New Solid-Gas Metathetical Synthesis of Binary Metal Polysulfides and Sulfides at Intermediate Temperatures: Utilization of Boron Sulfides

Article abstract of DOI:10.1021/ja0392521

A new simple synthetic method for binary metal polysulfides and sulfides was developed by utilizing an in situ formation of boron sulfides and their subsequent reactions with metal-source oxides in a closed container at intermediate temperatures above 350 °C at which the boron sulfides react in a gaseous form. The versatility of the new method is demonstrated with oxides of various transition metals (Ti, V, Mn, Fe, Ni, Nb, Mo, Ru, and W) and rare-earth metals (Y, Ce, Nd, Sm, Eu, Tb, and Er) as starting materials that exhibit different chemical characteristics. Regardless of the oxidation states of metals in the starting materials, the sulfidation reactions occurred quantitatively with stoichiometric mixtures of boron and sulfur, and within 24 h the reactions yielded pure products of TiS₂, TiS₃, VS₄, FeS₂, NiS₂, NbS₃, MoS ₂, RuS₂, WS₂, Y₂S₃, and RS₂ (R = Ce, Nd, Sm, Eu, Tb, and Er) which were the thermodynamically stable phases under the reaction conditions. The scope and implications of the new sulfidation method are also discussed.

Full text of DOI:10.1021/ja0392521

Published on Web 03/23/2004
New Solid-Gas Metathetical Synthesis of Binary Metal
Polysulfides and Sulfides at Intermediate Temperatures:
Utilization of Boron Sulfides
Li-Ming Wu and Dong-Kyun Seo*
Contribution from the Department of Chemistry and Biochemistry, Arizona State UniVersity,
Tempe, Arizona 85287-1604
Received October 27, 2003; E-mail: dseo@asu.edu
Abstract: A new simple synthetic method for binary metal polysulfides and sulfides was developed by
utilizing an in situ formation of boron sulfides and their subsequent reactions with metal-source oxides in
a closed container at intermediate temperatures above 350 °C at which the boron sulfides react in a gaseous
form. The versatility of the new method is demonstrated with oxides of various transition metals (Ti, V, Mn,
Fe, Ni, Nb, Mo, Ru, and W) and rare-earth metals (Y, Ce, Nd, Sm, Eu, Tb, and Er) as starting materials
that exhibit different chemical characteristics. Regardless of the oxidation states of metals in the starting
materials, the sulfidation reactions occurred quantitatively with stoichiometric mixtures of boron and sulfur,
and within 24 h the reactions yielded pure products of TiS2, TiS3, VS4, FeS2, NiS2, NbS3, MoS2, RuS2,
WS2, Y2S3, and RS2 (R ) Ce, Nd, Sm, Eu, Tb, and Er) which were the thermodynamically stable phases
under the reaction conditions. The scope and implications of the new sulfidation method are also discussed.
1. Introduction
can be also prepared at higher temperatures by reacting H2S
with starting materials that contain the corresponding metal ions
of high oxidation states, but the problem of incomplete reactions
and/or of impurities still remains.5
Because of their wide range of semiconducting properties and
unique structural characteristics, binary sulfides and polysulfides
of transition metals and rare-earth metals are widely used in
industrial applications such as catalysis, lubrications, battery
fabrication, refractories, pigments, and optical and magnetic
devices.1 While metal sulfides can be prepared by employing
various sulfidation methods, the synthesis of their polysulfide
analogues has not been as straightforward because many
polysulfides decompose at elevated temperatures and the source
of the polyanions are relatively scarce. H2S alone is not an
efficient source of S0, and the thermodynamics of the thermal
decomposition of H2S is not favorable at low temperatures. For
example, at temperatures below 550 °C, the equilibrium
concentration of sulfur is less than 1%, and even at 900 °C it is
only 13%.2 Reactions with elemental sulfur often require a
pressurized reaction container and/or a multistep procedure.3
More recently, solid-state metathesis and/or solution methods
have been used for the preparation of disulfides of Fe, Co, Ni,
La, and Pr by employing Na2S2, K2S2, or Na2S5.4 FeS2 and CoS2
Recently, we have found that boron sulfides, B2S3, BS2, and
their mixtures, could be versatile sources of sulfur that can
operate at intermediate temperatures in their gaseous forms.6
B2S3 does not have a well-defined melting point, but begins to
sublime at temperatures no higher than 300 °C, as found in the
literature7 and from our experience. Although BS2 melts
congruently at 417 °C under atmospheric pressure, our experi-
ences show that a significant amount of BS2 evaporates even
at 300 °C under vacuum. While all sulfur atoms in B2S3 have
a formal oxidation state of -2, the structure of crystalline BS2
exhibits dimerized S- ions in addition to S2-.8 The gaseous
boron sulfides are corrosive in nature, and indeed the preparation
of boron sulfides, crystalline or vitreous (v), requires a heavy
carbon-coating on silica reaction vessels over 800 °C.9 The
composition and equilibrium behavior of the boron sulfide vapor
are exceedingly complex because of the existence of polymeric
species Sn(g), (BS2)n(g), and (B2S3)n(g).10 However, previous
mass spectrometric studies have concluded that stoichiometric
B2S3(s) vaporizes congruently to give B2S3(g) and its polymers,
(1) See, for example: (a) Sulfur. Its Significance for Chemistry, for the Geo-,
Bio- and Cosmosphere and Technology; Mu¨ller, A., Krebs, B., Eds.;
Elsevier: Amsterdam, 1984. (b) Transition Metal Sulfur Chemistry;
Biological and Industrial Significance; Stiefel, E. I., Matsumoto, K., Eds.;
ACS Symposium Series 535; American Chemical Society: Washington,
DC, 1996.
