A novel chemical reduction route towards the synthesis of crystalline nickel nanoflowers from a mixed source

Article abstract of DOI:10.1002/ejic.200500453

A novel chemical reduction route was designed for the synthesis of nickel nanocrystals with distinct flowery shapes by using a mixture of two complexes of Ni(N₂H₄)₃²⁺ and Ni(dmg)₂ (nickel dimethylglyoximate) as the nickel source. Formation of these flowers, comprising spherical centers and swordlike petals, was related to the varying stability and structural properties of the two precursors as well as the reaction conditions. Investigations of the time-dependent shape-evolution process indicated that spherical particles were formed first as flower centers, and then swordlike petals grew radially from the particle surfaces. A rational mechanism of formation was proposed on the basis of a range of contrasting experiments. Compared with bulk nickel, these nanoflowers exhibited an enhanced coercivity (M_s); and a decreased saturation magnetization (M _s); this fact is attributed to their peculiar morphology. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500453

FULL PAPER
DOI: 10.1002/ejic.200500453
A Novel Chemical Reduction Route towards the Synthesis of Crystalline Nickel
Nanoflowers from a Mixed Source
Xiaomin Ni,[a] Qingbiao Zhao,[a] Huagui Zheng,*[a] Beibei Li,[a] Jimei Song,[a]
Dongen Zhang,[a] and Xiaojun Zhang[a]
Keywords: Nickel / Nanostructures / Chemical reduction / Crystal growth
A novel chemical reduction route was designed for the syn-
formed first as flower centers, and then swordlike petals
grew radially from the particle surfaces. A rational mecha-
nism of formation was proposed on the basis of a range of
contrasting experiments. Compared with bulk nickel, these
nanoflowers exhibited an enhanced coercivity (Hc) and a de-
creased saturation magnetization (Ms); this fact is attributed
to their peculiar morphology.
thesis of nickel nanocrystals with distinct flowery shapes by
2+
using a mixture of two complexes of Ni(N2H4)3
and
Ni(dmg)2 (nickel dimethylglyoximate) as the nickel source.
Formation of these flowers, comprising spherical centers and
swordlike petals, was related to the varying stability and
structural properties of the two precursors as well as the reac-
tion conditions. Investigations of the time-dependent shape-
evolution process indicated that spherical particles were
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
In addition to the effect of templates, modification of
the reaction steps also has great influence on the crystal
configuration. Generally, the formation processes of crys-
tals include nucleation and growth, which is practically a
continuous process. However, in the two-step, seed-medi-
ated synthesis technique, the nucleation and growth are
purposely separated for adjusting the crystal morphology.
In the first step, the seed nanocrystals, whose characteristic
symmetry and structure are critical for directing the final
crystal shape, are synthesized and added to the synthetic
system. Then, precursors to the desired structures are chem-
ically reduced in the presence of capping molecules and
seed nanocrystals that serve as nucleation sites.[4] Following
this methodology, a series of different shaped nanocrystals
were fabricated. For example, uniform Au and Ag nano-
rods, dumbbell-shaped Au–Ag core-shell nanocrystals were
all created from the seed-mediated growth in solution.[5a–5c]
Oriented arrays of ZnO nanorods were produced from cit-
rate solution in the presence of nanosized ZnO seeds.[5d]
Likewise, using Ag nanoparticles as seeds, a peculiar netlike
Ag skeleton could be synthesized by reduction of silver ions
with Raney nickel.[5e] These results indicate this synthetic
approach has great potential for the anisotropic growth of
inorganic nanocrystals, but the interface issues between the
seed and growing materials have not been well resolved, and
further research is required for their general applicability
and extension to the synthesis of complex nanostruc-
tures.[4b]
Introduction
For many years, control of the morphology of nanoma-
terials has been of great interest for their shape-dependent
properties and potential applications in nanoelectronics,
photonics, magnetics, catalysis, and so forth.[1] Exploration
of various shape-controlling synthetic methods and studies
on their unusual properties will inevitably drive the progress
in nanotechnology. Templates such as macroporous mem-
branes, carbon nanotubes, aluminum anode oxides (AAO),
