Metal Nanotubes and Nanowires with Rhombohedral Cross-Section

Article pubs.acs.org/Langmuir

Metal Nanotubes and Nanowires with Rhombohedral Cross-Section Electrolessly Deposited in Mica Templates Falk Muench,* Ulrike Kunz, Hans F. Wardenga, Hans-Joachim Kleebe, and Wolfgang Ensinger TU Darmstadt, Department of Materials and Geoscience, Alarich-Weiss-Straße 2, 64287, Darmstadt, Germany ABSTRACT: Electroless plating is a facile wet-chemical process for the creation of metal thin films on arbitrary substrates, which can be used to produce intricate nanomaterials. In this study, we demonstrate how nanotubes and nanowires can be electrolessly deposited in the rhombohedral pores of ion-track etched muscovite mica templates. Mutual optimization of the activation and plating reactions proved to be essential for the fabrication of well-defined nanostructures of an aspect ratio (lengthto-diameter) of up to approximately 70. By repeating the activation procedure utilizing the redox couple Sn(II) and Ag(I), a high density of Ag nanoparticle seeds could be deposited on the template surface, which was required to initiate metal film nucleation with nanoscale homogeneity. Furthermore, it was necessary to adapt the plating reaction to ensure sufficient diffusion of the reagents into the depth of the template pores. To prove the flexibility of the process and to evaluate the effect of the intrinsic film morphology on the shape of the resulting nanostructures, three different plating reactions were applied (Ag, Au, Pt). If the size of the deposited metal particles approached the dimension of the template pores, only wire-like structures of moderate shape conformity were obtained. Electroless plating protocols which yield homogeneous deposits consisting of small nanoparticles allowed exact replication of the pore shape. Under consideration of the above-mentioned requirements, electroless plating displays an effective and versatile route toward the fabrication of parallel arrays of angular metal nanotubes and nanowires in the chemically and thermally robust mica templates. By simply immersing the templates in aqueous plating solutions for an appropriate time, well-defined metal nanomaterials for application in, for example, plasmonics, catalysis, or molecular separation are obtained.

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utilized processes12 and to further improve the synthetic control (e.g., to enable the creation of extreme aspect ratios,12 well-defined surface features,13 or new nanostructure cross sections).14 The two perhaps most commonly employed types of passive, hard templates are ion-track etched polymers4 and porous anodized alumina.15 Both template types contain parallel pore arrays, which are generated by self-organization in the case of anodized alumina and by selective removal of the material around the damaged zone of the parallel ion trajectories in the case of ion-track etched polymer membranes. By filling these templates with different materials, replicas of the pore structure are obtained, which remain mostly limited to rounded cross sections, although this generic morphology can be modified in a number of ways along the pore axis (e.g., branched pores,16 conical or biconical pores,4 cigar-shaped pores,4 pores with additional sheet-like cavities).17 For the fabrication of one-dimensional nanostructures with angular cross-section, ion-track etched muscovite mica templates are especially well suited.18−21 This crystalline material has a layered structure with pronounced anisotropy.22 Owing to its long-range order, straight fronts defined by the direction of the slowest etch rate are obtained once the severely damaged material alongside the linear ion-tracks is selectively

INTRODUCTION Highly anisotropic metal nanostructures of the 1-D type such as nanotubes (NTs) and nanowires (NWs) have attracted considerable attention1−4 due to their interesting functional properties which can substantially outperform common spherical metal nanoparticles (NPs). Contrary to other inorganic, readily NT-forming materials such as C, BN, or MoS2,5,6 metals with their mostly very symmetric crystal lattices tend to avoid thermodynamically unfavorable, one-dimensional morphologies with high surface area.7 To enforce the directed growth of these structures, the application of template-based methods has proven particularly useful.4,6,8,9 While metal deposition in templates allows an adjustment of important compositional and morphological parameters quite independently, in a large range and with relative ease,4,6,8,9 alternative approaches such as shape-controlled, colloidal NP synthesis,7,10 or the exploitation of intrinsic anisotropy11 rely on more complex mechanisms which are difficult to control. Furthermore, they are generally less transferrable to other systems and often yield complex product mixtures, while templates enable the fabrication of morphologically comparable NTs or NWs consisting of various metals or alloys.4,9 However, as important benefits of colloidal approaches, improved scalability and the often excellently defined crystallinity of the products can be named.10 Remarkable advances have been achieved in the template-based fabrication of metal NTs and NWs3,4,6 since the advent of this approach.8 Nevertheless, considerable efforts are undertaken to enhance the simplicity and reliability of the © XXXX American Chemical Society