(5) (a) Griffith, R. H.; Morcom, A. R. J. Chem. Soc. 1945, 786. (b) Iwakura,
C.; Isobe, N.; Tamura, H. Electrochim. Acta 1983, 28, 269.
(6) Wu, L.-M.; Sharma, R.; Seo, D.-K. Inorg. Chem. 2003, 42, 5798.
(7) (a) Brauer, G., Ed. Handbook of PreparatiVe Inorganic Chemistry, 2nd
ed.; Academic Press: New York, 1963; Vol. 1, p 788 and references therein.
(b) Greene, F. T.; Gilles, P. W. J. Am. Chem. Soc. 1964, 86, 3964. (c)
Chen, H.-Y., Gilles, P. W. J. Phys. Chem. 1972, 76, 2035.
(8) (a) Diercks, H.; Krebs, B. Angew. Chem., Int. Ed. Engl. 1977, 16, 313. (b)
Krebs, B.; Huerter, H. U. Angew. Chem., Int. Ed. Engl. 1980, 19, 478.
(9) Martin, S. W.; Bloyer, D. R. J. Am. Ceram. Soc. 1990, 73, 3481.
(10) Chen, H.-Y.; Gilles, P. W. J. Am. Chem. Soc. 1970, 92, 2309.
(2) Kaloidas, V. E.; Papayannakos, N. G. Int. J. Hydrogen Energy 1987, 12,
403.
(3) Webb, A. W.; Hall, H. T. Inorg. Chem. 1970, 9, 1084.
(4) (a) Bonneau, P. R.; Shibao, R. K.; Kaner, R. B. Inorg. Chem. 1990, 29,
2511. (b) Chen, J. H.; Dorhout, P. K. J. Solid State Chem. 1995, 117, 318.
(c) Shaw, G. A.; Morrison, D. E.; Parkin, I. P. J. Chem. Soc., Dalton Trans.
2001, 1872. (d) Guidotti, R. A.; Reinhardt, F. W.; Dai, J.; Reisner, D. E.
Mater. Res. Soc. Symp. Proc. 2002, 730, 207.
9
4676
J. AM. CHEM. SOC. 2004, 126, 4676-4681
10.1021/ja0392521 CCC: $27.50 © 2004 American Chemical Society

Synthesis of Binary Metal Polysulfides and Sulfides
A R T I C L E S
diagrams at the designated reaction temperatures. To ensure a complete
reaction of metal oxides, sulfur was loaded 10% more than the
stoichiometric amounts. The two tubes were subsequently situated inside
a larger silica tube container (10 mm i.d.), with the tube containing
the metal oxide on top of the other (Figure 1). After the whole container
was evacuated and flame-sealed, it was gradually heated at 50 °C/h to
the designated reaction temperature, kept for 24 h, and subsequently
cooled radiatively to room temperature. The reaction conditions and
results are summarized in Table 1. After the reactions, the lower tube
was empty, which indicated complete consumption of boron. The silica
reaction tubes were intact, and there was no visible indication of
corrosion on the inner surface of the tubes. The products were cleaned
several times with CS2 and deionized water (or methanol) to wash off
unreacted sulfur, boron sulfides, and the byproduct B2O3.13 The clean
powder samples were found to be boron-free based on our atomic
absorption spectrometric analysis within the detection limit (5 ppm)
of the instrument (Varian SpectrAA-400 Flame). The phase identifica-
tion was carried out from the powder X-ray diffraction patterns obtained
by use of a Siemens X-ray diffractometer. The JADE program (version
6.1.3) was used for the unit cell refinements. The lattice parameters
were matched with the values from the literature usually within much
less than 0.2% for all of the sulfide products except for CeS2 (0.5%
for the b-axis). All of the products were X-ray pure without any
impurities including the starting materials, as found from their powder
patterns (Supporting Information). SEM/EDX analysis was conducted
on a Hitachi S-4700-II high-resolution scanning electron microscope,
and the obtained compositions were well in agreement with the
previously reported data.
