The preparation and phase transformation of nanocrystalline cobalt sulfides via a toluene thermal process

Article abstract of DOI:10.1021/ic980177i

Nanocrystalline cobalt sulfides were prepared by the reactions between cobalt chlorides and sodium polysulfide via a toluene thermal process in the temperature range 120-170°C. Two single phases of Co₉S₈ and CoS₂ were obtained. TEM microphotos showed that the Co₉S₈ and CoS₂ particles were both spherical in shape with sizes of about 20 nm. Chemical analysis gave the formulas Co₉S_7.93 and CoS_1.97, respectively. The transformations among cobalt sulfides (Co₉S₈, Co₃S₄, and CoS₂) with changing reaction conditions and precursors were studied.

Full text of DOI:10.1021/ic980177i

Inorg. Chem. 1999, 38, 2621-2623
2621
The Preparation and Phase Transformation of Nanocrystalline Cobalt Sulfides via a
Toluene Thermal Process
X. F. Qian, X. M. Zhang, C. Wang, Y. Xie, and Y. T. Qian*
Chemistry Department, University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
ReceiVed February 19, 1998
Nanocrystalline cobalt sulfides were prepared by the reactions between cobalt chlorides and sodium polysulfide
via a toluene thermal process in the temperature range 120-170 °C. Two single phases of Co9S8 and CoS2 were
obtained. TEM microphotos showed that the Co9S8 and CoS2 particles were both spherical in shape with sizes of
about 20 nm. Chemical analysis gave the formulas Co9S7.93 and CoS1.97, respectively. The transformations among
cobalt sulfides (Co9S8, Co3S4, and CoS2) with changing reaction conditions and precursors were studied.
Introduction
lower than 200 °C; however, only mixed phases of Fe3S4 and
FeS2 were obtained.
There are many phases of cobalt sulfides such as CoS, Co3S4,
CoS2, and Co9S8.1 Among them, Co9S8 and CoS2 have attracted
attention for their properties for a long time.2 Co9S8 is of
importance in hydrodesulfurization catalysts and magnetic
devices.3 Pyrite CoS2 is a metallic ferromagnet with a Curie
temperature of about 120 K.4
In this paper, nanocrystalline Co9S8 and CoS2 have been
successfully prepared via a toluene thermal process using sodium
polysulfide and cobalt chlorides as the starting materials. The
phase transformations among cobalt sulfides with changing
reaction conditions and redox atmosphere are discussed.
Crystalline transition-metal sulfides were synthesized using
solid state reactions between stoichiometric amounts of the
constituent elements in evacuated silica tubes in the temperature
range 500-1200 °C,4,5 but intermittent grinding and reheating
were often necessary to obtain single phases of these sulfides.
Hydrogen sulfide is another sulfur source to prepare transition-
metal sulfides. For instance, Co9S8 was prepared with anhydrous
cobalt sulfate salt in a flowing gas of hydrogen sulfide and
hydrogen at 525 °C.6 CoS2 was prepared by the reaction between
anhydrous hexammine cobalt(II) with hydrogen sulfide7 at low
temperature, and further treatment was necessary for crystal-
lization at 800 °C. Recently, pyrolysis of metal complexes8,9
having Co-S bonds inside has been carried out to prepare CoS2.
Overall, with those technologies, the products were highly
crystalline.
Experimental Section
Analytical grade solvent and reagents were purchased from Shanghai
Chemistry Co. Ltd. Na2S3 was prepared according to the literature.12
Preparation of Nanocrystalline Co9S8. Appropriate amounts of
CoCl2‚6H2O and Na2S3 (50% excess over CoCl2‚6H2O) were added to
a Teflon-lined autoclave of 120 mL capacity. The autoclave was filled
with toluene up to 75% of the total volume, maintained at 120 °C for
24 h, and then cooled to room temperature naturally. A black precipitate
was collected after being washed with carbon disulfide, absolute ethanol,
and distilled water. The final product was dried in a vacuum box at 80
°C for 4 h.
