Synthesis of MWNTs using Fe-Mo bimetallic catalyst by CVD method for field emission application

Article abstract of DOI:10.1016/j.ssc.2007.08.016

Multiwalled carbon nanotubes (MWNTs) were synthesized by chemical vapor deposition technique using MgO supported bimetallic Fe-Mo catalyst. The role of Mo on the quality of as-synthesized CNTs and their application in field emission has been investigated. The field emission properties of the device fabricated from the CNTs synthesized with Fe-Mo catalyst was found to be better than the similar device prepared with CNTs synthesized with Fe catalyst alone.

Full text of DOI:10.1016/j.ssc.2007.08.016

Solid State Communications 144 (2007) 498–502
www.elsevier.com/locate/ssc
Synthesis of MWNTs using Fe–Mo bimetallic catalyst by CVD method for
field emission application
B.K. Singha,∗, Sung-Wook Choa, K.S. Bartwalb, Nguyen Duc Hoac, Hojin Ryub
a
Minerals & Materials Processing Research Division, Korea Institute of Geoscience & Mineral Resources, Gajeong-dong 30, Yuseong-gu,
Daejeon 305-350, South Korea
b
Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, South Korea
c
Department of Materials Science and Engineering, Chungnam National University, 220 Gung-dong, Yuseong, Daejeon 305-764, South Korea
Received 13 February 2007; received in revised form 27 June 2007; accepted 16 August 2007 by T. Ando
Available online 23 August 2007
Abstract
Multiwalled carbon nanotubes (MWNTs) were synthesized by chemical vapor deposition technique using MgO supported bimetallic Fe–Mo
catalyst. The role of Mo on the quality of as-synthesized CNTs and their application in field emission has been investigated. The field emission
properties of the device fabricated from the CNTs synthesized with Fe–Mo catalyst was found to be better than the similar device prepared with
CNTs synthesized with Fe catalyst alone.
c
ꢀ 2007 Elsevier Ltd. All rights reserved.
PACS: 81.07.De; 81.16.Hc; 68.55.Jk
Keywords: A. Carbon nanotubes; B. Chemical vapor deposition; D. Raman spectroscopy; D. Field emission
1. Introduction
concerned, various synthetic methods e.g. arc discharge [5],
laser ablation [6], plasma enhanced chemical vapor deposition
(PECVD) [7] and chemical vapor deposition (CVD) [8] are
currently practiced. Among these methods, CVD is one of the
most commonly used techniques for laboratory and commercial
scale synthesis of CNTs. Using the CVD method, CNTs can
either be grown by the floating catalyst method [9,10] or
over catalysts deposited on solid supports [11,12]. In both the
cases, transition metal catalyst nanoparticles are considered to
be the nuclei and potential sites for the growth of CNTs at
moderate temperatures [13]. Individually Fe, Co and Ni were
commonly employed for the formation of CNTs by chemical
vapor deposition technique; however, bimetallic catalysts like
Fe–Mo [14,15], Co–Mo [16,17] Co–Ni [18] and Fe–Co [19,20]
have also been effectively utilized. Also, the use of the solid
support to the active catalyst has a detrimental effect on
the yield, morphology and purity of the CNTs produced.
Various solid supports such as alumina [21], zeolite [22],
silicas [23] and MgO [24] with different hydrocarbons viz.,
CO, ethane, methane, acetylene etc., as carbon source have been
investigated.
Carbon nanotubes (CNTs) are interesting materials in view
of their wide range of applications in hydrogen storage,
chemical sensor, catalytic support, biochemical applications
and field emission [1]. Such tremendous applications are due
to unique geometry, high chemical stability, high mechanical
strength and excellent thermal and electrical conductivity [2,3].
Specifically, for applications related to field emission device,
the small size and sharp tip morphology of carbon nanotubes
is useful as it can act as an electron gun. The electrons
emitted from the carbon nanotubes can be used in flash
memories, electron microscopy, X-ray tubes, light-sources and
field emission display devices. However, for the practical
application it is crucial to achieve economical mass production
of CNTs of right morphology. In recent years, commercial
field emission displays [4] are being synthesized by low-cost
screen-printing technique. As far the synthesis of CNTs is
∗
Corresponding author. Tel.: +82 42 868 3606; fax: +82 42 861 9727.
E-mail address: bks he@yahoo.co.in (B.K. Singh).
c
0038-1098/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.08.016

