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or Bi addition on microstructure and thermoelectric properties of Cu0.05Bi2Te3
Author’s Accepted Manuscript Influence of Se-doping and/or Bi addition on microstructure and thermoelectric properties of Cu0.05Bi2Te3 Song Chen, Kefeng Cai, Shirley Shen www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)31731-X http://dx.doi.org/10.1016/j.ceramint.2016.09.200 CERI13860
To appear in: Ceramics International Received date: 14 July 2016 Revised date: 27 September 2016 Accepted date: 28 September 2016 Cite this article as: Song Chen, Kefeng Cai and Shirley Shen, Influence of Sedoping and/or Bi addition on microstructure and thermoelectric properties of Cu0.05Bi2Te3, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.09.200 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Se-doping and/or Bi addition on microstructure and thermoelectric properties of Cu0.05Bi2Te3 Song Chen1,2, Kefeng Cai1*, Shirley Shen3 1
Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education; School of
Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China 2
School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350108, China 3
CSIRO Manufacturing, Gate 3, Normanby Road, Clayton VIC 3190, Australia *
Corresponding author. [email protected]
Abstract Cu0.05Bi2SexTe3-x (x=0.2, 0.3, 0.4) bulk samples were prepared by hot pressing corresponding nanopowders synthesized by a gas-induced reduction method. Se doping significantly increases the carrier concentrations and hence the electrical conductivities of the samples. Based on this, a Cu0.05Bi2Se0.3Te2.7 sample with a higher power factor was chosen as a pristine sample; some of the Cu0.05Bi2Se0.3Te2.7 nanopowders were mixed with various amounts of Bi nanoparticles (NPs) and then hot pressed into bulk samples. The addition of Bi NPs promotes the sintering of samples. When the content of Bi NPs added is low (≤1 wt%), the electrical conductivity is higher than the pristine sample, and all the Bi NPs added samples have higher Seebeck coefficient than the pristine sample, hence, the power factor has been improved. A maximum power factor, µWcm-1K-2, has been obtained for the 2 wt.% Bi NPs added sample. Keywords: Bi2Te3; thermoelectric; doping; nanostructure.
1
1. Introduction Bi2Te3 based alloy is the best thermoelectric (TE) material system at around room temperature. It has been reported that nanostructuring can improve TE figure of merit, ZT(= where is the Seebeck coefficient, electrical conductivity, thermal conductivity of the materials and T is the absolute temperature) value of Bi2Te3 based alloys [1-7]. Two approaches have been used to obtain nanostructured Bi2Te3 based alloys: top-down and bottom-up. Top-down approaches, such as melt-spinning followed by spark-plasma-sintering [4], have been reported to produce p-type bulk Bi2Te3-based materials with high ZT values. Bottom-up approaches have also been reported to prepare Bi2Te3-based materials with high ZT values [5-7]. For instance, Mehta et al. [7] reported Bi2Te3-based materials with high ZT values (>1 at 300 K) prepared by a microwave-stimulated wet-chemical technique followed by cold pressing and then vacuum sintering. Recently, we developed a novel and simple gas-induced-reduction (GIR) method to synthesize chalcogenide nanostructures [8-9], which is also applicable to Bi2Te3 based alloys [10]. As nanopowders have very high specific surface area and will adsorb oxygen on their surface when they are exposed to air. The adsorbed oxygen will make Bi2Te3 based alloys be slightly oxidized during hot-pressing at elevated temperatures [11]. Mehta et al. [7] reported that sulfur doping can inhibit the oxidation of Bi2Te3-based materials. Recently, we [12] found that Cu doping (CumBi2Te3, m=0, 0.01, 0.025, 0.05) can also inhibit the oxidation of Bi2Te3-based materials as well as improve the TE properties (the Cu0.05Bi2Te3 sample has better TE properties than the other samples). Therefore, in this work, we chose Cu0.05Bi2Te3 as the pristine sample, and firstly studied the effect of Se doping on the electrical transport properties of the Cu0.05Bi2Te3 [Cu0.05Bi2SexTe3-x (x=0.2, 0.3, 0.4)] and then the effect of addition of Bi nanoparticles (NPs) on the electrical transport properties of Cu0.05Bi2Se0.3Te2.7. 2
2. Experimental The precursor solution for synthesis of Cu0.05Bi2SexTe3-x (x=0.2, 0.3, 0.4)
was prepared as
