Shape-selective electroless plating within expanding template pores

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Shape-selective electroless plating within expanding template pores: Etching-assisted deposition of spiky nickel nanotube networks Tim Böttcher, Sandra Schaefer, Markus Antoni, Tobias Stohr, Ulrike Kunz, Michael Dürrschnabel, Leopoldo Molina-Luna, Wolfgang Ensinger, and Falk Muench Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00030 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Shape-selective electroless plating within expanding template pores: Etching-assisted deposition of spiky nickel nanotube networks Tim Boettcher,∗,† Sandra Schaefer,†,‡ Markus Antoni,† Tobias Stohr,† Ulrike Kunz,† Michael Dürrschnabel,†,¶ Leopoldo Molina-Luna,† Wolfgang Ensinger,† and Falk Muench† †Technische Universität Darmstadt, Department of Materials and Geoscience, Alarich-Weiss-Straße 2, 64287 Darmstadt, Germany ‡CEST Kompetenzzentrum f¨ ur elektrochemische Oberflächentechnologie GmbH, Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria ¶Karlsruher Institut f¨ ur Technologie (KIT), Institut f¨ ur Angewandte Materialien, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany E-mail: [email protected] Phone: +49 6151 16-21993. Fax: +49 6151 16-21991

Abstract

sured by the reducing agent hydrazine. Iminodiacetic acid is not only used as a complexing agent during synthesis but apparently acts also as a capping agent and limits random nucleation on the spike facets. Finally, we apply our synthesis to templates with interconnected pores to obtain free-standing spiky nickel nanotube networks, demonstrating its ability to homogeneously coat substrates with extended inner surfaces and to operate in nanoscale confinement.

Nanoobjects are favored structures for applications such as catalysis and sensing. Although they already provide a large surfaceto-volume ratio, this ratio can be further increased by shape-selective plating of the nanostructure surfaces. This process combines the conformity of autocatalytic deposition with the defined nucleation and growth characteristics of colloidal nanoparticle syntheses. However, many aspects of such reactions are still not fully understood. In this study, we investigate in detail the growth of spiky nickel nanotubes in polycarbonate template membranes. One distinctive feature of our synthesis is the simultaneous growth of nanospikes on both the inside and outside of nanotubes while the tubes are still embedded in the polymer. This is achieved by combining the plating process with locally enhanced in situ etching of the poylmer template, for which we propose a theory. Electron microscopy investigations reveal twinning defects as driving force for the growth of crystalline nanospikes. Deposit crystallinity is en-

Introduction Nanostructures are present in many applications such as catalysis, sensing, energy conversion and storage, and magnetics 1–9 . Compared to their bulk counterparts, nanostructures provide a larger surface-to-volume ratio and thus enable a more efficient material use. Also, they often possess properties that are different from the bulk properties 10,11 . Commonly, nanostructures are formed by bottom-up“ processes ei” ther template-free (e.g. electrospinning 12 ) or

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by use of a template that acts as a framework. The most popular template materials are anodic aluminum oxide (AAO), which can be manufactured easily and provides a regular alignment of pores 13–15 , and ion track-etched polymer templates 16–18 . Ion track-etched templates are manufactured by irradiating polymer foils with swift heavy ions followed by a chemical etching step 19 . While ion track-etched templates are more complicated to manufacture and require more expensive equipment, they are also more versatile than AAO templates when it comes to the assembly of network templates or hierarchical templates 17,20 . Although nanostructures already offer a higher surface-to-volume ratio than their bulk counterparts, this ratio can be increased even more. Conventional techniques such as dealloying, galvanic exchange 21 or acid leaching 22 often bear the disadvantage of a reduced mechanical stability since in these processes, parts of the nanostructures are dissolved. Shapeselective plating of nanostructures also yields higher surface-to-volume ratios 11 but does not lead to less mechanical stability as this is an additive process. The obtained results are heavily influenced by the applied precursor, the applied ligand, environmental conditions, and the metal that shall be plated itself. Due to the vast amount of parameters, shape-selective plating is only poorly understood nowadays 11 . In this study, we will use ion track-etched polycarbonate membranes to manufacture spiky nickel nanotubes with spikes growing simultaneously on the inside and outside of the tubes. The outside spike growth is accomplished by in situ template etching during the plating process and thus just providing enough space to grow spikes without the nanostructures collapsing. We investigate the spike growth and the required prequisites using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For this, we take a closer look on every component of our electrolyte and propose a theory that explains the local enhanced etching rate at the polymer-metal interface between template and nickel tube. In the end, we successfully apply our synthesis to polycarbonate network templates to obtain a free-standing

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spiky nickel nanotube network.

