... Echegoyen et al. ... X-ray photoelectron spectroscopy (XPS) was measured using a Thermo ESCALAB 250 instrument with a monochromatic Al Kα source (hν = 1486.6 eV) at 2.2 × 10 - 9 Torr. ... The probe fluid was deionized water with a droplet volume of 3 μL. ...