Yolk-shell structured Fe3O4@void@TiO2 as a photo-Fenton-like catalyst for the extremely efficient elimination of tetracycline

Herein, we demonstrate the yolk-shell structured Fe3O4@[email protected]2 sphere as an efficient heterogeneous catalyst for the photo-Fenton-like degradation of tetracycline (TC). The composite was synthesized by first successively coating amorphous SiO2 and TiO2 shells around a Fe3O4 core via the sol-gel strategy to form Fe3O4@SiO2@TiO2, then crystallizing TiO2 and removing SiO2 through calcination and the ultrasonic ammonia-etching treatment, respectively. The particles are monodisperse and uniform in size (ca. 180 nm), where the diameter of the core is ca. 100 nm and the thickness of the shell is ca. 10 nm. This composite possesses anatase TiO2 shell with high crystallinity, superparamagnetic core with a saturation magnetization value of 28.71 emu/g and high specific surface area of 101 m2 g−1. It shows extremely high activity towards the degradation of TC (40 mg/L) in a wide pH range as demonstrated by almost 100% elimination efficiency at pH = 3 and ca. 75% at pH = 9 within 6 min. Moreover, benefitting from the superparamagnetism of the core, this composite could be magnetically recovered and reapplied in the TC degradation without significant loss of activity after 5 recycling (ca. 10%). The degradation curve can be well fitted by pseudo-first order model, where a kinetic constant of 0.51 min−1 is achieved, much higher than those of Fe3O4@TiO2 (0.24 min−1), hollow TiO2 (0.17 min−1), Fe3O4@SiO2@TiO2 (0.14 min−1) and Fe3O4 (0.11 min−1). The high activity is attributed to the efficient enrichment and confinement of reactants (TC and hydroxyl radicals) in the nanocavity of the yolk-shell structure, and the efficient reduction of Fe3+ to Fe2+ by the photo-generated electrons from the TiO2 shell.