Single-Walled Carbon Nanotube Paper as a Sorbent for Organic

Mar 2, 2006 - Abstract. Single-walled carbon nanotubes were examined as an adsorptive material for a thermally desorbed preconcentrator for organic va...
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Anal. Chem. 2006, 78, 2442-2446

Technical Notes

Single-Walled Carbon Nanotube Paper as a Sorbent for Organic Vapor Preconcentration Feng Zheng, David L. Baldwin, Leonard S. Fifield, Norman C. Anheier, Jr., Christopher L. Aardahl, and Jay W. Grate*

Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352

Single-walled carbon nanotubes were examined as an adsorptive material for a thermally desorbed preconcentrator for organic vapors. The nanotubes were processed into a paper form and packed into a metal tube for flowthrough sampling. Adsorbed vapors were released by a temperature-programmed desorption method and detected downstream with a flexural plate wave vapor sensor. The tested vapors, methyl ethyl ketone, toluene, and dimethyl methylphosphonate, were released from the packed column at different temperatures. The vapors were retained more strongly than previously observed for the widely used Tenax porous polymer, indicating a significant affinity of the single walled nanotubes for organic vapors. Although there has been a great deal of experimental and theoretical work focused on the adsorption of inorganic gas molecules by carbon nanotubes (CNTs), the interactions of CNTs with organic vapors has been studied to a lesser extent. Chakrapani et al. ultrasonicated HiPCO (high-pressure carbon monoxide synthesis process) single-walled carbon nanotubes (SWNTs) in liquid acetone and characterized the resulting modified surfaces with X-ray photoelectron spectroscopy (XPS) and temperatureprogrammed desorption of chemisorbed species.1 Without ultrasonication, “not much” increase in surface oxygen was observed by XPS after exposure to liquid acetone. Yang et al. obtained adsorption isotherms for methanol and ethanol on HiPCO SWNTs.2 Hilding et al. measured butane adsorption isotherms on multiwall nanotubes (MWNTs) and determined that multilayer surface condensation accounts for most of the butane sorption.3 The heat of adsorption of butane on MWNTs has also been measured.4 Eklund et al. examined the effect of alcohol adsorption on the electrical properties of bundled SWNTs.5 Practical applications of CNT sorbents in trace gas preconcentration and detection have also started to attract interest. Penza * E-mail: [email protected]. (1) Chakrapani, N.; Zhang, Y. M.; Nayak, S. K.; Moore, J. A.; Carroll, D. L.; Choi, Y. Y.; Ajayan, P. M. J. Phys. Chem. B 2003, 107, 9308-9311. (2) Yang, C.-M.; Kanoh, H.; Kaneko, K.; Yudasaka, M.; Iijima, S. J. Phys. Chem. B 2002, 106, 8994-8999. (3) Hilding, J.; Grulke, E. A.; Sinnott, S. B.; Qian, D.; Andrews, R.; Jagtoyen, M. Langmuir 2001, 17, 7540-7544. (4) Hilding, J. M.; Grulke, E. A. J. Phys. Chem. B 2004, 108, 13688-13695. (5) Romero, H. E.; Sumanasekera, G. U.; Kishore, S.; Eklund, P. C. J. Phys.: Condens. Matter 2004, 16, 1939-1949.

