The trend of ammonia evolution rate steeply increases from pure Cs2SnBr6 (about 50 µmol g - 1 h -1 ) to > 200 µmol g -1 h -1 for the composites including an intermediate amount of perovskite (20-25%) and reaching the highest value of 266 μmol g -1 h -1 for the composition Cs2SnBr6 35 wt%/g-C3N4 65 wt% before decreasing for composites with a larger amount of g-C3N4. Note that pure g-C3N4, under these reaction conditions, provides an ammonia evolution rate of 2.7 μmol g -1 h -1 . The data reported in Figure demonstrate, first of all, that the perovskite alone is already quite active in the nitrogen reduction reaction and that such activity can be boosted through the heterojunction formation, achieving a value, for the optimal composition, which is about 4 times that of pure Cs2SnBr6. The absolute value of ammonia production obtained for the Cs2SnBr6 35 wt%/ g-C3N4 65 wt% composition is nearly 8 times larger than what previously reported on perovskite-based systems. The good stability of the ammonia production has been tested on the top performing composite for three successive cycles as reported in Figure . Post-catalysis XRD data are shown in Figure , indicating the stability of the system. UV-Vis absorption spectra and the corresponding Tauc plots for Cs2SnBr6/g-C3N4 system are reported in Figure . The band gap of Cs2SnBr6 is 2.75 eV and that of g-C3N4 is 2.78 eV, in agreement with previous reports. Being the two bandgaps very close, no relevant shifts or trend are observed as a function of composition. The normalized photoluminescence spectra of the Cs2SnBr6/g-C3N4 system are reported in Figure . The emission is dominated by the broad and intense features of graphitic carbon nitride, even at very high perovskite loading. Such broad spectra, that completely covers the weak perovskite emission, derive from the superimposition of multiple electronic transitions. In particular, three transitions have been modeled in pristine g-C3N4: a high energy one, centered at around 2.81 eV, which derives from the relaxation of δ* electrons to the LP state, and two low energy transitions centered at 2.70 and 2.42 eV. To access these low energy transitions a non-radiative mechanism to populate the π* state is required, thus the structure of the broadband spectrum depends on the efficiency of this process. In previous works, the interplay between the density of defects and the non-radiative population of π* state have been studied and related to the photocatalytic properties of g-C3N4. 29 A close inspection of the spectral region around the emission band peak (inset of Figure )