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–) 3 Si–H > (b) ( Si–O–) 3 SiOH > (c)？(HO–) n Si( Si) m(–O Si) 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 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 T 2 relaxation parameter simultaneously with background suppression.