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Electrochemical deposition of thermoelectric SbxTey thin films and nanowires
Journal of Alloys and Compounds 485 (2009) 362–366
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrochemical deposition of thermoelectric Sbx Tey thin films and nanowires K. Park a,∗ , F. Xiao b , B.Y. Yoo c , Y. Rheem b , N.V. Myung b,∗∗ a
Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering, University of California-Riverside, Riverside, CA 92521, USA c Department of Metallurgy and Materials Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 12 January 2009 Received in revised form 21 May 2009 Accepted 21 May 2009 Available online 28 May 2009 Keywords: Thermoelectric materials Electrical transport Scanning electron microscopy SEM Thermoelectric Microstructure
a b s t r a c t Sbx Tey thin films and nanowires were electrochemically deposited on a Pt/Si substrate and a Au seed layer, respectively, from aqueous nitric acid solutions at room temperature. As the applied potential increased, the Te content in the films and nanowires decreased. Stoichiometric Sb2 Te3 thin films and nanowires were grown at an applied voltage of −140 mV. The grain size and morphology of the Sb2 Te3 films strongly depended on applied voltage and film thickness. The as-prepared Sbx Tey films were amorphous, whereas ¯ structure. We fabricated reproducibly the as-annealed films were crystallized in the rhombohedral R3m continuous and dense Sb2 Te3 nanowires at −140 mV, allowing potential materials for high-performance thermoelectric nanodevices. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A2 B3 -type compounds such as Sb2 Te3 , Bi2 Te3 , and Bi2 Se3 are narrow-band gap semiconductors with a homologous layered crystal structure. These compounds have been extensively studied because of their potential applicability for efficient thermoelectric devices [1]. Various methods, including evaporation [2–4], sputtering [5], metalorganic chemical vapor deposition (MOCVD) [6,7], electrochemical deposition (ECD) [8–12], and molecular beam epitaxy (MBE) [13] have been used for the deposition of A2 B3 -type thin films and nanowires. Among these processes, ECD is widely investigated because of its many advantages, including cost-effectiveness, rapid deposition rate, and ease of control of the film thickness and nanowire diameter. Antimony telluride (Sb2 Te3 ) and its doped derivatives are considered promising candidates for near room temperature thermoelectric materials. However, only a few studies have been conducted on the electrodeposition of Sb2 Te3 thin films and nanowires [14–16]. Because the Sb3+ tends to hydrolyze and precipitate, it is very difficult to obtain the Sbx Tey thin films and nanowires with a sufficiently high concentration of Sb, especially Sb2 Te3 , in aqueous solution by the ECD technique [16,17]. Until now,
∗ Corresponding author. Tel.: +82 2 3408 3777; fax: +82 2 3408 4342. ∗∗ Corresponding author. Tel.: +1 951 827 7710; fax: +1 951 827 5696. E-mail addresses: [email protected] (K. Park), [email protected] (N.V. Myung). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.106
there has been little information about appropriate chelating agents and their concentrations. In particular, the electrodeposition of the nanowires is both attractive and challenging because of their unique thermoelectric properties in comparison with the bulk and thin films. Leimkühler et al. [14] investigated the electrodeposition of Sbx Tey thin films on an indium tin oxide (ITO) substrate from an aqueous acidic solution, which was made from SbCl3 and TeO2 with HCl. They reported that the pH, temperature, composition, and deposition potential of the bath significantly affected the morphology and crystal structure of Sbx Tey films. Wang et al. [15] deposited Sbx Tey thin films on a Si substrate by ECD technique at room temperature with the hydrochloric acid aqueous solution containing Sb2 Te3 and TeO2 . They studied the relation between the applied potential (−1 to −4 V) and counter electrode-working electrode capacitance during ECD, and reporting that the ECD technique provides a possible way to prepare Sbx Tey films. Jin et al. [16] prepared nanowire arrays of Sb2 Te3 by the ECD technique with the electrolyte solutions consisting of 0.075 M HTeO2 + and 0.05 M SbO+ . The above studies show the feasibility to the ECD technique of Sbx Tey thin films and nanowires, but there is a lack of systematic studies explaining the deposition mechanisms and the effect of electrodeposition conditions on the composition, crystal structure, and morphology of Sbx Tey thin films and nanowires. In this work, the ECD process of Sbx Tey thin films and nanowires was systematically investigated to find an optimal condition and a deposition mechanism for synthesizing the high-quality films and nanowires.
