Template-Assisted Electrodeposition of Porous FeâNiâCo Nanowires

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Template-Assisted Electrodeposition of Porous FeNi-Co Nanowires with Vigorous Hydrogen Evolution Deyang Li, and Elizabeth Podlaha-Murphy Nano Lett., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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Direct Electrodeposition of Porous Fe-Ni-Co Nanowires 318x83mm (150 x 150 DPI)

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Template-Assisted Electrodeposition of Porous FeNi-Co Nanowires with Vigorous Hydrogen Evolution Deyang Lia and Elizabeth J. Podlaha*,a,b

*Corresponding

aDepartment

author and ACS Member

of Chemical Engineering, Northeastern University, Boston, Massachusetts

02115, USA

bDepartment

of Chemical & Biomolecular Engineering, Clarkson University, Potsdam,

New York 13699, USA

KEYWORDS porous nanowires, H2 bubbles, Fe-Ni-Co, template-assisted electrodeposition


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ABSTRACT

A novel method to fabricate porous Fe-Ni-Co nanowires directly by electrodepositing into polycarbonate membranes is reported, when the electrolyte pH  0.5. Hydrogen bubbles are used as a dynamic porous template created by operating in electrolytes with very low pH to drive the proton reduction reaction. The electrolyte pH was adjusted with sulfuric acid, and the added sulfate ions are thought to help reduce bubble coalescence, but not detachment at the electrode surface, to facilitate metal deposition within the nanopores. Porous nanowires were obtained when the electrolyte pH was less than 1.0. The average alloy composition, was found to be pH sensitive, which shifted from an Ferich porous alloy to a Ni-rich porous alloy as the electrolyte pH decreased.

TEXT

Electrodeposition can be used to fabricate, at low cost, porous nanowires using nanotemplates through a two-step method. First, an alloy with segregated or discrete phases is deposited, and then one region is selectively etched.1-6 For example, this method has been reported for the preparation of noble metal porous structures, including Au2-4 and Pt,5-6 by etching a less noble constituent. In


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the first deposition step, the hydrogen (H2) evolution side reaction can pose a challenge for the reduction of metal ions from aqueous electrolytes into nanotemplates as gas bubbles can coalesce and block the pores of the template, halting deposition. For the reduction of noble metal ions, conditions can be selected to avoid the hydrogen side reaction from both proton reduction and the water splitting reaction, however, for non-noble metal ion reduction, such as Fe(II), Ni(II) and Co(II), their reduction occurs in the region where hydrogen gas evolution cannot be avoided. Strategies that have been used to circumvent pore blockage by gas bubble growth is to use pulse plating that provides time for diffusion of gas bubbles out of the pores and species redistribution,711

as well as using a surfactant that reduces surface tension to avoid gas bubbles blocking the

electrode surface.12-17 If gas bubbles can be deterred from growing and coalescing, but remain at the electrode surface to act as an additional template, within the nanotemplate, it is possible to directly deposit a porous nanowire, without the subsequent etching step needed. Presented here is the first demonstration of porous Fe-Ni-Co nanowires prepared directly by electrodeposition, without a subsequent etching step. This achievement relies upon restricting the generated hydrogen gas bubble diameter to the nanometer range during the metal electrodeposition side reaction. Recently, the use of H2 gas bubbles as a dynamic template accompanied with metal species reduction has been used to create porous metal thin films, not porous nanowires, where the metal deposits within the space between bubbles.18-31 An advantage of this technique is that it can be applied to a wide range of metals such as Cu,19-22 Ag,23-24 Au,25 Ni,26-28 Ni-Co,29 and Fe-Ni-Co.30 The disadvantage of these 3-D films is that there is a change of micro-scale porosity in the direction of growth, that tends to increase as the gas bubbles grow and coalesce, and this internal void structure with increasing film thickness, is often not accessible or not efficiently used as


