1796
J. Phys. Chem. C 2010, 114, 1796–1799
Microsphere Bouquets of Bismuth Telluride Nanoplates: Room-Temperature Synthesis and Thermoelectric Properties Tie Wang,† Rutvik Mehta,† Chinnathambi Karthik,† P. Gopal Ganesan,† Binay Singh,† Wei Jiang,† N. Ravishankar,†,‡ Theo Borca-Tasciuc,§ and Ganpati Ramanath*,† Materials Science and Engineering Department and Department of Mechanical Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, and Materials Research Centre, Indian Institute of Science, Bangalore 560012, India ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: NoVember 16, 2009
We report the synthesis and properties of sphere-shaped microscale aggregates of bismuth telluride nanoplates. We obtain porous microspheres by reducing bismuth chloride and orthotelluric acid with hydrazine in the presence of thioglycolic acidswhich serves as the shape-and size-directing agentsfollowed by roomtemperature agingswhich promotes nanoplate aggregation. Thin film assemblies of the nanoplate microspheres exhibit n-type behavior due to sulfur doping and a Seebeck coefficient higher than that reported for assemblies of chalcogenide nanostructures. Adaptation of our scalable approach to synthesize and hierarchically assemble nanostructures with controlled doping could be attractive for tailoring novel thermoelectric materials for applications in high-efficiency refrigeration and harvesting electricity from heat. Nanostructuring is a promising means to realize factorial increases in thermoelectric figure of merit ZT (defined as (R2σ/ κ)T, where σ is the electrical conductivity, κ the thermal conductivity, R the Seebeck coefficient, and T the absolute temperature) for developing materials for solid-state cooling and electrical power generation from heat.1-3 Nanoscale confinement not only facilitates a decrease in κ by enhancing boundary scattering, but also allows for increases in σ and R through quantum effects.4-11 Several works12-19 have reported the synthesis of nanostructured building blocks of various shapes, e.g., nanoparticles, nanorods, and nanoplates, of chalcogenides materials that exhibit high ZT in the bulk to explore further increases in ZT by introducing spatial confinement in one to three dimensions. One of the major challenges is to produce macroscopic samples from such nanoscale blocks for applications. Here, we report the synthesis of microscale sphere-shaped aggregates of size-controlled Bi2Te3 nanoplates that could be attractive for scalably producing macroscale samples of nanostructured thermoelectric materials. Our synthesis involves the reduction of BiCl3 and orthotelluric acid with hydrazine in the presence of thioglycolic acid (TGA), followed by roomtemperature aging to induce aggregation. Thin film assemblies of these chalcogenide nanoplate microspheres show n-type behavior due to sulfur doping and a higher Seebeck coefficient than assemblies of spherically shaped chalcogenide nanoparticles. Our scalable approach is adaptable to hierarchically assemble nanostructures of thermoelectric materials across multiple length scales for designing new materials for emerging applications in high-efficiency refrigeration and power generation from heat. A low-magnification scanning electron microscopy (SEM) image from a sample aged for 48 h (Figure 1A) shows a large * To whom correspondence should be addressed. E-mail: Ramanath@ rpi.edu. † Materials Science and Engineering Department, Rensselaer Polytechnic Institute. ‡ Indian Institute of Science. § Department of Mechanical Engineering, Rensselaer Polytechnic Institute.
Figure 1. (A) Low- and (B) high-magnification SEM images of Bi2Te3 microspheres. (C) A cross-section showing that the microspheres are comprised of interconnected 4 ( 2 nm nanoplates. SEM micrographs from bismuth telluride microspheres synthesized using (D) 0.25 and (E) 2.50 vol % TGA capture the effect of the TGA concentration on the nanoplate thickness.
number of porous microspheres with diameters ranging between 500 nm and 4 µm. High-magnification SEM images (Figure
10.1021/jp908727b 2010 American Chemical Society Published on Web 01/07/2010
Microsphere Bouquets of Bismuth Telluride Nanoplates
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1797
Figure 2. X-ray diffractogram from a bismuth telluride microsphere sample plotted on a logarithmic ordinate to accentuate the smaller peaks. The Bragg peaks correspond to trigonal bismuth telluride with lattice parameters a ) 0.4385 nm and c ) 3.0483 nm. The peaks marked with an asterisk are from bismuth sulfide telluride.
