Yoshihiro Nakato

Bio: Yoshihiro Nakato is an academic researcher from Osaka University. The author has contributed to research in topics: Electrode & Silicon. The author has an hindex of 42, co-authored 217 publications receiving 6557 citations.

Mechanism for Visible Light Responses in Anodic Photocurrents at N-Doped TiO2 Film Electrodes

Abstract: Nitrogen doping of anatase TiO2 powder extended the photocurrent action spectrum for water oxidation from the UV-light region (≤400 nm) to the visible-light region (≤ ∼550 nm), as reported. Investigations of the effect of the addition of reductants such as methanol, SCN-, Br-, I-, and hydroquinone to the electrolyte have for the first time given clear experimental evidence to the mechanism that visible-light responses for N-doped TiO2 arise from an N-induced midgap level, formed slightly above the top of the (O-2p) valence band. The investigations, in combination with the above mechanism, have also shown that photocatalytic oxidation of organic compounds on N-doped TiO2 under visible illumination mainly proceed via reactions with surface intermediates of water oxidation or oxygen reduction, not by direct reactions with holes trapped at the N-induced midgap level.

Primary intermediates of oxygen photoevolution reaction on TiO2 (Rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements.

Abstract: Primary intermediates of oxygen photoevolution (water photooxidation) reaction at the TiO2 (rutile)/aqueous solution interface were investigated by in situ multiple internal reflection infrared (MIRIR) absorption and photoluminescence (PL) measurements. UV irradiation of TiO2 in the presence of 10 mM Fe3+ in the solution caused the appearance of a new peak at 838 cm-1 and a shoulder at 812 cm-1. Detailed investigations of the effects of solution pH, the presence of methanol as a hole scavenger, and isotope exchange in water (H216O→H218O) on the spectra have shown that the 838- and 812-cm-1 bands can be assigned to the O−O stretching mode of surface TiOOH and TiOOTi, respectively, produced as primary intermediates of the oxygen photoevolution reaction. The results give strong support to our previously proposed mechanism that the oxygen photoevolution is initiated by a nucleophilic attack of a H2O molecule on a photogenerated hole at a surface lattice O site, not by oxidation of surface OH group by the hole...

In situ FTIR studies of primary intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions.

Abstract: Multiple internal reflection infrared spectroscopy was applied to in situ investigations of surface intermediates of photocatalytic reactions on nanocrystalline TiO2 films in contact with aqueous solutions. UV irradiation in the presence of dissolved O2 caused the appearance of new bands peaked at 943, 838, and 1250−1120 cm-1 together with intensity changes in other bands. Investigations of influences of the solution pH, the presence or absence of hole and electron scavengers, and isotopic H2O → D2O exchange on the spectral changes have revealed that the primary step of photocatalytic O2 reduction is the formation of the surface peroxo species, Ti(O2), giving the 943 cm-1 band, probably with the surface superoxo species, TiOO·, as a precursor, in neutral and acidic solutions. The surface peroxo species is then transformed to the surface hydroperoxo, TiOOH, giving the 838 and 1250−1120 cm-1 bands, by protonation in the dark. This is, to our knowledge, the first direct in situ spectroscopic detection of pri...

Electrocatalytic activity of amorphous RuO2 electrode for oxygen evolution in an aqueous solution

Abstract: The electrocatalytic activity of amorphous and crystalline RuO2 thin films for oxygen evolution in an aqueous solution was investigated. The RuO2 films were prepared on FTO substrates by electrodeposition or RF magnetron sputtering technique. The obtained films were annealed at various temperatures. In both cases, the as-prepared films or the 200 °C annealed film had an amorphous structure, whereas the films annealed at 300 °C and over were crystallized to rutile structure. The analysis of the Tafel slope indicated that the rate-determining step in the oxygen evolution reaction on the amorphous RuO2 was the combination of the adjacent Ru–OH groups, whereas that on crystalline RuO2 was the dissociation of O–H bond in Ru–OH group. The onset potentials of the amorphous RuO2 films for oxygen evolution were shifted toward the negative side by 0.06–0.03 V from those for the rutile crystalline samples. The shift of the onset potential is probably attributed to the structural flexibility which is characteristic of the amorphous surface. This result suggested that the electrocatalytic activity of amorphous RuO2 for oxygen evolution was higher than that of rutile crystalline RuO2.

