Hydroxyapatite nanostructure material derived using cationic surfactant as a template

Abstract

This article reports the successful synthesis of mesoporous hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ (denoted HA) using cationic surfactant as the template. The wide-angle (2θ > 10°) diffraction data revealed characteristic peaks of HA, where a hexagonal lattice structure can be deduced. The lattice structure is found in space group P6₃/m, the parameters of which are in excellent agreement with reference data; i.e.a = b = 0.9418 nm, c = 0.6884 nm. For small angle diffraction (2θ < 10°), the characteristic peaks occur at 2θ values of 3.30, 5.75, 7.25 and 8.30°, indicating the presence of atomic planes with a periodical spacing of 2.677 nm. Nitrogen adsorption indicated a pore size distribution of approximately 3 nm, and a corresponding pore volume of 0.0113 cm³ g⁻¹, hence the volume ratio of the mesopores was found to be ∼0.036. SEM micrographs reveal a rod-like structure of HA, possessing a thickness of about 50–100 nm and length ranging from 500 to 1000 nm, while TEM micrographs revealed that nano-channels are formed within the rod-like structure. These nano-channels align in a lengthwise direction within the rods, consistent with the cavities being generated by the removal of “organic” CTAB templating structure during calcination. The channels have dimensions of around 3.5 nm and the spaces between the nano-channels are filled with an ordered crystalline HA structure. Comparing the pore size and the spacing between neighboring channels, the pore volume ratio of each rod was calculated to be ∼0.029, which is in agreement with the result from gas adsorption. A probable mechanism is that CTAB–PO₄³⁻ mixtures form rod-like micelles, which contain many PO₄³⁻ groups on the surface, and in the presence of Ca²⁺, Ca₉(PO₄)₆ clusters are preferentially condensed on the rod-shaped micellar surface due to the conformation compatibility between the identical hexagonal shapes of the micelles and Ca₉(PO₄)₆ clusters. The micelles act as nucleating points for the growth of HA crystals. During the thermal incubation stage, CTAB–HA complexes are produced and they coalesce to form a stable three-dimensional rod-like structure. The morphology of the final product shows about 10 layers of HA crystal grows on one micelle during the reaction in water.