Flower-like nanostructure and thermoelectric properties of hydrothermally synthesized La-containing Bi2Te3 based alloys

Materials Chemistry and Physics 103 (2007) 484–488

Flower-like nanostructure and thermoelectric properties of hydrothermally synthesized La-containing Bi2Te3 based alloys Y.H. Zhang a,b , T.J. Zhu a , J.P. Tu a , X.B. Zhao a,∗ a

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China b Academy of Equipment Command & Technology, Beijing 102249, PR China

Received 13 June 2005; received in revised form 30 November 2006; accepted 19 February 2007

Abstract Single-phase La-containing Bi2 Te3 based nanopowders have been synthesized via a hydrothermal route using BiCl3 , LaCl3 , selenium and tellurium powders as the reactants, NaBH4 as the reductant, NaOH as the pH-value controller and ethylenediaminetetraacetic disodium salt (EDTA) as the additive. The synthesized powders have a flower-like morphology assembled by the side-by-side arrangement of the nano-sheets. Rietveld refinement analyses of crystal structure indicated that the alloyed lanthanum substituted bismuth at the 6c lattice position of Bi2 Te3 based compounds. The thermoelectric property measurements of the hot-pressed samples showed that the substituting lanthanum in the Bi2 Te3 based compounds was an n-type dopant. A ZT value of about 0.58 has been obtained for the La0.2 Bi1.8 Te3 alloy at 480 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Thermoelectric materials; Hydrothermal synthesis; Crystal structure

1. Introduction Thermoelectric (TE) materials are a class of semiconductor functional materials, which can be used to convert directly heat energy to electricity or reversely. They are of interest for applications in solid-state cooling devices and electrical power generators due to many attractive features such as long life, no moving parts, no emissions of toxic gases, low maintenance, and high reliability [1,2]. Bismuth telluride based alloys are known as one of the best thermoelectric materials currently available in the temperature range of 200–400 K [3]. However, their highest dimensionless figures of merit, ZT = (α2 σ/κ)T, have been close to unity until now [4], where α, σ and κ are respectively the Seebeck coefficient, electrical and thermal conductivity, and T is the temperature. Many efforts have been made to further enhance their thermoelectric properties [5,6]. Both modifying energy band structure by alloying and making the materials nanostructured should be most possible ways to increase the figures of merit of the materials. In the last decade, the preparations and properties of low dimensional TE materials have been intensively investigated, especially those of superlattice thin films and nanowire

arrays. A high ZT value of about 2.4 has been reported recently on Bi2 Te3 /Sb2 Te3 superlattices [7]. ¯ crystal As well-known, Bi2 Te3 has a rhombohedral R3m structure with van der Waals bonds between two neighboring hexagonal layers [8], which make possible the interposition of other atoms with suitable sizes and hence the improvement of the TE properties by modifying the energy band structure and density of state. For example, the low-temperature performances of the alloy was improved dramatically by alloying it with alkali metal cesium [9]. It is reported that the special electronic structures of some rare earth metals can lead to an unusually intermediate valence state in a rare earth intermetallic compound, which may result in a high TE power factor α2 σ [1,10,11]. In our previous work, bismuth telluride nano-powders alloyed with rare earth elements have been successfully prepared by the solvothermal synthesis [12–14]. In this study, we prepared nano-sized Lax Bi2−x Te3 and Lax Bi2−x Sey Te3−y powders by the hydrothermal synthesis method. The microstructure and TE properties of the alloys were experimentally investigated and presented here. 2. Experimental

∗

Corresponding author. Tel.: +86 571 87951451; fax: +86 571 87951451. E-mail address: [email protected] (X.B. Zhao).

