Published papers

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. We report herein one of our recent studies on nanostructured ZnO electrodes for application in dye-sensitized solar cells, focusing on achieving a higher open-circuit voltage (VOC). ZnO films were obtained through solution-processed routes including pyrolytic conversion of layered hydroxide zinc acetate (LHZA) films deposited on a fluorine-doped tin oxide-coated conducting glass substrate by a chemical bath deposition method. The morphology of the initial LHZA and the converted ZnO films was tuned from a thick (approximately 12 μm) flower bed-/lawn-like bilayer structure to a thin (1.2 μm) lawn-like quasi-monolayer structure by decreasing the Zn source concentration in the chemical bath. VOC was found to be enhanced with this morphological change from 0.692 (the bilayer structure) to 0.735 V (the quasi-monolayer structure). Fine tuning of the quasi-monolayer structure by introducing the grain growth effect led to VOC of the cell as high as 0.807 V, although a short-circuit photocurrent density (JSC) remained low. Further attempts were then made to increase JSC while maintaining the high VOC. When the thickness of the lawn-like monolayer film was increased up to approximately 5 μm, the resultant cell showed VOC = 0.750 V, JSC = 6.20 mA cm−2 and a conversion efficiency (η) of 2.83%. The film with a modified flower bed-/lawn-like bilayer structure approximately 11 μm in thickness finally yielded VOC = 0.741 V, JSC = 13.6 mA cm−2, and η = 5.44%.

© 2018 Elsevier Ltd and Techna Group S.r.l. A brucite-type hydroxide of CazCo1−z(OH)2 with a high Ca content was synthesized and utilized as a single-source precursor for the fabrication of bulk ceramics of a semiconducting layered calcium cobaltite CayCoO2. Plate-like CazCo1−z(OH)2 particles with z up to 0.22 were reproducibly obtained by a reverse co-precipitation method using KOH as an alkaline source. Uniaxial pressing of the plate-like CazCo1−z(OH)2 particles (z = 0.22), which was calcined at 373 K beforehand, at a pressure as high as 500 MPa produced a highly dense green pellet where the plate-like particles were stacked along the pressing direction. The green pellet was then converted by a heat treatment at 923 K into an oxide ceramic of preferentially (010)-oriented CayCoO2 via a topotactic-like pyrolytic reaction maintaining edge-sharing CoO6 octahedra. Although the ceramic also contained a minor insulating Co3O4 phase (16.7 ± 0.7 mol% estimated from thermal analysis), the electrical measurements proved its p-type semiconducting behavior resulting from the CayCoO2 phase with enhanced Seebeck coefficient exceeding 160 μV/K at room temperature.

© 2019 Elsevier B.V. Hollow and aggregated microspheres of an Sm 3+ -doped CeO 2 phosphor material were synthesized by liquid processes using organic materials as a template and a structure-directing agent, respectively The microstructure of both the microspheres obtained were characterized as their crystallite size and specific surface area because they were comprised of CeO 2 :Sm 3+ nanocrystals approximately 20 nm in size. The microspheres showed orange luminescence due to electronic transitions of the Sm 3+ ions upon excitation with ultraviolet light. The luminescence intensity was decreased when the microspheres were treated with an aqueous L(+)-ascorbic acid solution as a reductant. The intensity was then recovered by treating the microspheres with an H 2 O 2 solution as an oxidant. Such the redox-responsive luminescence switching is promising for sensing applications if the sensitivity can be controlled well. The relationship between the structural features of the microspheres and the luminescence switching property was then examined in detail.

Pt nanoparticles were loaded on Y2WO6:Eu3+ phosphor microspheres to enhance hydrogen sensitivity at low temperatures through turn-off luminescence. Mesoporous Y2WO6:Eu3+ microspheres were synthesized first by a hydrothermal method. Pt loading on the surface of the Y2WO6:Eu3+ microspheres was then carried out by a method of the self-regulated reduction of surfactants using aqueous K2PtCl4 solutions. Structural and morphological properties of unloaded and Pt-loaded Y2WO6:Eu3+ microspheres were investigated by various techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The relative photoluminescence intensity of the Pt-loaded Y2WO6:Eu3+ microspheres, due to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) electronic transitions of doped Eu3+ ions, was found to decrease as low as 22% after the microspheres were kept in the hydrogen gas atmosphere at a low temperature of 150 °C for 15 min. Meanwhile, mechanisms underlying this turn-off luminescence of the Pt/Y2WO6:Eu3+ microspheres in response to hydrogen gas are illustrated in detail.

