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Kinetically Controlled Side-Wall Functionalization of Carbon Nanotubes by Nitric Acid Oxidation Hao Yu,* Yuguang Jin, Feng Peng,* Hongjuan Wang, and Jian Yang School of Chemical and Energy Engineering, South China UniVersity of Technology, Guangzhou 510640, China ReceiVed: December 21, 2007; In Final Form: February 28, 2008
The nitric acid oxidation followed by sodium hydroxide washing was applied for functionalization of singlewalled carbon nanotubes (SWCNTs), few-walled CNTs (FWCNTs), and multiwalled CNTs (MWCNTs). The surface functionalization of CNTs via acidic oxidation was studied by transmission electron microscopy, Raman scattering spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The oxidation of CNTs involves the exfoliation of functionalized carbon layers into smaller carbonaceous fragments (CF), which might influence the performance of composite materials based on modified CNTs. Optimizing oxidation time increased the selectivity to thermodynamically unstable functionalized CNTs. Thicker CNTs, including FWCNTs and MWCNTs, are better for high-density surface functionalization, due to higher structure stability. It was proposed that a rational selection of the oxidation process and type of CNTs is important for the applications involving CNT-related composite materials.
1. Introduction In many applications based on carbon nanotubes (CNTs), CNTs must be modified to introduce chemical groups on their surfaces.1-8 Usually, CNTs are oxidized by acids for carboxyl groups, and then further grafted with polymeric, bio, and other functional molecules.9-13 This approach has been widely used to increase the affinity between CNTs and matrixes for better dispersion.14,15 In such CNT-reinforced composites, stress/charge is transported from matrixes to CNT fillers via surface covalent bonds.13,14,16-20 Hence, the performance of composites will strongly depend on the surface groups of CNTs. Effective surface functionalization will enhance the mechanical/conductive properties of composites.4,13,14,20,21 Many reported protocols for CNT surface modification start with wet oxidation of CNTs.11,12 By refluxing raw CNTs in HNO3,22,23 HNO3/H2SO4,24 or other oxidative agents,25 amorphous carbon and residual catalysts can be removed. Carboxyl, carbonyl, hydroxyl, and other oxygen-containing groups can be formed on the side walls and ends. However, a recent report by Salzmann et al.26 shows that the process of acid oxidation might be more complex than we expected. The delicate experiment demonstrated that the carboxyl groups produced by nitric acid treatment mainly are on carboxylated carbonaceous fragments (CFs) absorbed on single-walled CNTs (SWCNTs), but not on the side walls of SWCNTs. Obviously, the different hosts of carboxyl groups will influence the applications in which the interaction between carbon species and matrixes is essential. The performances of composites will be difficult to predict, when stress/charge transports not only through groups on CFs but also the interface between CNTs and CFs. This probably causes the scattering of performance of reported CNT-based composites or devices. Thus, it is highly desirable to address the following issues: (i) how CFs are formed during the acid oxidation process of CNTs; (ii) why chemical groups are * Corresponding author. E-mail:
[email protected] (H.Y.); cefpeng@ scut.edu.cn (F.P.).
