Preparation and characterization of CuO nanorods by thermal decomposition of CuC2O4 precursor

Article abstract of DOI:10.1016/S0025-5408(02)00848-6

Synthesis of copper oxide (CuO) nanorods was achieved by thermal decomposition of the precursor of CuC₂O₄ obtained via chemical reaction between Cu(CH₃COO)₂·H₂O and H₂C₂O₄·2H₂O in the presence of surfactant nonyl phenyl ether (9)/(5) (NP-9/5) and NaCl flux. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM) were used to characterize the structure features and chemical compositions of the as-made nanorods. The results showed that the as-prepared nanorods is composed of CuO with diameter of 30-100 nm, and lengths ranging from 1 to 3 μ m. The mechanism of formation of CuO nanorods was also discussed.

Full text of DOI:10.1016/S0025-5408(02)00848-6

Materials Research Bulletin 37 (2002) 2365±2372
Preparation and characterization of CuO nanorods by
thermal decomposition of CuC O precursor
2
4
*
Congkang Xu, Yingkai Liu, Guoding Xu, Guanghou Wang
National Laboratory of Solid State Microstructure, Department of Physics, Nanjing University,
Nanjing 210093, PR China
(
Refereed)
Received 2 February 2002; accepted 7 June 2002
Abstract
Synthesis of copper oxide (CuO) nanorods was achieved by thermal decomposition of the
precursor of CuC O obtained via chemical reaction between Cu(CH COO) ÁH O and
2
4
3
2
2
H C O Á2H O in the presence of surfactant nonyl phenyl ether (9)/(5) (NP-9/5) and NaCl
2
2
4
2
¯
ux. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectro-
scopy, X-ray photoelectron spectroscopy (XPS), selected-area electron diffraction (SAED)
and high-resolution TEM (HRTEM) were used to characterize the structure features and
chemical compositions of the as-made nanorods. The results showed that the as-prepared
nanorods is composed of CuO with diameter of 30±100 nm, and lengths ranging from 1 to
3
#
mm. The mechanism of formation of CuO nanorods was also discussed.
2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Nanostructure; A. Oxides; C. X-ray diffraction
1
. Introduction
In recent years, quasi one-dimensional (1-D) solid nanostructures (nanowires or
nanorods) have stimulated considerable interest for scienti®c research because of
their importance in mesoscopic physics and their potential applications. Compared
with micrometer-diameter whiskers and ®bers, these nanostructures are expected to
have remarkable optical, electrical, magnetic, and mechanical properties. Different
*
Corresponding author. Tel.: ‡86-25-3595082; fax: ‡86-25-3595535.
E-mail address: wangqun@nju.edu.cn (G. Wang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 8 4 8 - 6

