Herein, unique three-dimensional (3D) hierarchically structured carbon nanofiber (CNF)/metal oxide/conducting polymer composite materials were successfully synthesized by combinations of various experimental methods. Firstly, base CNFs were synthesized by carbonization of electrospun PAN/PVP fibers to attain electric double-layer capacitor (EDLC) characteristics. To further enhance the capacitance, tin oxide (SnO₂) and iron oxide (Fe₂O₃) were coated onto the CNFs via facile hydrothermal treatment. Finally, polypyrrole (PPy) was introduced as the outermost layer by a dispersion polymerization method under static condition to obtain 3D-structured CNF/SnO₂/PPy and CNF/Fe₂O₃/PPy materials. With each synthesis step, the morphology and dimension of materials were transformed, which also added the benign characteristic for supercapacitor application. For the practical application, as-synthesized CNF/SnO₂/PPy and CNF/Fe₂O₃/PPy were applied as active materials for supercapacitor electrodes, and superb specific capacitances of 508.1 and 426.8 F g⁻¹ (at 1 A g⁻¹) were obtained (three-electrode system). Furthermore, an asymmetric supercapacitor (ASC) device was assembled using CNF/SnO₂/PPy as the positive electrode and CNF/Fe₂O₃/PPy as the negative electrode. The resulting CNF/SnO₂/PPy//CNF/Fe₂O₃/PPy device exhibited excellent specific capacitance of 101.2 F g⁻¹ (at 1 A g⁻¹). Notably, the ASC device displayed a long-term cyclability (at 2000 cycles) with a retention rate of 81.1%, compared to a CNF/SnO₂//CNF/Fe₂O₃ device of 70.3% without an outermost PPy layer. By introducing the outermost PPy layer, metal oxide detachment from CNFs were prevented to facilitate long-term cyclability of electrodes. Accordingly, this study provides an effective method for manufacturing a high-performance and stable supercapacitor by utilizing unique 3D hierarchical materials, comprised of CNF, metal oxide, and conducting polymer.

Hollow TiO₂ nanoparticles (HNPs) capped with conducting polymers, such as polythiophene (PT), polypyrrole (PPy), and polyaniline (PANI), have been studied to be used as polarizability-tunable electrorheological (ER) fluids. The hollow shape of TiO₂ nanoparticles, achieved by the removal of the SiO₂ template, offers colloidal dispersion stability in silicone oil owing to the high number density. Conducting polymer shells, introduced on the nanoparticle surface using vapor deposition polymerization method, improve the yield stress of the corresponding ER fluids in the order of PANI < PPy < PT. PT-HNPs exhibited the highest yield stress of ca. 94.2 Pa, which is 5.0-, 1.5-, and 9.6-times higher than that of PANI-, PPy-, and bare HNPs, respectively. The improved ER response upon tuning with polymer shells is attributed to the space charge contribution arising from the movement of the charge carriers trapped by the heterogeneous interface. The ER response of studied ER fluids is consistent with the corresponding polarizability results as indicated by the permittivity and electrophoretic mobility measurements. In conclusion, the synergistic effect of hollow nanostructures and conducting polymer capping effectively enhanced the ER performance.

Light detection and ranging (LiDAR) sensors utilize a near-infrared (NIR) laser with a wavelength of 905 nm. However, LiDAR sensors have weakness in detecting black or dark-tone materials with light-absorbing properties. In this study, SiO₂/black TiO₂ core/shell nanoparticles (SBT CSNs) were designed as LiDAR-detectable black materials. The SBT CSNs, with sizes of 140, 170, and 200 nm, were fabricated by a series of Stöber, TTIP sol-gel, and modified NaBH₄ reduction methods. These SBT CSNs are detectable by a LiDAR sensor and, owing to their core/shell structure with intrapores on the shell (ca. 2–6 nm), they can effectively function as both color and NIR-reflective materials. Moreover, the LiDAR-detectable SBT CSNs exhibited high NIR reflectance (28.2 R%) in a monolayer system and true blackness (L* < 20), along with ecofriendliness and hydrophilicity, making them highly suitable for use in autonomous vehicles.

A facile oxygen (O₂) atmospheric plasma treatment is applied to a polyvinyl alcohol (PVA) matrix to enhance its wettability and hydrophilicity. The optimal plasma treatment conditions are determined by varying the applied plasma power and plasma treatment time. A PVA matrix treated with a plasma power of 120 W for 5 s shows the most hydrophilicity owing to successful formation of carbonyl (–CO, >C=O) functional groups without any structural degradation. The plasma-treated PVA matrix is used as the gel-polymer electrolyte of a solid-state supercapacitor (SSC) by immersing solid matrix into various liquid electrolytes, such as sodium sulfate (Na₂SO₄), sulfuric acid (H₂SO₄), and potassium hydroxide (KOH). Compared with the pristine PVA-based device, PVA-120W5/Na₂SO₄-, PVA-120W5/H₂SO₄-, and PVA-120W5/KOH-based SSCs show 2.03, 2.05, and 2.14 times higher specific capacitances, respectively. The plasma-treated PVA matrix shows increased specific capacitance owing to the increased wettability, which in turn increases the ion transportation and reduces the electrical resistance. This study successfully demonstrates that the electrochemical performance of a SSC can be readily enhanced through plasma treatment for a short time (≤5 s).

Surface modification of dielectric materials changes the dipole-dipole interactions under electric fields, thereby controlling the electrorheological (ER) response. The introduction of metal oxides onto mica templates and further coating of dyes is expected to simultaneously improve the color clarity and ER performance. Dye-coated TiO₂ platelets on mica are synthesized for high-performance colorful ER fluids. A sol-gel method is utilized to grow TiO₂ on mica to prepare precursor light-colored mica/TiO₂ materials, which are coated with appropriate dyes to enhance the vividness as determined by the Commission Internationale de clairage L*a*b* color system. The color expression and color clarity improvement are explained via the light interference effect and the presence of chromophores. The uniform TiO₂ layers can be obtained under low pH conditions with controlled nucleation kinetics. The addition of dyes to TiO₂ increases the surface area and porosity of ER materials and introduces heteroatoms that act as positive factors. In practical ER applications, dye-coated TiO₂-based ER fluids exhibit higher ER performances compared with the corresponding light-colored TiO₂-based ER fluids. The vivid-colored ER fluids could provide an easy selection for a wide range of rheological systems requiring a specific magnitude of stress by confirming the color.

Herein, the electrorheological (ER) performances of ER fluids were correlated with their colors to allow for the visual selection of the appropriate fluid for a specific application using naked eyes. A series of TiO₂-coated synthetic mica materials colored white, yellow, red, violet, blue, and green (referred to as color mica/TiO₂ materials) were fabricated via a facile sol–gel method. The colors were controlled by varying the thickness of the TiO₂ coating layer, as the coatings with different thicknesses exhibited different light interference effects. The synthesized color mica/TiO₂ materials were mixed with silicone oil to prepare colored ER fluids. The ER performances of the fluids decreased with increasing thickness of the TiO₂ layer in the order of white, yellow, red, violet, blue, and green materials. The ER performance of differently colored ER fluids was also affected by the electrical conductivity, dispersion stability, and concentrations of Na⁺ and Ca²⁺ ions. This pioneering study may provide a practical strategy for developing new ER fluid systems in future.