difficult to form on the side walls of SWCNTs; and (iii) how to produce side-wall functionalized CNTs for further applications. In this paper, we report the kinetic course of oxidizing CNTs in hot nitric acid. The formation of CFs is explained based on a kinetic account. We also show the possibility to facilitate the formation of side-wall functionalized CNTs via the selection of the proper type of CNTs. 2. Experimental Section Purified SWCNTs were purchased from Shenzhen Nanotech Port Co. Ltd. The SWCNTs are less than 2 nm in diameter and about 5-15 µm in length. SWCNTs were annealed at 1100 °C under argon for 5 h before experiment to remove the possible surface groups produced by purification. Few-walled CNTs (FWCNTs) and multiwalled CNTs (MWCNTs) were provided by Tsinghua University as received. FWCNTs were synthesized by CH4 decomposition, using CoMo/MgO as the catalyst, and washed with dilute HNO3 to remove residual catalyst. Highresolution transmission electron microscopic (HRTEM) measurement shows that the FWCNTs consist of 2-6 layers of graphene. The MWCNTs were synthesized by propylene decomposition, using FeMo/Al2O3 as the catalyst, and then purified to achieve 99.9 wt % MWCNTs by high-temperature annealing at 2000 °C in vacuum to remove catalyst particles.27 HRTEM images of SWCNTs, FWCNTs, and MWCNTs used in this work are shown in the Supporting Information. Salzmann’s protocol26 was applied to modify the CNT samples. Typically, pristine CNTs (denoted as p-CNTs) were ultrasonicated for 30 min, and then refluxed in 9 M HNO3 in a round-bottom glass flask at 100 °C for 1-24 h. The resulting oxidized products (denoted as o-CNTs) were filtered and washed to pH 7 by distilled water, then dried at 110 °C overnight. o-CNTs were stirred in 8 M NaOH solution in a PTEF container under nitrogen at 100 °C for 48 h. The products (denoted as w-CNTs) were filtered and washed by distilled water thoroughly, then dried at 110 °C overnight. The dark brown filtrate contains CFs. After neutralizing the filtrate with HCl, the filtrate was
10.1021/jp711975a CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008
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Figure 1. The process of CNT oxidation. The left and right panels represent the functionalization processes of SWCNTs, and FWCNTs or MWCNTs. The oxidation and NaOH washing phases of Salzmann’s protocol26 are delivered from top to bottom in an increasing time series. The defects from the sp3 carbon atoms in the graphene lattice are first attacked for oxidation. Consequential oxidation of these sites etches CNTs, and then CFs exfoliates. Thin SWCNTs are easy to cut, thus functionalities on side walls will be washed away by NaOH. The higher stability of FWCNTs and MWCNTs allows for the functionalities leaving defects or holes on side walls after NaOH washing.
concentrated for further characterizations of CFs by evaporating water in a rotary evaporator and then separating NaCl crystals. The w-CNTs have been reacted with 8 M NaOH again for 48 h, and only clean filtrate can be obtained, indicating that the NaOH washing step separated CFs thoroughly. The samples were characterized by N2 adsorption, transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermalgravimetric analysis (TGA). The surface areas of samples were obtained by N2 adsorption at 77 K in a Micromeritics ASAP 2010M apparatus. TEM images of CFs were taken in a JEM 2010 transmission electron microscope operated at 200 kV. The Raman spectra were recorded with a Renishaw RM2000 spectrometer excitated at 785 or 514 nm. XPS spectra were recorded with a Quantum-2000 Scanning ESCA Microprobe with Al KR radiation, calibrated internally by carbon deposit C1s. TGA data were recorded with Netzsch STA449C under nitrogen at a ramping rate of 10 deg/min. 3. Results and Discussion The major idea of our experimental was based on an analysis of oxidation reaction of carbon materials, as illustrated in Figure 1. As stated by Salzmann et al.,26 the acidic oxidation of CNTs is a quite complex sequential process in which CNTs are gradually oxidized into many oxygen-containing intermediates.