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et al. demonstrated detection of alcohol vapors by acoustic wave and optical fiber sensors coated with SWNT films applied by the Langmuir-Blodgett technique.6 Snow et al. reported the use of a random network of SWNTs in a thin film transistor configuration as a sensor for dimethyl methylphosphonate (DMMP), a nerve agent stimulant.7 In addition, chemiresistor flow cell sensors were fabricated from carbon nanotubes grown by chemical vapor deposition on the inside surface of the flow cell tube.7 A capacitortype chemical sensor has also been described in which a SWNT network serves as one plate of the capacitor; chemical vapor responses were enhanced by thin coating of a functionalized sorptive polymer.8 Li et al. evaluated 60-80 mesh particles of purified MWNTs as a column packing for gas chromatography (GC) and as a solid adsorbent for a vapor preconcentrator.9,10 The preconcentrator tube was rapidly heated to 250 °C to release organic vapors to a GC column. Mitra et al. reported a preconcentraor “microtrap” for trace organic vapors based on a steel capillary tube with MWNTs deposited on their inside wall by chemical vapor deposition.11 The preconcentrator was rapidly heated to 350 to 400 °C to release vapors as a sharp injection pulse to a GC. In this paper, we report the use and characterization of SWNTs as an adsorptive material for a thermally desorbed vapor preconcentrator for organic vapors, in which the SWNTs are processed into carbon nanotube paper12-17 prior to packing a metal preconcen(6) Novak, J. P.; Snow, E. S.; Houser, E. J.; Park, D.; Stepnowski, J. L.; McGill, R. A. Appl. Phys. Lett. 2003, 83, 4026-4028. (7) Penza, M.; Cassano, G.; Aversa, P.; Antolini, F.; Cusano, A.; Cutolo, A.; Giordano, M.; Nicolais, L. Appl. Phys. Lett. 2004, 85, 2379-2381. (8) Snow, E. S.; Perkins, F. K.; Houser, E. J.; Badescu, S. C.; Reinecke, T. L. Science 2005, 307, 1942-1945. (9) Li, Q.-L.; Yuan, D.-X.; Lin, Q.-M. J. Chromatogr., A 2004, 1026, 283288. (10) Li, Q.; Yuan, D. J. Chromatogr., A 2003, 1003, 203-209. (11) Saridara, C.; Brukh, R.; Iqbal, Z.; Mitra, S. Anal. Chem. 2005, 77, 11831187. (12) Whitten, P. G.; Spinks, G. M.; Wallace, G. G. Carbon 2005, 43, 18911896. (13) Vohrer, U.; Kolaric, I.; Haque, M. H.; Roth, S.; Dettlaff-Weglikowska, U. Carbon 2004, 42, 1159-1164. (14) Spinks, G. M.; Wallace, G. G.; Fifield, L. S.; Dalton, L. R.; Mazzoldi, A.; De Rossi, D.; Khayrullin, I. I.; Baughman, R. H. Adv. Mater. 2002, 14, 17281732. (15) Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; RodriguezMacias, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A: Mater. Sci. Process. 1998, 67, 29-37. 10.1021/ac051524q CCC: $33.50

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Figure 2. Schematic diagram of the test system for delivering a constant flow rate of a diluted vapor stream to the preconcentrator tube with downstream monitoring of the vapor concentration using an FPW sensor. The back-pressure regulator (BPR) provides a constant pressure upstream from the rotameter needle valve that regulates flow to the device under test. This back pressure is measured with a pressure guage (G). Figure 1. Schematic diagram of the processing of carbon nanotube paper into a preconcentrator tube, including an SEM microscopy (LEO 982 field-emission SEM) of the as-synthesized SWNT paper.

trator tube. This approach is illustrated in Figure 1. The preconcentrator tube is heated to temperatures as high as 460 °C in a preprogrammed thermal ramp to desorb vapors. The nanotube adsorbent binds organic vapors significantly more strongly than the Tenax porous polymer investigated previously by similar methods. EXPERIMENTAL SECTIONS Multiwalled carbon nanotubes used as a powder in preliminary experiments were obtained from Nanostructured and Amorphous Materials. As-produced HiPCO single walled carbon nanotubes (SWNTs) were obtained from Carbon Nanotechnologies, Inc. (Houston, TX) and used as received. To produce a paper form, a batch of 37 mg of SWNTs was ultrasonicated in 100 mL of toluene for 30 min. The SWNT suspension was then filtered through a 37-mm-diameter, 10-µm-pore PTFE membrane (LC10, Millipore) and allowed to dry. The free-standing mat was then peeled from the membrane. A small-volume vapor preconcentrator tube was created by packing SWNT paper into an Inconel alloy tube 15 mm long by 3.8 mm i.d. fitted with inlet and outlet tubes and a resistive heater. The gas inlet and outlet tubes were fitted onto the ends by welding (before packing) or by a press fit (after packing). A glass-coated, fine-gauge, nickel 200 wire wound around the body of the preconcentrator tube served as a resistive heater and simultaneously as an RTD element for heater feedback control. A type-K thermocouple was positioned between the wound heater and the body of the preconcentrator to independently monitor the heater temperature. The wire wound heater was encapsulated using a high-temperature, electrically insulating, and thermally conductive zircon-based cement. This metal preconcentrator assembly and control electronics are capable of 480 °C continuous and 530 °C peak operation and can maintain the adsorbent preconcentrator at a constant programmed temperature during the vapor adsorption process. (16) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 12531256. (17) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340-1344.

