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Optimization of the Electrodeposition Process of High-Performance Bismuth Antimony Telluride Compounds for Thermoelectric Applications Jekaterina Kuleshova,† Elena Koukharenko,‡ Xiaohong Li,§ Nicole Frety, Iris S. Nandhakumar,*,† John Tudor,‡ Steve P. Beeby,‡ and Neil M. White‡
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† School of Chemistry, University of Southampton, Southampton, SO17 1BJ, U.K., ‡School of Electronics and Computer Science, University of Southampton, Southampton, SO17 1BJ, U.K., §School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, U.K., and Institut Charles Gerhardt-Equipe PMOF, Universit e Montpellier II, Montpellier, France
Received May 15, 2010. Revised Manuscript Received September 8, 2010 High-quality films of bismuth antimony telluride were synthesized by electrodeposition from nitric acid electroplating baths. The influence of a surfactant, sodium ligninsulfonate, on the structure, morphology, stoichiometry, and homogeneity of the deposited films has been investigated. It was found that addition of this particular surfactant significantly improved the microstructural properties as well as homogeneity of the films with a significant improvement in the thermoelectric properties over those deposited in the absence of surfactant. A detailed microprobe analysis of the deposited films yielded a stoichiometric composition of Bi0.35Sb1.33Te3 for the films electrodeposited in the absence of surfactant and a stoichiometry of Bi0.32Sb1.33Te3 for films deposited in the presence of surfactant.
1. Introduction One of the challenges for modern society is the pursuit of alternative energy resources, necessitated by the depletion of the limited supply of natural resources and a variety of broader ecological problems. One of the main approaches to solving this problem is to make greater use of unused energy sources within the environment and convert these into useful electrical power; solar, wind, vibration, and thermoelectric conversion are common examples. Approximately 90% of the world’s power is generated by technologies that use fossil fuel combustion as a heat source, and these typically operate with only 30-40% efficiency.1 In this context, solid-state thermoelectric materials can offer great potential in many applications by converting waste heat to electricity. Thermoelectric (TE) generation systems offer a viable method of power harvesting by converting heat energy directly into electrical energy without moving parts, with silent operation, long lifetime, no emission of toxic gases, no maintenance and high reliability.2 The drawbacks of existing TE generators include their low efficiency, e.g., commercially available devices have an efficiency of ∼10% with figures of merit ZT ∼ 1, and require a high operational temperature gradient (50-70 °C).2,3 The efficiency of thermoelectric materials is defined by the figure of merit, ZT, which can be expressed as ZT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively.4 Enhancing the ZT value hence requires a high Seebeck coefficient and electrical conductivity and a low thermal conductivity. Bismuth-telluride-based alloys, such as Bi2Te2.7Se0.3 and Bi0.5Sb1.5Te3 forming n-type and p-type thermoelectric materials, *To whom correspondence should be addressed. (1) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Enhanced thermoelectric performance of rough silicon nanowires. Nature 2008, 451, 163–168. (2) Rowe, D. M. CRC Handbook of Thermoelectrics; CRC Press: Boca Raton, FL, 1995. (3) Tellurex Home Page. http://www.tellurex.com. (4) Goldsmid, H. J. Thermoelectric Refregeration; Plenum: New York, 1964.
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respectively, are known as the best commercially available thermoelectric materials for applications near room temperature.2,5 Until now, different fabrication techniques, such as single crystal growth (Czochralski, Bridgman methods),6 thin film technologies (metal organic vapor deposition (MOVPD), molecular beam epitaxy (MBE) including liquid phase epitaxy),7 and bulk powder syntheses8 have been employed for bismuth-telluride-based compounds. These commonly used methods are either costly or difficult to realize. Electrochemical deposition provides an attractive alternative route to the fabrication of high-quality bismuth-telluride films with promising thermoelectric properties offering several advantages9 over other methods. These include low cost, high controllability, compatibility with silicon microfabrication processes, as well as room-temperature fabrication. While the electrodeposition mechanism has been investigated in great detail for n-type binary Bi2Te3 and n-type ternary Bi-Te-Se compounds by many research groups,9,10 there is still (5) Nolas, G. S.; Sharp, J.; Goldsmid, H. J. Thermoelectrics: Basic Principles and New Materials Developments; Springer: New York, 2001. (6) (a) Svechnikova, T. E.; Chizhevskaya, S. N.; Maksimova, N. M.; Polikarpova, N. V.; Konstantinov, P. P.; Alekseeva, G. T. Doping of the Bi2Te2.85Se0.15 solidsolution with germanium. Inorg. Mater. 