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Diameter and composition modulated bismuth telluride nanowires by galvanic displacement reaction of segmented NiFe nanowires
Electrochimica Acta 75 (2012) 201–207
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Diameter and composition modulated bismuth telluride nanowires by galvanic displacement reaction of segmented NiFe nanowires Hoyoung Suh a,1 , Hyunsung Jung b,1 , Carlos M. Hangarter b , Hosik Park b , Youngin Lee c , Yongho Choa c , Nosang V. Myung b,∗ , Kimin Hong a,∗ a
Department of Physics, Chungnam National University, Daejeon 301-150, Republic of Korea Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering, University of California-Riverside, CA 92521, United States c Department of Bionanotechnology, Hanyang University, Ansan 426-791, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 6 February 2012 Received in revised form 24 April 2012 Accepted 25 April 2012 Available online 3 May 2012 Keywords: Bismuth telluride Galvanic displacement reaction Dumbbell-like structure NiFe Nanowires
a b s t r a c t Dumbbell-like BiTe nanowires with segmentally tailored composition and dimension were synthesized by galvanic displacement reaction of multi-segmented NiFe nanowires with Ni-rich and Fe-rich segments. The composition and dimension of each segment were determined by the tuned sacrificial segments to control the galvanic displacement reaction rate. The composition and dimension of each segment were determined by the tuned sacrificial segments to control the galvanic displacement reaction rate. For examples, the bismuth content in Bix Te1−x adjusted from 32 to 60 at.% by controlling the Fe content in NiFe sacrificial nanowires. The ability to tune the dimension of segments was demonstrated by the diameter variation from 99 to 551 nm with the segment length from 97 nm to 1 m. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nanostructured metals, semimetals, and semiconductors have been extensively studied for use in various electronic, photonic, catalytic, and magnetic applications because of its ability to “tune” the properties by engineering the dimensions and composition. In many cases, these materials were synthesized by physical/vacuum processes or wet chemical reactions, which have limited control to modulate composition within a single construct. Additional control over nanowire features such as shape or geometric features is consequently a significant hurdle for one-dimensional (1-D) nanostructures, often requiring complex processing for just a single radial modulation. Although template directed electrodeposition with anodized aluminum oxide templates permits axial modulation, coordinating shape and dimension control of these scaffolds with compositional variation would be difficult for most materials due to concentration changes along the pore length and consequently has yet to be demonstrated with significant periodicity [1–7]. Recently, we demonstrated the ability to synthesize complex nanostructures (i.e., nanopeapods) by coupling template-directed
∗ Corresponding authors. Tel.: +1 951 827 7710; fax: +1 951 827 5696. E-mail addresses: [email protected] (N.V. Myung), [email protected] (K. Hong). 1 Both authors contributed equally to this work. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.090
electroplating with galvanic displacement reaction (GDR), in which a single galvanic reaction occurs in the presence of noble metal constituents, demonstrating potential for extended periodicity in nanowires [8]. In addition to industrial relevance for coating metallic layers without aids of external power source, GDR has been studied to create hollow nanostructures and to alter the surface of materials, particularly in catalysis [9,10]. Furthermore, GDR is a versatile method to synthesize various functional materials including metals (e.g., Au, Ag, Pb and Pt) and few semiconductors (e.g., Te and BiTe) at near room temperatures [11–27]. Herein, we demonstrate a facile synthesis of bismuth telluride nanowire containing compositional and structural modulation utilizing galvanic displacement reaction. High precision periodicity is readily achieved with control of segment length, dictated by electrodeposition, and diameter, determined by two different sacrificial materials. Corrosion engineering, by compositional change in the sacrificial materials, was used to control reaction rates and driving force to generate distinct diameters and compositions, which may be a general route to create advanced semiconducting nanostructures. Bi2 Te3 was examined in this study for its unique electron transport properties and high thermoelectric figure-of-merit at near room temperature. Various processes have been employed to synthesize Bi2 Te3 for the purposes of structure analysis [28,29], thermal conductivity measurements [30], and thermoelectric power measurement [31–33]. In addition, a micro- and
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Fig. 1. SEM images and elemental EDS line scan of segmented NiFe nanowires with tailored composition and length: (a, b) segmented NiFe nanowires with Ni-rich Ni75 Fe25 segment in length of 1.5 ± 0.07 m and Fe-rich Ni36 Fe64 segment in length of 1.4 ± 0.04 m, and (c, d) Ni-rich Ni77 Fe23 segment in length of 2.5 ± 0.12 m and Fe-rich Ni33 Fe67 segment in length of 3.1 ± 0.16 m.
