Formation of cobalt nitrate hydrate, cobalt oxide, and cobalt nanoparticles using laser vaporization controlled condensation

Article abstract of DOI:10.1021/jp0370831

In this paper, we report for the first time the synthesis of cobalt nitrate hexahydrate, cobalt oxide, and cobalt particles formed from a high purity cobalt metal by a novel laser vaporization controlled condensation (LVCC) method under controlled pressures of N₂ and O₂. The metal vapor produced from a cobalt target in the presence of 50% N₂ and 50% O₂ results in the formation of cobalt nitrate. We also explored the possibilities of forming cobalt oxide and cobalt nanoparticles by altering the ratio of N₂ and O₂ present. For example, the synthesis of pure cobalt oxide (CoO) nanoparticles is of importance and challenging since a simple chemical route is complex. We believe that this work will be of significant importance since the present method is promising for the synthesis of metal mono oxides.

Full text of DOI:10.1021/jp0370831

9604
J. Phys. Chem. B 2004, 108, 9604-9607
Formation of Cobalt Nitrate Hydrate, Cobalt Oxide, and Cobalt Nanoparticles Using Laser
Vaporization Controlled Condensation
†
‡
,§
Garry P. Glaspell, Paul W. Jagodzinski, and A. Manivannan*
Department of Chemistry, West Virginia UniVersity, Morgantown, West Virginia 26506, Department of
Chemistry & Geochemistry, Colorado School of Mines, Golden, Colorado 80401, and Department of Physics,
West Virginia UniVersity, Morgantown, West Virginia 26506
ReceiVed: October 11, 2003; In Final Form: April 13, 2004
In this paper, we report for the first time the synthesis of cobalt nitrate hexahydrate, cobalt oxide, and cobalt
particles formed from a high purity cobalt metal by a novel laser vaporization controlled condensation (LVCC)
method under controlled pressures of N
of 50% N and 50% O results in the formation of cobalt nitrate. We also explored the possibilities of forming
cobalt oxide and cobalt nanoparticles by altering the ratio of N and O present. For example, the synthesis
2 2
and O . The metal vapor produced from a cobalt target in the presence
2
2
2
2
of pure cobalt oxide (CoO) nanoparticles is of importance and challenging since a simple chemical route is
complex. We believe that this work will be of significant importance since the present method is promising
for the synthesis of metal mono oxides.
Introduction
Recently, synthesis of various semiconductor and intermetallic
produced using high purity cobalt metal target at various N2
and O2 pressures.
1-12
nanoparticles via LVCC has been of particular interest.
This
Experimental Section
method utilizes well defined conditions to produce nanoparticles
of controlled size and composition without the need for chemical
precursors or heat treatments. Specifically, a laser is used to
produce a supersaturated metal vapor from a target inside a
chamber. Convection currents, resulting from the top plate of
the chamber being cooled and the bottom plate heated, affect
the rate of nucleating metal vapor. Particles generated by this
procedure are predominately spherical and have significantly
less fluctuation in size compared to most reduction procedures.
This can be achieved by the choice of laser power, chamber
pressure, and temperature gradient. El-Shall and co-workers have
explored the synthesis of a variety of metal oxide nanoparticles
The cobalt target (99.9%) was purchased from Alpha Aesar
Ward Hill, MA). The N2 (99%) and O2 (99%) were obtained
(
from Air Gas (Westover, WV) and used without any further
purification. The LVCC chamber was home-built and consists
of two metal plates modified to provide uniform heating and
cooling. To provide convection, the top plate was cooled with
liquid nitrogen and the bottom plate was heated with circulating
water. A Laserphotonics Nd:YAG laser (Orlando, Florida) was
used to ablate the metal target. Altering the laser power, chamber
pressure, and temperature gradient between the two plates results
in the formation of particles of various sizes. The size and shape
of the particles were confirmed using a JEOL 1220 transmission
electrom microscopy (TEM) operating at 80 KeV. X-ray
diffraction (XRD) patterns of the powder sample were measured
at room temperature with a Rigaku Diffractometer (DMAX-B)
and Cu KR radiation (λ ) 1.5418 Å). The samples were
mounted on a silicon plate for X-ray measurements. IR spectra
were obtained using a MIDAC M2000 series FT-IR (Costa
Mesa, CA).
1
3,14
using LVCC by adding oxygen to the reaction chamber.
This method provides insight into the physical chemistry of the
formation of these materials. In this regard, we attempted to
synthesize metal nitrates from a metal target in the presence of
N2 and O2 at controlled pressures. For the first time, we have
successfully synthesized cobalt nitrate hexahydrate. We have
also explored how the ratio of N2 and O2 controls the physical
chemistry of the various types of cobalt compounds produced.
Surprisingly, we could also synthesize cobalt oxide (CoO)
nanoparticles. It is important to mention that pure CoO cannot
be synthesized easily by a simple chemical route since this
approach typically produces a mixture of Co3O4 with small
amounts of CoO. Therefore, our objective is to optimize the
conditions to synthesize pure CoO nanoparticles and other
related cobalt compounds.
Results and Discussion
We have carried out numerous experiments by varying the
O2 and N2 pressures in the chamber in order to obtain various
compounds of Co. As a technique, LVCC has been used to make
nanoparticles of various semiconductors and metal oxides. In
our experiment, it has been demonstrated how various com-
pounds of Co are formed by simply varying the O2 and N2 ratios
during synthesis. Moreover, the physical chemistry of the
formation of cobalt nitrate is intriguing. The cobalt metal vapor
formed via the laser simply reacts with the N2 and O2 to produce
cobalt nitrate. The details of the formation of various Co
compounds are described below.
In this report, we describe the LVCC method for the synthesis
of cobalt nitrate hydrate, cobalt oxide, and cobalt nanoparticles
*
To whom correspondence should be addressed. E-mail: amanivan@
wvu.edu.
†
Department of Chemistry, West Virginia University.
Colorado School of Mines.
Department of Physics, West Virginia University.
‡
§
The nanoparticles generated in pure nitrogen are primarily
cobalt and are also the primary product at low concentrations
1
0.1021/jp0370831 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/08/2004

