Supporting Information
1. Catalyst preparation and characterization
For preparation Fe and Fe:Mo catalysts supported on Al2O3 powder a solution in
methanol of the weighted amounts of ammonium heptamolybdate tetrahydrate (Sigma
Aldrich) and/or iron (II) sulphate heptahydrate (Sigma Aldrich) was added to a methanol
suspension of the alumina powder (DEGUSSA Corporation, BET surface area ~100 m2
g-1). The solvent was evaporated under nitrogen gas flowing, and the resultant cake
heated to 100-120 C for 3h. The fine powders were then calcined for 1h at 500 ºC.
Finally, the resulted powder was then heat treated with Ar gas flowing for 30 min at 820
C and re-grounded before loading into the CVD equipment. The BET surface area of
final catalyst was 43 m2 g-1. The catalyst composition was confirmed using Energy
Dispersive X-ray analysis in a scanning electron microscope.
Because of the catalyst preparation procedure, the small iron particles are buried
inside the pores of the support material (Al2O3) and therefore, direct study of the catalyst
nanoparticles by TEM is extremely difficult. For estimation of resulted catalyst particles
size (r) we used the magnetic characterization as a alternate method described in [A.R.
Harutyunyan et al., “Evolution of catalyst particle size during carbon single-walled
nanotube growth and its effect on the tube characteristics”, Journal of Applied Physics
100, 044321 (2006)]. We use the blocking temperature
TB= K<V>/25kB
and the plot of magnetization of the catalyst nanoparticle as a function of H/T using a
Langevin function fitting:
M/Ms = coth(H/kBT)-kBT/H
where K-is the magnetic anisotropy constant, kB=1.38 x10-23J is the Boltzmann constant,
and <V(r))> is the volume of the particle, Ms is the saturation magnetization,  is the true
magnetic moment of each particle, H is the external magnetic field and T is the absolute
temperature. For determination of blocking temperature (TB), the temperature
dependence of the magnetization under zero field cooling (ZFC) and field cooling (FC)
conditions was measured.
2. CVD synthesis
The synthesis of SWNTs was performed in a CVD apparatus with a MS (Thermo
Star GSD 300T) attached at the gas outlet (Figure S1). In a regular experiment, ~100 mg
of catalyst were introduced in the reactor (2 inch diameter quartz tube) on a quartz boat.
The temperature was then increased to 500 ºC under a flow of H2 and He gases (40 and
100 sccm, respectively), and maintained for 1h. After that, the gases were switched to Ar
1
(200 sccm) and the furnace ramped up to 820 ºC at 10 ºC/min. Once the temperature was
reached, CH4 gas (60 sccm) was added to the Ar stream, for the desired time. After the
synthesis, the CH4 gas was turned off and the reactor was cooled down under Ar gas (200
sccm). The synthesis time was defined as the period of time that CH4 was supplied into
the reactor. The MS attached at the gas outlet of the reactor allowed us to monitor in situ
the catalyst activity during the synthesis of SWNTs, by following the H 2 formed in the
decomposition of CH4 (CH42H2+C, H=74.4 KJ/mol). In addition, the system was
also connected to a gas chromatograph (30-m porapak-Q capillary column in a Shimadzu
GC-17A), for sampling the gas stream at regular intervals to identify other reaction
products than H2. Along with H2 and non-reacted CH4, we detected very small amounts
of C2H4 and C2H6 hydrocarbons. Therefore, the H2 detected during the synthesis could be
correlated with the catalyst activity, and therefore with the carbon formation.
Before introducing the gases into the reactor for the CVD synthesis of SWNTs,
they were passed through a purification cartridge (Praxair) to trap residual O2 and H2O.
After catalyst reduction the reactor was thoroughly purged for about three hours by Ar
gas in order to remove residual H2 and He from the reactor. For comparison within
independent measurements by mass spectroscopy, N2 gas, added at the gas outlet, was
used as a standard gas. In addition to the H2, CH4 and H2O molecules were monitored.
