Synthesis of Bi2Te3 and (Bi: XSb1- x)2Te3 nanoparticles using the novel IL [C4mim]3[Bi3I12]

Article abstract of DOI:10.1039/c6dt02361d

The novel Bi-containing reactive ionic liquid [C₄mim]₃[Bi₃I₁₂], which was synthesized in quantitative yield by equimolar reaction of BiI₃ and [C₄mim]I, was used as a novel Bi-source for the ionothermal synthesis of Bi₂Te₃ nanoparticles by reaction with (Et₃Si)₂Te in the ionic liquid [C₄mim]I. The solid state structure of [C₄mim]₃[Bi₃I₁₂] was determined by single crystal X-ray diffraction. In addition, the ionothermal synthesis of the single source precursor (Et₂Sb)₂Te and [C₄mim]₃[Bi₃I₁₂] yielded the ternary (Bi_xSb_1-x)₂Te₃ (x = 0.25, 0.5, 0.75) nanoparticles. The chemical composition and phase purity of the tetradymite-type materials were determined by EDX and XRD and the surface composition of the nanoparticles was further investigated by IR and XPS. In addition, the morphology of the nanoparticles was investigated by SEM and TEM.

Full text of DOI:10.1039/c6dt02361d

Showcasing research from the Schulz material research
lab at the University of Duisburg-Essen and Center for
Nanointegration Duisburg-Essen (CENIDE).
As featured in:
Volume 45 Number 39 21 October 2016 Pages 15263–15686
Dalton
Synthesis of Bi Te and (Bi Sb ) Te nanoparticles using the
2
3
x
1−x
2
3
Transactions
novel IL [C mim] [Bi I ]
A
n
wwin.r
t
sc.org/d
e
r
n
a
t
ioal
n
ton
al
journal of inorganic chemistry
4
3
3 12
Thermal decomposition of the single source precursor (Et Sb) Te
2
2
and the reactive IL [C mim] [Bi I ] yields crystalline tetradymite-
4
3
3 12
type (Bi Sb ) Te (x = 0.25, 0.5, 0.75, 1.0) nanoparticles at
x
1–x 2
3
moderate temperatures without formation of Bi-rich material
phases.
ISSN 1477-9226
PAPER
Patrick Eisenberger, Rebecca L. Melen, Cathleen M. Crudden, Douglas W.
Stephan et al.
Chiral carbene–borane adducts: precursors for borenium catalysts for
asymmetric FLP hydrogenations
See S. Schulz et al., Dalton Trans.,
2016, 45, 15326.
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PAPER
Synthesis of Bi Te and (Bi Sb ) Te
2
3
x
1−x 2
3
nanoparticles using the novel IL [C mim] [Bi I ]†
4
3
3 12
Cite this: Dalton Trans., 2016, 45,
15326
a
a
b
a
c
a
M. Loor, G. Bendt, U. Hagemann, C. Wölper, W. Assenmacher and S. Schulz*
The novel Bi-containing reactive ionic liquid [C
by equimolar reaction of BiI and [C mim]I, was used as a novel Bi-source for the ionothermal synthesis
of Bi Te nanoparticles by reaction with (Et Si) Te in the ionic liquid [C mim]I. The solid state structure of
mim] [Bi 12] was determined by single crystal X-ray diffraction. In addition, the ionothermal synthesis
of the single source precursor (Et Sb) Te and [C mim] [Bi 12] yielded the ternary (Bi Te (x = 0.25,
4 3 3
mim] [Bi I12], which was synthesized in quantitative yield
3
4
2
3
3
2
4
[C
4
3
3
I
2
2
4
3
3
I
x
Sb1−x
)
2
3
Received 14th June 2016,
Accepted 1st September 2016
0.5, 0.75) nanoparticles. The chemical composition and phase purity of the tetradymite-type materials
were determined by EDX and XRD and the surface composition of the nanoparticles was further
investigated by IR and XPS. In addition, the morphology of the nanoparticles was investigated by SEM
and TEM.
DOI: 10.1039/c6dt02361d
www.rsc.org/dalton
Seebeck coefficients combined with glass-like low thermal con-
ductivities. They are isostructural and crystallize in the tetra-
Introduction
4
The synthesis of highly efficient thermoelectric materials for dymite-type structure, with the Te atoms building a closed
technical application as thermoelectric generators TEG, which packing structure perpendicular to the c-axis in R ˉ3 m:H and the
allow the conversion of waste thermal energy into electric stacking sequence chh in Jagodzinski-symbols. The stacking
1
2
energy, has attracted considerable attention in recent years.
sequence (AγB□AγBαC□BαCβA□Cβ) results in the formation of
Even though radioisotope thermoelectric generators (RTG), double layers of edge-sharing Bi(Sb)Te6 octahedra consisting
which were used in specific technical applications such as of 5 atom layers (quintuple layer) with the sequence Te1–
3
energy supply of satellites and space probes, have been known Bi(Sb)–Te2–Bi(Sb)–Te1 and the composition (Bi(Sb))
2
Te
3
. The
since the 1950’s, their low thermoelectric efficiency, which is bonding inside the quintuple layer can be described as mixed
2
given by the dimensionless figure of merit ZT = (α σ/λ)T covalent-ionic, while only weak van der Waals bonding is
4
(
α = Seebeck coefficient, σ = specific electrical conductivity, observed between the quintuple layers.
