Crystal growth and structural analysis of hydroxyapatite nanofibers synthesized by the hydrothermal microwave-assisted method

Hydroxyapatite nanofibers were synthesized by the hydrothermal microwave-assisted method at 170 °C using a mixture of CaNO3, KOH, K2HPO4 and glutamic acid as precursors. The reaction took place inside of pressurized Teflon vessels at 80 bars. The reaction time was varied from 15 up to 45 min meanwhile temperature, microwave power and precursor composition were kept constant. Crystal phase composition, functional groups, vibrational modes, surface morphology and nanostructure were studied by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, Scanning Electron Microscopy and High Resolution Transmission Electron Microscopy, respectively. Hydroxyapatite nanofibers, with diameters in the order of nanometers and lengths of micrometers, were obtained; their surface seems to be smooth with well-defined shapes and edges. The length increased as the reaction time, so fibers as long as 70 µm were obtained with a diameter the order of 450 nm. According to the XRD patterns, preferential crystalline orientations in the [211] and [300] directions were observed, which are attributed to the glutamic acid concentration used in the reacting mixture as well as to the reaction time. SEM images showed a hexagonal needle-like morphology of nanofibers and studies by TEM and HRTEM indicate that those nanofibers grow in a preferential crystalline direction along the “c” axis of the HAp structure.