spectroscopy. The silver promoters were loaded on the surface of NS-C3N4 by an impregnation method from an aqueous solution containing AgNO3 or an in situ photodeposition method. Impregnation of NS-C3N4 with 2.0 wt% Ag followed by reduction with H2 at 473 K (further modified with RuP) resulted in the highest photocatalytic activity, giving a turnover number of 5700 (based on RuP), which was the greatest value among the formate-generating hybrid systems with a mononuclear complex. While the optimized photocatalyst contained highly dispersed Ag2O-like nanoclusters as the major silver species, experimental results suggested that highly dispersed Ag0 species are more important for enhancing CO2 reduction activity, that is, the obtained experimental results led us to conclude that there are two major factors affecting activity: one is the feature size of silver species (smaller is better), and the other is the oxidation state of silver (metallic is better).

DOI： 10.1039/c8ta03245a

Nitrogen/fluorine-codoped rutile titania as a stable oxygen-evolution photocatalyst for solar-driven Z-scheme water splitting Reviewed

Akinobu Miyoshi, Junie Jhon M. Vequizo, Shunta Nishioka, Yuma Kato, Muneaki Yamamoto, Shunsuke Yamashita, Toshiyuki Yokoi, Akihide Iwase, Shunsuke Nozawa, Akira Yamakata, Tomoko Yoshida, Koji Kimoto, Akihiko Kudo, Kazuhiko Maeda

Sustainable Energy & Fuels 2 2025 - 2035 2018

Engaging the flux-grown La1-xSrxFe1-yTiyO3 crystals in visible-light-driven photocatalytic hydrogen generation Reviewed

Mirabbos Hojamberdiev, Kenta Kawashima, Mahesh Kumar, Akira Yamakata, Kunio Yubuta, Aleksander Gurlo, Masashi Hasegawa, Kazunari Domen, Katsuya Teshima

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 42 ( 44 ) 27024 - 27033 2017.11

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Perovskite LaFeO3 is regarded as one of the promising photocatalysts for solar splitting of water to hydrogen and oxygen due to its suitable band edge positions, visible light absorbance, and high chemical stability. To further improve its photocatalytic performance of LaFeO3, the effects of solute concentration in the KCl-flux growth and partial A- and B-site substitution are investigated in this work. Controlling the solute concentration in the range of 1-20 mol% is crucial to grow phase-pure LaFeO3 crystals with idiomorphic shape. With increasing the solute concentration, the flux-based growth route gradually changes to a solid state-based growth route because of a decrease in solubility and an increase in the crystallization core number. The La1-xSrxFe1-yTiyO3 (x,y = 0,0; 0,0.15; 0.15,0; 0.1125,0.0375; 0.0375,0.1125; and 0.075,0.075) crystals were also synthesized by a KCl-flux method to explore the effect of partial Sr2+-to-La3+ and/or Ti4+-to-Fe2-4+ substitution on photocatalytic performance of LaFeO3. The Sr2+ Ti4+ co-substitution is found to enhance the photocatalytic performance of LaFeO3 as compared with the corresponding individual substitution (Sr2+ or Ti4+). The highest photocatalytic hydrogen generation rate (83.2 mu mol h(-1)) was observed for Pt-photodeposited La0.925SF0.075Fe0.925Ti0.075O3 crystals in 5-h reaction due to the improvement of both bulk properties and photoactivity and the reduction in both grain boundaries and lattice defects stemmed from the Sr2+-Ti4+ co-substitution and KCl flux growth. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2017.09.036

Cation-dependent restructure of the electric double layer on CO-covered Pt electrodes: Difference between hydrophilic and hydrophobic cations Reviewed

