Exciting Opportunities for Solid-State Mo NMR Studies of MoS Nano-structures in Materials Research from Low to Ultra-high Magnetic Field (35.2 T).

Solid-state, natural-abundance Mo NMR experiments of four different MoS materials have been performed on a magnet = 19.6 T and on a new Series Connected Hybrid (SCH) magnet at 35.2 T. Employing two different 2H-MoS (2H phase) materials, a "pseudo-amorphous" MoS nano-material, and a MoS layer on the AlO support of a hydrodesulphurization (HDS) catalyst have enabled introduction of solid-state Mo NMR as an important analytical tool in studies of MoS nano-materials. Mo spin-lattice relaxation time ( ) studies of 160- and 4-layer 2H-MoS samples at 19.6 and 35.2 T show their relaxation rates (1/ ) increase in proportion to . This is in accord with chemical shift anisotropy (CSA) relaxation being the dominant (Mo) mechanism, with a large Mo CSA = 1025 ppm determined for all four MoS nano-materials. The dominant CSA mechanism suggests the MoS band-gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source ( τ << 1) for Mo CSA in 2H-MoS. A decrease in (Mo) is observed for an increase in field and for a decrease in the number of 2H-MoS layers. All four nano-materials exhibit identical Mo electric field gradient (EFG) parameters. The results account for the several failures to retrieve Mo spectral EFG and CSA parameters for multilayer 2H-MoS samples in the pioneering solid-state Mo NMR studies performed during the past two decades (1990-2010), because of the extremely long (Mo) = ~200-250 s observed at low (~9.4 T) used at that time. Much shorter (Mo) values are observed even at 19.6 T for the "pseudo-amorphous" and the HDS catalyst (MoS-AlO support) MoS nano-materials. These allowed useful solid-state Mo NMR spectra for these two samples to be obtained at 19.6 T in a few to < 24 h. Most importantly, this research led to observation of an impressive Mo MAS spectrum for an average of 1-4 thick MoS-layers on a AlO support, i.e., the first MAS NMR spectrum of a low natural-abundance, low-γ quadrupole-nucleus species layered on a catalyst support. While a huge gain in NMR sensitivity, factor ~ 60, is observed for the Mo MAS spectrum of the 160-layer sample at 35.2 T compared to 14.1 T, the MAS spectrum for the 4-layer sample is almost completely wiped out at 35.2 T. This unusual observation for the 4-layer sample (crumpled, rose-like and defective Mo-edge structures) is due to an increased distribution of the isotropic Mo shifts in the Mo MAS spectra at up to 35.2 T upon reduction of the number of sample layers.