Macroscopic Alignment of Silver Nanoparticles in Reverse

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Macroscopic Alignment of Silver Nanoparticles in Reverse Hexagonal Liquid Crystalline Templates

2002 Vol. 2, No. 12 1403-1407

Martin Andersson,*,† Viveka Alfredsson,§ Per Kjellin,† and Anders E. C. Palmqvist*,†,‡ Department of Applied Surface Chemistry, and Competence Centre for Catalysis, Chalmers UniVersity of Technology, SE-412 96 Go¨ teborg, Sweden, and Physical Chemistry 1, Lund UniVersity, SE-221 00 Lund, Sweden Received June 10, 2002; Revised Manuscript Received September 24, 2002

ABSTRACT A flexible method of preparing and macroscopically aligning nanoparticles of crystalline silver into millimeter long fibers is presented. The approach utilizes the dual functionality of a reverse hexagonal liquid crystalline template containing a built-in reducing agent facing the aqueous domain. The method is advantageous in that its slow kinetics allows for a thorough introduction of a silver salt into the liquid crystal before the reduction takes place, allowing for an efficient loading of the template and a retained mesoscopic ordering as evidenced by SAXS. It was confirmed by 1H NMR that the oxyethylene groups of the amphiphilic polymer reduce the silver ions while being oxidized to aldehydes. The silver nanoparticles are uniform in size and in the same size range as the diameter of the aqueous domain of the liquid crystal (3 nm), further supporting that the silver particles form inside the liquid crystal. TEM images confirm the macroscopic alignment of silver nanoparticles into fibrils and the packing of fibrils into millimeter long fibers. The diameter of the fibrils and fibers ranges from 30 nm to several hundreds of micrometers. Electron diffraction analysis of a collection of silver nanoparticles confirms their crystallinity as three diffraction rings could be indexed to the face centered cubic structure of silver. A key to the successful macroscopic alignment of the nanoparticles is that the particles are formed inside the liquid crystal, thus minimizing the need for their diffusion into and inside the liquid crystal.

Nanostructured materials gain a lot of interest because of their different properties compared to the corresponding bulk materials, and during recent years much effort has been devoted to develop methods to prepare, for example, nanorods, nanowires, dots, and nanotubes of inorganic materials.1,2 Both size and shape have been shown to greatly affect properties of nanomaterials.3-5 In addition, collective properties of assembled nanoparticles have also been reported recently as a result of the geometric arrangement of nanoparticles.6-9 It is hence of great importance to be able to control not only the size and shape of the nanoparticles but also the way in which they are spatially arranged in respect to each other. Nanostructured noble metals are of special interest, since their physiochemical properties have a plethora of applications in fields as diverse as photovoltaics, catalysis, electronic and magnetic devices, etc.10-12 A range of physical and chemical methods have been utilized to prepare nanosized noble metals, and several of these are based on the use of a * Corresponding authors: [email protected]; anders.palmqvist@ surfchem.chalmers.se † Department of Applied Surface Chemistry. ‡ Competence Centre for Catalysis. § Lund University. 10.1021/nl0256412 CCC: $22.00 Published on Web 10/31/2002

© 2002 American Chemical Society

structure-directing template. The use of polymeric films to produce nanowires,13 the formation in single walled carbon nanotubes,14 templating by oleate vesicles,15 the use of DNA,16 the Langmuir technique,17 and templating with the use of mesoporous silica18 are just a few of the recent examples for the preparation of nanostructured noble metals. Among the wet-chemical methods, those based on the use of self-assembled amphiphilic templates have proven to be particularly successful in the preparation of nanostructured noble metals. Surfactant aggregates in the form of microemulsions have been extensively studied for the preparation of noble metal nanoparticles, and it has been found that the size of the droplets is correlated with that of the prepared nanoparticles.19-21 Hexagonal and cubic liquid crystalline templates are well-known templates for the synthesis of mesoporous oxides22-26 but have also been used in the preparation of mesoporous noble metals.27 The ordering of silver nanoparticles in one and two dimensions has also been studied using various methods.6-9 Among these, Qi et al.6 showed that a lamellar liquid crystalline phase consisting of C12EO4 surfactants and water can be used to produce silver nanoparticles with a narrow size distribution. They also

