Kyeong Youl Jung

Anatase-phase titania: preparation by embedding silica and photocatalytic activity for the decomposition of trichloroethylene

Abstract Nanophase titania particles were prepared by the sol–gel process using two different precursors; titanium isopropoxide(TTIP) and titanium ethoxide (TEOT). Silica-embedded titania particles was also prepared from TEOT and tera-ethyl-ortho-silicate (TEOS). In the case of nanophase titania particles prepared from TTIP, the rutile/anatase mixed phase had higher photoactivity than the pure anatase in the decomposition of TCE. However, in the nanophase titania prepared from TEOT, the photoactivity was increased with the heat treatment temperature until rutile phase began to be formed. The surface area was decreased with the heat treatment temperature. The photoactivity of the pure anatase titania prepared from TEOT was higher than that of Degussa P25 and the anatase/rutile mixed titania prepared from TTIP. Therefore, we concluded that, in order to achieve high photocatalytic activity, it was important to prepare titania particles at high temperature, preferably without forming rutile phase but not necessarily. This conclusion was confirmed by the experimental result that the silica-embedded titania particle of pure anatase phase had higher photoactivity than that of Degussa P25 and the pure anatase titania prepared from TEOT. The embedding of small amount of silica into anatase titania matrix enhanced the thermal stability of nanophase titaina particle resulting in the suppression of the phase transformation from anatase to rutile phase. This thermal stability enables us to calcine the silica-embedded particles at higher temperature without accompanying the phase transformation and to reduce the bulk defects, which are responsible for the low photocatalytic activity.

Linear relationship between the crystallite size and the photoactivity of non-porous titania ranging from nanometer to micrometer size

The dependency of the photoactivity on the crystallite size of anatase titania for the decomposition of trichloroethylene (TCE) was investigated. The crystallite size of anatase-phase was controlled at a fixed surface area and particle size by the spray pyrolysis. The surface area was also changed by the gas-phase decomposition of titanium-tetra-iso-propoxide (TTIP) by keeping the crystallite size in a similar range. The photoactivity of titania particles of the same surface area and particle size increased linearly with increasing the crystallite size of anatase-phase as long as no significant rutile phase was formed. It was proved that the photoactivity of titania is supposed to be enhanced by increasing the surface area while keeping a similar crystallite size. The lab-made titania particles of nanometer size and pure anatase-phase had higher photoactivity than that of Degussa P25 due to the larger crystallinity, although their surface area was smaller than that of Degussa P25. For all titania samples, the photoactivity normalized by the surface area was linearly increased with increasing the crystallite size of anatase-phase, regardless of the preparation method; the particle size ranging from nanometer to micrometer.

Morphology Control and Optimization of Luminescent Property of YBO 3 : Tb Phosphor Particles Prepared by Spray Pyrolysis

YBO 3 :Tb fine particles with high photoluminescence were prepared by a spray pyrolysis process. The emission intensity of YBO 3 :Tb particles under vacuum ultraviolet (VUV) excitation was optimized by controlling the Tb content, excess quantity of boron, and heat-treatment temperature. The highest luminescent intensity under VUV excitation was obtained when 10 atom % Tb with respect to yttrium and 25% excess boron of the stoichiometric quantity were used at 1150°C post-treatment temperature. It was tried to simultaneously control the morphological and luminous properties of YBO 3 :Tb particles via the modification of the precursor solution to be sprayed. The YBO 3 :Tb particles prepared from a nitrate aqueous solution which was obtained by only dissolving yttrium nitrate, terbium nitrate, and boric acid in purified water showed rod-like morphology. When the nitrate aqueous solution was modified by NH 4 OH, however, the prepared particles had not only a spherical-like shape, but also fine and more uniform size distribution than the rod-like particles prepared and the commercial YBO 3 :Tb particles. In addition, the YBO 3 :Tb particles prepared from the NH 4 OH-assisted spray solution showed improved luminescent intensity compared with the particles with a rod-like shape. From X-ray diffraction analysis, it was found that an enhancement of the crystallinity of YBO 3 :Tb was achieved by using NH 4 OH as a chemical additive. Finally, the photoluminescence intensity of YBO 3 :Tb particles optimized in terms of the composition, heat-treatment condition, and morphology was higher than that of the commercial YBO 3 :Tb particles as well as Zn 2 SiO 4 :Mn.

