facile route to straight sno2 nanowires and their optical properties

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Journal of Alloys and Compounds
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Facile route to straight SnO
2
nanowires and their optical properties
P.G. Li
a,∗
,M.Lei
a
, W.H. Tang
a
,X.Guo
a
,X.Wang
b
a
Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, China
b
Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China
article info
Article history:
Received 19 September 2008
Received in revised form 8 October 2008
Accepted 15 October 2008
Available online xxx

Keywords:
Nanostructured materials
Gas–solid reactions
Transmission electron microscopy
Optical property
abstract
Rutile SnO
2
nanowires were fabricated by a simple chemical vapor method using as-synthesized SnO
2
nanoparticles as starting material. These nanowires with unusual [12-1] growth direction are very
straight and uniform in diameter and length. Self-catalytic vapor–liquid–solid (VLS) mechanism should
be responsible for the growth of the nanowires. The photoluminescence (PL) spectrum exhibits a wide
yellow emission centered at 576 nm with a relatively small orange emission at 629 nm. The Raman
spectrum exhibits four additional modes that are not allowed by rutile-type structure in first-order
Raman-scattering at the zone center. The possible reasons for the unusual PL and Raman spectrum are
proposed.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
In the past decade, one-dimensional nanostructures, such as
nanotubes, nanowires and nanobelts, have attracted consider-
able attentions because of their peculiar structure characteristics
and excellent physical properties [1–3]. As an important n-
type semiconductor with wide bandgap (E
g
= 3.6 eV at 300 K),
SnO
2
have been widely used for transparent conductors, gas
sensor solar cells, lithium-ion batteries, and electronic devices

[4–10].Recently,one-dimensional nanowire structure has attracted
increasing attentions, owing to its enhanced surface to volume
ratio and promising applications for gas sensors [11,12] and
electronic nanodevices [13] Up to now, various methods includ-
ing direct-oxidized growth [14–16], molten-salt synthesis [17,18],
hydrothermal method [19,20], laser-ablation synthesis [21], car-
bothermal reduction [22,23], and template method [24], etc. have
been developed to fabricate SnO
2
nanowires. However, the as-
synthesized nanowires easily bend and length and diameter is
not uniform, which limits their promising applications. So, fabri-
cation of straight SnO
2
nanowires with uniform size and smooth
surface is still a challenge up to now. In this work, we devel-
oped a novel chemical vapor method to synthesize large-scale
SnO
2
nanowires with uniform size using SnO
2
nanoparticles as
starting materials. The structure property and growth mecha-
nism were investigated in detail. In addition, some interesting
∗
Corresponding author. Tel.: +86 571 86843468; fax: +86 571 86843222.
E-mail address: (P.G. Li).
optical features of the SnO
2
nanowires were presented in the

paper.
2. Experimental
The starting material is SnO
2
nanoparticles synthesized by a hydrothermal
method reported by literature [25]. In the experiment, an alumina boat contain-
ing 5 g SnO
2
nanoparticles was loaded into the center of a horizontal alumina
tube and 10 mm × 10 mm-sized 6H-SiC substrate for growth of SnO
2
nanowires was
placed on the downstream end of the alumina tube. Direct thermal evaporation of
SnO
2
nanoparticles was performed at 1550
◦
C for 90 min with an Ar flow rate of
300 SCCM. Finally, the furnace was cooled to room-temperature and white products
were deposited on 6H-SiC substrate.
Powder X-ray diffraction (XRD) of the product was characterized by PaNalytical
X’Pert Pro MPD X-ray diffractometer with Cu K␣ radiation. The morphology of the
as-synthesized product was examined by field-emission scanning electron micro-
scope (FEI XL30 S-FEG). The transition electronic microscopy (TEM) images and
high-resolution TEM (HRTEM) of samples were collected on the JEOL 2010F trans-
mission electron microscope. The X-ray photoelectron spectra (XPS) are recorded
on a VGESCALAB MKII X-ray photoelectron spectrometer, using nonmonochroma-
tized Mg K␣ X-ray as the excitation source. Raman measurement was performed on
a multichannel modular triple Raman system (JY-T64000) using a 532 nm laser as
excitation source. Photoluminescence (PL) spectrum of the nanowires was collected