B.K. Singh et al. / Solid State Communications 144 (2007) 498–502
499
In the present work, we describe the synthesis of CNTs
over Fe- and Mo-based bimetallic catalytic system on MgO
support using acetylene (C2H2) as a carbon source. We have
also investigated the efficacy of the as-synthesized CNTs for
their application in the field emission device fabricated by
screen-printing method. The effect of transition metal ions Fe
and Mo individually and in combination for the synthesis of
CNTs and the consequent field emission devices has also been
investigated.
Fig. 1. Schematic diagram of the emission testing circuit for MWNTs vacuum
diode.
2. Experimental details
ambient conditions using a Bru¨ker FRA-106/S with the
excitation laser source of Nd:YAG (wavelength: 1.064 µm)
remaining constant and with an optical power of 100 mV at
the sample position.
2.1. Preparation of carbon nanotubes
The carbon nanotubes were synthesized by Chemical Va-
por Deposition (CVD) method using MgO/Fe–Mo as catalyst
and C2H2 gas as a carbon source. The use of MgO was re-
ported to be particularly advantageous in terms of ease in the
purification step of as-formed CNTs due to the high solubility
of MgO in dilute hydrochloric acid that does not damage the
CNTs produced [25]. Different metal ion compositions were
supported on MgO keeping the support to metal ratio 1:1. We
prepared seven catalysts with a general molecular formula of
MgOFexMoy where the values of x and y were varied from 0
2.2. Fabrication of field emission device
Fig. 1 shows the schematic diagram of the diode-type
field emitter. The field emitter device was fabricated by an
inexpensive and simple screen-printing method. The mixture
of 100 mg purified CNT powder synthesized with different
compositions of catalyst MgOFe1.0Mo0.0, MgOFe0.9Mo0.1
,
MgOFe0.7Mo0.3, MgOFe0.6Mo0.4, 20 ml of organic vehicle
(Trepineol dihydro) and 100 mg of organic binder (Ethyl-
cellulose) were premixed through solder paste softener for
10 min. We denoted S1, S2, S3, and S4 for devices which were
fabricated using CNT powders synthesized with composition
to 1.0 i.e. MgOFe0.0Mo1.0, MgOFe0.5Mo0.5, MgOFe0.6Mo0.4
,
MgOFe0.7Mo0.3 MgOFe0.8Mo0.2 MgOFe0.9Mo0.1 and
,
,
MgOFe1.0Mo0.0. Here we describe the method for the syn-
thesis of CNTs with catalyst composition MgOFe0.6Mo0.4
as a representative synthesis method. The same procedure
with variations in ingredients composition was followed for
the synthesis of CNTs with other catalyst composition. In a
typical synthesis of CNTs, analytical grade Fe(NO3)2·9H2O
(2.1720 g, 0.005 mol), (NH4)6Mo4O24 (2.5774 g, 0.002 mol),
and Mg(OH)2 (0.7233 g, 0.012 mol) were mixed with ethanol
(4 ml) in an agate mortar and the slurry was grounded well
during which the ethanol got evaporated. All operations were
carried out in an efficient fu◦me hood. The resulting powder
was dried in an oven at 100 C for 10 h and grounded again
in an agate mortar. The fine powder (0.1 g) thus obtained was
then uniformly spread in an alumina boat and was placed in the
central region of a horizontal 100 cm long quartz tube furnace
having an inner diameter of 4.5 cm. The furnace was flushed
of catalyst MgOFe1.0Mo0.0, MgOFe0.9Mo0.1, MgOFe0.7Mo0.3
,
MgOFe0.6Mo0.4, respectively. After mixing the CNT with
organic vehicle and organic binder, the three-roll mill process
was carried out for mixing and dispersion of CNT powders in
organic vehicle as polymer matrix. The well-dispersed CNT
paste was then printed onto an indium tin oxide ITO/Glass
substrate using the printing-mask with 2 × 2 cm in size. After
that the samples were dried on a hot plate at 100 ◦C for
30 min to obtain the uniform thickness of CNT paste films.
The residue of the organic vehicle and binder in the paste were
removed in order to obtain the stable emission characteristics by
annealing the CNT paste films at 400 ◦C in air for 1 h. The field
emission properties of diode-type emitter were characterized in
the vacuum chamber of 10−6 Torr. The spacer has a thickness
of ∼150 µm, the anode is phosphor/glass. The data (I–V)
was automatically recorded by using a programmable software
(Keithley model 248 high voltage supply).
◦
with argon gas◦while it was heated at a rate of 10 C/min till
it attained 800 C. After the attainment of the desired temper-
ature, H2 gas was introduced into the furnace at a flow rate of
100 cc/min for 60 min so as to generate active metallic (Fe, Mo)
or bimetallic (Fe–Mo) nanoparticles on MgO support. Subse-
quently, C2H2 gas in different quantities varying from 10 to
50 cc/min was introduced into the furnace for 1 h maintaining
the flow of H2 gas constant. We optimized the condition and the
maximum yield (0.4 gm) of CNTs at C2H2 flow was obtained
at 15 cc/min.
3. Results and discussion
The morphology of the as-synthesized carbon nanostruc-
tures was investigated by scanning electron microscopy (SEM)
images. Fig. 2 shows the SEM image of the sample gown with
MgOFe0.0Mo1.0 as catalyst. It is evident from the figure that
there are no fibrous structures present in the sample. The ab-
sence of any CNT-like structure suggests that Mo itself is not
suitable as catalyst for the growth of CNTs [26].
The morphology and microstructure of as-synthesized
carbon materials were characterized by scanning electron
microscopy (Philips XL 30S), high-resolution transmission
electron microscopy (HRTEM, Techni GII, FEI 20), EDS
(JMS-6700 F) and FT-Raman spectra were recorded under
The SEM image of the sample grown with MgOFe1.0Mo0.0
as catalyst is shown in Fig. 3(a). Though the sample shows

