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Synthesis, processing, and thermoelectric properties of bulk nanostructuredbismuth telluride (Bi2Te3)

Mohsin Saleemi,a Muhammet S. Toprak,*a Shanghua Li,a Mats Johnssonb and Mamoun Muhammeda

Received 10th August 2011, Accepted 10th October 2011

DOI: 10.1039/c1jm13880d

Bismuth telluride (Bi2Te3) is the best-known commercially used thermoelectric material in the bulk

form for cooling and power generation applications at ambient temperature. However, its

dimensionless figure-of-merit-ZT around 1 limits the large-scale industrial applications. Recent studies

indicate that nanostructuring can enhance ZT while keeping the material form of bulk by employing an

advanced synthetic process accompanied with novel consolidation techniques. Here, we report on bulk

nanostructured (NS) undoped Bi2Te3 prepared via a promising chemical synthetic route. Spark plasma

sintering has been employed for compaction and sintering of Bi2Te3 nanopowders, resulting in very

high densification (>97%) while preserving the nanostructure. The average grain size of the final

compacts was obtained as 90 � 5 nm as calculated from electron micrographs. Evaluation of transport

properties showed enhanced Seebeck coefficient (�120 mVK�1) and electrical conductivity compared to

the literature state-of-the-art (30% enhanced power factor), especially in the low temperature range. An

improved ZT for NS bulk undoped Bi2Te3 is achieved with a peak value of �1.1 at 340 K.

1. Introduction

Thermoelectric (TE) materials have been the topic of intensive

research due to their unique dual capability of directly converting

heat into electricity or electrical power into cooling or heating.

These materials can play an important role in reducing carbon

emission by converting waste heat into electricity.1 Bismuth

telluride (Bi2Te3) and its alloys are the best-known TE materials

for ambient temperature applications.2 The performance of TE

energy conversion devices depends upon the dimensionless figure

of merit (ZT) which is defined as:

ZT¼ S2s

kT (1)

where T is the temperature, S is the Seebeck coefficient, s is the

electronic conductivity and k is the total thermal conductivity

which is the sum of electronic (ke) and lattice thermal (kL)

conductivities.3 There have been continual efforts to improve the

ZT values since 1950s, but the peak ZT value of commercial TE

materials based on Bi2Te3 has increased only marginally, from

about 0.6 to about 1 at ambient temperature.4 During the past

decade, the interest in TE materials was revived by the utilization

of nanotechnology tools in this field. By nanostructuring, s and k

can be de-coupled; s or S can be enhanced by changing the

aDivision of Functional Materials, Royal Institute of Technology (KTH),Electrum 229, Isafjordsgatan 22, SE-16 440 Kista, Sweden. E-mail:toprak@kth.se; Tel: +4687908344bDepartment of Materials and Environmental Chemistry, StockholmUniversity, SE-106 91 Stockholm, Sweden. E-mail: mats.johnsson@mmk.su.se; Fax: +468152187; Tel: +468162169

This journal is ª The Royal Society of Chemistry 2012

density of the states5 and k can be greatly reduced by increased

phonon scattering at the grain boundaries.6 Theoretical predic-

tions from the Dresselhaus group7,8 and the following experi-

mental studies on low dimensional nanostructures, superlattices

of Bi2Te3/Sb2Te3,9 quantum dots of PbTe/PbSnSeTe10 and

quantum well structures of PbTe/Pb1�xEuxTe11 have shown

higher ZT (2–2.5),9,11 due to the quantum confinement effect.

These advanced low-dimensional nano-architectures are not

practical for industrial-scale commercial use because they are

fabricated by atomic layer deposition techniques such as

molecular beam epitaxy, making them feasible for small scale or

high-end applications and rather expensive for large area/scale

applications.5 There are also a lot of difficulties associated with

TE module/device fabrication technology using these materials.12

Therefore, there is still great need for high-performance TE

materials based on the bulk form.