follows: 4 mmol Bi(NO3)3·5H2O [analytical reagent (AR)], 0.1 mmol Cu(NO3)2·H2O (AR), 2x mmol SeO2 (AR), 6-2x mmol TeO2 (AR), 1.5 mL nitric acid and 80 mL ethylene glycol (EG) (AR) were added to a 100-mL beaker sequentially. The mixture was then heated to 100 oC with magnetic stirring until it became transparent. Alumina crucible cleaned with nitric acid and distilled water and then dried in air was placed at the bottom of a Teflon-lined autoclave. 20 mL of the precursor solution was poured into the alumina crucible, and 2 mL of 85 vol.% hydrazine hydrate was injected into the bottom of the Teflon container. The autoclave was sealed and heated to 250 oC at a heating rate of 3 oC /min, and held for 24 h, then naturally cooled to room temperature. The precipitates were collected and washed with deionized water and absolute ethanol in sequence for several times, then separated by centrifugation at 3900 rpm for 5 min and finally dried in vacuum at 70 oC. Bi NPs (~100 nm) were prepared by dissolving Bi(NO3)3·5H2O in EG and through microwave assisted reactions (in a 700 W microwave oven) with a working frequency of 2.45 GHz. Some of the Cu0.05Bi2Te3 powders were mixed with 0.5, 1, or 2 wt% of the Bi NPs using an agate motar. All the powders were hot pressed into pellets (10 mm in diameter and about 1 mm in thickness) at 723 K for 2 h at 80 MPa. The pellets were cut into rectangles (10 × 2 × 2 mm3) for electrical conductivity and Seebeck coefficientmeasurements. The measurements were carried out using a home-made computer control test system under argon atmosphere. The measurement was performed by a steady-state four-probe technique with a square wave current (~10 mA in amplitude). The Seebeck coefficient value was determined by the slope of the linear relationship 3
between the thermal electromotive force and temperature difference (~10 K) between the two ends of each sample. The powder and bulk samples were examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation (λ = 1.5406 Å). The microstructure of the samples was examined by field-emission scanning electron microscopy (FESEM, Quanta 200; FEG). For elemental analyses of the bulk samples, ICP-OES (Optima 8000, PerkinElmer) was used.
3. Results and discussion 3.1 Preparation and TE Properties of Se doped Cu0.05Bi2Te3 XRD analysis (see supporting materials) indicates that the XRD peaks for Se-doped Cu0.05Bi2Te3 can be indexed to standard data for Bi2Se0.3Te2.7（JPCDS card: 50-0954). The lattice parameters (a and c) of the samples calculated from the XRD data both decrease as the doping amount of Se increases (see supporting materials). It suggests that Se atoms substitute Te atoms in the Bi2Te3 crystal structure. As the radius of Se atom is smaller than that of Te atom, the lattice constant decreases after Se doping. FESEM observation (not shown here) reveals that as the Se content increases, the samples become denser and that the grain sizes of the samples become bigger. Fig.1 shows temperature dependence of electrical transport properties of the Cu0.05Bi2SexTe3-x samples from RT to 550 K. The electrical conductivity of the Cu0.05Bi2SexTe3-x samples decreases with the increasing of temperature in the whole temperature range, exhibiting a metallic conducting behavior. At a given temperature, the electrical conductivity increases with increasing of Se content. The increase in electrical conductivity is mainly because of the increase of the carrier concentration (see Fig.2 hereinafter).
4
Fig. 1 Temperature dependence of TE properties (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor of the Cu0.05Bi2SexTe3-x samples
Fig.1 b shows temperature dependence of Seebeck coefficient of the samples. The Seebeck coefficient values are negative, indicating that they are n-type semiconductors. As the Se content increases, the Seebeck coefficient value decreases. This is because the carrier concentration increases after Se doping (see Fig. 2), and the Seebeck coefficient () is inversely proportional to the carrier concentration, n, as indicated by the following simple model of electron transport [13]: 2
8 2 k B2 * 3 mT 3eh 2 3n
(1)
where kB, h, and m* are the Boltzmann constant, Planck constant, and effective mass of the carrier,
5
respectively. The Seebeck coefficient of the samples first increases with temperature, and when T >430 K it turns to decrease with temperature. The decreasing of the Seebeck coefficient value at elevated temperatures is due to the start of intrinsic excitation and two-band conduction from both electrons and holes. Two-band conduction tends to reduce the total Seebeck coefficient, as is evident from the Mott relation [14]: nnppnp where αn and αp are the negative and positive Seebeck coefficients, respectively, and σn and σp are the corresponding conductivities of each band. The power factor of the sample is shown in Fig 1(c). The Cu0.05Bi2Se0.3Te2.7 sample has a higher power factor among the studied samples with a maximum power factor of µWcm-1K-2. Fig.2 shows the carrier concentrations and motilities of the samples at RT. Obviously, the carrier concentration increases with the Se content increasing. It may be because Se doping introduces a donor doping-level and hence increases the carrier concentration. However, the mobilities of the samples are very similar.
