Microwave-assisted synthesis, characterization, and thermal decomposition of rare-earth metal disulfides RES2 (RE = La, Pr, Nd)

Article abstract of DOI:10.1002/zaac.201200221

Microwave-assisted metathesis reactions from anhydrous rare-earth metal trichlorides and alkali metal polychalcogenides in dimethylformamide in the temperature interval 160 °C ≤ T ≤ 220 °C yield sub-micron and nanometer scaled particles of the rare-earth

Full text of DOI:10.1002/zaac.201200221

ARTICLE
DOI: 10.1002/zaac.201200221
Microwave-Assisted Synthesis, Characterization, and Thermal
Decomposition of Rare-Earth Metal Disulfides RES2 (RE = La, Pr, Nd)
Christian Bartsch,[a] Eike Ahrens,[a] and Thomas Doert*[a]
Keywords: Polychalcogenides; Rare-earth metals; Microwave-assisted synthesis; Nanostructures; Thermal decomposition
Abstract. Microwave-assisted metathesis reactions from anhydrous
Raman and UV/Vis spectroscopy. The products adopt the monoclinic
rare-earth metal trichlorides and alkali metal polychalcogenides in di- α-RES2 type (space group P21/a) and form desert rose-like agglomer-
methylformamide in the temperature interval 160 °C Յ T Յ 220 °C ates. The nano-scaled samples show significantly reduced optical
yield sub-micron and nanometer scaled particles of the rare-earth metal bandgaps compared to the bulk material. The thermal decomposition
disulfides RES2 (RE = La, Pr, Nd). The samples were characterized by in air at 550 °C Յ T Յ 1000 °C yields rare-earth metal oxide sulfates
X-ray powder diffraction, electron microscopy, chemical analysis, and RE2O2SO4.
20RES2.00(s) Ǟ 20RES1.90(s) + S2(g),
Introduction
40RES1.90(s) Ǟ 40RES1.85(s) + S2(g),
Polysulfides RES2–δ (RE = Y, La–Nd, Sm, Gd–Lu; 0 Յ δ Ͻ
25RES1.85(s) Ǟ 25RES1.77(s) + S2(g),
0.3) of trivalent rare-earth metals have been intensively studied
in the last years with focus on synthesis, crystal structures,
7.41RES1.77(s) Ǟ 7.41RES1.50(s) + S2(g).
thermal properties, and thermodynamic data.[1–7] The struc-
The stability of a given rare-earth metal polysulfide structure
tures of these compounds generally consist of puckered [RES]+
type is structurally correlated to the size of the rare-earth metal
double slabs and planar [S]1–δ layers and can be related to the
tetragonal ZrSSi type (P4/nmm, a0 ≈ 3.8 Å, c0 ≈ 8 Å[8]). The
number of chalcogen defects and their assembly determine,
which kind of superstructure is realized. The planar sulfide
layers in the rare-earth metal disulfides RES2 consists of S22–
dinuclear anions arranged in a herringbone-like pattern. Two
polymorphs are known for these disulfides: The monoclinic
α-RES2 type (space group P21/a, lattice parameters a ≈ 8 Å,
b ≈ 4 Å, c ≈ 8 Å, β ≈ 90°)[3,5,7,9] and the orthorhombic β-type
(Pnma; a ≈ 8 Å, b ≈ 16 Å, c ≈ 3 Å).[3,10] The chalcogen layers
of the compounds RES2–δ with δ Ͼ 0 exhibit defects and iso-
lated S2– anions besides dinuclear S22– anions. Two commensu-
rate superstructures of the aristotype are known for sulfur de-
ficient rare-earth metal polysulfides RES2–δ: the CeS1.9 type, a
tenfold superstructure with δ = 0.1 (i.e. ten percent chalcogen
defects in the planar layers)[1,11] and the Gd8Se15 type, a 24-
fold superstructure ideally with δ = 0.125 (12.5% chalcogen
defects in the planar layers).[6]
cations RE3+. With decreasing atom size antibonding interac-
tions between the S22– and S2– anions in the planar sulfide lay-
ers increase, leading to a destabilization of the crystal struc-
ture.[6] As a result, only disulfides of the larger trivalent rare-
earth metals La, Ce, Pr, and Nd are accessible by conventional
syntheses under ambient pressure conditions.
