## 2 Mechanisms Suitable for Gas Sensing of Nanostructured Materials  
Chemiresistive gas sensors operate based on the principle that exposure to target gas molecules induces changes in the electrical resistance of the sensing material, typically a semiconductor or metal oxide [35]. This process is influenced by the adsorption and desorption of gas molecules on the sensor surface, which modulates the concentration, mobility, and spatial distribution of charge carriers within the material. The mechanism can be broadly understood as a surface-mediated process, where interactions between the sensor surface and gas molecules directly affect the electron or hole density, causing measurable variations in resistivity (Figures 1 and 2) [36; 37]. When the sensing material is exposed to a gas, the molecules are adsorbed onto the surface, often in a manner governed by the Langmuir adsorption isotherm, which describes how the surface coverage of adsorbed molecules depends on the partial pressure of the gas and the affinity of the adsorbate to the surface. In this context, the Langmuir isotherm provides insight into the equilibrium adsorption state, which is particularly relevant for understanding sensor response at varying concentrationsThe adsorption process can lead to charge transfer between the adsorbed molecules and the sensing material, resulting in a depletion or accumulation of charge carriers in the sensor's space-charge region. For instance, in n-type metal oxides like ZnO and SnO\({}_{2}\), reducing gases (such as CO) tend to donate electrons, increasing the charge carrier concentration, while oxidizing gases (such as NO\({}_{2}\)) withdraw electrons, creating a depletion layer that reduces conductivity (Figure 3). This balance between adsorption and desorption is essential for establishing a steady-state response, as well as for sensor recovery. The space-charge layer model further explains the impact of gas adsorption on the electronic structure of the sensing material. In this model, adsorbed molecules influence the distribution of charge carriers by altering the surface potential, which extends into the bulk in the form of a space-charge region. This phenomenon is particularly pronounced in materials with high surface-to-volume ratios, such as nanostructured oxides, where surface effects strongly dominate over bulk properties. The width of the space-charge layer and the position of the Fermi level shift dynamically in response to surface interactions, modulated by factors such as gas concentration, temperature, and humidity. As a result, the sensor's resistive behavior can vary significantly, influenced by the extent of band bending, and carrier modulation in the space-charge layer is another critical parameter that affects gas sensor performance. Elevated temperatures can enhance the adsorption and desorption kinetics, allowing sensors to achieve faster response and recovery times. However, excessive temperatures can also lead to an unwanted desorption of gas molecules, reducing sensitivity. For example, metal oxide-based chemiresistive sensors typically operate in the range of 200-400 \({}^{\circ}\)C, where sufficient thermal energy allows for optimal reaction kinetics without compromising stability [37]. Furthermore, temperature affects the energy barrier at the surface, altering the likelihood of charge transfer interactions with adsorbed molecules. The crystalline structure, morphology, and dopant types play an essential role in dictating sensor response and selectivity. Nanostructures, such as nanowires, nanosheets, and  
Figure 1: Schematic diagram of the sensing mechanism of n-type semiconducting metal oxide nanostructures for reducing gas (reproduced from [36], MDPI, 2020). nanoparticles, with high surface-to-volume ratios provide more active sites for adsorption, potentially enhancing sensitivity. Additionally, doping or compositional modifications can tailor the band structure to favor interaction with specific gas molecules, improving selectivity.