Wet-chemical synthesis of different bismuth telluride nanoparticles using metal organic precursors-single source vs. dual source approach

Article abstract of DOI:10.1039/c5dt02072g

Thermolysis of the single source precursor (Et₂Bi)₂Te 1 in DIPB at 80 °C yielded phase-pure Bi₄Te₃ nanoparticles, while mixtures of Bi₄Te₃ and elemental Bi were formed at higher temperatures. In contrast, cubic Bi₂Te particles were obtained by thermal decomposition of Et₂BiTeEt 2 in DIPB. Moreover, a dual source approach (hot injection method) using the reaction of Te(SiEt₃)₂ and Bi(NMe₂)₃ was applied for the synthesis of different pure Bi-Te phases including Bi₂Te, Bi₄Te₃ and Bi₂Te₃, which were characterized by PXRD, REM, TEM and EDX. The influence of reaction temperature, precursor molar ratio and thermolysis conditions on the resulting material phase was verified. Moreover, reactions of alternate bismuth precursors such as Bi(NEt₂)₃, Bi(NMeEt)₃ and BiCl₃ with Te(SiEt₃)₂ were investigated.

Full text of DOI:10.1039/c5dt02072g

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DOI: 10.1039/C5DT02072G
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ARTICLE
Wet-chemical Synthesis of different Bismuth
Telluride Nanoparticles using Metal Organic
Precursors - Single Source vs. Dual Source
Approach
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
a
a
a
b
Georg Bendt, Anna Weber, Stefan Heimann, Wilfried Assenmacher,
a
a
Oleg Prymak and Stephan Schulz *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract. Thermolysis of the single source precursor (Et
2
Bi)
2
Te
1
in DIPB at 80 °C yielded phase-
pure Bi
temperatures. In contrast, cubic Bi
Et BiTeEt in DIPB. Moreover, a dual source approach (hot injection method) using the reaction
of Te(SiEt and Bi(NMe was applied for the synthesis of different pure Bi-Te phases including
Bi Te, Bi and Bi Te , which were characterized by PXRD, REM, TEM and EDX. The
4
Te
3
nanoparticles, while mixtures of Bi
4 3
Te and elemental Bi were formed at higher
2
Te particles were obtained by thermal decomposition of
2
2
3
)
2
2 3
)
2
4
Te
3
2
3
influence of reaction temperature, precursor molar ratio and thermolysis conditions on the
resulting material phase was verified. Moreover, reactions of alternate bismuth precursors such
2 3 3 3 3 2
as Bi(NEt ) , Bi(NMeEt) and BiCl with Te(SiEt ) were investigated.
intense research has been done on this field, leading to several
preparation and optimization strategies.
Introduction
Scheele et al. showed that nanosized Bi Te particles can be
In the modern society, we are surrounded by waste heat
emitters such as combustion engines, electronic components
and even the human body. Since the amount of emitted heat is
often only small, utilization of this energy is laborious but
possible by thermoelectric generators TEG, which allow the
conversion of thermal energy into electric energy and therefore
2
3
synthesized in a twoꢀstep process by reducing bismuth acetate
with oleylamine to elemental bismuth particles in dodecanthiole
and further treatment with trioctyltellurophosphorane TOPTe,
which is a widely used soluble Teꢀsource, at 110 °C. The
resulting rhombohedral bismuthꢀrich Bi Te ꢀparticles showed
2
3
[
1]
reduced thermal conductivity after SPSꢀtreatment and a high
provide a wearꢀ and noiseless power source. The efficiency of
a thermoelectric material is given by the dimensionless figure
ꢀ
2
ꢀ1 [4]
power factor (5 µWꢁK ꢁcm ).
synthetic protocol also allowed the synthesis of (Sb Bi ) Te
3
Modifying this general
2
of merit (ZT= (α σ/λ)T), where α is the Seebeck coefficient, σ
x
1ꢀx 2
nanoparticles. Stavila et al. investigated the thermolysis of
TOPTe and bismuth oleate and investigated the role of the
reaction temperature on the resulting particle shape. Asꢀ
obtained particles reached ZT values of 0.38 at room
is the specific electrical conductivity, λ is the thermal
conductivity as sum of the electronic λel and the lattice λla
contribution and T the absolute temperature in Kelvin.
Unfortunately, the transport coefficients are linked together and
typically can't be optimized independently from each other. For
instance, metals show high electrical and thermal
conductivities, whereas both values are small for insulators
such as ceramics. The best choices for technical applications
are semiconducting materials with heavy elements due to their
high effective masses. For technical applications near room
temperature, Sb Te and Bi Te as well as the solid ternary
[
5]
temperature. Moreover, bismuth nitrate and TOPTe were
thermolyzed in octadecene and oleic acid by Ruan et al.,
resulting in the formation of hexagonal und rodꢀshaped
[
6]
particles, while Borca-Tasciuc et al. obtained hexagonal
Bi Te3 plates from the reaction of BiCl3 and TOPTe in
2
[
7]
thioglycolic acid in a microwave assisted synthesis. The
resulting material, which was subꢀatomically doped with
sulphur, showed a remarkably high ZT value (1.1).
2
3
2
3
solutions (Sb Bi ) Te are the most effective materials due to
x
1ꢀx 2
3
Even though TOPTe is a suitable Teꢀprecursor for the solutionꢀ
based synthesis of bismuth tellurides, alternative Teꢀprecursors
their high electrical conductivities and high Seebeck
coefficients combined with glassꢀlike low thermal
[
2]
have been investigated.