Bismuth telluride (Bi2Te3) and its alloys are the best TE

materials available today near room temperature with the high-

est ZT as reported recently.13 The ZT 1 barrier for bulk Bi2Te3TE alloys is broken by Poudel et al.14 and Heremans et al.15

Poudel reported that Sb doped nanocrystalline BixSb(2�x)Te3synthesized by a conventional mechanical alloying method fol-

lowed by ball milling and sintering by a conventional hot-

pressing technique exhibited a peak ZT of 1.4 at 373 K.14 Their

synthesis method is scalable however the energy cost is relatively

high and the chance to introduce the impurities is higher.

Furthermore, the conventional hot-pressing technique applied

for sintering the powder causes significant grain growth accom-

panied with recrystallization and/or diffusion. To address these

issues, a novel compaction method spark plasma sintering (SPS)

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has been introduced.16 In the SPS technique a pulsed direct

current is conducted through the graphite die under hydrostatic

uniaxial pressure, which consolidates the material. SPS has been

generally recognized as a better sintering technique than

conventional hot-press since it requires relatively shorter

dwelling times because of faster heating rates under high pres-

sure.17 This technique can achieve very high densification that

approaches to theoretical density18 while preserving the nano-

structures during the sintering process.19

Bi2Te3 has shown high potential in improving the ZT during

the last decade because of novel structural modification and

advanced processing strategies. Nanoscale TE material can be

synthesized by several methods including vapour deposition,

pulsed laser ablation, mechanical alloying and conventional

powder metallurgy routes.20 These methods are expensive and

face scale up challenges. Many solution-based methods are

established by using organic precursors for the production of

nanocrystalline Bi2Te3.21,22 Nevertheless, their peak ZT value is

low (in the range of 0.6) due to the presence of organic residues

coming from solvents or stabilizing ligands.23,24 Ritter had

proposed a two-step process for the preparation of poly-

crystalline Bi2Te3 by reduction of bismuth and tellurium

oxides.25,26

In our previous works we reported the enhancement of TE

transport properties of bulk skutterudites (based on CoSb3family) by nanostructuring.27–29 In this paper we focus on the

large scale synthesis of nanostructured undoped bulk NS Bi2Te3materials by employing a novel bottom-up solution-based

chemical approach. A precursor material with the desired

composition and high production yield has been synthesized.

Thermochemical processes via calcination and reduction (under

H2 gas) steps produced the final material. Detailed consolidation

of prepared NS material was studied by SPS and transport

properties of compacted bulk NS Bi2Te3 are presented.

2. Experimental

2.1 Synthesis

The synthesis method was tailored from the chemical alloying

method we have reported earlier.30 Thermodynamic modeling

was performed prior to the synthesis to select the pH range for

the co-precipitation of Bi and Te. Modeling results showed co-

precipitates of bismuth oxide (BiONO3) and tellurium oxide

(TeO2) appear in the pH range of 2–3. All reagents used in this

work were of analytical grade. Bismuth nitrate pentahydrate (Bi

(NO3)2$5H2O, 99.99%, Merck) and tellurium oxide (TeO2,

99.999%, Sigma-Aldrich) were used as starting materials for the

co-precipitation process. Both chemicals were weighed according

to the stoichiometry (Bi2Te3) and separate metal ion solutions of

1 M were prepared by dissolving them in concentrated HNO3

(65 wt%). A stock solution of 3 M sodium hydroxide (NaOH)

was prepared for the pH adjustment. Both metal ion solutions

were mixed in a burette (solution 1) and another burette was

filled with NaOH solution (solution 2). Solutions 1 and 2 were

mixed simultaneously at room temperature under controlled

hydro-dynamic conditions and complete co-precipitation was

obtained by stirring the suspension for 15 minutes. Obtained

white precipitates were filtered off, washed sequentially with de-

726 | J. Mater. Chem., 2012, 22, 725–730

ionized water and ethanol and then dried in an oven at 80 �Covernight. Dried precursor was calcined in a furnace at 250 �Cfor thermo-mechanical grinding. In the end, the oxides of

bismuth and tellurium were reduced in 99% pure hydrogen gas

(H2) at 400�C for 2 hours.

