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Synthesis of Hollow Ag-Au Bimetallic Nanoparticles in Polyelectrolyte Multilayers Xin Zhang, Guangyu Zhang, Bodong Zhang, and Zhaohui Su Langmuir, Just Accepted Manuscript • DOI: 10.1021/la400728k • Publication Date (Web): 03 May 2013 Downloaded from http://pubs.acs.org on May 5, 2013
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Synthesis of Hollow Ag-Au Bimetallic Nanoparticles in
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Polyelectrolyte Multilayers
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Xin Zhang, Guangyu Zhang, Bodong Zhang, Zhaohui Su*
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State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
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Chinese Academy of Sciences, Changchun 130022, P. R. China
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ABSTRACT: Ag nanoparticles of ~20 nm size and rather uniform size distribution were synthesized in
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polyelectrolyte multilayers (PEMs) via an ion-exchange/reduction process in two stages (seeding and
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growth), which were used as sacrificial templates to fabricate Ag-Au bimetallic hollow nanoparticles via
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galvanic replacement reaction. The reaction process was monitored by UV-vis spectroscopy. The
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morphology and structure of the nanoparticles were characterized by transmission electron microscopy
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(TEM) and energy dispersive X-ray spectroscopy, which confirmed the formation of hollow Ag-Au
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bimetallic nanoparticles. UV-vis absorbance spectroscopy and TEM results indicated that both size and
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optical properties of the Ag nanoparticles in the PEM can be controlled by manipulating ion content in
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the PEM and the number of the ion-exchange/reduction cycle, whereas that of Ag-Au bimetallic
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nanoparticles were dependent on size of the Ag templates and the replacement reaction kinetics. The
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hollow Ag-Au bimetallic nanoparticles exhibited a significant red shift in the surface plasmon resonance
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to the near-infrared region. The strategy enables facile preparation of hollow bimetallic nanoparticles in
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situ in polymer matrices.
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KEYWORDS: polyelectrolyte multilayer, gold, silver, bimetallic nanoparticles, hollow
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INTRODUCTION
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Metallic nanoparticles have been the focus of intense research in recent decades because of their
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attractive properties and potential applications in areas such as electronics, photonics, filters, sensors,
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catalysis, information storage, and surface-enhanced Raman scattering (SERS).1,2 In particular,
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nanoparticles of noble metals such as Au and Ag have attracted growing attention due to their strong
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surface plasmon resonance (SPR), a property drastically different from that of the bulk materials and
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strongly dependent on their size, shape, structure, composition, and the dielectric properties of the
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surrounding medium.3,4 As a result, these nanoparticles can display intense colors when dispersed in
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liquid media or embedded in solid supports.5-7
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Among the nanoparticles of various shapes and structures, hollow nanoparticles have generated
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special interest because they offer some advantages over the corresponding counterparts, such as
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increased surface area, lower density, saving of materials, reduction in cost, and tunable optical
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properties.8,9 Hollow metallic or oxide nanoparticles can be fabricated through the nanoscale Kirkendall
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effect,10 and Ostwald ripening,11 but the main synthetic route to hollow nanostructures is to use various
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sacrificial templates such as polystyrene microspheres,12 silica microspheres,13 and different metallic
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nanoparticles14-21 such as Co, and Ag. In particular, galvanic replacement reaction is an effective and
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simple method for fabrication of metallic nanostructures with scalable cavity where the additional step
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of removal of the sacrificial core via calcination or chemical etching is not needed. For example, Xia
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and coworkers reported the syntheses via galvanic replacement reaction of metallic nanoboxes,
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nanoshells, nanotubes, and nanocages, which may find potential biomedical application such as
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diagnosis and photothermal treatment.15-18 Bai and coworkers have prepared hollow Pt nanoparticles
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using Co sacrificial templates, and reported their good electrocatalytic activity for methanol oxidation.14
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Murphy and coworkers have synthesized hollow Ag-Au nanowires with high SERS performance.19 Lee
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and coworkers have prepared hollow Ag-Au nanoshells with tunable optical properties.20 Zhou and
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coworkers reported the experimental and theoretical simulation investigation of hollow Au-Ag alloy
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nanoshells.21 Most of the synthesis of hollow nanoparticles reported to date are carried out in liquid ACS Paragon Plus Environment
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media. However, for many practical applications, such as catalysis, SERS, optoelectronic and sensor
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devices, the nanoparticles often need to be immobilized on a substrate or embedded in a matrix.22
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Therefore in situ synthesis of hollow nanoparticles within solid films is highly desirable.