A
promising candidate is
conductivities. The thermal conductivity of a given material
can be further lowered by nanostructuring approaches, which
result in an increased phonon scattering at grain boundaries
bis(triethylsilyl)tellurane (Et Si) Te, whose general capability
3
2
to serve as rather lowꢀtemperature Teꢀprecursor for the
deposition of Sb Te and Bi Te thin films by atomic layer
[
3]
smaller than the phonons mean free path. As a consequence,
2
3
2
3
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DOI: 10.1039/C 2072G
Jou5DrnT0al Name
[
8]
deposition (ALD) as well as for the wetꢀchemical synthesis of
Results and discussion
[
9]
GeTeꢀnanoparticles was recently demonstrated. In addition,
bis(trimethylsilyl)tellurane (Me Si) Te in combination with
bismuth trisamides Bi(NR ) was also successfully applied as
3
2
Thermolysis of (Et Bi) Te 1
2
2
2
3
Since (Et Sb) Te was previously shown to exhibit superior
lowꢀtemperature MOCVD precursor for the deposition of
2
2
[
10]
properties as SSP for the solutionꢀbased synthesis of highꢀ
Bi Te thin films. Surprisingly, to the best of our knowledge,
2
3
[
20]
quality hexagonal Sb Te nanoplates, the heavier homologue
an analogous solutionꢀbased synthesis of Bi Te nanoparticles
using these specific precursors has not yet been reported.
Bi Te crystallizes in the tetradymiteꢀtype structure with five
membered stacks of alternating tellurium and bismuth layers
2
3
2
3
(
Et Bi) Te
1
, which is readily accessible by insertion reaction
of elemental Te into the weak BiꢀBi bond of
tetraethyldibismuthine in ꢀpentane at low
2
2
2
3
n
4
Et Bi2
[
23]
1
2
1
temperature, was investigated.
along the cꢀaxis with the sequence Te ꢀBiꢀTe ꢀBiꢀTe , in which
the indices correspond to tellurium atoms in different
environments. Each stack has the composition Bi Te and the
(
(Scheme 1 here))
Scheme 1. Synthesis of 1.
is a red solid, whose thermal properties were studied by
2
3
[
2]
hexagonal unit cell (
R
ꢀ3
m
:H) contains 3 formula units.
A
general problem in the wetꢀchemical synthesis of Bi Te3
2
1
nanoparticles is their strong tendency to form soꢀcalled antisite
defects, the occupation of either Te sites by Bi atoms or Bi sites
by Te atoms in the tetradymite lattice, resulting in the formation
differential scanning calorimetry (DSC). The endothermic peak
at 51.3 °C indicates the melting point of , followed by a broad
exothermic peak between 60 and 100 °C, corresponding to its
thermal decomposition (SI, Fig. S3). From the resulting product
mixture, Et Bi and Et Te were clearly identified. Et Bi (170 °C)
and Et Te (225 °C) decompose at higher temperatures, which
was confirmed by comparison with the DSC curves of the pure
compounds (SI Fig. S1, Fig. S2). These findings were
confirmed by thermolysis studies of
capillary, showing melting of at 53 °C followed by a colour
change from red to black upon further temperature increase. In
addition, Et Bi and Et Te were clearly identified as
decomposition products of
1
of either
pꢀ or nꢀdoped materials. This wellꢀknown process
strongly influences the carrier concentration and therefore the
[
11]
3
2
3
thermoelectric properties of the resulting material. Moreover,
the facile incorporation of excess bismuth into the tetradymite
lattice leads to a series of sandwichꢀlike structures of the
general form (Bi ) (Bi Te ) , in which the quintuple layers are
2
2
n
2
3 m
12]
[
1 in a sealed melting point
separated by Biꢀbilayers
as can be observed in tsumoite
1
BiTe, pilsenite Bi Te and hedleyite Bi Te , respectively.
4
3
7
3
Single source precursors (SSPs), which contain the elements of
the desired material already preꢀformed in a single molecule,
are promising alternative precursors for the synthesis of
3
2
1 in a thermolysis experiment in a
[
13]
sealed NMRꢀtube. According to these studies, the
decomposition of occurs by a different and more complex
reaction mechanism, which most likely includes the formation
of radicals, compared to that of (Et Sb) Te, which proceeded in
nanoparticles and thin films.
They are typically easier to
1
handle, their thermal properties can be modified by the
precursor design and they typically exhibit lower
decomposition temperatures, which render them very attractive
as lowꢀtemperature precursors. While the use of SSPs for the
synthesis of several main group and transition metal
2
2
a stoichiometric reaction with subsequent formation of Sb Te
2
3
[
20]
and four equivalents of SbEt3.
(Figure 1 here))
only been recently developed. Sb Te nanoplates were obtained Figure 1. Xꢀray diffractogram of Bi
[
14]
chalcogenides including M E nanoparticles (M = Sb, Bi; E
2
3
(
[
15]
=
S, Se) is well established,
suitable SSPs for Sb Te have
2 3
Te
nanoparticles as
2
3
4
3
from aerosol assisted chemical vapor deposition (AACVD) obtained by thermal decomposition of
1
at 80 °C in DIPB and
i
[16]
[17]
using [Sb{(TeP Pr ) N} ].
