研究者詳細 - 藤原　忍

論文 - 藤原　忍

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Ni-doping effect on thermoelectric properties of c-axis-oriented CuFeO<inf>2</inf> ceramics

Hagiwara M., Tato M., Shiojiri D., Iida T., Fujihara S.

Journal of Alloys and Compounds （Journal of Alloys and Compounds） 905 2022年06月

ISSN 09258388

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.

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Grain-size-insensitive dielectric properties of Sr<inf>0.6</inf>Ba<inf>0.4</inf>Nb<inf>2</inf>O<inf>6</inf> relaxor ferroelectric ceramics with tetragonal tungsten bronze structure

Tsuchiya M., Fujihara S., Hagiwara M.

Ceramics International （Ceramics International） 48 （ 5 ） 6819 - 6825 2022年03月

ISSN 02728842

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.

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Redox-induced dual optical switching of CaTiO<inf>3</inf>:Pr3+ phosphor nanoparticles synthesized by sol–gel method

Takahashi H., Hagiwara M., Fujihara S.

Journal of Sol-Gel Science and Technology （Journal of Sol-Gel Science and Technology） 2022年

ISSN 09280707

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.]

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Fabrication of meso- and macro-porous Y<inf>2</inf>WO<inf>6</inf>:Eu3+ phosphor thin films by Pechini-type sol–gel dip-coating method and their characteristic optical properties

Ye H., Hagiwara M., Fujihara S.

Journal of Sol-Gel Science and Technology （Journal of Sol-Gel Science and Technology） 100 （ 2 ） 232 - 243 2021年11月

ISSN 09280707

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.]

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Fabrication of p-type semiconducting NiCo<inf>2</inf>O<inf>4</inf> thin films using hydroxide nanoplatelet precursors and their application to N749-sensitized photocathodes

Miyachi M., Hagiwara M., Fujihara S.

Journal of the Ceramic Society of Japan （Journal of the Ceramic Society of Japan） 129 （ 7 ） 348 - 354 2021年07月

ISSN 18820743

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.

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Fabrication of mesostructured Y<inf>2</inf>O<inf>3</inf>:Eu3+ materials from metal–organic frameworks and their H<inf>2</inf>O<inf>2</inf>-sensitive turn-off luminescence

Sakamawari K., Hagiwara M., Fujihara S.

Optical Materials （Optical Materials） 116 2021年06月

ISSN 09253467

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.

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Fabrication of bismuth silicate Bi<inf>2</inf>SiO<inf>5</inf> ceramics as a potential high-temperature dielectric material

Sakamoto K., Hagiwara M., Taniguchi H., Fujihara S.

Journal of Materials Science （Journal of Materials Science） 56 （ 14 ） 8415 - 8426 2021年05月

ISSN 00222461

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.

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Effects of dual lanthanide ions doping on optical and electrical properties of barium stannate with Ba<inf>1–x–y</inf>La<inf>x</inf>Sm<inf>y</inf>SnO<inf>3</inf> compositions

Yamauchi T., Hagiwara M., Fujihara S.

Journal of Alloys and Compounds （Journal of Alloys and Compounds） 861 2021年04月

ISSN 09258388

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.

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Comparative hydrothermal synthesis of CeO<inf>2</inf>crystals for use in light-scattering layers of dye-sensitized solar cells

Shoji T., Hagiwara M., Fujihara S.

CrystEngComm （CrystEngComm） 23 （ 6 ） 1415 - 1422 2021年02月

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.

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Fabrication of highly (1 1 1)-oriented Cu<inf>2</inf>O films on glass substrates by repeated chemical bath deposition

Tonagi D., Hagiwara M., Fujihara S.

Journal of Crystal Growth （Journal of Crystal Growth） 551 2020年12月

ISSN 00220248

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.

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Hydrothermal growth of c-axis oriented ferroelectric (Bi<inf>1/2</inf>K<inf>1/2</inf>)TiO<inf>3</inf> films on metal substrates

Yamamoto M., Sakurai R., Hagiwara M., Fujihara S.

Thin Solid Films （Thin Solid Films） 713 2020年11月

ISSN 00406090

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.

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Effect of micro-/mesoporous structures on H2O2 sensing ability of YVO4:Eu3+ phosphor particles

Motomiya K., Hagiwara M., Fujihara S.

Journal of the Ceramic Society of Japan （Journal of the Ceramic Society of Japan） 128 （ 8 ） 532 - 538 2020年08月

ISSN 18820743

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.

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Ferroelectric and piezoelectric properties of (Bi<inf>1/2</inf>K<inf>1/2</inf>)(Zr<inf>x</inf>Ti<inf>1−</inf><inf>x</inf>)O<inf>3</inf> lead-free ceramics

Ito M., Hagiwara M., Fujihara S.

Materials Letters （Materials Letters） 271 2020年07月

ISSN 0167577X

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.

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(Bi<inf>1/2</inf>K<inf>1/2</inf>)TiO<inf>3</inf>–SrTiO<inf>3</inf> solid-solution ceramics for high-temperature capacitor applications

Shiga M., Hagiwara M., Fujihara S.

Ceramics International （Ceramics International） 46 （ 8 ） 10242 - 10249 2020年06月

ISSN 02728842

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.

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Effect of particle size and morphology on the performance of BiFeO<inf>3</inf>-PDMS piezoelectric generators

Yoshiyama K., Mori M., Hagiwara M., Fujihara S.

CrystEngComm （CrystEngComm） 22 （ 17 ） 2919 - 2925 2020年05月

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.

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Chemical bath deposition of transparent ZnO films incorporated with erythrosine B molecules and their synergetic electro/photochromic properties

Yumiyama R., Hyakutake D., Hagiwara M., Hosono E., Matsuda H., Fujihara S.

CrystEngComm （CrystEngComm） 22 （ 14 ） 2447 - 2453 2020年04月

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.

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Fabrication of highly textured Ca<inf>3</inf>Co<inf>4</inf>O<inf>9</inf> ceramics with controlled density and high thermoelectric power factors

Shimonishi R., Hagiwara M., Fujihara S.

Journal of the European Ceramic Society （Journal of the European Ceramic Society） 40 （ 4 ） 1338 - 1343 2020年04月

ISSN 09552219

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.

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Synthesis of Pt-Loaded Y<inf>2</inf>WO<inf>6</inf>:Eu3+Microspheres and Their Hydrogen-Sensitive Turn-Off Luminescence

Ye H., Hara R., Hagiwara M., Fujihara S.

ACS Omega （ACS Omega） 5 （ 12 ） 6697 - 6704 2020年03月

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.

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Reactive Templated Grain Growth and Thermoelectric Power Factor Enhancement of Textured CuFeO<inf>2</inf> Ceramics

Tato M., Shimonishi R., Hagiwara M., Fujihara S.

ACS Applied Energy Materials （ACS Applied Energy Materials） 3 （ 2 ） 1979 - 1987 2020年02月

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.

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Biphasic Sol-Gel Synthesis of Microstructured/Nanostructured YVO<inf>4</inf>:Eu3+ Materials and Their H<inf>2</inf>O<inf>2</inf> Sensing Ability

Motomiya K., Sugita K., Hagiwara M., Fujihara S.

ACS Omega （ACS Omega） 4 （ 23 ） 20353 - 20361 2019年12月

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.

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