J. Phys. Chem. B 2004, 108, 8737-8741
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Noncovalent Functionalization of Carbon Nanotubes with Molecular Anchors Using Supercritical Fluids† Leonard S. Fifield,‡ Larry R. Dalton,§ R. Shane Addleman,‡ Rosemary A. Galhotra,‡ Mark H. Engelhard,‡ Glen E. Fryxell,‡ and Christopher L. Aardahl*,‡ Pacific Northwest National Laboratory, Richland, Washington 99352, and Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 ReceiVed: December 22, 2003; In Final Form: April 9, 2004
In this article, we describe a facile and effective method for the modification of multiwall carbon nanotubes with molecular anchor molecules using supercritical fluids (SCFs). Through choice of deposition conditions, the degree of loading in these nanotube-anchor structures can be controlled to achieve sub-monolayer, monolayer, or greater-than-monolayer coverage. This level of control represents a potential advantage of SCFs over liquid solvents for anchor deposition. Employment of the described technique is expected to enable the direct addition of desired chemical functionality to many carbon nanotube structures for a variety of applications.
1. Introduction The controlled chemical modification of carbon nanotubes is important for the incorporation of this class of materials into devices that can take advantage of their interesting properties.1 Much progress has been made since the discovery of carbon nanotubes2-4 in the development of methods to covalently bond desired chemical groups to the ends and surfaces of the tubes. 5,6 Advances have also been made to enable confirmation and quantification of the changes made to nanotube structures and properties through chemical reaction.7 A drawback of covalent chemical modification of the conjugated backbone of nanotubes is a corresponding loss of the strength8 and conductivity characteristic of the pristine nanotube structure. One way to avoid loss of intrinsic carbon nanotube properties upon chemical modification is to functionalize nanotubes by noncovalent means. Some success has been achieved on this front through the use of polymeric surfactants9 and polymer wraps.10-12 Alternatively, nondestructive modification of the surfaces of carbon nanotubes can be accomplished through the use of molecular anchors.13-21 First used on graphite,22 “bifunctional” anchor molecules preferentially adhere to the aromatic rings of a nanotube surface through π stacking or hydrophobic interactions, do not physically disrupt the conjugation of the nanotube backbone, and can provide programmed chemical functionality. This π-stacked molecular anchor approach is general to any graphitic surface and thus facilitates the modification of easier to produce, but more difficult to chemically modify,23 multiwall carbon nanotubes. In one of the first demonstrations of carbon nanotube modification by molecular anchors, Chen and co-workers used a bifunctional molecule containing succinylimide ester and a pyrene moiety to bind proteins to the nanotube surface.14 Subsequent uses of a pyrene anchor have facilitated the †
Part of the special issue “Alvin L. Kwiram Festschrift”. * To whom correspondence may be addressed.
[email protected]. ‡ Pacific Northwest National Laboratory. § University of Washington.
E-mail:
adherence of gold nanoparticles to nanotube surfaces,16 polymerization of monomers on a nanotube surface,15 the dissolution of nanotubes in water,17 and attachment of nanotubes to DNA molecules for DNA-templated nanotube localization.18 Each of these applications has involved stirring and/or sonicating nanotube samples in organic solutions of anchor molecules followed by rinsing to remove any unbound anchors. Efforts to confirm the success of these solution methods in modifying the surfaces of carbon nanotubes have included the evaluation of change in thickness between modified and unmodified nanotubes (or nanotube bundles) and the detection of the presence or absence of metal particles associated with attached anchors.14,15,18 Raman spectroscopy has also been used to observe charge transfer between nanotubes and metal particles attached to the tubes with bifunctional linkers.16 Here we attach molecular anchors to multiwall carbon nanotubes using supercritical fluid (SCF) deposition and attempt to quantify the level of surface coverage with the use of thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and electron spectroscopy for chemical analysis (ESCA). SCFs have previously been used for efficient deposition of selfassembled monolayers onto many high surface area materials because of their gaslike diffusivity and liquidlike density.24,25 Despite their liquidlike solvation power, SCFs have poor solvent shielding. This serves to promote self-assembly of solvated molecules onto favorable substrates. The solubility of unsubstituted pyrene in supercritical carbon dioxide is well known26-31 and increases with the use of modifiers such as methanol and cosolvents such as propane.32 The solubility of multiwall nanotubes in these SCFs is negligible. We have demonstrated the attachment of a number of commercially available pyrenebased anchors including 1-pyrenecarboxaldehyde, 1-bromoacetyl pyrene, and 1-pyrenemethanol to multiwall carbon nanotubes in both supercritical carbon dioxide and in supercritical propane. The method described is generally applicable to a variety of nanotube structures and capable of providing a dynamic range of surface coverage. It is also preferable to the reported methods
10.1021/jp037977l CCC: $27.50 © 2004 American Chemical Society Published on Web 05/04/2004
8738 J. Phys. Chem. B, Vol. 108, No. 25, 2004
Figure 1. Plot of change in mass with temperature for BAP/nanotube samples in an inert atmosphere. A, B, and C are TGA mass loss curves for 5, 15, and 45 wt % anchor fractions in initial reaction mixtures, respectively.
