Chang-Min Yoon

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.

Electrorheological performance of multigram-scale mesoporous silica particles with different aspect ratios

Mesoporous silica (mSiO2) particle-based electrorheological (ER) fluids were examined to determine the effect of the particles’ aspect ratio on ER performance. Multigram-scale mSiO2 particles were fabricated with different aspect ratios using the sol–gel method. The ER performance of various mSiO2 particle-based ER fluids improved with an increasing aspect ratio due to the better flow resistance and mechanical stability. Moreover, an incremental increase in the aspect ratio enhanced the interfacial polarization of the material. Thus, mSiO2 materials with a high aspect ratio exhibited the highest ER performance due to combined contributions from geometrical effects and their dielectric properties. In addition, various mSiO2-based ER fluids showed potential for large-scale production and a wide applicable range of electric field strengths (up to 10.0 kV mm−1).

Enhanced Electroresponse of Alkaline Earth Metal-Doped Silica/Titania Spheres by Synergetic Effect of Dispersion Stability and Dielectric Property

A series of alkaline earth metal-doped hollow SiO2/TiO2 spheres (EM-HST) are prepared as electrorheological (ER) materials via sonication-mediated etching method with various alkaline earth metal hydroxides as the etchant. The EM-HST spheres are assessed to determine how their hollow interior and metal-doping affects the ER activity. Both the dispersion stability and the dielectric properties of these materials are greatly enhanced by the proposed one-step etching method, which results in significant enhancement of ER activity. These improvements are attributed to increased particle mobility and interfacial polarization originating from the hollow nature of the EM-HST spheres and the effects of EM metal-doping. In particular, Ca-HST-based ER fluid exhibits ER performance which is 7.1-fold and 3.1-fold higher than those of nonhollow core/shell silica/titania (CS/ST) and undoped hollow silica/titania (HST)-based ER fluids, respectively. This study develops a versatile and simple approach to enhancing ER activity through synergetic effects arising from the combination of dispersion stability and the unique dielectric properties of hollow EM-HST spheres. In addition, the multigram scale production described in this experiment can be an excellent advantage for practical and commercial ER application.

Enhanced electrorheological activity of polyaniline coated mesoporous silica with high aspect ratio.

Polyaniline-coated mesoporous silica (PANI/mSiO2) materials with different aspect ratios (L/D=1, 5, and 10) were fabricated by a vapor deposition polymerization (VDP) method to investigate the geometric effect on electrorheological (ER) activity. The PANI/mSiO2 materials were dedoped by a facile NH4OH treatment to reduce the conductivity to a level appropriate for ER applications. Notably, the PANI/mSiO2-based ER fluids exhibited enhanced ER performance with increasing aspect ratio. In particular, the PANI/mSiO2 material with the highest aspect ratio manifested the highest ER activity, which was attributed to geometric effects on flow resistance and mechanical stability. Moreover, the ER materials with higher aspect ratios showed improved dielectric properties of large achievable polarizability and short relaxation time. Hence, the synergistic contribution of geometric effects and dielectric properties resulted in enhanced ER activity. Consequently, this study provides insight into an effective method to improve ER performance by simple manipulation of the particle geometry.

High-Performance Three-Dimensional Mesoporous Graphene Electrode for Supercapacitors using Lyophilization and Plasma Reduction

In this study, a three-dimensional (3D) mesoporous plasma-reduced graphene oxide web (mPrGO web) was fabricated via lyophilization of graphene oxide (GO) solution and subsequent plasma reduction. The lyophilized graphene oxide web (GO web) was successfully reduced by a short plasma treatment (<2 s) using a commercially available plasma apparatus. The degree of reduction of the mPrGO web was determined by the applied plasma power (W) of the apparatus; the optimum power level for effective reduction was identified. The as-synthesized mPrGO web showed a high degree of reduction and robust graphitic characteristics, with a unique crack-like mesoporous structure created on corrugated graphene sheets. In addition to the above characteristics, the mPrGO web possessed a 3D web-like architecture that provided enhanced surface area along with ion-transportable channels derived from lyophilization. Owing to the synergistic effect of lyophilization and plasma reduction, the mPrGO web exhibited high electrical conductivity (87 S cm-1) and increased surface area (642 m2 g-1). Accordingly, the mPrGO web showed outstanding specific capacitance of 253.8 F g-1 at 0.2 A g-1 along with the excellent rate capability (76% capacitance retention at 5 A g-1). Furthermore, the assembled all-solid-state symmetric supercapacitor also exhibited remarkable electrochemical performances, demonstrating the potential applicability of the mPrGO web as an effective supercapacitor electrode material.

Fabrication of a silica/titania hollow nanorod and its electroresponsive activity

In this study, a 1D oriented hollow SiO2/TiO2 (HST) rod-like material was successfully fabricated via a sequential combination of sol–gel use, TiO2 incorporation, and a sonication-mediated etching and redeposition method. This carefully manipulated new material has numerous advantageous physical and intrinsic properties, such as increased surface area, pore volume, interfacial polarization, and dielectric properties introduced from each synthetic step. The synthesized HST rod was adopted as an electrorheological (ER) material for practical examination of these characteristics. The HST rod materials exhibited 1.5- and 3-fold higher ER performance than a non-metal SiO2 rod and a non-hollow SiO2/TiO2 core/shell (ST/CS) rod, which are interim synthetic steps. Moreover, the HST rod exhibited remarkable 6-fold increased ER efficiency relative to a sphere-shaped hollow SiO2/TiO2 particle synthesized using a similar experimental method. These notable enhancements in ER performance are attributed to incorporation of the experimentally designed characteristics of the HST rod: 1D structure, metal oxide incorporation, and creation of a hollow cavity. For future study, we expect that these versatile HST rod materials can be applied in a range of fields including drug delivery, photo-catalysis, and as building blocks.

