Light-Induced Formation of Silver Particles and Clusters in

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J. Phys. Chem. B 2004, 108, 14850-14857

Light-Induced Formation of Silver Particles and Clusters in Crosslinked PVA/PAA Films G. A. Gaddy,†,⊥ A. S. Korchev,† J. L. McLain,† B. L. Slaten,*,§ E. S. Steigerwalt,|,∇ and G. Mills*,† Department of Chemistry and Department of Consumer Affairs, Auburn UniVersity, Auburn, Alabama 36849, and Department of Chemistry, Vanderbilt UniVersity, NashVille, Tennessee 37235 ReceiVed: January 16, 2004; In Final Form: March 29, 2004

Thin films able to sustain an efficient photoreduction of Ag+ ions with 350 nm photons in air were prepared by crosslinking poly(vinyl alcohol) with glutaraldehyde in the presence of poly(acrylic acid). Standard colloid techniques served as screening methods for the selection of polymer compositions yielding films with desired properties. When present at high concentrations, Ag+ ions were reduced at room temperature in the films by poly(vinyl alcohol) but poly(acrylic acid) inhibited the slow dark reaction. Optical signals with maxima above 400 nm resulted from both reduction processes. The available evidence confirmed that they originated from nanometer-sized metal particles and is inconsistent with results for other proposed chromophores. An additional absorption centered at 280 nm that formed only under illumination was assigned to Ag3+ clusters. Small Ag crystallites with similar size distributions and with an average diameter of 5 nm were the main product of the photoreduction in non-crosslinked or crosslinked films. Larger particles were detected less frequently, and in the former films they consisted predominantly of crystallite aggregates. These results along with the longterm stability of the photogenerated Ag3+ clusters are consistent with a particle nucleation process based on diffusion and coalescence of mobile metal atoms in the films.

Introduction Silver particles with dimensions in the nanometer range possess characteristics that are size-dependent and different from those of bulk Ag. In this domain the metal properties are a function of the number of atoms present in the solid, or nuclearity. The most known size effects are the nuclearityinduced changes in the optical properties of spherical silver nanoparticles and of Ag clusters consisting of a few atoms.1,2 Alterations in the thermal and photochemical reactivities of the metal crystallites and clusters are evidence that the thermodynamic properties of silver evolve with nuclearity.2-8 These sizedependent properties of silver are of technological significance; for instance, reactivity differences between Ag particles and clusters are of critical importance in photographic imaging.9 Other technologies that rely on size effects of Ag are thermally developed photographic materials (TDPM),10 photosensitive glasses,11 and photochromic glasses.12,13 From the scientific point of view photochromic glasses are very interesting since they undergo reversible changes in properties under light exposure and are prototypic examples of photoresponsive systems. Reversible photogeneration of metal crystallites has been found to occur within gels prepared by swelling crosslinked poly(diallyldimethylammonium chloride) with methanolic solutions of AuCl4-.14 Photolysis of the metal complex yields nanometer-sized particles that are unstable toward oxidation by air, reforming AuCl4- in the dark. These findings were the * Corresponding authors. E-mail: [email protected] (B.L.S.); [email protected] (G.M.). † Department of Chemistry, Auburn University. § Department of Consumer Affairs, Auburn University. | Department of Chemistry, Vanderbilt University. ⊥ Present address: United States Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783. ∇ Present address: FCSC, 100 Plumley Dr., Paris, TN 38242.

conceptual starting point in efforts to prepare photoadaptive polymer fibers that respond reversibly to light. Among the desired properties was the ability to reflect infrared radiation (IR) upon photogeneration of metal crystallites inside the fibers, and to switch back to their initial nonreflecting state once the particles were oxidized in the absence of light. The first task consisted of identifying polymers having a macromolecular backbone able to donate electrons, which would facilitate a fast photoreduction of the metal ions. Considering that solid-state reactions are generally slow, efficient particle formation in the polymers was anticipated only under high photon fluxes. This is a desirable effect because the polymeric materials were intended as IR barriers against exposures to intense light. Furthermore, photogeneration of particles by ambient (indoor) light was undesirable since continuous oxidation of the polymer matrix would inevitably have adverse effects on the mechanical properties of the fibers. These requisites were fulfilled by fibers containing Ag+ ions made from blends of poly(vinyl alcohol), PVA, and poly(acrylic acid), PAA, using dimethyl sulfoxide (DMSO) as a crosslinking agent.15 Fast formation of Ag particles occurred upon exposing the fibers to high fluxes of 350 nm photons or direct sunlight, but no such process took place under ambient illumination. Selection of silver seemed logical considering that photoreduction of Ag+ requires only one reducing equivalent. Also, monitoring the formation kinetics of Ag nanoparticles with spectrophotometry seemed feasible due to their intense surface plasmon between 380 and 500 nm.1 Selection of PVA as a matrix component was based, in part, on the ability of this polymer to serve as an electron donor during the reduction of metal ions.16 In addition, optically transparent films of PVA have been employed to study the optical properties of Ag particles.17 Incorporation of Ag+ into solid films was found to be more reproducible when PAA was present, probably owing

