Ligand-optimized electroless synthesis of silver nanotubes and their activity in the reduction of

4-nitrophenol

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Nanotechnology 22 (2011) 415602 (9pp) doi:10.1088/0957-4484/22/41/415602

Ligand-optimized electroless synthesis ofsilver nanotubes and their activity in thereduction of 4-nitrophenol

Falk Muench, Markus Rauber, Christian Stegmann,Stefan Lauterbach, Ulrike Kunz, Hans-Joachim Kleebeand Wolfgang Ensinger

Department of Materials and Geoscience, Technische Universitat Darmstadt,Petersenstraße 23, 64287 Darmstadt, Germany

E-mail: muench@ca.tu-darmstadt.de

Received 6 May 2011, in final form 19 August 2011Published 14 September 2011Online at stacks.iop.org/Nano/22/415602

AbstractA facile electroless plating procedure for the controlled synthesis of nanoscale silver thin filmsand derived structures such as silver nanotubes was developed and the products werecharacterized by SEM, TEM and EDS. The highly stable plating baths consist of AgNO3 as themetal source, a suitable ligand and tartrate as an environmentally benign reducing agent. Nextto the variation of the coordinative environment of the oxidizing component, the influence ofthe pH value was evaluated. These two governing factors strongly affect the plating rate and themorphology of the developing silver nanoparticle films and can be used to adapt the reaction tosynthetic demands. The refined electroless deposition allows the fabrication of homogeneoushigh aspect-ratio nanotubes in ion track etched polycarbonate. Template-embedded metalnanotubes can be interpreted as parallelled microreactors. Following this concept, both thesilver nanotubes and spongy gold nanotubes obtained by the use of the silver structures assacrificial templates were applied in the reduction of 4-nitrophenol by sodium borohydride,proving to be extraordinarily effective catalysts.

1. Introduction

Electroless plating represents a technologically modest andhighly flexible wet-chemical method for the fabrication ofmetal films [1]. The two main components of electrolessplating solutions are a reducing agent and a metal salt forminga metastable redox pair to avoid homogeneous nucleation.Metal precipitation selectively occurs on a substrate surfaceand is maintained by the autocatalytic effect of the depositedmetal. If the substrate is not initially active towards thedeposition reaction, it has to be modified, for instance bycovering it with metal nanoparticles (NPs). As the reductionusually takes place in the presence of water and oxygen, mostlynoble or semi-noble metals with adequate oxidation resistanceand a sufficient thermodynamic driving force towards theelemental state are used for the preparation of pure metal films(e.g. Cu [2], Ag [3], Au [3], Pd [4], Pt [4]).

If electroless plating is applied to the synthesis ofnanomaterials, higher requirements in terms of reactioncontrol have to be met. Due to the constriction ofmass transport, the fabrication of homogeneous metal filmson substrates exhibiting extended cavities is especiallydemanding [3, 5]. Despite the increased intricacy of synthesis,the resulting metal structures like high aspect-ratio nanotubes(NTs) [6], nanowires [7] or intensely roughened films [8]are of special scientific and practical interest. Sensing,nanoelectrochemistry [6, 9], catalysis [10] and separation [9]can be pointed out as general fields of application. Forinstance, porous GaN electrolessly covered with Ag wasintroduced as a surface enhanced Raman spectroscopy (SERS)substrate of high and uniform activity [8], Pd nanowire arraysproved to be efficient ethanol electrooxidation catalysts [11],and Au–NTs were implemented in molecular filtration [9, 12].Ag nanostructures are also often used as sacrificial templates toderive structures such as pentagonal Au–NTs [13, 14], hollow

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Nanotechnology 22 (2011) 415602 F Muench et al

Ag–Au-nanorods [15] or Pt-containing rods [16] and tubes [17]acting as highly active electrocatalysts.

In the case of electroless Ag deposition, many proceduresrely on the ammonia complex of AgI [3, 8, 18–20] as theoxidizing component, although unfavourably high reactionrates [3] and bath instability have been observed, leading tohomogeneous nucleation and Ag precipitation [19]. Despitethese experiences, electroless Ag deposition in non-activatednanoporous alumina, which should have an inactive surface,interestingly leads to the formation of highly crystalline low-diameter NTs [21]. Rather than forming polycrystalline thinfilms as observed in standard electroless plating experiments,directed growth along the nanochannels was observed, and thetubes continued to grow even beyond the template surface.However, this extraordinary reactivity has not been observedin polymer templates.

