Single-crystal hexagonal disks and rings of ZnO: Low-temperature, large-scale synthesis and growth mechanism

Article abstract of DOI:10.1002/anie.200460783

Solution-phase synthesis of single-crystal ZnO disks and rings was achieved in high yield at low temperature (70-90°C) by using an anionic surfactant as a template. The reaction can be controlled by means of the growth temperature and the molar ratio of reagents to favor formation of disks or rings. A growth mechanism is proposed on the basis of structural information provided by SEM and TEM.

Full text of DOI:10.1002/anie.200460783

Communications
ZnO Nanostructures
Single-Crystal Hexagonal Disks and Rings of
ZnO: Low-Temperature, Large-Scale Synthesis
and Growth Mechanism**
Feng Li,* Yong Ding, Puxian Gao, Xinquan Xin, and
Zhong L. Wang*
The optical response of a particle depends on its particular
size, shape, and local dielectric environment.[1] Synthesis of
size- and shape-controlled nanostructures is important in
controlling their chemical and physical properties.[2,3] ZnO,
with a wide band gap of 3.37 eV, is a candidate for
[*] Dr. F. Li
Department of Chemistry
University of New Orleans
New Orleans, LA 70148 (USA)
Fax: (+1)504-280-6860
E-mail: fli@uno.edu
Dr. Y. Ding, P. Gao, Prof. Dr. Z. L. Wang
School of Materials Science and Engineering
Georgia Institute of Technology
Atlanta, GA 30332-0245 (USA)
Fax: (+1)404-894-9140
E-mail: zhong.wang@mse.gatech.edu
Prof. X. Xin
Department of Chemistry
Nanjing University
Nanjing 20093 (P. R. China)
[**] The authors gratefully acknowledge the financial support of the
National Science Foundation of China (No.90101028) and the
National Science Foundation.
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DOI: 10.1002/anie.200460783
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Chemie
applications in electronics, photoelectronics, and sensors.[4–9]
Among the developed approaches, solid–vapor phase growth
(SVG) at high temperature is favored for its simplicity and
high-quality in producing ZnOnanowires, nanobelts, nano-
cantilevers, and nanonails.[6–17] In addition to fabrication of
rings by lithography[1,18–20] and self-assembly[21,22], our recent
significant progress based on the SVG approach is the
synthesis of single-crystal ZnOrings from self-assembled
polar nanobelts.[23,24] The SVG approach has been used for
discovering a diversity of nanostructures,[25] but its major
limitation is low yield.
Solution phase synthesis (SPS), including microemulsion
and hydrothermal growth, has also proved effective and
convenient in preparing ZnOnanowires, nanorods, and
helical rods at low temperature on a large scale.[4,5,26–32] In
this approach, it is believed that the shape of micelles plays a
key role in the controlled growth of one-dimensional (1D)
nanostructures in solution.[26] Since surfactants can be made
to form micelles with diverse shapes from spheres to rods to
ellipsoids to disks, and even much more complex shapes, by
adjusting experimental parameters,[33] we can further control
the nanostructures by means of the SPS approach. Here we
report a rational synthesis of single-crystal hexagonal ZnO
nanodiscs and rings on a large scale by the SPS approach, and
demonstrate its potential for low-temperature, large-scale,
controlled synthesis of single-crystal ZnOnanostructures. The
approach may be extendible to the synthesis of a wide range
of nanomaterials. The hexagonal disks and rings are new
members in the family of ZnOnanostructures. On the basis of
structural information provided by electron microscopy, a
growth mechanism is proposed for the formation of disks and
rings.
Figure 1. a) Low-magnification SEM image recorded from the as-syn-
thesized ZnO sample prepared at 708C, demonstrating the predomi-
nance of hexagonal disks. b), c) Enlarged SEM images displaying
detailed structures of the hexagonal disks.
