CVD fabrication of carbon nanotubes on electrodeposited flower-like Fe nanostructures

Article abstract of DOI:10.1016/j.jallcom.2010.07.216

Galvanostatic method was used to electrodeposit Fe nanostructures on platinum electrodes as catalysts. Scanning electron microscopy (SEM) revealed flower-like Fe deposits with high surface area. Carbon nanotubes were grown on flower-like Fe nanostructures

Full text of DOI:10.1016/j.jallcom.2010.07.216

Journal of Alloys and Compounds 507 (2010) 494–497
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
CVD fabrication of carbon nanotubes on electrodeposited flower-like
Fe nanostructures
a,e,∗
b,e
c
d
Saeid Zanganeh
, Morteza Torabi , Amir Kajbafvala , Navid Zanganeh ,
b
b
b
e
M.R. Bayati , Roya Molaei , H.R. Zargar , S.K. Sadrnezhaad
a
Department of Electrical and Computer Engineering, University of Connecticut, 371 Fairfield Way, U-2157 Storrs, CT 06269-2157, USA
Department of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box 16845-161, Tehran, Iran
Department of Materials Science and Engineering, North Carolina State University, 911 Partner’s Way, Raleigh, NC 27695-7907, USA
Chemical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran
b
c
d
e
Department of Materials Science and Engineering, Center of Excellence for Production of Advanced Materials, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Galvanostatic method was used to electrodeposit Fe nanostructures on platinum electrodes as catalysts.
Scanning electron microscopy (SEM) revealed flower-like Fe deposits with high surface area. Carbon
nanotubes were grown on flower-like Fe nanostructures by chemical vapor deposition. The structure
of the synthesized carbon nanotubes was investigated by scanning electron microscopy, transmission
electron microscopy, Raman spectroscopy and X-ray diffraction. According to X-ray diffraction patterns,
Fe was the only detected constituent of the deposited coating. The carbon nanotubes had small wall-
thickness and wide hollow core.
Received 31 March 2010
Received in revised form 30 July 2010
Accepted 30 July 2010
Available online 8 August 2010
Keywords:
Nanostructure
Carbon nanotubes
Electrodeposition
Chemical vapor deposition
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
nanosized catalysts to promote the CNT growth is attributed to their
catalytic activity of decomposition of carbon feedstock, formation
of metastable carbides and subsequent diffusion of carbon atoms
and formation of graphitic sheets [22]. Techniques for preparation
of the catalysts have therefore become an important issue. Trying
to efficiently screen the preparation conditions of CNT catalysts,
several studies have used combinatorial approaches involving dry
[23,24] and wet [25,26] processes. Galvanostat method is a simple
way, requiring shorter times and yielding greater rates.
The latest results obtained about production of flower-like
nanostructured Fe catalysts deposited electrochemically on plat-
inum electrodes (as a suitable catalyst for synthesis of carbon
nanotubes) are presented in this paper. CVD is used as a convenient
way of deposition of CNTs on the flower-like Fe catalyst substrate.
Carbon nanotubes (CNTs) have tubular structure and graphitic
lattice [1]. Due to unique atomic structure, extraordinary mechan-
ical strength, desirable electronic properties [2–4], high material
storage capacity for special applications such as drug delivery and
optics [5,5-b,5-c,6], CNTs have inspired wide interest in the current
nanoscience/nanotechnology progressing versatile fields. CNTs can
be produced via carbon arc discharge [7], laser ablation [8], pyrol-
ysis [9,10], chemical vapor deposition (CVD) [11,12] and plasma
enhanced chemical vapor deposition [13,14]. CVD seems to be a
promising technique because of its low cost, high purity, high yield,
simple configuration and flexibility in adjustment of the influen-
tial parameters such as temperature and presence of hydrocarbon
gases [15] which can be used to control the structure of the CNTs
produced [16–18].
2
.
Experimental procedure
The electrolyte contained 0.1 M FeSO4·7H2O (99.5%, M = 278.02 g mol , Merck,
A key factor in controlling the growth of CNT by CVD is utiliza-
tion of a suitable nanosized catalyst. Nanosized metallic catalysts
such as iron, cobalt and nickel [19] have been widely reported as
nuclei for growth of CNTs by CVD [20,21]. The peculiar ability of the
−1
−
1
Germany) in an aqueous solution of 0.1 M Na2SO4 (99.99%, M = 142.04 g mol
,
Merck, Germany). The platinum sheets were carefully polished and cleaned before
the experiments. All the electrochemical processes were carried out in a potentio-
stat/galvanostat equipment (Autolab PGSTAT30) using a standard three electrode
cell. Electrochemical potentials were recorded versus an Ag/AgCl reference elec-
trode. Linear sweep voltammetry was applied in the potential range of −0.1 to
−
1.8 V vs. Ag/AgCl in order to deposit nanostructured iron on Pt electrode. To
∗ Corresponding author at: Department of Electrical and Computer Engineering,
University of Connecticut, 371 Fairfield Way, U-2157 Storrs, CT 06269-2157, USA.
E-mail address: SAZ@engr.uconn.edu (S. Zanganeh).
understand the mechanism of electrodeposition of iron, different electrochemi-
cal measurements including linear sweep voltammetry (LSV), chronopotentiometry
and chronoamperometry were carried out on the samples. The morphology of the
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.07.216

