Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

We developed organocatalyst systems to promote the cleavage of stable C-H bonds, such as formyl, α-hydroxy, and benzylic C-H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7'-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π-π interactions to form electron donor-acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7'-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7'-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon-hydrogen (C-H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.

DOI： 10.1021/jacs.2c01705

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

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have...

DOI： 10.1039/d1sc06054f

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acscatal.1c04547

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DOI： 10.1038/s41467-022-28686-x

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DOI： 10.1002/CHEM.202200875

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

Pulverization followed by annealing treatment improved the activity of BiVO₄ although the annealing treatment had a negative impact on the non-milled sample.

DOI： 10.1039/d2se00065b

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

DOI： 10.1039/D2TA02183H

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DOI： 10.1039/D2TA02604J

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Terminal-group halogenation is an effective strategy for tuning energy levels, improving light-harvesting ability, and enhancing the intermolecular stacking of nonfullerene acceptors (NFAs) for organic photovoltaic (OPV) devices. Understanding the influence of different halogen atoms on their film structure and photophysical and photovoltaic properties is crucial for designing NFAs. To address this issue, three acceptor-donor-acceptor (A-D-A)-type NFAs, named NTTIC-F, NTTIC-Cl, and NTTIC, were designed. All three NFAs consisted of a naphthalene-containing fused-ring as a D core unit and dihalogenated or nonhalogenated 1,1-dicyanomethylene-3-indanone groups as terminal A units. NTTIC-Cl exhibited broadened absorption and downshifted energy levels compared to NTTIC-F and NTTIC. Meanwhile, the singlet exciton lifetimes in solution increased in the order of NTTIC < NTTIC-F < NTTIC-Cl, but these were shortened in the pristine films and rather comparable. From grazing-incidence wide-angle X-ray scattering measurements, the tendency of NTTIC-F < NTTIC < NTTIC-Cl was found for the formation of the face-on-oriented packing structures. To the best of our knowledge, this is the first observation of a change in the molecular orientation of NFAs by the type of the halogenation atom at the terminal A unit. When blended with J71 as a conjugated polymer donor, the NTTIC-Cl-based OPV device showed a higher short-circuit current density (JSC, 20.5 mA cm-2) than the NTTIC-F- and NTTIC-based devices (19.6 and 15.6 mA cm-2, respectively). Overall, the power conversion efficiencies of the NTTIC-F- and NTTIC-Cl-based devices were similar (10.6 and 10.5%, respectively) but higher than that of the NTTIC-based device. The former resulted from the higher open-circuit voltage (VOC, 0.812 V) and fill factor (0.666) of the NTTIC-F-based device than those of the NTTIC-Cl-based device (0.793 V and 0.646, respectively).

DOI： 10.1021/acsaem.1c02816

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/pssa.202100363

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0065074

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsaem.1c01539

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

In the development of oxynitride photocatalysts, thermal ammonolysis of a metal oxide precursor has often been conducted by varying the reaction conditions (e.g., temperatures, reaction times, and ammonia gas flow rates) to obtain high-quality oxynitride particles that efficiently function as photocatalysts. However, this approach may suffer from undesirable changes in the physicochemical properties of the resulting oxynitride, leading to the lowering of the photocatalytic activity. Here, we show that it is possible to control the photocatalytic activity of Ruddlesden-Popper metastable layered oxynitride K2LaTa2O6N, obtained from the Dion-Jacobson phase KLaTa2O7 through a topochemical ammonolysis reaction, by controlling the quality of the KLaTa2O7 template. During the ammonolysis of KLaTa2O7, in the presence of K2CO3, to K2LaTa2O6N, the structural properties (e.g., degree of crystallinity and particle size) of the oxide precursor were replicated in the resulting oxynitride. Namely, the use of KLaTa2O7, possessing a higher degree of crystallinity, led to larger K2LaTa2O6N particles being formed. By increasing the crystallinity of KLaTa2O7, the photocatalytic activity of the resulting K2LaTa2O6N for H2 evolution was improved for reaction in aqueous NaI solution under visible light irradiation. This improvement in performance was due to the longer lifetime of the photogenerated mobile electrons in high-crystallinity K2LaTa2O6N compared with that in the low-crystallinity analogue, as confirmed by femtosecond transient absorption spectroscopy. However, the photocatalytic activity of K2LaTa2O6N derived from well-grown larger KLaTa2O7 particles was an order of magnitude lower than that of the best-performing material. Physicochemical measurements revealed that the large K2LaTa2O6N particles contained a relatively high density of anionic defects on the surface, which shortened the lifetime of the photogenerated charge carriers, leading to lower photocatalytic activity.

DOI： 10.1021/acs.chemmater.1c01726

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/jacs.1c03555

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/jacs.1c04861

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/ejoc.202100276

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/ejoc.202100276

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acsomega.1c01458

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41929-021-00605-1

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Other Link： http://www.nature.com/articles/s41929-021-00605-1

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

<p>A well-dispersed Pt cocatalyst obtained using Na as a promoter provides enhanced photocatalytic hydrogen evolution over BaTaO₂N under visible light.</p>

DOI： 10.1039/d1ta01162f

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acsaem.0c03055

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acscatal.0c05104

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>Oxynitride photocatalysts hold promise for renewable solar hydrogen production via water splitting owing to their intense visible light absorption. Cocatalyst loading is essential for activation of such oxynitride photocatalysts. However, cocatalyst nanoparticles form aggregates and exhibit weak interaction with photocatalysts, which prevents eliciting their intrinsic photocatalytic performance. Here, we demonstrate efficient utilization of photoexcited electrons in a single-crystalline particulate BaTaO₂N photocatalyst prepared with the assistance of RbCl flux for H₂ evolution reactions via sequential decoration of Pt cocatalyst by impregnation-reduction followed by site-selective photodeposition. The Pt-loaded BaTaO₂N photocatalyst evolves H₂ over 100 times more efficiently than before, with an apparent quantum yield of 6.8% at the wavelength of 420 nm, from a methanol aqueous solution, and a solar-to-hydrogen energy conversion efficiency of 0.24% in Z-scheme water splitting. Enabling uniform dispersion and intimate contact of cocatalyst nanoparticles on single-crystalline narrow-bandgap particulate photocatalysts is a key to efficient solar-to-chemical energy conversion.

