Sb Multilayered

Sep 9, 2008 - High-density thermoelectric Bi2Te3/Sb multilayered nanowire arrays with a minimum period of 10 nm are successfully fabricated by a ...
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15190

J. Phys. Chem. C 2008, 112, 15190–15194

Manipulating Growth of Thermoelectric Bi2Te3/Sb Multilayered Nanowire Arrays Wei Wang, Genqiang Zhang, and Xiaoguang Li* Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: April 14, 2008; ReVised Manuscript ReceiVed: August 1, 2008

High-density thermoelectric Bi2Te3/Sb multilayered nanowire arrays with a minimum period of 10 nm are successfully fabricated by a template-assisted pulsed electrodeposition process based on the deep investigation of Bi-Sb-Te electrolyte solution. Large-scale, solid, and continuous nanowire arrays with multisegment characters and uniform diameters can be observed by FE-SEM and TEM, and the randomly selected neighboring segments were characterized to be Bi2Te3 and Sb, respectively, by SAED and EDS. It is demonstrated that the segment length and length ratios can be effectively tuned by manipulating the deposition time, which presents good candidates for further thermoelectric applications. I. Introduction There is a growing interest for searching thermoelectric materials with high performance because of the increasing demands in energy but decreasing fuel supplies.1-3 The thermoelectric materials with the figure of merit (ZT) larger than 3 are promising building blocks for the thermoelectric device, which shows a performance competitive with that of traditional refrigeration systems.4,5 For this purpose, many efforts have been made, and it is evidenced by theoretical and experimental results that the low-dimensional thermoelectric materials can have a higher performance than the bulk materials due to the sharper density of states for larger Seebeck coefficient and enhanced phonon scattering for lower thermal conductivity.6-8 For example, the highest ZT of 2.4 is realized in the Bi2Te3/Sb2Te3 superlattice film.9 Recently, the most important progress has been obtained in silicon nanowires with rough surface, and the corresponding ZT is a factor of 60 larger than that of the bulk silicon around room temperature,10-12 showing a bright future for commercial markets. Therefore, as a combination of the advantages of superlattice and one-dimensional nanostructures, it is predicted that the superlattice nanowire has the ability for the further improvement of ZT because of its zero-dimensional structure.13,14 To fabricate the multilayered thermoelectric heterostructure nanowires, several kinds of strategies have been successfully developed. On the basis of a vapor-liquid-solid (VLS) process, the pulsed laser ablation/chemical vapor deposition (PLA-CVD) method was used to fabricate the Si/SiGe heterostructure nanowires with a reduced thermal conductivity.15,16 In another route, Bi/Sb and Bi-Te/Bi-Sb-Te multilayered nanowire arrays can be synthesized by porous anodic alumina (PAA) template-assisted pulsed electrodeposition technology.17,18 However, few reports have a deep investigation on the fabrication of high-density multilayered nanowires with modulated periods, which are very important and desirable for minimizing thermal conductivity and thus improving ZT.19 To meet the increasing requirements for searching ideal thermoelectric materials (phononglass/electron-crystal) as well as the practical device applica* Corresponding author. Phone: +86-551-3603408. Fax: +86-5513603408. E-mail: [email protected].

Figure 1. Cyclic voltammogram results of (a) Bi-Sb-Te electrolyte solution with different Sb contents, and (b) separated Bi-Te (-) and Sb (- - -) electrolyte solutions.

tions, it is still a challenge and of great significance to develop thermoelectric multilayered nanowires. Bismuth tellurium-based compounds are a typical kind of thermoelectric material, which exhibit excellent thermoelectric performance around room temperature.20-22 On the other hand, antimony plays an indispensable role in optimizing the thermoelectric properties in various systems, and the Sb-rich nanostructured precipitates help bulk materials (such as AgPbmSbTe2+m, Pb9.6Sb0.2Te10-xSex systems) to obtain a high ZT.23-25 In this work, we have a deep study focused on the synthesis of the Bi2Te3/Sb multilayered nanowires utilizing the PAA template-assisted pulsed electrodeposition technology. The Bi2Te3/Sb multilayered nanowires with tunable

10.1021/jp803207r CCC: $40.75  2008 American Chemical Society Published on Web 09/09/2008

Bi2Te3/Sb Multilayered Nanowires

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15191 followed by adding 1 M C6H8O7 · H2O, 0.5 M K2C6H5O7 · H2O, and 0.004 M Bi(NO3)3 · 5H2O. Finally, 0.1 M Sb2O3 was dissolved in this solution by heating until the solution turned clear. The high H+ concentration was needed to stabilize the Bi3+ and HTeO2+ ions, and C6H8O7 acted as a complexing agent to stabilize the Sb3+ ion. The electrodeposition was carried out in a three-electrode electrochemical cell at room temperature, where a piece of graphite was used as the counter electrode, and the PAA template fabricated by a two-step anodic oxidation process was coated with a thin gold layer to serve as the working electrode.26,27 The cyclic voltammograms experiments were performed on the CHI 760c electrochemical station. Fieldemission scanning electron microscopy (FE-SEM, JEOL JSM6700F) was employed to observe the morphologies of the asprepared nanowire arrays after partly dissolving the PAA template in 5 M NaOH, and the microstructure of dispersed nanowires after dissolving the whole PAA template was studied using transmission electron microscopy (TEM, JEOL 2010). The structure and composition analyses were characterized by selected-area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS).

