NANO LETTERS
Original Single Walled Nanotubules Based on Weakly Interacting Covalent Mineral Polymers, 1∞[Nb2PS10-] in N-Methylformamide
2002 Vol. 2, No. 4 403-407
Franck Camerel, Jean-Christophe P. Gabriel,* and Patrick Batail* Sciences Mole´ culaires aux Interfaces, CNRS FRE 2068, Nantes, France
Patrick Davidson and Bruno Lemaire Laboratoire de Physique des Solides, UMR 8502 CNRS, Baˆ t. 510, UniVersite´ d’Orsay, F-91405 Orsay, France
Marc Schmutz† Institut de Ge´ ne´ tique et de Biologie Mole´ culaire et Cellulaire, (INSERM,CNRS/ULP) 1, rue Laurent Fries, BP 163, F-67404 Illkirch, France
Thadde´e Gulik-Krzywicki Centre de Ge´ ne´ tique Mole´ culaire, UPR 2167 CNRS, Baˆ t. 26, AV. de la Terrasse, F-91198 Gif sur YVette, France
Claudie Bourgaux Laboratoire pour l’Utilisation du Rayonnement Electro-magne´ tique, UMR 130 CNRS, UniVersite´ Paris Sud, Baˆ timent 209D, Centre UniVersitaire, F-91405 Orsay, France Received November 26, 2001; Revised Manuscript Received February 6, 2002
ABSTRACT This paper reports the synthesis of a new one-dimensional phase, NaNb2PS10, which is found to be soluble in polar organic solvents such as N-methylformamide (NMF) or dimethylformamide (DMF). Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) studies have revealed that these covalent polymer coils fold into unprecedented single wall monodispersed nanotubules in NMF (external diameter 10 nm; wall thickness ) 1.6 nm; length from 10 nm to over one micrometer). Cooperative weak bonds involving the hydrogen bond donor/acceptor ability of the primary amide solvent at the organic−inorganic interface are likely to assemble the flexible, charged covalent mineral polymer and indeed stabilize the nanotubule wall.
An active field of research in materials chemistry is to synthesize structurally well-defined building blocks of the nanometer length scale which are of interest for the synthesis of designed catalysis, photonic band gap materials, nanoscale electronic devices, and chemical separation media.1 In the large variety of shapes and dimensionalities in which these building blocks can be shaped, tubular structures are of particular interest because of their inherent mechanical * To whom correspondence should be addressed Present address for J.-C.P.G.: Covalent Materials, Inc. 1295 A, 67th Street, Emeryville, CA 94608 (USA). Fax: (510) 658 0425. E-mail:
[email protected]. Present address for P.B.: FRE 2447, CNRS-Universite´ d’Angers, Baˆt. K, UFR Sciences, 2, Boulevard Lavoisier, 49045 Angers, France. Fax: +33 241 735 011. E-mail:
[email protected]. † Present address: Institut Charles Sadron UPR 22 CNRS, 6, rue Boussingault, F-67083 Strasbourg, France. 10.1021/nl010090l CCC: $22.00 Published on Web 03/08/2002
© 2002 American Chemical Society
strength,2 their electronic transport properties,3 and their ability to act as containers or capsules.4 Although such structures are ubiquitous in living systems and are increasingly common in supramolecular organic chemistry,5 there are not many examples of purely inorganic nanotubes reported (MoS2 and WS2,6 BN,7 BxCyNz,8 NiCl2,9 V2O5,10 InS,11 Bi12). The majority of these inorganic nanotubes synthesis are performed at high temperature in the gas phase. Therefore, rational synthesis strategies based on low-temperature solution-phase chemistry should have intrinsic advantages such as, for instance, the direct production of suspensions of nanotubules. In this context, we have recently shown that the use of small, highly polar amides like dimethylformamide (DMF) and N-methylformamide (NMF) as powerful molecular functions to dissolve or dismantle
Figure 1. View of the 1∞[Nb2PS10]- chain (ORTEP; thermal ellipsoids at the 90% probability level) showing the atom labeling and selected Nb-Nb bond lengths.
