Yasumichi Matsumoto

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.

Simple photoreduction of graphene oxide nanosheet under mild conditions

Graphene oxide (GO) nanosheets were reduced by UV irradiation in H2 or N2 under mild conditions (at room temperature) without a photocatalyst. Photoreduction proceeded even in an aqueous suspension of nanosheets. The GO nanosheets reduced by this method were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. It was found that epoxy groups attached to the interiors of aromatic domains of the GO nanosheet were destroyed during UV irradiation to form relatively large sp2 islands resulting in a high conductivity. I-V curves were measured by conductive atomic force microscopy (AFM; perpendicular to a single nanosheet) and a two-electrode system (parallel to the nanosheet). They revealed that photoreduced GO nanosheets have high conductivities, whereas nonreduced GO nanosheets are nearly insulating. Ag+ adsorbed on GO nanosheets promoted the photoreduction. This photoreduction method was very useful for photopatterning a conducting section of micrometer size on insulating GO. The developed photoreduction process based on a photoreaction will extend the applications of GO to many fields because it can be performed in mild conditions without a photocatalyst.

Synthesis of hexagonal nickel hydroxide nanosheets by exfoliation of layered nickel hydroxide intercalated with dodecyl sulfate ions.

One-nanometer-thick nickel hydroxide nanosheets were prepared by exfoliation of layered nickel hydroxides intercalated with dodecyl sulfate (DS) ions. The shape of the nanosheets was hexagonal, as was that of the layered nickel hydroxides intercalated with DS ions. The nickel hydroxide nanosheets exhibited charge-discharge properties in strong alkaline electrolyte. The morphology of the nanosheet changed during the electrochemical reaction.

Photoluminescence of perovskite nanosheets prepared by exfoliation of layered oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion).

Luminescent perovskite nanosheets were prepared by exfoliation of single- or double-layered perovskite oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion). The thickness of the individual nanosheets corresponded to those of the perovskite block in the parent layered compounds. Intense red and green emissions were observed in aqueous solutions with Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets, respectively, under UV illumination with energies greater than the corresponding host oxide band gap. The coincidence of the excitation spectrum and the band gap absorbance indicates that the visible emission results from energy transfer within the nanosheet. The red emission intensity of the Gd1.4Eu0.6Ti3O10-nanosheets was much stronger than that of the La0.90Eu0.05Nb2O7-nanosheets reported previously. The strong emission intensity is a result of a two-step energy transfer cascade within the nanosheet from the Ti-O network to Gd(3+) and then to Eu(3+). The emission intensities of the Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets can be modulated by applying a magnetic field (1.3-1.4 T), which brings about a change in orientation of the nanosheets in solution. The emission intensities increased when the excitation light and the magnetic field directions were perpendicular to each other, and they decreased when the excitation and magnetic field were collinear and mutually perpendicular to the direction of detection of the emitted light.

Proton Conductivities of Graphene Oxide Nanosheets: Single, Multilayer, and Modified Nanosheets

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop-cast method) are larger than those of single-layer GO (prepared by either the drop-cast or the Langmuir-Blodgett (LB) method). At 60% relative humidity (RH), the σ value increases from 1×10(-6) S cm(-1) in single-layer GO to 1×10(-4) and 4×10(-4) S cm(-1) for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.

Electrodeposition of TiO2 photocatalyst into nano-pores of hard alumite

Abstract TiO2 was electrodeposited into the pores of hard alumite (Al/Al2O3/TiO2) by alternative electrolysis in (NH4)2[TiO(C2O4)2] solution, where the alumite was prepared by anodic oxidation of aluminum in sulfuric acid (Al/Al2O3). When the electrolysis was carried out in an (NH4)2[TiO(C2O4)2] solution under ac bias for the Al/Al2O3, a cathodic current due to the reduction of H+ and/or H2O was observed at about −10 V in measurement of the Lissajous figure, leading to the deposition of TiO2. Consequently, the TiO2 was deposited during an increase in pH due to the electrochemical reduction reactions of H+ and/or H2O in the pores of the alumite. The deposited TiO2 was highly dispersed in the pores of the alumite. The prepared TiO2 in the pores (Al/Al2O3/TiO2) showed high photocatalytic activity for the decomposition of acetaldehyde compared to TiO2 deposited directly on an aluminum surface (Al/TiO2).

