Growth Mechanisms and Kinetics of Photoinduced Silver

J. Phys. Chem. C 2010, 114, 8679–8687

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Growth Mechanisms and Kinetics of Photoinduced Silver Nanoparticles in Mesostructured Hybrid Silica Films under UV and Visible Illumination Yann Battie,† Nathalie Destouches,*,† Laurence Bois,‡ Fernand Chassagneux,‡ Alexandre Tishchenko,† Ste´phane Parola,‡ and Aziz Boukenter† UniVersite´ de Lyon, F-42023 Saint-Etienne, France, CNRS, UMR 5516, Laboratoire Hubert Curien, 18 rue Pr. Lauras F-42000 Saint-Etienne, France, UniVersite´ de Saint-Etienne, Jean-Monnet, F-42023 Saint-Etienne, France, and Laboratoire Multimate´riaux et Interfaces, UniVersite´ de Lyon, UniVersite´ Claude Bernard Lyon 1, Bat Berthollet, 69622 Villeurbanne, France ReceiVed: January 23, 2009; ReVised Manuscript ReceiVed: March 25, 2010

The growth of silver nanoparticles in mesostructured hybrid silica films under laser illumination is investigated by optical absorption spectroscopy and transmission electron microscopy. At low laser doses in the UV range, a blue-shift of the plasmon resonance is observed for increasing particle size. This size dependence is interpreted on the basis of the Mie theory assuming a two-layer core-shell model resulting from the localization of the nanoparticles in the block copolymer part of the film. At higher laser doses, the observed red-shift of the resonance wavelength and decrease of the absorbance level at resonance are attributed to the formation of a high refractive index shell around the nanoparticles that thickens slowly with the dose. The growth kinetics of silver nanoparticles is also studied for different illumination wavelengths in the UV and visible ranges. Surprisingly, the nanoparticles also grow under visible illumination. This is partly attributed to the release of electrons by the degrading copolymer under illumination. The decrease of the photoreduction process rate with the incident wavelength increase has been evaluated quantitatively. We propose an autocatalytic model that fits well with the experimental data and suggest that the absorption of some incident wavelengths by the generated small silver clusters boosts their own growth. 1. Introduction Noble metal nanoparticles exhibit strong surface plasmon resonances that are increasingly utilized in many fields of applied sciences, such as sensors, optoelectronics, optical filtering, information storage, and biomedical analysis.1-4 Their fabrication and characterization in diverse host materials have led to extensive studies both in bulk and thin film forms. The fabrication methods currently used for metal-doped glasses are compound powder melting, sol-gel, chemical vapor deposition, sputtering, ion exchange, and ion implantation. Among the latter, the sol-gel process has the advantages of being made at room pressure and temperature, to be cost-effective, and to produce either bulk solids or deposited films. The metal-nanoparticlereduction processes can be thermal,5 chemical,6 optical,7 by X-ray8 or gamma-ray9 irradiation, or even via electron beam irradiation.10 Contrary to the above-mentioned processes, except electron beam and X-ray lithography, optical illumination can be space-selective, which implies that the nanoparticle growth can be photoactivated locally to form spatially resolved microoptical elements such as optical waveguides or diffraction gratings. The control of the morphological parameters and of the size distribution of metal nanoparticles is crucial to control the optical properties of the films. Among the host matrixes elaborated by sol-gel processes, mesoporous silica films have been used in the past few years as a mold for controlling the shape and the size of nanoparticles grown in situ by chemical reductive * To whom correspondence should be addressed. E-mail: nathalie. [email protected]. † Universite´ de Lyon, CNRS, and Universite´ de Saint-Etienne. ‡ Universite´ de Lyon, Universite´ Claude Bernard Lyon 1.

treatments.11 However, no photoreduction of silver nitrate in mesoporous silica films was reported to date, probably because of the low efficiency of such a photoreduction process. In addition, narrow size distributions of silver nanoparticles have been obtained recently by photoreduction in block copolymer films using the photoactivated reductive action of the copolymer.12,13 In the present paper, we combine both host matrix technologies and demonstrate that such a mold effect can also be obtained with mesostructured hybrid silica films containing, in addition to the silica walls, both triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide ((PEO)106(PPO)70(PEO)106, F127) and a silver salt. Such silica films offer both advantages of high photosensitivity via the presence of the copolymer12,13 and of a hard silica matrix to confine and isolate the nanoparticles. The joint use of a laser reduction technology and of a silver-containing mesostructured dielectric sol-gel film can be a simple and low cost way to create 3D submicrometer optical elements with well controlled optical properties. The development of 3D optical components is not dealt with in the frame of the present paper, which focuses on the fundamental study of the nanoparticlegrowth mechanisms that occur when mesostructured silica films doped with silver salts are illuminated with continuous laser light. The size increase is monitored from the very beginning stage of the nanoparticle formation to the complete reduction of silver precursor in the film. During the growth process, a blue-shift of the plasmon resonance is observed that we interpret assuming a core-shell model where the silver nanoparticles are surrounded with an adsorbed O2 single layer and an outer shell of block copolymer in the silica matrix. At higher laser doses, the observed red-shift of the resonance wavelength and the decrease of the absorbance level at resonance are attributed to

10.1021/jp9046903  2010 American Chemical Society Published on Web 04/22/2010

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Figure 1. TEM images on the top view (a) and on the cross section (b) of the AgNO3-containing mesostructured film before any reduction treatment.

