- Email: [email protected]
Electrochemistry of composite films of C60 and multiwalled carbon nanotubes: A robust conductive matrix for the fine dispersion of fullerenes
Chemical Physics Letters 413 (2005) 346–350 www.elsevier.com/locate/cplett
Electrochemistry of composite films of C60 and multiwalled carbon nanotubes: A robust conductive matrix for the fine dispersion of fullerenes Hua Zhang a, Louzhen Fan b
a,*
, Yueping Fang b, Shihe Yang
b,*
a Department of Chemistry, Beijing Normal University, Beijing 100875, China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Received 8 July 2005; in final form 21 July 2005 Available online 22 August 2005
Abstract The electrochemical behavior of a C60/MWCNT (multi-walled carbon nanotube) film cast on GC electrode in acetonitrile solution was investigated. Repeated cyclic voltammetric scans transformed the precursor to a C60–MWCNT composite film which exhibited reversible electron-transfer reactions and superpose the redox feature of the bare MWCNTs with monotonic charge injection over the whole potential range. This is in contrast to both C60 film and peapods of C60 inside carbon nanotubes but resembles the behavior of C60 dissolved in organic solutions. The formation and structure of the C60–MWCNT composite films were studied by UV–Vis, FT-IR, and TEM. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction C60 and carbon nanotubes as novel all-carbon pelectron systems have increasingly invited exploration of their outstanding new physical and chemical properties. There is keen interest in preparing composite films of fullerenes and carbon nanotubes from both fundamental and practical points of views [1,2]. However, one of the key challenges is to overcome the high aggregation tendency of these nanoscale carbon spheres and fibers [3]. C60 can be cathodically reduced in aprotic electrolyte solution towards C60 , C260 , . . . , C660 in six reversible oneelectron steps [4], while thin solid films of C60 exhibit irreversible reductions, with a reconstruction of the film structure [5,6]. The electrochemistry of carbon nanotubes *
Corresponding authors. Fax: +86 01 58802075 (L. Fan). E-mail addresses: [email protected] (L. Fan), [email protected] (S. Yang). 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.009
is dominated by double-layer charging [7,8]. As both fullerenes and carbon nanotubes show specific redox responses, it is tempting to explore the redox behavior of their combinations and address some fundamental problems of electronic properties of carbon nanostructures in general. One approach is to insert C60 into CNT (carbon nanotube) to form as called peapods [9–11]. However, the results showed that the electrochemistry of peapods was dominated by their capacitive charging without distinct faradaic processes. Furthermore, in situ Vis–NIR spectra of the peapods exhibited similar features to those of the empty carbon nanotubes [9–11]. In this Letter, we report on the creation of a C60– MWCNT composite film by cyclic voltammetry. Remarkably, the composite film displays electrochemical behaviors of the constituent C60, which are very similar to those of C60 homogeneously dissolved in organic solutions but superpose the redox features of bare MWCNTs with a monotonic charge injection over the whole potential range in our study.
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
2. Experimental
15.0
347
10.0
3. Results and discussion The freshly cast C60/MWCNT mixture film on a GC electrode looked black and smooth. It was subjected to cyclic voltammetric (CV) scans. After two or three potential scans between 0 and 2.0 V (vs. Ag/AgCl), a typical CV profile of the C60–MWCNT composite film on GC electrode in the acetonitrile solution of 0.1 mol l 1 TBAPF6 is obtained as shown in Fig. 1 (curve a). Notice that a distinction is made here between the C60/ MWCNT mixture and the C60–MWCNT composite; the former is characterized by p–p stacking, whereas the latter involves stronger interactions between C60 and MWCNT induced by the CV scans. Interestingly,
10.0
5.0 2.5
a
0.0 -2.5
0.0
-0.5 -1.0 -1.5 E / V (Ag/AgCl)
-2.0
b
5.0
i / µA
C60 (>99.9%) was purchased from Peking University. Multi-walled carbon nanotubes (MWCNTs) (Carbon Nanotechnologies, Inc., USA) were further purified prior to use by stirring in concentrated nitric acid for 12 h. TBAPF6 (Aldrich) was dried for 6 h in vacuum before use. Acetonitrile (CH3CN) (HPLC grade, Labscan Asia Co., Thailand) was used as purchased. Toluene (C6H5CH3) (Park Co. Dublin, Ireland) was dried with sodium, refluxed for 6 h, and then distilled. The purified toluene was stored in the presence of sodium. Immediately before use, a glassy carbon electrode (3 mm diameter) was polished to a mirror finish with emery paper and alumina slurry (1.0 and 0.3 lm), ultrasonically cleaned in toluene for 5 min, and dried in a high-purity nitrogen stream. Purified MWCNTs and C60 (MWCNTs:C60 = 2:1) with a total amount of about 1 mg were dispersed in 10 ml toluene in an ultrasound bath for 30 min to give a 0.1 mg ml 1 suspension. To prepare the C60/MWCNT mixture film, 15 ll of the suspension was directly cast on a glassy carbon (GC) electrode surface and the solvent was allowed to evaporate at room temperature. UV–Vis and FT-IR spectra were measured on a Cintra 10 e UV–Vis Spectrometer (GBC, Australia) and AVATAR 360 FT-IR (Nicolet, USA), respectively. Transmission electron microscopy (TEM) measurements were conducted on Philips CM20 and JEOL 2010F transmission electron microscopes with an accelerating voltage of 200 kV. Cyclic voltammetry (CV) (CHI610B, Inc., Austin, USA) was performed with a three-electrode configuration in acetonitrile solution containing supporting electrolyte (TBAPF6, 0.1 mol l 1). The GC or the C60/MWCNT mixture film on GC was used as the working electrode. A Pt wire electrode served as the auxiliary electrode, and a Ag/AgCl electrode was used as the reference. All electrochemical experiments were performed in a high purity N2 atmosphere at ambient temperature (22 ± 1 °C).
