Synthesis and magnetic properties of nickel nanocrystals

Article abstract of DOI:10.1016/j.materresbull.2005.09.019

Size-controlled synthesis of nickel nanocrystals by the decomposition of nickel acetylacetone in a noncoordinating reagent, oleylamine, was studied. Three processes were employed to synthesize nanocrystals with diameters in the range of 20-60 nm. The resultant nanoparticles were characterized to be single-phase nanocrystalline nickel with a face-centered cubic type. Mechanism of the size-controlled nanocrystals formation was studied and found that relative monodispersed nanocrystals could be formed by the separation of nucleation and growth process while the size could be controlled through modulating growth period. Magnetic measurements showed that samples were still ferromagnetic, and that their saturation magnetization and coercivity are size-dependent.

Full text of DOI:10.1016/j.materresbull.2005.09.019

Materials Research Bulletin 41 (2006) 495–501
www.elsevier.com/locate/matresbu
Synthesis and magnetic properties of nickel nanocrystals
b
a
H.T. Zhang , G. Wu , X.H. Chen , X.G. Qiu
a
a,
*
a
Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics,
University of Science and Technology of China, Hefei, Anhui 230026, China
b
National Laboratory for Superconductivity, Institute of Physics, Center for Condensed Matter Physics,
Chinese Academy of Sciences, Beijing 100080, China
Received 20 May 2005; received in revised form 15 September 2005; accepted 23 September 2005
Available online 7 October 2005
Abstract
Size-controlled synthesis of nickel nanocrystals by the decomposition of nickel acetylacetone in a noncoordinating reagent,
oleylamine, was studied. Three processes were employed to synthesize nanocrystals with diameters in the range of 20–60 nm. The
resultant nanoparticles were characterized to be single-phase nanocrystalline nickel with a face-centered cubic type. Mechanism of
the size-controlled nanocrystals formation was studied and found that relative monodispersed nanocrystals could be formed by the
separation of nucleation and growth process while the size could be controlled through modulating growth period. Magnetic
measurements showed that samples were still ferromagnetic, and that their saturation magnetization and coercivity are size-
dependent.
#
2005 Elsevier Ltd. All rights reserved.
PACS: 61.46; 75.50.Cc; 81.10.Dn; 87.64.Dz
Keyword: C. Electron microscopy; D. Magnetic properties
1
. Introduction
Much work has been done on the synthesis and characterization of nanoscale magnetic metal nanocrystals due to
their unusual chemical and physical properties, and technological applications [1–7]. Magnetic nanoparticles are
being used widely in conducting paints [8], rechargeable batteries [9–10], chemical catalysts [11], optoelectronics
[
12], magnetic recording media [13], ferro-fluids [14], magnetic resonance imaging contrast enhancement and drug
delivery [15–17], and so on. To cater to the requirement of magnetic nanocrystals with tuning properties, a great many
methods have been developed to synthesize particles with controlled size and shape. The most popular methods are
photolytic reduction, [18] radiolytic reduction [19], sonochemical method [20], solvent extraction reduction [21],
microemulsion technique [22–23], polyol process [24], and alcohol reduction [25]. Recently, the use of organometallic
precursors has recently been proven interesting since it leads to particles with controlled size, surface coordination,
and crystallinity that are monodisperse to Æ1 atomic shell [26].
Among the transition metal magnetic nanoparticles, nickel nanocrystals are difficult to be prepared because they
are easily oxidized. To obtain pure nickel nanocrystals, many methods were performed in organic media to avoid the
*
Corresponding author. Tel.: +86 551 3601654; fax: +86 551 3601654.
E-mail address: chenxh@ustc.edu.cn (X.H. Chen).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.09.019