the formation of a high refractive index shell of silver oxide or silver sulfide that slowly thickens with the laser dose. The growth kinetics of the silver nanoparticles up to the complete reduction of silver salt is also studied at different wavelengths in the UV and visible ranges. The decrease of the photoreduction process rate upon the incident wavelength increase is quantitatively evaluated, and we propose an autocatalytic model whereby the absorption of some incident wavelengths by the generated small silver clusters boosts the growth of the nanoparticles. 2. Experimental Methods The mesostructured hybrid silica films are synthesized at room temperature by a sol-gel process. The polymeric silica sol is prepared by mixing 4 g of TEOS with 1.76 g of hydrochloric acid (0.055 M) for 30 min. A 1.14 g portion of triblock copolymer F127 (PEO106-PPO70-PEO106) dispersed in 18 g of ethanol is then added to the sol, which is then stirred for 15 min. The final solution is obtained after adding 1.7 g of silver nitrate and stirring for 15 min. The films are deposited by dip coating on cleaned glass substrates using a speed of 3.5 cm/ min and are dried at room temperature for 12 h. The synthesis is performed under yellow light in order to prevent the spontaneous formation of silver nanoparticles in the sol. The deposited films are transparent, colorless, and crack-free, and their average thickness, measured with a Dectack3ST VEECO profilometer, is about 170 nm (the film thickness is not a critical parameter in what follows). At nanometer scale, the films exhibit a mesostructure resulting from the self-assembly of block copolymer F127 micelles, which appear brighter than the silica matrix on the TEM pictures. Figure 1 shows a top view (a) and a cross section (b) of the AgNO3-containing mesostructured film before any reduction treatment. The precipitation of silver nanoparticles in the films is produced by laser illumination. The lasers used are a doubled argon laser emitting at 244 nm wavelength, a helium cadmium laser at 325 or 442 nm, and an argon laser at 488 or 514 nm. The laser beam is expanded to 5 mm diameter using a convergent lens, and the samples are illuminated through a circular hole of 2 mm diameter centered on the expanded beam so as to get quasi-homogeneous intensity. The laser beam intensity on the samples was fixed at 64 mW/cm2 at 325 nm wavelength, 230 mW/cm2 at 442 nm, 6.4 W/cm2 at 488 nm, and 9.5 W/cm2 at 514 nm; at these wavelengths, the laser dose (J/cm2) received by the samples was increased by the sole increase of the exposure time. At 244 nm wavelength, the laser intensity was varied from 2 to 218 mW/cm2 by varying the exit power of the laser; two series of measurements were performed: one with increasing laser dose at the fixed laser intensity of 50

Battie et al. mW/cm2 (increasing exposure time) and one with different intensities but at fixed dose (exposure time decreases as intensity increases). The absorbance spectra of the illuminated areas were recorded in the UV and visible ranges with a lambda 900 PerkinElmer spectrometer, the absorbance being defined as the logarithm of the inverse transmission coefficient. The illumination at 244 nm wavelength quickly gives the films a yellowish color due to the occurrence of an absorption band in the blue, characteristic of the surface plasmon resonance of silver nanoparticles.14 This resonance phenomenon strongly depends on several parameters that are considered in this paper such as the nanoparticle size and the dielectric function of the nanoparticles in their close environment. The absorption band is characterized by the absorbance level at resonance (the maximum value of the absorbance spectrum), the resonance wavelength (the wavelength at which the absorbance is maximum), and the full width at half-maximum (fwhm). The transmission electron microscope (TEM) pictures were recorded on a TOPCON EM002B instrument operating at 200 kV. To permit TEM observations, film fragments of noncontrolled thickness were stripped off from the substrate by slightly scratching the samples with a razor blade and were deposited onto a carbon-coated copper grid. Some TEM measurements were also performed on the film cross section. For such measurements, the film was deposited onto a polymer substrate and then cut with an ultramicrotome to get 30 nm thick cross sections. The TEM picture of Figure 1b was recorded on a Philips CM 120 instrument operating at 120 kV after slicing the sample by ultramicrotomy. To determine whether nonreduced silver remains in the film after exposure, the samples were immersed in a freshly prepared sodium borohydride (NaBH4) solution (50 mM) for 1 min, rinsed with deionized water, and dried at 100 °C for 15 min in an air atmosphere. So strong a reductive solution is able to reduce silver precursors as well as silver oxide or sulfide. As a consequence, an increase of the absorbance level after the NaBH4 soaking is the evidence that there still was some nonreduced silver in the film. Therefore, the criterion for a complete reduction of the silver precursor during the photoreduction process is the constancy of the absorbance level through the NaBH4 soaking process. 3. Results 3.1. Spectral Variations and Nanoparticle-Size Evolution under UV Light at 244 nm Wavelength. At fixed laser intensity, the absorbance level at resonance rapidly grows with the illumination time. In what follows, the intensity of the 244 nm wavelength laser beam is fixed at 50 mW/cm2. Figure 2a shows a selection of recorded spectra after different exposure times corresponding to different laser doses on the sample. Figure 2b represents the values of the absorbance level at resonance and of the resonance wavelength for increasing dose. These graphs illustrate that the absorbance level at resonance rapidly increases with the dose at low doses (below 30 J/cm2) and then tends to stabilize for doses from about 30 to 100 J/cm2 before slowly decreasing above 100 J/cm2. The increase of the absorbance level at resonance goes with a nonmonotonic variation of the resonance wavelength which first shifts toward the blue by about 20 nm at the very beginning of the exposure and then slightly shifts toward the red. With the decrease of the absorbance level at resonance (above about 100 J/cm2), the yellowish color of the samples tends to vanish. The absorbance level at resonance slowly decreases, while the resonance wavelength shifts toward the red to reach about 440 nm. The

Growth of Photoinduced Silver Nanoparticles

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Figure 2. (a) Optical absorbance spectra recorded after exposing the films at 244 nm wavelength with an intensity of 50 mW · cm-2 for increasing exposure times ti. The inserted caption gives the dose (intensity by time) received by each film. (b) Plot of the absorbance level at resonance (black squares) and of the resonance wavelength (gray triangles) versus the laser dose. The open squares and triangles correspond, respectively, to the absorbance level at resonance and the resonance wavelength measured when illuminating the films under the same exposure conditions but in a vacuum chamber under a residual pressure of 3 × 10-5 mbar.

