Wet-chemical catalyst deposition for scalable synthesis of vertical aligned carbon nanotubes on metal substrates

Article abstract of DOI:10.1016/j.cplett.2011.06.027

A scalable process for carbon nanotube (CNT) growth on metallic substrates has been developed including dip-coating steps for the wet-chemical catalyst and co-catalyst layer deposition and a subsequent chemical vapor deposition step. Organic metal salt/2-propanol solutions were applied as precursors for alumina co-catalyst thin films and the actual Fe (Co, Mo) catalyst layer. Vertical aligned carbon nanotube forests were obtained on catalyst-coated nickel foil in a thermal CVD process at atmospheric pressure and 730 °C using ethene as carbon source. The influence of the catalyst composition on growth rate, density and structure of resulting CNT films was investigated.

Full text of DOI:10.1016/j.cplett.2011.06.027

Chemical Physics Letters 511 (2011) 288–293
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Wet-chemical catalyst deposition for scalable synthesis of vertical aligned
carbon nanotubes on metal substrates
b
b
c
a
a,b
S. Dörfler a, , A. Meier , S. Thieme , P. Németh , H. Althues , S. Kaskel
⇑
a Fraunhofer Institute for Material and Beam Technology (IWS) Dresden, Winterbergstraße 28, 01277 Dresden, Germany
b Institute for Inorganic Chemistry, Dresden, University of Technology, Bergstraße 66, 01069 Dresden, Germany
c Hungarian Academy of Sciences, Chemical Research Center, Institute of Nanochemistry and Catalysis, Department of Surface Modifications and Nanostructures,
1025 Budapest, Pusztaszeri ut 59-67, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
A scalable process for carbon nanotube (CNT) growth on metallic substrates has been developed includ-
ing dip-coating steps for the wet-chemical catalyst and co-catalyst layer deposition and a subsequent
chemical vapor deposition step.
Organic metal salt/2-propanol solutions were applied as precursors for alumina co-catalyst thin films
and the actual Fe (Co, Mo) catalyst layer. Vertical aligned carbon nanotube forests were obtained on cat-
alyst-coated nickel foil in a thermal CVD process at atmospheric pressure and 730 °C using ethene as car-
bon source. The influence of the catalyst composition on growth rate, density and structure of resulting
CNT films was investigated.
Received 17 February 2011
In final form 11 June 2011
Available online 16 June 2011
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
catalyst layer on nickel foils. The catalyst compositions were tested
to achieve an optimum in the CNT APCVD process using ethene as
The efficient synthesis of vertical aligned carbon nanotube
(VA-CNT) films via CVD growth was discovered in 2004 by Hata
et al. [1,2] and has been intensively studied since then. Most prom-
ising applications are field emission displays [3–5], lithium ion
cells, optical polarizers [2,6], gas sensors [7], heat conductors and
electrochemical double layer capacitors [2]. For the scalable
production of such devices atmospheric pressure deposition
techniques and direct synthesis on conductive substrates need to
be developed.
For the CVD growth, substrates are typically covered by a buffer
layer with the actual catalyst layer on top. Most frequently used
are low pressure physical vapor techniques for the deposition of
Al2O3 buffer layers and Fe thin films as catalyst [1,8–11]. An alter-
native route is the wet-chemical catalyst deposition using metal
nitrates, chlorides and acetates in polar organic solvents like etha-
nol and methanol [12–16] deposited mostly for low-pressure CVD
processes. Precursor solutions can be applied via dip-coating which
is essential for the development of scalable processes. However, for
the direct deposition on metal substrates, challenges like poor wet-
ability and sufficient homogeneity need to be overcome.
precursor. Growth rates and resulting CNT properties were
investigated.