The SWNT preconcentrator was tested for trace organic vapor uptake and release in a flow system that continuously delivered either clean dry nitrogen or test vapor in clean dry nitrogen to the preconcentrator under preprogrammed computer control.18 Test vapors were generated from bubbler sources and diluted using carrier gas flows regulated by electronic mass flow controllers. This system can deliver either single pure vapors diluted in carrier gas or diluted vapor mixtures. The vapor sources were liquid toluene (TOL), methyl ethyl ketone (MEK), and dimethyl methylphosphonate (DMMP), all obtained from Sigma-Aldrich. A poly(dimethylsiloxane)(PDMS)-coated 8-MHz flexural plate wave (FPW) microsenor in series downstream from the preconcentrator, as shown in Figure 2, detected released vapors as a change in oscillation frequency.18 An FPW device is a type of acoustic wave sensor;19,20 these respond to added mass from the sorption of vapor into the polymer coating with a decrease in the resonant frequency. This vapor delivery system generates a large volumetic flow rate of diluted vapor, which is then split into two paths via a tee junction, as shown in Figure 2. A rotameter with a needle valve on one path is used to set the flow rate to the device or sensor under test, and a back-pressure regulator on the other path vents the large volume of excess diluted vapor while maintaining a constant pressure on the upstream side of the rotameter and, hence, a constant flow to the device under test. The flow characteristics of CNT-packed preconcentrators were observed upon connecting them to this system, since a flow restriction will appear as a decrease in flow rate on the rotameter. In other words, given a constant total pressure drop from the upstream side of the rotameter needle valve to the preconcentrator outlet at atmospheric pressure, the greater the pressure drop across the preconcentrator, the less the pressure drop across the rotameter needle valve, and, hence, the lower the flow rate. With the preconcentrator at room temperature, a known dilute concentration of vapor was delivered to the preconcentrator for 2 min at 30 mL/min. After a 10-s delay, the preconcentrator temperature was raised at a preprogrammed ramp rate (TPD).18 At the top of the thermal ramp, the temperature was maintained (18) Grate, J. W.; Anheier, N. C.; Baldwin, D. L. Anal. Chem. 2005, 77, 18671875. (19) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 940A948A. (20) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 987A996A.

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for a preselected hold period to allow for the test vapors to fully desorb. The flow rate during the breakthrough experiments was also 30 mL/min. RESULTS AND DISCUSSION SWNT-Containing Preconcentrator. Significant flow restrictions were observed in exploratory experiments in which commercial MWNT powders were packed into the metal preconcentrator tubes described in the Experimental Section. A flow restriction in the preconcentrator tube is revealed on connecting the device to the output of the vapor delivery system downstream from the rotameter, as shown in Figure 2 (no sensor attached). With the back-pressure set at 2 psi and the rotamter needle valve set to give an initial flow rate of 100 mL/min without a preconcentrator connected, it was observed that the flow rate dropped to just 2-3 mL/min upon connecting a tube packed with 27 mg of MWNT powder. This type of flow restriction is impractical for either laboratory experiments or for use in detection devices using small pumps. A desirable feature of a preconcentrator is that one can efficiently flow a large volumes of gas through the device to collect vapors for subsequent desorption into a much smaller volume. These results prompted us to consider the use of nanotube papers as a more structured nanotube-based material that might allow gas flow more easily than randomly packed fine powders. Nanotube papers, also called “bucky paper”, have been prepared using a filtration method.12-17 SWNTs have been the primary starting material for nanotube papers described in the literature. In our own experience, nanotube paper can be readily assembled from SWNTs but is much more difficult to produce consistently from the more rigid MWNTs. Accordingly, we prepared SWNT paper as described in the Experimental Section. The morphology of the SWNT paper was characterized by scanning electron microscopy, and a representative micrograph is included in Figure 1. The paper consists of a fibrous mat of nanotube bundles and ropes. The paper was typically 60 µm thick as measured with a micrometer and had density of ∼0.6 g/cm3 on the basis of the mass, area, and thickness of the paper. The specific surface area of the resulting SWNT paper was ∼700 m2/g as determined by BET measurements (Quantachrome AutoSorb-6). This value is quite similar to the surface area of 680 m2/g obtained by Vohrer et al., who also prepared SWNT paper from HiPCO SWNTs from Carbon Nanotechnologies, Inc. These results from different laboratories suggest good reproducibility in a critical parameter for adsorptive applications. SWNT paper was loaded into the preconcentrator tube as shown schematically in Figure 1: 3-mm-wide strips of varying length were cut from the circular mat of 60-µm-thick SWNT paper. Each strip was rolled up lengthwise and compressed into the preconcentrator tube with the “plane” of each flattened roll perpendicular to the tube axis. Consecutive rolls were rotated relative to one another (around the tube axis) to minimize the possibility of channeling around the packing. The SWNT paper method was empirically more successful in terms of producing sorbent beds for flow through applications. We found that we could pack preconcentrator tubes that did not produce the flow restrictions previously observed for packed nanotube powders. For example, an SWNT-paper preconcentrator tube containing 25 mg of material did not reduce the flow rate by more than a 2444 Analytical Chemistry, Vol. 78, No. 7, April 1, 2006