Figure 3. TEM pictures of the films after exposure at 244 nm wavelength with constant laser intensity (50 mW/cm2) and increasing laser doses: (a) 4 J · cm-2; (b) 17 J · cm-2; (c) 42 J · cm-2; (d) 100 J · cm-2. The scale bar is 10 nm in each case. Histograms e-h give the size distribution calculated from four different TEM pictures of the same sample (only one is shown above) for each laser dose. TEM picture i shows the film cross section after illumination at 244 nm at a dose of 90 J · cm-2.

last measurements were carried out in air. Now, making the same experiment in a vacuum chamber results in a strikingly different dose dependence of the absorbance level and of the resonance wavelength: as shown by the open squares and triangles in Figure 2b, they both remain practically constant even at high laser dose. As a consequence, the formed silver nanoparticles are stable when the film is illuminated at high doses under vacuum and the film does not bleach. This experiment evidences that a chemical reaction occurs between silver nanoparticles and ambient air diffused in the film under high doses that modifies the surface plasmon resonance of the nanoparticles. The simulations in section 4.2 will show that this chemical reaction leads to the formation of a high refractive index shell around the metallic-silver nanoparticles, which may be made of silver oxide. TEM pictures recorded for different doses between 4 and 100 J · cm-2 on samples processed in air are shown in Figure 3a-g. They allow one to follow the size evolution of silver nanopar-

ticles when the absorbance level at resonance rises up and tends to stabilize. These TEM pictures do not concern the dose range at which the films bleach. The TEM observation of the nanoparticles requires a large magnification (above 400 000) at which the film mesostructure in Figure 3a-d can hardly be identified. The silver nanoparticles observed after different UV doses have a spherical shape. At a dose of 4 J · cm-2 (Figure 3a,e), corresponding to the very beginning of the growth process at the left of the graph in Figure 2b where the wavelength at resonance is blue-shifted, most of the silver nanoparticles are smaller than 3 nm. At a dose of 17 J · cm-2, the density of particles increases (Figure 3b) and so does the proportion of particles of diameter in the 2-3.5 nm range (Figure 3f). At a dose of 42 J · cm-2 (Figure 3c,g), which corresponds to the beginning of the top plateau of the absorbance level at resonance in Figure 2b, the particle diameter rises predominantly in the 2-5 nm range. Finally, at a dose of 100 J · cm-2, which corresponds to the end of the top plateau, there are fewer

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Figure 4. Absorbance spectra of a film having received 320 J · cm-2 (obtained by illuminating the film with an intensity of 178 mW/cm2 for 30 min) or 42 J · cm-2 (intensity of 175 mW/cm2 for 4 min) at 244 nm wavelength, before and after soaking in NaBH4.

nanoparticles but these are larger (Figure 3d); the particle diameter has a broader distribution with a greater proportion in the 6-8 nm range (Figure 3h). The nanoparticles have a crystalline structure that matches well with the {111} lattice planes of cubic face centered metallic silver (high resolution TEM images and diffraction pattern not shown). Figure 3a-d shows top views of film fragments of noncontrolled thickness. They seem to show a high density of nanoparticles, but these nanoparticles are actually located at different depths within the film, as evidenced by TEM observations of the film cross section on 30 nm thick fragments cut by ultramicrotomy (Figure 3i). Therefore, the nanoparticles are dispersed rather homogeneously in the film thickness and are relatively far from each other. The fact that silver is well dispersed within the film thickness was also confirmed by Rutherford backscattering measurements. The measurements performed on non-irradiated films showed the presence of a slightly decreasing gradient of silver concentration in the film depth.15 Unlike the chemical reduction reported in ref 15, the UV-assisted reduction of the silver precursor performed here did not significantly change the silver distribution in the film depth, confirming that nanoparticles grow within the film material and not at its interfaces. One can conclude from Figure 3a-g that the dose increase leads to an increase of the number of reduced silver ions and to the growth of silver nanoparticles. As demonstrated recently on platinum nanocrystals, this growth is likely to result from a combination of two processes: the attachment of monomer species and the coalescence of particles.16 The nanoparticle growth is accompanied by an increase of the absorbance level at resonance up to the full consumption of the silver precursor. Once the latter is consumed, the absorbance level at resonance stabilizes but the TEM pictures show that the nanoparticles keep on growing to the detriment of their density (Figure 3c,d). This may characterize an aggregation of the formed nanoparticles, or an Ostwald ripening leading to a growth of larger nanoparticles at the expense of smaller ones. At higher doses (above 100 J · cm-2), the films bleach, which leads to a decrease of the absorbance level at resonance, as seen in Figure 2. This photobleaching that does not occur under vacuum (as explained previously) results from the reappearance of nonreduced silver in the film, which can be evidenced by soaking the film in a strong reducer like NaBH4. Figure 4 illustrates the difference between the absorbance spectra of a bleached film and those of a film exposed to a dose of 42 J/cm2 (at which the absorbance level at resonance stabilizes at a maximum value in Figure 2) before and after soaking in NaBH4. The reductive treatment of the two illuminated films has little influence on the film exposed to 42 J/cm2 but strongly increases