MeSb(TeBu)2 were recently shown to be promising SSPs for
the wetꢀchemical synthesis of Sb Te nanoparticles and the
In addition, Ph SbTeEt
and reference (green: Bi Te , JCPDS 33ꢀ216).
2
2
3
2
4
3
[
18]
The thermolysis reaction of
1
was performed with 200 mg
2
3
(
Et Bi) Te 1, which was dissolved in 10 mL DIPB, slowly
2
2
CVD deposition of Sb Te thin films. While early AACVD
2
3
heated to 170 °C and stirred for 1 h. The red solution turned
into a black suspension around 80 °C. After cooling to ambient
temperature, macroscopic, shiny metallicꢀlike particles were
isolated from the colourless liquid by centrifugation and were
shown by EDX studies to consist of pure bismuth metal. The
thermolysis temperature was therefore lowered to 80 °C,
yielding a black powder after work up. The chemical
composition of the material asꢀdetermined by EDX analysis
revealed a bismuth to tellurium ratio of 56:44 at%, which is
close to the theoretical value for Bi Te (57:43). All observed
studies with Et SbTeEt and Te(SbEt ) failed to give Sb Te
2
2 2
2
3
[
19]
films, we successfully demonstrated the promising potential
[
20]
of Te(SbEt ) to serve as SSP for the solution based and gas
2
2
[
21]
phase based
synthesis of highly stoichiometric Sb Te3
2
nanoparticles and thin films with high Seebeck coefficients.
These studies clearly demonstrated that the formation of antisite
defects in Sb Te nanoplates can be significantly reduced by
2
3
use of this specific SSP. Moreover, the studies showed that the
ligand design of the precursors is of high interest in order to
control the specific decomposition mechanism and therefore the
level of impurities incorporated into the resulting material. In
remarkable contrast, single source precursors for the solutionꢀ
based synthesis of highꢀquality Bi Te nanoparticles have not
4
3
peaks in the PXRD pattern (Fig. 1) can be indexed on the basis
of rhombohedral Bi Te3 (JCPDS 33ꢀ216). The lattice
4
parameters were refined to (a = 4.43(1) Å, c = 41.84(1) Å and
2
3
3
V = 415.5(4) Å ) (SI Fig. S10. The broad full width half
been reported to the best of our knowledge, while Read et al.
recently demonstrated that the bismuth chloride telluroether
complex [BiCl (TeBu ) ] is a suitable single source precursor
maximum (FWHM) of the peaks indicate a small particle size
of roughly 20 nm.
3
2 3
[
22]
for the MOCVD deposition of highꢀquality Bi Te thin films.
TEM studies of the nanoparticles (Fig. 2) show the formation of
2
3
Herein we report on our attempts to prepare suitable single agglomerated particles of roughly 200 nm in size, consisting of
source and dual source precursors for the wetꢀchemical smaller particles with diameter ranging from 10 to 40 nm. The
synthesis of crystalline Bi Te3 nanoparticles at rather mild particles are crystalline and lattice fringes of the randomly
2
reaction condition below 100 °C.
orientated crystals are easily resolved in HRTEM. A dꢀspacing
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT020 G
of roughly 310 pm is often observed, which refers to the 100% 14 h. The resulting black suspension was centrifuged and the
peak (107) in the PXRD. The corresponding ring diffraction black solid repeatedly washed (3x) with chloroform.
pattern (SI Fig. 14) from an area with several crystals is in good
(
(Figure 4 here))
Figure 4. Xꢀray diffractogram of black powders as obtained by
thermal decomposition of at 100 °C in DIPB and reference
red: Bi Te, JCPDS 42ꢀ540).
agreement with the PXRD result and proves the formation of
[
24]
crystalline Bi Te nanoparticles by simulation
using 30526ꢀ
4
3
[
25]
ICSD.
(Figure 2 here))
Figure 2. TEM images of Bi Te nanoparticles as obtained by
2
(
2
(
The chemical composition of resulting materials was analysed
by EDX (Bi:Te ratio 65:35 at% (DIPB) and 62:38 at% (OA)).
These results again show the formation of Biꢀrich materials. Xꢀ
ray diffraction studies confirmed the EDX results, proving the
formation of phaseꢀpure Bi Te in DIPB (Fig. 4), while the
material formed in OA contained elemental Bi as second phase
4
3
thermal decomposition of
1 at 80 °C in DIPB.
In an attempt to suppress the agglomeration of the particles, the
2
thermolysis reaction of according to scheme 1 was performed
1
at 80 °C in DIPB in the presence of 3 wt % of poly(1ꢀ
vinylpyrolidone)ꢀgraftꢀ(1ꢀtriacontene) PVP* as capping agent.
After workup, a black powder was obtained. TEM analysis of
the powder showed less agglomeration and the size of the
crystalline particles (Fig. 3) increased as can be seen by the
diffraction contrast of the grains, which however showed no
distinct morphology. The SAED (SI Fig. S15) pattern again
(
SI, Fig S19). Any attempts to increases the Teꢀcontent in the
material by adding TOPTe as soluble Teꢀsource failed. In all
cases, Bi Te was formed as was proven by EDX and XRD,
respectively.
2
SEM pictures of the resultant black particles proved the
formation of pseudoꢀcubic particles with edge lengths of
several 100 nm (Fig. 5). With increasing reaction time, the
particle size increases to 500 nm, proving an Ostwald ripening
of the initially formed nanoparticles.
corresponds well to the calculated pattern of Bi Te .
