Ammonium Hydroxide Mediated Hydrothermal Crystallization of Hydroxyapatite Coatings on Titanium Substrate

Controlled growth of hydroxyapatite (HAp) coatings on titanium substrate plays an important role in the fabrication of the composites for bone tissue engineering. We describe the synthesis of the crystalline hydroxyapatite coatings on the Ti/TiO2 substrate through a hydrothermal method by using ethylenediamine tetraacetic acid disodium salt (Na2EDTA) and varying concentrations of ammonium hydroxide (NH4OH) in calcium-phosphate precursor solution. Na2EDTA serves as a chelating agent, while NH4OH is used as an alkaline source and crystal growth modifier. We characterized the HAp coatings using x-ray diffraction, scanning electron microscopy, and Raman spectroscopy. We also performed the elemental chemical analysis by means of a particle induced x–ray emission method. Our results show that there is a pH limit for which the hydrothermal deposition of HAp on titanium occurs. Moreover, we observed that NH4OH had a measurable influence on the coating thickness as well as on the size and shape of the HAp crystals. We found that with the increase of NH4OH concentration, the thickness of the Hap layer increases and its morphology changes from irregular flakes to well-defined hexagonal rods.