In fact reducing the production cost of CNTs growth in

In fact, reducing the production cost of CNTs growth in large-scale requires quantitative and qualitative optimization studies of CVD parametric effects on CNTs growth, a concept which has not been fully developed. The present work is an optimization study of synthesis parameters involved in carbon nanotubes growth over the bimetallic (FeCo) catalyst supported on CaCO3. In this study, rather than fractional factorial experiments, statistical approach of DOE employed in CNTs synthesis was developed by full factorial design to control the yield and quality properties of the CNTs. The article is focused on the determination of the optimum growth conditions of CNTs. Properties of the CNTs were also investigated to determine how the preparation parameters affect the CNTs quality properties.
Materials and methods
Results and discussion The synthesized FeCo/CaCO3 catalyst exhibited properties that made it Phos-tag Biotin suitable for CNTs growth. The properties of the catalyst and its suitability for CNT growth was investigated using different characterization techniques including XRD, TGA, HRTEM-SEM/EDX and BET which were used to study the catalyst\'s crystallinity, thermal behaviour, morphology and surface area respectively. There has been conflicting opinions regarding the phases present in the final FeCo/CaCO3 catalyst material. In this study, the characterization results suggest that the crystalline phase, CoFe2O4 was more likely present in the catalyst material. The XRD pattern showed that the solid catalyst is polycrystalline with different crystal sizes resulting in a number of peaks as presented in Fig. 1. The thermal decomposition of the catalyst conducted in nitrogen environment is shown in the TGA profile presented in Fig. 2. The catalyst thermal decomposition showed four weight loss regimes. The first slope is attributed to loss of unbound water, the next two weight losses are due to conversion of Fe and Co nitrates to form a ternary metal oxide, most likely, CoFe2O4. The final weight loss represents the decomposition of CaCO3 to evolve CO2 and form CaO. The SEM micrograph of the catalyst and its corresponding EDX presented in Fig. 3(a) and (b) showed the morphology and qualitative elemental compositions of the sample. The SEM/EDX is evidence that the Fe and Co nanoparticles were well-dispersed in the CaCO3 matrix, possibly present as cobalt ferrite, CoFe2O4. A high resolution transmission electron microscopy (HRTEM), and the corresponding selected area electron diffraction (SAED) pattern was collected for determination of different phases present in the sample. Fig. 4 shows highly crystalline phases dispersed in the whitish CaCO3 support matrix. The crystalline phase in the HRTEM further suggests the possible formation of CoFe2O4 in the final catalyst. The wet impregnation method used in catalyst preparation has proved to be effective in dispersing the metal nanoparticles onto the support matrix. This was aided by stirring of the slurry mixture prior to heat treatments. Though, Fig. 4 revealed some zones where metal particles and CaCO3 support were in isolation from each other, excellently dispersed crystals of the metal particles in the support are illustrated by Fig. 5(a) and (b). The regions of metal particles are the active part or sites of the catalyst on which CNTs would grow, while white bulk regions are the support-dominated regions. The BET method was used to analyze the specific surface area of the catalyst sample and a value of 3.9 m2/g was obtained. It was observed that the catalyst has surface area similar to that of the CaCO3 powder used as support. The Fourier transform infrared (FTIR) spectroscopy of the catalyst as presented in Fig. 6 shows the functionalities that were present in the sample. By comparison with standards, the stretches, bends and sharp bands were used to identify groups that are present. As presented in Fig. 6, three functional groups were identified; the hydroxyl (OH) from unbound water, carbonate (CO32−) from the CaCO3 and the nitrates (NO32−) from Fe3+ and Co2+ salts. Calcium oxide (CaO) was not present because CaCO3 would not decompose until above the calcination temperature of 400 °C. The FeCo/CaCO3 catalyst possessed good properties that made it suitable for synthesis of Carbon nanotubes.