Fabrication and characterization of three-dimensional
macroscopic all-carbon scaffolds
Gaurav Lalwani, Andrea Trinward Kwaczala, Shruti Kanakia, Sunny C. Patel, Stefan Judex,
Department of Biomedical Engineering, Stony Brook University, Stony Brook, NY 11794-5281, United States
A R T I C L E I N F O
Received 8 August 2012
Accepted 15 October 2012
Available online 24 October 2012
A B S T R A C T
We report a simple method to fabricate macroscopic, 3-D, free standing, all-carbon scaf-
folds (porous structures) using multiwalled carbon nanotubes (MWCNTs) as the initial
materials. The scaffolds prepared by radical initiated thermal crosslinking, and annealing
of MWCNTs possess macroscale interconnected pores, robust structural integrity, stability,
and electrical conductivity. The porosity of the three-dimensional structure can be con-
trolled by varying the amount of radical initiator, thereby allowing the design of porous
scaffolds tailored towards specific potential applications. This method also allows the fab-
rication of 3-D scaffolds using other carbon nanomaterials such as single-walled carbon
nanotubes, fullerenes, and graphene indicating that it could be used as a versatile method
for 3-D assembly of carbon nanostructures with pi bond networks.
? 2012 Elsevier Ltd. All rights reserved.
The development of three-dimensional (3-D) all carbon scaf-
folds (porous structures) could lead to significant advance-
ments in the field of energy storage, electronic devices, high
performance catalysts, super capacitors, photovoltaic cells,
field emission devices, smart sensors, and biomedical devices
and implants [1–6]. 3-D microscopic scaffolds using carbon
nanotubes have been successfully assembled by ‘‘bottom-
up’’ (e.g. chemical vapor deposition) or ‘‘top-down’’ (e.g. capil-
lary-induced self-assembly) approaches [7–12]. Using these
strategies, microscopic 3-D random or patterned structures
comprised of either aligned or entangled carbon nanotubes
have been synthesized. Macroscopic scale (>1 mm in two or
all three dimensions) structures of vertically aligned or entan-
gled networks of pristine CNTs and graphene have also been
fabricated [13–22]. However, the suitability of these ap-
proaches to control the porosity of the 3-D CNT structures
or to form covalent bonds between CNTs, an important fea-
ture for many applications  still has to be demonstrated.
Furthermore, the potential of these techniques to synthesize
3-D macroscale structures using other carbon nanomaterials
such as fullerenes and graphene still needs to be investigated.
Additionally, these approaches may present a practical chal-
lenge to develop macroscopic-scale (>1 mm in all 3 dimen-
sions) carbon devices; either due to scalability issues, or
high operational cost.
Towards the goal of fabricating 3-D all-carbon devices
with macroscopic dimensions, we report the synthesis, and
characterization of macroscopic, structurally-stable 3-D, all-
carbon scaffold using MWCNTs. We also demonstrate that
this facile method can in general be applied to fabricate 3-
D, all-carbon scaffolds with different architectures (such as
cylinders, disk etc.) using other carbon nanomaterials such
carbon nanotubes, and
0008-6223/$ - see front matter ? 2012 Elsevier Ltd. All rights reserved.
* Corresponding author.
E-mail address: email@example.com (B. Sitharaman).
C A R B O N 53 ( 2 01 3 ) 90 –1 0 0
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
2. Materials and methods
2.1.Fabrication of 3-D crosslinked carbon scaffolds
659258), single walled carbon nanotubes (Sigma–Aldrich, Cat
No. 519308), fullerenes (Sigma–Aldrich, Cat No. 483036), ben-
zoyl peroxide (BP, Luperox?, Sigma–Aldrich, Cat No. 179981)
and chloroform (CHCl3, Fisher Scientific, Cat No. BPC297) were
used as purchased. Graphene nanoplatelets were synthesized
and characterized by a literature method, and have been re-
ported elsewhere . The MWCNT scaffolds were fabricated
by mixing MWCNT andBP
(MWCNT:BP = 1:0.5, 1:1, 1:2, 1:3 and 1:4). 1 ml CHCl3was added
to the mixture to dissolve, and ensure uniform dispersion of
BP (see Supplementary information Fig. S2 for the dispersion
state of MWCNTs). The fullerenes, SWCNT and graphene
nanoplatelet scaffolds were prepared by mixing BP with these
carbon nanomaterials in the ratio 1:1. The BP-carbon nano-
material mixture was subjected to bath sonication (30 min,
Ultrasonicator FS30H, Fischer Scientific, Pittsburgh, PA),
poured in custom machined Teflon molds (length = 1.2 mm,
diameter = 0.5 mm), and incubated at 60 ?C for 24 h. Post incu-
bation, the molds were disassembled to obtain the cross-
linked three-dimensional carbon scaffolds. Five scaffolds
were prepared for each experimental group. As a purification
step after crosslinking, scaffolds were placed at 150 ?C for
20 min to remove the excess BP.
carbon nanotubes (Sigma–Aldrich,Cat No.
at differentmass ratios
Raman analysis was performed using a WITec alpha300R Mi-
cro-Imaging Raman Spectrometer using a 532 nm Nd-YAG
excitation laser. Point spectra were recorded between 50 and
3750 cm?1at room temperature.
