## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Molybdenum Oxide (MoO\({}_{3}\))  
Molybdenum oxide (MoO\({}_{3}\)) is gaining attention in gas sensing applications due to its high sensitivity to both oxidizing and reducing gases, including ammonia (NH\({}_{3}\)), hydrogen (H\({}_{2}\)), and nitrogen oxides (NO\({}_{\rm x}\)). Its layered structure and bandgap, typically around 2.9-3.1 eV, provide unique electronic properties that facilitate strong gas adsorption and  
Figure 10: (**top**): (**a**) TEM images of the In\({}_{2}\)O\({}_{3}\) thin film deposited at 500 \({}^{\circ}\)C. (**b**) SAED of the In\({}_{2}\)O\({}_{3}\) thin film deposited at 500 \({}^{\circ}\)C. (**bottom**): Repeatability of indium oxide thin films sprayed at 500 \({}^{\circ}\)C toward 50 ppm toluene (reproduced from [92], ACS, 2021). high catalytic activity, making MoO\({}_{3}\) suitable for sensitive and selective gas detection. The layered arrangement of MoO\({}_{3}\) contributes to its exceptional adsorption capacity, as it allows gases to intercalate between layers, which maximizes the interaction area and promotes effective electron transfer upon gas exposure [95]. Nanostructuring MoO\({}_{3}\) into forms such as nanorods, nanosheets, and nanoflakes greatly enhances its surface area and reactivity. These nanostructures allow more active sites for gas adsorption and create efficient electron pathways, resulting in faster response and recovery times. For example, nanorods provide a high surface-to-volume ratio with directional electron transport, which is particularly beneficial for detecting gases like H\({}_{2}\) and NO\({}_{2}\) at low concentrations. Nanosheets and nanoflakes, with their high surface area and open edges, enable rapid adsorption and desorption cycles, leading to more reliable, repeatable sensing [96; 97]. The performance of MoO\({}_{3}\)-based sensors is further optimized through doping with elements such as vanadium (V) and chromium (Cr). These dopants modify the band structure of MoO\({}_{3}\), introducing defect sites and oxygen vacancies that enhance the sensor's interaction with target gases. V-doped MoO\({}_{3}\), for instance, shows improved sensitivity to NH\({}_{3}\) and NO\({}_{\rm x}\) gases by promoting stronger chemical interactions at the sensor surface, while Cr doping enhances MoO\({}_{3}\)'s response to reducing gases like H\({}_{2}\) by facilitating electron transfer processes. Doping with transition metals creates localized electronic states that improve charge carrier mobility and reactivity, which leads to faster response and recovery times [98; 99]. In addition to doping, combining MoO\({}_{3}\) with other metal oxides, such as titanium dioxide (TiO\({}_{2}\)), has proven effective for improving sensitivity and selectivity. MoO\({}_{3}\)-TiO\({}_{2}\) composites, for instance, benefit from the formation of heterojunctions that facilitate charge separation and increase selectivity for specific gases. TiO\({}_{2}\) enhances the catalytic activity of MoO\({}_{3}\), particularly under UV light, allowing the composite to perform well in detecting volatile organic compounds (VOCs) and NOx gases. These composites take advantage of the synergistic effects between MoO\({}_{3}\) and TiO\({}_{2}\), leading to enhanced sensitivity and stability, even under fluctuating environmental conditions [100]. The gas sensing mechanism in MoO\({}_{3}\) relies on the modulation of electrical conductivity upon gas adsorption. For oxidizing gases like NO\({}_{2}\), MoO\({}_{3}\) interacts by trapping electrons from the conduction band, leading to an increase in resistance. For reducing gases like H\({}_{2}\) and NH\({}_{3}\), the interaction results in the donation of electrons to the MoO\({}_{3}\) surface, decreasing its resistance. This resistance modulation is reversible, enabling MoO\({}_{3}\) sensors to operate in real-time applications. MoO\({}_{3}\)-based sensors can detect even trace levels of gases due to this effective charge transfer mechanism, making them ideal for applications in environmental monitoring and industrial safety [101].