Synthesis in ionic liquids: [Bi2Te2Br](AlCl 4), a direct gap semiconductor with a cationic framework

Article abstract of DOI:10.1021/ja107483g

The Lewis acidic ionic liquid EMIMBr-AlCl₃ (EMIM = 1-ethyl-3-methylimidazolium) allows a novel synthetic route to the semiconducting layered metal chalcogenides halide [Bi₂Te ₂Br]-(AlCl₄) and its Sb analogue. [Bi₂Te ₂Br](AlCl₄) is a direct band gap, strongly anisotropic semiconductor and consists of cationic infinite layers of [Bi₂Te ₂Br]₊ and [AlCl₄] anions inserted between the layers.

Full text of DOI:10.1021/ja107483g

Published on Web 10/04/2010
Synthesis in Ionic Liquids: [Bi Te Br](AlCl ), a Direct Gap Semiconductor with
2
2
4
a Cationic Framework
†
†
†
‡
†
Kanishka Biswas, Qichun Zhang, In Chung, Jung-Hwan Song, John Androulakis,
‡
,†,§
Arthur J. Freeman, and Mercouri G. Kanatzidis*
Department of Chemistry and Department of Physics and Astronomy, Northwestern UniVersity, EVanston,
Illinois 60208, United States, and Materials Science DiVision, Argonne National Laboratory, Argonne,
Illinois 60439, United States
Received August 26, 2010; E-mail: m-kanatzidis@northwestern.edu
The black compound [Bi
by the reaction of elemental Bi with elemental Te in EMIMBr-
2 2 4
Te Br](AlCl ) (1) was synthesized
Abstract: The Lewis acidic ionic liquid EMIMBr-AlCl
-ethyl-3-methylimidazolium) allows a novel synthetic route to the
semiconducting layered metal chalcogenides halide [Bi Te Br]-
AlCl ) and its Sb analogue. [Bi Te Br](AlCl ) is a direct band gap,
strongly anisotropic semiconductor and consists of cationic infinite
3
(EMIM )
1
22
AlCl at 165 °C for 7 days. The reaction produced black
3
2
2
needle-like crystals of 1, and the structure was determined from
single-crystal X-ray diffraction data collected at 100 K on a
STOE 2 X-ray diffractometer. The isostructural Sb analogue
(
4
2
2
4
+
-
layers of [Bi
layers.
2 2 4
Te Br] and [AlCl ] anions inserted between the
[
Sb
2 2 4
Te Br](AlCl ) (2) was also synthesized similarly. Com-
23
pounds 1 and 2 both crystallize in the space group C2/c. Figure
1
a shows the crystal structure of 1 down the b-axis. The structure
+
-
2 2 4
consists of cationic infinite layers of [Bi Te Br] with [AlCl ]
Ionic liquids are unique alternatives to traditional aqueous or
organic solvents. Room-temperature ionic liquids generally are
1
anions inserted between the layers. The layers themselves are
2
+
made of infinite chains of [Bi
2
Te
2
]
which are connected by
organic salts with high thermal stability, wide liquidus range,
negligible vapor pressure, good ionic conductivity, and tunable
solubility of both organic and inorganic molecules. Uses of ionic
liquids in solvent extraction and organic catalysis have been
extensive, but their use in the synthesis of inorganic materials is
-
Br ions along the crystallographic c-axis (Figure 1b). The chains
2
+
of [Bi
2
Te
2
]
2
are made of (BiTe) rhombic units having Te-
Bi-Te angles ranging from 87.06(2) to 92.16(2)°. Bi1 and Bi2
atoms are five-fold-coordinated by three telluriums and two
bridging bromides to form distorted square-pyramidal geometry.
The oxidation state of Bi atoms is 3+, and the inert lone pair of
electrons is clearly stereochemically expressed. The Bi-Te bond
2,3
relatively recent. Recently, ionic liquids have been used as media
4
5
for the syntheses of zeolites, metal-organic frameworks (MOFs),
6
7
clathrates, metal nanoparticles and nanorods, and microporous
and mesoporous carbon and graphene, as well as in electrochemi-
cal nanotechnology and polymer science.
Although the synthesis of metal chalcogenides in molten salts
i.e., inorganic ionic liquids) is well established,
2
0
8,9
lengths range from 2.9475(8) to 2.9779(8) Å.