nanostructured Bi2 Te3 composite prepared by hot pressing of microstructured BiTe powders with galvanically displaced BiTe nanopowders has demonstrated the highest figure of merit, suggesting GDR is a promising route for synthesis of such materials [8–27]. 2. Experimental The plating electrolyte to fabricate compositionally modulated NiFe wires consisted of 0.9 M FeCl2 , 0.6 M NiCl2 , 1.0 M CaCl2 , and 0.03 M l-ascorbic acid in deionized water. Commercially available anodized alumina with nominal pore size of 200 nm (Whatman Inc.) and polycarbonate membrane (Whatman Inc.) with the nominal pore size of 50 and 100 nm were utilized as templates to synthesize nanowires with different diameter. Alternating Ni-rich and Fe-rich segments were grown by applying two plating current densities alternately in the bath; 5 mA cm−2 (2.5 mA cm−2 ) for Nirich segment and 20 mA cm−2 (40 mA cm−2 ) for Fe-rich segment. Length of each segment was controlled by the plating time. The operating temperate and agitation were fixed at 40 ◦ C and 200 rpm, respectively. Nanowires were suspended by dissolving the templates. The solution for dissolution of AAO was 1.0 M NaOH for 2 h and that for polycarbonate was 99.5% 1-methyl-2-pyrrolidinone for 1 h. Composition of the segments was analyzed with TEM-EDX. Diameter of the segmented NiFe wires was 298 ± 13 nm. GDR electrolyte consisted of 0.02 M Bi3+ and 0.01 M HTeO2 + in 1.0 M HNO3 . The mutli-segmented NiFe nanowires were immersed into the electrolyte solution at room temperature, where the reaction time was controlled to be 4, 6, 8, and 10 min. To determine the
open circuit potential during GDR, Ni-rich and Fe-rich thin films were electrodeposited on 100 nm thick Pt-coated 300 nm thick silicon oxide/silicon substrate and immersed into the electrolyte. Three electrode configurations were utilized to measure OCP where Pt wire and SCE (saturated calomel electrode) were utilized as the counter and reference electrodes, respectively. Biologic multipotentiostat (VMP2, Princeton Applied Research) was used to measure the OCP. Morphology, crystallinity and composition of the synthesized nanowires were investigated by SEM (XLG-30FEG, Philips), TEM (JEM-2100F, JEOL) with high resolution-TEM and SAED analyses, and energy dispersive spectroscopy (EDS) which was conducted with TEM at the acceleration voltage of 200 kV (Inca x-stream, OXFORD) and SEM at the acceleration voltage of 20 kV (Phoenix). XRD patterns of thin films were analyzed by X-ray diffractometer (D8 Advanced Diffractometer, Bruker). 3. Results and discussion The synthesis process initiated with template directed electrodeposition of multi-segmented nanowires with alternating Ni-rich and Fe-rich sections. Sacrificial NiFe wires were prepared by electroplating into porous AAO (nominal diameter of 200 nm) and polycarbonate (nominal diameter of 50 nm) membranes. Different membranes were utilized to investigate this effect on the formation of nanostructures. Alternating Ni-rich and Fe-rich segments were achieved by changing the applied current density, where Ni-rich and Fe-rich segments were electrodeposited at 5 and 20 mA cm−2 , respectively [6,7]. The composition and length of each segment in
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standard reduction potentials; E◦ (Ni2+ /Ni◦ ) = −0.26 V (vs. SHE), E◦ (Fe2+ /Fe◦ ) = −0.41 V (vs. SHE), E◦ (Bi3+ /Bi◦ ) = 0.31 V (vs. SHE), and E◦ (HTeO2 + /Te◦ ) = 0.55 V (vs. SHE). The interaction of the BiTe electrolyte with metal (M) can be represented as described in Eq. (1). + (s) 2Bi3+ (aq) + 3HTeO+ 2 (aq) + 9M (s) + 9H
→ Bi2 Te3 (s) + 9M2+ (aq) + 6H2 O (aq)
Fig. 2. XRD patterns of NiFe thin films: (a) Fe-rich NiFe thin film deposited at the charge density of 20 mA cm−2 and (b) Ni-rich NiFe thin film deposited at the charge density of 5 mA cm−2 .