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and 1 M HNO3 starts to appear at ∼−250 mV. In the electrolyte containing SbO+ , when applied potential reaches ∼−140 mV, a current density significantly increased. This was caused by the induced deposition of Sb onto a Te layer deposited initially on the substrate [20]. The overall Sb2 Te3 deposition reaction can be described as follows: 2SbO+ + 3HTeO2 + + 13H+ + 18e− → Sb2 Te3 + 8H2 O
Fig. 1. Linear sweep voltammograms for Sbx Ty electrodeposition in solutions consisting of 0.7 mM TeO2 , 33 mM tartaric acid, 1 M HNO3 , and 0–3.2 mM Sb2 O3 without agitation.
2. Experimental Prior to electrodeposition, linear sweep voltammograms were obtained with a standard three-electrode cell (EG&G Potentio/Gavalnostat 273A) in solutions consisting of 0.7 mM TeO2 , 33 mM tartaric acid, 1 M HNO3 , and 0–3.2 mM Sb2 O3 at 1000 rpm. The temperature was kept at room temperature and the scan rate was fixed at 10 mV/s. Voltammetries were performed with a rotating disk platinum electrode (RDE: 3 mm diameter) as a working electrode. The RDE was embedded in a cylindrical Teflon holder. A Pt wire and SCE (saturated KCl) were used as a counter and a reference electrode, respectively. Sbx Tey films were electrodeposited on a Pt/Si substrate using an aqueous nitric acid electrolyte at room temperature. The electrolytes were composed of 1.6 mM Sb2 O3 , 0.7 mM TeO2 , 33 mM tartaric acid, and 1 M HNO3 . First of all, TeO2 powders were dissolved by concentrated nitric acid in beaker. Next, Sb2 O3 and tartaric acid powders were dissolved by nanopure water in another beaker. Once the powders in the two separate beakers were dissolved, they were mixed together and nanopure water was added to reach the final volume. The pH value of the electrolyte was adjusted to 0.3. The reference and counter electrodes were SCE and platinum-coated titanium stripe, respectively. The applied potential voltages were in the range of −80 to −180 mV, in steps of 20 mV. During electrodeposition, the electrolytes were magnetically stirred at 500 rpm. After electrodeposition, Sbx Tey films were rinsed with nanopure water, dried in air, and then annealed in forming gas (7.2% H2 in Ar) at 250 ◦ C for 2 h to improve the crystallinity of the deposited films. For the preparation of Sbx Tey nanowires, Sbx Tey nanowire bundles were electrodeposited on a Au seed layer at room temperature based on the electrolyte solutions and processing conditions used for the preparation of the Sbx Tey thin films. The Au seed layer was sputtered onto one side of the commercially available alumina templates (Whatman ANODISCTM 25) to serve as the working electrode. After electrodeposition, the obtained Sbx Tey nanowire bundles were rinsed with nanopure water and then dried in air at room temperature. The composition of the deposited Sbx Tey thin films and nanowires was determined by atomic absorption spectroscopy (AAS; PerkinElmer, Analyst 800). Surface morphologies of the deposited Sbx Tey films and nanowires were investigated using scanning electron microscopy (SEM; XL 30, Philips) attached with energy dispersive spectroscopy (EDS; DX-4, Philips). The crystal structure of the films and nanowires was investigated with an X-ray diffractometer (XRD; Bruker D8 Advance). In this work, the influence of applied potential on the composition and microstructure of Sb2 Te3 thin films was studied to find optimal electrodeposition conditions for highquality Sb2 Te3 thin films. Also, high-density and large-area Sb2 Te3 nanowires were electrochemically deposited on a Au seed layer in alumina template at an applied potential voltage of −140 mV.
