# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 7 Other Metal-Oxide Nanomaterials
### ZnO-Based  
Zinc oxide (ZnO) has been known for centuries, primarily as a pigment and additive in products like ceramics, paints, and ointments. Its photochemical properties, however, began to gain attention in the early 20 th century when scientists first observed its semiconducting and photoactive behaviors. The development of solid-state physics, in the 1930s, led to the discovery of the semiconducting nature of ZnO. Its wide bandgap (\(\sim\)3.37 eV) and high exciton binding energy (\(\sim\)60 meV) made it a subject of interest for electronic and optical applications. Early photocatalytic studies were primarily focused on understanding its photoconductivity and interaction with ultraviolet (UV) light. These investigations laid the groundwork for using ZnO in photochemical reactions. ZnO gained more importance in photocatalysis during the 1970s and 1980s, driven by the global energy crisis and increased environmental concerns. Researchers were exploring photocatalytic water splitting and pollutant degradation as solutions to energy and environmental challenges. Nowadays, ZnO is one of the most extensively studied nanomaterials for photocatalysis, owing to its excellent photocatalytic, electrical, and optical properties [186]. ZnO wide band gap limits its adsorption capacity in the visible light range, thus constraining its application as a photocatalyst in indoor environments. To enhance its visible light adsorption properties, ZnO can be modified using dopants [187] or combined with other semiconductors to form composites [188]. Due to its polar nature, ZnO promotes rapid separation and transfer of photogenerated charge carriers. When irradiated with UV light, ZnO generates photogenerated electron centers (Zn\({}^{+}\)) and hole centers (O\({}^{-}\)) [189]. Research has examined the degradation of formaldehyde using a photocatalytic ZnO air filter, both with and without UV light irradiation [190]. Notably, the ZnO filter exhibited a higher formaldehyde removal efficiency without UV light. When irradiated with UV light, photogenerated electron-hole pairs react with H\({}_{2}\)O molecules to form hydroxyl ions, leading to the photocatalytic adsorption and degradation of formaldehyde. However, under UV radiation, fewer pollutant molecules are adsorbed on the surface of the ZnO  
Figure 22: Conversion of toluene and concentration of CO\({}_{2}\) using no catalyst (**a**) and TCN IO (**b**) in dark; bulk-TiO\({}_{2}\) (**c**), inverse opal TiO\({}_{2}\) (TiO\({}_{2}\) IO) (**d**), CNDQs in situ-modified TiO\({}_{2}\) inverse opal (TCN IO) (**e**) and P25 (**f**) under irradiation [183]. filter, resulting in lower removal efficiency. Conversely, the ZnO filter without UV light demonstrates higher removal efficiency due to its greater adsorption capacity, as more formaldehyde molecules are captured on its surface in the presence of condensed H\({}_{2}\)O, given formaldehyde's hydrophilic nature [190]. In another study [191] (Figure 23), ZnO nanorods (ZnO-NRs) were synthesized through hydrothermal growth of ZnO nanoparticles and assessed for the photocatalytic degradation of formaldehyde. These ZnO-NRs were incorporated into the nanofibrils of a polytetrafluoroethylene (PTFE) membrane (ZnO-NRs@PTFE), commonly used for air purification. The ZnO-NRs@PTFE filter achieved a formaldehyde removal rate of 53%, while the PTFE membrane alone exhibited no formaldehyde removal. Furthermore, the deposition of silver (Ag) nanoparticles on the ZnO nanorods improved photocatalytic removal, reaching a maximum degradation rate of 60%. The enhanced photocatalytic activity of the ZnO-NRs@PTFE filter can be attributed to the increased specific surface area of the membrane and the reduced spacing between its fibrils due to the presence of ZnO nanorods. Additionally, Ag nanoparticles act as electron traps, promoting charge carrier separation and decreasing the recombination rate [191]. As mentioned earlier, ZnO can be doped to enhance its adsorption properties under visible light.