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J. Phys. Chem. C 2009, 113, 19005–19010

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Silver Nanoparticles Obtained in PAH/PAA-Based Multilayers by Photochemical Reaction Giovanna Machado,*,†,‡ Marisa M. Beppu,§ Adriano F. Feil,# Carlos A. Figueroa,† Ricardo Rego Bordalo Correia,# and Se´rgio R. Teixeira# Departamento de Engenharia Quı´mica, UniVersidade de Caxias do Sul, UCS, Caxias do Sul, Rio Grande do Sul, Brazil, CETENE - Centro de Tecnologias Estrate´gicas do Nordeste, Recife - PE, Brazil, Faculdade de Engenharia Quı´mica, UniVersidade Estadual de Campinas, USP, Campinas, Sa˜o Paulo, Brazil, and Instituto de Fı´sica, UniVersidade Federal do Rio Grande do Sul, UFRGS, Porto Alegre, Rio Grande do Sul, Brazil ReceiVed: June 27, 2009; ReVised Manuscript ReceiVed: September 9, 2009

Carboxylic acid groups in PAH/PAA-based multilayers bind silver cations by ion exchange with the acid protons. The aggregation and spatial distribution of the nanoparticles proved to be dependent on the process used to reduce the silver acetate aqueous solution. The reducing method with ambient light formed larger nanoparticles with diameters ranging from 4-50 nm in comparison with the reduction method using UV light, which gave particles with diameters of 2-4 nm. The high roughness of samples reduced by ambient light is a result of two population distributions of particle sizes caused by different mechanisms when compared with the UV light process. According to these phenomena, a judicious choice of the spectral source can be used as a way to control the type and size of silver nanoparticles formed on PEMs. Depending on the energy of the light source, the Ag nanoparticles present cubic and/or hexagonal crystallographic structures, as confirmed by XRD. Beyond the kinetically controlled process of UV photoinduced cluster formation, the annealing produced by UV light allowed a second mechanism to modify the growth rates, spatial distribution, and phases. Introduction Nanotechnology has brought some new challenges, such as the development of quality synthesis processes regarding nanostructures based on inorganic and organic substances. Much attention has been given to nanoparticle (NP) synthesis due to their properties and unusual applications. The microemulsion technique,1 polyol process,2 photolytic reduction,3 and layerby-layer approach (L-b-L) are some of the methods of synthesis that can be used to prepare metal NPs. Among these methods, the polyelectrolyte multilayer (PEM) processes are highlighted, as they consolidate routes to obtain NPs with specific sizes through colloidal methodology.4-9 The interest in producing NPs with a specific size and shape is related to the properties generated from the electronic confinement effects, which can be applied in catalytic, photonic, and magnetic systems.10-13 The process of PEM involves the deposition of weak electrolytes from dilute aqueous solution based on electrostatic interactions of opposite polymer charges or on specific substrates (glass, silicon, mica). Rubner and Cohen13-15 studied PEMs produced from weak polyelectrolytes that were used to bind inorganic ions. Different metal ions present different binding activities that depend on the type of polyelectrolyte and pH. Metal nanoparticles in PEMs are obtained by reducing the ions using several processes, such as ultraviolet (UV) light, a H2 atmosphere, or reducing agents such as sodium tetrahydridoborate (NaBH4) or dimethylamine borane (DMAB). Special control during NP reduction can be obtained by using a polyelectrolyte that is not able to bind inorganic ions.15 * Corresponding author. E-mail: [email protected]. † Universidade de Caxias do Sul. ‡ Centro de Tecnologias Estrate´gicas do Nordeste. § Universidade Estadual de Campinas. # Universidade Federal do Rio Grande do Sul.

