CVD growth of high density SWNTs network on surface using iron phosphide nanorods as catalyst precursor

Article abstract of DOI:10.1016/j.cplett.2008.04.128

In this Letter, we demonstrated that FeP nanorods synthesized by thermal decomposition of iron carbonyl under the protection of surfactants can be used as catalyst for SWNTs growth. The domains structure of FeP nanorods or uniform dispersion of the nanorods by self-assembly on surface can be controlled by chemical modification of the surface and concentration of the nanorods solution. Domain structure of SWNTs network or extremely high density SWNTs film with uniform diameter can be generated using carbon monoxide chemical vapor deposition (CO-CVD). The uniform diameter of the SWNTs is believed to be due to the in situ formation of uniform Fe nanoparticles from the decomposition of FeP nanorods at high temperature. The use of nanorods or nanowires as catalyst precursor for SWNTs growth could open a new method to generate extremely high density SWNTs with uniform diameter for SWNTs-based nanoelectronics device applications.

Full text of DOI:10.1016/j.cplett.2008.04.128

Chemical Physics Letters 464 (2008) 49–53
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
CVD growth of high density SWNTs network on surface using iron phosphide
nanorods as catalyst precursor
a
a
a
b
Shaoming Huang a, , Yanhu Bai , Jinagyin Chen , Shun Wang , Zhimin Wang
*
a Nanomaterials and Chemistry Key Laboratory, Wenzhou University, Xue Yuan Road 256, Zhejiang Province, Wenzhou 325027, PR China
b Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this Letter, we demonstrated that FeP nanorods synthesized by thermal decomposition of iron carbonyl
under the protection of surfactants can be used as catalyst for SWNTs growth. The domains structure of
FeP nanorods or uniform dispersion of the nanorods by self-assembly on surface can be controlled by
chemical modification of the surface and concentration of the nanorods solution. Domain structure of
SWNTs network or extremely high density SWNTs film with uniform diameter can be generated using
carbon monoxide chemical vapor deposition (CO-CVD). The uniform diameter of the SWNTs is believed
to be due to the in situ formation of uniform Fe nanoparticles from the decomposition of FeP nanorods at
high temperature. The use of nanorods or nanowires as catalyst precursor for SWNTs growth could open a
new method to generate extremely high density SWNTs with uniform diameter for SWNTs-based nano-
electronics device applications.
Received 7 February 2008
In final form 17 April 2008
Available online 13 May 2008
Ó 2008 Published by Elsevier B.V.
1. Introduction
the aggregation of metal nanoparticles at high temperature, very
low concentration of nanoparticles solution or low loading concen-
Chemical vapor deposition (CVD) is one of the main methods for
growing single-walled carbon nanotubes (SWNTs). It has some
great advantages in term of large scale production, controllability
and in situ fabrication of SWNT-based nanoelectronic devices.
Many metal catalysts either nanoparticles or supported metal cat-
alysts have been widely used for SWNTs growth. Recent years,
growing SWNTs on substrate surface particularly Si wafer has been
received much attention, because of the desirability of nanodevices
fabrication [1–3]. The main catalysts for surface growth of SWNTs
used are uniform metal nanoparticles mainly iron-family elements
including metal Fe, Co, Ni and their alloys such as Fe/Mo nanopar-
ticles and Fe/Mo molecular cluster [4–12]. Most recently, it has
been found other metal nanoparticles rather than iron-family
including Cu, Au, Ag, Pd and Pt with diameter less than 6 nm can
act as effective catalysts for SWNT growth [13,14]. The catalysts
could be from nanoparticles solution, metal precursors or metal
film [6,8,15–22]. CO, CH4/C2H2 and alcohols are the main carbon
sources for SWNTs growth by CVD method. It is noted that the par-
ticles size plays very important role in generating isolated SWNTs
and controlling the diameter of the SWNTs on substrate. Because of
tration of catalyst precursor are normally used. Most recently,
heating thin metal film with nano-scale thickness on wafer has
been applied to generate nano-sized nanoparticles for aligned
SWNTs growth although this method was used for multi-walled
carbon nanotubes growth a few years ago [17–22]. The advantages
for thin metal layer approach are to form the nanoparticles in situ
at high temperature which is favor for SWNTs growth.
