Satoru Takeshita

Effects of citrate additive on transparency and photostability properties of YVO4: Bi3+, Eu3+ nanophosphor

YVO 4 :Bi 3+ ,Eu 3+ nanoparticles were prepared by the wet chemical synthesis in various concentrations of sodium citrate, 15 ≤ x cit ≤ 90, where x cit , is the nominal molar percentage of citrate relative to the sum of metallic ions, Y 3+ , Bi 3+ , and Eu 3+ . Mean primary nanoparticle diameter and mean hydrodynamic size in the aqueous colloidal solution have minimum values, 21 and 36 nm, respectively, for the sample prepared at x cit = 50. The transparency of the aqueous colloidal solution in the visible region increases with decreasing the mean hydrodynamic size, and the sample prepared at x cit = 50 shows the highest transparency. YVO 4 :Bi 3+ ,Eu 3+ nanoparticles show the characteristic photobleaching behavior and the photoreduction of V 5+ to V 4+ under the irradiation of near-UV light, where the fraction of photobleach becomes larger at large x cit . These results suggest that the citrate ion works not only as an effective dispersion agent, but also as a reducing agent against V 5+ of YVO 4 :Bi 3+ ,Eu 3+ .

Optical Properties of Transparent Wavelength-Conversion Film Prepared from YVO4 : Bi3 + , Eu3 + Nanophosphors

The optical properties of the transparent wavelength-conversion film containing 38.8 wt % YVO4:Bi 3+ ,Eu 3+ nanoparticles of 10.8 ± 1.6 nm in size are compared to that of the film containing 30.0 wt % micron-sized particles of 1.12 ± 0.48 μm in size. The photoluminescence intensity at 619 nm corresponding to the f-f transition of Eu 3+ for the film containing nanoparticles increases with increasing the film thickness up to 400 μm, whereas that of the film containing micron-sized particles reaches the maximum at the film thickness of ~40 μm. The transmittance at 619 nm for the former is ~96% irrespective of the film thickness, whereas that is less than 0.17% for the latter. These results suggest that the transparent film of YVO 4 :Bi 3+ ,Eu 3+ nanoparticles has markedly low light-scattering loss.

Mixed-solvent strategy for solvothermal synthesis of well-dispersed YBO3:Ce3+,Tb3+ nanocrystals

We investigated the influence of the composition of a 1,4-butanediol–water mixed solvent on the structural and particulate properties in the synthesis of well-dispersed YBO3:Ce3+,Tb3+ nanocrystals, without assembly, using a solvothermal method. YBO3:Ce3+,Tb3+ nanocrystals were synthesized from trimethyl borate and acetates of yttrium, cerium(III), and terbium(III) via a solvothermal reaction at 250 °C for 6 h in the mixed solvent. The presence of water promoted YBO3 crystallization. The size of the primary nanocrystals increased from ∼5 to ∼40 nm as the water content of the mixed solvent increased from 2.5 to 100 vol%. The primary nanocrystals formed disk-like secondary assembled particles for water contents below 75 vol%, but formed polyhedral secondary particles in pure water. In contrast, well-dispersed primary nanocrystals with a mean hydrodynamic size of ∼35 nm were obtained when the water content was 75 vol%. These results indicate that not only the size of the primary nanocrystals, but also their assembly can be controlled by changing the composition of the mixed solvent. The obtained well-dispersed nanocrystals were transparent to the naked eye and showed green photoluminescence corresponding to 4f → 4f transitions of Tb3+ by energy transfer from Ce3+ to Tb3+ under near-UV irradiation.

Electrophoretic deposition and characterization of transparent nanocomposite films of YVO4:Bi3+,Eu3+ nanophosphor and silicone-modified acrylic resin.

