Selective Electrodeposition and Growth Mechanism of Thermoelectric

Apr 20, 2010 - By increasing the deposition time, the needlelike particles become numerous thin sheets, which grow upright and interlace with one anot...
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J. Phys. Chem. C 2010, 114, 8256–8263

Selective Electrodeposition and Growth Mechanism of Thermoelectric Bismuth-Based Binary and Ternary Thin Films Mustafa Bic¸er, Hilal Ko¨se, and I˙lkay S¸is¸man* Department of Chemistry, Arts and Sciences Faculty, Sakarya UniVersity, 54187, Sakarya, Turkey ReceiVed: February 8, 2010; ReVised Manuscript ReceiVed: April 6, 2010

Different morphologies of thermoelectric Bi1-xSbx and Bi2-xSbxTe3 thin films, including rods, dendrites, thin sheets, and spherical particles, were selectively obtained by an electrodeposition method at room temperature (25 °C). Cyclic voltammetry was used for determination of the deposition potentials of thin films. The influences of the deposition potential and the electrolyte composition on the films were studied. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were employed to characterize the thin films. It was found that the different shaped Bi-rich Bi1sxSbx films can be obtained by tuning the electrolyte composition. However, the Bi2sxSbxTe3 films can be fabricated by changing the deposition potential. In underpotentials (UPD), the reduction reactions of Bi3+, Te4+, and Sb3+ take place to form (Bi0.5Sb0.5)2Te3. When the potential shifts into overpotentials (OPD), Bi0.5Sb1.5Te3 is formed on the electrode surface. The EDS data indicates that the composition of the films is consistent with XRD results. The SEM investigations show that the growth mechanism of deposited Bi1sxSbx and Bi2sxSbxTe3 films depends on the electrolyte composition and the deposition potential, respectively. 1. Introduction Thermoelectric (TE) materials are special types of semiconductors that can directly convert heat to electricity.1 The binary compounds of type VA-VIA are commonly used for TE devices such as thermoelectric generators, coolers, radiation detectors, and so forth.2 Among these, Bi2Te3 and its derivative compounds are considered to be the best materials used in TE refrigeration at room temperature.3 The performance of TE devices depends on the figure of merit (ZT) of the material. Alloying Bi2Te3 with Sb2Te3 is expected to lead an enhancement of the ZT.4 In comparison with bulk TE materials, thin film TE materials offer tremendous scope for ZT enhancement.5 In the formation of high-quality thermoelectric devices, several thin film formation methodologies are used, including molecular beam epitaxy (MBE),6 chemical vapor deposition (CVD),7 and sputtering.8 In general, these methods are performed in vacuum and are thermal methods achieving compound formation by heating reactants and substrate. However, electrochemical deposition of thin films is a convenient alternative to vacuum-based methods because of simplicity, low cost, and the ability to work at ambient temperature and pressure. In addition, low-temperature electrochemical deposition is desirable to avoid heat-induced interdiffusion of adjacent layers in a structure. Semiconductor thin films can be formed via surface-limited reactions by using atomic layer epitaxy (ALE). ALE involves the growth of compound thin films using surface-limited reactions to form single atomic layers of each element in a cycle. Electrochemical surface-limited reactions are generally referred to as underpotential deposition (UPD).9 The origin of the UPD phenomena is the free energy of formation of a surface compound between the depositing element and the substrate surface (electrode). UPD is a surface phenomenon that depends on the substrate structure, substrate physical-chemical char* To whom correspondence should be addressed. Tel: +90-264-2956063. Fax: +90-264-2955950. E-mail: [email protected].