2.3.Thermogravimetric analysis (TGA)
TGA was performed using a Pyris Perkin Elmer diamond TGA
instrument at the Center for Functional Nanomaterials (CFN),
Brookhaven National Laboratory, New York. Measurements
were conducted on samples in alumina pan from 50 to
800 ?C with a heating rate of 10 ?C/min under an air flow of
Mechanical properties of purified MWCNT scaffolds were
determined using nanoindentation (Triboindenter; Hysitron,
Minneapolis, MN) with a Berkovich indenter tip. MWCNT
scaffolds were attached to metal disks using cyanocryolate
and mounted into the indenter. The points of indentation
were selected at a distance no less than 100 lm away from
each other. Samples were indented 7 times to determine elas-
tic modulus (Er) and material hardness (H). The tip area func-
tion was calibrated from indentation analysis on fused
quartz, and drift rates in the system were measured prior to
each indentation using standard indentation testing proce-
dures . First, a preload of 3 lN was applied to the system
followed by a constant loading rate (10 lN/s). Then a hold seg-
ment at a fixed system load was applied, followed by a con-
stant unloading rate to retract the tip (?10 lN/s), then
another hold segment was imposed (3 lN). The sample was
indented with peak loads ranging from ?15 lN to 100 lN
. The elastic response was calculated from the 20% to
90% portion of the unloading curve using methods previously
Micro-CT analysis was performed to quantify the 3-D poros-
ity of MWCNT scaffolds. A Scanco Medical microCT-40
(Scanco Medical AG, Bassersdorf, Switzerland) was used at
45 kV, 177 lA current and 900 ms integration time. A 3-D
Gaussian filter was applied to the images and a global
threshold separated carbon nanotubes from noise . The
threshold value was determined by visual comparison be-
tween the thresholded and the raw gray-scale image and
was optimized to accurately represent the raw images of
scaffolds. For a 150 · 150 · 150 voxel cube, total volume
(TV), carbon nanotube volume (CNV) and scaffold volume
fraction (CNV/TV) were determined. Three voxel cubes per
scaffold were analyzed and the average of the three regions
and its standard deviation is reported. The regions of analy-
sis were selected in the center of the scaffold to eliminate
the edge artifacts. The porosity of the scaffolds was calcu-
1 ? scaffoldvolume fractionCNV
Scanning electron microscopy (SEM) was performed using
JOEL 7600F Analytical high resolution SEM at the Center for
Functional Nanomaterials (CFN), Brookhaven National Labo-
ratory, New York. Crosslinked carbon nanotube specimens
were placed on a conductive, double sided, carbon adhesive
tab (PELCO, Ted Pella), and imaged at 1 and 5 kV accelerating
voltages using a secondary electron imaging (SEI) detector.
Transmission electron microscopy (TEM) was performed
using FEI BioTwinG2TEM at Stony Brook University. The sam-
ples were imaged at 80 kV using 300 mesh size, holey lacey
carbon grids (Ted Pella, Inc.).
Image processing toolbox in MATLAB was used to quantify
the porosity values of the crosslinked specimens. SEM images
at various magnifications were cropped to remove the leg-
ends, and the scale bar, and were subjected to image process-
ing steps such as edge detection, thresholding, median
filtration, erosion and dilation followed by quantification of
region properties. Porosity was calculated using n = 5 images
as the ratio of the total area of voids to the total area of the
Areaof voids=area of the image
C A R B O N 5 3 (2 0 1 3) 9 0 –10 0
2.8.Liquid extrusion porosimetry
Liquid extrusion porosimetry (LEP) was performed on purified
MWCNT scaffolds using the PMI liquid extrusion porosimeter
at Porous Materials Inc., Ithaca, NY. The CNT scaffolds were
placed on a membrane and the sample chamber was filled
with Galwick?(wetting liquid, surface tension ?0, propene,
1,1,2,3,3,3-hexafluoro, oxidized, polymerized) which pene-
trates into the pores of the sample. An inert gas under pres-
sure was applied to extrude the liquid from the pores of the
MWCNT scaffold. The volume and weight of the extruded li-
quid was measured, and porosity and median pore diameter
were calculated as described previously [29,30].
2.9.Four point resistivity measurements
Bulk resistivity was assessed by a four probe resistance mea-
surement technique (Signatone S302-4, SP-4 probe) at Center
for Functional Nanomaterials (CFN), Brookhaven National
Laboratory, New York. Four point resistance measurements
assess planar resistances for a theoretically infinitesimal
thickness of sample. Thus, bulk material resistance can be
derived from sheet resistance with a correction factor (F) to
account for the thickness of the sample. The four, spring-
loaded probes were equally spaced at 1.25 mm distances, with
the two outer probes providing current and inner probes mea-
suring voltage. Sheet resistance values for each MWCNT scaf-
fold was measured at three different regions. Resistivity of the
MWCNT scaffold was calculated by:
where q is the bulk resistivity, Rsheetis the sheet resistance, w
is the thickness of the sample (0.5 cm), and F is the correction
factor. The conductivity was then obtained by calculating the
q ¼ Rsheet? w ?
