Electroless plating is a powerful wet-chemical method for the fabrication of metal thin films on arbitrarily shaped substrates [1]. Despite its relative simplicity just involving the immersion of a work piece in a deposition solution, electroless plating is suitable for the creation of macroscopic [1] aswell as intricate nanoscale structures [2–5]. Depending on the type of substrate and depositedmetal, the obtained materials can be utilized in various fields, including electronics, wear and corrosion resistance, medical technology and catalysis [1,6]. The special properties of electrolessly plated metal nanomaterials give rise to particularly interesting applications such as molecular separation [4] or microreactors

1. Surface & Coatings Technology 242 (2014) 100–108
Contents lists available at ScienceDirect
Surface & Coatings Technology
journal homepage: www.elsevier.com/locate/surfcoat
Polymer activation by reducing agent absorption as a flexible tool for the
creation of metal films and nanostructures by electroless plating
Falk Muench a,⁎, Adjana Eils a, Maria Eugenia Toimil-Molares b, Umme Habiba Hossaina,b, Aldin Radetinac a,
Christian Stegmann a, Ulrike Kunz a, Stefan Lauterbach a, Hans-Joachim Kleebe a,Wolfgang Ensinger a
a TU Darmstadt, Department of Materials and Geoscience, Alarich-Weiss-Straße 2, 64287 Darmstadt, Germany
b GSI Helmholtz Centre for Heavy Ion Research GmbH, Department of Materials Research, Planckstraße 1, 64291 Darmstadt, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received 25 November 2013
Accepted in revised form 18 January 2014
Available online 24 January 2014
Keywords:
Polymer swelling
Polymer activation
Metal nanoparticles
Electroless plating
Ion-track etched polymers
Metal nanotubes
The ability to modify the activity of polymer substrates for subsequent electroless plating is of fundamental im-portance
for both the homogeneous metallization of macroscopic work pieces and the fabrication of advanced
nanomaterials. In this study, we demonstrate the high flexibility of a polymer activation technique based on
the absorption of reducing agents, followed by metal nanoparticle precipitation in the presence of metal salt so-lutions.
We show that the process can be applied to polymers with very different chemical properties, namely
polycarbonate, poly(ethylene terephthalate), acrylonitrile butadiene styrene and polyvinyl alcohol. Furthermore,
the effects of important reaction parameters (type of reducing agent, metal precursor, concentration of the
sensitization and activation solutions) on the seeding properties (particle size, shape and density) are evaluated
extensively. Based on these results, synthetic guidelines are provided on how the substrate activity can be
tailored for the successful fabrication of free-standing metal nanotubes in ion-track etched polycarbonate
templates. In addition, we demonstrate that the process can be applied to prepare polymeric films and
three-dimensional structures for the electroless deposition of metal thin films.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Electroless plating is a powerful wet-chemical method for the fabri-cation
of metal thin films on arbitrarily shaped substrates [1]. Despite its
relative simplicity just involving the immersion of a work piece in a
deposition solution, electroless plating is suitable for the creation of
macroscopic [1] aswell as intricate nanoscale structures [2–5]. Depend-ing
on the type of substrate and depositedmetal, the obtained materials
can be utilized in various fields, including electronics, wear and cor-rosion
resistance, medical technology and catalysis [1,6]. The special
properties of electrolessly plated metal nanomaterials give rise to
particularly interesting applications such as molecular separation
[4] or microreactors [5].
During electroless plating, thework piece is continuouslymetallized
by the autocatalytic, surface-selective reaction of a metastable pair
formed by a metal complex and a reducing agent [1,6]. In order to initi-ate
the metal film growth, the substrate surface must be catalytically
active towards the plating reaction. Work pieces consisting of inactive
materials such as polymers require pretreatments which introduce
metal nanoparticles (NPs) to the substrate surface to act as seeds for
the metal deposition. These so-called activation processes determine
the film nucleation in the following plating step and are thus crucial
for the quality of the resulting products [7–12]. Therefore, much
research is focused on the optimization of existing and the development
of newactivation reactions [7–14]. Especially inthefield of nanomaterial
fabrication, high demands concerning the seeding have to be met. For
instance, plating of highly miniaturized circuit paths requires methods
ensuring precisely patterned activation [12], while the deposition of
metal nanotubes (NTs) within the pores of hard templates depends on
the nanoscale homogeneity of the activation on complex shaped sub-strates
[7,8].
