First Principles Calculations on the Diffusion of Cu, Ag and Au Atoms or Aggregates on the Bulk and Surface of Titania

摘要

This report results from a contract tasking Universita Milano Bicocca as follows: The Grantee will investigate with first principle quantum chemical methods the adsorption and diffusion of Cu, Ag and Au species (atoms or small aggregates) on the surface and in the bulk of titania. The Grantee will consider both stoichiometric and reduced titania, with particular attention to the interaction of the metal species with defects and irregularities in the oxide. We will investigate the effect of external electric fields on the diffusion of charged species in the attempt to better understand the behavior of this material in memory/switching devices. The nature of Ag(111)/TiO2 rutile and anatase interfaces, of interest for the design of memristors, has been studied by means of density functional theory (DFT) calculations using various computational approaches. We have considered interfaces where the lattice mismatch of the oxide crystalline phase and the metal electrode does not result in excessive strain. The bonding at the interface is very weak and the charge transfer is negligible for stoichiometric oxides. The formation of O vacancies has a lower cost at the interface with Ag than on the bare titania surface and results in stronger adhesion between the Ag electrode and the reduced TiO2-x oxide. The diffusion of Ag and O atoms/ions across the interface is a thermodynamically unfavorable process which can occur only at high temperatures or under the effect of an external electric field. Once Ag atoms are incorporated in the bulk of TiO2 they can be stabilized in interstitial (more favorable) or substitutional to Ti positions. In both cases Ag is ionized and transfers the valence electron to the host crystal with formation of Ti3+ states. The Ag atoms remain positively charged even when extended Ag chains are formed (nanofilaments).