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Layer-by-layer assembly is a simple, inexpensive and versatile approach by which thin films can be
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conveniently prepared.23 Using this approach, hybrid films containing inorganic nanoparticles and
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nanowires can be fabricated directly by consecutive deposition of polyelectrolyte and inorganic particles
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onto solid substrates,24 which can then be used in a large variety of applications ranging from catalysis25
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to antireflection and antifogging coatings.26 Alternatively, polyelectrolyte multilayers (PEMs) can be
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employed as nanoreactors for preparation of organic-inorganic nanocomposites.27-38 A broad range of
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metallic nanoparticles, such as Au,28 Ag,29-32 Pt,33 and Pd34 monometallic nanoparticles and Au-Ag36-38
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bimetallic nanoparticles, have been synthesized in PEMs. However, the size and size distribution of the
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nanoparticles produced in PEM nanoreactors in general is not well-controlled compared to that in liquid
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media, especially for that prepared via photochemical method or thermal reduction.
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Recently we demonstrated that counterions that are universally present in PEMs can be utilized for
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introduction of charged species into PEMs via ion exchange,39 which can undergo further reactions to
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yield nanoparticles in situ.32,33 Using this strategy, monometallic nanoparticles as well as bimetallic
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Au@Ag and Au@Pt core-shell nanoparticles have been synthesized.40-42 In this work, we report the
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fabrication in PEMs of spherical Ag nanoparticles with well-controlled sizes via a two-stage process
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(seeding and growth), which are then used as sacrificial templates to synthesize hollow Ag-Au
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bimetallic nanoparticles via galvanic replacement reaction. The geometry of the Ag-Au bimetallic
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nanoparticles, including the size of the cavity, can be readily controlled, and SPR characteristics of the
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particles in the PEMs are tunable in the near infrared region.
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EXPERIMENTAL SECTION
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Materials. Chloroauric acid tetrahydrate (HAuCl4·4H2O) and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium chloride (NaCl) was purchased from Beijing
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Chemical Reagents Company. Ascorbic acid was purchased from Huishi Biochemical Co., Ltd.
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Poly(diallyldimethylammonium chloride) (PDDA, 20 wt% in water, MW~200k-350k), poly(styrene
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sulfonate) (PSS, MW~70k) and sodium boronhydride (NaBH4) were purchased from Aldrich. All
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chemicals were used as received without further purification. Ultrapure water (18.2 MΩ cm) was
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purified with a PGeneral GWA-UN4 system and used in all experiments.
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Preparation of (PDDA/PSS)n film. Quartz and glass slides were cleaned in a boiling piranha solution
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(H2SO4/H2O2=70:30 v/v) and subsequently rinsed with copious amounts of ultrapure water. A
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(PDDA/PSS)n multilayer film, denoted PEMn, was assembled by sequential dipping of the substrate into
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PDDA (1.0 mg/mL) and PSS (1.0 mg/mL) aqueous solutions for 30 min each until the desired number
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of bilayers (n) was obtained. Every dipping was followed by sufficient water rinsing. NaCl of 1.5 M
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concentration was maintained in all polyelectrolyte solutions.
