| STUDY |
Rubner's group has shown, for example, that polyelectrolyte multilayered films of poly(acrylic acid) and poly(allylamine hydrochloride) can be employed as nanoreactors for preparing silver nanoparticle composites. The work was carried out in collaboration with MIT chemical engineering professor Robert E. Cohen. The team also demonstrated that the size of the nanoparticles and the overall metal concentration within the films can be systematically controlled by the polyelectrolyte solution pHs and other processing conditions. "One implication of the control over the silver content in the multilayers is the ability to systematically change the optical properties of these nanocomposite films," the authors suggested. |
| UPDATE | 05.02 |
| AUTHOR |
MIT's - Cohen Robert E. - Rubner Michael F. |
| LITERATURE REF. |
Langmuir, 18 (8), 3370 -3375, 2002. 10.1021/la015725a S0743-7463(01)05725-0 Web Release Date: March 7, 2002 Copyright © 2002 American Chemical Society Polyelectrolyte Multilayer Nanoreactors for Preparing Silver Nanoparticle Composites: Controlling Metal Concentration and Nanoparticle Size Tom C. Wang, Michael F. Rubner,* and Robert E. Cohen* Departments of Chemical Engineering and Materials Science and Engineering, and the Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received December 11, 2001 In Final Form: January 29, 2002 Abstract: The size of in situ prepared silver nanoparticles and the overall metal concentration within poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) multilayer films are systematically controlled through multilayer processing conditions. Carboxylic acid groups in the PAH/PAA-based multilayers bind silver cations by ion exchange with the acid protons. Subsequent reduction forms metallic nanoparticles. Because PAA has a pH-dependent degree of ionization, the multilayer film can be fabricated with different concentrations of free acid groups that are available to bind silver cations depending on the multilayer assembly pH. We show that nanoparticle size and silver concentration, examined through a combination of UV-visible spectroscopy, transmission electron microscopy, and elemental analysis, can be controllably increased by reducing the assembly pH of PAH/PAA-based multilayers. Nanoparticles with diameters of 2 to 4 nm at volume fractions of 4 to 8% are obtained for multilayers assembled from PAA and PAH solution pHs of 4.5 to 2.5. Furthermore, since the metal-binding carboxylic acid groups are reprotonated upon nanoparticle formation, the synthesis methodology can be repeatedly cycled to incorporate more silver cations. Up to five cycles of silver cation exchange and reduction have been accomplished to produce nanoparticles with average diameters up to 9 nm at a volume fraction of 24%. -------------------------------------------------------------------------------- Introduction Nanocomposites of inorganic nanoparticles embedded within a polymer matrix have attracted much interest, particularly over the past decade, for their broad range of potential applications in catalysis,1,2 magnetics,3,4 and photonics.5-8 Nanometer-sized inorganic particles have unique properties stemming from quantum confinement effects and their large surface areas relative to their volumes.9-11 To manipulate and process these particles into technologically useful forms, techniques for synthesizing nanoparticles within easily processible organic, and particularly polymeric, templates have been developed.12-16 For example, Cohen and co-workers have employed microphase-separated block copolymers (BCPs) as nanostructured templates for preparing inorganic nanoparticles.17,18 We have recently demonstrated the in situ preparation of inorganic nanoparticles in polyelectrolyte multilayer (PEM) films formed from weak polyelectrolytes;19 others have used strong polyelectrolyte based films to prepare inorganic nanoparticles.20 PEMs, formed from the layer-by-layer adsorption of oppositely charged polyelectrolytes, have attracted much attention over the past decade as easy to fabricate, robust thin films with tunable architectures (i.e., film composition and physical and chemical microstructure).21 The chemical and physical properties (e.g., film thickness, degree of interpenetration between layers, and relative concentration of functional groups) of PEMs