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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
Nano Energy (]]]]) ], ]]]–]]]
http://dx.doi.org/12211-2855/& 2014 P
nCorresponding auE-mail address: c
Please cite this arthttp://dx.doi.org/
RAPID COMMUNICATION
Power conversion of hybrid Bi2Te3/sithermoelectric nanocomposites
Cheng-Lun Hsinn, Yue-Yun Tsai
Department of Electrical Engineering, National Central University, Taoyuan 32001, Taiwan
Received 1 October 2014; received in revised form 24 November 2014; accepted 25 November 2014
KEYWORDSThermoelectric;Nanowire;Bi2Te3;Nanocomposite;Power conversion
0.1016/j.nanoen.2ublished by Elsevi
thor. Tel.: +886 [email protected]
icle as: C.-L. Hsin,10.1016/j.nanoen
AbstractNanostructured thermoelectric materials are promising for future energy conversion and harvesting.Recently, cost-effective and roughened Si nanowire has been demonstrated to be a good candidate.In this work, we report a solution process to synthesize composite nanostructures of Si nanowires andBi2Te3 nanoparticles by chemical-solution reactions for future thermoelectric applications. Bi2Te3nanoparticles can be coated among the nanowires of 30 μm in length, till a uniform coating andwrapping around nanowires. Structural characterization of the composite was identified by X-raydiffraction technique and standard microscopy practices. The power conversion of the nanocompo-site at different temperatures was measured, demonstrating the harvesting of thermal energy. Thisapproach provides an applicable route to produce advanced high-performance thermoelectricnanocomposites at micro-scale for future thermal scavenging and cooling in electronics.& 2014 Published by Elsevier Ltd.
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Introduction
For the purpose of converting waste heat into useful electri-city, thermoelectric materials have attracted much attentionin the past decades [1–3]. The dimensionless thermoelectricfigure-of-merit is defined by ZT= S2T/ρκ, where κ, S, ρ, and Tare the thermal conductivity, Seebeck coefficient, electricalresistivity, and absolute temperature, respectively [4–6]. Atroom temperature, Bi2Te3 and related alloys still hold therecord of high ZT and better thermoelectric performance
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4227151 34520..tw (C.-L. Hsin).
Y.-Y. Tsai, Power conversion of hyb.2014.11.053
around room temperature [7–10]. In practical applications togenerate electricity, a temperature gradient will be applied onthe thermoelectric materials. In this case, the dimensionalscale of the materials along the temperature gradient shouldbe large enough to match the temperature difference. How-ever, the production cost for Bi2Te3–based thermoelectricmicro-scaled thick film makes the commercialization difficult[11]. On the other hand, centimeter-scaled thermoelectricbulk materials may outrange the volume restriction forportable electronics [12]. Thus, the possible applications onminiaturized electronic products are limited. Recently, Sinanowires with roughened surface have been demonstratedto be a promising candidate due to low synthesis costand feasible material processing [13,14]. The rough surface
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Fig. 1 Schematic illustration of the formation process. (a) Si nanowire etching, (b) hydrazine monohydrate infiltration, (c) Bi2Te3nanoparticle synthesis, and (d) nanocomposite formation.
Fig. 2 (a) Cross-sectional SEM image of as-formed Si nanowires by chemical etching, (b) Top-view SEM image of the nanowires,showing the small interval between the nanowires, (c) TEM image of single Si nanobelt, and (d) the HRTEM image of the nanobelt.The inset is the corresponding FFT electron diffraction pattern.