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catalytically active sites.31 Thus, a collection of porous nanowires may be a more efficient engineered catalyst. In contrast to planar electrode surfaces, where H2 bubbles can continuously grow and coalesce freely, bubbles produced inside deep recesses are confined by the template walls that restrict their maximum diameters to the size of the nanopores, and it is necessary that the bubble diameters remain below the diameter of the pore wall. While a sufficient hydrogen evolution reaction is required to build a porous template, the size of the bubble must not only be nanoscale but also dispersed. Gas bubbles that aggregate in the center of a template pore can be useful to create nanotubes,32-34 but not a porous nanowire. Here a methodology is presented to prepare porous Fe-Ni-Co nanowires. In a previous study from our group, Li and Podlaha,15 showed that Fe-Ni-Co nanowires were not formed within 60 m deep alumina templates with very low electrolyte pH (e.g., pH 0.5) without surfactant in the electrolyte. The unsuccessful nanowire formation attempt was attributed to a blocked electrode surface by large, coalesced gas bubbles and adsorbed hydrogen. Surfactant addition to an electrodeposition electrolyte is known to promote the detachment of the gas bubbles and smaller bubble diameters.16,17 Adding sodium lauryl sulfate (SLS), a surfactant, to the electrolyte permitted solid nanowire growth but not a porous wire. Taking into account these two extremes, how can one design a porous Fe-Ni-Co nanowire using hydrogen gas as a template within a nanoporous template? Bubble coalescent should be deterred but it is desired that bubbles remain at the electrode surface, not detach to form a template. Eliminating SLS from the electrolyte from our past work would indeed keep bubbles at the surface, though not prevent large bubble coalescence. Thus, a strategy for decreasing bubble size and coalescence is to add a salt.35 Zhang et al.36 reported that the addition of sulfate ions to a copper electrolyte in the form of Na2SO4 or (NH4)2SO4 helped


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to reduce the pore size in electrodeposited copper foams created by H2 templating. In a similar approach, sulfate ions are used here in the electrolyte to reduce coalescence, but unlike films, with the use of nanostructured membranes as a secondary template, an additional challenge is to avoid nanotube formation. To explore hydrogen templating within a nanoporous template, a sulfate-boric acid electrolyte, in the absence of a surfactant, was used for the development of porous nanowires. A high hydrogen evolution reaction rate can be reached by lowering bulk pH (i.e., increasing H+ concentration), but may promote the dissolution of alumina templates. Thus, polycarbonate (PC) templates were employed. The commercial PC templates are thinner than commercial alumina ones, having the advantage of minimizing mass transport effects of the metal deposition rate at low overpotentials in order to avoid nanotube formation. Compared to smooth surfaces, porous structures exhibit larger surface area to facilitate charge transfer across the electrode/electrolyte interface, advantageous for enhanced sensing, electrocatalysis and desalination. Experimental Methods Fe-Ni-Co nanowires were electrodeposited potentiostatically into polycarbonate (PC), nanoporous membranes purchased from GSV Filter Technology. The membranes had a nominal pore size of 200 nm in diameter and were 10 µm in thickness that served as the template for deposition. Prior to electrodeposition, a thin Au layer, about 20 nm, was sputtered onto one side of the templates with a Hummer 6.2 Anatech sputter instrument. A three-electrode cell was used with the template/Au serving as the cathode. A platinum mesh was used as the anode, placed parallel above of the cathode. A saturated calomel electrode (SCE) was employed as a reference electrode. The electrodeposition was controlled by a potentiostat (Solartron, Analytical, Hampshire, UK, Model # SI 1287) at a constant potential of -1.20 V vs. SCE, where it has been identified that solid wires


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at pH 2.0 occur in alumina templates,15 and that the metal deposition reduction reactions are under a kinetic control.15,37 The electrolyte which was used to deposit Super Invar alloy nanowires consisted of 0.72 M nickel sulfate, 0.155 M ferrous sulfate, 0.005 M cobalt sulfate, 0.5 M boric acid and 0.011 M ascorbic acid. The electrolyte pH was adjusted by concentrated sulfuric acid from 1.5 to 0. The temperature of deposition was conducted at 40 oC and a stir bar was utilized to minimize the local concentration gradients near the template surface. After deposition, single Fe-Ni-Co nanowires were obtained by dissolving PC templates in dichloromethane (DCM) and stored in ethanol to prevent further oxidation. The average alloy composition prior to removing PC templates was analyzed by X-ray fluorescence (XRF) (Omicron, Kevex) at 40 kV and 1.5 mA in air condition. The average value was obtained by testing three randomly selected locations on the template, and error bars represent the standard deviation. The morphology of free nanowires was characterized by transmission electron microscopy, TEM (JEOL JEM-1010) and scanning electron microscopy, SEM (Hitachi S-4800). Results Figure 1 presents the current transient curves during the potentiostatic electrodeposition of Fe-NiCo nanowires at different pH values in PC templates over a deposition time of 1200 s. All current density transient curves have a similar shape with an initial non-steady state region followed by a plateau. The time to reach steady state changes with pH; it increased from 79 to 322 s as the electrolyte pH dropped from 1.5 to 0. Thus, the transient region is directly related to the hydrogen evolution reaction. The observed fluctuations, which are larger as the electrolyte pH is lowered, are ascribed to the periodic detachment of H2 from the template surface. To assess the transport


