Anal. Chem. 2005, 77, 7094-7097
Chromatography on Self-Assembled Carbon Nanotubes Chutarat Saridara† and Somenath Mitra*
Department of Chemistry and Environmental Science, New Jersey Institute Of Technology, Newark, New Jersey 07102
Stationary phases that provide high resolutions and are stable at high temperatures are of significant importance in chromatographic analysis. Carbon nanotubes (CNTs) are known to have high thermal and mechanical stability and have the potential to be high-performance separation media that utilize the nanoscale interactions. Here, we report the first application of self-assembled CNTs in long capillary tubes for the development of gas chromatography columns. A film of CNTs was deposited by chemical vapor deposition (CVD) to form the stationary phase in the open tubular format. High-resolution separation of a number of compounds has been achieved. Altering the CVD conditions can vary the thickness and the morphology of the CNT film, which opens the possibility of selectivity tuning. The ability to fabricate long tubes coated with CNTs can be readily employed in other gas- and liquidphase separations as well. Carbon-based sorbents are used in a variety of industrial- and laboratory-scale applications, such as, gas cleaning, water treatment, and separation of solutes. They are also used extensively in analytical applications, such as chromatographic stationary phases. Consequently, the developments in carbon-based sorbents have been an ongoing endeavor that has been of great interest to the scientific communities. In recent years, carbon nanotubes (CNTs) have been the subject of intense research because of their novel physical, chemical, and electrical properties.1 This development provides unique opportunities for the development of higher performance separation techniques that utilize the nanoscale interactions on a material known to have high thermal and mechanical stability. The CNTs have the potential to extend the applicability of carbonbased sorbents in many applications. The nanotubes are composed of two fullerenes halves and a cylinder made of a rolled up graphene sheet. They exist as single-walled nanotubes (SWNTs) or multiple-walled nanotubes (MWNTs) and also as open- or closeended structures of different morphology and diameter. The adsorption can occur on the outside surface, on the curved graphene planes, in the interstices between tubes that are bundled together, and on the inside when they are open-ended.1 There also exist the possibility of a molecular sieve effect on the interstitial spaces, where the large molecules could be excluded. † Permanent address: Rajamangala University of Technology, Thyanaburi, Thailand. * Corresponding author: (phone) (973) 596 5611; (e-mail)
[email protected]. (1) Wei, B. Q.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Nature 2002, 416, 495-496.
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Chromatography is widely used for high-resolution separation and in quantitative analysis. It is conceivable that chromatographies of various kinds stand to benefit from the developments in CNT synthesis and their assembly on micro/macrostructures. Chromatographic separation involves the differential partitioning of solutes (or analytes) between a stationary and a mobile phase.2 Typical solid phases for gas chromatography (GC) include porous polymers, silica, and activated carbons, where adsorption is the dominant retention mechanism and high temperatures are often used to vary the retention in GC. Carbon-based sorbents are usually used for the separation of small organic and inorganic molecules and are commercially available in various particle/pore sizes and with different specific surface areas. Typically, these particles are packed into a tube, although open tubular carbon phases are also available.2 In chromatography, the physical/ chemical affinity between the sorbate and the sorbent needs to be optimum to achieve separation within a reasonable time and at high resolutions. CNTs synthesized in different forms, sizes, and with different