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.