Bunsho Ohtani

Pristine Simple Oxides as Visible Light Driven Photocatalysts: Highly Efficient Decomposition of Organic Compounds over Platinum-Loaded Tungsten Oxide

Tungsten oxide loaded with nanoparticulate platinum is demonstrated to exhibit high activity for the decomposition of organic compounds both in liquid and gas phases; the activity was almost comparable to that of TiO2 under UV light irradiation and much higher than that of nitrogen-doped TiO2 under visible irradiation.

Visible-light-induced photocatalysis through surface plasmon excitation of gold on titania surfaces

Fifteen commercial titania (titanium(iv) oxide; TiO(2)) powders were modified with gold by photodeposition to prepare photocatalysts that work under irradiation with light in the visible range (vis). The gold-modified titania (Au/TiO(2)) powders were characterized by diffuse reflectance spectroscopy (DRS), field-emission scanning electron microscopy (FE-SEM), scanning transmission microscopy (STEM) and X-ray powder diffraction (XRD). It was shown that all tested powders could absorb visible light with an absorption maximum at localized surface plasmon resonance (LSPR) wavelengths (530-600 nm) and that the size and shape of gold nanoparticles determined the absorption ranges. The photocatalytic activity of Au/TiO(2) powders was examined both under ultraviolet and vis irradiation (mainly >450 nm) for acetic acid and 2-propanol photooxidation. It was found that the activity depended strongly on gold and titania properties, such as particle size and shape, surface area and crystalline form. Under vis irradiation, large rutile particles loaded with gold particles of a wide range of sizes showed the highest level of photocatalytic activity, possibly due to greater light absorption ability in a wide wavelength range resulting from transverse and longitudinal LSPR of rod-like gold particles. Action spectrum analyses showed that visible-light-induced oxidation of organic compounds by aerated gold-titania suspensions was initiated by excitation of LSPR absorption of gold. Although photocatalytic activity of nanosized gold particles under vis irradiation with a wavelength of ca. 430 nm and catalytic activity of gold-modified titania during dark reactions were also found, it was shown that the activities of Au/TiO(2) particles originated from activation of LSPR of gold by light of wavelength of 530-650 nm. Participation of molecular oxygen as an electron acceptor and titania as a conductor of electrons is suggested by comparing with results obtained under deaerated conditions and results obtained using a system containing gold-deposited silica instead of gold-titania, respectively. On the basis of these results, the mechanism of visible-light-induced oxidation of organic compounds on gold-titania is proposed.

Photocatalytic degradation of organic compounds in aqueous systems by transition metal doped polycrystalline TiO2

Abstract Some probe catalytic photooxidation reactions with aliphatic and aromatic organic compounds having different acid strengths, i.e. methanoic acid, ethanoic acid, benzoic acid and 4-nitrophenol, were carried out in aqueous systems by using polycrystalline TiO2 powders doped with various transition metal ions (Co, Cr, Cu, Fe, Mo, V and W). The Co-doped powder showed to be more photoactive than the bare TiO2 for methanoic acid degradation while the behaviour of TiO2/Cu and TiO2/Fe was similar to that of the support. TiO2/W was the most efficient sample for the photodegradation of benzoic acid and 4-nitrophenol, TiO2 the most active powder for ethanoic acid. A tentative explanation is provided by taking into account: (i) the dissociation constants (Ka) of the different acids used as substrates; (ii) their aliphatic or aromatic nature; (iii) the points of zero charge (PZC) of the photocatalysts; (iv) their relative rate constants for photoelectron–hole recombination (kr) determined by femtosecond pump-probe diffuse reflectance spectroscopy.

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 transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions : Correlation with electron-hole recombination kinetics

Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (<100 fs). The second-order rate constant (kr) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H2 and PCA production under deaerated conditions depended strongly on kr, but the relation between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different kr dependences of photoactivity on the reaction kinetics governed by e−–h+ recombination were attributed to the presence of O2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.

Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

The molar amounts of defective sites (Md) in several titanium(IV) oxide (TiO2) powders were determined using photoinduced reactions of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. Measurements of pH dependence of typical anatase and rutile TiO2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO2 in ranges of 0–0.35 V for anatase and 0–0.25 V for rutile. A linear relation of Md with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that Md could be a quantitative parameter of recombination between a photoexcited electron and a positive hole. The fact that there was no linear relation between Md and the specific surface area suggests that the surface area was not directly reflected on Md. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and Md revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO2 powders in this reaction system.

Visible-Light-Induced Water Splitting Based on Two-Step Photoexcitation between Dye-Sensitized Layered Niobate and Tungsten Oxide Photocatalysts in the Presence of a Triiodide/Iodide Shuttle Redox Mediator

Water splitting into H2 and O2 under visible light was achieved using simple organic dyes such as coumarin and carbazole as photosensitizers on an n-type semiconductor for H2 evolution, a tungsten(VI) oxide (WO3) photocatalyst for O2 evolution, and a triiodide/iodide (I3(-)/I(-)) redox couple as a shuttle electron mediator between them. The results on electrochemical measurements revealed that the oxidized states of the dye molecules having an oligothiophene moiety (two or more thiophene rings) in their structures are relatively stable even in water and possess sufficiently long lifetimes to exhibit reversible oxidation-reduction cycles, while the carbazole system required more thiophene rings than the coumarin one to be substantially stabilized. The long lifetimes of the oxidized states enabled these dye molecules to be regenerated to the original states by accepting an electron from the I(-) electron donor even in an aqueous solution, achieving sustained H2 and I3(-) production from an aqueous KI solution under visible light irradiation when they were combined with an appropriate n-type semiconductor, ion-exchangeable layered niobate H4Nb6O17. The use of H4Nb6O17 loaded with Pt cocatalyst inside the interlayer allowed the water reduction to proceed preferentially with a steady rate even in the presence of a considerable amount of I3(-) in the solution, due to the inhibited access of I3(-) to the reduction site, Pt particles inside, by the electrostatic repulsion between the I3(-) anions and the negatively charged (Nb6O17)(4-) layers. It was also revealed that the WO3 particles coloaded with Pt and IrO2 catalysts exhibited higher rates of O2 evolution than the WO3 particles loaded only with Pt in aqueous solutions containing a considerable amount of I(-), which competitively consumes the holes and lowers the rate of O2 evolution on WO3 photocatalysts. The enhanced O2 evolution is certainly due to the improved selectivity of holes toward water oxidation on IrO2 cocatalyst, instead of undesirable oxidation of I(-). Simultaneous evolution of H2 and O2 under visible light was then achieved by combining the Pt/H4Nb6O17 semiconductor sensitized with the dye molecules having an oligothiophene moiety, which can stably generate H2 and I3(-) from an aqueous KI solution, with the IrO2-Pt-loaded WO3 photocatalyst that can reduce the I3(-) back to I(-) and oxidize water to O2.

Correlation of the crystal structure of titanium dioxide prepared from titanium tetra-2-propoxide with the photocatalytic activity for redox reactions in aqueous propan-2-ol and silver salt solutions

Titanium dioxide (TiO2) has been prepared by the hydrolysis of titanium tetra-2-propoxide, followed by calcination at various temperatures (Tc) up to 1000 °C. The content and crystallite size of anatase in the TiO2 powders increased upon increasing Tc up to 550 °C. In the Tc range 550–600 °C a mixture of anatase and rutile was obtained. A further increase in Tc resulted in TiO2 of rutile structure only. The photocatalytic activities of these TiO2 powders for redox reactions were evaluated in the following systems: (1) aqueous propan-2-ol solution, (2) aqueous Ag2SO4 solution and (3) aqueous Ag2SO4 solution containing propan-2-ol. The anatase TiO2 showed photocatalytic activity in all these systems, the activity increasing with crystal growth. In aqueous propan-2-ol solution the activity is dramatically enhanced by partial coverage of the TiO2 with platinum black. The photocatalytic activity of the rutile TiO2 powder was comparable to or even greater than that of anatase when the reaction system included the silver salt, but was negligibly small for aqueous propan-2-ol solution regardless of the partial Pt coverage.

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.