In this work, we generally refined the AgI-tartratedeposition system for nanotechnological applications. Toenhance reaction control, we pursued a coordination-chemicalstrategy to adjust the activity of the metal component byadding suitable ligands. Using optimized plating baths, highaspect-ratio Ag–NTs were fabricated in ion track etchedpolymer templates. This proves the homogeneous depositionof nanoscale Ag films on complex shaped substrates, extendingthe range of electroless Ag plating towards demanding taskssuch as the fabrication of functionalized micro- and nanofluidicdevices [22–24].

Finally, the template-embedded NTs and porous Auderivatives obtained by cementation were applied as hetero-geneous catalysts in the reduction of 4-nitrophenol. Thisconvenient model reaction can be easily monitored by means ofUV–vis spectroscopy [25] and is frequently applied to evaluatethe catalytic activity of metal–NPs [26–31]. The presentedsynthetic scheme is interesting for several reasons. Concerningthe utilization of porous templates with aligned channels, theAg–NTs can be directly implemented as microreactors withoutthe need for additional assembly steps. In contrast to setupsinvolving colloidal NP systems [27–30], the reaction mediumflows through the template-supported NTs. Anchoring thecatalyst to the polymer membrane renders recovering stepsunnecessary, similar to strategies such as immobilizing NPswithin polymer films [31] or on ion exchange beads [26].Furthermore, high reactivities can be expected due to the neatsurface of the electrolessly grown Ag nanostructures, whereasprotecting agents [27] or composite matrices [31] required toensure NP dispersion are often detrimental to the catalyticperformance.

2. Experimental details

2.1. General, chemicals

Glassware was cleaned with concentrated nitric acid priorto use. The sensitization, activation and plating solutionswere freshly prepared. All procedures were performed withpurified Milli Q 18 M� water. The following chemicalswere used without further purification: 4-nitrophenol (Fluka,puriss. p.a.), AgNO3 (Grussing, p.a.), ammonia 33% in

water (Merck, puriss.), AuCl3 (Aldrich, 99.99% trace metalbasis), dichloromethane (Sigma-Aldrich, puriss. p.a.), ethanol(Sigma-Aldrich, G Chromasolv), ethylenediamine (Fluka,puriss. p.a.), methanol (Aldrich, 99.8%), pyridine (Grussing,purum 99%), potassium sodium tartrate tetrahydrate (Fluka,puriss. p.a.), SnCl2 × 2H2O (Sigma-Aldrich, ACSreagent), sodium borohydride (Merck, for synthesis), sodiumhydroxide solution 32% in water (Fluka, puriss. p.a.),tetraoctylammonium bromide (Aldrich, 98%), trifluoroaceticacid (Riedel-de Haen, >99%).

2.2. Preparation of ion track etched templates

Polycarbonate foils (Makrofol®, Bayer MaterialScience AG,nominal thickness 30 μm) were irradiated with Au ions(energy 11 MeV per nucleon, ion fluence 1 × 108 n cm−2)at the Helmholtz Centre for Heavy Ion Research (GSI).Subsequently, they were irradiated with UV light in thepresence of air (1 h per side, UV source provides 1.5 and 4 Wm−2 in the ranges 280–320 nm and 320–400 nm, respectively)and etched in stirred sodium hydroxide solution (6 M, 50 ◦C,the time depending on the desired diameter). The as-preparedtemplates were thoroughly washed with water and dried. Formass gain experiments, unetched polycarbonate foils wereused.

2.3. Electroless Ag deposition

Prior to the electroless plating, the polycarbonate substrateswere activated with Ag–NPs to ensure initial catalyticactivity [3]. For this purpose, the polymer foils were storedin a sensitization solution for at least 45 min (42 mM SnCl2,71 mM trifluoroacetic acid in water:methanol = 1:1), leadingto the adsorption of SnII. After thorough washing withethanol, the substrate was transferred to the activation solution(59 mM AgNO3, 230 mM NH3 in water) for 3 min. Agprecipitation was indicated by the appearance of a slightbrown colour. After washing with ethanol and water, thefreshly activated substrates were transferred to aqueous platingsolutions containing AgNO3 (17 mM), potassium sodiumtartrate (120 mM) and different amounts of complexing agents(100 mM in the case of ammonia and ethylenediamine and50 mM in the case of pyridine). Adjustment of pH wasperformed with sodium hydroxide and trifluoroacetic acid. Thelatter component was chosen to avoid interference of the acid-introduced anion with AgI and elemental Ag. All depositionstook place at 8 ◦C. After the desired reaction time, the Ag-covered templates were removed, washed with water and driedin air.

