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Polycarbonate activation for electroless platingby dimethylaminoborane absorption and subsequentnanoparticle deposition
Falk Muench • Sebastian Bohn • Markus Rauber • Tim Seidl •
Aldin Radetinac • Ulrike Kunz • Stefan Lauterbach •
Hans-Joachim Kleebe • Christina Trautmann • Wolfgang Ensinger
Received: 26 July 2013 / Accepted: 23 October 2013 / Published online: 22 November 2013
� Springer-Verlag Berlin Heidelberg 2013
Abstract Electroless plating of metal films on polymer
substrates usually requires the presence of metal particles
acting as catalytically active nuclei for the deposition
reaction. Herein, we present a novel and versatile approach
towards the activation of polycarbonate substrates with
metal nanoparticles. It is based on the diffusion of dim-
ethylaminoborane into the polymer matrix, followed by
reaction of the sensitized substrates with metal salt solu-
tions. The reducing agent uptake was controlled by
changing the duration of the sensitization and the dimeth-
ylaminoborane concentration in the sensitization solution.
Different seed types (Ag, Au, Pd, Pt and Rh) were
deposited by variation of the activation solution. The pro-
posed mechanism was confirmed with FTIR and TEM
measurements. In addition, AFM revealed that apart from a
slight roughening in the nanometer range, the surface
morphology of the polymer remained unchanged, render-
ing the method viable for template-based nanomaterial
fabrication. Due to its pronounced variability, the new
technique allows to tailor the activity of polymer substrates
for consecutive electroless plating. The feasibility and
nanoscale homogeneity of the process were proven by the
electroless fabrication of well-defined Au and Pt nanotubes
in ion-track etched polycarbonate templates. The combi-
nation of features (use of simple and easily scalable wet-
chemical processes, facile seed variation, high activation
quality on complex surfaces) renders the outlined tech-
nique promising for the fabrication of intricate nanomate-
rials as well as for the metallization of macroscopic work
pieces.
1 Introduction
Electroless plating (EP) is a facile, solution-based
approach towards metal thin films based on the surface-
selective, autocatalytic reduction of metal complexes by
chemical agents. This class of reactions is used to create
materials for various applications, including surface pro-
tection, electronics, membrane and medical technology,
catalysis and lightweight components [1–3]. Contrary to
the related electrochemical deposition of metals, EP
allows to coat insulating substrates, e.g. to metallize
polymers [2]. In addition, very complex structures can be
covered with films of excellent homogeneity. This
remarkable flexibility concerning both the material and
shape of the substrate as well as the high deposit quality
allowed the synthesis of precisely defined metal micro-
and nanostructures. For instance, three-dimensional sub-
strates obtained by two-photon polymerization [4], ion-
track etched polymer membranes [5], diatom shells [6]
and surfaces patterned with self-assembled monolayers [7]
could be decorated by EP to yield arbitrary shaped metal
structures [4], metal nanotubes (NTs) [5], replica of
intricate biotemplates [6] and nanoscale circuits [7],
respectively.
Electronic supplementary material The online version of thisarticle (doi:10.1007/s00339-013-8119-z) contains supplementarymaterial, which is available to authorized users.
F. Muench (&) � S. Bohn � M. Rauber � T. Seidl �A. Radetinac � U. Kunz � S. Lauterbach � H.-J. Kleebe �C. Trautmann � W. Ensinger
Department of Materials and Geoscience, TU Darmstadt,
Alarich-Weiss-Straße 2, 64287 Darmstadt, Germany
e-mail: muench@ca.tu-darmstadt.de
M. Rauber � T. Seidl � C. Trautmann
Department of Materials Research, GSI Helmholtz Centre for
Heavy Ion Research GmbH, Planckstraße 1, 64291 Darmstadt,
Germany
123
Appl. Phys. A (2014) 116:287–294
DOI 10.1007/s00339-013-8119-z
However, as a prerequisite for utilization in EP, the
surface of a substrate has to be catalytically active
towards the plating reaction [2, 3]. Once a suitable work
piece is immersed in the plating bath, the deposition
reaction is initiated by catalytic centers present at the
solution-substrate interface. After a metal layer is formed,
the autocatalytic nature of the plating reaction leads to a
continuing film growth. Most substrates besides metals
are not intrinsically active, thus efficient pretreatment
processes are required to enable further EP processing.