Formation of Cobalt Compounds
J. Phys. Chem. B, Vol. 108, No. 28, 2004 9605
Figure 1. XRD showing (A) cobalt (PDF 15-0806) as the primary product formed when the ratio of O
PDF 15-0806) and the emergence of cobalt oxide (PDF 43-1004) when the ratio of O to N is 2.3%; (C) cobalt oxide (PDF 43-1004) as the
primary product formed when the ratio of O to N is 3%; (D) Alfa Aesar 95% CoO having mixed oxide phases matching with CoCo (PDF
1-1152) and CoO (PDF 43-1004).
2 2
to N is 0.3%; (B) the decline of cobalt
(
2
2
2
2
2 4
O
0
TABLE 1: Standard Crystallographic Peak Assignments for
Cobalt Nitrate Hexahydrate Produced via LVCC
that the nanoparticles of CoO possess the same crystal structure
as bulk CoO. The approximate crystallite size for the CoO was
4 nm determined via TEM (Figure 2). This is a significant result
since the commercially available CoO (Alfa-Aesar) has mixed
oxide phases as indicated by X-ray diffraction (Figure 1D).
Strong peaks are observed at 2θ angles 31.63, 36.64, 38.90,
2
-Theta LVCC
h
k
l
2-Theta LVCC
h
k
l
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
3
3
5.1
0
-2
0
-2
2
2
-4
-2
0
-2
0
0
1
-2
0
-2
4
0
0
1
1
1
0
0
1
1
0
2
0
2
2
2
2
1
2
2
1
1
1
2
2
3
3
4
0
4
1
0
2
2
1
38.42
38.97
40.46
40.95
41.07
41.32
42.76
43.06
43.93
46.66
48.4
48.73
49.67
53.23
58.37
63.65
-2
-4
-4
-2
6
0
0
-2
-4
0
2
2
1
2
1
2
0
1
2
0
0
0
1
2
0
1
0
0
5
2
5
4
0
5
4
6
6
6
5
4
2
7
8
8
5.84
6.28
9.1
45.10, 56.01, 59.71, and 65.50 that can be assigned to scattering
3
from the 111, 200, 222, 400, 422, 511, and 533 planes,
3.91
5.34
6
7.06
8.15
9.09
0.54
1.51
2.1
1
5
respectively, of the CoCo2O4 crystal lattice.
Formation of a mixture of cobalt nitrate tetrahydrate and
cobalt oxide is observed when the percentage of O2 is raised to
6
.6% (Figure 3 A). The h, k, and l indicies are not reported for
cobalt nitrate tetrahydrate as they are not available in the
1
5
database (PDF 43-1004). It is believed that hydrates are
formed due to exposure of the nanoparticles to residual moisture
that is present as they are collected. We cannot rule out surface
6
-4
-4
0
2.84
3.24
4.26
moisture residing in the chamber even after the chamber was
evacuated and purged repetitively with nitrogen.1
6,17
Cobalt
of O2 below 0.3% (Figure 1A). It is evident from the XRD
pattern of the Co nanoparticles that the diffraction peaks at
scattering angles (2θ) of 44.31 and 51.65 are assigned to
oxide peaks continue to diminish as the O2 concentration is
increased to the 33-50% range. The primary product is then
cobalt nitrate hexahydrate as shown in Figure 3B with the major
peaks labeled. The standard crystallographic peak assignments
for the XRD pattern is given in Table 1 which indicates that
cobalt nitrate hexahydrate produced via LVCC possesses the
same crystal structure as its bulk counterpart. The XRD pattern
of cobalt nitrate hexahydrate, purchased from Aldrich, was
recorded (Figure 3C) and compared to cobalt nitrate hexa-
hydrate produced via LVCC. It is significant to note that the
two spectra possess minor differences. Specifically, the XRD
for cobalt nitrate produced via LVCC matches closely to the
reference PDF. The absence of the peak at 2θ angle of 19
15
scattering from the 111 and 200 planes of the Co crystal lattice.
This would indicate that nanoparticles having the same crystal
structure as bulk Co have been formed. When the percentage
of O2 is increased to 2.3%, we begin to observe the formation
of cobalt oxide (CoO) together with trace amounts of cobalt as
confirmed by XRD (Figure 1B). The CoO peaks are observed
at scattering angles 36.53, 42.41, and 61.64 and are assigned to
scattering from the 111, 200, and 220 planes of the CoO crystal
lattice.15 The trace amounts of cobalt disappear as the O2
concentration is increased to 3% (Figure 1C). It is also observed
15

