New insights into the mechanisms of TiB2(001) thermal oxidation combining molecular dynamics and density functional theory calculations

Using reactive force field molecular dynamics and Density Functional Theory, this study unravels the early stages of TiB2 oxidation. Overall, it is found that the TiB2 boron sheets hinder the oxidation until a temperature of 800 K is reached, above which a complex sequence of chemical mechanisms leads to the formation of a titanium oxide at the top surface and agglomeration of pure boron underneath, preventing further TiO2 growth below 900 K. It is demonstrated that titanium oxidation is possible after the softening and distortion of boron sheets due to oxygen adsorbate presence; mostly BO radicals, no B2O3-like structure is formed. The local distortions of boron aromatic rings allow oxygen insertion into the subsurface, which is followed by titanium extraction and migration towards the outer surface in contact with the molecular oxygen atmosphere. Mechanistic details of these complex extraction-oxidation processes and their relation to cooperative oxygen atoms and molecules are detailed in terms of pathways and corresponding energetics. Interestingly different reaction stages are identified, that do not exceed 2 eV activation. The proposed hierarchical scenario of oxidation of titanium and boron in TiB2 might benefit to the general understanding of metal diboride oxidation.