Unraveling the core-shell structure of ligand-capped Sn/SnOx nanoparticles by surface-enhanced nuclear magnetic resonance, Mössbauer, and X-ray absorption spectroscopies.
A particularly difficult challenge in the chemistry of nanomaterials is the detailed structural and chemical analysis of multicomponent nano-objects. This is especially true for the determination of spatially resolved information. In this study, we demonstrate that dynamic nuclear polarization surface-enhanced solid-state NMR spectroscopy (DNP-SENS), which provides selective and enhanced NMR signal collection from the (near) surface regions of a sample, can be used to resolve the core-shell structure of a nanoparticle. Li-ion anode materials, monodisperse 10-20 nm large tin nanoparticles covered with a ∼3 nm thick layer of native oxides, were used in this case study. DNP-SENS selectively enhanced the weak 119Sn NMR signal of the amorphous surface SnO2 layer. Mössbauer and X-ray absorption spectroscopies identified a subsurface SnO phase and quantified the atomic fractions of both oxides. Finally, temperature-dependent X-ray diffraction measurements were used to probe the metallic β-Sn core and indicated that even after 8 months of storage at 255 K there are no signs of conversion of the metallic β-Sn core into a brittle semiconducting α-phase, a phase transition which normally occurs in bulk tin at 286 K (13 °C). Taken together, these results indicate that Sn/SnOx nanoparticles have core/shell1/shell2 structure of Sn/SnO/SnO2 phases. The study suggests that DNP-SENS experiments can be carried on many types of uniform colloidal nanomaterials containing NMR-active nuclei, in the presence of either hydrophilic (ion-capped surfaces) or hydrophobic (capping ligands with long hydrocarbon chains) surface functionalities.
Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich CH-8093, Switzerland.
Loredana
L
Protesescu
Aaron J
AJ
Rossini
Dominik
D
Kriegner
Maxence
M
Valla
Antoine
A
de Kergommeaux
Marc
M
Walter
Kostiantyn V
KV
Kravchyk
Maarten
M
Nachtegaal
Julian
J
Stangl
Bernard
B
Malaman
Peter
P
Reiss
Anne
A
Lesage
Lyndon
L
Emsley
Christophe
C
Copéret
Maksym V
MV
Kovalenko