NANO LETTERS
Composite Nanotubes Formed by Self-Assembly of PbS Nanoparticles
2003 Vol. 3, No. 4 569-572
Epameinondas Leontidis,*,† Maria Orphanou,† Tasoula Kyprianidou-Leodidou,† Frank Krumeich,‡ and Walter Caseri§ Department of Chemistry, UniVersity of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus, Laboratory of Inorganic Chemistry, ETH Zu¨ rich, HCI-G105, CH-8093 Zu¨ rich, Switzerland, and Institut fu¨ r Polymere, ETH Zentrum, CH-8092 Zu¨ rich, Switzerland Received February 28, 2003
ABSTRACT Unusual arrangements of nanoparticles were observed during the synthesis and subsequent dissolution of PbS particles under the action of the surfactant sodium dodecyl sulfate (SDS) in solutions containing hydrophilic polymers. At the selected polymer and surfactant concentrations, the surfactant forms the insoluble lead dodecyl sulfate salt (Pb(DS)2), which competes with the formation of the PbS particles. Under the action of the surfactant the PbS particles form bending self-assembled layers; a novel type of metastable nanotubes arises then, the walls of which consist of layers of ordered PbS nanoparticles which are, most likely, separated by bilayers of surfactant molecules.
Nanotubes are structures adopted by many materials under special conditions. Starting with their discovery in 1991, carbon nanotubes have reached a central position in current materials science, because, for example, of their unique mechanical or electrical properties.1 In recent years, nanotubes of other materials have also been discovered and extensively investigated.2 The walls of most nanotubes investigated to date comprise a layered structure. Often the thickness of the layers forming the wall skeleton lies in the atomic range, while the spacing between successive layers may be considerably larger, in particular if the nanotubes contain organic spacer molecules.2-6 In marked contrast to the nanotubes described so far, we report here on selfassembled nanotubes, the walls of which enclose nanoparticles as building units. Such nanotubes evolve as metastable structures in systems of polymers, surfactants, and PbS nanoparticles. The fact that nanoparticles can form such superstructures by self-assembly may open a window of understanding for a range of nanotube-formation processes but also a possible avenue for the creation of new materials with unique structures and properties. We have recently demonstrated that relatively dilute solutions of poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS) may serve as interesting media for inorganic crystallization reactions.7 Our previous studies have focused * Corresponding author. Tel. +357 22 892185; Fax +357 22 339063; E-mail
[email protected]. † University of Cyprus. ‡ ETH Zu ¨ rich. § ETH Zentrum. 10.1021/nl034124w CCC: $25.00 Published on Web 03/12/2003
© 2003 American Chemical Society
on the formation of lead sulfide in solutions containing watersoluble polymers, such as poly(ethylene oxide) (PEO), and sodium dodecyl sulfate (SDS). In the appropriate concentration ranges, surfactant micelles associate almost exclusively with the polymer chains, creating a unique environment for the nucleation and growth of inorganic crystals.7,8 This particular system is governed by an intricate network of chemical equilibria which can lead to unexpected products and to a variety of remarkable metastable structures.8,9 Over a wide range of reaction conditions, the product favored by thermodynamics appears to be lead dodecyl sulfate, Pb(DS)2.8 We have found that PbS, which forms initially because of the large local concentration excesses in the vicinity of the negatively charged SDS micelles, slowly transforms to Pb(DS)2, going through a variety of metastable structures.8,9 Recently, we have observed an intriguing phenomenon that occurs in these systems already after short reaction times for a small stoichiometric excess of Pb2+ over S2-. It appears that PbS nanoparticles assemble into layered superstructures, which transform into nanotubes under the action of the surfactant present in the system.10 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images clearly show the tubelike character of these structures (Figure 1). The tube length can grow up to a few hundred nanometers, and the diameter is typically about 50 nm. These nanotubes appear to evolve from regions containing agglomerates of PbS particles and flat Pb(DS)2 crystals (Figure 2), similar to growth phenomena observed in “ordinary” nanotubes whose walls are not formed by nanoparticles.3,11 At high magnifica-
Figure 1. PEO/SDS system 5-6 weeks after reaction onset. (a) SEM image of a cluster of nanotubes, evolving from a region consisting of PbS particles. (b) TEM image of a well-developed nanotube.
Figure 3. HRTEM picture of a nanotube showing the layered wall structure. The PbS layer width (layers with dark contrast) is 2 nm and the surfactant interlayer is also 2-3 nm wide.
Figure 2. TEM image of nanotubes evolving from a region containing a cluster of PbS nanoparticles. PVA/SDS system 1 day after reaction onset.
