- 1. CARBON 4 9 ( 2 0 1 1 ) 2 3 5 2 2 3 6 1 available at
www.sciencedirect.comjournal 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
Chena,*aDepartment of Chemistry and Biochemistry, University of
Wisconsin-Milwaukee, Milwaukee, WI 53211, USAbDepartment of
Chemistry, Michigan Technology University, Houghton, MI 49931, USAA
R T I C L E I N F OA B S T R A C TArticle history: Stable carbon
nanotube (CNT) aerogels were produced by forming a
three-dimensionalReceived 18 December 2010assembly 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 signicantlyAvailable
online 25 February 2011improve 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 ($12 S/cm) and large specic
surface area ($590680 m2/g). 2011 Elsevier Ltd. All rights
reserved.1.Introduction range from 25 to 33 wt.%) are strong and
electrically conduc- tive ($102 S/cm) [9]. 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 [15]. Aerogels haveSWCNTs, resorcinol, and
formaldehyde at 1050 C underalready demonstrated orders of
magnitude faster response nitrogen [1012]. These carbon-reinforced
SWCNT aerogelsfor sensing, energy storage, and energy conversion
than otherare mechanically robust and highly electrically
conductivepore-solid architectures [68]. Carbon nanotubes (CNTs)
rep- (up to 1.12 S/cm) and show specic surface area up toresent a
rare material that exhibits a number of outstanding 184 m2/g, which
are excellent llers for high-performanceproperties in a single
material system, such as high aspect ra-polymer composites [12].
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 extraordinarylimited in the amount of DWCNTs (up to 8
wt.%) that couldproperties of CNTs with those of aerogels, a new
class ofbe 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 nd applications in fuel cells, [13]. 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 [14].
These anisotropicchemical and biological sensors, etc.MWCNT
aerogels are electrically conductive (up toBryning and coworkers
created CNT aerogels from wet1.9 102 S/cm) and have specic 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 loadingsCNT
sponges containing large-diameter MWCNTs (3050 nm) * Corresponding
author: Fax: +1 414 229 5530. E-mail address: [email protected] (J.
Chen).0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights
reserved.doi:10.1016/j.carbon.2011.02.001
2. CARBON 4 9 ( 20 1 1 ) 2 3 5 223 6 12353by a chemical vapor
deposition method [15]. These MWCNTgels were prepared in
chlorobenzene according to our previ-sponges display exceptional
structural exibility, excellent ous procedure [17]. The mass ratio
of CNT:chemical cross-electrical conductivity ($1.7 S/cm), and good
specic surfacelinker (CCL) was kept at 1, 2, and 4, respectively
(Table 1).area (300400 m2/g). While this paper was in
preparation,The freestanding monolithic gel was soaked in
anhydrousZou and coworkers reported the synthesis of an
ultralightethanol for solvent exchange to remove the
chlorobenzene.MWCNT aerogel, which shows large specic surface
areaThe resulting wet gel in ethanol was transferred to a
Tousimis(580 m2/g) and has an electrical conductivity of 3.2 102 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 [16].
the ethanol. The CO2 supercritical drying of the wet gel wasIn 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[17], followed by a CO2
supercritical drying to create stable350 C until the mass loss
reached either $2025 wt.% (an-aerogels; (2) thermal annealing of
these aerogels in air to sig- nealed I CNT aerogels) or $4143 wt.%
(annealed II CNT aero-nicantly 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
aerogelsThe surface and porosity data of CNT aerogel samples
werecontaining small-diameter CNTs such as SWCNTs andcalculated by
BrunauerEmmettTeller (BET) and BarrettDWCNTs, which are
mechanically stable and stiff, highly por- JoynerHalenda (BJH)
methods based on N2 adsorptionous ($99%), and exhibit excellent
electrical conductivity ($1 desorption isotherms at 77 K obtained
using an ASAP 20202 S/cm) and large specic surface area ($590680
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)
weresurement. For an accurate characterization of the
microporoussynthesized and characterized according to literature
meth-region, a separate measurement was performed at low
