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Effect of the substrate on the electrodeposition of Bi2Te3−ySey thin films
Materials Research Bulletin 43 (2008) 1808–1813 www.elsevier.com/locate/matresbu
Effect of the substrate on the electrodeposition of Bi2Te3ySey thin films Luxia Bu a, Wei Wang a,*, Hui Wang b a
Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China b Analytical Center, Tianjin University, Tianjin 300072, PR China Received 3 June 2006; received in revised form 14 June 2007; accepted 3 July 2007 Available online 10 July 2007
Abstract The potentiostatic electrodeposition of n-type Bi2Te3ySey thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi2Te3ySey films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi2Te3ySey films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H2SeO3 in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO2+ and Bi3+ to form Bi2Te3ySey alloy. On stainless steel substrates, HTeO2+ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H2SeO3 and Bi3+ to form Bi2Te3ySey alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; C. X-ray diffraction; D. Crystal structure
1. Introduction Bismuth telluride (Bi2Te3) and its derivative selenide compounds are considered to be the best materials for use in thermoelectric devices at room temperature [1]. Traditionally, bulk Bi2Te3 based materials have been prepared by the solid-state reactions at elevated temperatures [2–4]. However, an increasing interest is expected to develop with advances and discoveries in micro-thermoelectric devices [5]. Electrodeposition is one of the potential techniques to fabricate thin films for micro-systems. Compared with many other methods including chemical vapor deposition (CVD) [6,7], flash evaporation [8,9] and co-evaporation [10], electrodeposition offers the advantages of simple, easy operation and low cost. Moreover, the growth rate and composition of the films can also be easily controlled through
* Corresponding author. Tel.: +86 22 27402895; fax: +86 22 27409483. E-mail address: [email protected] (W. Wang). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.07.002
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adjusting the electrodepositing parameters. Recently, several studies on the electrodeposition of Bi2Te3 thin films onto Pt, Mo, Ni, Ti and stainless steel, have been studied by many researchers [11–16]. Michel et al. have prepared Bi2Te2.7Se0.3 thin films on stainless steel substrates by the chemical deposition method [5]. The formation of Bi2Te3ySey films has been claimed, in general, but no discussion on the possible influence of the substrate on the properties of the electrodeposit has been brought about. However, it is well known that the nature of the substrate plays a very important role on the initial formation of any electrodeposit, with consequences on their morphological and structural properties. The choice of stainless steel substrates is due to a typical support for use in preparation of thermoelectric films [5,15]. Au can be used as a substrate to fabricate thermoelectric micro-generator [17], presenting interesting properties, such as good adhesion for electrodeposition and high overpotential for hydrogen evolution in acid aqueous solutions. In this paper, we report a comparative study on the characteristics of the electrodeposited Bi2Te3ySey films onto stainless steel and gold substrates. The effect of the substrate on the morphology, structure and thermoelectric performance of the electrodeposited Bi2Te3ySey films was investigated in details. 2. Experimental 2.1. Substrates Gold and stainless steel sheets with geometric area of approximately 0.64 cm2 were used as the substrates for the electrodeposition of Bi2Te3ySey films. In order to get good adhesion between the electrodeposited films and the substrates, a special attention was paid to the pretreatment of the substrates. The substrates were mechanically polished to mirror surface, and then degreased by acetone, etched in concentrated HNO3 later, and rinsed with redistilled water at last. The pretreatments were performed just before the substrates were immersed into the solution to begin electrodeposition or cyclic voltammetry measurements. 