## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Tin Oxide (SnO\({}_{2}\))  
Tin oxide (SnO\({}_{2}\)) is one of the most widely researched n-type semiconductors for gas sensing applications, known for its excellent electronic properties, chemical stability, and versatility in detecting a wide range of gases. SnO\({}_{2}\) has a direct bandgap of around 3.6 eV, which allows it to respond quickly to reducing gases such as hydrogen (H\({}_{2}\)), carbon monoxide (CO), volatile organic compounds (VOCs), and ammonia (NH\({}_{3}\)). This wide bandgap also ensures that the sensor remains stable and highly resistive when not exposed to reducing gases, making it ideal for selective gas sensing [77; 78]. Nanostructured SnO\({}_{2}\), such as in nanowires, nanoparticles, and thin films, offers an improved sensitivity due to its enhanced surface-to-volume ratio, which increases the active sites for gas adsorption. Nanowires, for instance, provide a direct pathway for electron transport, leading to faster response and recovery times for hydrogen and CO gases, as well as the sensitive monitoring of VOCs under various environmental conditions [79; 80; 81]. Thin films, on the other hand, allow for precise control over thickness and morphology, enabling stable sensor performance and long-term reliability. Doping SnO\({}_{2}\) with noble metals like platinum (Pt) and palladium (Pd) further enhances its selectivity, response time, and overall performance. These dopants act as catalytic agents that lower the activation energy required for gas interactions at the sensor surface, leading to faster adsorption-desorption dynamics and improved sensitivity. Pd-doped SnO\({}_{2}\), for example, has demonstrated superior response to hydrogen due to Pd's high catalytic activity in dissociating hydrogen molecules into atomic hydrogen, which enhances the electron transfer process. Pt doping, on the other hand, improves sensitivity to carbon monoxide and methane by promoting the adsorption of these gases and facilitating charge transfer to the SnO\({}_{2}\) surface, resulting in significant changes in conductivity [82] (Figure 9). The gas sensing mechanism in SnO\({}_{2}\) relies on the changes in surface resistivity upon interaction with target gases. In the presence of a reducing gas, SnO\({}_{2}\) undergoes a reduction in surface resistance, as electrons are transferred from the adsorbed gas molecules to the SnO\({}_{2}\) lattice, increasing charge carrier concentration. For oxidizing gases like nitrogen dioxide (NO\({}_{2}\)), the opposite effect occurs: the gas molecules capture electrons from the SnO\({}_{2}\) surface, reducing the conductivity. This reversible interaction with gases allows SnO\({}_{2}\)-based sensors to operate in real time and makes them highly useful in detecting fluctuating concentrations of gases, especially in environmental and industrial settings [83]. Temperature plays a crucial role in SnO\({}_{2}\) gas sensing performance, as it influences the rate of the adsorption and desorption of gas molecules on the sensor surface. SnO\({}_{2}\) sensors typically operate within a temperature range of 150-400 \({}^{\circ}\)C, with the optimal temperature depending on the specific gas and nanostructure. Higher temperatures accelerate gas diffusion and improve sensitivity, but can also increase the desorption rate, potentially reducing sensitivity. Doped SnO\({}_{2}\), particularly with Pd or Pt, can achieve improved sensitivity at lower temperatures, which is advantageous for applications requiring low-power operation, such as wearable devices or portable gas monitors [83; 84]. Light activation has also been explored to enhance the low-temperature performance of SnO\({}_{2}\) sensors, especially in applications requiring energy-efficient operation. By using UV light, for example, it is possible to activate the SnO\({}_{2}\) surface at lower temperatures, as UV photons generate electron-hole pairs that facilitate gas interactions without the need for thermal excitation.