First-Principles Study of Hydrogen Dynamics in Monoclinic 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-principles calculations, the preferences are studied for the hydrogen absorption sites and diffusion paths between them. The oxygen vacancies are found to be the 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 using the nudge elastic band method. Several competitive channels are identified, with barrier heights varying from 2.87 to 3.71 eV, that are 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 toward oxygen vacancy sites). These results suggest that hydrogen may persist in oxygen vacancy sites up to high enough temperatures.