Atomic-scale de-passivation mechanisms of anatase TiO₂ induced by corrosive halides based on density-functional theory

By using density-functional theory calculations, we systematically investigated the localized corrosion behavior of anatase TiO₂ by chloride (Cl)/fluoride (F) via different mechanisms. Energetics for adsorption and substitution pointed to a more aggressive influence of F over Cl, revealing that F was more likely to induce TiO₂ de-passivation by “adsorption-thinning” mechanism, though surface hydroxylation can promote Cl etching via this mechanism. Whereas Cl accessed to TiO₂ interior with nearly negligible diffusion energy barrier, facilitated by dramatic relaxations via mechanism of Cl residing at oxygen vacancies (V_O), accompanied by a reduction in passivity. Moreover, the annihilation/creation of V_O at the TiO₂ (1 0 1) surface was dominated by competition of adsorbed species. While a subsurface V_O can move towards the outermost surface by exchanging with lattice O, a surface V_O can spontaneously re-combine with a surface O promoting re-passivation. Co-adsorption of Cl/F with O can significantly inhibit V_O–O combination by directly occupying the V_O site or by reducing the energy to create a V_O. Our studies outlined the non-negligible interactions between corrosive species and defects in TiO₂ and the mechanisms concerning Cl/F/O inducing re(de)-passivation, the understanding of which is meaningful in mitigating Ti corrosion in environments containing halides.