Figure 1. Reaction container (10 mm i.d.) for the sulfidation of metal oxides
from in situ formation of boron sulfides (see text).
while a B-S system richer in sulfur than B2S3 evaporates
incongruently into (BS2)n(g) and (B2S3)n(g) and the composition
of the vapor depends on the overall ratio of boron and sulfur.10
No other sulfur-richer species have been reported in the
literature, and hence we suspect that any gaseous mixtures with
a higher sulfur content (S/B > 2) should contain elemental sulfur
vapor in addition to the (BS2)n(g) and/or (B2S3)n(g) species.
The coexistence of S- and S2- ions in gaseous boron sulfide
species provides a unique possibility for employing boron
sulfides as a sulfiding agent that can provide both S- and S2-
ions. Metathetical reactions of the boron sulfides are favored
particularly when metal oxides are used as a metal source. The
large formation energy of the coproduct B2O3 (∆Gf°(v-B2O3)
) -1182.5 kJ/mol),11 as compared to those of boron sulfides
(∆Gf°(v-B2S3) ) -247.6 kJ/mol; ∆Gf°(v-BS2) ) -120 kJ/
mol),12 may allow sulfidation of various metal oxides. In this
paper, we report the synthesis of metal polysulfides and sulfides
by utilizing an in situ formation of boron sulfides to convert a
variety of metal oxides that contain transition metals (Ti, V,
Mn, Fe, Ni, Nb, Mo, Ru, and W) or rare-earth metals oxides
(Ce, Nd, Sm, Eu, Tb, and Er). It is emphasized that this work
is intended for demonstration of the utility of the boron sulfide
chemistry, rather than for exhaustive examination of all of the
opportunities of the chemistry which will be carried out in due
course.
3. Results and Discussion
3.1. Transition Metal Polysulfides and Sulfides. The
oxidation states of the transition metal ions in the starting
materials range from +6 to +2, and the boron sulfides either
reduce or oxidize the metal ions into the stable oxidation states
in the corresponding polysulfides and sulfides (Table 1). The
sulfidation reactions of TiO2 at two different temperatures
provided different phases, TiS3 (at 400 °C)14 and TiS2 (at 800
°C),15 both in a pure form. The lattice parameters of our TiS2
product indicate that its S/Ti ratio is close to 1.94 due to a sulfur-
deficiency in the structure. This nonstoichiometry is consistent
with the previous studies in which the same amount of sulfur-
deficiency was found in the most sulfur-rich product from the
reactions between the elements at 800 °C.15,16
The formation of VS4 is interesting because its synthesis has
not been very successful in previous efforts partly due to its
decomposition above 400 °C.17-19 V5S8 appears to be the most
sulfur-rich vanadium sulfide that can be prepared by reactions
between the elements.17 V2S3 has been used to prepare crystal-
line VS4 by the reaction with elemental sulfur at 400 °C, but
the sulfidation was incomplete even after a 4-week reaction
period.18,19 By utilization of the boron sulfide method in our
experiments, however, V2O5 could be converted to pure VS4 at
350 °C within 24 h, as found from the X-ray powder diffraction
2. Experimental Section
Amorphous boron powder (99.99%, 325 mesh), sulfur powder
(99.999%), and metal oxides (99.9% or higher) were purchased from
Alfa Aesar. The reaction materials were loaded in the scale of mmol
in a glovebox or in air. In a general reaction scheme, a metal oxide in
powder was placed in a short fused silica tube (Figure 1). Stoichiometric
amounts of boron and sulfur powders were mixed and placed in a
separate fused silica tube of the same size. The loaded ratios of the
starting oxide:boron:sulfur are shown in Table 1, and they are indeed
the reaction coefficients of the reactants in the individual balanced
chemical equations for the oxides and target (poly)sulfides. The reactant
ratios were chosen to provide the most sulfur-rich phases in the phase
(13) While most of the metal (poly)sulfides are practically insoluble in water
and/or alcohols, B2O3, the reaction byproduct, easily absorbs moisture in
air and consequently dissolves in water (10-50 g/L), methanol (200 g/L),
glycerol (280 g/L), and 95% ethanol (<1 g/L): Weast, R. C., Astle, M. J.,
Beyer, W. H., Eds. CRC Handbook of Chemistry and Physics, 67th ed.;
CRC Press: Boca Raton, FL, 1986.
(14) Fureseth, S.; Brattas, L.; Kjekshus, A. Acta Chem. Scand. A 1975, 29, 623.
(15) Benard, J.; Jeannin, Y. AdV. Chem. Ser. 1963, 39, 191.
(16) Mckelvy, M. J.; Glaunsinger, W. S. J. Solid State Chem. 1987, 66, 181.
(17) Nakano-Onada, M.; Yamaoka, S.; Kato, K.; Kamada, I. J. Less-Common
Met. 1976, 44, 341.
(18) Pedersen, B. F. Acta Chem. Scand. 1959, 13, 1050.
