Article
pubs.acs.org/cm
Controlling Carbon Nanotube Type in Macroscopic Fibers
Synthesized by the Direct Spinning Process
Víctor Reguero, Belén Alemán, Bartolomé Mas, and Juan José Vilatela*
IMDEA Materials Institute, c/Eric Kandel 2, Getafe, 28960, Madrid, Spain
S Supporting Information
*
ABSTRACT: We report on the synthesis of kilometers of
continuous macroscopic fibers made up of carbon nanotubes
(CNT) of controlled number of layers, ranging from singlewalled to multiwalled, tailored by the addition of sulfur as a
catalyst promoter during chemical vapor deposition in the
direct fiber spinning process. The progressive transition from
single-walled through collapsed double-walled to multiwalled is
clearly seen by an upshift in the 2D (G′) band and by other
Raman spectra features. The increase in number of CNT layers
and inner diameter results in a higher fiber macroscopic linear
density and greater reaction yield (up to 9%). Through a
combination of multiscale characterization techniques (X-ray photoelectron spectroscopy, organic elemental analysis, highresolution transmission electron microscopy, thermogravimetric analysis, and synchrotron XRD) we establish the composition of
the catalyst particles and position in the isothermal section of the C−Fe−S ternary diagram at 1400 °C. This helps explain the
unusually low proportion of active catalyst particles in the direct spinning process (<0.1%) and the role of S in limiting C
diffusion and resulting in catalyst particles not being in thermodynamic equilibrium with solid carbon, therefore producing
graphitic edge growth instead of encapsulation. The increase in CNT layers is a consequence of particle coarsening and the
ability of larger catalyst particles to accommodate more layers for the same composition.
■
INTRODUCTION
Among the ever increasing plethora of nanoparticles with
interesting properties superior to those observed in bulk,
carbon nanotubes remain one of the most studied so far due to
their exceptional combination of properties, close to those of
in-plane graphite or an individual graphene layer. They have
high tensile strength, Young’s modulus1 and electrical (when
metallic2) and thermal conductivity.3 On a mass basis, these
properties are superior to those of reference engineering
materials used up to date such as carbon fiber and copper.
Hence the interest in producing macroscopic CNT-based
materials that exhibit a significant fraction of the properties of
the CNT building blocks and can therefore outperform current
high-performance materials.
One of the most successful strategies in this direction
consists in assembling the CNTs in fibers, with the CNTs
parallel to each other and to the fiber axis in a natural
embodiment to exploit their axial properties. This route follows
basic principles from the early 1950s of obtaining good fiber
properties from highly aligned extended polymer molecules,4,5
and which has resulted in commercial fibers such as polyester,
aramid (Kevlar), and ultrahigh-molecular weight polyethylene
(Dyneema/Spectra). CNT fibers can be produced by three
main methods: drawing from an array of aligned CNTs,6 wet
spinning from a liquid crystalline7 dispersion or a polymer
dispersion,8 and by direct spinning of a fiber from the gas-phase
during CNT growth by CVD.9 The fibers produced by these
© 2014 American Chemical Society
methods have mechanical properties already in the highperformance range and mass-basis electrical conductivity close
to copper but still modest compared to the properties of
individual CNTs. Further improvements in properties require
fibers made up of CNTs as similar as possible in terms of
diameter, number of layers, metallicity, and chiral angle. Similar
CNT diameter, for example, results in better packing and
therefore more efficient intertub stress transfer,10 while having
all metallic tubes is expected to benefit electrical conductivity.
The challenge is to produce kilometers of fiber where the ∼107
CNTs in its cross section have identical molecular structure and
to have the possibility of covering the whole CNT “molecular”
spectrum, from single-walled nanotubes (SWNTs) to multiwalled nanotubes (MWNTs). It is also as important to tailor
the type of CNTs as it is to understand how this is achieved.
