Designing Tetraoxa[8]circulenes To Serve as Hosts and Sensors.

In D4-symmetric tetraoxa[8]circulenes, alternating fused benzene and furan rings form an octagonal array. These compounds are little known despite their novel properties, which include extended planar π-conjugation and a formally antiaromatic cyclooctatetraene core. Tetraoxa[8]circulenes can be formed by acid-induced cyclocondensations of suitable quinones, but existing methods often give very low yields. In addition, π-stacking of simple tetraoxa[8]circulenes reduces solubility and limits opportunities to form homogeneous mixtures or cocrystals with other compounds. To help make tetraoxa[8]circulenes more useful, we have developed better ways to synthesize them, and we have used these methods to produce awkwardly shaped derivatives with large concave electron-rich aromatic surfaces. These compounds crystallize to form open structures that can accommodate various guests, including C60. Analysis of the structures shows that the cyclooctatetraene core of the hosts exhibits surprising variations in C-C bond lengths and conjugation, which appear to be related to the gain or loss of aromaticity. This allows tetraoxa[8]circulenes to serve as sensitive probes of local molecular environment and to be used as sensors of electron-deficient species such as nitroaromatic compounds.

Ion Implantation as an Approach for Structural Modifications and Functionalization of Ti3C2Tx MXenes.

MXenes are a young family of two-dimensional transition metal carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti3C2Tx (where Tx is a surface termination) thin films. Damage and structural defects caused by the implantation process are characterized at different depths by XPS on Ti 2p core-level spectra, by ToF-SIMS, and with electron energy loss spectroscopy analyses. Results show that the ion-induced alteration of the damage tolerant Ti3C2Tx layer is due to defect formation at both Ti and C sites, thereby promoting the functionalization of these sites with oxygen groups. This work contributes to the inspiring approach of tailoring 2D MXene structure and properties through doping and defect formation by low-energy ion implantation to expand their practical applications.

Evidence for Symmetry Reduction in Ti3(Al1-δCuδ)C2 MAX Phase Solid Solutions.

Ti3[Al1-δCuδ]C2 MAX phase solid solutions have been synthesized by sintering compacted Ti3AlC2-Cu composites produced by mechanical milling. Using X-ray and neutron diffraction techniques, it is demonstrated that the Cu mixing into the Al site is accompanied by lattice distortion, which leads to symmetry reduction from a hexagonal to a monoclinic structure. Such symmetry reduction likely results from this mixing through deviation of the A-site position from the special (0, 0, 1/4) position within the P63/mmc space group of the original Ti3AlC2 structure. Moreover, it is demonstrated that the Cu admixture into the A site can be adjusted from the composition of the reactant mixture. The lattice parameter variation of the solid solution compounds, with 10-50 atom % Cu in the A site, is found to be consistent with Vegard's law.

Site-projected electronic structure of two-dimensional Ti3C2 MXene: the role of the surface functionalization groups.

The role of the surface groups T (T = OH, O or F) in the chemical bonding in two-dimensional Ti3C2Tx MXene is directly evidenced combining electron energy-loss spectroscopy in a transmission electron microscope and simulations based on density functional theory. By focusing on the 1s core electrons excitations of the C and (F, O) atoms, the site projected electronic structure is resolved. The Electron Energy-Loss Near Edge Structures (ELNES) at the C-K edge are shown to be sensitive to the chemical nature and the location of the T-groups on the MXene's surface and thereby allow for the characterization of the MXene's functionalization on the nanometre scale. In addition, the ELNES at the C and F-K edges are shown to be determined by the hybridizations of these atoms with the Ti d bands: these edges are thus relevant probes of the Ti d density of states close to the Fermi level which is of particular interest since it drives most of the Ti3C2Tx electronic properties. Finally, the crucial role in the MXene's functionalization of the etchant used for its synthesis is evidenced by locally determining the [O]/[F] concentration ratio using the corresponding K edges. This ratio is shown to be drastically increased from 1.4 to 3.5 when using HF or LiF/HCl respectively.