(19) Naman, S. A. Int. J. Hydrogen Energy 1997, 22, 783.
(11) Thermochemical Data of Pure Substances, 3rd ed.; Barin, I., Ed.;
Weinheim: Germany, 1995.
(12) Thermodynamic Data for Inorganic Sulphides, Selenides and Tellurides;
Mills, K. C., Ed.; Butterworth Co.: London, England, 1974.
9
J. AM. CHEM. SOC. VOL. 126, NO. 14, 2004 4677

A R T I C L E S
Wu and Seo
Table 1. Reaction Conditions and Results of the Metal Oxide Sulfidationa
starting
oxide
loaded ratio
(oxide:B:S)
reaction
temp (°C)
product
(space group)
lattice parameters
in the literature
color of the
product
lattice parameters
a ) 4.957(5) Å
ref
TiO2
3:4:9
400
TiS3 (P21/m)
a ) 4.958(2) Å
grey
14
b ) 3.397(4) Å
c ) 8.788(8) Å
â ) 97.23(5)°
b ) 3.401(2) Å
c ) 8.778(4) Å
â ) 97.32(4)°
a ) b ) 3.407(1) Å
c ) 5.695(1) Å
a ) 6.775(5) Å
b ) 10.42(1) Å
c ) 12.11(1) Å
â ) 100.8(2)°
a ) b ) c ) 5.22(1) Å
a ) b ) c ) 5.428(1) Å
a ) b ) c ) 5.428(1) Å
a ) b ) c ) 5.677(1) Å
a ) 4.963(2) Å
b ) 6.730(2) Å
c ) 9.144(4) Å
â ) 97.17(1)°
a ) 4.963(2) Å
b ) 6.730(2) Å
c ) 9.144(4) Å
â ) 97.17(1)°
TiO2
V2O5
3:4:9
800
350
TiS2 (P3hm1)
VS4 (I2/c)
a ) b ) 3.407(1) Å
c ) 5.703(1) Å
a ) 6.782(3) Å
b ) 10.421(4) Å
c ) 12.101(5) Å
â ) 100.71(5)°
a ) b ) c ) 5.221(1) Å
a ) b ) c ) 5.426(1) Å
a ) b ) c ) 5.421(1) Å
a ) b ) c ) 5.683(1) Å
a ) 4.961(6) Å
b ) 6.732(9) Å
c ) 9.15(1) Å
black
black
15
20
3:10:24
MnO2
Fe2O3
Fe3O4
NiO
3:4:6
1:2:4
3:8:18
3:2:6
3:2:9
550
550
550
550
550
MnS (Fm3hm)
FeS2 (Pa3h)
FeS2 (Pa3h)
NiS2 (Pa3h)
NbS3 (P1h)
black
black
black
black
black
30
25
25
26
32
NbO
â ) 97.3(1)°
Nb2O5
3:10:18
550
NbS3 (P1h)
a ) 4.963(5) Å
b ) 6.728(6) Å
c ) 9.144(9) Å
â ) 97.2(1)°
black
32
MoO2
MoO3
3:4:6
1:2:2
800
550
MoS2 (P63/mmc)
MoS2 (P63/mmc)
a ) b ) 3.162(3) Å
c ) 12.281(8) Å
a ) b ) 3.159(4) Å
c ) 12.284(1) Å
a ) b ) c ) 5.611(2) Å
a ) b ) 3.154(4) Å
c ) 12.33(1) Å
a ) 17.50(1) Å
b ) 4.007(2) Å
c ) 10.183(4) Å
â ) 98.62(4)°
a ) 8.101(4) Å
b ) 4.054(3) Å
c ) 8.081(5) Å
â ) 89.86(7)°
a ) 8.028(5) Å
b ) 4.002(2) Å
c ) 8.001(5) Å
â ) 90.00(1)°
a ) b ) 3.160(1) Å
c ) 12.294(4) Å
a ) b ) 3.160(1) Å
c ) 12.294(4) Å
a ) b ) c ) 5.610(1) Å
a ) b ) 3.153(1) Å
c ) 12.323(5) Å
a ) 17.523(1) Å
b ) 4.011(1) Å
c ) 10.174(1) Å
â ) 98.60(1)°
black
black
36
36
RuO2
WO3
3:4:6
1:2:2
550
550
RuS2 (Pa3h)
WS2 (P63/mmc)
black
grey
27
37
Y2O3
1:2:3
1:2:4
1:2:4
800
550
550
Y2S3 (P21/m)
CeS2(P21/c)
NdS2(P21/c)
light yellow
42
Ce2O3
Nd2O3
a ) 8.107 Å
grayish yellow
grayish yellow
38
38
b ) 4.075 Å
c ) 8.084 Å
â ) 89.80°
a ) 8.015 Å
b ) 4.012 Å
c ) 7.988 Å
â ) 90.0°
Sm2O3
Eu2O3
Tb4O7
1:2:4
550
550
550
SmS2(P42/n)
EuS2(P4/nmm)
TbS2(P21/m)
a ) b ) 8.781(5) Å
c ) 15.84(1) Å
a ) b ) 7.842(7) Å
c ) 8.066(8) Å
a ) 3.871(2) Å
b ) 3.865(2) Å
c ) 7.880(4) Å
â ) 90.05(5)°
a ) b ) 8.796 Å
c ) 15.889 Å
grayish yellow
grayish yellow
grayish yellow
38
43
44
1:2:4
a ) b ) 7.871 Å
c ) 8.040 Å
3:14:24
a ) 3.872(1) Å
b ) 3.860(1) Å
c ) 7.887(1) Å
â ) 90.1(1)°
Er2O3
1:2:4
550
ErS2 (P4/nmm)
a ) b ) 7.649(5) Å
c ) 7.827(7) Å
a ) b ) 7.636(1) Å
c ) 7.811(6) Å
grayish yellow
39
a The JADE program was employed for X-ray powder diffraction pattern analysis and unit cell refinement.
