Synthesis of carbon nanotubes by CVD: Effect of acetylene

pressure on nanotubes characteristics

Mariano Escobar a,b, M. Sergio Moreno c,d, Roberto J. Candal a,d,M. Claudia Marchi a, Alvaro Caso b, Pablo I. Polosecki b,

Gerardo H. Rubiolo b,d,e, Silvia Goyanes b,d,*a Instituto de Quımica Fısica de los Materiales, Medio Ambiente y Energıa, CONICET-UBA, Ciudad Universitaria,

Pab2, (C1428EHA) Buenos Aires, Argentinab LP&MC, Dep. de Fısica, FCEyN-UBA, Ciudad Universitaria, Pab1, (C1428EHA) Buenos Aires, Argentina

c Centro Atomico Bariloche, 8400 S.C. de Bariloche, Argentinad Consejo Nacional de Investigaciones Cientıficas y Tecnicas (CONICET), Argentina

e Unidad de Actividad Materiales, CNEA, Av. Gral. Paz 1499, (1650) San Martın, Buenos Aires, Argentina

Available online 10 July 2007

www.elsevier.com/locate/apsusc

Applied Surface Science 254 (2007) 251–256

Abstract

The effect of acetylene partial pressure on the structural and morphological properties of multi-walled carbon nanotubes (MWCNTs)

synthesized by CVD on iron nanoparticles dispersed in a SiO2 matrix as catalyst was investigated. The general growing conditions were: 110 cm3/

min flow rate, 690 8C synthesis temperature, 180 Torr over pressure and two gas compositions: 2.5% and 10% C2H2/N2. The catalyst and nanotubes

were characterized by HR-TEM, SEM and DRX. TGA and DTAwere also carried out to study degradation stages of synthesized CNTs. MWCNTs

synthesized with low acetylene concentration are more regular and with a lower amount of amorphous carbon than those synthesized with a high

concentration. During the synthesis of CNTs, amorphous carbon nanoparticles nucleate on the external wall of the nanotubes. At high acetylene

concentration carbon nanoparticles grow, covering all CNTs’ surface, forming a compact coating. The combination of CNTs with this coating of

amorphous carbon nanoparticles lead to a material with high decomposition temperature.

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.07.�b; 81.15.Gh

Keywords: Carbon nanotubes; Chemical vapor deposition; Acetylene decomposition; Iron catalysts; X-ray diffraction; Scanning and transmission electron

microscopy; Thermal analysis

1. Introduction

Since their discovery in the early 1990s, carbon nanotubes

(CNTs) have captured the attention of researchers worldwide.

A very important amount of work has been done in order to

identify their unique structural, mechanical, chemical and

electrical properties. However, a complete development of this

* Corresponding autor at: Universidad de Buenos Aires, Facultad de Ciencias

Exactas y Naturales, Departamento de Fısica, LPyMC, Pabellon 1,

(C1428EHA) Buenos Aires, Argentina. Tel.: +54 11 4576 3353;

fax: +54 11 4576 3357.

E-mail address: goyanes@df.uba.ar (S. Goyanes).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.07.044

material will not be reached until their synthesis can be

optimized and controlled.

Carbon nanotubes are prepared by electric arc discharge [1],

laser ablation [2], and chemical vapor deposition (CVD) of

hydrocarbon gases over a catalytic material [3].

The most widely used catalyst for CNTs synthesis by CVD

method is iron. Iron nanoparticles can be prepared by different

methods [4], the most employed consists in iron nanoparticles

dispersed in a SiO2 matrix. The sol–gel synthesis method has

been reported to ensure a highly homogeneous distribution of

transition metal ions in the silica matrix [5].

The objective of this work is to describe the effect of

acetylene partial pressure on the structural and morphological

properties of CNTs synthesized by CVD on iron nanoparticles

dispersed in a SiO2 matrix as catalyst.

Fig. 1. TEM image of the catalyst after fired at 450 8C for 10 h in air.

