Tapered BiTe nanowires synthesis by galvanic displacement reaction of compositionally modulated NiFe nanowires

Electrochimica Acta 90 (2013) 582–588

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Tapered BiTe nanowires synthesis by galvanic displacement reaction of compositionally modulated NiFe nanowires Hoyoung Suh a , Kyung-Ho Nam a , Hyunsung Jung b , Chang-Yeon Kim c , Jin-Gyu Kim c , Chang-Soo Kim d , Nosang V. Myung b,∗∗ , Kimin Hong a,∗ a

Department of Physics, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of Chemical and Environmental Engineering, University of California-Riverside, Riverside, CA 92521, USA c Division of Electron Microscopic Research, Korea Basic Science Institute, 169-148 Gwahangno, Yuseong-gu, Daejeon 305-806, Republic of Korea d Center for Materials Measurement, Division of Industrial Metrology, Korea Research Institute of Standards and Science, 267 Gajeong-Ro, Yuseong-Gu, Daejeon 305-340, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 5 October 2012 Received in revised form 5 December 2012 Accepted 6 December 2012 Available online 21 December 2012 Keywords: Galvanic displacement reaction Electrodeposition Bismuth telluride Nanowire Tapered structure

a b s t r a c t Tapered BiTe nanowires with controlled periodicity and composition were synthesized by galvanic displacement reaction (GDR) of compositionally modulated NiFe nanowires. The Fe content in NiFe nanowires was controlled by sweeping the applied deposition potentials where more negative cathodic potential resulted in Fe-rich NiFe nanowires. Although the Bi/Te ratio at the shell of nanowires was uniform, the Bi/Te ratio altered from ∼0.7 to ∼1.5 as the Fe content of sacrificial NiFe increased from 22% to 78%. The periodicity with the BiTe nanowire was precisely controlled from 1.15 ␮m to 2.30 ␮m by adjusting the sweep rate from 2 mV/s to 0.5 mV/s. This work demonstrates the ability to create complex shaped semiconducting nanowires by engineering the sacrificial nanowires. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction One-dimensional (1-D) nanostructures such as nanowires, nanotubes, and nanoribbons have been of great interest in both scientific and industrial points of view because of their unique electromagnetic, thermoelectric, and optical properties mostly originating from the quantum size effect [1–5]. In recent years, there have been great efforts to synthesize 1-D heterostructures consisting of dissimilar materials and structures to further enhance the properties [6–11]. However, it is difficult to control heterostructures from conventional physical and chemical depositions and lithographic methods. Recently, galvanic displacement reaction (GDR) have been attracting attention as an alternative route to synthesize 1-D heterostructures including nanopeapods, dumbbell-like nanowires, and hollow nanotubes because of its ability to modulate composition and structure [12–18]. Bi2 Te3 is a narrow band gap (∼0.15 eV) semiconductor which has been applied to thermoelectric energy harvester or cooler near

∗ Corresponding author. Tel.: +82 42 821 5456; fax: +82 42 822 8011. ∗∗ Co-corresponding author. E-mail addresses: [email protected] (N.V. Myung), [email protected] (K. Hong). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.011

room temperature because of excellent thermoelectric figure-ofmerit, ZT = S2 T/K19 (where S is the Seebeck coefficient,  is the electrical conductivity, T is the absolute temperature, and K is the thermal conductivity). In bulk and thin films, the thermopower (S2 ) typically increased with decrease in thermal conductivity to obtain higher ZT. In this sense, size reduction to 1-D and nanocomposite material with artificial defects are one of promising routes to further enhance ZT because they lead to marked decrease in thermal conductivity by phonon scattering than electrical conductivity [19]. Recently, Bi2 Te3 has drawn new interests as a topological insulator permitting conduction of charges on the surface [20–22]. 1-D nanostructures have a good advantage for studying surface property of topological insulator due to large surface-to-volume ratio [24]. In this study, we demonstrated the ability to create tapered BiTe nanostructures with controlled periodicity and composition by utilizing compositionally modulated NiFe nanowires as sacrificial nanowires. Compositionally modulated NiFe nanowires were electrochemically synthesized using template directed electrodeposition where the deposited Fe contents were altered continuously along the length of the wire by sweeping the deposition potential. GDR of the compositionally modulated NiFe wires with an electrolyte containing Bi and Te ions resulted in tapered BiTe nanowires due to the difference of displacement rates between

H. Suh et al. / Electrochimica Acta 90 (2013) 582–588

Fig. 1. NiFe deposition potentials as a function of deposition time. Scan rate was changed between 0.5 and 2 mV/s to alter the length of the segments.

