# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 7 Other Metal-Oxide Nanomaterials
### Wo\({}_{3}\)-Based  
Tungsten trioxide (WO\({}_{3}\)) was first identified in the 19 th century and has long been known for its distinctive blue color and various industrial applications, such as in the production of catalysts and as a component in electrochromic devices. WO\({}_{3}\)'s photocatalytic properties, however, were not recognized until much later. Its potential for photocatalysis in environmental applications, especially for air pollution control, became an area of growing interest only in the late 20 th century. Early studies focused on WO\({}_{3}\)'s ability to degrade organic compounds under UV light, similar to other photocatalysts like TiO\({}_{2}\), but with the added benefit of WO\({}_{3}\)'s potential visible light activity. WO\({}_{3}\)-based nanomaterials have garnered attention in photocatalysis due to their narrow band gap of approximately 2.6 eV and the high oxidation capacity of their photogenerated holes, which can oxidize H\({}_{2}\)O to O\({}_{2}\)[195]. WO\({}_{3}\) is frequently utilized as a co-catalyst to enhance the photocatalytic activity of other semiconductors because of its narrow band gap. The introduction of WO\({}_{3}\) can significantly improve the photocatalytic efficiency of the composite under visible light radiation due to the unique optical and electronic properties of the modified photocatalyst. Studies have shown that combining WO\({}_{3}\) with TiO\({}_{2}\) enhances photocatalytic activity under both UV and visible light. The conduction band of WO\({}_{3}\) is more positive than that of TiO\({}_{2}\), facilitating the transfer of photogenerated electrons from TiO\({}_{2}\) to WO\({}_{3}\) under UV light, thereby increasing the overall photocatalytic activity. Under visible light, only WO\({}_{3}\) is excited, allowing the adsorbed pollutant molecules on TiO\({}_{2}\) to react with the photogenerated charge carriers from WO\({}_{3}\). This synergistic effect between the two semiconductors significantly enhances the photocatalytic activity [195]. Research has examined the photocatalytic degradation of NO using WO\({}_{3}\)/TiO\({}_{2}\) composites with varying proportions of TiO\({}_{2}\) under both visible light and UV radiation [195]. The results indicated that incorporating TiO\({}_{2}\) into the WO\({}_{3}\) photocatalyst increases the photocatalytic degradation of NO under both light conditions. The presence of TiO\({}_{2}\) likely promotes charge carrier separation due to the formation of a heterostructure, leading to enhanced photocatalytic activity. Specifically, the highest photocatalytic activity was observed with 70% TiO\({}_{2}\) under UV light and 80% TiO\({}_{2}\) under visible light. However, further increasing the TiO\({}_{2}\) content resulted in reduced photocatalytic activity, likely due to the formation of recombination centers [195]. In another study [120] (Figure 25), nanosheets of N-doped TiO\({}_{2}\)/WO\({}_{3}\) (WO\({}_{3}\)-N-TNSs) composites were synthesized with varying WO\({}_{3}\) content to investigate their photocatalytic degradation of vaporous hexane. The band gap of the samples was determined from UV-vis absorption spectra, revealing that the WO\({}_{3}\)-N-TNS samples exhibited a significant increase in adsorption capacity with band gaps ranging from 2.75 to 2.91 eV, compared to N-TNS (3.08 eV) and undoped TNS (3.17 eV). It was also observed that increasing the WO\({}_{3}\) content resulted in greater visible light shifts in the WO\({}_{3}\)-N-TNS samples. These composites demonstrated higher photocatalytic efficiency compared to N-doped TNS (N-TNS) and undoped TNS, attributed to improved adsorption efficiency, enhanced charge carrier separation, and the combined effects of WO\({}_{3}\) and N doping. Photoluminescence spectra indicated a lower intensity for the WO\({}_{3}\)-N-TNS samples compared to the others, suggesting a lower recombination rate. The optimal amount of WO\({}_{3}\) in this photocatalyst was determined based on trends in specific surface area and pore volume.