10.1021/jp0497561 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/04/2004

Ag Particles in Cross-Linked PVA/PAA Films to the known ability of this polymer to bind silver ions.18 Although preparation of transparent fibers was possible, optical measurements with these materials were found to be difficult, and an alternative strategy was conceived that employed thin films for these determinations. Kinetic studies showed that particle photogeneration in films was similarly fast for PVA/PAA blends either non-crosslinked or crosslinked with DMSO but slower when glutaraldehyde (GA) was the crosslinking agent.15 Although these results validated the approach employed in the preparation of the polymer systems, the films exhibited several drawbacks. For instance, films of non-crosslinked PVA or those crosslinked with DMSO are not suitable for practical applications since they dissolve in hot water. Also, acceptable reproducibility of crystallite formation was difficult to achieve in view of the numerous steps involved in the preparation of films dry cast on glass surfaces. The present paper is centered on subsequent studies that addressed these issues using an alternative procedure for film deposition together with a modified method to crosslink PVA/PAA blends with GA. The modifications yielded chemically stable films exhibiting the efficient particle formation with high-intensity photon fluxes and the long-lived metal clusters found in the earlier investigation.15 Kinetic information about the crystallite generation process inside the films is presented in a companion article.40 Experimental Section PAA (average molar mass ) 2 × 103 g mol-1), PVA (average molar mass ) 8.9-9.8 × 104 g mol-1), AgNO3, AgClO4, and NaNO3 were from Aldrich, whereas CH3OH, H2O2 (50 wt %), glutaraldehyde (25 wt %), HCl, and HNO3 were obtained from Fisher. The chemicals were used without further purification. All glassware was treated with aqua regia; all solutions were prepared using deionized water obtained from an ion-exchange resin deionizer (U. S. Filter Service). Crosslinking of PVA/PAA blends with GA by curing at 60 °C yields films that are chemically stable and of good optical quality.19 A slight modification of this method improved the cross-link reproducibility as compared with that of blends cured at room temperature.15 The resulting flexible films were able to swell when immersed in aqueous or methanolic solutions without losing PAA, which allowed extraction of unreacted GA and of the acid crosslinking catalyst from the polymeric matrixes as well as subsequent incorporation of Ag+ ions. In the first step of film preparation, aqueous solutions of 2.14 M PVA (M in terms of monomer units, 8.6 wt %, 25 mL) and 0.7 M PAA (4.8 wt %, 20 mL) made separately at 73 °C were combined after 45 min, and the mixture was maintained at this temperature for 30 min. The solution was then allowed to cool to room temperature, treated with 3 mL of a 0.1 M (3 wt %) aqueous GA solution and with 3 mL of 0.5 M HCl (or HNO3) after 5 min. Constant stirring throughout these steps ensured homogeneous mixing of the components. The mixture was poured after 2 min onto clean glass substrates; uniform films were obtained using a Gardner knife (Model AP-G08, setting of 1.3 mm). Heating the films in an oven between 55 and 60 °C for 3 h accelerated crosslinking of PVA. After the films were cooled to room temperature, crosslinking was quenched by exposure to large volumes of CH3OH for 24 h to extract excess GA and acid. Particular care was taken to eliminate all HCl from crosslinked films prior to incorporation of Ag+ ions; otherwise AgCl crystalllites formed inside the polymeric materials turn them sensitive to ambient light. Immersion of films in a NaNO3 solution overnight and ensuing potentiometric analysis

