2.3. Instrumental and analytical conditions
High resolution transmission electron microscopy (HRTEM) images were obtained using JEM-2100F transmission electron microscope (JEOL, Japan). Scanning electron microscopy (SEM) images were taken using a Quanta 200 scanning electron microscope (FEI, Holland). Nitrogen sorption experiments were carried out at 77 K by using JW–BK surface area and pore size analyzer (JWGB Sci. & Tech., Beijing, China). Ultraviolet–visible (UV–vis) spectrum was recorded on a B-500 spectrophotometer (Metash Instruments Co., Ltd., Shanghai, China).