Preparation of nickel, iron, and cobalt nanopowders via reduction of salts with sodium dissolved in liquid ammonia

Article abstract of DOI:10.1023/B:INMA.0000037925.93214.d4

Nickel, iron, and cobalt powders are prepared via reduction of salts with sodium dissolved in liquid ammonia. The low temperature of the process and the ability of liquid ammonia to stabilize dangling chemical bonds notably reduce the growth rate of metal

Full text of DOI:10.1023/B:INMA.0000037925.93214.d4

Inorganic Materials, Vol. 40, No. 8, 2004, pp. 809–814. Translated from Neorganicheskie Materialy, Vol. 40, No. 8, 2004, pp. 928–934.
Original Russian Text Copyright © 2004 by Novikov, Pan’kov, Kunitskii.
Preparation of Nickel, Iron, and Cobalt Nanopowders
via Reduction of Salts with Sodium Dissolved in Liquid Ammonia
V. P. Novikov, V. V. Pan’kov, and L. I. Kunitskii
Belarussian State Technological University, ul. Sverdlova 13a, Minsk, 220050 Belarus
e-mail: root@bstu.unibel.by
Received June 30, 2003; in final form, December 31, 2003
Abstract—Nickel, iron, and cobalt powders are prepared via reduction of salts with sodium dissolved in liquid
ammonia. The low temperature of the process and the ability of liquid ammonia to stabilize dangling chemical
bonds notably reduce the growth rate of metal particles and prevent their aggregation. The magnetic properties
of the nanopowders are investigated.
INTRODUCTION
for the synthesis of nanopowders. The low temperature
of the process in liquid ammonia (–33 to –50°C) and
the ability of liquid ammonia to stabilize free radicals
Nanopowders of iron-group metals possess a high
saturation induction and are potentially attractive for
producing magnetic fluids and composites. Dispersions
of iron-group metal nanoparticles are used as engine oil
additives for reclaiming worn parts of automotive and
other engines directly during operation [1]. Such nano-
particles can also be used as magnetic-recording media,
for manufacturing permanent magnets, in magnetic
cooling systems, as magnetic sensors, in drug delivery
systems, as magnetic resonance imaging contrast
media, and in other applications [2].
[
7] substantially reduce the growth rate of metal parti-
cles, preventing aggregation of the resulting precipitate.
The objective of this work was to develop a process for
preparing powders of iron-group metals via reduction
of appropriate chlorides with Na dissolved in liquid
ammonia.
EXPERIMENTAL
Metal nanoparticles were synthesized using appro-
priate chlorides or sulfates, Na metal, and anhydrous
liquid ammonia. The chlorides were dehydrated in a
Submicron powders of iron-group metals can be
prepared via pyrolysis of appropriate volatile carbon-
yls. The chief drawbacks to this process are the high
toxicity of carbonyl precursors and the relatively large
size of the resultant particles. Nanopowders can be pre-
pared by reducing appropriate salts with borohydrides
or hypophosphates in aqueous solutions [3, 4], but the
resulting metal nanoparticles are unstable to oxidation
and aggregation in the solution. Moreover, the metals
obtained with the use of such reductants contain signif-
icant amounts of boron or phosphorus, which are
extremely difficult to remove from the precipitate. Iron-
group metals can also be obtained by reducing their
salts with aqueous hydrazine. The resultant metals con-
tain no difficult-to-remove impurities, but the reduction
potential of hydrazine is not very high and cannot
ensure a thermodynamic supersaturation high enough
for producing metal nanoparticles [5]. Thus, the devel-
opment of new processes for the preparation of metal
nanopowders is of great practical importance.
2
vacuum of 10 Pa at 180°C for 24 h. Liquid ammonia
was prepared via condensation of predried ammonia
1
1
40
20
3
.5°
100
80
6
4
2
0
0
0
0
As shown earlier [6], one efficient way of preparing
Fe nanopowders is by reducing Fe salts with Na dis-
solved in liquid ammonia. This process has a number of
important advantages. Na solutions in liquid ammonia
have a very high reduction potential and can, therefore,
ensure high thermodynamic supersaturations necessary
3
.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6
θ, deg
2
Fig. 1. Small-angle x-ray scan of Fe particles prepared by
reducing iron(II) sulfate with sodium dissolved in liquid
ammonia.
0
020-1685/04/4008-0809 © 2004 MAIK “Nauka/Interperiodica”

