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Hollow Silicon Nanostructures via Kirkendall effect Yoonkook Son, Yeonguk Son, Min Choi, Minseong Ko, Sujong Chae, Noejung Park, and Jaephil Cho Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02842 • Publication Date (Web): 04 Sep 2015 Downloaded from http://pubs.acs.org on September 5, 2015
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Hollow Silicon Nanostructures via Kirkendall effect Yoonkook Son1, ‡, Yeonguk Son1, ‡, Min Choi2, Minseong Ko1, Sujong Chae1, Noejung Park3,* and Jaephil Cho1,* 1
School of Energy and Chemical Engineering, Ulsan National Institute of Science and
Technology (UNIST), 689-798, Ulsan, South Korea 2
Department of Chemistry and 3Department of Physics, School of Natural Science Center for
Multidimensional Carbon Materials, Ulsan National Institute of Science and Technology (UNIST), 689-798, Ulsan, South Korea.
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KEYWORDS: Kirkendall effect, hollow nanostructure, self-organization, silicon, lithium ion batteries
ABSTRACT: The Kirkendall effect is a simple, novel phenomenon that may be applied for the synthesis of hollow nanostructures with designed pore structures and chemical composition. We demonstrate use of the Kirkendall effect for silicon (Si) and germanium (Ge) nanowires (NWs) and nanoparticles (NPs) via introduction of nanoscale surface layers of SiO2 and GeO2, respectively. Depending on the reaction time, Si and Ge atoms gradually diffuse outward through the oxide layers, with pore formation in the nanostructural cores. Through the Kirkendall effect, NWs and NPs were transformed into nanotubes (NTs) and hollow NPs, respectively. The mechanism of the Kirkendall effect was studied via quantum molecular dynamics calculations. The hollow products demonstrated better electrochemical performance than their solid counterparts because the pores developed in the nanostructures resulted in lower external pressures during lithiation.
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Strategies for synthesizing hollow nanostructures can be classified into two categories, the sacrificial template and self-organization methods1,2. The former consists of two steps (deposition of the materials onto a template and removal of the template) and requires a template upon which to construct the hollow nanostructures. The self-organization method, a one step process, requires a thermodynamically preferred reaction for the formation of hollow nanostructures. Self-organization methods have been widely used in applications as varied as catalysis, medicine, and energy storage3,4. As a promoter of self-organization, the Kirkendall effect has been reported and spotlighted as a prominent technique. The Kirkendall effect is observed for atoms in two adjacent phases with different diffusivities: i.e., the atoms in an inner layer diffuse quickly into an outer layer, from which atoms move slowly into the inner layer. During the diffusion processes, the supersaturation of lattice vacancies develops into an interior pore which becomes the inner part of the final hollow structure4-7. This strategy has been extensively used for noble and transition metals, metal oxides, and sulfides
5,8,9
. However, the Kirkendall effect for Si or Ge has been rarely
investigated because their diffusivities were expected not as high as the Kirkendall effect induced. Although a few trials for Kirkendall effect of Si or Ge were reported10-12, they found the Kirkendall effect mostly in only thin film structure and the mechanism of the effect for such elements has remained elusive. Especially, the Kirkendall effect of Si element has not been apparent hitherto. Herein, we report hollow Si and Ge structures prepared by the Kirkendall reaction between Si and Ge nanostructures and their oxide layers respectively. The depth of the diffusion layer which leads to the internal empty spaces could be controlled as a function of reaction time. In addition, the diffusion mechanism for such Kirkendall reactions was modeled using computational simulations. To the best of our knowledge, we introduce the template-free synthesis of hollow SiOx nanostructure with clear diffusion 3 ACS Paragon Plus Environment
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mechanism of the Kirkendall effect for the first time. The Kirkendall reaction between Ge nanostructure and its oxide layer is also worthy of close attention. Furthermore, significant electrochemical performance of hollow Si and Ge structure is illustrated, and the potential of the Kirkendall effect for application in lithium ion batteries (LIB) anodes is highlighted. SiNWs, GeNWs, and SiNPs were prepared by the vapor-liquid-solid (VLS) method using SiH4 gas or the solution-liquid-solid (SLS) method using GeCl4 and commercial Si nanopowder (< 100 nm particle size, >99% trace metals basis), respectively (Figure 1a and S1a)13,14. Both of high-resolution TEM (HR-TEM) images and the corresponding fast Fourier-transform (FFT) patterns of the SiNWs revealed crystalline structure surrounded by a thin amorphous native oxide layer (~5 nm) (Figure S2). The transformation of NWs and NPs into the hollow nanostructures required two stages: development of the thick oxide layer and thermal treatment which induced the Kirkendall diffusion (Figure 1b). Thus, SiNWs and SiNPs were heated at 800°C for 1 h in air to introduce a SiO2 layer of 10–30 nm thickness (Figure S3a, S4). Then, the samples were heated hydrothermally at 200°C in a Teflon-lined reactor for 4–6 days (Figure 1c and S5) (See the experimental section for the details and the Ge case). As shown in Figure 1c and S5, the SiNWs and SiNPs were transformed completely into hollow Si oxide nanostructures (results for GeNWs are presented in Figure S1b). The exterior diameters of the NTs and hollow NPs were slightly larger than the pristine nanomaterials due to the Kirkendall effect. The average values of the outer diameters of the SiOxNTs and GeOxNTs were found to be similar to each other with a value of ~230 nm, while the average diameters of SiNWs and GeNWs were ~200 nm. It was revealed that the mean pore diameter of SiOxNTs was 80 nm and that of GeOxNTs was 120 nm. In the case of the hollow SiOxNPs (H-SiOxNPs), the maximum outer diameter was also changed from 200 to 230 nm and the maximum pore diameter was 180 nm (Figure S5).