In a previous study, we have presented that the absorption of
dimethylaminoborane (DMAB) by slightly swollen polycarbonate (PC)
followed by metal NP deposition is a highly versatile activation proce-dure
for nanomaterial fabrication [8]. From a mechanistic point of
view, thismethod can be distinguished fromotherwet-chemical activa-tion
processes such as the commonly employed aqueous solutions
of Sn(II) and Pd(II), which rely on the limited superficial adsorption of
the reducing agent [1,15] instead of an uptake by the polymer matrix.
Using physical absorption during the sensitization, the reducing power
of the substrate can be adjusted in a wide range [8], and no specific
requirements regarding the surface functionality of the polymer are
set (e.g. providing sites for the complexation of metal ions [10,14]).
Also, possibly detrimental contaminationwith Sn like in the conventional
Abbreviations: NP, nanoparticle; NT, nanotube; PC, polycarbonate; PET, poly(ethylene
terephthalate); ABS, acrylonitrile butadiene styrene; PVA, polyvinyl alcohol; DMAB,
dimethylaminoborane.
⁎ Corresponding author. Tel.:+49 6151 166387; fax: +49 6151 166378.
E-mail address: muench@ca.tu-darmstadt.de (F. Muench).
0257-8972/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.surfcoat.2014.01.024

2. F. Muench et al. / Surface & Coatings Technology 242 (2014) 100–108 101
process can be avoided [15], and seed distributions which are dense and
homogeneous on the nanoscale can be achieved easily [8].
PC and DMAB were chosen as model polymer and reducing agent,
respectively, to provide a proof of principle for the proposed activation
mechanism[8]. However, the two fundamental steps of the technique –
polymer swelling and metal NP precipitation in the presence of reduc-ing
agents – are very common phenomena. Therefore, the process
should be transferrable to a variety of other systems, as we will show
in this paper. Furthermore, we will examine in detail the dependence
of the metal film nucleation on the seeding properties in order to pro-vide
synthetic guidelines for the optimization of activation processes,
emphasizing the flexibility of the swelling-based activation process
and pointing out the importance of the activation conditions for the
consecutive synthesis of metal nanomaterials.
2. Material andmethods
2.1. Chemicals
Milli-Q water (N18 MΩ cm at room temperature) was employed in
all procedures. The following chemicals were applied without further
purification: 1,4-dioxane (Sigma Aldrich, ACS reagent); acetone (vwr,
technical); AgNO3 (Grüssing, p.a.); borane dimethylamine complex
(Fluka, purum); CH2Cl2 (Sigma Aldrich, puriss. p.a.); CuSO4 pentahydrate
(Fluka, purum p.a.); ethanol (Brenntag, 99.5%); ethylene glycol (Roth,
p.a.); ethylenediamine (Fluka; puriss.); formaldehyde solution 36.5%
stabilized with methanol (Fluka, puriss. p.a.); K2PtCl4 (Aldrich, ≥99.9%
tracemetal basis); methanol (Aldrich, 99.8%); NaBH4 (Merck, for synthe-sis);
NaOH solution 32% in water (Fluka, puriss. p.a.); (NH4)3[Au(SO3)2]
stock solution (Galvano Goldbad from Schütz Dental GmbH, 15 g Au
99.9% per liter); N2H4 monohydrate solution 80% in water (Merck, for
synthesis); polyvinyl alcohol (Aldrich, MW: 89,000–98,000); potassium
sodium tartrate tetrahydrate (Fluka, puriss. p.a.); and trifluoroacetic acid
(Riedel-de Haën, N99%).