Scheme 1. Schematic Representation of the Oxidation of an Ethyleneoxide Group in the Block Copolymer Pluronic P123 Explaining the Appearance of Aldehydes during the Formation of Silver Nanoparticles

indicated that the ordering of the particles was templated by the liquid crystalline phase, but no macroscopic alignment was shown. In this study a reverse hexagonal liquid crystal is instead used as a template for the formation and macroscopic alignment of silver nanoparticles. The reverse hexagonal phase offers the advantage of restricting the assembly of particles in two dimensions instead of one as in the lamellar phase. In addition, experimental support for the proposed reaction mechanism is given in the present study. The approach utilizes the dual functionality of a reverse hexagonal liquid crystalline template containing a built-in reducing agent facing the aqueous domain. The liquid crystal was loaded with a silver nitrate solution, and the slow reduction of the silver ions by the reducing groups of the surface-active polymer resulted in the formation of 3 nm sized silver particles, i.e., in the same size regime as the diameter of the aqueous domain of the liquid crystal.28 The nanoparticles were subsequently aligned by the liquid crystal forming macroscopic silver/liquid crystal-composite fibers of several millimeter in length. Specifically, the surfactant used in this study was the commercially available surfactant Pluronic P123, which is a poly (ethyleneoxide)-block-poly(propyleneoxide)-block-poly(ethyleneoxide), with the composition EO20PO70EO20. Mixtures of P123, butyl acetate, and water have been thoroughly studied, and phase diagrams are available in the literature.28 The phase used in this study is composed of hexagonally packed water channels surrounded by a continuous oil domain. The water channels are separated from the oil by a monolayer of the polymer. The hydrophilic polyethylene groups point toward the aqueous domain and the silver ions. It has been reported that EO-type nonionic surfactants can reduce silver ions to metallic silver through oxidation of their oxyethylene groups.29 The rate of oxidation of the surfactants is, however, relatively slow,29 enabling the liquid crystalline phase to form and to work as a direct template when the reduction of the silver commences (Scheme 1). The method presented here is advantageous in that its slow kinetics allows for the introduction of the silver ions into the liquid crystal before the reduction takes place, allowing for an efficient loading of the template. The synthesis procedure of the macroscopically aligned silver nanoparticles was as follows. The surfactant Pluronic P123 (pharmaceutical grade, BASF Corporation) was mixed with butyl acetate (spectrophotometric grade, Aldrich) and silver nitrate (pro analysi, MERCK) dissolved in water in the weight ratio of 10 P123: 7 CH3COOC4H9: 0.03 AgNO3: 2.97 H2O. The silver concentration used was 1 wt %, and 1404

Figure 1. Indexed SAXS diffractograms of the reversed hexagonal liquid crystalline phase (a) in the absence of AgNO3 and (b) in the presence of AgNO3, with insets showing magnified parts of the diffractograms. The reversed hexagonal phase is retained but swelled upon addition of AgNO3.

the water was of MilliQ purity. The mixture obtained was in the form of a clear, highly viscous pale yellow gel. The gel was left to equilibrate at room temperature in a covered glass beaker for six months. During this time the yellow color of the mixture successively darkened as a result of the reduction of silver ions. The mixture was subsequently washed with ethanol and the solid product was collected by filtering. The recovered product was in the form of millimeter-long fibers with an aspect ratio of 20-40 and showing metallic luster. The liquid crystalline synthesis gel was stored for three days prior to being analyzed by small-angle X-ray scattering (SAXS) experiments on a Kratky compact small angle system equipped with a position sensitive wire detector (OED Nano Lett., Vol. 2, No. 12, 2002

Figure 2. 1H NMR spectra of the liquid crystalline phase (a) in the absence of AgNO3 and (b) in the presence of AgNO3 showing, in enlargement between 9 and 10 ppm shift, the appearance of aldehyde peaks during the formation of silver nanoparticles.

