Journal of ELECTRONIC MATERIALS, Vol. 38, No. 7, 2009
Special Issue Paper
DOI: 10.1007/s11664-009-0711-9 Ó 2009 TMS
Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi0.5Sb1.5Te3 Compared with its Solvothermally Prepared Counterpart CHIA-JYI LIU,1,2 GAO-JHIH LIU,1 CHUN-WEI TSAO,1 and YO-JHIH HUANG1 1.—Department of Physics, National Changhua University of Education, Changhua 50007, Taiwan. 2.—e-mail:
[email protected];
[email protected]
We report on the successful hydrothermal synthesis of Bi0.5Sb1.5Te3, using water as the solvent. The products of the hydrothermally prepared Bi0.5 Sb1.5Te3 were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi0.5Sb1.5Te3 were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi0.5Sb1.5Te3 at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 9 1018 cm3 and 1.42 9 1019 cm3 for the hydrothermally prepared Bi0.5Sb1.5Te3 and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 lW/cm K2 and 2.9 lW/cm K2 for the hydrothermally prepared Bi0.5Sb1.5Te3 and solvothermally prepared sample, respectively. Key words: Bismuth telluride alloy, hydrothermal synthesis, solvothermal synthesis, power factor, thermoelectrics
INTRODUCTION Thermoelectric materials are viewed as energy and environmentally friendly materials, since they could be used as power generators via the Seebeck effect or as chillers via the Peltier effect without the usage of coolants. The figure of merit (Z) of thermoelectric materials is determined by three transport parameters and can be expressed as Z = rS2/j, where r, S, and j are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. Bismuth alloys show the highest dimensionless figure of merit ZT around room temperature and are one of the most commercialized thermoelectric materials. Turning bulk thermoelectric materials into nanomaterials might help improve the thermoelectric figure of merit. The density of states of nanomaterials is expected to increase the Seebeck coefficient; phonon (Received June 30, 2008; accepted February 3, 2009; published online February 27, 2009)
scattering of nanomaterials is expected to reduce the thermal conductivity.1,2 Epitaxial growth of Bi2Te3 nanoplatelets from the surface of a Te tube wall has been achieved via a two-step process using the solvothermal process.3,4 The hot-pressed nanocomposite exhibited an enhanced thermoelectric figure of merit, which was ascribed to the reduced thermal conductivity as a result of the efficient phonon blocking effect.5 We previously reported on the preparation of Bi0.5Sb1.5Te3 mats containing nanosized sheet-tubes by the solvothermal method, using dimethylformamide (DMF) organic solvent.6 Since DMF has a nasty odor and might cause liver damage, we attempted to use water to replace DMF as the reaction medium and successfully synthesized Bi0.5Sb1.5Te3. In this paper, we describe the hydrothermal synthesis of Bi0.5Sb1.5Te3 and compare the thermoelectric characteristics of the sintered nanoplatelets with those of the solvothermally synthesized samples. We demonstrate that the power factor of the hydrothermally synthesized Bi0.5Sb1.5Te3 is 1499
1500
C.-J. Liu, G.-J. Liu, Tsao, and Huang
approximately four times better than that of its solvothermally synthesized counterpart. EXPERIMENTAL PROCEDURE Nanoplatelets of Bi2xSbxTe3 were synthesized by the hydrothermal or solvothermal method. For the samples synthesized by the solvothermal method, the procedure has been described elsewhere.6 In the case of x = 1.5, synthesized by the hydrothermal method, Te powders (6 mmol), BiCl3 (1 mmol), SbCl3 (3 mmol), ethylenediamine tetra-acetic acid (EDTA, 8 mmol), and de-ionized water (70 mL) were quantitatively added into a polytetrafluoroethylene (PTFE) cylindrical container and stirred well at room temperature for 1 h. KOH (70 mmol) was then added and the solution was stirred for 1 h, followed by the addition of NaBH4 (24 mmol) and stirring for another 0.5 h. The PTFE container with the solution was transferred to a high pressure bomb and heated at 140°C for 36 h in a box furnace. NaBH4 was required to inhibit the formation of Bi2O3. The resulting powders were washed in de-ionized (DI) water and then