Figure 4. Electron diffraction pattern from a region containing nanotubes. Because of the very small size of the crystallites composing the nanotube walls, only very diffuse diffraction rings at 3.05 and 2.11 C, corresponding to the [200] and [220] reflections, are visible.
tions, it can be verified that the self-assembled nanotubes consist indeed of layers of very small PbS nanocrystals (Figure 3). The presence of PbS nanoparticles was confirmed by electron diffraction from a region containing nanotubes (Figure 4). The PbS nanoparticles are not larger than 2-4 nm in diameter. The spacing between the nanoparticle layers is also in this size range (2-3 nm), suggesting the presence of an interdigitated bilayer of surfactant molecules. As mentioned before, the formation of layered superstructures starts already from reaction onset and continues for several weeks as long as the originally formed PbS is not depleted and remains in solution. According to our current understanding, key requirements for the formation of the nanotubes are (a) the formation of clusters of PbS nanoparticles of roughly uniform size, and (b) a strong association between surfactant molecules and metal ions in the system. Requirement (a) is satisfied in polymer-surfactant systems in which the surfactant molecules associate exclusively on polymer coils, attract Pb2+ ions, and create nucleation domains for
the subsequent reaction between Pb2+ and HS-. Pb(DS)2 is virtually insoluble,8,12 and at conditions around neutral pH values this compound may become the major precipitating product. Thus PbS must dissolve in the equilibrium state of the system in favor of the platelike crystals of Pb(DS)2. This process can occur in several different ways,8,9 but, especially at the surfaces of clusters, the surfactant ions that are decorating the surfaces of individual particles apparently interdigitate, forcing a parallel alignment of the particles on the surfaces of surfactant bilayers. At the same time the surfactant creates an unfavorable environment for PbS ripening processes, since it interacts very strongly with Pb2+. Given the strong interaction of the surfactant with Pb2+ ions on the PbS surfaces, we assume that bending and (eventually) nanotube formation occur when stresses arise because of the geometric mismatch of the highly curved PbS colloids and the planar bilayers (see Figure 5 for a demonstration of this bending). The probable mechanism of nanotube formation outlined above is presented schematically in Figure 6.
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Figure 5. TEM image showing that nanoparticle layers bend under the action of bridging surfactant bilayers. PEO/SDS system, 1 day after reaction onset.
that we are observing is the reverse of experiments in which quantum particles are generated by appropriate reactions in preexisting lamellar surfactant phases.13,14 In other respects the formation of PbS superstructures is also a reverse biomineralization process, in which a lipidic component actually breaks down a mineral. A special feature of this process is that nanorods or other compact structures are not formed, as is often the case with many potentially nanotubeforming processes.2 However, it must be recalled that the nanotubes prepared in this way are metastable structures, forming a bridge between a typical inorganic ionic crystal (PbS) and a layered precipitate (Pb(DS)2). This work appears to be strongly related to several similar systems in which the growth of different crystals competes in the presence of a surfactant. A reaction of Zn(OH)2 particles with carboxylic acids led to the formation of fibrous particles through bending of particle layers.15 The formation of layered iron oxide structures obtained by H2O2 oxidation in the presence of SDS,14 the formation of CdS and CdSe nanotubes in the presence of surfactant,16 and the production of copper sulfide nanotubes by amine-assisted processes17 may follow related mechanisms. Furthermore, the presence of long-chain amines are necessary to build up vanadium oxide nanotubes, which represent another example for such a tubular composite.3 The nanotube formation process presented here may also show the way for the production of novel nanotube structures in other systems by dissolving or organizing inorganic nanocrystals with the aid of appropriate surfactants. The main requirement for such processes, as highlighted in the present system, is a strong interaction of the surfactant with the metal involved and with the surface of the nanocrystals. References
Figure 6. Schematic of the proposed mechanism of nanotube formation in this system. The initial PbS precipitation in polymersurfactant rich domains is followed by adsorption of surfactant on particle surfaces, ordering and alignment of the particles by surfactant bilayers and bending of the layers to form nanotubes. The black lines represent layers containing Pbs particles.