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C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
approaches have been used for the preparations of quasi 1-D solid nanostructures,
e.g. the vapor±liquid±solid (VLS) growth [1±11], solution±liquid±solid (SLS)
method [12], template-mediated growth method [13±15], electron-beam lithography
(
EBL) [16] and scanning tunneling microscopy (STM) techniques [17], and other
methods [14±18] of nanowire production. Exploration of novel methods for the
large-scale synthesis of 1-D nanostructures is a challenging research area. As a p-
type semiconductor with a narrow band gap and the basis of several high-Tc
superconductors, copper oxide (CuO), has received a considerable attention over
last few years because of many applications in various ®eld, especially, CuO
nanostructured materials are expected to possess properties having photothermal,
photoconductive, and gas sensors applications [19±22]. However, up to now, many
methods have been developed to synthesize CuO nanoparticles and ®lms, few
attempts have been made to prepare CuO nanorods. In this work, we report a simple
and novel approach to the fabrication of CuO nanorods by thermal decomposition of
the precursor of CuC O obtained via chemical reaction between Cu(CH COO) Á-
2
4
3
2
H O and H C O Á2H O in the presence of surfactant nonyl phenyl ether (9)/(5)
2
2
2
4
2
(
NP-9/5) and NaCl ¯ux.
2
. Sample preparation and experimental procedure
Staring materials are copper acetate Cu(CH COO) ÁH O, oxalic acid H C O Á-
3
2
2
2 2 4
2
H O, nonyl phenyl ether (9)/(5)(NP-9/5), and NaCl. Among them, Cu(CH COO) Á-
2 2 2 4 2
2
3
2
H O, H C O Á2H O, and NaCl are of analytical purity, NP-9/5 is up to 99.0%. A total
of 3.99 g of Cu(CH COO) ÁH O and 2.53 g of H C O Á2H O according to 1:1 molar
3
2
2
2
2
4
2
rate were mixed with 5 ml of NP-9/5 in a mortar, ground for several minutes and kept
in a thermostat oven at 50±608C for 6 h to prepare the precursor, the product was
washed several times with distilled water and acetone to remove remaining reactants,
NP-9/5 and by-product, and then dried in an oven at 70±808C for 12 h. The obtained
product was collected for the fabrication of CuO nanorods. Two grams of CuC O ,
2
4
was simultaneously mixed with 8 g of NaCl powder, and then ground for several
minutes. The ground mixture was annealed at 9508C for 2 h in a porcelain crucible
that was placed in the middle of the alumina tube; the reaction chamber is of alumina
tube with a length of 1500 mm and a diameter of 60 mm. After heat treatment, it was
gradually cooled to room temperature (5 8C/min), the as-prepared product was
washed with distilled water one time, ethanol three times and then ethyl ether
one time using an ultrasonic bath and a centrifuge.
The as-prepared product was identi®ed by X-ray powder diffraction (XRD)
employing a scanning rate of 0.028/s in a 2y range from 30 to 758, using a Japanese
Rigaku D/max-gA K X-ray diffractometer equipped with graphite monochroma-
II
tized Cu Ka radiation. The morphologies and dimensions of the products were
observed by transmission electron microscopy (TEM), which were taken on a JEOL-
2010 transmission electron microscope using an accelerating voltage of 200 kV.
Meanwhile, the as-made products were also characterized by X-ray photoelectron

C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
2367
spectroscopy (XPS), which was carried out on an ESCALAB M K X-ray photo-
II
electron spectrometer, using Mg Ka X-ray as the excitation source; Raman spectro-
scopy was obtained using a RFS 100 spectrometer from 100 to 700 cm at room
À1
temperature. The 1064 nm line of the laser was used as the excitation source, with the
capability of supplying 200 mW. For high-resolution TEM (HRTEM) and TEM
observation, the as-prepared product was ground in a mortar and suspended in ethanol
using an ultrasonic bath for half an hour. A drop was then placed on a holey-carbon
copper grid.
3
. Results and discussion
The general TEM morphologies of the as-synthesized product are shown in Fig. 1.
It can be seen that the product mainly consists of solid rod-like structures. The length
of the rods, as shown in Fig. 1a, can be up to several micrometers. The typical
nanorod, as can be seen in Fig. 1b, are around 30 nm in diameter.
Fig. 1. TEM image of the as-prepared CuO nanorods. (a) Morphologies of the as-prepared CuO
nanorods. (b) Morphologies of the as-prepared single CuO nanorod.

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C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
Fig. 2. The XRD pattern of the as-prepared CuO nanorods.
Fig. 2 shows the XRD of the as-prepared products obtained by thermal decom-
position of the precursor of CuC O .
The reaction is as follows:
2
4
2
À
‡ Cu2‡ ! CuC O ;
C O
2
(1)
(2)
4
2
4
CuC O ! CuO ‡ CO ‡ CO :
2
4
2
Its diffraction peaks were quite identical to those of pure CuO, which can be
Ê
Ê
Ê
indexed as the monoclinic structure CuO (a ˆ 4:68 A, b ˆ 3:42 A, c ˆ 5:12 A) and
diffraction data were in agreement with JCPDS card of CuO (JCPDS 5-0661). No
characteristic peaks of impurities such as NaCl, CuC O , Cu O, and other precursor
2
4
2
compounds were observed. Thus, the results clearly showed that the as-prepared
product is single phase monoclinic CuO.
Fig. 3 presents Raman spectra of the as-prepared products. CuO belongs to the
6
C space group with two molecules per primitive cell. One can ®nd for the zone
center normal modes:
2h
G ˆ 4Au ‡ 5Bu ‡ Ag ‡ 2Bg:
There are three acoustic modes (Au ‡ 2Bu), six infrared active modes (3Au ‡ 3Bu),
and three Raman active modes (Ag ‡ 2Bg).
À1
As Fig. 3 shown, three Raman peaks at 621.4, 339.8, and 295.4 cm were
observed, with ®rst one being broad and the second one being weakest. In comparison
À1
with the vibrational spectra of a CuO single crystal [23], the peak at 295.4 cm is
À1
assigned to the Ag mode and the peak at 339.8, and 621.4 cm to the Bg modes. The
results are in agreement with the literature reference value for nanocrystal CuO [24].
As a consequence, the Raman spectra showed that the as-made product is composed
of single phase CuO with monoclinic structure.

