**5. Conclusions**

This chapter provided a concise overview on the use of graphene and graphene derivatives coupled to nanocrystalline semiconductors of metal sulfides in heterogeneous photocatalysis. For these applications, pure metal-sulfide nanoparticles show some limitations, which depending on the semiconductor include limited harvesting of photons in the visible region, low photocatalytic quantum yield, fast recombination of photogenerated charge carriers, and photocorrosion. Hence, several chemical strategies have been reported to improve photoefficiency and performance of these nanoparticles as photocatalysts. Metal-sulfide phases coupled with graphitic materials have been employed to prevent the photocorrosion of the chalcogenide semiconductor and to increase the photocatalytic efficiency of the resultant hybrid nanostructures. Particularly, graphene (and its derivatives) have shown great merits in improving the photogenerated charge-carriers separation and migration, also extending the light absorption range and the adsorption capacity of the photocatalysts. Furthermore, their use as supporting substrates in aqueous suspensions also inhibits the agglomeration of the particles, thus keeping exposed a high surface area to the photoactive semiconductors. The design of graphene-based materials decorated with metal sulfides as photocatalysts requires the assessment of the several parameters that might contribute to their performance in specific contexts. Hence, it has been reported that these hybrid nanostructures show optimal compositional features on the carbon nanomaterial and semiconductor, depending on the target pollutant and operational conditions. Furthermore, the surface functionalization of graphene materials also plays an important role in the development of such photocatalysts, namely the content load in the metal sulfide and their defect structure. Finally, the cost-effective production of graphene-based semiconductor nanocomposites at a large scale, envisaging their application as photostable and efficient heterogeneous photocatalysts, is still a great challenge. In the chemical design of such photocatalysts, eco-friendly up-scale strategies should be also addressed by researchers, to guarantee its future commercialization for environmental applications.
