Metal Oxide Nanowires for Gas Sensing: because Size Matters !

Metal-oxide based gas sensors, whose response rely on the change of their electrical resistance upon the interaction with the target gas [1], have been widely studied and commercialized. The use of nanostructured materials is useful mainly for its huge surface-to-volume ratio, which leads to improved gas response. The use of single nanowire (NW) devices allows to investigate their gas sensing properties without any average effect on different nanowires. It also avoids the potential effect of grain boundaries, allowing to focus on the properties of the nanostructure under investigation. Because of their tiny diameter, comparable to the Debye length, monocrystalline nanowires' electronic properties are strongly influenced by the processes that take place at their surface. This means that a smaller diameter leads to higher gas response. Even though the space charge model has been confirmed in many works [2], there are still few reports on the diameter-dependence of nanowires gas response. In the present work single nanowires with different diameter size are characterized as nitrogen dioxide sensors. Gas sensor response of the devices is investigated as a function of working temperature, gas concentration and nanowire diameter. All the devices present an optimum working temperature around 250-350°C. Their response is linear towards gas concentration up to 500 parts per million, then starts to saturate, reaching a maximum response of 18.9 to 1000 ppm NO2 for the best device (thinnest nanowire). Limit of detection is 7.2 ppm for the same device, while response time and recovery are very fast (3 seconds each @ 400°C) and the percentage recovery degree is about 2%. These working parameters make single nanowires-based devices ideal candidates for real time gas sensors. Gas sensing properties (response, response time and recovery time) are studied as a function of the nanowires diameter, in order to investigate the depletion layer model which is 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.