Nanocrystalline tin as a preparative tool: Synthesis of unprotected nanoparticles of SnTe and SnSe and a new route to (PhSe)4Sn

Article abstract of DOI:10.1021/ic020272y

The reactions of nanocrystalline tin metal (Sn*) with elemental selenium and tellurium and with the diaryldichalcogenides Ph₂Se₂ and Ph₂Te₂ have been investigated. Reaction of Sn* with the soluble tellurium source Ph₂Te₂ led to a clean formation of nanoparticles of cubic SnTe. Dependent on the concentration of Ph₂Te₂, the particles sizes could be varied between 15 and 60 nm in average, whereas the reaction of Sn* with Ph₂Se₂ formed molecular Sn(SePh)₄ in high yield. The latter molecular compound was thermolyzed at 300°C, yielding nanocrystalline SnSe with a broader distribution of size. The nanoparticles of SnTe were thoroughly investigated by transmission electron microscopy (TEM) and powder X-ray diffraction. The reactions of Sn* with elemental selenium and tellurium gave single-phase but microcrystalline SnSe and SnTe.

Full text of DOI:10.1021/ic020272y

Inorg. Chem. 2002, 41, 6001−6005
Nanocrystalline Tin as a Preparative Tool: Synthesis of Unprotected
Nanoparticles of SnTe and SnSe and a New Route to (PhSe)4Sn
Sabine Schlecht,* Michael Budde, and Lorenz Kienle
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
Received April 15, 2002
The reactions of nanocrystalline tin metal (Sn*) with elemental selenium and tellurium and with the diaryldichal-
cogenides Ph2Se2 and Ph2Te2 have been investigated. Reaction of Sn* with the soluble tellurium source Ph2Te2
led to a clean formation of nanoparticles of cubic SnTe. Dependent on the concentration of Ph2Te2, the particles
sizes could be varied between 15 and 60 nm in average, whereas the reaction of Sn* with Ph2Se2 formed molecular
Sn(SePh)4 in high yield. The latter molecular compound was thermolyzed at 300 °C, yielding nanocrystalline SnSe
with a broader distribution of size. The nanoparticles of SnTe were thoroughly investigated by transmission electron
microscopy (TEM) and powder X-ray diffraction. The reactions of Sn* with elemental selenium and tellurium gave
single- phase but microcrystalline SnSe and SnTe.
Introduction
Experimental Section
The IV/VI semiconductor compounds such as PbSe, SnSe,
and SnTe are widely applied in IR detectors1 and as
thermoelectric materials,2 and SnS is discussed as a possible
photoactive layer in solar cells.3-5 In contrast to the multitude
of syntheses for nanoparticles of II/VI semiconductors6,7 there
is only a small variety of synthetic routes to IV/VI
semiconductor nanoparticles. Some of them involve the use
of an autoclave and liquid ammonia or other amines as
solvents to activate the chalcogen.8-10 We have developed a
new approach to tin chalcogenide nanoparticles starting from
nanocrystalline-activated tin metal. So far, similar elemental-
direct reactions11 leading to nanoparticles have only been
reported for II/VI semiconductors.11,12
All manipulations were carried out under dry, oxygen-free argon,
except where stated otherwise. Li[Et3BH], 1 M in tetrahydrofuran
(THF), diethylene glycol dimethyl ether (anhydrous), diphenyl
diselenide, and diphenyl ditelluride were purchased from Aldrich,
and SnCl2 (anhydrous), selenium, and tellurium were obtained from
Merck. All chemicals were used without further purification. THF
was purchased from Merck and distilled over CaH2 under an argon
atmosphere.
Powder X-ray diffraction data were collected using a STOE-
Stadi P diffractometer (germanium monochromator, Cu KR radia-
tion, λ ) 1.540 56 Å, linear position sensitive detector, silicon as
an external standard).
The sample was characterized by high-resolution electron
microscopy (HREM) and selected area electron diffraction (SAED).
The particles were dispersed in n-butanol. One drop of the
suspension was placed on a perforated carbon/copper net, which
was dried carefully, leaving the particles in random orientations.