In the case of direct spinning from the gas phase during CNT
synthesis by CVD, early work showed the effect of different
carbon sources on fiber mechanical properties and provided a
first evidence that tensile properties (in specific units) benefit
from CNT of a few layers.11 Similarly the use of acetoneethanol mixtures was shown to produce concentric aerogels of
mainly DWNTs.2 It was also recognized that S plays a critical
role in the reaction by limiting C diffusion to the catalyst
Received: April 3, 2014
Revised: May 6, 2014
Published: May 7, 2014
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Figure 1. Images of fiber samples at different length scales. (a) Photograph of a bobbin with approximately one kilometer of CNT fiber produced
over a couple of hours of continuous spinning. (b) Optical micrographs of fibers densified by interaction with acetone at their exit of the reactor. The
use of higher S precursor in the reaction produces larger diameter fibers. (c) SEM micrograph of an individual capillary-densified CNT fiber with a
noncircular cross section.
atmosphere at 1250 °C, using a precursor feed rate of 5 mL/h and a
winding rate of 7−9 m/min.
Raman spectroscopy was performed using a Jasco NRS-5100, using
laser lines of 532 and 785 nm, corresponding to 2.33 and 1.58 eV,
respectively. Laser power was fixed at 5.5 mW and exposure time to 15
s. CNT diameters were calculated as 248/(radial breathing mode
Raman shift), which provided the best fit according to the Kataura plot
(S1). 2D peaks were fit with a single Lorentzian peak. For
determination of the 2D position, all parameters were kept constant
to avoid laser-induced peak shifts. Thomas Swan Ltd. Elicarb MWNT
and SWNT were used as comparators.
Thermogravimetric (TGA) analysis was carried out with a Q800 TA
Instruments with a ramp of 10 °C/min. Sulfur/carbon content was
measured by dynamic flash combustion of the sample using a Flash
2000 Thermo Fisher organic elemental analyzer.
Fiber linear density was determined by producing a known length of
fiber established from winding rate and collection time, typically 10−
80 m, and weighing it on a COBOS ES 125 SM balance. Variations
between samples were below 10%. Instrumental error was obtained
from the uncertainty in the determination of fiber length based on
winding speed and duration and used as deviation from the mean
value.
Electron microscopy was carried out with an FIB-FEGSEM Helios
NanoLab 600i (FEI) at 30 kV and a JEOL JEM 3000F TEM at 300
kV. Catalyst structure was determined from the Fast-FourierTransform of lattice-resolved micrographs.
XPS data were collected in a SPECS GmbH electron spectroscopy
system provided with a PHOIBOS 150 9MCD analyzer. The
deconvolution shown is fitted by Gaussian/Lorentzian peak shapes
and Shirley background profile subtraction.
XRD measurements were performed at the Noncrystalline
Diffraction beamline at Alba Light Source using a wavelength of 1
Å. 2D wide-angle X-ray scattering patterns were azimuthally integrated
to obtain radial profiles. The data shown are after sample−detector
distance calibration.
surface and accelerating CNT growth.12 Sundaram et al.
produced fibers made of predominantly armchair CNTs by
using carbon disulfide (CS2) instead of the more common
thiophene as S source. The synthesis of SWNTs was favored by
limiting catalyst particle size to 1−2 nm through the availability
of S earlier in the reaction (CS2 decomposes at lower
temperature than thiophene), which also resulted in the
predominance of armchair chiralities.13 More recently Jung et
al. systematically explored the parameter space for fiber
spinning varying precursor concentrations (ferrocene, thiophene, and ethanol), feed rate, hydrogen flow rate, and
temperature. They proposed that low Fe concentrations favor
the production of SWNTs and a decrease in the ratio of Raman
intensities of the D over G bands (ID/IG).14 There is
comparatively much more understanding of “traditional”
CNT synthesis by CVD (see for example15) but which is
difficult to relate to the direct spinning method. It corresponds
typically to growth on substrates, in the absence of sulfur and at
lower temperatures (<900 °C) at which the catalyst is not
molten16 and therefore less active.17
Here we show a controlled process for making continuous
(up to kilometers) fibers made up of specific types of nanotubes
across the range SWNT-collapsed DWNT-MWNT. We
present new evidence of sulfur in the catalyst by X-ray
photoelectron spectroscopy (XPS) and organic elemental
analysis (OEA), which combined with synchrotron X-ray
diffraction (XRD), high-resolution transmission electron
microscopy (HRTEM), and thermogravimetric analysis
(TGA) of the fiber provides us with an estimate of the
concentration of Fe, C, and S in the catalyst. The catalyst
composition is then related to the isothermal section of the
ternary phase diagram at 1400 °C to explain the unusually small
proportion of active particles in the direct spinning process and
the special role of sulfur in controlling CNT morphology. Our
observations point to the ratio S/C as the most critical one,
controlling the evolution of the CVD reaction either into
inactive particles or active particles that produce CNTs with
morphology depending on the degree of particle coarsening.