2. Experimental
2.1. Aluminum-complex for Al2O3 deposition
The stable aluminum(2,4-pentadionate)x(iso-propoxide)x Al
(AcAc)x(iPrO)3 complex in 2-propanol is prepared by hydrolysis
of aluminum isopropoxide (Al(iPrO)3) (MERCK) [17]. Al(iPrO)3 is
added to a 2 molꢀlꢁ1 solution of acetylacetone (AcAcH, VWR) in
2-propanol with a molar ratio of 0.5 according to AcAcH. The pre-
cursor solution is stirred for 2 h followed by water hydrolysis of the
aluminum complex with a molar ratio of 0.85:1 H2O:Al(iPrO)3.
Afterwards, the concentration of Al(iPrO)3 is adjusted to 60 gꢀlꢁ1
with 2-propanol and the resulting complex is agitated for further
3 h. Finally, concentrated nitric acid (65%, ACROS) is slowly added
to the solution forming a clear, slightly yellow aluminum complex.
Before dip coating, 0.05 wt.% (regarding the aluminum complex)
cetyltrimethylammonium bromide (CTAB, ACROS
ORGANICS) as sur-
In this Letter, wet chemical methods based on metal complex
solutions were developed to deposit the buffer as well as the
factant is added to enhance the substrate wet-ability.
2.2. Synthesis of carboxylate solutions
2.2.1. Mx(2-ethylhexanoate)x-complexes
⇑
Corresponding author. Fax: +49 351 83391 3300.
E-mail addresses: susanne.doerfler@iws.fraunhofer.de (S. Dörfler), andreas.
meier@chemie.tu-dresden.de (A. Meier), peter.nemeth@chemres.hu (P. Németh),
holger.althues@iws.fraunhofer.de (H. Althues), stefan.kaskel@chemie.tu-dresden.de
(S. Kaskel).
Fe(2-ethylhexanoate)3 (ALFA
Co(2-ethyhexanoate)2 (SIGMA
A
ESAR, 50% in mineral spirits) and
A
LDRICH, 65% in mineral spirits) are
0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.06.027

S. Dörfler et al. / Chemical Physics Letters 511 (2011) 288–293
289
dissolved in 2-propanol (BERKEL). For optimizing the Fe:Co ratio and
2.5. Characterization methods
the growth kinetics an entire catalyst concentration of 0.22 molꢀlꢁ1
in iPrOH/mineral spirit is used. For controlling the carbon nano-
tube diameter the concentration is varied starting with 0.022
molꢀlꢁ1 and ending with 0.22 molꢀlꢁ1. Experiments with Mo addi-
The CNT morphology is characterized by a scanning electron
microscope (SEM) (Zeiss type DSM 982 GEMINI) with a heat-able
field emission tungsten cathode. All samples are tilted by 45° so
that CNT lengths appear shortened in the SEM images. For the
investigation of the CNT film homogeneity substrates with
2.6 cm maximum length could be analyzed by SEM due to the size
limitation of the substrate holder.
For transmission electron microscopy (TEM) samples are
crushed under ethanol and deposited Cu grids covered by carbon
supporting films. TEM data are acquired with a MORGAGNI 268D
TEM (100 kV; W filament, top-entry; line-resolution = 0.3 nm).
The CNT structure is investigated by a Raman spectrometer
tion are done with Mo(2-ethylhexanoate)4 (STREM
CHEMICALS).
For the deposition procedure on the metallic substrates rough
nickel foil (Ra = 25 nm) (ALFA
AESAR, 99% Ni) are cut into pieces of
3.5 ꢂ 3.5 cm2 and primarily coated with Al2O3 and than with the
respective Fe/Co/Mo combination.
2.2.2. M(Oleate)x synthesis
Different metal oleates are synthesized by a simple phase-trans-
fer reaction similar to that described by Park et al. [18]. For the prep-
aration of a 0.28 molꢀlꢁ1 metal oleate solution in toluene (VWR)
20 mmol of the correspondent metal salt are dissolved in a mixture
of distilled water and ethanol. Following metal salts are applied:
FeCl3ꢀ6H2O (GRÜSSING), Cr(NO3)3ꢀ9H2O (GRÜSSING), MnCl2ꢀ2H2O
RENISHAW type RENISHAW 3000. The Ramanshift on each sample is
measured three times at two different locations using laser beam
wave lengths of 785 and 514 nm. The software GRAMS/32 Version
4.1 averages over three measured spectra.
