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J. Phys. Chem. B 2005, 109, 138-141
Film Formation of Ag Nanoparticles at the Organic-Aqueous Liquid Interface Jill K. Sakata,† Andrew D. Dwoskin,† John L. Vigorita,‡ and Eileen M. Spain*,† Departments of Chemistry and Physics, Occidental College, 1600 Campus Road, Los Angeles, California 90041 ReceiVed: August 7, 2004; In Final Form: September 27, 2004
We report a wet-chemical method to make films by spontaneous assembly of passivated Ag nanoparticles at the organic-aqueous liquid interface. The interfacial films exhibit a blue opalescence and are characterized with transmission electron microscopy and UV-vis spectrophotometry. Measurements indicate that nanoparticles in the interfacial film can form superlattices and in some cases nanostructures.
A plethora of recent work on the synthesis and physical characterization of metallic nanoparticles and nanostructures is driven by a prime goal: purposeful synthesis of a particular composition, size, shape, and crystallinity of nanostructure in high yield.1-19 To capitalize on the distinctive optical and nearinfrared properties of anisotropic metallic nanostructures simply adds to the keen excitement in this area of study.4,5,9,11,20 Another attractive scientific avenue to pursue is formation of superlattices composed of nanoparticles that do not aggregate,21-27 where interparticle distance is controllable.28,29 Such control allows the opportunity to access novel charge-transport phenomena30 such as the transition from a Mott insulator to a metallic conductor.31,32 In addition to using nanoparticle precursors in solution to form nanostructures and superlattices at air-solid and air-liquid interfaces, it is useful to develop means to assemble these structures at liquid-liquid interfaces33,34 where reversible induction of novel optical or electronic properties may occur with relative ease and efficiency. In this paper, we report a simple wet-chemical method to assemble Ag nanoparticles spontaneously at the organic-aqueous liquid interface. Langmuir-Blodgett techniques have been used to prepare 2-D superlattices, or monolayers, of Ag or other metallic nanoparticles at the air-liquid interface.31,32 Characterization of these arrays showed that an insulator to metal transition was induced as the interparticle distance decreased. Subsequent work to assemble nanoparticles at the air-solid interface has involved variations of the drop-cast method,24,26 and has shown promise for assembly of particles into multilayers and ordered arrays.35 For example, Saponjic et al.26 reported a mirrorlike film that was produced when surfactant-capped Ag nanoparticles selfassembled onto a glass substrate. The recent work of Reincke and co-workers34 discussed the assembly of charged gold nanoparticles at the water-oil interface. Reincke et al. infer from their data that the particles assemble at the interface to form a monolayer. In addition, they propose that changes in the interfacial energy, electrostatic energy, or van der Waals interactions among the particles, water, and oil determine if spontaneous film formation will occur at the interface. Our work supports that by Reincke et al. in what appears to be a general method for film synthesis using metal nanoparticles; however, we observe differences that make our system unique. Most * To whom correspondence should be addressed. E-mail: emspain@ oxy.edu. Phone: (323) 259-2940. Fax: (323) 341-4912. † Department of Chemistry. ‡ Department of Physics.
Figure 1. Photographs of (a, c) perpendicular and (b) side views of vials containing Ag nanoparticles dissolved in chloroform (bottom phase) in contact with an ethanol/water (60:40, v/v) mixture (top phase). The vials at the right (2) in (a) and (b) contain mercaptoundecanoic acid in the aqueous phase. As a result, a blue opalescent film is formed at the organic-aqueous liquid interface in (a) and (b). In (c), a film that appears reflective and silver-colored is our most recent observation.
notably, our system requires the addition of an aqueous-phase ligand to form multilayered interfacial films and, in some cases, leads to the formation of nanostructures. In Figure 1, photographs display two reaction vials each with a chloroform solution of passivated Ag nanoparticles (lower phase) in contact with an aqueous mixture (upper phase). A film is formed at the interface in the right reaction vials (labeled 2) in Figure 1a,b, and the film can be characterized by a vivid opalescence in the blue region. Neither a film nor any color is visible when 11-mercaptoundecanoic acid (MUA) is absent from
10.1021/jp046439l CCC: $30.25 © 2005 American Chemical Society Published on Web 12/14/2004
Ag Nanoparticle Film at the Liquid-Liquid Interface
Figure 2. UV-vis spectra of (4) Ag nanoparticles, washed once and resuspended in chloroform, (0) a film formed at the liquid-liquid interface and transferred to glass, and (O) the film subsequently dissolved in 200 proof ethanol.