J. Phys. Chem. B, Vol. 108, No. 39, 2004 14851 with a Cl- ion selective electrode (ISE) verified that no residual HCl remained in the solid polymer matrixes after the CH3OH treatment. Using the Gardner knife method instead of dry casting allowed deposition of highly uniform films and eliminated photogeneration of crystallite gradients. Crosslinked films were insoluble in boiling water, they were optically transparent above 300 nm, and their average thickness was 70 µm as determined with a TMI 49-60 micrometer. Neglecting the small amount of incorporated GA, the film composition was 70.1 wt % PVA and 29.9 wt % PAA. Films with masses of (5-9) × 10-2 g were subsequently exposed for 24 h to 100 mL of 1 × 10-2 M AgClO4 or AgNO3 in methanol. After rinsing the polymer surfaces briefly with CH3OH, the films were dried in a vacuum oven for 2 h at room temperature. The content of Ag+ in crosslinked films (1.4-3.4 wt %) was determined by dissolving them in 5 mL of 3 M HNO3 at 80-90 °C, followed by treatment of the solutions with base to reach pH 1 and dilution to 25 mL. Potentiometric determinations of [Ag+] were carried out with Radiometer instrumentation consisting of a PHM 95 pH meter, an Ag+ ISE (Model FK 1502), and Hg/Hg2SO4 reference electrode (Model REF 601). Films of PVA alone were made as outlined above but without the crosslinking step. Non-crosslinked films of the PVA/PAA blends were obtained similarly, except that the polymer solutions were dry cast onto glass substrates. Both types of films swelled in CH3OH but were water-soluble; non-crosslinked PVA/PAA matrixes were less uniform in thickness (≈150 µm) than the crosslinked materials. Ag+ ions were incorporated into noncrosslinked polymer matrixes by means of methanolic solutions as described before for crosslinked films. After preparation, all films were stored at -80 °C to ensure preservation of their properties for extended periods of time. Replacement of HCl by HNO3 during crosslinking, or of AgClO4 by AgNO3, produced no significant differences in the rates of the photoreactions. Deviations in the reaction rates were within 30% for crosslinked films but were at least 45% in the case of polymer matrixes not treated with GA. For optical density (OD) measurements and illuminations performed in air, the films were located between two quartz windows held together with a metal frame. A limited number of air-free experiments were conducted by placing films vertically inside a 1 mm quartz cell fused to a female graded glass joint (10/7). The cell was sealed with the corresponding male joint fused to a T-shaped glass tube having airtight stopcocks on each end, and was purged with Ar for 1 h prior to illumination. UV-vis absorption spectra were acquired on a Shimadzu UV-2501-PC spectrophotometer. Irradiations were carried out by positioning the films near the center and at midheight of a cylindrical Rayonet 100 photochemical reactor equipped with RPR-3500A lamps that produced 350 ((17) nm photons. The temperature inside the illuminated reactor was 29 °C; irradiations occurred at a typical light intensity (I0) of 1.2 × 10-1 M (hν) min-1 as estimated with 1 mm quartz cells using the ferrioxalate actinometer.20 In a few cases PVA/PAA films were exposed to the full UV-vis output of a 150 W Xe lamp employing a PTI A-1010 S system and a 10 cm cylindrical water filter with quartz windows to eliminate the IR component. Ag colloids were made by treating the PVA solutions (with or without PAA) that served for film preparation as described above with enough solid AgClO4 to attain [Ag+] ) 2 × 10-51 × 10-4 M. The resulting solutions were illuminated with 350 nm light (I0 ) 2.4 × 10-3 M (hν) min-1) in 1 cm fluorescence quartz cells until the Ag plasmon absorption remained constant

