CARBON 4 9 ( 2 0 1 1 ) 2 3 5 2 –2 3 6 1 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbonPreparation of stable carbon nanotube aerogels with highelectrical conductivity and porosityRyan R. Kohlmeyer a, Maika Lor a, Jian Deng a, Haiying Liu b, Jian Chen a,*a Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USAb Department of Chemistry, Michigan Technology University, Houghton, MI 49931, USAA R T I C L E I N F O A B S T R A C TArticle history: Stable carbon nanotube (CNT) aerogels were produced by forming a three-dimensionalReceived 18 December 2010 assembly of CNTs in solution to create a stable gel using a chemical cross-linker, followedAccepted 1 February 2011 by a CO2 supercritical drying. Thermal annealing of these aerogels in air can signiﬁcantlyAvailable online 25 February 2011 improve their electrical and mechanical properties, and increase their surface area and porosity by re-opening the originally blocked micropores and small mesopores in the as- prepared CNT aerogels. Thermally annealed CNT aerogels are mechanically stable and stiff, highly porous ($99%), and exhibit excellent electrical conductivity ($1–2 S/cm) and large speciﬁc surface area ($590–680 m2/g). Ó 2011 Elsevier Ltd. All rights reserved.1. Introduction range from 25 to 33 wt.%) are strong and electrically conduc- tive ($10À2 S/cm) . Worsley and coworkers fabricated car-Aerogels are highly porous, low-density materials comprising bon-reinforced single-walled CNT(SWCNT) aerogels (SWCNTa solid, three-dimensional (3D) nanoscale network com- loading up to 55 wt.%) by pyrolysis of a dried gel mixture ofpletely accessible to ions and molecules [1–5]. Aerogels have SWCNTs, resorcinol, and formaldehyde at 1050 °C underalready demonstrated orders of magnitude faster response nitrogen [10–12]. These carbon-reinforced SWCNT aerogelsfor sensing, energy storage, and energy conversion than other are mechanically robust and highly electrically conductivepore-solid architectures [6–8]. Carbon nanotubes (CNTs) rep- (up to 1.12 S/cm) and show speciﬁc surface area up toresent a rare material that exhibits a number of outstanding 184 m2/g, which are excellent ﬁllers for high-performanceproperties in a single material system, such as high aspect ra- polymer composites . Worsley and coworkers were alsotio, small diameter, light weight, high mechanical strength, able to use the similar approach to incorporate double-walledhigh electrical and thermal conductivities, and unique optical CNTs (DWCNTs) into a carbon aerogel, which was, however,and optoelectronic properties. By combining extraordinary limited in the amount of DWCNTs (up to 8 wt.%) that couldproperties of CNTs with those of aerogels, a new class of be incorporated into the carbon aerogel framework and inmaterials becomes accessible with unique multifunctional its ability to achieve monolithic densities below 70 mg/cm3material properties, which may ﬁnd applications in fuel cells, . Kwon and coworkers fabricated multi-walled CNTsuper capacitors, 3D batteries, advanced catalyst supports, (MWCNT)-based aerogels with aligned porous structuresenergy absorption materials, multifunctional composites, using an ice-templating process . These anisotropicchemical and biological sensors, etc. MWCNT aerogels are electrically conductive (up to Bryning and coworkers created CNT aerogels from wet 1.9 · 10À2 S/cm) and have speciﬁc surface area up to 181 m2/CNT-surfactant gel precursors, and they showed that polyvi- g. Very recently, Gui and coworkers synthesized highly porousnyl alcohol-reinforced CNT aerogels (typical CNT loadings CNT sponges containing large-diameter MWCNTs (30–50 nm) * Corresponding author: Fax: +1 414 229 5530. E-mail address: email@example.com (J. Chen).0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2011.02.001
CARBON 4 9 ( 20 1 1 ) 2 3 5 2–23 6 1 2353by a chemical vapor deposition method . These MWCNT gels were prepared in chlorobenzene according to our previ-sponges display exceptional structural ﬂexibility, excellent ous procedure . The mass ratio of CNT:chemical cross-electrical conductivity ($1.7 S/cm), and good speciﬁc surface linker (CCL) was kept at 1, 2, and 4, respectively (Table 1).area (300–400 m2/g). While this paper was in preparation, The freestanding monolithic gel was soaked in anhydrousZou and coworkers reported the synthesis of an ultralight ethanol for solvent exchange to remove the chlorobenzene.MWCNT aerogel, which shows large speciﬁc surface area The resulting wet gel in ethanol was transferred to a Tousimis(580 m2/g) and has an electrical conductivity of 3.2 · 10À2 S/ SAMDRI-PVT-3D critical point dryer. The ethanol in the wetcm that can be further increased to 0.67 