Tadashi Ishigaki

Hydrothermal synthesis of meso/macroporous BiVO4 hierarchical particles and their photocatalytic degradation properties under visible light irradiation

An ordered hierarchical meso/macroporous monoclinic bismuth vanadate (BiVO4) particle was fabricated for the first time by a simple two-step melamine template hydrothermal method followed by calcination. The physiochemical parameters of as-prepared porous materials were characterized by means of X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Raman, Barrett–Emmett–Teller, and UV–vis techniques. The nitrogen adsorption–desorption measurement and pore size distribution curve suggest that meso/macropores exist in these hierarchical microarchitectures. Further, it is found that melamine plays a significant role in the formation of porous BiVO4 particles, and when a known amount of melamine was added, the surface area and pore size of such porous BiVO4 particles were increased. The photocatalytic activities of the as-prepared hierarchical BiVO4 samples were measured for the photodegradation of Congo red aqueous dye solution under visible light irradiation. Surprisingly, the porous BiVO4 particles showed outstanding photocatalytic activities than polycrystalline BiVO4 sample. The possible enhancement of such catalytic performance has also been further discussed.

Site engineering concept of Ce 3+ -activated novel orange-red emission oxide phosphors

Abstract: Novel Sr6(Y1-xCex)2Al4O15 (0.005 ≤ x ≤ 0.20) phosphors were synthesized in a single phase form by a conventional solid-state reaction method. These phosphors can be achieved the emission color tunable from blue to orange-red by controlling the Ce3+ doping site in the Sr6Y2Al4O15 lattice and exhibit orange-red emission centered on 600 nm by blue light irradiation as the Ce3+ concentration was increased. To the best of our knowledge, this is the first report of Ce3+ doping oxide phosphor exhibiting orange-red emission centered on 600 nm under blue light excitation.

Improved synthesis of SrLiAl3N4:Eu2+ phosphor using complex nitride raw material

Narrow band deep-red emission SrLiAl3N4:Eu2+ phosphor was synthesized using complex nitride Sr3Al2N4 and Li3AlN2 as raw materials. The SrLiAl3N4:Eu2+ phosphor has the oxoplumbate type triclinic structure as the main phase. The phosphor exhibited deep-red emission peaking at 654 nm under excitation at 450 nm and showed an excellent thermal stability on the thermal quenching effect.

Synthesis and the Luminescent Properties of Silicate NaAlSiO4:Eu2+ Phosphor Using SiO Powder as a Silica Source

A yellow-emitting NaAlSiO4:Eu2+ phosphor was synthesized using SiO powder as a reducing agent and a silica source material. The emission intensity of the NaAlSiO4:Eu2+ phosphor synthesized using SiO powder is higher than that of a sample produced using a conventional silicate phosphor synthesis reaction using SiO2 powder. The ratio of non-reduced Eu3+ ions in the NaAlSiO4:Eu2+ phosphor synthesized using SiO powder was decreased by the reduction effect of SiO powder.

Improvement of luminescence properties of rubidium vanadate, RbVO3, phosphors by erbium doping in the crystal lattice

Greenish-white emitting Rb1−xErxVO3 (0 ≤ x ≤ 0.10) phosphors were synthesized by a conventional solid state reaction method. The thermal quenching effect of the RbVO3 phosphors was effectively improved by Er3+ doping into the RbVO3 lattice. Consequently, the emission peak intensity of the phosphors was successfully enhanced by Er3+ doping, and the internal quantum efficiency of Rb0.99Er0.01VO3 under excitation at 360 nm was 80%.

