Carbon nanosheets by the graphenization of ungraphitizable isotropic pitch molecules

We report a tremendous structural evolution of carbon nanosheets (CNSs) through the graphenization of isotropic pitches prepared by reforming commercially available naphtha cracking bottom oil. CNSs with thicknesses of 2-13 nm were prepared by spin-coating on quartz substrates directly without a catalytic material, followed by two successive thermal treatments consisting of stabilization at 270 °C in air and carbonization at up to 1200 °C in H2/Ar. The development of well-ordered graphene layers around pebble-like structures with increasing carbonization temperature was observed in only the 13-nm thick CNSs by transmission electron microscopy, whereas thin CNSs did not reveal well-developed fringe growth. In addition, Raman, UV-vis, and X-ray photoelectron spectroscopies along with mass spectrometry unanimously showed clear evidence indicating that unusually well-developed crystalline structure was formed in the thick CNSs. It should be noted that isotropic pitches, which are intrinsically non-graphitizable carbon sources, evolved into highly ordered structures at a relatively low carbonization temperature (1200 °C). We believe that the graphenization of spin-coated films of pitch molecules resulted from the condensation of small pitch molecules and their self-assembled structure during carbonization under the anisotropic pressure induced by significant shrinkage in the vertical axis.

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