Reaction temperature plays an important role in initiating the hydrolytic depolymerization process of cellulose and eventually enhancing the chemical degradation of cellulose macromolecule into the nano-dimensional. The crystallinity index of yielded nanocellulose affected by reaction temperature is shown in Figure 1a. The reaction temperature was set at different levels (20, 40, 60, 80 and 100 °C) while other operating parameters were fixed, including Cr(NO3)3 concentration (0.8 M), reaction time (1.5 h) and solid–liquid ratio (1:30). The crystallinity index of nanocellulose was significantly increased (62.1% to 86.7%) with the rise of temperature from 20 to 80 °C. The increment was mainly contributed by the increased diffusion rate of H3O+ and Cr3+ ions in aqueous solution and penetration into loosely-packed non-crystalline regions of cellulose to cleave the glycosidic bonds. In fact, the hydrolysis reaction is generally more kinetically favorable, which means that a higher reaction temperature could significantly enhance the degree of progressive removal of defective crystalline domains in the cellulose matrix, releasing the individual crystallite segments . This led to the breakage of the glycosidic linkage in cellulose long polymeric chains into a smaller dimension. However, the extremely high temperature of 100 °C resulted in the destruction of the crystalline structure of cellulose under excessive heat energy, and eventually promoted the formation of unfavorable side products, such as levulinic acid and HMF . Therefore, the optimal reaction temperature of 80 °C was selected to conduct further experiments with the advantages of energy-saving, ability to produce higher crystallinity product and, most importantly, the reaction was more manageable under normal conditions of atmospheric pressure.