Akira Yamakata, Masatoshi Osawa

JOURNAL OF ELECTROANALYTICAL CHEMISTRY 800 19 - 24 2017.9

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

Structural changes in water layers on a CO-covered Pt electrode triggered by rapid pulsed laser heating of the interface are studied using potential transient measurements. When the laser energy density is below 20 mJ cm(-2), the water layers undergo a change in orientation, which causes a negative shift in the rest potential that recovers within 20 ps with the cooling of the interface. In contrast, when the laser intensity exceeds 20 mJ cm(-2), the CO is desorbed and the rest potential first experiences a positive shift, which is followed by a negative shift. This positive shift is caused by replacement of the CO by water and subsequent restructuring of the water layer. The restructuring rate depends strongly on the electrolyte cation: the rest potential reaches a maximum value within 100 ps for hydrophilic cations such as H+ and Li+, whereas it takes >10 ms for hydrophobic cations such as Et4N+ and Bu4N+. Surface-enhanced IR absorption measurements suggest that the water molecules around the hydrophobic cations are more strongly hydrogen-bonded than those around the hydrophilic cations. Because the restructuring involves a reforming of the hydrogen-bonding network at the electric double layer, Et4N+ and Bu4N+, which have more strongly hydrogen-bonded hydration shells, require more time than H+ and Li+. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jelechem.2016.12.034

Elucidating the impact of A-site cation change on photocatalytic H-2 and O-2 evolution activities of perovskite-type LnTaON(2) (Ln = La and Pr) Reviewed

Mirabbos Hojamberdiev, Maged F. Bekheet, Judy N. Hart, Junie Jhon M. Vequizo, Akira Yamakata, Kunio Yubuta, Aleksander Gurlo, Masashi Hasegawa, Kazunari Domen, Katsuya Teshima

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 19 ( 33 ) 22210 - 22220 2017.9

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

Transition metal (oxy) nitrides with perovskite-type structures have been regarded as one of the promising classes of inorganic semiconductor materials that can be used in solar water splitting systems for the production of hydrogen as a renewable and storable energy carrier. The performance of transition metal (oxy) nitrides in solar water splitting is strongly influenced by the crystal structure-related dynamics of photogenerated charge carriers. Here, we have systematically assessed the influence of A-site cation exchange on the visible-light-induced photocatalytic H-2 and O-2 evolution activities, photoanodic response, and dynamics of photogenerated charge carriers of perovskite-type LnTaON(2) (Ln = La and Pr). The structural refinement results reveal the orthorhombic Imma and Pnma structures for LaTaON2 and PrTaON2, respectively; the latter has a more distorted crystal structure from the ideal cubic perovskite due to the smaller size of Pr3+ cations. Compared with LaTaON2, PrTaON2 exhibits lower photocatalytic H-2 and O-2 gas evolution activities and photoanodic response owing to an excessive amount of intrinsic defects associated with anionic vacancies and reduced tantalum species stemming from a long high-temperature nitridation process under reductive NH3 atmosphere. Transient absorption signals evidence the faster decay of photogenerated electrons (holes) in Pt (CoOx)-loaded LaTaON2 than that in Pt (CoOx)-loaded PrTaON2, consistent with the photocatalytic and photoelectrochemical performance of the two photocatalysts. This study suggests that in addition to selecting a suitable A-site cation, it is prerequisite to synthesize LnTaON(2) (Ln = La and Pr) crystals with a low defect density to improve their photo-conversion efficiency for solar water splitting.

DOI： 10.1039/c7cp03714g

Interfacial Manipulation by Rutile TiO2 Nanoparticles to Boost CO2 Reduction into CO on a Metal-Complex/Semiconductor Hybrid Photocatalyst Reviewed

Keisuke Wada, Chandana Sampath Kumara Ranasinghe, Ryo Kuriki, Akira Yamakata, Osamu Ishitani, Kazuhiko Maeda

ACS APPLIED MATERIALS & INTERFACES 9 ( 28 ) 23869 - 23877 2017.7

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

Metal-complex/semiconductor hybrids have attracted attention as photocatalysts for visible-light CO2 reduction, and electron transfer from the metal complex to the semiconductor is critically important to improve the performance. Here rutile TiO2 nanoparticles having 5-10 nm in size were employed as modifiers to improve interfacial charge transfer between semiconducting carbon nitride nanosheets (NS-C3N4) and a supramolecular Ru(II)-Re(I) binuclear complex (RuRe). The RuRe/TiO2/NS-C3N4 hybrid was capable of photocatalyzing CO2 reduction into CO with high selectivity under visible light (lambda > 400 nm), outperforming an analogue without TiO2 by a factor of 4, in terms of both CO formation rate and turnover number (TON). The enhanced photocatalytic activity was attributed primarily to prolonged lifetime of free and/or shallowly trapped electrons generated in TiO2/NS-C3N4 under visible-light irradiation, as revealed by transient absorption spectroscopy. Experimental results also indicated that the TiO2 modifier served as a good adsorption site for RuRe, which resulted in the suppression of undesirable desorption of the complex, thereby contributing to the improved photocatalytic performance. This study presents the first successful example of interfacial manipulation in a metal-complex/semiconductor hybrid photocatalyst for improved visible-light CO2 reduction to produce CO.