Preparation of Nanocrystalline CoS2. Appropriate amounts of
anhydrous CoCl2 and Na2S3 (50% excess over CoCl2) were added to a
Teflon-lined autoclave. The autoclave was filled with toluene up to
75% of the total volume, maintained in the temperature range 140-
170 °C for 12 h, and then cooled to room temperature naturally. The
washing procedures were the same to those for Co9S8.
To obtain single phases of transition-metal sulfides, it is
important to investigate phase tranformation during the prepara-
tion process. Hiromichi et al.10 reported the transformation
between Co9S8 and CoS in a H2S atmosphere above 600 °C.
Yamaguchi et al.11 investigated the phase transformations of
iron sulfides during hydrothermal processes at temperatures
Phase transformations were investigated by adding Zn or I2 to adjust
the redox atmosphere. The washing procedures were similar to those
for Co9S8 with the exception of adding dilute HCl (0.1 mol/L) in some
cases to remove impurities. The detailed processes and the as-prepared
samples are listed in Table 1.
The X-ray powder diffraction (XRD) patterns were recorded on a
Japan Rigaku Damax γA X-ray diffractometer with Cu KR radiation
(λ ) 1.541 78 Å). Transmission electron microscopy (TEM) images
were taken with a Hitachi H-800 transmission electron microscope.
The contents of the single-phase cobalt sulfides were determined by
chemical analysis.6
* Corresponding author.
(1) Wold, A.; Dwight, K. Solid State Chemistry; Chapman & Hall, Inc.;
New York, 1993.
(2) Wold, A.; Dwight, K. J. Solid State Chem. 1992, 96, 53.
(3) Pecoraro, T. A.; Chianelli, R. R. J. Catal. 1981, 67, 430.
(4) Morris, B.; Johnson, V.; Wold, A. J. Phys. Chem. Solids 1967, 28,
1565.
(5) Schneemeyer, L. F.; Sienko, M. J. Inorg. Chem. 1980, 19, 789.
(6) Pasquariello, D. M.; Kershaw, R.; Passaretti, J. D.; Dwight, K.; Wold,
A. Inorg. Chem. 1984, 23, 872.
(7) Passaretti, J. D.; Dwight, K.; Wold, A.; Croft, W. J.; Chianellli, R. R.
Inorg. Chem. 1981, 20, 2631.
(8) Singhal, G. H.; Botto, L. D.; Colle, K. S. J. Solid State Chem. 1994,
109, 166.
(9) Abboudi, M.; Mosset, A. J. Solid State Chem. 1994, 109, 70.
(10) Kiuchi, H.; Nakamure, I. Nippon Kosyo Kaishi 1983, 99, 401.
(11) Yamaguchi, S. Colloid Interface Sci. 1969, 31 (4), 578.
Results and Discussion
Figure 1a shows the XRD pattern of sample 1 (in Table 1).
All of the peaks can be indexed to the single phase of Co9S8.
After refinement, the cell constant a ) 9.92 Å is close to that
reported in the JCPDS cards (19-364). The size of Co9S8
(12) Draves, C. Z.; Tarter, H. V. J. Am. Chem. Soc. 1926, 48, 1527.
10.1021/ic980177i CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/06/1999

2622 Inorganic Chemistry, Vol. 38, No. 11, 1999
Qian et al.
Table 1. Reaction Conditions for the Preparation of Cobalt Sulfides
no.
precursors
additive
solvent
temp (°C)
time (h)
products
1
2
3
4
5
6
7
Na2S3 + CoCl2‚6H2O
Na2S3 + CoCl2‚6H2O
Na2S3 + CoCl2‚6H2O
toluene
toluene
toluene
toluene
toluene
toluene
toluene
120
150-170
170
120
170
170
120
24
12
12
4-12
4
12
12
Co9S8
Co9S8 + Co3S4 + CoS2
Co9S8
a
Zn
I2
I2
b
c
Co9S8
Co9S8 + Co3S4
b
Co9S8
Co9S8
CoS2 + Co3S4
b
I2
CoS2
CoS2
Na2S3 + CoCl2
a The sample prepared was washed with HCl additionally to remove Zn and Zn(OH)2. b Co9S8 is first formed by a toluene thermal process at 120
°C for 24 h. Without any posttreatment, I2 is added. c The amount of Co3S4 in the mixture is about 80%.