4
3
((Figure 3 here))
Figure 3. TEM images of Bi Te nanoparticles as obtained by
4
3
thermal decomposition of
wt % of PVP*.
1
at 80 °C in DIPB in the presence of ((Figure 5 here))
3
Figure 5. SEM images of Bi Te nanoparticles as obtained by
2
thermal decomposition of
2
at 100 °C in DIPB (a) and OA (b).
Thermolysis reactions at higher temperatures between 100 and
30 °C yielded mixtures of Bi Te and elemental Bi as a second
crystalline phase as was proven by PXRD (SI, Fig. S18).
Analogous findings were observed for thermolysis reactions in
this temperature range in DIPB in the absence of any capping
agent. These findings show, that the decomposition mechanism
1
4
3
In summary, all thermolysis reactions using
the formation of telluriumꢀpoor phases, while Bi Te was not
1 and 2 resulted in
2
3
formed. These findings point to different decomposition
mechanisms of and compared to analogous antimony
compounds as well as bismuth selenolates (R Bi) Se and
1
2
2
2
of
1 and its lighter homologue (Et Sb) Te strongly differ at
2 2
R BiSeR', which have been reported to decompose with
2
enhanced temperatures, which most likely has its origin in the
low BiꢀTe bond energy, rendering homolytic bond cleavage
with subsequent formation of radical species more likely.
formation of trialkylbismuthanes R Bi and Bi Se or
3
2
3
[27]
RBi(SeR') , respectively,
and may be attributed to the
2
decreasing BiꢀE bond energy with increasing atomic number of
the chalcogen atom. Breunig et al. investigated the
decomposition mechanism of
Thermolysis of Et BiTeEt 2
2
1 and 2 by mass spectroscopy and
Since the decomposition of
only yielded the Biꢀrich binary bismuthꢀtellurium phase Bi Te3 decomposition mechanism. According to Pauling's postulate,
1
, in which the Bi:Te ratio is 2:1, detected multiple species in the gas phase, indicating a complex
[
28]
4
(
pilsenite), we became interested in the thermal decomposition that heteronuclear bonding is always stronger than homonuclear
[
29]
of a precursor with lesser amount of Bi. Et BiTeEt
2
, which bonding, it seems reasonable to assume that the BiꢀTe bond
and , since the electronegativity
difference between Bi and Te is less compared to that between
Bi and C as well as Te and C. and are therefore expected to
2
[
26]
was prepared by reaction of Et Bi and Et Te , was therefore is the weakest bond within
1
2
4
2
2
2
chosen as promising single source precursor
(Scheme 2 here))
Scheme 2. Synthesis of
The reaction of Et Bi and Et Te in tolueneꢀd was monitored
.
1
2
(
undergo homolytic BiꢀTe bond breakage reactions, even though
the corresponding metalꢀcentered radicals have not been
identified directly by ESR experiments to the best of our
knowledge, to date. However, the same is true for
tetraalkyldistibines and ꢀdibismuthines, for which homolytic
SbꢀSb and BiꢀBi bond cleavage reactions with subsequent
formation of metal centered radicals are widely accepted,
despite the lack of broad experimental proof. To the best of our
knowledge, only one persistend Biꢀcentered radical, which was
2
.
4
2
2
2
8
1
by temperature dependent HꢀNMR spectroscopy (SI, Fig. S5).
Below 75 °C, no reaction was observed, while new signals with
growing intensity appeared at temperatures above 75 °C,
proving the formation of Et BiTeEt
2
, which decomposes at 85
2
°
C with formation of Et Bi and Et Te as well as insoluble solid
3
2
material (SI; Fig. S6). According to our DSC study (SI, Fig.
S4), decomposition of yields Et Bi and Et Te, which
decompose at 270 °C and 225 °C, respectively.
Thermolysis reactions of were performed in DIPB and in
oleylamine (OA) for 3 and 14 h, respectively, in order to
investigate the role of solvent and reaction time on the
composition and morphology of the resulting material. For
formed by
a homolytic bond cleavage reaction of a
2
3
2
dibismuthine containing sterically [ demanding organic
30]
substituents, was clearly identified.
In contrast, ESR
2
experiments with Me Bi2 as well as Ph Bi2 failed to give
signals due to the formation of Ph Bi radicals, even though
reactivity studies agreed with their formation.
4
4
2
[
31]
Compounds of the type R BiTeR' have been previously
described to decompose in solution at elevated temperature
with formation of BiR and RBi(TeR') , but mechanistic studies
these studies,
2
was freshly prepared in situ by stirring a
2
solution of Et Bi and Et Te in the respective solvent at ambient
4
2
2
temperature for 30 minutes, followed by heating to 100 °C for
3
2
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DOI: 10.1039
J
/
o
C
u
5DrnT0
a
2
l
0
N
72
a
G
me
[
27,32]
were not presented.
In contrast, our DSC and NMR studies With increasing reaction temperature, the peaks in the PXRDs
using
1 and 2 only proved the formation of Et Bi and Et Te, became sharper, pointing to the formation of larger crystallites,
3 2
while no indication for the formation of EtBi(TeEt) was found. and revealed Bi Te to be present as dominate phase as well as
2
2
3
Any attempts to detect radical intermediates by addition of small contributions of Bi Te (Fig. 7). The SEM studies showed
4
3
TEMPO to the reaction solution remained without success. We increasing particle size and a starꢀlike morphology of the
assume, that the formation of temperature stable Et Te, which nanoparticles. EDX analysis revealed the formation of Biꢀrich
2
was experimentally proven by DSC and NMR studies, is an material (Bi:Te ratio 44:56 at%), which clearly deviates from
efficient process for the transport of Te out of the reaction the expected 40:60 ratio for phaseꢀpure Bi Te .