■
■
RESULTS AND DISCUSSION
The synthesis of CNT fibers is carried out by direct spinning of
a CNT aerogel directly from the gas-phase during CNT
growth9 using ferrocene, thiophene, and butanol as Fe catalyst,
S promoter, and C source, respectively. The CNT aerogel is
collected either as a film by overlapping the material on top of
itself during winding or it can be densified online by capillary
forces upon exposure to acetone to produce a continuous
filament. Figure 1 shows typical samples of CNT fiber material:
a photograph of a bobbin with around 1 km of wound material
(Figure 1a) and optical micrographs of densified filaments
EXPERIMENTAL SECTION
The CNT fibers were synthesized by the direct spinning method9
using ferrocene, thiophene, and butanol as catalyst, promoter, and
carbon source, respectively. The reaction was carried out in hydrogen
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produced under different concentration of S promoter (Figure
1b). In both cases, their structure is composed of a network of
CNTs hierarchically associated in bundles and subfilaments
with preferential orientation along the fiber axis as a
consequence of the drawing action (Figure 1c). For this
study, we have used fiber samples produced under reproducible
and stable reaction conditions that ensure continuous spinning
maintainable for hours (see discussion on sample uniformity
(S2) and a video of nearly 2 h of continuous spinning in
Supporting Information).
By varying the concentration of thiophene in the precursor
mixture it is possible to tailor the type of CNTs that make up
the fiber. Figure 2a shows the normalized Raman spectra of
fibers produced under different thiophene concentrations in the
feedstock. The spectra start at low thiophene concentrations
with the typical fingerprint of SWNTs; radial breathing modes,
split of G peak into G+ and G− components, and a very low ID/
IG ratio. As thiophene is increased both RBMs and G− signals
gradually disappear and the ID/IG ratio increases. For samples
produced with 0.8 wt % thiophene the Raman spectrum is very
much indicative of MWNTs, albeit highly graphitized. The
spectra are normalized for comparison of finer details, but in
absolute units (Figure 2a inset) the transition from SWNT to
MWNTs is also clear as a dramatic drop in G band intensity
due to a lower resonance.18 Note that this effect also makes ID/
IG go from around 0.005 to 0.23, but without it necessarily
implying a lower degree of graphitization. The trend presented
in Figure 2a is supported by measurements on samples from
different batches and confirmed using additional excitation
energies (785 nm). Figure 2b presents a map of all observed
SWNTs (calculation in S1), shown as filled circles with gray
scale intensity correspondence, superimposed on all the
possible RBMs expected for the two excitation energies (as
crosses) up to the cutoff filter frequency of the spectrometer
used.
The evolution from SWNTs to MWNTs is further
manifested as an upshift of the 2D (G′) band for increasing
thiophene concentrations, plotted in Figure 2c. This shift is due
to the increasing number of CNT layers, widely observed and
reported in graphene samples19 (where a peak split also
occurs). Here, the upshift covers the range of SWNT to
MWNT obtained in commercial CNT samples.