Nitrogen adsorption is carried out with a surface area and pore
(GRÜSSING), Ni(NO3)2ꢀ6H2O (SIGMA
A
LDRICH) and CoCl2ꢀ6H2O (ABCR).
size analyzer QUANTACHROME
QUADRASORB SI. The specific surface area
An appropriate amountof sodium oleate (>82%, RIEDELde HAËN) is sus-
pended in toluene. The volume ratio between water, ethanol and
toluene is 0.43:0.57:1. After combining the solutions the mixture
is heated to 70 °C for 4 h under agitation. The resulting colored upper
organic layer is washed three times with 15 ml distilled water using
a separatory funnel.
(SSA) is calculated by using the values between 0.05 and 0.1 p/p0.
3. Results and discussion
The applied process chain includes the catalyst (oxidized iron,
cobalt and molybdenum) and buffer (Al2O3) coating procedure
and the subsequent CVD process by using ethene as carbon
precursor.
2.3. Dip coat procedure
The substrates are coated using a dip coater type LTB 300 VA
(WALTER LEMMEN). Before coating, the substrates are cleaned with
3.1. Aluminum oxide films
ethanol and annealed at 300 °C. The aluminum oxide layer is
deposited via 1.0 mmꢀsꢁ1 lifting speed; the resulting film is an-
nealed 5 min at room temperature and 5 min at 300 °C. Subse-
quently, the catalytic coating is obtained by moving the Al2O3
coated substrate out of the metal carboxylate solution with a speed
of 2.0 mmꢀsꢁ1 and a subsequent thermal treatment at room tem-
perature (5 min) and 350 °C (5 min). A dip-coated layer with
0.22 mol/l catalyst concentration has a thickness of approximately
45 nm on 30–40 nm Al2O3.
An Al2O3 supporting layer is found to be crucial for the growth
of at least 5 lm high VA-CNT via CVD. This layer acts both as co-
catalyst for the hydrocarbon decomposition [20] and as diffusion
barrier for the metal particles [11]. In this work, the aluminum
oxide film is deposited by a sol–gel synthesis developed by Yang
et al. [17] and a dip-coating process. The Al2O3 precursor is an
aqueous/alcoholic alumina sol synthesized by hydrolysis of
Al(iPrO)3 with acetylacetonate (AcAc) as chelating agent to avoid
the precipitation of aluminum hydroxide.
Figure
1 shows infrared transmission spectra of the as
2.4. CVD process
synthesized aluminum complex, Al(iPrO)3 and aluminum triacetyl-
acetonate Al(AcAc)3. The spectrum of the synthesized complex rep-
resents peaks both of the spectrum of Al(AcAc)3 and of Al(iPrO)3.
Nass et al. [21] reported a resulting mixed chelate complex using
aluminum-sec-butoxide, 2-propanol and AcAcH as educts.
CNT growth by APCVD is performed with a quartz tube (40 mm
diameter) positioned in a split tube furnace (HTM REETZ). The sub-
strate is moved into the quartz tube and annealed (65 K/min) to
the process temperature (730 °C). 1.0 slm (standard liter per min-
ute) argon (5.0, LINDE AG) as carrier gas, 0.17 slm ethene (3.5, LINDE
AG) as carbon precursor, 0.67 slm hydrogen (5.0, LINDE AG) as
in situ reductive for the oxidized catalyst layer are used for the
CNT deposition. A small water amount (85 ppm) as soft oxidant
for in situ emerging amorphous carbon plays an important role
for the catalyst activity enhancement [1,19]. Water is transported
in the entire gas flow by a stainless steel bubbler with argon as car-
rier gas.
