3.3. Results and discussion
3.3.1 Effect of acetonitrile: N,N’-dimethyl formamide ratio
3.3.1.1 Effects of varying acetonitrile ratio (sp hybridised nitrogen source)
D1A0 (DMF alone) consisted mainly of agglomerated ‘spaghetti-like’ N-CNTs (Fig. 3.1a) whilst D1A4 and D1A5 had noticeable amounts of amorphous carbon but D1A3 had a slight quantity (green circles in Fig. 3.1d-f). From the Fig. 3.1, it can be deduced that addition of acetonitrile increased the amount of amorphous carbon in the product. Additionally, the D1A1 N-CNTs were more aligned unlike the rest (green rectangle on Fig. 3.1b and supplementary information Fig. 3.S1). The Fe:C ratio (Table 3.S1 in supplementary material), decreased with addition of acetonitrile, and in the feedstock for D1Ax it was not responsible for increase in amorphous carbon in the product (Fig. 3.S1).
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Fig. 3.1. The representative images for the D1Ax samples (a) D1A0, (b) D1A1, (c) D1A2, (d) D1A3, (e) D1A4 and (f) D1A5.
All samples displayed visible bamboo compartments (Fig. 3.2), a preliminary indicator of foreign atom doping and in this case nitrogen. The appearance of bamboo compartments is caused by incorporation of nitrogen in the graphitic structure culminating in a curvature graphitic layer. This correlates with several works reported on N-CNTs synthesised using iron-
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based catalyst [3.15,3.26]. Iron metal residues, qualitatively determined by EDX and XPS, were noticeable on both tube walls and encapsulated inside N-CNTs (Fig. 3.2a and e-f). As the metal catalyst is deposited onto the N-CNT walls, it can be inferred that the injection rate does not match growth dynamics, i.e. the rate was too high and hence excess is deposited [3.15].
Considering that injection rate was the same in synthesis of all samples, the absence of iron residues on the D1A1, D1A2 and D1A3 N-CNT walls possibly points out on the changes in growth dynamics as a consequence of different ratios. The ratio of nitrogen sources influenced the product, for instance, D1A0 had some tubes with few bamboo compartments and the sizes of the compartments varied greatly (Fig. 3.2a). Addition of acetonitrile, in sample D1A1, produced N-CNTs with irregular shaped walls and fishbone-like shapes (Fig. 3.2b).
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Fig. 3.2. Representative TEM images for the D1Ax samples (a) D1A0, (b) D1A1, (c) D1A2, (d) D1A3, (e) D1A4 and (f) D1A5.
The general observation in the D1Ax series was N-CNTs were bent and a possible explanation is the fact that radicals involved in the N-CNT growth are from sources with different hybridisation. Hence, pentagonal and heptagonal structures are formed [3.27,3.28] from the different associated reaction kinetics and this culminates in different growth dynamics. From
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the dimensional analysis (Table 3.S2) the average wall thickness, obtained by subtracting average inner diameter (ID) from outer diameter (OD), increased from 9.3 nm (D1A0) to 40 nm in D1A1. The wall thickness thereafter decreased, although it was thicker than D1A0 in all samples, with increase in sp ratio except for sample D1A5. Whilst Thurakitseree et al., [3.29] reported reduction of diameters of N-CNTs upon introduction of sp-hybridised-N to ethanol, the current study showed an opposite trait. In the current study, this is possibly due to the involvement of sp3-hybridised-N source in the reagents for N-CNT synthesis culminating in active species different from the former.
All samples, as noticed from the SEM and TEM images, had high iron metal residues and this is a setback in applications were the metal interferes with functionality. There is a possibility of residual iron to be bound to nitrogen and therefore removal of the metal may result in reduction of nitrogen content [3.3]. However, the main reason of presenting the products as synthesised, before purification, in the current work was to get the traits of catalyst deactivation in the obtained N-CNTs. More studies will be done on the purified N-CNTs and physicochemical properties are also expected to change. The focus of the study was to elucidate how the mixing ratios of the two-small nitrogen/carbon sources with sp and sp3 hybridised nitrogen, acetonitrile and DMF, influence the ultimate product as synthesised, without further purification. The thrust was to communicate the best ratio with respect to residual catalyst wt.%, nitrogen content and functionality, and overall physicochemical properties of the obtained products.
From thermogravimetric analysis, N-CNTs from D1A1 had the highest metal residues (70%) followed by D1A2 (38%) then D1A5 (32%) (Fig. 3.3a1 and b1). A plausible elucidation is that the N-CNT growth rate compete with catalyst deactivation [3.30] and hence, reagents mixtures (carbon sources) of the aforementioned samples facilitated high catalyst deactivation.
On the other hand, D1A3 (84%), then D1A0 (82%) and D1A4 (79%) had the largest wt.% of N-CNTs. This means by controlling the ratios of reagents the composition of the product, particularly the wt.% of metal residues, can be altered. In addition, the occurrence of a sharpest derivative weight curve for D1A0 suggest the highest level of sample homogeneity and the almost flat curve for D1A1 means there was minimal carbonaceous material in the product (Fig. 3.3a2). Fig. 3b1-2 (green circle in Fig 3.3b2) shows that D1A4 and D1A5 had noticeable amount of amorphous carbon and D1A3 had a slight amount. The results corroborated SEM observations and a possible reason is that high ratio of sp hybridised nitrogen source results in formation of some radicals not favourable for N-CNT growth.
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Fig. 3.3. Representative TGA (a1 and b1) thermograms and (a2 and b2) derivative weight curve for the D1Ax samples.
The N-CNTs decomposition range was between 450 and 700 °C (Fig. 3.3a2 and b2). The derivative weight curve shoulders above 550 °C in all samples (arrows in Fig. 3.3a2 and b2) and SEM images (Fig. 3.1), suggest the presence of either two different species of sp2 hybridized carbon networks, carbon nanotubes and platelets, or existence of defective and non- defective sp2 hexagonal structures. Several possibilities exist in this regard, such as either existence of doped and un-doped CNTs or the presence of various nitrogen functionalities.
There was no clear trend on thermal stability but D1A1 was the most thermally stable mainly because of the limited exposure of the N-CNTs to air during decomposition due to high iron content. Again, the existence of N-CNTs with different levels of defects is a possible reason for lack of pronounced trend.
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