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Enhancement of anomalous codeposition in the synthesis of Fe–Ni alloys in nanopores
Electrochimica Acta 106 (2013) 392–397
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhancement of anomalous codeposition in the synthesis of Fe–Ni alloys in nanopores Ángela Llavona a,∗ , Lucas Pérez a,b , M. Carmen Sánchez a , Víctor de Manuel a,1 a b
Departamento de Física de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain Instituto de Sistemas Optoelectrónicos y Microtecnología, Universidad Politécnica de Madrid, 28040 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 31 January 2013 Received in revised form 14 May 2013 Accepted 16 May 2013 Available online 13 June 2013 Keywords: Electrodeposition Anomalous codeposition Fe–Ni alloys Nanopores
a b s t r a c t In this paper we report an enhancement of anomalous codeposition in NiFe electrodeposition when using nanoporous templates. Depending on the deposition potential it is possible to obtain homogeneous nanowires of a high Fe concentration or of a concentration similar to the one of the electrolyte used. In addition, nanowires with a decreasing Fe concentration are found for intermediate overpotentials. Voltammetric studies relate this effect with a possible higher block of the cathode surface by adsorption of intermediated species in nanopores. A study of the effect of hydrogen bubbles shows a change in the composition in the areas surrounding the bubbles, and yields an increase in the deposition rate, which results in longer nanowires. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Electrodeposition is a technique extensively used in the industry that has gained increased interest over the last decades since it offers a unique opportunity to synthesize materials at the nanometer scale. In particular, the use of nanoporous templates is very effective in the synthesis of nanowires [1,2] because of the relatively easy control of diameter and length, the low production costs and the possibility of nanostructuration which allows tailoring the properties of the grown nanomaterials [3]. However, electrodeposition of alloys formed by metals from the iron-group is not so simple due to the so-called anomalous codeposition: when plating an alloy of two metals belonging to this group, the less noble metal deposits preferentially to the more noble one, thus making it difficult to control the composition of the alloy [4]. In the case of NiFe alloys, this results in a fraction of Fe in the resulting alloy, which is greater than the ratio [Fe2+ ]/([Fe2+ ] +[Ni2+ ]) in the solution. Various efforts have been made, using both theoretical and experimental approaches, to understand this unusual effect. First steps were taken by Dahms and Croll in 1965, who suggested that an increase of the pH in the vicinity of the cathode surface could cause anomalous codeposition [5]. However, later experimental studies showed that the increase of pH was not significant enough
∗ Corresponding author. Tel.: +34 913944788. E-mail address: angelallavona@fis.ucm.es (Á. Llavona). 1 Present address: Institut für Experimentelle und Angewandte Physik, ChristianAlbrechts-Universität, 24098 Kiel, Germany. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.116
to explain the observed effects [6–8]. A new perspective was proposed by Matlosz [9], who proposed that anomalous codeposition was caused by a competitive absortion mechanism involving adsorbed intermediate ions. This idea was experimentally supported by Baker [10,11] and in other studies [12,13]. Although the origin of anomalous codeposition is still under debate, the idea of a partial surface blocking due to adsorbed or precipitated species is, nowadays, the most plausible and widely accepted explanation. We note that all studies realized so far have been focused on planar cathodes, however, up to now, no attention has been paid to anomalous codeposition in setups with nanometer-sized cathodes. Permalloy (Ni80 Fe20 ) is one of the most widely used magnetic materials due to its very soft magnetic properties and near zero magnetostriction [14]. These properties make this material specially suitable for many different magnetic applications, including magnetic field sensing [15], recording heads [16] or microinductors [17]. In recent years, after the birth of nanotechnology, Permalloy is also playing a key role in the development of new nanodevices, like the race-track memories [18], domain wall logic devices [19] and other nanodevices [20,21], as well as in magnonics [22] and spintronics [23]. Due to its wide-spread technical applicability, the study of the synthesis of NiFe nanostructures has become an increasingly active field of research in the last years. Since the properties of Permalloy are extremely dependent on the particular composition Ni80 Fe20 , it is important to study the effect of anomalous codeposition when plating in nanoporous templates in order to control the composition of NiFe electrodeposited nanowires.
Á. Llavona et al. / Electrochimica Acta 106 (2013) 392–397 Table 1 Characteristics of the membranes used in this work. The table summarizes the nominal values supplied by Sterlitech. Pore diameter Pore density Thickness
30 nm 6 × 108 cm−2 6 m
100 nm 4 × 108 cm−2 6 m
In this work we report an enhancement of the anomalous codeposition when nanopore templates are used as cathodes for electrodeposition instead of planar substrates. We find that changes in the geometry of the electrochemical setup modify the plating conditions. This leads to changes in the chemical composition, which can be understood in terms of a partial blocking of the cathode surface by adsorbed species. 2. Experimental We have electrodeposited arrays of NiFe nanowires using nanoporous membranes as templates. The electrolyte used is composed of NiSO4 (0.7 M) and NiCl2 (0.02 M) as Ni2+ sources, FeSO4 (0.03 M) as Fe2+ source and H3 BO3 (0.4 M) and saccharine (0.016 M) as additives. This composition is similar to the one typically used for the electrodeposition of Permalloy [24,25]. All chemicals were of analytical grade and they were used without further purification and mixed in deionized water. The pH was adjusted to 2.3 by dropping H2 SO4 10% vol. The synthesis was carried out at room temperature, in a threeelectrode-cell. A platinum mesh was used as counter electrode and a Ag/AgCl electrode as reference electrode. All potential values quoted in this work are referenced with respect to this electrode. Hydrophilic (polyvinilpyrrolidone coated) polycarbonate membranes supplied by Sterlitech were used as templates for the synthesis of nanowires. Their characteristics are summarized in Table 1. Before electrodeposition, a thin Au film was thermally evaporated on one side of the membrane. The diameter of the nanowires depends on the diameter of the membrane pores, whereas their length is controlled by the electrodeposition time. In addition, thin films were grown on Si wafers, previously coated with Au.
