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Dispersion of carbon nanotubes by carbazole-tailed amphiphilic imidazolium ionic liquids in aqueous solutions
Journal of Colloid and Interface Science 356 (2011) 190–195
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Dispersion of carbon nanotubes by carbazole-tailed amphiphilic imidazolium ionic liquids in aqueous solutions Bin Dong a, Yijin Su a, Yonghui Liu a, Jie Yuan a, Jingkun Xu b, Liqiang Zheng a,⇑ a b
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology, Normal University, Nanchang 330013, China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Accepted 28 December 2010 Available online 3 January 2011 Keywords: Carbon nanotubes Dispersion Surfactant Carbazole moiety Imidazolium ionic liquids
a b s t r a c t Surfactants with a polycyclic aromatic moiety and a long hydrocarbon chain, carbazole-tailed amphiphilic imidazolium ionic liquids 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [CzCnMIm]Br (n = 10 and 12), were designed to disperse carbon nanotubes (CNTs) in aqueous solutions. UV–vis–NIR spectra were performed to determine the dispersion of CNTs and the optimal concentration (Copt) of [CzCnMIm]Br. Compared with [CnMIm]Br, [CzCnMIm]Br was more effective with the smaller Copt and more individual CNTs, reflecting the effect of the carbazole moiety. The adsorption of [CzCnMIm]Br molecules on CNTs was investigated by zeta-potential, surface tension, fluorescence, and 1H NMR. Having zeta-potentials higher than 15 mV contributed to the long-term stability of aqueous CNT dispersions. The significant fluorescence quenching and the upfield shift of carbazole protons support the p–p stacking interaction between carbazole moieties and the p-networks of CNTs. Meanwhile, the upfield shift of imidazolium protons indicates the cation–p interaction between the imidazolium ions and the p-networks of CNTs. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Single-wall carbon nanotubes (SWCNTs) [1] and multiwall carbon nanotubes (MWCNTs) [2] have attracted a great deal of attention due to their extraordinary electronic, thermal, optical, and mechanical properties [3,4]. Unfortunately, the van der Waals tube–tube attraction makes their existence in aggregated bundles and hampers their practical applications because of their extremely low solubilities. So far, two main routes, i.e., covalent [5,6] and noncovalent [7–9] functionalizations, have been suggested to disperse CNTs as individual tubes to improve their solubilities. The former approach disrupts the p-networks of CNTs, which creates possible losses in their mechanical and electrical properties. The latter approach is based on the adsorption of appropriate molecules on the CNT surface, usually surfactants [7,8], aromatic compounds [9], and polymers [10–12], which preserves their desired properties. An early report of CNT shortening and surfactant-stabilized CNT dispersions has opened the door for surfactants in the solubilization of CNTs in water [13]. Up to now, ionic (e.g., sodium dodecyl sulfate, SDS [14–16]; cetyl trimethylammonium bromide, CTAB [17,18]) and non-ionic (e.g., Triton X-100 [19,20]; Tween 20 [21]) surfactants have been employed, and various dispersion parameters have been
⇑ Corresponding author. Fax: +86 531 88564750. E-mail address: [email protected] (L. Zheng). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.12.080
attempted to select the best surfactant and best processing conditions. The main driving force for surfactant-stabilized CNT dispersions in water is the hydrophobic/hydrophilic interaction, in which the hydrocarbon chains of surfactants interact with the CNT surface while the hydrophilic heads orient toward water for dissolution [7,8]. The ionic surfactant sodium dodecylbenzene sulfonate (SDBS) consisting of a benzene ring enhances the solubilization of CNTs in water compared with commonly used surfactants (e.g., SDS) [17,19,22]. The p–p stacking interaction between the benzene ring and the CNT surface is believed to significantly increase the binding of surfactant molecules onto CNTs. Indeed the delocalized p-electrons of CNTs could be utilized to promote the adsorption of various aromatic molecules on CNTs through the p–p stacking interactions [23–25]. Thus the solubilization of CNTs in aqueous solutions using polycyclic aromatic compounds carrying pyrene [26–29] or naphthalene [27,30] has been reported. Unlike conventional surfactants, there are no long hydrocarbon chains in these polycyclic aromatic compounds. Either surfactants or polycyclic aromatic compounds can improve CNTs’ solubilities in aqueous solutions. There is no report on the use of a polycyclic aromatic compound with a long hydrocarbon chain, although some compounds with a benzene ring and a long hydrocarbon chain (e.g., SDBS and TX-100) were utilized to disperse CNTs in water. In our previous report, we designed and synthesized three long-chain carbazole-tailed imidazolium ionic liquids (ILs), 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [CzCnmIm]Br (n = 6, 10, and 12), that incorporate a carbazole
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moiety at the terminal of the hydrocarbon chain [31]. From this concept, the ability of [CzCnMIm]Br to disperse CNTs will be amazing since the hydrophobic interaction and the p–p stacking interaction are together involved. Reports describing the solubilization of CNTs in water with the aid of short-chain ILs (1-aminoethyl-3-methylimidazolium bromide, [C2NH2MIm]Br; 1-(2-aminoethyl)-pyridinium bromide, [C2NH2Py]Br [32]) and longchain ILs (1-dodecyl-3-methylimidazolium bromide, [C12MIm]Br [33]; 1-hexadecyl-3-vinylimidazolium bromide, [C16VIm]Br [34]; butyl-a,b-bis(dodecylimidazolium bromide, [C12-C4-C12Im]Br2 [35]) have been published. The ionic nature and the strong polarity of ILs are proved to be beneficial to the dispersion of CNTs. In this paper, carbazole-tailed amphiphilic imidazolium ILs [CzCnMIm]Br (n = 10 and 12) were used to disperse CNTs in water. The effect of the hydrocarbon chain as well as the carbazole moiety on the solubilization of CNTs was examined by UV–vis–NIR spectra. Zeta-potential, surface tension, fluorescence, and 1H NMR were employed to explore the interaction mechanism between [CzCnMIm]Br and CNTs. The introduction of polycyclic aromatic moieties into the surfactants brings about the p–p stacking interaction between carbazole moieties and p-networks of CNTs and thus results in more dispersed CNTs, opening new possibilities for the solubilization of CNTs in aqueous solutions.
