LETTER pubs.acs.org/NanoLett
Highly-Ordered Supportless Three-Dimensional Nanowire Networks with Tunable Complexity and Interwire Connectivity for Device Integration Markus Rauber,*,†,‡ Ina Alber,‡ Sven M€uller,‡ Reinhard Neumann,‡ Oliver Picht,‡ Christina Roth,† Alexander Sch€okel,† Maria Eugenia Toimil-Molares,‡ and Wolfgang Ensinger† † ‡
Department of Material- and Geo-Sciences, Technische Universit€at Darmstadt, Petersenstrasse 23, 64287 Darmstadt, Germany Materials Research Department, GSI Helmholtzzentrum f€ur Schwerionenforschung GmbH, Planckstrasse 1, 64291 Darmstadt, Germany
bS Supporting Information ABSTRACT: The fabrication of three-dimensional assemblies consisting of large quantities of nanowires is of great technological importance for various applications including (electro-) catalysis, sensitive sensing, and improvement of electronic devices. Because the spatial distribution of the nanostructured material can strongly influence the properties, architectural design is required in order to use assembled nanowires to their full potential. In addition, special effort has to be dedicated to the development of efficient methods that allow precise control over structural parameters of the nanoscale building blocks as a means of tuning their characteristics. This paper reports the direct synthesis of highly ordered large-area nanowire networks by a method based on hard templates using electrodeposition within nanochannels of ion track-etched polymer membranes. Control over the complexity of the networks and the dimensions of the integrated nanostructures are achieved by a modified template fabrication. The networks possess high surface area and excellent transport properties, turning them into a promising electrocatalyst material as demonstrated by cyclic voltammetry studies on platinum nanowire networks catalyzing methanol oxidation. Our method opens up a new general route for interconnecting nanowires to stable macroscopic network structures of very high integration level that allow easy handling of nanowires while maintaining their connectivity. KEYWORDS: Nanowire assembly, 3D architecture, electrocatalysis, electrochemical deposition, ion track-etched membranes, nanowire network, platinum nanostructures
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or the realization of new functional devices based on nanowires, the development of methods that allow the organization of nanostructures into integrated arrangements is crucial. Very high integration levels, which are required for many applications including energy harvesting, (electro-)catalysis, and optoelectronics, can only be achieved by three-dimensional (3D) assemblies.14 Consequently, research in patterning of 1D nanostructures into various device architectures has led to the synthesis of 3D nanowire superstructures such as arrays, networks, and hierarchical structures.57 Nanowire networks (NWNs) are of special interest, since each individual nanowire can be directly associated in different ways with a varying number of wires, providing a large diversity of potential kinds of interconnectivity. General synthesis strategies for nanowire networks are based on direct growth of complex nanowire arrangements or the assembly of nanowires. Direct synthetic approaches are more suitable for the fabrication of 3D architectures. Among these direct methods, most notably the use of hard template materials proved to be an efficient approach to control morphology and dimensions of the building blocks precisely, whereas templateless r 2011 American Chemical Society
methods, such as epitaxial growth or vaporliquidsolid processes, demonstrated the capability to produce highly ordered nanowire superstructures.811 Although some of the methods reported so far highlight the possibility to adjust more than one structural parameter, it is, however, often not possible to independently and simultaneously control several of the parameters, defining both (i) characteristics of individual nanowires, such as diameter, length, and composition, and (ii) the arrangement of nanowires into networks, determined by nanowire orientation and integration level. To overcome these limitations and apply nanowires in real-world devices, it is essential to develop new effective strategies that enable the controlled synthesis of nanowire network structures with adjustable complexity. Current mesoporous silica templates exhibit uniform but limited pore dimensions resulting in networks with thin nanowires that measure only a few nanometers in diameter.8 As a consequence, these nanowire structures often Received: February 16, 2011 Revised: May 10, 2011 Published: May 24, 2011 2304
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Figure 1. Schematic of the template fabrication and formation of a 3D nanowire network. A polymer foil (a) is irradiated in several steps from different directions, in each case at an angle R, as indicated by arrows. Subsequent etching leads to the formation of a 3D nanochannel network (b) that can be filled with the desired material. After removing the polymer matrix from the embedded nanostructures (c), a freestanding 3D nanowire network (d) is obtained.
