Particuology 10 (2012) 783–788
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Particuology
journal homepage: www.elsevier.com/locate/partic
Hydrothermal synthesis and characterization of NiS flower-like architectures
Hai Zhou a,c , Baoliang Lv a,∗ , Dong Wu a , Yuhan Sun a,b
a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
Low Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, China
c
Graduate University of the Chinese Academy of Science, Beijing 100049, China
b
a r t i c l e
i n f o
Article history:
Received 15 December 2011
Received in revised form 9 March 2012
Accepted 13 March 2012
Keywords:
Nanostructures
Chemical synthesis
Electron microscopy
Magnetic properties
a b s t r a c t
Under the influence of thiocyanate anions (SCN− ) and cetyltrimethyl ammonium bromide (CTAB), NiS
flower-like architectures were successfully synthesized by a one-step hydrothermal method. The synthesized flower-like architectures, with a multilayered and highly ordered texture, have diameters of
several micrometers. X-ray powder diffraction (XRD) shows that the NiS flower-like architectures are
rhombohedral crystalline. On the basis of condition-dependent experiments, the diffusion-limited aggregation (DLA) model and cage effect were used to explain the growth process of rhombohedral crystalline
NiS flower-like architectures. Magnetic measurements showed that the coercivity (Hc ) of the as-obtained
NiS flower-like architectures was 102.14 Oe.
© 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of
Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction
In recent years, great efforts have been invested in the synthesis of higher ordered metal sulfide nanomaterials with specific
sizes, shapes, and hierarchies, since the potential to design new
materials and devices in various fields (Duan, Huang, Cui, Wang, &
Lieber, 2001; Park et al., 2005; Wang, Cai, & Xia, 2005; Whitesides
& Grzybowski, 2002; Xia et al., 2003) are foreseeable. The 3D
transition metal sulfide has attracted considerable interest due to
its microwave absorption ability, magnetothermal effect, which
make these materials widely used in the field of space technology,
nuclear magnetic resonance, particle accelerator, as superconducting and alternative materials in Li-ion batteries, etc. (Li &
Qian, 2008; Paolella et al., 2011). Nickel sulfide, as a typical 3D
transition metal sulfide, also displays metal-insulator behavior,
paramagnetic–antiferromagnetic phase changes and can be used
as a hydrodesulfurization catalyst, in solar energy storage, as cathode and/or anode materials of lithium batteries and as possible
transformation toughener in window glass (Barry & Ford, 2001;
Fernandez, Nair, & Nair, 1993; Kriven, 1990; Qu et al., 2011; Sparks
& Komoto, 1963; Wang, Zhu, Tao, & Su, 2011). NiS was traditionally prepared by high temperature solid-state reaction, vapor
phase reaction and solvothermal methods (Han et al., 1999; Wold
& Dwight, 1992; Yu et al., 1998).
Nickel sulfide NiS of various morphologies has been synthesized, including nanorods (Meng, Peng, Yu, & Qian, 2002; Shen
∗ Corresponding author. Tel.: +86 0351 4049859; fax: +86 0351 4041153.
E-mail address: [email protected] (B. Lv).
et al., 2003), triangular nanoprisms (Ghezelbash, Sigman, & Korgel,
2004), hollow spheres (Hu, Chen, Chen, & Li, 2004), thin films
(Yu & Yoshimura, 2002) etc. Nano-flower architectures have been
reported by several groups (Cao et al., 2010; Li et al., 2007; Wu, Pan,
Li, Yang, & Xie, 2007), but most works simply proposed mechanisms
though a few of them revealed the growth process of flower-like NiS
architectures in the light of microcosm. We will report a one-step
hydrothermal route to synthesize rhombohedral NiS with uniform and discrete flower-like structures, using thiocyanate anions
(SCN− ) as coordination ions and S source. Meanwhile, the influence of CTAB on the structure of NiS and the magnetic properties
of synthesized materials were investigated, and finally, possible
formation mechanism was proposed on the growth process of the
NiS flower-like architectures.
2. Experimental
All reagents were used without further purification:
Ni(NO3 )2 ·6H2 O (98.5 wt%, analytical reagent (A.R.), Tianjin
Beichen Fangzheng Chemicals Co., Ltd), NaSCN (98 wt%, A.R.,
Tianjin Tianda Chemicals Co., Ltd), CTAB (99.0 wt%, A.R., Shanghai
Sangon Biological Engineering Co., Ltd). Double-distilled water
was used throughout the experiment.
