137
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.
138
by increasing synthesis temperatures. Number of N-CNT walls from sp3 nitrogen source, DMF, can be controlled by varying the synthesis temperature. Both synthesis temperature and sp: sp3 mixing ratio have an influence in the number of tube walls. The enhancement of N- doping in N-CNTs by mixing sp3 and sp hybridised N sources is subject to synthesis temperature. The main N functionality in the N-CNTs reported herein was pyrrolic. N-CNTs synthesised at 900 °C from both sp: sp3 hybridised nitrogen source ratios of 1:0 and 1:3 had the lowest residual iron and highest N-CNTs composition. Therefore, according to the current work, DMF and acetonitrile in 1:3 ratio and 900 °C were the best synthesis conditions for N- CNTs.
Acknowledgements
The authors wish to thank the University of KwaZulu-Natal (UKZN) for the access to facilities used in this work. Edwin Tonderai Mombeshora is grateful to the UKZN Nanotechnology Platform for funding this work. The XPS analysis was supported by the National Research Foundation of South Africa (Grant Number 93205). Otherwise, this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
References
[3.1] G. Keru, P.G. Ndungu, V.O. Nyamori, Nitrogen-doped carbon nanotubes synthesised by pyrolysis of (4-{[(pyridine-4-yl)methylidene]amino}phenyl)ferrocene, Journal of Nanomaterials, 2013 (2013) 1-7.
[3.2] E.N. Nxumalo, V.O. Nyamori, N.J. Coville, CVD synthesis of nitrogen doped carbon nanotubes using ferrocene/aniline mixtures, Journal of Organometallic Chemistry, 693 (2008) 2942-2948.
[3.3] Y. Tang, S.C. Burkert, Y. Zhao, W.A. Saidi, A. Star, The effect of metal catalyst on the electrocatalytic activity of nitrogen-doped carbon nanotubes, The Journal of Physical Chemistry C, 117 (2013) 25213-25221.
[3.4] L.M. Ombaka, P.G. Ndungu, V.O. Nyamori, Tuning the nitrogen content and surface properties of nitrogen-doped carbon nanotubes synthesized using a nitrogen-containing ferrocenyl derivative and ethylbenzoate, Journal of Materials Science, 50 (2014) 1187- 1200.
139
[3.5] K. Chizari, A. Vena, L. Laurentius, U. Sundararaj, The effect of temperature on the morphology and chemical surface properties of nitrogen-doped carbon nanotubes, Carbon, 68 (2014) 369-379.
[3.6] R.M. Yadav, J. Wu, R. Kochandra, L. Ma, C.S. Tiwary, L. Ge, G. Ye, R. Vajtai, J. Lou, P.M. Ajayan, Carbon nitrogen nanotubes as efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions, ACS Applied Materials and Interfaces, 7 (2015) 11991-12000.
[3.7] Z. Zang, A. Nakamura, J. Temmyo, Nitrogen doping in cuprous oxide films synthesized by radical oxidation at low temperature, Materials Letters, 92 (2013) 188-191.
[3.8] M. Hafeez, L. Gan, H. Li, Y. Ma, T. Zhai, Chemical vapor deposition synthesis of ultrathin hexagonal rese2 flakes for anisotropic raman property and optoelectronic application, Advanced Materials, 28 (2016) 8296-8301.
[3.9] E.N. Nxumalo, N.J. Coville, Nitrogen doped carbon nanotubes from organometallic compounds: a review, Materials, 3 (2010) 2141-2171.
[3.10] W. Lv, K. Shi, L. Li, S. Shao, Nitrogen-doped multiwalled carbon nanotubes and their electrocatalysis towards oxidation of NO, Microchimica Acta, 170 (2010) 91-98.
[3.11] X.F. Li, K.Y. Lian, L. Liu, Y. Wu, Q. Qiu, J. Jiang, M. Deng, Y. Luo, Unraveling the formation mechanism of graphitic nitrogen-doping in thermally treated graphene with ammonia, Scientific Reports, 6 (2016) 23495.
