Pretreatment of Chitosan: an appropriate amount of Chitosan was soaked in an oxalic acid solution with a mass ratio of 2 wt.% for 1 h. The solution was then rinsed with ultrapure water until neutral, and the remaining powder was placed in a vacuum drying oven and dried at 40–50 °C for 12 h. The light-yellow powder was subsequently obtained by grinding.
Preparation of NCC materials: Firstly, the mixture of 10 g of pretreated sugar and NH4Cl was ground for 10 min and heated to 1000 °C in a N2 tube furnace and then held for 3 h until cooled to room temperature, and the black swelling was collected and ground into a black powder in an agate mortar, which is the NCC powder, as shown in