2.1. Synthesis of nickel sulfide flower-like architectures
For the preparation of NiS flower-like architectures, a typical
synthesis process was as follows: 1.8 mM of NaSCN, 2.58 mM CTAB
and 1.7 mM of Ni(NO3 )2 ·6H2 O were dissolved in 80 mL of distilled water under ultrasonic radiation. The as-formed bright green
1674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2012.03.004
784
H. Zhou et al. / Particuology 10 (2012) 783–788
Fig. 1. XRD patterns for (a) NiS flower-like architectures, and (b) JCPDS 12-0041.
solution was sealed in a Teflon-lined autoclave of 150 mL capacity,
and maintained at 220 ◦ C for 24 h. After cooling to room temperature, the products were collected by centrifugation, and washed
with distilled water three times and absolute ethanol twice to
remove impurities. The final products were dried in air at 80 ◦ C. In
order to investigate the growth process of the flower-like NiS, different experiments, including time-dependent experiments, were
carried out. To investigate the influence of CTAB, the mole ratio
of CTAB to Ni2+ was varied from 0.28 to 1, 1.5 and 2. To observe
the growth process of the NiS architectures, the reaction time was
altered from 4 to 8, 16 and 24 h.
2.2. Characterizations
The products were characterized by transmission electron
microscopy (TEM, JEOL JEM-1011) and high-resolution TEM
(HRTEM, JEM2010), scanning electron microscopy (SEM, XL30
S-FEG), powder X-ray diffraction (XRD, Rigaku D/max-rB diffractometer using Cu K␣ radiation, = 0.15408 nm), and vibrating
sample magnetometer (VSM, Lakeshore 7407).
3. Results and discussion
3.1. Morphology and structure of the NiS flower-like architectures
Fig. 1 shows the XRD pattern of the as-prepared flowerlike architectures which consists of NiS (JCPDS 12-0041) with
no characteristic peaks of other impurities, thus indicating that
only rhombohedral crystalline NiS was obtained via the simple
hydrothermal process.
Fig. 2 shows the SEM, TEM and HRTEM carried out for both
morphology and microstructure of the as-prepared NiS flower-like
architectures. Clearly, the produced samples consisted of uniform
and discrete flower-like structures, consisting of multilayered and
highly ordered texture. Most of the flower-like particles are several
micrometers in size. The NiS flower-like architectures are composed of nanopetals and nanorods, as shown in Fig. 2(b) and (c).
Fig. 2(e) shows an enlarged image of the area marked by white
frame in Fig. 2(d), the corresponding fast Fourier transform (FFT)
image. It can be seen clearly that the distances between adjacent
lattice fringes are about 0.22 and 0.25 nm, corresponding to the
(0 3̄ 1) and (0 4 0) planes of rhombohedral phase NiS, respectively.
3.2. Effect of synthesis conditions and plausible mechanism
The amount of dispersants have important effect on the morphology of the final product (Li, Li, Yang, & Li, 2003; Li, Li, Yang, & Li,
2004; Rana, Zhang, Yu, Mastai, & Gedanken, 2003; Yu et al., 2005);
so several contrast experiments were carried out to determine
the role of CTAB in the formation of NiS flower-like architectures.
These experiments were carried out at the reaction temperature of
220 ◦ C, using 1.8 mM of NaSCN and 1.7 mM of Ni(NO3 )2 , by varying
only the amount of CTAB, as a result, leading to the corresponding SEM images shown in Fig. 3. When the mole ratios of CTAB to
Ni2+ were only 0.28 and 1, irregular particles with some flowerlike structures were formed, and the percentage of the flower-like
architectures increased with further increase of CTAB (Fig. 3(a)
and (b)). When the mole ratio of CTAB to Ni2+ was 1.5, uniform
and discrete flower-like architectures consisting of nano-petals
and nano-rods were formed (Fig. 3(c)). The composition of the asprepared materials was rhombohedral crystalline NiS (Fig. 1(a)).
However, when the mole ratio of CTAB to Ni2+ was increased
to 2, flower-like architectures with many pompon-like structures
appeared (Fig. 3(d)). The above results show that appropriate
amount of CTAB is required in order to obtain uniform and discrete
flower-like architectures, that is, the chosen mole ratio of CTAB
to Ni2+ of 1.5.
