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Superconducting Nanocrystalline Tin Protected by Carbon Vilas G. Pol,*,† P. Thiyagarajan,‡ Somobratra Acharya,§ Katsuhiko Ariga,§ and Israel Felner| IPNS, Argonne National Laboratory, Argonne, Illinois 60439, Office of Basic Energy Sciences, Department of Energy, SC-22.1/Germantown Building, 1000 Independence AVenue SW, Washington, D.C. 20585, World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and The Racah Institute of Physics, Hebrew UniVersity of Jerusalem, 91904 Israel ReceiVed December 10, 2008. ReVised Manuscript ReceiVed January 13, 2009 Nanosized pure Sn crystals protected by in situ formed carbon synthesized by the thermolysis of allyltriphenyltin in an inert atmosphere under its autogenic pressure in closed reactor showed superconductivity at 3.7 K.
Nanosized (2 nm) pure Sn crystals protected by in situ formed carbon synthesized by the thermolysis of allyltriphenyltin in an inert atmosphere under its autogenic pressure in a closed reactor shows superconductivity at 3.7 K. It has been predicted and also observed that finite size effects can have a significant influence on the superconducting properties of materials when their dimensions are below the bulk values of their London penetration depth λL(T) and the coherence length ξ(T).1,2 A smaller penetration depth, a noticeable change in Tc, and an enhancement in zero temperature critical fields by more than 2 orders of magnitude were reported for 15 nm Sn nanoparticles.3a Various morphologies of superconducting Sn such as nanoparticles,3b nanodisks,3c nanoplates,3d nanorods,3e and nanowires4 are known but found to be unsuitable because of their high sensitivity to oxidation. Therefore, stabilized nanostructures with coatings to protect them from oxidation will be required when studying their fundamental physical and chemical properties for implementation as nano building blocks in devices such as electron field emission sources or superconducting devices.5 The superconducting Sn nanowires covered with graphitic carbon layers with diameters in the range of 15-35 nm had a 30-fold higher critical magnetic field6 than did metallic Sn because of their reduced diameters. However, XRD and 119Sn Mo¨ssbauer spectroscopy revealed 59% tin dioxide in the core-shell 1D morphologies prepared via catalytic decomposition of acetylene at 700 °C.6 Thus, for practical applications there is an urgent need for an easy, low cost, reproducible method to * Corresponding author. E-mail:
[email protected]. † Argonne National Laboratory. ‡ Department of Energy. § National Institute for Materials Science (NIMS). | Hebrew University of Jerusalem. (1) Moshchalkov, V. V.; Gielen, L.; Strunk, C.; Jonckheere, X.; Van Haesendonck, Q. C.; Bruynseraede, Y. Nature 1995, 373, 319. (2) Guo, Y.; Zhang, Y. F.; Bao, X.-Y.; Han, T.-Z.; Tang, Z.; Zhang, L.-X.; Zhu, W.-G. E.; Wang, G.; Niu, Q.; Qiu, Z. Q.; Jia, J.-F.; Zhao, Z.-X.; Xue, Q.-K. Science 2004, 306, 1915. (3) (a) Yeh, V.; Wu, S. Y.; Li, W.-H. Colloids Surf., A 2008, 313, 246. (b) Hsu, Y.-J.; Lu, S.-Y.; Lin, Y.-F. Small 2006, 2, 268. (c) Salzemann, C.; Urban, J.; Lisiecki, I.; Pileni, M.-P. AdV. Funct. Mater. 2005, 15, 1277. (d) Lu, W.; Ding, Y.; Chen, Y.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2005, 127, 10112. (e) Orendorff, C. J.; Hankins, P. L.; Murphy, C. J. Langmuir 2005, 21, 2022. (4) Tian, M.; Wang, J.; Kurtz, J. S.; Liu, Y.; Chan, M. H. W. Phys. ReV. B 2005, 71, 104521. (5) Nagaoka, K.; Yamashita, T.; Uchiyama, S.; Yamada, M.; Fujii, H.; Oshima, C. Nature 1998, 396, 557. (6) Jankovicˇ, L.; Gournis, D.; Trikalitis, P. N.; Arfaoui, I.; Cren, T.; Rudolf, P.; Sage, M.-H.; Palstra, T. T. M.; Kooi, B.; Hosson, J. D.; Karakassides, M. A.; Dimos, K.; Moukarika, A.; Bakas, T. Nano Lett. 2006, 6, 6.
