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

The surface structure and adjacent interior of commercially available silicon nanopowder (np-Si) was studied using multinuclear, solid-state NMR spectroscopy. The results are consistent with an overall picture in which the bulk of the np-Si interior consists of highly ordered (crystalline) silicon atoms, each bound tetrahedrally to four other silicon atoms. From a combination of H-1, Si-29 and H-2 magic-angle-spinning (MAS) NMR results and quantum mechanical Si-29 chemical shift calculations, silicon atoms on the surface of as-received np-Si were found to exist in a variety of chemical structures, with apparent populations in the order (a) (Si-O-)(3)Si-H > (b) (Si-O-)(3)SiOH > (c) (HO-)(n)Si(Si)(m)(-OSi)(4-m-n) approximate to (d) (Si-O-)(2)Si(H)OH > (e) (Si-O-)(2)Si(-OH)(2) > (f) (Si-O-)(4)Si, where Si stands for a surface silicon atom and Si represents another silicon atom that is attached to Si by either a Si-Si bond or a Si-O-Si linkage. The relative populations of each of these structures can be modified by chemical treatment, including with O-2 gas at elevated temperature. A deliberately oxidized sample displays an increased population of (Si-O-)(3)Si-H, as well as (Si-O-)(3)SiOH sites. Considerable heterogeneity of some surface structures was observed. A combination of H-1 and H-2 MAS experiments provide evidence for a substantial population of silanol (Si-OH) moieties, some of which are not readily H-exchangeable, along with the dominant Si-H sites, on the surface of as-received np-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 T-2 relaxation parameter simultaneously with background suppression.