block copolymers, microemulsions, polymers, and complex-
ing agents are often utilized during synthesis to control the
shape of nanomaterials.[2] Among these templates, utilizing
coordination complexes as precursors has attracted particu-
lar attention for its simplicity and convenience. A range of
novel-shaped nanocrystals have been fabricated by this
method, for example Cu2O nanowires, cubic FeS2 crystal-
lites, and several different shaped ZnO nanostructures in-
cluding hollow spheres, ringlike nanosheets standing on
spindlelike rods, nanoparticles nanoribbons, flowerlike
cupped-end microrod bundles, branched spindles, and pris-
matic whiskers.[3] In this technique, the morphology of the
target product was closely associated with the structure and
shape of the complex precursors. Upon removal of the li-
gand molecules in a rational and controllable manner, inor-
ganic nanomaterials with the desired size and shape could
be synthesized.[3e]
Nanosized particles of nickel have diverse applications in
the fields of catalysis, magnet recording, medical diagnosis,
and conduction.[6] In the past decade, nickel nanocrystals
with the following shapes have been fabricated: nanotubes,
[a] Department of Chemistry, University of Science and Technol-
ogy of China,
Hefei, Anhui, 230026, People’s Republic of China
Fax: +86-551-3601600
E-mail: hgzheng@ustc.edu.cn
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Synthesis of Crystalline Nickel Nanoflowers from a Mixed Source
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hollow spheres, nanobelts, nanorods, nanoprisms, and hex- ure 2c. Besides the flowerlike nanostructures, it was noted
agonal flakes. However, the creation of nickel nanoflowers that a small fraction of hexagonal flakes (about 1%, as ar-
has seldom been reported.[7] In this paper, we have designed rowed in Figure 2a) emerged in the final product. Shown in
a new reaction route which combines coordination control Figure 2d is a TEM image of two overlapping flakes with a
with the step modulation technique and yields nickel nano- side length of about 100 nm. The corresponding selected
crystals with a novel flower shape. This strategy involves area electron diffraction (SAED) pattern indicated that they
the monitoring of crystal nucleation and growth through are also due to fcc nickel.
chemical reactions. In contrast to previous methods using
2+
a single-source precursor, two complexes, Ni(N2H4)3 and
Ni(dmg)2, comprise the nickel source. The reaction process
2+
involves two steps: firstly, Ni(N2H4)3 is reduced, and the
resulting spherical nanocrystals serve as the flower centers.
Then, Ni(dmg)2 yields swordlike nanocrystals which grow
radially on the existing spherical centers as petals. A series
of experiments were conducted to investigate the possible
formation process.
Results and Discussion
The phase and purity of the as-prepared product was
determined by X-ray diffraction (XRD), as shown in Fig-
ure 1. All the diffraction peaks could be indexed as face-
centered cubic (fcc) nickel (JCPDS 01–1260). No character-
istic peaks due to the impurities of nickel oxides or hydrox-
tion SEM image; (b) a high-magnification SEM image of an indi-
Figure 2. SEM images of as-prepared samples: (a) a low-magnifica-
ides were detected, indicating that pure crystalline nickel
was fabricated under such conditions.
vidual flower (the inset is a typical petal); TEM images of the sam-
ple: (c) several typical nickel flowers; (d) two overlapping hexagonal
flakes of fcc Ni.
More details of the flowerlike structure were investigated
by high resolution transmission electron microscopy
(HRTEM) and SAED. A typical sharp-ended petal (Fig-
ure 3a) with a diameter of about 20 nm was chosen as the
object of investigation. The HRTEM image in Figure 3b
shows three sets of lattice spacings of Ϸ 0.20, 0.18, and
0.12 nm that are in accordance with the separation between
the (111), (200), and (220) planes of fcc Ni, respectively.
From the HRTEM image, the growth direction of the petal
¯
could be indexed as along [011]. The corresponding ED
pattern in Figure 3c was recorded with the electron beam
along the [011] zone axis, revealing the single crystalline na-
ture of the petal. It was noted that in the SAED pattern
additional diffraction spots along the directions of [111]
and [200] appeared, which implied the presence of five su-
perlattices in both directions.
Figure 1. XRD pattern of as-prepared flowerlike nickel crystals.