2
3
mixture, always resulting in the formation of Teꢀpoor (Biꢀrich)
materials. However, the exact decomposition mechanism for
((Figure 8 here))
the formation of Bi Te or Bi Te using either
1 or 2 as SSPs
4
3
2
Figure 8. Xꢀray diffractograms of black powders as obtained
remain still unclear and are expected to depend on the specific
organic substituents as was previously reported by Bochman33n]
by reaction of Bi(NMe ) and Te(SiEt ) in DIPB in 2:3 (a), 4:3
2
3
3 2
(
b) and 1:1 (c) molar ratios at 100 °C and references (red:
et al. for several trisꢀselenolatoꢀbismuthines Bi(SeR)3.[
Bi Te , JCPDS 015ꢀ0863; green: Bi Te , JCPDS 33ꢀ216; blue:
2
3
4
3
Thermal decomposition reactions of Bi(SeR)3 in solution
BiTe, JCPDS 44ꢀ0667).
preferentially proceeded with formation of elemental Bi due to
the favorable breakage of the BiꢀSe bond compared to the SeꢀC
bond. However, the decomposition mechanism was found to
Influence of thermolysis procedure and precursor ratio
.
Since
only biphasic bismuth tellurides were formed as described
before, the thermolysis conditions were slightly modified. The
Te precursor was dissolved in DIPB and heated to 100 °C.
Bi(NMe2)3 was then rapidly injected (hot injection) to this
solution, immediately resulting in the formation of a black
powder, which was isolated by centrifugation after 3 h. PXRD
studies proved the formation of phaseꢀpure Bi Te (Fig. 8a) The
strongly depend on the organic substituents. (2,4,6ꢀ
tꢀ
Bu C H Se) Bi was found to decompose with elimination of
3
6
2
3
the selenane R Se and formation of a mixture of elemental
2
bismuth and Bi Se , while (2,4,6ꢀMe C H Se) Bi yielded
2
3
3
6
2
3
elemental Bi and the diselenane R Se .
2
2
2
3
refined lattice parameters for the nanosized Bi Te particles as
Reaction of Bi(NMe ) with Te(SiEt )
3 2
2
3
2
3
determined by Rietveld analysis (Fig. S11) for Bi Te (a =
2
3
Since the thermal decomposition reactions of both single source
precursors and failed to give the desired Bi Te phase, we
3
4
.399(1) Å, c = 20.297(3) Å and V = 507.7(1) Å ) are in good
1
2
2
3
agreement with values reported for Bi Te (JCPDS 015ꢀ0863).
2
3
became interested in the development of a suitable dual source
approach. ALD studies have revealed that Bi(NMe2)3 and
Te(SiEt ) are suitable precursors for the formation of Bi Te
Since a multiplicity of bismuth telluride phases with different
bismuth concentration exists, we carefully investigated
thermolysis reaction of Bi(NMe ) and Te(SiEt ) also with two
3
2
2
3
[
10]
2 3
3 2
thin films,
so we expanded our studies on the reactions of
different precursor molar ratios (1:1; 4:3). The solution of
Te(SiEt ) in DIPB was heated to 100 °C, followed by addition
Bi(NR ) and Te(SiEt ) . Addition of Bi(NMe ) to a solution
2
3
3 2
2 3
3
2
of Te(SiEt ) in DIPB at ambient temperature resulted in an
3
2
of different amounts of a solution of Bi(NMe ) in DIPB, also
2
3
immediate darkening, indicating a fast reaction. Monitoring the
yielding black suspensions, from which black powders were
isolated by centrifugation after 3 h. Depending on the precursor
ratio, two different phases with Bi:Te molar ratios of 1:1 (BiTe)
and 4:3 (Bi Te ) were obtained (Fig. 8b,c). In each case, the
1
reaction in THFꢀd by HꢀNMR spectroscopy proved that the
8
reaction
proceeded
with
elimination
of
N,Nꢀ
dimethylaminotriethylsilane Me NSiEt , which is the only
2
3
4
3
detectable product after 15 minutes (SI, Fig. S7). According to
these studies, the reaction occurs by the following mechanism:
chemical composition is close to the theoretical value with
respect to the experimental error. The refined lattice parameters
for the nanosized particles as determined by Rietveld analysis
((Scheme 3 here))
3
for BiTe (a = 4.420(5) Å, c = 24.09(2) Å and V = 407.5(9) Å ;
Scheme 3. Reaction of Bi(NMe ) and Te(SiEt )
2
3
3 2
Fig. S12) and Bi Te (a = 4.462(1) Å, c = 41.84(1) Å and V =
4
3
3
7
21.1(4) Å , Fig. S13) are in good agreement with values
Influence of reaction temperature
.
Since the reaction
reported in the literature for these binary bismuthꢀtellurium
phases (JCPDS No. 015ꢀ0863 (Bi Te ), No. 015ꢀ0863 (Bi Te )
temperature is known to play an important role for the
formation of nanoparticles and has major impacts on size, shape
and crystallinity of the resulting nanoparticles, we
systematically varied the reaction temperature from 30 °C to
2
3
4
3
and No. 44ꢀ0667 (BiTe).