HRTEM observations confirm Raman results and rule out
the possibility, for example, of the strong SWNT resonance
masking other CNTs in the low thiophene samples. Figures 3
(a-c) show representative images of samples produced using
low (0.1 wt %), mid (0.2 wt %), and high (1.5 wt %) thiophene
concentration. The micrographs confirm the predominance of
SWNTs and MWNTs at the two ends of the sulfur range. For
mid concentration (0.2 wt %) the nanotubes are predominantly
large diameter of a few layers which therefore auto collapse20,21
and form stacks of adjacent layers at turbostratic separation.
An important aspect of controlling CNT type is to achieve
this without increasing the amount of impurities in the fiber,
especially in the form of soot or poorly graphitized carbon
tubules often observed as byproducts of the CVD synthesis.
The SEM micrographs in the insets of Figure 3 show that the
fibers in this work produced at different sulfur concentrations
have similarly low contents of the carbonaceous impurities.
This is also confirmed by TGA (S3).
A key observation during operation of the fiber spinning
reactor is that increasing the amount of thiophene precursor
produces a thicker aerogel at the exit of the reactor and a fiber
Figure 2. Raman spectroscopy data showing the evolution from
SWNT to MWNT with increasing S promoter during CNT fiber
synthesis. (a) Normalized full spectra showing SWNT features (RBM,
G split) at low thiophene concentration. (b) Map of all observed
SWNTs using 2.33 eV and 1.58 eV excitation energies. Crosses
represent possible SWNTs detectable with 532 and 785 nm laser lines,
in green and red, respectively. (c) Plot of Raman 2D (G′) peak
position for samples produced with different amounts of sulfur
precursor. Error bars represent standard deviation. The upshift with
increasing sulfur is due to the increment in number of layers of the
CNTs.
of higher linear density. Interestingly, this increase in linear
density is due almost entirely to the increase in CNT inner
diameter and number of layers. We have determined average
CNT inner and outer diameters and number of layers from
around 150 TEM images with different thiophene concentrations (S4). Figure 4a shows the mass increase of individual
CNTs (CNT linear density) compared to a plot of fiber linear
density, for different thiophene concentrations. The close
correspondence is quite revealing because it indicates that the
use of more sulfur promoter does not result in the formation of
more CNTs but only on the increase in their diameter and
number of layers. Instead of increasing nucleation density the
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Figure 3. Electron micrographs of CNT fibers showing the effects of increasing sulfur precursor. TEM images show the evolution from SWNT (a)
through collapsed CNT (b) to MWNT (c). SEM micrographs in the insets indicate that the samples have similarly low amounts of carbonaceous
impurities.
We note that from knowledge of the fiber linear, winding rate
(fixed at 7m/min) and precursor throughput a reaction yield
can be calculated, which in terms of injected carbon/output
carbon goes from around 0.7 to 9% (S5) for the data in Figure
4a (yield (%) = linear density (g/km) * 17.11 (km/g)). An
increasing reaction yield could probably be achieved by further
addition of thiophene or, at fixed thiophene concentration, by
reinjection of the outlet gas mixture.
A final critical observation is that varying the concentration
of Fe in the feedstock by even an order of magnitude does not
significantly affect the type of CNTs or the carbon conversion
in the reaction. At a fixed low thiophene concentration of
(0.2%), increasing ferrocene concentration from 0.25 to 2.1 wt
% does not change the SWNT Raman fingerprints of the fiber,
producing mainly redundant catalyst that ends up trapped in
the fiber (S6). This highlights the ratio S/C as being the most
relevant to control the reaction.
The question then moves on to the structure and
composition of the catalyst and the effect of sulfur on it. We
analyze in detail the catalyst particles ex situ and carry out
different elemental analysis on the fiber in order to establish the
composition of the catalyst particles, which is then related to
the isothermal section of Fe−S−C ternary diagram at 1400 °C.
