Hyo-Jin Oh

Effect of post treatments on the structure and thermal stability of titanate nanotubes

TiO2 sol was prepared hydrothermally in an autoclave from aqueous TiOCl2 solutions as a starting precursor. Titanate nanotubes were obtained when the sol?gel-derived TiO2 sol was treated chemically with a 10?M NaOH solution and subsequently heated in the autoclave at 150??C for 48?h. The samples were characterized using XRD, TEM, SEM, EDX, Raman spectroscopy, and a BET surface area analyser. The effect of post treatments, such as washing with and without hydrochloric acid and calcination, on the phase structure, shape and morphology, pore structures, and BET surface area of the titanate nanotubes was investigated. When a sample containing 7.08?wt% Na (after washing only with water) was calcined at different temperatures from 300 to 900??C, it showed the formation of a mixture of sodium trititanates and sodium hexatitanates and was found to preserve the tubular morphology at higher temperatures. However, a sample containing 0.06?wt% Na obtained after prolonged washing with hydrochloric acid followed by heat treatment showed the formation of TiO2 anatase involving TiO2 (B) as an intermediate at lower temperatures and anatase was further transformed to the rutile phase when the temperature was raised. On the basis of different observations, a general formula NaxH2?xTi3O7?nH2O has been proposed for the trititanate nanotubes.

Preparation and Characterization of Silica/Polyamide-imide Nanocomposite Thin Films

The functional silica/polyamide-imide composite films were prepared via simple ultrasonic blending, after the silica nanoparticles were modified by cationic surfactant—cetyltrimethyl ammonium bromide (CTAB). The composite films were characterized by scanning electron microscope (SEM), thermo gravimetric analysis (TGA) and thermomechanical analysis (TMA). CTAB-modified silica nanoparticles were well dispersed in the polyamide-imide matrix, and the amount of silica nanoparticles to PAI was investigated to be from 2 to 10 wt%. Especially, the coefficients of thermal expansion (CET) continuously decreased with the amount of silica particles increasing. The high thermal stability and low coefficient of thermal expansion showed that the nanocomposite films can be widely used in the enamel wire industry.

Structure-tunable synthesis of titanate nanotube thin films via a simple hydrothermal process

In this study, titanium metal acts as the titanium source to prepare oriented titanate nanotube thin film. The effects of preparation parameters, such as reaction temperature, duration and post-treatment conditions, on the film morphology and stability have been examined. A general formation mechanism for oriented titanate nanotube thin film is proposed on the basis of detailed observations of the products using two-dimensional surface SEM studies and TEM images. The overall formation of titanate nanotubes can be summarized as a sequence of four steps: (a) titanium dissolution and alkali titanate hydrogel formation; (b) alkali titanate hydrogel dissolution, increased TiO32−, TiO2(OH)22− or TinO2n+m2m− concentration and layered Na2Ti3O7 formation; (c) layered Na2Ti3O7 growth; (d) nanotube formation via Na2Ti3O7 splitting and the multilayer scrolling process. The Na2Ti3O7 lamellar structures split between the (010) planes into nanosheets.

Photocatalytic Properties of Nanotubular-Shaped Powders with Anatase Phase Obtained from Titanate Nanotube Powder through Various Thermal Treatments

Photocatalytic properties of nanotubular-shaped titanate powders, synthesized by hydrothermal treatment of , were studied after calcinations at various temperatures and times under , air, and H2 atmospheres. Their photocatalytic properties were measured with checking decomposition of 4-chlorophenol under UV-A irradiation, together with physical characterizations using SEM, TEM, XRD, BET, and Raman spectroscopy. With increasing calcination temperature and time, nanotubular-shaped titanate particles became short, rod-like, or irregular by oversintering, and their crystalline structure was changed into anatase phase. The photocatalytic ability in decomposition of 4-chlorophenol using the calcined powders showed the highest value due to obtainment of high specific area by maintaining nanotubular shape, together with transformation from titanate to anatase phase having perfect crystallinity in or air atmosphere.

Photoelectrochemical properties of Fe 2 O 3 supported on TiO 2 -based thin films converted from self-assembled hydrogen titanate nanotube powders

A photoanode was fabricated using hematite (α-Fe2O3) nanoparticles which had been held in a thin film of hydrogen titanate nanotubes (H-TiNT), synthesized by repetitive self-assembling method on FTO (fluorine-doped tin oxide) glass, which were incorporated via dipping process in aqueous Fe(NO3)3 solution. Current voltage (I-V) electrochemical properties of the photoanode heat-treated at 500°C for 10 min in air were evaluated under ultraviolet-visible light irradiation. Microstructure and crystallinity changes were also investigated. The prepared Fe2O3/H-TiNT/FTO composite thin film exhibited about threefold as much photocurrent as the Fe2O3/FTO film. The improvement in photocurrent was considered to be caused by reduced recombination of electrons and holes, with an appropriate amount of Fe2O3 spherical nanoparticles supported on the HTiNT/ FTO film. Nanosized spherical Fe2O3 particles with about 65wt% on the H-TiNT/FTO film showed best performance in our study.

