Kyeong Min Cho

A highly photoactive, visible-light-driven graphene/2D mesoporous TiO2 photocatalyst

In this paper, we have reported a new, visible-light-driven photocatalyst composed of 2D mesoporous TiO2 on reduced graphene oxide, which enables enhanced absorption of visible light, showing outstanding charge separation ability and a large surface area. Graphene–2D mesoporous TiO2 (GOMTi) was prepared via a simple hydrothermal process by ternary self-assembly of metal oxide precursors, graphene oxide, and a surfactant. The structural strength and large interfacial contact between the graphene and two dimensional TiO2 sheet lead to a high performance photocatalyst, as compared to a conventional graphene–TiO2 nanostructure composite. In the test for degradation of methylene blue, the photodegradation rate of GOMTi was ∼9 times higher than the rate of common TiO2 nanoparticles (p25), due to the enhanced synergetic effects of graphene and TiO2.

High quality reduced graphene oxide through repairing with multi-layered graphene ball nanostructures

We present a simple and up-scalable method to produce highly repaired graphene oxide with a large surface area, by introducing spherical multi-layered graphene balls with empty interiors. These graphene balls are prepared via chemical vapor deposition (CVD) of Ni particles on the surface of the graphene oxides (GO). Transmission electron microscopy and Raman spectroscopy results reveal that defects in the GO surfaces are well repaired during the CVD process, with the help of nickel nanoparticles attached to the functional groups of the GO surface, further resulting in a high electrical conductivity of 18,620 S/m. In addition, the graphene balls on the GO surface effectively prevent restacking of the GO layers, thus providing a large surface area of 527 m2/g. Two electrode supercapacitor cells using this highly conductive graphene material demonstrate ideal electrical double layer capacitive behavior, due to the effective use of the outstanding electric conductivity and the large surface area.

Complete magnesiothermic reduction reaction of vertically aligned mesoporous silica channels to form pure silicon nanoparticles

Owing to its simplicity and low temperature conditions, magnesiothermic reduction of silica is one of the most powerful methods for producing silicon nanostructures. However, incomplete reduction takes place in this process leaving unconverted silica under the silicon layer. This phenomenon limits the use of this method for the rational design of silicon structures. In this effort, a technique that enables complete magnesiothermic reduction of silica to form silicon has been developed. The procedure involves magnesium promoted reduction of vertically oriented mesoporous silica channels on reduced graphene oxides (rGO) sheets. The mesopores play a significant role in effectively enabling magnesium gas to interact with silica through a large number of reaction sites. Utilizing this approach, highly uniform, ca. 10 nm sized silicon nanoparticles are generated without contamination by unreacted silica. The new method for complete magnesiothermic reduction of mesoporous silica approach provides a foundation for the rational design of silicon structures.

Highly Enhanced Fluorescence Signals of Quantum Dot-Polymer Composite Arrays Formed by Hybridization of Ultrathin Plasmonic Au Nanowalls.

Enhancement of the fluorescence intensity of quantum dot (QD)-polymer nanocomposite arrays is an important issue in QD studies because of the significant reduction of fluorescence signals of such arrays due to nonradiative processes in densely packed polymer chains in solid films. In this study, we enhance the fluorescence intensity of such arrays without significantly reducing their optical transparency. Enhanced fluorescence is achieved by hybridizing ultrathin plasmonic Au nanowalls onto the sidewalls of the arrays via single-step patterning and hybridization. The plasmonic Au nanowall induces metal-enhanced fluorescence, resulting in a maximum 7-fold enhancement of the fluorescence signals. We also prepare QD nanostructures of various shapes and sizes by controlling the dry etching time. In the near future, this facile approach can be used for fluorescence enhancement of colloidal QDs with plasmonic hybrid structures. Such structures can be used as optical substrates for imaging applications and for fabrication of QD-LED devices.

Combining the silver nanowire bridging effect with chemical doping for highly improved conductivity of CVD-grown graphene films

Here, the conductivity of graphene films was significantly enhanced, with only a negligible loss in optical transmittance, by controlling the intrinsic electrical properties and domains through a combination of chemical doping and the deposition of a few one-dimensional (1D) silver nanowires (AgNWs). Ultraviolet photoelectron spectrophotometry (UPS) results and Raman spectra showed that the transparent conducting performance of the CVD-grown graphene film can be attributed to the chemical doping effect and the electric pathways provided by the AgNW bridging between graphene domains. Importantly, it was found that the transparent conducting performance of the AgNW modified CVD graphene films was significantly influenced by the types and sequence of chemical doping. Various combinations were investigated, including: (i) Au pre-treatment and then AgNW deposition, (ii) AgNW deposition and then Au post-treatment, (iii) acid (HNO3) pre-treatment and then AgNW deposition, and (iv) AgNW deposition and then acid (HNO3) post-treatment. Electrical conductivity and transmittance results showed that the combination of Au pre-treatment and subsequent AgNW deposition onto the graphene films is the most effective way to enhance the conductivity of graphene films for a variety of optoelectronic device applications, demonstrating a large reduction in the sheet resistance of the graphene film (ΔRs ≈ 80%) without significant loss of transmittance. This is due to independent evolution of the chemical doping effect of Au and the bridging effect of the AgNWs on the poly-domain graphene. For deposition of AgNWs and subsequent Au post-doping, on the other hand, transmittance was reduced by more than 5% after Au post-treatment of the graphene films on which AgNWs had already been deposited.