the initial formation of a thin layer of a black melt at the top of
the oxide sample prohibited a complete sulfidation of the
material underneath.
Numerous synthetic methods have been proposed for pyrite,
FeS2, because of its importance particularly in recent thermal
battery and lithium primary battery applications.21 FeS2 starts
to lose sulfur above 550 °C and decomposes at 743 °C.22 In
addition, it transforms into a marcasite structure below 445 °C.23
Figure 2. X-ray powder diffraction pattern of the product from the reaction
of V2O5. The pattern matches well with the calculated powder pattern of
VS4 and does not exhibit any Bragg reflection peaks of the starting V2O5.
(20) Kutoglu, A.; Allmann, R. Neues Jahrb. Mineral., Monatsh. 1972, 339.
(21) (a) Pimenta, G.; Kautek, W. Thin Solid Films 1994, 238, 213. (b) Raturi,
A. K.; Ndjeli, L.; Rabah, K. Renewable Energy 1997, 11, 191. (c) Qian,
X.; Xie, Y.; Qian, Y. Mater. Lett. 2001, 48, 109. (d) Yang, S.-H.; Steve,
O.; Quinn, C. H. J. Electrochem. Soc. 2002, 149, A1499. (e) Au, M. J.
Power Sources 2003, 115, 360.
pattern of the product in Figure 2, which matches well with the
previous data.20 The starting oxide powder needed to be well
dispersed on the surface of the reaction container; otherwise
(22) Chivers, T.; Hyne, J. B.; Lau, C. Int. J. Hydrogen Energy 1980, 5,
499.
(23) Kubaschewsky, O. Iron Binary Phase Diagrams; Springer: Berlin, 1982.
9
4678 J. AM. CHEM. SOC. VOL. 126, NO. 14, 2004

Synthesis of Binary Metal Polysulfides and Sulfides
A R T I C L E S
significant fusion of the sulfide particles once they were formed
initially and hence afforded the sulfide product particles much
smaller than the original oxide particles. The particle size of
our RuS2 product is in fact much smaller than the particle sizes
(<50 nm) in a previous report in which crystalline RuS2
nanoparticles were obtained after annealing of the amorphous
product from the aqueous reaction of RuCl3 and H2S.31
It is emphasized that the sulfidation reactions were efficient
regardless of the initial oxidation states of the metal atoms in
the starting materials. For instance, both Fe2O3 and Fe3O4
provided FeS2, with proper amounts of boron and sulfur to react
with. Such flexibility in selection of starting materials is
demonstrated more dramatically in our preparation of NbS3. In
addition to Nb2O5, the sulfidation was also carried out by
employing NbO (Nb2+) as a testing example because of the
extensive Nb-Nb bonds in its structure which is in contrast to
Nb2O5 in which Nb atoms are in a fully oxidized state (Nb5+).
Under exactly the same reaction temperature, nevertheless, both
of the oxides provided pure NbS3 (Nb4+) as indicated from the
X-ray powder diffraction analysis (Table 1).32 In other words,
by controlling the loading ratios of boron and sulfur, the Nb
atoms could be oxidized or reduced during the sulfidation
processes. The nanoscopic structural nature of the NbS3 product
is worth mentioning. The reaction products from both NbO and
Nb2O5 exhibit a fiberlike morphology, and some of the products
were found on the surface of the silica jacket outside of the
container in which the original oxide was located, while most
the product remained in the container. More detailed work is
in progress.