relativeods [18,19]. Puried 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 purication. Fc-PPECNT and Fc-PPETECNT 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). 3. 2354CARBON4 9 ( 2 0 1 1 ) 2 3 5 2 2 3 6 1Table 1
Properties of CNT aerogels.Sample no.a CNTCCL (wt.%)bSBETVmesoDmeso
Porositycrd (m2/g)(cm3/g) (nm) (%) (S/cm)1as-preparedSWCNTFc-PPE
(20%)327 0.8811.2 99.4Fragilee1annealed I SWCNT635 0.93 9.4
Fragile2as-preparedDWCNTFc-PPETE (20%)464 1.2011.4
99.3Fragile2annealed I DWCNT679 1.44 9.2
Fragile3as-preparedSWCNTFc-PPE (33%)176 0.4913.8 98.61.81
1013annealed I SWCNT507 0.63 9.2 1.963annealed IISWCNT596 0.85 9.9
8.85 1014as-preparedDWCNTFc-PPE (33%)237 0.7614.4 99.12.91
1014annealed IIDWCNT684 1.22 8.6 1.785as-preparedDWCNTFc-PPETE
(33%)276 0.8413.5 99.11.22 1015annealed I DWCNT447 1.0810.8
1.585annealed IIDWCNT587 1.3210.4 1.106as-preparedSWCNTFc-PPE
(50%)145 0.5017.3 98.93.37 1027as-preparedDWCNTFc-PPE (50%)141
0.4615.8 98.82.57 102a Annealed I samples: mass loss $2025 wt.%
relative to as-prepared aerogels; annealed II samples: mass loss
$4143 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 [39]
and DWCNT [40] 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 theare lled with organic liquid. We found that SWCNTs
func-computer controlled LabVIEW program. Electrical contacts
totionalized 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 [17]. When the concentrationScanning electron microscopy
(SEM) was performed using a of the Fc-PPESWCNT is sufciently high,
the ferrocenylHitachi S-4800 eld emission scanning electron
microscopegroups act as anchoring units to cross-link SWCNTs and
en-(accelerating voltage: 3 kV). SEM samples were imaged
withoutable 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 microscope3D nanotube network, therefore avoiding the
nanotube pre-(operated at 300 kV). Attenuated total
reectance-Fouriercipitation 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 discussionto investigate
effects of different CCLs (Fc-PPE vs. Fc-PPETE, Fig. 1) and
different mass ratios of CNT:CCL on the stability3.1. CNT
organogelsof 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-PPECNTous, 3D
interconnected CNT aerogels [14,17,2029]. Pristineorganogels are
more robust than Fc-PPETECNT 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 oftween the CNT organogels and corresponding CNT
aerogels.weak physical interactions among CNTs. Fc-PPECNT aerogels
consistently show better mechanicalWe recently developed a
versatile and nondamagingstability than corresponding Fc-PPETECNT
aerogels.chemistry platform that enabled us to engineer specic
CNTThe mass ratio of CNT:CCL has even more dramatic effectssurface
properties, while preserving CNTs 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 functionalizeprepared CNT aerogels. Pristine
CNTs (0 wt.% of CCL) do notand solubilize CNTs, and disperse CNTs
homogeneously inform freestanding organogels. When the mass ratio
is 4polymer matrices [17,3038]. In an organogel, gelling agents (20
wt.% of CCL), CCLCNT organogels are quite fragile. As(gelators)
form a brous 3D network whose interstitial spacesthe mass ratio
decreases to 2 (33 wt.% of CCL), CCLCNT 4. CARBON4 9 ( 20 1 1 ) 2 3
5 223 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), CCLCNT
organogelsboth 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 specic surfaceThe CO2 supercritical drying of CNT
organogels prevents thearea of an individual SWCNT is generally
much larger than an3D nanotube network from signicant shrinkage and
leadsindividual DWCNT [41]. According to SEM of CNT aerogelsto
low-density (716 mg/cm3), highly porous ($99%), mono-(Fig. 2) and
TEM of as-received, puried CNTs (Fig. 4), however,lithic CNT
aerogels (Table 1). SEM conrms the highly porous SWCNTsHiPco form
much larger bundles than DWCNTs. Theo-3D nanotube network in CNT
aerogels (Fig. 2). As-preparedretical calculation shows that the
external specic surface areaCNT aerogels are quite conductive
considering the extremelydecreases dramatically with the increase
of the CNT bundlelow density: $0.10.3 S/cm for CNT aerogels with 33
wt.% ofsize [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 decreasesarea and porosity
observed in this study.with the increase of the semiconducting CCL.
Both SWCNT and DWCNT materials contain some impuri-The specic
surface area (SBET) of an as-prepared CNTties. 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.% (
