Surface oxidation process of a diamond-like carbon film analyzed by difference X-ray photoelectron spectroscopy

Difference X-ray photoelectron spectroscopy (D-XPS) revealed the surface oxidation process of a diamond-like carbon (DLC) film. Evaluation of surface functional groups on DLC solely by the C 1s spectrum is difficult because the spectrum is broad and has a secondary asymmetric lineshape. D-XPS clarified the subtle but critical changes at the DLC surface caused by wet oxidation. The hydroxyl (C?OH) group was dominant at the oxidized surface. Further oxidized carbonyl (CO) and carboxyl (including carboxylate) (COO) groups were also obtained; however, the oxidation of CO to COO was suppressed to some extent because the reaction required C?C bond cleavage. Wet oxidation cleaved the aliphatic hydrogenated and non-hydrogenated sp(2) carbon bonds (C?H sp(2) and C?C sp(2)) to create a pair of C?OH and hydrogenated sp(3) carbon (C?H sp(3)) bonds. The reaction yield for C?H sp(2) was superior at the surface, suggesting that the DLC film was hydrogen rich at the surface. Oxidation of aromatic sp(2) rings or polycyclic aromatic hydrocarbons such as nanographite to phenols did not occur because of their resonance stabilization with electron delocalization. Non-hydrogenated sp(3) carbon (C?C sp(3)) bonds were not affected by oxidation, suggesting that these bonds are chemically inert. Copyright (c) 2014 John Wiley & Sons, Ltd.