Among inert materials, polymers are of particular
importance for plating, being used e.g. for electronic
devices [8] (including advanced designs such as flexible
circuitry [9]), for the template-based synthesis of nano-
structures [5] and for the fabrication of composite
materials [10].
Accordingly, considerable effort is focused on the
development and optimization of procedures to introduce
catalytic activity to polymer surfaces [3, 8–20]. In all cases,
metal seeds on the polymer surfaces act as nuclei for
consecutive EP. Activation approaches most commonly
employed can be divided into three categories:
1. A reducing agent on a polymer surface causes pre-
cipitation of metal particles in situ when the substrate
is brought into contact with a solution containing metal
cations. The perhaps most commonly used activation
procedure, which relies on the reduction of noble metal
salts such as Ag(I) or Pd(II) by surface-bound Sn(II),
belongs to this category [2, 3, 11]. In addition, the
polymer surface itself can be rendered reducing, as it
was performed by covalent attachment of hydrazine
[12, 13] or reduction of the polymer itself [14, 15].
2. Reversely, the polymer surface can be covered or
infiltrated with metal cations, which are reduced in a
second step. For instance, the surface of polyimide was
treated with hydroxide to yield carboxylate moieties,
allowing the formation of complexes with Cu(II) [16]
or Ag(I) [17]. The conversion of the complexes to Cu
and Ag nanoparticles (NPs) occurred by chemical
reduction [16] and annealing [17], respectively. In an
analogous manner, etched acrylonitrile-butadiene-styr-
ene films absorb Cu(II) ions which can be subse-
quently reduced to yield Cu particles [18]. If no
suitable binding sites are available, polymer surfaces
can be modified with other materials containing ligand
functionalities. For example, chitosan was used to
anchor Ni(II) ions [8], while grafting allowed the
localized attachment of Cu(II) [9] and thus patterned
EP after the formation of Cu NPs. Similarly, methac-
rylate spheres were modified with different amines to
improve the seed density [19].
3. Finally, pre-synthesized metal seeds can be deposited
on the polymer surface. This strategy includes
activation with Pd aerosols [10] or printing of Pd
colloids [20].
Each of the approaches described above has distinct
advantages and disadvantages. Disadvantages include
laborious and time-consuming fabrication steps [12, 13, 16,
19] or the need of special chemicals and reaction condi-
tions [10, 15]. If excess amounts of expensive metals such
as Pd are required, the commercial utilization of the tech-
niques can be constricted [8, 18]. Methods such as aerosol
activation and printing are unable to homogeneously acti-
vate complex shaped, three-dimensional substrates [9, 10,
20], and many processes require defined chemical func-
tionalities in the polymers [8, 11–19]—e.g. polar surface
functionalities forming complexes with metal ions or
groups allowing to modify the chemical structure of the
polymer.
These challenges evidence the need to develop efficient
and flexible activation procedures. In this study, we intro-
duce a surprisingly simple and versatile route to achieve
homogeneous attachment of metal NPs on polycarbonate
(PC) substrates. The method is based on the uptake of the
reducing agent dimethylaminoborane (DMAB) by the
polymer matrix and belongs to the first above-mentioned
category. DMAB is a quite versatile chemical, which is
often found in electroless plating bath formulations [2]. But
like in our case, it can also be applied in activation pro-
cedures [16, 18]. The excellent homogeneity of the new
activation procedure at the nanoscale is proven for complex
shaped polymer templates by fabricating metal NTs inside
ion-track etched membranes.
