Selective Co and Sn co-doped black phosphorene for hydrogen storage: first-principles insights

In this study, long-range van der Waals (vdW) and effective Coulomb interaction (Ueff) corrected density functional theory
(DFT + vdW + Ueff) calculations were performed to explore the effect of co-doping (of Co and Sn) on the crystal structure,
electronic, magnetic and chemical (H2 adsorption) properties of phosphorene. The stability of the doped structures was
determined with formation energy (Ef) and molecular dynamics (MD) simulations. The dopants were found to form strong
bonds with phosphorus (P) atoms. The introduction of Co/Sn-atoms slightly altered the local geometry of phosphorene,
resulting in new electronic characteristics. For instance, unlike pristine phosphorene (P48), the co-doped phosphorene
(P45CoxSny) can exhibit ferrimagnetic (FIM) or ferromagnetic (FM) coupling in the ground state. The doped structures
exhibit integral magnetic moments, mainly contributed by the Co-atoms. The P45Co1Sn2 and P45Co2Sn1 structures show
metallic band structure, while P45Co3 becomes a small band gap (Eg) semiconductor (Eg =0.2 eV). The adsorption of the
H2 molecules was investigated at various sites on un-doped and co-doped phosphorene. It was found that H2 molecule
is weakly adsorbed on un-doped phosphorene, whereas moderate adsorption was observed for co-doped systems. The
adsorption energy (Ea) was found to be -0.03, -0.35, -0.48, and − 0.65 eV, respectively for P48, P45Co1Sn2, and P45Co2Sn1
and P45Co3. The DOS plots confirmed that the observed adsorption is due to the s-d interaction between H-atom and the
doped Co-atom. The Bader charge and charge density difference (CDD) analysis showed that H2 molecule acts as a charge
acceptor and doped phosphorene as a charge donor. In addition, the effect of strain on adsorption was also considered. The
adsorption capacity of the doped systems decreased with the number of H2 molecules. These findings show that co-doped
phosphorene can be used for spin-based nanodevices and hydrogen capture for energy storage applications.