Materials Sciences and Applicatio ns, 2011, 2, 1307-1312
doi:10.4236/msa.2011.29177 Published Online September 2011 (http://www.SciRP.org/journal/msa)
1307
Synthesis of Cobalt Nano Crystals in Aqueous
Media and Its Characterization
Pamela Alex*, Sanjib Majumdar, Jugal Kishor, Inderkumar G. Sharma
Materials Processing Division, Bhabha Atomic Research Centre, Trombay, India.
Received January 21st, 2011; revised March 10th, 2011; accepted June 22nd, 2011.
ABSTRACT
The work demonstrates the production of nano crystals of cobalt in bulk quantities in aqueous medium using hydrazine
as the reducing agent. Preparation of Co nano powders of 30 - 70 nm of 99.99% purity was accomplished from 0.25 - 2
M CoSO4 solutions in batch scale of 0.1 to 1 kg. The results of characterization studies using XRD, SEM, TEM indicate
the formation of finer particles with increase in concentration of cobalt ions in solution and dominance of fcc cobalt in
room temperature reduction. VSM results revealed a higher saturation magnetization of the nano cobalt at 100 K to be
comparable to that of the bulk metal.
Keywords: Magnetic Properties, Magnetic Materials, Thermogravimetric Analysis (Tga), Electron Microscopy, X-Ray
Diffraction (Xrd)
1. Introduction
The enhanced mechanical, electronic and magnetic prop-
erties [1] of metallic nano particles have triggered pro-
posals for applications as high-density magnetic storage
devices [2], hall sensors [3], soft magnetic materials ex-
hibiting higher permeability/lower coercivity [4] and
heterogeneous catalysts [5]. Transition metallic (Fe, Co,
Ni) nano particles have found wide application in cata-
lysts, solar energy absorption and magnetic recording [6-
8]. Of the three transition metals that are normally fer-
romagnetic, the nano particles of pure cobalt probably
have special significance in both theory and technology
[9-11], because the metal possesses hexagonal close-
packed (HCP or α-cobalt) structure besides face-centered
cubic (FCC or h-cobalt) structure.
Metallic cobalt nanoparticles have been available from
wet-phase synthesis methods for more than 50 years [12],
today offering good control of product size and shape.
The wide range of synthetic, mainly liquid based, prepa-
ration methods offers access to cobalt nano particles of
primary particle sizes starting from only several nano-
meters up to submicron sized materials. Applications for
cobalt nano powders generally involve their magnetic
properties and include catalysts and magnetic recording
and in medical sensors and bio medicine as a contrast
enhancement agent for magnetic resonance imaging.
Cobalt nano powders are also being tested for site spe-
cific drug delivery agents for cancer therapies and in
coatings, fuel cells, batteries, DNA circuits and chrome
replacement in sporting goods, luxury and consumer
products [13].
Many methods, such as decomposition of dicobalt oc-
tacarbonyls, pulse current electro-deposition, gas vapor
condensation, salts reduced by reducing agent NaBH4 or
H2, are used in the preparation of cobalt nano particles
[10-17]. Cobalt nano particles are also prepared by re-
duction of cobalt salt in hydrazine alkaline system [18]
involving the application of ultrasound initiative and also
in ethanol medium [19]. In a recent publication by
Kalyan et al. [20], has reported synthesis of nano cobalt
from a cobalt hydrazine complex in 1gm scale. However
production of nano sized cobalt powder in aqueous me-
dium using hydrazine as reductant in bulk quantities has
not yet been investigated.