Iron phosphides, low band-gap semiconductors, are technolog-
ically important as phosphorescence devices and electronic com-
ponents [23]. Bulk iron phosphides had been prepared by
various methods. They include high temperature reaction of
iron/organometallic precursors with red phosphorus, Na3P or me-
tal chloride under various conditions [24–27]. In this Letter, we
describe a new route to grow SWNTs on surface by CVD method
using iron phosphide nanorods as catalyst precursor which are
prepared from the decomposition of Fe(CO)5 with the mixed
TOPO/TOP surfactants [28]. By heating the FeP nanorod under
H2 atmosphere iron nanoparticles will form from the decomposi-
tion of the FeP nanorods. It was found that the surface property of
the wafer will affect the self-assembly of FeP on surface. Either
FeP nanorods domains or uniform assembled nanorods can be
formed on surface by controlling the surface properties of the
wafer surface, which can be used as catalyst precursor for SWNTs
network or high density nanotubes film on surface. This could
provide a new way leading to controlled synthesis of SWNTs net-
work with high density and uniform diameter for nanotube-based
nanodevices applications.
* Corresponding author.
E-mail address: smhuang@wzu.edu.cn (S. Huang).
0009-2614/$ - see front matter Ó 2008 Published by Elsevier B.V.
doi:10.1016/j.cplett.2008.04.128

50
S. Huang et al. / Chemical Physics Letters 464 (2008) 49–53
2.4. Characterization
2. Experimental
SEM images were taken on a scanning electron microscopy
2.1. Synthesis of FeP nanorod
(SEM FEI at 5 kV), AFM images were taken on a Nanoscope IIIA sys-
tem with a multi-mode AFM. Raman spectra were collected from
the quartz substrate using a Dilor Raman spectrometer with triple
spectrograph with excitation wavelength of 514.5 nm using an Ar-
ion laser source at a power of 120 mW. The beam size was 3 lm in
diameter. TEM images were taken on a Philip TEM.
FeP nanorods were prepared by the thermal decomposition of
their carbonyl complexes in solution under a N2 atmosphere. Trioc-
tylphosphine oxide (TOPO, 99%, Aldrich) worked as stabilizing sur-
factant and also as the solvent [19-31].. Typically, five grams of
trioctylphosphine oxide (TOPO) was heated to a desired tempera-
ture (>300 °C) under N2 atmosphere. The precursor, 0.2 mL iron
pentacarbonyl (Fe(CO)5 99.999%, Aldrich) was quickly injected into
the rapidly stirred, hot TOPO solution. The temperature dropped
with the injection. Keeping the lower temperature for 30 min in or-
der that a complete thermal decomposition of the precursor could
be achieved to generate spherical seeds. 0.2 mL of aliquots of the
reaction mixture was extracted from the solution for TEM observa-
tion to monitor the growth process. Then, 0.2 mL portion of addi-
tional cold stock solution, iron pentacarbonyl/trioctylphosphine
(TOP, 90% aldrich) (Fe(CO)5/TOP V/V: 0.5 mL/4.5 mL), was added
into the reaction system each 30 min intervals to prepare nano-
rods. Finally, the solution was precipitated in acetone and re-dis-
persed in hexane.
3. Results and discussion
Fig. 1a shows the TEM image of the original uniform Fe nano-
particles from the decomposition of Fe(CO)5 under the existence
of TOPO. However, it was very interesting to find that the nanopar-
ticles become nanorods with ratio of length/diameter 5–10/1(typ-
ically 3–5 nm in diameter and 15–30 nm in length) after the
continuous addition of Fe(CO)5/TOP as shown in Fig. 1b. EDX and
XPS analysis indicate that the chemical composition of the nano-
rods is FeP (the atomic ratio of P/Fe is 1:1.1) [28]. Slightly higher
content of phosphorus may be due to the small amount of surfac-
tant, which was not removed completely by the washing process.
From the binding energy of iron and phosphorus the iron is in its
metallic state. XRD and HRTEM confirmed that the nanorods is
crystalline monophosphide FeP with orthorhombic structure space
group Pnma62 (lattice constant a = 5.191, b = 3.099 and c = 5.792)
[31]. It is believed that TOP worked not only as the solvent but also
as the P sources for the formation of FeP nanorods. And TOPO
played the role of capping reagents since TOPO was more easily
coordinated with the Fe atom on the surface of FeP than TOP.