2.2. Substrate preparation
For the fabrication of polymeric thin films, the correspondingma-terials
(polyvinyl alcohol (PVA): powder from the chemical supplier;
acrylonitrile butadiene styrene (ABS): Lego® bricks) were dissolved
under stirring (PVA: water as solvent; ABS: acetone as solvent) until
viscous, homogeneous solutions were obtained. Subsequently, the
polymer solutions were dripped onto a cleaned glass plate and
streaked slowly with a squeegee. After solvent evaporation in air
for 24 h, the polymer samples were carefully removed from the
glass substrate. To obtain ion-track etched polymer membranes
with channel-shaped pores, PC foils (Makrofol N®, Bayer Material
Science AG, nominal thickness: 30 μm) and poly(ethylene tere-phthalate)
(PET) foils (Hostaphan®, Hoechst AG, nominal thickness:
36 μm) were irradiated with Au ions (energy= 11.1 MeV per nucle-on,
fluence = 1 × 108 ions cm−2, initial charge state: Au25+) at the
GSI Helmholtzzentrum für Schwerionenforschung GmbH (Darmstadt,
Germany). Subsequently, the PET foil was irradiated with UV light
(Privileg UVA lamp, 105 W, 90 min irradiation per side) to increase
etching selectivity. Finally, the polymer foils were etched in stirred
NaOH solutions (50 °C, PC: 6 M NaOH, PET: 4.8 M NaOH, time de-pending
on desired pore diameter), thoroughly rinsed with water
and dried.
2.3. Sensitization, activation, electroless plating
Glassware was cleaned with aqua regia prior to use. Sensitization
was performed by immersion of the polymer substrate in a solution of
a reducing agent (type and concentration is specified in the text). De-pending
on the polymer, different solvents are used for this procedure
(PVA, PC and ABS: methanol; PET: 1,4-dioxane). After 30 min, the
substrates were removed and washed (PVA: ethanol; ABS, PC and
PET: water). Then, the sensitized substrates were transferred to the ac-tivation
solutions, which contained AgNO3 or K2PtCl4 to produce Ag or
Pt seeds, respectively. As solvent, water was used in all cases except
PVA,whichwas activated in ethylene glycol. Unless otherwise specified,
the concentration of the metal salts was held constant at 59 mM, the
same concentration which is applied in the standard activation reaction
of ion-track etched PC [7]. After 15 min activation, the substrates were
washed with water and stored in ethanol for 2 h to remove residual
reducing agent from the sensitization step. Subsequently, the sub-strates
were immersed in the electroless plating baths. Details
regarding the plating reactions can be found in the literature [5,7].
Briefly, the Ag plating bath consisted of 120 mM tartrate (reducing
agent), 17 mM AgNO3 (metal source), 100 mM ethylenediamine
(ligand) and 9.5 mM trifluoroacetic acid (pH adjustment). The Cu
plating bath was composed of 1.2 M formaldehyde (reducing
agent), 100 mM CuSO4 (metal source), 220 mM tartrate (ligand),
240 mM ethylenediamine (ligand) and 230 mM NaOH (pH adjust-ment).
The Au bath used for the PVA foils was not based on water
as the solvent, but on ethylene glycol, containing 600 mMformalde-hyde
(reducing agent) and 7 mM (NH4)3[Au(SO3)2] (metal source,
added in the form of electroplating stock solution).
2.4. Characterization
SEM (JSM-7401F microscope (JEOL), 5–10 kV acceleration voltage):
for the measurement of the nanostructures fabricated in the ion-track
etched PC by electroless plating, the polymerwas dissolvedwith dichlo-romethane
and the remaining material was collected on Si wafer pieces
sputter-coated with Au to improve the conductivity. The metallized
PVA, ABS and PET substrates were analyzed in the presence of the poly-mermatrix.