i / µA
7.5
0.0
-5.0
-10.0 0.0
-0.5
-1.0
-1.5
-2.0
E / V (Ag/AgCl) Fig. 1. Cyclic voltammograms of: (a) C60–MWCNT composite film on GC electrode in acetonitrile solution containing 0.1 mol l 1 TBAPF6; (b) C60 dissolved in acetonitrile and toluene (v:v = 1:4) solution containing 0.1 mol l 1 TBAPF6 on GC electrode; and (inset) C60 film on GC electrode in acetonitrile solution containing 0.1 mol l 1 TBAPF6. Scan rate: 50 mV s 1.
well-defined redox responses are observed, which resemble those of C60 dissolved in an acetonitrile/toluene solvent mixture on a GC working electrode (curve b of Fig. 1) but superpose on the redox curve of the bare MWCNT with monotonic charge injection over the whole potential range [7,8]. Namely, the composite film exhibits reversible multiple-step electron-transfer reactions corresponding to C60 =C60 , C60 =C260 , C260 =C360 , and C360 =C460 . This is in sharp contrast with the reduction waves of C60 film in an acetonitrile solution containing 0.1 mol l 1 TBAPF6, where there are large splittings between the first two reduction waves and corresponding reoxidation waves (see inset of Fig. 1) [5,6]. This is also different from the case of peapods formed by insertion of C60 into CNTs, where the CV is analogous to that in empty CNT but no distinct redox couples of C60 was observed [9–11]. The C60–MWCNT composite films were found to be stable on the electrode. Both the potentials and currents of the redox waves were essentially unchanged after several continuous scans at a rate of 50 mV s 1. Significantly, even after this film electrode was rinsed with toluene several times, the currents of the redox waves remained unchanged. We also added the resulting C60– MWCNT composites into toluene and subjected the suspension to ultrasonication for several hours, but the toluene solution did not turn purple, a characteristic color of C60. These results indicate that C60 has been strongly bounded with MWCNTs. Early studies found that bare C60 films, in addition to having large splittings between the first two reduction waves and their corresponding reoxidation waves, are easy to fall off soon after potential cycling over the third reduction wave [5]. However,
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
the C60–MWCNT composite films we obtained are intact on GC electrode even when the potential was scanned over the forth reduction wave. Scan-rate dependence was measured for the C60/MWCNT composite film electrode between 0.01 and 0.1 V s 1. For the reduction waves of C60–MWCNT composite films, the peak currents increase linearly with the increase of the scan rate, indicating that the redox processes were confined on the electrode surface [12]. We have followed the formation process of the C60– MWCNT composites and studied the interaction between C60 and MWCNTs. First, C60 was added to toluene to form a purple C60 solution. Immediately after adding MWCNTs accompanied by ultrasonication, the colour of the solution was found to become more faint even with direct visual inspection. Several solvent suspensions with the same amount of C60 and MWCNT were prepared at the same time, followed by ultrasonication and precipitation. UV–Vis measurements of the clear solutions above these precipitates were carried out at different times from the addition of the MWCNTs. The results are shown in Fig. 2. Clearly, with the elapse of time, the heights of typical absorption peaks of C60 at 338 and 407 nm in the solutions decrease, suggesting that the dissolved C60 was continuously adsorbed on MWCNTs. After 48 h, the height of this absorption peak no longer decreases, indicating an adsorption saturation of C60 on the MWCNTs. Next, the ethanol suspension of a C60–MWCNT composite film obtained after a number of redox cycling scans together with that of pristine MWCNTs were also characterized by UV–Vis absorption spectroscopy (see Fig. 3). A pronounced peak at 333 nm in the absorption spectrum of the C60–MWCNT composites (curve b) is in sharp contrast with that of the MWCNTs in the region of 300–900 nm (curve a), which is essentially featureless.