Although these oxygen-containing groups are usually represented by carboxyl groups, carbonyl, hydroxyl,and other oxygencontaining groups also exist, as shown in Figure 8. Overoxidation might cause CNT collapse and produce carbon species in the form of CFs. This process also produces so-called cutting SWCNTs.22,28 With a strong enough oxidant, carbon will be fully gasified to form CO2. This process can be described with the following equation: k1
CNTs 98 CNTssCOOH + CNTsdO + k2
CNTssOH ... 98 CFssCOOH + CFsdO + k3
CFssOH ... 98 CO2v Since the CNTssCOOH, CNTsdO, and CNTssOH are thermodynamically unstable, a proper kinetic control favors to achieve high yield of these intermediates, especially when k2/ k1 > 1. We conducted Salzmann’s protocol26 that includes nitric acid oxidization and NaOH washing steps to control surface functionalities of CNTs by altering different oxidation times. When this consequential process is terminated before the formation of CFs, the high yield of functionalized CNTs can be achieved. An alternative method to produce the high-yield functionalized CNTs is to increase the stability of functionalized intermediates and/or lower the oxidation rate from functionalized
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Yu et al. However, it is insensitive to CNTsdO and CNTssOH. In this work, we used TGA to measure the number of surface groups and Raman spectroscopy to characterize the CNT samples. A parameter f was defined to describe the selectivity of functionalized CNTs during the oxidation reaction:
f)
Figure 2. Raman spectra of p-SWCNTs, o-SWCNTs, and w-SWCNTs, excited at 785 nm and averaged by 3-5 randomly selected 2 µm spots. The intensities have been normalized to the G+ peaks.
CNTs to CFs or gaseous products, thereby the yield of desired functionalized CNTs can be promoted. We modified few-walled CNTs (FWCNTs) and multiwalled CNTs (MWCNTs), and expected that their large sizes and resulting stabilities could prevent them from collapsing in a limited time, thus chemical groups can remain on CNTs after NaOH washing and form sidewall functionalized CNTs. Figure 2 shows the Raman spectra of pristine, oxidized by HNO3 for 24 h, and NaOH washed SWCNTs (denoted as p-SWCNTs, o-SWCNTs, and w-SWCNTs, respectively). HNO3 oxidation resulted in a significant increase of D band at ∼1300 cm-1, caused by vacancies, substitutional heteroatoms, finitesize effects, and bending, indicating serious damage on the walls of SWCNTs.25,29,30 After refluxing in NaOH solution, the D band of w-SWCNTs returned to that of p-SWCNTs approximately, as predicted by Salzmann’s protocol. TEM observation showed that the separated carbonaceous substance is in the form of amorphous CFs, as shown in Figure 3. These results continue to support the point of view that chemical groups induced by oxidation are mainly on CFs, which can be separated by NaOH washing, despite the source of SWCNTs. The defects of starting SWCNT sample will influence the amount of CFs as products of over-oxidation significantly. If the SWCNTs went through the Salzmann’s protocol without annealing, the yield of CFs would increase by 2-fold compared with the counterpart annealed SWCNTs. This demonstrates that the oxidation takes place at defects first. Salzmann et al. suggested that one of the sources of CFs is the amorphous carbon in raw SWCNT samples. However, it was assessed that the yield of CFs separated by NaOH washing was about 5% of p-SWCNTs. Using w-SWCNTs through one run of Salzmann’s protocol as starting materials for the oxidation-NaOH washing process, about 5% yield of CFs was measured again. These results indicate that carbon impurities could not be the exclusive source of CFs. There must have been considerable CFs converted from SWCNTs, implying a dynamic course of CFs formation predicted in Figure 1. A further quantitative analysis needs a parameter to represent the efficiency of this method on the surface functionalization of side walls of CNTs. IR spectroscopy has been widely used for functionalized CNTs.30,31 However, it is neither a quantitative nor a robust technique for characterization of surface groups due to intrinsic IR-active modes of CNTs.26,30,32 XPS is surfacesensitive and can identify the carbon atoms at different states.33 But the quantification of these species depends on the thickness of CNTs when over two layers exist. The NH3-chemsorption technique can be used to titrate surface acidic groups.34
Rw-CNTs - Rp-CNTs Ro-CNTs - Rp-CNTs