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) with a diamond knife (DKK)). AFM
(Cypher microscope (Asylum Research)): Unetched PC foils were used
as the samples to provide a planar geometry. The measurements were
performed in tapping (AC) mode operating in the repulsive regime. Al
coated Si cantilevers (PPP-Zeihr-50 (Nanosensors)) with resonance
frequencies ranging from 98 to 177 kHz were used.
3. Results and discussion
All the polymer treatments presented in this work are based on
two reaction steps, sensitization and activation [8]. First, in the sen-sitization
step, the substrate is immersed in a solution containing
a solvent which is able to cause a slight swelling. The second compo-nent
of the solution – a reducing agent – diffuses into the widened
polymer matrix. In the activation step, the polymer substrate is
transferred to a metal salt solution. At the surface of the substrate,
themetal ions and the reducing agent react, leading to the precipita-tion
of metal NPs. These NPs constitute the seeds for consecutive
electroless plating.
The success of a given activation protocol is influenced by each of the
steps involved. In the following, we therefore present the results of the
systematic study of important reaction conditions on the seeding prop-erties
of ion-track etched PC membranes (Section 3.1). In Section 3.2,
differently activated PC templates are used to evaluate the effect of
the seeding on the fabrication of metal NTs. Finally, it will be shown
that the activation process can be transferred to other polymer types
and substrate shapes (Section 3.3). The deposition of the respective
metals in the reactions presented here (Ag and Pt particles after
activation; Ag, Au and Cu films after plating) was confirmed by EDS
(data not shown).

3. 102 F. Muench et al. / Surface & Coatings Technology 242 (2014) 100–108
3.1. Influence of sensitization and activation solution composition on
seeding
3.1.1. PC-sensitization with various reducing agents
We have sensitized ion-track etched PC templateswith a pore diam-eter
of approx. 500 nm and a thickness of 30 μm with four different
methanolic solutions of reducing agents (2 M), namely (i) hydrazine,
(ii) NaBH4, (iii) formaldehyde and (iv) DMAB. In all experiments, the
composition of the activation solution remained unchanged with
59 mM AgNO3 as the metal precursor.
(i) Hydrazine sensitization led to severe embrittlement and frag-mentation
of the polymer foils and was thus not applicable. We
attribute this reactivity to the high susceptibility of PC to the
chemical attack of hydrazine [16].
(ii) With NaBH4, the templates could be stored in the sensitization
solution without significant damage, but no color change was
observed in the activation step, suggesting an insufficient up-take
of the reducing agent. Ionic species such as the reducer
tetrahydroborate have to carry counter-ions with them in
order to maintain charge neutrality, which could hinder
their diffusion into the polymer structure. Diffusion of reduc-ing
agents consisting of neutral molecules, such as DMAB or
formaldehyde, should instead be favored and facilitate their
uptake.
(iii) Accordingly, the uncharged formaldehyde which is also less
aggressive than hydrazine could be successfully implemented
as a reducing agent for the activation of PC alternatively to
DMAB. Fig. 1a,b displays TEM images of etched ion-tracks in
PC after sensitization with 2 M formaldehyde in methanol
and activation with AgNO3. Themicrotome cutting of the acti-vated
template was performed approximately perpendicular
to the long axis of the tubular pores, therefore the pore
cross-sections exhibit sometimes an elliptical shape. Because
Fig. 1. TEMimages ofmicrotome-cut ion-track etched PC templates after Ag activation (left column: survey images; right column:magnified images of individual pore cross-sections). a),
b) Sensitization with 2Mformaldehyde, activation with 59mMAgNO3. c), d) Sensitization with 2MDMAB, activation with 59mMAgNO3. e), f) Sensitization with 2MDMAB, activation
with 10 M AgNO3.

4. Fig. 2. AFM characterization of the PC samples activated with Ag (a–c) and Pt (e–g) using different amounts of DMAB in the sensitization step (0.25 M, 1 M and 2 M DMAB). The scale bar on the left refers to all AFM images. The AFM studies are
complemented by representative TEM images of the Ag (d) and Pt (h) seeds deposited after sensitization with 2 M DMAB.