To prepare stoichiometric disulfides of the lighter rare-earth
metals and to overcome the increasing decomposition pressure,
two general approaches seem possible: the application of high-
pressure synthesis conditions or a considerable reduction of
the reaction temperature. The first possibility has recently
proven its capability again.[5,7] The general obstacle for the
low-temperature routes is the allocation of a sufficient amount
of activation energy. It has, however, been demonstrated al-
ready in the 1970ies, that metathesis is a possible reaction
route to synthesize transition metal sulfides in aprotic solvents
at moderate temperatures, cf. Ref. [12], e.g., and microwave-
assisted syntheses have been successfully applied for the prep-
aration of sulfides of other trivalent metals like Bi2S3 and
Sb2S3.[13] The synthesis of nano-scaled rare-earth metal
sulfides and oxide-sulfides is described in the literature.[14]
However, literature featuring nano-scaled polychalcogenides
of trivalent rare-earth metals is scarce.[15]
The ranges of existence of the binary polysulfides RES2–δ
depends on the sulfur vapor pressure. The phases undergo a
stepwise incongruent decomposition upon heating until the
sesquisulfides RE2S3 are finally formed as could be shown in
Ref. [5], e.g.:
We performed a series of microwave-assisted metathesis re-
actions in organic solvents in order to explore, if rare-earth
metal disulfides might also be accessible under these specific
low-temperature conditions. Phase pure crystalline samples of
the disulfides of La, Pr, and Nd could be prepared and the
results are presented.
* Prof. Dr. Th. Doert
Fax: + 49-351-463-37287
E-Mail: thomas.doert@chemie.tu-dresden.de
[a] Fachrichtung Chemie und Lebensmittelchemie
Technische Universität Dresden
Helmholtzstr. 10
01062 Dresden, Germany
Z. Anorg. Allg. Chem. 2012, 638, (15), 2491–2497
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. Bartsch, E. Ahrens, Th. Doert
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In accordance with the lanthanide contraction, the diffraction
angles increase from α-LaS2 to α-NdS2, as shown in Figure 3.
Results and Discussion
General Results
The microwave assisted syntheses starting from anhydrous
rare-earth metal trichlorides and sodium polysulfides yields
ocher-orange colored powder samples. After leaching out the
sodium chloride with an ethanol/water mixture, the powder
X-ray diffraction patterns for RE = La, Pr, and Nd reveal re-
flections of the monoclinic α-RES2 type samples, cf. Figure 1.
[3]
The crystal structure of α-NdS2 is shown in Figure 2 as an
example. Sulfur, generated as by-product during the synthesis,
deposits most probably amorphously and is usually rinsed
completely during the washing process if Na2S2 was chosen as
sulfide source. This observation has also been made for con-
ventional metathesis reactions.[16] For reactions starting with
Na2S4 sulfur is sometimes found as by-product in the powder
X-ray patterns of the respective samples (cf. also Figure 11 and
Figure 13). Like the bulk material, the sub-micron and nano-
structured α-RES2 samples are not sensitive to atmospheric
conditions. The broad reflections in the X-ray pattern can be
taken as a first evidence for the formation of small particles.
Moreover, the peak broadening observed in the power dia-
grams is clearly hkl-dependent: reflections with l = 0 show a
significantly lower full width at half maximum (FWHM) (cf.
Figure 1, e.g.) indicating, that the samples should consist of
anisotropic particles with their smallest dimension along [001].
Figure 2. Crystal structure of α-NdS2, projection of the unit cell
(right), planar layer with dinuclear S22– anions (left).[3]
Figure 3. Diffraction patterns of nano-structured samples of α-LaS2,
α-PrS2, and α-NdS2.
SEM and TEM images confirm the plate-like shape of the
RES2 particles (Figure 4, Figure 5 and Figure 6). The sizes of
the particles depend on the individual reaction conditions.