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Synthesis of Ag nanoparticles. A PEM3 film was dipped into a AgNO3 solution (10 mL, 10 mM) for
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5 min, removed and rinsed with water, and then treated with a freshly prepared aqueous solution of
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NaBH4 (10 mL, 10 mM, for the first reduction only) or ascorbic acid (10 mL, 0.1 M, for all subsequent
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reductions) for 5 min. This ion-exchange/reduction reaction cycle was repeated 3 times to produce a
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PEM loaded with Ag nanoparticle seeds. Then additional PDDA and PSS layers were deposited onto the
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seeded PEM3 film following the assembly procedure described above until a desired number of bilayers
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was reached. Next the film was subjected to the ion-exchange/reduction cycle described above using
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only ascorbic acid as the reducing agent. The thus-prepared films loaded with Ag nanoparticles are
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denoted PEM3Ag3/PEMnAgx for short, where n and x is the number of the additional PDDA/PSS
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bilayers deposited and the number of the ion-exchange/reduction cycles carried out for Ag, respectively.
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Fabrication of hollow Ag-Au bimetallic nanoparticles. The PEMs loaded with Ag nanoparticles
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were immersed into a HAuCl4 solution (10 mL, 0.1 mM) with different dipping times to allow the
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galvanic replacement reaction to proceed. The films were then removed and rinsed with water
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thoroughly and dried in a nitrogen stream.
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Characterization. UV-vis spectra of the PEMs containing nanoparticles deposited on quartz slides
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were acquired on a TU-1901 spectrometer (Beijing Purkinje General Instrument Co., Ltd.).
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Transmission electron microscopy (TEM) measurements were carried out on a JEOL JEM-1011
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microscope operating at an accelerating voltage of 100 kV. A small piece of PEM film loaded with
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nanoparticles was peeled off from the substrate in hydrofluoric acid, floated in ultrapure water, and
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transferred to carbon-coated copper grids for TEM characterization. Energy dispersive X-ray (EDX)
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spectra were acquired on a Tecnai F20 microscope (Philips) operating at 200 kV.
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RESULTS AND DISCUSSION
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Fabrication of metal nanoparticles in PEM nanoreactors has been explored extensively, which
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involves various methods including chemical reduction,31 thermal reduction,31 and photochemical
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reaction.43 The nanoparticles prepared however, in general are very small and with a rather broad size
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distribution. To mitigate this drawback, we adopted a two step protocol. First, Ag seeds are introduced
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into a PEM3 film via the ion-exchange/reduction reaction.32 A PEM consisting of only 3 bilayers is used
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here so that the counterion content is low, and hence only a small number of Ag seeds are produced.32
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Then additional PDDA and PSS layers are deposited on this seeded PEM, which dramatically increases
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the counterion content of the film,32 so that much more Ag metal can be produced in each ion-
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exchange/reduction cycle and bigger particles can be synthesized in fewer cycles. In addition, a weak
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reducing agent, ascorbic acid, is used after the first cycle so that the Ag reduced in all subsequent cycles
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would grow the existing nanoparticles (produced in the first cycle) rather than generate new ones,
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leading to more uniform particle sizes.42
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Figure 1 shows typical TEM images of the Ag nanoparticles synthesized in the PEMs under different
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conditions. It can be seen that the small Ag nanoparticles are roughly spherical with a good dispersion,
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and when they grow bigger some impinge into one another, and the particles become slightly elongated.
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The size distribution of the large nanoparticles was rather uniform (Figure 1, c~f) compared to those
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previously reported for nanoparticles synthesized in PEMs, with a relative standard deviation of ~14%
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for the particle size (Supporting Information). The average size of the Ag seeds was ~10 nm (Figure 1a).
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For PEM3Ag3/PEMnAg2 (n=1~3), where additional 1~3 PDDA/PSS bilayers were deposited on the
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PEM3Ag3 so that the total numbers of bilayers of the PEMs became 4~6, respectively, and two ion-
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exchange/reduction cycles were carried out, the average sizes of the Ag nanoparticles were 13.6, 16.9,
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22.0 nm (Figure 1, b~d), exhibiting a linear dependence on the number of bilayers in the PEM. In
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addition, for PEM3Ag3/PEM3Agx, where the number of bilayers was the same (6 total), the average size
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of the Ag particles was 22.0, 26.6, and 29.8 nm when the additional ion-exchange/reduction cycles (x)
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carried out was 2, 4, and 6, respectively (Figure 1, d~f). Therefore, it is clear that the size of the
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nanoparticles in the PEM can be effectively controlled by varying the bilayer number of the PEM and/or
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the number of exchange/reduction cycle.
