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Isolated and Linear Arrays of Surfactant-Encapsulated Polyoxometalate Clusters on Graphite Peng Wu,† Dirk Volkmer,‡ Bjo¨rn Bredenko¨tter,‡ Dirk G. Kurth,§ and J. P. Rabe*,| UniVersity of Kentucky, Department of Chemical & Materials Engineering, Lexington, Kentucky 40506, UniVersity of Ulm, Department of Inorganic Chemistry, Albert-Einstein-Allee 11, D-89081 Ulm, Germany, Max Planck Institute of Colloid & Interfaces, D-14424 Postdam, Germany and National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Humboldt UniVersity Berlin, Department of Physics, Newtonstr. 15, D-12489 Berlin, Germany ReceiVed October 17, 2007. In Final Form: December 11, 2007 We report on the self-assembly of several surfactant-encapsulated clusters (SECs) on the basal plane of graphite consisting of the doughnut-shaped tungstophosphate anion [Na(H2O)P5W30O110] covered by a hydrophobic shell of surfactants. Well-ordered rodlike structures are observed using scanning force microscopy. No such ordering is observed if the surfactant methyltrioctadecylammonium is used for encapsulation, suggesting that the density of alkyl chains around the polyoxometalate cluster is an important factor in determining the order of SEC assemblies on graphite. Coadsorption of tetratetracontane (n-C44H90) and (DODA)14[Na(H2O)P5W30O110] results in single, isolated SECs on a buffer layer of tetratetracontane, as determined by scanning tunneling microscopy.
Introduction Polyoxometalates (POMs) are atomically defined early transition metal oxygen clusters, which exhibit a wide diversity of molecular architectures, surface charge densities, and electroand photochemical as well as magnetic properties.1,2 Generally, POMs are isolated as crystalline solids, and, therefore, it is hard to exploit the value-adding properties of POMs for devices and materials.3 A general approach to engineer the surface-chemical properties of POMs is to encapsulate the negative cluster in a shell of positively charged amphiphilic molecules.4 The anionic cluster is nested within a shell of surfactants, thus leading to discrete, electrostatically neutral, hydrophobic core-shell particles, which are soluble in nonpolar solvents. The so-called surfactant-encapsulated clusters (SECs) form mesophases and monolayers at the air-water interface, which can be transferred onto solid substrates.5,6 Surfactants exhibit a plethora of phases including spherical micelles and vesicles depending on the concentration of the solution, temperature, molecular structure, and other experimental factors.7 Therefore, we anticipate that the self-assembly of SECs should strongly depend on the properties of the surfactants. SECs have been shown to form ordered 2-D arrays at the air-water interface as observed by TEM.8 Finally, surfactants are expected * Corresponding author. Email:
[email protected]. Fax: (+49)30-20937632). † University of Kentucky. ‡ University of Ulm. § Max Planck Institute of Colloid & Interfaces, Germany, and National Institute for Materials Science, Japan. | Humboldt University Berlin. (1) Yamase, T. Chem. ReV. 1998, 98, 307. (2) Long, D. L.; Burkholder, E.; Cronin, L. Chem. Soc. ReV. 2007, 36, 105. (3) Liu, S. Q.; Volkmer, D.; Kurth, D. G. Pure Appl. Chem. 2004, 76, 1847. (4) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Colfen, H.; Koop, M. J.; Muller, A.; Du Chesne, A. Chem.sEur. J. 2000, 6, 385. (5) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Muller, A.; Schwahn, D. J. Chem. Soc., Dalton Trans. 2000, 3989. (6) Clemente-Leon, M.; Coronado, E.; Soriano-Portillo, A.; Mingotaud, C.; Dominguez-Vera, J. M. AdV. Colloid Interface Sci. 2005, 116, 193. (7) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401. (8) Volkmer, D.; Du Chesne, A.; Kurth, D. G.; Schnablegger, H.; Lehmann, P.; Koop, M. J.; Muller, A. J. Am. Chem. Soc. 2000, 122, 1995.
