Chemical State Analysis of Entrapped Nitrogen in Carbon Nanohorns Using Soft X-ray Emission and Absorption Spectroscopy

The chemical state of nitrogen in nitrogen-entrapped carbon nanohorns (N-CNHs) synthesized by submerged arc discharge in liquid nitrogen was determined by soft X-ray emission and absorption spectroscopy using synchrotron radiation. X-ray absorption spectra (XAS) and X-ray emission spectra (XES) in the CK and NK regions of the N-CNHs were measured at the Advanced Light Source. From fingerprint analysis in NK-XAS and by referring to nitrogen-containing aromatic compounds, the suggested chemical state of nitrogen in N-CNHs is imine. Theoretical analysis of NK-XES using the discrete variational X alpha molecular orbital calculations elucidated that the local structure of N-CNHs is chemisorption of N-2, forming pentagonal and/or hexagonal rings with carbon atoms at the graphitic layer edges, which assume an imine structure. Therefore, the entrapped nitrogen in N-CNHs is chemisorbed at the edges of the graphitic layers.