2.2 Characterization

X-Ray powder diffraction (XRD) was performed on a Philips

X’pert pro super diffractometer with Cu Ka (l ¼ 1.5418 �A). A

Zeiss Ultra 55 scanning electron microscope was used for scan-

ning electron microscopy (SEM) and energy-dispersive X-ray

(EDX) analysis (at a standard working distance of 8 mm and an

accelerating voltage of 20 kV) analysis. A JEOL TEM-2100F

field-emission microscope was employed for transmission elec-

tron microscopy (TEM) and selected-area electron diffraction

(SAED) analysis at an accelerating voltage of 200 kV. Samples

for TEM were prepared by dropping few drops of the ground

Bi2Te3 pellet dispersed in ethanol on C coated TEM grids.

Particle and grain size analyses were performed on several

micrographs counting a minimum of 300 particles per grain.

2.3 Spark plasma sintering

Spark plasma sintering (SPS) was performed under argon

atmosphere by using Dr Sinter 2050 SPS (Sumitomo Coal

Mining Co., Tokyo, Japan). Graphite sheet was used to cover the

graphite die with an inside diameter of 12mm. Typically 2–3 g of

Bi2Te3 was loaded into the die during the sintering experiments.

Different SPS parameters for the compaction of Bi2Te3 were

optimized by applying different pressures, holding times and

temperatures. All the samples were prepared with a constant

heating rate of 50 �C min�1, temperature in the range of 375–450�C, and pressure range was 50–70 MPa. Different holding times

from 0 to 4 minutes were applied to check their effect on relative

densities and grain size. Compacted pellets were mechanically

ground and polished by using three different grades of sand

paper in a sequence of 1000, 2500 and 4000.

2.4 Thermoelectric transport property measurements

The thermoelectric properties were measured in the temperature

range of 325–475 K. The Seebeck coefficient and electrical

conductivity were measured on compacted samples by temper-

ature gradient and four probe methods, respectively.31 A laser

flash apparatus, Netzsch LFA 427, was employed to obtain the

thermal diffusivity and a differential scanning calorimeter

(DSC), Netzsch DSC404, was used to measure the specific heat,

Cp. Thermal conductivity was calculated by the product of

thermal diffusivity, specific heat and measured density.

3. Results and discussion

The synthesis method developed here is tailored from the

chemical alloying method that we have reported in our earlier

work, after optimization of the co-precipitation conditions. In

order to design the new route for the synthesis of the precursor,

thermodynamic modeling was performed by Chemical Equilib-

rium Diagrams software.32 Fig. 1(a) and (b) show the modeling

results of H+, Bi3+, NO3�, Na+ and Te4+ systems. According to

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 Thermodynamic modeling of metal nitrate precursors for: (a) bismuth precipitates and (b) tellurium precipitates.

Fig. 2 SEM micrographs of reduced Bi2Te3 powder.Fig. 3 XRD patterns of (a) calcined (B and * are oxide phases of

bismuth JCPDS# 27-0050 and tellurium JCPDS# 01-0870 nanopowder,

respectively) and (b) reduced Bi2Te3 nanopowder (JCPDS# 85-0439).

Fig. 4 SEM fractographs of SPS compacted Bi2Te3 samples consoli-

dated under different conditions: (a) reduced Bi2Te3 powder; sintered

powders at 400 �C for a holding time of (b) 0 min, (c) 2 min and (d) 4 min.