show poor mechanical stability and restricted accessibility due to agglomeration. Here we combine specifically designed template materials, featuring controlled arrangement of nanochannels, with electrochemical deposition to produce highly ordered nanowire networks. The network structures consist of well-defined interconnected nanowires with controlled morphology and composition. Networks made of Pt were investigated by cyclic voltammetry (CV) to determine the active surface area and the activity as electrocatalyst for methanol oxidation. Compared to commercially available Pt nanoparticle catalysts, the electrocatalytic activity proves to be remarkably high despite the relatively low specific surface area. Furthermore, NWNs can be easily manipulated, readily provide interfaces between nanowires and electrical contacts, and thus are suitable for device integration. Results and Discussion. The basic concept of our approach relies on the ion-track template electrodeposition method that was used to grow arrays of parallel aligned nanowires with excellent control over the wire morphology and crystallinity.12,13 Although this synthesis route has been known for decades, further development mainly focused on the growth process to produce novel or improved 1D nanostructures using template materials with parallel aligned nanochannels; only very few reports deal with nanowire assemblies deviating from arrays.14 Starting from this conventional fabrication technique, we extended the method to organize nanowires into more complex structures by modifying the template fabrication process. The main steps of NWN fabrication are schematically shown in Figure 1. In brief, a polymer foil is through-irradiated systematically with energetic heavy ions in several steps (each from a different direction at an incident angle R with respect to the surface of the polymer) in such a way as to enable crossing of ion tracks. Subsequent chemical etching of the tracks to nanochannels with the desired diameter leads to the formation of a 2D or 3D nanochannel network, serving as template. By electrochemical deposition of a chosen material into the nanochannels, the shape of the template is adopted, and thus well-defined interconnected nanowires are created. Afterward, the arising network structure can be liberated from the template material by means of an organic solvent, resulting in a freestanding nanowire network. According to requirements, the metal layer, which served as cathode for nanowire deposition, can be selectively removed. Note that two steps of the fabrication process are decisive with regard to the overall structure: (i) the irradiation step defines the integration density and the absolute and relative orientations of
the nanowires, and (ii) the electrochemical deposition process has a strong influence on the composition and crystal structure of the nanowires. We conducted our study on platinum but also demonstrate the method’s general applicability by fabricating NWNs consisting of different metals and semiconductor nanowires (Au, Pt, CdTe, Supporting Information, Figures S3S5). In addition, the templates can be filled by electroless deposition processes resulting in the creation of nanotube networks. A two-step solution process, which is based on the initial formation of nanoparticles followed by the deposition onto the template surface in the second step, was employed to produce nanotube networks consisting of an iron oxide compound. These nanotube structures are often less robust than networks of nanowires and are not stable after the polymer dissolution (Supporting Information, Figure S7). It is indicated that for the selection of the templates and of the network materials the wide range of materials reported for nanowire arrays by template-based methods can be used, including metals, semiconductors, and composite materials.15,16 In the case of Pt, NWNs have been synthesized using soft and hard templates, and other approaches such as solution-based reduction processes.2,1720 Characterization by field-emission scanning electron microscopy (FESEM, Figure 2ac) reveals that the surface of the Pt network structures, as shown in Figure 2a, appears homogeneous at low magnification; individual nanowires, creating an open porous network, can be identified at higher magnification (inset of Figure 2a). The 1D building blocks are directly connected to each other by junctions consisting of material, which is an inherent part of each crossing nanowire forming a metallic bond. Consequently, the wires act not only as functional elements but also as interconnects. These stable connections can be observed in Figure 2b. Several cross-junctions, representing a fundamental kind of interconnection that is necessary for complex network structures, are visible. The mechanical stability depends strongly on nanowire diameter, network dimensions, and degree of crosslinking. As demonstrated by imaging a cross-sectional area in Figure 2c, even very thin networks, consisting of nanowires with an average diameter of only 35 nm, are stable, if the integration density is sufficiently high. Optical images (Figure 2d,e) prove that the networks exist as macroscopic objects without the need for a support. A typical sample is depicted in Figure 2d with lateral dimensions of more than 1 cm2 and 30 μm in thickness. Because of the high mechanical stability, the freestanding NWNs can easily be 2305
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Figure 2. Images of Pt nanowire networks. (a) Low magnification FESEM image of a piece cut from a larger NWN. The inset shows an area with higher magnification. (b) Interconnected nanowires of a network; junctions of 1D building blocks, illustrating the connectivity, are clearly visible. (c) Cross-section of a thin and mechanical stable NWN (nanowire diameter is 35 nm). (d) Optical image of a nanowire network of macroscopic size. (e) Robust network structure that can be manipulated with tweezers. (f) Water contact angle measurements of a Pt NWN.