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control of the H2 evolution reaction, a diffusion boundary layer is needed. The nanoporous template is no more than 10 m (0.001 cm) deep but the added convection by the stir bar may not maintain the bulk concentration at the pore mouth. To assess the order of magnitude of the effective boundary layer at the bottom of the pore, the long-time expansion to the linear diffusion problem having finite boundary conditions, presented by Bond et al.38 was used assuming only proton diffusion. For example, at a pH of 0.5, using a diffusion coefficient of 9.3 × 10-5 cm2 s-1 for protons in dilute solutions,39 an effective boundary layer length is between 0.02-0.03 cm, (Figure S1, supporting information) much larger than the pore length, provides a current density that has a matching order of magnitude of the current density provided in Figure 1. Thus, in addition to linear diffusion within the pore there is expected hemispherical diffusion from the bulk to the pore mouth, as described by Valizadeh et al.40 and Blanco et al.41 In addition, in Figure 1, the time transients are considerably longer than ones expected for only diffusion. Due to the large volume of gas evolution, these transients may reflect the changes of the bubble convection on the proton transport control. It is evident that the total cathodic current density at pH 1.0 was lower than pH 1.5, an unexpected result if only considering the a diffusion controlled side reaction since there is more H+ reactant available with pH 1.0 vs 1.5. Thus, this difference may be is due to the higher metal rate at pH 1.5 compared to pH 1.0. The average alloy composition, over a collection of wires inside the PC templates is shown in Figure 2 for different electrolyte pH values. Overall, the deposition composition is sensitive to electrolyte pH. The Fe content is highest at the very high pH of 1.5 and decreases substantially with lower pH values. Thus, the wires show more of the anomalous codeposition behavior,42,43 a preferential deposition of Fe over that of Ni, at pH 1.5. The mass ratio of the Fe to Ni (gFe/gNi) in the electrolyte is 0.2, while at lower pH values the mass ratio in the deposit is closer to this ratio,


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but completely inverted at pH 1.5, which can suggest a change in the reducing species. It is expected that at very low pH values, pH < 0.5, there is an insignificant amount of hemihydroxide metal species, MOH+ locally at the electrode surface, where M represents Fe, Ni or Co. However, these species begin to form when the bulk pH is higher, even in acid electrolytes (pH > 1.5). 44,15 Since the local pH at the electrode surface is expected to be a little higher due to the consumption of the proton, the changing alloy composition in Figure 2 may be a reflection of the local OHgenerated and the resulting amount of electroactive MOH+, leading to a more anomalous electrodeposition behavior. It is noted that the average composition of Fe-Ni-Co nanowires obtained at pH 1.5 is close to the Super Invar alloy (64 wt% Fe, 31 wt% Ni, and 5 wt% Co), which is expected to have a low coefficient of thermal expansion at room temperature. Solid, high Fe content nanowires have been previously observed with similar deposition conditions and are polycrystalline with multiple phases.37 Figure 3 shows the TEM images of the electrodeposited Fe-Ni-Co nanowires at various electrolyte pH values from 1.5 to 0. The insets in Figure 3 (a-e) shows a lower magnification image to capture the wire lengths. Comparing the insets in Figure 3(a) and (b), where the wires were electrodeposited at pH values of 1.5 and 1.0, respectively, the wire lengths are about 2.5 times longer at pH 1.5, consistent with Figure 1. At a higher magnification, the wires fabricated at a pH of 1.5 are solid and dense (Figure 3 (a)), and at a pH of 1.0 (Figure 3 (b)), short porous regions were observed at the ends of the wires and in some cases tubes. Figure 3 (c-e), at pH values of 0.5, 0.25, 0, show bright spots randomly distributed along the whole wire, indicating the presence of nanoporous structures. Figure 4 presents a higher resolution TEM of porous nanowires prepared at an electrolyte pH of 0.5 and a distribution of the pore sizes. The largest number of pore sizes range from 21 to