C.-L. Hsin, Y.-Y. Tsai2
Please cite this article as: C.-L. Hsin, Y.-Y. Tsai, Power conversion of hybrid Bi2Te3/si thermoelectric nanocomposites, Nano Energy (2014),http://dx.doi.org/10.1016/j.nanoen.2014.11.053
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3Power conversion of hybrid Bi2Te3/si thermoelectric nanocomposites
provided a route for phonon scattering and hence decreasedthe thermal conductivity. The breakthrough on the figure ofmerit of Si has shed a light on many applications on heatconversion at room temperature. Nevertheless, higherfigure of merit is still necessary to reach the expectationand contact electrode for nanowire is also another issue.One successful route to further improve the performance ofthe roughened Si nanowires is though nanomaterials engi-neering with efficient thermoelectric materials [15]. In thisstudy, we developed a cost effective and feasible approachto obtain nanocomposite thick film using chemical solutionmethod. Si nanowires were fabricated by wet-chemicaletching and then Bi2Te3 nanoparticles were synthesizedand coated around the as-formed Si nanowires. The 30-μm-thick nanocomposite thermoelectric materials wereconfirmed by various structural identification techniques.The power conversion of the nanocomposite at differenttemperatures was demonstrated and the power densitywas estimated. These results prove that this method isa feasible route to produce high quality thermoelectriccomposite nanomaterials for future thermal scavenging andcooling [16,17].
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Sample preparation, characterization, andmeasurement procedures.
All chemicals were obtained from Sigma Aldrich and usedwithout further purification. Fig. 1 is the schematic illustra-tion of the fabrication process. (001) Si wafers were etched
Fig. 3 (a) The TEM image of the Bi2Te3 nanoparticles, (b) the conanoparticles, indicating the chemical composition.
Please cite this article as: C.-L. Hsin, Y.-Y. Tsai, Power conversion of hybhttp://dx.doi.org/10.1016/j.nanoen.2014.11.053
by mixed solution of 0.4 M AgNO3 and HF for 30 min. Afterthe etching process, the by-products (Ag nanostructures)were removed by nitric acid, leaving the nanowire bundles(Fig. 1(a)). The samples were cleaned by deionized waterand dilute HF is used to remove the surface oxide. Thendifferent amount of hydrazine monohydrate (98%) was con-trolled and infiltrated through the as-formed nanowires(Fig. 1(b)). A solution of mixing BiCl3 (0.2 M) in nitric acidand H6TeO6 (0.15 M) in deionized water was prepared,followed by the addition of thioglycolic acid for the forma-tion of the mixed complex at room temperature. Then thesolution temperature was raised to 90 1C and Bi2Te3 nano-particles were synthesized when dipping the hydrazine-coated samples in the as-prepared solution for the reductionof the Bi2Te3 (Fig. 1(c)). Different controlling parameterswere conducted to elucidate the reaction process and obtaindifferent morphologies (Fig. 1(d)). The as-synthesized sam-ples were then washed with excess acetone and driedby N2 blow.
Sample morphology was imaged by a JEOL 6500 field-emission scanning electron microscope (FESEM). JEOL 2010transmission electron microscope (TEM) was used to char-acterize the microstructures and determine chemical com-position. X-ray diffractometer and Energy dispersion spe-ctrometer (EDS) was conducted to identify the chemicalelements. Fast Fourier transform technique was used in thehigh resolution TEM (HRTEM) image to analyze the diffractionpattern. The power conversion was measured by Keithley2602b from 40 1C to 180 1C on a homemade temperaturecontroller.
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123rresponding diffraction pattern, and (c) EDS spectrum of the
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Results and discussion
Fig. 2(a) is the cross-sectional SEM image of as-formed Sinanowires with length greater than 30 μm. The nanowires areso long that they would be inclined and become bundle-like.The gaps between the nanowires and bundles range fromseveral tens of nanometers to micrometers (Fig. 2(b)). Bes-ides, the aspect ratio of the structure is around 100�1000.Fig. 2(c) depicts the TEM image, showing the belt-like nan-owires of about 150 nm width. The dark fringe could be due tonon-uniform thickness. HRTEM image in Fig. 2(d) reveals thesurface roughness of the nanowires. Lattice planes are clearlyevident in the image. The FFT pattern in the inset of Fig. 2(b) from the HRTEM image confirms the crystallographicdirection of the nanowire along the [0 0 1] direction.
Fig. 3(a) is the as-synthesized Bi2Te3 nanoparticles andFig. 3(b) is the corresponding diffraction pattern, respec-tively. The nanoparticles were synthesized by adding thehydrazine monohydrate into the as-prepared solutions.Prominently, the nanoparticles are uniform in size withfacets. The diameter of the precipitate is around 10 nm in
Fig. 4 (a) Cross-sectional SEM image of the Si nanowires with Bmagnification image highlighted with an mark in (a), (c) Correspospectrum of the nanocomposite. The inset is the table of the compoand Te.
