Synthesis conditions in tailoring morphology of reticular particles in porous nickel powders prepared by template-assisted polyol process

Article abstract of DOI:10.1016/j.jallcom.2009.08.001

Porous nickel powders having reticular structure were synthesized with polyvinylpyrrolidone (PVP) as a soft template through a polyol process. PVP addition brought structural change of nickel particles from agglomerates to interconnected particle network.

Full text of DOI:10.1016/j.jallcom.2009.08.001

Journal of Alloys and Compounds 487 (2009) L8–L11
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Synthesis conditions in tailoring morphology of reticular particles in porous
nickel powders prepared by template-assisted polyol process
∗
Joon Hwan Choi , Dong-Hyun Kim, Jae-Cheol Yoon, Yong-Jin Kim
Department of Powder Materials, Korea Institute of Materials Science, 531 Changwondaero, Changwon 641-831, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous nickel powders having reticular structure were synthesized with polyvinylpyrrolidone (PVP) as
a soft template through a polyol process. PVP addition brought structural change of nickel particles
from agglomerates to interconnected particle network. Surface morphology of the particles was changed
from prickly to smooth structure with increasing reaction temperature and was also dependent on PVP
polymerization degree. The reticular structure formed above a certain level of Ni concentration and neck
region of the interconnected particles became thicker at higher Ni concentration. The porosity of the
powders increased with increasing PVP content and was saturated at a certain amount of PVP, which is
required to obtain a maximum porosity of the powders.
Received 17 June 2008
Accepted 1 August 2009
Available online 8 August 2009
Keywords:
Chemical synthesis
Metals and alloys
Microstructure
©
2009 Elsevier B.V. All rights reserved.
Surface and interface
1
. Introduction
Particles with rough surface are suitable for catalytic applica-
tions because of their high surface area and powders with reticular
structure are preferable for applications requiring high porosity
such as electrodes of MCFC and batteries. The filamentary particles
having high aspect ratio are also a good candidate as filler materi-
als for conductive polymer/metal composites because it can render
high electrical conductivity by the addition of smaller fraction of the
materials than particles having low aspect ratio such as spherical
particles. Therefore, it is very important to control the morphology
and microstructure such as particle size, pore structure, porosity,
and surface morphology of the particles for practical applications.
In this work, we synthesized porous nickel powders having
three-dimensional network structure by a polyol process with
multi-functional ligand polyvinylpyrrolidone (PVP) as a template
and investigated the effects of reaction temperature, nickel con-
centration, PVP content, and polymerization degree of PVP on
morphology and surface structure of the nickel particles and poros-
ity of the powders. We found there are strong effects of the process
conditions on the particle morphologies and could synthesize fil-
amentary nickel powders having various surface structures and
microstructures of the particles. We also suggested a possible
explanation for the effect of PVP template on the reticular structure
formation of the nickel particles.
Porous materials have attracted much interest because of their
potential applications [1–3]. Especially, porous nickel powders
have been widely investigated because they provide a degree
of chemical activity such as a catalyst or adsorption surface for
molecular storage and engineering. Their excellent properties
lead to unique opportunities in multi-functional applications such
as electrodes for molten carbonate fuel cell (MCFC), recharge-
able lithium batteries, etc. [4]. Recently, high aspect, sub-micron
and nanoscale, filamentary nickel powders, known commercially
as NanostrandsTM, fabricated by atmospheric pressure chemical
vapor deposition process were introduced for many potential
applications including filler materials for electrical conductive
polymer/metal composites [5].
Generally, nickel powders can be prepared via vapor phase
method, spray pyrolysis, chemical solution routes, etc. [6–10].
Among these methods, the chemical solution process is a cost-
effective and promising approach for large-scale production and
control of structure and morphology of the particles [11,12].
Recently, some polyol processes have been used to prepare metal
powders with porous nanostructure [13,14]. However, there have
been few works on synthesis conditions in tailoring morphology of
particles in porous nickel powders prepared by template-assisted
polyol process.
2
.
Experimental
All chemicals were of analytical grade. Nickel chloride hexahydrate (NiCl2·6H2O)
and PVP (average molecular weight, Mw = 10,000, 55,000, and 1,300,000) were
received from Aldrich. 80% hydrazine monohydrate (N2H4·H2O; A.R.) solution and
ethylene glycol were purchased from Junsei chemical. The water used throughout
this study was deionized with purification system. The procedure for synthesis of
the nickel powders is as follows: ethylene glycol (300 mL) was heated to reaction
∗ Corresponding author. Tel.: +82 55 280 3573; fax: +82 55 280 3392.
E-mail address: jchoi@kims.re.kr (J.H. Choi).
0
925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.08.001

