Electroless Growth of Silver Nanoparticles into Mesostructured Silica Block Copolymer Films

pubs.acs.org/Langmuir © 2010 American Chemical Society

Electroless Growth of Silver Nanoparticles into Mesostructured Silica Block Copolymer Films )

Laurence Bois,*,† Fernand Chassagneux,† C
edric Desroches,† Yann Battie,‡ Nathalie Destouches,‡ Nicole Gilon,§ St
ephane Parola,† and Olivier St
ephan

)

† Laboratoire des Multimat
eriaux et Interfaces, UMR CNRS 5615, B^ at. Berthollet, Universit
e Claude Bernard - Lyon 1, 43 Bd 11 novembre 1918, 69622 Villeurbanne, France, ‡ Laboratoire Hubert Curien, CNRS UMR 5516 - Universit
e Jean Monnet, 18 Rue Prof. B. Lauras, Bat F, F-42000 Saint-Etienne, France, §Laboratoire des Sciences Analytiques, Universit
e Claude Bernard - Lyon 1, UMR CNRS 5180, 43 Bd 11 novembre 1918, 69622 Villeurbanne, France, and Laboratoire de Spectrom
etrie Physique, UMR 5588, Universit
e Joseph Fourier, Grenoble I, France

Received November 27, 2009. Revised Manuscript Received February 16, 2010 Silver nanoparticles and silver nanowires have been grown inside mesostructured silica films obtained from block copolymers using two successive reduction steps: the first one involves a sodium borohydride reduction or a photoreduction of silver nitrate contained in the film, and the second one consists of a silver deposit on the primary nanoparticles, carried out by silver ion solution reduction with hydroxylamine chloride. We have demonstrated that the F127 block copolymer ((PEO)106(PPO)70(PEO)106), “F type”, mesostructured silica film is a suitable “soft” template for the fabrication of spherical silver nanoparticles arrays. Silver spheres grow from 7 to 11 nm upon the second reduction step. As a consequence, a red shift of the surface plasmon resonance associated with metallic silver has been observed and attributed to plasmonic coupling between particles. Using a P123 block copolymer ((PEO)20(PPO)70(PEO)20), “P type”, mesostructured silica film, we have obtained silver nanowires with typical dimension of 10 nm
100 nm. The corresponding surface plasmon resonance is blue-shifted. The hydroxylamine chloride treatment appears to be efficient only when a previous chemical reduction is performed, assuming that the first sodium borohydride reduction induces a high concentration of silver nuclei in the first layer of the porous silica (film/air interface), which explains their reactivity for further growth.

1. Introduction Metallic nanoparticles exhibit surface plasmon resonance, which opens numerous applications such as optical data storage, an enhancement of Raman scattering, and nonlinear optics.1-3 Metallic nanoparticle arrays provide opportunities to observe new collective optical phenomena.4 The optical spectra of arrays are influenced by particle shape and size and interactions between particles, and a general understanding of how these factors influence optical spectra has been developed by numerical methods such as the discrete approximation method (DDA).4-9 It has been shown that the optical answer to a 2D array of silver nanoparticles may be tuned by their interparticle spacing.4,5 In the case of large crystals, quadrupole resonance may also appear that is blue-shifted compared to the dipole resonance.9 Arrays of *To whom correspondence should be addressed. E-mail: laurence.bois@ univ-lyon1.fr. Phone: 33 04 72 43 14 00. Fax: 33 04 72 44 06 18. (1) Faraday, M. Philos. Trans. R. Soc. 1857, 147, 145. (2) Mulvaney, P. Langmuir 1996, 12, 788. El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. Eustis, S.; El-Sayed, M. A. Chem. Soc. Rev. 2006, 35, 209. Link, S.; El-Sayed, M. A. Annu. Rev. Phys. Chem. 2003, 54, 331. (3) P
erez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. Rev. 2005, 249, 1870. Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L. F.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (4) Zhao, L. L.; Kelly, K. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 7343. (5) Portales, H.; Pinna, N.; Pileni, M.-P. J. Phys. Chem. A 2009, 113, 4094. (6) Pinna, N.; Maillard, M.; Courty, A.; Russier, V.; Pileni, M. P. Phys. Rev. B 2002, 66, 045415. (7) Zhao, J.; Pinchuk, A. O.; McMahon, J. M.; Li, S.; Ausman, L. K.; Atkinson, A. L.; Schatz, G. C. Acc. Chem. Res. 2008, 12, 1710. (8) Haynes, C. L.; McFarland, A. D.; Zhao, L. L.; Van Duyne, R. P.; Schatz, G. C.; Gunnarsson, L.; Prikulis, J.; Kasemo, B.; K€all, M. J. Phys. Chem. B 2003, 107, 7337–7342. (9) Jin, R.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901.

