RESUMO
We report a combined experimental/theoretical approach to study the connection of S-vacancies and wrinkling on MoS2 layers, and how this feature produces significant changes in the electronic structure and reactivity of this 2D material. The MoS2 material, when used as a catalyst in operative conditions, was found to be mainly composed of thin and short 1-5 layer sheets instead of a poorly crystalline structure, as it was previously assumed. Notably wrinkled structures with S-vacancies were also found through transmission electron microscopy. Atomistic simulations revealed a natural connection between sulfur-vacancies, wrinkling and folding. Density functional calculations further revealed that such curved structures present a lower electronic band-gap and a higher reactivity towards thiophene compared to the planar MoS2 counterpart.
RESUMO
Water-mineral interfaces are important for several environmental, industrial, biological, and geological processes. Gypsum, CaSO4·2H2O, is a widespread mineral of high technological, medical, and environmental relevance, but little is known about its surface structure and its interaction with water. A molecular-level understanding of gypsum/water interface is given here by a combined experimental/theoretical study. We investigate the structure and dynamics of water adsorbed from vapor on the gypsum (010) single-crystal surface at room temperature, combining sum-frequency generation (SFG) vibrational spectroscopy experiments and ab initio molecular dynamics (AIMD) simulations. The SFG spectra of gypsum at low relative humidity (RH) show an anisotropic arrangement of structural water molecules and the presence of dangling OH groups. The AIMD simulations allow a detailed assignment of the SFG spectra and show that the cleaved (010) surface rearranges to have only 25% of the OH groups pointing away from the surface. At higher RHs, the first adsorbed water layer binds to these OH groups and forms an anisotropic arrangement, but with the amount of free OH groups significantly suppressed and without any significant diffusion. Upon adsorption of a second water layer, although the topmost layer of molecules is more disordered and dynamic than the previous one, its structure is still influenced by the gypsum surface underneath because it has a much reduced amount of free OH groups with respect to the free surface of water, and a slower surface diffusion with respect to bulk water. The theoretical results corroborate the experimental ones and provide an accurate atomic characterization of the surface structure.
RESUMO
Gas-phase Ag-Pd clusters in the size range of 38-100 atoms are studied via a combined density-functional/empirical-potential (DF-EP) approach. Many-body EPs describing Pd-Pd, Ag-Ag, and Ag-Pd interactions are reparametrized and used in thorough global optimization searches at sizes N=38, 60, and 100 and compositions 25%, 50%, and 75%. The results are analyzed in terms of structural families, whose lowest-energy isomers are reoptimized at the DF level to investigate the crossover among structural motifs. It is found that the reparametrized EPs show a better qualitative and quantitative agreement with DF results when compared to the original potentials taken from literature: Both methods agree on which is the lowest-energy isomer at each size and composition, and the energy differences in the various isomers are in good qualitative agreement, especially for 60- and 100-atom clusters. The reparametrized potentials should thus be applicable to large clusters, where DF calculations are not feasible any more.