Shell Nanoparticles - The

Article pubs.acs.org/JPCC

Size-Dependent Strain of Sn/SnOx Core/Shell Nanoparticles Nikolas Oehl,* Peter Michalowski, Martin Knipper, Joanna Kolny-Olesiak, Thorsten Plaggenborg, and Jürgen Parisi Energy and Semiconductor Research Laboratory, Institute of Physics, Carl-von-Ossietzky Universität, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany S Supporting Information *

ABSTRACT: The lattice constants of metallic nanoparticles shrink with respect to that of a bulk material. This behavior affects the properties of nanoscaled crystallites and can influence their application potential. In this work, we investigate the size-dependent lattice parameters of core/ shell Sn/SnOx nanoparticles, synthesized via a simple chemical reduction method. Therein, the use of appropriate surface ligands, reaction temperature, and reaction time allows us to tune the mean particle size from 6 to 104 nm. X-ray powder diffraction revealed that the ß-Sn reflections shift toward higher angles for smaller particles, showing a size-dependence of the lattice constants. The change in the lattice constants varies, depending on the direction, and can be described as an inverse function of the diameter of the crystallites. The different degree of deformation can be explained by the direction dependency of the bulk modulus K and the interface energy γ of the monocrystalline tin nanoparticles.

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Results by Yu et al.25 who determined size-dependent lattice constants for Sn and Bi show that the lattice parameters a and c decrease for smaller grain sizes. Nevertheless, a theoretical interpretation of this data is missing. There have also been reports about lattice expansions in nanocrystalline material due to grain boundaries.26 Shin et al.27 reported a size-dependent lattice dilatation in tetragonal Sn nanowires, which, according to the authors, originates from a strong anisotropy of the surface stress. In this article we investigate the size-dependent lattice parameters of spherical tin nanoparticles in the size range between 6 and 104 nm. While there are numerous reports on the synthesis of Sn nanoparticles,28−35 such about the control of the size in this relatively large range are missing. Therefore, we modified an existing synthetic method35 in order to extend the range of possible sizes. The samples were studied by powder X-ray diffraction to obtain the lattice parameters for the particles with different diameters. For the calculation of the tetragonal lattice constants a and c, all reflections of the observed Sn pattern were taken into account. The dissimilar change of the lattice constants in a- and c-directions can be explained by the direction-dependent bulk modulus K and interface energy γ of the Sn particles following a method presented by Ouyang et al.36

INTRODUCTION Tin nanoparticles are of strong interest for several technological applications such as lead free solders,1 highly conductive inks,2 or gas sensing.3 Recently, it has also been proposed to use tin nanostructures as an anode material in lithium ion batteries.4−6 Schmülling et al.7 showed that Sn nanocrystals can be applied as a high energy density anode material. Furthermore, this study demonstrated that the size of the nanoparticles affects their electrochemical behavior. It is well-known that the size of a nanoparticle affects its lattice parameters and therefore induces a strain.8 Works on electrocatalysis show that the lattice strain of metal surfaces influences the reactivity of a metal.9 Consequently, the lattice strain could be a parameter to tune the properties of tin as an anode material.10 Lattice strain can cause phase transformation, e.g., in gold (face-centered cubic to body-centered tetragonal)11 and in silver (face-centered cubic to face-centered tetragonal).12 It had been observed that small Sn nanocrystals in lithium ion battery anodes can recrystallize from ß-Sn (body-centered tetragonal) to the α-Sn (bodycentered cubic) at room temperature depending on the size of the crystallite.7,13,14 The shrinkage of the lattice constants with the size of the crystallites was described already in 1968 by Vermaak et al.15 Since then the change in lattice constants was observed and calculated successfully for silver,16−19 gold/platinum,20 bismuth,21 copper/nickel,22 and palladium.23,24 Apai et al.22 showed that the shrinkage of the Cu−Cu neighbor distance could reach up to 9% for the smallest clusters observed. Nevertheless, most of these studies were done on cubic systems, and sometimes only single reflections were considered. © 2014 American Chemical Society

Received: September 23, 2014 Revised: November 27, 2014 Published: December 2, 2014 30238

dx.doi.org/10.1021/jp5096147 | J. Phys. Chem. C 2014, 118, 30238−30243

The Journal of Physical Chemistry C

Article

Figure 1. Size histograms, TEM images, HR-TEM images, and corresponding FFTs with the assignment to lattice planes of ß-Sn for Sn/SnOx samples with volume-weighted mean diameters of (a−d) 6 nm; (e−h) 26 nm; and (i−l) 56 nm.

