Hee Jo Song

RbBaPO4:Eu2+: a new alternative blue-emitting phosphor for UV-based white light-emitting diodes

A novel blue-emitting phosphor on a phosphate-based host matrix, RbBaPO4:Eu2+ (RBP), was synthesized by a solid-state reaction. This phosphor could be excited efficiently by ultra violet-visible light in the 220–420 nm range to exhibit a highly efficient emission peak in the range of 420–440 nm. To investigate the possibility of commercial use, it was compared with the commercial blue phosphors of BaMgAl10O17:Eu2+ (BAM) and Sr3MgSi2O8:Eu2+ (SMS) to assess the photoluminescence properties. Compared to commercial phosphors, RBP displayed enhanced photoluminescence properties, in this case a high quantum efficiency and excellent thermal stability under near-Ultra Violet (n-UV) excitation. Furthermore, bright white light-emitting diodes (LEDs) were fabricated by integrating the RBP phosphor, along with commercial green/red phosphors, into n-UV LEDs. These results strongly suggest that the newly discovered RBP phosphor can be commercially utilized in UV-based white LEDs.

Tailoring uniform γ-MnO2 nanosheets on highly conductive three-dimensional current collectors for high-performance supercapacitor electrodes

Recent efforts have focused on the fabrication and application of three-dimensional (3-D) nanoarchitecture electrodes, which can exhibit excellent electrochemical performance. Herein, a novel strategy towards the design and synthesis of size- and thickness-tunable two-dimensional (2-D) MnO2 nanosheets on highly conductive one-dimensional (1-D) backbone arrays has been developed via a facile, one-step enhanced chemical bath deposition (ECBD) method at a low temperature (∼50 °C). Inclusion of an oxidizing agent, BrO3−, in the solution was crucial in controlling the heterogeneous nucleation and growth of the nanosheets, and in inducing the formation of the tailored and uniformly arranged nanosheet arrays. We fabricated supercapacitor devices based on 3-D MnO2 nanosheets with conductive Sb-doped SnO2 nanobelts as the backbone. They achieved a specific capacitance of 162 F·g−1 at an extremely high current density of 20 A·g−1, and good cycling stability that shows a capacitance retention of ≈92% of its initial value, along with a coulombic efficiency of almost 100% after 5,000 cycles in an aqueous solution of 1 M Na2SO4. The results were attributed to the unique hierarchical structures, which provided a short diffusion path of electrolyte ions by means of the 2-D sheets and direct electrical connections to the current collector by 1-D arrays as well as the prevention of aggregation by virtue of the well-aligned 3-D structure.

Enhanced Photocatalytic Activity of Ultrathin Ba5Nb4O15 Two-Dimensional Nanosheets.

Anisotropic two-dimensional (2D) nanosheets of the layered perovskite, Ba5Nb4O15, with thicknesses of 5-10 nm and lateral sizes of 300-1200 nm, were synthesized by a hydrothermal route. The influences of the 2D morphology of the material on the crystal and electronic structures, light absorption properties, and photocatalytic activity were investigated. The ultrathin nanosheets showed much-enhanced photocatalytic activity compared to both thick nanosheets (∼30 nm) and micrometer-sized particles for the evolution of H2 from water splitting under UV light illumination. This enhanced activity is predominantly attributed to the larger surface area, higher optical absorption, and charge separation ability of the 2D nanosheet, which results from the variation of the local crystal structure arising from the ultrathin morphology of the Ba5Nb4O15.

Cerium-doped yttrium aluminum garnet hollow shell phosphors synthesized via the Kirkendall effect.

We report, for the first time, the synthesis of the Y3Al5O12:Ce(3+) hollow phosphor particles with a uniform size distribution via the Kirkendall effect, characterized by using a combination of in situ X-ray diffraction and high-resolution transmission electron microscopy analyses as a function of calcination temperature. The formation of hollow Y3Al5O12:Ce(3+) particles was revealed to originate from the different diffusivities of atoms (Al and Y) in a diffusion couple, causing a supersaturation of lattice vacancies. The optical characterization using photoluminescence spectroscopy and scanning confocal microscopy clearly showed the evidence of YAG (yttrium aluminum garnet) hollow shells with emission at 545 nm. Another advantage of this methodology is that the size of hollow shells can be tunable by changing the size of initial nanotemplates that are spherical aluminum hydroxide nanoparticles. In this study, we synthesized the hollow shell particles with average diameters of 140 and 600 nm as representatives to show the range of particle sizes. Because of the unique structural and optical properties, the Y3Al5O12:Ce(3+) hollow shells can be another alternative to luminescence materials such as quantum dots and organic dyes, which promote their utilization in various fields, including optoelectronic and nanobio devices.

Hierarchical assembly of TiO2-SrTiO3 heterostructures on conductive SnO2 backbone nanobelts for enhanced photoelectrochemical and photocatalytic performance.

Heterostructures can play a role in enhanced photoinduced electrochemical and catalytic reactions due to the advantageous combination of two compounds. Herein, we demonstrate the fabrication of Sb:SnO2@TiO2-SrTiO3 3D heterostructures via a simple hydrothermal method using a conductive Sb:SnO2@TiO2 nanobelt electrode as a template. XRD, FESEM, and TEM analyses confirm that a well-dispersed and crystalized SrTiO3 layer is formed on the surface of TiO2 nanorods. The photoelectrochemical (PEC) performance of the heterostructure is optimized by controlling the reaction time. Details about the effect of the hydrothermal reaction time on the PEC performance are discussed. The optimized Sb:SnO2@TiO2-SrTiO3 heterostructure exhibited a higher onset potential and a saturated photocurrent in comparison to the Sb:SnO2@TiO2 nanostructure. The result is attributed to a Fermi level shift and a blocking layer effect caused by the SrTiO3. Furthermore, the photocatalytic degradation of methylene blue was significantly enhanced on the optimized Sb:SnO2@TiO2-SrTiO3. This work demonstrates that a synergetic effect between three-dimensional nanoarchitecturing and a heterojunction structure is responsible for enhanced PEC as well as improved photocatalytic performance levels, both of which can be extended to other metal-oxide and/or ternary compounds.