absolute ethanol, followed by drying at 80°C. The resulting powders were compacted into parallelepipeds and sealed in an evacuated Pyrex ampoule for sintering at temperatures of 300°C for 10 h. For the solvothermal method, using DMF as solvent, we adopted a similar procedure, except that we used a different molar ratio of NaBH4/Te. Powder x-ray diffraction (XRD) patterns were obtained with a Shimadzu XRD-6000 diffractometer equipped with Fe Ka radiation. The morphology of the samples was observed with a field emission scanning electron microscope (FE-SEM; Hitachi 4300). The thickness of the nanoplatelets was determined by an atomic force microscope (AFM) using AC mode (Molecular Imaging, PicoScan 2100) by oscillation of the cantilever assembly at or near the cantileverÕs resonant frequency using a piezoelectric crystal, for which a Si tip (Nanosensors, force constant 48 N/m; resonance frequency 190 kHz) was vibrated vertically while scanning above the film surface. The composition of the nanoplatelets was determined with a JEOL JEM-2010 transmission electron microscope (TEM) equipped with an Oxford INCAx sight energy dispersive x-ray (EDX) spectrometer detector and INCAx stream pulse processor. Electrical resistivity was measured by the standard four-probe technique, by reversal of the current sources to cancel thermoelectric voltages between 300 K and 10 K, in an Oxford closed cycle cooler cryostat. A Cernox sensor was used to monitor the ambient temperature of the sample. The temperature-dependent Seebeck coefficient was obtained between 300 K and 80 K by steady state techniques. The thermally generated Seebeck voltage across the sample was measured with a Keithely 2182 nanovoltmeter. The temperature gradient across the sample was monitored by a
type E differential thermocouple. Temperature gradients were typically between 0.5 K and 1 K. The thermoelectric power (TEP) of the sample was obtained by subtraction of the TEP of Cu Seebeck probes. The carrier concentration was determined by Hall measurements using the van der Pauw method under an applied field of 0.55 T (Ecopia: HMS-3000). RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of as-prepared Bi2xSbxTe3 synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5). The diffraction peaks can be indexed using a rhombohedral lattice (#166). All the samples with the space group of R3m show a small amount of Te impurity. The impurity of Bi in the solvothermally synthesized sample (see Fig. 1b) could be eliminated either by decreasing the molar ratio of Te/NaBH4 from 3/2 to 1/4 in the starting precursors or by sintering in an evacuated ampoule. Figure 2a shows an SEM image of hydrothermally synthesized Bi0.5Sb1.5Te3 powders. The dominant morphology of the powders is hexagonal nanoplatelets. These hexagonal nanoplatelets have various sizes, with edges of 200–1500 nm and thicknesses of 30–50 nm. In contrast, there are two different kinds of morphology for the solvothermally synthesized Bi0.5Sb1.5Te3 powders. In Fig. 2b one can see the sheet-tubes-like microstructure, which resembles that of the Bi2Te3 sheets that grow along the [003] direction.4 However, the nanosheets growing along the Te tube are more irregular in shape and are warped (akin to leaves growing on a stalk). The other morphology shown in Fig. 2c are the nanoplatelets less than 100 nm in size. For characterization of the transport properties, all the hydrothermally synthesized and solvothermally synthesized powders were compacted into
Fig. 1. Powder x-ray diffraction pattern of sintered Bi2xSbxTe3 samples which are synthesized (a) hydrothermally for x = 1.5; (b) solvothermally for x = 1.5; (c) solvothermally for x = 1.0; (d) solvothermally for x = 0.
Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi0.5Sb1.5Te3 Compared with its Solvothermally Prepared Counterpart
1501
Fig. 3. Temperature dependence of the electrical resistivity for sintered Bi2xSbxTe3 samples which are synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5).
Fig. 4. Temperature dependence of the Seebeck coefficient for sintered Bi2xSbxTe3 samples which are synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5).