8
10
NOVIKOV et al.
2
σ, A m /kg
1
20
1
00
8
6
4
2
0
0
0
0
0
1
00
300
500
700
900
1100 T, K
Fig. 2. Specific saturation magnetization as a function of temperature for Fe nanoparticles prepared by reducing iron(II) sulfate with
sodium dissolved in liquid ammonia.
χ–1 × 10 , g/cm
–2
3
5
4
3
2
1
00
00
00
00
00
0
200
400
600
800
1000
T, K
Fig. 3. Reciprocal magnetic susceptibility as a function of temperature for Fe O prepared via oxidation of Fe nanopowder.
2
3
gas. The ammoniacal solutions and liquid ammonia etry MCl (NH ) . The salt concentration was 0.16 M.
2
3 6
were stored in glass Dewar flasks. The Na solution Sodium was present in excess (5–10%) of the stoichio-
(
0.24 M) in ammonia was prepared immediately before metric amount for the full reduction of the metal by the
2+
+
experiments by adding pieces of metallic Na to vigor-
ously stirred liquid ammonia. The solution was deep
blue. According to current views, sodium dissolved in
overall reaction å + 2Na
å + 2Na . To reduce the
metal, a metal ammine chloride solution (suspension)
was added to the Na solution with vigorous stirring. In
ammonia is fully ionized and can be thought of as a the course of the reaction, the solution changed color
mixture of sodium ions and solvated electrons. The lat- from blue (sodium dissolved in ammonia) to black (sol
ter play a central role in determining the redox and of Co metal). The metal powders were prepared, puri-
other properties of the solution.
fied, and stored in a dry argon atmosphere.
Metal-salt suspensions were prepared by reacting
The metal was recovered by magnetic separation or
the anhydrous chlorides or sulfates and liquid ammo- ultrafiltration. After washing with liquid ammonia and
nia. The reactions between the chlorides and ammonia ethanol, the filtrate was transferred to a quartz cell filled
yielded ammines, as a rule, with the general stoichiom- with anhydrous toluene. The powders thus prepared
INORGANIC MATERIALS Vol. 40 No. 8 2004

PREPARATION OF NICKEL, IRON, AND COBALT NANOPOWDERS
811
Ni
Ni
O
0
2
4
6
8
10
Energy, keV
Fig. 4. X-ray emission spectrum (electron-beam excitation) of the Ni powder after holding in an oxygen-poor atmosphere; the oxy-
gen content of the sample is 15–20 at %.
were characterized by x-ray diffraction (XRD), mag- ically can be accounted for by a superparamagnetic
netic measurements, and electron microscopy.
state of the particles. The curve in Fig. 2 shows an
anomaly characteristic of a diffuse Curie transition at
9
70 K, which is somewhat lower than the Curie temper-
RESULTS AND DISCUSSION
ature of bulk Fe.
–
1
Iron powder. The synthesized Fe powder was pyro-
The reciprocal magnetic susceptibility χ of the
phoric: when exposed to air, it self-ignited. The com- powders was measured as a function of temperature by
bustion of the powder led to sintering of the forming the Faraday method in a magnetic field of 640 kA/m.
oxide particles. To prevent this process, the powder was The force acting on the sample in a nonuniform mag-
oxidized in an oxygen-poor (2–5 vol %) atmosphere for netic field was measured using a Sartorius BP 310 S
several hours. In this process, the powder changed color electronic analytical balance. The variable temperature
from black (fine-particle Fe) to yellow-brown (iron
oxide).
2
σ, A m /kg
After washing and drying, the Fe powder was exam-
ined by XRD, which showed no reflections from crys-
talline phases, indicating that the sample was amor-
phous and consisted of nanoparticles. The latter was
confirmed by small-angle x-ray scattering: an x-ray
scan showed a broad band centered at 2θ = 3.5°, with a
full width at half maximum of about 3° (Fig. 1).
2
5
It is known that small-angle x-ray scattering data
can be used to evaluate the average particle size of the
material and the variance of the particle size [8].
Using the procedure described by Svergun and Feigin
0
[
8], the average particle size of the powders prepared
by the sodium-in-ammonia process was determined to
be 2–3 nm.
Figure 2 shows magnetization versus temperature
data for the Fe powder prepared by the sodium-in-
ammonia process. The decrease in magnetization with
increasing temperature is characteristic of ferromag-
netic materials. The fact that σ decreases nonmonoton-
–
25
–320
–160
0
160
320
H, kA/m
Fig. 5. Magnetic hysteresis loop of the Ni powder.
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8
12
NOVIKOV et al.
1
5
20
25
30
35
40
45
50
55
60
65
70
75
2θ, deg
Fig. 6. XRD pattern of nanoparticles prepared by reducing cobalt(II) chloride with sodium in liquid ammonia.
attachment we used enabled the temperature to be var- (1–5%) atmosphere for 0.5 h suppressed the pyrophoric
ied continuously in the range 80–1200 K, without properties and markedly inhibited subsequent oxida-
changing the sample position. Above room tempera- tion of the Ni powder. Electron probe x-ray microanal-
ture, susceptibility was measured in a dry argon atmo- ysis showed that the aggregates were covered with a
sphere.
stable oxide layer, which prevented the sample from
further oxidation.
Since the Fe powder was amorphous, it was rather
difficult to ascertain which iron oxide was formed on
the surface of the Fe nanoparticles in the initial stages
of oxidation. After heat treatment at 500°C in air, the
XRD pattern of the powder showed reflections from
hematite. Using temperature-dependent magnetic sus-
ceptibility measurements, we revealed the main mag-
netic phase transitions of α-Fe O (Fig. 3).
The x-ray emission spectrum of the Ni powder is
shown in Fig. 4. After air exposure, the powder parti-
cles had a composite structure: Ni/Ni O or
2
3
Ni/Ni(OH) . This was also evidenced by XRD analysis.
2
2
3
2
σ, A m /kg
At 250 K, we observed an anomaly due to an anti-
ferromagnetic–antiferromagnetic phase transition.
Above 600 K, a ferromagnetic component of magneti-
zation appeared as a result of partial destruction of the
antiferromagnetic order. Above 900 K, the powder
underwent a transition to a paramagnetic state. All
these transitions are characteristic of α-Fe O [9].
3
0
2
3
Nickel powder. Ni metal was prepared by the same
process as Fe. In contrast to the Fe powder, Ni was
obtained in the form of aggregates about 1 µm in size.
The likely reason is that one possible intermediate in
the reaction between nickel and ammonia is the rela-
tively stable amide Ni(NH ) , which has time to crys-
0
2
2
tallize in the ammoniacal solution. At room tempera-
ture, this amide decomposes after ammonia evapora-
tion. The aggregate size seems to be determined by the
crystal size of Ni(NH ) .
–
30
320
–
–160
0
160
320
2
2
H, kA/m
The Ni powder was also pyrophoric, but to a lesser
extent than the Fe powder. Exposure to an oxygen-poor
Fig. 7. Magnetic hysteresis loop of the Co powder.
INORGANIC MATERIALS Vol. 40 No. 8 2004