sacrificial NiFe nanowires can be readily controlled by adjusting the applied charge density in single bath with both electroactive species of Ni and Fe as shown in Fig. 1. SEM images showed the morphologies of the electrodeposited segmental NiFe nanowires (Fig. 1(a) and (b)) and EDS line scan confirmed Ni-rich and Fe-rich NiFe segmental nanowires with tailored length (Fig. 1(c) and (d)). The galvanic displacement reaction of the sacrificial NiFe nanowires with tailored composition and length can determine the segmental composition and dimension of displaced BiTe nanowires. XRD analysis of Ni-rich and Fe-rich part was conducted with thin film deposits as described in Fig. 2. Ni-rich NiFe and Fe-rich NiFe thin film in thickness of 2 m were electrodeposited at applying the charge density of 5 mA cm−2 and 20 mA cm−2 , respectively, corresponding with those for segmented NiFe nanowires. The XRD patterns of electrodeposited films showed the Fe peak of (1 1 0) plane with body centered cubic (BCC) structure in Fe-rich NiFe film and the Ni peak of (1 1 1) plane with the faced centered cubic (FCC) structure in Ni-rich NiFe film. Two theta peak-shift around ∼44.5◦ in the films confirmed that Fe peak of (1 1 0) plane in Fe-rich NiFe film and Ni peak of (1 1 1) plane in Ni-rich NiFe film corresponded to Fe (1 1 0) plane of 44.67◦ (JCPDS #06-0696) and Ni (1 1 1) plane of 44.50◦ (JCPDS #04-0850). The grain size of 24.2 nm and 13.9 nm in Fe-rich and Ni-rich NiFe thin films was determined by analysis of Fe (1 1 0) plane and Ni (1 1 1) plane using Scherrer’s equation, respectively. The segmented NiFe nanowires were immersed to an acidic nitric electrolyte containing Bi and Te ions to displace Ni and Fe (Fig. 3), in which the reaction is driven by the difference of
(1)
where M represents sacrificial metals (e.g., Ni or Fe). Compared to Ni-rich NiFe which has faced centered cubic (FCC) structure with high corrosion resistance, wheras body centered cubic (BCC) Ferich NiFe segment would have a greater driving force due to more negative redox and fast corrosion rate. Typical electrodeposited multi-segmented NiFe nanowire prepared by single bath method is shown in Fig. 4. Bright-field TEM image of nanowires shows that the diameter of nanowire is approximately 300 nm with the segment length of Fe-rich and Nirich segments to be approximately 500 and 200 nm, respectively (Fig. 4(a)). EDS point analysis indicated that the compositions of the Ni-rich and Fe-rich segments are Ni70 Fe30 and Ni21 Fe79 , respectively (Fig. 4(b)). The EDS line scan and elemental mapping clearly show the composition modulation within nanowires (Fig. 4(c) and (d)). The electrodeposited NiFe multi-segmented nanowires were glavanically displaced in an acidic nitric electrolyte consisted of Bi3+ , HTeO2 + in 1 M HNO3 . Unlike sacrificial nanowires which show smooth surface morphology without the modulated diameter, the galvanically displaced BiTe nanostructure show a periodic modulation of diameter with different shape morphology at individual segments (Fig. 5). Each segment displayed porous crystal structures, as reported previously [8]. Compared to the sacrificial nanowires (the average diameter of ∼300 nm), the diameter of Fe-rich sections increased substantially to ∼500 nm while that of Ni-rich sections exhibited little or no change in the diameter. HRTEM images and SAED patterns of both thin Ni-rich (‘1 in Fig. 5) and thick Fe-rich (‘2 in Fig. 5) segments display a polycrystalline nature for Bi2 Te3 with (0 1 5) plane as a dominant crystal plan with a d-spacing of ∼0.323–0.332 nm. The polycrystallinity of Bi2 Te3 is also confirmed by the ring patterns in SAED analysis; mostly (0 1 5) planes and auxiliary (1 0 1 0), (1 1 0), and (0 0 1 5) planes. The EDS analysis of the segments indicated remains of Ni and Fe in polycrystalline BiTe. The randomly distributed Ni and Fe nanocrystals in BiTe matrix may enhance thermoelectric properties by increasing phonon scattering and charge filtering effects at the interface [34–43]. As shown in Fig. 5, the GDR-synthesized BiTe nanowires show distinct structures for the different Ni-rich and Fe-rich sections. To investigate the structural/morphological evaluation
Fig. 3. A segmented NiFe nanowire with Ni- and Fe-rich sections (a) is galvanically displaced by Bi3+ and HTeO2 + ions. A diameter-modulated BiTe nanowire was synthesized because of difference in displacement reaction rates caused by different in mixed potentials and corrosion rate (b). Further GDR drives the formation of the diameter-modulated nanostructure (c).
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Fig. 4. TEM-EDS analysis of electrodeposited multi-segmented NiFe nanowire: (a) bright-field TEM image, (b) EDS point analysis of each segments, (c) EDS line scan (c) and EDS elemental mapping (d) of Ni and Fe along the axis of nanowire.