Fig. 2. SEM image of Bi0.5Sb1.5Te3 (a) hydrothermally synthesized with de-ionized water as the solvent, (b) and (c) solvothermally synthesized with DMF as the solvent.
parallelepipeds and sintered at 300°C for 10 h. Figure 3 shows the temperature dependence of the electrical resistivity of the sintered Bi2xSbxTe3 samples which were synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5). The electrical resistivity decreases with decreasing
temperature below 250 K for all the samples, which is a characteristic of metal-like temperature dependence. The x = 0 sample shows the lowest resistivity in this series of materials, whereas the x = 1.0 sample shows the maximum resistivity. This trend seems to be opposite to the electrical conductivity in the direction perpendicular to the trigonal c-axis of (Bi,Sb)2Te3 crystals.7 In addition, it can be readily seen that the electrical resistivity of the hydrothermally prepared Bi0.5Sb1.5Te3 sample is much smaller than that of the solvothermally prepared one in all the investigated temperatures. The size of the resistivity is 5.7 mX-cm and 13.8 mX-cm at 290 K for the hydrothermally prepared Bi0.5Sb1.5Te3 and solvothermally prepared sample, respectively. Figure 4 shows the temperature dependence of the Seebeck coefficient of sintered Bi2xSbxTe3 samples which were synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5). The
1502
C.-J. Liu, G.-J. Liu, Tsao, and Huang
Fig. 5. Power factor for sintered Bi2xSbxTe3 samples which are synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5).
absolute value of the Seebeck coefficient decreases with decreasing temperature below 250 K for all the samples, which is a characteristic of metal-like temperature dependence. Both of the x = 0 and 1.0 samples exhibit negative thermopower. In contrast, the x = 1.5 sample synthesized either hydrothermally or solvothermally shows a positive Seebeck coefficient. The size of the Seebeck coefficient of x = 0 is smaller than that of the hydrothermally synthesized and hot-pressed sample.8 However, the room-temperature resistivity of our x = 0 sample is slightly lower than the latter. The size of the Seebeck coefficient for x = 1.0 is twice that in the direction perpendicular to the trigonal c-axis of a BiSbTe3 crystal.7 The negative Seebeck coefficient of the x = 1.0 sample is unusual, and could be because the real composition of the sample deviated from the nominal composition or the inhomogeneity of the composition. The Seebeck coefficient of the hydrothermally prepared Bi0.5Sb1.5Te3 is significantly larger than that of its solvothermally prepared counterpart in all the investigated temperatures. The size of the Seebeck coefficient is 245.4 lV/K and 200.2 lV/K at 290 K for the hydrothermally prepared Bi0.5Sb1.5Te3 and its solvothermally prepared counterpart, respectively. Figure 5 shows the temperature dependence of the power factor of sintered Bi2xSbxTe3 samples which were
synthesized solvothermally (x = 0, 1.0 and 1.5) or hydrothermally (x = 1.5). It can be readily seen that the power factor of the hydrothermally prepared Bi0.5Sb1.5Te3 has been significantly improved when compared with that of its solvothermally prepared counterpart at all the investigated temperatures. The size of the power factor is 10.4 lW/cm K2 and 2.9 lW/cm K2 at 290 K for the hydrothermally prepared Bi0.5Sb1.5Te3 and its solvothermally prepared counterpart, respectively. Table I shows the results of the Hall effect measurements. The hydrothermally prepared Bi0.5Sb1.5Te3 has a lower carrier concentration but higher mobility than its solvothermally prepared counterpart. The carrier concentration is 9.47 9 1018 and 1.42 9 1019 for the hydrothermally prepared Bi0.5Sb1.5Te3 and its solvothermally prepared counterpart, respectively. The mobility is 114.9 cm2/V s and 32 cm2/V s at room temperature for the hydrothermally prepared Bi0.5Sb1.5Te3 and its solvothermally prepared counterpart, respectively. The Hall measurements confirm that the majority carrier of both samples is p-type, which is consistent with the results of the Seebeck coefficient measurements. The higher hole concentration of the solvothermally synthesized sample could be due to a larger concentration of anti-structure defects generated in the sample, since the hole concentration of the p-type (Bi,Sb)2 Te3 can be attributed to anti-structure defects by occupation of Te sites by Bi or Sb atoms.9 Therefore, the larger resistivity of the solvothermally synthesized sample is caused by a significantly larger mobility due to more charge carrier scattering by the anti-structure defects than in its hydrothermally synthesized counterpart. The diffusion thermopower can be simplified by10 S¼
kB ðd þ C ln nc Þ e
(1)
where kB, e, nc, d, and C are the Boltzmann constant, electronic charge, carrier concentration, scattering parameter, and constant, respectively. The higher thermopower of the hydrothermally synthesized sample could be attributed to the lower hole concentration, according to Eq. 1. The effects of sintering temperature on the thermoelectric characteristics of the hydrothermally synthesized sample are under investigation.
