Fumiaki Amano

Visible light responsive pristine metal oxide photocatalyst: enhancement of activity by crystallization under hydrothermal treatment.

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

Photocatalytic activity of octahedral single-crystalline mesoparticles of anatase titanium(IV) oxide

Octahedral titanium(IV) oxide (TiO(2)) crystallites with exposed anatase [101] facets exhibited relatively high photocatalytic activity for oxidative decomposition of organic compounds and low activity for hydrogen evolution in the absence of molecular oxygen, probably due to the characteristics of the anatase [101] surface.

Fabrication and photoelectrochemical property of tungsten(VI) oxide films with a flake-wall structure

Efficient visible light-induced photoelectrochemical oxidation of water was achieved using a tungsten(iv) oxide (WO(3)) film composed of perpendicularly oriented plate-like crystallites, a flake-wall film, prepared on a transparent conductive substrate by controlling anisotropic crystal growth of tungsten oxide hydrate (WO(3).H(2)O) followed by calcination.

Correlation between surface area and photocatalytic activity for acetaldehyde decomposition over bismuth tungstate particles with a hierarchical structure.

The photocatalytic oxidative decomposition of gaseous acetaldehyde (AcH) in air under visible light irradiation (wavelength >400 nm) was driven by bismuth tungstate (Bi(2)WO(6)) polycrystalline particles with a hierarchical structure, flake-ball particles, prepared by hydrothermal reaction at various temperatures. Complete decomposition of AcH into CO(2) was proven for all of the flake-ball particle photocatalysts. The rate of CO(2) liberation was increased in proportional to the specific surface area for flake-ball particles with similar high degrees of crystallinity. Kinetic analysis assuming Langmuirian adsorption of AcH revealed that the initial rate of photocatalytic decomposition could be reproduced by first-order kinetics with respect to the amount of surface-adsorbed AcH. A linear relationship between the photocatalytic activity and surface area of photocatalysts under conditions in which other physical properties such as the photoabsorption property, crystalline content, exposed crystal facets, and secondary particle size are almost the same was experimentally revealed.

Gold–Titanium(IV) Oxide Plasmonic Photocatalysts Prepared by a Colloid-Photodeposition Method: Correlation Between Physical Properties and Photocatalytic Activities

Colloidal gold (Au) nanoparticles were prepared and successfully loaded on titanium(IV) oxide (TiO(2)) without change in the original particle size using a method of colloid photodeposition operated in the presence of a hole scavenger (CPH). The prepared Au nanoparticles supported on TiO(2) showed strong photoabsorption at around 550 nm due to surface plasmon resonance (SPR) of Au and exhibited a photocatalytic activity in mineralization of formic acid in aqueous suspensions under irradiation of visible light (>ca. 520 nm). A linear correlation between photocatalytic activity and the amount of Au loaded, that is, the number of Au nanoparticles, was observed, indicating that the activity of Au/TiO(2) plasmonic photocatalysts can be controlled simply by the amount of Au loading using the CPH method and that the external surface area of Au nanoparticles is a decisive factor in mineralization of formic acid under visible light irradiation. Very high reaction rates were obtained in samples with 5 wt % Au or more, although the rate tended to be saturated. The CPH method can be widely applied for loading of Au nanoparticles on various TiO(2) supports without change in the original size independent of the TiO(2) phase. The rate of CO(2) formation also increased linearly with increase in the external surface area of Au. Interestingly, the TiO(2) supports showed different slopes of the plots. The slope is important for selection of TiO(2) as a material supporting colloidal Au nanoparticles.

Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes with High Photoelectrode Activity

Tungsten trioxide (WO(3)) thin film electrodes with platelike structures were prepared by a facile hydrothermal reaction of tungsten sheets in a dilute nitric acid solution at 100-180 °C and subsequent calcination at 450 °C. The calcination step facilitated the transformation of the crystal structure from tungsten oxide hydrates (WO(3)·H(2)O or WO(3)·2H(2)O) to monoclinic WO(3) without significant modification to the platelike structures. The photoelectrochemical performance of the thin film electrodes for water splitting that took place in a dilute sulfuric acid was strongly dependent on both temperature and the time used for the hydrothermal reaction. This suggests that the thickness of the film influences the process of photoexcited electron transport. The time required for the hydrothermal reaction under higher temperatures was reduced in the generation of thin film electrodes with high photoelectrode activity, because the crystal growth is accelerated at high temperatures and the electron transport is restricted by a relatively thick compact layer that is comprised of WO(3) nanoparticulates. The electrode exhibited sensitivity to the violet portion of the visible light spectrum due to the bandgap of 2.8 eV and high photoelectrode efficiency, as well as an incident photon-to-current conversion efficiency (IPCE) of 66.2%, for the photoelectrochemical oxidation of water.