2.2. Deposition conditions Concentrated nitric acid was used to dissolve Bi(NO3)35H2O, H2TeO3 and H2SeO3, and the solution was subsequently diluted to 1 M HNO3. The concentration of Bi3+, HTeO2+ and H2SeO3 in the solution was 10, 9.5 and 3 mM, respectively. Bi2Te3ySey films were prepared by potentiostatic electrodeposition at room temperature using a conventional three-electrode cell consisting of gold and stainless steel sheets as working electrode, Pt plate as the auxiliary electrode and saturated calomel electrode (SCE) as the reference electrode. CHI660B electrochemical working station was used to keep the deposition potential. Cyclic voltammograms were recorded with CHI660B electrochemical station at scan rate of 10 mV/s without stirring. All potentials were measured and expressed relative to SCE. 2.3. Characterization of the electrodeposited films The morphology and composition of the deposited films were studied using a environment scanning electron microscope (ESEM, PHILIPS XL30) equipped with energy dispersive spectroscopy (EDS, OXFORD ISIS300). The Xray diffraction (XRD) patterns of the as-deposited Bi2Te3ySey thin films were investigated in the diffracting angle range 20–1108 with a film X-ray diffractometer (X’Pert Pro) using Co Ka radiation (l = 0.17903 nm). The Seebeck coefficient of the films was measured with the thermoelectric performance measurement system (TPMF-100) developed by Tianjin University. Electrical resistivity (r) was measured by the four-point probe method. And the Seebeck coefficient and electrical resistivity were measured along the direction parallel to the surface of the films at room temperature. 3. Results and discussion 3.1. Voltammetric studies The purpose of the cyclic voltammetric study was to define, for each substrate, the potential region in which the reactions occur. The voltammetric curves (the first cycle) measured by gold and stainless steel substrates are shown in
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Fig. 1. Cyclic voltammogram of the gold sheet in the solution containing 10 mM Bi3+, 9.5 mM HTeO2+ and 3 mM H2SeO3.
Figs. 1 and 2. It can be found that the shapes of the two curves are quite different, which indicates the difference of the deposition potential electrodeposited on the gold and stainless steel substrates. It can also be found that the peak current density of the cyclic voltammogram measured by gold sheets is much lower than that of stainless steel sheets. And these imply that the reduction rate on gold sheets is slower than that on stainless steel sheets and their reduction process are also different. During the cathodic scanning process in Fig. 1, a pink deposit was firstly observed on the surface of gold electrode and its color was gradually converted to the dark with the negative shift of the sweeping potential. It is well known that the pink deposit is reducing product of Se4+. There are two clear reduction peaks that can be observed from Fig. 1 at about potentials 0.03 and 0.02 V, which reveals two different reducing reactions existed. The two reactions can be described as: H2 SeO3 þ 4e þ 4Hþ ! Se þ 3H2 O
(1)
ðyÞSe þ ð3 yÞHTeO2 þ þ 2Bi3þ þ ð9 3yÞHþ þ ð18 4yÞe ! Bi2 Te3y Sey þ ð6 2yÞH2 O
(2)
During the cathodic scanning process in Fig. 2, the surface color of the stainless steel sheet was immediately converted to the dark due to the replacement once the sheet was immersed into the electrolyte. The dark deposit is a larger of atomic Te, the reducing product of HTeO2+ (elemental Te was determined by EDS). Thus, the cathodic
Fig. 2. Cyclic voltammogram of the stainless steel sheet in the solution containing 10 mM Bi3+, 9.5 mM HTeO2+ and 3 mM H2SeO3.
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scanning process on the stainless steel sheet had been converted to a cathodic scanning process on Te pre-deposited electrode. The whole deposition process can be described as [18]: HTeO2 þ þ 2Fe þ 3Hþ ! Te þ 2Fe2þ þ 2H2 O
(3)
ð3 yÞTe þ 2Bi3þ þ ðyÞH2 SeO3 þ ð4yÞHþ þ 18e ! Bi2 Te3y Sey þ ð3yÞH2 O
(4)
3.2. Morphology characterization In order to investigate the effect of substrate on the morphology of electrodeposited Bi2Te3ySey films, the stainless steel sheets and gold sheets were used as substrates to prepare the films separately. The ESEM images of the films electrodeposited at potential 0.04 Vare shown in Fig. 3. It can be seen that the film electrodeposited on stainless steel sheet (Fig. 3(a)) is less compact than that on gold electrode (Fig. 3(b)). Differences are also noticed on the crystallite morphology (Fig. 3(c and d)). For the films prepared on stainless steel sheet, the surface shows a jujube pit-like morphology and the average size of the particles is about 10 mm, while the surface of the film deposited on gold sheet presents grains-like morphology with the average grain size of about 4 mm. These results clearly show that the surface morphology of the films is sensitive to the substrate material. Combined with the above analyses for Figs. 1 and 2, the current density during the cathodic process measured by the stainless steel sheet is higher than that by the gold sheet which indicates that the electrodepositing rate on the stainless steel sheet is quicker than that on the gold sheet. Generally the surface morphology of the films is much looser while the reducing rate is increased. So the more compact structure of the films electrodeposited on the gold sheet can be ascribed to the lower reducing rate.