Received: April 4, 2014 Revised: August 18, 2014

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etched out.21,22 X-ray diffraction investigations revealed that the resulting walls of the typically observed pores with rhombohedral cross-section correspond to the oxygen-terminated {110} planes in the muscovite lattice.21 One-dimensional metal nanostructures with asymmetric cross-section exhibited a pronounced polarization dependence of their optical properties.14,20 Also, metal nanostructures with morphological features of high curvature (e.g., the edges of angular NWs) often show improved electromagnetical field enhancement.23 The possibility to obtain noncylindrical NWs and NTs therefore renders mica templates useful for the fabrication of nanostructures for plasmonic sensing.24 Aside their unique pore shape, mica templates are also interesting because of their excellent chemical, thermal, and electrical stability. For instance, iontrack etched mica was used for microfiltration under aggressive conditions25,26 and could be applied to examine the magnetic properties of embedded Ni NWs up to a temperature of 700 K.21 This also extends the synthetic possibilities to deposit materials inside ion-track etched mica. Polymer templates can only be used in a limited temperature range and are relatively susceptible to chemical attack,17 while alumina is etched by both strong acids and bases due to its amphoteric character. In contrast, special conditions are required to dissolve mica (e.g., using hydrofluoric acid, such as that applied in the ion-track etching process).21,22 To our best knowledge, all papers concerned with the fabrication of metal nanostructures in mica templates apply electrodeposition, a method which favors the creation of NWs.18,20,21 Accordingly, there are no reports on mica-derived metal NTs, which would display a particularly interesting, hollow, and angular morphology. In this study, we will therefore examine the applicability of electroless plating to ion-track etched mica, a method well suited for the creation of NTs in porous substrates.9,17,24 In contrast to the electrodeposition technique, electroless plating does not need conducting substrates and special instrumentation, is easily scalable, and promotes tube formation due to its growth mode which is based on metal deposition simultaneously occurring on the whole substrate surface.27 However, complex shaped templates such as ion-track etched membranes possessing narrow and extended pores cannot be homogeneously metalized using conventional electroless plating reactions.28,29 Furthermore, to initiate the metal deposition, a substrate usually has to be covered with catalytically active NPs acting as seeds for the consecutive metal film deposition.9,30,31 These pretreatments severely affect the quality of the deposited nanostructures and depend on the surface chemistry and porosity of the particular substrate. Therefore, the utilization of new template types for the electroless fabrication of nanomaterials necessitates adapted activation procedures.32 In the following, we will demonstrate how these challenges can be overcome to produce metal NTs and NWs of uncommon shape and high aspect ratio within ion-track etched mica membranes. The flexibility of the process regarding the deposited metal type is verified by using three different plating baths, yielding Ag, Au, and Pt nanomaterials. Because of the remarkable dependence of a nanostructures’ properties on its shape,10,24 synthetic guidelines will be provided as how to obtain well-defined nanostructures of varying morphology. Since only a few papers are concerned with the mechanistic details of electroless NT and NW deposition,33,34 the insights provided by this study are also of general value and can be

helpful to, for example, transfer the results to other metals, template types, or pore geometries. The outlined approach just involves the immersion of the templates in different reaction solutions. It is highly versatile and outperforms recent template-based 1D metal nanomaterial syntheses in terms of experimental effort and morphological control. For instance, the wall thickness of electrodeposited Au NTs was adjusted by an intricate multistep process including the predeposition of collapsing polymer cores,35 while the same parameter can be addressed in electroless plating by simply changing the deposition time. NTs created by sputter-coating remain limited to very small aspect ratios,36 and conventional, quickly proceeding chemical deposition reactions result in irregular wall structures and relatively short NTs and nanorods not spanning the whole template length.37 On the contrary, optimized electroless plating represents a facile route toward extended, homogeneous and self-supporting NTs and NWs.