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limit of the instrument, and thus, the excitation slit width was fixed at 15 nm and a 1% T attenuator model was used [31]. 1H NMR spectra were run at 400.13 MHz on a Bruker Avance 400 spectrometer equipped with pulse field gradient module (Z axis) using a 5-mm broadband observe (BBO) probe. The chemical shifts were referred to 4,4-dimethyl-4-silapenfane sodium sulfonate (DSS) as the external standard. 3. Results and discussion 3.1. Solubilization of CNTs by [CzCnMIm]Br Bundled CNTs are hardly active in the UV–vis–NIR region while individual CNTs are active in this region due to the van Hove singularities [16,37]. Therefore, it is pertinent to compare the amount of individually dispersed CNTs through the absorption intensity. The UV–vis–NIR spectra of aqueous CNT dispersions using different [CzC10MIm]Br concentrations were measured to verify the ability of [CzC10MIm]Br to disperse CNTs and ascertain its optimal concentration (Copt). As shown in Fig. 1A, the increase of [CzC10MIm]Br
2. Materials and methods 2.1. Materials CNTs (chemical vapor deposition method) were purchased from Shenzhen Nanotech Port Co., Ltd., and were used as received, which contain 60% SWCNTs and 35% MWCNTs with the residuals as catalyst and amorphous carbon. [CzCnMIm]Br (n = 10 and 12) and [C12MIm]Br were synthesized and purified as described elsewhere [31,36]. 2.2. Dispersion of CNTs in water To obtain surfactant-stabilized CNT dispersions in aqueous solutions, the following experimental steps were performed. First, surfactant aqueous solutions at concentrations of 0–2.0 mM for [CzC10MIm]Br, 0–0.8 mM for [CzC12MIm]Br, and 0–40.0 mM for [C12MIm]Br were prepared. Second, CNTs (2 mg) were added to the vials with surfactant aqueous solutions (6.0 mL). Finally, well-dispersed CNT suspensions were observed after ultrasonication (100 W and 40 kHz for 2 h, KQ-250DB, Analytical Instrument Inc., China) and centrifugation (4000 rpm for 10 min, LG10-2.4A, Beijing Lab Centrifuge Co., Ltd., China), and then the dark-gray supernatant was carefully decanted. Unless noted, the aqueous CNT dispersions were stored for 2 months before characterization. 2.3. Characterizations The solubilization of CNTs in aqueous solutions was evaluated by UV–vis–NIR spectroscopy (Hitachi U-4100, Japan). The corresponding surfactant aqueous solution was used as the blank. Zeta-potentials of aqueous CNT dispersions were measured on a Malvern Zeta Master apparatus (Zeta Master 3, Malvern Instrument Ltd., Malvern, UK). Surface tension measurements were carried out by a Krüss K12 tensiometer (Switzerland, accuracy ±0.01 mN/m). The fluorescence emission spectra were conducted on a PerkinElmer LS-55 spectrofluorometer (PE Company, UK). For aqueous CNT dispersions, the slit widths were fixed at 10 and 10 nm for the excitation and the emission, respectively. Under the same experimental conditions, the emission intensities of the corresponding surfactant aqueous solutions exceeded the upper
Fig. 1. (A) UV–vis–NIR spectra of aqueous CNT dispersions using different [CzC10MIm]Br concentrations. (B) UV–vis–NIR spectra of 1.0 mM [CzC10MIm]Brstabilized CNT dispersions in aqueous solutions diluted by a factor of 20. The blank was the corresponding [CzC10MIm]Br aqueous solution.
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initially resulted in an increase of the absorption intensity; however, with the sequential increase of [CzC10MIm]Br, the absorption intensity decreased. The value of Copt for [CzC10MIm]Br was determined to be 1.0 mM, which was slightly bigger than its critical micelle concentration (cmc) (0.7 mM determined by surface tension measurements [31]). And thus, it is assumed that most [CzC10MIm]Br molecules in the dispersions adsorb on the CNT surface. The addition of [CzC10MIm]Br more than Copt means wasting material and may lead to undesired results. A similar phenomenon was reported by Shin et al., who found that the values of Copt for surfactants (SDS, CTAB, and Igepal CO-990) were slightly bigger than their own cmc and CNTs aggregated and formed bundles at the surfactant concentrations bigger than Copt [18]. Fig. 1B shows the UV–vis–NIR spectra of 1.0 mM [CzC10MIm]Brstabilized CNT dispersions in aqueous solutions diluted by a factor of 20. Evidently, the spectra exhibit well-resolved peaks due to the van Hove transitions of metallic and semiconducting CNTs [33,38], which reflects a high degree of individual CNTs [17,39]. The peaks at 900–1260 nm are assigned to the first van Hove transitions of semiconducting CNTs (S11). The peaks at 550–800 nm are attributed to the second van Hove transitions of semiconducting CNTs (S22); and the peaks at 400–600 nm are ascribed to the first van Hove transitions of metallic CNTs (M11). The retention of S11, S22, and M11 confirms that the electronic properties of CNTs are still retained after the nanotube sidewall functionalization with [CzC10MIm]Br, which is a feature of the noncovalent functionalization of CNTs [7–9]. [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions were black and homogeneous without any precipitation (Fig. S1 in Supporting material). Transmission electron microscopy (TEM) was further performed to corroborate the dispersion of CNTs in aqueous solutions. Mostly individual nanotubes are visibly observed in the TEM picture, pointing to a fairly good unbundling activity of [CzC10MIm]Br on aggregated CNTs (Fig. S1 in Supporting material). As evidenced by UV–vis–NIR and TEM, CNTs can be exfoliated and dispersed by [CzC10MIm]Br molecules after the sonication step. The bundles, carbonaceous impurities, and residual catalyst can be effectively removed by the following centrifugation step, leaving mainly individual CNTs in aqueous solutions [33,40]. Following the same procedure as [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions, the value of Copt for [CzC12MIm]Br was determined to be 0.6 mM (Fig. S2 in Supporting material), which was also slightly bigger than its cmc (0.2 mM determined by surface tension measurements [31]). Compared with [CzC10MIm]Br, the value of Copt for [CzC12MIm]Br is much smaller while the amount of the individual CNTs in aqueous solutions is much higher, indicating that [CzC12MIm]Br is more effective because of its longer hydrocarbon chain. Longer hydrocarbon chains render higher spatial volume and more steric hindrance giving greater repulsive forces among individual CNTs [41]. Islam et al. reported that sodium butylbenzene sulfonate has the same ring and head group as SDBS but performed poorly in the dispersion of CNTs and they specified that the substantially longer hydrocarbon chain improves the surfactant energetics, which is probably energetically favorable for the hydrocarbon chain to adsorb on the CNT surface [19]. 3.2. Comparison of [CzCnMIm]Br and [CnMIm]Br on the dispersion of CNTs Since the aromatic molecules can disperse CNTs through p–p stacking interactions [26–30], one may predict that [CzCnMIm]Br with a carbazole moiety in its hydrophobic chain should also be effective in this application. For comparison, [C12MIm]Br without the terminal carbazole moiety was employed. The value of Copt for [C12MIm]Br was determined to be 20.0 mM (Fig. S3 in
Fig. 2. UV–vis–NIR spectra of aqueous CNT dispersions using different amphiphilic imidazolium ILs at their own optimal concentrations.
