Pyrrolic nitrogen-doped multiwall carbon nanotubes using ball-milled slag-SiC mixtures as a catalyst by aerosol assisted chemical vapor deposition

We demonstrated that the ball-milled slag-SiC mixture is an effective catalyst to grow pyrrolic nitrogen-doped multiwall carbon nanotubes (N-MWCNTs) by aerosol assisted chemical vapor deposition (AACVD) method. N-MWCNTs synthesized at 800 °C, 850 °C and 900 °C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, x-ray powder diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). TEM characterizations revealed the presence of a bamboo-like structure, a typical feature of nitrogen-doped carbon nanotubes. The presence of nitrogen was confirmed by the N1s XPS spectrum. Furthermore, a deconvolution of the N1s spectra revealed the presence of N-pyrrolic defects. This nitrogen functionality is investigated concerning the presence of silicon carbide material. Giant nanotubes with large diameters were obtained when SiC was added to the slag to be used as a substrate for N-MWCNTs synthesis. From Raman spectroscopy, the appearance of the D-band was observed, indicating the presence of topological defects that were also observed by TEM. XRD and TEM characterizations demonstrated the presence of Fe3C and α-Fe nanoparticles. The N-MWCNTs fabricated here could be used into (electro)catalytic applications or for reinforcing ceramic nanomaterial or polymers.


Experimental
The slag material used in this work comes from Minera Mexico, a mining industry located in San Luis Potosí, Mexico. As-received slag was grounded as follows: the slag was immersed in liquid nitrogen and hit with a hammer to reduce the size of the material; later, they were ground in a mortar to produce fine powder material. Following this, the resulting powders were sieved through a 32 microns mesh, and 4.5 grams of these powders were introduced into milling steel vials (6.4 cm high and 5 cm outer diameter and 3.3 cm internal diameter) together with 12 ml of ethanol and crushed for 120 min. The steel balls were four of 6.33 mm diameter and four of 12.7 mm diameter. The resultant slag particles exhibited sizes ranging from 100-500 nm long. This sample is denoted as SG-2h. Another ball-milled powder sample was obtained by adding 0.045 grams of SiC (K D Unipress) to the 4.5 grams of slag. The same milling procedure was performed in this sample. This sample was called SG-SiC. N-MWCNTs were synthesized by the AACVD method [24] using SG-2h or SG-SiC nanoparticles. The figure S1 is available online at stacks.iop.org/MRX/7/025602/mmedia (supplementary information) shows a drawing of the CVD setup used in this work. Two different procedures were employed to fabricate three different kinds of N-MWCNTs, as following: (1) one procedure consisted of a mixed solution of benzylamine (C 6 H 5 CH 2 NH 2 , Sigma-Aldrich, 99%) with 6% by wt. of SG-2h for the sprayer. This mixture is subjected to an ultrasonic bath for 30 min to obtain a well-homogenized solution. The final solution is then introduced into the ultrasonic mist generator. An aerosol cloud is generated and carried by argon flow (Praxair 99%), which flows at a rate of 2.5 l min −1 through a quartz tube (1 inch and 1.1 inch are the internal and external diameters, respectively). Such quartz tube is inside a tubular furnace where N-MWCNTs are grown (see figure  S1); (2) the second procedure involved two kind N-MWCNTs, these consisted in placing the SG-2h (or SG-SiC) inside the quartz tube within the first furnace, then benzylamine alone was introduced into the ultrasonic mist generator, and the synthesis was carried out. In the three cases, the synthesis was done at three different temperatures (800°C, 850°C, and 900°C). The samples were labeled as CNx-800, CNx-850, and CNx-900 for the procedure (1), and the other two kinds of samples obtained by the procedure (2) were labeled as CNx-T-SG when SG-2h was used as catalyst, and when SG-SiC was used the samples where CNx-T-SiC (T=800°C, 850°C and 900°C). A detailed scheme of the N-MWCNT fabrication process can be seen in figure S2. Samples were dispersed using a Cole Palmer ultrasonic processor (500 watts 20 kHz) for the characterization. The produced N-MWCNTs were characterized by SEM, scanning transmission electron microscopy (STEM), and energy dispersive spectroscopy (EDAX) using a Philips FEG-XL30 microscope and an HRTEM 300 kV FEI TECNAI F30 STWIN G2. X-ray powder diffraction patterns (XRD) were obtained using a Bruker XD8 ADVANCE, and Raman spectra were obtained using a Micro Raman Renishaw with excitation wavelengths of 633 nm and 532 nm. X-ray photoelectron spectroscopy was performed using a PHI 5000 VersaProbe II equipment with a monochromatic Al-κα source (1486.7 eV). The deconvolution analysis was performed using the software Spectral Data Processor SDP 4.1. In addition, thermogravimetric analysis (TGA) was performed for all samples using an STA 6000 Perkin-Elmer under the following conditions: samples were heated in simultaneous thermal analyzer in a temperature range of 50 to 950°C with a heating speed of 10°C min −1 under a dynamic flow of oxygen (20 ml min −1 ). Measurements of CNT diameter were obtained using ImageJ (free software for image analysis from the National Institutes of Health) from at least 5 SEM (50,000X) images of each sample. Crystallographic Toolbox software was used to obtain zone-axis analysis and diffraction pattern recognition.
Although the results of EDX analysis performed on the samples are only qualitative, they can be served as a guide for the analysis. For example, from table 1, considering the case of slag without SiC, the Fe content is around 10 at%. However, when the SiC was added, it seems that Fe concentration is relatively reduced to ∼3 at % with higher concentrations of oxygen (∼45%), and Si concentration near to the 10%. Figure 2 shows the elemental mapping of the main detected chemical elements that were performed on a particle of SG-2h. Notice that all considered elements are homogeneously distributed on this sample. XRD patterns of the original slag, SG-2h, and SG-SiC also were performed, observing the presence of silicon carbide (SiC), magnetite (Fe 3 O 4 ), iron carbonyl sulfide (Fe 3 (CO) 10 S) among other more complex crystal structures (see figures S4(a)) and (b)).
