3.3. Results and discussion
3.3.2 Temperature effects on N-MWCNTs
3.3.2.2 Temperature effects on D1A3
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In the elemental quantification, hydrogen decreased with increase in synthesis temperature from 900 °C to 1000 °C (Table 3.2). A possible motive is that at higher temperatures, dangling bonds are minimal. The highest nitrogen content was achieved at 800 and 900 °C (Table 3.2) and these samples corresponded with smallest N-CNT diameters (Table 3.S3 and Fig. 3.S11- 12 in supplementary material). The decrease in nitrogen content at 950 and 1000 °C corroborated the works of Yadav et al. [3.6] In addition, from the study, it is clear that DMF is a suitable carbon and nitrogen source for the synthesis of N-CNTs with between 2.5 and 6
% nitrogen depending on temperature of synthesis
Table 3.2: Elemental analysis and Raman spectroscopy data of D1A0
Sample Temperature H N ID/IG
D1A0
800 0.90 5.86 0.79
850 1.15 4.88 0.19
900 0.07 5.87 1.27
950 0.04 2.98 1.13
1000 0.00 2.47 1.19
Raman data (Table 3.2) suggest that lower temperatures of synthesis were associated with low defect concentrations in N-CNTs. Also, from critical examination of the data from TEM analysis (Table 3.S3 in supplementary information), elemental composition (Table 3.2) and textural characteristics (Fig. 3.17), it was eminent that high nitrogen doping using a sp3 source induced a decrease in ID, OD and wall thickness and increase in BET surface area of N-CNTs.
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This is because at high temperatures iron particles agglomerated due to high collision rates and therefore carbon sphere formation was facilitated [3.1].
Fig. 3.18. The representative SEM images for the D1A3 samples (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
D1A3 synthesised at 800 °C had N-CNTs with lower intensities of bamboo compartments (Fig.
3.19a) and at 850 °C N-CNTs appeared to have wrinkled surfaces. At 900 °C defective tubes
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walls were also observed whilst at 950 °C some Y shaped tubes were obtained (Fig. 3.19c-d).
Defective walls at 900 °C were due to inclusion of heptagons and pentagons in the graphitic structures of N-CNTs.
Fig. 3.19. The representative TEM images for the D1A3 samples (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
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Temperature influenced morphology and dimensions, for instance synthesis at 850 °C produced cup shaped tube ends but at 1000 °C cap shaped tube ends were noticed.
Additionally, OD, wall thickness and number of tube walls increased in the synthesis temperature range of 800-900 °C (Table 3.S2 in supplementary information). Beyond 900 °C, the number of N-CNT walls decreased. The different synthesis temperature produced different product composition and physical properties, and therefore decompose at different temperatures. Hence, different TGA profiles were obtained for the 5 samples shown in Fig.
20 a.
D1A3 samples at different synthesis temperatures had similar thermal stabilities (Fig. 3.20a), however, they had different residual iron wt. %. Residual iron wt.% decreases with increase in synthesis temperatures from 800 °C to 900 °C then increases at 950 °C (Fig. 3.20a). The reason for the decrease of iron residue wt.%., more inclined thermogram and a wider decomposition temperature for D1A3 sample synthesised at 1000 °C (Fig. 3.20) was attributed to formation of both carbon spheres and N-CNTs. The decomposition temperature region of the D1A3 N-CNTs was between 400 and 700 °C. This means beyond 700 °C all the carbonaceous material would have decomposed, and the catalyst residues are exposed to heat and oxygen. This led to formation of a metal oxides and hence, weight increased. Compared to D1A0, D1A3 samples decomposed over a wider range of temperature and had lower amounts of amorphous carbon (derivative weight curve on Fig. 3.20b). The composition of amorphous carbon decreased with increase in temperature and the parallel broadening of the derivative weight peak is an indication of decrease in sample homogeneity. This was attributed to the formation of both N-CNTs and carbon spheres at 1000 °C as well as increase in both OD and ID between 800 and 950 °C due catalyst agglomeration.
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Fig. 3.20. Representative TGA (a) thermograms and (b) derivative weight curve for the D1A3 N-CNTs synthesised at different temperatures.
