A step toward control of the surface structure of biomimetic hydroxyapatite nanoparticles: Effect of carboxylates on the {010} P-rich/Ca-rich facets ratio

Two types of biomimetic hydroxyapatite (HA) nanoparticles were prepared by acid-base neutralization reactions, using Ca(OH)₂ or Ca(CH₃COO)₂ as a calcium source, to evaluate the effect of acetate anions on particle formation. High-resolution transmission electron microscopy observations provided evidence that in both cases nanoparticles are elongated along the c-axis, but to a more limited extent when prepared in the presence of acetates, and are mainly limited by {010} facets. IR spectra of nanoparticles containing adsorbed CO revealed that the actual termination of these are both of the {010}-Ca-rich and {010}-P-rich type, the latter being significantly more abundant for HA nanoparticles grown in the medium containing CH₃COO^- species. Moreover, these nanoparticles appeared to be more sensitive toward aggregative stacking by thermal treatment, resulting in a significant decrease in specific surface area, while retaining the size of primary particles.