The morphology of the sample was investigated by field
emission scanning electron microscopy (FE-SEM) and
transmission electron microscopy (TEM). A low-magnifica-
tion image, shown in Figure 2a, demonstrates that the prod-
uct exhibits novel flowerlike shapes. A magnified image of
an individual crystallite indicated that the flower comprised
dozens of swordlike petals radiating from the center, which
can be seen in Figure 2b. The petals with sharp ends and
protruding surface ridges have sizes in the range of 30–
80 nm in diameter, 20–50 nm in thickness, and 0.5–2.0 μm
in length. Sonication for 10 min did not break this nanos-
tructure into discrete particles, indicating that the nano-
flowers were actually integrated, and were not only aggrega-
tions of spherical and swordlike particles. TEM images of
the nanoflowers with petals assembled at the center, agree-
Figure 3. (a) TEM image of the tip of a typical petal. (b) HRTEM
image taken from the petal. (c) The corresponding SAED pattern,
taken from the [011] zone axis. The additional diffraction spots
along [111] and [200] implied the presence of five superlattices of
ing well with the FE-SEM observations, can be seen in Fig- nickel in these two directions.
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To probe into the growth process of the nanoflowers, we transfer onto the surfaces of the existing particles which
studied the shape evolution of the product over time by FE- served as seed sites for further growth. Because of the
SEM. Figure 4 shows the images of three samples hydro- higher stability of Ni(dmg)2, the reaction rate at this stage
thermally treated for 3 h, 7 h, and 11 h, respectively (Fig- slowed down, and the concentration of nickel atoms was
ure 4a, b, and c). It was shown that spherical nickel par- not enough for the former spherical particles to grow from
ticles with diameters of about 100 nm emerged as the initial the circumference. This would lead to undersaturation
product after the mixture was heated at 120 °C for 3 h. With around the existing particles, and the continuous addition
time, short rods grew out on these spherical particles, and of nickel atoms to the as-formed particle surface would
some particles developed into flowery crystals. After 11 h, preferentially occur at the active sites of the circumferential
most of the product had evolved into flower-shaped crystal- edges which had relatively higher free energies than other
lites with swordlike petals.
sites on the surface. As a result, a flowerlike structure was
formed because of the lack of mass transport of nickel
atoms to the seed particles for their further growth.[10] The
reduced particles grew around each seed particle until all
the Ni(dmg)2 was reduced, eventually resulting in radially
oriented petals on the surfaces. It is worth noting that the
growth of nickel nanocrystals from Ni(dmg)2 proceeded in
a different manner relative to that from Ni(N2H4)32+. As a
special planar complex, Ni(dmg)2 imposes obvious confine-
ment on the growth of the resultant particles, orienting
them along a certain direction.[7f] Therefore, anisotropic pe-
tals with swordlike shapes were produced. A schematic
pattern of the formation process is shown in Scheme 1.
However, it is interesting that the petals did not grow into
hexagonal flakes as before, but mainly into swordlike rods,
which may be attributed to the effect of the spherical nano-
crystals formed at the initial stage. Since the seed particles
greatly affected the shape of the crystals, the nickel nano-
crystals grown in the presence of spherical particles may
develop into a different shape.[11] In fact, if the spherical
nanoparticles were prepared first and then dispersed into
the system with Ni(dmg)2 and hydrazine, a similar flower-
like nanostructure (as seen in Figure 5c) could also be pro-
Figure 4. SEM images of the samples obtained at different reaction
stages. (a) 3 h, (b) 7 h, (c) 11 h.