Fig.2 The carrier concentrations and mobilities of the Cu0.05Bi2SexTe3-x (x=0, 0.2, 0,3, 0.4) samples at RT
3.2 Preparation and properties of the Bi NPs added Cu0.05Bi2Se0.3Te2.7 6
Since the Cu0.05Bi2Se0.3Te2.7 sample has a higher power factor than other samples (Fig. 1), we further study the effect of Bi NPs addition on the TE properties of the Cu0.05Bi2Se0.3Te2.7. Fig.3 presents the XRD patterns of the Cu0.05Bi2Se0.3Te2.7 nanopowders mixed with various amount of Bi NPs after hot pressing. The XRD peaks of the samples can still be indexed to the standard data of Bi2Se0.3Te2.7（JPCDS card: 50-0954) and no XRD peak for Bi is detected. It may be because that the amount of Bi NPs added is below the resolution of the instrument and/or the Bi reacted with the Cu0.05Bi2Se0.3Te2.7 to form solid solution during hot pressing. The intensity of (00l) diffraction peaks increases with the increasing of the content of Bi NPs added, indicating that the addition of Bi NPs is beneficial to the preferential growth of Cu0.05Bi2Se0.3Te2.7 grains. The degree of orientation was evaluated in terms of the Lotgering factor f, which is calculated using the following equation [15]: f
p p0 1 p0
(3),
where p0=∑I0(00l)/∑I0(hkl) and p=∑I(00l)/∑I(hkl). I0 and I are the intensities of each of the diffraction peaks in XRD patterns as presented in ICSD data (JPCDS card: 50-0954) and those determined experimentally, respectively. The degree of orientation, f, is 7.33%, 10.12%，17.02% and 18.53% for the samples with 0, 0.5, 1, and 2% Bi NPs added, respectively. With the increasing of the content of Bi NPs added, the orientation degree of the samples increases. It may be because the melting point of Bi is only 544.44 K, which is much lower than the hot pressing temperature (723 K), the Bi NPs were in molten state during the hot pressing. Namely, the Bi NPs added samples were sintered with liquid-phase; hence the grains were easier to grow up preferentially.
7
Fig. 3 XRD patterns of the hot pressed samples with 0.5, 1, 2 % Bi NPs added
Fig. 4 shows the cross-section morphologies of the hot pressed samples. As the content of Bi NPs added increases, the average gain size of the samples increases. Each grain, in fact, consists of many nanosheets (thickness <100 nm), which is clearly shown in the high magnification FESEM image (see Fig. 4d).
Fig. 4 FESEM images of the hot pressed samples with (a) 0.5, (b) 1, (c)2% Bi NPs added, (d) enlarged image corresponding to the area marked by the white square in (c)
8
Fig. 5(a) shows the electrical conductivity of the samples from RT to 523 K. The samples with 0.5, 1 wt.% Bi NPs added have higher electrical conductivity than the pristine sample, whereas the one with 2 wt.% Bi NPs added has a lower electrical conductivity when the temperature lower than 450 K. At a given temperature, the electrical conductivity increases when the content of Bi NPs added is low (≤1 %). This might be because of the increasing degree of orientation, which is beneficial to electrical conductivity. However, for the 2% Bi NPs added sample, its degree of orientation is similar to that of the 1% Bi NPs added sample, and its mobility is significantly low (see supporting materials) probably due to containing nano- to submicro-pores (see Fig. 4d), leading to its lower electrical conductivity. It is seen from Fig. 5a that the electrical conductivity of all the Bi NPs added samples first decreases with temperature and then increases with temperature. The electrical conductivity turning to increase at higher temperatures may be due to intrinsic excitation of the samples. The turning point is about 475, 450, and 400 K for samples with 0.5, 1, and 2 wt.% Bi NPs added, respectively, indicating that the band gap for the samples becomes narrow as the content of Bi NPs added increases. This suggests that the added Bi NPs reacted with Cu0.05Bi2Se0.3Te2.7 to form a solid solution. Fig. 5 (c) shows that Seebeck coefficients of the samples from RT to 523 K. Seebeck coefficient of all the samples is negative, indicating that the samples still maintain the n-type conduction. The absolute Seebeck coefficient of the samples increases with increasing of the content of Bi NPs added at T < ~450 K and it is about 210 V/K at RT for the 2 wt.% Bi NPs added sample. The added Bi NPs melted during the hot pressing and some of the Bi might enter the crystal structure of Cu0.05Bi2Se0.3Te2.7 as interstitial atoms (since the position of the XRD peaks almost did not shift). The interstitial atoms as well as the above-mentioned pores scatter carriers so that the 9
Seebeck coefficient increases. The absolute Seebeck coefficients for all the samples reduce at higher temperatures and converge to ~about 160V/K. The reduction of the Seebeck coefficient value at high temperatures is due to the start of intrinsic excitation and two-band conduction from both electrons and holes as equation (2) described. Note that the chemical formulas of the samples given above are nominal ones. According to ICP-OES analysis, the chemical composition (wt.) of the 2 wt.% Bi NPs added sample is: Cu 0.11 %, Bi 56.76%, Se 1.03%, Te 42.28%. If the total atomic number of Se and Te is fixed to 3, the chemical formula of the sample is calculated to be Cu0.015Bi2.37Se0.038Te2.962. The atomic number of Cu and Se is smaller than we originally designed, and the latter should be because Se is relatively easier to volatilize during hot pressing; the atomic number of Bi is higher than 2, which should be due to additional Bi NPs added. The power factors of the samples are shown in Fig. 5 (c). Generally, the power factor of all the samples decreases with increasing temperature. Around RT, it increases obviously with the content of Bi NPs added. A maximum power factor, µWcm-1K-2, being 75% higher than that of the pristine sample, has been obtained for the 2 wt.% Bi NPs added sample. It indicates that adding Bi NPs is very efficient to improve the electrical transport properties. Since the composition of the 2% Bi NPs added sample (Cu0.015Bi2.37Se0.038Te2.962) is more complicated than that of the Cu0.05Bi2Te3 sample, which has a thermal conductivity of ~0.95 W/mK at RT [12]; it is deduced that the 2 wt.% Bi added sample should have a thermal conductivity not higher than 0.95 W/mK, and ZT value of this sample is estimated to be higher than 0.97.
