hou2014

Original Paper
Cryst. Res. Technol. 49, No. 11, 899–906 (2014) / DOI 10.1002/crat.201400231
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 films 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 different substrates. Dense ZnO
films consisted of sunken prisms can also be achieved by
controlling the concentration of CTAB. The influences of
reactant concentrations, types of the substrates and precoated ZnO nanoparticles on the formation of ZnO films
were examined. XRD patterns indicated the Wurtzite structure of ZnO and the preferred growth direction is [001]. The
role of CTAB in CBD process was discussed and the evolution
of different 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 concentration of CTAB can improve the ultraviolet emission.
1 Introduction
The shape, size and crystalline structure of semiconductors 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 material 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 optoelectronic material in the UV region [3]. Other important
properties including size-dependent surface luminescence [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 morphologies of ZnO nanostructures. Several interesting
structures have been obtained via solution methods,
such as hexagonal discs and rings [13], doughnutshaped 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 solution [20], is effective for the preparation of orientated
ZnO nanorod and nanotube arrays which have important 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 nanostructures. 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 Manufacturing, Harbin Institute of Technology, 2 Yikuang Street, Harbin,
150080, P. R. China
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Original Paper
Y. Hou and M. Yang: Fabrication of hollow ZnO nanostructures by a CTAB-assisted . . .
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 Kα radiation. XRD data were collected 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 electron 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 )6 N4 ) and CTAB
in a bottle with an autoclavabled screw cap. A glass substrate, which was washed with distilled water and absolute 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 according to the method described as before [23].
3 Results and Discussions
Fig. 1 XRD patterns of ZnO precipitates on the glass substrate obtained by employing each reactant concentration 0.1 M at 90 °C
with different 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 exhibited a typical hexagonal morphology and showed a
bowl-like characteristic. Based on the careful observation 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 characteristic 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 compared with other positions and the center is protuberant. Typically, a hexagram lies on the top of the hexagonal prisms as shown in the inset of figure 2b2. Further
reaction made the center of the hexagonal end more protuberant (figure 2c1 and c2). It seems that there is a disk
lying on the top of the hexagonal prisms. The inset in figure 2c2 clearly reveals that the formation of such disks
was due to the helical crystal growth.
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 orientation along the c-axis. Figure 2 shows SEM images of
ZnO precipitates on glass substrates obtained with different reaction time. Uniform ZnO twinning rods, connected together by two prisms with length ca. 1 μm and
width from 1.5 μm to 2 μm have been obtained after 12 h
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3.2 Influence 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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Original Paper
Cryst. Res. Technol. 49, No. 11, 899–906 (2014)
Fig. 2 SEM images of ZnO precipitates on the glass substrate obtained by employing each reactant concentration 0.1 M at 90 °C with
different 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.
1 μm 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 μm to 1.5 μm and length
more than 5 μm (figure 4a). The aspect ratio of seallike ZnO was also improved when compared with that
formed under the high concentration. If the concentration 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 precipitates on Si (100) wafer was relatively low compared with
that obtained on the glass substrate as shown in figure 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 morphologies. The flower-like ZnO was formed by the assembly
of half-baked hexagonal ZnO tubes. As indicated by arrows in figure 4c and 4d, some ZnO prisms can grow
from the inner of ZnO hollow structures. We can also observe some ZnO hollow structures with multilayer walls
(figure 4e and 4f). The inboard wall stems from the bottom 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 . . .
Fig. 3 SEM images of ZnO films 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 magnification and (b) high magnification.
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 crystal seeds before the crystal growth process. Such a precoated 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 surface 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 reaction. 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 structures. 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 hollow 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 crystal faces. When the reaction time was increased to 4 h,
ZnO bowls with the intact bottom can be found as main
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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 dissolution 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 arrays, 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 explain 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)4 2- and CTAB [25c, 25d]. In previous work, the use
of CTAB can generally produce ZnO rods or their assemblies 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− ) on the
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 decomposes slowly in heated aqueous solutions to yield ammonia 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. However, the effect of substrate also has to be considered.
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Original Paper
Cryst. Res. Technol. 49, No. 11, 899–906 (2014)
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 °C for 12 h.
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 further 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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Y. Hou and M. Yang: Fabrication of hollow ZnO nanostructures by a CTAB-assisted . . .
Fig. 5 SEM images of ZnO precipitates on the glass substrate obtained by employing each reactant concentration 0.1 M at 90 °C with
reaction time (a-d) 1h, (e) 4h and (f) TEM image of ZnO bowls. Thin, regular and thick arrows indicated the different morphologies in the
initial reaction stage.
reaction process. As CTAB is a kind of strong-acid-weakbase salt, it could accelerate the decomposition of HMTA
which produced more OH− in the system, resulting in the
increase of pH value. We know that the level of supersaturation is important in determining the final crystal
morphology. When pH value was 5.5, the supersaturation
was low and the spiral growth of ZnO dominated. When
pH value was increased to 7.0 by adding CTAB, the supersaturation was also increased and two-dimensional
(2D) growth may predominate. The probability of occurrence of 2D nucleation is far greater near to edges and
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corners of crystal faces [20]. The first formation of ZnO
rings in our system can be therefore explained as a result
of the predomination of 2D growth. At the same time, the
high supersaturation can induce secondary nucleation
process, which led twinning to occur. The stripes on the
crystal surface as shown in figure 5c and 5d was a result
of the simultaneous slow spiral growth.