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Scanning transmission electron microscopy (STEM)–energy dispersive X-ray spectroscopy (EDS) line scans of the Si and oxygen (O) atoms in the SiOxNTs at three different points toward the inner core revealed that the atomic ratio of Si to O changed from 52:48 to 91:9, showing a stoichiometric change from SiO0.92 to SiO0.1, respectively (Figure 1d). A similar result was confirmed for GeOxNTs (after 5 h treatment) (Figure S1c, S6, and S7). These trends revealed that O atoms continuously diffused from the outer shell into vacant sites or were substituted into Si or Ge sites of the SiNWs or GeNWs, and that Si or Ge atoms conversely diffused outward. The X-ray photoelectron spectroscopy (XPS) results for the Si and Ge nanostructures were consistent with the EDS analyses; the XPS spectra showed that the SiOxNTs and GeOxNTs contained the same specific quantities of O, whereas the SiNWs and GeNWs were almost pure Si and Ge (Figure 2a and S8a)15-18. Additionally, the SiOxNTs and GeOxNTs were both found to be amorphous after the Kirkendall reaction based on powder X-ray diffraction (XRD) patterns, consistent with the HR-TEM results (Figure 2b, S8b, and S9). They were uncommon phenomena, when compared to the previous reports for Kirkendall effect which had shown the crystalline transition metal or noble metal alloy structures after Kirkendall reactions5,6,19. The temperatures of Si Kirkendall reaction were quite low, compared to that of crystalline Si or Si oxide nanostructure synthesis. Hence, the crystallization of diffused Si and Ge atoms in amorphous SiOx and GeOx should need the higher reaction temperature, compared to our synthetic condition20. On the contrast, it is favorable for the transition metal oxide to get crystalline structure after Kirkendall reaction21. The H-SiOxNPs generally afforded a Si-to-O atomic ratio of about 1:1 (Figure S10). This may be due to the comparatively larger surface areas of the NPs compared to the NWs (the SiNPs have significantly smaller radii than the SiNWs), resulting in an increased amount of the oxide layer. We also confirmed that the SiNWs were eventually transformed into silica 5 ACS Paragon Plus Environment
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(SiO2) NTs when hydrothermally heated at 200 °C over 15 days (Figure S11), indicating that prolonged thermal treatment induces enormous O diffusion from external water into the Si core. Based on our studies, hydrothermal treatment for 6 days was found to be optimal for forming hollow structures with the lowest O content. The O content in 6-day-treated SiOxNTs indicates that Si diffusion into the SiO2 matrix is faster than O diffusion from water to the SiO2 surface and in the SiO2 bulk. To analyze the Kirkendall process, VLS-grown SiNWs (~50 mg) in distilled water (~20 mL) were annealed in an autoclave at 200 °C for 1, 4, and 6 days to produce SiOxNTs. Similarly, SiNPs were subjected to the same conditions but for reaction times of 1, 3, and 4 days to produce H-SiOxNPs. The degree of pore expansion increased with elapsing reaction time (Figure 3).
The images in Figure 3 show the progression of the diffusion process: the
formation of small pores (10–50 nm) in the SiNWs (Figure 3b), pore expansion (40–80 nm, Figure 3c), and attainment of final state (60–100 nm, Figure 3d). The Kirkendall effect occurs through a mutual diffusion process at the interface between two materials such that pores are produced to compensate for the unbalanced atomic flows. In this work, the driving force of the Kirkendall effect is the different diffusion velocities of the Si or Ge atoms in the cores of the reactants and the O atoms in the oxide shells. The slow diffusion of O atoms in the oxide layer is attributed to the higher formation energy of the oxide shell, concomitant with the outward diffusion of both Si (or Ge) atoms and O atoms (Figure S12). Previously, the calculation of the Kirkendall effect was limited to kinetic aspects of atoms in two adjacent phases22. However here the mechanism of the Kirkendall effect in the SiNWs was simulated using ab initio quantum mechanical molecular dynamics calculations considering electronic as well as kinetic aspects, as shown in Figure 3e to h (see the SI for detailed method). The image in Figure 3e shows the initial state of a SiNW with a pure Si 6 ACS Paragon Plus Environment
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core and SiO2 shell. We assumed that there was no additional O supply from an external source such as water. Then, at 1800 K, Si atoms diffused via SiO2 with the formation of SiOx (0