DOI： 10.1038/s41467-021-21284-3

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Other Link： http://www.nature.com/articles/s41467-021-21284-3

Authorship：Lead author,　Corresponding author  

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

<p>The Fe^III/Fe^II redox couple in the amorphous FeO_x cocatalyst on the Bi₄TaO₈Cl photocatalyst captures photoexcited electrons and facilitates the multielectron (6-electrons) reduction of IO₃⁻, thus boosting O₂ evolution under visible light.</p>

DOI： 10.1039/D1TA01703A

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

The hydrogen evolution activity of Ga-doped La₅Ti₂Cu_0.9Ag_0.1O₇S₅ (Ga-LTCA) was boosted by loading a Rh cocatalyst having uniform dispersion and intimate contact.

DOI： 10.1039/d1ta08770c

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acsaem.0c02427

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Publishing type：Research paper (scientific journal)  

DOI： 10.1149/MA2020-02563873mtgabs

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

Copyright © 2020 American Chemical Society. The molecular catalyst sensitized system (MCSS), where an excited molecular catalyst adsorbed on a semiconductor such as TiO2 injects electrons to the conduction band of the semiconductor leading to hydrogen evolution/CO2 reduction coupled with an oxidation of water on the molecular catalyst, has been one of the most probable candidates in the approach to artificial photosynthesis. For a full utilization of visible light, however, a serious light scattering of the aqueous suspension of TiO2 in the visible region, which is generally experienced, should be avoided. Here, we report a preparation of optically transparent colloidal dispersion of TiO2 by the sol/gel reaction of TiCl4 through progressive hydrolysis/condensation under the basic condition without any calcination processes. The TiO2 nanoparticles (TiO2(NPs)) obtained were characterized as an amorphous particle (∼10-15 nm) having a microcrystal domain of anatase within several nm by XRD, Raman spectroscopies, XRF, XAFS, TG/DTA, and HRTEM, respectively. The energy-resolved distribution of carrier electron traps in TiO2(NPs) as a fingerprint of TiO2 was characterized through reversed double-beam photo-acoustic spectroscopy to have a close similarity to that of TiO2(ST-01) as well as the observation of carrier traps by transient absorption spectroscopy. Though the powder TiO2(NP) itself was not dispersed well in aqueous solution, the wet TiO2(NPs) as prepared before being dried up provided a completely transparent aqueous dispersion under the acidic condition (1 M HCl). Addition of methanol enabled the colloidal dispersion (TiO2(NPs, MeOH/H2O, 0.1 M HCl)) to keep the optical transparency for longer than 1 year (550 days), which is the first example of TiO2 dispersion storable for such a long period. TiO2(NPs, MeOH/H2O) exhibited a moderate photocatalytic reactivity of H2 evolution with a quantum yield of ∼2.6% upon 365 nm light irradiation. An optically transparent thin film of TiO2(NPs, MeOH/H2O) was also successfully prepared on a glass plate to exhibit an enhanced hydrophilicity upon UV light irradiation.

DOI： 10.1021/acsami.0c12951

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jcat.2020.07.025

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.inorgchem.0c01607

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/anie.202006747

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/anie.202006747

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

<p>Coloading of a Ni–Ru composite cocatalyst on a 700 nm-class single-crystalline particulate selenide photocatalyst improves its hydrogen evolution activity.</p>

DOI： 10.1039/D0SC01167C

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Authorship：Lead author,　Corresponding author  

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

Here we show that nanoparticulate CoAl2O4 spinel serves as an efficient cocatalyst for photocatalytic oxidation of water. The O-2 evolution activity of CoAl2O4/g-C3N4 from an aqueous solution of AgNO3 under visible light irradiation was similar to 10 times that of unmodified g-C3N4 and also exceeded that of an analogue modified with a well-known Co3O4 cocatalyst for each optimal preparation condition. Transient absorption spectroscopy indicated that the CoAl2O4 cocatalyst with Co2+ as the main cobalt species at the A site of the spinel possessed better hole-capturing properties than the Co3O4 cocatalyst, which was the main reason for the improved water-oxidation performance.

DOI： 10.1021/acscatal.0c00944

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jpcc.0c03369

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.cej.2020.124616

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

<p>Nitrogen/fluorine-codoped rutile TiO₂ nanoparticles worked more efficiently as a water oxidation photocatalyst in visible-light Z-scheme water splitting than a bulk analogue.</p>

DOI： 10.1039/d0ta04450d

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0010121

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.apcatb.2019.118333

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Authorship：Lead author,　Corresponding author  

DOI： 10.1021/acsaem.9b02262

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Copolymerized carbon nitride nanosheets (NS-C3N4) are synthesized by heating a mixture of urea and 2-aminobenzonitrile (ABN) at 823 K for 2 h in air. The visible light-absorption capability of the copolymerized NS-C3N4 can be improved with an increase in the ABN content in the starting material, while maintaining the rather negative conduction band potential of NS-C3N4. With the aid of a ruthenium(II) complex catalyst, the copolymerized NS-C3N4 becomes active for CO2 reduction into formate (>90% selectivity) under visible light (lambda > 400 nm) in the presence of triethanolamine as an electron donor. The activity is enhanced with increasing the starting ABN content to reach a maximum at a certain amount, beyond which it declines. The optimized material, modified with a silver promoter and a phosphonate-functionalized ruthenium(II) catalyst, gives a high turnover number of 6000 (vs Ru catalyst) for formate production. Physicochemical analyses indicate that increasing the starting ABN concentration improves the visible light-absorption capability of the copolymerized NS-C3N4, but increases the number of trap states, which can work as recombination centers of photogenerated electrons and holes. Therefore, an appropriate adjustment of the ABN comonomer amount is essential to obtain copolymerized NS-C3N4, which shows high photocatalytic activity.