to direct the self-assembly of SECs on solid surfaces.9,10 Also, thermotropic mesophases, micelles, and honeycomb architectures have been realized using SECs.11-14 Tungsten POMs are known for their high stability, redoxchemistry, photochemical reactions, and their catalytic properties.15 Therefore, we have chosen for this study the wellestablished Preyssler anion [Na(H2O)P5W30O110]14- encapsulated in a shell of either dimethyldioctadecylammonium (DODA), dimethylditetradecylammonium (DTDA), or methyltrioctadecylammonium surfactants (Scheme 1). Here, we present the structures that are formed on the basal plane of graphite using scanning probe methods. By the addition of tetratetracontane molecules (C44H90) to the SECs solution, SECs are individualized on the surface, which are investigated by scanning tunneling microscopy (STM). Experimental Section Materials and Methods. The compounds (DODA)14[Na(H2O)P5W30O110], (DTDA)14[Na(H2O)P5W30O110], and (TODA)14[Na(H2O)P5W30O110] (TODA ) methyltrioctadecylammonium) were synthesized according to a previously published procedure5 and characterized by elemental analysis. (DODA)14[Na(H2O)P5W30O110]. Anal. calcd for C532H1122N14NaO111P5W30, M ) 15185.7 g mol-1: H, 7.45%; C, 42.08%; N, 1.29%. Found: H, 7.81%; C, 41.93%; N, 1.31%. (DTDA)14[Na(H2O)P5W30O110]. Anal. calcd for C420H898N14NaO111P5W30, M ) 13614.7 g mol-1: H, 6.65%; C, 37.05%; N, 1.44%. Found: H, 7.01%; C, 37.23%; N, 1.41%. TODA)14[Na(H2O)P5W30O110]. Anal calcd for C770H1598N14NaO111P5W30, M ) 18524.0 g mol-1: H, 8.70%; C, 49.93%; N, 1.06%. Found: H, 9.09%; C, 49.99%; N, 1.15%. Tetratetracontane (purity >99%, (9) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681. (10) Severin, N.; Rabe, J. P.; Kurth, D. G. J. Am. Chem. Soc. 2004, 126, 3696. (11) Li, W.; Yi, S. Y.; Wu, Y. Q.; Wu, L. X. J. Phys. Chem. B 2006, 110, 16961. (12) Nyman, M.; Ingersoll, D.; Singh, S.; Bonhomme, F.; Alam, T. M.; Brinker, C. J.; Rodriguez, M. A. Chem. Mater. 2005, 17, 2885. (13) Bu, W. F.; Li, H. L.; Sun, H.; Yin, S. Y.; Wu, L. X. J. Am. Chem. Soc. 2005, 127, 8016. (14) Li, H. L.; Sun, H.; Qi, W.; Xu, M.; Wu, L. X. Angew. Chem., Int. Ed. 2007, 46, 1300. (15) Hiskia, A.; Mylonas, A.; Papaconstantinou, E. Chem. Soc. ReV. 2001, 30, 62.
10.1021/la7032143 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/15/2008
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Scheme 1. Self-Assembly of the Negatively Charged Polyoxometalate Cluster and the Surfactants Results in Formation of the Surfactant-Encapsulated Cluster (SEC)a
a
The structure of the Preyssler anion, [Na(H2O)P5W30O110]14-, is shown at the bottom
Aldrich) was obtained and used without further purification. The SECs were deposited by spin coating from dilute toluene solutions (3.23 × 10 -6 mol/L) onto freshly cleaved highly oriented pyrolytic graphite (HOPG) at 40 rps. Scanning force microscopy (SFM) was carried out at roomtemperature in air using a commercial instrument (Nanoscope IIIa, Digital Instruments, CA) and silicon cantilevers (Olympus) with a resonance frequency from 200 to 400 kHz and a force constant of 42 N/m. All measurements were taken in the tapping mode. Scanning tunneling microscopy (STM) was performed in situ at the liquid-solid interface using a home-built beetle-type STM interfaced with a commercial controller (Omicron). A drop (about 0.05 mL) of almost saturated solution of tetratetracontane and (DODA)14[Na(H2O)P5W30O110] in a 1:1 molar ratio in 1,2,4trichlorobenzene was deposited on HOPG surface before measuring. STM tips were mechanically cut from a 0.25 mm thick Pt/Ir (80%/ 20%) wire. STM current images were recorded in quasi-constant height mode.
