Increasing the structural and compositional diversity of ion-track templated 1D nanostructures through multistep etching, plastic deformation, and deposition

In this experiment, a polycarbonate foil containing cylindrical nanopores was stretched by applying a constant force under heating to facilitate plastic deformation. The fabrication scheme and the metal replicas of the stretched pore ends, including the surface layer are depicted in figure 1.

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After the initial etching, cylindrical pores of about 1 μm diameter are obtained. The mean diameters of the nanostructures were estimated based on the pore dimensions, which are highly reproducible given constant and well-defined etching conditions, and thus can be calculated based on the etching rate, with a precision of few tens of nanometers [29]. During mechanical stretching, they shrink in one dimension to 600 nm (variance $\sigma$ = ±0.1 nm), while being extended in the other to roughly 2100 nm (variance $\sigma$ = ±0.2 nm). With this roughly two-fold diameter elongation strongly elliptical pores with an aspect ratio of ∼3.5 are obtained.

This stretching procedure provides a novel route towards membranes inclosing elliptical pores, which can be used to synthesize 1D nano-and microstructures with an additional degree of anisotropy, or to improve the functionality of the membrane itself. For instance, elliptical pores have been found to result in improved flux while maintaining size exclusion in filtration experiments, and the mechanical properties of block copolymer membranes have been shown [42].

Two etching steps targeting biconical and cylindrical pores were consecutively applied to generate a funnel shaped hybrid pore structure (figure 2). In the first step, ion-tracks containing polycarbonate were etched conically, then in the second step funnel shaped structures were produced by following cylindrical etching process. It is important to note that the conical pores do not penetrate through the membrane fast, so that the initial track is maintained, and can be etched out in a second step to form the cylinder section.

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For successful track etching, it is necessary that the track etch rate v_T exceeds that of the isotropic bulk etch rate v_B. The track etch rate v_T correlates with the linear energy transfer of the particle along the track and is higher on the ion entry side than on the exit side. The resulting pore geometry is influenced by the etch rate ratio v_T/v_B and can be adjusted according to it. While v_T 》v_B, then small etch rate ratios lead to produce conical pore geometries. External parameters such as the concentration and temperature of the etching solution, or any applied voltage, can cause conical pores via the etch rate ratio [42].

In this part a strong KOH bath containing methanol was used to produce conical pores on both sides of the polymer membrane. In the second etching step with aqueous NaOH, prior to the breakthrough of the conical pores, results in a double funnel structure, alternatively 'bow tie' or 'dumbbell' structure. Dissolved KOH is a stronger etchant than NaOH for polycarbonate at a given temperature and concentration [43]. By decreasing the interface energy and alcoholytic breakage of ester groups, methanol increases the bulk etching rate v_B of the etching solution [44]. Given an initial nominal template thickness of 60 μm, the metal pore replicas are only 45.3 μm long, achieving an approximate KOH bulk etching rate v_B of 12.9 μm min⁻¹.

A distinct necking can be observed at the transition between cone and cylinder of the metal structures. The tube diameter decreases at the neck to about 0.3 μm. These tapered tube ends can also be observed in the other samples produced, as other research groups have observed in their samples [45, 46]. The reason for this morphology is suggested to be (1) an altered surface layer due to film manufacture [47], (2) the influence of a secondary electron cascade during track formation or (3) an amplifying effect of the etching products [46]. Since micrometers of our surface layer were removed in the initial methanol-KOH etchant, and, more importantly, the necking of the cylindrical pore section occurred in the interior of the template, below the conical pores, we can certainly exclude reasons (1) and (2), since an initial electron cascade as well as altered polymer surface properties cannot explain altered etching properties deep within the template membrane. Since analyses have suggested that etching products as well as surfactants reduce the etching rate [47], another mechanistic factor might be in play, but based on our observations, the origin of the necking cannot be surface related.

The base diameter D_b of the cones is about 4.3 μm, at a length L of 5.1 μm. From this a track etching rate v_T of 24.9 μm min⁻¹ can be derived. The excess of v_T over v_B determines the cone.

$$\begin{eqnarray*}&&{\rm{Half}}-{\rm{angle}}\,\theta ={\tan }^{-1}\left.\left(\,\tfrac{{D}_{b}-{d}_{t}}{2L}\right.\right).\end{eqnarray*}$$

If it is small, the angle becomes large [47]. With an assumed tip diameter d t of 0.1 μm, the calculated apex angle is 44.8°, in close agreement with the average measured angle of 45°. Apex angles of roughly 30° [48] and 40° [49] achieved with other methods. Other work on optimizing the cone angle has focused more on a smaller angle (<20°) and a sharp tip, aimed at using these structures as cold cathodes in field emission devices [50, 51].