handled and manipulated by tweezers. The assemblies allow the investigation of macroscopic effects like the wettability (Figure 2f). Pt NWNs are very hydrophobic with a water contact angle of 129°, which was measured by a video contact angle instrument. The method allows adjusting the diameter, length, orientation, integration level, and degree of connectivity of the nanowires at once. Thereby, the network porosity can be tuned over a wide range. No other existing method can achieve all this together. Using this approach, continuously connected structures with lateral size of several square centimeters and up to 60 μm in height can readily be synthesized. The fundamental limits of the approach in terms of template dimensions are defined by the projected ion range in the template material, which depends on the ion species used for irradiation and its kinetic energy E. In the case of heavy ions like Au with E = 10 MeV/u, the template cannot be much thicker than 100 μm if the angle of incidence R = 45°. The smallest nanochannel diameter that typically can be achieved in polycarbonate is ≈10 nm. However, due to the etching process it is not possible to obtain nanochannels of cylindrical geometry for an aspect ratio of diameter to length, which is larger than approximately 1:1000. The networks with the highest integration level in this work consisted of more than 1 1013 NWs/cm3 and a by multiples higher number of branching points. Increasingly complex structures arise with the number of integrated
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Figure 3. FESEM images of Pt nanowire networks. (a,b) Differently oriented nanowires demonstrating specifically generated branching geometries. Arrows correspond to the growth directions of the wires. The network templates have been irradiated from 4 and 8 directions, respectively. (c) Highly ordered nanowire assemblies that were disconnected from the NWN maintain the orientation of the wires. (d) Node of a NWN consisting of segmented wires synthesized by a pulse-reverse electrodeposition method. (e) NWN consisting of nanowires with an average diameter of 20 nm. The wires lost the orientation after template removal, but are still interconnected to a macroscopic object. The inset depicts the network at higher magnification.
nanowires, leading to intensified interwire communication and the creation of new functionalities. These structures, consisting only of nanowires, provide the excellent capability to investigate how different combinations of nanowires act in comparison to individual nanowires. The branching geometries are determined by the irradiation protocol, which had been applied during the template fabrication process. Adjusting the total number of irradiation steps and the angle and direction of incidence of the heavy ions that travel through the polymer foil during each irradiation step allows precise control over the branching geometries. The formation of a simple cross-junction, needed for a 2D network, requires two irradiation steps. More complex branching geometries are obtained by irradiating repeatedly. Panels a and b of Figure 3 display NWNs whose fabrication involved 4 and 8 irradiation steps, respectively. From these images, the different growth directions of the wires, indicated by arrows, can be observed. Furthermore, the maximum number of nanowires crossing in one junction may be derived. The anisotropy induced by the growth directions is also evident in nanowire arrangements that were disconnected from the total network probably due to imposed mechanical stress (Figure 2c). Assemblies originating from networks that were grown in templates, irradiated in 4 evenly distributed 2306
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Figure 4. TEM images taken from the edge of a Pt NWN. (a) Junction of two crossing nanowires. (b) HRTEM image presenting grains of the polycrystalline wires. The inset shows a representative SAED pattern.
directions with R = 45°, are often organized into octahedrons. Note that no isolated nanowires are found that leave the network. This aspect is important regarding safety issues, since separated nanowires can be internalized by living cells, where they may have toxic effects.21 Networks consisting of nanowires with small average diameter and relatively low integration density (such as the NWN depicted in Figures 3e, composed of nanowires with an average diameter of 20 nm) cannot maintain the orientation given by the template. The nanowires bend, but they are still interconnected and form a macroscopic object that can be handled comfortably. Furthermore, investigations by FESEM reveal well-defined cylindrical nanowires with a diameter distribution that is distinctly narrower than 10% for all regarded samples. The Pt network nanowires that were produced by direct current deposition show a very smooth surface. Controlled deviations from cylindrical geometry can be introduced by pulse-reverse electrodeposition, which influences the local electrolyte distribution during the growth process.22 Consequently, we also fabricated networks consisting of single-element segmented nanowires (Figure 3d). In addition, the method reveals its outstanding richness by providing the possibility to adopt additional structuring techniques, reported for unconnected nanowires, to produce multilayered nanowire networks (e.g., Pt/Co NWNs, Supporting Information, Figure S6).23 Images of NWNs obtained by transmission electron microscopy (TEM) exemplify not only the precise replication of the template shape but also show particularly well how nanowires are interconnected. A representative micrograph of an isolated node taken from the edge of a nanowire network is depicted in Figure 4a. The network nanowires exhibit an average diameter of 13 nm. At higher magnification, individual grains of the polycrystalline nanowires can be identified (Figure 4b). The randomly oriented face-centered cubic Pt crystallites have an average size of
