Mid Sweden University

This thesis explores the development and performance of semiconducting metal oxide (SMOX)-based gas sensors prepared by different methods, specifically targeting hazardous gases like hydrogen sulfide (H₂S) and in some cases methyl mercaptan (CH₃SH). These gases pose significant risks to human health and the environment, even at low concentrations. Therefore, developing sensitive, reliable, and cost-effective sensors is crucial for industrial safety, environmental monitoring, and healthcare. 

The compact SnO₂ layers prepared by Ultrasonic Spray Pyrolysis (USP) demonstrated effective H₂S detection at an optimal operating temperature of 450°C. This method resulted in uniform, dense layers with high crystallinity and minimal impurities, ensuring a reliable sensor response. However, the sensor’s selectivity was limited by the presence of other interference gases, especially in humid environments. To enhance performance, ZnO/SnO₂ heterostructures were incorporated, fabricated by controlling precursor ratios during the USP process. These heterostructures showed improved sensor response and selectivity for detecting H₂S compared to pure SnO₂. 

The Flame Spray Pyrolysis (FSP) method successfully produced porous SnO₂ structures, which excelled in detecting low concentrations of H₂S and CH₃SH at an optimal operating temperature of 250°C. The highly porous morphology increased the surface area, yielding a remarkable gas response down to 20 ppb and enabling efficient gas diffusion, making it suitable for detecting sub-ppb levels of toxic gases.

Additionally, screen printing was employed to create ZnO/SnO₂ porous heterostructure sensors. The sensor with a 3:4 SnO₂/ZnO ratio achieved a limit of detection (LOD) of 140 ppt at an optimal operating temperature of 325°C, outperforming single-component sensors and demonstrating the effectiveness of the simple screen-printing method in producing scalable, high-performance gas sensors.

In summary, this thesis underscores the significance of material design and fabrication techniques in enhancing the performance of SMOX-based gas sensors. The findings highlight that utilizing porous structures and heterojunction engineering offers substantial advantages in sensor response and selectivity, making these sensors well-suited for real-world applications in hazardous gas detection.