M. Escobar et al. / Applied Surface Science 254 (2007) 251–256252

2. Experimental

2.1. Synthesis and characterization of the catalyst

Tetraethyl ortho silicate (Aldrich) (5 ml) was mixed with

0.9 M iron nitrate (Riedel de Hagen) aqueous solution (7.5 ml)

and ethanol (10 ml) by magnetic stirring for 20 min. A few

drops of concentrated hydrogen fluoride (0.4 ml) were then

added, and the mixture was stirred for another 20 min. The

mixture was then dropped onto a borosilicate glass plate, and air

dried to form a film of thickness 0.30–0.50 mm. After that, the

mixture was dried overnight at 80 8C to remove the excess of

water and ethanol. During this drying process the gel cracks

into small pieces of area 5–20 mm2. Finally the substrates were

placed in a ceramic boat and introduced into the chamber of a

tube furnace. Substrates were fired at 450 8C for 10 h in air and

then reduced at 600 8C for 5 h in a flow of 9% hydrogen in

nitrogen at 180 Torr.

X-ray diffraction patterns (XRD) were recorded for substrates

at different synthesis stages. X-ray diffractometer Siemens

D5000, with Cu Ka radiation and a graphite monochromator was

employed. Some samples were also characterized by transmis-

sion electron microscopy (TEM-EM Philips 301).

2.2. Synthesis and characterization of carbon nanotubes

Carbon nanotubes were synthesized by the catalytic

decomposition of acetylene over the iron catalyst previously

described. Two gas compositions 2.5% and 10% C2H2/N2 were

studied. After the reduction of the catalyst, a 110 cm3/min flow

of the acetylene/nitrogen gas mixture was introduced into the

chamber at 600 8C with an over pressure of 180 Torr.

Immediately, the temperature was raised at 690 8C in 10 min

and hold at this value for 5 min. Finally the temperature

controller was fixed at 600 8C during 3 h. After the growth of

carbon nanotubes, the furnace was cooled at room temperature

under the flow of nitrogen gas.

The acetylene conversion and carbon yield were calculated

following the procedure of Ref. [6]. High resolution transmis-

sion electron microscopy (HR-TEM Philips CM200) and field

emission scanning electron microscopy (FEG-SEM Zeiss LEO

982 GEMINI) were used to analyze the particle size, catalyst

morphology, and shape, diameter and wall structure of

synthesized CNTs. Thermogravimetric analysis (Shimadzu

TGA-51) and differential thermal analysis (Shimadzu DTA-50)

were performed on 15 mg samples with a heating rate of 10 K/

min and air flowing of 50 cm3/min. When it was necessary, the

samples of nanotubes were detached from the catalyst by

ultrasonic dispersal in ethanol.

3. Results

3.1. TEM and HR-TEM images

Fig. 1 shows TEM images of the catalyst. The iron

containing particles appear as dark spots dispersed in the SiO2

matrix (light grey) with a wide size distribution. The particles

appear spherical in shape with a bimodal distribution of

diameters. Both modes can be fitted by a lognormal distribution

function, shorter diameters have the most probable value

around 6 nm and a distribution width of 0.4 nm, while for large

diameters these values are 40 nm and 1 nm, respectively. These

results are in agreement with previous works reported by Pan

et al. [5] and del Monte et al. [7].

Fig. 2(a) and (c) shows HR-TEM and TEM images,

respectively of nanotubes obtained with 2.5% C2H2/N2.

Synthesized nanotubes display multiple walls, external and

internal diameters values can be fitted by a lognormal

distribution with the most probable value around 17 nm and

5 nm, respectively, and the same width of 0.3 nm. The inner

channel is very regular. The walls were mostly crystalline with

very low content of amorphous carbon adhered on the external

surface. Amorphous carbon aggregates in the form of non-

crystalline nanoparticles, as can be seen in Fig. 3(a).

Fig. 2(b) and (d) shows HR-TEM and TEM images,

respectively of nanotubes obtained with 10% C2H2/N2. Also a

lognormal distribution can represent the set of values obtained

for their external and internal diameters; in this case the most

probable values are around 28 nm and 6 nm, respectively, and the

same width of 0.3 nm. Nanotubes’ walls are formed by a 5 nm

thick crystalline layer, covered by a 5–10 nm thick amorphous

layer. A closer examination of nanotubes’ structure indicates that

non-crystalline carbon is deposited on the nanotube’s surface in

the form of nanoparticles that cover the entire surface. This

phenomenon can be clearly seen in Figs. 2(d) and 3(b).

