(HRTEM, JEM-2100). Thermogravimetric analysis (TGA) was
carried out to monitor the mass loss of products at a heating rate
of 10 C min 1 from 25 to 700 C under an air atmosphere
(Mettler Toledo, TGA/SDTA851e). N2 adsorption–desorption
isotherms were measured on a QuadraSorb SI apparatus at 77 K.
The surface areas were calculated by the Brunauer–Emmett–
Teller (BET) method, and the pore size distribution was calcu-
lated from the desorption branch using the Barrett–Joyner–
Halenda (BJH) theory.