Fig. 3. ESEM images of Bi2Te3ySey films electrodeposited on stainless steel (a and c) and gold substrates (b and d).
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Fig. 4. XRD patterns of Bi2Te3ySey films electrodeposited on stainless steel (a) and gold (b) substrates.
3.3. Crystal structure deposits characterization XRD patterns of the films prepared at potential 0.04 V on stainless steel and gold substrate under the same depositing conditions are shown in Fig. 4. A comparison of the XRD relative peak intensity (I/Imax)/% among the electrodeposited Bi2Te3ySey films and Bi2Te3ySey standard pattern (00-051-0643) is shown in Table 1. It can be found from Table 1 that the films exhibit strong preferential orientation along (1 1 0) plane, but the strength of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. These results indicate that substrate materials have an important influence on the structure of electrodeposited Bi2Te3ySey films. Generally, the films prepared by the electrodeposition method exhibit preferential orientation along (1 1 0) plane because of much lower electrical resistivity perpendicular to the surface of the substrate. However, the films prepared by physical or chemical techniques often exhibit preferential orientation along (0 1 5) plane [8,10]. Moreover the Seebeck coefficient of the films prepared by physical or chemical techniques is often higher than that of the films prepared by electrodeposition methods. These imply that the Seebeck coefficient of the films electrodeposited on the stainless steel substrates can be higher than that on the gold substrate. And the effect of the substrate on the performance of the films will be further discussed as follows. 3.4. Performance of the electrodeposited films In order to investigate the effect of substrates on the electrodeposition of Bi2Te3ySey thin films in details, the films were deposited at potential 0.04 V onto stainless steel and gold sheets. The composition, thickness, electrical resistivity and Seebeck coefficient of the films were measured and the results are shown in Table 2. It can be found that
Table 1 Effect of substrates on the XRD relative peak intensity (I/Imax)/% of electrodeposited Bi2Te3ySey films (h k l)
(1 0 1) (0 1 5) (1 0 1 0) (1 1 0) (2 0 5) (0 2 1 0) (1 2 5)
XRD relative peak intensity percentage Bi2Te3ySey films on stainless steel substrate
Bi2Te3ySey films on gold substrate
Bi2Te3ySey standard pattern
33 81 17 100 35 13 22
53 77 12 100 27 11 14
6 100 42 40 20 13 15
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Table 2 Material parameters for the films electrodeposited on stainless steel and gold substrates Parameter
Stainless steel support
Gold support
Composition Thickness (mm) Electrical resistivity (mV cm) Seebeck coefficient (mV K1)
Bi2Te2.39Se0.77 38.46 14.68 62.24
Bi2Te2.45Se0.85 36.68 8.46 48.58