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the results a complete co-precipitation reaction of bismuth oxy-

nitrate (BiONO3) and tellurium oxide (TeO2) is expected in the

pH range of 2–4. Further decomposition of BiONO3 gives

bismuth oxide in the final product. The precipitated powder was

exposed to thermochemical treatment including calcination and

reduction steps. After optimization of the co-precipitation,

calcination, and reduction steps, a large batch up to 30 grams of

NS bismuth telluride could be obtained with a yield of more than

90 percent. The presented method is a bottom-up strategy,

allowing fast processing and consumes fewer chemicals, which is

also superior to physical methods such as ball-milling, since it

offers better control over impurities, size, size distribution, and

morphology of the nanoparticles. More importantly, the new

method developed in this work rendered smaller particle size with

fine plate-like morphology (Fig. 2) of NS bismuth telluride

compared to the earlier reported method.33 Observed hexagonal

plate-like morphology may originate from the crystal habit of

hexagonal Te crystals, forming by the reduction of TeO2 under

H2, which in turn form the final desired Bi2Te3 phase upon

reaction with in situ formed Bi atoms.

XRD patterns of as-synthesized nanopowder at different

processing steps are shown in Fig. 3. After calcination, the XRD

pattern given in Fig. 3(a) was indexed to Bi2O3 (JCPDS no. 27-

0050) and TeO2 (JCPDS no. 01-0870) phases, respectively, which

shows that the decomposition of Bi precursor has taken place.

Peak broadening originates from the small crystallite size of the

This journal is ª The Royal Society of Chemistry 2012

materials; using the Scherrer equation, the crystallite size was

estimated to be ca. 70 nm for TeO2 and 10 nm for Bi2O3 in the

calcined powder mixture. All the diffraction peaks of the reduced

samples (Fig. 3(b)) were indexed to pure rhombohedral Bi2Te3

J. Mater. Chem., 2012, 22, 725–730 | 727

Fig. 5 XRD patterns of (a) reduced bulk Bi2Te3; sintered powders at

400 �C for (b) 0 min and (c) 4 min holding time.

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phases (JCPDS no. 85-0439) with an average crystallite size

estimated to be 50 � 5 nm from peak broadening. Further SEM

analysis of the reduced Bi2Te3 nanopowder in Fig. 4(a) shows

plate-like morphology with an average particle size of 80 � 5 nm

and thickness of 5–10 nm. Energy dispersive X-ray spectroscopy

(EDS) confirmed the atomic ratio between Bi and Te to be 2 : 3 in

accordance with the desired stoichiometry.

In order to perform screening of SPS parameters on the

compaction of nanopowder, it is essential that nanopowders

have the same history, which requires fabrication of large

quantities of the same material with the same purity and particle

size. As there is no detailed information available in the literature

on the optimum SPS consolidation conditions for bulk NS

Bi2Te3 we performed a series of experiments to optimize the SPS

parameters. The critical parameters are sintering temperature,

pressure and holding time to optimize SPS consolidation to

obtain highly compacted powder. The holding time at a partic-

ular temperature and sintering temperature were altered while

the pressure was kept constant at 70 MPa. Table 1 lists the

experimental results of SPS compacted samples from NS Bi2Te3,

relative density and average grain size of NS Bi2Te3 before and

after sintering is also reported. The density of compacted samples

was measured by the Archimedes principle. The compaction

density increases at higher holding time and temperature, as

shown in Table 1. The grain size also increased by increasing the

holding time, as shown in Fig. 4(b)–(d). However, the grain size

first slightly decreased when the temperature was increased from

375 �C to 400 �C, then increased dramatically when the

temperature was increased again from 400 �C to 450 �C. Veryhigh densification (>97%) was achieved for most of the condi-

tions except for the sample sintered at 375 �C with no holding

time. SEM micrographs of these compact samples were taken

from the cleaved surfaces and are shown in Fig. 4(b)–(d). The

average grain size increased to �300 nm with an increase of

holding time at a sintering temperature of 400 �C. After opti-

mization, a protocol containing a sintering temperature of

400 �C, 70 MPa pressure, and no holding time was established as

the best sintering conditions for as-prepared NS bulk Bi2Te3.