1.6
b
0.06
Absorbance
348
0.04
0.02
a 0.00 300
450
600
Fig. 3. UV–Vis spectra of ethanol suspension solution of: (a) a C60– MWCNT composite film after redox scans; (b) a bare MWCNT film.
This is a clear evidence showing the dispersion of C60 on MWCNTs. Compared with the absorption of C60 at 338 nm in Fig. 2, a blue shift of about 5 nm is observed and can be attributed to the interaction between C60 and MWCNTs. This interaction, if indeed present, should also be reflected in FT-IR. As shown in Fig. 4, pristine MWCNTs do not show any meaningful peaks as reported before (Fig. 4, curve a) [11], while the spectrum of bare C60 exhibits four clear absorptions at 527, 577, 1180, and 1430 cm 1, respectively (Fig. 4, curve b). However, in the spectrum of the C60–MWCNT composite film (Fig. 4, curve c), two main peaks are observed at 1380 and 1460 cm 1, which are thought to be derived from the peaks of bare C60 at 1180 and 1430 cm 1, respectively. The relative shifts of the peaks are ascribable to the interaction between C60 and MWCNTs. Similar observations were reported in the functionalization
c
0.8
c
Transmitance (%)
Absorbance
1460 1380 b
900
Wavelength (nm)
a
1.2
750
d
0.4
1430
b
577
1180
526
a
0.0 300
400
500
600
700
800
900
Wavelength(nm) Fig. 2. UV–Vis spectra of the clear solution of C60 after adding MWCNT for: (a) 15 min; (b) 8 h; (c) 16 h; (d) 48 h.
1500
1350
1200
1050
900
750
600
-1
Wavenumbers (cm ) Fig. 4. FT-IR spectra (KBr pellet) of: (a) MWCNTs; (b) C60; (c) C60– MWCNT composite film.
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
of C60 previously, but here the functional group is the MWCNT [14,15]. It follows that, in the C60–MWCNT composite film prepared by CV, C60 is most probably attached to the MWCNTs by covalent interaction, which is in line with the existing literature reports [13–15]. In order to understand the role of cyclic voltammetric (CV) scans in conferring the reversible redox behavior to the C60–MWCNT composite, transmission electron microscopy (TEM) was employed to examine the surface morphologies of the C60 and MWCNT mixture film before and after CV scans. The pristine MWCNTs exhibit no anchored nanoparticles at all as expected (see Fig. 5a). For the C60/MWCNT mixture film, the form before CV scans, aggregates of C60 with diameters of about 10 nm are observed (Fig. 5b). However, the TEM image of the C60–MWCNT composite film obtained after several potential cycling scans clearly shows that smaller nanoparticles of C60 with diameters from 2 to 4 nm are uniformly dispersed on the MWCNTs (Fig. 5c). It is likely that the C60–MWCNT composites are formed in the following way. To begin with, C60 molecules react with MWCNTs at those sites on the surface of MWCNTs where the reaction is easier to take place. However, because C60 moieties are more reactive
349
than MWCNTs, the additional free C60 molecules will tend to aggregate with the already bounded C60 on MWCNT to form C60 clusters. Conceivably, the CV scans to negative potentials provide an electron-rich environment on the MWCNT surfaces, especially at defect sites, for nucleophilic addition to the electro-deficient C60 to form a covalent bond. This leads to uniform dispersion of C60 on MWCNT surfaces. This proposal is supported by the CV studies. We found that the initial CV scan for the freshly prepared C60/ MWCNT mixture films is clearly more complicated (see Figure S1 in Supporting Information). Its reduction peaks comprise the reductions of both C60 films with aggregation and C60–MWCNT composite films. This implies that C60 initially adopts on the surface of MWCNTs with different forms. After two or three cyclic voltammetric scans, stable and reversible CV responses such as the curve a shown in Fig. 1 present. This is a manifestation of the rearrangement of C60 on the surfaces of MWCNTs to generate uniform C60–MWCNT composites, where C60 is most likely covalently bonded to the MWCNTs. The redox wave potentials and the differences between the reduction wave potentials and the correspond-
Fig. 5. TEM images of: (a) a pristine MWCNT film; (b) a C60/MWCNT film before redox scan; (c) a C60–MWCNT composite film after redox scan.