Battie et al. the absorbance level at resonance of the bleached film. This experiment confirms that, in a film whose absorbance spectrum exhibits a stabilization of the resonance level at a maximum value, the whole silver precursor is reduced. It also shows that nonreduced silver reappears in the film when it bleaches at higher doses. 3.2. Kinetics in the UV and Visible Ranges. The growth kinetics of the nanoparticles has been compared under different illumination wavelengths by measuring the rate at which the absorbance level at resonance increases. We want to point out that, for each wavelength, only the dose range that does not lead to photobleaching is considered in this section: only the data that correspond to an increasing or a stabilizing absorbance level at resonance with the dose are considered. In this dose range and under 244 nm wavelength illumination, the silver nanoparticles grow, as shown by the TEM observations of section 3.2. As the growth rate of the absorbance level at resonance with the dose decreases with increasing illumination wavelength, the considered dose range has to be increased with wavelength to give rise to similar values of the absorbance level at resonance. As the different lasers used to change the wavelength have various output powers (some are fixed) and consequently deliver various intensities on the sample, we first ensured, with one laser having an adjustable output power, that the laser intensity has no influence on the changes of the absorbance level at resonance with the dose. This was made by varying the output power of the 244 nm wavelength laser beam. The intensity was varied from 2 to 218 mW/cm2, and the exposure time was adjusted to expose the film at the same dose. The measurements (not shown) show that the reached absorbance level at resonance remains constant whatever the laser intensity in the considered range when the dose and the wavelength are fixed. This result is quite different from what was observed with high power pulsed lasers (a few hundreds of MW/cm2) where the precipitation of silver nanoparticles is very dependent on the peak power per pulse.17 At such high peak powers per shot, the sample locally warms up, which in turn promotes the reduction of silver ions. In the present case, the thermal effects are negligible and the reduction of silver ions only depends on the number of photons per unit area received by the film, at a given wavelength. According to this result at 244 nm, the measurements performed at other wavelengths with different intensities can be compared. Silver particles are usually considered to grow under UV light and not under visible light. The following study proves that visible light can be used to form silver nanoparticles as well and allows one to quantitatively estimate the decrease of the photoreduction process rate upon an increase of the incident wavelength from the ultraviolet to the visible range. At each wavelength, the nanoparticle size increases with the dose and leads to a raise of the absorbance level at resonance. The photoreduction process rates are compared by following the increase of the absorbance level at resonance with the dose at different wavelengths. Their plots at wavelengths of 244, 325, 442, 488, and 514 nm using the intensity values given in section 2 are shown in Figure 5 (dots). At the first three wavelengths, the absorbance level at resonance rises up to a stabilization. According to the results detailed at 244 nm wavelength, this stabilization means that the whole silver precursor has been consumed. The laser dose needed to reach a given level of absorbance at resonance increases with the wavelength. At the 488 and 514 nm wavelengths, the doses used are about 3 orders of magnitude larger than those used at λ ) 244 nm and the absorbance does not attain a plateau under the exposure



Figure 5. Absorbance level at resonance versus laser dose for (a) 244 and 325 nm, (b) 442 nm, and (c) 488 and 514 nm wavelengths. The solid lines are the fits of the experimental data (dots) calculated with values of Table 1 in the frame of the kinetic model proposed.

TABLE 1: Values of the Parameters of the Kinetic Model Used to Fit the Experimental Data and Measured Absorption Coefficients Normalized by the Value at 244 nm Wavelength of Silver Nitrate (βAgNO3) and Copolymer F127 (βF127) wavelength (nm) 244 325 442 488 514

Af

g (cm2/J) -1

R′

βAgNO3

0.20 2.5 × 10 0.0 1 0.20 2 × 10-1 37.6 7.5 × 10-2 0.20 3 × 10-3 1.3 1.4 × 10-3 0.20 5 × 10-5 0.0 8.5 × 10-4 0.18 7 × 10-6 7.2 × 10-4

βF127 1 3.5 × 10-1 1.8 × 10-1 1.5 × 10-1 1.3 × 10-1

conditions used. It can be noted that the shape of the curve at λ ) 325 nm differs distinctly from the others. A quantitative study of these curves will be undertaken in the next section on the basis of a modeling of the kinetic equations governing the formation of silver nanoparticles. 4. Discussion 4.1. Interpretation of the Results Obtained under 244 nm Wavelength Illumination. The evolution of the absorbance spectra of the growing metallic nanoparticles can in principle be modeled by means of the Mie theory, more so as the form of the nanoparticles is close to spherical as Figure 3 shows. However, this evolution as a function of the received laser dose cannot be simply explained as the growth of silver nanoparticles in a homogeneous silica matrix, as will be seen later. In order to interpret the observed spectral variations, we will distinguish two phases: first, the dose range for which the absorbance level at resonance increases up to stabilization and, second, the dose range for which the absorbance level at resonance decreases with the dose increase (photobleaching). 4.1.1. Nanoparticle Growth in Mesostructured Hybrid Silica Films. During the first phase, as the dose increases, the resonance wavelength first shifts to the blue and then slightly comes back to the red, while the full width at half-maximum always narrows down. The experimental results of Figure 2 are represented in Figure 6 (see the crosses) by plotting the fwhm versus the resonance wavelength for laser doses below 96 J/cm2 for the purpose of comparing them with the theoretical calculations. According to the TEM measurements of Figure 3, we know that the size rises up to about 8 nm with the dose in this range. The blue-shift reported here for very low doses (below 30 J/cm2) is not commonly observed for embedded silver nanoparticles, where no shift or red-shift is usually reported for particle size increasing from 0 to 10 nm.18-22 The absence of shift or the red-shift of the resonance position for increasing size is generally described as the net result of the competition between two opposite trends induced by two surface effects: a blue-shift (for increasing size) induced by the free electron spillout beyond the cluster surface (quantum effect) and a redshift that results from the lower polarizability of the outermost

Figure 6. Plot of the full width at half-maximum (fwhm) of the absorbance spectra versus the resonance wavelength for several models of embedded nanoparticles. The parameter within each theoretical curve is the nanoparticle radius. The parameter within the experimental curve is the laser dose.

ionic cores.18-22 In order to interpret the rarely observed blueshift, few authors quoted by Cai et al. in ref 23 proposed other interpretations. The latter,23 taking into account the lattice contraction of Ag particles with increasing size combined with the free path effect of electrons in a simple classical Mie calculation, predicts the blue-shift of the resonance without involving the surface skin of ineffective polarizability. The lattice contraction with increasing size was confirmed experimentally by the same authors24 for Ag particles in silicate glass subjected to specific synthesis conditions and subsequent thermal treatment. At the same time, the authors23,24 have reported size dependences similar to the ones observed in our first phase corresponding to the growth of nanoparticles in mesoporous silica films doped with silver nitrate and heat treated. In that case, they interpret their results using a multilayer core-shell model based on the interaction of the particle surface with ambient air and on the presence of porosity at the particle-matrix interface.25 This model can be adapted to our material that differs from the silver-containing mesoporous silica films in that the pores are filled with copolymer instead of air. The hydrophilic PEO part of the copolymer used in the present study is known to complex Ag+ ions;26 therefore, it is reasonable to deduce that the silver salt in the film is stabilized in the PEO phase. PEO is also known to photochemically reduce Ag+ to metallic Ag nanoparticles and to degrade under UV illumination.26 These properties have been recently used by Li and co-workers27 to grow ordered silver nanoparticles with well controlled size in polymer films containing self-assembled PEO domains. The latter fixed the size of the silver nanoparticles following a block copolymer photolithography process, in which the silver nanoparticles fill the former PEO domains. In our experiments, IR measurements (not shown) prove that the copolymer degrades in the film under UV illumination and TEM pictures show that the silver nanoparticles grow up to about 8 nm, which corresponds to the size of the triblock copolymer

