Effect of ferrocene catalyst particle size on structural and morphological characteristics of carbon nanotubes grown by microwave oven

The influence of catalyst particle size on the formation and diameter of carbon nanotubes (CNTs) is investigated. Ferrocene catalyst with an average diameter of 19.7, 21.4, 23.6 and 27.0 µm is used for the growth of CNTs by a cost-effective and facile method using microwave oven. Morphological observations by transmission electron microscopy and field emission scanning electron microscopy reveal consistently that smaller catalyst diameter generates CNTs with smaller diameter. Raman spectroscopy indicates that the full width at half maximum of G-, D- and 2D-bands decreases gradually with increasing CNTs diameter; meanwhile, G-band/D-band intensity ratio is found to be sensitive to crystal defects, showing a drop for CNTs diameter in the range 25–40 nm then followed by a slight increase for higher diameters. This may be associated with CNTs curvature and strain which developed along tube walls. X-ray diffraction analysis demonstrates an increase in d (002) interlayer spacing with decreasing CNTs diameter. Furthermore, CNTs diameter is found to be inversely proportional to (002) linewidth. Finally, the energy band gap estimated from UV–NIR–Vis measurements increases slightly with CNTs diameter, 5.69–5.84 eV.