Hidehisa Hagiwara

Preparation of p-Type CaFe2O4 Photocathodes for Producing Hydrogen from Water

An (hk0)-oriented p-type CaFe2O4 (E(g): 1.9 eV) photocathode was prepared, and hydrogen and oxygen gases were produced from a photocell short-circuited by connecting the CaFe2O4 and n-type TiO2 electrodes under illumination without applying an external voltage. The open-circuited voltage was 0.97 V and the short-circuit current was about 200 μA/cm(2), and the amount of evaluated hydrogen and oxygen gases after 2 days of reaction were about 70 and 4 μmol, respectively.

Synthesis and Photocatalytic Activity of Rhodium-Doped Calcium Niobate Nanosheets for Hydrogen Production from a Water/Methanol System without Cocatalyst Loading

Rhodium-doped calcium niobate nanosheets were synthesized by exfoliating layered KCa(2)Nb(3-x)Rh(x)O(10-δ) and exhibited high photocatalytic activity for H(2) production from a water/methanol system without cocatalyst loading. The maximum H(2) production rate of the nanosheets was 165 times larger than that of the parent Rh-doped layered oxide. The quantum efficiency at 300 nm was 65%. In this system, the methanol was oxidized to formaldehyde (main product), formic acid, and carbon dioxide by holes, whereas electrons cause the reduction of water to H(2). The conductivity of the parent layered oxide was decreased by doping, which indicates the octahedral RhO(6) unit in the lattice of the nanosheet functions as an electron trap site. The RhO(6) units in the nanosheet probably also act as reaction sites for H(2) evolution.

Potential gradient and photocatalytic activity of an ultrathin p-n junction surface prepared with two-dimensional semiconducting nanocrystals.

The creation of p-n junction structure in photocatalysts is a smart approach to improve the photocatalytic activity, as p-n junctions can potentially act to suppress the recombination reaction. Understanding the surface conditions of the junction parts is one of the biggest challenges in the development of photocatalyst surface chemistry. Here, we show a relationship between the photocatalytic activity and potential gradient of the junction surface prepared from two-dimensional crystals of p-type NiO and n-type calcium niobate (CNO). The ultrathin (ca. 2 nm) junction structure and the surface potential were analyzed using low energy ion scattering spectroscopy and Kelvin probe force microscopy. The photocatalytic H2 production rate for the n-p (CNO/NiO) junction surface was higher than those for p-n (NiO/CNO) junction, p, and n surfaces. The surface potential of the CNO/NiO junction part (surface: CNO) was lower than that of the CNO crystals in the same CNO crystal surface. These potential gradients result in specially separated reaction sites, which suppress the recombination reaction in the CNO nanosheet. Photo-oxidation and photoreduction sites in the junction structure were confirmed using the photodeposition reaction of MnO(x) and Ag.

Spacer effects in metal-free organic dyes for visible-light-driven dye-sensitized photocatalytic hydrogen production†

Metal-free organic dyes containing benzo[b]phenothiazine were synthesized and effectively used for dye-sensitized visible-light-driven photocatalytic hydrogen production. The materials exhibited high stability and hydrogen production when numerous π-conjugated bridges were inserted as spacers between the donor and the anchor moiety. Photocatalytic hydrogen production was investigated in a TiO2/dye/Pt structure using triethanolamine as the sacrificial reagent. Compound dye 3, which had the longest spacer between the donor and the acceptor, exhibited the best hydrogen production performance of the series examined in this study. It displayed a turnover number (TON) of 4460, a turnover frequency of 278 after 16 h, and a photo-quantum efficiency of 1.65% at 420 nm. Furthermore, it showed the longest electron injection lifetime because its coordination structure was considered to be vertically standing on the TiO2 surface by theoretical calculations. On the other hand, dye 1 showed the lowest hydrogen production performance with a TON of 483 and very short electron injection lifetime. This observation is confirmed by the computation results, which showed the lying geometry of 1 with monodentate coordination of the dye with respect to the TiO2 surface. This spacer effectproperty relationship study may provide a good strategy for the development of metal-free organic dyes for dye-sensitized photocatalytic water splitting.

Charge‐Transfer Mechanism in Pt/KTa(Zr)O3 Photocatalysts Modified with Porphyrinoids for Water Splitting

The mechanism of photocatalytic splitting of H(2)O into H(2) and O(2) on Pt/KTa(Zr)O(3) modified with various porphyrinoids was investigated. The photocatalytic activity of KTaO(3) catalysts is improved by dye modification. Cyanocobalamin (vitamin B(12)) is the most effective for improving water-splitting activity, and the formation rates of H(2) and O(2) achieved values of 575 and 280 micromol g(cat.) (-1) h(-1), respectively. X-ray photoelectron spectroscopy spectra of KTa(Zr)O(3) photocatalysts showed that Pt loaded onto dye-modified KTaO(3) was slightly oxidized and had low catalytic activity for the H(2) oxidation reaction. Photoluminescence (PL) spectra of KTaO(3) catalysts suggested that excitation energy was transferred between KTaO(3), tetraphenylporphyrinatochromium(III) (Cr-TPP), and the Pt cocatalyst. The wavelength dependence of the activity of dye-modified KTa(Zr)O(3) photocatalysts indicated that excitation of both KTa(Zr)O(3) and the dye was essential for achieving increased photocatalytic activity. This result suggests that two-step excitation occurred in the dye-modified KTa(Zr)O(3) photocatalysts. Because the lifetime of the charge-separated state increased, this study reveals that modification with porphyrinoids is effective for increasing water-splitting activity.