Supporting material), which was also slightly bigger than its cmc (10.9 mM determined by surface tension measurements [36]). As shown in Fig. 2, [CzC12MIm]Br is more effective in the dispersion of CNTs with the smaller Copt and more individual CNTs than [C12MIm]Br. Even [CzC10MIm]Br is more effective than [C12MIm]Br, although its hydrocarbon chain length is shorter. The value of Copt for [C12MIm]Br was bigger than those of SDS (9.0 mM [18]) and CTAB (1.5 mM [18]). However, the introduction of the carbazole moiety enhances the ability to disperse CNTs in aqueous solutions; that is, the values of Copt for [CzC10MIm]Br and [CzC12MIm]Br are much smaller than those of SDS and CTAB. These results verify that [CzCnMIm]Br with a terminal carbazole moiety is more effective than [CnMIm]Br, virtually showing the effect of the carbazole moiety. The interaction between [CnMIm]Br and CNTs may be weaker than that of [CzCnMIm]Br since the p–p stacking interaction between carbazole moieties and the p-networks of CNTs enhance the binding and surface coverage of [CzCnMIm]Br molecules to the CNT surface. The proposed mechanism of [CzCnMIm]Br to disperse CNTs in aqueous solutions will be discussed afterward. 3.3. Adsorption of [CzCnMIm]Br on CNTs The UV–vis–NIR spectra were monitored over time to assess the stability of [CzCnMIm]Br-stabilized CNT dispersions in aqueous solutions (data not shown). During the observation time period (10 months), no precipitate was produced and the absorption intensity remained almost unchanged, showing their long-term stability [19]. The zeta-potential is usually used to acquire the surface charge and as an index of the magnitude of electrostatic interactions among colloidal particles. Particles with zeta-potentials smaller than 15 mV or bigger than 15 mV are deemed to be stabilized by electrostatic repulsion interactions [42]. [CzCnMIm]Br molecules are expected to adsorb on the CNT surface and prevent the reaggregation of CNTs by the electrostatic repulsion. Actually, the zeta-potentials of [CzCnMIm]Br-stabilized CNT dispersions in aqueous solutions were bigger than 15 mV (e.g., 40.7 mV for 0.6 mM [CzC10MIm]Br-stabilized CNT dispersions, 41.1 mV for 0.5 mM [CzC12MIm]Br-stabilized CNT dispersions), contributing to their long-term stability. Further evidence for the adsorption of [CzCnMIm]Br molecules on CNTs can be provided by surface tension measurements and the dependence of cmc for [CzCnMIm]Br on the presence of CNTs can also be obtained, which is a crucial point for understanding the solubilization of CNTs. Fig. 3 shows the surface tension (c) as a function of concentration (C) for [CzCnMIm]Br in the sole [CzCnMIm]Br aqueous solutions and [CzCnMIm]Br-stabilized CNT
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Fig. 3. Surface tension as a function of concentration for [CzCnMIm]Br in the sole [CzCnMIm]Br aqueous solutions and [CzCnMIm]Br-stabilized CNT dispersions.
dispersions. In the presence of CNTs or not, the value of c progressively decreases with increasing the concentration of [CzCnMIm]Br and then reaches a plateau region. The impurity is always a concern when using surface tension measurements to examine the surfactant system [43,44]. There is no minimum around the breakpoint and so some useful information can be concluded from the c– C curves. The concentration of the breakpoint corresponds to cmc where micelles begin to form in solution and the concentration of [CzCnMIm]Br at the air/water interface remains constant. Meanwhile, the difference is visible after the addition of CNTs. First, the values of c are bigger at the same initial [CzCnMIm]Br concentrations, although this difference becomes little at high [CzCnMIm]Br concentrations. Second, the cmc is reached at higher [CzCnMIm]Br concentrations, from 0.7 and 0.2 to 0.8 and 0.4 mM for [CzC10MIm]Br and [CzC12MIm]Br, respectively. These differences confirm the adsorption of [CzCnMIm]Br molecules on CNTs, resulting in a decrease of the effective [CzCnMIm]Br concentration in bulk solutions, and thus, higher concentrations will be needed to reach the cmc [22]. 3.4. Fluorescence properties of [CzCnMIm]Br-stabilized CNT dispersions Carbazole is a typical fluorophore and so the variation in the fluorescence spectra after the addition of CNTs would supply fundamental information on the interaction between [CzCnMIm]Br and CNTs. Fig. 4 shows the emission spectra of [CzC10MIm]Br and [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions (the relevant information of [CzC12MIm]Br shown in Fig. S4 of Supporting material). In order to obtain well-defined spectra, different experimental conditions were applied as described under Section 2. It is worth noting that the emission intensity of [CzC10MIm]Br-sta-
Fig. 4. Emission spectra of [CzC10MIm]Br and [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions. The experimental conditions for aqueous CNT dispersions and the corresponding surfactant aqueous solutions were different as described under Section 2.