Bi/(Te + Se) atomic ratios in the film deposited on the stainless steel sheet is much higher than that on the gold sheet. It can also be seen that the electrical resistivity and Seebeck coefficient of the film measured by the stainless steel sheet are higher than those by the gold sheet. The lower electrical resistivity measured by the gold sheet can be mainly ascribed to the more compact structure of the films electrodeposited on the gold sheet as it has been discussed in Fig. 3. And the higher Seebeck coefficient of the films measured by the stainless steel sheet can be mostly ascribed to the stronger relative peak intensity along (0 1 5) direction which has been discussed in Fig. 4. And the results are also consistent with the analyses above. In addition, the stoichiometry of the electrodeposited films also has an important effect on the performance of the films. Altogether, the performance of the films prepared under the same depositing conditions is strongly dependent on the substrates. 4. Conclusions Bi2Te3ySey thin films have been successfully prepared by the potentiostatic electrodeposition technique onto stainless steel and gold substrates. Voltammetric studies imply the different mechanisms of Bi3+, HTeO2+ and H2SeO3 deposited on both substrates. XRD and ESEM data show that the film crystalinity and morphology are both sensitive to the substrate materials. The properties of the films deposited on stainless steel substrates are also comparable to those of the films deposited on gold substrates. The electrical resistivity and Seebeck coefficient of the film measured by stainless steel substrates are higher than those measured by gold substrates. Acknowledgment This work was supported by the National Natural Science Foundation of China under Grant No. 50071040. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
B. Yim, F. Rosi, Solid State Electron. 15 (1957) 1121. D. Vasilevskiy, A. Sami, J.-M. Simard, R. Masut, J. Appl. Phys. 92 (2002) 2610. V. Damodara Das, S. Selvaraj, Solid State Commun. 111 (1999) 43. J.Y. Yang, T. Aizawa, A. Yamamoto, T. Ohta, J. Alloys Compd. 312 (2000) 326. S. Michel, N. Stein, M. Schneider, C. Boulanger, J.-M. Lecuire, J. Appl. Electrochem. 33 (2003) 23. A. Giani, A. Boulouz, B. Aboulouz, F. Pascal-Delannoy, A. Foucaran, A. Boyer, A. Mzerd, J. Cryst. Growth 204 (1999) 91. A. Al Bayaz, A. Giani, M.C. Artaud, A. Foucaran, F. Pascal-Delannoy, A. Boyer, J. Cryst. Growth 241 (2002) 463. V. Damodara Das, S. Selvaraj, J. Appl. Phys. 86 (1999) 1518. K.-W. Cho, I.-H. Kim, Mater. Lett. 59 (2005) 966. H. Zou, D.M. Rowe, G. Min, J. Cryst. Growth 222 (2001) 82. K. Tittes, A. Bund, W. Plieth, A. Bentien, S. Paschen, M. Plo¨tner, H. Gra¨fe, W.-J. Fischer, J. Solid State Electrochem. 7 (2003) 714. Y. Miyazaki, T. Kajitani, J. Cryst. Growth 229 (2001) 542. S. Golia, M. Arora, R.K. Sharma, A.C. Rastogi, Curr. Appl. Phys. 3 (2003) 195. 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. D. Del Frari, S. Diliberto, N. Stein, C. Boulanger, J.-M. Lecuire, Thin Solid Films 483 (2005) 44. M. Takahashi, M. Kojima, S. Sato, N. Ohnisi, A. Nishiwaki, K. Wakita, T. Miyuki, S. Ikeda, Y. Muramatsu, J. Appl. Phys. 96 (2004) 5582. J.-P. Fleurial, G.J. Snyder, J.A. Herman, P.H. Giauque, W.M. Philips, M.A. Ryan, P. Shakkottai, E.A. Kolawa, M.A. Nicolet, in: Proceedings of the 18th International Conference on Thermoelectrics, 1999, p. 294. [18] M. Martı´n-Gonza´lez, G. Jeffrey Snyder, A.L. Prieto, R. Gronsky, T. Sands, A.M. Stacy, Nano Lett. 3 (2003) 97.
