Flower-like hierarchical nickel microstructures: Facile synthesis, growth mechanism, and their magnetic properties

Article abstract of DOI:10.1016/j.materresbull.2012.04.079

Flower-like hierarchical nickel microstructures were prepared by a facile chemical reduction method requiring 4 h at temperature of 85°C without any template or external magnetic field. Nickel (II) sulfate hexahydrate was used as nickel source and hydrazine hydrate acted as the reducing agent. XRD study confirmed the highly crystalline with face-centered cubic (fcc) phase. SEM images revealed that the individual flower-like microstructures have an average diameter of 1-2 μm and are composed of sword-like nanopetals growing radially from the core of the spherical particles. HRTEM image and SAED pattern of the single petal show that the lattice spacing is 0.203 nm corresponding to the (1 1 1) plane of fcc nickel and the growth orientation is along [0 1 1] direction. A rational formation process of nickel micro-flowers was proposed. Magnetic hysteresis measurements revealed that the hierarchical nickel microstructures possess ferromagnetic behavior with an enhanced coercivity value of about 203.3 Oe.

Full text of DOI:10.1016/j.materresbull.2012.04.079

Materials Research Bulletin 47 (2012) 1839–1844
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Flower-like hierarchical nickel microstructures: Facile synthesis, growth
mechanism, and their magnetic properties
c
Huiyu Chen a,b, , Chunju Xu , Chuang Chen a,b, Guizhe Zhao a,b, Yaqing Liu a,b
*
a Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, People’s Republic of China
b College of Materials Science and Engineering, North University of China, Taiyuan 030051, People’s Republic of China
c School of Advanced Materials Science & Engineering (BK21), Sungkyunkwan University, Suwon 440746, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Flower-like hierarchical nickel microstructures were prepared by a facile chemical reduction method
requiring 4 h at temperature of 85 8C without any template or external magnetic field. Nickel (II) sulfate
hexahydrate was used as nickel source and hydrazine hydrate acted as the reducing agent. XRD study
confirmed the highly crystalline with face-centered cubic (fcc) phase. SEM images revealed that the
Received 18 July 2011
Received in revised form 5 March 2012
Accepted 18 April 2012
Available online 25 April 2012
individual flower-like microstructures have an average diameter of 1–2
mm and are composed of sword-
like nanopetals growing radially from the core of the spherical particles. HRTEM image and SAED pattern
of the single petal show that the lattice spacing is 0.203 nm corresponding to the (1 1 1) plane of fcc
nickel and the growth orientation is along [0 1 1] direction. A rational formation process of nickel micro-
flowers was proposed. Magnetic hysteresis measurements revealed that the hierarchical nickel
microstructures possess ferromagnetic behavior with an enhanced coercivity value of about 203.3 Oe.
ß 2012 Elsevier Ltd. All rights reserved.
Keywords:
A. Nanostructures
A. Magnetic materials
B. Chemical synthesis
B. Crystal growth
B. Magnetic properties
1. Introduction
challenge to develop simple and practical route for the direct
preparation of magnetic hierarchical nanomaterials without any
In the past decades, the fabrication of micro-/nanomaterials
with controllable shape and size has attracted increasing attention,
because it is widely accepted that the intrinsic properties of these
materials have a close relationship with their morphology, size,
size distribution, and crystalline [1–3]. Recently, many efforts have
been devoted to preparing hierarchical structures assembled by
low-dimensional materials within nanoscale in that such three-
dimensional (3D) assemblies are believed to exhibit novel
properties different from their bulk counterparts and have
potential applications in nanodevices [4,5]. Different driving
mechanisms were proposed for the self-assembly of highly
organized building blocks of metals and semiconductors [6–8].
As for the assembling magnetic nanocrystals into desired
architectures, template or external magnetic fields are usually
necessary [9,10]. However, the above methods still have some
limitations. For example, the self-assembly route is considerably
complicated and the usage of template inevitably adds to the
complexity in the synthetic procedures. The magnetic-field-
induced growth approach is inappropriate for industries due to
the reliability on external magnetic field. Currently, it is still a big
external assistance so as to fully exploit their peculiar properties
and unique applications.
