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Synthesis of one-dimensional silver oxide nanoparticle arrays and silver nanorods templated by Langmuir monolayers
Journal of Colloid and Interface Science 314 (2007) 297–303 www.elsevier.com/locate/jcis
Synthesis of one-dimensional silver oxide nanoparticle arrays and silver nanorods templated by Langmuir monolayers Hong-Guo Liu a,∗ , Fei Xiao a , Chang-Wei Wang a , Qingbin Xue a , Xiao Chen a , Yong-Ill Lee b , Jingcheng Hao a , Jianzhuang Jiang a a Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China b Department of Chemistry, Changwon National University, Changwon 641-773, South Korea
Received 9 February 2007; accepted 17 May 2007 Available online 24 May 2007
Abstract One-dimensional (1D) silver oxide nanoparticle arrays were synthesized by illuminating the composite Langmuir–Blodgett monolayers of porphyrin derivatives/Ag+ and n-hexadecyl dihydrogen phosphate (n-HDP)/Ag+ deposited on carbon-coated copper grids with daylight and then exposing them to air. Transmission electron microscopy (TEM) observation shows that the nanoparticle size is around 3 nm, with the separation of about 2–3 nm. High-resolution TEM (HRTEM) investigation indicates that the particles are made up of Ag2 O. Ag nanorods with the width of 15–35 nm and the length of several hundreds of nanometers were synthesized by irradiating the composite Langmuir monolayers of porphyrin derivatives/Ag+ and n-HDP/Ag+ by UV-light directly at the air/water interface at room temperature. HRTEM image and selected-area electron diffraction (SAED) pattern indicate that the nanorods are single crystals with the (110) face of the face-centered cubic (fcc) silver parallel to the air/water interface. The formation of the 1D arrays and the nanorods should be attributed to the templating effect of the linear supramolecules formed by porphyrin derivative or n-HDP molecules in Langmuir monolayers through non-covalent interactions. © 2007 Elsevier Inc. All rights reserved. Keywords: Ag nanorod; 1D array; Ag2 O nanoparticle; Langmuir monolayer; Air–water interface
1. Introduction 1D ordered arrays of metallic and semiconductor nanoparticles have aroused much attention recently due to their unique electronic and optical properties different from single nanoparticle, such as surface plasmon resonance coupling, arising from the interaction between the nanoparticles. These 1D chains can serve as a “plasmon waveguide,” and have potential applications in optoelectronic nanodevices [1,2]. Various methods have been developed to synthesize and fabricate these ordered arrays, including template and template-free methods [3]. Two strategies have been adopted to prepare 1D arrays of metal nanoparticles by using of template method. One is to fabricate the preformed nanoparticles onto the linear templates, includ* Corresponding author. Fax: +86 531 88564750.
E-mail address: [email protected] (H.-G. Liu). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.05.048
ing biological substrates of DNA [4–6], RNA [7], and peptides [8], polymers [9–11], and carbon nanotubes [12], etc. via weak interactions and covalent bonds [11]. The other is to reduce the precursors that are combined with the linear templates [13–15]. Alternatively, anisotropic 1D metallic nanoparticles, such as nanorods [16–21], nanowires [22–25], and nanotubes [26–28] have attracted a great deal of attention, too, due to their peculiar properties and potential applications. Most of these 1D metallic nanoparticles were synthesized in solution through epitaxial growth in the presence of surfactants or templates and through oriented attachment of nanoparticles and ripening in the presence of templates. Synthesis and assembly 1D arrays and anisotropic nanoparticles at the interfaces are convenient for their practical applications in nanodevices. Langmuir monolayer technique is a versatile method to prepare and assembly nanoparticles at the interfaces, which falls into two categories: synthesis and/or as-
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sembly directly at the air/water interface, and treatment of the transferred Langmuir–Blodgett (LB) monolayer and multilayers. For example, anisotropic nanocrystals, such as PbS triangular nanoplates [29] and CdS nanorods [30] were synthesized at the air/water interface, and 1D arrays of metal sulfide nanoparticles [31,32] and 2D ordered arrays of silver nanoparticles [33] were synthesized and assembled at the air/water interface in a one-step process. In addition, Tl2 O3 nanowires were fabricated at the air/water interface by assembling the Tl2 O3 nanoparticles in the subphase onto the linear polymer templates [34], and 2D ordered arrays of semiconductor or metallic nanoparticles were fabricated by spreading the surfactant-modified nanoparticles on pure water surface [35–38]. For the treatment of the transferred films, ordered layered structures of semiconductors [39] and pyramid-shaped nanocrystals [40] were produced by treating the ordered LB films. However, most work focuses on 2D assemblies of nanocrystals or 2D nanoplates. The study on synthesis and assembly of 1D chains and 1D nanoparticles is very rare by Langmuir monolayer technique [31,32,41]. As revealed in our previous studies, porphyrin derivatives [42,43] and n-HDP [44] can form linear supramolecules at the air/water interface through non-covalent π–π attractions and hydrogen bonds, respectively. These supramolecules were applied as templates to prepare silver oxide nanoparticle 1D arrays and silver nanorods in this paper, by irradiating the composite LB monolayers on copper grids transferred from the air/AgNO3 aqueous solution interface and the composite monolayers at the air/AgNO3 aqueous solution interface, respectively. 2. Experimental 2.1. Chemicals Porphyrin derivatives TAPP, TDPP and TSPP (Chart 1) and n-HDP were synthesized according to the literature methods [45,46], respectively. Ethyl stearate (ES) was purchased from Aldrich. AgNO3 (99.9%) was purchased from Shanghai Reagent Co. The water used is highly purified with the resistivity 18.0 M cm. All chemicals were used as received without further purification.
Chart 1. Molecular structures of TAPP, TDPP, and TSPP.
