Copyright © 2008 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 8, 4688–4691, 2008
Free-Standing ZnO Nanorods and Nanowalls by
Aqueous Solution Method
Dae-Hee Kim1 , Sam-Dong Lee1 , Kyoung-Kook Kim2 , Gyeong-Su Park3 ,
Ji-Myon Lee4 , and Sang-Woo Kim1 ∗
1
RESEARCH ARTICLE
School of Advanced Materials and System Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk 730-701, Korea
2
Semiconductor Device Laboratory, Samsung Advanced Institute of Technology, Yongin, Gyeonggi 446-712, Korea
3
Analytical Engineering Center, Samsung Advanced Institute of Technology, Yongin, Gyeonggi 446-712, Korea
4
Department of Materials Science and Metallurgical Engineering, Sunchon National University, Suncheon, Jeonnam 540-742, Korea
Large quantity of free-standing ZnO nanorods and nanowalls were synthesized at low temperature
Ingenta to:
zinc nitrate by
hexahydrate,
and hexamethylenetetramine by using
of below 100 C using zinc acetate,Delivered
a simple aqueous solution method.
TheKyun
generalKwan
morphology
of the grown ZnO nanostructures which
Sung
University
include nanorods and nanowalls was strongly
influenced by growth conditions. It was found that the
IP : 115.145.199.17
grown ZnO nanorods are of a single-crystalline
hexagonal
structure and preferred c-axis growth
Fri, 22 Oct 2010
01:46:25
orientation. ZnO nanorods were of better crystallinity than ZnO nanowalls, due to the higher growth
temperature used to grow ZnO nanorods. Strong free exciton emission bands with relatively weak
deep level emission were clearly observed from ZnO nanorods and nanowalls, indicating their good
optical properties.
Keywords: ZnO Nanorods, ZnO Nanowalls, Free Standing, Aqueous Solution Method.
1. INTRODUCTION
The compound semiconductor ZnO has attracted much
research interest due to its wide band-gap of 3.37 eV
at room temperature (RT) with the capability of emitting
light in the ultraviolet spectral region. Additionally, large
exciton binding energy of 60 meV allows for efficient excitonic lasing even at RT.1 Recently, ZnO nanostructures
have been of great interest because of their excellent applications in optoelectronics, sensors, and photovoltaics.2–4
Among various ZnO nanostructures, one-dimensional (1D)
ZnO nanocrystals, especially ZnO nanorods, have been
extensively investigated due to their great potential in
fundamental studies and industrial applications. Twodimensional (2D) ZnO nanostructures such as nanowalls5 6
with a high surface-to-volume ratio have also attracted a
lot of attention recently because of their high potential
for application in chemical and biological sensors, energystorage devices, and solar cells.
Chemical vapor deposition (CVD) is generally used to
grow high quality ZnO nanostructures. Studies on fabrication and characterization of ZnO nanostructures such
as nanorods and nanowalls on substrates at high temperature over 500 C in a CVD process via vapor-solid or
∗
Author to whom correspondence should be addressed.
4688
J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 9
vapor-liquid-solid mechanism have been reported.5–8 However, high-temperature growth processes limit the applicable substrate materials and lead to the generation of a
number of point defects in nanostructures. In this regard,
recently, an aqueous solution process with many advantages, such as; low temperature processing, large area uniformity, and potentially inexpensive manufacturing, has
been regarded as a promising method for realization
of various kinds of ZnO nanostructures. ZnO nanorods
grown by the aqueous solution method have been actively
reported,9–11 while growth of ZnO nanowalls using the
solution method is rare. Moreover, to the best of our
knowledge there has been no study on the optical properties of ZnO nanowalls grown using a wet chemical process. One of the most important issues to consider in the
fabrication of ZnO-based nanoscale devices is the mass
production of ZnO nanostructures such as nanorods and
nanowalls. In this study we report on mass production
and characterization of free-standing ZnO nanorods and
nanowalls via a simple low temperature aqueous solution
route without any introduction of substrates.
2. EXPERIMETAL DETAILS
ZnO seeds for growth of free-standing ZnO nanorods
and nanowalls were prepared into 10 mM zinc acetate
1533-4880/2008/8/4688/004
doi:10.1166/jnn.2008.IC42
Kim et al.
Free-Standing ZnO Nanorods and Nanowalls by Aqueous Solution Method
[Zn(C2 H3 O2 )2 ] dissolved acetone solution at 90 C for
5 min. After ZnO seed formation, free-standing nanorods
and nanowalls were formed by the continuous supply of
zinc ions and hydroxyl radicals into the aqueous solution
consisting of 25 mM zinc nitrate hexahydrate [Zn(NO3 )2 ·
6H2 O], 25 mM hexamethylenetetramine [C6 H12 N4 ]
(HMT), and de-ionized (DI) water. The main growth of
ZnO nanorods and nanowalls was carried out at 98 and
40 C for four hours, respectively. All chemicals used in
this experiment were regent grade. The following reactions
are involved in the formation of ZnO seeds.
