Keun-Young Shin

Micropatterning of Graphene Sheets by Inkjet Printing and Its Wideband Dipole‐Antenna Application

PURPOSE: A graphene sheet using inkjet printing and a wideband dipole antenna application are provided to minimize the loss of raw material and time. CONSTITUTION: Waterborne graphene oxide nano particle solution is manufactured and is used for conductive ink of inkjet printing. The chemical characteristic of the waterborne graphene oxide nano particle solution is changed in order to form a detailed pattern on a supporting material. The waterborne graphene oxide nano particle solution is injected to a printer head, and an oxide graphene thin film is formed. The supporting material including the oxide graphene thin film is located in a vapor deposition reactor, and an oxidation-reduction reaction is executed. A graphene sheet based broadband dipole antenna electrode is connected to an antenna analysis device, and the performance of the antenna is measured.

Heavy metal ion adsorption behavior in nitrogen-doped magnetic carbon nanoparticles: Isotherms and kinetic study

To clarify the heavy metal adsorption mechanism of nitrogen-doped magnetic carbon nanoparticles (N-MCNPs), adsorption capacity was investigated from the adsorption isotherms, kinetics and thermodynamics points of view. The obtained results showed that the equilibrium adsorption behavior of Cr(3+) ion onto the N-MCNPs can be applied to the Langmuir model and pseudo-second-order kinetics. It indicated that the fabricated N-MCNPs had the homogenous surface for adsorption and all adsorption sites had equal adsorption energies. Furthermore, the adsorption onto N-MCNPs taken place through a chemical process involving the valence forces. According to the thermodynamics, the adsorption process is spontaneous and endothermic in nature which means that the adsorption capacity increases with increasing temperature due to the enhanced mobility of adsorbate molecules. The effects of the solution pH and the species of heavy metal ion on the adsorption uptake were also studied. The synthesized N-MCNPs exhibited an enhanced adsorption capacity for the heavy metal ions due to the high surface area and large amount of nitrogen contents.

Highly Sensitive and Multifunctional Tactile Sensor Using Free-standing ZnO/PVDF Thin Film with Graphene Electrodes for Pressure and Temperature Monitoring

We demonstrate an 80-μm-thick film (which is around 15% of the thickness of the human epidermis), which is a highly sensitive hybrid functional gauge sensor, and was fabricated from poly(vinylidene fluoride) (PVDF) and ZnO nanostructures with graphene electrodes. Using this film, we were able to simultaneously measure pressure and temperature in real time. The pressure was monitored from the change in the electrical resistance via the piezoresistance of the material, and the temperature was inferred based on the recovery time of the signal. Our thin film system enabled us to detect changes in pressure as small as 10 Pa which is pressure detection limit was 103-fold lower than the minimum level required for artificial skin, and to detect temperatures in the range 20–120°C.

A strategy for fabricating single layer graphene sheets based on a layer-by-layer self-assembly

A novel approach to synthesize graphene nanosheet is explored on the basis of the layer-by-layer (LbL) self-assembly of polyallylamine (PAA) and poly(styrene-sulfonate) (PSS) multi-layer with a metallic dopant.

Evaluation of anti-scratch properties of graphene oxide/polypropylene nanocomposites

Graphene oxide/polypropylene (GO/PP) nanocomposites were fabricated by varying the concentration of GO in the presence of octyltriethoxysilane (OTES) using an internal batch mixer to enhance the anti-scratch properties of pristine PP. Mechanical properties, including hardness, elastic modulus, and fracture toughness, of the prepared GO/PP nanocomposites were analyzed by nanoindentation. As a compatibilizer, OTES increased dispersibility, interfacial interactions, and mechanical interlocking between the GO nanofiller and the PP matrix, improving mechanical strength. A 286% increase in hardness, 127% improvement of elastic modulus and 117% enhancement on fracture toughness are achieved by addition of only 1.0 wt% of GO and OTES. For scratch tests, several GO/PP nanocomposite samples were analyzed according to various scratch parameters. Furthermore, scratch hardness, elastic recovery, and tangential forces acting on the scratch probe were investigated to provide insight into scratch behavior. The outstanding resistance to scratch deformation such as a maximum increase in scratch hardness of 189% was likely due to the excellent mechanical properties of the GO/PP nanocomposites.

Graphene size control via a mechanochemical method and electroresponsive properties.

Highly dispersible graphene oxide (GO) sheets of uniform submicrometer size were successfully fabricated from pristine graphite using a simple mechanochemical process. The GO flake morphology was transformed into a spherical form, and the density was decreased slightly via the ball-milling process. Ball-milled GO can be used as an electrorheological (ER) material because of its small particle size, low conductivity, and outstanding dispersibility in silicone oil. We found that the 2-h ball-milled GO-based ER fluid had the best ER performance (shear stress of 78.5 Pa and 630% ER efficiency), which was double that of the nonmilled GO-based ER fluid. The response time to form a fibrillar structure along the applied electric field direction and the recovery time to the starting level decreased with increasing ball-milling time. Additionally, the retarded settling velocity of isolated GO sheets and the electrostatic repulsion between oxygen functional groups on the GO sheets combined to improve the antisedimentation property. The ability to control the size of graphene sheets is a great opportunity to advance graphene commercialization in a high-quality, scalable production setting.

