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

Abstract

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.

Solid‐State NMR and DFT Combined for the Surface Study of Functionalized Silicon Nanoparticles

“…Assignments of 29 Si chemical shifts from previously published work on various non-functionalized Si NP systems [4,24,25] show accordance with assignments for similar PS systems. [35] Peaks observed at approximately d( 29 Si) = À14, [25] À50, À74, [25] À84, À89, [25] À91, À102, and À109 ppm are attributed to (HO)Si(Si) 3 or (HO) 2 Si(Si) 2 or (HO)Si(Si) 2 Owing to the freshly etchedn ature of this Si NP system,t hese results show that the initial stage of oxidationo ft hese Si NPs involves back-bond oxidation( as observed for similarS iN W systems [36] )a nd that the hydrogenc apping of the surfacer emains mostly intact in this first stage. This is to be expected because the SiÀHb ond is stronger than the SiÀSi bond.…”

Endogenous Stable Radicals for Characterization of Thermally Carbonized Porous Silicon by Solid-State Dynamic Nuclear Polarization 13C NMR

“…−62 ppm corresponding to T m species [T m = Si(OSi) m (OH) 3−m R, m = 1−3 where R corresponds to C in this particular case], 41 between −80 and −110 ppm corresponding to Q n species [Q n = Si(OSi) n (OH) 4−n , n = 2−4] and/or hydrogenated silicon, 42 and between −40 and +20 ppm, assigned to Si atom from Si 1−y C y (0 < y < 1) created upon thermal carbonization with acetylene and/or oxidized silicon species such as [Si(Si) n (OH) 4−n , n = 2−3] expected at −12/− 14 ppm or Si(Si) 2 (OH)(H) or Si(Si)(OSi)(OH)(H)] expected at −22/−24 ppm. 10,12 The Si 2p and C 1s core-level XPS spectra of TCPSi samples were also recorded ( Figure 5). The Si 2p core-level spectrum shows a peak at 99.57 eV, at the lowest binding energy, corresponding to Si−Si bonds.…”

Room-Temperature Reactivity Of Silicon Nanocrystals With Solvents: The Case Of Ketone And Hydrogen Production From Secondary Alcohols: Catalysis?

“…It was affected by 1 H- 29 Si CP/MAS decoupling with different contact times, suggesting that this peak was due to the silicon atoms on the surface. 33 The CP technique is based directly 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Figure 10. The two possible mechanisms for Acetone production, but only the mechanism B reproduce our results that were probed by NMR and pH measurements where the surface the nanoparticles is made of disilene bonds.…”