Fabrication and optical properties of ordered sea urchin-like ZnO
nanostructures by a simple hydrothermal process
Yi Zhou
a,
n
, Ce Liu
a
, Mengyao Li
a
, Hongyan Wu
a
, Xian Zhong
a
, Dang Li
a
, Difa Xu
b
a
Department of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, China
b
Department of Science and Technology, Changsha University, Changsha 410003, China
article info
Article history:
Received 28 March 2013
Accepted 29 April 2013
Available online 8 May 2013
Keywords:
Nanocrystalline materials
Sea urchin-like ZnO
Crystal growth
Solar energy materials

3D ZnO nanostructures have been prepared, such as jack-like ZnO
[9,10], flower-like ZnO [11,12], and hedgehog-like ZnO [13]. These
structures have 1D nanoscale with 3D architectures combining
properties of 3D and 1D materials, which may be used as an
interesting alternative with higher specific surface and porosity
than those of simple arrays of ZnO nanowires [14], especially for
application in DSSCs [15].
In this paper, new type 3D ordered urchin-like single-crystal
ZnO nanostructures were fabricated and their photoelectric prop-
erties in the field of DSSCs were studied and compared with those
of ZnO nanorod arrays. 3D ordered urchin-like single-crystal ZnO
nanostructures are assembled by a central nucleus and many
needle-like ZnO which grew radially from the nucleus. The photo-
electric properties of such an ordered architecture are better than
those of ZnO nanorod arrays.
2. Experimental
Ordered sea urchin-like zinc oxide (ZnO) nanostructures were
prepared through a simple hydrothermal process in a Teflon-
sealed autoclave. Zinc nitrate hexahydrate (Zn(NO
3
)
2
Á 6H
2
O) and
hexamethylenetetramine (C
6
H
12
N

http://dx.doi.org/10.1016/j.matlet.2013.04.102
n
Corresponding author. Tel./fax: +86 731 85258328.
E-mail addresses: [email protected], [email protected] (Y. Zhou).
Materials Letters 106 (2013) 94–96
JEOL JEM-2100F) equipped with selected area electron diffraction
(SAED). The photocurrent–voltage (I–V) characteristics were mea-
sured with a computer-controlled digital source meter (Keithley,
model 2400) under illumination with a Newport solar simulator
(AM1.5, 100 mW cm
2
). The X-ray diffraction patterns were
recorded on an X-ray diffraction system (SIEMENS D5000).
UV–visible optical absorption was measured by UV–visible spec-
trophotometer (Beijing,TU-1901).
3. Results and discussion
Fig. 1a shows the XRD pattern of the sea urchin-like ZnO by a
simple hydrothermal process at relatively mild conditions. All the
diffraction peaks are labeled and can be readily indexed to hexago-
nal phase of ZnO (JCPDS card no. 361451). No other diffraction peaks
are presented, which demonstrates that high-purity ZnO products
have been synthesized. Besides, the diffraction peaks are sharp,
which confirm the well-crystallized particles of prepared ZnO.
Fig. 1bshowstheUV–visible optical absorption characteristics of
the sea urchin-like ZnO. Excitonic absorption peak of the ZnO
sample is at about 360 nm in the ultraviolet region, which is blue-
shifted due to the nanometric size effect when compared with the
absorption edge of bulk ZnO which appears at 400 nm [17].From
Fig. 1b, it is also found that the average excitonic absorption in the
ultraviolet region is up to 75%.

This suggests that the photoanode with a highly branched net-
work possesses higher photoconversion efficiency when com-
pared with the 1D nanostructures. The higher photoconversion
sea urchin-like ZnO nanostructures efficiency is ascribable to the
contributions from electron through the enrichment of dye load-
ing without sacrificing electron-transport properties [18,19]. In
addition, the geometry of ordered sea urchin-like nanostructures
possesses larger pores, which can provide an effective path for
electrolyte diffusion; thus light harvesting and overall efficiency
are improved.
The conversion efficiency of the sea urchin-like ZnO nanos-
tructures is lower than the highest value reported in the literature
[20]. This may relate to the difference of cell configurations and
the quality of the dye. However, in the same condition, the
conversion efficiency of sea urchin-like ZnO nanostructures is
superior to that of ZnO nanorod arrays. This shows that the sea
urchin-like ZnO nanostructures fabricated via a simple hydrother-
mal process at relatively mild conditions have potential as a
photoelectrode for the DSSCs.
4. Conclusions
Ordered sea urchin-like zinc oxide (ZnO) nanostructures were
fabricated via a simple hydrothermal process at relatively mild
conditions. XRD shows the sea urchin-like ZnO crystals as hex-
agonal wurtzite monocrystals. TEM shows that the ZnO crystals
were assembled by a central nucleus and many needle-like ZnO
which grow radially from the nucleus. When using the sea urchin-
like ZnO nanostructures as the anode of dye-sensitized solar cells
(DSSC), the efficiency of power conversion (η) was increased 77.6%
over those of ZnO nanorod arrays.
Acknowledgments

[17] Duan XF, Lieber CM. Adv Mater 2000;12:298–302.
[18] Matt L, Lori EG, Justin CJ, Richard S, Yang PD. Nat Mater 2005;4:455–9.
[19] Chen HN, Li WP, Hou Q, Liu HC, Zhu LQ. Electrochim Acta 2011;56:8358–
64.
[20] Chen LY, Yin YT. Nanoscale 2013;5:1777–80.
Fig. 3. I–V characteristics of the sea urchin-like ZnO nanostructures based DSSCs.
Y. Zhou et al. / Materials Letters 106 (2013) 94–9696