© 2018 The Ceramic Society of Japan. All rights reserved. A facile synthesis method of delafossite-type CuYO2 using a layered yttrium hydroxide (LYH) was developed. LYH microplates and Cu2O nanoparticles were both prepared from aqueous solutions and used as raw materials for the solid-state reaction to form CuYO2. It was found that CuYO2 could be obtained by heating LYHCu2O mixed powder at 900°C for 4 h under a N2 atmosphere. Although the product contained a slight amount of Y2O3, the heating time used in the proposed method was much shorter than that required for the conventional synthesis method via the Cu2Y2O5 precursor phase. Analysis of the X-ray diffraction pattern showed that the CuYO2 powder synthesized using LYH contained the 3R-type polymorph as the major content, whereas a larger content of the 2H-type polymorph was included in the powder synthesized by the conventional method. Based on these results, a topotactic reaction maintaining the layered structure of LYH was proposed as the formation mechanism for CuYO2 in LYHCu2O mixed powder.

The relaxor state of (Bi1/2K1/2)TiO3 exhibits promising dielectric properties for high-temperature capacitor applications, but the spontaneous phase transition into the low-temperature ferroelectric state and the excessively high dielectric maximum temperature (Tm) at around 360 °C are the main drawback to this material. In this study we examined solid solutions of (Bi1/2K1/2)TiO3 with SrTiO3 to improve the temperature stability of the dielectric properties. As precursors to fabricate the sold-solution ceramics, fine powders of (Bi1/2K1/2)TiO3 and SrTiO3 were both synthesized by the hydrothermal method. Dense (1 − x)(Bi1/2K1/2)TiO3–xSrTiO3 ceramics with x up to 0.5 were then obtained by reaction sintering of the powders. A crystal structure analysis revealed that the average symmetry of the solid-solution ceramics changes from tetragonal to cubic with increasing the SrTiO3 content. Dielectric measurements showed that the incorporation of SrTiO3 into (Bi1/2K1/2)TiO3 stabilizes the relaxor state to shift Tm largely toward lower temperatures. As a result, the sample with x = 0.5 exhibited a temperature-stable dielectric permittivity of 1700 ± 15% over a wide temperature range from room temperature up to 260 °C. The electric-field and temperature dependences of the energy-storage properties of the sample were also investigated.

Microstructured/nanostructured YVO4:Eu3+ powders and films were synthesized through a biphasic sol-gel method, aiming at their application as H2O2 sensing materials based on the turn-off luminescence of Eu3+ ions. The synthesis was typically carried out at temperatures of 80 °C or lower by using organic solutions to dissolve vanadium alkoxide and aqueous solutions to dissolve yttrium and europium salts together with sodium carboxylates. The resultant crystalline YVO4:Eu3+ powders and films were characterized as containing micrometer-sized particles comprising primary nanoparticles with high specific surface areas. A comparative study was performed on the H2O2-responsive turn-off luminescence properties for the above samples and those synthesized by a single-phase sol-gel or a conventional solid-state reaction method. The results indicated that the microstructural feature of the samples from the biphasic sol-gel method was effective for detecting H2O2 through its adsorption on the particle surface and quenching of the Eu3+ luminescence. The film samples showed repeatable and quantitative turn-off luminescence, thereby demonstrating their suitability as solid-state H2O2 sensors.