2 Experimental
2.1 General, chemicals
Glassware was cleaned with aqua regia prior to use. All
solutions were freshly prepared. Milli-Q water
([18 MX cm at room temperature) was employed in all
procedures. The following chemicals were applied without
further purification:
4-(dimethylamino)pyridine (Fluka, puriss.); AgNO3
(Grussing, p.a.); AuCl3 hydrate 50 % Au (Fluka, purum);
borane dimethylamine complex (Fluka, purum); CH2Cl2(Sigma Aldrich, puriss. p.a.); ethanol (Brenntag, 99.5 %);
ethylenediamine (Fluka, puriss.); formaldehyde solution
36.5 % stabilized with methanol (Fluka, puriss. p.a.); H2PtCl6solution 8 % in water (Sigma Aldrich); methanol (Aldrich,
99.8 %); N2H4 monohydrate solution 80 % in water (Merck,
for synthesis); NaOH solution 32 % in water (Fluka, puriss.
p.a.); Na2SO3 (Merck, p.a.); NH3 solution 33 % in water
(Merck, puriss.); (NH4)3[Au(SO3)2] solution (Galvano Au
288 F. Muench et al.
123
bath from Schutz Dental GmbH, 15 g Au 99.9 % per liter);
PdCl2 (Fluka, purum); RhCl3 hydrate (Aldrich, 38–40 % Rh
content); SnCl2 dihydrate (Sigma-Aldrich, ACS reagent); tri-
fluoroacetic acid (Riedel-de Haen,[99 %).
2.2 Fabrication of ion-track etched polymer templates
PC foils (Makrofol� from Bayer Material Science AG,
nominal thickness: 30 lm) were irradiated with Au ions
(energy = 11 MeV nucleon-1, fluence = 1 � 108 ions cm-2)
at the GSI Helmholtz Centre for Heavy Ion Research. The
polymer foils were etched in stirred soda lye (50�C, NaOH
(6 mol L-1), etching time depending on desired pore
diameter). Finally, the templates were rinsed with water
and dried. As the pores created by the track etching process
already provide an excellent adhesion of metal films to the
substrate, further surface roughening steps which are usu-
ally crucial for EP reactions [3, 18] are not required.
2.3 Template activation and plating
In the case of the conventional Pt NT synthesis, the ion-track
etched PC template was activated with Ag NPs by consecutive
immersion in a sensitization solution (SnCl2 (42 mmol L-1)
and CF3COOH (71 mmol L-1) in water:methanol = 1:1) and
an activation solution (AgNO3 (59 mmol L-1) and NH3
(230 mmol L-1) in water). In between, the polymer foil was
washed with ethanol. The procedure was repeated three times
to ensure a high seed density [11].
In the case of the new activation procedure, the tem-
plates were immersed in methanol containing defined
amounts of DMAB. Unless otherwise mentioned, a sensi-
tization time of 30 min was applied. Consecutively, the
templates were washed with water twice before being
transferred to aqueous activation solutions prepared with
RhCl3, PdCl2, H2PtCl6, AgNO3 and AuCl3 (metal con-
centration: 59 mmol L-1). After 15 min reaction, the
templates were washed with water and stored in ethanol for
2 h to remove residual DMAB. All activation reactions
were performed at room temperature.
The activated templates were transferred to the EP baths
after a final washing step with water. Details concerning
the applied EP reactions can be found in the literature [5,
11, 21].
2.4 Analysis
SEM (JSM-7401F (JEOL), 5–10 kV acceleration voltage):
For sample preparation, the polymer was dissolved with
dichloromethane and the metal nanostructures grown
inside the templates were collected on Si wafer pieces
sputter-coated with Au.
TEM (CM20 microscope (FEI, Eindhoven, The Neth-
erlands), 200 kV acceleration voltage, LaB6 cathode): The
NT-containing templates were embedded in Araldite 502�
(polymerization at 60 �C for 16 h) and examined as
ultrathin sections [70 nm thickness, Ultracut E ultrami-
crotome (Reichert-Jung) with a diamond knife (DKK)]. In
combination with TEM, EDS was performed [Oxford
Model 6767 EDS system (England)].
AFM [MFP-3D microscope (Asylum Research)]: the
samples were mounted on flat glass substrates with adhe-
sive tape and probed in tapping (AC) mode operating in the
attractive regime with low force. Pt-coated Si cantilevers
(from NT-MDT) with resonance frequencies ranging from
140 to 390 kHz were used.
FTIR (Magna-IR 550 spectrometer (Nicolet), 2 cm-1
spectral resolution): the spectra were recorded in trans-
mission mode using 16 coadded scans. The area of the
analyzed absorption bands was determined with the base-
line method using a peak fitting software (fityk V0.9.8,
open source).