S/cm by a high- gel was exchanged with liquid CO2 several times to removecurrent pulse method . the ethanol. The CO2 supercritical drying of the wet gel was In this article we report a new approach to the synthesis of carried out for 24 h above the critical temperature and pres-stable CNT aerogels. Our method involves following two dis- sure of CO2 (31.1 °C, 1072 psi) and then the chamber pressuretinctive aspects: (1) 3D chemical assembly of CNTs in solution was slowly released overnight to obtain the aerogel. No signif-to form a stable gel using a chemical cross-linker such as fer- icant sample shrinkage was observed after supercritical dry-rocene-grafted poly(p-phenyleneethynylene) (Fc-PPE, Fig. 1) ing. The as-prepared CNT aerogels were annealed in air at, followed by a CO2 supercritical drying to create stable 350 °C until the mass loss reached either $20–25 wt.% (an-aerogels; (2) thermal annealing of these aerogels in air to sig- nealed I CNT aerogels) or $41–43 wt.% (annealed II CNT aero-niﬁcantly improve their electrical and mechanical properties gels) relative to the original mass of the as-prepared CNTand enhance their surface area and porosity. We have demon- aerogel.strated the preparation of thermally annealed CNT aerogels The surface and porosity data of CNT aerogel samples werecontaining small-diameter CNTs such as SWCNTs and calculated by Brunauer–Emmett–Teller (BET) and Barrett–DWCNTs, which are mechanically stable and stiff, highly por- Joyner–Halenda (BJH) methods based on N2 adsorption–ous ($99%), and exhibit excellent electrical conductivity ($1– desorption isotherms at 77 K obtained using an ASAP 20202 S/cm) and large speciﬁc surface area ($590–680 m2/g). surface area and porosimetry analyzer (Micromeritics Instru- ment Corporation). Samples were heated at 100 °C under2. Experimental section vacuum for at least 12 h to remove any potential adsorbed species such as air, water, or organic solvents prior to the mea-Two chemical cross-linkers (Fc-PPE and Fc-PPETE, Fig. 1) were surement. For an accurate characterization of the microporoussynthesized and characterized according to literature meth- region, a separate measurement was performed at low relativeods [18,19]. Puriﬁed SWCNTsHiPco and DWCNTs were pur- pressure (P/P0 < 0.01), and the micropore volume was calcu-chased from Carbon Nanotechnologies Inc. and were used lated by t-plot theory. The two-point probe measurement forwithout further puriﬁcation. Fc-PPE–CNT and Fc-PPETE–CNT direct current electrical conductivity study was performedFig. 1 – Chemical structures of two chemical cross-linkers used in this study: (1) ferrocene-grafted poly(p-phenyleneethynylene) (Fc-PPE); (2) ferrocene-grafted poly[(p-phenyleneethynylene)-alt-(2,5-thienyleneethynylene)] (Fc-PPETE).
2354 CARBON 4 9 ( 2 0 1 1 ) 2 3 5 2 –2 3 6 1Table 1 – Properties of CNT aerogels. Sample no.a CNT CCL (wt.%)b SBET Vmeso Dmeso Porosityc rd (m2/g) (cm3/g) (nm) (%) (S/cm) 1as-prepared SWCNT Fc-PPE (20%) 327 0.88 11.2 99.4 Fragilee 1annealed I SWCNT 635 0.93 9.4 Fragile 2as-prepared DWCNT Fc-PPETE (20%) 464 1.20 11.4 99.3 Fragile 2annealed I DWCNT 679 1.44 9.2 Fragile 3as-prepared SWCNT Fc-PPE (33%) 176 0.49 13.8 98.6 1.81 · 10À1 3annealed I SWCNT 507 0.63 9.2 1.96 3annealed II SWCNT 596 0.85 9.9 8.85 · 10À1 4as-prepared DWCNT Fc-PPE (33%) 237 0.76 14.4 99.1 2.91 · 10À1 4annealed II DWCNT 684 1.22 8.6 1.78 5as-prepared DWCNT Fc-PPETE (33%) 276 0.84 13.5 99.1 1.22 · 10À1 5annealed I DWCNT 447 1.08 10.8 1.58 5annealed II DWCNT 587 1.32 10.4 1.10 6as-prepared SWCNT Fc-PPE (50%) 145 0.50 17.3 98.9 3.37 · 10À2 7as-prepared DWCNT Fc-PPE (50%) 141 0.46 15.8 98.8 2.57 · 10À2a Annealed I samples: mass loss $20–25 wt.% relative to as-prepared aerogels; annealed II samples: mass loss $41–43 wt.% relative to as-prepared aerogels. b CCL (wt.%): the chemical cross-linker and its loading. c Porosity: calculated from the aerogel density, assuming a density of 1.1 g/cm3 for SWCNT  and DWCNT  and 1.2 g/cm3 for Fc-PPE and Fc-PPETE. d Electrical conductivity. e The sample is fragile and the conductivity cannot be measured reliably.using a Keithley 2400 source meter instrument through the are ﬁlled with organic liquid. We found that SWCNTs func-computer controlled LabVIEW program. Electrical contacts to tionalized by Fc-PPE could act as gelators to gelate commonaerogel samples were made with silver paste. The Lucas Labo- organic solvents to form a freestanding organogel that cannotratories Pro4 system was used for the four-point probe mea- be redispersed in any organic solvents, indicating the robust-surement to verify the two-point probe measurement. ness of a 3D nanotube network . When the concentrationScanning