A thermoelectric copper-iodide composite from the pyrolysis of a well-defined coordination polymer

Article abstract of DOI:10.1039/c8dt00090e

Coordination polymer 1 was prepared from CuI and a flexible [SNS] ligand L in acetonitrile. The thermal decomposition of 1 yielded a CuI-rich thermoelectric carbon composite 2, which is relatively light, thermally stable and robust. Composite 2 possesses high Seebeck coefficients (700-950 μV K^-1) from rt to 204 °C after an optimization cycle.

Full text of DOI:10.1039/c8dt00090e

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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A thermoelectric copper-iodide composite from pyrolysis of a
well-defined coordinaꢀon polymer†
Shi-Qiang Bai,*a,b Ivy Hoi Ka Wong,a Ming Lin,a David James Younga,c and T. S. Andy Hor*b,d
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Coordination polymer 1 was prepared from CuI and a flexible low thermopower. Conversely, plastic (organic) materials
[SNS] ligand L in acetonitrile. Thermal decomposition of 1 yielded typically possess low electrical and thermal conductivities and
17
a CuI-rich thermoelectric carbon composite 2, which is relatively relatively large thermopower values.15
Thermoelectric
−
light, thermally stable and robust. Composite 2 possesses high material development is, therefore, a problem of optimization
Seebeck coefficients (700−950 ꢀV/K) from rt to 204°C after an for particular applications. Performance evaluation is
optimization cycle.
determined on thin films or pellets, and the latter is ideally
made from a powder sample with good thermal stability and
low density for most practical applications. Metal coordination
polymers assembled from organic ligands and metal
precursors can offer fascinating structures with diverse
applications such as luminescent sensors, molecular magnets,
Thermoelectric materials are substances that can directly
harness a temperature change to electrical potential or vice
versa.1 All materials exhibit a certain level of thermoelectric
3
−
effect, albeit negligible in most cases, but a number of
traditional inorganic compounds (e.g. Bi2Te3, SnSe, ZnO and
CuI) and organic conductive polymers (e.g. PEDOT, polyaniline
and poly(alkylthiophene)) exhibit useful applicable cooling or
heating when an electrical potential is applied, or a usable
separation membranes, thin film devices and thermoelectric
materials.18 Inexpensive copper iodide (CuI) is a transparent
26
−
semiconductor with a wide band gap.6 It is also an attractive
inorganic precursor to coordination clusters and polymers
supported by various organic ligands for photochemical,
electrical potential from a temperature gradient generated by,
14
for example, the waste heat from a combustion engine.4
−
29
photophysical and other applications.27
−
However, these materials still face the challenge of relatively
low power conversion efficiency, which is defined by the figure
A thin film of CuI was recently reported to exhibit a room
temperature ZT value of 0.21.29 Although still too low for
practical applications, this discovery inspired researchers in the
field to explore other CuI-based thermoelectric materials. We
of merit (ZT), where ZT = S2σT/
and electrical conductivity (
contribute to the ZT value while thermal conductivity (
κ
. The Seebeck coefficient (S)
) are intensive properties that
) is
σ
κ
have previously synthesized a large range of functional
coordination complexes and polymers30 and herein report a
38
−
inversely proportional to ZT. Significant effort has been
devoted to materials with high Seebeck coefficients and
electrical conductivity, and low thermal conductivity. However,
new coordination polymer [L2Cu4I4]n (1) consisting of [Cu4I4]
clusters bridged by flexible amine-dithioether ligands (L =
bis(2-(cyclohexylthio)ethyl)amine)). The application of ligand L
in molecular catalysis and crystal engineering has been
these three determinative parameters (S,
σ, κ) show
interdependent relationships in a single material, such that an
increase in one may decrease another. Metallic (inorganic)
materials are always electrically and thermally conductive with
41
reported previously by our group and by others.39
This
−
coordination polymer
composite material (
1
was thermally degraded to a CuI-based
2
) that could be pelletized under 10 bar of
pressure (ESI†). The density of each pellet decreased with
increasing thickness to an average value of 4.24 g/cm3 (Fig. S1,
ESI†). Temperature-dependent electrical conductivity and
Seebeck coefficients of the composite pellets were optimized
and evaluated by multi-cycle testing with four temperature
gradients (Fig. 5 and Fig. S2 ESI†). Subjecting commercial CuI to
the same thermal treatment and pelletization gave a material
that was more dense (~5.20 g/cm3) and easily disassembled
during handling.
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The single-crystal XRD structure (ESI†) of
1 indicated that each TEM images likewise indicated that composite 2 is a mixture of
Cu(I) centre is tetrahedral with geometry index42 ι4 values as crystalline CuI and amorphous carbon (Fig. 3). A typical
0.87 and 0.90, for Cu1 and Cu2 respectively. The multiple crystalline CuI particle is shown in Fig 3b, which is oriented
donor coordination environment of Cu1 slightly reduces its along the [111] zone axis. The measured lattice spacing of 0.21
geometry index compared to Cu2. All three donor atoms (SNS) nm corresponds to the distance between (220) planes (Fig. 3b
in the ligand L coordinate to Cu(I) centres via NS chelating and and Fig. 2a). EFTEM images revealed the presence and
thioether S bridging. This enables each ligand to connect three distribution of each element in
metal atoms and provide maximum stabilization to this and CuI is shown in Fig. 3e-f. There is about 4.25 wt% carbon in
triumvirate of metal centres. The only other linking ligand is composite based on the microanalyꢀcal data (ESI†). No
iodide ion which 2-bridges singly across Cu1 and Cu2, and carbon was detected in the powder XRD patterns, presumably
doubly across Cu2 and Cu2A (Fig. 1a). The latter [Cu2I2] because of its amorphous nature (Fig. 2a).
configuration thus constitutes binuclear unit with an Coordination polymer was stable up to about 250
inversion centre in a Z-shaped [Cu1(Cu2)2Cu1] tetranuclear and 260 C under N2. Two obvious weight losses were
aggregate (Fig. 1a). The associated connection of each [Cu4I4] observed in both TGA curves (Fig. 1d). The first between 250
cluster to four neighbouring ones through two Cu1 centres and and 400 C (air) and between 260 and 460 C (N2) can be
