Effect of processing conditions on the structure and collective magnetic properties of flowerlike nickel nanostructures

Article abstract of DOI:10.1016/j.materresbull.2010.03.002

To acquiring more insights into the relationship between the morphology and magnetic properties of magnetic nanocrystallites, uniform flowerlike Ni nanostructures with different branch lengths were fabricated via a simple solvothermal reduction route based on a series of comparative experiments. The formation mechanism of the Ni flowers was proposed. Moreover, the magnetic properties of the products were evaluated. Results indicate that the morphology of the Ni particles strongly depends on reaction temperature, and the branch length of the flowerlike Ni particles strongly depends on the initial concentration of Ni²⁺ ions. The flowerlike Ni particles with a longer branch length show higher coercivity value, which may be attributed to the peculiar microstructure. We believe the present research may provide an ideal example for the synthesis of assembled magnetic nanostructures with controllable morphology and magnetic properties.

Full text of DOI:10.1016/j.materresbull.2010.03.002

Materials Research Bulletin 45 (2010) 682–687
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Effect of processing conditions on the structure and collective magnetic properties
of flowerlike nickel nanostructures
c
a
a,
Chunhong Gong a,b, Juntao Tian a, Jingwei Zhang a, , Xuefeng Zhang , Laigui Yu , Zhijun Zhang
*
*
a Key Laboratory of Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, China
b College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China
c Industrial Materials Institute, National Research Council Canada, Quebec J4B 6Y4, Canada
A R T I C L E I N F O
A B S T R A C T
Article history:
To acquiring more insights into the relationship between the morphology and magnetic properties of
magnetic nanocrystallites, uniform flowerlike Ni nanostructures with different branch lengths were
fabricated via a simple solvothermal reduction route based on a series of comparative experiments. The
formation mechanism of the Ni flowers was proposed. Moreover, the magnetic properties of the products
were evaluated. Results indicate that the morphology of the Ni particles strongly depends on reaction
temperature, and the branch length of the flowerlike Ni particles strongly depends on the initial
concentration of Ni2+ ions. The flowerlike Ni particles with a longer branch length show higher coercivity
value, which may be attributed to the peculiar microstructure. We believe the present research may
provide an ideal example for the synthesis of assembled magnetic nanostructures with controllable
morphology and magnetic properties.
Received 7 July 2009
Received in revised form 3 February 2010
Accepted 2 March 2010
Available online 9 March 2010
Keywords:
A. Metals
B. Magnetic materials
C. Chemical synthesisl
D. Magnetic properties
ß 2010 Elsevier Ltd. All rights reserved.
1. Introduction
larger coercivity (HC) than common materials [7,8]. However, it
still remains a challenge to efficiently and conveniently control the
It is well-known that nanoscale magnetic materials have
potential applications in areas such as electromagnetic wave
shielding or/and absorbance, magnetic fluids, magnetic recording
devices, and magnetic drug delivery [1]. Since the magnetic
properties of magnetic nanocrystals depend strongly on structure
and morphology, it is imperative to study novel shape-controlling
synthetic strategies and hence fabricate novel magnetic nanos-
tructures with unusual properties [2]. So far, a large number of
magnetic nanostructures with peculiar shapes and magnetic
properties have been successfully fabricated [3–6]. To name a
few, various shape-controllable Ni nanostructures, e.g., Ni nano-
wires electrodeposited on anodic aluminum oxide (AAO) tem-
plates [3], Ni nanobelts grown with the assistance of complexing
agents and surfactants [4], Ni hollow spheres formed on
microemulsion template [5], and Ni nanorods created by thermal
decomposition of organic compounds have been fabricated using
different precursors and routes [6].
morphology of anisotropy magnetic nanostructures. In a previous
research we prepared one dimensional (1D) Ni nanocrystallite
fibers with different lengths using magnetic field-induced self-
assembly and found that a relatively high intensity of the magnetic
field and large concentration of Ni2+ ions favored the generation of
Ni fibers with increased length and magnetic properties [9].