The process could be simply described in following reaction
2.2. Chemical modification of substrate
Typically, the substrates, silicon wafer with 1000 nm thick silica
on top, were first cleaned by a Piranha solution (a mixture of 98%
H2SO4 and 30% H2O2 at 7:3 v/v) at 70 °C for ca. 30 min, followed
by thoroughly rinsing with deionized water. Silicon wafer was fur-
ther chemically modified with a methyl terminated self-assembled
monolayer (SAM) by sealing the substrate with
a drop of
1,1,1,3,3,3-hexamethyldisilazane (98%, Acros) in a steel chamber
(200 mL in volume) and heated at 150 °C overnight followed by
thorough rinsing with ethanol before use.
FeðCOÞ5 þ TOP ! ðCOÞxFeðPðC8H17Þ3Þy þ ðCOÞ
nðCOÞ FeðPðC8H17Þ Þ ! ðFePÞn þ nðCOÞ þ nð5Cꢀ8xH17Þ Þ
x
3
y
x
3 y
Iron is one of the most effective catalysts for SWNTs growth on
surface. Nano-sized iron nanoparticles can be either from pre-pre-
pared particles by various methods or from the in situ formation
from thin metal layer on surface [3,18]. We find that the FeP nano-
rods can be changed to nanoparticles for SWNTs growth when the
nanorods dispersed on substrate followed by the treatment under
H2/Ar at 700–900 °C and exposed to CO/H2 atmosphere which is
normally used for SWNTs growth [9–12]. Fig. 2 give the typical
SEM images of the nanotubes which are several to several tens
2.3. Growth of SWNTs
A drop of hexane solution containing FeP nanorods was droped
onto the silicon wafer or quartz substrate. After dried in ambient
condition, the substrate was treated at 700 °C for 10 min under
H2/Ar (200 sccm/1000 sccm) and then heated to 900 °C. After turn-
ing off the Ar, CO (800 sccm) was introduced into the system and
the reaction last for 10–30 min. Then cooling under H2.
Fig. 1. TEM image of Fe nanoparticles (a) and FeP nanorod with ratio of length/diameter 5–10/1 (b).

S. Huang et al. / Chemical Physics Letters 464 (2008) 49–53
51
Fig. 2. (a) Low magnification SEM image of the SWNTs network domain structure on substrate grown from FeP nanorods under CO/H2 at 900 °C, scale bar: 20 l; and (b) high
magnification of the SWNTs domain, scale bar: 5 l.
micrometers in length. It was very interesting to find that the
nanotubes form a domain network as shown in Fig. 2a. The domain
size was estimated to be 1–5 l. Raman spectroscopy measurement
indicates that the nanotubes are single-walled carbon nanotubes
(Fig. 3). A strong G-mode vibration is observed at 1584 cmꢀ1 and
1868 cmꢀ1, which related to the in-plane vibration of high curved
graphene sheet. The characteristic single-walled carbon nanotube
radial breathing mode is observed at 176 cmꢀ1. The average diam-
eter of the SWNTs is calculated to be about 1.27 nm based on the
equation c (cmꢀ1) = 223.75/d. AFM high measurement shown in
Fig. 4a is also in good agreement with the Raman data. AFM mea-
surement also indicates that most of the nanotubes are individual.
What is interesting is that the SWNTs using FeP nanorods as pre-
cursor has much narrow distribution in diameter than that using
1600
1200
800
400
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Raman Wavenumber (cm-1
)
Fig. 3. Raman spectrum of SWNTs network from FeP nanorods.
Fig. 4. (a) AFM image of SWNTs, scale bar: 1 l and (b) diameter distribution of the SWNTs.

52
S. Huang et al. / Chemical Physics Letters 464 (2008) 49–53
Fig. 5. (a) SEM images of the SWNTs network domain structure, scale bar: 20 l and (b) extremely high density SWNTs film, scale bar: 20 l.
