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 Fe 3 O 4 @void@TiO 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 SiO 2 and TiO 2 shells around a Fe 3 O 4 core via the sol-gel strategy to form Fe 3 O 4 @SiO 2 @TiO 2 , then crystallizing TiO 2 and removing SiO 2 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 TiO 2 shell with high crystallinity, superparamagnetic core with a saturation magnetization value of 28.71 emu/g and high specific surface area of 101   m 2   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 Fe 3 O 4 @TiO 2 (0.24   min −1 ), hollow TiO 2 (0.17   min −1 ), Fe 3 O 4 @SiO 2 @TiO 2 (0.14   min −1 ) and Fe 3 O 4 (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 Fe 3+ to Fe 2+ by the photo-generated electrons from the TiO 2 shell.