Experimental General, Chemicals All the solutions were freshly prepared and the glassware was cleaned with boiling aqua regia prior to use. For the preparation of all solutions, purified water (Milli-Q >18.2 MΩ cm) and the following chemicals were used without further purification: Borane dimethylamine complex (DMAB) (Aldrich, 97 %), dichloromethane (Merck, >99.8 %), hydrazine monohydrate 80 % in water (Merck, for synthesis), iminodiacetic acid (Fluka Analytical, ≥98 %), methanol (AppliChem, ≥99.5 %), nickel(II) sulfate heptahydrate (Sigma-Aldrich, ≥99 %), nitrilotriacetic acid (NTA) (Sigma-Aldrich, ≥99 %), palladium(II) chloride (Alfa Aesar, 99.9 % metal basis), potassium chloride (Fluka Analytical, ≥99 %), sodium hydroxide solution 32 % in water (Sigma-Aldrich), tin(II) chloride dihydrate (Sigma-Aldrich, 98 %), sodium citrate tribasic dihydrate (Sigma-Aldrich, >99 %), trifluoroacetic acid (Sigma-Aldrich, 99 %).

Template preparation Polycarbonate foils (Makrofol, Bayer MaterialScience AG) with a nominal thickness of 30 µm were irradiated at certain angles with swift heavy ions (energy: 11.4 MeV u−1 ) at the Helmholtz Center for Heavy Ion Research (GSI) in Darmstadt. Subsequently, the membranes were etched in stirred 6 M sodium hydroxide solution for 12 min at a temperature of 50 ◦C. The as-prepared templates were thoroughly rinsed with water and dried. This process is schematically illustrated in Figure 1.

Nanostructure synthesis Prior to the deposition of nickel nanostructures, the templates were seeded by a sensitization and activation process 23 . After each step, the templates were rinsed in water. For sensitization, the templates were immersed in a Sn(II) solution for 45 min, which consisted of 0.042 M


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ence solution, iminodiacetic acid was replaced by an equivalent amount of nitrilotriacetic acid and an additional two equivalents of sodium hydroxide to compensate the extra acid group of nitrilotriacetic acid. The electrolyte was used under the same plating conditions as the first hydrazine-based plating solution. After the deposition of each nickel layer, the metal surface films were removed by cautious grinding. By using dichloromethane, the polycarbonate template was dissolved and thereby the metallic structures were exposed.

Figure 1: Process of template preparation. Irradiation with swift heavy ions leads to the formation of latent damage tracks. These can be etched subsequently using alkaline solution to form pores inside the polymer material. Depending on the angle of incidence of ions, different superstructures such as parallel pores (upper pathway) or 3D networks (lower pathway) can be realized.

Characterization Characterization of the structures was performed using SEM, TEM and scanning transmission electron microscopy (STEM). For SEM, the exposed metal structures were collected on a Si wafer piece which was attached to a sample holder with carbon glue to ensure electrical conductivity. Investigations were conducted on a Philips XL30 FEG operated at 15 kV accelerating voltage. TEM and STEM analyses were carried out at a JEOL ARM200F operated at 200 kV. Samples were prepared by embedding the nanostructurecontaining template foils in Araldite 502 resin. Ultrathin slices with a thickness of roughly 70 nm were prepared by ultramicrotomy using a Reichert-Jung Ultracut E ultramicrotome equipped with a DKK diamond knife.