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The morphology of the nanowires was studied with a Scanning Electron Microscope (SEM) JEM 6335 F. Before measuring the nanowires, the polycarbonate template was removed with dichloromethane. The average composition of the samples was measured by Energy Dispersion X-Ray Spectroscopy (EDX) using a SEM Microscope JEM 6335 F with an EDX Oxford Mod 6506. For thin films the signal was recorded in the center of the sample. In the case of nanowires, the measurement was done in an area with a high density of wires and, therefore, the composition represents an averaged value over a large group of nanowires. In all cases, several measurements were performed in different areas of the samples so the values provided in this work correspond to the mean values. The composition along the length of individual nanowires was measured with a Transmission Electron Microscope (TEM) JEOL JEM 2000FX. Measurements were made at both ends of the wires and in some points in between. In order to release the nanowires, the gold layer was removed by contact with mercury and the samples were washed thoroughly with deionized water. Afterwards, the polycarbonate membrane was dissolved with dichloromethane. The suspension was centrifuged and new dichloromethane was added. Finally, some drops of this solution were placed on a TEM grid. 3. Results and discussion 3.1. Anomalous codeposition in nanopores During the electrodeposition of NiFe alloys, H2 evolution reaction takes place simultaneously, an effect which we observed in nanowires and thin films. In both cases, the presence of H2 bubbles locally affects the composition of the electrodeposited alloys. In the first part of this section we present the results of our study of the composition in areas located far away from the H2 bubbles, areas which represent the majority of the sample. Afterwards, we focus on how the presence of H2 at the surface of the templates influences the resulting composition. We have grown thin films and nanowires using growth potentials from E = −0.9 V to E = −1.4 V. When using nanoporous membranes we obtain a large density of nanowires with both templates (100 nm and 30 nm diameter), as low-magnification SEM images show in Fig. 1a–d. The nanowires are quite homogeneous
Fig. 1. SEM images of nanowires grown in a 100 nm template at (a) E = −0.9 V and (b) E = −1.4 V and grown in a 30 nm template at (c) E = −0.9 V and (d) E = −1.4 V.
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Fig. 2. (a) Fe concentration as a function of the plating potential for the different samples studied in this work. The horizontal red line corresponds to the relative Fe2+ concentration in the electrolyte which is included to show the presence of anomalous codeposition in all the samples. (b) Fe concentration profile for nanowires electrodeposited at E = −0.9 V, E = −1.1 V and E = −1.4 V.
and they present a large aspect ratio. We fixed the electrodeposition time to grow nanowires with lengths not larger than 1.5 m. Considering than the nanopores in the template are 6 m long, less than one third of the nanopore is filled. Therefore, the electrodeposition takes places far from the bulk solution and the depletion of the metal ions should be similar during the entire growth process. The composition of the samples as a function of the growth potential is plotted in Fig. 2a. There is clear evidence for the presence of anomalous codeposition in all samples, as the relative Fe concentration in the samples is much higher than the relative concentration of Fe2+ in the electrolyte, which clearly indicates that Fe deposits preferentially. The data in Fig. 2 show that all Fe concentration curves display the same qualitative behaviour. One can distinguish between three different growth potential regimes. First, at low overpotentials, an increase of the Fe content with increasing overpotential takes place, until the Fe concentration reaches a maximum. Second, at larger overpotentials, the Fe content in the samples decreases. In the third potential regime, at even higher overpotential values, all curves approach a constant Fe concentration, whose value is close to the [Fe2+ ]/([Ni2+ ]+[Fe2+ ]) ratio of the electrolyte. The behaviour of the Fe concentration in the different regions can be explained as follows. The partial current of Fe2+ is higher at lower overpotentials providing that Ni2+ is present in the electrolyte as a consequence of the anomalous codeposition [9]. This
results in an increase of the Fe concentration in the deposited sample in the first region. This concentration increases with increasing overpotential until it reaches a maximum, when the diffusion of Fe2+ ions in the electrolyte becomes the rate determining step. At this potential the maximum partial current of Ni2+ is not yet achieved and, in the second region, for more negative plating potentials, the Ni2+ current continues to increase, thereby decreasing the relative Fe concentration in the sample. Finally, when the Ni partial current reaches its diffusion limit, both Fe2+ reduction and Ni2+ reduction are diffusion controlled. In this regime, the concentration of the samples becomes independent of the plating potential and assumes a value very similar to the one of the electrolyte. Although these three regions have been previously observed in setups based on planar working electrodes [26], a new feature shows up when nanoporous templates are used. One can see from Fig. 2a that the maximum in the Fe content, the limit between zones 1 and 2, moves towards higher overpotentials as the pore diameter decreases. Therefore, a larger overpotential is needed to reach the diffusion limit for Fe2+ in the electrolyte as the pore diameter decreases. Our results indicate that the anomalous behaviour is steeper in nanopores and dominates the growth in a broader range of growth potentials. Having established the importance of anomalous codeposition for plating in nanoporous electrodes, it is as a subsequent step important to study how this process affects the composition profile along single nanowires. There should be clear differences when plating in the different regions previously identified in Fig. 2a. To