Exciting Opportunities for Solid-State 95Mo NMR Studies of MoS2 Nanostructures in Materials Research from a Low to an Ultrahigh Magnetic Field (35.2 T)

Solid-state, natural-abundance Mo-95 NMR experiments of four different MoS2 materials have been performed on a magnet at B-0 = 19.6 T and on a new series-connected hybrid magnet at 35.2 T. Employing two different 2H-MoS2 (2H phase) materials, a pseudo-amorphous MoS2 nanomaterial and a MoS2 layer on an Al2O3 support of a hydrodesulfurization (HDS) catalyst, has enabled the introduction of solid-state Mo-95 NMR as an important analytical tool in the study of MoS2 nanomaterials. Mo-95 spin-lattice relaxation time (T-1) studies of 160- and 4-layer 2H-MoS2 samples at 19.6 and 35.2 T show their relaxation rate (1/T-1) increase in proportion to B-0(2). This is in accord with chemical shift anisotropy (CSA) relaxation, which is the dominant T-1(Mo-95) mechanism, with a large Mo-95 CSA of 1025 ppm determined for all four MoS2 nanomaterials. The dominant CSA mechanism suggests that the MoS2 band gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (omega(o)tau(c) << 1) for Mo-95 CSA in 2H-MoS2. A decrease in T-1(Mo-95) is observed for an increase in the B-0 field and for a decrease in the number of 2H-MoS2 layers. All four nanomaterials exhibit identical Mo-95 electric-field gradient (EFG) parameters. The T-1 results account for the several failures in retrieving the Mo-95 spectral EFG and CSA parameters for multilayer 2H-MoS2 samples in the pioneering solid-state Mo-95 NMR studies performed during the past 2 decades (1990-2010) because of the extremely long T-1(Mo-95) = similar to 200-250 s observed at a low B-0 (similar to 9.4 T) used at that time. Much shorter T-1(Mo-95) values are observed even at 19.6 T for the pseudo-amorphous and the HDS catalyst (MoS2-Al2O3 support) MoS2 nanomaterials. These allowed obtaining useful solid-state Mo-95 NMR spectra for these two samples at 19.6 T in a few to <24 h. Most importantly, this research led to the observation of an impressive Mo-95 MAS spectrum for an average of 1-4 layer thick MoS2 on an Al2O3 support, that is, the first MAS NMR spectrum of a low-natural-abundance, low-gamma quadrupole-nucleus species layered on a catalyst support. While a huge gain in NMR sensitivity, by a factor of similar to 60, is observed for the Mo-95 MAS spectrum of the 160-layer sample at 35.2 T as compared to 14.1 T, the MAS spectrum of 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-95 shifts in the Mo-95 MAS spectra at B-0 up to 35.2 T upon reduction of the number of sample layers.

Automated summary

Solid-state Mo-95 NMR experiments were conducted on four different MoS2 materials at different magnetic field strengths, revealing the dominance of chemical shift anisotropy relaxation mechanism and the delocalization of MoS2 band gap electrons in lattice-layer structures. The studies showed that Mo-95 spin-lattice relaxation time decreased with increasing magnetic field strength and decreasing number of 2H-MoS2 layers, leading to useful NMR spectra acquisition for MoS2 nanomaterials.