DFT study of BaTiO 3 (001) surface with O and O 2 adsorption

Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional theory (DFT), we present an electronic and structural ab initio study of a BaTiO 3 (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers. Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO 3 (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered. By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO 2 termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O 2 molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O 2 molecule adsorbed in various geometrical configurations are also analyzed. For O 2, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 Å. By comparison, the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated results. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200773.20.Hb Impurity and defect levels; energy states of adsorbed species, 71.15.Mb Density functional theory, local density approximation, gradient and other corrections,

Adsorption and co-adsorption of CH 3 and H on flat and defective nickel (111) surfaces

We use a periodic density functional theory (DFT) code to study the adsorption of CH3 and H, as well as their co-adsorption on a Ni(111) surface with and without Ni ad-atom, at a surface coverage of 0.25 monolayer (ML). We systematically investigate the site preference for CH3 and H. Then we combine CH3 and H in many co-adsorbed configurations on both surfaces. Methyl and hydrogen adsorption on a flat Ni(111) surface favours the hollow site over the top site. The presence of a Ni ad-atom stabilizes the adsorption of CH3 better than a flat surface, while hydrogen is more stable on a flat Ni(111) surface. When H and CH3 are co-adsorbed at nearest Ni neighbours on the (111) surface, their interaction is always repulsive. However, the dissociative adsorption of CH4 is stabilised when the fragments are infinitely separated. For the co-adsorbed fragments CH3 and H, in the presence of an ad-atom, the repulsive interaction is lowered, so that the dissociative form of CH4 is locally stable

Genomic and functional characterisation of IncX3 plasmids encoding blaSHV-12 in Escherichia coli from human and animal origin

Microbial Biotechnolog