Characterization of Sn@C core–shell NWs

Scanning electron microscopic (SEM) images of samples A and B are shown in Figure 4.1.

In the low magnification views Figure 4.1a and 4.1c, numerous one-dimensional NWs with lengths up to tens of micrometers are observed. Magnified images in Figure 4.1b and 4.1d suggest that diameters of the NWs in A and B are hundreds of nanometers. The insets reveal that the NWs contain an apparent core–shell tubular structure. It is easy to perceive the contrast differences displayed within these images, from the side-view and the top-view of the NWs in Figure 4.1b and 4.1d, respectively. An energy dispersive X-ray (EDX) spectrum of A displayed in Figure 4.2 indicates that the core–shell NWs contain both C and Sn. The presence of metallic Sn (JCPDS card file no. 86-2265) in both A and B is confirmed by the XRD patterns shown in Figure 4.3³¹. Product B, probably due to large original SnO2 sizes, still contains unreacted SnO2

as suggested by its diffraction pattern observed in the XRD (JCPDS card file no. 72-1147).³¹ In Figure 4.4, Raman spectra of A and unprocessed SnO2 are compared. For A, two broad bands at 1328 cm^-1 and 1583 cm^-1 are observed. They correspond respectively to D band and G band, with a D/G intensity ratio of 1.46, of a less-ordered carbon material.^32–34 Because no signals of SnO² are observed, we conclude that A does not contain any unreacted oxide. The quantities of Sn and C in A are estimated using the results from a thermogravimetric analysis (TGA) shown in Figure 4.5. After A was heated to 1273 K in an O2 environment, obvious weight decrease was observed above 850 K. By assuming all Sn and C in A were fully oxidized, we estimate that the Sn and C contents are 8.1 at% and 91.9 at%, respectively. Morphology of the samples prepared under other reaction conditions is shown in the SEM images in Figure4.6. From Figure 4.6b, the lowest NW growth temperature is determined to be 923 K.

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Figure 4.1 SEM images. (a) Low and (b) high magnification views of product A. (c) Low and (d) high magnification views of product B. The core–shell structures can be observed in the regions marked in the insets.

Figure 4.2 SEM image and energy dispersive X-ray spectrum (EDX) of product A.

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Figure 4.3 XRD patterns of products A and B.

Figure 4.4 Raman spectra of product A and SnO2. (Excitation wavelength: 785 nm, power: 5 mW)

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Figure 4.5 Thermogravimetric analysis (TGA) profile of A. The Sn and C contents are estimated to be 46.8 wt % (8.1 at %) and 53.2 wt % (91.9 at %), respectively. The analysis was taken in O2 using a heating rate of 10 K min^-1.

Figure 4.6 SEM images of samples prepared at (a) 873 K for 60 min, (b) 923 K for 30 min, (c) 973 K for 30 min, (d) 1073 K for 30 min, and (e) 1123 K for 30 min. C2H2 was flowing at 3 sccm under atmospheric pressure.

Results of transmission electron microscopy (TEM) studies of A are shown in Figure 4.7.

From the images of several samples, Figure 4.7a–c, it is confirmed that the NWs have a core–

shell structure with diameters 200–350 nm and shell thicknesses 30–70 nm. Inside the shell, in some cases, the inner core fills the space completely, as shown in Figure 4.7a. On the other hand, more partially filled NWs are formed, as shown in a couple of examples in Figure 4.7b.

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From a section of a broken NW, low and high magnification views are shown in Figure 4.7c and d, respectively. From Figure 4.7d, selected area electron diffraction (SAED) patterns are taken and shown in Figure 4.7e and f. They correspond to the shell and the core, respectively.