The electron microscopy studies were performed with a Philips
CM30/ST instrument equipped with a LaB6 cathode. At 300 kV,
the point resolution is 0.19 nm. SAED patterns were obtained using
a diaphragm that limited the diffraction to a circular area of 125
nm in diameter. Kinematical SAED patterns were calculated with
the EMS program package. Working with the Philips CM30/ST,
X-ray analyses (mapping and EDX point measurements) were
carried out using a Noran SiLi detector and the Vantage DSI system
(Noran). EDX analyses were also performed applying the SE mode
* To whom correspondence should be addressed. E-mail: schlecht@
jansen.mpi-stuttgart.mpg.de.
(1) Unger, K. Verbindungshalbleiter; Akademische Verlagsanstalt: Leipzig,
Germany, 1986.
(2) Dresselhaus, M. S.; Dresselhaus, G.; Sun, X.; Zhang, Z.; Cronin, S.
B.; Koga, T.; Ying, J. Y.; Chen, G. Microscale Therm. Eng. 1999, 3,
89.
(3) Nair, M. T. S.; Nair, P. K. Semicond. Sci. Technol. 1991, 6, 132.
(4) Prince, M. B. J. Appl. Phys. 1955, 26, 534.
(5) Loferski, J. J. J. Appl. Phys. 1956, 27, 777.
(6) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843.
(7) Malik, M. A.; O’Brien, P.; Revaprasadu, N. S. Afr. J. Sci. 2000, 96,
55.
(8) Henshaw, G.; Parkin, I. P.; Shaw, G. J. Chem. Soc., Dalton Trans.
1997, 231.
(9) Li, Y.; Wang, Z.; Ding, Y. Inorg. Chem. 1999, 38, 4737.
(10) Wang, W.; Geng, Y.; Yan, P.; Liu, F.; Xie, Y. J. Am. Chem. Soc.
1999, 121, 4062.
(11) Li, Y.; Liao, H.; Ding, Y.; Fan, Y.; Zhang, Y.; Qian, Y. Inorg. Chem.
1999, 38, 1382.
(12) Li, Y.; Ding, Y.; Qian, Y.; Zhang, Y.; Yang, L. Inorg. Chem. 1998,
37, 2844.
10.1021/ic020272y CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/15/2002
Inorganic Chemistry, Vol. 41, No. 23, 2002 6001

Schlecht et al.
of a Philips XL30 ESEM scanning electron microscope (acceleration
voltage, 25 kV). For the latter analyses, a small amount of the tin
chalcogenide was placed on an adhesive graphite foil.
was stored in air at room temperature. Powder diffraction data were
in accordance with JCPDS 48-1224 for SnSe.
Results and Discussion
Elemental Direct Reaction of Nanocrystalline Tin with
Selenium or Tellurium. A 190 mg sample of SnCl2 was dissolved
in 60 mL of THF and reduced at room temperature by 2.0 mL of
a 1.0 M solution of Li[Et3BH] in THF.13 The reaction mixture was
stirred for 15 min, and then the THF was decanted and substituted
by 60 mL of diglyme. Then 77 mg (1.0 mmol) of elemental
selenium was added and the suspension was heated to 125 °C for
24 h. It was allowed to cool to room temperature, and the precipitate
was filtered off and washed twice with 10 mL of THF. The SnSe
was dried under vacuum and stored at room temperature in air.
Solvochemically activated tin (Sn*) was obtained from
SnCl2 and Li[Et3BH] as a reducing agent.13 As-prepared
activated tin was reacted at low temperatures with elemental
selenium and tellurium (Scheme 1).
Scheme 1
The reaction of Sn* with tellurium was performed by following
the same protocol but with a reaction temperature of 85 °C.