14852 J. Phys. Chem. B, Vol. 108, No. 39, 2004 with time. Reduction of the Ag+ ions was completed at this point since no AgCl formed upon addition of NaCl to the solutions. The metal colloids exhibited an absorption maximum that varied slightly between 410 and 430 nm and optical densities that increased linearly in the range 2 × 10-5 e [Ag+] e 8 × 10-5 M. Extinction coefficient () values of the metal crystallites (per mole of Ag atoms) were 5 × 103 M-1 cm-1 at 420 nm and 1.1 × 103 M-1 cm-1 at 280 nm. The ratio (280)/ (420) ) 0.22 was very close to the value of 0.23 obtained for Ag colloids prepared in 2-propanol.8 Transmission electron microscopy (TEM) of non-cross-linked films was performed in a Philips CM-20T instrument operating at 200 kV. Samples were made by dissolving in water films exposed to light and placing a drop of the resulting solution onto Cu grids coated with holey carbon films (SPI), followed by evaporation under ambient conditions. Photolyzed crosslinked films were microtomed and collected on Cu-mesh grids. Sections were examined with a Zeiss EM10 TEM microscope operating at 60 kV. Enlarged prints of TEM images were employed to determine average diameters of very small crystallites (2-4 nm). X-ray diffraction (XRD) data was acquired by placing films directly in the sample holder of a Scintag X1 powder diffractometer that used Cu KR radiation. Results and Discussion Photoreduction of Ag+ in PVA/PAA films yielded the typical surface plasmon of Ag particles, but additional optical signals extending sometimes beyond 500 nm were noticed at certain polymer blend compositions. Similar absorptions have been detected during the transformation of Ag+ ions to metal particles in aqueous solutions containing poly(acrylate) ions induced by radiolysis,21,22 NaBH4,23 or UV photolysis.18 Although silver clusters seem to be the origin for some of the signals, the chemical composition of these species remains uncertain because the results of different studies diverge. In the case of PVA/ PAA films, formation of broad absorptions at wavelengths longer than the particle plasmon band was not entirely reproducible. The presence of these signals also hampered attempts to evaluate optically the formation kinetics of Ag crystallites. For these reasons, blend compositions best suited for the objectives of the present study were those producing no broad Ag absorptions at longer wavelengths. An interesting finding was that for a given blend composition similar optical signals resulted by photogeneration of Ag particles in the PVA/PAA solutions and in films made from them. Therefore, optimization of the blend composition of the solid matrixes was achieved using the optical data gathered from the colloid experiments, which required a much simpler preparation technique than for films. Presented in Figure 1 are optical spectra obtained during illumination experiments with different solutions; curve 1 depicts the spectrum of unirradiated solutions containing 1.19 M PVA with and without 0.31 M PAA. Irradiation of a solution containing only PVA and 1 × 10-4 M Ag+ ions resulted in a broad absorption centered at 430 nm that sharpened and shifted continuously to 410 nm as photolysis proceeded. The solution turned yellow and at longer times a second absorption appeared as a shoulder at about 520 nm; curve 2 of Figure 1 recorded after 175 min of irradiation is a typical spectrum showing both signals. PVA solutions containing a [PAA] lower than 0.12 M yielded both bands simultaneously, which merged into the signal at 420 nm upon further illumination. Very broad absorptions extending beyond 700 nm were obtained at higher [PAA]. Further studies showed that none of the bands at longer wavelengths were present in solutions with [Ag+] < 1 × 10-4

Gaddy et al.

Figure 1. Changes in optical spectra after photolysis (350 nm, I0 ) 2.4 × 10-3 M (hν) min-1) of aqueous solutions containing 1.19 M PVA, 0.31 M PAA, and AgClO4. Curve 1, prior to illumination; curve 2, after 175 min, [Ag+] ) 1 × 10-4 M and no PAA; curve 3, after 120 min, [Ag+] ) 6 × 10-5 M. Curve 4 (inset), 0.31 M PAA and 1 × 10-4 M Ag+ irradiated for 2150 min.