C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
2369
Fig. 3. The Raman spectra of the as-prepared CuO nanorods.
Fig. 4a and b shows XPS spectra taken from the Cu and O regions of the sample.
The peaks at about 933.1 and 952.9 eV are attributed to Cu 2p3/2 and 2p1/2 (Fig. 4a),
respectively, which are close to the data for Cu (2p) in CuO; the gap between the Cu
2
spectrum of Cu. On the other hand, Cu (2p) peak is sharp, as implying the existence of
p1/2 and 2p3/2 level is 19.8 eV that is approximately the same as in the standard
2‡
Cu , the peak at 933.1 eV with two shake-up satellites at about 9 and 10.1 eV higher
in binding energy than that of the main peak. The existence of strong satellite features
for Cu (2p) rules out the possibility of the presence of Cu O phase. From Fig. 4b, it is
2
seen that the O (1s) XPS is asymmetric, indicating that at least two oxygen species are
present in the nearby region. The peak at about 530 eV is due to oxygen in the CuO
crystal lattice, which corresponds to O±Cu bonds, whereas the peak at about 532 eVis
due to chemisorbed oxygen caused by surface hydroxyl, which corresponds to O±H
bonds. The atomic composition of Cu and O were calculated by using the integrated
peak area and sensitivity factors, the atomic ration of Cu:O is nearly 1:1. Thus the
XPS results clearly proved that the sample is composed of CuO.
Fig. 5 exhibits the selected-area electron diffraction (SAED) patterns, taken from
the same nanorod shown in Fig. 1b, can be indexed to the re¯ection of monoclinic
CuO structure, which is consistent with the above XRD result. Clearly, it also
demonstrates that the nanorod is single crystal.
As a rule, the chemical route often uses direct, very simple, chemical reactions to
produce a product in powder form. These techniques have in common the fact that
they do not offer underlying of physical or chemical principles. Thus, the general-
ization of the processes involved must require the understanding of the formation
mechanism of nanorods. There usually exist two models that explain the mechanism
of growth for nanorods [18]. One is the VLS growth mechanism; the other is the

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C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
Fig. 4. The XPS analysis of the as-prepared CuO nanorods. (a) Cu region. (b) O region.
conventional spiral mechanism, that is, VS mechanism. In our product, because no
dropletswereobservedonanyendofthenanorods, whichisthemostremarkablesignof
the VLS mechanism, no evidence indicated that the CuO nanorod growth matches VLS
mechanism. On the contrary, there exists a conical tip at the end of nanorod (Fig. 1b),
which is evidence of the spiral growth mechanism, hence, the growth mechanism of the
CuO nanorods is most likely controlled by the VS growth mechanism.
Apparently, the formation of the nanorods must be affected by the character of the
starting material, such as, the particle size and/or chemical activity in that the

C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
2371
Fig. 5. SAED of one single crystalline CuO nanorods.
dissolution rate of the material depend upon these characters. The viscosity of the ¯ux
during calcination and the eutectic temperature of the system also affect the formation
of the nanorods. We found that the CuO nanorods fail to form in the absence of NaCl
and surfactant NP-9/5 or either of them. Only in this way can the CuO nanorods be
formed in the presence of surfactant NP-9/5 and NaCl ¯ux. We speculated that NaCl
may signi®cantly decrease the viscosity of the melt, and thus make mobility of
components in the ¯ux become easier, i.e. provide a favorable environment for the
growth of nanorods, the surfactant NP-9/5 is favorable to form ®ne particle and make
a ``shell'' surrounding the particles to prevent them from aggregating to larger
particles during the grinding process of the precursor, on the other hand, during
the formation of CuO nanorods, surfactant was thought to be able to act as a template,
with the template action resulting in the expitaxial growth of the product. It is
interesting to note that the precursor takes the shape of rod-like structure if the
surfactant NP-9/5 is suf®cient. A detailed study of mechanism of CuO nanorods is in
progress.
4
. Conclusions
In summary, CuO nanorods with diameters of 30±100 nm and lengths of several
micrometers have been successfully prepared by a simple and novel method. The as-
prepared CuO nanorods are structurally uniform, single crystalline. The growth
mechanism of the CuO nanorods is most likely controlled by the VS growth
mechanism. Particularly, surfactant NP-9/5 is of critically importance for the for-
mation of CuO nanorods, if surfactant is chosen properly, this method may be
extended to other oxide nanorods.
Acknowledgments
This work was ®nancially supported by the National Nature Science Foundation of
China (no. 29890210, 10023001, 10074024).