charged, anisotropic covalent mineral compounds, can yield complex mineral fluids, which eventually exhibit transient or permanent liquid crystalline textures, leading to some of the rare cases of mineral liquid crystal (MLC).13 There is increasing evidence that such small amides, in addition to their polar character and high dielectric constants, also act as a structuring agent at the organic-inorganic interface within these complex fluids. For example, DMF plays a clear structuring role in the dispersion/fragmentation/rearrangement sequence which transforms KNiPS4 chains into the concave inorganic cycle [(NiPS4)3]3-.14 We now report on the structural influence of such small amides and demonstrate their ability to organize the title semiflexible covalent polymer chains up to a very large length scale to form original mineral nanotubules. We derived the synthesis of NaNb2PS10 15 from the one previously reported for KNb2PS10 16 using the flux method17 from P2S5, Nb, and Na2S3.18 Pure powder in bulk quantities could be obtained in good yields at 500 °C. However, because the powder X-ray diffraction pattern did not match that of MNb2PS10 (M ) K, Rb, Cs),16 we performed an X-ray structure determination on a single-crystal grown at 650 °C.19 The overall framework of the 1∞[Nb2PS10]- chain is retained within this novel, monoclinic modification of the former triclinic KNb2PS10 structure. This chain is best described as based on bicapped trigonal distorted biprism building blocks20 that are bridged via thiophosphate groups (Figure 1). As expected, NaNb2PS10 is soluble at room temperature in various polar solvents such as formamide (FA), Nmethylformamide, dimethylformamide, or dimethyl sulfoxide (DMSO), leading to clear dark red solutions.21 In the following, we shall essentially consider the solutions of NaNb2PS10 in NMF. Indeed, NaNb2PS10 dissolves slowly in NMF, within minutes to hours depending upon the concentration. Quite remarkably, at a concentration larger than 0.018 mol‚L-1, these solutions, observed in polarized light between crossed polarizers, display flow birefringence. Such flow birefringence can arise either from a preferred orientation of anisotropic rigid molecules induced by the flow (like logs aligned with the flow in a river)22 or from stretching of entangled, folded, semiflexible polymers (like drawn spaghetti). No permanently birefringent fluid solutions could be obtained in NMF, even in concentrated solutions, which would have otherwise strongly suggested the liquid crystal404
line ordering of rigid objects. Therefore, at this stage, one cannot decide in favor of one or other explanation. Nevertheless, the observed flow birefringence indicates the existence of persistent 1∞[Nb2PS10]- chains in NMF solutions. Moreover, X-ray diffractograms at high angles of concentrated solutions display broad peaks at 0.38 and 0.28 nm corresponding to the Nb-Nb distances within the chain. Again, this strongly suggests that the 1∞[Nb2PS10]- polymers are preserved in solution. Furthermore, it should be noted that months after their preparation there is no evidence for a degradation of these solutions provided dry solvents are used and solutions are previously degassed with argon or nitrogen before sealing the vials. To understand the origin of this flow birefringence, a study by small-angle X-ray scattering (SAXS) was performed on the D24 and ID2 experimental stations at the LURE and ESRF synchrotrons, respectively.23 We first examined the scattering from semi-concentrated samples (0.071 > c > 0.004 mol‚L-1). Typical SAXS curves versus scattering vector modulus q (q ) (4π/λ) sin θ, where 2θ is the scattering angle and λ is the wavelength) are shown in Figure 2a. The scattering signal can be divided into three components: (i) a strong small angle scattering (0.1 < q < 0.5 nm-1) proves the existence of large