Papers by Author: Joong Hee Lee

Abstract: A facile approach for the reduction of graphene oxide (GO) using of Tin (Sn) powder and dilute hydrochloric acid at room temperature (RT) and at 50 °C upto 0.5-3 h has been reported. The as-prepared high quality crystalline graphene has been characterized using various analytical techniques. The rate of reduction scales with the reaction time for both RT and 50 °C as evidenced from electrical conductivity data. The graphene samples reduced at RT shows high electrical conductivity value due to the presence of un-reacted Sn-particles. resulting the enhancement in the However, the graphene prepared at 50 °C shows low electrical conductivity in comparison to the room temperature reduced graphene. XPS elemental analysis is in good agreement with this observation.

Abstract: Preparation of functionalized graphene by electrochemical exfoliation of graphite rod using sodium dodecyl benzene sulfonate (SDBS) is reported. SDBS solution has been used as the electrolyte as well as functional groups. SDBD is an anionic surfactant which helps to provide uniform dispersion in water and prevents the π-π π-π stacking as well. XRD result indicates the formation of graphene whereas; the functionalization of graphene was confirmed through the FT-IR spectrum, which shows presence of peaks corresponding to SO₃^-. UV-vis spectroscopy demonstrates the dispersibility of SDBS-functionalized graphene, and peaks of SDBS and graphene appeared at 225 nm and 260nm, respectively. Raman spectroscopy show I_D/I_GIDIG ratio is < 1. It means that defects of SDBS-functionalized graphene are reduced.

Abstract: The development of sulfonated poly (ether ether ketone)-based (SPEEK) membranes by the incorporation of layered double hydroxide (LDH) as inorganic nanofiller has been achieved and its potential application in proton exchange membrane fuel cells (PEMFCs) has been investigated. The physical properties and proton conductivity of SPEEK matrix incorporated layered double hydroxide (LDH) were analyzed and found that the incorporation of LDH to SPEEK improves the thermal stability, water uptake and proton conductivity compared to the virgin SPEEK. The proton conductivity of the SPEEK/LDH composite membrane containing 3 wt% LDH has been found to be 229.9 mS/cm where as the virgin SPEEK shows the proton conductivity value 147.6 ms/cm at 80 °C and 100 %RH.

Abstract: Graphene/CeO₂ nanocomposite has been successfully prepared by directly growing CeO₂ nanoparticles on graphene sheets via in-situ reduction of graphene oxide containing the metal precursor. The presence of cetyltrimethyl ammonium bromide (CTAB) results the formation of CeO₂ nanoparticles with a narrow size distribution. The structural, morphological, particles size and optical properties of the synthesized products were investigated through X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FT-IR) and UVvis absorbance spectroscopy, respectively. The XRD pattern shows that graphene/CeO₂ nanocomposite is highly crystalline in nature. Growth of CeO₂ nanoparticles with size in range of 5-18 nm on the graphene sheet were observed by TEM measurement. Optical energy band gap was calculated to be ~3.30 eV corresponding to direct transition. The catalytic activity of the synthesized nanocomposite was investigated taking hydrazine hydrate as a model system. Significant enhancement in the peak current with respect to CeO₂ was observed on graphene/CeO₂ nanocomposite-based electrode demonstrating the higher catalytic activity of graphene/CeO₂ nanocomposite-based electrode.

Abstract: The nanocomposites containing linear low density polyethylene (LLDPE) and dodecyl amine (DA) modified graphene (DA-G) as nanofiller have been prepared by solution-mixing method and characterized. Transmission electron microscopy analysis of the nanocomposites exhibits homogeneous dispersion of graphene in the LLDPE matrix. Thermal stability of the nanocomposites with 1 wt.% DA-G are superior as compared to neat LLDPE. Gas barrier properties of the nanocomposites are also much better than that of the neat LLPDE. Co-efficient of thermal expansion values of the nanocomposites decreases with the addition of functionalized graphene. Keywords: Functionalized graphene; UV-Vis spectra; Nanocomposites; Gas barrier property; Mechanical property

