Jyongsik Jang

Fabrication of amorphous carbon-coated NiO nanofibers for electrochemical capacitor applications

Amorphous carbon-coated nickel oxide nanofibers (NiC NFs) were fabricated using vapor deposition polymerization (VDP) on electrospun nickel oxide nanofibers (NiO NFs), followed by carbonization. To decorate the surface with amorphous carbon, the NiO NF starting materials were prepared by electrospinning a PVP solution containing a nickel oxide precursor (NiAc2·4H2O) and calcining the electrospun NFs. Then, polypyrrole (PPy)-coated NiO nanofibers (NiP NFs) were fabricated as intermediate materials using the pyrrole monomer in the VDP method. Finally, carbonization of the NiP NFs converted the PPy into amorphous carbon and thereby formed the NiC NFs. According to X-ray diffraction (XRD) and Barrett–Joyner–Halenda (BJH) analyses, the NiO structure was maintained during the PPy coating and heat treatment processes. Furthermore, a new pore structure was formed with each fabrication step. The NiC NFs were used as electrochemical capacitors (ECs) with 1 M KOH as the electrolyte. The electrochemical results show that NiC NFs with a thin coating (NiC_L) had a higher specific capacitance (288 F g−1 at 0.3 A g−1) and longer cycle stabilization (89% capacitance maintained after 3000 cycles) than pristine NiO NFs (221 F g−1 at 0.3 A g−1; 56% capacitance maintained after 3000 cycles). Herein, the synthetic methodology is an effective route to obtain hybrid core (inorganic)–shell (organic) nanostructures for electrochemical applications.

Size-controlled SiO2 nanoparticles as scaffold layers in thin-film perovskite solar cells

Perovskite-based solar cells have received much recent research attention for renewable-energy applications because of their high efficiency and long-term stability. Here, we report perovskite solar cells formed using a scaffold layer composed of size-controlled SiO2 nanoparticles (NPs). The infiltration of perovskite into the scaffold layer depended strongly on the size of the SiO2 NPs. We investigated the effects of scaffold layers comprised of SiO2 NPs that were 15, 30, 50, 70, and 100 nm in diameter on the properties of perovskite films. The performance of perovskite solar cells based on 50 nm diameter SiO2 NPs exhibited a current density (Jsc) of 16.4 mA cm−2, a open-circuit voltage (Voc) of 1.05 V, and a power-conversion efficiency (PCE) of 11.45%, which represent a significant improvement compared with perovskite solar cells fabricated using a TiO2 scaffold layer, where Jsc = 17.3 mA cm−2, Voc = 0.94 V, and the PCE was 10.29%.

Antimicrobial polymer nanostructures: Synthetic route, mechanism of action and perspective

Protection against bacterial infections is an important research field in modern society. Antimicrobial polymers have received considerable attention as next-generation biocides because they represent an ecologically friendly approach that does not promote resistance. In the last decade, many authors have reported the development of nano-sized antimicrobial polymers with enhanced bactericidal performance by increasing the active-area of biocides. This review presents several suitable methods of synthesis of antimicrobial polymer nanomaterials with various shapes, including a nanosphere and fibrous and tubular structures. We also discuss the antimicrobial mechanisms of these polymers. In addition, antimicrobial polymer thin films, which can inhibit bacterial adhesion, are introduced briefly with examples. Our aim is to present synthetic routes and formation mechanisms of various antimicrobial polymer nanostructures.

Enhanced electrorheological performance of a graphene oxide-wrapped silica rod with a high aspect ratio

In this study, the influence of particle geometry on electrorheological (ER) activity is examined by varying the aspect ratio of graphene oxide (GO)-wrapped silica materials. The GO-wrapped silica material-based ER fluid exhibits a high shear stress as the aspect ratio of the particle increases; this is attributed to the flow resistance and mechanical stability of the fluid. Additionally, the dielectric loss model is used to investigate the dielectric properties of ER fluids, which have been shown to be associated with the enhancement of ER activity. The GO-wrapped silica material with a higher aspect ratio exhibits a higher dielectric constant and shorter relaxation times for interfacial polarization, due to greater polarizability. Thus, the aspect ratio of GO-wrapped silica materials plays a prominent role in the enhancement of ER performance.

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.

Three‐Dimensional Scaffolds of Carbonized Polyacrylonitrile for Bone Tissue Regeneration

Carbon-based materials have been extensively studied for stem cell culture. However, difficulties associated with engineering pure carbon materials into 3D scaffolds have hampered applications in tissue engineering and regenerative medicine. Carbonized polyacrylonitrile (cPAN) could be a promising alternative, as cPAN is a highly ordered carbon isomorph that resembles the graphitic structure and can be easily processed into 3D scaffolds. Despite the notable features of cPAN, application of cPAN in tissue engineering and regenerative medicine have not been explored. This study, for the first time, demonstrates the fabrication of microporous 3D scaffolds of cPAN and excellent osteoinductivity of cPAN, suggesting utility of 3D cPAN scaffolds as synthetic bone graft materials. The combination of excellent processability and unique bioactive properties of cPAN may lead to future applications in orthopedic regenerative medicine.

Fabrication of Ag-coated AgBr nanoparticles and their plasmonic photocatalytic applications

Ag-coated AgBr nanoparticles (Ag@AgBr) were fabricated via an aqueous, one-pot route. In this system, poly(vinyl alcohol) (PVA) acted as a stabilizer for the formation of nanoscale AgBr composites through interaction with Ag ions and their hydroxyl (–OH) groups. The mild reducing agent L-arginine was used for partial reduction of AgBr to form metallic Ag nanoparticles on the AgBr surface. The metallic Ag nanoparticles enhanced light absorption in the visible region due to surface plasmon resonance (SPR). The size of the synthesized nanoparticles was controlled by varying the reaction temperature, and was found to influence the light absorption of Ag@AgBr nanocomposites. The prepared nano-photocatalysts exhibited excellent photocatalytic activities under both visible light and direct sunlight.

Enhanced electrorheological performance of barium-doped SiO2/TiO2 hollow mesoporous nanospheres

Barium-doped SiO2/TiO2 hollow mesoporous nanospheres (Ba-HNSs) exhibited 22.5 times higher maximum yield stress than commercial BaTiO3 nanopowders, and 4 times faster ER response than pristine. These superior ER properties were originated from the unique hollow mesoporous structure and high dielectric properties of BaTiO3 in the Ba-HNSs.