50M from MBraun, Graz) containing 1024 channels of width 53.0 µm. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID 3000 X-ray generator operating at 50 kV and 40 mA. The synthesis mixture was dissolved in CDCl3 and analyzed with 1H NMR at 20 °C using a Varian 400 MHz spectrometer. The fibers were analyzed by transmission electron microscopy (TEM), and the samples were prepared either by lightly crushing the fibers under acetone and dispersing them onto holey carbon copper grids or by embedding the fibers in an epoxy resin (Spurr’s resin) and subsequently cutting them into thin sections (30-50 nm) using an ultramicrotome with a diamond knife. Electron micrographs were recorded on a Philips CM120 Bio TWIN microscope at an accelerating voltage of 120 kV. Electron diffraction patterns and EDS spectra were obtained with a JEM2000FX microscope operated at 200 kV and equipped with a LINK EDS system. Figure 1 shows the small-angle X-ray diffractograms for the liquid crystalline template in the absence and in the presence of silver nitrate. Both diffractograms show the presence of four diffraction peaks, one strong (100) and three weak ones (110), (200), and (210) that obey the relationship 1:x3:2:x7, which is characteristic of reverse hexagonal liquid crystals.28 This shows that the formation of the liquid crystalline template was successful and that its ordered hexagonal mesostructure was maintained also upon introduction of silver nitrate. The introduction of the silver salt led to an increase in intensity of the diffraction peaks by a factor of 10 (Figure 1b). This is an expected result for a reverse hexagonal liquid crystal filled with silver nitrate due to the increased scattering contrast between the heavy atom in the inorganic salt and the organic liquid crystal. The increase in intensity of the diffraction peaks is clear evidence of a successful incorporation of silver nitrate inside the aqueous domain of the liquid crystal. In addition, the diffraction peaks are slightly shifted to lower angles upon introduction of the silver nitrate (Figure 1b), and this is attributed to a swelling of the liquid crystal as a result of silver nitrate incorporation. From the diffraction angles the lattice spacing of the reverse hexagonal unit cell was calculated to increase from 13.9 to 14.6 nm upon introduction of the silver nitrate. As the size of the water domains is affected by the structure of the amphiphilic polymer,30 it is possible to control the diameter Nano Lett., Vol. 2, No. 12, 2002

Figure 3. CCD camera image of macroscopically aligned silver nanoparticles formed in a reverse hexagonal liquid crystalline phase appearing as fibers with metallic luster, here collected on a filter paper.

of the water domains and thus potentially the size of the silver particles by choice of polymer in the template. As the silver ions are being reduced inside the reverse hexagonal liquid crystalline template, a corresponding oxidation occurs. It has previously been found that oxyethylene groups in PEO-type nonionic surfactants can reduce silver ions to metallic silver,29 so it is most likely that the oxyethylene groups in the pluronic P123 can also act as reducing agents for silver in the present study. To verify the proposed mechanism of silver reduction 1H NMR was used to study the formation of aldehyde (Scheme 1), which is one of the products resulting from the oxidation of ethylene oxide.31 Figure 2 shows the 1H NMR spectrum of the filtrate from the filtered final solid product of silver fibers prepared in the reverse hexagonal liquid crystal. A large number of peaks are present in the 1H NMR spectrum, but the peaks appearing at a chemical shift of 9.7-9.8 are indicative of the presence of aldehydes in the liquid. However, from the spectrum it is not possible to deduce exactly which aldehydes were present, but it is anticipated that a whole range of 1405

