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Ramon A. Alvarez-Puebla, G.-Abbas Nazri and Ricardo F. Aroca*. Received 22nd February ..... 7 H. W. Rollins, F. Lin, J. Johnson, J. J. Ma, M. H. Tu,. D. D. Desmarteau and ... 11 M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.

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

Fabrication of stable bimetallic nanostructures on Nafion membranes for optical 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(tetrafluoroethylene) (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 conductivity and moderate thermal stability (up to 200 uC in 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 Materials and Surface Science Group, Faculty of Sciences, University of Windsor, Windsor, ON, Canada N9B 3P4. E-mail: [email protected]

This journal is ß The Royal Society of Chemistry 2006

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 polymer], and one negative (e.g. Nafion ionomers). The Nafion membrane is then consecutively immersed into both nanoparticle 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 discussed, 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. J. Mater. Chem., 2006, 16, 2921–2924 | 2921

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 ˚ s21) was monitored using an XTC deposition rate (0.5 A 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 UVVis 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 KAuCl4 solution, 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 2922 | J. Mater. Chem., 2006, 16, 2921–2924

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 (#).

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 nanopores 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 This journal is ß The Royal Society of Chemistry 2006

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.

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 ATRFTIR 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 ATRFTIR, 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 This journal is ß The Royal Society of Chemistry 2006

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.

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. 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 pointby-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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