Letter pubs.acs.org/Langmuir
Surfactant-Mediated Electrodeposition of Bismuth Telluride Films and Its Effect on Microstructural Properties Andrew J. Naylor,† Elena Koukharenko,‡ Iris S. Nandhakumar,*,† and Neil M. White‡ †
School of Chemistry and ‡School of Electronics and Computer Science, University of Southampton, Southampton, Hampshire, SO17 1BJ, United Kingdom ABSTRACT: We report the synthesis of highly crystallographically textured films of stoichiometric bismuth telluride (Bi2Te3) in the presence of a surfactant, sodium lignosulfonate (SL), that resulted in the improved alignment of films in the (110) plane and offered good control over the morphology and roughness of the electrodeposited films. SL concentrations in the range 60−80 mg dm−3 at a deposition potential of −0.1 V vs SCE (saturated calomel electrode) were found to yield the most improved crystallinity and similar or superior thermoelectric properties compared with results reported in the literature.
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of close to −0.1 V vs saturated calomel electrode (SCE), have indicated that such films can be prepared with the preferred (1 1 0) crystallite orientation, a feature not observed in films grown at more cathodic potentials.8 This preferred orientation is considered to help yield the best TE performance of Bi−Te materials, which exhibit strong anisotropy in their physical properties, as their TE properties along the a-axis are superior to those along the c-axis, due to the R3̅m rhombohedral unit cell structure, consisting of the Te(1)-Bi-Te(2)-Bi-Te(1) sequence, along the c-axis.9,10 It is well-known from metal plating processes11−13 that additives such as surfactants and optical brighteners can be an effective way to control the crystal-building processes in electrochemical deposition, and we have therefore made it the focus of the present study to investigate the effects of one particular surfactant, sodium lignosulfonate (SL), on the microstructural properties of Bi−Te films, as this has previously yielded p-type Bi0.5Sb1.5Te3 films with enhanced microstructural and TE properties.14 SL, the basic structure of which is shown in Figure 1, is a water-soluble polyelectrolyte which comes from the group of anionic sulfonated lignin compounds. To the best of our knowledge, the effects of additives on the crystallinity and other microstructural properties of electrodeposited Bi−Te films from nitric acid baths have only been reported in two studies in the literature to date,15,16 and more work is clearly needed in this direction with a view to improving their TE properties. The current work investigated the effects of the addition of various concentrations of SL to nitric acid electrolytes on the electrodeposition of Bi−Te films, their crystallinity, morphol-
INTRODUCTION Thermoelectric (TE) devices offer a promising route to efficient and sustainable electrical power harvesting from waste heat, providing solid-state operation, zero emissions, zero maintenance, and long lifetime.1 This is, however, contingent on the ability to fabricate materials with improved TE performance over those that are currently available. The TE efficiency of materials is governed by the ZT figure of merit, which is defined as: ZT = (S2σ/κ)T, where S (V K−1) is the Seebeck coefficient, σ [=1/ρ] (S m−1) is the electrical conductivity, κ (W m−1 K−1) is the thermal conductivity, and T (K) is the absolute temperature. The best commercially available TE materials to date are bismuth telluride (Bi−Te) compounds which have a ZT value close to 1 at room temperature, which makes them only suitable for power generation within niche applications such as space missions, laboratory equipment, and medical applications.1,2 Theoretical studies,3 however, have predicted that 1D nanorods of bismuth telluride could yield ZT values as high as 14 and, as a result, have sparked a major thrust of research focused on fabricating nanostructured TE materials. In contrast, very little attention has been directed at investigating the microstructural properties (crystallinity, crystallite size, and chemical composition) of such materials, even though these have been shown to influence their performance. In fact, films with the preferential crystalline orientation in the (1 1 0) plane, optimized crystallite size, and a composition of Bi2Te3 are required to achieve optimum TE properties.4−6 Electrochemical synthesis has been shown to provide a costeffective route to fabrication of high-quality Bi−Te materials at room temperature, which is scalable and offers good control over properties such as composition, crystallinity, and morphology.1,5 Preliminary results, obtained in our research group and as reported by Ma et al.,7 for the electrodeposition of bismuth telluride from nitric acid baths at an applied potential © 2012 American Chemical Society
Received: April 3, 2012 Revised: May 14, 2012 Published: May 14, 2012 8296
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Letter
Figure 1. Simplified structure of the polyelectrolyte surfactant sodium lignosulfonate.
ogy, and composition. TE properties for films grown with the optimal SL concentration are reported.