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large particles may produce very narrow quadrupole resonance.10 For applications, it is often necessary to generate metallic nanoparticles assemblies within a stable host matrix.11 The formation of metallic nanostructures generally requires complex methods such as lithography (e-beam, photo-, or X-ray lithography).12 Microcontact printing methods have also been proposed.13 Other lithography methods relying on a self-assembly process have been developed: lithography using a nanosphere monolayer as a mask14 or the self-assembly of diblock copolymer micelles.15-20 In this field, approachs combining both microcontact printing (10) Evanoff, D. D., Jr.; Chumanov, G. ChemPhysChem 2005, 6, 1221. (11) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (12) Corrigan, T. D.; Guo, S.-H.; Szmacinski, H.; Phaneuf, R. J. Appl. Phys. Lett. 2006, 88, 101112. Corbierre, M. K.; Beerens, J.; Lennox, R. B. Chem. Mater. 2005, 17, 5774. (13) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (14) Jensen, T. R.; Duval, M. L.; Kelly, K. L.; Lazarides, A. A.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 9846. (15) Spatz, J. P.; M€ossmer, S.; Hartmann, C.; M€oller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. Aizawa, M.; Buriak, J. M. Chem. Mater. 2007, 19, 5090. Deshmukh, R. D.; Composto, R. J. Chem. Mater. 2007, 19, 745. (16) Leong, W. L.; Lee, P. S.; Lohani, A.; Lam, Y. M.; Chen, T.; Zhang, S.; Dodabalapur, A.; Mhaisalkar, S. G. Adv. Mater. 2008, 20, 2325. Li, J.; Kamata, K.; Watanabe, S.; Iyoda, T. Adv. Mater. 2007, 19, 1267. (17) Lohmueller, T.; Bock, E.; Spatz, J. P. Adv. Mater. 2008, 20, 2297. (18) Lopes, W. A.; Jaeger, H. M. Nature 2001, 13, 735. Mitsuichi, M.; Ishifuji, M.; Endo, H.; Tanaka, H.; Miyashita, T. Polym. J. 2007, 39, 411. Peponi, L.; Tercjak, A.; Gutierrez, Jl.; Stadler, H.; Torre, L.; Kenny, J M.; Mondragon, I. Macromol. Mater. Eng. 2008, 293, 568. (19) Bockstaller, M. R.; Mikiewicz, R. A.; Thomas, E. L. Adv. Mater. 2005, 17, 1331. Jung, Y. S.; Ross, C. A. Small 2009, 5, 1654. (20) Sohn, B.-H.; Choi, J.-M.; Yoo, S. I.; Yun, S.-H.; Zin, W.-C.; Jung, J. C.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368. Warren, S. C.; Messina, L. C.; Slaughter, L. S.; Kamperman, M.; Zhou, Q.; Gruner, S. M.; DiSalvo, F. J.; Wiesner, U. Science 2008, 320, 1748.