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H.T. Zhang et al. / Materials Research Bulletin 41 (2006) 495–501
formation of oxide or hydroxide. Organometallic precursors, such as Ni(CO) and Ni(Cp) , have been used for the
4
2
synthesis of nickel nanoparticles [27], and Ni(COD) was also used to synthesize nickel particles by spontaneous
2
decomposition in CH Cl in the presence of poly(vinylpyrrolidone) [28,29]. More interesting, trigonal nickel
2
2
nanoparticles could be prepared through the reaction of Ni(COD) in tetrahydrofuran with tetra-n-octylammonium
2
carboxylates as reducing and stabilizing [30]. Especially the newly developed high-temperature solution-phase
method, which can form particle samples that are uniform in size, composition, shape, internal structure, and surface
chemistry, allowing the properties of nanoscale magnetic materials to be mapped [26]. The nickel nanocrystals with
diameters of 20–100 nm were rarely synthesized uptil now.
Here we report one simple route to synthesize size-controlled nickel nanocrystals with diameters in the range of 20–
0 nm through decomposition of nickel acetylacetone in oleylamine (OA), which is modified compared to the method
6
in synthesizing of Ni@Pd core-shell nanoparticles [31]. Particle size was modulated through controlling the
nucleation and growth stages.
2
. Experimental
Organometallic precursor, Ni(acac) , was prepared following the method reported previously [32]. The
2
nanocrystals were formed through three different processes: direct thermolysis, seed-assisted and hot injection.
Samples a and b were obtained by the thermolysis of Ni(acac) solved in OA at 215 8C for 30 min and 240 8C for
2
1
0 min, respectively. Sample c was formed in a seed-assisted synthesis procedure as reported in the reference [33]; a
solution including 0.5 mmol Ni(acac) solved in 40 mL OA was heated at 220 8C in oil bath under drastically stirring
2
for 20 min in a N atmosphere. The reaction mixture turned black, resulting from the nucleation and growth of nickel
nanocrystals. Then the solution was removed from oil bath and cooled to room temperature naturally. Four millimoles
2
of Ni(acac) was added into the cooled solution, and maintained the solution at 195 8C for 3 h. Sample d was obtained
2
in a hot injection process, 2 mmol Ni(acac) dissolved in 10 mL OA at 100 8C was injected into 30 mL OA at 220 8C
2
under flowing N gas, then the solution was heated rapidly to 220 8C and maintained for 20 min. All nanocrystals were
2
separated upon the addition of equal volume alcohol, centrifuged, and washed using a mixture of alcohol and
dichloromethane solvent. The offspring H O vapor was confirmed by unhydrated CuSO .
2
4
Transmission electron microscope (TEM) and selected-area electron diffraction (SAED) studies were conducted
with a Hitachi H-800 instrument. Products were characterized by a Rigaku D/max-A X-ray diffractometer (XRD) with
graphite monochromatized Cu Ka radiation in the 2u range of 20–808 with the step of 0.028 at room temperature. The
magnetic measurements were carried out with a superconducting quantum interference device SQUID magnetometer
(
MPMS-5).
3
. Results and discussion
Typical TEM images for nickel nanocrystals formed at different reaction conditions are shown in Fig. 1. Fig. 1A
and B show the TEM images of samples a and b formed through direct thermolysis of organometallic precursors,
Ni(acac) , at 215 8C and 240 8C, respectively. Their mean diameters are 22 nm and 35 nm, and standard deviation is
2
2
8% and 20%, respectively, indicating that the mean diameter of nanocrystals increases with increasing reaction
temperature and standard deviation decreases with increasing reaction temperature. To get monodispersed
nanocrystals, seed-assisted and hot injection processes were also employed. Fig. 1C shows the TEM image of nickel
sample c formed in a seed-assisted process as described in Section 2. The nanocrystals have mean diameters of 46 nm
and standard deviation of 15%. Fig. 1D shows nanocrystals d formed by a hot injection process, which have a mean
diameter of 59 nm and standard deviation of 20%.
Typical XRD pattern of sample c is shown in Fig. 2. Three characteristic peaks (2u = 44.58, 51.9, 76.54) for face-
centered cubic (fcc) nickel (JCPDS, no. 03–1051) marked by their indices (1 1 1), (2 0 0) and (2 2 2) were observed.
This revealed that the resultant nanocrystals were pure nickel. The diameter of the nanocrystals calculated by the
Scherrer formula is 43 nm on the base of (1 1 1) peak, the result is in good agreement with the TEM results. In the same
way, calculated diameters of nanocrystals formed in the other processes are consistent with the observed results.
Accordingly, it could be concluded that the nanocrystals prepared in this work were of pure nickel with fcc structure.
To understand the mechanism of nanocrystal formation, we obtained TEM images on samples after heating at
various temperatures and for various times in different reaction processes. After aging at 180 8C for 3 h, no

