Insights on Surface and Subsurface Differences in C 1s Core-Level Shifts of Metal Carbides

Core-level binding energy (BE) shifts from X-ray photoelectron spectroscopy (XPS) provide valuable insights into the electronic structure and reactivity of materials. Metal carbides widely used as catalysts in reactions like Fischer-Tropsch synthesis involve carbon in the catalytic cycle. Accurate characterization and prediction of C 1s BE shifts, especially for surface C, are crucial to understanding their reactivity. However, experimental and theoretical surface C 1s BE often shows significant discrepancies. To address this, we examine C 1s XPS spectra of metal (Fe, Co, Ni) carbide films of varying thicknesses, comparing surface and subsurface signals with angle-resolved XPS and theoretical simulations. Our results show that C 1s BE increases from Fe₂C to Co₂C and reaches its highest values in Ni₂C, reflects a progressive reduction in electron density on carbon atoms, consistent with reduced charge transfer and a more metallic bonding environment. Yet, surface carbon in experiments exhibits a smaller shift than theoretical simulations. We identify initial state effects─specifically electrostatic potential variations at the surface─as a key factor in these shifts. Accounting for surface-localized electrostatic potential differences is essential to accurately represent surface carbon's electronic structure and reactivity. These findings provide a method to reconcile experimental and theoretical XPS values and offer deeper insights into surface carbon's electronic properties, aiding in more precise catalytic predictions.