Takashi Ogi

Novel rare-earth-free tunable-color-emitting BCNO phosphors

We present a facile synthesis of novel, rare-earth (RE)-ion-free boron carbon oxynitride (BCNO) phosphors. The preparation method, chemical composition, luminescent properties and emission mechanisms, as well as current trends in BCNO phosphors are reviewed. The novel BCNO phosphors were synthesized from inexpensive and environmentally friendly raw materials by a straightforward route using liquid precursors at low temperatures under atmospheric pressure. The newly developed BCNO phosphors demonstrated tunable color emission, high quantum efficiency, and long-duration afterglow. The color emission of these phosphors can be tuned across almost the entire visible light spectrum by varying the molar ratios of the raw materials.

Role of C-N Configurations in the Photoluminescence of Graphene Quantum Dots Synthesized by a Hydrothermal Route.

Graphene quantum dots (GQDs) containing N atoms were successfully synthesized using a facile, inexpensive, and environmentally friendly hydrothermal reaction of urea and citric acid, and the effect of the GQDs’ C–N configurations on their photoluminescence (PL) properties were investigated. High-resolution transmission electron microscopy (HR-TEM) images confirmed that the dots were spherical, with an average diameter of 2.17 nm. X-ray photoelectron spectroscopy (XPS) analysis indicated that the C–N configurations of the GQDs substantially affected their PL intensity. Increased PL intensity was obtained in areas with greater percentages of pyridinic-N and lower percentages of pyrrolic-N. This enhanced PL was attributed to delocalized π electrons from pyridinic-N contributing to the C system of the GQDs. On the basis of energy electron loss spectroscopy (EELS) and UV-Vis spectroscopy analyses, we propose a PL mechanism for hydrothermally synthesized GQDs.

Mesopore-Free Hollow Silica Particles with Controllable Diameter and Shell Thickness via Additive-Free Synthesis

Mesopore-free hollow silica particles with a spherical shape, smooth surface, and controllable diameter (from 80 to 300 nm) and shell thickness (from 2 to 25 nm) were successfully prepared using an additive-free synthesis method. Different from other hollow particle developments, a mesopore-free shell was produced because of the absence of additive. Although common reports pointed out the importance of the additional additive in pasting and growing silica on the surface of a template, here we preferred to exploit the effect of the template charge in gaining the silica coating process. To form the silica, basic amino acid (i.e., lysine) was used as a catalyst to replace ammonia or hydrazine, which is harmless and able to control the silica growth and produce hollow particles with smooth surfaces. Control of the particle diameter was drastically achieved by altering the size of the template. The flexibility of the process in controlling the shell thickness was predominantly attained by varying the compositions of the reactants (i.e., silica source and catalyst). The present mesopore-free hollow particles could be efficiently used for various applications, especially for thermal insulator and optical devices because of their tendency not to adsorb large molecules, as confirmed by adsorption analysis.

Effect of the Carbon Source on the Luminescence Properties of Boron Carbon Oxynitride Phosphor Particles

The effect of the carbon source on the optimization of the photoluminescence (PL) performance of boron carbon oxynitride (BCNO) phosphor particles was systematically investigated. Ethylene glycol, tetraethylene glycol (TEG), and poly(ethylene glycol) of various Mw values were used as the organic sources. When TEG was used, the BCNO phosphors exhibited high PL performance under excitation at 365 nm with a quantum efficiency of up to 60%. The emission spectrum peak of the prepared BCNO particles was affected by the Mw of the carbon sources. An additional study investigated in detail the effects of the carbon/boron and nitrogen/boron molar ratios on PL properties. The emission spectra with a wavelength peak ranging from 380 nm (near UV) to 570 nm (near red) could be adjusted by varying the carbon/boron or nitrogen/boron ratios.

Nanometer to submicrometer magnesium fluoride particles with controllable morphology.

Magnesium fluoride particles with controllable size (from several nanometers to submicrometers) and morphology (spherical and cubic forms) were successfully prepared via liquid-phase synthesis. The particles were synthesized from the reaction of MgCl(2) and NH(4)F in an aqueous solution at 75 degrees C for 1 h under a nitrogen atmosphere. Control of particle size was accomplished mainly by changing the concentration of the reactants, which could be qualitatively explained by conventional nucleation theory. Flexibility of the process in controlling particle morphology, from a spherical to a cubical form, was predominantly achieved by varying the concentration of MgCl(2). Since the same XRD pattern was detected in particles with varying morphologies, the shape transformation was due to changes in particle growth. With the ability to control particle size and morphology, the creation of other inorganic particles is possible and has potential for many field applications.

