Synthesis and properties of cobalt sulfide phases: CoS2 and Co9S8

Article abstract of DOI:10.1002/zaac.201300649

Single phase cobalt disulfide (CoS₂) nanoparticles were prepared by thermal decomposition of cobalt-thiourea complex at a low temperature (400 °C). CoS₂ nanoparticles exhibit ferromagnetic ordering at 122 K below which the temperatur

Full text of DOI:10.1002/zaac.201300649

Job/Unit: Z13649
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Date: 25-02-14 11:05:39
Pages: 7
ARTICLE
DOI: 10.1002/zaac.201300649
Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and Co9S8
Nitesh Kumar,[a] Natarajan Raman,[b] and Athinarayanan Sundaresan*[a]
Dedicated to Professor C. N. R. Rao on the Occasion of His 80th Birthday
Keywords: Cobalt sulfide; Ferromagnetism; Resistivity; Nanoparticles; Thermal decomposition
Abstract. Single phase cobalt disulfide (CoS2) nanoparticles were pre-
pared by thermal decomposition of cobalt-thiourea complex at a low
temperature (400 °C). CoS2 nanoparticles exhibit ferromagnetic order-
tance effect (6.5%). A mixture of CoS and Co9S8 phases were obtained
between the temperature interval 400 °C Ͻ T Ͻ 1000 °C. At 1000 °C,
a pure bulk Co9S8 phase was obtained. It exhibits magnetic hysteresis
ing at 122 K below which the temperature dependent resistivity of cold typical of a ferromagnet at room temperature and a peak in magnetiza-
pressed nanoparticles deviates from metallic behavior and shows a
broad maximum. Just below TC, it also exhibits a large magnetoresis-
tion at low temperature with a strong relaxation indicating possible
spin glass state.
ticles of phases like CoS2, CoS, and Co9S8 are carried out
hydrothermally at relatively lower temperatures (120–230 °C).
Bao et al. demonstrated bio-assisted CoS nanowires synthesis
by hydrothermal method using cysteine as the sulfur source.[10]
The same sulfur source is used to synthesize CoS2 nanopar-
ticles on graphene surface.[11] CoS nanocrystals are grown on
graphene hydrothermally starting with thioacetamide as the
sulfur source for supercapacitor applications.[12] Other sulfur
sources used for the synthesis of cobalt sulfide nanostructures
are thiourea,[13] H2S,[14] Na2S,[15,16] Na2S2O3,[17] etc. High
electrial conductivity in many cobalt sulfide phases has made
these materials ideal for many energy devices like dye sensi-
tized solar cells (DSC), Li-ion batteries, and supercapacitors.
Phases like CoS2, CoS, and Co9S8 have been used as counter
electrode in DSC, where it collects electrons from the external
circuit and speeds up the reduction of I3– for dye degrada-
tion.[18–22] CoS2 and Co9S8 are also used as anode material in
Introduction
Transition metal sulfides are very important class of materi-
als known for their rich structural diversities along with inter-
esting and technologically relevant electronic and magnetic
properties.[1] One additional advantage is that they are cheap
and abundant, many of which are found in nature in the form
of minerals such as heazlewoodite (Ni3S2), chalcocite (Cu2S),
pyrite (FeS2) etc. Among all, sulfides which crystallize in py-
rite structure (FeS2, CoS2, NiS2, CuS2, and ZnS2) show great
variation in electronic properties by changing transition metal
ion and also by substituting Se or Te in the anion site.[2-4]
According to the band theory FeS2 and ZnS2 are semiconduct-
ing while CoS2 and CuS2 are metallic.[2] NiS2 is a Mott insu-
lator.[5] In CoS2, Co2+ remains in the low spin state with one
electron in the eg sub-band. Because of the intermediate
strength of the eg electron correlation, ferromagnetic transition
temperature (Curie temperature, TC) is found to be less (120 K)
with fractional magnetic moment (0.85 B.M.).[6–8] The elec- Li-ion battery.[23,24] Wang et al. have shown discharge capacity
as large as 1210 mAhg–1 and good cyclability for CoS2
hollow spheres.[23] Cobalt sulfides can also store charge as
pseudocapacitors in redox processes during charging and dis-
charging. Different morphologies and composite of cobalt sulf-
ide with graphene have been tried for supercapacitor applica-
tions.[12,25–29] Nanocrystalline CoS2 phase has also been tried
to be used as a material for electrocatalysis of oxygen re-
duction reaction in fuel cell.[30]
Pyrite type CoS2 is particularly interesting in the cobalt-
sulfur phase diagram for its interesting magnetic and electronic
properties. Properties of bulk polycrystals and single crystals
have been extensively studied but studies of corresponding
properties of nanocrystalline CoS2 are scarce in the literature.
