Eui Jung Kim

Chemical functionalization of graphene sheets by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone

We report a simple and effective method for reducing and functionalizing graphene oxide into chemically converted graphene by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone (NMP). Graphene oxide sheets were functionalized by free radicals generated during heating of NMP in the presence of air. The degree of functionalization was easily controlled by manipulating the reduction time. High functionalized solvothermally reduced graphene oxide (STRG) shows superior dispersibility in various organic solvents, while slightly functionalized STRG shows excellent electrical conductivity. The superior dispersibility of highly functionalized STRG in organic solvents was attributed to the steric effect of functionalized groups on the surface of STRG sheets. Free-standing STRG paper that was reduced for 1 h exhibited electrical conductivity as high as 21600 S m−1, while the dispersibility of STRG that was reduced for 5 h was as high as 1.4 mg mL−1.

Influence of calcination temperature on structural and optical properties of TiO2 thin films prepared by sol-gel dip coating

TiO2 thin films were prepared by sol–gel dip coating and their structural and optical properties were examined at various calcination temperatures. The X-ray diffraction (XRD) results showed that TiO2 thin film calcined at 300 °C was amorphous, and transformed into the anatase phase at 400 °C, and further into rutile phase at 1000 °C. The phase transformation temperature has been dependent upon the concentration of catalyst HCl. The crystallite size of TiO2 thin films was increased with increasing calcination temperature. The micro-particles in the films grew by intra-agglomerate densification below 1000 °C, whereas they grew by inter-agglomerate densification above 1000 °C. The transmittance of the films calcined at 1000 and 1100 °C was reduced significantly in the wavelength range of about 300–700 nm due to the change of crystallite phase and composition in the films. The refractive index of TiO2 thin films was increased with increasing calcination temperature, and the film thickness and the porosity of TiO2 thin films were decreased.

Highly Conductive Poly(methyl methacrylate) (PMMA)-Reduced Graphene Oxide Composite Prepared by Self-Assembly of PMMA Latex and Graphene Oxide through Electrostatic Interaction

We report a simple, environmentally friendly approach for preparing highly conductive poly(methyl methacrylate)-reduced graphene oxide (PMMA-RGO) composites by self-assembly of positively charged PMMA latex particles and negatively charged graphene oxide sheets through electrostatic interactions, followed by hydrazine reduction. The PMMA latex was prepared by surfactant-free emulsion polymerization using a cationic free radical initiator, which created the positive charges on the surface of the PMMA particle. By mixing PMMA latex with a graphene oxide dispersion, positively charged PMMA particles easily assembled with negatively charged graphene oxide sheets through electrostatic interaction. The obtained PMMA-RGO exhibited excellent electrical properties with a percolation threshold as low as 0.16 vol % and an electrical conductivity of 64 S/m at only 2.7 vol %. Moreover, the thermomechanical properties of PMMA-RGO were also significantly improved. The storage modulus of PMMA-RGO increased by about 30% at 4.0 wt % RGO at room temperature while the glass transition temperature of PMMA-RGO increased 15 °C at only 0.5 wt % RGO.

Synthesis of a highly conductive and large surface area graphene oxide hydrogel and its use in a supercapacitor

In this report, we describe the structure of a robust and highly conductive 3D graphene oxide hydrogel. The reduced graphene oxide hydrogel or rGH is fabricated by a crosslinking reaction with ethylene diamine followed by a hydrazine reduction. The material showed a high electrical conductivity of 1351 S m−1 and a specific surface area of 745 m2 g−1 with 10.3 MPa break strength. When used as electrodes for a supercapacitor, it showed a high specific capacitance of 232 F g−1.

Chemical reduction of an aqueous suspension of graphene oxide by nascent hydrogen

One of the major challenges in the chemical reduction of graphene oxide is increasing the C/O atomic ratio of the chemically converted graphene. In this paper, we report a simple and effective method to reduce aqueous suspensions of graphene oxide using nascent hydrogen generated in situ by the reaction between Al foil and HCl, Al foil and NaOH and Zn powder and NaOH. The nascent hydrogen-reduced graphene oxides (nHRGOs) were characterized by elemental analysis, UV-vis spectra, Raman spectra, X-ray photoelectron spectroscopy, thermogravimetric analysis and electrical conductivity measurements. The reduction efficiency of graphene oxide strongly depended on the reaction medium and the rate of nascent hydrogen generation. The best nHRGO achieved a C/O atomic ratio greater than 21 and a bulk electrical conductivity as high as 12 500 S m−1, corresponding to the nascent hydrogen generated from the reaction between Al foil and HCl. Since nascent hydrogen could be produced on a metal surface upon oxidation in solution, other metals with low standard reduction potentials, such as Mg, Mn, and Fe, can be applied to reduce graphene oxide.