during GDR, the reaction time was varied from 4 to 10 min. Fig. 6 shows SEM images of BiTe heterostructures formed from NiFe multi-segmented nanowires with 300 nm in diameter as a function of reaction time, where the length of Ni-rich and Fe-rich segments was equal (i.e., 500 nm). During the early stage, the GDR is predominantly occurred at Fe-rich segments which lead to the formation of bamboo like nanowires (Fig. 6(a)). As the reaction continued (6 and 8 min), the shape altered to a dumbbell-like structure (Fig. 6(b) and (c)). Finally, when the reaction time was further increased to 10 min, bells continue grow to form bead-like structure (Fig. 6(d)). The changes in the diameter as a function of reaction time for both
Ni-rich and Fe-rich sections are shown in Fig. 7. The diameter of BiTe from Ni-rich sections remained unchanged from that of the sacrificial NiFe wire (298 ± 13 nm) during a 4–6 min range of reaction and decreased by 69% (207 ± 14 nm) after 8–10 min. On the other hand, the Fe-rich sections exhibited a steady increase in diameter up to 94% (578 ± 27 nm) by 6 min, stagnating thereafter. The observed trend might be attributed to more cathodic redox potential of Fe compared to Ni, which resulted in a rapid replacement rate of Fe-rich sections. Moreover, it might be explicated by the different GDR reaction rate between Fe-rich and Ni-rich phases where Fe-rich phase has BCC structure with lower corrosion
Fig. 5. TEM, HR-TEM, and SAED image of GDR BiTe nanostructures: (a) bright-field TEM images and (b) HR-TEM images and SAED patterns, and (c) elemental EDS analyses of thin and thick segments (denoted by ‘1 and ‘2 in the figures).
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Fig. 6. Reaction time dependent SEM images of galvanically displaced Bix Te1−x nanowires: (a) 4, (b) 6, (c) 8, and (d) 10 min.
resistance compare to Ni-rich phase which consists of FCC structure with higher corrosion resistance. The rapid reaction rate and more negative reaction potential of Fe-rich segments are confirmed by OCP measurement of electrodeposited Ni-rich and Fe-rich NiFe thin films in same GDR electrolyte (Fig. 8). As shown in Fig. 8, the OCP of 2 m thick Fe-rich NiFe thin films were significantly more negative than Ni-rich NiFe thin films. The complete conversion time of sacrificial Fe-rich NiFe thin films, which can be determined from the rapid increase in OCP, was approximately 3 times faster than Ni-rich NiFe thin films. These observations are consisted with the corrosion behaviors of NiFe thin films in acidic media where BCC Fe-rich NiFe thin films have a
two to three orders of magnitude lower corrosion resistance than FCC Ni-rich NiFe thin films [44,45]. Since the governing factor for GDR of the multi-segmented NiFe wire is the difference of their redox potentials, the ratio of Fe and Ni rich segment length allows one to arbitrarily engineer the displaced BiTe segments. Additionally, changing the diameter of the sacrificial NiFe wires resulted in changes in the relative thickness of displaced segments from Ni-rich and Fe-rich sections. Fig. 9(a) and (b) is the displaced BiTe nanowires from NiFe nanowires of 200- and 50-nm in diameters, respectively. The ratio of diameters for the two sections of BiTe is greater in the nanowire with diameter of 99 nm than that in the nanowire with diameter of 456 nm.
Fig. 7. Diameter variation of segmental BiTe nanowires in Ni-rich and Fe-rich segments during GDR.
Fig. 8. Open circuit potential (OCP) of Ni-rich and Fe-rich NiFe thin films in an electrolyte containing 0.02 M Bi3+ and 0.01 M HTeO2 + in 1.0 M HNO3 .
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Fig. 9. SEM images of GDR BiTe nanowires with different scarifical diameters (a, b) and segmented lengths (c, d): (a) nominal diameter of 456/551 nm, (b) nominal diameter of 99/175 nm, (c) nanowires with longer Fe-rich segments, and (d) nanowires with longer Ni-rich segments.
Additionally, as the length of each segment was varied, different shapes were obtained, as shown in Fig. 9(c) and (d).
supported in part by the National Research Foundation of Korea Grant (NRF-2010-013-C00013).
