Besides the reaction temperature, hydrolysis time also presents a significant effect on the crystallinity index of nanocellulose, which is illustrated in Figure 1b when the hydrolytic depolymerization of cellulose was conducted at 80 °C, 0.8 M Cr(NO3)3 concentration and solid–liquid ratio of 1:30. The nanocellulose yield showed a gradual decline trend while the crystallinity index of nanocellulose was increased over time. The increment reached a maximum of 86.8%, followed by a decreasing trend when the reaction was further prolonged to 2.5 h. The decrement was due to unfavorable hydrolysis process occurring in the crystalline cellulose, which led to the damage of high crystalline segments. However, insufficient reaction time (0.5 h) resulted in incomplete hydrolysis process of breaking down the strong bonding network of cellulose macromolecules, contributing to a low crystallinity index (79.6%). It is widely accepted that increasing contact time between hydrolyzing catalyst and reactant would eventually cause the swollen cellulose and even the enlargement of the intra- and inter-fiber pores of treated matrix, so that the degradation of cellulose network structure can happen effectively. This would also allow the catalyst to penetrate into the interior part of the cellulose pulp to liberate the nanoscale cellulose segments to accelerate the production of nanocellulose. For these reasons, it is very important to choose an appropriate hydrolysis time to maximize the removal of non-crystalline areas in cellulose, while preserving the crystalline parts. Therefore, hydrolysis time of 1.5 h was chosen as the optimal preparation condition and was used in the subsequent experiments, as the extension of time would cause a negative effect in enhancing the crystallinity of nanocellulose.