TY - JOUR
T1 - 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)
AU - Jakobsen, Hans J.
AU - Bildsøe, Henrik
AU - Bondesgaard, Martin
AU - Iversen, Bo B.
AU - Brorson, Michael
AU - Larsen, Flemming H.
AU - Gan, Zhehong
AU - Hung, Ivan
N1 - Publisher Copyright:
© 2021 American Chemical Society.
Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.
PY - 2021/4
Y1 - 2021/4
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=85104927554&partnerID=8YFLogxK
U2 - 10.1021/acs.jpcc.0c10522
DO - 10.1021/acs.jpcc.0c10522
M3 - Journal article
C2 - 34262634
AN - SCOPUS:85104927554
SN - 1932-7447
VL - 125
SP - 7824
EP - 7838
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
IS - 14
ER -