Since the formation of the desired Bi Te3 phase was of
2
1
20 °C. PXRD studies of the resulting materials obtained at low
particular interest, the particles were further analysed by TEM,
showing small pseudoꢀcubic particles with edge lengths of
roughly 40 nm (Fig. 9). The SAED pattern (Fig. S16) of these
particles is in good agreement with the literature data and
clearly shows the formation of phase pure Bi Te as proven by
temperatures showed broad peaks, which did not allow the
detailed assignment of any specific phases, and SEM studies
showed the formation of small particles with undefined
morphology (Fig. 6).
2
3
comparison with calculated EDꢀpattern using data from ICSDꢀ
(
(Figure 6 here))
[34]
4
2546.
Figure 6. SEM images of Bi Te nanoparticles as obtained by
reaction of Bi(NMe ) and Te(SiEt ) at 120 °C in DIPB.
2
3
((Figure 9 here))
2
3
3 2
Figure 9. TEM images of Bi Te nanoparticles as obtained by
2
3
(
(Figure 7 here))
reaction of Bi(NMe ) and Te(SiEt ) (2:3 molar ratio) in DIPB
2
3
3 2
Figure 7. Xꢀray diffractogram of black powders as obtained by at 100 °C.
reaction of Bi(NMe ) and Te(SiEt ) at different temperatures.
Bi Te ((black: JCPDS 015ꢀ0863) and Bi Te (red, JCPDS 33ꢀ
2
2
3
3 2
Figure 10 displays a HRTEM image of a Bi Te nanoparticle
2
3
2
3
4
3
from Bi(NMe ) and Te(SiEt ) (2:3 molar ratio) in DIBP at
2
3
3 2
16).
1
00 °C in [110] zone axis orientation, thus perpendicular to the
4
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT0
A
2
R
0
T
72IC
G
LE
[
39]
[40]
stacking of the five membered Bi Te3 building units. The Bi(NMe2)3,
Bi(NEt ) and Bi(NEtMe)3
2 3
were synthesized
2
contrasts in the image change strongly with increasing according to literature methods. All synthetic steps were
thickness as common in HRTEM, but the Fourier transform of performed under argon atmosphere using standard Schlenk
the whole image shown in figure 10b) indicates that the techniques.
periodicity is perfect. The Fourier transform matches perfect
with the simulation of the electron diffraction for Bi Te (Fig. Thermolysis of (Et
Bi)
Te 1
2
2
3
2
1
0c), and as no additional peaks or streaking is observed, an
2
00 mg (0.3 mmol) (Et Bi) Te were dissolved in 10 mL of
2
2
intergrowth of different phases of the BiꢀTe system can be
excluded. A closer inspection of the thin areas of the sample
shows periodic 1 nm stacks perpendicular to the
Bi Te . Each of the stacks has five rows of well resolved
contrast maxima, which match the positions of the atoms of
Bi Te in this projection. This is illustrated by an overlay of a
DIPB. The solution was heated to 80 °C for 14 h. 10 mL of
MeOH were added after cooling the solution to ambient
temperature and the precipitate was isolated by centrifugation
and purified by repeated washing with chloroform (2ꢀ3 times).
cꢀaxis of
2
3
Yield: 53 mg (28% based on Bi content in 1) Bi Te .
4 3
2
3
ball and stick plot of Bi Te in the same orientation and proofs
2
3
Thermolysis of Et BiTeEt 2
2
the synthesis of pure Bi Te (Fig. 10a).
2
3
1
06.8 mg (0.2 mmol) Et Bi and 62.7 mg (0,3 mmol) Et Te
4 2 2 2
were dissolved in either 15 mL of DIPB or OA at ambient
temperature. Attempts to increase the Te content were
performed with 15.4 mg (0.03 mmol) of TOPTe (Te excess of
(
(Figure 10 here))
Figure 10. HRTEM image of a Bi Te3 nanoparticle from
Bi(NMe ) and Te(SiEt ) (2:3 molar ratio) in DIBP at 100 °C.
a) Zoom of the boxed area with structure model as overlay; b)
Fourier transform of the whole HRTEM image; c) FT with
simulation of the diffraction pattern in [110] as overlay.
2
2
3
3 2
1
5 %). The red solution was heated to 100 °C and stirred at this
temperature for either 3 or 14 h. The black solid was isolated as
described before. Yield: 55 mg (55.4% based on Bi content in
2
) Bi Te.
2
Variation of the Bi-precursor
. Since the reaction of Bi(NMe2)3
Thermolysis of Bi(NMe ) and Te(SiEt )
3 2
4
and Te(SiEt ) yielded phaseꢀpure Bi Te , we expanded our
2 3
3
2
2
3
studies on analogous reactions of alternate bismuth triamides.