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and self-assembled block copolymers have already been reported.21 Mesostructured inorganic materials, such as mesoporous silica films formed through the self-assembly of surfactants, can also be used.22-28 To a much lesser extent, hybrid mesostructured silica films obtained from block copolymers have been considered to be templates for silver nanoparticle growth.29-31 The preparation of silver nanoparticles or silver nanowires using a two-step reduction process inside a mesostructured silica film is described in this article. This new process is a combination of the block copolymer self-assembly method29-31 and hydroxylamine growth.17,32 Silver nanoparticles are produced within the host matrix. Subsequent hydroxylamine growth of the metallic seeds then occurs. As established, hydroxylamine chloride is thermodynamically able to reduce Ag(I) to Ag(0). It is noticed that the redox reaction is catalyzed at metallic surfaces (Ag). On this basis, we have investigated the enlargment of silver nanoparticles previously trapped within silica-based matrices.32 To generate the silver seeds, three main strategies can be considered:33 a chemical reduction involving reducers such as sodium borohydride (NaBH4)31 or formaldehyde, photoreduction,34 or thermal treatment.31 To prepare large particles with plasmonic coupling, we have focused our attention on the comparison of the results obtained with seeds coming from chemical reduction and photoreduction combined with a growth step involving reduction with hydroxylamine. (21) Cong, Y.; Fu, J.; Zhang, Z.; Cheng, Z.; Xing, R.; Li, J.; Han, Y. J. Appl. Polym. Sci. 2006, 100, 2737. (22) Bronstein, L. M. Top. Curr. Chem. 2003, 226, 55. Zhu, J.; K
onya, Z.; Puntes, V. F.; Kiricsi, I.; Miao, C. X.; Ager, J. W.; Alivisatos, A. P.; Somorjai, G. A. Langmuir 2003, 19, 4396. Hornebecq, V.; Antonietti, M.; Cardinal, T.; Treguer-Delapierre, M. Chem. Mater. 2003, 15, 1993. Park, J.-H.; Park, J. K.; Shin, H. Y. Mater. Lett. 2007, 61, 156. Adhyapak, P. V.; Karandikar, P.; Vijayamohanan, K.; Athawale, A. A.; Chandwadkar, A. J. Mater. Lett. 2004, 58, 1168. (23) Sun, J.; Ma, D.; Zhang, H.; Liu, X.; Han, X.; Bao, X.; Weinberg, G.; Pfander, N.; Su, D. J. Am. Chem. Soc. 2006, 128, 15756. Lin, D.-H.; Jiang, Y.-X.; Wang, Y.; Sun, S.-G. J. Nanomater. 2008, doi:10.1155/2008/473791. Worboys, L. M.; Edwards, P. P.; Anderson, P. A. Chem. Commun. 2002, 2894. Xie, Y.; Quinlivan, S.; Asefa, T. J. Phys. Chem. C 2008, 112, 9996. (24) Plyuto, Y.; Berquier, J.-M.; Jacquiod, C.; Ricolleau, C. Chem. Commun. 1999, 17, 1653. Gacoin, T.; Besson, S.; Boilot, J. P. J. Phys.: Condens. Matter. 2006, 18, S85. Besson, S.; Gacoin, T.; Jacquiod, C.; Ricolleau, C.; Boilot, J.-P. Mater. Res. Soc. Symp. Proc. 2002, 707, 119. Besson, S.; Gacoin, T.; Ricolleau, C.; Boilot, J.-P. Chem. Commun. 2003, 9, 360. (25) Wang, L. Z.; Shi, J.-L.; Zhang, W.-H.; Ruan, M.-L.; Yu, J.; Yan, D.-S. Chem. Mater. 1999, 11, 3015. Renard, C.; Ricolleau, C.; Fort, E.; Besson, S.; Gacoin, T.; Boilot, J.-P. Appl. Phys. Lett. 2002, 80, 300. Huang, M. H.; Choudrey, A.; Yang, P. Chem. Commun. 2000, 1063. (26) Eustis, S.; Krylova, G.; Smirnova, N.; Eremenko, A.; Tabor, C.; Huang, W.; El-Sayed, M. A. J. Photochem. Photobiol., A 2006, 181, 385. Krylova, G.; Eremenko, A.; Smirnova, N.; Eustis, S. Int. J. Photoenergy 2005, 7, 193. Krylova, G. V.; Eremenko, A. M.; Smirnova, N. P.; Eustis, S. Theor. Exp. Chem. 2005, 41, 105. (27) Fuertes, M. C.; Marchena, M.; Marchi, M. C.; Wolosiuk, A.; Soler-Illia, G. J. A. A. Small 2009, 5, 272. (28) Bois, L.; Bessueille, F.; Battie, Y.; Chassagneux, F.; Destouches, N.; Hubert, C.; Boukenter, A.; Parola, S. J. Colloids Surf. 2008, 325, 86. (29) Dag, O.; Samarskaya, O.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 2003, 13, 328. (30) Valverde-Aguilar, G.; Renteria, V.; Garcia-Macedo, J. A. Proc. SPIE 2007, 6641, 66411W-1–66411W-7. Garcia-Macedo, J. A.; Valverde, G.; Lockard, J.; Zink, J. I. Proc. SPIE 2004, 5361, 117. Garcia-Macedo, J.; Franco, A.; Valverde, G.; Zink, J. I. Proc. SPIE 2004, 5520, 206. (31) Bois, L.; Chassagneux, F.; Parola, S.; Bessueille, F.; Battie, Y.; Destouches, N.; Boukenter, A.; Moncoffre, N.; Toulhoat., N. J. Solid. State. Chem. 2009, 182, 1700. Destouches, N.; Battie, Y.; Ouerdane, Y.; Boukenter, A.; Bois, L.; Chassagneux, F.; Parola, S.; Moncoffre, N.; Toulhoat N. J. Nanopart. Res. DOI 10.1007/s11051-0099794-8. (32) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726. Meltzer, S.; Resch, R.; Koel, B. E.; Thompson, M. E.; Madhukar, A.; Requicha, A. A. G.; Will, P. Langmuir 2001, 17, 1713. Leopold, N.; Lendl, B. J. Phys. Chem. B 2003, 107, 5723. Ca~namares, M. V.; Garcia-Ramos, J. V.; Sanchez-Cortesa, S.; Castillejo, M.; Oujja, M. J. Colloid Interface Sci. 2008, 326, 103. Prevo, B. G.; Esakoff, S. A.; Mikhailovsky, A.; Zasadzinski, J. A. Small 2008, 4, 1183. (33) Walters, G.; Parkin, I. P. J. Mater. Chem. 2009, 19, 574. (34) Canamares, M. V.; Garcia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Langmuir 2007, 23, 5210. Muniz-Miranda, M. J. Raman Spectrosc. 2004, 35, 839. Bjerneld, E. J.; Murty, K.V.G. K.; Prikulis, J.; K€all, M. ChemPhysChem. 2002, 1, 116. Lau, D.; Furman, S. Appl. Surf. Sci. 2008, 255, 2159. Henley, S. J.; Silva, S. R. P. Appl. Phys. Lett. 2007, 91, 023107-1.

