The Surface of Nanoparticle Silicon as Studied by Solid-State NMR.

The surface structure and adjacent interior of commercially available silicon nanopowder (-Si) was studied using multinuclear, solid-state NMR spectroscopy. The results are consistent with an overall picture in which the bulk of the -Si consists of highly ordered ("crystalline") silicon atoms, each bound tetrahedrally to four other silicon atoms. From a combination of ¹H, Si and ²H magic-angle-spinning (MAS) NMR results and quantum mechanical Si chemical shift calculations, silicon atoms on the of "as-received" -Si were found to exist in a variety of chemical structures, with apparent populations in the order (a) (-O-)₃-H > (b) (-O-)₃OH > (c) (HO-)()(-O) ≈ (d) (-O-)₂(H)OH > (e) (-O-)₂(-OH)₂ > (f) (-O-)₄, where stands for a surface silicon atom and represents another silicon atom that is attached to by either a - bond or a -O- linkage. The relative populations of each of these structures can be modified by chemical treatment, including with O₂ gas at elevated temperature. A deliberately oxidized sample displays an increased population of (-O-)₃-H, as well as (-O-)₃OH sites. Considerable heterogeneity of some surface structures was observed. A combination of ¹H and ²H MAS experiments provide evidence for a substantial population of silanol (-OH) moieties, some of which are not readily H-exchangeable, along with the dominant -H sites, on the surface of "as-received" -Si; the silanol moieties are enhanced by deliberate oxidation. An extension of the DEPTH background suppression method is also demonstrated that permits measurement of the T₂ relaxation parameter simultaneously with background suppression.