Surface hydration and cationic sites of nanohydroxyapatites with amorphous or crystalline surfaces: A comparative study

This paper is an extension of previous work devoted to the characterization of platelet-like hydroxyapatite (HA) nanoparticles constituted by a crystalline core coated by an amorphous surface layer 1-2 nm thick (Bertinetti et al. J. Phys. Chem. C. 2007, 111, 4027 -4035). By increasing the preparation temperature, the platelet morphology was retained but HA nanoparticles exhibited a higher degree of crystallinity (evaluated by X-ray diffractometry). High-resolution transmission electron microscopy revealed that, in this case, the crystalline order was extended up to the particles' surfaces, which were of the (010), (100), and (001) types. IR spectroscopy was used to investigate the surface hydration of both materials, in terms of adsorbed H₂O molecules and surface hydroxy groups, as well as the Lewis acidity of surface cations, by removing water and adsorbing CO. For both features, strong similarities between amorphous and crystalline surfaces were found.