functionalities will provide variable affinity and selectivity and thus can be used for the separation of a wide range of solutes. It is therefore conceivable that CNTs will emerge as highly selective, high-temperature sorbents for chromatography. Recently, chromatography on CNTs has been reported, where multiple-wall nanotubes have bee used as column packing.3 It is well known that CNTs in powder form cluster together and form large aggregates and because of which, much of their nanoscale characteristics may be lost. The objective of this research is to self-assemble CNTs to serve as a separation medium in gas chromatography. Based on current developments, techniques for CNT synthesis may be classified under three major categories, namely, laser ablation,4 catalytic arc discharge,5 and chemical vapor deposition (CVD).6-8 While, the first two methods are excellent for largescale production, they cannot be used for the self-assembly on different substrates. Catalytic CVD appears to be the method of choice for the direct deposition on a device or a structure. Over (2) Skoog, D. A.; Holler, F. J.; Niemann, T. A. Principles of Instrumental Analysis; Saunders College Publishing: Philadelphia, PA, 1998. (3) Li, Q.; Yuan, D. J. Chromatogr., A 2003, 100, 203-209. (4) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Smalley, R. E. Science 1996, 273, 483-487. (5) Journet, C.; Matser, W. K.; Bernier, P.; Laiseau, L.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756-758. (6) Makris, T. D.; Giorgi, L.; Giorgi, R.; Lisi, N.; Salernitano, E. Diamond Relat. Mater. 2005, 14, 815-819. (7) Nasibulin, A. G.; Moisala, A.; Brown, D. P.; Jiang, H.; Kauppinen, E. I. Chem. Phys. Lett. 2005, 402, 227-232. (8) Flahaut, E.; Laurent, C.; Peigney, A. Carbon 2005, 43, 375-383. 10.1021/ac050812j CCC: $30.25
© 2005 American Chemical Society Published on Web 09/29/2005
Figure 1. (a) Schematic diagram of the CVD setup; (b) gas chromatography setup for the evaluation of the columns. Gas samples were injected using a 10-port gas sampling valve.
the past decade, a variety of nanotube architectures have been fabricated using CVD.6-13 Recently we have reported the development of a microtrap for analyte preconcentration by CVD of MWNTs.14 However, assembling CNTs on macrostructures remains a challenge. A variety of catalysts have been used for the CVD synthesis of CNTs, which have been deposited by injecting particles, liquid solution, or via electrochemical methods.6-13 Here we report the first application of self-assembled CNTs in long capillary tubing to serve as gas chromatography columns. CNTs are deposited via CVD over long lengths of the tubing to form effective gas chromatography columns for analytical separations. EXPERIMENTAL SECTION The self-assembly of CNTs in 500-µm-i.d. capillaries was carried out in a tube furnace by the CVD method. This technique has been described previously,14,15 was modified for growth in long tubes, and is shown in Figure 1. A variety of carbon sources including CO, CH4, ethanol, and C2H4 were tried for the CVD of a uniform CNT layer. Ethylene was found to consistently provide high density, uniform films that were deemed useful for GC separation. The other precursors were not studied further. Depositing a uniform catalyst layer on the inside surface of a long capillary tubing can be relatively difficult. Two types of microcapillaries were tried here: one was silica-lined stainless steel and (9) Lee, W. Y.; Liao, T. X.; Juang, Z. Y.; Tsai, C. H. Diamond Relat. Mater. 2005, 13, 1232-1236. (10) Yabe, Y.; Ohtake, Y.; Ishitobi, T.; Show, Y.; Izumi, T.; Yamauchi, H. Diamond Relat. Mater. 2004, 13, 1292-1295. (11) Liu, R.-M.; Ting, J.-M. Mater. Chem. Phys. 2003, 82, 571-574. (12) Emmenegger, C.; Bonard, J.-M.; Mauron, P.; Sudan, P.; Lepora, A.; Grobety, B.; Zu ¨ ttel, A.; Schlapbach, L. Carbon 2003, 41, 539-547. (13) Lee, C. J.; Park, J.; Yu, J. A. Chem. Phys. Lett. 2002, 360, 250-255. (14) Saridara, C.; Brukh, R.; Iqbal, Z.; Mitra, S. Anal. Chem. 2005, 77, 11831187. (15) Sharma, R.; Iqbal, Z. Appl. Phys. Lett. 2004, 84, 990-992.