As an important ferromagnetic material, nickel has received
much attention and has been the focus of intense research because
of their diverse applications as catalyst, memory storage, magnetic
fluid, and biomedical diagnosis [11–15]. Until now, nickel with
different morphologies including nanoparticles [16], nanobelts
[17], nanowires [18], nanochains [19], nanotubes [20], hollow
spheres [21], and other 3D complex [22–24], has been synthesized
by a variety of methods. Typically, nickel nanobelts were fabricated
with the assistance of sodium dodecylbenzenesulfonate (SDBS)
[17]. Nickel nanotubule array with 35
by electrodeposition in the pores of an alumina membrane [20]. Fu
and coworkers prepared Ni dendritic nanostructures via
mm in length were obtained
a
hydrothermal reduction approach in the presence of surfactant
CTAB [25]. Nickel nanowires and nanoflowers were prepared by
introducing a magnetic force into the reaction system [10].
Therefore, it is highly expected to synthesize controlled Ni
hierarchical structures by a facile and practical method such as
the variation of reaction time, concentrations, and so forth, without
employing the external assistance including template or magnetic
force.
In this work, we reported a simple chemical reduction method
to the fabrication of flower-like hierarchical nickel microstructures
by reducing nickel sulfate hexahydrate with hydrazine in the
absence of any template or external magnetic field. The ligand
* Corresponding authors at: College of Materials Science and Engineering, North
University of China, Taiyuan 030051, People’s Republic of China.
Tel.: +86 351 3557256; fax: +86 351 3557256.
E-mail addresses: hychen09@sina.com (H. Chen), zfflyq98@163.com (Y. Liu).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.04.079

1840
H. Chen et al. / Materials Research Bulletin 47 (2012) 1839–1844
agent of ethylenediamine played a key role for the formation of
nickel micro-flowers, and two-stage grow process was rational
proposed for the formation mechanism on the basis of a series of
controlled experiments. Magnetic measurements demonstrate
that as-synthesized fcc phase nickel with a hierarchical micro-
structure show ferromagnetic behavior with an enhanced coer-
civity compared to bulk counterpart.
(111)
(222)
(311)
(200)
2. Experimental
(220)
2.1. Materials
30
40
50
60
70
80
90
100
All chemical reagents were purchased from Sigma–Aldrich and
used without further purification. Nickel (II) sulfate hexahydrate,
ethylenediamine, sodium hydroxide, and hydrazine hydrate were
used as received.
2θ/degree
Fig. 1. XRD pattern of the obtained flower-like nickel microstructures.
2.2. Synthesis of flower-like nickel microstructures
(Lakeshore 7407, USA). Magnetization curve was recorded at room
temperature by first saturating the sample powder in a field of
5000 Oe, and then the saturation magnetization (Ms), the remanent
magnetization (Mr), and the coercivity (Hc) were determined for
the sample.
In a typical synthesis, 12 g of NaOH was dissolved in deionized
water (20 mL) by intensively magnetic stirring, then 1 mL of
NiSO4Á6H2O (0.1 mol/L) solution and 0.35 mL of ethylenediamine
(EDA) were introduced. After the mixture was continuously stirred
for 20 min, 0.25 mL of N2H4ÁH2O (hydrated hydrazine 30%) was
added dropwise to the above solution. The obtained precursor was
stirred for another 10 min, and then transferred to a 50 mL glass
bottle followed by heating to 85 8C and holding for 4 h. After the
heat treatment, the black fluffy product floating on the solution
was collected by magnetic bar, rinsed with distilled water and
absolute ethanol several times to remove any alkaline and
byproduct that remained in the final products, and then finally
dried in air at 40 8C for 3 h. Controlled experiments were carried
out by changing the reaction time and dosage of ethylenediamine,
respectively, while kept other synthetic parameters and procedure
the same as those of typical reaction.
3. Results and discussion
The phase structure and purity of the as-obtained Ni sample
was examined by XRD. As the XRD pattern shown in Fig. 1 that five
characteristic peaks can be indexed as the (1 1 1), (2 0 0), (2 2 0),
(3 1 1), and (2 2 2) planes of the face-centered cubic (fcc) phase Ni
˚
with lattice constant a = 3.522 A, which is in good agreement with
˚
the reported data (JCPDS No. 04-0850, a = 3.524 A). It was worth
noting that the relative diffraction peak intensity ratio of the
{1 1 1} facets to {2 0 0} facets for the sample (3.33) is higher than
that for the conventional value (1.92). The significantly intensified
{1 1 1} diffraction peak implies the orientation of the {1 1 1}
crystallographic plane. No characteristic peaks due to the
impurities such as nickel oxides or hydroxides were detected,
which indicated that metallic Ni product was obtained under the
current synthesis conditions.