The advantage of using PAH (poly(allyl amine hydrochloride)) and PAA (poly(acrylic acid)) as weak polyelectrolytes to construct PEMs is that these polymers present linear charge densities that can be changed by adjusting the pH of the dipping solution, which allows control over the number of carboxyl groups and, therefore, control of the layer thickness and the organization of the adsorbed polymer chains.15-18 Wang et al.15,19 verified that multilayer thickness is directly proportional to the number of loadings and that an increase in charge density at low pH is also observed for a high number of reducing cycles. In this study, silver NPs were obtained by the reduction of silver acetate (AgCH3CO2) under ambient and UV light, and we will show that the silver nanoparticle crystalline structure can be defined. In fact, the process induced by the ambient light (AL) revealed an important new scheme for the growth of nanometric particles in L-b-L structures. This made it possible, especially on the 20 L-b-L structure, to define the nucleation of different particles in opposite directions across the film width, as promoted by the formation of metal NPs with significantly different average sizes along the two interfaces. Each distribution contributes different reflectivity close to the surface and also distinct plasmon resonances for extinction. This tool may be used to devise an all-optical assembling method for nanoparticle production, for example, with aimed spatial distribution across a planar photonic device. Jin et al.20 used the photoinducement method for converting large quantities of silver nanospheres into nanoparticle with triangular shape. This process resulted in a colloid with typical optical properties that were related to the nanoprism shape of the particles. There are several methods that have been used to reduce silver nanoparticles or to incorporate metal nanoparticles in the PEMs. In this context, some works describes the formation of metal nanoparticles using chemical reduction agents,21 using visible light,22,23 or describe a route using a different polymer system I (PDDA-PSS) by

10.1021/jp906018p CCC: $40.75  2009 American Chemical Society Published on Web 10/13/2009

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chemical reduction agent.22 However, they do not explore how to control the size and crystalline structure of NPs. This paper describes an innovative method that allows the control of the size and crystalline structure of Ag NPs by spectrally selecting the light used in the photoreduction processes and also the number of layers in the substrate (PAA-PAH LbL). Experimental Section Materials. A glass substrate was immersed in 2% detergent solution under ultrasonication for 15 min, followed by rinsing and ultrasonication for 15 min in water. Afterward, the glass substrate was immersed in an aqueous solution of poly(allyl amine hydro chloride) PAH (Mw ) 70 000, Aldrich-Sigma; 10-2 M) for 15 min and then withdrawn from the solution and rinsed in deionized water by dipping for 2, 2, and 1 min, consecutively. The substrate was then immersed in a 25% aqueous solution of poly(acrylic acid) PAA (Mw ) 90 000, Polysciences; 10-2 M) for 15 min and then rinsed in deionized water as in the previous step. Both PAH and PAA aqueous solutions were prepared at pH 3.5; the polyelectrolyte’s solution was adjusted to the desired pH (0.01) with 1 M HCl or 1 M NaOH. The PEMs were obtained with 5 and 20 bilayers. These PEMs were immersed into a silver aqueous solution (5 mM, pH 6.5) for 1 h and then removed and rinsed 3 times in deionized water for 1 min each time. The reduction was carried out in ambient light (AL) and under UV radiation with exposure for 20 h, leading to the formation of Ag(0) nanoparticles. The reduction carried out with UV radiation exposed the samples to 365 nm UV light generated from a lamp (Ushio Inc., UER20-172 V), while the reduction with AL was performed with a conventional 40 W fluorescent lamp. XRR and WAXD. X-ray reflectivity (XRR) measurements were performed in a Philips XPERT MRD X-ray diffractometer. The X-ray source was a copper sealed tube with KR radiation (λ ) 1.5406 Å) operating at 40 kV and 45 mA. The incident and reflected beams were collimated with slits of 0.09° and 0.04°. The reflectivity measurements consisted of coupled 2θ-ω scans acquired with a scan of 0.2-5.0°, a step size of 0.005°, and 10 s per step. Interpretation of XRR results was performed by fitting the reflectivity curves using X’Pert Reflectivity software. Wide-angle X-ray diffraction (WAXD) measurements were performed in a Philips X_PERT MRD X-ray diffractometer equipped with a curved graphite crystal. The diffraction data were collected at room temperature in Bragg-Brentano θ-2θ geometry with Cu KR radiation (λ ) 1.5406 Å). The equipment was operated at 40 kV and 40 mA with a scan range between 10° and 90°. The simulations of Bragg reflections and Rietveld’s refinement were performed with a pseudo-Voigt function using the FULLPROF code.24 AFM. The morphology and roughness of the multilayers were characterized by atomic force microscopy (AFM). The AFM measurements were performed in air using the tapping mode (Dimension 3000 Veeco Instruments). TEM. For transmission electron microscopy (TEM), the multilayer films were deposited on polystyrene (PS) coverslips. This material was then immersed in silver acetate and reduced under AL. Thin strips of PS coverslip samples were cut and embedded in an epoxy resin that was cured at 50 °C for 48 h. The embedded specimens were first trimmed with a glass knife, and then ultrathin cross sections were obtained by using a cryogenic Diatome diamond knife at 45° at room temperature. The ultrathin sections of approximately 80 nm thickness were floated on a deionized water surface and immediately mounted

