Selective synthesis of various nanoscale morphologies of hydroxyapatite via an intermediate phase

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.