Local Structural Disorder and Relaxation in SnO2 Nanostructures

Mar 10, 2011 - Local Structural Disorder and Relaxation in SnO2 Nanostructures Studied by 119Sn MAS NMR and 119Sn Mössbauer Spectroscopy. Sylvio Indr...
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Local Structural Disorder and Relaxation in SnO2 Nanostructures € ssbauer Spectroscopy Studied by 119Sn MAS NMR and 119Sn Mo Sylvio Indris,†,* Marco Scheuermann,† Sebastian M. Becker,†,‡ Vladimir Sepelak,† Robert Kruk,† Jens Suffner,†,§ Fabian Gyger,‡,|| Claus Feldmann,‡,|| Anne S. Ulrich,‡,^,# and Horst Hahn†,‡,§ †

Institute of Nanotechnology, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany DFG-Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology, Wolfgang-Gaede-Str. 1A, 76131 Karlsruhe, Germany § Joint Research Laboratory Nanomaterials, Technical University Darmstadt - Karlsruhe Institute of Technology, Petersenstr. 32, 64287 Darmstadt, Germany Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesser Str. 15, 76131 Karlsruhe, Germany ^ Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany # Institute of Biological Interfaces (IBG-2), Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany

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‡

ABSTRACT: We studied the local structural disorder and relaxation in different nanostructures of SnO2 by using 119Sn MAS NMR in combination with 119Sn M€ossbauer spectroscopy. We investigated nanocrystalline powders with an average crystallite size of 8 nm as well as hollow spheres with a wall thickness of 3 nm and a diameter of 14 nm, and compared the results to coarse-grained materials. Whereas the uniform SnO6 octahedra in the coarse-grained material show a well-known distortion and thus large electric field gradients, the nanocrystalline SnO2 exhibits a structural relaxation leading to a distribution of local environments and more symmetric octahedra. The SnO2 hollow spheres show strong local disorder in combination with highly asymmetric environments around the Sn atoms.

’ INTRODUCTION Nanostructured materials often exhibit new macroscopic properties in comparison to their coarse-grained counterparts. The reduced structural dimensions can have strong influence on the mechanical,1,2 electrical,3-6 magnetic,7,8 optical,9,10 catalytic,11-13 and thermodynamic14 properties, but also on the diffusion in these materials.15-19 Many of these macroscopic properties are dominated by local structural disorder in the near-surface or interfacial regions. Tin dioxide, SnO2, is a material with many different fields of application like gas sensors,20-23 catalysis,24,25 dye-sensitized solar cells,26,27 and electrode materials for lithiumion batteries.28-37 With respect to applications as electrode materials, nanostructures offer the advantage of short diffusion pathways and increased surface area, which in turn enables fast Li insertion and thus higher power densities.38 Figure 1 shows the tetragonal structure of SnO2.39 It crystallizes in the space group P42/mnm with two formula units per unit cell and lattice constants a = 4.737 Å and c = 3.185 Å. Chains of edge-sharing SnO6 octahedra oriented along the c axis are connected via common corners. All Sn atoms are crystallographically equivalent. The SnO6 octahedra are asymmetric, that is compressed along the c axis, resulting in large electric field gradients at the sites of the Sn nuclei. In this article, we investigate how the local structure in this material changes when the structural dimensions are reduced to a r 2011 American Chemical Society

few nanometers. We use 119Sn magic angle spinning (MAS) nuclear magnetic resonance (NMR) as well as 119Sn M€ossbauer spectroscopy. Both methods are sensitive to changes in the local structure around the Sn probe nuclei.40 The long-range structure was investigated with X-ray diffraction (XRD) and the crystallite sizes/shapes are studied by transmission electron microscopy (TEM).

’ EXPERIMENTAL SECTION SnO2 nanoparticles were prepared by means of chemical vapor synthesis (CVS) using tetramethyltin (TMT, Sn(CH3)4, >99.0%, Sigma-Aldrich) as precursor material.41 Liquid TMT was placed in a bubbler and held at 303 K to compensate for the heat of evaporation. Twenty-five sccm were fed together with 1500 sccm of oxygen and 150 sccm of helium to a hot wall reactor (sccm denotes standard cubic centimeter per minute at standard temperature and pressure). The vapor phase reaction takes place at a temperature of 1473 K in an alumina tube with an inner diameter of 17 mm and a heated length of 35 cm. All gas flows were controlled by means of thermal mass flow controllers (MKS Instruments). The pressure was regulated to a value of 15 mbar Received: January 21, 2011 Revised: February 17, 2011 Published: March 10, 2011 6433