Unfortunately, there have been very few reports on the fabrication
of three dimensional (3D) nickel nanostructures as a kind of
important shape anisotropy materials [10–12], possibly because
complicated multi-step processes are usually involved and it
requires rigorous conditions such as high temperature and use of
surfactants and/or external magnetic field to effectively tailor the
size and morphology of the products. In other words, it also
remains a challenge to develop simple and reliable routes for the
self-assembly of magnetic nanoparticles with controllable multi-
dimensional morphology.
With those perspectives in mind, by properly adjusting reaction
conditions, 3D flowerlike Ni particles with different branch lengths
were successfully prepared via a convenient solvothermal process
in the absence of any surfactants or external magnetic forces. The
magnetic properties of the products at room temperature were
systemically evaluated. Hopefully, the present research is to
provide some insights into the relationship between the morphol-
ogy and magnetic properties of Ni nanostructures.
Several theories are currently available for analyzing shape-
dependent magnetic properties, and it has been widely recognized
that magnetic particles with large shape anisotropy usually have a
* Corresponding authors.
E-mail address: jwzhang0378@yahoo.cn (J. Zhang).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.03.002

C. Gong et al. / Materials Research Bulletin 45 (2010) 682–687
683
2. Experimental
3. Results and discussion
All reagents are of analytical grade and used without further
purification. In a typical experiment, an appropriate amount of
nickel chloride (NiCl2ꢀ6H2O) (0.01–0.10 M) was dissolved directly
in 30 mL of ethylene glycol. Then 3 mL of ammonia solution (25–
28 wt%) and 3 mL of hydrazine hydrate solution (N2H4ꢀH2O,
80 wt%) were added into the above solution under continuous
stirring, allowing the generation of a red mixture. Finally, the red
mixture was transferred into a 50-mL Teflon-lined autoclave and
sealed and held at 90–150 8C for 7 h, followed by cooling to room
temperature. The reactions can be described as follows:
As a typical example, the sample obtained via the solvothermal
process of 0.10 M Ni2+ and 3 mL of ammonia solution and 3 mL of
hydrazine at 100 8C for 7 h was analyzed by using FESEM and TEM,
and the results are shown in Fig. 1. It is seen that the flowerlike Ni
particles have a diameter of 4–6
mm and show good uniformity
(see Fig. 1(a)). At the same time, the flowery particle consists of a
sphere-like core attached to various nanostructure branches, and
most of the branches have radiating arrangement around the core.
Such structural features are also reflected in corresponding TEM
image (Fig. 1(b)). Moreover, the TEM image of an individual branch
of the flowerlike particles shows signs of tapering off from top to
bottom of a tip (Fig. 1(c)). And it is likely that the branch has a
2þ
Ni2þ þ NH3 ¼ ½NiðNH3Þ6ꢁ
(1)
(2)
length of about 1
mm and width from several nanometers to about
2Ni2þ þ N2H4 þ 4OHꢂ ¼ 2Ni þ N2 " þ 4H2O
200 nm. Fig. 1d shows a typical selected area electron diffraction
(SAED) pattern obtained along a typical individual branch. The
spots pattern indicates that the as-prepared Ni sample has single
crystalline structure and can be indexed to pure cubic phase nickel.
Fig. 2 shows the typical XRD and EDS spectra of the as-prepared
3D Ni nanostructures obtained under the same conditions as in
Fig. 1. The XRD pattern shows obvious signs of broadening,
indicating that the as-prepared Ni sample may consist of
nanocrystallites (Fig. 2a). Besides, the XRD pattern can be indexed
as face-centered cubic (fcc) nickel (JCPDS 01-1260) [13]. Although
it is conceivable that Ni nanostructures are very active and might
react with oxygen in air forming oxides, no nickel oxide or any
other impurities have been detected by XRD, which indicates that
nickel crystalline fabricated under the present conditions has a
very high purity. Moreover, the EDS spectrum (Fig. 2b) also
illustrates that the products contain only nickel, which corre-
sponds well to the SAED pattern shown in Fig. 1(d).
where Ni2+ ions are reduced into Ni and the evolved nitrogen can
help to prevent nascent nickel nanocrystallites from oxidation.