2.2. Formation and characterization of 1D arrays of silver oxide nanoparticles and silver nanorods TAPP, TDPP, TSPP, and HP/ES (1:2) were dissolved in chloroform/DMF (v/v: 80/20), chloroform/ethanol (v/v: 95/5), chloroform, and chloroform, respectively. It was reported that n-HDP cannot form stable monolayer at the air/water interface alone [44,46]. In fact, n-HDP can dissolve in water, and the Gibbs monolayers were studied recently [47–49]. So the mixed system of n-HDP with ES (1:2 in molar ratios) was chosen. Aqueous solution of AgNO3 with the concentration of 1 × 10−4 mol L−1 was used as subphase. Langmuir monolayers were formed by spreading the corresponding solutions on the subphase surface filled in a NIMA 611 trough and compressing the monolayers to 20 mN m−1 after evaporating the organic solvents. For the formation of 1D arrays, the composite monolayers were transferred onto carbon-coated 230-mesh copper grids by using Langmuir–Schaefer method, and then illuminated with natural daylight for several hours and stored in a dark place. The samples were characterized by TEM (JEM100CXII, accelerating voltage of 100 kV) and HRTEM (GEOL2010, accelerating voltage of 200 kV). For the formation of Ag nanorods, the Langmuir monolayers of TSPP/Ag+ and nHDP/ES/Ag+ at 20 mN m−1 at the air/water interface were illuminated by UV-light (λ = 254 nm) directly for several minutes or hours, then transferred onto copper grids and investigated through TEM and HRTEM. 3. Results and discussion 3.1. 1D ordered arrays Under a TEM, large numbers of linear arrays were observed, as shown in Fig. 1. The arrays may extend for lengths of from 100 to a few hundreds of nanometers and are made up of discrete nanoparticles. The diameter of the particles is found to be 3.32 ± 0.82, 3.54 ± 1.11, 2.86 ± 0.84, and 3.07 ± 1.29 nm for TAPP, TDPP, TSPP, and n-HDP/ES monolayers, respectively, and the interparticle distance is ca. 2–3 nm. The average diameter of the particles in TSPP monolayer is less than those in TDPP, TAPP, and n-HDP/ES monolayers, which should be attributed to different mean Ag+ densities in these monolayers. The HRTEM image in Fig. 2 shows clear lattice fringes. To our great surprise, the lattice distance is measured to be 0.271 nm, which is close neither to 0.236 nm, the interplanar distance of {111} facets, nor to 0.293 nm, the interplanar distance of {110} facets of fcc Ag, indicating that the formed nanoparticles are not composed of Ag(0). However, the measured lattice spacing is very close to 0.272 nm, the calculated lattice spacing between the {111} facets of cubic Ag2 O based on the lattice parameter of 0.4726 nm [50], indicating that the nanoparticles in the 1D chains may be composed of Ag2 O. It was reported that the transformation between silver and silver oxide occurs facilely when exposing silver to air and illuminating silver oxide by light, respectively [51], especially for the smaller nanoparticles without capping agents. Based
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Ag nanoparticles transformed to Ag2 O nanoparticles when the samples were preserved in a dark place and exposed to air. The 1D chain structure keeps during the transformation process. The IFFT image shown in Fig. 2 also reveals the crystalline structure of the particles. 3.2. Nanorods
Fig. 1. 1D arrays of silver oxide nanoparticles templated by TAPP, TDPP, TSPP, and n-HDP–ES Langmuir monolayers and the corresponding size distribution histograms.
When the composite Langmuir monolayers were irradiated directly by UV-light at the air–water interface, nanorods were produced instead of 1D chains. Fig. 3 shows the morphologies and the formation procedure of the nanorods under TSPP Langmuir monolayers. After 1 min irradiation, scattered nanoparticles appear (not shown); 5 min later, 1D nanostructures made up of loosely packed nanocrystals appear, which serve as the “embryos” for nanorods (Fig. 3a); 15 min later, nanorods with the width of ca. 17 nm appear (Fig. 3b). The width of the nanorods falls in 15–35 nm within the irradiation time from 15 min to 3 h, and the aspect ratio is more than 20. The SAED pattern shown in Fig. 3g gives distinct diffraction rings that could be indexed to (111), (200), (220), and (311) faces of fcc Ag, respectively. Under the same experimental conditions, Ag nanorods can be also prepared under n-HDP/ES Langmuir monolayers (Fig. 3h). However, no nanorod forms without Langmuir monolayer of the porphyrin derivatives or n-HDP (Fig. 3i). Fig. 4 shows the HRTEM micrographs of the nanorods, together with the corresponding ED pattern. It can be seen from Fig. 4a and the enlarged image in Fig. 4b that 2D lattice appears clearly, indicating that single crystalline nature of the nanorods. The interplanar distances were measured to be 0.231, 0.200, and 0.148 nm, as labeled in Fig. 4b, corresponding to the spacings between {111}, {200}, and {220} facets of fcc Ag crystal, respectively. The nanorods grow along [100] direction. The diffraction spots corresponds to (111) and (200) reflections, as labeled in Fig. 4c, indicating that (110) is the basal plane, which is parallel to the air–water interface. It can be also seen from the micrograph that there are some atom-level grooves parallel to (111) faces in the (110) surface of the nanorod. The larger grooves have the width of ca. 1 nm and the depth of ca. 5–6 Å, as shown in the HRTEM micrograph (Fig. 4d) and the enlarged image (Fig. 4e). As we know, the surface free energies of the crystal faces of fcc crystals obey the following order, σ (110) > σ (100) > σ (111). It was reported that strips or grooves would appear in (110) plane through surface reconstruction in order to lower the surface energy, the new produced faces are (111) ones [52]. 3.3. Formation mechanisms
Fig. 2. HRTEM image and IFFT pattern of the nanoparticles in 1D arrays.
on the HRTEM result and the related literatures, we can infer that Ag nanoparticles formed first when the composite LB monolayers were irradiated by natural daylight, then the formed
Amphiphilic porphyrin derivatives can form stable Langmuir monolayers at the air–water interface [43]. Although nHDP cannot form stable Langmuir monolayers at the air–water interface by itself, the stable composite Langmuir monolayers of n-HDP/ES can be formed due to the hydrophobic interactions between the long alkyl chains in the two components [44]. According to our previous studies [43,44], the porphyrin and n-HDP molecules form linear supramolecular structures in the
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Fig. 3. TEM micrographs of nanorods formed under TSPP (a–f) and n-HDP/ES (h) monolayers by UV-light irradiation and the corresponding ED pattern (g), and TEM image of particles formed at the air–pure AgNO3 aqueous solution interface (i).