350
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
Table 1 Redox potentials of C60–MWCNT composite film and C60 solution Era (V) 1 C60–MWCNT composite film C60 solution a b
DEpb (mV) 2
0.613 0.511
3 0.879 0.953
4 1.356 1.423
1 1.785 1.932
2 182.1 109.4
3 66.5 130.7
4 94.5 132.3
79.7 171.7
Er, reduction wave potentials/V vs. Ag/AgCl. Ep = Er E0 (mV), where E0 is the corresponding reoxidation wave potentials.
ing reoxidation wave potentials (DEp) of the C60– MWCNT composite in acetonitrile solution were further compared with that of C60 dissolved in a toluene and acetonitrile mixture solution. As shown in Table 1, with the C60–MWCNT composite electrode, except for the first reduction, all of the observed reduction wave potentials shift more positively. This is an expected result if C60 is covalently bonded to the MWCNT framework, where sp2 carbon atoms undergo sp3 conversion [15]. Accordingly, C60 in the C60–MWCNT composite tends to get electrons more easily than that in the organic solution. Moreover, except for the first reduction, the corresponding DEp values are also smaller for the C60– MWCNT composites than those for C60 dissolved in solutions. This suggests a faster electron transfer kinetics in the C60–MWCNT composite because the fine dispersion, nanometer size, large accessible surface to volume ratio, and low resistance offer a favorite microenvironment for electron transfer between C60 and the underlying electrode. As for the first reduction, the more negative potential and the relatively large splitting between the reduction and reoxidation waves are thought to result from the initial adsorption configuration, which is less favorable for electron transfer. Once electron transfer has occurred, C60 may reposition itself on the MWCNT accompanied by counter ion diffusion, making the subsequent reduction processes more easily, although more studies are necessary.
4. Summary and conclusion In summary, uniform C60–MWCNT composite films have been prepared by dispersing C60 in a MWCNT matrix through CV cycling. The composite films present reversible redox behavior which is similar to that of C60 dissolved in a toluene and acteonitrile mixture solution but is very different from those of either C60 films, CNT films, or peapod films. On the basis of the spectroscopic and TEM results we obtained, it is presumed that these novel properties come from the covalent anchorage of C60 to the MWCNTs in a uniform fashion. Further studies on the mechanistic details and functional properties of these unique all-carbon composites are currently being investigated in our laboratory, which
may pave the way for their extensive potential applications in nanoscience and nanotechnology.
Acknowledgements This work is financially supported by National Natural Science Foundation of China (20473014) and NNSFC-RGC administrated by the UGC of Hong Kong (N_HKUST604/04).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.cplett.2005.08.009.
References [1] J. Chlistunoff, D. Cliffel, A.J. Bard, Thin Solid Films 257 (1995) 166. [2] F. Diederich, M. Gomez, Chem. Soc. Rev. 28 (1999) 263. [3] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158. [4] Q. Xie, E. Perez, L. Echegoyen, J. Am. Chem. Soc. 114 (1992) 3978. [5] C. Jehoulet, Y.O. Obeng, Y.T. Kim, F. Zhou, A.J. Bard, J. Am. Chem. Soc. 114 (1992) 4237. [6] P. Janda, T. Krieg, L. Dunsch, Adv. Mater. 17 (1998) 1434. [7] L. Kavan, P. Rapta, L. Dunsch, M.J. Bronikowski, P. Willis, R.E. Smalley, J. Phys. Chem. B 105 (2001) 10764. [8] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [9] L. Kavan, L. Dunsch, H. Kataura, Chem. Phys. Lett. 361 (2002) 79. [10] L. Kavan, L. Dunsch, H. Kataura, A. Oshiyama, M. Otani, S. Okada, J. Phys. Chem. B 107 (2003) 7666. [11] L. Cai, J.L. Bahr, Y. Yao, J.M. Tour, Chem. Mater. 14 (2002) 4235. [12] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 2001. [13] X. Li, L. Liu, Y. Qin, W. Wu, Z. Guo, L. Dai, D. Zhu, Chem. Phys. Lett. 377 (2003) 32. [14] J.L. Bahr, J. Yang, D.V. Kosynkin, M. Bronikowski, R.E. Smalley, J. Tour, J. Am. Chem. Soc. 123 (2001) 6536. [15] P. Boul, J. Liu, E.T. Mickelson, C.B. Huffman, L. Ericson, I. Chiang, K. Smith, D. Colbert, R. Hauge, J. Margrave, R.E. Smalley, Chem. Phys. Lett. 310 (1999) 367.