M. The sharpest plasmon bands were obtained in the presence of 1.19 M PVA and 0.31 M PAA; spectrum 3 in Figure 1 resulted after complete photoreduction of 6 × 10-5 M Ag+ in such a solution. Irradiation generated a single absorption with λmax ) 410 nm, which turned stronger with time. Reduction of Ag+ induced by 350 nm photons also took place in PAA solutions, but as shown in the inset of Figure 1, only a weak Ag plasmon was noticed after extended exposures. In agreement with the trends observed in the colloid experiments, films made from solutions containing 1.19 M PVA and 0.31 M PAA yielded the best results. Photolysis of these films (composed of 70.1 wt % PVA and 29.9 wt % PAA) turned them yellow and induced growth of an absorption with λmax ) 430 nm but without other signals at longer wavelengths. Figure 2a illustrates the spectral evolution with time of a crosslinked film; the spectra became progressively sharper as illumination proceeded without any change in the wavelength of the maximum of absorption. Subtraction of the spectrum measured prior to illumination from the optical data gathered at different photolysis times resulted in the corrected spectra shown in Figure 2b. In contrast, dry cast non-crosslinked films exhibited broader spectra centered initially at about 480 nm that blueshifted to 445 nm as photolysis proceeded. Similar observations were made in our previous study using films deposited with the dry cast method.15 Depicted in the inset of Figure 2b is a spectrum of a film containing only PVA. Extensive illuminations were needed to induce changes, generating multiple metal signals. The inefficiency of the photoreaction can be attributed, in part, to the low silver salt concentration present in films without PAA and to variations in the amounts of silver ions incorporated. No such problems were encountered with the blends because incorporation of Ag+ into the films was more reproducible in the presence of PAA, which binds the metal ions.18 In addition, PVA films exposed to solutions with [AgClO4] > 1 × 10-2 M that were rinsed and dried suffered from slow metal ion reductions in the dark. Very broad optical signals with λmax ≈ 500 nm resulted after several days at room temperature, and the thermal process was significantly less reproducible than the photoreaction. Binding of the metal ions to PAA inhibited their dark reduction by PVA; for blends with 29.9 wt % polymeric acid this reaction was noticed only after treating the films with [AgClO4] g 0.1 M. Hence, the slow thermal particle formation seemed to involve

Ag Particles in Cross-Linked PVA/PAA Films

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Figure 2. (a) Evolution of spectra during illumination (350 nm, I0 ) 1.2 × 10-1 M (hν) min-1) of a crosslinked film containing 70.1 wt % PVA and 29.9 wt % PAA after exposure to 1 × 10-2 M AgClO4. Irradiation times are indicated in the figure. (b) Normalized spectra obtained from the data shown in (a). The inset is a normalized spectrum from a film made without PAA irradiated for 70 min.

reduction of silver species not bound to PAA. Extensive film darkening occurred at 57 °C, demonstrating that metal generation was accelerated when the temperature increased. Strong absorptions centered at 430-450 nm have been reported for PVA films containing AgNO3 made by drying at elevated temperatures solutions of the chemicals.24,25 The signal was assigned to a chelate presumed to form when the metal salt binds to OH groups of PVA. However, several observations from the present and from earlier studies raised doubts about such an interpretation. For instance, illumination of blends containing AgClO4 or AgNO3 produced the same absorptions, which were bleached upon exposing the films to solutions with [H2O2] g 8 × 10-2 M. Also, unirradiated films with or without Ag+ ions exhibited identical spectra, which according to Figure 2a were free of any signal above 300 nm. Since binding of Ag + to CO2- groups of PAA produced no absorptions in this spectral

range, it seems unlikely that bands with maxima at 430-450 nm originated from the proposed AgNO3-PVA chelate. Furthermore, these absorptions resemble Ag surface plasmons recorded in a variety of matrixes containing metal particles,1 including films of PVA and of the polymeric alginic acid.26,27 Initial efforts to identify the species generated in the films employed XRD. In agreement with previous attempts,25 films exposed to light for a few hours showed no metal signals. This result is not unexpected given the low sensitivity of the XRD method and the well-known broadening of diffractions from nanometer-sized Ag crystallites.8 Photolysis for 24 h circumvented such problems, inducing darkening of films with or without PAA. Figure 3 is a diffractometer trace from an illuminated PVA/PAA film showing reflections from the {111}, {200}, {220}, and {311} lattice planes of fcc Ag. The relatively broad signals centered at about 40° and 53° were also detected

14854 J. Phys. Chem. B, Vol. 108, No. 39, 2004

Figure 3. XRD powder pattern of a PVA/PAA film containing AgClO4 irradiated for 24 h (I0 ) 8 × 10-4 M (hν) min-1). The bottom trace corresponds to the raw data; the top trace was obtained after background subtraction. Reference positions for the diffraction signals of metallic Ag are identified by vertical lines.

