Fabrication of Functional Silver Nanobowl Arrays via Sphere

Physikalisches Institut and Center for Nanotechnology (C198eNTech), Westfälische Wilhelms-Universität Münster, D-48149 Münster, Germany. § Depart...
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Fabrication of Functional Silver Nanobowl Arrays via Sphere Lithography Miaojun Xu,†,§ Nan Lu,*,† Hongbo Xu,† Dianpeng Qi,† Yandong Wang,† and Lifeng Chi*,†,‡ †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, P. R. China, ‡Physikalisches Institut and Center for Nanotechnology (C198eNTech), Westf€ alische Wilhelms-Universit€ at M€ unster, D-48149 M€ unster, Germany, and §Department of Chemistry, College of Science, Northeast Forestry University, 150040, Harbin, P. R. China Received June 17, 2009. Revised Manuscript Received August 31, 2009

We report a low-cost and high-throughput method to fabricate large-area silver nanobowl arrays via thermal evaporation of silver on a self-assembled monolayer of nanospheres. The nanobowl array is a hierarchical structure, composed of silver nanoparticles with average diameter size of ca.10 nm, which can serve as a reaction container and catalyst. The optical absorption spectra indicates that surface plasmon resonance of silver nanoparticles exists on the nanobowl array, and it can serve as an excellent surface enhanced Raman scattering (SERS)-active substrate.

Introduction Recently, periodic nanostructures have attracted increasing attention because of their potential applications in photonic crystals,1,2 data storage,3,4 and biosensors.5,6 Taking a selfassembled monolayer of polystyrene (PS) sphere as a template to create periodic nanostructures is one of the most effective procedures used, and structures such as nanobowl,7-11 nanorod,12 nanohoneycomb,13 and nanoring14-16 arrays as well as other nanostructures17-21 have been fabricated. Among them, periodic nanobowl structure arrays are of great interest because of their potential applications in nanoparticles selection,7 biomedical and nanofluidic devices, coercivity enhancement,10 plasmonic devices for enhancing electromagnetic fields,22 and electrocatalytic activities.23 The metallic arrays are also good candidates for surface *Corresponding author. E-mail: [email protected]; [email protected]. (1) Wanke, M. C.; Lehmann, O.; Muller, K.; Wen, Q.; Stuke, M. Science 1997, 275, 1284. (2) Kuo, C.; Shiu, J.; Cho, Y.; Chen, P. Adv. Mater. 2003, 15, 1065. (3) Hehn, M.; Ounadjela, K.; Buncher, J. P.; Rousseaux, F.; Decanini, D.; Bartenlian, B.; Chappert, C. Science 1996, 272, 1782. (4) Cheng, J. Y.; Ross, C. A.; Chan, V. Z. H.; Thomas, E. L.; Lamertink, R. G. H.; Vancso, G. J. Adv. Mater. 2001, 13, 1174. (5) Haes, A. J.; van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (6) Lee, K. B.; Park, S. J.; Mirkin, C. A.; Smirth, J. C.; Mrksich, M. Science 2002, 295, 1702. (7) Wang, X. D.; Gruagnard, E.; King, J. S.; Wang, Z. L.; Summers, C. J. Nano Lett. 2004, 4, 2223. (8) Wang, X. D.; Lao, C.; Gruagnard, E.; Summers, C. J.; Wang, Z. L. Nano Lett. 2005, 5, 1784. (9) Li, Y.; Li, C. C.; Cho, S. O.; Duan, G. T.; Cai, W. P. Langmuir 2007, 23, 9802. (10) Srivastava, A. K.; Madhavi, S.; White, T. J.; Ramanujan, R. V. J. Mater. Chem. 2005, 15, 4424. (11) Chen, T. H.; Tsai, T. Y.; Hsieh, K. C.; Chang, S. C.; Tai, N. H.; Chen, H. L. Nanotechnology 2008, 19, 465303. (12) Wang, X. D.; Neff, C.; Graugnard, E.; Ding, Y.; King, J. S.; Pranger, L. A.; Tannenbaum, R.; Wang, Z. L.; Summers, C. J. Adv. Mater. 2005, 17, 2103. (13) Kei, C. C.; Chen, T. H.; Chang, C. M.; Su, C. Y.; Lee, C. T.; Hsiao, C. N.; Chang, S. C.; Perng, T. P. Chem. Mater. 2007, 19, 5833. (14) Kosiorek, A.; Kandulski, W.; Glaczynsha, H.; Giersig, M. Small 2005, 1, 439. (15) Li, J. R.; Garno, J. C. ACS Appl. Mater. Interfaces 2009, 1, 969. (16) Sun, F. Q.; Yu, J. C.; Wang, X. C. Chem. Mater. 2006, 18, 3774. (17) Pacifica, J.; Gomez, D.; Mulvaney, P. Adv. Mater. 2005, 17, 415. (18) Sun, F.; Cai, W.; Li, Y.; Duan, G.; Nichols, W. T.; Liang, C.; Koshizaki, N.; Fang, Q.; Boyd, I. W. Appl. Phys. B: Lasers Opt. 2005, 81, 765. (19) Li, Y.; Cai, W. P.; Duan, G. T. Chem. Mater. 2008, 20, 615. (20) Trujillo, N, J.; Baxamusa, S. H.; Gleason, K. K. Chem. Mater. 2009, 21, 742. (21) Chen, X.; Wei, X.; Jiang, K. Microelectron. Eng. 2009, 86, 871. (22) Wang, S.; Pile, D. F. P.; Sun, C.; Zhang, X. Nano Lett. 2007, 7, 1076. (23) Shin, C.; Shin, W.; Hong, H. G. Electrochim. Acta 2007, 53, 720.