Abstract: Nafion-clay nanocomposite membrane has been prepared by dispersing unmodified and acid activated Laponite XLS in Nafion 20% dispersion. The resulting membranes possess better proton conductivity and mechanical strength as compared to the virgin membrane. Acid activation of the nanoclay leads to the in-situ generation of H₃PO₄ by the hydrolysis of the peptizer present on the surface of the nanoclay. The in-situ generated H₃PO₄ helps in improving all the technical properties of the nanocomposite including the water uptake and proton conductivity of the nanocomposite, containing acid activated clay compared to the nanocomposite, containing unmodified clay. The maximum proton conductivity of 270.2 mS/cm is achieved at 110 °C for the nanocomposite membrane containing 3% acid-activated Laponite compared to 136.2 mS/cm for the virgin Nafion. Keywords: Nafion, clay, nanocomposite, peptizer, polymer electrolyte membrane fuel cell (FEMFC), proton conductivity, membrane

Abstract: Among different sterilization methods, heat-treatment of bone is recognized as one of the simple and practical methods to lower the human immunodeficiency virus (HIV) infection and overcome the risks of rejection and disease transfer from allograft and xenograft during bone transplantation. In order to best characterize the micro-structural mechanical property of bone after heat treatment, the nanoindentation technique was applied in this study to measure the localized elastic modulus and hardness for interstitial lamellae and osteons lamellae of bovine cortical bones at temperature 23°C (room temperature-pristine specimen), 37°C, 90°C, 120°C and 160°C, respectively. The elastic modulus (E) and hardness (H) of interstitial lamellae obtained higher values as compared with osteons lamellae which show that interstitial lamellae are more stiff and mineralized than osteons. Moreover, as a specimen pre-heat treated at 90°C, the E and H values of interstitial lamellae and osteons were closed to a pristine specimen. For a specimen pre-heat treated at 120°C, both interstitial lamellae and osteons obtained an increase in E and H values. As a specimen pre-heat treated at 160°C, the interstitial lamellae and osteons obtained a slight decrease in E and H values. These findings are correlated to results reported by other researchers [1, 2] that calcified collagen molecules starts to degenerate at about 120°C and complete at 160°C. Interestingly, when a specimen was pre-heat treated at 37°C, both interstitial lamellae and osteons obtained significant decreases in E values of 57% and 40%, respectively as compared to the pristine specimen; while in H values, there was a decrease of 27.4% and 15%, respectively. Thus, this paper will investigate the mechanical properties of bovine cortical bones under various temperature ranges by nanoindentation technique.

Abstract: Polydicyclopentadiene (PDCPD) is a kind of engineering plastic which possesses excellent mechanical performance. It can be made by dicyclopentadiene (DCPD) through Ring-opening metathesis polymerization. In this paper, a supported catalyst of WCl6-phenol on montmorillionite (MMt) was prepared, formed a high efficienct, low-cost and more stable in air, tagether with Et2AlCl as activator, to polymerize the DCPD monomer through ring-opening metathesis polymerization mechanism, and obtained a Polydicyclopentadiene/MMt Nanocomposites. The Molding of PDCPD was finally made by Gusmer-decker’s RIMcell Lt Machine, and the products were characterized by X-ray diffraction, TEM, TGA, SEM, DMA. Mechanical properties were also measured. The result showed that the supported catalyst had good catalytic activity in polymerizing DCPD. MMt was exfoliated in the matrix after the in-situ polymerization, according to the XRD and TEM, which brought about improved properties of PDCPD with small amount MMt dispersed in the matrix.

Abstract: A bisphenol-A glycidol ether epoxy resin was toughened in cryogenic temperature using reactive macroglycol as modifiers. The properties of modified epoxy resin were characterized by SEM and DMA. Phase-separated structure formed during curing process in the PPG and PTMG modified epoxy resins system, and did not occurred in the PEO modified epoxy resins system. The impact strength of epoxy resin increased at both room temperature (RT) and cryogenic temperature (CT, 77 K) using PEO as modifier. The DMA results confirm that the introduction of PEO chains in the structure of the epoxy increases the mobility of the molecular segment of epoxy network at both RT and CT.

Abstract: SDBS modified graphene was prepared by electrochemical method using Sodium dodecylbenzenesulfonate (SDBS) as electrolyte and graphite rod as electrode. The anode graphite rod was corroded and deposited at the bottom of the electrolyte solution. The obtained graphene was characterized by Atomic force microscopy (AFM), Raman and Fourier transform infrared spectra (FT-IR). AFM images indicated that most of the layers had the thickness of less than 2 nm, suggesting the fromation of single layer of graphene. The resulting graphene showed very good dispersion stability both in water and in organic solvents (ethanol, acetone).