Template-free synthesis of monodispersed Y3Al5O12:Ce3+ nanosphere phosphor

Monodispersed Ce3+-doped Y3Al5O12 (YAG:Ce3+) nanospheres were synthesized through forced hydrolysis using a urea method, followed by thermal calcination processes, and their luminescence properties were examined. The crystallization of the YAG:Ce3+ phase and morphological evolutions after subsequent heat treatment were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDS) analysis revealed that the amorphous aluminum oxide layer played an important role in preventing necking between the particles during heat treatment. As a result, stand-alone YAG:Ce3+ nanospheres with an amorphous aluminum oxide layer shell were synthesized while maintaining monodispersity with an average particle diameter of about 33 nm. These nanospheres had a dense structure and smooth surface with relatively good crystallinity after annealing at 1075 °C. They absorbed light efficiently in the visible region of 400–500 nm, and showed a single broadband emission peak at 536 nm with a luminescence quantum efficiency (QE) of 33% and relatively good photostability.

High-power and long-life supercapacitive performance of hierarchical, 3-D urchin-like W18O49 nanostructure electrodes

We report the facile, one-pot synthesis of 3-D urchin-like W18O49 nanostructures (U-WO) via a simple solvothermal approach. An excellent supercapacitive performance was achieved by the U-WO because of its large Brunauer–Emmett–Teller (BET) specific surface area (ca. 123 m2·g–1) and unique morphological and structural features. The U-WO electrodes not only exhibit a high rate-capability with a specific capacitance (Csp) of ~235 F·g–1 at a current density of 20 A·g–1, but also superior long-life performance for 1,000 cycles, and even up to 7,000 cycles, showing ~176 F·g–1 at a high current density of 40 A·g–1.

An approach to flexible Na-ion batteries with exceptional rate capability and long lifespan using Na2FeP2O7 nanoparticles on porous carbon cloth

As post-Li-ion batteries (LIBs), rechargeable Na-ion batteries (NIBs) are considered as one of the potential candidates for large-scale energy storage systems because of the abundance and low cost of sodium resources, and similar electrochemical behavior of Na ions to Li ions for intercalation in cathodes. While there exist many challenges in the fabrication of cathodes, a polyanionic compound, Na2FeP2O7, has been in the spotlight as a potential cathode material for NIBs because of its rate capability, cyclability, and thermal stability. In this study, Na2FeP2O7 nanoparticles (NFP-NPs) embedded in carbon were prepared via a citric acid-assisted sol–gel method, followed by a post-heat treatment process. For the first time, NFP-NPs exhibited a reversible capacity close to the theoretical value (95 mA h g−1) over the voltage range of 2.0–4.0 V (vs. Na/Na+). Moreover, they displayed a superior rate capability of 77, 70, 66 and 65 mA h g−1 even at high rates of 10, 20, 30 and 60C, respectively. Equally notable is their exceptional long-term cyclability at high rates. At the rate of 10 and 60C, the capacity retention after 10 000 cycles is 83 and 84%, respectively. In addition, NFP-NPs uniformly loaded on the surface of flexible porous carbon cloth (NFP-NPs@PCC) electrodes without any conductive agents and polymeric binders also exhibit excellent rate capability and long-term cyclability at a high rate of 10C (56 mA h g−1 after 2000 cycles). We show high-performance free-standing NFP-NPs@PCC electrodes for possible application in flexible NIBs.

Ta-substituted SnNb2−xTaxO6 photocatalysts for hydrogen evolution under visible light irradiation

Ta-substituted SnNb2−xTaxO6 was successfully prepared via a solid-state reaction to study the effect of Ta insertion in Nb sites on the crystal structure, photophysical properties, and photocatalytic activities for hydrogen evolution. Analyses of X-ray diffraction patterns and Raman spectra revealed that the substitution of Ta caused not only a more tightly packed atomic structure with greater crystal structural distortion, but also a shorter M–O bond length in MO6 octahedra. Additionally, we observed a gradual increase in the band gap, changing the photoabsorption property and conduction band electronic structure. The SnNb1.4Ta0.6O6 photocatalyst showed enhanced hydrogen evolution compared to pristine SnNb2O6. This result was mainly attributed to better transport ability of the photo-generated charge carriers.

Biomineralized Multifunctional Magnetite/Carbon Microspheres for Applications in Li-Ion Batteries and Water Treatment

Advanced functional materials incorporating well-defined multiscale architectures are a key focus for multiple nanotechnological applications. However, strategies for developing such materials, including nanostructuring, nano-/microcombination, hybridization, and so on, are still being developed. Here, we report a facile, scalable biomineralization process in which Micrococcus lylae bacteria are used as soft templates to synthesize 3D hierarchically structured magnetite (Fe3O4) microspheres for use as Li-ion battery anode materials and in water treatment applications. Self-assembled Fe3O4 microspheres with flower-like morphologies are systematically fabricated from biomineralized 2D FeO(OH) nanoflakes at room temperature and are subsequently subjected to post-annealing at 400 °C. In particular, because of their mesoporous properties with a hollow interior and the improved electrical conductivity resulting from the carbonized bacterial templates, the Fe3 O4 microspheres obtained by calcining the FeO(OH) in Ar exhibit enhanced cycle stability and rate capability as Li-ion battery anodes, as well as superior adsorption of organic pollutants and toxic heavy metals.