electron microscopy (SEM) was performed using a of the Fc-PPE–SWCNT is sufﬁciently high, the ferrocenylHitachi S-4800 ﬁeld emission scanning electron microscope groups act as ‘‘anchoring’’ units to cross-link SWCNTs and en-(accelerating voltage: 3 kV). SEM samples were imaged without able the formation of the 3D nanotube network, which, incoating to avoid potential metal coating artifacts. Energy-dis- turn, gelates the organic solvent. It appears that the strong,persive X-ray spectroscopy (EDS) was performed with the same yet noncovalent interaction between the ferrocenyl groupSEM instrument and was calibrated with ferrocenecarboxylic and the neighboring nanotube surface allows the concertedacid. Transmission electron microscopy (TEM) was performed cross-linking among SWCNTs during the formation of theusing a Hitachi H 9000 NAR transmission electron microscope 3D nanotube network, therefore avoiding the nanotube pre-(operated at 300 kV). Attenuated total reﬂectance-Fourier cipitation from solution, which is a common and highlytransform infrared (ATR-FTIR) measurements were obtained undesirable competing process in chemical cross-linking ofon a Nexus 670 FTIR spectrometer with a Smart OMNI-Sampler nanotubes in solution.accessory containing a Germanium crystal. In this study, we used this gelation method to prepare a series of SWCNT and DWCNT organogels, which allowed us3. Results and discussion to investigate effects of different CCLs (Fc-PPE vs. Fc-PPETE, Fig. 1) and different mass ratios of CNT:CCL on the stability3.1. CNT organogels of CNT organogels and corresponding aerogels. We found that the Fc-PPE could solubilize CNTs better than the Fc-PPETE atStable CNT gels are critical precursors to stable, highly por- the same nanotube concentration. As a result, Fc-PPE–CNTous, 3D interconnected CNT aerogels [14,17,20–29]. Pristine organogels are more robust than Fc-PPETE–CNT organogels.SWCNTs and DWCNTs are not soluble in most solvents and We observed a strong correlation in mechanical stability be-do not form stable, freestanding monolithic gels because of tween the CNT organogels and corresponding CNT aerogels.weak physical interactions among CNTs. Fc-PPE–CNT aerogels consistently show better mechanical We recently developed a versatile and nondamaging stability than corresponding Fc-PPETE–CNT aerogels.chemistry platform that enabled us to engineer speciﬁc CNT The mass ratio of CNT:CCL has even more dramatic effectssurface properties, while preserving CNT’s intrinsic proper- on the mechanical stability of CNT organogels and corre-ties. We discovered that rigid conjugated macromolecules sponding CNT aerogels and surface area and porosity of as-such as PPEs could be used to noncovalently functionalize prepared CNT aerogels. Pristine CNTs (0 wt.% of CCL) do notand solubilize CNTs, and disperse CNTs homogeneously in form freestanding organogels. When the mass ratio is 4polymer matrices [17,30–38]. In an organogel, gelling agents (20 wt.% of CCL), CCL–CNT organogels are quite fragile. As(gelators) form a ﬁbrous 3D network whose interstitial spaces the mass ratio decreases to 2 (33 wt.% of CCL), CCL–CNT
CARBON 4 9 ( 20 1 1 ) 2 3 5 2–23 6 1 2355organogels become mechanically stable. As the mass ratio mation from Fig. 3 clearly indicates that, as the CCL increases,further decreases to 1 (50 wt.% of CCL), CCL–CNT organogels both micropores and small mesopores are blocked, hence con-become mechanically robust. Similarly, mechanical proper- siderably reducing the SBET of as-prepared CNT aerogels.ties of corresponding CNT aerogels increases as the mass ra- Table 1 also reveals another clear but somewhat unexpectedtio of CNT:CCL decreases. trend: a DWCNT aerogel consistently outperforms the corre- sponding SWCNT aerogel with the same type of CCL and load-3.2. As-prepared CNT aerogels ing in SBET and Vmeso (mesopore volume) when the CCL loading is at either 33 wt.% or 20 wt.%. The theoretical speciﬁc surfaceThe CO2 supercritical drying of CNT organogels prevents the area of an individual SWCNT is generally much larger than an3D nanotube network from signiﬁcant shrinkage and leads individual DWCNT . According to SEM of CNT aerogelsto low-density (7–16 mg/cm3), highly porous ($99%), mono- (Fig. 2) and TEM of as-received, puriﬁed CNTs (Fig. 4), however,lithic CNT aerogels (Table 1). SEM conﬁrms the highly porous SWCNTsHiPco form much larger bundles than DWCNTs. Theo-3D nanotube network in CNT aerogels (Fig. 2). As-prepared retical calculation shows that the external speciﬁc surface areaCNT aerogels are quite conductive considering the extremely decreases dramatically with the increase of the CNT bundlelow density: $0.1–0.3 S/cm for CNT aerogels with 33 wt.