10
Fig.5 temperature dependence of the electrical conductivity (a) Seebeck coefficient (b) and power factor (c) of the samples with various amounts of Bi NPs added
4. Conclusions In summary, after Se doping, the carrier concentration of the Cu0.05Bi2Te3 sample increases significantly and hence the electrical conductivity increases. The Cu0.05Bi2Se0.3Te2.7 sample has a higher power factor than the other samples. The electrical transport properties of the Cu0.05Bi2Se0.3Te2.7 sample have been furtherly improved by addition of Bi NPs. The addition of Bi NPs can promote the sintering of the samples. When the content of added Bi NPs is low (≤1 wt%), the electrical conductivity is higher than the pristine sample. However, all the Bi added samples have higher Seebeck coefficient than that of the pristine sample. The power factors have been 11
improved significantly. A maximum power factor, µWcm-1K-2, has been obtained for the sample with 2 wt.% added Bi NPs, indicating it is an efficient method to improve the electrical transport properties of Bi2Te3 based materials.
Acknowledgments This work has been supported by National Natural Science Foundation of China (51271133), the 973 Project under Grant no. 2013CB632500, and the foundation of the State Key Lab of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, Grant No. 2016-KF-2).
References [1] B. Poudel, Q. Hao, Y. Ma, Y. C. Lan, A. Minnich, B. Yu, Y. Xiao, D. Z. Wang, A. Muto, D.Vashaee, X. Y. Chen, J. M. Liu, M. S. Dresselhaus, G. Chen, and Z. F. Ren, High thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys, Science. 320 (5876) (2008) 634–638. [2] S. II Kim, K. H. Lee, H.A. Mun, H.S. Kim, S.W. Hwang, J.W. Roh, D. J. Yang, W. H. Shin, X. S. Li, Y. H. Lee, Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics, SCIENCE 348( 6230)(2015) 109-114 [3] Y. Xiao, J.Y. Yang, Q.H. Jiang, L.W. Fu, Y. B. Luo, D. Zhang, Z. W. Zhou, Synergistic tuning of carrier and phonon scattering for high performance of n-type Bi2Te2.5Se0.5 thermoelectric material J. Mater. Chem. A 3(2015): 22332-22338 [4]W.J. Xie, X.F. Tang, Y.G. Yan, Q.J. Zhang, T.M. Tritt, Unique Nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys, Appl. Phys. Lett. 94 (10) (2009) 102–111. [5] Y.Q. Cao, X.B. Zhao, T.J. Zhu, X.B. Zhang, J.P. Tu, Syntheses and thermoelectric properties of 12
Bi2Te3/Sb2Te3 bulk nanocomposites with laminated nanostructure, Appl. Phys. Lett. 92 (14) (2008) 143106. [6] Liu CJ, Lai HC, Liu YL, Chen LR. High thermoelectric figure-of-merit in p-type nanostructured. (Bi,Sb)2Te3 fabricated via hydrothermal synthesis and evacuated-and-encapsulated sintering. Journal of Materials Chemistry 22(2012)4825-31 [7] R.J. Mehta，Y. Zhang，C. Karthik，B. Singh，R.W. Siegel，T. Borca-Tasciuc，G.Ramanath, A new class of doped nanobulk high-figure-of-merit thermoelectrics by scalable bottom-up assembly, Nat. Mater. 11 (2012) 233-240. [8] X. Wang, K.F. Cai, H. Liu, Novel gas-induced-reduction route to chalcogenide nanostructures taking Sb2Se3 as an Example, Crys. Growth Des. 11 (2011) 4759–4767. [9] X. Wang, K.F. Cai, F. Shang, S. Chen, Preparation and electrical transport properties of nanostructured Sb2Se3 films fabricated by combining spin-coating and gas-induced reduction, J. Nanopart. Res. 15 (4) (2013) 1-8. [10] X. Wang, K. F. Cai, B. J. An, S. Shen, Gas induced reduction synthesis of Sb2Te3 and Bi0.5Sb1.5Te3 nanosheets and their evolvement mechanism, J. Materiomics, 1(2015)316-324. [11] Y. Du, K. F. Cai, H. Li, and B. J. An. The influence of sintering temperature on the microstructure and thermoelectric properties of n-type Bi2Te3-xSex nanomaterials, J. Electron Mater. 