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 degrees C, yield fluorinated nano-onions (F-NO) of approximately C10.1F, C3.3F, and C2.3F 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.