In order to observe the growth process of the as-prepared NiS
flower-like architectures, time-dependent experiments were carried out, by altering the synthesis time from 4 through 8 and 16
to 24 h, as shown in the corresponding SEM images in Fig. 4, that
is, from near-spherical particles to flower-like architectures. For
synthesis time of 4 h (Fig. 4(a)), near-spherical particles could be
observed, and the composition of as-obtained sample was NiS with
a bit of Ni0.96 S (JCPDS 50-1791) (Fig. 5(a)). For reaction time of
8 h, nanorods and nanopetals could be observed on these nearspherical particles (Fig. 4(b)–(d)). The major components of the
resulting products were still NiS and Ni0.96 S, and the crystallinity of
the as-obtained samples was not too well (Fig. 5(b)). When the reaction time reached 16 h, basically only flower-like structures formed
and the major component of the product was rhombohedral crystalline NiS with very small amounts of Ni0.96 S (Figs. 4(e) and 5(c)).
When the reaction time was prolonged to 24 h, NiS flower-like
architectures with perfect morphology and crystallinity prevailed
(Figs. 4(f) and 5(d)).
On the basis of the characterization results from conditiondependent experiments, the growth process of NiS flower-like
structures could be explained in terms of the diffusion-limited
aggregation (DLA) process (Meakin, 1983; Meakin, Kertész, &
Vicsek, 1988) and of the cage effect (Fu, Shen, & Yao, 1990, chap.
11) as displayed in Fig. 6.
Owing to the molar ratio of SCN− to Ni2+ to be about 1:1, there
might exist [Ni(SCN)x ]2−x (0 ≤ x ≤ 4) in the clear and apple green
transparent solution before the addition of CTAB. After adding
enough amounts of CTAB into the system, the solution became a
white emulsion. In a microcosmic sense, large amounts of CTAB
would separate the continuous system into massive small regions,
that is, plenteous cages (micelles) when the concentration of CTAB
reached the critical micelle concentration (CMC), and these cages
might contain limited amounts of Ni2+ and [Ni(SCN)x ]2−x (0 ≤ x ≤ 4)
(Buscaglia et al., 2008). S2− released from [Ni(SCN)x ]2−x (Pang, Lu,
Li, & Gao, 2009; Zhang, Xu, Tang, Li, & Qian, 2005) would collide with uncombined Ni2+ to form Nix S in any cage during the
diffusion process of Ni2+ and [Ni(SCN)x ]2−x or from one cage to
another due to the ambient high temperature and high pressure.
In the initial stage, there would exist both of Ni0.96 S and NiS in the
product, and after a while the increasing rate of Ni0.96 S was larger
than that of NiS though they rose at the same time, as revealed
by XRD patterns from 4 to 16 h (Fig. 5(a)–(c)). This is because the
concentration of uncombined Ni2+ in a cage was insufficient. As
reaction proceeded, the quickly decreasing concentration of Ni2+
could no longer meet the requirement for forming NiS, and neither
H. Zhou et al. / Particuology 10 (2012) 783–788
785
Fig. 2. SEM, TEM and HRTEM images of the as-prepared NiS flower-like architectures: (a) and (b) SEM images of the sample; (c) TEM images of the sample; (d) HRTEM
images of the sample; (e) enlarged image of the area marked by white frame in (d), to show corresponding FFT pattern.
could [Ni(SCN)x ]2−x release enough uncombined Ni2+ to sustain
the reaction. During this process, Nix S nuclei gradually formed and
developed into near-spherical particles. The electronegativity of S
is 2.58 while that of Ni is 1.91 (periodic table of electronegativity
using the Pauling scale), that is, Nix S (x < 1) is negative and could
attract the positively charged side of the CTAB, causing excess CTAB
to surround and adsorb on the particles, thus limiting the particles
to grow along one-dimension to form petals or rods on the surface
of particles. However, when CTAB was in excess, the in between
space became smaller thus leading to the formation of small rodlike
structures, and finally, the pompon-like structures (see Fig. 3(d)).
This shows that CTAB played an important role in the growth process of NiS flower-like architectures. With prolonged reaction time,
the diffusion process of Ni2+ and [Ni(SCN)x ]2−x went on consecutively, and the concentration of uncombined Ni2+ and S2− in a
cage might meet the need of forming NiS, while these petals and
rods might grow further, and larger petals and rods might split into
smaller ones to better satisfy the spatial requirements of the crystal
growth process during recrystallization. Finally, the rhombohedral crystalline NiS flower-like architectures came into being. This
growth mechanism might seem similar to that reported by Li et al.