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Bois et al. Table 1. Sample Description block copolymer used reduction method

F127

P123

NaBH4 NaBH4 and hydroxylamine photoreduction photoreduction and hydroxylamine

FNB FNBH FUV FUVH

PNB PNBH PUV PUVH

Silica films mesostructured with triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide, PEO-PPOPEO (P123 or F127),35 have been chosen as host matrices. This kind of polymer acts in different ways. First, the ethylene oxide functional groups form a complex with silver salt. This complexation allows the introduction of a higher silver level within the silica film. Then, the polymer can slowly reduce the silver salt, probably into very small silver clusters. Moreover, the polymer, having amphiphilic properties, self-assembles into cylindrical or spherical micelles. Then the silver nanoparticles’ localization is at least partially controlled by the micelle organization. By using block copolymer F127, (PEO)106(PPO)70(PEO)106, a body-centeredcubic type of arrangement of spherical micelles is usually formed and block copolymer P123, (PEO)20(PPO)70(PEO)20, is appropriate to form a 2D hexagonal organization of cylindrical micelles. Silver salt is introduced during film synthesis. A first, the in situ reduction of Agþ is performed either with NaBH4 or UV irradiation. The corresponding films are denoted FNB or PNB if they have been obtained with F127 or P123 after chemical reduction and FUV or PUV upon photoreduction (Table 1). After the hydroxylamine growth step, the corresponding samples are denoted as FNBH, PNBH, FUVH, and PUVH, respectively (Table 1).

2. Experimental Section 2.1. Synthesis. A sol is prepared by mixing tetraethoxysilane (4 g), F127 block copolymer (PEO)106(PPO)70(PEO)106 (1.14 g), ethanol (11.8 g), and water acidified with nitric acid (pH 1.4) (1.76 g), according to ref 35. Silver nitrate (325 mg) is added to this sol. After 1 h of mixing at room temperature, films have been prepared by dip-coating at 40 mm/min and subsequent drying at room temperature for an additional period of 24 h. The first chemical reduction step can be performed by immersing the film in an aqueous sodium borohydride solution (20 mL, 50 mM) for 30 s. An intense yellow coloration is obtained for the corresponding FNB films. A short washing step is performed by dipping the film in water for 1 s. Otherwise, insolation can be performed for 2 min using a UV lamp (P = 500 W). In this case, a yellowish coloration is observed for the corresponding FUV films. A second additional chemical reduction step (hydroxylamine growth) can therefore be performed by immersing the samples in an aqueous solution (40 mL) containing silver nitrate (40 mg) and hydroxylamine hydrochloride (100 μL, 0.1 M) for 15 min. A duration of 15 min has been chosen because longer times induce the formation of large silver crystals. After the hydroxylamine treatment, a short washing step is performed by dipping the film in water for 1 s. After this second reduction step, a yellow-reddish coloration has been obtained for the FNBH samples when immersing FNB films. No color change for FUVH is observed if this second reduction step follows a UV reduction (immersion of FUV films). A similar procedure has been performed using the P123 copolymer. A sol is prepared by mixing tetraethoxysilane (7 g), block copolymer P123 (PEO)20(PPO)70(PEO)20 (1.75 g), ethanol (29.4 g) and water acidified with nitric acid (pH 1) (2.8 g). Silver (35) Dourdain, S.; Bardeau, J. F.; Colas, M.; Smarsly, B.; Mehdi, A.; Ocko, B. M.; Gibaud, A. Appl. Phys. Lett. 2005, 86, 11. Gibaud, A., Henderson, M. J., Colas, M., Dourdain, S., Bardeau, J.-F. White, J. W. AzoNano 2005, 1, a0109/1-7.