When the SG-2h material is mixed with benzylamine to synthesize N-MWCNTs, the effect of synthesis temperature is very important to define the carbon nanotube diameter. For example, the average diameter observed in CNx-800 (figure 3(a)) is found in the range of 20-60 nm, but some larger diameter can be observed (see the white arrow and figure S5(a)). However, when the temperature of synthesis rises to 850°C (figure 3(b)), Figure 1. Energy-dispersive x-ray spectroscopy (EDX) chemical analysis of slag before ball milling process (a); 2 h ball-milled slag material (b); and 2 h ball-milled slag+SiC mixed material. Apparently, the main contributions in the (a) and (b) cases come from Fe, O, Si, and Ca. In the case of (c) also C seems to have a significant contribution.  the diameter distribution is bimodal with diameters around 330 nm and 580 nm ( figure S5(b)). At 900°C, the CNx-900 possesses large diameters around 400 nm and 600 nm (figure 3(c)), but also a smaller diameter near 100 nm (figure S5(g)). It is possible to observe Y junctions and branches or kinks, which could be due to the presence of sulfur contained in the SG-2h. Previous experimental reports demonstrated that sulfur induces branched or Y-shaped morphologies in MWCNTs [31,78]. A completely different morphology is found when the SG-2h is placed in the quartz tube for the N-MWCNTs synthesis. Although CNx-800-SG (figure 3(d)) presents a variety of diameters from 20 nm to 160 nm, most of them have diameters less than 100 nm. At this temperature, the size of the catalytic nanoparticles is not uniform, and the diameter of CNx-800-SG also did not present a preferential size. When the temperature is raised to 850°C (figure 3(e)), the distribution of CNx-850-SG diameter exhibits a Lorentzian-like distribution with a peak around 30 nm (figure S5(e)). If the temperature is increased to 900°C (figure 3(f)), the diameter distribution of CNx-900-SG is also Lorentzian-like with a peak at 70 nm (see figure S5(h)).
The morphology of CNx-850-SG (figure 3(e)) is like the CNx-800-SG but with a larger diameter probably as a consequence of a higher temperature synthesis. In the case of CNx-900-SG (figure 3(f)), the surface is damaged. It is observed that some of them appear to be longitudinally cut. Although this CNx-T-SG (T=800°C, 850°C, 900°C) group of N-MWCNTs presents many defects, they have a uniform appearance at these SEM images.
The morphologies of the CNx-T-SiC group (T=800°C, 850°C, 900°C) are similar to the CNx-T-SG group of N-MWCNTs, but some peculiarities deserve to be mentioned. For example, in figure 3(g) (CNx-800-SiC case), there are open carbon nanostructures (white arrows) with non-uniform diameters. There are also many agglomerates of N-MWCNTs. The same circumstance happens with CNx-850-SiC and CNx-900-SiC, figures 3(h), and (i), respectively. It is worth mentioning that in the case of CNx-T-SG and CNx-T-SiC, the carbon nanotubes are regularly attached to the catalyst substrate (∼2 or 3 μm of size). It is probably that some irregular morphologies of CNx-T-SiC are due to the additional SiC powder in slag. It is possible that SiC powders interacted with traces of Fe, O, Ca, Cu, Al, Mo, and Mg, forming a complex catalyst that provokes the formation of atypical N-MWCNTs.

TEM characterization
The TEM image of CNx-800 (see figure 4(a)) confirmed the presence of the typical bamboo-like feature, as previously reported [77]. Figure 4(b) displays a nanoparticle inside of a CNx-800. The presence of the graphitic layers with an interlayer spacing of 3.4 Å and iron-carbide nanoparticle reveals an interlayer spacing of 2 Å, which corresponds to the (200) plane of Fe 3 C (figure 4(c)). We observed that CNx-800 exhibited large catalytic nanoparticles, which were found mainly within nanotubes with relatively small diameters (around 20 nm). Figure 4(d) shows bamboo shape N-MWCNts from CNx-850 exhibiting a broken tip. It can be observed that compartment dimensions in the bamboo-shape structures change alongside the CNT. Figure 4(e) shows the structure of borders between compartments. The acute geometrical structure observed could be explained due to the possible existence of N-pyrrolic chemical species. The typical distance of 3.4 Å between graphitic layers is shown in figure 4(f). For a synthesis temperature of 900°C, in CNx-900 sample kink morphologies were identified (figure 4(g)), these morphologies could be attributed to deficient concentrations of non-carbon elements such as sulfur [78,79]. However, in some cases, the produced CNx-900 exhibits a poor graphitization on the walls (see figure 4(g)). This effect could be generated due to the multi-element catalyst precursor. It is well known that defects close to the surface of CNTs that contain foreign atoms can present such behavior in the carbon walls [52]. Nevertheless, some of the CNx-900 presents very well graphitization, as it is shown in figure 4(h), although superficial cavities also are formed (red arrow). The distance between the carbon layers was 3.4 Å in this sample.
To identify the chemical elements contained in the catalytic particles, we performed an energy dispersive spectra (EDS) line profile of different nanoparticles inside CNx-900 (see figures 5(a) and (b)). The EDS line profiles indicate that the nanoparticles contain mostly Fe (see figures 5(d) and (e)). The carbon line profile does not exhibit a significant drop in the intensity. This fact suggests that the interior of the nanoparticle is composed of iron carbide (Fe 3 C). Figure 6 shows TEM images of different features of CNx-800-SG nanomaterial. Figure 6(a) shows a cluster of nanoparticles that were agglomerated in apparently amorphous carbon material. It is possible to observe a carbon flake below the cluster (red arrow). Figure S7(a) exhibits the same cluster but with a blue circle to point the zone where an EDX chemical analysis was performed (figure S7(b)). Notice that almost the same elements of original slag were detected. In figure 6(b), an irregular small diameter CNT entangled with a curved largediameter CNT. Note that two large nanoparticles (red arrows) could be responsible for such complex amassment. The inset (red square) in this figure shows another example of this structure; in this case, there are four nanoparticles involved (yellow arrows). This type of morphology seems to be a constant when the SG-2h is set into the quartz tube for the CNT synthesis. Two kinds of nanotubes could be mainly detected. One with a large external diameter (∼130 nm) and relatively small thickness of multiwall carbon layers (∼30 nm). The relation between the outer diameter (O.D.) divided by internal diameter (I.D.) being more than four. It is possible that these kinds of CNT (red arrow in figure 6(c)) are generated by large nanoparticles. Interestingly, in some areas of N-MWCNT nanostructure, there are double or triple groups of multiwall carbon layers (see yellow arrow in figure 6(c)). The bamboo shape of this giant N-MWCNTs is not uniform, and mostly the compartments are divided by few carbon layers. Another kind of structure found is shown in figure 6(d), where an irregular tubular curved carbon nanostructure is exhibited. The rugged surface of this kind of CNT is severely affected due to small nanostructures that are attached (red arrows in figure 6(d)). A closer image shows such surface nanostructures (red arrows in figure 6(e)). Besides, the carbon walls are only locally ordered and have a thickness around 17 nm, considering that the diameter of the nanotube is ∼35 nm, then O.D./I.D.∼2. This morphology property promotes irregular compartments (figure 6(e)).