Single-phase SnSe (Pbnm) and SnTe (Fm3hm) were
obtained in quantitative yield after 24 h. These products
represent the thermodynamically stable modifications of the
respective compounds.15 The given reaction temperatures of
85 °C for tellurium and 125 °C in the case of selenium
represent the lowest possible temperatures at which a
complete conversion into the tin chalcogenide occurred. In
contrast to the analogous synthesis of SnS,16 the products
are not nanocrystalline but microcrystalline. Our experience
with the synthesis of SnS nanoparticles suggested that the
chalcogen that is used in the reaction needs to be soluble in
diglyme (diethylene glycol dimethyl ether) solvent to give a
nanocrystalline product. Therefore, we used diphenyl ditel-
luride, Ph2Te2, and diphenyl diselenide, Ph2Se2, as soluble
chalcogen sources. Reaction of Ph2Te2 with nanocrystalline
tin metal at 165 °C gave single-phase nanocrystalline SnTe
and diphenyl telluride, Ph2Te (Scheme 2). When the same
Synthesis of SnTe Nanoparticles. Nanocrystalline tin metal was
obtained according to ref 13. A 190 mg (1.0 mmol) amount of
SnCl2 was dissolved in anhydrous THF, and 2.0 mL of a 1.0 M
solution of Li[Et3BH] was added. Immediate precipitation of
nanocrystalline tin was observed. The reaction mixture was stirred
for 15 min and then allowed to settle. The supernatant was decanted,
and the tin was washed with THF and dried under vacuum. The
black powder (1.0 mmol) was suspended in 60 mL of anhydrous
diglyme. A 409 mg (1.0 mmol) amount of diphenyl ditelluride were
added and dissolved immediately. The suspension was heated to
165 °C (gentle boiling of the diglyme solvent) for 2 h. The color
of the solution gradually changed from light orange to orange-red.
The reaction mixture was allowed to cool, and the precipitate was
filtered off and washed with 3 × 5.0 mL of THF. The SnTe
nanoparticles were dried under vacuum and stored in air at room
temperature.
The same sequence of reactions was conducted starting from 19
mg (0.1 mmol) of SnCl2 in 100 mL of anhydrous THF and
subsequent conversion of as-prepared tin with 41 mg (0.1 mmol)
of Ph2Te2 in 60 mL of diglyme at 165 °C for 2 h. Thus obtained
smaller SnTe nanoparticles were centrifuged from the solution,
washed, and dried under vacuum.
Scheme 2
reaction was conducted in boiling THF (68 °C), a conversion
to SnTe still took place but the product was amorphous.
The nanoparticles can be separated from the solution of
the liquid byproduct by simple filtration. The nanosized SnTe
particles were characterized by X-ray powder diffraction
(XRD), EDX analysis, and transmission electron microscopy
(TEM).
When nanocrystalline SnTe is synthesized from nano-
crystalline tin and a soluble source of tellurium, the question
arises how the particle size and shape of the product depend
on the size of the tin particles and the concentration of the
tellurium source. We tried to vary the particle size of the
nanocrystalline tin by a change of the concentration of SnCl2
in THF before the reduction with Li[Et3BH]. Strikingly, no
effect on the particle size was observed when the concentra-
tion of the solution of SnCl2 was reduced from 10 to 1 mM.
In both cases, tin particles of 40-50 nm size were obtained
(see Supporting Information).
Amorphous SnSe was obtained from activated tin metal (1.0
mmol) and Ph2Se2 (1.0 mmol) by following the same protocol.
Synthesis of Sn(SePh)4. A 119 mg sample (1.0 mmol) of
activated tin was suspended in 50 mL of THF. A 624 mg (2.0
mmol) amount of diphenyl diselenide was added, and the reaction
mixture was heated under reflux for 15 h. It was allowed to cool
to room temperature, a very small amount of residual tin was filtered
off, and the solvent was evaporated to dryness under vacuum.
Sn(SePh)4 was obtained as a yellow powder in 94% yield. Elemental
analysis, powder diffraction data, and NMR spectroscopic data were
in full accordance with the literature14 and with the composition
C24H20Se4Sn.
Pyrolysis of Sn(SePh)4. A 372 mg (0.5 mmol) amount of
Sn(SePh)4 was placed at the bottom of a long Schlenk tube equipped
with a bubbler for pressure release. The bottom half of the Schlenk
tube was immersed in an electrical oven and heated to 300 °C.