Please cite this article as: C.-L. Hsin, Y.-Y. Tsai, Power conversion of hyhttp://dx.doi.org/10.1016/j.nanoen.2014.11.053
diameter. From the image contrast in Fig. 3(a), it is believedthat the nanoparticle is single crystalline. EDS spectrum inFig. 3(c) confirmed the stoichiometry of Bi and Te.
By controlling the experimental parameters, the proce-dures of the filling process can be elucidated. The cross-sectional SEM image in Fig. 4(a) demonstrated the infiltra-tion of the reactant and thus the production of nanoparti-cles. The Bi2Te3 nanoparticles can be observed either at thetop or bottom of the intervals with uniform size distribu-tion. Fig. 4(b) is the high magnification SEM image high-lighted in (a). Although the aspect ratio of the interval ofthe bundles or nanowires is much higher than that of theconventional interconnect through/trench structure in theintegrated circuit, the overhang and blocking at the top wasnot observed. The concern of the as-synthesized Bi-Tecompound phase can be revealed in the XRD spectrum inFig. 4(c). The characteristic peaks can be identified to beBi2Te3 nanoparticles. EDS spectrum (Fig. 4d) carried out inthe SEM confirmed the chemical elements and the table inthe inset indicated the stoichiometry of the Bi2Te3 nano-particles. The as-formed nanoparticle can be seen clearly
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i2Te3 nanoparticle infiltrated within the bundles, (b) The highnding XRD spectrum of the as-formed nanocomposite, (d) EDSsition of each element, indicating the stoichiometric ratio of Bi
brid Bi2Te3/si thermoelectric nanocomposites, Nano Energy (2014),
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Fig. 5 (a) HRTEM image of the Bi2Te3 nanoparticle on Si nanowire.(b) Low-magnification TEM image showing the uniform coverage ofthe nanoparticles, (c) EDS spectrum of the nanocomposite.
Fig. 6 A series of SEM images presenting the structure of the nanocom(b) Nanocomposite with Bi2Te3 filling and wrapping the Si nanowire bunimages highlighted the (d) top, (e) middle, and (f) bottom of the nanocom
5Power conversion of hybrid Bi2Te3/si thermoelectric nanocomposites
on the Si nanowire surface, as shown in the HRTEM image ofFig. 5(a). In the low-magnification TEM image, nanoparticleshave covered the surface of the nanowire uniformly.Besides, the size distribution of the nanoparticle is identicalto the observation of the SEM image in Fig. 4(b). Fig. 5(c) isthe EDS spectrum of the corresponding TEM image, showingthe composition of the nanocomposite.
The synthesis of nanocomposite can be demonstratedby Bi2Te3 nanosheath, which is encircling and filling thebundles. A series of SEM images of the nanocomposite is inthe Fig. 6. To compare with Fig. 2(b) and Fig. 6(a), Bi2Te3has filled in the interval of nanowires (inside the bundle). Indifferent process, the amount of the Bi2Te3 was controlledand covered the bundles, as shown in the SEM image ofFig. 6(b). Fig. 6(c) is the high-magnification SEM image of(b), proving that Bi2Te3 has also filled inside the bundle.Fig. 6 (d), (e) and (f) are the cross-sectional SEM images of(b). It is evident that the filling and covering of Bi2Te3 isuniform from the top to the bottom. Moreover, there is nooverhang or void observed within the bundles. This struc-ture demonstrated that a 30-μm-thick film of thermoelec-tric nanocomposite can be achieved cost-effectively by thisapproach. In addition, the nanoparticle-composed Bi2Te3sheath synthesized at low temperature would be a promis-ing structure. The higher performance of the nanoengi-neered bulk thermoelectric was credited by its lower-ed thermal conductivity due to phonon scattering at the
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posite. (a) A top-view SEM image showing the filling of the Bi2Te3.dles. (c) High-magnification SEM image of (b). Cross-sectional SEMposite, proving that there is no overhang or void within the bundles.
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Fig. 7 The plot of power conversion at different tempera-tures. The inset depicted the schematic configuration ofthe setup.
