Fabrication of SiO2-ZnO Core-Shell Urchin-Like Structure by Hydrothermal Method Using Self-Assembled Particles as Nuclei and Application to UV-Activated Gas Sensors

This study aims to improve the efficiency of gas sensors with a zinc oxide (ZnO) structure by widening the surface area for reaction and using UV-activation. The silica (SiO2)-ZnO core-shell urchin-like structure is a promising candidate to achieve this aim, due to its broad surface area and electrically insulated formation. The higher resistivity of silica prevents the escape of electrons and recombination during reaction with gas; thus, improving its sensitivity. The structure was fabricated by a two-step process. First, ZnO-silica core-shell structures were produced. ZnO nanoparticles (φ≤ 34 nm) self-assembled to form a shell around a core comprising silica particles (φ5 μm). Gravity sedimentation was then used to obtain the silica particles, while the ZnO particles were obtained by dropping and drying of the suspension. Closely packed structures were obtained due to the meniscus attraction between the particles at the drying stage of the suspension. Second, ZnO urchin-like structures were synthesized on the silica particles using the hydrothermal method, with the originally placed ZnO nanoparticles as the nuclei. The method is a simple material synthesis involving the crystal growth process in a sealed container, in which substrates and precursors are stored and maintained at an elevated temperature. The obtained structure (or morphology) changed depending on the nucleation and growth conditions. The appropriate conditions were clarified through systematic experiments. Finally, the gas sensor performance was examined.