Figure reports the results in terms of hydrogen and ammonia evolution rates (top and bottom graphs, respectively), expressed as μmol g -1 h -1 , obtained by photocatalytic experiments under simulated solar light (for details see Methods section). The two production rates follow opposite trends as a function of composition, with perovskite-rich samples performing better for the hydrogen generation and perovskite-poor ones running the nitrogen reduction more efficiently. The trend of hydrogen evolution rate (HER), deriving from HCl splitting, steeply increases from pure Cs2AgBiCl6 to the composite including 10wt% of g-C3N4, reaching a value of 625 μmol g -1 h -1 which is more than double with respect to pure perovskite. Note that pure g-C3N4, under these reaction conditions, provide an HER of 4 μmol g -1 h -1 . The reported data confirm a synergistic effect of the composite in enhancing the photocatalytic HCl splitting ability of Cs2AgBiCl6, thanks to the formation of an effective heterojunction (see later in the text). The present results, while being the first related to the hydrogen generation by Cs2AgBiCl6, can be compared to analogous sytems involving double perovskites. To date, only the HBr splitting by Cs2AgBiBr6 has been explored in heterojunctions with reduced graphene oxide, NiCoP or nitrogen-doped carbon. The best HER has been reported for the Cs2AgBiBr6/2.5%RGO system providing around 490 μmol g -1 h -1 of hydrogen under solar illumination in HBr. With the present Cs2AgBiCl6/10% g-C3N4 composite we are reporting the highest hydrogen photogeneration, around 625 μmol g -1 h -1 for a system including a lead free double perovskite through the efficient HCl splitting which could be of more applicative potential with respect to the splitting of HBr or HI reported so far. The reduction of nitrogen to ammonia, as mentioned, follows a different trend with respect to the composition (Figure ), with a gradual increase of the NH3 generation rate starting from pure Cs2AgBiCl6 up to the Cs2AgBiCl6/90% g-C3N4 composite, reaching a maximum value of about 40 μmol g -1 h -1 . Pure g-C3N4 nanosheets or lower amounts of perovskite in the composite (see the point at 5wt%) perform significantly less effectively. Also in this case, by looking at the nitrogen fixation results of the pure compounds, it is possible to unveil an effective synergy between the two semiconductors. As mentioned, there are no previous reports about nitrogen reduction by an MHP-based heterojunction. However, g-C3N4 has been reported as a suitable material for this reaction, in particular thanks to the presence of nitrogen vacancies. Therefore, g-C3N4 nanosheets used to prepare the present samples have been synthesized to promote the presence of such defects (see Methods), as determined by Electron Paramagnetic Resonance (EPR) experiments (see Figure ). The presence of nitrogen vacancies result in the appearance of an EPR signal related to the number of unpaired electron directly correlated to the defect formation. We estimated 4.5×10 paramagnetic centers per gram of sample.