*2.3. Characterization*

The powder X-ray diffraction (XRD) analysis was measured by Bruker-axs D-8 advance diffractometer (Cheshire, UK) with Cu Kα radiation. The morphology and element composition were recorded by using Scanning electron microscope (FE-SEM, Hitachi Regulus 8220, Tokyo, Japan). Raman measurements were acquired on a Bruker Senterra R200-L Raman spectrometer (Ettlingen, Germany). The optical adsorption behavior of the samples was performed on a Cary 5000 UV-vis-NIR spectrophotometer (Agilient Technologies, SantaClara, CA, USA). The absorption spectra were obtained by analyzing the reflectance measurement with Kubelka-Munk (KM) emission function, *F(R*∞*)*. Optical band gap energy (*E*g) can be determined from the plot between *E* = <sup>1240</sup>/λAbsorp.Edge and [*F(R*∞*)h*υ]<sup>1</sup>/<sup>2</sup> where *E* is the photonic energy in eV and *h*υ is the energy of an incident photon. X-ray photoelectron spectroscopy (XPS) analysis was analyzed by a Kratos Axis Ultra DLD (Manchester, UK) with Al Kα X-ray source (1486.6 eV). A Fluorescence Spectrophotometer (JASCO FP-6500, Tokyo, Japan) was used for photoluminescence (PL) measurement at the excitation wavelength of 420 nm.
