Cryst. Res. Technol. 49, No. 11, 899–906 (2014) / DOI 10.1002/crat.201400231
Original Paper
Fabrication of hollow ZnO nanostructures by a CTAB-assisted
chemical bath deposition method
Ying Hou and Ming Yang∗
Received 8 July 2014, revised 4 September 2014, accepted 15 September 2014
Published online 9 October 2014
ZnO lms consisted of hollow nanostructures were prepared
by a CTAB-assisted chemical bath deposition (CBD) method.
ZnO rings, bowls and assemblies of hollow structures were
successfully obtained on dierent substrates. Dense ZnO
lms consisted of sunken prisms can also be achieved by
controlling the concentration of CTAB. The inuences of
reactant concentrations, types of the substrates and pre-
coated ZnO nanoparticles on the formation of ZnO lms
were examined. XRD patterns indicated the Wurtzite struc-
ture of ZnO and the preferred growth direction is [001]. The
role of CTAB in CBD process was discussed and the evolution
of dierent ZnO nanostructures was studied based on the
observation of SEM. A plausible crystal growth mechanism
was proposed for the formation of ZnO rings and bowls. The
investigation of optical properties showed that high con-
centration of CTAB can improve the ultraviolet emission.
1 Introduction
The shape, size and crystalline structure of semi-
conductors are important elements in determining
their chemical and physical properties [1]; to meet
the actual technological applications, the rational
control over these factors are essential in today’s ma-
terial science [2]. The wide direct band-gap of ZnO
(3.37 eV) and large exciton binding energy (ca. 60 meV
at room temperature) makes it a promising optoelec-
tronic material in the UV region [3]. Other important
properties including size-dependent surface lumines-
cence [4], room-temperature storage of hydrogen [5],
photocatalysis [6], nanoelectricity generators [7] have
also been demonstrated for ZnO-based materials.
Accordingly, various ZnO nanostructures have been
synthesized including nanowires [8], nanotubes [9],
nanosprings [10], nanonails [11], tetrapods [12], and
etc. using high-temperature physical methods. The
solution-based technique provides an environment
friendly and economical way in controlling the mor-
phologies of ZnO nanostructures. Several interesting
structures have been obtained via solution methods,
such as hexagonal discs and rings [13], doughnut-
shaped microparitlces [14], nanowalled microboxes [15],
helical rod-like structures [16], nanopyramids [17],
nanosheets [18], and hierarchical structures [19]. The
chemical bath deposition (CBD) of metal oxide thin
films, which is based on controlled precipitation on a
substrate via hydrolysis and/or condensation reactions
of metal ions and/or complexes from aqueous solu-
tion [20], is effective for the preparation of orientated
ZnO nanorod and nanotube arrays which have impor-
tant applications in nanoscale devices [21]. However,
there are very limited investigations on the influence
of surfactants during the CBD process, which may play
an important role in the formation of ZnO nanostruc-
tures. Further, hollow nanostructures can have special
advantages when compared with solid structures due
to their higher specific surface area [22]. In this study,
a cetyltrimethylammonium bromide (CTAB)-assisted
CBD process was used to controllably prepare ZnO with
different morphologies, such as ZnO rings and bowls.
ZnO films consisted of dense sunken prisms can also be
obtained. Based on the study of the influence of different
reaction conditions, the role of CTAB in CBD process was
discussed and a possible formation mechanism of these
ZnO structures was proposed. The photoluminescence
(PL) spectra of ZnO samples were also investigated.
∗Corresponding author: e-mail: [email protected]
Key Laboratory of Microsystems and Micronanostructures Manu-
facturing, Harbin Institute of Technology, 2 Yikuang Street, Harbin,
150080, P. R. China
899
C2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Y. Hou and M. Yang: Fabrication of hollow ZnO nanostructures by a CTAB-assisted . . .
Original Paper
2 Experimental Section
2.1 Physical Methods
Powder X-ray diffraction (XRD) analysis was performed
on a Rigaku D/MAX 2500/PC diffractometer with
graphite-filtered Cu Kαradiation. XRD data were col-
lected over 20–80 °with a step interval of 0.02 °and
a preset time of 1.2 s per step at room temperature.
Scanning electron microscope (SEM) was performed on
JSM-6700F electron microscope and transmission elec-
tron microscope (TEM) was performed on JEM-3010
electron microscope. PL spectra were measured using a
He-Cd laser excited at 325 nm at room temperature.
2.2 Materials and Preparation Methods
All reagents (analytic grade) were purchased from Beijing
Chemical Co. Ltd and used as received without further
purification. ZnO precipitates were obtained in 40 mL
of equimolar (0.1 M) aqueous solution of zinc nitrate,
hexamethylenetetramine (HMTA, (CH2)6N4)andCTAB
in a bottle with an autoclavabled screw cap. A glass sub-
strate, which was washed with distilled water and ab-
solute ethanol before air drying, was placed inside. The
bottle was then heated at a constant temperature of 90 °C
for a certain time in a conventional laboratory oven. The
as-prepared ZnO samples were thoroughly washed with
distilled water to remove any possible contaminations.