study this influence, we have measured the changes of Fe concentration along the length of individual nanowires grown in the first (E = −0.9 V), second (E = −1.1 V) and third (E = −1.4 V) regions. The concentration profile of the different nanowires is shown in Fig. 2b. For nanowires grown in the first region, the Fe concentration is practically constant along the nanowires for both templates. However, nanowires electrodeposited in region 2 present a clear decrease in the Fe concentration along the nanowire, with a composition gradient that increases with increasing pore size. When using a potential in the third region, the nanowires are again homogeneous but this time with a Fe content closed to the Fe2+ concentration in the electrolyte. These results are consistent with the previously explained behaviour in the different regions. For low overpotentials (region 1), the growth is kinetically controlled for both ions, so at any time during electrodeposition there is no change in the composition. When the overpotential increases and enters into region 2, the first stages of the growth are kinetically controlled and, as in region 1, the nanowires will be Fe-rich. As the diffusion process takes place, the ion concentration profile into the pores tends to the steady state when the process becomes controlled by the mass transport of Fe2+ . This leads to a decrease of Fe concentration along the wire until it reaches an homogeneous concentration. When both ions are controlled by diffusion, the composition of the nanowires becomes homogeneous again. When the electrodeposition takes place in region 2, larger differences between the growth potential and the peak potential lead to a larger gradient in the Fe concentration because the kinetics of Ni2+ increases with potential. Indeed, this effect is observed in the case of nanowires deposited in 100 nm and 30 nm templates grown at −1.1 V. 3.2. Effect of hydrogen bubbles As mentioned above, H2 evolution takes places simultaneously with NiFe electrodeposition. Therefore, during the electrodeposition process some H2 bubbles are formed on top of the cathode surface as a result of the hydrogen evolution reaction. We found these bubbles both for thin films as well as nanotemplates. In order to study a possible change in the composition due to the presence
Á. Llavona et al. / Electrochimica Acta 106 (2013) 392–397
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Fig. 3. (a) SEM image of the fingerprint of an H2 bubble after electrodepositing a NiFe thin film. The red letters indicate the places at which composition was measured. (b) Schematics of the electrolyte stream profile around an H2 bubble.
Fig. 4. (a) SEM image of nanowires grown at E = −0.96 V in an area close to an H2 bubble. The areas marked as A, B, C and D are shown enlarged in the figures (b), (c) and (d). (e) Iron content measured by EDX in the different marked areas.
3.3. Cyclic voltammetry We have carried out electrochemical measurements to understand the influence of the template on the enhancement of the 39
100nm -0.9V 30nm -0.9V 100nm -1.1V 30nm -1.1V 100nm -1,4V 30nm -1.4V
36 33 30 27
[Fe] (wt. %)
of the bubbles, we have measured the Fe composition in the areas where the hydrogen bubbles have been observed. Fig. 3 shows the pit left behind by a H2 bubble in a NiFe thin film. We have measured the composition in the different zones marked in Fig. 3a. At the center of the H2 bubble (zone A) we observed a lack of Fe and Ni indicating that the bubble is formed early during growth and prevents the Fe–Ni deposition. At the border of the bubble (zone B) the Fe concentration is 7%, and in zones C and D is 23% and 25%, respectively. Clearly, anomalous codeposition is less pronounced close to the bubble. Previous studies carried out in thin films showed changes in the composition in the vicinity of the hydrogen bubbles [27]. In particular, these changes were observed when an external magnetic field was applied during the electrodeposition process. When a bubble is formed, this results in a distortion of the current lines that are no longer perpendicular to the surface [28], as shown schematically in Fig. 3b. This distortion in the current line profile creates an area in which the electrolyte moves more slowly and, as a consequence, the composition process is controlled by mass transport. Therefore, the composition tends to be the one observed in the third zone described in the previous section, in which both ions, Fe2+ and Ni2+ , are limited by diffusion. It is also worth to mention that, although in the previous studies this effect was observed when plating in a magnetic field, we observe the same effect in our study, where no magnetic field was applied. In the case of nanowires, we see growth material outside the templates in the areas where a bubble was attached to the membrane (zone A of Fig. 4a). The length of the nanowires decreases, as they are located further away form the bubble. Images 4b–d are magnifications of the areas marked with letters in Fig. 4a. The composition of the alloy in these areas is displayed in Fig. 4e. One observes a clear increase in the Fe concentration, as the distance from the bubble increases. The effect is similar to the one observed in thin films, indicating that close to the bubbles the deposition is less anomalous, favoring the Ni deposition. The previously discussed study of the composition along individual nanowires was realized using wires, which were grown far away from H2 bubbles. These wires represent the majority of electrodeposited nanowires of our sample. In Fig. 5, we show the results of the same analysis performed with wires grown close the bubbles. It can be seen that the results are qualitatively the same for a potential of E = −1.4 V, constant composition and Fe content similar to the one of the Fe2+ in the electrolyte, and for E = −1.1 V, gradient of composition. Nevertheless, for E = −0.9 V the composition changes for the 30 nm nanowires, whereas it is constant for the 100 nm wires. Therefore, for the 30 nm membranes, the diffusion of Fe2+ ions is dominant at lower overpotentials in the neighboring regions of the hydrogen bubbles, because region 2 is reached at lower overpotentials.
