Shintaro Ueno

Fabrication of Porous Cubic Architecture of ZnO Using Zn-terephthalate MOFs with Characteristic Microstructures

A method for synthesizing porous cubic-shaped ZnO particles a few tens of micrometers in size is described on the basis of a pyrolytic conversion of Zn-terephthalate metal-organic frameworks (MOFs). MOF crystals were initially grown in solutions containing Zn(NO3)2·6H2O and terephthalic acid as solutes and N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) as a solvent under a solvothermal condition. It was the key to controlling the microstructure of MOF cuboids for their use as an intermediate compound for ZnO. Actually, many cracks were formed and hence the cubic microstructure was somewhat destroyed in the pyrolytic conversion from dense MOF crystals (grown in the DMF solution) to ZnO. In contrast, mesocrystal-like MOF cuboids (grown in the DEF solution) could maintain their shape during the pyrolysis because of the relaxation against a MOF-to-ZnO volume change. The resultant ZnO with a highly porous cubic structure showed intense visible photoluminescence upon irradiation with ultraviolet light.

Electric field induced lattice strain in pseudocubic Bi(Mg1/2Ti1/2)O3-modified BaTiO3-BiFeO3 piezoelectric ceramics

Contributions to the piezoelectric response in pseudocubic 0.3BaTiO3-0.1Bi(Mg1/2Ti1/2)O3-0.6BiFeO3 ceramics were investigated by synchrotron X-ray diffraction under electric fields. All of the lattice strain determined from the 110, 111, and 200 pseudocubic diffraction peaks showed similar lattice strain hysteresis that was comparable to the bulk butterfly-like strain curve. It was suggested that the hysteresis of the lattice strain and the lack of anisotropy were related to the complex domain structure and the phase boundary composition.

Revealing the role of heat treatment in enhancement of electrical properties of lead-free piezoelectric ceramics

The structural and electrical properties of the virgin, annealed, and quenched samples of 0.80BiFeO3–0.20BaTiO3 lead-free ceramics were investigated. The crystal structures investigated with the structure refinement method exhibited a rhombohedral structure for all the samples. The significant enhancement of ferroelectric and piezoelectric properties is ascribed to the relaxation of the lattice strain induced in the samples after the heat treatment. The bond-length calculated from the final structure refinement results revealed the increment of the A-O bond-length after the heat treatment, while the B-O bond-length was maintained in all the virgin, annealed, and quenched samples suggesting the specific influence of the lattice strain at the A-site of the material system.

Influence of Sintering Behavior of ZnO Nanoparticles on J–V Characteristics of ZnO-Based Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are one of the promising photovoltaic devices because of their lower manufacturing cost and higher energy conversion efficiency. Wide-gap, porous semiconducting metal oxides are generally used as electrode materials of DSSCs. Previously we utilized ZnO as DSSC electrodes and achieved a high conversion efficiency of 6.58% by improving the structure of ZnO films. However, open-circuit voltage (VOC), one of the factors to determine the performance of DSSCs, was still at a lower level (≈ 0.60 V) than that of common TiO2-based cells. We believe that the lower VOC is due mainly to the occurrence of recombination. In this work, we tried to enhance VOC by controlling the heating process of electrodes and suppressing the recombination for further development of ZnO-based DSSCs. As a result, we have achieved a higher VOC of 0.725 V, confirming that the sintering behavior (grain growth and/or necking of grains) influenced largely the characteristics of DSSCs.

Structural and electrical characteristics of potential candidate lead-free BiFeO3-BaTiO3 piezoelectric ceramics

The crystal structures and electrical properties of lead-free BiFeO3-BaTiO3 [(1-x)BFxBT] piezoelectric system are investigated as a function of BaTiO3 concentration. The well-saturated P-E hysteresis loop was observed in the 0.80BiFeO3–0.20BaTiO3 composition system, while a less hysteretic strain-electric field curve was exhibited by the 0.70BiFeO3–0.30BaTiO3 with a strain hysteresis of 16%, the value comparable to PZT-based piezoelectric ceramics. The crystal structures investigated under the synchrotron radiation X-ray diffraction exhibited a rhombohedral structure for BFBT system with x = 0.10–0.25 and a pseudo-cubic structure for BFBT system with x = 0.30–0.40. The structural phase diagram for the BiFeO3-BaTiO3 system is suggested based on the results of temperature-dependent synchrotron radiation X-ray diffraction measurement and investigated electrical properties.