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Because of the stoichiometric Sb2 Te3 formation free energy (−57.5 kJ/mol), the Sb reduction potential would shift as an amount of E = G/nF = 0.2 V [21]. The free energy for Sbx Tey compounds with higher Sb content should be between 0 and −57.5 kJ/mol, which would decrease the potential shift. This free energy also shifts the Te bulk deposition potential positively as shown in Fig. 1. The concentration of SbO+ did not cause significantly change in the current density over the applied potential range of −140 to −200 mV, indicating that the mass transfer rate of SbO+ had no significant influence on the deposition rate over this potential range. Based on the obtained LSV, it is apparent that appropriate applied potential for the deposition of Sbx Tey thin films and nanowires is in the range of −80 to −180 mV. Fig. 2 shows the current density during the electrodeposition of Sbx Tey films under the applied potential voltages of −80 to −180 mV as a function of time. At the initial stage of deposition time, there was a significant fluctuation of the responding current when the applied potential was −120 mV. The fluctuation of the responding current strongly implies that there is progressive nucleation. At an applied potential of −120 mV, an island-like structure was found, indicating strong evidence for progressive nucleation. The islandlike structure will be shown below. Fig. 3 shows the Te content in Sbx Tey thin films and the deposition rate of the films as a function of applied potential. We found that applied potential substantially affects the chemical composition of the thin films. More negative applied potential yielded lower Te content in the films. This is because in strongly acidic solution, the standard reduction potential of Sb (∼0.204 V) from its 3+ oxidation state (SbO+ ) is more negative than that of Te (∼0.57 V). For example, when the applied potential was −80 mV, the Te content in the films was 82.3 at.%. And when the applied potential was −180 mV, the Te content in the films was 53.4 at.%. We found that the optimal potential for obtaining the stoichiometric Sb2 Te3 was −140 mV, wherein the deposited Te content was 59.8 at.%. In addition, the deposition rate of the Sbx Tey films gradually increased with an increase in the negative applied potential, reaching a maximum
3. Results and discussion In order to understand the electrochemical reactions in the bath, linear sweep voltammograms (LSV) for electrolytes consisting of 0–3.2 mM Sb2 O3 , 0.7 mM TeO2 , 33 mM tartaric acid, and 1 M HNO3 were obtained, as shown in Fig. 1. The identification of the current peaks has been previously reported [18,19]. The Te reduction peak from Te only electrolyte of 0.7 mM TeO2 , 33 mM tartaric acid,
Fig. 2. Current density during the electrodeposition of Sbx Ty films under the applied potential voltages of −80 to −180 mV as a function of time.
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Fig. 3. Te content in the Sbx Tey thin films and the deposition rate of the films as a function of applied potential.
value (3.9 × 10−4 mg/s) at −160 mV, and then decreasing for higher negative applied potential. Fig. 4(a)–(f) shows SEM images from the surfaces of the asprepared Sbx Tey films deposited at applied potentials of −80 to −180 mV, in steps of 20 mV, respectively. Their film thickness was in the range 2.8–6.0 m. The grain size and morphology of the asdeposited Sbx Tey films depend strongly on the applied potential,
i.e., the film composition and the film thickness. For example, the Te-rich Sbx Tey films deposited at −80 mV consist of the clusters of ∼0.5–1.5 m. An individual cluster is composed of fine grains of ∼200–300 nm in size with the granular structure. In particular, the Sb2 Te3 films, which were deposited at −140 mV, showed a needle-like structure. For Sb2 Te3 films, with increasing film thickness, the grain morphology changed from the island-like granular to the needle-like structure. Fig. 5 shows the grain morphology at the initial stage of deposition, i.e., very small film thickness. From Figs. 4 and 5, the nucleation and subsequent grain growth depend on the applied potential and the film thickness, resulting in a different grain morphology. Furthermore, we found no noticeable change in the surface morphology of the film after annealing (Fig. 6). For example, the SEM image from the surfaces of the as-annealed Sb2 Te3 films deposited at applied potentials of −140 mV. Fig. 7(a)–(f) shows the cross-sectional SEM images from the asprepared Sbx Tey films deposited at the applied potentials of −80 to −180 mV, respectively. The Sbx Tey films and the Pt layer bonded well and the interface was sharp and clear. The thickness of the films was relatively homogeneous. No phases have been observed along the interface region, implying that there was no reaction between them during the electrodeposition. We obtained XRD patterns from the surface of the as-prepared and annealed Sbx Tey thin films in order to identify the crystal structure. Fig. 8 shows the typical XRD patterns for one of the samples, Sb2 Te3 , which was obtained at the applied potential of −140 mV.
Fig. 4. SEM images from the surfaces of the as-prepared Sbx Tey films deposited at applied potentials of (a) −80, (b) −100, (c) −120, (d) −140, (e) −160, and (f) −180 mV.
K. Park et al. / Journal of Alloys and Compounds 485 (2009) 362–366
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Fig. 5. SEM image from the surfaces of the as-prepared Sbx Tey films at the initial stage of Sbx Tey deposition at an applied potential of −140 mV.
Fig. 6. SEM image from the surface of the as-annealed Sbx Tey films deposited at applied potentials of −140 mV.
The as-prepared Sbx Tey films were amorphous. The diffraction peaks were sharpened as a consequence of annealing, indicating improved crystallinity. This change gives rise to an enhancement of carrier mobility and thermoelectric properties. The pattern of annealed Sb2 Te3 can be indexed as the rhombohedral structure ¯ (1 6 6) [22]. (a = 4.262 Å and c = 30.450 Å) with the space group R3m In particular, the intensity of the (0 1 5) peak is strong, indicative
of the texturing along the [0 1 5] direction. This is the orientation yielding a high thermoelectric performance [23]. By applying the Debye–Scherrer equation to the full width at half maximum of the diffraction (0 1 5) peak [24], the calculated average crystallite size of Sb2 Te3 thin films was 58 nm. The Debye–Scherrer equation can be expressed as follows: D = 0.89/{ˇ(2)cos }, where ˇ(2) is the
Fig. 7. Cross-sectional SEM images from the as-prepared Sbx Tey films deposited at applied potentials of (a) −80, (b) −100, (c) −120, (d) −140, (e) −160, and (f) −180 mV.
