2.2 Preparation of the SCG
The SCG was dried in a 105 °C oven for 24 h, and then the dried SCG was heat treated in a tube furnace under nitrogen and air atmosphere at 240 °C for 1.5 h with the heating speed of 5 °C·min-1, respectively. The obtained SCG were named as NSCG and ASCG corresponding to the heat treatment atmosphere of nitrogen and air separately.