2.4. Synthesis of porous Au–NTs

A glass syringe attached to a membrane filtration holder(Nucleopore® sample kit for diagnostic cytology) was usedto purge the polycarbonate template-embedded Ag–NTs withthe reaction solution consisting of tetrachloroauric acid(3.9 mM, prepared by dissolving AuCl3 in diluted HCl)and tetraoctylammonium bromide (6.1 mM) in ethanol.Subsequently, the NT-containing template was flushed with

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ethanol and ammonia solution (one part of 33% ammoniasolution to two parts of water) to remove reactants and Aghalide residues formed during the Ag exchange.

2.5. Catalysis

The reaction solution was prepared by mixing a solutioncontaining 4-nitrophenol with a freshly prepared sodiumborohydride solution. The final solution contained 8.4 ×10−4 M 4-nitrophenol and a large excess of sodiumborohydride (60 mM) to ensure pseudo-first order kinetics.For flow-through catalysis, 3 ml of the solution was pressedthrough the NTs using the setup described in section 2.4.

2.6. Characterization

For TEM (FEI CM20, 200 kV acceleration voltage, LaB6-cathode) the NT-containing templates were embedded inAraldite 502© (polymerization at 60 ◦C for 16 h) andexamined as ultrathin sections (70 nm, Reichert-Jung UltracutE ultramicrotome, DKK diamond knife). For SEM (JEOLJSM-7401F, 5–10 kV acceleration voltage), prior to themeasurement the template was removed with dichloromethane.The freed metal structures were collected on silicon waferpieces sputter-coated with gold. The UV–vis spectra wererecorded with an ATi Unicam UV/vis Spectrometer UV4 usinga scan rate of 240 nm min−1.

3. Results and discussion

3.1. Synthetic strategy

Figure 1 outlines the fabrication scheme of the Ag–NT-containing membranes. The templates are prepared byirradiating polymer foils with heavy ions (step 1), followedby etching out the ion-induced linear damage zones (step2). By variation of the polymer foil, the irradiationand the etching procedure, the nanochannel density andmorphology (orientation, length, diameter, shape) can be wellcontrolled [32]. This high degree of control has led tothe widespread application of ion track etched templates inthe synthesis of anisotropic nanomaterials [32, 33]. Besideshomemade templates, several porous polymer membranesare commercially available (e.g. Nucleopore®, Nuclepore™,Poretics® or Cyclopore™ membrane filters). By consecutivetreatment with solutions containing SnII (step 3) and AgI

(step 4), the templates are covered with small Ag–NPs. Theactivated templates catalyze the electroless plating reactionand thus initiate the growth of the Ag film (step 5).During deposition, Ag–NTs are formed inside the template.The obtained Ag nanostructures can be used as sacrificialtemplates. In our case, AuIII was employed to synthesizeAu–NTs (step 6). The reaction scheme can be easilytransferred to other substrates. For instance, the Ag thinfilm syntheses described in section 3.2 were performed withunetched polycarbonate foils.

Concerning the optimization of the electroless platingstep, the desired reduced reaction rates are better achieved byvariation of the redox couple than by reduced concentrations

Figure 1. Nanotube fabrication scheme starting from polycarbonatefoil which is irradiated with heavy ions (1). Consecutively, the iontracks are selectively etched out with NaOH to yield thenanochannel-containing template (2). By sensitization, SnII isanchored to the template surface (3), leading to the precipitation ofAg–NPs in the activation step (4). The Ag–NPs act as nuclei for theelectroless deposition of nanoscale Ag films and NTs (5). Bycementation with AuIII, porous Au structures can be obtained (6).

of the reactive components. Highly reactive species willquickly decrease in concentration alongside the nanochannelsby consumption at their catalytic surfaces. The concentrationgradient formed causes inhomogeneous deposition withincreasing film thickness at the outer template regions [3, 5].Since the reactivity of a metal cation strongly depends onits coordinative environment, the introduction of ligands is astraightforward means to vary the oxidizing component.

Appropriate ligands for electroless plating must formwater-soluble complexes fulfilling the metastability criterion.Further conditions of practical interest are the availability andtoxicity of the ligand. If catalytic applications are considered,poisoning components such as sulfur have to be avoided. In aligand screening involving different common amines, pyridinesand polyaminoacids, ethylenediamine (en) and pyridine (py)fulfilled the mentioned requirements and were therefore chosenfor the following experiments.

