Gas sensing performance of nanostructured metal oxides depending on their size and dimensionality

Semiconducting gas sensors are nowadays being widely investigated. Nanostructured metal oxides are better candidate than their thick or thin film counterpart, mainly for their huge surface-to-volume ratio which gives higher number of adsorption sites and enhances their gas response. Their tiny size and high surface-to-volume ratio make their electrical properties sensitive even to few gas molecules. The use of single nanostructures as architecture of sensing devices allows to study their gas sensing properties in better detail without averaging effects on several different structures and avoiding grain boundaries. As the sensing part of the material is the surface, a thinner nanostructure should show higher gas response. Such behaviour, intuitively following the space charge model, has been confirmed in several works, but the reports on the diameter-dependence of nanowires gas response are still few. We will present the growth and characterization of sensors based on tin oxide single nanowires with different diameters. The effects of working temperature, gas concentration and particularly nanowire diameter on the sensing devices performance are investigated [1]. An optimal working temperature of 250-350°C has been found for all the devices. The sensor response as a function of gas concentration is linear for all the devices up to 500 parts per million, then starts to saturate. Limit of detection (the lowest concentration detectable) is few parts per million or lower. All the sensors demonstrate rapid detection, with very short response and recovery times (around 3 seconds each @ 400°C). The stability of the devices is good, with a percentage recovery degree around 2%. These sensing performance make single nanowires-based devices ideal candidates for real time gas sensors. The single nanowire based sensors characteristics (sensor response, response time and recovery time) are investigated as a function of the nanowire diameter. This study verifies the depletion layer model, commonly used to explain the sensing mechanism of monocrystalline metal oxide nanostructures. A good confirmation of the model is found, with a depletion layer depth of around 14 nanometres. Analogous performance investigation and geometrical approximation are used to examine hydrogen and liquid petroleum gas (LPG) sensing properties of multiple one- and two-dimensional ZnO nanostructures. As expected, their larger cross section reduces their sensor response, but increases the intrinsic conductance, thus lowering the limit of detection of the sensing devices [2]. Such findings demonstrate that there is no "best nanostructure" to fabricate a gas sensor, but one should plan carefully the sensing device focusing on his specific requirements.