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allowing potential materials for high-performance thermoelectric nanodevices. 4. Conclusion
Fig. 8. XRD patterns of (a) as-prepared and (b) as-annealed Sb2 Te3 thin films obtained at the applied potential of −140 mV.
Sbx Tey thin films and nanowires were potentiostatically electrodeposited at room temperature in aqueous nitric acid electrolyte solutions consisting of 1.6 mM Sb2 O3 , 0.7 mM TeO2 , 33 mM tartaric acid, and 1 M HNO3 . More negative applied potential yielded lower Te content in the films and nanowires. The morphology of the asdeposited Sbx Tey films depended strongly on the applied potential, i.e., the film composition and the film thickness. At an applied voltage of −140 mV, we obtained stoichiometric high-quality Sb2 Te3 thin films and nanowires. For Sb2 Te3 films, the grain morphology in the early stage of growing was a granular structure, and it changed to a needle-like structure with increasing film thickness. Even though we observed no significant difference in the surface morphology of the films after annealing, the crystallinity of the film was significantly improved. The as-prepared Sbx Tey films were amorphous, whereas the annealed Sbx Tey films had the rhombohe¯ structure. The Sb2 Te3 films showed a preferred orientation dral R3m along the [0 1 5] direction. We fabricated reproducibly continuous and dense Sb2 Te3 nanowire arrays at −140 mV, allowing potential materials for high-performance thermoelectric nanodevices. References
Fig. 9. Cross-sectional SEM image of Sb2 Te3 nanowire arrays in alumina template.
width of the pure diffraction profile in radians, is the wavelength of the X-rays (0.15406 nm), 2 is the diffraction angle of the (0 1 5) peak, and D is the average diameter of the crystallite. We successfully prepared nanowire arrays of Sbx Tey with a ∼200 nm diameter from the same as solutions and processing condition used for the deposition of the Sbx Tey films using commercially available alumina templates. Fig. 9 shows a cross-sectional SEM image of the Sb2 Te3 nanowire arrays grown at an applied potential of −140 mV. The chemical composition of the nanowire arrays grown at an applied potential of −140 mV was basically the same as that of the films deposited at an applied potential of −140 mV. The nanowires began to grow up on the Au seed layer. We found that the nanowires were dense and their diameters were homogeneous. Most nanowires were filled into the pores of the templates. A very small gap between the nanowires and the pore walls of the templates was observed, indicating non-wetting behavior between them. In the solution and processing condition used here, we fabricated reproducibly high-quality Sb2 Te3 nanowires,
[1] B.Y. Yoo, C.-K. Huang, J.R. Lim, J. Herman, M.A. Ryan, J.-P. Fleurial, N.V. Myung, Electrochim. Acta 50 (2005) 4371. [2] H. Zou, D.M. Powe, G. Min, J. Vac. Sci. Technol. A 19 (2001) 899. [3] H. Zou, D.M. Powe, G. Min, J. Cryst. Growth 222 (2001) 82. [4] C. Shafai, M.J. Brett, J. Vac. Sci. Technol. A 15 (1997) 2798. [5] H. Böttner, J. Nurnus, A. Gavriko, G. Kühner, M. Jägle, C. Kunzel, D. Eberhard, G. Plescher, A. Schbert, K.H. Schlereth, J. Microelectromech. Syst. 13 (2004) 414. [6] R. Venkatasubramanian, T. Colpitts, E. Watko, M. Lamvik, N. El-Masry, J. Cryst. Growth 170 (1997) 817. [7] A. Giani, A. Boulouz, F. Pascal-Delannoy, A. Foucaran, E. Charles, A. Boyer, Mater. Sci. Eng. B 64 (1999) 19. [8] M. Takahashi, Y. Katou, K. Nagata, S. Futura, Thin Solid Films 240 (1994) 70. [9] M.S. Martin-Gonzalez, A.L. Prieto, R. Gronsky, T. Sands, A.M. Stacy, J. Electrochem. Soc. 149 (2002) C546. [10] Y. Miyazaki, T. Kajitani, J. Cryst. Growth 229 (2001) 542. [11] M. Takahashi, Y. Muramatsu, T. Suzuki, S. Sato, M. Watanabe, K. Wakita, T. Uchida, J. Electrochem. Soc. 150 (2003) C169. [12] M. Takahashi, Y. Oda, T. Ogino, S. Futura, J. Electrochem. Soc. 140 (1993) 2550. [13] Y. Kim, A. DiVenere, G.K.L. Wong, J.B. Ketterson, S. Cho, J.R. Meyer, J. Appl. Phys. 91 (2002) 715. [14] G. Leimkühler, I. Kerkamm, R. Reineke-Koch, J. Electrochem. Soc. 149 (2002) C474. [15] C. Wang, Q. Wang, L. Chen, X. Xu, Q. Yao, Electrochem. Solid-State Lett. 9 (2006) C147. [16] C. Jin, G. Zhang, T. Qian, X. Li, Z. Yao, J. Phys. Chem. B 109 (2005) 1430. [17] M. Martín-González, A.L. Prieto, R. Gronsky, T. Sands, A.M. Stacy, Adv. Mater. 15 (2003) 1003. [18] F. Xiao, B. Yoo, M.A. Ryan, K.