J.H. Choi et al. / Journal of Alloys and Compounds 487 (2009) L8–L11
L9
Fig. 1. X-ray diffraction patterns of nickel powders prepared at different reaction
◦
◦
◦
temperatures with 0.1 M PVP (Mw = 55,000): (a) 100 C, (b) 135 C, (c) 165 C, and (d)
◦
1
90 C.
◦
temperature (100–190 C) and maintained for 10 min at the temperature. Hydrazine
monohydrate (60 mL) adjusted to 50 vol.% was added slowly to the ethylene glycol
and maintained for 10 min. Nickel chloride hexahydrate solution (150 mL) having
molar concentration of 0.05–1.0 M and PVP solution (300 mL) having molar concen-
tration of 0–0.5 M were mixed and added to the solution under magnetic stirring.
Following reaction for 2 h, the solution with precipitates was centrifuged and rinsed
with ethanol more than five times repeatedly. The residual ethanol was removed in
◦
vacuum oven maintained at 60 C. X-ray diffraction (XRD) analysis of the samples
was conducted using a Rigaku diffractometer with Ni-filtered Cu-K␣ radiation. The
morphology and microstructure of Ni particles were characterized by field emission
scanning electron microscopy (FESEM; JEOL JSM-5400). The porosity of the powders
was measured by mercury intrusion porosimeter (AutoPore IV 9510, Micromeritics).
3
. Results and discussion
Fig. 1 shows XRD patterns of the Ni powders prepared at
different reaction temperatures with Ni and PVP (Mw = 55,000) con-
centrations of 0.1 M. The XRD results represent a typical pattern of
nickel crystal without any other peak from nickel oxides or precur-
sor compounds and the peak intensity increased with increasing
reaction temperature. This seems to be due to the effect of reac-
tion temperature on the extent of crystallinity and grain size of the
synthesized powders.
Fig. 2 shows SEM microstructures of the Ni powders and Fig. 2e
and f are the magnified images of the particles synthesized at
◦
◦
1
00 C and 190 C showing the surface morphology of the particles,
◦
respectively. The nickel particles obtained at 100 C had irregular
particle shape with the prickly surface morphology. Nickel pow-
ders obtained at 190 C, however, showed spherical particles with
◦
smooth particle surface.
The effect of Ni concentration on the morphology and pore
structure of the porous nickel powders was also investigated.
Figs. 3 and 2f show the shape change of the nickel particles syn-
◦
thesized with different Ni concentrations at 190 C. The particles in
the 0.05 M Ni sample did not form fully three-dimensional network,
but interconnected structure mostly having one-dimensionally
assembled particles (Fig. 3a). This means 0.05 M is not enough Ni
concentration to generate three-dimensional reticulation of the
particles. When increasing the Ni molar concentration to 0.1 M,
the reticular structure started to form with weakly connected neck
between the particles (Fig. 2f). With increasing the Ni concentra-
tion, the neck region became thicker and the neck of the particles
was almost disappeared at the Ni concentration of 1.0 M (Fig. 3c).
At high Ni concentration, residual Ni2 ions can move to the neck
region following the network formation of Ni particles because nor-
mally the neck is energetically favorable site for nucleation and
growth of the particles.
Fig. 2. SEM micrographs of Ni powders prepared at different reaction temperatures
◦
◦
◦
with 0.1 M PVP (Mw = 55,000): (a and e) 100 C, (b) 135 C, (c) 165 C, and (d and f)
◦
1
90 C.
Effects of polymerization degree of PVP on particle morphol-
+
ogy are shown in Figs. 4, 2d, and 2f. The powders were synthesized
◦
at 190 C using Ni concentration of 0.1 M. The powders prepared
using PVP with lower polymerization degree (Mw = 10,000 and
55,000) showed smooth particle surface with similar microstruc-

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J.H. Choi et al. / Journal of Alloys and Compounds 487 (2009) L8–L11
Fig. 3. SEM micrographs of Ni powders prepared with different Ni concentrations
with 0.1 M PVP (Mw = 55,000): (a) 0.05 M, (b) 0.5 M, and (c) 1.0 M.
Fig. 4. SEM micrographs of Ni powders prepared with 0.1 M PVP having different
polymerization degree: (a and c) Mw = 10,000 and (b and d) Mw = 1,300,000.
tures (Figs. 4c and 2f). The particles synthesized using PVP with
higher polymerization degree (Mw = 1,300,000), however, showed
rough surface with agglomeration of small particles (Fig. 4d).
Fig. 5 is an SEM microstructure of the Ni powder synthesized
without the PVP template showing severe agglomeration of spher-
ical particles. The Ni concentration and reaction temperature were
◦
0
.1 M and 190 C, respectively. The size of the individual particle
in Fig. 5 was similar to that in the powder synthesized with PVP
shown in Fig. 2f. The powders without PVP, however, did not have
any reticular structure of the particles. This indicates the PVP as a
soft template strongly affects the formation of three-dimensional
network structure.
Nickel nanoparticles are known to have a tendency to connect
to each other by their magnetic dipole interaction [15]. There-
fore, it is not easy to prepare isolated magnetic nanoparticles
and the particles can be agglomerated easily (Fig. 5). The sur-
face atoms of metal particles can coordinate with oxygen atoms
in PVP, which means PVP can act as a soft template to form one-
dimensional nanomaterials such as nanowires and nanorods. Gao
Fig. 5. SEM micrograph of Ni powder prepared without PVP addition.