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EXPERIMENTAL SECTION
Bi−Te films were grown potentiostatically for 2 h in a conventional three-electrode cell, with electrolytes prepared by the dissolution of elemental Bi (7.5 mM, Alfa Aesar, 99.999%) and Te (10 mM, Alfa Aesar, 99.999%) in 1 M HNO3 (Fisher, 70%) with varying concentrations of SL (Aldrich, product 370975, CAS number 8061− 51−6) up to 140 mg dm−3 (pH 0 mg dm−3, 0.19; 80 mg dm−3, 0.18). The electrochemical deposition potential was controlled by an AutoLab PGSTAT30 potentiostat/galvanostat. A large-area platinum gauze electrode and an SCE served as the counter and reference electrodes, respectively. We employed DVD-R disks (Delkin Devices Archival Gold Inkjet) with a 100-nm-thick gold layer as working electrodes, which were cut into approximately 1 cm2 area samples. After each deposition, samples were washed with deionized water and allowed to dry in air. The morphology of the deposited films was investigated by scanning electron microscopy (SEM; JEOL JSM6500F) with film thicknesses being determined by SEM (Zeiss EVO LS25 ESEM) of cross sections of Bi−Te films deposited onto silicon substrates with a 200-nm-thick evaporated gold layer. The composition and crystallinity of the electrodeposited films were determined by energy-dispersive X-ray spectroscopy (EDX; Oxford Inca 300) and X-ray diffraction (XRD; Siemens D5000, Cu Kα radiation: λ = 1.5406 Å), respectively. Hall Effect measurement equipment (HMS 300, Ecopia) was used to measure transport properties. Prior to each measurement, electrical contacts were prepared by soldering 0.2-mm-thick copper wires (RS Components Ltd.) onto each corner of the square sample, resulting in contact resistance of less than 0.2 Ω. The Seebeck coefficient of the film on the substrate was measured by a custom-made Seebeck measurement unit.
Figure 2. Cyclic voltammograms for the electrodeposition of Bi−Te recorded on DVD-R gold substrates (∼1 cm2) with varying concentrations of SL up to 1.5 g dm−3. The first cycle is shown starting at +0.3 V with potential limits of −0.4 V and +0.8 V. The scan rate was 20 mV s−1.
more compact deposit of 7.38 μm thickness. Thus, the surfactant aids in the growth of well-leveled films that are expected to exhibit improved Seebeck coefficients and transport properties. Table 1 gives the composition, determined by EDX analysis, of samples electrodeposited in the absence and in the presence of SL up to 100 mg dm−3. While the films deposited in the absence of and in the presence of 80 mg dm−3 SL give compositions closest to stoichiometric (Bi2Te3), desired for best TE properties, it is important to note, however, that all the samples have compositions close to the optimum within the margin of error for EDX measurements (∼5%). This suggests that, contrary to the common notion that composition of a deposited alloy is a function of deposition potential, an increase in the surfactant concentration has little effect on the composition of the Bi−Te films, possibly due to complexation of the metal ions by the surfactant, bringing the electrode potentials of the metals closer. Figure 4 shows the powder XRD patterns obtained from asdeposited Bi−Te films grown in the absence of and with 80 mg dm−3 and 120 mg dm−3 SL in the electrolyte, selected to demonstrate the transition observed upon addition of the surfactant. An improved orientation toward the preferred (1 1 0) crystallographic texture of the films with increased SL concentration is observed. Orientation toward other planes such as (1 0 10) is inhibited relative to the (1 1 0) plane which is not observed to such a great extent in previous studies such as that by Ma et al.7 An optimum concentration of 80 mg dm−3 is required to give the greatest texturing, but at higher concentrations, this tendency is reduced, thought to be due to an unfavorable arrangement of the surfactant at the electrode at higher concentrations. This alignment, desired for the best TE properties,4 shows that the surfactant can be used to optimize the crystallinity of electrodeposited Bi−Te films. An increase in surfactant concentration reduces the crystallite size (calculated by using the Scherrer equation on the (1 1 0) peak) from 38.5 nm when depositing in the absence of SL to 20.8 nm in the presence of 140 mg dm−3 of SL, which provides evidence that the films are nanostructured. The crystallite size
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RESULTS AND DISCUSSION Figure 2 shows cyclic voltammograms for the electrodeposition of Bi−Te recorded on DVD-R gold substrates with varying concentrations of SL up to 1.5 g dm−3. The voltammograms clearly show that the addition of SL shifts the deposition peak potential toward negative potentials, with the largest shift of 90 mV being observed at a concentration of 1.5 g dm−3. This is thought to be due to the reduced surface energy at the cathode in the presence of the surfactant, hence offering a way of controlling the deposition potential and affecting the nucleation and growth processes of Bi−Te. Electrodeposition of Bi−Te was performed at a potential of −0.1 V vs SCE in accordance with results reported by Ma et al.7 that suggested that close to this deposition potential highly oriented deposits in the preferred (1 1 0) plane can be formed. SEM images of as-deposited bismuth telluride samples deposited at a potential of −0.1 V vs SCE in the absence of and with 100 mg dm−3 of SL are shown in Figure 3. Dendritic films, of thickness 18.35 μm, with a dense layer at the base of the deposit grow in the absence of the surfactant while the addition of 100 mg dm−3 to the electrolyte yields a 8297
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Figure 3. SEM images of as-deposited bismuth telluride films electrodeposited over 2 h at a potential of −0.1 V vs SCE (a) in the absence of SL and (b) in the presence of 100 mg dm−3 SL. Cross-sectional SEM images of the respective films are presented as insets.