% of size [41,42]. Therefore the much smaller bundle sizes ofCCL and $0.03 S/cm for CNT aerogels with 50 wt.% of CCL (Ta- DWCNTs are probably responsible for their superior surfaceble 1). The electrical conductivity of CNT aerogels decreases area and porosity observed in this study.with the increase of the semiconducting CCL. Both SWCNT and DWCNT materials contain some impuri- The speciﬁc surface area (SBET) of an as-prepared CNT ties. According to calibrated EDS, the SWCNT material hasaerogel increases with the decrease of CCL loading (Table 1): $9.5 wt.% (2.2 at.%) of Fe and the DWCNT material has$140 m2/g for SWCNT and DWCNT aerogels with 50 wt.% of $0.2 wt.% (<0.1 at.%) of Fe. Bright-ﬁeld TEM shows the majorCCL; $180 m2/g for SWCNT aerogels and $240–280 m2/g for impurity in SWCNTsHiPco to be iron-rich nanoparticles thatDWCNT aerogels with 33 wt.% of CCL; $330 m2/g for SWCNT produce dark contrast with white Fresnel fringe at their rimaerogels and $460 m2/g for DWCNT aerogels with 20 wt.% of (Fig. 4c), while the main impurity in the DWCNT material isCCL. Similarly, Worsley and coworkers reported that carbon- hollow, graphitic carbon nanoparticles (Fig. 4f). Both impuri-reinforced SWCNT aerogels (SWCNT loading up to 55 wt.%) ties have a negative impact on the SBET of CNT aerogels be-exhibited speciﬁc surface area up to 184 m2/g [10–12]. cause such nanoparticle impurities have higher densities According to IUPAC, micropores are pores with diameters and lower speciﬁc surface areas than SWCNTs and DWCNTs.less than 2 nm, mesopores have diameters between 2 and It is worth mentioning that the nitrogen adsorption–50 nm, and macropores have diameters greater than 50 nm. desorption analysis mainly measures micropores and mesop-The N2 adsorption and desorption isotherms of as-prepared ores, therefore the large macropores observed in SEM (Fig. 2)CNT aerogels at 77 K are shown in Fig. 3a and b. The isotherms can not be experimentally evaluated using this analytical ap-of as-prepared CNT aerogels resemble type IV IUPAC isotherms proach .with a small hysteresis, suggesting pore structures are pre- Although CNT aerogels with 20 wt.% of CCL demonstratedominated by mesopores. A steep increase in N2 adsorption the best SBET and Vmeso among as-prepared CNT aerogels (Ta-at low relative pressure (P/P0 < 0.01) is characteristic of the ble 1), their mechanical weakness makes them less attractivepresence of micropores. As shown in Fig. 3a and b, both micro- materials. Conversely, CNT aerogels with 50 wt.% of CCL dis-pore and mesopore volumes signiﬁcantly decrease with the in- play the best mechanical robustness, but have low speciﬁccrease of CCL. Fig. 3c and d show the pore size distribution in surface areas. Hence as-prepared CNT aerogels with 33 wt.%as-prepared CNT aerogels. When the CCL increases from 20% of CCLs are the focus of further investigations in this study.to 33% and 50%, small mesopores (2–3 nm in SWCNT aerogels;2–4 nm in DWCNT aerogels) signiﬁcantly diminish in as-pre- 3.3. Thermally annealed CNT aerogelspared CNT aerogels (Fig. 3c and d), which also leads to the in-crease of average mesopore size (Dmeso) of as-prepared CNT While the CCL is crucial to form stable CNT organogels andaerogels with the increase of CCL (Table 1). The collective infor- aerogels, we found that it could substantially block micropores Fig. 2 – SEM images of (a) the 3as-prepared SWCNT aerogel and (b) 4as-prepared DWCNT aerogel. Scale bar: 200 nm.
2356 CARBON 4 9 ( 2 0 1 1 ) 2 3 5 2 –2 3 6 1Fig. 3 – N2 adsorption and desorption isotherms of (a) as-prepared SWCNT aerogels and (b) as-prepared DWCNT aerogels. Poresize distributions of (c) as-prepared SWCNT aerogels and (d) as-prepared DWCNT aerogels.(<2 nm) and small mesopores (2–4 nm) and reduce the speciﬁc that the removal of polymer side chains predominates in thesurface area and mesopore volume of CNT aerogels. To solve annealing I step with typical 20–25 wt.% loss. The averagethis dilemma, we developed a simple yet effective post- mass-loss rate of annealed I CNT aerogels in the annealingannealing approach, which can signiﬁcantly enhance the II step is much slower: $0.3 wt.% loss/min for the 3annealed Ielectrical and mechanical properties and enhance the surface SWCNT aerogel and $0.2 wt.