We fabricated nanocomposite films from an aqueous suspension of red-emitting YVO4:Bi(3+),Eu(3+) nanoparticles (hydrodynamic size: 22 ± 6 nm) and silicone-modified acrylic resin nanoparticles of (60 ± 15 nm) by electrophoretic deposition under application of a constant voltage. The nanocomposite films were formed from these negatively charged nanoparticles on ITO-coated glass substrates on the anodic side at the volume ratio of nanophosphor:resin ∼ 40:60. According to transmission electron microscopy observations, the YVO4:Bi(3+),Eu(3+) nanoparticles are well-dispersed around the resin nanoparticles. The fabricated films are transparent to the naked eye under white light because both nanoparticles show no absorption and low light scattering in the visible region. A silicone-modified acrylic resin film without the nanophosphor exhibits no absorption in the UV region (>300.0 nm). However, the fabricated nanocomposite films show near-UV absorption owing to the interband transition between the valence band and the conduction band of the YVO4:Bi(3+),Eu(3+) nanoparticles. A sharp emission peak corresponding to the (5)D0 → (7)F2 transition of Eu(3+) is observed at 619.5 nm, under 365.0 nm excitation, for each nanocomposite film. The photoluminescence intensity at 619.5 nm under 365.0 nm excitation is proportional to 1-10(-OD) (OD: optical density at 365.0 nm) for film thicknesses ≤6 μm. This is attributed to the low light scattering from both nanoparticles in the nanocomposite film. Conversely, the observed photoluminescence intensity for film thicknesses >6 μm is higher than the value expected from the proportional relationship. This suggests that the excitation of the nanophosphors efficiently occurs due to multiple scattering of excitation light.

Combinatorial optimization of the atomic compositions for green-emitting YBO3:Ce3+,Tb3+ and red-emitting YBO3:Ce3+,Tb3+,Eu3+ phosphors using a microplate reader

Microplate readers are versatile devices that can rapidly measure the photoluminescence intensities of multiple samples, and are widely used in biological chemistry. In this work, using a commercial microplate reader, we attempted to optimize the atomic compositions of green-emitting phosphor Y1−x−yCexTbyBO3 and red-emitting phosphor Y1−x−y−zCexTbyEuzBO3. We filled 48 individual wells of an alumina microplate with aqueous solutions of nitrates of Y3+, Ce3+, Tb3+, and Eu3+ with different compositions, and then added an aqueous solution of boric acid to each well. After drying, the microplate was heated at 550 °C for 2 h in air, and then at 1100 °C for 3 h in a reducing atmosphere. Y1−x−yCexTbyBO3 absorbed near ultraviolet light through 4f → 5d transitions of Ce3+ and emitted green fluorescence corresponding to 4f → 4f transitions of Tb3+ through Ce3+ → Tb3+ energy transfer. Moreover, Y1−x−y−zCexTbyEuzBO3 emitted red fluorescence corresponding to 4f → 4f transitions of Eu3+ through Ce3+ → Tb3+ → Eu3+ energy transfer under near-ultraviolet light. Measurement of the photoluminescence intensity of each well by a microplate reader revealed that the optimized green and red phosphors were Y0.835Ce0.025Tb0.14BO3 and Y0.535Ce0.005Tb0.45Eu0.01BO3, respectively.

Wet chemical synthesis and photoluminescence properties of YVO4:Bi3+,Eu3+ nanophosphors

YVO4:Bi3+,Eu3+ nanoparticles were prepared by the wet chemical synthesis in various concentrations of sodium citrate, 15 ≤ xcit ≤ 90, where xcit is the nominal molar percentage of citrate relative to the sum of metallic ions, Y3+, Bi3+, and Eu3+. Mean primary particle size and mean hydrodynamic size in the aqueous colloidal solution have minimum values, 21 and 36 nm, respectively, for the sample prepared at xcit = 50 mol%. The transparency of the aqueous colloidal solution in the visible region increases with decreasing the mean hydrodynamic size, and hence the sample prepared at xcit = 50 mol% shows the highest transparency. YVO4:Bi3+,Eu3+ nanophosphor synthesized by the citrate route shows the photobleaching behavior, i.e., the decrease in photoluminescence intensity under the continuous irradiation of excitation light. This photobleaching property is remarkably suppressed by the washing post-treatment combined with centrifugation and the hydrothermal post-treatment. A relation between the fraction of photobleach and the amount of citrate ions coordinating to metallic ions at the surface of nanoparticles verifies that the photobleaching behavior of YVO4:Bi3+,Eu3+ nanophosphor originates from the redox reaction between the citrate ions and V5+ in YVO4 host crystal.

