Single-source approach to cubic FeS2 crystallites and their optical and electrochemical properties

Article abstract of DOI:10.1021/ic049049m

Cubic FeS₂ crystallites have been synthesized via a single-source approach using iron diethyldithiocarbamate as precursor under hydrothermal conditions. The sample is characterized by XRD, IR, TEM, and FESEM. The optical properties of the as-prepared FeS₂ reveal that there exists a red shift compared with that of bulk materials. Meanwhile, the electrochemical properties of FeS₂ demonstrate that it delivers a large discharge capacity, which might find possible application as an electrode material in lithium cells. It is also found that the reaction temperature is of importance to the formation of cubic FeS₂ crystallites.

Full text of DOI:10.1021/ic049049m

Inorg. Chem. 2005, 44, 951−954
Single-Source Approach to Cubic FeS2 Crystallites and Their Optical
and Electrochemical Properties
Xiangying Chen, Zhenghua Wang, Xiong Wang, Junxi Wan, Jianwei Liu, and Yitai Qian*
Structure Research Laboratory and Department of Chemistry, UniVersity of Science and
Technology of China, Hefei, Anhui 230026, P. R. China
Received July 17, 2004
Cubic FeS2 crystallites have been synthesized via a single-source approach using iron diethyldithiocarbamate as
precursor under hydrothermal conditions. The sample is characterized by XRD, IR, TEM, and FESEM. The optical
properties of the as-prepared FeS2 reveal that there exists a red shift compared with that of bulk materials. Meanwhile,
the electrochemical properties of FeS2 demonstrate that it delivers a large discharge capacity, which might find
possible application as an electrode material in lithium cells. It is also found that the reaction temperature is of
importance to the formation of cubic FeS2 crystallites.
Introduction
as the cathode for high-energy-density batteries.9,10 and as
depolarizer anode for hydrogen production.11 Considerable
progress has been made since Wo¨hler12 first prepared
artificial pyrite by reaction of Fe2O3 with liquid sulfur and
NH4Cl in an open system. Many different methods were put
forward to prepare FeS2 crystals such as the toluene-thermal
process,13 in situ formation of boron sulfides and their
subsequent reactions with metal-source oxides (Fe2O3 or
Fe3O4) at 550 °C,14 the plasma-sprayed approach,15 and the
thermal spray method.16
To search for more green or environmentally benign
processes, it is necessary to reduce the amounts of organic
solvents and hazardous substances.17 From the viewpoint of
green chemistry, the hydrothermal approach is a good
candidate since the reaction can proceed at a mild temper-
ature in H2O under a sealed environment.
During the past decades, much attention has been focused
on the fabricating nanoscale inorganic materials, which have
exhibited fascinating chemical and physical properties and
potential applications in the future devices.1-3 Furthermore,
transition metal sulfides are of great interest for industrial
applications such as catalysis, lubrications, refractories,
pigments, battery fabrication, and optical and magnetic
devices, because of their wide range of semiconducting
properties and unique structural characteristics.4-6
FeS2 crystallizes in the pyrite structure with the space
group of Pa3, in which the Fe atoms are octrahedrically
coordinated by six S atoms, while the S atoms are tetrahe-
drally coordinated to three Fe atoms and one S atom. It has
received growing attention in recent years owing to its
potential as a useful material for solar-energy applications,7,8
Herein, we present a single-source approach to fabricate
cubic FeS2 crystallites by the thermal decompostion of an
iron diethyldithiocarbamate complex obtained by mixing
sodium diethyldithiocarbamate (NaS2CNEt2 or NaDDTC)
and FeCl3 in absolute ethanol. It was reported that NaDDTC
* Author to whom correspondence should be addressed. E-mail:
ytqian@ustc.edu.cn. Tel.: +86-551-3606647. Fax: +86-551-3607402.
(1) Sardar, K.; Rao, C. N. R. AdV. Mater. 2004, 16, 425.
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State Chem. 2003, 31, 5.
(3) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32,
435.
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(10) Au, M. J. Power Sources 2003, 115, 360.
(11) Lalvani, S. B.; Shami, M. J. Electrochem. Soc. 1986, 133, 1364.
(12) Wo¨hler, W. Liebigs Ann. 1836, 17, 260.
(5) Ferrer, I. J.; Caballero, F.; Delas, H. C.; Sarchez, C. Solid State
Commun. 1994, 89, 349.
(13) Qian, X. F.; Zhang, X. M.; Wang, C.; Xie, Y.; Wang, W. Z.; Qian,
Y. T. Mater. Sci. Eng., B 1999, 64, 170.
(14) Wu, L. M.; Seo, D. K. J. Am. Chem. Soc. 2004, 126, 4676.
(15) Guidotti, R. A.; Reinhardt, F. W.; Dai, J. X.; Roth, J.; Reisner, D. E.
J. New Mater. Electrochem. Syst. 2002, 5, 273.
(16) Reisner, D. E.; Xiao, T. D.; Ye, H.; Dai, J.; Guidotti, R. A.; Reinhardt,
F. W. J. New Mater. Electrochem. Syst. 1999, 2, 279.
(17) Clark, J. H. Green Chem. 1999, 1, 1.
(6) Tang, K. B.; Qian, Y. T.; Zeng, J. H.; Yang, X. G. AdV. Mater. 2003,
15, 448.
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K.; Bronold, M.; Hopfner, Ch.; Tributsh, H. Sol. Energy Mater. Sol.
Cell 1993, 29, 289.
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10.1021/ic049049m CCC: $30.25
Published on Web 01/19/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 4, 2005 951