acteristics, and deposit (adatom)-substrate interactions. As a result of the UPD process, which takes place at more positive potentials than the deposition equilibrium potential (Nernst potential), the electrode surface is partially or completely (up to an atomic layer) covered by a deposit. However, the overpotential deposition (OPD or bulk deposition) process is determined by electrode potential (overpotential), deposit growth kinetics and mechanism (2D or 3D), electroactive species concentration, and deposit-substrate and deposit-deposit interactions. OPD takes place at more negative potentials than the Nernst equilibrium potential. Briefly, UPD may involve deposition onto substrate while OPD would involve deposition onto a substrate surface modified by an atomic layer, which was formed during the UPD process. Generally, deposits reach more than one atomic layer in the OPD regions.10 Electrochemical atomic layer epitaxy (ECALE), developed by Gregory and Stickney, is the result of combining UPD with the principles of ALE to form a deposition cycle.11 Atomic layers of a compound’s component elements are deposited at underpotentials in a cycle to directly form a compound. However, this method is very time-consuming and produces a large amount of dilute wastewater because of the rinsing of the substrate after each deposition. Automated deposition systems by ECALE were developed to overcome these problems.12 The electrodeposition of semiconductor thin films has been proposed to occur by the so-called induced codeposition mechanism, where both elements are deposited at the same time from the same solution.13 Stoichiometry is maintained by having the more noble element as the limiting reagent. Codeposition holds great promise if greater control can be achieved. At present, the main points of control are solution composition and ¨ znu¨lu¨er et al.14 have the deposition potential. Recently, O developed an electrodeposition method, on the basis of codeposition from the same solution of the precursors of the target compound at a constant potential, which is determined from the UPD potential of each precursor. This method has been

10.1021/jp101221u  2010 American Chemical Society Published on Web 04/20/2010

Bismuth-Based Binary and Ternary Thin Films successfully applied to obtain thin films of PbS,14 CdS,15 and Bi2Te3sySey16 and nanowires of CdS.17 While there are some reports on both Bi1-xSbx thin films18,19 and nanowires20,21 and Bi2-xSbxTe3 (x ∼ 1.5) thin films22-24 and nanowires25,26 using electrochemical methods, there is only one report found, to the best of the authors’ knowledge, on the Bi2sxSbxTe3 nanotubes with x ∼ 1.27 However, no work has been reported on the electrodeposition and growth mechanism of the binary and ternary bismuth-based compounds under the conditions of UPD-OPD. In our previous work, Bi2Te3sySey TE thin films were successfully deposited under the conditions of UPD only.16 As a different work on TE thin films, in this paper, we report the selective electrodeposition and growth mechanism of thin films of Bi1sxSbx and Bi2sxSbxTe3 on Au(111) substrates under the conditions of both UPD and OPD. Thin films of Bi1sxSbx and Bi0.5Sb1.5Te3 were obtained in the OPD regions of Bi and Sb, respectively. However, (Bi0.5Sb0.5)2Te3 thin films were attained in the UPD regions of each element. Bi1sxSbx alloys are among the best TE materials, and the alloy with x ) 0.04-0.22 is considered to be the best n-type material for thermoelectric cooling at temperature ∼ 100 K.28 (Bi0.5Sb0.5)2Te3 is a narrow band gap semiconductor and exhibits anomalous characteristics and irreversible behavior upon heating.29 On the other hand, Bi0.5Sb1.5Te3 exhibits the best TE properties at room temperature.30 The appropriate codeposition potentials were determined by cyclic voltammetry. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were employed for the characterization of the thin films. 2. Experimental Section Cyclic voltammetry and electrodeposition experiments were performed using a PAR model 2273 potentiostat/galvanostat connected to a three-electrode cell (K0269A Faraday Cage, PAR) at room temperature (25 °C). The Au(111) working electrode, similar to a ball-shaped droplet, was (111)-oriented single-crystal gold (Alfa-Johnson Matthey, 99,995%) prepared as previously described by Hamelin.31 An Ag/AgCl/3 M NaCl was used as the reference electrode, and a platinum wire was used as the counter electrode. Solutions were prepared with deionized water (i.e., >18 MΩ). The Bi3+ solutions were prepared by dissolution of Bi(NO3)3 · 5H2O (analytical reagent) in concentrated HNO3 aqueous solution followed by diluting the HNO3 to 0.1 M. The Te4+ solutions were obtained in the same way by the dissolution of TeO2 (analytical reagent). The Sb3+ solution was prepared by dissolution of SbCl3 (analytical reagent) in concentrated tartaric acid (C4H6O6) solution followed by adding with concentrated HNO3 and diluting the tartaric acid and HNO3 to 0.2 and 0.1 M. Tartaric acid was used as a complexing agent to improve the solubility of Sb in water through the formation of a Sb-tartaric complex. The electrolyte solutions were deaerated by nitrogen bubbling through them for 15 min prior to each experiment. Solutions were not stirred during all the electrochemical measurements and depositions. The deposition potentials of films were determined from cyclic voltammetry data of each element. Bi0.8Sb0.2 thin films were electrodeposited at -0.10 V, which stands for the OPD region of Bi, from a solution of 2 mM Bi(NO3)3 · 5H2O, 1 mM SbCl3, 0.2 M C4H6O6, and 0.1 M HNO3 (solution 1). However, the synthesis of Bi0.9Sb0.1 films was performed at the same potential from a solution of 3 mM Bi(NO3)3 · 5H2O, 1 mM SbCl3, 0.2 M C4H6O6, and 0.1 M HNO3 (solution 2). On the other hand, (Bi0.5Sb0.5)2Te3 thin films were obtained from a solution of 0.2 mM Bi(NO3)3 · 5H2O, 1 mM

