XPS spectra were acquired using a hemispherical electron analyzer SPECS Leybold EA 11 MCD in ultrahigh vacuum conditions with a base pressure of 5×10 -10 mbar. The analyzer has a wide acceptance angle for emitted electrons (cone of 15° half-angle) and the size of the analyzed area is ≈ 1 mm x 4 mm. The X-ray source is non-monochromatic Mg Kα with hν= 1253.6 eV. The XPS core level spectra are measured at 29.925 eV pass energy. The electron-energy scale of the instrument is calibrated with respect to the reference Au 4f7/2 core-level signal at 83.8 eV.