Electrochemistry of composite films of C60 and multiwalled carbon nanotubes: A robust conductive matrix for the fine dispersion of fullerenes Hua Zhang a, Louzhen Fan b
a,*
, Yueping Fang b, Shihe Yang
b,*
a Department of Chemistry, Beijing Normal University, Beijing 100875, China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Received 8 July 2005; in final form 21 July 2005 Available online 22 August 2005
Abstract The electrochemical behavior of a C60/MWCNT (multi-walled carbon nanotube) film cast on GC electrode in acetonitrile solution was investigated. Repeated cyclic voltammetric scans transformed the precursor to a C60–MWCNT composite film which exhibited reversible electron-transfer reactions and superpose the redox feature of the bare MWCNTs with monotonic charge injection over the whole potential range. This is in contrast to both C60 film and peapods of C60 inside carbon nanotubes but resembles the behavior of C60 dissolved in organic solutions. The formation and structure of the C60–MWCNT composite films were studied by UV–Vis, FT-IR, and TEM. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction C60 and carbon nanotubes as novel all-carbon pelectron systems have increasingly invited exploration of their outstanding new physical and chemical properties. There is keen interest in preparing composite films of fullerenes and carbon nanotubes from both fundamental and practical points of views [1,2]. However, one of the key challenges is to overcome the high aggregation tendency of these nanoscale carbon spheres and fibers [3]. C60 can be cathodically reduced in aprotic electrolyte solution towards C60 , C260 , . . . , C660 in six reversible oneelectron steps [4], while thin solid films of C60 exhibit irreversible reductions, with a reconstruction of the film structure [5,6]. The electrochemistry of carbon nanotubes *
Corresponding authors. Fax: +86 01 58802075 (L. Fan). E-mail addresses: [email protected] (L. Fan), [email protected] (S. Yang). 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.08.009
is dominated by double-layer charging [7,8]. As both fullerenes and carbon nanotubes show specific redox responses, it is tempting to explore the redox behavior of their combinations and address some fundamental problems of electronic properties of carbon nanostructures in general. One approach is to insert C60 into CNT (carbon nanotube) to form as called peapods [9–11]. However, the results showed that the electrochemistry of peapods was dominated by their capacitive charging without distinct faradaic processes. Furthermore, in situ Vis–NIR spectra of the peapods exhibited similar features to those of the empty carbon nanotubes [9–11]. In this Letter, we report on the creation of a C60– MWCNT composite film by cyclic voltammetry. Remarkably, the composite film displays electrochemical behaviors of the constituent C60, which are very similar to those of C60 homogeneously dissolved in organic solutions but superpose the redox features of bare MWCNTs with a monotonic charge injection over the whole potential range in our study.
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
2. Experimental
15.0
347
10.0
3. Results and discussion The freshly cast C60/MWCNT mixture film on a GC electrode looked black and smooth. It was subjected to cyclic voltammetric (CV) scans. After two or three potential scans between 0 and 2.0 V (vs. Ag/AgCl), a typical CV profile of the C60–MWCNT composite film on GC electrode in the acetonitrile solution of 0.1 mol l 1 TBAPF6 is obtained as shown in Fig. 1 (curve a). Notice that a distinction is made here between the C60/ MWCNT mixture and the C60–MWCNT composite; the former is characterized by p–p stacking, whereas the latter involves stronger interactions between C60 and MWCNT induced by the CV scans. Interestingly,
10.0
5.0 2.5
a
0.0 -2.5
0.0
-0.5 -1.0 -1.5 E / V (Ag/AgCl)
-2.0
b
5.0
i / µA
C60 (>99.9%) was purchased from Peking University. Multi-walled carbon nanotubes (MWCNTs) (Carbon Nanotechnologies, Inc., USA) were further purified prior to use by stirring in concentrated nitric acid for 12 h. TBAPF6 (Aldrich) was dried for 6 h in vacuum before use. Acetonitrile (CH3CN) (HPLC grade, Labscan Asia Co., Thailand) was used as purchased. Toluene (C6H5CH3) (Park Co. Dublin, Ireland) was dried with sodium, refluxed for 6 h, and then distilled. The purified toluene was stored in the presence of sodium. Immediately before use, a glassy carbon electrode (3 mm diameter) was polished to a mirror finish with emery paper and alumina slurry (1.0 and 0.3 lm), ultrasonically cleaned in toluene for 5 min, and dried in a high-purity nitrogen stream. Purified MWCNTs and C60 (MWCNTs:C60 = 2:1) with a total amount of about 1 mg were dispersed in 10 ml toluene in an ultrasound bath for 30 min to give a 0.1 mg ml 1 suspension. To prepare the C60/MWCNT mixture film, 15 ll of the suspension was directly cast on a glassy carbon (GC) electrode surface and the solvent was allowed to evaporate at room temperature. UV–Vis and FT-IR spectra were measured on a Cintra 10 e UV–Vis Spectrometer (GBC, Australia) and AVATAR 360 FT-IR (Nicolet, USA), respectively. Transmission electron microscopy (TEM) measurements were conducted on Philips CM20 and JEOL 2010F transmission electron microscopes with an accelerating voltage of 200 kV. Cyclic voltammetry (CV) (CHI610B, Inc., Austin, USA) was performed with a three-electrode configuration in acetonitrile solution containing supporting electrolyte (TBAPF6, 0.1 mol l 1). The GC or the C60/MWCNT mixture film on GC was used as the working electrode. A Pt wire electrode served as the auxiliary electrode, and a Ag/AgCl electrode was used as the reference. All electrochemical experiments were performed in a high purity N2 atmosphere at ambient temperature (22 ± 1 °C).