monolayers due to the π–π interactions between the porphyrin rings and hydrogen bonds between P=O and P–OH groups, respectively, as shown in Scheme 1. In this study, the soft supramolecular systems were employed as templates. Fig. 5 shows the π–A isotherms of TSPP and n-HDP/ES (1:2) at pure water and AgNO3 aqueous solution surfaces. It can be seen that the molecular areas decrease at the air–aqueous solution interfaces and the monolayers become more condensed compared with those at the air–pure water interface, indicating the interaction between the amphiphilic molecules and Ag+ ions. It is very interesting that different ordered structures can be obtained under different experimental conditions. The formation mechanisms were illustrated in Scheme 2. Because one carboxylic group in the porphyrin molecule or one hydroxyl group in the n-HDP molecule just binds one Ag+ , the number of Ag+ ions in the composite Langmuir–Blodgett monolayers transferred on the copper grids is limited. When the composite monolayers were illuminated by daylight, Ag+ transformed to silver atoms, resulting in the creation of (Ag)n clusters, which arrange along the linear supramolecules, leading to produce 1D linear chains
made up of discrete Ag nanoparticles, as shown in Scheme 2A. The transformation of Ag to Ag2 O takes place when exposing the nanostructures to air. However, when UV-light was used to illuminate the Langmuir monolayers at the air–water interface, Ag+ ions were reduced to Ag atoms rapidly, resulting in the creation of much more nuclei at the beginning of the reaction that align along the linear porphyrin supramolecules, as shown in Fig. 3a. More and more Ag atoms form with time because of sufficient supply of Ag+ ions from the subphase, leading to rapid growth of the particles. Due to the direction of the linear structure of porphyrin supramolecules, particle epitaxial growth, oriented attachment of the particles, and possible ripening process, nanorods were obtained at last, as shown in Scheme 2B. It is reasonable to propose these processes involved in the nanorod formation. As can be seen from Figs. 3a and 3e, the lengths of the “embryo” and the final nanorod are several tens of nanometers and more than one micrometer, respectively, the former is much less than the latter. Maybe at the early stage of the nanorod formation, the linear supramolecules play a key
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Fig. 5. π –A isotherms of TSPP (a, b) and n-HDP/ES (c, d) at the air–water (a, c) and air–AgNO3 aqueous solution interfaces (b, d).
Fig. 4. HRTEM image and SAED pattern of the nanorod formed under TSPP Langmuir monolayer.
Scheme 2. Formation mechanisms of 1D nanoparticle arrays (A) and nanorods (B).
Scheme 1. Linear supramolecular structures formed at the air–water interface by TAPP, TSPP, and n-HDP.
role in directing the nucleation and growth of the nanoparticles. During the particle growth process, the nanoparticles attached to each other and fuse to form a shorter rod. Fig. 6 gives a HRTEM micrograph of a nanorod, in which a defect can be clearly seen, probably arising from the incomplete fusion. After the shorter rod forms, the nanorod grows according to its intrinsic crystal symmetry, with the adsorption of the amphiphilic molecules to reduce the higher surface energy to stabilize the nanoparticles. It was demonstrated that epitaxial growth and attachment/fusion are the main possible processes for the nanorod formation. In general, two kinds of metal nanorods have been prepared, which grow along [110] or [100] direction with the side faces bounded by {100} [16,53] or {110} facets [54–56], respectively. It was said that the presence of the {110} facets is a
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Fig. 6. HRTEM image of the nanorod formed under TSPP Langmuir monolayer.
unique feature of metal nanorods and is not commonly observed because of its higher surface energy [52]. In the present case, the silver nanorods with the {110} basal planes were produced at the air–water interface. It seems that the adsorption of 1D supramolecules on the Ag nanoparticles not only directs the growth of the nanoparticles with a certain geometry, but also stabilizes the surface atoms of the {110} facets. Because {110} facets have the highest surface energy among the crystal facets of {100}, {111}, and {110}, the atoms on these surfaces could form stronger bonds with the supramolecules at the air–water interface. The preferential adsorption results in the formation of the (110) plane parallel to the interface. Wang et al. investigated the structure of the less stable higher energy {110} surfaces of gold nanorods. It was found that these surfaces exhibit the missing-row reconstruction, rows of atoms are missing along [110], the surface shows an irregular “saw-tooth” structure, the (110) surface transforms into strips of {111} facets [52]. Klabunde et al. also reported similar result [57]. Similar phenomena were observed in this paper. As shown in Figs. 4 and 5, the strips or grooves arising from the missing-row reconstruction appeared directly on the (110) surface. 4. Conclusion 1D arrays of metal or semiconductor nanoparticles and 1D anisotropic nanoparticles can be prepared via interfacial reaction at the interfaces templated by linear supramolecules formed by amphiphilic molecules through weak interactions at the air–water interface. It is a facile way to prepare various nanostructures by design and create supramolecules in Langmuir monolayers. Acknowledgments The authors thank the financial support of Education Ministry of China, and NSFC (20373025, 20304006 and 20428101). References [1] Y. Kang, K.J. Erickson, T.A. Taton, J. Am. Chem. Soc. 127 (2005) 13800. [2] S. Lin, M. Li, E. Dujardin, C. Girard, S. Mann, Adv. Mater. 17 (2005) 2553. [3] Z. Tang, N.A. Kotov, Adv. Mater. 17 (2005) 951.