bilized CNT dispersions was much lower than that of the sole [CzC10MIm]Br aqueous solutions. Similar results were reported in the dispersion of CNTs using ionic pyrene and naphthalene derivatives in aqueous solutions [27,28] as well as using a phenylene vinylene polymer [45], zinc protoporphyrin IX [46], and anthracene derivatives [23,25] in organic solvents. The carbazole moiety is a rigid and planar structure with no free rotation of the ring. This natural structure is ideal for its mapping to the CNT surface and thus no red shifting of the emission peaks was observed [25]. The significant fluorescence quenching implies an efficient energy transfer from the carbazole moieties to CNTs derived from the p–p stacking interaction via vibrational coupling. Although the fluorescence behaviors in the noncovalent functionalization of CNTs with aromatic molecules remain complex [28], the variation in the fluorescence spectra supports the effect of the carbazole moiety in the interaction between [CzCnMIm]Br and CNTs. For the sole [CzC10MIm]Br aqueous solutions at concentrations of 0.6 and 2.0 mM, the peaks at the high wavelengths (ca. 415 and 440 nm) are assigned to excimers/exciplexes after the formation of micelles [47]. However, for [CzC10MIm]Br-stabilized CNT dispersions, these peaks disappear at a concentration of 0.6 mM and appear again at a concentration of 2.0 mM. These results are in accordance with those determined by surface tension measurements, which have demonstrated that micelles are formed at higher [CzCnMIm]Br concentrations owing to the adsorption of [CzCnMIm]Br molecules on CNTs. 3.5. Proposed mechanism of CNT solubilization 1
H NMR is a very sensitive technique at the molecular level; however, it is not often applied in the noncovalent functionalization of CNTs [45,48]. Fig. 5 shows the proton assignments and 1 H NMR spectra of 1.0 mM [CzC10MIm]Br and 1.0 mM [CzC10MIm]Br-stabilized CNT dispersions in D2O (the chemical structure shown in Fig. 6). After the addition of CNTs, the chemical shifts of carbazole protons (H12) move upfield and the peaks become broad, supporting the p–p stacking interaction between carbazole moieties and the p-networks of CNTs. The upfield shift of imidazolium protons (H4,5) is also remarkable since CNTs can orient the imidazolium ions on their p-networks by a possible cation–p interaction [49]. Fukushima et al. mixed imidazolium ionbased ILs with CNTs and found that the ‘‘cation–p’’ interaction plays an important role in the formation of ‘‘bucky gels’’ [50]. Three most probable configurations of how the adsorbed surfactant molecules arrange on CNTs are in dispute: CNTs encapsulated within cylindrical micelles [51], or coated with hemispherical micelles [19], or covered with randomly adsorbed molecules [15]. Consequently, Fig. 6 just depicts the schematic illustration of solubilization of CNTs by [CzC10MIm]Br. The hydrophobic interaction between the long hydrocarbon chains and the CNTs and the p–p stacking interaction between the carbazole moieties and the
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Fig. 5. Proton assignments and 1H NMR spectra of 1.0 mM [CzC10MIm]Br and 1.0 mM [CzC10MIm]Br-stabilized CNT dispersions in D2O.
surfactant with a polycyclic aromatic moiety and a long hydrocarbon chain. The encouraging results will provide insight into a new synthesis to disperse CNTs considering tunable ILs and various polycyclic aromatic moieties, which would be beneficial in applications of CNTs from the construction of novel colloid materials as sensors or as drug carriers in water. Acknowledgments This work was supported by Natural Scientific Foundation of China (50972080, 20773081), Natural Scientific Foundation of Shandong Province (Z2007B06), the key scientific project from Education Ministry of China, and National Basic Research Program (2007CB808004, 2009CB930101). Appendix A. Supplementary material
Fig. 6. Schematic illustration of solubilization of CNTs in aqueous solutions by [CzC10MIm]Br. Circles denote the hydrophilic heads. Solid lines denote the hydrocarbon chains. Rectangles denote the terminal carbazole moieties.