1
0
11
1
2,13
(
syntheses in
organic or acidic ionic liquids are very few. Jiang and Zhu used
ethylene glycol, aqueous acid, and ionic liquid as a reaction medium
14
2 3 2 3
to prepare single-crystalline Bi S and Sb S nanorods. Recently,
Rao and co-workers synthesized binary metal-chalcogenides
nanoparticles and nanorods in different imidazolium-based ionic
1
5
liquids, but research to discover new compounds in such media
has been scarce. Recently, we reported the synthesis of the novel
7 8 2 4 3
cluster compound [Sb S Br ](AlCl ) with cationic chalcogenide
1
6
clusters in ionic liquids.
Here, we report the discovery of [Bi
chalcogenide-halide solid with an extended layered structure and
2
2 4
Te Br](AlCl ), a unique
Figure 1. (a) Crystal structure of compound 1, viewed down the b-axis.
+
(
2 2
b) View of cationic [Bi Te Br] down the a-axis. The Bi-Br bridging
semiconducting properties, prepared in an ionic liquid containing
distances range from 3.1983(15) to 3.2205(15) Å.
Lewis acid or strong acceptors, EMIMBr-AlCl
3
(EMIM ) 1-ethyl-
3
-methylimidazolium). The acidic character of this ionic liquid
+
arises from the AlCl
3
. This is the first example of a cationic
The layers of [Bi
2
Te
2
Br] stack along the crystallographic a-axis
chalcogenide framework synthesized in acidic organic ionic liquids.
Chalcogenide polycationic compounds and clusters are a very
interesting subgroup, and they adopt a variety of structures. There
are several methods for their synthesis, including the use of
with an interlayer distance of 10.474 Å. The tetrachloroaluminate
ions have a moderately distorted tetrahedral structure (Figure 1a)
in which the Al-Cl bond lengths range from 2.139(4) to 2.171(4)
17
Å and the Cl-Al-Cl angles from 104.75(17) to 117.29(16)°.
-
electrophilic and acidic media, such as liquid SO
2
and H
2
SO
4
, or
The [AlCl
4
]
ions render the compounds air- and moisture-
1
8
19
strong oxidizing agents, such as WCl
5
,
SbF
5
, and AsF
5
,
or in
sensitive because of the hydrolytic instability of Al-Cl bonds. The
electronic absorption spectrum of compound 1 (taken with care to
minimize air exposure) shows an absorption edge at ∼0.8 eV
(Figure 2), consistent with the black color. Thermogravimetric
2
0
21
the presence of strong acceptors, such as AlCl
3
4
and ZrCl .
†
Department of Chemistry, Northwestern University.
Department of Physics and Astronomy, Northwestern University.
Argonne National Laboratory.
‡
§
2
analysis (TGA) under N atmosphere (Figure S2, Supporting
1
4760 9 J. AM. CHEM. SOC. 2010, 132, 14760–14762
10.1021/ja107483g  2010 American Chemical Society

C O M M U N I C A T I O N S
tions with spin-orbit interaction (SOC) included give a direct
band gap of 0.43 eV, which is smaller than the experimentally
estimated value of ∼0.8 eV. The projected angular-momentum-
resolved density of states for Bi, Te, and Br p-orbitals (Figure
3
b) shows the p-p mixing effect on the electronic structure.
All states contribute significantly to both the valence and
conduction band edges, but with slightly higher Te and Br
p-orbital contribution to the valence band maximum due to the
ionic characteristics. Therefore, the bad gap transition shown in
Figure 2 is mainly associated with interlayer excitations from
Te and Br atoms to Bi atoms.
Figure 2. Electronic absorption spectrum of compound 1. The energy gap
is indicated. The electronic spectra were obtained as described elsewhere.
24
Bismuth/antimony telluride-based materials have not only diverse
structures but also fascinating properties, such as thermoelectric
Information) showed that compound 1 starts to lose weight at 350
2
6
and topological insulation. The chalcogenides,[Bi
2 2 4
Te Br](AlCl )
K and undergoes a sharp weight loss at 600 K. The loss is attributed
1
x
6
and [Sb Te Br](AlCl ), with novel cationic two-dimensional frame-
2
2
4
to AlCl
eq 1).
3
or AlCl3-xBr
x
and gives rise to a BiTeBr Cl(1-x) species
works and anisotropic electrical transport properties, can be
synthesized in a Lewis acidic organic ionic liquid. Organic acidic
ionic liquids enhance the reactivity of normally inert reactants such
as Bi, Sb, and Te to generate saturated solutions. Thus, they provide
a promising synthetic strategy toward new chalcogenides with
cationic frameworks rather than the familiar anionic frameworks
observed in molten salt reactions.