4. Conclusions
References
We have successfully demonstrated a new strategy to tailor dimensions, morphology, and crystallinity of nanowires by galvanic displacement multi-segmented nanowires. Specifically, we utilized composition-modulated NiFe nanowires as sacrificial materials and obtained geometrically tailored BiTe nanowires. The composition, diameter and length of segmented BiTe nanowires could be controlled by changing three parameters; relative composition of Ni and Fe in the sacrificial NiFe segments, dimensions of the sacrificial NiFe nanowires, and reaction time of the sacrificial wire in the GDR electrolyte. The dumbbell-shaped BiTe nanowires with tailored composition and dimension were driven by the difference of standard redox potentials and corrosion resistance with different crystal structures between Ni-rich and Fe-rich NiFe sacrificial materials. The created BiTe heterostructures may have potential applications including electronic devices, since p-n junctional nanowires can be demonstrated by tuning of composition. Additionally, the BiTe nanostructures consisting of Ni and Fe composites in the BiTe matrix may have improved thermoelectric properties. Acknowledgements
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This research work was supported by the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0013323) funded by the Ministry of Education, Science and Technology (MEST) and the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. HS and KH acknowledge financial
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Diameter and composition modulated bismuth telluride nanowires by galvanic displacement reaction of segmented NiFe nanowires Hoyoung Suh a,1 , Hyunsung Jung b,1 , Carlos M. Hangarter b , Hosik Park b , Youngin Lee c , Yongho Choa c , Nosang V. Myung b,∗ , Kimin Hong a,∗ a
Department of Physics, Chungnam National University, Daejeon 301-150, Republic of Korea Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering, University of California-Riverside, CA 92521, United States c Department of Bionanotechnology, Hanyang University, Ansan 426-791, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 6 February 2012 Received in revised form 24 April 2012 Accepted 25 April 2012 Available online 3 May 2012 Keywords: Bismuth telluride Galvanic displacement reaction Dumbbell-like structure NiFe Nanowires
a b s t r a c t Dumbbell-like BiTe nanowires with segmentally tailored composition and dimension were synthesized by galvanic displacement reaction of multi-segmented NiFe nanowires with Ni-rich and Fe-rich segments. The composition and dimension of each segment were determined by the tuned sacrificial segments to control the galvanic displacement reaction rate. The composition and dimension of each segment were determined by the tuned sacrificial segments to control the galvanic displacement reaction rate. For examples, the bismuth content in Bix Te1−x adjusted from 32 to 60 at.% by controlling the Fe content in NiFe sacrificial nanowires. The ability to tune the dimension of segments was demonstrated by the diameter variation from 99 to 551 nm with the segment length from 97 nm to 1 m. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nanostructured metals, semimetals, and semiconductors have been extensively studied for use in various electronic, photonic, catalytic, and magnetic applications because of its ability to “tune” the properties by engineering the dimensions and composition. In many cases, these materials were synthesized by physical/vacuum processes or wet chemical reactions, which have limited control to modulate composition within a single construct. Additional control over nanowire features such as shape or geometric features is consequently a significant hurdle for one-dimensional (1-D) nanostructures, often requiring complex processing for just a single radial modulation. Although template directed electrodeposition with anodized aluminum oxide templates permits axial modulation, coordinating shape and dimension control of these scaffolds with compositional variation would be difficult for most materials due to concentration changes along the pore length and consequently has yet to be demonstrated with significant periodicity [1–7]. Recently, we demonstrated the ability to synthesize complex nanostructures (i.e., nanopeapods) by coupling template-directed
∗ Corresponding authors. Tel.: +1 951 827 7710; fax: +1 951 827 5696. E-mail addresses: [email protected] (N.V. Myung), [email protected] (K. Hong). 1 Both authors contributed equally to this work. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.090
electroplating with galvanic displacement reaction (GDR), in which a single galvanic reaction occurs in the presence of noble metal constituents, demonstrating potential for extended periodicity in nanowires [8]. In addition to industrial relevance for coating metallic layers without aids of external power source, GDR has been studied to create hollow nanostructures and to alter the surface of materials, particularly in catalysis [9,10]. Furthermore, GDR is a versatile method to synthesize various functional materials including metals (e.g., Au, Ag, Pb and Pt) and few semiconductors (e.g., Te and BiTe) at near room temperatures [11–27]. Herein, we demonstrate a facile synthesis of bismuth telluride nanowire containing compositional and structural modulation utilizing galvanic displacement reaction. High precision periodicity is readily achieved with control of segment length, dictated by electrodeposition, and diameter, determined by two different sacrificial materials. Corrosion engineering, by compositional change in the sacrificial materials, was used to control reaction rates and driving force to generate distinct diameters and compositions, which may be a general route to create advanced semiconducting nanostructures. Bi2 Te3 was examined in this study for its unique electron transport properties and high thermoelectric figure-of-merit at near room temperature. Various processes have been employed to synthesize Bi2 Te3 for the purposes of structure analysis [28,29], thermal conductivity measurements [30], and thermoelectric power measurement [31–33]. In addition, a micro- and
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Fig. 1. SEM images and elemental EDS line scan of segmented NiFe nanowires with tailored composition and length: (a, b) segmented NiFe nanowires with Ni-rich Ni75 Fe25 segment in length of 1.5 ± 0.07 m and Fe-rich Ni36 Fe64 segment in length of 1.4 ± 0.04 m, and (c, d) Ni-rich Ni77 Fe23 segment in length of 2.5 ± 0.12 m and Fe-rich Ni33 Fe67 segment in length of 3.1 ± 0.16 m.