The encapsulated nanoparticle observed in figure 6(d) (red circle) is α-Fe (see inset of figures 6(f) and S8) with spheroidal-shape (figure 6(f)). The inset of this figure is the FFT that define a [-110] zone-axis . Figure 6(f) shows a faint ring surrounded the nanoparticle. This ring is associated with iron oxide [80], but here according to the EDX results, this ring probably also contains SiO 2 (see figure S9). Figure 6(g) is a TEM image exhibiting another short CNx-800-SG with a large diameter (∼250 nm). Due to the active sonication process, it is very likely that some of these CNTs with large diameters were unzipped, yielding graphite sheets of few layers (see red arrow in figure 6(h)). Notice that although the structure of CNx-800-SG of figure 6(e) is not uniform, the carbon layers exhibit a good grade of crystallinity (figure 6(i)).
The effect of raising the synthesis temperature to fabricate CNx-850-SG is shown in figure 7. The clusters of curled CNx-850-SG (figure 7(a)) are present. At this temperature of 850°C, the catalytic nanoparticles are spherical and larger. This morphology is regularly not present in typical N-MWCNTs synthesis with benzylamine and ferrocene, where the catalyst nanoparticles are conical and more abundant [80]. The carbon structures shown in figure 7(b) are present in almost all temperatures of CNx-T-SG (T=800, 850, and 900°C). The majority of these CNTs exhibit irregular kink structures and no homogeneous compartments in size and shape (figure 7(c)). The presence of non-carbon elements, such as sulfur or nitrogen, could induce this irregular morphology [31]. Note, however, that the crystalline of CNx-850-SG is apparently not affected. It could be detected 200 nm-diameter CNx-850-SG with curly surface and large tip catalyst (see red arrow in figure 7(d)). In addition, long nanotubes without bamboo shape also were found (yellow arrow in figure 7(d)). Fe 3 C nanoparticle encapsulated within the nanotubes were observed (figure 7(e)) with a [011] zone-axis . Figure S10 shows the general procedure to define the zone-axis, and the crystal structure is displayed. In figure S11, its respective EDX analysis is presented; notice that the Fe and C peaks are the most intense. Another aspect of this CNx-850-SG is the small attached material to the outer walls (figure 7(f) and inset), which exhibit a more complex morphology. CNx-850-SG also contains several kinds of nanoparticles that probably were partially responsible for its irregular morphology. The inset of figure 7(g) corresponds to the FFT (yellow arrow) of Fe 3 C nanoparticle, in which the [0-23] crystall direction could be identified (see figure S12). Figure S12 shows the complete analysis carried out for the identification of this crystall direction and the crystal structure of this internal Fe 3 C nanoparticle. High crystallinity of Fe 3 C (red arrow) nanomaterial is observed in the inferior part of the same figure 7(g) with distances of ∼2.4 Å and ∼4.8 Å. Figure S13 also shows the whole analyses performed on this nanomaterial to determine the crystal structure associated with the TEM image were the crystall plane was [010]. The EDX analysis is shown in figure S14(b), where several elements were detected. It looks that the O, Si, Ca, Mn elements could be a component of the material around such Fe 3 C nanoparticles (see figure S14(a)). Some carbon nanoribbon-like structures were also observed (figure 7(h)). Figure 7(i) exhibits another large CNx-850-SG with 100 nm of diameter and a large catalyst nanoparticle presenting a sharp end. This nanoparticle is covered with a wide graphitic shell.   Figure 8(b) shows another group of CNx-900-SG presenting thick walls (∼50 nm) and open tips. Interestingly, this CNx-900-SG is decorated with small (∼5.3 nm) nanomaterial (figure 8(c)). In inset 1 and inset 2, this type of decoration is observed at the tip or in the lateral surface of CNx-900-SG, respectively. Also, in inset 1 can be seen that the big nanoparticle is surrounded by a thin film of another material different than carbon. This catalyst nanoparticle (figure 8(d)) is monocrystalline α-Fe with a [-110] zone-axis as observed by FFT (see inset figures 8(d) and S15). In figure 8(e) are shown two types of such anchored nanomaterial, one it seems different than carbon with polygonal morphologies (red arrow) and another probably carbon nanomaterial (yellow arrow). Figure 8(f) shows a fullerene-type attached to CNx-900-SG with an interlayer distance of ca. 3.7 Å. An EDX analysis on the nanoparticle (figure S16(a)) demonstrates that it contains Fe (see figure S16(b)). The Fe 3 C nanoparticle showing a [-4-12] zone-axis is exhibited in figure S17. Figure 9 depicts the effect of adding SiC powder to the slag to grow N-MWCNTs at 800°C. There are two types of CNTs: one with a large diameter (red arrow) and another with a smaller diameter and a highly curled morphology (figure 9(a)). The morphology does not change when the slag is altered with SiC. It seems that only the diameter distribution changes. Internal nanoparticles of large diameter CNx-800-SiC are Fe 3 C or α-Fe. These nanotubes present an internal structure with double or triple wall groups (figure 9(b)). Unusual morphologies can be found, as shown in figure 9(c), where a giant nanotube with an enormous tip compared with the body is exhibited. An EDS analysis performed at the interface between carbon and a catalytic nanoparticle (magenta circle in figure S18(a)) shows Fe and oxygen content. The oxygen comes from the thin iron oxide film formed around the Fe nanoparticle [80]. Also, copper (from the grid), silicon, and carbon were detected in this CNx-800-SiC. In figure S18(b), the EDS analysis taken at the green circle on the center of nanoparticle shows the presence of O and Fe. Notice that the oxygen signal is smaller concerning to Fe signal. The CNx-800-SiC exhibited in figure 9(d) could be a distinctive nanomaterial found in this sample and not in other cases. The external diameter is approximately 150 nm and a multiwall thickness of around 75 carbon layers. These kinds of CNTs could have a better contribution to high oxygen reduction reaction activity [81]. They also have large inner diameters. Eswaramoorthi and Hwang et al [82] fabricated CNTs with large diameters but using anodic titanium oxide. Slag has a diopside containing titanium that could be the causing of such structures, but it is necessary to deepen the investigation to clarify such a phenomenon. Unlike those manufactured by Eswaramoorthi and Hwang [82,83], our CNx-800-SiC nanotubes have internal structures, probably due to nitrogen. CNTs with huge diameters (5 μm) have been produced using laser or electric arc techniques [84]. The structure of the small diameter and corrugated CNTs (figure 9(e)) are like the CNx-T-SG cases, namely, disordered compartments along with curly CNT structure (figure 9(f)). Large diameter CNx-800-SiC with two groups of carbon walls, as shown in figure 9(g), is another morphology that has probably never been reported. The mixture of nanoparticles forming the catalyst probably promoted these two groups of multiwall (see figure 9(g)). Figure 9(h) shows more clearly two groups of multiwall morphology, where the internal nanoparticle (red arrow) possibly generated the internal wall (yellow arrows). In this class of nanomaterial, it was found the morphology presented in figure 9(i), which is spherical nanoparticle (α-Fe) surrounded by silicon oxide, forming a core-shell nanostructure. The core nanoparticle is α-Fe with a [311] zone-axis (see figure S19). The shell part is SiO 2 with a [−132] zone-axis, as is showed in figure S20. Figure 10 shows the TEM and HRTEM images of CNx-850-SiC nanostructures. Figure 10(a) exhibits an agglomerated CNTs. Figure 10(b) shows an individual CNx-850-SiC with an internal nanoparticle ∼40 nm at the tip (red arrow). Figure 10(c) shows this type of CNx-850-SiC exhibiting highly curved, rugged surface and and (c) exhibiting a large relation between internal diameter (ID) and outer diameter (OD). (e) A small diameter curved CNx-800-SiC nanotube. (f) section of CNx-800-SiC showed in (e) that is similar to CNx-800-SG (figure 6(e)). (g) Giant CNx-800-SiC with a large internal nanoparticle; this enormous CNx-800-SiC presents the double-shell multiwall morphology. (h) HRTEM of (g) where it easy to see the double-shell multiwall with no uniform growing. (i) core-shell material where the core is α-Fe nanoparticle with a [311] zone-axis (figure S19) and the shell is SiO 2 (see figure S20).
