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

Abstract

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 (VO), accompanied by a reduction in passivity. Moreover, the annihilation/creation of VO at the TiO2 (101) surface was dominated by competition of adsorbed species. While a subsurface VO can move towards the outermost surface by exchanging with lattice O, a surface VO can spontaneously re-combine with a surface O promoting re-passivation. Co-adsorption of Cl/F with O can significantly inhibit VO–O combination by directly occupying the VO site or by reducing the energy to create a VO. 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.