[4] A. Ongaro, F. Griffin, P. Beecher, L. Nagle, D. Lacopino, A. Quinn, G. Redmond, D. Fitzmaurice, Chem. Mater. 17 (2005) 1969. [5] G. Wang, R.W. Murray, Nano Lett. 4 (2004) 95. [6] Z. Deng, Y. Tian, S.-H. Lee, A.E. Ribbe, C. Mao, Angew. Chem. Int. Ed. 44 (2005) 3582. [7] A.Y. Koyfman, G. Braun, S. Magonov, A. Chworos, N.O. Reich, L. Jaeger, J. Am. Chem. Soc. 127 (2005) 11886. [8] L.-s. Li, S.I. Stupp, Angew. Chem. Int. Ed. 44 (2005) 1833. [9] H. Lee, S.H. Choi, T.G. Park, Macromolecules 39 (2006) 23. [10] C. Minelli, C. Hinderling, H. Heinzelmann, R. Pugin, M. Liley, Langmuir 21 (2005) 7080. [11] Q. Dai, J.G. Worden, J. Trullinger, Q. Huo, J. Am. Chem. Soc. 127 (2005) 8008. [12] M.A. Correa-Duarte, J. Perez-Juste, A. Sanchez-Iglesias, M. Giersig, L.M. Liz-Marzan, Angew. Chem. Int. Ed. 44 (2005) 4375. [13] L. Berti, A. Alessandrini, P. Facci, J. Am. Chem. Soc. 127 (2005) 11216. [14] T.M. Day, P.R. Unwin, N.R. Wilson, J.V. Macpherson, J. Am. Chem. Soc. 127 (2005) 10639. [15] D.S. Kim, T. Lee, K.E. Geckeler, Angew. Chem. Int. Ed. 45 (2006) 104. [16] C.J. Johnson, E. Dujardin, S.A. Davis, C.J. Murphy, S. Mann, J. Mater. Chem. 12 (2002) 1765. [17] B. Nikoobakht, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) 8635. [18] F. Kim, J.H. Song, P. Yang, J. Am. Chem. Soc. 124 (2002) 14316. [19] C.J. Orendorff, P.L. Hankins, C.J. Murphy, Langmuir 21 (2005) 2022. [20] O.R. Miranda, T.S. Ahmadi, J. Phys. Chem. B 109 (2005) 15724. [21] A.J. Mieszawska, F.P. Zamborini, Chem. Mater. 17 (2005) 3415. [22] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165. [23] X. Sun, Y. Li, Adv. Mater. 17 (2005) 2626. [24] Z. Wang, J. Liu, X. Chen, J. Wan, Y. Qian, Chem. Eur. J. 11 (2005) 160. [25] J.-U. Kim, S.-H. Cha, K. Shin, J.Y. Jho, J.-C. Lee, Adv. Mater. 16 (2004) 459. [26] P. Gao, C. Zhan, M. Liu, Langmuir 22 (2006) 775. [27] T. Sehayek, M. Lahav, R. Popovitz-Biro, A. Vaskevich, I. Rubinstein, Chem. Mater. 17 (2005) 3743. [28] Y. Sun, B. Mayers, T. Herricks, Y. Xia, Nano Lett. 3 (2003) 955. [29] J. Yang, F.C. Meldrum, J.H. Fendler, J. Phys. Chem. 99 (1995) 5500. [30] J. Yang, J.H. Fendler, J. Phys. Chem. 99 (1995) 5505. [31] A. Berman, N. Belman, Y. Golan, Langmuir 19 (2003) 10962. [32] N. Belman, Y. Golan, A. Berman, Cryst. Growth Des. 5 (2005) 439. [33] A. Swami, P.R. Selvakannan, R. Pasricha, M. Sastry, J. Phys. Chem. B 108 (2004) 19269. [34] J.F. Liu, K.Z. Yang, Z.H. Lu, J. Am. Chem. Soc. 119 (1997) 11061. [35] K.M. Gattas-Asfura, C.A. Constantine, M.J. Lynn, D.A. Thimann, X. Ji, R.M. Leblanc, J. Am. Chem. Soc. 127 (2005) 14640. [36] X. Ji, C. Wang, J. Xu, J. Zheng, K.M. Gattas-Asfura, R.M. Leblanc, Langmuir 21 (2005) 5377. [37] F. Kim, S. Kwan, P. Yang, J. Am. Chem. Soc. 123 (2001) 4360. [38] S. Acharya, S. Efrima, J. Am. Chem. Soc. 127 (2005) 3486. [39] S. Guo, L. Konopny, R. Popovitz-Biro, H. Cohen, H. Porteanu, E. Lifshitz, M. Lahav, J. Am. Chem. Soc. 121 (1999) 9589. [40] H. Gong, M. Yin, M. Liu, Langmuir 19 (2003) 8280. [41] B.M. Rybak, M. Ornatska, K.N. Bergman, K.L. Genson, V.V. Tsukruk, Langmuir 22 (2006) 1027. [42] H.-G. Liu, D.-J. Qian, Q.-B. Xue, X.-S. Feng, K.-Z. Yang, Langmuir 16 (2000) 5079. [43] H.-G. Liu, X.-S. Feng, L.-J. Zhang, G.-L. Ji, D.-J. Qian, Y.-I. Lee, K.-Z. Yang, Mater. Sci. Eng. C 23 (2003) 585. [44] H.-G. Liu, X.-S. Feng, Q.-B. Xue, G.-L. Ji, S.-Y. Wang, X.-Y. Wang, D.-J. Qian, K.-Z. Yang, Mater. Lett. 58 (2004) 688. [45] A. Ruaudel-Teixier, A. Barraud, B. Belbeoch, M. Rouillay, Thin Solid Films 99 (1983) 33. [46] W. He, C. Jiang, F. Liu, Z. Tai, Y. Liang, Z. Guo, L. Zhu, J. Colloid Interface Sci. 246 (2002) 335. [47] Md.M. Hossain, K.-I. Iimura, T. Kato, J. Colloid Interface Sci. 306 (2007) 391.
H.-G. Liu et al. / Journal of Colloid and Interface Science 314 (2007) 297–303
[48] Md.M. Hossain, K.-I. Iimura, T. Kato, J. Colloid Interface Sci. 298 (2006) 348. [49] Md.M. Hossain, T. Suzuki, T. Kato, Colloids Surf. A 284–285 (2006) 119. [50] C.-M. Li, I.M. Robertson, M.L. Jenkins, J.C. Hutchison, R.C. Doole, Micron 36 (2005) 9. [51] L.A. Peyser, A.E. Vinson, A.P. Bartko, R.M. Dickson, Science 291 (2001) 103. [52] Z.L. Wang, R.P. Gao, B. Nikoobakht, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) 5417.
303
[53] H.-Y. Wu, H.-C. Chu, T.-J. Kuo, C.-L. Kuo, M.H. Huang, Chem. Mater. 17 (2005) 6447. [54] Z.L. Wang, M.B. Mohamed, S. Link, M.A. El-Sayed, Surf. Sci. 440 (1999) L809. [55] P.L. Gai, M.A. Harmer, Nano Lett. 2 (2002) 271. [56] J. Hernadez, J. Solla-Gullon, E. Herrero, A. Aldaz, J.M. Feliu, J. Phys. Chem. B 109 (2005) 12651. [57] S.I. Stoeva, V. Zaikovski, B.L. Prasad, P.K. Stoimento, C.M. Sorenson, K. Klabunde, Langmuir 21 (2005) 10280.
