V. A. Ranea

Rotation assisted diffusion of water trimers on Pd{111}

Diffusion barriers for a cluster of three water molecules on Pd{111} have been estimated from ab-initio Density Functional Theory. A model for the diffusion of a cluster of three water molecules (trimer) based in rotations yields a simple explanation of why the cluster can diffuse faster than a single water molecule by a factor ≈ 102 [1]. This model is based on the differences between the adsorption geometry for the three molecules forming the trimer. One member interacts strongly with the surface and sits closer to the surface (d) while the other two interact weakly and stay at a larger separation from the surface (u). The trimer rotates nearly freely around the axis determined by the d-like monomer. Translations of the whole trimer imply breaking the strong interaction of the d-like molecule with the surface with a high energy cost. Alternatively, thermal fluctuations can exchange the position of the molecule sitting closer to the surface with a lower energetic cost. Rotations around different axis yield a diffusion mechanism where the strong interaction is maintained along the diffusion path, therefore lowering the effective activation barrier.

The structure of the bulk and the (001) surface of V2O5. A DFT+U study

GGA (PW91) + U is applied to the calculation of the structure (lattice parameters) and the electronic structure of the V2O5 bulk and its (001) surface for different values of U eff used in the literature (0.0, 3.0 and 6.6 eV). Similar surface lattice parameters are calculated for the (001) surface and for the bulk, as well as similar electronic structures. The calculated lattice parameters (a and b for the surface and a, b and c for the bulk) are in good agreement with experimental results. It seems that there is no strong correlation between the calculated lattice parameters and the value of U eff . The calculated width of the valence band keeps the value of for the three studied U eff . However, the energy gap between the valence and the conduction bands increases with the value of U eff . U eff = 3.0 eV seems to be the most adequate value to describe the energy gap after comparison with experimental results. Electronic density contour plots indicate that for a larger (smaller) U eff the accumulated charge in the V-O(1) bond is overestimated (underestimated). The contour plots (in the a direction) show that the charge distribution V-O(3) is less correlated with U eff than the charge distribution V-O(1), whereas charge distribution V-O(2) seems not to be corretaled with U eff . The energy gap between the valence and the conduction bands seems to be strongly related with the charge distribution in the V-O(1) bond. The V-O(1) bond stability seems to be correlated with U eff . However, the stability of the V-O(2) and V-O(3) bonds seems not to be strongly affected by U eff .