where R ) ID/IG. This definition is based on the consensus that R can be used to probe defects in graphene. The defects come from either the functionalization process or the inherent defects of pristine CNT samples, reflected by Rp-CNTs. In o-CNTs, the defects are from all the possible carbon species, including CNTs and CFs. Because the functionalized CFs are removed in the NaOH washing step, the graphene defects of w-CNTs are contributed from the sp3 sites where CNT walls are functionalized and the inherent ones. Thus, f means the quotient of the defective degree caused by chemically functionalized groups on CNT walls and that caused by the groups on either CNT walls or CFs. Since R always increases with defectiveness, the defects from functionalized sites in a certain sample (with a fixed level of inherent defects) can be measured by R. On the basis of the above considerations, f will be a measure of the fraction of functionalized sites on CNT side-walls in all the sites contributed from CNT walls and CFs produced by the oxidation process. Although f is not strictly equal to such a ratio, one can evaluate the selectivity of forming side-wall functionalized CNTs of the nitric acid oxidation process by the factor f. The acid oxidation always increases R. Some authors have reported that hydroxides (e.g., KOH and NaOH) can react with CNTs to form multiple hydroxyl group modified SWCNTs35 or to increase porosity.36 These reactions take place under solid-phase mechanochemical conditions35,36 or at high temperature.36 There is no evidence that washing with 8 M NaOH will create new defects on CNTs. Thus, f should be a number ranging from 0 to 1. When f ) 0, all the functionalities are on CFs and the side walls of CNTs cannot be modified. The bigger f is, the more groups are grafted on the side-walls of CNTs. Figure 4 shows f of SWCNTs as a function of oxidation time. The variation of f has a typical character of consequential reaction. A maximum of f was achieved at 4 h of oxidation. After 4 h, f decreased sharply, indicating that SWCNTs were over-oxidized into functionalized CFs after long enough exposure. This tendency was validated by TGA, as shown in Figure 4. The largest weight loss at 900 °C of w-SWCNTs was detected in the sample for 4 h, validating that a consequential oxidation did present and the f factor can be used to evaluate this process. More importantly, we demonstrated that the oxidation of SWCNTs can be terminated by a proper kinetic control before the formation of CFs. It strongly suggests that a rational kinetic control is necessary for functionalization of CNTs, which is absent in many empirical purification and modification procedures for CNTs. To verify our viewpoint further, we tried to use thick-wall CNTs to increase the structural stability of intermediates of the oxidation reaction and accordingly decrease the kinetics constant forming CFs. Figure 5a,b shows the TEM images of separated carbonaceous species obtained by washing o-FWCNTs and o-MWCNTs with NaOH. Amorphous CFs similar to that of SWCNTs in Figure 3 were observed for FWCNTs and MWCNTs, indicating that the nitric acid oxidation of different types of CNTs obeys a similar consequential mechanism suggested by Figure 1. As shown in Figure 6, the oxidization
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Figure 3. TEM images of CFs separated from o-SWCNTs by 8 M NaOH reflux for 48 h. The inset in top left of panel a shows the optical photograph of the concentrated filtrate containing CFs. Panels a and b represent the images of CFs in the upper solution and the black precipitate, respectively.
Figure 4. f factor of SWCNTs and TGA weight loss at 900 °C of w-SWCNTs as functions of HNO3 refluxing time.
increased the D band of FWCNTs significantly after 24 h of nitric acid oxidation, leading to R ) 1.9. However, a thorough NaOH washing did not take the D band back to the value of p-FWCNTs. Since refluxing in NaOH solution only removes CFs absorbed on the side-walls of FWCNTs, this strongly indicates that more chemical groups can be bonded on FWCNTs than SWCNTs. A similar situation was observed as the thickness of CNT samples was further increased. As shown in Figure 7,
the D bands of o-MWCNTs and w-MWCNTs were both obviously higher than that of p-MWCNTs. Although the MWCNT sample has been annealed at 2000 °C and was highly perfect in crystallite,27 nitric acid oxidation did damage the walls of MWCNTs. Meanwhile, the high stability of the MWCNTs also decreased the yield of CFs. As shown in the inset of Figure 5b, only a yellowish filtrate can be obtained after the same NaOH washing and concentration operations for MWCNTs, implying a high selectivity to form functionalized MWCNTs. f parameters and TGA weight losses of SWCNTs, FWCNTs, and MWCNTs after a 24 h of the oxidation-neutralization process are compared in Table 1. Because functionalities are bonded on the outer shell of CNTs, the number of groups on different CNT samples should be normalized to the specific surface area for a fair comparison. The order of f and weight loss based on surface area is SWCNTs < FWCNTs < MWCNTs, agreeing well with the prediction of the dependence of surface functionalization on structural stability. From SWCNTs to FWCNTs, f parameters increased by 1 order of magnitude from 0.017 to 0.64, and TGA weight loss increased from 0.34 to 0.99 mg/m2. (It should be noted that the TGA weight losses could be contributed by functionalities on both side-walls and the end. This might over-estimate the degree of side-wall functionalization of SWCNTs.) Thus, it is effective to increase the fraction of surface functionalized CNTs by using thicker and more stable CNTs. Moreover, it is also demonstrated that the f factor is applicable for evaluating the degree of side-wall functionalization by nitric acid oxidation across different types of CNTs.