λ = thermal conductivity = sum of electronic λel and lattice λla
contribution, T = absolute temperature [K]), inhibited their the last century as a promising method for increasing the thermo-
Nanostructuring has been identified in the late nineties of
5
broad technical application.
electric efficiency (zT) of a given material, resulting from the
Semiconducting materials with heavy elements are very decreased thermal conductivity of the material due to an
promising materials due to their high effective masses. efficient phonon scattering at boundaries and interfaces, and
Among many different systems, which have been investigated a simultaneously increased Seebeck coefficient due to both
in the last decades, Sb Te , Bi Te and the solid ternary solu- quantum confinement effects and the modification of the elec-
2
3
2
3
6
tions (Sb Bi ) Te are still some of the most effective materials tronic band structure. In addition, ternary solid solutions
x
1−x 2
3
for technical applications operating near room temperature. (Bi
x 2 3
Sb1−x) Te showed enhanced zT values compared to the
These materials show high electrical conductivities and high corresponding binary materials (Bi Te , Sb Te ) as was demon-
2
3
2
3
−3
−1
strated for (Bi Sb ) Te single crystals (z = 3.2 × 10
room temperature ) and p-type (Bi0.2Sb0.8) Te nanocomposites
K
at
x
1−x 2
3
7
2 3
a
Institute of Inorganic Chemistry and Center for Nanointegration Duisburg-Essen
CENIDE), University of Duisburg-Essen, Universitätsstr. 5-7, D-45117 Essen,
−3
−1
8
(
z of 3.52 × 10
K
at room temperature ). Poudel et al.
(
reported record-high zT values of 1.4 at 373 K for bulk
Germany. E-mail: stephan.schulz@uni-due.de
2
a
b
Interdisciplinary Center for Analytics on the Nanoscale (ICAN), NETZ, Carl-Benz-
(Bi
observed a maximum zT value of 1.56 at 300 K for the p-type
Bi0.26Sb ) Te material, which is roughly a 50% improve-
x 2 3
Sb1−x) Te with embedded nanostructures and Xie et al.
Str. 199, 47047 Duisburg, Germany
c
Institute of Inorganic Chemistry, University of Bonn, Römerstr. 164, D-53117 Bonn,
(
0.74 2
3
Germany
9
2 3
ment compared to commercial Bi Te .
†
Electronic supplementary information (ESI) available. CCDC 1479636. For ESI
As a consequence, the synthesis of nanoparticles and thin
films of tetradymite-type materials has received increasing
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6dt02361d
15326 | Dalton Trans., 2016, 45, 15326–15335
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interest. Unfortunately, their strong tendency to form antisite ing tetradymite-type nanoparticles were investigated by IR,
defects, which refers to the occupation of Te sites by Bi atoms EDX, XPS, XRD, SEM and TEM.
or Bi sites by Te atoms and, as a consequence, the formation
of either p- or n-type doped materials, often diminish their
1
0
thermoelectric performance. In addition, the facile incorpor-
Results and discussion
ation of excess bismuth into Bi Te yield Bi-rich material
2 3
phases, which typically adopt sandwich-like structures of the
general form (Bi ) (Bi Te ) , in which the quintuple layers are
2 3
Our previous studies on the synthesis of Bi Te nanoparticles
2
n
2
3 m
1
in organic solvents showed
a strong tendency for the
1
separated by Bi-bilayers as can be observed in tsumoite BiTe,
pilsenite Bi Te and hedleyite Bi Te , respectively.
The synthesis of bulk and nanostructured p-type
formation of Bi-rich materials, most likely resulting from the
thermal lability of the Bi precursor. Therefore, we became
interested in a more stable Bi precursor and began to investi-
gate the use of complex bismuth halide anions such as
4
3
7
3
2
a,12
(Bi
x
Sb1−x
)
2
Te
3
has been widely investigated.
Bismuth-rich
Bi
2
Te
3
and (Sb
x
Bi1−x
)
2
3
Te particles were obtained from the
3+
23–26
[
Bi
we synthesized the novel reactive IL [C
yield by reaction of [C mim]I and BiI (Scheme 1).
3
I
12] , which have been synthesized in the past.
Hence,
reduction of bismuth/antimony acetate with oleylamine (OA)
in dodecanethiole and subsequent reaction with trioctyltelluro-
4
mim] [Bi 12] 1 in high
3
3
I
4
3
1
3
phosphorane (TOPTe) as well as by reaction of TOPTe and
1, which shows a melting point of 98 °C, was obtained as a
1
4
bismuth oleate. In addition, hexagonal Bi Te plates were
2
3
bright yellow powder at ambient temperature. The yellow color
obtained upon thermolysis of bismuth nitrate and TOPTe in
of [C mim] [Bi I ] 1 was found to intensify upon cooling to
4
3
3 12
1
5
octadecene and oleic acid and by reaction of BiCl
3
and
−196 °C; whereas it turns red and finally becomes metallic-
1
6
TOPTe in thioglycolic acid in a microwave assisted synthesis.
like, almost black-purple upon heating to 250 °C (Fig. S8†).