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EXPERIMENTAL SECTION

Chemicals. Milli-Q water (> 18 MΩ cm at room temperature) was employed in all procedures. The following chemicals were applied without further purification: 4-(dimethylamino)pyridine (Fluka, puriss.); AgNO3 (Grüssing, p.a.); ethanol (Brenntag, 99.5%); ethylenediamine (Fluka, puriss.); formaldehyde solution 36.5% stabilized with methanol (Fluka, puriss. p.a.); HF aqueous solution 40% (Sigma-Aldrich, puriss. p.a.); H2PtCl6 solution 8% in water (Sigma-Aldrich); methanol (Aldrich, 99.8%); N2H4 monohydrate solution 80% in water (Merck, for synthesis); Na2SO3 (Merck, p.a.); NH3 solution 33% in water (Merck, puriss.); (NH4)3[Au(SO3)2] solution (Galvano Gold Bath from Schütz Dental GmbH, 15 g Au 99.9% per liter); potassium sodium tartrate tetrahydrate (Fluka, puriss. p.a.); SnCl2 dihydrate (Sigma-Aldrich, ACS reagent); trifluoroacetic acid (Riedel-de Haën, >99%). Template Preparation. Single-crystalline mica sheets (Richard Jahre GmbH) of ∼16 μm thickness were irradiated with Au ions (kinetic energy = 11.4 MeV per nucleon, fluence = 108 ions cm−2, initial charge state, Au25+) at the GSI Helmholtzzentrum für Schwerionenforschung GmbH (Darmstadt, Germany). Subsequently, the mica sheets were etched with 20% HF at room temperature to create rhombohedral pores. Extreme caution is required when working with HF (protection equipment and emergency medication have to be considered), as it easily penetrates the skin and is highly toxic. The etching time was adjusted depending on the desired pore diameter (etching rates under the applied conditions: approximately 20 nm min−1 for the long and 12 nm min−1 for the short pore axis). Template Pretreatment and Electroless Plating. Before electroless plating, the etched mica templates were subjected to a two-step procedure to introduce Ag NPs on the template surface.9 First, the mica template was immersed in a sensitization solution (42 mM SnCl2 and 71 mM trifluoroacetic acid in MeOH/water = 1:1) for 45 min. After the sensitized template was washed twice with ethanol, it was immersed in an activation solution for 3 min (59 mM AgNO3 and 230 mM NH3 in water). After the template underwent two washing steps with ethanol, the procedure was repeated to increase the Ag NP loading (the number of iterative activation cycles is stated in the text). Finally, the activated templates were subjected to electroless plating by storing them in previously established plating baths optimized for NT synthesis. Details concerning the properties and preparation of these solutions can be found elsewhere.1,28,29 Briefly, the Pt bath1 was composed of hexachloroplatinic acid (metal source), ethylenediamine (ligand), and hydrazine (reducing agent), the Au bath28 consisted of disulfitoaurate (metal source), Na2SO3 (excess ligand), formaldehyde (reducing agent), and 4-dimethylaminopyridine (reaction moderator), and the Ag bath29 contained AgNO3 (metal source), ethylenediamine (ligand), tartrate (reducing agent), and trifluoroacetic acid (pH adjustment). The Ag and Pt plating reactions were performed at 8 °C and room temperature, respectively. Au plating was performed at either 8 °C or at room temperature as stated in the text. B