objects in suspension; (ii) superimposed on this small angle scattering, a broad peak labeled A is also observed. Because its position shifts to lower q upon decreasing concentration, it reveals the existence of an average distance, dA, between the moieties in suspension in the solvent. The dependence of dA upon concentration, dA ∝ c-1/2, is that expected for the 2D swelling of 1D cylinders;24 (iii) another broad peak labeled B, around q ≈ 2π/7 nm-1, is also observed together with its higher orders (Figure 2a, inset). Because the position of this intensity modulation barely changes with concentration, it must be due to interferences within each object. At this stage, two assumptions may be made to explain these interferences: they may either arise from the form factor of monodisperse moieties in solution or from a superstructure of the chains in solution such as a helical pitch for instance. This distance cannot be explained from the chemical structure of the chain and is not observed in the X-ray powder diagrams of the starting materials. Upon increasing concentration in NMF (c g 0.14 mol‚L-1), the SAXS patterns start displaying very thin diffraction lines superimposed on the previously described broad peaks (Figure 2b). These diffraction lines can be indexed with a 2D hexagonal lattice and therefore point to the appearance of a columnar mesophase (H) of hexagonal symmetry. In other words, the cylinders orient in the same direction and pack on a 2D hexagonal lattice perpendicular to their main axis. Because no (hkl) (h * 0, k * 0, and l * 0) reflection is observed, this phase has no 3D correlations and, in principle, the cylinders can slide past each other, as expected for a mesophase. The integrated intensities of the SAXS signals show that the proportion of the hexagonal mesophase increases with concentration, indicating that the isotropic (I)/ hexagonal phase transition is first-order (i.e., with phase coexistence). Unfortunately, the pure hexagonal mesophase Nano Lett., Vol. 2, No. 4, 2002
Figure 3. Arrays of nanotubules as observed by FFEM of a 0.140 mol‚L-1 solution of NaNb2PS10 in NMF: (a) the cryofracture plane makes an arbitrary angle with the nanotubule axes; (b) the cryofracture plane is roughly perpendicular to the nanotubule axes.
Figure 2. (a) Evolution of the SAXS patterns versus concentration of solutions of NaNb2PS10 in NMF. (0, 0.071 mol‚L-1; O, 0.036 mol‚L-1; 4, 0.018 mol‚L-1; 3, 0.009 mol‚L-1; ], 0.004 mol‚L-1). The scattering signal can be divided into three components: (i) a strong small angle scattering (0.1 < q < 0.5 nm-1), (ii) a broad peak also located at low q and labeled A whose position shifts to lower q upon decreasing concentration, and (iii) another broad peak, labeled B, located at q ≈ 2π/7 nm-1. Its higher orders are also observed (inset). (b) SAXS patterns of a solution of NaNb2PS10 in NMF (0.143 mol‚L-1) showing additional sharp diffraction lines that can be indexed with a 2D hexagonal lattice. (c) SAXS pattern of the previous sample subjected to a shear rate γ˘ ) 283 s-1. Inset: SAXS patterns showing the melting transition of the hexagonal phase upon increasing shear rate.
could not be obtained without any isotropic solution because the saturation (0.16 mol‚L-1) of the suspension was reached beforehand. It should be noted that the occurrence of a Nano Lett., Vol. 2, No. 4, 2002