3 Results and discussion
3.1 Synthetic scheme
It is known from ion-track template preparation that the
presence of methanol in the etchant increases the etch rate
and favors the development of (bi)conical channels [22],
probably due to an improved accessibility of the polymer
strains for the decomposing chemicals caused by a slight
swelling. In a similar way, swelling can be used to enhance
the transport of other species into or out of a polymer
matrix [23].
The presented activation approach similarly utilizes the
improved permeability of polymers treated with suitable
solvents. First, a reducing agent is absorbed by a swollen
polymer. In the second step, the modified polymer is
transferred to a metal salt solution. Here, the reducing
agent is released again. At the polymer-solution interface,
the reducer and the metal cations react, causing the pre-
cipitation of metal NPs on the polymer surface. These NPs
then act as seeds for consecutive electroless plating. The
overall scheme is summarized in Fig. 1.
The outlined process must fulfill following require-
ments. First, the selected solvent has to cause the swelling
of a polymer substrate without damaging its structure.
Second, a reducer is needed which can be dissolved in the
swelling agent and transferred into the polymer matrix.
Furthermore, the absorption should be reversible, and when
released to the metal salt solution, it has to be strong
enough to efficiently reduce the metal ions and create metal
NPs.
Polycarbonate activation for electroless plating 289
123
In the following sections, we will provide experimental
support for the proposed reaction scheme. FTIR and AFM
investigations are presented to discuss changes of the
polymer structure and composition, while TEM measure-
ments are used to prove the creation of NP seeds during the
activation step. Finally, it will be illustrated how the novel
activation reactions can be employed to create sophisti-
cated metal nanostructures in ion-track etched polymer
membranes with EP. These templates are fabricated by
irradiation of polymer foils with swift heavy ions, which
completely penetrate through the material. Given by the
large amount of deposited energy, each ion produces a
nanometric cylindrical damage zone along its trajectory
which can selectively be dissolved and converted into an
open high-aspect ratio nanocapillary/channel/pore [24].
The pore length is given by the foil thickness and the
diameter is controlled by the etching process. Because each
ion leads to the formation of one pore, the pore density is
directly correlated to the ion fluence, which can be freely
adjusted. Therefore, templates with strongly differing yet
precisely defined pore structures can be created, marking
the importance of this template class for the fabrication of
nanomaterials [24, 25]. Due to the particular relevance of
ion-track etched PC [11, 25], this polymer was chosen for
the following experiments. Besides its privileged role in
electroless plating [2, 16, 18], DMAB was selected as a
model reducing agent because it displays IR bands which
can be easily detected in the presence of PC (see Sect. 3.2)
3.2 Infrared analysis of the DMAB transport
To verify the proposed uptake of reducing agent by the
polymer substrate, the interactions between the reducing
agent, the solvent and the polymer were studied by means
of FTIR. For this purpose, the spectra of a pristine ion-track
etched PC template and of a sensitized template were
recorded. Sensitization was performed by soaking the
polymer membrane in a solution of DMAB (1 mol L-1) in
methanol for 30 min. In addition, a polymer sample treated
with DMAB-free methanol was measured to evaluate the
influence of the pure solvent. No significant differences
between the spectra of pristine and methanol-swollen PC
membranes were found, confirming that the solvent itself
does not affect the chemical structure of the polymer (see
Supplementary Material, Fig. S1). However, if the reducing
agent was added to the methanol, DMAB-related absorp-
tion bands appeared (see Fig. 2). Clearly distinguishable
peaks in the range of 3,300–3,200 cm-1 and 2,400–2,300
cm-1 are visible in the otherwise polymer-dominated
spectrum.