3.2. Impact of the ligand on deposition speed and productmorphology

The commonly used NH3–AgI system [3, 8, 18–21] served asa reference for studying the influence of the ligand. The pHof the plating solution containing en was adjusted to the pHof the NH3 solution (11.3), while the natural pH of the pysolution (8.6) was not altered due to precipitation occurring inthe case of NaOH addition (probably due to Ag2O formation).Film morphology and reaction progress were characterized bySEM and mass gain due to Ag film formation normalized tothe surface area of the polycarbonate substrate, respectively.For calculation of error propagation, a mass error of 0.1 mgwas assumed.

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At the beginning of the depositions, an induction periodis found during which the activated polycarbonate surface isfirst covered with electrolessly plated Ag. Subsequently, theobserved mass gain rate is approximately constant accordingto a deposition reaction which has reached steady-stateconditions. At later reaction stages, the mean Ag mass gainrate slightly decreases due to reagent depletion. To quantifythe reaction speed, the initial mass gain rates of the depositionswere considered. As they are not directly accessible byexperiment, they were calculated by fitting the data sets withy = A − e−Bx (y represents the mass gain, x the reactiontime, and A and B are the fitting variables). This fittingfunction describes a mass gain whose rate is decreasing withtime and which is approaching a maximum value of A (due tocompletion of the reaction). At low x values, the function isasymptotic to a line with a slope of B . This fitting variablerepresents the linear mass gain rate at the beginning of thereaction and was therefore chosen for comparing the differentexperiments.

As expected, in the presence of NH3 the highest depositionrate was observed. The substitution of NH3 by en leads to asignificant rate reduction, whereas the deposition is even moredampened in the presence of py. Compared to NH3, the initialmass gain rate in the en experiment is approximately reducedby 85% (from 2.3×10−4 to 3.5×10−5 mg cm−2 min−1), whilea rate decrease of 98.4% (to 3.7 × 10−6 mg cm−2min−1) isobserved in the py experiment (figure 2(a)). In figure 2(b), thedecrease of the mean mass gain rate during the course of thereaction is shown. Besides the ligand, the pH of the solutionhas a distinct impact on its reactivity. Typical reducing agentsapplied in electroless plating baths are more easily oxidizedat higher pH values [34]. The same trend was observed inthe AgI-tartrate system. In addition to causing high reactionrates, increased pH values are detrimental to the stability ofthe plating solutions towards homogeneous nucleation. Forinstance, en baths, which are very stable at moderately basicpH values, quickly decompose to form a grey Ag suspensionat a pH value of 12.7. The aim of attaining intermediatedeposition rates in the py and NH3 deposition systems (pHdecrease of NH3 baths and pH increase of py baths) failed dueto the formation of Ag-containing precipitates. Unlike in thecase of ammonia and py, the deposition characteristics of theen baths can be extensively adjusted by alteration of the pH(figure 2(c)). If the pH is raised from 10.1 to 11.6, the initialmass gain rate is increased by a factor of approximately 20.

Both the observed inability of py to keep AgI dissolvedat raised pH values and the negligible impact of addition ofpy to en baths indicate en to be the stronger ligand. Indeed,the reactivity found is supported by the complex formationconstants of the corresponding systems. For the first ligandaddition reaction AgI + L → [AgL]+, the complex formationconstants log(β) at room temperature are 1.96 for py [35],3.44 for NH3 [36] and 13.15 for en [37] (probably presentas the dimeric complex [Ag2en2]2+). The second additionreaction [AgL+] + L → [AgL2]+ has log(β) values of2.21 for py [35], 6.92 for NH3 [36] and 7.64 for en [37].Considering the relative position of py as the weakest of thethree ligands used for electroless Ag plating, the severely

Figure 2. (a) Ag mass gain normalized to the surface of thepolycarbonate foil including fits and the values of the fittingparameter representing the initial mass gain rate. (b) Time-dependentmean mass gain rates of the experiments in (a). (c) ThepH-dependent initial mass gain rates in the en deposition system.

dampened reduction kinetics are most probably related to therelatively low pH of the plating bath, not to an effectivethermodynamic stabilization of AgI by py. As expected byits efficient complex formation, en has a distinct stabilizingeffect on AgI as can be seen by the decrease in the depositionrate under otherwise identical conditions (NH3 and en infigures 2(a) and (b)).

