Our DFT calculations of the g-C3N4/Cs2AgBiCl6 heterostructure shows a strong binding of the g-C3N4 on the (Ag0.5Bi0.5)Cl2-terminated Cs2AgBiCl6 surface with a formation energy of -4.22 eV/nm 2 , mediated by formation nitrogen-metal bonds with average bond length of 3.09 and 3.23 Å for N-Ag and N-Bi, respectively, see Figure . Consequently, reactive VCl surface sites are passivated upon increase of g-C3N4 and further proton transport to the Cs2AgBiCl6 surface may be effectively hindered, reducing the evolution of H2. Note that already a small wt% of g-C3N4 may be sufficient to cover the Cs2AgBiCl6 surface to a large extent, as we estimated from a toy model based on geometrical considerations, see Figure . Controlling the wt% of g-C3N4 consequently results in an ideal composition that balances efficient charge separation and carrier lifetime as well as the access to active catalytic sites, thereby maximizing the hydrogen evolution rate.