40(2011) 518-522. [12]. S. Chen, K. F. Cai and F. Y. Li, The effect of Cu addition on the system stability and thermoelectric properties of Bi2Te, J. electron. Mater., 43(2014) 1966-1971. [13] G. J. Snyder, E.S. Toberer, Complex Thermoelectric Materials. Nat. Mater. 7(2008) 105–114. [14] J. Yang, D.T. Morelli, G.P. Meisner, W. Chen, J.S. Dyck, C. Uher, Effect of Sn substituting for Sb on the low-temperature transport properties of ytterbium-filled skutterudites, Phys. Rev. B. 67 13
(16) (2003) 165207. [15] F. K. Lotgering, Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures, J. Inorg. Nucl. Chem. 9 (1959) 113–123.
Supporting materials
SFig.1 XRD pattern for the hot pressed Cu0.05Bi2SexTe3-x with （a）x=0.2,(b)x=0.3,(c)x=0.4
14
SFig.2
Parameters for the Cu0.05Bi2SexTe3-x samples as a function of x value, (a) a-axis, (b) c-axis.
SFig.3 Carrier concentration and mobility of the samples with different amounts of Bi NPs added
15
PII: DOI: Reference:
S0272-8842(16)31731-X http://dx.doi.org/10.1016/j.ceramint.2016.09.200 CERI13860
To appear in: Ceramics International Received date: 14 July 2016 Revised date: 27 September 2016 Accepted date: 28 September 2016 Cite this article as: Song Chen, Kefeng Cai and Shirley Shen, Influence of Sedoping and/or Bi addition on microstructure and thermoelectric properties of Cu0.05Bi2Te3, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.09.200 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Se-doping and/or Bi addition on microstructure and thermoelectric properties of Cu0.05Bi2Te3 Song Chen1,2, Kefeng Cai1*, Shirley Shen3 1
Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education; School of
Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China 2
School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350108, China 3
CSIRO Manufacturing, Gate 3, Normanby Road, Clayton VIC 3190, Australia *
Corresponding author. [email protected]
Abstract Cu0.05Bi2SexTe3-x (x=0.2, 0.3, 0.4) bulk samples were prepared by hot pressing corresponding nanopowders synthesized by a gas-induced reduction method. Se doping significantly increases the carrier concentrations and hence the electrical conductivities of the samples. Based on this, a Cu0.05Bi2Se0.3Te2.7 sample with a higher power factor was chosen as a pristine sample; some of the Cu0.05Bi2Se0.3Te2.7 nanopowders were mixed with various amounts of Bi nanoparticles (NPs) and then hot pressed into bulk samples. The addition of Bi NPs promotes the sintering of samples. When the content of Bi NPs added is low (≤1 wt%), the electrical conductivity is higher than the pristine sample, and all the Bi NPs added samples have higher Seebeck coefficient than the pristine sample, hence, the power factor has been improved. A maximum power factor, µWcm-1K-2, has been obtained for the 2 wt.% Bi NPs added sample. Keywords: Bi2Te3; thermoelectric; doping; nanostructure.