By fitting, the maxima of the relevant peaks were
determined as 3,280, 3,246, 2,364 and 2,304 cm-1. The
first two signals can be assigned to N–H stretching vibra-
tions, the others to B-H stretching vibrations, agreeing well
Fig. 1 Scheme of the swelling-based activation process, showing the
use of ion-track etched polymer templates to create tubular metal
structures with EP. 1 The polymer substrate is immersed in a solution
of a reducer, leading to a swelling of its surface and diffusion of the
reducing agent into it. 2 The sensitized substrate is transferred to a
solution containing metal cations. Metal reduction causes NP
formation on the surface. 3 The pretreated substrate is ready for
electroless plating, in which a metastable redox pair involving a metal
complex and a reducing agent is applied. The species only react on
the catalytically active surface, leading to the growth of a metal film
Fig. 2 IR spectra of a untreated and b sensitized ion-track etched PC
membranes. Consequently to sensitization, the PC membrane was
washed shortly with water to remove superficial sensitization solution
and dried
290 F. Muench et al.
123
with the values reported for pure DMAB [26] and similar
aminoboranes [27]. The corresponding force constants of
the chemical bonds were calculated to be approximately
580-590 N m-1 (14N-1H, 3,280 and 3,246 cm-1) and
290–300 N m-1 (11B-1H, 2,364 and 2,304 cm-1). Due to a
strong overlap with polymer bands, an additional B-H
mode could not be effectively isolated. Both the excellent
correlation of the peaks to pure DMAB and the absence of
changes in the polymer background lead to the conclusion
that the reducing agent is physically bound within the
polymer matrix.
To investigate the reversibility of the DMAB transport,
FTIR measurements were carried out with templates lea-
ched in water or methanol after the methanol-induced
infiltration of DMAB (Fig. 3). In both cases, the intensity
of the DMAB-related peaks was reduced. To analyze the
treatment-dependent relative loss D crel of DMAB, semi-
quantitative calculations were performed under the
assumption of Beer-Lambert’s law using integrated peak
absorbances A:
Dcrel ¼ADMAB
APC� ADMAB;leached
APC;leached
ADMAB
APC
ð1Þ
For the polymer- and DMAB-related absorbances, a
pronounced and well separated polymer absorption band at
600-520 cm-1 and the previously described N-H and B-H
peaks were integrated, respectively. Using linear back-
grounds, the decrease of the sum of the peak areas assigned
to N-H and B-H corresponds to DMAB losses of 54 %
(water-leaching) and 94 % (methanol-leaching). From
these results, it can be stated that the storage of the DMAB-
infused templates in solvents leads to a diffusion of the
reducing out of the polymer whose extent strongly depends
on the nature of the solvent. In the case of methanol, the
swelling of the PC is maintained, leading to an enhanced
mobility of the DMAB in the widened matrix which
facilitates diffusion. With non-swelling solvents such as
water, slower exchange is observed. In addition, the
methanol experiment proves that a nearly complete
removal of DMAB can be achieved. This result strongly
supports the proposed mechanism involving the reversible,
noncovalent absorption of the reducing agent.
Finally, the dependence of the reducing agent uptake on
the sensitization time and the sensitization solution con-
centration was evaluated (see Supplementary Material, Fig.
S2). In the time-dependent DMAB absorption by the PC
membrane, a fast saturation was observed. After 10 min,
more than 50 % of the DMAB band intensity of the
maximum sensitization time (180 min) was reached.
Probably, the transfer of DMAB into the substrate quickly
decreases with the diffusion layer advancing into deeper
polymer layers. Considering the effect of the sensitization
solution concentration, a pronounced increase of the
DMAB uptake was found throughout the whole investi-
gated concentration range (0.1–3 mol L-1). To adjust the
DMAB content and thus the reducing power of the PC
substrate in the following activation step, it is therefore
most efficient to change the concentration of the sensiti-
zation solution while using a sufficiently long sensitization
time.
3.3 Morphological changes during sensitization
The influence of the sensitization procedure on the PC
surface morphology was investigated with AFM. Due to
the production process, the PC foil has a rough and a
smooth side. Also, optical microscopy investigations show
many scratches, spikes and calderas on both sides. For the
AFM investigation, an area on the smooth polymer surface
with nearly none of these artifacts was chosen to minimize
the background roughness.
