Abstract
Nitrogen doping is an important strategy in tuning the properties and functions of carbonaceous materials. But the chemical speciation of the nitrogen groups in the sp2-carbon framework has not been firmly established. Here we address two important questions in nitrogen doping of carbonaceous materials from a computational approach: the relative stability of different nitrogen groups and their X-ray photoelectron spectrum (XPS) signatures of the core-level (N 1s) electron binding energies. Four types of nitrogen groups (graphitic, pyrrolic, aza-pyrrolic, and pyridinic) in 69 model compounds have been examined. Computed formation energies indicate that pyrrolic and pyridinic nitrogens are significantly more stable (by about 110 kJ/mol) than graphitic and aza-pyrrolic nitrogens. This stability trend can be understood from the Clar’s sextet rule. Predicted N 1s binding energies show relatively high consistency among each dopant type, thereby offering a guide to identify nitrogen groups. The relative stability coupled with predicted N 1s binding energies can explain the temperature-dependent change in the experimental XPS spectra. The present work therefore provides fundamental insights into nitrogen dopants in carbonaceous materials, which will be useful in understanding the applications of nitrogen-doped carbons in electric energy storage, electrocatalysis, and carbon capture.
