Exciting Opportunities for Solid-State ⁹⁵Mo NMR Studies of MoS₂ Nano-structures in Materials Research from Low to Ultra-high Magnetic Field (35.2 T).

ABSTRACT: Solid-state, natural-abundance ⁹⁵Mo NMR experiments of four different MoS₂ 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-MoS₂ (2H phase) materials, a "pseudo-amorphous" MoS₂ nano-material, and a MoS₂ layer on the Al₂O₃ support of a hydrodesulphurization (HDS) catalyst have enabled introduction of solid-state ⁹⁵Mo NMR as an important analytical tool in studies of MoS₂ nano-materials. ⁹⁵Mo 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 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(⁹⁵Mo) mechanism, with a large ⁹⁵Mo CSA = 1025 ppm determined for all four MoS₂ nano-materials. The dominant CSA mechanism suggests the MoS₂ band-gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (ω _oτ_c << 1) for ⁹⁵Mo CSA in 2H-MoS₂. A decrease in T ₁(⁹⁵Mo) is observed for an increase in B ₀ field and for a decrease in the number of 2H-MoS₂ layers. All four nano-materials exhibit identical ⁹⁵Mo electric field gradient (EFG) parameters. The T ₁ results account for the several failures to retrieve ⁹⁵Mo spectral EFG and CSA parameters for multilayer 2H-MoS₂ samples in the pioneering solid-state ⁹⁵Mo NMR studies performed during the past two decades (1990-2010), because of the extremely long T ₁(⁹⁵Mo) = ~200-250 s observed at low B ₀ (~9.4 T) used at that time. Much shorter T ₁(⁹⁵Mo) values are observed even at 19.6 T for the "pseudo-amorphous" and the HDS catalyst (MoS₂-Al₂O₃ support) MoS₂ nano-materials. These allowed useful solid-state ⁹⁵Mo 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 ⁹⁵Mo MAS spectrum for an average of 1-4 thick MoS₂-layers on a Al₂O₃ 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 ⁹⁵Mo 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 ⁹⁵Mo shifts in the ⁹⁵Mo MAS spectra at B ₀ up to 35.2 T upon reduction of the number of sample layers.

SUBMITTER: Jakobsen HJ 

PROVIDER: S-EPMC8276973 | biostudies-literature | 2021 Apr

REPOSITORIES: biostudies-literature