In situ hydrothermal crystallization of hexagonal hydroxyapatite tubes from yttrium ion-doped hydroxyapatite by the Kirkendall effect

An in situ hydrothermal crystallization method with presence of glutamic acid, urea and yttrium ions was employed to fabricate hexagonal hydroxyapatite (HAp, Ca₅(PO₄)₃(OH)) tubes with length of 200 nm–1 μm. Firstly, yttrium ion-doped HAp (Y-HAp, Ca_5−xY_x(PO₄)₃(OH)) was synthesized after hydrolysis of urea and HPO₄²⁻ ions at 100 °C with a dwell time of 24 h. The shift of X-ray diffraction peaks of HAp to high angle was caused the substitution of Ca²⁺ ions by small-sized Y³⁺ ions. At 160 °C, further hydrolysis reactions of urea and HPO₄²⁻ ions resulted in the generation of ample OH⁻ and PO₄³⁻ ions, which provided a high chemical potential for the dissolution of Y-HAp and recrystallization of HAp and YPO₄. Finally, HAp tubes were formed in situ on Y-HAp according to the Kirkendall effect as a result of the difference of diffusion rate of cations (Ca²⁺ ions, outward and slow) and anions (OH⁻ and PO₄³⁻ ions, inward and fast). The formation process of HAp tube was simulated by the encapsulation of fluorescein molecules in precipitates. Photoluminescence properties were enhanced for HAp tubes with thick and dense walls. This novel tubular material could find wide applications as carriers of drugs, dyes and catalysts. - Highlights: • Hexagonal HAp tubes with adjustable sizes are prepared by a hydrothermal method. • A dissolution-recrystallization process occurs during hydrothermal treatment. • The formation mechanism is explained by the Kirkendall effect. • The crystallization is simulated by the encapsulation and release of fluorescein