Enhanced Water Splitting by Fe2O3-TiO2-FTO Photoanode with Modified Energy Band Structure

The effect of TiO2 layer applied to the conventional Fe2O3/FTO photoanode to improve the photoelectrochemical performance was assessed from the viewpoint of the microstructure and energy band structure. Regardless of the location of the TiO2 layer in the photoanodes, that is, Fe2O3/TiO2/FTO or TiO2/Fe2O3/FTO, high performance was obtained when α-Fe2O3 and H-TiNT/anatase-TiO2 phases existed in the constituent Fe2O3 and TiO2 layers after optimized heat treatments. The presence of the Fe2O3 nanoparticles with high uniformity in the each layer of the Fe2O3/TiO2/FTO photoanode achieved by a simple dipping process seemed to positively affect the performance improvement by modifying the energy band structure to a more favorable one for efficient electrons transfer. Our current study suggests that the application of the TiO2 interlayer, together with α-Fe2O3 nanoparticles present in the each constituent layers, could significantly contribute to the performance improvement of the conventional Fe2O3 photoanode.

Microstructural Effect of Carbon Blacks for the Application in Lithium Ion Batteries

Carbon blacks commercially available (Super P, SP and Ketjen black, KB) and synthesized by a liquid phase plasma process (SC) were compared for the lithium ion battery applications as an anode material. All the carbon black samples were spherical with sizes in the range of 30–50 nm. The Brunauer–Emett–Teller (BET) specific surface areas of the SP, KB, and SC samples were measured to be 62, 1452, and 895 m2/g, respectively. The overall fraction of the ordered structure, represented by the ratio of the G-band to the D-band (G/D raio) from Raman spectra, was highest for the SC sample. A large specific surface area of the samples was found to play an important role in storing lithium ions, contributing to high initial charge capacities, 2050 mAh/g for KB and 1542 mAh/g for SC. The initial charge–discharge coulombic efficiency of the samples was strongly influenced by the solid electrolyte interface (SEI) formation behavior. The behavior of SEI formation seemed to be affected by the microstructural characteristics of the carbon blacks such as crystallinity and G/D ratio. The SC sample having a high G/D ratio and a slight variation without a peak of dQ/dV with potential showed a small initial capacity irreversibility.

Fabrication of Fe2O3/TiO2 Photoanode for Improved Photoelectrochemical Water Splitting

A thin layer of TiO2 with photocatalytic property was employed as an interlayer to facilitate electron transfer in a conventional photoanode consisting of Fe2O3 and fluorine-doped tin oxide (FTO) films. The photoelectrochemical performance was compared between the samples with and without the TiO2 interlayer. The photoanode with the interlayer showed a greatly improved performance under 100 mW/cm2 UV–vis light illumination; the onset voltage for the photocurrent shifted to a 20% smaller value of about 0.75 (V vs real hydrogen electrode) and the photocurrent density of 4.35 mA/cm2 at 1.23 (V vs RHE) was observed, which is 3.5 times larger than that obtained from the photoanode without the TiO2 interlayer. The effect of the TiO2 interlayer on the photoanode performance was systematically investigated, and an effective way to improve the photoanode was suggested.

Water-splitting performance of Cu-In-S compounds/1D-Na2Ti6O13/FTO photoelectrodes

An approximately 700 nm thick film of 1D-Na2Ti6O13 nanorods, synthesized by heating a mixture of Natitanate nanotubes and NaCl at 810°C, was fabricated using LBL-SA technique. To improve the photoactivity of the coated film, CuxS, In2S3 and CuInS2 were synthesized and incorporated into the film by the polyol process. The thicknesses of CuxS, In2S3 and CuInS2 films incorporated into the film were 2 µm, 2 µm, and 40 µm, respectively. A deterioration of photocurrent was observed with the film incorporated with In2S3 particles. However, the films with CuxS or CuInS2 showed improved photocurrent, and the onset voltage was also reduced to 0.4 V. The performance of the films incorporated with 40 µm thick CuInS2 were affected by the light direction; a higher photocurrent was measured with the light illuminated through opposite side of the film.

P-239: Synthesis and Photoluminescence Properties of Tb3+-Doped Lanthanide Orthophosphates (La1−xGdx)0.94Tb0.06PO4 (0.5≤x≤1.0)

The photoluminescence properties of the (La1−xGdx)0.94Tb0.06PO4 (0.5≤x≤1.0) phosphors synthesized by ultrasonic spray pyrolysis were investigated under VUV irradiation. In order to understand the effect of Gd addition on the photoluminescence properties, we obtained the excitation and emission spectra of the (La1−xGdx)0.94Tb0.06PO4 phosphors with various Gd contents. Annealed (La1−xGdx)0.94Tb0.06PO4 phosphors showed a smooth, regular, and spherical morphology. The XRD spectra of the (La1−xGdx)0.94Tb0.06PO4 showed the solid solutions of the constituent lanthanide phosphates, which crystallized in a monoclinic monazite. The XRD peaks shifted slightly toward higher angles with increasing Gd content, indicating that the lattice parameter slightly decreased with an increase in Gd content. The phosphors emitted the green light due to the transition from 5D4 to 7Fj (j=3−6) at 489, 543, 585, and 621nm, respectively. (La0.25Gd0.75)0.94Tb0.06PO4 showed the strongest emission intensity.