tin(II) chloride, and 0.071 M trifluoroacetic acid in a methanol-water-mix (1:1). The unbound Sn(II) was removed by washing the templates twice in water. Afterwards, they were immersed in the palladium activation solution for 4 min. This solution was composed of 0.011 M palladium(II) chloride, and 0.033 M potassium chloride in water. Due to the precipitation of palladium nanoparticles on the templates its color turned slightly brownish. Subsequently, the templates were used for nickel deposition. The first deposition solution for the thin semi-amorphous nickel layer on the template surface consisted of 0.1 M nickel(II) sulfate, 0.1 M sodium citrate, and 0.1 M borane dimethylamine in water. The deposition took place at room temperature for 10 min. Afterwards, the template was rubbed to remove most of the superficial nickel deposit. The spikes were generated in a second deposition step in a differently composed nickel plating solution (0.040 M nickel(II) sulfate, 0.080 M iminodiacetic acid, 0.212 M sodium hydroxide, and 1 M hydrazine in water). The deposition was performed at a temperature of 80 ◦C for 60 min. For comparison, a second hydrazine-based deposition solution was used with a related aminopolycarboxylic acid ligand. In this refer-

Results and Discussion The formation of nickel spikes was observed before, but - to our best knowledge - never investigated in detail 1,24 . As a starting point, we synthesized spiky nickel nanotubes in polycarbonate templates with nickel spikes growing on the inside and outside of the tubes while the polymer membrane is etched simultaneously. The overall process is schematically depicted in Figure 2. A polymer foil - in this case polycarbonate - is irradiated with swift heavy ions. Interactions between these ions and the polymer foil lead to the formation of latent damage tracks along the ion trajectories 25–27 . These damage


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tracks are etched in alkaline solution to form a nanoporous template, where the pore diameter is depending on the etching time, amongst others (Figure 2, Step a, in more detail see Figure 1). The template is then seeded by a sensitization and activation step (Figure 2, Steps b and c), where Sn2+-ions are loosely bound to the template surface by electrostatic interactions and used to reduce Pd2+ ions to form metallic nanoparticle seeds: Pd2+ + Sn2+

Pd↓ + Sn4+

(1)

These palladium nuclei are responsible for the initiation of the surface-selective autocatalytic nickel plating process 1 . After 10 min of processing in a nickel-borane dimethylamine plating solution, a thin nickel film has grown homogeneously on the template surface and concentrically inside the template pores (Figure 2, Step d). This metal layer is used as substrate layer for the nickel spike formation and as a protective layer to maintain template integrity during plating and will be later on referred to as base tube. Subsequently, the outer template surfaces are thoroughly rubbed with a wipe, so that the nickel film only persists inside the template pores (Figure 2, Step e). This ensures that the metal-polymer interface that is required in the next step is still accessible. Furthermore, it prevents the growth of a dense nickel film on the outer template surfaces and thus clogging of the pores. In the last synthesis step the formation of the spiky nickel film takes place in a nickelhydrazine plating solution (Figure 2, Step f). This plating solution provides compounds for both, the shape selective nickel plating and the in situ etching of the polymer membrane, so that metal deposition can occur on the inner and outer surfaces of each tube while the polymer membrane provides the mechanical stability for the nanostructures.

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: Process of Ni spike deposition. The etched template foil (Step a) is sensitized (Step b) and activated (Step c) which leads to the formation of noble metal seeds on the template surface and inside the pores. Afterwards, a nickel film is deposited (Step d). By cautious grinding, most of the Ni film that formed outside the pores is removed (Step e). On the remaining Ni structures, Ni spikes are created (Step f). ter the first deposition step. These base tubes have a length of 30 µm and an outer diameter of approximately 760 nm. The wall thickness amounts to slightly less than 50 nm after a deposition time of 10 min. As it can be seen, the tube walls are dense and have a constant thickness across the complete tube length. After the second deposition step (Figure 3b), the tube length remains at 30 µm but its outer diameter has increased up to approximately 1400 nm including the grown nanospikes. This is a first indication for a template etching process that occurs simultaneously to the plating step. It is also observed that the average spike length is constant along the tube length, which means that the pore widening occured with the same rate along the whole tube. A closer investigation of the nanospikes re-

The nanostructures obtained in this synthesis were investigated using SEM and TEM. Figure 3a shows SEM images of the nanotubes af-


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(a)