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Fig. 2. SEM micrographs of electroplated NiFe wire bundles.

2.2. Synthesis of tapered BiTe nanowires Ni and Fe. By adjusting the electrochemical process parameters, we could tailor geometrical and structural characteristics of the BiTe nanowires. 2. Experiment details 2.1. Synthesis of sacrificial NiFe nanowires A conventional electroplating method was used to prepare sacrificial NiFe nanowires. A platinum plate and Ag/AgCl electrode were used as a counter and a reference electrode, respectively [23]. Gold was evaporated onto one side of anodized aluminum oxide (AAO) membrane (Whatman Inc.) for use as a working electrode. Nominal pore size of the membrane was approximately 200 nm. NiFe was electrodeposited without agitation into the pores of the membrane using an electrolyte composed of 0.9 M FeCl2 ·4H2 O, 0.6 M NiCl2 ·6H2 O, 1.0 M CaCl2 , and 0.03 M l-ascorbic acid with pH 0.7 at 40 ◦ C. In order to change composition of wire segments, the deposition potential was continuously scanned from −0.85 V to −0.60 V which decreased Fe content from ∼80% to ∼20% and increased Ni content from ∼20% to ∼80%. By changing the scan rate from 2 mV/s to 0.5 mV/s, the length of individual segments was altered. Fig. 1 shows the applied deposition potentials used for the electrodeposition; scan rates of 0.5, 1.0 and 2.0 mV/s were used to adjust segmental lengths. After electrodeposition, the gold layer and AAO were selectively removed and NiFe wires were harvested.

GDR was conducted between the sacrificial NiFe nanowire and an electrolyte containing 0.02 M Bi(NO3 )3 ·5H2 O, 0.01 M TeO2 , and 1.0 M HNO3 for 5–10 min at room temperature. After reactions, products were washed with de-ionized water. To measure open circuit potential (OCP) during GDR, NiFe thin films with 2 ␮m thickness were electrodeposited on the platinum disk electrode and soaked into electrolyte for 1000 s, where stainless steel rod and Ag/AgCl electrode were used as the counter and reference electrode. Both of electrochemical deposition and OCP measurement were conducted with a potentiostat (VSP, Bio-Logic). The morphology, composition, and crystalline structure of NiFe and BiTe nanowires were investigated by SEM (S-4800, Hitachi) and TEM (JEM-2100F, JEOL) with selected area electron diffraction (SAED) analysis and energy dispersive X-ray spectroscopy (EDS). The crystallinity of NiFe thin films was examined by X-ray diffractometer (Ru-200B, RIGAKU). 3. Results and discussion Fig. 2 shows the NiFe wire bundles prepared by electroplating before GDR process. Fig. 3 shows the compositional variation in the electroplated NiFe wires measured with a TEM–EDX. When the scan rate of the plating potentials were 0.5, 1.0 and 2.0 mV/s, lengths of the NiFe segments were 1.15, 1.74, and 2.30 ␮m, respectively, as shown in Fig. 3(a)–(c). When the diameter of NiFe wires

Fig. 3. TEM–EDX line scan results of NiFe wires of different segmental lengths: (a) 1.15 ␮m, (b) 1.74 ␮m, and (c) 2.30 ␮m.

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Fig. 4. Composition and length variation of NiFe wires: (a) decrease of Fe concentration in NiFe segment, and (b) a relation between deposition time and segmental length.