Figure 5. TEM images of CFs separated from o-FWCNTs (a) and o-MWCNTs (b) by 8 M NaOH reflux for 48 h. The insets in top left of panels a and b show the optical photograph of the concentrated filtrate containing CFs. The samples were taken from the upper solutions.
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Figure 6. Raman spectra of p-FWCNTs, o-FWCNTs, and w-FWCNTs, excited at 785 nm and averaged by 3-5 randomly selected 2 µm spots.
Figure 7. Raman spectra of p-MWCNTs, o-MWCNTs, and wMWCNTs, excited at 514 nm and averaged by 3-5 randomly selected 2 µm spots.
TABLE 1: Comparison among Surface Densities of Functionalities on w-SWCNTs, w-FWCNTs, and w-MWCNTs. samples f specific surface area (m2/g) TGA wt loss (mg/m2)
w-SWCNTs w-FWCNTs w-MWCNTs 0.017 457.9 0.34
0.64 196.9 0.62
0.79 179.2 0.99
XPS was employed to examine the nature of surface species of w-MWCNTs. The asymmetric C1s spectrum of w-MWCNTs is shown in Figure 8. Since the C1s spectrum of graphite is an asymmetric peak with a long tail extended to the higher binding energy region,37 the recorded C1s spectrum was deconvoluted with an asymmetric Gaussian-Lorentzian sum function centered at 284.4 eV and several symmetric Gaussian peaks, representing the contributions from graphite-like walls and functionalized carbon atoms, respectively. The peak at 285.2 eV is contributed from sp3-hybridized carbon atoms.38 Its intensity correlates with the degree of oxidation on side walls.33 The deconvoluted C1s spectrum of w-MWCNTs clearly demonstrated that the surface of MWCNTs was functionalized by various oxygenous groups. The peaks at 286.2, 287.4, and 290.1 eV can be assigned as -CsO- (alcohol, ether), -CdO (ketone, aldehyde), -COO(carboxylic acid, ester) species, respectively.33 A semiquantitative evaluation of the ratio of peak areas shows that about 22% of the carbon atoms detected were bound to oxygen atoms. These results support that CNTs can be oxidized by nitric acid and oxygenous groups can be decorated on the side-walls even after NaOH washing, as suggested in Figure 1.
Figure 8. Deconvoluted C1s XPS spectra of w-MWCNTs. The assignments of deconvoluted peaks are marked with arrows.