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B.K. Singh et al. / Solid State Communications 144 (2007) 498–502
Fig. 2. SEM image of sample obtained using the MgOFe
M
0.0 1.0
catalyst.
Fig. 4. (a) SEM image of the CNTs grown over MgOFe Mo
catalyst
0.9
0.1
(b) HRTEM image of MWNTs.
nanofibers embedded with uneven sized lumps of catalyst and
carbon nanotubes. The carbon nanofibers have large diameter
of 72 nm with no hollow core when the carbon nanotubes have
smaller outer diameter of about 20 nm (Fig. 3(b) inset). The
CNTs appear to be multiwalled carbon nanotubes with the wall
thickness of about 5 nm. The interplanar distance between two
graphite layers is estimated to be about 0.345 nm, which is
higher than the value reported for the graphite phase (0.335).
In the SEM image of the sample grown with MgOFe0.9Mo0.1
(bimetallic) catalyst (Fig. 4(a)), we see the presence of smooth,
fairly uniform and continuous carbon nanotubes with a length
running to several micrometers. The presence of the catalyst at
the tips of the CNTs can also be clearly seen in its HRTEM
images (Fig. 4(b)). The EDS of this sample (not show here)
clearly indicates the presence of both Fe and Mo. Also the size
of the catalyst crystallite seems to be fairly uniform.
Morphological features observed for the samples grown
individually with Mo and Fe and with Fe–Mo were analyzed. It
is to be noted that Mo by itself is not favourable for the growth
of CNTs but helps in the dispersion of the Fe catalyst which
is responsible for the uniform growth of CNTs in the case of
Fe–Mo catalyst.
In order to optimize the maximum yield of CNTs by
changing the composition of the catalyst, CNTs yield was
calculated by weighing the sample using the following
Fig. 3. (a) SEM image of the CNTs grown over MgOFe Mo
(b) HRTEM images of MWNTs of (a) at low and high magnification (inset).
catalyst,
1.0
0.0
CNT-like structure, the presence of different sizes of CNTs
together with the agglomeration of amorphous carbon and
catalyst indicates uneven dispersion of the active catalyst. The
TEM micrograph of MgOFe1.0Mo0.0 is shown in Fig. 3(b). The
TEM image of this sample confirms the presence of carbon