MoS2 and WS2 do not exhibit sulfur-sulfur bonds and yet
are the most sulfur-rich crystalline phases in their binary phase
diagrams.33 Solution reaction routes have provided sulfur-richer
WS5 and MoSx (x ) 5-6) in an amorphous state.34 Figure 6
shows the X-ray powder diffraction patterns of the products
from MoO2 and WO3 obtained from the boron sulfide reactions
at 550 °C. The MoS2 product from MoO3 showed a similar
powder pattern (see Supporting Information). The Bragg reflec-
tion peaks are broad, comparable to previous studies based on
various reaction routes,35 and yet the relatively sharp (002)
reflection peaks (2θ ≈ 14°) imply that the disulfide layers are
well stacked. There are no impurities found in the powder
patterns.36,37 Although not shown here, the products from the
reactions at 800 °C essentially show the same type of broad
peaks in the X-ray powder patterns, yet with sharper (002)
reflection peaks.
Figure 3. X-ray powder diffraction patterns of the products from the
reactions of Fe2O3 (top) and Fe3O4 (bottom). The patterns match well with
the calculated powder pattern of FeS2 and do not exhibit any Bragg reflection
peaks of the starting oxides.
The reaction between the elements has not been successful.24
The reactions of FeCl2 or Fe2O3 with H2S are not complete for
the temperature region where the pyrite structure is stable.5b
Low-temperature solution synthesis in previous studies often
resulted in amorphous products and/or the marcasite phase as
an impurity.21d Figure 3 shows the X-ray powder diffraction
patterns of the products from the reactions of Fe2O3 and Fe3O4
with appropriate amounts of boron and sulfur that were carried
out at 550 °C. Both of the patterns match well with that of the
previously known stoichiometric pyrite,25 and no impurities were
observed. The sulfidation of NiO and RuO2 also provided pure
NiS2 and RuS2 of the pyrite-type structure under the same
reaction condition for the FeS2. The calculated unit cell
parameters are in agreement with those of the known stoichio-
metric compounds.26,27 An isotypic compound, MnS2, is not
stable under our reaction conditions and decomposes into R-MnS
(rock-salt structure) around 260 °C.28 Two other polymorphic
forms (â, zinc blend; γ, wurtzite) are known, but only R-MnS
is thermodynamically stable.28a It is reasonable, therefore, that
our sulfidation of MnO at 550 °C provided pure R-MnS as the
product, and the refined unit cell parameters match well with
the known values.29 Interestingly, the RuS2 product was found
to consist of nanosized particles (<15 nm), as indicated from
our SEM and TEM studies (Figure 4). The Bragg reflection
peaks in the X-ray powder diffraction pattern of the product
(Figure 5) were very broad, and the estimated average particle
size was 13 nm, which is consistent with the findings in our
electron microscopy studies. The starting RuO2 particles were
very small as well, with an average diameter of 26 nm as
estimated from our powder diffraction.30 It is suspected that the
high melting point of RuS2 (>1200 °C) does not allow a
3.2. Rare-Earth Metal Disulfides and Sulfides. All of the
rare-earth metal elements are known to form a disulfide as the
most sulfur-rich phase in their binary system, although the
stability and compositional range of the disulfides can vary with
different metal elements.38 In particular, YS2 can be prepared
(31) Ashokkumar, M.; Kudo, A.; Sakata, T. J. Mater. Sci. 1995, 30, 2759.
(32) Rijnsdorp, J.; Jellinek, F. J. Solid State Chem. 1978, 25, 325.
(33) Desk Handbook: Phase Diagrams for Binary Alloys; Okamoto, H., Ed.;
Materials Park, OH: ASM Internation, 2000.
(34) (a) Hibble, S.; Walton, R. I.; Feaviour, M. R.; Smith, A. D. J. Chem. Soc.,
Dalton Trans. 1999, 2877. (b) Afanasiev, P.; Bezverkhy, I. Chem. Mater.
2002, 14, 2826.
(35) Wildervanck, J. C.; Jellinek, F. Z. Anorg. Allg. Chem. 1964, 328, 309.
(36) Bronsema, K. D.; de Boer, J. L.; Jellinek, F. Z. Anorg. Allg. Chem. 1986,
540, 15.
(37) Schutte, W. J.; de Boer, J. L.; Jellinek, F. J. Solid State Chem. 1987, 70,
207.
(24) Pimenta, G.; Kautek, W. Thin Solid Films 1992, 219, 37.
(25) Finklea, S.; Cathey, L.; Amma, E. L. Acta Crystallogr., Sect. A 1976, 32,
529.
(26) Nowack, E.; Schwarzenbach, D.; Hahn, T. Acta Crystallogr., Sect. B 1991,
47, 650.
(27) Zeng, Y.; Holzwarth, N. A. W. Phys. ReV. B 1994, 50, 8214.
(28) (a) Furuseth, S.; Kjekshus, A. Acta Chem. Scand. 1965, 19, 1405. (b) Biltz,
W.; Wiechmann, F. Z. Anorg. Allg. Chem. 1936, 228, 268.
(29) Sweeny, J. S.; Heinz, D. L. Phys. Chem. Miner. 1993, 20, 63.
(30) The particle size of RuO2 was not specified by the manufacturer.
