## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Zinc Oxide (ZnO)  
Zinc oxide (ZnO) has emerged as a highly promising material for gas sensing applications, largely due to its intrinsic semiconducting properties, wide bandgap of 3.37 eV, and high exciton binding energy of approximately 60 meV, which contribute to its stability and versatility in various sensing environments [41]. ZnO is particularly sensitive to changes in ambient oxygen levels and responds well to a broad range of gases, including hydrogen, methane, ammonia, nitrogen oxides, and volatile organic compounds (VOCs). Its sensitivity and selectivity to specific gases can be tuned by manipulating factors such as dopant incorporation, morphology, and operating temperature, allowing ZnO-based sensors to be tailored to diverse applications in environmental monitoring, industrial safety, and medical diagnostics. The active sites in ZnO-based gas sensors are typically located at the surface of the material. These sites are responsible for the adsorption and interaction with gas molecules. Modifications to ZnO influence the nature and number of active sites. Surface defects, such as oxygen vacancies, act as active sites for gas adsorption. The increased concentration of oxygen vacancies leads to a greater number of active sites, enhancing sensitivity. The type of defect also influences the interaction with specific gas molecules, affecting selectivity. The introduction of specific dopants or surface functional groups can enhance the adsorption of the target gas, improving selectivity. For instance, Ni-doped ZnO nanowire arrays show high selectivity for H\({}_{2}\)S [42], while Na-doped ZnO nanoneedles exhibit enhanced selectivity for specific VOCs [43]. The use of different transition metal dichalcogenides in heterojunctions with ZnO [44] can further alter selectivity. Doping ZnO with elements such as aluminum (Al), gallium (Ga), and indium (In) has proven effective in modifying the electronic structure and enhancing its gas sensing properties. Al-doped ZnO, for example, has shown increased conductivity and improved response times, particularly for reducing gases like hydrogen and hydrocarbons. Ga and In doping have similar effects, with In-doped ZnO exhibiting superior sensitivity to ammonia and nitrogen oxides by adjusting the electronic states at the surface, which enhances adsorption properties and can lead to a 62% response to 0.1% CH\({}_{4}\), which is 4.7 times higher than undoped ZnO [39; 40]. The dopants act by creating additional oxygen vacancies or by modifying the carrier density, which directly influences the sensor's ability to detect specific gas molecules. This process increases the material's reactivity and helps stabilize the response in real-world conditions where gas concentrations may vary widely. Nanostructuring ZnO into forms like nanowires, nanoparticles, and thin films has been another critical strategy for improving gas sensing performance, since gas sensors based on ZnO nanostructures show a high sensitivity, reversible response, and good selectivity towards HCHO under UV light. Nanoscale ZnO structures, with their high surface-to-volume ratio, offer more active sites for gas molecule adsorption and exhibit unique facet-dependent properties. For instance, ZnO nanowires with exposed polar (001) facets have been shown to exhibit enhanced sensitivity to VOCs, largely due to the increased electron density and surface oxygen availability on these facets, which promote gas-surface interactions [45]. Nanoparticles provide similar advantages, as their size and shape can be engineered to maximize adsorption sites and facilitate faster response and recovery times. Thin films, particularly when synthesized with specific crystallographic orientations, also show improved gas sensing performance, as they provide a stable platform with enhanced carrier mobility, contributing to consistent sensor response across different operating conditions [46; 47].