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 ¹H, 29Si and ²H magic-angle-spinning (MAS) NMR results and quantum mechanical 29Si 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-)₃Si-H > (b) (Si-O-)₃SiOH > (c) (HO-)nSi(Si)m(-OSi)4-m-n ≈ (d) (Si-O-)₂Si(H)OH > (e) (Si-O-)₂Si(-OH)₂ > (f) (Si-O-)₄Si, 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 O₂ gas at elevated temperature. A deliberately oxidized sample displays an increased population of (Si-O-)₃Si-H, as well as (Si-O-)₃SiOH 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 (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 T₂ relaxation parameter simultaneously with background suppression.