Densification and Photoluminescence Improvement of Y 2 O 3 Phosphor Particles Prepared by Spray Pyrolysis

Spray pyrolysis was used to prepare high luminous Y 2 O 3 :Eu phosphor particles with spherical shape. In this work, we introduce a simple but effective preparation strategy for enhancing the photoluminescence intensity of Y 2 O 3 :Eu particles. The key idea was the solution technique used to densify the porous structure and simultaneously enhance the crystallinity of Y 2 O 3 :Eu particles. When the yttrium nitrate solution was modified using the organic additive, the nonhollow particles could be obtained but they were very porous and the luminescence intensity was not improved. To solve this drawback, we used a drying control chemical additive (DCCA) as a secondary additive. It was found that the surface area was greatly reduced and the crystallite size was increased by the use of DCCA. As a result, the densified Y 2 O 2 :Eu particles showed great improvement in the photoluminescence intensity.

Amorphous GeOx-Coated Reduced Graphene Oxide Balls with Sandwich Structure for Long-Life Lithium-Ion Batteries

Amorphous GeOx-coated reduced graphene oxide (rGO) balls with sandwich structure are prepared via a spray-pyrolysis process using polystyrene (PS) nanobeads as sacrificial templates. This sandwich structure is formed by uniformly coating the exterior and interior of few-layer rGO with amorphous GeOx layers. X-ray photoelectron spectroscopy analysis reveals a Ge:O stoichiometry ratio of 1:1.7. The amorphous GeOx-coated rGO balls with sandwich structure have low charge-transfer resistance and fast Li(+)-ion diffusion rate. For example, at a current density of 2 A g(-1), the GeOx-coated rGO balls with sandwich and filled structures and the commercial GeO2 powders exhibit initial charge capacities of 795, 651, and 634 mA h g(-1), respectively; the corresponding 700th-cycle charge capacities are 758, 579, and 361 mA h g(-1). In addition, at a current density of 5 A g(-1), the rGO balls with sandwich structure have a 1600th-cycle reversible charge capacity of 629 mA h g(-1) and a corresponding capacity retention of 90.7%, as measured from the maximum reversible capacity at the 100th cycle.

Effect of Calcination Temperature and Addition of Silica, Zirconia, Alumina on the Photocatalytic Activity of Titania

Nanophase titania was prepared by sol-gel method and spray pyrolysis. We tried to elucidate the relationship between the photoactivity and the crystallite size of anatase phase. To better understand the changes in the bulk and the surface of titania as the calcination temperature is changed, EPR and photoluminescence analysis were carried out. The effect of the secondary metal oxide embedded into titania matrix on the photoactivity was also investigated. It was found that the photoactivity of titania has a linear relationship to the crystallite size. For the analysis of EPR and photoluminescence for pure titania, the increase of photoactivity with increasing the calcination temperature is due to the formation of surface active sites such as O- as well as the increase of crystallinity resulting from the removal of bulk defects. For silica/titania mixed oxide, it was found that the improvement of the thermal stability of anatase phase is important to enhance the photoactivity of titania because the prepared catalyst was calcined at a higher temperature than 700 °C without forming rutile phase. It was also concluded that the simultaneous increase of the surface area and the crystallinity promises to improve the photoactivity achieved by increasing the content of silica up to 60%. By the analysis of EPR and photoluminescence, it was found that the embedding of silica into titania matrix suppresses the formation of Ti3+ and produces a new active site of Ti-O-Si, which easily interacts with the oxygen. In the investigation of zirconia/titania and alumina/titania mixed oxide, it was found that the increase of the surface OH is essential to positively affect of the improved thermal stability on the photoactivity.

Macroporous Fe3O4/carbon composite microspheres with a short Li+ diffusion pathway for the fast charge/discharge of lithium ion batteries.

