Zinc oxide (ZnO) is a versatile semiconductor with a wide bandgap of approximately 3.37 eV, making it highly suitable for gas sensing applications. Its sensitivity to gases such as nitrogen dioxide (NO2), carbon monoxide (CO), and hydrogen (H2) stems from its surface chemistry, electronic properties, and the ability to tailor its nanostructure for enhanced performance. The interaction between these gases and ZnO surfaces involves adsorption, charge transfer, and subsequent changes in electrical resistance, which form the basis for detection.

The surface of ZnO plays a critical role in gas sensing. When exposed to air, oxygen molecules adsorb onto the ZnO surface, capturing free electrons from the conduction band and forming oxygen ions (O2−, O−, or O2−, depending on temperature). This creates a depletion layer near the surface, increasing the material’s resistance. Upon exposure to reducing or oxidizing gases, the surface reactions alter the depletion layer, leading to measurable resistance changes.

For NO2, an oxidizing gas, the interaction with ZnO involves the adsorption of NO2 molecules, which further extract electrons from the conduction band. This deepens the depletion layer and increases resistance. Studies show that ZnO sensors exhibit high sensitivity to NO2 at moderate temperatures (200–300°C), with response values (defined as Rgas/Rair or Rair/Rgas) reaching up to 100 for 10 ppm NO2 when using nanostructured ZnO. The selectivity toward NO2 can be improved by doping ZnO with elements like indium or tin, which modify surface reactivity.

In contrast, CO and H2 are reducing gases. When CO interacts with pre-adsorbed oxygen on ZnO, it reacts to form CO2, releasing trapped electrons back into the conduction band and reducing resistance. The sensing mechanism for H2 involves dissociation on the ZnO surface, where hydrogen atoms react with oxygen ions to form water vapor, also releasing electrons. The response to these gases is highly dependent on operating temperature, with optimal performance typically observed between 300–400°C for CO and 150–250°C for H2.

Selectivity remains a challenge for ZnO-based gas sensors, as similar resistance changes can occur for different gases. Strategies to enhance selectivity include doping, surface functionalization, and the use of heterostructures. For example, palladium (Pd) doping improves H2 sensitivity due to Pd’s catalytic effect on hydrogen dissociation, while copper (Cu) doping enhances CO response by promoting oxidation reactions.

Nanostructuring ZnO significantly enhances gas sensing performance by increasing surface area and providing more active sites for gas adsorption. Common nanostructures include nanowires, nanorods, and porous thin films. Nanowires, with their high surface-to-volume ratio and single-crystalline nature, exhibit fast response and recovery times. For instance, ZnO nanowire sensors demonstrate response times of less than 10 seconds for NO2 and recovery times under 30 seconds at 250°C. Porous ZnO films, fabricated by techniques like sol-gel or sputtering, further improve sensitivity due to their interconnected pore networks facilitating gas diffusion.

The table below summarizes key performance metrics for ZnO-based gas sensors:

Gas Optimal Temp (°C) Response (Rgas/Rair or Rair/Rgas) Response Time (s)
NO2 200–300 Up to 100 (10 ppm) <10
CO 300–400 5–20 (100 ppm) 20–60
H2 150–250 10–50 (1000 ppm) 10–30

Doping and composite formation further refine sensor performance. For example, ZnO-SnO2 heterostructures exhibit improved NO2 selectivity due to the formation of electron depletion layers at the interface. Similarly, Au-functionalized ZnO shows enhanced CO sensitivity owing to spillover effects.

Despite these advances, challenges such as long-term stability and humidity interference persist. However, ongoing research in surface engineering and advanced nanostructuring continues to push the boundaries of ZnO-based gas sensors, making them viable for industrial safety, environmental monitoring, and automotive applications. The future lies in optimizing material compositions and device architectures to achieve reliable, selective, and sensitive detection in real-world conditions.