The morphology of samples was characterized using transmission electron microscopy (TEM, Philips Technai 12) equipped with energy dispersive X-ray spectroscopy. ICP-atomic emission spectroscopy (ICP-OED) measurements were conducted on Optima 8000 spectrometer. Samples were digested in hot concentrated HNO3 for 1 h and diluted to desired concentrations. UV-vis spectrum was performed on a Shimadzu 1700 spectrophotometer in ethanol solution with a concentration of 1×10 -5 mol mL -1 . The X-ray photoelectron spectroscopy data were collected on a Thermo ESCALAB 250Xi spectrometer equipped with a monochromatic AlK radiation source (1486.6 eV, pass energy 20.0 eV). The data were calibrated with C 1s 284.6 eV. Raman spectra were collected using a LabRAM HR800 laser confocal micro-Raman spectrometer with a laser wavelength of 514.5 nm. Scanning transmission electron microscopy was characterized on a double spherical-aberration-corrected FEI Themis Z microscope at 60 kV.