The baselines were recorded using an FTO glass and a BaSO4 plate as references for transmittance and reflectance, respectively. Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) elemental mapping were performed using an NVision 40 (Zeiss Microscopy, Germany) scanning electron microscope equipped with an Octane Elite (EDAX, USA) EDX system. Photoluminescence (PL) spectra were recorded on an RF-6000 spectrofluorophotometer (Shimadzu, Japan) using excitation wavelength of 360 nm with a 400 nm cut-off filter placed in front of the emission detector. Attenuated total reflection Fourier transform infrared (ATR-IR) spectroscopy was performed by the FT-IR spectrometer (Alpha II, Bruker, Germany). X-ray photoelectron spectroscopy (XPS) measurements were performed with monochromatized Al Kα radiation using a PHI Quantera SXM system (ULVAC-PHI, Japan). The binding energies were calibrated based on C 1s peak of adventitious carbon (284.8 eV).