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EXPERIMENTAL METHODS Chemicals. We used tin (II) chloride (SnCl2, 99%, SigmaAldrich), polyvinylpyrrolidone (PVP, molecular weight (MW) 40 000/360 000, Alfa Aesar), tetraethylene glycol (TEG, 99%, Alfa Aesar), and sodium borohydride (NaBH4, 99% Acros). All chemicals were used as received. Synthesis of 11 nm ß-Sn nanospheres. For a typical batch of tin particles with a mean diameter of 11 nm, 0.275 g of SnCl2 and 0.75 g of PVP (MW 360 000) were dissolved at room temperature in 75 mL of TEG. The stirred solution was heated up to 110 °C under argon atmosphere using the Schlenk technique. Meanwhile, 0.4 g of NaBH4 was dissolved in 10 mL of TEG. This solution was added dropwise (within t = 1 min) to the stirred SnCl2 solution. The reaction solution turned black already after the addition of a few drops. The resulting solution was cooled down to room temperature, diluted with acetone and centrifuged (5500 rpm, 5 min, 3 times) to isolate the Sn/SnOx core/shell nanoparticles. Afterward, they were dispersed and stored in ethanol. By modifying the reaction temperature, the molecular weight of PVP, and the reaction time t, one can achieve a desired particle size (see Supporting Information for detailed reaction conditions). Characterization. Powder X-ray diffraction (XRD) data were collected using a PANalytical X’Pert Pro diffractometer operating with Cu−Kα radiation in Bragg−Brentano θ−2θ geometry, using a goniometer with 240 mm radius and an automatic divergence slit. The parameters for instrumental line broadening were refined, using 660b LaB6 standard reference material from the National Institute of Standards and Technology (NIST). Therefore, the instrumental parameters

for asymmetry, i.e., the Caglioti function, the Gaussian contribution, as well as the Kα1/Kα2 and Kα1/Kß intensity ratio, were refined and saved for all further analysis.37,38 Tin samples were prepared by drop casting solution of the nanoparticles on low background silicon sample holders. An automatic divergence slit was used to illuminate a constant 10 × 10 mm2 area on the sample holder to achieve constant volume conditions during the 2θ scan. Transmission electron microscopy (TEM) measurements were performed using a Zeiss EM 902A microscope with an acceleration voltage of 80 kV. High-resolution TEM (HR-TEM) images were collected on JEOL 2010F with an electron acceleration voltage of 200 kV. TEM samples were prepared by drop casting diluted nanoparticle solution on high transparent carbon-coated Cu TEM grids.

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RESULTS AND DISCUSSION TEM. The TEM images reveal that all nanoparticles of a particular batch are mainly spherical and exhibit a distinct size distribution. In general, such a distribution can be described by a log-normal function.39 Therefore, several hundreds of particles were measured for every nanoparticle sample, leading to the histograms in Figure 1a. For reasons of comparability with XRD, where the contribution of a crystalline domain to the signal relates to its volume, the mean diameter of the nanoparticles was calculated as the volume-weighted mean diameter D4,3.40 D4,3 = 30239

∑ D4 ∑ D3

(1)



Article

with D denoting to the whole particle (core and shell). The values for D4,3 are summarized in Table 1.

Sn, SnO, and SnO2 phases were considered to calculate the diffraction pattern. The initial structural parameters for all three phases were taken from the American Mineralogist Crystal Structure Database.41 The database reference codes for the mentioned phases were Sn(0011248), SnO(0011634), and SnO2(0011761). The scale factor and the unit cell parameters (a, c) were refined first, to match peak intensity and position. Line broadening was fitted at the end to determine crystallite sizes. Figure 3 shows an XRD pattern and the results of the Rietveld refinement of 12 nm particles. The experimental data is a superposition of the Sn, SnO, and SnO2 patterns. Strongly broadened SnOx reflections are visible in the range of 25° to 35° and from 50° to 60°. This pronounced broadening results from the small size of the crystalline SnOx domains. The quantitative-analysis reveals that the sample consists of 50 wt % Sn, 23 wt % SnO2, and 27 wt % SnO. The phase parameters used for the Rietveld refinement are listed in Table 2. All other structural parameters were not changed or refined. The size of the crystallites was determined by the Rietveld refinement of the ß-Sn pattern and is listed in Table 1. The size of the crystallite L (obtained from XRD) and the size of the particle D4,3 (determined by TEM) are in good agreement, which implicates that the nanoparticles are monocrystalline. The stronger deviation between L and D4,3 for the largest particles might be due to the fact that larger crystalline domains show less broadening, making the determination of their size more inaccurate. Also, the fraction of Sn increases toward larger particles. This trend is in good agreement with the TEM observations of the nearly constant thickness of the SnOx shell and the increasing surface to volume ratio for smaller particles. In Figure 4, the change in lattice spacings Δd = d0 − d with respect to the bulk lattice distance (ICCD database: 03-0657657) is plotted against the mean particle diameter for three different lattice planes. The lattice spacing of the (101) plane does not change with the diameter of the particles. This plane has also the smallest angle toward the (001) plane (c-axis) of the tetragonal lattice. Planes perpendicular to the c-axis, e.g., the (001) plane, are not visible in the XRD pattern since their spacing is too small for the wavelength used to fulfill the Bragg condition in the 2θ range measured. The planes oriented orthogonal to the a- or b-axis like (200) show a strong decrease in lattice spacings. The lattice constants a and c for a tetragonal crystal can be calculated from

