Complex ZnO-TiO₂ Core-Shell Flower-Like Architectures with Enhanced Photocatalytic Performance and...

ZnO-TiO₂ core-shell photocatalysts of a complex flower-like architecture were synthesized, using a well-controlled sol-gel coating reaction of presynthesized ZnO flower-like structures. The samples were characterized by X-ray diffraction, field emission scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, diffuse reflectance UV-vis and attenuated total reflectance-Fourier transform infrared spectroscopy, nitrogen adsorption-desorption measurements, and photoluminescence measurements. Well-defined, core-shell flowers with a wurtzite ZnO core and anatase TiO₂ shells, with variable shell thickness, were acquired by appropriately adjusting the ZnO/Ti precursor mass feed ratio in the reaction. Moreover, hollow TiO₂ flowers were obtained, and they retained their morphology following the etching of the ZnO core in an acidic solution. The photocatalytic performance of the core-shell and hollow semiconductors was evaluated via the decoloration of a methylene blue dye solution under UV-vis irradiation. The core-shell flowers exhibited a higher decoloration rate, when compared with bare ZnO flowers, TiO₂ particles, and hollow TiO₂ flowers, and the photoactivity was dependent on the TiO₂ shell thickness. This was attributed to the efficient separation of the photogenerated holes and electrons at the ZnO-TiO₂ interface. Moreover, the most photoactive core-shell catalyst exhibited excellent reusability and stability for at least three photocatalytic cycles and excellent superhydrophilicity without UV irradiation, which is due to the increased roughness of the flower-like structures.