Letter pubs.acs.org/NanoLett
n-Type Nanostructured Thermoelectric Materials Prepared from Chemically Synthesized Ultrathin Bi2Te3 Nanoplates Jae Sung Son,†,‡ Moon Kee Choi,† Mi-Kyung Han,§ Kunsu Park,† Jae-Yeol Kim,∥ Seong Joon Lim,⊥ Myunghwan Oh,† Young Kuk,⊥ Chan Park,∥ Sung-Jin Kim,§ and Taeghwan Hyeon*,† †
World Class University (WCU) Program of Chemical Convergence for Energy and Environment (C2E2), Institute of Chemical Processes, and School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea ‡ Creative Research Center, Creative and Fundamental Research Division, Korea Electrotechnology Research Institute (KERI), Changwon, Gyeongsangnam-do, 641-120, Korea § Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea ∥ Department of Materials Science and Engineering and ⊥Department of Physics and Astronomy, Seoul National University, Seoul 151-744, Korea S Supporting Information *
ABSTRACT: We herein report on the large-scale synthesis of ultrathin Bi2Te3 nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi2Te3 nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ∼1 nm, which corresponds to a single layer in Bi2Te3 crystals. Bi2Te3 nanostructured bulk materials were prepared by sintering of surfactant-removed Bi2Te3 nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi2Te3 nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis. KEYWORDS: Chemical synthesis, Bi2Te3, nanoplate, spark plasma sintering, thermoelectrics
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properties. Furthermore, nanostructuring can improve the Seebeck coefficient by quantum confinement effect4 or filtering of low energy electrons at the interface.5 Among various recent nanostructured thermoelectric materials, several kinds of nanostructured bulk materials with high ZT have been fabricated by the hot-pressing of nanoparticles.6 This approach has been considered as a successful route for the large-scale production of highly efficient thermoelectric materials. However, the nanoparticles prepared by top-down physical methods such as ball-milling or melt-spinning process generally exhibit polydispersed nanostructures with relatively large size. Recently, bottom-up colloidal synthetic processes have been used for synthesizing large-quantities of high-quality nanoparticles.7 These chemical methods can produce uniform-sized
hermoelectric materials have attracted tremendous attention because the direct conversion between heat and electricity in thermoelectric modules can find various technological applications such as power generation from waste heat and environmentally friendly refrigeration.1 The efficiency of the thermoelectric materials is generally estimated in terms of thermoelectric figure-of-merit, ZT = (σS2/k)T, where σ is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, and T is the absolute temperature. Over the past three decades (1960−1990), only a small increase in ZT in bulk materials has been achieved, thereby imposing a limitation on the practical use of thermoelectric materials to various applications.2 Recently, the introduction of nanostructures in various thermoelectric materials has led to a significant improvement of ZT of >1.3 This increase in thermoelectric efficiency has mainly attributed to the reduction of thermal conductivity caused by the increased interfaces to scatter phonons with a minimal detrimental effect on the electrical © 2012 American Chemical Society
Received: September 29, 2011 Revised: January 20, 2012 Published: January 23, 2012 640
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nanoparticles with controlled sizes and shapes,8 which provide the possibility of achieving further improvement in thermoelectric efficiency because phonon scattering in nanostructured materials is dependent on the size and shape of the nanostructures.9 However, only very recently these chemically synthesized nanoparticles have been investigated for thermoelectric applications.10 Several methods, including annealing, pressing, and sintering, have been used for fabricating nanostructured bulk materials from the compaction of nanoparticles. Especially, spark plasma sintering (SPS) is known to be very useful hot-pressing process for preparing nanostructured bulk materials owing to the very fast heating and cooling rates,11 which enable fast sintering, making it possible to prevent unwanted grain growth arising from a long sintering process at high temperatures. Furthermore, the grain growth and densification during SPS can be controlled by varying the SPS conditions, allowing the investigation of the effect of grain size and porosity on the thermoelectric properties of nanostructured bulk materials. Consequently, the SPS process provides an effective method for the compaction of colloidal nanoparticles, and the systematic study on the SPS of colloidal nanoparticles will offer various insights that would contribute to increasing the thermoelectric efficiency of nanostructured bulk materials in terms of a controlled grain size, porosity, and interface properties.12 Bi2Te3 nanostructures provide an excellent model system for studying the preparation of nanostructured thermoelectric materials because bulk Bi2Te3-based materials are known to exhibit very high ZT values over the