# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 6 Carbon-Based Nanomaterials
### Graphitic Carbon Nitrides  
Graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\)) is a carbon-based two-dimensional conjugated polymer that has recently gained attention in photocatalysis. Although it has a relatively low specific surface area and a limited porous structure, its adsorption properties can be enhanced through modifications with metal oxides that increase its surface area [155]. g-C\({}_{3}\)N\({}_{4}\) is a semiconductor with an energy band gap of 2.7 eV, featuring conduction band and valence band potentials of \(-\)0.85 eV and 1.85 eV, respectively [171]. When exposed to radiation with sufficient energy, it can photogenerate electron and hole pairs. Due to its excellent stability, metal-free composition, and environmental friendliness, g-C\({}_{3}\)N\({}_{4}\) is utilized as a photocatalyst for the removal of volatile organic compounds (VOCs). However, its photocatalytic activity is hindered by a low density of active sites and low charge carrier mobility, which leads to a high recombination rate of electron-hole pairs [172].  
To enhance the adsorption and photocatalytic efficiency of g-C\({}_{3}\)N\({}_{4}\), various strategies have been explored, including doping [173], surface modifications [174], and the creation of heterojunctions with other semiconductors [175]. For instance, a composite photocatalyst combining g-C\({}_{3}\)N\({}_{4}\), TiO\({}_{2}\), and waste zeolites has been synthesized for the photocatalytic removal of formaldehyde (Figure 21) [95]. In this study, it was observed that the emission peak of g-C\({}_{3}\)N\({}_{4}\) in the photoluminescence spectrum is intense and broad, indicating rapid charge carrier recombination. However, when combined with TiO\({}_{2}\) and waste zeolites, the photoluminescence intensity of these samples decreases significantly and experiences a blue shift, likely due to enhanced contact with TiO\({}_{2}\).  
Figure 21: (a) Photoluminescence spectra of g-C\({}_{3}\)N\({}_{4}\)-TiO\({}_{2}\) with various amounts of g-C\({}_{3}\)N\({}_{4}\), P25, and g-C\({}_{3}\)N\(4\) and (b) degradation of formaldehyde using g-C\({}_{3}\)N\({}_{4}\)-TiO\({}_{2}\)/waste zeolites with various amount of g-C\({}_{3}\)N\({}_{4}\), P25 and a sample of g-C\({}_{3}\)N\({}_{4}\)/TiO\({}_{2}\)[95].  
Figure 20: (a) UV–vis absorption spectra of TiO\({}_{2}\) and rGO/TiO\({}_{2}\) and (b) degradation of formaldehyde using no catalyst, TiO\({}_{2}\), rGO, and rGO/TiO\({}_{2}\)[90].  
Furthermore, it was noted that the total pore volume and micropore volume of the samples slightly increase as the amount of g-C\({}_{3}\)N\({}_{4}\) in the composite increases, probably due to the formation of low-density g-C\({}_{3}\)N\({}_{4}\) on TiO\({}_{2}\). Under visible light, photogenerated electrons in g-C\({}_{3}\)N\({}_{4}\) are transferred to the conduction band of TiO\({}_{2}\) through a heterojunction, reducing the charge carrier recombination rate and increasing photocatalytic activity. The photogenerated electrons in the conduction band of TiO\({}_{2}\) react with oxygen molecules to form reactive oxygen species (ROS), while the photogenerated holes in the valence band of g-C\({}_{3}\)N\({}_{4}\) react with water molecules to produce hydroxyl (-OH) radicals. The photocatalytic degradation of formaldehyde was investigated, revealing that most of the prepared samples exhibited higher photocatalytic activity compared to P25 [95].