Figure 3. Detected vapors desorbed from the SWNT preconcentrator in single vapor tests: DMMP at 165 mg/m3, toluene at 5190 mg/m3, methyl ethyl ketone at 13 400 mg/m3. Linear portion of thermal ramp rate is 3.9 °C/s (120-s ramp).

milliliter per minute or two. Note that this mass is similar to the 27 mg of powdered MWNT that produced a severe drop in flow in our apparatus. The final preconcentrator had a sorbent bed consisting of 24.9 mg of HiPCO SWNTs packed in a tube interior volume of 170 mm3. The volume of this mass of paper is 42 mm3, so the SWNTpaper is estimated to occupy about 25% of the tube interior volume. Assuming 15% Fe in the HiPCO SWNTs, the tube contained 21.2 mg of carbon nanotube material; given a density of 1.35 g/cm3,21 the volume of actual CNTs in the tube was 15.7 mm3. Therefore, the volume of the nanotubes themselves is estimated to be about 10% of the tube interior volume, leaving ∼90% void volume. For comparison, the Tenax-containing preconcentrators in our prior published work contained 25 mg of Tenax, in which the porous polymer particles were packed in a highly porous metal foam inside Teflon tubing assemblies.18,22 Vapor Absorption and Release. The SWNT preconcentrator was tested for organic vapor uptake and release as described in the Experimental Section using a PDMS-coated FPW vapor sensor in series downstream from the preconcentrator to detect desorbed vapors or vapors in breakthrough experiments (see Figure 2). The detected release profiles of individual vapors that were desorbed with a TPD ramp rate of 3.9 °C/s are shown together in Figure 3 along with the thermal ramp profile. The MEK, TOL, and DMMP desorption peaks from the packed bed were observed by the downstream sensor as a decrease in frequency; at the test vapor concentrations and the specified thermal ramp rate, the peaks appeared downstream in the sensor response when the preconcentrator tube was at 210, 360, and 420 °C, respectively. Although methyl ethyl ketone and toluene can be completely desorbed with a linear temperature ramp to 460 °C, the DMMP (21) Gao, G. H.; Cagin, T.; Goddard, W. A. Nanotechnology 1998, 9, 184191. (22) The metal preconcentrator tubes used in the present SWNT study can hold 25-30 mg of Tenax particles.

Table 1. Methyl Ethyl Ketone Adsorption Capacities of the SWNT Preconcentrator

Figure 4. Methyl ethyl ketone breakthrough curves of the SWNTpreconcentrator at 30 mL/min flow rate (circles, methyl ethyl ketone inlet concentration 55 900 mg/m3; diamonds, 33 500 mg/m3; squares, 22 400 mg/m3. On the y axis, C/Co is the exit concentration detected by the FPW sensor divided by the challenge or inlet concentration.