Different reactant concentrations were used during the
CBD process. The Si (100) wafer was also used to deposit
ZnO films to investigate the substrate effect. The ZnO
nanoparticles were pre-coated on Si (100) wafer accord-
ing to the method described as before [23].
3 Results and Discussions
3.1 Morphological and Structural Characterizations
Figure 1 shows XRD patterns for the as-prepared ZnO
products. All of the peaks can be indexed to hexagonal
ZnO (JCPDC card No. 36–1451). Along with the increase
of reaction time, the dominance of the (002) peak can be
detected more obviously, indicating strong preferred ori-
entation along the c-axis. Figure 2 shows SEM images of
ZnO precipitates on glass substrates obtained with dif-
ferent reaction time. Uniform ZnO twinning rods, con-
nected together by two prisms with length ca. 1 μmand
width from 1.5 μmto2μm have been obtained after 12 h
Fig. 1 XRD patterns of ZnO precipitates on the glass substrate ob-
tained by employing each reactant concentration 0.1 M at 90 °C
with dierent reaction time (a) 12 h, (b) 24 h, and (c) 36 h.
reaction (figure 2a1). The high-magnification SEM image
(figure 2a2) shows that the as-obtained ZnO crystals ex-
hibited a typical hexagonal morphology and showed a
bowl-like characteristic. Based on the careful observa-
tion on these ZnO bowls, it was found that the bowls
have six vaulted walls with thickness ca. 200 nm. When
the reaction time was further increased to 24 h, the char-
acteristic of ZnO bowls disappeared. As shown in figure
2b1 and 2b2, the end of ZnO prisms showed a seal-like
morphology, which has the same dimension as the ZnO
bowls. Six corners of the hexagonal end are sunken com-
pared with other positions and the center is protuber-
ant. Typically, a hexagram lies on the top of the hexag-
onal prisms as shown in the inset of figure 2b2. Further
reaction made the center of the hexagonal end more pro-
tuberant (figure 2c1 and c2). It seems that there is a disk
lying on the top of the hexagonal prisms. The inset in fig-
ure 2c2 clearly reveals that the formation of such disks
was due to the helical crystal growth.
3.2 Inuence of Reactant Concentrations, Substrates
and Pre-coated ZnO Nanoparticles
Although the introduction of CTAB into CBD process
can produce unique ZnO nanostructures, the density of
ZnO products on the glass substrate is relatively low.
By changing the concentration of CTAB from 0.1 M to
0.05 M, the density of ZnO film was improved distinctly.
The low-magnification SEM image (figure 3a) shows that
the glass substrate was covered with large-scale uniform
ZnO crystals when the concentration of CTAB drops to
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Cryst. Res. Technol. 49, No. 11, 899–906 (2014)
Original Paper
Fig. 2 SEM images of ZnO precipitates on the glass substrate obtained by employing each reactant concentration 0.1 M at 90 °Cwith
dierent reaction time (a1, a2) 12 h, (b1, b2) 24 h, and (c1, c2) 36 h.
0.05 M. A high-magnification SEM observation (figure
3b) shows that the film was consisted of ZnO sunken
prisms. The diameter of ZnO prisms is uniformly ca.
1μm and the thickness of the wall is ca. 50 nm.
When the concentration of each reactant was reduced
from 0.1 M to 0.05 M, we can obtain seal-like ZnO with
diameters in the range from 1 μmto1.5μm and length
more than 5 μm (figure 4a). The aspect ratio of seal-
like ZnO was also improved when compared with that
formed under the high concentration. If the concentra-
tion of each reactant was further reduced to 0.01 M, the
nucleation of ZnO was inhibited.
To investigate the influence of different substrates on
the morphology of ZnO, we adopted Si (100) wafer as
substrate instead of glass. The density of ZnO precipi-
tates on Si (100) wafer was relatively low compared with
that obtained on the glass substrate as shown in fig-
ure 4b, which indicated Si (100) wafer is not suitable for
the nucleation of ZnO in our reaction conditions. The
ZnO crystals tended to adopt a dendritic morphology.
Figure 4c and 4d shows the typical flower-like morpholo-
gies. The flower-like ZnO was formed by the assembly
of half-baked hexagonal ZnO tubes. As indicated by ar-
rows in figure 4c and 4d, some ZnO prisms can grow
from the inner of ZnO hollow structures. We can also ob-
serve some ZnO hollow structures with multilayer walls
(figure 4e and 4f). The inboard wall stems from the bot-
tom of outward hollow structures. To improve the density
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Y. Hou and M. Yang: Fabrication of hollow ZnO nanostructures by a CTAB-assisted . . .
