Synthesis and characterization of Bi 2 Te 3 /polyaniline composites Yong Li, Qing Zhao, Yin-gang Wang n , Ke Bi College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China. article info Available online 15 March 2011 Keywords: Thermoelectric Bi 2 Te 3 Polyaniline Mechanical blending. abstract Bi 2 Te 3 flakes and rice-like polyaniline particles have been synthesized using hydro- thermal and chemical oxidation routes, respectively, and the Bi 2 Te 3 /polyaniline com- posites were prepared by mechanical blending. The Bi 2 Te 3 /polyaniline composites exhibit an n-type conduction, and its Seebeck coefficient is similar to that of the Bi 2 Te 3 while the conductivity of the composites is almost the same with the polyaniline. As the synergistic effect of the Seebeck coefficient and the conductivity, the power factor of Bi 2 Te 3 /polyaniline composites is lower than that of the Bi 2 Te 3 or polyaniline, and remains almost unchanged with temperature. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Thermoelectric materials as one of the new energy conversation materials have experienced a resurgence since the new concept that the disordered components through alloying could reduce the phonon thermal con- ductivity rather than electrical conductivity [1]. There are several new systems of thermoelectric materials, such as the skutterudites, the clathrates, the Half-Heusler inter- metallic compounds and so on [2–4]. Various works have been made to improve the thermoelectric properties [5–7]. To date, the most widely used materials with the best thermoelectric figure of merit (ZT) are still the bismuth telluride alloys, which have been developed for many years [8]. As there is no upper limit of ZT theore- tically, one has been carrying out studies to optimize the thermoelectric properties [9]. The composition of inorganic materials and conduct- ing polymers has been striking great interests since it can combine the merits of the two materials [10]. The con- ducting polymers possess the features of cheap, light- weight, flexible, low thermal conductivity and the elec- trical conductivity could be improved through doping [11], which make them another promising thermoelectric materials. However, there is few work reported on ther- moelectric polymer materials. In this article, we first synthesized Bi 2 Te 3 nanoparticles and polyaniline using hydrothermal and chemical oxidation routes, respectively, then prepared the Bi 2 Te 3 /polyaniline nanocomposites by mechanical blending, and finally studied the structure and the formation mechanism of the product with emphasis on comparing the electric performance of the samples. 2. Experimental Bi 2 Te 3 was prepared by a hydrothermal route. In a typical experimental procedure, a mixture of Te powder and BiCl 3 with the mole ratio of 2:3 was put into a glass beaker filled with 140 ml distilled water. The pH value was adjusted using NaOH and controlled to be 13. A sufficient amount of NaBH 4 was put into the solution as reductant. The mixture was transferred to the 200 ml autoclave, which was then sealed and maintained at 473 K for 24 h with stirring. After the system was natu- rally cooled down to room temperature, the resulting product was gathered by centrifugation, washed several times by distilled water and absolute ethyl alcohol and then dried in vacuum at 333 K. Chemical oxidation was used to synthesize the poly- aniline product. 4 ml distilled aniline was dissolved in 100 ml of 1.5 M concentrated HCl aqueous solutions. The mixture was transferred to a three-necked flask. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/mssp Materials Science in Semiconductor Processing 1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.02.019 n Corresponding author. Tel./fax: þ86 25 52112626. E-mail addresses: [email protected] (Y.-G. Wang), [email protected] (K. Bi). Materials Science in Semiconductor Processing 14 (2011) 219–222
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing
Materials Science in Semiconductor Processing 14 (2011) 219–222
1369-80
doi:10.1
n Corr
E-m
bike@n
journal homepage: www.elsevier.com/locate/mssp
Synthesis and characterization of Bi2Te3/polyaniline composites
Yong Li, Qing Zhao, Yin-gang Wang n, Ke Bi
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China.
Bi2Te3 flakes and rice-like polyaniline particles have been synthesized using hydro-
thermal and chemical oxidation routes, respectively, and the Bi2Te3/polyaniline com-
posites were prepared by mechanical blending. The Bi2Te3/polyaniline composites
exhibit an n-type conduction, and its Seebeck coefficient is similar to that of the Bi2Te3
while the conductivity of the composites is almost the same with the polyaniline. As the
synergistic effect of the Seebeck coefficient and the conductivity, the power factor of
Bi2Te3/polyaniline composites is lower than that of the Bi2Te3 or polyaniline, and
remains almost unchanged with temperature.