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enhanced Raman scattering (SERS) active substrates because of their periodic characteristics and nanosized structure. For example, metal nanoparticle arrays and metallic coating arrays have been synthesized and applied for SERS.24-26 Baumberg and coworkers27 showed that the uniform gold nanocavity structure could be used as a substrate for SERS spectroscopy, which was highly reproducible via sphere-template-directed electrochemical deposition. Dintinger and co-workers28 reported that there was a strong coupling between the surface plasmon polaritons and the J-aggregates of dye molecules in the subwavelength metallic hole arrays. So the hierarchical structure arrays with mirco- and nanoscaled building blocks are expected to present promising SERS properties. However, the generation of large-area metallic nanostructures is still limited by the high cost and low throughput of the proposed techniques. For example, the nanobowl structures are fabricated either via very expensive atomic layer deposition followed by ion-milling,7,8 or using the chemical process.9,10 Although the chemical methods can offer many advantages compared to physical methods, such as creating three-dimensional nanobowl structures, high aspect ratio nanostructures, and synthesis various alloy compositions, the experimental process is very complicated. Therefore, much effort has been devoted to developing techniques for fabricating metallic nanostructures. Herein, we present a low-cost and high-throughput method to fabricate large-area metallic nanobowl arrays via thermal evaporation of silver on a self-assembled monolayer of nanospheres. The nanobowls can serve not only as reaction containers and catalysts for a catalytic reduction reaction of nitro-compound by sodium borohydride, but also as a SERS-active substrate.

Experimental Section Materials. Monodispersed 10 wt % PS spheres with a diameter of 580 nm and 1-dodecanethiol were obtained from SigmaAldrich. Ethanol, acetone, chloroform, tetrahydrofuran (THF), sodium dodecyl sulfate (SDS), p-nitroaniline (PNA), and sodium (24) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B. 2003, 107, 7426. (25) Zhang, X.; Yonzon, C. R.; Van Duyne, R. P. J. Mater. Res. 2006, 21, 1083. (26) Lu, L.; Randjelovic, I.; Capek, R.; Gaponik, N.; Yang, J.; Zhang, H.; Eychmuller, A. Chem. Mater. 2005, 17, 5731. (27) Baumberg, J. J.; Kelf, T. A.; Sugawara, Y.; Cintra, S.; Abdelsalam, M. E.; Bartlett, P. N.; Russell, A. E. Nano Lett. 2005, 5, 2262. (28) Dintinger, J.; Klein, S.; Bustos, F.; Barnes, W. L.; Ebbesen, T. W. Phys. Rev. B. 2005, 71, 035424.

