Temperature driven morphological changes of chemically precipitated hydroxyapatite nanoparticles

Hydroxyapatite (HA) is synthesized by a wet chemical route using calcium hydroxide and ortho-phosphoric acid at various temperatures (40, 80, and 100°C). X-ray diffraction of the precipitate particles revealed HA as the predominant phase (>99%) with a small amount of β-tricalcium phosphate. Fourier transform infrared spectroscopy indicated the presence of carbonate substitution, which decreased with increasing temperature. Transmission electron microscopy observations revealed needle-shaped particles with a high aspect ratio at 40°C, which changed to spheroidal when the precipitation temperature was increased to 100°C. The changes in the morphology with temperature were analyzed taking into account the driving force for the HA precipitation and the supersaturation level of Ca²⁺ and PO ₄^3- ions with respect to HA. The analysis indicated that the supersaturation level of the reactants, especially the concentration of Ca²⁺ ions, played a predominant role on the precipitate morphology for this classical acid-base reaction.