Template-free synthesis of zinc oxide hollow microspheres in aqueous solution at low temperature

A novel template-free hydrothermal method is developed to prepare large-scale ZnO hollow microspheres. The key strategy for the successful realization is that we employ a high concentration of trisodium citrate to control the nucleation and growth rate of ZnO crystal forming microspheres with core/shell structures and the groper pH of an alkaline solution to remove the cores generating hollow microspheres. The morphology, structure, and composition of the microspheres have been investigated with field-emission scanning electron microscopy and high-resolution transmission electron microscopy for the growth mechanism. A series of controlled experiments have also been carried out to better understand the formation mechanism of ZnO hollow microspheres and the function of reactants used in the experiments. The infrared absorption spectra indicate that annealing can lead to the decomposition of impurity groups in the as-prepared sample. Both room-temperature Raman and photoluminescence results demonstrate that the annealed ZnO microspheres are of good crystalline wurtzite structure with high optical quality.