-H. Lee, N.V. Myung, Electrochim. Acta 52 (3) (2006) 1101. [19] B. Yoo, F. Xiao, K.N. Bozhilov, J. Herman, M.A. Ryan, N.V. Myung, Adv. Mater. 19 (2) (2007) 296. [20] D. Del Frari, S. Diliberto, N. Stein, C. Boulanger, J.-M. Lecuire, Thin Solid Films 483 (1–2) (2005) 44. [21] G. Leimkuhler, I. Kerkamm, R. Reineke-Koch, J. Electrochem. Soc. 149 (10) (2002) C474. [22] JCPDS file No. 15-0874. [23] H. Scherrer, S. Scherrer, in: D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, 1995. [24] Q. Pang, J. Shi, Y. Liu, D. Xing, M. Gong, N. Xu, Mater. Sci. Eng. B 103 (2003) 57.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrochemical deposition of thermoelectric Sbx Tey thin films and nanowires K. Park a,∗ , F. Xiao b , B.Y. Yoo c , Y. Rheem b , N.V. Myung b,∗∗ a
Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering, University of California-Riverside, Riverside, CA 92521, USA c Department of Metallurgy and Materials Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 12 January 2009 Received in revised form 21 May 2009 Accepted 21 May 2009 Available online 28 May 2009 Keywords: Thermoelectric materials Electrical transport Scanning electron microscopy SEM Thermoelectric Microstructure
a b s t r a c t Sbx Tey thin films and nanowires were electrochemically deposited on a Pt/Si substrate and a Au seed layer, respectively, from aqueous nitric acid solutions at room temperature. As the applied potential increased, the Te content in the films and nanowires decreased. Stoichiometric Sb2 Te3 thin films and nanowires were grown at an applied voltage of −140 mV. The grain size and morphology of the Sb2 Te3 films strongly depended on applied voltage and film thickness. The as-prepared Sbx Tey films were amorphous, whereas ¯ structure. We fabricated reproducibly the as-annealed films were crystallized in the rhombohedral R3m continuous and dense Sb2 Te3 nanowires at −140 mV, allowing potential materials for high-performance thermoelectric nanodevices. © 2009 Elsevier B.V. All rights reserved.
1. Introduction A2 B3 -type compounds such as Sb2 Te3 , Bi2 Te3 , and Bi2 Se3 are narrow-band gap semiconductors with a homologous layered crystal structure. These compounds have been extensively studied because of their potential applicability for efficient thermoelectric devices [1]. Various methods, including evaporation [2–4], sputtering [5], metalorganic chemical vapor deposition (MOCVD) [6,7], electrochemical deposition (ECD) [8–12], and molecular beam epitaxy (MBE) [13] have been used for the deposition of A2 B3 -type thin films and nanowires. Among these processes, ECD is widely investigated because of its many advantages, including cost-effectiveness, rapid deposition rate, and ease of control of the film thickness and nanowire diameter. Antimony telluride (Sb2 Te3 ) and its doped derivatives are considered promising candidates for near room temperature thermoelectric materials. However, only a few studies have been conducted on the electrodeposition of Sb2 Te3 thin films and nanowires [14–16]. Because the Sb3+ tends to hydrolyze and precipitate, it is very difficult to obtain the Sbx Tey thin films and nanowires with a sufficiently high concentration of Sb, especially Sb2 Te3 , in aqueous solution by the ECD technique [16,17]. Until now,
∗ Corresponding author. Tel.: +82 2 3408 3777; fax: +82 2 3408 4342. ∗∗ Corresponding author. Tel.: +1 951 827 7710; fax: +1 951 827 5696. E-mail addresses: [email protected] (K. Park), [email protected] (N.V. Myung). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.106
there has been little information about appropriate chelating agents and their concentrations. In particular, the electrodeposition of the nanowires is both attractive and challenging because of their unique thermoelectric properties in comparison with the bulk and thin films. Leimkühler et al. [14] investigated the electrodeposition of Sbx Tey thin films on an indium tin oxide (ITO) substrate from an aqueous acidic solution, which was made from SbCl3 and TeO2 with HCl. They reported that the pH, temperature, composition, and deposition potential of the bath significantly affected the morphology and crystal structure of Sbx Tey films. Wang et al. [15] deposited Sbx Tey thin films on a Si substrate by ECD technique at room temperature with the hydrochloric acid aqueous solution containing Sb2 Te3 and TeO2 . They studied the relation between the applied potential (−1 to −4 V) and counter electrode-working electrode