With this protocol, a very high densification (97% of theoretical

density) and a smallest grain size of 90 � 5 nm have been ach-

ieved after SPS which was not significantly larger than the

particle size of 80 � 5 nm before SPS. The SPS technique in this

work has been demonstrated successfully in maintaining nano-

Table 1 Sintering conditions, relative densities and average grain size of SP

Sample. no.Sinteringtemperature/�C

Pressure/MPa

Holdingtime/min

Relative densitcm�3

1 375 70 0 7.022 375 70 4 7.623a 400 70 0 6.274 400 70 2 7.645 400 70 4 7.626 425 70 0 7.607 425 70 4 7.648 450 70 0 7.659 450 70 4 7.66

a Sample on which TE transport property measurements are performed.

728 | J. Mater. Chem., 2012, 22, 725–730

sized grains while achieving a high densification, which is not

possible by a conventional hot-pressing technique as reported by

Poudel et al.14,34 The XRD pattern of the compact shown in

Fig. 5 confirmed that no new phases were introduced by SPS,

however relative peak intensity on certain planes has been

changed as reported earlier by L.D. Zhao et al.35 For example,

Fig. 5(b) and (c) show peaks from planes (220), (011) and (208),

which are not obvious in the reduced samples presented in Fig. 5

(a). This indicates that extended grain growth has taken place in

these crystal planes in agreement with earlier report.35

The detailed morphology and phase of the nano-grains after

sintering at 400 �C with no holding time were examined by TEM

and few micrographs are presented in Fig. 6. Sintered NS-Bi2Te3exhibits thin plate-like morphology with nano-sized and highly

crystalline grains. The indexed selected area diffraction pattern

(SAED), inset of Fig. 6(a), confirms that it is polycrystalline and

the inter-planar distance calculated from the SAED pattern as

well as those measured from high-resolution TEM micrographs

are in agreement with d values obtained from XRD peaks.

TE transport property measurements have been performed on

a sample sintered at 400 �C for 0 min holding time. SPS conditions

resulting in the highest relative density values obtained which is

97% of the theoretical density. These compaction conditions

preserve the grain size at the nanoscale (below 100 nm). This

sample was selected to study the effect of compacted nano-

structures on transport properties. Fig. 7 shows the temperature

dependence of S, s, the power factor (S2s), k, and ZT, in the

Sed Bi2Te3 samples

y/gDensification(theoreticaldensity ¼ 7.73 g cm�3)

Particle size beforesintering/nm

Grain size aftersintering/nm

91.25% 100 + 20 160 � 1598.96% 100 + 20 230 � 2597.29% 80 + 5 90 � 1598.22% 80 + 5 160 � 2099.96% 80 + 5 200 � 2098.32% 110 � 25 170 � 2598.83% 110 � 25 200 � 1598.97% 110 � 25 300 � 2099.10% 110 � 25 380 � 25

This journal is ª The Royal Society of Chemistry 2012

Fig. 6 (a) TEM and (b) HRTEM micrograph of sintered Bi2Te3.

Selected area electron diffraction (SAED) pattern of the sample is given

in the inset of (a).

Fig. 7 Thermoelectric transport measurements vs. temperature SPSed

bulk NS Bi2Te3: (a) Seebeck coefficient (S), (b) electrical conductivity

(s), (c) power factor (S2s), (d) thermal conductivity (k) and (e) figure of

merit (ZT).

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investigated temperature range of 325–450 K. The negative value