% loss/min for the 4annealed IIarea and porosity of CNT aerogels. Thermal annealing is par- DWCNT aerogel. This suggests that the decomposition ofticularly effective for CNT aerogels because aerogels have 3D polymer backbones, carbon impurities associated with CNTs,interconnected porous networks, which ensure a uniform and defective CNTs probably predominates in the annealing IIheat treatment. stage. Thermal decomposition of dialkoxy-PPEs in air involves The FTIR spectroscopy has proven to be an important tooltwo steps: a rapid cleavage of polymer side chains starting to investigate the interaction between CNTs and molecules/at 220 °C followed by a slow decomposition of conjugated macromolecules. We found previously that the infrared vibra-polymer backbones [44,45]. As shown in Figs. 5 and 6, as- tions of adsorbed molecules/macromolecules that give rise toprepared CNT aerogels with 33% of Fc-PPE could undergo uni- dipole changes parallel to the highly polarizable CNT surfaceform and stepwise annealing in air at 350 °C, which is below are diminished signiﬁcantly in intensity . The CNT surfacethe decomposition temperatures of puriﬁed SWCNTs and attenuated infrared absorption (CNT SAIRA) therefore pro-DWCNTs [46,47], to generate annealed I and II CNT aerogels vides a mechanism to probe and compare the overall surfacewith either reduced or similar density, respectively. The aver- qualities of various bulk CNT materials as well as the interac-age mass-loss rate of as-prepared CNT aerogels with 33% of tion strength between CNTs and molecules/macromoleculesFc-PPE in the annealing I step is relatively fast and insensitive [34,48]. The IR spectrum of pure Fc-PPE shows a number ofto CNT materials: $1.2 wt.% loss/min for the 3as-prepared characteristic vibration modes (Fig. 7): (1) mas(CH2) (2922 cmÀ1)SWCNT aerogel and $1.4 wt.% loss/min for the 4as-prepared from polymer side chains; (2) m(C@O) (1713 cmÀ1), side chains;DWCNT aerogel. As-prepared CNT aerogels with 33% of Fc- (3) mas(COC) (1275 and 1213 cmÀ1), side chains; (4) m(cyclopenta-PPE have $28 wt.% of polymer side chains. These data suggest dienyl ring (Cp ring)) (1136 cmÀ1), side chains; (5) ds(CH2)
CARBON 4 9 ( 20 1 1 ) 2 3 5 2–23 6 1 2357 Fig. 4 – TEM images of (a–c) SWCNTsHiPco and iron nanoparticles, and (d–f) DWCNTs and graphitic carbon nanoparticles.Fig. 5 – Stepwise annealing of a 3as-prepared SWCNT aerogel with 33 wt.% of Fc-PPE (left, mass: 29.6 mg; density: 9.9 mg/cm3) inair at 350 °C, which led to the 3annealed I aerogel (middle, mass: 22.8 mg; density: 9.3 mg/cm3) and 3annealed II aerogel (right,mass: 17.4 mg; density: 9.8 mg/cm3), sequentially.Fig. 6 – Stepwise annealing of a 4as-prepared DWCNT aerogel with 33 wt.% of Fc-PPE (left, mass: 29.3 mg; density: 9.8 mg/cm3) inair at 350 °C, which led to the 4annealed I aerogel (middle, mass: 21.9 mg; density: 8.9 mg/cm3) and 4annealed II aerogel (right,mass: 17.3 mg; density: 8.0 mg/cm3), sequentially.(1463 cmÀ1), side chains; (6) ds(CH3) (1388 cmÀ1), side chains; (7) and, therefore, their IR intensities are reduced dramatically.m(C@C) (1514 and 1429 cmÀ1), backbones. As compared to pure Most importantly, the relatively sharp m(C@O) and m(Cp ring)PPE, most of IR absorptions arising from Fc-PPE in the modes, which are associated with ferrocenyl groups at the3as-prepared SWCNT aerogel diminish signiﬁcantly in intensity end of polymer side chains (Fig. 1), also diminish signiﬁcantly.(Fig. 7a), thanks to the CNT SAIRA . Since the mas(CH2) mode This indicates a substantial interaction between ferrocenylcauses primarily a dipole change perpendicular to the CNT groups and neighboring SWCNT surfaces in the 3as-preparedsurface, its IR intensity is expected to remain unchanged. In SWCNT aerogel.contrast, the m(C@C), ds(CH2), ds(CH3), and mas(COC) modes all The CNT SAIRA is highly sensitive to the distance betweengive rise to net dipole changes parallel to the CNT surface adsorbed molecules/macromolecules and the CNT surface as
2358 CARBON 4 9 ( 2 0 1 1 ) 2 3 5 2 –2 3 6 1Fig. 7 – ATR-FTIR spectra of pure Fc-PPE and (a) 3as-prepared SWCNT aerogel and (b) 4as-prepared DWCNT aerogel before and afterthermal annealings. The spectra of pure Fc-PPE and as-prepared CNT aerogel were normalized at the 2922 cmÀ1 peak andwere offset vertically for better visual comparison in each series, respectively.well as the degree of nanotube conjugation and surface clean- lently attached to CNT surfaces, or could arise from residualliness . Unlike SWCNTsHiPco, as-received, puriﬁed carbon impurities (e.g. oxidized amorphous carbon after puri-DWCNTs have detectable amount of carbonyl groups based ﬁcation) that are noncovalently adsorbed on CNT surfaceson previous study . These carbonyl groups could be cova- . The existence of these carbonyl groups, which show aFig. 8 – N2 adsorption and desorption isotherms of (a) as-prepared SWCNT aerogels and (b) as-prepared DWCNT aerogelsbefore and after thermal annealings. Pore size distributions of (c) as-prepared SWCNT aerogels and (d) as-prepared DWCNTaerogels before and after thermal annealings.