Synthesis of one-dimensional silver oxide nanoparticle arrays and silver nanorods templated by Langmuir monolayers Hong-Guo Liu a,∗ , Fei Xiao a , Chang-Wei Wang a , Qingbin Xue a , Xiao Chen a , Yong-Ill Lee b , Jingcheng Hao a , Jianzhuang Jiang a a Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China b Department of Chemistry, Changwon National University, Changwon 641-773, South Korea
Received 9 February 2007; accepted 17 May 2007 Available online 24 May 2007
Abstract One-dimensional (1D) silver oxide nanoparticle arrays were synthesized by illuminating the composite Langmuir–Blodgett monolayers of porphyrin derivatives/Ag+ and n-hexadecyl dihydrogen phosphate (n-HDP)/Ag+ deposited on carbon-coated copper grids with daylight and then exposing them to air. Transmission electron microscopy (TEM) observation shows that the nanoparticle size is around 3 nm, with the separation of about 2–3 nm. High-resolution TEM (HRTEM) investigation indicates that the particles are made up of Ag2 O. Ag nanorods with the width of 15–35 nm and the length of several hundreds of nanometers were synthesized by irradiating the composite Langmuir monolayers of porphyrin derivatives/Ag+ and n-HDP/Ag+ by UV-light directly at the air/water interface at room temperature. HRTEM image and selected-area electron diffraction (SAED) pattern indicate that the nanorods are single crystals with the (110) face of the face-centered cubic (fcc) silver parallel to the air/water interface. The formation of the 1D arrays and the nanorods should be attributed to the templating effect of the linear supramolecules formed by porphyrin derivative or n-HDP molecules in Langmuir monolayers through non-covalent interactions. © 2007 Elsevier Inc. All rights reserved. Keywords: Ag nanorod; 1D array; Ag2 O nanoparticle; Langmuir monolayer; Air–water interface
1. Introduction 1D ordered arrays of metallic and semiconductor nanoparticles have aroused much attention recently due to their unique electronic and optical properties different from single nanoparticle, such as surface plasmon resonance coupling, arising from the interaction between the nanoparticles. These 1D chains can serve as a “plasmon waveguide,” and have potential applications in optoelectronic nanodevices [1,2]. Various methods have been developed to synthesize and fabricate these ordered arrays, including template and template-free methods [3]. Two strategies have been adopted to prepare 1D arrays of metal nanoparticles by using of template method. One is to fabricate the preformed nanoparticles onto the linear templates, includ* Corresponding author. Fax: +86 531 88564750.
E-mail address: [email protected] (H.-G. Liu). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.05.048
ing biological substrates of DNA [4–6], RNA [7], and peptides [8], polymers [9–11], and carbon nanotubes [12], etc. via weak interactions and covalent bonds [11]. The other is to reduce the precursors that are combined with the linear templates [13–15]. Alternatively, anisotropic 1D metallic nanoparticles, such as nanorods [16–21], nanowires [22–25], and nanotubes [26–28] have attracted a great deal of attention, too, due to their peculiar properties and potential applications. Most of these 1D metallic nanoparticles were synthesized in solution through epitaxial growth in the presence of surfactants or templates and through oriented attachment of nanoparticles and ripening in the presence of templates. Synthesis and assembly 1D arrays and anisotropic nanoparticles at the interfaces are convenient for their practical applications in nanodevices. Langmuir monolayer technique is a versatile method to prepare and assembly nanoparticles at the interfaces, which falls into two categories: synthesis and/or as-
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sembly directly at the air/water interface, and treatment of the transferred Langmuir–Blodgett (LB) monolayer and multilayers. For example, anisotropic nanocrystals, such as PbS triangular nanoplates [29] and CdS nanorods [30] were synthesized at the air/water interface, and 1D arrays of metal sulfide nanoparticles [31,32] and 2D ordered arrays of silver nanoparticles [33] were synthesized and assembled at the air/water interface in a one-step process. In addition, Tl2 O3 nanowires were fabricated at the air/water interface by assembling the Tl2 O3 nanoparticles in the subphase onto the linear polymer templates [34], and 2D ordered arrays of semiconductor or metallic nanoparticles were fabricated by spreading the surfactant-modified nanoparticles on pure water surface [35–38]. For the treatment of the transferred films, ordered layered structures of semiconductors [39] and pyramid-shaped nanocrystals [40] were produced by treating the ordered LB films. However, most work focuses on 2D assemblies of nanocrystals or 2D nanoplates. The study on synthesis and assembly of 1D chains and 1D nanoparticles is very rare by Langmuir monolayer technique [31,32,41]. As revealed in our previous studies, porphyrin derivatives [42,43] and n-HDP [44] can form linear supramolecules at the air/water interface through non-covalent π–π attractions and hydrogen bonds, respectively. These supramolecules were applied as templates to prepare silver oxide nanoparticle 1D arrays and silver nanorods in this paper, by irradiating the composite LB monolayers on copper grids transferred from the air/AgNO3 aqueous solution interface and the composite monolayers at the air/AgNO3 aqueous solution interface, respectively. 2. Experimental 2.1. Chemicals Porphyrin derivatives TAPP, TDPP and TSPP (Chart 1) and n-HDP were synthesized according to the literature methods [45,46], respectively. Ethyl stearate (ES) was purchased from Aldrich. AgNO3 (99.9%) was purchased from Shanghai Reagent Co. The water used is highly purified with the resistivity 18.0 M cm. All chemicals were used as received without further purification.
Chart 1. Molecular structures of TAPP, TDPP, and TSPP.
