How dimensionality and size of metal oxide nanostructures affect their gas sensing performance?

Nanostructured metal oxides are ideal candidate for the development of conductometric gas sensors, mainly for their large surface-to-volume ratio which leads to enhanced gas response. As the sensing part of the material is its surface, a thinner nanowire should show higher gas response. Such behavior, 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 different metal oxide nanostructures, and their use in gas sensors. Tin oxide single nanowires with different diameters, used as NO2 sensors, while zinc oxide hexagons are studied as 2D nanostructures. The effect of working temperature, gas concentration, nanostructure dimensionality and size on the sensing performance is investigated. As expected from geometrical approximations, 2D nanostructures show worse sensor response, but a better limit of detection: their larger cross section lowers their sensor response, but increases the intrinsic conductance, lowering the limit of detection. The single nanowire based sensors performance (sensor response, response and recovery times) are investigated in detail as a function of the nanowire diameter. This study verifies the depletion layer model which is used to explain the sensing mechanism of monocrystalline metal oxide nanowires.