Short reaction times (t ≈ 20 min) and lower reaction tempera-
tures (T ≈ 160 °C) generally favor the formation of smaller
particles of lateral dimensions about 25–100 nm and heights
of about 5–10 nm. Reaction times of about 90 min and tem-
peratures in the range 180 Յ T Յ 200 °C yielded plates of
approximate dimensions of (200ϫ200ϫ10) nm3. Moreover,
the particle anisotropy deduced from the X-ray powder pat-
terns is clearly confirmed in the micrographs. The α-RES2 par-
ticles were generally found agglomerated, in most cases desert
rose-like structures are formed (cf. Figure 4 and Figure 6). Due
to the agglomeration the determination of a particle size distri-
bution by dynamic light scattering (DLS) did not yield reliable
results. The application of the Scherrer formula[17] to extract
particle sizes is also of limited scientific value. As stated
above, the α-RES2 compounds crystallize in the monoclinic
space group P21/a with nearly tetragonal unit cell dimensions
of a ≈ 8 Å, b ≈ 4 Å, c ≈ 8 Å, and β ≈ 90° and hence largely
overlapping reflections are recorded. A mathematical peak de-
convolution is thus difficult, especially taking the considerably
broad reflections and the hkl-dependent line broadening into
account. The reported particle sizes in this contribution thus
Figure 1. Diffraction pattern of a NdS2 sample; top: measured pattern
of the nano-structured sample; bottom: pattern calculated for α-
NdS2.[3]
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Microwave-Assisted Synthesis of Rare-Earth Metal Disulfides RES2
stem from SEM and TEM micrographs. N. B.: As the dedi-
cated formation of monodisperse RES2 nanoparticles was not
the aim of this work, improvements of nucleation and growth
processes as well as optimization procedures to determine the
particle size distribution were not undertaken.
Figure 4. SEM images of nano-structured α-NdS2 (left) and α-LaS2
(right) samples.
Figure 7. Relevant section of the Raman spectra showing the stretch-
ing mode of the disulfide anions S22– for α-LaS2 and α-NdS2 samples
obtained under different reaction conditions.
The size of the particles also influences the electronic prop-
erties of the α-RES2 compounds. Rare-earth metal disulfides
are semiconductors and macroscopic crystals are of deep red
color (cf.[3] e.g.). In accordance with the quantum size effect
the optical absorption edge shows a blue shift from roughly
450 nm for samples consisting of micrometer-sized particles
obtained from conventional solid state reaction to about
350 nm for the nano-sized samples. This corresponds to an
increase of the bandgap from approximately 2.5 to 3.5 eV,
respectively, as shown for α-NdS2 in Figure 8.
Figure 5. TEM image of agglomerated particles of α-NdS2.
Figure 8. UV/Vis-spectra of different α-NdS2 samples illustrating the
shift of the optical absorption edge.
Figure 6. SEM-images of microcrystalline α-LaS2 (left) and nano-
structured α-PrS2 with similar intergrowth of the particles.
EDX analyses as well as ICP-OES data confirm the metal/
Raman spectra of the α-RES2 samples show the typical sig- sulfur ratio within experimental errors. Both methods indicate,
nal for the S–S stretching mode at about 400 to 420 cm–1 (Fig- however, a considerable amount of carbon (up to 20 atom-%)
ure 7) consistent with literature data for LaS2, e.g.[18]. Probably and oxygen (up to 50 atom-%) in the α-RES2 samples. As no
due to scattering effects in the small particles the Raman sig- evidence for rare-earth oxides (RE2O3), oxide–sulfides
nals appear somewhat broadened as compared to the bulk sam- (RE2O2S or RE10OS14) or carbon containing phases are found
ples.
by X-ray powder diffraction and in the spectroscopic investi-
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C. Bartsch, E. Ahrens, Th. Doert
ARTICLE
gations, we assume contamination of the sample surfaces due min of reaction although the X-ray pattern exhibits only broad
to the reaction conditions and the rinsing process.
reflections and a poor signal/background ratio. However, side
products were not detected. The optimized reaction time was
found to be 90 min. Shorter as well as longer reaction times
give lower levels of crystallinity. Figure 11 shows the powder
X-ray patterns of four α-NdS2 samples prepared at different
reaction times. It is not yet clear why a prolongation of the
reaction time decreases the crystallinity of the sample. This
observation is, however, also supported by SEM images, in
which the faceting of the particles is found less pronounced in
the respective samples.