Published on Web 09/09/2009

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borohydride were purchased from commercial sources in the highest available purity. All solvents and chemicals were used without further purification. The tape used in this work is a common sticking tape, which is transparent and more flexible. Ultrapure water (18.2 MΩ 3 cm) was used directly from a Millipore System (Marlborough, France). The glass substrates of 2 cm  2 cm were cut from an ordinary glass, and were sonicated consecutively in a bath of acetone, chloroform, ethanol, and ultrapure water for 5 min each and dried with nitrogen. Then the glass substrates were treated with oxygen plasma etching on the Plasma System 100 PVA Tepla, Germany, with O2 (100 mL min-1) at a power density of 300 W for 3 min to get them thoroughly clean and kept in 10% SDS aqueous solution for 5 h. Fabrication of the PS Spheres Template. The PS spheres monolayers were prepared on cleaned glass substrates by the selfassembling process.29 Briefly, the 10 wt % aqueous suspension of PS microspheres were diluted with an equal volume of ethanol and ultrasonicated for 10 s to improve the mixing. A 2 wt % SDS solution was introduced to consolidate the particles. The prepared substrate was tilted into a glass Petri dish, which was filled with 80 mL of water, constructing the system for fabricating the PS monolayer. A 5 μL PS mixed suspension was dispersed onto the substrate. Some domains of the PS sphere monolayer were initially formed on the water surface, and, after adequately standing, the highly ordered PS monolayer was dip coated onto the substrate from the water surface by draw-backing the substrate. After being dried at room temperature, the highly ordered template of the PS spheres was fabricated onto a glass substrate. Deposition of Silver Film. A silver film of 300 nm in thickness was deposited on the template by using a commercial thermal evaporation system at a pressure of 5  10-4 Pa (Shenyang City Keyou Institute of Vacuum Technology, China). Reduction Reaction of PNA. To ensure the reaction only occurs inside the nanobowls, the edges of the nanobowls were initially modified with 1-dodecanethiol by immersing the substrate-bearing silver nanobowl arrays into a 0.5 mM 1-dodecanethiol ethanol solution. After the formation of the Ag-S bond between silver and 1-dodecanethiol, the PS spheres were removed with THF. The modified nanobowl array substrate was first immersed in a 1 mM PNA ethanol solution for 1 h and then thoroughly rinsed with ethanol. Finally, the arrayed substrate was immersed in a 1 mM sodium borohydride aqueous solution for 1 h and completely rinsed with water. Characterization. Atomic force microscopy measurements were taken on a Multimode Nanoscope IIIa instrument (Digital Instrument, Santa Barbara, CA) operating in tapping mode with silicon cantilevers (resonance frequency in the range 280340 kHz). Scanning electron microscope (SEM) micrographs were taken with a JEOL JSM 6700F field-emission SEM with a primary electron energy of 3 kV, and the samples are sputtered with a layer of Pt (ca. 2 nm thick) prior to observation to improve conductivity. Optical absorption spectra were measured on a Shimadzu UV-3600 spectrometer. SERS spectra were measured on a Nicolet 960 FT-Raman spectrometer equipped with a liquidnitrogen-cooled Ge detector and a Nd:VO4 laser (1064 nm) as the excitation source (the laser power used was about 250 mW at the samples) and a Renishaw Raman system model 1000 spectrometer with the 514.5 nm argon ion laser exciting source (the laser power at the sample position was typically 400 μW with an average spot size of 1 μm in diameter). The spectral resolution was 4 cm-1 at the excitation wavelength. The typical accumulation time used in this study was 30 s.

Results and Discussion The fabrication procedure of a silver nanobowl array is schematically illustrated in Figure 1. According to the ref 29, a (29) Liz-Marzan, L. M.; Giersig, M. Low-Dimensional Systems: Theory, Preparation, and Some Applications; Kluwer Academic Publishers, Dordrecht, The Netherlands, 2003; pp 163-172.

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Figure 1. Schematic illustration of the procedure for fabricating a silver nanobowl array.

monolayer of PS spheres (580 nm in diameter) was fabricated on a glass substrate by the self-assembly technique (Figure 1a). The self-assembled PS spheres were hexagonally ordered in long-range on the glass substrate, which could be confirmed by the atomic force microscopy measurement (See Supporting Information, Figure S1). A 300 nm-thick film of silver was directly deposited on the template by thermal evaporation (Figure 1b). Then the silver film was peeled off together with PS spheres with sticking tape (Figure 1c) and immersed into a THF solution to dissolve the PS spheres. Finally, a larger area of the silver nanobowl array was obtained, as presented in Figure 1d. The SEM images shown in Figure 2 demonstrate the results of each step described above. Figure 2a shows the monolayer of selfassembled PS spheres coated with the silver film, which is composed of silver particles produced by the thermal evaporation process. The spacing between the spheres is partially filled as a result of the silver evaporation. Subsequently, the PS spheres coated with silver were peeled off from the glass substrate by applying a sticking tape on top of the silver film. A top view of the transferred PS spheres and silver film on the tape is shown in Figure 2b. After removing the PS spheres, the silver nanobowl array was created, as demonstrated in Figure 2c. The nanobowl is composed of silver nanoparticles with an average size of ca. 10 nm, as depicted in Figure 2d. During the thermal evaporation process, the silver particles passed through the spacing between the spheres and deposited onto the glass substrate when the deposition speed was low (0.1 nm s-1). Thus, the nanobowls are in hexagonal arrangement, and every nanobowl has six symmetric triangular protrusions of silver particles (see Supporting Information, Figure S2a). On the contrary, only a few silver particles can deposit onto the glass substrate as a result of the spacing between the spheres being filled with too much silver particle and being quickly blocked when the evaporation speed is high (0.3 nm s-1), leading to few triangular films generated (See Supporting Information, Figure S2b) Netti and Coyle30,31 have demonstrated that the metal cavity effectively couples with incident light and completely laterally confines the surface plasmons, thus multiple plasmon modes covering a wide spectral range may be excited. Figure S3 (Supporting Information) presents a comparison of the optical absorption spectra of the silver nanobowl array film (line a) and the silver film on a glass substrate (line b) prepared with the same vacuum deposition conditions. The unique (30) Netti, M. C.; Coyle, S.; Baumberg, J. J.; Ghanem, M. A.; Birkin, P. R.; Bartlett, P. N.; Wittaker, D. M. Adv. Mater. 2001, 13, 1368. (31) Coyle, S.; Netti, M. C.; Baumberg, J. J.; Ghanem, M. A.; Birkin, P. R.; Bartlett, P. N.; Whittaker, D. M. Phys. Rev. Lett. 2001, 87, 176801.