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.059

Analytical grade of chemicals LaCl3 , BiCl3 and 5N pure Se and Te powder was used as the precursors for the synthesis of lanthanum-containing Bi2 Te3

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Fig. 1. Observed (grey dotted line), calculated (thin solid line) and difference patterns at the bottom of the figures obtained by Rietveld structural refinement from the powder XRD data of the samples LBT and LBST, vertical bars in the figures indicate the positions of the calculated Bragg peaks. based alloys. The precursors with the nominal compositions of La0.2 Bi1.8 Te3 and La0.2 Bi1.8 Se0.3 Te2.7 were respectively put into a Teflon-lined autoclave filled with about 200 ml distilled water. About 6 g NaBH4 was added as the reductant, 5 g NaOH as the pH-value controller and 0.4 g ethylenediaminetetraacetic disodium salt (EDTA) as the organic complexing reagent. The autoclave was then sealed and maintained at 423 K for 24 h. During the reaction, the solution was stirred by a stainless steel stirrer with a rotational speed of about 360 rpm. After the system was cooled down to room temperature, the dark grey powders in the autoclave was filtered, washed with distilled water, ethanol and acetone for several times to remove the residual precursors and by-products, and dried in vacuum at 373 K for 6 h. The bulk samples were prepared from the synthesized nanopowders by hot uniaxial pressing in a graphite die with the diameter of 16 mm at 523 K for 15 min at a pressure of 75 MPa under a vacuum of about 1 Pa. The phase structures of the powders were investigated by X-ray diffraction (XRD) on a Rigaku D/MAX-2550P diffractometer using Cu K␣ radiation (λ = 0.154056 nm). The data were collected in the 2θ range of 6–120◦ at a step size of 0.02◦ and a counting time of 4 s. The diffraction patterns were analyzed by Rietveld method using Retica software [15] for the refinement of the crystal structures, phase abundance and the lattice parameters. The powder morphology was observed by field emission scanning electron microscopy (FESEM) on a FEI-Sirion microscope. The electric conductivity and Seebeck coefficient of the bulk samples were simultaneously measured using a computer-assisted device. Thermal diffusivities a and special heats CP were measured with a Netzsch LFA-427 laser flash and DSC-404 analyzer, respectively, in Institute of Materials Research, German Aerospace Center (DLR), Germany. To calculate the thermal conductivity, κ = aρCP , the sample density ρ was measured using the Archimedes method. The figures of merit, ZT, of all the samples were cal-

culated by interpolating the measurement data of α, σ and κ at given temperatures.

3. Results and discussion Fig. 1(a) and (b) give the XRD patterns of the hydrothermally synthesized La-containing powders with the nominal compositions La0.2 Bi1.8 Te3 (LBT) and La0.2 Bi1.8 Se0.3 Te2.7 (LBST), respectively. The calculated and the difference patterns between the measured and calculated patterns by Rietveld structural refinement are also plotted in Fig. 1. From the difference patterns, it is clear that the calculated patterns are well consistent with the measured ones. The measured XRD patterns in Fig. 1 indicate that the samples LBT and LBST have a single phase ¯ rhombohedral structure as that of binary Bi2 Te3 . Since of R3m there are no visible diffraction peaks of La and Se or their compounds in the XRD patterns, it is believed that they have entered the lattice of Bi2 Te3 . The lattice parameters of sample LBT and LBST were calculated using Rietveld refinement. The occupancies of La, Bi, Te and Se atoms in the lattices were obtained by minimizing the square differences between the calculated diffraction patterns and the measured XRD patterns. The Rietveld refinement results for both alloys were given in Table 1. LBT and LBST

Table 1 Rietveld refinement results for the samples LBST and LBT synthesized hydrothermally Nominal composition La0.2 Bi1.8 Te3 Space group Atom Wyckoff site Occupancy x y z ˚ Lattice parameters (A) Theoretical density (g cm−3 ) ˚ 3) Unit cell volume (A Pattern factor, RP Weighted pattern factor, RWP Standard deviation, S.D.