capacitance during ECD, and reporting that the ECD technique provides a possible way to prepare Sbx Tey films. Jin et al. [16] prepared nanowire arrays of Sb2 Te3 by the ECD technique with the electrolyte solutions consisting of 0.075 M HTeO2 + and 0.05 M SbO+ . The above studies show the feasibility to the ECD technique of Sbx Tey thin films and nanowires, but there is a lack of systematic studies explaining the deposition mechanisms and the effect of electrodeposition conditions on the composition, crystal structure, and morphology of Sbx Tey thin films and nanowires. In this work, the ECD process of Sbx Tey thin films and nanowires was systematically investigated to find an optimal condition and a deposition mechanism for synthesizing the high-quality films and nanowires.
K. Park et al. / Journal of Alloys and Compounds 485 (2009) 362–366
363
and 1 M HNO3 starts to appear at ∼−250 mV. In the electrolyte containing SbO+ , when applied potential reaches ∼−140 mV, a current density significantly increased. This was caused by the induced deposition of Sb onto a Te layer deposited initially on the substrate [20]. The overall Sb2 Te3 deposition reaction can be described as follows: 2SbO+ + 3HTeO2 + + 13H+ + 18e− → Sb2 Te3 + 8H2 O
Fig. 1. Linear sweep voltammograms for Sbx Ty electrodeposition in solutions consisting of 0.7 mM TeO2 , 33 mM tartaric acid, 1 M HNO3 , and 0–3.2 mM Sb2 O3 without agitation.
2. Experimental Prior to electrodeposition, linear sweep voltammograms were obtained with a standard three-electrode cell (EG&G Potentio/Gavalnostat 273A) in solutions consisting of 0.7 mM TeO2 , 33 mM tartaric acid, 1 M HNO3 , and 0–3.2 mM Sb2 O3 at 1000 rpm. The temperature was kept at room temperature and the scan rate was fixed at 10 mV/s. Voltammetries were performed with a rotating disk platinum electrode (RDE: 3 mm diameter) as a working electrode. The RDE was embedded in a cylindrical Teflon holder. A Pt wire and SCE (saturated KCl) were used as a counter and a reference electrode, respectively. Sbx Tey films were electrodeposited on a Pt/Si substrate using an aqueous nitric acid electrolyte at room temperature. The electrolytes were composed of 1.6 mM Sb2 O3 , 0.7 mM TeO2 , 33 mM tartaric acid, and 1 M HNO3 . First of all, TeO2 powders were dissolved by concentrated nitric acid in beaker. Next, Sb2 O3 and tartaric acid powders were dissolved by nanopure water in another beaker. Once the powders in the two separate beakers were dissolved, they were mixed together and nanopure water was added to reach the final volume. The pH value of the electrolyte was adjusted to 0.3. The reference and counter electrodes were SCE and platinum-coated titanium stripe, respectively. The applied potential voltages were in the range of −80 to −180 mV, in steps of 20 mV. During electrodeposition, the electrolytes were magnetically stirred at 500 rpm. After electrodeposition, Sbx Tey films were rinsed with nanopure water, dried in air, and then annealed in forming gas (7.2% H2 in Ar) at 250 ◦ C for 2 h to improve the crystallinity of the deposited films. For the preparation of Sbx Tey nanowires, Sbx Tey nanowire bundles were electrodeposited on a Au seed layer at room temperature based on the electrolyte solutions and processing conditions used for the preparation of the Sbx Tey thin films. The Au seed layer was sputtered onto one side of the commercially available alumina templates (Whatman ANODISCTM 25) to serve as the working electrode. After electrodeposition, the obtained Sbx Tey nanowire bundles were rinsed with nanopure water and then dried in air at room temperature. The composition of the deposited Sbx Tey thin films and nanowires was determined by atomic absorption spectroscopy (AAS; PerkinElmer, Analyst 800). Surface morphologies of the deposited Sbx Tey films and nanowires were investigated using scanning electron microscopy (SEM; XL 30, Philips) attached with energy dispersive spectroscopy (EDS; DX-4, Philips). The crystal structure of the films and nanowires was investigated with an X-ray diffractometer (XRD; Bruker D8 Advance). In this work, the influence of applied potential on the composition and microstructure of Sb2 Te3 thin films was studied to find optimal electrodeposition conditions for highquality Sb2 Te3 thin films. Also, high-density and large-area Sb2 Te3 nanowires were electrochemically deposited on a Au seed layer in alumina template at an applied potential voltage of −140 mV.
