Figure 6.

Atomic structure analysis and hydrogen adsorption

Atomic structures and ΔG_H∗ values of H adsorption at each N-site of SV+3N and DV+4N (A) without Ca, (B) with a Ca single atom, and (C) with an extra H adsorbed onto Ca (In the side views some C atoms blocking N and H atoms from our view are not shown). Each ΔG_H∗ value is for H adsorption onto the N atom indicated by ∗. When two H atoms are adsorbed onto DV+4N they prefer to adsorb at two N in diagonal positions [N₁, N₃] rather than neighboring positions [N₁, N₂], as the diagonal configuration is more stable due to longer separation between the two H∗ (Fujimoto and Saito, 2014). The structures in black boxes are planar, with very strong H binding (very negative ΔG_H∗ values). Adsorbing one more H (below black boxes) or depositing a Ca atom (dashed blue boxes) significantly increases ΔG_H∗ values to very positive values by changing N-H bond from sp²-like to sp³-like and breaks the charge density interaction between H and other N atoms. Further adsorbing an H onto Ca (blue boxes) reduces the influence of Ca to graphene structure, hence reduces ΔG_H∗ to less positive values and makes them more suitable for HER.