Binding of an oxide layer to a metal: the case of Ti(10-10)/TiO2(100)

TL;DR

This study investigates the chemical bonding and stability of Ti/TiO2 interfaces, revealing mainly ionic bonds with significant charge transfer, which explains the oxide layer's protective nature on titanium surfaces.

Contribution

It provides a detailed first-principles analysis of Ti/TiO2 interface bonding, highlighting the ionic character and charge transfer effects that enhance interface stability.

Findings

Interface bonding is mainly ionic.

Charge transfer from Ti to O is greater at the interface.

The interface has stronger binding than bulk materials.

Abstract

We study the chemical nature of the bonding of an oxide layer to the parent metal. In order to disentangle chemical effects from strain/misfit, Ti(10$\bar{1}$0)/TiO$_{2}$(100) interface has been chosen. We use the density functional pseudopotential method which gives good agreement with experiment for known properties of bulk and surface Ti and TiO$_2$. Two geometries, a film-like model (with free surface in the structure) and a bulk-like model (with no free surface in the structure) are used to simulate the interface, in each case with different terminations of Ti and TiO$_2$. For the single-oxygen interfaces, the interface energy obtained using these two models agree with each other; however for the double-oxygen ones, the relative stability is quite different. The disturbance to the electronic structure is confined within a few atomic layers of the interface. The interfacial bonding is mainly ionic, and surprisingly there is more charge transfer from Ti to O in the interface than in the bulk. In consequence the Ti/TiO$_2$ interface has stronger binding than the bulk of either material. This helps to explain why the oxide forms a stable, protective layer on Ti and Ti alloys.