Hajime Wagata

An aqueous solution process and subsequent UV treatment for highly transparent conductive ZnO films

High electric conductivity was achieved in ZnO films prepared by a low-temperature (<100 °C) wet-chemical process and subsequent UV treatment by a commercial blacklight-blue lamp with a central wavelength of 360 nm and output power of 2.0 mW cm−2. The UV treatment to the as-deposited film successfully decreased the electric resistivity of the film by three orders of magnitude from 11 to 4.4 × 10−3 Ω cm. The resistivity was not restored to the initial value even after dark storage for 50 days, indicating that the decrease of the resistivity has not mainly originated from conventional persistent photoconductivity. The results of Hall measurement and Fourier transformed infrared measurement showed that an increase of carrier concentration and decomposition of organic impurities of the films occurred simultaneously, which indicates that the phenomenon was caused by introduction of donor impurities such as hydrogen atoms, carbon species and oxygen vacancies into the ZnO lattice by the photocatalytic decomposition of residual organic impurities remaining in the film.

Flux growth of Sr2Ta2O7 crystals and subsequent nitridation to form SrTaO2N crystals

Highly crystalline, idiomorphic Sr2Ta2O7 and SrTaO2N crystals were successfully grown by a SrCl2 flux cooling method and followed by a nitriding treatment using NH3 gas, respectively. The flux-grown Sr2Ta2O7 crystals had a columnar structure with flat and well-developed {010}, {061}, and {150} faces. The lattice parameters of the Sr2Ta2O7 crystals were determined to be a = 0.398, b = 2.716, and c = 0.570 nm. The TEM images indicated that the flux-grown Sr2Ta2O7 crystals had a mature columnar structure with high crystallinity. The shapes and sizes of the SrTaO2N crystals were in good agreement with the original Sr2Ta2O7 crystals, and they consisted of numerous small crystals with approximate dimensions of 50 nm and high crystallinity. The lattice parameters of the SrTaO2N crystals were a = 0.570 and c = 0.809 nm. The optical absorption edges of the Sr2Ta2O7 and SrTaO2N crystals were approximately 275 and 600 nm, respectively, and the band gaps were estimated to be located at 4.5 and 2.1 eV, respectively.

The cross-substitution effect of tantalum on the visible-light-driven water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method

Various perovskite-type transition metal oxynitride photocatalysts have been intensively investigated for photocatalytic water splitting under visible light. The band gap engineering and the formation of solid solution have been demonstrated to be one of the beneficial approaches to achieve a high solar energy conversion efficiency. As a member of the niobium-based oxynitride family, BaNbO2N has a small band gap energy of 1.79 eV and light absorption up to 690 nm. Here, we have investigated the cross-substitution effect of tantalum on the photocatalytic water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method. It was found that with increasing the amount of tantalum substituted for niobium in BaNb1−xTaxO2N (x = 0, 0.25, 0.50, 0.75, and 1), the average crystal size and the intensity of background absorption gradually reduced and the O2 evolution rate increased monotonically for BaTaO2N crystals due to the shift of the top of the valence band to a more positive side and the lowered densities of mid-gap states associated with defects. The CoOx/flux-grown-BaTaO2N/Ta photoanode exhibited a photocurrent of about 0.85 mA cm−2 at 1.2 VRHE, which decreased gradually by about 35% after 24 h, evidencing that the flux-grown BaTaO2N crystals are highly stable for photoelectrochemical water oxidation. BaTaO2N modified with CoOx (2 wt% Co) as an O2 evolution cocatalyst exhibited an apparent quantum yield (AQY) of 0.24% at 420 nm for an O2 evolution reaction in the presence of Ag+ ions as sacrificial electron acceptors. In addition, the results from the first-principles DFT calculations conform to the experimentally obtained data on the photocatalytic water oxidation activity of the BaNb1−xTaxO2N crystals.

Low-temperature growth of spinel-type Li1+xMn2−xO4 crystals using a LiCl–KCl flux and their performance as a positive active material in lithium-ion rechargeable batteries

Low-temperature growth of idiomorphic spinel-type Li1+xMn2−xO4 (x = 0.09, 0.14) crystals was achieved by using a LiCl–KCl flux. The flux growth driven by rapid cooling resulted in truncated octahedral Li1+xMn2−xO4 crystals surrounded by both dominating {111} and minor {100} faces. The chemical compositions, sizes, and shapes of the Li1+xMn2−xO4 crystals could be tuned by simply changing the growth conditions. Among the various products, the crystals grown at a low temperature of 600 °C showed a small average size of 0.2 μm. The small Li1+xMn2−xO4 crystals grown at 600 °C showed better rate properties than the large crystals grown at 900 °C, when used as a positive active material in lithium-ion rechargeable batteries.

