Oxidative corrosion potential vs. pH diagram for single-walled carbon nanotubes

Abstract

The oxidative corrosion of single-walled carbon nanotubes (SWCNTs) was investigated, and the diagram of oxidative corrosion potential vs. pH was obtained. The results will contribute to fundamental understanding of oxidative corrosion reactions on sp² type carbons.

---

The oxidative corrosion of carbon is an urgent problem because carbon is widely used as a platform electrode to immobilize catalysts. One of the factors in the gradual decrease of output power in fuel cells such as a polymer electrolyte fuel cell is the oxidative corrosion of the carbon supports.¹ The oxidative corrosion of carbon is a complicated process that includes parallel oxidation pathways.² Furthermore, the following electrochemical oxidation reaction (1) of carbon by water molecules occurs at a much more negative potential than that thermodynamically expected, although the rate of this reaction is very slow.^1–3

C + 2H₂O = CO₂ + 4H⁺ + 4e⁻, 0.207 V vs. NHE (1)

A considerable amount of electrochemical research has been directed towards elucidating the mechanism of the oxidation of the hexagonal plane sp² carbon family, such as highly oriented pyrolytic graphite (HOPG), carbon nanotubes and graphene as model reaction systems.³ Recently, a number of studies using ab initio molecular orbital calculations have focused on the oxidation steps of hexagonal plane sp² carbons.⁴ However, the detailed oxidation mechanism of such sp² carbons is still unclear. Furthermore, we should also give attention to other sp² carbon structures. Hexagonal plane sp² carbon materials consist of 6-membered rings; however, such structures are probably not perfect.⁵ 5-, 7-, and 8-membered rings would be included as other structures in the hexagonal plane sp² carbons. Quite importantly, such other structures are easily oxidized in comparison with 6-membered rings because of their instability. Furthermore, it is thought that the oxidized defect sites promote further oxidation of carbon.⁴ The oxidation reactions of 5-, 7- and 8-membered rings in sp² carbons have not been investigated beyond theoretical investigations because such structures are few and localized on the sp² carbon sheet. Single-walled carbon nanotubes (SWCNTs) are described as rolled up single graphite structure sheets with each end capped by six corannulene structures.^6,7 Corannulene is composed of five 6-membered rings fused to a central 5-membered ring and has a bowl-shaped π-conjugated system.⁸ In the case of CNTs, it is well known that these corannulene-like end-caps are more easily oxidized than the side-wall consisting of 6-membered rings. Thus, it is easily expected that oxidative corrosion would be promoted from defect sites derived from end-cap oxidation. Knowledge of the onset oxidation potential of CNTs gives insight into the oxidative corrosion reactions of sp² carbons including 5-, 7-, and 8-membered ring structures. In this study, the oxidative corrosion of SWCNTs in an aqueous solution has been investigated. In the corrosion processes, we separated three types of SWCNTs: non-oxidized, end-cap oxidized (end-cap eliminated and opened) and side-wall oxidized SWCNTs, to obtain the diagram of the oxidative corrosion potential vs. pH. The diagram will be useful as a model system of sp² carbon and its defect sites (Fig. 1).

Fig. 1 Process of the oxidative corrosion of a SWCNT (showing non-oxidized, end-cap oxidized (end-cap eliminated) and side-wall oxidized SWCNTs).

The SWCNTs used in this work were synthesized onto a gold wire surface using a CVD method according to our previous reports (see Experimental section, ESI†).⁹ The fresh SWCNT electrodes were immediately immersed into an electrolyte solution (ionic strength = 0.1) composed of HClO₄ and NaOH. To oxidize the SWCNTs, controlled-potential electrolysis (CPE) was carried out for 30 min using a conventional three-electrode system. Prior to CPE, a potential of −0.6 V (vs. Ag/AgCl/saturated KCl) was applied to the SWCNT electrode to remove molecular oxygen bound to its surface by reduction reaction.⁹ Usually, the detection of end-cap eliminated SWCNTs is quite difficult using analysis methods such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and infrared (IR) spectroscopy because of the huge aspect ratio of SWCNTs. Thus, we paid attention to the molecular encapsulation behaviors of the SWCNTs. β-Carotene is well known as an organic molecular encapsulation agent.¹⁰ β-Carotene molecules encapsulate into the inside of SWCNTs if the end-cap is oxidized and then eliminated. After CPE, the SWCNTs were immersed in hexane containing 2 mmol dm⁻³ β-carotene and then refluxed for 10 h under an argon atmosphere. The SWCNTs were characterized by Raman spectroscopy using a Horiba (Jobin Yvon) LabRAM HR-800 instrument with 514 nm (2.41 eV) laser excitation. The G-band/D-band intensity ratio indicated that the synthesized pristine SWCNTs were highly crystalline sp²-hybridized carbon structures. The diameter of the SWCNTs from TEM images and radial breathing mode (RBM) in Raman spectra was estimated to be 0.9–1.6 nm, which was large enough for β-carotene to be encapsulated (see Fig. S2–S4, ESI†).¹⁰

