Ryu Abe

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.

Efficient Nonsacrificial Water Splitting through Two-Step Photoexcitation by Visible Light using a Modified Oxynitride as a Hydrogen Evolution Photocatalyst

A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO(2)/TaON species (H(2) evolution photocatalyst), an O(2) evolution photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was investigated. Among the O(2) evolution photocatalysts and redox mediators examined, Pt-loaded WO(3) (Pt/WO(3)) and the IO(3)(-)/I(-) pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO(2)/TaON achieved stoichiometric water splitting into H(2) and O(2) under visible light, achieving an apparent quantum yield of 6.3% under irradiation by 420.5 nm monochromatic light under optimal conditions, 6 times greater than the yield achieved using a TaON analogue. To the best of our knowledge, this is the highest reported value to date for a nonsacrificial visible-light-driven water splitting system. The high activity of this system is due to the efficient reaction of electron donors (I(-) ions) and acceptors (IO(3)(-) ions) on the Pt/ZrO(2)/TaON and Pt/WO(3) photocatalysts, respectively, which suppresses undesirable reverse reactions involving the redox couple that would otherwise occur on the photocatalysts. Photoluminescence and photoelectrochemical measurements indicated that the high activity of this Z-scheme system results from the moderated n-type semiconducting character of ZrO(2)/TaON, which results in a lower probability of undesirable electron-hole recombination in ZrO(2)/TaON than in TaON.

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.

Facile Fabrication of an Efficient Oxynitride TaON Photoanode for Overall Water Splitting into H2 and O2 under Visible Light Irradiation

The porous oxynitride TaON film electrode prepared on conducting glass (FTO) showed significantly high quantum efficiency (IPCE = ca. 76% at 400 nm at 0.6 V vs Ag/AgCl) in an aqueous Na(2)SO(4) solution, after loading of IrO(2) x nH(2)O nanoparticles as a cocatalyst for water oxidation. Overall water splitting into H(2) and O(2) under visible light was demonstrated using an IrO(2) x nH(2)O-loaded TaON photoanode combined with a Pt electrode under an externally applied bias (0.6-1 V).

Highly Stable Water Splitting on Oxynitride TaON Photoanode System under Visible Light Irradiation

Highly stable photoelectrochemical water splitting is demonstrated for the first time on a tantalum oxynitride (TaON) photoanode under visible light irradiation. Highly dispersed CoO(x) nanoparticles on the TaON photoanode efficiently scavenge photogenerated holes and effectively suppress self-oxidative deactivation of the TaON surface, resulting in a stable photocurrent. The use of highly dispersed CoO(x) cocatalyst on TaON together with phosphate solutions significantly increased the photocurrent due to the formation of a cobalt/phosphate phase. This enabled us to stably split water into H(2) and O(2) under visible light irradiation at a relatively low applied bias (0.6 V vs Pt counter electrode).

A new type of water splitting system composed of two different TiO2 photocatalysts (anatase, rutile) and a IO3−/I− shuttle redox mediator

Abstract A new photocatalytic reaction that splits water into H2 and O2 was designed by a two-step photoexcitation system composed of a IO3−/I− shuttle redox mediator and two different TiO2 photocatalysts, Pt-loaded TiO2-anatase for H2 evolution and TiO2-rutile for O2 evolution. Simultaneous gas evolution of H 2 (180 μ mol/h) and O 2 (90 μ mol/h) was observed from a basic (pH=11) NaI aqueous suspension of two different TiO2 photocatalysts under UV irradiation (λ>300 nm, 400 W high-pressure Hg lamp).

Fabrication of efficient TaON and Ta3N5 photoanodes for water splitting under visible light irradiation

Efficient TaON and Ta3N5 photoanodes for water splitting were fabricated on conducting glass support (FTO). A necking treatment, which forms effective contacts between TaON (or Ta3N5) particles, afforded a significant increase in the photocurrent. Furthermore, loading of IrO2·nH2O nanoparticles as a cocatalyst for water oxidation improved the photocurrent of the TaON (or Ta3N5) photoanode. The incident photon to charge carrier efficiencies (IPCEs) of the TaON and Ta3N5 photoanodes were calculated to be ca. 76% at 400 nm and ca. 31% at 500 nm, respectively, at 1.15 V vs. reversible hydrogen electrode (RHE) in aqueous Na2SO4 solution. Overall water splitting into H2 and O2 under visible light was demonstrated using an IrO2·nH2O-loaded TaON (or Ta3N5) photoanode combined with a Pt electrode under an externally applied bias (TaON: > 0.6 V, Ta3N5: > 1.0 V).

SrNbO2N as a water-splitting photoanode with a wide visible-light absorption band.

Strontium niobium oxynitride (SrNbO(2)N) particles were coated on fluorine-doped tin oxide (FTO) glass and examined as a photoelectrode for water splitting under visible light in a neutral aqueous solution (Na(2)SO(4), pH ≈ 6). SrNbO(2)N, which has a band gap of ca. 1.8 eV, acted as an n-type semiconductor and generated an anodic photocurrent assignable to water oxidation upon irradiation with visible-light photons with wavelengths of up to 700 nm, even without an externally applied potential. Under visible light (λ > 420 nm) with an applied potential of +1.0-1.55 V vs RHE, nearly stoichiometric H(2) and O(2) evolution was achieved using a SrNbO(2)N/FTO electrode modified with colloidal iridium oxide (IrO(2)) as a water oxidation promoter. This study presents the first example of photoelectrochemical water splitting involving an n-type semiconductor with a band gap smaller than 2.0 eV that does not require an externally applied potential.

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.

Significant effect of iodide addition on water splitting into H2 and O2 over Pt-loaded TiO2 photocatalyst: suppression of backward reaction

Abstract Direct water splitting into H 2 and O 2 over Pt-loaded semiconductor photocatalysts such as Pt–TiO 2 was investigated. We found that addition of a small amount of iodide anion, I − , into the aqueous suspension of Pt–TiO 2 –anatase photocatalyst significantly improved the splitting into H 2 and O 2 with a stoichiometric ratio. The iodide anion was adsorbed preferentially onto Pt co-catalyst as iodine atom, I. This iodine layer effectively suppressed the backward reaction of water formation from H 2 and O 2 to H 2 O over the Pt surface.