increased, the segmental lengths were reduced because the wire length (and the wire volume) depended on the total amount of charges employed in the plating process. However, the composition of the segments did not change because it depended on the plating voltage. Fe content in each segment of the wires decreased from 78% to 22%, as shown in Fig. 4(a), which is consistent with our previous work on NiFe thin films [25]. The stable variation of composition did not deteriorate even after deposition of over 20 cycles. Quite naturally, the deposition time was shorter for a smaller segment length; for segmental lengths of 1.15, 1.74, and 2.30 ␮m, the corresponding deposition time was approximately 400, 800, and 1600 s, respectively, as shown in Fig. 4(b). We have investigated the microstructures of the NiFe nanowires. Fig. 5 shows XRD spectra of NiFe thin films prepared with constant deposition potential between −0.60 and −0.85 V. Ni-rich thin films, electrodeposited at a relatively higher potential, consisted of fcc structure with mainly Ni(1 1 1) orientation. However, as Fe content increased to ∼60%, the crystalline orientation altered to bcc structure of Fe(1 1 0), which is coincident with previous finding [26]. Selected area electron diffraction (SAED) patterns of NiFe nanowire revealed similar results. Fig. 6(a) shows ring patterns in Ni-rich part, exhibiting (1 1 1), (2 0 0), (2 2 0), and (3 1 1) of Ni. It indicates that Ni-rich part was composed of small grains of fcc structure without preferential direction. On the other hand, Ferich part contained relatively larger grains of bcc structure with Fe planes of (1 1 0), (2 0 0), and (2 1 1), as can be seen in Fig. 6(b).

Fig. 5. XRD spectra of electroplated NiFe thin films as the content of Ni changed from 78% to 24%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The NiFe nanowires were converted to BiTe nanowires via GDR with the electrolyte containing Bi3+ and HTeO2 + ions [14]. In our case Ni and Fe having lower redox potentials (i.e., E 0 2+ 0 = Ni

−0.26 V (vs. SHE), E 0

Fe2+ /Fe0 with Bi and Te (i.e., E 0 3+ 0 Bi /Bi

/Ni

= −0.41 V (vs. SHE)) could be displaced = 0.31 V (vs. SHE), E 0

HTeO2 + /Te0

= 0.55 V

(vs. SHE)) as following equation. 2Bi3+ (aq) + 3HTeO2 + (aq) + 9M(s) + 9H+ → Bi2 Te3 (s) + 9M2+ (aq) + 6H2 O(aq)

(1)

where M represents sacrificial metals such as Ni or Fe. Geometrical shape and surface morphology of displaced BiTe nanowires are shown in Fig. 7. As can be seen in Fig. 7(a), wires seemed to be made of repetition of tapered segments. The length of the taper was the same as that of a NiFe segment. When we examined the wire at a higher magnification, Fig. 7(b), we could observe that each segment was made of two parts: one was a smooth and straight part (Fig. 7(c)) whose diameter did not change from the sacrificial NiFe wire. The other was a relatively rough and tapered part (Fig. 7(d)) whose diameter increased from ∼275 nm to ∼385 nm. In most cases, observable change in diameter started near the center of each segment and gradually increased to the highest thickness. The transformation of a straight NiFe wire to a tapered BiTe wire can be qualitatively explained in terms of local difference of GDR rates. Since the driving force of the displacement reaction is the difference in redox potentials, Fe with more negative potential than Ni in a common electrolyte can react faster with the electroactive species in the electrolyte. We have previously shown that increasing the relative contents of Fe in the sacrificial NiFe wires resulted in the thicker diameter of the displaced BiTe wires [17]. OCP (open circuit potential) measurement is a useful tool to compare reaction rates of dissimilar metallic thin films [29]. Fig. 8 is OCP measurement results of 2 ␮m thick NiFe thin films of varying compositions in the GDR electrolyte. When Fe content in a NiFe film was 80%, metallic dissolution or oxidation started at −0.24 V and completely dissolved in ∼80 s. As the Fe content in the thin film decreased to ∼50%, the ionization potential increased to −0.21 V and removal of the metals took ∼140 s. As for the Ni-rich thin film, i.e. 20% of Fe in the film, the ionization potential decreased to −0.14 V and ionization was much slower and it took ∼160 s to complete the reaction. This result indicates that the displacement rate of a NiFe thin film is proportional to Fe concentration. We think that, due to the dependence of displacement rate on Fe content, the diameter of BiTe nanowire gradually increased from Ni-rich area to Fe-rich area and finally a tapered structure was obtained. Compositions and crystalline structures were investigated using TEM with EDS and SAED analyses. Fig. 9 is the STEM micrographs of thin and thick cross sectional areas cut by focused ion beam (FIB).

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Fig. 6. Selected area electron diffraction (SAED) results: (a) Ni-rich segment and (b) Fe-rich segment.