were
chosen, which are oily liquids at ambient temperature, while
is a solid. The reaction of and with Te(SiEt ) at ambient
temperature in THFꢀd8 were monitored by HꢀNMR
spectroscopy. Compared to the reaction of , which is finished
after 15 min, the reaction of , which proceeds with formation
Influence of reaction temperature. 53.6 mg (0.15 mmol)
Bi(NEt2)
3
as well as the asymmetric Bi(NMeEt2)3
5 6
Te(SiEt ) were added to a yellow solution of 31.5 mg (0.1
3 2
4
mmol) Bi(NMe ) in 15 mL of DIPB, immediately yielding a
2
3
5
6
3
2
greenish suspension. This suspension was heated to the desired
temperature and stirred for 3 h at this temperature. After
cooling to ambient temperature, 10 mL of MeOH were added
and the black solid was separated from the liquid by
centrifugation and repeatedly washed with chloroform (2ꢀ3
times). Biphasic mixtures of Bi Te (major phase, > 95 %
1
4
5
of the expected elimination product N,Nꢀdiethylaminoꢀ
triethylsilane, is decelerated and incomplete even after 20 h (SI,
2
3
Fig X). Surprisingly,
temperature even after 20 h (Fig. S9). One the basis of the
NMR results, and are less suitable bismuth precursors for
the synthesis of bismuth telluride nanoparticles than
6 didn't show any reaction at ambient
according to PXRD) and Bi Te3 in different molar ratios
4
depending on the reaction temperature were obtained.
5
6
Influence of precursor ratio
. A solution of 107.4 mg (0.3
4
.
mmol) Te(SiEt ) in 15 mL of DIPB was heated to 100 °C and
3
2
a solution of either 68.2 mg (0.2 mmol), 102.4 mg (0.3 mmol)
or 136.5 mg (0.4 mmol) Bi(NMe ) in 2 mL of DIPB was added
to the hot solution (hot injection method). The resulting black
suspension was stirred at 100 °C for 3 h. After cooling to
ambient temperature, 10 mL of MeOH were added and the
solid was isolated from the solution by centrifugation and
Finally, we investigated the reaction of BiCl with Te(SiEt ) ,
3
3 2
2
3
which was expected to proceed with elimination of Et SiCl
3
(
dehalosilylation reaction). Since BiCl is not soluble in nonꢀ
3
coordinating solvents such as DIPB, we used oleylamine (OA)
as solvent. The thermolysis reaction was performed at 160 °C
for 14 h. The SEM of the resulting product showed very small
particles without defined structure (Fig S17). The Bi:Te ratio
washed with chloroform (2ꢀ3 times). Bi Te3: Yield: 79.9 mg
2
(
0.094 mmol, 94%); BiTe: Yield: 96.9 mg (0.29 mmol, 96%);
(
42 at% : 58 at%) is close to the expected value for Bi Te .
2 3
Bi Te3: Yield: 114 mg (0.093 mmol, 93%).
4
These results were confirmed by PXRD, which proved the
formation of Bi Te as the only crystalline product (Fig. 11).
Influence of Bi-precursor. 107.4 mg (0.3 mmol) Te(SiEt3)2
and either 87.1 mg (0.2 mmol) Bi(NEt2)3 or 76.7 mg (0.2
2
3
The lattice parameters for the Bi Te particles (a = 4.405(1) Å
2
3
mmol) (Bi(NEtMe) , respectively, were dissolved in 15 mL of
3
and c = 20.284(4) Å) are in good agreement with values
reported in the literature for Bi Te (JCPDS No. 015ꢀ0863).
DIPB and heated to 120 °C. The solution was stirred for 3 h at
2
3
this temperature. Due to the insolubility of BiCl in DIPB, the
3
thermolysis reaction of 63.1 mg (0.2 mmol) BiCl and 107.4
((Figure 11 here))
3
mg (0.3 mmol) was performed in 15 mL of OA. The solution
was heated to 160 °C for 3 h and the isolation and purification
of the resulting black solid was performed as described before.
Figure 11. Xꢀray diffractogram of the material as obtained by
reaction of BiCl and Te(SiEt ) in OA at 160 °C and reference
3
3 2
(
red: Bi Te , JCPDS 015ꢀ0863).
2 3
Characterization
Experimental
X-ray Powder Diffraction (PXRD). PXRD patterns were
obtained at ambient temperature (25±2°C) using a Bruker D8
Advance powder diffractometer in Bragg−Brentano mode with
Synthetic procedures, Materials and methods
PVP* (SigmaꢀAldrich) was used as received while 1,3ꢀ
diisopropylbenzene (DIPB, SigmaꢀAldrich) was carefully dried
over Na/K alloy and oleylamine (OA, Acros) was degassed
Cu K radiation (λ= 1.5418 Å, 40 kV, and 40 mA). The powder
α
samples were investigated in the range of 5 to 90° 2θ with a
[38] step size of 0.01° 2θ and a counting time of 0.3 s. Rietveld
[
35]
[36]
[37]
prior to use. (Et Bi) Te,
Te(SiEt3)2,
Et Bi ,
Et Te ,
2 2
2
2
4
2
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Jou al Name
5DrnT0
1
refinement was performed with the program package TOPAS Electronic Supplementary Information (ESI) available: [DSC studies, Hꢀ
4
.2 from Bruker.
NMR spectroscopy studies, Rietveld refinements, SAED pattern, SEM
pictures]. See DOI: 10.1039/b000000x/
SEM Analysis. Scanning electron microscopy (SEM) studies
were carried out on a Jeol JSM 6510 equipped with an energyꢀ
dispersive Xꢀray spectroscopy (EDX) device (Bruker Quantax
1
2
M. H. Elsheikh, D. A. Shnawah, M. F. M. Sabri, S. B. M. Said, M. H.
Hassan, M. B. A Bashir and M. Mohamad, Renewable Sustainable
Energy Rev., 2014, 30, 337.
4
00).
