# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 7 Other Metal-Oxide Nanomaterials
### CeO\({}_{2}\)-Based  
Cerium oxide (CeO\({}_{2}\)), commonly referred to as ceria, is a p-type semiconductor that serves as an effective photocatalyst due to its chemical stability, low toxicity, affordability, high adsorption capacity under UV radiation, and impressive photocatalytic activity. The compound was first identified in the early 19 th century and gained early industrial significance for applications in catalysis, particularly in automotive catalytic converters. Its redox-active nature, where cerium can exist in both +3 and +4 oxidation states, made CeO\({}_{2}\) a valuable catalyst for various reactions, but its photocatalytic properties were not extensively studied until later. in the late 1990s and early 2000s. Early studies identified CeO\({}_{2}\)'s ability to generate reactive oxygen species (ROS), such as hydroxyl radicals (-OH),  
Figure 25: (**a**) Photocatalytic efficiency of hexane removal using TNS, N–TNS, and WO\({}_{3}\)–N–TNSs containing various amounts of WO\({}_{3}\) and (**b**) photoluminescence spectra of TNS, N–TNS, and WO\({}_{3}\)–N-TNSs containing various amount of WO\({}_{3}\)[120]. under UV light, similar to more conventional photocatalysts like TiO\({}_{2}\). This opened up new avenues for using CeO\({}_{2}\) in photocatalytic air purification, especially for degrading volatile organic compounds (VOCs), nitrogen oxides (NOx), and ozone (O\({}_{3}\)). When combined with an n-type semiconductor, CeO\({}_{2}\) displays photocatalytic properties comparable to those of the widely used TiO\({}_{2}\). However, its photocatalytic applications are restricted by a wide band gap, a high charge carrier recombination rate, and a low charge transfer rate. To enhance its functionality, CeO\({}_{2}\) has been coupled with other semiconductors to form composites that narrow its band gap. Furthermore, CeO\({}_{2}\) can improve the stability of the catalysts it is combined with, possesses a substantial oxygen storage capacity, and can effectively release and store oxygen to drive photocatalytic reactions [143]. CeO\({}_{2}\) has a stable fluorite structure containing both Ce\({}^{4+}\) and Ce\({}^{3+}\) ions, which can lead to the creation of oxygen vacancies. The introduction of metal heteroatoms as dopants can further promote the formation of these vacancies [196] (Figure 26). One study investigated a photocatalyst composed of CeO\({}_{2}\) and BiVO\({}_{4}\) (CeO\({}_{2}\)-BiVO\({}_{4}\)) coupled through a heterojunction to assess its photocatalytic degradation of formaldehyde under visible light [197]. This coupling resulted in a narrowing of the CeO\({}_{2}\) band gap and an enhancement of its photocatalytic activity. UV-vis adsorption spectra revealed that the introduction of BiVO\({}_{4}\) significantly increased CeO\({}_{2}\)'s adsorption in the visible range. Additionally, the photoluminescence intensity of the composite was lower than that of pure CeO\({}_{2}\) and BiVO\({}_{4}\), indicating a reduced charge carrier recombination rate [197]. The CeO\({}_{2}\)-BiVO\({}_{4}\) composite demonstrated a maximum formaldehyde degradation of 78%, outperforming both pure CeO\({}_{2}\) and BiVO\({}_{4}\). The optimal ratio of the two photocatalysts was determined to be 4:1 for CeO\({}_{2}\)-BiVO\({}_{4}\)[197]. Figure 26: (a) UV–vis spectra of CeO\({}_{2}\) and Eu/CeO\({}_{2}\) containing different amounts of Eu and (b) photocatalytic degradation of formaldehyde using P25, CeO\({}_{2}\), and Eu/CeO\({}_{2}\) containing different amounts of Eu [196]. In another study, the photocatalytic activity of europium-doped CeO\({}_{2}\) nanosheets was examined for formaldehyde degradation. Different amounts of Eu were used to dope CeO\({}_{2}\), enhancing defects on its surface and promoting the formation of oxygen vacancies. UV-vis adsorption spectra indicated that the visible range adsorption increased with higher Eu concentrations, with all Eu/CeO\({}_{2}\) samples showing superior adsorption compared to undoped CeO\({}_{2}\).