1994, 30(2), 161–164. (b) Ashida, M.; Hamachiyo, T.; Hasezaki, K.; Matsunoshita, H.; Kai, M.; Horita, Z. Texture of bismuth telluride-based thermoelectric semiconductors processed by high-pressure torsion. J. Phys. Chem. Solids 2009, 70(7), 1089–1092. (7) (a) Boulouz, A.; Giani, A.; Pascal-Delannoy, F.; Boulouz, M.; Foucaran, A.; Boyer, A. J. Cryst. Growth 1998, 194, 336. (b) Peranio, N.; Eibl, O.; Nurnus, J. Structural and thermoelectric properties of epitaxially grown Bi2Te3 thin films and superlattices. J. Appl. Phys. 2006, 100(11), 10. (8) Wenjie, X.; Xinfeng, T.; Yonggao, Y.; Qingjie, Z.; Tritt, T. M. High thermoelectric performance BiSbTe alloy with unique low-dimensional structure. J. Appl. Phys. 2009, 105(11), 113713. (9) (a) Miyazaki, Y.; Kajitani, T. Preparation of Bi2Te3 films by electrodeposition. J. Cryst. Growth 2001, 229(1), 542–546. (b) Li, S.; Toprak, M. S.; Soliman, H. M. A.; Zhou, J.; Muhammed, M.; Platzek, D.; Muller, E. Chem. Mater. 2006, 18, 3627. (c) Huang, Q.; Wang, W.; Jia, F.; Zhang, Z. J. Univ. Sci. Technol. Beijing 2006, 13(3), 277. (d) Frari, D.; Diliberto, S.; Stein, N.; Boulanger, C.; Lecuire, J.-M. Thin Solid Films 2005, 483, 44–49. (e) Lim, S.-K.; Kim, M.-Y.; Oh, T.-S. Thin Solid Films 2009, 517, 4199–4203. (10) (a) Marin-Gonzalez, M. S.; Prieto, A. M.; Gronsky, R.; Sands, T.; Stacy, A. M. J. Electrochem. Soc. 2002, 149(11), C546–C554. (b) Wen, S.; Cordeman, R. R.; Seker, F.; Zhang, A.-P.; Denault, L.; Blohm, M. L. J. Electrochem. Soc. 2006, 153(9), C595–C602.
Published on Web 10/05/2010
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a lack of information regarding the microstructural and physical properties of p-type Bi0.5Sb1.5Te3 electroplated compounds,9c,11 which are essential for the realization of functional thermoelectric devices. In addition, there is only a very limited amount of research that correlates microstructure with physical properties in p-type electrodeposited compounds. Koukharenko et al.12 have shown that the optimum microstructural properties (e.g., grain size, chemical composition, and composition homogeneity across a sample) play an important role in the performance of thermoelectric materials. A highly homogeneous material composition as well as small (submicrometer range) grain sizes are required to yield optimal thermoelectric properties.2,13 In an earlier study14 we reported the influence of the electrodeposition conditions on composition, homogeneity, crystallinity and transport properties for p-type nanowire arrays of bismuth antimony telluride. A result of this study was the observation that under some conditions the deposited films appeared to be rough, exhibiting protrusions and pinholes on the surface. These defects can potentially adversely affect the thermoelectric properties of the films. The adsorption of additives from solution onto metals has received some attention in the past as an effective and simple methodology to control the electrochemical processes at the substrate/electrolyte, which in turn could potentially lead to more uniform and homogeneous electrodeposited films of bismuth antimony telluride.15-17 It is also known that the addition of surfactants to solutions lowers the interfacial tension and enables the easy release of gas bubbles from the electrode surface, thereby avoiding the formation of pinholes and pitting in the deposited films. To the best of our knowledge, there has so far been only one paper by Li et al.18 that describes the electrodeposition of Bi2Te3 in the presence of a surfactant. The authors reported that stoichiometric bismuth telluride films with thicknesses up to 350 μm could be obtained in the presence of ethylene glycol. In the present study, we have demonstrated for the first time that highly homogeneous p-type bismuth antimony telluride (Bi-SbTe) films can be electrodeposited in the presence of a specific surfactant, namely sodium ligninsulfonate. In particular, we have investigated the effect of sodium ligninsulfonate on the structure, morphology, stoichiometry, and thermoelectric properties of the deposited films with the aim of developing a methodology for the reproducible production of homogeneous and stoichiometric electrodeposited thin films of p-type Bi-Sb-Te. The thermoelectric properties of these films were measured and compared to the films deposited at similar conditions but in the absence of the surfactant.
Figure 1. CVs recorded at a gold disk working electrode (1 mm diameter) in electrolyte solutions containing 0.001 M Bi3þ, 0.01 M HTeO2þ, 1 M HNO3, 0.02 M Sb3þ, 0.1 M H3Cit, and 0.05 M Na3Cit in the presence (black line) and absence (red line) of ligninsulfonate (60 mg dm-3). The voltammograms were obtained at room temperature of 20-25 °C at a scan rate of 50 mV s-1. In each case, the first cycle was recorded.
