The XPS analysis was conducted using a VG Scientific Escalab
MK II spectrometer equipped with a monochromatic Mg K X-ray
source (1253.6 eV). Pass energy was set as 187.85 and 29.35 eV for
survey and high-resolution spectra, respectively. The XPS spectra
were calibrated by taking the graphitic peak as 284.6 eV. The surface
atomic concentrations were calculated from the survey spectra
after correcting the relative peak areas by sensitivity factors.