Fe/Mo nanoparticles as catalyst according to the AFM measure-
ments. More than 100 SWNTs were measured and the diameters
of the SWNTs were indicated in Fig. 4b. It is well-known that metal
nanoparticles attend to aggregate together at high temperature. As
we have demonstrated that the narrow distribution in diameter of
SWNTs can be achieved by using Fe/Mo molecular cluster which
has uniform molecular structure of 1.2 nm as catalyst [12]. It has
been an effective way to gain nanoparticles on substrate in situ
by thermo-treatment of thin metal film on substrate at high tem-
perature for nanotubes growth. We believe that the same situation
happened in the case of the FeP nanorod. It is well-known that the
P-riched iron phosphate can become Fe-riched species, because of
the release of phosphorus at high temperature. Once the nano-
sized iron particles are formed they are active for nanotube growth
at the same time. The formation of iron nanoparticles from FeP
nanorods may be due to either the decomposition of FeP and/or re-
moval of PH3 under H2/Ar atmosphere at high temperature (700–
900 °C). The nanoparticles from the nanorods serve as catalyst
for SWNTs growth, because FeP nanorods are only 3–5 nm in diam-
eter. Iron nanoparticles with smaller diameter could be formed
after P was removed, which is favor size for SWNTs growth under
the CO/H2 conditions. Different from the Fe/Mo molecular cluster,
the narrow distribution in diameter of the SWNTs is believed to be
due to the formation of Fe nanoparticle in situ at high temperature,
which reduces the possible aggregation of the Fe nanoparticles.
The domain structure formation of the SWNTs network on sur-
face is believed to be due to the domain formation of nanorods on
the silicon surface which has 1 l silica layer on top. It is well-
known that nanorods have very strong tendency to self-assemble
together. In our case, the nanorods encapsulated by a surfactant
(POTO/TOP) are hydrophobic in outer surface, which does not
match the hydrophilic surface of the silicon wafer. So the nanorods
tend to form domain structures on the surface after the evapora-
tion of the solvent, resulting in the domain structure of the iron
nanoparticles. Thus domain structure of SWNTs network formed
after growth process as shown in Fig. 2a. Basically, the density
and the size of the nanotube domain can be controlled by varying
the concentration of the nanorods in solution. Fig. 5a shows a low
density of the SWNTs network by using low concentration FeP
nanorods solution. In order to prove this we changed the silicon
surface to be hydrophobic by surface chemical modification using
1,1,1,3,3,3-hexamethyldisilazane, which reacts with –OH group
to be –OCH3 group on the surface. (The contact angle measurement
shows that the contact angle changes from 40° on bare wafer (trea-
ted with H2SO4 and H2O2 solution) to 90° after 24 h treatment at
120 °C with 1,1,1,3,3,3-hexamethyldisilazane) SEM showed that
uniform SWNTs film with extremely high density rather than main
structure was formed on the surface after growth process under
the same condition (Fig. 5b). This indicated the nanorods were uni-
formly dispersed on the surface after the substrate become hydro-
phobic. These results open a new way to generate extremely high
density SWNTs with uniform diameter on substrate for SWNTs-
based nanoelectronics devices applications.
Basically the diameter and higher aspect ratio of FeP nanowires
(10–200) can be prepared by controlling the ratio of TOPO/TOP. FeP
nanorods can be also self-assembled into certain ordered struc-
tures or patterns by various technology so that location-defined
SWNTs geometry with narrow diameter distribution could be gen-
erated for various SWNTs-based nanodevice application purposes.
4. Conclusions
In summary we have demonstrated that FeP nanorods can be
synthesized by thermal decomposition of iron carbonyl under the
protection of surfactants including TOPO and TOP. The FeP nano-
rods can be changed to be isolated nanoparticles on surface under
H2/Ar at high temperature, which subsequently can be used as cat-
alyst for SWNTs growth. The domains structure of FeP nanorods or
uniform dispersion of the nanorods by self-assembly on surface
can be controlled by chemical modification of the surface and con-
centration of the nanorods solution. Domain structure of SWNTs
network or extremely high density SWNTs film with uniform
diameter can be generated based on self-assembled FeP nanorod
by using carbon monoxide chemical vapor deposition (CO-CVD).
The uniform diameter of the SWNTs is believed to be due to the
in situ formation of uniform Fe nanoparticles from the decomposi-
tion of FeP nanorods at high temperature. The use of nanorods or
nanowires as catalyst precursor for SWNTs growth could open a
new method to generate extremely high density SWNTs with uni-
form diameter for SWNTs-based nanoelectronics device
applications.