Figure 4: SEM image of several nickel spikes. The dashed lines in the inset mark the facet edges of one single spike. ticles 28 or as an enhanced in-plane growth of metal nanoplates 29 . It can also be observed that nucleation on the spike surfaces is strongly restrained which fortifies the anisotropic growth. STEM measurements conducted on one single spike (Figure 5b) show a long-range order and yield a diffractogram from which a zone axis orientation of h110i can be inferred using fast fourier transform (FFT). This yields a twinning defect that propagates along [113] direction and spike surfaces with low indexes such as {111}. An easy explanation for the presence of twinning defects would be a connection between the nickel spike film and the underlying nickel base tube as a substrate. However, nickel spike growth was also observed on polycrystalline platinum 24 and polymer substrates decorated with silver- or palladium nanoparticles 1,24 . This indicates an indepedency between spike growth and the underlying substrate.

(b)

Figure 3: SEM-images of the smooth semiamorphous initial nickel tube (a) and the spiky nickel nanotubes (b). The insets show the surface morphology in more detail. veals that the spike length amounts up to 150 nm after a deposition time of 60 min. As it can be seen from Figure 4, all nanospikes share one general growth direction and are slightly tilted with respect to that direction. Also, the lengths and shapes differ to some degree. An interesting detail that is also pointed out in the inset is that several spikes show facets on their surfaces and thus a pyramid-like shape. Investigation with TEM and STEM unveils the crystalline structure of single spikes. As shown in Figure 5a, each nanospike possesses a central twinning defect that ranges from the bottom of the spike all the way to its tip. This indicates that spike formation is apparently driven by an anisotropic growth along a twinning defect. Such growth was also observed for the formation of spiky platinum nanopar-

To verify this independency and to elucidate the origin of the twinning defect, nickel nanospikes were grown on a silver-seeded polyethylene terephthalate (PET) template based on the procedure described by Muench et al. 1 and investigated with TEM after various plating durations. The obtained TEM images are shown in Figure 6. As it can be seen, after 10 min of deposition, a thin film of nickel grains has grown. After a duration of 18 min, the nickel grains have formed a film and the


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(a)

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Figure 6: TEM images of spike growth after different plating durations. The images were taken after 10 min, 18 min, and 45 min of deposition time.

Figure 5: (a) TEM-images of nickel spikes. The arrows indicate the presence of a twin boundary in the middle of each spike. (b) Higher magnification of a single spike with marked twin boundary, outer surface, and corresponding FFT pattern.

rubbing steps. In the late deposition stages of this example, the spikes almost completely fill up the tube interior. Such distinct nanoscale confinement can be desirable to increase the dwell time of reactants and intermediates in close proximity to the metal surface, which allows tuning catalyst selectivity and yield 30 . While template-supported nanotubes can be employed as monolithic catalysts without internal flow 31 , they are particularly suited for application as miniaturized flow reactors. In this case, larger inner diameters (e.g., 500 nm or more) are preferrable in order to reduce flow resistance 23 .

first few spikes can be observed. In the further course, the nickel film slightly grows in thickness and the amount and length of nanospikes increases. This coincides with the observations of Muench et al. 1 and proves the independency of spike growth from the substrate. Instead, the twinning defect responsible for spike growth forms spontaneously during the deposition of the nickel film. Figure 6 also shows that structure growth has only occured towards the center of each template pore. This is due to the fact that PET is less prone to chemical attack as compared to polycarbonate and thus, also no or only little pore widening occurs. Also, in this experiment, we omitted the base tube plating and

Since we have shown that nanospike growth does not depend on the substrate, it must originate from the applied electrolyte and its compounds. These amount to hydrazine, sodium hydroxide and iminodiacetic acid. Hydrazine is introduced as a reducing agent. Compared to other reducing agents such as sodium borohydride (NaBH4) or DMAB, hydrazine enables the deposition of pure nickel without any incorporation of boron. Also, hydrazine-based deposits are more crystalline, while the other two reducing agents lead to more amorphous depositions 1,32,33 . Furthermore, hydrazine shifts the pH value of the electrolyte to more alkaline regions. Sodium hydroxide is added for two reasons: Firstly, it enables the chemical etching of poly-

(b)