composed of weak polyelectrolytes such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) are particularly sensitive to the pH of the polyelectrolyte solution during multilayer assembly.22,23 One method for synthesizing inorganic nanoparticles in microphase-separated BCPs employs carboxylic acid containing domains as "nanoreactors" that can bind metal cations from an aqueous solution. Post-binding chemistries, for example, reduction or sulfidation, produce nanoparticles from the cationic precursors.24,25 Analogously, in our in situ nanoparticle synthesis methodology, we use PEMs formed from PAH and PAA to bind metal precursors for subsequent nanoparticle synthesis. By fabricating of the PEMs at an assembly pH where the PAA is only partially ionized, a fraction of its carboxyl groups remain protonated (i.e., "free" acid groups) in the multilayer film. Therefore, the acid protons can be subsequently exchanged for metal cations. Upon reduction, for example, zerovalent metal nanoparticles are formed. Moreover, nanoparticles can be selectively prepared and spatially positioned within PEMs in lamellar-type structures consisting of nanoparticle-containing multilayers and nanoparticle-free multilayers by adsorbing a fully ionized polyanion such as sulfonated polystyrene (SPS) instead of PAA during PEM assembly. The SPS sulfonate groups become completely bound to the PAH ammonium groups during multilayer assembly leaving no ion-exchangeable groups for metal cation binding. Besides binding metal cations within PEMs to form nanoparticles, we have also demonstrated the selective binding of palladium complex cations to PAH/PAA-based multilayer surfaces to activate them for electroless nickel plating.26 While others have assembled multilayer films of prefabricated nanoparticles and other inorganic materials with polyelectrolytes,27-33 the nanoparticles usually must be modified to promote assembly. Moreover, systematic control over inorganic concentration within the film is difficult during assembly, and it is not possible to manipulate concentration after assembly. With our in situ synthesis methodology, it should be possible in principle to vary both the concentration and size of the nanoparticles by manipulating the multilayer processing conditions. In this paper, we extend our work in preparing inorganic nanoparticles within PAH/PAA-based PEMs, using silver as a model inorganic material; our objective is to demonstrate the systematic manipulation of inorganic concentration and particle size. PEM films with a range of concentrations of metal-binding carboxylic acid groups were prepared from different polyelectrolyte solution pHs. As the pH of the polyelectrolyte solutions during PEM assembly is increased, the carboxyl groups become increasingly ionized. Fewer free acid groups remain in the final film and available for subsequent silver cation binding. At higher multilayer assembly pHs, with a smaller concentration of bound silver cations, smaller nanoparticle sizes and volume fractions were observed from the optical absorbance of the film, elemental analysis, and electron microscopy. Moreover, the ion exchange and reduction of silver cations to zerovalent Ag nanoparticles can be repeatedly cycled to increase metal concentration in the film. Using this approach, the nanoparticle volume fraction in the PEM can be increased by an order of magnitude. Experimental Section PAH (Mw = 70 000), SPS (Mw = 70 000), and silver(I) acetate (Ag(ac)) were obtained from Sigma-Aldrich (St. Louis, MO). PAA (Mw = 90 000) was obtained from Polysciences (Warrington, PA). Hydrogen gas (grade 4.7) was obtained from BOC Gases (Murray Hill, NJ). All chemicals were used without further purification. Deionized water (>18 M cm, Millipore Milli-Q), with an unadjusted pH of approximately 5.5, was exclusively used in all aqueous solutions and rinsing procedures. PAH/PAA-based multilayers were assembled on polystyrene (PS) tissue-culture substrates (corona-treated; Nalge Nunc International, Naperville, IL), glass microscope slides, or polished single-crystal silicon wafers (<100>) using an automated Zeiss HMS slide stainer as previously described.23 Silicon and glass substrates were degreased in a detergent solution followed by air plasma treatment (5 min at 100 W; Harrick Scientific PDC-32G plasma cleaner/sterilizer) prior