Figure 1. (a) Powder XRD of SPC, (b) Mossbauer spectrum of SPC, and (c) EDX of the SPC.
synthesize well-defined low-dimensional superconducting materials. This letter reports a facile, reproducible method that yields pure nanocrystalline superconducting Sn particles protected by carbon (SPC) prepared via thermolysis of allyltriphenyltin under its autogenic pressure in a closed reactor. The novelty of our method is that it is a one-pot synthesis and does not involve any template, catalyst, or solvent, which leads to the synthesis of Sn
10.1021/la804076k CCC: $40.75 2009 American Chemical Society Published on Web 02/03/2009
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Figure 2. (a) SEM image of SPC, (b) LR-TEM of SPC (bar ) 10 nm), (c) HR-TE image of SPC, bar ) 2 nm (inset, ED pattern), and (d) HR-TEM of SPC, where the arrow indicates the poor ordering of the surrounding carbon.
nanoparticles coated with a layer of carbon rendering them totally protected in air for the first time. In a typical synthesis of superconducting SPC, 1 g of allyltriphenyltin [C21H20Sn] was placed into a 5 mL stainless steel (SS) reactor of 1/2 in. diameter sealed on one side. The SS reactor containing the precursor was closed tightly with a cap and placed at the center of a box furnace for uniform heating. The temperature of the furnace was ramped to 700 °C at a rate of 40 °C/min and maintained for 30 min. After the SS reactor was cooled (∼3 h) to ambient temperature, a 0.55 g yield of SPC was collected. It is known that the allyltriphenyltin complex with active allyl groups polymerizes at ∼400 °C, which further results in a Sn/hydrocarbon composite.7 The solid-state pyrolysis of allyltriphenyltin in a sealed quartz tube in vacuum at 10-7 Pa at 700 °C resulted in a black powder, which was further purified by sonication and centrifugation to obtain 5 to 20 nm hollow Sn particles encapsulated with carbon.7 In our system, the precursor allyltriphenyltin is placed in a closed SS reactor at atmospheric pressure in an inert atmosphere, which was dissociated under autogenic pressure formed at 700 °C. Because Sn has a low melting point (232 °C), it is anticipated that at T > 300 °C the formed liquid-phase Sn fraction will uniformly disperse into the organic gas phase to form an aerosol. Concurrently, further reactions can occur on the surface of the Sn particles to form a carbon shell at 700 °C. Upon cooling, the carbon-covered liquid Sn is converted to solid Sn, resulting in the formation of nanosized Sn particles covered by semigraphitic carbon. One of our controlled experiments also confirms that the allyltriphenyltin precursor indeed fabricates very small (2 to 3
nm) particles with much higher stabilities because of the higher concentration of carbon in the precursor compared to tetramethyl tin as the precursor. Powder X-ray diffraction (XRD), Mo¨ssbauer spectroscopy, and energy-dispersive X-ray analysis (EDS) were employed to investigate the purity of tin in the SPC. The XRD pattern of the obtained SPC showed the typical diffraction peaks at 30.98, 32.18, 43.18, and 45.18 for metallic Sn with a tetragonal cell (a ) 0.582 Å and c ) 0.317 Å, Figure 1a). The carbon did not show any peak in the XRD pattern because of its amorphous/ semigraphitic nature, and was further confirmed by Raman spectroscopy. 119Sn Mo¨ssbauer spectroscopy served as a powerful complementary tool to XRD to identify and characterize the local environment of Sn in the protected carbon. Mo¨ssbauer measurements were carried out at room temperature using a constant acceleration spectrometer and a 5 mCi Ba119mSnO3 source. The obtained (Figure 1b) spectrum is composed of a singlet (line width of 0.9 mm/s) with an isomer shift of 2.60(1) mm/s relative to BaSnO3. This clearly indicates the presence of only pure β-Sn. The superconducting Sn nanowires encapsulated in carbon nanotubes reported by Jankovicˇ et al. showed two quadrupole doublets6 in Mo¨ssbauer, indicating the presence of β-Sn and SnO2 phases. Furthermore, our EDS analysis of SPC did not detect any elements except C and Sn, further confirming the purity of formed Sn nanoparticles. However, the atomic ratio of these two components cannot be quantified owing to the contributions of carbon from the lacey supported carbon. Taken together, our synthesis route indicates the formation of air-stable Sn nanoparticles owing to the presence of protecting carbon
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Figure 3. Superconducting properties of SPC. (a) Magnetization versus temperature curve, taken at 25 Oe, showing a diamagnetic behavior below the bulk superconducting transition temperature for Sn (inset shows the exact Tc ) 3.7 K). (b) Magnetization versus magnetic field curve taken at 2 K.
