Design and characterization of graphite-like and hybrid nanomaterials for advanced and sustainable systems

Graphitic carbon nitride (g-C3N4) has attracted great interdisciplinary attention among carbon based materials as a challenging metal-free polymeric semiconductor. Currently, a huge number of studies is carried out focused on the synthesis and application of g-C3N4 as a metalfree catalyst. This is due to its appealing electronic band structure, strong redox properties, high physicochemical stability, easy synthesis via simple thermal polymerisation of cheap and abundant, nitrogen-rich precursors (like urea, melamine, etc.). Ultimately, g-C3N4 can bridge the gap between purely carbon-based and purely metal-based materials for the development of environmentally friendly and highly efficient catalytic processes, aiming to eliminate or reduce waste generation, including toxic metals. Despite its promising potential as a functional material, its intrinsic limitations, such as charge recombination, limited light absorption, and low surface area, have required significant efforts to overcome. These include innovative strategies such as morphology control, doping and defect engineering, heterojunction design, while also focusing on scaling-up production. Considering the economic and environmental benefits, several strategies, like constructing porous 3D structures, membranes, or designing magnetic heterojunctions to address the recycling and reuse challenges, were also attempted. Based on this, g-C3N4 can be considered a suitable candidate to be integrated in a circular economy paradigm. In fact, together with its metal-free nature, the possible synthesis from urea precursor, which can, in turn, be produced from urban waste streams, the g-C3N4 applications in the agricultural sector as a fertiliser, may provide the basis of further considerations on its end-of-life reusability. In response to the circularity challenge, g-C3N4 shows potential applications in waste-toresource transformations and environmental remediation fields, thus converting greenhouse gases into high-value energy sources and valuable products or removing pollutants from water and air. In conclusion, g-C3N4 allows not only promising applications as a functional material, but also a potential shift from linear "take-make-dispose" models to circular systems, by aligning with the circular economy principles. In the present thesis, both aspects have been addressed. The synthesis and the physicochemical characterisation of pristine g-C3N4 obtained from different precursors was carried out, together with its synergistic interaction with TiO2- and ZnO-based inorganic nanomaterials. In particular, g-C₃N₄/TiO₂ hybrid systems were designed using shape-controlled titania nanostructures, including nanotubes, nanosheets and nanoparticles, highlighting the role of g- C₃N₄ domains in preserving the system morphology. Similarly, g-C₃N₄/ZnO-tetrapods heterostructures were developed and tested for UV-activated NO₂ gas sensing applications, demonstrating improved sensitivity and stability. The sustainability assessment of g-C3N4 has been undertaken by means of LCA method, which allows for evaluating the environmental impacts of the adopted raw materials together with the energy demand and emission release during the proposed synthesis method. Concerning this, two projects have been proposed for the sustainable development of pure g-C3N4-based nanoarchitectures and their applications in the photodegradation of model organic pollutants and of hybrid-g-C3N4-ZnO- based nanomaterials for sensor applications. A multi-technique characterisation of the materials was carried out, including spectroscopy (FTIR, ATR, O-PTIR, Raman, UV-Vis, PL, XPS), surface investigation by means of probe molecule, advanced imaging (AFM-IR-sSNOM), microscopy techniques (HRTEM, FESEM, AFM), X-ray diffraction (XRD), thermal analysis (TGA), surface area measurements, computational modelling together with the environmental impact study via LCA approach