Published papers

NiCo2O4 is promising as one of the p-type oxide semiconductors for practical applications in optoelectronic devices. In this study, we have elaborated NiCo2O4 hexagonal nanoplatelet films and examined their photovoltaic properties. The film fabrication was actually achieved by four steps: the synthesis of hexagonal nanoplatelets of a Ni1/3Co2/3(OH)2 precursor, the dispersion of the nanoplatelets in liquid media, the self-assembly of the nanoplatelets on dipped substrates, and the topological conversion of the hydroxide to the oxide by the heat treatment. The resultant NiCo2O4 thin films had structural features such as the crystallographic (111) orientation, the high specific surface area, and the high light-scattering ability. As a result, the NiCo2O4 thin films loaded with an N749 dye could work effectively as photocathodes in the configuration of dye-sensitized solar cells.

We report the effect of Zr substitution for Ti on the ferroelectric and piezoelectric properties of lead-free (Bi1/2K1/2)TiO3 (BKT) ceramics. Dense ceramics of (Bi1/2K1/2)(ZrxTi1−x)O3 with x up to 0.06 were fabricated by sintering of hydrothermally derived fine powders. The investigation of the crystal structure and dielectric properties revealed that the Zr substitution decreased the tetragonal lattice distortion of BKT and stabilized the relaxor state. The coercive electric field was found to be decreased monotonically with increasing the Zr content, while the largest remanent polarization of 26 μC cm−2 was observed at x = 0.04. As a result, the ceramic with x = 0.04 showed an enhanced piezoelectric d33 constant of 140 pC N−1.

Undoped and Ln3+-doped (Ln3+ = Eu3+, Tb3+) CaWO4 phosphor particles were synthesized by a hydrothermal method, aiming at generating fluorochromic properties in response to redox reactions. The phosphor particles obtained were first treated with an aqueous L(+)-ascorbic acid solution as a reductant at room temperature, which resulted in a considerable decrease in photoluminescence (PL) intensity of both CaWO4 (the host-lattice blue emission) and CaWO4:Ln3+ (the 4f-4f red or green emission). The quenched PL could then be recovered by treating the particles with a sodium hypochlorite solution as an oxidant also at room temperature. This PL switching behavior was predominantly caused by redox reactions between the solid particles and the redox species in the solutions based on a change of W6+/W5+ valance states of the CaWO4 host crystal. More interestingly, CaWO4:Eu3+ exhibited a change of emission colors, depending on the excitation wavelength, due to different sensitivity of the host emission and the Eu3+ emission to the redox treatment. Such a changeable fluorochromism is regarded as a newly developed function of the phosphor materials for use in facile optical detectors and sensors.

The influence of grain size on the dielectric properties of Sr0.6Ba0.4Nb2O6 (SBN60) ceramics with a tetragonal tungsten bronze structure was examined. The conventional and two-step sintering procedures were used to fabricate dense and phase-pure SBN60 ceramics with grain sizes ranging from 2.2 μm to 17.3 μm. In the temperature dependence of the dielectric permittivity, all samples showed a relaxor-like broad dielectric peak with a maximum at around 60 °C. The maximum dielectric permittivity and dielectric maximum temperature of the SBN ceramics were found to be less dependent on the grain size. The reason for the grain-size-insensitive dielectric properties of the SBN60 ceramics is discussed based on the analysis of the dielectric properties and Raman spectra.

Ferroelectric films fabricated on metal substrates are desirable for use in piezoelectric vibration energy harvesters. In this report, we demonstrate that dense films of lead-free ferroelectric (Bi1/2K1/2)TiO3 (BKT) with preferred c-axis orientation can be grown on nickel-based alloy substrates by utilizing the hydrothermal synthesis method. A buffer layer of LaNiO3 (LN) was formed on the substrates by the sol-gel spin coating method, and then BKT was grown by a hydrothermal reaction at a low temperature of 150 °C. The surface structure and thickness of the final BKT films were found to strongly depend on the numbers of the deposition cycles for LN and BKT, respectively. As a result, highly c-axis oriented BKT films with a smooth surface and a thickness up to 830 nm were obtained. Although the as-deposited BKT films were suggested to contain hydroxyl groups in their crystal lattice, a post-annealing process at 650 °C in air was found to be effective to remove them. The polarization response of the annealed BKT film proved the ferroelectric polarization switching in the film.