Herein, we have demonstrated the synthesis of CoS2 nanopar-
ticles from a single source precursor, namely, cobalt-thiourea
complex, by decomposing it at 400 °C. The electrical and mag-
trons, which are responsible for ferromagnetic interaction, also
give rise to electronic conduction in CoS2.
The phase diagram of cobalt sulfide is fairly complex con-
taining Co4S3, Co9S8, CoS, Co3S4, Co2S3, and CoS2 phases.[9]
Bulk polycrystalline materials of these phases are synthesized
by heating elemental sulfur and cobalt together in silica am-
poules. Corresponding single crystals are prepared by adding
a flux such as CoBr2 to the elemental reactants. Recently,
chemical synthesis of nanoparticles and submicron sized par-
* Dr. A. Sundaresan
Fax: +91-80-2208 2766
E-Mail: sundaresan@jncasr.ac.in
[a] Chemistry and Physics of Materials Unit
Jawaharlal Nehru Centre for Advanced Scientific Research
Bangalore-560064, India
[b] Department of Chemistry
VHNSN College
Virudhunagar-626 001, India
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netic properties of the CoS2 nanoparticles resemble that of the
bulk sample. We have also investigated electrical and magnetic
properties of bulk Co9S8 prepared by heating the complex at
1000 °C.
Results and Discussion
Several thiourea complexes of cobalt have been reported in
the literature. Cotton et al. carried out detailed synthesis
and characterization of Co(CH4N2S)2Cl2, Co(CH4N2S)2Br2,
Co(CH4N2S)4(NO3)2,
Co(CH4N2S)3SO4,
and
Co(CH4N2S)4(ClO4)2. Herein, we choose the most volatile of
them, Co(CH4N2S)4(NO3)2, (b.p. 127 °C) for thermal decom-
position in order to obtain different phases of cobalt sulfide.
Figure 1a shows the TGA data of Co(CH4N2S)4(NO3)2 in ni-
trogen atmosphere obtained by ramping the temperature at a
speed of 10 °C·min–1. Till 200 °C the complex is almost stable
apart from a small weight loss because of removal of absorbed
water and/or alcohol. Near 200 °C there is a sudden weight
loss because of the decomposition of the complex with the
evolution of gases containing nitrogen and sulfur. This is con-
tinued till 280 °C after which the weight loss becomes gradual.
The region between 280 °C and 580 °C is perhaps due to the
removal of sulfur from already formed cobalt sulfide phases.
After 600 °C, the weight loss is very slow which implies that
low sulfur content phases like CoS and Co9S8 would be stable
for wide range of temperature. In contrast, high sulfur contain-
ing phase like CoS2 could be stabilized for a narrow range of
temperature because of the steeper change in the weight loss
at low temperature.
The complex exhibits paramagnetic behavior down to the
lowest measured temperature (2 K). The susceptibility and in-
verse susceptibility plots are shown in Figure 1b. The effective
paramagnetic moment per formula unit is calculated at 300 K
by using the relation: μeff = 2.83 (χmT)1/2. The observed μeff is
4.84 B.M., which is close to the earlier report of 4.75 B.M. for
high spin Co2+.[31] This confirms the absence of any magnetic
impurity as well as the unreacted starting material.
Figure 1. (a) TGA and (b) magnetic susceptibility of cobalt-thiourea
complex.
is obtained. Therefore, we have investigated the electrical and
magnetic properties of CoS2 and Co9S8 phases.
The thermal decomposition of the cobalt-thiourea complex
is carried out at different temperatures in order to obtain dif-
ferent phases of cobalt sulfide. Such strategy to obtain metal
sulfides which involves single precursor solution-free ap-
proach is not very well explored in the literature. We have
recently demonstrated phase evolution of different nickel sulf-
ide phases using this strategy.[32] When the complex is heated
at 400 °C for 2 h in nitrogen atmosphere a cubic CoS2 phase
is obtained. For making other phases of cobalt sulfide, we de-
composed the complex at higher temperatures till 1000 °C.
Figure 2 shows the XRD patterns of samples obtained at dif-
ferent temperatures. At 500 °C, CoS2 phase almost disappears
with major contributions from an unknown phase and NiAs
type hexagonal CoS. This unknown phase does not match to
any of the reported phase in the Co-S phase diagram and could
be attributed to a nonstoichiometric composition. CoS is the
only major phase with a small amount of Co9S8 at 600 °C. At
900 °C Co9S8 becomes the major phase with small amount of
Figure 2. XRD patterns of cobalt sulfide phases obtained at different
CoS as the secondary phase. At 1000 °C, pure phase of Co9S8 temperatures.
