論文 - 藤原　忍

Chemical solution deposition of magnetoelectric ZnO–La<inf>2</inf>CoMnO<inf>6</inf> nanocomposite thin films using a single precursor solution

Saito M., Hagiwara M., Fujihara S.

Materials Chemistry and Physics （Materials Chemistry and Physics）  236 2019年10月

ISSN  02540584

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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.

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Hydrothermal synthesis of monodispersed CePO<inf>4</inf>:Tb³⁺ porous microspheres and their redox-responsive luminescence

Takasu S., Hagiwara M., Fujihara S.

SN Applied Sciences （SN Applied Sciences）  1 （ 8 ）  2019年08月

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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.

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Synthesis of hollow and aggregated CeO <inf>2</inf> :Sm ³⁺ microspheres and their redox-responsive luminescence

Umehara T., Hagiwara M., Fujihara S.

Journal of Alloys and Compounds （Journal of Alloys and Compounds）  787   1074 - 1081 2019年05月

ISSN  09258388

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© 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.

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Controlled 90° domain wall motion in BaTiO<inf>3</inf> piezoelectric ceramics modified with acceptor ions localized near grain boundaries

Miyaura A., Kawaguchi T., Hagiwara M., Fujihara S.

SN Applied Sciences （SN Applied Sciences）  1 （ 4 ）  2019年04月

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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.

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Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured Ca<inf>y</inf>CoO<inf>2</inf> thermoelectric ceramics

Shimonishi R., Hagiwara M., Fujihara S.

Ceramics International （Ceramics International）  45 （ 3 ） 3600 - 3607 2019年02月

ISSN  02728842

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© 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.

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Fabrication and refractive index control of transparent and luminescent HfO<inf>2</inf>:Ln³⁺ (Ln³⁺ = Eu³⁺, Tb³⁺) thin films for enhanced surface emissions

Muroi M., Hagiwara M., Fujihara S.

ECS Journal of Solid State Science and Technology （ECS Journal of Solid State Science and Technology）  8 （ 12 ） R169 - R175 2019年

ISSN  21628769

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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.

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Structural improvement of ZnO electrodes through solution-processed routes for enhancing open-circuit voltage in dye-sensitized solar cells

Tanaka E., Nurdiwijayanto L., Hagiwara M., Fujihara S.

Journal of Solid State Electrochemistry （Journal of Solid State Electrochemistry）  22 （ 10 ） 3119 - 3127 2018年10月

ISSN  14328488

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© 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%.

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Sol-gel processing of fluoride and oxyfluoride materials

Fujihara S.

Handbook of Sol-Gel Science and Technology: Processing, Characterization and Applications （Handbook of Sol-Gel Science and Technology: Processing, Characterization and Applications）     333 - 359 2018年07月

ISSN  9783319320991

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© 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.

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A novel synthesis method of delafossite-type CuYO<inf>2</inf> using a layered yttrium hydroxide as an yttrium source

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

Journal of the Ceramic Society of Japan （Journal of the Ceramic Society of Japan）  126 （ 5 ） 286 - 291 2018年05月

ISSN  18820743

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© 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.

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Fluorochromic properties of undoped and Ln³⁺-doped CaWO<inf>4</inf> phosphor particles

Tsuchiya Y., Hagiwara M., Fujihara S.

ECS Journal of Solid State Science and Technology （ECS Journal of Solid State Science and Technology）  7 （ 5 ） R50 - R56 2018年

ISSN  21628769

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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.

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A Biphasic Sol-Gel Route to Synthesize Anatase TiO₂ Particles under Controlled Conditions and Their DSSC Application

奈良部徹, 萩原 学, 藤原 忍

Journal of Asian Ceramic Societies （Elsevier）  5 （ 4 ） 427 - 435 2017年12月

研究論文（学術雑誌）, 共著, 査読有り

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Effect of Thermal History on Stability of the Relaxor State in (Bi_1/2K_1/2)TiO₃ Ceramics

萩原 学, 藤原 忍

Japanese Journal of Applied Physics （応用物理学会）  56 （ 10S ） 10PC03 (5 pages) 2017年09月

研究論文（学術雑誌）, 共著, 査読有り

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Relaxor-Ferroelectric Crossover in (Bi_1/2K_1/2)TiO₃: Origin of the Spontaneous Phase Transition and Effects of External Fields

HAGIWARA, MANABU; EHARA, YOSHITAKA; NOVAK, NIKOLA; KHANSUR, NEAMUL H.; AYRIKYAN, AZATUHI; WEBBER, KYLE G.; FUJIHARA, SHINOBU

Physical Review B （American Physical Society）  96 （ 1 ） 014103 (9 pages) 2017年07月

研究論文（学術雑誌）, 共著, 査読有り

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Hydrothermal Synthesis of Lead-free Perovskite (Bi_1/2K_1/2)(Zr_xTi_1-x)O₃ Powders

伊藤 洵，萩原 学, 藤原 忍

Journal of the Ceramic Society of Japan （日本セラミックス協会）  125 （ 6 ） 454 - 457 2017年06月

研究論文（学術雑誌）, 共著, 査読有り

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Fabrication of Luminescent Antireflective Coatings with CaMoO₄:Eu³⁺/Ag Composite Structure

松本和浩, 萩原 学, 藤原 忍

Coatings （MDPI）  7 （ 6 ） 74 (10 page) 2017年06月

研究論文（学術雑誌）, 共著, 査読有り

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Size-controlled Synthesis of ZIF-8 Particles and Their Pyrolytic Conversion into ZnO Aggregates as Photoanode Materials of Dye-sensitized Solar Cells

榎本泰啓, 上野慎太郎, 細野英司, 萩原 学, 藤原 忍

CrystEngComm （Royal Society of Chemistry）  19 （ 21 ） 2844 - 2851 2017年06月

研究論文（学術雑誌）, 共著, 査読有り

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Fabrication of Luminescence-sensing Films Based on Surface Precipitation Reaction of Mg-Al-Eu LDHs

八神高史, 萩原 学, 藤原 忍

Journal of Sol-Gel Science and Technology （Springer）  82 （ 2 ） 380 - 389 2017年05月

研究論文（学術雑誌）, 共著, 査読有り

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Defects and Microstructure of a Hydrothermally Derived (Bi_1/2K_1/2)TiO₃ Powder

萩原 学, 伊藤 洵，藤原 忍

Journal of Asian Ceramic Societies （Elsevier）  5 （ 1 ） 31 - 35 2017年03月

研究論文（学術雑誌）, 共著, 査読有り

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Solvent-assisted Microstructural Evolution and Enhanced Performance of Porous ZnO Films for Plastic Dye-sensitized Solar Cells

大橋ひとみ, 萩原 学, 藤原 忍

Journal of Power Sources （Elsevier）  342   148 - 156 2017年02月

研究論文（学術雑誌）, 共著, 査読有り

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固相法によるZn₂(Ti_xSn_1-x)O₄:Eu蛍光体材料合成のための初期粒子サイズ制御

古谷彰平, 中野裕美，藤原 忍

粉体工学会誌 （粉体工学会）  53 （ 12 ） 779 - 784 2016年12月

研究論文（学術雑誌）, 共著, 査読有り

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