F. Muench et al. / Surface & Coatings Technology 242 (2014) 100–108 103

5. 104 F. Muench et al. / Surface & Coatings Technology 242 (2014) 100–108
of the similar composition of the embedding resin and the poly-mer
matrix, the channels can only be identified by the presence
of metal NPs on their walls. It can be seen on the images that a
relatively low amount of NPs was found (size ~50 nm), which
made identifications of channel wallsmore challenging. Interest-ingly,
a halo with a contrast differing from the polymer matrix
was observed near the pores (Fig. 1a). This phenomenon was
also noted in related experiments (see Fig. 1c) and is probably
caused by a change of the polymer in the region inwhich the dif-fusion
process during sensitization takes place.
(iv) In Fig. 1c,d, the results sensitization with uncharged DMAB,
followed by activation with 59 mM AgNO3 are shown. In
this case, a higher number of Ag NPs were deposited, and
the channel walls could be identified more easily. Similar to
the formaldehyde-based activation, relatively large NPs in
the size range of some tens of nanometers were obtained.
We attribute the higher amount of deposited Ag to the higher re-ducing
efficiency of DMAB, as the complete oxidation of one mole-cule
of DMAB (Eq. (1)) yields three times more electrons than the
oxidation of one formaldehyde molecule (Eq. (2)). In addition, elec-trochemical
experiments indicate that DMAB is more easily oxidized
on Ag surfaces than formaldehyde [17], suggesting an improved con-version
of the former reducing agent on the Ag seeds evolving during
activation.
BH3
−NðCH3
Þ
2H þ 3H2O→BðOHÞ
3
þ ½NðCH3
Þ
2H2
þ þ 5H
þ þ 6e
− ð1Þ
HCHO þ H2O→½HCOO− þ 3H
þ þ 2e
− ð2Þ
Fig. 3. SEMimages of the Ag nanostructures obtained fromdifferently activated ion-track etched PC templates. a), b) Sensitization with 0.25MDMAB and activation with a) Ag and b) Pt
(b). c), d) Sensitization with 1 M DMAB and activation with c) Ag and d) Pt. e), f) Sensitization with 2 M DMAB and activation with e) Ag and f) Pt.

6. F. Muench et al. / Surface Coatings Technology 242 (2014) 100–108 105
3.1.2. Influence of the AgNO3 activation solution concentration
In an additional experiment, we significantly increased the concen-tration
of the Ag activation solution from 59 mM AgNO3 to 10 M
AgNO3, while keeping the DMAB concentration in the sensitization
step constant at 2 M. Optically, we observed that the color change of
the PC substrate after activation was stronger using the higher concen-tration
of AgNO3. Fig. 1e,f displays TEM images of cross-sections of the
activated PC membrane, evidencing that a large amount of relatively
small NPs was formed completely covering the channel walls. This be-havior
might be explained by the pronounced supersaturation of Ag
caused by the reduction of the very concentrated AgNO3 solution by
DMAB diffusing out of the polymer. While at low Ag concentration
few NPs are formed which grow during ongoing metal reduction, high
supersaturation favors a large number of nucleation events, with less
material deposited on each nucleus [18].
3.1.3. Influence of the DMAB sensitization solution concentration
The DMAB concentration in the sensitization solution affects the up-take
of the reducing agent by the polymer substrate [8] and thus should
allow modification of the amount of metal precipitated during activa-tion
in a controlled manner. We investigated this issue by applying
two different activation solutions (59 mM AgNO3 and 59 mM K2PtCl4)
on three PC templates each sensitized with a different concentration
of DMAB, namely 0.25 M, 1 M and 2 M.
The shape, size and density of the resulting seeds were studied by
TEM and AFM (Fig. 2). Fig. 2a–c shows AFM images of the Ag-activated
samples, in Fig. 2e–g the results for the Pt activation are
displayed. In each row, the DMAB concentration is rising from left to
right. In Fig. 2d and h, TEM images of Ag and Pt seeds obtained with 2
M DMAB are found.