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Figure 7. Schematic drawing of the multilayer core-shell model for Ag particles embedded in the hybrid silica film. R, particle radius; d, thickness of a surface monolayer; ds, thickness of the copolymer shell.

micelles.28,29 As a result, the silver nanoparticles are assumed to grow in place of a part of the copolymer micelles trapped in the mesostructured silica film thickness. Furthermore, it has already been reported that the nanoparticles can be confined in the mesopores of mesoporous silica matrixes6,9 especially when the silica walls are thicker than 3-5 nm.30 In our case, the silica walls are about 6-7 nm thick. As a consequence, they are expected to be thick enough to confine and isolate the nanoparticles. Following Cai’s hypothesis,25 we propose to describe our system by an ensemble of independent spherical nanoparticles surrounded by two successive shells and embedded in a silica matrix (Figure 7), and to calculate the spectral variations of its extinction cross section in the framework of the Mie theory for core-shell particles (spherical heterosystems).14 It has to be noted that the nanoparticles are small enough and distant enough (the distance between adjacent nanoparticles is at least equal to the period of the mesostructure that is about 14-15 nm) not to significantly interact with each other. In the case of so small silver nanoparticles (100 J/cm2), it was observed that the film bleaches. Experiments have



Figure 8. Plot of the theoretical extinction cross sections (solid lines) and wavelength values (dotted lines) at resonance versus thickness of the outer shell, for an external shell filled with air (a) and silver oxide or silver sulfide (b).

shown that this bleaching is due to the reappearance of nonreduced silver in the film. This implies that the size of the metallic-silver nanoparticles must somehow decrease. The changes of the extinction cross section due to this size reduction can be calculated using a model similar to that used previously and described in Figure 7. Only the nature of the external shell of thickness ds will be changed and thus its dielectric function. Indeed, this shell was composed of copolymer in the previous section dealing with the growth of silver nanoparticles. However, as said previously,26,27 the copolymer degrades under UV light and does not exist anymore in a pore containing a silver nanoparticle of 8 nm diameter. Therefore, if the nanoparticle size now slightly decreases, the increasing external shell cannot be filled with copolymer. We will then consider two hypotheses for the nature of this external shell. In the first hypothesis, the silver nanoparticles are assumed to be progressively surrounded by a low index shell of air diffused in the pores of the film, the pores being left by Ag+ diffusing in the film. For the calculation, the dielectric function of the external shell of thickness ds is set to 1 and the size of silver nanoparticles is assumed to decrease from 8 to 2 nm in a pore of fixed diameter (8 nm) which progressively fills with air. The calculated extinction cross section at resonance and the resonance wavelength are reported in Figure 8a versus the air-shell thickness ds. This graph shows that the decrease of the metallic-nanoparticle diameter leads in that case first to a blue-shift of the resonance wavelength and then to a red-shift. During the blue-shift, the initial value of the extinction cross section is divided by about 7. These results obviously do not agree with the experiment that shows above 100 J/cm2 in Figure 2b only a slight decrease of the absorbance level at resonance accompanied by a marked red-shift of the resonance wavelength. The second hypothesis that will be considered allows one to simulate both the decrease of the extinction cross section at resonance and the red-shift of the resonance wavelength. It consists of using a high refractive index for the external shell of thickness ds. Figure 8b shows the extinction cross section at resonance and resonance wavelength versus the external shell thickness calculated by taking into account the complex dielectric functions of silver sulfide and silver oxide, respectively. Ag2S33 and Ag2O34 have a refractive index greater than 2 and a non-negligible extinction coefficient in the visible domain that were taken from Bennett’s measurements33 and Pettersson’s results on silver coatings deposited by e-beam evaporation,34 respectively. The calculations show that, from small thickness values, the decrease of the extinction cross section goes with a marked red-shift of the resonance wavelength in agreement with the experimental results of Figure 2b. The presence of such a high index shell now needs to be further discussed. The experimentally shown stability of the nanoparticles formed under a vacuum confirms that, in air and under high

laser doses, the silver nanoparticles react chemically with the air diffused in the film. The nanoparticles may get slowly oxidized or sulfurized by the ambient oxygen or sulfur atoms diffused in the film. On the one hand, sulfurization is currently observed on silver nanostructures created at surfaces exposed to air35 and may occur in our material too. Nonetheless, sulfur was not detected in the films by energy dispersive X-ray (EDX) measurements. EDX measurements were performed during the TEM observations with a probe diameter of 30 nm (the smallest one providing a measurable signal) in order to identify the film content. On the other hand, the oxidation of silver nanoparticles was frequently observed with heat treatments at increasing temperature.5,36-40 This oxidation is all the easier as the nanoparticles are small.36,37 As in the studies of refs 36-38, the formed thin shells are not clearly distinguishable on the TEM pictures because they are too small to be crystallized and because they are all the more difficult to observe as the nanoparticles are buried in the silica matrix. In our experiments, the silver oxidation is likely to result from a local heating around the nanoparticles occurring after exposures to laser doses greater than about 100 J/cm2 at 244 nm wavelength. 4.2. Kinetic Modeling for the Silver-Nanoparticle Growth. It has been shown in section 3 that the growth of silver nanoparticles and the increase of the absorbance level at resonance with continuous UV light essentially depend on the incident dose that is the number of photons per unit area. Actually, the nanoparticle growth depends on the dose absorbed by the film that strongly decreases with increasing wavelength in the visible range. Therefore, as shown in Figure 5, the incident dose needed to reduce the silver salt and to initiate the nanoparticle growth increases with the wavelength. The decrease of the growth rate with wavelength observed experimentally can be quantitatively evaluated involving an ad hoc kinetic model. In Figure 5a, the shape of the curve recorded at 325 nm wavelength (Figure 5) differs from the others at low doses with a sigmoidal characteristic that can be attributed to autocatalytic kinetics.41 This particular shape can be modeled considering the following sequence of reactions42 (eq 3):