The first evidence of the incorporation of S in the catalyst
comes from XPS measurements on the fibers. Figure 5a shows
the Fe fingerprints in the XPS spectrum for a 0.2 wt %
thiophene fiber. The Fe 2p signal is evident, showing spin−
orbit splitting at 707.1 eV (Fe 2p3/2) and 720.3 eV (Fe 2p1/2)
corresponding to metallic Fe. There is a clear broadening of
both peaks toward higher binding energies with increasing
thiophene concentration (inset), suggesting that the two
emerging contributions to the spectra at 712 and 725 eV are
related to Fe−S compounds23 in the catalyst particles. The
observation of S 2p core levels (Figure 5b) provides further
confirmation of sulfur compounds. Figure 5c then presents a
detail spectrum of C 1s core level showing a high-intensity
sharp peak at 284.5 eV from sp2 CC, a small shoulder at
285.7 eV related to a low amount of sp3 C−C carbon, and a
broad peak centered at 290.5 eV related to π−π*transitions
attributed to the interaction between the CNTs in the
bundles.24 No changes in the peaks in the region (280−300
eV) are observed with an increasing amount of thiophene
precursor. These measurements rule out the possibility of S
Figure 4. Plots of the effect of increasing sulfur precursor on fiber
linear density (a) and on the relative mass per unit length of individual
CNTs (CNT linear density) determined from TEM (b). The close
correspondence implies that fiber linear density increases almost
exclusively due to the increment in CNT number of layers and
diameter but at an almost constant number of CNTs. Schematic
representations of the different CNTs, showing the evolution SWNT
to collapsed CNT to MWNT, are also included.
increase in thiophene makes larger catalyst nuclei due to
particle coarsening through collisions in the gas-phase,22 as
discussed later in the text. (This argument assumes equal CNT
length, which is difficult to measure due to the exceptionally
long length >1 mm of the CNTs produced by this process,12
but our extensive TEM observations so far have not suggested
the contrary.)
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Figure 5. Composition analysis of the CNT fibers by XPS showing the
presence of sulfur associated with the catalyst but not incorporated
into the CNTs as a dopant. (a) Fe is observed in a metallic state,
associated with S. (b) The signal from S further confirms its presence
in the sample. (c) Deconvolution of C-related peaks showing the
predominance of sp2 CC and the absence of S doping.
doping by direct incorporation into the CNTs and establish
that all S in the sample is associated with the catalyst.
The amount of S in the fiber is measured by organic
elemental analysis through analysis of gas evolution during
combustion. Such measurements give a weight ratio of S/C ≈
0.005 in the fiber. Combined with TGA values for Fe/C,
typically around 0.1 depending on synthesis conditions (S7), it
is possible to determine the full concentration of Fe, C, and S in
the fibers. These values are listed in Table S7a, but clearly the
more relevant concentrations are those in the catalyst particle,
not in the whole fiber. This requires knowing the amount of C
in the catalyst, which we approximate by measuring the amount
of C segregated by the catalyst particles using HRTEM and
from knowledge of the catalyst crystallographic phase obtained
by synchrotron XRD.
Figure 6a presents a typical XRD pattern of a sample
produced with 1.5 wt % of thiophene, with strong peaks at 3.35
Å, 2.13 Å, and 2.07 Å, related to (002) and (100) reflections of
CNTs and Fe FCC (111), respectively. The (002) reflection
from CNTs is broadened by contributions at higher d-spacing
from imperfect CNT stacking, which in fact extend up to the
small-angle region.25 We also observe an increase in the ratio of
intensities CNT(100)/FCC(111) with increasing thiophene
concentration (Figures 6b and 6c), confirming the formation of
more graphitic layers per amount of Fe catalyst when using
more S promoter.
Figure 6. XRD patterns showing the presence of FCC Fe and
graphitic CNTs. (a) (111) reflection from FCC Fe and (002) and
(100) from CNTs. (b), (c) Increasing thiophene concentration results
in a higher CNT(100) intensity relative to FCC Fe(111), suggesting
that more graphitic layers per catalyst are formed.