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Figure 1. Cyclic voltammograms of Bi from 2.0 mM and 0.2 mM Bi(NO3)3 in a 0.1 M HNO3 solution on a Au(111) electrode. The scanning rate is 50 mV/s.

SbCl3, 0.3 mM TeO2, 0.2 M C4H6O6, and 0.1 M HNO3 (solution 3) at -0.10 V since this potential value is well suited to the UPD of each element. However, Bi0.5Sb1.5Te3 thin films were deposited from the same electrolyte at -0.22 V since this potential value is well suited to the OPD region of Sb. After electrodeposition, the obtained thin films were rinsed with deionized water and then were dried in air at room temperature. Characterization of the films was carried out with different techniques. The crystal structure of the films was investigated by an X-ray diffractometer (Rigaku, D-max 2200, Japan) using Cu KR radiation (λ ) 1.54050 Å). Surface morphologies of the deposited films were observed with scanning electron microscopy (SEM), JEOL, JSM-6060LV. The chemical compositions of the films were determined by an energy-dispersive X-ray spectrometer (EDS) attached to the SEM. The working conditions for EDS analysis include an accelerating voltage of 15 kV, a beam current of 5 nA, a working distance of 15 mm, and a live time of 60 s for each run. 3. Results and Discussion 3.1. Electrodeposition of Binary Thin Films. To determine the codeposition potentials for the elements, cyclic voltammetry experiments were performed on Au(111) substrates. The cyclic voltammogram of a Au(111) electrode in a solution of 2.0 mM Bi(NO3)3 · 5H2O and 0.1 M HNO3 is shown in Figure 1 (thick curve). The reductive peak C1 corresponding to the peak of Bi UPD and C2 is the OPD (bulk deposition) peak of Bi, where the oxidative stripping peaks of A2 and A1 correspond to C2 and C1, respectively. The voltammetric behavior of Bi on Au(111) is in good agreement with what has been reported in the literature.10 The bulk deposition starts at about -0.01 V in this voltammogram. If the Au(111) electrode is immersed in 0.2 mM Bi(NO3)3 + 0.1 M HNO3 solution, peaks C2/A2 disappear (thin curve). The total charge density under peaks C1/A1 in the 0.2 mM Bi(NO3)3 solution is the same as that in 2.0 mM Bi(NO3)3 solution, and increasing concentrations of Bi3+ from 0.2 to 2.0 mM cause only the bulk redox process to become more intensive. Since the UPD process is surface-limited, the charge involved in the UPD is independent of concentration.

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Bic¸er et al. TABLE 1: Atomic Percentages of Films composition of solutions (mM) solution 2 solution 1 solution 3 solution 3

Figure 2. Cyclic voltammograms of Au(111) electrode in the solutions containing 0.1 M HNO3, 0.2 M C4H6O6, and (a) 1 mM SbCl3 and (b) 1 mM SbCl3 and 0.2 mM Bi(NO3)3 · 5H2O. Scanning rate 50 mV/s.