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Characterization. Scanning electron microscopy (SEM) (JSM7401F microscope (JEOL), 5−10 kV acceleration voltage): To isolate the nanostructures for the SEM measurements, the mica matrix was selectively removed with 20% HF by extended etching at room temperature (special caution required, see preceding section). The remaining material was collected on Si wafer pieces. Transmission electron microscopy (TEM) (CM20 microscope (FEI, Eindhoven, The Netherlands), 200 kV acceleration voltage, LaB6 cathode): The nanostructure-containing templates were embedded in Araldite 502 (polymerization at 60 °C for 16 h) and examined as ultrathin sections (70 nm thickness, Ultracut E ultramicrotome (Reichert−Jung) equipped with a diamond knife (DKK)). X-ray photoelectron spectroscopy (XPS) (PHI 5700 spectrometer (Physical Electronics), monochromatic Al Kα radiation with hν = 1486.6 eV): The mica foils were directly mounted on a sample holder. Aside an ion-track etched reference sample, mica membranes were analyzed which had been subjected to either a single sensitization step or one full activation cycle just before the measurements. Because of the the insulating nature of the substrate, the measurements were carried out with a charge neutralizer, and all spectra were calibrated by using the C 1s peak at 284.6 eV as a constant reference.

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RESULTS AND DISCUSSION Activation of the Mica Templates. Electroless metal deposition reactions are based on metastable solutions containing a metal complex and a reducing agent. Because metal reduction is kinetically hindered, homogeneous precipitation such as in the case of colloidal NP syntheses10 is prevented. To start heterogeneous metal reduction, catalytically active sites are required on the substrate surface. Once a metal film has been plated, the reaction is carried on by autocatalysis, leading to continuous, surface-conformal deposition. In the case of substrates which do not initiate electroless plating, pretreatments are applied to them which introduce metal NPs acting as seeds for consecutive plating. These so-called activation reactions are also needed for electrolessly plating mica.38 In our syntheses, we applied a two-step activation technique based on the precipitation of Ag NPs using the redox couple Sn(II)−Ag(I), which was introduced to the template-based nanomaterial fabrication by Martins group.39 The effect of each reaction step on the surface chemistry of the mica membranes was analyzed by XPS (Figure 1). Initially, a reference measurement was performed to identify the surface composition of the ion-track etched mica membrane (Figure 1a). Besides the peaks of the elements constituting the ideal muscovite formula K2Al4[Si6Al2O20](OH,F)4,40 additional peaks relating to C, Mg, and Co were found. The latter signals were explained by carbonaceous species of atmospheric origin, Mg cations which are commonly incorporated in the mica lattice41 and Co impurities within the mineral. As the F peak is most pronounced in the case of the reference sample, it is probable that the surface of the material was enriched with fluoride-containing species during the track-etching process. In the first synthetic step, the template was immersed in a sensitization solution containing SnCl2 as a source of Sn(II) ions, followed by thorough washing during which excess solution and insufficiently bound species were removed. The sensitization solution was acidified to reduce the oxidation of Sn(II) to Sn(IV)42 and to prevent the precipitation of oxidic Sn compounds. Because the pores created by ion-track etching mica are terminated by oxide ions,21 oxygen-bridged surface complexes with Sn(II) are probably formed during sensitization.42,43 Another possible mechanism of Sn uptake is the exchange of intercalating K cations between the alumosilicate

Figure 1. XPS spectra of differently treated mica foils, including (a) ion-track etched mica, (b) a sensitized sample, and (c) a sensitized and activated sample. For the sake of clarity, each peak is only denoted at first appearance. In panels b and c, the 3d regions of the elements introduced during the sensitization and activation steps are highlighted.