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Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and Co9S8
Figure 4. (a) ZFC-FC magnetizations and (b) isothermal magnetiza-
tion of CoS2 nanoparticles with inset showing the enlarged view of
magnetization near the origin.
Figure 3. (a) XRD pattern and (b) TEM image of CoS2 nanoparticles
with electron diffraction pattern in the inset.
inside the bulk remain parallel to each other. The effect of
The XRD pattern of CoS2 nanoparticles is shown in Fig- surface anisotropy is also observed in the magnetic hysteresis
ure 3a. It crystallizes in cubic pyrite type structure (space of nanocrystalline CoS2 as shown in Figure 4b. Inset of Fig-
¯
group, Pa3). Crystallite size of the CoS2 nanoparticles is ob- ure 4b shows the enlarged view of the magnetization near the
tained to be 23 nm using Scherrer’s formula.TEM image of origin. The value of coercive field (HC) is about 100 Oe at
CoS2 nanoparticles is shown in Figure 3b. The average size of 2 K, which is otherwise non hysteretic in the bulk form. The
the particles is ca. 20 nm with no defined morphology. Spots value of HC decreases as the temperature is increased and al-
in the electron diffraction pattern reveal the crystalline nature most vanishes near TC. Unlike the bulk phase where the satura-
of the particles.
tion magnetization (MS) is 0.84 B.M., CoS2 nanocrystals exhi-
Figure 4a shows the zero field cooled (ZFC) and field co- bit a value of 0.61 B.M. which is around 25% less. Such a
oled (FC) magnetizations of CoS2 nanoparticles obtained at an decrease in the magnetization is often observed in the ferro-
applied magnetic field of 100 Oe. It undergoes a ferromagnetic magnetic nanoparticles because of the surface disorder.
transition at 122 K (TC) which is consistent with the previous
CoS2 is known to be a metallic ferromagnet. Figure 5a
studies of bulk CoS2 phases. Surprisingly, Lei et al.[33] reported shows the resistivity of CoS2 nanoparticles as a function of
a TC of 140 K for a composition CoS1.93 and attributed the temperature in the cold pressed form using four probe tech-
increase in the TC to the sulfur vacancies as compared to the nique. Interestingly, connectivity among the nanoparticles is
bulk CoS2. There are no further reports of magnetic properties sufficient enough to exhibit metallic behavior down to the
of CoS2 nanoparticles. The fact that the TC of the present sam- magnetic transition temperature. Such a behavior is rare in the
ple is 122 K rules out the possibility of nonstoichiometry. En- bulk electrical resistivity measurements of otherwise intrin-
ergy dispersive X-ray spectroscopy (EDX) shows the composi- sically conducting nanoparticles and they are often observed
tion to be CoS2.04 for our sample. Another main feature of the to be showing semiconducting property because of electron
ZFC-FC magnetizations is the large divergence at low tem- hopping between loosely connected grains. We observe an
perature compared to the bulk material, which can be attributed anomaly in the resistivity corresponding to the ferromagnetic
to the surface anisotropy of the nanoparticles. According to transition (120 K). A broad hump is observed because of the
Coey and co-workers,[34] due to the competition between sur- increase in the resistivity below the transition temperature
face and bulk magnetocrystalline anisotropy the spins at the which again decreases at low temperature. Similar but much
surface tend to orient normal to the surface while the spins sharper hump is observed in bulk CoS2. The origin of this
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Figure 5. (a) Resistivity as a function of temperature and (b) magne-
toresistance at fixed temperatures.
Figure 6. (a) XRD pattern and (b) temperature dependent resistivity
of Co9S8.
anomaly is because of a very interesting property of CoS2, octahedral hole. Figure 6a shows the XRD pattern of the sam-
namely, the transformation to a half metal below TC. Turning ple obtained at 1000 °C, which can be fitted with Co9S8 phase.