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44 nm. Figure 5 presents the SEM images of these nanowires. Figure 5 (a) shows the image taken at a low magnification, where a cluster of forest-like nanowires is observed. At a higher magnification, as shown in Figure 5 (b-d), holes and voids are clearly evident on the Fe-Ni-Co nanowires, producing a foam-like surface. Discussion The Fe-Ni-Co nanowire morphology was found to be a function of electrolyte pH: i. long and solid wires were obtained at a pH of 1.5, while short, and partial hollow tubes are observed at the ends of wires at pH of 1.0, and ii. porous wires were observed at pH values from 0.5 to 0. i. Previous reports of Fe-Ni-Co nanowires having an Fe-rich composition with similar electrolytes to the one reported here, but at a pH of 2, demonstrated solid nanowires.10,11 At electrolyte pH values from 1.5 to 1.0, as shown in Figure 4, the morphology of Fe-Ni-Co nanowires transitions from solid wires into tubes at the end of the wires. Literature studies observed tubes at the other end of the wires near the substrate surface, formed at the beginning of the deposition process due to partial pore filling of the sputtered, conductive layer on the back of the template.32-34 For example, in the electrodeposition of copper into PC membranes with partial pore filling of a sputtered gold layer on the membrane backside, Davis and Podlaha34 reported that Cu tubes were formed when the deposition conditions were selected to create a low current efficiency, but wires formed when the operating conditions were designed to maximize the current efficiency. Thus, the tubes that form in Figure 4 (b,c) at the opposite end of the substrate surface can be due to the changing current efficiency, getting worse as the deposition progresses. As the metal grows inside the nanoporous membrane, the boundary layer for the mass transport of the proton in the H2 evolution reaction decreases and hence the H2 evolution reaction is larger near the end of the deposition compared to the beginning, contributing to tube growth.


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ii. The lower pH electrolytes were achieved by adding concentrated sulfuric acid, which introduced sulfate ions into the solution. Consistent with Zhang et al.36 the addition of added sulfate ions may have been responsible for preventing large bubble coalescence in order to make the porous nanowires possible. The addition of inorganic salts to water is indeed known to prevent coalescence, and one school of thought is that this proceeds from a hydrophobic effect as a result of the electrolyte's ionic influence on water structure. However, Craig et al.35 has observed that H2SO4 has little to no effect on bubble coalescence, as other metal salts with sulfate ions such as MgSO4 and Na2SO4 do inhibit coalescence. In addition to a reduction in the hydrophobic attraction, bubble coalescence can also be due to the Gibbs-Marangoni effect (a result of surface tension gradients) and a decrease of gas solubility, all which are influenced by the ionic strength of the electrolyte.45 Thus, the addition of sulfuric acid to adjust and lower the electrolyte pH contributes to an enhanced ionic strength in the electrolyte. As reported in our previous work,15 there were no wires formed at all in alumina membranes at a pH of 0.5, attributed to pore blockage by the generated, coalesced gas bubbles, in contrast to the porous wires obtained in PC templates presented here, despite having the same electrolyte concentrations. A significant difference between these two cases is that the PC templates are thinner, having a template depth of ~10 µm, about 1/6 of the previous alumina templates (60 µm), which minimizes the diffusion effect of the metal ions and keeps the local metal ion concentration high. Thus, we suspect that the mass transport vs kinetic behavior of the metal reduction impacts the degree of bubble coalesce by altering the ionic strength at the electrode surface, and also decreases the available surface for adsorption. Under a metal ion reduction kinetic control, a competitive adsorption model was presented,15 consisting of absorbed metal species (𝜃𝑀 ), gas bubbles (𝜃𝐺), protons (𝜃𝐻 + ), and surfactant, such as SLS (𝜃𝑆𝐿𝑆) for Fe-Ni-Co