p-networks of CNTs, together with the cation–p interaction between the imidazolium ions and the p-networks of CNTs, are involved in the system of [CzCnMIm]Br and CNTs, resulting in the solubilization of CNTs in aqueous solutions. This study employed carbazole-tailed amphiphilic imidazolium ILs to disperse CNTs in aqueous solutions, which is the first attempt on the use of a surfactant with a polycyclic aromatic moiety and a long hydrocarbon chain, although the use of anionic and non-ionic surfactants with an aromatic moiety was reported.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2010.12.080. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusions Carbazole-tailed amphiphilic imidazolium ILs 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [CzCnMIm]Br (n = 10 and 12) behaved satisfactorily effective in the dispersion of CNTs in aqueous solutions. The optimal concentration (Copt) to disperse CNTs follows the sequence [CzC12MIm]Br < [CzC10MIm]Br < [C12MIm]Br, while the most individual CNTs were dissolved by [CzC12MIm]Br, indicating the effect of the hydrocarbon chain and the carbazole moiety. Together with fluorescence and 1H NMR spectra, the hydrophobic interaction, the p–p stacking interaction, and the cation–p interaction were demonstrated to contribute to the exfoliation and dispersion of CNTs by [CzCnMIm]Br. Although surfactants with a benzene ring and a long hydrocarbon chain (e.g., SDBS and TX-100 [19]) were employed to disperse CNTs in aqueous solutions, this study is the first attempt on the use of a
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Dispersion of carbon nanotubes by carbazole-tailed amphiphilic imidazolium ionic liquids in aqueous solutions Bin Dong a, Yijin Su a, Yonghui Liu a, Jie Yuan a, Jingkun Xu b, Liqiang Zheng a,⇑ a b
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology, Normal University, Nanchang 330013, China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Accepted 28 December 2010 Available online 3 January 2011 Keywords: Carbon nanotubes Dispersion Surfactant Carbazole moiety Imidazolium ionic liquids
a b s t r a c t Surfactants with a polycyclic aromatic moiety and a long hydrocarbon chain, carbazole-tailed amphiphilic imidazolium ionic liquids 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [CzCnMIm]Br (n = 10 and 12), were designed to disperse carbon nanotubes (CNTs) in aqueous solutions. UV–vis–NIR spectra were performed to determine the dispersion of CNTs and the optimal concentration (Copt) of [CzCnMIm]Br. Compared with [CnMIm]Br, [CzCnMIm]Br was more effective with the smaller Copt and more individual CNTs, reflecting the effect of the carbazole moiety. The adsorption of [CzCnMIm]Br molecules on CNTs was investigated by zeta-potential, surface tension, fluorescence, and 1H NMR. Having zeta-potentials higher than 15 mV contributed to the long-term stability of aqueous CNT dispersions. The significant fluorescence quenching and the upfield shift of carbazole protons support the p–p stacking interaction between carbazole moieties and the p-networks of CNTs. Meanwhile, the upfield shift of imidazolium protons indicates the cation–p interaction between the imidazolium ions and the p-networks of CNTs. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Single-wall carbon nanotubes (SWCNTs) [1] and multiwall carbon nanotubes (MWCNTs) [2] have attracted a great deal of attention due to their extraordinary electronic, thermal, optical, and mechanical properties [3,4]. Unfortunately, the van der Waals tube–tube attraction makes their existence in aggregated bundles and hampers their practical applications because of their extremely low solubilities. So far, two main routes, i.e., covalent [5,6] and noncovalent [7–9] functionalizations, have been suggested to disperse CNTs as individual tubes to improve their solubilities. The former approach disrupts the p-networks of CNTs, which creates possible losses in their mechanical and electrical properties. The latter approach is based on the adsorption of appropriate molecules on the CNT surface, usually surfactants [7,8], aromatic compounds [9], and polymers [10–12], which preserves their desired properties. An early report of CNT shortening and surfactant-stabilized CNT dispersions has opened the door for surfactants in the solubilization of CNTs in water [13]. Up to now, ionic (e.g., sodium dodecyl sulfate, SDS [14–16]; cetyl trimethylammonium bromide, CTAB [17,18]) and non-ionic (e.g., Triton X-100 [19,20]; Tween 20 [21]) surfactants have been employed, and various dispersion parameters have been
⇑ Corresponding author. Fax: +86 531 88564750. E-mail address: [email protected] (L. Zheng). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.12.080
attempted to select the best surfactant and best processing conditions. The main driving force for surfactant-stabilized CNT dispersions in water is the hydrophobic/hydrophilic interaction, in which the hydrocarbon chains of surfactants interact with the CNT surface while the hydrophilic heads orient toward water for dissolution [7,8]. The ionic surfactant sodium dodecylbenzene sulfonate (SDBS) consisting of a benzene ring enhances the solubilization of CNTs in water compared with commonly used surfactants (e.g., SDS) [17,19,22]. The p–p stacking interaction between the benzene ring and the CNT surface is believed to significantly increase the binding of surfactant molecules onto CNTs. Indeed the delocalized p-electrons of CNTs could be utilized to promote the adsorption of various aromatic molecules on CNTs through the p–p stacking interactions [23–25]. Thus the solubilization of CNTs in aqueous solutions using polycyclic aromatic compounds carrying pyrene [26–29] or naphthalene [27,30] has been reported. Unlike conventional surfactants, there are no long hydrocarbon chains in these polycyclic aromatic compounds. Either surfactants or polycyclic aromatic compounds can improve CNTs’ solubilities in aqueous solutions. There is no report on the use of a polycyclic aromatic compound with a long hydrocarbon chain, although some compounds with a benzene ring and a long hydrocarbon chain (e.g., SDBS and TX-100) were utilized to disperse CNTs in water. In our previous report, we designed and synthesized three long-chain carbazole-tailed imidazolium ionic liquids (ILs), 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [CzCnmIm]Br (n = 6, 10, and 12), that incorporate a carbazole
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moiety at the terminal of the hydrocarbon chain [31]. From this concept, the ability of [CzCnMIm]Br to disperse CNTs will be amazing since the hydrophobic interaction and the p–p stacking interaction are together involved. Reports describing the solubilization of CNTs in water with the aid of short-chain ILs (1-aminoethyl-3-methylimidazolium bromide, [C2NH2MIm]Br; 1-(2-aminoethyl)-pyridinium bromide, [C2NH2Py]Br [32]) and longchain ILs (1-dodecyl-3-methylimidazolium bromide, [C12MIm]Br [33]; 1-hexadecyl-3-vinylimidazolium bromide, [C16VIm]Br [34]; butyl-a,b-bis(dodecylimidazolium bromide, [C12-C4-C12Im]Br2 [35]) have been published. The ionic nature and the strong polarity of ILs are proved to be beneficial to the dispersion of CNTs. In this paper, carbazole-tailed amphiphilic imidazolium ILs [CzCnMIm]Br (n = 10 and 12) were used to disperse CNTs in water. The effect of the hydrocarbon chain as well as the carbazole moiety on the solubilization of CNTs was examined by UV–vis–NIR spectra. Zeta-potential, surface tension, fluorescence, and 1H NMR were employed to explore the interaction mechanism between [CzCnMIm]Br and CNTs. The introduction of polycyclic aromatic moieties into the surfactants brings about the p–p stacking interaction between carbazole moieties and p-networks of CNTs and thus results in more dispersed CNTs, opening new possibilities for the solubilization of CNTs in aqueous solutions.
