## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Mixed Metal Oxides and Other Systems
Similarly, adding nickel (Ni) to SnO\({}_{2}\)-TiO\({}_{2}\) systems has been shown to improve sensitivity to VOCs by promoting rapid electron transfer and enabling selective adsorption under lower operating temperatures, which is critical for applications that require energy-efficient, low-temperature operation [108]. Nanostructuring plays a crucial role in further enhancing the performance of MMO-based gas sensors. Structuring MMOs into nanoscale morphologies, such as nanorods, nanosheets, and nanocomposites, significantly increases their surface area and active sites, promoting higher gas adsorption rates and faster electron transport. Nanorod arrays, for example, provide a high surface-to-volume ratio and directional pathways for electron flow, improving sensitivity and response time. In NiO\({}_{2}\)-CeO\({}_{2}\) nanostructures, this structure facilitates the rapid adsorption and desorption cycles necessary for detecting NO\({}_{2}\) and H\({}_{2}\) gases at low concentrations. The decoration of NiO on the CeO\({}_{2}\) surface develops a built-in potential at the interface of NiO and CeO\({}_{2}\), which plays a vital role in the superior sensing performance of the NiO/CeO\({}_{2}\) sensor. Sharp response and recovery times (15 s/19 s) were observed for the NiO/CeO\({}_{2}\) sensor towards 100 ppm isopropanol at room temperature [110]. Nanosheets and nanoflakes, with large surface areas and open edges, allow for efficient charge transfer and enhanced interaction with target gases, which is particularly useful for detecting low-concentration CO and NO\({}_{2}\)[109]. In addition to structural and doping modifications, photoactivation has emerged as a method for enhancing MMO sensor performance. Under UV or visible light, electron-hole pairs are generated, which improve the adsorption and desorption kinetics at lower operating temperatures. This activation method has shown particular promise in ZnO-CuO- and TiO\({}_{2}\)-based MMOs, where light exposure can reduce the sensor's response time, lower power consumption, and extend its applicability to portable and wearable devices. In light-activated SnO\({}_{2}\)-TiO\({}_{2}\) sensors, for example, UV-induced electron-hole pairs enhance gas sensitivity by increasing the number of reactive oxygen species on the surface, allowing for the efficient detection of NO\({}_{2}\) and VOCs without the need for elevated temperatures [110]. MMOs are also highly customizable to complex and fluctuating environmental conditions, which is crucial for real-world applications in air quality monitoring, industrial safety, and environmental control. The robustness of these materials is enhanced by their ability to maintain stability over long periods, even under varied humidity and temperature conditions. For instance, Fe\({}_{2}\)O\({}_{3}\)-SnO\({}_{2}\) composites demonstrate exceptional stability in humid environments while retaining high sensitivity to target gases. This robustness is largely attributed to the synergistic interaction between Fe\({}_{2}\)O\({}_{3}\) and SnO\({}_{2}\), which strengthens the material's resistance to degradation and maintains consistent performance across a wide range of conditions [111; 112]. The gas sensing mechanisms in MMOs are typically based on the modulation of electrical resistance or conductance when exposed to target gases. Upon the adsorption of reducing gases, such as H\({}_{2}\) or CO, the electrons from these gases transfer to the conduction band of the n-type metal oxide component (e.g., SnO\({}_{2}\) in SnO\({}_{2}\)-TiO\({}_{2}\)), decreasing resistance. Conversely, when exposed to oxidizing gases like NO\({}_{2}\), electrons are withdrawn from the conduction band, increasing resistance. This electron modulation is highly reversible, which is crucial for applications that require continuous, real-time monitoring [111; 112; 113; 114; 115; 116; 117; 118; 119; 120].