Side reaction in catalytic CVD growth of carbon nanotubes: Surface pyrolysis of a hydrocarbon precursor with the formation of lateral carbon deposits

Here we have studied the kinetics of the ethylene non-catalytic decomposition on the surface of multi-walled carbon nanotubes (MWCNTs) leading to the formation of lateral carbon deposits (LCD). Despite the comprehensive data on the gas-phase pyrolysis and a clear presence of surface impurities formed of carbon deposits, this process has not gained any proper attention. Within the temperature range studied (650-750 degrees C), we have found the rate of the reaction to be proportional to the MWCNT surface area and ethylene partial pressure. The activation energy of carbon deposition is close to that for the collision of two ethylene molecules to form C2H5 and C2H3 radicals. Based on the data obtained, we have proposed the radical mechanism of the formation of the deposits. High-resolution TEM, synchrotron radiation XRD, Raman spectroscopy, differential thermal analysis, temperature dependence of conductivity, and surface area measurements were used to characterize the structure and properties of the carbon-carbon composite produced. We have shown the deposition of lateral carbon to be a soft non-destructive technique for MWCNT surface functionalization allowing one to tune the properties of 3D-structured nanotube materials (e.g. arrays or aerogels). As an example, the LCD coating gradually changes the conductivity mechanism to 3D variable range hopping. (C) 2018 Elsevier Ltd. All rights reserved.