In situ Raman spectroscopy study of oxidation of nanostructured carbons

The ability to synthesize carbon nanostructures, such as fullerenes, carbon nanotubes, nanodiamond, and mesoporous carbon; functionalize their surface; or assemble them into three-dimensional networks has opened new avenues for material design. Carbon nanostructures possess tunable optical, electrical or mechanical properties, making them ideal candidates for numerous applications ranging from composite structures and chemical sensors to electronic devices and medical implants. Unfortunately, current synthesis techniques typically lead to a mixture of different types of carbon rather than a particular nanostructure with defined size and properties. In order to fully exploit the great potential of carbon nanostructures, one needs to provide purification procedures that allow a selective separation of carbon nanostructures, and methods which enable a control of size and surface functionalization. Oxidation is a frequently used method for purification of carbon materials, but it can also damage or destroy the sample. In situ Raman spectroscopy during heating in a controlled environment allows a time-resolved investigation of the oxidation kinetics and can identify the changes in material structure and composition, thus helping to accurately determine optimal purification conditions. However, while carbon allotropes such as graphite and diamond show unique Raman signals and allow a fast and straightforward identification, the interpretation of Raman spectra recorded from nanostructures containing mixtures of sp, sp2 and sp3 bonded carbon is more complex and the origin of some peaks in Raman spectra of nanocarbons is not yet fully understood. In this study we applied in situ Raman spectroscopy to determine conditions for selective oxidation of carbon nanostructures, such as nanodiamond, nanotubes, carbidederived carbon and carbon onions; accurately measure and control the crystal size; and improve the fundamental understanding of effects of temperature, quantum confinement and surface chemistry on Raman spectra of nanocrystalline materials. Thermogravimetric analysis, X-ray diffraction and high-resolution transmission electron microscopy were used to complement Raman spectroscopy in order to facilitate the analysis and the interpretation of the results. This work has improved our understanding of oxidation of carbon materials, especially selectivity of the oxidation process to different carbon structures in a broad temperature range. The results of this study have been used to develop simple and environmentally friendly procedures for purification and surface functionalization of carbon nanoparticles and nanoporous materials.