Figure 1. Reflectivity curves for films of 5 and 20 L-b-L on glass substrates loaded in silver acetate (AgCH3CO2).

onto 200 mesh copper grids and dried in a desiccator. Finally, they were examined using a JEOL 200CX TEM operating at an accelerating voltage of 200 kV. UV-vis. The UV-visible absorption spectra of the abovedescribed samples deposited on the 1 mm thick optical glass substrates were captured on a Cary 5000 spectrophotometer in the range 300-800 nm. The spectra of PEMs immersed into silver acetate aqueous solution were compared to unloaded PEMs. Results and Discussion The multilayer and silver nanoparticle assembled structure was characterized by XRR, XRD, AFM, TEM, and UV-vis analyses. In XRR, the intensity is measured as a function of the incidence angle, and the data are analyzed from the positions of Kiessig fringes at low angles, allowing calculation of the total film thickness. These fringes arise from the interference of X-ray beams reflected at the substrate-film and film-air interfaces.25-29 The results of XRR for PAH/PAA at pH ) 3.5 for 5 L-b-L and 20 L-b-L on glass substrates are shown in Figure 1. Kiessig fringes were well-defined for both 5 and 20 L-b-L. The calculated total thickness was 32 and 60 nm for 5 and 20 L-b-L, respectively, but when the PEMs were loaded in silver acetate aqueous solution, the total thickness increased from 32 to 38 nm and from 60 to 132 nm, as observed Figure 1. An increase in thickness was observed, probably because of the higher quantity of silver ions adsorbed. The metal loading capacity of these films is attributed to CH3COO- groups present in the chemical structure of the matrix.30 For 20 L-b-L, when these films were loaded in silver acetate aqueous solution, the Kiessig fringes indicated a growth of electron density with the adsorption of silver ions. The distance between these fringes is

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Figure 2. (a) XRD diffractograms for 5 and 20 L-b-L films loaded in silver acetate and reduced by ambient light. (b) Rietveld’s refinement for 20 L-b-L films loaded with silver acetate and reduced by ambient light. (c) XRD diffractogram for 20 L-b-L films loaded in silver acetate and reduced by ambient light and UV radiation.

TABLE 1: Lattice Parameters from Rietveld Refinement phase

space group

a (Å)

b (Å)

c (Å)

β (ο)

fcc hcp

Fm 3m P 63/m m c

4.06(3) 2.906(4)

4.055(3) 2.906(4)

4.055(3) 10.483(9)