hexagonal mesophase implies a low polydispersity in the cylinder diameter. Shearing an H/I biphasic sample (0.143 mol‚L-1) in a Couette shear cell resulted in the partial alignment of the hexagonal MLC. However, beyond a shear rate of 240 s-1 the H phase melted and its diffraction lines disappeared (Figure 2c). This melting transition under shear is reversible as the diffraction lines reappeared when the shearing was stopped. Such behavior was first reported recently for the hexagonal mesophase of a surfactant25 and our experiments nicely extend this result to MLCs. In summary, the existence of a hexagonal mesophase is more compelling evidence for the cylinder-like conformation of 1 [Nb PS ]- chains in NMF solution. ∞ 2 10 To directly examine the conformation of the chains, concentrated solutions of 1∞[NaNb2PS10] in NMF (0.018 < c < 0.140 mol‚L-1) were studied by freeze fracture electron microscopy (FFEM).26 At a concentration of 0.140 mol‚L-1, FFEM images show cylinders of 10 ( 1 nm in diameter, separated by about 15 ( 1 nm (Figure 3a). These microscopic observations are therefore in very good agreement with the SAXS studies, which infer the existence of 1D moieties separated by an average distance of 16 nm. Furthermore, this 10 nm diameter accounts for the B modulation observed by SAXS all the more because this diameter appears fairly monodisperse by FFEM.27 Besides, areas where the cylinders are regularly packed were also observed and most probably correspond to small domains of the hexagonal phase detected by SAXS (Figure 3b). The stability of the cylinders upon dilution in NMF was tested by low dose TEM,28 which shows their existence even at low concentration, indicating that the superstructure is not induced by cylinder-cylinder interactions (Figure 4). It should be noted here that the length of the cylinders seems to be largely polydisperse, with the small rods usually adsorbed perpendicular to the microscope grid (corresponding to rings on the photographs) and the longer ones lying along the grid29 (Figure 4a). Isolated, very straight tubes of more than one micron in length could even be observed (Figure 4b). The much stronger absorption contrast generally observed from the sides of the cylinders compared to their cores, the thickness of the wall (1.6(1) nm), which is close to the diameter of the NaNb2PS10 chains (1.5 nm), and the circular extremities suggest that these objects are single-wall tubes. 405
Figure 4. Image obtained by low dose TEM of very dilute solutions of NaNb2PS10 in NMF (a) showing single-wall nanotubules, consisting mainly of 10 nm diameter tubes (1), a few 7 nm tubes (2), a nanotubule presenting a defect allowing its diameter to vary from 7 to 10 nm (3), and small rings corresponding to tubes seen along their axis (4). (b) Image of an isolated and very straight tube of more than one micron in length.
Figure 5. Globular bodies of about 10 nm in diameter observed by FFEM of a 0.124 mol‚L-1 solution of NaNb2PS10 in DMF.
We now turn to the properties of 1∞[Nb2PS10]- chains dissolved in DMF. Quite unexpectedly, these solutions are much less viscous than those in NMF and never show any flow birefringence. Furthermore, the SAXS patterns of these suspensions only display the A broad peak due to interferences between objects, but not the B peak that is typical of the nanotubule superstructure. This suggests that there are important differences in chain conformations in the solvents. This is reflected by the dependence of dA upon concentration in DMF, dA ∝ c-1/3, which is that awaited for the 3D swelling of globular objects.24 In addition, the replicas of freeze fracture samples (0.124 mol‚L-1) did not present any image of nanotubules, but instead confirmed the existence of globular bodies about 10 nm in diameter (Figure 5). These results prove that solvent-chain interactions play a key role in the conformation adopted by the 1∞[Nb2PS10]- chains in solution. This key role of the solvent is further confirmed 406