The AFM measurements clearly evidence that the roughness of the
metallized polymer surface increases with the DMAB concentration in
the sensitization solution (Fig. 2a-c, e-g). Because the rootmean square
roughness of the bare PC surface is less than 1 nm even after sensitiza-tion
[8], the observed structures can be related to themetal NPs depos-ited
during activation. This argumentation is supported by TEM images
of NPs created during activation (Fig. 2d,h): The particle morphology
visualized by TEMresembles the surface structure of the corresponding
AFMimages.While Ag tends to form faceted, relatively large and isolat-ed
NPs (Fig. 2d), Pt activation yields aggregates consisting of smaller
NPs (Fig. 2h).
With the lowest DMAB concentration of 0.25 M, only few small NPs
were reduced on the polymer during activation with both Ag and Pt
(Fig. 2a,e). A DMAB concentration of 1 M led to a stronger uptake of
the reducing agent by the polymer and thus enabled the formation of
a much more dense layer of metal NPs (Fig. 2b,f). Compared to Pt, Ag
activation yielded a higher amount of deposited metal and larger NP
sizes. With the most concentrated sensitization solution (2 M DMAB),
even larger, but still mostly isolated particles were found in the case of
Ag (Fig. 2c), while extended clusters were obtained with Pt activation
(Fig. 2g).
3.2. Influence of the substrate activity on the electroless synthesis of metal
NTs
In this section, we demonstrate that the seed metal type and seed
density can be utilized to tailor the substrate activity to affect the
consecutive electroless plating of metal NTs. We fabricated etched ion-track
PC membranes with ~600 nm diameter channels, and activated
them using DAMB, AgNO3 and PtCl4, using identical conditions to the
experiments conducted in Section 3.1.3. Thus we obtained both Ag
and Pt seeds with three different NP loadings. After activation, all tem-plates
were subjected to electroless Ag plating for 20 h.
Fig. 4. a), b) SEM images of Cu NTs fabricated in an ion-track etched PC template sensitized with 1 M DMAB and activated with Pt seeds. c) Magnified SEM image (secondary electron
mode) of two Ag NTs (sensitization: 2 M DMAB, activation: Pt). d) SEM image of the sample shown in c) (backscattered electron mode). The Pt seeds on the outer Ag NT surface can
be clearly identified by the atomic number contrast.

7. 106 F. Muench et al. / Surface Coatings Technology 242 (2014) 100–108
Fig. 3 shows Ag nanostructures obtained sensitization with 0.25
DMAB in the case of a) Ag and b) Pt seeds. In both cases no continuous
Ag NTswere obtained due to the lowdensity of seeds created during ac-tivation
(see Fig. 2a,e). However, relatively dense Ag surface films were
formed on the outer, planar surfaces of the template foil. This result is in
agreement with previous reports [5,19,20], corroborating that the for-mation
of 2D films is easier than the homogeneous coverage of extend-ed
inner surfaces poorly accessible to the reactants.
Fig. 3c,e presents free-standing Ag NTs spanning the complete thick-ness
of the template (~30 μm). These tubes are obtained when a higher
density of Ag seeds is created by sensitization with more concentrated
DMAB solutions, namely 1 M and 2 M.
Fig. 3d,f presents Ag nanostructures obtained with increased seed
density in the case of Pt-seeded experiments. In these cases, only
short NTs are formed adjacent to the planar Ag film on the outer tem-plate
surface. To exclude the possibility that an inhomogeneous seed
distribution with low particle densities within the template channels
was responsible for this result, we performed electroless Cu plating on
an ion-track etched membrane activated with Pt seeds after sensitiza-tion
with 1 M DMAB. Fig. 4a,b evidences the formation of well-defined
Cu NTs. Because an identical amount and distribution of seeds did not
suffice to enable the formation of extended free-standing Ag NTs
(Fig. 3d), the Pt NPs initiated metal film nucleation more reliablywithin
the depth of the template pores when comparing Cu with Ag plating.
