Hollow Silicon Nanostructures via the Kirkendall Effect

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 the 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.

Electrolytic Formation of Crystalline Silicon/Germanium Alloy Nanotubes and Hollow Particles with Enhanced Lithium‐Storage Properties

“…Interestingly,t he high-temperature environment of molten chlorides and the comparable crystallographic structures between crystalline Si and Ge can facilitate solid diffusion process in Ge@Si governed by the nanoscale Kirkendall effect to form SiGe alloy. [9,[35][36][37][38] Thed riving force of solid diffusion in ab inary alloy is generally explained as minimization of total Gibbs free energy and the presence of Ge at the surface of the SiGe alloy is energetically favorable because Ge has lower surface energy than Si. [20] Hence,t he outward diffusion of Ge species is faster than the inward diffusion of Si species in the Ge@Si core-shell nanowires,thus generating internal voids.Asaresult, the transformation from the core-shell Ge@Si nanowires to SiGe alloy nanotubes is favorable.…”

Ordered CaSi2 Microwall Arrays on Si Substrates Induced by the Kirkendall Effect

“…After removing the upper CaSi region, the CaSi 2 microwalls grown in vertical directions and those rooted to the Si substrate remained as shown in Figure 8d.T he Kirkendall effect of Si has been rarely investigated, however,d iffusion of Si atoms into aS iO 2 layer was used as an effective strategy to produce ah ollow SiO x structure. [24] In this case, Si diffuses outward isotropically in the SiO 2 nanotube. In contrast, the phenomena focused on in this study are based on the crystallographic symmetry of Si to control the morphologya nd growth direction of CaSi 2 on the Si substrate.…”

“…On the Si(111) surface, the inclined angles of Si(11 ), (1 1), and ( 11) are the same so that the three directional CaSi 2 microwall arrays were formed as shown in Figures and . After removing the upper CaSi region, the CaSi 2 microwalls grown in vertical directions and those rooted to the Si substrate remained as shown in Figure d. The Kirkendall effect of Si has been rarely investigated, however, diffusion of Si atoms into a SiO 2 layer was used as an effective strategy to produce a hollow SiO x structure . In this case, Si diffuses outward isotropically in the SiO 2 nanotube.…”

“…[22] It is cleart hat various kinds of hollow nanoparticles and tubes can be fabricated by using the Kirkendall effect,r egardless of whether the final structures consist of ac ompound or as olid solution. [23] The Kirkendall effect of Si was reported in only af ew cases [24,25] because the diffusivity of Si is generally believed to be low.F or silicide materials,K irkendall void formation was effectively appliedt o nickel silicides to produce hollow nanocrystals, [26] and to Li-Si for lithium-ion batteries. [27] However,t he Kirkendall void formation process has not been positively applied to CaSi 2 .…”

Kirkendall void formation in reverse step graded Si1−xGex/Ge/Si(001) virtual substrates

“…Due to the extremely low inter-diffusion rates of Si and Ge [7,8], this effect is reported to occur mostly in nanowire and nanosphere structures [9] that use extended annealing [10,11] or electro-reduction of SiO 2 and GeO 2 (e.g. to create SiGe nanotubes for Li-ion battery electrode applications [12]) with minor developments in thin film [13].…”