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An oxidation with a degree of 50% generates nine possible configurations among which only six are stable. As illustrated in Figure 1.9, the unit cell comprises four P-atoms assigned as P₁, P₂, P₃, and P₄ and two O-atoms, namely, OA and OB. Oxygen atoms bind to two P-atoms on the same side (more precisely, they are attached either to the up-side or to the down-side of the surface and or they are in opposite sides, namely . The index U and D referred to the up and the down side of the P-atoms. For example, to get , one should place the OA-atom up on P₁ and the OB down on P₂ (OB is in the opposite side of OA) forming a fashion on either side of the plane. The resulting new derivatives can be divided onto three main groups. The first group contains the structures and that have only dangling bonds P = O. In three conformers constituting the second group, dangling oxygen motif and bridging bond alternate, respectively. The third class concerns the configurations exhibiting only bridging bonds which are energetically less favorable (see [56] for more details). All the half-oxidized conformers exhibit a high buckling parameter confirming the anisotropic behavior of their properties. It was found, using different methods [24, 56], that half-oxidation of phosphorene (P₄O₂) allows to build a stable material.

Half O-functionalization influences significantly the electronic structure of phosphorene mono-layer as depicted in Figure 1.10. The oxidation induces a band gap modulation with the highest value observed in the bridge structures [56]. In all structures the band gap ranges from 0.54 (1.19) to 1.57 (2.88), calculated by GGA (GW) approximation. Moreover, a half O-functionalization tunes the band gap from direct to indirect in all the conformers, except P₂O_D which presents a direct band gap.

Figure 1.9 Configurations of 50% oxidized phosphorene: (a) dangling structures and (b) bridge structures.

Figure 1.10 GGA and GW band structures of half-oxidized structures.

POs can provide oxygen in its solid-phase to valve regulated Li-O₂ batteries. In addition, when the Li atom is absorbed at the surface of the POs, it binds strongly to the O atoms indicating a strong ionic characteristic of the bond between oxygen and lithium [75]. The absolute values of binding energies of the Li atom adsorbed on the PO surface are greater than those of the Li atom on pure phosphorene, MoS₂ and graphene [76–78]. POs promise high diffusivity owing to the anisotropy of POs cathode barrier that is reduced by half with respect with the armchair axis for Li diffusion on POs. Besides, Li-PO structures with a number of Li atoms lower than O atoms show stable discharge products for PO cathodes [75].