H.T. Zhang et al. / Materials Research Bulletin 41 (2006) 495–501
497
Fig. 1. TEM images of nanocrystals formed in different reaction processes: (A) sample a obtained through direct thermolysis of precursor at 215 8C
for 30 min; (B) sample b obtained through direct thermolysis of precursor at 240 8C for 10 min; (C) sample c obtained in the seed-assisted process
aging at 200 8C for 3 h; (D) sample d formed by aging 240 8C for 20 min in a hot injection process.
nanoparticles were formed, while aging at 180 8C for 4 h produced highly polydispersed nanoparticles with diameters
ranging from 10 nm to 80 nm, as shown in Fig. 3. The TEM image of samples, formed at 215 8C for 30 min, showed
that polydispersed nanocrystals with mean diameter of 22 nm were formed. Aging at 240 8C for 10 min resulted in
nanocrystals with mean diameter of 35 nm and a narrower size distribution as compared with those formed at 215 8C,
narrower sized distribution of nanocrystals with a mean diameter of 46 nm were formed in the seed-assisted process.
Larger nanocrystals with a mean diameter of 59 nm were formed in hot injected process with aging at 240 8C for
2
0 min.
From the TEM measurement, we can propose the following formation mechanism for the nanocrystals. Nucleation
occurs at about 180 8C triggered by the dissociation of acetylacetone from the Ni(acac) precursor with H O
2
2
elimination. The major growth stage occurs at about ꢀ215 8C or higher. Consequently, we were able to synthesize a

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H.T. Zhang et al. / Materials Research Bulletin 41 (2006) 495–501
Fig. 2. XRD pattern of sample c formed in the seed-assisted process.
relatively narrow-sized distribution of nanocrystals by the separation of nucleation and growth stages, which tends to
take place at different temperatures.
Nickel is known to be one of the most important magnetic material. To further investigate the magnetic properties
of nanocrystals, magnetic measurements on three samples (a,c,d), which were synthesized at different processes, were
carried out. Magnetization versus magnetic field (M–H loops) at 5 K is shown in Fig. 4. All the three samples show
hysteresis behavior, revealing that the nanocrystals are ferromagnetic. The saturation magnetization (M ), remanent
s
magnetization (M ), and coercivity (H ) for all samples are listed in Table 1. It is easily seen that the M , M and H are
c
r
c
s
r
size-dependent. The saturation magnetization (M ) of sample d determined at 5 K in a field of 1 T is very close to that
s
of bulk nickel (57.6 emu/g) [34], indicating the metallic characteristic of those nanocrystals. The relatively lower
saturation magnetization of 28.68 emu/g and 47.84 emu/g for samples a and c, respectively, could be due to the
smaller particle size and the accompanied increase in specific surface area. The saturation magnetization (M ) of
s
Fig. 3. TEM image of product formed at 180 8C for 4 h.