There is an alternative, quite different mechanism, which can partly explain the phenomenon of PbS nanocrystal alignment. One can speculate that PbS grows epitaxially on initially formed Pb(DS)2 lamellar crystals. However, it is difficult to understand the bending behavior that leads to nanotubes using this second mechanism. Proving or disproving mechanisms in these complex systems and following the nanotube formation process step-by-step is unfortunately quite difficult since several different kinds of particles coexist at every point in time in solution. This process of nanotube formation with self-assembled nanoparticles as wall-forming material has not been observed so far, to our knowledge. In some respects, the phenomenon Nano Lett., Vol. 3, No. 4, 2003
(1) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; World Scientific: Singapore, 1998. (2) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (3) Spahr, M. E.; Bitterli, P.; Nesper, R.; Muller, M.; Krumeich, F.; Nissen, H. U. Angew. Chem., Int. Ed. Engl. 1998, 37, 1263-1265. Krumeich, F.; Muhr, H.-J.; Niederberger, M.; Bieri, F.; Schnyder B.; Nesper R. J. Am. Chem. Soc. 1999, 121, 8324. Niederberger, M.; Muhr, H.-J.; Krumeich, F.; Bieri, F.; Gunther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995. (4) Zhu, Y. Q.; Hsu, W. K.; Terrones, H.; Grobert, N.; Chang, B. H.; Terrones, M.; Wei, B. Q.; Kroto, H. W.; Walton, D. R.; M. Boothroyd, C. B.; Kinloch, I.; Chen, G. Z.; Windle, A. H.; Fray, D. J. J. Mater. Chem. 2000, 10, 2570. (5) Rothschild, A.; Popovitz-Biro, R.; Lourie, O.; Tenne, R. J. Phys. Chem. B 2000, 104, 8976-8981. (6) Jiang, X.; Xie, Y.; Lu, J.; Zhu, L.; He, W.; Qian, Y. AdV. Mater. 2001, 13, 1278. (7) Leontidis, E.; Kyprianidou-Leodidou, T.; Caseri, W.; Kyriacou, K. Langmuir 1999, 15, 3381. (8) Leontidis, E.; Kyprianidou-Leodidou, T.; Robyr, P.; Krumeich, F.; Caseri, W.; Kyriacou, K. J. Phys. Chem. B 2001, 105, 4133. (9) Leontidis, E.; Kyprianidou-Leodidou, T.; Caseri, W.; Kyriacou, K. Prog. Colloid Polym. Sci. 2001, 118, 57. (10) In a typical experiment, 30.0 mL of a 4 gL-1 aqueous solution of SDS were mixed with 30.0 mL of a filtered 0.5% w/w aqueous PEO solution. Other water-soluble polymers have also been successfully used at similar concentrations (for example, poly(vinyl alcohol) is used in the experiment described in Figure 2). The weight average molecular weight of the polymer was typically 105 - 2 × 105 g mol-1. 3.0 mL of a 0.1 M lead acetate solution was added next, whereupon clouding was observed because of Pb(DS)2 formation. Upon subsequent addition of 3.0 mL of a 0.1 M solution of Na2S‚ 9H2O, the solution turned to brown-black, as PbS was formed. The 571
pH of the slightly basic solution was adjusted to 7.0 with addition of dilute HNO3, if necessary. 1-2 h after reaction onset, and subsequently at specific time intervals, a few drops of the reaction medium were placed on a carbon-coated holey foil supported on a copper grid, which was examined by TEM (Philips CM30 microscope operated at 300 kV) after solvent evaporation. The SEM investigation of the as-synthesized sample was performed on a LEO 1530 Gemini instrument, operated at 1 kV. (11) Zhu, Y. Q.; Hsu, W. K.; Grobert, N.; Chang, B. H.; Terrones, M.; Terrones, H.; Kroto, H. W.; Walton, D. R. M. Chem. Mater. 2000, 12, 1190. (12) Sreenivasarao, K.; Doyle, F. M. Separ. Pur. Tech. 1997, 12, 157.
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(13) Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1995, 99, 3736. Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. Guo, S.; Popovitz-Biro, R.; Arad, T.; Hodes, G.; Leiserowiz, L.; Lahav, M. AdV. Mater. 1998, 10, 657. (14) Tolbert, S. H.; Sieger, P.; Stucky, G. D.; Aubin, S. M. J.; Wu, C.-C.; Hendrickson, D. N. J. Am. Chem. Soc. 1997, 119, 8652. (15) Ogata, S.; Tagaya, H.; Karasu, M.; Kadokawa, J. J. Mater. Chem. 2000, 10, 321. (16) Rao, C. N. R.; Govindaraj, A.; Leonard Deepak, F.; Gunari, N. A. Appl. Phys. Lett. 2001, 78, 1853. (17) Lu, Q.; Gao, F.; Zhao, D. Nano Lett. 2002, 2, 725.
NL034124W
Nano Lett., Vol. 3, No. 4, 2003
