The large binding energy presents an interesting hypothesis that may explain the observed drop in the H2 production rate when adding a small weight percentage of g-C3N4. To estimate the surface coverage, we derive a simple toy system relying on pure geometric considerations, shown in Figure . We assume cubic nanocrystals of Cs2AgBiCl6. The nanocrystal surface area is given by 6L , where L represents the nanocubes dimension. The g-C3N4 is modeled as 2D rectangular layers, which may passivate the surfaces of the perovskite nanocubes. The given wt% of g-C3N4 allows us to calculate the surface ratio of the Cs2AgBiCl6 to the g-C3N4, as shown in Figure . Our model system indicates that substantial coverage of the nanocrystal surface can already be achieved at low wt%. This suggests that the passivation of the surface may inhibit the catalytic active site of Cs2AgBiCl6 due to a small amount of g-C3N4.