S. Zanganeh et al. / Journal of Alloys and Compounds 507 (2010) 494–497
495
Fe-coated platinum substrates obtained was characterized by a scanning electron
microscope (SEM, VEGA, TESCAN) equipped with energy dispersive X-ray (EDX). X-
ray diffraction analysis by Philips X’Pert XRD diffractometer using Cu-K␣ radiation
(
ꢀ = 1.5405 A˚ ) was used to identify the crystalline structure of the deposited layer.
BET was used to investigate the specific surface area of the prepared catalyst (Gemini
Micrometrics 2375).
The nanostructured flower-like Fe catalyst obtained via linear sweep voltamme-
try is utilized to synthesize carbon nanotubes via CVD process. In order to fabricate
CNTs on the catalyst, the Fe-coated platinum substrate was placed at the center of a
◦
quartz tube reactor heated to about 700 C under flowing nitrogen/hydrogen (10:1
◦
molar ratio) gas mixture. After reaching to the desired temperature (700 C), a flow
of acetylene (C2H2) gas was introduced into the reactor with a rate of 60 ml/min for
about 30 min. A black coating covered the surface of the catalyst which was analyzed
using transmission electron microscope (TEM, Philips CM200), X-ray diffractometer
(
XRD, Philips X’Pert diffractometer) and Raman spectroscope (Thermo Nicolet).
3
. Results and discussion
3.1. Production of flower-like nanostructured Fe catalyst
Fig. 1A indicates the onset potential of ca −1.0 V vs. Ag/AgCl for
iron deposition. No reduction peak can be observed in the figure. It
can, therefore, be concluded that the mechanism of the reduction
reaction is charge-transfer which is controlled by increasing the
potential which culminates at elevated densities of the current.
Transition time (ꢁ) vs. current density measurements can pro-
vide valuable information about the reaction mechanism. Fig. 1B
represents the Galvanostatic transient for deposition of Fe from
2
the electrolyte at 10 mA/cm . It shows a sharp peak which relates
to the double-layer capacitance at initial part of the curve. This is
related to the coalescence of charges on the surface of the Pt elec-
trode. Reduction in the potential right after the initial-part sharp
peak corresponds to the charge-transfer processes which occur
during the Fe deposition process. This finding is in consistence
with the results of linear sweep voltammetry which approved the
dominance of charge-transfer processes in deposition of Fe layer.
Following the reduction step in the chronopotentiometry diagram
(
Fig. 1A), the potential changes towards the nobler potential sec-
tion (∼−1.08002 V vs. Ag/AgCl) which corresponds to the transient
stage, slowly increases to −1.028 V vs. Ag/AgCl with the growth
continuation. It can, therefore, be concluded that the deposition
observed in this system is controllable through charge-transfer
mechanism.
For confirmation of the charge-transfer mechanism in the sys-
tem, chronoamperometric (I–t transient) measurements were done
at −1.0 V vs. Ag/AgCl. Fig. 1C shows the I–t transient curve of the
system. As it is clearly seen, the diagram has one well-defined rec-
ognizable maximum current (first peak) followed by a sharp fall
and subsequent broad noisy peak. Such a particular shape for the
I–t transient curve clearly shows that a nucleation and growth pro-
cess is involved in the electrodeposition of Fe and charge-transfer is
the rate-limiting process. The oscillations observed in this diagram
can be attributed to the instability of the system.
Fig. 1. (A) The linear voltammogram related to electrodeposition of iron on platinum
substrate in the required potential range from −0.1 to −1.8 V, (B) galvanostatic E–t
transients for deposition of Fe from electrolyte and (C) potentiostatic I–t transients
for deposition of Fe from electrolyte.
As it is seen in the first stage of the Fe electrodeposition, instabil-
ities such as nucleation and double-layer effect occur. Furthermore,
in the galvanostatic process, the stability is reached after about
2
00 s. A longer deposition time is required for synthesis of a more
uniform catalyst layer. About 300 s is, hence, used for this purpose.
The electroactive-type concentration of the solution was
changed during the experiment. The changes were, however, not
sufficient and the current density decreased very little. Using Pt
caused the decrease of the H evolution and increase of the cathodic
2
current efficiency. It could, therefore, be expected that the current
density is consumed during the ferrous reduction.
X-ray diffraction spectrum of the deposited layer is presented in
Fig. 2. The curve shows different peaks attributed to the platinum
substrate and the Fe deposited layer. The average crystallite size
of the deposited Fe layer was calculated through Scherrer formula
Fig. 2. XRD pattern of the deposited Fe on the platinum substrate.