temperature range of 200− 400 K. Furthermore, a dramatic increase in ZT has been reported in Bi2Te3-based nanostructured bulk materials prepared by the hot-pressing of ball-milled nanoparticles. Although pure bulk Bi2Te3 exhibited ZT of ∼0.5 at room temperature,13 ZT values of 1.04 for n-type BiTe3−ySey nanostructured bulk alloy6d and 1.4−1.6 for p-type BixSb2−xTe nanostructured bulk alloy6a,e were achieved. Most chemically synthesized Bi2Te3 nanocrystals have twodimensional plate-shapes because of their crystal structure.14 A bulk Bi2Te3 crystal has a layered structure that is composed of stacked two-dimensional layers of Te(1)−Bi−Te(2)−Bi−Te(1) with a Te(1)−Te(1) weak van der Waals interaction, leading to ease of cleavage along planes perpendicular to the c-axis. Recently, various single-layered nanostructures of metal chalcogenides, including Bi2Te3, have been prepared by a topdown exfoliation process.15 Nonetheless, ultrathin Bi2Te3 nanocrystals composed of a single layer or a few layers have been rarely synthesized using bottom-up chemical methods. Herein, we report a colloidal synthesis of ultrathin Bi2Te3 nanoplates with the thickness of 1−3 nm and subsequent SPS for fabricating n-type nanostructured bulk thermoelectric materials. We found that the grain size and density of Bi2Te3 nanostructured bulk materials were strongly dependent on the SPS temperature. Consequently, the thermoelectric properties of Bi2Te3 nanostructured bulk materials sintered at various temperatures were characterized to find the optimum sintering condition for highly efficient nanostructured thermoelectric materials. The highest ZT of 0.62 was achieved in sample sintered at 250 °C, which is one of the highest value among those of reported n-type nanostructured thermoelectric materials prepared from chemically synthesized nanoparticles. Bi2Te3 nanoplates were synthesized by the reaction between bismuth dodecanethiolate and tri-n-octylphosphine telluride in the presence of oleylamine. The transmission electron
microscopy (TEM) image (Figure 1a) shows ultrathin Bi2Te3 nanoplates with various shapes such as disk, ellipse, and angled
Figure 1. (a) TEM image of Bi2Te3 nanoplates. Inset shows the lateral view of stacked plates. (b) Electron diffraction pattern of Bi2Te3 nanoplate. (c) HRTEM image of Bi2Te3 nanoplate in a lateral view. (d) XRD pattern of Bi2Te3 nanoplates. The vertical lines indicate the pattern of rhombohedral bulk Bi2Te3 crystal.
plates. Some of the nanoplates exhibited rolled and folded structures due to the plate morphology. The electron diffraction (ED) pattern (Figure 1b) reveals the crystalline nature of the Bi2Te3 nanoplates. High-resolution transmission electron microscopy (HRTEM) analysis in the lateral view of the stacked nanoplates (Figure 1c) revealed that most of the nanoplates had the thickness of ∼1 nm, which matched to that of a single layer in the rhombohedral Bi2Te3 crystals. Lattice fringes of 0.32 nm corresponding to the (015) facets of rhombohedral Bi2Te3 (Figure 1c) were observed. A small fraction of the nanoplates had a relatively large thickness of 2−3 nm. The width of the Bi2Te3 nanoplates ranged from 10 to 30 nm. To further confirm the ultrathin thickness, the Bi2Te3 nanoplates were characterized by noncontact mode atomic force microscopy (AFM). The AFM image and the height profile (shown in Supporting Information, Figure S1) shows that a single Bi2Te3 nanoplate has the thickness of ∼1 nm, which matches very well with the TEM results. X-ray diffraction pattern (XRD) (Figure 1d) shows that the nanoplates have a rhombohedral Bi2Te3 structure (JCPDS 82−0358). Interestingly, the (110) peak for the ab plane was distinct, whereas the (003) and (006) peaks for the c axis could not be observed, revealing the very small thickness of Bi2Te3. In the synthesis of Bi2Te3 nanoplates, two important factors, dodecanethiol concentration and the kind of coordinating solvent used, were critical for the formation of the twodimensional nanoplates. When the molar ratio of dodecanethiol/bismuth neodecanoate was higher than 3, highly aggregated flower-like structures were generated (shown in Supporting Information, Figure S2a). Because the dangling bonds of Bi cations exist only at the edges of the layers, 641
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negatively charged dodecanethiol surfactants were expected to coordinate very strongly to the edges in the Bi2Te3 nanoplates. Therefore, the edges in Bi2Te3 nanoplates were stabilized by dodecanethiol surfactants. Then, higher concentrations of dodecanethiol seemed to generate a greater number of edges in Bi2Te3 nanocrystals during the growth, consequently producing flowerlike structures. Various solvents were screened to synthesize nanoplates, and it was found that only alkylamines could produce plate-shaped nanocrystals. When the synthesis carried out using a noncoordinating or weakly coordinating solvent such as octadecene or dialkyl ether, flowerlike structures having a small size (∼10 nm) were generally produced (shown in Supporting Information, Figure S2b). Furthermore, irregular aggregated structures with a large size (∼100 nm) were generated by using strongly coordinating solvents such as oleic acid (shown in Supporting