The sensing reaction on the Ni-NiO (111) surface at atomic and molecule level and migration of electron

•NiO octahedrons with exposed clean {111} facets show enhanced sensing properties.•Density of the surface 3-coordinated Ni atoms is increased by removing Cl− ions.•The 3-coordinated Ni atoms are regarded to act as a sensing reaction active site.•They can adsorb oxygen, generate free electrons and catalyze the sensing reaction.•An atomic and molecule level mechanism for sensing reaction is presented firstly.