Synthesis and nucleation-growth mechanism of almost catalyst-free carbon nanotubes grown from Fe-filled sphere-like graphene-shell surface

This finding focuses on the optimization of synthesis time for the transformation of Fe-filled spherical-like graphene shell (GS) to almost catalyst-free carbon nanotube (CNT) structure using two-stage catalytic chemical vapor deposition apparatus. The camphor oil and ferrocene were used as carbon precursor and catalyst respectively, following the variety growth of graphene-family nanomaterials for 2, 4, 6, 8, 10, 30, and 60 min at 800°C synthesis temperature. The graphene-family nanomaterial properties were characterized using field emission scanning electron microscope, high resolution transmission electron microscope, micro-Raman spectrometer, thermogravimetric, and carbon-hydrogen-nitrogen-sulfur/oxygen (CHNS/O) analyzer. The result of field emission scanning electron microscopy analysis reveals that the CNTs were observed with high aspect ratio at 60-min synthesis time. The dependence of integrated intensity ratio of D-band and G-band (ID/IG) presented that ID/IG ratio sharply decreases with longer synthesis time. At higher synthesis time, thermogravimetric and CHNS/O analysis of CNT can obviously improve with decreases of non-carbonaceous material and transition metal catalyst. The nucleation-growth model of Fe-filled spherical-like GS to almost catalyst-free CNT has been highlighted to explain the change in growth mode.