The different bath compositions also seriously affectthe morphology of the plated films. All Ag films arepolycrystalline with a dense and relatively smooth structureconsisting of merged edge crystals in the case of NH3

addition (figure 3(a)). In the presence of en, similar Agfilms with larger and more defined grains are obtained(figure 3(b)). Plating solutions with slow deposition kinetics(en at reduced pH values and py baths) yield incomplete films

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Figure 3. SEM images of Ag-covered polycarbonate films grown under different conditions: (a) ligand = 100 mM NH3, pH = 11.3;(b) ligand = 100 mM en, pH = 11.3; (c) ligand = 100 mM en, pH = 10.1; (d) ligand = 50 mM py, pH = 7.5. Corresponding magnificationsare shown on the right. All scale bars represent 1 μm in the left and 200 nm in the right images (magnifications of 9400× and 56 000×,respectively).

of polyhedral, poorly connected NPs with a preference forplate-like morphologies in the case of en (figure 3(c)) andmore globular structures in the case of py (figure 3(d)). Inboth cases, crystals with the characteristic morphologies arefound on top of a film on the polycarbonate surface consistingof relatively small particles. It seems that in the case of thetwo latter depositions, the reduction reaction does not leadto the evolution of many new Ag nuclei and continues ononly a few particles. This can be explained by an increasedselectivity of the Ag deposition in the case of decreasingreactivity. While at a high reaction speed many of the initial Agnuclei grow to form closed films, low reaction speed correlateswith a deposition focused on a few favoured sites. Additionalheterogeneous nucleation is uncommon and the few expandingcrystals maintain a relatively regular shape.

Also, the differing nanostructures of the Ag thin filmsare noticeable in their optical appearance. On flat surfaces,both the NH3 and the en films are shiny, while the first is ofa bright silver, the second of a light golden colour. Even atadvanced reaction times, py films and en films grown at lowpH values are dark grey in colour and exhibit some dullness,corresponding to the relatively high roughness and the badconnection of the individual NPs.

3.3. Synthesis of Ag–NTs

To evaluate the impact of the composition of the plating bathon the challenging synthesis of high aspect-ratio NTs, 30 μmthick templates with relatively narrow pores of approximately250 nm diameter at their entrances were used. They were

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Figure 4. SEM images of Ag–NTs synthesized by varying the ligand (magnifications used: (a) 11 000×, (c) 1800×, other images 22 000×).(a) Ligand = 100 mM NH3. (b) Closed tube openings in (a). (c) Ligand = 100 mM en. (d) Magnification of broken tubes (compare (c)).(e) Ligand = 50 mM py.

identically activated and put in reaction solutions containingthe three ligands NH3, en and py.

Our attempts to fabricate NTs utilizing the [Ag(NH3)2]+complex as the Ag source agreed with previous results,revealing that the AgI-tartrate-NH3 plating system is unable tohomogeneously cover surfaces featuring extended cavities [8].In a comparable experiment of electroless Ag deposition onion track etched polycarbonate membranes with a pore size of1 μm, a tube wall thickness of approximately 150 nm wasreached after a reaction time of 2 h, and limited diffusionof reagents hindered the deposition inside the template atlater stages [3]. SEM images of our results are shown infigures 4(a) and (b). After a plating time of just 15 min, thetemplate was already covered with a shiny Ag film. The depositconsists of an incomplete surface film and short fragmentsof Ag–NTs (figure 4(a)). Some of the pore openings arealready blocked (figure 4(b)), rendering further growth ofnanostructures inside the template impossible. Also, ammoniabaths are sensitive to homogeneous nucleation, often showingclouding and development of a brownish colour even at earlystages of the reaction.

In figures 4(c)–(d), the results of the en depositionare shown. During a reaction time of 26 h, Ag–NTswere formed which spanned the whole template thicknessof 30 μm (figure 4(c)). Figure 4(d) illustrates the NTstructure, comprising dense walls which reproduce the smoothpolycarbonate surface and a rough inner wall surface which isdominated by polygonal crystals similar to those obtained inthe thin film experiments (figure 3(b)).

Consistent with the low reaction rate (figures 2(a) and (b)),the py experiments required extended reaction times to yieldtubular structures. Figure 4(e) shows an Ag surface film anda fragment of a NT obtained after 69 h of deposition. As inthe case of en, the porous structure resembles those of the Agdeposits grown on a polycarbonate film.