Both processes are fully reversible. Even though the reason for
this thermochromic behavior is not yet clear, we believe that
the color change upon cooling and heating results from a
phase transition of 1. Phase transition reactions are well
known for bismuth(III)iodides, for which more than 60 com-
pounds have been structurally characterized and which show a
large structural diversity. To date, almost 20 different struc-
The resulting material, which was sub-atomically doped with
sulfur, showed a remarkably high zT value (1.1). In addition,
n
Reid et al. recently demonstrated that [BiCl (Te Bu ) ] is a suit-
3
2 3
able single source precursor for the MOCVD (metal organic
chemical vapor deposition) deposition of high-quality Bi Te
thin films. In addition to the widely applied Te source
TOPTe, bis(triethylsilyl)tellurane (Et Si) Te has been demon-
strated to be a promising low-temperature Te-precursor. Sb Te
and Bi Te thin films were deposited by ALD process (atomic
2
3
1
7
3
2
27
2
3
tural types have been reported. The structures were found to
2
3
largely depend on the specific cation. A templating effect of
the cations has been identified as the major structure deter-
mining factor that induces the formation of various metal–
1
8
19
layer deposition) and by low-temperature MOCVD process.
In addition, (Et Si) Te was successfully applied for the wet
chemical synthesis of multiple Bi–Te phases including
3
2
4
halide networks. In the case of (2-MIm)BiI , a change of the
2
0
Bi
2
Te
Due to our general interest in thermoelectric materials, we
started only recently to investigate both gas phase deposition
3
.
electron lone pair activity of the bismuth atom was also found
27b
to play a major role in the phase transition mechanism.
A DSC study revealed an exothermic process around −50 °C
and the melting point was also observed (Fig. S9†).
1
8b,c
19c,d
of thin films using ALD
and MOCVD processes
as well
Te and
and in ionic
liquids (ILs). ILs were shown to be very promising solvents for
the synthesis of Sb Te nanoparticles with very high zT values
as wet chemical approaches for the synthesis of Sb
2
3
1
is highly soluble in strong polar, aprotic solvents such as
2
0,21
Bi Te3 nanoparticles in organic solvents
2
acetonitrile and DMSO and barely soluble in less polar
solvents, e.g. ethanol and methanol, while it is insoluble in
non-polar solvents such as pentane and hexane as well as in
2
3
2
2
of up to 1.5. However, the limited thermal stability of metal
organic bismuth precursors such as bismuth amides or
bismuth alkyl compounds often resulted in the formation of
water. In comparison to the NMR spectra of [C
4
mim]I the
aromatic signals of 1 are high-field shifted by 0.2 ppm in the
1
13
H NMR and by 4 ppm in the C NMR spectra, while the
2
0
elemental bismuth or Bi-rich material phases. Therefore, we
became interested in the development of alternate Bi
precursors, which should be thermally stable to avoid simple
thermal decomposition but whose reactivity should still be
high enough to produce the desired materials at rather low
temperatures.
resonances due to the aliphatic groups show a low-field shift
by the same amounts (Fig. S1 and S2†), indicating stronger
electronic interactions between the positively charged aromatic
−3
system with the [Bi
3
I
12
]
anion compared to the iodine anion,
hence resulting in an increased electron density in the aro-
matic system but a decreased electron density in the aliphatic
chain. The IR spectrum of 1 shows the typical absorption
We herein report on the synthesis and solid state structure
of the novel reactive IL [C mim] [Bi I12] and its promising
4 3 3
potential to serve as alternate Bi source for the synthesis of
phase-pure binary (Bi Te ) and – together with the single
2
3
source precursor (Et
2
Sb)
2
x 2 3
Te – ternary ([Bi Sb1−x] Te ) bismuth
telluride nanoparticles in an IL-based wet chemical approach.
The composition, phase purity and morphology of the result- Scheme 1 Synthesis of [C mim] [Bi I12] 1.
4 3 3
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Table 1 Chemical composition of the Bi
by EDX spectroscopy
2
Te
3
nanocrystals determined also with formation of hexaethyldisilane Si
whose formation points to a homolytic cleavage of the weak
Te–Si bond. As formed Et Si radicals then easily recombine to
Si Et . These findings also explain the presence of small
2 6
Et (Fig. S4a–c†),
3
a
b
c
[
4
C mim][]
[C
4
mim]I/OA
4
[C mim]I/OA
2
6
Bi (%)
Te (%)
29.00 ± 1.00
71.00 ± 1.64
40.73 ± 1.41
59.27 ± 1.36
38.93 ± 1.25
61.07 ± 1.33
amounts of elemental Te in the resulting material, which can
be removed by washing the material with TOP as was pre-
a
b
20
Thermal treatment to 150 °C. Thermal treatment to 150 °C with OA. viously demonstrated. In contrast, thermolysis of (Et
Thermal treatment to 150 °C microwave irradiation.