2. Experimental Procedure Materials. Bi powder (Alfa Aesar, 99.999%), Te powder (Alfa Aesar, 99þ%), SbCl3 (Sigma-Aldrich 99þ%), sodium ligninsulfonate (Sigma-Aldrich), C6H8O7 3 H2O (H3Cit, Sigma-Aldrich (11) (a) Li, F.; Wang, W. Appl. Surf. Sci. 2009, 255, 4225–4231. (b) Takashiri, M.; Tanaka, S.; Takiishi, M.; Kihara, M.; Miyazaki, K.; Tsukamoto, H. J. Alloys Compd. 2008, 351–355. (12) Koukharenko, E.; Frety, N.; Shepelevich, V. G.; Tedenac, J. C. J. Cryst. Growth 2001, 222(4), 773–778. (13) Ivanova, L. D.; Brovikova, S. A.; Zusmann, G.; Rainshaus, P. Inorg. Mater. 1994, 30(6), 770–775. (14) Li, X.; Koukharenko, E.; Nandhakumar, I. S.; Tudor, J.; Beeby, S. P.; White, N. M. High density p-type Bi0.5Sb1.5Te3 nanowires by electrochemical templating through ion-track lithography. Phys. Chem. Chem. Phys. 2009, 11(18), 3584–90. (15) Fulop, G. F.; Taylor, R. M. Annu. Rev. Mater. Sci. 1985, 15, 197. (16) Schlesinger, M.; Paunovic, M. Modern Electroplating; Wiley: New York, 2000. (17) Pletcher, D.; Zhou, H.; Kear, G.; Low, C. T. J.; Walsh, F. C.; Wills, R. G. A. J. Power Sources 2008, 180, 621–629. (18) Li., S.; Toprak, M. S.; Soliman, H. M. A.; Zhou, J.; Muhammed, M.; Platzek, D.; M€uller, E. Chem. Mater. 2006, 18, 3627.
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Figure 2. XRD patterns of Bi-Sb-Te films deposited at -0.08 V vs SCE at a gold working electrode in the absence (a) and presence (b) of surfactant.
99.5%), Na3C6H8O7 3 2H2O (Na3Cit, Sigma-Aldrich, 99.5%) and HNO3 (Fisher, 70%) were used as received. All solutions were prepared using water from a Purite Select Fusion 160 (Ondeo) water purification system (resistivity 18.2 MΩ cm). The working DOI: 10.1021/la101952y
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electrodes for the film’s deposition were prepared by evaporating 15 nm of chromium onto glass slides followed by a 200 nm layer of gold. Prior to deposition, all gold slides were cleaned by sonication in isopropanol for 10 min, washed with deionized water, and dried in air. The prepared substrates were then masked using insulating adhesive polyimide tape to leave a 2 cm2 area exposed for electrodeposition. The polyimide tape was in contact with the electrolyte solution; however, given the excellent chemical resistance of this material and the fact that it was firmly attached to the substrate, the chances of solution leakages were minimal. Electrodeposition. Electrochemical deposition experiments were carried out using a three-electrode setup in combination with an Autolab potentiostat/galvanostat. Large-area platinum gauze and saturated calomel electrodes (SCEs) were used as counter and reference electrodes, respectively. The dependence of the film’s composition on deposition potential was studied previously.14 The electrochemical deposition conditions in this study were chosen in accordance with those reported in our earlier investigation.14 Bismuth antimony telluride films were electrodeposited at -0.08 V vs SCE at room temperature from a stirred electroplating bath containing 0.001 M Bi3þ, 0.01 M HTeO2þ, 1 M HNO3, 0.02 M Sb3þ, 0.1 M H3Cit, and 0.05 M Na3Cit. The buffer solution (H3Cit and Na3Cit) was necessary to promote dissolution of antimony via complexation of Sb3þ. Initially, various amounts of the surfactant (20, 40, 60, and 100 mg dm-3) were Table 1. Chemical Composition of Bi0.5Sb1.5Te3-Electroplated Film Surfaces (without Surfactant) by EDX and Microprobe Analyses chemical composition characterization of bismuth antimony telluride films deposited without surfactant type of analysis
EDX
microprobe
Bi concentration (atom %) Sb concentration (atom %) Te concentration (atom %) analyzed composition
14 30 56 Bi0.75Sb1.65Te3
7.4 28 65 Bi0.35Sb1.33Te3
Table 2. Chemical Composition of Bi0.5Sb1.5Te3-Electroplated Film Surfaces (with Surfactant) by EDX and Microprobe Analyses chemical composition characterization of bismuth antimony telluride films deposited with surfactant type of analyses Bi concentration (atom %) Sb concentration (atom %) Te concentration (atom %) analyzed composition
EDX
microprobe
16 26 60 Bi0.8Sb1.3Te3
6.8 28 65 Bi0.32Sb1.33Te3