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limit of the instrument, and thus, the excitation slit width was fixed at 15 nm and a 1% T attenuator model was used [31]. 1H NMR spectra were run at 400.13 MHz on a Bruker Avance 400 spectrometer equipped with pulse field gradient module (Z axis) using a 5-mm broadband observe (BBO) probe. The chemical shifts were referred to 4,4-dimethyl-4-silapenfane sodium sulfonate (DSS) as the external standard. 3. Results and discussion 3.1. Solubilization of CNTs by [CzCnMIm]Br Bundled CNTs are hardly active in the UV–vis–NIR region while individual CNTs are active in this region due to the van Hove singularities [16,37]. Therefore, it is pertinent to compare the amount of individually dispersed CNTs through the absorption intensity. The UV–vis–NIR spectra of aqueous CNT dispersions using different [CzC10MIm]Br concentrations were measured to verify the ability of [CzC10MIm]Br to disperse CNTs and ascertain its optimal concentration (Copt). As shown in Fig. 1A, the increase of [CzC10MIm]Br
2. Materials and methods 2.1. Materials CNTs (chemical vapor deposition method) were purchased from Shenzhen Nanotech Port Co., Ltd., and were used as received, which contain 60% SWCNTs and 35% MWCNTs with the residuals as catalyst and amorphous carbon. [CzCnMIm]Br (n = 10 and 12) and [C12MIm]Br were synthesized and purified as described elsewhere [31,36]. 2.2. Dispersion of CNTs in water To obtain surfactant-stabilized CNT dispersions in aqueous solutions, the following experimental steps were performed. First, surfactant aqueous solutions at concentrations of 0–2.0 mM for [CzC10MIm]Br, 0–0.8 mM for [CzC12MIm]Br, and 0–40.0 mM for [C12MIm]Br were prepared. Second, CNTs (2 mg) were added to the vials with surfactant aqueous solutions (6.0 mL). Finally, well-dispersed CNT suspensions were observed after ultrasonication (100 W and 40 kHz for 2 h, KQ-250DB, Analytical Instrument Inc., China) and centrifugation (4000 rpm for 10 min, LG10-2.4A, Beijing Lab Centrifuge Co., Ltd., China), and then the dark-gray supernatant was carefully decanted. Unless noted, the aqueous CNT dispersions were stored for 2 months before characterization. 2.3. Characterizations The solubilization of CNTs in aqueous solutions was evaluated by UV–vis–NIR spectroscopy (Hitachi U-4100, Japan). The corresponding surfactant aqueous solution was used as the blank. Zeta-potentials of aqueous CNT dispersions were measured on a Malvern Zeta Master apparatus (Zeta Master 3, Malvern Instrument Ltd., Malvern, UK). Surface tension measurements were carried out by a Krüss K12 tensiometer (Switzerland, accuracy ±0.01 mN/m). The fluorescence emission spectra were conducted on a PerkinElmer LS-55 spectrofluorometer (PE Company, UK). For aqueous CNT dispersions, the slit widths were fixed at 10 and 10 nm for the excitation and the emission, respectively. Under the same experimental conditions, the emission intensities of the corresponding surfactant aqueous solutions exceeded the upper
Fig. 1. (A) UV–vis–NIR spectra of aqueous CNT dispersions using different [CzC10MIm]Br concentrations. (B) UV–vis–NIR spectra of 1.0 mM [CzC10MIm]Brstabilized CNT dispersions in aqueous solutions diluted by a factor of 20. The blank was the corresponding [CzC10MIm]Br aqueous solution.
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initially resulted in an increase of the absorption intensity; however, with the sequential increase of [CzC10MIm]Br, the absorption intensity decreased. The value of Copt for [CzC10MIm]Br was determined to be 1.0 mM, which was slightly bigger than its critical micelle concentration (cmc) (0.7 mM determined by surface tension measurements [31]). And thus, it is assumed that most [CzC10MIm]Br molecules in the dispersions adsorb on the CNT surface. The addition of [CzC10MIm]Br more than Copt means wasting material and may lead to undesired results. A similar phenomenon was reported by Shin et al., who found that the values of Copt for surfactants (SDS, CTAB, and Igepal CO-990) were slightly bigger than their own cmc and CNTs aggregated and formed bundles at the surfactant concentrations bigger than Copt [18]. Fig. 1B shows the UV–vis–NIR spectra of 1.0 mM [CzC10MIm]Brstabilized CNT dispersions in aqueous solutions diluted by a factor of 20. Evidently, the spectra exhibit well-resolved peaks due to the van Hove transitions of metallic and semiconducting CNTs [33,38], which reflects a high degree of individual CNTs [17,39]. The peaks at 900–1260 nm are assigned to the first van Hove transitions of semiconducting CNTs (S11). The peaks at 550–800 nm are attributed to the second van Hove transitions of semiconducting CNTs (S22); and the peaks at 400–600 nm are ascribed to the first van Hove transitions of metallic CNTs (M11). The retention of S11, S22, and M11 confirms that the electronic properties of CNTs are still retained after the nanotube sidewall functionalization with [CzC10MIm]Br, which is a feature of the noncovalent functionalization of CNTs [7–9]. [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions were black and homogeneous without any precipitation (Fig. S1 in Supporting material). Transmission electron microscopy (TEM) was further performed to corroborate the dispersion of CNTs in aqueous solutions. Mostly individual nanotubes are visibly observed in the TEM picture, pointing to a fairly good unbundling activity of [CzC10MIm]Br on aggregated CNTs (Fig. S1 in Supporting material). As evidenced by UV–vis–NIR and TEM, CNTs can be exfoliated and dispersed by [CzC10MIm]Br molecules after the sonication step. The bundles, carbonaceous impurities, and residual catalyst can be effectively removed by the following centrifugation step, leaving mainly individual CNTs in aqueous solutions [33,40]. Following the same procedure as [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions, the value of Copt for [CzC12MIm]Br was determined to be 0.6 mM (Fig. S2 in Supporting material), which was also slightly bigger than its cmc (0.2 mM determined by surface tension measurements [31]). Compared with [CzC10MIm]Br, the value of Copt for [CzC12MIm]Br is much smaller while the amount of the individual CNTs in aqueous solutions is much higher, indicating that [CzC12MIm]Br is more effective because of its longer hydrocarbon chain. Longer hydrocarbon chains render higher spatial volume and more steric hindrance giving greater repulsive forces among individual CNTs [41]. Islam et al. reported that sodium butylbenzene sulfonate has the same ring and head group as SDBS but performed poorly in the dispersion of CNTs and they specified that the substantially longer hydrocarbon chain improves the surfactant energetics, which is probably energetically favorable for the hydrocarbon chain to adsorb on the CNT surface [19]. 3.2. Comparison of [CzCnMIm]Br and [CnMIm]Br on the dispersion of CNTs Since the aromatic molecules can disperse CNTs through p–p stacking interactions [26–30], one may predict that [CzCnMIm]Br with a carbazole moiety in its hydrophobic chain should also be effective in this application. For comparison, [C12MIm]Br without the terminal carbazole moiety was employed. The value of Copt for [C12MIm]Br was determined to be 20.0 mM (Fig. S3 in
Fig. 2. UV–vis–NIR spectra of aqueous CNT dispersions using different amphiphilic imidazolium ILs at their own optimal concentrations.