24 21 18 15 12 9 6 3 0
0
500
1000
1500
2000
2500
3000
L / nm Fig. 5. Fe concentration profile for nanowires electrodeposited close to a H2 bubble at E = −0.9 V, E = −1.1 V and E = −1.4 V.
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on electrodeposition of Si nanowires into the pores of polycarbonate templates from an ionic solvent, the observed pre-peak was related to adsorption of silicon ions onto the gold surface and/or to the gold surface reconstruction [30]. The point in common of these works is the idea of a partial blocking of the surface by adsorption of chemical species. In our opinion, the peak P1 can be related also to some adsorbed chemical species, probably HADS . In fact, the relation between anomalous codeposition and a partial blocking of the cathode surface has already been established [9,12,10,11,13]. As Fig. 6a shows, this pre-peak is more pronounced in the case of small nanopores and appears at lower overpotentials. This would mean that there is a stronger blocking of the cathode surface in the case of small nanopores that could indicate that the kinetics of the reaction would be the limiting factor for a higher range of potentials. This blocking would lead to a displacement of the potential at which the diffusion of Fe 2+ ions becomes the rate determining step towards higher overpotential, explaining the enhancement of anomalous codeposition seen in Fig. 2 for the case of nanopores. This also explains why the nanowires show a higher iron content than thin films deposited under the same conditions. 4. Conclusions
Fig. 6. Cyclic voltammetry scans for (a) thin films and nanopores in NiFe electrolyte and for (b) 30 nm nanopores in NiFe electrolyte for scans reversals at different potentials. Note that the curves corresponding to thin films and 100 nm nanopores have been rescaled.
anomalous codeposition. Cyclic voltammetry scans, measured from the potential of E = 0.05 V in cathodic direction at a scan rate of 10 mV/s, exhibit two cathodic peaks labelled P1 and P2 in Fig. 6a. There are clear differences when using different working electrodes. First of all, the current density at the peak positions increases when the diameter of the pores decreases (note that the curves corresponding to thin films and 100 nm nanopores have been rescaled). In addition, both peaks appear at lower overpotentials for the case of 30 nm pores, moving towards more negative growth potentials for 100 nm pores and thin films. These differences in the voltammograms between the different cathode configurations imply changes in the deposition process. Further insight into the growth process can be gained by analyzing the cyclic voltammograms obtained by reversing the curves at different potentials. Fig. 6b shows the results for the 30 nm pores. Similar results are observed using the other templates. It can be seen that no anodic peaks appear when the sweep is reversed in the region of the peak P1 while anodic peaks appear when reversing in the region between P1 and P2, as well as for potentials, which are more negative than the peak P2. The origin of this pre-peak at low overpotentials is still unclear. In a study focused on the electrodeposition of Co on Au, Flis-Kabulska related the pre-peak she found to a monolayer deposition of Co[29]. She suggested that the prepeak may be associated with the hindrance to the surface diffusion of Co adatoms and to nucleation processes due to the adsorption of chemical species, mainly of adsorbed hydrogen, HADS . The peak P1 dissapears if more than one cycle is performed. In Mallet’s work
In summary, we have shown that the composition of NiFe electrodeposited nanowires is dramatically different from the composition of thin films electrodeposited under the same conditions due to an enhancement of the anomalous codeposition in nanopores. This leads to a much higher iron concentration in nanowires than in thin films. Depending on the deposition potential, nanowires can be homogeneous with a high content of Fe, they can show a gradient in the Fe concentration or they can be homogeneous with a Fe concentration similar to the Fe 2+ concentration of the electrolyte. We have also observed changes in the alloy composition in the areas close to where hydrogen bubbles are attached to the cathode surface. For nanowires, hydrogen bubbles also cause a faster deposition, leading to longer nanowires. We suggest that the enhancement of the anomalous codeposition in nanowires is linked to a higher blocking of the cathode surface by adsorption of chemical species in nanopores, as compared to the case of thin films. Acknowledgements We thank Alicia Prados for valuable discussions and suggestions. We acknowledge partial financial support of this work by the Spanish Ministry of Science (projects MAT2010-21553-C02-01 and MAT2011–28751-C02) and the Universidad Complutense de Madrid. References [1] C.R. Martin, Nanomaterials: A Membrane-based Synthetic Approach, Science 266 (1994) 1961. [2] T.M. Whitney, P.C. Searson, J.S. Jiang, C.L. Chien, Fabrication and Magnetic Properties of Arrays of Metallic Nanowire, Science 261 (1993) 5126. [3] L. Sun, Y. Hao, C.L. Chien, P.C. Searson, Tuning the Properties of Magnetic Nanowire, Journal of Research and Development 49 (2005) 79. [4] A. Brenner, Electrodeposition of Alloys: Principles and Practice, number v. 1 in Electrodeposition of Alloys: Principles and Practice, Academic Press Inc., New York, 1963. [5] H. Dahms, I.M. Croll, The Anomalous Codeposition of Iron-Nickel Alloys, Journal of the Electrochemical Society 112 (1965) 771. [6] S. Hessami, C.W. Tobias, Insitu Measurement of Interfacial pH Using a Rotatingring-disk Electrode, AIChE Journal 39 (1993) 149–162. [7] H. Deliglianni, L.T. Romankiw, Insitu Surface pH Measurement During Electrolysis Using a Rotating pH Electrode, IBM Journal of Research and Development 37 (1993) 85–95. [8] J.D. Gangasingh, J. Talbot, Anomalous Electrodeposition of Nickel-Iron, Journal of the Electrochemical Society 138 (1991) 3605–3611.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhancement of anomalous codeposition in the synthesis of Fe–Ni alloys in nanopores Ángela Llavona a,∗ , Lucas Pérez a,b , M. Carmen Sánchez a , Víctor de Manuel a,1 a b