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Figure 1. TEM images of Ag nanoparticles synthesized in the PEMs under different conditions. (a)
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PEM3Ag3, (b) PEM3Ag3/PEM1Ag2, (c) PEM3Ag3/PEM2Ag2, (d) PEM3Ag3/PEM3Ag2, (e)
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PEM3Ag3/PEM3Ag4, and (f) PEM3Ag3/PEM3Ag6. Insets are corresponding size distribution.
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The Ag nanoparticles embedded in the PEM were characterized by UV-vis absorption spectroscopy. As seen in Figure 2a, the intensity of the SPR peak for Ag nanoparticles increases with the number of ACS Paragon Plus Environment
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bilayers, a clear indication of higher Ag loadings in thicker PEMs, where counterions are more abundant
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and a greater amount of Ag can be produced in each cycle. This is accompanied by a slight red shift in
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the peak position (from 428 to 446 nm as the bilayer number increases from 3 to 6), as a result of the
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particle size increase. Similar trends are observed in Figure 2b, where the Ag loading and particle size
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are grown by increasing the number of ion-exchange/reduction cycle. In addition the SPR peak in Figure
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2b becomes rather broad, where two components appear to be present. This might be due to the fact that
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some of the particles are elongated rather than being perfectly spherical, as seen in Figure 1 e and f.
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Figure 2. UV-vis absorption spectra of Ag nanoparticles prepared (a) in PEMs of different bilayer number, and (b) via different ion-exchange/reduction cycles.
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Figure 3. UV-vis absorption spectrum as a function of reaction time for (a) PEM3Ag3/PEM3Ag2, (b)
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PEM3Ag3/PEM3Ag4, and (c) PEM3Ag3/PEM3Ag6 film treated with HAuCl4 solution. Dashed lines
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indicate estimated Au SPR peak positions assuming Ag template nanoparticles are completely converted
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into Au spheres.
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The synthesis of hollow nanoparticles in solution phase has been extensively reported. We adopted the
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galvanic replacement reaction for in situ fabrication of hollow nanoparticles in PEMs. Using the Ag
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nanoparticles prepared as sacrificial templates, hollow Ag-Au nanoparticles4,20 were synthesized in the
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PEMs via a galvanic replacement reaction between the Ag nanoparticles and a HAuCl4 solution
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(3Ag(s)+AuCl4-(aq)Au(s)+3Ag+(aq)+4Cl-(aq)).7,16 More specifically, when Ag particles are immersed
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in a HAuCl4 solution, they are oxidized into silver ions due to the difference in standard reduction
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potential between AuCl4-/Au (0.99 V vs SHE) and Ag+/Ag (0.80 V vs SHE). In the galvanic
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replacement reaction, a Au layer is deposited on the surface of the Ag nanoparticle, and with the reaction
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time increasing, more Ag atoms are oxidized and removed from the interior of the particle. Also from
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the reaction equation and density data, it is clear that the volume of Au produced is about 1/3 of Ag
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consumed. Therefore, hollow nanoparticles of smaller sizes than the Ag templates are formed in the
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process. The reaction process was monitored by UV-vis spectroscopy. Figure 3 displays the UV-vis
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spectra as functions of reaction time for Ag nanoparticles of different sizes. It can be seen that the SPR
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peak decreases gradually and becomes broader with time, with the peak position shifting to longer
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wavelength, and when the reaction time is longer than 20 min, the SPR peak shifts back to ~550 nm.
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This observation is consistent with the replacement of Ag by Au and the formation of hollow
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nanoparticles. The weakening of the SPR peak, i.e. the decrease in the extinction cross-section is due to
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two reasons, as documented in the literature.20 First, Au generally has smaller extinction cross-sections
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than Ag particles because there is less plasmon damping in Ag than in Au. Second, upon the
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replacement of Ag by Au, there is less material left. With the progress of the galvanic replacement
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reaction, Au is deposited on the nanoparticles, and both the amount of hollow particles and the cavity
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size in the hollow particles increase with time, resulting in progressive red-shift in the plasmon
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resonance. Eventually the hollow structures collapse to form Ag-Au nanoparticles. The process is also
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evidenced in Figure 4, where TEM images of the nanoparticles at different reaction times are displayed.