DOI： 10.1002/solr.201900461

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DOI： 10.1021/acs.est.9b05044

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DOI： 10.1021/acsami.9b14802

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

Inducing oxygen vacancy defects in SrTiO3 powders via high temperature treatment in the presence of a mixture of Ar, O-2, dry air, and H-2 ambient gases is a promising strategy to produce homogeneously defective SrTiO3 photocatalysts with a remarkable 40-fold enhancement of H-2 evolution activity. Electron doping of SrTiO3 due to oxygen vacancies triggers the development of a highly active SrTiO3 photocatalyst; however, the photodynamic processes involved in these modifications of SrTiO3 have not been fully elucidated yet. In this work, we investigated the impact of high temperature treatment based on the dynamics of photocarriers by transient absorption spectroscopy (TAS). TAS results revealed that upon band gap excitation of SrTiO3, most of the photoexcited electrons in non-reduced SrTiO3 are deeply trapped in the intrinsic defects as evident from the strong broad absorption signals peaking at 11000 cm(-1) (909 nm, 1.36 eV) and 20000 cm(-1) (500 nm, 2.48 eV), whereas the absorption intensities in this wavenumber region largely decreased in highly reduced SrTiO3, suggesting a possible electron filling of deeply trapped states via reduction treatment (or electron doping). Interestingly, the photoexcited electrons in oxygen-deficient SrTiO3 preferably occupy the shallower electron traps. The lowest energy limit of the electron trap filled by photoexcited electrons is estimated to be at the absorption edge located at 1000 cm(-1) (similar to 0.12 eV below the conduction band), which is much shallower than that of non-reduced SrTiO3 (>0.7 eV). Furthermore, it was found that the electron population in the shallow traps in highly reduced SrTiO3 is nearly 2 orders of magnitude higher compared to that in non-reduced SrTiO3, indicating a large improvement in the electron lifetime. The findings herein offer significant insights into the crucial impact of the reduction of SrTiO(3)via induced oxygen vacancy defects to provide available photoexcited electrons that can be readily utilized for the H-2 generation reaction.

DOI： 10.1039/C9TA08216F

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The improvement of photocatalytic activity of BaTi O to overall H O splitting was investigated. From the evaluation of co-catalysts, Rh Cr O was preferable. However, this co-catalyst was degrading with the progress of the photocatalytic reaction. When CoO was co-loaded as the co-catalyst for O evolution reaction over the surface, the steady progress of the photocatalytic reaction can be confirmed. Here, effective way and condition for loading CoO co-catalyst as well as effective preparation method of BaTi O were demonstrated in order to improve the photocatalytic activity of BaTi O with Rh Cr O and CoO co-catalysts to the overall H O splitting. As the results, the photocatalyst prepared by preferable method and condition showed high photocatalytic activity to the overall H O splitting, where the apparent quantum yield (AQY) at 313 nm reached to 41 %. 4 9 2 0.7 1.3 3 x 2 x 4 9 4 9 0.7 1.3 3 x 2 2

DOI： 10.1002/cctc.201901564

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

Mixed-anion compounds (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidation, but most of them suffer from oxidative degradation by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidation photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (lambda <520 nm) with a rather negative photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte solution. Stable photoanodic current was observed even without loading a water oxidation promoter such as CoOx,. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm(-2)), stable water oxidation to form O-2 was achieved using the optimized Pb2Ti2O5.4R1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochemical water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidation promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidation promoter.

DOI： 10.1021/jacs.9b06570

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DOI： 10.1002/cctc.201901669

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

Rutile TiO2 codoped with tantalum and nitrogen (TiO2:Ta,N) was assessed as a water oxidation photocatalyst for Z-scheme water splitting driven by visible light. This material was prepared by thermal ammonolysis of TiO2:Ta with dry NH3 at 773 K, while samples of the oxide precursor were synthesized using a microwave-assisted solvothermal technique, applying various conditions. The photocatalytic activity of the TiO2:Ta,N during water oxidation to O-2 from an aqueous FeCl3 solution was found to be greatly affected by the synthesis parameters. The rate of O-2 evolution was increased upon increasing the level of Ta doping in conjunction with a highly-crystalized TiO2:Ta precursor resulting from applying a higher temperature during synthesis of the oxide. IrO2 loading of the TiO2:Ta,N photocatalyst also improved the O-2 evolution activity. The optimized IrO2/TiO2:Ta,N photocatalyst was applied to a Z-scheme water splitting system in combination with Ru/SrTiO3:Rh and in the presence of redox mediators (Fe3+/2+ or [Co(bpy)(3)](3+/2+)). Under AM1.5G simulated sunlight, this system exhibited a maximum solar-to-hydrogen energy conversion efficiency of 0.039%, which was nearly twice as high as the previously reported system with the use of RuO2/TiO2:Ta,N (0.021%).

DOI： 10.1039/C9SE00289H

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Publishing type：Research paper (scientific journal)  

© 2019 American Chemical Society. The importance of electron concentration in rutile TiO2 photocatalysts motivated us to apply H2 reduction treatment to visible-light-responsive TiO2 photocatalysts sensitized by chromium ion doping. We found that H2 reduction treatment of rutile TiO2 particles codoped with Ta and Cr (TiO2:Ta/Cr) enhanced photocatalytic activity for O2 evolution by water oxidation under visible irradiation (>2.2 eV). The enhanced visible light activity of H2-treated TiO2:Ta/Cr was attributed to the increase of electron concentration, which was confirmed by UV-vis diffuse reflectance and electron spin resonance (ESR) spectroscopy. The H2-treated TiO2:Ta/Cr photocatalyst was repeatedly used in aqueous media in spite of the presence of doped electrons. Photoluminescence and transient absorption spectroscopies revealed that electron doping with H2 treatment decreased the midgap states working as deep traps of photoexcited electrons and increased the accumulation of the photoexcited electrons in the conduction band. The optimized H2 reduction temperature was decreased with an increase in the amount of higher valence Ta5+ used as a donor-type dopant. This study shows that the precise control of the bulk electronic structure of rutile TiO2 by a combination of codoping and H2 reduction treatment improves the visible-light-driven photocatalytic activity because of the decrease of the trapping sites at deep energy levels and the recombination between deeply trapped electrons and valence band holes.