nanostructured Bi2 Te3 composite prepared by hot pressing of microstructured BiTe powders with galvanically displaced BiTe nanopowders has demonstrated the highest figure of merit, suggesting GDR is a promising route for synthesis of such materials [8–27]. 2. Experimental The plating electrolyte to fabricate compositionally modulated NiFe wires consisted of 0.9 M FeCl2 , 0.6 M NiCl2 , 1.0 M CaCl2 , and 0.03 M l-ascorbic acid in deionized water. Commercially available anodized alumina with nominal pore size of 200 nm (Whatman Inc.) and polycarbonate membrane (Whatman Inc.) with the nominal pore size of 50 and 100 nm were utilized as templates to synthesize nanowires with different diameter. Alternating Ni-rich and Fe-rich segments were grown by applying two plating current densities alternately in the bath; 5 mA cm−2 (2.5 mA cm−2 ) for Nirich segment and 20 mA cm−2 (40 mA cm−2 ) for Fe-rich segment. Length of each segment was controlled by the plating time. The operating temperate and agitation were fixed at 40 ◦ C and 200 rpm, respectively. Nanowires were suspended by dissolving the templates. The solution for dissolution of AAO was 1.0 M NaOH for 2 h and that for polycarbonate was 99.5% 1-methyl-2-pyrrolidinone for 1 h. Composition of the segments was analyzed with TEM-EDX. Diameter of the segmented NiFe wires was 298 ± 13 nm. GDR electrolyte consisted of 0.02 M Bi3+ and 0.01 M HTeO2 + in 1.0 M HNO3 . The mutli-segmented NiFe nanowires were immersed into the electrolyte solution at room temperature, where the reaction time was controlled to be 4, 6, 8, and 10 min. To determine the
open circuit potential during GDR, Ni-rich and Fe-rich thin films were electrodeposited on 100 nm thick Pt-coated 300 nm thick silicon oxide/silicon substrate and immersed into the electrolyte. Three electrode configurations were utilized to measure OCP where Pt wire and SCE (saturated calomel electrode) were utilized as the counter and reference electrodes, respectively. Biologic multipotentiostat (VMP2, Princeton Applied Research) was used to measure the OCP. Morphology, crystallinity and composition of the synthesized nanowires were investigated by SEM (XLG-30FEG, Philips), TEM (JEM-2100F, JEOL) with high resolution-TEM and SAED analyses, and energy dispersive spectroscopy (EDS) which was conducted with TEM at the acceleration voltage of 200 kV (Inca x-stream, OXFORD) and SEM at the acceleration voltage of 20 kV (Phoenix). XRD patterns of thin films were analyzed by X-ray diffractometer (D8 Advanced Diffractometer, Bruker). 3. Results and discussion The synthesis process initiated with template directed electrodeposition of multi-segmented nanowires with alternating Ni-rich and Fe-rich sections. Sacrificial NiFe wires were prepared by electroplating into porous AAO (nominal diameter of 200 nm) and polycarbonate (nominal diameter of 50 nm) membranes. Different membranes were utilized to investigate this effect on the formation of nanostructures. Alternating Ni-rich and Fe-rich segments were achieved by changing the applied current density, where Ni-rich and Fe-rich segments were electrodeposited at 5 and 20 mA cm−2 , respectively [6,7]. The composition and length of each segment in
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standard reduction potentials; E◦ (Ni2+ /Ni◦ ) = −0.26 V (vs. SHE), E◦ (Fe2+ /Fe◦ ) = −0.41 V (vs. SHE), E◦ (Bi3+ /Bi◦ ) = 0.31 V (vs. SHE), and E◦ (HTeO2 + /Te◦ ) = 0.55 V (vs. SHE). The interaction of the BiTe electrolyte with metal (M) can be represented as described in Eq. (1). + (s) 2Bi3+ (aq) + 3HTeO+ 2 (aq) + 9M (s) + 9H
→ Bi2 Te3 (s) + 9M2+ (aq) + 6H2 O (aq)
Fig. 2. XRD patterns of NiFe thin films: (a) Fe-rich NiFe thin film deposited at the charge density of 20 mA cm−2 and (b) Ni-rich NiFe thin film deposited at the charge density of 5 mA cm−2 .
