Masanobu Izaki

Crystalline oxide microfeatures offer optical, electronic, catalytic, and interfacial functions, but their fabrication often requires templates, patterned scaffolds, or serial machining. A template-free route converted an electrodeposited Cu₂O coating on Au(111)/Si into substrate-supported vertical microfeatures by anodization at a nominal cell voltage of 10 V in 0.002 mol L⁻¹ Na₂S₂O₈ at 277 K. FE-SEM showed progressive widening of the pre-existing intercolumnar network and narrowing of the retained features. This spatially non-uniform removal identifies preferential dissolution along the intercolumnar network as the principal removal pathway at the coating-morphology scale. From 1 to 8 min, the within-image mean and median feature widths decreased by 36.0% and 45.9%, respectively. The dominant out-of-plane Cu₂O(111) diffraction signature was retained while mean visible reflectance decreased. The Cu-H₂O potential-pH framework provides a qualitative thermodynamic guide to possible oxidative pathways that are evaluated against the experimental evidence. C 1s-referenced XPS provides direct ex situ evidence of an anodization-associated Cu(II)/CuO-like contribution at the outermost surface. Chopped photoelectrochemical measurements showed the largest condition-level light–dark current density contrast after short anodization, providing a secondary functional comparison of the completed coatings.

Research on electromagnetic wave absorption materials that can be used at high frequencies above the GHz band has been attracting attention. For this application, in our development of soft magnetic Fe–B particles embedded in epoxy resin composite films, this study aims to improve the particle content and dispersion of Fe–B particles within epoxy films. To prevent oxidation and to enhance the dispersion of Fe–B particles, they were coated with silica using the liquid-phase hydrolysis method. The submicron-sized silica-coated Fe–B particles are expected to exhibit lower eddy current losses. Importantly, the silica coating prevented the oxidation of Fe–B particles, thereby suppressing magnetic properties degradation. Fe–B/epoxy composite films were fabricated using silica-coated Fe–B particles via the layer-by-layer assisted composite plating method. The composite film produced with an Fe–B particle concentration of 10 g/l achieved a particle content of 90.1 wt. % and exhibited a real part of complex permeability (μ′) of 1.5 at 1 GHz.

Chemical reductions of Cu(OH)₂/Cu(O,S) and Cu(O,S)/Cu(O,S) bilayers fabricated on glass substrates by chemical bath deposition (CBD) was investigated through electrochemical measurements, X-ray diffraction, and evaluations of the electrical properties and adhesion strength by a tape-peeling test. The potential change during the chemical reduction in NaBH₄ aqueous solutions was measured and discussed based on a mixed potential theory using the equilibrium potential calculated with chemical thermodynamics. The sheet resistance decreased from over 1 × 10⁸ W/□ to 0.24 W/□, mainly due to the increase in the thickness, with the resistivity estimated to be 4.8 × 10^–6 W cm. The metallic Cu layer showed adhesion strengths of 6.4 and 4.4 N cm⁻¹ for maximum and average values respectively by tape-peeling test, without any detachment, but the Cu sheet peeled off from the glass substrate after the complete chemical reduction of the Cu(OH)₂/Cu(O,S) bilayer. The Cu fragments formed by the chemical reduction of the Cu(O,S)/Cu(O,S) bilayer peeled off from the glass substrate, but the Cu(O,S)/Cu(O,S) bilayer showed the adhesion strengths of 6.7 and 4.0 N cm⁻¹ for maximum and average values, respectively, by the tape-peeling test. Both the Cu(OH)₂ and Cu(O,S) layers were indispensable to secure the Cu layer on the glass substrate.

The surface state of soda-lime glasses before and after ultrasonication pretreatments with water, acetone, ethanol, water/ethanol/NaOH solutions, and UV/O₃ treatment were estimated using atomic force microscopy（AFM）and Fourier-transformed infrared spectroscopy with attenuated total reflection mode（FT-IR-ATR）. Ultrasonication with water/ethanol/NaOH solutions and UV/O₃ treatments drastically decreased the size and number of surface contaminant particles and enhanced absorption of the isolated silanol and hydrogen-terminated silanol. The Cu（OH）₂/Cu（O,S）bilayers were prepared using chemical bath deposition（CBD）in aqueous solutions containing copper（II）nitrate hydrate, ammonium nitrate, and urea or thiourea on glass substrates treated with water/ethanol/NaOH solutions. Top metallic Cu layers were then formed chemically by simple immersion of the bilayers into a NaBH₄ aqueous solution. The sheet resistance decreased with increasing immersion time, leading to 0.68 Ω/□ sheet resistance and 3.3×10⁻⁵ Ω cm resistivity after 10 min immersion. The adhesion strength was estimated using the tape peeling off test with a tensile testing machine and adhesion tape. The Cu/Cu（OH）₂/Cu（O,S）bilayer fabricated by chemical reduction for 10 min exhibited maximum and average adhesion strengths of 13.3 N/cm and 11.0 N/cm, respectively. The adhesivity was inferred to have originated from hydrogen bonding between the oxygen ions in Cu（O,S）and -OH- in silanol of the glass substrate.

A metallic Cu layer was fabricated on a glass substrate using chemical bath deposition of Cu（OH）₂/Cu（O,S）bilayer in aqueous solutions containing copper（II）nitrate hydrate, ammonium nitrate, and thiourea or urea, followed by chemical reduction in a sodium borohydride solution. The Cu layer possessed resistance of 0.68 Ω/□ and maximum peeling strength of 13.4 N/cm.