i / µA
7.5
0.0
-5.0
-10.0 0.0
-0.5
-1.0
-1.5
-2.0
E / V (Ag/AgCl) Fig. 1. Cyclic voltammograms of: (a) C60–MWCNT composite film on GC electrode in acetonitrile solution containing 0.1 mol l 1 TBAPF6; (b) C60 dissolved in acetonitrile and toluene (v:v = 1:4) solution containing 0.1 mol l 1 TBAPF6 on GC electrode; and (inset) C60 film on GC electrode in acetonitrile solution containing 0.1 mol l 1 TBAPF6. Scan rate: 50 mV s 1.
well-defined redox responses are observed, which resemble those of C60 dissolved in an acetonitrile/toluene solvent mixture on a GC working electrode (curve b of Fig. 1) but superpose on the redox curve of the bare MWCNT with monotonic charge injection over the whole potential range [7,8]. Namely, the composite film exhibits reversible multiple-step electron-transfer reactions corresponding to C60 =C60 , C60 =C260 , C260 =C360 , and C360 =C460 . This is in sharp contrast with the reduction waves of C60 film in an acetonitrile solution containing 0.1 mol l 1 TBAPF6, where there are large splittings between the first two reduction waves and corresponding reoxidation waves (see inset of Fig. 1) [5,6]. This is also different from the case of peapods formed by insertion of C60 into CNTs, where the CV is analogous to that in empty CNT but no distinct redox couples of C60 was observed [9–11]. The C60–MWCNT composite films were found to be stable on the electrode. Both the potentials and currents of the redox waves were essentially unchanged after several continuous scans at a rate of 50 mV s 1. Significantly, even after this film electrode was rinsed with toluene several times, the currents of the redox waves remained unchanged. We also added the resulting C60– MWCNT composites into toluene and subjected the suspension to ultrasonication for several hours, but the toluene solution did not turn purple, a characteristic color of C60. These results indicate that C60 has been strongly bounded with MWCNTs. Early studies found that bare C60 films, in addition to having large splittings between the first two reduction waves and their corresponding reoxidation waves, are easy to fall off soon after potential cycling over the third reduction wave [5]. However,
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
the C60–MWCNT composite films we obtained are intact on GC electrode even when the potential was scanned over the forth reduction wave. Scan-rate dependence was measured for the C60/MWCNT composite film electrode between 0.01 and 0.1 V s 1. For the reduction waves of C60–MWCNT composite films, the peak currents increase linearly with the increase of the scan rate, indicating that the redox processes were confined on the electrode surface [12]. We have followed the formation process of the C60– MWCNT composites and studied the interaction between C60 and MWCNTs. First, C60 was added to toluene to form a purple C60 solution. Immediately after adding MWCNTs accompanied by ultrasonication, the colour of the solution was found to become more faint even with direct visual inspection. Several solvent suspensions with the same amount of C60 and MWCNT were prepared at the same time, followed by ultrasonication and precipitation. UV–Vis measurements of the clear solutions above these precipitates were carried out at different times from the addition of the MWCNTs. The results are shown in Fig. 2. Clearly, with the elapse of time, the heights of typical absorption peaks of C60 at 338 and 407 nm in the solutions decrease, suggesting that the dissolved C60 was continuously adsorbed on MWCNTs. After 48 h, the height of this absorption peak no longer decreases, indicating an adsorption saturation of C60 on the MWCNTs. Next, the ethanol suspension of a C60–MWCNT composite film obtained after a number of redox cycling scans together with that of pristine MWCNTs were also characterized by UV–Vis absorption spectroscopy (see Fig. 3). A pronounced peak at 333 nm in the absorption spectrum of the C60–MWCNT composites (curve b) is in sharp contrast with that of the MWCNTs in the region of 300–900 nm (curve a), which is essentially featureless.
