Nucleation and growth of hydroxyapatite nanocrystals by hydrothermal method

Hydroxyapatite (HA) was synthesized by using a hydrothermal method with Ca(NO3)2·4H2O and H3PO4. We use x-ray diffraction and field-emission scanning electron microscopy to investigate how pH, reaction temperature, hydrothermal-reaction time, and calcium-ion concentration affects the microstructure and the growth of HA crystals. In addition, we discuss the growth mechanism. The results show that the crystals grow more completely and that the aspect ratio tends to increase with increasing hydrothermal-reaction time, reaction temperature, and calcium-ion concentration. The pH of the system strongly impacts the growth of HA crystals. With increasing pH, the HA crystal grain size and aspect ratio decrease significantly. By using 1 mol/L calcium-ion concentration, pH = 10, and a hydrothermal reaction at 200 °C for 8 h, we obtain high crystallinity and crystal clear of the growth polarity with hexagonal 60–100-nm-long columnar HA, 30–40 nm in diameter. The mechanisms producing this growth may be the effect of growth conditions on ion concentration, thereby changing the HA crystal growth rate along the different crystal axes.