The optimal water vapor concentration in the entire gas flow
has been investigated by a laser diode spectrometer (Fraunhofer
IWS) containing a laser with a small frequency band width in the
near infra-red. For this investigation polished silicon with cata-
lyst/buffer layer deposited by pulsed laser technique was used as
an ‘ideal’ substrate. An optimum of 85 ppm water vapor could be
observed resulting in the highest CNT film thicknesses.
20 min growth time is applied for the investigation of the opti-
mal catalyst composition and the experiments regarding the differ-
ent CNT diameters. CNT lengths are controlled by variation of the
growth time from 5 to 40 min.
Figure 1. Infrared transmission spectra of the synthesized aluminum complex, Al
(AcAc)3 and Al(iPrO)3.

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S. Dörfler et al. / Chemical Physics Letters 511 (2011) 288–293
The complex/2-propanol solution was modified by adding
cetyltrimethylammonium bromide (CTAB) as surfactant to
enhance the wet-ability on the metallic foil. The resulting CNT
layer on nickel foil becomes significantly inhomogeneous without
CTAB addition.
3.2. CNT films
Catalytic active transition metal (Fe, Co, Ni, Mo) [22] complex
mixtures are subsequently deposited on the Al2O3 coated sub-
strates and tested as catalyst in the CVD process at 730 °C.
Starting with Fe as catalyst metal, several catalyst complexes or
rather salts in 2-propanol, ethanol, water, heptane, toluene
and pentane are screened for the catalyst film coating: Fe(NO)3,
Fe(acac)3 [23], FeCl3 [24], Fe(oleate)3 and Fe(2-ethylhexanoate)3.
The combination of Fe(2-ethylhexanoate)3 and 2-propanol results
in the best macroscopic catalyst and with it CNT layer
homogeneity.
Figure 3. CNT film thickness vs. growth time.
Homogeneous black coatings on the metal substrates are ob-
served for all samples after at least 5 min CVD process indicating
efficient CNT growth. However, orientation and height of the films
after 20 min growth rate differ for the different catalyst composi-
tions. The optimal Co content in the pure Fe catalyst layer is inves-
tigated. Figure 2 shows that the highest carbon nanotube forests
in the tube interior so that they become ineffective. Figure 5 shows
TEM images of scratched-off CNTs with dark catalyst particles lo-
cated in the middle of the tubes.
For 2.6 cm substrate length the CNT film after 20 min growth
time is much more homogenous (176
5 lm) than the CNT layer
after 40 min (207 25 m) probably due to gas diffusion limita-
l
(170
lm) are observed with 40 mol% Fe and 60 mol% Co catalyst
tions or catalyst inactivation during further growth time.
Several groups describe homogeneous VA-CNT growth on sub-
strates with a size up to 2.5 ꢂ 2.5 cm2 [13,27,9]. Yasuda et al.
[19] reported CNT CVD growth on an A 4 sized substrate coated
by Fe via sputter technique and being not specified in more detail.
Guzmán de Villoria et al. [28] presented a semi-continuous lab-
scaled process for CNT growth but did not described in detail nei-
ther the maximum coated substrate size nor the film homogeneity.
In summary, none of these as-mentioned groups investigated the
CNT films alongside the substrate length by SEM.
The synthesized CNT are multi-walled with average diameters
between 10 and 20 nm as determined by TEM. Figure 6 shows
the nitrogen adsorption isotherm of the scratched-off CNTs grown
after 20 min by FeCo 2:3 catalyst composition shown in Figure 5c:
the specific surface area (SSA) is 248 m2/g (BET) and in good agree-
ment with the theoretically determined geometrical surface of
12 nm thick tubes with a density of 1.35 g/cm3 (246.9 m2/g). The
theoretical specific surface area SSAtheo was calculated by Eq. (1):
qMWNT describes the density of one single MWNT and dMWNT the
diameter of one MWNT.
composition. CNT heights of about 130
l
m are obtained by a cata-
lyst layer containing 100% Fe. Only a thin layer of randomly
oriented CNTs is achieved with pure Co. A reason for this observa-
tion could be the higher eutectic point of Co and C compared with
Fe and C being crucial for the liquid state of the catalyst particle
and therefore for the efficient CNT growth as well [25]. With
increasing Co content in the iron layer the melting point of the sys-
tem drops so that a ratio close to 1:1 Co:Fe showed the highest CNT
deposition rate. The addition of Mo to the optimized Fe:Co system
affects the CNT growth rate in remarkable manner negatively.