This result points out that the activation parameters of the substrates
used in the electroless fabrication of nanomaterials must be optimized
with respect to the applied plating solution.
We found that in the case of the employed electroless Ag plating re-action,
Pt seeding led to less Ag deposition within the template com-pared
to Ag seeding under identical plating conditions (Fig. 3). This
indicates a slower formation of the Ag filmin the Pt-activated template,
which can be explained by a reduced activity of the Pt seeds in the
Fig. 5. Characterization of themetal films plated on different polymers after sensitization with DMAB and activationwith Ag. a), b) SEM images of the nanostructured Ag film on ion-track
etched PET. c) SEMimage of the Ag film deposited on ABS foil. d) Photograph of a Lego® brick after electroless Ag plating. The light golden color of the work piece is often observed in the
case of Ag films roughened on the nanoscale [5,29]. e), f) SEMimages of the Au film deposited on PVA foil. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)

8. F. Muench et al. / Surface Coatings Technology 242 (2014) 100–108 107
plating reaction. Ideally, metal deposition should occur faster on the
seed particles than on the plated metal film, because in this case
substrate areas where film nucleation has not started yet are preferred
deposition sites [21]. Due to their improved activity compared to the
Pt seeds, Ag seeds are therefore preferred to obtain free-standing Ag
NTs of high aspect ratio in the reaction conditions described here.
Despite the fact that no continuous Ag NTs were obtained in the Pt
activation experiments (Fig. 3b,d,f), it is important to note that the
outlined reaction scheme can be used to create metal NTs decorated
with NPs on their outerwalls consisting of a secondmetal (Fig. 4c,d). Bi-nary
NTs represent an upcoming catalyst systembenefiting fromboth a
unique morphology and synergistic effects [22,23], thus activation with
a seed metal differing from the NT material is an interesting and
straightforward route towards metal NTs with improved functionality.
3.3. Activation protocol for different polymers
In the study introducing the swelling activation technique [8], PC
was used because of its privileged role as a template for the deposition
of high aspect ratio nanomaterials such as nanowires [24] and NTs [7].
To prove the universality of the activation scheme, it is now shown
how the process can be transferred to other polymers. Three polymer
types with significantly deviating properties were chosen, namely
(i) PET, (ii) ABS and (iii) PVA. For all three polymers, we used technical
polymer data sheets, and it was of high importance to choose a sensiti-zation
solvent which causes slight swelling of the substrate surface
without damaging themorphology of thework piece or even dissolve it.
(i) PET is a polyester like PC, but is characterized by a higher chem-ical
stability and thus can be applied undermore aggressive con-ditions
[25]. Given the stability of PET in methanol, 1,4-dioxane
was chosen as solvent for the sensitization of PET membranes.
Using DMAB as the reducing agent, Ag nuclei were deposited in
the activation step, followed by electroless Ag plating. During
activation, the PET template turned brownish, indicating the cre-ation
of Ag seeds. After electroless plating, a metallic gray luster
appeared. The creation of a homogeneous thin film composed
of the relatively large NPs typical for electroless Ag [5] was con-firmed
by SEM (Fig. 5a,b). The porous structure of the template
was fully preserved during the treatment (Fig. 5a,b). As can be
seen in Fig. 5b, Ag deposition also took place on the inner surfaces
of the PET substrate.
(ii) ABS is an industrially important polymer frequently used for elec-troless
plating [1,26]. Due to its hydrophobic nature, etching and
oxidizing steps are usually required to provide adhesion for plat-ed
metal films and to introduce polar surface functionalities for
activation procedures [26,27]. The sensitization procedure used
for PC could directly be transferred to ABS. After being sensitized
in amethanolic solution of DMAB, activation in an aqueous solu-tion
of AgNO3 yielded intensely brown-colored ABS substrates
which could be evenlymetallized by consecutive electroless plat-ing
(Fig. 5c,d). Dense Ag films were obtained, as expected from
the high seed loading indicated by the strong color shift during
activation [7]. The presence of a nanostructured Ag film was con-firmed
with SEM (Fig. 5c). Both home-made ABS foils (Fig. 5c)
and complex shaped, macroscopic work pieces such as Lego®
bricks (Fig. 5d) were successfully plated.