Acknowledgements
The author would like to thank Wenzhou STB (H2005B022),
DST of Zhejiang Province (2006C24010), 973(2007CB616901) and
863 project (2006AA02Z111) from MST of PRC, and NSFC
(50772076) for their partially financial support.
References
[1] J. Kong, H. T Soh, A.M. Cassell, C.F. Quate, H.J. Dai, Nature 395 (1998) 878.
[2] X. Liu, C. Lee, C. Zhou, Appl. Phys. Lett. 79 (2001) 3329.
[3] H.J. Dai, J. Phys. Chem. B 103 (1999) 11246.
[4] P. MeEuen, Yi-Yong.Yi-Yong Park, MRS Bull. (2004) 272.
[5] A. Javey, J. Guo, Q. Wang, M. Lundstrom, H. Dai, Nature 424 (2003) 654.
[6] T. Durkop, S.A. Getty, E. Cobas, M.S. Fuhrer, Nanoletters 4 (2004) 35.
[7] H. Ago, S. Ohshima, K. Uchida, M. Yumura, J. Phys. Chem. B 105 (2001) 10453.
[8] C.L. Cheung, A. Kurtz, H. Park, C.M. Lieber, J. Phys. Chem. B 106 (2002) 2429.
[9] L.X. Zheng, M.J. O’Connell, S.K. Doorn, X.Z. Liao, Y.H. Zhao, et al., Nat. Mater. 3
(2004) 73.
[10] S. Huang, X. Cai, J. Liu, J. Am. Chem. Soc. 125 (2003) 5636.

S. Huang et al. / Chemical Physics Letters 464 (2008) 49–53
53
[11] S.M. Huang, M. Woodson, R. Smalley, J. Liu, Nanoletters 4 (2004) 1025.
[12] L. An, J.M. Owens, L.E. McNeil, J. Liu, J. Am. Chem. Soc. 124 (2002) 13688.
[13] D. Takagi, Y. Homma, H. Hibino, S. Suzuki, Y. Kobayashi, Nanoletters 6 (2006)
2642.
[14] W. Zhou, Z. Han, J. Wang, Y. Zhang, et al., Nanoletters 6 (2006) 1987.
[15] L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen, M. Meyyappan, Chem.
Phys. Lett. 348 (2001) 368.
[16] J. Lu, S. Yi, T. Kopley, C. Qian, J. Liu, E. Gulari, J. Phys. Chem. B 110 (2002) 6655.
[17] Y. Li, D. Mann, M. Rolandi, W. Kim, A. Ural, et al., Nanoletters 4 (2004) 317.
[18] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306
(2004) 1362.
[22] L. Qu, L. Dai, J. Mater. Chem. 17 (2007) 3401.
[23] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press,
New York, 1994. p. 563.
[24] D.E.C. Corbridge, Phosphorus, An Outline of its Chemistry, Biochemistry and
Technology, fifth edn., Elsevier, 1995, p. 82.
[25] J.C. Fitzmaurice, A.L. Hector, I.P. Parkin, J. Mater. Sci. Lett. 13 (1994) 1.
[26] J.D. Sweet, D.J. Casadonte, Ultrason. Sonochem. 8 (2001) 97.
[27] Y.L. Gu, F. Guo, Y.T. Qian, H.G. Zheng, Z.P. Yang, Mater. Res. Bull. 37 (2002)
1101.
[28] C. Qian, F. Kim, L. Ma, F. Tsui, P. Yang, J. Liu, J. Am. Chem. Soc. 126 (2004)
1195.
[19] Y. Xu, E. Flor, M.J. Kim, B. Hamadani, H.H. Hauge, et al., J. Am. Chem. Soc. 128
(2006) 6560.
[20] T. Hiraoka, et al., J. Am. Chem. Soc. 128 (2006) 13338.
[21] M. Cantoro, S. Hofmann, S. Pisana, V. Scardaci, A. Parvez, et al., Nanoletters 6
(2006) 1107.
[29] S.J. Park et al., J. Am. Chem. Soc. 122 (2000) 8581.
[30] X.G. Peng, J. Wickham, A.P. Alivisatos, J. Am. Chem. Soc. 120 (1998)
5343.
[31] S. Rundqvist, Acta Chem. Scand. 16 (1962) 287.