The ZnOrings and disks were synthesized by a solution-
phase approach. The procedure involves 1) preparation of an
oil-in-water microemulsion with surfactant, namely, sodium
bis(2-ethylhexyl) sulfosuccinate (NaAOT), in a water/1-
butanol/Zn(NO3)2 system, and 2) subsequent growth of
ZnOrings and disks in microreactors. Figure 1a shows a
typical low-magnification SEM image of ZnOnanostructures
grown at 708C. The as-synthesized product is pure and
uniform, and is dominated by hexagonal-based thin disks with
uniform size and well-defined shape. The disks are 2–3 mm in
diameter and 50–200 nm thick. An enlarged image shows the
back-to-back paired stacking of the disks (Figure 1b), with
one side of the disk surface smoother, and the other rougher.
The two adjacent contacting surfaces are rather smooth.
Some of the disks have a hole at the middle, and are thus
hexagonal rings (Figure 1c).
Figure 2. a) Low-magnification SEM image recorded from the as-syn-
thesized ZnO sample prepared at 908C; the higher yield of hexagonal
rings is evident. b)–d) Enlarged SEM images displaying detailed struc-
tures of the hexagonal rings.
The morphology of the disks can be modified by adjusting
the growth temperature. Increasing the growth temperature
from 70 to 908C dramatically increases the yield of hexagonal
rings (Figure 2a). Most of the rings have a hexagonal base and
their morphology is fairly uniform. The size of the inner holes
varies from ring to ring, and they are mostly hexagonal
(Figure 2b, c). In some cases, the shape of the hole is irregular
(Figure 2d).
rough side had a higher “etching” rate (see below), so that the
surface is nonuniform, and there is a small hole at the middle
(Figure 3b). The nonuniform morphology of the top surface
can be best seen by tilting the disk (Figure 3c), which reveals
the grooved surface structure associated with the crystal
symmetry on one side (Figure 3d). In contrast, the backside of
the ring is rather flat. The hole appeared to have been created
from the rougher side and extends half way through the
thickness (Figure 3e). For a small hole that passes throughout
Detailed examination revealed that the top and bottom
surfaces of the hexagonal rings and disks are quite different;
one side is smooth, and the other rough (Figure 3a). The
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Figure 4. a), b) Dark-field TEM images and corresponding diffraction
pattern of a hexagonal disk, showing planar defects in the disk, which
are likely to be low-angle grain boundaries induced during crystal
growth. c) Bright-field TEM image of a hexagonal ring and its corre-
sponding diffraction pattern, showing that the ring has a top/bottom
Figure 3. SEM images recorded from ZnO hexagonal disks showing
the structural features on the rougher and smoother surfaces of the
disks.
¯
(0001) surfaces and inner/outer {1100} surfaces. d) Dark-field TEM
¯¯
image of the ring, displaying defects closely parallel to {2110}.
the thickness, a rough edge at the inner surface is apparent
(Figure 3 f).
e) High-resolution TEM image recorded from the inner edge marked in
c), showing clean and sharp surfaces.
The TEM analysis of the hexagonal disk showed a single-
¯
crystal structure, with (0001) top/bottom surfaces, and {1100}
side facets (Figure 4a). The dark-field image of the disk shows
defects, which are distributed radially with a higher concen-
tration at the center. The defects are most likely low-angle
grain boundaries (Figure 4b). Some thickness fringes are
observed, which correspond to the reduced thickness towards
the center.
with AOTÀ directly to form white precipitates in water.
4) Further investigations revealed that the molar ratio of Zn2+
to NH3·H2Ois one of the dominant parameters for the
formation of well-structured disks and rings. Hexagonal disks
and rings are produced at Zn2+/NH3·H2Omolar ratios of = ,
1
3
1
1
1
= , = , and = , but no ZnOparticles were observed in a
4
5
6
The hexagonal ring also shows a single-crystal structure,
reaction carried out at a molar ratio of 1= . Thus, in the oil-in-
2
¯
with (0001) top/bottom surfaces and {1100} side surfaces
water microemulsion system, the reaction temperature and
the concentration of NH3·H2Oare the dominant parameters
in the formation of hexagonal rings and disks.