The as-synthesized solid products were precipitated, separated,
washed with ethanol for several times, and dried in a vacuum oven
at 60 8C for 24 h. The structure and morphology of the resultant
dried samples were characterized by means of X-ray diffraction
(XRD, Philips X’ Pert Pro X-ray diffractometer; Cu–K radiation,
a
l
= 0.15418 nm), scanning electron microscopy (SEM, JEOL JSM-
5600LV; acceleration voltage of 20 kV), field-emission scanning
electron microscopy (FESEM, Hitachi FE-SEM, S-4800; carried out
with a field-emission scanning electron microanalyzer (JEOL-
6300F, 15 kV)), and transmission electron microscopy (TEM, JEOL
JEM-2010; accelerating voltage 200 kV). The main constituent
elements of the typical products were also determined by means of
energy-dispersive X-ray spectrometry (EDS) attached to the SEM
(JEOL JSM-5600LV). The magnetic hysteresis loops (M–H loops) of
the products at room temperature were measured using a
vibrating sample magnetometer (Lake Shore 7410 VSM, USA).
It is well-known that the morphology of Ni nanostructures is
highly dependent on many factors such as temperature, concen-
tration of reactants, and the types of solvents and surfactants, etc.
Fig. 1. FESEM image of Ni flowerlike sample (a); TEM images of the Ni flowerlike sample (b) and the Ni branch (c); and SAED pattern of a Ni branch (d).

684
C. Gong et al. / Materials Research Bulletin 45 (2010) 682–687
Fig. 2. XRD (a) and EDS (b) spectra of as-prepared 3D Ni nanostructures obtained under the same conditions as in Fig. 1.
For example, we recently found that the microstructure of Ni
nanocrystallites was closely dependent on the type of the solvents
and only alcohol-type solvents containing double hydroxyl groups
might be helpful to the formation of Ni nanocrystallites with shape
anisotropy [14]. To further understand the self-organization
mechanism and growth process of flowerlike Ni nanostructures,
the products obtained at different parameters were investigated in
detail. This, hopefully, is to help to shed light on the relationship
among the morphology of Ni nanostructures and the concentration
of initial Ni2+ ions and reaction temperature as well.
images, it can be seen that the three samples prepared at Ni2+ ion
concentration of 0.02 M, 0.06 M, and 0.10 M (see the insets in
Fig. 3(b–d)) have typical branches length of 100–300 nm, 300–
600 nm, and 800–1500 nm, respectively. Moreover, it is likely that
a higher concentration of Ni2+ ions, for instance, 0.06 M and 0.10 M,
is beneficial to the formation of flowerlike particles with longer
branches (see Fig. 3(c–d)).