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Figure 2. SEM images of (a) the monolayer of PS spheres coated with silver, (b) the silver-coated PS spheres transferred onto a tape, (c) the generated silver nanobowl array on the tape, and (d) an enlarged SEM image of the nanobowls.

“fingerprint” for quasispherical silver nanoparticles can only be observed on line a, which is at 410 nm and originates from the well-known surface plasmon resonance of silver nanoparticles.32 The morphology of the nanobowl depicted in Figure 2d also confirmed that the nanobowls were formed by the quasispherical silver nanoparticles. The silver film on the glass substrate obtained with the same vacuum physical vapor deposition is relatively flat, and the silver particles are aggregated together, as revealed in Figure S4. With the same deposition conditions, the density of the silver particles deposited on the glass substrate is higher than that of those deposited inside the nanobowls because the surface area of the nanobowls is larger than the flat glass substrate. The aggregation of silver particles decreased the surface roughness of the silver film on the glass substrate. As a result, surface plasmon resonance of the flat silver film does not occur. Previous investigations have proven that the metal nanoparticles are excellent catalyst in organic synthesis, fuel cell and pollution treatments because of their quantum effect, high surface-tovolume ratio, and surface energy.33-35 By using impregnation and (32) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B. 2003, 107, 668. (33) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (34) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B. 2005, 109, 12663. (35) Haruta, M. Chem. Rec. 2003, 3, 75.

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photoreduction techniques, Kundu et al.36 prepared silver nanoparticles on a silica solid matrix, and the silver nanoparticles showed excellent catalytic activity for the reduction of nitrocompounds. To investigate the function of the nanobowls, the catalytic reduction reaction of PNA by sodium borohydride was carried out inside the nanobowl arrays, and the SERS spectra of the reactant and product molecules were measured. The silver nanoparticles were employed for the catalytic reduction of PNA by NaBH4 due to the simplicity and high efficiency of the reaction with the presence of metallic surfaces.37 This reaction can not happen without metallic catalysts, even in a period of several days. To elucidate the catalysis and SERS activity of the fabricated silver nanobowl array, the SERS spectra of the PNA molecules and the product molecules p-phenylenediamine of catalytic reduction PNA adsorbed on the silver nanobowl array were measured. For comparison, the results of Raman spectra of the solid PNA sample and the PNA molecules adsorbed on a silver film prepared on the glass substrate by the vacuum physical vapor deposition were also collected (See Figure 3). As revealed in line a of Figure 3, the spectrum of the reagent of the solid PNA sample exhibits dominant bands at 1590, 1508, 1108, and 864 cm-1. The bands at (36) Kundu, S.; Mandal, M.; Ghosh, S. K.; Pal, T. J. Colloid Interface Sci. 2004, 272, 134. (37) Zhou, Q.; Qian, G. Z.; Li, Y.; Zhao, G.; Chao, Y. W.; Zheng, J. W. Thin Solid Films. 2008, 516, 953.

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Figure 3. The Raman spectra excited at 514.5 nm of a solid PNA sample (a), PNA molecules absorbed in the silver nanobowl array (b), the product molecules of p-phenylenediamine of the catalytic reduction of PNA absorbed in the silver nanobowl array (c), and the Raman spectra (multiplied 102 times) of the PNA molecules adsorbed on a rough silver film obtained by thermal evaporation deposition on the glass substrate (d).