Similar characteristics of the UPD dependence on the Bi ion concentration have been observed with the results in an ECALE synthesis of Bi-Te as reported by Zhu et al.5 According to the cyclic voltammogram of 0.2 mM Bi(NO3)3, the Bi monolayer might form more negative potentials through -0.15 V. The cyclic voltammograms (CVs) for a Au(111) substrate in solution with 1 mM SbCl3, 0.2 M C4H6O6, and 0.1 M HNO3 are shown in Figure 2a. For the convenience of discussion, the CVs are divided into regions of UPD (from -0.03 to -0.20 V) and OPD (-0.20 V and more negative). When the potential of the Au(111) electrode is scanned between 0.60 and -0.20 V, the reductive peak C1(C2) appears at -0.03 V for a surfacelimited deposition (UPD) of Sb (thin curve). As shown in the thin curve, there is only one cathodic peak but two anodic peaks (A2 and A1) on the cyclic scanning; obviously, the peak C1(C2) is the union of two different Sb UPD structures similar to the observation of Sb on Au reported by previous researchers.32,33 In other words, the stripping peaks A2 and A1 correspond to the dissolution of the composite C1(C2) peak. If the potential of the Au(111) working electrode is scanned through more negative potentials than -0.20 V, a second reductive peak C3 appears at -0.25 V for the deposition of bulk Sb in the OPD region, and the corresponding oxidative peak A3 occurs at 0.0 V (thick curve). According to the CVs, if the potential of the electrode is kept constant at a potential within a range of -0.03 to -0.20 V (UPD region), which is more positive than the Nernst potential of Sb, an atomic layer of Sb is deposited at the electrode. If the electrode potential is shifted toward potentials less than -0.20 V (OPD region), more atomic layers of Sb is deposited on the substrate than what is normally expected for UPD region.33 In comparison with Figures 1 and 2a, the CVs for a Au(111) electrode in 1 mM SbCl3, 0.2 mM Bi(NO3)3 · 5H2O, 0.1 M C4H6O6, and 0.1 M HNO3 electrolyte are shown in Figure 2b. Sweeping negatively from an initial potential of 0.60 V, two

Bi(NO3)3

SbCl3

3 2 0.2 0.2 0.2

1 1 1 1 1

potential (V) TeO2

0.3 0.3

atomic (%) Bi

-0.10 -0.10 -0.10 -0.10 -0.22

92.1 80.6 48.6 19.5 9.2

Sb

Te

7.9 19.4 51.4 21.1 59.4 31.6 59.2

reductive peaks (labeled 1 and 2) are observed. When the potential sweep is reversed at -0.25 V and is scanned positively, four stripping features, 3-6, are observed. It can be seen that the CVs are different from the CVs of Sb3+ without Bi3+. As shown in the thin curve (UPD region), there is only one cathodic peak, at -0.04 V, on the cyclic scanning; obviously, the peak can be attributed to the codeposition of the Bi and Sb. On the other hand, it can be regarded that peak 5 comes from the oxidation of Bi UPD as compared with the cyclic voltammogram of Bi3+. If the potential of the working electrode is scanned at more negative potentials through -0.25 V, the Bi bulk deposition does not occur (thick curve). According to the cyclic voltammograms of 0.2 mM Bi(NO3)3 · 5H2O, 1 mM SbCl3, 0.2 M C4H6O6, and 0.1 M HNO3 electrolyte, the Bi1sxSbx (x ∼ 0.5) film might be formed at a potential range of -0.04 to -0.20 V, which stands for a region between the Bi and Sb UPD, and Bi and Sb will be deposited simultaneously at the electrode (Table 1). Therefore, it should promote the atom-by-atom growth of the Bi1sxSbx at the substrate. On the other hand, if the electrodeposition is performed from solution 1 or solution 2 at a potential range of -0.04 to -0.20 V, which stands for Bi OPD region (Figure 1), Bi-rich Bi1sxSbx films will be deposited at the electrode according to the following reaction.19

(1 - x)Bi3 + + (x)Sb3 + + 3e- f Bi1-xSbx

(1)