TEM Analysis. TEM studies were conducted on transmission
electron microscopes (i) FEI Philips CM30 T/LaB operated at
3
kV. Both microscopes are equipped Gatan CCD’s for image
recording and with Thermo NSS systems for EDS analysis
using a Si(Li) Nanotrace and a HPꢀGe EDS detector,
respectively.The samples were prepared on perforated carbon
foils without further grinding.
6
00 kV and (ii) FEIꢀPhilips CM300 UT/FEG operated at 300
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We investigated the suitability of two possible single source
precursors, (Et Bi) Te
based synthesis of Bi Te3 nanoparticles. The thermal
5
V. Stavila, D. B. Robinson, M. A. Hekmaty, R. Nishimoto, D. L.
1 and Et BiTeEt 2, for the solutionꢀ
2
2
2
Medlin, S. Zhu, T. M. Tritt and P. A. Sharma, ACS Appl. Mater.
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(
DSC). Thermolysis of
1
in DIPB led to the formation of Bi Te3
4
at 80 °C and mixtures of Bi Te and elemental Bi at higher
4
3
temperatures, while thermolysis of in situ generated
2 in DIPB
8
a) K. Knapas, T. Hatanpää, M. Ritala and M. Leskelä, Chem. Mater.,
or OA gave pseudoꢀcubic Bi Te particles, whose size increase
2
2
010, 22, 1386; b) S. Zastrow, J. Gooth, T. Boehnert, S. Heiderich,
with increasing reaction time. To the best of our knowledge,
this is the first report of the solution based formation of Bi Te.
For all thermolysis reactions, the formation of tellurium poor
W. Toellner, S. Heimann, S. Schulz and K. Nielsch, Semicond. Sci.
Technol., 2013, 28, 035010; c) C. Bae, T. Bohnert, J. Gooth, S. Lim,
S. Lee, H. Kim, S. Heimann, S. Schulz, H. Shin and K. Nielsch,
Semicond. Sci. Technol., 2014, 29, 064003.
2
phases were observed.
1 and 2 are therefore unsuitable for the
synthesis of Bi Te , which may be attributed to the comparable
2
3
low TeꢀBi binding energy and complex decomposition
mechanism. We assume that the formation of temperature
9
S. Schulz, S. Heimann, K. Kaiser, O. Prymak, W. Assenmacher, J. T.
Brüggemann, B. Mallik and A.ꢀV. Mudring, Inorg. Chem., 2013, 52
4326.
,
stable Et Te is an efficient process for the transport of Te out of
2
1
the reaction mixture, leading to Teꢀpoor (Biꢀrich) materials.
In contrast to the single source precursors, the dual source 10 a) T. J. Groshens, R. W. J. Gedridge and C. K. LoweꢀMa, Chem.
approach using the reaction of Te(SiEt ) and Bi(NMe ) was
Mater. 1994, 6, 727; b) T. Groshens, R. Gedridge, R. Scheri and T.
3
2
2 3
successfully applied for the synth esis of multiple BiꢀTe phases
including Bi Te depending on the precursor ratio. In contrast,
Cole, in Fifteenth International Conference on Thermoelectrics
Proceedings ICT ’96, IEEE, 1996, p 430.
2
3
analogous reactions of Bi(NEt2)3 and Bi(NMeEt)3 occurred
with formation of a mixture of elemental tellurium and bismuth
and a mixture of Bi Te and tellurium, respectively, while the
1
1 Z. Stary, J. Horak, M. Stordeur and M. Stölzer, J. Phys. Chem.
Solids, 1988, 49, 29.
4
3
reaction of BiCl and Te(SiEt ) gave Bi Te nanoparticles. The 12 J. W. G. Bos, H. W. Zandbergen, M.ꢀH. Lee, N. P. Ong and R. J.
3
3 2
2
3
reaction conditions leading to phase pure material are
summarized in figure 11.
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((Figure 12 here))
4
Figure 12. Summary of the ideal reaction conditions for the
formation of phaseꢀpure binary bismuthꢀtellurium materials.
1
2
R. V. S. R. Pullabhotla, K. Ramasamy and P. O'Brien, Thin Solid
Films, 2010, 519, 197; c) N. O. Boadi, M. Azad Malik, Paul O'Brien
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Acknowledgements
S. Schulz thanks the Deutsche Forschungsgemeinschaft (DFG)
for financial support of this work within the priority program
SPP 1708 (Material Synthesis near Room Temperature).
1
5 a) N. Maiti, S. H. Im, C.ꢀS. Lim and S. I. Seok, Dalton Trans., 2012,
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Notes and references
Institute of Inorganic Chemistry and Center for Nanointegration
a
DuisburgꢀEssen
(CeNIDE),
University
of
DuisburgꢀEssen,
Universitätsstr. 5ꢀ7, Dꢀ45117 Essen, Germany. Fax: 44 0201 1833830;
Tel: 44 0201 1834635; Eꢀmail: stephan.schulz@uniꢀdue.de
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Jou5DrnT0al Name
Graphical and textual abstract
Thermolysis of metal organic single source and dual source precursors yielded phaseꢀpure Bi Tey nanoparticles at low
x
temperatures.
8
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2x1mm (300 x 300 DPI)

Dalton Transactions
Page 22 of 23
View Article Online
DOI: 10.1039/C5DT02072G
1
2x1mm (300 x 300 DPI)

Page 23 of 23
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT02072G
1
2x1mm (300 x 300 DPI)