Selective Synthesis of Various Nanoscale Morphologies of Hydroxyapatite via an Intermediate phase

Abstract

Dicalcium phosphate (DCP) was found to be a suitable precursor for nanoscopically controlled hydroxyapatite (HAp) crystals. Nanoscale needles, fibers, and sheets of HAp were selectively prepared through the hydrolysis of a solid precursor crystal of DCP in an alkali solution by varying the pH and ion concentrations. An oriented array of bundled nanoneedles of HAp elongated in the c axis was obtained under a highly basic condition at pH 11–13. The ordered architecture originated from the spatially periodic nucleation of HAp seeds on the DCP surface through topotactic solid–solid transformation. Long HAp fibers were observed under a relatively mild basic condition at pH 9–10. The fibrous morphology evolved from the nanoneedles produced by the solid–solid transformation with the elongation of the c -axis through a dissolution−precipitation route. Flaky HAp nanosheets consisting of a parallel assembly of nanoneedles were observed with an excess amount of phosphate ions under mild basic conditions. The presence of phosphate ions suppressed the solid–solid transformation and promoted the formation of a two-dimensional structure with the dissolution−precipitation process.