Eun Woo Shin

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.

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.

Temperature-dependent photoluminescence from chemically and thermally reduced graphene oxide

Temperature-dependent photoluminescence (PL) of graphene oxide (GO) reduced with hydrazine and heat has been measured to investigate the effect of reduction type on the bandgap of the reduced GO. Nitrogen functionalities formed in the hydrazine-treated GO were responsible for a strong localization of carriers that caused in a fluctuation in PL peak position with temperature. The intensity of C-OH peak was relatively low in the heat-treated GO, indicating that raising temperature facilitated the removal of hydroxyl groups, resulting in larger sp2 domain size and smaller bandgap energy.

Synthesis of hierarchical rose bridal bouquet- and humming-top-like TiO2 nanostructures and their shape-dependent degradation efficiency of dye

Novel hierarchical rose bridal bouquet- and humming-top-like nanostructured TiO(2) were successfully prepared by the simple process with the hydrothermal temperature as the morphology-controlling factor. The gradual transformation from layered titanate to brookite phase was well consistent with the formation mechanism of the hierarchical morphologies. The three-dimensional flower bouquets built from the bunches of roses with surrounding fern fronds displayed the best adsorptivity and completely degraded methylene blue within 60 min under UV irradiation, whereas the humming-top geometry composed of anisotropically elongated spindle-like crystallites was detrimental to the dye photodegradation.

A catalytic and efficient route for reduction of graphene oxide by hydrogen spillover

In this paper, an efficient catalytic route for the reduction of graphene oxide (GO) is described using hydrogen spillovered platinum (Pt) nanoparticles at room temperature. Pt nanoparticles were decorated on GO sheets using a hydrogen reduction of chloroplatinic acid. The Pt nanoparticles served as a catalyst for the process of hydrogen disassociation to atomic hydrogen, which spillovered onto the GO sheets which acted as a strong reducing agent for the reduction of GO. Reduced graphene oxide (RGO) obtained in this manner has a C/O atomic ratio as high as 22, which is one of the highest values ever reported for chemically converted graphenes. The electrical conductivity was greater than 8000 S m−1. Electrochemical studies revealed that the RGO–Pt hybrid exhibits excellent electrocatalytic activity toward the methanol oxidation reaction, with high CO tolerance and long-term stability.

Adsorption of tetracycline on La‐impregnated MCM‐41 materials

We prepared La‐impregnated mesoporous silicates to investigate the adsorption of tetracycline in aqueous solution. Nitrogen adsorption–desorption isotherm tests confirmed the mesoporosity of the prepared materials, resulting in a decline in surface area and pore volume along with La loading amount. In adsorption tests, however, La impregnation improved the adsorption capacity of materials for tetracycline even though the surface area and the pore volumes were lower after La impregnation. Adsorption capacities for tetracycline showed a maximum value with La loading amounts. The highest adsorption capacity for tetracycline was 303.3 mg g−1, which was acquired from the fitting of isotherm data of 10 wt% La‐impregnated mesoporous silicates to the Langmuir model. Changes in FTIR patterns of tetracycline adsorbed on La‐impregnated mesoporous silicates implied that the negatively charged oxygen of the tricarbonylamide group of tetracycline chemically interacted with La hydroxide species in materials in the neutral condition.

Low-Temperature Preparation of Highly Conductive Thin Films from Acrylic Acid-Stabilized Silver Nanoparticles Prepared through Ligand Exchange