¯ (166) R3m La Bi 6c 6c 0.0919 0.9081 0 0 0 0 0.4007 0.4007 a = 4.3872, c = 30.4558 7.74 507.7 2.56% 8.71% 1.84

La0.2 Bi1.8 Se0.3 Te2.7 Te(1) 6c 1.0023 0 0 0.2083

Te(2) 3a 0.9895 0 0 0

¯ (166) R3m La Bi 6c 6c 0.1476 0.8524 0 0 0 0 0.3986 0.3986 a = 4.3819, c = 30.4652 7.47 506.9 2.45% 7.5% 1.75

Se(1) 6c 0.0729 0 0 0.2098

Te(1) 6c 0.9271 0 0 0.2098

Se(2) 3a 0.2909 0 0 0

Te(2) 3a 0.7091 0 0 0

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Fig. 2. SEM images of La-containing Bi2 Te3 based alloys powders hydrothermally synthesized for LBT (a and b) and LBST (c and d).

have a layered hexagonal structure similar to that of Bi2 Te3 , in which atom layers arrange along the c-axis direction in the order of –Te(1) –Bi–Te(2) –Bi–Te(1) –, here the superscripts of Te denote the two different coordinative types of tellurium atoms. Te(2) atoms are octahedrally surrounded by Bi atoms. Three of Te(1) atoms in a same layer bond with Bi atoms, and another three of them bond with the Te(1) atoms in the neighboring layer via van der Waals force, where a Bi2 Te3 crystal is easy to cleave. As shown in Table 1, La substitutes for Bi at the 6c position and Se substitutes for Te at the 6c and 3a positions in the lattice structure of the samples LBT and LBST, showing that the LBT and LBST alloys prepared by the hydrothermal synthesis in the present work are lanthanum substituting Bi2 Te3 based compounds. The result is different from the case for CsBi4 Te6 alloys, where the Cs ions lie between the [Bi4 Te6 ] layers and undergo considerable “rattling” motion [9]. The lattice parame˚ c = 30.45(6) A; ˚ ters of LBT are calculated to be: a = 4.387(2) A, ˚ c = 30.46(5) A. ˚ Comand those of LBST are: a = 4.381(9) A, pared to the lattice parameters of binary Bi2 Te3 given by JCPDS ˚ c = 30.44(1) A, ˚ the parameters a are 82-0358, a = 4.395(3) A, slightly shorter but c are almost same for the samples LBT and LBST. The little change for the alloyed crystal structures can be contributed to the small differences between the covalent ˚ and Bi atoms (1.69 A), ˚ and Se (1.16 A) ˚ radii of La (1.46 A) ˚ and Te atoms (1.36 A). From the occupancy percent in Table 1, the chemical formula of LBT is La0.18 Bi1.82 Te3 , which agrees well with the nominal composition La0.2 Bi1.8 Te3 . However, the chemical formula of LBST is La0.29 Bi1.71 Se0.44 Te2.56 , which is different from the nominal composition La0.2 Bi1.8 Se0.3 Te2.7 ,

and has higher contents of La and Se. It is possibly because there is higher intensity at the (1 1 0) diffraction peak than at the (1 0 1 0) in the observed pattern of LBST, but it is reverse in the calculated one. The SEM images in Fig. 2 show that the hydrothermally synthesized LBT and LBST powders have three levels of morphologies: micron-sized flower-like agglomerates, petal-like polygons with a thickness of 30 nm, and fine nano-sheets crystals. It can be seen from Fig. 2(a) and (c) that the nano-sheets with a thickness of tens of nanometers of the samples LBT and LBST arrange almost parallelly, and connect each other to form about 1 ␮m petals in the lateral size. Then these petals assembly to large flower-like clusters. It should be noted that the clusters of the sample LBST (Fig. 2(c)) are more complete and regular than those of the sample LBT (Fig. 2(a)). The formation of the unique flower-like La-containing Bi2 Te3 based alloys during the hydrothermal synthesis should be related to the layered anisotropic lattice structure. During the synthesis process, a free Te atom or Te2− ion tends to bond with the atoms on the growing crystal surface via covalent bonding. A single Te atom or Te2− ion from the solution attaching itself to a Te(1) layer crystal surface will probably jump back into the solution, since a van der Waals bonding is not strong enough to hold the atom on the atomic surface. Therefore, a Bi2 Te3 crystal will grow faster in the a- and b-axis directions than in the c direction [16]. This growth mechanism should lead to a sheet-like crystal morphology during the hydrothermal synthesis process as shown in Fig. 2. EDTA added as a chelating agent during the hydrothermal synthesis can combine with Bi3+ ions to form large molecu-