Information, Figure S2c). These results demonstrated that the optimal coordination strength of alkylamines was critical for the synthesis of Bi2Te3 nanoplates. To prepare Bi2Te3 nanostructured bulk materials, we sintered the Bi2Te3 nanoplate powder by SPS after the removal of surfactants. The removal of organic surfactants is an imperative step for the thermoelectric applications because they act as electrically insulating barriers, eventually decreasing the electrical conductivity significantly. Furthermore, they can block the effective contact between the nanoparticles, which has an adverse effect on sintering. To remove the organic surfactant coating on the surface of the nanoplates, we used the modified version of the method reported by the Weller group,10e which is “surfactant exchange with oleic acid and subsequent ammonia treatment”. The oleic acid surfactant on the Bi2Te3 nanoplates was removed by the reaction with ammonia to form ammonium oleate. The carbon content characterized by the CHN analysis was 3.9% in the assynthesized nanoplates and 0.85% in the nanoplates after the surfactant removal process, demonstrating the effective removal of organic surfactants. During the sintering process, the organic residue content was further decreased to 20 min increased the density by just 1−2% relative to the sample sintered for 5 min. Because the powder began to melt at 350 °C, the sintering temperature was varied from room temperature to 325 °C with keeping the holding time constant at 5 min. The XRD patterns of Bi2Te3 nanostructured bulk materials with various sintering temperatures (Figure 2a) shows that the peak width narrows with an increase in the sintering temperature, revealing that the grain size of Bi2Te3 nanostructured bulk materials increased with the sintering temperature. In all the XRD patterns, no peak related to the crystalline phase change, oxidation, or impurity was observed. The density of Bi2Te3 nanostructured
Figure 2. (a) XRD patterns of Bi2Te3 nanostructured bulk materials sintered at various temperatures. The vertical lines indicate the pattern of rhombohedral bulk Bi2Te3 crystal. (b) Graph of relative densities (blue square) and grain sizes (red circle) calculated by the XRD patterns using the Scherrer equation versus sintering temperature.
bulk materials was found to be strongly dependent on the sintering temperature. The measured relative density and grain size calculated by the XRD patterns using the Scherrer equation are plotted versus the sintering temperature in Figure 2b. When the sintering temperature increased, the relative density began to increase at lower temperature, followed by an increase in the grain size at higher temperature, which is consistent with the typical trend in SPS.11 The maximum relative density of 94% was achieved for sample sintered at 325 °C. The grain growth was negligible below 200 °C and it dramatically increased at the temperatures above 200 °C. The grain size of Bi 2 Te 3 nanostructured bulk materials varied from 20 to 50 nm, which is much smaller than that of conventional nanostructured bulk materials on the scale of several hundred nanometers. This could be attributed to the ultrathin dimension of Bi2Te3 nanoplates. Moreover, the random aggregation of nanoplates and the organic residue could interfere with the grain growth. The scanning electron microscopy (SEM) images of Bi2Te3 nanostructured bulk materials (Figure 3) shows that they were well sintered and composed of nanometer-sized grains, and that the grain size increased with an increase in the sintering temperature. The thermoelectric properties of Bi2Te3 nanostructured bulk materials sintered at temperatures higher than 200 °C were measured at temperatures ranging from 300 to 400 K. Samples sintered below 200 °C exhibited a room-temperature electrical 642
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Figure 3. SEM images of Bi2Te3 nanostructured bulk materials sintered at (a) 200, (b) 250, (c) 300, and (d) 325 °C.
conductivity of 12 μW cm−1 K−2, which is one of the highest values reported among chemically synthesized n-type nanostructured thermoelectric materials. The electrical properties of thermoelectric materials are known to be strongly dependent on the carrier concentrations. For metals or degenerate semiconductors, the Seebeck coefficient is given by S ∼ n−2/3,3a where n is the carrier concentration. Furthermore, the electrical conductivity (σ) is calculated by σ = enμ, where e is the electron charge and μ is the carrier mobility. Therefore, the electrical conductivity is 643
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Table 1. The Electron Concentration and Mobility of Bi2Te3 Nanostructured Bulk Materials Sintered at Various Temperatures sintering temperature (°C) electron concentration (cm−3) electron mobility (cm2 V−1 s−1)
200 1.03 × 1020 12.66
250 6.80 × 1019 36.49
300 4.68 × 1019 62.63
325 1.43 × 1019 274.0
Figure 5. Temperature dependence of (a) thermal conductivities, (b) lattice thermal conductivities, (c) ZT, and (d) the ratios of electrical to thermal conductivity of Bi2Te3 nanostructured bulk materials sintered at 200 °C (green square), 250 °C (red circle), 300 °C (blue upward triangle), and 325 °C (purple downward triangle).