of the Seebeck coefficient, shown in Fig. 7(a), confirms n-type

semiconductor behavior. At room temperature the maximal

measured S value is�120 mVK�1 that is two times higher than the

reportedvalue inM.Scheele et al.23Thes, shown inFig. 7(b), starts

at 2000 S cm�1 and decreases with increasing temperature from

This journal is ª The Royal Society of Chemistry 2012

325 K to 450 K, which shows metallic or semi metallic behavior.

Compared to the undoped bulk Bi2Te3 ingots, s of our sample is

around 50% higher and it is also about two-to-three times higher

than the state-of-the-art nanostructured bulk bismuth telluride

prepared by ball milling and SPS compaction.13 The electronic

conductivity decreased sharply after 425 K, which is probably

related to metallic behavior that enhances thermal conductivity

and decreases electrical conductivity. Relative density/porosity

plays an important role on the electrical and thermal transport

properties as reported byMedlin and Synder.36High densification

achieved by SPS is probably accounting for the high electrical

conductivities and thermopower measured. The measured power

factor as shown in Fig. 7(c) is about 2 times higher as compared to

recently reported bulk Bi2Te3 prepared by the ball milling and SPS

consolidation method.37A key reason to employ NS TEmaterials

is to have a low k due to extensive phonon scattering at the grain

boundaries.38Themeasuredk (Fig. 7(d)) of the sample shows lower

values than that of bulk ingot Bi2Te3 (1.5 W mK�1). It should be

noted that the k of our sample is also comparable to the state of the

art NS ball milled SPS compacted Bi2Te3 samples (0.7 W mK�1

(ref. 13) and 1–1.2 W mK�1 (ref. 33)). Moreover, thermal

conductivity in this sample confirms a dramatic reduction from

bulk values due to increased phonon scattering. Also the amount

of porosity has been minimized by obtaining >97% compaction of

NSBi2Te3 by using SPS. It has been reported39 that porositymight

have played an important role which can be deduced by the low

electrical conductivity.38 Thus, the sufficient densification with

smaller grain size decreases k, enhancess andS, giving really a high

s/k for nanostructuredBi2Te3 sinteredbySPS, especially in the low

temperature region.

Thermal conductivity, k, is composed of lattice thermal

conductivity, kL, and electronic thermal conductivity, ke (k ¼ kL+ ke). ke can be calculated by using the Wiedemann–Franz law,

ke ¼ LosT (ref. 40) where Lo is the Lorentz number, s is the

electrical conductivity and T is the absolute temperature. ke and

kL were calculated by using Lo 1.5 � 10�8 W U K�2 (data are not

shown) for non-degenerate semiconductors. In our case calcu-

lated kL is even less than zero, which is physically not possible;

this reveals widely accepted Lo value does not apply to nano-

structured bulk material. It has been reported by Minnich et al.41

that the Lo can decrease due to enhancement in the resonant level

which basically increases the Seebeck coefficient and reduces the

ke. Other authors42 explained that materials with higher elec-

tronic density of states can reduce the electronic thermal

conductivity to zero.43 Limitations of the Wiedemann–Franz law

that have been discussed in a recent paper44 may help to explain

the observed anomaly.

Finally, the figure of merit ZT as a function of temperature is

shown in Fig. 7(e), which shows improved values starting from

a lower temperature range than the previous state-of-the-art

undoped Bi2Te3,13 achieving a peakZT value of 1.1 at 340 K. The

measured ZT value is about 30% (ZT ¼ 0.6)33 and 70% (ZT ¼0.4)44 higher than recently reported, ball milled and SPS com-

pacted Bi2Te3 samples.

4. Conclusion

In summary, we successfully fabricated bulk nanostructured

undoped Bi2Te3 with a high phase purity using a promising

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chemical synthetic route—feasible for large-scale production.

SPS has been effectively used and optimized for the consolida-

tion of bulk NS Bi2Te3, yielding a very high densification (>97%)

without significant grain growth. Fine plate-like NS Bi2Te3 with

a particle size of �80 nm and a grain size of �90 nm has been

obtained and showed improved TE transport properties.

Transport measurements showed increased Seebeck coefficient

and electrical conductivity, which resulted in about 30%

improved power factor. A figure of merit ZT of 1.1 at 340 K is

achieved, that is slightly higher than the state-of-the-art reported

value for undoped Bi2Te3, especially at ambient temperatures.

This is attributed to the high electrical conductivity, Seebeck

coefficient as well as reduced thermal conductivity.

Acknowledgements

The authors are grateful to Dr C. Stiewe, Dr D. Platzek, and Dr

E. Muller from German Aerospace Center (DLR) for the ther-

moelectric measurements and to Dr Wubeshet Sahle for TEM

analysis.

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