Calculation of Graphene Work Function Variations Due to Alkali Metal + Oxygen Adsorption

Graphene has a high work function of 4.6 eV, and it can't be directly used as thermionic emission material. The graphene work function shows a decreasing trend in alkali metal/oxygen (AM/O) coadsorption experiments. Anderson–Newns (AN) model is a classical theoretical model for calculating the work function of adsorption surface. The present study, aiming at AM/O adsorption system, modifies and optimizes the AN model by redefining the calculation method of adsorption bond length λ and establishes the relationship between material work function, thermionic emission performance, and surface coverage. Based on the modified model, the influence of work function decline and thermionic emission performance of cesium/oxygen, potassium/oxygen, and sodium/oxygen graphene adsorption systems is studied. The results show that the work function of graphene affected by the three AM/O adsorbents is basically the same with the increase of coverage, and all of them quickly decrease to the lowest value and then increase slightly. The maximum work function declines are 3.51, 3.30, and 2.83 eV, respectively. At 1000 K, the maximum output current densities are 340, 36, and 0.15 A cm−2, respectively.