4
96
S. Zanganeh et al. / Journal of Alloys and Compounds 507 (2010) 494–497
Fig. 4. XRD pattern of the CNTs grown on the flower-like Fe nanostructures elec-
trodeposited on platinum electrode.
Transmission electron microscopy (TEM) images of the pro-
duced CNTs show a randomly entangled network of long and curved
CNTs grown in various diameters between 9 nm and 35 nm which
have small wall-thickness and therefore a large inner space (Fig. 5a
and b). These wide hollow cores and small wall-thicknesses pro-
vide appropriate conditions for transfer through empty nanotube
channels and improve the required potential for both storage and
drug delivery applications.
High-resolution images of all tubes formed in CVD experi-
ments (Fig. 6) reveal well-ordered graphitic layers grown on the
Fig. 3. (a) SEM image, Fe mapping and (b) EDS results of the deposited Fe layer.
(
d = 0.9ꢀ/B cos ꢂ: d is the average crystallite size of the deposited
layer, ꢀ is the wavelength of Cu-K␣ radiation, B is the full width at
half maximum intensity of the sharpest Fe peak and ꢂ is the Bragg
angle). The result obtained was ∼28.7 nm.
Fig. 3 illustrates the SEM morphology of the nanostructured Fe
layer. The layers formed were composed of exfoliated Fe petal-like
nanostructures which make a flower-like nanostructure. Elemental
mapping reveals homogenous distribution of the Fe atoms. More-
over, the EDS results show that Fe is the major component of the
catalyst (99.1 wt%). Oxygen and carbon may be attributed due to
contamination of the surface or slight iron oxides. The BET sur-
face area of the Fe catalyst that was synthesized electrochemically
2
is about 287 m /g. This rather high surface area is a result of the
flake-like morphology of the Fe catalyst.
3.2. CVD growth of CNTs on flower-like Fe nanostructures
Fig. 4 shows the XRD pattern of the black material obtained on
the surface of the catalyst after the CVD process. A strong sharp
peak occurs at 2ꢂ = 25.20 corresponding to the (0 0 2) reflection of
the graphitic carbon deposited on the flower-like Fe nanostructure.
XRD results clearly indicate the well graphitized highly pure CNTs
fabricated in this research.
Fig. 5. TEM image of the CNTs grown on flower-like Fe nanostructures electrode-
posited on platinum electrode by CVD method at (a) lower magnification and (b)
higher magnification.

S. Zanganeh et al. / Journal of Alloys and Compounds 507 (2010) 494–497
497
the analysis of Kasuye et al. [27], the structure of 1540–1600 cm 1
can be understood by z1-folding of the graphite phonon dispersion
relation.
−
4. Conclusions
This paper investigates the application of a special shape of
iron nanostructures as a catalyst for high-yield preparation of high
purity CNTs with desirable nanostructure. Flower-like Fe nanos-
tructure was produced as an appropriate catalyst for preparation of
highly pure CNT via electrodeposition from iron sulfate electrolyte
on a platinum electrode. The growth mechanisms were admin-
istered via linear sweep voltammetry, chronopotentiometry and
chronamperometry. It was found that the electrodeposition was
charge-transfer controlled. BET confirmed high surface area of the
Fe catalyst. Elemental mapping and EDS results showed uniform
distribution of Fe in the catalyst with negligible impurities. The
structure of the synthesized carbon nanotubes, and flower-like Fe
nanostructure were investigated by scanning electron microscopy,
transmission electron microscopy, Raman spectroscopy and X-ray
diffraction. Using the CVD method, long carbon nanotubes with
small wall-thicknesses and wide hollow cores in various diameters
between 9 nm and 35 nm were obtained.
Fig. 6. High-resolution TEM image of the synthesized walls of CNTs grown on the
Fe nanostructures at 8,500,000 magnification.
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