Monodispersed CePO4:Tb3+ microspheres, which were formed by aggregation of primary nanoparticles, were synthesized by a hydrothermal reaction method assisted with ethylenediaminetetraacetic acid. The microspheres were found to have a hierarchically porous structure as well as a high specific surface area according to structural and chemical analyses. The microspheres exhibited green photoluminescence (PL) due to 4f–4f electronic transitions of Tb3+ ions doped in the CePO4 crystal upon irradiation with ultraviolet light. The microspheres were then treated with an oxidant (potassium permanganate) and a reductant (l(+)-ascorbic acid) in aqueous solutions at room temperature to examine their effect on the PL property. The PL intensity of the microspheres was once decreased by the oxidation and then increased by the reduction. Such the change in the PL intensity was also shown to be quantitative against the concentration of the oxidant and the reductant. Thus the monodispersed CePO4:Tb3+ porous microspheres were demonstrated to be a facile luminescence-sensing material.

The combined effect of chemical doping and grain orientation on the thermoelectric properties of delafossite-type CuFeO2 ceramics was investigated. Ni was chosen as the dopant, whereas the reactive templated grain growth method was used for the fabrication of grain-oriented bulk ceramics. Nanorods of Ni-doped α-FeOOH with a nominal Ni content of up to 5 mol% were synthesized by the hydrothermal method and were then used as the template to fabricate Ni-doped CuFeO2 ceramics with a preferred c-axis orientation. The electrical conductivity, Seebeck coefficient, and thermal conductivity of the Ni-doped CuFeO2 ceramics along the in-plane direction (perpendicular to the orientation axis) were measured at temperatures between 300 and 1100 K. Ni doping was found to effectively increase the electrical conductivity and decrease the thermal conductivity, whereas it maintained a high Seebeck coefficient of around 400 μV K−1. Consequently, the c-axis-oriented CuFeO2 ceramics doped with 1 and 3 mol% Ni showed an enhanced dimensionless figure of merit of 0.08 at 900 K.

The delafossite-type CuFeO2 (CFO) is a promising p-type thermoelectric oxide reported to show a large Seebeck coefficient, but the lack of a texturing method for CFO ceramics limits their thermoelectric performance. Here, we show that textured CFO ceramics with an enhanced thermoelectric power factor can be obtained by a facile process based on the reactive templated grain growth (RTGG) method. Nanorods of α-FeOOH were synthesized by a hydrothermal reaction and used as templates for the RTGG process. Textured CFO ceramics with a high c-axis orientation were obtained by ordinal uniaxial pressing of a mixed powder of the α-FeOOH nanorods and Cu2O particles and subsequent reaction sintering process. The best CFO ceramic with the highest orientation degree (Lotgering factor) of 0.71 was obtained at a sintering temperature of 1020 °C. It was found that textured CFO ceramics were formed through a topotactic conversion process via an intermediate α-Fe2O3 phase. The textured CFO ceramic showed a high in-plane electrical conductivity, leading to an enhanced thermoelectric power factor of 1.4 × 10-4 W m-1 K-2 at 700 K, which is the highest value ever reported for undoped CFO ceramics.

Red-emitting CaTiO3:Pr3+ phosphor nanoparticles were synthesized by a sol–gel method with a final heat treatment at a low temperature of 650 °C. For comparison, the CaTiO3:Pr3+ phosphor was also synthesized by a conventional solid-state reaction method conducted at 1250 °C. The two kinds of resultant CaTiO3:Pr3+ samples were revealed to differ largely in their microstructure and optical properties. Diffuse reflectance and photoluminescence measurements suggested that the sample from the solid-state reaction possessed a larger number of defects, which would deteriorate the optical properties, due to heating at high temperature. On the other hand, the sol–gel-derived sample exhibited much better optical properties and thus it was used for evaluating optical-switching phenomena upon redox treatments. In photoluminescence, three excitation bands were observed at 265, 335, and 375 nm for the red emission of the CaTiO3:Pr3+ sample, and one of them (375 nm) was more effective for inducing luminescence quenching by a reduction treatment at room temperature. A body color of the CaTiO3:Pr3+ sample was also changed from white to light yellow by the reduction due to the enhanced visible-light absorption. Such the dual luminescence/absorption switching was shown to be reversible with a subsequent oxidation and repeatable with a consecutive redox treatment. [Figure not available: see fulltext.]

© Springer International Publishing AG, part of Springer Nature 2018. Producing fluoride and oxyfluoride materials is not easy because of the potential toxicity of fluorinated compounds and gases. Sol-gel processing has been used to produce fluoride and oxyfluoride materials in two ways. In the first case, the composition is formed from oxide precursors, followed by a reactive treatment in HF to fluorinate the composition. In the second case, trifluoroacetate precursors or metal alkoxides in trifluoroacetic acid are used to form the gel or thin films directly. The principal motivation for using sol-gel routes to prepare glassy fluorides and oxyfluorides is that they have useful optical applications.