3.2. Electrodeposition of Ternary Thin Films. The cyclic voltammetric behavior of a Au(111) electrode in 3 mM TeO2 and 0.1 M HNO3 electrolyte is shown in Figure 3 (thick curve). In the range of the electrode potential between 0.90 and -0.1 V, the voltammogram is characterized by three cathodic features, labeled C1-C3, as well as by three anodic peaks, labeled A3-A1. The cathodic peaks C1 and C2 correspond to the Te UPD, while peaks A1 and A2 correspond to the anodic processes of C1 and C2. Other peaks (C3/A3) correspond to the deposition and dissolution of bulk Te in the OPD region. As shown in Figure 3, the bulk Te deposition feature does not occur until -0.02 V in 3 mM TeO2 and 0.1 M HNO3 electrolyte, which suggests that an atomic layer of Te might be formed at more positive potentials than -0.02 V. According to the results, the UPD potentials of Sb and Te do not overlap on Au(111) electrode. If the Au(111) electrode is immersed in a 0.3 mM TeO2 + 0.1 M HNO3 solution, the cathodic peaks C1 and C2 shift to negative potentials indicating that the deposition process is significantly more slow with the Te ion concentration (thin curve). In addition, other peaks (C3/A3) disappear in this voltammogram. From Figure 3, it can also be seen that the total charge density of stripping peaks A1 and A2 in the 0.3 mM TeO2 solution is the same as that in 3 mM TeO2 solution. The coverage of the Te monolayer is the same at various Te ion concentrations, and thus increasing the concentrations from 0.3 to 3 mM causes only the bulk redox process to become more intensive. Because the UPD process is surface-limited, the charge involved in the UPD is independent of concentration.

Bismuth-Based Binary and Ternary Thin Films

Figure 3. Cyclic voltammograms of Te from 3 mM and 0.3 mM TeO2 in a 0.1 M HNO3 solution on a Au(111) electrode at a scan rate of 50 mV/s.

Zhu et al.34 have reported similar characteristics of the UPD dependence on the Te ion concentration with a deposition system of Te on Au. According to the cyclic voltammogram of 0.3 mM TeO2, the Te monolayer might be formed at more negative potentials through -0.20 V. Comparing with Figure 2b, the UPD potentials of Bi, Sb, and Te overlap on Au(111) electrode. The electrochemical behaviors of solution 3 investigated by using cyclic voltammetry are shown in Figure 4. The relatively broad cathodic peak between 0.44 and 0.17 V, labeled 1, corresponds to the deposition of the first Te UPD. Another broad reductive peak at about -0.05 V is considered to be the result of the second Te UPD peak as well as both the Sb UPD peak and the Bi UPD peak. When the potential sweep is reversed at -0.20 V and is scanned positively, three stripping features, 5-7, are observed. In comparison with their counterpart peaks, in Figures 1-3, these anodic peaks correspond to the oxidation of Sb, Bi, and Te in the UPD region, respectively. In contrast to the behavior observed in Figure 3, the oxidation of Te UPD in the ternary system results in one peak (labeled 7). If the potential of the working electrode scanned more negative potentials through -0.25 V (OPD region), a third reduction peak, labeled 3, and its stripping peak, 4, appear. In comparison with their counterpart peak pair, in Figure 2a, the 3/4 peak pair is associated with Sb bulk deposition and oxidation. It can also be seen that peaks 5-7 do not change with prolonged polarization verifying that they are the oxidative stripping peaks of Sb, Bi, and Te UPD. On the basis of the above results, if the potential of the electrode in solution 3 is kept constant at -0.10 V, which is suitable for the UPD regions of each element, Bi, Sb, and Te will be deposited simultaneously at the electrode surface. Therefore, it should promote the atom-by-atom growth of the mixture of Bi2Te3 and Sb2Te3 or Bi2sxSbxTe3 with x ∼ 1 at the substrate. Since the value of this potential is not enough for the bulk deposition of Bi, Sb, and Te, deposition of Bi3+ on Bi, SbO+ on Sb, or HTeO2+ on Te will not occur. As a

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Figure 4. Cyclic voltammograms of Au(111) electrode in the solution containing 0.2 mM Bi(NO3)3 · 5H2O, 1 mM SbCl3, 0.3 mM TeO2, 0.2 M C4H6O6, and 0.1 M HNO3. The scanning rate is 50 mV/s.

