A NEXAFS DETERMINATION OF THE OXIDATION-STATE OF VANADIUM CARBIDE ON V(110) - OBSERVATION OF CHARGE-TRANSFER FROM VANADIUM TO CARBON

We have applied the near-edge X-ray absorption fine structure (NEXAFS) technique to investigate the electronic and structural properties of thin vanadium carbide films on a vanadium (110) single-crystal surface. The carbon K-edge features of vanadium carbides were characterized by two relatively sharp resonances at 285.5 and 287.5 eV, and by one broad peak centered at 295.0 eV. A comparison of NEXAFS data by means of measuring the fluorescence yield (bulk sensitive) and electron yield (surface sensitive) suggested that the 285.5 and the 287.5 eV resonances could be related to the bulk carbide and surface carbide, respectively, while the 295.0 eV feature was contributed from both bulk and surface carbides. Such an assignment was also supported by comparing the NEXAFS data recorded at normal and glancing incidences. Finally, we have obtained a NEXAFS calibration curve correlating the peak positions of vanadium L-edge features and the vanadium oxidation states of a series of model oxide compounds. A linear relationship, 0.68 +/- 0.03 eV per oxidation state, was determined by the NEXAFS measurements. By using this calibration curve, the oxidation state of vanadium carbide was estimated to be in the range of V-1.2+/-0.2. Th, fact that vanadium is positively charged in the carbide indicates that the direction of charge transfer is from vanadium to carbon.