Figure 3. TEM microphoto of nanocrystalline cobalt sulfide pow-
ders: (a) TEM microphoto of nanocrystalline Co9S8 powder prepared
by the reaction between CoCl2‚6H2O and Na2S3 for 24 h at 120 °C;
(b) TEM microphoto of nanocrystalline CoS2 powder prepared by the
reaction between anhydrous CoCl2 and Na2S3 in the range of 120 °C
for 12 h.
cards No. 41-1471). The average size is about 20 nm estimated
by half-width of peaks using the Scherrer equation. This result
Figure 1. XRD patterns of nanocrystalline cobalt sulfide powders (C
also can be confirmed by the TEM image (Figure 3b) in which
) Co9S8, L ) Co3S4, P ) CoS2): (a) the single phase of Co9S8 prepared
the as-prepared CoS2 particles are in a uniform spherical shape
with an average size of about 20 nm.
by the reaction between CoCl2‚6H2O and Na2S3 for 24 h at 120 °C;
(b) a mixed phase of Co9S8, CoS2, and Co3S4 prepared by the reaction
between CoCl2‚6H2O and Na2S3 in the range 150-170 °C for 12 h;
(c) a mixed phase of Co3S4 and Co9S8 prepared through the reaction of
Co9S8 and I2 for 4-12 h at 120 °C; (d) a mixed phase of Co3S4 and
CoS2 prepared through the reaction of Co9S8 and I2 for 4 h at 170 °C;
(e) the single phase of CoS2 prepared by the reaction between Co9S8
and I2 for 12 h at 170 °C.
Figure 1c is the XRD pattern of sample 4, the reflections at
d spacings of 2.85, 1.67, and 2.36 Å corresponding to the 311,
440, and 400 planes of Co3S4. The reflections at d spacings of
1.76, 2.98, and 1.91 Å correspond to the 440, 311, and 311
planes of Co9S8. The amount of Co3S4 is about 80% estimated
by the relative intensity of peaks in the XRD pattern.
Chemical analysis was used to detect the sulfur contents of
the as-prepared single phases Co9S8 and CoS2. These samples
were heated to constant weight in a stream of oxygen. According
to ref 6, to prevent overoxidation of CoO, the heating temper-
ature should not exceed to 900 °C. Samples prepared by the
above procedure corresponded to the formulas Co9S7.93 and
CoS1.97, respectively.
In the case of the formation of Co9S8, the reaction could be
described as follows:
9CoCl2‚6H2O + 9Na2S3 f
Co9S8 + 18NaCl + 19S + 54H2O (1)
Figure 2. XRD patterns of nanocrystalline CoS2 prepared by the
reaction between anhydrous CoCl2 and Na2S3 at 120 °C for 12 h.
The optimum condition for preparing Co9S8 was about 120
°C for 24 h. The water of crystallization in the precursor CoCl2‚
6H2O is critical in the process. As anhydrous CoCl2 was used
as the precusor, only CoS2 was obtained. The reaction could
be described as follows:
particles is about 18 nm calculated by half-width of peaks using
the Scherrer equation.13 The TEM microphotograph (Figure 3a)
of the sample shows that the Co9S8 particles are in a uniform
spherical shape with an average size of about 20 nm.
The XRD pattern of sample 7 is shown in Figure 2. All of
the peaks can be indexed as the single phase of CoS2 (JCPDS
CoCl2 + Na2S3 f CoS2 + 2NaCl + S
(2)
With the presence of the water of crystallization in the CoCl2‚
6H2O, the temperature and duration of the reaction and the redox
(13) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; Wiley:
New York, 1974.