Supporting material), which was also slightly bigger than its cmc (10.9 mM determined by surface tension measurements [36]). As shown in Fig. 2, [CzC12MIm]Br is more effective in the dispersion of CNTs with the smaller Copt and more individual CNTs than [C12MIm]Br. Even [CzC10MIm]Br is more effective than [C12MIm]Br, although its hydrocarbon chain length is shorter. The value of Copt for [C12MIm]Br was bigger than those of SDS (9.0 mM [18]) and CTAB (1.5 mM [18]). However, the introduction of the carbazole moiety enhances the ability to disperse CNTs in aqueous solutions; that is, the values of Copt for [CzC10MIm]Br and [CzC12MIm]Br are much smaller than those of SDS and CTAB. These results verify that [CzCnMIm]Br with a terminal carbazole moiety is more effective than [CnMIm]Br, virtually showing the effect of the carbazole moiety. The interaction between [CnMIm]Br and CNTs may be weaker than that of [CzCnMIm]Br since the p–p stacking interaction between carbazole moieties and the p-networks of CNTs enhance the binding and surface coverage of [CzCnMIm]Br molecules to the CNT surface. The proposed mechanism of [CzCnMIm]Br to disperse CNTs in aqueous solutions will be discussed afterward. 3.3. Adsorption of [CzCnMIm]Br on CNTs The UV–vis–NIR spectra were monitored over time to assess the stability of [CzCnMIm]Br-stabilized CNT dispersions in aqueous solutions (data not shown). During the observation time period (10 months), no precipitate was produced and the absorption intensity remained almost unchanged, showing their long-term stability [19]. The zeta-potential is usually used to acquire the surface charge and as an index of the magnitude of electrostatic interactions among colloidal particles. Particles with zeta-potentials smaller than 15 mV or bigger than 15 mV are deemed to be stabilized by electrostatic repulsion interactions [42]. [CzCnMIm]Br molecules are expected to adsorb on the CNT surface and prevent the reaggregation of CNTs by the electrostatic repulsion. Actually, the zeta-potentials of [CzCnMIm]Br-stabilized CNT dispersions in aqueous solutions were bigger than 15 mV (e.g., 40.7 mV for 0.6 mM [CzC10MIm]Br-stabilized CNT dispersions, 41.1 mV for 0.5 mM [CzC12MIm]Br-stabilized CNT dispersions), contributing to their long-term stability. Further evidence for the adsorption of [CzCnMIm]Br molecules on CNTs can be provided by surface tension measurements and the dependence of cmc for [CzCnMIm]Br on the presence of CNTs can also be obtained, which is a crucial point for understanding the solubilization of CNTs. Fig. 3 shows the surface tension (c) as a function of concentration (C) for [CzCnMIm]Br in the sole [CzCnMIm]Br aqueous solutions and [CzCnMIm]Br-stabilized CNT
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Fig. 3. Surface tension as a function of concentration for [CzCnMIm]Br in the sole [CzCnMIm]Br aqueous solutions and [CzCnMIm]Br-stabilized CNT dispersions.
dispersions. In the presence of CNTs or not, the value of c progressively decreases with increasing the concentration of [CzCnMIm]Br and then reaches a plateau region. The impurity is always a concern when using surface tension measurements to examine the surfactant system [43,44]. There is no minimum around the breakpoint and so some useful information can be concluded from the c– C curves. The concentration of the breakpoint corresponds to cmc where micelles begin to form in solution and the concentration of [CzCnMIm]Br at the air/water interface remains constant. Meanwhile, the difference is visible after the addition of CNTs. First, the values of c are bigger at the same initial [CzCnMIm]Br concentrations, although this difference becomes little at high [CzCnMIm]Br concentrations. Second, the cmc is reached at higher [CzCnMIm]Br concentrations, from 0.7 and 0.2 to 0.8 and 0.4 mM for [CzC10MIm]Br and [CzC12MIm]Br, respectively. These differences confirm the adsorption of [CzCnMIm]Br molecules on CNTs, resulting in a decrease of the effective [CzCnMIm]Br concentration in bulk solutions, and thus, higher concentrations will be needed to reach the cmc [22]. 3.4. Fluorescence properties of [CzCnMIm]Br-stabilized CNT dispersions Carbazole is a typical fluorophore and so the variation in the fluorescence spectra after the addition of CNTs would supply fundamental information on the interaction between [CzCnMIm]Br and CNTs. Fig. 4 shows the emission spectra of [CzC10MIm]Br and [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions (the relevant information of [CzC12MIm]Br shown in Fig. S4 of Supporting material). In order to obtain well-defined spectra, different experimental conditions were applied as described under Section 2. It is worth noting that the emission intensity of [CzC10MIm]Br-sta-
Fig. 4. Emission spectra of [CzC10MIm]Br and [CzC10MIm]Br-stabilized CNT dispersions in aqueous solutions. The experimental conditions for aqueous CNT dispersions and the corresponding surfactant aqueous solutions were different as described under Section 2.
