Plasma modification of vanadium oxynitride surfaces: Characterization by in situ XPS experiments and DFT calculations

Plasma modification of transition metal nitride/oxynitride (MO_xN_y) surfaces for enhanced surface properties is highly desirable, given the scalability of such methods and limitations of thermal treatments. In situ x-ray excited photoelectron spectroscopy demonstrates that the O₂ plasma oxidation of VO_xN_y films generates non-lattice N1s surface features with binding energies near 396.5 eV, which are associated with the nitrogen reduction reaction activity but not observed upon thermal oxidation. The NH₃ plasma generates N1s surface features near 400.5 eV binding energy. The O₂+NH₃ plasma generates both types of N1s features. Annealing in UHV to <1000 K reverses plasma-induced changes to N1s spectra. Density functional theory (DFT) calculations integrated with the experiments indicate that the plasma-induced N1s features at ∼396.5 eV and 400.5 eV are V≡N: and V-NH₂ sites, respectively, with significantly lower thermal stabilities than lattice N sites. These results provide practical insight regarding the plasma modification of MO_xN_y surfaces for important applications.