[3.12] Q. Wei, X. Tong, G. Zhang, J. Qiao, Q. Gong, S. Sun, Nitrogen-doped carbon nanotube and graphene materials for oxygen reduction reactions, Catalysts, 5 (2015) 1574-1602.
[3.13] T. Susi, Z. Zhu, G. Ruiz-Soria, R. Arenal, P. Ayala, A.G. Nasibulin, H. Lin, H. Jiang, O. Stephan, T. Pichler, A. Loiseau, E.I. Kauppinen, Nitrogen-doped SWCNT synthesis using ammonia and carbon monoxide, Physica Status Solidi (b), 247 (2010) 2726-2729.
[3.14] E.T. Mombeshora, V.O. Nyamori, A review on the use of carbon nanostructured materials in electrochemical capacitors, International Journal of Energy Research, 39 (2015) 1955-1980.
[3.15] J. Liu, Y. Zhang, M.I. Ionescu, R. Li, X. Sun, Nitrogen-doped carbon nanotubes with tunable structure and high yield produced by ultrasonic spray pyrolysis, Applied Surface Science, 257 (2011) 7837-7844.
140
[3.16] C.-d. Kim, H.-R. Lee, H.T. Kim, Effect of NH3 gas ratio on the formation of nitrogen- doped carbon nanotubes using thermal chemical vapor deposition, Materials Chemistry and Physics, 183 (2016) 315–319.
[3.17] C. Tang, Y. Bando, D. Golberg, F. Xu, Structure and nitrogen incorporation of carbon nanotubes synthesized by catalytic pyrolysis of dimethylformamide, Carbon, 42 (2004) 2625-2633.
[3.18] U. Ritter, N.G. Tsierkezos, Y.I. Prylutskyy, L.Y. Matzui, V.O. Gubanov, M.M. Bilyi, M.O. Davydenko, Structure–electrical resistivity relationship of N-doped multi-walled carbon nanotubes, Journal of Materials Science, 47 (2011) 2390-2395.
[3.19] H.T. Chung, P. Zelenay, A simple synthesis of nitrogen-doped carbon micro- and nanotubes, Chemical Communications (Camb), 51 (2015) 13546-13549.
[3.20] E.N. Nxumalo, P.J. Letsoalo, L.M. Cele, N.J. Coville, The influence of nitrogen sources on nitrogen doped multi-walled carbon nanotubes, Journal of Organometallic Chemistry, 695 (2010) 2596-2602.
[3.21] M. Arjmand, K. Chizari, B. Krause, P. Pötschke, U. Sundararaj, Effect of synthesis catalyst on structure of nitrogen-doped carbon nanotubes and electrical conductivity and electromagnetic interference shielding of their polymeric nanocomposite, Carbon, 98 (2016) 358–372.
[3.22] G. Ćirić-Marjanović, I. Pašti, S. Mentus, One-dimensional nitrogen-containing carbon nanostructures, Progress in Materials Science, 69 (2015) 61–182.
[3.23] S. Hussain, R. Amade, E. Jover, E. Bertran, Nitrogen plasma functionalization of carbon nanotubes for supercapacitor applications, Journal of Materials Science, 48 (2013) 7620-7628.
[3.24] Y. van de Burgt, Y. Bellouard, R. Mandamparambil, Kinetics of laser-assisted carbon nanotube growth, Physical Chemistry Chemical Physics, 16 (2014) 5162-5173.
[3.25] E.N. Nxumalo, V.P. Chabalala, V.O. Nyamori, M.J. Witcomb, N.J. Coville, Influence of methylimidazole isomers on ferrocene-catalysed nitrogen doped carbon nanotube synthesis, Journal of Organometallic Chemistry, 695 (2010) 1451-1457.
[3.26] S.V. Dommele, A. Romero-Izquirdo, R. Brydson, K.P.D. Jong, J.H. Bitter, Tuning nitrogen functionalities in catalytically grown nitrogen-containing carbon nanotubes, Carbon, 46 (2008) 138-148.