∼2-3 nm carbon nanoparticles anchored to its surface (red arrows). Notice these CNx-850-SiC present Fecontaining (figure S21) nanoparticles within the cores at the tip (figure 10(d)). Another CNx-850-SiC observed is shown in figure 10(e). This carbon nanostructure consists of a large no-uniform diameter with semi-tubular morphology and nanomaterial with irregular morphology anchored to its surface (figure 10(f)). Analogous to the common N-MWCNTs fabricated with benzylamine and ferrocene [85,86], they also exhibit a type of bamboo-shape structure with compartments of around 30 nm (see red double arrow in figure 10(g)). Also, there are empty spaces (red double arrow in figure 10(h)) between two carbon groups of walls, which resemble two concentric separated multiwalled-CNTs ( figure 10(h)). The phase of the encapsulated nanoparticle in this CNx-850-SiC is α-Fe, as observed in figure 10(i), where FFT analysis (inset) shows the zone-axis of an α-Fe nanoparticle. A more detailed analysis can be consulted in figure S22. Giant tubular carbon structures with large diameters (∼400 nm) and several sets of multiwall morphology are also present in this CNx-850-SiC group (figure 10(j)). A big nanoparticle is generating a large diameter, but other internal nanoparticles are also generating the other interior walls. Notice that the compartments are not the typical bamboo shape ( figure 10(k)). The attached material on the surface of CNx-850-SiC is also present (figure 10(l)), and according to an EDS analysis, they contain O, Fe, Al, Si, S, (see figure S23(a)). Figure S23(b) shows the EDS analysis of the encapsulated nanoparticle being Fe the main element found.
Different aspects of the carbon nanomaterial obtained from the CNx-900-SiC group are exhibited in figure 11. Clusters of irregular CNT with curling shape are revealed in this sample ( figure 11(a)). Large diameter multiple sets of multiwall carbon giant structures were detected in this group of CNT (red arrow in figure 11(b)). The diameters of these CNTs are approximately 300 nm. A huge catalyst nanoparticle with a dimension of 570 nm long and 300 nm of diameter are associated with their synthesis (red squared). Notice the tiny CNT below this giant CNT (green arrow). Graphene-like flakes (blue arrow) and extremely large dimension CNx-900-SiC (yellow square) structures were observed. This image also exhibits a straight CNx-900-SiC nanotube with 80 nm of diameter (yellow arrow). Figure 11(c) also shows several large diameters (∼100 nm) CNx-900-SiC with sizable nanoparticles distributed alongside. Note the massive structure that is accompanying these CNTs (red arrow). Figure 11(d) exhibits the different carbon structures found in the CNx-850-SiC group, namely large-diameter multi-wall, small diameter, and curly CNTs. Another tremendous CNx-900-SiC structure is displayed in figure 11(e). Its diameter is close to 275 nm. These giant carbon structures can be useful to the catalysis process at the nanometric level [82,83]. Because they have several compartments at the same level could be applied to have molecules with different functional activity. They can be carefully broken to fabricate large graphene sheets using the Elias et al method [87]. Figure 11(f) shows graphene-like transparent material identified in this sample, which can indicate that it is a material of a few carbon layers. Another kind of CNTs can be further found, as is showed in figure 11(g). Note as the upper CNx-900-SiC (red arrow) start with a like-normal multiwall morphology with an internal diameter of one-third of external diameter. However, after some distance, a double set of multiwall starts continuing to the end. This last part presents a disordered structure with several internal nanoparticles alongside the walls. Probably their electrical properties are different when the electrons change from one zone to the other. The lower CNx-900-SiC (blue arrow) presents a very pronounced folded close to the tip. Important to mention that the bamboo shape in this kind of carbon structure is not well defined, as can be seen in figure 11(h), which is a magnification of the upper CNx-900-SiC of figure 11(g). Note that the compartments are irregular and the borders consist of few carbon layers. Figure S24 shows two EDS spectra of colored nanoparticles from figure 11(h), red (a), and green (b). According to these spectra, both nanoparticles contain Fe. Figure 11(i) displays another N-MWCNT from the CNx-900-SiC group presenting a bamboo shape of a large diameter. However, their compartments are not uniform and suddenly stop appearing (see red arrow). Besides, graphene-like structures could be detected in this TEM image. Figure 12 shows the x-ray diffractograms for all the samples obtained from the different synthesis conditions. Figure 12(a) presents CNx-800 (red), CNx-800-SG (blue) and CNx-800-SiC (black) cases. Fe 3 C (asterisks) and C (rhombs) are clearly identifiable. In CNx-800-Sg also there is a peak associated with β-SiO 2 (star) and in both CNx-800-SG and CNx-800-SiC α-Fe phase (inverse filled black triangles) is present. There is a broad peak observed between 10-15 2θ-degrees, which is likely due to the presence of intercalated graphite oxide. Figure 12(b) corresponds to the 850°C temperature synthesis cases. CNx-850 presents a shoulder at around 21 2θ-degrees correlated to β-SiO 2 . In this sample, the Fe 3 C peaks (asterisks) were not resolved. The only difference between CNx-800-SG (CNx-800-SiC) and CNx-850-SG (CNx-850-SiC) was the existence of a peak at 24 degrees related to the expanded graphite (EG). This situation could be related to the phenomenon observed in figure S17, where the significant distance between carbon layers where observed When the N-MWCNTs are synthesized at 900°C the β-SiO 2 and EG phases are not present ( figure 12(c)). The SiC phase was not detected in any CNx-T-SiC (T=800°C, 850°C, and 900°C). Figure 13 depicts the Raman spectra for the N-MWCNTs obtained at 800°C (figure 13(a)), 850°C ( figure 13(b)), and 900°C ( figure 13(c)). CNx-800 (red curve in figure 13(a)) presents an I D /I G =1.14, probably associated with the rugged surfaces of this sample. The I D /I G for CNx-800-SG (blue curve in figure 13(a)) is 0.83, which is associated with the purity of CNT or low degree of disorder in CNT [88]. According to SEM and TEM characterization of this CNx-800-SG, they show a dramatic curved structure with some defective surface and complicated internal morphology. However, HRTEM shows that even such accidental and irregular framework of such CNT, these are formed with well-defined graphitic layers. The I D /I G for CNx-800-SiC (black curve in figure 13(a)) is 0.89, a close value to the CNx-800-SG case. Due to the morphologies of CNx-800-SiC and CNx-800-SG are very similar, probably this increase in I D /I G for CNx-800-SiC could be associated with the small nanoparticles attached to the CNx-800-SiC surfaces (figure 6(e)). The Raman spectra of N-MWCNTs for the three cases studied at 850°C synthesis temperature ( figure 13(b)) exhibit a comparable behavior with the 800°C case. The value of I D /I G for CNx-850 is 1.28, being larger than the CNx-800 case. According to figure 3(d), these CNT present larger diameters and rougher surfaces, also there are more entangled small diameters CNx-850, which could be associated with this increase. CNx-850-SG (blue curve) presented I D /I G =0.81 very similar than CNx-850. Also, the values of I D /I G for CNx-850-SiC do not differ from CNx-800-SiC being, in this case, 0.82. CNx-900-SG and CNx-900-SiC exhibit ID/IG values of 0.83 and 0.96, respectively. In these two latter cases, the structures showed in figure 11(d) (yellow squared) possible are to attribute for the increase in I D /I G in the case of CNx-900-SiC.