the other was untreated stainless steel. Preliminary attempts with silica-lined tubing showed nonuniform and often sparse CNT growth, and it was not used further. Nanocrystals of iron are well known to catalyze the growth of CNTs,16 which were generated on the steel surface to provide the catalytic activity. Appropriate surface treatment was necessary to enhance the catalytic activity of the iron in the steel tubing; mainly amorphous carbon was formed in its absence. Nanostructured iron was generated on the tube surface as follows. First the capillary was heat-treated in air at 500 °C for 30 min at a flow rate of 10 mL/ min to oxidize the surface, and then the surface was reduced in a 10 mL/min flow of H2 at 500 °C for 30 min. The oxidationreduction led to the formation of a catalytically active surface.16,17 After this, CVD was carried out in a flow of C2H4 at 700 °C for anywhere between 1 and 3 h. This process formed the GC column with a thin film of CNTs along with amorphous and other nontubular carbons. These were eliminated by thermal oxidation in the presence of oxygen at 350 °C at a flow rate of 300 mL/min. This step selectively removed the carbonaceous layer while leaving the nanotubes intact.18 Depending upon the CVD conditions, the nanotube layer could be anywhere from 1 to 50 µm thick. Thickness of the CNT film was measured by cutting the column at different points and making measurements using a SEM (LEO 1530). Chromatographic separations were carried out using a HewlettPackard 5890 gas chromatograph with a flame ionization detector. Injections were made with a Valco gas sampling valve. RESULTS AND DISCUSSION The oxidation-reduction led to the formation of nanostructured iron crystals on the steel tubing as shown in Figure 2a. CVD with C2H4 as the precursor formed a thin CNT film (micrometer scale) along with some nonnanotubular carbonaceous materials on top. The CNTs were anchored on the metal surface, while the amorphous carbon formed a layer of loose particles. The CNTs have significantly higher thermal stability than the amorphous and other nontubular carbons. So, these can be selectively eliminated by thermal annealing in the presence of oxygen while leaving the nanotubes intact. This is seen from the SEM images presented in Figure 2d and e. In Figure 2e, the amorphous layer has been removed, and now the CNTs alone are the exposed surface for adsorption during chromatographic separation. Similar results were obtained for the CO CVD of CNTs. As can be seen in Figure 2c, the CNT phase was sparse compared to the C2H4 CVD. The CNTs deposited were MWNTs and were radially aligned inside the tube. Previous studies have reported similar CNT formation.9 Raman spectra taken from the inside surface of the cut capillary did not show the characteristic peaks associated with the strongly diameter-dependent radial breathing mode of SWNTs below 300 cm-1, which suggested that SWNTs were not formed under the prevailing conditions. Changing the CVD precursor and the deposition conditions could vary the thickness of the CNT films grown in the capillary. This is an important parameter that (16) Karwa, M.; Iqbal, Z.; Mitra, S. Carbon. Submitted. (17) Peng, X.; Li, Y.; Luan, Z.; Di, Z.; Wang, H.; Tian, B.; Jia, Z. Chem. Phys. Lett. 2003, 376, 154-158. (18) Osswald, S.; Flahant, E.; Ye, H.; Gogotsi, Y. Chem. Phys. Lett. 2005, 402, 422-427.
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Figure 2. SEM images from CNT self-assembly: (a) nanostructured iron on tube surface after surface treatment; (b) typical CNTs covered with amorphous carbon from CO CVD; (c) CNTs from CO CVD after the oxidative annealing; (d) typical CNTs covered with amorphous carbon from C2H4 CVD; (e) CNTs from C2H4 CVD after the oxidative annealing.