Most of the literature refers to preparation of Co nano
powders in micro to few gram scale studies only, without
mentioning any details of the reproducibility of results in
increased batch scales. In this paper, we describe an easy
and inexpensive chemical synthesis route in alkaline me-
dium and a moderately high metal concentration is ap-
plied. The preparation of pure cobalt nano particles is
conducted in a hydrazine alkaline system, at ambient
temperature and completed instantaneously without in-
troduction of a nucleator. We predict that the process

Synthesis of Cobalt Nano Crystals in Aqueous Media and Its Characterization
1308
could be applied for production of Co nano powders in
bulk quantities. The size and structure of the resultant
nano particles were characterized by transmission elec-
tron microscopy (TEM), X-ray diffraction (XRD), Scan-
ning electron microscopy (SEM) and Thermo gravimet-
ric analysis. The powder was also subjected to magnetic
evaluation.
2. Experimental Procedure
Cobalt sulphate, Hydrazine, and Sodium hydroxide used
in the experiments were the guaranteed reagent of E.
Merck. Typically, an appropriate amount of cobalt sul-
phate (0.25 M to 2 M) was dissolved directly in demate-
rialized water. Then, required amount of hydrazine (2 - 4
M) and NaOH (0.5 to 2.5 M) was added in sequence. For
each experiment, the entire Co2+ ion in the solution was
allowed to precipitate. The reduced powder was washed
thoroughly with dilute HCl to remove traces of alkali and
finally with ethanol, dried under vacuum, weighed and
was stored in propanol solution. Extensive studies were
carried out to establish ideal condition of precipitation of
cobalt nano powders. The reduction reaction could be
expressed as
2
242 2
2CoNH 4OH2CoN 4HO

  (1)
XRD and SEM analysis were done after separating the
precipitate from solution using a permanent magnet, then
washing the precipitates using ethanol, and finally vac-
uum drying at room temperature. XRD measurements
were performed on a Rigaku D/max III X-ray diffracto-
meter using Mo Kα radiation (λ = 0.07093 nm). The par-
ticle size was measured by the X-Ray Diffraction method
using the Scherer Formula. The standard sample of Ceria
was taken along with the cobalt sample and the diffrac-
tion pattern was obtained. The FWHM, d and the 2
 were
obtained from the diffraction patterns of both the stan-
dard and the Co powder. Considering the equation
22
tan
F
WHM UW  , the regression curve was plot-
ted between the 2
F
WHM and tan2  for the standard
Ceria sample and the curve fitting was done. The values
of the constants U, V and W obtained from that curve
fitting were then used to calculate the FWHM (Bs) at
different
 values, at which the peaks were obtained form
the Co powder. The actual broadening (B) was calculated
using the relationship 221
(
es
BBB 2 ), where Be the
experimental FWHM value for the cobalt sample, and Bs
is the calculated FWHM value for the standard. The
value of the B so obtained was then put in the Scherer
Formula to determine the value of particle size.
SEM analysis was carried out using Camscan MV
2300CT/100. The particle sizes were determined by
TEM using a TITAN model JEM-1200EX at 160 kV.
The samples for TEM analyses were obtained by diluting
the dispersed solution with ethanol and then placing a
drop of the diluted solution onto a Formvar-co- vered
copper grid and evaporated in air at room temperature.
Before the samples were withdrawn, the cobalt nanopar-
ticle-dispersed in ethanol solutions were sonicated for 1
min to obtain the better particle dispersion on the copper
grid. Thermo gravimetric analysis of the powder was
conducted using Setaram make thermo balance (Model
Setsys Evolution). Powder was kept in an alumina cruci-
ble and heated in inert atmosphere at about 600˚C. Mag-
netic studies of the cobalt powder were carried out using
Vibrating Sample Magnetometer of Oxford Make.
3. Results and Discussion
Concentration of alkali in the cobalt solution played a
major role in controlling the precipitation reaction. This
was clearly visible in the preliminary experiments we
carried out, by using a solution of 1M CoSO4 and a pH
greater than 12 [21]. As observed, Co forms a complex
of Co(OH)4+ with NaOH and precipitates to form
Co(OH)2 subsequently. Our investigations were aimed at
selecting the required alkali concentration for the com-
plete precipitation of Co from high strength Co solutions.
