How Size and Shape of Metal Oxide Nanostructures affect their gas sensing properties

Resistive metal oxide 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 enhanced gas response. The use of single nanostructures in the fabrication of sensing devices allows to study their gas sensing properties in better detail, without average effects on different structures. Furthermore, single monocrystalline nanowires do not present grain boundaries, allowing thus to explore the properties of the nanostructure itself. Thanks to their tiny size and high surface-to-volume ratio, a few gas molecules are sufficient to greatly affect the electrical properties of the sensing element. As the sensing part of the material is the surface, a thinner nanowire should show higher gas response. Such behaviour, intuitively following the space charge model, has been confirmed in many works, but still few are the reports on the diameter-dependence of nanowires gas response. This work presents the growth and characterization of tin oxide single nanowires with different diameters, used as nitrogen dioxide sensors. The effect of working temperature, gas concentration and nanowire diameter on the sensing devices response is investigated. An optimum operating temperature of 250-350°C has been found for all the devices. The sensor response toward 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 as low as 7.2 ppm. 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 of about 2%. These sensing properties make single nanowires-based devices ideal candidates for real time gas sensors. The single nanowire based sensors performance (sensor response, response time and recovery time) are investigated as a function of the nanowire diameter. This study verifies the depletion layer model which is commonly used to explain the sensing mechanism of monocrystalline metal oxide nanowires. A good confirmation of such model is found, with a depletion layer depth of around 14 nanometres. Similar performance investigation and geometrical approximation are used to examine the hydrogen sensing properties of multiple two-dimensional ZnO nanostructures. As expected, their larger cross section lowers their sensor response, but increases the intrinsic conductance, thus lowering the limit of detection of the sensing devices.