3 2
Si) Te
c
and [C mim] [Bi I ] in a solution of 10 mL [C mim]I and
4
3
3 12
4
3
mL oleylamine (OA) at 150 °C yielded pure Bi
particles according to EDX studies (Scheme 2).
bands for the [C mim] cation and additional sharp absorp- We recently demonstrated that OA reacts with (Et Si) Te
2 3
Te nano-
+
4
3
2
−
1
−1
tion bands between 1150 cm and 930 cm , which can be at ambient temperature with formation of silylamine (Et SiOA)
3
2
8
ascribed to the Bi–I-stretching vibrations (Fig. S3†).‡
and tellurium polyanions, which then consequently react
Single crystals of 1 were obtained by re-crystallization from with [C mim] [Bi I ] with the formation of Bi Te .
4
3
3 12
2
3
ethanolic solution. [C
clinic space group P2 /c. The asymmetric unit comprises three this technique was found to be very promising for the gene-
cations at general positions and two anions at special posi- ration of pure Sb Te nanoparticles. A solution of (Et Si) Te
4
mim]
3
[Bi
3
I12] crystallises in the mono-
We also investigated a microwave-assisted synthesis, since
1
2
2
2
3
3
2
tions (centre of inversion) as was previously observed for the and [C mim] [Bi I ] in 10 mL of [C mim]I and 3 mL of OA
4
3
3 12
4
3
majority of the known [Bi I12] polyanions. The two indepen- was heated to 150 °C by microwave irradiation and the result-
dent anions lead to an occupation of all vertices and face- ing particles were treated as mentioned before. However,
centres of the unit cell similar to a cubic closest packing of slightly Te-rich materials were formed according to EDX
spheres. However, their anisotropic shape – they are best studies (Table 1).
described as a linear arrangement of three face-sharing BiI6
We further investigated the presence of any kind of organic
octahedra – prevents cubic symmetry. The cations fill the voids molecules, i.e. solvent molecules or capping agents, on the
where the ones labelled X1x and X3x can be found in the surface of the materials as-obtained from thermolysis reac-
pseudo-tetrahedral gap, while the X2x-labelled and its sym- tions of the precursors in [C mim]I and OA. IR spectroscopy
4
metry equivalent share the pseudo-octahedral gap.
2 3
clearly demonstrated that the resulting Bi Te nanoparticles
Bond lengths and angles of the cations show typical values. do not contain OA (capping agent) on the surface as can
The conformation of the butyl group of the cations differs clearly be seen by comparing the IR spectra of the nano-
depending on the space available to fill (Cx5–Cx6–Cx7–Cx8: particles and that of pure OA (Fig. 2). In addition, neither the
−170(2)° (x = 1), 66(2)° (x = 2), 175(2)° (x = 3)). The range of IL nor acetonitrile (washing solvent) bind to the nanoparticles.
Bi–I bond lengths matches well with those previously reported According to these IR studies, the nanoparticles are considered
by Carmalt et al. and others for [Bi
3
I
12
]
and [Bi
4
I
16
]
to be almost capping-agent free.
2
3–25
polyanions.
These polyanions can be described as central
The resulting materials were investigated by powder X-ray
3
−
BiI6 octahedral units with neutral BiI moieties capping two diffraction (PXRD), which clearly proved the formation of
3
of its trans oriented faces. The independent anions are roughly Bi Te in all experiments (Fig. 3). The peaks can be indexed on
2
3
perpendicular to one another (Bi11⋯Bi12/Bi21⋯Bi22 the basis of phase-pure Bi Te (PDF 15-863), but the materials
2
3
7
4.22(1)°) and parallel to the (100) plane. Inter-halide inter- obtained in ionothermal syntheses in [C mim]I in the absence
4
actions lead to the formation of layers parallel to this plane of oleylamine as well as in microwave assisted reactions
(
Table S1†). These layers are interconnected by non-classical in [C mim]I and OA showed additional reflexes at 23°, 38°,
4
2
6
hydrogen bonds between anion and cations (Table S2†).
and 44°, respectively, which indicate the presence of small
impurities of elemental tellurium. In contrast, the ionothermal
Synthesis of Bi Te nanoparticles
2
3
synthesis in a mixture of [C
Te
2 3
The lattice parameters for the nanosized Bi Te particles,
4
mim]I and oleylamine yielded
Thermolysis of (Et Si) Te and [C mim] [Bi I ] in a solution of phase pure Bi
1
.
3
3
2
4
3
3 12
2
0 mL of [C mim]I at 150 °C yielded a black precipitate, which
4
was isolated by centrifugation and purified by repeatedly which were refined to a = 4.388(8) Å, c = 30.517(7) Å and
washing with acetonitrile and hexane (3x). The elemental com-
position of the material as determined by EDX proved the for-
2 3
mation of slightly Te-rich Bi Te materials (Table 1). NMR
1
13
spectroscopic studies ( H, C) showed that the thermolysis
not only occurred with the expected formation of Et SiI but
3
‡1
H, C NMR and IR spectra of [C
13
4
mim]I and [C
4 3 3
mim] [Bi I12] 1; in situ NMR
spectroscopic studies of the reaction of (Et
3
Si) Te and 1; thermal behavior of 1
2
upon heating and cooling; central structural parameters of 1 and EDX results
from STEM spot analyses on single crystals.
