Size-Dependent Crystalline to Amorphous Uphill Phase Transformation of Hydroxyapatite Nanoparticles

The room temperature Ca(C2H3O2)(2)-K3PO4-H2O equilibrium system was examined for the preparation of hydroxyapatite nanopowder with sizes less than 10 nm. The reaction products were characterized with X-ray diffraction, transmission electron microscopy (TEM), nitrogen-adsorption surface area, helium pycnometry, thermogravimetric analysis, and Karl Fisher titration methods. TEM revealed that similar to 5 nm nanopowders could be successfully prepared with this synthesis approach. However, the vast instability of these powders brought upon by the method of sample separation or the characterization method itself made it impossible to use other conventional methods of characterization to validate TEM data. This study has identified key processing steps that control the order and disorder of these nanomaterials, as well as the conditions that lead to surface area reduction. The most unique phenomenon from this work is the observed crystalline to amorphous phase transformation when washed or unwashed nanopowders are aged for 5 months in 30% relative humidity. This transformation, the first of its kind to be reported in the literature, is accompanied by a surface area loss by a factor of 3 or greater. The uphill phase transformation from the nanocrystalline to amorphous state appears to be driven by the reduction of the large positive surface energy inherent in the as-crystallized similar to 5nm nanopowder.