Kazuhiro Sayama

Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst

The photocatalytic splitting of water into hydrogen and oxygen using solar energy is a potentially clean and renewable source for hydrogen fuel. The first photocatalysts suitable for water splitting, or for activating hydrogen production from carbohydrate compounds made by plants from water and carbon dioxide, were developed several decades ago. But these catalysts operate with ultraviolet light, which accounts for only 4% of the incoming solar energy and thus renders the overall process impractical. For this reason, considerable efforts have been invested in developing photocatalysts capable of using the less energetic but more abundant visible light, which accounts for about 43% of the incoming solar energy. However, systems that are sufficiently stable and efficient for practical use have not yet been realized. Here we show that doping of indium-tantalum-oxide with nickel yields a series of photocatalysts, In1-xNixTaO4 (x = 0–0.2), which induces direct splitting of water into stoichiometric amounts of oxygen and hydrogen under visible light irradiation with a quantum yield of about 0.66%. Our findings suggest that the use of solar energy for photocatalytic water splitting might provide a viable source for ‘clean’ hydrogen fuel, once the catalytic efficiency of the semiconductor system has been improved by increasing its surface area and suitable modifications of the surface sites.

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO2, Nb2O5, ZnO, SnO2, and In2O3 with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400‐600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochromesensitized ZnO solar cell with an I ‐ /I3 ‐ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99mWcm ‐2 ) reached 2.5% with a short-circuit photocurrent density (Jsc) of 7.44mA cm ‐2 , a open-circuit photovoltage (Voc) of 0.52 V, and a fill factor (ff) of 0.64. The Jsc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the Voc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.

Design of new coumarin dyes having thiophene moieties for highly efficient organic-dye-sensitized solar cells

We have developed new coumarin dyes having thiophene moieties in order to improve the photovoltaic performance of dye-sensitized nanocrystalline TiO2 solar cells based on the organic dyes as photosensitizers. A solar-energy-to-electricity conversion efficiency (η) of 7.7% was attained under AM 1.5 irradiation (100 mW cm−2) with a short-circuit current density (Jsc) of 14.3 mA cm−2, an open-circuit voltage (Voc) of 0.73 V, and a fill factor (ff) of 0.74.

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).

A coumarin-derivative dye sensitized nanocrystalline TiO2 solar cell having a high solar-energy conversion efficiency up to 5.6%

It is found that newly synthesized coumarin derivatives work as highly efficient photosensitizers for dye-sensitized nanocrystalline TiO2 solar cells producing a 5.6% solar-light-to-electricity conversion efficiency, the highest efficiency so far among organic dye-sensitized solar cells, with a short-circuit current density of 13.8 mA cm−2, an open-circuit photovoltage of 0.63 V, and a fill factor of 0.63 under standard AM 1.5 irradiation (100 mW cm−2).

Highly efficient photoelectrochemical water splitting using a thin film photoanode of BiVO4/SnO2/WO3 multi-composite in a carbonate electrolyte

The solar energy conversion efficiency considering the energy loss by the external bias for water splitting reached ca. 0.9 or 1.35% using single- or double-stacked photoanodes, respectively, of BiVO(4)/SnO(2)/WO(3) multilayers in a highly concentrated carbonate electrolyte aqueous solution.

Effect of carbonate salt addition on the photocatalyticdecomposition of liquid water over Pt–TiO2catalyst

It has been found that an addition of carbonate salts to Pt-loaded TiO 2 suspensions led to highly efficient stoichiometric photocatalytic decomposition of liquid water into H 2 and O 2 . Neither the pH nor cation directly contributes to the water splitting, and the presence of a high concentration of carbonate ions is essential for the catalytic photodecomposition of water. The carbonate ion affects both the Pt particles and the TiO 2 surface. The Pt was covered with some titanium hydroxide compounds and, therefore, the rate of the back reaction (H 2 O formation from H 2 and O 2 ) on the Pt was suppressed effectively in the presence of carbonate ions. On the other hand, the TiO 2 surface was readily covered with several types of carbonate species. It is considered that these carbonate species aid desorption of O 2 from the TiO 2 surface. A new reaction mechanism involving peroxocarbonates has been proposed.

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.

Photocatalytic decomposition of water into H2 and O2 by a two-step photoexcitation reaction using a WO3 suspension catalyst and an Fe3+/Fe2+ redox system

H2 and O2 gas evolution in a stoichiometric ratio (H2:O2 = 2:1) has been realized over a RuO2-WO3 photocatalyst suspended in an aqueous solution containing an Fe3+/Fe2+ redox system. O2 gas was evolved and Fe3+ ions were reduced to Fe2+ ions on a WO3 powder catalyst excited by visible light at less than 460 nm and UV light. Fe2+ ions excited by UV light at less than 280 nm were oxidized back to Fe3+ ions with the evolution of H2 gas. The reaction mechanism is similar to the Z-scheme in photosynthesis.

Effect of carbonate addition on the photocatalytic decomposition of liquid water over a ZrO2 catalyst

Abstract The stoichiometric photodecomposition of water into H 2 and O 2 proceeds over ZrO 2 powder suspended in aqueous solutions in the absence of loaded metals such as Pt, RuO 2 and NiO x . The activity is affected significantly by the pressure of the reaction system, the types of additive and the pH of the solution. Specifically, the addition of carbonate, such as NaHCO 3 and Na 2 CO 3 , leads to a remarkable increase in the activity and stability of the gas evolution rate vs. the reactor pressure. The surface of ZrO 2 was covered with carbonate species by the addition of carbonate salt. It is thought that carbonate species on ZrO 2 play an important role in the desorption of O 2 via the carbonate radical. In contrast, the promotion effect of platinum loading on water splitting is negligible. This behaviour can be explained in terms of the backward reaction (H 2 O formation from H 2 and O 2 ) on Pt and the barrier height of the semiconductor-metal junction at which electrons in the conduction band migrate. Furthermore, it is concluded that the suppression of the backward reaction on Pt is essential for overall water splitting into H 2 and O 2 , and this is one of the important functions of NaHCO 3 and Na 2 CO 3 . Pt on ZrO 2 in carbonate solutions exists as an oxide; therefore the effect of the backward reaction is suppressed.