XPS characterization
Different chemical functional groups attached to the surface or edges of N-MWCNTs were analyzed by x-ray photoelectron spectroscopy (XPS). According to these results, the total atomic concentrations of carbon, nitrogen, oxygen, and silicon depend on the synthesis temperature and the type of CNx (see table 2).
Due to the chemical characteristics of samples, when the XPS is performed in CNx-T-SG or CNx-T-SiC (T=800°C, 850°C, and 900°C), the surveys present several peaks that were not considered for simplicity of analysis. We only present the results were carbon, nitrogen, oxygen, and silicon are involved. For instance, CNx-800 presents a concentration of carbon, nitrogen, oxygen, and silicon of 64.3 at%, 2.0 at%, and 22.6 at%, and 8.9 at%, respectively. However, CNx-800-SG and CNx-800-SiC contain more than 76 at% of C1s, which is around 15% more than the CNx-800 case. This situation is inverse when the synthesis temperature is increased to 850°C where the C1s concentration in CNx-850 is the highest (77.1%) comparing with CNx-850-SG (64.1%) and CNx-850-SiC (75.8%). The previous situation is repeated for CNx-900 and CNx-900-SG, where C1s concentration is 78.8% and 61.0%, respectively. However, in the case of CNx-900-SiC, the C1s concentration increase to 84.1 being the highest. In the case of N1s, the concentration oscillates between 1.3% (CNx-850-SG) and 3.6% (CNx-900-SiC). This last feature could be the cause of morphologies, as presented in figure 11(b). When slag is mixed with SiC, the N1s contain is relatively high of 2.4% in CNx-800-SiC, 1.8% in CNx-850-SiC, and 3.6% CNx-900-SiC. However, the lowest values over the N1s average are for CNx-T-SG, where T=800 is 1.7%, T=850 is 1.3%, and T=900 is 2.2%. Really large values of O1s were found in most of the cases being the highest for CNx-850-SG (26.7%) and the lowest for CNx-900-SiC (10.4%). SG-2h in benzylamine provokes high O1s to contain in N-MWCNTs which could be very important to electrochemistry studies [89]. This phenomenon has been studied, and it is attributed to the possible oxidation of benzylamine [90,91]. At higher temperatures, this process is less than possible. Contrary, in the case of CNx-900-SG, the XPS results show an increasing of O1s concentration with the temperature. Here the oxygen might come from the metal oxides that are not in the CNT,probably due iron oxide (Fe 3 O 4 ). From the SEM and TEM result, it was possible to observe that CNx-850-SG are attached to large nanoparticles formed by similar elements than the original slag. At higher temperatures, these nanoparticles are more prone to accept oxygen atoms. Although in the case of SiC is added to slag also CNx-T-SiC attached to large nanoparticles are observed, the cases of CNx-T-SiC have completely different behavior than CNx-T-SG. Also, slag in some cases foments concentration of Si2p since 1.2% in the CNx-900-SiC case to high as 11.1% or 10.1% for CNx-800 and CNx-900-SG, respectively. It has been reported nanostructured Si for high energy lithium-ion batteries due to it has ten times higher specific capacity than traditional carbon anodes [92]. The percentage of Si2p considerably decreases in CNx-T as T increase from T=800°C (11.1%) to T=850°C (5.9%), then at 900°C (6.1%) remains approximately constant. Contrary to the previous case, CNx-T-SG increases the content of Si2p as the synthesis temperature raises. For example, for CNx-800-SG Si2p is 1.7%, for CNx-850-SG is 2.6% and for CNx-900-SG is 10.1%. The content of Si is probably associated with the encapsulated or attached nanoparticles, which were detected by EDS in these samples. It is very probable that also the change of morphology exhibited in figures 6, 7 and 8 are related to the interaction between Si, N, and Fe in the CNT growth process. When additional SiC is involved in the catalyst for CNx fabrication the Si2p % content, also increase from 3.7% in CNx-800-SiC to 4.7% in CNx-850-SiC. The decrease of Si2p in CNx-900-SiC could be correlated with the increase in temperature where possible SiC is sublimated. Therefore, SiO 2 is the main Si-nanomaterial that could interact with C, N, and Fe. SiO 2 nanomaterials were found at the shell of Fe, or Fe3C encapsulated nanoparticles (figures S9 and S20).

CNx-T cases (T=800, 850 and 900°C)
According to the C1s spectrum, there is a large amount of sp 3 hybridized carbons revealed by the XPS characterization for the three synthesis temperatures studied (figures 14(a), (d), and (g)), which agrees to the Raman characterization where all these samples presented low quality of graphitization. Interesting is that the number of C-O-C functionalities is considerably large, which are related to the ether group (ethoxy, methoxy, or epoxy) generally formed in defective surfaces or edges of carbon nanomaterials [93].