2.2. Formation and characterization of 1D arrays of silver oxide nanoparticles and silver nanorods TAPP, TDPP, TSPP, and HP/ES (1:2) were dissolved in chloroform/DMF (v/v: 80/20), chloroform/ethanol (v/v: 95/5), chloroform, and chloroform, respectively. It was reported that n-HDP cannot form stable monolayer at the air/water interface alone [44,46]. In fact, n-HDP can dissolve in water, and the Gibbs monolayers were studied recently [47–49]. So the mixed system of n-HDP with ES (1:2 in molar ratios) was chosen. Aqueous solution of AgNO3 with the concentration of 1 × 10−4 mol L−1 was used as subphase. Langmuir monolayers were formed by spreading the corresponding solutions on the subphase surface filled in a NIMA 611 trough and compressing the monolayers to 20 mN m−1 after evaporating the organic solvents. For the formation of 1D arrays, the composite monolayers were transferred onto carbon-coated 230-mesh copper grids by using Langmuir–Schaefer method, and then illuminated with natural daylight for several hours and stored in a dark place. The samples were characterized by TEM (JEM100CXII, accelerating voltage of 100 kV) and HRTEM (GEOL2010, accelerating voltage of 200 kV). For the formation of Ag nanorods, the Langmuir monolayers of TSPP/Ag+ and nHDP/ES/Ag+ at 20 mN m−1 at the air/water interface were illuminated by UV-light (λ = 254 nm) directly for several minutes or hours, then transferred onto copper grids and investigated through TEM and HRTEM. 3. Results and discussion 3.1. 1D ordered arrays Under a TEM, large numbers of linear arrays were observed, as shown in Fig. 1. The arrays may extend for lengths of from 100 to a few hundreds of nanometers and are made up of discrete nanoparticles. The diameter of the particles is found to be 3.32 ± 0.82, 3.54 ± 1.11, 2.86 ± 0.84, and 3.07 ± 1.29 nm for TAPP, TDPP, TSPP, and n-HDP/ES monolayers, respectively, and the interparticle distance is ca. 2–3 nm. The average diameter of the particles in TSPP monolayer is less than those in TDPP, TAPP, and n-HDP/ES monolayers, which should be attributed to different mean Ag+ densities in these monolayers. The HRTEM image in Fig. 2 shows clear lattice fringes. To our great surprise, the lattice distance is measured to be 0.271 nm, which is close neither to 0.236 nm, the interplanar distance of {111} facets, nor to 0.293 nm, the interplanar distance of {110} facets of fcc Ag, indicating that the formed nanoparticles are not composed of Ag(0). However, the measured lattice spacing is very close to 0.272 nm, the calculated lattice spacing between the {111} facets of cubic Ag2 O based on the lattice parameter of 0.4726 nm [50], indicating that the nanoparticles in the 1D chains may be composed of Ag2 O. It was reported that the transformation between silver and silver oxide occurs facilely when exposing silver to air and illuminating silver oxide by light, respectively [51], especially for the smaller nanoparticles without capping agents. Based
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Ag nanoparticles transformed to Ag2 O nanoparticles when the samples were preserved in a dark place and exposed to air. The 1D chain structure keeps during the transformation process. The IFFT image shown in Fig. 2 also reveals the crystalline structure of the particles. 3.2. Nanorods
Fig. 1. 1D arrays of silver oxide nanoparticles templated by TAPP, TDPP, TSPP, and n-HDP–ES Langmuir monolayers and the corresponding size distribution histograms.
When the composite Langmuir monolayers were irradiated directly by UV-light at the air–water interface, nanorods were produced instead of 1D chains. Fig. 3 shows the morphologies and the formation procedure of the nanorods under TSPP Langmuir monolayers. After 1 min irradiation, scattered nanoparticles appear (not shown); 5 min later, 1D nanostructures made up of loosely packed nanocrystals appear, which serve as the “embryos” for nanorods (Fig. 3a); 15 min later, nanorods with the width of ca. 17 nm appear (Fig. 3b). The width of the nanorods falls in 15–35 nm within the irradiation time from 15 min to 3 h, and the aspect ratio is more than 20. The SAED pattern shown in Fig. 3g gives distinct diffraction rings that could be indexed to (111), (200), (220), and (311) faces of fcc Ag, respectively. Under the same experimental conditions, Ag nanorods can be also prepared under n-HDP/ES Langmuir monolayers (Fig. 3h). However, no nanorod forms without Langmuir monolayer of the porphyrin derivatives or n-HDP (Fig. 3i). Fig. 4 shows the HRTEM micrographs of the nanorods, together with the corresponding ED pattern. It can be seen from Fig. 4a and the enlarged image in Fig. 4b that 2D lattice appears clearly, indicating that single crystalline nature of the nanorods. The interplanar distances were measured to be 0.231, 0.200, and 0.148 nm, as labeled in Fig. 4b, corresponding to the spacings between {111}, {200}, and {220} facets of fcc Ag crystal, respectively. The nanorods grow along [100] direction. The diffraction spots corresponds to (111) and (200) reflections, as labeled in Fig. 4c, indicating that (110) is the basal plane, which is parallel to the air–water interface. It can be also seen from the micrograph that there are some atom-level grooves parallel to (111) faces in the (110) surface of the nanorod. The larger grooves have the width of ca. 1 nm and the depth of ca. 5–6 Å, as shown in the HRTEM micrograph (Fig. 4d) and the enlarged image (Fig. 4e). As we know, the surface free energies of the crystal faces of fcc crystals obey the following order, σ (110) > σ (100) > σ (111). It was reported that strips or grooves would appear in (110) plane through surface reconstruction in order to lower the surface energy, the new produced faces are (111) ones [52]. 3.3. Formation mechanisms
Fig. 2. HRTEM image and IFFT pattern of the nanoparticles in 1D arrays.
on the HRTEM result and the related literatures, we can infer that Ag nanoparticles formed first when the composite LB monolayers were irradiated by natural daylight, then the formed
Amphiphilic porphyrin derivatives can form stable Langmuir monolayers at the air–water interface [43]. Although nHDP cannot form stable Langmuir monolayers at the air–water interface by itself, the stable composite Langmuir monolayers of n-HDP/ES can be formed due to the hydrophobic interactions between the long alkyl chains in the two components [44]. According to our previous studies [43,44], the porphyrin and n-HDP molecules form linear supramolecular structures in the
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Fig. 3. TEM micrographs of nanorods formed under TSPP (a–f) and n-HDP/ES (h) monolayers by UV-light irradiation and the corresponding ED pattern (g), and TEM image of particles formed at the air–pure AgNO3 aqueous solution interface (i).