Reaction Temperature
The overall crystallinity – as judged by absolute peak inten-
sities and reflection/background ratios – of the synthesized
samples can be influenced by the reaction temperature (cf. Fig-
ure 9). The variation of the reaction temperature between
160 °C and 220 °C was tested for the Discover microwave sys-
tem keeping the other reaction parameters constant [reaction
time t = 2 h, V(DMF) = 15 mL, reactant amounts: 1 mmol].
An optimized reaction temperature of T = 180 °C was made
out. Samples prepared at lower temperatures were found
with lower overall crystallinity while samples prepared at
T Ն 220 °C generally show good crystallinity but are in some
cases accompanied by side-products like rare-earth metal ox-
ides and sulfur. The particle size and shape is not much affec-
ted if the synthesis is conducted within the temperature interval
180 Յ T Յ 200 °C, as can be seen from Figure 10.
Figure 11. Diffraction patterns of α-NdS2 samples synthesized with
different reaction times. Reflections of sulfur as by-product in the 3-
h-sample are indicated by arrows.
Solvent Volume and Reactant Concentration
As can be expected, the solvent volume and the effective
reactant concentration also influence the crystallinity of the
products. As shown for α-NdS2 in Figure 12, the crystallinity
decreases rapidly with increasing solvent volume (decreased
reactant concentration). For absolute solvent volumes less or
equal to 5 mL in a 35 mL vessel (Discover SP system) the
Figure 9. Diffraction patterns of α-LaS2 samples synthesized at 220 °C
(grey) and 180 °C (black).
Figure 10. SEM images of α-LaS2 samples synthesized at 220 °C (left)
and 180 °C (right).
Reaction Time
The reaction time for the synthesis was varied systematically
for the Discover microwave system between t = 20 min. and
t = 3 h keeping the remaining reaction parameters constant
[T = 180 °C, V(DMF) = 10 mL, reactant amounts: 1 mmol]. A
Figure 12. Diffraction pattern of α-NdS2 samples synthesized with dif-
formation of the target product can be affirmed yet after 20 ferent solvent volumes.
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Microwave-Assisted Synthesis of Rare-Earth Metal Disulfides RES2
desired products could not be obtained because of complete
evaporation of the solvent and a lack of reaction media.
Figure 13 shows the powder X-ray patterns of two lantha-
num samples, synthesized from initial concentrations of
1 mmol and 2 mmol of LaCl3 and Na2S2, respectively, [Dis-
cover SP system, T = 180 °C, V(DMF) = 10 mL, t = 1.5 h].
Significant differences can’t be made out although the sample
obtained from the higher concentrated starting mixture exhibits
slightly broader reflections and traces of sulfur.
Figure 14. Diffraction pattern of an oxidized sample (La2O2SO4 from
α-LaS2 after heat treatment at T = 850 °C for 24 h, calculated pattern
according to reference[19]).
Figure 13. Diffraction patterns of α-LaS2 samples synthesized with
different initial reactant concentrations, the sample obtained from the
higher concentration shows some additional peaks of sulfur as marked
by arrows.
Thermal Decomposition
Figure 15. SEM image of an oxidized sample (La2O2SO4 from α-LaS2
after heat treatment at T = 850 °C for 24 h; same material as used for
Figure 14).
