First-Principles Study of Hydrogen Dynamics in Monoclinic TiO

The existence of intrinsic vacancies in cubic (monoclinic)TiOsuggests opportunity for hydrogen absorption, which was addressedin recent experiments. In the present work, based on first-principlescalculations, the preferences are studied for the hydrogen absorptionsites and diffusion paths between them. The oxygen vacancies are foundto be the primary hydrogen traps with absorption energy of -2.87eV. The plausible channels for hydrogen diffusion between adjacentvacancy sites (ordered in the monoclinic TiO structure) are comparedwith the help of calculations using the nudge elastic band method.Several competitive channels are identified, with barrier heightsvarying from 2.87 to 3.71 eV, that are high enough to ensure relativestability of trapped hydrogen atoms at oxygen vacancy sites. Moreover,the possibility of adsorption of molecular hydrogen was tested andfound improbable, in the sense that the H-2 molecules penetratingthe TiO crystal are easily dissociated (and released atoms tend toproceed toward oxygen vacancy sites). These results suggest that hydrogenmay persist in oxygen vacancy sites up to high enough temperatures.

Automated summary

Based on first-principles calculations, this study investigates the preferences for hydrogen absorption sites and diffusion paths in cubic (monoclinic) TiO with intrinsic vacancies. Oxygen vacancies are identified as the primary hydrogen traps, while several competitive diffusion channels with barrier heights ranging from 2.87 to 3.71 eV are found. Adsorption of molecular hydrogen is unlikely, as the H-2 molecules easily dissociate upon penetrating the TiO crystal. These results suggest that hydrogen can persist in oxygen vacancy sites up to high temperatures.