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Thermoelectric materials as one of the new energyconversation materials have experienced a resurgencesince the new concept that the disordered componentsthrough alloying could reduce the phonon thermal con-ductivity rather than electrical conductivity [1]. There areseveral new systems of thermoelectric materials, such asthe skutterudites, the clathrates, the Half-Heusler inter-metallic compounds and so on [2–4]. Various works havebeen made to improve the thermoelectric properties[5–7]. To date, the most widely used materials with thebest thermoelectric figure of merit (ZT) are still thebismuth telluride alloys, which have been developed formany years [8]. As there is no upper limit of ZT theore-tically, one has been carrying out studies to optimize thethermoelectric properties [9].
The composition of inorganic materials and conduct-ing polymers has been striking great interests since it cancombine the merits of the two materials [10]. The con-ducting polymers possess the features of cheap, light-weight, flexible, low thermal conductivity and the elec-trical conductivity could be improved through doping [11],which make them another promising thermoelectric
ll rights reserved.
6.
G. Wang),
materials. However, there is few work reported on ther-moelectric polymer materials. In this article, we firstsynthesized Bi2Te3 nanoparticles and polyaniline usinghydrothermal and chemical oxidation routes, respectively,then prepared the Bi2Te3/polyaniline nanocomposites bymechanical blending, and finally studied the structure andthe formation mechanism of the product with emphasison comparing the electric performance of the samples.
2. Experimental
Bi2Te3 was prepared by a hydrothermal route. In atypical experimental procedure, a mixture of Te powderand BiCl3 with the mole ratio of 2:3 was put into a glassbeaker filled with 140 ml distilled water. The pH valuewas adjusted using NaOH and controlled to be 13.A sufficient amount of NaBH4 was put into the solutionas reductant. The mixture was transferred to the 200 mlautoclave, which was then sealed and maintained at473 K for 24 h with stirring. After the system was natu-rally cooled down to room temperature, the resultingproduct was gathered by centrifugation, washed severaltimes by distilled water and absolute ethyl alcohol andthen dried in vacuum at 333 K.
Chemical oxidation was used to synthesize the poly-aniline product. 4 ml distilled aniline was dissolvedin 100 ml of 1.5 M concentrated HCl aqueous solutions.The mixture was transferred to a three-necked flask.
Fig. 1. XRD patterns of (a) Bi2Te3 and (b) polyaniline.
Table 1Conductivity of three kinds of polyaniline.
Sample Supplied
current (mA)
Voltage
(mV)
Resistivity rv
(O cm)
Conductivity
s(S cm�1)
PANI 1 2.08 0.375 2.67
PANII 1 2.21 0.398 2.51
PANS 1 2.60 0.468 2.14
Fig. 2. SEM images of (a) Bi2T
Y. Li et al. / Materials Science in Semiconductor Processing 14 (2011) 219–222220
Quantitative amount of (NH4)2S2O8 was dissolved in 35 mlHCl aqueous solution, which was then transferred to thedropping funnel and slowly added dropwise to the three-necked flask with stirring. The mixture was reacted for 5 hat room temperature, the resulting product was filtered andwashed with 1.5 M concentrated HCl aqueous solution,distilled water and absolute ethyl alcohol several times tillthe filtrate became colorless, and dried under a vacuum at333 K to get the HCl-doped polyaniline (Emeraldine saltform), which is referred as PANI. For comparison, we alsosynthesized PANII and PANS. After the as-prepared PANIwas de-doped by soaking in ammonia solution for 24 h,HCl aqueous solution and sulfosalicylic acid were usedto re-dope the obtained powder to get PANII and PANS,respectively.
The Bi2Te3/polyaniline composites were fabricated bya mechanical blending method. The mixture of Bi2Te3 andpolyaniline powders and ethyl alcohol was milled for 3 hin a Teflon jar using ZrO2 as the grinding media. Theobtained slurries were dried at 333 K in vacuum for 24 h.
The crystal structures of the powders were character-ized by X-ray diffraction(XRD) using a Bruker D8 systemwith Cu K-a radiation (l¼0.15405 nm). Surface morphol-ogies of the product were analyzed by scanning electronmicroscopy using a LEO-1550 instrument. Four-point-probe method was adopted for the Seebeck coefficientand electric conductivity measurements where the sam-ples were pressed to tablets by a cold pressing process.
3. Results and discussion
X-ray patterns of Bi2Te3 and polyaniline are shown inFig. 1. All the reflections of the patterns can be wellindexed to the hexagonal Bi2Te3 (JCPDS 15-0863) inFig. 1(a), no remarkable diffractions of other phases canbe found, which indicates that the pure Bi2Te3 has beenobtained. From Fig. 1(b), we can see that the polyanilinepowders exhibit three weak crystalline peaks at 2y¼151,20.51 and 25.31, which indicate the low crystallinity.
Table 1 shows the conductivity of all three kinds ofpolyaniline measured at room temperature. It can be seenthat the conductivity of the three samples is similar andclose to that reported [12]. Considering the complexity of
e3 and (b) polyaniline.
