Won-Jae Lee

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.

Surface morphologies and electrical properties of antimony-doped tin oxide films deposited by plasma-enhanced chemical vapor deposition

Antimony-doped tin oxide (ATO) films were deposited on Corning glass 1737 substrates by a plasma-enhanced chemical vapor deposition (PE-CVD) technique using a gas mixture of SnCl4–SbCl5–O2–Ar. Electrical properties and surface morphologies of these films were studied by varying the deposition temperature, input gas ratio, R[=(PSbCl5/PSnCl4)], and RF power. The PE-CVD method effectively enhanced the deposition rate and also improved the surface roughness of the deposit compared with thermal CVD. The antimony doped tin oxide films which had relatively good electrical properties were obtained at a deposition temperature of 450°C, an input gas ratio of R=1.12, and a RF power of 30 W. In addition, the studies on the morphological development of the films by AFM analysis suggested that higher input gas ratio and lower deposition temperature led to a decrease in the surface roughness of the deposited films.

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.

Photonic decomposition of ultrafine, rutile phased TiO2 powder in aqueous 4-chlorophenol solutions

The redox properties of a homogeneously precipitated TiO2 rutile powder with a BET surface area of ∼200 m 2 g −1 ,c onsisting of acicular primary particles, were characterized usin ga photocatalytic reaction in aqueous 4-chlorophenol, Cu- and Pb-EDTA solutions under ultraviolet irradiation. Results were compared to those of commercial P25 TiO2 powder with spherical primary particles as well as those of home-made anatase TiO2 powder with ∼200 m 2 g −1 BET surface area. Here, the anatase powder also includes mainly the primary particles very similar to the acicular shapes of the rutile TiO2 powder. The rutile powder showed the fastest decomposition rate and the largest amount of product in the photoredox, compared with the anatase or P25 powder, while the anatase powder unexpectedly showed the slowes tr at ea nd the smallest amount in the same experiments regardless of having almost the same surface area. The excellent photoredox abilities of this rutile powder appear to be due to a specific powder preparation method, for example a homogeneous precipitation leading to direct crystallization from the solution, regardless of their crystalline structures, even when the powders have similar particle shapes and surface areas. (Some figures in this article are in colour only in the electronic version)

Characteristics of piezoelectric multilayer devices containing metal-oxide multicomponent electrode

The objective of this study is to investigate sintering behavior of silver/palladium electrode powders with TiO 2 nano-particle addition and their matching characteristics to a piezoelectric ceramics, Pb(Zr, Ti)O 3 . The electro-mechanical properties of the piezoelectric multilayer devices with the silver-TiO 2 particle electrode was studied. The densification of the electrodes was investigated as a function of sintering temperature and the reaction between the ceramic matrix and electrodes was studied by employing mechanical bonding test and scanning electron microscopy. The densification of the nanoparticle TiO 2 -added electrode paste followed the TiO 2 solid state diffusion-controlled mechanism upon sintering process. Reaction between ceramic and electrode layers with the TiO 2 nano-particle powder allows internal stress to be reduced and mechanical bonding strength to be increased. High adhesive strength and good electrical conductivity of more than 10 4 /Ω cm could be obtained in the multilayer ferroelectric structure being in form of stacking ceramics layer and electrode layer containing the nano-particles. In order to understand the effectiveness of the nano-TiO 2 doped electrode for multilayer device, the multilayer ceramic actuators containing Ag-based electrode with nano-sized TiO 2 powder were fabricated and evaluated each other. Both the samples exhibited similar piezoelectric and dielectric properties.

Electrochemical Properties of Carbon Nano-tube as the Counter Electrode of Dye-sensitized solar cell

Studies on porous oxide electrode, dye and electrolyte for dye-sensitized solar cells have been intensively carried out until now. However, counter electrode have not been much studied so far. Accordingly, it is needed to investigate new counter electrode materials with superior catalyst property and to substitute for Pt electrode. In this case, carbon nano-tubes (CNTs) are one of alternatives for counter electrodes as following merits: low resistivity, excellent electron emission property, large surface area and low cost due to development of mass production technique. Such advantages gave us to select multiwalled CNTs (MWCNT) as counter electrode for dye-sensitized solar cell. Also, cyclic voltammetry and impedance spectroscopy were used to investigate electrochemical properties of both CNT electrode and Pt electrode. It was found that sheet resistance of CNT electrode was similar to that of Pt electrode, also, electrochemical properties of CNT electrode was superior to that of Pt electrode on the basis on the measurement of CV and impedance spectrum. It was found that CNT is likely to be a very promising electrode material for dye solar cells.

