Evolution of Pb1–xSnxTe Thin Films from Dendrites to Nanoparticles

Article pubs.acs.org/Langmuir

Evolution of Pb1−xSnxTe Thin Films from Dendrites to Nanoparticles on Gold Substrates by Electrodeposition Mustafa Biçer and Il̇ kay Şişman* Department of Chemistry, Faculty of Arts and Sciences, Sakarya University, 54187 Sakarya, Turkey S Supporting Information *

ABSTRACT: Dendritic and nanostructured Pb1−xSnxTe thin films were synthesized on gold substrates from acidic solutions through a simple electrodeposition route. The deposition potential of thin films was determined using cyclic voltammetry. All of the thin films were deposited in both the absence and presence of cetyltrimethylammonium bromide (CTAB) as a cationic surfactant. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Fourier transform infrared (FT-IR) spectroscopy were employed to characterize the deposits. XRD results showed that the diffraction peaks shift to larger angles as mole fraction x increases, indicating the formation of Pb1−xSnxTe alloy. Morphological analysis revealed that the obtained thin films in the absence of CTAB were composed of dendrites, while the obtained thin films in the presence of CTAB were made of nanoparticles. Growth mechanisms for the dendritic and nanostructured thin films were discussed. The optical absorption studies show that the band gap of Pb1−xSnxTe thin films grown with short deposition times could be tuned from 0.21 to 0.35 eV by adding only the surfactant to the deposition solution.

1. INTRODUCTION Pb1−xSnxTe alloys are well-known as thermoelectric and infrared optoelectronic materials.1,2 Thermoelectric materials are special types of semiconductors that can directly convert heat to electricity.3 In comparison with bulk materials, nanostructured materials may enhance the thermoelectric efficiency.4 Similarly, optical properties of semiconductor nanomaterials differ from the corresponding bulk materials.5 In the formation of ternary Pb1−xSnxTe alloys, a number of methods are used, including molecular beam epitaxy (MBE),2,6 temperature difference method under controlled vapor pressure (TDM-CVP),7 mechanical alloying,1,8 and powder metallurgical processing.9 However, there are only a few reports on synthesis of nanostructured Pb1−xSnxTe materials. Ji et al.10 synthesized Pb0.75Sn0.25Te nanorods through an alkali metal hydrothermal treatment and reported the nanorods have an enhancement in thermoelectric efficiency relative to their bulk systems. Arachchige and Kanatzidis11 also reported a lowtemperature colloidal synthesis of Pb1−xSnxTe nanocrystals and found their optical band gap energies are significantly blueshifted as a result of quantum confinement. Semiconductor bulk or nanostructured materials can be formed via electrodeposition. Electrodeposition has shown the powerful ability to control the crystallization engineering, and it presents a simple, quick, and economical method for the preparation of large area thin films and has the advantage of allowing the controllable growth of patterned nanostructures.3 In literature, it was reported that the surfactants are frequently © 2012 American Chemical Society

used in metal electrodeposition to control the metallic crystal shape and size.12,13 The adsorption of surfactants aggregates onto electrodes has large effects on the kinetics of the electron transfer. The effect on the electron transfer rates includes blocking of the active sites by the surfactants, and electrostatic interactions between electroactive species and adsorbed surfactants.14 Consequently, adsorbed surfactants have provided grain refinement and suppression of dendritic growth.15 While there are a some reports on thermoelectric Bi2Te3 films16−19 using surfactant-assisted electrodeposition methods, there is only a report found, to the best of authors’ knowledge, on the Bi−Sb−Te films.20 To the best of our knowledge, electrodeposition of thermoelectric Pb1−xSnxTe has not been reported before. Recently, we have reported electrodeposition of thermoelectric Bi2Te3‑ySey, Bi1−xSbx, Bi2−xSbxTe3, and Bi2−xSbxSe3 thin films.21−23 As a different work on thermoelectric thin films, in this paper, we first report electrodeposition of dendritic and nanostructured Pb1−xSnxTe thin films on gold substrates in the absence and presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), respectively. The electrodeposition potential was determined by cyclic voltammetry studies. The compositional, structural, morphological, and optical properties of the thin films are presented and discussed. Received: September 16, 2012 Revised: October 19, 2012 Published: October 19, 2012 15736