Departamento de Física de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain Instituto de Sistemas Optoelectrónicos y Microtecnología, Universidad Politécnica de Madrid, 28040 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 31 January 2013 Received in revised form 14 May 2013 Accepted 16 May 2013 Available online 13 June 2013 Keywords: Electrodeposition Anomalous codeposition Fe–Ni alloys Nanopores
a b s t r a c t In this paper we report an enhancement of anomalous codeposition in NiFe electrodeposition when using nanoporous templates. Depending on the deposition potential it is possible to obtain homogeneous nanowires of a high Fe concentration or of a concentration similar to the one of the electrolyte used. In addition, nanowires with a decreasing Fe concentration are found for intermediate overpotentials. Voltammetric studies relate this effect with a possible higher block of the cathode surface by adsorption of intermediated species in nanopores. A study of the effect of hydrogen bubbles shows a change in the composition in the areas surrounding the bubbles, and yields an increase in the deposition rate, which results in longer nanowires. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Electrodeposition is a technique extensively used in the industry that has gained increased interest over the last decades since it offers a unique opportunity to synthesize materials at the nanometer scale. In particular, the use of nanoporous templates is very effective in the synthesis of nanowires [1,2] because of the relatively easy control of diameter and length, the low production costs and the possibility of nanostructuration which allows tailoring the properties of the grown nanomaterials [3]. However, electrodeposition of alloys formed by metals from the iron-group is not so simple due to the so-called anomalous codeposition: when plating an alloy of two metals belonging to this group, the less noble metal deposits preferentially to the more noble one, thus making it difficult to control the composition of the alloy [4]. In the case of NiFe alloys, this results in a fraction of Fe in the resulting alloy, which is greater than the ratio [Fe2+ ]/([Fe2+ ] +[Ni2+ ]) in the solution. Various efforts have been made, using both theoretical and experimental approaches, to understand this unusual effect. First steps were taken by Dahms and Croll in 1965, who suggested that an increase of the pH in the vicinity of the cathode surface could cause anomalous codeposition [5]. However, later experimental studies showed that the increase of pH was not significant enough
∗ Corresponding author. Tel.: +34 913944788. E-mail address: angelallavona@fis.ucm.es (Á. Llavona). 1 Present address: Institut für Experimentelle und Angewandte Physik, ChristianAlbrechts-Universität, 24098 Kiel, Germany. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.116
to explain the observed effects [6–8]. A new perspective was proposed by Matlosz [9], who proposed that anomalous codeposition was caused by a competitive absortion mechanism involving adsorbed intermediate ions. This idea was experimentally supported by Baker [10,11] and in other studies [12,13]. Although the origin of anomalous codeposition is still under debate, the idea of a partial surface blocking due to adsorbed or precipitated species is, nowadays, the most plausible and widely accepted explanation. We note that all studies realized so far have been focused on planar cathodes, however, up to now, no attention has been paid to anomalous codeposition in setups with nanometer-sized cathodes. Permalloy (Ni80 Fe20 ) is one of the most widely used magnetic materials due to its very soft magnetic properties and near zero magnetostriction [14]. These properties make this material specially suitable for many different magnetic applications, including magnetic field sensing [15], recording heads [16] or microinductors [17]. In recent years, after the birth of nanotechnology, Permalloy is also playing a key role in the development of new nanodevices, like the race-track memories [18], domain wall logic devices [19] and other nanodevices [20,21], as well as in magnonics [22] and spintronics [23]. Due to its wide-spread technical applicability, the study of the synthesis of NiFe nanostructures has become an increasingly active field of research in the last years. Since the properties of Permalloy are extremely dependent on the particular composition Ni80 Fe20 , it is important to study the effect of anomalous codeposition when plating in nanoporous templates in order to control the composition of NiFe electrodeposited nanowires.
Á. Llavona et al. / Electrochimica Acta 106 (2013) 392–397 Table 1 Characteristics of the membranes used in this work. The table summarizes the nominal values supplied by Sterlitech. Pore diameter Pore density Thickness
30 nm 6 × 108 cm−2 6 m
100 nm 4 × 108 cm−2 6 m
In this work we report an enhancement of the anomalous codeposition when nanopore templates are used as cathodes for electrodeposition instead of planar substrates. We find that changes in the geometry of the electrochemical setup modify the plating conditions. This leads to changes in the chemical composition, which can be understood in terms of a partial blocking of the cathode surface by adsorbed species. 2. Experimental We have electrodeposited arrays of NiFe nanowires using nanoporous membranes as templates. The electrolyte used is composed of NiSO4 (0.7 M) and NiCl2 (0.02 M) as Ni2+ sources, FeSO4 (0.03 M) as Fe2+ source and H3 BO3 (0.4 M) and saccharine (0.016 M) as additives. This composition is similar to the one typically used for the electrodeposition of Permalloy [24,25]. All chemicals were of analytical grade and they were used without further purification and mixed in deionized water. The pH was adjusted to 2.3 by dropping H2 SO4 10% vol. The synthesis was carried out at room temperature, in a threeelectrode-cell. A platinum mesh was used as counter electrode and a Ag/AgCl electrode as reference electrode. All potential values quoted in this work are referenced with respect to this electrode. Hydrophilic (polyvinilpyrrolidone coated) polycarbonate membranes supplied by Sterlitech were used as templates for the synthesis of nanowires. Their characteristics are summarized in Table 1. Before electrodeposition, a thin Au film was thermally evaporated on one side of the membrane. The diameter of the nanowires depends on the diameter of the membrane pores, whereas their length is controlled by the electrodeposition time. In addition, thin films were grown on Si wafers, previously coated with Au.
