First-principles investigations into the effect of oxygen vacancies on the enhanced reactivity of NiO via Bader charge and density of states analysis

Vacancy defects are important parameters in determining the reactivity of metal oxides. Different areas such as catalysis, fuel cell, battery, and other energy conversion technologies (e.g., chemical looping combustion) can benefit from fundamental studies on the effect of vacancy defects on metal oxide reactivity. The effect of oxygen vacancies on the reactivity of NiO, which is a prominent and promising material in the aforementioned areas, is studied in this work. Using density functional theory, a comprehensive analysis of oxygen vacancy effect on the hydrogen chemisorbed state of NiO (100) is provided with Bader charge, charge redistribution, and density of states analyses. By inclusion of an oxygen vacancy, adsorption sites change from Ni and O sites on NiO to O and O vacant sites on NiO1−x. Adsorbed hydrogen on the positively charged oxygen vacant site has an enhanced interaction with two neighboring Ni atoms. As revealed by our analysis, change in the magnitude of H–Ni interaction is the main contributor to the enhanced reactivity of NiO by the addition of O vacancy. The results establish an understanding of the enhanced reactivity by the creation of O vacancies and elucidate the changes in the bonding nature as a result of the vacancy.