The transformation of single-crystal calcium phosphate ribbon-like fibres to hydroxyapatite spheres assembled from nanorods

Two precursors of ribbon-like anhydrous dicalcium phosphate (DCPA) and calcium phosphate fibres were successfully synthesized at 85 deg. C through a simple and mild pathway from Ca(NO3)2·4H2O and (NH4)2HPO4 upon the hydrolysis of urea. Different molar concentrations of urea resulted in different precursors, including precursor I (DCPA phase) and precursor II (calcium phosphate with DCPA, octacalcium phosphate (OCP) and hydroxyapatite (HAp) phase). By immersing the two precursors in ammonium hydroxide solution (pH = 12), the transformation from precursors to hydroxyapatite could be achieved. X-ray diffraction (XRD) results combined with transmission electron microscopy (TEM) indicated that DCPA fibres (precursor I) were transformed to HAp nanorods in transformation I. In transformation II, dandelion-like spheres assembled by HAp nanorods were obtained from calcium phosphate (precursor II). The mechanisms of transformations I and II are also proposed and discussed based on the XRD and TEM results