Table 1. Particle Diameter, Crystallite Size, and Fraction of Metallic Tin for All Synthesized Sn/SnOx Nanoparticles D4,3a (nm)

Lb (nm)

w(Sn)c (wt %)

6(2) 12(2) 15(2) 19(2) 26(2) 33(2) 56(4) 86(4) 104(4)

7(1) 11(1) 11(1) 18(1) 22(1) 33(1) 48(1) 91(2) 133(2)

18(2) 50(1) 65(1) 51(1) 79(1) 83(1) 73(1) 91(1) 94(1)

a

D4,3 denotes the volume-weighted mean diameter calculated on the basis of the TEM measurements. bL denotes the average size of the crystalline Sn domains derived by the Rietveld refinement of the XRD data. cw(Sn) denotes the fraction of Sn in the crystalline material derived by the Rietveld refinement.

As can be seen in Figure 1g,k, the tin nanoparticles consist of a crystalline tin core covered by a thin shell. EDX measurements (see Supporting Information) reveal that the nanoparticles contain a significant fraction of oxygen; therefore, we conclude that the outer layer of the tin nanoparticle becomes oxidized after exposed to air, which is in agreement with previous reports on the formation of such a SnOx shell.29,30 This SnOx layer suppresses the diffusion of oxygen and tin to the phase boundary, and consequently, the further oxidation of the core slows down. However, no signals in the FFTs could definitely be attributed to SnOx phases, most likely because of the low crystallinity and the small thickness of the shell material. An amorphous SnOx shell on the Sn core is also likely. XRD Analysis. XRD measurements (Figure 2) show reflections that can be attributed to tetragonal ß-Sn. Toward

1 2

(dhkl)

=

h2 + k 2 l2 + 2 2 a c

(2)

where dhkl is the lattice spacing calculated from the peak position, and h, k, and l are the Miller indices. For the calculation of the lattice constants, all detectable lattice planes in the 2θ range from 20° to 90° from the XRD pattern were used. In Figure 5, the relative change of the lattice constants is plotted against D4,3. The lattice parameters of bulk Sn x0 serve as the reference points. Ouyang et al.36 reported that the change in the lattice spacing is inversely proportional to the diameter. Thus, the change in the bulk lattice constant x0 can be expressed as

Figure 2. XRD patterns of Sn/SnOx nanoparticles with different sizes. The lattice planes of ß-tin, giving rise to the observed reflections, are indicated.

smaller nanoparticles, reflections from ß-Sn broaden and their intensity decreases due to the smaller crystalline domains and due to an increased strain. Additional reflections originating from SnOx are visible in the range from 25° to 35° (see also Figure 3). In order to analyze composition and microstructure at the same time, Rietveld refinement with the Maud software version 2.3.337 was applied to fit the complete pattern. In all cases the background was refined with a polynomial of the fifth degree.

x0 − x 4γ = x 3DK 30240

(3)



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Figure 3. (a) XRD pattern, Rietveld refinement, and residual plot of Sn/SnOx core/shell nanoparticles with a volume-weighted mean diameter of 12 nm. The weighted R-factor Rw for this refinement is 1.88%. (b) Size histogram with TEM image of the corresponding 12 nm Sn/SnOx sample.

Table 2. Parameters for Rietveld Refinement from the Pattern in Figure 3 symmetry, space group ß-Sn SnO SnO2

tetragonal, I41/amd:1 tetragonal, P4/nmm:1 tetragonal, P42/mnm

a (Å)

c (Å)

La (nm)

wb (wt %)

5.8227(3)

3.1824(2)

10.9(4)

50(1)

3.816(4)

4.99(1)

1.4(5)

27(2)

4.486(7)

3.34(1)

1.4(5)

23(2)

a

L denotes the average size of the crystalline domains. bw denotes the amount of the phase in the crystalline material.