On the basis of previous studies of nickel nanocrystals
from different complexes and the evolution of nickel flowers
described above, the possible mechanism of formation of
the nickel nanoflowers was detailed. It is known that Ni2+
ions can react with dimethylglyoxime (dmgH) and hydra-
zine (N2H4) in aqueous solution to form the complexes of
Ni(dmg)2 and Ni(N2H4)32+, respectively. In basic solution,
2+
the Ni(N2H4)3 (with an instability constant of 4.5×10–8)
can react with OH– in the following manner:
2+
2 Ni(N2H4)3 + 4 OH– Ǟ 2 Ni + N2 + 4 H2O + 5 N2H4
When the pH of the solution was above 12 and the tem-
perature was higher than 85 °C, the reaction proceeded very
fast and produced spherical particles.[8] Because of its
higher stability (with an instability constant of 4×10–24
)
and special planar square structure, Ni(dmg)2 reacted with
hydrazine at a relatively slow rate and yielded nickel nano-
crystals with a hexagonal flakelike shape.[9,7f] In our system,
about 60 mol-% Ni2+ ions were coordinated by dmgH and
the rest yielded Ni(N2H4)32+. These two complexes coex-
isted in one system and comprised the nickel source. How-
ever, because of their different stability and structure, they
were reduced at different reaction stages and resulted in dif-
ferently shaped particles. Ni(N2H4)32+ was reduced first and
yielded spherical particles, which were dispersed in the sys-
tem. As the reaction proceeded, Ni(dmg)2 was attacked,
Scheme 1. Schematic illustration of the formation process of the
nickel nanoflowers. (a) mixture of Ni(dmg)2 and Ni(N2H4)32+ com-
plexes in the system. (b) spherical nickel nanoparticles were formed
which came from the reduction of Ni(N2H4)3 (c) part of the
spherical nickel particles had evolved into small flowers with the
reduction of Ni(dmg)2. (d) nickel nanoflowers emerged as the final
2+
and the newly formed nickel atoms tended to spontaneously product.
2+
Figure 5. SEM and TEM images of the samples from the system with different molar ratios (ω) of Ni(N2H4)3 to Ni(dmg)2 (a) ω =
20:80 (b) ω = 80:20, (c) TEM image of the sample in which the spherical particles were added externally to the system.
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Synthesis of Crystalline Nickel Nanoflowers from a Mixed Source
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duced, which supported the two-step formation process
proposed above.
Following the formation of the nickel flowers, it was
rationally proposed that the size of the flower centers and
petals could be adjusted by changing the molar ratio (ω) of
2+
Ni(N2H4)3 to Ni(dmg)2, considering their different con-
tribution to the flowerlike structure. This supposition was
validated by the FE-SEM images of two samples obtained
at different ω values while the other reaction conditions
were kept the same. Figure 5a and b shows the images of
two samples from the systems with ω values of 20:80 and
80:20, respectively. It is clear that increasing the proportion
of Ni(dmg)2 obviously promoted the size of petals, while a
higher proportion of Ni(N2H4)32+ led to flowers with bigger
centers.
In all the samples, a small quantity of hexagonal nickel
flakes appeared regardless of the reaction time and the ratio
of the two Ni sources, as shown in Figure 2, Figure 4, and
Figure 5. It was believed that they originated solely from
the precursor of Ni(dmg)2, and not through the two-step
process observed above for the flowers. When Ni(dmg)2 is
the only source of nickel, hexagonal flakelike nickel crystals
can be produced exclusively. The current system, however,
became inhomogeneous because of flocculent Ni(dmg)2
suspended in the solution, which caused differences in the
local concentrations of the reactants. In places where the
concentration of hydrazine was relatively high, part of
Figure 6. TEM images of the samples prepared from the other
2+
mixed nickel sources. (a) Ni(NH3)62+-Ni(dmg)2; (b) Ni(N2H4)3
-
Ni(C4H2O6)2– (tartrate); (c) Ni(N2H4)32+- Ni(C9H7NO)2 (nickel 8-
2+
Ni(dmg)2 would be reduced simultaneously with Ni(N2H4)3
2+
hydroxyquinoline); (d) Ni(NH3)62+-Ni(en)3 (ethylenediamine).
and directly developed into flakes. Careful observations
indicated that small hexagonal flakes appeared even when
the mixture was only heated for 3 h (shown in Figure 4a),
which well validated our supposition.
measured at room temperature (Figure 7), showing coerciv-
ity (Hc), saturation magnetization (Ms), and remanent mag-
netization (Mr) values of approximately 173.2 Oe, 30.8
emug–1, and 9.9 emug–1, respectively. Compared with that
of the bulk nickel (100 Oe, 55 emug–1, 2.7 emug–1), the
Hc value was much enhanced. As ultrafine ferromagnetic
particles often exhibit enhanced coercivity relative to the
corresponding bulk material, the small size and shape an-
isotropy of the flowers may be responsible for the increased
Hc value.[12] However, it was noted that this value was much
lower than that of the 1D nanorods (332 Oe) or nanobelts
(640 Oe), which was possibly attributed to the special 3D
The present synthetic route monitored crystal nucleation
and growth through two matching chemical reactions in
one system. For the two complexes, their different stability
resulted in reduction reactions at different stages, and the
different structures resulted in particles with different
shapes. Cooperation of the two precursors was responsible
for the formation of the unique flowerlike nanostructure.
