Preparation of Silver− Latex Composites

Institute of Physics, Chemnitz UniVersity of Technology, D-09107 Chemnitz, Germany ... tion techniques or sputtering onto latex beads.8,10 These ap- p...
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J. Phys. Chem. B 2000, 104, 7278-7285

Preparation of Silver-Latex Composites A. B. R. Mayer,† W. Grebner, and R. Wannemacher* Institute of Physics, Chemnitz UniVersity of Technology, D-09107 Chemnitz, Germany ReceiVed: February 11, 2000; In Final Form: May 24, 2000

Several in-situ chemical reduction methods were systematically evaluated in view of the formation of silverlatex composites, and in particular with respect to the task of coating colloidal latex spheres with uniform thin layers of silver. Such nanocomposite materials are of profound interest, due to their expected novel optical properties. The samples were investigated by transmission electron microscopy and UV-vis spectroscopy. A range of silver particle features was obtained, including even and uniform silver coatings in the nanometer size regime on the latex particles.

Introduction Composite materials incorporating metal particles or layers in the nanometer size regime are intriguing, since they can exhibit unique optical, electronic, magnetic, or catalytic properties.1-4 Spherical nanoparticles with thin metal coatings are especially interesting in view of their unusual optical properties. Due to coupling of surface plasmons corresponding to the two interfaces of the silver shell, it is possible to shift the Mie resonances to any desired wavelength in the visible spectral range by choosing core size, layer thickness, and coating material. The scattering of evanescent waves, generated by total internal reflection, by metal-coated spherical particles has been investigated theoretically in a recent publication,5 and strong enhancement of higher-order multipolar resonances in the scattering and extinction spectra has been predicted. To obtain sharp resonances, the core particles should be spherical, the thickness of the silver layer homogeneous, and the coating should contain as few grain boundaries as possible. Monodisperse latex particles can be produced very reproducibly with coefficients of variance in the range of a few percent and are hence suitable as core particles. The aim of the present work was the preparation of latex spheres coated homogeneously with silver. Such composite particles are also appropriate substrates for organic molecules in surface-enhanced Raman scattering studies.6 In this case, the deposition of silver nanoparticles instead of smooth silver layers is generally desirable, due to the strong near-field enhancement in the vicinity of small silver grains. The methods for coating latex particles with silver are of special interest, and several routes have been explored and described in the literature.6-10 Frequently, the preparations involve the activation of the latex surface by “seeds” of a different metal, such as palladium,7 followed by the deposition of the desired metal. Even a thin layer of palladium with a thickness of about 1 nm, however, turns out to strongly damp the optical resonances mentioned above, due to the large imaginary part of the dielectric constant of palladium. Other wet-chemistry methods involve the immersion of the latex particles into a solution of the respective silver precursor, and * Phone: ++40 371 531 3009. Fax: 0371 531 3060. E-mail: [email protected]. † Present address: Department of Chemistry, State University of New York Albany, NY 12222. E-mail: [email protected].

subsequent reduction.6 However, in most cases, either relatively thick metal coatings in the micrometer size regime have been obtained,7 or only a small portion of the latex particles has been covered by a metal layer of irregular thickness, leaving a large number of noncoated latex particles.6 The binding of silver precursor ions onto latex surfaces by complex or ion pair formation prior to reduction has been reported as well.7,9 This method requires the surface modification of the latex with groups capable of interacting with the metal precursor ions.7,9 In these cases, separate silver nanoparticles have been immobilized onto latex surfaces. However, no dense packing of the nanoparticles, resulting in a “nanocoating” of regular thickness, has been obtained. Further preparation methods employ thermal evaporation techniques or sputtering onto latex beads.8,10 These approaches result in the coverage of each latex particle in a sample by the metal. However, merely metal islands and partial coverage are usually achieved by these methods. The uniform and complete coverage of each latex particle in a sample by a defined silver coating below 20 nm thickness thus still remains a challenge and is the basis for several fundamental investigations involving optical phenomena. Therefore, further deposition methods need to be explored and evaluated for their usefulness for the preparation of optically interesting materials. To deposit silver evenly on the entire particle, the preparation of the composite materials by chemical reductions in solution was chosen in the present investigations. A selection of in-situ reduction methods was systematically studied in view of the deposition of silver nanoparticles and dense silver packings onto latex supports, without the activation by additional “seed metals”. In the present investigations, a large range of silver particle features was obtained. The composite materials were investigated by transmission electron microscopy (TEM) to demonstrate the nanosize dimensions, the nanoparticle features, and the location of the silver particles on the latex surface. UV-vis spectroscopy was employed for further qualitative characterization of the optical properties of the materials. Experimental Section Materials. Silver nitrate (AgNO3; 99+%) and silver acetate (AgCH3COO; Ag(ac); 99.999%), and the reducing agents potassium tetrahydroborate (KBH4; 99.99%), hydrazine (N2H4‚ H2O; 35 wt % in water), and sodium hypophosphite hydrate

