Surface engineered hollow hydroxyapatite microspheres: Hydrothermal synthesis and growth mechanisms

Well-organized hollow hierarchical hydroxyapatite microspheres were prepared hydrothermally via a template free process. Citrate was used as chelating or morphology regulating agent in this study. In the presence of regulating agent two-dimensional monetite phase evolved into three-dimensional hollow hydroxyapatite mi-crospheres with an average diameter of 4 mu m. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed the importance of Citrate/Calcium (Cit/Ca) ratio in regulating the particle morphology. The results indicate that the surface morphology of microspheres can be engineered from nano sheets to nanorods by regulating agent concentration. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy confirmed crystallinity and purity in the synthesized powders. Brunauer-Emmett-Teller (BET) indicated an increase in the specific surface area with the change from hydroxyapatite microspheres with nanosheets (HAMNS) to hydroxyapatite microspheres with nanorods (HAMNR) on the surface. Thermodynamic and kinetic theories are proposed to account for the observed changes in the surface morphology and crystal growth.