One-pot soft-templating method to synthesize crystalline mesoporous tantalum oxide and its photocatalytic activity for overall water splitting.

Crystalline mesoporous Ta2O5 has been successfully synthesized by a one-pot route using P-123 as the structure directing agent (SDA). A series of crystalline mesoporous Ta2O5 samples has been prepared by changing the calcination temperature. The surface area decreased and the pore size increased with the increasing calcination temperature, which were the results of crystallite growth. At the same time, the pore volume was well maintained, which means limited shrinkage during the calcination of elevated temperature. The porous structure and crystal structure of as-synthesized mesoporous Ta2O5 were characterized by XRD, TG-DTA, SEM, TEM, and N2 sorption techniques. The photocatalytic activity of the as-synthesized mesoporous Ta2O5 with the cocatalyst NiOx for overall water splitting under ultraviolet (UV) light irradiation was systematically evaluated. The photocatalytic activity of crystalline mesoporous Ta2O5 showed about 3 times that of commercial Ta2O5 powder and 22 times that of amorphous mesoporous Ta2O5.

Ni-Fe Nitride Nanoplates on Nitrogen-Doped Graphene as a Synergistic Catalyst for Reversible Oxygen Evolution Reaction and Rechargeable Zn-Air Battery

Obtaining bifunctional electrocatalysts with high activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is a main hurdle in the application of rechargeable metal-air batteries. Earth-abundant 3d transition metal-based catalysts have been developed for the OER and ORR; however, most of these are based on oxides, whose insulating nature strongly restricts their catalytic performance. This study describes a metallic Ni-Fe nitride/nitrogen-doped graphene hybrid in which 2D Ni-Fe nitride nanoplates are strongly coupled with the graphene support. Electronic structure of the Ni-Fe nitride is changed by hybridizing with the nitrogen-doped graphene. The unique heterostructure of this hybrid catalyst results in very high OER activity with the lowest onset overpotential (150 mV) reported, and good ORR activity comparable to that for commercial Pt/C. The high activity and durability of this bifunctional catalyst are also confirmed in rechargeable zinc-air batteries that are stable for 180 cycles with an overall overpotential of only 0.77 V at 10 mA-2 .

Black-colored nitrogen-doped calcium niobium oxide nanosheets and their photocatalytic properties under visible light irradiation

A black-colored nitrogen-doped calcium niobium oxide nanosheet with a thickness of around 1 nm was prepared by exfoliating nitrogen-doped layered oxide (KCa2Nb3O10−xNy), which exhibited photocatalytic activities superior to those of the parent material under visible light irradiation. It is suspected that the ultrathin thickness is suppressing the recombination.

Application to Photocatalytic H2 Production of a Whole-Cell Reaction by Recombinant Escherichia coli Cells Expressing [FeFe]-Hydrogenase and Maturases Genes

A photocatalytic H2 production system using an inorganic-bio hybrid photocatalyst could contribute to the efficient utilization of solar energy, but would require the development of a new approach for preparing a H2 -forming biocatalyst. In the present study, we constructed a recombinant strain of Escherichia coli expressing the genes encoding the [FeFe]-hydrogenase and relevant maturases from Clostridium acetobutylicum NBRC 13948 for use as a biocatalyst. We investigated the direct application of a whole-cell of the recombinant E. coli. The combination of TiO2 , methylviologen, and the recombinant E. coli formed H2 under light irradiation, demonstrating that whole cells of the recombinant E. coli could be employed for photocatalytic H2 production without any time-consuming and costly manipulations (for example, enzyme purification). This is the first report of the direct application of a whole-cell reaction of recombinant E. coli to photocatalytic H2 production.

Impact of alkoxy chain length on carbazole-based, visible light-driven, dye sensitized photocatalytic hydrogen production

Alkoxyphenyl-substituted carbazole-based metal-free organic dyes were synthesized and effectively used for dye-sensitized, visible-light-driven, photocatalytic hydrogen production. Photocatalytic hydrogen production was investigated using a TiO2/dye/Pt structure with triethanolamine as the sacrificial reagent. The dye-loaded TiO2 photocatalyst exhibited a high yield of hydrogen production when the length of the alkoxy chain was long enough to sufficiently improve the hydrophobicity at the interface between the dye-loaded TiO2 and the water medium. In the alkoxyphenyl-substituted carbazole dyes, the dye with the longest alkoxy chain (C22) exhibited the best hydrogen production performance, but it had a yield only slightly better than that of the dye with the second longest chain length (C16). The dye C22 displayed a turnover number (TON) of 3094 after 24 h of visible light irradiation (>420 nm). However, the compound with no hydrophobic substituent (C1), exhibited the lowest hydrogen production performance with a TON of 1497. Thus, a 207% increase in the hydrogen production yield was observed when hydrophobic substituents were present. Analysis of time-resolved absorption spectra, impedance spectra and incident photon conversion efficiency spectra revealed that the alkoxy chain has a hydrophobic effect at the interface between the dye-loaded TiO2 and the water. Specifically, the hydrophobicity of the dye improved the charge-recombination lifetime for electron injection from the dye into the TiO2 surface in the water for hydrogen production.