Chen et al.
Figure 1. Possible framework of the metal complex structure.
Figure 2. IR spectra of the as-prepared Fe-DDTC complex (b) and pure
NaDDTC (a).
could be used as complex reagent to form Sb2S3 and Bi2S3
nanorods,18,19 but there is no report on the preparation of
FeS2 formed with the aid of NaDDTC. The optical properties
of the as-prepared cubic FeS2 crystallites reveal that there
exists a red shift compared with that of bulk materials.
Furthermore, the electrochemical properties of FeS2 dem-
onstrate that it delivers a large discharge capacity, which
might find possible application as an electrode material in
lithium cells. The single-source approach may open the
opportunity to fabricate other transition metal sulfides or
polysulfides simply using NaDDTC as complex reagent and
sulfur source.
microscopy (TEM) images were characterized by Hitachi H-800
transmission electron microscope with a tungsten filament and an
accelerating voltage of 200 kV. Field emission scanning electron
microscopy (FESEM) images were taken with a JEOL JSM6700F
scanning electron microscope.
Optical absorption spectra of the sample at ambient temperature
were recorded at optical wavelength between 1200 and 1500 nm
with a Shimadzu UV-365 spectrophotometer. The electrochemical
performances were examined with metallic lithium as the anode at
room temperature. The positive electrodes were fabricated by
pasting slurries of the as-prepared FeS2 crystallites (85 wt %),
carbon black (Super P, 10 wt %), and polyvinylidene (PVDF, 5 wt
%) dissolved in N-methylpyrrolidinone (NMP) on Al foil strips by
the doctor blade technique. Then the strips were dried at 160 °C
for 24 h in an air oven, pressed under 20 MPa pressure, and kept
at 120 °C for 12 h in a vacuum. The electrolyte was 1 M LiPF6 in
a 1:1 mixture of ethylene carbonate (EC)/diethyl carbonate (DEC);
the separator was Celgard 2500. The cells were assembled in the
glovebox filled with highly pure argon gas. The cells were
galvanostatically cycled in the 2.5-1.0 V range at a current density
Experimental Section
All the analytical reagents were purchased from Shanghai
Chemical Reagents Company and used without further purification.
Synthesis of Iron Diethyldithiocarbamate Complex. The iron
diethyldithiocarbamate complex was prepared from stoichiometric
amounts of 1 mmol of FeCl3‚6H2O and 3 mmol of NaDDTC in 50
mL of absolute ethanol. The obtained black precipitate was filtered
off, washed with distilled water and absolute ethanol several times,
and dried at room temperature. The reaction for the preparation of
iron diethyldithiocarbamate complex can be formulated as follows:
of 0.2 mA‚cm-2
.
Results and Discussion
FeCl3 + 3NaS2CNEt2 f Fe(S2CNEt2)3 + 3NaCl
IR spectra of the as-prepared Fe-DDTC complex (b) and
pure NaDDTC (a) are given in Figure 2. In Figure 2b, the
characteristic absorption peaks at 1490 and 987 cm-1 can
be assigned to the stretching vibrations of C-N and C-S,
respectively,21 and the frequency of the C-N stretching
vibration of the complex shifted from 1477 cm-1 in the pure
NaDDTC (Figure 2b) to 1490 cm-1. The shift of the C-N
stretching vibration implies that the sulfur in the diethylth-
iocarbamate coordinates with the metal ion in the complex.22
On the basis of the IR spectra, it is believed that the
Fe(DDTC)3 complex is successfully synthesized via the
reaction of NaDDTC and FeCl3‚6H2O in absolute ethanol.
Figure 3 shows the XRD patterns of FeS2 crystallites
obtained at different reaction temperatures. When the reaction
was carried out at 160 °C for 12 h, the products were of
mixture of orthorhombic phase FeS2 (JCPDS Card No. 37-
0475, marked with “9”) and cubic phase FeS2 (JCPDS Card
On the basis of previous reports,19,20 a possible framework for the
metal complex structure is demonstrated in Figure 1.
Synthesis of Cubic FeS2 Crystallites. In a typical procedure,
0.4 g of Fe(S2CNEt2)3 and 40 mL of distilled water were added to
a Teflon-lined stainless steel autoclave with a capacity of 50 mL.
The autoclave was sealed, maintained at 180 °C for 12 h, and cooled
to room temperature naturally. The precipitate was filtered off,
washed with distilled water and absolute ethanol several times, and
dried in a vacuum at 50 °C for 6 h.
Instruments and Characterization. X-ray powder diffraction
(XRD) patterns were determined using a Philips X’Pert PRO
SUPER X-ray diffractometer equipped with graphite-monochro-
matized Cu KR radiation (λ ) 1.541 874 Å). Fourier transform
infrared (FTIR) spectra of the as-prepared products were recorded
at room temperature with a KBr pellet on a VECTOR-22 (Bruker)
spectrometer ranging from 400 to 4000 cm-1. Transmission electron
(18) Xie, G.; Qiao, Z. P.; Zeng, M. H.; Chen, X. M.; Gao, S. L. Cryst.
Growth Des. 2004, 4, 513.
(19) An, C. H.; Tang, K. B.; Yang, Q.; Qian, Y. T. Inorg. Chem. 2003,
42, 8081.
(21) Monteiro, O. C.; Nogueira, H. I. S.; Trindade, T. Chem. Mater. 2001,
13, 2103.
(22) Yamaguchi, A.; Penland, R. B.; Mizushima, S.; Lane, T. J.; Curran,
C.; Quagliano, J. B. J. Am. Chem. Soc. 1958, 80, 527.
(20) Kunkely, H.; Vogler, A. Inorg. Chem. Commun. 2002, 5, 730.
952 Inorganic Chemistry, Vol. 44, No. 4, 2005