Results and Discussion First, we study the self-assembly of (DODA)14[Na(H2O)P5W30O110] on the basal plane of graphite. In particular, we aimed at preparing films with submonolayer coverage, so we can clearly image any self-assembled structure. Surprisingly, we observe rodlike structures with a length ranging from 10 to 80 nm. The rods are packed parallel to each other into rafts or domains with three predominant orientations (Figure 1). The angle between the domains is about 120°, which reflects the symmetry of the underlying graphite lattice. The average width of one rod can be estimated from the ordered domains to be 5.50 ( 0.20 nm. The central POM core, [Na(H2O)P5W30O110]14-, is a doughnut-shaped anion with C5v symmetry. The diameter along the long axis (perpendicular to C5 axis) is 1.71 nm. The length of a fully extended DODA molecule is approximately 2.25 nm. Therefore, the diameter of a single SEC adsorbed on a surface can be as large as 6.2 nm, which is a little more than the experimentally
determined value with SFM indicating that the alkyl chains are not completely extended. Perpendicular to the graphite surface (z axis), the apparent height of one rod is 1.4 ( 0.2 nm, which is obtained from the cross section analysis (Figure 1). It is worth mentioning that although the measured height value depends on the humidity and on the SPM tip shape16 it is obvious that the SECs are somewhat flattened due to the interactions between the flexible alkyl chains and the graphite surface. Moreover, the small height value indicates that the rods are composed of single SECs packing one by one next to each other along the substrate surface and no aggregation occurs in the direction perpendicular to the graphite surface (z axis). It has been reported that salts of [NaP5W30O110]14- form crystals on a graphite surface and big islands are observed using SFM,17 but to the best of our knowledge, it is the first example of SEC nanorods to be observed. To exclude the possibility that the rods are composed of pure surfactant molecules a control experiment was carried out. Compared to the SEC solution (ca. 4.52 × 10 -5 mol/L), a 14fold amount of pure DODA‚Br was applied to the graphite surface. However, only lamellar monolayers are observed (Figure 2). The lamella periodicity is approximately 4.6 nm, corresponding to a bilayer of fully extended DODA‚Br. In addition, we used STM to investigate the (DODA)14[Na(H2O)P5W30O110] assemblies in situ at the solid-liquid interface. No stable assembly is formed at the surface if we use a neat solution of the SECs. To immobilize the SECs at the solidliquid interface, tetratetracontane was added to the SEC solution since it is well known that long-chain alkanes have a strong tendency to form well-ordered lamellae at the graphite-liquid interface.18 Also, the formation of an organic adlayer increases (16) Zhuang, W.; Ecker, C.; Metselaar, G. A.; Rowan, A. E.; Nolte, R. J. M.; Samori, P.; Rabe, J. P. Macromolecules 2005, 38, 473. (17) Kaba, M. S.; Song, I. K.; Duncan, D. C.; Hill, C. L.; Barteau, M. A. Inorg. Chem. 1998, 37, 398. (18) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424.
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Figure 1. SFM height image of (DODA)14[Na(H2O)P5W30O110] on graphite, exhibiting straight nanorods oriented parallel to each other in small domains. The angle R between rods is 120°, reflecting the symmetry of the underlying substrate lattice. The line scan shows that the height of one cylinder along the direction perpendicular to a graphite surface is 1.4 ( 0.2 nm.
the lateral diffusion barrier and can therefore acts as template for the adsorption of other species.9,19,20 From the STM images (Figure 3) well-ordered alkane lamellae can be identified while single spherical objects are observed scattered randomly across the alkane monolayer. While the alkane lamellae are imaged very clearly, it is not possible to resolve the SECs with higher resolution. The formation of the film is characterized by the competitive adsorption of alkanes and SECs. Apparently, the interaction between alkane molecules and graphite is stronger than that of SECs and graphite. Therefore, the alkane lamellae probably form first, thereby providing an organic monolayer on graphite. As a result of the increased lateral diffusion barrier across the monolayer, the SECs are immobilized strong enough so that they can be imaged by STM. Since the adsorption of SECs on the alkane lamellae is dominated by van der Waals forces and (19) Askadskaya, L.; Rabe, J. P. Phys. ReV. Lett. 1992, 69, 1395. (20) Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, S. X.; Wang, H. N.; Xu, S. D.; Bai, C. L. J. Am. Chem. Soc. 2000, 122, 5550.
