Phosphate-driven microstructural evolution of hydroxyapatite

The microstructure, composition, and size of hydroxyapatite (HA) are key factors influencing its physicochemical properties and potential applications. However, research on the impact of the intrinsic physicochemical properties of phosphates on the HA microstructure remains limited. Herein, one-dimensional (1D) HA nanofibers and three-dimensional (3D) microspheres were successfully synthesized using ricinoleic acid-assisted solvothermal synthesis with three different phosphate sources (NaH₂PO₄·2H₂O, Na₅P₃O₁₀, and (NaPO₃)₆). XRD, SEM, and TEM analyses revealed that NaH₂PO₄·2H₂O favors the formation of ultralong flexible HA nanofibers that preferentially grow along the c-axis, whereas (NaPO₃)₆ inhibits c-axis growth and promotes the formation of nanorod self-assembled microspheres. Based on these findings, a mechanism was proposed to explain the phosphate-mediated regulation of HA microstructural evolution from 1D nanofibers to 3D microspheres.