ZnO-Based nanostructures for gas sensing applications.

Metal oxide chemical sensors based on nanomaterials are gaining popularity and finding extensive use in automotive industries, process control and environmental monitoring. ZnO, a semiconducting metal oxide has attracted great interest over the years for its sensitivity to a variety of gases. Nanostructured sensing materials, such as thin films, nanowires, tetrapods, nanoflackes offer an inherently high surface area, reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this thesis, ZnO nanostructures have been tested as chemical sensors and a detailed study on the effect of different process parameters such as grain size, roughness, surface-to-volume ratio, depletion layer, temperature, gas concentration and material properties on gas sensitivity is presented. Initially, ZnO nanodevices were prepared with a variety of techniques, such as RF sputtering, electrodeposition, hydrothermal growth, chemical vapour deposition, thermal evaporation and controlled oxidation. The structural characterization of the nanodevices has been done by a FEI QUANTA 3D dual beam SEM/FIB machine and by a Dimension 3100 Atomic Force Microscope (AFM) (Digital Instruments) in tapping mode. X-ray diffraction (XRD) spectra were recorded on an AXS D8 diffractometer (Bruker) with a Cu Kα X-ray tube. The gas sensor substrate based on alumina consisted of Pt grid of 50nm thickness and golden contacts of 200nm thickness creating an alumina patterned substrate. The sensor deposition area was coated with ZnO nanostructures to form the sensing material. Sensing measurements are performed in a closed steel chamber where air and tested gases have been inserted. ZnO based nanostructures’ response was measured in different concentrations of Ethanol, CO and NO2. Initially the role of grain size and roughness has been investigated in several thin film based nanodevices. Grain size is decreasing with increasing RF sputtering power and increasing by post-annealing treatment. Roughness instead is increasing with both the increasing of RF sputtering power and post-annealing treatment. High response was observed for those films with smaller grain size, while the roughness seems to influence very little the response of the sensor. For all thin films, the response is increasing with ii temperature and gas concentration. Recovery time and response time seem to follow a non-linear behavior with the above parameters. Extended studies have investigated the role of surface-to-volume ratio and depletion layer in the sensing performance. It has been observed that the increase of surface-to volume ratio has an important effect on the sensitivity, increasing, more than twice the response of such a device in respect to another that is based on a ZnO thin film. On the other hand, the dimensions of a nanostructure play the most crucial role in the depletion layer width in respect to the sensing properties. The diameter of a nanowire should be comparable with its depletion layer width. In this case the depletion layer has strong effect, which makes the sensor’s response depend also on it. The sensing properties of all fabricated structures have been compared to find the optimum sensor that could face the demands of automotive industries. All fabricated structures have been compared in different configurations to find out which one presents the best sensing performance. To that direction sensors based on thin film, tetrapods, nanowires, nanoflackes have been tested in same environmental conditions. Advanced nanostructures present better sensing properties. Sensing response of every advanced nanostructure presents more than double sensing response than every thin film-based nanostructure. Comparing the advanced nanostructures with each other, tetrapods based sensor has higher response and recovery time, while the sensitivity is slightly higher for the nanowires-based sensor. Theoretical studies have been performed by ab-initio simulations in NO2 environment. They have revealed that the sensing mechanism is driven almost exclusively by competitive adsorption between NO2 and atmospheric oxygen mediated by temperature change. The influence of the NO2 on the electronic properties of ZnO has been assessed and it is in accordance with the experiments. Our future work is the investigation of other materials for the development of sensing nanodevices targeting to develop more sensitive nanosensors in the same or lower cost. Additionally, the investigation of other growth techniques that could develop more complicated structures in low cost is another point of interest for the future.