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carbonate at elevated temperatures, and secondly, it enhances the reducing strength of hydrazine as it becomes more active with increasing alkalinity of the electrolyte 34 . The last component, iminodiacetic acid (IDA), acts as complexing agent and aims at preventing the formation of Ni(OH)2 in alkaline solution 35 . It is well known that the formation of a complex kinetically stabilizes the involved compounds. In this case, Ni2+ ions are stabilized by IDA. To verify whether a kinetic stabilization is sufficient, depositions of nickel were carried out using the same electrolyte as for the deposition of nickel spikes but replacing IDA by NTA. The obtained results are depicted in Figure 7. As it can be seen, instead of nickel spikes, tubes made of coarse nickel grains are formed. A comparison of IDA and NTA yields, that the latter one contains one more carboxy group and forms a stronger complex with nickel (log K = 11.54) 36 than IDA (log K = 8.26) 37 . As NTA forms the stronger complex, kinetic limitation cannot be the reason for the formation of spikes. Apart from forming a complex with Ni2+ ions, the ligand may also interact with the other electrolyte compounds and the formed metal film. An example for the first point might be an interaction with hydrazine, which can also act as complexing agent 38 . This however seems unlikely as then parts of the reducing agent will be consumed before a reduction reaction can take place and the overall reaction will be slowed down. Thus, an interaction of the ligand with the metal film is more likely. In this case, IDA may attach to the formed nickel film or spikes as a surfactant and prevent further nucleation. This ligand-dependent growth was also reported for the formation of spiky platinum nanoparticles 28 and spiky nickel/silver core-shell nanoparticles where only hydrazine was used 39 and which fortifies this theory. As a short summary, the following factors are mandatory for nickel spike growth: the formation of a twinning defect, a reducing agent that leads to crystalline deposits, and a complexing agent/surfactant that restricts nucleation on the deposited film. This breakdown

Figure 7: Non-spiky nanotubes obtained by electroless plating with hydrazine as reducing agent and NTA as complexing agent. of prerequisites denotes a starting point for the development of new shape-selective plating reactions. However, a direct transfer remains challenging since many parameters that influence shape-selective plating (e.g. redox potential, coordination chemistry, adsorbate chemistry, (auto)catalytic properties, and nucleation and defect energies etc.) differ from element to element, likewise to colloidal nanoparticle syntheses 40 . After determining and explaining the mandatory factors of spiky nickel deposition, the reason for pore widening during the deposition in polycarbonate templates needs to be identified. The chemical etching of irradiated polycarbonate membranes has been performed and optimized for several years and is often carried out by immersing the irradiated foil in warm sodium hydroxide solution 27,41,42 . The etching process is based on a nucleophilic substitution on the polyester chains driven by the alpha effect of the OH– ion acting as a nucleophile 43 .


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To find out whether OH– is the only substance that etches the polymer, we conducted two more etching experiments: We immersed slices of irradiated but non-etched polycarbonate membranes into two etching solutions for 25 min at 80 ◦C. The first etching solution consisted of the electrolyte for nickel spike deposition but without the nickel sulfate, the second one also omitted the IDA. After 25 min, we investigated the obtained pores under SEM and calculated pore widening rates by dividing the measured diameter by the immersion time. The corresponding images are shown in Figure 8.

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10 nm min−1 . A comparison of the first two values leads to the conclusion that OH– ions are mainly responsible for the etching as by adding IDA, some of the OH– ions are consumed and thus, less ions are available for etching. The even lower pore widening rate which was calculated from the deposition can be explained as follows: On the one hand side, the dense nickel base tube limits the access towards the polymer film around the tube length, on the other hand side, OH– ions are also consumed during the hydrazine oxidation reaction (see below). Thus, the local concentration of OH– ions is lower than in the bulk solution. However, this does not explain why the etching rate at the polymer-metal film interface (vpore , Figure 9a) is much higher than for the bulk polymer (vbulk ) as it was observed earlier in this article. Previous experiments showed that although nanostructures are electrochemically deposited inside the template pores there are still small gaps between nanostructure and polymer allowing the diffusion of reactive species in these gaps 44,45 . But still, if etching along the metal-polymer interface would be in the same order of magnitude as for the bulk, spikes on the outer edges of the nanotube would be much larger than in the center. We assume that in our case such remaining voids provide an initial path for the bath to creep into the polymer template, and initiate etching alongside the tube walls. A theory on this local etching can be formulated based on the oxidation mechanism of hydrazine as published by van den Meerakker in 1981 46 :