(2007), though our proposed mechanism could not only explain
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H. Zhou et al. / Particuology 10 (2012) 783–788
Fig. 3. SEM images of samples obtained at different mole ratio of CTAB to Ni2+ : (a) 0.28, (b) 1, (c) 1.5 and (d) 2.
the transformation process of the composition at different stages,
but also introduced the concept of cages in explaining the growth
process of NiS flower-like architectures due to the use of CTAB in
synthesis.
3.3. Magnetic properties of the NiS flower-like structures
The magnetic properties of nanomaterials are highly dependent
on the structure, morphology and geometry of the material, such as
diameter and aspect ratio (Pang et al., 2009). In addition, presence
of anisotropy will significantly enhance the magnetic properties
(Leslie-Pelecky & Rieke, 1996) and higher aspect ratio will favor
the increase of coercivity. To investigate the magnetic activity of
the as-prepared NiS flower-like architectures, data of remanent
magnetization (Mr ) and coercive force (Hc ) were acquired at room
temperature. Fig. 7 shows that Mr and Hc of NiS flower-like architectures were 0.00405 emu/g and 102.14 Oe (the average value
of 122.39 Oe and 81.89 Oe), respectively. These results show that
the as-obtained material has some weak ferromagnetic interactions, due to the strong interaction between adjoining electrons.
Fig. 4. SEM images of samples obtained at the synthesis time of (a) 4 h, (b)-(d) 8 h, (e) 16 h, and (f) 24 h.
H. Zhou et al. / Particuology 10 (2012) 783–788
787
Fig. 5. XRD pattern of samples obtained at the synthetic time of (a) 4 h, (b) 8 h, (c)
16 h, and (d) 24 h.
This is different from the magnetic behavior of bulk NiS (Sparks &
Komoto, 1968) though similar to that of ␣-NiS reported by Zhang,
Wu, and Chen (2005) of remanent magnetization of 0.00012 emu/g
and coercivity of 155 Oe, that of ␣-NiS obtained by Salavati-Niasari,
Davar, and Mazaheri (2009) of Mr as 0.0011 emu/g and coercivity
of 154 Oe, and that of NiS reported by Cao et al. (2010) of Mr as
0.006 emu/g and coercivity of 178 Oe. It can be seen that the weak
ferromagnetic interactions of our NiS flower-like architectures is
slightly higher than those reported both by Zhang et al. and by
Salavati-Niasari et al., and lower than those of Cao et al. Furthermore, the magnetic hystersis loop of the as-obtained sample was
not symmetrical due to exchange bias (Kiwi, 2001), that is to say,
the NiS flower-like architectures with anisotropy might be due to
an antiferromagnetic phase with paramagnetic contribution from
the surface (Hansen & Morup, 1988; Kodama, 1999). The origin of
the ferromagnetic property may be attributed to the size confinement effect of materials (Wang & Zhang, 1994, chap. 3). The size of
NiS flower-like architectures, composed of nanorods or nanopetals,
was within the limit of nanoscale which could greatly contribute
Fig. 7. Magnetization curves of NiS flower-like architectures. Inset (bottom right):
enlargement at low field strength.
to the magnetic properties of the materials. In other words, the
magnetic behavior of the as-obtained NiS flower-like architectures
might arise from finite-size effect (Kodama, Makhlouf, & Berkowitz,
1997).
4. Conclusions
In summary, NiS flower-like architectures, having a multilayered and highly ordered texture, have been successfully
synthesized by a one-step hydrothermal method. On the basis of
condition-dependent experiments, diffusion-limited aggregation
(DLA) model and cage effect were applied to explain the growth
process of NiS flower-like architectures. The results of magnetic
measurements showed that the as-obtained flower-like architectures NiS displayed magnetic behavior.
Fig. 6. Proposed growth mechanism of as-prepared NiS flower-like architectures.
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H. Zhou et al. / Particuology 10 (2012) 783–788
Acknowledgements
We are grateful for grants from the National Natural Science
Foundation of China (No. 21003147), Natural Science Foundation
of Shanxi (2011011007-3), Distinguished Young Scholar Project
of Institute of Coal Chemistry, Chinese Academy of Sciences
(2011SJCRC07) and the State Key Laboratory of Coal Conversion
(SKLCC) in-house project (No. 2011BWZ005).
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