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Figure 2. XRD patterns of microcrystalline SnO2, nanocrystalline SnO2, and SnO2 hollow spheres. Figure 1. Tetragonal structure of SnO2 with chains of edge-sharing SnO6 octahedra oriented along the c axis that are connected via common corners.

using a sliding vane and roots pump (Leybold Vacuum), a Baratron pressure manometer, and a butterfly valve (both MKS Instruments). The as-synthesized particles were separated from the gas stream behind the hot wall reactor in a thermophoretic collector through a thermal gradient created by means of water-cooled walls and the center heated by quartz lamps. SnO2 hollow spheres were prepared in a water-in-oil microemulsion utilizing cetyltrimethylammoniumbromide (Aldrich, 95%) as a surfactant, hexanol (Fluka, >98%) as a cosurfactant, and n-dodecane (Aldrich, >99%) as the nonpolar oil-phase.42 In addition, a 5:1 mixture of methanol (Seulberger, 99%) and demineralized water was added as the polar phase. To this micellar system a solution of Sn(Ot-Bu)4 (ABCR, 99.99%) in dodecane was added dropwise at ambient temperature without stirring. Subsequently, the reaction mixture was left to react for 12 h. Accordingly, hydrolysis of the tin alcoholate occurred at the liquid-to-liquid phase boundary and hollow spheres encapsulating the polar phase were formed. By addition of diethylene glycol the reaction was terminated. The SnO2 hollow spheres were then separated by centrifugation. Finally, the resulting colorless precipitate was washed with ethanol by resuspending and centrifugation and dried in a chamber furnace. Transmission electron microscopy (TEM) of the SnO2 nanoparticles was performed on a CM20 super twin microscope (FEI) operated at an acceleration voltage of 200 kV. Nanoparticles were dispersed in ethanol and dispensed on a carboncoated copper grid. TEM on the SnO2 hollow spheres was performed with a Philips CM200 FEG/ST microscope at an acceleration voltage of 200 kV. XRD measurements were performed with a Philips X’Pert diffractometer using Cu-KR radiation. 119 Sn MAS NMR spectra were acquired with 2.5 mm rotors at a spinning speed of 35 kHz. The magnetic field was 11.7 T corresponding to a resonance frequency of 186.5 MHz. A rotor-synchronized hahn-echo pulse sequence was used with a π/2 pulse length of 2.2 μs and a recycle delay of 60 s. Spectra were referenced to microcrystalline SnO2 with an isotropic chemical shift of -604.3 ppm.43,44 119 Sn M€ossbauer spectroscopic measurements were carried out in transmission mode at room temperature. 119Sn-enriched

Figure 3. Transmission electron micrographs of a) nanocrystalline SnO2 and b) SnO2 hollow spheres. The insert on the left side shows the diffraction pattern.

CaSnO3 was used as a γ-ray source. The velocity scale was calibrated with SnO powder which has an isomer shift of 2.63 mm/s and a quadrupole splitting of 1.30 mm/s.45,46 For each sample 10 mg of powder were deposited on an area of 1 cm2, which corresponds to a number of 3.4  1018 119Sn nuclei per cm2.

’ RESULTS AND DISCUSSION Figure 2 shows the XRD patterns of microcrystalline SnO2, nanocrystalline SnO2, and SnO2 hollow spheres. The pattern of the microcrystalline sample reveals narrow peaks and thus good crystallinity. In contrast to that, the nanocrystalline sample shows strong line broadening typical of small crystallite sizes. Using Scherrer’s equation, a crystallite size of about 5 nm can be estimated from the line width. The SnO2 hollow spheres give rise to even broader peaks that can hardly be resolved any more. Figure 3 shows the TEM images of nanocrystalline SnO2 and SnO2 hollow spheres. For the nanocrystalline SnO2 (part a of Figure 3), the TEM image reveals well-crystallized small particles with an average crystallite size d0 of 7.7 nm. The particle size distribution obtained from TEM can be described with a lognormal distribution47   1 ln d - ln d0 gðdÞ ¼ pffiffiffiffiffiffi exp 2ln2 σd 2π 3 d 3 ln σ d 6434

ð1Þ

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Table 1. 119Sn M€ ossbauer Parameters of Microcrystalline SnO2, Nanocrystalline SnO2, and SnO2 Hollow Spheresa isomer shift

quadrupole splitting

-0.0233 ( 0.0003

0.527 ( 0.001

1.053 ( 0.002

nano -0.0206 ( 0.0055 hollow spheres -0.0117 ( 0.0016

0.487 ( 0.023 0.600 ( 0.004

1.078 ( 0.029 0.975 ( 0.007

micro

a

line width

All values are given in mm/s.

