morphology control of zno bilayer structure by low-temperature

Morphology control of ZnO bilayer structure by low-temperature
hydrothermal process
Yanyan Lou
a,b
, Shuai Yuan
a,
n
, Yin Zhao
a
, Pengfei Hu
b
, Zhuyi Wang
a
,
Meihong Zhang
a
, Liyi Shi
a
a
Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, China
b
Laboratory for Microstructures, Shanghai University, 99 Shangda Road, Shanghai 200444, China
article info
Article history:
Received 7 April 2013
Accepted 17 May 2013
Available online 5 June 2013
Keywords:
Semiconductors
Thin film
Multilayer structure

ZnO
abstract
Morphology control of ZnO bilayer structure films has been obtained via low-temperature hydrothermal
process without any surfactant-assistance. ZnO bilayer structures including nanoflower layer on nanorod
array structure, nanodendrite layer over nanorod array structure, and double nanorods layers structures
were successfully fabricated by controlling the ammonia concentration in the initial hydrothermal
solution and the immersing ways. The shape evolution of bilayer structure consisting of nanodendrite
layer on the top and nanorod array at the bottom was elucidated by growing–renucleating mechanism.
The higher ammonia concentration in the initial hydrothermal solution promotes the secondary
nucleation and growth on the surface defects of backbones, resulting in the formation of needle-like
nanorods in ZnO nanodendrite–nanorod structure. The ZnO film with nanodendrite–nanorod structure
has better optical properties and lesser defects compared to that of nanoflower–nanorod structures.
& 2013 Elsevier B.V. All rights reserved.
1. Introduction
ZnO is a direct band gap semiconductor with wide energy band
gap (3.37 eV), high electron mobility (115–155 cm
2
V
−1
s
−1
) and
simple fabrication of ZnO nanostructures which make it useful for
photoelectron devices such as light emitters [1], transparent
conducting oxides (TCOs) [2], dye sensitized solar cells [3] and
gas sensors [4]. It is necessary to fabricate ZnO nanostructures and
control its size and shape for specific functional purpose. Various
ZnO nanostructures such as nanorods [5], nanoflowers [6] and
nanosheets [7] have been prepared by hydrothermal processes,
allowing for low cost and readying extension to large scale

fabrication. Previously, ZnO nanorods array could be obtained by
tuning the pH [8], process temperature [9], and concentration
of precursors [10,11]. In this work, nanoflower–nanorod,
nanodendrite–nanorod and double nanorods layers structures
were fabricated by controlling growing condition via low-
temperature hydrothermal process. The possible shape evolution
mechanism of nanodendrite–nanorod structure was discussed.
2. Experimental details
Firstly ZnO seed layer was prepared by dip-coating an FTO
substrate in a solution of ehylene glycol monomethyl ether with
1:1 M ratio of 0.375 M zinc acetate and ethanolamine, and
annealing at 550 1C for 90 min. The nanostructures were fabri-
cated by using hydrothermal method in Teflon-lined autoclaves,
where the substrates with the seed layer were immersed into
75 ml aqueous solution of 0.12 M Zn(NO
3
)
2
Á 6H
2
O and NH
3
Á H
2
O.
The autoclaves were sealed and heated at 90 1C for 4 h; then the
solution was refreshed and the growth was repeated for another
4 h. Samples A and B were faced upward and immersed slantways
in 0.96 M NH
3

Á H
2
O and 1.2 M NH
3
Á H
2
O, respectively; sample C
was faced downward and immersed in 1.2 M NH
3
Á H
2
O. The
morphologies and crystal structure of different ZnO nanostruc-
tures were characterized by a JEOL JSM-6700F and Rigaku D/max
2500 X ray diffractometer with CuKα radiation. The photolumi-
nescence spectrum was excited by a 330 nm Horiba HR800 UV
laser at room temperature.
3. Results and discussion
Fig. 1 shows the top and side-view SEM images of samples A–C.
Sample A is the nanoflower–nanorod structure consisting of
nanoflower layer on top and nanorod array at bottom. The
flower-shaped nanostructure is 10 mm in diameter and is com-
posed of many hexagonal plane-tipped nanorods with diameters
of 200–300 nm and lengths of 5 mm. The bottom layer of sample A
is a nanorods array about 6 mm in thickness. Sample B has a
nanodendrite–nanorod structure with nanodendrite layer on top
and nanorod array at bottom. The nanodendrite shape is quite
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Materials Letters

0167-577X/$ -see front matter & 2013 Elsevier B.V. All rights reserved.
/>n
Corresponding author. Tel./fax: +86 21 66134852.
E-mail address: (S. Yuan).
Materials Letters 107 (2013) 126–129
similar to the flower shape of sample A. The branches of nanoden-
drite structure are needle-like nanorods with diameters of 100 nm
and lengths of 800 nm, while the backbones have 400–500 nm
diameters and 5–6 mm lengths. Compared to sample A, the back-
bones of nanodendrite structure are twice as large as the petals of
sample A, while the bottom nanorods layer is also about 6 mmin
thickness.
Sample C has a bilayer nanorods structure built with double
nanorods layers. The top nanorods layer of sample C is estimated
to be about 8 mm in thickness, while the bottom layer is about
4 mm in thickness. Fig. 1I shows that the tips of nanorods on the
top layer of sample C can be identified as two kinds of nanorods:
the large one with a tip of 100–200 nm diameter and the small one
with a tip of around 50 nm diameter. Fig. 1G and I shows that the
small nanorods come from the backbones of large nanorods,
which are similar to needle-like branches in the nanodendrite
structure (Fig. 1F). Sample C shares the same reaction solution as
that of sample B, except for different immersing ways. Sample C is
faced downward while sample B is faced upward. Multilayer
nanorods array is usually obtained by hydrothermal method when
the substrate is faced downward in reaction solution [12,13].
However, both the needle-like branches in sample B and the
small nanorods in sample C indicate the secondary nucleation
and growth occur in this reaction solution with high ammonia
contents, compared to that of sample A.