DOI： 10.1021/acsaem.9b00126

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DOI： 10.1021/acsami.9b03747

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DOI： 10.1016/j.ceramint.2019.05.080

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Metal-complex/semiconductor hybrids are promising photocatalysts for visible-light CO2 reduction with high selectivity for the formation of a desired product. Herein we applied nanoparticulate Ta3N5/SiO2 as the semiconductor component of a hybrid system with the aid of a binuclear ruthenium(II) complex. The Ta3N5/SiO2 material was prepared by the nitridation of nanoparticulate Ta2O5/SiO2 (which had been previously synthesized by a sol-gel method) under a flow of NH3 gas at 973-1223 K. The synthesized hybrid reduced CO2 into formate with very high selectivity (>99 %) under irradiation with visible light (lambda>480 nm). The activity increased with nitridation temperature up to 1023 K, beyond which it began to drop. The optimized photocatalyst, which consisted of oxygen-doped Ta3N5 as revealed by UV-visible diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy, exhibited 6 times higher activity than that for an analogous hybrid constructed with bulk Ta3N5. Physicochemical analyses indicated that reducing the defect density by high-temperature nitridation contributed to the suppression of charge recombination, which resulted in higher activity of Ta3N5/SiO2, while a nitridation temperature that was too high was undesirable because of a decrease in the reactivity of photogenerated holes due to a loss of the doped oxygen content in Ta3N5.

DOI： 10.1002/cptc.201900120

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.apcatb.2019.01.046

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

A mixed valence compound, sodium titanium oxide bronze (NaxTiO2-B), combines intriguing properties of high electric conductivity and good chemical stability together with a unique one-dimensional tunnel crystal structure available for cation storage. However, this compound has not been studied for a long period because of the strongly reductive condition at high temperature required for its preparation, which limits the morphological control such as the preparation of nanocrystals. For the first time in this paper, the topotactic synthesis of nano-sized NaxTiO2-B with high specific surface area (>130 m(2) g(-1)) from TiO2(B) nanoparticles has been demonstrated. The reaction of metastable TiO2(B) with NaBH4 allows carrier electrons to be doped simultaneously with incorporation of Na+ ions into the interstitial sites of the host Ti-O lattice at relatively low temperature. An electrochemical investigation of Li+- and Na+-ion storage behaviors suggests that the incorporated Na+ ions are mainly placed in the 6-fold coordination sites of bronze. In addition, optical measurements including time-resolved transient spectroscopy revealed that the doped electrons in the NaxTiO2-B nanoparticles are predominantly in the Ti3+ state and behave as a small polaron. The pelletized NaxTiO2-B nanoparticles shows a good electronic conductivity of 1.4 x 10(-2) S cm(-1) at 30 degrees C with an activation energy of 0.17 eV, which is attributable to the thermal barrier for the polaron hopping.

DOI： 10.1039/c8nr08372j

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

DOI： 10.1038/s41467-018-08226-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nature Publishing Group  

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer is a key mechanism for achieving artificial photosynthesis using the whole solar spectrum, even including the infrared (IR) region. In contrast to the explosive development of photocatalysts based on the plasmon-induced hot electron transfer, the hole transfer system is still quite immature regardless of its importance, because the mechanism of plasmon-induced hole transfer has remained unclear. Herein, we elucidate LSPR-induced hot hole transfer in CdS/CuS heterostructured nanocrystals (HNCs) using time-resolved IR (TR-IR) spectroscopy. TR-IR spectroscopy enables the direct observation of carrier in a LSPR-excited CdS/CuS HNC. The spectroscopic results provide insight into the novel hole transfer mechanism, named plasmon-induced transit carrier transfer (PITCT), with high quantum yields (19%) and long-lived charge separations (9.2 μs). As an ultrafast charge recombination is a major drawback of all plasmonic energy conversion systems, we anticipate that PITCT will break the limit of conventional plasmon-induced energy conversion.

DOI： 10.1038/s41467-018-04630-w

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

DOI： 10.1016/j.jphotochemrev.2018.12.001

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

Infrared (IR) light represents an untapped energy source accounting for almost half of all solar energy. Thus, there is a need to develop systems to convert IR light to fuel and make full use of this plentiful resource. Herein, we report photocatalytic H-2 evolution driven by near- to shortwave-IR light (up to 2500 nm) irradiation, based on novel CdS/Cu7S4 heterostructured nanocrystals. The apparent quantum yield reached 3.8% at 1100 nm, which exceeds the highest efficiencies achieved by IR light energy conversion systems reported to date. Spectroscopic results revealed that plasmon-induced hot electron injection at p-n heterojunctions realizes exceptionally long-lived charge separation (>273 mu s), which results in efficient IR light to hydrogen conversion. These results pave the way for the exploration of undeveloped low-energy light for solar fuel generation.

DOI： 10.1021/jacs.8b11544

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DOI： 10.1039/C8RA07830K

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DOI： 10.1021/acssuschemeng.8b03782

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

The Z-scheme CO2 reduction activity of metal complex-semiconductor hybrid photocatalysts was investigated in detail with a focus on the interfacial electron transfer process. Semiconductors of GaN:ZnO solid solutions, TaON, and Ta/N-codoped TiO2 were examined as components of the hybrid photocatalyst in combination with a binuclear Ru(II) complex. The (photo)physical properties of the semiconductor part were found to strongly affect the efficiency of interfacial electron transfer from/to the Ru complex photosensitizer unit, which was attached to the semiconductor surface. The photocatalytic activity of the hybrids showed a reasonable relationship with the efficiencies of forward and backward electron transfer. Among the three semiconductors, the highest activity was obtained with GaN:ZnO, which had the most negative conduction band potential among the semiconductors examined. The experimental results clearly demonstrated that analyses of the emission quenching process of the excited photosensitizer moiety of the binuclear Ru(II) complex allowed visualization of the interfacial electron transfer between the semiconductor and the Ru complex, giving us a rational guideline to improve the efficiency of the hybrid photocatalyst for Z-scheme CO2 reduction.

DOI： 10.1021/acscatal.8b03062

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Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

DOI： 10.1002/anie.201803931

PubMed

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

DOI： 10.1039/C8GC01746H

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

DOI： 10.1016/j.apcatb.2018.07.038

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DOI： 10.1021/acs.jpcc.8b03996

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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

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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

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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

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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

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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

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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

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

DOI： 10.1039/C8SE00191J

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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

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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 &gt;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

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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

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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 &gt; 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

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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 &gt; 420 nm) and under AM 1.5G simulated sunlight.