inversely proportional to the Seebeck coefficient. In addition to carrier concentrations, the potential barrier scattering of carriers at interfaces also strongly modifies the electrical properties in nanostructured thermoelectric materials. As mentioned above, the positive temperature dependence of electrical conductivity may come from the potential barrier scattering of carriers at the interfaces. Furthermore, the change in the interface density brought about by controlling the grain size and porosity could alter the number of scattering sites, consequently, varying the carrier mean free path and mobility. In particular, the potential barriers of interfaces are known to filter the low-energy carriers crossing the interface. Because the low-energy carriers are known to decrease the Seebeck coefficients, their filtering contribute to the enhancement of the Seebeck coefficients.5 Consequently, the increase in the interface density in nanostructured bulk materials could cause the increase in the total Seebeck coefficients. In the current Bi2Te3 nanostructured bulk materials, it is expected that both the carrier concentration and potential barrier scattering simultaneously influence their electrical properties because of the high interface density resulting from the small grain size. We measured electron concentrations at room temperature using a Hall measurement system. The electron mobilities were also calculated from electron concentration and electrical conductivity. Interestingly, with
an increase in the sintering temperature, the electron concentration of Bi 2Te 3 nanostructured bulk materials decreased dramatically from 1.03 × 1020 to 1.43 × 1019 cm−3, as shown in Table 1. Considering that the electrical conductivity is proportional to the electron concentration, this remarkable decrease in the electron concentration was totally unexpected. Furthermore, these unusual properties have not been observed in conventional nanostructured materials fabricated using top-down physical methods and bulk materials, which generally exhibited the increased electron concentration with increasing sintering or annealing temperature due to the formation of defects arising from the evaporation of tellurium. It was widely known that the Te(Bi) antisite defects, and Te vacancies in Bi2Te3 crystal are electron donors.16 The Bi2Te3 nanoplates have very high interface density derived from the ultrathin thickness of ∼1 nm. These interfaces already have many defects, dangling bonds, and organic residues to act as the doping agent before sintering. Several studies have reported on the doping effect of organic ligands in nanoparticles.10h,j,17 Consequently, the remarkable decrease in electron concentration can be explained by the doping effect of the defects and organic residues in the interfaces, and the decrease of interface density with an increase in the sintering temperature. Although electron concentrations decreased with the sintering temperature, the Seebeck coefficients unexpectedly decreased as the 644
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°C with a slightly low relative density of ∼82%. To correct the lattice thermal conductivity considering the porosity, the modified formulation of the effective medium theory suggested by Lee et al. was used: kp = kh(2 − Φ)/(2 + Φ), where kh is the thermal conductivity of the host, kp is the thermal conductivity of the porous material, and Φ is the porosity.22 The corrected lattice thermal conductivity was 0.37 W m−1 K−1, which was still considerably lower than that of bulk Bi2Te3 but similar to those of the values of conventional nanostructured Bi2Te3-based materials.6d,10e The dimensionless figure-of-merit, ZT, of Bi2Te3 nanostructured bulk materials (Figure 5c) increased with an increase in measurement temperature, which was due to the temperature dependent-power factors. The maximum ZT at room temperature was 0.45 for sample sintered at 300 °C, and that at 400 K was 0.62 for sample sintered at 250 °C, which is comparable to or higher than that of the bulk Bi2Te3 crystal,13 and is one of the highest among the values of the reported ntype nanostructured materials based on chemically synthesized nanoparticles. Interestingly, the maximum ZT was achieved in samples sintered at intermediate temperatures of 250 and 300 °C. Considering that the difference in the Seebeck coefficients among Bi2Te3 nanostructured bulk materials sintered at various temperatures was not large enough to significantly affect the ZT values, the current results revealed that Bi2Te3 nanostructured bulk materials sintered at low temperatures exhibited too low electrical conductivity whereas those sintered at high temperatures showed too high thermal conductivity. The ratios of electrical-to-thermal conductivities (σ/k) of samples sintered at 200 and 325 °C were lower than those of samples sintered at 250 and 300 °C (Figure 5d). Lee et al. predicted that the electrical conductivity in porous materials composed of nanosized grains increased more rapidly than the thermal conductivity as the density and grain size increases.22 According to this prediction, the σ/k of the current Bi2Te3 nanostructured bulk materials should increase as the sintering temperature increases, which is in disagreement with the current results. This is presumably due to the sintering temperature dependence of the electron concentration because Lee et al. calculated the properties by assuming fixed carrier concentrations. The change of the electron concentration in Bi2Te3 nanostructured bulk materials could leads to the decreased electrical conductivity of sample sintered at 325 °C and increased electrical conductivity of sample sintered at 200 °C relative to the theoretically expected values at fixed carrier concentrations. Consequently, the optimum sintering temperatures of 250−300 °C seemed to result from the electron concentrations as well as the effects of grain size and porosity. Furthermore, these properties could be considered as the unique properties of chemically synthesized nanoparticles with a high interface density and efficient doping. Moreover, the entire process of the preparation of Bi2Te3 nanostructured bulk materials from chemically synthesized Bi2Te3 nanoplates offers a cost-effective way for the large-scale production of efficient thermoelectric materials. In summary, we developed a new synthetic route of ultrathin Bi2Te3 nanoplates with thickness of ∼1 nm. Subsequently, nanostructured thermoelectric materials were fabricated by sintering of surfactant-removed Bi2Te3 nanoplates using an SPS process. We systematically investigated the effect of sintering temperature on the thermoelectric properties of Bi2Te3 nanostructured bulk materials. The electrical and thermal conductivities increased with an increase in the sintering