Statistical analysis was performed using a student’s ‘‘t’’ test
and one-way anova followed by Tukey Kramer post hoc anal-
ysis. A 95% confidence interval (p < 0.05) was used for all sta-
3.Results and discussion
MWCNTs were thermally crosslinked via radical-initiated
reaction using benzoyl peroxide. Briefly, a few drops of chloro-
form were added to the MWCNT-BP mixture (see method sec-
tion for details), and the slurry was poured into prefabricated
PTFE (Teflon?) molds (disk or cylinder molds), and incubated
at 60 ?C for 24 h. Benzoyl peroxide is a widely used initiator
in free radical polymerization reactions . It thermally
decomposes to yield phenyl or benzoyloxyl free radicals,
and CO2gas, and has been used for covalent functionalization
of carbon nanotubes [32,33]. Polymerization of formulations
with reactive double bonds initiated by temperature-, or radi-
ation-induced radicals is a widely-used method . In the
above reaction, the radicals react with the double bond net-
work on the MWCNT structure; thereby forming active cen-
ters, which serve as inter-nanotube cross-linking sites. This
results in the nanoscale crosslinking of carbon nanotubes,
yielding macroscopic 3-D carbon scaffolds. The un-reacted
BP and other volatiles (generated during the termination of
radical crosslinking reaction) were removed by annealing
the 3-D carbon scaffolds at 150 ?C for 20 min. Fig. 1 displays
the digital images of representative unpurified and purified
3-D MWCNT scaffolds prepared by mixing MWCNTs and BP
in the mass ratio 1:4. The unpurified scaffolds have a gray-
ish-black tint, due to some residual BP (red circles), while
purified scaffolds do not have this tint. The scaffolds are ro-
bust free-standing structures, and structurally stable; similar
to polymeric scaffolds (see supplementary information S7
3.1. Raman spectroscopy
The Raman spectra of the pristine MWCNT, the unpurified,
and purified MWCNT scaffolds (MWCNT:BP mass ratio = 1:4)
are presented in Fig. 2A. The pristine MWCNT used as the
starting material shows the characteristic D, G, and G’ bands
at 1355 cm?1, 1580 cm?1, and 2694 cm?1, respectively (Fig. 2A,
blue line). The ID/IGratio for pristine MWCNTs is 0.07. The G
band in the Raman spectra has been attribute to the intrinsic
vibration of sp2bonded graphitic carbon atoms, whereas the
D band corresponds to the defects induced in the nanotube
structure due to disruption of the sp2C@C bonds . The Ra-
man spectrum of the unpurified (Red line), and purified (green
line) MWCNT scaffolds (MWCNT:BP mass ratio = 1:4) shows a
Fig. 1 – Optical images of representative thermally-
crosslinked 3-D, macroscopic (A) unpurified and (B) purified
MWCNT scaffolds; prepared as cylinders (5 mm diameter,
10 mm length), and disks (5 mm diameter, 4 mm thickness).
C A R B O N 53 ( 2 01 3 ) 90 –1 0 0
substantial increase in the intensity of the D band. The ID/IG
ratio for the unpurified and purified MWCNT scaffolds is
0.85, and 0.14, respectively. The Raman spectrum of the unpu-
rified MWCNT scaffolds also shows additional minor peaks at
1000, 1230 and 1775 cm?1, which can be attributed to the
breathing mode (C–C stretching) of benzene ring, C–O bond
stretching (vibration of the peroxide chain) and C@O bond
stretching (aryl carbonate functional group), respectively
[37,38]. These peaks are routinely observed in the Raman
spectra of most radical functionalization reactions with BP
. The intensities of these peaks were relatively minor com-
pared to the D and G bands, and were repeatedly observed
only in the Raman spectra of unpurified MWCNT scaffolds.
The decrease in the ID/IGratio, and the absence of the minor
peaks in the Raman spectrum of the purified MWCNTs scaf-
folds compared to the purified MWCNTs scaffolds suggests
that the disruption of the sp2(C@C) bonds for the purified
MWCNTs scaffolds is due to crosslinked C–C bonds, covalent
carbonyl, benzoyloxyl and phenyl functional groups formed
during crosslinking reaction , non-covalent p–p interac-
tions between the MWCNTs and the aromatic groups of unre-
acted BP , and benzoyloxyl /phenyl radical by-products.
The annealing of the unpurified MWCNT scaffolds removes
the unreacted BP, and the reaction by-products which
Fig. 2 – (A) Representative Raman spectra of pristine multiwalled carbon nanotubes (blue trace) and the 3-D crosslinked
MWCNT scaffolds (MWCNT:BP mass ratio = 1:4) before (red trace) and after (green trace) purification. (B) TGA curves of pristine
MWCNTs, MWCNT scaffolds before- and after- purification. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
C A R B O N 5 3 (2 0 1 3) 9 0 –10 0
decompose between 100 and 150 ?C. The heating procedure
decomposes the unreacted BP and by-products, and partially
restores sp2(C@C) bonds decreasing the ID/IGratio. However,
the ID/IGratio of the purified MWCNT scaffolds is still more
than two orders greater than pristine MWCNTs indicating
the presence of C–C, C–O and C@O bonds. The above assess-
ment is further corroborated by TGA analysis.
3.2. Thermogravimetric analysis
Thermogravimetric analysis (TGA) has been widely used for
the characterization of carbon based nanomaterials [41–44].
The TGA spectra of the pristine MWCNT, the unpurified and
the purified MWCNT scaffolds (MWCNT:BP mass ratio = 1:4)
is presented in Fig. 2B. The TGA spectra of pristine MWCNTs
is similar to previous reports , and exhibit negligible
weight loss (0.05%) up to 700 ?C confirming its high thermal
stability, and purity. Thermal decomposition of unpurified
and purified MWCNT scaffolds can be divided into three tem-
perature zones, 0–150 ?C, 150–500 ?C and >500 ?C. In first tem-
perature zonebetween 0–150 ?C,
unpurified and purified MWCNT scaffolds was 43.06% and
0.03%, respectively. The high %weight loss observed for the
unpurified MWCNT scaffolds can be attributed to the removal
of residual water vapor, unreacted BP, and other volatiles
(possible benzoyloxyl, and phenyl adducts formed during ter-
mination of the crosslinking reaction). The purified MWCNT
scaffolds show negligible %weight loss indicating the high
temperature annealing completely removes the unreacted
BP, and other volatile by-products adsorbed on the unpurified
MWCNT scaffold. In the second temperature zone between
150 and 500 ?C, the %weight loss is similar for the unpurified
(16.51%), and the purified MWCNT (15.06%) scaffolds, and cor-
responds to the removal of functional groups attached to
MWCNTs . Finally, above 500 ?C, the observed %weight
loss for the unpurified and purified MWCNT scaffolds corre-
sponds to the thermal degradation of the MWCNT with sp2
and sp3carbon atoms [41–43].