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The morphology of the nanowires was studied with a Scanning Electron Microscope (SEM) JEM 6335 F. Before measuring the nanowires, the polycarbonate template was removed with dichloromethane. The average composition of the samples was measured by Energy Dispersion X-Ray Spectroscopy (EDX) using a SEM Microscope JEM 6335 F with an EDX Oxford Mod 6506. For thin films the signal was recorded in the center of the sample. In the case of nanowires, the measurement was done in an area with a high density of wires and, therefore, the composition represents an averaged value over a large group of nanowires. In all cases, several measurements were performed in different areas of the samples so the values provided in this work correspond to the mean values. The composition along the length of individual nanowires was measured with a Transmission Electron Microscope (TEM) JEOL JEM 2000FX. Measurements were made at both ends of the wires and in some points in between. In order to release the nanowires, the gold layer was removed by contact with mercury and the samples were washed thoroughly with deionized water. Afterwards, the polycarbonate membrane was dissolved with dichloromethane. The suspension was centrifuged and new dichloromethane was added. Finally, some drops of this solution were placed on a TEM grid. 3. Results and discussion 3.1. Anomalous codeposition in nanopores During the electrodeposition of NiFe alloys, H2 evolution reaction takes place simultaneously, an effect which we observed in nanowires and thin films. In both cases, the presence of H2 bubbles locally affects the composition of the electrodeposited alloys. In the first part of this section we present the results of our study of the composition in areas located far away from the H2 bubbles, areas which represent the majority of the sample. Afterwards, we focus on how the presence of H2 at the surface of the templates influences the resulting composition. We have grown thin films and nanowires using growth potentials from E = −0.9 V to E = −1.4 V. When using nanoporous membranes we obtain a large density of nanowires with both templates (100 nm and 30 nm diameter), as low-magnification SEM images show in Fig. 1a–d. The nanowires are quite homogeneous
Fig. 1. SEM images of nanowires grown in a 100 nm template at (a) E = −0.9 V and (b) E = −1.4 V and grown in a 30 nm template at (c) E = −0.9 V and (d) E = −1.4 V.
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Fig. 2. (a) Fe concentration as a function of the plating potential for the different samples studied in this work. The horizontal red line corresponds to the relative Fe2+ concentration in the electrolyte which is included to show the presence of anomalous codeposition in all the samples. (b) Fe concentration profile for nanowires electrodeposited at E = −0.9 V, E = −1.1 V and E = −1.4 V.
and they present a large aspect ratio. We fixed the electrodeposition time to grow nanowires with lengths not larger than 1.5 m. Considering than the nanopores in the template are 6 m long, less than one third of the nanopore is filled. Therefore, the electrodeposition takes places far from the bulk solution and the depletion of the metal ions should be similar during the entire growth process. The composition of the samples as a function of the growth potential is plotted in Fig. 2a. There is clear evidence for the presence of anomalous codeposition in all samples, as the relative Fe concentration in the samples is much higher than the relative concentration of Fe2+ in the electrolyte, which clearly indicates that Fe deposits preferentially. The data in Fig. 2 show that all Fe concentration curves display the same qualitative behaviour. One can distinguish between three different growth potential regimes. First, at low overpotentials, an increase of the Fe content with increasing overpotential takes place, until the Fe concentration reaches a maximum. Second, at larger overpotentials, the Fe content in the samples decreases. In the third potential regime, at even higher overpotential values, all curves approach a constant Fe concentration, whose value is close to the [Fe2+ ]/([Ni2+ ]+[Fe2+ ]) ratio of the electrolyte. The behaviour of the Fe concentration in the different regions can be explained as follows. The partial current of Fe2+ is higher at lower overpotentials providing that Ni2+ is present in the electrolyte as a consequence of the anomalous codeposition [9]. This
results in an increase of the Fe concentration in the deposited sample in the first region. This concentration increases with increasing overpotential until it reaches a maximum, when the diffusion of Fe2+ ions in the electrolyte becomes the rate determining step. At this potential the maximum partial current of Ni2+ is not yet achieved and, in the second region, for more negative plating potentials, the Ni2+ current continues to increase, thereby decreasing the relative Fe concentration in the sample. Finally, when the Ni partial current reaches its diffusion limit, both Fe2+ reduction and Ni2+ reduction are diffusion controlled. In this regime, the concentration of the samples becomes independent of the plating potential and assumes a value very similar to the one of the electrolyte. Although these three regions have been previously observed in setups based on planar working electrodes [26], a new feature shows up when nanoporous templates are used. One can see from Fig. 2a that the maximum in the Fe content, the limit between zones 1 and 2, moves towards higher overpotentials as the pore diameter decreases. Therefore, a larger overpotential is needed to reach the diffusion limit for Fe2+ in the electrolyte as the pore diameter decreases. Our results indicate that the anomalous behaviour is steeper in nanopores and dominates the growth in a broader range of growth potentials. Having established the importance of anomalous codeposition for plating in nanoporous electrodes, it is as a subsequent step important to study how this process affects the composition profile along single nanowires. There should be clear differences when plating in the different regions previously