Designed synthesis of hydroxyapatite nanostructures: bullet-like single crystal and whiskered hollow ellipsoid.

Hydroxyapatite (HAp) nanostructures of whiskered hollow ellipsoid and bullet-like single crystal were synthesized under mild reaction conditions by using a template-free "one pot" synthetic method. Immersing calcium carbonate precursor into ammonium phosphate solution resulted in the HAp phase. Formed HAp crystals were characterized by X-ray diffraction and transmission electron microscopy techniques. The stability and phase composition of calcium carbonate influenced the morphology and crystallinity of HAp. The transformation of the most stable calcite precursor yielded the bullet-like HAp single crystal of 300-600 nm in length, ~40 nm in tip diameter and ~80 to ~100 nm in bottom diameter. The metastable vaterite precursor showed the formation of the whiskered hollow ellipsoid nanostructures composed of HAp nanorods of ~10 nm in diameter. The driving force for the whole transformation process was the difference in solubility of calcium carbonate and HAp. At the same time, Kirkendall effect and Ostwald ripening played important roles in the formation of the different HAp nanostructures.