Zn(C2 H3 O2 )2 + CH3 COCH3 → ZnO + 4H2 O + 2H2 + 7C
C + O2 → CO2
The aqueous solution including grown free-standing
ZnO nanorods and nanowalls in large quantities was filtered by a micro filter paper (pore size is 1 m). Finally,
the free-standing ZnO nanorods and nanowalls that
remained on the micro filter paper were dried at 50 C for
several hours in air ambient. The grown ZnO nanorods and
nanowalls were characterized by field-emission scanning
electron microscopy (FE-SEM), X-ray diffraction (XRD),
transmission electron microscopy (TEM), and photoluminescence (PL) measurements in order to investigate their
morphological, structural, and optical properties.
3. RESULTS AND DISCUSSION
(b)
(a)
(b)
2 µm
(c)
1 µm
(d)
(c)
2 µm
Fig. 1. Schematic images showing the aqueous solution method process
the growth of ZnO nanorods and nanowalls. (a) Preparation of the main
growth and seed solution. (b) Adding of seed solution containing a large
number of ZnO nuclei to the main growth solution for formation of ZnO
nanorods and nanowalls. (c) Formation of free-standing ZnO nanorods
and nanowalls into growth solution as a function of growth temperature.
J. Nanosci. Nanotechnol. 8, 4688–4691, 2008
1 µm
Fig. 2. (a) and (b) Low- and high-magnification FE-SEM of freestanding ZnO nanorods. (c) and (d) Low- and high-magnification FESEM of free-standing ZnO nanowalls. The average length and diameter
of the nanorods are 1.2 m and 150 nm, respectively. A large number of
nanowalls are of narrow wall thickness below 60 nm.
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RESEARCH ARTICLE
The morphology of the grown ZnO nanorods and nanoZnO is formed by the reaction between zinc acetate
walls is strongly influenced by the growth conditions,
and acetone. In this stage, formed ZnO is not grown into
especially growth temperature. Free-standing ZnO nanoZnO nanorods or nanowalls, but exists as a nucleus for
rods were grown at 98 C, while nanowalls were synthethe main growth of nanorods or nanowalls. When ZnO
sized at to:
40 C. Figure 2 shows FE-SEM images of
Delivered
nuclei are added into the main ZnO nanorod and
nanowall by Ingenta
as-grown
free-standing ZnO nanorods and nanowalls. The
Sung
Kyun
University
2+
growth solution as shown in Figure 1, Zn
and
OH−Kwan
length
and
diameter of ZnO nanorods can be effectively
IP
:
115.145.199.17
adhere to the ZnO nuclei in all directions. ZnO nuclei and
controlled
by
growth time and temperature, which range
Fri,
22
Oct
2010
01:46:25
nanorods/nanowalls are separated in a filtering process.
from
1
to
6
m
and from 90 to 210 nm, respectively.
The following reactions are involved in the formation of
Figures
2(c
and
d)
show a large number of nanowalls
ZnO nanorods and nanowalls.
with significantly narrow wall thicknesses of below 60 nm
(NO3 )2 Zn · 6H2 O → (NO3 )2 Zn + 6H2 O
and uniform distribution of formed networks. Freestanding ZnO nanorods and nanowalls grown with no sub2+
−
(NO3 )2 Zn → Zn + 2(NO3 )
strates using an aqueous solution method can obtain much
C6 H12 N4 + 6H2 O → 6HCHO + 4NH3
larger quantities than ZnO nanorods and nanowalls grown
on substrates in a CVD process, indicating that this aque4+
−
NH3 + H2 O → NH + OH
ous solution method is very promising for mass production
Zn2+ + 2OH− → ZnOS + H2 O
of ZnO nanostructures at low temperature. A large quantity of free-standing ZnO nanorods and nanowalls were
collected with a mass ratio of acquisition/source material
(a)
Free-Standing ZnO Nanorods and Nanowalls by Aqueous Solution Method
Kim et al.
(a)
RESEARCH ARTICLE
(b)
Fig. 3. (a) XRD pattern of free-standing ZnO nanorods. (b) XRD patern
of free-standing ZnO nanowalls. The unindexed peaks labeled with asterisks (∗) originate from the HMT remaining into the ZnO nanowall
sample.
Fig. 4. (a) Low-magnification TEM image of free-standing ZnO
nanorods. (b)
to above 0.8, indicating mass productivity ofDelivered
ZnO nano- by Ingenta
to:HRTEM image of a single ZnO nanorod having a singlecrystalline
hexagonal structure and preferred c-axis growth direction.
structures in this process.
Sung Kyun Kwan
University
(c) FFT pattern taken from the nanorod shown in the HRTEM image.