Controlled experiments show that, keeping the rest of the
conditions the same, substitution of the two complexes with
other pairs of Ni sources such as Ni(NH3)62+-Ni(dmg)2,
2+
Ni(N2H4)32+-Ni(C4H2O6)2–
(tartrate),
Ni(N2H4)3
-
-
2+
Ni(C9NOH7)2 (nickel 8-hydroxyquinoline), or Ni(NH3)6
Ni(en)32+ (ethylenediamine) did not all result in nanoflower
formation (as seen in Figure 6). This lack of nanoflower
formation is possibly due to a mismatch of the stability or
structure of the two starting materials, which idd not meet
the conditions required for the formation of the flowers.
The role of the temperature was also studied, and it was
found that the flowers could be reproducibly prepared over
an optimized temperature range of 100–150 °C. Tempera-
tures above 150 °C led to spherical particles due to a
faster reducing rate, while at temperatures below 100 °C,
Ni(dmg)2 was not completely reduced.
It is known that the magnetic properties of nanomateri-
als are closely related to the sample size, shape, crystallinity
Figure 7. Magnetic hysteresis loop of as-prepared nickel nanoflow-
etc. The M–H hysteresis loop of the flowers obtained was ers measured at room temperature.
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were collected and washed, and finally dried in vacuum at 60 °C
for 4 h.
flowerlike structure. Because of the radial orientation of the
petals, it was difficult for all of them to be aligned simulta-
neously along the direction of the external magnetic field
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Conclusions
Flower-shaped nickel nanocrystals were successfully pre-
2+
pared by reduction of the mixed complexes of Ni(N2H4)3
and Ni(dmg)2 in alkaline solution with hydrazine hydrate.
2+
Spherical nickel nanocrystals formed first from Ni(N2H4)3
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swordlike petals, which originated from the reduction of
Ni(dmg)2. The different stability and structure of the two
complexes played an important role in the formation of the
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and petals could be adjusted by changing the molar ratio
of the two complexes. Due to their unique flowery shapes,
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Experimental Section
X-ray diffraction (XRD) patterns of the samples were recorded
with a Philips XЈpert diffractometer with Cu-Kα radiation (λ =
0.15418 nm). Field emission scanning electron microscopy (FE-
SEM) images were recorded with a JEOL JSM-6300F SEM. Trans-
mission electron microscopy (TEM) images were collected with a
Hitachi, H-800 electron microscope with an accelerating voltage of
200 kV. High-resolution transmission electron microscopy
(HRTEM) images and the selected area electron diffraction
(SAED) patterns were recorded with a JEOL-2010 TEM at an ac-
celeration voltage of 200 kV. M–H hysteresis loops were recorded
with a vibrating sample magnetometer (BHV-55) at an applied field
of 104 Oe and a magnetization scale of 2.5 emu on a 0.0352-g sam-
ple sealed in a 6.6×10–2 mL vessel.
Nickel chloride (NiCl2·6H2O), dimethylglyoxime (dmgH), sodium
hydroxide (NaOH) were all of analytical purity. In a typical experi-
ment, NiCl2·6H2O (0.166 g) was dissolved in distilled water
(25 mL) to give a green transparent solution. Then an ethanol solu-
tion (13 mL) containing 1 wt.-% dmgH was added dropwise into
the solution. Red flocculates emerged, indicating the formation of
Ni(dmg)2. Subsequently, N2H4·H2O (2.0 mL, 80 wt.-%) was added
under continuous stirring. The solution pH was adjusted to above
12 using sodium alkali. The mixture was stirred for 30 min and
transferred into a Teflon-lined autoclave with a capacity of 50 mL.
The autoclave was sealed, heated at 110 °C for 12 h, and then co-
oled naturally to room temperature. The resulting black powders
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Received: May 19, 2005
Published Online: October 25, 2005
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