Synthesis of Si nanowire/nanosheet complexes from CaSi₂ crystals by thermal annealing under MnCl₂/NH₄Cl vapors

Abstract

Si-based nanowire/nanosheet bundle complexes were synthesized from CaSi 2 crystal powders by thermal annealing under MnCl 2 /NH 4 Cl vapors. The formation of the complexes is a result of two processes that simultaneously occurred. One is the nanosheet bundle synthesis by Ca-atom extraction from the CaSi 2 crystals. Another is the growth of the Si nanowires by thermal annealing using MnCl 2 . The mixing of these materials resulted in the successful synthesis of the nanowire/nanosheet bundle complexes. In addition to the formation of the complex, nanotubes were also synthesized by bottom-up nanostructure synthesis technique, which resulted by the thermal annealing of the catalytic Mn-related materials under a higher vapor pressure using NH 4 Cl. The growth mechanism of the nanotubes is discussed based on the inhomogeneous Mn-related catalyst deposition and the surface diffusion of Si atoms.

Synthesis of MnSi1.7 nanosheet bundles from CaSi2 crystal powders using MnCl2 in molten salt

“…The density of the nucleation sites of the Si crystals by Ca atom extraction on the side wall of the CaSi 2 crystals increased with increasing thermal annealing temperatures, thereby leading to the formation of thinner nanosheets. 49) The morphology evaluation indicated that the thickness of MnSi 1.7 sheets is thinner than that of Mg 2 Si nanosheet bundles. 27) A small void space was formed between the nanosheets owing to the reduction in volume caused by the Ca atom extraction.…”

“…In this case, MnCl 2 , MnSi 1.7 , or other Mnbased compounds acted as catalyst for the growth of Si nanowires when the excess Si atoms were supplied. 10,49,50) It is interesting to compare the results with and without NH 4 Cl source for the nanosheets synthesized at 600 °C for 10 h (Samples 5 and 6). When NH 4 Cl was used as the source material, the dissociation of NH 4 Cl provided HCl formation, similar to the case of using deliquescent chloride; the HCl molecules were significantly transported to the CaSi 2 crystals at elevated temperatures.…”

Transformation of CaSi overgrowth domains to the CaSi2 crystal phase via vacuum annealing

“…To obtain mono or multilayered silicene or nanostructured Si by extracting the Ca from the CaSi 2 crystal, several chemical treatment methods have been adopted. [27][28][29][30] Recently, An et al claimed that a bulk CaSi 2 crystal could be transformed to polycrystalline Si while maintaining its morphology (outer shape of crystallites) after heating above 800 °C in a vacuum. 31) In comparison to those methods, as speculated from the results, the Ca extraction process (shown as Fig.…”