Template-free sonochemical synthesis of hierarchically porous NiO microsphere

•Hierarchically porous NiO microspheres have been fabricated by a template-free sonochemical technique.•Compared to the mechanical stirring, the ultrasonic radiation can accelerate the decomposition of urea.•The specific surface area of NiO microsphere is 103.20m2/g.•The synthesized NiO microspheres were mesoporous materials with a high fraction of macropores. A novel template-free sonochemical synthesis technique was used to prepare NiO microspheres combined with calcination of NiO2.45C0.74N0.25H2.90 precursor at 500°C. The NiO microspheres samples were systematically investigated by the thermograviometric/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), fourier-transformed infrared spectroscopy (FT-IR), Brunnauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms, laser particle size analyzer, and ultraviolet–visible spectroscopy (UV–Vis). The morphology of the precursor was retained even after the calcination process, and exhibited hierarchically porous sphericity. The morphology changed over the ultrasonic radiation time, and the shortest reaction time was 70min, which was much less than 4h for the mechanical stirring process. The mechanical stirring was difficult to form the complete hierarchically porous microsphere structure. The BET specific surface area and the median diameter of the hierarchically porous NiO microspheres were 103.20m2/g and 3.436μm, respectively. The synthesized NiO microspheres were mesoporous materials with a high fraction of macropores. The pores were resulted from the intergranular accumulation. The ultraviolet absorption spectrum showed a broad emission at the center of 475nm, and the band gap energy was estimated to be 3.63eV.