During the reaction process, white precipitates can be
observed in the solution. These precipitates were composed of CTAB and Zn(OH)2 . Similar phenomena were
reported before [25c]. The possible formation processes
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Original Paper
Cryst. Res. Technol. 49, No. 11, 899–906 (2014)
Scheme 1 Reaction process for the formation of Zn(OH)2 and ZnO.
Fig. 6 PL spectra of ZnO films obtained under different CTAB
concentrations excited by a He-Cd laser at 325 nm at room
temperature: (a) 0.01 M, (b) 0.05 M and (c) 0.10 M employing
[Zn2+ ] = [HMTA] = 0.10 M and reaction time 24 h.
3.4 PL Spectra Investigation
Scheme 2 Concept diagram of the growth of ZnO nanostructures.
of Zn(OH)2 and ZnO are given in Scheme 1. Along with
the reaction proceeding, more precipitates will appear.
The adding of surfactants such as CTAB into CBD system will decrease the surface tension of the solution system. Accordingly, the interfacial energy between solid
and solution will be decreased. In this case, precipitation will more easily occur in the solution compared
with that when no CTAB was present (this can also explain why dense ZnO film can only be obtained with
low concentration of CTAB). The concentration of Zn2+
will decrease due to the consumption during the formation of Zn(OH)2 , which will decrease the surpersaturation and make spiral growth preferable to 2D nucleation.
This change will induce the crystal growth in the middle
of the hexagonal prisms, resulting in the filling of cavities. The formation of seal-like ZnO may be due to the
collective effect of 2D nucleation and spiral growth near
to the corners of crystal faces. When reaction time was
long enough (36 h), the middle part of ZnO prisms will
become more protuberant and shows the helical growth
habit directly. Scheme 2 shows a concept diagram of the
growth of ZnO nanostructures.
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The effect of the concentrations of CTAB on the photoluminescence (PL) spectra was shown in figure 6. PL spectra generally showed two peaks for each sample. The ultraviolet emission is the characteristic near-band-edge
transition of wide band-gap, intrinsic ZnO [26]. The visible emission has been associated with a range of defects
[27]. The ratio of the UV to visible emission intensities
(IUV : IVI ) provides one measure of the material quality.
The concentrations of CTAB were found to have obvious
effect on IUV : IVI . As shown in figure 6, the concentrations of CTAB had little effects on the positions of both
ultraviolet and visible emissions. Generally, the PL spectra consisted of one peak centered at around 390 nm and
another around 510 nm. However, we can see that when
the concentration of CTAB was increased from 0.01 M to
0.10 M, IUV :IVI was increased from 0.18 to 0.94. The above
result indicated that the high concentration of CTAB can
reduce the amounts of defects and improve the quality of
ZnO nanostructures.
4 Conclusions
In summary, ZnO films with unique hollow nanostructures were prepared by a CTAB-assisted CBD method.
ZnO rings and bowls can be obtained respectively according to the reaction time. Dense ZnO films consisted
of uniform sunken prisms were obtained by controlling
the concentration of CTAB. ZnO with dendritic morphologies consisted of the assembly of hollow structures,
can be obtained when the glass substrate was replaced
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Y. Hou and M. Yang: Fabrication of hollow ZnO nanostructures by a CTAB-assisted . . .
by Si (100) wafer. It was shown that the adding of CTAB
into the CBD system can adjust the reaction process by
changing the level of supersaturation. Time-dependent
SEM observation indicated that in the CTAB-assisted
CBD system, 2D growth was firstly predominant,
whereas, spiral growth will be preferred along with the
increase of the reaction time. The investigation on optical properties indicated that high CTAB concentration
can improve the quality of ZnO nanostructures.
Acknowledgements. Y. H. would like to thank the financial support from HIT 100-talent program (Grant No. AUGA5710006813) and
Fundamental Research Funds for the Central Universities (Grant
No. HIT IBRSEM. A. 201405); M. Y. would like to thank the financial support from the National Natural Science Foundation of
China (Grant No. 21303032), China Postdoctoral Science Foundation
(Grant No. 2014M550184), Heilongjiang Postdoctoral Science Foundation (Grant No. LBH-Q13074), HIT Young Talent Program (Grant
No. AUGA5710050613), and Fundamental Research Funds for the
Central Universities (Grant No. HIT. IBRSEM. A. 201406).
Key words. Zinc oxide, hollow nanostructures, chemical bath
deposition.
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
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