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

Abstract

The structure of synthetic nanodiamond has been characterized by (13)C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range (1)H-(13)C dipolar couplings and (13)C relaxation times. The surface layer of these approximately 4.8-nm diameter carbon particles consists mostly of sp(3)-hybridized C that is protonated or bonded to OH groups, while sp(2)-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 (1)H-(13)C distance measurements, based on (13)C{(1)H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains approximately 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 (15)N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location of unpaired electrons (approximately 40 unpaired electrons per particle) was studied in detail, based on their strongly distance-dependent effects on T(1,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.

Paramagnetic centers in detonation nanodiamonds studied by CW and pulse EPR

“…Data on the g-factor as well as on the concentration of paramagnetic centers are consistent with the literature within the experimental errors [5,8,9]. According to recent NMR data [10,11], these paramagnetic centers are located mainly in the bulk.…”

“…According to [35,36], there is a core with a perfect diamond lattice which is surrounded by a shell of compressed diamond lattice, and this core-shell structure is enveloped in a nondiamond carbon. Recent NMR data [11] showed the presence of nitrogen atoms in the shell of nanodiamond particle, not in the core. It is possible to propose that the P1-centers are located in the compressed diamond shell.…”

“…Due to the short spin-lattice relaxation time (<10 μs [8]) of these radicals there is little microwave saturation at a microwave power up to 200 mW. The assignment of this spectrum to the dangling C-C bonds on the diamond cluster surface [5] or to the unpaired electrons located in the bulk of the nanodiamond particle [10,11] is under discussion.…”

Understanding structure and porosity of nanodiamond-derived carbon onions

“…Nanodiamonds [28] are an intriguing precursor for the gram-scale synthesis [29] of control over the resulting structure and properties via thermal annealing [27]. The typical size of nanodiamonds used for OLC synthesis is around 5 nm [28] with dangling bonds on the surface that are typically stabilized by functional groups, mostly carboxyls [28,30], and locally terminated by a thin layer of sp 2 -hybridized carbon [31][32][33]. These two surface features contribute to the notorious aggregation of individual nanodiamond particles forming clusters in the range of tens to hundreds of nanometer [34]: individual grains are engulfed by thin layers of graphitic carbon or covalently linked by bridging functional groups [28,34], leading to the formation of so-called tight agglomerates.…”

Hierarchical porous graphitic carbon monoliths with detonation nanodiamonds: synthesis, characterisation and adsorptive properties

“…The single digit nanodiamond (SDND) experienced a significant weight loss of 7.1 % up to 160°C due to the presence of water and other volatiles on its surface. DND is known to be a highly hygroscopic material [54,55]. The carbon monoliths underwent 3 main stages of weight loss.…”