Abstract
Solid-state, natural-abundance (95)Mo NMR experiments of four different MoS(2) materials have been performed on a magnet B (0) = 19.6 T and on a new Series Connected Hybrid (SCH) magnet at 35.2 T. Employing two different 2H-MoS(2) (2H phase) materials, a "pseudo-amorphous" MoS(2) nano-material, and a MoS(2) layer on the Al(2)O(3) support of a hydrodesulphurization (HDS) catalyst have enabled introduction of solid-state (95)Mo NMR as an important analytical tool in studies of MoS(2) nano-materials. (95)Mo spin-lattice relaxation time (T (1)) studies of 160- and 4-layer 2H-MoS(2) samples at 19.6 and 35.2 T show their relaxation rates (1/T (1)) increase in proportion to B (0) (2). This is in accord with chemical shift anisotropy (CSA) relaxation being the dominant T (1)((95)Mo) mechanism, with a large (95)Mo CSA = 1025 ppm determined for all four MoS(2) nano-materials. The dominant CSA mechanism suggests the MoS(2) band-gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (ω (o)τ(c) << 1) for (95)Mo CSA in 2H-MoS(2). A decrease in T (1)((95)Mo) is observed for an increase in B (0) field and for a decrease in the number of 2H-MoS(2) layers. All four nano-materials exhibit identical (95)Mo electric field gradient (EFG) parameters. The T (1) results account for the several failures to retrieve (95)Mo spectral EFG and CSA parameters for multilayer 2H-MoS(2) samples in the pioneering solid-state (95)Mo NMR studies performed during the past two decades (1990-2010), because of the extremely long T (1)((95)Mo) = ~200-250 s observed at low B (0) (~9.4 T) used at that time. Much shorter T (1)((95)Mo) values are observed even at 19.6 T for the "pseudo-amorphous" and the HDS catalyst (MoS(2)-Al(2)O(3) support) MoS(2) nano-materials. These allowed useful solid-state (95)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 (95)Mo MAS spectrum for an average of 1-4 thick MoS(2)-layers on a Al(2)O(3) 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 (95)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 (95)Mo shifts in the (95)Mo MAS spectra at B (0) up to 35.2 T upon reduction of the number of sample layers.