Hydrothermal Growth Mechanism of Controllable Hydrophilic Titanate Nanostructures on Medical NiTi Shape Memory Alloy

In this study, various titanate nanostructures were grown on medical NiTi alloy surface via a hydrothermal process. A combined SEM and XRD investigation of the reaction products reveal that different structures (e.g. nanoflake, nanorod, nanograin, nanofiber, microwhisker) are formed by using 5 ~ 20 M NaOH at 90 ~ 150 °C for 6 ~ 72 h. A general formation mechanism of titanate nanostructures was proposed in terms of the growth units model of anion coordination-polyhedra. The overall formation of titanate nanostructures can be summarized, as a sequence of growth units nucleation, formation and combination. Furthermore, with increasing duration, nanoflake transform into nanofiber or microwhisker due to the alteration of reaction conditions. Compared with NiTi surface, the resulting surface displays hydrophilicity at different levels. The remarkable controllability of this process make it widely accessible as an encouraging technique for biomedical surface modification.