1
1. Introduction Bi2Te3 based alloy is the best thermoelectric (TE) material system at around room temperature. It has been reported that nanostructuring can improve TE figure of merit, ZT(= where is the Seebeck coefficient, electrical conductivity, thermal conductivity of the materials and T is the absolute temperature) value of Bi2Te3 based alloys [1-7]. Two approaches have been used to obtain nanostructured Bi2Te3 based alloys: top-down and bottom-up. Top-down approaches, such as melt-spinning followed by spark-plasma-sintering [4], have been reported to produce p-type bulk Bi2Te3-based materials with high ZT values. Bottom-up approaches have also been reported to prepare Bi2Te3-based materials with high ZT values [5-7]. For instance, Mehta et al. [7] reported Bi2Te3-based materials with high ZT values (>1 at 300 K) prepared by a microwave-stimulated wet-chemical technique followed by cold pressing and then vacuum sintering. Recently, we developed a novel and simple gas-induced-reduction (GIR) method to synthesize chalcogenide nanostructures [8-9], which is also applicable to Bi2Te3 based alloys [10]. As nanopowders have very high specific surface area and will adsorb oxygen on their surface when they are exposed to air. The adsorbed oxygen will make Bi2Te3 based alloys be slightly oxidized during hot-pressing at elevated temperatures [11]. Mehta et al. [7] reported that sulfur doping can inhibit the oxidation of Bi2Te3-based materials. Recently, we [12] found that Cu doping (CumBi2Te3, m=0, 0.01, 0.025, 0.05) can also inhibit the oxidation of Bi2Te3-based materials as well as improve the TE properties (the Cu0.05Bi2Te3 sample has better TE properties than the other samples). Therefore, in this work, we chose Cu0.05Bi2Te3 as the pristine sample, and firstly studied the effect of Se doping on the electrical transport properties of the Cu0.05Bi2Te3 [Cu0.05Bi2SexTe3-x (x=0.2, 0.3, 0.4)] and then the effect of addition of Bi nanoparticles (NPs) on the electrical transport properties of Cu0.05Bi2Se0.3Te2.7. 2
2. Experimental The precursor solution for synthesis of Cu0.05Bi2SexTe3-x (x=0.2, 0.3, 0.4)
was prepared as
follows: 4 mmol Bi(NO3)3·5H2O [analytical reagent (AR)], 0.1 mmol Cu(NO3)2·H2O (AR), 2x mmol SeO2 (AR), 6-2x mmol TeO2 (AR), 1.5 mL nitric acid and 80 mL ethylene glycol (EG) (AR) were added to a 100-mL beaker sequentially. The mixture was then heated to 100 oC with magnetic stirring until it became transparent. Alumina crucible cleaned with nitric acid and distilled water and then dried in air was placed at the bottom of a Teflon-lined autoclave. 20 mL of the precursor solution was poured into the alumina crucible, and 2 mL of 85 vol.% hydrazine hydrate was injected into the bottom of the Teflon container. The autoclave was sealed and heated to 250 oC at a heating rate of 3 oC /min, and held for 24 h, then naturally cooled to room temperature. The precipitates were collected and washed with deionized water and absolute ethanol in sequence for several times, then separated by centrifugation at 3900 rpm for 5 min and finally dried in vacuum at 70 oC. Bi NPs (~100 nm) were prepared by dissolving Bi(NO3)3·5H2O in EG and through microwave assisted reactions (in a 700 W microwave oven) with a working frequency of 2.45 GHz. Some of the Cu0.05Bi2Te3 powders were mixed with 0.5, 1, or 2 wt% of the Bi NPs using an agate motar. All the powders were hot pressed into pellets (10 mm in diameter and about 1 mm in thickness) at 723 K for 2 h at 80 MPa. The pellets were cut into rectangles (10 × 2 × 2 mm3) for electrical conductivity and Seebeck coefficientmeasurements. The measurements were carried out using a home-made computer control test system under argon atmosphere. The measurement was performed by a steady-state four-probe technique with a square wave current (~10 mA in amplitude). The Seebeck coefficient value was determined by the slope of the linear relationship 3
between the thermal electromotive force and temperature difference (~10 K) between the two ends of each sample. The powder and bulk samples were examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation (λ = 1.5406 Å). The microstructure of the samples was examined by field-emission scanning electron microscopy (FESEM, Quanta 200; FEG). For elemental analyses of the bulk samples, ICP-OES (Optima 8000, PerkinElmer) was used.
3. Results and discussion 3.1 Preparation and TE Properties of Se doped Cu0.05Bi2Te3 XRD analysis (see supporting materials) indicates that the XRD peaks for Se-doped Cu0.05Bi2Te3 can be indexed to standard data for Bi2Se0.3Te2.7（JPCDS card: 50-0954). The lattice parameters (a and c) of the samples calculated from the XRD data both decrease as the doping amount of Se increases (see supporting materials). It suggests that Se atoms substitute Te atoms in the Bi2Te3 crystal structure. As the radius of Se atom is smaller than that of Te atom, the lattice constant decreases after Se doping. FESEM observation (not shown here) reveals that as the Se content increases, the samples become denser and that the grain sizes of the samples become bigger. Fig.1 shows temperature dependence of electrical transport properties of the Cu0.05Bi2SexTe3-x samples from RT to 550 K. The electrical conductivity of the Cu0.05Bi2SexTe3-x samples decreases with the increasing of temperature in the whole temperature range, exhibiting a metallic conducting behavior. At a given temperature, the electrical conductivity increases with increasing of Se content. The increase in electrical conductivity is mainly because of the increase of the carrier concentration (see Fig.2 hereinafter).