Pristine, unetched PC foil and an identical sample sen-
sitized for 30 min in a methanolic solution of DMAB
(1 mol L-1) were measured and compared. The corre-
sponding AFM images can be found in the Supplementary
Material (Fig. S3). The pristine polymer possesses a very
smooth and homogeneous surface with a root mean squared
roughness (RRMS) of 0.3 nm and a maximum height dif-
ference of 3.3 nm. The surface of the DMAB-treated PC
Fig. 3 IR spectra of ion-track etched PC membranes sensitized as in
Fig. 2b after a leaching in water for two hours and b leaching in
methanol for two hours
Polycarbonate activation for electroless plating 291
123
appears rougher, but is still extremely uniform. The eval-
uation yields a RRMS value of 0.6 nm and a maximum
height difference of 5.7 nm. This indicates that sensitiza-
tion only causes a slight surface roughening, which allows
applying it to nanostructured polymer templates without
losing a significant amount of morphological information.
However, the increased roughness could affect the nucle-
ation of nanoparticles and enhance their anchoring to the
polymer surface in the following reaction step.
3.4 Creation of metal seeds during activation
When the DMAB-soaked ion-track etched PC templates
were brought into contact with metal salt solutions, ele-
ment-specific color changes occured which were ascribed
to the formation of metal NPs (Au: red, Ag and Pd: gray-
ish-brown, Pt and Rh: gray; for examples, see the Sup-
plementary Material, Fig. S4). The intensity of the color
which is associated to the amount of reduced metal
increased with increasing DMAB content of the PC
membranes.
To verify the presence of metal NPs, cross-sections of
the activated templates were analyzed with TEM. Fig. 4
shows the results for the activation with different metal
precursor solutions (metal ion concentration:
59 mmol L-1) after sensitization with DMAB
(2 mol L-1). Although some particle dislocation occured
during the sample preparation, it can be seen that NPs were
deposited on the channel walls within the polymer which
appear elliptical in the cross sections due to the microtome
cutting not performed perfectly perpendicular to the
cylindrical pores. EDS confirmed the presence of the
expected metal types (not shown). In the case of Ag and
Pd, the NPs were facetted, whereas Au, Pt and Rh yielded
more globular or irregularly shaped NPs. While mostly
isolated particles were obtained with Ag, Au and Pd, par-
ticle clusters were found in the case of Pt and Rh. Also, the
particle size strongly depended on the seed metal. While
the Ag, Au and Pd particles were in the size regime of
some tens of nanometers, the Rh and Pt seeds were
significantly smaller than 10 nm (magnified images of the
different particles are found in the Supplementary Material,
Fig. S5). This may be explained with the lower critical
nucleus size of the latter two metals which have the highest
melting points of the used element ensemble.
3.5 Fabrication of metal NTs
By covering the walls of the cylindrical channels of ion-
track etched polymer templates with nanoscale metal films,
NTs are obtained [11]. These one-dimensional nanostruc-
tures have a number of desired properties such as high
intrinsic activity or straightforward functionalizability,
which led to their implementation, e.g., in heterogeneous
catalysis [5, 21, 28], molecular separation [29] or sensing
[29, 30].
For homogeneous NT formation it is crucial to achieve a
high density of equally distributed seeds during the acti-
vation process [11]. Thus, a detailed morphological char-
acterization of NTs fabricated by EP is a convenient way to
evaluate the quality of an activation procedure. In addition,
the conventional activation approach for ion-track etched
polymer templates has several drawbacks such as moderate
seed activity or contamination of the resulting NTs with
elements stemming from the template pretreatments [11].
Improved activation schemes are thus valuable to solve
synthetic problems related to the fabrication of these highly
interesting, anisotropic nanomaterials.
The possibility to employ different seed metals offered
by the presented activation technique can be used to
optimize the results of EP. As an example, we examined
the fabrication of Pt NTs by deposition with a known
system based on Ag-activated ion-track etched PC [21].
In the reported synthesis, a relatively high amount of Ag
seeds applied during repeated reaction of surface-bound
Sn(II) with Ag(I) was required to ensure a sufficiently
homogeneous Pt deposition [11, 21]. This reactivity
complicates the formation of stable Pt NTs with closed
walls, as can be seen by porous, partly fragmented tubes
obtained with the standard activation (Fig. 5a, b). The
Fig. 4 TEM images of cross-sections of activated templates. a Rh seeds. b Pd seeds. c Pt seeds. d Ag seeds. e Au seeds
292 F. Muench et al.
123
nanostructures are composed of Pt grains which evolve
from the catalytic nuclei stemming from activation.