The Role of Layer-Controlled Graphene for Tunable Microwave Heating and Its Applications to the Synthesis of Inorganic Thin Films

In this paper, we present the first method for precisely controlling the heat generated by microwave heating by tuning the number of graphene layers grown by chemical vapor deposition. The conductivity of the graphene increases linearly with the number of graphene layers, indicating that Joule heating plays a primary role in the temperature control of the graphene layer. In this method, we successfully synthesize TiO2 and MoS2 thin films, which do not interact well with microwaves, on a layer-controlled graphene substrate for a very short time (3 min) through microwave heating.

Universal Method for Creating Hierarchical Wrinkles on Thin-Film Surfaces

One of the most interesting topics in physical science and materials science is the creation of complex wrinkled structures on thin-film surfaces because of their several advantages of high surface area, localized strain, and stress tolerance. In this study, a significant step was taken toward solving limitations imposed by the fabrication of previous artificial wrinkles. A universal method for preparing hierarchical three-dimensional wrinkle structures of thin films on a multiple scale (e.g., nanometers to micrometers) by sequential wrinkling with different skin layers was developed. Notably, this method was not limited to specific materials, and it was applicable to fabricating hierarchical wrinkles on all of the thin-film surfaces tested thus far, including those of metals, two-dimensional and one-dimensional materials, and polymers. The hierarchical wrinkles with multiscale structures were prepared by sequential wrinkling, in which a sacrificial layer was used as the additional skin layer between sequences. For example, a hierarchical MoS2 wrinkle exhibited highly enhanced catalytic behavior because of the superaerophobicity and effective surface area, which are related to topological effects. As the developed method can be adopted to a majority of thin films, it is thought to be a universal method for enhancing the physical properties of various materials.

Selective Molecular Separation on Ti3C2Tx–Graphene Oxide Membranes during Pressure-Driven Filtration: Comparison with Graphene Oxide and MXenes

In this work, we prepared 90 nm thick Ti3C2Tx-graphene oxide (GO) membranes laminated on a porous support by mixing GO with Ti3C2Tx. This process was chosen to prevent the penetration of target molecules through inter-edge defects or voids with poor packing. The lattice period of the prepared membrane was 14.28 Å, as being swelled with water, resulting in an effective interlayer spacing of around 5 Å, which corresponds to two layers of water molecules. The composite membranes effectively rejected dye molecules with hydrated radii above 5 Å, as well as positively charged dye molecules, during pressure-driven filtration at 5 bar. Rejection rates were 68% for methyl red, 99.5% for methylene blue, 93.5% for rose Bengal, and 100% for brilliant blue (hydrated radii of 4.87, 5.04, 5.88, and 7.98 Å, respectively). Additionally, the rejections of composite membrane were compared with GO membrane and Ti3C2Tx membrane.

Facile Synthesis of Composition-Controlled Graphene-Supported PtPd Alloy Nanocatalysts and Their Applications in Methanol Electro-Oxidation and Lithium-Oxygen Batteries

A new and simple approach is reported for the synthesis of uniformly dispersed PtPd alloy nanocatalysts supported on graphene nanoplatelets (GNPs) (PtPd-GNPs) through the introduction of bifunctional materials, which can modify the GNP surface and simultaneously reduce metal ions. With the use of poly(4-styrenesulfonic acid), poly(vinyl pyrrolidone), poly(diallyldimethylammonium chloride), and poly(vinyl alcohol) as bifunctional materials, PtPd-GNPs can be produced through a procedure that is far simpler than previously reported methods. The as-prepared nanocrystals on GNPs clearly exhibit uniform PtPd alloy structures of around 2 nm in size, which are strongly anchored and well distributed on the GNP sheets. The Pt/Pd atomic ratio and loading density of the nanocrystals on the GNPs are controlled easily by changing the metal precursor feed ratio and the mass ratio of GNP to the metal precursor, respectively. As a result of the synergism between Pt and Pd, the as-prepared PtPd-GNPs exhibit markedly enhanced electrocatalytic performance during methanol electro-oxidation compared with monometallic Pt-GNP or commercially available Pt/C. Furthermore, the PtPd-GNP nanocatalysts also show greatly enhanced catalytic activity toward the oxygen reduction/evolution reaction in a lithium-oxygen (Li-O2 ) process, resulting in greatly improved cycling stability of a Li-O2 battery.

Ultrathin graphene oxide membranes on freestanding carbon nanotube supports for enhanced selective permeation in organic solvents

Among the various factors required for membranes in organic solvent separations, the stability of membrane supports is critical in the preparation of membranes with universal chemical stability, mechanical flexibility, and high flux. In this study, nanoporous freestanding carbon nanotube (CNT) films were fabricated and utilized as supports for enhanced permeation in organic solvents. The excellent chemical stability of the CNT support allowed it to withstand various organic solvents such as toluene, acetone, and dimethylformamide. In addition, the structural stability and high pore density of CNT supports allowed the deposition of an ultrathin selective layer for an enhanced-flux membrane. Membrane performance was demonstrated by depositing a thin graphene oxide (GO) layer on the CNT support; GO was selected because of its high chemical stability. CNT-supported GO membranes effectively blocked molecules with molecular weight larger than ~800 g mol−1 while allowing the fast permeation of small molecules such as naphthalene (permeation was 50 times faster than that through thick GO membranes) and maintaining selective permeation in harsh solvents even after 72 hours of operation. We believe that the developed CNT support can provide fundamental insights in utilizing selective materials toward organic solvent membranes.