CARBON 4 9 ( 20 1 1 ) 2 3 5 2–23 6 1 2359very broad IR absorption in the region of 1800–1650 cmÀ1 , Fig. 8 shows that thermal annealing has a dramatic impacton DWCNT surfaces can considerably interfere with the p–p on pore structures of CNT aerogels. As shown in Fig. 8a and b,interaction between Fc-PPE and the nanotube surface and, thermal annealing substantially increases the micropore vol-as a result, increase their distance of separation. Therefore ume at low relative pressure (P/P0 < 0.01) in CNT aerogels, par-DWCNTs could only partially reduce IR intensities of ad- ticularly in the thermal annealing I phase, which issorbed Fc-PPE in the 4as-prepared DWCNT aerogel between predominated by the removal of polymer side chains. The1800 and 1000 cmÀ1 (Fig. 7b). The substantial intensity of the data in Fig. 8a and b as well as Fig. 3a and b indicate clearlyrelatively sharp m(C@O) and m(Cp ring) modes, which are asso- that polymer side chains of CCL block most micropores andciated with ferrocenyl groups (Fig. 7b), suggests the interac- thermal annealing re-opens these blocked micropores bytion between ferrocenyl groups and neighboring DWCNT removing polymer side chains. The micropore volumes forsurfaces is weaker in the 4as-prepared DWCNT aerogel than that 3annealed II SWCNT aerogel and 4annealed II DWCNT aerogelin the 3as-prepared SWCNT aerogel (Fig. 7a). There is little free are 0.17 and 0.12 cm3/g, respectively. Although thermalFc-PPE in the 4as-prepared DWCNT aerogel because the Fc-PPE annealing signiﬁcantly increases the micropore volumes,is only 33 wt.%, which is far less than the experimental satura- the pore structures in annealed aerogel samples are still pre-tion loading ($50 wt.% of Fc-PPE). Hence it is unlikely that IR sig- dominated by mesopores.nals observed in the region of 1800–1000 cmÀ1 in the 4as-prepared Thermal annealing’s effects on mesopore sizes of CNTDWCNT aerogel arise primarily from free Fc-PPEs. aerogels are also signiﬁcant and, again, most dramatic After the thermal annealing I, the mas(CH2) mode, which is changes occur in the thermal annealing I phase (Fig. 8c andassociated with polymer side chains and not affected by the d). After the annealing I in air at 350 °C, small mesoporesCNT SAIRA effect, disappears as expected. This lends further (2–3 nm in SWCNT aerogels; 2–4 nm in DWCNT aerogels) re-support to the notion that the removal of polymer side chains appear, thanks to the removal of polymer side chains. Uponpredominates in the annealing I step. The interpretation of further heat treatment in the annealing II, the peak pore sizethe change in the region of 1800–1000 cmÀ1 is complicated of small mesopores become larger in SWCNT aerogels (fromby the CNT SAIRA effect. We do notice, however, that all sharp 1.8 to 2.5 nm) but smaller in DWCNT aerogels (from 2.7 toIR signals below 1800 cmÀ1 in the DWCNT aerogel either dis- 2.4 nm). The origin of such difference is not fully understoodappear and/or convert to new broad features, which are not at present.due to DWCNTs, after the thermal annealing I (Fig. 7b). These Fig. 8 reveals that thermal annealing in air is a simple yetnew broad features disappear after the thermal annealing II. effective method for re-opening the originally blockedThe IR spectra of CNT aerogels after the thermal annealing micropores and small mesopores in as-prepared CNT aero-II are essentially identical to those of as-received, puriﬁed gels. As a result, speciﬁc surface areas of annealed CNT aero-SWCNTs and DWCNTs, respectively. gels increase dramatically by $50–240% (Table 1).Fig. 9 – A 3annealed II SWCNT aerogel (left, 17.4 mg, density: 9.8 mg/cm3); the same sample supporting a 20 g weight (middle,$1150 times its own weight); the same sample after removal of the weight (right).Fig. 10 – A 4annealed II aerogel (left, 17.3 mg, density: 8.0 mg/cm3); the same sample supporting 15 g of total weights (middle,$870 times its own weight); the same sample after removal of weights (right).
