Download - Fabrication of stable bimetallic nanostructures on Nafion membranes for optical applications
Fabrication of stable bimetallic nanostructures on Nafion membranes foroptical applications
Ramon A. Alvarez-Puebla, G.-Abbas Nazri and Ricardo F. Aroca*
Received 22nd February 2006, Accepted 28th April 2006
First published as an Advance Article on the web 1st June 2006
DOI: 10.1039/b602626e
Novel stable crystalline bimetallic silver–gold nanostructures homogeneously dispersed on Nafion
were prepared by galvanic substitution of a vacuum evaporated silver island film with gold. The
method allows control of the composition of the bimetallic nanostructures and tuning of optical
properties. Formation of the nanoalloy was monitored by UV-Vis absorption, SEM-EDX, XRD,
AFM, ATR-FTIR, and Raman scattering. Bimetallic nanostructure growth was monitored by
UV-Vis absorption and surface-enhanced Raman scattering (SERS), by casting an aliquot from a
dilute solution of 2-naphthalenethiol (2-NAT) on the composite surface. SERS intensity increases
with galvanic substitution, reaching a maximum, and providing a material that delivers SERS
enhancement several times higher than those obtained with regular silver and gold island films.
Optical enhancement is also fairly homogeneous throughout the treated Nafion surface; this is
demonstrated by mapping the average SERS intensity of a mixed Langmuir–Blodgett monolayer
bis(benzimidazo)perylene and stearic acid excited with 514 and 633 nm laser lines.
1. Introduction
Nafion is well-known as an ionomer membrane and as
a solid proton-conducting electrolyte in electrochemical
technology. Many modern or potential energy devices such
as fuel cells, electrochromic displays, and solar cells use this
polymer.1 Nafion contains an hydrophobic poly(tetra-
fluoroethylene) (PTFE) backbone with regularly spaced,
short, perfluorovinyl ether side-chains, each terminated with
a highly hydrophilic sulfonate group.2a These membranes
have many interesting properties, including high ionic con-
ductivity and moderate thermal stability (up to 200 uCin air),2b high mechanical strength, chemical inertness, and
nanoporous structure.
Metal nanostructures have been extensively studied for
many decades because of their use in applications such as
catalysis, photography, optics, electronics, optoelectronics,
information storage, biological and chemical sensing, and
surface-enhanced spectroscopy.3–5 Correspondingly, metal
nanoparticles have been prepared in Nafion under different
experimental conditions.6 These composite films present many
advantages. First, the Nafion membrane provides a stable
matrix to prevent the agglomeration and corrosion of the
nanostructures.7 Second, the optical, electrical, and catalytic
properties of the nanoparticles embedded in the template may
be modified.8 Another advantage is that the nanoparticles
embedded in Nafion membranes are easy to handle and recycle
for catalytic purposes.6
Methods of nanostructure deposition on Nafion usually
include ion-exchange reactions for the retention of ions, since
the Nafion structure is composed of numerous hydrophilic
ionic clusters (pores) with diameters in the order of 4–5 nm,2a,9
with posterior treatments for the oxidation or reduction of the
retained cations. A disadvantage of this method is that the ion
retention is restricted by the maximum retention capacity of
the membrane. On the other hand, the retention of cations
into the Nafion nanopores leads to the formation of small
nanoparticles because of pore size restrictions. This small
nanoparticle size leads to a to decrease in the efficiency of
this composite material for both catalytic10 and optical
properties.11 Another method proposed to load Nafion
membranes with metallic nanoparticles is layer-by-layer self
assembly.12 In this case, metallic nanoparticles are prepared
and stabilized with two different agents, one positive [e.g.
poly(diallymethylammonium chloride) (PDDA) ionic poly-
mer], and one negative (e.g. Nafion ionomers). The Nafion
membrane is then consecutively immersed into both nano-
particle suspensions, giving rise to the controlled growth of a
nanoparticle film. This method allows an increase in the degree
of nanoparticle loading through an increase in the number of
bilayers. However, the use of stabilizing agents hinders the
Nafion–nanoparticle contact, as well as the nanoparticles–
adsorbate, making catalytic, electrochemical and spectroscopic
processes more difficult.