for this purpose in that organic solvents such as N,N-dimethylformamide and toluene are eliminated from the deposition process. 2. Experimental Section 2.1. SCF Deposition Process. In a typical experiment, 10 mg of 1-bromoacetyl pyrene (BAP) purchased from SigmaAldrich (St. Louis, MO) is stirred with 90 mg of powder-form multiwall carbon nanotubes (20-40 nm diameter) purchased from Nanostructured and Amorphous Materials (Los Alamos, New Mexico). This dry mixture is then added to a 15-mL vessel and placed in a 25-mL high-pressure chamber (both stainless steel) held at 150 °C. The chamber is flushed with carbon dioxide and pressurized with the gas to a level of 7500 psi. Under these pressure and temperature conditions, the supercritical CO2 has a density of 0.7 g/mL33 and an estimated unsubstituted pyrene solubility of 2.5 wt % (∼45 mg).29 The mixture of anchor molecules and nanotubes is allowed to interact at these conditions for 10-15 min. After reaction, the supercritical CO2 is exhausted from the chamber, carrying with it pyrene molecules that have not adhered to the nanotubes. Loading of the nanotubes can be controlled by adjusting the ratio of molecular anchor to nanotubes used in the reaction. 2.2. TGA and DSC. Anchor content of carbon nanotube samples modified by SCF deposition was determined by heating the samples to 500 °C at 10 degrees per minute in a N2 atmosphere. Mass loss and heat flow into the samples were monitored simultaneously using a Netzsch STA 409 TGA/DSC instrument. 2.3. ESCA. Bromine atomic % values for the carbon nanotube samples modified with Br-containing anchor molecules were determined using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe with a focused monochromatic Al KR 1486.7-eV X-ray source and a spherical section analyzer. 3. Results and Discussion Organic anchor molecules in the modified samples are assumed to be responsible for observed mass loss since the nanotubes were confirmed to be thermally stable in inert atmosphere over the temperatures scanned. To compensate for the fact that the mass of anchor-containing samples did not reach a steady state by the end of the instrument-limited temperature ramp, anchor content was calibrated by the mass loss of pure anchors tested in the same way. Heating-induced mass loss reveals the relationship between anchor fraction in reaction mixture and anchor content in modified nanotube products (Figure 1).
Fifield et al.
Figure 2. ESCA results indicate a strong correspondence between BAP anchor fraction in initial reaction mixture and Br content in the modified nanotube samples.