J.H. Choi et al. / Journal of Alloys and Compounds 487 (2009) L8–L11
L11
soft template. The addition of the PVP template in the process
brought structural change of the particles from particle agglom-
erates to interconnected particle networks. The morphology of
the porous Ni powders was strongly dependent on the reaction
temperature, Ni concentration, and polymerization degree of PVP.
◦
The nickel powder obtained at 100 C had irregular particle shape
with the prickly surface morphology and the powder obtained at
◦
1
90 C showed interconnected structure of spherical particles with
smooth particle surface. The reticular structure started to form with
weakly connected neck at Ni molar concentration of 0.1 M. With
increasing the Ni concentration, the neck region became thicker
and the neck of the particles was almost disappeared at the Ni
concentration of 1.0 M. The particles using PVP with higher poly-
merization degree (Mw = 1,300,000) showed rougher surface with
agglomeration of small particles than those with lower polymeriza-
tion degree (Mw = 10,000 and 55,000). The porosity increased with
increasing PVP content and was saturated at a certain amount of
PVP (0.1 M in this study) which is required to obtain a maximum
porosity of the powders.
Fig. 6. Porosity of Ni powders prepared with different PVP (Mw = 55,000) concen-
trations.
et al. reported non-magnetic silver nanowires with linear shape
could be synthesized through polyol process in the presence of
PVP [16]. Therefore, the difference between the microstructures in
Figs. 2fand 5 isthoughttobe ascribedtothe combiningeffects ofthe
magnetic interaction of nickel particles and preferential nucleation
and growth with soft template. Thus, the nickel particles synthe-
sized with PVP did not grow into one-dimensional linear structure,
but formed three-dimensional network probably because of the
combining effects.
Acknowledgement
This work was supported by a grant funded by Korea Institute
of Materials Science through the Fundamental Research Program.
References
[
[
1] A.H. Lu, F. Schuth, Adv. Mater. 18 (2006) 1832–1837.
2] J.W. Lee, J.Y. Kim, T.H. Hyeon, Adv. Mater. 18 (2006) 2073–2094.
As mentioned above, PVP plays an important role to make the
reticular structure of the nickel particles. Therefore, the effect of
PVP contents was also investigated. Fig. 6 shows porosity change of
the powders prepared with different PVP concentrations (0–0.5 M)
[3] O.D. Velev, A.M. Lenhoff, Curr. Opin. Colloid Interface Sci. 5 (2000) 56–63.
[4] E. Simonetti, R.L. Presti, J. Power Sources 160 (2006) 816–820.
[
[
5] G. Hansen, SAMPE J. 41 (2005) 2–11.
6] J.H. Hwang, V.P. Dravid, M.H. Teng, J.J. Host, B.R. Elliot, D.L. Johnson, et al., J. Am.
Ceram. Soc. 88 (2005) 303–307.
◦
using 0.1 M Ni at 190 C. The powder synthesized without PVP had
[7] S. Stopic, J. Nedeljkovic, S. Rakocevic, D. Uskokovic, J. Mater. Res. 14 (1999)
059–3065.
3
very little pore volume in good agreement with the SEM results
shown in Fig. 5. The porosity increased with adding more PVP and
was saturated at 0.1 M. A certain amount of the template material,
therefore, is required to obtain a maximum porosity of the powders.
[
[
8] F. Fievet, J.P. Lagier, M. Figlarz, MRS Bull. 14 (1989) 29–33.
9] Y.D. Li, C.W. Li, H.R. Wang, L.Q. Li, Y.T. Qian, Mater. Chem. Phys. 59 (1999) 88–90.
[10] A. Degan, J. Maeek, Nanostruct. Mater. 12 (1999) 225–228.
11] J.Y. Choi, Y.K. Lee, S.M. Yoon, H.C. Lee, B.K. Kim, J.M. Kim, et al., J. Am. Ceram.
Soc. 88 (2005) 3020–3023.
[
[
12] D.H. Chen, S.H. Wu, Chem. Mater. 12 (2000) 1354–1360.
4
. Conclusions
[13] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426.
[
[
[
14] J. Bao, Y. Liang, Z. Xu, L. Si, Adv. Mater. 15 (2003) 1832–1835.
15] D. Li, S. Komarneni, J. Am. Ceram. Soc. 89 (2006) 1510–1517.
16] Y. Gao, P. Jiang, D.F. Liu, H.J. Yuan, X.Q. Yan, Z.P. Zhou, et al., J. Phys. Chem. B 108
(2004) 12877–12881.
The porous Ni powders with reticular structure were synthe-
sized by the polyol process with polyvinylpyrrolidone (PVP) as a