C.-L. Hsin, Y.-Y. Tsai6
grain boundaries. For the nanocomposite in this study, lowthermal conductivity was expected due to the rough inter-face of the Si nanowires and Bi
2Te3 filling. However, thethermal budget of the manufacturing process in thisapproach is much lower than that of the nanoengineeringprocess.
The power conversion of the nanocomposite at differenttemperatures was measured and the setup was depicted inthe inset of Fig. 7. The sample was placed on two pedestalsof the temperature controller to introduce a temperaturedifference. Two tungsten probes with tip of 70 nm indiameter were positioned on the top of a bundle-likenanocomposite as in Fig. 6(a). The power conversion ofthe nanocomposite was shown in Fig. 7, where the tem-perature was defined by the hot terminal. Normally, thetemperature difference between two probes (� 10 μm)would be increased due to the increase of the temperatureat the hot terminal. However, the temperature differencewas estimated to be less than 1 1C even when the pedestalwas set at 180 1C. This is because the distance between twoprobes is much less than the gap. The output power wasincreased at higher temperatures and reached a max-imum�1 nW at 180 1C. If the heat conduction is out-of-plane and sample surface is air-cooled, the temperaturedifference across the nanocomposite could be up to 8 1C.When the Seebeck coefficient (� 200 μV/K), the volume ofinterest (10 μmn10 μmn30 μm) and the series resistivity(4 mΩ-cm) of the nanocomposite were taken into consid-eration, the power conversion density of a centimeter-scaled sample was estimated to be up to�0.2 W/cm2 underthis condition. The power generated should be applicablefor portable electronics in the future.
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Conclusion
In summary, a solution process to synthesize compositenanostructures of Si nanowire and Bi2Te3 nanoparticle forthermoelectric applications was reported. Bi2Te3 nanopar-ticles are filled between the nanowires with high aspectratio and coated on the nanowire bundle surface. A 30-μm-thick film of thermoelectric nanocomposite can be achievedcost-effectively by this approach. Structural characteriza-tion of the composite was identified thoroughly using X-ray
Please cite this article as: C.-L. Hsin, Y.-Y. Tsai, Power conversion of hyhttp://dx.doi.org/10.1016/j.nanoen.2014.11.053
diffraction technique and standard microscopy practices.The power conversion of the nanocomposite from roomtemperature to 180 1C was demonstrated. The power con-version density of a centimeter-scaled sample was esti-mated to be up to�0.2 W/cm2. This approach provides anapplicable route to synthesize advanced high performancethermoelectric composite materials at low temperature andthe nanocomposite is expected to extend its potentialapplication to commercial electronics.
Acknowledgment
The authors acknowledge the support from Ministry of Scienceand Technology, Taiwan Grants MOST 103–2633-E-008-001-.
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Cheng-Lun Hsin received his B.S. degree andPh.D. degree in Dept. of Materials Science andEngineering from National Tsing-Hua University,Taiwan in 2002/06 and 2008/05, respectively.From 2009 to 2012, he was a PostdoctoralResearch Fellow in National Chiao Tung Uni-versity and University of California, Berkeley.Since 2012/8, he has been an Assistant Pro-fessor with the Electrical Engineering Depart-ment, National Central University, Taiwan. His
research interests include electronics, thermoelectrics and plasmonics.
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7Power conversion of hybrid Bi2Te3/si thermoelectric nanocomposites
Dr. Hsin has won the Exploration Research Award of Pan Wen YuanFoundation, Taiwan on 2013 and is a member of the Chinese Institute ofElectrical Engineering.
Yue-Yun Tsai received the B.S. degree inElectrical Engineering from National Kaoh-siung First University of Science and Tech-nology, Taiwan, in 2013. He is currentlyworking toward the master degree in Elec-trical Engineering at National Central Uni-versity, Taiwan. His research interestincludes the development and measurementof thermoelectric devices and fundamentalstudy of metal oxide nanostructures.
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Please cite this article as: C.-L. Hsin, Y.-Y. Tsai, Power conversion of hybrid Bi2Te3/si thermoelectric nanocomposites, Nano Energy (2014),http://dx.doi.org/10.1016/j.nanoen.2014.11.053