Figure 5. XRD pattern of Bi1sxSbx rods synthesized from solution 1 at -0.10 V for 3 h.

consequence, the stoichiometric 1:1:3 ratio of Bi:Sb:Te is expected under the above experimental conditions. On the other hand, if the electrodeposition is performed from the same electrolyte at potential range of -0.20 V to more negative, which stands for Sb OPD region, Sb-rich Bi2sxSbxTe3 films will be deposited at the electrode. 3.3. Structural, Compositional, and Morphological Characterization of Bi1sxSbx and Bi2sxSbxTe3 Films. The XRD pattern of the Bi1sxSbx thin film deposited from solution 1 at -0.10 V (OPD region of Bi) is presented in Figure 5. It can be seen that all diffraction peaks can be indexed to the rhombohedral Bi (JCPDS, 85-1331). The sharp and narrow peaks indicate that the film has good crystalline. However, these peaks are slightly shifted to higher 2θ values from the positions expected for a pure Bi, which is consistent with the formation of a Bi-Sb alloy (the film is approximately 80% Bi and 20%

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Figure 6. SEM images of Bi1sxSbx rods deposited from solution 1 at -0.10 V for various deposition times: (a) 1, (b) 1.5, and (c) 3 h.

Sb as determined by EDS; see Table 1 and Figure S1 of the Supporting Information). Figure 6 presents the SEM images of Bi1sxSbx thin films on Au(111) substrates electrodeposited at -0.10 V from the same solution for 1, 1.5, and 3 h. As can be seen in Figure 6a, the Au(111) surface is completely covered by well-separated nanorods with diameters in the range of 75-100 nm and with lengths of up to 250 nm. As the time for deposition increases to 1.5 h, new rods form at the surface (Figure 6b). Figure 6c shows the SEM image obtained after the electrodeposition for 3 h. By increasing the deposition time, the film surface consists of rodlike particles. It can be seen that the Bi-rich Bi1sxSbx films show 3D growth characteristics. Figure 7 presents the SEM images of Bi1sxSbx films on Au(111) substrates electrodeposited from solution 2 at -0.10

Bic¸er et al.

Figure 7. SEM images of Bi1sxSbx dendrites obtained from solution 2 at -0.10 V for various deposition times: (a) 0.5 and (b, c) 3 h. Panel c shows a high magnification image of dendritic crystals that compose the dendrites shown in panel b.

V for 0.5 and 3 h. When the deposition is performed for 0.5 h, numerous nonuniform dendritelike crystals were obtained on the electrode surface (Figure 7a). As the deposition proceeds further, the dendritelike crystals were transformed to bigger fernshaped dendrites in a large quantity and in good uniformity (Figure 7b, c). The chemical composition of the dendrites was determined to be roughly 90% Bi and 10% Sb by EDS (Table 1 and Figure S1 of the Supporting Information). The Bi-rich Bi1sxSbx dendrites look like a long main trunk with short side branches all decorated by small leaves. The length of the trunk is up to 5 µm. Furthermore, all the side branches grow from the main trunk with an angle of ca. 60° implying that the Bi-

Bismuth-Based Binary and Ternary Thin Films

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8261 3HTeO2+ + (2 - x)Bi3 + + xSbO + + (9 + 2x)H + + 18e- f Bi2-xSbxTe3 + (6 + x)H2O

(2)

Figure 9 presents the SEM images of (Bi0.5Sb0.5)2Te3 thin films on Au(111) substrates electrodeposited from solution 3 at -0.10 V for 0.5, 1, and 3 h. It can be seen in Figure 9a that the Au(111) surface is almost covered as smoothly by the (Bi0.5Sb0.5)2Te3 film with only a few pits. When the deposition time is increased to 1 h, numerous spherical nanoparticles about 100 nm in diameter were formed on the top of the (Bi0.5Sb0.5)2Te3 overlayer (Figure 9b). We performed SEM experiments to investigate the films forming at higher deposition times. The film surface

Figure 8. XRD patterns of the Bi2sxSbxTe3 thin films deposited from solution 3 at potentials of (a) -0.10 and (b) -0.22 V.