monolayers due to the π–π interactions between the porphyrin rings and hydrogen bonds between P=O and P–OH groups, respectively, as shown in Scheme 1. In this study, the soft supramolecular systems were employed as templates. Fig. 5 shows the π–A isotherms of TSPP and n-HDP/ES (1:2) at pure water and AgNO3 aqueous solution surfaces. It can be seen that the molecular areas decrease at the air–aqueous solution interfaces and the monolayers become more condensed compared with those at the air–pure water interface, indicating the interaction between the amphiphilic molecules and Ag+ ions. It is very interesting that different ordered structures can be obtained under different experimental conditions. The formation mechanisms were illustrated in Scheme 2. Because one carboxylic group in the porphyrin molecule or one hydroxyl group in the n-HDP molecule just binds one Ag+ , the number of Ag+ ions in the composite Langmuir–Blodgett monolayers transferred on the copper grids is limited. When the composite monolayers were illuminated by daylight, Ag+ transformed to silver atoms, resulting in the creation of (Ag)n clusters, which arrange along the linear supramolecules, leading to produce 1D linear chains
made up of discrete Ag nanoparticles, as shown in Scheme 2A. The transformation of Ag to Ag2 O takes place when exposing the nanostructures to air. However, when UV-light was used to illuminate the Langmuir monolayers at the air–water interface, Ag+ ions were reduced to Ag atoms rapidly, resulting in the creation of much more nuclei at the beginning of the reaction that align along the linear porphyrin supramolecules, as shown in Fig. 3a. More and more Ag atoms form with time because of sufficient supply of Ag+ ions from the subphase, leading to rapid growth of the particles. Due to the direction of the linear structure of porphyrin supramolecules, particle epitaxial growth, oriented attachment of the particles, and possible ripening process, nanorods were obtained at last, as shown in Scheme 2B. It is reasonable to propose these processes involved in the nanorod formation. As can be seen from Figs. 3a and 3e, the lengths of the “embryo” and the final nanorod are several tens of nanometers and more than one micrometer, respectively, the former is much less than the latter. Maybe at the early stage of the nanorod formation, the linear supramolecules play a key
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Fig. 5. π –A isotherms of TSPP (a, b) and n-HDP/ES (c, d) at the air–water (a, c) and air–AgNO3 aqueous solution interfaces (b, d).
Fig. 4. HRTEM image and SAED pattern of the nanorod formed under TSPP Langmuir monolayer.
Scheme 2. Formation mechanisms of 1D nanoparticle arrays (A) and nanorods (B).
Scheme 1. Linear supramolecular structures formed at the air–water interface by TAPP, TSPP, and n-HDP.
role in directing the nucleation and growth of the nanoparticles. During the particle growth process, the nanoparticles attached to each other and fuse to form a shorter rod. Fig. 6 gives a HRTEM micrograph of a nanorod, in which a defect can be clearly seen, probably arising from the incomplete fusion. After the shorter rod forms, the nanorod grows according to its intrinsic crystal symmetry, with the adsorption of the amphiphilic molecules to reduce the higher surface energy to stabilize the nanoparticles. It was demonstrated that epitaxial growth and attachment/fusion are the main possible processes for the nanorod formation. In general, two kinds of metal nanorods have been prepared, which grow along [110] or [100] direction with the side faces bounded by {100} [16,53] or {110} facets [54–56], respectively. It was said that the presence of the {110} facets is a
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Fig. 6. HRTEM image of the nanorod formed under TSPP Langmuir monolayer.
unique feature of metal nanorods and is not commonly observed because of its higher surface energy [52]. In the present case, the silver nanorods with the {110} basal planes were produced at the air–water interface. It seems that the adsorption of 1D supramolecules on the Ag nanoparticles not only directs the growth of the nanoparticles with a certain geometry, but also stabilizes the surface atoms of the {110} facets. Because {110} facets have the highest surface energy among the crystal facets of {100}, {111}, and {110}, the atoms on these surfaces could form stronger bonds with the supramolecules at the air–water interface. The preferential adsorption results in the formation of the (110) plane parallel to the interface. Wang et al. investigated the structure of the less stable higher energy {110} surfaces of gold nanorods. It was found that these surfaces exhibit the missing-row reconstruction, rows of atoms are missing along [110], the surface shows an irregular “saw-tooth” structure, the (110) surface transforms into strips of {111} facets [52]. Klabunde et al. also reported similar result [57]. Similar phenomena were observed in this paper. As shown in Figs. 4 and 5, the strips or grooves arising from the missing-row reconstruction appeared directly on the (110) surface. 4. Conclusion 1D arrays of metal or semiconductor nanoparticles and 1D anisotropic nanoparticles can be prepared via interfacial reaction at the interfaces templated by linear supramolecules formed by amphiphilic molecules through weak interactions at the air–water interface. It is a facile way to prepare various nanostructures by design and create supramolecules in Langmuir monolayers. Acknowledgments The authors thank the financial support of Education Ministry of China, and NSFC (20373025, 20304006 and 20428101). References [1] Y. Kang, K.J. Erickson, T.A. Taton, J. Am. Chem. Soc. 127 (2005) 13800. [2] S. Lin, M. Li, E. Dujardin, C. Girard, S. Mann, Adv. Mater. 17 (2005) 2553. [3] Z. Tang, N.A. Kotov, Adv. Mater. 17 (2005) 951.