1590 and 1108 cm-1 can be assigned to the C-C stretching and C-H in-plane bending, respectively, and the bands at 1508 and 864 cm-1 can be assigned to the -NO2 asymmetric stretching and bending, respectively.38,39 Spectrum line b shown in Figure 3 demonstrates that the intensity strongly increased at 1606, 1536, 1388, 1318, and 1148 cm-1 when the PNA molecules absorbed on the silver nanobowl array. Furthermore, their positions shifted to higher wave numbers. The SERS behaviors of the PNA molecules adsorbed on silver nanoparticles have been well-documented in the literature.40 The bands have been assigned to the b2 modes of the PNA molecules and can only be enhanced by the chargetransfer mechanism.41 As presented in Figure 2d, the surface of the nanobowls is composed of silver nanoparticles with average size of ca. 10 nm and very rough. It has been suggested that the enhancement of the b2 modes of the PNA molecules is largely dependent on the nanosized structure of silver and the assembly structure of the silver nanoparticles. The SERS spectrum of the product of the catalytic reduction of PNA (Figure 3, line c) shows a great change in the feature band compared with that of the PNA absorbed on the silver nanobowl array (Figure 3, line b). The bands at 1536 and 851 cm-1 related to the -NO2 asymmetric stretching and bending disappeared, which suggests that the PNA has been catalytic reduced completely. The silver nanoparticles function as the electron relay between the adsorbed PNA molecules and sodium borohydride.37 So the reaction only occurs on the surface of the silver nanoparticles for the electron transfer process during the reduction from the reagent PNA to the product p-phenylenediamine. The SERS signal (multiplied 102 times) of the reagent PNA molecules absorbed on the silver film obtained on the glass substrate is very weak, as revealed in line d in Figure 3; however, that of those absorbed on the hierarchical structured surface exhibits a very strong SERS signal (see Figure 3, line b). The (38) Tanaka, T.; Nakajima, A.; Watanabe, A.; Ohno, T.; Ozaki, Y. J. Mol. Struct. 2003, 661, 437. (39) Ma, W.; Fang, Y. J. Colloid Interface Sci. 2006, 303, 1. (40) Miranda, M. M.; Neto, N. Colloids Surf., A 2004, 249, 79. (41) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935.

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strong SERS signal of the silver nanobowl array can be attributed to the bowl-like structure, the periodic structure, and the hierarchical surface roughness. First, as a result of the excitation of the metal surface plasmon and the confinement of the silver cavity to the electromagnetic field, the electromagnetic field inside the silver cavities should be greater than the incident field. The enhancement of the Raman scattering of the molecules therefore could be similar to or greater than that obtained on the spherical metal particles. This point has recently been demonstrated by Baumberg and co-workers.27 Second, according to Gaponenko,42 the redistribution of photon density of states may easily occur in the periodic structure, which has also been proved by the calculations,43 resulting in an increase of the density of optical modes and thus the enhancement of the Raman scattering of the detected molecules. Third, the nanoscaled surface roughness and the size of silver particles in the nanobowl array could bring stronger SERS, because the UV-visible absorption band at about 410 nm only can be observed on the Ag nanobowl array film, which originates from the surface plasmon resonance of silver nanoparticles.

Conclusion In conclusion, we present a simple technique for fabricating functional silver nanobowl arrays by thermally evaporating silver on the sphere monolayer-masked substrate. This method provides an alternative approach to produce nanobowl arrays on large areas with low cost and high throughput; the size and period of the nanobowl array can be adjusted by varying the size of the PS spheres. The materials for generating nanobowls can be readily extended to a wide range. The whole nanobowl array is composed of silver nanoparticles with an average diameter size of ca. 10 nm, which allows for application as the reaction container and catalyst. The optical absorption spectra indicate that surface plasmon resonance of silver nanoparticles exists on the nanobowl (42) Gaponenko, S. G. Phys. Rev. B. 2002, 65, 140303. (43) Zuev, V. S.; Frantsesson, A. V.; Gao, J.; Eden, J. G. J. Chem. Phys. 2005, 122, 214726.

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array. The Raman spectra measurement shows that the nanobowl array is not only an excellent catalyst, but also an excellent SERSactive substrate. Acknowledgment. Financial support was given by the National Natural Science Foundation of China (20773052, 20373019), the Program for New Century Excellent Talents in

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University, the National Basic Research Program (2007CB808003, 2009CB939701), and the Program 111. Supporting Information Available: The process of the reduction reaction of PNA by sodium borohydride and SERS measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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