2.3. Characterizations
The phase purity of the Ni product was examined by X-ray
powder diffraction (XRD) with a Bruker D8 focus diffractometer at
a voltage of 40 kV and a current of 40 mA with a Cu K
a
radiation
The morphology of nickel product was investigated by SEM and
a typical SEM image with panoramic view was displayed in Fig. 2a,
from which we could clearly observe that the sample possessed
(
l
= 0.15406 nm), employing a scanning rate of 48/m in the 2u
range from 308 to 1008. Field emission scanning electron
microscopy (FESEM) images were taken on a JEOL JSM-6700F
scanning electron microscope. The transmission electron micros-
copy (TEM and HRTEM) images and the corresponding selected
area electron diffraction (SAED) patterns were captured on a JEOL
JEM-2100F transmission electron microscope at an acceleration
voltage of 200 kV. The magnetic property of the flower-like Ni
sample was measured in a vibrating sample magnetometer (VSM)
uniform flower-like shape with an average diameter of 2
mm.
Further observation from high magnification SEM image (Fig. 2b)
demonstrated that the nickel micro-flowers consisted of a
spherical core attached with many sword-like petals. These
petals grew radially from the spherical core with diameters of
150–250 nm at the root and lengths of 200–400 nm from the root
to the tip.
Fig. 2. SEM images of the obtained flower-like nickel microstructures with (a) panoramic view and (b) high magnification view.

H. Chen et al. / Materials Research Bulletin 47 (2012) 1839–1844
1841
Fig. 3. TEM images of (a) nickel micro-flowers with panoramic view and (b) individual sample with enlarged view.
Fig. 3a shows a representative TEM image with low magnifica-
tion view. It could be observed more clearly that the nickel micro-
flowers have a quasi spherical core with plenty of sword-like petals
protruding out of the core surface and radiating to various
directions. The average size of individual micro-flower is about
amount of microparticles with fine hierarchical nanoarchitectures
were greatly increased at the expense of smaller particles. The
petals were initially developed at this stage as the SEM image was
shown in Fig. 4c. The size of nickel microparticles with fine
hierarchical nanoarchitectures would grow gradually. Eventually,
nickel micro-flowers composed entirely of nanopetals were
formed (Figs. 2 and 4d) when the time was increased to 4 h.
Similar hierarchical microstructures were previously prepared for
several kinds of metals. The formation mechanism involved the
surfactant assistance, or included that smaller nanoparticles self-
assembled into complicated microstructures [26–28]. However, in
our case, the whole evolution process is consistent with the
previous reports of so called two-stage growth process [29–31],
during which the time factor is the most important one and four
types of samples could be obtained with different time intervals:
(1) particles with size ranging from a few hundred nanometers to
micrometer scale; (2) smaller particles and microparticle with fine
hierarchical nanoarchitectures; (3) microparticle with fine hierar-
2
mm, consistent with the observation from SEM images.
Interestingly, the flower-like structure cannot be destroyed into
discrete petals even under a long period of time of ultrasonication,
indicating that the complex architectures are actually integrated
and not made up of loosely aggregated nanopetals through
magnetic dipole interactions. The microstructure of the petals was
studied by high-resolution TEM (HRTEM). Fig. 3b is an enlarged
TEM image of a partial micro-flower with the petals observed
clearly. The corresponding HRTEM image (Fig. 3c) was taken from
the position enclosed in a white box in Fig. 3b. The regular spacing
of the observed lattice plane was ca. 0.203 nm, which was
consistent with the separation of {1 1 1} planes of fcc nickel.
Fig. 3d is the corresponding selected area electron diffraction
(SAED) pattern, which exhibits a regular and clear diffraction spot
array. Therefore, a single crystal nature of the petal could be
concluded. Combined the HRTEM and SEAD analysis, it is evident
that the sword-like petal of flower-like nickel microstructure
grows along [0 1 1] direction and agrees well with the XRD result.