2360 CARBON 4 9 ( 2 0 1 1 ) 2 3 5 2 –2 3 6 1 Despite losing $41–43 wt.% of the original mass, the light-  Pierre AC, Pajonk GM. Chemistry of aerogels and theirdensity, annealed II SWCNT and DWCNT aerogels could still applications. Chem Rev 2002;102:4243–65.support the same amount of weight as corresponding as-  Rolison DR. Catalytic nanoarchitectures-the importance of nothing and the unimportance of periodicity. Scienceprepared aerogels do without deformation. This corresponds 2003;299:1698–701.to $1150 times the annealed II SWCNT aerogel’s weight and  Long JW, Rolison DR. Architectural design, interior$870 times the annealed II DWCNT aerogel’s weight, respec- decoration, and three-dimensional plumbing en route totively (Figs. 9 and 10). The thermal annealing also substan- multifunctional nanoarchitectures. Acc Chem Restially increases the electrical conductivity of CNT aerogels 2007;40:854–62.by a factor of 6–13 (Table 1). The two-probe measurement re-  Long JW, Dunn B, Rolison DR, White HS. Three-dimensionalsults have been conﬁrmed by the four-probe measurement. battery architectures. Chem Rev 2004;104:4463–92.  Aerogels 7. In: Rolison DR, editor. Proceedings of the 7thFor example, the two-point probe/four-point probe conductiv- International Symposium on aerogels. J Non-Cryst Solidsities of the 4annealed II DWCNT aerogel are 1.78/1.84 S/cm. 2004;350:1–404.These combined observations strongly suggest the annealing-  Rolison DR, Dunn B. Electrically conductive oxide aerogels:induced reinforcement of CNT–CNT junctions in CNT aerogels, new materials in electrochemistry. J Mater Chemwhich will be a subject of future study. 2001;11:963–80.  Bryning MB, Milkie DE, Islam MF, Hough LA, Kikkawa JM, Yodh AG. Carbon nanotube aerogels. Adv Mater4. Summary 2007;19:661–4.  Worsley MA, Kucheyev SO, Satcher JH, Hamza AV, BaumannWe have developed a new approach to the synthesis of stable TF. Mechanically robust and electrically conductive carbonCNT aerogels. The approach involves three steps: (1) 3D chem- nanotube foams. Appl Phys Lett 2009;94:073115.ical assembly of CNTs in solution to form a stable gel using a  Worsley MA, Pauzauskie PJ, Kucheyev SO, Zaug JM, Hamzachemical cross-linker; (2) CO2 supercritical drying of CNT gels AV, Satcher JH, et al. Properties of single-walled carbon nanotube-based aerogels as a function of nanotube loading.to create stable aerogels; (3) thermal annealing of these aero- Acta Mater 2009;57:5131–6.gels in air to signiﬁcantly enhance their electrical and mechan-  Worsley MA, Kucheyev SO, Kuntz JD, Hamza AV, Satcher JH,ical properties, and enhance their surface area and porosity. Baumann TF. Stiff and electrically conductive composites ofWe have demonstrated the preparation of thermally annealed carbon nanotube aerogels and polymers. J Mater ChemCNT aerogels containing small-diameter CNTs such as 2009;19:3370–2.SWCNTs and DWCNTs, which are mechanically stable and  Worsley MA, Satcher JH, Baumann TF. Synthesis andstiff, highly porous ($99%), and exhibit excellent electrical con- characterization of monolithic carbon aerogel nanocomposites containing double-walled carbonductivity ($1–2 S/cm) and large speciﬁc surface area ($590– nanotubes. Langmuir 2008;24:9763–6.680 m2/g). We have found that thermal annealing in air is a  Kwon SM, Kim HS, Jin HJ. Multiwalled carbon nanotubesimple yet effective method for re-opening the originally cryogels with aligned and non-aligned porous structures.blocked micropores and small mesopores in the as-prepared Polymer 2009;50:2786–92.CNT aerogels.  Gui X, Wei J, Wang K, Cao A, Zhu H, Jia Y, et al. Carbon nanotube sponges. Adv Mater 2010;22:617–21.  Zou J, Liu J, Karakoti AS, Kumar A, Joung D, Li Q, et al.Acknowledgements Ultralight multiwalled carbon nanotube aerogel. ACS Nano 2010;4:7293–302.J.C. thanks the ﬁnancial support from the National Science  Chen J, Xue C, Ramasubramaniam R, Liu HY. A new methodFoundation (DMI-06200338), UWM start-up fund, UWM Re- for the preparation of stable carbon nanotube organogels. Carbon 2006;44:2142–6.search Growth Initiative award, and the Lynde and Harry  Xue C, Chen Z, Wen Y, Luo FT, Chen J, Liu HY. Synthesis ofBradley Foundation. We thank Magda Salama in the Materials ferrocene-grafted poly(p-phenylene-ethynylenes) and controlResearch Institute at the Pennsylvania State University for of electrochemical behaviors of their thin ﬁlms. Langmuircollecting all of the surface area and porosity data and invalu- 2005;21:7860–5.able discussions. We thank Prof. Marija Gajdardziska-  Pang Y, Li J, Barton TJ. Processible poly[(p-Josifovska for TEM access at the HRTEM Laboratory of Univer- phenyleneethynylene)-alt-(2,5-thienyleneethynylene)]s ofsity of Wisconsin-Milwaukee and invaluable discussions, and high luminescence. their synthesis and physical properties. J Mater Chem 1998;8:1687–90.Donald Robertson for TEM analysis. We also thank the  Fukushima T, Kosaka A, Ishimura Y, Yamamoto T, Takigawaﬁnancial support from the National Science Foundation T, Ishii N, et al. Molecular ordering of organic molten salts(CHE-0723002) for the acquisition of a ﬁeld emission scanning triggered by single-walled carbon nanotubes. Scienceelectron microscope (Hitachi S-4800) at UWM. 2003;300:2072–4.  Kovtyukhova NI, Mallouk TE, Pan L, Dickey EC. Individual single-walled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes. J Am Chem SocR E F E R E N C E S 2003;125:9761–9.  Hough LA, Islam MF, Janmey PA, Yodh AG. Viscoelasticity of single wall carbon nanotube suspensions. Phys Rev Lett  Gesser HD, Goswami PC. Aerogels and related porous 2004;93:168102. materials. Chem Rev 1989;89:765–88.  Sabba Y, Thomas EL. High-concentration dispersion of  Husing N, Schubert U. Aerogels-airy materials: chemistry, ¨ single-wall carbon nanotubes. Macromolecules structure, and properties. Angew Chem Int Ed 1998;37:23–45. 2004;37:4815–20.
