Papers by Author: J.G. Kim

Abstract: Transition metal-doped TiO2 powders as a photocatalyst were prepared by sol-gel process and Sb, Bi and Nb were introduced into them as dopants. The photocatalytic behaviors of the doped TiO2 powder were studied as a function of dopant, doping concentration and preparation conditions. X-ray diffraction, FT-Raman, B.E.T. and scanning electron microscopy were applied for structural and microstructural studies. Optical properties of the doped TiO2 powders were studied by UV-Visible Spectrometer and photocatalytic activity of the doped TiO2 was characterized in terms of the degradation of 1,4-dichlorobenzene. X-ray difraction analysis showed that doping with a transition metal ion suppresses anatase-to-rutile phase transition compared with the pure TiO2. The Sb and Nb-doped TiO2 powders did not exhibit any other diffraction peaks except those belonging to TiO2. On the other hand, a diffraction peak of Bi4Ti3O12 appears for 5 at.% Bi-doped samples. All of the doped TiO2 powders had higher specific surface area than undoped TiO2. Surface area increased with increasing dopant concentration depending on the dopant, from 33.9 m2/g to 55.4m2/g. The UV-visible absorption spectra of doped samples were red-shifted by 20~50nm according to the doping level. Also transition metal doped TiO2 powders exhibited better photocatalytic activity than the undoped TiO2. The increase in photoactivity is probably due to the increase in the interfacial electron transfer, red shifts, and better crystallinity.

Abstract: The nanostructural and optical features of hydrogenated nanocrystalline silicon (nc-Si:H) thin films, which were prepared by plasma enhanced chemical vapor deposition (PECVD), were investigated as a function of deposition conditions. It was found that the crystallite size varied with the relative fraction of Si-H3 bonds in the films, [ ] eger n n n H Si H Si int 3 1 3 / ] [ = = ∑ − − , which was sensitively related with the flow rate of SiH4 reaction gas. The silicon nanocrystallites in the films enlarged from ~2.0 to ~8.0 nm in their size with increasing gas flow rate, while the PL emission energy varied from 2.5 to 1.8 eV; the relative fractions of the Si-H3, Si-H2, and Si-H bonds in the amorphous matrix were also varied sensitively with the SiH4 flow rate. A model for the nanostructure of the nc-Si:H films was suggested to discribe the variations in the size and chemical bonds of the nanocrystallites as well as the amorphous matrix depending on the deposition conditions.

Abstract: Bioactive ORMOSILS (organically modified silicate), PDMS-CaO-SiO2-P2O5 with five different P2O5 content (0, 0.01, 0.03, 0.06, 0.09 mol%) have successfully been synthesized by sol-gel process. The hybrids have been prepared with polydimethylsiloxane (PDMS), tetraethoxysilane (TEOS), calcium nitrate tetrahydrate [Ca(NO3)2
4H2O] and triethyl phosphate (TEP) as starting materials and subsequently soaked into the simulated body fluid (SBF) for different period of time and the bioactivity of hybrids was determined by examining the apatite formation on the surface of the specimen by FT-IR, Thin-Film X-ray Diffraction, and Scanning Electron Microscopy (SEM). All of the prepared samples with different P2O5 content showed in vitro bioactivity. It was observed that the increase in P2O5 content up to 0.03 mole % increases the apatite formation compared to P2O5- free hybrids. However, further increase in P2O5 concentration slows down the formation of the apatite layer most probably due to the decrease of pH of SBF by dissolution of a large amount of phosphate ions.

Abstract: [SiO2]x[TiO2]1-x(x = 0.1, 0.2, 0.3, 0.4) powders were prepared by hydrothermal reaction of titanium isopropoxide (TIP) and tetraethyl-orthosilicate (TEOS) stabilized in acidic ethanol/water solution. The size and phase of TiO2 particles were controlled by adjusting the SiO2 content and the composition of the solvent system. The microstructure and photocatalytic activity of the prepared SiO2-TiO2 nanoparticles were characterized by XRD, TEM, BET, Raman and UV visible spectroscopic techniques. The major phase for all the prepared samples was anatase. The particle size of 0.3SiO2-0.7TiO2 powders prepared with excess amount of ethanol (REtOH = EtOH/alkoxide molar ratio = 120) was about 8.7 nm, while that of the sample prepared with small amount of ethanol (REtOH = 4) was 9.9 nm. The photocatalytic activity of these composite powders under excess ethanol was 1.5 times higher than that of pure TiO2 with respect to the decomposition of 1,4-dichlorobenzene (DCB).