The precursor might play a very important role in terms of the
morphology of the Ni nanostructures, and the above observations
could be rationally understood by taking into account the influence
of reduction rate on the nucleation and typical Ostwald ripening
progress. In the Ostwald ripening process, the larger particles will
grow at the cost of the small ones due to the difference of energy
between large and small particles of a higher solubility based on
the Gibbs–Thompson law [15]. Namely, in the first step, Ni2+ ions
In order to investigate the effect of the concentration of Ni2+
ions on the morphology of Ni products, the reduction of Ni2+ ions
with varied initial concentration from 0.01 M to 0.10 M by 3 mL of
hydrazine hydrate solution in the presence of 3 mL of ammonia
solution was performed at 100 8C. Fig. 3 shows the SEM images of
the resulting Ni products. It is clearly seen that the morphology of
the Ni products is highly dependent on the initial concentration of
Ni2+ ions. The Ni sample obtained at an initial Ni2+ ions
concentration of as low as 0.01 M appears as microspheres,
showing relatively smooth surface and non-uniform size of 0.2–
2+
2+
can form complex precursors Ni(NH3)6
and Ni(N2H4)x
in
solution containing ammonia and hydrazine, leading to a sharp
decrease in the concentration of free Ni2+ ions. Subsequently, Ni
complex precursors were slowly dissolved and reduced by
hydrazine hydrate forming Ni atoms. Since a minimum number
of atoms are required to form a stable nucleus and a collision must
occur among several atoms during nucleation, the atoms formed at
that period might participate mainly in collisions with already
formed nuclei, instead of forming new nuclei [16]. This would
result in precipitation of nuclei and their quick growth into
2
m
m (Fig. 3(a)). When the initial concentration of Ni2+ ions is
higher than 0.01 M, Ni nanostructures with 3D flowerlike
morphology is obtained and the branch length of the flowerlike
particles gradually increases with increasing initial concentration
of Ni2+ ions (Fig. 3(b–d)). By carefully viewing the magnified SEM
Fig. 3. SEM images of the samples obtained at 100 8C from the reaction systems with different concentrations of Ni2+ ions: (a) 0.01 M, (b) 0.02 M, (c) 0.06 M, and (d) 0.10 M.
The insets are higher magnification images.

C. Gong et al. / Materials Research Bulletin 45 (2010) 682–687
685
Fig. 4. SEM images of Ni samples obtained at different reaction temperature: (a) 90 8C, (b) 120 8C, (c) 130 8C, and (d) 150 8C.
primary particles. In the follow-up step, the primary particles
might serve as nucleation sites of the 3D structure and finally form
flowerlike structure by way of Ostwald ripening of Ni atoms and
nanoparticles. Without a preferential tropism, Ni branches could
grow in any directions, and subsequently, 3D flowerlike nickel
nanostructures were harvested at an extended reaction time. And
the branch length of the 3D nanostructures increased with
increasing reaction time until the metal salt was completely
consumed. Therefore, the higher the concentration of free Ni2+
ions, the longer the branches of the 3D nanostructures.
To investigate the effect of reaction temperature on the
morphology of Ni products, we carried out temperature-depen-
dent experiments with a fixed initial concentration of Ni2+ ions
(0.10 M), 3 mL of ammonia solution, and 3 mL of hydrazine hydrate
solution within a solvothermal temperature range of 90–150 8C.
The typical SEM morphologies of four samples obtained at
different solvothermal temperature are shown in Fig. 4. At a lower
temperature, e.g., 90 8C, a mixture of black product and blue stuff
was obtained even after an extended reaction time of more than
7 h, implying that the reducing reaction could not be completed.
The corresponding black product obtained by magnetic separation
methods, a mixture of flowerlike particles and erose structure with
slower reduction rate. At the same time, ethylene glycol solution
with a high viscosity largely restrains the diffusion of Ni nuclei,
which facilitates the aggregation of Ni crystals and growth of
anisotropic dendritic crystal in the early period of reduction
(Fig. 4(a–b)). With the increase of temperature, the viscosity of the
reaction solution was decreased and the reduction rate increased,
favoring the generation of more nuclei and formation of smaller Ni
spherical particles. Moreover, due to Ostwald ripening, more
energy can be provided to the reaction system at a higher
temperature, which also favors the re-dissolving of anisotropic
dendritic crystals, resulting in spherical particles with smooth
surfaces (Fig. 4(c–d)). Therefore, aside from the great dependence
of branch length of Ni nanostructures on the concentration of Ni2+
ions, the flowerlike morphology of the Ni nanostructures also
strongly depends on the reaction temperature.