(a)

(b)

Figure 8: Obtained pores by etching for 25 min at 80 ◦C using a) a solution containing 1 M NaOH, 212 mM N2H4, and 80 mM IDA, b) a solution containing 1 M NaOH and 212 mM N2H4. The obtained pore widening rates amount to a) 25 nm min−1 , b) 60 nm min−1 .

N2H4

metal

N2H3 + OH– N2H3OH N2H2OH + OH–

For the electrolyte containing only hydrazine and sodium hydroxide, this yields a pore widening rate of 60 nm min−1 , for the electrolyte also containing IDA, a pore widening rate of 25 nm min−1 was calculated. Also, a pore widening rate was calculated for the nickel spike deposition reaction which amounts to

2H + 2 OH–

N2H3 + H

(2)

N2H3OH + e– (3) metal

N2H2OH + H (4) N2H2(OH)2 + e– N2↑ + 2 H2O (5) 2 H2O + 2 e–

(6)

The presence of a metal surface leads to a homolysis of hydrazine (Equation 2). The formed N2H3-radical reacts with an incoming hydroxide ion to hydroxyhydrazine and provides one


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electron for the nickel reduction reaction (Equation 3). Further homolysis of the hydroxyhydrazine (Equation 4) and reaction with another hydroxide ion leads to the formation of dihydroxyhydrazine and another electron for the nickel reduction reaction. The dihydroxyhydrazine is then split into two water molecules and one nitrogen molecule (Equation 5). More electrons can be gained if the formed hydrogen radicals react with hydroxide ions (Equation 6). The other possibility is the combination of two hydrogen radicals to a desorbing hydrogen molecule. While these reactions are generally enabled in the presence of metal surfaces, metals like nickel and cobalt especially boost this reaction as they provide a strong negative reaction potential for hydrazine oxidation 47 . In our case, the plating bath can be considered as metastable. This means that the activation energy barrier for the oxidation of hydrazine is high enough to prevent a spontaneous reaction in the electrolyte. However, the presence of metal surfaces lowers this barrier so that locally at the nickel base tube, the hydrazine oxidation occurs. As explained by van den Meerakker 46 , the hydrazine oxidation involves several intermediates which, at least in the case of the formed radicals, are highly reactive and thus may damage the polymer film directly adjacent to the metal film. This induced damage by the hydrazine oxidation may then facilitate an etching attack by the OH– ions and resulting in a locally increased etching rate around the nickel metal tube and thus explain the pore widening. Furthermore, hydrazine itself can also act as a nucleophile and damage the polymer by a nucleophilic substitution as shown in Figure 9b. The fact that many electroless plating baths are alkaline and contain additional corrosive reagents such as hydrazine or reaction intermediates indicates that the pore widening process is not limited to nickel but is also available to other metals 46 . From only this point of view, the processes described in this paper can be transferred to systems based on other metals. As mentioned before, shape-selective structure growth has also been reported for other systems which further encourages transferability.