3.2. SEM images

Fig. 4 shows SEM images of nanotubes. Images correspond-

ing to lower acetylene partial pressure show thinner nanotubes

in agreement with TEM observations. Interestingly, most of

nanotubes synthesized under both acetylene concentrations are

closed by one extreme, as can be seen in Fig. 4(d).

Fig. 2. Images of a MWCNT synthesized with 2.5% C2H2/N2: (a) HR-TEM; (b) TEM. Images of MWCNTs synthesized with 10% C2H2/N2: (c) HR-TEM; (d) TEM.

M. Escobar et al. / Applied Surface Science 254 (2007) 251–256 253

3.3. XRD measurements

Fig. 5 shows XRD patterns of Fe-SiO2 composites,

containing 0.9% mol/mol Fe/Si, annealed under different

Fig. 3. HR-TEM images of amorphous carbon aggregates in the form of non-

crystalline nanoparticles. MWCNTs synthesized with: (a) 2.5% C2H2/N2 and

(b) 10% C2H2/N2.

conditions. The raw composite is amorphous and does not

display a diffraction pattern (not shown). After annealing at

450 8C for 10 h in air, iron oxide nanoparticles crystallized and

the sample displays a typical g-Fe2O3 pattern (Fig. 5(a)). Peaks

are wide and poorly defined as consequence of the small

particle size (4.8 nm as calculated from Scherrer equation).

These observations are in agreement with previous works

reported by del Monte et al. in similar systems [7].

A new crystalline phase is detected when the sample is

further annealed at 600 8C during 4 h in 9% H2/N2 flow

(Fig. 5(b)). This phase correspond to laihunite, an iron-silicate

where iron is present as Fe(II) and Fe(III).

After the synthesis of MWCNTs by catalytic CVD of

acetylene (Fig. 5(c) and (d)), the sample shows a new

crystalline phase identified as fayalite where iron is present as

Fe(II). The peak centered around 26 degrees corresponds to the

MWCNTs diffraction as suggested by Huiqun et al. [8].

Metallic iron was not identified in neither of the studied

conditions, suggesting that oxidation of the nanoparticles

occurred immediately after exposition of the samples to air.

3.4. TGA and DTA analysis

Fig. 6(a) shows traces of TGA and DTA corresponding to

samples of nanotubes obtained under the two different C2H2/N2

compositions. In both cases, losses of mass correspond to

endothermic processes, as can be deduced from DTA traces.

Derivatives of TGA traces shown in Fig. 6(b) indicate that in

each case the loss of mass can be considered as the combination

of two independent processes. Table 1 shows values of carbon

conversion, carbon yield and the starting temperature for the

Fig. 4. SEM images of MWCNTs synthesized with: (a) and (c) 2.5% C2H2/N2; (b) and (d) 10% C2H2/N2.

M. Escobar et al. / Applied Surface Science 254 (2007) 251–256254

loss of mass processes of MWCNTs obtained for both synthesis

conditions.

The carbon yield increases with the proportion of acetylene,

while the carbon conversion remains constant. These results

indicate that the proportion of acetylene decomposed is the

same in both regimens; consequently, the amount of carbon

deposited increases with the amount of acetylene.

4. Discussion

The distribution of internal diameters of synthesized

nanotubes is very similar to the distribution of iron/silica

Fig. 5. XRD patterns of Fe-SiO2 composites annealed under different condi-

tions. See details in the text.

nanoparticles of the catalyst under both conditions of acetylene

flow rate. There is no evidence of nanotubes grown with

internal diameters around the 40 nm which is the most probable

diameter of the second mode of the particles size distribution in

the catalyst.

The initial crystalline phase in the supported catalyst

obtained after annealing in air atmosphere is maghemite, in the

form of poorly crystalline nanoparticles. After treatment at

600 8C under reducing atmosphere (9% H2/N2) two simulta-

neous processes take place, partial reduction of Fe(III) to Fe(II)

and crystallization of a new iron-silicate phase. A more reduced

phase (Fe2SiO4, fayalite) is obtained after the synthesis of

nanotubes, when the Fe-SiO2 matrix is exposed to N2/C2H2

mixture at 600 8C. The nitrogen acetylene mixture provides a

slightly reductive atmosphere that completely reduces the

Fe(III) to Fe(II). The close interaction between iron and the

silica matrix leads to the formation of iron silicates. The

existence of fayalite in reduced matrixes of Fe-SiO2 was

reported previously in systems containing high percentages of

iron (28.5%) annealed under H2 atmosphere [9].

