Synthesis of Fiber-like Monetite without Organic Additives and Its Transformation to Hydroxyapatite

Monetite (CaHPO4, DCPA) and hydroxyapatite (Ca-10(PO4)(6)(OH)(2), HAp) are promising biomaterials with wide application in the biomedical field. In this study, we propose a novel method to prepare fiber-like DCPA without organic additives for the first time and show that nano structured HAp can be synthesized without changing the macroscopic morphology of DCPA. Our results show that the temperature, pH, calcium to phosphate ratio (Ca/P), and concentration of NH4+ are essential factors in the formation of fiber-like DCPA. The synthesized materials were characterized by powder X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy, and the DCPA -> HAp phase transformation process was examined. The growth of HAp on the DCPA crystal can be regulated by the composition of the hydrolyzing solution, forming HAp with its c-axis perpendicular [HAp (001)//DCPA (100)] or parallel [HAp (001)//DCPA (010)] to the elongation direction of the DCPA crystal. At a high concentration of Ca2+ in NH4OH solution, octacalcium phosphate was observed as an intermediate phase with its c-axis parallel to elongation of the initial DCPA crystal. The HAp crystals formed after complete transformation of the precursor phases have a preferred crystallographic orientation with c-axis coaxial to the elongation direction of DCPA. In the concentrated NH4OH solution, the direct transformation of DCPA to HAp was observed. At initial stages of the hydrolysis, the c-axis of newly formed HAp is perpendicular to the elongation direction of DCPA. This study provides a new insight into regulation of the crystal growth of DCPA without organic additives and a facile route to form HAp with hierarchical structures.