90 120

related to the film thicknesses (38 and 132 nm, for 5 and 20 L-b-L, respectively). The pH of polyelectrolyte solutions used in this study guaranteed that free carboxylic acid groups in the PAH/PAA bilayers were available for binding silver ions.15,19 In addition, it is well-known that the diffusion coefficient decreases significantly when the thickness decreases, probably because of the alignment of polymer chains or a decrease of their mobility, resulting in low electron density for small L-b-L numbers.31-33 In general, weak polyelectrolytes such as PAH/ PAA show an increase in thickness with an increase in the number of bilayers deposited,15-18 and consequently, a larger quantity of silver ions are adsorbed. Figure 2 shows XRD patterns of samples irradiated by ambient and UV light. It can be seen that different products were obtained (see Figure 2a). In the case of irradiation with AL, silver crystallizes in face-centered-cubic (fcc) and hexagonalclose-packed (hcp) arrangements (see Figure 2b). However, irradiation with UV yielded only the NPs with fcc structure.20,34,35 Conversely, the material irradiated with ambient light yields a mixture of 50% fcc and 50% hcp (Figure 2b). Figure 2a displays X-ray diffractograms for PEM with 5 and 20 L-b-L loaded in silver acetate and reduced by AL. From Rietveld’s refinement (Figure 2b), it is possible to confirm the existence of two different structural phases for Ag(0). The first phase has a cubic-face-centered (fcc) configuration, and the most representative Bragg reflections were found at 38.7° and 44.9°, corresponding to the (111) and (200) indexed planes, respectively. The second phase is hexagonal-closepacked (hcp) with Bragg reflections at 35.82°, 37.01°, and 40.04°, corresponding to indexed reflections of the (100), (101), and (102) planes. The lattice parameters obtained for both phases were quite similar to the values of JCPDS file numbers 03-0931 and 87-0598. These parameters are summarized in Table 1. For comparison, Figure 2c shows both diffractograms reduced by ambient and UV light. Note the clear difference between the two samples. The curves with pattern continue and the dot line corresponds to 20 L-b-L samples deposited on Si (100) and glass substrates, respectively. The broad peak observed at around 35° for the 20 L-b-L sample, dot line, represents the amorphous halo of glass substrate which did not appear when a Si (100) substrate was used.

The XRD pattern of the hcp phase showed a sharp crystalline peak at 37.01°. The tight peak suggests the presence of larger silver particles. However, the 20 L-b-L sample reduced by UV radiation produced only the fcc phase (Figure 2c). This behavior suggests that the levels of intensity of energy provided by the different wavelengths found in both UV light and AL may induce different kinetics in the reduction mechanisms in each case. In the case of samples reduced by AL, two phases were identified through XRD, supporting the presence of two different products. The silver can crystallize in either fcc or hcp structures under standard conditions of pressure and temperature. Moreover, the fcc phase is the most stable structure (thermodynamic product).20 In the reduction mechanism (Ag+ f Ag(0)), the silver cations are reduced by the promoting electrons to the conduction band due to the energy involved during irradiation.36 In fact, these electrons come from the oxidation of organic molecules.37 In addition, the oxidation of organic molecules depends on quantity and energy of incident photons. Therefore, the driving force of photo reduction of Ag+ ions is the intensity and wavelength of the incident radiation that can improve the rate of electron transfer.38 According to this mechanism, higher incident energy not only induces the reaction but also thermally agitates the matrix.39,40 This generates conditions to achieve the best transition geometry in the activated process for product formation. Thus, the thermodynamic product should be obtained. Theoretical simulations using classical nucleation theory revealed that a random-hcp structure is the pure kinetic product.37 Moreover, Celik et al.38 have shown a pressure effect on the structural properties of amorphous silver during isothermal annealing. These molecular dynamics simulations demonstrated the evolution from the hcp to fcc structure as a function of pressure (matrix effect). Our work shows that silver reduction by UV radiation produces an fcc structure that is obtained as the thermodynamic product, while silver reduction by AL gives both structures (fcc and hcp). By taking into account the experimental results and these simulations, different silver NP crystalline structures could be obtained depending on the energy involved in the irradiation. The process with UV radiation could, in principle, provide enough energy to activate a mechanism that could rapidly and efficiently produce only the fcc phase. Figure 3a,b depicts the AFM images of the 5 and 20 L-b-L films, respectively. Figure 3c,d presents AFM images of the 5 and 20 L-b-L films loaded with silver acetate and reduced by ambient light, while Figure 3e,f shows images of the 5 and 20

