## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Indium Oxide (In\({}_{2}\)O\({}_{3}\))  
Indium oxide (In\({}_{2}\)O\({}_{3}\)) is a promising material for gas sensing applications due to its wide bandgap of approximately 2.9 eV, high electrical conductivity, and exceptional chemical and thermal stability, which enable it to maintain performance under varied environmental conditions. These properties make In\({}_{2}\)O\({}_{3}\) especially effective for detecting oxidizing gases, such as nitrogen dioxide (NO\({}_{2}\)), ozone (O\({}_{3}\)), and carbon dioxide (CO\({}_{2}\)), as its n-type semiconducting nature readily facilitates charge transfer with these gas molecules. The high reactivity of In\({}_{2}\)O\({}_{3}\) towards oxidizing gases is due to its propensity for electron donation and reduction upon gas exposure, which induces measurable changes in conductivity, allowing for fast, sensitive gas detection [88]. The nanostructuring of In\({}_{2}\)O\({}_{3}\) into forms such as nanofibers, nanorods, and nanoparticles has been shown to significantly enhance its gas sensing performance. For instance, nanofibers provide a high surface area-to-volume ratio, improving the density of active sites for gas adsorption, which results in faster response and recovery times. Nanorods and nanoparticles, with their unique surface morphologies, offer additional active sites and pathways for efficient electron transport, leading to improved sensitivity, particularly for low-concentration gas detection. Nanostructured In\({}_{2}\)O\({}_{3}\) has exhibited remarkable sensitivity to NO\({}_{2}\), which can be detected at parts-per-billion (ppb) levels, demonstrating its suitability for applications requiring precise gas measurements, such as environmental monitoring and industrial safety [89; 90]. Doping In\({}_{2}\)O\({}_{3}\) with metals like tin (Sn) and zinc (Zn) has further optimized its gas sensing performance by modifying its electronic structure, enhancing both sensitivity and selectivity. For example, Sn-doped In\({}_{2}\)O\({}_{3}\) sensors have shown improved selectivity towards oxidizing gases such as NO\({}_{2}\) and O\({}_{3}\), as Sn atoms create additional oxygen vacancies that promote stronger interactions between the target gas molecules and the In\({}_{2}\)O\({}_{3}\) surface. Similarly, Zn doping has been observed to shift the bandgap and enhance response to specific gases by introducing donor levels, which facilitate electron transfer and enhance response times at lower gas concentrations. This doping strategy allows In\({}_{2}\)O\({}_{3}\)-based sensors to achieve high performance even in complex and fluctuating environments, where multiple gases may be present [91; 92]. In\({}_{2}\)O\({}_{3}\) sensors also exhibit the advantage of operating at relatively low temperatures, which makes them more energy-efficient compared to other metal oxide sensors that require higher operating temperatures (typically above 200 \({}^{\circ}\)C). This low-temperature operability not only reduces energy consumption, but also broadens the potential applications of In\({}_{2}\)O\({}_{3}\) sensors to include portable and wearable devices, where energy efficiency is essential. Additionally, low-temperature operation reduces the risk of interference from thermal noise and degradation, ensuring stable performance over extended periods [93]. The sensing mechanism of In\({}_{2}\)O\({}_{3}\) is predominantly based on the modulation of resistance upon gas adsorption. In the presence of oxidizing gases like NO\({}_{2}\), electrons from the conduction band of In\({}_{2}\)O\({}_{3}\) are transferred to gas molecules, leading to the formation of negatively charged adsorbates on the sensor surface. This electron withdrawal decreases the free electron concentration in the n-type In\({}_{2}\)O\({}_{3}\), resulting in an increase in resistance. When the oxidizing gas desorbs, the electrons return to the In\({}_{2}\)O\({}_{3}\), restoring its original conductivity.