DOI： 10.1021/acsami.7b07484

Solar-driven Z-scheme water splitting using tantalum/nitrogen co-doped rutile titania nanorod as an oxygen evolution photocatalyst Reviewed

Akinobu Nakada, Shunta Nishioka, Junie Jhon M. Vequizo, Kanemichi Muraoka, Tomoki Kanazawa, Akira Yamakata, Shunsuke Nozawa, Hiromu Kumagai, Shin-ichi Adachi, Osamu Ishitani, Kazuhiko Maeda

JOURNAL OF MATERIALS CHEMISTRY A 5 ( 23 ) 11710 - 11719 2017.6

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

A visible-light-driven water-splitting system that involves two-step photoexcitation (Z-scheme) was constructed using rutile TiO2 nanorod doped with Ta and N (TiO2:Ta/N) as an O-2 evolution photocatalyst. The Ta-doped TiO2 nanorods, prepared by a solvothermal synthesis, underwent nitridation to possess visible-light absorption under mild conditions, even at 623 K under an ammonia flow. The TiO2: Ta/N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation for producing O-2 in the presence of reversible electron acceptors (IO3- or Fe3+), while TiO2: N exhibited negligible activity. The results of time-resolved infrared absorption spectroscopy indicated that co-doping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Simultaneous H-2 and O-2 evolution via water splitting was achieved using a combination of RuO2-modified TiO2: Ta/N, Ru-loaded SrTiO3:Rh and an Fe3+/Fe2+ redox couple under visible-light irradiation (lambda > 420 nm) and under AM 1.5G simulated sunlight.

DOI： 10.1039/c6ta10541f

Trapping-Induced Enhancement of Photocatalytic Activity on Brookite TiO2 Powders: Comparison with Anatase and Rutile TiO2 Powders Reviewed

Junie Jhon M. Vequizo, Hironori Matsunaga, Tatsuya Ishiku, Sunao Karnimura, Teruhisa Ohno, Akira Yamakata

ACS CATALYSIS 7 ( 4 ) 2644 - 2651 2017.4

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

Brookite TiO2 is a promising material for active photocatalysts. However, the principal mechanism that determines the distinctive photocatalytic activity between anatase, rutile, and brookite TiO2 has not yet been hilly elucidated. Therefore, in this work, we studied the behavior of photogenerated electrons and holes in these TiO2 powders by using femtosecond to millisecond time-resolved visible to mid-IR absorption spectroscopy. We found that most of the photogenerated electrons in brookite TiO2 are trapped at powder defects within a few ps. This electron trapping decreases the number of surviving free electrons, but it extends the lifetime of holes as well as the trapped electrons because the probability of electrons to encounter holes is decreased by this electron-trapping. As a result, the number of surviving holes increases, which is beneficial for photocatalytic oxidation. In contrast, the reactivity of electrons is decreased to some extent by trapping, but they still remain active for photocatalytic reductions. Electron trapping also takes place on anatase and rutile TiO2 powders, but the trap-depth in anatase is too shallow to extend the lifetime of holes and that of rutile is too deep than the thermal energy (kT) at room temperature for the electron consuming reactions. As a result of the moderate depth of the electron trap in brookite, both electrons and holes are reactive for photocatalytic,reductions and oxidations. These results have clearly demonstrated that the presence of an appropriate depth of the electron trap can effectively contribute to enhance the overall photocatalytic activity.