at RT using the 325 nm line of a He–Cd laser as the excitation source.
3. Results and discussion
Fig. 1a shows the SEM image of the SnO
2
nanoparticles syn-
thesized by a hydrothermal method. The average size of these
nanoparticles is about 5 nm, indicating the SnO
2
nanoparticles
can be decomposed at relatively low temperature comparing with
micropowders. SEM images of the product deposited on 6H-SiC
substrate are shown Fig. 1b and c. Fig. 1b shows that straight
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Fig. 1. (a) SEM image of the SnO
2
nanoparticles synthesized by hydrothermal method. (b and c) SEM images of the SnO
2
nanowires. (d) EDS analysis of the SnO
2
nanowires.
nanowires with high density are distributed over the entire sur-
face of the substrate. SEM image with higher magnification (Fig. 1c)
clearly indicates that these nanowires are of uniform size and
smooth surface, and the average diameter and length of these

nanowires are 80 nm and 5 ␮m, respectively. The energy disper-
sive X-ray spectroscopy (EDS) spectrum (Fig. 1d) indicates that the
sample only consists of Sn and O element. The average O/Sn ratio
is 1.92:1, close to the chemical composition of SnO
2
. Fig. 2 shows
a typical XRD pattern of the nanowires deposited on 6H-SiC sub-
strate. All the diffraction peaks can be well indexed as rutile SnO
2
(ICDD-PDF No. 41-1445). The diffraction peaks are sharp and no
other impure peaks are detected, confirming the good crystallinity
of the nanowires. The composition of the Product can be further
determined by XPS spectra (Fig. 3). The binding energy centered
at 530.75, 486.88, 495.38 eV for O1s, Sn3d
5/2
and Sn3d
3/2
, respec-
tively, are in good agreement with the value of the bulk SnO
2
.
Quantification of the Sn3d and O1s peaks gives an average Sn/O
atomic ratio of 1:1.95, indicating the O-deficient formation of the
SnO
2
nanowires.
Fig. 2. XRD pattern of the SnO
2
nanowires.
Fig. 3. XPS spectra of the obtaine d SnO

2
nanowires: (a) O1s region, (b) Sn3d region.
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Fig. 4. (a) TEM image of the SnO
2
nanowires. (b) TEM image of a single nanowire. The corresponding FFT pattern is shown in the inset. (c) TEM-based EDS spectrum of the
single nanowire. (d) HRTEM image of the nanowire.
TEM image (Fig. 4a) clearly indicates that nanowires are of
smooth surface and rather uniform size along the growth direction.
Fig. 4b shows a typical nanowire with a clear surface. TEM-
based EDS analysis of the nanowire (Fig. 4c) confirms that the
nanowire mainly consists of Sn and O element, and average Sn/O
atomic ratio of 1:1.94, which exhibits the O-deficient condition of
the nanowire. The corresponding FFT pattern and HRTEM image
clearly show that the nanowire is single crystalline and grows
along [12-1] direction, which is different from common [1 0 1]
and [1 1 0] growth direction [26,27]. No obvious defects and dis-
locations are observed, and the interplanar space is 0.356 nm
and 0.236 nm, which corresponds to the (101) and (200) plane
of the rutile crystalline SnO
2
(Fig. 4d), further confirming rutile
structure of the nanowire. Based on the experimental results, con-
ventional vapor–liquid–solid (VLS) cannot dominate the growth
of the nanowires due to no metal catalyst such as tin particle
attached on the tip of nanowire. Vapor–solid (VS) mechanism also

cannot explain the growth process because SnO
2
powder unavoid-
ably decomposes at high temperature. We deduce that the growth
of the nanowires follows a self-catalytic VLS process. First, SnO
2
nanoparticles decomposed into Sn and SnO vapor. Sn and SnO vapor
subsequently are transported to low temperature zone and form
liquid droplets. The liquid droplets gradually absorb oxygen and
are further oxidized into SnO
2
droplets. The enhanced absorption
and diffusion of tin oxides occurred at SnO
2
liquid tip will finally
form SnO
2
nanowires.
A typical room-temperature photoluminescence (PL) spectrum
is shown in Fig. 5. The spectrum is dominated by a strong yellow
emission centered at 576 nm with a small orange emission shoul-
der at 629 nm. Near band edge (NBE) emission (centered at around
320 nm) is not detected, which is ascribed to strong surface effects
due to the larger surface-aspect ratio and the more surface defects
[28]. In this work, the deep-level (DL) emission such as yellow and
orange emission may be induced by bulk defects such as oxygen
vacancy (V
o
), and tin interstitial (Sn
i