to a half metal implies that due to exchange splitting the den- Cell parameter of Co9S8 as obtained from Lebail fitting is
sity of states for minority spins near the Fermi level decreases 9.9282(9) Å. We cold-press the powder and measure the resis-
drastically, which therefore increases the resistivity. Effect of tivity as a function of temperature by employing four probe
magnetic field on resistivity of bulk CoS2 has been studied in technique. It shows metallic behavior throughout the tempera-
detail but the same has not been explored in nano- ture range with a resistivity value of 3.5 mΩ·cm–1 at 300 K as
particles.[6,35–37] Figure 5b shows magnetoresistance of cold shown in Figure 6b. Figure 7a shows ZFC-FC magnetizations
pressed CoS2 nanoparticles in a configuration where the mag- of Co9S8 at 100 Oe applied field. Unlike the finding of Lotger-
netic field is applied perpendicular to the current direction. ing et al.,[38] wherein Curie-Weiss law is observed between
Magnetoresistance of 6.5% is observed at 115 K and 5 T, 150 and 300 K, we find an entirely different behavior. There
which varies almost linearly with respect to field. At 70 K is a strong irreversibility between ZFC-FC curves and hystere-
there is a negative contribution at low field but as the field is sis at room temperature indicating ferromagnetism at room
increased positive contribution takes over. At a temperature temperature. The observation of high magnetic ordering tem-
significantly higher than the TC, resistivity does not depend on perature in transistion metal sulfides is uncommon. Therefore,
the magnetic field. The negative contribution is due to the spin the observed ferromagnetism could be due to cobalt metal.
orientation effect of H, while the positive contribution can be However, we do not see any impurity peak in X-ray diffraction
explained by the Lorentz force magnetoresistance. It has also pattern. Moreover, the obtained saturation magnetization at
been shown that magnetoresistance diverges near the transition room temperature is ca. 10% of the cobalt metal. Such a high
temperature.[6] Co9S8 is the high temperature phase in the amount of cobalt metal should be seen in X-ray diffraction.
Co-S phase diagram, which is formed peritectically at 835 °C. EDX analysis for the compound at different regions provides
We obtain a pure phase of Co9S8 on decomposing the Co- an average composition of Co9S7.95, which again rules out the
thiourea complex at 1000 °C. Co9S8 crystallizes in a face cen- possibility of ca. 10% Co metal impurity in the sample. It
¯
tered cubic structure (space group, Fm3m). It consists of cubic requires further investigation to know the exact ordering tem-
closed packed sulfur atoms with one cobalt atom in each of perature and origin of magnetic ordering. At low temperature
the tetrahedral holes and the ninth cobalt atom occupies an (about 5 K), a peak in the ZFC magnetization is observed. To
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Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and Co9S8
Experimental Section
Synthesis of [Co(tu)4.(NO3)2]: Synthesis of [Co(tu)4.(NO3)2] was car-
ried out following a previously reported procedure.[31] In a typical syn-
thesis, Co(NO3)2·6H2O (2.91 g, 0.01 mol) was dissolved in red hot
butanol (25 mL). Thiourea (3.04 g, 9.04 mol) was added and the mix-
ture was heated to boiling, until all solid had dissolved. The color of
the solution changed from red to blue. On cooling, a blue solid sepa-
rated. This was suction filtered, washed with diethyl ether, dried under
vacuum, and recrystallized from ethyl acetate. This produced large
green-blue crystals, which were dried in vacuum at 100 °C.
Synthesis of Cobalt Sulfide Phases: The complex was heated at
different temperatures between 400 and 1000 °C for 2 h in nitrogen
atmosphere to obtain several phases of cobalt sulfide.
Characterization: X-ray diffraction patterns were obtained with
Bruker D8 Discover and Rigaku 99 diffractometers. Rietveld refine-
ment was carried out using FullProf software. Transmission electron
microscopy images were taken from JEOL JEM 3010 fitted with Gatan
CCD camera operating at accelerating voltage of 300 kV. Magnetic
properties were studied with SQUID VSM, Quantum Design, USA.
DC electrical resistivity measurements were carried out in Physical
Property Measurement System (PPMS), Quantum Design, USA. Ex-
periments for thermogravimetric analysis (TGA) were conducted with
a Mettler Toledo Star system.
Acknowledgements
NR thanks Jawaharlal Nehru Centre for Advanced Scientific Research
(JNCASR), Bangalore for providing all the necessary research facili-
ties and Visiting Fellowship.
Figure 7. (a) ZFC-FC magnetization of Co9S8 at 100 Oe with inset
showing room temperature hysteresis and (b) magnetization vs. time
plot in ZFC and FC conditions at 2 K and 200 Oe.
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not observe any relaxation under ZFC conditions, which con-
firms the low temperature peak (5 K) to be due to spin freez-
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Received: December 14, 2013
Published Online: ᭿
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Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and Co9S8
N. Kumar, N. Raman, A. Sundaresan* ............................. 1–7
Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and
Co9S8
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