Appl Phys A (2011) 105:847–854 DOI 10.1007/s00339-011-6646-z Multiple activation of ion track etched polycarbonate for the electroless synthesis of metal nanotubes F. Muench · M. Oezaslan · T. Seidl · S. Lauterbach · P. Strasser · H.-J. Kleebe · W. Ensinger Received: 15 June 2011 / Accepted: 6 October 2011 / Published online: 4 November 2011 © Springer-Verlag 2011 Abstract In our study, we examined the formation of thin 1 Introduction films of silver nanoparticles on polycarbonate and the in- fluence of the silver loading on the electroless synthesis of In the early 1990s, Martin and coworkers developed several methods for the template-supported synthesis of metallic metal nanotubes. Control of the silver film thickness oc- micro and nanotubes (NTs), focusing on electroless plating curred by consecutive dipping of the polymer template in and ion track etched polycarbonate [1–3]. Ion track etched tin(II) and silver(I) solutions. The deposition progress was polymer templates contain stochastically distributed chan- studied using UV-Vis spectroscopy. The reaction mecha- nels whose density, diameter, and shape can be substantially nism relies on the adsorption of reactive ions on the poly- varied by the production conditions [4–6]. Wires and tubes mer template as well as on the silver nanoparticles. The are obtained by depositing metals in these channels [7–10]. initial catalytic activity of silver-covered ion track etched These virtually one-dimensional nanostructures have been effectively implemented in relevant fields such as nanoelec- polycarbonate is an important governing factor for the elec- trochemistry [3], field emission [7], selective chemical trans- troless synthesis of metal nanotubes with desired thickness port [8] and flow-through [9], or fuel cell [10] catalysis. and shape. Therefore, the presented method allows specific In controlled reactions, electroless plating leads to a uni- template preparation according to given synthetic demands. form metal deposition on the whole template surface. There- High aspect ratio copper, gold, and platinum nanotubes were fore, it is especially suited for the homogeneous coverage of produced by the combination of sufficiently activated tem- complex shaped templates such as nanochannel-containing plates with optimized electroless plating procedures. membranes with extended inner surfaces. NTs are formed by deposition of a metal film on the nanochannel walls. Due to this growth mode, electroless plating is a straightfor- ward method for the synthesis of tubular metal structures. In contrast, electrochemical plating leads to a linear growth F. Muench () · T. Seidl · S. Lauterbach · H.-J. Kleebe · of metal starting from the cathode side. This method prefers W. Ensinger clearly the formation of wires [7]. Electroplating relies on Department of Materials and Geoscience, Technische Universität the reduction of metal cations present in the plating solution Darmstadt, Petersenstraße 23, 64287 Darmstadt, Germany by cathode-supplied electrons. In electroless baths, the reac- e-mail: [email protected] Fax: +49-6151-166378 tion is maintained by a chemical reducing agent. These plat- ing solutions are thermodynamically instable with respect to M. Oezaslan · P. Strasser decomposition to the metal and a corresponding oxidation Department of Chemistry, Technische Universität Berlin, Straße product [11, 12]. des 17. Juni 124, 10623 Berlin, Germany To obtain surface-selective deposition, the reaction must T. Seidl be kinetically suppressed in the bulk solution and enforced Material Research Group, GSI Helmholtz Centre for Heavy Ion on the template surface. The first condition is met by intro- Research GmbH, Planckstraße 1, 64291 Darmstadt, Germany ducing metastable redox pairs (an appropriate combination 848 F. Muench et al. of one or more metal sources, reducing agents and additives) 50% in water (Sigma-Aldrich); trifluoroacetic acid (Riedel- which are insensitive toward homogeneous nucleation [11, de Häen, >99%). A commercial electroplating solution (El- 12]. Depending on the metal to be plated, various baths have Form Galvano Goldbad, Schütz Dental GmbH) was used been developed, comprising Co [14], Ni [14], Cu [13–15], as the Au source (15 g 99.9% Au per liter, present as Ag [16], Au [8, 9], Pd [17], or Pt [10]. The critical chal- (NH4)3(Au(SO3)2)). All procedures were conducted with lenge is the stability of these baths [12, 13]. The second purified water (Milli Q 18-M water). requirement is achieved by using templates which catalyze the decomposition reaction. After initiation of the electro- Template synthesis Polycarbonate foils (Makrofol®, Bayer less plating, the formed metal film sustains the deposition MaterialScience AG, nominal thickness 30 µm) were irradi- reaction by autocatalysis [11, 12]. The bare polymer tem- ated with Au ions (energy: 11 MeV per nucleon; fluence: plates mostly do not promote the deposition reaction. There- 1 × 108 ions per cm2) at the GSI Helmholtz Centre for fore, an appropriate pre-treatment of polymer templates is of Heavy Ion