Concentration of NaOH was varied from 0.5 M - 2.5 M
(Figure 1), when the color of the cobalt hydroxide pre-
cipitate changes from dark pink to bluish violet and fi-
nally precipitates as grayish Co powder.
Once, the bulk precipitation is initiated the rate of re-
duction is fast and completes within few minutes. The
reduction fraction in Figure 1 essentially indicates the
percentage ratio of precipitated Co metal to starting Co
ions. It can be seen that increase in alkali concentration
highly accelerates the reduction of Co ions and a com-
Figure 1. Effect of NaOH Concentration on reduction of
Co 2+ to Co.

Synthesis of Cobalt Nano Crystals in Aqueous Media and Its Characterization1309
plete reduction of Co ions is possible with an effective
concentration of 2.4 M NaOH in the solution.
Figure 2 shows the reduction rate of Co ions with the
increase in concentration of hydrazine. The rate increases
linearly up to 4 M, and becomes constant. Hence a
minimum concentration of 4 M N2H4 was required for
the complete precipitation of cobalt ions from the solu-
tion. The ratio of Co to hydrazine was maintained at 1:16
for any strength of Co solution.
The Co2+ ions in solution also contributed to the rate of
reduction and size of Co nano particles. It was observed
that lower concentration of Co ions in solution exhibited
very slow kinetics and finer particles. However when
concentration of Co in solution was increased from 0.25
M to 2 M, the particle size increased up to a Co concen-
tration of 0.5 M, and then started giving finer particles at
a very fast rate (supported by XRD Figure 4). This
anomalous behavior of Co nano powder produced at as
high a concentration as that of 1 M, can be due to faster
nucleation and faster nucleation kinetics of Co particles
at higher concentrations and is advantageous in produc-
ing nano powders in bulk. However, further increase in
Co concentration resulted in severe filtration problems
and was discontinued.
The effect of temperature of the reaction on rate of re-
action can be seen from Figure 3. The rate of reaction
was slow at lower temperatures and increases steadily at
temperatures greater than 50˚C and stabilizes above 70˚C.
This is due to the highly exothermic nature of the reduc-
tion reaction. At lower temperatures the nucleation time
is more and kinetics of the reaction is slow. Increase in
temperature and availability of higher surface area with
formation of cobalt metal, accelerates the reduction reac-
tion. A temperature of 70˚C not only increases the reac-
tion rate, when bulk quantities of Co has to be produced,
Figure 2. Dependence of Co precipitation on Hydrazine.
Figure 3. Effect of Temp. on Cobalt reduction.
but also results in much finer particle at as high a con-
centration as 1 M. In fact it requires only 10 minutes
preparing Co nano powder in kilogram scale. The pro-
duction of Co nano powder can be easily accomplished
by maintaining a cobalt concentration as high as 1 M, at
pH greater than 13 and at a temperature of 70˚C. 1 kg of
CoSO4 gives roughly 200 g of Co metal powder.
Most of the reported work on Co nano powder prepa-
ration is in mg or gm scale of Co while we have at-
tempted to make Co nano powder in larger scale. The
ease with which the nano powder could be prepared in
this investigation can be scaled up to large scale produc-
tion and its future application in various fields.
Characterization Studies
The detailed XRD analysis carried out using the Co
powder produced at different conditions is presented in
Figure 4. Mostly, the peaks from both the hcp and fcc
phases are detected. It is observed that the peak broaden-
ing is more for the Co powder produced using 0.25M
concentration of cobalt solution as compared to that pro-
duced using 0.5 M solutions at 70˚C.
This indicates the formation of finer particles at lower
cobalt ion concentration. The particle size calculated
using Scherrer formula is around 24 nm and 49 nm, re-
spectively for 0.25 M and 0.5 M solutions. The domi-
nance of one particular phase is also detected in the Co
powder produced at room temperature. Figure 5 repre-
sents the calculated size of Co particles using the XRD
data presented in Figure 4. It is observed from Figure 5
that the cobalt particle size is reduced with increase in
the cobalt concentration in the solution beyond 0.5 M.
