Template-free synthesis of zinc citrate yolk-shell microspheres and their transformation to ZnO yolk-shell nanospheres

A novel, facile, green and template-free approach was developed for the fabrication of amorphous zinc citrate yolk-shell microspheres and crystalline ZnO yolk-shell nanospheres. In this approach, the amorphous zinc citrate yolk-shell microspheres were prepared through a single chemical reaction at low temperature (90 degrees C) and with room temperature ageing. The zinc citrate yolk-shell microspheres have an average size of about 1.25 mu m. The average diameter of the inner cores and the average thickness of the outer shells are 500 nm and 150 nm, respectively. The effect of ageing time on the morphology of the zinc citrate microstructures was analysed. As the ageing time increases from 0 to 12 h, the zinc citrate microstructure transforms from a solid microsphere through a yolk-shell microsphere to a hollow microsphere. The ZnO solid nanospheres, yolk-shell nanospheres and hollow nanospheres can be prepared via the perfect morphology inheritance of the zinc citrate solid microspheres, yolk-shell microspheres and hollow microspheres, by calcination at 600 degrees C for 2 h. The ZnO yolk-shell nanospheres show the largest visible emission and the highest photocatalytic activity.