The formation processes of the 3D flowerlike Ni particles are
schematically illustrated in Scheme 1. In the initial stage of
2+
solvothermal reaction, Ni complex precursor Ni(NH3)6
and
Ni(N2H4)x2+ are formed and reduced into small nickel nanocrystals
in the presence of hydrazine hydrate as the reducing agent (stage
I). In the second stage, small nickel nanocrystals grow into Ni
nuclei, providing favorable environment for the nucleation and
growth of more nickel nanocrystals (stage II). In the third stage,
nickel nanocrystals are gradually attached with each other to
generate spherical particles, accompanied by the formation of
spearlike branches via seed-induced growth (stages II and III), and
spiked morphology, has a mean diameter of about 1
mm (Fig. 4(a)).
At 120 8C, the product is a mixture of submicrometer-sized spheres
and rough structure with spiked morphology and it has a mean
diameter of about
1 mm (Fig. 4(b)). At a higher reaction
temperature of 130 8C, microspheres with a relatively smooth
surface were obtained, and the Ni particulates had non-uniform
size of 0.5–3
mm (Fig. 4(c)). When the temperature was further
increased to 150 8C, the reaction could be completed within an
hour, resulting in uniform smooth Ni microspheres with a
diameter of about 0.5
mm (Fig. 4(d)), which could be attributed
to a faster reaction rate thereat. It seems that a moderate reaction
temperature favors the formation of Ni nanostructures with good
uniformity. In other words, only when the reaction temperature
was fixed at 100 8C, well-defined flowerlike particles were
harvested (Fig. 1). And once the reaction temperature was over
100 8C, the flowerlike morphology was destroyed.
The above-mentioned results might be explained as follows. At
a lower temperature (90–120 8C), Ni complex precursors were
dissolved and reduced by hydrazine hydrate slowly, leading to a
Scheme 1. Schematic illustration of the morphological evolution process of Ni
flowerlike nanostructures: (I) Small nickel nanocrystals were formed from Ni
precursors. (II) Small nickel nanocrystals were gradually attached with each other
to generate bigger spherical nickel nanoparticles, accompanied by the formation of
spearlike branches via seed-induced growth. (III) Spherical nickel particles were
partially transformed into flowers with the reduction of Ni2+. (IV) Nickel flowers
emerged as the final product.

686
C. Gong et al. / Materials Research Bulletin 45 (2010) 682–687
108.4 Oe, 118.5 Oe, and 159.9 Oe, respectively. Since the branches
have higher shape anisotropy than spherical particles, the increase
of HC with increasing branch length might be attributed to the
presence of a hierarchical structure which changes the magnetiza-
tion reversal mechanism [19]. This conforms well to what have
been reported elsewhere in that the samples with high shape
anisotropy often possess high coercivity [7–13]. In addition, our
experimental result gives a hint that Ni2+ concentration has an
obvious effect on the morphologies and magnetic properties of
nanocrystallites Ni products. And the improved magnetic proper-
ties of the 3D flowerlike Ni nanostructures may be very valuable to
their application in magnetic recording materials.
4. Conclusions
A simple solvothermal route has been established to prepare
flower-shaped nickel nanostructures in ethylene glycol in the
absence of templates or expensive precision equipments. It has
been found that the assembly process and morphology of the Ni
nanostructures are highly dependent on the reaction conditions.
By properly controlling the concentration of Ni2+ ions and
solvothermal temperature, flowerlike Ni nanostructure and Ni
microspheres are readily obtained, and it is feasible to adjust the
branch length of the flowerlike particles within 200–1500 nm. The
HC value of the 3D Ni nanostructures increases with increasing
branch length, which may be closely related to the unique flowery
shape and is critical to the application of Ni crystallites in magnetic
recording, catalysis, conduction, etc. Hopefully, the present
synthetic method is to be accepted as an efficient model for
preparing magnetic assembly nanostructures with controllable
morphology and magnetic properties via simply controlling
reaction conditions. And it may be an ideal model for acquiring
more insights into the relationship between the morphologies and
magnetic properties of Ni nanostructures.