Erythrosine B (EB) dye molecules were embedded in a transparent ZnO semiconductor film, aiming at fabricating an organic/inorganic chromic material. A red-coloured, 3.4 μm thick ZnO film was obtained through a chemical bath deposition method conducted at a low temperature of 90 °C. The EB dye was confirmed to be distributed uniformly inside the ZnO film. Spectroelectrochemical measurement was performed using a three-electrode system with the EB/ZnO film employed as the working electrode. The red colour of the film started to disappear at an applied potential of -0.9 V in a negative scan of cyclic voltammetry. The decolourized film slowly became coloured again at +1.9 V and above in a positive scan. The electrochemically decolourized film was also coloured under fluorescent lighting without applying the potential. Because the EB dye dissolved in water did not show such a reversible colour change, the observed electro/photochromism is attributed to the EB/ZnO hybrid structure where the organic molecules are electronically functionalized by the metal oxide semiconductor.

We report a chemical solution deposition method for fabricating magnetoelectric ZnO–La2CoMnO6 (LCMO) nanocomposite films on platinum coated silicon (Pt/Si) substrates. A single precursor coating solution containing all constituent cations (Zn2+, La3+, Co2+, and Mn2+) was prepared by dissolving acetate and nitrate raw materials in 2-metoxyethanol in the presence of monoethanolamine. Investigations of decomposition and crystallization behaviors of a dried powder prepared from the ZnO–LCMO precursor solution revealed that a crystalline phase of ZnO first appeared at a low temperature below 400 °C followed by crystallization of LCMO at a much higher temperature. A film coated on a ZnO-seeded Pt/Si substrate was found to have a characteristic vertical nanocomposite structure composed of c-axis oriented ZnO columns and LCMO nanograins. The ZnO–LCMO film exhibited ferromagnetic properties originating from the LCMO phase. A converse magnetoelectric measurement at a temperature of 10 K demonstrated that the magnetization of the film at 45 kOe was decreased by 9% by application of an external electric field of 100 kV/cm.

Submicrometer-sized CeO2crystals were synthesized by a hydrothermal reaction method, aiming at their use as light-scattering layers in dye-sensitized solar cells (DSSCs). The morphology of the crystals was varied among a round-cornered cubic, a distinct cubic, or an octahedral shape by changing the synthetic conditions. The film formability and the light-scattering ability of the CeO2crystals, which were coated on glass substrates by a squeegee method, were found to be dependent on their morphology according to the microstructural observation and the diffuse reflectance measurement. The effects of the CeO2light-scattering layer were then examined for DSSCs using nanoparticulate ZnO electrodes with different thicknesses and sensitizer dyes, namely N-719 and N-749, with different absorption spectra. A short-circuit photocurrent density could be enhanced, for example, from 11.20 to 14.15 mA cm−2, by stacking the CeO2layer on the ZnO electrode. The effectiveness of the light-scattering layer in the CeO2/ZnO electrodes was discussed based on the CeO2crystal morphology and the CeO2film microstructure after the systematic investigation.

The irreversible motion of non-180° ferroelectric domain walls is a predominant cause for the increased hysteretic loss in piezoelectric ceramics at elevated temperatures. In this study we designed piezoelectric ceramics modified with acceptor ions localized near grain boundaries for the purpose of suppressing the irreversible domain wall motion at elevated temperatures without diminishing the contribution from the reversible domain wall motion. Particles of undoped BaTiO3 (BT) were coated with Mn-doped BT by a coprecipitation method and then sintered to form a Mn-modified BT ceramic. Structural and dielectric characterizations indicated that the Mn ions in the resulting ceramic were localized near grain boundaries. The measurement of the unipolar electric-field-induced strain was conducted for the Mn-modified and unmodified (pure) BT ceramics at temperatures up to 120 °C (just below the Currie temperature) to discuss the hysteretic behavior in their piezoelectric response. The results showed that the effective piezoelectric coefficient d33* of the Mn-modified BT ceramic was comparable to the unmodified BT ceramic. The hysteresis of the Mn-modified BT ceramic was retained below 40% in the whole temperature range whereas that of the unmodified BT ceramic increased significantly at elevated temperatures over 100 °C. The results in this study demonstrate that the localized Mn ions effectively pin the domain walls to suppress their irreversible motion at elevated temperatures, while the reversible motion inside the grains remains possible.

