Functionalization of carbon nano-onions by direct fluorination

Carbon nano-onions (CNO) are made of concentric fullerene-like shells and range from double- and triple- to multilayered structures. They remain the least studied allotrope of carbon yet. In the present study, a one-step process for functionalization of CNO (50-100 nm diameter) by addition of fluorine through direct fluorination at variable temperatures is reported. The reactions of CNO at three different temperatures, 350, 410, and 480°C, yield fluorinated nano-onions (F-NO) of approximately C_10.1F, C_3.3F, and C_2.3F stoichiometry, respectively, representing a new family of nanoscale fluorocarbon materials. The F-NO were characterized by a set of materials characterization methods including FTIR, Raman, UV-vis, and X-ray photoelectron spectroscopy, scanning (SEM) and transmission electron microscopy (TEM), X-ray diffraction, and thermal gravimetric analysis. SEM and TEM images show that even after breaking of the graphene layers in CNO by fluorine, the F-NO products retain the spherical onion-shaped morphology. The subsequent defluorination of F-NO by hydrazine treatment results in remarkable "healing" of broken graphene layers which rejoin within the sphere to substantially restore the CNO microstructure. In comparison with pristine CNO, fluorinated nano-onions show dramatically improved solubility in organic solvents, e.g., alcohols and DMF, enabling their processing for lubricating coatings, paints, nanocomposites, and biomedical applications.