Nanoindentation was performed on purified MWCNT scaf-
folds (MWCNT:BP mass ratio = 1:1 and 1:2). Table 1 summa-
rizes values of elastic modulus (Er) and hardness (H)
measured by 7 indents (at least 100 lm distance between each
indent). Representative force–displacement curve is pre-
sented in Fig. 3 (MWCNT:BP mass ratio = 1:2). Er and H values
1.82 ± 0.54 MPa, respectively. MWCNT scaffold (1:2) exhibited
Er of 45.72 ± 18.78 MPa and H of 3.47 ± 1.73 MPa, higher than
1:1 MWCNT:BP scaffold. These elastic modulus values for
MWCNT scaffolds are much higher than the values measured
for various polymeric, graphene and CNT based foams
[15,21,45]. For example, the CNT assembly reported by Xu
et al. possessed storage modulus of 1 MPa and loss modulus
of 0.3 MPa . Young’s modulus of 3-D graphene assemblies
as reported by Zhang et al. was 1.2–6.6 MPa , Shi et al. was
? 290 kPa  and Wang et al. was ?260 kPa . The rela-
tively high values of elastic modulus and hardness of MWCNT
scaffolds further corroborates the formation of nanoscale,
covalent crosslinks between MWCNTs necessary to achieve
the measured mechanical strengths at a macroscopic scale.
were38.45 ± 14.42 MPaand
Quantitative XPS chemical composition analysis, and high
resolution carbon 1s and oxygen 1s analysis of the purified
MWCNT scaffolds (MWCNT:BP mass ratio = 1:4) was also per-
formed (see Supplementary information). The quantitative
XPS chemical composition analysis showed that carbon
(94.1%) and oxygen (5.54%) were the primary elements in
the scaffolds. The high resolution carbon 1s and oxygen 1s
analysis indicate that oxygen is mainly present as an element
of carboxyl functional group. The carboxyl groups could be
due to the presence of trace amounts of benzoyloxyl moieties
that get covalently attached to the MWCNTs during the radi-
cal initiation reaction, and/or carboxylic acid groups formed
due to reaction of active radical sites on the MWCNTs with
oxygen impurities during the radical termination reaction.
Furthermore, the bulk electrical conductivity of purified
MWCNT scaffolds (cylinders, 6 mm length, 5 mm diameter,
MWCNT:BP mass ratio = 1:4) was calculated to be 2 · 10?1 -
S cm?1from four point resistivity measurements , and
satisfy the conductivity requirements for a large number of
electrical applications . This electrical conductivity value
is similar or higher than a large number of thin films prepared
using carbon nanotubes or graphene with large networks of
sp2carbon atoms, and scattered regions of sp3carbon atoms,
but lower than thin films of carbon nanotubes or graphene
Table 1 – Mechanical properties of MWCNT scaffolds determined by nanoindentation.
Indent #MWCNT:BP 1:1 MWCNT:BP 1:2
Er (MPa)H (MPa)Er (MPa)H (MPa)
Mean ± SD38.45 ± 14.421.82 ± 0.5445.72 ± 18.78 3.47 ± 1.73
C A R B O N 53 ( 2 01 3 ) 90 –1 0 0
Fig. 3 – Representative loading–unloading curve during nanoindentation of MWCNT scaffold (MWCNT:BP mass ratio = 1:2).
The red dots are raw data, green dots are analyzed data. The slope of the best fit line (blue) was used to calculate elastic
modulus. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
Fig. 4 – Representative low (A, B), and high (C, D) resolution scanning electron microscopy images of unpurified MWCNT
scaffold cross-sections (MWCNT:BP mass ratio = 1:4).
C A R B O N 5 3 (2 0 1 3) 9 0 –10 0
with only sp2carbon networks [50–52]. Thus, the Raman,
TGA, XPS, and conductivity results taken together implies
that the chemical composition of the purified MWCNT scaf-
folds mainly comprises of sp2carbon networks with sp3car-
bon junctions at the crosslinking sites.
SEM was performed on the MWCNT scaffolds to characterize
their structure, and confirm the cross-linking of the nano-
tubes (Fig. 4). Fig. 4A and B shows low resolution SEM images
of a representative unpurified MWCNT scaffold prepared by
mixing MWCNT and BP in a ratio of 1:4. The cross-sections
clearly show interconnected MWCNT networks that form
the macroscopic 3-D architecture. The high resolution SEM
in Fig. 4C and D also displays the crosslinking between indi-
vidual MWCNTs, and the formation of junctions (red arrows,
Fig. 4D). Unlike polymer chains that coil together tightly with
no inter-chain space or air pockets, the cross-linked MWCNT
network is highly porous. The pores are irregular shaped and
inter-connected. (Representative TEM images (Supplementary
information, Fig. S1) display the formation of crosslinks be-
Micro computed tomography (micro-CT) and SEM
The porosity and pore size of the unpurified and purified
MWCNT scaffolds was further evaluated by microCT and
SEM image analysis. No statistically significant difference
was observed in the porosity and pore size values for unpuri-
fied, and purified. Thus, only the analysis of purified MWCNTs
is presented. MicroCT is a well-established method used to
characterize the macroporosity of 3-D crosslinked scaffolds
. Fig. 5A displays a 3-D reconstructed microCT image of
a 1.23 mm · 1.23 mm · 1.23 mm section of a representative
unpurified MWCNT (MWCNT:BP = 1:0.5) scaffold. Fig. 5B–D
shows the top, middle, and bottom section of the 3-D image
displayed in Fig. 5A, and clearly confirm the presence of pores
(blue color represents the voids). These observations were
consistent throughout all individual cross-sections of the mi-
croCT reconstructed images. The analysis of the microCT
slices determined the pore sizes to be between 100 and
300 lm. The pores were interconnected, and distributed
throughout the structure (see Supplementary information
S8 movie for a representative 360? view of 3-D microCTrecon-
structed MWCNT scaffold. The scaffolds can be examined
from any angle of view at up to 6 lm resolution by shifting,
Fig. 5 – (A) Representative 3-D reconstructed microCT image of unpurified MWCNT scaffold, and the (B) top, (C) middle and (D)
bottom microCT slice of the reconstructed 3-D MWCNT scaffold image. The blue color in the images represents void spaces.