Effect of the substrate on the electrodeposition of Bi2Te3ySey thin films Luxia Bu a, Wei Wang a,*, Hui Wang b a
Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China b Analytical Center, Tianjin University, Tianjin 300072, PR China Received 3 June 2006; received in revised form 14 June 2007; accepted 3 July 2007 Available online 10 July 2007
Abstract The potentiostatic electrodeposition of n-type Bi2Te3ySey thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi2Te3ySey films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi2Te3ySey films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H2SeO3 in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO2+ and Bi3+ to form Bi2Te3ySey alloy. On stainless steel substrates, HTeO2+ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H2SeO3 and Bi3+ to form Bi2Te3ySey alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; C. X-ray diffraction; D. Crystal structure
1. Introduction Bismuth telluride (Bi2Te3) and its derivative selenide compounds are considered to be the best materials for use in thermoelectric devices at room temperature [1]. Traditionally, bulk Bi2Te3 based materials have been prepared by the solid-state reactions at elevated temperatures [2–4]. However, an increasing interest is expected to develop with advances and discoveries in micro-thermoelectric devices [5]. Electrodeposition is one of the potential techniques to fabricate thin films for micro-systems. Compared with many other methods including chemical vapor deposition (CVD) [6,7], flash evaporation [8,9] and co-evaporation [10], electrodeposition offers the advantages of simple, easy operation and low cost. Moreover, the growth rate and composition of the films can also be easily controlled through
* Corresponding author. Tel.: +86 22 27402895; fax: +86 22 27409483. E-mail address: [email protected] (W. Wang). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.07.002
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adjusting the electrodepositing parameters. Recently, several studies on the electrodeposition of Bi2Te3 thin films onto Pt, Mo, Ni, Ti and stainless steel, have been studied by many researchers [11–16]. Michel et al. have prepared Bi2Te2.7Se0.3 thin films on stainless steel substrates by the chemical deposition method [5]. The formation of Bi2Te3ySey films has been claimed, in general, but no discussion on the possible influence of the substrate on the properties of the electrodeposit has been brought about. However, it is well known that the nature of the substrate plays a very important role on the initial formation of any electrodeposit, with consequences on their morphological and structural properties. The choice of stainless steel substrates is due to a typical support for use in preparation of thermoelectric films [5,15]. Au can be used as a substrate to fabricate thermoelectric micro-generator [17], presenting interesting properties, such as good adhesion for electrodeposition and high overpotential for hydrogen evolution in acid aqueous solutions. In this paper, we report a comparative study on the characteristics of the electrodeposited Bi2Te3ySey films onto stainless steel and gold substrates. The effect of the substrate on the morphology, structure and thermoelectric performance of the electrodeposited Bi2Te3ySey films was investigated in details. 2. Experimental 2.1. Substrates Gold and stainless steel sheets with geometric area of approximately 0.64 cm2 were used as the substrates for the electrodeposition of Bi2Te3ySey films. In order to get good adhesion between the electrodeposited films and the substrates, a special attention was paid to the pretreatment of the substrates. The substrates were mechanically polished to mirror surface, and then degreased by acetone, etched in concentrated HNO3 later, and rinsed with redistilled water at last. The pretreatments were performed just before the substrates were immersed into the solution to begin electrodeposition or cyclic voltammetry measurements. 2.2. Deposition conditions Concentrated nitric acid was used to dissolve Bi(NO3)35H2O, H2TeO3 and H2SeO3, and the solution was subsequently diluted to 1 M HNO3. The concentration of Bi3+, HTeO2+ and H2SeO3 in the solution was 10, 9.5 and 3 mM, respectively. Bi2Te3ySey films were prepared by potentiostatic electrodeposition at room temperature using a conventional three-electrode cell consisting of gold and stainless steel sheets as working electrode, Pt plate as the auxiliary electrode and saturated calomel electrode (SCE) as the reference electrode. CHI660B electrochemical working station was used to keep the deposition potential. Cyclic voltammograms were recorded with CHI660B electrochemical station at scan rate of 10 mV/s without stirring. All potentials were measured and expressed relative to SCE. 2.3. Characterization of the electrodeposited films The morphology and composition of the deposited films were studied using a environment scanning electron microscope (ESEM, PHILIPS XL30) equipped with energy dispersive spectroscopy (EDS, OXFORD ISIS300). The Xray diffraction (XRD) patterns of the as-deposited Bi2Te3ySey thin films were investigated in the diffracting angle range 20–1108 with a film X-ray diffractometer (X’Pert Pro) using Co Ka radiation (l = 0.17903 nm). The Seebeck coefficient of the films was measured with the thermoelectric performance measurement system (TPMF-100) developed by Tianjin University. Electrical resistivity (r) was measured by the four-point probe method. And the Seebeck coefficient and electrical resistivity were measured along the direction parallel to the surface of the films at room temperature. 3. Results and discussion 3.1. Voltammetric studies The purpose of the cyclic voltammetric study was to define, for each substrate, the potential region in which the reactions occur. The voltammetric curves (the first cycle) measured by gold and stainless steel substrates are shown in
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Fig. 1. Cyclic voltammogram of the gold sheet in the solution containing 10 mM Bi3+, 9.5 mM HTeO2+ and 3 mM H2SeO3.