1B,C) reveal that the microspheres are comprised of a bouquet of ∼4 nm thick petal-like nanoplates. The characteristic spacing between the nanoplates, i.e., the pore size in the spheres, ranges between 25 and 300 nm. Baseline experiments show that TGA plays an important role in Bi2Te3 nanoplate formation. Synthesis experiments without TGA result in Bi2Te3 crystals with irregular shapes and a wide size distribution (see Figure S1 in the Supporting Information). Increasing the TGA concentration from 0.25 to 2.5 vol % during synthesis increases the nanoplate thickness from 3 ( 2 to 20-40 nm (Figure 1D,E). These results suggest that TGA amplifies the inherent anisotropy in the layered structure12,20-22 of trigonal Bi2Te3, leading to the formation of plate-shaped structures. X-ray diffractograms (see Figure 2) reveal that the nanoplates in the microspheres consist of trigonal Bi2Te3 with small amounts of the hexagonal bismuth sulfide telluride phase.23 The latter indicates that sulfur is injected into the Bi2Te3 crystal, likely through partial hydrolysis of the mercaptan in TGA. We could not analyze the as-prepared microspheres by transmission electron microscopy (TEM) because of the large sphere diameters (see Figure S2 in the Supporting Information). The microspheres disintegrated upon electron beam exposure for even only a few seconds, allowing us to examine the fragments, which typically consisted of faceted and fused grains or dispersed nanorods. Diffraction patterns and high-resolution TEM images (Figure S2) of the grains reveal trigonal Bi2Te3, consistent with the XRD results. Core-level spectra obtained from the microsphere dispersions using X-ray photoelectron spectroscopy (XPS) reveal a S 2p3/2 sub-band at 160.8 eV corresponding to ligated sulfur, indicating that TGA is anchored to the nanoplate surfaces via the mercaptan moiety24 (Figure 3A,B). The presence of the C 1s carboxyl signature at 287.6 eV is consistent with TGA passivation of the Bi2Te3 nanostructures. The nanosphere surfaces consist of both oxidized and unoxidized forms of bismuth and tellurium, evident from the presence of Bi 4f7/2 and Te 3d5/2 sub-bands at 159.2 and 575.5 eV, respectively, besides the unoxidized states at 158.2 and 572.3 eV (Figure 3B,C). The oxidized states could arise from surface oxide and/or thioligation and cannot be discerned from our measurements because TGA contains both oxygen and sulfur. To obtain insights into the mechanism of nanoplate formation and aggregation into microspheres, we examined the products at different conditions and stages of the synthesis (see Figure S3 in the Supporting Information). Reduction of orthotelluric acid with hydrazine in the absence of BiCl3 forms Te nano-
Figure 3. Core-level bands of (A) C 1s, (B) Bi 4f, and (C) S 2p acquired by XPS scans from as-prepared bismuth telluride microsphere films.
crystals instantaneously. The presence of BiCl3 slows Te2oxidation, forming Te nanocrystals in only ∼5-10 min, which then transform to Bi2Te3 as the reaction proceeds. In contrast, hydrazine reduction of BiCl3 in the absence of orthotelluric acid produces no solid products before 5 h; we observe Bi precipitation after ∼9 h. These observations suggest that Bi2Te3 forms by the reaction of Te2- and Bi3+ ions in solution. The initially high Te/Bi molar ratio decreases with aging time to stabilize at about 1.5, indicating that a Te-rich product forms first and gradually transforms to Bi2Te3. Increasing the hydrazine concentration leads to smaller and more equiaxed Bi2Te3 nanoparticles (Figure 4B,C), indicative of the strong influence of the reduction rate on nanostructure shape selection. Aging experiments carried out under different stirring conditions show that microsphere formation via nanoplate aggregation is sensitive to the stirring rate. Vigorous stirring leads to the formation of loose aggregates of packs of nanoplate stacks, indicating a high rate of face-to-face collisions in solution (Figure 4D). In unstirred solutions or at low stirring rates, the plates aggregate into spheres due to multiple random collisions in all possible orientations to yield porous spherical aggregates. These results are consistent with the higher sticking probability
1798
J. Phys. Chem. C, Vol. 114, No. 4, 2010
Figure 4. (A) Te/Bi molar ratio at different reaction times. SEM images of porous Bi2Te3 microspheres synthesized with (B) 2.9 and (C) 2.61 vol % hydrazine. The inset in (C) shows the finer feature sizes at higher hydrazine concentration. (D) SEM image showing loosely packed clusters of face-to-face stacks of nanoplates when vigorous stirring was employed during room-temperature aging.
