Figure reports the room temperature X-ray diffraction (XRD) patterns of some selected compositions investigated. Bottom part of Figure shows the pattern of Cs2SnBr6 vacancy-ordered double perovskite which is also reported against the calculated pattern in Figure . The perovskite displays a cubic symmetry with Fm-3m space group (#225) and a lattice parameter of 10.7761(2) Å, in agreement with previous results. A sketch of the cubic unit cell is reported in Figure . At the top of Figure , the X-ray pattern of pure g-C3N4 nanosheets is reported. The typical broad peaks around 13° and 28° corresponding to the (100) and (002) reflections are clearly visible. In all the patterns of the composites (cf. Figure ) the diffraction peaks of Cs2SnBr6 are clearly visible due to its high scattering power with respect to g-C3N4. Morphological inspection of Cs2SnBr6, g-C3N4, and selected composites has been carried out by scanning electron microscopy (SEM) and the results are reported in Figure . g-C3N4 nanosheets have also been characterized by transmission electron microscopy (TEM) and representative images showing their nanostructure are shown in Figure .