Figures 3 and 4a–d show the CNT film thickness control via dif-
ferent growth times: Interestingly, in the first 5 min CNT heights
up to 35
to a CNT height of 170
tube synthesis becomes less efficient (5
l
m (7
l
m/s) can be achieved. 20 min growth time lead
m (8.5 m/s). After 20 min, carbon nano-
m/s) compared to the
l
l
l
first 20 min for several reasons: On the one hand, catalysts could
be poisoned by more and more emerging amorphous carbon [26]
so that the constant water content in the entire gas flow is insuffi-
cient for in situ oxidation. On the other hand, higher CNT forests
could also limit ethene diffusion [10,19] Furthermore, the longer
the carbon nanotubes, the more catalyst particles could be soaked
4000
qMWNT ꢀ dMWNT
SSAtheo
¼
ð1Þ
One approach to decrease the MWNT diameter is to reduce the cat-
alyst complex concentration in the dip-coating solution and there-
fore, to drop the catalyst layer thickness on nickel substrate [29]. In
this work the catalyst film thickness for the Fe:Co (2:3) system is
lowered by decreasing the catalyst precursor concentration in the
dip-coating solution. For the production of a dense vertical aligned
CNT forest by the described coating technique concentration of at
least 0.110 molꢀlꢁ1 is necessary. For this concentration a resulting
(oxidized) film thickness of 20 nm was determined by ellipsometry
on silicon substrates. Lower concentrations lead to decreased
homogeneity and density of the films and are not investigated
further.
Interestingly, a slight addition of 2–6 mol% Mo to the pure iron
complex without cobalt leads to lower, but denser nanotube for-
ests. Nishino et al. [27] reported a similar result by using wet-
chemically prepared and deposited catalyst particles. Sugime
et al. [30] observed that Mo addition suppresses the surface
Figure 2. Dependency of the CNT length on the Fe:Co catalyst ratio.

S. Dörfler et al. / Chemical Physics Letters 511 (2011) 288–293
291
Figure 4. (a–d) SEM images of dense CNT forest on rough nickel foil grown after 5–40 min.
Figure 5. (a and d) TEM images of scratched-off CNTs grown by Fe:Co 2:3 ratio (a and b) and by Fe:Mo 47:3 ratio (c and d).
diffusion and hence, Ostwald ripening of Co catalyst atoms because
Mo atoms have a larger radius than Co or rather Fe. Similar effects
could be the reason for the density increase with 6 wt.% Mo con-
tent in the iron catalyst layer, since Fe and Co atom radii are similar
(Figure 7).
The resulting CNT layers synthesized by three different catalyst
compositions were analyzed by Raman spectroscopy: The ratio of
the graphitic peak (G peak) and the peak representing disordered
carbon (D-peak) varies between 0.72 for the Fe:Co 2:3 composi-
tion, 1.02 for pure Fe, and 1.33 for the Fe:Mo 47:3 system using
a laser wave length of 514 nm. No so-called ‘radial breathing
modes’ (RBM) representing the single and double walled nano-
tubes were observed with this wavelength for all three samples.
Measuring with kLaser = 785 nm smaller G/D peak ratios compared
to kLaser = 514 nm could be observed. The difference regarding the
G/D ratios could be due to better resonance conditions with
514 nm, but the Raman spectroscopy of multi-walled nanotubes
has not been well investigated up to now [31]. For the samples
with pure Fe layer and the optimized Fe:Co composition no RBMs
could be observed. All samples with 2–8% Mo addition in pure Fe as
catalyst layer show small RBM intensities. The highest RBM inten-
sity was detected for the 6% Mo sample due to the herein before
mentioned suppression of the catalyst particle ripening. Figure 5c
and d show TEM images of scratched-off CNTs grown by 47:3
Fe:Mo catalyst composition: A slight different CNT/particle mor-
phology between catalyst system FeCo 2:3 and Fe:Mo 47:2 could
be observed, respectively. The diameter of 70 CNT at five different
sample locations was investigated. For the Fe:Co sample, the

292
S. Dörfler et al. / Chemical Physics Letters 511 (2011) 288–293
Table 1
Density and heights of the CNT forest grown by different catalyst combinations.