Single-Source Approach to Cubic FeS2 Crystallites
Figure 5. Optical absorption spectrum of FeS2 crystallites measured at
ambient temperature.
Figure 3. XRD patterns of FeS2 crystallites obtained at different reaction
temperatures: (a) 180 °C; (b) 160 °C; (c) 140 °C; (d) 120 °C.
Figure 6. Discharge curve of cubic FeS2 crystallites recorded with a current
density of 0.2 mA/cm2.
to the direct band gap (0.93 eV) of bulk FeS2,23,24 there exists
a red shift between them. It is probably caused by the cubic
morphology of FeS2 crystallites since Wei et al.24 reported
that the structure, crystallization, and stoichiometry remark-
ably influence the optical performance of the as-obtained
pyrite. As for the light absorption at 1500 nm and probably
below, it is generally assigned to the absorption of minor
water in the absolute ethanol.
Electrochemical performance of the as-prepared cubic FeS2
crystallites in the cell configuration of Li/FeS2 was evaluated.
Figure 6 shows the discharge curve of cubic FeS2 crystallites
on the first cycle with cutoff voltage of 1.0 V at a current
density of 0.2 mA/cm2. The discharge capacity of cubic FeS2
crystallites can reach about 756 mA h/g, which is ap-
proximately equivalent to that of natural FeS2 cell (750 mA
h/g) or synthetic FeS2 cell (775 mA h/g) reported by Horn
et al.25,26 The high capacity of cubic FeS2 crystallites makes
it attractive as a possible cathode for rechargeable lithium
cells in the future.
Figure 4. FESEM (a, b) and TEM (c) images of cubic FeS2 crystallites
obtained at 180 °C.
No. 71-2219, marked with “*”). When the reaction temper-
ature is increased to 180 °C, pure cubic phase FeS2 with a
lattice constant of a ) 5.414 Å can be obtained, which is
consistent with the reported value (JCPDS Card No. 71-2219,
a ) 5.417 Å). Meanwhile, the XRD patterns in Figure 3c,d
recorded at the reaction temperature of 140 or 120 °C are
indiscernible.
Figure 4 a,b shows the FESEM images of cubic FeS2
crystallites with different magnifications. It is seen that there
exists a large number of cubic FeS2 crystallites with the
average diameter of 500 nm. The representative TEM image
of the sample is shown in Figure 4c, consistent with those
of a sample observed by FESEM.
It was reported by Joris et al.27 that a noncatalyzed
decomposition of the DTC- ion could occur by means of a
proton transfer from nitrogen to sulfur according to the
following equations: However, there is no free proton for
The optical properties of FeS2 crystallites dispersed in
absolute ethanol by sonication were investigated with ambi-
ent temperature absorption spectroscopy (Figure 5). The
product shows strong light absorption in the lower energy
region. The FeS2 crystallites possess a well-defined, broad
optical absorption located at 1420 nm (0.87 eV). In contrast
(23) Yang, T. R.; Yu, J. T.; Huang, J. K.; Chen, S. H.; Tsay, M. Y.; Huang,
Y. S. J. Appl. Phys. 1995, 77, 1710.
(24) Wan, D. Y.; Wang, Y. T.; Wang, B. Y.; Ma, C. X.; Sun, H.; Wei, L.
J. Cryst. Growth 2003, 253, 230.
(25) Horn, Y. S.; Horn, Q. C. Electrochim. Acta 2001, 46, 2613.
(26) Horn, Y. S.; Osmialowski, S.; Horn, Q. C. J. Electrochem. Soc. 2002,
149, A14994.
Inorganic Chemistry, Vol. 44, No. 4, 2005 953