This temperature was held for 30 min. The formation of a black
solid from the Sn(SePh)4 melt and the condensation of the volatiles
in the top part of the Schlenk tube started around 130 °C. The
Schlenk tube was allowed to cool to room temperature, and the
black powder (SnSe) at the bottom was taken out mechanically. It
For the conversion of Sn* to nanocrystalline SnTe two
different reaction conditions, named A and B in the follow-
ing, were applied (Scheme 3).
(13) Bo¨nnemann, H.; Brijoux, W.; Joussen, T. Angew. Chem., Int. Ed. Engl.
1990, 29, 273.
(14) Barton, D. H. R.; Dadoun, H. New J. Chem. 1982, 6, 53.
(15) Wiedemeier, H.; von Schnering, H.-G. Z. Kristallogr. 1978, 148, 295.
(16) Schlecht, S.; Kienle, L. Inorg. Chem. 2001, 40, 5719.
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Nanocrystalline Tin as a PreparatiWe Tool
size of 60 nm was derived from the reflections using
Scherrer’s equation.17
The TEM micrograph in Figure 1 gives an overview of
the round SnTe nanoparticles. It confirms the average particle
size obtained from powder diffraction data. No amorphous
byproducts can be found according to electron diffraction
and TEM micrographs. The EDX analyses confirm the
expected composition (SnTe); no inhomogenities were
observed in the nanocrystals using X-ray mapping.
Most of the particles are highly crystalline and do not
exhibit any crystal defects. Hence, the electron diffraction
patterns are characterized by Bragg reflections only. A high-
resolution electron micrograph of a SnTe nanoparticle and
the corresponding SAED pattern (zone axis [110]) are
depicted in Figure 4. Patterns taken from thin regions of the
nanocrystals agree convincingly with simulated kinematical
patterns (see Figure 4a,b). In rare cases twin boundaries were
observed in the nanocrystals. A typical example is shown in
Figure 5 for zone axis [110]. All indices refer to the zone
axis orientations of the single domains. The positions of the
Bragg intensities in the SAED pattern of the twinned crystal
can be constructed by superimposing two patterns with [110]
zone axis (see Figure 5b for the experimental pattern and
Figure 5a for the construction). The mmm-symmetry of the
superimposed pattern offers two possibilities for the twin
law: a reflection perpendicular to the directions [122]* or
[111h]* of the reciprocal space (see Figure 5a,b). For the
unambiguous determination of the twin law the direct space
can be analyzed using HREM. Figure 5c indicates the
orientations of both twin domains (see arrows marking [001])
which are connected by a reflection perpendicular to [111h].
Between the two domains a narrow region of overlapping
domains is observed.
Figure 1. Typical micrograph of round SnTe nanocrystals (60 nm average
size).
Figure 2. Typical micrograph of star-shaped agglomerates of SnTe needles
(15 × 40 nm average size).
The smaller, more needlelike particles prepared by route
B were also investigated by TEM and compared to the ones
obtained via route A. Figure 6 shows a high-resolution
electron micrograph of one of the needles and the corre-
sponding SAED pattern (zone axis [110]). Like the round
particles, the needles are also highly crystalline and exhibit
very few defects.
Scheme 3
In contrast to the tellurium system, the analogous reaction
of Sn* with Ph2Se2 at 165 °C for 2 h led to amorphous tin
selenide SnSe in quantitative yield. To obtain nanocrystalline
SnSe the reaction had to be conducted in two separate steps
(Scheme 4).
Under conditions A, when 1.0 mmol of nanocrystalline
tin and 1.0 mmol of Ph2Te2 in 60 mL of diglyme were used,
large agglomerates of randomly oriented particles of about
60 nm size were obtained (Figure 1). Under conditions B,
with only 0.1 mmol of Sn* and Ph2Te2 in the same amount
of solvent, the reaction yielded smaller star-shaped ag-
glomerates of needles with an average size of 15 × 40 nm
(Figure 2).
