Figure 2: Ice-templating mechanism underlying the V₂O₅ nanofibre scaffold formation.

(a) The ice crystal growth along the temperature gradient direction can be divided into three different zones. The liquid zone exhibits randomly oriented V₂O₅ nanofibres that form hydrogen bonds with the surrounding water molecules via oxygen-functionalities on the nanofibres’ surface. This functionality likewise interacts with the ice crystal surface, guiding the assembly of nanofibres in the freezing zone. The nanofibre arrangement becomes trapped and compacted, as the ice crystal front proceeds, represented by the frozen zone. The ice crystal growth is two to three orders of magnitude faster along the a-axis directions, as compared to the perpendicular c-axis direction of the plates, yielding a layered structure. Furthermore, the temperature gradient overlies the lateral expansion, leading to highly anisotropic ice crystal plates. (b) Arrangement and trapping of the nanofibres between the crystal tips at the front of the growing ice crystal plates creates the pillars. (c) Schematic illustration of the cross-section of an ice platelet in a-axis direction, with the fibre assembly directed by the structural features of the ice crystal plates. The SEM image shows a V₂O₅ nanofibre scaffold cross-section, where the replica of individual ice platelets are marked by dashed yellow lines. It can be seen that the pillars are arranged within each single replica of the ice platelet as a result of the fibres trapped within the plates.