two thioether S2 bridges results in two-dimensional assigned to the decomposition of ligand L and the formation of
2. The co-existence of carbon
2
ꢀ
a
1
°C in air,
°
°
°
a
polymeric structure in the bc plane (Fig 1b). The intra- CuI with weight losses of 42.9 and 45.0%, respectively (calcd.
tetranuclear Cu1⋅⋅⋅Cu2 and Cu2⋅⋅⋅Cu2 distances are 4.61 and 44.2%). The second decomposition stages occur from 450 to
2.62 Å, respectively.
590
can be ascribed to CuI conversion to CuO in an air flow at high
temperature. The TGA trace of indicated minor weight loss (<
1 %) on heating to 430 C (air) or 500 C (N2) (Fig. 2b). Further
. (only joining atoms S2B, increasing the temperature let to conversion of copper iodide
S2D, Cu1C and Cu1E from neighbouring clusters are shown for to copper oxide (Fig. 2b), which occurred more readily in air.
clarity) (b) Two-dimensional polymeric structure of . (c)
Powder XRD patterns of . (T = theoretical profile referenced
to the experimentally determined single-crystal XRD pattern; E
= experimental data) (d) TGA curves of
°C (air) and from 500 to 890 °C (N2), respectively. These
<< Fig. 1 Here>>
2
°
°
Fig. 1 (a) Molecular structure of
1
1
1
<< Fig. 4 Here>>
1
.
Fig. 4 Uv-vis reflectance spectroscopy of 1, 2 and CuI. Inset
showing a magnification of the spectrum of 2.
<< Fig. 2 Here>>
A pressed pellet of coordination polymer
. (b) TGA curves bar at room temperature and it was not electrically
. (c and d) Low-magnification FESEM images of conductive. Reflectance Uv-vis spectroscopy of indicated a
commensurately large band gap (3.55 eV) (Fig. 4, ESI†). The
The measured X-ray diffraction pattern of coordination corresponding pellet of composite formed under the same
polymer matched well with the simulated pattern, conditions was electrically conductive with a similar band gap
supporting phase purity (Fig. 1c). The powder XRD of value (2.84 eV) to a control sample of CuI (2.89 eV) (Fig. 4),
composite indicated the predominance of CuI crystallites which is in the range of reported band gap values of copper
1 was prepared at 10
Fig. 2 (a) Powder XRD patterns of composite
of
2
2
2.
1
2
1
2
with cubic crystal system according to the PDF card CuI-00- iodide.6
006-0246 (Fig. 2a). There is no major difference between the
XRD patterns generated from as-prepared powder and the
pressed pellet after thermoelectric evaluation cycles (Fig. 2a).
<< Fig. 5 Here>>
The low-magnification FESEM images of a powder sample of
2
Fig. 5 Temperature-dependent thermoelectric properties of
(pellet A, density: 4.32 g/cm3) evaluated from
revealed nano- and micro-particle aggregating to micro-sized composite
2
block structures (Fig. 2c,d).
<< Fig. 3 Here>>
twelve-cycle measurements: (a) electrical conductivity, (b)
Seebeck coefficient, and (c) power factor.
Two pressed pellets, A and B, of composite
. (b) performance evaluation using a ZEM3 system under vacuum
High-resolution TEM image showing the lattice structure of CuI (ESI†). Twelve- and nine-cycle programming tests were
in . Inset showing the Fast Fourier Transform (FFT) of the performed on pellets A and B, respectively (Fig. 5 and Fig. S2,
structures. (c-f) EFTEM images of composite using the (d) C- ESI†). Room temperature data were collected. Eight
K, (f) Cu-K and (f) I-M edges. White areas indicate the presence temperature points (35, 60, 85, 110, 135, 160, 185 and 210 C)
of individual elements. The areas highlighted by arrows in with four temperature gradients ( T) of 5, 10, 15 and 20
2 were used for
Fig. 3 (a) Low-magnification TEM image of composite
2
2
2
°
ꢁ
°C
image (d) indicate the distributions of carbon.
were used for each cycle. Both pellets (A and B) demonstrated
similar behaviours in their first measurement cycles, which
were quite different from the rest of the cycles (the 2nd 12th
−
2 | J. Name., 2012, 00, 1-3
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cycles of pellet A in Fig. 5 and the 2nd 9th cycles of pellet B in
−
1
2
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electrical conductivity and Seebeck coefficients. The first cycle
of measurement of each pellet can therefore be viewed as a
pre-process/optimization cycle. The electrical conductivity
values varied over small ranges (0.0007 to 0.0047 S/cm for A
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relatively high Seebeck coefficients over a narrow range (743
3
4
5
6
to 942
gave calculated power factor (PF) values over relatively small
ranges (0.045 to 0.416
W/mK2 for A and 0.119 to 0.637
W/mK2 for B) based on the relationship PF = S2σ. Composite
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aminedithioether ligands. Coordination polymer has a well-
defined structure and good thermal stability with obvious
decomposition of ligand L between 260 and 460 C under
nitrogen. Thermal treatment of under an argon atmosphere
provided composite with dominant crystalline CuI and minor
amounts of amorphous carbon. Composite was less dense
than CuI and could be easily pressed into a robust pellet. The
electrical conductivity of at different temperatures was
reproducible after an optimization cycle. This electrical
conductivity was low, but the Seebeck coefficient of was high
from room temperature to 204 C. We propose that the
1 consisting of [Cu4I4]
1
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°
presence of amorphous carbon enhances the pellet stability
and contributes to relatively small density, but decreases the
electrical conductivity. This work demonstrated the potential
of using inorganic-organic hybrid molecular materials including
coordination polymers, particularly of copper-iodide clusters,
as precursors for pyrolytically derived composites for
thermoelectric applications.
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Conflicts of interest
There are no conflicts to declare.
25 N. Abdullah, R. Hashim, L. N. Ozair, Y. Al-Hakem, H.
Samsudin, A. Marlina, M. Salim, S. M. Said, B. Subramanian
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The X-ray diffraction experiments of
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National University of Singapore with grateful assistance from L.
L. Koh and G. K. Tan. Funding support from the Institute of
Materials Research and Engineering (A*STAR) (SERC Grant No.
1527200020) is acknowledged.
29 C. Yang, D. Souchay, M. Knei
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a
c
1E
1T
10
20
30
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50
ο
2θ /
b
d
100
1 in air
1 in N
2
50
0
200
400
600
800
T / oC