XRD also shows the negligible content of cementite (Fe3C)
in the sample (difficult to determine unambiguously by
HRTEM); an important observation for our determination of
C content by HRTEM. From HRTEM observation of catalyst
particles (S7) we find that all inactive particles are encapsulated
by several graphitic layers (Figure 7b).26 We assume the carbon
of these layers to have diffused in the catalyst during CVD and
then precipitated either at the point of reaction or upon
cooling.26 Thus, the carbon fraction (in wt %) in these particles
can be estimated from the number of encapsulating layers/
particle diameter, (α), which is approximated as 3να/ρFe, where
ν (0.75 × 10−6 kg/m2) is the areal density of a graphene sheet,
and ρFe the volumetric density of Fe (S5). Our HRTEM
analysis gives values of α of 0.2−0.64 nm−1, higher than 0.2−
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Figure 7. (a) Isothermal section of the Fe−C−S phase diagram at 1400 °C showing the location of inactive catalyst particles based on experimentally
approximated composition and where active catalyst composition and CNT growth are expected. HRTEM micrographs of (b) an inactive FCC Fe
catalyst particle encapsulated by a couple of graphene layers and (c) an active particle that catalyzed the growth of a MWNT, showing also FCC Fe
crystallographic structure (FFT in inset).
0.3 nm−1 reported for encapsulated catalyst particles produced
in low temperature CNT synthesis.26
The concentration C/Fe estimated from TEM is then
combined with the ratio of S/Fe (same in the fiber and the
catalyst), to calculate the complete elemental composition of
the catalyst particles (details in S7). The results for these
inactive particles give an approximate atomic composition
Fe:C:S of 70:26:4, only slightly dependent on reactant
concentration (Table S7b).
Figure 7a shows the location of these inactive catalyst
particles in the iron-rich corner of the isothermal 1400 °C
section of the Fe−C−S ternary diagram.12 Their composition is
where the S-rich and C-rich liquids (L1,L2) coexist with solid
carbon (i.e., graphite), toward the boundary with cementite.
Relating these observations to the actual catalyst particles
from which CNT grew is, however, a significant challenge.
Unlike most CNT substrate growth processes by lowtemperature CVD where there is a large proportion of active
particles (up to 20%27 and 84% upon addition of water
vapor28) easily accessible by inspection of the tip or root of the
array, in our method the proportion is estimated to be around
0.05% (S8). Thus, the probability of finding an active particle in
2000 inactive or the end of a 1 mm CNT in between the ∼107
CNT per fiber cross section is rather low.
Still, in Figures 7b and 7c we present HRTEM images of a
typical inactive particle (Figure 7b) and an example of one of
the active particles found (Figure 7c). Overall, our postreaction
comparison has not exposed significant differences between
them, other than their shape. Active particles have the typical
elongated shape widely observed in CNT growth, whereas
inactive ones are mainly spherical. Both have predominantly the
structure of FCC iron, evidenced in Figures 7b,c by the {200}
planes with 1.9 Å d-spacing.
The emerging model is that all Fe particles saturate rapidly
with C in their trajectory through the reactor. Most of them
encounter S when their concentration of C is already very high,
approaching the phase-diagram region (Figure 7a) that does
not favor CNT growth because the two immiscible liquids (one
S-rich the other C-rich) are in equilibrium with solid carbon;
therefore, encapsulation of the particles is more likely than
lifting off of the graphitic cap that starts CNT growth. Remnant
C in the supersaturated particle precipitates upon cooling.26
Following this argument, the very small proportion of active
particles would be those in which S diffusion occurred at lower
C concentrations corresponding to the regions of the ternary
phase diagram in Figure 7a where the liquid(s) is (are) not in
equilibrium with C(S) and therefore the incoming carbon would
upon saturation of the catalyst particle get extruded to form a
CNT. This argument is in agreement with S limiting the
diffusion of C in Fe, as observed both in CNT growth and in
steel literature,12 and is also supported by our finding that the
ratio S/C plays a far more central role in controlling CNT
growth than Fe/C. The postsynthesis TEM analysis, by
showing the similarity between active and inactive particles,
highlights the subtle role of sulfur as a promoter, which even at
small quantities (5 < at.%) plays a key role in controlling CNT
growth not only by limiting C diffusion but also assisting C
reconstruction and edge growth.12
The increase in number of layers and tube size can then be
explained by the increase in size of the active catalyst particles.