sacrificial NiFe nanowires can be readily controlled by adjusting the applied charge density in single bath with both electroactive species of Ni and Fe as shown in Fig. 1. SEM images showed the morphologies of the electrodeposited segmental NiFe nanowires (Fig. 1(a) and (b)) and EDS line scan confirmed Ni-rich and Fe-rich NiFe segmental nanowires with tailored length (Fig. 1(c) and (d)). The galvanic displacement reaction of the sacrificial NiFe nanowires with tailored composition and length can determine the segmental composition and dimension of displaced BiTe nanowires. XRD analysis of Ni-rich and Fe-rich part was conducted with thin film deposits as described in Fig. 2. Ni-rich NiFe and Fe-rich NiFe thin film in thickness of 2 m were electrodeposited at applying the charge density of 5 mA cm−2 and 20 mA cm−2 , respectively, corresponding with those for segmented NiFe nanowires. The XRD patterns of electrodeposited films showed the Fe peak of (1 1 0) plane with body centered cubic (BCC) structure in Fe-rich NiFe film and the Ni peak of (1 1 1) plane with the faced centered cubic (FCC) structure in Ni-rich NiFe film. Two theta peak-shift around ∼44.5◦ in the films confirmed that Fe peak of (1 1 0) plane in Fe-rich NiFe film and Ni peak of (1 1 1) plane in Ni-rich NiFe film corresponded to Fe (1 1 0) plane of 44.67◦ (JCPDS #06-0696) and Ni (1 1 1) plane of 44.50◦ (JCPDS #04-0850). The grain size of 24.2 nm and 13.9 nm in Fe-rich and Ni-rich NiFe thin films was determined by analysis of Fe (1 1 0) plane and Ni (1 1 1) plane using Scherrer’s equation, respectively. The segmented NiFe nanowires were immersed to an acidic nitric electrolyte containing Bi and Te ions to displace Ni and Fe (Fig. 3), in which the reaction is driven by the difference of
(1)
where M represents sacrificial metals (e.g., Ni or Fe). Compared to Ni-rich NiFe which has faced centered cubic (FCC) structure with high corrosion resistance, wheras body centered cubic (BCC) Ferich NiFe segment would have a greater driving force due to more negative redox and fast corrosion rate. Typical electrodeposited multi-segmented NiFe nanowire prepared by single bath method is shown in Fig. 4. Bright-field TEM image of nanowires shows that the diameter of nanowire is approximately 300 nm with the segment length of Fe-rich and Nirich segments to be approximately 500 and 200 nm, respectively (Fig. 4(a)). EDS point analysis indicated that the compositions of the Ni-rich and Fe-rich segments are Ni70 Fe30 and Ni21 Fe79 , respectively (Fig. 4(b)). The EDS line scan and elemental mapping clearly show the composition modulation within nanowires (Fig. 4(c) and (d)). The electrodeposited NiFe multi-segmented nanowires were glavanically displaced in an acidic nitric electrolyte consisted of Bi3+ , HTeO2 + in 1 M HNO3 . Unlike sacrificial nanowires which show smooth surface morphology without the modulated diameter, the galvanically displaced BiTe nanostructure show a periodic modulation of diameter with different shape morphology at individual segments (Fig. 5). Each segment displayed porous crystal structures, as reported previously [8]. Compared to the sacrificial nanowires (the average diameter of ∼300 nm), the diameter of Fe-rich sections increased substantially to ∼500 nm while that of Ni-rich sections exhibited little or no change in the diameter. HRTEM images and SAED patterns of both thin Ni-rich (‘1 in Fig. 5) and thick Fe-rich (‘2 in Fig. 5) segments display a polycrystalline nature for Bi2 Te3 with (0 1 5) plane as a dominant crystal plan with a d-spacing of ∼0.323–0.332 nm. The polycrystallinity of Bi2 Te3 is also confirmed by the ring patterns in SAED analysis; mostly (0 1 5) planes and auxiliary (1 0 1 0), (1 1 0), and (0 0 1 5) planes. The EDS analysis of the segments indicated remains of Ni and Fe in polycrystalline BiTe. The randomly distributed Ni and Fe nanocrystals in BiTe matrix may enhance thermoelectric properties by increasing phonon scattering and charge filtering effects at the interface [34–43]. As shown in Fig. 5, the GDR-synthesized BiTe nanowires show distinct structures for the different Ni-rich and Fe-rich sections. To investigate the structural/morphological evaluation
Fig. 3. A segmented NiFe nanowire with Ni- and Fe-rich sections (a) is galvanically displaced by Bi3+ and HTeO2 + ions. A diameter-modulated BiTe nanowire was synthesized because of difference in displacement reaction rates caused by different in mixed potentials and corrosion rate (b). Further GDR drives the formation of the diameter-modulated nanostructure (c).
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Fig. 4. TEM-EDS analysis of electrodeposited multi-segmented NiFe nanowire: (a) bright-field TEM image, (b) EDS point analysis of each segments, (c) EDS line scan (c) and EDS elemental mapping (d) of Ni and Fe along the axis of nanowire.
