## 1 Introduction  
Direct Z-scheme photocatalysis mechanisms, obtained by combining two different photocatalysts (PC), have been proven to be a promising option for enhancing photocatalytic reactions efficiency [1]. In this configuration, inspired by the study of natural photosynthesis, photogenerated electrons in the conduction band (CB) of a PC I can recombine with photogenerated holes in the valence band (VB) of a PC II: this results in a charge-separated state where holes are left on the VB of PC I and electrons on the CB of PC II, separated in space. This inhibits recombination, thus enhancing lifetimes and boosting the yield of redox processes employing them. Z-schemes can be implemented using two semiconductors with opportunely aligned band edges and offers the possibility of using materials with different bandgaps, with a double advantage: (1) materials that are good oxidants but poor reductants can be used as PC I and the reverse can be used for PC II, thus maximizing the activity of their redox potentials; and (2) a broader portion of sunlight spectrum can be exploited. For these reasons, Z-schemes are increasingly popular in photoactive materials and are increasingly studied for a variety of applications, from water splitting to wastewater remediation [2, 3]. Graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\))-based materials recently attracted great attention due to, among other things, useful optical properties, high chemical stability, and ease of fabrication [4]. g-C\({}_{3}\)N\({}_{4}\)-based materials could also be used as Fenton-like catalysts, adsorbents, disinfectants, and membrane materials [5; 6; 7]. Despite these advantages, g-C\({}_{3}\)N\({}_{4}\) also has significant drawbacks, such as poor UV-visible absorption (<450 nm), low surface area, and fast recombination kinetics for photogenerated electron-hole couples, limiting its photocatalytic performance. Different methodologies for modifying g-C\({}_{3}\)N\({}_{4}\) have been investigated in order to improve its performance, for example by doping g-C\({}_{3}\)N\({}_{4}\) with different elements: TiO\({}_{2}\)[8; 9; 10; 11; 12], Cu [13], SiC [14], Ag [15; 16], ZnO [17], graphitized polyacrylamide[18], CdO [19], and WO\({}_{3}\)[20]. However, most of these dopings comprise nanomaterials in powdered form and are used for colloidal photocatalysis. Much less investigated is the possibility of utilizing g-C\({}_{3}\)N\({}_{4}\) (either alone or in composites) as thin films over substrate surfaces, for example FTO [21; 22; 23; 24]. In water purification applications, the use of colloidal suspensions still represents the main research field due the available high surface areas. However, the recovery of the material, after use, constitutes an additional problem [25]. From an industrial perspective, working with immobilized photocatalysts, such as thin films, allows the facile removal at the end of the process and displays enhanced compatibility with flow reactor installations [26; 27]. On the other hand, the application of \(Z\)-schemes or other useful heterojunction-based design principles is quite straightforward and well established for nanopowders but much more difficult for thin-films. Indeed, for such a material to be effective in photocatalysis, both the components need to have a solid-liquid interface at which redox reactions with substrates can occur, and this is not possible with the layered composites generally obtained with most thin-film fabrication methods. In photocatalysis, g-C\({}_{3}\)N\({}_{4}\) composites have been recently investigated for the degradation of emerging pharmaceutical contaminants (like sulphonamides, accylovir) as well as of pesticides [28; 29; 30; 31; 32; 33]. Only a few papers deal with the synthesis of FeOx/C\({}_{3}\)N\({}_{4}\) composites for oxidation processes, water splitting, electro-catalysis, dye degradation, or artificial photosynthesis [34; 35; 36].