layers, which is less likely due to the large differences in the ionic radii and the mild reaction conditions.43 XPS characterization of the obtained product clearly proved the presence of Sn on the mica surface (Figure 1b). However, due to the vanishing difference between the binding energy of surfacebound Sn(II) and Sn(IV)42 and the error introduced by the charge neutralization, detailed information regarding the chemical speciation could not be obtained. A binding energy of 486.2 eV was found for the Sn 3d5/2 peak of the sensitized mica sample, which is close to the value of 486.5 eV reported for both Sn(II)- and Sn(IV)-modified glass surfaces.42 The absence of significant amounts of superficially bound Cl indicates a full solvolysis of the SnCl2 precursor, resulting in an oxygen-dominated coordination environment of the Sn ions. This result concurs with sensitization experiments performed on glass.42 In the second step, the sensitized mica template was transferred to an activation solution containing Ag(I). Here, a slight color shift to brown occurred, indicating the precipitation of Ag NPs on the template surface according to Sn(II) + 2 Ag(I) → Sn(IV) + 2 Ag. In accordance with this observation, XPS measurements of the activated sample verified the presence of Ag (Figure 1c). A 3d5/2 binding energy of 367.9 eV was observed, which is in the range of values reported for elemental Ag in other nanomaterials.44,45 Additionally, it was found that the Sn which was accumulated during sensitization basically remained on the substrate surface (Figure 1c). The Sn binding energy adopted a value of 486.2 eV and thus did not significantly change during activation. As discussed above, this value corresponds to cationic Sn, but does not allow the identification of a distinct oxidation state. C

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Because the surface chemistry of mica differs from that of other template types, the activation process had to be optimized to ensure a sufficient seed density for the plating of nanoscale materials.9,30,31 This was achieved by repeating the two-step procedure described above several times (Figure 2).

Figure 3. TEM images of mica templates activated (a) twice, (b) fourfold, and (c) six-fold. The unusual pore shape in panel a is caused by percolation of individual pores. NP deposits are marked with arrows. In panel d, a magnified image of a pore wall of the six-fold activated sample is shown.

on the morphology of the resulting nanostructures will be demonstrated using electroless Au plating. In the beginning, one of these factors is optimized, while the second is held in a nonoptimized condition. Finally, it will be shown that with successive optimization of both parameters, well-defined onedimensional nanostructures of high aspect ratio can be obtained. At low seed densities, the structures obtained by electroless plating tend to consist of isolated or just slightly connected metal islands instead of closed nanofilms.9,30,31 Accordingly, using Au plating on a two-fold activated template containing few seeds (Figure 3a), only ill-defined NTs were found aside poorly linked, cluster-like NPs (Figure 4 a,b). The pore size (hereafter defined as the long distance between the two pore corners with acute angles) was ∼220 nm in these experiments. Besides sufficient seed density, a relatively low plating rate is required to homogeneously cover the extended inner surface of porous templates.28,29 If the deposition reaction is too fast, the plating reagents are consumed on the well accessible parts of the substrate (outer surface, pore openings) to a large extent and thus are depleted in the inner template regions. This leads to inhomogeneous metal deposition alongside the pores with a reduced amount of metal deposited in the deeper regions.28,29,34 Figure 4c,d shows an example of Au NTs of ∼450 nm diameter fabricated using an unfavorably quick deposition which occurred when performing the plating reaction at room temperature. After few hundred nanometers, the NT walls became porous and brittle, indicating a reduced amount of plated metal compared to the closed and mechanically robust surface film (Figure 4d). Therefore, the NTs broke off after removal of the template matrix, and free-standing fragments of only short aspect ratio could be obtained.

Figure 2. Scheme of the activation process applied to the mica templates, showing a cross-section of a pore created by ion-track etching. During sensitization, the substrate surface is covered with Sn(II), which reduces Ag(I) in the activation step, leading to the formation of Ag NPs. The process can be repeated n times to increase the amount of deposited Ag.

To gain insight into the seed deposition, cross sections of mica templates treated using varying numbers of activation cycles (n = 2, 4, 6) were analyzed by TEM (Figure 3). Owing to the brittleness of the mica template, the material heavily fragmented during microtome cutting in the absence of metal NTs or NWs. Nevertheless, typical pores of rhombohedral shape with angles of ∼60° and 120° resulting from the iontrack etching could be identified,19,21 which allowed an examination of the density and size of the NPs deposited on the pore walls. With two-fold activation, nearly no NPs were found (Figure 3a). This result contrasts the behavior of polycarbonate templates, which already are covered with a dense layer of Ag NPs after a single activation step,9 facilitating the reliable plating of for example, Au NTs.9,39 Compared to this substrate type, the activation of mica proceeds with a lower efficiency, probably caused by a reduced adsorption of the reagents used in sensitization or activation. However, by applying more activation cycles, a good coverage with NP seeds was achieved (Figure 3b−d). The NPs usually have a maximum size of ∼10 nm (Figure 3d), but in some cases, larger particles with a size of some tens of nanometers were found (Figure 3c, arrow). Synthesis Optimization. In this section, the effect of two most important parameters (seed loading, deposition kinetics) D