Herein we demonstrate that the side-wall functionalization of CNTs by nitric acid is kinetically controlled either by the oxidation duration or by the stability of starting CNT materials. This insight to the nitric acid oxidation process offers a base of rational design of modified CNT-based functional materials. For SWCNTs, the major task of functionalization is to control the depth of the oxidation reaction to avoid the damage of their fragile walls. Our results show that a proper oxidation time is significant to obtain side-wall functionalized SWCNTs without the formation of over-oxidized CF products. Generally, the methods by which the strength of oxidation reaction can be facilely controlled might be of potential use for achieving reasonable side-wall functionalities, such as by accurately controlled time, at lower temperature, as well as with mild oxidants. Although we have not investigated all of the conditions in the present work, this deduction is supported by a recent work by Tchoul et al.39 They showed that diluted HNO3 at reflux and concentrated HNO3 with sonication at room temperature both performed well to oxidize SWCNTs and to form soluble SWCNT samples. To establish a robust side-wall functionalization protocol, thicker CNTs are attractive, because they provide more effective side-wall functionalization without significant damage to the walls. Taking into account the more side-wall functionalities on FWCNTs and their mechanical and electrical properties parallel to SWCNTs,40,41 we suggest that FWCNTs could be an important material for composite applications, for which the high density of side-wall functionalities and the intrinsic strength/conductance are both important for the performance of the final material. 4. Conclusion It was demonstrated that the over-oxidation of functionalized CNTs leads to exfoliated CFs. The majority of surface groups in oxidation products of SWCNTs are on CFs that can be removed by NaOH washing. By using the f factor defined with Raman spectra of p-CNTs, o-CNTs, and w-CNTs, the selectivity of forming side-wall functionalities in this consequential oxidation reaction was evaluated. Since thin SWCNTs are more unstable structurally, the oxidation tends to form CFs, therefore the yield of functionalized SWCNTs is low. To increase the selectivity of CNT oxidation to these thermodynamically unfavored intermediates, the oxidation time must be controlled based on a kinetics consideration. It is also effective to increase the functionality on CNTs by using thicker CNTs, including FWCNTs and MWCNTs, due to their higher structural stability.
Side-Wall Functionalization of Carbon Nanotubes It is suggested that a rational selection of the type of CNTs should be a necessary dimension of design of CNT composites or devices. As a prediction, to obtain high-performance functional composites based on modified CNTs, FWCNTs might be a better compromise between surface functionality and weight, despite the better intrinsic mechanical/conductive properties of SWCNTs. Acknowledgment. We thank Ms. Lin Hu and Dr. Qiang Zhang in the Department of Chemical Engineering, Tsinghua University, for help with the TEM measurements. This work was supported by the Guangdong Provincial Science and Technology Project (No. 2006A10903002) and the Guangzhou Civic Science and Technology Project (No. 2007Z3-D2101). Supporting Information Available: HRTEM images of SWCNTs, FWCNTs ,and MWCNTs used in this work (Figures S1-S3) and Raman spectra of SWCNTs oxidized by nitric acid for different times and their corresponding samples washed by NaOH for 48 h (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Science 2001, 293, 1299. (2) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002. (3) Bahr, J. L.; Yang, J. P.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (4) Blake, R.; Gun’ko, Y. K.; Coleman, J.; Cadek, M.; Fonseca, A.; Nagy, J. B.; Blau, W. J. J. Am. Chem. Soc. 2004, 126, 10226. (5) Chen, R. J.; Zhang, Y. G.; Wang, D. W.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 3838. (6) Dalton, A. B.; Ortiz-Acevedo, A.; Zorbas, V.; Brunner, E.; Sampson, W. M.; Collins, L.; Razal, J. M.; Yoshida, M. M.; Baughman, R. H.; Draper, R. K.; Musselman, I. H.; Jose-Yacaman, M.; Dieckmann, G. R. AdV. Funct. Mater. 2004, 14, 1147. (7) Bellayer, S.; Gilman, J. W.; Eidelman, N.; Bourbigot, S.; Flambard, X.; Fox, D. M.; De Long, H. C.; Trulove, P. C. AdV. Funct. Mater. 2005, 15, 910. (8) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Small 2006, 2, 182. (9) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Langmuir 2001, 17, 5125. (10) Hill, D. E.; Lin, Y.; Rao, A. M.; Allard, L. F.; Sun, Y. P. Macromolecules 2002, 35, 9466. (11) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17. (12) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105.
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