3.4. Application of NTs in flow-through catalysis

Template-embedded metal NTs constitute parallelled micro-or nanofluidic devices [38, 39], and Au–NTs have beensuccessfully implemented as microreactors in the reductionof 4-nitrophenol by sodium borohydride by simply pressing a

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Figure 5. Morphological and compositional analysis of the nanostructures applied in the catalytic experiments. TEM images of (a) an Ag–NTfield obtained by quick deposition at a pH of 11.6 and (b) a single Ag–NT obtained at a pH of 11.1 (inset shows a magnified particle).(c) SEM image of porous Au–NTs. TEM images of Ag–NT walls with (d) beginning (arrow shows evolving Au structures) and (e) completedAg–Au exchange. EDS spectra of (f) Ag–NTs and (g) the fully developed porous Au structures.

reaction solution through them [22]. Here, we applied Ag–NTs in analogous flow-through experiments. In addition, theusability of Ag–NTs as sacrificial templates was displayed bythe displacement of the Ag structures with Au.

For catalyst membrane preparation, en plating bathsproved to be most suitable in terms of deposition speed,catalytic activity and tube stability after the Ag–Au exchangeand were therefore chosen for the experiments described here.A relatively large pore size of 500 nm was chosen to ensureeasy flow. To evaluate the influence of the plating rate, andtherefore the tube homogeneity, on the catalytic activity, twodifferent pH values were tested. TEM images of microtomecuts of the resulting polymer-embedded Ag–NTs are shown infigure 5(a) (high deposition speed, pH 11.6) and figure 5(b)(reduced deposition speed, pH 11.1). The elliptical appearanceof the NTs in the TEM images is due to transverse cuttingof the cylindrical structures. The microtome cuts both stem

from NT regions located close to the outer template surfacewhere both syntheses are comparable in terms of NP density,morphology and size (approx. 30 nm mean Ag–NP diameter).Figure 5(c) depicts porous Au–NTs yielded by reaction of theAg–NTs with a HAuCl4 solution. At the beginning of thereaction, small Au–NPs evolve on the surface of the polygonalAg grains (figure 5(d)). After completion of the reaction, amesoporous network of segments approximately 15 nm thickis formed (figure 5(e)). The initial structures solely consist ofAg (figure 5(f)). EDS reveals that even after full conversion ofthe precursor NPs some residual Ag is found (figure 5(g)). Inall spectra, the remaining signals (C, O, Cu and Si) originatefrom the polymer and the TEM setup.

The progress of the applied model reaction can be easilymonitored by UV–vis spectroscopy due to the intense colour ofthe 4-nitrophenolate ion (strong absorption at 400 nm) whichdoes not interfere with 4-aminophenolate, the reaction product

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(absorption at 300 nm). From a mechanistic point of view,the reaction follows Langmuir–Hinshelwood kinetics. It isof pseudo-first order towards the 4-nitrophenol concentrationgiven that borohydride is present at large excess, leading to anexponential decay of the reaction educt. Therefore, apparentrate constants for the reaction can be calculated by linearlyfitting the natural logarithm of the normalized 4-nitrophenolconcentration over time [25]. Next to its straightforwardexamination, the presented reaction has a general syntheticimportance: the catalysed reduction of aromatic nitrocompounds by either hydrogen gas or metal hydrides yieldingaromatic amines as relevant synthetic intermediates is of highindustrial interest [40]. For instance, nitrophenol reductionplays a role in the pharmaceutical industry, e.g. for theproduction of acetaminophen (paracetamol), the well-knownanalgesic and antipyretic.

Figure 6 summarizes the results of the performedcatalytic experiments. In figure 6(a), the typical changeof the UV–vis spectrum as the reaction time proceeds(which corresponds to the number of flow-throughs) isillustrated on the basis of the slowly grown Ag–NT membrane(figure 5(b)). As the absorbance at 400 nm is proportionalto the 4-nitrophenolate concentration, the decrease of thepeak intensity documents the educt conversion, while thedeveloping peak at 300 nm indicates product formation. Therelatively low aminophenolate absorption is related to theextinction coefficient of the product, while the presence ofisosbestic points indicates that the reaction does not lead tobyproducts [25]. The catalytic activities of the Ag–NTs and theAu–NTs derived from them are shown in figure 6(b). Whilethere is little difference between the relatively solid Ag–NTsgrown at a given pH and their porous Au–NT counterparts, theactivities of the two pH series vary strongly: the slowly grownNTs are approximately seven times more potent as catalysts.Since both NT series have similar morphologies at theirentrances (see figures 5(a) and (b)), the significantly increasedactivity in case of the slower growth can be attributed to thehigher metal content due to the more homogeneous structurein the central template regions. This result again points out theimportance of slow and controlled electroless plating reactionsfor the synthesis of high aspect-ratio nanomaterials.