YVO4:Eu3+ phosphor particles are proposed as an H2O2 sensing material based on their turn-off luminescence. The YVO4:Eu3+ particles having various surface microstructures were synthesized through liquid processes and their H2O2 sensing ability was evaluated by immersing them in an aqueous H2O2 solution with a constant volume at a fixed concentration. The microstructures were characterized in detail not only by electron microscopy but also by nitrogen adsorption to elucidate factors governing the H2O2 sensing ability. It was then indicated that micro-/mesoporous structures, which were introduced in the micrometer-scaled spherical particles during their synthesis by a biphasic solgel method, were a decisive factor for achieving the effective H2O2 sensing. The YVO4:Eu3+ particles synthesized under the improved condition could show a 37% decrease of luminescence intensity after the H2O2 treatment.

Piezoelectric oxide-polymer composite generators have attracted increasing attention for sensoring and energy-harvesting applications. In this work, the effect of the size and morphology of BiFeO3(BFO) particles on the performance of BFO-poly(dimethylsiloxane) (PDMS) composite generators is revealed. Three kinds of BFO particles—nanoparticles, microspheres, and microplates—were synthesizedviasolution synthesis routes and applied to BFO-PDMS composite films. The BFO microplates were found to be preferentially oriented along the pseudocubic <100> direction in the composite film due to their anisotropic morphology, while the nanoparticles and microspheres were not oriented. The composite film fabricated from the BFO microplates generated a higher open-circuit voltage in response to an applied compressive force compared to the other films. The effect of the particle size and morphology on the open-circuit voltage was discussed in terms of the dispersion and polarized states of the particles.

Two kinds of trivalent lanthanide ions (Ln3+) were substitutionally doped in the cubic perovskite-type BaSnO3 and their effects on optical and electrical properties were investigated. La3+ was fixed as the first dopant because of its electron-donor nature, while the second dopant was selected from luminescent Ln3+ ions, namely, Pr3+, Sm3+, Eu3+, or Tb3+. In preliminary screening, Sm3+ was found to exhibit strong photoluminescence (PL) through an efficient O2––Sm3+ charge transfer excitation. Then, powders and sintered pellets of Ba1–x–yLaxSmySnO3 compositions (x = 0–0.04 and y = 0.001–0.06) were prepared through a polymerized complex method and their PL intensity and electrical resistivity were measured and compared among the samples. The PL intensity of the powder samples increased with the increasing Sm3+ content y at any La3+ content x. However, at the fixed Sm3+ content y, the PL intensity decreased with the increasing La3+ content x. The electrical resistivity of the pellet samples decreased with the increasing La3+ content x, while it increased considerably with the increasing Sm3+ content y. The dependence of the above physical properties on the La3+ and Sm3+ contents was discussed in association with the formal valence change of Sn4+ after the dual La3+/Sm3+ doping in BaSnO3.

Dense and porous thin films of luminescent Ln3+-doped HfO2 materials (Ln3+ = Eu3+ and Tb3+) were fabricated on silica glass substrates by a sol-gel dip-coating method using different kinds of coating solutions. The refractive index of the HfO2:Ln3+ thin films was controlled by changing their porosity based on the Lorentz-Lorenz relationship. Pores could be introduced systematically into the thin films with the addition of different amounts of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer to the coating solutions and the subsequent heat-treatment at 700°C. While photoluminescence (PL) of the dense HfO2:Ln3+ thin films with higher refractive index was inefficient mainly due to the reflection problem, the porous HfO2:Ln3+ thin films with lower refractive index exhibited much improved PL intensity by suppressing total internal reflection of the emitted light. The existence of the pores was also beneficial to an increase in the absorption of the excitation light due to its multiple scattering inside the thin films. Our results then demonstrate that the porous luminescent thin films are suitable for applications where the transparency and the strong surface emission are both necessary.

