# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 7 Other Metal-Oxide Nanomaterials
### MnO\({}_{2}\)-Based  
Manganese dioxide (MnO\({}_{2}\)) has long been known as a mineral, most notably as the naturally occurring pyrolusite. Its use has been common in batteries, ceramics, and as a catalyst in chemical processes, such as the oxidation of gases. Until the early 20 th century, MO\({}_{2}\) was primarily seen as a material with industrial significance in oxidative reactions, rather than for photocatalysis. While titanium-based photocatalysts dominated the early years of photocatalysis research, MnO\({}_{2}\) began to gain attention in the 1970s and 1980s as a potential material for oxidizing air pollutants under UV light. The real breakthrough for MnO\({}_{2}\)-based photocatalysts in environmental applications came in the 1990s and early 2000s with the advent of nanotechnology. Research into nanomaterials had rapidly expanded during this period, and scientists began investigating how the unique properties of MnO\({}_{2}\) nanostructures could be exploited for photocatalysis. Nowadays, manganese dioxide (MnO\({}_{2}\))-based nanomaterials have gained significant attention as alternatives to noble metal-based photocatalysts due to their advantageous properties. These materials are studied in photocatalysis because of their high specific surface area, pore characteristics, and oxygen storage potential. Additionally, MnO\({}_{2}\) is low-cost and environmentally compatible, making it an attractive option for various applications. This material exhibits significant photoadsorption properties for volatile organic  
Figure 23: (**A**) UV–visible absorption spectra of various photocatalysts and (**B**) degradation of formaldehyde using various photocatalysts under visible-light irradiation [192]. compounds (VOCs) and demonstrates an important photothermal effect. While MnO\({}_{2}\) shows high absorbance under UV and visible light irradiation, its adsorption properties are relatively lower in the near-infrared radiation range. The photocatalytic activity of MnO\({}_{2}\) can be further enhanced through doping or by forming junctions with other semiconductors [193]. Layered manganese oxide, commonly known as bimessite, has been extensively studied for the removal of formaldehyde [194]. This material has achieved a remarkable formaldehyde removal rate of up to 93%. Research indicates that H\({}_{2}\)O molecules within bimessite enhance the adsorption of formaldehyde, with increased humidity levels (0% to 33% and then to 65%) positively affecting this process. However, at higher humidity levels of 92%, a decrease in the degradation rate was observed, likely due to competition between H\({}_{2}\)O and formaldehyde for the active sites on the photocatalyst. In another study (Figure 24) [193], a graphene-based MnO\({}_{2}\) composite (MnO\({}_{2}\)-G) was evaluated for photocatalytic removal of formaldehyde under UV, visible, and infrared radiation. The MnO\({}_{2}\) exhibited superior formaldehyde removal capabilities compared to graphene, primarily due to its enhanced adsorption capacity; graphene was observed to desorb the pollutant instead. Notably, the graphene sample did not yield any CO\({}_{2}\), indicating that it only adsorbed the pollutant without facilitating its degradation. Conversely, the MnO\({}_{2}\)-G composite demonstrated the highest photocatalytic activity for degrading formaldehyde, achieving a conversion rate of 80% into CO\({}_{2}\), significantly higher than that of MnO\({}_{2}\) alone. This enhancement in photocatalytic activity can be attributed to the synergistic photothermal effect between graphene and MnO\({}_{2}\), where photothermal energy is transferred from graphene to MnO\({}_{2}\), leveraging graphene's high conductivity. The combined photothermal effect and efficient electron transfer activate the active sites on MnO\({}_{2}\), facilitating the oxidation of the pollutant [193]. Apart from being low-cost and abundant, MnO\({}_{2}\) is a non-toxic, environmentally friendly material.