Macroporous Fe3O4/carbon composite and core-shell Fe3O4@carbon composite microspheres have been prepared by means of one-pot spray pyrolysis. The addition of polystyrene (PS) nanobeads to a spray solution containing an iron salt and poly(vinylpyrrolidone) (PVP) led to macroporous Fe3O4/carbon composite microspheres, the carbon and iron components of which are uniformly distributed over the entire composite microsphere. The pore-size distribution curve for the macroporous Fe3O4/carbon composite shows distinct peaks at around 10 and 80 nm. An electrode prepared from the macroporous Fe3O4/carbon composite microspheres showed better cycling and rate performances than an electrode formed from core-shell Fe3O4@carbon composite microspheres. The initial discharge and charge capacities of the macroporous Fe3O4/carbon composite microsphere electrode were determined to be 1258 and 908 mA h g(-1) at 2 A g(-1), respectively, and the corresponding initial coulombic efficiency was 72 %. The composite microsphere electrode cycled 500 times at 5 A g(-1) showed a high discharge capacity of 733 mA h g(-1).

Preparation of Fine-Sized SrSi2O2-δ N2 + 2 / 3δ : Eu2 + Phosphor by Spray Pyrolysis and its Luminescent Characteristics

The spray pyrolysis process was applied to prepare oxynitride phosphor (SrSi 2 O 2-δ N 2+2/3δ :Eu 2+ ), and the luminescence properties were investigated. The prepared particles had a green emission and good excitation characteristics in the region of 400-450 nm. The SrSi 2 O 2-δ N 2+2/3δ :Eu 2+ particles had an oxygen-rich phase (8 ≈ 0). On the basis of the concentration quenching data, the dipole-dipole interaction was dominantly involved in the energy transfer between the Eu 2+ ions, and the critical distance was determined to be about 15.1 A. The oxynitride phosphor with a fine size and good luminescent properties could be successfully prepared by the spray pyrolysis.

Improved Photoluminescence of Sr5 ( PO 4 ) 3Cl : Eu2 + Phosphor Particles Prepared by Flame Spray Pyrolysis

Flame spray pyrolysis has been utilized in order to improve the photoluminescence (PL) intensity of blue-emitting Sr 5 (PO 4 ) 3 Cl:Eu 2+ phosphor particles for light emitting diode applications. When Sr 5 (PO 4 ) 3 Cl:Eu 2+ phosphor particles were prepared by conventional spray pyrolysis, the PL intensity was much lower than that of commercial Sr 5 (PO 4 ) 3 Cl:Eu 2+ phosphor particles. However, Sr 5 (PO 4 ) 3 Cl:Eu 2+ phosphor particles prepared by the flame spray pyrolysis showed a high PL intensity compared with the commercial one. The morphology and the crystallinity of Sr 5 (PO 4 )Cl:Eu 2+ phosphor particles are strongly affected by the flame condition and the post-treatment environment. Sr 5 (PO 4 ) 3 Cl:Eu 2+ particles prepared by flame spray pyrolysis were of smaller size than commercial ones, although their spherical morphology was not maintained after the post-treatment at 1000°C with NH 4 Cl flux. The addition of NH 4 Cl flux during post-treatment greatly enhances the PL intensity of prepared Sr 5 (PO 4 ) 3 Cl:Eu 2+ phosphor particles due to increased crystallinity. It was found that the phosphor particles with larger crystallite size have higher PL intensity regardless of the method of preparation.

Modification of titania particles by ultrasonic spray pyrolysis of colloid

Composite fine particles such as ZnO-TiO2, CdS-TiO2, ZnS-TiO2 and Ag-TiO2 were prepared by ultrasonic spray pyrolysis of metal salt aqueous solution in which TiO2 (Degussa P25) particles were suspended. The crystallinity, chemical compositions, and morphologies of the produced particles were investigated by XRD, centrifugal particle size analyzer, and SEM/EDAX. The photocatalytic activity of the modified particles was investigated for the decomposition of trichloroethylene (TCE). The prepared particles were micro-porous and had spherical shapes of 0.3–1.5 μm in diameter. The size and size distribution of the prepared particles were not significantly changed by the type and loading of modifier. The crystallinity and crystallite size of TiO2 was not changed by these modifications. The photocatalytic activities of modified particles was slightly higher than pure titania when the loading was low. However, as the loading was further increased, the photocatalytic activity was sharply decreased and became almost constant.