Role of the active layer thickness on the sensitivity of WO3 gas sensors

Sensitive materials in gas sensors are often polycrystalline semiconducting oxides such as WO3, SnO2, CuO or ZnO. They are most often composed of nanometric grains. They can be deposited either as thin or thick films. The film thickness plays an important role in the response stability and sensitivity of sensors. It is now well accepted that the relationship between the surface and volume of the sensitive layer plays a major role in the efficiency of detection. Many experimental and theoretical works were reported in explaining the experimental sensitivity vs. thickness relationships reported for the gas sensors prepared by different fabrication techniques. In addition, significant changes can be expected by adding catalytic nanograins in small quantities on the surface of the sensitive layers. For example, cobalt nanograins deposited on the surface of WO3 sensors produce an important change in the WO3 conductance. Indeed, cobalt changes the conduction type of the sensors from n- to p-type. This paper describes the effect of reducing the size of the sensors and nanostructured sensitive materials on the sensor response.