Thermal treatments of the submicron- and nano-scaled
α-RES2 samples in air were performed to check, if rare-earth
metal oxides RE2O3 can be obtained under retention of the
plate-like appearance of the disulfide particles. Interestingly,
the oxides could not be obtained by heat treatment up to
Conclusions
Phase pure samples of the monoclinic α modifications of
LaS2, PrS2, and NdS2 (space group P21/a) were prepared by
microwave assisted metathesis reactions starting from anhy-
drous rare-earth trichlorides and sodium polysulfides in di-
methylformamide. According to SEM micrographs the prod-
ucts consist of plate-like particles. The sizes of the primary
particles vary approximately between (25ϫ25ϫ5) nm3 and
(200ϫ200ϫ10) nm3. The α-RES2 plates tend to form larger,
sand-rose like agglomerates which hinders the determination
of a meaningful size distribution. The optimized reaction pa-
rameters to get phase pure material of adequate crystallinity
were determined to: T = 180 °C, t = 1.5 h, V(DMF) = 10 mL,
n(RECl3) = n(Na2Sx) = 1 mmol; 35 mL silica vessel in a Dis-
cover SP microwave system. With these parameters, particles
of about 200 nm lateral dimensions and thicknesses of about
10 nm were found by SEM. Raman spectra confirm the pres-
ence of S22– dinuclear anions by their characteristic stretching
S–S mode and a blue shift of the optical absorption edge for
[19]
1000 °C. Instead rare-earth metal oxide sulfates RE2O2SO4
resulted in the oxidation at temperatures T Ն 550 °C as can be
seen in the X-ray powder pattern in Figure 14. The reaction
proceeds via the oxide sulfide RE2O2S[20] in the intermediate
temperature range 350 °C Յ T Յ 550 °C as Bragg reflections
of these phases can be detected in the X-ray powder patterns
of the respective samples. Due to the high surface area of the
nano-structured α-RES2 particles and possibly due to the ox-
idic surface contamination the oxidation is quick. The ocher-
colored samples of α-LaS2 turn white almost immediately
when placed in a pre-heated furnace even at moderate tempera-
tures of T = 350 °C. The particle morphology of the oxidized
products is retained in the first place. After prolonged thermal
treatment at T Ն 850 °C the faceting of the particles is, how-
ever, less pronounced and further agglomeration towards com-
pact samples can be observed (Figure 15).
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C. Bartsch, E. Ahrens, Th. Doert
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Table 1. Reaction variables and system parameters of the used microwave systems.
MARS 5
Discover SP
Vessel type
PTFE and silica with PTFE cap
120 / 80 mL
160–220 °C
0–3 bar excess pressure
30–50 mL
1–2 mol
Silica with PTFE cap
35 mL
Vessel volume
Temperature range
Pressure range
Solvent volume
Product amount
Reaction time
5–20 mL
0.5–2 mol
0.33–8 h
1–24 h
equal sample amounts) was used for a better comparability of the sam-
ple characteristics. Only the measured angular range was changed in
some cases to allow for different overall scanning times. Theoretical
patterns were calculated with the WinXPow software package.[21]
the nano-sized samples was observed in UV/Vis spectra. The
synthesis of α-RES2 samples by microwave assisted synthesis
for Ce, Sm, and Gd was not successful yet. Heat treatments of
the α-RES2 samples in air in the interval 550 °C Յ T Յ
1000 °C gave the rare-earth metal oxide sulfates RE2O2SO4 as
oxidation products.
Electron Microscopy and Energy Dispersive X-ray Spectroscopy:
Scanning electron micrographs of the samples were recorded with a
DSM 982 Gemini or with an UltraPlus SEM (both Carl Zeiss NTS).
Energy dispersive X-ray spectroscopy (EDX) was performed with a
Si(Li) detector (Voyager, Noran) attached to the Zeiss 982. Transition
electron microscopy (TEM) studies were conducted with a Philips EM-
208 (Philips) with an acceleration voltage of 100 kV.
Experimental Section
Synthesis: All manipulations with the starting reagents were carried
out in purified argon (99.999%, Air Liquide) in a glove box (Unilab,
MBraun). Nano-scaled RES2 are obtained by metathesis reactions of
adequate amounts of anhydrous rare-earth metal trichloride (LaCl3,
PrCl3, NdCl3, all: Ͼ99.9%, ABCR) and sodium polysulfides (Na2S2
or Na2S4) in dried dimethylformamide (DMF) by microwave-assisted
metatheses reactions according to the scheme:
Chemical Analyses: Besides EDX determinations, the compositions
of selected samples were also determined by atomic emission spec-
troscopy (ICP-OES). For that, samples were sealed into capillaries,
which were cracked in an autoclave with PTFE inlay and digested
using a 4 m HNO3/HCl solution. The standard solutions as reference
for the rare-earth metals and sulfur were taken from Johnson Matthey.