141
[3.27] A. Shaikjee, N.J. Coville, The role of the hydrocarbon source on the growth of carbon materials, Carbon, 50 (2012) 3376–3398.
[3.28] J.H. Lehman, M. Terrones, E. Mansfield, K.E. Hurst, V. Meunier, Evaluating the characteristics of multiwall carbon nanotubes, Carbon, 49 (2011) 2581-2602.
[3.29] T. Thurakitseree, C. Kramberger, P. Zhao, S. Aikawa, S. Harish, S. Chiashi, E.
Einarsson, S. Maruyama, Diameter-controlled and nitrogen-doped vertically aligned single-walled carbon nanotubes, Carbon, 50 (2012) 2635-2640.
[3.30] G. Chen, R.C. Davis, H. Kimura, S. Sakurai, M. Yumura, D.N. Futaba, K. Hata, The relationship between the growth rate and the lifetime in carbon nanotube synthesis, Nanoscale, 7 (2015) 8873-8878.
[3.31] G. Zhang, D. Mann, L. Zhang, A. Javey, Y. Li, E. Yenilmez, Q. Wang, J.P. McVittie, Y. Nishi, J. Gibbons, H. Dai, Ultra-high-yield growth of vertical single-walled carbon nanotubes: Hidden roles of hydrogen and oxygen, Proceedings of the National Academy of Science USA, 102 (2005) 16141-16145.
[3.32] V.O. Nyamori, N.J. Coville, Effect of ferrocene/carbon ratio on the size and shape of carbon nanotubes and microspheres, Organometallics, 26 (2007) 4083–4085.
[3.33] F. Li, Y. Wang, D. Wang, F. Wei, Characterization of single-wall carbon nanotubes by N2 adsorption, Carbon, 42 (2004) 2375-2383.
[3.34] K. Ghosh, M. Kumar, T. Maruyama, Y. Ando, Tailoring the field emission property of nitrogen-doped carbon nanotubes by controlling the graphitic/pyridinic substitution, Carbon, 48 (2010) 191-200.
[3.35] I. Stamatin, A. Morozan, A. Dumitru, V. Ciupina, G. Prodan, J. Niewolski, H. Figiel, The synthesis of multi-walled carbon nanotubes (MWNTs) by catalytic pyrolysis of the phenol-formaldehyde resins, Physica E: Low-dimensional Systems and Nanostructures, 37 (2007) 44-48.
[3.36] G. Zhou, C. Xu, W. Cheng, Q. Zhang, W. Nie, Effects of oxygen element and oxygen- containing functional groups on surface wettability of coal dust with various metamorphic degrees based on xps experiment, Journal of Analytical Methods in Chemistry, 2015 (2015) 467242.
142
[3.37] T. Susi, T. Pichler, P. Ayala, X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms, Beilstein Journal of Nanotechnology, 6 (2015) 177-192.
[3.38] L.M. Ombaka, P.G. Ndungu, B. Omondi, J.D. McGettrick, M.L. Davies, V.O. Nyamori, A facile approach towards increasing the nitrogen-content in nitrogen-doped carbon nanotubes via halogenated catalysts, Journal of Solid State Chemistry, 235 (2016) 202- 211.
[3.39] S. Latil, S. Roche, D. Mayou, J. Charlier, Mesoscopic transport in chemically doped carbon nanotubes Physical Review Letters, 92 (2004) 256805.
[3.40] W.P. Wright, V.D. Marsicano, J.M. Keartland, R.M. Erasmus, S.M.A. Dube, N.J.
Coville, The electrical transport properties of nitrogen doped carbon microspheres, Materials Chemistry and Physics, 147 (2014) 908-914.
[3.41] E.T. Mombeshora, P.G. Ndungu, A.L.L. Jarvis, V.O. Nyamori, Oxygen-modified multiwalled carbon nanotubes: physicochemical properties and capacitor functionality, International Journal of Energy Research, (2017).
