Multilayered core–shell structure of polyol-stabilized calcium fluoride nanoparticles characterized by NMR

Diethyleneglycol (DEG)-stabilized calcium fluoride (CaF2) nanoparticles with a mean diameter of about 20 nm consist of an inorganic crystalline core, covered by a roughly monomolecular layer of organic material according to standard physico-chemical analysis. Multinuclear NMR experiments under magic angle spinning were used to identify the constituent 13C, 1H and 19F atoms. All resonances were assigned (DEG, EtOH, H2O, H3O+, OH−, F−, CaF2), and different populations could be discriminated in terms of their mobilities and correlations with neighboring atoms. We observed several different layers: mobile EtOH on the particle surface, an “interphase” containing immobile DEG molecules and fluoride ions, plus the single-crystalline CaF2 core. It was thus possible for the first time to characterize by NMR all layered components of such core–shell particles, and the thickness of the newly discovered interphase could be estimated from the NMR intensities.

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► Diethylene-glycol-stabilized CaF2 nanoparticles are studied by 13C, 1H, and 19F NMR.
► The surface contains mobile EtOH, and H3O+ and/or OH− ions from water.
► At least two types of fluoride ions are observed in the nanoparticles.
► Core 19F ions are >1 nm from the surface, hence there must be an interphase.
► The interphase layer is estimated to be about 1.6 nm thick.