In order to investigate the formation process of such novel
flower-like nickel microstructures, time-dependent experiments
were carried out while other synthetic parameters were kept
unchanged. The samples were collected at different time intervals
as the SEM images shown in Fig. 4. At the early stage with reaction
time of only 30 min, the nickel product was dominated by particles
chical nanoarchitectures; and (4)
a spherical hierarchical
nanoarchitectures with high yield. This growth mechanism
included a fast nucleation of primary particles followed by a slow
aggregation and crystallization of primary particles. In our work,
Ni2+ was first coordinated with ethylenediamine (EDA) to form a
relative stable complex [Ni(EDA)3]2+, then this complex was
reduced to form nuclei and quickly grew into the primary particles.
In the subsequent growth stage, the primary particles aggregated
into microspheres that became the core of hierarchical nanoarch-
itectured microparticles. At the same time, EDA was released and
acted as surfactant to assist formation of finally flower-like nickel
microstructures.
with diameters ranging from 0.3 to 0.6
mm (Fig. 4a). The sample
collected 1.5 h later was composed of microparticles with fine
hierarchical nanoarchitectures and the average size was about
As discussed above, EDA acted not only as a ligand to produce
[Ni(EDA)3]2+ but also as a surfactant when it was released. We
believe EDA plays important role in the formation of hierarchical
nickel micro-flowers and Fig. 5 shows the SEM images of nickel
1.5
mm (Fig. 4b). Meanwhile, particles with diameter of ca. 0.5 mm
were also coexisted. If we continued to prolong the time to 3 h, the

1842
H. Chen et al. / Materials Research Bulletin 47 (2012) 1839–1844
Fig. 4. SEM images of nickel samples synthesized with reaction time of (a) 30 min, (b) 1.5 h, (c) 3 h, and (d) 4 h.
products prepared with various amount of EDA. The sample was
dominated by microparticles with mean size of ca. 1 m when no
[Ni(EDA)3]2+ can decrease the free Ni2+ concentration in the
solution and result in the slow generation of Ni nanoparticles.
The reaction rate can therefore be tuned, which can regulate the
kinetics of nucleation and growth of the products and further
efficiently control the shape and structures of the final product. On
the other hand, the EDA released from the complex [Ni(EDA)3]2+
after the reduction reaction was considered to kinetically control
the growth rates of different crystallographic facets of Ni micro-
flowers through preferentially adsorbing and desorbing on these
facets.
m
EDA was employed (Fig. 5a). Irregular microparticles coexisted
with some undeveloped flower-like structures in the final product
when 0.1 mL was introduced to the synthetic system (Fig. 5b). It
was obvious that the quantity of EDA is not enough for the current
synthesis of nickel micro-flowers. However, if we further increased
EDA to 0.35 mL or more, separate flower-like hierarchical
microstructures were obtained as the dominant product. Especial-
ly, when EDA with amount of 1 mL was added, some Ni flowers
possess fewer petals compared to those obtained with 0.35 mL
EDA in the typical synthesis, while such assembled petals have
large diameters and longer length (Fig. 5c). The formation of
The dosage of reducing agent was another important factor to
influence the final morphology of nickel sample. An experiment
was carried out with larger amount (2 mL) of hydrated hydrazine
Fig. 5. SEM images of nickel samples prepared with (a) 0 mL, (b) 0.1 mL, (c) 1 mL ethylenediamine, and (d) SEM image of nickel sample synthesized with 2 mL hydrated
hydrazine and 0.35 mL ethylenediamine.

H. Chen et al. / Materials Research Bulletin 47 (2012) 1839–1844
1843
40
20
0
However, this value is lower than that of one-dimensional Ni
nanorods (ca. 332 Oe) [36] or nanobelts (ca. 640 Oe) [17] with high
anisotropy. The magnetic properties of nanomaterials have been
widely accepted to be highly dependent on the sample morpholo-
gy, size, crystallinity, magnetization direction, and so on [24]. The
nickel micro-flowers prepared in the present work possess
hierarchical structure leading to the surface/volume ratio and
enhanced shape anisotropy. Hence, higher coercivity value was
observed compared to that of bulk nickel and hollow nickel
spheres.