), etc. It is interesting to observe
that the orange emission disappears after annealing in air at 850
◦
C
for 3 h, whereas no change happens as annealing at Ar and N
2
atmo-
sphere. These results indicate that the yellow emission is related to
the V
o
, whereas orange emission originates from Sn
i
[29,30]. Due to
the synthesis process is in the O-deficient condition, V
o
is unavoid-
able exist. As a native defect of the n-type SnO
2
, The V
o
cannot
be eliminated during the above annealing process. Nevertheless,
Sn
i
can be oxidized and thus removed by annealing in air or oxy-
gen atmosphere. So, orange emission is not commonly detected in
Fig. 5. Room-temperature photoluminescence spectrum of the SnO
2
nanowires.
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Fig. 6. Room-temperature Raman scattering spectrum of the SnO
2
nanowires.
the O-rich synthesis condition [28,31,32]. However, details on these
bulk defects such as their distributing forms need to be further
investigated.
Fig. 6 shows the R aman spectra of the as-synthesized SnO
2
nanowires. Rutile SnO
2
belongs to the point group D
14
4h
and space
group P4
n
/mnm. According to the group theory, the active Raman
modes B
1g
,E
g
,A
1g
, and B
2g
can be observed in first-order spectrum.

A
1g
and B
2g
modes vibrate in the plane perpendicular to the c-axis
while the E
g
mode vibrates in the direction of the c-axis [33].As
shown in Fig. 5, three active Raman scattering peaks at 482.3, 638.3
and 779.1cm
−1
can be assigned to E
g
,A
1g
and B
2g
mode, respec-
tively, in good agreement with those of rutile SnO
2
single crystal
[34]. Nevertheless, the four additional modes are also observed,
all of which are not allowed by rutile-type structure in first-order
Raman-scattering at the zone center. Among them, the mode peak
at 699.7 cm
−1
reported in previous article [27] is caused by the
finite-size effects of SnO
2
. The other additional peaks at 548.7, 591.1

and 733.8 cm
−1
have not been reported up to now. We deduce that
these additional modes may be in line with the defect-induced
phonon modes due to the surface disorder and large amount of O
vacancies and Sn interstitial. However, the exact mechanism needs
to be further investigated.
4. Conclusions
Large-scale rutile SnO
2
nanowires were fabricated by a simple
chemical vapor method using SnO
2
nanoparticles as starting mate-
rials. These straight nanowires are uniform in diameter and length,
and grow along the unusual [12-1] direction. The PL spectrum
exhibits a wide yellow emission centered at 576 nm with a small
orange emission shoulder at 629 nm. The yellow emission is related
to the oxygen vacancies (V
o
), whereas orange emission originates
from tin interstitial (Sn
i
). The Raman spectrum presents some new
features of the nanowires. The additional mode at 699.7 cm
−1
is
attributed to finite-size effects of SnO
2
. The peaks at 548.7, 591.1,

and 733.8 cm
−1
, respectively, were related to the defect-induced
phonon modes due to the surface disorder and large amount of O
vacancies and Sn interstitial.
Acknowledgements
This work was supported by the key project of the National Nat-
ural Science Foundation of China (60571029, 50672088) and the
Zhejiang Provincial Natural Science Foundation (Z605131).
References
[1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949.
[2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445.
[3] M. Lei, H. Yang, P.G. Li, W.H. Tang, Appl. Surf. Sci. 254 (2008) 1947–1952.
[4] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Alye, S.K. Kulkarni,
Thin Solid Films 295 (1997) 271–276.
[5] J. Watson, Sens. Actuators 5 (1984) 29–42.
[6] G. Faglia, C. Baratto, G. Sberveglien, M. Zha, A. Zappettini, Appl. Phys. Lett. 86
(2005) 011923–011925.
[7] M. Turrion, J. Bisquert, P. Salvador, J. Phys. Chem. B 107 (2003) 9397–9403.
[8] P.H. Wei, G.B. Li, S.Y. Zhao, L.R. Chen, J. Electrochem. Soc. 146 (1999) 3536–3537.
[9] M.W.J. Prins, K.O. Grosse-Holz, J.F.M. Cillessen, L.F. Feiner, J Appl. Phys. 83 (1998)
888–893.
[10] T.W. Kim, D.U. Lee, J.H. Lee, Y.S. Yoon, Solid State Commun. 115 (2000) 503–507.
[11] Q. Kuang, C.L. Lao, Z.L. Wang, X. Xie, L.S. Zheng, J. Am. Chem. Soc. 129 (2007)
6070–6071.
[12] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Nano Lett. 5
(2005) 667–673.
[13] V.V. Sysoev, J. Goschnick, T. Schneider, E. Strelcov, A. Kolmakov, Nano Lett. 7
(2007) 3182–3188.
[14] Z.R. Dai, J.L. Gole, J.D. Stout, Z.L. Wang, J. Phys. Chem. B 106 (2002) 1274–1279.