Research (Darmstadt). Subsequently, they were major importance for electroless plating. Usually, they are irradiated with UV light in the presence of air (1 h per side, activated by anchoring Ag- [3, 9, 10] or Pd-nanoparticles UV source provides 1.5 and 4 W m2 in the range between (NPs) [13] on their surface. However, sparse effort has been 280 nm–320 nm and 320 nm–400 nm) and etched in stirred ◦ spent on examining the nature of the Ag structures formed sodium hydroxide solution (6 M, 50 C, time depending on on polycarbonate templates as well as their reactivity in con- desired nanotube diameter). The as-prepared templates were secutive electroless plating. New methods to enhance and thoroughly washed with purified water and dried in air be- tune the activity of these templates are required to improve fore use. or to enable the challenging synthesis of high aspect ratio metal-NTs. Covering the template with Ag-nanoparticles Firstly, the In general, the successful preparation of metal-NTs by etched polycarbonate membranes were stored in a Sn(II)- electroless plating depends on the combination of a suffi- solution for at least 45 min (0.042 M SnCl2, 0.071 M = ciently activated substrate with a plating bath ensuring slow CF3COOH in methanol:water 1:1). To remove exces- and controlled metal deposition. Activation as the starting sive Sn(II), the impregnated foils were washed twice with point of the metal film growth is a crucial step for both the ethanol before immersing them in Ag(I)-solution for 3 min homogeneity and density of the obtained NTs [11], while (0.059 M AgNO3, 0.230 M NH3). These procedures were low deposition rates are essential to ensure sufficient mass performed at room temperature. For multiple activation, the transport of the reactants to the poorly accessible regions in- templates were washed twice with ethanol prior to repeating side the template [8, 14]. Additionally, the metal deposit has the described procedure. to be uniform on the nanoscale. In this work, we provide synthetic guidelines for the electroless fabrication of Au-, Electroless synthesis of metal nanotubes The last washing Pt-, and Cu-NTs using tailored Ag-NP activation. step before the transfer to the growth solution was conducted with ethanol, followed by water. All depositions took place at 8◦C. After the desired reaction time, the metal-covered templates were taken out, washed with water and dried in 2 Experimental details air. Au and Pt deposition solutions were prepared accord- ing to reported procedures [9, 10]. They contained Au(I), General, chemicals All glassware was cleaned with aqua sulfite, 4-(dimethylamino)pydidine and formaldehyde in the regia prior to use. All reaction solutions were freshly pre- case of the Au solution and Pt(IV), chloride, ethylenedi- pared. Following chemicals were used without further pu- amine and hydrazine for the Pt solution. The Cu deposition rification: 4-(dimethylamino)pyridine (Fluka, puriss.); sil- solution contained 0.10 M Cu(II) (introduced as copper sul- ver nitrate (Grüssing, p.a.); ammonia 33% in water (Merck, fate), 0.22 M tartrate (introduced as potassium sodium salt), puriss.); copper sulfate pentahydrate (Fluka, purum p.a.); 0.24 M ethylenediamine and 1.2 M formaldehyde. It was ethylenediamine (Fluka, puriss. p.a.); formaldehyde solu- adjusted to a pH value of 11.5–12.0. tion 37% in water, methanol stabilized (Grüssing, p.a.); hexachloroplatinic acid 8% in water (Fluka); hydrazine Analytics TEM (FEI CM 20 microscope (Netherlands) hydrate 80% in water (Merck, for synthesis); methanol with a LaB6 cathode, operated at 200 kV, equipped with (Sigma-Aldrich, laboratory reagent); potassium sodium tar- an Oxford Model 6767 EDS-system (England); FEI Tecnai 2 trate tetrahydrate (Fluka, puriss. p.a.); sodium chloride G S-TWIN microscope with a LaB6 cathode, a Gatan MS (Merck, suprapur); sodium hydroxide solution 32% in wa- 794 P CCD camera and EDS detector, operated at 200 kV): ter (Fluka, puriss. p.a.); sodium sulfite (Merck, p.a.); tin(II) The tube-containing templates were embedded in Araldit chloride (Merck, for synthesis); tin(II) methanesulfonate 502© (polymerization at 60◦C for 16 h) and examined as Multiple activation of ion track etched polycarbonate for the electroless synthesis of metal nanotubes 849 ultrathin sections (70 nm, Reichert-Jung Ultracut E ultrami- crotome, DKK diamond knife). SEM (JEOL JSM-7401F, 5– 10 kV acceleration voltage): Prior to the measurement, the polymer template was removed with dichloromethane. The freed metal structures were collected on silicon wafer pieces sputter-coated with a thin Au film. XRD (Seifert PTS 3003): Investigations on Cu-NT containing membranes were per- formed using a Cu anode and an x-ray mirror primary to the sample. At the secondary side, a slit system and a soller 45 slit was used. UV-Vis spectroscopy (ATi Unicam UV/Vis Spectrometer UV4): The measurements were performed on twice ethanol washed template strips in ethanol-filled quartz Fig. 1 Reaction route for the electroless synthesis of metal-NTs: Ion cuvettes.