(38) Vasilyeva, I. G. Polysulfides. In Handbook on the Physics and Chemistry
of Rare Earths; Gschneidner, K. A., Jr., Eyring, L., Lander, G. H., Eds.;
Elsevier: Amsterdam, 2001; Vol. 32 and references therein.
9
J. AM. CHEM. SOC. VOL. 126, NO. 14, 2004 4679

A R T I C L E S
Wu and Seo
Figure 4. Left: A representative SEM image of the RuS2 product. Center: A representative TEM image of the RuS2 product in which the RuS2 particles
of <20 nm diameter are aggregated. Right: A zoomed image of the square box shown in the image at the center. The well arranged lattice fringes indicate
a good crystallinity of the RuS2 nanoparticles.
corresponding sesquisulfides were treated with elemental sulfur
over 600 °C, but still in a low yield.41 LaS2 and PrS2 have been
prepared from the reactions of the corresponding chlorides and
bromides with K2S2 at temperatures as low as 300 °C after which
the unreacted starting materials were washed away.4b
In our experiments (Table 1), the sulfidation reactions of R2O3
(R ) Y, Ce, Nd, Sm, Eu, and Er) and Tb4O7 with boron sulfides
resulted in pure products of their corresponding disulfides at
550 °C, with the exception of the Y2O3. The reaction of Y2O3
at 550 °C provided Y2S3 with a large amount of unreacted
starting material after a 24-h reaction period. The same reaction
condition at 800 °C resulted in pure Y2S3. The powder X-ray
diffraction patterns matched well with those from the literature
for all of the rare-earth disulfides and sulfide.38,42-44 The
compositions obtained from the EDX studies are also in good
agreement with the literature,37 in that the light rare-earth
disulfides (R ) Ce and Nd) are stoichiometric (within (0.2%),
while significant amounts of sulfur-deficiency were found in
Figure 5. X-ray powder diffraction pattern of the product from the reaction
of RuO2. The pattern matches well with the calculated powder pattern of
RuS2 and does not exhibit any Bragg reflection peaks of the starting RuO2.
The Bragg reflection peaks are broad with an estimated particle size of 13
nm.
the samples of the heavy rare-earth disulfides (SmS1.93, EuS1.91
,
TbS1.88, and ErS1.87). The values correspond to the lower limits
found in the samples reported previously in the literature.38 The
refined unit cell parameters also match well with the literature
values with a maximum deviation of 0.5% and typically with
deviations below 0.2% (Table 1).38,42-44 The lattice parameters
in the literature are rather scattered because of the synthetic
problems as well as the structural changes by nonstoichiometries
and synthetic conditions.38 More detailed studies are in progress
on the relationships among the lattice parameters, compositions,
and reaction temperatures of the rare-earth disulfides based on
this new boron-sulfide metathesis method.
3.3. Scope of the Method. As shown in the previous sections,
the new sulfidation method was found to be efficient for various
metal oxide compounds that contain transition metals or rare-
earth metals in different oxidation states. Although not described
here, our other experiments indicated that the reaction temper-
atures could be varied without much difference in the results.
In another set of experiments for polysulfide synthesis, reactions
were repeated with the amounts of sulfur less than required for
the stoichiometric reactions. The results were the incomplete
Figure 6. X-ray powder diffraction patterns of the products from the
reactions of MoO2 (top) and WO3 (bottom). The patterns match well with
the calculated powder patterns of hexagonal (shown here) or rhombohedral
MoS2 and WS2 and do not exhibit any Bragg reflection peaks of the starting
oxides.
only under a high-pressure condition (>35 kbar) over 500 °C,39a
and the disulfides of heavy rare-earth metals (Eu-Yb) exhibit
significant sulfur-deficiencies (5-15%).38 The reaction of La
and Nd oxides with H2S begins only at 700 °C,39b and the
reaction temperature could be lowered only to 500 °C under a
high-pressure condition.40 In the previous studies of others,
crystals of the rare-earth disulfides could form when the
(40) Le Rolland, B.; Molinie´, P.; Colombet, P.; McMillan, P. F. J. Solid State
Chem. 1994, 113, 312.
(41) See, for example: Vasilyeva, I. G.; Belyaeva, E. I. J. Solid State Chem.
1999, 142, 261.
(42) Schleid, T. Inorg. Chem. 1968, 7, 2282.
(43) Eliseev, A. A.; Sadovskaya, O. A.; Nguyen, V. T. IzV. Akad. Nauk. SSSR
Neorg. Mater. 1974, 10, 2134.
(39) (a) Webb, A.; Hall, T. Inorg. Chem. 1970, 9, 1084. (b) Aloman, A. ReV.
Chim. Romaˆn. 1968, 19, 507.
(44) Kuzmicheva, G. M.; Eliseev, A. A.; Orlova, I. G.; Mukhin, B. V. Zh. Neorg.
Khim. 1983, 28, 1337.