Intense green and yellow emissions from electrospun BCNO phosphor nanofibers

BCNO phosphor nanofibers, composed of polycrystalline-BCN and B2O3 crystal, were prepared by electrospinning followed by calcination at 700 °C. These showed intense green and yellow emissions under UV-light irradiation that could be seen by the naked eye. The prepared nanofibers were uniform, non-agglomerate, thermally resistant, and had a good atomic distribution.

Direct white light emission from a rare-earth-free aluminium–boron–carbon–oxynitride phosphor

White light-emitting diodes offer the possibility of efficient, safe, and reliable solid-state lighting, and thus have various applications. Reported white light-emitting phosphors usually contain expensive rare-earth metals and are generally prepared by high-energy processes (e.g., >1000 °C, H2 and CO reduction atmospheres). These factors limit their applications. Therefore, preparing cost-effective white light-emitting phosphors from environmentally friendly processes is an important challenge. Herein, a direct white light-emitting aluminium–boron–carbon–oxynitride (AlBCNO) phosphor, which can be economically produced using low-energy methods (<900 °C, atmospheric conditions), is reported. To the best of our knowledge, this is the first reported rare-earth-free white light-emitting phosphor. AlBCNO emission spans the entire visible spectrum and its broad excitation spectrum is comparable to that of near-UV light-emitting diodes. Increasing the relative concentrations of B or Al in AlBCNO enables emission tuning to yellow or blue, respectively. These findings have implications for new methods of preparing white light-emitting phosphors.

Influences of Porous Structurization and Pt Addition on the Improvement of Photocatalytic Performance of WO3 Particles

Tungsten trioxide (WO3) displays excellent performance in solar-related material applications. However, this material is rare and expensive. Therefore, developing efficient materials using smaller amounts of WO3 is inevitable. In this study, we investigated how to create high photocatalytic performance of WO3 particles containing platinum (Pt, as a co-catalyst) and homogeneously spherical macropores (as a medium to enable access of large molecules and light penetration into the remote internal regions of the catalyst). The present particles were prepared by spray drying of a precursor solution containing WO3 nanoparticles, Pt solution, and polystyrene (PS) spheres (as a colloidal template). Photocatalytic studies showed that changes in particle morphology (from dense with smooth surfaces, to dense with rough surfaces, to porous structures) and added Pt effectively improved the photocatalytic performance over WO3 nanoparticles. Our results showed that the best precursor (prepared using a PS/WO3 mass ratio of 0.32 and containing Pt co-catalyst) provided WO3 particles with a photocatalytic rate of more than 5 times that of pure 10 nm WO3 nanoparticles. Moreover, the catalyst can be effectively recycled without an apparent decrease in its photocatalytic activity. The experimental results were also supported by a proposal mechanism of the photocatalytic reaction phenomenon.

Influences of Surface Charge, Size, and Concentration of Colloidal Nanoparticles on Fabrication of Self-Organized Porous Silica in Film and Particle Forms

Studies on preparation of porous material have attracted tremendous attention because existence of pores can provide material with excellent performances. However, current preparation reports described successful production of porous material with only partial information on charges, interactions, sizes, and compositions of the template and host materials. In this report, influences of self-assembly parameters (i.e., surface charge, size, and concentration of colloidal nanoparticles) on self-organized porous material fabrication were investigated. Silica nanoparticles (as a host material) and polystyrene (PS) spheres (as a template) were combined to produce self-assembly porous materials in film and particle forms. The experimental results showed that the porous structure and pore size were controllable and strongly depended on the self-assembly parameters. Materials containing highly ordered pores were effectively created only when process parameters fall within appropriate conditions (i.e., PS surface charge ≤ -30 mV; silica-to-PS size ratio ≤0.078; and silica-to-PS mass ratio of about 0.50). The investigation of the self-assembly parameter landscape was also completed using geometric considerations. Because optimization of these parameters provides significant information in regard to practical uses, results of this report could be relevant to other functional properties.

Transient nature of graphene quantum dot formation via a hydrothermal reaction

A facile, economic and environmentally friendly one-step approach for the preparation of highly luminescent graphene quantum dots (GQDs) was developed using a hydrothermal reaction between citric acid and urea. Unlike previous reports, we focused on the effect of the transient nature of GQD formation on the photoluminescence (PL) properties and molecular structure changes of the products. We found that the GQDs have an optimum reaction time and require an effective precursor to achieve excellent luminescent properties. The PL, ultraviolet-visible (UV-vis) absorption, zeta potential, and nuclear magnetic resonance (NMR) analyses of the GQDs prepared at various reaction times revealed that the molecular structures responsible for the luminescence of the GQDs are aggregates or condensation products of citric acid amides. We found that urea addition to the precursor drastically enhances the PL intensity of the GQDs, and it is 40 times higher than those prepared using the pure citric acid precursor. Additionally, a GQDs–polyvinyl alcohol composite achieved an excellent quantum yield (QY) of 43.6%.