Formation and Scrolling Behavior of Metal Fluoride and Oxyfluoride Nanosheets

We report 1–2 unit-cell-thick CaF 2 nanosheets, which can be converted topochemically into LaF 3–2 x O x nanosheets that scroll spontaneously. The formation of CaF 2 nanosheets is achieved through interlayer confinement and templating within CaSi 2 during reaction with aqueous HF. The structure and morphology of these nanosheets are characterized by HRTEM, AFM, and powder XRD. Solid-state MAS and solution 19 F NMR spectroscopies provide further information about interstitial fluoride sites within CaF 2 nanosheets as well as help identify side products of the CaSi 2 + HF reaction. CaF 2 nanosheets react with lanthanide salts at room temperature to yield nanostructured hexagonal LnF 3 (Ln = Ce, Pr, Nd, Sm, Eu), orthorhombic LnF 3 (Ln = Gd, Dy, Ho, Er, Yb), and cubic YbF 3– x products. Furthermore, the reaction of CaF 2 nanosheets with lanthanum salts is unique in producing LaF 3–2 x O x . The evidence for this composition includes powder XRD, EDS, XPS, and 19 F NMR data. The structure of LaF 3–2 x O x differs from hexagonal LaF 3 only in the replacement of two fluorides by one oxygen. While this topochemical transformation preserves the two-dimensional morphology it also causes lattice strain that initiates scrolling. The resulting product consists of remarkable ∼20 × 5 nm scroll-like tubes of LaF 3–2 x O x that are unique among metal fluoride materials. These results demonstrate novel metal fluoride nanochemistry and a new scrolling mechanism.