Figure 4.7e presents a pattern composed of three diffused rings. Starting from the most inside, these are assigned to the reflections from (005), (105) and (202) planes of the less ordered C. A dot pattern displayed in Figure 4.7f suggests that the core can be indexed to the 0 0 1 zone axis of single crystalline Sn. From the image, the lattice parameters a and c of a tetragonal crystal system are estimated to be 0.58 nm and 0.32 nm, respectively.³¹ Figure 4.7g shows a high resolution lattice image of the core. Two sets of parallel fringes, spaced at 0.29 nm and 0.21 nm, are observed. These are assigned correspondingly to the spacings of {0 2 0} and {2 2 0}

planes of crystalline Sn. Combined with the SAED result, the growth of the Sn core is determined to be along the [2 4 0] axis. An EDX spectrum shown in Figure 4.7h confirms that the NW contains both C and Sn. In a series of TEM images presented in Figure 4.8, it is suggested that the Sn core inside the C shell was liquefied upon continuous electron beam irradiations, as shown by the extending Sn boundary in the void displayed in the images. The presence of the cavities inside C shells is known to accommodate enormous volume variations during Li alloying and dealloying processes in anodes fabricated from Sn containing materials.^9,10

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Figure 4.7 TEM studies of product A. (a) A completely filled core–shell NW, (b) a pair of partially filled NWs, (c) a section of a broken NW, (d) a high magnification image from the pointed region in (c), (e) SAED pattern from the rectangle area in (c) and (f) SAED pattern from the end of the core in (d). (g) HRTEM image from the region circled in (d). (h) EDX spectrum from (c).

Figure 4.8 TEM images of a Sn@C core–shell NW. (a) Low magnification image and (b) high magnification view of the area marked by the arrow in (a) images taken after irradiating the circled area in (b) by an electron beam for (c) 60 s and (d) 180 s. The void shown in the circled area in (b) was filled by melted Sn after the electron beam irradiation.

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4.3.2 Proposed reaction pathway

Growth of the Sn@C core–shell NWs is further discussed below to illustrate the important features of the reaction. The overall reaction stoichiometry between C2H2 and SnO2 to produce Sn and C could be either

2 C2H2 + SnO2  Sn + 4 C + 2 H2O Gr° = -372.84 kJ/mol or

2 C2H2 + 2 SnO2  2 Sn + 3 C + CO2 + 2 H2O Gr° = -247.33 kJ/mol Both reactions are thermodynamically favored due to the negative standard Gibbs free energies of reaction (G^or).³⁵ In addition, we suggest that the core–shell NWs are grown via a VSRG pathway shown in Scheme 4.1. It is analogous to the ones proposed before for the growths of 1D CaF2@α-C NWs and SiC tubes.^29,30 The reaction happens at the interface between the vapor phase C2H2 and the surface of the SnO2 solid. From the SEM image displayed in the inset in Figure 4.6a, it is discovered that the oxide can be reduced to metallic Sn at 873 K.³⁶ Due to their large difference in surface energies, the as-formed products are composed of phase-separated Sn and C nanoparticles (NPs). As the Sn NPs coalesce to generate a core, the C NPs covered the Sn surface to form a shell. After more Sn and C products migrate and incorporate into this heterostructured seed, the high melting point C shell solidifies even though the low melting point Sn core stays in the liquid state. Since the solidified C shell possesses a certain width, incorporation of more products into the growing heterostructure can only extend one dimensionally into the observed core–shell NW morphology. In the experiments, growth of small diameter core–shell NWs is observed at 923 K, as shown in Figure 4.6b. It is known that as the size of a nanomaterial increases, its melting point increases as well.

Thus, we anticipate that the NWs grown at higher temperatures should have larger diameters.

This is supported by the observations shown in Figure 4.1b and 4.1d and the data summarized in Table 4.2. The diameters of the NWs in A, grown at 973 K, are wider than those of B, grown at 923 K. The images in Figure 4.6b–d also demonstrate that as the growth temperature is

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increased from 923 K to 1123 K, more and wider NWs are formed. It is worthwhile to mention that the proposed VSRG reaction proceeds via steps very different from the ones in typical vapor–liquid–solid (VLS) type growths.³⁷

Scheme 4.1 Proposed VSRG pathway to form Sn@C core–shell NWs.