Doris Vogtenhuber

Ab initio study of atomic Cl adsorption on stoichiometric and reduced rutile TiO2 (110) surfaces

The (1 × 2) adsorption of atomic Cl on the stoichiometric and the reduced rutile TiO 2 (110) surface was investigated from ab initio by applying the full-potential augmented plane wave method. According to the calculated adsorption energies of the fully relaxed systems, the most favourable adsorption site is at O defects of the reduced surface. On the stoichiometric surface, Cl is preferentially adsorbed on top of the undercoordinated Ti atoms. Cl 3s and 3p states lie ∼2 eV above and on the upper edge of the O 2s and p bands, respectively. The uppermost state of the p band is emptied upon adsorption of Cl on the stoichiometric substrate. The work functions are increased by 1-2 eV, depending on substrate stoichiometry and adsorption geometry. STM images, calculated according to the model of Tersoff and Hamann are in good agreement with experiment.

Saddle Point Energies from Ab Initio Jump Profiles in MC Simulation of Order Kinetics in Intermetallics.

Kinetic Monte Carlo (KMC) simulation is a valuable tool to investigate configu-ration changes in intermetallic compounds. The elementary process is the jump of an atomfrom a lattice site to a neighboring vacancy. In classical transition state theory the jump ratecontains the energy difference between the original equilibrium state and the saddle point (=transition) state. In traditional KMC the saddle point has mostly received rather careless treat-ment, setting it constant or relating it to the type of jumping atom. In the present work, saddlepoint heights were considered explicitly. Taking L12 ordered Ni3Al as an example, jump energyprofiles for various atom environments were calculated ab initio in relaxed configurations ofa 3x3x3 supercell, employing the Nudged Elastic Band method where necessary. From theseresults, effective ’pure’ saddle point heights were extracted. To show the effect on kinetics,simulations of order-order transitions were done with jump probabilities based on these results.When compared to the old assumption of constant saddle point heights, both overall kineticsand detailed jump statistics result considerably changed.

Ab-initio study of the adsorption of Cu on Pt(100)

Abstract Electrochemical and gas-phase deposition experiments show a conflicting dependence of the adsorption geometries on the adsorption conditions for Cu on Pt(100). To clarify the experimental findings, the adsorption was investigated by an ab-initio method. According to the calculated results, (1 × 1) adsorption is favoured over the c (2 × 2) adsorption for the deposition from the gas phase as well as from electrolyte solution. It seems that, in the electrochemical experiments, adsorbate-size effects are responsible for the observed c (2 × 2) geometry. The calculated adsorption energy of 0.25 eV/Cu for c (2 × 2) coverage compares well with the experimentally measured underpotential. The atomic positions were relaxed for the topmost three layers. It turns out that the relaxation energy of -0.45 eV gives a very important contribution to the adsorption energy of -0.25 eV of the system Cu c (2 × 2)/Pt(100). Upon Cu adsorption the work function of Pt is decreased already for half a...