Fig. 2 shows the Raman spectra of SWCNTs that underwent CPE at different potentials in a neutral solution followed by encapsulation treatment. In the case of CPE at 0.9–1.1 V (vs. Ag/AgCl) for 30 min, the spectra indicated that the SWCNT end-caps had been oxidized and eliminated because the characteristic vibrational frequency for the encapsulated β-carotene was observed at ca. 945 cm⁻¹ as the v₄ mode assigned to out-of-plane C–H wagging modes. The large intensity enhancement of the v₄ mode signal provides strong evidence that β-carotene was stabilized inside the SWCNTs.¹⁰ Three other Raman peaks representing the v₁, v₂, and v₃ modes were observed for the β-carotene encapsulated SWCNTs. These β-carotene v₁, v₂, and v₃ modes are assigned to the stretching modes of the conjugated C=C bonds, the mixture of C=C and C–C bond stretching modes with C–H bending modes, and the stretching modes of C–CH₃ bonds between the main-chain and the side methyl carbons, respectively (Fig. S3, ESI†).¹⁰ Here, we note that the observation of these three peaks is not conclusive of whether the β-carotene was encapsulated inside the SWCNTs. We were able to identify that the SWCNT end-caps were not eliminated when CPE was carried out at potential lower than 0.8 V because no v₄ mode peak was observed, although the v₁, v₂, and v₃ peaks were observed. Interestingly, in the cases of CPE at 1.2 and 1.4 V, no enhancement of the v₄ mode peak intensity was observed, even though the end-cap of the SWCNTs had already been oxidized and eliminated. One possible reason for this behavior is that the side-wall of the SWCNTs would be oxidized at such higher potentials. In fact, the intensity of the D-band was enhanced after CPE at 1.2 and 1.4 V in comparison with that of the spectra obtained at lower potentials. Also, the RBM around 100–300 cm⁻¹ almost disappeared, which also supports the decomposition of SWCNT side-walls. These results indicated that β-carotene cannot be stabilized inside such well-oxidized SWCNTs, resulting in the intensity of the v₄ mode peak being decreased and/or not observed. To summarize the results for a pH 7 solution, we were able to identify the following: (1) no oxidation potential below ca. 0.8 V, (2) the oxidation potential for elimination of the end-cap moiety at 0.9–1.1 V, and (3) the oxidation potential for both side-walls and end-caps at >1.2 V. The results of dynamics simulations indicate that the end-cap is much more susceptible to oxidation than the remainder of the CNT.¹¹ Our results provide strong evidence for this theoretical expectation. Previously, the results of the redox reaction behavior of metallic Ni nanoparticles included into CNTs after partial side-wall oxidation were reported as such proof.¹² Our method using β-carotene encapsulation is more sensitive because of the ease of detecting end-cap elimination.

Fig. 2 Raman spectra of SWCNTs after CPE followed by β-carotene encapsulation treatment.

We identified three types of SWCNT oxidation reactions after CPE at various pH values to obtain a diagram of oxidative corrosion potential vs. pH, as shown in Fig. 3. It is noteworthy that the oxidative corrosion behavior shows a biphasic behavior in the diagram. The potential at which the end-cap moiety was oxidized and eliminated was determined to be ca. 1.0 V when pH < 9, and was not dependent on pH. In contrast, pH dependence was observed when pH > 9. Here, a model for the reaction steps of SWCNT oxidation when pH < 9 might be shown as in Fig. 4 (see Scheme S1, ESI†).^1–3 Previous studies have shown reaction steps 1–3 to be reversible reactions, and steps 1 and 2 include the 1-proton-1-electron reaction.^2,3 Here, importantly, these oxidation reactions do not result in elimination of the end-cap from the SWCNT. The end-cap can only be eliminated when reaction step 4 occurs. This reaction is not proton-dependent. Therefore, we concluded that the potential of reaction step 4 was ca. 1.0 V. This potential is in good agreement with that previously obtained for a highly oriented pyrolytic graphite.³ This fact led us to conclude that reaction step 4 would require higher potential than the reaction steps 1–3, and/or would be the rate-determining step in the elimination of the end-cap. Without any barrier, the following reactions, steps 5 and 6, would occur at the same time as step 4 because they would be expected to occur at <0.6 V.³ In fact, an increase in O=C–O species was not detected in the XPS results for SWCNTs after CPE at 1.0 V in pH 7 in comparison with pristine SWCNTs (see Table S1, ESI†). In the region of pH > 9, the slope of the onset oxidation potential/pH was evaluated as ca. 0.12 V, which indicated that some parallel oxidation pathways including ion interactions may have occurred.^2,13,14 Further investigation is required.

Fig. 3 Diagram of potential vs. pH for the oxidative corrosion of SWCNTs (crosses: non-oxidized SWCNTs, circles: end-cap eliminated SWCNTs, squares: sidewall and end-cap oxidized SWCNTs).

Fig. 4 Schematic illustration of the reaction steps of SWCNT oxidative corrosion.