Chen et al.
recorded at the reaction temperature of 140 or 120 °C are
also indiscernible, which are shown in Figure 3c,d. Therefore,
it is believed that the reaction temperature is of importance
to prepare pure cubic FeS2 crystallites.
Conclusions
In summary, we have synthesized cubic FeS2 crystallites
through a single-source approach using iron diethyldithio-
carbamate as precursor under hydrothermal conditions. The
as-prepared products are composed of a large number of
cubic FeS2 crystallites with the average diameter of 500 nm.
The optical properties of the products show a red shift in
contrast to that of bulk FeS2. The electrochemical properties
of FeS2 demonstrate that it delivers a large discharge
capacity. The experimental results show that the reaction
temperature is of importance to the formation of cubic FeS2
crystallites. This single-source approach might open an
opportunity to prepare other transition metal sulfides in the
future.
the hydrogen bond between nitrogen and sulfur in the Fe-
DDTC precursors (shown in Figure 2), and the sulfur atoms
in the dialkyldithiocarbamate are coordinated to the metal
(Fe) ion. The formation process of the cubic FeS2 crystallites
perhaps proceeds as follows: A hydrogen bond probably
forms between the nitrogen and sulfur with the aid of the
proton from H2O suggested by Xie et al.,18 and then the
hydroxide anion will attack the electrophilic site of the ligand.
Followed by the possible cleavage of Fe-S and C-S bonds
under hydrothermal conditions, a simultaneous oxidation and
reduction (Fe3+ f Fe2+; S2- f S-) might occur in the
autoclave, which further results in the formation of FeS2
crystallites.
Acknowledgment. The National Natural Science Foun-
dation of the P. R. China and the 973 Program are gratefully
acknowledged for their financial help. The authors thank Mr.
Fanqing Li for his help in FESEM observations.
To investigate the influences of reaction conditions on the
structure and morphology of the product, many experiments
were conducted. When the reaction is carried out at 160 °C
for 10 h, pure FeS2 cannot be obtained by the characterization
of XRD (shown in Figure 3b). When the reaction temperature
is increased to 200 °C, pure FeS2 can be obtained as that
prepared at 180 °C. At the same time, the XRD patterns
IC049049M
(27) Joris, S. J.; Aspila, K. I.; Chakrabarti, C. L. Anal. Chem. 1970, 42,
647.
954 Inorganic Chemistry, Vol. 44, No. 4, 2005