*2.3. Characterization of Material*

Field emission scanning electron microscopy (FESEM, JSM-7000 F, JEOL Ltd., Tokyo, Japan) was used to determine the morphology of the samples. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a TEM (FEI Tecnai G2 F30 S-Twin TEM, Georgia Tech, Atlanta, GA, USA) instrument. Raman spectrum was obtained using a Raman spectrometer (LabRamHR800, HORIBA, Ltd., Kyoto, Japan) that was excited by an Ar laser at 514.5 nm under 500 μW. The crystal phase properties of the samples were analyzed by a Bruker D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA) with Ni-filtered Cu Kα radiation at 40 kV and 40 mA and 2θ from 10◦ to 60◦ with a scan rate of 0.02◦. X-ray photoelectron spectroscopy (XPS) analysis (PHI5000Versaprobe, ULVAC-PHI, Inc. Chigasaki, Kanagawa, Japan) was used to determine the chemical composition of the products.