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Figure 3. AFM images (a) and (b) correspond to the morphology of 5 and 20 L-b-L, respectively. Images (c) and (d) are films that were loaded in silver acetate and reduced by ambient light for 5 and 20 L-b-L, respectively, and images (e) and (f) show films reduced by UV radiation for 5 and 20 L-b-L, respectively. (g) rms roughness for PAH/PAA multilayer as a function of the number of deposited layers when load in silver acetate aqueous solution and reduced by ambient and UV light. (h) rms roughness for PAH/PAA multilayer as a function of the number of deposited layers. The value 3.5 in this legend corresponds to the pH for both polyelectrolytes (PAH/PAA), and the values 1, 1.5, 2, 5, 20, and 20.5 correspond to the number of deposited layers.

L-b-L films reduced by UV radiation. The multilayer showed an increase of roughness with the deposition of subsequent layers of PAH/PAA (see Figure 3g). Additionally, multilayers with the last layer made with PAH were smoother than films that ended with PAA (Figure 3h). The images obtained by AFM confirm the results observed from XRR and XRD, which indicated that when PAA is deposited after the formation of the tenth PAH layer, voids start to be filled in, and, consequently, a higher degree of interpenetration can be observed. These results agree with the findings by Kolasinska and Bosto.41,42 They observed holes in AFM images for a few layers, but with an increase in the number of layers, the surface of the film became more uniform. However, multilayers with PAH on the top were smoother than films that ended with PAA, which can be a consequence of the presence of more loops and tails in the PAA layer. TEM images of 5 L-b-L and 20 L-b-L loaded in silver acetate and reduced by ambient light are presented in Figure 4a,b,

respectively. These micrographs show two populations of silver nanoparticles with size distributions between 4-10 nm and 12-50 nm, respectively (see the histograms of the NPs size distribution). In the present study, the size difference between 5 and 20 L-b-L can be attributed to the low thickness of PEM (5 L-b-L), which retains less adsorbed silver ions, and, consequently, NPs with sizes greater than 10 nm experience difficult adsorption and are not observed. Larger PEM thicknesses (20 L-b-L) contribute to a wider size distribution, as observed in the bimodal histogram (Figure 4b). Similar morphology and distribution of NPs, as observed by TEM (Figure 4b), were also reported by Russel et al.,43 but in their case, the NPs were reduced by annealing at 185 °C (Ag+ f Ag(0)), and the block copolymer PS-b-PMMA added into the silver solution film was characterized by Rutherford backscattering spectroscopy (RBS). They observed that, for the as-cast film, the silver NPs were uniformly distributed, but after 2 h of annealing, the Ag NPs aggregated

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Figure 4. TEM images for 5 and 20 L-b-L loaded with silver acetate and reduced by ambient light (a,b), respectively; (c) TEM image for 20 L-b-L loaded with silver acetate and reduced by UV radiation; highlighted is the histogram with the mean size distribution.

on the surface. Figure 4c represents the same previous step for the 20 L-b-L sample, but now reduced with UV radiation. For these samples, a drastic reduction in particle size was observed, and the histogram shows a mean particle size of around 3.2 nm. According to Kamat,44 it was possible to disaggregate larger silver nanoclusters of 50-60 nm into smaller ones of 5-20 nm in diameter with 365 nm laser-pulse excitation. Selective fragmentation of silver clusters into smaller ones can be achieved with the choice of excitation wavelength. The idea that some wavelengths can interfere in nanoparticle aggregate size is very reasonable, and this can probably be a way of controlling the nanoparticle sizes deposited by the methods presented in this work. Linnert et al.45 observed the dissolution of silver nanoclusters irradiated with UV light. Thus, the dissolution of silver as evidenced by small nanoparticles is a possibility in the present experiment. A major photochemical event seems to be the initiator of the breaking of larger nanoparticles, as demonstrated here when one compares the reduction method with ambient light, which formed larger nanoparticles (diameter distribution ranging from 4-50 nm), with the reduction method using UV radiation, which produced nanoparticles with a diameter distribution of around 2-4 nm. Wang19 showed that is possible to control the size and concentration of silver nanoparticles by varying the dipping solution pH and reaction cycles. In the present study, only one wavelength is used in the case of UV radiation. On the other hand, using conventional fluorescent light, many wavelengths compose the visible light spectrum. Hence, a different quantity of energy that is spectrally divided is provided to the PEM and silver system when it is irradiated

