C sp(2)/sp(3) hybridisations in carbon nanomaterials - XPS and (X)AES study

Shape of the C KLL (KVV) Auger spectrum provides a measure of C sp(2)/sp(3) hybridisations, alternative to C 1s spectra fitting. Due to a smaller kinetic energy of C KLL electrons than C 1s photoelectrons the investigated information depths are attributed to lower or higher surface sensitivity, respectively. Shape of the KLL Auger spectrum of carbon nanostructures reflects density of electronic states (DOS) and contains contributions of sp(2) (graphite) and/or sp(3) (diamond) hybridisations, whereas for oxygen and hydrogen containing nanostructures this shape reflects chemical effects. C sp(2)/sp(3) content is evaluated from parameter D, which is defined as an energy difference between the maximum and minimum of the first-derivative C KLL spectrum, where dependence of parameter D on C sp(2)/sp(3) hybridisations is assumed to be linear between the D values of graphite and diamond. Derived values of parameter D and therefore C sp(2)/sp(3) hybridisations were found to be influenced by procedure of smoothing the Auger spectrum and D values used for reference materials with pure sp(3) and pure sp(2) hybridisations. Purpose of this work was to estimate reliability of C sp(2)/sp(3) hybridisations derived from parameters D determined for a set of carbon nanomaterials and study the chemical and morphological effects on the measured parameter D values. Presence of an inhomogeneous distribution of hybridisation as a function of depth from the surface was identified mainly in graphene oxides in contrast to graphite and reduced graphene oxide. The largest influence on parameter D and then evaluated C sp(2)/sp(3) content resulted from oxygen and hydrogen at the surface and applied smoothing procedure in contrary to structural properties of carbon nanomaterials (crystallinity, grain size). Values of parameter D for C sp(3) and C sp2 hybridisations, i.e. 13.2 eV and 23.1 eV, respectively, are recommended to be used for linear interpolation proposed by Lascovich et al. (C) 2018 Elsevier B.V. All rights reserved.