sintering temperature increased from 250 to 325 °C, demonstrating that the electron scattering process at interfaces of potential barrier dominated over electron concentration contribution in the electrical transport in Bi2Te3 nanostructured bulk materials. The calculated electron mobilities (Table 1) increased from 12.7 to 274 cm2 V−1 s−1 with an increase in the sintering temperature. This increased mobility was predictable considering the fact that the reduced interface density will increase the mean free path of the carriers. Furthermore, the increase in the electron mobility with the sintering temperature is responsible for the increased electrical conductivity. The thermal conductivity (k) was calculated using the equation k = αcpρ, where α is the thermal diffusivity that was measured by LFA, cp is the specific heat capacity that was measured by DSC, and ρ is the density. It should be noted that the thermal conductivities of Bi2Te3 nanostructured bulk materials were measured in a direction parallel to the press direction, whereas their electrical properties were characterized in the perpendicular direction. Because it is known that the crystallographic orientations of hot-pressed Bi2Te3 can be adapted to the pressing direction, the measurements of electrical and thermal properties have to be performed with a great care. In a single-crystalline Bi2Te3, the anisotropy in the thermal and electrical transport between the ab-plane and the caxis of crystal was 1.5−3.18 However, in nanostructured bulk materials prepared from nanoparticles, the anisotropies of thermoelectric properties were significantly decreased due to the random orientation of nanoparticles. Poudel et al. even reported the isotropic thermoelectric properties of nanostructured SbBiTe alloys.6a Zhang et al. calculated the ZT values of hot-pressed nanocomposites prepared from Bi2Te3 microparticles and Bi2Te3−Te nanoparticles using a correction factor of ∼0.8 for the anisotropic properties.19 In the current work, assuming that the anisotropic properties of Bi2Te3 nanostructured bulk materials were not dependent on the sintering temperature, the thermal conductivities were estimated from the measured values using a correction factor of 1.2, based on the results by the Chen group.6d With an increase in the sintering temperature, the estimated roomtemperature thermal conductivities (Figure 5a) increased from 0.44 to 1.13 W m−1 K−1, which were significantly reduced values as compared to the values of 1.5−2.5 W m−1 K−1 of bulk Bi2Te3.18 This significantly reduced thermal conductivity can be explained by the increased number of interfaces that scatter phonons. Furthermore, the sintering temperature dependence of thermal conductivity shows the controllability of the thermal transport in the nanostructured materials. To analyze the phonon contribution to the thermal conductivity, the lattice thermal conductivity (kL) was calculated using the relationship: k = ke + kL, where electronic thermal conductivity (ke) is estimated by the Wiedemann− Franz Law (k/σ = LT, where L is the Lorentz number). The Lorentz number of 2.0 × 10−8 V2 K−2 for a degenerate semiconductor was used.20 The calculated lattice thermal conductivity (Figure 5b) exhibited a trend similar to that of the total thermal conductivity, increasing with the sintering temperature. The lowest value of as low as 0.28 W m−1 K−1 was observed for sample sintered at 200 °C, which is slightly lower than the predicted minimum thermal conductivity of 0.31 W m−1 K−1 in nanograined Bi2Te3 with the full density calculated using the Debye−Callaway model.21 This extremely low thermal conductivity can be understood by considering a very small grain size and many pores in sample sintered at 200 645
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Efficiency and Resources program (20112010100100) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP). M.-K.H. and S.-J.K. acknowledge financial support from the Basic Science Research Program through the National Research Foundation of Korea (20110003767) and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education, Science, and Technology (20110030147).
temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficient ranged from −130 to −160 μV K−1 and roughly decreased with the sintering temperatures. Interestingly, the electron concentrations and mobility were found to change dramatically with the sintering temperature. On the basis of these results, we proposed the effect of interfaces on the electrical transport in Bi2Te3 nanostructured bulk materials, which were the potential barrier scattering at interfaces and the doping effect of the defects and the organic residue on the interfaces. The highest ZT values of 0.45 at room temperature and 0.62 at 400 K were achieved for Bi2Te3 nanostructured bulk materials sintered at 300 and 250 °C, respectively, which are comparable to or higher than that of bulk Bi2Te3 crystal and one of the highest value among those of reported n-type nanostructured materials based on chemically synthesized nanoparticles. This enhancement of ZT showed the possibility for preparing highly efficient nanostructured thermoelectric materials from chemically synthesized nanoparticles. Furthermore, the effect of sintering temperature on the thermoelectric properties of Bi2Te3 nanostructured bulk materials showed the controllability of the electric and thermal transport in nanostructured materials, which provides a way to improve ZT by means of controlling the grain size, porosity, and doping. In particular, we believe that the precise control of the doping content by chemical or physical doping, alloying, and mixing with appropriate nanoparticles will significantly increase the Seebeck coefficient of the nanostructured materials. Finally, the current synthesis for Bi2Te3 nanoplates provides a new way to produce ultrathin two-dimensional nanostructures in a large scale.