As the amount of available sulfur in the reactor is increased
more particles become active, but the probability of them
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coalescing increases as well and so does their size22 (S9). Their
larger diameter implies that they can accommodate a few
additional layers for the same composition as smaller ones.
During CNT growth the rate of incoming C scales as particle
radius squared12 whereas the rate of extrusion scales as number
of layers (n) times radius, thus with increasing the size of the
active catalyst particle the greater availability of C can produce
the formation of additional layers at the same elemental
composition.
An implication of the model is that the kinetics of
decomposition of the S and C precursors is key to control
the reaction yield and proportion of active catalyst, but the
kinetics of particle collisions are also central for the control of
CNT structure. Further work is directed at decoupling these
two effects to increase control of the reaction.
■
Article
ASSOCIATED CONTENT
S Supporting Information
*
Additional sample characterization data, TGA curves, Raman
spectra, HRTEM data and calculations, and a video of nearly 2
h of continuous spinning. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We are grateful to M. E. di Falco and R. Marcilla for OEA
measurements and to A. Windle and Y. Cui for useful
discussions. Synchrotron XRD experiments were performed
at NCD beamline at ALBA Synchrotron Light Facility with the
collaboration of ALBA staff. TEM characterization was carried
out at “ICTS-Centro Nacional de Microscopiá Electrónica”.
Financial support from MINECO (Spain) and FP7-PeopleMarie Curie Action-CIG is acknowledged.
CONCLUSIONS
Our results show that it is possible to produce continuous CNT
fibers on a kilometer scale with control of the type of CNTs by
adjusting the concentration of S precursor in the reaction. At
low S concentrations CNTs are predominantly SWNT of
approximately 1 nm diameter. Increasing S makes catalyst
particles larger and increases inner diameter and number of
layers of CNTs, producing at mid-S collapsed CNTs and at
higher S, MWNTs. This transition from SW up to MW is
clearly seen in Raman, particularly as an upshift in the 2D band
due to the increase in number of adjacent layers, similar to
observations in graphene.
The increase in CNT number of layers and inner diameter is
responsible for a higher fiber linear density and reaction yields.
A higher intensity ratio of the graphite (100) reflection over the
FCC Fe (111) reflection observed by synchrotron XRD
confirms the increase in graphitic layers at turbostratic
separation.
Through a combination of TGA, XPS, OEA, and HRTEM
we estimate the compositions of catalyst particles and find that
inactive particles, which represent over 99.9% of those
produced in the reactor and aggregated in the fiber, are
encapsulated due to their high C content. Encapsulation occurs
because at high C contents the catalyst is in equilibrium with
solid C, i.e. graphite. In contrast, active particles have an earlier
arrival of sulfur that limits carbon solubility. Upon C arrival and
saturation the catalyst particle is not in thermodynamic
equilibrium with solid C, thus, instead of forming an
encapsulating layer C gets extruded as a CNT and the particle
remains active.
At higher S concentrations the amount of active catalyst in
the gas-phase increases, enlarging particles through collisions
and coalescence. As a result, the inner and outer diameter and
number of layers of the CNTs increase too.
Overall, the results show control over CNT growth and
thermodynamic arguments to support the observed mechanisms. They also evidence the importance of the point of
availability of S, which further studies are directed at studying in
detail. An implication of our results is that the exceptionally fast
CNT growth rate in this method (∼1 mm/s) is related not only
to enhanced diffusion and faster kinetics due to the high
temperature but also to the scarcity of active catalyst particles
and abundance of available C.
■
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NOTE ADDED AFTER ASAP PUBLICATION
This paper was published ASAP on May 20, 2014, with an error
in Figure 7 and in the related text. The corrected version was
published ASAP on May 21, 2014.
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