(
∆
[
Bi Te Br](AlCl )
9
8 2BiTeBr Cl + AlCl3
(1)
2
2
4
0.5 0.5
Differential thermal analysis (DTA) (Figure S2, Supporting
Information) indicates a melting event at ∼844 K. The powder
X-ray diffraction (PXRD) pattern measured after the DTA can be
best matched with the theoretical PXRD pattern of the compound
BiTeBr (ICSD 79363), but the pattern is shifted to a higher 2θ
Acknowledgment. Financial support from the National Science
Foundation (DMR-0801855) is gratefully acknowledged.
angle, indicating the incorporation of chlorine, BiTeBr
Figure S3, Supporting Information).
The room-temperature electrical conductivity measured along the
x
Cl(1-x)
Supporting Information Available: Experimental methods, physical
characterization, and X-ray crystallographic files (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(
crystallographic b-direction (parallel to the layers) was found to
be ∼40 S/cm, while along the a-direction (perpendicular to the
References
-5
layers) it was negligible, at ∼8 × 10 S/cm (see Supporting
Information). This high anisotropy is consistent with the lamellar
structure of the material.
(
1) Welton, T. Chem. ReV. 1999, 99, 2071.
(
2) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed.
2004, 43, 4988.
(
(
(
3) Ma, Z.; Yu, J.; Dai, S. AdV. Mater. 2010, 22, 261.
4) Morris, R. E. Angew. Chem., Int. Ed. 2008, 47, 442.
5) See, for example: (a) Dybtsev, D. N.; Chun, H.; Kim, K. Chem. Commun.
2
2
004, 1594. (b) Lin, Z.; Slawin, A. M. Z.; Morris, R. E. J. Am. Chem. Soc.
007, 129, 4880.
(
(
6) Guloy, A. M.; Ramlau, R.; Tang, Z.; Schnelle, W.; Baitinger, M.; Grin, Y.
Nature 2006, 443, 320.
7) See, for example: (a) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner,
P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228. (b) Itoh, H.;
Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026. (c) Migowski, P.;
Dupont, J. Chem.sEur. J. 2007, 13, 32. (d) Wang, Y.; Yang, H. J. Am.
Chem. Soc. 2005, 127, 5316.
(8) Lee, J. S.; Wang, X.; Luo, H.; Baker, G. A.; Dai, S. J. Am. Chem. Soc.
2
009, 131, 4596.
(
9) Lu, J.; Yang, J.-X.; Wang, J.; Lim, A.; Wang, S.; Loh, K. P. ACS Nano
009, 3, 2367.
2
Figure 3. (a) Electronic band structure and (b) projected density of states for
the p-orbital of the individual elements (Bi, Te, and Br) of compound 1.
(10) Emdres, F. Chem. Phys. Chem. 2002, 3, 144.
11) Lodge, P. Science 2008, 321, 50.
12) (a) McCarthy, T. J.; Kanatzidis, M. G. Inorg. Chem. 1995, 34, 1257. (b)
Chondroudis, K.; McCarthy, T. J.; Kanatzidis, M. G. Inorg. Chem. 1996,
(
(
3
5, 840. (c) Liao, J. H.; Kanatzidis, M. G. 1993, 5, 1561. (d) McCarthy,
To better understand the charge transport properties, the
electronic band structure of compound 1 was calculated using
the full potential linearized augmented plane wave (FLAPW)
method with the screened-exchange local density approximation
T. J.; Kanatzidis, M. G. Chem. Mater. 1993, 5, 1061.
(
13) (a) Dhingra, S.; Kanatzidis, M. G. Science 1992, 258, 1796. (b) Kanatzidis,
M. G.; McCarthy, T. J.; Tanzer, T. A.; Chen, L. H.; Iordanidis, L.; Hogan,
T.; Kannewurf, C. R.; Uher, C.; Chen, B. X. Chem. Mater. 1996, 8, 1465.
(c) Kanatzidis, M. G.; Park, Y. Chem. Mater. 1990, 2, 99.
(
14) Jiang, Y.; Zhu, Y.-J. J. Phys. Chem. B 2005, 109, 4361.
(
sX-LDA) and the Hedin-Lundqvist (LDA) forms of the
(15) Biswas, K.; Rao, C. N. R. Chem.sEur. J. 2007, 13, 6123.
(16) Zhang, Q.; Chung, I.; Jang, J. I.; Ketterson, J. B.; Kanatzidis, M. G. J. Am.