Photobleaching Properties of YVO4:Eu3+ Nanophosphors Synthesized by Hydrothermal Treatment

1. Introduction YVO 4 :Eu 3+ has a high quantum efficiency of red emission under UV light irradiation. We expect that YVO 4 :Eu 3+ nanoparticles can be used in some fields such as lighting and security, by taking advantage of transparent property in comparison with micron-sized particles. YVO 4 :Eu 3+ nanoparticles have been prepared by hydrothermal treatment [1] and by a low-temperature wet chemical process in the presence of citrate ions for limiting the growth of particles [2]. Few works on the photo-stability of nanophosphors except quantum dots has been done so far. Here we report photobleaching properties of YVO 4 :Eu 3+ nanophosphors synthesized by hydrothermal treatment in the presence of citrate ions.

Composites of Eu3+-Doped Calcium Apatite Nanoparticles and Silica Particles: Comparative Study of Two Preparation Methods

We synthesized composites of Eu(3+)-doped calcium apatite (CaAp:Eu(3+)) nanoparticles and silica particles via two methods: (i) in situ synthesis of CaAp:Eu(3+) in the presence of silica particles and (ii) electrostatic adsorption of CaAp:Eu(3+) nanoparticles on silica particle surfaces. In both methods, submicrometer spherical silica particles were covered with CaAp:Eu(3+) nanoparticles without forming any impurity phases, as confirmed by X-ray diffractometry, Fourier-transform infrared spectroscopy, and scanning electron microscopy. In method i, part of the silica surface acted as a nucleation site for apatite crystals and silica particles were inhomogeneously covered with CaAp:Eu(3+) nanoparticles. In method ii, positively charged CaAp:Eu(3+) nanoparticles were homogeneously adsorbed on the negatively charged silica surface through electrostatic interactions. The bonds between the silica surface and CaAp:Eu(3+) nanoparticles are strong enough not to break under ultrasonic irradiation, irrespective of the synthetic method used. The composite particles showed red photoluminescence corresponding to 4f → 4f transitions of Eu(3+) under near-UV irradiation. Although the absorption coefficient of the forbidden 4f → 4f transitions of Eu(3+) was small, the red emission was detectable with a commercial fluorescence microscope because the CaAp:Eu(3+) nanoparticles accumulated on the silica particle surfaces.

An anomalous downsizing of glycothermally-synthesized YBO3 crystals by Ce3+ doping

We have systematically investigated the effects of rare-earth doping on the size and morphology of YBO3 crystals prepared by a glycothermal method. Spheroidal particles of YBO3 crystals doped with 1 at% of various rare-earth ions are prepared from trimethyl borate and acetates of rare-earths via a glycothermal reaction at 300 °C for 2 h in 1,4-butanediol. Ce3+-doped YBO3 shows remarkably small particles of ~180 nm in diameter, while undoped and other rare-earth-doped YBO3 consist of spheroidal particles of ~1 μm in diameter. This downsizing phenomenon for Ce3+-doped YBO3 is attributed to neither local lattice distortions nor surface adsorbed species. The downsizing is also observed when other cerium compounds, i.e., cerium nitrate, chloride, acetylacetonato, and oxide, are used as a cerium source. Investigation on the crystal growth process of Ce3+-doped YBO3 reveals that cerium ions promote the nucleation of YBO3 crystals at the early stage of the reaction, resulting in small particles.