Scheme 4
The powder diffraction diagram for tin telluride nanopar-
ticles prepared by route A and the reflections for bulk cubic
SnTe (JCPDS 46-1210) are shown in Figure 3. All reflections
in the powder diffraction diagram for the nanoparticles were
indexed, and the obtained lattice parameter of a ) 6.329(2)
Å is identical within the standard deviation with the published
value of a ) 6.3275(5) Å for bulk SnTe. No other crystalline
phase was observed. EDX analysis of the nanoparticles
confirmed a 1:1 ratio of tin and tellurium. A mean crystallite
In the first step, nanocrystalline tin metal was reacted with
2 equiv of diphenyl diselenide, Ph2Se2, at only 65 °C in THF,
and under these conditions the tin was cleanly converted into
the molecular addition product Sn(SePh)4. As in the alterna-
tive synthesis of Sn(SePh)4,15 the product is formed as a
(17) Guinier, A. X-ray Diffraction in Crystals, Imperfect Crystals, and
Amorphous Bodies; Dover: New York, 1994; Chapter 5, p 121.
Inorganic Chemistry, Vol. 41, No. 23, 2002 6003

Schlecht et al.
Figure 3. Powder diffraction pattern for 60 nm SnTe nanoparticles with reflections indexed for cubic SnTe (JCPDS 46-1210).
Figure 5. Twinning of SnTe nanocrystals: (a) construction of the
superimposed patterns; (b) experimental pattern; (c) HREM micrograph
indicating the orientation of the twin boundary, where black arrows mark
the region with overlapping domains 1 and 2.
Figure 4. HREM micrograph and SAED pattern for a round 60 nm SnTe
nanoparticle, zone axis [110]: (a) simulated pattern; (b) experimental pattern;
(c) HREM micrograph.
6004 Inorganic Chemistry, Vol. 41, No. 23, 2002

Nanocrystalline Tin as a PreparatiWe Tool
Prolonged pyrolysis of the molecular precursor Sn(SePh)4
at 300 °C led to big annealed particles that were no longer
nanocrystalline. For all types of SnSe products (amorphous,
nanocrystalline, and microcrystalline SnSe) EDX analysis
confirmed a Sn:Se ratio of 1:1.
None of the reactions reported herein could be performed
with commercial microcrystalline tin powder under the given
reaction conditions.
Summary
We have shown that activated nanocrystalline tin can be
used as a preparative tool for the synthesis of tin chalcogenide
nanoparticles nc-SnX with X ) S,16 Se, and Te. The whole
family of chalcogenides could be prepared by this general
synthetic approach, starting from an activated group 14
element instead of an activated chalcogen. In the case of
the SnTe nanoparticles it could be shown that particle size
and shape depend on the concentration of the reactants and
can therefore be controlled by an appropriate choice of these
concentrations. More detailed investigations of this concen-
tration dependence and of the nucleation and growth of the
SnTe particles in this type of reaction are currently under
way.
Acknowledgment. We are especially grateful to Viola
Duppel for her help with the TEM micrographs and to Prof.
Dr. Arndt Simon for providing the transmission electron
microscope. S.S. thanks the Fonds der Chemischen Industrie
and the BMBF for a Liebig-Fellowship and Prof. Dr. Martin
Jansen for his continuous support.
Figure 6. HREM micrograph and SAED pattern for a SnTe needle, zone
axis [110]: (a) HREM micrograph; (b) SAED experimental pattern.
mixture of both its known polymorphs but with much higher
yield (94% compared to 60%). In a second step, a short
pyrolysis of Sn(SePh)4 at 300 °C led to nanocrystalline SnSe
and to the elimination of the organoselenium compounds
Ph2Se2 and Ph2Se.
In contrast to the SnTe nanoparticles produced from
activated tin and Ph2Te2, these SnSe nanoparticles show a
broad dispersion with particle sizes from 3 up to 50 nm.
Supporting Information Available: Several TEM micrographs
and SAED patterns of the tin nanoparticles that were used as starting
materials. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC020272Y
Inorganic Chemistry, Vol. 41, No. 23, 2002 6005