10.1021/jp000568u CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000

Preparation of Silver-Latex Composites (NaH2PO2‚H2O) were obtained from Aldrich. Tannins was obtained from VEB Laborchemie Apolda. Aqueous solutions of formaldehyde (HCHO) and ammonia (NH3) were employed. The surfactant sodium dodecylbenzenesulfonic acid (SDS; ∼80%) was purchased from Fluka. Carboxylated polystyrene latex dispersion (0.45 µm; 2.5 wt %) was obtained from Polysciences, polystyrene latex (0.605 µm; 10 wt %) from Sigma, polystyrene latex (0.230 µm; 10 wt %) from Plano, and polydisperse polystyrene latex (35 wt %) from MonomerPolymer & Dajac Laboratories. All latex dispersions were obtained as aqueous dispersions. Preparation of Silver Colloids and Coatings. An aqueous solution (1.5 mL) containing the silver precursors (AgNO3 or AgCH3COO; 0.01 M) was combined with 1.5 mL of the aqueous latex dispersion (0.35 wt %), resulting in a mass ratio of latex/silver ) 2:1. Alternatively, 3 mL of the respective silver salt solution was combined with 1.5 mL of latex dispersion, to yield a mass ratio of latex/silver ) 1:1. Several samples were prepared using a 0.001 M aqueous solution of the silver precursors, resulting in a mass ratio of latex/silver ) 20:1. The reaction mixture (0.5 mL) was stirred at room temperature, and the following reduction methods were subsequently employed: Method 1 (reduction with formaldehyde): 0.04 mL of aqueous NH3 (3.7%) was added to the reaction mixture and stirred for 3-5 min. Subsequently, 0.04 mL of aqueous HCHO (12%) was added, and the mixture was further stirred at room temperature until completion of the reduction. Some samples were reduced in the presence of the surfactant SDS. In these cases, 0.25 or 1 mL of an aqueous solution of SDS (5 × 10-3 M) was added to the reaction mixture before performing the reduction. Furthermore, some samples were placed into an ultrasonic bath during reduction. Method 2 (reduction with potassium tetrahydroborate): 0.02 mL of a freshly prepared aqueous solution of KBH4 (0.001 g of KBH4 in 1.0 mL of H2O) was rapidly added to the stirred reaction mixture. An immediate color change indicated the rapid reduction of the silver precursor. Method 3 (reduction with hydrazine): 0.015 mL of N2H4 in water (35 wt %) was rapidly added to the stirred reaction mixture, and stirring was continued until completion of the reduction. An immediate color change was observed. Method 4 (reduction with tannins): 0.04 mL of aqueous NH3 (3.7%) was added, and the mixture was stirred for 3-5 min. Subsequently, 0.04 mL of an aqueous solution of tannins (0.002 g tannins in 1.0 mL of H2O) was added and was stirred until completion of the reduction. Method 5 (two-step reduction inVolVing sodium hypophosphite): 0.05 mL of a freshly prepared aqueous solution of NaH2PO2 (0.001 g in 1.0 mL of H2O) was added to the stirred reaction mixture. The mixture was stirred for another 5 min before a second reducing agent was added. Some samples (containing the AgNO3 precursor) were stored at room temperature in the dark for 1 day, before addition of the second reducing agent. The second reducing agent (HCHO, KBH4, N2H4, or tannins) was added to the stirred reaction mixture (containing the previously added NaH2PO2) according to the procedures described for methods 1-4. In the case of the silver acetate precursor, one sample was prepared without the addition of a second reducing agent. Method 6 (multiple deposition of silVer): 0.4 mL of an aqueous solution of the silver precursor AgCH3COO (0.01 M) was added to 0.4 mL of a latex dispersion already containing immobilized silver nanoparticles (samples 94 and 99). Alternatively, 0.04 mL (sample 85a) or 0.015 mL (sample 85b) of

