Synthesis of mesoporous hydroxyapatite nanoparticles using a template-free sonochemistry-assisted microwave method

A facile template-free sonochemistry-assisted microwave method was successfully developed for the synthesis of mesoporous hydroxyapatite nanoparticles (MHN) in a short time. The prepared MHN were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and N₂ adsorption–desorption isotherms. TEM images showed a clear rod-like morphology with length of 50–100 nm and width of about 20 nm. XRD and FTIR indicated a typical hydroxyapatite phase with high crystallinity. N₂ adsorption–desorption isotherms revealed an irregular mesoporous structure. Judging from the values calculated from N₂ isotherms, the specific surface area and pore volume obviously decreased after the sintering process. In a typical example, the specific surface area, pore volume, and pore size of MHN before and after calcination were 79.74 m²/g, 0.46 cm³/g, 2.7 nm and 45.41 m²/g, 0.22 cm³/g, 2.8 nm, respectively. In addition, MHN could be synthesized in a period of time as short as 10 min. Further investigation indicated that microwave radiation played a dominant role in the emergence of mesoporous structure, while ultrasound irradiation acted as a supporting role. Based on the above results, a possible mechanism of formation of mesoporous hydroxyapatite has been proposed.