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C. Xu et al. / Materials Research Bulletin 37 (2002) 2365±2372
References
[
[
1] A.M. Morales, C.M. Lieber, Science 279 (1998) 208.
2] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, Y.H. Tang, G.W. Zhou, Z.G. Bai, Z. Zhang, S.Q. Feng, D.P.
Yu, Solid State Commun. 105 (1998) 405.
[
[
3] G.W. Zhou, Z. Zhang, Z.G. Bai, S.Q. Feng, D.P. Yu, Appl. Phys. Lett. 73 (1998) 677.
4] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong,
H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458.
[
[
[
5] H.Z. Zhang, D.P. Yu, Y. Ding, Z.G. Bai, Q.L. Hang, S.Q. Feng, Appl. Phys. Lett. 73 (1998) 3396.
6] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci. Technol. B 15 (1997) 554.
7] D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, Y.H. Zhou, W. Qian, G.C. Xiong,
S.Q. Feng, Appl. Phys. Lett. 73 (1998) 3076.
[
[
8] Y.Q. Zhu, W.B. Hu, W.K. Hsu, M. Terrones, N. Grobert, T. Karali, H. Terrones, J.P. Hare, P.D.
Townsend, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 844.
9] K. Hiruma, M. Yazawa, T. Katsuyama, K. Ogawa, K. Haraguchi, M. Koguchi, H. Kakibayashi, J.
Appl. Phys. 77 (1995) 447.
[
[
[
10] G.W. Meng, D. Zhang, Y. Qin, F. Phillipp, S.R. Qiao, H.M. Guo, S.Y. Zhang, Chin. Phys. Lett. 9
1998) 689.
11] X.T. Zhou, N. Wang, H.L. Lai, H.Y. Peng, I. Bello, N.B. Wong, C.S. Lee, Appl. Phys. Lett. 74
1999) 3942.
12] T.J. Trentler, K.M. Hickman, S.C. Geol, A.M. Viano, P.C. Gibbons, W.E. Buhro, Science 270
1999) 1791.
(
(
(
[
[
[
[
[
[
13] H. Dai, E.W. Wong, Y.Z. Yu, S.S. Fan, C.M. Lieber, Nature 375 (1999) 769.
14] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287.
15] W.Q. Han, S.S. Fan, Q.Q. Li, B.L. Gu, Appl. Phys. Lett. 71 (1997) 2271.
16] E. Leobandung, L. Guo, Y. Wang, S.Y. Chou, Appl. Phys. Lett. 67 (1997) 938.
17] T. One, H. Saitoh, M. Esashi, Appl. Phys. Lett. 70 (1997) 1852.
18] H.Z. Zhang, Y.C. Kong, Y.Z. Wang, X. Du, Z.G. Bai, J.J. Wang, D.P. Yu, Y. Ding, Q.L. Hang,
S.Q. Feng, Solid State Commun. 109 (1999) 677.
19] A.E. Rakhshni, Solid State Electron. 7 (1986) 29.
[
[
[
[
20] T. Ishihara, M. Higuchi, T. Takagi, M. Ito, H. Nishiguchi, T. Takita, J. Mater. Chem. 8 (1998) 2037.
21] T. Ishihara, K. Kometani, M. Hashida, Y. Takita, J. Electrochem. Soc. 138 (1991) 173.
22] J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, N. Yamazoe, Sens. Actuators B 49
(
23] J.C. Irwin, J. Chrzanowski, T. Wei, Phys. C 166 (1990) 456.
1998) 121.
[
[
24] J.F. Xu, W. Ji, X. Shen, S.H. Tang, J. Solid State Chem. 147 (1999) 516.