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: raroca1@cogeco.ca

PAPER www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 2921–2924 | 2921

Publ

ishe

d on

01

June

200

6. D

ownl

oade

d by

Hei

nric

h H

eine

Uni

vers

ity o

f D

uess

eldo

rf o

n 13

/01/

2014

19:

05:0

8.

View Article Online / Journal Homepage / Table of Contents for this issue

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 (#).

2922 | J. Mater. Chem., 2006, 16, 2921–2924 This journal is � The Royal Society of Chemistry 2006

Publ

ishe

d on

01

June

200

6. D

ownl

oade

d by

Hei

nric

h H

eine

Uni

vers

ity o

f D

uess

eldo

rf o

n 13

/01/

2014

19:

05:0

8.

View Article Online

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.

This journal is � The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 2921–2924 | 2923

Publ

ishe

d on

01

June

200

6. D

ownl

oade

d by

Hei

nric

h H

eine

Uni

vers

ity o

f D

uess

eldo

rf o

n 13

/01/

2014

19:

05:0

8.

View Article Online

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.

References

1 D. M. Xing, B. L. Yi, F. Q. Liu, Y. Z. Fu and H. M. Zhang, FuelCells (Weinheim, Ger.), 2005, 5, 406–411.

2 (a) A. L. Rollet, O. Diat and G. Gebel, J. Phys. Chem. B, 2002,106, 3034–3036; (b) L. G. Lage, P. G. Delgade and Y. Kawano, J.Thermal Anal. Calorim., 2004, 75, 521–530.

3 Nanofabrication towards biomedical applications: Techniques, tools,applications and impact, ed. C. S. S. R. Kumar, J. Hormes andC. Leuschner, Wiley-VCH, Weinheim, 2005.

4 Nanoparticles: From theory to application, ed. G. Schmid, Wiley-VCH, Weinheim, 2005.

5 Metal Nanoparticles: Synthesis, characterization and applications,ed. D. L. Feldheim and C. A. Foss, Jr., Marcel Dekker, Inc.,New York, 2002.

6 P. Pathak, M. J. Meziani, Y. Li, L. T. Cureton and Y.-P. Sun,Chem. Commun., 2004, 1234–1235.

7 H. W. Rollins, F. Lin, J. Johnson, J. J. Ma, M. H. Tu,D. D. Desmarteau and Y. P. Sun, Langmuir, 2000, 16, 8031–8036.

8 A. Heilmann, Polymer Films with Embedded Metal Nanoparticles,Springer-Verlag, Berlin, Heidelberg, 2003.

9 T. D. Gierke, G. E. Munn and F. C. Wilson, J. Polym. Sci., Polym.Phys. Ed., 1981, 19, 1687–1704.

10 K. J. J. Mayrhofer, B. B. Blizanac, M. Arenz, V. R. Stamenkovic,P. N. Ross and N. M. Markovic, J. Phys. Chem. B, 2005, 109,14 433–14 440.

11 M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.12 H. Tang, M. Pan, S. Jiang, Z. Wan and R. Yuan, Colloids Surf., A,

2005, 262, 65–70.13 Y. Sun and Y. Xia, Nano Lett., 2003, 3, 1569–1572.14 C. J. L. Constantino, T. Lemma, P. A. Antunes and R. Aroca,

Anal. Chem., 2001, 73, 3674–3678.15 L. Rivas, S. Sanchez-Cortes, J. V. Garcia-Ramos and G. Morcillo,

Langmuir, 2000, 16, 9722–9728.16 I. Srnova-Sloufova, F. Lednicky, A. Gemperle and J. Gemperlova,

Langmuir, 2000, 16, 9928–9935.17 M. P. Mallin and C. J. Murphy, Nano Lett., 2002, 2, 1235–1237.18 S. Ivanova, V. Pitchon, C. Petit, H. Herschbach, A. Van Dorsselaer

and E. Leize, Appl. Catal., A, 2006, 298, 203–210.19 B. Ren, X.-F. Lin, Z.-L. Yang, G.-K. Liu, R. Aroca, B.-W. Mao

and Z.-Q. Tian, J. Am. Chem. Soc., 2003, 125, 9598–9599.20 J. Greeley and M. Mavrikakis, J. Phys. Chem. B, 2005, 109,

3460–3471.21 M. B. Sigman, Jr., A. E. Saunders and B. A. Korgel, Langmuir,

2004, 20, 3, 978–983.22 F. K. Liu, P. W. Huang, Y. C. Chang, F. H. Ko and T. C. Chu,

Langmuir, 2005, 21, 2519–2525.23 G. De and C. N. R. Rao, J. Phys. Chem. B, 2003, 107,

13 597–13 600.24 K. Kimura, M. Mannami, A. Kyoshima and M. Matsushita, Jpn.

J. Appl. Phys., 1980, 19, 733–736.25 H. Z. Shi, L. D. Zhang and W. P. Cai, J. Appl. Phys., 2000, 87,

1572–1574.26 J. Etheve, P. Huguet, C. Innocent, J. L. Bribes and G. Pourcelly,

J. Phys. Chem. B, 2001, 105, 4151–4154.27 A. Gruger, A. Regis, T. Schmatko and P. Colomban, Vib.

Spectrosc., 2001, 26, 215–225.28 R. F. Aroca, D. J. Ross and C. Domingo, Appl. Spectrosc., 2004,

58, 324A–338A.29 R. A. Alvarez-Puebla, D. S. Dos Santos, Jr and R. F. Aroca,

Analyst, 2004, 129, 1251–1256.30 D. S. dos Santos, Jr, R. A. Alvarez-Puebla, O. N. Oliveira, Jr. and

R. F. Aroca, J. Mater. Chem., 2005, 15, 3045–3049.31 P. J. G. Goulet, D. S. dos Santos, Jr., R. A. Alvarez-Puebla,

O. N. Oliveira, Jr. and R. F. Aroca, Langmuir, 2005, 21,5576–5581.

32 J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov and K. Sieradzki,Nature, 2001, 410, 450–3.

33 Y. Ding and J. Erlebacher, J. Am. Chem. Soc., 2003, 125,7772–7773.

34 G. C. Schatz and R. P. Van Duyne, in Handbook of vibrationalspectroscopy, ed. J. M. Chalmers and P. R. Griffiths, John Wileyand Sons Ltd., Chichester, 2002, vol. 1.

2924 | J. Mater. Chem., 2006, 16, 2921–2924 This journal is � The Royal Society of Chemistry 2006

Publ

ishe

d on

01

June

200

6. D

ownl

oade

d by

Hei

nric

h H

eine

Uni

vers

ity o

f D

uess

eldo

rf o

n 13

/01/

2014

19:

05:0

8.

View Article Online