bilized CNT dispersions was much lower than that of the sole [CzC10MIm]Br aqueous solutions. Similar results were reported in the dispersion of CNTs using ionic pyrene and naphthalene derivatives in aqueous solutions [27,28] as well as using a phenylene vinylene polymer [45], zinc protoporphyrin IX [46], and anthracene derivatives [23,25] in organic solvents. The carbazole moiety is a rigid and planar structure with no free rotation of the ring. This natural structure is ideal for its mapping to the CNT surface and thus no red shifting of the emission peaks was observed [25]. The significant fluorescence quenching implies an efficient energy transfer from the carbazole moieties to CNTs derived from the p–p stacking interaction via vibrational coupling. Although the fluorescence behaviors in the noncovalent functionalization of CNTs with aromatic molecules remain complex [28], the variation in the fluorescence spectra supports the effect of the carbazole moiety in the interaction between [CzCnMIm]Br and CNTs. For the sole [CzC10MIm]Br aqueous solutions at concentrations of 0.6 and 2.0 mM, the peaks at the high wavelengths (ca. 415 and 440 nm) are assigned to excimers/exciplexes after the formation of micelles [47]. However, for [CzC10MIm]Br-stabilized CNT dispersions, these peaks disappear at a concentration of 0.6 mM and appear again at a concentration of 2.0 mM. These results are in accordance with those determined by surface tension measurements, which have demonstrated that micelles are formed at higher [CzCnMIm]Br concentrations owing to the adsorption of [CzCnMIm]Br molecules on CNTs. 3.5. Proposed mechanism of CNT solubilization 1
H NMR is a very sensitive technique at the molecular level; however, it is not often applied in the noncovalent functionalization of CNTs [45,48]. Fig. 5 shows the proton assignments and 1 H NMR spectra of 1.0 mM [CzC10MIm]Br and 1.0 mM [CzC10MIm]Br-stabilized CNT dispersions in D2O (the chemical structure shown in Fig. 6). After the addition of CNTs, the chemical shifts of carbazole protons (H12) move upfield and the peaks become broad, supporting the p–p stacking interaction between carbazole moieties and the p-networks of CNTs. The upfield shift of imidazolium protons (H4,5) is also remarkable since CNTs can orient the imidazolium ions on their p-networks by a possible cation–p interaction [49]. Fukushima et al. mixed imidazolium ionbased ILs with CNTs and found that the ‘‘cation–p’’ interaction plays an important role in the formation of ‘‘bucky gels’’ [50]. Three most probable configurations of how the adsorbed surfactant molecules arrange on CNTs are in dispute: CNTs encapsulated within cylindrical micelles [51], or coated with hemispherical micelles [19], or covered with randomly adsorbed molecules [15]. Consequently, Fig. 6 just depicts the schematic illustration of solubilization of CNTs by [CzC10MIm]Br. The hydrophobic interaction between the long hydrocarbon chains and the CNTs and the p–p stacking interaction between the carbazole moieties and the
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Fig. 5. Proton assignments and 1H NMR spectra of 1.0 mM [CzC10MIm]Br and 1.0 mM [CzC10MIm]Br-stabilized CNT dispersions in D2O.
surfactant with a polycyclic aromatic moiety and a long hydrocarbon chain. The encouraging results will provide insight into a new synthesis to disperse CNTs considering tunable ILs and various polycyclic aromatic moieties, which would be beneficial in applications of CNTs from the construction of novel colloid materials as sensors or as drug carriers in water. Acknowledgments This work was supported by Natural Scientific Foundation of China (50972080, 20773081), Natural Scientific Foundation of Shandong Province (Z2007B06), the key scientific project from Education Ministry of China, and National Basic Research Program (2007CB808004, 2009CB930101). Appendix A. Supplementary material
Fig. 6. Schematic illustration of solubilization of CNTs in aqueous solutions by [CzC10MIm]Br. Circles denote the hydrophilic heads. Solid lines denote the hydrocarbon chains. Rectangles denote the terminal carbazole moieties.
p-networks of CNTs, together with the cation–p interaction between the imidazolium ions and the p-networks of CNTs, are involved in the system of [CzCnMIm]Br and CNTs, resulting in the solubilization of CNTs in aqueous solutions. This study employed carbazole-tailed amphiphilic imidazolium ILs to disperse CNTs in aqueous solutions, which is the first attempt on the use of a surfactant with a polycyclic aromatic moiety and a long hydrocarbon chain, although the use of anionic and non-ionic surfactants with an aromatic moiety was reported.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2010.12.080. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusions Carbazole-tailed amphiphilic imidazolium ILs 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [CzCnMIm]Br (n = 10 and 12) behaved satisfactorily effective in the dispersion of CNTs in aqueous solutions. The optimal concentration (Copt) to disperse CNTs follows the sequence [CzC12MIm]Br < [CzC10MIm]Br < [C12MIm]Br, while the most individual CNTs were dissolved by [CzC12MIm]Br, indicating the effect of the hydrocarbon chain and the carbazole moiety. Together with fluorescence and 1H NMR spectra, the hydrophobic interaction, the p–p stacking interaction, and the cation–p interaction were demonstrated to contribute to the exfoliation and dispersion of CNTs by [CzCnMIm]Br. Although surfactants with a benzene ring and a long hydrocarbon chain (e.g., SDBS and TX-100 [19]) were employed to disperse CNTs in aqueous solutions, this study is the first attempt on the use of a
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