Preparation and characterization of carbon nano-onions by nanodiamond annealing and functionalization by radio-frequency Ar/O₂ plasma

ABSTRACT

Carbon nanomaterials can be prepared by several methods having in common that need a carbon source and often require high energies. In this study, we report the synthesis and characterization of carbon nano-onions by annealing of commercially available nanodiamonds and explore for the first time their functionalization with a radio frequency Ar/O₂ plasma. Heat treatment of nanodiamonds at 1200°C for 6 hours under argon atmosphere afforded small spherical carbon nano-onion particles of 3–4 nm diameter and 5–6 graphitic shells. The prepared CNOs were visualized by HRTEM and showed the characteristic XRD and Raman features. The results have been compared with a sample prepared by annealing at 1600°C. Plasma functionalization in Ar/O₂ atmosphere was used to introduce oxygen moieties into the surface of synthesized CNOs. X-ray photoemission spectroscopy showed that oxygen-containing groups like C─O, C═O and O─C═O were introduced on the surface of CNOs, a process that is accompanied by a surface reorganization as evidenced by the change of I_D/I_G ratios in the Raman spectra, indicating a conversion of sp² to sp³ as a result of functionalization in the surface of CNO.

KEYWORDS:

• carbon nano-onion
• nanodiamond
• annealing
• radio frequency plasma

Acknowledgments

JPB thanks the Departament d'Enginyeria Química of Universitat Rovira i Virgili for a predoctoral scholarship. Financial support from Ministerio de Economía y Competitividad, Spain (Grant BIO2012-30936 to AF), is gratefully acknowledged.