Figure 3 shows XRD patterns of the Free-standing
ZnO
IP : 115.145.199.17
(d) Schematic image showing the growth mechanism of free-standing
nanorods and nanowalls. All the peaks in the Fri,
XRD22
pattern
Oct 2010
01:46:25
ZnO
nanorods with preferred c-axis growth direction.
of the nanorods correspond to wurzite ZnO with calculated cell parameters of a = 325 Å and c = 521 Å in
of an ideal ZnO nanorod. Growth in the 001 direction
the accordance with the standard values for bulk ZnO. No
has the fastest growth rate originating from the preferred
diffraction peaks from Zn or other phases were observed,
c-axis growth behavior of ZnO, while the growth rate in
indicating their good structural properties. On the other
the 100 direction is lowest. Thus, a possible mechanism
hand, the intensity of ZnO-related peaks from the nanowall
is that the formation of hexagonal shaped ZnO nanorods is
sample is relatively weak compared to that of the nanorod
attributed to the preferred c-axis growth behavior of ZnO.
sample. In addition, other peaks which are not related to
The relative growth rate of these crystal faces will deterZnO are also observed in the nanowall sample. It might
mine the aspect ratio of the ZnO nanorods. Thus, it is conbe suggested that the peaks irrelevant to ZnO originate
cluded that the formation of the ZnO nanorods is attributed
from the HMT remaining into the ZnO nanowall sample.
to the significant difference of growth rates that depend
Thus, we could conclude that ZnO nanorods are of better
on crystal planes of ZnO. Although the formation mechcrystallinity than ZnO nanowalls due to the higher growth
anism of free-standing ZnO nanowalls is not described
temperature used in ZnO nanorod growth.
clearly in the present study, a helical-columnar growth
Figure 4 shows TEM images of a single free-standing
mode10 12 as another form of anisotrophic growth may play
ZnO nanorod and the corresponding fast Fourier transforan important role in the formation of the free-standing ZnO
mation (FFT) pattern taken from the nanorod shown in
nanowalls. As for the precise formation mechanism of the
the high-resolution (HR) TEM image. The lattice distance
ZnO nanowalls in this study, more detailed investigation
measured from lattice fringes along the growth axis direcis required.
tion of the ZnO nanorod is 0.52 nm, which corresponds
Figure 5 shows RT PL results of the free-standing ZnO
to the c-axis spacing of the (002) atomic planes, showing
nanorods and nanowalls with the 325 nm line of a He-Cd
the preferred growth direction of [001]. The anisotropic
laser as an excitation source. The peaks around 3.22 eV
growth of the ZnO crystal along the [001] direction
(nanorods) and 3.27 eV (nanowalls) are usually attributed
is caused by the inherent polar properties along the
to recombination of free exciton, that is, near band-edge
c-axis. The FFT analysis of the free-standing ZnO nanorod
emission. The broad deep-level emission bands are located
demonstrates the single-crystalline nature of the nanorods
at about 2.18 eV (nanorods) and 2.22 eV (nanowalls).
grown along the [001] direction. HRTEM study of freeAs shown in Figure 5, the free exciton emission bands
standing ZnO nanowalls will be reported elsewhere.
from both nanorods and nanowalls are dominant. GenThe growth mechanism of ZnO nanorods and nanowalls
erally, very broad deep level emissions prevail over free
in aqueous solution will now be discussed. From
exciton-related emissions in the solution-grown ZnO
Figures 2(a and b), the free-standing ZnO nanorods have
nanostructures.13 It is known that deep level emission is
a hexagonal cross section and the faceted prismatic morphology at the tip. Figure 4(d) shows a schematic image
mainly due to deep states in the band gap which originate
4690
J. Nanosci. Nanotechnol. 8, 4688–4691, 2008
Kim et al.
Free-Standing ZnO Nanorods and Nanowalls by Aqueous Solution Method
of a single-crystalline hexagonal structure and preferred
c-axis growth orientation in HRTEM measurements. All
the peaks in the XRD pattern from the nanorods with
no diffraction peaks from Zn or other phases clearly correspond to wurzite ZnO, indicating their good structural
properties. However, the intensity of ZnO-related peaks
from the nanowall sample is relatively weak compared
to that of the nanorod sample. Strong free exciton emission and relatively weak deep level emission from both
nanorods and nanowalls were observed in RT PL measurements, indicating their good optical properties.
(a)
(b)
References and Notes
Delivered by Ingenta to:
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Fig. 5. RT PL spectra obtained from free-standing ZnO nanorods (a)
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6.
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formation of the small number of defects in the nanostructures.
7.
4. CONCLUSION
Free-standing ZnO nanorods and nanowalls were obtained
as a function of growth temperature at low temperature below 100 C via a simple aqueous solution process. We can confirm that the grown ZnO nanorods are
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J. Nanosci. Nanotechnol. 8, 4688–4691, 2008
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RESEARCH ARTICLE
Acknowledgment: This work was supported by the
Korea Research Foundation Grant funded by the Korean
Government (MOEHRD, Basic Research Promotion Fund)
(KRF-2006-331-D00312).