H.T. Zhang et al. / Materials Research Bulletin 41 (2006) 495–501
499
Fig. 4. Magnetization versus magnetic field for samples formed at different processes at 5 K.
samples a and c was reduced to 50% and 83% of that for the bulk nickel, respectively [34]. The decrease of the
saturation magnetization with decreasing the nanoparticles size is consistent with the previous report [35]. It is well
known that the thermal fluctuations will significantly reduce the total magnetic moment at a given field when a
magnetic nanoparticle’s energy becomes comparable to the thermal energy [36]. In addition to thermal fluctuation, the
disordered structure at interfaces provides less magnetic moment per unit mass than that of ferromagnetic core regions,
which also leads to a decrease in the M [37]. Furthermore, the magnetic molecules’ incomplete coordination and
s
disordered spins, and electron exchange between ligand and surface atoms also quench the magnetic moment [36].
Accordingly, the saturation magnetization (M ) of all samples is lower than that of the corresponding bulk materials,
s
especially for the sample a, which has the least diameter of 22 nm. It has been widely reported that M decreases with
s
decreasing particle size [38]. In order to understand that the saturation magnetization decreases with decreasing the
size in proportion to the specific surface area of the particles, a magnetically dead layer theory has been developed
[
39]. In the dead layer theory, the saturation magnetization for nanoparticles follows the formula:
M = M (bulk)(1 À 6t/d), where t is the thickness of magnetically inactive layer and d is the diameter of
s
s
nanoparticles [39]. The data in Table 1 can be fitted by the formula. It is found that the dead layer thickness t increases
with decreasing particle size, and t is 1.83 nm, 1.30 nm and 0.76 nm for the samples a, c, and d, respectively. As the
surface-to-volume ratio increases with decreasing nanoparticle size, the magnetically inactive layers fraction increases
with decreasing particles size too. Therefore, the M of the nickel nanoparticles decreases with decreasing nanoparticle
s
diameter.
As reported previously [40–43], the magnetic saturation and coercivity values of nanoscale materials were
significantly scattered for the nickel samples formed under different methods. As listed in Table 1, the M and M of
current nanocrystals increase with the increasing particle size, while H decreases with the increasing particle size. It
s
r
c
has been reported that H increases with decreasing particle size, then it reaches a maximum value at the critical size
c
for single domain, and decreases with further decreasing of the size [35]. That is, H has a peak around critical
c
diameter for transition from multidomain to a single domain particle [44]. However, the critical size of the nickel,
2
1.2 nm, has been calculated by the magnetic domain theory [35]. It indicates that the particle size for the all current
Table 1
s r c
Saturation magnetization (M ), remanent magnetization (M ), coercivity (H ) and magnetically inactive layer thickness (t) for three samples
synthesized at different processes
Sample
Diameter (nm)
M
s
(emu/g)
M
r
(emu/g)
H
c
(Oe)
t (nm)
a
c
d
22
46
59
28.68
47.84
53.31
10.66
16.29
19.74
352
340
280
1.83
1.30
0.76

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H.T. Zhang et al. / Materials Research Bulletin 41 (2006) 495–501
samples is larger than the critical size. It is expected that the H should increase with decreasing the nanoparticle size,
c
which is consistent with the results listed in Table 1. It should be pointed out that the formation of oxide layers (such as
NiO) cannot be avoided due to the thermodynamic instability although the nanocrystals are coated with OA [35].
Therefore, the H is retained high as the oxidation layer induces exchange coupling between ferromagnetic Ni and
c
antiferromagnetic NiO [41,43]. Smaller nanocrystals have more of an oxidation layer because of their higher surface-
to-volume ratios, so that the H increases with the decreasing particle size. This is why the dead layer thickness t
c
increases with the decreasing particle size, as listed in Table 1. As analyzed above, the nanocrystals’ diameters are just
equal to and larger than the theoretical single domain size. Therefore, the H of nanocrystals decreases with increasing
particle size.
c
4
. Conclusion
Size-controlled synthesis of nickel nanocrystals has been achieved by the decomposition of Ni(acac) in oleylamine
2
under flowing nitrogen gas through three different processes: direct thermolysis, hot injection, and seed-assisted
process. The resultant nanocrystals have been confirmed by XRD and TEM analysis as being single-phase
nanocrystalline nickel with of the fcc type. Nickel nanocrystals (20–60 nm) with a narrow size distribution could be
attained by the separation of nucleation and growth stages. The nickel nanoparticles with different sizes would be
applicable in many technical areas. Magnetic measurement revealed that smaller nanocrystals had higher coercivity
and smaller saturation magnetization, reflecting the size effect of nanocrystals. Using metal acetylacetone as
precursors to prepare nanocrystalline metal could be applicable to the other transition metals, such as cobalt.
Acknowledgements
This work was supported by the National Science Foundation of China, and the Knowledge Innovation Project of
Chinese Academy of Sciences.
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