[15] Y.Q. Chen, X.F. Cui, K. Zhang, D.Y. Pan, S.Y. Zhang, B. Wang, J.G. Hou, Chem. Phys.
Lett. 369 (2003) 16–20.
[16] P. Nguyen, H.T. Ng, J. Kong, A.M. Cassell, R. Quinn, J. Li, J. Han, M. McNeil, M.
Meyyappan, Nano Lett. 3 (2003) 925–928.
[17] W. Wang, C. Xu, X. Wang, Y. Liu, Y. Zhan, C. Zheng, F. Song, G. Wang, J. Mater.
Chem. 12 (2002) 1922–1925.
[18] D. Wang, X.F. Chu, M.L. Gong, Sens. Actuators B 117 (2006) 183–187.
[19] B. Liu, H.C. Zeng, J. Phys. Chem. B 108 (2004) 5867–5874.
[20] A.A. Firooz, A.R. Mahjoub, A.A. Khodadadi, Mater. Lett. 62 (2008) 1789–1792.
[21] Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, C. Zhou, Adv. Mater.
15 (2003) 1754–1757.
[22] C. Xu, X. Zhao, S. Liu, G. Wang, Solid State Commun. 125 (2003) 301–304.
[23] S. Thanasanvorakun, P. Mangkorntong, S. Choopun, N. Mangkorntong, Ceram.
Int. 34 (2008) 1127–1130.
[24] M.J. Zheng, G.H. Li, X.Y. Zhang, S.Y. Huang, Y. Lei, L.D. Zhang, Chem. Mater. 13
(2001) 3859–3862.
[25] H.Y. Jin, Y.H. Xu, G.S. Pang, W.J. Dong, Q. Wan, Y. Sun, S.H. Feng, Mater. Chem.
Phys. 85 (2004) 58–62.
[26] Z.L. Wang, Adv. Mater. 15 (2003) 432–436.
[27] Z.W. Chen, J.K.L. Lai, C.H. Shek, Phys. Lett. A 345 (2005) 391–397.
[28] D. Calestani, M. Zha, A. Zappettini, L. Lazzerini, G. Salviati, L. Zanotti, G. Sberveg-
lieri, Mater. Sci. Eng. C 25 (2005) 625–630.
[29] J.Q. Hu, X.L. Ma, N.G. Shang, Z.Y. Xie, N.B. Wong, C.S. Lee, S.T. Lee, J. Phys. Chem.
B 106 (2002) 3823–3826.
[30] J.Q. Hu, Y. Bando, Q. Liu, D. Golberg, Adv. Funct. Mater. 13 (2003) 493–496.
[31] X. Wu, B. Zou, J. Xu, B. Yu, G. Tang, G. Zhang, W. Chen, Optik 104 (1997) 142–144.
[32] J.Q. Hu, Y. Bando, D. Goldberg, Chem. Phys. Lett. 372 (2003) 758–762.
[33] R.S. Katiyar, P. Dawson, M.M. Haargreave, G.R. Wilkinson, J. Phys. C: Solid State
Phys. 4 (1971) 2421–2431.
[34] L. Abello, B. Bochu, A. Gaskov, S. Koudryavtseva, G. Lucazeau, M. Roumyantseva,

J. Solid State Chem. 135 (1998) 78–85.