The local structure around the Sn atoms was investigated with Sn MAS NMR spectroscopy. The results for the three samples are shown in Figure 4. All samples show a single symmetric peak centered at around -604 ppm, which is characteristic of the SnO6 octahedron in the tetragonal structure of bulk SnO2.43,44 For the microcrystalline sample, the peak is very narrow with a width of 1.7 ppm. This reveals a high crystallinity with all Sn atoms being located in the same local environment. The nanocrystalline SnO2 and the SnO2 hollow spheres give rise to much broader widths of 6.7 ppm and 54 ppm, respectively. This result is indicative that in these samples a large variety of SnO6 octahedra is present, with a broad distribution of bond angles and bond lengths. The center of these NMR signals is still at about -604 ppm, indicating that the average electron density around the Sn nuclei in the nanostructured samples is identical to the microcrystalline sample. Figure 5 shows the results of 119Sn M€ossbauer experiments. The ground state of the 119Sn nucleus has a nuclear spin I = 1/2 and thus no nuclear quadrupole moment (this is also true for the other NMR active isotopes of Sn, namely 115Sn and 117Sn). Therefore, NMR experiments on this isotope are not sensitive to electric field gradients at the site of the Sn nucleus. In contrast to that, the M€ossbauer experiments, making use of the transition from an excited nuclear state with I = 3/2 to the ground state, are indeed sensitive to such field gradients. For each sample, the M€ossbauer spectrum has to be described in terms of a doublet with overlapping Lorentzian lines. The parameters that result from the corresponding fits (solid lines in Figure 5) are listed in Table 1. For all three samples, the isomer shift is close to 0 mm/s, which is characteristic of the SnO2 structure and in good agreement with the NMR results showing the same average chemical shift for all samples. The microcrystalline sample reveals a quadrupole splitting of 0.53 mm/s, which is caused by the intrinsic asymmetry of the SnO6 octahedra in the regular structure of well-crystallized SnO2. This splitting is significantly smaller in the nanocrystalline sample (0.49 mm/s) than in the bulk, even in view of the larger error associated with this value. This finding suggests that the SnO6 octahedra in the nanocrystalline sample are more symmetric on average, even though the local environment around the Sn atoms has a broader distribution according to NMR. In contrast, the SnO2 hollow spheres show an increased quadrupole splitting of 0.60 mm/s. This parameter reveals strongly distorted SnO6 octahedra, which might indicate a high concentration of point defects. It is remarkable that the overall M€ossbauer transmission of the samples (note the different y scales in Figure 5) differ vastly for the three materials. Whereas the effect is about 15% for microcrystalline SnO2 (Figure 5), it is much smaller for nanocrystalline SnO2 (about 1%) and SnO2 hollow spheres (about 4%). This hints at a strongly reduced Debye-Waller factor in the nanostructured samples and thus to a softening of the crystal lattice. This is in good agreement with the NMR results that show increased structural disorder for the nanostructured samples. 119

Figure 4. 119Sn MAS NMR spectra of microcrystalline SnO2, nanocrystalline SnO2, and SnO2 hollow spheres.

Figure 5. 119Sn M€ossbauer spectra of microcrystalline SnO2, nanocrystalline SnO2, and SnO2 hollow spheres.

with a narrow width σd of 1.1. There is no evidence of amorphous surface layers formed during CVS. The specific surface area of the nanocrystalline SnO2 is 118 m2/g. The hollow spheres (part b of Figure 3) have an outer diameter of 14 nm and a wall thickness of about 3 nm, a dimension corresponding to less than 10 octahedra. They show good crystallinity in TEM and their surface area is 400 m2/g,48 which indicates that the shells are of porous nature.

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The Journal of Physical Chemistry C Figure 6. Summary of the different trends obtained for BET, XRD, NMR, and M€ossbauer investigations on microcrystalline SnO2, nanocrystalline SnO2, and SnO2 hollow spheres.

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more symmetric, possibly because the entire crystal lattice softens due to the surface distortions. In the SnO2 hollow spheres, however, the octahedra experience both stronger local distortions and a higher degree of asymmetry, which may be attributed to a high abundance of points defects. In summary, we showed by combining 119Sn NMR and 119Sn M€ossbauer spectroscopies, that various structural relaxation processes take place in the prominent surface regions of the nanostructured materials.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ49-721-608-28312, fax: þ49-721-608-28312, e-mail: [email protected].

a

XRD line width is determined for the (101) peak.

