## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Titanium Oxide (TiO\({}_{2}\))  
Titanium dioxide (TiO\({}_{2}\)) has garnered significant attention as a gas sensing material due to its notable properties, including strong oxidizing power, high chemical stability, non-toxicity, and excellent optical properties--particularly its high refractive index and UV absorption capabilities [51]. TiO\({}_{2}\)-based gas sensors are well suited for detecting volatile organic compounds (VOCs), nitrogen oxides (NO\({}_{x}\)), hydrogen sulfide (H\({}_{2}\)S), and other harmful gases. The semiconducting nature of TiO\({}_{2}\), with a bandgap of approximately 3.2 eV for anatase and 3.0 eV for rutile, allows for efficient charge transfer upon interaction with target gas molecules, especially under UV activation, enhancing sensitivity and response speed. The nanostructuring of TiO\({}_{2}\) into morphologies such as nanotubes, nanorods, and nanosheets has proven especially effective in enhancing gas sensing performance. These nanostructures provide a high surface-to-volume ratio, increasing the number of active  
Figure 5: Synthesis of hierarchical nanostructured ZnO nanorods decorated with Pd, and the proposed ethanol sensing mechanism (reproduced from [50], MDPI, 2023). sites for gas adsorption, which is crucial for detecting gases at low concentrations. For instance, TiO\({}_{2}\) nanotubes have shown increased sensitivity to VOCs and NOx due to their hollow structure, which promotes gas diffusion and increases the interaction area. SnO\({}_{2}\)-coated TiO\({}_{2}\) nanotube layers attained a higher sensing response than a reference SnO\({}_{2}\) sensor. Specifically, an 8 nm SnO\({}_{2}\)-coated TiO\({}_{2}\) nanotube layer recorded up to ten-fold enhancement in response compared to blank nanotubes for the detection of 1 ppm NOx at an operating temperature of 300 \({}^{\circ}\)C with 0.5 V applied bias. This is attributed to the SnO\({}_{2}\)/TiO\({}_{2}\) heterojunction effect [52]. Nanorods, on the other hand, facilitate faster response and recovery times due to their one-dimensional structure, which provides a direct pathway for charge carrier transport, minimizing grain boundary effects and enhancing conductivity [52; 53]. Nanosheets, with their large surface area and exposed reactive facets, further improve TiO\({}_{2}\)'s ability to interact with gas molecules, making them particularly effective for applications requiring rapid detection and high sensitivity. Doping TiO\({}_{2}\) with elements such as nitrogen (N) and platinum (Pt) has resulted in substantial improvements in its gas sensing capabilities, particularly in terms of sensitivity and response time. Nitrogen doping introduces mid-gap states that reduce the bandgap energy, allowing TiO\({}_{2}\) to be activated under visible light or low UV intensity. This enhances its gas sensing performance in ambient conditions, where UV light sources are either weak or absent. N-doped TiO\({}_{2}\) has shown increased sensitivity to NOx gases and VOCs, as nitrogen atoms create additional adsorption sites and facilitate charge transfer upon interaction with target gases [54; 55; 56]. It was revealed that a s-TiO\({}_{2}\) sensor without additional N\({}_{2}\) treatment showed superior H\({}_{2}\)-sensing performance than a N\({}_{2}\)-treated s-TiO\({}_{2}\) sensor when operated in a N\({}_{2}\) atmosphere. In addition, the response to N\({}_{2}\)-diluted H\({}_{2}\) was larger than that to air-diluted H\({}_{2}\), and the interference from water vapor was negligible in N\({}_{2}\) atmosphere. The inclusion of noble metals, such as Pt, further improves the response and recovery characteristics of TiO\({}_{2}\) gas sensors. Pt doping enhances the catalytic activity on the TiO\({}_{2}\) surface by providing active sites for gas dissociation, particularly under UV or visible light activation.