In summary, three types of oxidative corrosion of SWCNTs, resulting in non-oxidized, end-cap oxidized (end-cap eliminated) and side-wall oxidized SWCNTs, have been sensitively separated from Raman spectra using the encapsulation of β-carotene into the SWCNTs, and the diagram of oxidative corrosion types of potential vs. pH has been obtained. We believe that this analysis method and the obtained diagram will be useful for further understanding the oxidative corrosion of sp² carbons.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research (24550159) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Notes and references

1. R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon and D. Woode, et al., Chem. Rev., 2007, 107, 3904.
2. (a) K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, John Wiley & Sons, New York, 1988; (b) M. Noel and P. N. Anantharaman, Surf. Coat. Technol., 1986, 28, 161.
3. (a) H. Chang and A. J. Bard, J. Am. Chem. Soc., 1991, 113, 5588; (b) M. Nose, T. Kinumoto, H.-S. Choo, K. Miyazai, T. Abe and Z. Ogumi, Fuel Cells, 2009, 9, 284; (c) M. Matsumoto, T. Manako and H. Imai, J. Electrochem. Soc., 2009, 156, B1208; (d) R. L. McCreery and M. T. McDermott, Anal. Chem., 2012, 84, 2602; (e) S. Ohmori and T. Saito, Carbon, 2012, 50, 4932; (f) J. Kang, J. Wen, S. H. Jayaram, X. Wang and S.-K. Chen, J. Power Sources, 2012, 234, 208; (g) D. Wei, Y. Liu, L. Cao, H. Zhang, L. Huang, G. Yu, H. Kajiura and Y. Li, Adv. Funct. Mater., 2009, 19, 3618.
4. (a) T. Sun and S. Fabris, Nano Lett., 2012, 12, 17; (b) X. Qi, X. Guo and C. Zheng, Appl. Surf. Sci., 2012, 259, 195; (c) R. Backreedy, J. M. Jones, M. Pourkashanian and A. Williams, Faraday Discuss., 2001, 119, 385.
5. (a) R. Zan, Q. M. Ramasse, U. Bangert and K. S. Novoselov, Nano Lett., 2012, 12, 3936; (b) J.-C. Charlier, Acc. Chem. Res., 2002, 35, 1063.
6. (a) S. Iijima, Nature, 1991, 354, 56; (b) P. M. Ajayan, Chem. Rev., 1999, 99, 1787; (c) P. R. Wallace, Phys. Rev., 1947, 71, 622; (d) R. Saito, G. Dresselhaus and M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998.
7. (a) L. Dai, A. Patil, X. Gong, Z. Guo, L. Liu, Y. Liu and D. Zhu, ChemPhysChem, 2003, 4, 1150; (b) A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183; (c) R. L. McCreery, Chem. Rev., 2008, 108, 2646.
8. (a) C. Bruno, R. Benassi, A. Passalacqua, F. Paolucci, C. Fontanesi, M. Marcaccio, E. A. Jackson and L. T. Scott, J. Phys. Chem. B, 2009, 113, 1954; (b) M. Baumgarten, L. Gherghel, M. Wagner, A. Weitz, M. Rabinovitz, P.-C. Cheng and L. T. Scott, J. Am. Chem. Soc., 1995, 117, 6254.
9. (a) M. Tominaga, S. Sakamoto and H. Yamaguchi, J. Phys. Chem. C, 2012, 116, 9498; (b) M. Tominaga, A. Iwaoka, D. Kawai and S. Sakamoto, Electrochem. Commun., 2013, 31, 76; (c) S. Sakamoto and M. Tominaga, Chem.–Asian J., 2013, 8, 2680.
10. (a) K. Yanagi, Y. Miyata and H. Kataura, Adv. Mater., 2006, 18, 437; (b) P. Horn and M. Kertesz, J. Phys. Chem. C, 2010, 114, 12139; (c) K. Yanagi, K. Iakoubovskii, S. Kazaoui, N. Minami, Y. Maniwa, Y. Miyata and H. Kataura, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 74, 155420.
11. D. J. Mann and W. L. Hase, Phys. Chem. Chem. Phys., 2001, 3, 4376.
12. A. Ambroshi and M. Pumera, Chem.–Eur. J., 2012, 18, 3338.
13. A. Doepke, C. Han, T. Back, W. Cho, D. D. Dionysiou, V. Shanov, H. B. Halsall and W. R. Heineman, Electroanalysis, 2012, 24, 1501.
14. (a) A. M. Rao, P. C. Eklund, S. Bandow, A. Thess and R. E. Smalley, Nature, 1997, 388, 257; (b) G. U. Sumanasekera, J. L. Allen, S. L. Fang, A. L. Loper, A. M. Rao and P. C. Eklund, J. Phys. Chem. B, 1999, 103, 4292; (c) S. Lefrant, M. Baibarac, I. Baltog, J. Y. Mevellec, L. Mihut and O. Chauvet, Synth. Met., 2004, 144, 133.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02875a

---