1.6
b
0.06
Absorbance
348
0.04
0.02
a 0.00 300
450
600
Fig. 3. UV–Vis spectra of ethanol suspension solution of: (a) a C60– MWCNT composite film after redox scans; (b) a bare MWCNT film.
This is a clear evidence showing the dispersion of C60 on MWCNTs. Compared with the absorption of C60 at 338 nm in Fig. 2, a blue shift of about 5 nm is observed and can be attributed to the interaction between C60 and MWCNTs. This interaction, if indeed present, should also be reflected in FT-IR. As shown in Fig. 4, pristine MWCNTs do not show any meaningful peaks as reported before (Fig. 4, curve a) [11], while the spectrum of bare C60 exhibits four clear absorptions at 527, 577, 1180, and 1430 cm 1, respectively (Fig. 4, curve b). However, in the spectrum of the C60–MWCNT composite film (Fig. 4, curve c), two main peaks are observed at 1380 and 1460 cm 1, which are thought to be derived from the peaks of bare C60 at 1180 and 1430 cm 1, respectively. The relative shifts of the peaks are ascribable to the interaction between C60 and MWCNTs. Similar observations were reported in the functionalization
c
0.8
c
Transmitance (%)
Absorbance
1460 1380 b
900
Wavelength (nm)
a
1.2
750
d
0.4
1430
b
577
1180
526
a
0.0 300
400
500
600
700
800
900
Wavelength(nm) Fig. 2. UV–Vis spectra of the clear solution of C60 after adding MWCNT for: (a) 15 min; (b) 8 h; (c) 16 h; (d) 48 h.
1500
1350
1200
1050
900
750
600
-1
Wavenumbers (cm ) Fig. 4. FT-IR spectra (KBr pellet) of: (a) MWCNTs; (b) C60; (c) C60– MWCNT composite film.
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
of C60 previously, but here the functional group is the MWCNT [14,15]. It follows that, in the C60–MWCNT composite film prepared by CV, C60 is most probably attached to the MWCNTs by covalent interaction, which is in line with the existing literature reports [13–15]. In order to understand the role of cyclic voltammetric (CV) scans in conferring the reversible redox behavior to the C60–MWCNT composite, transmission electron microscopy (TEM) was employed to examine the surface morphologies of the C60 and MWCNT mixture film before and after CV scans. The pristine MWCNTs exhibit no anchored nanoparticles at all as expected (see Fig. 5a). For the C60/MWCNT mixture film, the form before CV scans, aggregates of C60 with diameters of about 10 nm are observed (Fig. 5b). However, the TEM image of the C60–MWCNT composite film obtained after several potential cycling scans clearly shows that smaller nanoparticles of C60 with diameters from 2 to 4 nm are uniformly dispersed on the MWCNTs (Fig. 5c). It is likely that the C60–MWCNT composites are formed in the following way. To begin with, C60 molecules react with MWCNTs at those sites on the surface of MWCNTs where the reaction is easier to take place. However, because C60 moieties are more reactive
349
than MWCNTs, the additional free C60 molecules will tend to aggregate with the already bounded C60 on MWCNT to form C60 clusters. Conceivably, the CV scans to negative potentials provide an electron-rich environment on the MWCNT surfaces, especially at defect sites, for nucleophilic addition to the electro-deficient C60 to form a covalent bond. This leads to uniform dispersion of C60 on MWCNT surfaces. This proposal is supported by the CV studies. We found that the initial CV scan for the freshly prepared C60/ MWCNT mixture films is clearly more complicated (see Figure S1 in Supporting Information). Its reduction peaks comprise the reductions of both C60 films with aggregation and C60–MWCNT composite films. This implies that C60 initially adopts on the surface of MWCNTs with different forms. After two or three cyclic voltammetric scans, stable and reversible CV responses such as the curve a shown in Fig. 1 present. This is a manifestation of the rearrangement of C60 on the surfaces of MWCNTs to generate uniform C60–MWCNT composites, where C60 is most likely covalently bonded to the MWCNTs. The redox wave potentials and the differences between the reduction wave potentials and the correspond-
Fig. 5. TEM images of: (a) a pristine MWCNT film; (b) a C60/MWCNT film before redox scan; (c) a C60–MWCNT composite film after redox scan.
