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.