2 3
Scheme 2 Synthesis of Bi Te nanoparticles.
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Fig. 1 Solid state structure of [C
4
mim]
3
[Bi
3
I12] 1. Probability ellipsoids
Fig. 3 PXRD of the Bi Te ionothermal and microwave (MW)
2
3
are displayed at 50% probability levels and hydrogen atoms are omitted
for clarity. Symmetry generated parts are displayed in pale colours. #1
2 3
approaches and reference for Bi Te (PDF 15-863). Peaks marked with
29
an * correspond to elemental Te.
−
x, 2 − y, −z; #2 1 − x, 1 − y, −z.
2 3
Table 2 Size of the Bi Te nanocrystals derived from the PXRD data
using the Scherrer equation
[
4
C mim]I
4
[C mim]I + OA
4
[C mim]I (MW)
a
Crystal size /nm
44 ± 2
55 ± 3
37 ± 2
a
The determination of the crystal size of anisotropic particles such as
thin plates by using the Scherrer equation is always problematic.
Values should therefore be taken with care.
3
0
for Bi
2
Te
3
.
After one day of exposure to air additional peaks
due to the presence of both bismuth oxide at 159 eV and tellur-
ium oxide at 576.6 eV binding energy can be observed. Oxide
intensities increase dramatically after 1 month, so that more
than 50% of the surface metal atoms (Bi 53%, Te 64%) are oxi-
dised (Table 3). These findings agree with the results very
recently observed for surface oxidation reactions of binary and
ternary bismuth chalcogenides, in which Bi Te and Bi Te Se
Fig. 2 IR spectra of the Bi
2
Te
3
nanoparticles synthesized with OA, pure
OA, acetonitrile, and [C
4
mim]I.
2
3
2
2
were found to easily oxidise upon expose to air while Bi
V = 509.0(7) Å , are in good agreement with the values reported significantly more stable toward oxidation. As a consequence,
2
Se
3
was
3
31
for Bi Te (PDF 15-863). The broadening of the full width half the nanoparticles have to be stored and handled under inert
2
3
maximum (FWHM) of the peaks indicate an average crystal gas conditions to avoid surface oxidation reactions.
size of roughly 55 nm (Table 2). According to SEM studies, the Bi Te materials as-obtained
We then investigated the stability of the Bi Te nano- in a solution of [C mim]I and oleylamine (OA) contain Bi Te
3
2
3
2
3
4
2
particles toward oxidation reactions upon exposure to air by nanoplates with an average size of about 50 nm and a more or
X-ray photoelectron spectroscopy (XPS). less hexagonal shape (Fig. 5c). These nanoplates are highly
A freshly prepared Bi Te sample was shown to contain an agglomerated as was expected due to the lack of any capping
2
3
oxygen-free surface. After expose to air for different period of agents on the surface as was demonstrated by IR spectroscopy.
times, the materials were again investigated by XPS. The Hence, TEM bright field images show plate-like particles with
changes of the Bi 4f and Te 3d signals are depicted in Fig. 4. It a size ranging from 20 to 100 nm (Fig. 5a and b), which are
can clearly be seen, that the particles are fairly stable for an heavily inter-grown, reminiscent of sintered powder. The par-
hour, showing only the metallic peaks at 157.2 eV for Bi 4f and ticles show diffraction contrast and are thus considered as
5
72 eV for Te 3d. These values agree with the literature values crystalline. Selected area electron diffraction (SAED; Fig. 5d)
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2 3
Fig. 4 Change of the Bi 4f and Te 3d XPS signals of Bi Te nanoparticles upon exposure to air.
Table 3 Percentage of oxidised metal atoms after exposure to air as formation of Bi-rich materials such as BiTe or Bi Te , which
4
3
determined by XPS
was often observed in reactions of metal organic precursors as
a result of the often thermally rather unstable Bi precursors, is
completely avoided in the reaction with the reactive Te-source
Exposure to air
None
Bi-oxide/%
Te-oxide/%
0
0
22
53
0
0
27
65
(Et
3 2
Si) Te, indicating a constant release of Bi under the
1
1
1
hour
day
month
specific reaction conditions.
Synthesis of ternary (Bi Sb ) Te nanoparticles
x
1−x 2
3
After the successful synthesis of phase-pure Bi
particles, we became interested in the synthesis of ternary
solid solutions of (Bi Te . Since the thermolysis of the
single source precursor (Et Sb) Te was previously established
for the synthesis of highly stoichiometric Sb Te nano-
2 3
Te nano-
x
Sb1−x
)
2
3
2
2
2
3
2
1,22
plates,
different amounts of the reactive IL [C
0 mL of [C mim]I. After thermolysis at 150 °C, the resulting
we investigated its thermolysis in the presence of
4
mim] [Bi 12] 1 in
3
3
I
1
4
black solid was isolated by centrifugation and purified by
repeatedly washing with acetonitrile (Scheme 3).
