Characterization and photocatalytic performance of hollow zinc oxide microspheres prepared via a template-free hydrothermal method

Highlights: • Highly porous hollow ZnO microspheres were synthesized by a template-free hydrothermal method. • Well-crystallized ZnO spheres with a uniform diameter could be obtained by using appropriate calcination temperature. • The large open pores and mesoporous pores provided facilitate permeation of the dye solution into the spheres. • Crystallinity, energy band gap and texture of hollow structure had strongly influence to photocatalytic activity of ZnO. -- Abstract: Hollow ZnO microspheres were synthesized via a hydrothermal method. Samples calcined at 400, 500 and 600 °C were characterized using XRD, SEM, TEM, HRTEM, SAED, BET-BJH, FTIR and UV–visible spectroscopic techniques. The results showed that all the samples possessed a hexagonal wurtzite structure and their crystallite sizes increased with their heat treatment temperature. SEM and TEM images revealed that the samples were uniform hollow spheres. On increased calcination temperature from 400 to 500 °C, the average diameter of these particles decreased from 2.17 to 2.09 μm, while the pore diameter was changed from 8.65 to 15.25 nm. The band gap energy (E_g) of the calcined samples was evaluated using optical absorption spectra and was found to be in the range of 3.16–3.19 eV. The performance of ZnO microspheres in catalyzing the photodegradation of methylene blue under UVA irradiation was investigated and the results showed that crystallite size, porous structure, and E_g value played a crucial role in photocatalytic activity of the hollow spheres.