PREPARATION OF NICKEL, IRON, AND COBALT NANOPOWDERS
813
2
σ, A m /kg
3
2
1
0
0
0
(a)
Cooling
Heating
0
0
0
0
0
0
Cooling
8
6
4
2
(b)
Heating
1
1
1
1
60
40
20
00
(c)
Cooling
8
6
4
2
0
0
0
0
Heating
0
200
400
600
800
1000
1200
1400
1600
T, K
Fig. 8. Specific saturation magnetization as a function of temperature for Co particles (a) 30, (b) 60, and (c) 80 nm in size.
In addition, the sample contained a small amount of sor, we used CoCl · 6H O, which was dehydrated in a
2
2
2
nickel nitride. The diffraction peaks from all of the
phases present were broadened, attesting to a small par-
ticle size. From the width of XRD peaks, the particle
size was evaluated to be 30–50 nm.
vacuum of 10 Pa at 180°C and then reduced in the
same way as the Ni precursor.
The main structural and magnetic properties of the
Co powder prepared by reducing CoCl (Figs. 6, 7) are
2
Figure 5 shows the magnetic hysteresis loop of the
Ni powder. The coercive field evaluated from this loop
is 8–16 kA/m. This value is substantially lower than the
coercive field in bulk Ni, suggesting that the shape-
anisotropy and mechanical-stress contributions are
very small [9].
very similar to those of the Ni powder. A noteworthy
feature is that the coercive fields of both the Ni and Co
powders are substantially smaller than those of bulk Ni
and Co. Moreover, all of the phase transitions in the Ni
and Co powders occur at lower temperatures.
The specific magnetization of Co nanoparticles was
Cobalt powder. Metallic Co was prepared by a pro- measured in the range 77–1450 K by the Faraday
cedure similar to that for Fe and Ni. As the Co precur- method in a field of 640 kA/m during both heating and
INORGANIC MATERIALS Vol. 40 No. 8 2004

8
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NOVIKOV et al.
cooling. The results (Fig. 8) demonstrate that increas- the absence of diffraction peaks in their XRD patterns,
ing the particle size from 30 to 80 nm increases the spe- and the observed small-angle x-ray scattering peak.
cific magnetization of the nanoparticles. During heat- The small particle size of the powders shows up in their
ing, the magnetization of the three samples increases magnetic properties. The chief advantage of the
sharply at ꢀ600 K. The increase in specific magnetiza- described process is the high reduction potential of
tion during the first heating to above 600 K is irrevers- sodium solutions in ammonia.
ible. Measurements during cooling and subsequent
heating reveal virtually a classic σ(T) behavior of ferro-
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decreasing nanoparticle size.
The Curie temperature of the nanoparticles studied
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We believe that the nitrogen and hydrogen dissolved
in the metal powders prepared in this study play a cen-
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analysis shows that the amorphous powders contain up
to 20 at % N. The incorporation of nitrogen seems to be
the result of catalytic ammonia decomposition on the
surface of nanoparticles and/or decomposition of unsta-
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2
+
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CONCLUSIONS
The sodium-in-ammonia process described in this
work offers the possibility of preparing amorphous
metal nanoparticles, as exemplified by the synthesis of
iron, nickel, and cobalt powders. An important point is
that these metals were obtained in an amorphous state
without introducing difficult-to-remove components.
The small particle size of the synthesized powders is
evidenced by their sedimentation stability in ammonia,
8
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INORGANIC MATERIALS Vol. 40 No. 8 2004