9606 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Glaspell et al.
Figure 2. TEM of CoO nanoparticles. The average particle size was
calculated to be ∼4 nm.
TABLE 2: Reaction Conditions Showing the Primary
Products and How the Number of Water Molecules
Coordinated to the Metal Nitrate Hydrate Change as the
Ratio of Oxygen Changes
%
O
2
products
2
no. of coordinated H O’s
0
.3
.7
.3
.7
Co
1
2
2
3
6
0
3
0
CoO/Co
CoO/Co
CoO
CoO/CoNO
3
OH
/CoO
/CoO
1
4
4
6
6
.6
Co(NO
Co(NO
Co(NO
Co(NO
3
3
3
3
)
)
)
)
2
2
2
2
2
3
5
and the presence of peaks 24.48 and 41.78 2θ (indicated by *’s
in the XRD spectrum) in the Aldrich sample may be due to
impurities.
We have summarized all of the results in Table 2 indicating
all of the reaction conditions responsible for the products that
are produced at various concentrations of O2 and N2. It should
be noted that the number of water molecules coordinated to
the metal nitrate hydrate changes as the ratio of oxygen changes.
This can be explained by the amount of oxygen available in
the chamber. When oxygen is present in the 3-33% range, the
amount of cobalt oxide surrounding the cobalt nitrate can vary
significantly. In that range, the number of water molecules in
contact with cobalt nitrate may be controlled by the amount of
CoO which depends on the ratio of oxygen to nitrogen. Thus,
oxygen plays a major role in this process of producing CoO,
cobalt nitrate hydrate, as well as the number of water molecules
bound to it. It is also important to note that the major product
was cobalt nitrate hydrate with a maximum of six water
molecules for the 33-50% oxygen range.
Figure 3. XRD showing (A) a mixture of cobalt oxide (PDF 43-
1
004) and cobalt nitrate tetrahydrate (PDF 18-0425) produced when
the ratio of O
2
to N
2
is 6.6%, (B) cobalt nitrate hexahydrate (PDF 25-
1
219) as the primary product formed when the ratio of O
2
to N is
2
50%, (C) cobalt nitrate hexahydrate (PDF 25-1219) purchased from
Aldrich.
We have also carried out IR studies in order to investigate
the cobalt nitrate compound. Figure 4 compares the IR spectra
of cobalt nitrate hexahydrate formed by LVCC and cobalt nitrate
hydrate purchased from Aldrich. The nitrate group has six
normal vibatrations that are IR active. These are the antisym-
metric stretching V4 (A′), symmetric stretching V1 (A′), totally
symmetric stretching V2 (A′), out of plane bending V6 (A′′),
antisymmetric in-plane bending V5 (A′), and the symmetric in-