ESCA data for a series of BAP anchor/nanotube loadings confirm a strong correlation between anchor fraction in initial reaction mixture and bromine content in modified nanotube sample (Figure 2). The Br signal is seen to steadily increase at low BAP loadings consistent with monolayer formation. Considering the mean-free path of liberated electrons in such a material, ESCA only interrogates the first 5-10 nm of the carbon nanotube sample surface.34 This likely causes the ESCAdetermined Br atomic percent to be overestimated at low anchor loadings, since much of the carbon in the 5-35 nm thick nanotube walls is not included and approaches that of the pure anchor for higher loadings when only the anchors are observed. The multiwall carbon nanotube samples used here consist of randomly oriented, intercontacting carbon cylinders rather than an ideal flat carbon surface. For this reason, the ESCA Br d-line values are difficult to use for quantitative measurement of the total Br atomic % present in the modified nanotube samples. Future developments in the interpretation of ESCA performed on nonideal surfaces may aid in the understanding of the plateau transition apparent in ESCA data between the low and high loading regions. The ESCA Br values for the anchor loadings tested do offer an orthogonal confirmation of the TGA data exhibiting a direct relationship between the fraction of anchor in the reaction mixture of a sample and the content of BAP in the modified sample. Analysis of the presence and magnitude of bulk pyrene phase transitions (melting point) in DSC data of anchor/nanotube samples enables an estimation of the amount of anchor bound to the surface of the nanotubes. Anchor molecules directly attached to nanotubes surfaces, and perhaps anchor molecules in nearest neighbor proximity to nanotube-attached molecules, do not contribute to the observed phase transition that corresponds to the melting of the bulk anchor. This is because attached molecules in the first and/or second deposited layers have a sufficiently different local environment from bulk anchor molecules to not exhibit the same solid-to-liquid phase transitions as the bulk molecules. This understanding is confirmed by comparing the TGA/DSC data for two different samples: one in which anchor molecules and nanotubes have merely been stirred together and another in which the anchor molecules and nanotubes have undergone the SCF surface deposition process (Figure 3). For identical total anchor content, the unreacted sample displays the melting point of the bulk anchor, whereas the reacted sample with monolayer or lower coverage shows no bulk-phase melting point. This strongly supports the conclusion that all anchors present in the reacted sample are located on surface sites. For very low levels of anchor loadings on carbon nanotubes, no melting-point transition of the pyrene anchors is observed.
Noncovalent Functionalization of Carbon Nanotubes
Figure 3. Plot of DSC (A) and TGA (B) data for anchor/nanotube samples with 5 wt % anchor content in which BAP has merely been mixed with the carbon nanotubes (solid line) and in which the anchor has been deposited onto the nanotube surface using the SCF method (dashed line).
Figure 4. Plot of DSC data (exothermic direction is up) for a series of BAP weight content (as indicated). For low-loading levels (much lower than complete monolayer coverage), no bulk-type anchor is observed.
This is presumably because all of the anchor molecules present in the sample are attached to the surface of a nanotube. It is likely that small clusters of molecules and/or unusual surface sites related to the morphology of intercontacting nanotubes or surface defects provide a local bulk-type environment for a significant number of molecules since bulk-phase transitions have been observed in nanotube samples confidently below the wt % levels of complete monolayer coverage. The magnitude of observed melting-point transitions increases dramatically at loading levels (around 30 wt %), corresponding to approximately twice the wt % of complete monolayer coverage since most or all of the surface sites have been occupied at these levels. The area under the phase-transition peak in the DSC heat flow data enables relation of anchor content in reaction mixture to bulktype anchor content in modified nanotube sample (Figure 4). The combination of TGA and DSC data yields values for the amount of anchor molecules bound on the modified carbon nanotube surface for any initial anchor/nanotube reaction mixture (Figure 5). The anchor content in a modified carbon nanotube sample, as determined by TGA, is confirmed to have a direct relationship with the anchor fraction in the initial reaction mixture for that sample. The portion of anchor in a modified carbon nanotube sample present in the bulk state (each anchor molecule surrounded by many other anchor molecules) may be determined by integrating the endothermic phase transition observed for that sample, determined by DSC, corresponding to bulk anchor melting. The difference between total anchor present in the modified carbon nanotube sample and the amount of anchor present that contributes to the phase transition observed for bulk anchor yields a reasonable estimate of the amount of anchor present in the modified carbon nanotube sample that is closely associated with the nanotube surface
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8739
Figure 5. Plot of the BAP content of modified carbon nanotubes vs BAP fraction in initial reaction mixture. (b) Total amount of anchor in modified nanotube samples as determined using TGA; (O) amount of the total anchor that is closely associated with the nanotube surface. Trend lines are shown to guide the eye.