(a)
-20
-40
4. Conclusions
-5000
-2500
0
2500
5000
In summary, hierarchical nickel micro-flowers were synthe-
sized at a relatively low temperature of 85 8C without using any
temperature or external magnetic field. The individual flower-like
H (Oe)
30
15
0
microstructures have an average diameter of 1–2
mm and are
(b)
composed of sword-like nanopetals growing radially from the core
of the spherical particles. The formation mechanism of such nickel
micro-flowers was governed by a two-stage growth process. It was
found that synthetic parameters such as reaction time, dosage of
ligand ethylenediamine and hydrazine hydrate were crucial to the
formation of flower-like nickel. The magnetic measurement
indicated that the as-obtained product had a ferromagnetic
behavior with a significantly enhanced coercivity value compared
to that of bulk nickel. The present facile method can be extended to
fabricate other micro-/nanomaterials with similar hierarchical
structures.
-15
-30
-500
-250
0
250
500
Acknowledgment
H (Oe)
The authors gratefully acknowledge the financial support from
High-level Scientific Research Foundation for the Introduction of
Talent through North University of China.
Fig. 6. (a) Magnetic hysteresis loop of the flower-like hierarchical nickel
microstructures measured at room temperature and (b) the hysteresis loop at
low field.
References
for a comparison. We could clearly observe from Fig. 5d that the
sample was dominated by irregular microparticles with very broad
size distribution. Some micropatches were also produced. The
reaction rate was so high with 2 mL of N2H4ÁH2O used that
the nucleation step could not be effectively separated from the
subsequent growth stage and resulted in the formation of irregular
microparticles. Therefore, minute amount of hydrated hydrazine is
beneficial to obtain hierarchical nickel microflowers in the current
synthesis.
[1] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan,
Adv. Mater. 15 (2003) 353.
[2] K. Nagaveni, A. Gayen, G.N. Subbanna, M.S. Hegde, J. Mater. Chem. 12 (2002) 3147.
[3] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176.
[4] W. Alejandro, A. Onur, P.V. Braun, J. Am. Chem. Soc. 127 (2005) 16356.
[5] V. Lionel, S. Conny, S.M. Butorin, Adv. Mater. 17 (2005) 2320.
[6] G. Kaltenpoth, M. Himmelhaus, L. Slansky, F. Caruso, M. Grunze, Adv. Mater. 15
(2003) 1113.
[7] J. Hu, L. Ren, Y. Guo, H. Liang, A. Cao, L. Wan, C. Bai, Angew. Chem. Int. Ed. 44 (2005)
1269.
Magnetic measurement of the as-prepared flower-like nickel
microstructures were conducted at room temperature in an
applied magnetic field sweeping from À5000 to 5000 Oe, and
the hysteresis loop is shown in Fig. 6a. The sample displays
hysteresis behavior, revealing that the nickel microstructures are
ferromagnetic. The saturation magnetization (Ms) value is ca.
33.5 emu/g, which is lower than that of the bulk nickel (55 emu/g)
[32]. The reduced Ms value may be owing to a very thin layer of NiO,
which cannot be clearly detected by diffraction methods, formed
on the surface. Tracy et al. [33] reported the growth of thin NiO
shells on nickel nanoparticles and confirmed that the Ms values
were decreased due to the NiO formation, and the detailed Ms
values were dependent on the Ni core size and NiO shell thickness.
It was found from the magnified hysteresis loop at low applied field
(Fig. 6b) that the remanent magnetization (Mr) and the coercivity
(Hc) for the sample are 12.2 emu/g and 203.3 Oe, respectively.
Compared to the coercivity value of bulk Ni (ca. 0.7 Oe) [17] and
that of hollow nickel submicrometer spheres (ca. 32.3 Oe) [34],
hollow Ni nanospheres (102 Oe) [21], or dandelion-like Ni
nanostructures (ca. 130 Oe) [35] at room temperature,
the flower-like Ni microstructures exhibit an enhanced value.
[8] P. Gao, Z. Wang, J. Am. Chem. Soc. 125 (2003) 11299.
[9] L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000) 12700.
[10] F.L. Jia, L.Z. Zhang, X.Y. Shang, Y. Yang, Adv. Mater. 20 (2008) 1050.