Mechanism of Water Photooxidation Reaction at Atomically Flat TiO2 (Rutile) (110) and (100) Surfaces: Dependence on Solution pH

Abstract: The mechanism of water photooxidation reaction at atomically flat n-TiO2 (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements. The PL bands, which peaked at around 810 and 840 nm for the (110) and (100) surfaces, respectively, were assigned to radiative transitions between conduction-band electrons and surface-trapped holes (STH), [Ti−O=Ti2]s+, formed at triply coordinated (normal) O atoms at the surface lattice. The PL intensity (IPL) decreased stepwise with increasing solution pH, namely, it sharply decreased at around pH 4, near the point of zero charge of TiO2 (about 5), and then rapidly decreased to zero near pH 13. The first sharp decrease around pH 4 is attributed to the increased rate of nucleophilic attack of a water molecule to a hole at a site of surface bridging oxygen (Ti−O−Ti), the density of which increases with increasing pH. The nucleophilic attack is regarded as the main initiating step of the water oxidation reaction in ...

Heterogeneous photocatalyst materials for water splitting

Abstract: This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Solar Water Splitting Cells

Abstract: Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

The surface science of titanium dioxide

Abstract: Titanium dioxide is the most investigated single-crystalline system in the surface science of metal oxides, and the literature on rutile (1 1 0), (1 0 0), (0 0 1), and anatase surfaces is reviewed This paper starts with a summary of the wide variety of technical fields where TiO 2 is of importance The bulk structure and bulk defects (as far as relevant to the surface properties) are briefly reviewed Rules to predict stable oxide surfaces are exemplified on rutile (1 1 0) The surface structure of rutile (1 1 0) is discussed in some detail Theoretically predicted and experimentally determined relaxations of surface geometries are compared, and defects (step edge orientations, point and line defects, impurities, surface manifestations of crystallographic shear planes—CSPs) are discussed, as well as the image contrast in scanning tunneling microscopy (STM) The controversy about the correct model for the (1×2) reconstruction appears to be settled Different surface preparation methods, such as reoxidation of reduced crystals, can cause a drastic effect on surface geometries and morphology, and recommendations for preparing different TiO 2 (1 1 0) surfaces are given The structure of the TiO 2 (1 0 0)-(1×1) surface is discussed and the proposed models for the (1×3) reconstruction are critically reviewed Very recent results on anatase (1 0 0) and (1 0 1) surfaces are included The electronic structure of stoichiometric TiO 2 surfaces is now well understood Surface defects can be detected with a variety of surface spectroscopies The vibrational structure is dominated by strong Fuchs–Kliewer phonons, and high-resolution electron energy loss spectra often need to be deconvoluted in order to render useful information about adsorbed molecules The growth of metals (Li, Na, K, Cs, Ca, Al, Ti, V, Nb, Cr, Mo, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) as well as some metal oxides on TiO 2 is reviewed The tendency to ‘wet’ the overlayer, the growth morphology, the epitaxial relationship, and the strength of the interfacial oxidation/reduction reaction all follow clear trends across the periodic table, with the reactivity of the overlayer metal towards oxygen being the most decisive factor Alkali atoms form ordered superstructures at low coverages Recent progress in understanding the surface structure of metals in the ‘strong-metal support interaction’ (SMSI) state is summarized Literature is reviewed on the adsorption and reaction of a wide variety of inorganic molecules (H 2 , O 2 , H 2 O, CO, CO 2 , N 2 , NH 3 , NO x , sulfur- and halogen-containing molecules, rare gases) as well as organic molecules (carboxylic acids, alcohols, aldehydes and ketones, alkynes, pyridine and its derivates, silanes, methyl halides) The application of TiO 2 -based systems in photo-active devices is discussed, and the results on UHV-based photocatalytic studies are summarized The review ends with a brief conclusion and outlook of TiO 2 -based surface science for the future

Semiconductor-based Photocatalytic Hydrogen Generation

Abstract: 2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519