Figure 2. (a) Applied pulsed potential waveform for the deposition of Bi2Te3/Sb multilayered nanowire arrays, and (b) the corresponding current response during the deposition process.

periods are obtained through manipulating the deposition time, and the minimum period of the high-density multilayered nanowires is about 10 nm. II. Experimental Methods The electrolyte solutions consisted of different amounts of Bi(NO3)3 · 5H2O, Sb2O3, TeO2, C6H8O7 · H2O, K3C6H5O7 · H2O, and HNO3, which were of analytical grade and were used without further purification. The detailed composition of each electrolyte solution is presented in Table 1. In a typical synthesis, 0.012 M TeO2 was dissolved in the 2 M HNO3 solution,

III. Results and Discussion For the growth of Bi2Te3/Sb multilayered nanowire arrays, the cyclic voltammograms (CVs) of the Bi-Sb-Te electrolyte solution as well as the separated Bi-Te solution and Sb solution were studied in detail. The CVs results of Bi-Sb-Te electrolyte solution with different Sb contents (as marked in Table 1) clearly show that two reduction waves, see Figure 1a (labeled as peaks A and B), appear around -0.06 and -0.32 V, respectively, in the cathodic scanning process with a scanning rate of 40 mV/s, and three oxidation peaks can be observed around 0.08, 0.28, and 0.55 V (labeled as peaks C, D, and E), respectively, in the reversal scanning process. As compared to the CVs results of the separated Bi-Te and pure Sb electrolyte solutions in Figure 1b, it can be regarded that peak A mainly comes from the

Figure 3. (a) FE-SEM image of the as-prepared Bi2Te3/Sb multilayered nanowire arrays, (b) TEM image of the dispersed nanowires, (c) close-up of a single multilayered nanowire, and SAED and corresponding EDS results for (d) segment I and (e) segment II.

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Wang et al.

TABLE 1: Detailed Composition of the Electrolyte Solutions 1 2 3 4 5

Bi(NO3)3 · 5H2O

TeO2

Sb2O3

C6H8O7 · H2O

K3C6H5O7 · H2O

HNO3

0.004 M 0.004 M 0.004 M 0.004 M

0.012 M 0.012 M 0.012 M 0.012 M

0.025 M 0.05 M 0.1 M

1M 1M 1M 1M 1M

0.5 M 0.5 M 0.5 M 0.5 M 0.5 M

2M 2M 2M 2M 2M

0.1 M

deposition of Bi-Te alloy, while the codeposition of Bi, Sb, and Te contributes to the appearance of peak B.28 The oxidation peaks originate from the combination of oxidation processes of Bi-Te alloy and individual Sb. However, it is noticed that the value of the first reduction wave (-0.06 V) is more negative than the one in the Bi-Te system (-0.02 V). It is consistent with the previous report for the deposition of Bi2-xSbxTe3 nanowire arrays,29 which may come from the contribution of the slight deposition of Sb in -0.06 V. Thus, it is considered that -0.02 V vs Ag/AgCl is suitable for the deposition of Bi-Te alloys with minimization of Sb impurity. On the other hand, the reduction wave around -0.32 V (peak B) in Bi-Sb-Te solution is much more positive as compared to the reduction wave around -0.47 V (peak G) of solution containing Sb only. This phenomenon comes from the previous formation of a Bi-Te compound on the surface of the electrode, which shifts

Figure 4. (a-d) TEM images of multilayered Bi2Te3/Sb nanowires deposited in different conditions labeled in the bottom of each corresponding figure, and (e) the comparison of the corresponding five kinds of Bi2Te3/Sb multilayered nanowires deposited with different time.