Although direct comparison with the results of othergroups exploring Ag catalysts is difficult due to inconsistentreaction conditions (reactant concentrations, catalyst loading,temperature) and the diverse reaction setup, it can be statedthat the activities of the synthesized NT membranes are veryhigh. For instance, the determined apparent rate constantsof Ag–NP covered anion exchange beads [26] and Ag–NP–dendrimer nanocomposites [27] were 3.19 × 10−3 s−1 (at15 ◦C) and 1.22 × 10−3 s−1 (room temperature), respectively.The activity of Ag–NP–chitosan nanocomposites was in therange of the preceding literature [28]. With other metals suchas Pd, much higher apparent rate constants have been reached(e.g. 4.07 × 10−1 s−1 for Pd–dendrimer nanocomposites [27]or 6.58 × 10−2 s−1 for conducting polymer supported Pd–NPs [29]). Compared to our experiments utilizing electrolesslysynthesized fine-grained Au–NTs [22], the slowly grownAg–NT membranes and the corresponding porous Au–NT

Figure 6. (a) UV–vis spectra of the reaction solution (start curve)and the solutions obtained by consecutive flow through the slowlygrown Ag–NT membrane. (b) Time-dependent change of the 400 nmabsorbance of the reaction solutions and calculated apparent rateconstants (slope of linear fits).

membranes are about five times as active. A first factor tobe considered is the relatively high metal loading comparedto the more filigree Au–NTs [22]. As the replacement ofAg by Au did not significantly change the NT performance(although the accessible catalyst surface area should beincreased by the formation of the spongy Au–NP network), theelevated effectiveness of the NTs is probably also related to apronounced intrinsic activity of the Ag structures.

In contrast to conventional setups utilizing dispersedNPs, no catalyst-recycling steps are required in the case ofNT membranes. Next to the good performance and theconvenient handling, catalyst stability is another importantbenefit of the presented system. The particles forming theNT walls are partly connected and anchored to the polymersurface leading to quite resilient structures. Despite the stresscaused by the high solution flow (up to 7 ml min−1) and thehydrogen evolution, the NT membranes are insensitive towardsNP loss. Also, experimental problems related to particlestability [26, 28] and agglomeration [27] are less relevant incase of metal NTs.

4. Conclusion

Coordination chemistry provides valuable strategies forthe optimization of electroless plating procedures in the

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challenging field of nanomaterial synthesis. In this work,the basic nitrogen ligands ethylenediamine and pyridine wereused to refine electroless Ag deposition. Next to the ligand,the pH value is an important controlling factor for both thespeed of the deposition and the structure of the evolving thinfilm. In the case of low pH values and the presence of therelatively strong ligand ethylenediamine the plating reaction isslowed down, leading to rough and porous films at very lowdeposition rates, while smoother films are obtained at moderateand high deposition rates. Depending on the desired physicalproperties of the Ag nanostructures, both regimes can be useful(e.g. compact films in the case of electrical conductivity [41]or rough films in the case of super-hydrophobicity [42]). Also,low deposition rates are positively correlated to the stability ofthe plating baths towards homogeneous nucleation.

The successful synthesis of high aspect-ratio Ag–NTsin ion track etched polymer templates proves that evenvery irregular substrates can be homogeneously covered withnanoscale Ag films. Since this aspect is a common problem inelectroless Ag deposition [3, 8], the new plating proceduresare valuable for the synthesis of advanced nanomaterials.Given sufficient synthetic control, electroless plating is asimple yet powerful method to fabricate metal thin filmsconsisting of NPs of tunable shape, size and density.Besides catalytic applications, the results presented here areparticularly valuable for the fabrication of SERS substrates,where reliable approaches towards morphologically tunable,unprotected Ag surfaces are in high demand [43].

Acknowledgments

We thank Professor Reinhard Neumann and Dr ChristinaTrautmann (GSI Helmholtz Centre for Heavy Ion Research) forsupport with the irradiation experiments and for the possibilityto use the SEM and UV–vis instruments of the materialsresearch group.