DOI： 10.1021/acscatal.7b00131

The contrasting effect of the Ta/Nb ratio in (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 crystals on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives Reviewed

Mirabbos Hojamberdiev, Maged F. Bekheet, Ehsan Zahedi, Hajime Wagata, Junie Jhon M. Vequizo, Akira Yamakata, Kunio Yubuta, Aleksander Gurlo, Kazunari Domen, Katsuya Teshima

DALTON TRANSACTIONS 45 ( 31 ) 12559 - 12568 2016.8

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

The effect of the Ta/Nb ratio in the (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 (0 <= x <= 4) crystals grown by a KCl flux method on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives BaNb1-xTaxO2N (0 <= x <= 1) was investigated. The Rietveld refinement of X-ray data revealed that all Ba5Nb4-xTaxO15 samples were well crystallized in the space group P (3) over bar m1 (no. 164). Phase-pure BaNb1-xTaxO2N (0 <= x <= 1) porous structures were obtained by nitridation of the flux-grown oxide crystals at 950 degrees C for 20, 25, 30, 35, and 40 h, respectively. The absorption edge of BaNb1-xTaxO2N (0 <= x <= 1) was slightly shifted from 720 to 690 nm with the increasing Ta/Nb ratio. The O-2 evolution rate gradually progressed and reached the highest value (127.24 mu mol in the first 2 h) with the Ta content up to 50 mol% but decreased at 75 and 100 mol% presumably due to the reduced specific surface area and high density of structural defects, such as grain boundaries acting as recombination centers, originated from high-temperature nitridation for prolonged periods. Transient absorption spectroscopy provided evidence for the effect of the Ta/Nb ratio on the behavior and energy states of photogenerated charge carriers, indicating a direct correlation with photocatalytic water oxidation activity of BaNb1-xTaxO2N.

DOI： 10.1039/c6dt02095j

Photocatalytic property of metal ion added SrTiO3 to Overall H2O splitting Reviewed

Yoshihisa Sakata, Yoshiko Miyoshi, Tatsuya Maeda, Kohki Ishikiriyama, Yuki Yamazaki, Hayao Imamura, Yeilin Ham, Takashi Hisatomi, Jun Kubota, Akira Yamakata, Kazunari Domen

APPLIED CATALYSIS A-GENERAL 521 227 - 232 2016.7

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

The effects of the metal ion addition to SrTiO3 to the overall H2O splitting were investigated to improve the photocatalytic activity. The addition of metal ions was carried out by impregnation method. The preparation condition of metal ion added SrTiO3 was examined with applying Na+ ion as additive and SrTiO3 prepared by polymerizable complex method. The photocatalytic activity was markedly improved by the addition of Na+ ion and calcined above 1173 K. The preferable state of the photocatalyst was Na ion (2 atm%) added SrTiO3 obtained by calcination at 1273 K and the photocatalytic activity with a Rh0.3Cr1.7O3 (Rh:0.3 wt%) co-catalyst was 16 mmol/h for H-2 and 8 mmol/h for O-2 production. The apparent quantum yield (AQY) was 16% under the irradiation of 360 nm. The photocatalytic performance of Na+ ion added various SrTiO3 and the effects of various metal ion addition to SrTiO3 to the photocatalytic activity of the overall H2O splitting were investigated to discuss the influence of the added metal ions to the photocatalytic activity. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apcata.2015.12.013

Enhancement of photoelectrochemical activity of SnS thin-film photoelectrodes using TiO2, Nb2O5, and Ta2O5 metal oxide layers Reviewed

Junie Jhon M. Vequizo, Masanori Yokoyama, Masaya Ichimura, Akira Yamakata

APPLIED PHYSICS EXPRESS 9 ( 6 ) 067101 2016.6

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

Tin sulfide (SnS) fine photoelectrodes fabricated by three-step pulsed electrodeposition were active for H-2 evolution. The incident-photon-conversion-efficiency increases from 900 nm and offers a good fit with the absorption spectrum. The activity was enhanced by 3.4, 3.0, and 1.8 times compared to bare SnS by loading Nb2O5, TiO2, and Ta2O5, respectively. Nb2O5 was most efficient because its conduction band is low enough to facilitate effective electron transfer from SnS; it also has sufficiently high potential for H-2 evolution. The overall activity is determined by the competitive interfacial electron transfer between SnS/metal-oxide and metal-oxide/water. Therefore, constructing appropriate heterojunctions is necessary for further improving photoelectrochemical systems. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/APEX.9.067101