to multilayer assembly. PS substrates were used as received. PAH and PAA aqueous solutions were adjusted to the desired pH (±0.1) using either 1 M HCl or 1 M NaOH. Briefly, PEMs were formed by first immersing substrates into the PAH solution (10-2 M by repeat unit) for 15 min followed by three 2 min immersions into water as rinsing steps. The substrates then were immersed into the PAA solution (10-2 M) for 15 min followed by identical rinsing steps. The adsorption and rinsing steps were repeated until the desired number of bilayers was obtained. Multilayers of PAH/SPS were similarly prepared except that 0.1 M NaCl was added to the polyelectrolyte solutions prior to pH adjustment. One bilayer is defined as a single adsorption of polycation followed by an adsorption of polyanion. The PEM was finally dried in air (room temperature) and stored at ambient conditions. Details of the in situ nanoparticle synthesis methodology can be found in ref 19. Figure 1 shows schematically the synthesis process flow. Silver acetate was previously found to be a good source for silver binding to PAA carboxylic acid groups in PEMs.19 The PEM films were first immersed in Ag(ac)aq (5 mM, nominally neutral pH) for 30 h, where the acid protons of PAA were exchanged for silver cations. The films were then washed in water and dried in air. The Ag(I)-containing PEMs were reduced in a hydrogen atmosphere (2 atm, 85 C) for 30 h, forming Ag(0) nanoparticles and regenerating the carboxylic acid protons. Note that the long ion-exchange and reduction durations were conservatively employed to ensure completeness of the reactions and were not optimized; shorter durations (e.g., less than 1 h) have also been successfully used to prepare nanoparticles. Additional silver was incorporated into the PEM by repeating the process of ion exchange and reduction. -------------------------------------------------------------------------------- Figure 1 Schematic of the metal-ion exchange and reduction process flow (not drawn to scale). -------------------------------------------------------------------------------- For cross-sectional transmission electron microscopy (TEM) imaging, multilayer films assembled on PS substrates were cut along a direction normal to the film plane using a RMC MT-X ultramicrotome with a diamond knife (Diatome, Fort Washington, PA) at room temperature. Approximately 50 nm thick cross sections of the samples were obtained. Cut sections were floated onto a trough of deionized water, immediately picked up with copper TEM grids, and blotted dry. TEM was performed on ultramicrotomed samples using a JEOL JEM-2000FX operated at 200 kV. While imaging, samples were tilted until the electron beam was oriented normal to the cross-sectional plane. Diameters of approximately 100 particles for each sample were measured from digitized and magnified TEM images to obtain histograms and average diameters. The reported histograms and averages are likely biased toward the larger (i.e., most easily distinguishable) particles and hence should be treated as upper-bound estimates. Film thicknesses were measured by profilometry (Tencor P10) or extracted from cross-sectional TEM images. Film thickness was determined by profilometry from the step made from removing part of the film from the substrate. Silver-free PEM thicknesses were also measured using ellipsometry (Gaertner) at wavelengths of 633 and 830 nm and at 70 incidence. Thickness measurements differed by less than 10% across the different methods. Optical transmission measurements were performed using a Varian Cary 5E or Cary 500i spectrophotometer. Near-normal reflectivities (fixed 7 off normal to the plane of the film) were measured using an absolute (VW configuration) specular reflectance accessory on the spectrophotometers. Because specular reflectance became significant with multiple silver cation exchange and reduction cycles, both transmittance and reflectance spectra were measured to calculate a corrected absorbance spectrum. In addition, back reflections from the substrate were accounted for in the transmittances and reflectances.34 Quantitative elemental analyses of silver, obtained by acid digestion of the nanocomposite film and atomic emission spectroscopy, were performed by Galbraith Laboratories (Knoxville, TN) and Schwarzkopf Laboratory (Woodside, NY). Results PEMs of 20.5 bilayers of PAH/PAA (i.e., with PAH next to both the substrate and the free surface) were assembled at five different pH conditions: 2.5, 3.0, 3.5, 4.0, and 4.5. Dry film thicknesses were approximately 94, 89, 90, 110, and 140 nm, respectively. All future references to the PEMs in this paper will be denoted as "(PAHx/PAAx)y+Agn" where x is the polyelectrolyte solution pH during PEM assembly, y is the number of bilayers, and n is the number of silver cation exchange and reduction cycles. It is important to emphasize that the Ag(ac) aqueous solutions used for silver cation binding were prepared at nominally neutral pH for all the PEM films. Bright-field, cross-sectional TEM images of (PAHx/PAAx)20.5+Ag1 films, in Figure 2a-c for selected PEM assembly pH conditions, show spherical Ag particles that are uniformly and randomly distributed throughout the film. The particles have diameters ranging from approximately 2.1 nm for multilayers assembled at pH 4.5 to 3.8 nm for those assembled at pH 2.5. Since it is not possible to quantitatively determine silver concentration from the TEM images, UV-vis transmission measurements and elemental analysis are used to quantify the changes in silver concentration with PEM assembly pH and with n, the number of silver cation exchange and reduction cycles. -------------------------------------------------------------------------------- Figure 2 Cross-sectional TEM images of (PAHx/PAAx)20.5+Ag1 with multilayers assembled at polyelectrolyte solution pHs of x = (a) 4.5, (b) 3.5, and (c) 2.5 and (PAHx/PAAx)20.5+Ag5 with multilayers assembled at x = (d) 4.5, (e) 3.5, and (f) 2.5. Nanoparticle diameter histograms (in nanometers) are insets; average diameters are (a) 2.1 ± 0.5 nm, (b) 3.1 ± 0.4 nm, (c) 3.8 ± 0.9 nm, (d) 6.5 ± 1.4 nm, (e) 7.4 ± 1.2 nm, and (f) 9.3 ± 2.1 nm. -------------------------------------------------------------------------------- The UV-vis transmission spectra for films assembled at different polyelectrolyte solution pHs and with n = 1 are shown in Figure 3. The optical spectrum of small, isolated particles of silver embedded in a dielectric matrix is dominated by a strong dipole absorption mode due to the collective oscillation of conduction electrons, so-called surface plasmons.35 Spherical particles with diameters less than several hundred nanometers exhibit a single, strong absorption peak in the visible region of the spectrum.36 In our measured spectra, the characteristic surface plasmon absorbance of the Ag nanoparticles dominates the visible wavelengths with a maximum centered around 435 nm; the other spectral feature is the UV absorption of the PEM matrix and PS substrate. The existence of a single absorption peak in the visible, and no additional peaks as far as 1500 nm, is consistent with the observation from TEM images that the Ag nanoparticles are essentially spherical. The flat transmission response at longer wavelengths to the red of the surface plasmon absorption edge indicates that scattering is minimal; the nanoparticles in the PEM films are much smaller than the probe wavelengths, and the concentrations are relatively low with n = 1. -------------------------------------------------------------------------------- Figure 3 UV-vis transmission spectra, normalized to film thickness, of (PAHx/PAAx)20.5+Ag1 assembled at pHs of x = (·) 2.5, () 3.0, () 3.5, () 4.0, and () 4.5. -------------------------------------------------------------------------------- The surface plasmon absorbance maximum, normalized to film thickness, increases with decreasing PEM assembly pH. It is not possible, however, to separate the effect of the concentration of absorbance centers from the effect of the extinction cross section of those centers on the absorbance maximum. Since the extinction cross section scales with particle volume35 and the Ag nanoparticle diameter as determined by TEM increases as PEM assembly pH decreases from 4.5 to 2.5, the stronger surface plasmon absorbance at low assembly pH can be partly attributed to larger extinction cross sections of the Ag nanoparticles. To determine the silver concentration directly, elemental analysis of the nanocomposite films was performed. Elemental analysis of silver in the PEM, tabulated in Table 1, shows an increasing weight fraction of silver with