Effect of pH on the Structure of Calcium Phosphate Crystals Synthesized using a Hydrothermal Process

We investigated the effect of pH on the structure of calcium phosphate crystals synthesized using a hydrothermal process. The as-prepared samples were characterized by using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, and transmission electron microscopy analyses. We found that the structure and the shape of the calcium phosphate crystals depended significantly on the pH of the precursor solution. At pH < 6, rectangular disk-shaped dicalcium phosphate anhydrous (DCPA) microcrystals were formed; on the other hand, at pH > 8, hydroxyapatite (HA) nanorods were predominantly formed. This can be explained by a self-setting mechanism associated with the role of hydroxide species during the hydrothermal process. Both DCPA and HA crystals were single crystalline, with preferred orientations along the (001) planes of monoclinic DCPA and hexagonal HA.

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