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Dec 13, 2006 - in a nanoporous polycarbonate membrane from a nonaqueous ionic liquid electrolyte. 1 Physics Department, Shahid Chamran University, ...
Appl. Phys. A 86, 373–375 (2007)

Applied Physics A

DOI: 10.1007/s00339-006-3783-x

Materials Science & Processing

i. kazeminezhad1,2 a.c. barnes2 j.d. holbrey3 k.r. seddon3 w. schwarzacher2,u

Templated electrodeposition of silver nanowires in a nanoporous polycarbonate membrane from a nonaqueous ionic liquid electrolyte 1

Physics Department, Shahid Chamran University, Ahvaz, Iran H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK 3 QUILL, School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, UK 2

Received: 16 June 2006/Accepted: 24 October 2006 Published online: 13 December 2006 • © Springer-Verlag 2006 ABSTRACT Template electrodeposition has been used to prepare a wide range of nanostructures but has generally been restricted to aqueous electrolytes. We report the deposition of silver nanowires in a commercial nuclear track-etched polycarbonate template from the nonaqueous ionic liquid, 1-butyl-3methylimidazolium hexafluorophosphate ([bmim][PF6]) using silver electrochemically dissolved from the anode. Transmission electron microscopy (TEM) shows that the nanowires have a very high aspect ratio with an average diameter of 80 nm and length of 5 µm. Ionic liquid electrolytes should greatly extend the range of metals that can be electrodeposited as nanowires using templates. PACS 81.15.Pq;

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81.07.-b

Introduction

Research on nanoscale materials has intensified recently from the perspective of both basic science and applied research, due to the unique physical and chemical properties presented by materials on this length scale. In particular, pure metal and alloy nanowires have shown interesting electronic and optical properties [1–3] which could be of interest for applications such as magnetic sensors [4]. Electrodeposition inside nanoporous membrane templates [5, 6] has proved to be a versatile approach to fabrication of freestanding metallic nanowires. In general, nanoporous templates are widely available and relatively inexpensive: templates permit the preparation of materials with a high degree of homogeneity and reproducibility. Many different metals have been electrodeposited using anodic alumina and nuclear track-etched polycarbonate. For example, the electrodeposition of silver nanowires has been reported from aqueous solution using anodic aluminium oxide templates [7–9]. However, an even greater range of metals could be electrodeposited in future through the use of room temperature ionic liquid electrolytes. The importance of room temperature ionic liquids (ILs) has grown significantly over the u Fax: +44 117 9255624, E-mail: [email protected]

past decade. In general, most of the research on ionic liquids has concentrated on electrochemical processes [10–12], and their use as reaction media for organic synthesis [13], catalysis [14], analysis [15], and chemical separations [16]; although many other materials applications have been described including, for example, their use as lubricants [17]. Interest in this class of solvent stems from the properties exhibited by the liquids, and the ease by which many of these properties may be varied. The electrodeposition of pure metals and alloys from ionic liquids has been extensively investigated [18]. Most ionic liquids studied have low vapour pressure, and good thermal stability compared to molecular organic solvents. The stability of ionic liquids above 100 ◦ C can be of advantage for electrodeposition compared to aqueous electrolytes, as crystalline materials deposited at higher temperatures generally have fewer defects. Additionally, the absence of water means that the electrochemical window (the range of potentials that may be applied to the working electrode without electrolysing the solvent) can be greater than 4 V. This makes it possible to electrodeposit metals, such as niobium-rich alloys that cannot be deposited from aqueous electrolytes [19], and reactive metals such as caesium [20]. Silver, the subject of this report has also been electrodeposited from a range of ionic liquids [21, 22] With the notable exception of choline chloride systems, which have been demonstrated as potential alternatives to chromate baths for chrome plating [23], most ionic liquids remain relatively expensive, which restricts their use in conventional electrodeposition. However, when the total sample size and the amount of material deposited are both small, this is much less of a consideration, which makes the investigation of nanofabrication techniques a particularly promising area of application for electrodeposition from ionic liquids. To our knowledge ionic liquids have not previously been used for electrodeposition with nanoporous membrane templates. Here we report a first step in this direction, showing that it is possible to electrodeposit silver metal nanowires from small volumes of the ionic liquid 1-butyl3-methyl-imidazolium hexafluorophosphate ([bmim][PF6 ]) using a commercially available nuclear track-etched polycarbonate nanoporous membrane, with nominal pore diameters of 30 nm as template.