Fig. 5. Hysteresis loops at room temperature for the samples obtained at 100 8C
from the reaction systems with different concentrations of Ni2+ ions: (a) 0.01 M, (b)
0.02 M, (c) 0.06 M, and (d) 0.10 M. The upper left inset shows the hysteresis loops of
the four samples between ꢂ150 and 150 Oe. Magnetic properties of the four
samples and the branch lengths of the three flowerlike samples are summarized in
the lower right inset.
the length of the branches is dominated by the Ni2+ ions
concentration. Finally, Ni flowers emerge at extended reaction
duration (stage IV). However, due to the complex growth
conditions involved, the detailed mechanism for the formation
of final Ni nanostructures still remains unknown. And in particular,
the growth process of the Ni branches can hardly be well described
herewith. This is because, aside from the Ostwald ripening and
diffusion, several other factors, including electrostatic and/or
magnetic dipolar attraction, van der Waals forces, and so on, may
also affect the self-assembly [17]. Thus further work is needed to
reveal the self-assembly growth mechanism of the Ni nanos-
tructure.
It is known that the magnetic properties of nanomaterials are
closely related to size, morphologies, crystallinity, and so on. To
investigate the influence of microstructure on the magnetic
properties, the magnetic properties of four samples (The same
as those shown in Fig. 3) were evaluated using a vibrating sample
magnetometer. The M–H loops measured at room temperature are
shown in Fig. 5, where the upper left inset is the magnified
hysteresis loops measured at a low intensity of applied magnetic
field and the saturated magnetization (MS) and coercivity (HC)
values are listed in lower-right inset. The hysteresis loops show
ferromagnetic behavior with a saturation magnetization of 49.8–
52.7 emu/g, which is slightly smaller than 55 emu/g, that of the
corresponding bulk material [9]. Due to the decrease in particle
size, the surface-disordered layer could serve as a nonmagnetic
layer and decrease the value of MS [18]. Moreover, the use of
ethylene glycol might result in the formation of a protective layer
on the particle surface, and the electron exchange between ligand
and surface atoms might quench the magnetic moment, both
leading to decrease of MS [10].
Acknowledgments
We acknowledge financial support from the National Basic
Research Program of China (project of ‘‘973’’ Plan, grant No.
2007CB607606) and National Science Foundation of China (Grant
Nos. 50902045/E0213 and 20971037/B0111).
References
[1] (a) X.F. Zhang, X.L. Dong, H. Huang, B. Lv, J.P. Lei, C.J. Choi, J. Phys. D: Appl. Phys. 40
(2007) 5383–5387;
(b) C.H. Gong, Y.P. Duan, J.T. Tian, Z.S. Wu, Z.J. Zhang, J. Appl. Polym. Sci. 110
(2008) 569–577;
(c) G. Leem, S. Sarangi, S. Zhang, I. Rusakova, A. Brazdeikis, D. Litvinov, T.R. Lee,
Cryst. Growth Des. 9 (2009) 32–34;
(d) M.P. Pileni, Adv. Funct. Mater. 11 (2001) 323–336;
(e) V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115–2117.
[2] Y. Sahoo, M. Cheon, S. Wang, H. Luo, E.P. Furlani, P.N. Prasad, J. Phys. Chem. B 108
(2004) 3380–3383.
[3] (a) I.Z. Rahman, A. Boboc, K.M. Razeeb, M.A. Rahman, J. Magn. Magn. Mater. 290–
291 (2005) 246–249;
(b) K.S. Napolskii, A.A. Eliseev, N.V. Yesin, A.V. Lukashin, Y.D. Tretyakov, N.A.
Grigorieva, S.V. Grigoriev, H. Eckerlebe, Physica E 37 (2006) 178–183.