in unirradiated samples and are due to the amorphous polymer matrix. Complementary TEM experiments were performed with PVA/PAA films exposed to light for 70 min. The intensities at 430 nm of the samples (Figure 2a) were close to those of unirradiated PVA films prepared at 60-80 °C,24,25 implying that the concentrations of the light-absorbing species were similar in both systems. Nanometer-sized particles were detected; although characterization data will be presented later, the relevant finding is that electron diffraction studies confirmed the metallic nature of the crystallites. Hence, the fact that Ag+ is reduced thermally within solid poly(vinyl alcohol), together with the above-mentioned optical, XRD, and TEM data, demonstrate that the absorptions at 430-450 nm of heated PVA films containing silver ions correspond to the surface plasmon of Ag particles. Another interesting observation made during the photoreduction of Ag+ in the films was the large increase in absorbance below 310 nm depicted in Figure 2a. The normalized spectra shown in Figure 2b revealed that an additional signal centered at 280 nm emerged in parallel with the Ag plasmon, and that the former was predominant during the earlier stages of the photoreaction. A similar absorption has been noticed in silver alginate films,27 and in our dry cast films;15 we attributed the signal to the Ag42+ cluster, which in solution is known to absorb strongly in this spectral region.2 However, an absorption with λmax ) 280 nm was also detected upon exposure to unfiltered UV light of air-saturated solutions of sodium polyacrylate in the absence of Ag+ ions.18a Radiolysis investigations have identified the chromophores as acetylacetone-like products of the free radical oxidation of PAA.28 The light-absorbing species originated from extensive degradation, including decarboxylation, of the polymer that occurred only in the presence of oxygen. Evidence has been found that absorption of UV light (below 300 nm) by the carboxylate functions of PAA yields polymer radicals.28a Thus, the high-energy photons used in the earlier study apparently induced photooxidations of PAA that generated acetylacetone-like products.18a In view of these findings, efforts were made to elucidate the origin of the 280 nm absorption exhibited by the films. It should

Gaddy et al. be noted that the high-energy absorption was generated exclusively by the action of light since no such signal was detected during the thermal metal formation in the polymers. Exposure of films free of Ag+ ions to 350 nm light for extended periods of time failed to alter their spectra because, as is shown in Figure 2a, the unirradiated polymers do not absorb significantly at this wavelength. Interestingly, photolysis of PVA/PAA films containing Ag+ in the presence of Ar produced optical data very close to that of Figure 2b, although no acetylacetone-like products resulted when PAA was oxidized in the absence of O2.28 Also, an absorption at 275 nm was photogenerated in PVA films containing AgClO4 (inset of Figure 2b), but was absent during the oxidation of this polymer.29 In addition, photolysis of cross-linked PVA/PAA films free of Ag+ ions with the full UV-vis output of a 150 W Xe lamp produced only a slight increase in OD over the entire wavelength range without any maximum. Given that these conditions simulated as close as possible those of the solution experiments that yielded acetylacetone-like products,18a the present results demonstrate that formation of these chromophores was inhibited in blend films containing PAA. Furthermore, the two strong optical signals shown in Figure 2b vanished after irradiated PVA/PAA films were immersed in solutions containing [H2O2] g 8 × 10-2 M. Considering that the 430 nm band corresponds to the plasmon of Ag particles, the results suggest that the 280 nm signal originated from additional products of the Ag+ reduction that were susceptible to oxidation by the peroxide. None of the absorptions appeared after subjecting unirradiated films to a similar treatment, meaning that acetylacetone-like products were not formed after exposure of the PAA present in the solid blends to H2O2. Thus, all the information gained in this study supported the assumption that the absorption with λmax ) 280 nm of Figure 2b originated from a silver species. A transient precursor of metal particles exhibiting a signal with maxima between 265 and 280 nm has been frequently detected during the reduction of Ag+ in several solvents including alcohols.2,5,8 On the basis of kinetic information the chromophore was initially identified as the Ag42+ cluster, but subsequent studies called to question this interpretation.30 Additionally, agents able to bind Ag+ and cationic clusters induced changes in the steps leading to particle formation; in these systems the transient absorption at 280 nm was proposed to originate from the Ag3+ cluster.31 In the absence of free Ag+ ions generation of crystallites occurred fast, with an analogous transient signal that was assigned to the Ag2 cluster.2 Interestingly, the cluster previously identified as Ag42+ exhibits an additional absorption at 460 nm when bound to poly(acrylate) ions in solution.21 A different cluster must have formed in illuminated PVA/PAA films because their spectra were free of such a signal (Figure 2a). Resistance measurements yielded a negligible mobility of Ag+ ions in the films. Hence, the environment of the solid polymers seems to favor formation either of Ag2, or of Ag3+ via trapping of the former clusters by Ag+ ions bound to PAA. It should be noted that the species identified earlier as Ag42+ persisted for several days under air in solutions containing poly(acrylate) anions.21 Photogenerated clusters have remained unchanged after storing PVA/PAA films for over a year in the presence of air, highlighting the stabilizing effect exerted by the solid matrixes on these otherwise unstable species. TEM experiments were performed on samples made by dissolving non-crosslinked films in H2O after illumination. Large amounts of polymers were left on the grids after solvent