Ternary materials of the general type (Bi Sb ) Te were
x
1−x 2
3
obtained in all cases, even though they showed a slight tellur-
ium deficit. In addition, a general trend for bismuth-rich par-
ticles was observed (Table 4). The reactions probably proceed
Fig. 5 TEM bright field images (a, b); SEM image (c) and contrast
inverted selected area electron diffraction pattern of Bi Te (Philips
CM30 T @300 keV) and simulated ring pattern as overlay SAED (d) of
2
3
Scheme 3 Thermolysis of the single source precursor (Et
tion of (Et Sb) Te and [C mim] [Bi for the synthesis of
(Bi Te nanoparticles.
2 2
Sb) Te; reac-
2
2
4
3
3 12
I ] 1
3
2
Bi
2
Te
3
nanoparticles.
x
Sb1−x
)
2
3
x 2 3
Table 4 EDX results from the (Bi Sb1−x) Te nanoparticles
taken from a crystal covered region gives a powder ring pattern
with d-values as expected for Bi Te .
2
3
x
Bi (%)
Sb (%)
Te (%)
The synthesis of phase-pure Bi
2
Te
3
nanoparticles proves the
3 12
mim] [Bi I ]
0
0
.25
.5
12.4 ± 0.6
22.5 ± 0.9
30.1 ± 1.2
28.3 ± 0.9
19.3 ± 0.5
11.3 ± 0.5
59.3 ± 1.8
58.2 ± 1.5
58.7 ± 1.6
promising potential of the novel reactive IL [C
to serve as
4
3
1
a
Bi-source in materials synthesis. The 0.75
15330 | Dalton Trans., 2016, 45, 15326–15335
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2 3
Fig. 7 SEM image of (Bi0,5Sb0,5) Te .
As was observed for the Bi Te nanoplates, the as-prepared
(Bi Sb ) Te materials also consist of hexagonal nanoplates,
x 1−x 2 3
2
3
Fig. 6 Powder X-ray diffractogram of (Bi
x
Sb1−x
)
2
Te
3
(x = 0.25, 0.5, 0.75,
33
1
2 3
) nanoparticles and reference for (Bi0.5Sb0.5) Te (PDF 072-1853).
which again are highly agglomerated. The average size of the
nanoparticles according to SEM studies ranges from roughly
5
0 nm to 100 nm (Fig. 7).
TEM studies confirm the hexagonal plate-like shape of the
with the initial formation of small Sb Te nanoparticles, in
which the antimony content is consequently substituted by
bismuth successively provided from the reactive IL 1.
PXRD studies proved the formation of phase pure ternary
x 2 3
materials (Bi Sb1−x) Te (Fig. 6). The successful substitution of
bismuth by antimony in the tetradymite-type material can be
monitored by a slight shift of the reflections toward higher
angles with increasing antimony concentration, resulting in
decreasing lattice parameters as was expected due to the repla-
cement of the large bismuth atoms by smaller antimony
atoms. This is confirmed by the refinement of the lattice para-
meters, which show a monotonically decreasing cell volume
2
3
(
Bi Sb ) Te materials with the growth direction perpendicu-
x 1−x 2 3
lar to the c-axis and a broad size distribution. Although some
of the larger hexagonal plates consist of an intergrowth of
several crystalline domains, crystals from 10 to 200 nm in size
are present in each of the (Bi Sb ) Te materials (Fig. 8a–c).
x
1−x 2
3
EDX spot analyses in the STEM mode on single crystals
Table S3†) confirm the composition corresponding to the
sum formula (Bi Sb ) Te with x = 0.25, 0.5 and 0.75 and
(
x
1−x 2
3
(
Table 5).
Compared to the phase-pure Bi
of the as-prepared ternary solid solutions (Bi
2
Te
3
nanoplates, the XRDs
Te
3
x
Sb1−x
)
2
showed increasing peak broadening, clearly indicating a stea-
dily decreasing size of the crystalline domains with increasing
bismuth concentration (Fig. 6). In addition, anisotropic peak
broadening becomes more dominant, underlining the prefer-
ential growth along the ab-plane perpendicular to the c-axis,
which manifests itself in a relatively sharp (110) reflection at
4
1.12° (2θ) compared to the other reflections, i.e. the neighbor-
ing (1010) reflex at 37.8°.
Table 5 Refined lattice parameters and crystallite sizes for
(Bi Sb1−x) Te nanoparticles
x 2 3
3
a
x
a [Å]
c [Å]
V [Å ]
Size [nm]
1
0
0
0
4.388(8)
4.360(6)
4.319(9)
4.291(9)
30.517(7)
30.449(2)
30.496(2)
30.580(4)
509.0(7)
501.4(1)
492.8(6)
487.8(3)
38.3(9)
33.6(6)
25.7(2)
22.9(3)
.75
.5
.25
a
Fig. 8 TEM bright field images of (Bi
x
Sb1−x
)
2
Te
3
with (a) x = 0.25; (b) x =
Te
3
The determination of the crystal size of anisotropic particles such as
thin plates by using the Scherrer equation is always problematic. 0.5; (c) x = 0.75. (d) Electron diffraction pattern of a (Bi0.25Sb0.75
)
2
Values should therefore be taken with care.
crystal along c*.