[43.2] I. Kruusenberg, N. Alexeyeva, K. Tammeveski, J. Kozlova, L. Matisen, V. Sammelselg, J. Solla-Gullón, J.M. Feliu, Effect of purification of carbon nanotubes on their electrocatalytic properties for oxygen reduction in acid solution, Carbon, 49 (2011) 4031-4039.
[3.43] Y. Tian, R. Xue, X. Zhou, Z. Liu, L. Huang, Double layer capacitor based on active carbon and its improved capacitive properties using redox additive electrolyte of anthraquinonedisulphonate, Electrochimica Acta, 152 (2015) 135-139.
143
Appendix: Supporting information for Chapter 3
Table 3.S1: Elemental composition in reagents Mass
ratios
Mol ratios % in the reagent/catalyst mixture
Ratios in the catalyst/reagent mixture
C% N% O% H% Fe% Fe:C ratio
Fe:N ratio
Fe:O ratio
O:C ratio D1A0 D1A0 49.60 18.73 21.55 9.48 0.64 0.0128 0.00068 0.0295 0.4345 D1A1 D0.36A0.64 52.82 24.00 13.84 8.70 0.64 0.0121 0.00050 0.0460 0.2620 D1A2 D0.21A0.78 54.38 26.44 10.23 8.31 0.64 0.0117 0.00044 0.0622 0.1881 D1A3 D0.16A0.84 55.16 27.90 8.18 8.11 0.64 0.0115 0.00041 0.0778 0.1483 D1A4 D0.12A0.87 55.85 28.78 6.82 7.92 0.64 0.0114 0.00039 0.0934 0.1221 D1A5 D0.1A0.90 56.24 29.46 5.84 7.82 0.64 0.0113 0.00038 0.1090 0.1038
D0A1 D0A1 58.58 33.37 0.18 7.24 0.64 0.0109 0.00033 3.4800 0.0031 D1A1 D0.36A0.64 52.82 24.00 13.84 8.70 0.64 0.0121 0.00050 0.0460 0.2620 D2A1 D0.53A0.47 51.55 21.95 16.87 8.99 0.64 0.0123 0.00056 0.0377 0.3273 D3A1 D0.63A0.37 51.07 20.98 18.23 9.09 0.64 0.0125 0.00059 0.0349 0.3369 D4A1 D0.69A0.31 50.77 20.49 18.91 9.19 0.64 0.0125 0.00061 0.0337 0.3725 D5A1 D0.74A0.26 50.48 20.20 19.40 9.29 0.64 0.0126 0.00062 0.0328 0.3843
Table 3.S2: The average outer-diameter (OD), inner-diameter (ID) and compartment sizes
Sample OD (nm) ID (nm) Thickness
(nm)
Number of walls
Compartment size (nm)
D0A1 136.78 41.55 95.22 141 36.32
D1A1 115.07 74.93 40.14 60 45.47
D2A1 31.75 20.82 10.94 17 26.60
D3A1 51.40 39.40 12.00 19 25.05
D4A1 37.26 24.12 13.15 20 23.11
D5A1 51.95 37.64 14.31 22 29.13
D1A0 30.79 21.50 9.29 15 21.52
D1A1 115.07 74.93 40.14 60 45.47
D1A2 42.70 23.12 19.58 30 31.36
D1A3 55.71 36.43 19.28 29 29.78
D1A4 59.95 42.35 17.59 27 44.23
D1A5 47.05 28.84 18.20 28 32.25
144
Table 3.S3: The average OD, ID and compartment sizes for samples D1A0 and D1A3.
Sample Temperature OD (nm) ID (nm) Thickness
(nm)
Number of walls
Compartment size (nm)
D1A0
800 23.29 14.29 9.00 13 20.54
850 42.43 30.30 12.13 19 43.99
900 31.18 17.75 13.43 21 20.80
950 47.13 30.07 17.06 26 21.23
1000 82.14 53.20 28.94 44 36.46
D1A3
800 34.85 21.81 13.03 20 30.99
850 55.02 37.37 17.64 27 31.91
900 55.71 36.43 19.27 29 29.78
950 68.29 50.88 17.41 27 36.58
1000 53.67 36.48 17.20 26 29.18
Fig. 3.S1. The distribution of products of (a) D1Ax, (b) DxA1, (c) D1A0 and (d) D1A3.
