In situ Study of the Activation Bimetallic Catalyst for Multi-walled Carbon Nanotubes Growth
Despite significant efforts, the CCVD synthesis of multi-walled carbon nanotubes with tailored characteristics is still challenging. Catalyst nature and reaction conditions embedded in multi-step mechanism create a complex puzzle of experimental data.
Using in situ XRD and in situ XPS, we separately studied the reduction of active metal species, their sintering, and carbon dissolution in catalyst. We found that the rate of the sintering of active particles determines the diameter of forthcoming nanotube. Due to the segregation, carbon species tend to concentrate on the surface or at subsurface layers of metallic species, that leads to insignificant carbon content in the core of small catalyst particles (<50 nm).
Using Raman spectroscopy, we studied a ratio of intensities of 2D (two-phonon scattering) and D (disorder-induced) bands (I2D/ID) as a function of catalyst composition, size, synthesis conditions. Each type of catalyst provides the linear dependence with its own specific slope. The difference in slope can be explained in terms of differences in kinetics and energy parameters of the main steps of MWCNT growth for different type catalysts. We consider the graphene fragments to form a mosaic grain-boundary structure of nanotube walls [S.N. Bokova-Sirosh et al. / Journal of Nanophotonics, (2016), 012526].
The data obtained can be used to provide the process kinetic model development and to optimize the synthesis conditions to produce MWCNT with controlled properties.