This anomalous behavior could be due the faster kinetics
of the reduction process with increasing the cobalt ion
concentration. The growth of the individual particles is

Synthesis of Cobalt Nano Crystals in Aqueous Media and Its Characterization
1310
Figure 4. XRD plots of the cobalt nano powder produced
using di ffer ent c obal t co nce ntr atio n and te mper at ure.
Figure 5. Change in particle size with cobalt concentra-
tion obtained from XRD results.
restricted due to the faster reaction kinetics at higher
concentrations.
The SEM image of the agglomerated Co particles is
presented in Figure 6. The single domain nano size fer-
romagnetic cobalt particles have the tendency to align in
particular direction.
Figure 7 shows the TEM image of some of the parti-
cles indicating an average size of around 70 nm. Diffi-
culties were faced to disperse the individual particles due
to the strong magnetic characteristics. The diffraction
pattern (inset in Figure 7) indicated the presence of both
hcp and fcc phases in the powder. The inner ring con-
taining diffraction spot doublets is due to hcp phase
Figure 6. SEM image of the agglomerate showing the
alignment of particles forming leaf like structure.
Figure 7. TEM image of the cobalt nano particles; inset
SAD pattern confirming both HCP and FCC rings due to
nano size.
and the next ring is for the fcc phase. Since, an aqueous
process has been adopted to prepare cobalt nano powder,
a marginal amount of gases are adsorbed as indicated by
slight weight loss during heating in TG studies (Figure
8).
Figure 9 shows the magnetic hysteresis loops of the as
prepared Co material recorded at 298 and 100 K. Both
loops showed a very small coercivity (100 Oe) together
with a high saturation magnetization. While saturation
magnetization at 298 K of 124 emug–1 lay below the
reference value for bulk cobalt metal (164 - 165 emug–1),
at 100 K the saturation magnetization was within 98% of
reference value (164 - 165 emug–1) [22]. The saturation
magnetization at room temperature was enhanced, fa-
voring the use of the present material for magnetic ap-
plications. The data at 100 K suggested a high content of
magnetic cobalt (>85%). The reduced saturation mag-
netization at 298 K indicated a strongly temperature de-

Synthesis of Cobalt Nano Crystals in Aqueous Media and Its Characterization1311
Figure 8. TG plot showing a very little change in weight
due to evaporation of adsorbed gases.
Figure 9. Magnetic hysteresis of as prepared Co nano
powder measured at 298 and 100 K exhibiting small co-
ercivity and a large saturation magnetization (124 - 164
emug–1).
pendent magnetic response which agrees with prior ob-
servations [23]. The low coercivity of 100 Oe is also
characteristic of single domain magnetization.
4. Conclusions
A detailed study on the preparation of nano size Co
powder from CoSO4 salts by varying different parame-
ters indicated that, a hydrazine concentration of 4 M was
sufficient to precipitate Co nano powder of 30 - 70 nm
from highly alkaline cobalt solution of 1 M, at a tem-
perature of 70°C. A reduction in particle size was identi-
fied with increase in cobalt ion concentration, which is
not reported in earlier findings. Dominance of fcc phase
was observed in the nano cobalt particles produced at
room temperature. The powder due to its highly ferro-
magnetic nature was agglomerated and formed a flower
like image as visualized in SEM. The TEM analysis in-
dicated a particle size of 50 - 70 nm was produced.
However, XRD analysis by Scherer formula gave the
size to be around 24 - 49 nm. The magnetic studies re-
vealed a very high content of magnetic cobalt.
5. Acknowledgements
The authors wish to thank Dr. Madangopal Krishnan and
Dr. R. Tiwari of Material Science Division, BARC, for
their support in XRD and TEM analysis.
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