Jong-Dal Hong

Buildup of ultrathin multilayer films by a self-assembly process: III. Consecutively alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces

A solid substrate with a positively charged planar surface is immersed in a solution containing an anionic polyelectrolyte and a monolayer of the polyanion is adsorbed. Since the adsorption is carried out at relatively high concentrations of polyelectrolyte, a large number of ionic residues remain exposed to the interface with the solution and thus the surface charge is effectively reversed. After rinsing in pure water the substrate is immersed in the solution containing a cationic polyelectrolyte. Again a monolayer is adsorbed but now the original surface charge is restored. By repeating both steps in a cyclic fashion, alternating multilayer assemblies of both polymers are obtained. The buildup of the multilayer films was followed by UV/vis spectroscopy and small angle X-ray scattering (SAXS). It is demonstrated that multilayer films composed of at least 100 consecutively alternating layers can be assembled.

Fabrication of Highly Ordered Multilayer Films Using a Spin Self‐Assembly Method

This work was supported by the Ministry of Education through the Brain Korea 21 Program at Seoul National University and by the National Program for Tera-level Nano-devices of the Ministry of Science and Technology as one of the 21st century Frontier Programs as well as by the Korean Ministry of Science and Technology (MOST) under Grant 99-07. X-ray reflectivity experiments performed at the Pohang Light Source (PLS) were supported in part by MOST and POSCO. We are very grateful to S.-H. Lee, H. Kang, J. Koo, and B. H. Seung for their assistance during the X-ray reflectivity experiments.

Highly Compressible Macroporous Graphene Monoliths via an Improved Hydrothermal Process

An improved hydrothermal process is developed to fabricate macroporous graphene monoliths (MGMs) using a soft template of organic droplets. The MGMs are constructed from closed-cell distorted spherical pores. This unique microstructure makes MGMs that have low weight densities, good electrical conductivities, and excellent elasticity with rapid recovery rates.

Layer-by-Layer Self-Assembled Multilayer Films Composed of Graphene/Polyaniline Bilayers: High-Energy Electrode Materials for Supercapacitors

Multilayer assemblies of uniform ultrathin film electrodes with good electrical conductivity and very large surface areas were prepared for use as electrochemical capacitors. A layer-by-layer self-assembly approach was employed in an effort to improve the processability of highly conducting polyaniline (PANi) and chemically modified graphene. The electrochemical properties of the multilayer film (MF-) electrodes, including the sheet resistance, volumetric capacitance, and charge/discharge ratio, were determined by the morphological modification and the method used to reduce the graphene oxide (GO) to reduced graphene oxide (RGO) in the multilayer films. The PANi and GO concentrations could be modulated to control the morphology of the GO monolayer film in the multilayer assemblies. Optical ellipsometry was used to determine the thickness of the GO film in a single layer (1.32 nm), which agreed well with the literature value (~1.3 nm). Hydroiodic acid (HI), hydrazine, or pyrolysis were tested for the reduction of GO to RGO. HI was found to be the most efficient technique for reducing the GO to RGO in the multilayer assemblies while minimizing damage to the virgin state of the acid-doped PANi. Ultimately, the MF-electrode, which could be optimized by fine-tuning the nanostructure and selecting a suitable reduction method, exhibited an excellent volumetric capacitance, good cycling stability, and a rapid charge/discharge rate, which are required for supercapacitors. A MF-electrode composed of 15 PANi/RGO bilayers yielded a volumetric capacitance of 584 F/cm(3) at a current density of 3.0 A/cm(3). Although this value decreased exponentially as the current density increased, approaching a value of 170 F/cm(3) at 100 A/cm(3), this volumetric capacitance is one of the best yet reported for the other carbon-based materials. The intriguing features of the MF-electrodes composed of PANi/RGO multilayer films offer a new microdimensional design for high energy storage devices for use in small portable electronic devices.

Performance enhancement of a graphene–sulfur composite as a lithium–sulfur battery electrode by coating with an ultrathin Al2O3 film via atomic layer deposition

A graphene–sulfur (G–S) composite was conformally coated with an ultrathin Al2O3 film via atomic layer deposition (ALD) and used as the cathode of a lithium–sulfur (Li–S) battery. The G–S composite cathode with an ALD-Al2O3 coating delivered a high specific capacity of 646 mA h g−1 after 100 charge–discharge cycles at 0.5 C, and this value is about twice that of the bare G–S composite. The rate capability and coulombic efficiency of the G–S composite electrode were also greatly increased. The ALD-Al2O3 coating worked as an artificial barrier to suppress the dissolution of polysulfides and alleviate the shuttle effect; thus, it effectively improved the performance of a G–S composite cathode in a Li–S battery.

A graphene wrapped hair-derived carbon/sulfur composite for lithium–sulfur batteries

A hair-derived carbon/sulfur composite was prepared via a facile melt-diffusion strategy and successively wrapped with reduced graphene oxide (rGO) sheets by electrostatic self-assembly. This composite was used as the sulfur electrode for lithium–sulfur (Li–S) batteries, exhibiting high capacity, good rate capability, and excellent cyclability. The electrode containing 69.0% (by weight, wt%) sulfur delivered an initial discharge capacity of 1113.2 mA h g−1 and 989.2 mA h g−1 after 300 cycles at a current density of 0.2 C with an average coulombic efficiency of 99.3%. Its capacity retention at 2 C was measured to be 62% with respect to the capacity achieved at 0.2 C. The high-performance of this electrode in Li–S batteries can be attributed to the porous carbon infrastructure, inherent nitrogen-doping and graphene protection. Taking into account the low-cost of raw materials and easy scalable processes, this work features a promising approach to prepare sulfur/carbon composites for high-performance Li–S batteries.