The bind of C with metals is only manifest in Fe 3 C at 282.6 eV for the CNx-800 case. N1s spectrum of CNx-800 indicates the presence of pyridinic nitrogen at 398.89 eV, pyrrolic nitrogen at 400.58 eV, and graphitic nitrogen at 401.4 eV. The pyridinic oxide nitrogen is also found at 402.3 eV (see figure 14(b)). CNx-850 yielded the following binding energies: pyridinic 399.12 eV, pyrrolic 400.68 eV, graphitic 401.86 eV, and pyridinic oxide at 403 eV. The percentage of this latter type of nitrogen is considerably reduced. We found the same four types of nitrogen defects with slightly different binding energies for synthesis at 900°C ( figure 14(j)). In general, all samples shown high concentrations of N-pyrrolic, which is not common in a conventional synthesis of N-MWCNTs [94]. It is notable that in all the cases, pyrrolic N is the most abundant functionality in CNx-T (around 50%). This situation could be correlated with the increase of disorder and the presence of SiC in the sample. It seems that pyrrolic functionality is correlated with the corrugated surface and large diameter of the CNx-T cases. This situation could be attributed to silicon carbide material contained in the original slag (see figure S4), note that a relatively large concentration of Si was detected in the slag material (around 15.4%). Therefore, Si atoms could act as anchor points for the nitrogen-atoms, as experimental reports have demonstrated that the SiC surface favored the formation of pyrrolyl groups due to the Si-N linkage [95,96]. The high-resolution O1s spectrum of CNx-800 presents a significant contribution from carbonyl carbons or hydroxyl groups. There is a relatively substantial contribution from metal oxides, which is concordant with the x-ray results. This type of bond considerably diminished in CNx-850 and CNx-900 (see figures 14(g) and (k)). The ether functionalities increase their percentage and decrease the carbonyl functional groups or phenolic functional groups contribution also for CNx-850 and CNx-900 (see table S1). It is characteristic in all synthesis temperatures that the morphologies of CNT are complex with junctions and curved shapes. Deconvolution of high-resolution XPS Si2p spectrum exhibits in CNx-800 the signal of siloxane (Si-O-Si) accompanied by a large percentage of SiO 2 and SiOx together with signals of following other chemical states Si 2 O 2 /SiC and SiC ( figure 14(d)). In the case of CNx-850, the reduction of both SiO 2 and SiOx is evident, and the siloxane group is not present anymore. Now the most important bands are related to the interaction between SiC and Si 2 O 2 and the SiC itself. For CNx-900, the signal of SiC almost disappears being the most crucial SiO 2 /SiC signal ( figure 14(l)).

CNx-T-SG cases (T=800, 850, and 900°C)
XPS results for CNx-T-SG samples are presented in figure 15. In all three the cases considered, C1s spectra revealed the presence of sp 2 (C=C) and sp 3 (C-C) hybridizations with binding energies of ∼284 and ∼285 eV, respectively. Besides, metal carbide is present but only in the CNx-800-SG case. Phenolic, carbonyl, and carboxyls are also present at ∼286 eV, ∼287 eV, and 288 eV.
C1s spectra for CNx-850 and CNx-900 (figures 15(e) and (i)) are very similar, with slightly contribution differences for the distinct functional carbon groups. In these two latter cases, there was not detected metal carbide. Although the contribution of sp 3 is still significant here for all CNx-T-SG (T=800°C, 850°C, and 900°C), the C=C also is important. For N1s spectra, N-pyridine, N-pyrrolic, N-graphite or quaternary, and N-O bonds in this carbon material were observed (figures 15(b), (f), and (j)). Like the CNx-T case, here, high concentrations of N-pyrrolic are one of the most important hybridizations. Graphitic N is also abundant in the CNx-850-SG and CNx-900-SG cases at the expense of the decrease of N-pyrrolic as can be seen in figures 15(f) and (j). These results are concomitant with Raman spectroscopies issues (see figure 13(b)). From O1s, highresolution signals can be constructed five main contributions. At around 531 eV, the contribution could be attributed to magnetite (Fe 3 O 4 ) that was detected by x-ray characterization. At around 531 eV and 532 eV, the oxygen contribution can be associated with C=O (carbonyls) and ester (COO-), respectively. C-O-C ether (∼533 eV) and -COOH (oxygen in carboxyl groups) is around 534 eV. In this case, CNx-800-SG presents an appreciable amount of oxygen in carboxyl groups (-COOH), which is not in CNx-850-SG and CNx-900-SG. This functionality increases the acidic behavior of the sample and their potential to be used in the adsorption of positive ions for Li-ion rechargeable batteries [97] and metallic contaminants removal [98]. Also, -COOH functionality helps in the formation of linking amine-carboxylic bonds to incorporate polymeric structures to create cross-linked CNT structures [99] and the functionalization with enzymes [100]. However, there is a Si2p detectable contribution to the survey spectrum of CNx-T-SG samples (see figures 15(d), (h), and (l)). Silicon carbide (∼100 eV) or SiC-β (∼101 eV) were identified. Also, oxides alone (SiOx) and oxides interacting with SiC as Si2O3/SiC at ∼104 eV and ∼103 eV were revealed (see figures 15(d), (h), and (l)). In CNx-850-SG and CNx-900-SG, the interaction between Si 2 O 3 and SiC is more significant, as can be appreciated in figures 15(h) and (l). Figure 16 shows the deconvolutions obtained from the high-resolution XPS of C1s (first column), N1s (second column), O1s (third column), and Si2p (fourth column). C1s spectra consist of four contributions, namely, the presence of sp 2 (C=C) and mostly sp 3 hybridizations (C-C) at ∼284 and ∼285.2 eV, respectively. Notice that the  (c)). In the case of N1s, in CNx-800-SiC N-graphitic atoms present a significant contribution to the high-resolution spectrum than N-pyrrolic or N-pyridinic ( figure 16(b)). There are more N-pyridinic than the component of N-pyrrolic. The high-resolution N1s spectrum of CNx-850-SiC is different from the CNx-800-SiC case. Here the concentration of N-pyrrolic is the highest, followed by N-pyridinic and then N-graphitic. Synthesis of N-MWCNTs carried out at 900°C does not appreciably change the concentration of N-pyrrolic but modify the concentration of N-graphitic being now the second-largest concentration that appears in the high-resolution N1s spectrum (see figure 16(j)). The O1s signal for CNx-800-SiC consists of five contributions. The lowest energy corresponds to metallic oxides (Fe 3 O 4 ) at ∼530 eV, O=C at ∼531 eV (carbonyls) followed for COO-corresponding to ester, amides, and anhydrides at ∼532 eV, then C-O-C about ether group at 533-534 eV and finally COOH which is the carboxylic family at 534 eV. In the case of CNx-850-SiC, this situation changes because now only four chemical species were deconvoluted mainly, namely Fe 3 O 4 , O=C, COO-and C-O-C, no deconvolution of COOH was possibly found. Probably due to the temperature, this chemical group was not formed at the CNT surface. CNx-800-SiC show a Si2p contribution by five chemical bindings (see figures 16(d), (h), and (l)), specifically SiOx (minimal contribution) at ∼105 eV, SiO 2 at ∼104 eV, Si 2 O 3 /SiC at ∼103 eV, SiC-β (minimal contribution) at ∼102 eV and SiC at 101 eV. In CNx-850-SiC and CNx-900-SiC, the panorama change for SiCβ, which considerably augments probably due to the increase of temperature. These high resolutions XPS for CN X -T-SiC demonstrate that SiC global participation in the Si2p spectrum (Si 2 O 3 /SiC, SiC-β, and SiC components) is considerably larger than in CNx-T-SG.