layers. It is worth mentioning that the protected narrow Sn particles are stable in air at room temperature even after a year. Thus, our single-step synthesis process is very robust in producing stable superconducting nanoparticles. The scanning electron micrograph (SEM) of SPC shows the overall morphology of the protected tin particles in a carbon matrix (Figure 2a). The spherical aggregates are observed with dark and bright contrast. The tin core particles are surrounded by the faint graphitic layers, which is also confirmed by selectedarea EDS analysis. To gain information on exact particle size, shape, structure, and morphology, SPC was further investigated by employing high-resolution transmission electron microscopy (HR-TEM). The HR-TEM was taken on the dispersed particles. TEM reveals tiny dark dispersed particles surrounded by faint carbon (Figure 2b). The HR-TEM image of a 2 nm Sn particle shows clear lattice fringes with high crystallinity of the Sn particles. The highlighted lattice fringes corresponding to the (020) planes of β-Sn with d spacings of 0.29 nm (Figure 2c). The selected-area electron diffraction (SAED) pattern clearly revealed the (020) and (011) lattice planes of β-Sn with d spacings of 0.29 and 0.27 nm, respectively (inset of Figure 2c). The surrounded semigraphitic carbon marked by an arrow can be seen around the crystalline particle that stabilizes the 2 nm Sn particles. In SPC, the nature of the carbon layer is further confirmed by Raman
Letters
spectroscopy. In the Raman-shift range of 1200-1800 cm-1, two peaks are observed at 1347 and 1598 cm-1 corresponding to D (disordered) and G (graphitic) bands, respectively, with a relative intensity (ID/IG) of 0.86. The G-band corresponds to the tangential stretching (E2g) mode of highly oriented pyrolytic graphite (HOPG) and provides information on the degree of graphitization, whereas the D band at 1347 cm-1 originates from the disorder in the sp2-hybridized carbon atoms.8 Magnetization measurements were conducted to investigate the superconducting properties of the SPC. Figure 3a depicts the temperature-dependent zero-field and field-cooled magnetization curves (measured at 25 Oe) for 14.6 mg of SPC. The precursor, C21H20Sn, is composed of C (64.5 wt %), H (5.1 wt %), and Sn (30.4 wt %) initially. On the basis of this, we estimate the amount of Sn in the SPC to be ∼30 wt %. The data showed the typical diamagnetic behavior of a superconductor with an onset at 3.7 K (Figure 3a), which is very close to Tc ) 3.73 K reported for bulk β-Sn.9 The magnetization drops rapidly below the bulk Tc, but a small tail in the magnetization remains at temperature as high as 3.8 K (inset to Figure 3a). It has been demonstrated that single-crystalline superconductors, with structural characteristic length smaller than the corresponding temperature-dependent coherence length, do not experience the significant suppression in superconducting transition temperature commonly observed for polycrystalline or amorphous superconductors.9 Recently, the negligible suppression in the superconducting transition temperature was reported for 10 nm Sn nanowires.10 The linear isothermal magnetization at 2 K yields a slope of dM/dH ) 0.0175 emu/g (Figure 3b, assuming 7.3 g/cc to be the density of Sn), which is 73% (considering 30% Sn in SPC) of the expected -1/4π for a bulk superconductor. An analogous synthesis process was previously employed for the fabrication of superconducting MgCNi311 and MgB212 nanoparticles. In conclusion, the thermolysis of a single precursor, allyltriphenyltin, in a closed reactor in an inert atmosphere under its autogenic pressure produced pure 2 nm Sn nanocrystals layered by in situ formed carbon. Nanosized superconducting stable tin crystals that are produced in a one-pot process without using catalyst or solvent show superconductivity at 3.7 K. Employing Mo¨ssbauer, XRD, and EDS techniques, the high purity of narrow Sn particles is confirmed. Acknowledgment. This work benefited from the use of facilities at IPNS, CNM, and EMC at ANL, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357 by UChicago Argonne, LLC. LA804076K (7) Cui, G.; Hu, Y.-S.; Zhi, L.; Wu, D.; Lieberwirth, I.; Maier, J.; Mu¨llen, K. Small 2007, 3, 2066. (8) Li, W. Z.; Zhang, H.; Wang, C. Y.; Zhang, Y.; Xu, L.; Zhu, W. K.; Xie, S. S. Appl. Phys. Lett. 1997, 70, 2684. (9) Tian, M.; Wang, J.; Snyder, J.; Kurtz, J.; Liu, Y. Appl. Phys. Lett. 2003, 83, 1620. (10) Hsu, Y.-J.; Lu, S. Y. J. Phys. Chem. B 2005, 109, 4398. (11) Rana, R. K.; Pol, V. G.; Felner, I.; Meridor, E.; Frydman, A.; Gedanken, A. AdV. Mater. 2004, 16, 12. (12) Pol, V. G.; Pol, S. V.; Felner, I.; Gedanken, A. Chem. Phys. Lett. 2006, 433, 115.