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electrodeposition. The value of 𝜃𝑀 and 𝜃𝐺 is proportional to the metal ion and proton concentration at the electrode surface, respectively. Since the hydrogen evolution reaction follows a VolmerHeyrovsky mechanism, 𝜃𝐻 + is relatively low and can be treated as 0. Therefore, in the absence of a surfactant, SLS (𝜃𝑆𝐿𝑆 = 0), it the adsorption of gas 𝜃𝐺 competes with adsorbed metal intermediates, 𝜃𝑀. Under a mixed kinetic-mass transport control of metal ion reduction, the metal ion adsorbed intermediate, 𝜃𝑀 ,becomes smaller than when only kinetic control occurs. A more mass transport dominated reaction then creates a situation where 𝜃𝑀  0. Also, when mass transport limitations occur for the metal deposition, there is a gradient of metal ions distributed within the membrane, with a very low concentration at the electrode surface, promoting bubble coalescence due to a lower ionic strength. These two factors then can contribute to pore blockage and no deposition, as is the case for deposition into the AAO membrane at a pH of 0.5 at an applied potential of -1.2 V vs SCE (as observed in ref 15). The choice of the applied potential in this study was to ensure a kinetic control for the metal deposition, despite having a transport control for the proton reduction reaction, and by doing so, the adsorption of metal intermediates, 𝜃𝑀, is high, and due to the higher metal ion concentration at the electrode surface, along with the added sulfate or formed bisulfate ions, increases the ionic strength and helps to inhibit bubble coalescence. Conclusions Porous Fe-Ni-Co nanowires were successfully fabricated within porous PC templates by electrodeposition at low pH values, where nanosized hydrogen gas bubbles serve as a dynamic porous template. In contrast, partial hollow tubular structures and solid wires were observed at a higher pH of 1.0 and 1.5, respectively. A high proton concentration and the addition of sulfuric acid are thought to be critical to maintain the porous hydrogen bubble template within the


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nanopores at the electrode surface. A high ionic strength, created by the addition of sulfuric acid, was proposed to play an important role in deterring bubble coalescence, and also affected the type of metal ion species that was reduced that changed the deposit composition with pH. The deposit was less anomalous at the lower pH values. The results suggest the following design rules for depositing porous nanowires: (1) metal ion deposition should follow a kinetic control to maximize both the ionic strength and to keep the adsorption of metal ion intermediates large at the electrode surface, (2) common surfactants used in electrodeposition that deter bubbles from sticking to the electrode surface, such as SLS should not be used and (3) the pH < 1.0 to have sufficient bubble generation. Supporting Information. Supporting information includes order of magnitude estimate of an effective boundary layer. Author Information. *Corresponding

Author: Elizabeth J. Podlaha, [email protected]

Author Contributions. The manuscript was written through contributions of both authors. Both authors have given approval to the final version of the manuscript. ‡These authors contributed equally: Deyang Li‡ and Elizabeth J. Podlaha*‡

Notes. The authors declare no competing financial interests. Acknowledgements


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The authors acknowledge and thank Roche Diagnostics and US National Institutes of Health Grant # 1R21hg006278-01 for support of this project. The authors also thank the Electron Microscopy Facility at Northeastern University Center for use of their equipment and Dr. W. Fowle for training D.L. on the use of the instruments. References 1. Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450453. 2. Ji, C.; Searson, P. C. Appl. Phys. Lett. 2002, 81 (23), 4437-4439. 3. Ji, C.; Searson, P. C. J. Phys. Chem. B 2003, 107 (19), 4494-4499. 4. Liu, Z.; Searson, P. C. J. Phys. Chem. B 2006, 110 (9), 4318-4322. 5. Strasser, P.; Koha, S.; Greeley, J. Phys. Chem. Chem. Phys. 2008, 10, 3670-3683. 6. Zhang, X.; Lu, W.; Da, J.; Wang, H.; Zhao, D.; Webley, P. Chem. Comm. 2009, 0 (2), 195197. 7. Tourillon, G.; Pontonnier, L.; Levy, J. P.; Langlais,V. Electrochem. Solid St. 2000, 3 (1) 20-23. 8. Kim, H.; Soper, S. A.; Podlaha-Murphy, E. J. ECS Transactions 2013, 53 (11), 9-14. 9. Samanifar, S.; Kashi, M. A.; Ramazani, A.; Alikhani, M. J. Magn. Magn. Mater. 2015, 378, 73-83. 10. Geng, X.; Podlaha, E. J. Nano Lett. 2016, 16 (12), 7439-7445. 11. Geng, X.; Liang, W.; Podlaha, E. J. J. Electrochem. Soc. 2017, 164 (4), D218-D224. 12.

