ZnO hierarchical microsphere for enhanced photocatalytic activity

As an important semiconductor material for photocatalysis, zinc oxide (ZnO) has been prepared by various methods to enhance its photocatalytic activity. Herein, hierarchically porous ZnO microspheres were synthesized by hydrothermal treatment of precursor zinc salts and subsequent low-temperature (300 °C) calcination for 2 h, and the photocatalytic properties of the ZnO microspheres were examined by degrading Rhodamine B under simulated solar light. The type of the precursor zinc salts (nitrate, acetate, sulfate and chloride) had a great impact on the morphological, textural, optical and photoelectrochemical properties of the as-prepared ZnO microspheres, which collectively affected the photocatalytic performance. The as-prepared hexagonal wurtzite ZnO exhibits unique hierarchical porous microsphere morphology with diameters in the range of 3–6 μm. The photocatalytic activity of the ZnO microspheres also depended on the zinc precursor and followed the order: zinc sulfate > zinc nitrate > zinc acetate > zinc chloride. The high photocatalytic activity of ZnO microspheres prepared from zinc sulfate was due to its higher specific surface area (91 m2 g−1), better visible light absorption, more oxygen defects and higher charge separation and transfer efficiencies, as well as the hierarchical pore structure that promotes the diffusion of reactant molecules. Moreover, electron paramagnetic resonance spectroscopy identified hydroxyl radicals and superoxide radical anions as reaction intermediates, shedding light on the mechanism of the dye photodegradation reaction.