Ag Particles in Cross-Linked PVA/PAA Films

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Figure 5. Bright field images from microtomed, crosslinked PVA/ PAA films irradiated for 70 min. (a) Overview image of a sectioned film (marker length ) 100 nm); the arrows indicate areas where small crystallites exist next to large particles. (b) Magnified image of the region identified by the left arrow in (a); length of marker ) 25 nm.

Figure 4. TEM images from a non-crosslinked PVA/PAA film exposed for 70 min to 350 nm photons. (a) Bright field image of a region with several aggregates; marker length ) 25 nm. (b) Electron diffraction pattern from crystallites within an aggregate. (c) Size histogram including small crystallites (diameter e10 nm) and aggregates, which predominate for sizes >10 nm.

evaporation, which affected the image contrast. Spherical metal crystallites with diameters between 2 and 9 nm were present individually and also formed part of the larger irregular aggregates. Figure 4a is a bright field image from a region containing individual small Ag crystallites and aggregates. Parallel dark field measurements corroborated that particles with sizes above 10 nm were predominantly polycrystalline aggregates. Shown in Figure 4b is a typical electron diffraction

pattern of silver crystallites within an aggregate; the strongest signals were indexed as reflections from the {111}, {200}, {220}, and {311} lattice planes of Ag. Depicted in Figure 4c is a histogram of particle diameters; the longest axis of irregularly shaped aggregates was selected as representative of their sizes. For the small spherical crystallites the statistical average diameter was 5 nm. Dark field results indicated that the size distributions were similar for small crystallites present individually or within the aggregates. Since both aggregates and spherical particles were detected in the size range of 10-25 nm, they were included together in the histogram resulting in an average diameter of 13 nm. TEM analysis of crosslinked films microtomed after illumination yielded a size distribution that matched well the histogram presented in Figure 4c. On the other hand, significant differences were found between the two types of films. According to the bright field image depicted in Figure 5a, particles with sizes in

14856 J. Phys. Chem. B, Vol. 108, No. 39, 2004 the range of 30-70 nm formed in crosslinked films. However, the larger particles were detected much less frequently than those with smaller diameters. In addition, as indicated by the arrows in the figure, small crystallites were usually found close to particles with sizes >15 nm. An example of this behavior is presented in Figure 5b, which is a magnified image of the region identified by the left arrow in Figure 5a. Furthermore, no evidence was found in bright field images of crosslinked films that particles with sizes larger than 10 nm were polycrystalline. In fact, most of these particles were fairly spherical and several of them exhibited stress fringes usually associated with single crystals. Repeated attempts to probe the larger particles by means of dark field and electron diffraction measurements failed. Extensive heating of the specimen by the electron beam occurred during such determinations, which induced sudden shifts of the sectioned polymers and prevented capture of clear images. The divergent TEM results of non-crosslinked and crosslinked films agree well with the differences in optical data recorded during particle formation in the two matrixes. Aggregation of small spherical Ag crystallites into particle networks is known to red-shift, broaden, and decrease the strength of the plasmon resonance.32 The red-shifted (λmax ) 480 nm) and broad Ag signals of non-crosslinked films are consistent with the TEM finding that crystallite aggregates form. During photolysis the absorptions turned stronger and blue-shifted as crystallites fused inside the aggregates, yielding large single particles possessing an intense plasmon centered at shorter wavelengths.32 Illumination of crosslinked films produced stronger plasmon bands that remained centered at 430 nm (Figure 2a). These observations are in agreement with the lack of significant aggregate formation noticed in the TEM images. Dry cast PVA/PAA blends crosslinked with 5% GA displayed broad surface plasmons that blue-shifted during photolysis,15 whereas the spectra of films treated in the same way but prepared with the Gardner knife method resembled those of Figure 2a. Particle aggregates were also produced during the UV photoreduction of Ag+ in dry cast PVA films.26 Hence, the above-mentioned differences resulted from the combined effects of chemical crosslinking and film deposition methods. Aggregates can originate in solution through diffusion and agglomeration of small crystallites/clusters. This sequence seems to operate when photographic emulsions are immersed in liquid developers,33 or during melting of TDPM media at the high processing temperature of silver carboxylate systems.34 The fact that small clusters are stable in the PVA/PAA films argues against any formation mechanism of metal particles/aggregates based on diffusion of clusters or small Ag crystallites. Coalescence of mobile Ag atoms is a more likely particle nucleation pathway in the films; the resulting clusters then grow by further atom addition. The photogeneration of Ag particles in silver carboxylates have also been rationalized in terms of metal atom diffusion.27,35 Ag atoms coalesce in a few seconds inside viscous epoxy polymers because their diffusion coefficient, 7.6 × 10-10 cm2 s-1,36 is about 5 × 103 times lower than the values for atom migration at the silver-water interface.37 An even lower mobility of the metal atoms is expected for PVA/PAA films, which explains the slower growth of the Ag plasmon resonance in these matrixes. Since Ag2 clusters are expected to exhibit mobility similar to that of metal atoms, stabilization of such clusters by the PVA/PAA matrix appears unlikely. Thus, we attribute the optical signal centered at 280 nm to Ag3+ clusters since these species are anticipated to be stabilized by the carboxylate groups of PAA.