with ambient light. This means that different products can be obtained if the system presents processes with activation energies at the level of the intensity of the light used. Furthermore, the size, concentration, and type of crystalline structure of silver NPs can be easily controlled by choosing the proper excitation spectrum for photoreduction. This may be because AL includes a wide range of wavelengths, including visible light, and even a faint portion of near UV emission. In other words, many mechanisms may be activated when using AL, retarding the reduction of silver nanoparticles. In Figure 5, the observed spectra were compared with the transmission of the multilayer optical samples while taking into account the extinction spectra of the 5 and 20 L-b-L average diameters of the spherical shapes of Ag nanoparticles, respectively. Thin film optical extinction spectra of these structured films show the corresponding silver plasmon resonances as compared to “unloaded” thin films. The changes in this extinction resonance’s central wavelength, width, and amplitude are dependent upon spatial distribution of the metallic nanoparticles and their size, as observed in Figure 4, and on the effective dielectric function of the composite in which they are embedded. The films, when reduced by UV radiation, present transmission spectra maxima for the respective 38 and 132 nm thick samples that agree with the 1:4 ratio between layers and display the same fwhm of 122 nm for the 3.25 nm average nanoparticles. The resonant plasmon peak position for the 5 L-b-L structure at 432 nm suggests an effective value for the dielectric function of the composite ε ) n2 ) (1.58).2 This is due to the large concentration of nanoparticles (∼7% vol),13 which affects the dielectric

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Figure 5. UV-vis image for 5 and 20 L-b-L loaded with silver acetate and reduced by ambient and UV light.

composition of the PAA and PAH. An observed shift to 444 nm of this maximum for 20 L-b-L represents, in these circumstances, the interference of multiple reflections within this single thicker film, as devised by Wang for photonic applications in polymeric multilayer devices. For the L-b-L structures containing nanoparticles reduced by ambient light, additional behavior in the extinction was observed because of the bimodal size dispersion of silver nanoparticles and their migration within the layer. In that case, a dielectric inhomogeneous medium with depth dependence must be taken into account. Therefore, shifted and broader maxima (150 nm fwhm) are expected either for 5 or 20 L-b-L because of these contributions. Conclusions Silver nanoparticles were formed after the reduction of PEMs deposited on glass substrates by ambient light and UV radiation. The phases formed and the aggregation of nanoparticles proved to be very dependent on the process used to reduce the silver acetate aqueous solution. The high roughness of samples reduced by ambient light is a consequence of two population distributions of particle sizes caused by different mechanisms of reduction when compared with the UV light process. Most likely, the proper choice of excitation light can be used as a way to control the type of silver nanoparticles formed on PEMs, allowing for the manipulation of growth and spatial distribution of the nanoparticles. This is corroborated by the good agreement among the results obtained by XRR, XRD, AFM, UV, and TEM. Acknowledgment. This paper was supported in part by CNPq, CAPES, and FAPERGS Brazilian financial agencies. Parts of this work were carried out in the Department of Chemical Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology (MIT), Cambridge, and Robert E. Cohen and Michael F. Rubner are gratefully acknowledged. References and Notes (1) Osseo-Asare, K.; Arriagada, F. J. Ceram. Trans. 1990, 12, 3.

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