Electrical conductivities of SrFeO3−δ and BaFeO3−δ perovskites

Abstract The conductivities and the Seebeck coefficients of the SrFeO3−δ and the BaFeO3−δ perovskites were measured in air, O2, and N2 in the temperature range from 650°C to room temperature. The carrier densities and the mobilities were calculated by using a small polaron hopping model. All the samples of the BaFeO3−δ show the hopping conduction, while the conductions of the SrFeO3−δ in air and O2 will be near metallic because the carrier densities and the mobilities are very large. The mobilities rather than the carrier densities decreased with the increase of the amounts of the oxygen vacancy, δ, in the range of 0

Electrophoretic deposition assisted by soluble anode

The rate of cathodic electrophoretic deposition (EPD) of silicon monoxide particles was accelerated by using a soluble anode such as stainless steel or zinc in an acetone bath containing iodine. In contrast, the use of an inert anode such as Pt inhibited deposition. In the former cases, anodic dissolution or corrosion of the metal plate occurred due to the presence of iodide ions in the bath. The metal ions produced adsorbed onto surface hydroxyl groups of oxide particles together with protons. As a result, a positive suspension ζ-potential was achieved. Deposition using the Zn anode was about 10 times larger than that for the stainless steel anode, attributable to the difference in the solubility of the substrate. The Zn anode is likely to be more soluble than the stainless steel anode because stainless steel is covered with more protective passive film. The electrophoretic deposition mechanisms are discussed briefly, and the utility of a soluble anode in EPD is demonstrated.

Origin of the silver doping effects on superconducting oxide ceramics

The origin of silver doping effects on Y‐Ba‐Cu‐O superconducting ceramics, such as a high‐temperature shift of the point at zero resistivity in resistivity measurements and an increase in critical current density, was sought by transmission electron microscopy observations. Sintering was strongly promoted together with suppression of the formation of the liquid phase in the grain boundaries by silver. Consequently, the liquid phase, which is not a superconductor, was not present in almost all the grain boundaries of the silver‐doped ceramics, while this phase was present in almost all the grain boundaries in pure ceramics.

Dynamic Control of Photoluminescence for Self‐assembled Nanosheet Films Intercalated with Lanthanide Ions by Using a Photoelectrochemical Reaction

Semiconductor oxide nanosheets synthesized by exfoliation of layered oxides are two-dimensional crystals with a thickness of about 1 nm. New layered materials and their films can be reassembled by electrostatic self-assembly deposition (ESD) and by layer-by-layer (LBL) techniques, respectively. Since the nanosheets have a negative charge in aqueous solution they can be used with various cationic species as the starting materials. Layered materials prepared from nanosheets and lanthanide (Ln) ions are promising as new functional materials because Ln ions have unique properties, such as luminescence and magnetic properties, that are attributable to the 4f electron orbital. For example, the titanate layered oxide intercalated with Eu ions prepared from titanate nanosheets and Eu ions has unique luminescence properties. The layered oxide gives a red emission from the Eu ions which is induced by energy transfer through excitation of the bandgap of the titanate nanosheet, and the emission from the Eu ions is promoted by intercalated water molecules. Furthermore, spectral hole burning caused by the intercalated water molecules was observed in the excitation spectra at room temperature. Nanosheets of TiOx, NbOx, and TaOx give a high photocurrent during the photoelectrochemical reaction under UV illumination with an energy higher than that of the bandgap. This finding indicates that a large charge separation is produced between the holes in the valence band and the electrons in the conduction band during excitation of the bandgap. Consequently, layered oxide materials intercalated with Ln ions simultaneously exhibit both photoluminescence and a photoelectrochemical reaction during excitation of the bandgap on illumination with UV light. The study reported herein demonstrates a new form of dynamic control over the photoluminescence of Ln ions intercalated in self-assembled nanosheet films of TiOx and NbOx. The photoluminescence properties of Ln ions are changed by factors such as a change in the pH value and the addition of anionic species. However, it is difficult to dynamically control the photoluminescence properties of Ln ions. In the present system, the emission intensities of the intercalated Eu and Tb ions can be readily controlled by varying the applied potential. The nanosheet/Ln (Ti1.81O4 nanosheet/Eu 3+ (TiO/Eu) and Nb6O17 nanosheet/Tb 3+ (NbO/Tb)) films were prepared and fixed on boron-doped diamond electrodes by the LBL technique. The chemical compositions of the TiO/Eu and NbO/Tb films were EuxTi1.81O4 (x = 0.20–0.30) and TbyNb6O17 (y= 1.30–1.50), respectively. These are close to the theoretical neutral compositions (Eu0.25Ti1.81O4 and Tb1.33Nb6O17). The Ln ions were sandwiched between nanosheets (see Figure S-1 in the Supporting Information). Figure 1 shows a sche-