Formation of Cobalt Compounds
J. Phys. Chem. B, Vol. 108, No. 28, 2004 9607
energy necessary for binding is quenched. The formation of only
cobalt oxide (CoO) could be achieved by keeping the O2 ratio
at 3%. When the O2 ratio was increased to the range 33-50%,
the resulting product was found to be cobalt nitrate hydrate with
six water molecules. Work is in progress to further understand
such processes with other metals such as Ni, Fe, and Mn and
to produce various particle sizes of these metal mono oxides
by varying laser power, chamber pressure, temperature gradient,
etc.
Acknowledgment. One of the authors (G.G.) expresses his
gratitude to Dr. Matthew Scanlon (Fairmont State College) and
Dr. Fred King (West Virginia University) for helpful discussions
and valuable consultations. TEM was measured by Diane
Schwegler-Berry at the National Institute for Occupational
Safety and Health, Morganotwn, WV.
Figure 4. Comparison of IR spectra of cobalt nitrate hydrate purchased
from Aldrich (A) and synthesized by LVCC (B), respectively. The
-
1
nitrate normal vibrations are symmetric stretching V
1
(1388 cm ),
-1
totally symmetric stretching V
2
(1052 cm ), symmetric in-plane bending
(1629 cm ), antisymmetric
(729 cm ), and the out of plane bending V (870
-
1
-1
V
3
(664 cm ), antisymmetric stretching V
4
-
1
in-plane bending V
5
6
References and Notes
-
1
cm ).
(
1) El-Shall, M. S.; Graiver, D.; Pernisz, U.; Baraton, M. I. Nanostruct.
Mater. 1995, 6, 297.
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Baraton, M. I. J. Phys. Chem. 1995, 51, 17805.
(3) Jonsson, B. J.; Turkki, T.; Strom; El-Shall, M. S.; Rao, K. V. J.
Appl. Phys. 1996, 79, 5063.
plane bending V3 (A′) observed at 1629, 1388, 1052, 870, 729,
(
-
1
18-20
-1
and 664 cm , respectively.
The broad peak at 3500 cm
-
1
is indicative of moisture and the cluster of peaks at 2400 cm
can be assigned to CO2. No significant differences were
observed in the spectra which again confirm the formation of
cobalt nitrate hexahydrate.
(
(
4) Li, S.; Silvers, S.; El-Shall, M. S. J. Phys. Chem. 1997, 101, 1794.
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(6) Li, S.; El-Shall, M. S. Nanostruct. Mater. 1999, 12, 215.
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A 2002, A329-A331, 92.
One possible explanation for the formation of the cobalt
nitrate hexahydrate is the high temperature generated at the metal
surface by the laser. This provides enough energy to form NO
from N2 and O2. The NO then reacts with O2 to form NO2 (a
brown gas which was observed in the chamber within minutes
of turning on the laser). NO2 in the presence of O2 then reacts
(
(
(
(
10) Pithawalla, Y. B.; Deevi, S.; Deevi, S. C.; El-Shall, M. S. AdV.
2
1
with the metal vapor to produce the resulting metal nitrate. It
is also important to indicate that the resulting product was Co3O4
when nitrogen was replaced by helium or argon. This is similar
to chemical methods, where optimum conditions to produce
CoO were not successful.
Powder Metall. Particulate Mater. 2001, 9, 99.
(11) Li, Shoutian; Germanenko, Igor N.; El-Shall, M. S. J. Cluster Sci.
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1
(
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1990, 51, 1199.
(
1
Conclusion
(
The results reported here clearly indicate that nanoparticles
of cobalt, cobalt oxide, and cobalt nitrate hydrate were generated
via LVCC by controlling the ratio of N2 and O2 present in the
reaction chamber. Nanoparticles of cobalt can be synthesized
in pure nitrogen and at low levels of O2 since the metal vapor
produced is cooled though collisions with N2 before it can
interact with O2. As the concentration of O2 increases (0.3%-
(
(
(
(
19) Choca, M.; Ferraro J. R.; Nakamoto, K. J. C. S. Dalton 1972, 2297.
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3
33.
21) Bunce, N. EnVironmental Chemistry; Werz Publishing Ltd: Win-
nipeg, Canada, 1994.
(
2
%), the metal vapor is able to interact with oxygen before the