Amorphous Silicon: New Insights into an Old Material

Amorphous silicon is synthesized by treating the tetrahalosilanes SiX4 (X=Cl, F) with molten sodium in high boiling polar and non-polar solvents such as diglyme or nonane to give a brown or a black solid showing different reactivities towards suitable reagents. With regards to their technical relevance, their stability towards oxygen, air, moisture, chlorine-containing reaction partners RCl (R=H, Cl, Me) and alcohols is investigated. In particular, reactions with methanol are a versatile tool to deliver important products. Besides tetramethoxysilane formation, methanolysis of silicon releases hydrogen gas under ambient conditions and is thus suitable for a decentralized hydrogen production; competitive insertion into the MeOH versus the MeOH bond either yields H- and/or methyl-substituted methoxy functional silanes. Moreover, compounds, such as MenSi(OMe)4−n (n=0–3) are simply accessible in more than 75 % yield from thermolysis of, for example, tetramethoxysilane over molten sodium. Based on our systematic investigations we identified reaction conditions to produce the methoxysilanes MenSi(OMe)4−n in excellent (n=0:100 %) to acceptable yields (n=1:51 %; n=2:27 %); the yield of HSi(OMe)3 is about 85 %. Thus, the methoxysilanes formed might possibly open the door for future routes to silicon-based products.