(1)
Because of the stoichiometric Sb2 Te3 formation free energy (−57.5 kJ/mol), the Sb reduction potential would shift as an amount of E = G/nF = 0.2 V [21]. The free energy for Sbx Tey compounds with higher Sb content should be between 0 and −57.5 kJ/mol, which would decrease the potential shift. This free energy also shifts the Te bulk deposition potential positively as shown in Fig. 1. The concentration of SbO+ did not cause significantly change in the current density over the applied potential range of −140 to −200 mV, indicating that the mass transfer rate of SbO+ had no significant influence on the deposition rate over this potential range. Based on the obtained LSV, it is apparent that appropriate applied potential for the deposition of Sbx Tey thin films and nanowires is in the range of −80 to −180 mV. Fig. 2 shows the current density during the electrodeposition of Sbx Tey films under the applied potential voltages of −80 to −180 mV as a function of time. At the initial stage of deposition time, there was a significant fluctuation of the responding current when the applied potential was −120 mV. The fluctuation of the responding current strongly implies that there is progressive nucleation. At an applied potential of −120 mV, an island-like structure was found, indicating strong evidence for progressive nucleation. The islandlike structure will be shown below. Fig. 3 shows the Te content in Sbx Tey thin films and the deposition rate of the films as a function of applied potential. We found that applied potential substantially affects the chemical composition of the thin films. More negative applied potential yielded lower Te content in the films. This is because in strongly acidic solution, the standard reduction potential of Sb (∼0.204 V) from its 3+ oxidation state (SbO+ ) is more negative than that of Te (∼0.57 V). For example, when the applied potential was −80 mV, the Te content in the films was 82.3 at.%. And when the applied potential was −180 mV, the Te content in the films was 53.4 at.%. We found that the optimal potential for obtaining the stoichiometric Sb2 Te3 was −140 mV, wherein the deposited Te content was 59.8 at.%. In addition, the deposition rate of the Sbx Tey films gradually increased with an increase in the negative applied potential, reaching a maximum
3. Results and discussion In order to understand the electrochemical reactions in the bath, linear sweep voltammograms (LSV) for electrolytes consisting of 0–3.2 mM Sb2 O3 , 0.7 mM TeO2 , 33 mM tartaric acid, and 1 M HNO3 were obtained, as shown in Fig. 1. The identification of the current peaks has been previously reported [18,19]. The Te reduction peak from Te only electrolyte of 0.7 mM TeO2 , 33 mM tartaric acid,
Fig. 2. Current density during the electrodeposition of Sbx Ty films under the applied potential voltages of −80 to −180 mV as a function of time.
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Fig. 3. Te content in the Sbx Tey thin films and the deposition rate of the films as a function of applied potential.
value (3.9 × 10−4 mg/s) at −160 mV, and then decreasing for higher negative applied potential. Fig. 4(a)–(f) shows SEM images from the surfaces of the asprepared Sbx Tey films deposited at applied potentials of −80 to −180 mV, in steps of 20 mV, respectively. Their film thickness was in the range 2.8–6.0 m. The grain size and morphology of the asdeposited Sbx Tey films depend strongly on the applied potential,
i.e., the film composition and the film thickness. For example, the Te-rich Sbx Tey films deposited at −80 mV consist of the clusters of ∼0.5–1.5 m. An individual cluster is composed of fine grains of ∼200–300 nm in size with the granular structure. In particular, the Sb2 Te3 films, which were deposited at −140 mV, showed a needle-like structure. For Sb2 Te3 films, with increasing film thickness, the grain morphology changed from the island-like granular to the needle-like structure. Fig. 5 shows the grain morphology at the initial stage of deposition, i.e., very small film thickness. From Figs. 4 and 5, the nucleation and subsequent grain growth depend on the applied potential and the film thickness, resulting in a different grain morphology. Furthermore, we found no noticeable change in the surface morphology of the film after annealing (Fig. 6). For example, the SEM image from the surfaces of the as-annealed Sb2 Te3 films deposited at applied potentials of −140 mV. Fig. 7(a)–(f) shows the cross-sectional SEM images from the asprepared Sbx Tey films deposited at the applied potentials of −80 to −180 mV, respectively. The Sbx Tey films and the Pt layer bonded well and the interface was sharp and clear. The thickness of the films was relatively homogeneous. No phases have been observed along the interface region, implying that there was no reaction between them during the electrodeposition. We obtained XRD patterns from the surface of the as-prepared and annealed Sbx Tey thin films in order to identify the crystal structure. Fig. 8 shows the typical XRD patterns for one of the samples, Sb2 Te3 , which was obtained at the applied potential of −140 mV.