Size-controlled synthesis of ZIF-8 particles and their pyrolytic conversion into ZnO aggregates as photoanode materials of dye-sensitized solar cells

We have proposed a fabrication process of submicrometer-sized zinc oxide aggregates with nano/microstructures through pyrolysis of ZIF-8 particles. Methanol solutions of zinc nitrate hexahydrate and 2-methylimidazole were used for synthesizing ZIF-8 with its characteristic rhombic dodecahedron morphology. The systematic addition of 1-methylimidazole to the above methanol solutions led to the formation of size-tunable ZIF-8 particles. Precise control of heating conditions was a key to successful conversion from ZIF-8 to ZnO without destroying the morphology. The ZnO aggregates thus obtained were evaluated as photoanode materials of dye-sensitized solar cells. A photoanode using a 9 μm thick ZnO aggregate film gave a power conversion efficiency of 3.37% due to its nano/microstructures being suitable for dye adsorption, electrolyte diffusion, and light harvesting.

Fabrication of Nanostructured Zinc Oxide Films on Plastic Substrates by Pyrolysis Method and their Application to Dye-Sensitized Solar Cells

t has been attempted to utilize plastic substrates for electrodes, instead of conventional glass substrates, to fabricate flexible, lightweight, and low-cost dye-sensitized solar cells (DSSCs). We examined a pyrolysis method by which preformed layered hydroxide zinc acetate (LHZA: Zn5(OH)8(CH3COO)2·2H2O) films were converted into ZnO films at temperatures below 120 oC. Since the pyrolyzed films still contained zinc acetate compounds, they were immersed in hot water aiming at removing such the impurities. This hot water treatment influenced the film morphology to a large extent. The energy conversion efficiency of 3.41% was obtained for the cell using an N719/ZnO electrode fabricated on an indium tin oxide-coated polyethylene naphthalate (ITO-PEN) substrate through the present pyrolysis method.

Fabrication of layered hydroxide zinc nitrate films and their conversion to ZnO nanosheet assemblies for use in dye-sensitized solar cells

Abstract Layered hydroxide zinc nitrate (LHZN; Zn5(NO3)2(OH)8·xH2O) films were fabricated on glass or plastic substrates by a chemical bath deposition method combined with a homogeneous precipitation in methanolic solutions. High- or low-temperature pyrolytic decomposition of the LHZN films having two-dimensional morphology was attempted to obtain porous ZnO nanosheet-assembly films. The LHZN films were converted into porous ZnO films by pyrolyzing at temperatures above 400 °C, while porous LHZN/ZnO hybrid films were obtained by pyrolyzing at a lower temperature of 120 °C without morphological changes. The pyrolyzed ZnO films were applied to dye-sensitized solar cells (DSSCs), resulting in the generation of relatively high open-circuit voltages. The low-temperature pyrolysis enabled us to fabricate the LHZN/ZnO film even on the plastic substrate. Actually a cell using the LHZN/ZnO film on an indium tin oxide-coated polyethylene naphthalate substrate showed an energy conversion efficiency of 2.08% with a high open-circuit voltage around 0.70 V.

Effective Sol-Gel Nanocoatings on ZnO Electrodes for Suppressing Recombination in Dye-Sensitized Solar Cells

Attempts have been made to improve the performance of dye-sensitized solar cells by forming metal-oxide nanocoating layers on ZnO electrodes by a sol-gel transformation. SiO2, Nb2O5, TiO2, or ZrO2 nanocoating layers could be formed by dipping ZnO films into metal alkoxide solutions of low concentrations and subsequent heat treatments. The performance of DSSCs using the coated ZnO electrodes depends strongly on the structure of coating layers such as the thickness and the surface coverage, which are significantly influenced by the coating conditions. In particular, SiO2 and Nb2O5 coating layers are effective to suppress the recombination by constructing the energy barrier at the ZnO/electrolyte interface and enhance energy conversion efficiency. It is found that the coating layers also hinder the grain growth of ZnO, contributing to the enhanced cell performance as well.

Role of structure gradient region on dielectric properties in Ba(Zr,Ti)O3–KNbO3 nanocomposite ceramics

Crystal structures of KNbO3 (KN)/BaZrxTi1−xO3 [BZT, (0.1 ≤ x ≤ 1)] nanocomposite ceramics have been investigated using synchrotron radiation X-ray powder diffraction, where BZT nanoparticles thinly coated with KN crystals through the heteroepitaxial interface are sintered. The Rietveld analysis based on the multicomponent model reveals that the ceramic grain has the core/multishell structure consisting of a BZT core, distorted BZT and KN multishells. The variations of the volume ratio of the distorted BZT shell region corresponding to the structure gradient interface region and the dielectric property of the ceramics show similar trends as a function of x. From these results, we propose that the structure gradient region is electrically soft, and provides a crucial contribution to the dielectric properties of the nanocomposite ceramics.