rich Bi1sxSbx dendritic crystals grow along a preferential direction. It can be seen that the dendrites grow on the electrode surface instead of on the rods at the same applied potential by increasing the Bi3+ concentration. Similar characteristics of the electrochemical formation of dendritic zinc film dependence on the zinc ion concentration have been observed by Lopez and Choi.35 XRD experiments were performed to investigate the crystal structures of Bi2sxSbxTe3 films electrodeposited at different potentials (Figure 8). The XRD pattern of the film electrodeposited on the Au(111) electrode at -0.10 V (UPD region of each element) for 3 h in solution 3 is shown in Figure 8a. The diffraction peak observed at 2θ ) 38.22° is due to the Au(111) substrate. All of the other broad and weak diffraction peaks observed at 2θ ) 27.94, 41.69, and 50.93° correspond to the (015), (110), and (205) planes of the (Bi0.5Sb0.5)2Te3 (JCPDS, 72-1835), respectively. In addition, no diffraction peaks from the elemental Bi, Sb, and Te were detected. This result could mean that the film is a mixture of Bi2Te3 and Sb2Te3 or that the Bi2sxSbxTe3 with x ∼ 1 alloy phase has formed in the UPD region (the composition is roughly 1:1:3 for Bi:Sb:Te as confirmed by EDS; see Table 1 and Figure S2 of the Supporting Information). The XRD pattern of the film deposited from the same electrolyte at -0.22 V (OPD region of Sb) for 3 h is presented in Figure 8b. The sharp and narrow diffraction peaks (2θ scale) at 28.12, 51.50, and 58.26° correspond to (015), (205), and (0210) reflections of the Bi0.5Sb1.5Te3 (JCPDS, 49-1713), respectively. It can be seen that Bi0.5Sb1.5Te3 grows on the substrate instead of (Bi0.5Sb0.5)2Te3. This indicates that the x value of Bi2sxSbxTe3 films strongly depends on the deposition potential. The chemical composition of the film was determined to be Bi0.46Sb1.58Te2.96 by EDS (Table 1 and Figure S2 of the Supporting Information). In addition, the sharpening of the diffraction peaks with the increasing of the deposition potentials indicates the improvement of the Bi2sxSbxTe3 crystallites. This change gives rise to an enhancement of carrier mobility and thermoelectric properties.36 Consequently, XRD results suggest that the electrodeposition carried out at the different codeposition potentials is able to induce ternary alloys formation that can be described as23

Figure 9. SEM images of Bi2sxSbxTe3 thin films obtained from solution 3 at -0.10 V for various deposition times: (a) 0.5, (b) 1, and (d) 3 h.

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Bic¸er et al. needlelike particles become numerous thin sheets, which grow upright and interlace with one another. As clearly seen in Figure 10, the films obtained at -0.22 V are shown in 3D growth characteristics. In this potential (OPD region of Sb), 3D growth begins rapidly, and eventually Sb-rich films form. Consequently, the grain size and morphology of the deposited Bi2sxSbxTe3 films depend strongly on the deposition potential. 4. Conclusions In summary, we reported here a selective and efficient electrodeposition route for the synthesis of Bi1sxSbx and Bi2sxSbxTe3 thin films on Au(111) substrates at room temperature. The cyclic voltammetry, XRD, and EDS analyses suggest that the x value of Bi2sxSbxTe3 films strongly depends on the deposition potential. In addition, SEM investigations indicate that the growth mechanism of Bi2sxSbxTe3 thin films changes at different deposition potentials. However, the shape of Birich Bi1sxSbx films could be altered from a rod to a fern leaf as the Bi3+ concentration increases. We expect that this method can be extended to synthesize other materials, and these products can be used in thermoelectric applications. Acknowledgment. Sakarya University is gratefully acknowledged for the financial support of this work. Supporting Information Available: EDS characterization of the Bi1sxSbx and Bi2sxSbxTe3 thin films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. SEM images of Bi2sxSbxTe3 thin films deposited from solution 3 at -0.22 V for different deposition times: (a) 0.5, (b) 1, and (c) 3 h.

consists of spherical (Bi0.5Sb0.5)2Te3 crystals with average size of 250 nm after the electrodeposition for 3 h (Figure 9c). SEM images recorded as a function of deposition time clearly indicate that the growth mode changes from the 2D to the 3D growth, and eventually well-separated spherical crystals form in the UPD region of each element. When the deposition potential is changed to -0.22 V for 0.5 h, the Au(111) surface is completely covered by the needlelike Bi0.5Sb1.5Te3 nanoparticles with average size of 100 nm (Figure 10a). As the time for deposition increases to 1 h, new needlelike particles with average size of 250 nm form on the top of former nanoparticles (Figure 10b). Figure 10c shows the SEM image obtained after the electrodeposition of Bi0.5Sb1.5Te3 for 3 h. By increasing the deposition time, the

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