Flexible-in-rigid polycrystalline titanium nanofibers: a toughening strategy from a macro-scale to a molecular-scale.

TiO 2 nanomaterials, especially one-dimensional TiO 2 nanofibers fabricated by electrospinning, have received considerable attention in the past two decades, for a variety of basic applications. However, their safe use and easy recycling are still hampered by the inherently subpar mechanical performance. Here, we toughened polycrystalline TiO 2 nanofibers by introducing Al 3+ -species at the very beginning of electrospinning. The resultant long-and-continuous TiO 2 nanofibers achieved a Young's modulus of 653.8 MPa, which is ca. 25-fold higher than that of conventional TiO 2 nanofibers. Within each nanofiber, amorphous Al 2 O 3 -based oxide effectively hindered the coalescence of TiO 2 nanocrystals and potentially repaired the surface groves. The solid-state 17 O-NMR spectra further revealed the toughening strategy on a molecular scale, where relatively flexible Ti-O-Al bonds replaced rigid O-Ti-O bonds at the interfaces of TiO 2 and Al 2 O 3 . Moreover, the modified TiO 2 nanofibers exhibited superb sinter-resistance, without cracking over 900 °C, which was dynamically monitored by TEM. Therefore, flexible-in-rigid TiO 2 fibrous mats can be facilely folded into 3D sponges through origami art. As a potential showcase, the TiO 2 sponges were demonstrated as a duarable and renewable filtrator with a high filtration efficiency of 99.97% toward PM 2.5 and 99.99% toward PM 10 after working for 300 min. This work provides a rational strategy to produce flexible oxide nanofibers and gives an in-depth understanding of the toughening mechanism from the macro-scale to the molecular-scale.