Scale bar: (A) 100 lm, (B–D) 300 lm (MWCNT:BP mass ratio = 1:4). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
C A R B O N 53 ( 2 01 3 ) 90 –1 0 0
rotating, and magnifying them in virtual space, and provide
further visual support of the interconnected pores).
The macroporosity of the scaffolds fabricated by mixing
MWCNTs with BP at different mass ratios (between 1:0.5
and 1:4) was determined from the microCT data, and is pre-
sented in Fig. 6A and Table 2. The results show that porosity
of MWCNT scaffolds decreased from 85% to 21% with increase
in the amount of BP added for crosslinking the MWCNTs. It
should be noted that the white and grey solid interconnected
structures (Fig. 5B–D) in the microCT images have nanometer
sized pores, which cannot be visualized due to the microCT’s
resolution limit of 6 lm. The macroporosity within these
structures can be clearly visualized in the images by SEM
(see Fig. 4). To further quantify the macroporosity, a widely-
used and accepted literature technique [54–57] was used to
perform image processing on a series of SEM images, and cal-
culate the porosity within the white solid structure structures
seen in the microCT images (see Section 2 for details). The
porosity calculated by this method corresponds to the surface
porosity, and has been used to estimate the porosity values
for sandstones, and tissue engineering polymeric scaffolds
[54–57]. The pore sizes from this analysis were determined
to be between 125 and 750 nm. The macroporosity of the var-
ious MWCNT scaffolds (MWCNT:BP mass ratios between 1:0.5
and 1:4) is presented in Fig. 6B and Table 3. The results show a
trend similar to the microCT porosity data with a decrease in
porosity from 43.42% to 23.62% with increase in MWCNT:BP
3.7. Liquid extrusion porosimetry (LEP)
In addition to microCT and SEM image processing, LEP was
performed to assess the porosity of MWCNT scaffolds. LEP
is a widely used, IUPAC recommended, non-hazardous (no
mercury) method to assess the porosity of ceramics, food
products and nonwoven fibrous filter media beds [58–61].
The porosity (%) and median pore diameter for all MWCNT
scaffolds (MWCNT:BP mass ratios between 1:0.5 and 1:4) is
presented in Fig. 5C and Table 4. The results show a decreas-
ing trend in porosity and average pore diameter as a function
of MWCNT:BP ratio, similar to microCT and SEM image anal-
ysis.The macro-porosityand medianpore diameter
Table 4 – Porosity and median pore diameter of MWCNT
scaffolds determined from liquid extrusion porosimetry.
Porosity (%) by
Fig. 6 – (A) and (B) are porosity of purified MWCNT scaffolds
fabricated with different mass ratios of BP (between 1:0.5
and 1:4) as determined by microCT and SEM image
processing analysis, respectively. (C) Porosity of purified
MWCNT scaffolds analyzed by liquid extrusion porosimetry.
Table 2 – Porosity of MWCNT scaffolds calcu-
lated from microCT analysis.
84.67 ± 1.70
79.26 ± 1.77
70.29 ± 2.34
68.80 ± 5.72
21.31 ± 1.52
Table 3 – Porosity of MWCNT scaffolds calcu-
lated from SEM analysis.
by SEM image
43.424 ± 2.88
44.121 ± 3.66
39.895 ± 2.72
32.389 ± 4.93
23.623 ± 2.02
C A R B O N 5 3 (2 0 1 3) 9 0 –10 0
decreased from 94.48% to 20.19% and 324.48 lm to 115.87 lm,
respectively, with increase in MWCNT:BP ratio. The microCT,
SEM porosity and LEP results taken together indicate that
the porosity of MWCNT scaffolds can be tuned by varying
the amount of crosslinking agent – BP. The higher amount
of BP leads to the increase in the amount of active sites on
the MWCNTs thereby inducing a higher crosslinking, and
thereby, alters the porosity .
The thermal cross-linking method discussed above to fab-
ricate 3-D MWCNT scaffolds can be easily adapted to fabricate
3-D scaffolds of various geometries (e.g. disks or cylinders)
with other carbon nanomaterials such 0-D fullerenes, 1-D sin-
gle-walled carbon nanotubes or 2-D graphene as starting
materials (see Fig. 7A). Fig. 7B–D shows the SEM images of
scaffolds fabricated using these nanomaterials. The SEM
cross-sections clearly show the macroscopic 3-D architec-
tures due the crosslinking of these carbon nanomaterials.
