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 (1)H, (29)Si and (2)H magic-angle-spinning (MAS) NMR results and quantum mechanical (29)Si 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) ≈ (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 (1)H and (2)H 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.