Y. Li et al. / Materials Science in Semiconductor Processing 14 (2011) 219–222 221
the preparation, the PANI powders were used to synthe-size the Bi2Te3/polyaniline composites.
Fig. 2 shows the SEM images of Bi2Te3 and polyaniline.Irregular flakes with a size ranging from �10to �600 nmcould be seen in Fig. 2(a). This may be contributed to themolecular structure. Bi2Te3 has a layered hexagonalstructure with the Te and Bi atom layers arranged inthe order of –Te(1)–Bi-Te(2)–Bi–Te(1)– along the c-axis.Between two Te(1) layers there are van-der-waals bonds,while all others are covalent bonds. As the energy of abroken covalent bond is much higher than that of adangling van-der-waals bond, a Bi2Te3 crystal grows
Fig. 3. Temperature dependence of (a) Seebeck coefficient a, (b) elec-
trical conductivity s and (c) power factor a2s for Bi2Te3, polyaniline and
Bi2Te3/polyaniline composites, respectively.
faster in the c-plane and thus the flake morphology wasformed through the natural growth. From Fig. 2(b) onecan see about 20 nm rice-like particles aggregated intoclusters whose diameter is about 50–200 nm and theseclusters aggregate further. The HCl-doped polyanilineincreases in the molecular polarity and the molecularchains interactions also increase, that’s why the particlesare severely aggregated.
The temperature dependence of Seebeck coefficient afor Bi2Te3, polyaniline (PANI) and Bi2Te3/polyaniline com-posites is shown in Fig. 3(a). The Seebeck coefficient canbe expressed as aEg� ln n, where g is the scatteringcoefficient and n is the carrier concentration [13]. Thesample Bi2Te3 and Bi2Te3/polyaniline composites exhibitan n-type conduction, as they have the similar negativeSeebeck coefficients. There is no obvious change in See-beck coefficients with temperature, which may be attrib-uted to the fact that the scattering coefficient decreasesas temperature increases. The Seebeck coefficient ofpolyaniline rises and falls as temperature increases. Thehighest value 166.9 mV K�1 is obtained at 300 K, and thenreduced sharply to �429.8 mV K�1, which may be causedby the de-doping of polyaniline at higher temperature.After that, the Seebeck coefficient is still unstable with ageneral trend of rising values, which could be due to thebuild up of more carriers on colder side.
The temperature dependence of electrical conductivitys is shown in Fig. 3(b). The conductivity of Bi2Te3 increasedgradually with temperature and reached 9.5 S mm�1 at473 K. This corresponds to an increase in the carriermobility (m) and concentration (n), respectively, as givenby the equation s¼nem [14]. Because of the effect of weakcrystal structure discussed above, a corresponding changein the carrier mobility appears in scattering processes.Although the carrier concentrations increase with tem-perature, the conductivity of polyaniline is low andremains the same. The conductivity of the composites isalmost the same as that of the polyaniline. Both theSeebeck coefficient and the conductivity of the compositesare relatively low as there was no heat treatment for thesample [15]. Here we only compare the electric perfor-mances of all the products.
The temperature dependence of power factor a2s isshown in Fig. 3(c). The power factor of Bi2Te3 increasedwith temperature, which indicates that the influenceof the carrier concentration increasing on the sample’selectric performance is bigger than that of the carrierscattering enhancing. The power factor of the polyani-line changes unsteadily and the highest value of5.02 mW m�1 K�2 appears at 326.5 K, which indicates thatit is appropriate for use at room temperature. The powerfactor of Bi2Te3/polyaniline composites is lower than thatof the Bi2Te3 or of the polyaniline, and remains almostunchanged with temperature. As the Seebeck coefficient ofthe composites is similar to that of the Bi2Te3, the lowpower factor is mainly caused by the low conductivity.The mechanical blended composites couldn’t improve theelectrical properties, which agrees well with the resultsreported [16–18], but the thermal conductivity of thepolyaniline is low [19], which may be contributed to ahigh thermoelectric performance.
Y. Li et al. / Materials Science in Semiconductor Processing 14 (2011) 219–222222
4. Conclusion
Naturally growing Bi2Te3 product has the flake mor-phology and the increasing molecular polarity causes asevere aggregation of the rice-like polyaniline particles.The power factor of Bi2Te3 increases as the temperatureincreases while that of the polyaniline changes unsteadilywith the highest value of 5.02 mW m�1 K�2 at 326.5 K. Themechanical blended composites have a lower power factorthan both the Bi2Te3 and the polyaniline, which remainsalmost unchanged with temperature. The electrical proper-ties cannot be improved by mechanical blending.
Acknowledgments
This work is supported by the Scientific Research andInnovation Foundation of NUAA and SRF for ROCS, SEM.
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