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Synthesis of 1-butyl-3-methyl-imidazolium iodide [C
2 mL (0.276 mol) 1-methylimidazole (99%, Sigma Aldrich)
was dissolved in 100 mL of acetonitrile and 1-iodobutane
0.308 mol, 35 mL) was added dropwise in the dark at 0 °C.
4
mim]I
2
(
The solution was stirred at ambient temperature for 12 h and
then all volatiles were removed under dynamic vacuum. The
resulting residue was washed with 150 mL of ethyl acetate.
After removal of the solvent, the remaining yellowish oil was
dried for 72 h under dynamic vacuum at 50 °C.
1
Yield: 59.47 g (81%). H NMR (300 MHz, 25 °C, DMSO-d ):
6
1
2
δ (ppm) 9.14 (s, 1H), 7.76 (dt, JH–H = 13.5 Hz, JH–H = 1.7 Hz,
= 7.2 Hz, 2H), 3.88 (s, 3H), 1.79 (dt, J
14.8 Hz, JH–H = 7.5 Hz, 2H), 1.29 (m, 2H), 0.93 (t, JH–H
2 3
Fig. 9 (a) HRTEM bright field image of (Bi0.75Sb0.25) Te in [100] orien-
3
1
tation; (b): Fourier filtered cutout with ball and stick model of (BiSb)
Te
2 3
2H), 4.19 (t, J
H–H
H–H
=
=
2
3
as overlay (green: Te, orange: Bi, Sb).
7.3 Hz, 3H).
Synthesis of [C mim] [Bi I ] 1
4
3
3 12
agree with those summarized in Table 4, which were obtained
from big agglomerates.
1
4.91 g (0.561 mol) [C mim]I and 27.57 g (0.468 mol) BiI were
4
3
added to 500 mL ethanol and the resulting suspension was
stirred at ambient temperature for 5 d. The resulting bright
yellow solid was separated via filtration, washed with 100 mL
of ethanol and carefully dried for 72 h under dynamic vacuum
at ambient temperature.
Fig. 8d shows an electron diffraction (ED) pattern of
Bi0.25Sb0.75) Te in [001] orientation. The d-values of the {100}
2 3
and {030} reflections are 2.19 Å and 1.22 Å, respectively,
and agree with the lattice parameters from PXRD and with
(
the reflection conditions for space group R ˉ3 m:H. The presence
Yield: 30.87 g (77.16%). Melting point: 98 °C. Elemental
analysis (EDX): Bi: 19.79 ± 0.95 at%, I: 80.21 ± 1.74 at%.
of Bi and Sb in small volumes proven by spot EDX and
the absence of any super-structure reflections in the
ED-patterns reveal the random distribution of antimony and
bismuth on the cation positions of the tetradymite-type
structure.
1
6
H NMR (300 MHz, 25 °C, DMSO-d ): δ (ppm) 9.11 (s, 1H),
1
2
3
7
7
.73 (dt, J
= 20.2 Hz, J
= 1.8 Hz, 2H), 4.16 (t, J
=
H–H
H–H
H–H
.2 Hz, 2H), 3.85 (s, 3H), 1.76 (m, 2H), 1.29 (m, 2H), 0.93
3
(
t, JH–H = 7.3 Hz, 3H).
Unfortunately, HRTEM studies of (Bi
dicular to the c-axis, which enables the depiction of the stack-
ing of the quintuple Bi Te layers, was difficult to perform
since the plate-like crystals were predominantly found lying in
001] orientation or they were most often too thick for HRTEM,
x 2 3
Sb1−x) Te perpen-
2
3
Synthesis of Bi Te nanoparticles
2
3
3
.4 g (3.9 mmol) of 1 was dissolved in 10 mL of [C mim]I or in
4
[
a mixture of 10 mL of [C mim]I and 3 mL of oleylamine in a
4
when the orientation was perpendicular to the c-axis. Fig. 9
centrifuge tube and the mixture was stirred for 1 h at 100 °C.
shows a HRTEM image of (Bi0.75Sb ) Te in [100] orien-
0.25 2
3
1
.24 g (3.47 mmol) (Et Si) Te were then added and the result-
3 2
tation. Although the atom columns are not well resolved, the
stacking of the quintuple layers of 10.2 Å is found in all areas.
This indicates that no additional Bi-bilayers are present and
confirms the Bi Te -type of structure.
ing black suspension was stirred for 12 h at 150 °C. The result-
ing colloidal solutions were centrifuged (2500 rpm), washed
with 6 × 15 mL acetonitrile and 2 × 15 mL hexane, and dried at
room temperature under vacuum.
2
3
x 2 3
Synthesis of (Bi Sbi1−x) Te nanoparticles
Experimental
In a centrifuge tube, an appropriate amount (see Table 6) of
[
C mim][Bi I ] was dissolved in 2 mL [C mim]I and stirred for
4 3 12 4
All synthetic procedures including the synthesis of the IL as
well as all thermolysis experiments were performed under
inert gas conditions (Ar atmosphere) in a glovebox or using
standard Schlenk techniques. Acetonitrile (99.9+%, Extra Dry,
Acros organics), ethanol (J. T. Baker 98%) and 1-iodobutane
1
h at 100 °C. 137 µl (1.44 mmol) (Et Sb) Te were then added
2 2
and the solution was stirred for 12 hours at 150 °C. The result-
ing black particles were washed with 6 × 15 mL acetonitrile.