the aqueous upper phase (shown in the left vials, labeled 1, of Figure 1a,b). Passivated Ag nanoparticles were prepared by a method similar to that described by Klabunde et al.36 Briefly, 0.957 g of tetra-n-octylammonium bromide (TAB; Lancaster) was dissolved by sonication (ca. 6 min) in 10 mL of anhydrous toluene. Next, 0.0283 g of AgNO3 (Strem Chemical) was added to the solution and again sonicated for 10 min until all AgNO3 crystals were dissolved. The mixture was stirred vigorously for 10 min to achieve a homogeneous mixture. The passivating agent, dodecanethiol (Sigma), was quickly added by syringe (12 µL) under vigorous mixing, which continued for 15 min. The Ag+ was reduced by adding 1.5 mL of 0.22 M NaBH4(aq). Immediately, an orange-brown color developed and persisted, and the reaction mixture was vigorously stirred under N2(g) for 3 h. Following the reduction, approximately 15 mL of ethanol (200 proof) was added to rinse away side products and some TAB, and precipitate the particles from solution. The particles were dried under N2(g) and resuspended in dry chloroform. All solvents were degassed with N2(g) for 3 h prior to synthesis.37 All solvents and reagents were used without further purification, unless indicated. Glassware was cleaned by immersion in a 2-propanol/KOH base bath before being rinsed with distilled water and dried in a 110 °C oven. Typically, we qualititatively monitor the size distribution of the once-washed Ag nanoparticles by observation of the surface plasmon band of the passivated nanoparticles dissolved in chloroform by ultraviolet-visible spectrophotometry (UV-vis). Here we report UV-vis spectra obtained with a double-beam scanning spectrophotometer (Shimadzu UV-2101PC). Figure 2 provides a typical spectrum for particles in chloroform, over the optical region, with 428 nm as the extinction maximum (the typical range for this synthesis is λmax ) 426-432 nm). Transmission electron microscopy (TEM) was employed to quantitatively characterize these particles and the films they form (Philips 430, 300 keV). Figure 3a displays the representative TEM and inset histogram of particles synthesized, washed once with ethanol, and resuspended in chloroform. The average particle diameter was determined as 5.0 ( 0.4 nm, by fitting the histogram to a Gaussian distribution. To prepare the film at the liquid-liquid interface, nanoparticles dissolved in chloroform are contacted to an equal volume of an ethanol/water mixture (60:40 or 50:50, by volume), containing 0.5-10 mM MUA at pH 1-2 (adjusted with HCl(aq)). Within minutes, a visibly thick film, best described as a skin when using higher MUA concentrations, is formed at the chloroform-aqueous interface. The film displays a vivid blue sheen, as shown in Figure 1a,b. The blue opalescence is
J. Phys. Chem. B, Vol. 109, No. 1, 2005 139 distinct from the orange-brown nanoparticle solution and originates at the interface; no film is formed at the interfacial region when MUA is absent from the aqueous layer, as seen in the left vials in Figure 1. In addition, without Ag nanoparticles in the organic solution, but with dodecanethiol and TAB present in considerable quantities, there is no opalescence observed at the organic-aqueous interface. These experiments indicate that the film is entirely dependent on the presence of Ag nanoparticles in chloroform. We note that these films retain their initial appearance for minutes, hours, or several days depending on the concentration of MUA, the proportions of ethanol and water in the aqueous phase, and the degree of nanoparticle size and polydispersity, as measured qualitatively by the full width at half-maximum of the surface plasmon extinction band. In the most recent film synthesis, we created a reflective, silver-colored film, shown in Figure 1c; no blue color was observed. We confirmed the silvery appearance was not due to colloidal silver formation by shaking the reaction vial following initial film formation. Identification of the critical factors of our nanoparticle synthesis that lead to this dramatic visual change in the prepared film is the subject of ongoing investigations. MUA and other passivating ligands have been used to modify the surface hydrophobicity/hydrophilicity of particles, providing a method to extract or transfer particles from the organic to the aqueous phase. A thorough review of phase-transfer recipes for nanoparticles is given by Sastry.38 Although MUA is used in our film preparation, we do not observe particle extraction when the particles are washed once with ethanol after synthesis. It is likely that, following synthesis, the surface of the Ag nanoparticles is passivated by both chemisorbed dodecanethiol and physisorbed TAB. In cases where the particles are thoroughly washed after synthesis, the addition of the MUA/ethanol/water mixture to the nanoparticle/chloroform mixture results in particle extraction to the aqueous phase (unpublished results). As proposed by Reincke et al., the decrease in surface charge density on the interfacial nanoparticle may be a key factor which leads to the formation of the interfacial film. If the TABsalso a phase-transfer agent used in organic synthesissis physisorbed to the Ag nanoparticle, it may serve as a transient ligand that desorbs from the nanoparticle at the organic-aqueous liquid interface, thus reducing the surface charge and allowing film formation. The question naturally arises regarding the role of MUA in the film formation. Considering work by Lin et al.33 and Reincke et al., the MUA may play a role in altering the interfacial energy among passivated particles, chloroform, and the aqueous solution, allowing the particles to assemble at that interface. Additionally, MUA may play another chemical role in directing nanostructure growth,10 as we observed changes of nanoparticle structures from spheres to prisms, evidenced with TEM. Films such as those pictured in Figure 1a,b were imaged by TEM. Briefly, the interfacial film, along with a small volume of both the organic and aqueous phases, was directed to the sidewall of the reaction vial. Before the two liquid phases evaporated, a holey carbon grid was contacted to the film and allowed to dry. TEM images of a film are shown in Figure 3b,c. In Figure 3b, large domains of ordered nanoparticle arrays are observed, and an edge of the film is seen at the top of the image. Some areas display layers of tightly packed particles not commensurate with each another, indicating a multilayered film or a slightly disordered superlattice. In some cases, Ostwald ripening and particle aggregation result in formation of various nanostructures, as observed in Figure 3c. When nanostructures were observed, the displaced film on glass was dissolved in
140 J. Phys. Chem. B, Vol. 109, No. 1, 2005
Sakata et al.
