Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)

Abstract

Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is explored as a useful characterization technique in the study of carbonization and graphitization of organic precursors. A model series of carbon materials was prepared from highly graphitizable petroleum pitch heat treated in the range 200–2730°C. Initial characterization was performed using the established techniques of X-ray diffraction (XRD), He pycnometry, TEM, electron diffraction and high-resolution lattice imaging (HREM). EELS in the TEM was then examined. Two routes are presented to quantify the change in the proportion of sp ² type hybridization accompanying the heat treatment as the material transforms to the graphitic state. Both routes suggest an initial relative sp ² content of ∼70%, rapidly increasing to ∼90% during mesophase development and carbonization, and then slowly increasing to 100% during graphitization. The peak position of the bulk valence plasmon (π + σ) is shown to be an excellent measure of the degree of graphitic character, and its fundamental dependence upon sample density (ρ) is confirmed. The appearance and definition of features within the core loss region representing the density of unoccupied σ* states are demonstrated to be an excellent measure of the extent of order. Finally, a method is established by which to extract the C–C bond length from core loss EELS spectra with an accuracy of ±0.1 pm. This method suggests an average bond length of 1.44 Å in samples with low heat treatment temperatures, decreasing to the theoretical length of 1.42 Å as both the heteroatom content and proportion of non-sp ²-type hybridized carbon atoms decrease.

Acknowledgements

The authors are grateful for research funding from EPSRC (for a studentship to HD) and to EPSRC and HEFCE for funding the electron microscope facilities. Also, thanks to Dr. C Hindmarsh who prepared some of the specimens and made the XRD and He density measurements.