(
99%, Acros organics) were commercially available and used as
Table 6 Amount of [C
4
mim]
3
[Bi
3
I
12
]
1
used for the synthesis of
received, while ethyl acetate (J. T. Baker) was redistilled prior
to use. Solvents were carefully dried according to standard
procedures and degassed prior to use. (Et Sb) Te was prepared
according to the literature method. Melting points were
determined with an Electrothermal IA9300 digital melting
point instrument from ThermoFisher Scientific.
(
Bi Te nanoparticles
x
Sb1−x
)
2
3
2
2
x
m 1 [mg]
n 1 [mmol ]
3
4
0
0
.25
.5
103
205
308
0.04
0.08
0.12
0.75
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Single crystal X-ray diffraction
samples were investigated in the range of 5 to 90° 2θ with a
step size of 0.01° 2θ and a counting time of 0.3 s. Rietveld
refinement was performed with the program package TOPAS
Crystallographic data of 1 were collected on a Bruker D8 Kappa
APEX2 diffractometer (MoK
α
radiation, λ = 0.71073 Å) at 100(1)
5.0 from Bruker. The instrumental correction was determined
K: [C H Bi I N ], M = 2567.40, red crystal, (0.145 × 0.094 ×
24
45 3 12 6
using LaB powder from NIST (National Institute of Standards
6
0
.047 mm); monoclinic, space group P2
1
/c; a = 17.5900(9) Å,
and Technology) as the standard reference material and taken
into account for each refinement.
b = 15.9186(7) Å, c = 19.3836(9) Å; α = 90°, β = 96.503(3)°, γ =
3
−1
9
3
0°, V = 5392.6(4) Å ; Z = 4; μ = 16.658 mm ; ρcalc =
.162 g cm− ; 87 677 reflexes (θmax = 28.408°), 13 458 unique
3
(
R
int = 0.1193); 415 parameters; largest max./min. in the final
−
3
−3
difference Fourier synthesis 2.683
e
Å
/−2.694
= 0.0423 (I > 2σ(I)), wR
.0864 (all data). The solid-state structure of 1 is shown
in Fig. 1. The structure was solved by direct methods
e
Å
;
Conclusions
max./min. transmission 0.41/0.75; R
1
2
=
The novel Bi source [C mim] [Bi I ] 1 containing the complex
4
3
3 12
0
[
Bi
1 is a promising Bi source for the preparation of phase pure
Bi Te material in ILs using ionothermal synthesis.
3
I12] trianion was synthesized and structurally characterized.
3
5
(
SHELXS-97) and refined anisotropically by full-matrix least-
2
36
2
3
squares on F (SHELXL-2014). Absorption corrections were
performed numerical based on indexed faces (Bruker AXS
APEX2). Hydrogen atoms were refined using a riding model or
rigid methyl groups.
The formation of Bi-rich material phases, as has been often
described for other synthetic routes, is completely avoided.
The Bi Te nanoparticles are surfactant free according to
2
3
IR spectroscopic studies and are easily oxidised upon exposure
to air.
In addition, co-thermolysis of 1 with the single source pre-
2 2 4 3
cursor (Et Sb) Te in different molar ratios in [C mim] I
The crystallographic data of 1 (excluding structural factors)
have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC 1479636.
allowed for the synthesis of a whole range of ternary solid solu-
tions of the general formula (Bi Sb ) Te , which were charac-
Instrumentation
x
1−x 2
3
IR spectroscopy. IR spectra were recorded in a glovebox terized by XRD, SEM, EDX and TEM.
using an ALPHA-T FT-IR spectrometer equipped with a single
reflection ATR sampling module.
1
13
1
NMR spectroscopy. H (300 MHz) and C{ H} (75.5 MHz)
NMR spectra (δ in ppm) were recorded using a Bruker Avance
Acknowledgements
DPX-300 spectrometer and were referenced to internal DMSO- S. Schulz gratefully acknowledges financial support by
1
13
d6 ( H: δ = 2.50; C: δ = 39.52).
the Deutsche Forschungsgemeinschaft (DFG) within the
XPS. XPS studies were performed using a Versaprobe II™ Priority Program SPP 1708 “Material Synthesis near Room
(
Ulvac-Phi) with monochromatic Al Kα light at 1486.6 eV Temperature” and the University of Duisburg-Essen.
photon energy. The emission angle between the analyzer and
sample is 45°. The Cu 2p signal at 932.67 eV binding energy of
a sputter–cleaned Cu foil was used as the binding energy refer-
ence. The foil and the powder were put onto insulating
double-sided tape and charging effects were compensated
using a dual-beam neutralizing approach using electrons and
slow moving argon ions.
Electron microscopy. Particle size and morphology as well as
the elemental composition of the powders were analyzed
by scanning electron microscopy (SEM) using a Jeol JSM
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3
0 (a) C. D. Wagner, A. V. Naumkin, A. Kraut-Vass,
J. W. Allison, C. J. Powell and J. R. Rumble Jr., NIST
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Dalton Trans., 2016, 45, 15326–15335 | 15335