Unraveling the Core-Shell Structure of Ligand-Capped Sn/SnOx Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mossbauer, 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 similar to 3 nm thick layer of native oxides, were used in this case study. DNP-SENS selectively enhanced the weak Sn-119 NMR signal of the amorphous surface SnO2 layer. Mossbauer 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 beta-Sn core and indicated that even after 8 months of storage at 255 K there are no signs of conversion of the metallic beta-Sn core into a brittle semiconducting alpha-phase, a phase transition which normally occurs in bulk tin at 286 K (13 degrees C). Taken together, these results indicate that Sn/SnOx nanoparticles have core/shelll/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.