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31. Chaudhari, N. K.; Jin, H.; Kim, B.; Lee, K. Nanoscale 2017, 9 (34), 1223112247. 32. Motoyama, M.; Fukunaka, Y.; Sakka, T.; Ogata, Y. Electrochim. Acta 2007, 53 (1), 205212. 33. Fu, J.; Cherevko, S.; Chung, C.-H. Electrochem. Commun. 2008, 10 (4), 514-518. 34. Davis, D.; Podlaha, E. Electrochem. Solid State Lett. 2005, 8 (2), D1-D4. 35. Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97 (39), 10192-10197. 36. Zhang, W.; Ding, C.; Wang, A.; Zeng, Y. J. Electrochem. Soc. 2015, 162 (8), D365-D370. 37. Xiaohua Geng X.; Podlaha, E. J. J. Electrochem. Soc. 2017, 1624 (4), D218-D224. 38. Bond, A.; Luscombe, D.; Oldham, K; Zoski, C. J. Electroanal. Chem. 1988, 249, 1-14. 39. Newman, J. Electrochemical Systems, Prentice-Hall, Englewood Cliffs, New Jersey, 1973; p. 230. 40. Valizadeh S.; George, J., Leisner, P.; Hultman L. Electrochim. Acta 2001, 47, 865–874. 41. Blanco, S.; Vargas,R.; Mostany, J. Borrás C; Scharifkerb B.R. J.Electrochem. Soc. 2014, 161 (8) E3341-E3347. 42. Brenner, A. Electrodeposition of Alloys, Academic Press, New York, 1963; volume 1, pp 77. 43. Landolt, D. Electrochim. Acta 1994, 39 (8-9), 1075-1090. 44. Smith. R. M.; Martell, A. E. Critical Stability Constants, Plenum Press, New York, 1989; volume 6. 45. Weissenborn, P. K.; Pugh, R. J. Langmuir 1995, 11 (5), 1422-1426.

List of Figures


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Figure 1. Current transient curves during the electrodeposition of Fe-Ni-Co nanowires at a potential of -1.2 V vs. SCE. Electrolyte pH was varied from 0 to 1.5.

Figure 2. Average alloy composition as a function of electrolyte pH. Symbols: ● Fe, □ Ni and ∆ Co.

Figure 3. TEM images of wires fabricated at various electrolyte pH values: (a) pH 1.5, (b) pH 1.0, (c) pH 0.5, (d) pH 0.25 and (e) pH 0.

Figure 4. Porous wires fabricated at an electrolyte pH of 0.5 (a) TEM and (b) distribution of pore size.

Figure 5. SEM images of wires obtained at pH of 0.5. (a) low magnification and (b) – (d) high magnification.


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Figure 1. Current transient curves during the electrodeposition of Fe-Ni-Co nanowires at a potential of -1.2 V vs. SCE. Electrolyte pH was varied from 0 to 1.5. 164x99mm (150 x 150 DPI)


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Figure 2. Average alloy composition as a function of electrolyte pH. Symbols: ● Fe, □ Ni and ∆ Co. 152x106mm (150 x 150 DPI)


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Figure 3. TEM images of wires fabricated at various electrolyte pH values: (a) pH 1.5, (b) pH 1.0, (c) pH 0.5, (d) pH 0.25 and (e) pH 0. 127x190mm (150 x 150 DPI)


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Figure 4. Porous wires fabricated at an electrolyte pH of 0.5 (a) TEM and (b) distribution of pore size. 271x100mm (150 x 150 DPI)


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Figure 5. SEM images of wires obtained at pH of 0.5. (a) low magnification and (b) – (d) high magnification. 204x144mm (150 x 150 DPI)


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