because nanotubule superstructures can only be observed for solutions in which the solvent is a strong hydrogen bond donor such as NMF or FA and not DMF or DMSO.30 In summary, we have disclosed unprecedented nanotubules formed by aggregation of flexible covalent charged inorganic polymers in N-methylformamide. Two factors are important in this respect: the flexibility of the covalent mineral chains and the local patterns of weak amide-chain interactions. The TEM results show that these nanotubules can be isolated because their structures are retained either after air or freezedrying, thus opening the way to their nanomanipulation.31 In addition, they self-assemble into domains of hexagonal packing, reminiscent of the organization of nanotubes into bundles. Also, although the structure of the walls in the present tubules remains to be fully solved, one can already point out a major difference with current inorganic nanotubes. In the latter, neighboring atoms are all covalently bonded to each other. In the present objects, since the walls arise from the folding or coiling of one-dimensional chains, cooperative weaker bonds, such as hydrogen bonds as well as van der Waals interactions, should probably be considered to understand the nanotubule wall structure.32 The formation of the organic-inorganic nanotubules reported herein is reminiscent of the nanotubule formation mechanism based on a conformation change and weaker interactions in the field of biopolymers, the helix-coil transition of polypeptides and block copolymers,33 triggered by a change in solvent or temperature. Acknowledgment. We would like to thank Drs. J. Sayettat, R. Brec, and S. Jobic for supplying us with a sample of KNb2PS10 on which we first observed solubility in hot DMSO, the LURE and the ESRF for the award of beamtime, Drs. P. Panine and T. Narayanan for technical support at ID2, Dr. S. Uriel for performing mass spectra and Dr. L. M. Bull for 31P MAS-NMR spectra. Financial support from the Ministry of Education (Ph.D. fellowship for F.C.), the Ecole Normale Supe´rieure and the Ecole Nationale des Ponts et Chausse´es (Ph.D. fellowships for B.L.), INSERM, l’Hoˆpital Universitaire de Strasbourg, the Re´gion Pays de Loire and the GDR-CNRS 690 FORMES is gratefully acknowledged. References (1) (a) Ozin, G. A. AdV. Mater. 1992, 4, 612. (b) Gates, B.; Qin, D.; Xia, Y. AdV. Mater. 1999, 11, 466-469. (c) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66-69. (2) Van Lier, G.; Van Alsenoy, C.; Van Doren, V.; Geerlings, P. Chem. Phys. Lett. 2000, 326, 181-185. (3) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. Nature 1998, 391, 62-64. (4) Meyer, R. R.; Sloan, J.; Dunin-Borkowski, R. E.; Kirkland, A. I.; Novotny, M. C.; Bailey, S. R.; Hutchison, J. L.; Green, M. L. H. Science 2000, 289, 1324-1326. (5) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. Engl. 2001, 40, 988-1011. (6) (a) Feldman, Y.; Wasserman, E.; Srolovita, D. J.; Tenne, R. Science 1995, 267, 222-225. (b) Tenne, R.; Homyonfer, M.; Feldman, Y. Chem. Mater. 1998, 10, 3225-3238. (7) Chopra, N. G.; Luyren, R. J.; Cherry, K.; Crespi, V. H.; Cohen, M. L.; Louis, S. G.; Zettl, A. Science 1995, 269, 966-967. (8) Stephan, O.; Ajayan, P. M.; Colliex, C.; Redich, Ph.; Lambert, J. M.; Bernier, P.; Lefin, P. Science 1994, 266, 1683-1685. (9) Hacohen, Y. R.; Grunbaum, E.; Sloan, J.; Hutchison, J. L.; Tenne, R. Nature 1998, 395, 336-337. Nano Lett., Vol. 2, No. 4, 2002
(10) Spahr, M. E.; Bitterli, P.; Nesper, R.; Mu¨ller, M.; Krumeich, F.; Nissen, H.-U. Angew. Chem., Int. Ed. Engl. 1998, 37, 1263-1265. (11) Hollingsworth, J. A.; Poojary, D. M.; Clearfield, A.; Buhro, W. E. J. Am. Chem. Soc. 2000, 122, 3562-3563. (12) Li, Y.; Wang, J.; Deng, Z.; Wu, Y.; Sun, X.; Yu, D.; Yang, P. J. Am. Chem. Soc. 2001, 123, 9904-9905. (13) (a) Davidson, P.; Gabriel, J.-C.; Levelut, A.-M.; Batail, P. Europhys. Lett. 1993, 21, 317-322. (b) Davidson, P.; Gabriel, J.-C.; Levelut, A.-M.; Batail, P. AdV. Mater. 1993, 5, 665-668. (c) Gabriel, J.-C. P.; Davidson, P. AdV. Mater. 2000, 12, 9-20. (d) Gabriel, J.-C. P., Boubekeur K., Uriel S., Batail P. Chem. ReV. 2001, 101, 20372066. (14) Sayettat, J.; Bull, L. M.; Gabriel, J.-C. P.; Jobic, S.; Camerel, F.; Marie, A.