145
Fig. 3.S2. The representative OD for the D1Ax samples (a) D1A0, (b) D1A1, (c) D1A2, (d) D1A3, (e) D1A4 and (f) D1A5.
Fig. 3.S3. The representative OD for the DxA1 samples (a) D0A1, (b) D1A1, (c) D2A1, (d) D3A1, (e) D4A1 and (f) D5A1.
146
Fig. 3.S4. The representative ID for the D1Ax samples (a) D1A0, (b) D1A1, (c) D1A2, (d) D1A3, (e) D1A4 and (f) D1A5.
Fig. 3.S5. Compartment sizes for the D1Ax samples (a) D1A0, (b) D1A1, (c) D1A2, (d) D1A3, (e) D1A4 and (f) D1A5.
147
Fig. 3.S6. The representative ID for the DxA1 samples (a) D0A1, (b) D1A1, (c) D2A1, (d) D3A1, (e) D4A1 and (f) D5A1.
Fig. 3.S7. Compartment sizes for the DxA1 samples (a) D0A1, (b) D1A1, (c) D2A1, (d) D3A1, (e) D4A1 and (f) D5A1.
148
Fig. 3.S8. The OD for D1A0 samples synthesised at (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
Fig. 3.S9. The ID for D1A0 samples synthesised at (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950
°C and (e) 1000 °C.
149
Fig. 3.S10. The compartment sizes for D1A0 samples synthesised at (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
Fig. 3.S11. The OD for D1A3 samples synthesised at (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
150
Fig. 3.S12. The ID for D1A3 samples synthesised at (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
Fig. 3.S13. The compartment sizes for D1A3 samples synthesised at (a) 800 °C, (b) 850 °C, (c) 900 °C, (d) 950 °C and (e) 1000 °C.
151
Fig. 3.S14. The C1s XPS spectra for representative N-CNT samples.
Fig. 3.S15. The Raman representative spectrum for N-CNT samples, D4A1 N-CNTs.
152
Fig. 3.S16. A representative thermogram of the purified N-CNT samples showing an almost zero residual catalyst mass.
153
Chapter Four
154
Effect of graphite/sodium nitrate ratio and reaction time on physicochemical properties of graphene oxide
Edwin T. Mombeshora,1 Patrick G. Ndungu2 and Vincent O. Nyamori1 *
1School Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban, 4000, South Africa
2Department of Applied Chemistry, University of Johannesburg, P.O. Box 17011, Doornfontein, Johannesburg, 2028, South Africa
Graphical abstract
155
Abstract
Graphene oxide (GO) synthesis was done by varying graphite: sodium nitrate ratio and the reaction time. The study aimed at investigating the optimum graphite: sodium nitrate ratio and reaction time for obtaining the highest oxygen content in GO and it also explained the effect of oxygen content on physicochemical properties. GO was characterized by transmission electron microscopy, scanning electron microscopy, atomic force microscopy, powder X-ray diffraction, Raman spectroscopy, infra-red spectroscopy, thermogravimetric analysis, ultraviolet-visible spectrophotometry, and elemental analysis. Increasing sodium nitrate ratio amplified elemental oxygen content, BET surface area, pore volume and pore size but reduced crystallite sizes in the GO samples. Variation in reaction time did not show a clear trend in terms of oxygen amount. Physicochemical properties such as d-spacing and defect intensity increased whilst thermal stability decreased with increase in oxygen elemental properties.
Varying graphite: sodium nitrate ratio and reaction time modifies physicochemical properties such as oxygen content, crystallinity, thermal stability and overall morphology.
Keywords: graphene oxide; oxygen content; reaction time; carbon; Hummer’s method
* Corresponding author: Vincent Nyamori, School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban, 4000, South Africa
Email: [email protected] Tel.: +27-31 2608256; Fax: +27-31 260 3091