decreasing PEM assembly pH, from 28% at pH 4.5 to 41% at pH 2.5. Using the bulk silver density (10.5 g/cm3) and a PEM density of 1.2, the calculated nanoparticle volume fraction increases from 4% to 8% as PEM assembly pH decreases from 4.5 to 2.5. During PEM assembly, the degree of PAA ionization is controlled by the pH of the polyelectrolyte solutions. It was previously reported that ~70% of PAA carboxyl groups are in acidic form for films prepared at pH 2.5 while only ~30% are acid groups at pH 4.5 as determined by FTIR.22 Hence, the amount of PAA carboxylic acid groups retained in the final film, and therefore the amount of silver cations that can be bound, can be systematically controlled. As the multilayer assembly pH was increased to 6.5, no Ag nanoparticles could be formed. During layer-by-layer assembly at this pH condition, all the PAA carboxyl groups and the PAH amine groups are ionized and fully pair up leaving no free acid groups for subsequent silver cation binding.23 To expand the range of concentrations and particle sizes that can be accessed by manipulating film assembly pH, the nanoparticle synthesis methodology can be repeatedly cycled to increase the silver content. After the reduction of exchanged silver cations to Ag(0) nanoparticles, the carboxylic acid groups are reprotonated permitting further metal cation binding. Figure 2d-f shows TEM images of PAH/PAA-based films for selected PEM assembly pH conditions with n = 5. A significant increase in the particle size is observed compared to films with n = 1. In films assembled at pH 4.5, where the lowest initial nanoparticle volume fraction was obtained, particle diameters increased from 2.1 to 6.5 nm after five cycles. Between n = 1 and n = 5 for films assembled at pH 2.5, the average particle diameter increased from 3.8 to 9.3 nm. Moreover, the nanoparticles continue to be uniformly distributed throughout the multilayer film after five cycles. The silver content from elemental analysis, reported as the mass ratio of silver to PEM, increases linearly with n as shown in Figure 4. For a PEM assembled at a given pH, the silver content increases by approximately the same amount with every cycle. This suggests that there are minimal physical (e.g., reduced mass transport due to blockage) or chemical (e.g., unfavorable local equilibrium) limitations to silver cation exchange into the PEM, at least up to five cycles (n = 5). The calculated nanoparticle volume fraction increases from 4% to 14% in pH 4.5 assembled films and from 8% to 24% in pH 2.5 assembled films after five cycles. The volume fraction can also be independently estimated from the increase in film thickness, assuming a constant film area. For PEMs assembled at pH 2.5, the thickness of the film increased approximately 25% after five cycles, comparable to the elemental analysis derived 24%. -------------------------------------------------------------------------------- Figure 4 Silver to PEM mass ratio as a function of n for (PAHx/PAAx)20.5+Ag1 assembled at pHs of x = (·) 2.5, () 3.0, () 3.5, () 4.0, and () 4.5. -------------------------------------------------------------------------------- The optical spectra of these films exhibit a broadening and eventual splitting of the surface plasmon absorbance peak as n increases from 1 to 5. The spectra of multilayers assembled at pH 3.5 are shown in Figure 5. Similar absorbance behavior is observed for the films assembled at the other pH conditions. The absorbance is broader and the splitting is slightly more pronounced, though still far from complete, at lower assembly pHs. The evolution in the shape of the surface plasmon absorbance is consistent with increasing interparticle interactions as n increases.36 -------------------------------------------------------------------------------- Figure 5 Absorbance spectra of (PAH3.5/PAA3.5)20.5+Agn with n = (·) 1, () 2, () 3, () 4, and () 5. The corrected absorbance curve is presented rather than the uncorrected transmission curve because specular reflection from the thin film becomes significant and overlaps with the surface plasmon absorbance as more silver is added. -------------------------------------------------------------------------------- Finally, while this paper has focused primarily on multilayers based on two polyelectrolytes, PAH and PAA, the