[4] Z.P. Liu, S. Li, Y. Yang, S. Peng, Z.K. Hu, Y.T. Qian, Adv. Mater. 15 (2003) 1946–1948.
[5] (a) J.C. Bao, Y.Y. Liang, Z. Xu, L. Si, Adv. Mater. 15 (2003) 1832–1835;
(b) Q. Liu, H.J. Liu, M. Han, J.M. Zhu, Y.Y. Liang, Z. Xu, Y. Song, Adv. Mater. 17 (2005)
1995–1999.
Usually, a moderate coercivity is desired for a magnetic
medium [18]. The HC value of the sample obtained at a lower
Ni2+ ions concentration (0.01 M) is only 96.8 Oe (spherical shape
particles), which is slightly smaller than 100 Oe, that of the
corresponding bulk material [9]. The 3D flowerlike Ni nanos-
tructures obtained at a higher Ni2+ ions concentration (0.02 M and
above) have increased HC with increasing branch length. Namely,
when the branch length of flowerlike structure is 100–300 nm,
300–600 nm, and 800–1500 nm, the corresponding HC value is
[6] N. Cordente, M. Respaud, F. Senocq, M. Casanove, C. Amiens, B. Chaudret, Nano
Lett 1 (2001) 565–568.
[7] (a) Y. Soumare, J.Y. Piquemal, T. Maurer, F. Ott, G. Chaboussant, A. Falquic, G. Viau,
J. Mater. Chem. 18 (2008) 5696–5702;
(b) X.M. Liu, S.Y. Fu, J. Cryst. Growth 306 (2007) 428–432;
(c) J.C. Bao, C.Y. Tie, Z. Xu, Q.F. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631–
1633.
[8] R.H. Wang, J.S. Jiang, M. Hu, Mater. Res. Bull. 44 (2009) 1468–1473.
[9] C.H. Gong, L.G. Yu, Y.P. Duan, J.T. Tian, Z.S. Wu, Z.J. Zhang, Eur. J. Inorg. Chem.
(2008) 2884–2891.

C. Gong et al. / Materials Research Bulletin 45 (2010) 682–687
687
[10] G.Q. Zhang, T. Zhang, X.L. Lu, W. Wang, J.F. Qu, X.G. Li, J. Phys. Chem. C 111 (2007)
12663–12668.
[14] C.H. Gong, C.Q. Du, Y. Zhang, Z.S. Wu, Z.J. Zhang, Chin. J. Inorg. Chem. 25 (2009)
1569–1574.
[11] X.M. Ni, Q.B. Zhao, D.E. Zhang, X.J. Zhang, H.G. Zheng, J. Phys. Chem. C 111 (2007)
601–605.
[15] G.S. Guo, F.B. Gu, Z.H. Wang, H.Y. Guo, Chin. Chem. Lett. 16 (2005) 1101–
1104.
[12] (a) X.M. Ni, Q.B. Zhao, H.G. Zheng, B.B. Li, J.M. Song, D.E. Zhang, X.J. Zhang, Eur. J.
Inorg. Chem. (2005) 4788–4793;
(b) F.L. Jia, L.Z. Zhang, X.Y. Shang, Y. Yang, Adv. Mater. 20 (2008) 1050–1054.
[13] C.H. Gong, J.T. Tian, T. Zhao, Z.S. Wu, Z.J. Zhang, Mater. Res. Bull. 44 (2009) 35–40.
[16] S.H. Wu, D.H. Chen, J. Colloid. Interf. Sci. 259 (2003) 282–286.
[17] C.H. Gong, J.T. Tian, Z.S. Wu, Z.J. Zhang, Chin. J. Inorg. Chem. 24 (2008) 964–970.
[18] A.H. Lu, E.L. Salabas, F. Schu¨ th, Angew. Chem. Int. Ed. 46 (2007) 1222–1244.
[19] L.L.P. Diandra, D.R. Reuben, Chem. Mater. 8 (1996) 1770–1783.