Gaddy et al. The proposed mechanism of nanoparticle generation is also useful to rationalize the different particle arrangements illustrated in Figures 4a and 5b. A simple particle nucleation model involves diffusion of metal atoms through spaces that exist between intertwined macromolecular chains. The concentration of Ag atoms builds up in localized polymer regions of lower atom mobility, facilitating particle nucleation. Reduced atom mobilities are expected in zones of extensive physical and chemical crosslinking as well as in domains of crystalline PVA or of high interpolymer hydrogen bonding, which are known to exist at the presently used PVA/PAA composition.38 Macromolecular segments will be displaced from the space occupied by the growing Ag particles, increasing the polymer density in surrounding areas where atom coalescence becomes now more probable. For the stiffer crosslinked films these zones are anticipated to span only a few nanometers away from the metal surface, reaching high densities for particles that have grown to significant sizes. Support for this interpretation is provided in Figure 5b, which shows thin darker polymer layers around particles with sizes exceeding 15 nm. Because the high-density layers are just a few nanometers thick, further small crystallites nucleate close to the larger particles, as is evident in this figure. In contrast, the lack of chemical crosslinks for blends not treated with GA allows them to dissipate pressure stresses induced by the growing metal particles over wider regions than in the case of crosslinked films. Thus, nucleation of additional small crystallites can occur for non-crosslinked blends in zones farther away from existing Ag particles. Particle aggregates form when multiple nucleation takes place in such regions followed by crystallite growth. Generation of aggregates is more probable for blends containing large amounts of occluded H2O because water acts as a plasticizer in crosslinked PVA films.39 Hence, the fact that particle aggregates predominate in all dry cast blends (independent if they are treated with GA or not) can be understood if larger amounts of H2O are retained in these thicker films. The film preparation method described herein has permitted the collection of reproducible kinetic data on the photogeneration of Ag particles. These results are presented in the companion article in this issue.40 Conclusions Preparation of water-insoluble PVA/PAA films exhibiting an efficient generation of nanometer-sized Ag particles with high fluxes of 350 nm photons was achieved using improved methods of PVA crosslinking and film deposition. Identification of polymer compositions suitable for monitoring the photoreaction spectrophotometrically was aided considerably with the use of conventional colloidal techniques. Results from chemical and physical analyses of the films confirmed that nanometer-sized Ag particles are the origin of the strong absorption centered at 430 nm. The available evidence supports the assignment of a second optical signal with a maximum at 280 nm to an Ag cluster consisting of two metal atoms. Formation of the metal cluster and of silver particles in the solid state is consistent with a pathway based on diffusion and nucleation of mobile Ag atoms. Acknowledgment. We thank S. Ruggs and X. Wang for helping us with sample preparations and analysis, M. E. Miller for his help with TEM measurements, and C. M. Lukehart (Vanderbilt University) for making TEM instrumentation available to us. Funding for this research was provided by DOC/ NTC via project No. M98-AIO.

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