CARBON 4 9 ( 20 1 1 ) 2 3 5 2–23 6 1 2361 Wang Z, Chen Y. Supramolecular hydrogels hybridized with  Pradhan B, Kohlmeyer RR, Setyowati K, Chen J. Electron single-walled carbon nanotubes. Macromolecules doping of small-diameter carbon nanotubes with exohedral 2007;40:3402–7. fullerenes. Appl Phys Lett 2008;93:223102. Srinivasan S, Babu SS, Praveen VK, Ajayaghosh A. Carbon  Pradhan B, Kohlmeyer RR, Setyowati K, Owen HA, Chen J. nanotube triggered self-assembly of oligo(p- Advanced carbon nanotube/polymer composite infrared phenylenevinylene)s to stable hybrid p-gels. Angew Chem Int sensors. Carbon 2009;47:1686–92. Ed 2008;47:5746–9.  Arnold M, Green A, Hulvat J, Stupp S, Hersam M. Sorting Klink M, Ritter H. Supramolecular gels based on multi-walled carbon nanotubes by electronic structure using density carbon nanotubes bearing covalently attached cyclodextrin differentiation. Nature Nanotech 2006;1:60–5. and water-soluble guest polymers. Macromol Rapid Commun  Green A, Hersam M. Processing and properties of highly 2008;29:1208–11. enriched double-wall carbon nanotubes. Nature Nanotech Moniruzzaman M, Sahin A, Winey KI. Improved mechanical 2009;4:64–70. strength and electrical conductivity of organogels containing  Peigney A, Laurent Ch, Flahaut E, Bacsa RR, Rousset A. carbon nanotubes. Carbon 2009;47:645–50. Speciﬁc surface area of carbon nanotubes and bundles of You YZ, Yan JJ, Yu ZQ, Cui MM, Hong CY, Qu BJ. Multi- carbon nanotubes. Carbon 2001;39:507–14. responsive carbon nanotube gel prepared via ultrasound-  Williams KA, Eklund PC. Monte Carlo simulations of H2 induced assembly. J Mater Chem 2009;19:7656–60. physisorption in ﬁnite-diameter carbon nanotube ropes. Oh H, Jung BM, Lee HP, Chang JY. Dispersion of single walled Chem Phys Lett 2000;320:352–8. carbon nanotubes in organogels by incorporation into  Zhang X, Chang D, Liu J, Luo Y. Conducting polymer aerogels organogel ﬁbers. J Colloid Interface Sci 2010;352:121–7. from supercritical CO2 drying PEDOT-PSS hydrogels. J Mater Chen J, Liu H, Weimer WA, Halls MD, Waldeck DH, Walker GC. Chem 2010;20:5080–5. Noncovalent engineering of carbon nanotube surfaces by  Moroni M, Le Moigne J, Luzzati S. Rigid rod conjugated rigid, functional conjugated polymers. J Am Chem Soc polymers for nonlinear optics. 1. Characterization and linear 2002;124:9034–5. optical properties of poly(aryleneethyny1ene) derivatives. Ramasubramaniam R, Chen J, Liu H. Homogeneous carbon Macromolecules 1994;27:562–71. nanotube/polymer composites for electrical applications.  Bunz UHF. Poly(aryleneethynylene)s: syntheses, Appl Phys Lett 2003;83:2928–30. properties, structures, and applications. Chem Rev Chen J, Ramasubramaniam R, Xue C, Liu H. A versatile, 2000;100:1605–44. molecular engineering approach to simultaneously  Chiang IW, Brinson BE, Huang AY, Willis PA, Bronikowski MJ, enhanced, multifunctional carbon nanotube-polymer Margrave JL, et al. Puriﬁcation and characterization of single- composites. Adv Funct Mater 2006;16:114–9. wall carbon nanotubes (SWNTs) obtained from the gas-phase Sankapal B, Setyowati K, Chen J, Liu H. Electrical properties of decomposition of CO (HiPco process). J Phys Chem B air-stable, iodine-doped carbon nanotube-polymer 2001;105:8297–301. composites. Appl Phys Lett 2007;91:173103.  Muramatsu H, Hayashi T, Kim YA, Shimamoto D, Kim YJ, Setyowati K, Piao MJ, Chen J, Liu H. Carbon nanotube surface Tantrakarn K, et al. Pore structure and oxidation stability of attenuated infrared absorption. Appl Phys Lett double-walled carbon nanotube-derived bucky paper. Chem 2008;92:043105. Phys Lett 2005;414:444–8. Pradhan B, Setyowati K, Liu H, Waldeck DH, Chen J. Carbon ´ ´  Kamaras K, Botka B, Pekker A, Ben-Valid S, Zeng A, Reiss L, nanotube-polymer nanocomposite infrared sensor. Nano et al. Surface-induced changes in the vibrational spectra of Lett 2008;8:1142–6. conducting polymer–carbon nanotube hybrid materials. Phys Yang L, Setyowati K, Li A, Gong S, Chen J. Reversible infrared Status Solidi B 2009;246:2737–9. actuation of carbon nanotube–liquid crystalline elastomer nanocomposites. Adv Mater 2008;20:2271–5.