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(1) (a) Rowe, D. M., Ed. Thermoelectric Handbook: Macro to Nano; Taylor & Francis Group, 2006. (b) Disalvo, F. J. Science 1999, 285, 703. (c) Bell, L. E. Science 2008, 321, 1457. (2) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z.; Fleurial, J. P.; Gogna, P. Adv. Mater. 2007, 19, 1043. (3) (a) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. (b) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2009, 48, 8616. (c) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Adv. Mater. 2010, 22, 3970. (d) Chen, G.; Dresselhaus, M. S.; Dresselhaus, G.; Fleurial, J. P.; Caillat, T. Int. Mater. Rev. 2003, 48, 45. (d) Minnich, A. J.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. Energy Environ. Sci. 2009, 2, 466. (e) Majumdar, A. Science 2004, 303, 777. (f) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (g) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najaria, M.; Majumdar, A.; Yang, P. Nature 2008, 451, 163. (h) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddar, W. A.; Heath, J. R. Nature 2008, 451, 168. (i) Harman, T. C.; Tavlor, P. J.; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229. (j) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Science 2004, 303, 818. (4) (a) Mahan, G. D.; Sofo, J. O. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 7436. (b) Humphrey, T. E.; Linke, H. Phys. Rev. Lett. 2005, 94, 096601. (5) (a) Martin, J.; Wang, L.; Chen, L.; Nolas, G. S. Phys. Rev. B 2009, 79, 115311. (b) Kishimoto, K.; Tsukamoto, M.; Koyanagi, T. J. Appl. Phys. 2002, 92, 5331. (c) Seto, J. Y. W. J. Appl. Phys. 1975, 46, 5247. (d) Kishimoto, K.; Koyanagi, T. J. Appl. Phys. 2002, 92, 2544. (e) Moyzhes, B.; Nemchinsky, V. Appl. Phys. Lett. 1998, 73, 1895. (6) (a) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Science 2008, 320, 634. (b) Ma, Y.; Hao, Q; Poudel, B.; Lan, Y.; Yu, B.; Wang, D.; Chen, G.; Ren, Z. Nano Lett. 2008, 8, 2580. (c) Joshi, G.; Lee, H.; Lan, Y.; Wang, X.; Zhu, G.; Wang, D.; Gould, R.; Cuff, D. C.; Tang, M. Y.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Nano Lett. 2008, 8, 4670. (d) Yan, X.; Poudel, B.; Ma, Y.; Liu, W. S.; Joshi, G.; Wang, H.; Lan, Y.; Wang, D.; Chen, G.; Ren, Z. F. Nano Lett. 2010, 10, 3373. (e) Xie, W.; Tang, X.; Yan, Y.; Zhang, Q.; Tritt, T. M. J. Appl. Phys. 2009, 105, 113713. (f) Xie, W.; He, J.; Kang, H. J.; Tang, X.; Zhu, S.; Laver, M.; Wang, S.; Copley, J. R. D.; Brown, C. M.; Zhang, Q.; Tritt, T. M. Nano Lett. 2010, 10, 3283. (g) Wang, X. W.; Lee, H.; Lan, Y. C.; Zhu, G. H.; Joshi, G.; Wang, D. Z.; Yang, J.; Muto, A. J.; Tang, M. Y.; Klatsky, J.; Song, S.; Dresselhaus, M. S.; Chen, G.; Ren, Z. F. Appl. Phys. Lett. 2008, 93, 193121. (7) (a) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (b) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553. (c) Son, J. S.; Wen, X.D.; Joo, J.; Chae, J.; Baek, S.-i.; Park, K.; Kim, J. H.; An, K.; Yu, J. H.; Kwon, S. G.; Choi, S.-H.; Wang, Z.; Kim, Y.-W.; Kuk, Y.; Hoffmann, R.; Hyeon, T. Angew. Chem., Int. Ed. 2009, 48, 6861. (8) (a) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Peng, X. Adv. Mater. 2003, 15, 459. (c) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310. (d) Kwon, S. G.; Hyeon, T. Acc. Chem. Res. 2008, 41, 1696. (e) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int.