Smart Fluid System Dually Responsive to Light and Electric Fields: An Electrophotorheological Fluid

Electrophotorheological (EPR) fluids, whose rheological activity is dually responsive to light and electric fields (E fields), is formulated by mixing photosensitive spiropyran-decorated silica (SP-sSiO2) nanoparticles with zwitterionic lecithin and mineral oil. A reversible photorheological (PR) activity of the EPR fluid is developed via the binding and releasing mechanism of lecithin and merocyanine (MC, a photoisomerized form of SP) under ultraviolet (UV) and visible (VIS) light applications. Moreover, the EPR fluid exhibits an 8-fold higher electrorheological (ER) performance compared to the SP-sSiO2 nanoparticle-based ER fluid (without lecithin) under an E field, which is attributed to the enhanced dielectric properties facilitated by the binding of the lecithin and SP molecules. Upon dual application of UV light and an E field, the EPR fluid exhibits high EPR performance (ca. 115.3 Pa) that far exceeds its separate PR (ca. 0.8 Pa) and ER (ca. 57.5 Pa) activities, because of the synergistic contributions of the PR and ER effects through rigid and fully connected fibril-like structures. Consequently, this study offers a strategy on formulation of dual-stimuli responsive smart fluid systems.

Synthesis of Hierarchical Silica/Titania Hollow Nanoparticles and Their Enhanced Electroresponsive Activity

Wrinkled silica nanoparticle (WSN)-based hollow SiO2/TiO2 nanoparticles (W-HNPs) with hierarchically arrayed internal surfaces were prepared via the combination of sol-gel, TiO2 coating, and etching of core template techniques. The hierarchical internal surface of W-HNPs was attained using WSNs as a core template. Compared with SiO2 sphere-templated hollow SiO2/TiO2 nanoparticles (S-HNPs) with flat inner surfaces, W-HNPs displayed distinctive surface areas, TiO2 loading amounts, and dielectric properties arising from the hierarchical internal surface. The unique properties of W-HNPs were further investigated as an electrorheological (ER) material. W-HNP-based ER fluids exhibited ca. 1.9-fold enhancement in the ER efficiency compared to that of S-HNP-based ER fluids. Such enhancement was attributed to the unique inner surface of W-HNPs, which effectively enhanced the polarizability by increasing the number of charge accumulation sites, and to the presence of the high-dielectric TiO2. This study demonstrated the advantages, in terms of practical ER applications, of hollow nanomaterials having uniquely arrayed internal spaces.

Enhanced Electrorheological Performance of Mixed Silica Nanomaterial Geometry

The mixed geometrical effect on the electrorheological (ER) activity of bimodal ER fluids was investigated by mixing SiO2 spheres and rods of different dimensions. To gain an in-depth understanding of the mixed geometrical effect, 12 bimodal ER fluids were prepared from 4 sizes of SiO2 spheres (50, 100, 150, and 350 nm) and 3 types of SiO2 rods with different aspect ratios (L/D = 2, 3, and 5). Five concentrations of SiO2 spheres and rods were created for each bimodal ER fluid, resulting in a total of 60 sets of comprehensive ER measurements. Some bimodal ER fluids exhibited enhanced ER performance, as high as 23.0%, compared to single SiO2 rod-based ER fluids to reveal the mixed geometrical effect of bimodal ER fluids. This interesting experimental result is based on the structural reinforcement provided by spheres to fibrillated rod materials, demonstrating the mixed geometrical effect on ER activity.

Dual electric and magnetic responsivity of multilayered magnetite-embedded core/shell silica/titania nanoparticles with outermost silica shell

Multilayered magnetite-embedded core/shell silica/titania (SiO2/TiO2) nanoparticles with an outermost silica shell (SiO2/TiO2@Fe3O4/SiO2) were synthesized and used to develop stimuli-responsive smart fluids. Benefiting from the incorporation of the various materials, these smart fluids demonstrated electrorheological (ER) and magnetorheological (MR) activities under applied external electric (E) and magnetic (H) fields, respectively, and electromagnetorheological (EMR) characteristics with the simultaneous application of E and H fields. The inner SiO2/TiO2 core nanoparticles, embedded Fe3O4 nanoparticles and the outer SiO2 shell served as electroresponsive, magnetoresponsive and preventative materials toward corrosion, sedimentation and aggregation. The EMR performance of these fluids depended on the direction of the applied E and H fields. Notably, a 6.6-fold enhancement in EMR activity was observed with parallel application of E and H fields compared to perpendicular direction. This study demonstrates an effective approach to precisely and spatially control the rheological/mechanical properties of dual-responsive smart fluids via both field-induced and directional control of external fields.