The sides of the cone are not curved and run largely straight. Thus, it can be assumed that v_T is approximately constant over time. A modification of this morphology could be achieved with a one-sided conical etching before cylindrical etching to produce metal structures consisting of a cylinder and only one cone [52].

In this part, electro- and electroless deposition techniques were applied to consecutively deposit metal nanowire trunks, which are then enclosed by nanotubes consisting of a different metal, i.e. a cylindrical core–shell structure. At the beginning, short Pt nanowires were electrodeposited in the cylindrical-pores of a polycarbonate membrane. Then, a second etching step was completed to enlarge the pore diameter, following by electroless Ni plating to obtain hybrid Pt–Ni 1D nanostructures.

Figure 3 shows the fabrication scheme of wire-in-tube samples and a SEM image of the segmented metal nanostructures. The length of the Pt wires is about 11 μm, at a diameter of about 240–300 nm. Then again, the surrounding Ni tubes have an outer diameter of 800–1000 nm and range across the entire template thickness. With a tube wall thickness of 60 nm, which can be further reduced over the deposition time, thin-walled tubes for special applications would also be possible. It is important to note that after the etching, the platinum wires might tilt and touch the pore walls, resulting in a non-connecting alignment, such as evident in figure 3 (hybrid wire-tube). Thus, is in accord with related synthetic approaches [53].

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The fact that etching performs well around the nanowires tells us that the nanowires do not completely seal the pores, and that etching can efficiently encroach the sides of the wires and release the wires from the template, making space for a second deposition step. Pore widening might even occur in aggressive deposition solutions, resulting in an in situ modification of the template membrane, affecting the forming nanostructures in the process [38]. The annular gap between the wire and the pore wall remaining after the second etching provides space for a further nanocasting step.

The electroless Ni plating created tubes over the entire thickness of the template, which included the Pt wires, demonstrating our deposition concept. The major advantage of this approach is that the dimensions of the core and the shell can be adjusted independently of each other. The mixed tube-wire morphologies with etching between the depositions ('electrodeposition-etch-electrodeposition') was carried out by Loh et al [54] to synthesize coaxial hetero-nanostructures in which they show an increase of the tube thickness start from below point to upwards and more deposition takes place on the upper side of the wire, which does not lead to any further elongation of the tubular structures than the wire. Another point is the conductive contact in the solution, in which the deposition of the tubes does not occur from all walls at the same time. Compared to their study, we used a different deposition strategy (electrodeposition-etch-electroless deposition) and we achieved homogeneous thickness of tubes. It is important to note that our different deposition method leads to different mechanism in where deposition of the tube is coming from the walls and reaching into the center of the pore in order to complete a certain structure of completely embedded long tubes, wires of independently controllable length, which was not observed in the existing literature. Care are must be taken, where appropriate, to ensure that the materials deposited are in the correct order and that they are inert to the later employed reaction media. For instance, we chose to first deposit Pt and then Ni to prevent galvanic displacement between the metals, which would have occurred if the metal order was reverted. If this issue is considered, several films of different compositions can coat the core in a multilayer coaxial hetero structure. Multilevel nanostructures characterized by interior substructuring are interesting for devising tailored microreactor or sensor architectures [55], and ion-track etching provides a flexible route for rationally constructing such materials.

Metallic bisegmented nanostructures composed of a Pt nanowire topped by a Ni nanotube cap were created in a similar fashion to the wire-in-tube nanostructures, but without the intermediate etching step. For demonstrating this approach, we started with Pt electrodeposition followed by Ni electroless plating to create one-sided Ni nanotubes partially filled with Pt. It is important to note that the Ni shell covers the Pt wire but in areas without Pt wire only the pure Ni shell remains.

The fabrication scheme and SEM images of the tube-on-wire sample are depicted in figure 4. The length of the Pt wires is about 2.2 μm, at a diameter of about 200 nm. Due to the missing etching step, the wires and tubes share the same diameter. The entire length of the pore is used to form the tube during electroless plating, but if electrodeposition is used, the tube length can be adjusted like the wire length via the deposition time. While in this work we focus on different metals and oxides, we want to note that electroless plating and electrodeposition represent versatile methods capable of depositing a range of metals in nanostructured form, as we have previously shown, e.g. in the fabrication of Au, Ag, Pt or Cu nanotubes [27, 36]. Also, other deposition reactions such as layer-by-layer method can be used to introduce different material classes into such nanostructures. Multi-segmented nanotubes and nanowires have been observed by Roy et al using polypyrene electrodeposition or layer-by-layer deposition to create organic nanotubes attached to the top of metal nanowires [7]. However, we are probably the first and show this deposition reaction with metals.

