BIF primarily consists of hematite and magnetite grains alternating to quartz grain bands from thin bands of sedimentary rocks [
76]. Luis and colleagues used BIF powder obtained from the Bundelkhand Craton, Northern India, as a catalyst source for the production of N-MWCNTs via aerosol-assisted catalytic chemical vapor deposition (AACCVD) [
77]. The growth process is represented in . Yield production as high as 340% wt./wt. was obtained by 1-hour ball-milled BIF powders, and this indicated that BIF powders can be employed for large-scale CNTs production. The produced CNTs had a diameter in the range of 20–200 nm. N-MWCNTs having diameter of 200 nm were found to have wrinkled or corrugated surfaces. Helical structures with diverse surface morphologies and diameters could be obtained, which could be due to the unevenly shaped catalytic particles. They discussed a possible scenario to explain N-MWCNT morphologies for the high CNT yield. Upon reduction, the Fe
2O
3 structure embedded inside the BIF nanoparticles is transformed into the α-Fe phase. In their system, small nanoparticles of the catalyst were found to have strong interaction with the SiO
2 substrate, which remain fixed at their respective positions, and this hindered their catalytic performance. Further, this affects the growth of carbon nanomaterials, resulting in the complex NMWCNTs morphology. However, they predicted that the fixed nanoparticles in the system could lead to the thinner MWCNTs with individual longitudinal clumps, separated by the different size of the fixed catalytic nanoparticle. The thick NMWCNTs’ growth could be catalyzed by the combination of Fe
3C and fixed catalytic nanoparticles inside the system. Due to the complexity of the catalytic materials, they tried to explain this phenomenon with several possible conditions (see ). However, they pointed out that these circumstances must be more thoroughly studied to understand this phenomenon. Further, they proposed the following hypothetical explanation for the high CNT yield obtained. They speculated that, first, the nitrogen coming from benzylamine decomposes at 950 °C and converts into benzyl and amino. They synthesized their samples at 850 °C and it is quite probable that at this temperature, benzylamine is not completely decomposed, resulting in only the interaction of benzyl with the nanocatalyst, leaving a fraction of amino. Minor Fe nanoparticles formed when the BIF sample underwent a reduction for 30 min. The superficial O
2 bond at the SiO
2 nanomaterial was weakened, and the O
2 avoided the negative effect of H
2 from benzyl, supporting the CNTs’ growth. They conceptualized that the CNTs yield could be enhanced by controlling the hydrogen activity with the oxygen from the SiO
2 nanoparticle. In the scenario of N
2, MWCNTs yield is decreased due to toluene. The obtained results exhibit that benzylamine increases the CeN which improves the yield of NMWCNTs. Next, they extended their activities to study the electrochemical response of NMWCNTs-composed electrodes in order to examine the obtained nanomaterial for energy storage and sensing applications (see
Section 4).
Figure 7. Schematic representation of the steps followed to obtain nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) using the banded iron formation (BIF) material as a catalyst. (
a) Initial BIF powder with red-brown color, (
b) Ball milling of 5 g BIF powder with 10 mL of ethanol was performed for 1 h, 2 h, and 3 h, (
c) the obtained ball-milled BIF powders with dark brown color, (
d) the tablet of the milled BIF powder by 20 t pressing, (
e) AACCVD reactor wherein the tablets were positioned for growth, and (
f) the as-obtained N-MWCNTs after growth [
77]. Copyright © 2020 Elsevier B.V. Reprinted with permission from Elsevier.