DOI： 10.1039/c6ta10541f

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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

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The effect of the Ta/Nb ratio in the (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 (0 &lt;= x &lt;= 4) crystals grown by a KCl flux method on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives BaNb1-xTaxO2N (0 &lt;= x &lt;= 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 &lt;= x &lt;= 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 &lt;= x &lt;= 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

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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

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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

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Language：Japanese  

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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

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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 &gt; 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 (&gt;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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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

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Language：English   Publisher：The Surface Science Society of Japan  

The behavior of photocarriers in anatase, rutile, and brookite TiO&lt;sub&gt;2&lt;/sub&gt; was studied by using transient absorption and FT-IR spectroscopy. In anatase, free electrons survive &gt; 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

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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

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Language：Japanese   Publisher：The Surface Science Society of Japan  

DOI： 10.14886/sssj2008.36.0_218

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Language：Japanese   Publisher：The Surface Science Society of Japan  

DOI： 10.14886/sssj2008.36.0_216

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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

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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

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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 &gt;= 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

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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

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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

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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

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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 (&lt;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

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DOI： 10.5796/electrochemistry.82.771

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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

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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

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DOI： 10.1021/jp4078882

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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

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We examined the effect of high temperature calcination on the photocatalytic activity and photoexcited electron behavior of rutile TiO 2 with a small specific surface area. Photocatalytic activity for H2 evolution was significantly decreased by calcination at temperatures higher than 500 C, although the specific surface area showed little change. This deactivation was attributed to fast charge carrier recombination. Samples deactivated by high temperature calcination were reduced by hydrogen at 500 or 700 C, increasing both the density of long-lived charge carriers and photocatalytic activity. These results suggest that density of oxygen vacancies is an important factor for photocatalytic reactivity. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2013.06.038

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Structural changes of the KcsA potassium channel fixed on gold electrode surface in the upright orientation were studied by surface-enhanced IR absorption spectroscopy (SEIRAS). Measurements were performed at neutral pH, where the activation gate is kept closed. Band intensities were enhanced for the asymmetric (1565 cm(-1)) and symmetric (1405 cm(-1)) OCO-carboxylate groups at negative electrode potentials in the K+ solution, but not in the Na+ solution. Even for the reverse-oriented channel, the enhanced OCO-carboxylate band was evident at negative potential. When TBA was loaded in the central cavity, the K+-specific OCO band was not elicited. These results indicate that the negative electrode potential renders the local K+ concentration accumulated at the vicinity of the electrode surface, and the KcsA channel bathed in high K+ changes conformation of the selectivity filter from the collapsed to the open, and OCO-carboxylate groups (D80 and E71) in the back of the filter were rearranged. Crown Copyright (C) 2013 Published by Elsevier B. V. All rights reserved.

DOI： 10.1016/j.chemphys.2013.02.035

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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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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

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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

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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

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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 &gt;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

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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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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

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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

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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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DOI： 10.1380/jsssj.30.68

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Photocatalytic activities of amorphous and crystal bismuth tungstate (Bi(2)WO(6))were investigated using oxidative decomposition of gaseous acetaldehyde under visible light irradiation (&gt;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

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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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The proton-coupled redox cycle of 4,4&apos;-bipyridine monolayer adsorbed on an Ag electrode catalytically enhances hydrogen evolution reactions in basic and neutral solutions. The specific adsorption via one Wend facilitates electrochemical formation of an unstable N,N-dihydro bipyridine and its decomposition yields H(2).

DOI： 10.1021/ja803446s

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The dynamic behavior of water molecules on CO-covered Pt electrode has been studied by laser induced temperature jump method. A 532-nm laser pulse quickly heats up the interface (Delta T approximate to 90 K) and perturbs the orientation of water, which results in a negative jump of the rest potential. The potential is relaxed with cooling the substrate (similar to 20 mu s). This behavior is identical to that on the CO-free surface, but the amplitude of the negative jump is more than 30 times larger on the CO-covered surface. This large difference can be ascribed to the differences in the electric field at the interface and also in the interactions of water-Pt and water-CO; water is more flexible on CO. At high laser power densities exceed a threshold, the negative potential jump is followed by a slightly slower (similar to 70 mu s) irreversible potential shift toward the positive direction. Infrared reflection absorption spectroscopy and picosecond two-pulse correlation experiments reveal that the subsequent positive shift arises from thermally activated desorption of CO. Adsorption of water and restructuring of the electric double layer after CO desorption is responsible to the positive shift. The rate of restructuring becomes slower as the initial potential becomes lower due to the increase in the electric field and the number of hydrated cations. The intense field and the hydrogen-bonding affect water structure in a long-range and enhance the collective motion of the water layers.

DOI： 10.1021/jp8018149

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Surface-enhanced infrared absorption spectroscopy (SEIRA) was used to examine the adsorption state of nitrogen monoxide (nitric oxide, NO) and the reduction of the adsorbed species. The SEIRA spectra gave two distinct bands at 1723-1733 and 1575-1607 cm(-1) with an additional weak band at 1656-1676 cm(-1) at 0.20 V, the frequencies of which are slightly dependent on the surface coverage. The former two bands are attributed to the on-top and bridged NO, respectively. While the on-top NO stably remained on the surface in the potential range of 0.05-0.60 V, the bridged NO decreased in its intensity with increasing electrode potential. The reduction of the adsorbed NO obeys first-order kinetics with respect to the adsorbed NO. The rate constants are 2.24 +/- 0.03 and 0.24 +/- 0.09 s(-1) at -0.10 V for the on-top and bridged NO, respectively. Tafel slopes obtained from the potential dependence of the rate constant indicate that the rate-determining step is the first electron-transfer process.

DOI： 10.1021/la703394z

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The adsorbed species formed from the nitrate ion were examined using surface-enhanced infrared absorption spectroscopy (SEIRAS). The main band was observed at 1547-1568cm(-1) at 0.2 V in 0.01 M NaNO3 + 0.1 MHClO4. Although this band is close to that assigned to the adsorbed NO in the literature, it is assigned to the N=O stretching vibration of the chelating bidentate form of nitrate for the following reasons. (i) Nitrate gives a single band, while NO has three bands independent of the coverage. (ii) The band intensity remained constant in the potential range from 0.1 to 0.6 V. while that of the bridged NO at around 1600 cm(-1) decreased in this range. (iii) The rate constants for the reduction and/or desorption at negative potentials are about 3 times higher than those of the bridged NO. (iv) The adsorbed species from nitrate is replaced with CO more easily than the bridged NO.