Figure 5. Relative change in lattice constants a and c for the tetragonal ß-Sn core with respect to the particle size D4,3. Bulk modulus K was taken from Vitos et al.,42 and the surface energy density γ was set as a free fitting parameter.

It is well-known that the bulk modulus and the surface energy density depend on the particle diameter as well.44 This influence becomes critical only for nanoparticles much smaller than 10 nm.45 Therefore, it could be neglected in the current study. The solid line in Figure 5 gives the fit according to eq 3 and is in good agreement with the lattice parameters determined. This indicates that eq 3 is not limited to simple nanoparticles and can be applied to core/shell particles, too.

Figure 4. Relative shift of a selection of lattice spacings d against the particle diameter D4,3 for three different lattice planes (hkl).

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CONCLUSIONS We prepared Sn/SnOx core/shell nanoparticles with sizes in the range from 6 to 104 nm. HR-TEM images reveal that all particles have core/shell structures with a SnOx shell around a monocrystalline ß-Sn core. These shells seem to protect the tin core from further oxidation. The fraction of ß-Sn and the size of the crystallite were determined by a Rietveld refinement of the XRD measurements. The crystallite size (obtained from XRD) and the particle size (determined by TEM) are in good agreement, which implicates that all the nanoparticles are monocrystalline. Also, the fraction of Sn increases and that of SnOx decreases toward larger particles. This trend is in good agreement with the TEM observations of the constant SnOx shell and the increasing surface to volume ratio for smaller particles. We calculated the lattice constants from the XRD patterns. The lattice constants of the tin core depend on the size of the crystallite. For the characterized nanoparticles, both lattice constants, a and c, of the tetragonal tin core change with the

where K is the bulk modulus of tin, γ the surface energy density, and D the diameter of the nanoparticle. The volume-weighted mean diameter D4,3 corresponds well with the size of the crystallites and is therefore a suitable parameter to describe the crystallite size distribution. The change in the lattice parameter Δx = x0 − x was also determined by a volume-weighted measurement, and therefore, D4,3 is used in eq 3. K and γ of monocrystalline particles are constants depending on the material and its lattice direction. The value for K for the (100) and the (001) direction is 53 and 77 GPa, respectively.43 Vitos et al.42 calculated γ of Sn−air interface to be 0.716 and 0.387 J/m2 for the (100) and the (001) direction, respectively. Because the Sn particles are covered with a thin oxide layer, the interface energy for the system Sn/SnOx has to be applied. Therefore, γ was set as a free fitting parameter, providing values of 0.516(3) and 0.110(4) J/m2 for the (100) and the (001) direction, respectively. This indicates that the surface energy of Sn is higher than the interface energy of Sn/SnOx. 30241