smaller crystallite size should contribute, along with the optimization of other factors, to improved TE performance.6 The measured Seebeck coefficients of the films showed little variation, with the highest value being −15 μV K−1 with a concentration of 20 mg dm−3 of the surfactant, compared to −13 μV K−1 in the absence of SL. The influence of the substrate on the measured Seebeck coefficient was found to be negligible. Depositing over a duration of 6 h gave a Seebeck coefficient of −18 μV K−1 for the film deposited with 60 mg dm−3 of the surfactant, 5 μV K−1 more negative than for the film deposited over 2 h. The highest reported Seebeck coefficient for electrodeposited Bi−Te films before annealing is −100 μV K−1.5,17 While the Seebeck coefficient is one of the most significant factors contributing to the ZT figure of merit, since it is a squared term, the carrier concentration and electrical and thermal conductivities must also be considered. The surfactant, therefore, has little effect on the Seebeck coefficient of the films. Transport property measurements for 60 and 80 mg dm−3 of the SL showed that the Hall coefficient, RH, was negative, and therefore, the semiconductor films are of n-type with electrical resistivity of 3 × 10−4 Ω cm and 1.3 × 10−3 Ω cm, respectively. The Hall mobility and the carrier concentration were calculated as 4 cm2 V−1 s−1 and 4.5 × 1021 cm−3 for 80 mg dm−3 and 35 cm2 V−1 s−1 and 2.5 × 1020 cm−3 for 60 mg dm−3. Thus, it appears that these transport property characteristics are either similar or improved in comparison with other electrodeposited Bi−Te films reported.5,18
Table 1. Chemical Composition, Determined by EDX Analysis, of the Bi−Te Films Deposited from Various Concentrations of Sodium Lignosulfonate in the Electrolyte EDX analysis [SL]/mg dm−3
Bi/at. %
Te/at. %
composition
0 20 40 60 80 100
39.84 41.96 39.31 39.40 39.85 41.54
60.16 58.04 60.69 60.60 60.15 58.46
Bi1.99Te3.01 Bi2.10Te2.90 Bi1.97Te3.03 Bi1.97Te3.03 Bi1.99Te3.01 Bi2.08Te2.92
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CONCLUSIONS It has been shown that the presence of SL lowers the deposition potential for the system and results in leveled and compact bismuth telluride deposits with composition close to stoichiometric Bi2Te3 at −0.1 V vs SCE. The presence of the surfactant reduces the crystallite size to 20.8 nm and drives growth in the preferred (1 1 0) orientation, while inhibiting growth toward other undesirable planes, with an optimum concentration for greatest texturing close to 80 mg dm−3. The transport properties show similarity or improvement, compared to those reported already in the literature, with the use of SL in the range 60−80 mg dm−3, which is very promising for thermoelectric applications.
Figure 4. Powder XRD patterns for as-deposited Bi−Te films electrodeposited over 2 h at −0.1 V vs SCE in the presence of (a) 0 mg dm−3, (b) 80 mg dm−3, and (c) 120 mg dm−3 SL in the electrolyte.
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calculated for the film deposited with the optimum SL concentration of 80 mg dm−3 is 23.2 nm. It is thought that a
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 8298
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Notes
electric N-Type Bi(2)Te(3) Thin Films. Electrochim. Acta 2005, 50, 4371−4377.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the Engineering and Physical Sciences Research Council of the United Kingdom, the School of Chemistry, and the School of Electronics and Computer Science at the University of Southampton.
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REFERENCES
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