repeated silver cation exchange and reduction cycles apply equally well to spatially defined heterostructures consisting of three polyelectrolytes, PAH, PAA, and SPS, where PAH/PAA multilayers alternate with PAH/SPS multilayers.19 Silver nanoparticles are spatially selective for and confined to the PAH/PAA multilayers over multiple exchange and reduction cycles. Figure 6 shows magnified portions of relatively thick, ~1.3 m, lamellar structures consisting of five (PAH3.0/PAA3.0)24 lamellae alternating with four (PAH3.0/SPS3.0)77 lamellae on a PS substrate after n = 1 and n = 5, where the Ag nanoparticles are confined to the PAH/PAA multilayers. Note that even with this thick stack, an order of magnitude thicker than those typically prepared (<0.1 m), Ag nanoparticles are uniformly distributed in the PAH/PAA multilayers and absent from the PAH/SPS multilayers. -------------------------------------------------------------------------------- Figure 6 Cross-sectional TEM images of [(PAH3.0/PAA3.0)24(PAH3.0/SPS3.0)77]4.5+Agn heterostructure with n = (a) 1 and (b) 5. Magnified, selected regions at the free surface and next to the substrate are shown. -------------------------------------------------------------------------------- Discussion and Conclusion PAA, with a solution pKa between 5 and 6 depending on polymer concentration and ionic strength,37 is less ionized at lower assembly pHs. Therefore more carboxyl groups remain acidified in the final PEM structure and are available to participate in subsequent ion exchange with silver cations. Not only are there more carboxylic acid groups relative to carboxylate groups on a PAA chain at lower assembly pHs, but more PAA chains relative to PAH chains are incorporated into the multilayer.23 During assembly at lower pHs, multilayers with a PAA surface are less ionized and hence have a lower surface charge, reducing the amount of PAH adsorbed. When PAA is adsorbed to a PAH surface at lower pHs, the PAA is less ionized and hence more chains are adsorbed onto the PAH surface for charge compensation. The result is a higher concentration of carboxylic acid groups at lower assembly pHs. In addition, the multilayer assembly pHs need not be the same for both polycation and polyanion solutions as presented in this paper; other pH combinations can be used to obtain a range of carboxylic acid concentrations with different physical architectures (e.g., adsorbed layer thickness) and morphologies.23,38 While the multilayer assembly pH is an independent parameter for controlling the amount of silver cations exchanged into the multilayer, other factors also influence the nanoparticle size. One hypothesis for the relatively small particle sizes with n = 1, diameters between 2.1 nm at pH 4.5 and 3.8 nm at pH 2.5, is the particle-stabilizing attributes of carboxylic acid groups. At low silver cation concentrations, particles grow until they reach a size with a surface area that can be stabilized by the polymer. This is analogous to the use of polymer concentration in solution synthesis of nanoparticles as a parameter for tuning particle size.39 Since the density of polymer, and hence of stabilizing ligands, is relatively high in the dry films, the particle size should be relatively small. Additionally, silver cation proximity in the multilayer film also appears to play a role in determining final particle size as seen in the small variation in particle diameters with pH at n = 1. At the lower assembly pHs where silver cation concentrations are higher, average particle diameters are larger while particle concentrations are smaller. From the nanoparticle volume fractions and average diameters (i.e., not accounting for size dispersity), nanoparticle concentrations are estimated to be 8.7 × 1018, 3.5 × 1018, and 2.6 × 1018 particles/cm3 at pH 4.5, 3.5, and 2.5, respectively. This observation is more pronounced with increasing n, where the cations are in much closer proximity to previously formed nanoparticles. Aggregation of the particles may occur, though no gross coagulation of particles is observed in the TEM images. During successive silver cation exchange and reduction cycles, the particle size after each cycle becomes larger than the amount of silver introduced per cycle as determined by elemental analysis. For example, the nanoparticle volume fraction