*3.5. Characterization*

Transmission electron microscopy (TEM) analysis was performed on a JEM 1200EX transmission electron microscope (Hitachi, Tokyo, Japan). Scanning electron microscopy (SEM) analysis was performed on a Tecnai G2 F20 scanning electron microscope (FEI, USA). The X-ray diffraction (D8 Advance X-ray diffractometer, Bruker, Karlsruhe, Germany) with a Cu Kα anode (λ = 0.15406 nm) at 40 kV and 40 mA was used to measure the crystal structures of the samples. FTIR spectra were obtained by a FTIR spectrometer (8700/Continuum XL Imaging Microscope, Nicolet, Waltham, MA, USA). The spectra were collected between 400 and 4000 cm<sup>−</sup>1. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (DTG-60A, Shimadzu, Japan) in the range of 30–600 ◦C under a nitrogen flow (heating rate of 10 ◦C min−1).