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Applied Physics A – Materials Science & Processing

Experimental

We used a nuclear track-etched polycarbonate membrane of diameter 13 mm as a template for nanowire fabrication. The thickness, pore diameter and pore density quoted by the supplier (Osmonics) were 6 µm, 30 nm and 6 × 108 cm−2 respectively. The shiny side of the membrane was coated with an evaporated 250 nm Au film to provide a conducting substrate (working electrode). This was placed into contact with a Cu plate connected to a potentiostat, and kapton tape was used to ensure that only the central area of the membrane was exposed to the electrolyte. The latter was [bmim][PF6 ], prepared from 1-methylimidazole by alkylation with bromoethane, followed by anion metathesis with sodium hexafluorophosphate using the procedure described by Cull et al. [24]. The ionic liquid was fully characterized (by techniques including 1 H and 13 C NMR spectroscopy, ion chromatography, elemental analysis, cyclic voltammetry, and Karl Fisher determination of water content [25]); the levels of water and halide were below that which would effect the results of the work described here. For convenience, the electrolyte was used without any additional silver salts. Ag entered the electrolyte through dissolution at the anode, which was an Ag plate held at a distance of 7 mm from the working electrode using plastic spacers. Other researchers have used silver tetrafluoroborate (AgBF4 ) dissolved in [bmim][PF6 ] for electrodeposition experiments [22]. The electrochemical cell was also of plastic, and had a diameter of 30 mm. Figure 1 is a schematic of the experimental arrangement. To deposit the nanowires, the potential of the working electrode was held at −1.2 V relative to the Ag anode. Ag nanowires of length ∼ 6 µm required a deposition time of ∼ 60 h. Samples for transmission electron microscopy (TEM) were prepared by dissolving the membrane in chloroform, and then collecting the deposited nanowires on a carbon-coated copper grid. The TEM used in this work was a Philips 430 operating at 250 keV, which was fitted with an energy-dispersive X-ray analysis (EDX) system. 3

ate matrix. EDX was used to confirm that they consisted of Ag. The maximum length of the wires is ∼ 5 µm and the average diameter ∼ 80 nm. This is significantly larger than the nominal pore diameter quoted by the manufacturer. Other workers have found similar results. For example, Ansermet and co-workers studied the pore size distribution for polycarbonate track-etched membranes from several companies by electrodepositing Ni and Co replicas, and found that membranes with a quoted diameter of 30 nm had a measured diameter of 57 ± 3 nm [26]. Interestingly, the measured diameters agreed well with diameters calculated from the manufacturers’ data for air and water flow through the membranes. Many of our wires were found to have lengths of less than 5 µm, possibly because they are easily broken during sample preparation. In order to observe the microstructure of the Ag nanowires, they were also examined at higher magnification. A typical image is shown in Fig. 3. The wire at the center of the figure has a diameter that decreases towards one end. Tapering at both ends was observed in previous studies of electrodeposited nanowires and attributed to the pores in which they grew also being tapered [27]. The variation in contrast along the length of this wire suggests that it is polycrystalline, with a grain size that varies from ∼ 10 to ∼ 200 nm. The largest grains appear to occupy the full width of the nanowire.

Results and discussions

Figure 2 is a low magnification TEM bright field image of the nanowires after dissolution of the polycarbon-

FIGURE 1

Schematic diagram of the electrodeposition apparatus

FIGURE 2

Low magnification TEM bright field image of Ag nanowires

FIGURE 3

High magnification TEM bright field image of a single Ag

nanowire

KAZEMINEZHAD et al.

Templated electrodeposition of silver nanowires in a nanoporous polycarbonate membrane

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Conclusion

Ag nanowires with an average diameter of 80 nm were successfully electrodeposited using [bmim][PF6 ] within the pores of a commercially-available track-etched polycarbonate membrane. EDX confirmed their composition and TEM showed that they were polycrystalline with grain sizes in the range 10 – 200 nm. This demonstrates the practicality of using small volumes of non-aqueous ionic liquid electrolyte for template electrodeposition. In future it could be of great interest to electrodeposit and study the physical properties of reactive metal and semiconductor nanowires that cannot be deposited from aqueous electrolytes. ACKNOWLEDGEMENTS The contributions of Matthew Haigh and Christopher Gregory to this work are gratefully acknowledged FIGURE 4

Selected area electron diffraction pattern of Ag nanowire

REFERENCES

FIGURE 5 Selected area electron diffraction pattern from a single grain. The structure is f.c.c. with 110 parallel to the electron beam

Selected area diffraction patterns of single nanowires also demonstrate their polycrystalline nature. For Fig. 4, the electron beam probes a region consisting of a large number of grains, giving rise to a pattern of bright spots lying on concentric circles in reciprocal space. For Fig. 5, in contrast, only a single grain was probed giving rise to a periodic diffraction pattern. In this case, the pattern is characteristic of a face-centered cubic (f.c.c.) metal, with 110 parallel to the electron beam. We calculated the lattice parameter as 4.03 ± 0.03 Å. Since Ag is f.c.c. with a lattice parameter of 4.086 Å, this result provides independent support for our conclusion from EDX data that the wires consist of Ag.

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