[11] J.G. Park, W.S. Nam, S.H. Seo, Y.G. Kim, Y.H. Oh, G.S. Lee, U.G. Paik, Nano Lett. 9
(2009) 1713.
[12] F.Y. Cheng, J. Chen, X.L. Gou, Adv. Mater. 18 (2006) 2561.
[13] R.Y. Zhang, L. Amlani, J. Baker, J. Tresek, R.K. Tsui, Nano Lett. 3 (2003) 731.
[14] L. Sun, P.C. Searson, C.L. Chien, Appl. Phys. Lett. 79 (2001) 4429.
[15] S. Pignard, G. Goglio, A. Radulescu, L. Piraux, S. Dubois, A. Declemy, J.L. Duvail, J.
Appl. Phys. 87 (2000) 824.
[16] Y. Hou, H. Kondoh, T. Ohta, S. Gao, Appl. Surf. Sci. 241 (2005) 218.
[17] Z.P. Liu, S. Li, Y. Yang, S. Peng, Z.K. Hu, Y.T. Qian, Adv. Mater. 15 (2003) 1946.
[18] X.R. Li, Y.Q. Wang, G.J. Song, Z. Peng, Y.M. Yu, X.L. She, J.J. Li, Nanoscale Res. Lett. 4
(2009) 1015.
[19] C.M. Liu, L. Guo, R.M. Wang, Y. Deng, H.B. Xu, S.H. Yang, Chem. Commun. 25 (2004)
2726.
[20] J.C. Bao, C.Y. Tie, Z. Xu, Q.F. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631.
[21] Q. Liu, H.J. Liu, M. Han, J.M. Zhu, Y.Y. Liang, Z. Xu, Y. Song, Adv. Mater. 17 (2005)
1995.
[22] J.G. Guan, L.J. Lin, L.L. Xu, Z.G. Sun, Y. Zhang, CrystEngComm 13 (2011) 2636.
[23] S. Senapati, S.K. Srivastava, S.B. Singh, K. Biswas, Cryst. Growth Des. 10 (2010)
4068.
[24] L.P. Zhu, G.H. Liao, W.D. Zhang, Y. Yang, L.L. Wang, H.Y. Xie, Eur. J. Inorg. Chem. 8
(2010) 1283.
[25] X.M. Liu, S.Y. Fu, J. Cryst. Growth 306 (2007) 428.
[26] B. Zhang, P. Xu, X.M. Xie, H. Wei, Z.P. Li, N.H. Mack, X.J. Han, H.X. Xu, H.L. Wang, J.
Mater. Chem. 21 (2011) 2495.

1844
H. Chen et al. / Materials Research Bulletin 47 (2012) 1839–1844
[27] C. Wang, X.J. Han, X.L. Zhang, S.R. Hu, T. Zhang, J.Y. Wang, Y.C. Du, X.H. Wang, P. Xu,
J. Phys. Chem. C 114 (2010) 14826.
[32] J.H. Hwang, V.P. Dravid, M.H. Teng, J.J. Host, B.R. Elliott, D.L. Johnson, T.O. Mason, J.
Mater. Res. 12 (1997) 1076.
[28] C. Wang, X.J. Han, P. Xu, J.Y. Wang, Y.C. Du, X.H. Wang, W. Qin, T. Zhang, J. Phys.
Chem. C 114 (2010) 3196.
[29] Y. Cheng, Y.S. Wang, Y.H. Zheng, Y. Qin, J. Phys. Chem. B 109 (2005) 11548.
[30] R.L. Penn, J. Phys. Chem. B 108 (2004) 12707.
[33] A.C. Johnston-Peck, J.W. Wang, J.B. Tracy, ACS Nano 3 (2009) 1077.
[34] J.C. Bao, Y.Y. Liang, Z. Xu, L. Si, Adv. Mater. 15 (2003) 1832.
[35] J.T. Tian, C.H. Gong, L.G. Yu, Z.S. Wu, Z.J. Zhang, Chin. Chem. Lett. 19 (2008) 1123.
[36] X.M. Ni, X.B. Su, Z.P. Yang, H.G. Zheng, J. Cryst. Growth 252 (2003) 612.
[31] L.P. Zhu, W.D. Zhang, H.M. Xiao, Y. Yang, S.Y. Fu, J. Phys. Chem. C 112 (2008) 10073.