added to the electrolyte solution, and the morphology of all films analyzed by scanning electron microscopy (SEM) measurements. Characterization. The morphology of the deposited films was analyzed by SEM using a Philips XL-30 SEM system equipped with energy dispersive X-ray (EDX) microanalysis (NEW XL-30 132-10). Cross sections for SEM analysis were prepared by cleaving the samples using a diamond scribe. X-ray diffraction (XRD) data were obtained on a Siemens D5000 X-ray diffractometer with Cu KR radiation (λ = 1.5406 A˚). In addition, the chemical composition and the homogeneity across the film surface were determined by microprobe analysis (Cameca, Cataing). The accuracy of this type of measurement was 0.1% (for heavy elements), and the analyzed volume was on the order of 1 μm3. The volume analyzed by the microprobe corresponded to a depth of approximately 1 μm from the film surface. The transport properties of the deposited films were measured using the standard van der Pauw technique with a direct current (dc) of 19 mA and a permanent magnetic field of 0.55 T at room temperature using a commercial Hall effect measurement system (HMS 300 from Ecopia). Prior to Hall effect measurements, each sample was cut into a square of 10 10 mm2 and then bonded to a commercial PCB sample board designed for this measurement unit. Commercial 0.2 mm thick copper wires (from RS Ltd.) were soldered onto each corner of the squared sample using an indium compound to ensure good ohmic contact. A differential method of thermopower measurements was used: the temperature difference ΔT between two points of the sample and the potential difference ΔV between the same two points were measured when the net current in the sample was zero, J = 0. Copper-constantan thermocouples were positioned directly onto the sample surface at “cold” and “hot” sides. The Seebeck coefficient S (μV K-1) was determined using a custom-made Seebeck measurement unit, which was calibrated against a polycrystalline Bi foil reference standard. The measurement accuracy was found to be within 5%, and the system was calibrated using copper-constantan thermocouples and a highprecision Keithley DMM 2000/E digital multimeter with 0.1% accuracy.
3. Results and Discussion In this work, a number of surfactants have been investigated for the deposition of p-type Bi-Sb-Te films, including sodium ligninsulfonate, Triton X100, ethylene glycol, and sodium dodecyl sulfate. Among these surfactants, sodium ligninsulfonate was found to significantly improve the film quality leading to a smooth, dense, and pinhole-free deposit. The addition of sodium dodecyl sulfate results in a rough deposit. On the other hand, the addition of Triton X100 has an inverse effect on the deposit, in
Figure 3. SEM images of Bi0.5Sb1.5Te3 film deposited at -0.08 V vs SCE without surfactant. 16982 DOI: 10.1021/la101952y
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which the film was dissolved back into the solution after deposition. As a consequence, all further discussion will focus on the results obtained for sodium ligninsulfonate as a surfactant. It was observed that, at the lower concentrations, the deposits had a rough surface and, at high concentrations, the growth of the film was significantly suppressed so only thin films could be produced. The 60 mg dm-3 of sodium ligninsulfonate was shown to be the optimum concentration for producing smooth, uniform, and relatively thick films. Hence this surfactant concentration was employed for the deposition of Bi0.5Sb1.5Te3 films. Figure 1 shows representative cyclic voltammograms (CVs) recorded at a gold working electrode immersed in an electrolyte solutions containing Bi3þ, HTeO2þ, and SbOþ ions with and without sodium ligninsulfonate. The plots show no significant differences between the voltammograms. Both CVs reveal three cathodic peaks at potentials of -0.09, -0.18, and -0.32 V vs SCE, which may be attributed to electrodeposition of Bi, Te, and Sb, respectively. This indicates that the region between 0 and -0.4 V is suitable for deposition of ternary films. According to our previous investigation,14 the films deposited at -0.08 V vs SCE were found to exhibit a stoichiometric composition corresponding to Bi0.5Sb1.5Te3 with an optimal crystalline orientation along the {110} plane. For this reason, a deposition potential of -0.08 V
Figure 4. Cross-sectional SEM image of Bi0.5Sb1.5Te3 film deposited without surfactant.
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vs SCE was chosen in the current studies. CVs were also recorded as a function of different sodium ligninsulfonate concentrations (cf. Supporting Information) but are not shown here as voltammetric features essentially remain unchanged. It was observed that, at high concentrations of the surfactant (>than 60 mg dm-3), the deposits had a rough surface, and, at low concentrations (
