Formation and Scrolling Behavior of Metal Fluoride and Oxyfluoride Nanosheets

Abstract

We report 1−2 unit-cell-thick CaF 2 nanosheets, which can be converted topochemically into LaF 3−2x 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 3−2x O x . The evidence for this composition includes powder XRD, EDS, XPS, and 19 F NMR data. The structure of LaF 3−2x 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 3−2x O x that are unique among metal fluoride materials. These results demonstrate novel metal fluoride nanochemistry and a new scrolling mechanism.

Synthesis of 2D Germanane (GeH): a New, Fast, and Facile Approach

“…In this context, fluorine is more electronegative than Ge in CaGe 2 , causing the fast formation of the calcium halide by‐product and hence the formation of germanane. This is in agreement with the findings of Ramachandran et al., [16] who reported that during the topotactic deintercalation of the Zintl phase of CaSi 2 , the HF reactivity is very high, while the use of HCl instead of HF results in a different solubility of the calcium halides formed. The formation of insoluble solid CaF 2 during etching is instant and thus helps to pull the reaction toward the product side due to the removal of CaF 2 from the system according to Le Chatelier's principle.…”

“…To remove the residual CaF 2 ,t he material was treated with as aturated aqueous solution of ethylenediaminetetraacetic acid (EDTA);E DTAi sh ighly water-soluble and acts as achelating agent that allows to extract the residual Ca 2+ from the sample.Inorder to understand the role of fluorine during the reaction, one must consider that the electronegativity of fluorine is higher than that of other halogens. [15] In this context, fluorine is more electronegative than Ge in CaGe 2 , causing the fast formation of the calcium halide by-product and hence the formation of germanane.T his is in agreement with the findings of Ramachandran et al, [16] who reported that during the topotactic deintercalation of the Zintl phase of CaSi 2 ,t he HF reactivity is very high, while the use of HCl instead of HF results in ad ifferent solubility of the calcium halides formed. Theformation of insoluble solid CaF 2 during etching is instant and thus helps to pull the reaction toward the product side due to the removal of CaF 2 from the system according to Le Chateliersp rinciple.…”

Surface defect‐enhanced conductivity of calcium fluoride for electrochemical applications

“…The resistance of the system corresponds to the intersection of the complex impedance semicircle with the x‐axis diminishing the reactance providing pure real contributions at lower frequencies. Fitting of the measured Nyquist plots had been carried out to obtain relevant material parameters . The applied equivalent circuit model for the electrolyte (inset in Figure B) composed of two constant phase elements (CPE1 and CPE2) and an ohmic resistor (R1) .…”

Rolled‐up Nanotechnology: Materials Issue and Geometry Capability

“…c) Schematic of the overall scrolling process, and TEM image of the LaF 3−2 x O x nanosheet and a single nanotube showing lattice fringing with a d ‐spacing of 3.23 Å that matches the (101) plane of the LaF 3 . Reproduced with permission . Copyright 2016, American Chemical Society.…”

“…Similar to the strain in heteroepitaxial crystalline bilayer, the topochemical transformation strain is also generated utilizing lattice mismatch. However, to the best of our knowledge, this method has only been reported once utilizing a specific topochemical transformation, in which a 2D material was rolled up into microtubes. As shown in Figure c, the CaF 2 nanosheets with the thickness of 1–2 unit cells react with lanthanum salts in water at ambient conditions, causing a topochemical conversion to yield partially oxidized lanthanum fluoride nanosheets.…”