Submicron particles of Co, Ni and Co-Ni alloys

Abstract

Magnetic sub-micron sized particles (with diameters in the range 100–600 nm) of Co, Ni and Co-Ni alloys, protected with polyvinylpyrrolidone
have been prepared in gram quantities using the polyol process. Experiments carried out with different metal precursors and
starting compositions have yielded reliable routes to produce particles of the desired diameters in the 100–600 nm range.
The particles were characterized with X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, thermogravimetric
analysis and magnetic measurements. The particles are found to be stable under ambient conditions indefinitely. The coercivity
values of the Co and Ni particles are ∼50% higher compared to the corresponding bulk values. The alloy particles follow a
trend similar to the bulk alloys.

Full text

Page 1

Bull. Mater. Sci., Vol. 24, No. 5, October 2001, pp. 515–521. © Indian Academy of Sciences.
515
Submicron particles of Co, Ni and Co–Ni alloys
P SARAVANAN, T A JOSE, P JOHN THOMAS and G U KULKARNI*
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore 560 064, India

MS received 8 August 2001

Abstract. Magnetic sub-micron sized particles (with diameters in the range 100–600 nm) of Co, Ni and Co–Ni
alloys, protected with polyvinylpyrrolidone have been prepared in gram quantities using the polyol process.
Experiments carried out with different metal precursors and starting compositions have yielded reliable routes
to produce particles of the desired diameters in the 100–600 nm range. The particles were characterized with
X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis
and magnetic measurements. The particles are found to be stable under ambient conditions indefinitely. The
coercivity values of the Co and Ni particles are ~ 50% higher compared to the corresponding bulk values.
The alloy particles follow a trend similar to the bulk alloys.

Keywords. Submicron particles; Co; Ni; Co–Ni alloys.


1. Introduction

Metal particles with size in the range of few nanometers
to several microns occupy the central place in catalysis,
surface coating, microelectronics and magnetic storage
and allied fields (Ferrier et al 1985; Zenia 1986). The
geometric effect of increased surface to volume ratio
imparts such attributes to the particle system that its vari-
ous properties undergo major changes at some critical
sizes (Edwards et al 2000). Thus, metal to nonmetal transi-
tion is seen in metal particles with diameters < 2–3 nm
(Vinod et al 1998), while a change in the magnetic
property is often seen at larger sizes, say several microns
(Cullity 1972). Moreover, the properties of a tiny particle
are affected by the ligand or the surfactant shell protecting
it (Bradley 1994; Osuna et al 1996), an issue which, so far
has not been understood in great detail.
Large-scale synthesis of metal particles is invariably
carried out using chemical routes. Of the many proce-
dures, the polyol process (Fievet et al 1989; Seshadri and
Rao 1994; Ayyappan et al 1996), involving the reduction
of metal salts with polyols such as ethylene glycol, di-
ethylene glycol, has been in wide use to obtain metal
particles, especially of large diameters (10 nm–1 micron).
Thus, particles of Co, Ni, Ag, Au and Pd (Ganapathi et al
1987; Silvert and Tekaia-Elhsissen 1995; Ayyappan and
Rao 1996; Ayyappan et al 1996, 1997; Tekaia-Elhsissen
et al 1999) and of alloys, Pd–Ni, Fe–Ni (Viau et al
1996a), Co–Ni (Viau et al 1996b) have been synthesized.
Uniform, spherical non-agglomerated particles with bulk
structures were obtained in the above cases. The magnetic
nanoparticles Co, Ni, Co–Ni, Co–Fe have been a focus of
attention due to their microwave absorption properties
(Viau et al 1994) and their potential uses as radiation
shields (electromagnetic absorbers).
We considered that a polymer coating such as with
polyvinylpyrrolidone (PVP) may improve the stability of
particles with respect to atmospheric oxygen and facilitate
easy handling of metal powders and accordingly, prepared
particles of Co, Ni and their alloys in the sub-micron
range. The particles were characterized using scanning
electron microscopy (SEM), energy dispersive X-ray
analysis (EDAX), powder X-ray diffraction (XRD) and
thermogravimetry (TGA). The magnetic properties of the
particles were examined using a vibrating sample magneto-
meter (VSM). It may be noted that Co and Ni form a solid
solution in the bulk, CoxNi1–x. For x < 0⋅75, the alloy
exists in hcp structure, while for x > 0⋅7, the alloy has a
fcc structure, with a mixed regime in the intermediate
compositions. The atomic moments in Co–Ni alloys
increase linearly with Ni content (Tebble and Craik
1969). We have found a similar trend with our work
on sub-micron sized alloy particles. Thus, the ligand
shell that dramatically alters the properties of small
particles (Bradley 1994; Osuna et al 1996), has few
undesirable effects in the case of sub-micron sized alloy
particles.
2. Experimental
Particles of Co and Ni were prepared by refluxing an
ethylene glycol solution of their acetates or sulphates con-
taining a desired amount of PVP (MW–40,000 gmol–1). In a

*Author for correspondence

Page 2

P Saravanan et al

516


typical reduction, 2 g of PVP, 0⋅5 g of the metal precursor
were dispersed in 50 ml of ethylene glycol and the result-
ing solution refluxed at 198°C for 10 h. In the case of Ni,
the pH was adjusted to 9–11 by addition of NaOH prior to
reduction. In all cases, the solution turned black within
a few minutes of reaching 198°C. The particles were
studied by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM).
By varying the Ni2+ : ethylene glycol mole ratio from
0⋅008 : 1 to 0⋅0045 : 1, the mean diameter of the particles
could be varied from 520 nm to 750 nm. Similarly, in the
case of Co, a change in the Co2+ : ethylene glycol ratio
from 0⋅005 to 0⋅0022 resulted in a change of the particle
diameters from 410 nm to 550 nm. A variation in
the metal : PVP weight ratios in the range 1 : 4–1 : 20
however, produced no change in the obtained particle dia-
meters. When Ni(SO4) was used instead of Ni(ac)2, very


Figure 1. Transmission electron micrograph of small PVP
covered Ni particles produced by reduction of Ni(SO4).
Figure 2. Typical SEM micrographs of Co, Ni and Co–Ni alloy particles. (a)–(b) Co
particles; (c)–(d) Ni particles; alloy particles of the composition (e) Co0⋅50Ni0⋅50 and
(f) Co0⋅90Ni0⋅10. The scale bars on the images correspond to 500 nm.

