Friday, February 06, 2009

J. Am. Chem. Soc., 2009, 131 (4), pp 1426–1435

Nonaromatic Core−Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy

XiaoWen Fang, JingDong Mao, E. M. Levin and Klaus Schmidt-Rohr

The structure of synthetic nanodiamond has been characterized by 13C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range 1H−13C dipolar couplings and 13C relaxation times. The surface layer of these ∼4.8-nm diameter carbon particles consists mostly of sp3-hybridized C that is protonated or bonded to OH groups, while sp2-hybridized carbon makes up less than 1% of the material. The surface protons surprisingly resonate at 3.8 ppm, but their direct bonding to carbon is proved by fast dipolar dephasing under homonuclear decoupling. Long-range 1H−13C distance measurements, based on 13C{1H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains ∼60% of all carbons, predominantly resonate more than +8 ppm from the 37-ppm peak of bulk diamond (i.e., within the 45−80 ppm range). Nitrogen detected in 15N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location of unpaired electrons (∼40 unpaired electrons per particle) was studied in detail, based on their strongly distance-dependent effects on T1,C relaxation. The slower relaxation of the surface carbons, selected by spectral editing, showed that the unpaired electrons are not dangling bonds at the surface. This was confirmed by detailed simulations, which indicated that the unpaired electrons are mostly located in the disordered shell, at distances between 0.4 and 1 nm from the surface. On the basis of these results, a nonaromatic core−shell structural model of nanodiamond particles has been proposed.

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