Gas inlet
Gas outlet
GC
Catalyst
Furnace
He
H2
Ar
CH4
MS
Mass flow controllers
Figure S1. Experimental CVD set-up used for the growth of carbon SWNTs, with
attached MS and GC.
3. Sample characterization
3.1. Thermogravimetrical analysis. The carbon up-take in the CVD experiments was
determined by Temperature Programmed Oxidation (TPO) in a TA instruments 2960
thermogravimetrical analyzer. For each experiment, ~10 mg of sample were placed in an
alumina boat, and were heated at 5 ºC/min until 1000 ºC with air flowing (100 sccm).
The instrument was calibrated prior to perform the experiments with a weight calibration
2
and a temperature calibration. For the temperature calibration, different standards (In, Sn,
Zn, Al, Ag and Cu) were used covering the whole temperature range of the experiment.
3.2. DSC measurements. Differential scanning calorimetric measurements (DSC) were
performed on a STA 449C (NETZSCH) and SDT 2960 (TA Instruments) instruments, in
order to measure the catalyst melting point. For each measurement, around 20 mg of
sample were placed in an alumina crucible. The temperature was increased with a heating
rate of 10 C/min to 1100 C, with Ar gas (200 sccm) flowing. A temperature and a heat
flow calibrations were performed. The temperature calibration was done using six
different standards (In, Sn, Zn, Al, Ag and Cu) to cover the temperature range of the
experiment. The heat flow was calibrated using a sapphire standard.
3.3. Raman measurements. The carbonaceous samples prepared were characterized
using Raman scattering with laser excitations of 532 and 785 nm (Thermo Nicolet
Almega Raman spectrometer equipped with a CCD detector).
4. Experiments with isotopic gases (12CH4 and 13CH4)
In order to study the catalyst lifetime for SWNTs growth, experiments with
sequential introduction of 12CH4 and 13CH4 were performed. The synthesis was similar to
the procedure described in section 2, except that after certain period of providing 12CH4,
the gas was switched to 13CH4. Some modifications of the CVD set-up were necessary for
these studies (Figure S2). The two gas isotopes were injected through different gas lines
using a three way valve, in order to minimize any discontinuity in the gas supply into the
reactor, and to be accurate in the time of the switching of gases. Prior to starting the
synthesis, the 13CH4, diluted in Ar (200 sccm), was run through the line. After a few
minutes, the valve was turned so the line for H2, He, Ar and 12CH4 was now the one
supplying gas into the reactor. The reactor was then purged with Ar to remove any
residual 13CH4 before starting the experiment. This way, the 13CH4/Ar mixture was ready
in the secondary line for the gas switching. When it was time to replace one gas for
another, a simple turn in the valve was all that was necessary. The “discontinuity”
observed in the insets of Fig. 1b has only “mechanical” nature, due to the slight pressure
changes during the switching of the gases. This change is obviously more “visible” when
the curve has a negative slope (hydrogen concentration is decreasing, third inset in Fig.
1b).
3
Three way
Gas inlet
valve
Gas outlet
GC
Catalyst
Furnace
Mixer
He
H2
Ar
CH4
12
Ar
13
CH4
MS
Mass flow controllers
Figure S2. Experimental CVD set-up used for the growth of carbon SWNTs with
sequential introduction of 12CH4 and 13CH4 isotopes.
5. Theoretical calculations
The simulations show that the interaction strength between dissolved carbon and the
Al2O3 substrate is not critical for the results presented here, and we make it identical to
that for Fe-Al2O3 (a more accurate description would require a many-body description).
Table S1. Potentials used in the simulations
Potentials
Fe
CD
Al2O3
Johnson
Morse
1.74 (Fe2
binding)
 = 0.35
D = 0.153
Johnson
LJ (12-6)
Born-Mayer
Fe
CD
 = 0.35
 = 0.00287
Morse
D = 0.153
All potential strength constants are in eV/bond.
CD: Dissolved Carbon
 : Well depth
4
Born-Mayer type repulsive and many-body attractive energy (1,2)