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a
b
2 (pellet after TE measurement)
2 in air
2 in N
2
50
2 (powder)
CuI-00-006-0246
0
200
400
T / oC
600
800
20
40
60
80
2
θ / o
c
d

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ꢁ
ꢂ
ꢄ
ꢃ
ꢅ
/
/ꢀ
L

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3.55 eV
2
CuI
15
10
2.89 eV
50
2.84eV
2.7
3.0
3.3
2.84 eV
3
4
Photon energy / eV
5

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1st
2nd
3rd
4th
ꢀꢁꢂ
0.4
0.2
5th
6th
7th
8th
9th
10th
11th
12th
0.0
50
100
150
150
150
200
T / oC
1st
2nd
3rd
4th
1500
5th
ꢀꢃꢂ
6th
7th
8th
9th
10th
11th
12th
1000
500
50
100
200
T / oC
1st
2nd
3rd
4th
ꢀꢄꢂ
6
3
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6th
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9th
10th
11th
12th
50
100
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200

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Table of Contents Entry
A thermoelectric copper-iodide composite from pyrolysis of a well-defined
coordination polymer
Shi-Qiang Bai,* Ivy Hoi Ka Wong, Ming Lin, David James Young and T. S. Andy Hor*
Robust and relatively light CuI-rich carbon composite derived from an inorganic-organic hybrid
molecular material demonstrates high Seebeck coefficient from room temperature to 204 °C.