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Fig. 3. SEM images of the hot-pressed LBT (a) and LBST (b) alloys.

lar groups, which facilitates the Bi2 Te3 nano-sheets to grow along the surfaces of EDTA agglomerates and provides the structure-directing to the growth of the hydrothermally synthesized Bi2 Te3 crystals [17]. The movement of these fine sheet-like crystals together with EDTA in the solution makes it possible for neighboring nano-sheets to laterally connect each other by the suspended bonds at edges and consequently form large petal-like sheets as shown in the amplified image in Fig. 2(d) taken from a cluster of the sample LBST. It can be also seen that the joints between the nano-sheets are imperfect as shown in Fig. 2(d), and there exist small interspaces, mismatches and steps among those petal-like sheet crystals. Thus, other nano-sheets can connect to the joints due to the existence of the suspended bonds at the imperfects and deflect by an angle as demonstrated in Fig. 2(d). The continuous nucleation, growth and coalescence of nano-sheets result in the flower-shaped morphology with a center projection as shown in Fig. 2(b) and (d). As mentioned above, the growth of Bi2 Te3 based alloys should be mainly in the a- and b-axis directions, i.e., along the base plane of the crystal. Therefore, various flower-like morphologies of the hydrothermally synthesized Bi2 Te3 based alloys consist of many petal-like sheet crystals with the side-by-side arrangement of many nano-sheets. The hydrothermal synthesized La-containing nanopowders were hot-pressed to the disk-like bulk samples. By comparing the measured densities of the hot-pressed samples of LBT and LBST, 7.11 and 7.20 g cm−3 , respectively, with the calculated densities by the Rietveld structural refinement, 7.74 and 7.47 g cm−3 , it is found that the relative densities of the hotpress samples LBT and LBST were respectively 91.9% and 96.4%. Fig. 3(a) and (b) shows the microstructure images of the hot-pressed LBT and LBST samples, respectively. It is evident that there are the similar sheet-shaped microstructures for both the samples. The sheet crystals significantly grew up during the hot pressing. However, the thickness of the sheet crystals is still lower than 100 nm, which should be beneficial to the improvement of the TE properties of Bi2 Te3 based alloys. Fig. 4 shows the temperature dependences of the TE properties of the hot-pressed LBT and LBST alloys. Both the samples exhibit n-type conduction as they have the negative Seebeck coefficients in the measured temperature range shown

in Fig. 4(a), indicating that La atoms substituting the 6c positions of Bi in Bi2 Te3 crystal cells should be a donor dopant. The intrinsic excitation of both the samples occurs at about 480 K, implying that they have the similar band gaps, which are higher than that of binary Bi2 Te3 , and that the energy band structure of the LBST alloy is not remarkably affected by the substitution of selenium for tellurium. The Seebeck coefficients of both the samples decrease with the increase of temperature above 480 K due to the rapid increase of the minor carriers. As can be seen from Fig. 4(b), the electrical conductivities of the samples LBT and LBST decrease with the increase in temperature, exhibiting the metallic conduction, which suggests high carrier concentrations in the LBT and LBST samples prepared by hot pressing of the hydrothermally synthesized nano-powders. The La-containing sample LBST has the higher Seebeck coefficients and electrical conductivities than the sample LBT. Although selenium element substituting for Te in the LBST alloys introduced a number of point defects, which are expected to effectively reduce the short-wave phonon scattering and hence thermal conductivity, the thermal conductivity of the Se-containing alloy LBST, in the present work, is higher than that of the LBT alloy, which is about 0.9 W m−1 K−1 at room temperature as shown in Fig. 4(c). This is contributed to the lower density of the bulk sample of LBT than LBST. On the other hand, the existence of La element in both the LBST and LBT alloys can also scatter the short-wave phonons strongly so as to weaken the contribution of Se element to the reduction of the phonon thermal conductivity of LBST. Fig. 4(d) summarizes the temperature dependence of the dimensionless figures of merit calculated from ZT = (α2 σ/κ)T. Because of the lower thermal conductivity of the sample LBT, the ZT values of LBT are higher than those of LBST over the whole measured temperature range. The highest ZT value for LBT is about 0.58 at 460 K, higher than that of LaBi2 Te3 alloys synthesized by the solvothermal process [18]. Although the ZT values of the two samples are inferior to the best Bi2 Te3 -based thermoelectric materials, the improvement of the TE properties of nanostructured Bi2 Te3 based thermoelectric materials hydrothermally or solvothermally synthesized remains desirable by the optimization of carrier concentration or doping. Especially, fundamental investigations about the effect of rare earth in Bi2 Te3 on ther-