Magnetically recyclable core–shell nanocatalysts for efficient heterogeneous oxidation of alcohols

We describe the designed fabrication of magnetically recyclable core–shell Pd nanocatalysts for the efficient oxidation of alcohols under base-free reaction conditions in water. The Pd NPs that are half-partitioned in the polymer matrix can provide not only high catalytic activity but also stabilization of the nanocatalysts under harsh reaction conditions. Furthermore the magnetic separation provides a convenient method for removing and recycling the active Pd nanocatalysts from the reaction mixture. The designed nanocatalysts can be readily synthesized in a large scale and were able to be reused for five consecutive cycles of the oxidation of cycloheptanol. The nanocatalysts present high catalytic activity in other types of catalytic reactions involving Pd NPs such as Suzuki cross-coupling and reduction of nitroarenes.

High electrothermal performance of expanded graphite nanoplatelet-based patch heater

A sub-kilogram scale (∼500 g) of expanded graphite nanoplatelet (EGnP) with multi-layered graphene sheets were successfully fabricated using a simple mild-oxidation of pristine graphite. In particular, a substantial amount of trapped water molecules in EGnP make it a good ionic conductor, while simultaneously allowing it to serve as an electrolyte with ion transport characteristics. Due to its high electrical and thermal conductivity, micro-patterned EGnP can be used to produce electro-heating elements for line heaters. We found that the surface resistance of EGnP-based films was two orders of magnitude smaller than that of graphene-based thin films. The EGnP-based line heater demonstrated efficient heat propagation with uniform temperature distribution, resulting in an energy savings of up to ca. 37% in comparison to the graphene-based flexible heater. Especially, the steady-state temperature increased as the applied voltage increased and it reached to 172.3 °C at a driving voltage of 14 V. In addition, the EGnP-based line heaters under a bending radius of 4 cm had a 25 °C higher temperature as compared with heaters under flat conditions. Most of all, screen printing technique provides the facile formation of shape and size, and makes it possible to be used as a cheap and lightweight patch heater for industrial applications.

Graphene/polyaniline/poly(4-styrenesulfonate) hybrid film with uniform surface resistance and its flexible dipole tag antenna application.

A graphene/polyaniline/poly(4-styrenesulfonate) (G/PANI/PSS)-based conducting paste is successfully fabricated by introducing a PANI/PSS nanofiller into a multilayer graphene matrix by mechanical blending. As a compatibilizer, the PSS binder increases the dispersibility, interfacial interactions, and mechanical interlocking between the multilayer graphene matrix and PANI, thereby allowing surface resistance with narrow distribution. High concentrations of this PSS binder, obtained using ex situ polymerization, further improve the adhesion of the hybrid film to a flexible substrate. The minimum surface resistance of the screen-printed G/PANI/PSS hybrid film is approximately 10 Ω sq(-1) for a 70 μm uniform thickness. When bent to angles of -30°, the flexible hybrid film exhibits an approximately 6% decrease in surface resistance. The surface resistance after 500 bending cycles increases by only 10 Ω sq(-1) , which is 14 times that of smaller, graphene-based thin films. The micropatterned, screen-printed G/PANI/PSS hybrid film is evaluated as a practical dipole tag antenna. High-resolution patterns are formed in the hybrid film by the inherently high surface tension and the properties of grains within the domain-based structure. The G/PANI/PSS-based dipole tag antenna has a bandwidth of 28.7 MHz, a high transmitted power efficiency of 98.5%, and a recognition distance of 0.42 m at a mean frequency of 910 MHz. These characteristics indicate that the G/PANI/PSS-based dipole tag antenna could be used as a signal-receiving apparatus, much like a radio-frequency identification tag, for detecting nearby objects.

Highly porous nanostructured polyaniline/carbon nanodots as efficient counter electrodes for Pt-free dye-sensitized solar cells

We report a novel method for synthesizing highly porous polyaniline (PANI) using carbon nanodots (CNDs) as a nucleating agent and demonstrate their use as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). CNDs surrounded with aniline act as efficient nuclei in the polymerization reaction. CNDs disrupt undesirable secondary growth reactions leading to the formation of an agglomerated structure, and organize the highly porous PANI structures with a large surface area (43.6 m2 g−1). Moreover, the presence of CNDs in the polymerization mixture facilitates generation of head-to-tail dimers, and enhances the degree of para-coupling in the molecular structure of PANI. As a result of these nucleation effects, the fabricated PANI-CND films exhibit an increased electrical conductivity of ca. 774 S cm−1. When used as a CE in DSSCs, PANI-CND CEs exhibit a superior power conversion efficiency (η = 7.45%) to those of conventional platinum (η = 7.37%) and pristine PANI CEs (η = 5.60%).