Page 3

Submicron particles of Co, Ni and Co–Ni alloys
517
small Ni particles in the 10 nm size range were obtained
(see TEM image in figure 1).
The Co–Ni alloy particles were obtained by co-
reducing the metal precursors in the desired molar
ratios with a fixed metal : PVP weight ratio of 1⋅0 : 4⋅5.
The total metal ion concentration was held constant at
0⋅1 M for all the preparations. Thus, to obtain alloy
particles of the composition Co0⋅9Ni0⋅1, 1⋅12 g of
Co(ac)2⋅4H2O and 0⋅124 g of Ni(ac)2⋅4H2O were dis-
solved in 50 ml of ethylene glycol and were refluxed at
198°C overnight.
In all the above cases, the particles could be precipi-
tated from the solution by adding excess water or acetone
and were isolated by centrifugation. The particles were
then washed with water several times to remove excess
PVP, and unreduced matter if any. Finally, the obtained
black precipitate was dried in vacuum to obtain waxy
solids.
Figure 3. Histograms showing the size distribution of the Co, Ni and Co–Ni alloy particles. (a) and (b) Co particles; (c) and (d) Ni
particles; alloy particles of the composition (e) Co0⋅50Ni0⋅50 and (f) Co0⋅90Ni0⋅10.

Page 4

P Saravanan et al

518
SEM and energy dispersive X-ray analysis (EDAX)
were performed with a Leica S-440i instrument. Samples
were prepared by depositing a drop of the toluene sol on
an aluminum stub. Thermogravimetric measurements
(TGA) were carried out with a Mettler–Toledo instrument
in the temperature range 100–900°C under a N2 or O2
atmosphere at a heating rate of 5°C/min using typically
25 mg of the solid in a porcelain boat. X-ray diffraction
(XRD) data were collected using a Seifert 3000 instru-
ment employing the Co–Ka radiation for pure Co and alloy
samples and Cu–Ka for the Ni samples. The magnetic
measurements were carried out employing a vibrating sam-
ple magnetometer (VSM 7300, Lakeshore) equipped with a
10 kOe magnet using typically 20 mg of the dried sols. The
metal content estimated from TG measurements was used to
calculate the magnetization in terms of emu/g.
3. Results and discussion
Typical SEM images of the Co, Ni and Co–Ni alloy parti-
cles are shown in figure 2. The images reveal that the par-
ticles are generally spherical with a uniform diameter. In
some portions of the images, we see agglomerated parti-
cles perhaps due to their magnetic nature. By picking out
over 100 particles in each case, histograms of the particle
diameter were obtained. Typical histograms are shown in
figure 3. The particles of various compositions were
monodisperse with s ~ 15–20%.
XRD measurements confirmed the crystalline nature
of the nanoparticles. All the Ni particles were found to
exhibit a fcc structure, similar to bulk Ni. The Co
nanoparticles on the other hand, exhibited a hcp structure.
The alloy nanoparticles, rich in Co exhibited both fcc and
hcp peaks, while those with higher Ni content exhibited
only fcc peaks. However, owing to the small difference in
the lattice spacings of Ni and Co (Co = 0⋅35477 nm,
Ni = 0⋅35238 nm), the peaks could not be definitively
assigned to a solid solution of Co and Ni. These results
are similar to the ones with uncoated Co–Ni alloy
nanoparticles obtained by other workers (Viau et al
1996b). The diameter of the nanocrystals could be esti-
mated from the width of the X-ray peaks, according to the
Table 1. Analysis of Co–Ni alloy particles.



Metal salts
feed ratio
particles by EDAX


Co0⋅10Ni0⋅90 Co0⋅13Ni0⋅87
Co0⋅30Ni0⋅70 Co0⋅31Ni0⋅69
Co0⋅50Ni0⋅50 Co0⋅53Ni0⋅47
Co0⋅70Ni0⋅30 Co0⋅69Ni0⋅31
Co0⋅90Ni0⋅10 Co0⋅88Ni0⋅12






Diameter


Composition of the

Phases as seen
by XRD SEM (nm) XRD (nm)






fcc
fcc
fcc
200
350
450
–
190
190
342
440
–
170
fcc + hcp
fcc + hcp









Figure 4. TG curves of Ni and Co0⋅30Ni0⋅70 alloy particles. The vertical lines demarcate
the regions of weight loss corresponding to the solvent at ~ 200°C and PVP and ~ 410°C.
Inset shows the TGA curve of PVP.

Page 5

Submicron particles of Co, Ni and Co–Ni alloys
519




Figure 5. The area around zero field in M vs H sweep, showing the coercive field for Co and Ni particles along with that of the
bulk. Inset shows the overlapping hysteresis loops of Co and Ni particles with their bulk analogues.
Figure 6. Hysteresis loops of Co–Ni alloy particles along with those obtained with ingots of pure
Co and Ni. Inset shows the change in magnetization value at 8⋅5 kOe field with composition.