J. Phys. Chem. B, Vol. 104, No. 31, 2000 7279 an aqueous solution of AgCH3COO (0.01 M) was added to 0.5 mL of a latex dispersion containing immobilized silver nanoparticles. The mixtures were stirred at room temperature, and the samples were reduced by HCHO as described for method 1. Doubly distilled water was used for all solutions. The glassware employed was either new, or was cleaned with concentrated nitric acid before use. Transmission Electron Microscopy. A selection of the latex-silver composite materials was investigated by transmission electron microscopy using a Philips CM20FEG instrument operated at an accelerating voltage of 200 kV. The TEM samples were prepared by placing drops of the colloidal dispersion on a Formvar/carbon-coated copper grid (200 mesh; placed onto a filter paper to remove excess solvent), and letting the solvent evaporate at room temperature. A selection of these samples was additionally investigated by electron energy loss spectroscopy (EELS) in order to obtain a silver map of the samples. UV-vis Spectroscopy. UV-vis spectra were obtained after reduction of the samples with a Shimadzu UV 3101 PC UVvis-NIR spectrophotometer in the range from 300 to 800 nm, using 10 mm path length quartz cuvettes. The samples were diluted (3.5 mL of water and 0.1 mL of sample). The transmission of an identical suspension of noncoated latex containing no silver species was measured as baseline in order to account for the scattering stemming from the latex particles. For the completely coated latex particles, these UV-vis spectra resulted in negative differential extinction, due to apparently altered scattering characteristics of the latex particles. This may be related to fluctuations of the concentration caused by partial precipitation or agglomeration of the latex particles accompanied by silver coating. Therefore, in these cases, either the concentration of the samples was increased (3.5 mL of water and 0.15 mL of sample) versus the baseline, or the spectra were recorded versus water. Results and Discussion Several in-situ chemical reductions were investigated for their effect on the nucleation and silver particle growth, and for their utility to densely immobilize silver nanoparticles on latex supports. Such materials are of profound interest for the investigation of the optical properties of composite materials. The prepared samples are listed in Table 1 for carboxylated polystyrene latex and in Table 2 for various nonmodified polystyrene latexes. The samples exhibited a variety of colors, ranging from intense yellow and orange to violet and dark brown-black. The colors depended on the dilution of the samples, as well as the silver particle features, agglomeration, and immobilization on the latex supports. The silver particle features varied depending on the preparation method and conditions. In several cases an even coating of all the latex particles in the respective sample with silver particles was obtained. The formation of metal nanoparticles is strongly governed by the balance of the nucleation rate and particle growth. For instance, a large number of nuclei is formed with the use of “rapid” reducing agents, such as tetrahydroborates. In these cases, smaller nanoparticle sizes are usually obtained. When employing “slower” reducing agents, such as, for instance, formaldehyde, fewer nuclei reaching the critical size for particle growth are initially formed. If the rate of particle growth is larger than that of nucleation, these nuclei grow to larger particle sizes. Therefore, the careful selection of the reducing agents and conditions is crucial for controlling the features and properties of the silver particles and coatings.

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Mayer et al.

TABLE 1: Latex-Silver Samples for Carboxylated Polystyrene Latex sample

precursor

reduction methoda

mass ratio latex/Ag

color

67a 67b 67c 67d 68a 68b 68c 70a 70b 70c 70d 84a 84b 85a 85b 87 88 89a 89b 89c 94 99 100b 100c 104b

AgNO3 AgNO3 AgNO3 AgNO3 AgNO3 AgNO3 AgNO3 Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) AgNO3 AgNO3 AgNO3 Ag(ac) Ag(ac) Ag(ac) Ag(ac) AgNO3

meth. 1 meth. 2 meth. 3 meth. 5 (tannins) meth. 1 meth. 2 meth. 3 meth. 1 meth. 2 meth. 3 meth. 4 meth. 1 (US)b) meth. 1 (SDS,US)c) meth. 6 meth. 6 meth. 1 (SDS)d) meth. 1 (SDS)e) meth. 5 (HCHO) meth. 5 (KBH4) meth. 5 (N2H4) meth. 6 meth. 6 meth. 5 (HCHO) meth. 5 (no 2nd agent) meth. 5 (N2H4)

2:1 2:1 2:1 2:1 20:1 20:1 20:1 2:1 2:1 2:1 2:1 1:1 1:1

violet disp. brownish-yellow disp. gray disp. black-brown disp. reddish-gray disp. yellow disp. bluish-gray disp. red-brown, fine precip. orange-yellow disp. dark gray disp. brownish-yellow disp. gray disp. and silver mirror dark brown disp. black-brown, fine precip. brown, fine precip. dark brown, fine precip. dark brown, fine precip. orange-brown, fine precip. black-brown, fine precip. black-brown, fine precip. black-brown, fine precip. black-brown, fine precip. black-brown, fine precip. black-brown, fine precip. black-brown, fine precip.