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TEM images including EDS measurements were recorded to verify the elemental composition of the nanotube-nanowire segments (figure 5). Which according to the synthesis scheme depicted in figure 4 should consist of nickel and platinum, respectively. Meeting expectations, mainly Pt can be found in the wire and Ni in the tube. Clear C and O signals stem from the resin, the Cu is assigned to the TEM grid. Embedded boron from the deposition solution can hardly be detected with this method. Pd contained in the tubes from catalytically active nuclei could not be verified because of its low quantity.

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Embedded in resin, it is extremely difficult to cut the nanostructure at a perfect angle in the longitudinal direction. As a consequence, areas corresponding to the nanowire and nanotube se nanotube sections of the hybrid 1D nanostructures were examined separately by EDS (figure 5).

Different synthesized samples were discussed by means of nickel and platinum structures using elelctro- and elelctroless deposition. By combining a template-assisted synthesis for metal nanotube arrays with the autocatalytic deposition of shape-controlled metal nanocrystals, the composition and morphology of the underlying material can be further altered. To illustrate the wide range of possibilities we have produced derivatives with Ni metals and oxide (TiO₂). In this work, we focused on Ni deposition because of the good mechanical stability of the resulting nanostructures, facilitating the production of free-standing architectures enduring the template removal. But the approach is likewise applicable to different metals and even metal combinations.

In the previous sections, the pore geometries of the template were replicated with rather smooth metal wires or coatings, resulting in nanostructures closely matching the original pore shape. Here, we want to showcase how the shape-controlled deposition of anisotropic nanocrystals onto template-assisted 1D nanostructures allows constructing hierarchical architectures. The requirement for the nanospike deposition to smoothly proceed is that the substrate acts as a catalyst in the anodic oxidation of the employed reducer [56], so that electrons are generated on the nanostructures, which are channeled into the reduction of the surrounding nickel complexes, causing the nanospikes to start growing. In the first step, an already etched polycarbonate film was deposited with a thin nickel film using electroless nickel plating bath for a smooth nickel nanotubes production. Then a further specified nickel plating bath (see experimental part) resulted in the formation of funnelled nanotube arrays, over coated with a dense layer of nickel nanospikes. It is noted that the formed Ni nanotubes are already catalytically active and therefore do not require prior seeding. Figure 6 shows the fabrication scheme (left side) and SEM image (right side) of a spiky Ni-nanotube sample.

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On top of the smooth nickel film, nickel spikes have grown on both sides of the structure from the specified deposition solution. Simultaneous etching of the membrane allows deposition also on the outside of the 1D nickel structures. This increases the surface-to-volume ratio while maintaining mechanical stability. Applications of nickel micro- and nanostructures, which depend on the surface size, can therefore benefit from decorating with spikes. Growing along twinning defects, they can be up to 150 nm long with a deposition time of 1 h [38]. When deposition takes more than twice as long, the spikes in our samples have grown up to 500 nm in length and are distributed homogeneously on the surface of the metal structures. Hydrazine, which acts as a reducing agent in the plating bath, also enhances pore widening during electroless deposition. Such a design is interesting to increase the surface area of an underlying architecture (e.g. for enhancing the amount of exposed interface for heterogeneous catalysis or sensing) [57, 58], to achieve bimetallic synergies, or to introduce novel functionalities.

The multi-deposition strategy is not limited to metals, another class of materials can also be deposited on the 1D nickel structures, in this case oxides. To demonstrate this possibility, we combined the Ni nanofunnel arrays obtained by electroless plating with a consecutive chemical bath deposition to overcoat them with a nanostructured titania coating. A fabrication scheme of TiO₂ deposition on Ni funnel nanotubes can be found in figure 7 (left upper side). First, conical etched polycarbonate film was coated with electroless Ni bath (step 1), then a chemical bath solution (step 2) contain Ti³⁺, which is continuously oxidized over time, resulting in a supersaturation of the solution with respect to Ti⁴⁺, resulting in the nucleation and growth of a TiO₂ nanocrystal film [36]. Due to its mechanism, this deposition is less selective than the previously employed autocatalytic nanospike modification reaction, resulting in homogeneous nucleation in the bulk solution alongside the growth of a nanostructured coating on the metal nanostructures. Removing the template reveals the core–shell structure with a rough TiO₂ inner surface due to low conductivity and a smooth Ni outer surface, which are easily discernible in SEM figures 7(a), (b).

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Both layers consistently reproduce the pore geometry. Hetero-nanostructures composed of a metals and metal oxides can exhibit synergies and multifunctionality and are interesting e.g. to create fuel cell catalysts [40] or reusable photocatalysts [59].