## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Iron Oxides (Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\))  
Iron oxides, particularly hematite (Fe\({}_{2}\)O\({}_{3}\)) and magnetite (Fe\({}_{3}\)O\({}_{4}\)), are widely studied materials in gas sensing due to their affordability, chemical stability, and favorable electronic properties. Fe\({}_{2}\)O\({}_{3}\), an n-type semiconductor with a bandgap of around 2.1 eV, and Fe\({}_{3}\)O\({}_{4}\), known for its unique mixed valency and half-metallic behavior, have proven effective in detecting gases such as hydrogen (H\({}_{2}\)), carbon monoxide (CO), nitrogen oxides (NO\({}_{x}\)), and ammonia (NH\({}_{3}\)). These iron oxides exhibit a strong affinity for oxygen, making them particularly sensitive to reducing gases that alter their charge carrier dynamics through redox reactions at the surface (Figure 8) [70; 71; 72]. Figure 7: (**a**) SEM and (**b**) HR-SEM images of the as-obtained samples. HR-SEM images of the samples annealed at (**c**) 205 \({}^{\circ}\)C and (**d**) 450 \({}^{\circ}\)C (reproduced from [66], MDPI, 2022). The gas sensing mechanism of iron oxides is primarily governed by changes in resistance that occur when reducing or oxidizing gases interact with the sensor surface. In the presence of reducing gases like H\({}_{2}\) or CO, these gases donate electrons to iron oxide, leading to an increase in conductivity, especially in n-type Fe\({}_{2}\)O\({}_{3}\). For oxidizing gases such as NO\({}_{\rm x}\), the opposite effect occurs, as these gases extract electrons, decreasing the concentration of charge carriers and leading to an increase in resistance. Fe\({}_{3}\)O\({}_{4}\), with its unique ability to exhibit both semiconducting and metallic-like behavior depending on conditions, has shown potential in detecting multiple gas types at varied temperatures, adding versatility to iron oxide-based sensors [71; 72]. Since Fe oxide-based sensors respond to multiple gasses, selectivity can be improved quite successfully by functionalization with Pt, Pd, or Au, or temperature modulation [70; 71; 72; 73; 74]. Nanostructuring iron oxides into forms like nanospheres, nanotubes, nanorods, and thin films has greatly improved their sensitivity and response times. Nanospheres, for example, offer a large surface area that enhances gas adsorption, increasing the rate at which the sensor responds to target gases. Nanotubes and nanorods, due to their one-dimensional structure, support direct pathways for charge transport, enhancing conductivity and leading to faster response and recovery times. In particular, Fe\({}_{2}\)O\({}_{3}\) nanorods have shown high sensitivity to nitrogen oxides due to their surface reactivity and stable resistance changes upon exposure to oxidizing gases [73; 74]. These unique morphologies maximize active sites for gas interaction, making nanostructured Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) highly suitable for applications in environmental monitoring and industrial safety. Doping iron oxides with metals like tin (Sn) and nickel (Ni) has proven beneficial in enhancing their selectivity, response rates, and stability under varied environmental conditions. Sn-doped Fe\({}_{2}\)O\({}_{3}\), for example, has shown improved sensitivity and selectivity towards reducing gases like CO and H\({}_{2}\) by increasing oxygen vacancy concentration and facilitating charge transfer between Fe\({}_{2}\)O\({}_{3}\) and the adsorbed gas molecules. Similarly, Ni-doped Fe\({}_{2}\)O\({}_{3}\) has been found to increase sensor response rates to ammonia and nitrogen oxides, largely due to nickel's catalytic effect, which enhances adsorption kinetics and the stability of the surface reaction intermediates. The inclusion of these dopants modifies the electronic structure and creates additional defect sites, effectively tuning the sensor's performance for specific applications [71; 75]. Other examples of Fe oxide-based sensors include force and pressure sensors, which were produced using carrageenan films in one study [73].