Unusual, broad red emission of novel Ce3+-activated Sr3Sc4O9 phosphors under visible-light excitation

Most conventional white light emitting diodes (white-LEDs) that are widely used as a new lighting system in next generation lights with Y3Al5O12:Ce3+ (YAG:Ce3+)-based phosphors have a low colour rendering index (CRI) because the YAG:Ce3+ phosphor shows a weak emission intensity in the red spectral region. Therefore, discovering a red-emitting phosphor with a high-efficiency is quite important to enhance the CRI of white-LEDs. In this study, we successfully discovered a novel, red-emitting Ce3+-activated Sr3Sc4O9 phosphor that can be excited by blue-light irradiation at 425 nm. A crystal structure of the host material was first determined by Rietveld refinement, which indicated that it should be isostructural with Ba3Ln4O9 (Ln = Sc, Y and Dy-Lu). As the Ce3+ content increased, the X-ray diffraction patterns shifted to a lower angle, which suggested that the Ce3+ ion could substitute Sc in the Sr3Sc4O9 host. Under UV and blue-light excitation, the Ce3+-activated Sr3Sc4O9 phosphor exhibited a broad emission band with a maximum peak at 620 nm, and its full width half maximum (FWHM) was 180 nm (4530 cm−1). The highest emission intensity was obtained for Sr3(Sc0.997Ce0.003)4O9, and the internal quantum efficiency of this phosphor under excitation at 425 nm was 53%. To the best of our knowledge, the emission band of the Ce3+-activated Sr3Sc4O9 phosphor is the longest wavelength for a reported Ce3+-activated oxide phosphor.

Novel Soft Chemical Synthesis Methods of Ceramic Materials

We report novel soft chemical synthesis method, solid hydratethermal reaction (SHR) method as a new soft chemistry. This method is very simple and can synthesize the ceramic materials just by storing the mixture of raw materials added a small amount of water in a reactor at low temperature below 373 K. For example, nanosize YVO4 (under 100 nm in diameter) was obtained using the SHR method.

Synthesis and the luminescent properties of NaAlSiO4:Eu2+ phosphor using SiO powder as a silica source

A yellow-emitting silicate phosphor, NaAlSiO4:Eu2+, was synthesized through a new reducing technique, using SiO powder as a silica source, during the preparation process. The addition of SiO powder as a silica source significantly enhanced the Eu2+ content, which led to an increase in the photoluminescence emission intensity of the NaAlSiO4:Eu2+ phosphor. The optimization of the composition resulted in obtaining the maximum emission intensity for NaAlSiO4:7 mol% Eu2+.

Redetermination of the low-temperature polymorph of Li2MnSiO4 from single-crystal X-ray data

Crystals of dilithium manganese(II) silicate were grown under high-temperature hydrothermal conditions in the system LiOH—MnO2—SiO2. The title compound crystallizes in the βII-Li3PO4 structure type. The coordination polyhedra of all cations are slightly distorted tetrahedra (m symmetry for MnO4 and SiO4), which are linked by corner-sharing to each other. The vertices of the tetrahedra point to the same direction perpendicular to the distorted hexagonal close-packed (hcp) array of O atoms within which half of the tetrahedral voids are occupied by cations. In comparison with the previous refinement from powder X-ray data [Dominko et al. (2006 ▶). Electrochem. Commun. 8, 217–222], the present reinvestigation from single-crystal X-ray data allows a more precise determination of the distribution of the Li+ and Mn2+ cations, giving a perfectly site-ordered structure model for both Li+ and Mn2+.

Combinatorial synthesis of phosphors using arc-imaging furnace

Abstract We have applied a novel ‘melt synthesis technique’ rather than a conventional solid-state reaction to rapidly synthesize phosphor materials. During a synthesis, the mixture of oxides or their precursors is melted by light pulses (10–60 s) in an arc-imaging furnace on a water-cooled copper hearth to form a globule of 1–5 mm diameter, which is then rapidly cooled by turning off the light. Using this method, we synthesized several phosphor compounds including Y3Al5O12:Ce(YAG) and SrAl2O4:Eu,Dy. Complex phosphor oxides are difficult to produce by conventional solid-state reaction techniques because of the slow reaction rates among solid oxides; as a result, the oxides form homogeneous compounds or solid solutions. On the other hand, melt reactions are very fast (10–60 s) and result in homogeneous compounds owing to rapid diffusion and mixing in the liquid phase. Therefore, melt synthesis techniques are suitable for preparing multi component homogeneous compounds and solid solutions.