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Synthesis of Binary Metal Polysulfides and Sulfides
A R T I C L E S
sulfidation that still provided polysulfides and left some amount
of oxides unreacted, rather than the formation of the corre-
sponding sulfides with less sulfur content. Regardless of the
chemical nature of the metal oxides, therefore, the reaction
products were indeed the most sulfur-rich compounds that are
expected thermodynamically at the reaction temperatures. In a
simplified picture, the reaction procedures can be broken down
into two subsequent reactions described by the following
equations, and yet it is noted that the formation of boron sulfides
in the first step is not necessarily complete before the second
step proceeds:
The precise role of boron in the reaction mechanism is not
clear yet and will require a thorough examination of the reactions
both experimentally and theoretically. As mentioned earlier, the
obvious benefit of boron is that the coproduct of the reactions,
B2O3(v), is strongly favored thermodynamically. This reminds
us of the well-established alkali sulfide solid-state metathesis
method which is primarily based on the simple exchanges of
ions during the reaction processes driven by the large enthalpy
of formation of alkali halides, the coproduct.46 However, the
reaction mechanism in the boron sulfide metathesis could be
more complicated because the boron sulfides react in a form of
gaseous molecules as H2S does in its much studied sulfidation
processes.47 This argument is supported by the large difference
in the bond enthalpies between B-O and B-S (228 kJ/mol).13
Accordingly, we suspect that understanding the reactions at the
molecular level could be important.
xB(s) + yS(g) f aB2S3(g) + bBS2(g) + cS(g)
m(metal oxide)(s) + aB2S3(g) + bBS2(g) + cS(g) f
p(metal sulfide)(s) + qB2O3(v)
While the new sulfidation method utilizes solid-gas reactions
between the metal source and gaseous boron sulfides, there is
a striking difference between the new method and the traditional
solid-gas synthesis based on the sulfiding gases, H2S and CS2;
the reactions are loaded with all solid-state materials (metal
source, boron, and sulfur) in a closed container. Given a target
sulfide, the starting materials are mixed at an appropriate ratio,
and the reaction proceeds to its completeness until the starting
materials are all consumed. There is no need for a continuous,
and sometimes prolonged, flow of a sulfiding gas for a complete
reaction, and the risk of harmful gases can be minimized. In
this regard, the new method has a great resemblance to the
sulfidation reaction between elements, that is, metal and sulfur.
Differently from sulfur, however, boron sulfides can act as either
a reducing or an oxidizing agent, depending on the oxidation
states of the metal ions in the starting materials and the loaded
ratios of boron and sulfur. This also contrasts the new boron
sulfide method with the recently proposed sulfidation method
that employs “gaseous” Y2S3 in which the oxidation state of
sulfur is fixed.45 In addition, the in situ preparation of boron
sulfides is greatly advantageous over the expensive Y2S3 method
in industrial applications; that is, the boron powder may not
even need a high purity. When the impurities, mostly magnesium
in industrial boron powder, react with sulfur, the resulting
sulfides are expected to remain in the original container because
of their typically high boiling point, while only boron sulfides
evaporate to react with the metal source compounds in a separate
container.
4. Concluding Remarks
As demonstrated in our experiments, the newly found use of
boron sulfides offers a convenient route for synthesizing a broad
range of crystalline metal polysulfides and sulfides in a pure
form at intermediate temperatures. We believe that the mild
reaction condition also can play an important role in preparation
of nanostructured materials of such compounds, as indicated
by our recent work on preparation of NdS2 nanoparticles.6 We
are currently extending our work to other various metal-source
materials that contain different ligands, with preliminary success.
It will not be impossible to apply the same preparative technique
for polyselenides and selenides as well, because boron selenides
are similar to boron sulfides in their characteristics.
Acknowledgment. D.-K.S. is grateful for financial support
from the National Science Foundation through his CAREER
Award (DMR - Contract No. 0239837). The authors thank Dr.
Lynda Williams for her help in using SEM/EDX.
Supporting Information Available: X-ray powder diffraction
patterns of the products listed in Table 1 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA0392521
(46) (a) Parkin, I. P. Transition Met. Chem. 2002, 27, 569. (b) Bonneau, P. R.;
Jarvis, R. F.; Kaner, R. B. Nature 1991, 349, 510.
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Sci., Ser. C: Sci. Chim. 1972, 274, 1768. (b) Larpin, J.-P.; Colson, J.-C.
Mater. Res. Bull. 1975, 10, 641. (c) Lee, B. S.; Rap, R. A. J. Electrochem.
Soc. 1984, 13, 2998. (d) Agnihotri, R.; Chauk, S. S.; Mahuli, S. K.; Fan,
L.-S. Chem. Eng. Sci. 1999, 54, 3443.
(45) Garch, J. E.; Barker, M. G.; Begley, M. J.; Batsanov, A. S. J. Chem. Soc.,
Dalton Trans. 1995, 23, 3825.
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