P-type CuO-Cu₂O nanocomposite semiconductors composed of Cu₂O-embedded CuO aggregations and Cu₂O aggregation consisting of space-filling CuO grains have been fabricated by photoelectrochemical high-frequency potential-switching of 100 to 1000 cycles in an aqueous solution containing copper(II) sulfate hydrate, tartaric acid, and sodium hydroxide, and the size of both the CuO and Cu₂O grains decreased from 40–44 nm to approximately 10 nm remaining the characteristic monoclinic and cubic lattices with an increase in cycle numbers. The bandgap energy of CuO components was almost a constant value of 1.5 eV, and the Cu₂O components showed a decrease in bandgap energy from 2.05 eV to 1.85 eV with an increase in cycle number due to the Cu²⁺ state incorporation, and the CuO-Cu₂O nanocomposites possessed an ionization energy of approximately 5.2 eV and work function of approximately 4.6 eV respectively, and were close to those for single CuO and Cu₂O layers.

Metal oxide semiconductor material has great potential to act as window layer in p–n heterojunction solar cell thin film owing to low production cost and significant properties in photovoltaic mechanism. In this work, n-TiO₂/ZnO bilayer thin film was effectively constructed by means of sol-gel spin coating technique in an effort to diminish the electron-hole recombination rate from single-layered thin film. Annealing time is one of the important parameters in the fabrication process and was varied to study the impact of annealing treatment towards the thin film characteristics as window layer. It was found that the optimum parameter for the n-TiO₂/ZnO bilayer was 500 °C with an annealing time of 2 h. High crystallinity of the n-(101)-TiO₂/(002)-ZnO bilayer thin film was obtained, which consists of anatase and a hexagonal wurtzite structure, respectively. Orientation of (002)-ZnO is essential for deposition with the (111) Cu₂O-absorbing layer due to a low different lattice mismatch between these two interfaces. The homogenous morphology of n-TiO₂/ZnO bilayer was observed with a compact and dense layer. The improvement of transmittance has also been achieved in a range of 60%–80%, which indicated that the incident light can penetrate throughout the thin film directly. In addition, a p-Cu₂O absorbing layer was successfully fabricated on top of n-TiO₂/ZnO bilayer thin film to form a p-n junction in order to visualize significant electrical rectification properties. The existence of p-Cu₂O was confirmed by a (111)-peak orientation and triangular shape in structural and morphological properties, respectively.

A metallic Cu layer with a sheet resistance of 0.19 Ω was chemically prepared on a glass substrate by chemical reduction of the intermediate Cu(OH)₂/Cu(O,S) bilayers using dimethylamine-borane (DMAB), while Cu(OH)₂, and Cu(O,S) layers were fabricated by a sequential chemical bath deposition (CBD) in aqueous solutions containing copper (II) nitrate hydrate and ammonium nitrate with urea and thiourea. The thickness of the Cu(OH)₂ and resultant Cu layers were strongly affected by the structure under the Cu(O, S) layer, and a metallic Cu layer was formed mainly from the Cu(OH)₂ layer. The reduction reactions of Cu(OH)₂ and Cu(O, S) were discussed based on thermodynamics.

Oxide nanowall arrays having aligned growth orientation and aligned wall crystal planes are a challenge to form on substrates without the use of templates. Here, oriented magnesium oxide (MgO) nanowall (NW) arrays with a [110] growth direction and (111) wall surfaces are prepared using a template-free spontaneous route: electrodeposition of magnesium hydroxide (Mg(OH)₂) and its topotactic-like pseudomorphic transformation (TPT). Oriented Mg(OH)₂ NWs were prepared on a conductive substrate using simple electrochemical deposition from a Mg(NO₃)₂ aqueous solution, and were characterized using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared (FTIR) spectroscopy and Raman spectroscopy. The obtained Mg(OH)₂ is a vertical nanowall array structure (∼660 nm height and 20–40 nm thickness) intersecting each other, with a [110] growth orientation and a (0001) wall surface. The size of NWs can be controlled by electrodeposition time and deposition current density. Heat treatment of the [110]-Mg(OH)₂(0001) NW arrays at 500 °C and 800 °C in air led to the formation of [110]-MgO(111) NW arrays via the TPT process. The chemical state analysis of NW surfaces using X-ray photoelectron microscopy and FTIR spectroscopy revealed that MgO NWs heated at 500 °C exhibit a superior CO₂ adsorption property compared to the Mg(OH)₂ NWs and MgO NWs heated at 800 °C.

Scintillators, which convert high-energy rays such as X-rays and positrons into visible and ultraviolet light, are incorporated as radiation detectors in X-ray computer tomography (X-ray CT) and positron emission tomography (PET), which system performances highly depends on the characteristics of the scintillator used. In this study, SrGa₂S₄:Eu/ZnO vertical nanowires (ZnO-vnws) bilayer scintillators were fabricated by electrodeposition of ZnO-vnws followed by electrophoresis deposition of SrGa₂S₄:Eu particles, and their structure, optical properties, and scintillator characteristics were evaluated. The ZnO-vnws were composed of vertically aligned ZnO nanowires with an excellent (001) out-of-plane orientation, and the SrGa₂S₄:Eu layer was composed of particles adhered to the ZnO-vnws. The SrGa₂S₄:Eu/ZnO-vnws bilayer scintillators emitted visible light at a wavelength of 533 nm with a lifetime of 0.389 μs and functioned as a scintillator for X-ray irradiation. The resolution and luminescence intensity of the SrGa₂S₄:Eu/ZnO-vnws bilayer scintillators were improved by the introduction of the ZnO-vnws layer, when compared with those of the ZnO-vnws-free-SrGa₂S₄:Eu layer, when the thickness ratio of SrGa₂S₄:Eu layer to ZnO-nws length were 1.1 ∼1.3. The SrGa₂S₄:Eu/ZnO-vnws bilayer scintillator revealed a spatial resolution of 6.8 μm, which was slightly larger than 5.4 μm of a ready-made LuAG single-crystal scintillator.