Ag+ + e- f Ag nAg f Agn

(3)

These general reactions express in a simplified manner how the nanoparticles form. First, the silver salt is reduced by an electron that may result from the dissociation of an excited state of silver nitrate obtained after the absorption of an incident photon or from the oxidation of the oxygen groups of the copolymer under UV illumination.12,13,26,27 Of course, other sources of electrons may exist, but the latter are among the most probable. Such mechanisms of electron generation strongly depend on wave-

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length, and their efficiency increases with the absorption coefficient of silver nitrate and copolymer, respectively. Second, the silver-nanoparticle growth results from the aggregation of reduced silver atoms. The aggregation process can be sped up with the light absorption of silver atoms or small silver clusters. The resulting autocatalytic kinetic model is described by eqs 4-6 in which all of the nanoparticles are assumed to have the same number of atoms, n.

d[AgNO3]/dt ) -k[AgNO3](1 + R[Ag])

(4)

[Ag] ) C0 - [AgNO3]

(5)

[Agn] ) (C0 - [AgNO3])/n

(6)

C0, [AgNO3], and [Ag], are the initial concentration of AgNO3 and the concentrations of AgNO3 and Ag at time t, respectively. k is a kinetic constant linked to the absorption coefficient of the film and to its ability to free electrons under exposure, which increases as the wavelength decreases. The relative absorption coefficients, normalized at 244 nm wavelength, of silver nitrate (0.1 mol/L in water) and of copolymer F127 (3 g dissolved in 12 g of ethanol at 40 °C) solutions were measured at the wavelengths used and are reported in Table 1. They show a rapid decrease with increasing wavelength. The ability of silver nitrate and copolymer F127 to free electrons is likely not to be directly proportional to their absorption coefficient, but no accurate information can be given. k is also assumed to be proportional to the incident intensity. R is an autocatalytic coefficient increasing with the absorption coefficient of silver atoms at the incident wavelength. The resolution of differential eq 4 gives an expression for the dose varying silver-nanoparticle concentration (eqs 7 and 8):

[Agn] ) Cf(exp(gW) - 1)/(exp(gW) + R′)

(7)

gW ) kt(1 + R′)

(8)

where Cf is the nanoparticle concentration once all of the silver precursors have been consumed, i.e., Cf ) C0/n, W is the laser dose of the incident wave, R′ is equal to RC0, and g, linked to k by eq 8, increases with the rate of the nanoparticle growth. Assuming that the absorbance level at resonance A is directly proportional to the nanoparticle concentration, we get the following expression for the absorbance level at resonance versus dose:

A ) Af(exp(gW) - 1)/(exp(gW) + R′)

(9)

where Af is the maximum value of absorbance level at resonance. Despite the approximations, this model allows one to fit rather accurately the experimental data for each of the illumination wavelengths (solid lines in Figure 8). The parameters found in each case are summarized in Table 1. The Af coefficient does not vary significantly between the samples, since it refers to the initial silver precursor content in the film. The parameter g allows one to quantitatively follow the decrease of the rate of the nanoparticle growth as the wavelength increases in the visible range. As expected, g is all the lower as the absorption coefficient of silver nitrate or copolymer F127 is low even if g decreases more quickly with wavelength. We see, for instance, that this kinetic parameter is 100 times smaller at 442 nm than at 244 nm. Nevertheless, the growth of silver nanoparticles also occurs in the visible and leads to changes in the absorbance spectrum on the condition that the dose is large enough, even at low light intensity. This may be an issue for any sample kept in daylight if all of the silver precursors were not previously consumed. To prevent such an uncontrolled yellowing of the

films, the samples may be kept in a fridge and in obscurity. On the contrary, we can report that the films in which the silver salt is fully consumed (the absorbance has reached a maximum) are very stable in time at room temperature and under daylight. The sigmoid shape of the curve at 325 nm wavelength can be explained further. This shape indicates that the rate of growth with dose (or the slope of the curve in Figure 5) is initially rather low and then abruptly increases to be nearly similar to the one at 244 nm wavelength. In the frame of the proposed model, such a shape is caused by the high value of R′ coefficient, which promotes the aggregation of silver atoms or silver clusters. The high value of R′ coefficient at 325 nm wavelength can be attributed to the high absorption of the incident wave by silver atoms or small silver clusters. They are indeed known to absorb in the 310-360 nm range,43 and it can be noted that outside this wavelength range the calculated R′ are much lower or null. Therefore, according to the model, one can conclude that the absorption of the incident light by the generated small silver clusters and silver atoms that occurs in the 310-360 nm range boosts the growth of nanoparticles. 5. Conclusion The photoreduction of ionic silver precursor under light exposure in mesostructured hybrid silica films is investigated. This photoreduction leads to the emergence of an absorption band due to the surface plasmon resonance of silver nanoparticles. The absorbance level at resonance of the films increases with the laser dose up to the total consumption of the silver salt, then stabilizes, and finally slowly decreases at high laser doses because of the reappearance of nonreduced silver in the film. The silver-nanoparticle size increases when the absorbance level at resonance rapidly increases with the dose. When the latter tends to stabilize, the nanoparticles go on growing, leading to an aggregation of the formed nanoparticles or to an Ostwald ripening. The nanoparticles grow inside the film mesostructure up to reaching the size of the copolymer micelles, around 8 nm, that degrade under exposure having a reductive action. During their growth, the resonance wavelength first shifts to the blue before slightly coming back to the red. This blue-shift is simulated using the Mie theory and a double-layer core-shell model that takes into account the strong reactivity of silver with ambient oxygen and the presence of copolymer in the silica film. At high laser doses, the film bleaching is also interpreted on the basis of the Mie theory. The observed red-shift of the resonance wavelength that goes with the bleaching is attributed to the growth of a high index shell around the particle that may be due to the formation of silver oxide. Finally, the paper emphasizes that visible light can be used to form silver nanoparticles and quantitatively estimates the decrease of the photoreduction process rate upon an incident wavelength increase from the ultraviolet to the visible range. The proposed kinetic model also points out the role of the absorption of the reduced atoms or small clusters on the rate of the nanoparticle growth. This absorption phenomenon especially increases the rate of the nanoparticle growth and gives a sigmoid shape to the kinetic curve at an incident wavelength of 325 nm. Acknowledgment. The authors thank Jean Lerme´, Laboratoire de Spectrome´trie Ionique et Mole´culaire, Universite´ Claude Bernard - Lyon I, France, for fruitful discussions and Annie Rivoire from the Centre Technologique des Microstructures, Universite´ Claude Bernard - Lyon I, for the cutting of the sample