DOI： 10.1021/la703476m

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Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode is used to examine the structure of water on a polycrystalline Pt electrode in H2SO4 and HClO4 as a function of applied potential. The electrode surface covered with CO is used as the reference in recording spectra, which enables us to obtain the absolute infrared spectrum of the interfacial water layer (monolayer or bilayer) in contact with the surface with negligible interference from the bulk water. The spectrum of the interfacial water is largely different from that of bulk water and changes around the potential of zero charge of the electrode. The spectral changes are ascribed to the potential-dependent reorientation of water molecules from a weakly hydrogen-bonded oxygen-up orientation at the negatively charged surface to a strongly hydrogen-bonded nearly flat orientation at the positively charged surface in agreement with theoretical simulations reported in the literature. Clear experimental evidence of the formation of a stable ice-like structured water on the positively charged surface is reported.

DOI： 10.1021/jp710386g

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DOI： 10.5796/electrochemistry.76.208

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DOI： 10.5111/bunkou.57.23

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Electron kinetics of nine TiO2 catalysts were compared in a microsecond time domain. Each catalyst was band-gap excited with an UV light pulse, and electron-induced absorption of mid infrared light was observed as a function of time delay. The probability of electron-hole recombination in the bulk, electron attachment to adsorbed oxygen, and hole attachment to adsorbed methoxy species was estimated. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemphys.2007.05.010

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Picosecond time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS) has been used for the first time to examine the potential jump at the electrochemical interface induced by a visible pulse irradiation. The potential dependent shift of the C-O stretching vibration of CO adsorbed on a Pt electrode was utilized to monitor the potential jump. A 6-cm(-1) red-shift was observed with a time delay of similar to 200 ps with respect to a visible pump-pulse irradiation (532 nm, 35 ps duration, 3 mJ cm(-2)). The observed red-shift is ascribed to the heating of the in-plane frustrated translational mode of CO and the negative shift of potential. These two contributions can be separated with the aid of the transient of the background reflectivity of the electrode surface. The heating of water layers near the surface is mainly responsible for the potential jump through the orientation change of water molecules. This method is promising as a tool to examine ultrafast electrode dynamics.

DOI： 10.1021/jp060387d

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The adsorption of 4,4'-bipyridine (BiPy) on Cu(111) has been investigated in 0.1 M HClO4 by cyclic voltammetry, electrochemical scanning tunneling microscopy (STM), and surface-enhanced infrared adsorption spectroscopy (SEIRAS). Cyclic voltammetry showed the double layer region extending from -0.2 to 0.26 V and a pair of redox waves superposing on hydrogen evolution wave at more negative potentials. Diprotonated BiPy, BiPyH22+, is adsorbed flat on the Cu(111) (1 x 1) surface and forms a well-ordered monolayer with a (3 x 4) symmetry in the double-layer potential region. At more negative potential, BiPyH22+ is reduced to its monocation radical, BiPyH2 center dot+, and forms another well-ordered structure in which the radicals are stacked in molecular rows with a face-to-face self-dimer as the building unit. The SEIRA spectra of both BiPyH22+ and BiPyH2 center dot+ are dominated by gerade modes which should be IR-inactive for the centrosymmetric species. The breakdown of the selection rule of IR absorption is ascribed to the vibronic coupling associated with charge transfer between BiPyH22+ and the surface and between the radicals.

DOI： 10.1021/la052765w

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Sulfur-doped TiO2 was prepared by two methods; one was simple oxidation annealing of TiS2, the other was mixing of titanium isopropoxide and thiourea. These two sulfur-doped TiO2, preparations showed fairly different photocatalytic activity under visible light. The dynamics of photogenerated charge carriers were studied by the transient absorption measurement in the region of mid-IR. In both samples, excitation by 532 nm pulse led to photocarrier generation to the same extent. Nevertheless, the reactivity of the photocarriers was totally different. Photogenerated electrons and holes transferred to reactant gas in the latter sample, whereas they did not in the former sample. We attributed the different carrier behavior to the difference in the distribution of S atoms or particle size. These observations can explain the difference in capability of photocatalysis under visible light. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jphotochem.2005.06.006

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The mechanism of temporal potential oscillations that occur during galvanostatic formic acid oxidation on a Pt electrode has been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS). Carbon monoxide (CO) and formate were found to adsorb on the surface and change their coverages synchronously with the temporal potential oscillations. Isotopic solution exchange (from (HCOOH)-C-13 to (HCOOH)-C-12) and potential step experiments revealed that the oxidation of formic acid proceeds dominantly through adsorbed formate and the decomposition of formate to CO2 is the rate-determining step of the reaction. Adsorbed CO blocks the adsorption of formate and also suppresses the decomposition of formate to CO2, which raises the potential to maintain the applied current. The oxidative removal of CO at a high limiting potential increases the coverage of formate and accelerates the decomposition of formate, resulting in a potential drop and leading to the formation of CO. This cycle repeats itself to give the sustained temporal potential oscillations. The oscillatory dynamics can be explained by using a nonlinear rate equation originally proposed to explain the decomposition of formate and acetate on transition metal surfaces in UHV.

DOI： 10.1021/jp055220j

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The present account describes how photochemical reactions over metal oxides are traced by time-resolved infrared (IR) absorption spectroscopy. The ac-coupled amplification of the IR signal allows detection of transient absorbance changes as small as 10(-6) with a time resolution of 50 ns. Band-gap excited electrons in TiO2 and NaTaO3 present a structureless absorption of IR light from 3000 to 1000 cm(-1). Reaction-perturbed decay of this absorption evidences the assignment to photoexcited electrons, not to holes. The efficiency of the water splitting reaction on NaTaO3-based catalysts correlates with the quantity of electrons detected by the IR absorption. A short-lived intermediate state of 2-propanol oxidation on TiO2 is identified by its vibrational band at 1640 cm(-1) superposed on the structureless absorption of electrons. Ru dye (N3) on a TiO2 film is irradiated with a 532-nm light pulse to simulate dye-sensitized solar cells. The neutralization rate of dye cations and the decay rate of electrons injected in the film are quantified, leading to a three-state model which describes the relaxation of injected electrons. These results demonstrate the ability of this method in tracing photochemical kinetics over metal oxides.