4
Fig. 1 Temperature dependence of TE properties (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor of the Cu0.05Bi2SexTe3-x samples
Fig.1 b shows temperature dependence of Seebeck coefficient of the samples. The Seebeck coefficient values are negative, indicating that they are n-type semiconductors. As the Se content increases, the Seebeck coefficient value decreases. This is because the carrier concentration increases after Se doping (see Fig. 2), and the Seebeck coefficient () is inversely proportional to the carrier concentration, n, as indicated by the following simple model of electron transport [13]: 2
8 2 k B2 * 3 mT 3eh 2 3n
(1)
where kB, h, and m* are the Boltzmann constant, Planck constant, and effective mass of the carrier,
5
respectively. The Seebeck coefficient of the samples first increases with temperature, and when T >430 K it turns to decrease with temperature. The decreasing of the Seebeck coefficient value at elevated temperatures is due to the start of intrinsic excitation and two-band conduction from both electrons and holes. Two-band conduction tends to reduce the total Seebeck coefficient, as is evident from the Mott relation [14]: nnppnp where αn and αp are the negative and positive Seebeck coefficients, respectively, and σn and σp are the corresponding conductivities of each band. The power factor of the sample is shown in Fig 1(c). The Cu0.05Bi2Se0.3Te2.7 sample has a higher power factor among the studied samples with a maximum power factor of µWcm-1K-2. Fig.2 shows the carrier concentrations and motilities of the samples at RT. Obviously, the carrier concentration increases with the Se content increasing. It may be because Se doping introduces a donor doping-level and hence increases the carrier concentration. However, the mobilities of the samples are very similar.
Fig.2 The carrier concentrations and mobilities of the Cu0.05Bi2SexTe3-x (x=0, 0.2, 0,3, 0.4) samples at RT
3.2 Preparation and properties of the Bi NPs added Cu0.05Bi2Se0.3Te2.7 6
Since the Cu0.05Bi2Se0.3Te2.7 sample has a higher power factor than other samples (Fig. 1), we further study the effect of Bi NPs addition on the TE properties of the Cu0.05Bi2Se0.3Te2.7. Fig.3 presents the XRD patterns of the Cu0.05Bi2Se0.3Te2.7 nanopowders mixed with various amount of Bi NPs after hot pressing. The XRD peaks of the samples can still be indexed to the standard data of Bi2Se0.3Te2.7（JPCDS card: 50-0954) and no XRD peak for Bi is detected. It may be because that the amount of Bi NPs added is below the resolution of the instrument and/or the Bi reacted with the Cu0.05Bi2Se0.3Te2.7 to form solid solution during hot pressing. The intensity of (00l) diffraction peaks increases with the increasing of the content of Bi NPs added, indicating that the addition of Bi NPs is beneficial to the preferential growth of Cu0.05Bi2Se0.3Te2.7 grains. The degree of orientation was evaluated in terms of the Lotgering factor f, which is calculated using the following equation [15]: f
p p0 1 p0
(3),
where p0=∑I0(00l)/∑I0(hkl) and p=∑I(00l)/∑I(hkl). I0 and I are the intensities of each of the diffraction peaks in XRD patterns as presented in ICSD data (JPCDS card: 50-0954) and those determined experimentally, respectively. The degree of orientation, f, is 7.33%, 10.12%，17.02% and 18.53% for the samples with 0, 0.5, 1, and 2% Bi NPs added, respectively. With the increasing of the content of Bi NPs added, the orientation degree of the samples increases. It may be because the melting point of Bi is only 544.44 K, which is much lower than the hot pressing temperature (723 K), the Bi NPs were in molten state during the hot pressing. Namely, the Bi NPs added samples were sintered with liquid-phase; hence the grains were easier to grow up preferentially.
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Fig. 3 XRD patterns of the hot pressed samples with 0.5, 1, 2 % Bi NPs added
Fig. 4 shows the cross-section morphologies of the hot pressed samples. As the content of Bi NPs added increases, the average gain size of the samples increases. Each grain, in fact, consists of many nanosheets (thickness <100 nm), which is clearly shown in the high magnification FESEM image (see Fig. 4d).