identified in Fig. 2a. To study this influence, we have measured the changes of Fe concentration along the length of individual nanowires grown in the first (E = −0.9 V), second (E = −1.1 V) and third (E = −1.4 V) regions. The concentration profile of the different nanowires is shown in Fig. 2b. For nanowires grown in the first region, the Fe concentration is practically constant along the nanowires for both templates. However, nanowires electrodeposited in region 2 present a clear decrease in the Fe concentration along the nanowire, with a composition gradient that increases with increasing pore size. When using a potential in the third region, the nanowires are again homogeneous but this time with a Fe content closed to the Fe2+ concentration in the electrolyte. These results are consistent with the previously explained behaviour in the different regions. For low overpotentials (region 1), the growth is kinetically controlled for both ions, so at any time during electrodeposition there is no change in the composition. When the overpotential increases and enters into region 2, the first stages of the growth are kinetically controlled and, as in region 1, the nanowires will be Fe-rich. As the diffusion process takes place, the ion concentration profile into the pores tends to the steady state when the process becomes controlled by the mass transport of Fe2+ . This leads to a decrease of Fe concentration along the wire until it reaches an homogeneous concentration. When both ions are controlled by diffusion, the composition of the nanowires becomes homogeneous again. When the electrodeposition takes place in region 2, larger differences between the growth potential and the peak potential lead to a larger gradient in the Fe concentration because the kinetics of Ni2+ increases with potential. Indeed, this effect is observed in the case of nanowires deposited in 100 nm and 30 nm templates grown at −1.1 V. 3.2. Effect of hydrogen bubbles As mentioned above, H2 evolution takes places simultaneously with NiFe electrodeposition. Therefore, during the electrodeposition process some H2 bubbles are formed on top of the cathode surface as a result of the hydrogen evolution reaction. We found these bubbles both for thin films as well as nanotemplates. In order to study a possible change in the composition due to the presence
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Fig. 3. (a) SEM image of the fingerprint of an H2 bubble after electrodepositing a NiFe thin film. The red letters indicate the places at which composition was measured. (b) Schematics of the electrolyte stream profile around an H2 bubble.
Fig. 4. (a) SEM image of nanowires grown at E = −0.96 V in an area close to an H2 bubble. The areas marked as A, B, C and D are shown enlarged in the figures (b), (c) and (d). (e) Iron content measured by EDX in the different marked areas.
3.3. Cyclic voltammetry We have carried out electrochemical measurements to understand the influence of the template on the enhancement of the 39
100nm -0.9V 30nm -0.9V 100nm -1.1V 30nm -1.1V 100nm -1,4V 30nm -1.4V
36 33 30 27
[Fe] (wt. %)
of the bubbles, we have measured the Fe composition in the areas where the hydrogen bubbles have been observed. Fig. 3 shows the pit left behind by a H2 bubble in a NiFe thin film. We have measured the composition in the different zones marked in Fig. 3a. At the center of the H2 bubble (zone A) we observed a lack of Fe and Ni indicating that the bubble is formed early during growth and prevents the Fe–Ni deposition. At the border of the bubble (zone B) the Fe concentration is 7%, and in zones C and D is 23% and 25%, respectively. Clearly, anomalous codeposition is less pronounced close to the bubble. Previous studies carried out in thin films showed changes in the composition in the vicinity of the hydrogen bubbles [27]. In particular, these changes were observed when an external magnetic field was applied during the electrodeposition process. When a bubble is formed, this results in a distortion of the current lines that are no longer perpendicular to the surface [28], as shown schematically in Fig. 3b. This distortion in the current line profile creates an area in which the electrolyte moves more slowly and, as a consequence, the composition process is controlled by mass transport. Therefore, the composition tends to be the one observed in the third zone described in the previous section, in which both ions, Fe2+ and Ni2+ , are limited by diffusion. It is also worth to mention that, although in the previous studies this effect was observed when plating in a magnetic field, we observe the same effect in our study, where no magnetic field was applied. In the case of nanowires, we see growth material outside the templates in the areas where a bubble was attached to the membrane (zone A of Fig. 4a). The length of the nanowires decreases, as they are located further away form the bubble. Images 4b–d are magnifications of the areas marked with letters in Fig. 4a. The composition of the alloy in these areas is displayed in Fig. 4e. One observes a clear increase in the Fe concentration, as the distance from the bubble increases. The effect is similar to the one observed in thin films, indicating that close to the bubbles the deposition is less anomalous, favoring the Ni deposition. The previously discussed study of the composition along individual nanowires was realized using wires, which were grown far away from H2 bubbles. These wires represent the majority of electrodeposited nanowires of our sample. In Fig. 5, we show the results of the same analysis performed with wires grown close the bubbles. It can be seen that the results are qualitatively the same for a potential of E = −1.4 V, constant composition and Fe content similar to the one of the Fe2+ in the electrolyte, and for E = −1.1 V, gradient of composition. Nevertheless, for E = −0.9 V the composition changes for the 30 nm nanowires, whereas it is constant for the 100 nm wires. Therefore, for the 30 nm membranes, the diffusion of Fe2+ ions is dominant at lower overpotentials in the neighboring regions of the hydrogen bubbles, because region 2 is reached at lower overpotentials.