aldehydes may result from the synthesis conditions employed according to Scheme 1. In addition, the concentration of the aldehydes was very low, and this rendered quantification of the exact amount of aldehydes in the solution not feasible. Figure 3 shows a CCD camera image of the filtered silver fibers collected on a filter paper. The fibers appear as needles with metallic luster and a length of several millimeters combined with a typical aspect ratio of 20-40. The fibers are quite brittle and fracture upon excessive applied force. They are, however, rigid enough to be handled and moved around, and do not spontaneously fall apart. Figures 4a-c show the TEM micrographs of the silver fibers that were dispersed in acetone. The broken silver fiber in Figure 4a, reveals at the fractured area the presence of the thinner fibrils of which the fiber is composed. The fibrils are shown in Figure 4b. A comparison of the contrast between the fibrils and the “untemplated” silver particles that were present also shows that the density in the fibrils is lower. This indicates that the fibrils are composed of smaller Ag particles that are loosely bound together. This is even more apparent in Figure 4c, which shows two fibrils from the embedded and sectioned sample (thickness approximately 40 nm). Here, the small particles that make up the fibrils are clearly resolved. The particle size is about 3 nm and the particle size distribution is narrow. However, it does not appear as if the particles have a mesoscopic ordering. Even so, the narrow size distribution, along with the particle size, clearly indicates that the particles were formed in a constrained space of the same size range as present in the liquid crystalline structure. On the other hand, the diameter of the fibrils and fibers ranges from 30 nm to several hundreds of micrometers. The aligned nanoparticles formed during the synthesis in the reverse hexagonal liquid crystal were crystalline. This is evident from Figure 5, which shows a typical electron diffraction pattern of a collection of silver nanoparticles present in a fiber. The pattern is rather diffuse due to the small particle size, but three diffraction rings were clearly seen and they could be indexed to the face centered cubic structure of silver as follows. The strongest ring and the one closest to the center is probably a combination of the {111} and {200} reflections. The second ring is likely the {222} reflection, whereas the outermost and weakest third ring is either the {420} and/or the {422} reflections. A further confirmation of the silver nanoparticles was obtained from the energy dispersive spectrometer (EDS) in the TEM instrument, which showed that the nanoparticles are made of silver. During the synthesis of the silver nanoparticles, the liquid crystal became successively less viscous. This is an expected result from the oxidation of the ethylene oxide chains of the pluronic P123, and it would indicate that the formation of silver particles is accompanied by the destruction of the reverse hexagonal liquid crystal and thus the loss of the mesostructuring template. However, the fact that the silver nanoparticles are of a uniform size similar to the diameter of the water domains of the liquid crystal28 and the fact that millimeter long fibers are formed suggest that there is still enough mesoscopic order present in the template throughout 1406

Figure 4. Transmission electron micrograph of (a) a broken silver fiber revealing at the fracture surfaces that the fiber is composed of thinner fibrils (scale bar ) 500 nm), (b) thinner silver fibers and fibrils showing at the surfaces the presence of aligned nanoparticles (scale bar ) 200 nm), and (c) two parallel fibrils from a silver fiber showing that the fibers are composed of macroscopically aligned 3 nm sized silver particles (scale bar ) 100 nm).

the synthesis for a macroscopic alignment of the silver nanoparticles to take place. A key to the successful macroscopic alignment of the nanoparticles is that the particles are formed inside the liquid crystal and that they nucleate rapidly in comparison to their growth rate. This minimizes the need for diffusion of the silver nanoparticles inside the liquid crystal and, in addition, yields a small particle size and a uniform particle size Nano Lett., Vol. 2, No. 12, 2002

Figure 5. Electron diffraction pattern of a collection of silver nanoparticles present in the fibers showing three indexed diffraction rings (arrowed) consistent with the face centered cubic structure of silver.

distribution. It is also essential that the reducing power of the polymer be in excess of the silver ions to avoid too big of a change in the structure of the template. Acknowledgment. This work was financially supported by the Swedish Foundation for Strategic Research (SSF) through its Colloid and Interface Technology program. A.E.C.P. acknowledges support from the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Administration and the member companies: AB Volvo, Johnson Matthey CSD, Saab Automobile AB, Perstorp AB, Eka Chemicals AB, MTC AB, and Swedish Space Corporation. Gunnel Karlsson, Lund University, is acknowledged for the TEM sample preparation, Katarina Flodstro¨m, Lund University, is acknowledged for help with SAXS, and Dan Lundberg, Chalmers University of Technology, is acknowledged for the 1H-NMR analysis. Supporting Information Available: TEM micrographs of silver nanoparticles formed in reverse hexagonal liquid crystal. The numbers in the micrographs indicate the particles measured. The particle sizes are presented in a table. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393-395.

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