increases from 8% to 24% after five cycles at pH 2.5. If the nanoparticle concentration is assumed to remain constant (i.e., all newly introduced silver is incorporated into existing particles), the nanoparticle diameter should be ~6 nm in contrast to the observed 9.3 nm average. This suggests that the particles are growing by a mechanism that incorporates previously existing nanoparticles as in aggregation or Ostwald ripening. A similar mechanism has been argued for nanoparticle synthesis in block copolymers.40 The increasing proximity between nanoparticles as n increases was also reflected in the broadening and eventual twinning of the surface plasmon absorbance. Interparticle interactions become significant as n increases. One study reports that silver nanoparticles in a two-dimensional periodic array begin to interact with one another at interparticle spacings of approximately one particle diameter.41 An estimate of interparticle spacing in the PEM matrix, defined as the shortest distance from the surface of one particle to the surface of another and normalized to particle diameter, can be made by assuming a face-centered cubic arrangement of same-sized, spherical nanoparticles. For pH 4.5 assembled films with 2.1 nm diameter particles at a volume fraction of 4%, the interparticle spacing is 3.4 nm (1.6 diameters). For pH 2.5 assembled films with 3.8 nm diameter particles at a volume fraction of 8%, the interparticle spacing is 4.4 nm (1.2 diameters). On the basis of these estimates, the interparticle interactions should be insignificant with n = 1, consistent with the single absorbance peak observed. With larger n, however, interparticle spacings are estimated to reduce to 4.9 nm (0.75 diameters) for pH 4.5 with n = 5 and 4.3 nm (0.47 diameters) for pH 2.5 with n = 5, where interactions become significant. It is important to note that our nanoparticles are arranged not in such a well-ordered structure in the film but rather randomly. Additionally, percolation, as determined by in-plane resistance measurements, was not observed for the (PAH2.5/PAA2.5)20.5+Ag5 film with 24% nanoparticle volume fraction. In conclusion, we have demonstrated that the silver nanoparticle size and the overall silver concentration can be systematically manipulated within PAH/PAA-based multilayers. This is accomplished by the control afforded by multilayer assembly pH over the number of free acid groups within the multilayer and by the flexibility of the silver cation exchange and reduction synthesis to facilitate repeated cycling. Moreover, the continued incorporation of silver through repeated ion exchange and reduction remains strongly confined to and uniformly distributed only within PAH/PAA regions of heterostructure multilayers. As we reported previously, the in situ synthesis methodology is applicable to a variety of other metals, only requiring the precursor to be in the form of a metal cation that can bind to the PAA carboxylic acid groups. In addition to reducing metal cations to zerovalent metal nanoparticles such as Au, Pd, and Pt, sulfidation has been used to prepare chalcogenide semiconductor nanoparticles such as CdS, PbS, and ZnS. The manipulation of PEM assembly pH and the number of metal cation exchange and reduction cycles discussed in this paper for silver should be readily generalized to these other inorganic systems. Moreover, one should be able to extend the multiple metal cation exchange and reduction scheme to form mixed metal or bimetallic nanocomposite systems, analogous to work in carboxylic acid containing block copolymer matrixes.42 Finally, one implication of the control over the silver content in the multilayers is the ability to systematically change the optical properties of these nanocomposite films. 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Silver Content within (PAHx/PAAx)20.5+Agn at Various PEM Assembly pHs (x) and Silver Cation Exchange and Reduction Cycles (n) PEM pH Ag wt % (±3%) Ag vol % (±0.3%) Ag/PEM mass ratio (±0.03) 2.5 n = 1 41 7.5 0.71 n = 3 65 17.7 1.9 n = 5 73 23.5 2.7 3.0 n = 1 33 5.3 0.49 n = 3 56 12.7 1.3 n = 5 67 18.7 2.0 3.5 n = 1 34 5.5 0.51 n = 5 64 16.8 1.8 4.0 n = 1 29 4.4 0.40 n = 3 51 10.6 1.0 n = 5 62 15.7 1.6 4.5 n = 1 28 4.2 0.38 n = 3 45 8.6 0.83 n = 5 58 13.8 1.4 |
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