Chem. Soc. 2009, 131, 9896.
(17) Brownridge, S.; Krossing, I.; Passmore, J.; Jenkins, H. D. B.; Roobottom,
H. K. Coord. Chem. ReV. 2000, 197, 397.
2
5
exchange-correlation potential. Figure 3a shows the electronic
band structure of compound 1. The nearly perfectly flat band
edges along the Γ-to-X direction (related to the a-direction in
(
(
18) Beck, J.; Wetterau, J. Inorg. Chem. 1995, 34, 6202.
the crystal) confirm the highly anisotropic transport properties.
19) Klap o¨ tke, T.; Passmore, J. Acc. Chem. Res. 1989, 22, 234.
-
(20) Beck, J.; Dolg, M.; Schl u¨ ter, S. Angew. Chem., Int. Ed. 2001, 40, 2287.
4
This comes from the fact that the tetrahedral anions [AlCl ]
(
(
21) Beck, J.; Hedderich, S. J. Solid State Chem. 2003, 172, 12.
22) Synthesis of [Bi Te Br](AlCl ) (1): 105 mg (0.5 mmol) of Bi, 64 mg (0.5
mmol) of Te, 250 mg (1.87 mmol) of AlCl , and 75 mg (0.39 mmol) of
do not play an important role in bonding with the layers, except
for making charge neutrality, and do not provide facile hopping
paths for carriers. This flat band results in very high effective
masses along the a-axis, which predicts that the out-of-plane
electronic conductivity is expected to be negligible compared
to in-plane, consistent with experiments. The sX-LDA calcula-
2
2
4
3
EMIMBr were loaded in a 20 mL glass vial inside a glovebox. The vial
was closed and put in a preheated oven at 165 °C for 7 days. After
completion of the reaction, the vial was opened inside the glovebox, and
the product was washed with chloroform. Black needle-like crystals were
obtained (71% yield). The compound is moisture-sensitive and stable in
air up to 1 h.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 42, 2010 14761

C O M M U N I C A T I O N S
(
23) Crystal data for [Bi
2
Te
2
Br](AlCl
4
) (1) at 100 (2) K: STOE IPDS2
(24) (a) Kort u¨ m, G. Reflectance Spectroscopy. Principles, Methods, Applications;
Springer: New York, 1969. (b) Chung, D. Y.; Choi, K. S.; Iordanidis, L.;
Schindler, J. L.; Brazis, P. W.; Kannewurf, C. R.; Chen, B. X.; Hu, S. Q.;
Uher, C.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 3060.
(25) Asahi, R.; Mannstadt, W.; Freeman, A. J. Phys. ReV. B 1999, 59, 7486.
(26) (a) Chung, D.-Y.; Hogan, T.; Brazis, P.; Rocci-Lane, M.; Kannewurf, C.;
Bastea, M.; Uher, C.; Kanatzidis, M. G. Science 2000, 287, 1024. (b) Zhang,
H.; Liu, C.-X.; Qi, X.-L.; Dai, X.; Fang, Z.; Zhang, S. C. Nature Phys.
2009, 5, 438.
diffractometer, Mo KR radiation, monoclinic; C2/c; Z ) 8; a ) 21.2247(13)
Å, b ) 8.4778(3) Å, c ) 13.2264(10) Å, ꢀ ) 99.280(5)°; V ) 2348.8(2)
3
3
Å . Collection and refinement data: 2θmax ) 55°; dcalc ) 5.214 g/cm ; F(000)
2
2
)
3088; 9479 total reflections; 2688 unique reflections [F
parameters; GOF ) 1.055; R1 ) 4.35% and wR2 ) 11.04% for I > 2σ(I).
Crystal data for [Sb Te Br](AlCl ) (2) at 100 (2) K: monoclinic; C2/c; Z
o
> 2σ(F
o
)]; 92
2
2
4
)
8; a ) 21.216(4) Å, b ) 8.2816(17) Å, c ) 13.126(3) Å, ꢀ ) 98.71(3)°;
3
V ) 2279.7(8) Å . Collection and refinement data: 2θmax ) 53.48°; dcalc
)
3
4
.355 g/cm ; F(000) ) 2576; 8378 total reflections; 2413 unique reflections
2
2
[
F
o
> 2σ(F
o
)]; 93 parameters; GOF ) 1.012; R1 ) 2.45% and wR2 )
6
.45% for I > 2σ(I).
JA107483G
1
4762 J. AM. CHEM. SOC. 9 VOL. 132, NO. 42, 2010