350
H. Zhang et al. / Chemical Physics Letters 413 (2005) 346–350
Table 1 Redox potentials of C60–MWCNT composite film and C60 solution Era (V) 1 C60–MWCNT composite film C60 solution a b
DEpb (mV) 2
0.613 0.511
3 0.879 0.953
4 1.356 1.423
1 1.785 1.932
2 182.1 109.4
3 66.5 130.7
4 94.5 132.3
79.7 171.7
Er, reduction wave potentials/V vs. Ag/AgCl. Ep = Er E0 (mV), where E0 is the corresponding reoxidation wave potentials.
ing reoxidation wave potentials (DEp) of the C60– MWCNT composite in acetonitrile solution were further compared with that of C60 dissolved in a toluene and acetonitrile mixture solution. As shown in Table 1, with the C60–MWCNT composite electrode, except for the first reduction, all of the observed reduction wave potentials shift more positively. This is an expected result if C60 is covalently bonded to the MWCNT framework, where sp2 carbon atoms undergo sp3 conversion [15]. Accordingly, C60 in the C60–MWCNT composite tends to get electrons more easily than that in the organic solution. Moreover, except for the first reduction, the corresponding DEp values are also smaller for the C60– MWCNT composites than those for C60 dissolved in solutions. This suggests a faster electron transfer kinetics in the C60–MWCNT composite because the fine dispersion, nanometer size, large accessible surface to volume ratio, and low resistance offer a favorite microenvironment for electron transfer between C60 and the underlying electrode. As for the first reduction, the more negative potential and the relatively large splitting between the reduction and reoxidation waves are thought to result from the initial adsorption configuration, which is less favorable for electron transfer. Once electron transfer has occurred, C60 may reposition itself on the MWCNT accompanied by counter ion diffusion, making the subsequent reduction processes more easily, although more studies are necessary.
4. Summary and conclusion In summary, uniform C60–MWCNT composite films have been prepared by dispersing C60 in a MWCNT matrix through CV cycling. The composite films present reversible redox behavior which is similar to that of C60 dissolved in a toluene and acteonitrile mixture solution but is very different from those of either C60 films, CNT films, or peapod films. On the basis of the spectroscopic and TEM results we obtained, it is presumed that these novel properties come from the covalent anchorage of C60 to the MWCNTs in a uniform fashion. Further studies on the mechanistic details and functional properties of these unique all-carbon composites are currently being investigated in our laboratory, which
may pave the way for their extensive potential applications in nanoscience and nanotechnology.
Acknowledgements This work is financially supported by National Natural Science Foundation of China (20473014) and NNSFC-RGC administrated by the UGC of Hong Kong (N_HKUST604/04).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.cplett.2005.08.009.
References [1] J. Chlistunoff, D. Cliffel, A.J. Bard, Thin Solid Films 257 (1995) 166. [2] F. Diederich, M. Gomez, Chem. Soc. Rev. 28 (1999) 263. [3] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158. [4] Q. Xie, E. Perez, L. Echegoyen, J. Am. Chem. Soc. 114 (1992) 3978. [5] C. Jehoulet, Y.O. Obeng, Y.T. Kim, F. Zhou, A.J. Bard, J. Am. Chem. Soc. 114 (1992) 4237. [6] P. Janda, T. Krieg, L. Dunsch, Adv. Mater. 17 (1998) 1434. [7] L. Kavan, P. Rapta, L. Dunsch, M.J. Bronikowski, P. Willis, R.E. Smalley, J. Phys. Chem. B 105 (2001) 10764. [8] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [9] L. Kavan, L. Dunsch, H. Kataura, Chem. Phys. Lett. 361 (2002) 79. [10] L. Kavan, L. Dunsch, H. Kataura, A. Oshiyama, M. Otani, S. Okada, J. Phys. Chem. B 107 (2003) 7666. [11] L. Cai, J.L. Bahr, Y. Yao, J.M. Tour, Chem. Mater. 14 (2002) 4235. [12] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 2001. [13] X. Li, L. Liu, Y. Qin, W. Wu, Z. Guo, L. Dai, D. Zhu, Chem. Phys. Lett. 377 (2003) 32. [14] J.L. Bahr, J. Yang, D.V. Kosynkin, M. Bronikowski, R.E. Smalley, J. Tour, J. Am. Chem. Soc. 123 (2001) 6536. [15] P. Boul, J. Liu, E.T. Mickelson, C.B. Huffman, L. Ericson, I. Chiang, K. Smith, D. Colbert, R. Hauge, J. Margrave, R.E. Smalley, Chem. Phys. Lett. 310 (1999) 367.