1:1 1:1 2:1 2:1 2:1 1:1 1:1 2:1

a Methods: 1, reduction with formaldehyde; 2, reduction with KBH ; 3, reduction with hydrazine; 4, reduction with tannins; 5, two-step reduction 4 involving sodium hypophosphite (the second reducing agent is given in parentheses); 6, multiple deposition of silver. b Prepared under the simultaneous application of ultrasound during reduction. c Prepared with the addition of the surfactant SDS before reduction, and the simultaneous application of ultrasound during reduction. d Prepared with the addition of 0.25 mL of aqueous solution of the surfactant SDS before reduction. e Prepared with the addition of 1.0 mL of aqueous solution of the surfactant SDS before reduction.

TABLE 2: Latex-Silver Samples for Polystyrene Latex sample

latex size (µm)

61 28/40 92a 92b 103a 103c 103d 103(r)

0.605 0.605 polydisp. polydisp. 0.230 0.230 0.230 0.230

precursor

reduction methoda

mass ratio latex:Ag

color

AgNO3 AgNO3 Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac) Ag(ac)

meth.1 (US)b) meth. 1 (SDS)c) meth. 1 meth. 2 meth. 1 meth. 5 (HCHO) meth. 5 (HCHO) meth. 5 (HCHO)

2:1 2:1 2:1 2:1 2:1 1:1 2:1 2:1

yellowish disp. violet-gray disp. orange-brown, fine precip. dark yellow disp. yellowish disp. black-brown, fine precip. black-brown, fine precip. brown, fine precip.

a Methods: 1, reduction with formaldehyde; 2, reduction with KBH ; 5, two-step reduction involving sodium hypophosphite (the second reducing 4 agent is given in parentheses). b Prepared under the simultaneous application of ultrasound during reduction. c Prepared with the addition of the surfactant SDS before reduction.

Figure 1. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by KBH4 reduction (mass ratio latex/silver ) 2:1). Average size is about 20 nm for free colloidal particles and 70 nm for silver particles attached to the latex surface.

In the present investigations, several general trends were observed for the different reduction methods. A large number of “free”, that is, nonimmobilized silver particles was typically found for the reductions by potassium tetrahydroborate. Figure 1 shows a TEM micrograph for a representative example, namely, for silver nanoparticles in the presence of carboxylated polystyrene latex (sample 67b). It can

be clearly seen that only a very small portion of the silver nanoparticles is attached to the latex surfaces. The same results were obtained regardless of the silver precursor (AgNO3 or AgCH3COO) used. The samples exhibited the typical yellow color of “free”, that is, isolated nanosized silver particles, with a plasmon-related extinction peak at 393.5 nm. A typical UVvis extinction spectrum is included in Figure 1. The same features were also observed for samples containing only a small amount of silver (mass ratio latex/silver ) 20:1), which were obtained as well. In these cases, the samples were of an equally intense yellow color, and the TEM micrographs showed a large number of very small nonimmobilized silver particles. Potassium tetrahydroborate is a rapid reducing agent, capable of inducing the initial formation of a large number of nuclei. It can be assumed that the reduction and formation of the silver particles is completed before diffusion or attachment to the latex surface can occur. In addition, the surface properties and thus the affinity to hydrophobic surfaces (such as the polystyrene latex surface) may be altered by the presence of the side products formed during reduction (such as boranates or silver-boron alloys).11 These side products could contribute to the stabilization of the individual silver colloids.

Preparation of Silver-Latex Composites

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Figure 3. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from Ag(ac) precursor in the presence of carboxylated polystyrene latex by tannins reduction (mass ratio latex/silver ) 2:1). Average size of silver particles 25 nm.

Figure 2. TEM micrographs and UV-vis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by hydrazine reduction (mass ratio latex/ silver ) 2:1). Sizes of silver particles vary between 50 and 300 nm.