The SWCNT scaffolds show topography similar to the
MWCNT scaffolds. The fullerene and graphene scaffolds
show topography that is distinctly different from the MWCNT
and SWCNT scaffolds. Additional studies are required, and
are currently underway to understand how the dimensional-
ity these nanoscale building blocks affects the structure, and
porosity of the 3-D scaffolds. Nevertheless, the fabrication of
these 3-D all carbon macro-sized scaffolds opens avenues for
further experimental and theoretical studies to elucidate the
structure–(geometry, porosity) function (thermal, mechanical,
electrical, and electromagnetic properties) relationships of
The introduction of carbon nanotechnology into large
number of macro-scale applications for energy storage
[21,63,64], thermal management , catalysis , electronic
devices , and biomedical implants  would require the
assembly of nanoscale building-blocks such as carbon
nanotubes, fullerenes, and graphene to be assembled in
structurally robust 3-D architectures. An important issue
affecting this development is the formation of covalent
junctions between the building blocks [20,23]. The results
of this work introduce a novel, facile, cheap, and scalable
method to fabricate 3-D carbon nanotubes with chemically
cross-linked junctions between sp2carbon atoms, which
can be easily adapted to other carbon nanostructures such
as fullerenes and graphene. Additionally, while the scaffolds
architectures presented in this work are disk-shaped or
cylindrical, one can also envision adapting this fabrication
method using molds with complex geometries to tailor
the shapes of the scaffolds. The insights from further struc-
ture–function relationship studies should provide the guid-
ingprinciples for the
macroscopic all-carbon devices with specific requirements
for applications in clean energy technology, information
technology, and healthcare.
We report a simple method to fabricate macroscopic, 3-D, free
standing, all-carbon scaffolds using multiwalled MWCNTs as
the starting materials. The scaffolds prepared via radical ini-
tiated thermal crosslinking, and annealing of MWCNTs pos-
sess macroscale interconnected pores, robust structural
integrity, stability, and conductivity. The porosity of the
three-dimensional structure can be controlled by varying
the amount of radical initiator. This method also allows fabri-
cation of 3-D scaffolds using other carbon nanomaterials
Fig. 7 – (A) Optical images of thermally-crosslinked 3-D,
macroscopic unpurified cylinder (5 mm diameter, 8 mm
thickness), and discs (5 mm diameter, 3 mm thickness)
fabricated using SWCNTs, fullerenes and graphene oxide
nanoplatelets as starting material. (B), (C) and (D) are
scanning electron microscopy images of unpurified
scaffolds fabricated using SWCNTS, fullerenes and
graphene oxide nanoplatelets, respectively.
C A R B O N 53 ( 2 01 3 ) 90 –1 0 0
such as single-walled carbon nanotubes, fullerenes, and
graphene indicating that it could be used as a versatile meth-
od for 3-D assembly of carbon nanostructures with pi bond
networks. Additionally, the fabrication process of the scaf-
folds is rapid, cheap, and scalable, and can be adapted to fab-
ricate scaffolds with various geometries (e.g. cylinders, disks)
thereby opening avenues for structure–function studies to-
wards the development of macroscopic all-carbon devices.
This work was sponsored by National Institutes of Health
(Grant No.: 1DP2OD007394-01). Four point resistivity measure-
ments were performed at CFN, BNL, which is supported by the
U.S. Department of Energy, Office of Basic Energy Sciences,
under Contract No. DE-AC02-98CH10886.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.carbon.
R E F E R E N C E S
 Dai H. Carbon nanotubes: synthesis, integration, and
properties. Acc Chem Res 2002;35(12):1035–44.
 Sun DM, Timmermans MY, Tian Y, Nasibulin AG, Kauppinen
EI, Kishimoto S, et al. Flexible high-performance carbon
nanotube integrated circuits. Nat Nanotechnol
 Fan Z, Yan J, Zhi L, Zhang Q, Wei T, Feng J, et al. A three-
dimensional carbon nanotube/graphene sandwich and its
application as electrode in supercapacitors. Adv Mater
 Xiong W, Du F, Liu Y, Perez Jr A, Supp M, Ramakrishnan TS,
et al. 3-D carbon nanotube structures used as high
performance catalyst for oxygen reduction reaction. J Am
Chem Soc 2010;132(45):15839–41.
 Ma L, Sines G. Fatigue of isotropic pyrolytic carbon used in
mechanical heart valves. J Heart Valve Dis 1996;5(Suppl.
 Sitharaman B, Shi X, Walboomers XF, Liao H, Cuijpers V,
Wilson LJ, et al. In vivo biocompatibility of ultra-short single-
walled carbon nanotube/biodegradable polymer
nanocomposites for bone tissue engineering. Bone
 Huang JQ, Zhang Q, Zhao MQ, Xu GH, Wei F. Patterning of
hydrophobic three-dimensional carbon nanotube
architectures by a pattern transfer approach. Nanoscale
 Ren ZF, Huang ZP, Xu JW, Wang JH, Bush P, Siegal MP, et al.
Synthesis of large arrays of well-aligned carbon nanotubes
on glass. Science 1998;282(5391):1105–7.
 Bennett RD, Hart AJ, Miller AC, Hammond PT, Irvine DJ, Cohen
RE, et al. Creating patterned carbon nanotube catalysts
through the microcontact printing of block copolymer
micellar thin films. Langmuir 2006;22(20):8273–6.
 De Volder M, Tawfick SH, Park SJ, Copic D, Zhao Z, Lu W, et al.
Diverse 3-D microarchitectures made by capillary forming of
carbon nanotubes. Adv Mater 2010;22(39):4384–9.