References

[1] Watanabe T 2004 Nano-Plating: Microstructure ControlTheory of Plated Film and Data Base of Plated FilmMicrostructure (Amsterdam: Elsevier) pp 121–32

[2] Hidber P C, Nealey P F, Helbig W and Whitesides G M 1996Langmuir 12 5209

[3] Demoustier-Champagne S and Delvaux M 2001 Mater. Sci.Eng. C 15 269

[4] Rao C R K and Trivedi D C 2005 Coord. Chem. Rev. 249 613[5] Gilliam R J, Thorpe S J and Kirk D W 2007 J. Appl.

Electrochem. 37 233[6] Martin C R 1994 Science 266 1961

[7] Barbic M, Mock J J, Smith D R and Schultz S 2002 J. Appl.Phys. 91 9341

[8] Williamson T L, Guo X, Zukoski A, Sood A, Dıaz D J andBohn P W 2005 J. Phys. Chem. B 109 20186

[9] Wirtz M and Martin C R 2003 Adv. Mater. 15 455[10] Haruta M 2007 ChemPhysChem 8 1911[11] Wang H, Xu C, Cheng F and Jiang S 2007 Electrochem.

Commun. 9 1212[12] Jirage K B, Hulteen J C and Martin C R 1997 Science 278 655[13] Sun Y and Xia Y 2004 Adv. Mater. 16 264[14] Bi Y and Lu G 2008 Nanotechnology 19 275306[15] Sieb N R, Wu N C, Majidi E, Kukreja R, Branda N R and

Gates B D 2009 ACS Nano 6 1365[16] Bi Y and Ye J 2010 Chem. Commun. 46 1532[17] Gu X, Cong X and Ding Y 2010 ChemPhysChem 11 841[18] Szabo N J and Winefordner J D 1997 Anal. Chem. 69 2418[19] Formanek F, Takeyasu N, Tanaka T, Chiyoda K,

Ishikawa A and Kawata S 2006 Appl. Phys. Lett. 88 083110[20] Huang H, Kim S H and Yang S M 2011 Lab Chip 11 87[21] Zhang S H, Xie Z X, Jiang Z Y, Xu X, Xiang J, Huang R B and

Zheng L S 2004 Chem. Commun. 9 1106[22] Muench F, Kunz U, Neetzel C, Lauterbach S, Kleebe H J and

Ensinger W 2011 Langmuir 27 430[23] Frost C G and Mutton L 2010 Green Chem. 12 1687[24] Sparreboom W, van den Berg A and Eijkel J C T 2009 Nature

Nanotechnol. 4 713[25] Wunder S, Polzer F, Lu Y, Mei Y and Ballauff M 2010 J. Phys.

Chem. C 114 8814[26] Jana S, Ghosh S K, Nath S, Pande S, Praharaj S, Panigrahi S,

Basu S, Endo T and Pal T 2006 Appl. Catal. A 313 41[27] Esumi K, Isono R and Yoshimura T 2004 Langmuir 20 237[28] Murugadoss A and Chattopadhyay A 2008 Nanotechnology

19 015603[29] Harish S, Mathiyarasu J, Phani K L N and Yegnaraman V 2009

Catal. Lett. 128 197[30] Lee J, Park J C and Song H 2008 Adv. Mater. 20 1523[31] Hariprasad E and Radhakrishnan T P 2010 Chem. Eur. J.

16 14378[32] Ferain E and Legras R 2003 Nucl. Instrum. Methods Phys. Res.

B 208 115[33] Rao C N R and Govindaraj A 2009 Adv. Mater. 21 4208[34] Ohno I, Wakabayashi O and Haruyama S 1985 J. Electrochem.

Soc. 132 2323[35] Lavallee D K and Doi J D 1981 Inorg. Chem. 20 3345[36] Kolodziej B 1983 Can. J. Chem. 61 383[37] Van Poucke L C 1976 Talanta 23 161[38] Kim B Y, Swearingen C B, Ho J A, Romanova E V,

Bohn P W and Sweedler J V 2007 J. Am. Chem. Soc.129 7620

[39] Liu L, Lee W, Huang Z, Scholz R and Gosele U 2008Nanotechnology 19 335604

[40] Tafesh A M and Weiguny J 1996 Chem. Rev. 96 2035[41] Asher T, Inberg A, Glickman E, Fishelson N and

Shacham-Diamand Y 2009 Electrochim. Acta 54 6053[42] Li Y, Li C, Cho S O, Duan G and Cai W 2007 Langmuir

23 9802[43] Park H K, Yoon J K and Kim K 2006 Langmuir 22 1626

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