DOI： 10.1007/s11244-005-3826-0

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Femtosecond time-resolved near-infrared spectra of TiO2 and Pt/TiO2 powders are measured at the wavelength region of 0.9-1.5 mum with the direct absorption method. Broad absorption bands of charge carriers, mainly free and trapped electrons, are observed for TiO2 and Pt/TiO2. The absorption band shape changes with a time constant of 160 A after the photoirradiation, which most probably reflects the trapping of the generated free electrons. Population decay curves of the carriers after 1 ps, obtained for three different pump light powers, are well-explained by the second-order decay kinetics with a common second-order rate constant, indicating nongeminate recombination of the electron-hole pairs. When Pt is loaded to TiO2, an additional decaying process of 2.3 ps is observed. This decay component represents the transfer of generated electrons from TiO2 to Pt, which is consistent with the known increase of overall catalytic activities by the Pt cocatalyst.

DOI： 10.1021/jp047531k

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Surface-enhanced infrared absorption spectroscopy with a Kretschmann-type attenuated total reflection configuration has been used to study hydrogen-bonded pairing between 6-amino-8-purinethiol, a thiol-derivatized adenine, immobilized on a gold electrode surface, and thymidine, a complimentary base derivative of adenine, in 0. 1 M NaClO4 aqueous solution as a function of applied potential. 6-Amino-8-purinethiol is adsorbed on a gold surface via a sulfur atom to form a S-Au bond. Nearly half of the adsorbed molecules are protonated, and the long axis of the adenine moiety is tilted from the surface normal at open circuit potential. As the potential increases, the acid-base equilibrium is shifted toward the unprotonated form and the adenine moiety is reoriented toward a nearly perpendicular configuration. The hydrogen bond interaction between the adsorbed 6-amino-8purinethiol with thymidine in solution is greatly affected by the protonation and orientation of the adenine moiety and is controllable by the applied potential. Due to steric hindrance, an adenine-thymine-type hydrogen bond pair is formed only at potentials more positive than 0. 1 V (vs SCE) where the unprotonated adenine moiety is perpendicularly oriented.

DOI： 10.1021/ac049303s

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The effect of annealing temperature on electron dynamics in Ru complex-sensitized TiO2 films was studied by highly sensitive measurement of transient IR absorption. The amount of electron injection and the back electron transfer rate were not influenced to a large extent by annealing temperature, but the distribution of deep trap sites was considerably influenced. The difference in solar cell efficiency due to annealing temperature was mainly attributed to the difference in the nature of deep trap sites. By extending the probe light window from the mid-IR (1000-4000 cm(-1)) to the near-IR (-10000 cm(-1)) region, we investigated deep trap sites directly and found that electrons in deep trap sites have absorption in the near-IR region whose peak locates around 7500 cm(-1). As a whole, the electron dynamics in the films annealed at higher than 400 degreesC were considerably different compared with those in the films annealed at lower than 400 degreesC.

DOI： 10.1021/jp035416o

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Electrons photoexcited in catalysts based on NaTaO3 were observed by time-resolved infrared (IR) absorption spectroscopy. The electrons excited by 266-nm pump pulse exhibited a monotonic absorption of mid-IR light. The recombination kinetics of the excited electrons with holes was monitored in a vacuum as a function of time delay ranging from 100 ns to 1 s. When the Na content and La dopant were optimized to achieve high activity in the steady-state reaction of water splitting, the recombinative decay was hindered to increase the amount of electrons. The electrons were transferred to NiO ultrafine particles, loaded as a cocatalyst, within 1 mus following the pulse excitation. Exposing the catalysts to water vapor affected the electron decay and efficient electron transfer to water via the cocatalyst was evidenced.

DOI： 10.1021/jp036473k

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The reaction kinetics of electrons photoexcited in a Pt/TiO2 catalyst were observed by time-resolved infrared absorption in the presence of methanol-water vapor mixtures. The recombinative decay of the electrons was completed in microseconds on the catalyst exposed to a vacuum. The decay was suspended in the catalyst exposed to pure methanol vapor due to an effective hole-consuming reaction by adsorbed methoxy species. The suspension was released as partial water pressure was increased, indicating that the holes reacted with the methoxy species without the aid of adsorbed water or hydroxyl species. The excess electrons remaining in the catalyst were consumed by adsorbed water in a time domain of 0.1 s. The water-induced promotion of the methanol oxidation reaction described in the literature was interpreted with these results.

DOI： 10.1021/jp034997e

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Language：Japanese   Publisher：The Surface Science Society of Japan  

Oxidation of 2-propanol to acetone on a Pt/TiO&lt;Sub&gt;2&lt;/Sub&gt; photocatalyst by UV pulse irradiation was observed by time-resolved IR spectroscopy (TR-IR). The hole transfer to the 2-propanol, which triggers the oxidation, was completed within 0.5 μs. Transient vibrational spectra showed a new band at 1640 cm&lt;Sup&gt;−1&lt;/Sup&gt;, which appeared immediately after the irradiation. This band gradually transformed to a band at 1700 cm&lt;Sup&gt;−1&lt;/Sup&gt;, which was assigned to the C=O stretching mode of acetone, the product of 2-propanol oxidation. Thus, the band at 1640 cm&lt;Sup&gt;−1&lt;/Sup&gt; was suggested to be the reaction intermediate that transforms to acetone. We discussed the origin of this band using density functional calculation, and an acceptable agreement was obtained with the anion radical of acetone adsorbed on the catalyst.

DOI： 10.1380/jsssj.24.563

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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

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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

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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

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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

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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.