-M.; Fourmigue´, M.; Batail, P.; Brec, R.; Inglebert, R.-L. Angew. Chem. 1998, 110 (12), 1773-1776. Sayettat, J.; Bull, L. M.; Gabriel, J.-C. P.; Jobic, S.; Camerel, F.; Marie, A.-M.; Fourmigue´, M.; Batail, P.; Brec, R.; Inglebert, R.-L. Angew. Chem., Int. Ed. Engl. 1998, 37 (12), 1711-1714. (15) NaNb2PS10 was prepared by the flux method from a stoichiometric reaction of Na2S3 (0.75 mmol, 107 mg), P2S5 (ALFA, 99.5%, 0.75 mmol, 167 mg), Nb (ALFA, 99.8%, 3 mmol, 279 mg), and S (FLUKA, 99.99%, 9 mmol, 288 mg). Na2S3 was made from the stoichiometric reaction of elemental Na (Aldrich, 99.5%) with S in liquid ammonia. Starting materials were loaded under N2 atmosphere into a quartz tube and then sealed under a residual N2 pressure of 10-3 Torr (1 Torr ) 133.32 Pa). Red needlelike single crystals of NaNb2PS10, suitable for X-ray diffraction and crystal resolution, were obtained by heating the mixture at 650 °C for 1 month (heating rate 10 °C/h; cooling rate 5 °C/h). Electron microprobe analysis was performed on an EDAX-equipped JEOL JSM-5800LV, for NaNb2PS10: found (calcd.) Na 3.9 ( 0.4 (4.10), Nb 33.8 ( 0.9 (33.16), P 5.3 ( 0.2 (5.53), S 56.9 ( 0.7 (57.39). Microcrystalline red powder could be obtained in good yield (85%) by heating the above mixture at 500 °C, as proved by X-ray powder diffraction diagram. 31P solidstate NMR display a single peak at 119.85 ppm with respect to 85% H3PO4 at 0 ppm. (16) (a) Do, J.; Yun, H. Inorg. Chem. 1996, 35, 3729-3730. (b) Sayettat, J. Ph.D. Thesis, Nantes, 1997. (17) Sunshine, S. A.; Kang, D.; Ibers, J. A. J. Am. Chem. Soc. 1987, 109, 6202-6204. (18) (a) Bauer, G. Handbook Prep. Inorganic Chemistry; Academic Press: New York, 1963. (b) Bensch, W.; Na¨ther, C.; Du¨richen, P. Angew. Chem., Int. Ed. Engl. 1998, 37, 133-135. (19) The structure was solved by single-crystal X-ray diffraction. Although most crystals were twinned, a red fragment of a single crystal (0.42 × 0.03 × 0.03 mm3) of the title compound could be selected for X-ray diffraction analysis and mounted on a Stoe IPDS single φ-axis diffractometer with a 2D area detector based on Imaging Plate technology. Crystal data for NaNb2PS10, M ) 560.436 g/mol, F(000) ) 2144, monoclinic C2/c (no. 15), a ) 24.799(5), b ) 7.8495(16), c ) 12.905(3) Å, β ) 90.87(3)°, V ) 2511.7(9) Å3, Z ) 8, graphite monochromated Mo KR radiation, λ ) 0.710 73 Å, T ) 293 K, Dc ) 2.964 Mg‚m-3. A total of 10 459 reflections were measured (5.44 e 2θ e 51.74°; -30 e h e 30, 0 e k e 9, 0 e l e 15), 2423 were unique (Rint ) 0.0592) and 1838 have I > 2σ(I); no decay was observed. Corrections were made for absorption (µ(Mo KR) ) 3.601) by semiempirical method from equivalents (max and min transmission 0.022 and 0.012). The structure was solved by direct-methods (SHELXS-86) and subsequent Fourier difference techniques and refined anisotropically, by full-matrix least-squares, on F2 (program SHELXL-93). The final ωR(F2) was 0.0621, with conventional R(F) ) 0.0296, for 127 parameters, gof ) 0.919, (∆/σ)max ) 0.001. Minimum and maximum ∆F were -0.628 and +0.687 e.Å-3, respectively, in the vicinity of Nb atoms. Further details of the crystal structure investigations may be obtained from the Fachinformationzentrum Karlsruhe D-76344 Eggenstein-Leopoldshafen, Germany (Fax: (+49) 7247-808-666; E-mail:
[email protected]) on quoting the depository number CSD-411728. (20) Bensch, W.; Du¨richen, P.; Na¨ther, C. Solid State Sci. 1999, 1, 85108. (21) Since washing of the crude powder with water induced degradation, solubilization was achieved using as-synthesized microcrystalline powder. NaNb2PS10 can be dissolved in dried polar solvents such as NMF, DMF, DMSO, or FA at room temperature, with measured saturation concentrations of 0.5 and 0.16 mol‚L-1 in DMF and NMF, respectively. These solutions can be further purified by centrifuging off the insoluble impurities (