desorption peak was not completely developed at the end of the temperature ramp under these conditions. At lower vapor test concentrations, the detection of all the desorbed vapors occurs at greater times after the onset of heating, resulting in peak maximums being observed on the sensor at times corresponding to higher temperatures on the preconcentrator. The temperatures required to desorb the vapors from the SWNT packed bed were significantly higher than their normal boiling points (methyl ethyl ketone 80 °C, toluene 111 °C, DMMP 181 °C) and also much higher than the temperature required for release from a porous polymer adsorbent. These vapors can all be desorbed from a Tenax-containing preconcentrator with a ramp and hold to a maximum temperature of just 200 °C.18 These results indicate that there are significant interactions between the organic molecules and the SWNT paper. Breakthrough. Breakthrough characteristics of the SWNT preconcentrator were also determined for methyl ethyl ketone vapor at 25 °C, which was the least strongly adsorbed of our test vapors. In this experiment, the vapor was delivered for a 10-min adsorption interval at 30 mL/min. The breakthrough data for three concentrations are shown as open symbols in Figure 4. Proper adsorptive preconcentrator behavior is observed with a delay in the detection of vapor downstream due to capture on the column and longer breakthrough times at lower concentrations. Even at the highest test concentration, the onset of significant breakthrough required over an hour. The adsorption capacities at 1%, 10%, and complete breakthrough were calculated from the mass of vapor delivered and retained on the column. The percent breakthrough was determined directly from the observed vapor concentrations downstream as measured by the FPW sensor. The mass of vapor delivered (based on the concentration, flow rate, (23) Nelson, G. O.; Correia, A. N. Am. Ind. Hyg. Assoc. J. 1976, 37, 514-525. (24) Morris, L.; Caruana, D. J.; Williams, D. E. Meas. Sci. Technol. 2002, 13, 603-612. (25) Lu, C.-J.; Zellers, E. T. Anal. Chem. 2001, 73, 3449-3457. (26) Shaffer, R. E.; Rose-Pehrsson, S. L.; McGill, R. A. Field Anal. Chem. Technol. 1998, 2, 179-192. (27) Dworzanski, J. P.; Kim, M.-G.; Snyder, A. P.; Arnold, N. S.; Meuzelaar, H. L. C. Anal. Chim. Acta 1994, 293, 219-35. (28) Mitra, S.; Feng, C.; Zhang, L.; Ho, W.; McAllister, G. J. Mass Spectrom. 1999, 34, 478-485.

Co,MEK (mg/m3)

W1% (mg/g)

W10% (mg/g)

W100% (mg/g)

22 350 33 530 55 880

53.1 37.1 68.8

71.6 79.3 94.6

122 133 147

and time to reach the specified amount of breakthrough) was corrected for the amount not retained as integrated from the sensor response. (It has been estimated that at 10% breakthrough concentration, ∼2% of the vapor has penetrated all the way through.23) At 100% breakthrough, the amount represents the adsorbed mass of vapor for a completely equilibrated bed of adsorbent at that concentration. The results, in the tens of milligrams per gram, are listed in Table 1. Vahdat et al. have reported that the methyl ethyl ketone capacity of Tenax GR (graphite loaded porous polymer) adsorbent was 0.4-1 mg/g at 1% breakthrough. Retention of volatile vapors is known to be poor on Tenax. The methyl ethyl ketone adsorption capacity of Carboxen 569, a commercial carbon molecular sieve with good volatile vapor capture, was reported to be 10-45 mg/g. Values of 175-222 mg/g were found for Carobsieve S-III. Because breakthrough values depend on concentration, flow rate, and specific column configuration, which vary between laboratories and studies, direct comparisons of such literature values with our results for SWNT paper should be taken as qualitative. Nevertheless, our results suggest that SWNT paper is a good sorbent similar to other high surface area carbon materials. For reference, the surface areas of Tenax GR, Carboxen 569, and Carbosieve S-III, as reported by Vahdat, were 24, 485, and 800 m2/g, respectively, as compared to the value of 700 m2/g that we detemined for our SWNT paper. Discussion. CNTs offer a substantially different architecture from most other sorbents, which are typically particles of porous materials. CNTs, in principle, have all of their surface on the outside, although access to nanotube interiors or aggregation into bundles and ropes may create enclosed surfaces and pores. Regardless, CNTs have high surface area, high thermal stability, and high thermal conductivity, all of which are favorable properties for use in adsorptive thermally desorbed vapor preconcentrators. As shown in this study, CNTs are also effective at retaining vapors, as indicated by the high temperatures required to release them and the effective retention (delayed breakthrough) of the volatile methyl ethyl ketone vapor. In contrast to prior CNT vapor preconcentrator studies, we used SWNTs instead of MWNTs, and we developed a novel packing approach using CNT paper in order to obtain a practical flow through configuration. CNT-containing preconcentrators have potential for use in ambient air monitoring for environmental and security applications, in which the collected vapors are desorbed to a multivariate analytical device, such as a gas chromatograph, mass spectrometer, or sensor array for detection.11,18,24-28 ACKNOWLEDGMENT This work was supported by U.S. Department of Energy LDRD funds, administered by the Pacific Northwest National Laboratory, Analytical Chemistry, Vol. 78, No. 7, April 1, 2006

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and the Office of Nonproliferation Research and Engineering within the National Nuclear Security Administration of the Department of Energy. Significant portions of this research were performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental

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Research and located at Pacific Northwest National Laboratory. PNNL is operated for the Department of Energy by Battelle. Received for review August 25, 2005. Accepted February 8, 2006. AC051524Q