Morphological Evolution of Hydroxyapatite Particles in the Presence of Different Citrate:Calcium Ratios

This work is focused on the potential role of citrate-derived species as tailoring agents for the morphology of hydroxyapatite particles during their synthesis. Under hydrothermal conditions (T = 180 °C), precursor solutions with different initial citrate:calcium molar ratios (R) allow the nucleation and growth of hydroxyapatite particles whose final morphology shows a significant dependence on the initial citrate amount and growth time. Increasing R from 3:1 to 7:1 causes the morphology to evolve from needlelike- and rodlike-shaped nanoparticles toward larger microparticles with hedgehoglike or bundlelike aspects that are composed of assembled primary units. A strong decay of the specific surface area from 55 m2/g for nanometric particles (R = 3:1) down to 17 m2/g for microparticles (R = 7:1) also corroborates the morphological evolution. The evolution of the particle morphology is accompanied by citrate chemistry changes. As confirmed by Fourier transform IR spectroscopy and thermal analysis, citrate ions undergo thermal degradation during the synthesis process to generate smaller carboxylate-based species adsorbed on the particles' surfaces. The concentration of such species and their mode of coordination to the hydroxyapatite surface is R-dependent. Accordingly, it is suggested that these organics interplay differently with the growth of the developing hydroxyapatite structures, thereby affecting the final particle morphology.