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2. EXPERIMENTAL SECTION 2.1. Electrochemistry. The cyclic voltammetry and electrodeposition experiments were carried out using a PAR model 2273 potentiostat/galvanostat connected to a three-electrode cell (Faraday Cage, PAR) at room temperature (25 °C). In the cyclic voltammetry studies, an Au(111) substrate was used as working electrode. The Au(111) working electrode, similar to a ball-shaped droplet, was (111)-oriented single-crystal gold (Alfa Aesar, 99,999%) prepared as previously described by Hamelin.24 Then, the polycrystalline regions were coated with a chemically inert epoxy (Epoxy-Patch). However, the electrodepositions were performed on both Au(111) and gold foil substrates. In all electrochemical experiments, An Ag/AgCl/3 M NaCl (Bioanalytical Systems, Inc., West Lafayette, IN) was used as the reference electrode, and a platinum wire was used as the counter electrode. All of the solutions used in this study were prepared with deionized water (resistivity ≥ 18 MΩ cm). Prior to each experiment, the solutions were purged with purified N2 gas. The solutions were not stirred during all the electrochemical experiments. The electrodeposition of the thin films was carried out in a solution containing 10 mM Pb(CH3COO)2 (Pb(CH3COO)2·3H2O, Merck), 5 mM TeO2 (Merck), and varying concentrations of SnCl2 (SnCl2·2H2O, Merck) and CTAB (Aldrich, 95%). The solution compositions are listed in Table 1. The pHs of the solutions were adjusted to 0.5 ± 0.1 by using

Table 1. Atomic Percentages of Films composition of solutions (mM) solution

Pb2+

Sn2+

HTeO2+

1 2 3 4 5 6

10 10 10 10 10 10

1.5 2.5 6 6 6 6

5 5 5 5 5 5

atom % CTAB

Pb

Sn

Te

0.4 1 3

40.1 29.3 15.9 13.6 16.7 14.6

8.6 18.5 34.4 36.2 35.4 35.9

51.3 52.2 49.7 51.2 47.9 49.5

Figure 1. Cyclic voltammograms of Au(111) electrode in 0.5 M HNO3 solution containing: (a) 10 mM Pb(CH3COO)2 and (b) 5 mM TeO2. The scanning rate is 0.1 V/s.

0.5 M HNO3 (Merck) solution. The electrodeposition potential was determined from the cyclic voltammetry studies. All of the thin films were deposited at a potential of −0.5 V, with respect to the reference electrode. After each electrodeposition, the obtained thin films were rinsed with deionized water and then dried in air at room temperature. 2.2. Characterization. Characterization of the films was carried out with different techniques. The obtained films were subjected to SEM analysis in a JEOL JSM-6060LV to observe the morphology of the surface. The compositions of the films were subsequently determined by using an energy-dispersive X-ray spectrometer (EDS) attached to the SEM. Morphologies of the films were also investigated by using a semicontact mode NTEGRA Aura AFM instrument (NTMDT Co.). All images were taken in air. Single crystal silicon cantilevers of 135 μm length, 1.5 μm thickness, and 30 μm width with a force constant of 1.74 N/m and a resonant frequency of 90 kHz were used as the AFM probe. The XRD patterns for the films were recorded via a Rigaku Advance Powder X-ray diffractometer (λ = 1.54050 Å) in the span of angle between 20° and 75°. The optical measurements of the films were performed in the wavelength range of 2500−10 000 nm using a Shimadzu FT-IR-8000 series spectrophotometer, and the band gap energies were calculated from the absorbance data.

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry. In order to determine the deposition potential, cyclic voltammetry experiments were performed on Au(111) substrates. The cyclic voltammogram (CV) of an Au(111) electrode in a solution of 10 mM Pb(CH3COO)2 and 0.5 M HNO3 is shown in Figure 1a. In the range of the electrode potential between 0.90 and −0.5 V, the voltammogram is characterized by two cathodic features, labeled 1 and 2, as well as two anodic peaks, labeled 3 and 4. The cathodic peak 1 corresponds to the Pb UPD (under-

Figure 2. Cyclic voltammograms of Au(111) electrode in the solutions containing 0.5 M HNO3 and (a) 10 mM Pb(CH3COO)2 and 5 mM TeO2 and (b) 1.5 mM SnCl2. Scan rate: 0.1 V/s.