(Figure 4c, d). An important feature observed in the TEM
¯ ¯
image is the presence of defects closely parallel to h2110i in
the ring. The disk has a wedge shape that becomes thinner
towards the center. A high-resolution TEM image recorded
from the inner edge of the ring (Figure 4e) shows surface
steps, atomically sharp and free of contamination. The TEM
image also reveals the decrease in disk thickness towards the
center.
On the basis of the information we have gathered, a
growth process of the hexagonal disks and rings can be
proposed. From the crystal structure, ZnOcan be described as
a number of alternating planes composed of tetrahedrally
coordinated O2À and Zn2+ ions, stacked alternatingly along
the c axis. The oppositely charged ions produce positively
Several experiments were carried out to determine the
parameters other than temperature that are important for the
formation of disks and rings: 1) No hexagonal ZnOdisks were
produced in the absence of NaAOT or when NaAOT was
replaced with sodium dodecyl sulfate (SDS). 2) The use of n-
butanol is also crucial to the formation of shape-controlled
ZnOdisks. No hexagonal ZnOdisks are produced in the
reaction if only water is used as solvent to dissolve NaAOT, or
n-butanol is replaced with octane. 3) Zn2+ cations can react
charged Zn2+ (0001) and negatively charged O2À (0001) polar
¯
surfaces. Therefore, the positively charged Zn2+ (0001) and
the negatively charged O2À (0001) surfaces can have very
¯
different self-catalysis properties,[11] and the spontaneous
polarization along the c axis leads to the formation of
nanosprings,[24] rings,[23] and nanobows[26] due to electrostatic
interaction. The large polar surface is generally energetically
unfavorable unless the surface charge is compensated by a
passivation agent. On the other hand, it was reported that the
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anionic surfactant NaAOT can be made to form micelles with
diverse shapes by adjusting experimental parameters.[33–36]
The self-assembled AOTÀ layers at the interface of water
and oil could act as template for growing ZnO. The hydro-
philic head of AOTÀ can form an anionic surface exposed to
water, and thus Zn2+ cations can directly attach to the
negatively charged AOTÀ template to initiate the first layer of
crystal growth [Eq. (1)]. The produced Zn(AOT)2 then can be
transformed into [Zn(NH3)4]2+ by reaction with NH3·H2O
[Eq. (2)], so that the positively charged Zn2+ (0001) surface of
ZnOtends to directly interface with the anionic surface of the
AOTÀ template in the subsequent hydration of [Zn(NH3)4]2+
at higher temperature [Eq. (3)].[29]
From the TEM image presented in Figure 4b, the density
of defects is highest at the center of the disk. A higher etching/
reaction rate is possible at the defect sites. Therefore, a high
density of planar defects at the center may result in a higher
local etching rate and eventually lead to the formation of a
hole at the center. The hole is initiated from the O2À (0001)
¯
surface and it may not penetrate the entire thickness if the
reaction time is insufficient (see Figure 3e), as presented in
Figure 5b. As the reaction proceeds, the etching can create a
hole through the disk (Figure 5c). As the hole becomes larger,
¯
its side surfaces eventually take on the lower energy {1100}
facets of ZnO, and the disk becomes a hexagonal ring
(Figure 5d). The grooved feature on the surface of the ring
¯ ¯
along h2110i may be due to the dependence of the local
ZnðNO3Þ2 ðaqÞ þ 2 NaAOT ! ZnðAOTÞ2 ðsÞ þ 2 NaNO3 ðaqÞ ð1Þ
etching rate on the crystal orientation. On the other hand, the
etching rate is sensitive to the growth temperature and the
molar ratio of reactants; thus rings were not formed at 708C,
but at 908C or higher.