A novel flux coating method for the fabrication of layers of visible-light-responsive Ta3N5 crystals on tantalum substrates

Layers of well-developed crystals of Ta3N5 were successfully fabricated on Ta substrates by using a novel flux coating method in a flow of NH3. The flux coating method is a simple one: the Ta substrates were coated with aqueous solutions of sodium compounds (= fluxes) and subsequently heated in a flow of NH3, whereupon the surfaces of the Ta substrates were dissolved in the flux, resulting in Ta3N5 crystal layers. The Ta in Ta3N5 was provided by the substrate. Therefore, crystal layers with good adhesion could be grown directly on the substrates. The shape of the individual crystals as well as the surface morphology of the layers formed was determined by the flux used. The crystals fabricated using NaCl–Na2CO3 as the flux were prismatic and had relatively smooth faces, covering the surface of the Ta substrate uniformly. The crystal growth field resulting from the use of this method yielded well-formed crystals, which presumably grew from a solution. Finally, it was confirmed that a thus-synthesized Ta3N5 crystal layer modified using Co-Pi as the co-catalyst generated a photoanodic current under visible-light irradiation.

Template-free solvothermal preparation of ZnO hollow microspheres covered with c planes

ZnO hollow microspheres were prepared by a solvothermal method, without using a template. The morphology and structure of the microspheres were investigated by scanning and transmission electron microscopies. A series of controlled experiments was carried out to better understand the formation mechanism. The sample prepared at 150 °C for 1 h consisted of ZnO microspheres 1–2 μm in diameter. Microspheres were composed of crystallites of tens of nm in diameter. The surfaces of the microspheres prepared at 150 °C for 3 h were surrounded by hexagonal pyramid-like crystals, of a few hundred nm in diameter. The sample prepared by reaction at 150 °C for 6 h consisted of hollow spheres, even in the absence of a template. The microsphere surface consisted of ZnO c planes. ZnO hollow microspheres prepared at 150 °C for 12 h exhibited intense narrow ultraviolet emission.

Effects of Thermal Treatment on Crystallographic and Electrical Properties of Transparent Conductive ZnO Films Deposited by Spin-Spray Method

ZnO films were deposited by a novel aqueous solution process named the spin-spray method and thermally treated at different temperatures from 100 to 300 C. All films had a high transmittance of 80% in the visible range and exhibited conductivity after UV illumination for 24h. The dependence of thermal treatment temperature before UV illumination on their crystallographic and electrical properties was investigated in this study. The XRD peak intensity increased and the full width at half maximum of the (101) peak decreased from 0.35 to 0.26 for the sample thermally treated at 100 C. From the Fourier transform infrared (FT-IR) spectra, decreases in Vas (COO) and Vs (COO) intensities with increasing thermal treatment temperature were observed. The resistivity after the UV illumination of the sample without thermal treatment was 4:1 10 2 � cm, and it decreased to 1:6 10 2 � cm in the sample thermally treated at 100 C. The sample with the lowest resistivity exhibited a relatively high mobility of 3.3cm 2 V 1 s 1

Amount of tungsten dopant influencing the photocatalytic water oxidation activity of LaTiO2N crystals grown directly by an NH3-assisted flux method

Solar water splitting technologies for hydrogen generation using visible-light-active (oxy)nitride photocatalysts have been intensively studied to achieve a greater supply of clean and renewable energy. Here, we have investigated the amount of tungsten dopant influencing the photocatalytic water oxidation activity of LaTiO2N (LTON) crystals grown directly by an NH3-assisted KCl flux method in the absence of a cocatalyst. With the maximum amount of tungsten dopant (x = 0.10), LaWO0.6N2.4 was formed as a minor phase to a W-doped LaTiO2N phase (W-LTON). The optimum W-doped LaTiO2N crystals were grown with the tungsten amount of x = 0.05, showing a nearly three-fold higher O2 evolution rate (51 μmol h−1) compared to pure LaTiO2N crystals (18 μmol h–1). The improvement in the O2 evolution rate of the LaTiO2N crystals was presumably due to the improved conductivity, enhanced light absorption and efficient charge separation stemmed from tungsten doping. According to first-principles density function theory (DFT) calculations, although no significant change in band gap by tungsten doping was observed, slight broadening of valence and conduction bands was noted in W-LTON which may improve the electron/hole conductivity due to reduction of the effective mass of electrons. Also, tungsten doping was beneficial in creating effective defects and in enhancing the photocatalytic water oxidation activity of the LaTiO2N crystals.

Photocatalytic Activity and Related Surface Properties of Transparent ZnO Films Prepared by a Low-temperature Aqueous Route

Transparent ZnO were prepared using solutions with various trisodium citrate concentrations by a spin‐spray method at 90°C. The morphological and structural characteristics, as well as photocatalytic activity of the resulting ZnO films were examined with respect to the added trisodium citrate concentration. Photocatalytic activities of the ZnO films were evaluated from photodecomposition of methylene blue (MB) in aqueous solution. With increasing citrate concentrations, the ZnO films came to have higher transmittances in visible region but lower MB decomposition rate. Both high transmittance and high photocatalytic activity were achieved in the ZnO film prepared in the citrate concentration of 0.5 mm. The possible mechanism for the difference in photocatalytic activity by the samples prepared with the various concentrations of citrate was discussed from the viewpoint of film texture, crystal orientation and surface chemical state.

Epitaxial growth of orthorhombic NaTaO3 crystals on SrTiO3 (100) surface by flux coating

Orthorhombic NaTaO3 crystals were epitaxially grown on a SrTiO3 (100) surface using flux coating. The selected-area electron diffraction results revealed that the well-oriented orthorhombic NaTaO3 crystals and SrTiO3 had an orientation relationship of (10−1)[010] NaTaO3//(100)[010] SrTiO3.