24 21 18 15 12 9 6 3 0
0
500
1000
1500
2000
2500
3000
L / nm Fig. 5. Fe concentration profile for nanowires electrodeposited close to a H2 bubble at E = −0.9 V, E = −1.1 V and E = −1.4 V.
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on electrodeposition of Si nanowires into the pores of polycarbonate templates from an ionic solvent, the observed pre-peak was related to adsorption of silicon ions onto the gold surface and/or to the gold surface reconstruction [30]. The point in common of these works is the idea of a partial blocking of the surface by adsorption of chemical species. In our opinion, the peak P1 can be related also to some adsorbed chemical species, probably HADS . In fact, the relation between anomalous codeposition and a partial blocking of the cathode surface has already been established [9,12,10,11,13]. As Fig. 6a shows, this pre-peak is more pronounced in the case of small nanopores and appears at lower overpotentials. This would mean that there is a stronger blocking of the cathode surface in the case of small nanopores that could indicate that the kinetics of the reaction would be the limiting factor for a higher range of potentials. This blocking would lead to a displacement of the potential at which the diffusion of Fe 2+ ions becomes the rate determining step towards higher overpotential, explaining the enhancement of anomalous codeposition seen in Fig. 2 for the case of nanopores. This also explains why the nanowires show a higher iron content than thin films deposited under the same conditions. 4. Conclusions
Fig. 6. Cyclic voltammetry scans for (a) thin films and nanopores in NiFe electrolyte and for (b) 30 nm nanopores in NiFe electrolyte for scans reversals at different potentials. Note that the curves corresponding to thin films and 100 nm nanopores have been rescaled.
anomalous codeposition. Cyclic voltammetry scans, measured from the potential of E = 0.05 V in cathodic direction at a scan rate of 10 mV/s, exhibit two cathodic peaks labelled P1 and P2 in Fig. 6a. There are clear differences when using different working electrodes. First of all, the current density at the peak positions increases when the diameter of the pores decreases (note that the curves corresponding to thin films and 100 nm nanopores have been rescaled). In addition, both peaks appear at lower overpotentials for the case of 30 nm pores, moving towards more negative growth potentials for 100 nm pores and thin films. These differences in the voltammograms between the different cathode configurations imply changes in the deposition process. Further insight into the growth process can be gained by analyzing the cyclic voltammograms obtained by reversing the curves at different potentials. Fig. 6b shows the results for the 30 nm pores. Similar results are observed using the other templates. It can be seen that no anodic peaks appear when the sweep is reversed in the region of the peak P1 while anodic peaks appear when reversing in the region between P1 and P2, as well as for potentials, which are more negative than the peak P2. The origin of this pre-peak at low overpotentials is still unclear. In a study focused on the electrodeposition of Co on Au, Flis-Kabulska related the pre-peak she found to a monolayer deposition of Co[29]. She suggested that the prepeak may be associated with the hindrance to the surface diffusion of Co adatoms and to nucleation processes due to the adsorption of chemical species, mainly of adsorbed hydrogen, HADS . The peak P1 dissapears if more than one cycle is performed. In Mallet’s work
In summary, we have shown that the composition of NiFe electrodeposited nanowires is dramatically different from the composition of thin films electrodeposited under the same conditions due to an enhancement of the anomalous codeposition in nanopores. This leads to a much higher iron concentration in nanowires than in thin films. Depending on the deposition potential, nanowires can be homogeneous with a high content of Fe, they can show a gradient in the Fe concentration or they can be homogeneous with a Fe concentration similar to the Fe 2+ concentration of the electrolyte. We have also observed changes in the alloy composition in the areas close to where hydrogen bubbles are attached to the cathode surface. For nanowires, hydrogen bubbles also cause a faster deposition, leading to longer nanowires. We suggest that the enhancement of the anomalous codeposition in nanowires is linked to a higher blocking of the cathode surface by adsorption of chemical species in nanopores, as compared to the case of thin films. Acknowledgements We thank Alicia Prados for valuable discussions and suggestions. We acknowledge partial financial support of this work by the Spanish Ministry of Science (projects MAT2010-21553-C02-01 and MAT2011–28751-C02) and the Universidad Complutense de Madrid. References [1] C.R. Martin, Nanomaterials: A Membrane-based Synthetic Approach, Science 266 (1994) 1961. [2] T.M. Whitney, P.C. Searson, J.S. Jiang, C.L. Chien, Fabrication and Magnetic Properties of Arrays of Metallic Nanowire, Science 261 (1993) 5126. [3] L. Sun, Y. Hao, C.L. Chien, P.C. Searson, Tuning the Properties of Magnetic Nanowire, Journal of Research and Development 49 (2005) 79. [4] A. Brenner, Electrodeposition of Alloys: Principles and Practice, number v. 1 in Electrodeposition of Alloys: Principles and Practice, Academic Press Inc., New York, 1963. [5] H. Dahms, I.M. Croll, The Anomalous Codeposition of Iron-Nickel Alloys, Journal of the Electrochemical Society 112 (1965) 771. [6] S. Hessami, C.W. Tobias, Insitu Measurement of Interfacial pH Using a Rotatingring-disk Electrode, AIChE Journal 39 (1993) 149–162. [7] H. Deliglianni, L.T. Romankiw, Insitu Surface pH Measurement During Electrolysis Using a Rotating pH Electrode, IBM Journal of Research and Development 37 (1993) 85–95. [8] J.D. Gangasingh, J. Talbot, Anomalous Electrodeposition of Nickel-Iron, Journal of the Electrochemical Society 138 (1991) 3605–3611.
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