Formation and Scrolling Behavior of Metal Fluoride and Oxyfluoride Nanosheets

We report 1-2 unit-cell-thick CaF2 nanosheets, which can be converted topochemically into LaF3-2xOx nanosheets that scroll spontaneously. The formation of CaF2 nanosheets is achieved through interlayer confinement and templating within CaSi2 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 F-19 NMR spectroscopies provide further information about interstitial fluoride sites within CaF2 nanosheets as well as help identify side products of the CaSi2 + HF reaction. CaF2 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 YbF3-x products. Furthermore, the reaction of CaF2 nanosheets with lanthanum salts is unique in producing LaF3-2xOx. The evidence for this composition includes powder XRD, EDS, XPS, and F-19 NMR data. The structure of LaF3-2xOx differs from hexagonal LaF3 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 similar to 20 X 5 nm scroll-like tubes of LaF3-2xOx that are unique among metal fluoride materials. These results demonstrate novel metal fluoride nanochemistry and a new scrolling mechanism.