Figs. 1 and 2. It can be found that the shapes of the two curves are quite different, which indicates the difference of the deposition potential electrodeposited on the gold and stainless steel substrates. It can also be found that the peak current density of the cyclic voltammogram measured by gold sheets is much lower than that of stainless steel sheets. And these imply that the reduction rate on gold sheets is slower than that on stainless steel sheets and their reduction process are also different. During the cathodic scanning process in Fig. 1, a pink deposit was firstly observed on the surface of gold electrode and its color was gradually converted to the dark with the negative shift of the sweeping potential. It is well known that the pink deposit is reducing product of Se4+. There are two clear reduction peaks that can be observed from Fig. 1 at about potentials 0.03 and 0.02 V, which reveals two different reducing reactions existed. The two reactions can be described as: H2 SeO3 þ 4e þ 4Hþ ! Se þ 3H2 O
(1)
ðyÞSe þ ð3 yÞHTeO2 þ þ 2Bi3þ þ ð9 3yÞHþ þ ð18 4yÞe ! Bi2 Te3y Sey þ ð6 2yÞH2 O
(2)
During the cathodic scanning process in Fig. 2, the surface color of the stainless steel sheet was immediately converted to the dark due to the replacement once the sheet was immersed into the electrolyte. The dark deposit is a larger of atomic Te, the reducing product of HTeO2+ (elemental Te was determined by EDS). Thus, the cathodic
Fig. 2. Cyclic voltammogram of the stainless steel sheet in the solution containing 10 mM Bi3+, 9.5 mM HTeO2+ and 3 mM H2SeO3.
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scanning process on the stainless steel sheet had been converted to a cathodic scanning process on Te pre-deposited electrode. The whole deposition process can be described as [18]: HTeO2 þ þ 2Fe þ 3Hþ ! Te þ 2Fe2þ þ 2H2 O
(3)
ð3 yÞTe þ 2Bi3þ þ ðyÞH2 SeO3 þ ð4yÞHþ þ 18e ! Bi2 Te3y Sey þ ð3yÞH2 O
(4)
3.2. Morphology characterization In order to investigate the effect of substrate on the morphology of electrodeposited Bi2Te3ySey films, the stainless steel sheets and gold sheets were used as substrates to prepare the films separately. The ESEM images of the films electrodeposited at potential 0.04 Vare shown in Fig. 3. It can be seen that the film electrodeposited on stainless steel sheet (Fig. 3(a)) is less compact than that on gold electrode (Fig. 3(b)). Differences are also noticed on the crystallite morphology (Fig. 3(c and d)). For the films prepared on stainless steel sheet, the surface shows a jujube pit-like morphology and the average size of the particles is about 10 mm, while the surface of the film deposited on gold sheet presents grains-like morphology with the average grain size of about 4 mm. These results clearly show that the surface morphology of the films is sensitive to the substrate material. Combined with the above analyses for Figs. 1 and 2, the current density during the cathodic process measured by the stainless steel sheet is higher than that by the gold sheet which indicates that the electrodepositing rate on the stainless steel sheet is quicker than that on the gold sheet. Generally the surface morphology of the films is much looser while the reducing rate is increased. So the more compact structure of the films electrodeposited on the gold sheet can be ascribed to the lower reducing rate.
Fig. 3. ESEM images of Bi2Te3ySey films electrodeposited on stainless steel (a and c) and gold substrates (b and d).