 Qu J, Zhao Z, Wang X, Qiu J. Tailoring of three-dimensional
carbon nanotube architectures by coupling capillarity-
induced assembly with multiple CVD growth. J Mater Chem
 Chakrapani N, Wei B, Carrillo A, Ajayan PM, Kane RS.
Capillarity-driven assembly of two-dimensional cellular
carbon nanotube foams. Proc Natl Acad Sci USA
 Endo M, Muramatsu H, Hayashi T, Kim YA, Terrones M,
Dresselhaus MS. Nanotechnology: ‘buckypaper’ from coaxial
nanotubes. Nature 2005;433(7025):476.
 Cao A, Dickrell PL, Sawyer WG, Ghasemi-Nejhad MN, Ajayan
PM. Super-compressible foamlike carbon nanotube films.
 Xu M, Futaba DN, Yamada T, Yumura M, Hata K. Carbon
nanotubes with temperature-invariant viscoelasticity from
?196 to 1000 ?C. Science 2010;330(6009):1364–8.
 Gui X, Wei J, Wang K, Cao A, Zhu H, Jia Y, et al. Carbon
Nanotube Sponges. Adv Mater 2010;22(5):617–21.
 Worsley MA, Kucheyev SO, Satcher JJH, Hamza AV, Baumann
TF. Mechanically robust and electrically conductive carbon
nanotube foams. Appl Phys Lett 2009;94(7):73115–23.
 Kim KH, Oh Y, Islam MF. Graphene coating makes carbon
nanotube aerogels superelastic and resistant to fatigue. Nat
 Schiffres SN, Kim KH, Hu L, McGaughey AJH, Islam MF, Malen
JA. Gas diffusion, energy transport, and thermal
accommodation in single-walled carbon nanotube aerogels.
Adv Funct Mater 2012. http://dx.doi.org/10.1002/
 Worsley MA, Pauzauskie PJ, Olson TY, Biener J, Satcher JH,
Baumann TF. Synthesis of graphene aerogel with high
electrical conductivity. J Am Chem Soc 2010;132(40):14067–9.
 Zhang X, Sui Z, Xu B, Yue S, Luo Y, Zhan W, et al.
Mechanically strong and highly conductive graphene aerogel
and its use as electrodes for electrochemical power sources. J
Mater Chem 2011;21(18):6494–7.
 Biener J, Stadermann M, Suss M, Worsley MA, Biener MM,
Rose KA, et al. Advanced carbon aerogels for energy
applications. Energy Environ Sci 2011;4(3):656–67.
 Hashim DP, Narayanan NT, Romo-Herrera JM, Cullen DA,
Hahm MG, Lezzi P, et al. Covalently bonded three-
dimensional carbon nanotube solids via boron induced
nanojunctions. Sci Rep 2012:2. http://dx.doi.org/10.1038/
 Paratala BS, Jacobson BD, Kanakia S, Francis LD, Sitharaman
B. Physicochemical characterization, and relaxometry
studies of micro-graphite oxide, graphene nanoplatelets, and
nanoribbons. PLoS One 2012;7(6):e38185. http://dx.doi.org/
 Oliver WC, Pharr GM. Improved techniques for determining
hardness and elastic modulus using load and displacement
sensing indentation experiments. J Mater Res
 Mesarovic SD, McCarter CM, Bahr DF, Radhakrishnan H,
Richards RF, Richards CD, et al. Mechanical behavior of a
carbon nanotube turf. Scr Mater 2007;56(2):157–60.
 Ozcivici E, Ferreri S, Qin YX, Judex S. Determination of bone’s
mechanical matrix properties by nanoindentation. Methods
Mol Biol 2008;455:323–34.
 Judex S, Garman R, Squire M, Donahue L-R, Rubin C.
Genetically based influences on the site-specific regulation of
trabecular and cortical bone morphology. J Bone Miner Res
 Jena A, Gupta K. Liquid extrusion techniques for pore
structure evaluation of nonwovens. Int Nonwovens J
C A R B O N 5 3 (2 0 1 3) 9 0 –10 0
 Jena A, Gupta K. Determination of pore volume and pore Download full-text
distribution by liquid extrusion porosimetry without using
mercury. In: 26th annual conference on composites,
advanced ceramics, materials, and structures: B: ceramic
engineering and science proceedings. John Wiley & Sons,
Inc.; 2008. p. 277–84.
 Braun D. Origins and development of initiation of free radical
polymerization processes. Int J Polym Sci 2009;2009. http://
 Ying Y, Saini RK, Liang F, Sadana AK, Billups WE.
Functionalization of carbon nanotubes by free radicals. Org
 Peng H, Reverdy P, Khabashesku VN, Margrave JL. Sidewall
functionalization of single-walled carbon nanotubes with
organic peroxides. Chem Commun (Cambridge) 2003;3:362–3.
 Moad G, Solomon DH. The chemistry of radical
polymerization. Amsterdam, Boston: Elsevier; 2006.
 Ishigami N, Ago H, Motoyama Y, Takasaki M, Shinagawa M,
Takahashi K, et al. Microreactor utilizing a vertically-aligned
carbon nanotube array grown inside the channels. Chem
Commun (Cambridge) 2007;16:1626–8.
 Dresselhaus MS, Dresselhaus G, Saito R, Jorio A. Raman
spectroscopy of carbon nanotubes. Phys Rep
 Zyat’kov IP, Rakhimov AI, Pitsevich GA, Gogolinskii VI,
Androsyuk ER, Sagaidak DI. Effect of fluorine-containing
substituents on spectralstructural characteristics of aroyl
peroxides. J Appl Spectrosc 1983;39(1):798–802.
