Temporal and Thermal Effects on the Microstructural and Compositional Evolution of Ultralong Flexible Hydroxyapatite Nanofibers

The oleic acid-assisted solvothermal method has been widely employed for synthesizing ultralong flexible hydroxyapatite (HA) nanofibers. However, a detailed exploration into the nucleation and crystal growth processes of HA nanofibers induced by oleic acid remains lacking. In this study, the evolution process of HA nanofiber microstructure and composition was systematically explored based on gradient changes in solvothermal temperature and time. Results revealed that the initially formed amorphous calcium phosphate (ACP) nanoparticles and low-crystallinity HA whiskers eventually transformed into ultralong flexible HA nanofibers with high crystallinity under high-temperature and high-pressure conditions (180 °C). Both insufficient reaction time (≤18 h) and low-temperature conditions (120–160 °C) led to some ACP remaining. Based on the evolving microstructure and composition of the products, a reasonable formation mechanism for HA nanofibers was proposed to enhance the understanding of their nucleation and crystal growth processes.