the potential,28 and it also implies the codeposition of Bi, Te, and Sb at this potential. To get a majority of Sb segment, the most facile and effective route is to increase the relative Sb concentration. Here, with doubling the Sb content in electrolyte solution each time, the corresponding reduction wave amplitude (peak B) significantly turns larger and larger, while the Bi-Te reduction wave amplitude (peak A) remains its minor value. Yet the further increasing of Sb content to 0.2 M makes the electrolyte solution unstable, and white precipitates will appear during the electrodeposition process because the change of pH value around counter electrode would induce the hydrolyzation of Sb3+ ion. Thus, electrolyte solution 3 in Table 1 is finally selected for fabricating Bi2Te3/Sb multilayered nanowire arrays, and -0.02 V vs Ag/AgCl and -0.3 V vs Ag/AgCl are employed for the depositions of Bi-Te alloy and Sb, respectively. Figure 2a shows a typical pulsed potential waveform, which is adjusted periodically between -0.02 V for 240 s and -0.3 V for 20 s, and Figure 2b gives the corresponding current response with a regular alternation for the deposition of Bi2Te3/ Sb multilayered nanowire arrays. In a single cycle, the positive current appears at the start of the applied low potential, indicating the partial dissolving of Sb at the beginning of the deposition of Bi2Te3 alloy. This phenomenon also occurs during the pulsed deposition processes in many other systems.30,31 Because the Sb concentration is relatively high, the current turns sharply to a high value at -0.3 V, which is a tens of folds increase over the deposition of Bi2Te3 alloy to make sure that the metal Sb is the main product. Subsequently, the current decreases steadily due to the reduction of ion concentration in the PAA nanochannels in this short period. Figure 3a shows the SEM image of the as-deposited Bi2Te3/ Sb nanowire arrays. The large-scale nanowires with uniform diameters of 60 nm can be observed, and the nanowires are straight with an average length of tens of micrometers. Different from the Co/Pt system,32 it is hard to see the segments in the SEM image because the nanowires are solid and continuous, which was confirmed by the corresponding TEM image shown in Figure 3b. Two kinds of segments can be easily distinguished due to the obvious heterogeneous contrast, which is alternately ordered. As marked in Figure 3b, the length of one kind of segment for a selected nanowire is about 175 nm, and another kind of segment is as long as 245 nm. SAED and EDS analyses were performed on a couple of neighboring segments in a randomly selected nanowire shown in Figure 3c for the structure and composition characterization; see Figure 3d and e. The ring patterns with some bright spots indicate the polycrystalline nature of each segment. The SAED pattern of segment I can be indexed as a hexagonal Bi2Te3 phase (JCPDS 82-0358) with (015), (1010), (110), and (205) planes, and the corresponding EDS spectrum shows the significant Bi and Te signals with a trace of Sb, which indicates that segment I is the alloy with Bi:Sb:Te ) 32:6:62, that is, a slightly Sb-doped Bi2Te3. The SAED pattern for segment II can be steadily indexed to (012), (110), and (116) planes as a hexagonal Sb phase (JCPDS 851324), and almost only Sb signals can be observed in the EDS spectrum, and the corresponding quantitative analysis of the

Bi2Te3/Sb Multilayered Nanowires

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Figure 5. (a and b) TEM images of multilayered Bi2Te3/Sb nanowires deposited in different conditions labeled in the bottom of each corresponding figure. (c and d) The corresponding high magnification TEM images.

spectrum reveals that the ratio of Bi:Sb:Te is about 1:86:13, indicating that the majority of segment II is Sb. It is well-known that the period of the multilayer is extremely important for the final thermal conductivity of the system,9,19 and it has been predicted that the ZT of the multilayered nanowire would increase generally with decreasing the period.13 Here, by simply halving the deposition time in each round, various Bi2Te3/Sb multilayered nanowires with different periods can be obtained, as shown in Figure 4a-d. Becasue of the different electron-dispersing abilities of Bi and Sb atoms and the partial dissolving of Sb in the beginning of the Bi-Te deposition, it can be concluded that the brighter and narrower segments are Sb segments and the darker ones are Bi2Te3 segments. As a comparison of the five kinds of as-deposited Bi2Te3/Sb multilayered nanowires in Figure 4e, it can be found by careful observation that the length of Sb segments changes more quickly than that of the Bi2Te3 segments. It is reasonable that the dissolving of Sb would have a stronger influence on the thinner Sb segments deposited in a shorter time. It should be pointed out that the change of deposition time can effectively and finely tune the length ratio of the segments, and thus the multilayered nanowires with a longer Sb segment were successfully fabricated, as shown in Figure 5a and b. The corresponding close-up of the as-prepared multilayered nanowires in Figure 5c and d shows that the well parallel interfaces can be maintained even in such thin segments, and the minimum period of the Bi2Te3/Sb multilayered nanowires can be reached at 10 nm (Bi2Te3 4 nm/Sb 6 nm). The results demonstrate that this method provides an effective route with simplicity and low cost to fabricate the high-density thermoelectric multilayered nanowires, which are well suitable for the further thermoelectric performance study.

IV. Conclusions In summary, the high-density Bi2Te3/Sb multilayered nanowire arrays are successfully fabricated using a PAA templateassisted pulsed electrochemical deposition technology based on the deep CVs investigation of the Bi-Sb-Te electrolyte solution. The as-prepared Bi2Te3/Sb multilayered nanowires are solid and continuous with a high aspect ratio, and the multisegment characters can be clearly distinguished in the TEM image due to the obvious heterogeneous contrast, which is alternately ordered. By modulating the deposition time, the Bi2Te3/Sb multilayered nanowire arrays with tunable periods can be obtained, and the minimum period can be reached at 10 nm, which presents a good candidate for further thermoelectric device applications. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50721061 and 50772111) and the National Basic Research Program of China (2006CB922005). References and Notes (1) DiSalvo, F. J. Science 1999, 285, 703. (2) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. AdV. Mater. 2007, 1, 9–1043. (3) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. (4) Sales, B. C. Science 2002, 295, 1248. (5) Rowe, D. M. CRC Handbook of Thermoelectric; CRC Press: Boca Raton, FL, 1995. (6) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 12727. (7) Lin, Y. M.; Sun, X. Z.; Dresselhaus, M. S. Phys. ReV. B 2000, 62, 4610. (8) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229.

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