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Fig. 4. Thermoelectric properties of La-containing Bi2 Te3 alloys hot pressed from hydrothermally synthesized nanopowders. (a) Seebeck coefficient; (b) electric conductivity; (c) thermal conductivity; (d) dimensionless figure of merit.

moelectric transport properties of Bi2 Te3 based TE alloys should be interesting and beneficial for developing novel TE materials.

the National “863” Hi-Tech Project of China with Contract No. 2002AA302406. References

4. Conclusions Nanostructured La-containing Bi2 Te3 based TE compounds were synthesized by the hydrothermal method. As-synthesized nanopowders exhibited a flower-like structure, which was formed by the assembly of nano-sheet crystals. The diffraction patterns of the hydrothermally synthesized powders analyzed by Rietveld refinement method indicated that lanthanum substituted bismuth at the 6c lattice positions of the Bi2 Te3 based compounds. The thermoelectric property measurements of the hot-pressed samples reveal that TE properties are deeply affected by the defects in thermoelectric alloys and the substituting lanthanum in the Bi2 Te3 based compounds is an n-type dopant. The present results show that the substituting lanthanum can indistinctively improve the TE properties of Bi2 Te3 based compounds. Further optimization for the compositions and doping is needed. Acknowledgments The authors would like to thank Dr. E. M¨uller and his colleagues at DLR, Germany for the thermal conductivity measurement. This work was supported by the National Natural Science Foundation of China under Grant No. 50471039, and

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

F.J. DiSalvo, Science 285 (1999) 703. B.C. Sales, Science 295 (2002) 1248. C.B. Vining, Nature 423 (2003) 391. T.M. Tritt, Science 283 (1999) 804. X.B. Zhao, X.H. Ji, Y.H. Zhang, J.P. Tu, X.B. Zhang, Appl. Phys. Lett. 86 (2005) 062111. J. Jiang, L.D. Chen, Q. Yao, Q. Wang, Mater. Trans. 46 (2005) 1. R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001) 597. J.R. Drabble, C.H.L. Goodman, J. Phys. Chem. Solids 5 (1958) 142. D.Y. Chung, T. Hogan, P. Brazis, M. Rocci-Lane, C. Kannewurf, M. Bastea, C. Uher, M.G. Kanatzidis, Science 287 (2000) 1024. D.M. Rowe, G. Min, V.L. Kuznestsov, Phil. Magn. Lett. 77 (1998) 105. J.F. Meng, D.A. Polvani, C.D.W. Jones, F.J. DiSalvo, Y. Fei, J.V. Badding, Chem. Mater. 12 (2000) 197. X.B. Zhao, Y.H. Zhang, X.H. Ji, Inorg. Chem. Commun. 7 (2004) 386. X.H. Ji, X.B. Zhao, Y.H. Zhang, B.H. Lu, H.L. Ni, J. Alloy Compd. 387 (2005) 282. X.H. Ji, X.B. Zhao, Y.H. Zhang, B.H. Lu, H.L. Ni, Mater. Lett. 59 (2005) 682. D.B. Wiles, R.A. Young, J. Appl. Cryst. 14 (1981) 149. X.B. Zhao, X.H. Ji, Y.H. Zhang, G.S. Cao, J.P. Tu, Appl. Phys. A 80 (2005) 1567. Y. Deng, C.W. Nan, G.D. Wei, L. Guo, Y.H. Lin, Chem. Phys. Lett. 374 (2003) 410. X.H. Ji, X.B. Zhao, Y.H. Zhang, B.H. Lu, H.L. Ni, Mater. Lett. 59 (2004) 682.