Figure 6 gives an overview of the different trends obtained from the results of BET, XRD, NMR, and M€ossbauer investigations on microcrystalline SnO2, nanocrystalline SnO2, and SnO2 hollow spheres. The BET surface area, the XRD line width, and the NMR line width show the same trend, that is they increase when going from the microcrystalline sample to the nanocrystalline sample and then to the hollow spheres. This fact shows that the long-range structure (probed by XRD) and the short-range structure (probed by NMR) show increasing disorder when the particle size is decreased. In contrast, the quadrupole splitting in the M€ossbauer spectra, which reflects the average electric field gradient at the site of the Sn nuclei and thus the average asymmetry of the SnO6 octahedra, shows a different trend. The asymmetry of these octahedra is smaller in the nanocrystalline sample when compared to the microcrystalline sample, but highest in the SnO2 hollow spheres. These findings suggest that a structural relaxation takes place in the near-surface regions of the nanocrystalline SnO2 particles. This leads to variations in the bond lengths and angles, that is to local structural disorder, but at the same time to more symmetric SnO6 units on average. The particular morphology of the SnO2 hollow spheres leads to an even higher degree of local disorder, yet the SnO6 octahedra become even more asymmetric in this case.

’ CONCLUSIONS The NMR results reveal that the nanostructured samples, compared to bulk SnO2 material, show increased local structural disorder. The broad NMR lines are caused by a distribution of bond lengths and bond angles at the Sn sites leading to a variation of electron densities and thus to a distribution of chemical shifts. This can be seen for the nanocrystalline SnO2 particles, and even more pronounced for the SnO2 hollow spheres. The M€ossbauer results show that the intrinsically distorted SnO6 octahedra, wellknown in the microcrystalline sample, become more symmetric in the nanocrystalline sample on average. Although there is more local structural disorder, the average environment within and around the SnO6 octahedra becomes

’ ACKNOWLEDGMENT We are grateful to the German Ministry for Education and Research as well as to the DFG Center for Functional Nanostructures (CFN) at the Karlsruhe Institute of Technology (KIT) for financial support. We thank Jens Kling (Structural Research Division, Technical University Darmstadt) for support with TEM experiments on the nanocrystalline SnO2. F.G. and C.F. are grateful to Dr. R. Popescu and Prof. Dr. D. Gerthsen for performing TEM analysis on the SnO2 hollow spheres. J.S. and H.H. are grateful for financial support by the State of Hesse. ’ REFERENCES (1) Gleiter, H. Acta Mater. 2000, 48, 1. (2) Valiev, R. Z.; Islamgaliev, R. K.; Alexandrov, I. V. Prog. Mater. Sci. 2000, 45, 103. (3) Pasqual, J. I.; Mendez, J.; Gomez-Herrero, J.; Baro, A. M.; Garcia, N.; Landman, U.; Luedtke, W. D.; Bogachek, E. N.; Cheng, H.-P. Science 1995, 267, 1793. (4) Alivisatos, A. P. Science 1996, 271, 933. (5) Indris, S.; Heitjans, P.; Roman, H. E.; Bunde, A. Phys. Rev. Lett. 2000, 84, 2889. (6) Indris, S.; Heitjans, P.; Ulrich, M.; Bunde, A. Z. Phys. Chem. 2005, 219, 89. (7) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316. (8) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (9) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (10) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (11) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (12) Bell, A. T. Science 2003, 299, 1688. (13) Indris, S.; Amade, R.; Heitjans, P.; Finger, M.; Haeger, A.; Hesse, D.; Gr€unert, W.; B€orger, A.; Becker, K. D. J. Phys. Chem. B 2005, 109, 23274. (14) Volokitin, Y.; Sinzig, J.; de Jongh, L. J.; Schmid, G.; Vargaftik, M. N.; Moiseev, I. I. Nature 1996, 384, 621. (15) Indris, S.; Heitjans, P.; Roman, H. E.; Bunde, A. Defect Diffus. Forum 2001, 194-199, 935. (16) Indris, S.; Heitjans, P. J. Non-Cryst. Solids 2002, 307-310, 555. (17) Heitjans, P.; Indris, S. J. Phys. Condens. Matter 2003, 15, R1257. (18) Heitjans, P.; Indris, S. J. Mater. Sci. 2004, 39, 5091. (19) Ulrich, M.; Bunde, A.; Indris, S.; Heitjans, P. Phys. Chem. Chem. Phys. 2004, 6, 3680. (20) Oyabu, T. J. Appl. Phys. 1982, 53, 2785. (21) Yamazoe, N. Sens. Actuators, B 1991, 5, 7. (22) Watson, J.; Ihokura, K.; Coles, G. S. V. Meas. Sci. Technol. 1993, 4, 711. 6436

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