However, it is very likely that some effort needs to be put in the optimization of related systems. Apart from the potential influence of hydrazine intermediates on the etching of polycarbonate, the overall etching and deposition reaction can be divided into two reaction interfaces: The first interface exists between polymer and electrolyte (interface PE) and is mainly driven by the etching process that leads to pore widening. The second interface exists between electrolyte and deposited metal (interface EM) and is mainly driven by the reduction of nickel. These two interfaces need to move with similar speed during the reaction. If the etching occurs faster than metal deposition (vPE > vEM ) the polymer template is dissolved before the deposited metal film has reached sufficient thickness to be mechanically stable and thus the metal film will collapse. If the deposition occurs faster than template etching (vPE < vEM ) the space between polymer and metal film will be filled with freshly deposited metal and thus, no spikes will be obtained. However, if the two interface speeds are similar, a controlled nanospike growth is possible. During the investigation of nanospike growth, the formation of a nickel film from which the spikes are emerging was observed. If applied to already formed nanotubes, this film will increase mechanical stability of the nanotubes. To further increase mechanical stability and obtain a free-standing network of nanotubes, we exploited the versatility of ion track-etching process 48 and grew spiky nickel nanotubes in a superstructured nanonetwork polycarbonate template which was irradiated four times with an angle of incidence of 45° and rotated by 90° after each irradiation as it can be seen in the lower pathway in Figure 1. SEM and TEM images of the fabricated structures are shown in Figure 10. Smooth-walled nanotubes obtained after the first plating step are shown in Figure 10a. The controlled template superstructure and the interconnections between single nanotubes remain intact also after the second plating step (see Figure 10b). The obtained TEM images (Figure 10c) prove


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H H

O R1

O

+ N N O

R1

H H

R2

H H + O N⊕ N

R2

H H O H H

H H

O R1

N N O R2

R1

N N O H H R2

H H

(a)

O R1

H + H O N N

R2

H H

(b)

Figure 9: Proposed etching mechanism at the interface of polymer template and Ni metal film. a) Schematical drawing of etching attack by hydrazine and sodium hydroxide, b) reaction pathway of nucleophilic substitution with hydrazine and destruction of polymer chain. The same reaction is possible for hydroxide ions. the presence of spikes on both the inner and outer surfaces of the nanotube network. This shows that our process also works in small confined volumes, challenging substrate geometries and under diffusion limitations. The advantage of nanotube networks (NTNWs) over their counterparts with individual nanotubes is their enhanced mechanical stability, defined open porosity and a continuous electric conduction path through the complete network. The ion track-etching technique allows for an independent variation of all relevant structural parameters (i.e., tube diameter, orientation, density, shape and hierarchy 48 ). Thus, plentiful opportunities exist for tailoring the resulting materials to the demands of specific applications.


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(a)

(b)

(c)

Figure 10: a) SEM image of nickel NTNWs obtained from 10 min deposition in the nickel-borane dimethylamine plating bath. b) SEM image of spiky nickel NTNWs obtained after the second plating step. c) Longitudinal section and cross section of spiky nanotubes indicating spike growth on both inner and outer surface of the smooth tubes.

Conclusion

simultaenous plating can be extended to other materials and templates. The obtained structures can be investigated in their respective application fields, namely catalysis, sensing and energy storage.

In this study, spiky nickel nanotubes were manufactured in ion track-etched polymer membranes. The decoration of the outer surfaces was achieved by a combination of metal plating and simultaneous chemical etching of the polymer template. Growth of the crystalline nanospikes occurs along a central twinning defect with restrained nucleation on the spike surfaces and was proven to be independent from the underlying substrate. Comparative tests with NTA as complexing agent revealed that IDA acts not only as complexing agent but also as capping agent which limits the nucleation on the spike surfaces. We found out that the main driving force for polymer etching are OH– ions complemented by hydrazine. An increased etching rate was observed in the surrounding area of the nickel base tubes indicating that the formation of intermediates during the oxidation of hydrazine, which acts as reducing agent in our electrolyte, accelerates etching. The simultaneous processes of etching and plating allow the formulation of two reaction interfaces which need to proceed at similar speeds for a successful reaction. Based on this study, further effort can be put in the clarification of the local etching mechanism. Also, the combination of etching and

Acknowledgement We would like to thank Prof. C. Trautmann and the material research group (Helmholtz Center for Heavy Ion Research (GSI), Darmstadt) for the irradiation experiments. Furthermore, we would like to thank Dr. Tom Jäpel from Elektronen-Optik-Service GmbH for the recording of 3D tomographies of our spiky Ni nanotubes. M. D. and L. M.-L. acknowledge financial support from the Hessen State Ministry of Higher Education, Research and the Arts via LOEWE Response. L. M.-L. acknowledges funding from the German Research Foundation under DFG project No. M03010/3-1. T. B. acknowledges funding from the German Research Foundation under DFG project No. 342145578.

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Graphical abstract


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Shape-selective electroless plating within expanding template pores

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