CNx-T-SiC cases (T=800, 850 and 900°C)
3.6. TGA results TGA curves for different N-MWCNTs studied here are shown in figure 17. From 50°C to 100°C all the samples lost material, probably corresponding to water evaporation or released of other attached molecules [101,102]. In the case, CNx-800 (black curve in figure 17(a)) reveals a weight loss of ∼40% at 900°C in oxygen atmosphere with two significant temperatures of oxidation (TO) present one at ∼300°C probably corresponding to amorphous carbon [29] and the other at ∼490°C corresponding to the CNx-800. In the CNx-800-SG (red curve in figure 17(a)) case, the loss of amorphous carbon (∼20%) is considerably smaller than the CNx-800 case. At ∼475°C this sample loss ∼60% of its weight corresponding to the CNx-800-SG. CNx-800-SiC (blue curve in figure 17(a)) loss only 40% of its weight at 900°C and the TO in this interval is at 448°C. Synthesis at 850°C produced CNx type with relatively different thermal stability in an oxidizing atmosphere, as can be seen in figure 17(b). CNx-850 (black curve) exhibits a typical behavior of doped CNTs [25]. The TO for CNx-850 is 481°C which is higher than reported by Cruz-Silva, et al for similar doped carbon nanotubes [25]. There is another TO for CNx-850 at 473°C probably associated with the other CNx-850 with smaller diameter, because these CNTs present a bimodal behavior for diameter distribution. It is probably that each diameter is related to each oxidation temperature. The remnant material is only ∼3%, which means that ∼88% correspond to CNx-850 nanotubes. The thermal stability of the CNx-850-SG (red curve in figure 17(b)) reach to 250°C with the first TO of ∼300°C, where probably amorphous or turbostratic carbon is oxidized [29]. The second interval of thermal stability (approximately 160°C) from 320°C to 480°C is observed then 5% of the mass is consumed. Around 35% is probably converted into oxides. The second TMO ∼515°C could be correlated with the CNx-850-SG showed in figure 7(c). Similar behavior is found for CNx-850-SiC sample (blue curve in figure 17(b)), but only two oxidation temperatures were noticed (see figure 17(b)), one at 276°C (functional groups and amorphous carbon) with a related mass loss of 23%. This sample was instable during 140°C, where mass was consumed gradually. The sample was not oxidized from ∼570°C to 900°C. The TO is ∼514°C which could associate with the same type of CNTs. These two last results are concomitant with the Raman spectra associated with these samples. Although the carbon structures like presented in figures 7(b), (c), 10(b), and (c) are curved, they exhibit well-ordered carbon layers that could be responsible for the larger TOs observed. The TGA results of samples synthetized at 900 are presented in figure 17(c). The sample CNx-900 (black curve) exhibits 3 TO (∼420°C, ∼453°C, and 460°C). In the first 100°C, the sample loss very few mass (∼3%). Then the mass was gradually decreased during the scan from∼170°C to 412°C. An abrupt mass was consumed (∼65%) in an interval of 45°C . During ∼125°C the mass has a linear diminution of 5% being the residual mass 12%. According to the Raman results, the crystallinity of this sample is no good, which is related to the result of the largest TOM found (∼460°C) to this sample. However, the CNx-900-SG (red curve in figure 17(c)) and CNx-900-SiC (blue curve in figure 17(c)) present a considerable residual mass of 60% and 80%, respectively. Above 450°C, these two samples also exhibit TOMs of 493°C and 472°C, respectively. These results are concomitant with the Raman spectroscopy I D /I G values. The reason for such results is that CNx-900-SG and CNx-900-SiC are firmly attached to the catalyst nanoparticles (see figure 11(c)) from which they are growing. Notice that CNx-900-SiC is increasing its weight after 600°C. CNx-900-SiC also presented two oxidation temperatures (441°C and 471°C) again, possibly associated with the two kinds of CNTs observed: one with a large diameter ( figure 11(b)) and the other with curved morphology ( figure 11(a)).
To clarify the TGA results of all CNx studied. We also performed TGA of 'pure' slag, 2h ball-milled slag, 2h ball-milled (slag+SiC), and only SiC used in the fabrication of different CNx samples. The results are shown in figure 18. Around 400°C 'pure' slag starts to oxidize, increasing its weight (red curve) being more than 2% of its initial weight at 950°C. Probably oxygen is incorporating with the temperature to the slag sample by oxidation of Fe, Si, Mo, and probably aluminum compounds. In the case of a 2h-milled slag (blue curve), the TGA curve starts to slowly decrease approximately at 80°C losing 1% of its weight due to mainly water evaporation, then at ∼300°C the weight increases almost linearly gaining 7% of its initial weight. Here the size reduction is fundamental for the oxidation process to increase the weight. Due to this size reduction, the reactivity of this sample has been augmented. When SiC is added to the slag, and the resultant sample is milled for 2h, the specimen presents a strange behavior when it is oxidizing. Instead of reducing its weight, this is also increased Figure 17. TGA curves corresponding to CNx-T (red curves), CNx-T-SG (blue curves), and CNx-T-SiC (black curves) synthesized at different temperatures (a) 800°C; (b) 850°C, and (c) 900°C. It is notable the significant amount of residual material when the slag is used as a catalytic substrate (blue and black curves). If the slag is used as a catalytic source, the residual material is considerably reduced, except in the 800°C case, see (a). This phenomenon could be because not all Fe included in the slag is not used in the synthesis because the temperature was not appropriated to synthesize them. more than the precedent sample reaching 950°C to 7.5% more weight than its initial mass. The reason for such behavior could be related to the size of SiC is considerably affected by the milling [107]. For SiC, the TGA curve (black) is monotonically reduced almost linearly until ∼620°C where a relatively large amount of loss weight happens, reaching ∼95% at 820°C. At the first 200°C, the reducing of mass must be because of the water evaporation [103]. The following 400°C, the mass loss could be associated with functional groups added to the SiC nanoparticle surface. A mass reduction of 3% happens between 600°C and 800°C related to the carbon oxidation [102]. After 800°C starts the formation of a thin film of silicon oxide [104] and the mass is increased.