The preparation of AcA-stabilized Ag nanoparticles and its application to make highly conductive thin films are reported. The AcA-stabilized Ag nanoparticles were prepared through a ligand exchange of original oleylamine (OLA)-coated Ag nanoparticles with acrylic acid (AcA), which acted as both an antisolvent and a modifying ligand during the ligand exchange process. Efficiencies of the ligand exchange as well as the properties of Ag nanoparticles were analyzed using various techniques including TEM, FT-IR, XPS, TGA, and UV-vis methods. The thin films were fabricated by annealing spin-coated AcA-stabilized Ag nanoparticles. Further, the effects of annealing temperature, time, and film thickness on both the film morphology and electrical conductivity have been investigated. In this work, due to the low boiling temperature of stabilizer (AcA) and adjustment of annealing conditions, high electrical conductivity was obtained for the Ag thin films. For example, when annealing at 175 °C for 30 min, a 70 nm thick film showed a maximum electrical conductivity of 1.12 × 10(5) S cm(-1). A conductive layer on a flexible polymer substrate (e.g., PET) sheet has been successfully prepared by annealing a spin-coated film at 140 °C for 30 min. The combined advantages of long-term stability of the AcA-stabilized Ag nanoparticles, low annealing temperature, and high conductivity of the prepared thin films make this relatively simple method attractive for applications in flexible electronics.

Uniform distribution of TiO2 nanocrystals on reduced graphene oxide sheets by the chelating ligands

Reduced graphene oxide-TiO(2) hybrids were successfully prepared by the hydrothermal approach using triethanolamine and acetylacetone as the chelating agents. Without any additive, large aggregated TiO(2) clusters were randomly distributed dominantly at the edge and less on the basil plane of coagulated reduced graphene oxide (RGO) layers. The presence of chelating ligands remarkably facilitated the selective growth and regular spread of TiO(2) nanocrystals onto individually exfoliated RGO sheet. Such sandwich-like structure with stronger coupling and chemical interaction resulted in the surface area increase, the rearrangement of energy level, the enhanced concentration of oxygen vacancies, leading to much higher adsorbability and photocatalytic degradation of Rhodamine B under both UV and visible irradiations. These RGO-TiO(2) hybrid systems are potentially beneficial for widely practical applications in air/water purification, electronic devices, batteries, solar cells or supercapacitors.

Three-dimensionally ordered macroporous spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts with highly enhanced catalytic performance for soot combustion

Three-dimensionally ordered macroporous (3DOM) spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts were successfully prepared by a colloidal crystal template strategy and characterized by means of XRD, FE-SEM, BET, FT-IR, H2-TPR and O2-TPD. The 3DOM catalysts exhibited superior activity to their corresponding bulk counterparts, which were used as reference benchmarks in this study, due to the enlarged contact between the catalyst and the soot particle and improved mass transfer caused by the unique well-defined 3DOM structures. Meanwhile, the factors influencing the catalytic activity of the 3DOM catalysts in soot combustion were also optimized, such as metallic cations and feed compositions. Furthermore, the 3DOM catalysts displayed strong durability against structural collapse due to the robust 3DOM structure, demonstrating their promising potential in view of the practical working conditions of diesel engines.

Mesoporous titanosilicate/reduced graphene oxide composites: layered structure, high surface-to-volume ratio, doping effect and application in dye removal from water

In this study, a new system of integrated photocatalytic adsorbents (IPCAs), three-dimensional wormhole-like mesoporous titanosilicate/reduced graphene oxide layered composites, were prepared with various Ti/Si ratios. The reduced graphene oxide (RGO) nanosheets played the role of giant two-dimensional (2D) platforms for the deposition, spreading and growth of mesostructured layers, leading to a significant enhancement in mesoporosity. The substitution of titanium species into the mesoporous silica/RGO significantly changed the textural, physicochemical and optical properties. Gradually increasing the Ti doping content dramatically promoted the adsorptivity and UV-photocatalytic degradation of the dye contaminant; however, an excess dosage (20 mol%) inhibited the UV activity. This could be attributed to the changes in the coordination geometry and electronic states of Ti species in the composites. The IPCAs that dominantly contained highly dispersed and isolated Ti-oxide species in tetrahedral coordination within a mesostructured framework exhibited much higher photoactivity than those composed of aggregated and octahedrally coordinated Ti species due to the formation of efficient charge-transfer excited states.