(iii) PVA is an extremely hydrophilic and biocompatible polymer [28].
The home-made PVA foils used for the experiments were water
soluble. Thus, all reaction steps (sensitization, activation, electro-less
plating) were performed in non-aqueous solvents. While
DMAB sensitizationwas performedwithmethanol, ethylene gly-col
was used for activation because of its improved solubility of
inorganic salts such as the AgNO3 used as metal source. After ac-tivation,
the brownish PVA foilswere placed in an ethylene glycol
based electroless Au plating bath. During the storage of the
polymer foil in the reaction solution, a color shift to blue was
observed in addition with a slight bronze-colored luster. SEM
measurements confirmed the presence of a porous nanoscale
Au film (Fig. 5e,f).
In all shown experiments, metal filmadhesionwhich displays an es-sential
issue in electroless plating [1,26] was unproblematic. No delam-ination
of the metal films was observed after plating as well as during
scotch tape tests. This is expected in the case of the ion-track etched
PC and PET templates, since the polymer foils contain a high density of
pores which reliably anchor themetal films on their surfaces. Consider-ing
the ABS substrates, a certain roughness was found after swelling
activation and plating (see Fig. 5c), which is probably the cause for the
good metal film adhesion. The bonding of the Au film to the PVA foil
probably benefited from its nanoscale structure, because it is well
known that ensuring metal adhesion is especially challenging for large
film thicknesses [1]. Consequently, in syntheses using extended plating
times and smooth PC foils not containing pores from ion-track etching,
we frequently observed metal film peeling. This is not surprising, since
the applied reaction conditions do not increase the surface roughness
of the utilized PC foils significantly [8], which is necessary to improve
mechanical bonding due to the otherwise mostly weak interactions be-tweenmetals
and polymers [1,26,27,30].While the conservation of fine
morphological features is important for template-based nanomaterial
fabrication, in such cases, surface-roughening pretreatments [1,30] are
required before sensitization.
4. Conclusion
In this study, the activation of polymer substrates sensitized by the
absorption of reducing agents in the presence of swelling solvents was
demonstrated to be a universal and highly flexible process. By choosing
appropriate solvents causing a controlled modification of the polymer
surface without destroying the substrate morphology, four polymers
with strongly differing chemical structures were successfully activated
for consecutive electroless plating. Also, the reducing agent and seed
metal could be varied.
The reaction conditions during sensitization and activation strongly
affected the formation of metal NPs. Highmetal loadings were obtained
by sensitization with concentrated solutions of the strong reducing
agent DMAB. The seed particle shape, size and distribution depended
on the composition of both the sensitization and activation solutions,
with a particularly pronounced impact of the metal precursor type.
Because the activation quality is of major relevance for the electro-less
synthesis of metal thin films and nanomaterials derived thereof,
the effect of the seeding parameters on the fabrication of free-standing
metal NTs was evaluated. To achieve this challenging prod-uctmorphology,
an appropriate seed loading proved to be necessary,
but not sufficient. Generally, an adequate density of seeds which are
active in the corresponding plating reaction and ensure reliable
nucleation is required to homogeneously cover complex shaped sub-strates
with nanoscale metal films.
Acknowledgments
We thank Prof. Dr. Christina Trautmann (GSI Helmholtzzentrumfür
Schwerionenforschung GmbH (Darmstadt, Germany)) for supportwith
the irradiation experiments and for providing access to the HRSEM of
the materials research group. The authors highly recognize the synthe-ses
performed by Sebastian Bohn.
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