the periodicity of the lamellae is too small to accommodate a single SEC, there are no preferential adsorption sites and hence no ordered SEC arrays are observed in this case. To study the effect of the shell on the ordering of the SECs, two other SECs with different surfactant shells, namely, (DTDA)14[Na(H2O)P5W30O110] and (TODA)14[Na(H2O)P5W30O110, were investigated. Similar to the previous example, (DTDA)14[Na(H2O)P5W30O110] self-assembles into nanorods with lengths ranging from 30 to 100 nm (Figure 4). The width of a nanorod is 4.8 ( 0.3 nm, which is lower than that observed for (DODA)14[Na(H2O)P5W30O110] as the chain length of the surfactant is shorter by four methylene groups. The width of a single (DTDA)14[Na(H2O)P5W30O110] is estimated to be as large as 5.2 nm. In contrast, no ordered structure is observed for (TODA)14[Na(H2O)P5W30O110] (Figure 5). The surfactant structure has a profound impact on the structure of the resulting assemblies including the relative cluster-surfactant organization, the orientation and tilt of the surfactant tails, the degree of interdigitation, and the inclusion of small solvent
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Figure 2. SFM phase image of pure DODA‚Br on graphite. Wellordered lamellae in three domains can be recognized.
Figure 3. STM current image of tetratetracontane and (DODA)14[Na(H2O)P5W30O110] coadsorbed on graphite. The alkane lamellae are clearly visible. The round objects are individual SECs. Inset: enlarged image showing individual alkane molecules in the lamellae. Typical tunneling conditions are It ) 1.0 nA, Ut ) 1.5 V.
molecules. The dialkyldimethylammonium compounds used here (DODA, DTDA) prefer the common bilayer architecture in aqueous solution and in bulk, and the same is observed with (DODA)2Mo6O19.21 Apparently, the stratified ordering as in lamellae is lost in the case of the large POM anion used here. Charge compensation and electrostatic interactions result in a core-shell structure. The alkyl density on the cluster surface is assumed to affect the aggregation of the SECs. In the case of high alkyl density, interdigitation of adjacent shells and interactions with the substrate are hindered, and as a result we observe (21) Ito, T.; Sawada, K.; Yamase, T. Chem. Lett. 2003, 32, 938.
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Figure 4. SFM height image of (DTDA)14[Na(H2O)P5W30O110] on graphite arranged in straight nanorods.
Figure 5. Assembly of (TODA)14[Na(H2O)P5W30O110] on graphite surface. No ordered rods are observed in the assembly.
very little ordering. On the other hand, in the case of (DTDA)14[Na(H2O)P5W30O110] and (DODA)14[Na(H2O)P5W30O110], there is apparently sufficient mobility in the surfactant shell to allow epitaxial ordering of the alkyl chains along the lattice axis as well as interactions among adjacent SECs giving rise to straight nanorods.
Conclusions The surfactant-encapsulated clusters of the Preyssler anion, [Na(H2O)P5W30O110]14-, organize into straight nanorods on the basal plane of graphite. On the other hand, if a trialkylmethylammonium surfactant is used as in (TODA)14[Na(H2O)P5W30O110], no ordering is observed. This surprising result is rationalized in terms of alkyl chain interactions with the surface
Surfactant-Encapsulated Polyoxometalate Clusters
and between adjacent SECs. A high alkyl chain density in the surfactant shell significantly reduces the interaction with the surface and neighboring SECs, and thus no ordering is observed. Also, we note that if the SECs are coadsorbed with a long-chain alkane no ordering of the SECs is observed but individual SECs can be imaged. We propose that the increase in lateral diffusion barrier immobilizes the SECs sufficiently strong to prevent the formation of ordered structures. The process of individualizing SECs on the surface offers a route to study the properties of
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single SECs on a surface, e.g., by scanning probe techniques. Work on adjusting the surfactant part to control the SEC assembly is in process, since ordered thin film based on SECs bear promising perspectives toward organic-inorganic hybrid materials. Acknowledgment. This work was supported by the Volkswagen-Stiftung ‘Complex Materials’. D.V. and D. G. K. thank Deutsche Forschungsgemeinschaft for financial support. LA7032143