Fig. 4. SEM images from the surfaces of the as-prepared Sbx Tey films deposited at applied potentials of (a) −80, (b) −100, (c) −120, (d) −140, (e) −160, and (f) −180 mV.
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Fig. 5. SEM image from the surfaces of the as-prepared Sbx Tey films at the initial stage of Sbx Tey deposition at an applied potential of −140 mV.
Fig. 6. SEM image from the surface of the as-annealed Sbx Tey films deposited at applied potentials of −140 mV.
The as-prepared Sbx Tey films were amorphous. The diffraction peaks were sharpened as a consequence of annealing, indicating improved crystallinity. This change gives rise to an enhancement of carrier mobility and thermoelectric properties. The pattern of annealed Sb2 Te3 can be indexed as the rhombohedral structure ¯ (1 6 6) [22]. (a = 4.262 Å and c = 30.450 Å) with the space group R3m In particular, the intensity of the (0 1 5) peak is strong, indicative
of the texturing along the [0 1 5] direction. This is the orientation yielding a high thermoelectric performance [23]. By applying the Debye–Scherrer equation to the full width at half maximum of the diffraction (0 1 5) peak [24], the calculated average crystallite size of Sb2 Te3 thin films was 58 nm. The Debye–Scherrer equation can be expressed as follows: D = 0.89/{ˇ(2)cos }, where ˇ(2) is the
Fig. 7. Cross-sectional SEM images from the as-prepared Sbx Tey films deposited at applied potentials of (a) −80, (b) −100, (c) −120, (d) −140, (e) −160, and (f) −180 mV.
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allowing potential materials for high-performance thermoelectric nanodevices. 4. Conclusion
Fig. 8. XRD patterns of (a) as-prepared and (b) as-annealed Sb2 Te3 thin films obtained at the applied potential of −140 mV.
Sbx Tey thin films and nanowires were potentiostatically electrodeposited at room temperature in aqueous nitric acid electrolyte solutions consisting of 1.6 mM Sb2 O3 , 0.7 mM TeO2 , 33 mM tartaric acid, and 1 M HNO3 . More negative applied potential yielded lower Te content in the films and nanowires. The morphology of the asdeposited Sbx Tey films depended strongly on the applied potential, i.e., the film composition and the film thickness. At an applied voltage of −140 mV, we obtained stoichiometric high-quality Sb2 Te3 thin films and nanowires. For Sb2 Te3 films, the grain morphology in the early stage of growing was a granular structure, and it changed to a needle-like structure with increasing film thickness. Even though we observed no significant difference in the surface morphology of the films after annealing, the crystallinity of the film was significantly improved. The as-prepared Sbx Tey films were amorphous, whereas the annealed Sbx Tey films had the rhombohe¯ structure. The Sb2 Te3 films showed a preferred orientation dral R3m along the [0 1 5] direction. We fabricated reproducibly continuous and dense Sb2 Te3 nanowire arrays at −140 mV, allowing potential materials for high-performance thermoelectric nanodevices. References
Fig. 9. Cross-sectional SEM image of Sb2 Te3 nanowire arrays in alumina template.
width of the pure diffraction profile in radians, is the wavelength of the X-rays (0.15406 nm), 2 is the diffraction angle of the (0 1 5) peak, and D is the average diameter of the crystallite. We successfully prepared nanowire arrays of Sbx Tey with a ∼200 nm diameter from the same as solutions and processing condition used for the deposition of the Sbx Tey films using commercially available alumina templates. Fig. 9 shows a cross-sectional SEM image of the Sb2 Te3 nanowire arrays grown at an applied potential of −140 mV. The chemical composition of the nanowire arrays grown at an applied potential of −140 mV was basically the same as that of the films deposited at an applied potential of −140 mV. The nanowires began to grow up on the Au seed layer. We found that the nanowires were dense and their diameters were homogeneous. Most nanowires were filled into the pores of the templates. A very small gap between the nanowires and the pore walls of the templates was observed, indicating non-wetting behavior between them. In the solution and processing condition used here, we fabricated reproducibly high-quality Sb2 Te3 nanowires,
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