In Situ Raman Spectroscopy of the Nanodiamond-to- Carbon Onion Transformation During Thermal Annealing of Detonation Nanodiamond Powder

Abstract

In this study, in situ Raman spectroscopy is utilized to investigate the onset of nanodiamond (ND) graphitization, the conversion of diamond (sp3) to graphitic (sp2) carbon, and the subsequent formation of carbon onions. Although the ND-to-carbon onion transformation through thermal annealing of ND in vacuum or inert atmosphere at high temperatures is a well-known phenomenon, the kinetics of the transformation and related structural changes in the nanocrystals are yet not fully understood. Using a high temperature stage under inert atmosphere, the Ultraviolet (UV) Raman spectra of ND were recorded during thermal annealing under isothermal and non-isothermal conditions to monitor the structural transformation of ND crystals at temperatures ranging from 25 to 1100 C. To complement the UV Raman spectroscopy studies, X-ray diffraction, high-resolution transmission electron microscopy, and thermogravimetric analysis were performed on bulk samples of annealed ND powders. The results obtained in this study demonstrate that the ND-to-carbon onion transformation starts with the surface graphitization of smaller ND crystals at temperatures as low as 600700 C. Between 9001000 C, the ND crystals begin to convert to carbon onions from the surface inward. Our study further revealed that the level of surface graphitization and the subsequent transformation of the ND during thermal annealing are strongly dependent on annealing temperature, annealing time, and ND crystal size. The gained knowledge does not only provide better understanding of the ND-to-carbon onion transformation mechanism and therefore allow for an optimization of the carbon onion synthesis process, but also enables the fabrication of ND with various degrees of surface graphitization. These hybrid ND/carbon onion particles have unique physical and chemical properties that are expected to lead to a completely new set of applications, particularly in energy storage.