Growth of Photoinduced Silver Nanoparticles cross sections and the TEM measurements recorded on the Philips CM 120. Pr. Olivier Parriaux is thanked for a careful reading of the manuscript. The Region Rhoˆne-Alpes is gratefully acknowledged for the financial support of Y.B. This work was supported by the Agence Nationale de la Recherche in the framework of PNANO POMESCO. References and Notes (1) Shipway, A. N.; Katz, E.; Willner, I. Nanoparticle Arrays on Surfaces for Electronic, Optical, and Sensor Applications. ChemPhysChem 2000, 1, 18–52. (2) Daniel, M. C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-size Related Properties, and Applications towards Biology, Catalysis and Nanotechnology. Chem. ReV. 2004, 104, 293–346. (3) Ghosh, S. K.; Pal, T. Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications. Chem. ReV. 2007, 107, 4797–4862. (4) Ropers, C.; Elsaesser, T.; Cerullo, G.; Zavelani-Rossi, M.; Lienau, C. Ultrafast optical excitations of metallic nanostructures: from light confinement to a novel electron source. New J. Phys. 2007, 9, 397. (5) Li, W.; Seal, S.; Megan, E.; Ramsdell, J.; Scammon, K.; Lelong, G.; Lachal, L.; Richardson, K. A. Physical and optical properties of sol-gel nano-silver doped silica film on glass substrate as a function of heattreatment temperature. J. Appl. Phys. 2003, 93, 9553–9561. (6) Bois, L.; Bessueille, F.; Chassagneux, F; Battie, Y.; Destouches, N.; Hubert, C.; Boukenter, A.; Parola, S. Silver nanoparticles growth in a mesoporous silica film templated with the F127 triblock copolymer. S. Colloids Surf., A 2008, 325, 86–92. (7) Qiu, J.; Shirai, M.; Nakaya, T.; Si, J.; Jiang, X.; Zhu, C.; Hirao, K. Space-selective precipitation of metal nanoparticles inside glasses. Appl. Phys. Lett. 2002, 81, 3040–3042. (8) Chen, S.; Akai, T; Kadono, K.; Yazawa, T. Reversible Control of Silver Nanoparticle Generation and Dissolution in Soda-Lime Silicate Glass through X-ray Irradiation and Heat Treatment. Appl. Phys. Lett. 2001, 79, 3687–3689. (9) Hornebecq, V.; Antonietti, M.; Cardinal, T.; Treguer-Delapierre, M. Stable Silver Nanoparticles Immobilized in Mesoporous Silica. Chem. Mater. 2003, 15, 1993–1999. (10) Hofmeister, H.; Thiel, S.; Dubiel, M.; Schurig, E. Synthesis of nanosized silver particles in ion-exchanged glass by electron beam irradiation. Appl. Phys. Lett. 1997, 70, 1694–1696. (11) Besson, S.; Gacoin, T.; Ricolleau, C.; Boilot, J. P. Silver nanoparticle growth in 3D-hexagonal mesoporous silica films. Chem. Commun. 2003, 3, 360–361. (12) Li, J.; Kamata, K.; Watanabe, S.; Iyoda, T. Template- and VacuumUltraviolet- Assisted Fabrication of a Ag-Nanoparticle Array on Flexible and Rigid Substrates. AdV. Mater. 2007, 19, 1267–1271. (13) Lee, K. J.; Park, J. T.; Goh, J. H.; Kim, J. H. Synthesis of amphiphilic graft copolymer brush and its use as template film for the preparation of silver nanoparticles. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3911–3918. (14) Kreibig, U.; Vollmer, M. Optical properties of metal clusters; Springer Series in Material Science 25, Springer: Berlin, 1995. (15) Battie, Y.; Destouches, N.; Bois, L.; Chassagneux, F.; Moncoffre, N.; Toulhoat, N.; Jamon, D.; Ouerdane, Y.; Parola, S.; Boukenter, A. Generation of an ordered layer of silver nanoparticles in mesostructured dielectric films. J. Nanopart. Res. 2009, 12, 1073–1082. (16) Zheng, H.; Smith, R. K.; Jun, Y.-W.; Kisielowski, C.; Dahmen, U.; 1; Alivisatos, A. P. Observation of single colloidal platinum nanocrystal growth trajectories. Science 2009, 324, 1309–1312. (17) Blondeau, J.-P.; Pellerin, S.; Vial, V.; Dzierzega, K.; Pellerin, N.; Andreazza-Vignolle, C. Influence of pulsed laser irradiation on precipitation of silver nanoparticles in glass. J. Cryst. Growth 2008, 311, 172–184. (18) Tiggesba¨umker, J. ; Ko¨ller, L.; Meiwes-Broer, K. H.; Liebsch, A. Blue shift of the Mie plasma frequency in Ag clusters and particles. Phys. ReV. A 1993, 48, 1749–1752. (19) Fedrigo, S.; Harbich, W.; Buttet, J. Collective dipole oscillations in small silver clusters embedded in rare-gas matrices. Phys. ReV. B 1993, 47, 10706–10715.