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Figure 1. TEM images of “F-like“ films after NaBH4 treatment before hydroxylamine growth, FNB: (a) top view and (b) cross section. TEM images of F-like films after NaBH4 treatment and after hydroxylamine growth, FNBH: (c) top view and (d) cross section. nitrate (286 mg) is added to this sol. The same process is used for the film preparation and reduction steps. These films are called, respectively, PNB and PNBH after NaBH4 reduction and hydroxylamine growth. A pale-yellow color has been obtained for PNB films, and an inhomogeneous green color has been obtained for the PNBH films. Using UV photoreduction, films are called PUV and PUVH, respectively. A pale-yellow color is seen for these two films. 2.2. Characterization. The TEM images were taken with a TOPCON EM002B transmission electron microscope operating at 200 kV. Samples for the transmission electron microscopy (TEM) characterization were prepared by scrapping the film. The fragments were then deposed directly onto a copper grid coated with a holey carbon film. UV-visible absorption spectra were recorded on a Perkin-Elmer lambda 35. X-ray diffraction (XRD) patterns were recorded on a Philips Xpert Pro diffractometer equipped with a monochromator using Cu KR radiation. SEM images were acquired on Hitachi S800 equipment at 15 kV after the films were covered with a gold-palladium layer. Silicon losses during chemical treatments have been measured by inductively coupled plasma-optical emission spectroscopy, ICP-OES, with a Shimadzu ICPE9000.

3. Results Typical TEM pictures of both FNB and FNBH films are presented in Figures 1 and 2. The mesostructure of the FNB polymer silica film appears to be relatively well ordered (Figure 1a). By referring to previous studies,36-38 in order to give a qualitative (36) Wu, C.-W.; Yamauchi, Y.; Ohsuna, T.; Kuroda, K. J. Mater. Chem. 2006, 16, 3091. (37) Tate, M. P.; Eggiman, B. W.; Kowalski, J. D.; Hillhouse, H W. Langmuir 2005, 21, 10112. Eggiman, B. W.; Tate, M. P.; Hillhouse, H. W. Chem. Mater. 2006, 18, 723. (38) Sanchez, C.; Boissiere, C.; Grosso, D.; Laberty, C.; Nicole, L. Chem. Mater. 2008, 20, 682.

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Figure 2. HR TEM image of the F-like film after NaBH4 treatment and after hydroxylamine growth.

representation of the film, we could note some similitude with the distortion of the Im3m body-centered cubic phase with spherical micelles. The distortion occurs upon film drying and results in a compression of the layer. One of the possible distortions has been identified as an R3m phase in which the (111) planes are parallel to the substrate.37 The TEM picture (Figure 1a) is compatible with a [111] view of this R3m structure. Nevertheless, GISAXS experiments are necessary to establish a clear structural assignation. We must point out that a regular array of silver nanoparticles is formed within the silica polymer mesostructure. The mean size of the silver nanoparticles is 7 nm, and the distance between particles is about 5 nm. The cross section presented in Figure 1b shows that silver nanoparticles form a single monolayer in the first layer of the mesostructured silica film. These results have already been reported in previous work.31 The 7 nm mean size of metallic DOI: 10.1021/la904491v