Although NTs are formed, they remain inhomogeneous
due to the relatively low density of seeds which are
capable of initiating EP. Utilizing the new protocol, Pd
seeds were attached to the polymer surface, facilitating
extremely homogeneous Pt deposition (Fig. 5c, d).
Smooth and continuous Pt films of very uniform thick-
ness were plated on the outer template surface, and well-
defined NTs were obtained in the template channels. This
behavior can be explained by the higher activity of Pd
compared to Ag in the oxidation of hydrazine [31],
allowing a more efficient supply of electrons for the
reduction of the Pt(IV) complex in the plating bath. Also,
the activity of the Ag seeds in the standard synthesis may
be negatively affected by the presence of chloride stem-
ming from the Pt precursor H2PtCl6.
Besides a more efficient activation, the new process
allows to avoid the contamination of the plated structures
with elements introduced during the activation step. For
instance, Sn and Cl were found in the Ag NPs deposited by
sensitization of surfaces with SnCl2 solution [11, 32]. Also,
due to the incompleteness of exchange reactions [33], it is
very likely that some Ag remains in Au structures created
by EP on Ag-activated templates. This issue may account
for the unexpected reactivity [34] of Au NTs in the oxi-
dation of CO [28]. Usually, catalytic activity is reported for
small, supported Au NPs, but it is known that much larger
Au nanostructures such as sponges or tubes remain highly
active in the presence of small amounts of Ag [34]. To gain
further insight into the mechanisms of Au catalysis, it
would be interesting to create Au NTs completely without
the use of Ag or other noble metals. The new activation
scheme allows the facile variation of the metal seed type
and thus can be used to solve this synthetic problem. The
fabrication of pure Au NTs was achieved by combination
of Au activation and an optimized electroless Au plating
bath [5]. Fig. 6 shows electron micrographs of the obtained
NTs. As expected by the avoidance of Ag in all reaction
steps, Ag could not be detected by means of EDS (not
shown).
4 Conclusion
In this study, we outlined a new activation process based on
the diffusion of DMAB into swollen PC, followed by metal
NP precipitation. The method circumvents many restric-
tions present in the case of alternative activation proce-
dures (e.g. exotic chemicals, complex instrumentation,
necessity of certain functional groups or the limitation to
planar substrates). Both the morphology and the chemical
integrity of the polymer were excellently preserved during
the treatment. Using different precursor solutions, the type
of the deposited metal NPs could be easily varied. This
result can be used to optimize materials created by EP, as it
was shown with Au and Pt deposition reactions.
Fig. 5 SEM images of Pt NTs obtained with the standard Ag
activation [11] (a, b) and the novel activation process using Pd seeds
(c, d). In both cases, PC templates with pore diameters of approx.
800 nm were used. In b, a Pt grain formed from an activation nucleus
is marked. The inset in c shows a cross section of the Pt film grown on
the polymer surface evidencing its enhanced smoothness
Fig. 6 a SEM image of template-freed Au NTs grown inside Au-
activated PC (approx. 800 nm pore diameter). b Corresponding TEM
image of a NT cross-section
Polycarbonate activation for electroless plating 293
123
Furthermore, the seeds can be deposited with nanoscale
homogeneity even on very complex shaped substrates,
allowing the plating of challenging structures such as metal
NTs.
In summary, a simple and flexible wet-chemical route
for the high-quality activation of polycarbonate substrates
is presented. It is easily scalable, does not need complex
instrumentation, can be applied to demanding, three-
dimensional templates and allows the plating of metal films
with excellent nanoscale homogeneity. To further expose
the broad potential of the outlined technique, in an
upcoming study, we will demonstrate the universality of
the activation process. Furthermore, it will be examined
how the formation of metal NTs by EP can be controlled by
variation of the density and composition of the NP seeds.
Acknowledgments The authors thank the materials research
department (GSI Helmholtz Centre for Heavy Ion Research, Darms-
tadt) for support with the irradiation experiments, Maria Eugenia
Toimil-Molares for discussion of the manuscript and the Center of
Smart Interfaces (CSI, Darmstadt) for making the AFM available to
us. Supply of Au solution by Schutz Dental GmbH and the synthetic
support by Adjana Eils are gratefully acknowledged.
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