In the present report, the fabrication of stable, crystalline,
bimetallic Ag–Au nanostructures homogeneously distributed
on Nafion membranes is achieved using a vacuum evaporated
silver film and a posterior galvanic substitution reaction13 with
a KAuCl4 solution. This method allows the control of the
composition of the bimetallic nanostructures, and therefore
permits optimization of the required properties. In addition,
although only data for silver–gold nanostructures are dis-
cussed, the approach may be easily applied to other systems
where Cu, Co or Fe could be used as sacrificial films and Pt, Ir,
Pd, Rh, etc., as oxidizing agents. The chemical stability and
optical properties of the membranes with embedded Ag–Au
nanostructures are also discussed.Materials and Surface Science Group, Faculty of Sciences, University ofWindsor, Windsor, ON, Canada N9B 3P4. E-mail: [email protected]
PAPER www.rsc.org/materials | Journal of Materials Chemistry
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2. Experimental
Silver island films of 9 nm thickness, on Nafion N-117
perfluorosulfonic acid–PTFE copolymer (Alfa Aesar), were
prepared in a Balzers BSV 080 glow discharge evaporation
unit. During the silver film deposition on to the Nafion
membrane, the background pressure was 1026 Torr, and the
deposition rate (0.5 A s21) was monitored using an XTC
Inficon quartz crystal oscillator. Ag–Au bimetallic films were
prepared through galvanic substitution by immersing the 9 nm
Ag films into a 50 mL of a 161023 M KAuCl4 solution during
different time intervals. Formation of the nanoalloy was
monitored by UV-Vis absorption spectra (Varian Cary 50 UV-
Vis spectrophotometer), scanning electron microscopy (SEM)
and energy dispersive X-ray (EDX) analysis (Hitachi S-4500
Field Emission Scanning Electron Microscope equipped with
an IXRF-EDS 2000 Energy Dispersive Spectrometer), X-ray
diffraction (Inel G3000 X-ray diffractometer equipped with a
CPS 120 Inel curved real time X-ray detector), atomic force
microscopy [(AFM) Digital Instruments NanoScope IV],
ATR-FTIR (Bruker Equinox) and Raman scattering
(Renishaw Invia system, equipped with Peltier CCD detectors
and a Leica microscope). AFM topographical measurements
were performed in tapping mode with a silicon cantilever
(NCH model, Nanosensors) operating at a resonant frequency
of 244 kHz. Images were collected at high resolution (512 lines
per sample) with a scan rate of 0.5 Hz. The data were collected
under ambient conditions, and each scan was replicated to
ensure that any features observed were reproducible.
The growth rate of the bimetallic nanostructures was also
followed by SERS. Samples were prepared by casting 10 mL of
a 161023 M 2-NAT solution on to Nafion films prepared
with different immersion times in the KAuCl4 solution. The
laser line 785 nm was focused using a 506 objective,
and Raman spectra of five different spots were collected
per sample. The homogeneity of the Nafion film surfaces
was studied by depositing an LB mixed monolayer of
bis(benzimidazo)perylene (AzoPTCD) and stearic acid (a
non-optically-interfering fatty acid matrix) in a 1 : 10 ratio
on to the Nafion–metal film.14
3. Results and discussion
According to the AFM data, [inset, Fig. 1(A)] silver film on
Nafion presents a distribution of islands ranging from 30
to 80 nm in size and 15 to 22 nm in height, with 6 nm of
roughness, and a surface plasmon resonance (SPR) maximum
at 510 nm [dashed line, Fig. 1(A)]. SPR intensity decreases
abruptly after immersion of the Ag films in the KAuCl4solution, giving rise to a shifted SPR peak at 536 nm and the
appearance of two extra absorption bands, at 309 and 253 nm.
The disappearance of the plasmon absorption band at 510 nm
during the formation of the nanoalloys, and the appearance of
only one red-shifted SPR band with an absorption maximum
shifting from 536 to 558 nm [Fig. 1(B)] as more gold is taken
up as a function of the immersion time [Fig. 1(C)]; this is not
consistent with the plasmon absorption expected for core–shell
growth. Non-alloy, or core–shell Ag–Au nanoparticles, exhibit
two characteristic absorbance peaks, in which one peak
increases in absorbance as that component’s concentration
increases, accompanied by a corresponding decrease in the
intensity of the second peak.15,16 The complete disappearance
of the sacrificial film plasmon together with the red shift of the
absorption maximum as more gold is taken up suggests the
formation of an Ag–Au nanoalloy, as has been observed
previously by Mallin and Murphy.17 The absorption bands at
309 and 253 nm also increased with immersion time. However,
while the band at 253 nm shifts to the red with immersion time
(to 266 nm after 144 h), the band centred at 309 nm remains
constant; this peak could be due to the electronic absorption of
non-reduced [AuCl4]2 ions18 adsorbed on the membrane.
Notably, the growth of the three bands continues after 144 h.
From this point onwards, no modification in the intensity or
position of the bands is observed. The plasmon at 253 nm is
unusual in silver, gold and their nanostructured alloys. The
SPR absorption at low wavelengths is likely to be due to the
formation of nanoalloyed ellipsoidal nanoparticles in the nano-
pores of Nafion membranes, where the nanostructure size is
restricted to approximately 4–5 nm, due to the size of the pores,
thus also avoiding the aggregation of these small structures.
The presence of this plasmon needs further investigation.
Notably, SERS in the UV region is a challenging proposition,
although it has been reported using Rh and Ru electrodes.19
SEM micrographs of the composite film after 144 h of
immersion [Fig. 2(A)] show large structures of between 200 nm
and several microns in size. According with EDX data, the
average composition of those structures is 85.6% gold and
12.1% silver, with trace amounts of chlorine (2.3%). Detailed
analysis by AFM [Fig 2(B)] shows these large structures to
consist of small cauliflower-like clusters of particles ranging
from 10 to 120 nm in size and 40 to 120 nm in height, with a
roughness of 29.3 nm [Fig. 2(C)]. These structures are
crystalline, as is revealed in the X-ray diffractogram (XRD)
[Fig. 2(D)]. The Nafion Ag–Au composite shows two broad
bands, characteristic of the amorphous structure of Nafion
membranes together with an X-ray diffraction pattern similar
Fig. 1 (A) Variation in the surface plasmon of a silver island film
with immersion time in an Au(III) solution. The dashed line is the
original surface plasmon due to the silver island film. Inset: AFM
micrograph of a sacrificial Ag island film on Nafion; (B) red shift of
the plasmons as a function of immersion time; (C) increasing
absorption intensity of Ag–Au films as a function of immersion time;
absorption bands at 558 (e), 309 (n) and 266 nm (#).