Both images exhibited two distinctive phases; dense outer shell and porous inner core. SAED analysis show that BiTe wire was mostly composed of (1 1 0) and (0 1 5) orientations, as shown in Fig. 10. Compared to the outer shell, inner core exhibits less distinctive orientations and numerous auxiliary grains. In addition, TEM–EDX line profiles, shown in Fig. 11, scanned along the radial direction showed higher intensities of Bi and Te at the outsides of the wires. The structural difference between inner core and outer shell can be explained by distinction in deposition processes. While the shell of a nanowire is galvanically deposited on the surface of a NiFe nanowire, inside of the nanowire undergoes dealloying process

[27,28] as well as GDR. The dealloying process removes part of the deposits produced by GDR and results in less dense structures. We have measured the compositional variation of Bi and Te at locations of the wires and the results are shown in Table 1. Contents of both Bi and Te of shell were ∼50% without any traces of Ni and Fe. This result is quite surprising since the relative content of Ni and Fe in each segment of the sacrificial wire changed continuously, as was shown in Figs. 3 and 4(a). We have repeated the measurements at numerous wires and spots of a wire and observed same results. On the other hand, contrary to shells, Bi to Te ratio of cores changed between 0.65 and 1.49. We think that this variation was caused by

Fig. 7. SEM micrographs of tapered BiTe nanowires obtained by GDR processes: (a) low magnification view of the wire bundles, (b) high magnification view of wires consisting of straight and tapered sections, (c) cross-sectional view of a straight section, and (d) porous interior of a tapered section. The inset in (a) is a bundle of the wires right after GDR process and before separation.

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H. Suh et al. / Electrochimica Acta 90 (2013) 582–588 Table 1 The sectional composition of tapered BiTe nanowire. Elements Thinner site Core Shell Thicker site Core Shell

Fig. 8. Open circuit potential measurement results of 2 ␮m thick NiFe thin films with different Ni and Fe contents.

Ni (at.%)

Fe (at.%)

Bi (at.%)

Te (at.%)

16 0

5 0

31 49

48 51

0 0

2 0

58 51

39 49

the dealloying process and, consequently, by the residual Ni and Fe left behind after GDR process as was previously reported in a similar experiment [13]. Morphological variation of tapered BiTe was observed as we varied segment length of NiFe nanowire and GDR time. As shown in Fig. 12, the segment length of tapered BiTe nanowire strongly depended on that of sacrificial NiFe nanowire which was controlled by electrodeposition time (Fig. 1). As the reaction time was increased, length of the tapered section increased at the sacrifice of the straight part. It can be explained by the higher reaction rate of Fe than that of Ni as was seen in the OCP measurement result (Fig. 8). Since the reaction rate is higher at the Fe-rich part, the

Fig. 9. STEM micrographs of BiTe nanowires: (a) thin section and (b) thick section.

Fig. 10. Microstructure analysis result of BiTe nanowires using TEM–SAED: (a) outer shell of a wire and (b) inner core of a wire.

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Fig. 11. EDX line profiles of tapered BiTe nanostructure scanned along the transverse direction; (a) thick and (b) thin areas of a wire.

Fig. 12. Morphological evolution of tapered BiTe nanowires depending on segment length of sacrificial NiFe nanowire and GDR time.

section of the large diameter (high Fe concentration) reacts very quickly and the reaction is finished [26]. On the other hand, Ni-rich section continues the displacement reaction and length of tapers increase.

4. Conclusion We demonstrated that composition of NiFe nanowire can be continuously varied by modulation of applied potential during

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electrodeposition. In addition, tapered BiTe nanowire with continuous change of diameter and porous interior was fabricated by GDR. The tapered structure was made by difference of OCP and corrosion resistance induced from composition variation of sacrificial NiFe nanowire and porosity of interior of BiTe nanowire was generated by dealloying process of NiFe nanowire. The segment length, taper length, and taper diameter could be controlled by electrodeposition and galvanic displacement reaction time. To the best of our knowledge, it is the first time to synthesize a tapered structure by using electroplating and GDR processes. The transformation of nanostructure by electrodeposition and GDR is expected to allow new route for fabrication of special nanostructure.

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Acknowledgment This research work was supported by the National Research Foundation of Korea Grant, NRF-2011-0013323.

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