*2.4. Characterization*

The FEI Tecnai F30 transmission electron microscope (TEM, FEI Corporation, Hillsboro, OR, USA) with a field emission gun operating at 200 kV was used to investigate the morphology of the samples. The elemental mappings were performed via energy-dispersive X-ray spectroscopy (EDX, FEI Corporation, Hillsboro, OR, USA) as the TEM mode. Raman spectra were collected with a Renishaw micro Raman spectrometer (λ = 785 nm, Renishaw, Edinburg, UK). Thermogravimetric analysis (TGA) was carried out on 10 mg samples at a heating rate of 10 ◦C/min from room temperature to 900 ◦C under air using a DTA-Q600 SDT (TA Instruments, New Castle, DE, USA). X-ray diffraction (XRD) was conducted on a Philips diffractometer (Malvern, Cambridge, UK) using Cu Kα radiation. The N2 adsorption/desorption isotherms were measured on a Micromeritics ASAP 2010M instrument (Micrometrics, Tewkesbury, UK) at liquid nitrogen temperature (77 K). To compute the specific surface area and pore size distribution, the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used, respectively.