2RECl3 + 3Na2S2 Ǟ 2RES2 + 6NaCl + 2S
Optical Spectroscopy: Raman spectra were recorded with a Renishaw
RM 2000 Raman microscope equipped with a 633 nm He/Ne laser in
the range from 100 nm to 500 nm.
Anhydrous DMF (Ͻ 0.005% water, stored over molecular sieve,
Sigma Aldrich) was degassed by a continuous argon flow for at least
30 min prior to use. Na2S2 and Na2S4 were obtained by reaction of
the elements (Na: ingot, 99.99%, Chempur, S: 99.95%, Alfa Aesar,
re-crystallized from CS2 and twice sublimed prior to use) in dried and
oxygen-free ethanol or tetrahydrofuran.
UV/Vis spectra using BaSO4 (Merck, white standard DIN 5033) as
standard were measured with a Varian Cary 4000. Measurements were
performed in the range from 350 nm to 900 nm with a Varian Praying
Mantis device for diffuse reflectometry. The size of the bandgap was
estimated from the onset-wavelength at the absorption edge.
The reactants were put into polytetrafluoroethylene (PTFE) or silica
microwave vessels in an argon atmosphere and the vessels were closed
with a PTFE cap. The reaction mixtures were heated with a microwave
system MARS 5 or Discover SP (both: CEM GmbH). Both systems
and their characteristics are listed in Table 1. A heating ramp of 5 min
to reach the reaction temperature T (160 Յ T Յ 220 °C) was used.
The reaction mixtures were held at this temperature between 30 min
and 24 h, while stirring. Afterwards, the vessels were cooled down to
room temperature by switching off the microwave energy. The re-
sulting ocher-orange colored powders were washed with a 1:1 ethanol/
water-mixture and dried under dynamic vacuum supported by an ultra-
sonic treatment.
Dynamic Light Scattering (DLS): Apart from particle size estima-
tions based on SEM images, dynamic light scattering (DLS) was per-
formed to confirm these results with a Malvern Zetasizer ZS at a wave-
length of λ = 688 nm. The samples were suspended in ethanol. How-
ever, the DLS results are heavily biased by particle agglomeration and
were deemed unrepresentative. The data are thus not given here.
Thermal Treatment: Thermal decompositions of the nano-structured
α-RES2 were performed in air at temperatures 350 °C Յ T Յ 1000 °C.
The samples were filled in alumina boats, which were placed in a
furnace for reaction times between 1 min and 24 h. After cooling, the
samples were checked by powder X-ray diffraction and SEM imaging.
Attempts to prepare samples of RES2 for RE = Ce, Sm, and Gd under
the experimental conditions stated above were not successful. After
rinsing, the X-ray powder pattern indicated the presence of RE2O3 or
REOCl and elemental sulfur in the respective samples.
Acknowledgements
This work was financially supported by the Deutsche Forschungsge-
meinschaft (DFG, Bonn, Germany). We thank Ellen Kern, Susanne
X-ray Powder Diffraction: X-ray powder diffraction patterns were Goldberg (Department of Chemistry and Food Chemistry, TU
recorded with a PANalytical X’Pert diffractometer with Cu-Kα1-radia-
tion and Bragg-Brentano setup. The instrument settings were kept
Dresden), and Regine Boldt (Leibnitz Institut für Polymerforschung,
Dresden) for REM imaging and EDX analyses, Gudrun Auffermann
identical and the same type of sample holders (to ensure approximate (Max Planck Institute for Chemical Physics of Solids, Dresden) for
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Received: May 29, 2012
Published Online: October 19, 2012
Z. Anorg. Allg. Chem. 2012, 2491–2497
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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