A different trend was found for hydrazine, which is also a rapid reducing agent. In these cases, silver agglomerates partially located on the latex surfaces were obtained. A typical example for silver obtained in the presence of carboxylated polystyrene latex by the hydrazine reduction is depicted in Figure 2 (sample 67c). Common for these samples was the peculiar cone shape of those silver agglomerates located on the latex surface. The enlarged TEM pictures revealed that these agglomerates consisted of smaller silver particles, which were obviously initially formed by the rapid reducing agent. However, in contrast to the KBH4 reduction, no side products are formed which could act as protective agents and thus stabilize the individual silver particles, and change the surface properties of the silver nanoparticles. In the case of the hydrazine reduction, the initially formed silver particles seem to possess a larger tendency for further agglomeration. Gray dispersions were obtained, a feature which stems from the relatively large agglomerated silver particles of up to about 0.15 µm in diameter. The UV-vis spectra typically showed an undefined broad extinction exhibiting two shoulders (see UV-vis spectrum included in Figure 2). Due to the partially altered scattering characteristics of the latex (apparently caused by partial coverage with silver), a negative UV-vis extinction was obtained if the same concentration of noncoated latex was used as baseline. Therefore, a slightly lower concentration of latex sample was used as reference for obtaining the UV-vis spectra. The samples containing a lower silver concentration (mass ratio latex/silver ) 20:1) were light gray dispersions. In these cases, the TEM micrographs showed a few larger agglomerates, deposited onto the latex surface. This could indicate that the agglomeration process occurs already in an early stage of the reduction. Probably several nonimmobilized silver particles are initially formed, similar to the rapid reduction by the tetrahydroborate. However, due to unchanged surface properties, agglomeration occurred more readily for the samples reduced by hydrazine. A somewhat slower reducing agent is tannins, which is known to reduce silver salt solutions in the presence of ammonia. Tannins, consisting of polyphenols of varying chemical composition, is a weak acid whose lower molecular-weight fraction is soluble in water as colloidal dispersion.12 Due to its relatively

Figure 4. Schematic illustration of the generation of silver particles in the presence of latex particles under various conditions: (a) presence of protective agents/surfactants (below cmc); (b) presence of surfactants associated with the latex, causing a physical surface modification; and (c) surface modification of latex by bound charged groups, capable of interaction with the silver precursor ions.

large size and amphiphilic character (aqueous tannins dispersions are regarded as negatively charged by forming organic anions), this material can act as a protective agent for metal colloids.12 This should influence nucleation and particle growth, and the tendency of the silver particles to be immobilized on the latex surface. A typical TEM micrograph for silver particles reduced by tannins is depicted in Figure 3 (sample 70d). In these cases, a large number of nonimmobilized particles was found, similar to the samples reduced by KBH4. This is most probably due to the protection of the silver nanoparticles by tannins, which could prevent further agglomeration and attachment to the latex surface. Figure 4a depicts a schematic illustration of the proposed mechanism of silver nanoparticle formation in the presence of both a latex particle and a protective agent/surfactant (below the critical micelle concentration, cmc). Due to interaction and adsorption of the protective agent on the silver-particle surface during reduction, the silver particles are stabilized, and their further attachment onto the latex surface is reduced. The samples were yellow-orange dispersions, with their UV-vis spectra exhibiting a plasmon peak at about 430 nm and a shoulder at about 370 nm (see representative UV-vis spectrum in Figure 3). Pure tannins dispersions exhibited a peak at about 305 nm. The red-shift (extinction at higher wavelength) relative to the samples reduced by potassium tetrahydroborate could indicate strong interactions with tannins. The sensitivity of nanosized silver particles and their UV-vis extinction characteristics toward adsorbed species has been frequently reported in the literature.13,14 Such interactions with adsorbed species are

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Figure 5. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by formaldehyde reduction (mass ratio latex/ silver ) 2:1). Average size is about 50-60 nm for aggregated silver particles and 10 nm for isolated silver particles attached to the latex surface.

often accompanied by a red-shift of the silver plasmon peak (matrix effect) and a broadening of the bandwidth, as was observed here for the samples reduced by tannins versus the silver particles reduced by KBH4. The relatively uniform particle size obtained with the tannins reduction furthermore points to the possibility of size control of the particles due to formation of colloidal aggregates of tannins. Such size control of metal or semiconductor nanoparticles by the use of aggregates (in these cases, spherical micelles) formed by surfactants or amphiphilic block copolymers has gained recent interest, and several examples have been documented in the literature.15-18 A further slower reducing agent is formaldehyde, which is typically employed in formulations for silver mirror formation. In contrast to tannins, formaldehyde and its redox products cannot act as effective protective agents for metal colloids. This caused different features of the silver colloids. The TEM investigations showed that coexisting features, “free” and immobilized silver particles, both as small individual particles and small agglomerates, were obtained for the samples prepared from the silver nitrate precursor. The TEM micrograph for one sample, containing carboxylated polystyrene latex, is depicted in Figure 5 (sample 67a). Even though some tendency for immobilization of the silver particles onto the latex surface was found, no smooth coating was formed. In addition, a large portion of latex particles was not covered at all, an observation which is similar to findings made by Barnickel and Wokaun.6 They employed this reduction method for the silver nitrate precursor in the presence of nonmodified polystyrene and poly(methyl methacrylate) latexes. The dispersions obtained in the present case were yellowish or gray-violet, depending on the size and degree of agglomeration of the silver particles. The UV-vis spectra showed very broad undefined extinction bands at about 400 nm (see example in Figure 5). Also, in this case, the spectrum had to be scanned at an increased sample concentration in order to account for altered latex scattering. At this point, however, it is not clear whether these broad extinction peaks stem from the larger and irregular particle sizes, from the coating of the latex with silver (which occurred to some extent), or from a combination of both. An influence stemming from the silver precursor was observed as well. Frequently, the use of silver acetate was preferable to silver nitrate for obtaining particles located on the latex surface. Figure 6 shows the TEM micrograph for silver particles obtained from silver acetate precursor in the presence of carboxylated polystyrene latex by reduction with formaldehyde (sample 70a). The reaction mixture was allowed to stir for 1 day in the dark before performing the reduction. As can be seen from the TEM micrograph each latex particle contained