potential deposition or surface-limited deposition), while the peak 4 corresponds to the anodic process of 1.25,26 Other peaks, labeled 2 and 3, correspond to the bulk Pb deposition and its dissolution in the OPD (overpotential deposition) region, respectively.26 The CV for an Au(111) substrate in solution with 5 mM TeO2 and 0.5 M HNO3 is shown in Figure 1b. The 15737

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Figure 3. Cyclic voltammograms of Au(111) electrode in the solution containing 0.5 M HNO3, 10 mM Pb(CH3COO)2, 5 mM TeO2, and varying concentration of SnCl2. The scanning rate is 0.1 V/s.

voltammogram is characterized by three cathodic peaks, labeled C1−C3, as well as three anodic peaks, labeled A3−A1. The reductive peaks C1 and C2 correspond to the Te UPD, where the oxidative stripping peaks of A2 and A1 correspond to C1 and C2. The peaks C3 and A3 are related to bulk Te deposition and its dissolution, respectively. The voltammetric behavior of Te on Au(111) is in good agreement with what has been reported in the literature.21,22,27−30 Figure 2a displays the CV of the Au(111) electrode in a solution of 10 mM Pb(CH3COO)2, 5 mMTeO2, and 0.5 M HNO3. The voltammogram is characterized by four cathodic peaks, labeled 1−4, as well as by four anodic peaks, labeled 5− 8. In comparison with their counterpart peaks, in Figure 1, the cathodic peaks 1 and 2 correspond to the Te UPD, while the peak 4 corresponds to the bulk Pb deposition. However, the cathodic peak 3 with a shoulder can be attributed to the codeposition of the Pb UPD and bulk Te deposition. The anodic peaks, labeled 5−8, correspond to the oxidation of Pb and Te, respectively. In contrast to the behavior observed in Figure 1b, the oxidation of Te in the binary system results in two peaks (labeled 7 and 8). For comparison with Figure 2a, the CV for an Au(111) electrode in 1.5 mM SnCl2 and 0.5 M HNO3 solution is shown in Figure 2b. The cathodic peaks C1 and C2 correspond to the multilayer and bulk Sn deposition, while the peaks A2 and A1 correspond to the anodic process of C1 and C2, respectively.30−32 It can be seen that the reduction peak C2 appears at the same potential (around −0.45 V) in comparison with the bulk Pb deposition peak, labeled 4. Figure 3 displays the electrochemical behavior of 10 mM Pb(CH3COO)2, 1.5 mM SnCl2, 5 mM TeO2, and 0.5 M HNO3 solution (black line). It can be seen that the cyclic voltammogram is almost similar with the CV of binary system in Figure 2a. As shown by the gray line, with increasing of the Sn ion concentration, the iv/v peak pair gets stronger whereas

Figure 4. SEM images for Pb1−xSnxTe thin films electrodeposited from solutions (a) 1, (b) 2, and (c) 3.

other peaks remain unchanged. This indicated that the Sn deposition and dissolution peaks are overlapped with bulk Pb deposition and dissolution peaks. On the other hand, this means that the Sn content in Pb1−xSnxTe can be changed by controlling the Sn ion concentration. From Figure 3, it can also be seen that the bulk reduction of each element begins at about −0.4 V. For ternary system, a CV was also performed in the presence of CTAB but the voltammetric behavior was not changed significantly (not shown here). According to these cyclic voltammograms, if the electrodeposition is performed from Pb2+, Sn2+, and HTeO2+ at potential range of −0.4 to −0.5 15738

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Figure 5. AFM images of dendritic Pb0.8Sn0.2Te films at different magnifications.