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slowed down by decreasing the pH of the plating bath.29 In the depositions performed here, the pH value was therefore adjusted to 10.9 with trifluoroacetic acid. The Pt plating reaction was regulated by complexing the Pt(IV) precursor with 2.4 mol equiv of ethylenediamine.1 As described above, Au plating was performed at a reduced temperature of 8 °C. The deposition of the respective metal type (Ag, Au, Pt) was confirmed by X-ray analysis (SEM-EDS, data not shown). Beginning with Au plating, even in the case of small pores of 220 nm diameter, the template structure is well reproduced (see Figure 4e,f). To gain more information about the morphology of the obtained NTs, TEM measurements were performed (Figure 5a). The results reveal some inhomogeneity

Figure 4. Electron micrographs of Au nanostructures deposited within mica templates. (a,b) TEM images of cross sections of a mica template containing ill-defined NTs and cluster-like Au NPs which were deposited using two-fold activation (plating temperature: 8 °C). (c,d) SEM images of template-freed NT fragments of low aspect ratio obtained in a four-fold activated template at a relatively high deposition rate (plating at room temperature). (e) SEM survey image of a field of free-standing Au NTs of ∼220 nm diameter plated at 8 °C in a four-fold activated template. (f) Magnified SEM image of a nanostructure shown in image e, revealing the tubular structure.

Figure 5. (a) TEM image of the cross-section of a small NT (diameter, ∼220 nm) obtained with optimized Au deposition. It appears that the microtome cutting compressed the NT from the top (indicated by arrow), laying the region around the upper pore wall open. (b) SEM survey image of a field of free-standing Au NTs with a large diameter of ∼1.6 μm. (c) Magnified SEM image of a nanostructure shown in image b. (d) TEM image of the cross-section of a Pt NT of ∼450 nm diameter with a very narrow remaining cavity in the center.

By combining sufficient activation and adapted deposition kinetics (four-fold activation, plating at 8 °C), well-defined and free-standing NTs could be fabricated even using a small pore diameter of ∼220 nm (Figure 4e,f). Considering the template thickness of ∼16 μm, this corresponds to a high aspect ratio (nanostructure diameter/pore length) of about 70, which is a dramatic increase compared to the experiments shown in Figure 4c,d. Fabrication of High Aspect Ratio NTs and NWs. In this section, the morphology of electrolessly synthesized Ag, Au, and Pt nanomaterials will be related to the characteristics of the corresponding plating reactions and the template pore size. In each case, optimized reaction conditions were used. For electroless Ag and Au plating, the mica templates were activated four-fold, while for Pt plating, six-fold activation was applied. The increased seed loading for Pt plating was required because of the less reliable nucleation of this metal on Ag NPs.9 Also, all deposition reactions were adapted. From previous experiments, it is known that electroless Ag plating can be

in the wall thickness, corresponding to the intrinsic roughness of the Au film consisting of merged Au clusters which nucleated on the Ag seeds (Figure 4b). However, the pore edges are accurately covered with a closed Au film, indicating a high density of nucleation sites (Figure 5a). Both the low nucleation distance and the Au film homogeneity which are vital for the realization of robust and well-defined NTs of low diameter clearly surpass previously published efforts.33,34 In templates with a significantly increased pore diameter of ∼1.6 μm, Au plating yields NTs which finely reproduce the pore shape (Figure 5b,c), because in this case the inherent roughness of the Au film is negligible compared to the dimension of the replicated structure. In the case of electroless Pt deposition optimized for NT fabrication, the synthesized metal films consist of small NPs of