 Vacque V, Sombret B, Huvenne JP, Legrand P, Suc S.
Characterisation of the O–O peroxide bond by vibrational
spectroscopy. Spectrochim Acta Part A 1997;53(1):55–66.
 Baibarac M, Baltog I, Lefrant S, Mevellec JY, Bucur C.
Vibrational and photoluminescence properties of the
polystyrene functionalized single-walled carbon nanotubes.
Diamond Relat Mater 2008;17(7–10):1380–8.
 Baskaran D, Mays JW, Bratcher MS. Noncovalent and
nonspecific molecular interactions of polymers with
multiwalled carbon nanotubes. Chem Mater
 Rinzler AG, Liu J, Dai H, Nikolaev P, Huffman CB, Rodrı ´guez-
Macı ´as FJ, et al. Large-scale purification of single-wall carbon
nanotubes: process, product, and characterization. Appl Phys
A Mater Sci Process 1998;67(1):29–37.
 Hou P, Liu C, Tong Y, Xu S, Liu M, Cheng H. Purification of
single-walled carbon nanotubes synthesized by the hydrogen
arc-discharge method. J Mater Res 2001;16(09):2526–9.
 Chen IWP, Richard L, Haibo Z, Ben W, Chuck Z. Highly
conductive carbon nanotube buckypapers with improved
doping stability via conjugational cross-linking.
Nanotechnology 2011;22(48):485708. http://dx.doi.org/
 Kosynkin DV, Higginbotham AL, Sinitskii A, Lomeda JR,
Dimiev A, Price BK, et al. Longitudinal unzipping of carbon
nanotubes to form graphene nanoribbons. Nature
 Gibson LJ, Ashby MF. Cellular solids: structure and
properties. Cambridge University Press; 1997.
 Xu Y, Sheng K, Li C, Shi G. Self-assembled graphene hydrogel
via a one-step hydrothermal process. ACS Nano
 Tang Z, Shen S, Zhuang J, Wang X. Noble-metal-promoted
three-dimensional macroassembly of single-layered
graphene oxide. Angew Chem Int Ed 2010;49(27):4603–7.
 Smits F. Measurement of sheet resistivities with the four-
point probe. Bell Syst Tech J 1958;37(3):711–8.
 Chung DDL. Electrical applications of carbon materials. J
Mater Sci 2004;39(8):2645–61.
 Lau C, Cervini R, Clarke S, Markovic M, Matisons J, Hawkins S,
et al. The effect of functionalization on structure and
electrical conductivity of multi-walled carbon nanotubes. J
Nanopart Res 2008;10:77–88.
 Sahoo NG, Rana S, Cho JW, Li L, Chan SH. Polymer
nanocomposites based on functionalized carbon nanotubes.
Prog Polym Sci 2010;35(7):837–67.
 Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM,
Zimney EJ, Stach EA, et al. Graphene-based composite
materials. Nature 2006;442(7100):282–6.
 Shi X, Sitharaman B, Pham QP, Liang F, Wu K, Edward Billups
W, et al. Fabrication of porous ultra-short single-walled
carbon nanotube nanocomposite scaffolds for bone tissue
engineering. Biomaterials 2007;28(28):4078–90.
 Guarino V, Guaccio A, Netti P, Ambrosio L. Image processing
and fractal box counting: user-assisted method for multi-
scale porous scaffold characterization. J Mater Sci Mater Med
 McCullen SD, Stevens DR, Roberts WA, Clarke LI, Bernacki SH,
Gorga RE, et al. Characterization of electrospun
nanocomposite scaffolds and biocompatibility with adipose-
derived human mesenchymal stem cells. Int J Nanomed
 Grove C, Jerram DA. JPOR: an image J macro to quantify total
optical porosity from blue-stained thin sections. Comp
 Hunt RKRKP. Mineralogy of fine-grained sediment by energy-
dispersive spectrometry (EDS) image analysis – a
methodology. Environ Geol 2002;42(1):32–40.
 Rouquerol Jean, Baron Gino, Denoyel Renaud, Giesche
Herbert, Groen Johan, Klobes Peter, et al. Liquid intrusion
and alternative methods for the characterization of
macroporous materials (IUPAC technical report). Pure Appl
 Miller B, Tyomkin I. Liquid porosimetry: new
methodology and applications. J Colloid Interface Sci
 Datta AK, Sahin S, Sumnu G, Ozge Keskin S. Porous media
characterization of breads baked using novel heating modes.
J Food Eng 2007;79(1):106–16.
 Hutten IMM. Handbook of nonwoven filter media. Elsevier;
 Manley TR, Qayyum MM. Crosslinked polyethylene at
elevated temperatures. Polymer 1972;13(12):587–92.
 Kolpak AM, Grossman JC. Azobenzene-functionalized carbon
nanotubes as high-energy density solar thermal fuels. Nano
 Lee SW, Yabuuchi N, Gallant BM, Chen S, Kim B-S, Hammond
PT, et al. High-power lithium batteries from functionalized
carbon-nanotube electrodes. Nat Nanotechnol
 Romo-Herrera JM, Terrones M, Terrones H, Dag S, Meunier
V. Covalent 2-D and 3-D networks from 1-D
nanostructures: designing new materials. Nano Lett
 Shi X, Sitharaman B, Pham QP, Spicer PP, Hudson JL, Wilson
LJ, et al. In vitro cytotoxicity of single-walled carbon
nanotube/biodegradable polymer nanocomposites. J Biomed
Mater Res A 2008;86(3):813–23.
C A R B O N 53 ( 2 01 3 ) 90 –1 0 0