Table I. Hole Concentration (n) and Mobility (l) of Sintered Bi22xSbx Te3, which is Synthesized Solvothermally (x = 0, 1.0 and 1.5) and Hydrothermally (x = 1.5) at Room Temperature x = 1.5
n (1/cm3) l (cm2/V s)
x=0
x = 1.0
Hydrothermal
Solvothermal
1.18 9 1019 203
1.99 9 1019 17.5
9.47 9 1018 114.9
1.42 9 1019 32
Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi0.5Sb1.5Te3 Compared with its Solvothermally Prepared Counterpart
CONCLUSIONS We succeeded in hydrothermally synthesizing Bi0.5Sb1.5Te3 with the morphology of hexagonal platelets. The platelets were various sizes, with edges of 200–1500 nm and thicknesses of 30–50 nm. In comparison with the solvothermally prepared Bi0.5Sb1.5Te3, the hydrothermally prepared Bi0.5 Sb1.5Te3 showed larger thermoelectric power factor. The resulting thermoelectric power factor at 290 K was 10.4 lW/cm K2 and 2.9 lW/cm K2 for the hydrothermally prepared Bi0.5Sb1.5Te3 and its solvothermally prepared counterpart, respectively. ACKNOWLEDGEMENT This work is supported by the National Science Council of the Republic of China (ROC) (Grant No. NSC 95-2112-M-018-006-MY3). REFERENCES 1. L.D. Hicks and M.S. Dresselhaus, Phys. Rev. B 47, 12727 (1993). doi:10.1103/PhysRevB.47.12727.
1503
2. L.D. Hicks and M.S. Dresselhaus, Physica B 47, 16631 (1993). doi:10.1103/PhysRevB.47.16631. 3. W. Lu, Y. Ding, Y. Chen, Z.L. Wang, and J. Fang, J. Am. Chem. Soc. 127, 10112 (2005). doi:10.1021/ja052286j. 4. Y. Deng, C.W. Cui, N.-L. Zhang, T.-H. Ji, Q.-L. Yang, and L. Guo, Solid State Commun. 138, 111 (2006). doi:10.1016/ j.ssc.2006.02.030. 5. X.B. Zhao, X.H. Ji, Y.H. Zhang, T.J. Zhu, J.P. Tu, and X.B. Zhang, Appl. Phys. Lett. 86, 062111 (2005). doi: 10.1063/1.1863440. 6. C.-J. Liu, G.-J. Liu, C.-W. Tsao, Y.-F. Lu, and L.-S. Chang, Proceedings of the 26th International Conference on Thermoelectrics, ICTÕ07, Jeju, Korea, 3–5 June 2007. 7. M. Stordeur, CRC Handbook of Thermoelectrics (Boca Raton, FL: CRC Press, 1995), p. 243. 8. H.L. Ni, T.J. Zhu, and X.B. Zhao, Physica B 364, 50 (2005). doi:10.1016/j.physb.2005.03.034. 9. K. Park, J.H. Seo, D.C. Cho, B.H. Choi, and C.H. Lee, Mater. Sci. Eng. B 88, 103 (2002). doi:10.1016/S0921-5107(01)009 12-6. 10. X.A. Fan, J.Y. Yang, W. Zhu, S.Q. Bao, X.K. Duan, C.J. Xiao, Q.Q. Zhang, and Z. Xie, J. Phys. D Appl. Phys. 39, 5069 (2006).
