2.3 Material characterization
The as-prepared materials were characterized by scanning electron microscopy (SEM, Quanta 200F), transmission electron microscopy (TEM, FEI F20), elementary analysis (Vario MICRO Cube CHNS analyzer), X-ray diffraction (XRD, Bruker D8 Advance),X-ray photoelectron spectroscopy (XPS, PHI Quantera Scanning X-ray Microprobe) and Raman spectroscopy (Renishaw RM2000, at 633 nm).