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nanoparticle is also measured on this cross-sectional view. In the depth of the layer, the dimension of the micelle can also be observed at 7 nm. The localization of silver nanoparticles in this silica mesostructure is not arbitrary because the TEM picture shows that they are localized between the silica walls at the position that was formerly occupied by the block copolymer before the NaBH4 reduction treatment. The block copolymer within the first monolayer is probably dissolved during the NaBH4 treatment and the concomittent growth of silver nanoparticles in this porous silica layer. After hydroxylamine growth, TEM pictures of the corresponding FNBH films show a size increase for the nanoparticles (Figure 1c). The mean size of the silver nanoparticles now ranges from 10 to 11 nm, and the mean distance between nanoparticles has decreased to around 2 to 3 nm (Figure 1c). The nanoparticle size increase up to 11 nm is also shown on the cross-sectional view in Figure 1d. Some silver nanoparticles are also observed in the layer depth, revealing that the silver ions’ diffusion toward the external surface was not completed. Moreover, in the depth of the layer, the micelle size is still measured at 7 nm, showing that the mesostructure has not been deeply modified during this treatment. Nevertheless, the metallic nanoparticle size increase is probably possible either because of the partial dissolution of the silica walls in the first sublayer39 or more probably because of the compression of the silica walls. ICP-OES measurements have been performed on the solutions used for the hydroxylamine reduction. Taking special care to avoid glassware contamination during ICP measurements, silicon was monitored on several sensitive analytical lines (Si 251 nm and Si 288 nm) in the solution at the end of the hydroxylamine process. No silicon was measured at the end of this process. The formation of crystallized silver nanoparticles is confirmed by HRTEM analysis in Figure 2 because the (111) lattice franges are clearly observed. It must be noticed that large crystals (dimension of around 40 nm) are also observed on the surface of the mesostructure, which reveals that the film quality could certainly be improved. Upon photoreduction (FUV films), much smaller silver nanoparticles have been obtained with sizes ranging from 1 to 3 nm. Typical TEM pictures of the corresponding samples are shown in Figure 3a. A higher silver nanoparticle density is obtained. In contrast to FNB films (NaBH4 reduction), there is no accumulation of silver in the first layer of the mesostructure.40 Moreover, silver nanoparticles have grown both in the hydrophobic copolymer zone and inside the silica walls that are mixed with the hydrophilic part of the polymer. The photoreduction process induces the growth of silver crystals with limited diffusion compared to what is observed in the case of chemical treatment. After hydroxylamine growth, TEM pictures are shown in Figure 3b,c. In this case, the silver nanoparticles that are inside the mesostructure do not seem to be modified by the hydroxylamine treatment, but the great modification is the apparition of large nanocrystals whose size is around 40 nm. Besides, the HRTEM picture in Figure 3c shows a decahedral multitwinned silver nanoparticle47 covered with an amorphous layer. This amorphous layer could be explained by the presence of the block copolymer that is partially oxidized during the photoreduction of silver salt. By referring to previous studies, the silica organization in the PNB film, observed by TEM (Figures 4a,b), shows some (39) Bass, J. D.; Grosso, D.; Boissiere, C.; Belamie, E.; Coradin, T.; Sanchez, C. Chem. Mater. 2007, 19, 4349–4356. Dunphy, D. R.; Singer, S.; Cook, A. W.; Smarsly, B.; Doshi, D. A.; Brinker, C. J. Langmuir 2003, 19, 10403–10408. (40) Battie, Y.; Destouches, N.; Bois, L.; Chassagneux, F.; Tishchenko, A.; Parola, S.; Boukenter, A. J. Phys. Chem. C, submitted for publication.

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Figure 3. TEM images of F-like films after photoreduction treatment: (a) before hydroxylamine growth, FUV; (b) after hydroxylamine growth, FUVH; and (c) HR TEM of the F-like film, FUVH.

similitudes with a phase that is a distortion of the hexagonal 2D phase with the p6mm group and results from compression during drying. In this c2mm phase, the (10) plane is oriented parallel to the substrate.36-38 Therefore, the channels inside the (10) planes are also parallel to the substrate, but they meander by free rotation around a direction perpendicular to the substrate. To simplify, we call it the “P-type“ structure because it probably comes from the disorganization of the classical hexagonal 2D structure. Nevertheless, once again GISAXS experiments are the only way to confirm this hypothesis. In this configuration, the micelles are cylindrical. Silver nanoparticles are formed inside the channels of the silica polymer mesostructure. The mean size of the silver nanoparticles is around 3 nm, and they are spaced from about 10 nm. They are also localized exclusively between the silica walls, occupying the previous position of the block copolymer, in this porous silica layer. The cross section (Figure 4b) shows that silver nanoparticles form a single monolayer in the first layer of Langmuir 2010, 26(11), 8729–8736

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Figure 4. TEM images of P-like films after NaBH4 treatment and before hydroxylamine growth, PNB: (a) top view and (b) cross section. TEM images of P-like films, after NaBH4 treatment and after hydroxylamine growth, PNBH: (c) top view and (d) cross section.

the mesostructured silica film, as in the case of the FNB film. After hydroxylamine growth, TEM pictures of the film PNBH are presented in Figure 4c,d. It looks like silver nanowires have been formed inside the channels (Figure 4c). Their mean size is around 10 nm
100 nm. The cross-sectional TEM image shows that silver nanowires are localized in the first monolayer of the film (Figure 4d). Some silver nanoparticles are still observed in the depth of the layer, revealing also that the diffusion of silver ions to the first monolayer is not perfect and that some nanoparticles have been retained in the layer during the very short chemical treatment (30 s). The HR TEM image presented in Figure 5a shows the (111) lattice fringes of crystallized silver on the cross section of the layer. The nanoparticles are very close to each other, and they have grown inside the silica layer, which can be clearly observed in Figure 5b. The dimensions of the silver nanoparticles from this cross-sectional view are between 10 and 14 nm, and the silica layer thickness is around 3.7 nm. It appears that what we have called silver nanowires could be more precisely described as silver nanowires with an irregular diameter. The second silver phase grows first around the silver nuclei and then extends between the swollen spherical nanoparticles when the diameter of the channel is reached. Moreover, the {111}, {200}, {220}, and {311} rings of Ag nanoparticles with CFC structure can be observed on the selected-area electron diffraction (SAED) pattern (Figure 5c). Very small silver nanoparticles with a size of around 1 nm and with a low density are formed in the PUV film (Figure 6a). They are localized both in the hydrophobic polymer part and also inside the silica walls. Because TEM grids are prepared by scrapping, we are not completely sure that some movement has not occurred during the grid preparation. Larger aggregates of about 15 nm are seen. After hydroxylamine growth, a TEM picture of PUVH is shown in Figure 6b. In this case, a few silver nanowires are inside Langmuir 2010, 26(11), 8729–8736

