Fabrication of TiO2 hollow microspheres by ammonia-induced self-transformation

• TiO2–HMSs were solvothermally synthesized in the presence of ammonia.
• Fluffy yolk/shell structured TiO2–HMSs show the highest photocatalytic activity.
• Ammonia-induced self-transformation mechanism was also proposed for TiO2–HMSs.

TiO2 hollow microspheres (TiO2–HMSs) have attracted great interest in recent years due to their unique physical and chemical properties such as low density, high surface area and effective light-harvesting property. In this paper, anatase TiO2–HMSs were synthesized by hydrothermal treatment of amorphous TiO2 microspheres precursor in the presence of ammonia solution at 180 °C. The prepared TiO2–HMSs were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscopy (SEM). It was found that the presence of ammonia accelerates the hollowing process of TiO2 microspheres, and the phase transformation from amorphous TiO2 to metastable ammonium titanate occurs firstly and then to anatase TiO2 phase, forming fluffy core/shell and yolk/shell structured microspheres. The photocatalytic activity of the photocatalyst was evaluated both by photocatalytic degradation of cationic dye (Rhodamine B) and anionic dye (Brilliant Red X-3B) under UV irradiation. The calcined fluffy core/shell structured TiO2 microspheres with a hydrothermal reaction time of 18 h showed the highest photocatalytic activity. Ammonia-induced self-transformation mechanism was also proposed for the formation of TiO2–HMSs.