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to that of the silver island films. The XRD of the Ag islands
can be readily indexed to face-centered cubic (FCC) belonging
to the Fm3m (no. 225) space group (JCPDS file No. 04-0783).
The XRD pattern of the Ag–Au film reveals the same
diffraction peaks as those observed for Ag, but slightly shifted
to the left (from 2h = 38.1 to 37.6u). The presence of the same
pattern implies that the bimetallic film can also be indexed as
FCC belonging to the Fm3m (no. 225) space group in keeping
with previous studies.20–22 The decrease in the d(111) intensity
together with the Bragg reflections (111), (200) and (220)
shifting slightly to the left with the addition of gold indicates
the formation of good solid solutions,23 as has been suggested
in the literature,24,25 since gold and silver have almost the same
lattice constant (0.408 versus 0.409 nm, respectively), which is
consistent with the UV-Vis data.
Nafion, and Nafion composite films, were also characterized
using vibrational spectroscopy [Fig. 3(A)]. Raman and ATR-
FTIR spectra of Nafion, and of Nafion supporting bimetallic
nanostructures, show the same vibrational features. The
Raman spectra present bands at 1060 cm21 for the v(SO32)
moiety, 971 cm21 for v(C–O), 804 cm21 for v(C–S), 730 cm21
for v(C–F), and 382 cm21 for r(CF2); while ATR-FTIR
spectra show bands at 1203 cm21 for v(C–F), 1147 and
1057 cm21 for v(SO32), 981 cm21 for v(C–O), and 626 cm21
for v(CF2).26,27 However, while Nafion and Nafion composite
spectra have similar Raman intensity, in the case of ATR-
FTIR, the Nafion composite gives a spectrum ca. three times
more intense than that of Nafion alone. This result may be
interpreted as surface-enhanced infrared absorption.28
The enhanced optical properties of the nanostructured film
were monitored using the intensity of SERS for 2-NAT.29–31
Fig. 3(B) shows the spontaneous Raman and SERS spectra of
2-NAT cast on the Nafion composite film (obtained after
144 h of immersion), excited with a 785 nm laser line. The
quality of the SERS of 2-NAT recorded with the 633 and
785 nm laser lines is comparable, but the 785 nm line was
chosen for reporting since it causes the least photobleaching.
The SERS intensity of spectra recorded on films formed at
different immersion times shows a hyperbolic trend [Fig. 3(C)],
increasing with immersion time and reaching a plateau after
96 h. From this point onwards the enhancement remains
constant. Notably, the SERS intensity from this nanostructure
(after 96 h of immersion) is higher than that obtained with
regular silver or gold island films. The increase in the SERS
signal when compared to regular gold or silver island films
may be linked to the generation of some nanoporosity, as gold
is reduced and silver oxidized, in the nanoalloy.32,33 The
nanoscale heterogeneities in the Nafion–metal film may
increase the local electromagnetic field under laser excitation,
and, correspondingly, the enhancement factor for Raman
scattering.34 In order to probe the optical properties and
Fig. 2 (A) SEM and (B) AFM micrographs, (C) particle size
distribution, and (D) XRD pattern of the Nafion Ag–Au composite
film after 144 h of immersion in gold solution. The XRD of an Ag
island film is also shown.
Fig. 3 (A) Raman and ATR-FTIR spectra for Nafion and Nafion
Ag–Au composite film after 144 h of immersion in gold solution;
(B) Raman and SERS spectra of 2-NAT (laser line [LL]: 785 nm);
(C) Variation of the SERS signal (band at 1378 cm21) with the
immersion time.
Fig. 4 SERS spectra of Azo-PTCD excited with 514 and 633 nm laser
lines and SERS mapping results (inset) for an LB film of 1 : 10 (molar
ratio) Azo-PTCD : stearic acid.
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homogeneity of the surface of the Nafion composite film, an
LB film containing a 1 : 10 molar ratio of Azo-PTCD and
stearic acid was fabricated. The LB films deposited on to the
Nafion–metal film were mapped with two laser lines (514 and
633 nm). Fig. 4 shows both the spectra recorded and the point-
by-point mapping obtained using the vibrational band at
1294 cm21 for spectra excited with the 633 nm laser line. The
mapping results confirm the fact that strong and homogeneous
average SERS signals are observed through the entire film
surface.
Acknowledgements
Financial assistance from GM Canada and the Natural
Science and Engineering Research Council of Canada
(NSERC) through CRDPJ 305716 is gratefully acknowledged.
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