V, Pb1−xSnxTe alloys will be formed at the electrode via following reaction. (1 − x)Pb2 + + (x)Sn 2 + + HTeO2+ + 3H+ + 6e− → Pb1 − x SnxTe + 2H 2O

(1)

3.2. Compositional, Morphological, Structural, and Optical Characterization. In order to investigate the effect of Sn2+concentration in the solution on the composition of the films, Pb1−xSnxTe films were electrodeposited on Au(111) substrates by changing only Sn2+ concentration. All of the thin films were deposited at −0.5 V for 30 min. The representative EDS spectrum of Pb1−xSnxTe with x ∼ 0.2 prepared from solution 1 (10 mM Pb(CH3COO)2 + 1.5 mM SnCl2 + 5 mM TeO2 + 0.5 M HNO3) is shown in Figure S1a (see the Supporting Information). When the concentration of Sn2+ in the deposition solution was elevated to 2.5 mM (solution 2), the atomic percentage of Sn element in the film was increased (Figure S1b in the Supporting Information). Other compositions of the films deposited at different Sn2+ concentrations are listed in Table 1. EDS analyses of different regions of the deposited films gave similar results. The results indicate that the content of Sn in the Pb1−xSnxTe films is strongly dependent on the concentration of Sn2+. Figure 4 presents the SEM images of Pb1−xSnxTe films (x = 0.2, 0.4, 0.7) on Au(111) substrates electrodeposited from solutions 1−3. All of the samples were electrodeposited at −0.5 V for 30 min without CTAB. Figure 4a shows the surface

Figure 6. SEM images for Pb1−xSnxTe thin films electrodeposited from solutions (a) 4, (b) 5, and (c) 6.

morphology of Pb0.8Sn0.2Te film prepared from solution 1. It can be seen that the nonuniform fern-shaped dendrites were formed on the electrode surface. The dendrites look like a long main trunk with short side branches all decorated by small leaves. The trunk has length up to 10 μm. When the concentration of Sn2+ is changed to 2.5 mM (solution 2), the size of side branches is shorter than that in solution 1 (Figure 4b). Figure 4c shows the SEM image of Pb1−xSnxTe film with x ∼ 0.7 prepared from the solution 3. It is obvious that the size of side branches is much shorter than that in Figure 4a and b. It can be clearly seen that the size decreases with increasing Sn2+ 15739

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Figure 7. XRD patterns of Pb1−xSnxTe thin films electrodeposited at −0.5 V for 30 min from solutions (a) 1, (b) 2, (c) 3, and (d) 6. The vertical lines indicate the corresponding reflection positions and intensities for bulk PbTe.

Figure 9. Plots of (a) absorption and (b) (αhν)2−hν of Pb0.3Sn0.7Te thin films electrodeposited in both the absence and presence of CTAB.

concentration in solution. Consequently, the sizes of dendrites can be controlled by changing Sn content. Typical AFM images of the deposited dendritic Pb0.8Sn0.2Te films in the absence of CTAB are shown with different magnifications in Figure 5. As shown in Figure 5a, the angle between the trunk and its side branches is approximately 60°. Further enlargement shows that the trunk and side branches are consist of hierarchical nanoparticles with the sizes of about 75− 150 nm (Figure 5b). From the above results in section 3.1, it can be seen that the electrodeposition can be performed at potential range of −0.4 to −0.5 V. When the deposition potential is increased from −0.4 to −0.5 V, Pb2+, Sn2+, and HTeO2+ ions are reduced faster than the transport rate of Pb2+,

Figure 8. AFM images of Pb0.3Sn0.7Te thin films electrodeposited for 30 s from solutions (a) 3 and (b) 6.