Mayer et al.

Figure 6. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from Ag(ac) precursor in the presence of carboxylated polystyrene latex by formaldehyde reduction (mass ratio latex: silver ) 2:1). Estimated average size of the silver particles is 25 nm.

Figure 7. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from Ag(ac) precursor in the presence of carboxylated polystyrene latex by formaldehyde reduction under simultaneous application of ultrasound (mass ratio latex/silver ) 1:1). Average size of silver particles 40-50 nm.

immobilized silver particles. The UV-vis spectrum exhibited a broad extinction band at about 500 nm, possibly exhibiting a light shoulder at about 600 nm (see UV-vis spectrum included in Figure 6). The differences found for the two precursors could be due to different degrees of hydrophobicity of the counterions acetate and nitrate. It could be also related to acetate being the corresponding anion of a weak acid, in contrast to nitrate, which is the corresponding anion to a strong acid. It has been shown by Cohen and co-workers19,20 for block copolymer-silver nanocomposites that this is of significance, especially if copolymers containing carboxylate groups are employed. The ion exchange and attachment of metal ions to carboxylate groups has been reported to proceed more readily with the use of metal acetate precursors.19,20 Further variations can be achieved by the use of ultrasound during reduction, or by the use of surfactants. The sample which was placed into an ultrasonic bath during reduction showed silver deposited onto the latex support. However, a large number of latex particles was not coated at all, as can be seen on the TEM micrograph shown in Figure 7 (sample 84a), along with the corresponding UV-vis spectrum. An undefined broad extinction starting from about 400 nm was observed for this gray dispersion (taken at an increased sample concentration), reflecting the agglomerated and irregular particle features. A corresponding sample (sample 84b) was prepared with addition of the surfactant SDS. Here, a clearly larger portion of the latex contained immobilized silver particles; however, also a very large number of “free”, nonimmobilized silver particles was found. In this case, the UV-vis spectrum revealed an additional plasmon peak at about 420 nm (stemming from the large number of “free” silver particles), in addition to the broad extinction in the red. The results indicate that the presence of a surfactant can be very influential. Evenly distributed immobilized silver particles

Preparation of Silver-Latex Composites

Figure 8. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from Ag(ac) precursor in the presence of polydisperse polystyrene latex by formaldehyde reduction (mass ratio latex: silver ) 2:1). Average size of silver particles 20 nm.

reduced by formaldehyde were also obtained for polydisperse polystyrene latex. The respective TEM micrograph is shown in Figure 8 (sample 92a). Such commercial latex dispersions usually contain anionic surfactants for stabilization, causing a “physical” surface modification of the latex, which in turn can cause the binding of the Ag+ precursor ions to the latex surface. The principle of such physical surface modification of the latex by attached surfactant molecules, and subsequent interaction with silver ions, is schematically illustrated in Figure 4b. Reduction of the attached silver ions leads to silver nuclei and nanoparticles deposition onto the latex surface. This sample exhibited an orange-brown color and formed a fine hydrophobic precipitate during reduction. The UV-vis spectrum (recorded versus water) showed a broad band at about 450 nm, extending into the longer wavelength region. In contrast, the addition of the surfactant SDS to the reaction mixture before reduction lead to a reduced tendency of the silver particles to be immobilized on the latex surface, for the sample placed into an ultrasonic bath during reduction (sample 84b, described above). A SDS concentration below (but not drastically below) the cmc (8 × 10-3 M for SDS),21 but comparable, that is, of the same magnitude, to the silver precursor ion concentration was chosen for these experiments. Such concentrations of SDS (equal or above the silver ion concentration, but below the cmc) have been reported by Henglein as resulting in stabilized colloidal silver.21 The stabilizing effect of SDS has been attributed to the long hydrocarbon chains adhering to the silver colloid surface.21 The schematic illustration shown in Figure 4a is also applicable for this case. The effect is suggested to be similar to an agent, such as tannins, acting as protective matrix for the silver nanoparticles, and preventing further attachment to the latex surface. Since the concentration of SDS was below the cmc but not drastically reduced (that is, reduced by orders of magnitude), it seems also plausible that micelle formation and thus stabilization of the silver colloids could be induced even below the cmc, once the hydrophobic interactions between the silver surface and SDS hydrocarbon chain can become effective. Thus, while the presence of surfactants (which are capable of protecting the metal colloids) seemed to reduce the tendency for deposition onto the latex surface under certain conditions (such as the application of ultrasound), this is different if such surfactants are already adsorbed on the latex surface before the addition of the silver precursors and their reduction (physical surface modification of the latex). However, further investigations are necessary to fully understand the role and influence of surfactant addition. It can be expected that various parameters, such as the concentration (for instance, above, below, or far below the cmc), the sequence of addition (for instance, addition before or simultaneously with the silver precursors), or the application of ultrasound can have significant effects. A further indication for the complexity is given by the results obtained