of the platelets colliding in the face-to-face configuration than the face-to-edge configuration. Drop-cast films with an average thickness of ∼11 µm comprised of Bi2Te3 microspheres reveal a high Seebeck coefficient R with a negative sign and linear current-voltage behavior (Figure 5). Prior to our measurements, the films were annealed at 250 °C for 2 h to desorb TGA23 and enhance conducting pathways. The electrical conductivity σ ≈ 40 Ω-1 m-1 of the annealed films is lower than that of bulk Bi2Te325 mainly attributable to interparticle resistance. No microscale morphological changes were detectable in the microspheres upon annealing,24 suggesting that the annealing treatment mainly alters the microsphere surfaces, enabling better intersphere contact. The microsphere film assemblies exhibit a room-temperature Seebeck coefficient R ) -131 µV/K, which is higher in magnitude than corresponding values reported for random assemblies of Ag(Pb1-ySny)mSbTe2+m nanocrystals, spherical or rod-shaped Bi2Te3 nanostructures, and Sb2Te3 nanoplates,22-24,26 but ∼57% lower than that of undoped25 Bi2Te3. The negative sign arises from n-type semiconductor behavior due to sulfur doping, consistent with our XRD and XPS results and reminiscent of prior reports2,23,24 on Sand Se-doped Bi2Te3. The magnitude of the Seebeck coefficient is very sensitive to the defect states and the local electronic structure near the Fermi level, which are expected to differ for doped and undoped chalcogenides, and is yet to be fully understood. In summary, we have synthesized microspheres of bismuth telluride nanoplate assemblies at room temperature. The nano-
Wang et al.
Figure 5. (A) Current-voltage and (B) Seebeck voltage characteristics from a thin film assembly of Bi2Te3 microspheres. The inset in (A) shows a photograph of a test device used for both measurements. The Bi2Te3 microsphere film is the dark patch in the center, and the microheater is the square pattern on the right. The inset in (B) shows an SEM cross-section of the thin film microsphere assembly; the scale bar is 10 µm.
plates form through the transformation of a Te-rich phase formed by the reduction of bismuth and tellurium precursors in the presence of thioglycolic acid. Porous microspheres form by nanoplate aggregation during room-temperature aging. Thin film assemblies of the microspheres exhibit n-type behavior and a high Seebeck coefficient arising from sulfur doping of Bi2Te3. Filling interplate voids in the microspheres or intersphere spacings in the film with other chalcogenides could be attractive for obtaining new composite microspheres and assemblies with novel thermoelectric properties. Experimental Section Bi2Te3 Microsphere Synthesis. Orthotelluric acid (H6TeO6; 99.99%), bismuth chloride (BiCl3), TGA, and hydrazine anhydrate were purchased from Aldrich and used without further purification. In a typical synthesis, 37 mg (0.162 mmol) was dissolved in 2 mL of water. An aqueous solution was prepared in another beaker with 33.5 mg (0.107 mmol) of BiCl3 and 0.4 mL of HNO3 in 1.5 mL of H2O. Adding 0.14 mL of TGA (2.16 mmol) transforms this initially transparent solution to a yellow color due to bismuth complexation. H6TeO6 dissolved in 2 mL of water was mixed with the yellow solution and 30 mL of water and sonicated for 5 min. A 6 mL (120 mmol) portion of hydrazine anhydrate was added drop by drop to the sonicated solution, followed by room temperature aging for 48 h. Samples (∼2 mL) were extracted from the reaction mixture for analyses at different reaction time intervals between 5 min and 48 h. Material Microanalysis. TEM images and diffraction patterns were obtained using a JEOL 2010 microscope operated at 200 kV. SEM images were taken using a LEO-SUPRA 55
Microsphere Bouquets of Bismuth Telluride Nanoplates field-emission instrument operated at 5 kV. The as-prepared microspheres were dispersed in water, sonicated for 2 min, dropcast over a carbon-coated copper grid for TEM, on a Si wafer piece for SEM and XPS, or on a glass slide for X-ray diffraction, and air-dried in all three cases prior to the respective measurements. X-ray diffractograms were obtained using a Scintag PAD-V diffractometer with a Cu KR probe beam. X-ray photoelectron spectra of bismuth telluride microspheres were obtained using a PHI 5400 instrument with a Mg KR probe beam. Thermoelectric Properties. A computer-controlled multiprobe system was used to measure σ and R of films of Bi2Te3 microsphere assemblies. The films were drop-cast from aqueous microsphere dispersions on a glass substrate with a lithographically patterned gold microheater and microelectrodes (left inset of Figure 5A). While σ was measured using a four-probe technique, R was determined by establishing a temperature gradient across the film by supplying electrical power to the heater. Electrodes buried underneath the thermoelectric film monitored the Seebeck voltage developed between the heat source and sink, and the temperature difference was measured using two thermistors. Acknowledgment. We gratefully acknowledge support from the Interconnect Focus Center funded by MARCO and New York state, the National Science Foundation under Grants DMR 0519081 and CTS 0348613, and a research initiation grant from Honda. Supporting Information Available: Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddard, W. A., III; Heath, J. R. Nature 2008, 451, 168. (2) Chen, G.; Dresselhaus, M. S.; Dresselhaus, G.; Fleurial, J. P.; Caillat, T. Int. Mater. ReV. 2003, 48, 45.