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It can be seen that while the particle size remains largely the same, the percentage of hollow
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nanostructures is 11%, 67%, and 92% for PEM3Ag3/PEM3Ag2 at 1, 5, and 10 min reaction time,
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respectively, and at 20 min the particles are much smaller and there is no hollow structure present.
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Compared to the Ag template nanoparticles, the hollow particles are smaller and more irregular in shape,
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with a slightly broader size distribution.
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Figure 4. TEM images of Ag-Au nanoparticles prepared from PEM3Ag3/PEM3Ag2 at reaction time of (a)
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1 min, (b) 5 min, (c) 10 min, and (d) 30 min. Insets are corresponding size distribution.
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For hollow nanostructures, the sizes of the particle and the cavity are important for their properties.
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Figure 5 shows the hollow nanoparticles in the PEM prepared from Ag templates of different sizes. We
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can see that a large portion of the particles in the PEM are hollow and spherical (the percentage of ACS Paragon Plus Environment
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hollow particles is 51%, 43%, and 47%, respectively), and the size of the hollow particles can be well
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controlled by using Ag template particles of different sizes. Furthermore, the size of the cavity can be
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tuned by varying the time of reaction between the Ag templates and the HAuCl4 solution. As seen in
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Figure 4 (a-c), with the reaction time increasing, the size of the cavity in the hollow particles becomes
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bigger.
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Figure 5. Hollow nanoparticles prepared from (a) PEM3Ag3/PEM3Ag2, (b) PEM3Ag3/PEM3Ag4, and (c)
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PEM3Ag3/PEM3Ag6 (reaction time 3 min). Insets are corresponding size distribution.
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Furthermore, elemental analysis by EDX on the nanoparticles revealed presence of significant
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amounts of Ag in additional to Au, even in the collapsed particles. The Au/Ag atomic ratio was 0.65 and
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2.13 for the hollow particles and the solid ones formed at 10 and 30 min reaction time, respectively
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(Supporting Information). Therefore, the hollow structure we synthesized in the PEM was Ag-Au
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hollow nanoparticles.
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Although the present study is focused on hollow Ag-Au bimetallic nanoparticles, the strategy is
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general and can be utilized to synthesize hollow bimetallic nanoparticles of other elements in multilayer
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thin films, as long as the standard reduction potential of the first element is lower than that of the second
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one.
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CONCLUSIONS
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We have synthesized Ag-Au hollow nanoparticles of different sizes in polymeric thin films. First Ag nanoparticles were synthesized in the PEM via an ion-exchange/reduction process in two stages, a
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seeding stage and a size growth stage, which resulted in bigger particles and rather uniform size
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distribution, the size readily tuned by manipulating the ion content in the PEM and the number of the
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ion-exchange/reduction cycle. The Ag nanoparticles obtained were then used as sacrificial templates to
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fabricate hollow Ag-Au bimetallic nanoparticles via a galvanic replacement reaction. The process was
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monitored by UV-vis absorption spectroscopy, and the hollow structure can be tuned by controlling the
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reaction kinetics. The SPR peak of the hollow nanoparticles exhibited a significant red shift and
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appeared in the near-infrared region. This work provides a general strategy for in situ fabrication in
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polymer matrix of hollow bimetallic nanoparticles that may find application in the fields of catalysis and
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optical materials.
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ASSOCIATED CONTENT
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Supporting Information. EDX spectra and size distribution data for metal NPs synthesized in PEMs.
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This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
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*Phone (+86)431-85262854; *Fax (+86)431-85262126; e-mail
[email protected].
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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This work is supported by the National Natural Science Foundation of China (21174145). Z.S. thanks
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the NSFC Fund for Creative Research Groups (50921062) for support.
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REFERENCES
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