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Sn2+, and HTeO2+ ions to the interface. Consequently, the shape of growing crystals begins to be governed by the mass transport of Pb2+, Sn2+, and HTeO2+ ions, resulting in unexpected morphologies as dendritic crystals. Similar characteristics of the electrochemical formation of dendritic SnTe, PbTe, and Zn film dependence on the deposition potential have been reported elsewhere.30,33,34 To investigate the effect of the surfactant, the electrodeposition was performed on Au(111) electrodes using a cationic surfactant, CTAB. Figure 6 shows the SEM images of the Pb1−xSnxTe films with x ∼ 0.7 prepared from different concentrations of CTAB, ranging from 0.4 to 3 mM. In the presence of CTAB, the morphology and size are completely different than that obtained in the absence of CTAB. It can also be seen that the particle size was gradually decreased with increasing amount of CTAB. When the concentration of CTAB is changed to 3 mM (solution 6), the particles size is about 100 nm (Figure 6c). This could be due to the adsorption of surfactant on Au(111) surface during the electrodeposition process. The surfactant inhibits the hierarchical crystal growth due to the electrostatic repulsion between electroactive species (Pb2+, Sn2+, and HTeO2+) and adsorbed surfactant (CTA+). Consequently, it causes an increase of nuclei number and growth of small size crystals. Similar results have been observed for the electrodeposition of Zn−TiO2 and Zn films in the presence of CTAB.13,35 To further investigate the effect of CTAB, the electrodeposition was performed using other surfactants such as sodium dodecyl sulfate (SDS) and polyvinylpyrrolidone (PVP). The SEM images of Pb1−xSnxTe films with x ∼ 0.7 grown in the presence of SDS and PVP are shown in Figure S2a and b (Supporting Information), respectively. It can be clearly seen that the smallest particle size is obtained in the presence of CTAB. Figure 7 shows the typical XRD patterns of Pb1−xSnxTe films on gold foil substrates electrodeposited from solutions 1−3 and 6. It can be seen that all the patterns can be easily indexed to rock salt PbTe (JCPDS 38-1435). No additional impurity phases could be detected by XRD. Compared with the reference data pattern, the diffraction peaks in the patterns of Pb1−xSnxTe films (x = 0.2, 0.4, 0.7) grown in the absence of CTAB are shifted to larger angles with the increase of Sn content (Figure 7a−c). The results indicate the Sn is incorporated into the PbTe crystal structure. In addition, the sharp and narrow peaks indicate that the films are well crystalline. However, the broader and weaker diffraction peak, indexed to the (200) plane, is observed in the presence of CTAB (Figure 7d). This indicates that the size and crystallinity of films is changed in the presence of CTAB. Similar results have been reported for the Cu2O films electrodeposited with CTAB.36 In order to investigate the growth behaviors of films, the electrodeposition was performed at −0.5 V for 30 s in both the absence and presence of CTAB and the films were imaged using AFM (Figure 8). In the absence of CTAB, the initial stage of a dendrite is obtained on Au(111) surface (Figure 8a). The growth mechanism of dendritic crystals may be explained by using diffusion-limited aggregation (DLA) model.37 It can be clearly seen that the dendritic crystal or the root of dendrite is formed by successive aggregations of nanoparticles. In contrast, the surface is completely covered by well-separated spherical nanoparticles with diameters less than approximately 5 nm in the presence of CTAB (Figure 8b). In the presence of the

surfactant, numerous nuclei are formed due to lower crystal growth and aggregation rate. Figure 9a shows the FT-IR absorption spectra of Pb1−xSnxTe films with x ∼ 0.7 electrodeposited on gold foil substrates at −0.5 V for 30 s in the absence and presence of the surfactant. It can be seen that the absorption band onset of the nanostructured film shifts to higher energy as compared with that of the dendritic film. The band gap values of the films were calculated by plotting (αhν)2 versus hν and extrapolating the linear portion of graph to the energy axis (Figure 9b). The band gap values of the dendritic and nanostructured thin films were found to be 0.21 and 0.35 eV, respectively. This blue shift is attributed to enhanced quantum confinement in the nanoparticles. Similar blue shifts have been observed for the colloidal synthesis of Pb1−xSnxTe nanocrystals.11

4. CONCLUSIONS In summary, we have demonstrated the first electrodeposition of Pb1−xSnxTe thin films in both the absence and presence of CTAB. Cyclic voltammetry, XRD, and EDS analyses suggest that the x content of Pb1−xSnxTe thin films strongly depends on the solution composition. Morphological investigations indicate that the films evolve from fern-shaped dendrites to nanoparticles by increasing the CTAB concentration from 0 to 3 mM. In addition, growth mechanisms for the dendritic and nanostructured thin films have been discussed. The optical absorption studies revealed that nanostructured films grown with short deposition times showed quantum confinement effect. We expect that this method can likely be utilized to synthesize other semiconductor materials with various sizes.

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ASSOCIATED CONTENT

S Supporting Information *

EDS spectra of Pb1−xSnxTe thin films (Figure S1). SEM images of of Pb1−xSnxTe thin films electrodeposited in the presence of SDS and PVP (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +90-264-2956063. Fax: +90-264-2955950. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Commission of Science Research (Project No. 2012-50-02-027), Sakarya University, is gratefully acknowledged.

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REFERENCES

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