Effect of pH and Temperature on the Morphology and Phases of Co-precipitated Hydroxyapatite

This paper reports a high-yield process to fabricate biomimetic hydroxyapatite nano-particles or nano-plates. Hydroxyapatite is obtained by simultaneous dripping of calcium chloride (CaCl2) and ammonium hydrogen phosphate (NH4)2HPO4 solutions into a reaction vessel. Reactions were carried out under various pH and temperature conditions. The morphology and phase composition of the precipitates were investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The analyses showed that large plates of calcium hydrophosphate are formed at neutral or acidic pH condition. Nanoparticles of hydroxyapatite were obtained in precipitates prepared at pH 9 to 11. Hydroxyapatite plates akin to seashell nacre were obtained at 40°C and pH 9. This material holds promise to improve the strength of hydroxyapatite containing composites for bone implant or bone cement used in orthopaedic surgeries. The thermodynamics of the crystal growth under these conditions was discussed. An assembly mechanism of the hydroxyapatite plates was proposed according to the nanostructure observations.