B.K. Singh et al. / Solid State Communications 144 (2007) 498–502
501
Fig. 6. Raman spectra of the as-synthesized MWNTs by catalytic
decomposition of acetylene over (a) MgOFe Mo and (b) MgOFe Mo
.
0.0
0.9
0.1
1.0
Fig. 5. The yield of CNTs (%) as a function of Mo catalyst loading during the
60 min growth time with a flow of H + C H (100 + 15 cc/min).
2
2 2
equation:
CNTS yield (wt.%) = [(MF − M1)/M1] × 100,
where MF is the total weight obtained after synthesis and
M1 is the initial weight of the powder after reduction. Fig. 5
clearly demonstrates that the maximum yield of the CNTs
was obtained with the catalyst composition, MgOFe0.9Mo0.1
.
With the use of 0.1 g of bimetallic catalyst, up to 0.4 g
CNTs were synthesized which is better than the yields obtained
with other catalyst compositions. Raman spectroscopy was
employed to characterize the quality and crystallinity of the
as-synthesized MWNTs using Fe–Mo and Fe on MgO support
as catalyst (Fig. 6). Both the Raman spectra show two strong
peaks at 1300 cm−1 and 1600 cm−1, which correspond to the
peaks of multiwalled carbon nanotubes [27]. The former band
corresponds to the defect-induced Raman band and is known as
the defect mode, A1g- or D-band associated with the disordered
graphite- or diamond-like carbon. The latter peak, referred
to as G-band, represents the Raman-allowed E2g2 mode of
graphite and is related to the vibration of C–C bonded in a
two-dimensional hexagonal lattice of graphite layer [10]. The
ratio of the intensities of the two bands is a reflection of the
quality of the synthesized material. The ID/IG ratio for the
MWNTs grown with MgOFe0.9Mo0.1 was found to be 1.15
(Fig. 6(a)) which indicates the presence of relatively better
quality CNTs in this sample as compared to MWNTs grown
with MgOFe1.0Mo0.0 (Fig. 6(b)) for which ID/IG ratio was
about 1.37. This observation also indicates that by adding small
amounts of Mo on the Fe catalyst it is possible to increase
the quality of the CNTs. However, the presence of D-peaks as
compared to G-peaks in both the samples shows that CNTs still
possess some amount of defects and impurities.
Fig. 7. Field emission behaviour of the CNTs, inset is the corresponding F–N
plots of the data.
synthesized with composition of catalyst, respectively. Fig. 7
shows the J–V characteristics of the fabricated diode emitters.
The turn-on electric field of the CNT-based emitter (defined as
the electric field at 10 µA/cm2) is the smallest for sample S2
of 2 V/µm and about 3 V/µm for others. The smaller turn-on
field is expected due to the smaller nanotubes. The maximum
current density of sample S1 is 0.52 mA/cm2, it is lower than
that of sample S2 (1.16 mA/cm2), S3 (0.64 mA/cm2), and S4
(0.62 mA/cm2). The inset in Fig. 7 is the Fowler–Nordheim
(F–N) plots of the emission properties. The F–N equation can
be written as [28]:
I = J × α = A × V 2 exp(−B/V )
,
where A = 1.42 × 10−6 × α × β2/Φ × exp(10.4/Φ1/2
)
and B = 6.44 × 107 × Φ3/2/β.
By using the work function of CNTs Φ = 4 eV, the calculated
value for the field enhancement factor β is 17.17×104, 19.75×
104, 18.81 × 104 and 16.94 × 104 for samples S1, S2, S3,
and S4, respectively. The field enhancement factors β of our
device is higher compared to the similar device reported in the
literature [29]. The higher value β of sample S2 indicates the
higher emission current density for a given applied voltage.
The synthesized carbon nanotubes were used to fabricate
the field emission diode structure. For the comparison of the
effect of the quality of the CNTs on the field emission property,
the emission diode structures were prepared from the CNTs
grown with the catalysts MgOFe1.0Mo0.0, MgOFe0.9Mo0.1
,
MgOFe0.7Mo0.3, MgOFe0.6Mo0.4, denoted as S1, S2, S3, and
S4 for devices which were fabricated using CNT powders

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B.K. Singh et al. / Solid State Communications 144 (2007) 498–502
The better field emission properties of the device are due to
the better quality of the CNTs.
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4. Conclusions
In conclusion, we have demonstrated that it is possible to
tune the Fe catalyst size and dispersion on MgO support by the
addition of small amounts of Mo for the improved synthesis
of CNTs. The as-synthesized CNTs using bimetallic catalyst
were found to be better for its application as field emitter with
respect to the CNTs synthesized with Fe catalyst alone. The
synthesis method described in the present study is simple and
is potentially scalable.
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