E   A exp 
i j

1/ 2


 rij

 rij
 
p  1     2 exp  2q  1 

 r0
 i  j
 r0
 
A = 0.13315 eV,  = 1.6179 eV, p = 10.50, q = 2.60,
First neighbor distance: r0 = 2.553 Å
Distance between ith Fe and jth C atoms: rij
Lennard Jones (12-6) potential
E = 4 [( / r)12 - ( / r)6],
 = 0.00287 eV
Hard sphere radius of the atom (distance at which E is zero):  = 3.469 Å
Johnson potential (3, 4)
E FeCD
  rij / r0  rc / r0  3  rij / r0  rc / r0  2 
  3
 H rc  rij 
    2
r
/
r

1
r
/
r

1
  c 0
i j

 c 0
 
Where
0 rc  rij  0
H rc  rij   
1 rc  rij  0
Well depth for Fe - dissolved C (CD):  = 0.35 eV
Equilibrium distance: r0 = 1.94 Å
Cut off radius: rc = 2.53 Å
Distance between ith Fe and jth C atoms: rij
5
Morse potential (5)
3
E  D exp  2 ( z i  z 0 )  2 exp   ( z i  z 0 )
i 1
Parameter values for triple Fe layers:
Well depth: D = 0.15274 eV
Parameter controlling the width of the potential well:
 = 1.26835 Å -1
Equilibrium distance from surface: z0 = 2.21861 Å
6. Molecular Dynamics Simulations
MD simulations are carried in the NVT ensemble using the Verlet algorithm with
a time step t =1.0 fs and Nos´e-Hoover thermostat. For the initial configurations we
search for the best possible energy minima by randomly arranging atoms in a spherical
nanoparticle, carefully optimizing the positions of iron and carbon atoms and finally
annealing the nanoparticles for 6×106 MD iterations (6 ns): each nanoparticle is first
heated to high temperature (from 1000 K to 1400 K depending on the size of the particle)
for 0.6×106 steps, kept at constant temperature for another 0.6×106 iterations, and finally
cooled to 0 K during the remaining 4.8×106 MD steps.
The melting phenomenon is analyzed by performing several MD simulations
starting at about 300 K below the expected melting point with temperature increments of
10 K for small (N < 100) and 20 K for large clusters (with 5 K upon approaching the
transition). Only the lowest temperature simulations begin from the annealed initial
structures: the others start from the final configurations (positions, forces, velocities) of
the preceding temperature simulation. Data gathering of the energies and other averages
are performed over 106 MD steps.
Several dynamical and thermodynamical properties such as total energy, specific
heats, Lindemann index, and Lindemann index statistical fluctuation are used to identify
the “liquidus lines” (max solid points) of the phase diagrams (see Ref. 6).
6
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08
0
2
4
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86
6
8
66
88
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X Axis Title
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4
2
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00
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7. Additional figures
44
Y Axis Title
8
6
4
0
2
4
8
Y Axis Title
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8
10
12
13
C
C
6
4
20min pure 12CH4 or 13CH4
2
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2
4
6
8
10
X Axis Title
20000
Y Axis Title
6
X Axis Title
0
25000 0
Raman int. (a.u.)
=785 nm
Fe:Mo:Al2O3
2
10
0
Growing
10
10
X Axis
Axis Title
Title
X
15000
###
10000
5000
13min12CH4+10min 13CH4
0
1000
1200
1400
1600
X Axis Title
20min 12CH4+10min 13CH4
1000
1200
1400
1600
Terminated
30min 12CH4+10min 13CH4
1000
1200
1400
1600
Raman shift (cm-1)
Figure S3. Raman spectra for SWNTs grown on Fe:Mo:Al2O3 catalyst by sequential
introduction of 12CH4 and 13CH4 gases for the durations indicated.
90 min
T(synthesis)
Exo
838 C
Heat Flow (a.u.)
20 min + heat treated
815 C
20 min
850 C
7 min + heat treated 5h
793 C
7 min + heat treated 83 min
790 C
7 min
867 C
5 min
798 C
3 min
807 C
Initial catalyst
890 C
Al2O3
400
x20
500
600
700
800
900
1000
Temperature (C)
Figure S4. DSC curves of pristine catalyst and catalyst after SWNTs growth for
different synthesis duration. The curves for samples with additional heat treatment are
also included.
7
H2 concentration (a.u.)
15
Al2O3
Fe:Al2O3
Error bar
10
5
0
0
10
20
30
60
70
80
Time (min)
Figure S5. Evolution of hydrogen concentration obtained passing a mixture of methane
diluted in Ar over Al2O3 and Fe:Al2O3=1:15 (molar ratio) catalysts at 820 ºC for 90 min.
Fe:Al2O3
Raman intensity (a.u.)
=785 nm
1.5 min
2 min
x 20
3 min
5 min
7 min
ID
IG
20 min
90 min
300
600
900
1200
1500
1800
Raman shift (cm-1)
Figure S6. Evolution of the Raman spectra with the synthesis time for Fe:Al2O3 catalyst
(molar ratio 1:15) at 820 ºC.
8
Raman intensity (a.u.)
Fe:Mo:Al2O3
10 s
15 s
30 s
20 min
90 min
300
600
900 1200 1500 1800
-1
Raman shift (cm )
Figure S7. Evolution of the Raman spectra with the synthesis time for Fe:Mo:Al2O3
catalyst (molar ratio 1:0.21:15) at 820 ºC.
100
Weight (%)
80
60
40
Raw material
Material after
HF treatment
Purified SWNTs
20
0
0
200
400
600
800
1000
Temperature ( C)
Figure S8. Temperature programmed oxidation curves for raw material, material after
HF treatment and purified SWNTs (selective oxidation to remove carbon coating from
catalyst surface and HCl treatment to remove metal catalyst) obtained with Fe:Mo
catalyst at 820 ºC and 30 min of synthesis time. Although the bimetallic catalyst is liquid
for longer than the pure Fe catalyst, it is inactive for SWNT growth after t>20min. We
attribute this to the simultaneous growth of SWNTs and other sp2 carbon structures,
which gradually cover the surface of the catalyst deactivating it. The fact that removing
metal catalyst from carbon SWNT sample by acid treatment requires preliminary
selective oxidation at T~300-400 ºC (oxidation temperature for disorder carbon) to
provide accessibility for the acid, independently confirms that particles are indeed
encapsulated by carbon.14
9
Supporting online references
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2. J.Stanek, G. Marest, H. Jaffrezic, and H. Binczycka, Phys. Rev. B 52, 8414 (1995).
3. R. A. Johnson, Phys. Rev. 134, A1329 (1964).
4. F. Ding, K. Bolton, and A. Rosén, J. Phys. Chem. B 108, 17369 (2004).
5. P. M. Morse, Phys. Rev. 34, 57 (1929).
6. A. Jiang, N. Awasthi, A. N. Kolmogorov, W. Setyawan, A. Börjesson, K. Bolton, A.
R. Harutyunyan, and S. Curtarolo, “Theoretical study of thermal behavior of free and
alumina-supported Fe-C nanoparticles”, in press, Phys. Rev. B, (2007).
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