ZnðAOTÞ2 ðsÞ þ 4 NH3 Á H2O ðaqÞ
ð2Þ
2þ
À
! ½ZnðNH3Þ4Š ðaqÞ þ 2 AO T ðaqÞ þ 4 H2O ðlÞ
The formation of paired disks, as seen in Figure 1b, may
be due to the fact that AOTÀ usually forms self-assembled
double layers and multilayers in lamellar micelles. The growth
of ZnOdisks and rings can take place simultaneously on both
sides of the template and thus result in back-to-back growth
of paired disks or rings. After removing the oil phase from the
system, a close attachment of the two disks at the smoother
surfaces is energetically favorable because of the van der
Waals interaction between the long tails of the self-assembled
AOTÀ monolayer on the surfaces.
2þ
½ZnðNH3Þ4Š ðaqÞ þ 3 H2O ðlÞ
ð3Þ
Ð ZnO ðsÞ þ 2 NHþ4 ðaqÞ þ 2 NH3 Á H2O ðaqÞ
¯ ¯
Fast growth along h2110i results in the formation of a
hexagonal disk (Figure 5a). Slight bending of the self-
assembled AOTÀ monolayer makes it possible to accommo-
date local strain in the disk by formation of small-angle grain
In summary, using NaAOT as template, we have demon-
strated a solution-based synthesis of ZnOrings and disks at
low temperature and on a large scale. The as-synthesized
materials are structurally uniform and pure. By controlling
growth temperature and molar ratio of reactants, the disks
could converted into rings. The growth mechanism of the
hexagonal disks is suggested to be due to charge compensa-
tion of the anionic AOTÀ template at the Zn2+ (0001) surface
¯ ¯
of ZnO; fast growth along h2110i forms hexagonal disks
¯
enclosed by {1010} facets. A higher density of defects at the
center of the disk results in a higher local reaction/etching rate
+
by NH4 and NH3. A through-thickness hole, as defined by
the {1010} facets at the center, leads to the formation of a
Figure 5. Proposed formation process of the ZnO hexagonal disks and
rings, as well as their back-to-back stacking.
¯
hexagonal ring. The technique demonstrated here could be
extended for synthesizing a wide range of nanomaterials.
Using such well-structured materials as building blocks, one
could design a diverse range of nanodevices, from nanoscale
lasers to sensors and photonic crystals.
¯ ¯
boundaries closely parallel to {2110}. Therefore, some of the
grown disks may have this defect.
The Zn2+ (0001) surface bonds strongly to AOTÀ due to
charge interaction, and the densely packed AOTÀ will protect
the surface from further “etching” or reaction, resulting in the
formation of the flat side of the disks (see Figures 1 and 3).
The AOTÀ template stabilizes the surface charge and the
structure.
Experimental Section
For the preparation of ZnOdiscs and rings, a NaAOT microemulsion
was first prepared by adding an aqueous solution of Zn(NO3)2·6H2O
(0.025m) to a solution of NaAOT (0.10m) in 1-butanol and vigorously
stirring for 2 h. The volume ratio of the aqueous phase to the organic
phase was 10:1. Then a concentrated aqueous solution of NH3·H2O
(17.65m) with 4:1 molar ratio to Zn(NO3)2·6H2Owas added dropwise
to the well-stirred microemulsion. After addition was complete,
stirring was continued for 3 h at room temperature. The resulting
milky white mixture was subsequently kept at 70 or 908C for 5 days. A
white suspension was obtained and centrifuged to separate the
precipitate, which was washed several times with distilled water and
The exposed negatively charged O2À (0001) surface of the
¯
disk may be reactive towards NH4+ and NH3·H2O, as shown in
Equation (3). The equilibrium will move in the reverse
direction on adding excess NH3·H2O. The rate toward the
reverse direction of Equation (3) also increases drastically
with increasing temperature; thus, etching of the O2À (0001)
¯
surface by NH4+ and excess NH3 results in the rougher surface
(see Figure 3a).
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absolute ethanol. Finally, a white powder was obtained by drying at
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a LEO1530 field-emission scanning electron microscope (FE-SEM).
Transmission electron microscopy was carried out at 400 kV with a
JEOL 4000EX.
Received: May 25, 2004
Keywords: crystal growth · nanostructures · oxides · zinc
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