Discussion
One of the main substances used in this CNT synthesis is the benzylamine (C 6 H 5 CH 2 NH 2 ), which is an organic chemical compound that is a source of NH 2 . When CNx-T (T=800°C, 850°C, and 900°C) is fabricated, the benzylamine is strongly affected because it interacts with the elements contained within the slag. It seems that the iron carbonyl sulfide acts on the benzylamine because it turns green with time. According to XPS results, all the sample CNx-800 contains high amounts of O1s (24.4%). This iron carbonyl sulfide could be the source of such oxygen in the solution, although also diopside or pyroxene could participate as oxygen sources. In the case of CNx-850 or CNx-900, the O1s were reduced (16.5% and 11.7%, respectively). Here the synthesis temperature plays an essential role in the decrease of this oxygen concentration in the samples. In consonance with SEM and TEM results the three kind CNx-T nanotubes present two diameters. Also, the CNx-T nanotubes with a large diameter exhibit small internal diameter. It is probably that oxygen participates actively in the formation of such CNTs as when ethanol is used. It is essential to mention that the CNx-T-SG and CNx-T-SiC samples are composed of substrate and carbon nanotubes (see TGA results). In the case of CNx-T-SG, the results are entirely different from the CNx-T. For instance, in CNx-T-SG group, the O1s concentration increases with synthesis temperature from 17.7% to 29.9% and then 41.6% to 800°C, 850°C, and 900°C, respectively. Figures 3(b), (e), and (h) show the CNx-T-SG attached to particles of material that probably contain similar material than the original slag. With the increasing synthesis temperature, these nanoparticles possibly grow due to coalescence. It is probably that some percentage of such substrate participate as a source of oxygen and an increase of O1s concentration. When SiC is mixed with the slag to be used as a substrate for N-MWCNTs fabrication, O1s content of samples CNx-800-SiC and CNx-850-SiC are similar than the CNx-800-T and CNx-850-T cases. However, at 900°C, the situation changes because at the surface of SiC nanoparticles probably starts to grow a SiO 2 thin film. Therefore, it is easier to pull apart the carbon nanotubes from the substrate and some parts of the O1s concentration decrease could come from there.
A possible scenario for the mechanism formation of the morphologies here presented is as following. In the case of CNx-T, the slag is transported from the sprayer vessel by Ar-H 2 together with benzylamine and deposited on the quartz tube. Then occurs the formation of Fe clusters impelled by the hydrogen starts. However, a lot of oxygen is participating in the process of synthesis, and the carbon layers are not well-formed. Although the growth is faster than usual and carbon layers grow more rapidly with the increase of temperature, the crystalline Figure 18. TGA curves of (1) SiC used to prepare the catalytic substrate for CNx-TEMP-SiC carbon nanostructures (black curve); (2) pure mine tailings before ball milling process (figure SI2b, red curve); 2 h ball-milled mine tailings (blue curve) and 2 h ball-milled mine tailings mixed with SiC powder (green curve).
grade of these CNT is affected as SEM and TEM images illustrated. Also, sulfur is acting all the time, forming Y and kink structures at higher temperatures [31]. This situation provokes the growth of carbon nanostructures with large diameters and disorder in the layers, inclusive of the generation of coaxial morphologies. The superficial corrugated structure of these samples could be related to the intervention of SiC contained in the slag. The situation is different in the case of substrates are SG or SG-SiC. In these cases, first, the hydrogen starts to reduce and morphologically change the Fe 2 O 3 nanoparticles on the slag resulting in Fe 3 C or Fe catalytic nanoparticles [105]. The slag is also interacting with the quartz tube. Inclusive SiC is also interacting with quartz since the beginning of synthesis. In this situation, small Fe-containing nanoparticles begin to catalyze carbon nanotubes like those in figures 6(d), 7(b), 9(e), and 10(b) with exaggerated curved morphology and rugged surface. However, the Fe-containing nanoparticles continue coalescing, and big nanoparticles can be formed [106]. The formation of long and irregular nanoparticles is typical when the amount of catalyst source is few. This kind of giant catalyst nanoparticles promotes the formation of giant nanotubes with an irregular internal structure and two or more sets of multiwall as figure 9(b). The participation of SiC is crucial as CNx-T-SiC contain such kind of CNTs.

Resume and conclusions
Three different kinds of CNx samples have been produced using ball-milled slag-SiC material. The elemental composition of the SG-2h revealed that the most abundant elements were Si and Fe. In most cases, the diameter distribution of CNTs presented a bimodal behavior, probably because of a complicated mechanism of agglomeration of catalyst nanoparticles. A spectrum of morphologies was observed depending on the route or synthesis temperature utilized. When SG-2h were used inside the benzylamine, the synthesized samples exhibited N-MWCNTs with large external diameters and short internal diameters possible because of the participation of oxygen in the solution. These synthesized CNx-T revealed the presence of branches or kinks when the synthesis temperature exceeded 800°C. This situation is like the case when ethanol is mixed with the benzylamine. When SG-2h or SG-SiC are used as catalyst on the quartz tube the morphology can be classified by three primary morphologies: (1) carbon nanotubes with relative small diameters and rugged surfaces with irregular and narrow compartments; (2) carbon nanotubes with large both external and internal diameters, but with two or three sets of multiwall forming complex compartments; (3) completely irregular tubular carbon structures. According to TEM characterizations, CNx-850-SiC presents a peculiar carbon nanotubes type (2). These giant carbon nanotubes present a decorated surface with pieces of nanomaterials composed of carbon or different materials. The encapsulated nanoparticles can be α-Fe or Fe 3 C of different shapes and sizes. A significant result is that the nitrogen doping is pyrrolic type in all cases when the synthesis temperature is higher than 850°C (except for CNx-850-SG). In general, the addition of SiC did not promote N-pyrrolic sites, but it seems that when the synthesis temperature is 850°C, the effect of SiC for producing pyrrolic nitrogen is crucial. There is no doubt that the best option for producing CNTs with nitrogen atoms immersed in the hexagonal network is by mixing SG-2h with benzylamine. Unfortunately, the benzylamine is damaged by SG-2h which does not allow systematic production using this route. Thus, the best way to manufacture N-MWCNT is by using SG-2h as the substrate preferably at temperatures close to 800. A significant result is the produced CNTs in these cases are attached to catalytic nanoparticles. This feature is not adequate for some applications. However, very recently has been demonstrated that carbon-metal hybrid nanomaterials could be useful for electrochemical applications. Another important result is that SiC could promote the growth of giant carbon nanotubes with unusual internal compartment morphologies. It has been demonstrated here that the catalytic substrate effects are significant to define the morphology of CNTs-if the content of such substrate is altered with SiC giant N-pyrrolic CNTs can be produced.