Original Paper
Fig. 3 SEM images of ZnO lms on the glass substrate obtained by employing [Zn2+]=0.1 M, [HMTA] =0.1 M, [CTAB] =0.05 M at 90 °C
with reaction time 12 h. (a) low magnication and (b) high magnication.
of ZnO film when Si (100) wafer was used as substrate,
ZnO nanocrystals with diameter about 5–10 nm were
spin-cast several times onto the Si (100) wafer as crys-
tal seeds before the crystal growth process. Such a pre-
coated process dramatically improved the density of ZnO
film as shown in figure 4g, whereas, the morphology was
also changed. The Si (100) wafer was covered with ZnO
interleaving walls with typical thickness below 100 nm
(figure 4h). This observation is different from previous
results [23]. The difference may be due to the existence of
CTAB in the system, which tended to absorb on the sur-
face of ZnO nanoparticles and thereby affecting further
growth.
3.3 Plausible Growth Mechanism
To investigate the formation mechanism of these unique
morphologies, we examined the morphologies of ZnO
products in the initial reaction stage. Figure 5a–5d shows
the morphologies of ZnO films obtained only after 1 h re-
action. The hexagonal ring-like ZnO with a hole in the
center as indicated by arrows in figure 5a and 5b, can
be observed. Thin arrows in figure 5a and 5b indicated
some ZnO fragments which would form ring-like struc-
tures. We can also observe some bowl-like ZnO, however,
the bottom part was not completely formed (indicated by
thick arrows in figure 5a and 5b). The high-magnification
SEM images (figure 5c and 5d) revealed that these hol-
low structures exhibited obvious stripes on the crystal
surface as a result of helical crystal growth. The rupture
may happen at the joint of each crystal face as shown
in figure 5c. Some ZnO nanostructures exhibiting helical
characteristic can be found as shown in figure 5d, which
may be due to the further growth of the half-baked crys-
tal faces. When the reaction time was increased to 4 h,
ZnO bowls with the intact bottom can be found as main
morphologies (figure 5e). Figure 5f shows a typical TEM
image of ZnO bowls. The different contrast between the
center and the edge confirmed the hollow nature of ZnO
bowls.
Previously, Wang et al. have reported the synthesis
of ZnO hexagonal disks and rings [13]. In their study,
the formation of ZnO rings was due to the dissolu-
tion of ZnO disks from the center where the density
of defects is highest. Similar dissolution phenomenon
can be found in the formation of ZnO micro-tube ar-
rays, which was obtained based on the dissolution of
metastable face of ZnO micro-rod arrays [24]. However,
such a dissolution mechanism may not be used to ex-
plain the formation of hollow structures in our system
within which they formed first. We believe the adding
of CTAB into CBD system is important for controlling
the morphologies of ZnO in our system. CTAB has been
widely used in the preparation of one-dimensional ZnO
in hydrothermal system [25]. CTAB can influence the
growth orientation of ZnO via the interaction between
Zn(OH)42- and CTAB [25c, 25d]. In previous work, the use
of CTAB can generally produce ZnO rods or their assem-
blies forming flower-like structures. NaOH was generally
used as base source. Ge et al investigated the influence
of the molar ratio of Zn2+to OH−(Zn2+/OH−)onthe
formation of flower-like 3D ZnO nanostructures [25c].
They found that Zn2+/OH−was a crucial factor in the
formation of flower-like ZnO. If Zn2+/OH−is more than
1:5, ZnO rods instead of flower-like ZnO will be obtained.
In our case, the base source is HMTA which decom-
poses slowly in heated aqueous solutions to yield am-
monia and formaldehyde. Ammonia can further react
with water to provide OH−. The amount of OH−in our
system was relatively small compared with that when
NaOH was used as the base source. This difference may
make the formation of flower-like ZnO not favored. How-
ever, the effect of substrate also has to be considered.
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Cryst. Res. Technol. 49, No. 11, 899–906 (2014)
Original Paper
Fig. 4 (a) SEM images of ZnO precipitate on the glass substrate obtained by employing each reactant concentration 0.05 M; SEM images
of ZnO precipitates (b-f) on bare Si (100) wafer and (g-h) on Si (100) wafer pre-coated with ZnO nanoparticles obtained by employing
[Zn2+]=0.1 M, [HMTA] =0.1 M, [CTAB] =0.05 M at 90 °Cfor12h.
As mentioned before, Si (100) wafer was not favored for
the nucleation of ZnO. In this case, the earlier formed
ZnO crystals on Si (100) wafer can serve as sites for fur-
ther nucleation and the flower-like morphologies can be
observed.
To investigate the role of CTAB in our system, we
have measured pH values of CBD system with or without
CTAB. The pH value of CBD system without CTAB was
ca. 5.5 compared with that of 7.0 when CTAB was used.
This pH value was nearly unchanged during the whole
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