Comparison of the stability of free-standing silicene and hydrogenated silicene in oxygen: a first principles investigation

The stability of free-standing silicene in oxygen is worthwhile discussing. In this letter, the oxygen adsorption and dissociation on free-standing silicene is studied using first principles. It is found that free-standing silicene is not stable in oxygen because O-2 molecules can be easily adsorbed and dissociated into O atoms on a silicene surface without overcoming any energy barrier. Moreover, dissociated oxygen atoms are difficult to migrate on and desorb from silicene surfaces, leading to the formation of Si-O compounds. To enhance the stability of free-standing silicene in oxygen, fully hydrogenated silicene is used as a stabiliser. Interestingly, compared with no energy barrier on pristine silicene, there are two minor energy barriers of O-2 molecule adsorption and dissociation on fully hydrogenated silicene, indicating that hydrogenated silicene has higher stability than free-standing silicene in oxygen. However, once the O-2 molecule dissociates into two O atoms on hydrogenated silicene, desorption of O atoms will be very difficult due to its high energy barrier. This work will be helpful to understand the detail of O-2 molecule dissociation and dissociation-induced O atoms adsorption on free-standing and hydrogenated silicene in oxygen and will be useful to the application of silicene.