Bismuth silicate Bi2SiO5 (BSO) is known as an interesting new type of ferroelectric oxide; however, bulk ceramics of BSO have never been obtained because it easily decomposes at high temperature. In this study, we fabricated BSO ceramics for the first time using a sol–gel technique and investigated their potential as a dielectric material for high-temperature capacitor applications. A precursor powder was prepared by a sol–gel synthesis route using tetraethyl orthosilicate and bismuth nitrate pentahydrate as raw materials and then sintered under a small uniaxial pressure of 5 MPa with an addition of CH3COOLi as a sintering aid. A BSO ceramic with the highest relative density of 88.5% was obtained at a very low sintering temperature of 620 °C with preventing significant thermal decomposition. A crystal structure analysis revealed that the obtained BSO ceramic belongs to the ferroelectric monoclinic phase. The dielectric permittivity of the ceramic showed a superior temperature stability (± 5%) at temperatures up to 200 °C, followed by a sharp peak attributed to the ferroelectric–paraelectric phase transition at around 390 °C. The BSO ceramic also showed slim and pinched polarization hysteresis loops due to the random grain orientation and domain wall clamping, leading to a high energy storage efficiency over 75% at temperatures up to 100 °C.

In this study, we fabricated Cu2O films on bare glass substrates by a chemical bath deposition technique, which consists of a seeding process and a subsequent film-growth process. Dense (1 1 1)-oriented Cu2O films were formed in an aqueous solution of copper (II) sulfate containing L-ascorbate and citrate ions at a low temperature of 80 °C. The film thickness and (1 1 1)-orientation degree (Lotgering factor) were both monotonically increased by simply repeating the film-growth process. As a result, a Cu2O film with a high (1 1 1)-orientation degree of 0.83 and a large thickness 1440 nm was obtained. Detailed chemical and structural characterizations suggested that as-deposited Cu2O films contained citrate ions and metallic Cu as impurities. We also found that a heat treatment at 500 °C under nitrogen flow was effective to remove these impurities.

Ca3Co4O9 ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca3Co4O9 ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)2 as a template and CaCO3 as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m−1 K−2.

Meso- and macro-porous Y2WO6:Eu3+ phosphor thin films were fabricated on silica glass substrates by a Pechini-type sol–gel dip-coating method. Precursor solutions for coating were prepared by mixing aqueous solutions of metal salts, ethylene glycol, and citric acid with water–alcohol solutions of a nonionic triblock copolymer (P-123) employed as a structure-directing agent. The kind of the alcohol solvents (methanol, ethanol, or isopropanol) and the amount of ethylene glycol were systematically examined because they were found to be major factors that could influence the shape and size of pores formed in the resulting thin films. The transmittance for the visible light was decreased in the thin films having macropores due to the light scattering. The red photoluminescence of the Eu3+ ions could be enhanced in these macro-porous thin films, especially from their surface, by a synergetic effect of increased multiple scattering and suppressed internal reflection of the emission light. The relationship between the microstructure and the optical properties of the porous Y2WO6:Eu3+ thin films was then discussed based on the experimental results. [Figure not available: see fulltext.]

Mesostructured Y2O3:Eu3+ phosphor particles were fabricated by a pyrolytic conversion from a pertinent metal–organic framework (MOF), namely, MOF-76(Y):Eu with 1,3,5-benzenetricarboxylic acid ligands, aiming at luminescence sensing of H2O2. Large rod-like MOF-76(Y):Eu particles 301 ± 150 μm in average length were initially obtained by a reaction at 80 °C for 24 h. They were converted to rod-like Y2O3:Eu3+ particles 122 ± 34 μm in average length after heating at 500 °C. The Y2O3:Eu3+ particles actually consisted of primary nanoparticles 7.55 ± 1.43 nm in average size and had a mesoporous structure with a high specific surface area of 80.8 m2 g−1. The photoluminescence intensity of the Y2O3:Eu3+ particles decreased after treating with an aqueous H2O2 solution, which was recognized as H2O2-responsive turn-off luminescence. Three other kinds of Y2O3:Eu3+ particles, including those obtained from other MOF particles synthesized at room temperature, were also prepared and the effects of the microstructure on the degree of turn-off luminescence were examined. It was then observed that the H2O2-sensitivity was enhanced more effectively by using the MOF-derived Y2O3:Eu3+ particles having the higher specific surface area.