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Behavior of the electrons and holes photogenerated in TiO&lt;Sub&gt;2&lt;/Sub&gt; 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&lt;Sup&gt;−1&lt;/Sup&gt;, 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&lt;Sub&gt;2&lt;/Sub&gt; 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

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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

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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

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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

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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

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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

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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

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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 (&lt;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

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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

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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

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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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The exchange reaction of adsorbed formate on Ni(110) with formic acid in gas phase was studied by the time-resolved infrared reflection absorption spectroscopy (TR-IRAS). It was found that the pre-adsorbed deuterated formate (DCOO(a)) was changed to HCOO(a) in the presence of HCOOD(g) at the temperature region from 240 to 300 K. In such a temperature region, the formate was stable on the surface in vacuum and the decomposition of formate was not observed. It was suggested that the pre-adsorbed DCOO(a) was desorbed as DCOOD accepting deuterium atom from HCOOD. The activation energy of the exchange reaction was estimated to be 23 +/- 2 kJ mol(-1). The reaction order with respect to formic acid pressure was 0.3 +/- 0.1, while that as to the coverage of DCOO(a), was unity. Taking into account all the experimental results, the exchange of pre-adsorbed formate with post-exposed formic acid is suggested to proceed via forming a hydrogen-bonded intermediate complex on the formate's site, because all the sites were already occupied by formate. The small activation energy of the exchange reaction is discussed on the basis of 'adsorption-assisted desorption'. We propose that when desorption and adsorption are coupled, the activation energy of the exchange reaction should be described as a sum of the potential energy profiles of a desorbing and an adsorbing molecule. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S1381-1169(98)00251-9

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The dynamic behavior of adsorbed formate on Ni(110) in the presence of gaseous formic acid was studied by time-resolved infrared reflection absorption spectroscopy. The preadsorbed formate (DCOO(a)) was replaced by the postexposed formic acid (HCOOD) to form HCOO(a) on Ni(110) in the temperature region between 230 and 300 K, where the formate was stable under vacuum and no decomposition of formate was observed. It was confirmed that the preadsorbed DCOO(a) desorbed as DCOOD accompanied by formation of HCOO(a) via the deuterium atom transfer from HCOOD.

DOI： 10.1021/jp981192b

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Publisher：The Surface Science Society of Japan  

The exchange reaction of formate (DCOO(a)) with gaseous formic acid (HCOOD) on Ni(110) was investigated below the decomposition temperature of formate (340 K) by means of time-resolved infrared reflection absorption spectroscopy (TR-IRAS). When HCOOD gas was admitted on DCOO(a)-covered Ni(110) at 300 K, the pre-adsorbed DCOO(a) was replaced by HCOO(a). Similar replacement was also observed by introducing H&lt;SUP&gt;12&lt;/SUP&gt; COOH gas onto H-&lt;SUP&gt;13&lt;/SUP&gt;COO(a)-covered Ni(110). In such a temperature region from 240 to 300 K, no decomposition of formate was observed. Thus it was confirmed that those pre-adsorbed formate desorbed as formic acid accompanied by the transfer of hydrogen (deuterium) atom from post-introduced formic acid. The kinetics of the exchange reaction is investigated and the mechanism of that is also discussed.

DOI： 10.1380/jsssj.19.441

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Language：Japanese   Publisher：日本表面科学会  

The decomposition of the surface formate on Ni(110) was studied by temperature programmed desorption (TPD) and timeresolved infrared reflection absorption spectroscopy (TR-IRAS). It was observed that dehydration proceeded simultaneously with dehydrogenation. The rate-determining step of dehydration was found to be the C-H bond cleavage of formate as is the case of dehydrogenation by analysis of the kinetic isotope effect using HCOO(a) and DCOO(a). These results suggest that the rate-determining steps of dehydration and dehydrogenation are common. We propose a mechanism in which the CO₂(a) species formed via the C-H bond cleavage of formate acts as the intermediate not only for dehydrogenation but also for dehydration; dehydration takes place by dissociation of the C-O bond of CO₂(a).

DOI： 10.1380/jsssj.18.417

CiNii Article

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

The dehydration of formate adsorbed on Ni(110) has been studied by the direct observation of the CO(a) produced on the surface by means of time-resolved infrared reflection absorption spectroscopy (TR-IRAS). By the quantitative analysis of the temperature-programmed desorption and TR-IRAS results, it was found that the dehydration proceeds simultaneously with the dehydrogenation. The primary kinetic isotope effect (k(H)/k(D)) using HCOO(a) and DCOO(a) was observed not only in the dehydrogenation but also in the dehydration (k(H)/k(D) = 3.3 at 300 K). This result suggests that the rate-determining step (RDS) is the C-H bond dissociation in both processes. These suggestions let us propose a possible mechanism of the dehydration of formate on Ni(110); the RDS of the dehydration is the C-H bond dissociation which is the same as that of the dehydrogenation, and the produced CO2(a) is the precursor of the production of CO(a) and O(a).

DOI： 10.1021/jp9703418

Web of Science

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

Kinetics of the CH/CD isotope exchange reaction of formate (HCOO(a)/DCOO(a)) on Ni(110) with gaseous D-2/H-2 was investigated below the decomposition temperature (360 K) by infrared reflection absorption spectroscopy (IRAS). The apparent activation energies of HCOO(a) --&gt; DCOO (a) and DCOO(a) --&gt; HCOO(a) reactions were derived as 52 +/- 5 and 58 +/- 5 kJ . mol(-1), respectively. A half-order dependence of the reaction rate on hydrogen pressure was observed, suggesting that the rate-determining step was the reaction of the adsorbed formate with reversibly adsorbed hydrogen atom. The reaction of HCOO(a) with D-2 was 2.1 +/- 0.3 times faster than that of DCOO(a) with H-2 at 300 K, and the origin of the isotope effect is discussed.

DOI： 10.1021/jp9611986

Web of Science

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

Infrared-visible sum frequency generation (IR-vis SFG) spectroscopy, infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD) spectroscopy were used to investigate the systems prepared by the adsorption of formic acid (HCOOH) onto a Pt(110)-(1 X 2) reconstructed surface. In the SFG spectra, we observed that several vibrational bands which can be assigned to the CH strerching, the OH stretching and combination modes of formic acid adsorbed in multilayers show a strong dose temperature dependence. From the IRAS measurements, it was found that the formic acid adsorbed in multilayers forms the chain structure which is called the alpha-crystalline form. By combining the results of the dose temperature dependence of SFG spectra with that of IRAS spectra, we conclude that the length of the chain, the orientation of the chain axis, and the size of the single crystalline region of formic acid strongly depend on the dose temperature. Furthermore, we found that the SFG signal of the C-H stretching band shows the intensity oscillation during the exposure to formic acid.

DOI： 10.1016/0039-6028(96)00239-7

Web of Science

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Responsible for pages：p78-88  

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Responsible for pages：p227-236  

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Responsible for pages：p267-280  

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Publisher：The Electrochemical Society  

DOI： 10.1149/ma2011-01/38/1837

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CiNii Article

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J-GLOBAL

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J-GLOBAL

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Publisher：The Electrochemical Society  

DOI： 10.1149/ma2008-02/47/2874

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J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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