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(10) Wu, J.; Li, P.; Pan, Y.-T. (Frank); Warren, S.; Yin, X.; Yang, H. Surface Lattice-Engineered Bimetallic Nanoparticles and Their Catalytic Properties. Chem. Soc. Rev. 2012, 41, 8066. (11) Diao, J.; Gall, K.; Dunn, M. L. Surface-Stress-Induced Phase Transformation in Metal Nanowires. Nat. Mater. 2003, 2, 656−660. (12) Sun, Y.; Ren, Y.; Liu, Y.; Wen, J.; Okasinski, J. S.; Miller, D. J. Ambient-Stable Tetragonal Phase in Silver Nanostructures. Nat. Commun. 2012, 3, 971. (13) Kim, C.; Noh, M.; Choi, M.; Cho, J.; Park, B. Critical Size of a Nano SnO 2 Electrode for Li-Secondary Battery. Chem. Mater. 2005, 17, 3297−3301. (14) Xu, L.; Kim, C.; Shukla, A. K.; Dong, A.; Mattox, T. M.; Milliron, D. J.; Cabana, J. Monodisperse Sn Nanocrystals as a Platform for the Study of Mechanical Damage during Electrochemical Reactions with Li. Nano Lett. 2013, 13, 1800−1805. (15) Vermaak, J. S.; Mays, C. W.; Kuhlmann-Wilsdorf, D. On Surface Stress and Surface Tension: I. Theoretical Considerations. Surf. Sci. 1968, 12, 128−133. (16) Wasserman, H. J.; Vermaak, J. S. On the Determination of a Lattice Contraction in Very Small Silver Particles. Surf. Sci. 1970, 22, 164−172. (17) Montano, P. A.; Schulze, W.; Tesche, B.; Shenoy, G. K.; Morrison, T. I. Extended X-Ray-Absorption Fine-Structure Study of Ag Particles Isolated in Solid Argon. Phys. Rev. B 1984, 30, 672−677. (18) Montano, P. A.; Zhao, J.; Ramanathan, M.; Shenoy, G. K.; Schulze, W.; Urban, J. Structure of Silver Microclusters. Chem. Phys. Lett. 1989, 164, 126−130. (19) Medasani, B.; Park, Y. H.; Vasiliev, I. Theoretical Study of the Surface Energy, Stress, and Lattice Contraction of Silver Nanoparticles. Phys. Rev. B 2007, 75, 235436. (20) Solliard, C.; Flueli, M. Surface Stress and Size Effect on the Lattice Parameter in Small Particles of Gold and Platinum. Surf. Sci. 1985, 156 (Part 1), 487−494. (21) Liang, L. H.; Li, J. C.; Jiang, Q. Size-Dependent Melting Depression and Lattice Contraction of Bi Nanocrystals. Phys. B: Condens. Matter 2003, 334, 49−53. (22) Apai, G.; Hamilton, J. F.; Stohr, J.; Thompson, A. Extended XRay-Absorption Fine Structure of Small Cu and Ni Clusters: BindingEnergy and Bond-Length Changes with Cluster Size. Phys. Rev. Lett. 1979, 43, 165−169. (23) Da Silva, E. Z.; Antonelli, A. Size Dependence of the Lattice Parameter for Pd Clusters: A Molecular-Dynamics Study. Phys. Rev. B 1996, 54, 17057−17060. (24) Lamber, R.; Wetjen, S.; Jaeger, N. I. Size Dependence of the Lattice Parameter of Small Palladium Particles. Phys. Rev. B 1995, 51, 10968−10971. (25) Yu, X. F.; Liu, X.; Zhang, K.; Hu, Z. Q. The Lattice Contraction of Nanometre-Sized Sn and Bi Particles Produced by an Electrohydrodynamic Technique. J. Phys.: Condens. Matter 1999, 11, 937. (26) Zhu, Y. F.; Zheng, W. T.; Jiang, Q. Modeling Lattice Expansion and Cohesive Energy of Nanostructured Materials. Appl. Phys. Lett. 2009, 95, 083110. (27) Shin, H. S.; Yu, J.; Song, J. Y.; Park, H. M.; Kim, Y.-S. Origins of Size-Dependent Lattice Dilatation in Tetragonal Sn Nanowires: Surface Stress and Growth Stress. Appl. Phys. Lett. 2010, 97, 131903. (28) Chou, N. H.; Schaak, R. E. A Library of Single-Crystal Metal− Tin Nanorods: Using Diffusion as a Tool for Controlling the Morphology of Intermetallic Nanocrystals. Chem. Mater. 2008, 20, 2081−2085. (29) Wang, X.-L.; Feygenson, M.; Aronson, M. C.; Han, W.-Q. Sn/ SnOx Core−Shell Nanospheres: Synthesis, Anode Performance in Li Ion Batteries, and Superconductivity. J. Phys. Chem. C 2010, 114, 14697−14703. (30) Kravchyk, K.; Protesescu, L.; Bodnarchuk, M. I.; Krumeich, F.; Yarema, M.; Walter, M.; Guntlin, C.; Kovalenko, M. V. Monodisperse and Inorganically Capped Sn and Sn/SnO2 Nanocrystals for HighPerformance Li-Ion Battery Anodes. J. Am. Chem. Soc. 2013, 135, 4199−4202.

particle diameter. However, the magnitude of the relative contraction is not isotropic, showing a larger relative change in the direction of the a- than in the direction of the c-axis. This behavior can be explained by the direction dependency of the bulk modulus K and surface energy γ of the tin crystal. In summary, the size of the tin crystallites is a parameter to tune their lattice properties. The predictions from theory are in good agreement with the experimental results. Therefore, eq 3 is a suitable tool to calculate the lattice parameters for nanoparticles of different size. This could prove valuable in the comprehension and development of catalysts and battery materials.

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ASSOCIATED CONTENT

S Supporting Information *

Reaction conditions and EDX measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 441 798 3007. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding of the EWENachwuchsgruppe by EWE AG Oldenburg. P.M. would like to thank the Systemintegration Erneuerbarer Energien for financial support.

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