during GDR, the reaction time was varied from 4 to 10 min. Fig. 6 shows SEM images of BiTe heterostructures formed from NiFe multi-segmented nanowires with 300 nm in diameter as a function of reaction time, where the length of Ni-rich and Fe-rich segments was equal (i.e., 500 nm). During the early stage, the GDR is predominantly occurred at Fe-rich segments which lead to the formation of bamboo like nanowires (Fig. 6(a)). As the reaction continued (6 and 8 min), the shape altered to a dumbbell-like structure (Fig. 6(b) and (c)). Finally, when the reaction time was further increased to 10 min, bells continue grow to form bead-like structure (Fig. 6(d)). The changes in the diameter as a function of reaction time for both
Ni-rich and Fe-rich sections are shown in Fig. 7. The diameter of BiTe from Ni-rich sections remained unchanged from that of the sacrificial NiFe wire (298 ± 13 nm) during a 4–6 min range of reaction and decreased by 69% (207 ± 14 nm) after 8–10 min. On the other hand, the Fe-rich sections exhibited a steady increase in diameter up to 94% (578 ± 27 nm) by 6 min, stagnating thereafter. The observed trend might be attributed to more cathodic redox potential of Fe compared to Ni, which resulted in a rapid replacement rate of Fe-rich sections. Moreover, it might be explicated by the different GDR reaction rate between Fe-rich and Ni-rich phases where Fe-rich phase has BCC structure with lower corrosion
Fig. 5. TEM, HR-TEM, and SAED image of GDR BiTe nanostructures: (a) bright-field TEM images and (b) HR-TEM images and SAED patterns, and (c) elemental EDS analyses of thin and thick segments (denoted by ‘1 and ‘2 in the figures).
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Fig. 6. Reaction time dependent SEM images of galvanically displaced Bix Te1−x nanowires: (a) 4, (b) 6, (c) 8, and (d) 10 min.
resistance compare to Ni-rich phase which consists of FCC structure with higher corrosion resistance. The rapid reaction rate and more negative reaction potential of Fe-rich segments are confirmed by OCP measurement of electrodeposited Ni-rich and Fe-rich NiFe thin films in same GDR electrolyte (Fig. 8). As shown in Fig. 8, the OCP of 2 m thick Fe-rich NiFe thin films were significantly more negative than Ni-rich NiFe thin films. The complete conversion time of sacrificial Fe-rich NiFe thin films, which can be determined from the rapid increase in OCP, was approximately 3 times faster than Ni-rich NiFe thin films. These observations are consisted with the corrosion behaviors of NiFe thin films in acidic media where BCC Fe-rich NiFe thin films have a
two to three orders of magnitude lower corrosion resistance than FCC Ni-rich NiFe thin films [44,45]. Since the governing factor for GDR of the multi-segmented NiFe wire is the difference of their redox potentials, the ratio of Fe and Ni rich segment length allows one to arbitrarily engineer the displaced BiTe segments. Additionally, changing the diameter of the sacrificial NiFe wires resulted in changes in the relative thickness of displaced segments from Ni-rich and Fe-rich sections. Fig. 9(a) and (b) is the displaced BiTe nanowires from NiFe nanowires of 200- and 50-nm in diameters, respectively. The ratio of diameters for the two sections of BiTe is greater in the nanowire with diameter of 99 nm than that in the nanowire with diameter of 456 nm.
Fig. 7. Diameter variation of segmental BiTe nanowires in Ni-rich and Fe-rich segments during GDR.
Fig. 8. Open circuit potential (OCP) of Ni-rich and Fe-rich NiFe thin films in an electrolyte containing 0.02 M Bi3+ and 0.01 M HTeO2 + in 1.0 M HNO3 .
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Fig. 9. SEM images of GDR BiTe nanowires with different scarifical diameters (a, b) and segmented lengths (c, d): (a) nominal diameter of 456/551 nm, (b) nominal diameter of 99/175 nm, (c) nanowires with longer Fe-rich segments, and (d) nanowires with longer Ni-rich segments.
Additionally, as the length of each segment was varied, different shapes were obtained, as shown in Fig. 9(c) and (d).
supported in part by the National Research Foundation of Korea Grant (NRF-2010-013-C00013).
4. Conclusions
References
We have successfully demonstrated a new strategy to tailor dimensions, morphology, and crystallinity of nanowires by galvanic displacement multi-segmented nanowires. Specifically, we utilized composition-modulated NiFe nanowires as sacrificial materials and obtained geometrically tailored BiTe nanowires. The composition, diameter and length of segmented BiTe nanowires could be controlled by changing three parameters; relative composition of Ni and Fe in the sacrificial NiFe segments, dimensions of the sacrificial NiFe nanowires, and reaction time of the sacrificial wire in the GDR electrolyte. The dumbbell-shaped BiTe nanowires with tailored composition and dimension were driven by the difference of standard redox potentials and corrosion resistance with different crystal structures between Ni-rich and Fe-rich NiFe sacrificial materials. The created BiTe heterostructures may have potential applications including electronic devices, since p-n junctional nanowires can be demonstrated by tuning of composition. Additionally, the BiTe nanostructures consisting of Ni and Fe composites in the BiTe matrix may have improved thermoelectric properties. Acknowledgements
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This research work was supported by the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0013323) funded by the Ministry of Education, Science and Technology (MEST) and the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. HS and KH acknowledge financial
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