The preparation methods

In brief, 1.58 ml of OTMS was placed into 201.6 ml of absolute ethanol, to which 8.4 g of P25 (TiO₂ NP_S, ~25 nm) was added and magnetically stirred for 2 h. Then, 8.4 g of titanium oxide nanoparticles (TiO₂ NP_S, ~100 nm) were added to the above mixture and stirring continued for 0.5 h. Finally, the above mixture was kept under ultrasonic irradiation with a power of 150 w for 2.5 h until a uniform paint-like suspension was formed.

Sodium hydroxide/urea solution system (7 wt%/12 wt%) was prepared and placed in a low temperature water tank for freezing until −12 °C was achieved. Thereafter, the absorbent cotton of 1.46 g was weighed and added to the above solution system slowly with the mechanical stirring for 2 h. The resulting homogeneous solution was poured into the cylindrical polystyrene mould. After regeneration in deionized water, the samples were placed into the freeze-drying system (Telstar LyoBeta, Spain) for drying 6 h to produce the cellulose sponge.

Subsequently, by using a high-pressure spray gun (with 0.2 Mpa N₂), the as-prepared paint-like suspension was sprayed onto the sponge surface, which has been treated by the spray adhesive (EVO-STIK) previously. The spraying process was repeated 10~15 times. Finally, the superhydrophobic TiO₂ NP_S coated cellulose sponge (superhydrophobic cellulose sponge) was obtained and dried in air for at least 2 h before testing.