determines the sorption capacity and mass transfer in the CNT film, thus offering the possibility of the fabrication of different types of columns for diverse applications. The challenge here was to develop a relatively uniform film of CNTs, which could serve as a GC column. The CVD process is strongly dependent upon the residence time of the free radicals generated during the flow of the precursors.16 Consequently, the CNT deposition and film thickness varied along the length of the long column. Typical variation is shown in Figure 3. In this particular case, the first 1.2 m served as an effective stationary phase for GC separation. The flow conditions and the CVD precursor may be varied to alter film thickness and morphology of the deposited CNT, thus altering the sorption capacity. For example, under similar conditions, CO and C2H4 generated CNTs that are significantly different in diameter and density.14 7096 Analytical Chemistry, Vol. 77, No. 21, November 1, 2005
The CNT film was an excellent chromatography stationary phase and was an effective separation medium. It was stable and was able to handle the temperature cycling associated with typical GC applications. The column was used for extended periods of time, there was no column bleed, and the baselines were stable. A wide range of organic molecules could be separated on these columns. Typical separation of a group of alkenes is shown in Figure 4, which represents the chromatography of highly volatile compounds that are normally carried out in packed GC columns. The CNT phase allows the same separation to be carried out in an open tubular format, which is known to result in significantly higher resolution.2 The same column could be used in the separation of relatively less volatile compounds, such as benzene and toluene. The chromatograms are not presented here for brevity. An advantage of the CNT is its stability at high temper-
Figure 3. Variation in film thickness (micrometers) as a function of length. The column was formed by CVD using C2H4 as the CNT precursor for a deposition time of 1 h. Figure 5. Height equivalent of theoretical plates as a function of flow rate for methane at 30 °C (9), and ethylene at 120 °C ([).
Figure 4. Typical chromatograms generated from the CNT column in Figure 3. (a) Chromatogram generated from a large-volume injection of 5 mL (temperature program from 30 to 325 °C at 50 °C/ min, flow rate 10 mL/min.). Large-volume injection was employed for the ppb-level standard used in this analysis; (b) 10-µL injection of a ppm-level standard (temperature program from 30 to 300 °C at 70 °C/min, flow rate 7 mL/min).
atures, which is important from the standpoint of gas chromatography, because that can increase the range of compounds that can be separated by GC. The CNT film exhibited classical chromatographic behavior. The efficiency of separation was estimated using the plate theory of chromatography.2 The height equivalent of a theoretical plate (H) varied as a function of flow rate. It passed through a minimum, which is usually the optimum flow rate for separation. Variations of H with flow rate are presented for methane and ethylene in Figure 5. Similar results were obtained for several other compounds. Optimum flow rate ranged between 4 and 8 mL/min, which is typical of open tubular columns of this internal diameter.2 The separation capability of the GC column was computed based on the number of theoretical plates. At optimum flow rate, the total number of plates on the column in Figure 3 were found to be 1800 and 2328 for ethylene and methane, respectively. The relative standard deviation in retention time and peak area were comparable to conventional columns and were between 0 and 2%. Based on the retention as measured by capacity factor decreased with temperature and also followed classical chromatographic
Figure 6. Variation in capacity factor with temperature (van’t Hoff’s plot) for methane (2) and ethylene (9).
behavior. The logarithm of k′ varied linearly with the reciprocal of temperature. This is shown in Figure 6. Conclusions. CNT-based open tubular GC stationary phases were fabricated via CVD on steel capillaries. This phase demonstrated classical chromatography behavior and high resolution although the film thickness was found to be nonuniform. A major advantage is the ease of fabrication by the self-assembly of CNTs directly on the tube surface. The nanostructured metal catalyst developed on the tube surface effectively anchors the CNTs, leading to the formation of a stable stationary phase. The high thermal stability of CNTs can allow separations at higher temperatures, extending the range of conventional chromatography. The thickness of the CNT film and its morphology can be tailormade by varying the CVD precursor, the catalyst preparation, and by chemical functionalization. This can lead to the development of a wide range of chromatographic columns with variable selectivity. ACKNOWLEDGMENT This work was supported by the US EPA STAR (Contract RD830901) program. Mr. Mahesh Karwa is acknowledged for his help with the SEM. Received for review May 10, 2005. Accepted August 24, 2005. AC050812J
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