First-principles study of hydrogen dynamics in monoclinic TiO

TL;DR

This study uses first-principles calculations to investigate hydrogen absorption, trapping, and diffusion in monoclinic TiO, revealing stable oxygen vacancy sites and high energy barriers for hydrogen movement, with limited molecular hydrogen adsorption.

Contribution

It provides detailed insights into hydrogen behavior in monoclinic TiO, including preferred absorption sites, diffusion pathways, and the improbability of molecular hydrogen adsorption, based on first-principles calculations.

Findings

Oxygen vacancies are primary hydrogen traps with -2.87 eV absorption energy.

Hydrogen diffusion barriers range from 2.87 to 3.71 eV, indicating stability at vacancy sites.

Molecular hydrogen is unlikely to adsorb and remains dissociated within TiO.

Abstract

The existence of intrinsic vacancies in cubic (monoclinic) TiO suggests opportunity for hydrogen absorption, which was addressed in recent experiments. In the present work, based on first principle calculations, the preferences are studied for the hydrogen absorption sites and diffusion paths between them. The oxygen vacancies are found to be primary hydrogen traps with absorption energy of -2.87 eV. The plausible channels for hydrogen diffusion between adjacent vacancy sites (ordered in the monoclinic TiO structure) are compared with the help of calculations done with the nudge elastic band method. Several competitive channels are identified, with barrier heights varying from 2.87 to 3.71 eV, that is high enough to ensure relative stability of trapped hydrogen atoms at oxygen vacancy sites. Moreover, the possibility of adsorption of molecular hydrogen was tested and found improbable, in the sense that the H2 molecules penetrating the TiO crystal are easily dissociated (and released atoms tend to proceed towards oxygen vacancy sites). These results suggest that hydrogen may persist in oxygen vacancy sites up to high enough temperatures.