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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