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Figure 9. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from Ag(ac) precursor in the presence of carboxylated polystyrene latex by formaldehyde reduction and the addition of 0.25 mL of aqueous solution of the surfactant SDS before reduction (mass ratio latex/silver ) 1:1). Average size of silver particles is 20 nm.

Figure 10. TEM micrographs, silver map obtained by EELS, and UVvis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by two-step reduction employing the sequence NaH2PO2-formaldehyde (mass ratio latex/ silver ) 2:1). The thickness of the silver shell is 11.5 nm, which is also the size of the silver grains within the shell.

with the addition of SDS, but without the application of ultrasound. Figure 9 shows the TEM micrograph for one example (sample 87). These samples exhibited UV-vis spectra containing two peaks: one located at about 400 nm and the second one being a shoulder at about 550-600 nm (see UVvis spectrum included in Figure 9). The different observations made for the samples containing SDS could be explained by an effect from the application of ultrasound, causing “desorption” of the surfactant from the latex surface, and resulting in a mechanism as illustrated in Figure 4a. Without the presence of ultrasound, the effect could be similar to that illustrated in Figure 4b, with the surfactant (and associated silver ions) adhering to the hydrophobic latex surface. An especially uniform silver particle deposition was obtained employing a two-step reduction procedure, involving NaH2PO2 as initial reducing agent (sample 89a). The TEM micrographs are shown in Figure 10a,b. Figure 10c depicts the silver map obtained by EELS. The diameter of the latex particles is 450 nm, and the size of the homogeneously deposited silver particles can be determined from the silver map as about 11.5 nm, exhibiting a quite narrow size distribution. Hypophosphites are frequently used in electroless deposition baths, especially for nickel.22 For the present investigations, NaH2PO2 was added

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Figure 11. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by two-step reduction with the sequence NaH2PO2-KBH4 (mass ratio latex/silver ) 2:1). Average size of silver particles is 35 nm.

Figure 12. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by two-step reduction with the sequence NaH2PO2-N2H4 (mass ratio latex/silver ) 2:1). Average size of silver particles is 25 nm.

first, and a second reducing agent was added after a certain time period. The coated latex particles precipitated during reduction, appeared orange-brown, and showed a pronounced hydrophobic character. The TEM investigations revealed that a smooth coating was obtained. Due to the strongly altered scattering characteristics of the latex particles caused by complete silver coating and agglomeration of the coated latex spheres, the UV-vis spectrum versus the same concentration of noncoated latex resulted in negative differential extinction. Versus water as a baseline, a broad extinction band at about 550 nm was obtained (see UV-vis spectrum in Figure 10). It is proposed that a balance of initial control of nucleation (by the first reducing agent, NaH2PO2) and subsequent control of particle growth (by the second reducing agent, HCHO) promote the formation of silver particles of equal thickness and their deposition and immobilization on the latex surface. This is similar to the use of several reducing agents in a sequence, for instance, for obtaining monodisperse gold nanoparticles for biomedical and staining applications.23 Thus, this principle can be transferred in order to obtain uniform silver coatings on latex substrates. Further TEM micrographs were recorded after storage of the sample for 1.5 months. Some slight changes could be detected: the silver particles coatings appearing somewhat less uniform, indicating some degree of agglomeration. However, it has to be taken into consideration that the sample was stored in air with no further precautions taken. This can cause some redispersion by Ostwald ripening upon exposure to oxygen. The use of hypophosphite in a two-step reduction process was further investigated in combination with the other reducing agents, KBH4, tannins, and N2H4. Some results obtained by TEM are shown in Figures 11(KBH4 as second reducing agent, sample 89b),12 (N2H4 as second reducing agent, sample 89c), and 13 (tannins as second reducing agent, sample 67d). Even

Mayer et al.