Fe(0) was not detected by XRD analysis because it is quickly

oxidized by the atmospheric O2. Temperature-programmed

reduction (TPR) experiments in hydrogen atmosphere were

carried out by others authors on the same catalyst [9]. They

show that the percentage of reduced iron over the total iron

content in the catalyst is around 60%.

We have observed from TEM images the presence of CNTs

growing from the catalyst surface, most of them are closed at

the end but there is no iron nanoparticles encapsulated in those

Fig. 6. TGA and DTA of MWCNTs obtained under the two different C2H2/N2

compositions.

M. Escobar et al. / Applied Surface Science 254 (2007) 251–256 255

ends. This is the Yarmulke mechanism proposed by Dai et al.

[10] and indicates that the base-growth mode prevails in

agreement with a strong interaction between the catalyst

particle and the support. This strong interaction is due to the

chemical affinity between the iron oxide nanoparticle precursor

and the SiO2 matrix.

We demonstrate that MWCNTs with regular inner channels

and low amount of amorphous carbon can be synthesized with

low acetylene content. In several cases nanoparticles of

amorphous carbon closely connected with the wall of the

nanotubes were detected. The amount of these amorphous

nanoparticles increases notably when the pressure of acetylene

is increased. Very likely, the amorphous nanoparticles nucleate

on nanotubes’ walls and, at high content of acetylene, grow

closely attached to the surface of the nanotube. The resulting

materials can be described as formed by a nanotube core coated

by amorphous carbon. As consequence, nanotubes grown with

10% acetylene are wider than those synthesized with 2.5%

acetylene.

Table 1

Effect of acetylene percentage on the temperature of carbonaceous materials

decomposition, carbon conversion and carbon yield

C2H2/N2 flow 2.5% C2H2/N2 10% C2H2/N2

Loss of mass starting temperature (8C) 410 415

497 578

Carbon conversion 4.5 4.6

Carbon yield 20 36

The values of carbon conversion indicate that the

percentage of acetylene decomposed by the catalyst is the

same in both regimes. However, HR-TEM results have shown

that a layer of amorphous nanoparticles is deposited on the

crystalline wall of nanotubes when high flow is used. It appears

that a higher partial pressure of acetylene deactivates the

catalyst.

The loss of mass detected at low temperature from DTA can

be associated with combustion of amorphous carbon [11,12]

while the second process centered at higher temperatures can

correspond to CNTs decomposition. It should be noticed that

while a similar temperature was observed for the combustion of

amorphous carbon on nanotubes synthesized under both

acetylene concentrations, a shift to higher temperature was

observed for the decomposition of CNTs synthesized under

higher percentage of acetylene. This result indicates that the

decomposition temperature of coated nanotubes is higher than

that of un-coated ones. A possible explanation for this

phenomenon can be found in the characteristics of the

amorphous carbon that coats these nanotubes. This coating

is formed by closely packed carbon nanoparticles which,

according to Shi et al. [11], decompose at 635 8C. This

temperature is higher than the decomposition temperature of

CNTs (440–690 8C) [6,11] or amorphous carbonaceous

materials [11,12].

5. Conclusions

The concentration of acetylene has an important influence

on the characteristic of the CNTs synthesized by CVD.

CNTs synthesized with low acetylene concentration are

more regular and with a lower amount of amorphous carbon

than those synthesized with high concentration. During the

synthesis of CNTs amorphous carbon nanoparticles nucleate on

the external wall of nanotubes.

At high acetylene concentration carbon nanoparticles grow,

covering all the surface of CNTs, in the form of a compact

coating.

The combination of CNTs with coating of amorphous

carbon particles leads to a material with high decomposition

temperature.

Consequently, the concentration of acetylene provides

another way to control the morphology of nanotubes.

Acknowledgements

This work was supported by Universidad de Buenos Aires,

Argentina (Investigation Project, X191); Consejo Nacional de

Investigaciones Cientıficas y Tecnicas (PIP 5215; PIP 5959)

and, Agencia Nacional de Promocion Cientıfica y Tecnologica

(PICT 10-25834; PICT 06-10621).

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