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

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  • Hans J. Jakobsen
  • Henrik Bildsøe
  • ,
  • Martin Bondesgaard
  • Bo B. Iversen
  • Michael Brorson, Haldor Topsoe AS
  • ,
  • Flemming H. Larsen, University of Copenhagen
  • ,
  • Zhehong Gan, National High Magnetic Field Laboratory, Tallahassee
  • ,
  • Ivan Hung, National High Magnetic Field Laboratory, Tallahassee

Solid-state, natural-abundance 95Mo NMR experiments of four different MoS2 materials have been performed on a magnet at B0 = 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 95Mo NMR as an important analytical tool in the study of MoS2 nanomaterials. 95Mo spin-lattice relaxation time (T1) studies of 160- and 4-layer 2H-MoS2 samples at 19.6 and 35.2 T show their relaxation rate (1/T1) increase in proportion to B02. This is in accord with chemical shift anisotropy (CSA) relaxation, which is the dominant T1(95Mo) mechanism, with a large 95Mo 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 (ωoτc ≈ 1) for 95Mo CSA in 2H-MoS2. A decrease in T1(95Mo) is observed for an increase in the B0 field and for a decrease in the number of 2H-MoS2 layers. All four nanomaterials exhibit identical 95Mo electric-field gradient (EFG) parameters. The T1 results account for the several failures in retrieving the 95Mo spectral EFG and CSA parameters for multilayer 2H-MoS2 samples in the pioneering solid-state 95Mo NMR studies performed during the past 2 decades (1990-2010) because of the extremely long T1(95Mo) = ∼200-250 s observed at a low B0 (∼9.4 T) used at that time. Much shorter T1(95Mo) 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 95Mo 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 95Mo 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-γquadrupole-nucleus species layered on a catalyst support. While a huge gain in NMR sensitivity, by a factor of ∼60, is observed for the 95Mo 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 95Mo shifts in the 95Mo MAS spectra at B0 up to 35.2 T upon reduction of the number of sample layers.

Original languageEnglish
JournalJournal of Physical Chemistry C
Pages (from-to)7824-7838
Number of pages15
Publication statusPublished - Apr 2021

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