For D1A3, BET surface area decreased with increase in synthesis temperature (Fig. 3.21a). and no clear trend was noticeable on pore volume and size (Fig. 3.21b-c). A possibly reason for this trend is similarly linked to size variations. The lowest values of pore volume and sizes noticed at 1000 °C (Fig. 3.21b-c) were due to presence of carbon spheres. Hence, the presented data clearly shows mixing DMF with acetonitrile, 1:3 ratios, clearly allowed tailoring of BET surface area of the obtained products.
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Fig. 3.21. Textural characteristics of D1A3 samples synthesized at different temperatures.
A possible source of hydrogen, determined by means of elemental analysis, in the products (Table 3.3) was the existence of dangling bonds. Nitrogen content increased from 800 °C to 900 °C then decreased at 950 °C and this an indication of temperature effect on nitrogen doping (Table 3.3). The decrease in nitrogen amount in the product in both D1A0 and D1A3 from 900°C to 950 °C corroborates with the report by Chazari et al., [3.5] Unlike the optimum temperature of 850 °C reported by Keru et al., [3.1] Ombaka et al., [3.4] and Yadav et al., [3.6]
highest nitrogen content in the current work was achieved at 900 °C. This suggests the dynamics in the effect of nitrogen availability from source, influenced by nitrogen hybridisation in reagent, in addition to catalyst and growth temperature. Upsurge of nitrogen at 1000 °C can also be a result of the presence of carbon spheres. A critical analysis of the Table 3.2 - 3.3, unlike earlier reports [3.17,3.26], shows that nitrogen content does not necessarily decrease with increase in temperature. In addition, the two tables clearly indicate that at 900-1000°C D1A3 N-CNTs had higher nitrogen content composition unlike at 800-850
°C for D1A0. This means that enhancement of N-doping in N-CNTs is not obvious upon mixing sp3 and sp nitrogen sources, DMF and acetonitrile, but is subject to the choice of synthesis temperature selected. A possible inference is the independence of N-doping to metal-
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nitride pre-existence in the current work (low Fe:N ratios in Table 3.S1 supplementary information). Additionally, a further manifestation of induced differences in growth dynamics, it may be noted that the link between decrease in diameters, enhancement in BET surface area and increase N-doping levels was disrupted upon mixing sp and sp3 sources in the current work.
Table 3.3: Elemental analysis and Raman spectroscopy data of D1A3
Sample Temperature H N ID/IG
D1A3
800 0.68 2.90 0.37
850 0.12 3.40 0.44
900 0.89 9.38 0.36
950 0.05 3.69 0.76
1000 0.35 4.16 0.33
Similarly, D1A3 was less defective than D1A0 at the temperatures under study despite the higher nitrogen content in D1A3 N-CNTs (Table 3.2-3.3) and this corroborates with the earlier deductions about lessening of defect concentration upon mixing reagents. Also, the peaks on the PXRD spectrum were similarly assigned as in Fig. 3.8. Upon comparing the graphitic peak at two theta of 25°, D1A3 N-CNTs were less crystalline than D1A0 at 900-1000 °C and this can be explained by the high nitrogen doping in D1A3 unlike at lower temperatures (Table 3.2 - 3.3). Whereas, an analysis of rest of the peaks showed an opposite trend probably due to the high defect nature in D1A0 than D1A3 N-CNTs. The general trend observed was increase in crystallinity of both Fe3C and N-CNTs with increase in temperature for both D1A0 and D1A3 (Fig. 3.22).
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Fig. 3.22. The powder X-ray diffraction spectrums for (a) D1A0 and (b) D1A3 N-CNT samples synthesised at different temperatures.
The current results show several attributes when compared to most similar studies in literature;
for instance, the nitrogen content was higher than most of the literature values obtained from analogous experimental set-ups [3.2,3.12,3.21]. The nitrogen content obtained was comparable to what Ombaka et al., [3.38] obtained after introducing oxygen to acetonitrile.
The selectivity towards formation of tubes rather than other shaped carbon nanomaterials (such as amorphous carbon, spheres and nano-rods, amongst others) was one of the key attributes of the current approach when compared to other works [3.1,3.2,3.4,3.8,3.18]. Also, it was a simpler strategy of tailoring physicochemical properties such as dimensions and chemical moieties, amongst others.