SEM imaging was performed using FEI Nova NanoSEM 450 system equipped with a field emission electron gun operating at 2 kV. XPS spectra were recorded using a Microlab 350 (Thermo Electron, East Grinstead, UK) spectrometer. For this purpose, non-monochromatic AlKα (hν = 1486.6 eV, power 300 W) radiation was used as the X-ray excitation source, with a lateral resolution of about 0.2 cm 2 . The high-resolution XPS spectra were recorded using the following parameters: pass energy 40 eV, energy step size 0.1 eV. Shirley function of background subtraction was used to obtain the XPS signal intensity. All the collected XPS peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The measured binding energies were corrected in reference to the energy of C 1s at 284.8 eV. CasaXPS data system software was used to process the data.