the mesostructure. A lot of large nanocrystals whose size is around 40 nm have appeared. Optical absorption spectroscopy spectra of the FNB and PNB films have been acquired (Figure 7). After the NaBH4 reduction, the FNB film spectrum (Figure 7a) consists of an absorption band at 437 nm that characterizes the surface plasmon resonance of silver nanoparticles. After hydroxylamine growth (Figure 7b), the absorption band is much more intense and a red shift is observed (λ = 482 nm) whereas a new absorption band at lower wavelength (λ = 370 nm) has appeared as a shoulder. This lower-wavelength band could be explained by the presence of uncoated silver nanoparticles41 whereas the red shift of the SPR may be due to dipolar coupling between particles. In the case of UV reduction, the spectrum of the FUV film (Figure 7c) consists of a broader band at λ = 425 nm with a lower absorbance. After the hydroxylamine treatment (Figure 7d), a very slight blue shift is noted (λ = 417 nm) but the absorbance remains almost unchanged. A shoulder is also observed at around λ = 370 nm. After the NaBH4 reduction, the spectrum of the PNB film (Figure 7e) consists of an absorption band at λ = 431 nm. After hydroxylamine growth, for the PNBH film (Figure 7g), the absorption band is much more intense and a blue shift is observed (λ = 396 nm), as well as a shoulder at 370 nm. By growing silver nanowires, we expected the appearance of a transverse mode and a longitudinal mode. We have noted that the longitudinal mode is not observed, maybe because these nanowires are very tortuous. Another explanation resides in the fact that what we have called a silver “nanowire” in fact consists of spherical particles embedded in a silver phase that has grown inside the channels, as shown in Figure 5b. Nevertheless, a slight increase in absorption is observed at the higher wavelengths. The intermediate stage obtained after a (41) Chatterjee, K.; Banerjee, S.; Chakravorty, D. Phys. Rev. B 2002, 66, 085421.

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Figure 6. TEM images of P-like films after photoreduction treatment: (a) before hydroxylamine growth, PUV and (b) after hydroxylamine growth, PUVH.

Figure 5. TEM images of P-like films after NaBH4 treatment and after hydroxylamine growth, PNBH: (a) HR TEM image on a cross section, (b) another HRTEM image on a cross section and (c) selected-area electronic diffraction (SAED).

30 s of hydroxylamine treatment is also shown in Figure 7f with an absorption band at 417 nm and two shoulders at 370 and 500 nm. The formation of this shoulder at around 500 nm could be explained by the appearance of a longitudinal mode. The spectrum of the PUV film (Figure 7h) consists of a broader band at λ = 398 nm with low absorbance. After hydroxylamine growth (Figure 7i), a slight blue shift is seen (λ = 395 nm) with increased absorbance. A shoulder is also noted at around λ = 370 nm. The XRD pattern reveals that after the first reduction no crystallization can be revealed (image not shown). After hydroxylamine treatment, the {111} reflection of metallic silver and the {200} reflection of cubic silver chloride are observed, respectively, at 2θ = 38, 10, and 32,25°. The appearance of a silver chloride phase results from the utilization of hydroxylamine chloride. To avoid the formation of this AgCl phase, it is possible to use the hydroxylamine solution after the exchange of chloride anions using an anionic exchange resin. In this case, the absorption spectra obtained are almost unchanged, whereas on XRD 8734 DOI: 10.1021/la904491v

patterns metallic silver is the only detected phase. The formation of large silver and silver chloride crystals, outside the film, is more important when UV photoreduction has been used for the first step. SEM analysis (Figure 8a,b) shows that many more crystals are observed for the FUVH film (Figure 8b) than for the FNBH film (Figure 8a). The SEM image of the FNBH film (Figure 8a) shows some large crystals between 100 and 200 nm and a second population of crystals at around 20 nm. In the SEM image of the FUVH film (Figure 8b), there are also some large crystals between 100 and 200 nm and a second population is also noted with size of around 30 nm; this second population almost completely covers the film surface. EDX analyses show that large crystals are due to both silver and silver chloride but smaller ones are due to metallic silver.