Figure 13. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from AgNO3 precursor in the presence of carboxylated polystyrene latex by two-step reduction with the sequence NaH2PO2-tannins (mass ratio latex/silver ) 2:1). Average size of silver particles is 20-30 nm.

though a less uniform coating was obtained in these cases, it is evident that the initial use of hypophosphite caused the deposition of silver onto the latex surface. This leads to the conclusion that H2PO2- interacts with the latex surface, inducing the formation of evenly dispersed silver nuclei on the latex surface. The second reducing agent is subsequently responsible for the uniformity of particle growth and coating. Also, the appearance of the samples is drastically altered. While the samples obtained by the mere use of KBH4 or tannins were intense yellow or orange-yellow, respectively, the samples obtained by the two-step reduction method were dark brown to black and showed the pronounced hydrophobic character as observed for the sample reduced by the sequence NaH2PO2HCHO. Similarly, the sample reduced by hydrazine was dark brown-black and hydrophobic. Thus, the generally observed feature for completely coated latex particles was their precipitation during reduction (due to coating with metal) and pronounced hydrophobic behavior. The samples coated entirely by a silver layer were visually of an orange-brown or dark brown color, depending on the uniformity of the silver coating, forming a fine “hydrophobic” precipitate. As can be seen from the UVvis spectra included in the respective figures, a broad extinction about 450 nm was usually obtained for these samples. The sample reduced by the sequence NaH2PO2-tannins exhibited three extinction peaks, located at about 365, 475, and 612 (broad) nm. This reflects the complex nature of silver particle arrangements in this sample, that is, the coexistence of immobilized, “free”, and grouped silver particles, as can be seen on the TEM micrograph in Figure 13. The quality and chemical composition of the latex surface could be influential as well. It has been already shown that the introduction of groups onto latex particles which are capable of interacting with the metal precursor species, for instance, by ion pair formation, leads to immobilized metal particles.9 In the present investigations, both polystyrene latex and carboxylated polystyrene latex were employed and compared. A higher tendency for the immobilization of silver particles was found for the surface-modified carboxylated latex. This could most drastically be seen for the two-step reduction method involving the hypophosphite and HCHO in the presence of nonmodified polystyrene latex. From the respective TEM micrograph, a clearly less uniform deposition of silver is evident. The presence of surface-carboxylate groups probably leads to facilitated interaction and attachment of silver precursor ions Ag+ onto the latex surface by ion exchange. This results in the preferred uniform initial formation of silver nuclei on the latex surface, which can serve as seeds for further particle growth. The schematic illustration for this case is shown in Figure 4c. However, to draw precise conclusions, the same latex sizes

Preparation of Silver-Latex Composites

J. Phys. Chem. B, Vol. 104, No. 31, 2000 7285 Future investigations should aim at the role and effect of surfactant addition, as well as the influence of size and shape of the substrates. Acknowledgment. The authors thank Dr. S. Schulze for assistance with the TEM micrographs and EELS. W.G. and R.W. gratefully acknowledge financial support provided by the Deutsche Forschungsgemeinschaft in the framework of the “Innovationskolleg Methods and Materials in the Nanometer Range” established at Chemnitz University of Technology.

Figure 14. TEM micrograph and UV-vis extinction of silver nanoparticles obtained from Ag(ac) precursor in the presence of carboxylated polystyrene latex by multiple reduction (second silver deposition) with formaldehyde. Average size of silver particles is 3050 nm.

should be compared for fully including size and curvature/shape effects into these considerations. Finally, the method of multiple deposition of silver onto latex substrates was explored. Some samples containing immobilized silver nanoparticles were treated with silver precursor salt and subsequent reduction for a second time. Figure 14 shows the sample depicted in Figure 6, after the second deposition of silver. It seems that the growth continued on particles already located on the latex surface, resulting in “fused” and rodlike features, and starting island formation. The UV-vis spectra of these samples typically exhibited a broad extinction band at about 450-500 nm with a slight shoulder at about 350 nm (recorded versus water). Such materials can be especially useful for applications in SERS. Conclusions The results show that a uniform silver deposition on spherical latex supports can be achieved by the proper choice of various parameters, such as the reduction method and conditions, the latex surface properties, and the silver precursor type. Such materials are of profound importance for the investigation of various optical phenomena and clarification of theoretical predictions. The variation of the reduction method allows the preparation of a large variety of silver-latex composite materials. Especially interesting are furthermore the materials obtained by multiple silver deposition, since these composites could be useful for SERS applications.

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