Preparation of Ultra-Thin Transparent TiO 2 Coated Film by Ink-Jet Printing Method

Dye sensitized solar cells(DSSC) are the most promising future energy resource due to their high energy efficiency, low production cost, and simple manufacturing process. But one problem in DSSC is short life time compared to silicon solar cells. This problem occurred from photocatalytic degradation of dye material by nanometer sized particles. To prevent dye degradation as well as to increase its life time, the transparent coating film is needed for UV blocking. In this study, we synthesized nanometer sized particles in sols by increasing its internal pressure up to 200 bar in autoclave at for 10 hrs. The synthesized sols were all formed with brookite phase and their particle size was several nm to 30 nm. Synthesized sols were coated on the backside of fluorine doped tin oxide(FTO) glass by ink jet printing method. With increasing coating thickness by repeated ink jet coating, the absorbance of UV region (under 400 nm) also increases reasonably. Decomposition test of titania powders dispersed in 0.1 mM amaranth solution covered with coating glass shows more stable dye properties under UV irradiation, compared to that with as-received FTO glass.

Electronic state and down-conversion effect of Y-doped ZnO

The electronic state of Y doped ZnO (YZO) was calculated using the density functional theory. In this study, the program used for the calculation on theoretical structures of ZnO and YZO was Vienna Ab-initio Simulation Package (VASP), which is a sort of pseudo potential method. The detail of electronic structure was obtained by the discrete variational Xα (DV-Xα) method, which is a sort of molecular orbital full potential method. The density of state and energy levels of dopant elements was shown and discussed in association with optical properties, especially related to down-conversion effect. The down-conversion effect of YZO was experimentally investigated by preparing thin films deposited on F doped SnO2 (FTO) glass substrates by sol-gel method using the spin-coating system. A homogeneous and stable solution was prepared by dissolving acetates in the solution added diethanolamine as sol-gel stabilizer. In order to confirm a ultraviolet ray interruption and down-conversion effects, the transmission spectrum and the fluorescent spectrum of YZO films were estimated. The results obtained by experiment were compared with the calculated structure.

Structure and Characteristics of Tandem Solar Cell Composed of Dye-sensitized Solar Cell and Thermoelectric Generator

The tandem solar cell composed of a dye-sensitized solar cell (DSC) and a thermoelectric generator (TEG) was designed. In such new cell, the characteristics of DSC and TEG were investigated. DSC uses the wavelength range of 380∼750 nm and has the maximum efficiency of below 10 ％. If the solar light transmitted through DSC can be converted to heat energy, TEG can generate electric energy using this heat energy. By this means, it is possible to utilize most of solar energy in the wavelength range of 350∼3000 nm for electric generation and it can be expected to obtain higher solar energy conversion efficiency exceeding the known limit of maximum efficiency. For this purpose we suggest the tandem solar cell constructed with DSC and TEG. In this structure, DSC has a carbon nanotube film as a counter electrode of DSC in order to collect the solar light and convert it to heat energy. We measured the I-V characteristics of DSC and TEG, assembled to the tandem cell. As a result, it was shown that DSC with carbon nanotube and TEG had the efficiency of 9.1 ％ and 6.2 ％, respectively. From this results, it is expected that the tandem solar cell of the new design has the possibility of enhanced conversion efficiency to exceed above 15 ％.

Aging Phenomena of Multilayered PMN-PZT Ceramic Actuator

Aging phenomena of 0.2PMN-0.8PZT multilayered ceramic actuators(MCA) have been investigated at the room temperature. The piezoelectric materials were synthesized as conventional ceramic process, and MCA were fabricatedby tape casting methods. The crystalline structures and lattice parameters were investigated by X-ray diffraction analysis, showing the structure was tetragonal and c/a was about 1.01. And, the effective electromechanical coupling coefficient keff and pseudo-piezoelectric constant were measured. Variable unipolar electric fields, , were applied to MCh to investigate the aging characteristics. After 2 kV/mm unipolar electric field, keff and were 0.454 and 4.44 respectively. The measured and simulated values using for aging phenomena analysis, had a good fit to the linear logarithmic stretched exponential law.