[4] A. Ongaro, F. Griffin, P. Beecher, L. Nagle, D. Lacopino, A. Quinn, G. Redmond, D. Fitzmaurice, Chem. Mater. 17 (2005) 1969. [5] G. Wang, R.W. Murray, Nano Lett. 4 (2004) 95. [6] Z. Deng, Y. Tian, S.-H. Lee, A.E. Ribbe, C. Mao, Angew. Chem. Int. Ed. 44 (2005) 3582. [7] A.Y. Koyfman, G. Braun, S. Magonov, A. Chworos, N.O. Reich, L. Jaeger, J. Am. Chem. Soc. 127 (2005) 11886. [8] L.-s. Li, S.I. Stupp, Angew. Chem. Int. Ed. 44 (2005) 1833. [9] H. Lee, S.H. Choi, T.G. Park, Macromolecules 39 (2006) 23. [10] C. Minelli, C. Hinderling, H. Heinzelmann, R. Pugin, M. Liley, Langmuir 21 (2005) 7080. [11] Q. Dai, J.G. Worden, J. Trullinger, Q. Huo, J. Am. Chem. Soc. 127 (2005) 8008. [12] M.A. Correa-Duarte, J. Perez-Juste, A. Sanchez-Iglesias, M. Giersig, L.M. Liz-Marzan, Angew. Chem. Int. Ed. 44 (2005) 4375. [13] L. Berti, A. Alessandrini, P. Facci, J. Am. Chem. Soc. 127 (2005) 11216. [14] T.M. Day, P.R. Unwin, N.R. Wilson, J.V. Macpherson, J. Am. Chem. Soc. 127 (2005) 10639. [15] D.S. Kim, T. Lee, K.E. Geckeler, Angew. Chem. Int. Ed. 45 (2006) 104. [16] C.J. Johnson, E. Dujardin, S.A. Davis, C.J. Murphy, S. Mann, J. Mater. Chem. 12 (2002) 1765. [17] B. Nikoobakht, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) 8635. [18] F. Kim, J.H. Song, P. Yang, J. Am. Chem. Soc. 124 (2002) 14316. [19] C.J. Orendorff, P.L. Hankins, C.J. Murphy, Langmuir 21 (2005) 2022. [20] O.R. Miranda, T.S. Ahmadi, J. Phys. Chem. B 109 (2005) 15724. [21] A.J. Mieszawska, F.P. Zamborini, Chem. Mater. 17 (2005) 3415. [22] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165. [23] X. Sun, Y. Li, Adv. Mater. 17 (2005) 2626. [24] Z. Wang, J. Liu, X. Chen, J. Wan, Y. Qian, Chem. Eur. J. 11 (2005) 160. [25] J.-U. Kim, S.-H. Cha, K. Shin, J.Y. Jho, J.-C. Lee, Adv. Mater. 16 (2004) 459. [26] P. Gao, C. Zhan, M. Liu, Langmuir 22 (2006) 775. [27] T. Sehayek, M. Lahav, R. Popovitz-Biro, A. Vaskevich, I. Rubinstein, Chem. Mater. 17 (2005) 3743. [28] Y. Sun, B. Mayers, T. Herricks, Y. Xia, Nano Lett. 3 (2003) 955. [29] J. Yang, F.C. Meldrum, J.H. Fendler, J. Phys. Chem. 99 (1995) 5500. [30] J. Yang, J.H. Fendler, J. Phys. Chem. 99 (1995) 5505. [31] A. Berman, N. Belman, Y. Golan, Langmuir 19 (2003) 10962. [32] N. Belman, Y. Golan, A. Berman, Cryst. Growth Des. 5 (2005) 439. [33] A. Swami, P.R. Selvakannan, R. Pasricha, M. Sastry, J. Phys. Chem. B 108 (2004) 19269. [34] J.F. Liu, K.Z. Yang, Z.H. Lu, J. Am. Chem. Soc. 119 (1997) 11061. [35] K.M. Gattas-Asfura, C.A. Constantine, M.J. Lynn, D.A. Thimann, X. Ji, R.M. Leblanc, J. Am. Chem. Soc. 127 (2005) 14640. [36] X. Ji, C. Wang, J. Xu, J. Zheng, K.M. Gattas-Asfura, R.M. Leblanc, Langmuir 21 (2005) 5377. [37] F. Kim, S. Kwan, P. Yang, J. Am. Chem. Soc. 123 (2001) 4360. [38] S. Acharya, S. Efrima, J. Am. Chem. Soc. 127 (2005) 3486. [39] S. Guo, L. Konopny, R. Popovitz-Biro, H. Cohen, H. Porteanu, E. Lifshitz, M. Lahav, J. Am. Chem. Soc. 121 (1999) 9589. [40] H. Gong, M. Yin, M. Liu, Langmuir 19 (2003) 8280. [41] B.M. Rybak, M. Ornatska, K.N. Bergman, K.L. Genson, V.V. Tsukruk, Langmuir 22 (2006) 1027. [42] H.-G. Liu, D.-J. Qian, Q.-B. Xue, X.-S. Feng, K.-Z. Yang, Langmuir 16 (2000) 5079. [43] H.-G. Liu, X.-S. Feng, L.-J. Zhang, G.-L. Ji, D.-J. Qian, Y.-I. Lee, K.-Z. Yang, Mater. Sci. Eng. C 23 (2003) 585. [44] H.-G. Liu, X.-S. Feng, Q.-B. Xue, G.-L. Ji, S.-Y. Wang, X.-Y. Wang, D.-J. Qian, K.-Z. Yang, Mater. Lett. 58 (2004) 688. [45] A. Ruaudel-Teixier, A. Barraud, B. Belbeoch, M. Rouillay, Thin Solid Films 99 (1983) 33. [46] W. He, C. Jiang, F. Liu, Z. Tai, Y. Liang, Z. Guo, L. Zhu, J. Colloid Interface Sci. 246 (2002) 335. [47] Md.M. Hossain, K.-I. Iimura, T. Kato, J. Colloid Interface Sci. 306 (2007) 391.
H.-G. Liu et al. / Journal of Colloid and Interface Science 314 (2007) 297–303
[48] Md.M. Hossain, K.-I. Iimura, T. Kato, J. Colloid Interface Sci. 298 (2006) 348. [49] Md.M. Hossain, T. Suzuki, T. Kato, Colloids Surf. A 284–285 (2006) 119. [50] C.-M. Li, I.M. Robertson, M.L. Jenkins, J.C. Hutchison, R.C. Doole, Micron 36 (2005) 9. [51] L.A. Peyser, A.E. Vinson, A.P. Bartko, R.M. Dickson, Science 291 (2001) 103. [52] Z.L. Wang, R.P. Gao, B. Nikoobakht, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) 5417.
303
[53] H.-Y. Wu, H.-C. Chu, T.-J. Kuo, C.-L. Kuo, M.H. Huang, Chem. Mater. 17 (2005) 6447. [54] Z.L. Wang, M.B. Mohamed, S. Link, M.A. El-Sayed, Surf. Sci. 440 (1999) L809. [55] P.L. Gai, M.A. Harmer, Nano Lett. 2 (2002) 271. [56] J. Hernadez, J. Solla-Gullon, E. Herrero, A. Aldaz, J.M. Feliu, J. Phys. Chem. B 109 (2005) 12651. [57] S.I. Stoeva, V. Zaikovski, B.L. Prasad, P.K. Stoimento, C.M. Sorenson, K. Klabunde, Langmuir 21 (2005) 10280.