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Fig. 4. XRD patterns of Bi2Te3ySey films electrodeposited on stainless steel (a) and gold (b) substrates.
3.3. Crystal structure deposits characterization XRD patterns of the films prepared at potential 0.04 V on stainless steel and gold substrate under the same depositing conditions are shown in Fig. 4. A comparison of the XRD relative peak intensity (I/Imax)/% among the electrodeposited Bi2Te3ySey films and Bi2Te3ySey standard pattern (00-051-0643) is shown in Table 1. It can be found from Table 1 that the films exhibit strong preferential orientation along (1 1 0) plane, but the strength of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. These results indicate that substrate materials have an important influence on the structure of electrodeposited Bi2Te3ySey films. Generally, the films prepared by the electrodeposition method exhibit preferential orientation along (1 1 0) plane because of much lower electrical resistivity perpendicular to the surface of the substrate. However, the films prepared by physical or chemical techniques often exhibit preferential orientation along (0 1 5) plane [8,10]. Moreover the Seebeck coefficient of the films prepared by physical or chemical techniques is often higher than that of the films prepared by electrodeposition methods. These imply that the Seebeck coefficient of the films electrodeposited on the stainless steel substrates can be higher than that on the gold substrate. And the effect of the substrate on the performance of the films will be further discussed as follows. 3.4. Performance of the electrodeposited films In order to investigate the effect of substrates on the electrodeposition of Bi2Te3ySey thin films in details, the films were deposited at potential 0.04 V onto stainless steel and gold sheets. The composition, thickness, electrical resistivity and Seebeck coefficient of the films were measured and the results are shown in Table 2. It can be found that
Table 1 Effect of substrates on the XRD relative peak intensity (I/Imax)/% of electrodeposited Bi2Te3ySey films (h k l)
(1 0 1) (0 1 5) (1 0 1 0) (1 1 0) (2 0 5) (0 2 1 0) (1 2 5)
XRD relative peak intensity percentage Bi2Te3ySey films on stainless steel substrate
Bi2Te3ySey films on gold substrate
Bi2Te3ySey standard pattern
33 81 17 100 35 13 22
53 77 12 100 27 11 14
6 100 42 40 20 13 15
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Table 2 Material parameters for the films electrodeposited on stainless steel and gold substrates Parameter
Stainless steel support
Gold support
Composition Thickness (mm) Electrical resistivity (mV cm) Seebeck coefficient (mV K1)
Bi2Te2.39Se0.77 38.46 14.68 62.24
Bi2Te2.45Se0.85 36.68 8.46 48.58
Bi/(Te + Se) atomic ratios in the film deposited on the stainless steel sheet is much higher than that on the gold sheet. It can also be seen that the electrical resistivity and Seebeck coefficient of the film measured by the stainless steel sheet are higher than those by the gold sheet. The lower electrical resistivity measured by the gold sheet can be mainly ascribed to the more compact structure of the films electrodeposited on the gold sheet as it has been discussed in Fig. 3. And the higher Seebeck coefficient of the films measured by the stainless steel sheet can be mostly ascribed to the stronger relative peak intensity along (0 1 5) direction which has been discussed in Fig. 4. And the results are also consistent with the analyses above. In addition, the stoichiometry of the electrodeposited films also has an important effect on the performance of the films. Altogether, the performance of the films prepared under the same depositing conditions is strongly dependent on the substrates. 4. Conclusions Bi2Te3ySey thin films have been successfully prepared by the potentiostatic electrodeposition technique onto stainless steel and gold substrates. Voltammetric studies imply the different mechanisms of Bi3+, HTeO2+ and H2SeO3 deposited on both substrates. XRD and ESEM data show that the film crystalinity and morphology are both sensitive to the substrate materials. The properties of the films deposited on stainless steel substrates are also comparable to those of the films deposited on gold substrates. The electrical resistivity and Seebeck coefficient of the film measured by stainless steel substrates are higher than those measured by gold substrates. Acknowledgment This work was supported by the National Natural Science Foundation of China under Grant No. 50071040. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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