NO reduction with CO in the presence of O2 over Al2O3-supported and Cu-based catalysts

NO reduction with CO in the presence of O2 was examined over various kinds of Al2O3-supported and Cu-based catalysts. The effects of loaded elements, supports and calcination temperatures on catalysis for the NO–CO–O2 reaction system were investigated. We found that the most effective catalyst was 0.5 wt.% Cu/γ-Al2O3 calcined at 773 K. The states of Cu species were characterized with XRD, UV/VIS/NIR, EPR, TPR and X-ray absorption spectroscopic techniques. Highly dispersed Cu2+ species on Al2O3 are active species for selective reduction of NO to N2. The aggregated Cu species preferentially oxidize CO without reducing NOx, and the appearance of the aggregated Cu species resulted in drastic reduction in N2 formation capability.

Photoelectrochemical properties of tungsten trioxide thin film electrodes prepared from facet-controlled rectangular platelets

The control of anisotropic crystal growth is critical for directing the orientation of crystal lattice planes, and it plays a key role towards understanding the effects of different planes on chemical reactions. Here, we report on the photoelectrochemical properties of plate-structured tungsten trioxide (WO3) thin films prepared from facet-controlled rectangular platelets of hydrotungstite (WO3·2H2O) and tungstite (WO3·H2O), which are directly grown on tungsten substrates. The WO3 thin films, prepared via WO3·2H2O platelets, show relatively stable current for photoelectrochemical water splitting and methanol oxidation. On the other hand, the photocurrent of the WO3 thin films prepared via WO3·H2O platelets was significantly decreased during the photoelectrochemical oxidation of water, which is likely due to the accumulation of partially oxidized intermediates such as peroxo species on the surface. These results indicate that the surface nanostructures of WO3 may have a significant influence on photoelectrode efficiency and selectivity for the catalytic oxygen evolution reaction.

What Are Titania Photocatalysts?―An Exploratory Correlation of Photocatalytic Activity with Structural and Physical Properties

Abstract This article reports on an exploratory statistical analysis of the photocatalytic activity of titanium(IV) oxide (TiO2; titania) using forty commercially available titania samples. Five photocatalytic reactions were examined as test reactions for the evaluation of photocatalytic activity of the samples: oxygen liberation along with silver deposition, methanol dehydrogenation, acetic acid decomposition, acetaldehyde decomposition and synthesis of pipecolinic acid from L-lysine. Six structural and physical properties of the samples were measured: specific surface area (BET), density of defective sites (DEF), primary particle size (PPS), secondary particle size (SPS) and existence of anatase (ANA) and rutile (RUT) phases. Correlations of photocatalytic activities for the five kinds of reactions with structural and physical properties of 35 (out of 40) titania samples were obtained, and photocatalytic activities were empirically reproduced by a linear combination of six properties with good to fair reliability. While a portion of the results could be interpreted using a conventional mechanism, significant activity dependence on properties, not yet disclosed, was suggested.

Rutile titanium dioxide prepared by hydrogen reduction of Degussa P25 for highly efficient photocatalytic hydrogen evolution

Degussa (Evonik) TiO2 P25 consisting of a mixture of anatase and rutile crystallites is a well-known commercial material with high photocatalytic efficiency. The mixture of anatase and rutile phases in P25 is reportedly more active than the individual polymorphs of TiO2. Contrary to this viewpoint, we demonstrate that H2-reduced rutile TiO2 is much more active than mixed-phase P25 for photocatalytic H2 evolution under ultraviolet irradiation. An important factor to improve the photocatalytic activity of rutile TiO2 is H2 reduction treatment. H2-reduced rutile TiO2 outperforms anatase-rich TiO2 because of the wider absorption range caused by its smaller band gap (3.0 eV for rutile and 3.2 eV for anatase). The apparent quantum efficiency for H2 evolution of H2-reduced rutile TiO2 was estimated to be 46% under 390 nm irradiation, which was 3.3 times higher than that of mixed-phase P25. This highly efficient rutile photocatalyst is easily fabricated from P25 by H2 reduction treatment at 700 °C for 2 h.