J. Phys. Chem. C, Vol. 114, No. 19, 2010 8687 (20) Kresin, V. V. Collective resonances in silver clusters: Role of d electrons and the polarization-free surface layer. Phys. ReV. B 1995, 51, 1844–1849. (21) Serra, Ll.; Rubio, A. Optical response of Ag clusters. Z. Phys. D 1997, 40, 262–264. (22) Lerme´, J.; Palpant, B.; Prevel, B.; Pellarin, M.; Treilleux, M.; Vialle, J. L.; Perez, A.; Broyer, M. Quenching of the Size Effects in Free and Matrix-Embedded Silver Clusters. Phys. ReV. Lett. 1998, 80, 5105–5108. (23) Cai, W.; Hofmeister, H.; Dubiel, M. Importance of lattice contraction in surface plasmon resonance shift for free and embedded silver particles. Eur. Phys. J. D 2001, 13, 245–253. (24) Dubiel, M.; Brunsch, S.; Mohr, C.; Hofmeister, H. HREM and EXAFS characterization of silver particles in glass. In Proceedings of XVIII Int. Cong. on Glass, San Francisco, 1998; Choudhary, M. K., Hu, N. T., Drummond, C. H., III.; American Ceramic Society: Westerville, OH, 1998; session C1, pp 119-124, on CD-ROM. (25) Cai, W.; Hofmeister, H.; Rainer, T. Surface effect on the size evolution of surface plasmon resonances of Ag and Au nanoparticles dispersed within mesoporous silica. Physica E 2001, 11, 339–344. (26) Luo, C.; Zhang, Y.; Zeng, X.; Zeng, Y.; Wang, Y. The role of poly(ethylene glycol) in the formation of silver nanoparticles. J. Colloid Interface Sci. 2005, 288, 444–448. (27) Li, J.; Kamata, K.; Iyoda, T. Tailored Ag nanoparticle array fabricated by block copolymer photolithography. Thin Solid Films 2008, 516, 2577–2581. (28) Boissiere, C.; Grosso, D.; Lepoutre, S. Q.; Nicole, L.; Brunet Bruneau, A.; Sanchez, C. Porosity and Mechanical Properties of Mesoporous Thin Films Assessed by Environmental Ellipsometric Porosimetry. Langmuir 2005, 21, 12362–12371. (29) Naik, S. P.; Yamakita, S.; Ogura, M.; Okubo, T. Studies on mesoporous silica films synthesized using F127, a triblock co-polymer. Microporous Mesoporous Mater. 2004, 75, 51–59. (30) Armelao, L.; Bottaro, G.; Campostrini, R.; Ischia, M.; Poli, F.; Tondello, E. Synthesis and structural evolution of mesoporous silica-silver nanocomposites. Nanotechnology 2007, 18, 155606. (31) Palik, E. D. Handbook of optical constants of solids; Academic Press: New York, 1985. (32) Hovel, H.; Fritz, S.; Hilger, A.; Kreibig, U.; Vollmer, M. Width of cluster plasmon resonances: Bulk dielectric functions and chemical interface damping. Phys. ReV. B 1993, 48, 18178–18188. (33) Bennett, J. M.; Stanford, J. L.; Ashley, E. J. Optical Constants of Silver Sulfide Tarnish Films. J. Opt. Soc. Am. 1970, 60, 224–231. (34) Pettersson, L. A. A.; Snyder, P. G. Preparation and characterization of oxidized silver thin films. Thin Solid Films 1995, 270, 69–72. (35) Graedel, T. E.; Franey, J. P.; Gualtieri, G. J.; Kammlott, G. W.; Malm, D. L. On the mechanism of silver and copper sulfidation by atmospheric H2S and OCS. Corros. Sci. 1985, 25, 1163–1180. (36) Bi, H.; Cai, W.; Kan, C.; Zhang, L.; Martin, D.; Tra¨ger, F. Optical study of redox process of Ag nanoparticles at high temperature. J. Appl. Phys. 2002, 92, 7491–7497. (37) Bi, H.; Cai, W.; Zhang, L.; Martin, D.; Tra¨ger, F. Annealing-induced reversible change in optical absorption of Ag nanoparticles. Appl. Phys. Lett. 2002, 81, 5222–5224. (38) Pal, S.; De, G. Reversible transformations of silver oxide and metallic silver nanoparticles inside SiO2 films. Mater. Res. Bull. 2009, 44, 355–359. (39) Mennig, M.; Schmitt, M.; Schmidt, H. Synthesis of Ag-Colloids in Sol-Gel Derived SiO2-Coatings on Glass. J. Sol-Gel Sci. Technol. 1997, 8, 1035–1042. (40) Renteria-Tapia, V. M.; Garcia-Macedo, J. Influence of Oxygen on the Optical Properties of Silver Nanoparticles. J. Nanosci. Nanotechnol. 2008, 8, 6545–6550. (41) Ghosh, S. K.; Kundu, S.; Mandal, M.; Nath, S.; Pal, T. Studies on the evolution of silver nanoparticles in micelle by UV-photoactivation. J. Nanopart. Res. 2003, 5, 577–587. (42) Zeng, H.; Qiu, J.; Jiang, X.; Zhu, C.; Gan, F. Effect of Al2O3 on the precipitation of Ag nanoparticles in silicate glasses. J. Cryst. Growth 2004, 262, 255–258. (43) Linnert, T.; Mulvaney, P.; Henglein, A.; Weller, H. Long-lived nonmetallic silver clusters in aqueous solution: preparation and photolysis. J. Am. Chem. Soc. 1990, 112, 4657–4664.

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Growth Mechanisms and Kinetics of Photoinduced Silver

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