酸化チタン光触媒のキャリアーダイナミクス－粉末におけるアナターゼとルチルの特異的な挙動－ Reviewed

山方啓

光化学 47 ( 1 ) 25 - 32 2016.4

Dynamics of Photogenerated Charge Carriers on Ni- and Ta-Doped SrTiO3 Photocatalysts Studied by Time-Resolved Absorption and Emission Spectroscopy Reviewed

Akira Yamakata, Masayuki Kawaguchi, Ryosuke Murachi, Masahiro Okawa, Itaru Kamiya

JOURNAL OF PHYSICAL CHEMISTRY C 120 ( 15 ) 7997 - 8004 2016.4

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

The behavior of photogenerated charge carriers on SrTiO3 photocatalysts doped with transition metals (such as Ni and Ta) was examined by time-resolved visible to mid-IR absorption and emission spectroscopy. When SrTiO3 was co-doped with Ni and Ta, the catalyst absorbed visible light and exhibited photocatalytic activity undervisible light irradiation. However, activity under UV light was decreased significantly compared to that before doping. The results of time-resolved measurements showed that monodoping of Ni or Ta accelerated the recombination but co-doping Ni with Ta increased the lifetime of charge carriers compared to those without doping. Furthermore, electrons excited by a visible laser pulse had longer lifetimes compared to those excited by a UV laser pulse. Time-resolved photoluminescence measurements suggested that doped Ni cations act as recombination centers, giving a luminescence peak at similar to 8000 cm(-1) due to the downward d-d transition at Ni2+. However, the lifetime of the emission was much shorter than that of free or shallowly trapped electrons. These results suggest that recombination at the Ni cations is not the dominant process. In addition, the reactivity of photogenerated electrons was decreased dramatically by doping; electrons did not react with exposed O-2, although holes maintained reactivity with MeOH. These results confirm that the decrease in the steady-state activity of doped SrTiO3 under UV light irradiation is responsible for the decrease in reactivity of photogenerated electrons.

DOI： 10.1021/acs.jpcc.6b01494

Nature-Inspired, Highly Durable CO2 Reduction System Consisting of a Binuclear Ruthenium(II) Complex and an Organic Semiconductor Using Visible Light Reviewed

Ryo Kuriki, Hironori Matsunaga, Takuya Nakashima, Keisuke Wada, Akira Yamakata, Osamu Ishitani, Kazuhiko Maeda

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 138 ( 15 ) 5159 - 5170 2016.4

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

A metal-free organic semiconductor of mesoporous graphitic carbon nitride (C3N4) coupled with a Ru(II) binuclear complex (RuRu') containing photosensitizer and catalytic units selectively reduced CO2 into HCOOH under visible light (lambda > 400 nm) in the presence of a suitable electron donor with high durability, even in aqueous solution. Modification of C3N4 with Ag nano particles resulted in a RuRu'/Ag/C3N4 photocatalyst that exhibited a very high turnover number (>33000 with respect to the amount of RuRu'), while maintaining high selectivity for HCOOH production (87-99%). This turnover number was 30 times greater than that reported previously using C3N4 modified with a mononuclear Ru(II) complex, and by far the highest among the metal-complex/semiconductor hybrid systems reported to date. The results of photocatalytic reactions, emission decay measurements, and time-resolved infrared spectroscopy indicated that Ag nanoparticles on C3N4 milliseconds from the conduction band of C3N4, which were transferred to photocatalytic CO2 reduction driven by two-step photoexcitation of C3N4 and Ag/C3N4 hybrid photocatalyst worked efficiently in water containing a proper nature of C3N4 and low solubility of CO2 in an aqueous environment. collected electrons having lifetimes of several the excited state of RuRu', thereby promoting RuRu'. This study also revealed that the RuRu'/electron donor, despite the intrinsic hydrophobic nature of C3N4 and low solubility of CO2 in an aqueous environment.

DOI： 10.1021/jacs.6b01997

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