4. Discussion The block copolymer solubilizes the metallic salt, reduces it to small clusters, and directs the silver localization inside the silica film. The localization of silver nanoparticles is very dependent on the process used to reduce silver further: either NaBH4 treatment or photoreduction. Important silver nanoparticle seggregation is clearly observed by employing treatment with a sodium borohydride solution. In this case, silver nanoparticles are localized exclusively at the positions that were previously occupied by the polymer. If a photoreduction process is involved, then silver nanoparticles are mostly freezed inside the polymer, but because the ethylene oxide block and silica are very imbricated and the diffusion process is limited, the seggregation of silver nanoparticles is less effective. By using NaBH4 reduction to form the silver nanoparticles inside the F127 block copolymer mesostructured silica film, the F structure, an important accumulation of silver nanoparticles Langmuir 2010, 26(11), 8729–8736

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Figure 7. Photographs of F-like films, FNB and FNBH, and UV-vis absorption spectra: (a) FNB, (b) FNBH, (c) FUV, and (d) FUVH. Photographs of P-like films, PNB and PNBH, and UV-vis absorption spectra: (e) PNB, (f) PNBH after 30 s of treatment, (g) PNBH, (h) PUV, and (i) PUVH.

inside the first monolayer of the silica block copolymer mesostructure, is obtained. This phenomenon can essentially be explained by the highly reductive power of borohydride (E0(BH4-/H2) = -0.96 V at pH 10) and also to a lesser extent by its anionic character, which explains that the silica surface, which is also negatively charged, will push out this species.31,42 The rapid formation of silver nuclei occurs when borohydride reacts with the first Agþ species in the subsurface of the layer. This nucleation step is then followed by aggregative growth. The ionic silver depletion of the subsurface induces a gradient causing the diffusion of ionic species from deep within the layer toward the subsurface. Therefore, silver diffusion occurs toward the first monolayer, which accompanies the loss of the block copolymer by the dissolution31 and growth of the silver nanoparticle exclusively between the silica walls of this first monolayer. The formed silver nanocrystals can then be used as seeds for the growth of larger nanoparticles when hydroxylamine is used. Then, the growth of silver nanoparticles occurs essentially inside the first layer of the mesostructure, and silver nanocrystals have been substituted into the block copolymer position. A high silver nanoparticle size (11 nm) is obtained after hydroxylamine growth. We wonder whether the partial dissolution of the silica walls during chemical treatment39 explains such a nanoparticle size dimension. TEM examination of the cross section shows that the micelle dimension does not seem to be modified by the chemical treatment and that a silica layer always covers the silver nanoparticle layer. Consequently, if the dissolution of the silica walls in the first few sublayers is invoked to explain such a size increase, then it is a very limited process. ICP-OES experiments have confirmed that silicon losses were negligible. The silica walls of the (42) Bois, L.; Chassagneux, F.; Battie, Y.; Bessueille, F.; Mollet, L.; Parola, S.; Destouches, N.; Toulhoat, N.; Moncoffre, N. Langmuir 2010, 26, 1199–1206.

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Figure 8. SEM images of (a) an FNBH film and (b) an FUVH film.

first few sublayers are probably compressed to explain the growth of the metallic nanostructure. This growth of silver nanoparticles from 7 to 11 nm is evidenced by TEM analysis and is also revealed by visible absorption spectroscopy because the band observed at 482 nm may indicate the existence of interparticle dipole-dipole coupling.43,44 SPR simulations have been performed with coupling dipole theory. Each nanoparticle is substituted for one dipole that interacts with other dipoles.45 A cubic array composed of 15
15 silver nanoparticles with a center-to-center spacing of (43) Hutter, J. E.; Fendler, H. Adv. Mater. 2004, 16, 1685. (44) Choi, B.-H.; Lee, H.-H.; Jin, S.; Chun, S.; Kim, S.-H. Nanotechnology 2007, 18, 075706. (45) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11, 1491. Purcell, E. M.; Pennypacker, C. R. Astrophys. J. 1973, 186, 705. (46) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165.

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Figure 9. Simulations of the SPR absorption for a cubic array of 15
15 silver nanoparticles with a diameter of d nm and spaced from 12.5 nm.

12.5 nm is considered. The diameter of each nanoparticle increases from 7 to 10 nm. The result presented in Figure 9 shows that the SPR increases from 430 to 470 nm. Such a red shift of the resonance does not occur for such a size increase when the nanoparticles are far from each other. Our result seems coherent with previous work in which a red shift has been calculated for a planar array of silver nanoparticle as the array spacing decreases, for the smaller spacing (