(surface bound anchor). A look at the trend of surface bound anchor content of modified nanotubes determined in this way with the anchor fraction in initial reaction mixtures enables an estimate of the point at which bulk-form anchors begin to accumulate on the surface. The complete transition to bulk behavior for BAP on the multiwall nanotubes used is seen to occur around 20-30 wt % loading. An important question for molecular-anchor-type modification of carbon nanotubes is: What is the maximum achievable monolayer coverage? Complete monolayer coverage would allow the most efficient use of anchors and available nanotube surface area while optimizing system capacity. A minimum value of 7 wt % for complete monolayer loading with BAP molecules lying flat on the surface can be estimated by considering the 0.7 nm2 footprint of the 323.2 g/mol anchor and the 112 m2/g surface area (determined by Brunauer, Emmett, and Teller (BET)) of the multiwall carbon nanotubes used. Lowest-energy packing conformations calculated for 1-pyrene butanoic acid succinimidyl ester (PBSE) on a carbon nanotube surface20 suggest a mild overlap of neighboring molecules rather than this simple side-by-side flat arrangement. Assuming the preferred arrangement determined by Han and co-workers for PBSE of 1 anchor molecule for every 24 graphene surface atoms, one can estimate an optimal BAP monolayer loading of 14 wt %. Two complete layers of such coverage would result in a 28 wt % loading that may correspond to the dramatic surface-to-bulk behavior transition observed by DSC as thirdlayer molecules consistently find themselves in bulklike local environments. In an effort to model the adsorption situation of the BAP organic molecular anchor on a nanotube surface, a BET-type approach35 was used. By analogy to a common form of the greater-than-monolayer BET equilibrium isotherm
O)
cMR (1 - R){1 - (1 - c)R}
where O is the weight fraction of anchor observed in the modified nanotube sample, c is an enthalpy-related constant, M is the sample weight fraction corresponding to a complete monolayer of coverage, and R is the weight fraction of anchor in the reacted sample (Figure 6). The BET-type model fits the observed data well only at low coverage. This could be anticipated since one of the BET model assumptions, that adsorbate molecules do not interact with one another, is not met in the case of pyrene anchors at high coverage. Linearization and graphing of the above equation
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Fifield et al. surface area applications in electrochemistry, catalysis, and chemical separation. The use of molecular anchors rather than covalently bound molecules may improve the performance of nanotube systems involving temperature or electric swings and for systems with stringent nanotube property requirements.
Figure 6. BET equilibrium isotherm model fit (dashed line) and TGAobtained BAP anchor loading data (O) vs anchor content of sample in initial reaction.
provide an estimated monolayer coverage (M) for the system of 17 wt %. The scatter of points on the linear fit confirms that the BET model has limited applicability in the current system and detracts from confidence in this monolayer level estimate as independently reliable. 4. Summary and Conclusions The surface area footprint calculation provides a lowest-level estimate for BAP monolayer loading on the multiwall carbon nanotubes used at 7 wt % and DSC observation of transition to bulk coverage seems to place an upper limit on monolayer coverage at around 30 wt %. BET isotherm fit of the loading data estimates a 17 wt % loading, while packing similar to that predicted for the PBSE molecule would produce a 14 wt % loading. These estimates would result in capacities of around 3 × 1020 equivalents per gram for pyrene anchors with one functional moiety loaded onto multiwall carbon nanotubes. This value may be multiplied by the use of more than one functional group per anchor molecule and may be dramatically increased through the use of higher surface area materials such as singlewall carbon nanotubes. Spectroscopic analysis of the presence of pyrene dimers28,36 and/or solid-state NMR37 of SCF anchormodified single-wall carbon nanotubes might give more information regarding the nature of the anchor molecule monolayer