Fig. 4 FESEM images of the hot pressed samples with (a) 0.5, (b) 1, (c)2% Bi NPs added, (d) enlarged image corresponding to the area marked by the white square in (c)
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Fig. 5(a) shows the electrical conductivity of the samples from RT to 523 K. The samples with 0.5, 1 wt.% Bi NPs added have higher electrical conductivity than the pristine sample, whereas the one with 2 wt.% Bi NPs added has a lower electrical conductivity when the temperature lower than 450 K. At a given temperature, the electrical conductivity increases when the content of Bi NPs added is low (≤1 %). This might be because of the increasing degree of orientation, which is beneficial to electrical conductivity. However, for the 2% Bi NPs added sample, its degree of orientation is similar to that of the 1% Bi NPs added sample, and its mobility is significantly low (see supporting materials) probably due to containing nano- to submicro-pores (see Fig. 4d), leading to its lower electrical conductivity. It is seen from Fig. 5a that the electrical conductivity of all the Bi NPs added samples first decreases with temperature and then increases with temperature. The electrical conductivity turning to increase at higher temperatures may be due to intrinsic excitation of the samples. The turning point is about 475, 450, and 400 K for samples with 0.5, 1, and 2 wt.% Bi NPs added, respectively, indicating that the band gap for the samples becomes narrow as the content of Bi NPs added increases. This suggests that the added Bi NPs reacted with Cu0.05Bi2Se0.3Te2.7 to form a solid solution. Fig. 5 (c) shows that Seebeck coefficients of the samples from RT to 523 K. Seebeck coefficient of all the samples is negative, indicating that the samples still maintain the n-type conduction. The absolute Seebeck coefficient of the samples increases with increasing of the content of Bi NPs added at T < ~450 K and it is about 210 V/K at RT for the 2 wt.% Bi NPs added sample. The added Bi NPs melted during the hot pressing and some of the Bi might enter the crystal structure of Cu0.05Bi2Se0.3Te2.7 as interstitial atoms (since the position of the XRD peaks almost did not shift). The interstitial atoms as well as the above-mentioned pores scatter carriers so that the 9
Seebeck coefficient increases. The absolute Seebeck coefficients for all the samples reduce at higher temperatures and converge to ~about 160V/K. The reduction of the Seebeck coefficient value at high temperatures is due to the start of intrinsic excitation and two-band conduction from both electrons and holes as equation (2) described. Note that the chemical formulas of the samples given above are nominal ones. According to ICP-OES analysis, the chemical composition (wt.) of the 2 wt.% Bi NPs added sample is: Cu 0.11 %, Bi 56.76%, Se 1.03%, Te 42.28%. If the total atomic number of Se and Te is fixed to 3, the chemical formula of the sample is calculated to be Cu0.015Bi2.37Se0.038Te2.962. The atomic number of Cu and Se is smaller than we originally designed, and the latter should be because Se is relatively easier to volatilize during hot pressing; the atomic number of Bi is higher than 2, which should be due to additional Bi NPs added. The power factors of the samples are shown in Fig. 5 (c). Generally, the power factor of all the samples decreases with increasing temperature. Around RT, it increases obviously with the content of Bi NPs added. A maximum power factor, µWcm-1K-2, being 75% higher than that of the pristine sample, has been obtained for the 2 wt.% Bi NPs added sample. It indicates that adding Bi NPs is very efficient to improve the electrical transport properties. Since the composition of the 2% Bi NPs added sample (Cu0.015Bi2.37Se0.038Te2.962) is more complicated than that of the Cu0.05Bi2Te3 sample, which has a thermal conductivity of ~0.95 W/mK at RT [12]; it is deduced that the 2 wt.% Bi added sample should have a thermal conductivity not higher than 0.95 W/mK, and ZT value of this sample is estimated to be higher than 0.97.
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Fig.5 temperature dependence of the electrical conductivity (a) Seebeck coefficient (b) and power factor (c) of the samples with various amounts of Bi NPs added
4. Conclusions In summary, after Se doping, the carrier concentration of the Cu0.05Bi2Te3 sample increases significantly and hence the electrical conductivity increases. The Cu0.05Bi2Se0.3Te2.7 sample has a higher power factor than the other samples. The electrical transport properties of the Cu0.05Bi2Se0.3Te2.7 sample have been furtherly improved by addition of Bi NPs. The addition of Bi NPs can promote the sintering of the samples. When the content of added Bi NPs is low (≤1 wt%), the electrical conductivity is higher than the pristine sample. However, all the Bi added samples have higher Seebeck coefficient than that of the pristine sample. The power factors have been 11
improved significantly. A maximum power factor, µWcm-1K-2, has been obtained for the sample with 2 wt.% added Bi NPs, indicating it is an efficient method to improve the electrical transport properties of Bi2Te3 based materials.
Acknowledgments This work has been supported by National Natural Science Foundation of China (51271133), the 973 Project under Grant no. 2013CB632500, and the foundation of the State Key Lab of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, Grant No. 2016-KF-2).
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Supporting materials
SFig.1 XRD pattern for the hot pressed Cu0.05Bi2SexTe3-x with （a）x=0.2,(b)x=0.3,(c)x=0.4
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SFig.2
Parameters for the Cu0.05Bi2SexTe3-x samples as a function of x value, (a) a-axis, (b) c-axis.
SFig.3 Carrier concentration and mobility of the samples with different amounts of Bi NPs added
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