The Improving Carbon Nanotube Synthesis by the Removal of Amorphous Carbon

In this study, carbon nanotubes (CNTs) were synthesized on Ni-coated sapphire substrates using conventional and water-assisted chemical vapor deposition (CVD and WA-CVD) methods to evaluate the effect of water vapor on amorphous carbon removal and catalyst activity at low temperatures. Reduced nickel nanocatalysts were prepared by the sol–gel method and activated in a hydrogen atmosphere. Raman spectroscopy confirmed that CNTs synthesized by WA-CVD exhibited a higher degree of graphitization (ID/IG ≈ 1.18) and the absence of amorphous carbon peaks around 794 cm⁻¹, indicating improved purity. X-ray diffraction (XRD) analysis revealed the formation of graphitic carbon (002) and Ni₃C crystalline phases, as well as a rightward shift of the (002) peak to 2θ = 26.2°, suggesting lattice contraction caused by water-vapor-induced stress. Transmission electron microscopy (TEM) images showed that CNTs synthesized under WA-CVD conditions were thinner (17–25 nm), longer (≥ 1 µm), and cleaner than those obtained by conventional CVD, which exhibited thick amorphous carbon coatings. These results demonstrate that the controlled addition of water vapor during CVD suppresses amorphous carbon formation, regenerates catalyst active sites, and significantly enhances CNT crystallinity and morphological uniformity. The findings provide an efficient approach for synthesizing high-purity, well-aligned CNTs suitable for thermal interface materials, nanocomposites, and electronic device applications.

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