Oxide-derived copper (OD-Cu) catalysts are promising candidates for the electrochemical CO 2 reduction reaction (CO 2 RR) due to the enhanced selectivity toward ethylene over methane evolution, which has been linked to the presence of subsurface oxygen (O sb ). In this work, O sb is investigated with theoretical methods. Although O sb is unstable in slab models, it becomes stabilized within a "manually" reduced OD-Cu nanocube model which was calculated by self-consistent charge density functional tight binding (SCC-DFTB). The results obtained with SCC-DFTB for the full nanocube were confirmed with subcluster models extracted from the nanocube, calculated with both density functional theory (DFT) and SCC-DFTB. The higher stability of O sb in the nanocube is attributed to the disordered structure and greater flexibility. The adsorption strength of CO on Cu(100) is enhanced by O sb withdrawing electron density from the Cu atom, resulting in reduction of the σ-repulsion. Hence, the coverage of CO may be increased, facilitating its dimerization.

We have investigated the structure and electronic structure of single-and double-walled imogolite nanotubes with Ge and Si as group IV element. While it is known from experiment, and in the case of single-walled tubes confirmed by theory, imogolite nanotubes are monodisperse in diameter. We show that imogolite tubes are also showing a preferred chirality (zigzag), resulting from the hydrogen-bond network on the tube surfaces, and that there is an exceptionally stable form of intertube interaction that supports the formation of monodisperse double-walled imogolite nanotubes. The strongest stabilization of double-walled tubes has been found for tube indexes with nine units of difference around the circumference, and the minimum structure is found for the (12,0)@(21,0) tube in the case of germanium imogolite and (9,0)@(18,0) for imogolite. The electronic structure is only slightly affected by these geometric factors, as are the mechanical properties, which show Young moduli of 320−370 GPa, thus being in the same range as other clay mineral nanotubes.

No abstract

Structural, electronic, and mechanical properties of single-walled chrysotile nanotubes have been investigated using the self-consistent charge density-functional tight-binding method (SCC-DFTB). The naturally occurring chrysotile nanotubes (NTs) are composed of brucite, Mg(OH) 2 , layer in the outer side and tridymite, SiO 2 , in the inner side. The zigzag (17,0)−(45,0) and armchair (9,9)−(29,29) chrysotile nanotubes, which correspond to the radii ranging from 16 to 47 Å, have been calculated. The SCC-DFTB results are in good agreement with available experimental and previously published theoretical results. The chrysotile nanotubes are estimated to be insulator with band gap of 10 eV independently of their chirality and size, and the Young's moduli are estimated to be in the range of 261−323 GPa. In addition, we have shown that the chirality of the NTs does not affect their stability, and the variant with brucite in the inner side and the tridymite in the outer side of the nanotube is indeed less stable with respect to the inverse case.

Imogolite is a single-walled aluminosilicate nanotube (NT) found in nature that can be easily synthesized, as well as its analogue aluminogermanate NT. Based on geometrical assumptions and pKa values, species such as H3PO4, H3PO3, H3AsO3, H3AsO4 could also be candidates to form imogolite-like structures. In the present work, we provide insights about the stability, electronic, structural and mechanical properties of possible imogolite like NTs by means of self-consistent charge density-functional tight-binding method (SCC-DFTB). Similarly to aluminogermanate, where the tetrahedral silicate groups are replaced by germanate, in this work tetrahedral silicate groups are substituted by phosphate, phosphite, arsenate and arsenite units in the imogolite structure. Detailed analysis is focused on structural properties, strain energy, band gap and Mulliken charges distribution. The calculated strain energy curves for all studied zigzag imogolite-like NTs present well-defined minima, which change as a consequence of composition variation. Moreover, the strain energy curves of armchair imogolite-like NTs also present minima, although in all cases less stable than zigzags by at least 2.2 meV per atom. The insulating NT behaviour changes after internal modification from silicate to phosphate, phosphite, arsenate and arsenite, as well as the charge distribution inside and outside the nanotubes.