To further investigate the morphology of as-prepared CNF, the SEM images of the freeze-dried column-shape samples’ surface with different magnifications are displayed in Fig. 3. The surface morphology of these three freeze-dried CNF exhibits highly porous structures consisting of numerous macro-voids with diame-ter larger than 50 um as shown in Fig. 3. These voids reveal that they are constructed by the very thin lamella. Further magnification of these lamella confirmed that they were composed of numerous random unorientated CNF with the diameter ranging from several nanometers to a few micrometers. The result is consistent with the aforementioned TEM examination. The formation of this hierarchical structures was ascribe to the growing ice crystal and sublimation during the process of freeze-drying, which has been widely reported in publications.