Catalyst layer
composition
CNT
heights
CNT forest
density
G/D peak
ratio
(785 nm)
G/D peak
ratio
(514 nm)
RBMs
(
lm)
(g cmꢁ3
)
100 mol% Fe
40 mol% Fe,
60 mol% Co
94 mol% Fe,
6 mol% Mo
130
170
0.097
0.065
0.3
0.3
1.0
1.0
No
No
100
0.128
0.4
1.3
Yes
ate synthesis can be generally applied for a wide range of different
metals and element combinations due to the high solubility of
most metal oleates in unpolar solvents. With pure Ni, Mn, and Cr
catalyst layers only thin films of randomly orientated CNTs were
obtained. Both, for the metal 2-ethylhexanoates and for the oleates
even only a small nickel addition to the Fe:Co system affects the
CNT density and length negatively.
Figure 6. BET isotherm (N2 adsorption) of scratched-off CNTs grown after 20 min
and by FeCo 2:3 catalyst composition.
4. Conclusion
In conclusion, efficient wet-chemical deposition techniques of
catalyst and buffer layer were developed and a CVD process for
the growth of CNT forests on flexible and conductive nickel sub-
strate at atmospheric pressure was applied. CNT growth could be
optimized regarding growth time and catalyst composition and
concentration. The chemical solution deposition allows thereby
for alloying catalysts just by mixing different metal complex solu-
tions. The precursors are affordable and available and the dip-coat-
ing process as well as the APCVD process has the potential for up
scaling in a continuous process.
The approach described demonstrates the feasibility of scalable
production techniques being an important step towards CNT based
nanoporous electrodes for various applications, such as novel en-
ergy storage devices e.g. supercaps and lithium sulfur batteries.
Figure 7. Raman spectra of CNTs grown by different catalyst combinations: pure
Fe, Fe:Mo 47:3 and Fe:Co 2:3.
Acknowledgments
This work has been supported by the European Commission un-
der FP7 Collaborative Research Project N2P Contract Number CP-IP
214134-2 and by the Free State of Saxony under ECEMP Project
(ENERCOAT Subproject).
average CNT diameter is 15.7 nm (standard deviation: 3.5 nm). In
contrast, a slightly decreased CNT diameter (13.0 nm) with a larger
standard deviation of 6.4 nm could be observed for the carbon
nanotubes grown by the Fe:Mo (47:3) catalyst system. Further-
more, larger quantities of small black catalyst particles at the end
and in the middle of the tubes compared to the CNTs synthesized
by the Fe:Co 2:3 catalyst layer were generated that needs further
investigations.
In contrast to the Fe:Co 2:3 system, by reducing the Fe:Mo 47:3
catalyst concentration in the dip coating process step the CNT film
homogeneity does not decrease until a concentration of 0.015 mol/
l so this proves the reduced Ostwald ripening with a slight Mo
addition, as well.
Table 1 summarizes the results for pure Fe, Fe:Mo 47:3 and
Fe:Co 2:3 as catalyst layer compositions. CNT layers grown with
the Fe:Co 2:3 system are higher compared to both other systems,
but the density is much lower.
Additionally, several mixtures of various catalyst metal oleates
such as Fe, Co, Cr, Mn and Ni are synthesized by a phase transfer
route and tested in the CVD process. The results are in good agree-
ment with the experiments on 2-ethylhexanoate complexes giving
highest growth rates with a 1:1 ratio of iron and cobalt. Homoge-
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