Variation of structural and optical properties of sol-gel TiO2 thin films with catalyst concentration and calcination temperature

TiO2 thin films were prepared on quartz glass by sol-gel process and their structural and optical properties were examined at various catalyst concentrations and calcination temperatures. The as-deposited TiO2 thin films are amorphous, and they transform into the anatase phase at 400–600 °C, and into the anatase–rutile phase at 800 °C, and further into the rutile phase at 1000 °C. The temperature of phase transformation is decreased with the concentration of catalyst HCl. The crystallite size of TiO2 thin films is increased with increasing calcination temperature and catalyst concentration. The secondary particles in the TiO2 thin films grow by intra-agglomerate densification below 800 °C, whereas they grow by inter-agglomerate densification above 800 °C. The deposited TiO2 thin films have high transparency in the visible range. The transmittance of the films calcined at 800 and 1000 °C are significantly reduced in the wavelength range of 300–800 nm due to the change of crystallite phase and increased particle size. The refractive index of TiO2 thin films is increased with increasing calcination temperature and catalyst concentration. On the other hand, the porosity of TiO2 thin film is decreased.

Fabrication of Au/Graphene-Wrapped ZnO-Nanoparticle-Assembled Hollow Spheres with Effective Photoinduced Charge Transfer for Photocatalysis

Heterostructures of gold-nanoparticle-decorated reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres (Au/rGO/ZnO) are synthesized using tetra-n-butylammonium bromide as a mediating agent. The structure of amorphous ZnO hollow spheres is found to be transformed from nanosheet- to nanoparticle-assembled hollow spheres (nPAHS) upon annealing at 500 °C. The ZnO nPAHS hybrids with Au/rGO are characterized using various techniques, including photoluminescence, steady-state absorbance, time-resolved photoluminescence, and photocatalysis. The charge-transfer time of ZnO nPAHS is found to be 87 ps, which is much shorter than that of a nanorod (128 ps), nanoparticle (150 ps), and nanowall (990 ps) due to its unique structure. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0% in comparison with rGO/ZnO (40.3%) and previously reported ZnO hybrids. The photocatalytic activities of the samples are evaluated by photodegrading methylene blue under black-light irradiation. The Au/rGO/ZnO exhibits excellent photocatalytic efficiency due to reduced electron-hole recombination, fast electron-transfer rate, and high charge-transfer efficiency.

Comparison of optical and photocatalytic properties of TiO2 thin films prepared by electron-beam evaporation and sol–gel dip-coating

Transparent TiO2 thin films were deposited on soda lime glass by electron-beam evaporation (EBE) and sol–gel dip-coating (SGD) methods. Optical and photocatalytic properties of the EBE and SGD films were compared. The transmittance of both films decreased with increasing calcination temperature from 300 to 500 °C as a result of the growth of particles. The refractive index of the SGD films increased significantly from 1.79 to 1.93 after heating at 500 °C, whereas that of the EBE films increased slightly from 2.09 to 2.13. The EBE films had better optical property than the SGD films. On the other hand, the SGD films exhibited much better photoactivity than the EBE films. This has been explained in terms of porosity and the formation of Ti3+ ions during calcination.

Superior conductive polystyrene – chemically converted graphene nanocomposite

The polystyrene–chemically converted graphene composite (PS-CCG) prepared by solution blending followed by compression molding, exhibited a percolation threshold as low as 0.19 vol.% and an electrical conductivity as high as 72.18 S m−1 at only ∼2.45 vol.%. The superior electrical conductivity of PS-CCG is the result of the combination of high electrical conductivity of CCG and the good dispersion of the nanofiller in PS matrix. The thermal properties of polystyrene were greatly improved upon addition of a small amount of CCG. The onset decomposition temperature of the PS-CCG increased by approximately 60 °C at 0.19 vol% of CCG loading. The mechanical properties of the PS-CCG were also affected by CCG loading. The storage modulus in the glassy region was enhanced by about 28% at 1.94 vol.% of CCG loading.

Novel conductive epoxy composites composed of 2-D chemically reduced graphene and 1-D silver nanowire hybrid fillers

In this study, 1-D Ag nanowires (NWs), 2-D chemically reduced graphene (CRG), and hybrid CRG–Ag NW fillers were investigated for use as conductive epoxy composites. By combining the 2-D CRG with 1-D Ag NWs, the percolation limit of the Ag NWs decreased from 30 wt% to 10 wt% and the electrical conductivity was dramatically enhanced because of the decreased tunneling resistance between the Ag NWs due to the thin 2-D conductive CRGs. Their thermal and mechanical properties were also improved due to the chemical crosslinking effects between CRGs and the hardener in the epoxy matrix as well as the physical crosslinking effects between nano-structures and polymer chains. The break strength of the CRG/Ag-NW/epoxy composite was 50% higher than that of the pure epoxy resin.