Enhancing Electron Emission of Hf with an Ultralow Work Function by Barium–Oxygen Coatings

Barium (Ba)–oxygen (O) coatings were experimentally found to be very efficient to reduce the work function of tungsten (W). Following this idea, in this work, we theoretically investigate the effect of BaxO coatings on the work function of the Hf (101̅2) surface using density functional theory. The results show that BaxO/Hf (101̅2) systems are thermodynamically stable when x is not more than 1 and the Ba0.5O coating can significantly lower the work function of the Hf (101̅2) surface from 3.51 to 1.88 eV, exhibiting an ultralow work function as compared to that of Cs (∼1.95 eV) and LaB6 (∼2.6 eV). Bader charge analysis reveals that charge transfer occurs between the substrate and the coating layer, and detailed analysis of dipole moment shows that the reduction of the work function is ascribed to the polarization effects on the substrate and coating layer. Our findings suggest that the Ba–O coating is a promising strategy for improving the electron emission of the Hf cathode.