Figure 3. Transmission electron micrographs of (a) Ag nanoparticles, washed once, resuspended in chloroform, and drop-cast onto a grid. The inset displays the nanoparticle size distribution. (b, c) Films from the liquid-liquid interface and transferred to a grid at lower and higher magnifications, respectively. (d) Film imaged in (c) dissolved in 200 proof ethanol and drop-cast onto a grid.
ethanol, drop-cast onto a grid, and imaged, as shown in Figure 3d. Clearly, both nanostructures and nanoparticles, once resident in the film, were readily dissolved in ethanol. The UV-vis spectrum of the dissolved film, shown in Figure 2 (O), shows an extinction feature at 416 nm (typical range is λmax ) 412418 nm). The absorbance is weaker and blue-shifted from the 428 nm absorbance maximum observed for the silver nanoparticles in chloroform (4), consistent with the change in solvent refractive index.39-41 We conclude from Figure 3d and the UVvis of Figure 2 that the growth of the nanostructures formed in the liquid-liquid region is quenched upon film displacement and that the integrity of these structures is maintained upon dissolution in ethanol. The displaced film on glass was also characterized by UVvis spectrophotometry. The solutions were removed from the vial, and the vial was recapped and then placed in the path of the beam. The resulting spectrum, shown in Figure 2 (0), indicates λmax ) 459 nm, which is weaker and red-shifted relative to the “as-synthesized” particles in chloroform (4). This broad, red-shifted optical spectrum is similar to that observed for multilayered nanoparticle films of Ag26 and Au,42 where the red shift is attributed to the interparticle coupling of closely packed nanoparticles. Additionally, the shift in λmax and broadening of the band can also be attributed to a range of surface plasmon bands from variously shaped and sized nanostructures.43 In summary, a method to spontaneously assemble Ag nanoparticles into a multilayered, blue opalescent film at the organic-aqueous interface is presented. Measurements from TEM and UV-vis indicate that the particles form multiple layers of tightly packed arrays, in some cases resulting in ripening to various nanostructures. We are currently examining this interfacial film in more detail, with the goal of stabilizing the film and determining its reflectivity. It may be possible that the
formation of a multilayered film permits some defects in the crystal structure not allowed in the formation of metallic 2-dimensional lattices investigated by Heath and co-workers.28,31,32,44 According to percolation theory, an alternative path may be mapped through the 3-dimensional crystal, when a point or line defect is encountered. Acknowledgment. This research is based upon work supported by the National Science Foundation under Grant Nos. 9703345 (PECASE, Chemistry) and 0243531 (REU, Chemistry). Additional financial support is provided by the Camille & Henry Dreyfus Foundation as a Henry Dreyfus Teacher-Scholar award (E.M.S.). We gratefully acknowledge Dr. Carol Garland, California Institute of Technology (Caltech), for her expertise and assistance in making the TEM measurements. We thank Dr. Nathan F. Dalleska, Director of Caltech’s Environmental Analysis Center, for use of the double-beam spectrophotometer. We kindly acknowledge Prof. Nicholas A. Melosh, Stanford University, for helpful conversations on this work. The Sherman Fairchild Foundation is acknowledged for its support of a 2004 summer stipend (J.L.V.). Finally, we thank the Undergraduate Research Center and the Chemistry Department at Occidental College for support. References and Notes (1) Ghezelbash, A.; M. B. Sigman, J.; Korgel, B. A. Nano Lett. 2004, 4, 537. (2) Filankembo, A.; Pileni, M.-P. J. Phys. Chem. B 2000, 104, 5865. (3) Machulek, A.; Oliveira, H. P. M. d.; Gehlen, M. H. Photochem. Photobiol. Sci. 2003, 2, 921. (4) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (5) Metraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano Lett. 2003, 3, 519. (6) Pastoriza-Santos, I.; Liz-Marzan, L. M. Nano Lett. 2002, 2, 903. (7) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9060.
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