ASSOCIATED CONTENT
S Supporting Information *
Experimental Section. AFM image and height profile of the single Bi2Te3 nanoplate. TEM images of Bi2Te3 nanostructures synthesized in the reaction condition of dodecanethiol/bismuth neodecanoate ratio of 3 and by using octadecene and oleic acid as a solvent. The room-temperature electrical conductivities of the Bi2Te3 nanostructured bulk samples sintered at various temperatures. A photograph showing 1.12 g and TEM image of Bi2Te3 nanoplates synthesized in large scale. A photograph showing 1.06 g of the surfactant-removed Bi2Te3 nanoplates. A photograph showing Bi2Te3 nanostructured bulk pellet prepared by SPS of the surfactant-removed Bi2Te3 nanoplates at 300 °C.This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.H. acknowledges financial support by the Korean Ministry of Education, Science, and Technology (MEST) through Strategic Research (2010-0029138) and World Class University (R3110013) Programs of National Research Foundation (NRF) of Korea. T.H. and J.S.S. acknowledge financial support by the Korean Ministry of Knowledge Economy through the New and Renewable Energy (2010T100100651) and the Energy 646
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Nano Letters
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Ed. 2007, 46, 4630. (f) Ithurria, S.; Dubertret, B. J. Am. Chem. Soc. 2008, 130, 16504. (g) Li, Z.; Peng, X. J. Am. Chem. Soc. 2011, 133, 6578. (9) (a) Yu, J.-K.; Mitrovic, S.; Than, D.; Varghese, J.; Heath, J. R. Nat. Nanotechnol. 2010, 5, 718. (b) Hopkins, P. E.; Reinke, C. M.; Su, M. F.; Olsson, R. H. III; Shaner, E. A.; Leseman, Z. C.; Serrano, J. R.; Phinney, L. M.; El-Kady, I. Nano Lett. 2011, 11, 107. (c) Poulikakos, D.; Zhang, X.; Goicochea, J. V.; Giapis, K. P.; Hu, M. Nano Lett. 2011, 11, 618. (d) Kim, W.; Wang, R.; Majumdar, A. Nano Today 2007, 2, 40. (e) Kang, C.; Kim, H.; Park, S.-G.; Kim, W. Appl. Phys. Lett. 2010, 96, 213114. (f) Li, D.; Wu, Y.; Kim, P.; Shi, L.; Yang, P.; Majumdar, A. Appl. Phys. Lett. 2003, 83, 2934. (10) (a) Wang, R. Y.; Feser, J. P.; Lee, J.-S.; Talapin, D. V.; Segalman, R.; Majumdar, A. Nano Lett. 2008, 8, 2283. (b) Scheele, M.; Oeschler, N.; Meier, K.; Kornowski, A.; Klinke, C.; Weller, H. Adv. Funct. Mater. 2009, 19, 3476. (c) Kovalenko, M. V.; Spokoyny, B.; Lee, J.-S.; Marcus, S.; Weber, A.; Perera, S.; Landry, D.; Talapin, D. V. J. Am. Chem. Soc. 2010, 132, 6686. (d) Son, J. S.; Park, K.; Han, M.-K.; Kang, C.; Park, S.-G.; Kim, J. H.; Kim, W.; Kim, S.-J.; Hyeon, T. Angew. Chem., Int. Ed. 2011, 50, 1363. (e) Scheele, M.; Oeschler, N.; Veremchuk, I.; Reinsberg, K.-G.; Kreuziger, A.-M.; Kornowski, A.; Broekaert, J.; Klinke, C.; Weller, H. ACS Nano 2010, 7, 4283. (f) Zhao, Y.; Dyck, J. S.; Hernandez, B. M.; Burda, C. J. Phys. Chem. C 2010, 114, 11607. (g) Zhao, Y.; Dyck, J. S.; Hernandez, B. M.; Burda, C. J. Am. Chem. Soc. 2010, 132, 4982. (h) Drimyer, M. R.; Martin, J.; Nolas, G. S.; Sen, A.; Badding, J. V. Small 2009, 5, 933. (i) Purkayatsha, A.; Kim, S.; Gandhi, D. D.; Pethuraja, G. G.; Borca-Tasciuc, T.; Ramanath, G. Adv. Mater. 2006, 18, 2958. (j) Purkayatsha, A.; Lupo, F.; Kim, S.; Borca-Tasciuc, T.; Ramanath, G. Adv. Mater. 2006, 18, 496. (k) Qiu, X.; Austin, L. N.; Muscarella, P. P.; Dyck, J. S.; Burda, C. Angew. Chem., Int. Ed. 2006, 45, 5656. (l) Lu, W.; Ding, Y.; Chen, Y.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2005, 127, 10112. (m) Liang, W.; Hochbaum, A. I.; Fardy, M.; Rabin, O.; Zhang, M.; Yang, P. Nano Lett. 2009, 9, 1689. (n) Hu, J. Z.; Zhao, X. B.; Zhu, T. J.; Zhou, A. J. Phys. Script. 