in these systems. A novel method for convenient noncovalent chemical modification of carbon nanotube surfaces was demonstrated. Noncovalent chemistry of this type may be preferable to covalent modification of carbon nanotubes for many applications, as it should not detrimentally affect the desired inherent properties of the nanotubes. The described SCF method is useful for the modification of a large number of nanotube forms (bulk, surface grown, paper, etc.) and nanotube types (single-wall, multiwall, etc.) with a variety of molecular anchors (functionalized pyrene, anthracene, etc.). We are currently pursuing a direct comparison, in terms of control of loading and maximum achievable loading, between the SCF deposition process described here, deposition in liquid CO2, and the conventional solution method previously reported for deposition of molecular anchors onto carbon nanotubes. The results of this additional study, using a common anchor and a common nanotube form, will reveal if the SCF method has the advantage of being more effective than conventional methods for loading anchors onto nanotube surfaces in addition to its advantages of convenient processing of powder samples and elimination of harmful process solvents. The efficient coating of nanotube surfaces, such as those in hierarchical nanotube-on-substrate structures, with tailored molecular anchors is expected to be especially useful for high
Acknowledgment. The authors would like to thank T. R. Hart for valuable assistance with TGA/DSC. This work was partially supported by the Joint Institute for Nanoscience, funded by the Pacific Northwest National Laboratory and the University of Washington, and the Air Force Office of Scientific Research. Portions of the work described in this paper were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated by Battelle for the Department of Energy under Contract DE-AC06-76RLO 1830. References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (2) Iijima, S. Nature 1991, 354, 56. (3) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (4) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (5) Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952. (6) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760. (7) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (8) Garg, A.; Sinnott, S. B. Chem. Phys. Lett. 1998, 295, 273. (9) Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Wong Shi Kam, N.; Shim, M.; Li, Y.; Kim, W.; Utz, P. J.; Dai, H. Proc. Nat. Acad. Sci. U. S. A. 2003, 100, 4984. (10) Star, A.; Liu, Y.; Grant, K.; Ridvan, L.; Stoddart, J. F.; Steuerman, D. W.; Diehl, M. R.; Boukai, A.; Heath, J. R. Macromolecules 2003, 36, 553. (11) in het Panhuis, M.; Munn, R. W.; Blau, W. J. Synth. Met. 2001, 121, 1187. (12) Dalton, A. B.; Blau, W. J.; Chambers, G.; Coleman, J. N.; Henderson, K.; Lefrant, S.; McCarthy, B.; Stephan, C.; Byrne, H. J. Synth. Met. 2001, 121, 1217. (13) Meyer, A. G.; Dai, L.; Chen, Q.; Easton, C. J.; Xia, L. New J. Chem. 2001, 25, 887. (14) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (15) Gomez, F. J.; Chen, R. J.; Wang, D.; Waymouth, R. M.; Dai, H. Chem. Commun. 2003, 190. (16) Liu, L.; Wang, T.; Li, J.; Guo, Z.-X.; Dai, L.; Zhang, D.; Zhu, D. Chem. Phys. Lett. 2003, 367, 747. (17) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem. Lett. 2002, 638. (18) Xin, H.; Woolley, A. T. J. Am. Chem. Soc. 2003, 125, 8710. (19) Chase, J. E.; Boerio, F. J. Proc. Annu. Meet. Adhesion Soc. 2003, 26, 258. (20) Han, S.; Cagin, T.; Goddard, W. A., III. Mater. Res. Soc. Symp. Proc. 2003, 772, M6.3.1. (21) Zhang, J.; Lee, J.-H.; Wu, Y.; Murray, R. W. Nano Lett. 2003, 3, 403. (22) Katz, E. J. Electroanal. Chem. 1994, 365, 157. (23) Dai, H. Acc. Chem. Res. 2002, 35, 1035. (24) Shin, Y.; Zemanian, T. S.; Fryxell, G. E.; Wang, L.-Q.; Liu, J. Microporous Mesoporous Mat. 2000, 37, 49. (25) Zemanian, T. S.; Fryxell, G. E.; Liu, J.; Mattigod, S.; Franz, J. A.; Nie, Z. Langmuir 2001, 17, 8172. (26) Chrastil, J. J. Phys. Chem. 1982, 86, 3016. (27) Bartle, K. D.; Clifford, A. A.; Jafar, S. A.; Shilstone, G. F. J. Phys. Chem. Ref. Data 1991, 20, 713. (28) Rice, J. K. J. Am. Chem. Soc. 1995, 117, 5832. (29) Miller, D. J.; Hawthorne, S. B.; Clifford, A. A.; Zhu, S. J. Chem. Eng. Data 1996, 41, 779.
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