| Synthesis and characterization of novel semiconducting Si-based nanocomposites with slit-like nanopores |
| Hiroshi Itahara |
| Toyota Central Research & Development Labs. |
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Keywords: Silicon, Silicide, porous material, semiconductor, direct band gap
Si-based nanostructured materials with a high surface-to-volume ratio (i.e., porous or hollow structure) are promising for various applications, such as Li-ion batteries, photovoltaics and catalyst support. The porous or hollow structured nanomaterials are conventionally prepared by using sacrificing templates [1]: The typical templates are anodized Al2O3, polymer fibers and small SiO2 particles, and they are removed by calcining or chemical etching at the final fabrication process. Despite of usefulness of those nanostructured materials, their synthesis complexity may hinder their mass production.
In contrast, we have developed a template-free fabrication method, a solid-state exfoliation reaction method using layered CaSi2 [2], for the production of Si-based nanocomposites with slit-like nanopores. For example, heating the mixture of layered CaSi2 and NiCl2 provided the composite powders comprised of ¡®Ni and CaxSi2 (Ca extracted CaSi2)¡¯ or ¡®nickel silicide and CaxSi2¡¯. It was suggested that their formation mechanism is based on a solid-state exfoliation reaction wherein the formation of CaCl2 promotes the extraction of Ca from CaSi2, thereby exfoliating the layered structure (Figure 1). The CaxSi2 particles are the agglomerates comprised of many plate-like particles with thickness of ~ 15 nm. The plate-like particles are stacked with the gap of ~ 4 nm between them.
The prepared nanocomposites of ¡®Ni and CaxSi2¡¯ or ¡®nickel silicide and CaxSi2¡¯ showed a high anode capacity for Li ion batteries [2-3]. The highest initial capacity was 1,020 mAh/g, which is three times higher than that of conventional graphite and twelve times higher than that of the raw CaSi2. Because Ni or nickel silicide does not react with Li, CaxSi2 is considered as Li storable materials. In addition, the plate-like structure of CaxSi2 might facilitate insertion and extraction of Li, thereby leading such high capacity. Furthermore, we found that the CaxSi2 showed direct optical band gap while the layered CaSi2, the raw material, shows metallic property. With changing the synthetic conditions, the band gap was changed from ~ 1.0 eV to ~ 2.0 eV. Because of its simplicity, the solid-state exfoliation reaction using layered CaSi2 would provide a versatile strategy for the Si-based functional nanocomposites with nanopores.
References:
[1] X. W. Lou, L. A. Archer, Z. Yang, Advanced Material, 20, 3987-4019 (2008).
[2] S.-Y. Oh, H. Imagawa, H. Itahara, Chemistry, Asian J., in press, DOI: 10.1002/asia.201402544.
[3] S.-Y. Oh, H. Imagawa, H. Itahara, J. Mater. Chem. A., 2, 12501-12506 (2014). |
| Acknowledgements : The authors acknowledge Ms. Akiko Ueki and Mr. Yusuke Akimoto of Toyota CRDL for conducting the microstructural observations. |
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