Nanocrystalline Cobalt Sulfides
Inorganic Chemistry, Vol. 38, No. 11, 1999 2623
atmosphere would also have effects on the formation of cobalt
sulfides and the transformations among them. If the reaction
temperature was higher than 150 °C, besides the phase of Co9S8,
the phases of CoS2 and Co3S4 were also present (Figure 1b),
which might be due to the high reactive activation of nano-
crystalline Co9S8 and an enriched sulfur environment in the
autoclave. The reactions could be as follows (CoSx represents
the phases of CoS2 and Co3S4 according to the literature14:
1c). If the reaction temperature reached up to 170 °C for 4 h,
only the mixed phases of Co3S4 and CoS2 (sample 5, Figure
1d) was obtained and no Co9S8 phase occurred. With prolonging
the reaction time up to 12 h at 170 °C, a single phase of CoS2
(sample 6, Figure 1e) was formed. The transformation may be
due to the presence of I2 which reacts with H2 to form HI and
decrease the amount of H2, thus making eq 4 transfer toward
the right and form CoSx (x > 1.06).14
H2 + I2 f 2HI
(7)
Na2S3 + 2H2O - H2S + 2NaOH + 2S
(3)
x > 1.06
(4)
The byproducts such as S, NaCl, and the excess Na2S3, Zn,
and I2 can be removed by washing with carbon disulfide,
absolute ethanol, hydrogen chloride (0.1 M), and distilled water.
In the preparation process, toluene is chosen due to its
appropriate boiling point (110.6 °C) and the pressure. The
pressure of the toluene15 is about 5 atm at 178 °C, which is
lower than that of water (about 10 atm) at the same temperature.
This low pressure makes the toluene thermal process safer than
the hydrothermal process. Furthermore, toluene is a poorly polar
organic solvent, and it can avoid the immediate reaction of Na2S3
and CoCl2‚6H2O at room temperature, which is beneficial to
control the rate of the reaction and the phase transformation.
Co9S8 + (9x-8)H2S T 9CoSx + (9x-8)H2
Through adjusting the redox atmosphere, the phase transfor-
mations among cobalt sulfides (Co9S8, Co3S4, and CoS2) could
be controlled.
A reductive atmosphere can be created by introducing Zn
powder to the reaction system. Within the temperature range
120-170 °C, the appearance of other cobalt sulfides such as
Co3S4 and CoS2 mentioned above vanished and only the single
phase of Co9S8 was obtained. This process could be explained
by the effect of a reductive atmosphere on the equilibrium in
eq 4. The reaction between zinc powder and crystalline water
of CoCl2‚6H2O can generate hydrogen as shown in eq 5, which
would prevent the equilbrium in eq 4 toward the right. Although
zinc powder itself can act as a reductive agent in many chemical
reactions directly, hydrogen was believed to be the main
reductive agent since it can disperse homogeneously in this
system, which is beneficial for the formation of the single phase
of Co9S8. At the same time Zn(OH)2 could react with H2S
generated in eq 3 to form ZnS, which could be removed easily
by dilute HCl solution.
Conclusion
Nanocrystalline cobalt sulfides were prepared by the reactions
between cobalt chlorides and sodium polysulfide via a toluene
thermal process in the temperature range 120-170 °C. Two
single phases of Co9S8 and CoS2 were obtained. TEM micro-
photos show that the Co9S8 and CoS2 particles are both spherical
in shape with sizes of about 20 nm. The ratios of Co to S for
Co9S8 and CoS2 are about 9:7.93 and 1:1.97, respectively,
determimed by chemical analysis. The phase transformations
among cobalt sufides (Co9S8, Co3S4, and CoS2) with changing
reaction conditions and redox atmosphere by adding Zn and I2
were studied. The interconversion reactions between the various
cobalt sulfides depending on the reaction conditions may be of
importance for other systems.
Zn + 2H2O f Zn(OH)2 + 2H2
Zn(OH)2 + 2H2S f ZnS + 2H2O
(5)
(6)
Acknowledgment. This work was supported by the Chinese
National Foundation of Natural Science Research (No. 59771017)
and Climbing Program Research-National Foundation Project.
If I2 was added to Co9S8 prepared according to reaction 1
and then reacted at 120 °C for 4-12 h, mixed phases of Co9S8
and Co3S4 (sample 4) were obtained, in which the amount of
Co3S4 is about 80% determined by the intensity of XRD (Figure
IC980177I
(14) Krabbes, G.; Oppermann, H.; Henke, J. Z. Anorg. Allg. Chem. 1978,
442, 79-85.
(15) Robert, C. W. CRC Handbook of Chemistry and Physics, 63rd. ed.;
CRC Press, Inc: Boca Raton, FL; p D-219.