Oxide semiconductors such as tin dioxide (SnO₂) and tungsten trioxide (WO₃) have emerged as critical materials for gas sensing applications due to their tunable electronic properties, chemical stability, and sensitivity to various gaseous species. Their ability to detect toxic and environmental gases like nitrogen dioxide (NO₂) and carbon dioxide (CO₂) makes them indispensable in industrial safety, environmental monitoring, and healthcare. The sensing mechanism relies on surface interactions between the semiconductor and gas molecules, governed by adsorption-desorption kinetics, selectivity mechanisms, and nanostructuring effects that enhance performance.

**Surface Adsorption-Desorption Kinetics**
The gas sensing process begins with the adsorption of target molecules onto the semiconductor surface, altering its electrical resistance. In n-type oxide semiconductors like SnO₂, oxygen species (O₂⁻, O⁻, O²⁻) pre-adsorbed on the surface play a crucial role. At elevated operating temperatures (typically 200–400°C), atmospheric oxygen captures electrons from the conduction band, forming ionic species and creating a depletion layer. When reducing gases like CO interact with these oxygen ions, they release electrons back into the conduction band, decreasing resistance. Conversely, oxidizing gases like NO₂ extract additional electrons, further increasing resistance.

The adsorption-desorption equilibrium is temperature-dependent. For SnO₂, optimal NO₂ sensing occurs near 200°C, where adsorption dominates, while higher temperatures favor desorption, reducing sensitivity. WO₃ exhibits similar behavior but often operates at slightly lower temperatures for NO₂ detection due to its narrower bandgap. The kinetics follow the Langmuir isotherm model at low gas concentrations, where surface coverage is proportional to gas partial pressure. At higher concentrations, interactions between adsorbed molecules complicate the kinetics, sometimes leading to Freundlich or Temkin isotherm behavior.

**Selectivity Mechanisms**
Selectivity remains a challenge in oxide semiconductor gas sensors due to cross-sensitivity among multiple gases. Strategies to enhance selectivity include doping, surface functionalization, and operating temperature modulation.

Doping with catalytic metals (Pt, Pd, Au) or heteroatoms (Sb, In) modifies surface reactivity. For example, Pt-doped SnO₂ shows higher selectivity to CO due to Pt’s catalytic oxidation of CO to CO₂, which alters the electron exchange process. WO₃ doped with Au nanoparticles selectively interacts with NO₂ by promoting charge transfer at lower temperatures.

Surface functionalization with organic or inorganic layers can block interfering gases. A SnO₂ sensor functionalized with a thin SiO₂ layer may suppress responses to humidity while maintaining NO₂ sensitivity. Similarly, WO₃ coated with a porous polymer can filter larger molecules, allowing selective diffusion of smaller gases like CO₂.

Temperature modulation exploits differences in the activation energies of gas-surface reactions. SnO₂ sensors cycled between 150°C and 300°C can distinguish NO₂ (responsive at lower temperatures) from ethanol (responsive at higher temperatures). WO₃ sensors use similar approaches to differentiate between NO₂ and NH₃.

**Nanostructuring Effects**
Nanostructuring enhances gas sensing by increasing surface-to-volume ratios and introducing quantum confinement effects. Common morphologies include nanoparticles, nanowires, nanofibers, and hierarchical structures.

SnO₂ nanoparticles (5–20 nm) exhibit high sensitivity to NO₂ due to their high surface area and abundant oxygen vacancies. Smaller particles (<10 nm) show improved response but may suffer from instability due to excessive surface energy. WO₃ nanowires provide directional electron transport, reducing grain boundary scattering and improving response times. Their one-dimensional structure also facilitates charge carrier separation, enhancing sensitivity to CO₂.

Hierarchical structures, such as SnO₂ nanoflowers or WO₃ nanospheres, combine high surface area with porous networks for rapid gas diffusion. These structures often outperform dense films in detecting low-concentration gases (1–10 ppm). For example, mesoporous WO₃ achieves sub-ppm NO₂ detection due to its interconnected pore structure, which maximizes gas accessibility to active sites.

**Specific Gas Interactions**
NO₂ detection relies on strong electron withdrawal from the conduction band of n-type oxides. SnO₂ exhibits a linear response to NO₂ in the 0.5–10 ppm range, with recovery times influenced by temperature and humidity. WO₃’s higher baseline resistance makes it more sensitive to trace NO₂ (<0.5 ppm), but recovery requires longer durations or UV illumination to desorb strongly bound NO₂ species.

CO₂ sensing is more complex due to its weak electron affinity. Undoped SnO₂ shows minimal response, but CuO-SnO₂ heterostructures catalyze CO₂ adsorption via carbonate formation, altering resistance. WO₃-based sensors rely on oxygen vacancy interactions, where CO₂ dissociates into CO and O⁻, modulating conductivity. However, achieving selectivity over humidity and other interfering gases remains challenging.

**Conclusion**
Oxide semiconductors like SnO₂ and WO₃ offer versatile platforms for gas sensing, with performance dictated by surface chemistry, nanostructuring, and material modifications. Understanding adsorption-desorption kinetics enables optimization of operating conditions, while doping and morphology control address selectivity challenges. Advances in nanostructuring continue to push detection limits, particularly for critical gases like NO₂ and CO₂. Future work may focus on defect engineering and hybrid materials to further improve sensitivity and stability under real-world conditions.