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1799 (3) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. (4) Dresselhaus, M. S.; Dresselhaus, G.; Sun, X.; Cronin, S. B.; Koga, T. Phys. Solid State 1999, 41, 679. (5) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 16631. (6) Liufu, S.-C.; Chen, L.-D.; Yao, Q.; Wang, C.-F. Appl. Phys. Lett. 2007, 90, 112106. (7) Majumdar, A. Science 2004, 303, 777. (8) Sales, B. C. Science 2002, 1248. (9) Sun, X.; Zhang, Z.; Dresselhaus, M. S. Appl. Phys. Lett. 1999, 74, 4005. (10) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (11) Wang, W.; Poudel, B.; Yang, J.; Wang, D. Z.; Ren, Z. F. J. Am. Chem. Soc. 2005, 13792. (12) 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. (13) Foos, E. E.; Stroud, R. M.; Berry, A. D. Nano Lett. 2001, 1, 693. (14) Garje, S. S.; Eisler, D. J.; Ritch, J. S.; Afzaal, M.; O’Brien, P.; Chivers, T. J. Am. Chem. Soc. 2006, 128, 3120. (15) Ma, Y.; Hao, Q.; Poudel, B.; Lan, Y.; Yu, B.; Wang, D.; Chen, G.; Ren, Z. Nano Lett. 2008, 8, 2580. (16) 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. (17) Poudeu, P.; D’Angelo, J.; Kong, H.; Downey, A.; Short, J. L.; Pcionek, R.; Hogan, T. P.; Uher, C.; Kanatzidis, M. G. J. Am. Chem. Soc. 2006, 128, 14347. (18) Shi, W.; Yu, J.; Wang, H.; Zhang, H. J. Am. Chem. Soc. 2006, 128, 16490. (19) Tang, X.; Xie, W.; Li, H.; Zhao, W.; Zhang, Q. Appl. Phys. Lett. 2007, 90, 012102. (20) Li, G.-R.; Zheng, F.-L.; Tong, Y.-X. Cryst. Growth Des. 2008, 8, 1226. (21) Lu, W.; Ding, Y.; Chen, Y.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2005, 127, 10112. (22) Shi, W.; Zhou, L.; Song, S.; Yang, J.; Zhang, H. AdV. Mater. 2008, 20, 1892. (23) Purkayastha, A.; Lupo, F.; Kim, S.; Borca-Tasciuc, T.; Ramanath, G. AdV. Mater. 2006, 18, 496. (24) Purkayastha, A.; Kim, S.; Gandhi, D. D.; Ganesan, P. G.; BorcaTasciuc, T.; Ramanath, G. AdV. Mater. 2006, 18, 2958. (25) Nolas, G. S.; Sharp, J.; Goldsmid, H. J. Thermoelectrics: Basic Principles and New Materials DeVelopments; Springer: New York, 2001. (26) Androulakis, J.; Hsu, K. F.; Pcionek, R.; Kong, H.; Uher, C.; D’Angelo, J. J.; Downey, A.; Hogan, T.; Kanatzidis, M. G. AdV. Mater. 2006, 18, 1170.
JP908727B