Photoresponsive ion gating function of an azobenzene polyelectrolyte multilayer spin-self-assembled on a nanoporous support.

The fabrication of a polyelectrolyte multilayer (PEM) on a porous membrane was successfully improved by using spin-coating electrostatic self-assembly. Surprisingly, the quality of the PEM film obtained on the nanoporous alumina substrate (i.e., its thickness and surface morphology) was comparable to that of a film deposited on silicon. An optical molecular switch that acts as an ion-gating channel was realized using a PEM membrane deposited layer-by-layer on an alumina support. One of the layer components of this device was a poly(acrylamide) copolymer containing an azobenzene chromophore, which is known to reveal strong voluminous expansion and contraction during light-induced reversible cis/trans isomerizations. The permeability of the bulk SO4(2-) ions was found to be sensitive to the changed channel sizes; for instance, the ion-permeation rate of SO4(2-) increased about 1.6 times after UV irradiation of the PEM, whereas that of the Cl- ion increased only 1.2 times. In the study, it was successfully demonstrated that the ion flow through the PEM membrane could be reversibly switched on and off over several azobenzene isomerization cycles.

Fabrication of flexible reduced graphene oxide–TiO2 freestanding films for supercapacitor application

This paper described the fabrication of a flexible composite film electrode (rGT) composed of reduced graphene oxide (rGO) and TiO2 in an aligned stacking structure of rGO/TiO2 layers using a simple vacuum-assisted filtration method. The SEM images of rGO and rGT films indicate that the rGO film (d = 5.6 ± 0.5 μm) was highly expanded into a layered structure of rGT (d = 7.2 ± 0.5 μm) intercalated by the TiO2 nanoparticles, which prevent the interlayer stacking of graphene sheets. The intensities of C–O and CO bonding peaks in X-ray photoelectron spectra of rGT films decreased dramatically, as the reduction temperature was increased from 100 to 600 °C, indicating almost complete removal of the oxygenated functional groups. Hence, the electrochemical properties of rGT film electrodes significantly relied on the reduction temperature of graphene oxide (GO)–TiO2 film. The rGT600 electrode (annealed at 600 °C) in 1.0 M Na2SO4 aqueous electrolyte exhibited the specific capacitance of 286 F g−1 in addition to the excellent cycling stability with 93% capacitance retention after 1000 continuous charge/discharge cycles at 1 A g−1. The specific capacitance of the rGT600 electrode in the expanded interlayer stacking structure of rGO and TiO2 particles was by 63% higher than that of the three-dimensional TiO2–graphene hydrogel electrode (175 F g−1, 1 A g−1) prepared using a one-step hydrothermal approach. The modification in the preparation of the composite films comprising rGO sheets and TiO2 particles optimized the supercapacitor electrodes into the expanded interlayer stacking rGO/TiO2 structure, exhibiting excellent electrochemical performance, which is required for the development of advanced micro-size and flexible electrodes in energy storage devices.

A triple thermoresponsive schizophrenic diblock copolymer

This article describes a new class of triple thermoresponsive ‘schizophrenic’ diblock copolymer that undergoes transitions from conventional micelles (or vesicles) via unimers to reverse micelles (or vesicles) and finally to a precipitated state upon heating. The various transition temperatures of this copolymer could be well controlled by the concentration of trivalent anion and pH that control the upper critical solution temperature of the poly(dimethylaminoethyl methacrtylate) (PDMAEMA) block or the type of poly(methoxy oligoethylenegylol methacrylate) (PmOEGA) to vary the lower critical solution temperature.

Morphology of multilayers assembled by electrostatic attraction of oppositely charged model polyelectrolytes

Abstract Alternating multilayers assembled stepwise with two prototype polyelectrolytes, i.e. poly(allylamine) hydrochloride (PAA) and poly(vinyl sulfate) potassium salt (PVS), have been examined by means of reflection–absorption infrared (RAIR) spectroscopy and atomic force microscopy (AFM). The growth step revealed by the RAIR spectroscopy was in good agreement with the ellipsometric data. The RAIR spectral data revealed that both macromolecules were randomly oriented in the (PVS/PAA) composite system without any preferential alignment. The atomic force microscopy measurements showed that the morphology of the (PVS/PAA) multilayers prepared in the absence of NaCl were quite homogeneous in agreement with a recent report. However, the morphology of the multilayers prepared in the presence of NaCl was observed to be significantly heterogeneous; the morphology was gradually roughened upon an increase in the number of bilayers. The highly coiled, compact macromolecules present in a NaCl solution seemed preferentially to anchor on the oppositely charged, aggregated surface sites without spreading over the interfacial region. Nonetheless, the morphology of the surface of the (PVS/PAA) system was hardly subjected to change once a stable film had been assembled, regardless of the presence of NaCl.