2007, T129, 120. (o) Fan, X. A.; Yang, J. Y.; Xie, Z.; Li, K.; Zhu, W.; Duan, X. K.; Xiao, C. J.; Zhang, Q. Q. J. Phys. D: Appl. Phys. 2007, 40, 5975. (p) Zhang, G.; Kirk, B.; Jauregui, L. A.; Yang, H.; Xu, X.; Chen, Y. P.; Wu, Y. Nano Lett. 2012, 12, 56. (11) (a) Munir, Z. A.; Anselmi-Taburini, U.; Ohyanagi, M. J. Mater. Sci. 2006, 41, 763. (b) Shen, Z.; Mats, J.; Zhao, Z.; Nygren, M. J. Am. Ceram. Soc. 2002, 85, 1921. (c) Chaim, R. Mater. Sci. Eng., A 2007, 443, 25. (d) Wang, S. W.; Chen, L. D.; Hirai, T. J. Mater. Res. 2000, 15, 982. (12) (a) Matsubara, I.; Funahashi, R.; Takeuchi, T.; Sodeoka, S. J. Appl. Phys. 2001, 90, 462. (b) Liu, Y.; Lin, Y.; Shi, Z.; Nan, C.-W. J. Am. Ceram. Soc. 2005, 88, 1337. (c) Yang, L.; Wu, J. S.; Zhang, L. T. J. Alloys Comp. 2004, 364, 83. (d) Zhao, L. D.; Zhang, B.-P.; Li, J.-F.; Zhang, H. L.; Liu, W. S. Solid State Sci. 2008, 10, 651. (e) Jiang, J.; Chen, L.; Bai, S.; Yao, Q.; Wang, Q. Scr. Mater. 2005, 52, 347. (f) Kurosaki, K.; Maekawa, T.; Muta, H.; Yamanaka, S. J. Alloys Comp. 2005, 397, 269. (f) Jiang, J.; Chen, L.; Bai, S.; Yao, Q.; Wang, Q. Mater. Sci. Eng., B 2005, 117, 334. (g) Zhou, Y.; Hirao, K.; Yamauchi, Y.; Kanzaki, S. Scr. Mater. 2003, 48, 1631. (h) Kuo, C.-H.; Hwang, C.S.; Jeng, M.-S.; Su, W.-S.; Chou, Y.-W.; Ku, J.-R. J. Alloys Comp. 2010, 496, 687. (13) Goldsmid, H. J.; Douglas, R. W. Brit. J. Appl. Phys. 1954, 5, 386. (14) (a) Lu, W.; Ding, Y.; Chen, Y.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2005, 127, 10112. (b) Garje, S. S.; Eisler, D. J.; Ritch, J. S.; Afzaal., M.; O’Brien, P.; Chivers, T. J. Am. Chem. Soc. 2006, 128, 3120. (c) Wang, W.; Poudel, B.; Yang, J.; Wang, D. Z.; Ren, Z. F. J. Am. Chem. Soc. 2005, 127, 13792. (d) Shi, W.; Zhou, L.; Song, S.; Yang, J.; Zhang, H. Adv. Mater. 2008, 20, 1892. (e) Xiao, F.; Yoo, B.; Lee, K. H.; Myung, N. V. J. Am. Chem. Soc. 2007, 129, 10068. (f) Zhang, G.; Yu, Q.; Li, X. Dalton Trans. 2010, 38, 993. (g) Yuan, Q; Radar, K; Hussain, M. M. Chem. Commun. 2011, 47, 12131. (15) (a) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (b) Coleman, J. N.; et al. Science 2011, 331, 568. (c) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Nat. Acad. Sci. U.S.A. 2005, 102, 10451.
(d) Teweldebrhan, D.; Goyal, V.; Rahaman, M.; Baladin, A. A. Appl. Phys. Lett. 2010, 96, 053107. (16) (a) Pecheur, P.; Toussaint, G. J. Phys. Chem. Solids 1994, 55, 327. (b) Dato, P.; Köhler, H. J. Phys. C: Solid State Phys. 1984, 17, 3711. (c) Liu, W.-S.; Zhang, Q.; Lan, Y.; Chen, S.; Yan, X.; Zhang, Q.; Wange, D.; Chen, G.; Ren, Z. Adv. Energy Mater. 2011, 1, 577. (17) (a) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86. (b) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko., E. V. Chem. Rev. 2010, 110, 389. (c) Urban, J. J.; Talapin, D. V.; Shevchenko, E. V.; Kagan, C. R.; Murray, C. B. Nat. Mater. 2007, 6, 115. (18) Kaibe, H.; Tanaka, Y.; Sakata, M.; Nishida, I. J. Phys. Chem. Solids 1989, 50, 945. (19) Zhang, Y.; Wang, H.; Kräemer, S.; Shi, Y.; Zhang, F.; Snedaker, M.; Ding, K.; Moskovits, M.; Snyder, G. J.; Stucky, G. D. ACS Nano 2011, 5, 3158. (20) (a) Goldsmid, H. J., Ed. Electronic Refrigeration; Pion Limited: London, 1986. (b) Sales, B. C.; Mandrus, D.; Chakoumakos, B. C.; Keppens, V.; Thompson, J. R. Phys. Rev. B 1997, 56, 15081. (21) Chiritescu, C.; Mortensen, C.; Cahill, D. G.; Johnson, D.; Zschack, P. J. Appl. Phys. 2009, 106, 073503. (22) Lee, H.; Vashaee, D.; Wang, D. Z.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. J. Appl. Phys. 2010, 107, 094308.
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