The higher ammonia content of sample B in initial solution
could lead to the slight dissolution of the surface on the backbones
of nanodendrite due to the amphoteric feature of ZnO materials
[14]. Crystal surface with defects tends to further lower their
energy through surface reconstruction, which provides active sites
for the secondary nucleation [15]. Fig. 2 proposes a possible shape
evolution of the bilayer nanodendrite–nanorod structure: (A) seed
layer deposition on the substrate; (B) growth of nanorods layer;
(C) growth of nanoflower-like top layer; and (D) secondary nuclea-
tion and growth from backbones of nanodendrite.
Firstly ZnO nanorods grow on the seed layer (Fig. 2A, B) at 90 1C
through the reactions of Zn
2+
cations with OH
−
anions as foolows:
NH
3
⋅H
2
O⇔NH
þ
4
þOH
−
ð1Þ
Zn
2þ
þOH
−

-½ZnðOHÞ
4

2−
-ZnO þ H
2
O þ OH
−
ð2Þ
Zn
2þ
þ NH
3
⋅H
2
O-½ZnðNH
3
Þ
4

2þ
þ H
2
O ð3Þ
ZnO þ H
2
O þ OH
−
-½ZnðOHÞ
4


2−
ð4Þ
In the reaction system, NH
3
Á H
2
O is partly combined with Zn
2+
and releases OH
−
slowly, resulting in a relatively stable environ-
ment for crystal growth. Due to the sufficient Zn
2+
concentration
of the refreshed solution, the flower-like structure was assembled
by nanorods on the top of nanorods array [13] (Fig. 2C). According
to Eq. (4), excessive alkaline environment in sample B may
partially dissolve the surface on the backbones of nanodendrite
which provides active sites for the renucleation and sequential
growth of nanorods, finally resulting in the formation of complex
Fig. 1. (A)—(C) are the side and top-view SEM images of sample A; (D)—(F) are the side and top-view SEM images of sample B; and (G)—(I) are the side and top-view SEM
images of sample C. The inset (i), (ii) and (iii) images are over-view SEM images of sample A, B and C, respectively.
Y. Lou et al. / Materials Letters 107 (2013) 126–129 127
three-dimensional ZnO nanodendrite–nanorod structure (Fig. 2D).
For the sample C, the secondary nucleation and growth occur on
the side of the large nanorods, resulting in mixed nanorod
structure. Therefore, the ammonia concentration in the initial
hydrothermal solution plays the key role for the formation of
needle-like nanorods in ZnO nanodendrite–nanorod structure.

Fig. 3 shows the photoluminescence spectra of samples A and B
which share the same growth condition except for different
ammonia contents. The PL spectra consisted of a weak near
band-energy UV emission at around 3.2 eV and a broad
visible emission in the range of 1.6–2.8 eV. The visible emission
which is the surface-state related defect emission [16] can be
Gaussian-resolved into three bands located at 1.95 eV, 2.20 eV and
2.39 eV (corresponding to the orange, low- and high-energy
green luminescence, respectively). It is believed that oxygen
vacancies (V
O
) and zinc vacancies (V
Zn
) contribute to the green
luminescence [17], while oxygen interstitials (O
i
) are responsible
for the orange luminescence [18]. On comparing the PL spectra,
sample B has lower visible emission and higher UV emission,
indicating better optical property and fewer defects with respect
to sample A. Less defects concentration in sample B indicates
secondary nucleation and growth on the surface defects of back-
bones which eliminate partial defects, resulting in the low visible
emission.
4. Conclusion
In summary, nanoflower–nanorod structure, nanodendrite–
nanorod structure and double nanorods layers structure were
obtained by controlling growth concentration and position via
low-temperature hydrothermal process. The shape evolution of
nanodendrite–nanorod structure was discussed. The ZnO film with

nanodendrite–nanorod structure has better optical properties and
lesser defects comparing to those of nanoflower–nanorod struc-
tures. High ammonia concentration in the initial hydrothermal
solution promotes secondary nucleation and growth on surface
defects of backbones, resulting in the formation of needle-like
nanorods in ZnO nanodendrite–nanorod structure.
Acknowledgment
The authors acknowledge the supports of the Shanghai Leading
Academic Discipline Project (S30107), National Natural Science
Foundation of China (Grant no. 51202138), Natural Science Foun-
dation of Shanghai (Grant no. 12ZR1410500), Research and Inno-
vation Projects of Shanghai Education Commission (11YZ22),
Project of Shanghai Environment Condition (10dz2252300),
Science Foundation for Excellent Youth Scholars of Shanghai
University, and Science Foundation for Excellent Youth Scholars
of Universities (Shanghai).
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